Preface
The American Institute of Chemical Engineers (AIChE) has a 30-year history
of involvement with process safety and loss control for chemical and petrochemical
plants. Through its strong ties with process designers, builders, and operators,
safety professionals and academia, the AIChE has enhanced communication and
fostered improvement in the high safety standards of the industry. Its publications
and symposia have become an information resource for the chemical engineering
profession on the causes of incidents and means of prevention.
The Center for Chemical Process Safety (CCPS), a directorate of AIChE, was
established in 1985 to intensify development and dissemination of the latest scientific
and engineering practices for prevention and mitigation of catastrophic incidents
involving hazardous materials; advance the state of the art of engineering practices
through research; and develop and encourage the use of undergraduate education
curricula that will improve the safety knowledge and consciousness of engineers.
Over 55 corporations from all segments of the chemical process industries
(CPI) support the Center in its work. They help fund the Center; they select CCPS
projects relevant to improved process safety; and they furnish the professionals who
give the Center's works technical direction and substance.
Since its founding, CCPS has cosponsored several international, technical symposia and has published seven volumes in its Guidelines series, a technical workbook,
and the proceedings of three meetings. Some of these publications have become
the core of new courses in AIChE's Continuing Education series for engineers.
CCPS research projects now in progress will yield new data for improved process
safety.
One of the first projects undertaken by CCPS was directed to undergraduate
engineering education. Its primary objective was to increase the student's awareness
of, interest in, and knowledge of safety, health, and loss prevention concepts in
chemical engineering, and to increase recognition of the engineer's responsibilities
in these areas. This project was also intended to help college and university engineering
schools meet accreditation requirements published in Criteriafor Accrediting Programs
in Engineering in the United States, Effective for Evaluations during the 19891990 Academic Year by the Accreditation Board for Engineering and Technology
(ABET).
Studies done by the Safety and Health Division of AIChE, by the CCPS
Undergraduate Education Subcommittee, and by ABET revealed that the most
effective way to introduce safety, health, and loss prevention concepts to undergraduate
engineering students was through their integration into existing courses. It was
found that adding a safety and health course as a core requirement to an already
crowded curriculum was next to impossible. As an elective course, ABET criteria
could not be satisfied. Consequently, the Subcommittee designed this material for
use in many existing courses leading to chemical and other engineering degrees.
The subcommittee identified 34 areas that contained important safety, health,
and loss prevention concepts. The concepts were divided into problematic and
nonproblematic categories. The former were developed into problems to illustrate
fundamentals of health and safety and that could be used in teaching traditional
engineering subjects, such as thermodynamics, fluid mechanics, or heat transfer.
Wherever possible, nonproblematic concepts were introduced in the problem description
or background material provided the student and instructor.
Ninety problems have been developed, covering many of the identified conceptual
areas for several basic engineering subjects. The material is presented in two books:
the book for students contains background material for each problem and the problem
itself; the Instructor's Guide has the student's material as well as descriptive material
for each problem about the concepts involved and the problem's solution.
Many of the problems were used and critiqued by engineering faculty in 20
colleges and universities nationwide. As a result of this testing and problem preparation
by some of the Chemical Engineering faculty at the University of Arkansas, the
instructional aids presented in these books can be conveniently and easily incorporated
into current engineering courses at all levels of the undergraduate engineering
curriculum.
The problems teach safety, health, and loss prevention'as an integral part of
many engineering solutions and can provide new engineers with insights to industrial
situations they are likely to encounter. This material may also be used as a reference
for graduates as they begin industrial careers and by industrial in-house courses for
new engineering employees. They further demonstrate that safety and health issues
can be handled by basic engineering principles and logic, and are not foreign to
basic engineering practices. Finally, through use of this material, we hope to instill
increased recognition and acceptance of the professional and ethical responsibilities
which engineers must have to provide safe chemical plants, processes and products.
Acknowledgments
'
The American Institute of Chemical Engineers (AIChE) wishes to thank the
Center for Chemical Process Safety (CCPS) and those involved in its operation,
including its many sponsors, whose funding made this project possible; the members
of its Technical Steering Committee, who conceived of and supported this project;
and the members of its Undergraduate Education Subcommittee for their dedicated
efforts, technical contributions, and the guidance necessary for the preparation of
this work.
The Chairman of the Undergraduate Education Subcommittee was F. Owen
Kubias, The Glidden Company. The Subcommittee members were Roger W. Bohl,
Dow Chemical U.S.A.; Guy Colonna, National Fire Protection Association; Daniel
A. Crowl, Wayne State University; John Davenport, Industrial Risk Insurers; Dr.
James A. Gideon, National Institute for Occupational Safety and Health; Stanley
S. Grossel, Hoffmann-LaRoche, Inc.; Dr. Walter B. Howard, consultant; Dr. Joseph
J. Levitzky, Yale University; J. F. Louvar, BASF Corporation; Gene I. Matsumoto,
S. C. Johnson & Son, Inc.; Robert Nelson, Industrial Risk Insurers; John Noronha,
Eastman Kodak Company; Gary A. Page, American Cyanamid Company; Robert
M. Rosen, BASF Corporation; Jerry Schroy, Monsanto Chemical Company; Richard
F. Schwab, Allied-Signal, Inc.; Dr. Klaus Timmerhaus, University of Colorado.
Thomas W. Carmody, Russell G. Hill, Lester H. Wittenberg, and Ray E. Witter
of the Center for Chemical Process Safety were responsible for the overall administration
and coordination of this project.
AIChE thanks Dr. J. Reed Welker and Dr. Charles Springer, faculty in the
Chemical Engineering Department, University of Arkansas, Fayetteville, Arkansas,
for using their expertise to provide advice and to prepare the problems, solutions,
and background information for these books. The University of Arkansas helped
support this project from its inception to its completion, and this support is deeply
appreciated.
AIChE also gratefully acknowledges the contributions of Murray Underwood,
Washington University, St. Louis, and J. Arnold Glass, retired, in helping to
develop the basic safety concepts and associated topics; Prenticti-Hall for permission
to use problems from their book, Process Safety: Fundamentals with Applications,
written by D. A. Crowl and J. F. Louvar; and Eastman Kodak Company for
permission to use some problems from their in-house training program.
The assistance and philosophic support of the project's goals, given by the
National Institute for Occupational Safety and Health and the U.S. Environmental
Protection Agency, were of considerable help and are gratefully acknowledged.
The members of the CCPS Undergraduate Education Subcommittee wish to
thank the faculty members of many colleges and universities for testing problems
in their classrooms and providing critiques to improve the material for future students.
Finally, the subcommittee members thank their employers for providing the time
to participate in this project.
UNDERGRADUATE EDUCATION
PROBLEM MATRIX
ENGINEERING COURSE
Problematic
Health and
Safety Concept
I . Properties
of Materials
Introl
Fund.
lomentun
transfer
Heat
Transfer
Mass
rransfer
68
2. Process
Design
3. Explosions
4. Toxic Exposure
Control.
Personal
Protective
Equipment
5. Process Control
Interlocks.
Alms
6 . Toxicology and
Industrial
Hygiene
7. Vapor
Releases
8. lnening and
Purging
9. Storing,
Handling, and
Transpon
10. Fire
Protection
Systems
11. Haz. Waste
Generation and
Disposal
12. Rupture Discs
and Relief
Valves
13. Process Hazard
Reviews
14. Static
Electricity
IS. Physical
Hazards
77
Glossary
This glossary defines many of the terms used on Material Safety Data Sheets (MSDS).
It also explains some of the significance of the terms related to safety, health, and loss
prevention. The glossary can provide substantial assistance in understanding terms commonly
used by safety and health professionals.
An adverse effect on a human or
animal body, with severe symptoms developing
rapidly and coming quickly to a crisis. See
also, "Chronic. "
Importance: How much and how long one is
exposed to a chemical is the critical factor to
how adverse the health effects will be.
ACUTE TOXICITY The adverse (acute) effects resulting from a single dose or exposure to a
substance.
Importance: Ordinarily used to denote effects
in experimental animals.
ACGIH American Conference of Governmental Industrial Hygienists; an organization of professional personnel in governmental agencies or
educational institutions engaged in occupational
safety and health programs.
Importance: ACGIH develops and publishes
recommended occupational exposure limits (see
TLV) for hundreds of chemical substances and
physical agents.
APPEARANCE AND ODOR The physical properties
of a chemical, such as color and smell.
Importance: Knowing what chemicals look
and smell l i e allows an employee to recognized
unsafe working conditions.
ASPHYXIANT A vapor or gas which can cause unconsciousness or death by suffocation (lack of
oxygen). Most simple asphyxiants are harmful
to the body only when they become so concentrated that they reduce oxygen in the air (normally about 21 percent) to dangerous levels
(19.5 percent or lower).
Importance: Asphyxiation is one of the principal potential hazards of working in confined
spaces.
BOILING POINT The temperature at which a liquid
changes to a vapor state, at a given pressure;
usually expressed in degrees fahrenheit at sea
level pressure (760 mmHg, or one atmosphere).
For mixtures, the initial boiling point or the
boiling range may be given.
Importance: The lower the degree for the boiling point, the faster a liquid evaporates, in-
ACUTE EFFECT
creasing the amount of vapor present at room
temperature for both health and fire exposures.
"c:'OR CEILING The letter " C or the word "ceiling" on the TLV or PEL shows the highest
airborne concentration of a specijic chemical
that is allowed in the workplace. This concentration should never be exceeded, evenfor short
periods of time. See also, "PEL" and "TLV. "
Importance: Chemicals that react rapidly in
the body, causing ill health effects carry this
value.
CARCINOGEN A cancer-causing material.
Importance: If a substance is known to be
cancer causing, a potential health hazard exists
and special protection and precaution sections
should be checked on the MSDS.
C.A.S. NUMBER Chemical Abstracts Service Number.
Importance: C.A.S. Numbers are used on
MSDS's to identify specific chemicals.
cc Cubic centimeter; a volume measurement in the
metric system, equal in capacity to 1 milliliter
(ml). One quart is about 946 cubic centimeters.
CHEMICAL FAMILY A group of single elements or
compounds with a common general name. Example: acetone, methyl ethyl ketone, and methyl
isobutyl ketone are of the "ketone" family; acrolein, furfural, and acetaldehyde are of the
"aldehyde" family.
Importance: Elements or compounds within a
chemical family generally have similar physical
and chemical characteristics.
CHEMTREC Chemical Transportation Emergency
Center; a national center established by the
Chemical Manufacturers Association in Washington, D.C. in 1970, to relay pertinent emergency information concerning specijic chemicals.
Importance: Chemtrec has an emergency 24hour toll free telephone number (800-4249300).
CHRONIC EFFECT An adverse effect on a human
or animal body, with symptoms which develop
slowly over a long period of time. Also, see
"Acute."
Importance: The length of time that a worker
is exposed is the critical factor. Long periods
of time pass, with repeated exposure to a chemical, before any ill effects are detected in a
worker.
CHRONIC TOXlClTY Adverse (chronic) effects resulting from repeated doses of or exposures to
a substance over a relatively prolongedperiod
of time.
Importance: Ordinarily used to denote effects
in experimental animals.
co Carbon monoxide, a colorless, odorless,
flammable and very toxic gas produced by the
incomplete combustion of carbon; also a byproduct of many chemical processes.
CO, Carbon dioxide, a heavy, colorless gas, produced by the combustion and decomposition of
organic substances and as a by-product of many
chemical processes. C02 will not burn and is
relatively non-toxic (although high concentrations, especially in conjined spaces, can create
hazardous oxygen-dejicient environments.)
Importance: CO and C02 are often listed on
MSDS's as hazardous decomposition products.
COMBUSTIBLE A term usedto classify certain liquids
that will burn, on the basis offlashpoints. Both
the National Fire Protection Association (NFPAJ
and the Department of Transportatton (DOT)
define "combustible liquids" as having a flash
point of 100°F (37'8°C) or higher. See also,
"Flammable."
Importance: Combustible liquid vapors do not
ignite as easily as flammable liquids; however,
combustible vapors can be ignited when heated,
and must be handled with caution. Class I1
liquids have flash points at or above 10O0F,but
below 140°F. Class 111 liquids are subdivided
into two subclasses:
Class IIIA: Those have flash points at or
above 140°F but below 200°F.
Class IIIB: Those having flash points at or
above 200°F.
CONCENTRATION The relative amount of a substance when combined or mixed with other substances. Examples: 2 pprn Xylene in air, or a
50 percent caustic solution.
Importance: The effects of overexposure depend on the concentration or dose of a hazardous
substance.
CORROSIVE AS dejined by DOT, a corrosive material is a liquid or solid that causes visible
destruction or irreversible changes in human
tissue at the site of contact on-in the case of
leakagefrom itspackaging-a liquid that has
a severe corrosion rate on sreel.
Importance: A corrosive material requires different personal protective equipment to prevent
adverse health effects.
Breakdown of a material or substance (by heat, chemical reaction, electrolysis,
decay, or other processes) into parts or elements
or simpler compounds.
Importance: Decomposition products often
present different hazards than the original material.
DERMAL Used on or applied to the skin.
Importance: Dermal exposure, as well as inhalation exposure, must be considered to prevent
adverse health effects.
DERMAL TOXICITY Adverse effects resulting from
skin exposure to a substance. Also referred to
as "Cutaneous toxicity. "
Importance: Ordinarily used to denote effects
in experimental animals.
EMERGENCY AND FIRST AID PROCEDURES Actions
that should be taken at the time of a chemical
exposure before trained medical personnel arrive.
Importance: These procedures may lessen the
severity of an injury or save a person's life if
done immediately following a chemical exposure.
EPA U.S.Environmental Protection Agency; Federal agency with environmentalprotection regulatory and enforcement authority.
Importance: EPA regulations must be met for
the disposal of hazardous materials, as well as
in spill situations.
EVAPORATION RATE A number showing how fast
a liquid will evaporate.
Importance: The higher the evaporation rate,
the greater the risk of vapors collecting in the
workplace. The evaporation rate can be useful
in evaluating the health and fire hazards of a
material.
FLAMMABILITY LIMITS The range of gas or vapor
amounts in air that will burn or explode f a
flame or other ignition source is present.
Importance: The range represents an unsafe
gas or vapor mixture with air that may ignite
or explode. Generally, the wider the range the
greater the fire potential. Also, see LEL, LFL,
UEL, UFL.
FLAMMABLE A "Flammable Liquid" is defined by
NFPA and DOT as a liquid with a jlash point
below 100°F (37.8OC).
Importance: Flammable liquids provide ignitable vapor at room temperatures and must
be handled with caution. Precautions such as
bonding and grounding must be taken. Flammable liquids are: Class I liquids and may be
subdivided as follows:
Class IA: Those having flash points below 73°F
and having a boiling point below
100°F.
Class TS: Those having flash points below 73°F
and having a boiling point at or above
100°F.
DECOMPOSITION
Class IC: Those having flash points at or above
73°F and below 100°F.
FLASH POINT The lowest temperature at which vapors above a liquid will ignite. There are several
flash point test methods, and flash points may
vary for the same material depending on the
method used. Consequently, so the test method
is indicated when thejash point is given (150'
PMCC, 200" TCC, etc.) A closed cup type test
is used mostfrequently for regulatory puvoses.
Flash point test methods:
Cleveland Open Cup (CC)
Pensky Martens Closed Cup (PMCC)
Setaflash Closed Tester (SETA)
Tag Closed Cup (TCC)
Tag Open Cup (TOC)
Importance: The lower the flash point temperature of a liquid, the greater the chance of
a fire hazard.
FORMULA The conventional scientijic designation
for a material (water is H20, sulfuric acid is
H2S04. Sulfur dioxide is SO2, etc.)
Importance: Chemical formulas identify specific materials.
GENERAL EXHAUST A system for exhausting air
containing contaminants from a general work
area. See also, "Local Exhaust. "
Importance: Adequate ventilation is necessary
to prevent adverse health effects from exposures
to hazardous materials and vapor accumulations
that can be a fire hazard.
g Gram; a metric unit of weight. One U.S. ounce
is about 28.4 grams.
g/kg Grams per kilogram; an expression of dose
used in oral and dermal toxicology testing to
indicate the grams of substance dosed per kilogram of animal body weight. See also, "kg."
Importance: A measure of the toxicity of a
substance.
HAZARDOUS MATERIAL In a broad sense, a hazardous material is any substance or mixture of
substances having properties capable of producing adverse effects on the health or safety
of a human being.
Importance: Knowing what a hazardous material is and what materials are hazardous is
important in preventing adverse health or safety
effects.
INCOMPATIBLE Materials which could cause dangerous reactions from direct contact with one
another are described as incompatible.
Importance: On a MSDS, incompatible materials are listed to prevent dangerous reactions
in the handling and storage of the material.
INGESTION The taking of a substance through the
mouth.
Importance: A route of exposure to a hazardous
material.
INGREDIENTS A listing of chemicals that are in a
mixture.
Importance: Knowing exactly what chemicals
and how much of each is in a mixture helps
you to understand the potential hazard a mixture
presents.
INHALATION The breathing in of a substance in
the form of a gas, vapor, fume, mist, or dust.
Importance: A route of exposure to a hazardous
material.
INWITOR A chemical which is added to another
substance to prevent an unwanted chemical
change from occurring.
Importance: Inhibitors are sometimes listed
on a MSDS, along with the expected time period
before the inhibitor is used up and will no longer
prevent unwanted chemical reactions.
IRRITANT A substance which, by contact in sufjicient concentration for a sufficient period of
time, will cause an inflammatory response or
reaction of the eye, skin, or respiratory system.
The contact may be a single exposure or multiple
explosures. Some primary irritants: chromic
acid, nitric acid, sodium hydroxide, calcium
chloride, amines, chlorinated hydrocarbons,
ketones, alcohols.
Importance: Knowing that a substance is an
irritant allows you to be aware of the signs and
symptoms of overexposure.
kg Kilogram; a metric unit of weight, about 2.2
U.S. pounds, See also, "glkg," "g," and "mg."
L Liter; a metric unit of capacity. A U.S. quart
is about 9/10 of a liter.
LC Lethal Concentration: A concentration of a
substance being tested which will kill a test
animal.
LC50 Lethal Concentration 50; The concentration
of a material in air which, on the basis of
laboratory tests, is expected to kill 50 percent
of a group of test animals when administered
as a single exposure (usually 1 or 4 hours).
The LC50 is expressed as parts of material per
million parts of air, by volume (ppm)for gases
and vapors, or as micrograms of material per
liter of air (pg/L) or milligrams of material
per cubic meter of air ( m g l d )for dusts and
mists, as well as for gases and vapors.
Importance: Both are measures of the toxicity
of a substance.
LD Lethal Dose; A concentration of a substance
being tested which will kill a test animal.
LDSO Lethal Dose 50; A single dose of a material
which on the basis of laboratory tests is expected
to kill 50% of a group of test animals. The
LD50 dose is usually expressed as milligrams
or grams of material per kilogram of animal
body weight (mglkg or glkg).
Importance: Both are measures of the toxicity
of a substance.
LEL OR LFL Lower Explosive Limit or Lower
Flammable Limit of a vapor or gas; the lowest
concentration (lowest percentage of the sub-
stance in air) that will produce a flash of j r e
when an ignition source (heat, arc, or&me)
is present. See also, "UEL."
Importance: At concentrations lower than the
LELILFL, the mixture is too "lean" to burn.
LOCAL EXHAUST A system for capturing and ex-,
hausting contaminantsfrom the air at the point
where the contaminants are produced (welding,
grinding, sanding, dispersion operations). See
also, "General Exhaust."
Importance: Adequate ventilation is necessary
to prevent adverse health effects from exposures
to hazardous materials and prevent vapor accumuIations that can be a fire hazard.
MATERIAL IDENTIFICATION The name of a chemical. It may be a trade name, chemical name
or any other name a chemical is known by. On
a MSDS this section also includes the name,
address, and emergency telephone number of
the distributing chemical company.
Importance: Proper identification of a chemical
allows an employee to get additional health
hazard and safety information.
m3 Cubic meter; a metric measure of volume, about
35.3 cubic feet or 1.3 cubic yards.
MELTING POINT The temperature at which a solid
substance changes to a liquid state. For mixtures, the melting range may be given.
Importance: The physical state of a substance
is critical in assessing its hazard potential, route
of exposure and method of control.
mg Milligram; a metric unit of weight. There are
1,000 milligrams in I gram (gl of a substance.
mgkg Milligrams per kilogram; an expression of
toxicological dose. See also, "$/kg."
Importance: A measure of the toxicity of a
substance.
mg/m3 Milligrams per cubic meter; a unit of measuring concentrations of dusts, gases, or mists
in air.
Importance: The effects of overexposure depend on the concentration or dose of a hazardous
substance.
mL Milliliter; a metric unit of capacity, equal in
volume to I cubic centimeter (cc), or about 11
16 of a cubic inch. There are 1,000 milliliters
in 1 liter (L).
mm ~g Millimeters (mm) of Mercury (Hg); a unit
of measurement for low pressures or partial
vacuums.
Importance: Vapor pressures are expressed in
mm Hg.
MUTAGEN A substance or agent capable of altering
the genetic material in a living cell.
Importance: If a substance is known to be a
mutagen, a potential health hazard exists, and
special protection and precaution sections should
be checked on the MSDS.
NOSH National Institute for Occupational Safety
and Health of the Public Health Service, U.S.
Department of Health and Human Services
(DHHS).
Importance: Federal agency which-among
other activities-tests and certifies respiratory
protective devices, recommends occupational
exposure limits for various substancesand assists
in occupational safety and health investigations
and research.
N*
Oxides of Nitrogen; undesirable air pollutants.
Importance: Often listed on a MSDS as a hazardous decomposition product.
OLFACTORY Relating to the sense of smell.
Importance: The olfactory organ in the nasal
cavity is the sensing element that detects odors
and transmits information to the brain through
the olfactory nerves. This sense of smell is a
"built in" vapor detector.
ORAL Used in or taken into the body through the
mouth.
Importance: A route of exposure to a hazardous
material.
ORAL TOXICITY Adverse effects resulting from
taking a substance into the body via the mouth.
Importance: Ordinarily used to denote effects
in experimental animals.
OSHA Occupational Safety and Health Administration of the U.S. Department of Labor.
Importance: Federal agency with safety and
health regulatory and enforcement authorities
for most U.S. industry and business.
OXIDIZING AGENT, OXIDIZER A chemical or substance which brings about an oxidation reaction.
The agent may (1) provide the oxygen to the
substance being oxidized (in which case the
agent has to be oxygen, or contain oxygen),
or (2) it may receive electrons being transferred
from the substance undergoing oxidation. DOT
defines an oxidizer or oxidizing material as a
substance which yields oxygen readily to stimulate combustion (oxidation)of organic matter.
Importance: If a substance is listed as an oxidizer on the MSDS, precautions must be taken
in the handling and storage of the substance.
Keep away from flammables and combustibles.
PEL Permissible Exposure Limit; an exposure established by OSHA regulatory authority. May
be a Time Weighted Average (TWA) limit or a
maximum concentration exposure limit. See
also, "Skin."
Importance: If a PEL is exceeded, a potential
health hazard exists, and corrective action is
necessary.
POISON, CLASS A A DOT term for atremedy dangerous poisons, that is, poisonous gases or
liquids of such nature that a very small amount
of the gas, or vapor of the liquid, mixed with
air is dangerous to life. Some examples: phosgene, cyanogen, hydrocyanic acid, nitrogen
peroxide.
POISON, CLASS B A DOT term for liquid, solid,
paste, or semisolid substances-other than
Class A poisons or irritating materials- which
are known (orpresumed on the basis of animal
tests) to be so toxic to man as to afford a hazard
to health during transporation.
Importance: If a substance is known to be a
poison, health and safety hazards exist and special protection and precaution sections should
be checked on the MSDS.
POLYMERIZATION A chemical reaction in which
one or more small molecules combine to form
larger molecules. A hazardous polymerization
is such a reaction which takes place at a rate
which releases large amounts of energy.
Importance: If hazardous polymerization can
occur with a given material, the MSDS usually
will list conditions which could start the reaction
and the time period before any contained the
inhibitor is used up.
ppm Parts per million; a unit for measuring the
concentration of a gas or vapor in air-parts
(by volume) of the gas or vapor in a million
parts of air. Also used to indicate the concentration of a particular substance in a liquid or
solid.
Importance: The effects of overexposure depend on the concentration or dose of a hazardous
substance.
ppb Parts per billion; a unit for measuring the
concentration of a gas or vapor in air-parts
(by volume) of the gas or vapor in a billion
parts of air. Usually used to express measurement of extremely low concentrations of unusually toxic gases or vapors. Also used to indicate the concentration of a particular
substance in a liquid or solid.
Importance: The effects of overexposure depend on the concentration or dose of a hazardous
substance.
REAC~ION A chemical transformation or change;
the interaction of two or more substances to
form new substances.
Importance: Knowledge of reactions can prevent unsafe chemical changes.
REACTIVITY A description of the tendency of a
substance to undergo chemical reaction with
itself or other materials with the release of
energy. Undesirable effects-such as pressure
buildup, temperature increase, formation of
noxious, toxic, or corrosive by-product-may
occur because of the reactivity of a substance
to heating, burning, direct contact with other
materials or other conditions in use or in storage.
Importance: Knowledge of what conditions to
avoid can prevent unsafe chemical reactions.
REDUCING AGENT In a reduction reaction (which
always occurs simultaneously with an oxidation
reaction) the reducing agent is the chemical or
substance which ( I )combines with oxygen, ( 2 )
loses electrons in the reaction. See also, "Oxidizing Agent. "
Importance: If a material is listed as a reducing
agent on the MSDS, precautions must be taken
in the handling and storage of the substance.
Keep separate from oxidizing agents.
RESPIRATORY SYSTEM The breathing system; includes the lungs and air passages (trachea or
"windpipe," larynx, mouth, and nose) to the
air outside the body, plus the associated nervous
and circulatory supply.
Importance: Inhalation is the most common
route of exposure in the occupational workplace.
SENSITIZER A substance which on first exposure
causes little or no reaction in man or test animals, bur which on repeated exposure may
cause a marked response not necessarily limited
to the contact site. Skin sensitization is the most
common form of sensitization in the industrial
setting, although respiratory sensitization to a
few chemicals is also known to occur.
Importance: Knowing that a substance is a
sensitizer allows you to be aware of the signs
and symptoms of overexposure.
%KIN" A notation, sometimes used with PEL or
TLV exposure data; indicates that the stated
substance may be absorbed by the skin, mucous
membranes, and eyes-either by airborne or
by direct contact-and that this additional exposure must be considered part of the total
exposure to avoid exceeding the PEL or TLV
for that substance.
Importance: Even if workplace concentrations
of a chemical do n9t exceed the TLV or PEL,
the risk to health may be severe because breathing and skin contact are combined. Skin protection is advised.
SKIN SENSITIZER See "Sensitizer. "
SKIN TOXIC~TY See "Dermal Toxiciry."
SOLUBILITY IN WATER A term expressing the percentage of a material (by weight) that will dissolve in water at ambient temperature.
Importance: Solubility information can be
useful in determining spill cleanup methods
and fire-extinguishing agents and methods for
a material.
SO. Oxides of Sulfur; undesirable air pollutants.
Importance: Often listed on a MSDS as a hazardous decomposition product.
SPECIAL PRECAUTIONS Instructions that describe
proper handling and storage procedures specijic
to that material.
Importance: Following these procedures would
prevent excessive employee exposure. These
procedures tell you additional information
needed to handle the material safely.
SPECIAL PROTECTION INFORMATION A description
of engineering precautions and personal protection that should be provided when working
with a chemical in order to reduce an employee's
exposure.
Importance: Reducing the potential for exposure reduces the risk to health and safety.
SPECIFIC GRAVITY The weight of a material compared to the weight of an equal volume of water;
an expression of the density (or heaviness) of
the material. Example: If a volume of a material
weighs 8 pounds, andan equal volume of water
weighs 10 pounds, the material is said to have
a specific gravity of 0.8.
Importance: Insoluble materials with specific
gravity of less than 1.0 will float in (or on)
water. Insoluble materials with specific gravity
greater than 1.0 will sink (or go to the bottom)
in water. Most flammableliquids have specific
gravity less than 1.0 and, if not soluble, will
float on water-an important considerationfor
fire suppression and spill clean-up.
SPILL OR LEAK PROCEDURES Steps that should be
taken if a chemical spill or leak occurs.
Importance: Proper removal of a chemical spill
or leak from the work area eliminates the potential accumulationof hazardous concentrations
of the chemical, reduces the risk of creating
an environmental pollution problem and conforms with local, state and federal regulations.
S T A B U ~ ~ YAn expression of the ability of a material
to remain unchanged.
Importance: For MSDS purposes, a material
is stable if it remains in the same form under
expected and reasonable conditions of storage
or use. Conditions which may cause instability
(dangerous change) are stated-for example,
temperatures above 150"F,shock from dropping.
STEL Short Term Exposure Limit; ACGIH terminology. See also, "TLV-STEL. "
SYNONYM Another name or names by which a
material is known. Methyl alcohol,for example,
is also known as methanol, or wood alcohol.
Importance: A MSDS will list common name(s)
to help identify specific materials.
TERATOCEN A substance or agent to which exposure of a pregnant female can result in malformations in the fetus.
Importance: If a substance is known to be a
teratogen, a potential health hazard exists and
special protection and precaution sections should
be checked on a MSDS.
TLV Threshold Limit Value; a term used by ACGIH
to express the airborne concentration of a material to which nearly allpersons can be exposed
day after day without adverse effects. ACGIH
expresses T L V s in three ways:
TLV-TWA: The allowable Time Weighted Average concentrationfor a normal &hour workday or 40-hour work week.
TLVSTEL: The Short-Term Exposure Limit, or
maximum concentration for a continuous 15minute exposure period (maximum offour such
periodsper day, with at least 60 minutes between
exposure periods, and provided that the daily
TLV-TWA is not exceeded).
n v - c : The Ceiling exposure limit-the con1. t hould not be exceeded even
instantaneously.
are reviewed and revised annually where
necessary by the ACGIH
Importance: If a TLV is exceeded, a potential
health hazard exists and corrective action is
necessary. Also see "Skin" relative to TLV's.
~oxrcrrvThe sum of adverse effects resultingfrom
exposure to a material, generally by the mouth,
skin, or respiratory tract.
Importance: Knowledge of the toxicity of a
material helps prevent adverse health effects
from exposure.
TRADE NAME The trademark name or commercial
trade name for a material.
Importance: A MSDS will list trade name(s)
to help identify specific materials.
WA
Time Weighted Average exposure; the airborne concentration of a material to which a
person is exposed, averaged over the total exposure time-generally the total work-day (8
to 12 hours). See also, "TLV."
UEL OR UFL Upper Explosive Limit or Upper
Flammable Limit of a vapor or gas; the highest
concentration (highest percentage of the substance in air) that will produce a Jash o f f r e
when an ignition source (heat, arc, orjame)
is present.
Importance: At higher concentrations, the
mixture is too "rich" to bum. See also, "LEL."
UNSTABLE Tending toward decomposition or other
unwanted chemical change during normal handling or storage.
Importance: A MSDS will list materials that
are unstable and conditions to avoid to prevent
decompositionor unwanted chemical changes.
V A ~ R D E N S ~ The
~ Y weight of a vapor or gas compared to the weight of an equal volume of air;
an expression of the density of the vapor or
gas. Materials lighter than air have vapor densities less than 1.0 (example: acetylene, methane, hydrogen). Materials heavier than air (examples: propane, hydrogen sulJide, ethane,
butane, chlorine, sulfur dioxide) have vapor
densities greater than 1.0.
Importance: All vapors and gases will mix
with air, but the lighter materials will tend to
rise and dissipate (unless confined). Heavier
vapors and gases are likely to concentrate in
low places-along or under floors, in sumps,
sewers and manholes, in trenches and ditches
-and can travel great distances undetected
where they may create fire or health hazards.
VAWR PRESSURE The pressure exerted by a saturated vapor above its own liquid in a closed
container.
Importance: The higher the vapor pressure,
the easier it is for a liquid to evaporate and fill
the work area with vapors which can cause
health or fire hazards.
VENTILA~ON See "General Exhaust," and "Local
Exhaust."
TLV'S
Problem No. 01
CHEMICAL ENGINEERING TOPIC: Fundamentals
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment; Toxicology and Industrial Hygiene
BACKGROUND: Ventilation is an extremely important method of reducing the
level of toxic airborne contaminants in the workplace. Since it is impossible to
eliminate absolutely all leakage, some method will always be needed to remove
toxic materials from the air when they are present in the process streams.
Ventilation includes "general ventilation" (which is also sometimes referred as
"dilution ventilation") and "local exhaust ventilation," which is a method of
removing contaminants before they enter the workplace air. Local exhaust ventilation is much preferred as a method of contaminant control as the following problem
should illustrate.
The Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit (PEL) of vinyl chloride (VC) at 1.0 parts per million (ppm)
as a maximum time-weighted average (TWA) for an eight hour work day, because
VC is believed to be a human carcinogen. (A carcinogen is an agent that causes or
promotes the initiation of cancer, therefore, exposure to a carcinogenic substance
may increase the likelihood of the subject developing cancer in the future.)
Exposure to VC on a long term basis (chronic exposure) may result in liver damage
as well as some other symptoms. Acute exposure (one time exposure to relatively
high concentrations) may cause central nervous system depression.
If VC escapes into the air, its concentration must be maintained at or below the
PEL. If dilution ventilation were to be used, we might estimate the required air
flow rate by assuming complete mixing in the workplace air, and then assume that
the volume of air flow through the room will carry VC out with it at the concentration of 1.0 ppm.
PROBLEM: We have an operation where VC will evaporate at a rate of 10 glmin
into the air.What flow rate of air will be necessary to maintain the PEL of 1.0 ppm
by dilution ventilation? That is, what volume rate of air carrying VC at 1.0 ppm will
be required to remove the VC at a rate of 10 glmin?
We must also correct for the fact that complete mixing will not be realized. A
recommended way to do this is to multiply the air flow rate by a safety factor. In
this case, use a factor of 10.
An alternative is to partially enclose the operation and use local exhaust ventilation. Assume that this operation can be carried out in a hood with an opening of
30 in. wide by 25 in. high. Imagine that this hood looks like the ones you see in your
2
SAFETY, HEALTH, AND LOSS PREVENTON IN CHEMICALPROCESSES
chemistry laboratory. If the "face velocity," that is the average velocity of air
entering the hood opening must be 100ftlmin to effectively capture the VC vapor
generated inside the hood, what air flow rate will be required?
Which way seems best to you? Explain why dilution ventilation is not recommended for maintaining air quality.
Problem No. 02
CHEMICAL ENGINEERING TOPIC: Fundamentals; Design
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment; Toxicology and Industrial Hygiene
BACKGROUND: Ventilation is an extremely important method of reducing the
level of toxic airborne contaminants in the workplace. Ventilation includes
"general ventilation" (which is also sometimes referred as "dilution ventilation")
and "local exhaust ventilation," which is a method of removing contaminants
before they enter the workplace air. The local exhaust ventilation is much preferred
as a method of contaminant control because it removes the contaminant before it
can enter the workplace air. It also will require much less air flow if properly
designed, which means less equipment and energy required for the job.
Many industrial operations involve the exposure of solvents to the air, and thus
a problem of evaporationwill occur. Most common solvents will display some sort
of toxic effect, some of them more severe than others. Trichloroethylene is an
excellent solvent for a number of applications, and is especially useful in degreasing. Unfortunately, tricloroethylene can lead to a number of harmful health effects.
It has been shown to be carcinogenic in animal tests. (Carcinogenic means that
exposure to the agent might increase the likelihood of the subject getting cancer
at some time in the future.) It is also an irritant to the eyes and respiratory tract.
Acute exposure causes depression of the central nervous system, producing
symptoms of dizziness, tremors, and irregular heartbeat, plus others.
PROBLEM: Trichloroethylene has a molecular weight of approximately 131.5, so
the vapors are much more dense than air. As a first thought, one would not expect
to find a high concentration of this material above an open tank because we would
assume that the vapor, being dense, would sink to the floor. If this were so, then
we would place the inlet of a local exhaust hood for such a tank near the floor.
However, Industrial Ventilation* points out that toxic concentrations of many
materials are not much more dense than the air itself, so where there can be mixing
with the air we may not assume that all the vapors will go to the floor. For the case
of trichloroethylene OSHA has established a time-weighted average 8 hr PEL of
100 ppm; a 15-min ceiling of 200 ppm; and a 5-min peak of 300 ppm.
*IndustrialVenti1ation:A Manual ofRecommended Practice, 19th ed. Cincinnati:
ACGIH, 1986.
S A I E l Y , HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
Determine the density of a mixture of trichloroethylenein air at each of these
limiting concentrations, as well as that of a saturated vapor at 2YC, and compare
the values with that of pure air at the same temperature. That is, determine the
specificgravity (relative to air) for each mixture. Which, if any, of these concentrations would you feel might readily sink to the floor, and which might circulate with
the normal air currents which we would find in a room?
Problem No. 03
CHEMICAL ENGINEERING TOPIC: Fundamentals: Gas Mixture Composition
and Amagat's Law of Partial Volumes
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: It may not always be obvious to the workers when a breathing
apparatus is required. Whenever workmen must enter a vessel for maintenance,
entry should not be made until the air within the vessel is tested for flammable
agents, oxygen content, and, ifneeded, for toxic agents. Lack of oxygen is the most
common cause of death in vessel entry. As an example of what can happen,
consider that in order to prevent fire or explosion, storage tanks or other vessels
that have contained flammablematerials are frequently purged with nitrogen prior
to required maintenance. Purging with nitrogen will prevent fires from welding or
spark-producing activities but if a worker should enter this vessel without a
breathing apparatus, he will be quickly overcome from lack of oxygen.
Normal air contains about 21% oxygen, by volume. A typical adult's total lung
volume is about 5.5 L. During normal breathing, each inspiration and expiration
involves about 500 ml of air. Of this 500 ml, about 150 ml occupies the
tracheobronchial tree, where no interchange of oxygen can take place with the
blood. Thus, only 350 ml of air is actually exchanged in each inhalation.
The alveolar air, that is, the air fromdeepwithin the lungs that is exhaled contains
only about 12% oxygen, but combined with that which remained in the
tracheobronchial tree, the net exhaled composition is about 16%.
When the concentrationof air being inhaled dropsbelow 16% oxygen, symptoms
of distress will appear. Because the lack of oxygen affects the central nervous
system first, loss of consciousness is an early consequence, and will occur at
concentrations below about 11to 12% oxygen. Breathing will cease if the oxygen
content drops below about 6%.
PROBLEM: If it is assumed that loss of consciousness occurs when the average
concentration of oxygen in the lungs and tracheobronchial tract drops below 11%,
estimate how many breaths a worker will be able to take when he enters a vessel
that contains 100% nitrogen before he loses consciousness. If help comes in time,
he may recover if he gets into fresh air before the average concentration drops
below about 6%. How much time is there to help him? (A person who is breathing
normally will inhale about 30 L/min at 500mllinhalation.) What might you conclude
about using air-purifyingrespirators rather than air-supplying respirators for such
a case?
'
Problem No. 04
CHEMICAL ENGINEERING TOPIC: Fundamentals: Ideal Gas Law; Mass
Balance
SAFETY AND HEALTH CONCEPT: Toxicology and Industrial Hygiene: Chronic
Toxicity
BACKGROUND: Most of the common solvents used in the laboratoryand industry
are either toxic, flammable, or both. They may be acutely toxic, in which case care
must be taken for even short exposures. There are other chemicals that may be
tolerated for short times with no apparent immediate health effects, but which may
cause serious health problems such as organ degeneration or cancer if people are
exposed to them for a long time at toxic concentrations that are relatively low.
These chemicals are classified as exhibiting chronic toxicity.
Federal standards, based on the toxicity of various chemicals, have been set for
the "Permissible Exposure Limit," or PEL. The PEL is the maximum level of
exposure permitted in the work place based on a time-weighted average (TWA)
exposure. The TWA exposure is the average concentrationpermitted for exposure
day after day without causing adverse effects. It is based on exposure for 8 hr per
day for the worker's lifetime. Of course, the normal concentration of a toxic
material is usually essentially zero, so that the worker is not exposed to the PEL
concentration for more than short periods under unusual conditions. A "Short
Term Exposure Limit" (STEL) is specified in the new standards and is based on
a 15-min exposure. Both the STEL and the PEL are lower than the concentrations
that are expected to cause injury. PELSand other TWA criteria may reflect either
acute or chronic toxicity effect and should not be used as a comparative measure
of toxicity except in a very broad way. A comparative end-point used in toxicology
for acute inhalation exposure is called the LCso (lethal concentration, 50%). This
is a statistically derived concentration that will kill 50% of a group of test animals
following a short term exposure, and it is used to estimate the acute toxicity to man.
Acute toxicity values are usually expressed for some animal species, and (not
surprisingly) there is very little human exposure data.
PROBLEM: Some decades ago benzene was thought to be a relatively innocuous
chemical with a somewhat pleasant odor and was widely used. It has been found
that benzene can cause chronic adverse blood effects such as anemia and possibly
leukemia with chronic exposure. Benzene has a PEL for 8-hr exposure of 1.0 ppm.
If liquid benzene is evaporating into the air at a rate of 2.5 mllmin, what must the
ventilation rate be to keep the concentration below the PEL? The ambient
temperature is 6S°F and the pressure is 740 mm Hg.
Problem No. 05
CHEMICAL ENGINEERING TOPIC: Fundamentals
HEALTH AND SAFETY CONCEPT: Vapor Releases
BACKGROUND:No matter how carefullyworkers do their jobs, the possibility of
accidents remains. The more planning and preparation that has gone into accident
anticipationand contingency planning, the better the chance of avoiding complications, injury, or property damage if and when an accident occurs. In planning for
possible accidents, one of the more likely occurrencesmight be a release of a large
quantity of toxic or flammablevapors or gases. Methods are available for estimating the resulting concentrations from such releases; and from such estimates it is
sometimes possible to predict what areas of a plant or of the area surrounding a
plant might have to be evacuated.
Computational methods used for such emission sources as power plant stacks
can also be used for accidental releases. Such computations estimate the effect of
dilution as a plume leaves the source location. A derivation of any of the methods
is beyond the scope of this problem statement.
One of the simpler models to predict dispersion is called the "Gaussian Plume
Model," and expresses the average concentration at a location downwind of a
continuous source.
where
m = y2/[2(uy)2]
n = (z - ~ > ~ / [ 2 ( u z ) ~ l
n' = (Z + H ) ~ / [ ~ ( O ~ ) ~ ]
C = the concentration at a selected point downwind, mg/m3.
u = wind velocity, m/s
x = distance downwind from the source to the point of interest, m or km
y = distance cross-wind away from the centerline to the point of interest, m
z = height above ground level to the point of interest, m
H = the height of the source above ground, m
uz = diffusion coefficient in thez direction (vertical), m
uy = diffusion coefficient in they direction (cross-wind), m
Q = source strength (emission rate), mgls
S A l T T Y , HEALTH,AND LOSS PREVENTIONIN CHEMICALPROCESSES
This equation is valid for windblown plumes across fairly level ground. It is based
on an assumption of how the components of the plume would be dispersed. The
diffusion coefficientsdepend upon the stabilityof the atmosphere and the distance
downwind from the source. The diffusion coefficients may be estimated for most
purposes from the following:
a, =arb
az=cxd +f
and
Values for the constants, a, c, d, andf are given in the following table. The value
of b is always 0.894 andx is expressed in kilometers:
Values of Constants to Compute Diffusion Coefficients
Stabiity
class
a
x c lkm
c
d
f
x > lkm
c
d
f
Note: The value of a is independent of the downwind distance, x.
The stability categories may be estimated from the following:
Wind speed
(4s)
<2
2-3
3-5
5-6
>6
DAYLIGHT
Sunlight intensity
strong moderate
weak
A
A-B
B
C
C
A-B
B
B-C
C-D
D
B
C
C
D
D
NIGHT
Cloudy Clear
E
E
D
D
D
F
F
E
D
D
Suggested stability classes are from Turner, D.B., "Workbook for
Atmospheric Dispersion Estimates," HEW, Washington, D.C., 1969.
PROBLEM NO. 5
9
PROBLEM: Emergency plans are being formulated so that rapid action can be
taken in the event of an equipment failure. It is predicted that if a particular
pipeline were to rupture it would release ammonia at a rate of 100 lblsec. Persons
exposed to 500 parts per million (ppm) of ammonia will be endangered and
anywhere that the concentration might be that high should be evacuated until
repairs are made. What recommendation would you make as to how far from the
rupture people should be evacuated iE
A. The wind is 6 miles per hour and the sun is shining brightly.
B. The night is overcast and the wind is 10 miles per hour.
Constants for the d i i s i o n coefficient estimates are from Martin, D.O., J. Air
Pollution ControlAssoc.,Vol. 26, No. 2 (1976).
Problem No. 06
CHEMICAL ENGINEERING TOPIC: Fundamentals: Gas Laws, Partial Pressures and Partial Volumes
HEALTH AND SAFETY CONCEPT: Toxicology and Industrial Hygiene
BACKGROUND: When toxic materials are used and/or produced by chemical
processes, it is necessary to ascertain that the workers are not exposed to the
material(s) to such an extent that they receive a harmful dose. Since the most
frequent route of entry for toxic materials is by inhalation, ascertaining the extent
of exposure or exposure potential often takes the form of determining the concentration of toxic material in the air that the workers breathe.
The methods of sampling, capturing, and analyzing the air vary a great dealdepending upon the nature of the contaminant, the method of analysis that is to be
used, and the time period over which the information is desired. Also, sometimes
we will want to know the concentrationvariations within a room- perhaps to study
the efficiency of contaminant control practices -whereas, at other times, it may be
desirable for the workers to carry "personal samplers," which are fastened to their
clothing, as close as can conveniently be to their faces.
Particulate samples may be collected on a filter, with air being aspirated through
the filter. Some method will be used to determine the sample volume, either a flow
meter or a pump that moves a known volume of air per unit of time. The particles
can be collected and analyzed as needed, although sometimes only the mass is
required.
Many of the potentially harmful agents are vapors that may be in the air. The
methods used for vapor concentration are quite varied. There are direct reading
instruments, which do not actually accumulate samples, but only need to have
sampled air pass through a detector. Usually these devices will output to arecorder
or a computer so that both the time-weighted average (TWA) as well as the
instantaneous maximum may be determined, and sometimes it may be helpful to
know the time cycling of the concentration. All direct reading devices will have
threshold concentrations below which the contaminant in question is not detectable. In these cases, and some others, it will be necessary to sample a large volume
of air and concentrate the contaminant.
For any of various reasons it may not be appropriate to employ direct reading
instruments, and samplingwill have to be performed in some other way. Especially,
it is not feasible to use direct reading instruments as personal samplers.
A sample taken over a very short time is called a "grab sample." A grab sample,
or instantaneous sample is frequently collected in an evacuated container, and the
contents subsequently analyzed by an instrument such as a gas chromatograph. In
PROBLEM NO. 06
11
such a procedure, the contaminant will have to be at such a concentration that
direct analysis is possible.
Sampling may be carried out over an extended time, and the results of the
subsequent analysis then represent an average, actually a time-weighted average,
concentration. Such a procedure is called "integrated sampling." Methods of
obtaining large samples over extended times involve passing the air samplethrough
a collector device which might be an absorption train similar to that used for stack
gas sampling, or might be an adsorption bed (e.g., a bed of granular material such
as activated charcoal or silica gel). In this way, a time-weighted average can be
obtained, and a contaminant in a large volume of air can be concentrated, in order
to increase the sensitivity of the determination. Some care must be exercised to
prevent exceeding the capacity of a concentratingcollector, and some means must
be availableto determine that the capacitywasnot exceeded. One common method
is to use two sampl~collectors in series. If the first collector is not overloaded, then
there will be no contaminant collected in the second. If, however, a quantity of the
contaminant is found on the second collector, the results of the analysis will be
rejected.
PROBLEM: A 2-L grab sample of air (33"C, 99 kPa, and 70% relative humidity)
was collected in a stainless steel container which had been evacuated to a hard
vacuum. The samplewas admitted to the container by opening a valve and allowing
the air to enter until the pressures were equalized, whereupon, clean dry helium
was admitted until the pressure was 500 kPa.
The sample was taken to a gas chromatography laboratory where the temperature was 23°C. The next day, a sample from the container was released to the
chromatograph until the pressure in the container was reduced to 400 kPa. On
analysis, the portion of the sample admitted to the instrument was found to contain
1.65 ng of benzene. What is the concentration of benzene in the originalworkroom
air (in mgfm3),and is it in excess of the permissible exposure limit (PEL) of 1part
per million (ppm) on a mole basis?
NOTE: A grab sample as considered in this problem would not usually be used for
determining compliance with the PEL, which is an 8-hr average limit.
Problem No. 07
CHEMICAL ENGINEERING TOPIC: Fundamentals: Mass Balance
SAFETY AND HEALTH CONCEPT: Inerting and Purging
BACKGROUND :Many chemical plant operationsrequire that vessels and piping
be inerted or purged. There are several reasons for this requirement: For example,
when a plant is first constructed, the piping and vessels will be filled with air. The
air may have to be purged because it interferes with the process; because it can
lead to flammable mixtures with the chemicals that flow through the piping; and
because flammable mixtures may be formed in vessels. If piping or vessels that
contain a flammable or toxic material must be taken out of service and repaired or
inspected, they must first be purged to remove the flammable or toxic material.
Otherwise, workers will be exposed to the hazard of working under unsafe conditions.
If piping or vessel is being purged "into service," the air that is in the system at
the start can be purged out with an inert material. Nitrogen is frequently used for
purging into service because it is relatively cheap, it can be easily obtained, and it
does not pollute the atmosphere during the purging process. If piping or vessel is
being purged "out of service," the process is usually a little more complicated. First,
an inert gas must be used to purge the flammable or toxic gas from the system.
Then, the inert gas itself must be purged from the system using air if people are to
enter the vessel. During the purging process, care must be taken that the exit
stream, which contains varying concentrations of flammable or toxic material, is
not discharged to the atmosphere. The hazardous material must be removed
before discharge to prevent possible danger to workers and neighbors.
Liquids in piping and vessels present special problems. Sometimes the liquid is
soluble in water and can simply be rinsed out. Sometimes a detergent solution can
be used to wash the equipment. Sometimes a solvent must be used to wash the
equipment and the solvent must then be rinsed or washed out with water. Other
methods can also be devised. In all cases, the material washed or rinsed from the
equipment must be disposed of properly. Elaborate procedures may have to be
devised for some systems.
Whatever procedure is used to purge equipment, the final step should be to
check the atmosphere in the equipment to make certain that the concentration of
flammable or toxic material has been reduced to safe levels. The measurement of
residual concentrations is sometimes required by law; it is always needed for good
practice. Detailed procedures must be prepared and followed if workers are to
enter a vessel. These include completion of a tank entry permit that lists the
detailed procedures to be followed.
PROBLEM NO.07
13
PROBLEM: A tank used for storing liquefied natural gas must be taken out of
service and inspectedinternally.All the liquidnatural gas that canbe pumped from
the tank is removed. The tank is then allowed to warm from its service temperature
of about -260°F to ambient temperature. The tank then contains only natural gas
(assumed to be pure methane) gas at ambient temperature and atmospheric
pressure. Purging is accomplished in two steps: first, liquid nitrogen is sprayed
gently onto the tank floor, where it vaporizes and displaces the methane. The cold
nitrogen vapor displaces the warm natural gas in a piston-like flow as the nitrogen
fills the tank. Once all the methane has been displaced, the nitrogen is allowed to
warm to ambient temperature. Air is then blown into the tank. It mixes with the
nitrogen rapidly and completely, so that the concentration of oxygen in the
air-nitrogen mixture leaving the tank is equal to that in the tank.
a. How many gallons of liquid nitrogen will be required to displace all the
methane from a tank with a total volume of 175,000barrels (bbl)?
b. How many cubic feet of air will be required to increase the oxygen concentration to 20% by volume?
Problem No. 08
CHEMICAL ENGINEERING TOPIC: Fundamentals
SAFETY AND HEALTH CONCEPT: Properties of Materials
BACKGROUND: Three things are commonly recognized as being required for a
fire to occur: (1) a fuel, (2) an oxidizing agent, and (3) an ignition source. The
combustion reaction normally occurs in the gas phase, and, in most cases, the
oxidizer is air. If a flammable gas is mixed with air, there is a minimum gas
concentration below which ignition will not occur. That concentration is called the
lower flammable limit (LFL), and it is usually expressed in terms of the mole
percent or volume percent of the flammable gas in air. If the gas concentration is
less than the LFL, the gas mixture will not ignite. The LFL depends on the
temperature of the gas-air mixture, and at temperatures as high as those in flames,
essentially all the gas will burn. The LFL is usually measured at ambient temperature (25°C) and 1.0 atm pressure. There is usually an upper limit gas concentration
above which ignition will not occur. It is called the upper flammable limit (UFL),
and it is also measured at ambient temperature and atmospheric pressure. The
range of concentrations between the LFL and the UFL is called the flammability
region. Some gases have very wide flammability regions, and others are much
narrower. The following data are examples of LFL and UFL for a few common
flammable materials:
MATERIAL
Methane
Propane
n-pentane
Hydrogen
Ammonia
LFL
UFL
MoIe Percent
5.0
15.0
2.2
9.5
1.5
7.8
4.0
75
16
25
These data were taken from the NFPAStandard325M,Properties of Flammable
Liquids published by the National Fire Protection Association.
The LFL has been measured for many common materials and is available in the
literature. Occasionally, it will be necessary to estimate the lower flammable limit
of a gas. One simple method of doing so is based on the observation that for many
hydrocarbons the LFL is about half the concentration required for stoichiometric
combustion of the gas in air.
PROBLEM NO.08
15
PROBLEM. Estimate the LFL for methane, propane, n-pentane, hydrogen, and
ammonia. Compare your results to the data given above. In making the calculation,
you may assume that air is 21 mole percent oxygen and the balance nitrogen.
Assume that if hydrogen and carbon appear in the fuel molecule they will burn to
water and carbon dioxide. If nitrogen is in the fuel molecule, the reaction products
may contain a variety of oxidation products of nitrogen. Generically, these are
labeled as NO,, and most of them are highly toxic. However, most of the nitrogen
oxidation products are formed at very low concentrations at flame temperatures.
Thus, for a fust approximation, it may be assumed that the combustion products
are pure nitrogen if nitrogen is present in the fuel molecule. A small part of the
nitrogen in the air is also oxidized to NO, in the fue, but again, the concentrations
are small enough that the effect can be neglected for these calculations.
Problem No. 09
CHEMICAL ENGINEERING TOPIC: Fundamentals
SAFETY AND HEALTH CONCEPT: Vapor Releases
BACKGROUND: Many industrial chemicals are toxic or flammable; frequently a
chemical is both toxic and flammable. Regardless of whether the chemical is toxic
or flammable, it can present a danger to plant operators and the public if it is
released from its containers. Substantial effort is taken to assure that toxic or
flammable materials are not spilled or released from containment. However, there
is always a chance that such materials might be released, and if they are, provisions
have to be made for protection of the plant operators and anyone who lives or
works nearby.
One method of providing protection for the public is to locate plants well away
from housing areas, shopping areas, or other public places. Sometimes, there is
not sufficient land available to provide such separation distances as are desired,
and sometimes people build houses and businesses near plants that are already
operating. In any case, there may be times when a release of flammable or toxic
material may occur under conditions that might endanger the public. If such an
event is possible, the plant management should provide (1)methods for warning
the public, and (2) a community emergency plan for the most likely situations so
emergency personnel will be prepared to take immediate action. As part of the
emergency response plan, there must be an estimate of the time available for the
emergency response actions, such as evacuation, to take place.
Alternative methods can be used to mitigate the effects of a hazardous material
release. They include rapid shutdown of process equipment to limit the amount of
hazardous material released, backup instrumentation to reduce the probability of
losing process control, and measures to reduce the impact of a release that has
occurred. If a liquid is released, one method of mitigation is to limit its spread by
providing impounding dikes. Chemicals can sometimesbe neutralized by chemical
reactions. Mitigation methods usually depend on the properties of the chemical
that is released. If the chemical is released as a gas, mitigation is harder because
the movement of the gas cannot be controlled.
If a toxic material is dispersed in the air, the engineer must know how high its
concentration can rise without danger to people. The Occupational Safety and
Health Administration has set a concentration level for many chemicals called the
permissible exposure limit (PEL). The PEL is the maximum time-weighted
average concentration that a worker may be exposed to for an 8-hr work day for
his lifetime. Another concentration that is frequently referred to is the concentra-
PROBLEM NO.09
17
tion that is immediately dangerous to life or health (IDLH). This concentration is
the level at or below which a person exposed for 30 min will not lose control and
will be able to put on protective equipment or take other protective action. The
IDLH concentration is higher than the PEL concentration, and people exposed at
the IDLH concentration must take action to protect themselves. An even higher
concentration frequently used in the LCso concentration.The LCso concentration
is a concentration at which 50% of a group of test animals would die if exposed to
the hazardous chemical during a standard test. This concentration is frequently
used as an estimateof the degree of danger to humans as well. (Most toxicity studies
are performed using test animals. Humans obviously cannot be exposed to lethal
concentrations of toxic chemicals for determining toxicity. There are some differences in tolerance for chemicals for man and animals caused by differences in
metabolism and other factors, so the animal toxicity tests are not always easy to
interpret. However, the results of animal toxicity tests are used to guide the
selection of acceptable exposure limits for humans.)
If the hazardous material spill is of short duration, there is a choice to be made
concerning evacuation. If people are in their houses, it will take some time before
the toxic material can enter the house. If the release is small, it may be more
dangerous to try to evacuate people than to have them remain inside (with the
doors and windows closed and air circulation stopped) until the danger is past.
This and other options should be considered as part of the emergency action plan.
PROBLEM: A chemical plant uses acrolein (acrylaldehyde) as an intermediate in
its chemical process. The nearest residencesto the plant are 3000 ft from the point
where a spill is most likely to occur. It has been estimated that under adverse
atmospheric conditions, if a release occurs at the plant, the concentration of
acrolein in air at the nearest residence can reach a maximum of 10ppm. The release
will be liquid, which will be contained in an impounding area where it will slowly
evaporate. The concentration at the nearest residence is well above the IDLH
concentration, and plans are made for the evacuation of the residents in the event
of a release. The wind speed under the adverse conditions is 2.2 mph. It can be
assumed that the houses admit air from the outside at a rate of three air changes
per hour. The air in the houses can be assumed to be well mixed, so any gas that
enters will be at a uniform concentration throughout the house.
1. Assuming the vapor from the acrolein spill moves at the same speed as the
wind, how much time is available for evacuatingpeople from the residencesbefore
the toxic vapor arrives?
2. How much additional time will be required for the concentration of acrolein
to increase to the IDLH concentration inside the residences? Begin by writing an
unsteady state mass balance for the acrolein in the house. Account for the acrolein
in the house as well as that entering from the outside and that leaving from the
inside. Assume the density of air in the house is constant.
18
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
3. Compare the results of Part 2 with those of Part 1. How might the difference
influence your judgment of the time operatorshave to complete mitigationprocedures?
4. Can you suggest simple hazard mitigation methods that might be used to
reduce the acrolein concentration at the residences?
Problem No. 10
CHEMICAL ENGINEERING TOPIC: Fundamentals: Stoichiometry
SAFETY AND HEALTH CONCEPT: Properties of Materials
BACKGROUND: Three criteria must be met if a fue is to occur: (1)there must be
fuel present; (2) there must be an oxidizer present; and (3) there must be an ignition
source. In many industrial processes, the materials that are stored, transported, or
reacted to manufacture new materials are flammable. Other chemicals are used as
fuels, so they must be flammable if they are to have any value. While using or
handling fuels can be done safely, care is required to assure that ignitiontakes place
only where and when it is wanted.
For most fuels, combustion takes place only in the gas phase. For example,
gasoline does not burn as a liquid. However, when gasoline is vaporized, it burns
readily. Controlled burning, such as occurs in an automobile engine, is beneficial
and can be used to generate work. Uncontrolled burning, such as might occur if
gasoline is spilled and ignited, is wasteful in all cases, and it is dangerous in many
cases. Other flammable liquids are similar. If a flammable gas is released into air,
it will also form flammable mixtures.
There is a minimum concentration of fuel in air that can be ignited. If the fuel
concentration is less than this lower flammable limit (LFL) concentration,ignition
will not occur. Above the LFL, the amount of energy required for ignition is quite
small. For example, a spark can easily ignite most flammable mixtures. There is
also a fuel concentration called the upper flammablelimit (UFL) above which the
fuel-air mixture cannot be ignited. Fuel-air mixtures in the flammable concentration region between the LFL and the UFL can be ignited. Both the LFL and the
UFL have been measured for most of the common flammable gases and volatile
liquids. The LFL is usually the more important of the flammability concentrations
because if a fuel is present in the atmosphere in concentrations above the UFL, it
will certainly be present within the flammable concentration region at some
location. LFL concentrations for many materials can be found in the NFPA
Standard 325M, "Properties of Flammable Liquids," published by the National
Fire Protection Association.
The LFL represents the minimum concentration of fuel that must be present in
air for the mixture to be flammable. It is usually expressed as a volume percent,
which is equal to the mole percent under conditions at which the LFL is measured
(atmospheric pressure and 25°C). There is also a minimum oxygen concentration
required for ignition of any fuel. It is closely related to the LFL and can be
calculated from the LFL. The theory on which the calculation is based is that the
20
SAFET"Y,HEALTH, AND LOSS PREVENTlONIN CHEMICALPROCESSES
minimum amount of oxygen required for ignition is the stoichiometric quantity
required for complete combustion of the fuel at the LFL concentration. This
minimum oxygen concentration is frequently used as the maximum permissible
oxygen concentration in storage tanks or other places where fuel vapors may be
present under ordinary circumstances. In most industrial applications, an experimentally measured minimum oxygen concentration is used.
The minimum oxygen concentration required for ignition can be estimated by
multiplying the LFL concentration by the ratio of the number of moles of oxygen
required for complete combustion to the number of moles of fuel being burned.
The minimum oxygen concentration estimated by this method may not be accurate
enough for all purposes, particularly for some especially reactive substances. For
example, gases such as acetylene can decompose by an exothermic reaction that
proceeds very much like a combustion reaction even though there is no oxygen
present. In such cases there is no limiting oxygen concentration, because the
reaction can occur without the presence of oxygen. The important thing to remember is that the properties of the particular substance being considered must be
known and accounted for in any system design.
The oxygen concentration in a flammablemixture can be reduced by adding fuel
vapor to the air or by adding an inert material such as nitrogen or carbon dioxide.
If nitrogen is the diluent, estimating the maximum permissible oxygen concentration from the stoichiometry of the reaction works quite well. However, if carbon
dioxide is the inerting gas, slightly higher maximum permissible oxygen concentrations are measured. The higher maximum permissible oxygen concentrations
measured when carbon dioxide is present are due to the higher specific heat of
carbon dioxide. Some other inerting agents, such as the halons (halogenated
hydrocarbons) are effective at inhibiting ignition at even higher oxygen concentrations. Their effect is due to chemical effects rather than physical effects.
PROBLEM: Estimate the maximum permissible oxygen concentration for nbutane. The LFL concentration for n-butane is 1.9 mole percent.
(This problem is based on a problem in the text Chemical P m e s s Safety:
Fundamentals with Applications, by D. A. Crow1 and J. F. Louvar, published by
Prentice Hall, Englewood Cliffs, NJ.)
Problem No. 11
CHEMICAL ENGINEERING TOPIC: Fundamentals: Mass Balance
SAFETY AND HEALTH CONCEPT: Inerting and Purging
BACKGROUND: Many chemical plant operations require that vessels and piping
be inerted or purged. If a vessel is to be opened for maintenance or repair, for
example, and if the vessel has contained either toxic or flammable materials,
purging is required before workers can enter the vessel. For a vessel entry, piping
leading to or from the vessel will have to be blanked off and at least the portions
of the vessel that are open to the pipe will have to be purged as well. Purging must
be continued until the atmosphere in the vessel is safe for entry.
If the chemical in the vessel is flammable, purging must be accomplished in two
steps: first, the flammable material is purged from the vessel with an inert gas such
as nitrogen, and the nitrogen is then purged with air. When a vessel that is to be
used for storing or processing flammable chemicals is initially put into service, it
must be purged with an inert gas before flammable chemicals are put in the vessel.
This step is required to assure that a flammable mixture of the chemical and the
air in the tank does not form.
If the vessel or piping contains liquids, purging is sometimes accomplished by
washing. Either water or a detergent solution is used to wash the vessel or piping.
Usually, the vessel must then be dried before it is returned to service to prevent
contamination of its contents.
Regardless of the method used to purge equipment, the final step should be to
check the atmosphere in the equipment to make certain that the concentration of
flammable or toxic material has been reduced to safe levels. For tank entry, the
oxygen concentration must also be checked before the tank is entered. The
measurement of residual concentrationsis sometimes required by law; it is always
needed for good practice. Detailed procedures must be prepared and followed if
workers are to enter a vessel. These procedures include completion of a tank entry
permit that lists the detailed procedures to be followed.
PROBLEM: A 150-ft3tank containing air is to be inerted to 1% oxygen concentration. Pure nitrogen is available for the job. The tank has a maximum allowable
working pressure of 150 psia, so either of two methods is possible. In the first
method, air is purged by a continuous sweep of nitrogen. The nitrogen is simply
allowed to flow into the tank at essentially atmospheric pressure. It is assumed that
the nitrogen mixes rapidly and completelywith the air in the tank, so the gas leaving
the tank has the same concentration of oxygen as the gas in the tank.
22
SAFETY,HEALTH,AND LOSS PREVENTION IN CHEMICALPROCESSES
In the second technique, the tank is pressurized, the pure nitrogen inlet stream
is turned off, and the gas mixture in the tank is then exhausted, lowering the
pressure in the tank to atmospheric pressure. If the pressurization technique is
used, multiple pressurization cycles may be required, with the tank returned to
atmospheric pressure at the end of each cycle. Complete mixing is assumed for
each cycle.
In this problem, you may assume that both nitrogen and air behave as ideal gases
and that the temperature remains constant at 80°Fthroughout the process. Determine the volume of nitrogen (measured at standard conditions of 1.0 atrn and 0°C)
required to purge the tank using each purging technique. For the pressurization
technique, assume the pressure in the tank is raised to 140 psig (a little below its
maximum working pressure) with nitrogen and then vented to 0 psig.
(This problem is based on a problem in the text Chemical Process Safety:
Fundamentals with Applications, by D. A. Crow1 and J. F.Louvar, published by
Prentice Hall,Englewood Cliffs, NJ.)
Problem No. 12
CHEMICAL ENGINEERING TOPIC: Fundamentals
SAFETY AND HEALTH CONCEPT: Explosions
BACKGROUND: There are many cases when it is good practice to vent an
enclosure or building so structural and mechanical damage is limited in the event
of a deflagration within the enclosure. The National Fire Protection Association
Standard 68, "Guide for Venting of Deflagrations," provides guidance on how the
enclosure should be built to provide proper venting. The discussion following is
based very closely on NFPA 68. The most recent version of NFPA 68 is the 1988
edition. The standard is updated periodically, as are all NFPA Standards.
The American Heritage Dictionaly defines an explosion as "The sudden rapid
violent release of mechanical, chemical, or nuclear energy from a confined region;
especially, such a release that generated a radially propagating shock wave accompanied by a loud, sharp report, flying debris, heat, light, and fire." NFPA 68
provides a much more restricted definition: "The bursting or rupture of an
enclosure or a container due to the development of internal pressure from a
deflagration." A deflagration is "Propagation of a combustion zone at a velocity
that is less than the speed of sound in the unreacted medium." A deflagration is
differentiated from a detonation, which is, in NFPA parlance, "Propagation of a
combustion zone at a velocity that is greater then the speed of sound in the
unreacted medium." Obviously, the technical definitions used by NFPA are intended to be more precise than those used in ordinary conversation.
We will be interested in three kinds of deflagrations: those involving gases, those
involving dusts, and those involving mists. Deflagrations can occur in any flammable gas if the concentration of the gas in air is within the flammable concentration range. The flammable concentrationrange of gases is a unique property of the
gas at a given temperature and pressure. Deflagrations in dusts are more difficult
to quantify. First, since the dust is in the form of fine particles (NFPA 68 defines
dust to be composed of particles 420 microns or less in diameter), the rate of
deflagration will depend on the size of the dust particles. The concentration of dust
required to sustain a deflagration will also depend on the size of the particles. Dusts
may adsorb or absorb moisture from the atmosphere, and the amount of moisture
may also affect the deflagration rate. Mists are fine liquid droplets dispersed in the
atmosphere. Their combustion properties also depend on the droplet size and
concentration.Many mists will ignite easily, even though the same liquid would not
ignite if in a pool. Combinations of gases, dusts, and mists also occur frequently,
leading to even more complex situations.
S A F E T Y , HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
24
Three things must be present for a deflagration to occur. There must be a fuel
in the proper concentration, an oxidant in great enough quantity to support the
combustion, and an ignition source strong enough to ignite the fuel-oxidant
mixture. The most frequent oxidant is oxygen in air, and we will limit our attention
to mixtures of fuel in air; but it should be kept in mind that other oxidants may be
present in special situations.The ignition source ordinarily need not be very strong.
A spark or a small open flame is usually all that is necessary to ignite a flammable
mixture.
NFPA 68 provides a simple venting equation for determining the vent area
required to prevent unusually large damage to a low pressure structure (less than
1.5 psig), for example a building. The equation is
where
Av = vent area (ft2 or m2)
C = venting equation constant
As = internal surface area of enclosure (ft2 or rn2)
Pred = maximum internal pressure that can bewithstood by the weakest struc-
tural element (psi or kPa)
The venting equation constant, C, is given in NFPA 68 for various classes of gases
and dusts, and is based on experiments run with the actual dust or gas involved. It
is important that the dust be tested to establish the value of C to ensure reliable
results from the venting equation. The constant C has units of (pressure)05. One
side of the enclosure is always assumed to be at atmospheric pressure, so the
pressure used in the equation is the gauge pressure. The internal surface area of
the enclosure includes the floor, the. ceiling, and the walls (i.e., all the area of the
inside of the enclosure). There are other cautions that must be taken for the final
design of the vents, including the design of the vent covering, the provision of
sufficient strength for reaction forces, and the location of structural members
within the enclosure. A detailed design will require close reference to NFPA 68
and careful attention to all its provisions. However, we can make a quick estimate
of the area required from the venting equation for a simple situation.
PROBLEM: A building is 200 ft long, 50 ft wide, and has a 15-ft-highceiling. It is
connected to a tee at one end that is 80 ft long and 30 ft wide, with a 15-ft-high
ceiling. The plan view is shown in the sketch on page 25. Corn starch and powdered
nonfat dry milk are being packaged in the building. Corn starch is in Dust Hazard
Class 2, which has a value of C of 0.12 (psi)05 and owdered nonfat dry milk is in
s
Dust Hazard Class 1,with a C value of 0.10 (psi)0
.
PROBLEM N0.12
Estimate the venting area required for the building for each of the products. A
structural analysis of the building has determined that it can withstand a maximum
internal overpressure of 0.25 psi.
Problem No. 13
CHEMICAL ENGINEERING TOPIC: Fundamentals
SAFETY AND HEALTH CONCEPT: Process Control, Interlocks, and Alarms
BACKGROUND: Almost all chemical engineering processes require some form
of process control in order to be operated safely and efficiently. In many cases, the
instrument that is used for detection or control is separated from the process area
because the analytical equipment must be located indoors or away from the
environment of the process area. In such cases, samples are frequently taken at a
process area and then transmitted to the analytical equipment through relatively
long runs of small diameter tubing. Thus, there is a delay between the time a process
variable changes and the time the change is detected by the analytical equipment.
The analyticalequipment may be used to raise process alarms as well as for process
control, and it may be a critical element in process control for emergency shutdown. There may be a significant difference between the time a process goes
outside of its normal control limits and the time emergency operationsbegin. Delay
in normal control can lead to delay in process changes, which may, in turn, lead to
product deterioration and safety problems. Process design should enable samples
to be taken and analyzed quickly enough for proper process control, as well as for
proper alarms and emergency shutdown.
PROBLEM: In a chemical plant, air samples from a process area are continuously
drawn through a %in. diameter tube to an analytical instrument located 125 ft
from the process area. The Win. tubing has an outside diameter of 0.25 in. 6.35
mm) and a wall thickness of 0.030 in. (0.762 mm). The sampling rate is 10 cm /sec
under ambient conditions of 22°C and 1.0 atm. The pressure drop in the transfer
line can be considered negligible. Chlorine gas is used in the process, and if it leaks
from the process, it can poison workers who might be in the area of the leak.
Determine the time required to detect a leak of chlorine in the process area with
the equipment currently installed. You may assume the analytical equipment takes
5 sec to respond once the gas reachesthe instrument. You may also assume that
samples travel through the instrument sample tubing without dilution by mixing
with the air ahead of the sample. Suggest methods of reducing the sampling time
if the current detection time seems too long to be acceptable.
How would your answer be affected if the sampling system is connected to a
sequential sampler which must sample five different streams, each of which is
drawn separately and has the same total sample delay time? If the samples are
I
PROBLEM NO.13
27
drawn to the detector continuously and the instrument response continues to be 5
sec, what is the maximum sampling delay time?
Would you fmd the system acceptable if the process contained a less toxic
material, such as ammonia?
Both ammonia and chlorine have good warning properties (they can be smelled
at very low concentrations). Would a material such as carbon monoxide, which
cannot be detected by human senses, require different considerations? Would a
diierent detection system be required?
Problem No. 14
CHEMICAL ENGINEERING TOPIC: Fundamentals
SAFETY AND HEALTH CONCEPT. Explosions
BACKGROUND: National Fire Protection Association Standard No. 68 (1988
edition) defines an explosion as "The bursting or rupture of an enclosure or
container due to the development of internal pressure from a deflagration." A
deflagration is "Propagation of a combustion zone at a velocity that is less than the
speed of sound in the unreacted medium." Detonation, which is "Propagation of
a combustion zone at a velocity that is greater than the speed of sound in the
unreacted medium," may also occur, and if the total energy release is the same, a
detonation will be more damaging.
Deflagrationsmay occurwhen three types of fuel-air mixtures are present: those
with fuels in the form of gases, mists, and dusts. The fuel must be within the
flammable range for the deflagration to be initiated. The oxygen in the air serves
as the oxidant, and a small spark or flame is all that is needed to ignite the mixture.
If a deflagration occurs in a closed space, such as a building, an explosion may
occur in which the building is destroyed. There are several methods available for
preventing an explosion, including prevention of formation of flammable mixtures
in the building. If, despite efforts at prevention, flammable mixtures do form and
they are ignited, the damage to the building may be reduced by venting the
deflagration. NFPA 68 provides a simple equation for determining the vent area
required to prevent unusually large damage to a building if a deflagration occurs
in the building. The equation is
where
Av = vent area (ft2 or m2)
C = venting equation constant
As = internal surface area of enclosure (ft2 or m2)
Pred = maximum internal pressure that can be withstood by the weakest struc-
tural element (psi or kPa)
The venting equation constant is given in NFPA 68 for various classes of gases and
dusts. It is based on an extensive set of experimental tests using the gas or dust in
question. The tests are difficult to perform, and the results vary somewhat from
test to test, so the NFPA 68 values are meant to be used as a guide to establish
PROBLEM NO.14
29
and it as
conservative design bases. The units of the constant are
always assumed that the outside of the enclosure is at atmosphericpressure, so the
pressure used in the equation is the gauge pressure. The internal surface area
includes the floor, the walls, and the ceiling. While a complete design will require
close reference to NFPA 68, we can make a simple estimate of the vent area
required from the venting equation for simple building layouts.
PROBLEM: A building housing natural gas compressors is 300 ft long, 75 ft wide,
and has walls 20 ft high. The roof is gabled and has a pitch of 3/12 (i.e., the roof
rises 3 ft for each 12 ft of horizontal run). Natural gas is primarily methane, for
which Cis 0.37 (kpa)O5 and the building is designed for a maximum overpressure
of 0.3 psi. Estimate the venting area required for the building.
Problem No. 15
CHEMICAL ENGINEERING TOPIC: Fundamentals
HEALTH AND SAFETY CONCEPT: Storing, Handling, and Transport; Static
Electricity
BACKGROUND: A number of accidents have occurred in the past due to discharge of electrical charges accumulated on the surface of objects or materials that
are not properly grounded. The accumulated charge of this nature is called "static
electricity." Static electricity is generated whenever objects of a different conductivity are brought together and then separated, or whenever the objects are rubbed
together. Flowing fluids could generate such static charges.
If accumulated electrical charge is not dissipated to the ground through lowresistance conductors,then there exists a possibility that the potential will build up
to such an extent that sparking will be produced. A common example is lightning,
which is the discharge of static charges from clouds.
Since it is frequently impractical to prevent the generation of static charge, it is
appropriate to provide means by which the charges can be dissipated without the
accumulation of large potential differences required for arcing.
Charge accumulation is usually prevented by a combination of techniques, which
include providing readily available grounding, the bonding of conducting equipment together (with electrical conductors), and the minimizing of generation by
use of appropriate flow patterns.
Another obvious method of preventing accidents is to avoid the presence of
flammableor explosive mixtures. While this may frequently be impractical, it might
sometimes be a safety practice that should be considered. In the problem below,
you are asked to determine what temperatures might be required in order to
prevent the formation of flammable mixtures in air for a number of flammable
solvents.
PROBLEM: In order to provide an added measure of protection against explosion
or fire due to static electricity discharging, you are asked to determine what
temperatures should be employed in drum-filling operations so that flammable
mixtures, that is concentrations equal to or greater than the lower flammable limit
(LFL), will not be produced by vapor air mixtures in equilibrium with the liquids.
The liquids being considered are the following:
Acetone
Methyl ethyl ketone
Benzene
Toluene
Ethyl benzene
m-Xylene
PROBLEM NO. 15
31
Your instructor will provide you with literature references for the LFL values,
or else will supply the data. To determine the maximum temperature of handling
so as not to create a concentrationgreater than the LFL, you should determine the
temperature where the vapor pressure is such as to just produce such a mixture.
As an additional precaution, it is desirable to prevent the generation of concentrations greater than 25% of the LFL. Determine also what temperatures this
would require.
Please make a judgment regarding which, if any, of these temperatures would
be practical for drum-filling operations. If you feel that the temperature control
method is impractical, try to suggest an alternative method for the prevention of
the formation of flammable mixtures for the drum-filling operation.
Problem No. 16
CHEMICAL ENGINEERING TOPIC: Fundamentals; Design
HEALTH AND SAFETY CONCEPT. Toxicology and Industrial Hygiene; Protective Equipment
BACKGROUND: A potential hazard of the workplace that is sometimes overlooked is excessive noise. Excessive noise has been found to cause several types of
physiological damage. Damage to hearing is perhaps the most important type of
damage, but there are others, including heart disease, ulcers, and other stress-related diseases. Excessive noise can also interfere with communication, interfere
with sleep or relaxation, and could be the cause of failure to hear an alarm.
Noise, of course, is sound. Sound is caused by rapid fluctuations of air pressure
on the eardrum of the listener. The sound pressure means the root mean square
value of the pressure changes above and below atmospheric.
The usual value cited for the threshold of audibility, that is, the minimum sound
level that can typically be heard is a sound pressure of about 0.00002 Pa. At the
upper limit, where there is physical pain associated with the sound, the sound
pressure is about 20 Pa. The range of interest then, is a million fold or more.
To accommodate such a wide range, one could not readily use a linear scale, so a
logarithmic scale is used. The scale used is called a decibel scale, and the sound
level units are called decibels.
The decibel scale is arranged so that the lower limit of audibility, that is, 0.00002
Pa sound pressure, corresponds to 0 decibels (dB). Then, the sound level, or
intensity, is defined by
where I is the sound pressure and l o is the reference level (i.e., 0.00002 Pa). The
20-Pa sound pressure corresponds to 120 dB.
Following are some typical sound levels for your comparison:
Approximate
Activity
sound level (dB)
Turbojet Engine
160
Compressor
120
Rock and Roll Band
112
Power Lawn Mower
95
Conversation
70
40
Quiet Room
PROBLEM NO.16
33
Due to the possibility of health effects as well as for comfort, the American
Conference of Governmental Industrial Hygienists (ACGIH) ,has established
acceptable "doses" of noise. The time of exposure is analogous to a dose of toxic
material. The acceptable time is related to the intensity according to the following
table:
Duration
(hrlday)
16
8
4
2
1
112
114
118
Sound level
(dB)
80
85
90
95
100
105
110
115
There is to be no exposure to either continuous or intermittent levels above 115
dB without hearing protection. If workers are exposed to more than one sound
source, then the cumulative effect is to be considered. The cumulative effect is
evaluated by dividing the actual time of exposure by the permissible exposure time
for each source. These quantities are then added. If the sum is greater than unity,
the exposure limit is exceeded.
This can be expressed in equation form as follows:
where T is the actual time of exposure and P is the permissible time of exposure.
If E is greater than or equal to unity then hearing protection would be required,
or the noise level must be reduced.
PROBLEM: A worker is required to spend his or her working day in four different
locations, each of which has different noise levels. The various locations are
described as follows:
Location A, where a compressor is operating with a noise level of 100 dB.
Location B, where a blower and a pump combine to produce a noise level of 95 dB.
Location C, where a packaging operation causes a noise level of 110 dB.
Location D, which is a computer room, where the noise level is 85 dB.
Walking in between locations, the noise level is usually about 80 dB. Approximately
20 min is spent in Location A and 45 min in Location B. The worker will spend 5
min in Location C, and 4.5 hr in Location D. The rest of the 8-hr work day is spent
34
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
walking between locations. There is also the half-hour lunch break, where the noise
level is about 75 dB.
Would hearing protection be required for this worker in order to satisfy the
ACGIH guidelines?
If hearing protection were supplied in locations A, B, and C, so that the worker
would be exposed to a constant level of 80 dB while in these locations, would that
be adequate?
What would you say about this person spending 2 hr in the evening at a rock
concert?
Problem No. 17
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: When designing a ventilation system, the engineer must bear in
mind that an adequate duct system must be provided. If the hoods pick up the
various contaminants, there will be nothing gained ifthe duct system is inadequate
to remove the material collected. Although a complete ventilation systemmay grow
by modification over time, it must still be an integrated unit capable of performing
its function.
A complete ventilation system consists of the hoods, where the contaminants are
picked up with the air; a duct system, which serves as a path for removing the
contaminants; air-moving equipment (blowers),which suppliesthe energy to move
the air; air inlets, which must be supplied with clean air, either fresh air or cleaned
recirculated air; and a discharge system. Often it is also appropriate to have an air
cleaner included before the blower or fan prior to discharge, and most certainly
prior to recirculation.
Particulate matter may settle out in the ducts if the velocity is not adequate to
transport the particles. For smokes, fumes, gases and vapors, any economicvelocity
is adequate. However, for larger or heavier particles, if the duct velocity is too low,
the particles will settle out and perhaps block the flow, or the load may be heavy
enough to cause part of the ductwork to collapse. The book, Industrial Ventilation - A Manual of Recommended Practice (19th ed., ACGIH, 1986), provides a
great deal of both practical and theoretical information on the design of ventilation
systems. Included in the book are recommended duct velocities, on which the table
on the next page is based.
Iron foundries, where cast iron products are manufactured, have particularly
difficultproblems with the ventilation scheme because there is a great deal of dust
generated from the grinding of castings. Transport of the grinding dust through
the ductwork requires a fairly high velocity. Also, silica, an especially undesirable
dust, is a large component of the grindings because the casting molds are made of
sand. Silica inhalation can lead to the disease known as "silicosis," an impairment
of lung function, frequently incapacitating. Thus, it is important that a ventilation
system function so as to minimize the exposure of workers to airborne silica.
PROBLEM: Foundry grindingswill be transported through a single duct from five
grinding work stations. Each work station will have a hood that requires 3000 CFM
36
SAFETY, HEALTH, AND LOSS PREVENTION IN CHEMICAL PROCESSES
of air flow. Determine the required duct diameter to assure adequate transport of
the dust. Find the power required for a combined motorblower efficiency of 40%.
The duct equivalent length is 400 ft and there is an entrance loss of 0.4 in. H20 at
the hood entrances. There is a cyclone air cleaner with a pressure drop of 4.1 in.
H20.
Motors are available in increments of 1hp up to 5 hp, and in increments of 5 hp
above that. Recommend a motor size. Refer to the duct velocity table, which
follows.
Recommended Duct Velocities (Minimum Velocities Recommended for the
Transport of Various Types of Contaminant)
Type of Contaminant
Examples
Recommended
Velocity (ftlmin)
Vapors, gases, smokes,
fumes
All vapors, gases,
metal oxide fumes
Any economic
velocity
Very fine light dust
Cotton lint, wood flour
2000-2500
Dry dusts and powders
Fine rubber dust, cotton
dust, light shavings
2500-3000
Average industrial dust
general foundry dust
Sawdust, grinding dust,
limestone dust
3500-4000
Heavy Dusts
Metal turnings, sand
blast dust, lead dust
4000-4500
Heavy or moist dust
(very heavy dust)
Lead dust with small
chips, moist cement dust
quick lime dust
4500 and up
Adapted from Industrial Ventilation-A Manual of Recommended Practice (19th
ed. Cincinnati: ACGIH, 1986), which should be consulted for additional detail.
Problem No. 18
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: Respirators are devices that are worn over the face to prevent
inhaling harmful material. Normal practice is to provide workplace air which is
suitable for breathing without any such protection, but there may be times when
the systems providing for clean air fail to do so for any of various reasons. At these
times, respirators may be the only method of protection available. They are
therefore very important safety equipment items, and it is vital that they be used
appropriately, in recognition of their limitations. Most of the other devices that
maybe used for worker protection have some kind of backup in the event of failure,
but for respirators, there is none.
Chemical cartridge respirators provide protection against vapors and gases
being inhaled. One type of device uses an adsorbent, such as charcoal to adsorb
organic vapors and thus to purify the air that the wearer inhales. The bed of
charcoal will remove essentially all of the contaminant until breakthrough occurs,
after which the concentration will rise very rapidly.
Respirators are frequently used in dusty conditions to purify the breathing air
of workers who must be there. If the only contaminant is a particulate material,
then chemical adsorption or reaction will not be required and only mechanical
filtration will be needed. A respirator that serves only to remove particulate from
the breathing air will continue to serve adequately until the pressure drop across
the filter element and the accumulated cake becomes excessive. The limitation on
pressure drop may be the worker and his respiratory capabilities, or it may be that
higher pressure drops (lower pressure under the facepiece) promote leakage, or
both.
Individuals diier a great deal in their ability to tolerate pressure drop through
a breathing device. A worker who feels that he or she is "gasping for air" will be
tempted to remove the respirator and inhale deeply. Thus, the pressure drop must
be kept low enough to prevent the sensation of being short of breath.
OSHA has established criteria for the pressure drop across filtering respirators.
Specifically, at a flow rate of 85 Llmin, the initial (clean filter) pressure drop may
be no more than 20 mm H20, and after a specified test, no more than 50 mm H20,
for dusts, fumes, and mists with a single use filter. (There are different requirements for some other situations.)
38
SAFETY,HEALTH, AND LOSS PREVENTION IN C H E M I C A
PROBLEM: A particular filter was tested for compliance by passing a test dust
through the filter medium at 32Llminfor 90 min. Before the test, the pressure drop
through the filter was 17 mm H20 at a flow rate of 85 Llmin and after the test was
43 mm Hz0 at a flow rate of 85 Llmin. The test dust was at a concentration of 54
mg/m3.
If this filter element were to be used by a worker for an extended time, breathing
at a rate of 40 Llmin (average), how often will the filter element have to be changed
if the worker's pressure drop tolerance is 35 mm Hz0 and if he is breathing a dust
which is at an average concentration of 16 mg/m3?
Note that if the worker's respiration rate is 40 Wmin, remember that he spends
only about half of the time inhaling, and the other half exhaling. Thus the flow rate
through the filter during inhalation is more than 40 Llmin.
Problem No. 19
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics
HEALTH AND SAFETY CONCEPT: Process Control, Interlocks and Alarms
BACKGROUND: Instrumentationis widely used in the modern chemical processing plant to maintain process variables within acceptable ranges. Keeping the
processvariableswithin range is also an absolute necessitywhen there are potential
hazards in the process.
So many of our modern practices involve high temperature and pressure that
instrumentation must be heavily depended on to provide the controls to prevent
accidents. However, instruments sometimes fail to function, and sometimes the
utilities to an instrument may fail. Electronic instruments will provide no protection
when the power supply fails, and pneumatic instruments will not function if the air
supplyfails. Thus, when disastrous consequences will result from instrument utility
failure, it is necessary to have backup.
Pneumatic instrumentsrequire compressed air as their utility. In a large or even
a modest-sized chemical complex, the compressed air will come from a centrally
located compressor, which will supply air for a large number of instruments. A
backup supply is often provided through the use of portable cylinders of compressed air, and occasionally nitrogen instead of air. In operation, pneumatic
instruments bleed air at a somewhat variable rate through an orifice. The orifice
is slightly restricted by a flapper, so that the position of the flapper relative to the
outlet of the orifice varies the pressure behind the orifice. It is this pressure
variation that is used to control the valves or other control elements.
PROBLEM: A small control room has 12 pneumatic controllers. During a plant
emergency, the instrument air compressor that supplies air for these controllers
suffered damage and no longer operates. Automatic tripping devices enabled the
emergency supply for this control room. The emergency supply consists of a
cylinder of compressed air. The cylinder volume is 1.6 ft3, and has air at 2200 psig.
We wish to estimate how long the supply will last.
Assume that each of the instruments has a 0.75-mm diameter orifice, and that
due to the presence of the flapper, the average effective orifice coefficient is 0.45.
The pressure behiid the orifice ranges from 5 to 20 psig, usually. Assume that
it is at 14 psig, as an average, during the emergency.
When the available pressure drops to no more than 25 psig, the devices will no
longer function.
40
SAFETY, HEALTH, AND LOSS PREVENTION IN CHEMICAL PROCESSES
Now assume that a maintenance crew had replaced the air with nitrogen at the
same temperature and pressure because the air was needed somewhere else.
Furthermore, since nitrogen will enter the control room, the oxygen content of the
air in the room will start to decrease. If a worker enters a room where nitrogen has
dispIaced air so that the oxygen content is below about 16%, he or she will be
overcome and lose consciousness, probablybefore realizing that anything is wrong.
If no outside air enters the room, but the air in the room bleeds out, can the oxygen
content be reduced to below 16% by the nitrogen? The room is 8 ft by 10 ft by 8 ft
high.
NOTE: OSHA regulations stipulate that the oxygen content must be at least 19%
for room entry. However, people have variable tolerances due to individual
differences and differences in physical condition.
Problem No. 20
CHEMICAL ENGINEERING TOPIC: Momentum Transfer: muid Mechanics
SAFETY AND HEALTH CONCEPT: Fire Protection
BACKGROUND: Although fires occur very infrequently in chemical operations,
when they do occur, they can be very destructive. The damage caused by a fire can
be kept to a minimum if a properly designed fire protection system is installed.
There are many facets to a well-designed fire protection system, including the
detection systems, the fire protection hardware, and the fire fighters. The fire
protection systems may rely on several kinds of fire extinguishing and control
agents, including dry chemicals, foams, carbon dioxide or halons, and water. Water
is included as the major fire protection agent in almost all situations (there are a
few cases where water may be incompatible with the equipment or materials being
protected). Water is generally cheaper than other agents, and it is generally
available in large quantities. The high heat capacity and high heat of vaporization
of water also contribute to its suitability as a fire control agent. Water is also inert
(with respect to most other materials and chemicals) and can be stored and
delivered relatively easily. Thus, water is supplied to most plant locations for fire
control.
There are some problems to be considered when using water as a fire control
agent. One of the first things to consider is that the quantity of water used to fight
a fire is usually many times the theoretical minimum needed for extinguishment or
control. Thus, there is a large amount of runoff water that must be disposed of. At
first thought, it would seem that the runoff water could just be allowed to flow
through drains and be disposed of as though it were rain water. In some cases, that
can be done, but in many cases the runoff water will be contaminated with
chemicals that are either process chemicals spilled during the fire or products of
pyrolysis or combustion during the fire. In those cases, the water may be so
contaminated that it cannot simply be discharged to a storm drain or sewage
treatment system. It must be collected and treated before discharge. In almost all
cases, the rate at which water is applied for fire protection is much greater than
the rate at which water will fall during a rainstorm. Thus, special drainage systems
must be provided to assure that the runoff water can be disposed of properly.
The design of fire water distribution systems is performed in the same way as
any set of hydraulic calculations, although variations in methodology are sometimes used to reduce the amount of work. In addition, the fire water system will
usually have several branches and loops so that the flow calculations are more
complicated than they would be for a single pipeline leading from one point to
42
SAFETY, HEALTH,AND LOSS PREVENITONIN CHEMICALPROCESSES
another. The design calculationsmay also have to account for the fact that the flow
may differ from time to time because a single fire water supply system will be used
for an entire plant, and it is not likely that the entire system will be in operation at
once.
PROBLEM: Water is supplied to a pump for distribution to a fire water spraying
system. The water is taken from an outdoor reservoir 100 m from the pump inlet,
and the water surface is 5 m above the inlet to the pump. The pump is to deliver
water to a water spray system on top of several liquefied gas storage tanks 500 m
from the pump, at a discharge elevation of 35 m. The pressure at the nozzle
manifold must be at least 700 kPa and water is to be delivered to the nozzle
manifold for the spraying system at a rate of 900 m3/hr. The net positive suction
head of the pump is 10 m. The pipeline from the pump to the nozzle manifold is
10-in.schedule40 steel pipe. Pressure losses caused by fittings and valves upstream
of the pump are equivaIent to 15m of piping, and pressure losses caused by fittings
and valves downstream of the pump are equivalent to 50 m of piping.
a. What is the minimum diameter for the supply piping?
b. What is the power required to drive the pump? Assume the pump has 80%
efficiency and give your answer in kilowatts.
c. For what outlet pressure should the pump be designed?
Problem No. 21
CHEMICAL, ENGINEERING TOPIC: Momentum Transfer: Fluid Mechanics
SAFETY AND HEALTH CONCEPT. Fire Protection
BACKGROUND: While very large fires occur at chemical plants occasionally,
most of the fires that occur are smaller and present less danger to the operators.
In most cases, the plant will have either a fire brigade or the operators will receive
some training in fire protection. Part of that training will usually include the use of
manually applied water sprays. Although there are other fire extinguishing and
control agents that can be used in chemical operations, water should always be
available. The water can be used either for direct extinguishment of the fxe or for
exposure protection. Exposure protection refers to the practice of spraying equipment near the fire so it will not be damaged, leading to failure, and thus spreading
the fire. Sometimes exposure protection will be required for long periods, especially if a fire is large, in which case, it may not be possible to extinguish it, and it
must simply be allowed to burn out as the fuel is exhausted. During the burning
period, the surroundings must be protected to minimize fire losses.
For some fires, particularly those where the fuel for the fire is an ordinary solid
material, water sprays are directed at the fuel, coolingit and extinguishing the fire.
However, in many cases, such as those where the fuel will react with water, or for
liquid fuels that do not mix with water, spraying the fuel with water will do more
harm than good. Then, it may not be possible to extinguish the fire. If the fire cannot
be extinguished or reduced in size through the direct applicationof water, the water
can still be used to minimize the damage that might otherwise occur. Equipment
and buildings near the fire can be sprayed with water to cool them and prevent
damage by the fire. In addition, water sprays are frequently used to shield
firefighters or emergency operations crews during fire fighting and emergency
operations.
Water may be applied either through fmed systems or through portable systems.
Fixed systems are made of permanently mounted piping and nozzles and do not
require that a firefighter remain in the immediate area. They are frequently
provided in critical areas where access is limited or where firefighters cannot gain
access. Manual fire fighting using water from hoselines is a traditional practice for
locations where smaller quantities of water are needed, where there are enough
trained personnel to man the hoses, and by fire departments or mutual aid groups
that assist in fire protection. Hoselines are made from fabric reinforced material
that can withstand high pressures. Many plants use standard 12/2-in.or 2b5-in.
44
SAFETY, HEALTH,AND LOSS PREVENTION IN CHEMICALPROCESSES
hoses with nozzles. These serve as the first line of defense because they can be put
into operation relatively quickly if personnel are available and well trained.
There are two problems associated with the design and use of fire water spray
systems. One is to design the nozzle and piping system so that the required rate of
water can be delivered. Such calculations can be made through the use of standard
orifice and nozzle equations. The second is to determine the forces that must be
resisted during discharge. The fire fighting nozzle will have a substantial reaction
force, and the systemmust be designed to withstand the force. Permanent supports
and reinforcing structures must be provided for fixed systems, and portable
systems must be designed with the idea in mind that they will have to be held by
fire fighters or be provided with temporary supports.
PROBLEM: The nozzle on a fire hose is designed to minimize friction losses. It
can be considered to be an orifice with a discharge coefficient of 0.97 if the nozzle
is designed to deliver a solid stream of water. Nozzle flow rates are frequently
calculated by
where q is the flow rate through the n o d e in gallons per minute; d is the nozzle
bore diameter in inches; and P is the pressure at the nozzle, in psig.
a. Starting with the basic equations for flow through an orifice, show that the
nozzle flow rate equation is applicable and the dimensional constant 28.95 is
correct. What assumptions are inherent in the equation? What are the dimensions
of the constant?
b. Find the bore diameter required for delivery of 100gaVmin at a nozzle pressure
of 100 psig.
c. Calculate the reaction force on the nozzle. Would you judge it to be tolerable
for a single, unaided firefighter?
Problem No. 22
CHEMICAL ENGINEERING TOPIC: Momentum Transfer: Fluid Mechanics
SAFETY AND HEALTH CONCEPT: Storage and Handling: Fluids
BACKGROUND: Large quantities of liquefied petroleum gas (LPG) are used
each year, primarily as a fuel. The use as a fuel is seasonal, with much more being
used during the winter than during the summer. However, the rate of production
is more uniform, so supplies are increased during the summer and reduced during
the winter. One method of storing the LPG is to inject it into a cavern in a salt
dome. In some locations, primarily in Texas and Louisiana, there are large natural
salt deposits. These deposits are broad and deep, and if a well is drilled into them
and water is injected, some of the salt will be dissolved. If injection of fresh water
is continued, a large cavern will be formed in the salt dome. (The salt "mined" by
this method is usually used for the production of chlorine.) The caverns may have
a volume of a million or more barrels (a barrel, as used in the petroleum industry,
is equal to 42 gallons). Because of the shape of the salt domes and the method of
solution mining used to form the caverns, the caverns are irregular in shape, but
tend to have a much smaller diameter than length. Figure 1shows an idealized
sketch of a cavern in a salt dome. It appears, ideally, as a long vertical cylinder.
The tubing entering the cavern is arranged so that the cavern can be kept full at
all times. If the cavern is not kept full of liquid or gas under pressure, it may
collapse. In the case of propane storage, the pressure in the cavern is high enough
for the propane to be kept as a liquid under the ambient temperature of the cavern.
At least two streams must be able to enter and/or leave the cavern. That is usually
accomplished by providing a concentric pipe system, with a large outer pipe and
a smaller inner pipe. Normal operation for storage of LPG begins with the cavern
full of saturated brine followingthe formation of the cavern. LPG is then pumped
into the cavern through the annular space between the inner and outer pipes. The
brine in the cavern is forced out the inner pipe. The inner pipe extends nearly to
the bottom of the cavern and the outer pipe ends near the top. Thus, when the LPG
is pumped into the cavern, it enters at the top. The LPG has a lower density than
the brine, so it floats on top of the brine in the cavern. The liquid levels are
monitored carefully to assure that LPG is not forced out of the cavern during filling.
The brine forced from the cavern is transferred to a large storage pit, where it
remains until the LPG is needed.
When LPG is needed, it is forced from the cavern by pumping brine back into
the cavern. The brine enters through the inner pipe near the bottom of the cavern,
and the LPG is forced out of the cavern through the annulus between the two pipes.
46
SAFEI17, HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
??m
PROPANE
Figure 1. Schematic of salt dome storage well.
Again, the process is carefully monitored to keep brine from being displaced
through the LPG piping.
There are a number of potential hazards associated with salt dome storage. They
are similar to those in other industries that handle flammable liquids and gases.
AII involve the flammability of the LPG (or other products) that are stored in the
PROBLEM NO. 22
47
cavern. Special precautions should be taken to assure that none of the product is
spilled from the piping. Care must also be taken to assure there is no leakage from
the cavern, either through the cavern walls or the piping that penetrates the salt
dome to the cavern. Automatic shutdown systems should be installed to isolate the
cavern and its contents in the event of a failure of the surface piping. Special rules
govern the period of operation before the well must be taken out of service and
tested. The piping is subject to high salt concentrations, and corrosion is always a
problem. Gas detectors and fire detection systems should be provided to monitor
for hazardous conditions. Personneltraining must be kept current, and emergency
response training should be practiced periodically. Other potential safety
problems are encountered as well, but accessibility and low cost make salt domes
an excellent location for storage of hydrocarbons. In fact, the Strategic Petroleum
Reserve is designed to utilize underground storage in caverns mined in salt domes.
Safety considerations play a large part in its design and operation.
PROBLEM: A cavern in a salt dome is used for storage of LPG. The LPG has the
properties of propane. The top of the cavern is 780 ft below the ground surface,
and the outer pipe extends 800 ft below ground level. The end of the inner pipe is
2300 ft below ground level. The cavern has an average diameter of 50 ft. All piping
is Schedule 160 steel; the inner pipe is nominal 8-in. diameter, and the outer pipe
is nominal 14-in. diameter. Corrosion has increased the surface roughness of the
pipe to 0.002 ft. LPG is to be injected and produced from the cavern at rates
consistent with production and demand,respectively. The brine may be considered
to be saturated with salt. At the conditions of the cavern, the brine has a specific
gravity of 1.194 and a viscosity of 2 cp. The LPG has a specific gravity of 0.49 and
a viscosity of 0.1 cp. The temperature rnay be assumed to be constant at 80°F
1. If the cavern is full, with the propane level just at the end of the inner pipe,
and no transfer is taking place, what will the pressure in the propane line be at the
well head (ground level)?
2. If propane is being injected into the well at a rate of 70,000 bbl/day, what
well-head pressure will be required at the time the propane just reaches the level
of the inner pipe?
3. If the propane pump is 85% efficient, how many horsepower will be required
for the pump used to fill the cavern?
Problem No. 23
CHEMICAL ENGINEERING TOPIC: Momentum Transfer: Fluid Mechanics
SAFETY AND HEALTH CONCEPT: Storage, Handling, and Transport
BACKGROUND:Many different chemicals are used in modern chemical processing operations. Some of the chemicals normally do not present a hazard to humans
in their chemical effects (air and water, for example). Others are flammable, or
toxic, or both, and can create hazards for humans and the environment if they are
not kept under careful control. Any material used within the plant must be moved
to the point where it is needed, used for a chemical process or unit operation, and
the product or used material moved away for storage, continued processing,
ultimate use, or disposal. In most plants, the most convenient way to transport
materials from one place to another is through piping. Even solids may be
transported through piping as fluidized gas-solid mixtures or as liquid-solid
slurries. Fluid transport is preferred because the materials can be prevented from
release to the atmosphere, they can be handled by machinery that is readily
controlled,and the pumps (or compressors) used for moving the fluid are relatively
simple in design and economical in operation. If fluids are moved through piping,
the pressure in the piping must be high enough to keep liquids in the liquid phase,
to provide sufficient pressure drop to move the fluid through the piping, and to
keep the fluid at the pressure required for the process. Thus, there is always a
chance that piping may rupture and a leak may occur.
Piping is designed and constructed to various codes that specify the maximum
allowable pressure for the piping. In larger piping or in cases where extremely
hazardous materials are transported, the codes may require that all the piping be
welded. For less hazardous materials, the piping may be be assembled with flanged
connections or screwed connections. The piping must also be inspected and
maintained to assure that there is no gradual deterioration caused by corrosion,
by weathering and slumping of supports, or by damage from an accident. If piping
is damaged, the portion of the plant involved must be shut down and the piping
repaired.
It is important to be able to estimate how much material may be lost if there is
a leak in the piping in order to plan for proper emergency operations. Both the
leak rate and the total amount leaked are important in forecasting the consequences of the leak.
PROBLEM: Benzene is being transferred through long, small-diameter plant
piping under a pressure of 100 psig. At 1:00 PM the operator notices a drop in
PROBLEM NO. 23
49
pressure in the piping. He immediately restores the pressure to 100 psig and sends
another operator to trace the piping to see if the cause of the pressure drop can
be determined. At 230 PM a 0.25-in. diameter hole is discovered in the piping. The
flow of benzene is stopped; the line is depressured; the section of piping containing
the hole is isolated; and the piping is drained, purged, repaired, and returned to
service. Estimate how many gallons of benzene are spilled before the flow is
stopped.
(This problem is based on a problem in the text Chemical Process Safety:
Fundamentals and Applications, by D. A. Crow1 and J. F. Louvar, published by
Prentice Hall, Englewood Cliffs, NJ.)
Problem No. 24
CHEMICAL ENGINEERING TOPIC: Momentum Transfer: Fluid Mechanics
SAFETY AND HEALTH CONCEPT: Storing, Handling, and Transport
BACKGROUND: All chemical manufacturing operations require the storage and
movement of process materials.Sometimes the quantity of material stored is small,
but sometimes it is quite large. Refineries may store millions of gallons of hydrocarbons, for example, whiie a company that manufactures semiconductor devices will
have relatively little inventory of process materials. In some cases the materials
stored normally do not present a hazard to humans, at least in the sense that they
are neither flammable nor toxic. (Sometimes, seemingly innocuous materials can
create a hazard. Many years ago, a large molasses tank failed in Boston, drowning
a number of people who had been walking in the street.) In many cases the
materials stored inprocessingplants are either toxic, or flammable, or both. Special
precautions are then required to assure there is no release of such materials so that
damage to the environment or injury to people will be prevented.
Large leaks from storage tanks are very infrequent. Smaller leaks occur at higher
frequency; they may be caused by corrosion, in which case timely inspection and
regular maintenance of the tanks can prevent them in most cases. Larger leaks are
more likely to be caused by some outside event such as puncturing the tank with
maintenance equipment or damage from severe storms. Some of these larger leaks
can be prevented by proper procedures and by better design.
Tanks are constructed to codes of standards that experience has shown to
provide safe operation. The tanks are tested using a variety of methods such as dye
penetrants, x-raying, vacuum boxes, and magnetic techniques. Before the tank is
filled with a toxic or flammable process fluid, it is usually hydrostatically tested at
a pressure equal to or greater than its design pressure. All of these testing methods
have the same goal: to assure that the tank will not fail in service. In addition, relief
devices are provided on the tank to prevent hazardous overpressure or underpressure.
In many cases in which a tank is to contain a flammable material, if air is allowed
to enter the tank while a flammable liquid is stored in the tank, a flammable
concentration of vapor can form in the air above the liquid. In such cases, an inert
gas, usually nitrogen, is used to fill the vapor space. This practice, called padding
the tank, is used to prevent flammable mixtures from occurring. Thus, the possibility of ignition inside the tank is eliminated, and the volatile liquid can be stored
more safely.
PROBLEM NO. 24
51
By followingstrict design procedures, the relevant codes, and good maintenance
practices, most tank leaks can be prevented. The following problem is an illustration of what can happen if a tank is involved in an accident. Proper procedures and
training can prevent such accidents, and the engineer has the responsibility not
only of designing the tank for its intended service, but also of making sure the tank
is tested and maintained.
Written permits must be issued if work is to be done either on the tank or in the
immediate vicinity so safe practices will be followed.
PROBLEM: A cylindrical pressure vessel 20 ft high and 8 ft in diameter is used
to store benzene. The tank is padded with nitrogen to a constant, regulated
pressure of 1atm gauge to prevent air entering and a subsequent explosion. The
liquid level within the tank is presently at 17 ft. A 1-in. puncture occurs in the tank
5 ft off the ground due to an accident.
Estimate: (a) the number of gallons of benzene spilled from the tank, (b) the
time required for the benzene to leak out, and (c) the maximum flow rate of
benzene through the leak. Assume that the nitrogen pressure remains at 1atm
gauge as long as benzene flows from the tank.
(This problem is based on a problem in the text Chemical Process Safety:
Fundamentals with Applications, by D. A. Crow1 and J. F. Louvar, published by
Prentice Hall, Englewood Cliffs, NJ.)
Problem No. 25
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics, Choked Flow
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: One of the very critical areas of protecting workers from
dangerous conditions is that of assuring that the air the workers breathe is in fact
suitable for breathing. Much attention must be given to minimizing the exposure
to harmful concentrations of toxic vapors, but sometimes it is a matter of assuring
an adequate level of oxygen in the breathing air.
The U.S. Occupational Safety and Health Administration (OSHA) is an organization charged with responsibility for assuring safety in the workplace. One of
the concerns is that of providing that no one will enter a room, or other enclosure
where the oxygen concentration is too low. Normal air is about 21% oxygen (mole
or volume percent), with the remainder being primarily nitrogen. OSHA regulations require that the air be at least 19.5% oxygen for entry into an enclosure.
Although people have a variable tolerance for lower than normal oxygen, this level
is believed to be safe for most people in reasonably good health, and some would
be able to tolerate a lower level, perhaps.
If the oxygen content of the breathing air were to drop to 16% or below, many
people would lose consciousness and eventually suffer great injury or death
because they would become incapacitated and not be able to escape without
assistance. A complication in this situation would be that the person would not
realize the difficulty until it became too late.
Although nitrogen is not toxic, it nevertheless can displace oxygen from the air
in a room or other enclosure, so that the air is no longer suitablefor breathing. The
following problem addresses such a concern.
PROBLEM: A large tank of nitrogen at a pressure of 200 psig and 80°F has
developed a leak which is equivalent to a 0.1-in.-diameter hole. Estimate the rate
that nitrogen will leak from this hole, and at this rate, how long would it take to
reduce the level of oxygen in a room to an average concentration less than 19.5%.
The room is a small control room, 14 ft by 10 ft by 8 ft high. Note, to provide a
result that is conservatively safe, assume that the nitrogen enters the room and
displaces the air by plug flow, that is, the displaced air will be 21% oxygen. The
barometric pressure may be taken as 1atm, and the temperature of the room is
80°F. Assume the discharge coefficient is 1.0.
(This problem is based on a problem in the text Chemical Process Safety:
Fundamentals with Applications, by D. A. Crow1 and J. F. Louvar, published by
Prentice Hall, Englewood Cliffs, NJ.)
Problem No. 26
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics
SAFETY AND HEALTH CONCEPT. Rupture Disks and Relief Valves
BACKGROUND: Some method of pressure relief is required on all pressure
vessels and for other process equipment where increasing pressure might rupture
the vessel. Much of the piping used in modern chemical operations also requires
overpressure protection. Either relief valves or rupture disks may be used for
pressure relief. In many cases, either a rupture disk or a relief valve can be used;
usually, one or the other is preferred. Relief valves are usually used for process
protection, and rupture disks are used for vessel protection. The relief valve or
rupture disk must be designed so it will operate at a known pressure and prevent
the pressure within the system from increasing. Thus, the flow rate the valve can
handle is a major concern in its design.
Sometimes the maximum flow rate the relief system must handle is based on
process flow conditions, such as the maximum flow rate a pump can deliver at the
relief pressure or the maximum flow rate for a compressor at the relief pressure.
In other cases, the relief system maybe designed to relieve the pressure in a reactor
if the reaction gets out of control. Another important consideration is the protection of vessels from overpressure during a fire. In most vessels, the contents will be
either gases or liquids. In either case, a rise in temperature will cause a rise in
pressure in the vessel, either through expansion of the gas or through increase of
vapor pressure of the liquid. In such cases, the relief system is designed to relieve
the pressure at a rate determined by the heat transfer rate to the vessel.
Relief valves and rupture disks have similar purposes, but their designs are
different. A relief valve has a spring-loaded valve stem. Rather than turning a
handle or using a control system to open or close the valve, the force exerted by
the pressure inside the valve is resisted by the force of a spring. When the pressure
increases to the set point of the valve, the spring can no longer resist the force
caused by the pressure and the valve begins to open. At a slightlyhigher pressure,
the relief valve will be fully open and allow maximum flow. As the contents of the
vessel are released, the pressure in the vessel will begin to decrease. Once the
pressure decreases to a level a little below the set point, the spring will be strong
enough to reset the valve; the pressure will no longer decrease. A relief valve may
open and close many times during a prolonged incident in which the pressure rises,
and the vessel is then vented to relieve the pressure. Relief valves must be
maintained carefully to ensure their proper function. Their calibration must be
checked periodically, and they must be kept clean. When improper maintenance is
54
SAFETY, HEALTH, AND LOSS PREVENTION IN CHEMICAL. PROCESSES
used, a relief valve may not fully reseat when the pressure is reduced. The valve
will continue to leak until it is repaired. Since the seat of the relief valve may be in
contact with the material in the vessel, some corrosion may occur. Leaking and
corrosion may cause failure: either the valve may open at lower pressures or it may
fail to open when the pressure increases. Either result can be dangerous and should
be avoided by careful inspection and maintenance. If properly sized, installed, and
maintained, relief valves have been shown to be reliable and to reduce the probabiity of damage caused by overpressure.
Relief valves may also be designed to fail at a given temperature. Sometimes a
combination relief valve will be used that will operate at either a given temperature
or a given pressure. Home water heater relief valves are a combiiation temperature-pressure relief valve, for example. The design of a relief valve depends on
the material to be vented. If a gas or liquid alone is to be vented, the design is
relatively simple, and the relief system can be designed on the basis of single phase
fluid flow. However, in many systems containing liquids under their own vapor
pressure, venting can be a combination of liquid and vapor. In such cases, the
two-phase fluid that is vented must be accounted for. The methods used to design
such relief systems are substantiallydifferent than those for single phase fluids.
PROBLEM: A tank containingbenzene is to be relieved if the pressure in the tank
reaches 15 psig. The tank is cylindrical with hemispherical heads. The overall tank
length is 60 ft and it is 12 ft in diameter. The relief valve is to be designed so that
the tank can be vented if a fire occurs. In order to keep the tank from being
overfilled, procedures are put in force to limit the liquid depth in the tank to 11ft.
The venting rate is to be consistent with the heat transfer rate given by
as given in National Fire Protection AssociationStandard 30, "Flammable Liquids
Code." In the heating rate equation, q is the net heating rate to the contents of the
tank, in Btuthr, and A is the area of the tank wetted by the liquid portion of the
tank contents, in ft2. Only the given units may be used. Calculate the nominal pipe
size of the relief valve, assuming (a) only liquid flows through the valve and (b) only
vapor flows through the valve. The friction loss coefficient K for the relief valve is
3.0. You may assume the vapor to be an ideal gas for flow calculations.
Problem No. 27
CHEMICAL ENGINEERING TOPIC: fluid Mechanics
SAFETY AND HEALTH CONCEPT: Rupture Disks and Relief Valves
BACKGROUND: Some method of pressure relief is required on all pressure
vessels and for other process equipment where increasing pressure might rupture
the vessel. Much of the piping used in modern chemical operations also requires
overpressure protection. Either relief valves or rupture disks may be used for
pressure relief. In many cases, either a rupture disk or a relief valve can be used;
usually one or the other is preferred. Relief valves are more frequently used for
process protection, and rupture disks are more frequently used for vessel protection. The relief valve or rupture disk must be designed so it will operate at a known
pressure and prevent the pressure within the system from increasing. Thus, the
flow rate the valve can handle is a major concern in its design.
Sometimes the maximum flow rate the relief system must handle is based on
process flow conditions, such as the maximum flow rate a pump can deliver at the
relief pressure or the maximum flow rate for a compressor at the relief pressure.
In other cases, the relief system maybe designed to relieve the pressure in a reactor
if the reaction gets out of control. Another important considei-ationis the protection of vessels from overpressure during a fire. In most vessels, the contents will be
either gases or liquids. In either case, a rise in temperature will cause a rise in
pressure in the vessel, either through expansion of the gas or through increase of
vapor pressure of the liquid. In such cases, the relief system is designed to relieve
the pressure at a rate determined by the heat transfer rate to the vessel.
Relief valves and rupture disks have similar purposes, but their designs are quite
different. A rupture disk is a simple device that consists essentially of a ihin disk
of material held in place between two flanges. The disk is usually made of metal,
although it may be made of other materials. The choice of material is important
because the rupture disk must be designed to close tolerances in order to operate
properly. In use, the disk ruptures when the pressure level rises to a chosen level.
The vessel is then vented and the pressure in the vessel eventually drops to
atmospheric pressure. The rupture disk is chosen to be large enough to vent the
vessel at the maximum rate required.
Relief devices may also be designed to fail at a given temperature. Sometimes a
combination relief device will be used that will fail at either a given temperature
or a given pressure. Home water heater relief valves are a combination temperature-pressure relief valve, for example.
56
SAFETY, HEALTH,AND LOSS PREVENTION IN CHEMICALPROCESSES
The design of a rupture disk depends on the material to be vented. If a gas or
liquid alone is to be vented, the design is relatively simple, and the relief system
can be designed on the basis of single phase fluid flow. However, in many systems
containing liquids under their own vapor pressure, venting can be a combination
of liquid and vapor. In such cases, the two-phase fluid that is vented must be
accounted for. The methods used to design such relief systems are substantially
different than those for single phase fluids.
PROBLEM: A tank containingbenzene is to be relieved if the pressure in the tank
reaches 15psig. The tank is cylindrical with hemispherical heads. The overall tank
length is 60 ft and it is 12 ft in diameter. The rupture disk is to be designed so that
the tank can be vented if a fire occurs. In order to keep the tank from being
overfilled, procedures are put in force to limit the liquid depth in the tank to 11ft.
The venting rate is to be consistent with the heat transfer rate given by
as given in National Fire Protection AssociationStandard 30, "Flammable Liquids
Code." In the heating rate equation, q is the net heating rate to the contents of the
tank, in Btuthr andA is the area of the tank wetted by the liquid portion of the tank
contents, in ft2. Only the given units may be used. Calculate the diameter of the
rupture disk required, assuming (a) only liquid flows through the orifice and (b)
only vapor flows through the orifice. You may assume the orifice coefficient to be
that of a sharp-edged orifice, and the vapor to behave as an ideal gas.
Problem No. 28
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics, Design
SAFETY AND HEALTH CONCEPT. Rupture Disks and Relief Valves
BACKGROUND: Runaway chemical reactions may occur for a variety of exothermic reactions. Figure 1shows a typical temperature-time curve for an exothermic
reaction. The reactor-reaction combination has a stable range where process
controls keep the reaction under control; an unstable range where the reaction
may be brought back under control by cooling, inhibiting, and quenching; and a
runaway range where the reaction is out of control and the reactor must be vented.
Normally, operation is typically kept in the stable range where the controls and the
process operate as designed. However, the reaction could get out of control for
reasons such as too much initiator, loss of cooling, or loss of mixing. Typical
behavior would then be for the reactor temperature to gradually rise at first, then
accelerate to the point that a runaway occurs. During the unstable range the
reaction might be restabilized by methods such as emergency cooling or inhibiting.
Condition:
Stable
I
Unstable
I
Process
Controls
I
I
I
!
I
Runaway
1
I
Deaignt
I
TIME
I
Restabilize
I Venting
Emergency
(determination of
Cooling
I vent size & settings,
Inhibitor
! mechanical forces,
Quenching
emission for specific
(add water)
reaction/reactor
systems)
Figure 1. Venting and restabilization concepts for chemical exothermic runaways.
57
58
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
However, at the point of runaway, the onlyrecourse is to vent thereactor to prevent
reactor failure. This emergency venting is frequently confused with explosion
venting which is used for vapor-air and dust-air deflagrations. They differ because
emergency venting is for venting of a reactor whereas explosion venting is for
venting an explosion that occurs inside a building.
Even though we take a number of measures to reduce the probability of a
runaway, we almost always have to provide some form of protection for the reaction
vessel in the event of a runaway; the only way to protect the vessel is emergency
venting. Emergency venting design relates to several process and equipment
considerations. For a given vessel and its vent design, we must limit the maximum
quantity of a "runaway" chemical, so that the maximum venting pressure is limited
to a certain acceptable level. Many users limit it to 110% of the vessel's maximum
allowable working pressure (MAWP).
The Design Institute for Emergency Relief Systems (DIERS) Users Group,
which is an affiliate of the American Institute of Chemical Engineers, has
developed some methodology to design emergency relief systems.' The DIERS
study was very extensive and complicated. It involved significant developments and
applications of complex theories and experiments. Some aspects were reaction
kinetics under runaway conditions and multiphase critical flashing flow for viscous
and nonviscous systems. A number of DIERS users have attempted to simplify the
DIERS technology.273PThe following two venting analyses represent the two
extreme cases, tempered aad gassy reactions. Tempered reactions are reactions
that have energy removed due to significant vaporization of the liquids. Here the
heat of vaporization cooling during vapor or two-phase flow venting is sufficient
to temper the reaction. An equation representing the relief behavior for a vent
length LID < 400 is3
where
Mo = allowable mass of the reactor mixture charge (kg) to limit the venting
overpressure to Pp (psig)
Dp = rupture disk diameter, inches
Ps = the allowable venting overpressure (psi), i.e., the maximum venting
pressure minus the relief device set pressure
Pp = maximum venting pressure (psig)
Ps = the relief device set pressure (psig). Note that the relief device set
pressure can range from the vessel's MAWP to significantlybelow the
MAWP.
Ts= the equilibrium temperature correspondingto the vapor pressure where
the vapor pressure is the relief device set pressure (K)
PROBLEM NO.28
59
dTldts = the reactor mixture self-heat rate ("Clmin) at temperature Ts (K) as
determined by a DIERS or equivalent test
C p = specific heat of the reactor mixture (cdg-K or Btu/lb-"F)
Note that the equation given above is a dimensional equation and the dimensions given in the nomenclature must be used.
Gassy reactions are reactions that are not tempered because significant noncondensible gas is formed, and the heat of vaporization during vapor or two-phase
flow venting is insufficient to temper the reaction at any point. Hence, to limit the
overpressure at a specific maximum pressure, the venting rate must equal the peak
volumetric generation rate. An equation representing the relief behavior for a
length LID < 400 is3
where
Vp = vessel total volume (gal)
P p = maximum allowable venting pressure (psia)
Ms = sample mass used in a DIERS test or equivalent test, g
dPldt = pressure rise in test (psilsec)
TT = maximum temperature in test (K)
up= P p - Pamb, psi
Pamb = ambient pressure at the end of the vent line, psia
This is also a dimensional equation and the dimensions given in the nomenclature must be used.
PROBLEM
Part 1-Tempered Reaction: A 750-gal reactor containing a styrene mixture has
an 8-in. rupture disk and a vent line with equivalent length LID = 400. The vessel
MAWP is 100psig and the rupture disk set pressure is 15psig. The styrenemixture
had a self heat rate of 50°C/min at 160°Cas it tempered in a DIERS venting test.
What is the allowable reactor mixture charge to limit the overpressureto 10% over
the set pressure?
Part 2 -Gassy Reaction: A nominal 750-gal reactor with a net volume of 880 gal
containing tetrazole mixture has an 8-in. rupture disk and a vent line with an
equivalent length LID = 400. The vessel MAWP = 100 psig and the rupture disk
set pressure is 15 psig. The DIERS venting test showed that the reaction was
"gassy". The test mass was 25 g, the peak rate of pressure rise was 500 psilmin and
the maximum test temperature was 250°C. What is the allowable reactor mixture
charge to limit the overpressure to 10% of the MAWP?
60
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
REFERENCES
1. H. G. Fisher. DIERS Research Program on Emergency Relief Systems. Chem Engr Pro&81(8),
33-36 (August 198.5).
2. H. K Fauske, G. H. Clare, and M. J. Creed. Laboratory Tool for Characterizing Chemical
Systems. Proceedings of the International Symposium on Runaway Reactions, Cambridge, MA,
March 7-9,1989. Center for Chemical Process SafetyIAIChE,New York, 1989,pp. 372-394.
3. J. A. Noronha, R J. Seyler, and A. J. Torres. Simplified Chemical and Equipment Screening for
Emergency Venting Safety Reviews Based on the DIERS Technology. Proceedings of the International Symposium on Runaway Reactions, Cambridge, MA, March 7-9,1989. Center for Chemical
Process SafetylAIChE, New York, 1989,pp. 660-680.
4. D. P. Mason. Highlightsof FM Inspection Guidelineson EmergencyRelief Systems.Proceedings
of the International Symposium on Runaway Reactions, Cambridge, MA, March 7-9,1989. Center
for Chemical Process SafetyIAIChE,New York, 1989,pp. 722-750.
5. H. K Fauske. A Quick Approach to Reactor Vent Sizing. PlanVOperations Progress, 3(3),
145-146 (1984).
6. Chemical Engineering Progress, 81(8), 33-62 (198.5).
(This problem was provided by Mr. John Noronha of Eastman Kodak Company.)
Problem No. 29
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics; Fundamentals
SAFETY AND HEALTH CONCEPT: Inerting and Purging
BACKGROUND: Many of the common materials used in chemical processing are
toxic or flammable; frequently they are both toxic and flammable. Special care must
be taken when such materials are used. Consider flammablematerials for example.
If a flammable liquid vaporizes, or if the material is normally a gas, fuel and air
may mix and result in a flammable mixture. Two concentrations of fuel in air are
important: the upper flammablelimit (UFL) and the lower flammablelimit (LFL).
The LFL is the smallest concentration of fuel in air that can be ignited. At lower
concentrations, there is too little fuel to ignite. If the concentration is above the
UFL, there is insufficient oxygen in the air for the fuel to ignite. Only concentrations of fuel in air between the LFL and the UFL will ignite. The flammabilitylimits
depend on the temperature, so if the fuel-air mixture is heated, the flammability
limits widen. At temperatures reachedin flames, virtually allthe fuel will be burned
if enough oxygen is present. Pressure also affects the flammabilitylimits for fuel-air
mixtures, but the effect is not important in most ignition problems because most
fires burn near atmospheric pressure. Data for the LFL and UFL of various
chemicalscan be found in Sax's Dangerous Properties of Industrial Materials;NFPA
325M, "Properties of Flammable Liquids"; and the NIOSH "Pocket Guide to
Chemical Hazards."
One method of preventing damage or injury caused by ignition of fuel-air
mixtures is to keep the mixtures from forming. That may be accomplished by
keeping all fuels from escaping into the air or by ventilating the location with
enough air to keep the concentration below the LFL. Usually, prevention is
preferable to control, so the goal is to prevent escape of flammable materials. If
accidental escape occurs, control must be used to reduce the concentration or to
keep the concentration from exceeding the LFL. The most frequent method of
control is ventilation. Ventilation is important enough that National Fire Protection Association Standard 69, "Explosion Prevention Systems," describes veniilation systems as a method of preventing explosions. NFPA 69 (1986 Edition)
describes equations for calculating the quantity of ventilating air required for
reducing the concentration of a fuel-air mixture or for preventing a flammable
mixture from forming. The methods used in the calculations are essentially
material balances, and they are limited in application to the conditions discussed.
Other material balances can be written and solved under other conditions. One
such different set of conditions is considered in the following problem.
62
SAFETY, HEALTH,AND LOSS PREVENTIONIN CHEMICAL PROCESSES
PROBLEM: A natural gas compressor station has several large compressorsused
to boost the pressure in a cross country pipeline. The capacity of the pipeline is
500 million standard cubic feet (measured at 1.0 atm and 60°F) per day. The
compressors are housed in a building 200 ft long, 80 ft wide, and 30 ft high at the
eaves. The building has a sloped roof, and if the volume of the compressors and
ancillary equipment is considered,thenet free air volume of the building is 510,000
ft3. The temperature in the building averages about 90°F, and the pressure about
1.0 atm. The building has a ventilation system designed to operate at two speeds;
the low speed provides a ventilation rate equal to 6 air changes per hour and the
high speed provides 20 air changes per hour. The ventilation rate is based on
entering air, measured at building temperature and pressure. The building is
normally ventilated at the low rate, but a gas-sensing system monitors the air in the building, and if gas is released in the building and reaches a concentration of 25%
of the LFL, the ventilation system automaticallyincreases to the high rate.
1. Assume there is aleak of 10,000standard cubic feet of natural gas (considered
to be pure methane) per minute in the building. How long will it take for an alarm
to sound if the alarm sounds when the ventilation system increases to the higher
speed at 25% of the LFL concentration?
2. What is the maximum concentration reached if the leak continues indefinitely? Ventilation will be at the higher speed.
3. If the gas leak is shut off when the concentration reaches 25% of the LFL,
how long will it be until the concentration has dropped to 5% of the LFL if (a)
ventilation is at the low rate and (b) ventilation is at the high rate?
4. If the leak detection alarm is given at 25% of the LFL, how long will it be
before the concentration reaches the LFL? Ventilation will be at the higher speed
as long as the gas concentration is more than 25% of the LFL. Is that sufficient
time for the source of gas to be found and shut down in time to prevent the
concentration from reaching the LFL?
5. Calculate the energy released if the natural gas, which has a heat of combustion of 1044 Btdstandard cubic ft, is ignited and completely burned when the
concentration of gas is at the LFL. Compare that energy to the equivalent mass of
TNT, if the explosive energy of TNT is 2000 Btullb.
In your calculations, you may assume the air and gas in the building to be
completely mixed and the air-gas mixture leaving the building to be the same
composition as the mixture in the building. Ideal gas behavior may be assumed to
apply because the pressure is quite low. Note that there is always a flammable
mixture somewhere because there must be time for the pure gas to mix with air.
The assumption of complete mixing is made to simplify the problem. The results
can be used for designing the ventilation system, but it must be kept in mind that
mixing is not truly instantaneous.
Problem No. 30
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics, Fundamentals, Thermodynamics
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control
BACKGROUND: Many of the materials used in industry are either toxic or
flammable, and some are both toxic and flammable. Toxic materials may cause
immediate health problems such as poisoning, and others may cause delayed health
problems such as cancer. The term "acute exposure" is used to describe a condition
where a single exposure to a material, usually at relatively high concentration,
causes immediate health effects. Exposures that occur over a long period and are
usually repeated periodically, eventually causing health effects at relatively low
concentrations, are called "chronic exposures." It is difficult to measure the effects
of either acute or chronic exposure because we cannot deliberatelyexpose humans
to the effects of toxic materials. Thus, our knowledge comes from either animal
experiments or from accidental exposures of humans. In either case, the applicability of the information can be questioned for use in describing human health
effects in many cases.
Even though our information may be imperfect, we must strive for a safe
workplace for workers and a safe environment for neighbors. Therefore, various
groups have instituted exposure standards for most of the materials used in
industry. The OccupationalSafety and Health Administration (OSHA) has established a permissible exposure limit (PEL) for many materials. The PEL is a
concentration to which a worker may be exposed on a continuing basis. The PEL
is a time-weighted average and is based on an 8-hour daily (40 hour weekly)
exposure over the worker's lifetime. Another higher limit is the immediately
dangerous to life and health (IDLH) concentration. The IDLH concentration
represents the concentration level from which one could escape within 30 minutes
without experiencing any escape-impairing or irreversible health effects. The PEL
concentration should be listed on the material safety data sheet (MSDS), which
should be reviewed by all workers who might be in the vicinity of the material. There
is also a concentration called the Emergency Response Planning Guide (ERPG)
concentration,provided by the American Industrial Hygienists Association. It can
be useful in planning for emergency operations in a plant.
Some of the most serious chances of exposure occur if a hazardous material
escapes into the atmosphere. When the toxic material mixes with the atmosphere,
it may expose workers to its toxic effect. In most cases where the materials are
extremely toxic, the acceptable exposure level can only be attained by preventing
64
SAFTXY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
the release. However, if an accidental release occurs, ventilation is usually the
easiest method for preventing a buildup of the hazardousmaterial inside a building.
Several references may be consulted to find the allowable exposure levels of
materials. The National Institute for Occupational Safety and Health (NIOSH)
has published a substantial body of information, and OSHA is the source for legally
mandated exposure levels. The NIOSH "Pocket Guide to Chemical Hazards" is a
compact listing of PEL and IDLH values as well as a substantial amount of other
information. It is a condensed summary of information taken in part from the
NIOSHIOSHA Occupational Health Guidelines for Chemical Hazards. Sax's
Dangerous Properties of IndustrialMaterials is another source of information. The
OSHA PEL values are published in Title 29 of the Code of Federal Regulations,
Part 1910, Subpart 2, "General Industry Standards for Toxic and Hazardous
Substances."
PROBLEM: Ammonia is used as a refrigerant. It is compressed outdoors, and the
ammonia is then liquefied and circulated through the refrigerator's evaporator
heat exchanger. The evaporator is indoors. The room containing the evaporator
has a net open volume of 1000 m3 and a ventilation rate equal to two air changes
per hour. A leak develops in an ammonia line that results in 50 g of ammonia being
released per minute.
1. How long will it take for the ammonia concentration to increase to the PEL
concentration?
2. How long will it take to reach the IDLH concentration?
3. If the flow of ammonia is stopped at the time the IDLH concentration is
reached, how long will it be before the concentration is reduced to the PEL
concentration?
You may assume that the mixtures of gases are ideal at the low pressures involved.
Also assume that the ammonia is mixed with the air in the room completely and
instantaneously and that the air-ammonia mixture leaving the room has the same
concentration as the mixture in the room.
Problem No. 31
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics
HEALTH AND SAFETY CONCEPT: Storing, Handling, and Transport
BACKGROUND: There are many occasionswhen the thrust forces caused by high
velocity, high rate flow will cause excessive forces on piping and associated
equipment. Sometimes the design of relatively simple devices can be complicated
by the possibiity of severe mechanical loads on piping equipment. If proper
precautions are not taken to prevent failure due to these forces, then very serious
accidents can occur.
Some examples of such situations might be in the design of safety relief systems
wherein there exists the possibiity of suddenly initiated, very high velocity flow,
with the consequent possibility of the discharge piping reacting with significant
movement, as for example, after the manner of a garden hose that is not being held.
Our experience tells us that the hose will move erratically about, dischargingwater
in many directions. One should be aware that even heavy steel piping can behave
similarly if it is not suitably constrained.
Another serious situation can develop from misuse of a rather common item that
exists in laboratories, perhaps in laboratories where you have worked. This is the
compressedgas cylinder. A typical situation might exist at your gas chromatograph,
for example, where air is being used in conjunction with hydrogen, in the flame
ionization detector. The air is usually supplied at a pressure in excess of 2000 lb/in2.
If the cylinder is not properly restrained and held, it can be easily knocked over
with possibly disastrous results if the valve is broken off in the fall.
The result of unexpectedly high thrust forces from flow may frequently be
disastrous because of the rapid, violent, and unpredictable motion of a pipe, or as
in the example above, a rather heavy gas cylinder. There is also the distinct
possibiity of equipment failure from the forceswhich would result in the discharge
of dangerous materials to the air.
In the problem below, you are asked to estimate some forces, the magnitude of
which might easily cause equipment failure with the consequent loss of large
quantities of a highly flammable material to the air. The result would almost
certainly be a serious gas cloud explosion and fire.
PROBLEM: A tank ship hauling Liquefied Natural Gas (LNG) is being unloaded
into a 600,000 bbl storage tank at a rate of 55,000 gaVmin through a 30-in. diameter,
schedule 10stainless steel pipe. The schematicdiagram of the pipe inside the tank
is shown on the next page. Determine the total upward force on the tank roof and
the horizontal and vertical forces on the splash plate.
66
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
Problem No. 32
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics; Thermodynamics
HEALTH AND SAFETY CONCEPT: Storing, Handling, and Transport
BACKGROUND:There are many occasions when the thrust forces caused by high
velocity, high rate flow will cause excessive forces on piping and associated
equipment. Sometimes the design of relatively simple devices can be complicated
by the possibility of severe mechanical loads on piping equipment. If proper
precautions are not taken to prevent failure due to these forces, then very serious
accidents can occur.
Some examples of such situations might be in the design of safety relief systems
wherein there exists the possibility of suddenly initiated, very high velocity flow,
with the consequent possibility of the discharge piping reacting with significant
movement, as for example, after the manner of agarden hose that is not being held.
Our experience tells us that the hose will move erratically about, dischargingwater
in many directions. One should be aware that even heavy steel piping can behave
similarly if it is not suitably constrained.
The result of unexpectedly high thrust forces from flow may frequently be
disastrous because of not only the rapid, violent, and unpredictable motion of a
pipe, as in the example above, but there is also the distinct possibility of equipment
failure from the forces which would result in the discharge of dangerous materials
to the air.
Another serious situation can develop from misuse of a rather common item that
exists in laboratories, perhaps in laboratories where you have worked. This is the
compressed gas cylinder. A typical situation might exist at your gas chromatograph,
for example, where air is being used in conjunction with hydrogen, in the flame
ionization detector. The air is usually supplied at apressure in excess of 2000 lb~in.~.
If the cylinder is not properly restrained and held, it can be easily knocked over
with possibly disastrous results if the valve is broken off in the fall. In the problem
below, you will have an opportunity to estimate some of the consequences of such
an accident.
PROBLEM:A compressedgas cylinder contains air that is intended as the oxidizer
for a hydrogen flame detector on your gas chromatograph. During the hook-up
procedure, the assistant (not you!) removes the safety cap from the cylinder and
begins to attach the pressure regulator when the cylinder slips from his grasp and
the bottom slides across the newly waxed floor. As the cylinder falls, the valve
strikes the table edge, and the valve is broken off, exposing an opening in the top
68
SAFEEY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
of the cylinder that is 5% in. in diameter. The air, which was originally at room
temperature (75°F) and 2250 1b/ia2will escape.
What initial flow rate will occur through the break?
As a consequence of the flow, what force will be exerted on the cylinder?
As a consequence of the force, what acceleration will be imparted to the
cylinder?
What velocity and what kinetic energy will the cylinder have when it gets to the
laboratory wall which is 20 ft away?
The cylinder weighs 140 lb and has an internal volume of 1.5 ft3. To simplify the
problem, you may assume that the air behaves as an ideal gas, and that the thrust
remains constant as long as the critical pressure ratio is exceeded. The shape of
the opening will not be like a sharp-edged orifice, but rather will be a converging
nozzle. Assume that the discharge coefficient is 0.9.
Problem No. 33
CHEMICAL ENGINEERING TOPIC: Fluid Mechanics
HEALTH AND SAFETY CONCEPT: Storing, Handling, and Transport
BACKGROUND: There are many occasionswhen the thrust forces caused by high
velocity, high rate flow will cause excessive forces on piping and associated
equipment. Sometimes the design of relatively simple devices can be complicated
by the possibility of severe mechanical loads on piping equipment. If proper
precautions are not taken to prevent failure due to these forces, then very serious
accidents can occur.
The result of unexpectedly high thrust forces from flow may frequently be
disastrous because of the rapid, violent, and unpredictable motion of a pipe. A
possible instance of the uncontrolled high velocity flow might be that which would
occur with the rupture of a pipe carrying a high pressure fluid. There is also the
distinct possibility of equipment failure from the forces which would result in the
discharge of dangerous materials to the air.
A very serious situation can develop from misuse of a rather common item that
exists in laboratories, perhaps in laboratories where you have worked. This is the
compressedgas cylinder. A typical situation might exist at your gas chromatograph,
for example, where air is being used in conjunction with hydrogen, in the flame
ionization detector. The air is usually supplied at a pressure in excess of 2000 lb/in2.
If the cylinder is not held upright, it can be easily knocked over with possibly
disastrous results if the valve is broken off in the fall.
Another example of such situations might be in the operation of safety relief
systemswherein there is the possibility of suddenlyinitiated,very high velocity flow,
with the consequent possibility of the discharge piping reacting with significant
movement, as for example, after the manner of a garden hose that is not being held.
Our experience tells us that the hose will move erratically about, dischargingwater
in many directions. One should be aware that even heavy steel piping can perform
similarly if it is not suitably constrained.In the problem below, you will be given an
opportunity to calculate the thrust force that might occur as a consequence of a
release by a pressure relief system.
PROBLEM: A 4-in. schedule 40 steel pipe rises vertically from an elbow off of a
horizontal pipe. The pipe is the discharge end of a pressure relief device from a
ill require a discharge velocity of 120 ft/sec
reactor vessel. If the system vents, it w
of a flashing liquid-vapor mixture which has a mean density of 17 lb/ft3.
How much lateral force will be exerted on the end of the pipe at the elbow?
Problem No. 34
CHEMICAL ENGINEERING TOPIC: Design
SAFETY AND HEALTH CONCEPT: Explosions
BACKGROUND: In order for combustion to occur, there is a need for the
simultaneous presence of oxygen (or air), an ignition source, and a combustible or
flammablevapor or dust. The combinations often occur in industry. Flammable or
combustible liquids do not cause combustion by themselves, but they can form
vapors that can cause combustion. Similarly, large-size solids or wet dusts do not
often support combustion, but fine, dry dusts do.
Deflagrations, unlike fires, are combustion phenomena associated with sudden
pressure rises where the pressure wave moves at a speed less than the speed of
sound. Detonations, also unlike fires, are combustion phenomena associated with
sudden pressure rise where the pressure wave moves at a speed more than the
speed of sound. It is often impossible to protect buildings or even a strong vessel
against a detonation.
There are three methods of protecting avessel, once a deflagrationoccurs. They
are explosion suppression, explosion venting, and deflagration pressure containment (DPC). In DPC, a vessel is built strong enough to withstand the pressures
generated due to vapor-air or dust-air deflagrations. The vessel can be designed
to allow deformation but to prevent rupture, or a stronger design can be provided
that will prevent deformation. The DPC method has been used widely in Europe
but has been used on a more l i i t e d basis in the United States. Muchgreater usage
is expected in the United States following the adoption of the 1986 edition of
National Fire Protection Association (NFPA) Standard 69 on Explosion Prevention Systems.
DPC can have several advantages over other explosion prevention and protection systems. Most important, it is a passive system. Hence, it is more reliable and
thus reduces the risk of personal injury, vessel damage, and subsequent business
loss. It may also lower overall capital, maintenance, and operating costs than other
alternatives. Therefore, it can make both good safety sense and good business
sense to use DPC. There are also some limitations of DPC. Chapter 5 of NFPA
Standard 69 should be consulted for details.
Section 5.3 of NFPA 69 provides the design bases for vessels required to
withstand a deflagration without having deformation or without rupturing. The
follow summary gives the methodology presented in NFPA 69. That methodology
is based on the thermodynamics of the deflagration and the mechanical design of
the vessel.
PROBLEM NO. 34
71
The design pressure of the vessel is to be high enough to prevent rupture (PI)
or to prevent deformation (Pd).The design pressure required to prevent deformation is higher than that to prevent rupture, because deformation of the vessel
occurs before rupture. Deformation occurs when the yield strength is exceeded
in the vessel walls, and rupture occurs when the ultimate strength is exceeded.
Since a vacuum can follow a deflagration, vessels designed for DPC must also be
designed to withstand full vacuum. The design pressure for the vessels is calculated
from
and
where
Pr =design pressure to prevent rupture due to internal deflagration, psig
P,j = design pressure to prevent deformation due to internal deflagration, psig
Pi =maximum initial pressure at which the combustible atmosphere exists, psig
R =ratio of the maximum deflagration pressure to the maximum initial pressure;
For gas-air mixtures, R is taken as 9.0; and for dust-air mixtures, R is taken
as 10.0
Fu = the ratio of the ultimate stress of the vessel to the allowable stress of the vessel
Fy = the ratio of the yield stress of the vessel to the allowable stress of the vessel
If the operating temperature is below 2YC, the value of R is adjusted by
where Ti is the operating temperature in OC. For vessels fabricated of low carbon
steel and low alloy stainless steel, Fu = 4.0 and Fy = 2.0.
PROBLEM: A carbon steel 285 vessel is used for avariety of processes using many
types of flammable liquids and dusts. The processes are typically run near atmospheric pressure. The relief device set pressure is 20 psig. It is sized to have a
maximum pressure accumulation of 20% during upset conditions. What should the
vessel design pressures be to prevent rupture if the minimum operating temperatures are either 25°C or O°C? What should they be to prevent deformation?
(This problem was provided by Mr. John Noronha, Eastman Kodak Company.)
Problem No. 35
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Vapor-Liquid Equilibrium
SAFETY AND HEALTH CONCEPT: Toxicology and Industrial Hygiene: Chronic
Toxicity
BACKGROUND: When people are exposed to certain chemicals at relatively low
but toxic concentrations, the toxic effects are only experienced after prolonged
exposures. Mercury is such a chemical. Chronic exposure to low concentrations of
mercury can cause permanent mental deterioration, anorexia, instability, insomnia, pain and numbness in the hands and feet, and several other symptoms. The
level of mercury that can cause these symptoms can be present in the atmosphere
without a worker being aware of it because such low concentrations of mercury in
the air cannot be seen or smelled.
Federal standards based on the toxicity of various chemicals have been set for
the "Permissible Exposure Limit," or PEL. These limits are set by the Occupational Safety and Health Administration (OSHA). The PEL is the maximum level of
exposure permitted in the workplace based on a time-weighted average (TWA)
exposure. The TWA exposure is the average concentrationpermitted for exposure
day after day without causing adverse effects. It is based on exposure for 8 hours
per day for the worker's lifetime.
The present Federal standard (OSHAIPEL) for exposure to mercury in air is
0.1 mg/m3 as a ceilin value. Workers must be protected from concentrations
Iif they are working in areas where mercury is being used.
greater than 0.1 mg/m!
PROBLEM: Mercury barometers are filled and calibrated in a repair shop.
Mercury is stored in a small room that has no ventilation. A mercury spill occurs
in the storeroom and is not completely cleaned up. What is the maximum mercury
concentration that can be reached in the storeroom if the temperature is 20°C?
You may assume that the room has no ventilation and that the equilibrium
concentration will be reached. What would the concentration be if the room had
a temperature of 1OoC?Is either level acceptable for worker exposure?
Problem No. 36
CHEMICALENGINEERING TOPIC: Thermodynamics: Constant Pressure Expansion
SAFETY AND HEALTH CONCEPT: Explosions: Unconfined Burning of Vapor
Cloud
BACKGROUND: If a flammable material is burned, there will be an increase in
either thevolume of thegas produced (if the pressure is constant) or in the pressure
in the container (if the volume is constant). The change in volume during isobaric
combustion is caused by two changes in the system: the number of moles of
products is usually greater than the number of moles of reactants, and the temperature of the system increases as the exothermic combustion reaction occurs. The
change in the volume of the system can be quite large. Most combustion systems
are designed so that the combustion reaction goes essentially to completion.
Otherwise, the energy produced by the reaction is partially wasted. Pollution by
release of fuel vapors also occurs if the reaction is incomplete. Carbon monoxide
and other toxic gases that may be difficult to detect are also produced and released.
The calculations for the product gas temperature and volume can be made by
assuming that all the gases behave ideally. The assumption does not cause any
significant error because the system pressure will be low, about 1.0 atm if burning
is at ambient pressure, and the system temperature will be quite high.
PROBLEM: Calculate the volume formed during the adiabatic combustion of 1
g-mol of n-butane in air.
1. Assume that the amount of air is the stoichiometric amount.
2. Assume that the amount of air is 150% of the stoichiometric requirement.
You may assume that the gases are ideal and that the combustion process is
carried out at atmospheric pressure. The specific heats will not be constant,
Assume that the reaction goes to completion. The reactants enter at 25OC.
Problem No. 37
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Adiabatic Expansion of
an Ideal Gas
SAFETY AND HEALTH CONCEPT: Explosions: Energy Release, Bursting Gas
Container
BACKGROUND: Gases may be compressed and stored in tanks under pressure.
If the pressure of the gas entering the tank is higher than the pressure that the
tank can withstand, and if the overpressureprotection devices normally provided
for a tank are either inoperative or have not been installed, the tank can fail. The
resulting explosion can damage the surroundings as the pressure wave from the
explosion hits objects near the tank. The amount of work done by the expanding
pressure wave can be estimated by assuming that the expanding gas does not mix
with the surrounding air and that no heat is transferred to the gas as it expands.
For such a case, the relationship between pressure and volume is
pVY = Constant
where
P = pressure in expanding gas, psia
V = molar vo!ume of expanding gas, ft3/lb-mole
y = CpL
Cp = specific heat at constant pressure, BtuPb-moleOR
Cv = specific heat at constant volume, BtuPb-moleOR
Once the amount of work done by the expanding gas has been calculated, the
effect of that work on the surroundings can be estimated. The estimation of the
effects of the pressure wave depends on the pressure in the wave at any point. The
best estimates can be made based on the experimental data taken from tests in
which a known quantity of TNT is detonated and the effects on the surroundings
are measured. The work done by the expandinggas is converted to the equivalent
work done by a quantity of TNT and the quantity of TNT is used to estimate the
damage to be expected.
PROBLEM NO. 37
75
(a) Show that the work done on the surroundings by the expanding gas when a
tank containing a compressed ideal gas explodes is given by
whe& PI,= burst pressure of the tank, psia
VT = tank volume, ft3
Pa = atmospheric pressure, psia
You may assume that the expanding gas is ideal, that the specific heats are
constant and that the process is isentropic.
(b) One pound of TNT releases about 2000 Btu of energy when it detonates.
Estimate the equivalent energy release rate in pounds of TNT if a tank having
a volume of 500 ft3 fails at 200 psig. The tank contains air, for which y = 1.4.
Problem No. 38
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Isentropic Expansion
of Pressurized Liquid
SAFETY AND HEALTH CONCEPT: Explosions: Rupture of Tank Containing
Superheated Liquid
BACKGROUND: Most pressure vessels are designed according to one of the
ASME pressure vessel codes. For tanks designed for use under conditions of
ordinary severity and moderate pressures, the design criterion is that the tank be
designed for pressures of four times the normal working pressure of the vessel. The
design basis is the tensile strength of the steel in the vessel, so there is a four-to-one
safety factor. Relief valves (or sometimes rupture disks) are provided to keep the
vessel from exceeding the working pressure. On occasion, the relief valve will fail
to open, or it may be taken off and replaced by a valve with a higher setting. The
consequence of having an improper or nonworking relief valve can be substantial.
Although most systems designed for heating liquids in tanks have thermostats to
limit the temperature in the system, the thermostat can fail and allow heating to
continue. If there is liquid in the tank, the pressure in the tank will rise as the
temperature rises, and the tank pressure will equal the vapor pressure of the liquid.
Thus, if the thermostat fails and the relief valve fails, the tank can rupture. If the
tank ruptures, the superheated liquid in the tank will partially flash to vapor. The
vapor and remaining liquid will expand rapidly, causing an overpressurewave that
can damage the surrounding buildings or equipment. Personnel in the area may
be injured or killed. Pressurized liquids can store substantial amounts of energy.
Although the explosion results in a rapidly expandingvapor-liquid system, most
of the pressure drop in the expanding cloud is at the system boundary. Thus, for a
simplified approach to the problem, the expansion can be considered isentropic.
The internal energy change of the system can then be calculated, and since the
expansion is also adiabatic, the work done against the surroundings during the
expansion can also be calculated. That work can be compared to the work done by
an equivalent amount of TNT during an explosion to estimatethe damage potential
of the energy released by the tank contents when the tank bursts.
PROBLEM: A 50-gallon hot water tank has a working pressure of 75 psia and a
burst pressure of 300 psig. During maintenance operations, the tank is emptied,
the tank and firing system are cleaned and repaired, and the tank is put back into
service. When returned to service, the relief valve that is installed is incorrectly set
at 500 psig. The tank is filled and the workers go to lunch after filling the tank.
When they leave for lunch, one worker closes both the fill valve and the exit valve.
PROBLEM NO. 38
77
Another worker decides to heat the water during lunch so that the system will be
hot and ready to test when they return. Neither tells the other what he has done.
Before they return, the maintenance personnel are called to perform some emergency repairs at another location in the plant. The water tank heating system
continues to heat the water until the water pressure reaches the failure pressure
of the tank. The tank explodes when the pressure reaches 300 psig.
Estimate the blast energy in terms of the TNT equivalent. You may assume that
the explosion is adiabatic. The water flashes to steam and water at 100°C;you may
assume the expansion is isentropic. Assume that no air mixes with the steam during
expansion and that the tank is just filled with liquid water which is saturated at 300
psig when the tank explodes. The work equivalent of explodingTNT is about 2000
Btullb.
Problem No. 39
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Constant Volume Gas
Phase Reaction
SAFETY AND HEALTH CONCEPT: Explosions: Pressure Rise for Enclosed
Combustion Reaction
BACKGROUND: Refineries and chemical plants use a variety of low pressure
vessels as knockout drums and seal drums. Most of these vessels are operated at
very low pressures, but they may contain flammable mixtures of vapor and air. It
is quite unlikely that ignition will occur in such a vessel because there is usually no
source of ignition. However, there is always a chance that ignition might occur, so
the American Petroleum Institute's Recommended Practice 521(API RP 521), 1st
edition, states: "Most knockout drums and seal drums will be operated at relatively
low pressures. To ensure safe conditions and sound construction, a minimum
design pressure of 50 psig is suggested. ...A vessel with 50 psig design pressure
should not rupture if an explosion occurs. Stoichiometric hydrocarbon-air mixtures can produce peak explosion pressures in the order of 7 to 8 times operating
pressure, most flare seal drums operate in the range of 0 to 5 psig, and ASME
code-allowable stresses are based on a safetyfactor of 4 to 1." Section 8, Division
1of the ASME (American Society of Mechanical Engineers) pressure vessel code
specifiesa safety factor of four to one (applicableat low pressures only). That safety
factor implies that a vessel with a stated mechanical design of 50 psig should not
rupture at pressures up to 200 psig.
PROBLEM: Show that the 50 psig design pressure suggested by API RP 521 will
contain the explosive combustion of a mixture of air and n-hexane with initial
conditions of 77°F (25°C) and 5 psig and stoichiometricconcentration of n-hexane
in air. Compare your result to the estimated pressure rise of "7 to 8 times operating
pressure" referred to in the API standard. You may assume that the reaction
proceeds to completion and that the products of combustion are carbon dioxide
and water.
(This problem was suggested by Mr. J. R. PhiIIips, a graduate student at the
University of Arkansas.)
Problem No. 40
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Combustion
HEALTH AND SAFETY CONCEPT: Hazardous Materials Generation and Disposal
BACKGROUND: The handling of hazardous waste materials is covered by a
number of laws and regulations that are intended to minimize the possibility of
hazardous materials being released to the environment where they might have an
adverse effect on the environment or people. Several methods have been
developed for the disposal of hazardous materials, and, of course, the nature of
the waste is important to the suitability of any particular method.
Among the methods used are l a n d f i g , which means burying the waste in the
ground; deep well disposal, which is pumping it into a deep underground formation; recovery and recycle, which is reclaiming the material for reuse; biological
treatment, which is allowing microorganisms to break down the waste into harmless, or less harmful materials; and incineration, which is burning it.
All the methods have their advantages and disadvantages, depending upon the
nature of the waste. Waste minimization is the preferred way, but when there is
waste, probably recovery and recycle would generally be preferable when it is a
practical solution.
If a material can be made nonhazardous by biological treatment, then this would
be a desirable disposal method if it could be carried out without release to the
environment, since this is usually a fairly inexpensive method and serves to break
down the waste, rather than just to provide perpetual storage as would be the case
with the landfilling.
Incineration is often a desirable option if the material can be made nonhazardous by thermal treatment. In general, incineration is a practical solution to the
disposal of organic materials, including halogenated materials. The Environmental
Protection Agency (EPA) has developed regulations concerning incineration,
including the temperatures and residence times required for the destruction of
particular types of waste. An approved incineration process will be specified both
with respect to the thermal conditions imposed, but to the degree of destruction
required as well. The hazardous waste incinerators, indeed, any incinerator, must
not emit more than specified amounts or concentrations of some combustion
products. Usually this will require the use of scrubbersto remove gaseous materials
and/or some particulates. Often filters (baghouses) or electrostatic precipitators
will be required to remove very fine particulates.
EPA regulations require that hazardous waste incinerators must have destruction and removal efficiencies such that 99.99% of the principal hazardous con-
80
SAFETY,HEALTH, AND LOSS PREVENTION IN CHEMICALPROCESSES
stituent is destroyed. Furthermore, the minimum temperature may be specified as
well. A typical destruction temperature is 2000°F. In a great many cases, these
conditions may be obtained only by the addition of an auxiliary fuel, such as natural
gas, since the heating value of the waste may prove to be inadequate to create the
necessary temperature.
The following problem deals with such a situation.
PROBLEM: A low concentration of a hazardous component is in solution in a
mixture of methyl alcohol and water. The hazardous component distributes between the alcohol and the water in such a manner that, if the alcohol and water
were to be separated by distillation, each product would be contaminated to such
an extent that it could not be reused. It has therefore been decided that the waste,
with the water and the alcohol, will be incinerated. A temperature of 2000°F is
required.
The mixture of methanol and water is 30% methanol by weight, and the amount
of contaminant is low enough that its heating value can be ignored. You are to
determine what natural gas (methane) rate will be required to incinerate this
material if the latter is burned with 100% excess air. You may assume that the
combustion is adiabatic.
Assume that all the entering streams are at 25°C. The water and alcohol is a
liquid solution.
Problem No. 41
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Vapor Liquid Equilibrium
SAFETY AND HEALTH CONCEPT: Properties of Materials: Flash Point
BACKGROUND: Most combustion reactions occur in the gas phase. In order for
any flammable material to burn, there must be both fuel and oxidizer present.
There must also be a minimum concentration of the flammable gas or vapor in the
oxidizer. For most fires to occur, minimum fuel concentration must exist in air at
ambient temperature. The minimum concentration at which ignition will occur is
called the lower flammable limit (LFL). If the flammable material is normally
liquid, the liquid must be warm enough to provide a vapor-air mixture equal in
fuel concentration to the LFL concentration. The liquid temperature at which the
vapor concentration reaches the LFL can be found experimentally. It is usually
measured using a standard method called a "closed cup flash point" test. The "flash
point" of a liquid fuel is thus the liquid temperature at which the concentration of
fuel vapor in air is large enough for a flame to flash across the surface of the fuel
if an ignition source is present.
The flash point and the LFL concentrationare closely related through the vapor
pressure of the liquid. Thus, if the flash point is known, the LFL concentration can
be estimated, and if the LFL concentration is known, the flash point can be
estimated. In most cases, the calculation can be made for pure liquids using
Raoult's law. However, if the liquid is a mixture, particularly one in which the
components are dissimilar, the liquid solution may be nonideal. Then the liquid
phase activity coefficients may need to be determined if an accurate estimate of
the relationship between flash point and LFL is to be made. The system total
pressure is ambient, so it is low enough for the vapor (or gas) phase above the liquid
surface to be considered ideal.
PROBLEM: Estimate the flash point of n-octane and compare it with the experimental value given in the literature. (See NFPA Standard 325M, "Properties
of Flammable Liquids" or Sax's Dangerous Properties of IndustrialMaterials.) Start
with the basic tenet that the fugacityin the liquid phase must equal that in the vapor
phase. Justify each assumption required to arrive at Raoult's law. Then use
Raoult's law to estimate the flash point. The lower flammable limit of n-octane is
1.0%.
Problem No. 42
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Vapor Liquid Equilibrium
SAFETY AND HEALTH CONCEPT: Properties of Materials: Flash Point
BACKGROUND: Most combustion reactions occur in the gas phase. In order for
any flammable material to burn, there must be both fuel and oxidizer present.
There must also be a minimum concentration of the flammable gas or vapor in the
oxidizer. For most fves to occur, a minimum fuel concentration must exist in air at
ambient temperature. The minimum concentration at which ignition will occur is
called the lower flammable limit (LFL). If the flammable material is normally
liquid, the liquid must be warm enough to provide a vapor-air mixture equal in
fuel concentration to the LFL concentration. The liquid temperature at which the
vapor concentration reaches the LFL can be found experimentally. It is usually
measured using a standard method called a "closed cup flashpoint" test. The "flash
point" of a liquid fuel is thus the liquid temperature at which the concentration of
fuel vapor in air is large enough for a flame to flash across the surface of the fuel
if an ignition source is present.
The flash point and the LFL concentration are closely related through the vapor
pressure of the liquid. Thus, if the flash point is known, the LFL concentration can
be estimated, and if the LFL concentration is known, the flash point can be
estimated. In most cases, the calculation can be made for pure liquids using
Raoult's law. However, if the liquid is a mixture, particularly one in which the
components are dissimilar, the liquid solution may be nonideal. Then the liquid
phase activity coefficients may need to be determined if an accurate estimate of
the relationship between flash point and LFL is to be made. The system total
pressure is ambient, so it is low enough for the vapor (or gas) phase above the liquid
surface to be considered ideal.
PROBLEM: The flash point of a liquid mixture can be estimated by finding the
temperature at which the equilibrium concentration of the flammable vapors in air
reaches a concentration such that
where yi is the vapor phase mole percent of component i and LFLi is the lower
flammable limit concentration of component i expressed in mole percent. Estimate
the flash point of a liquid mixture containing 60 mole percent n-octane, 15 mole
1
PROBLEM NO. 42
83
percent n-nonane, and 25 mole percent n-decane. The LFL values are 1.0% for
n-octane, 0.8% for n-nonane, and 0.8% for n-decane. Vapor pressure data can be
found in Perry's ChemicalEngineerslHandbook, Reid, Prausnitz, and Poliig's The
Properties of Gases and Liquids, or other literature sources.
Problem No. 43
CHEMICAL ENGINEERING TOPIC: Thermodynamics: Vapor Liquid Equiliirium
SAFETY AND HEALTH CONCEPT: Properties of Materials: Flash Point
BACKGROUND: Most combustion reactions occur in the gas phase. In order for
any flammable material to burn, there must be both fuel and oxidizer present.
There must also be a minimum concentration of the flammablegas or vapor in the
oxidizer. For most fires to occur, a minimum fuel concentration must exist in air at
ambient temperature. The minimum concentration at which ignition will occur is
called the lower flammable Limit (LFL). If the flammable material is normally
liquid, the liquid must be warm enough to provide a vapor-air mixture equal in
fuel concentration to the LFL concentration.The liquid temperature at which the
vapor concentration reaches the LFL can be found experimentally. It is usually
measured using a standard method called a6'closedcup flash point" test. The "flash
point" of a liquid fuel is thus the liquid temperature at which the concentration of
fuel vapor in air is large enough for a flame to flash across the surface of the fuel
if an ignition source is present.
The flash point and the LFL concentration are closely related through thevapor
pressure of the liquid. Thus, if the flash point is known, the LFL concentration can
be estimated, and if the LFL concentration is known, the flash point can be
estimated. In most cases, the calculation can be made for pure liquids using
Raoult's law. However, if the liquid is a mixture, particularly one in which the
components are dissimilar, the liquid solution may be nonideal. Then the liquid
phase activity coefficients may need to be determined if an accurate estimate of
the relationship between flash point and LFL is to be made. The system total
pressure is ambient, so it is low enough for thevapor (or gas) phase above the liquid
surface to be considered ideal.
PROBLEM: Estimate the flash point of a mixture made by mixing 600 ml of
methanol and 400 ml of water. The solution is not ideal, and the activitycoefficients
must be estimated. For the estimation of activity coefficients, first determine the
activity coefficients for methanol-water solutions from vapor-liquid equilibrium
data. Assume that the activity coefficients are a function of composition only, and
do not depend on the system pressure and temperature. Is such an assumption
justified? Vapor liquid equilibrium data can be found in Perry's Chemical
Engineers'Handbook. Vapor pressure data can be found in the Handbook as well.
The LFL of methanol can be found in NFPA 325M, "Properties of Flammable
Liquids" or Sax's Dangerous Properties of Industrial Materials. Once the mixture is
ignited, will it continue to burn?
Problem No. 44
CHEMICALENGINEERINGTOPIC: Thermodynamics Vapor LiquidEquilibrium
SAFETY AND HEALTH CONCEPT: Properties of Materials: Flash Point
BACKGROUND: Most combustion reactions occur in the gas phase. In order for
any flammable material to burn, there must be both fuel and oxidizer present.
There must also be a minimum concentration of the flammablegas or vapor in the
oxidizer. For most fires to occur a minimum fuel concentration must exist in air at
ambient temperature. The minimum concentration at which ignition will occur is
called the lower flammable limit (LFL). If the flammable material is normally
liquid, the liquid must be warm enough to provide a vapor-air mixture equal in
fuel concentration to the LFL concentration. The liquid temperature at which the
vapor concentration reaches the LFL can be found experimentally. It is usually
measured using astandard method called a "closed cup flash point" test. The "flash
point" of a liquid fuel is thus the liquid temperature at which the concentration of
fuel vapor in air is large enough for a flame to flash across the surface of the fuel
if an ignition source is present.
The flash point and the LFL concentrationare closely related through the vapor
pressure of the liquid. Thus, if the flash point is known, the LFL concentration can
be estimated, and if the LFL concentration is known, the flash point can be
estimated. In most cases, the calculation can be made for pure liquids using
Raoult's law. However, if the liquid is a mixture, particularly one in which the
components are dissimilar, the liquid solution may be nonideal. Then the liquid
phase activity coefficients may need to be determined if an accurate estimate of
the relationship between flash point and LFL is to be made. The system total
pressure is ambient, so it is low enough for thevapor (or gas) phase above the liquid
surface to be considered ideal.
PROBLEM: Estimate the flash point of n-decane that contains 5.0 mole percent
propane. Vapor pressures can be found in Perry's ChemicalEngineers'Handbook.
The LFL for propane is 2.2% and that for n-decane is 0.8%. The LFL of a mixture
of gases can be estimated by finding the concentrationsin the gas phase such that
where yi is the vapor phase mole percent of component i and LFLi is the lower
flammablelimit concentration of component i expressed in mole percent. You may
assume the liquid mixture is ideal and use Raoult's law, or you may use the K-factor
charts for estimatingylx or for finding the temperature for a knownylx for either
component.
Problem No. 45
CHEMICAL ENGINEERING TOPIC: Thermodynamics
SAFETY AND HEALTH CONCEPT: Explosions
BACKGROUND: Many gases are commonly compressed from normal atmospheric pressure to quite high pressures, either to facilitate transfer or storage or
to take part in a chemical reaction that is run more favorably at high pressures. As
anyone who has ever used a bicycle pump knows, as a gas is compressed its
temperature rises if no heat is removed. The reason is explained by the first law of
thermodynamics. If the gas is co~pressed,and if no heat is removed, the energy
put into the gas in the form of work to increase the pressure becomes part of the
internal energy of the gas. The increase in internal energy increases the temperature of the gas. For an ideal gas with a constant specific heat, for example, the
energy balance requires that, for an adiabatic process,
where AU = change in internal energy
W = work
Cv = specific heat at constant volume
AT = change in temperature
If the compression process is reversible as well as adiabatic, the entropy does not
change during compression of the gas, and, as a consequence,
where T I and Tz are beginning and ending temperatures, Pi and P2 are beginning
and endingpressures, and y is the ratio Cp/Cv,with Cp the specific heat at constant
pressure. If the specific heats are not constant, but the process is still reversible,
the temperature can be calculated by integrating the equation for the entropy
change
where A S is the entropy change, S2 - S I ,and R is the gas law constant.
PROBLEM NO. 45
87
If the process is not reversible, and if the entropy change can be calculated, the
temperature can still be calculated. The procedure becomes even more complicated if the gas is not ideal and the specific heat depends on the pressure as well
as the temperature.
If the compression ratio (P2tP1) is small, there is usually no consequence to the
temperature rise that accompanies compression. However, in some cases there
can be a potential hazard. Consider the compression of air in which there is a small
amount of fuel, for example. If the compression ratio is large enough, the mixture
of fuel and air will have a temperature high enough after compression that the
autoignition temperature of the fuel-air mixture will be reached and the mixture
will explode. (The autoignition temperature, sometimes called AIT, is the
temperature at which a fuel-air mixture will ignite without external energy being
applied. No spark or flame is needed for ignition because the temperature is high
enough to initiate combustion.) In fact, the diesel engine operates by injecting
diesel fuel into the combustion chamber after the air has been compressed to a
high pressure as the piston rises to the top of its stroke. The compression ratio in
a diesel engine is about 22 to 1.
In an air compressor where the final pressure is quite high, there can be a
possibility of explosion if there is flammable lubricating oil in the compressor or
in the discharge from the compressor. Thus, if very high pressures are required,
the air may have to be removed from the compressor at an intermediate pressure
and cooled before completing compression to the fmal pressure. Intercoolers also
make the compression process more efficient because they reduce the amount of
work required for compression to a given pressure.
An explosion might also occur if certain gases are compressed to high pressures
and attain high temperatures. Such gases as ethylene and acetylene that have
positive heats of formation may undergo spontaneous explosions at the temperatures found during compression. Combustiblegases can explode if they are drawn
into an air compressor and compressed to a pressure high enough to reach the
AIT.
PROBLEM: What is the final temperature reached after compressingethylene and
air from 14.7psia to 1000 psia if the initial temperature is 100°C?You may assume
the ratio of CplCvto be 1.22.The AIT of ethylene is 490°C. Will an explosion occur
if the mixture is a flammable concentration?
(This problem is based on a problem in the text Chemical Process Safety:
Fundamentals with Applications, by D. A. Crow1 and J. F. Louvar, published by
Prentice Hall, Englewood Cliffs, NJ.)
Problem No. 46
CHEMICAL ENGINEERING TOPIC: Thermodynamics
SAFETY AND HEALTH CONCEPT: Hazard Reviews; Explosions
BACKGROUND: Most people have heard the tale of the loss of a kingdom because
of the loss of a nail. In its simplest form, the tale tells that a horseshoe nail was lost
from the shoe of the king's horse. Because the nail was lost, the horseshoe was also
lost. The loss of the horseshoe meant the horse was no longer available for the king,
and the king in turn could not reach the site of an important battle. Without the
king, there was no one at the battle to direct the forces assembled there to protect
the kingdom. Without direction, the army lost the decisive battle, and the kingdom
was conquered by the opposing army.
While the tale may seem simple, a more recent true story illustratesthe potential
for damage resulting from a seemingly small occurrence. In 1969, a worker was
walking across a high walkway when he stumbled. To save himself from falling, he
grabbed a nearby valve stem. The valve stem was not strong enough to support the
stress imposed on it by the worker's weight, and it failed. Flammable liquid spewed
out of the broken valve and formed a cloud of flammable vapor that was ignited
when it reached a nearby truck. The resulting explosion and fire spread to other
equipment nearby, and the fire lasted for six days, completely destroying the plant
and doing more than $4 million worth of damage.
In order for a fire or explosion to occur, there must be a fuel, an oxidizer, and
an ignition source present. These three items are called the "fire triangle," and if
any one is missing, ignition is not possible. Most fire and explosion prevention
schemes attempt to keep either the fuel or the ignition source from being present.
Oxygen is always present in the open atmosphere. In closed systems, inert gases
are frequently used to preclude flammable mixtures from being formed, There is
a minimum fuel concentration required for ignition. It is called the lower flammable limit concentration(LFL), and it is thevolume or mole fraction of flammable
vapor in air that must be reached if ignition is to occur. There is also an upper
flammable limit (UFL) concentration. If the fuel concentration is above the UFL,
there is too little oxygen in the mixture to support combustion, and themixture will
not ignite. The flammability limits of many materials are included in references
such as the NIOSH Pocket Guide to Chemical Hazards; the National Fire Protection Association Standard 325M, "Properties of Flamable Liquids"; and Sax's
Dangerous Properties of Industrial Materials.
The concentration of a volatile material in air may be calculated easily using
basic thermodynamic relationships such as Raoult's law, if it is assumed that the
PROBLEM NO. 46
89
concentrationin air reaches equilibrium. Thermodynamic calculations can also be
used to determine the energy released if the fuel-air mixture is ignited. By making
such calculations, an engineer can estimate the potential for damage should a spill
and fire occur. Of course, the damage calculations show what might happen, not
what must happen. The calculationsbecome part of a hazard review, and a decision
must then be made to determine what action must be taken to reduce the consequence of a flammablematerial spill or to reduce the probabilitythe spillwill occur.
Experience has shown a number of methods of reducing spills and their consequences. They include the use of proper storage aod handling methods, the
elimination of ignition sources, reduction in the quantity of material stored and
handled, and installation of equipment to detect spills and shut down transfer
operations when spills occur. Substantial effort and resources are expended to
reduce the risk from flammable liquids.
PROBLEM: A worker is transporting a can containing 1.5 L of carbon disulfide
from one laboratory to another late on a Friday afternoon. He stops at his office
on the way between the labs and sets the can on his desk. Noticing the late hour,
he grabs his briefcase and rushes to join his car pool for the trip home. The lid on
the can is loose, and when the janitor arrives to clean up a short time later, he
notices an odor faintly like garlic in the office. He cleans up as quickly as possible
and leaves, closing the door behind him to keep the disagreeable odor from
spreading. An energy-saving policy has dictated the shutdown of air conditioning
and heating systems over the weekend, and the temperature in the office stays at
8S°Foverthe weekend. All the carbon disulfideevaporatesinto the air in the office.
There is no ventilation because the air conditioning system is shut off, so all the
carbon disulfide vapor remains in the office. The worker returns early on Monday
morning, and turns on the lights.
What surprise greets the worker when he turns on the lights?
Name a few simple precautions that might have been taken to prevent the
surprise.
The energy released by one pound of exploding TNT is about 2000 Btu. Is the
surprise likely to be noticed anywhere else in the building?
You will need some additional information to solve this problem, and you may
have to make some assumptions as well. The references Sited above will provide
flammability limit data. Physical property data can be found in the Chemical
Engineers'Handbook If the worker's office is about the usual size, it will be about
10 ft by 12 ft with a 9-ft-highceiling. Carbon disulfideburns easily in air, and it has
a very low ignition temperature. Some references list the ignition temperature as
80°C, and most authorities agree it is 100°Cor less. Carbon disulfide burns to form
carbon monoxide, carbon dioxide, sulfur dioxide, and sulfur trioxide, depending
90
SAFEVY, HEALTH,AND LOSS PREVENTION IN CHEMICAL PROCESSES
on the conditions of combustion. In this case, you may assume the combustion
products are carbon dioxide and sulfur dioxide. Since you need only an estimate
of the energy from the explosion, you may assume the reaction occurs at the
standard state, 25°C.
Problem No. 47
CHEMICAL ENGINEERING TOP'IC: Thermodynam
SAFETY AND HEALTH CONCEPT: Explosions
BACKGROUND: One of the fundamental findings of the application of the
concepts of thermodynamics is that as a gas is compressed its temperature increases. That increase in temperature can sometimes lead to problems in the
operation of air compressors. For example, the lubricating oils used for air
compressors are frequently based on mineral oils, and they may have autoignition
temperatures (AIT) as low as 500°F at atmospheric pressure. The autoignition
temperature decreases at higher pressures. There is usually little of the lubricating
oil present in the air stream leaving the compressor, so the chances of an explosion
are quite low. However, the small amount of lubricating oil in the air may collect
on the inside of the piping downstreamof the compressor and result in an explosion
that bursts the piping. Severe damage may occur. If workers are nearby, they may
be injured or killed.
Several methods may be used to prevent explosions in air compressors. They
include limiting the compression ratio, cooling the air between compressor stages,
using oils that have a higher ignition temperature, and keeping the system clean so
there is not enough oil on the piping walls to form a film thick enough to allow a
mist to form.
The temperature of the air leaving the discharge of a compressor may be
calculated through thermodynamic relationships. The energy and entropy balances can be used for the purpose. Quick estimates can be made using ideal gas
properties and the assumption of reversible operation.
PROBLEM: Air is compressed in a chemical plant. The air enters the compressor
at ambient temperature; for this problem, the ambient temperature may be assumed to be as low as 10°F and as high as 95°F. Air enters the compressor at
atmospheric pressure. What is the maximum pressure the air can reach without
causing an explosion if the explosion may occur at a compressor exit temperature
of 450°F? Make the calculation under two sets of conditions:
1. Assume the air to be an ideal gas with a constant specific heat of Cp = 7.0
caVgmole K and with the compression occurring adiabatically and reversibly.
2. Consider the air to be an ideal gas with Cp given by
92
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
where Tis inK and Cpis in caVgmole K. The compressorstill operates adiabatically
and reversibly.
Determine the outlet temperature for the air having properties of Part 2 if the
air is compressed to 150 psig. You may neglect the effect of the small amount of
oil that might be present in the air going through the compressor.
Problem No. 48
CHEMICAL ENGINEERING TOPIC: Thermodynamics
SAFETY AND HEALTH CONCEPT: Vapor Releases
BACKGROUND: Many flammable and toxic substances are used in chemical
processing plants. Some of the substances are used directly in the processes, and
others are used indirectly for such purposes as heat exchange or in separations.
Frequently, there is no viable alternative to using the flammable or toxic solvent or
intermediate material, although a search should always be made to find the least
hazardous material for a particular duty.
When it is necessary to use a flammable or toxic material (some chemicals are
both flammable and toxic), the properties of the material should be well known,
and the plant design and operation should be chosen to minimize the probability
that any of the hazardous material will be spilled or will leak from piping and tanks.
Then, even though stringent precautions have been taken to avoid releases, studies
of the consequencesof spillsor leaks should be made so that emergency operations
can be planned for any potential accident at the plant. The probability of a spill
can be estimated as part of the formal process hazard review that should be
performed for each plant. The process hazard review is part of the information
required for administrative decisions concerning the safety of plant personnel and
the cooperative effort needed to protect neighboring areas from potential accidents at the plant.
Releases of materials in the plant can be either as solids, liquids, or gases. Unless
the materialsare very fine dusts, solid releases will seldom have an immediate effect
outside the local area of the spill and are even less likely to leave the plant. Gas
releases cannot be prevented from leaving the area of the leak and eventually the
gas will leave the plant boundaries. Liquid releases occupy a middle position,
because as the liquid vaporizes, the vapor will be blown beyond the location of the
spill and then blown outside the plant by the wind. One of the important estimates
that needs to be made in determining the potential effects of a large liquid spill is
to estimate the rate of vaporization of the liquid.
Since further consideration of the possibility for ignition of a flammable vapor
cloud or the toxic effects of a vapor cloud depend strongly on the rate of the spill
and the rate of vaporization, it is important to place realistic bounds on the
estimates of vaporization rates. The simplest analysis to begin with is that which
can be determined with a simple adiabatic energy balance on the liquid stream.
For example, if a heated liquid is discharged from a pipe, the process will be
essentially adiabatic. Then, the enthalpy of the vapor-liquid mixture leaving the
pipe will be the same as the enthalpy of the liquid flowing through the pipe. The
94
SAFEI"Y, HEALTH, AND LOSS PREVENTION IN CHEMICALPROCESSES
problem arises in trying to determine the temperature of the liquid after it leaves
the pipe. Assuming the liquid is above its normal boiling point in the pipe, does it
cool to its boiling point on leaving the pipe? Or does it cool to the temperature of
the ambient air? Or may there be circumstances where the liquid will cool to a
temperature even lower than the ambient temperature?
Finally, is the effect of cooling of the liquid important in determiningthe fraction
of liquid that flashes to vapor? Some of these questions are explored in the
followingproblem.
PROBLEM: Two chemicals are being considered for use as a solvent in a processing plant, n-heptane and methyl alcohol. Both will be suitable for the purpose, and
a decision must be made on which to use in the plant. Both are flammable, of
course, and both are pollutants if released to the atmosphere or spilled onto the
ground. Precautions have been taken in the plant design to keep any of the solvent
from spilling, but the process hazard review group must assume that spillage has
occurred and determine the potential consequences. You are to estimate the rate
of vapor generation that could accompany a spill of the solvent. Regardless of
which solvent is used, it must be used at a relatively high temperature. Solvent flow
at some points in the process will be at normal rates of 500 galJmin. You have
estimated the flow through a broken pipe will be double the normal rate. The liquid
temperature in the flowing stream will be as high as 400°F.In order to help in the
choice of which solvent to use, you decide to estimate the fraction of the liquid that
will vaporize under three assumed conditions: (a) the liquid and vapor are at the
normal boiling point followingthe release, (b) the liquid and vapor cool to ambient
temperature of 80°Ffollowingthe release, and (c) the liquid and vapor cool to 25OF
following the release. The last estimate is based on an analysis of heat transfer
between the liquid-vapor mixture and the surrounding air. Your first task is to
determine the fraction of the liquid that will vaporize based on the three assumed
conditions. In order to compare the two candidate solvents, you then estimate the
volume of the vapor-air mixture that would be formed if the reIease continues for
10min. You do that by determining the vapor cloud size based on having a mixture
at its lower flammable limit, and you assume the mixtures of vapor and air will be
ideal because the mixture will be at a low pressure. The solvent that generates the
smaller vapor-air cloud will be chosen for the process. Using these criteria, which
solvent will you choose for the process? Might you also consider the heat of
combustion of each candidate solvent? Why?
Problem No. 49
CHEMICAL ENGINEERING TOPIC: Thermodynamics
SAFETY AND HEALTH CONCEPT. Storage, Handling, and Transport
BACKGROUND: Most of the materials stored, transported, and processed in
modern chemical plants are stable under the storage and handling conditions and
are unlikely to undergo any spontaneoustransformationsthat cause unsafe operations. However, some materials have the potential for chemical reaction when
mixed with other chemicals, such as occurs if a flammable material is mixed with
air. Spontaneous heating of solids is also known to occur when reactions such as
oxidation are possible and there is no pathway for the heat to escape.
There are also a few materials such as certain unsaturated hydrocarbons that
are stable at low temperatures, but as the temperature increases, they begin to
react. As they react, the energy released heats the mixture further until a rapid,
even explosive, reaction occurs. Acetylene and ethylene are two gases used frequently in organic chemical reactions. Either can be used safely, but either has the
potential for rapid decomposition. If the heat of formation of either acetylene or
ethylene is found in a table, it will be seen to be positive. A positive heat of formation
indicates that the substance will liberate energy if it is decomposed into its
constituent molecules. The system temperature will have to be raised high enough
for the reaction to begin, of course, and if the substance is kept cool and away from
ignition sources, it may be handled safely. However, if the decomposition reaction
begins, it will continue until equilibrium is reached. In many cases, equilibrium will
be reached only at avery high temperature or when the decomposition is essentially
complete.
The property of decomposition of a pure material is illustrated in the following
problem for acetylene. However, you should keep in mind that any material that
has a positive heat of formation may be capable of undergoing an exothermic
decomposition if the conditions under which it is held allow the reaction to start.
Such decomposition reactions are the basis for design of certain explosives, so it
is easy to see that the destructive effect may be quite large.
PROBLEM: Acetylene may decompose with a rather large liberation of energy.
You are to determine the temperature and pressure that would be reached by
acetylene that decomposes in a tank, starting with pure acetylene. Assume the
decomposition produces only carbon (in the form of graphite) and hydrogen. You
may also assume the starting temperature to be 25°C in the bulk of the acetylene,
even though a higher temperature, perhaps in the form of a spark, would usually
be required to initiate the decomposition reaction. The initial pressure in the tank
%
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
is 5 atm absolute. Once the decomposition begins it will be very rapid, so you may
assume the reaction is adiabatic. Keep in mind that there will be no increase in the
volume of gas or in the number of moles of gas in the tank and that the solid
produced will not influence the pressure to any great extent. However, if the
temperature rises, the pressure of the confined gas will also rise. You may assume
the gases to behave ideally, but you should justify your assumption. You should
also determine the composition of the gas in the tank at the end of the reaction.
Problem No. 50
CHEMICAL ENGINEERING TOPIC: Heat Transfer
SAFETY AND HEALTH CONCEPT. Process Control, Interlocks and Alarms
BACKGROUND: A complex chemical process requires the use of automatic
controlsto maintain a successful operation.The modern plant is much too complex
for human operators to maintain all process variables under adequate control.
Many students are aware of the difficulty of keeping even a very simple process
under control in the laboratory. For example, if an individual were trying to
maintain a constant liquid level in a tank with one stream leaving and one entering,
it would require his or her constant attention to maintain the level if the outlet
stream were to fluctuate at all with time. Only the very simplest chemical process
could be held within acceptable control without instrumentation and process
control devices.
Modern chemical processing relies heavily upon instrumentation, not only to
maintain product quality and to keep the production schedule, but to prevent
accidents as well.
Instrumentation for accident prevention takes many and varied forms, and only
examples can be mentioned here. A typical application would be controls to detect
and respond to a rapid temperature rise in a reactor in order that the feed may be
interrupted, if that would slow the reaction. Another application might be a level
sensor in a tank to prevent overfilling, since overfilling might cause overpressure
or spillage of a toxic or flammable material.
Instruments that function to control the input of material or energy to a process
are particularly critical. Whenever possible, such control loops should be designed
to be "fail-safe"; that is, in the event of failure of the instrument or a component
of the control loop, the manipulated variable would go to a safe condition. For
example, if an exothermic reaction is controlled, at least in part, by control of the
input of a reactant, then a fail-safe control loop would be one that would shut off
the flow of that reactant if the control equipment failed. Unfortunately, many of
the typical process control devices are not inherently fail safe.
When controls are used to prevent accidents, the response time is frequently
critical to the success of the prevention. The exothermicreaction is a good example
of such a case, because if the temperature begins to rise out of control, the reaction
rate increases exponentially with the temperature, so it is critical that the loss of
control be detected quickly. The following problem considers the response time
of two different temperature sensors.
98
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
PROBLEM: A thermocouple assembly is to be installed in a stirred tank reactor.
Two different designs are being considered, one is a cylinder that is made of 316
stainless steel; it is 7.0 mm in diameter, and is 100 mm long. The alternate design
is also a cylinder made of an alloy similar to bronze; it is 4.5 mrn in diameter and
80 mm long.
At the location where the thermocouples are to be installed, the velocity of the
stirred liquid is expected to be 0.3 d s , essentially at right angles to the thermocouples.
If the temperature of the vessel contentswere to suddenlyrise by 10°C,how long
would it take for each of these thermocouplesto respond by indicating 90% of the
increase, that is, how long will each take to increase by 9"C?
Some properties and model assumptions are required for this problem:
Assume that the thermocouples are solid metal, although that is not the case,
because the large diameter is really a shield around the actual sensor.
Density
Conductivity
Heat capacity
St. Steel
Bronze
8238
13.4
468
8800
52
420
kg/m3
W/m K
J/kg K
The liquid properties are:
Density
Heat capacity
Viscosity
Thermal conductivity
985
2.5
0.006
0.09
kglm3
W/kg K
kg/m s
Wlm K
It would be reasonable to expect that the stainless steel thermocoupk will last
longer and be Iess liabIe to damage than the bronze one. Which one would you
specify? Explain.
Problem No. 51
CHEMICAL ENGINEERING TOPIC: Heat Transfer; Mass Transfer
HEALTH AND SAFETY CONCEPT: Toxicology and Industrial Hygiene
BACKGROUND: One of the potential hazards to workers in an industrialenvironment is that of exposure to excessivelyhigh temperatures in the workplace. Among
the various consequencesthat may result from excessive heat stress are heat skoke,
heat exhaustion, and heat cramps. Heat stroke is a life threatening condition, and
recovery may not be complete. Other responses to heat stress are less severe.
The normal metabolic processes generate heat within the body. The magnitude
of the generation depends largely upon the level of physical activity, and also upon
the body mass of the individual. A typical resting value for heat generation is 350
B t l ~ /(103
l ~ W) for a man of about 150 to 155lb (68 to 70 kg) of body mass. Very
vigorous physical labor, such as working with an axe may have a heat generation
rate as high as 5500 Btu/hr (1612 W). It is probable that vigorously competing
athletes will have an even higher metabolic generation rate. The metabolic rate for
a machinist might be about 750 B t u h (220 W), which is approximately the same
for a person walking. Typical values for workers in an industrial activity would
range from about 700 to 1500 Btu/hr (205 to 440 W). A worker will rarely exceed
a rate of 1200 Btufhr (352 W) over an extended time.
In order to niaintain a proper body temperature, the heating and cooling rates
must be in balance over an extended time. A temporary rise of 2°F (l.l°C)
temperature is about the maximum that would be tolerable. We might note that a
2°F rise in body temperature for a man who weighed 150 lb would follow from an
increase in the energy storage within the body of about 300 Btu (316 kJ), since the
body's heat capacity is approximatelythat of liquid water.
Industrial hygienists use the followingrelation for the thermodynamicprocesses
concerned:
S=M+R+C-E
where
S = the rate of storage of energywithin the body (essentially the rate of change
of internal energy of the body)
M = the metabolic rate of generation (discussed above).
R = net rate of radiative interchange with the surroundings. (Here R will be
positive if the surroundings are warmer than the skin temperature.)
C = the rate of convective heating of the body by the surroundiig air
E = Rate of cooling due to evaporation of sweat.
All terms must be in consistent units such as Btu/hr or watts.
100
SAFETY, HEALTH, AND LOSS PREVENTION IN CHEMICALPROCESSES
It is noted that the M term must always be positive, but the i? and C terms may
be either positive or negative. The evaporative coolingwill always result in cooling
of the body, so the term must be positive in the equation above. The rate of storage
of energy within the body, S, can be either positive or negative.
The evaporative cooling is the primary mechanism by which the body maintains
proper temperature control.
A small cooling effect can be derived from the breathing process, that is, inhaling
less than saturated air, and exhaling air that is essentially saturated, but this will
normally be only a minor effect, less than 10 Btu/hr in most instances.
PROBLEM: A man is working in an environment where the temperature of the
surrounding air is 96°F (35.6"C). The man has a body mass of 160lb (72.6 kg), and
is working at a rate of 1050 Btulhr (308 W) (metabolic rate). A spot cooling fan is
set up to blow air past his body at a velocity of 10 ftlsec (3.05 d s ) . The air, also at
96°F has a relative humidity of 60%. What rate of evaporation of sweat would be
required to maintain this man's body temperature at a constant value?
Consider the mass and heat transfer rates that might be possible in these
circumstances and determine whether the man can work indefinitely or whether
his body temperature will rise over time. If his body temperature rises with time,
how long can he work in such circumstances before it rises by TF.?
Some model simplifications will be required to work this problem. First, assume
that radiation interchange is negligible. Next, we wish to make a reasonable
estimate of the skin temperature. We know it will be higher than the wet bulb
temperature, but it may not be much higher. Assume that it is 5°F (2.S°C) above
the wet bulb. Finally, we need an estimate of his surface area and his shape. We
could assume that he is a cylinder, 12 in. (0.305 m) in diameter, and 6 ft (1.83 m)
tall. (If you don't think this is a reasonable approximation, then you might make an
estimate of his arm dimensions, his leg dimensions, and his trunk dimensions.)
HINTS FOR SOLUTION: It will be necessary to find a suitable model to predict
the heat transfer coefficient for a cylinder in a cross-flow of air. From the heat
transfer model, the mass transfer coefficient can then be deduced from the
Chiton-Colburn analogy.
The body will be either heated or cooled by convection according to the heat
transfer coefficient, the area and the driving force. Evaporation will proceed
according to the mass transfer of water into the air from the body surface.
Most heat transfer texts will have a suitable relation for the heat transfer
coefficient. If you cannot find any other one, use Incropera and DeWitt, Fundamentals of Heat and Mass Transfer, 2nd ed. (John Wiley & Sons, New York,
1985). The Hilpert equation (Eq. 7.52 in that text) will be adequate.
Problem No. 52
CHEMICAL ENGINEERING TOPIC: Heat Transfer
HEALTH AND SAFETY CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKROUND: One of the potential hazards from which employees must be
protected is thermal burns of the skin. Burns are a frequent problem in foundries
where molten metals must be poured because these materials often splash if
incorrectly handled. For such cases, clothingmust be availablewhich will resist the
hot metal and simultaneously protect the employee from s k i burns.
Another situation where employees need protection from burns is where hot
items must be handled manually. For such cases, gloves are required. For high
temperatures where cotton or other natural fabrics would fail due to thermal
breakdown, asbestos was used for many years. Asbestos gloves do not have good
abrasion resistance, so they often do not last very long. Also, there is a general
reluctance to use asbestos because it is a possible contributor to lung disease,
including cancer.
One modern alternative material is fabric made of polybenzimidazole (PBI).
This fabric resists thermal degradationto quite high temperatures and is otherwise
suitable for making relatively comfortable gloves or other clothing items.
The degree of discomfort andlor injury from contact burns depends on a
complex relationship between time of contact and temperature of the contacting
surface. However, if the skin surface temperature reaches 48"C, then significant
discomfort at least, and probably tissue damage, will occur. Most people will be
able to tolerate more high-temperaturecontact on parts of the hand than on other
parts of the body, but the 48°C limit is a reasonably good guideline.
PROBLEM: A worker is wearing a glove made of a PBI material that is 4 mm thick.
How long can he safely grasp an object that is at 300°C?
Assume that the object is of high conductivity (metal) and the surface remains
at approximately 300°C. Assume that the glove is initially at 30°C and that the
worker's s k i temperature will be the same as the inside surface of the glove.
The following properties may be assumed for the PBI fabric:
Thermal conductivity: 0.389 W/m K
Density: 360 kg/m3
Heat capacity: 1298J/kg K
Problem No. 53
CHEMICAL ENGINEERING TOPIC: Heat Transfer
SAFETY AND HEALTH CONCEPT: Storing, Handling, and Transport
BACKGROUND: Most organic chemicals are flammable and many chemicals are
toxic. In order to prevent spread of hazardous liquids that might be spilled, storage
tanks are usually enclosed by a dike large enough to contain the tank contents.
Several tanks in a tank farm may be surrounded by a common dike.
While large spills of flammable liquids are rare, they sometimes occur. If a fire
occurs following a spill of flammable liquid into the dike, the tank contents can
begin to boil, and the vapor generated must be vented or the tank will fail with
catastrophic results. Large tanks are designed to operate at a pressure only slightly
above atmospheric pressure, so the fire emergency vents are designed to operate
at low pressures for venting high rates of vapor flow due to heat input from a fire.
(Tanks are also fitted with pressure and vacuum vents used for filling and emptying
the tank and for ambient temperature and pressure changes. Those vents are too
small to be used for emergency fire vents.) An alternative to installing large fire
emergency vents is to build the tank with a weak roof seam so the roof seam will
open and vent the tank in case of fire.
The rate of vapor generation, and therefore the venting rate, can be estimated
from heat transfer calculations. Heat can be transferred to the tank walls by both
radiation and convection from the fire that surrounds the tank. The rate of heat
transfer through the tank walls to the liquid inside the tank is much greater than
to the vapor because boiling heat transfer coefficients are much higher then vapor
phase heat transfer coefficients. In fact, the temperature gradient through a tank
wall is usually only a few percent of the difference between the flame temperature
and the temperature of the boiling liquid in the tank. Thus, in estimating the boiloff
rate, only the portion of the tank that is wetted by liquid needs to be taken into
account. The vent designer must assume that the tankis filled to its maximum liquid
level, because the fire may occur when the tank is full. The venting rate can be
estimated from basic heat transfer considerations. Heat is transferred to the tank
by radiation from the hot soot particles and gases in the flame and by convection
from the hot gases. The radiation from the fire varies in intensity because it
originates from both solid particles (soot) and from gases. Radiation from soot
particles is similar in spectral distribution to blackbody radiation; that from hot
gases is primarily in more narrow spectral bands. The complex nature of the
radiation and the fluctuations of temperature within the fire make exact flame
radiation calculations very difficult. Fortunately, it is possible to simplify the
calculations. In fact, it has been found that the radiation flux can be averaged, and
PROBLEM NO. 53
103
that for many hydrocarbon fuels the radiant flux emitted by a flame is about 30,000
~tu/hr-ft2.
The average accounts for fluctuations in temperature, emissivity, composition, and other variables in the flame.
Convective fluxes can be estimated on the basis of natural convection heat
transfer. Most of the surfaces of interest are large and vertical, so the appropriate
correlations are those for vertical plane surfaces. The flame can be assumed to
have transport properties equal to those of hot air because of the large amount of
excess air in the flame and the fact that most of the gas in the flame is nitrogen.
Flame temperatures may vary from fuel to fuel, but a temperature of 2200°F will
provide a reasonable average value for most purposes.
PROBLEM: A tank is to be used for storing 10,000 bbl of toluene. The tank will
be cylindrical and will be designed to API 650 standards with a cone roof. The tank
walls are to be 27 ft high, and there is to be a maximum liquid level of 26 ft in the
tank. For what venting rate should the tank be designed? How large a vent area
will be required if the maximum tank pressure can be 10in. of water gauge? Would
you recommend a weak seam roof? A standard barrel contains 42 gal.
Problem No. 54
CHEMICAL ENGINEERING TOPIC: Heat Transfer: Radiation and Convection;
Design
SAFETY AND HEALTH CONCEPT: Fire protection
BACKGROUND: Fires do not occur very frequently at chemical plants, but they
do occur occasionally. Fires maybe caused by relatively minor failure of equipment
or storage vessels in which a flammable material is spilled and then ignited. If the
fire occurs in a process area where there is a substantial amount of process
equipment, piping, valves, instruments, or the like. These pieces of equipment can
fail, leading to propagation of fire from one plant area to another. In order to
prevent this propagating damage from occurring, several types of fire protection
equipment are provided.
The fire protection system and equipment can be divided into two large
categories: passive and active. Active equipment includes such things as water
sprays, foam, and dry chemicals. It requires that some action be taken, either by
the plant operators and fire brigades or as a response by an automatic fire
protection system. Passive fire protection equipment does not require any action
at the time of the fire. It is designed and installed at the time the plant is built and
remains passively in place until needed. Only routine maintenance is required to
keep it operable.
One example of passive fire protection is insulating material (called fire-proofing) that is applied to steel structural members and equipment supports in the
plant. The time required for unprotected steel supports to fail during a fire is very
short. Fire-proofing can extend the failure time significantly and provide enough
time for fire fighters to reach the scene, apply cooling water to the supports, and
bring the fire under control.
Fire-proofing applied for structural fire protection is often provided only for the
structural supports 30 ft above the fire base. Higher elevations can also be exposed
to high heat fluxes. Individualjudgments must be made in specific cases.
The heat transfer rate in a fire depends on two mechanisms: convection and
radiation. Calculation of the heat transfer rate must be made by considering each
of the mechanisms separately and then combining the result. If the fire is large, it
will radiate at a constant flux; for most hydrocarbons and combustible chemicals,
the radiant flux can be taken as 30,000 ~tu/hr-ft2.
Most of the radiation inside fires
is due to emission from hot carbon particles. In fact, the characteristic red-orange
color of a flame is due to the emission from hot particles at visible wavelengths.
There is also some band radiation emitted by hot gases at discrete wavelengths.
The band radiation is usually invisible to humans, but can be detected by instru-
PROBLEM NO.54
LO5
ments. The radiation from the hot particles is emitted along a continuum that
approximates graybody radiation.
Convective heat transfer in a fire occurs at rates that can be estimated from
standard convective correlations. In making the estimation, the properties of
nitrogen can be used to approximate the flame properties. Flame temperatures
vary considerably from place to place within the flame and as a function of time at
a fured position in the flame. A reasonable average for convective heat transfer is
2200°F. The effective blackbody radiation temperature of a flame is much lower.
PROBLEM: An engineer has just arrived on a new job for which the process and
piping layout has been completed. She is asked to determine whether fire-proofing
will be required for the support structure for piping and equipment. One of the
process vessels is a large cylindrical tank with dished ends that is mounted
vertically. The tank is supported by a skirt at the bottom. The skirt surrounds the
entire vessel, and totally encloses the space beneath the vessel. The figure below
shows the cross-section of the bottom of the vessel and the supporting skirt. The
STEEL
SlPPORT
'1
1
106
SAFETY,HW'I'H, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
supporting skirt will fail when its temperature reaches 1100°F.The engineer is told
that at least 2 hr of fire resistance are required to assure that fire fighters have
enough time to arrive and begin active fire protection. Ultimately, her decision on
the kind and thickness of fire-proofing insulation to be used will depend on initial
cost, maintenance cost, and the reliability of the insulation during its lifetime. She
begins her study by assuming that the insulation has the thermal properties of
cinder concrete.
a. If there is no insulation on the supporting skirt, how long will it be before the
skirt fails? You may assume the steel skirt has a uniform temperature across its
thickness, even though the temperature increases as the skirt is heated by the fire.
The skirt diameter is large enough that it can be considered one-dimensional. The
transport properties required for estimating the convective coefficient may be
assumed to be constant, but should be taken at the film temperature. The fdm
temperature willvarywith time, souse an averagetemperature for the steelsupport
half way between the initial temperature of 80°F and the flame temperature.
Transport properties for the steel can be assumed constant at their ambient
temperature properties. All the data required can be found in the background
material or in the sixth edition of Perry's Chemical Engineers' Handbook.
b. What insulation thickness will be required to provide protection to the
structural members for 2 hr? The temperature of the structural member must
remain below 1100°F. Start your solution by writing a rigorous equation for the
temperature gradient in the insulation. Specify the boundary conditions on both
sides of the insulation and the initial condition. You will not be able to solve this
set of equations unless you are a computer whiz and can do difficult numerical
simulations.Even though you cannot solve the equations, the time required for the
skirt to fail must still be estimated. Therefore, simplify the problem by making it
into a pseudo-steady state problem. To do this, calculate the insulation surface
temperature assuming that the steel temperature is its average value between
ambient and the failure temperature. The exposed surface temperature will be a
weak function of insulation thickness when thus calculated. Use the heat required
to heat the steel to its failure temperature and the failure time to estimate the
average heating rate. Then use the average heating rate to estimate the thickness
of fire-proofinginsuiation required.
Problem No. 55
CHEMICAL ENGINEERING TOPIC: Heat Transfer: Radiation
SAFETY AND HEALTH CONCEPT: Fire Protection: Separation
BACKGROUND: Many large storage tanks for flammable liquids are used in the
chemical process industries. These tanks are usually enclosed by d i e s that are
designed to contain the contents of the tank in case of spill or tank failure. If a large
spill occurs and the liquid is ignited, a very large fire will result because the d i e
area is very large. (Dikes may be several hundred feet long on each side. For
example, a 6-ft high dike around a 100,000bbl tank would have to be 300 ft2.) Such
a large fire will emit radiation that can be transmitted through the atmosphere for
long distances, and the radiant flux incident on surrounding buildings and equipment may be large enough to damage or destroy them. If people are exposed to
the radiant energy, they may be killed or injured.
In order to protect people and equipment from the effects of thermal radiation,
the storage areas are frequently separated from other equipment and from inhabited buildings. Remote impounding is also used. In remote impounding, the
ground around the tank is graded so that any spill can be drained to an impounding
area where the tanks are not exposed. In order to determine the separation
distances required from the dike or remote impounding basin to other areas, an
estimate of the radiant heat flux at any point around the fire must be made. The
estimate is based on simple models of the fire and the transmission of radiant heat
through the atmosphere.
The radiant heat transfer from a fire can be estimated from
q
=
a t F ~ a 4T
where
q = absorbed radiant flux,~tulhr-ft2
a = absorptivity of the surface where the radiation is incident, unitless
z = transmissivity of the atmosphere, unitless
E = emissivity of the flame, unitless
F = radiation view factor, unitless
a = Stefan-Boltzmann constant, 0.1714(10~)~ t u / h r - f t ~ - ' ~ ~
T = absolute temperature, OR
The absorptivity of the surface receiving the radiation is usually near 1 for
nonmetallic materials. It depends on the wavelength of the incident radiation as
well. Flame radiation contains both the continuum radiation characteristic of a
blackbody or a graybody and the band radiation from gas emission. However, the
108
SAFETY,HEALTH,AND LOSS PREVENTION IN CHEMICALPROCESSES
band emission from gases is primarily due to emission from water and carbon
dioxide. That radiation can be absorbed by the atmosphere to a large extent. The
atmospheric transmissivity depends on the wavelength of the radiation, as well,
and the incidentradiation maybe reducedsubstantiallyif the atmosphere is humid.
The flame emissivity is closely coupled to the flame temperature. In fact, some
methods of measuring flame temperatures are actuallybased on the measurement
of radiation fluxes from the flame. If a thermocouple or resistance bead thermometer is used to measure the flame temperature, the result will be in the range
of 1800-2200°F for most fires. If the radiant flux from the fire is measured, it will
for most hydrocarbon-based fuels.
be in the range of 30,000 to 45,000 ~tu/hr-ft2
Thus, emissivities in the range of 0.3 to 0.7 can be expected.
A simplifiedmethod for calculatingradiant fluxes is simplyto combine the flame
emissivity, the atmospheric transmissivity, and the flame temperature into a single
term that represents the effective surface emittance of the flame. Then the absorbed radiant flux can be calculated as
The effective surface flux, qs,accounts for the effective surface flux from the flame.
The incident flux at any location around the fire is then
where qs is an approximate value of the effective surface flux of the fire modified
for the flame emissivity, atmospheric transmissivity, and flame temperature.
The radiation view factor, F, depends only on the geometry of the problem. It is
a function of the flame size and shape, the distance to the location where the
incident radiant flux is desired (the "target"), and the angles between the flame
and the target. (Fis sometimescalled thegeometricviewfactor or the angle factor.)
The flame size and angle can be estimated based on the results of modeling the
behavior of flames. Flame heights can be estimated from the work of Thomas
(Ninth International Symposium on Combustion, Academic Press, New York,
1963), for example. He found that the height of a flame was given approximately
by
L/D = 42(d,paQT)0'61
where
L = flame height, ft
D = flame diameter, ft
m = mass burning rate, lblsec
pa = air density, lblft3
g = gravitational acceleration,ft/sec2
For circular or square dikes, the flame diameter is the dike diameter or length. The
equivalent hydraulic diameter may be used for other dike shapes. The flame may
PROBLEM NO.55
109
also be tilted by the wind. Welker and Sliepcevich (University of Oklahoma
Research Institute Report No. OURI-1578-FR, Norman, OK, 1970) have found a
method of predicting the flame angle. The flame may be tilted substantially if the
wind velocity is high. Notice that the equation for flame height is written in
dimensionless form, so any consistent system of units can be used.
Once the flame size and angle are determined, the radiation view factor can be
determined from the information in standard radiant heat transfer texts. The view
factor will be given in the form of an equation or a graph. The usual rules for view
factor geometry apply to the view factors of flames just as they do to any similar
geometric shape.
Once the incident radiant flux is known at some location near the fire, a judgment
can be made as to whether it is tolerable for various activities or pieces of
equipment. For example, the Code of Federal Regulations (Title 49, Part 193)
specifies the acceptable radiant flux permitted at various locations around liquefied natural gas plants. The values specified are 10,000 Btu/hr-ft2 at the plant
property line, 6700 Btu/hr-ft2 at streets near the plant, 4000 ~tulhr-ft2for some
buildings near the plant, and 1600 Btu/hr-ft2 (excluding solar radiation) for outdoor public areas such as playgrounds. There are specificrules more detailed than
Sited here, of course, but the general idea is to protect the public from the danger
of fire following a potential spill of liquefied natural gas.
Substantial damage to most plant equipment and buildings can be expected at
incident fluxes of 10,000 Btu/hr-ft2, and at fluxes of 1600 Btuthr-ft2 human skin
will receive second degree burns if exposed for about half a minute. Wood
structures will be charred and damaged if exposed to radiant fluxes of 4000
Btu/hr-ft2 for long periods.
PROBLEM: A liquefied propane storage tank will have a net storage volume of
280,000 bbl. Storage is at atmospheric pressure and the normal boiling point of
about -44°F. The tank is to be constructed in the vicinity of an existing plant and
the safety department desires to keep the radiant flux from any potential fire in the
dike surroundingthe tank below 10,000~tu/hr-ft2
at any location within the existing
facility. The dike to be built surroundingthe tank cannot be more then 8 ft high. It
has been found (Welker and Cavin, Report No. DOEEP-0042, U.S. Department
of Energy, 1982) that the effective surface flux from propane fires is about 50,000
Btu/hr-ft .Estimate the minimum distance from the dike to the present facilities.
You may assume that the burning rate of the fire is equal to a decrease in the fuel
depth of 0.44 in./minute and that there is no wind.
It is also desired to keep the radiant flux from the fire below 1600 Btu/hr-ft2(not
including solar radiation) at an outdoor parking lot for the plant employees. How
far must the d i e be located from the parking lot? In all your calculations, assume
that the exposed surface receiving the radiation is vertical. For the no-wind
conditions, the flame will also be vertical.
Problem No. 56
CHEMICAL ENGINEERINGTOPIC:Heat Transfer;Design; Momentum Transfer: Fluid Mechanics
SAFETY AND HEALTH CONCEPT: Inerting and Purging
BACKGROUND: Many chemical plant operations require that vessels be purged.
There are severalreasons. For example,when a plant is first constructed,the piping
andvesselswill be filled with air. The air may have to be purged because it interferes
with the process or because it can lead to flammable mixtures with the chemicals
within the piping or vessels. If piping or vessels containing a flammable or toxic
material must be taken out of service and repaired or inspected, they must first be
purged to remove the flammable or toxic material. Otherwise, workers will be
exposed to the hazard of working under unsafe conditions.
Piping and vessels may also have to be cleaned following service. The piping or
vessel will then have to be purged out of service, cleaned, and purged into service
before the process can be continued. In some cases, steam is used both to purge
the system and to clean it. For the present, consider only the purging step. It is
assumed that most of the liquid or solid residue that might have been in a vessel
has been removed. It is also assumed that the remaining vapor is either toxic or
flammable. Thus, the vessel must be purged. Steam is available, and it is used to
purge the vessel to remove all the hazardous material. The concentration level of
hazardous material is monitored to assure it has all been removed and the steam
supply is then stopped. At this point, the vessel is hot and full of steam, with no air
or other noncondensible gas present. The vessel will gradually cool, and as it does,
the steam will condense. When the steam condenses, the pressure in the tank will
decrease. A vent is provided to supply air or nitrogen to prevent drawing excessive
vacuum in the vessel. Otherwise the vessel will collapse when the maximum
allowable vacuum in the tank is reached. Therefore, special care must be used when
using steam as a purge gas.
PROBLEM: An uninsulated steel process tank is purged with steam before it is
cleaned. The tank is 15 ft in diameter and 30 ft high, and it has walls and roof that
are 0.25 in. thick. The tank will begin to collapse when the vacuum reaches 0.3 psig.
What vent area is required to prevent vacuum collapse caused by steam condensation? The ambient temperature is 25°F and the wind is blowing at 20 mph.
Consider two cases:
Case A: The steam used to purge the tank is saturated at 40 psig when it arrives
at the valve entering the tank. After being expanded through the valve, the enthalpy
PROBLEM NO. 56
111
of the steam entering the tank will be the same as the enthlapy of saturated steam
at 40 psig. The pressure in the tank will be 1.0 atm absolute. Thus, the steam in the
tank will be superheated at 1.0 atm. The steam must be cooled to 212OF before it
begins to condense. Assume the steam in the tank is at a uniform temperature.
Case B:Assume that the steam in the tank following purging is saturated at 1.0
atm pressure (and therefore at 212OF).In this case, the steamwill begin condensing
immediately.
In either case, the heat transfer through the bottom of the tank can be neglected,
but heat transfer through the top and sides must both be considered. Make your
venting calculations assuming that the venting rate required will be the rate
calculated as soon as the steam is shut off. In both cases, start by calculating how
long it will be before the tank begins to collapse. Collapse will start when the tank
pressure is reduced to 0.3 psi below atmospheric pressure.
Problem No. 57
CHEMICAL ENGINEERING TOPIC: Heat Transfer
SAFETY AND HEALTH CONCEPT. Process Design
BACKGROUND: Many chemical engineering process designs require heating.
The process heaters and furnaces may cover a wide range of sizes and be used for
a variety of purposes, including generation of steam and direct heating of reactors.
Heaters and furnaces must be designed to generate the required amount of heat
and to transfer it to the place where it is needed. In addition,they must be designed
to operate efficiently and safely. Most heaters use either gas or liquid fuels, but a
signif~cantfraction use solid fuels. Some use a combination fuel, such as finely
powdered coal suspended in oil.
Regardless of the type of fuel or the rate of heat generation, most heaters and
furnaces have one thing in common: somewhere in the design, there is a fire. The
fire may be small or large, depending on the rate of heat generation,but the burner
temperature will nearly always be very hot. Temperatures inside the furnace or
heater will be above 2000°F, and the walls of the furnace or heater will frequently
be above 2000°F as well. Thus, the walls of the heater or furnace must be well
insulated to minimize heat loss.
There is also another reason for insulating the walls. Generally, plant operators
must be near the heaters or furnaces and they must be protected from the heat.
Protection takes at least two considerations. First, the environment must be cool
enough for operators to work in the area for whatever time is necessary. Second,
even though the environment near the heater or furnace is cool, the actual surfaces
may be hot enough to burn unprotected s k i .
PROBLEM: A furnace is designed to reach interior wall temperatures of 2500°F.
It is lined with firebrick 4 in. thick having an average thermal conductivity of 0.2
Btu/hr-ft-OF. The firebrickis attached to asteel shell 0.25 in. thick. The temperature
of the steel shell currently reaches 220°F, and workers must avoid the furnace
because of the danger of being burned. A new engineer is asked to determine the
thickness of a layer of insulation that will be applied to the outside of the steel shell
toreduce the outside temperature to 120°F.She knows the temperature of the steel
shell must not exceed 500°Fbecause the steel will begin to weaken and the bonding
agent used to attach the insulation to the steel will start to deteriorate. What
thickness of external insulation with a thermal conductivity of 0.35 Btukr-ft-OFwill
be needed? By what percent will the heat loss from the furnace be reduced? The
engineer assumes the temperature drop across the steel to be negligible because
the steel layer is thin and the thermal conductivity of steel is about 25 Btu/hr-ft-OF,
PROBLEM NO.57
113
which is about 100 times the value for the insulating materials. Once she has
determinedthe thickness for the outer insulation layer, she calculates the temperature drop across the steel layer. Was her assumption of negligible temperature
drop valid?
Problem No. 58
CHEMICAL ENGINEERING TOPIC: Heat Transfer
SAFETY AND HEALTH CONCEPT: Process Design
BACKGROUND: Although most materials that are processed in chemical plants
remain in the same phase throughout the process, there are some materials and
processes in which a change of phase occurs. Within the processes, a liquid may
be boiled to form avapor, or avapor condensed to a liquid. Crystalsmay be formed
from a liquid solution, and solids may be dissolved. If a change of phase occurs
outsidethe control of the operators and in circumstanceswhere no change of phase
was anticipated in the plant design, serious problems can occur. For example, if
liquid condenses in a l i e leading to a compressor and the liquid enters the
compressor, severe damage can be caused. Similarly, cavitation in a pump can
damage the pump. Most process equipment is designed to handle only a single
phase, and is unable to handle two phases efficiently.
Another problem can be encountered as well. Freezing of liquids in piping or
equipment can be a serious problem, particularly in colder climate. Sometimes,
freezing of liquid in piping can result in serious safety hazards, such as might be
encountered if cooling water flow was stopped for a critical process. Freezing
might also lead to overpressure in piping as pumps try to force liquid through a
pipe that is plugged by frozen material. One of the most common materials to be
used, water, also expands when it freezes. When it does, piping and equipment can
be damaged or destroyed.
Process piping is frequently "traced" with heating wires or heating coils to
prevent freezing. Steam tracing or electrical tracing is frequently used. Electrical
tracing has the advantage that it is not subject to freezing, whereas steam may
condense and freeze ifflow is not kept at the proper rate. However, steam tracing
may be cheaper if waste heat is available in the plant. In any case, careful attention
should be given to the potential for freezing in piping, and steps should be taken
to prevent it.
PROBLEM: Cyclohexane is used in a chemical processing plant. It is circulated
through 1-in. schedule 40 steel piping. Flow may be interrupted at any time. To
prevent freezing of the cyclohexane, the piping is insulated with a V2-in.-thick layer
of insulation having a thermal conductivity of 0.23 Btu/hr-ft-OF. An electrical
resistance wire between the insulation and the piping supplies heat to the piping
to keep the temperature from decreasing below 50°F.
a. Assuming that the ambient temperature can drop to -lO°F, how many watts
of electrical energy should be supplied to the piping per foot of length to keep the
PROBLEM NO. 58
115
temperature of the piping and cyclohexane from dropping below 45"F? Assume
both the piping and the cyclohexane are at 45°F and no heat flows to either.
b. If the thermostatic switch that controls the electrical heating fails and the
heating system fails to go on, how long will it take for the cyclohexane to begin to
freeze? You may assume the insulation has negligible heat capacity and that the
temperature of piping and cyclohexane are uniform throughout.
c. How long will it take for the cyclohexane to freeze completely?Assume there
is no flow in the piping when the power goes off and that the piping is full of
cyclohexane.
Problem No. 59
CHEMICAL ENGINEERING TOPIC: Heat Transfer
HEALTH AND SAFETY CONCEPT: Fire Protection Systems
BACKGROUND: Almost all commercialbuildings, and certainly buildings on site
at a chemical production facilitywill be equipped with some type of fire protection
system. Componentsof the system range from alarms to sprinklersto COz-dispensing devices.
An automatic system, that is a system that would respond to a fire in the absence
of human interaction,willbe some type of device that willrespond to a temperature
rise or perhaps to smoke, as for example the "smoke alarm" that is frequently used
in homes, schools, hotel rooms, and stores.
A relatively common type device is a sprinkler system that responds by flooding
the room or area with a water spray in the event of a fire. These types of systems
have been in use for many years, and are still widely used when the situation is one
in which one would use water to extinguish a fire.
One of the oldest, yet more dependable ways the automatic sprinkler system is
activated is by the melting of a "fusible link." Such a device is suggested by the
sketch of a typical release mechanism shown in Figure 1.
The two elements of the link are held together by a low melting point soldering
alloy, which, upon melting from the heat of a small fire, will release the lever arms,
which then release the valve, allowing water to be sprayed on the fire.
The fusible links are also frequently used to provide door closing automatically
so as to isolate fires.
There are a number of so called "fusible alloys," ranging in melting point from
just under 500°F to less than 120°F. One typical alloy melts at 158°F and consists
of 50% Bismuth, 26.7% lead, 13.3% tin, and 10% cadmium. In the problem that
follows, you are asked to determine how long an alloy that melts at 158°Fwill take
to respond to a sudden rise in temperature caused by a fire in the room where it is
being used as the fusible alloy in a sprinkler system.
PROBLEM: A fusible link in a fire protection system responds to a fire by melting
when its temperature reaches 158°F.When it melts, it releases a device similar to
that shown in Figure 1,thus allowing the opening of the valve of a water sprinkling
system. For the purposes of this problem, assume that if a fire breaks out in the
room, the ventilation system will remove heat fast enough to keep the temperature
from going above 220°F for several minutes.
If the room temperature reaches a temporarily steady220°F,how long will it take
for the fusible link to melt if its initial temperature is 78"F?
PROBLEM NO.59
117
In order to work this problem, you will need to know some properties of the
materials and some conditions in the room.
Assume that the metal link which is to separate consists of two strips of brass,
held together by a very small amount of the alloy. When the interface between the
brass strips reaches 158"F,they will separate and release the valve. The amount of
solder is small enough that the heat of fusion may be neglected.
Each of the brass strips is 314 iu. long, 114 in. wide and 1/16 in. thick. The
properties of the brass may be taken as given below:
Density: 8500 kglm3
Heat capacity: 380 J/kgK
Conductivity: 110 W/mK
The heat transfer coefficient between the room and the link is 10 ~ t u l h r - f t ~ " ~ .
Force Exerted by
Valve Body
Link
Pivot
(held together
by fusible alloy1
Force Exerted by
Water Pressure
Figure 1. Fusible link release mechanism.
Problem No. 60
CHEMICALENGINEERING TOPIC: Heat Transfer; Design
SAFETY AND HEALTH CONCEPT: Explosions
BACKGROUND: Almost all chemical processes require some storage of raw
materials, intermediates, and finished products. The nature of chemicalprocesses
requires that the materials be reactive. Many are toxic and many are flammable;
frequently they are both toxic and flammable. Modern practice usually follows the
tenet that the quantity of material stored should be reduced to a minimum. Storing
smaller quantities reduces the hazard in case an accidental release occurs. Smaller
inventories also reduce the cost of handling the materials, both because smaller
(and cheaper) storage tanks can be used and because money invested in inventory
can be minimized.
The probability of tank rupture or leakage is very low. However, the consequences of a release can be very costly, both in terms of human injury or death and in
terms of damage to the plant, damage to the surrounding neighborhood, and loss
of production. When the consequence of a release is costly, special protection may
be provided to prevent an accident from escalating from a small consequence to a
catastrophe. Such protection may be relatively simple control systems, operating
procedures, and other prevention methods. Control methods may include rupture
disks and relief valves to vent tanks and process vessels to prevent failure. Proper
design of the vessel is also required, along with proper testing before the vessel is
put into service and proper maintenance during the service life of the vessel.
If a spill of flammable material occurs despite the efforts to prevent it, and if a
fire occurs, fire protection systems may be provided to control or extinguish the
fire and prevent it from causing the damage to propagate from one area to another.
Design of fire protection and safety systems may require estimates of the potential
heat transfer rates in a fire. If storage vessels are located close together, failure of
one vessel can easily lead to failure of another, either through mechanical effects
or fire damage. The original design of a storage tank will affect its behavior in a
fire, and the design must be consistent with the intended use.
If a fire surrounds a tank, the heat transfer to the tank is due to both radiation
and convection. The radiant heat transfer is due primarily to the hot soot particles
in the fire, but there is also some radiation from hot gases. Heat transfer from the
fire varies in intensity because the fire fluctuates in size, shape, and temperature.
However, for a period of a minute or so, the average heat transfer rate can be
estimated because the average flame properties can be used. Radiation from the
soot in fires is similar in spectral distribution to blackbody radiation; radiation from
hot gases occurs primarily in narrower spectral bands. While the exact calculation
PROBLEM NO. 60
119
of radiant emission is very complicated, it has been found that for many hydrocarbon fuels the radiant flux emitted by a large fire is an average of about 30,000
Btulhr-ft2. That average accounts for the average temperature, the average emissivity, composition of the flame, and flame turbulence.
Convective heat transfer rates can be estimated on the basis of natural convection heat transfer correlations. The flame is assumed to have transport properties
equal to those of hot air because free-burning flames draw in excess air and most
of the gas in the combustion zone is nitrogen. Hame temperatures vary from fuel
to fuel, but for most purposes, a flame temperature of 2200°F will be a reasonable
average. Note that the temperature suggested for convection heat transfer calculations is different than the effective blackbody temperature that would correspond
to the radiant heat flux. A blackbody temperature of 2200°F would provide a
radiant flux nearly three times that found in measurements for flames. The difference indicates that a flame must not behave as a blackbody.
Likewise, an object exposed to flame radiation may not absorb all the radiation
that impinges on it. However, if the object is in the flame, it will collect soot on its
surface, and the absorption of radiation will be quite efficient. Thus, it is usually
acceptable to assume that an object surrounded by a fue absorbs all the radiation
incident of it.
PROBLEM: Tanks fabricated for the storage of liquefied petroleum gas (LPG)
are designed for a working pressure of 250 psig and a safety factor of four based
on the tensile strength of the steel used in the tank. The dimensions of the tank
depend on its design volume, but tanks are normally cylindrical. Their diameters
are usually less than their length because the steel thickness required for the tank
depends more on the tank diameter than the length. In this problem, you may
assume the tank has a wall thickness of 0.625 in., which is the wall thickness for a
tank about 10ft in diameter. Suppose there is a fue and that the fue surrounds the
LPG tank. Eventually, the temperature of the tank walls will be high enough to
cause the steel to weaken. The temperature at which the tank walls will weaken
enough to fail will be about 1200°F, which is the temperature at which the tensile
strength is about one-quarter of its ambient temperature value. If the fire is not
fully developed and flickers around the tank, the average radiant flux from the fire
will be less than the 30,000 Btu/hr-ft2 discussed above. For this problem, assume
the average incident radiant flux is 15,000Btulhr-ft2.There will be convective heat
transfer from the fire to the tank as well. The heat transfer coefficient can be
estimated from natural convection correlations for heat transfer from hot air to a
cooler surface. In this case, the heat transfer coefficient will be about 2 to 3
Btu/hr-ft2-OF.You may use a value of 2.5 Btu/hr-ft2-"Funless your instructor asks
you to find a more precise value from one of the correlations. The absorptivity of
the tank surface for flame radiation will be about 0.95, the same as its emissivity.
You are to determine how long it will take the fue to heat the tank from ambient
temperature, say 80°F, to the failure point.
120
SAFEI*Y, HEALTH, AND LOSS PREVENTION IN CHEMICAL PROCESSES
You can start by writing an energybalance on the tank shell. You know that the
shell will not heat very much if there is liquid in contact with the steel because
boiling heat transfer coefficients are large and the heat will be transferred to the
liquid. As the liquid boils, the tank pressure will rise, the tank vents will open, and
the pressure will be relieved. However, if the steel of the vapor space in the tank
is heated, the heat transfer rate to the vapor inside the tank will be quite slow, so
the tank will begin to heat. Your energy balance should include terms for the
radiant energy incident on the tank from the fire, the convective energy from the
fire, the energy reradiated from the tank to the surroundings as the tank heats, and
the energy to heat the tank itself. Assume the specific heat and density of the steel
remain constant. Heat transfer to the interior of the tank will usually be small and
can be neglected for our present purposes. When you've completed your energy
balance, you will have a first order differential equation with one term linear in
temperature and one term fourth order in temperature. It will have to be solved
numerically. A simple Euler technique will be satisfactory,but you may also use a
Runge-Kutta method or other method suggested by your instructor.
Problem No. 61
CHEMICAL ENGINEERING TOPIC: Heat Transfer
HEALTH AND SAFETY CONCEPT: Vapor Releases
BACKGROUND: Anyone who watches newscasts on television or reads a daily
newspaper is aware of frequent transportation accidents, which cause chemicals
to be spilled into the environment. Very often, the chemical is one that evaporates
at an appreciable rate, which then causes a vapor cloud that spreads either
downwind or down gradient (downhill) of the spill site. There are a number of
models available to predict the downwind concentration of the chemical if the
emission rate is available. For an evaporating pool of chemical, the rate at which
it emits vapor is, of course, the evaporation rate.
When the vapor cloud is at or near ambient temperature, the cloud will be
neutral density and will behave as a tracer gas with the wind. However, if the cloud
is quite dense compared to the air -due either to its being very cold or from the
formation of aerosol droplets, which cause the cloud to be dense- the cloud will
tend to move down gradient. In some cases, this down gradient movement will be
against the wind. Either type of problem may occur, and will depend on the type
of material being spilled, the conditions of storage, and the local topography and
weather conditions.
The processes involve the release of chemicals, flash evaporation of superheated
liquids, formation and growth of a liquid pool, transfer of heat to the pool by solar
radiation, either cooling or heating by radiative exchange with the sky, either
cooling or heating of the pool by convection with the ground or soil, convective
heating or cooling by the air movement across the pool, and cooling by evaporation
of the liquid.
The various heat transfer processes are coupled by an overall energy balance
and the mass transfer process. The mass balance for the system is then solved
simultaneouslywith the energy balance. Heat transfer and mass transfer models
for the various processes are generally available in the literature.
Emission rate models for such processes are considered by Hanna, Guidelines
forthe Use of Vapor Cloud Dispersion Models, published by the AIChE/CCPS. An
emissions from spills model by Wu and Schroy is available from the Chemical
Manufacturers Association.
The student will no doubt note that the process, taken as a whole, may be quite
complicated. However, it is instructive to consider certain aspects of such a
problem and find solutions based on some s i m p l i n g assumptions and on some
realistic zssumed conditions.
122
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
Except in rare instances, a pool of liquid that might remain after the initial period
of active discharge from a leak or spill has terminated will be at a temperature less
than its boiling point. An estimation of the actual temperature will normally require
a rigorous solution of the complete coupled energy and mass balance equations.
Nevertheless, many times it is observed that a pool of more volatile material will
frequently exist below the ambient temperature. Indeed, we often find pools of
water to be cooler than the surrounding air, even if there is a considerable influx
of solar radiation. A still more volatile material, such as a typical organic solvent,
might exist at a temperature well below the ambient.
PROBLEM: Given that a pool of benzene is contained within a diked reservoir
following a spill from a storage tank. The diameter of the pool is large compared
to its depth, which is 6 in. (0.15 m). At a time when the soil surface temperature is
60°F (15.6"C), and the pool temperature is 46°F (7A°C), estimate the rate at which
the pool is being heated (the heat flux) from contact with the ground.
Note that this is a very much simplified model. In reality, the soil temperature
will be changing with time, as will the pool temperature. A much more realistic
model element for soil temperature variation is that of a semi-infmite solid.
The temperature of the pool of liquid should be considered constant throughout,
and the convective heat transfer model appropriate to this case is that of natural
convection.
Problem No. 62
CHEMICAL ENGINEERING TOPIC: Heat Transfer
HEALTH AND SAFETY CONCEPT: Vapor Releases
BACKGROUND: We are all aware that various instances of equipment failure, or
accidents, especially transportation accidents, frequently result in the release of
volatile chemicals to the surroundiigs such that the chemicals evaporate, resulting
in a release of vapor to the atmosphere. If the chemical is one that evaporates at
an appreciable rate, the result will be a vapor cloud that spreads either downwind
or down gradient (downhill) of the spill site. There are a number of models
available to predict the downwind concentration of the chemical if the emission
rate is available. For an evaporating pool of chemical, the rate at which it emits
vapor is, of course, the evaporation rate.
When the vapor cloud is at or near ambient temperature, the cloud will be
neutral density and will behave as a tracer gas with the wind. However, if the cloud
is quite dense compared to the air- due either to its being very cold or from the
formation of aerosol droplets, which cause the cloud to be dense -the cloud will
tend to move down gradient. In some cases, this down gradient movement will be
against the wind. Either type of problem may occur, and will depend on the type
of material being spilled, the conditions of storage, and the local topography and
weather conditions.
The processes involve the release of chemicals, flash evaporation of superheated
liquids, formation and growth of a liquid pool, transfer of heat to the pool by solar
radiation, either cooling or heating by radiative exchange with the sky, either
cooling or heating of the pool by convection with the ground or soil, convective
heating or coolingby the air movement across the pool, and cooling by evaporation
of the liquid.
The various heat transfer processes are coupled by an overall energy balance
and the mass transfer process. The mass balance for the system is then solved
simultaneously with the energy balance. Heat transfer and mass transfer models
for the various processes are generally available in the literature.
Emission rate models for such processes are considered by Hanna, Guidelines
forthe Use of Vapor Cloud Dispersion Models, published by the AIChEICCPS. An
emissions from spills model by Wu and Schroy is available from the Chemical
Manufacturers Association.
The student will no doubt note that the process, taken as a whole, may be quite
complicated. However, it is instructive to consider certain aspects of such a
problem and find solutions based on some simplifying assumptions and on some
realistic assumed conditions.
124
SAFETY,HEALTH, AND LOSS PREWENI'IONIN CHEMICALPROCESSES
Except in rare instances, a pool of liquid that might remain after the initial period
of active discharge from a leak or spill has terminated will be at a temperature less
than its boiling point. An estimation of the actual temperature will normally require
a rigorous solution of the complete coupled energy and mass balance equations.
Nevertheless, many times it is observed that a pool of more volatile material wvill
frequently exist below the ambient temperature. Indeed, we often find pools of
water to be cooler than the surrounding air, even if there is a considerable influx
of solar radiation. A still more volatile material, such as a typical organic solvent,
might exist at a temperature well below the ambient. In the following problem
however, we consider a time period during a release when the liquid is at, or very
near, its normal boiling point, having been cooled by flash evaporation upon
exposure to the atmosphere.
PROBLEM: Ethylene oxide is a widely used industrial intermediate, being used
for the manufacture of ethylene glycol (antifreeze) and a number of other commercially important products. It is also used as a disinfecting fumigant. Its normal
boiling point is 10.7"C. If a discharge of ethylene oxide were to occur on a rather
warm day when the ambient temperature is about 30°C, it would probably remain
at about its boiling point during the course of the discharge, and would change
temperature slowly for a short time thereafter.
Ethylene oxide is a relatively toxic material, as one might expect from the fact
that it is used as a disinfectant. OSHA has established a permissible exposure limit
(PEL) of 50 ppm as a time weighted average. It has a lower flammability limit of
3% in air, and is subject to detonation decomposition so that its upper explosive
limit is 100%.
Assume that a spill of ethylene oxide is occurring from a ruptured containmernt
vessel. As it is in contact with the soil inside a retaining dike, it boils at 1atm. The
soilis initially at a temperature of 32°C. It maybe assumed that the soil temperature
is initially constant, but, of course it will change with time as the surface is cooled
by the boiling liquid. Determine the time required for the soil surface temperature
under the pool to drop to within OS°C of the boiling point of the liquid. You may
assume the temperature of the pool and the heat transfer coefficient remain
constant during this time.
The soil properties may be assumed to be as given below:
Thermal Conductivi : 0.52 W/m K
Density: 2050 kgfm
Heat Capacity: 1840 J/kg K
7
The heat transfer coefficients for boiling heat transfer are usually fairly high,
although it is not possible to predict them with a high degree of confidence. For
the purposes of this problem, you may assume that the coefficient is XKIO w/m2 K.
Problem No. 63
CHEMICAL ENGINEERING TOPIC: Heat Transfer
HEALTH AND SAFETY CONCEPT: Vapor Releases
.
BACKGROUND: We are all aware that various instances of equipment failure, or
accidents, especially transportation accidents, frequently result in the release of
volatile chemicals to the surroundings such that the chemicals evaporate, resulting
in a release of vapor to the atmosphere. If the chemical is one that evaporates at
an appreciable rate, the result will be a vapor cloud that spreads either downwind
or down gradient (downbill) of the spill site. There are a number of models
available to predict the downwind concentration of the chemical if the emission
rate is available. For an evaporating pool of chemical, the rate at which it emits
vapor is, of course, the evaporation rate.
When the vapor cloud is at or near ambient temperature, the cloud will be
neutral density and will behave as a tracer gas with the wind. However, if the cloud
is quite dense compared to the air -due either to its being very cold or from the
formation of aerosol droplets, which cause the cloud to be dense - the cloud will
tend to move down gradient. In some cases, this down gradient movement will be
against the wind. Either type of problem may occur, and will depend on the type
of material being spilled, the conditions of storage, and the local topography and
weather conditions.
The processes involve the release of chemicals, flash evaporation of superheated
liquids, formation and growth of a liquid pool, transfer of heat to the pool by solar
radiation, either cooling or heating by radiative exchange with the sky, either
cooling or heating of the pool by convection with the ground or soil, convective
heating or cooling by the air movement across the pool, and cooling by evaporation
of the liquid.
The various heat transfer processes are coupled by an overall energy balance
and the mass transfer process. The mass balance for the system is then solved
simultaneouslywith the energy balance. Heat transfer and mass transfer models
for the various processes are generally available in the literature.
Emission rate models for such processes are considered by Hanna, Guidelines
for the Use of Vapor Cloud Dispersion Models, published by the AIChE/CCPS. An
emissions from spills model by Wu and Schroy is available from the Chemical
Manufacturers Association.
The student will no doubt note that the process, taken as a whole, may be quite
complicated. However, it is instructive to consider certain aspects of such a
problem and find solutions based on some simplifying assumptions and on some
realistic assumed conditions.
126
SAFEI"Y, HEiALTH,AND LOSS PREVENTIONIN CHEMICALPROCESSES
Except in rare instances, a pool of liquid that might remain after the initial period
of active discharge from a leak or spill has terminated will be at a temperature less
than its boiling point. An estimation of the actualtemperature willnormally require
a rigorous solution of the complete coupled energy and mass balance equations.
Nevertheless, many times it is observed that a pool sf more volatile material will
frequently exist below the ambient temperature. Indeed, we often frnd pools of
water to be cooler than the surrounding air, even if there is a considerable influx
of solar radiation. A still more volatile material, such as a typical organic solvent,
might exist at a temperature well below the ambient. In the following problem
however, we consider a time period during a release when the liquid is at, or very
near, its normal boiling point, having been cooled by flash evaporation upon
exposure to the atmosphere. Since the pool is seen here as being in contact with
relatively warmer soil, we will assume that it continues to boil during the time
period under consideration in the problem.
PROBLEM: Monomethyl m i n e is an important intermediate in the manufacture
of some pharmaceuticals, certain pesticides, surfactants, rubber chemicals, and a
photographic developer. It is a strong irritant as well as a significant fire hazard.
The lower flammable limit is 4.95% in air. The Federal Standard for industrial
exposure (OSHA limit) is 10 ppm, time-weighted average for an 8-hr day. Upon
heating, it will evolve even more toxic nitrogen oxides.
If a pool of methylamine forms from a discharge as a result of an accident, it will
cause the moisture in the soil below the pool to freeze, because its boiling point is
-6°C. We wish to determine what time would be required for the soil to freeze to
a depth of 5 mm. If the soil under a pool of spilled liquid freezes, it will prevent the
penetration of the liquid into the soil, thus preventing its further contamination of
soil or, perhaps, of the groundwater in the region. However, the ultimate release
of vapor to the air will be greater by the amount that might have otherwise gone
into the soil.
You may neglect the heat of fusion of the moisture in the soil, although doing so
does introduce an error if the soil were to have, for example, a moisture content
of about 15% water, which would not be untypical.
You may assume that the soil is initially at a uniform temperature of 23°C.
The properties of a typical soil are given below. Although you are to use these
properties in the solution of this problem, you should understand that such
properties vary over a wide range of values for various soils.
Soil properties:
Thermal Conductivity: 0.55 W/m K
Heat Capacity 1100 J/kg K
Density 2100 kglm3
PROBLEM NO.63
127
Although you are to understand that boiling heat transfer coefficientscannot be
predicted with high confidence because of the complexity of the process, you are
to assume that the coefficient in this instance is quite high so that the surface
temperature of the soil is approximately constant at the boiling point of the
methylamine.
Problem No. 64
CHEMICAL ENGINEERING TOPIC: Heat Transfer
HEALTH AND SAFETY CONCEPT: Vapor Releases
BACKGROUND: We have all read of spill accidents due to equipment failure, or
accidents, especially transportation accidents, which result in the release of volatile
chemicals to the surroundings. When the chemicals evaporate, the result is a
release of vapor to the atmosphere. If the chemical is one that evaporates at an
appreciable rate, the result maybe a flammable or toxicvapor cloud, which spreads
either downwind or down gradient (downhill) of the spill site. Models are available
to predict the downwind concentration of the chemical if the emission rate can be
estimated. For an evaporating pool of chemical, the rate at which it emitsvapor is,
of course, the evaporation rate.
When the vapor cloud is at or near ambient temperature, the cloud will be
neutral density and will behave as a tracer gas with the wind. However, if the cloud
is quite dense compared to the air- due either to its being very cold or from the
formationof aerosol droplets which cause the cloud to be denseOthecloud will tend
to move down gradient. In some cases, this down gradient movement will be against
the wind. Either type of problem may occur, and will depend on the type of material
being spilled, the conditions of storage, and the local topography and weather
conditions.
The processes involve the release of chemicals, flash evaporation of superheated
liquids, formation and growth of a liquid pool, transfer of heat to the pool by solar
radiation, either cooling or heating by radiative exchange with the sky, either
cooling or heating of the pool by convection with the ground or soil, convective
heating or coolingby the air movement across the pool, and cooling byevaporation
of the liquid.
The various heat transfer processes are coupled by an overall energy balance
and the mass transfer process. The mass balance for the system is then solved
simultaneouslywith the energy balance. Heat transfer and mass transfer models
for the various processes are generally available in the literature.
Emission rate models for such processes are considered by Hanna, Guidelines
forthe Use of Vapor Cloud Dispersion Models, published by the AIChEICCPS. An
emissions from spills model by Wu and Schroy is available from the Chemical
Manufacturers Association.
The student will no doubt note that the process, taken as a whole, may be quite
complicated. However, it is instructive to consider certain aspects of such a
problem and find solutions based on some simplifying assumptions, and on some
realistic assumed conditions.
PROBLEM NO.64
129
In many instances, a pool of liquid that might remain after the initial period of
active discharge from a leak or spill has terminated will be at a temperature less
than its boiling point. An estimation of the actual temperature will normally require
a rigorous solution of the complete coupled energy and mass balance equations.
Common observations,however, suggest to us that a pool of evaporating liquid will
often be at a temperature below the ambient. Indeed, we often find pools of water
to be cooler than the surrounding air, even if there is a considerableinflux of solar
radiation. A still more volatile material, such as a typical organic solvent, might
exist at a temperature well below the ambient.
If the liquid boiling point is well below the temperature of the surfaceupon which
it is resting, it is possible that active boiling will take place for some time, until the
surfaceis sufficientlycool to reduce the heat input rate from that source. Arigorous
solution to this problem is complicated by the fact that the boiling coefficient will
vary with the temperature of the soil which we would presume to be changing with
time. Although the semi-infiite solid model is useful for a number of common
boundary conditions, the variable coefficient condition makes a rigorous solution
difficult.
In the present instance, we wish to consider an initial condition where it would
be likely that active boilingwill be taking place, and we will attempt to estimate the
magnitude of the boiling heat transfer coefficient between the surface on which
the liquid rests and the boiling pool.
PROBLEM: Ammonia has a number of important industrial uses as an intermediate in the manufacture of fertilizers, dyes, plastics and fibers, as well as a
number of other products. It is also widely used as a fertilizer in the anhydrous
form. The OSHA limit for exposure is 50 ppm, time-weighted average over an 8-hr
day. The American Conference of Governmental Industrial Hygienists (ACGIH)
has set the level that is immediately dangerous to life and health (IDLH) at 500
PPmConsider that a distributor for anhydrous ammonia has a small storage facility
that is filled from time to time from tank trucks. A storage tank at the facility is
surrounded by a concrete pad with a dike that is high enough so that the contents
of the tank could be retained witbin the diked enclosure. The diked enclosure is
25 ft by 30 ft and is high enough to retain the contents of the storage tank.
Now a tank truck, somewhat carelessly operated, has just backed into the tank
and has not only ruptured the tank, but has also damaged the delivery valve on the
truck. As a consequence, liquid ammonia has nearly filled the diked area and is
rapidly boiliug, even though it is not an especially hot day.
Estimate the release rate of ammonia vapor due to the heat input from the flat
concrete surface, when the surface temperature of the slab is at -lS°C.
You may have some difficulty with this problem due to the need for data that
are hard to find, if they exist at all. The main problem here is in estimating the
boiling heat transfer coefficient. If you use a model such as the Rohsenow equation,
130
SAFETY, HEALTH, AND LOSS PREVENTION IN CHEMICAL PROCESSES
you may have to guess at some of the surface-and fluid-specificparameters. As an
approximation, you could consider that the fluid-specific parameters might be
about the same as water. The concrete surface is quite rough, so you might liken it
to a roughened surface of some other material. If you use an equation such as the
McNelly equation, you do not need some of these data, but the results are probably
a little less reliable.
Problem No. 65
CHEMICAL ENGINEERING TOPIC: Mass Transfer; Fixed Bed Adsorption
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: Although respirators, which are devices that are worn over the
face to prevent inhaling harmful material, are the "last line of defense" against
exposure to airborne contaminants, they are nevertheless very important safety
equipment items, and it is vital that they be used appropriately, in recognition of
their limitations.
Chemical cartridge respirators provide protection against vapors and gases
being inhaled. One type of device uses an adsorbent, such as charcoal, to adsorb
organic vapors and thus to purify the air that the wearer inhales. The bed of
charcoal will remove essentially all of the contaminant until breakthrough occurs,
after which the concentration will rise very rapidly, and substantial exposure can
result in a short time. When there is a significant chance that a breakthrough might
occur, it would be preferable to use a supplied air respirator, where clean air is
delivered to the worker, rather than having a device that purifies the air.
The analysis of the performance of an &purifying respirator can be done by
the methods of analysis of any ftved bed adsorber. In this problem we will see what
effect the concentration of contaminant has on the service life of an adsorption
canister.
The generalized correlation of adsorption potential shows that the logarithm of
amount adsorbed is linear with the function (TlV)[log (fs 1f )] over a useful range
of values. In the above function, T is the temperature (K); Vis the molar volume
of liquid at the normal boiling point (cm31g mole); fs is the saturated fugacity
(approximate as vapor pressure); and f is the fugacity of the vapor (approximate
as partial pressure).
PROBLEM: For a particular charcoal, we have data as follows for
dichloropropane (DCP) adsorption capacity:
Amount Adsorbed
[cm3 (1iq)llOO g]
1.0
10.0
(TfV)[loglo (fs If )I
(units above)
21
11
132
SAFETY,HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
A respirator canister contains 75 g of this type of carbon, and tests have shown
that it will allow breakthrough when 82% of the adsorbent is saturated. (Unused
length is 18% of the total length of the packed section.)
Regulations permit charcoal canister, full-face-mask respirators at DCP concentrations up to 750 ppm. If a worker were using this respirator in a DCP
concentration of 750 ppm when the temperature is 80°F, how long might it be
before there is a breakthrough? Assume that the worker breathes at a rate of 45
L/&.
If, due to an accident, a worker is caught in a DCP concentration of 2000 ppm,
how long might he have before breakthrough?
Note: A simple adsorbent canister respirator is not adequate for the 2000 ppm
level, which has been established as the level that is immediately dangerous to
life and health (IDHL). One would choose to use such a device only in an
emergency when nothing better were available to assist in escaping the exposure. For such cases, a self-containedbreathing apparatus (SCBA) would be
preferred. The SCBA is a device much like the SCUBA device worn by divers.
What would be the consequences if the respirator had been used a few days
earlier and the cartridge had not been changed?
Suppose the respirator were to be used in an atmosphere that contained
hydrogen cyanide (HCN) at 750 ppm? HCN will not be so readily adsorbed by the
carbon cartridge. Assume the adsorption capacity to be only about 5% of that for
DCP and recompute the time a worker would have in such an atmosphere.
The IDHL for HCN is 50 ppm. It is extremely toxic in acute exposure, and death
can result from breathing concentrationsthat are not detectable by human olfactory senses. That is, by the time the victim smells the gas, it is probably too late to
avoid death. For this reason, the air-purifyingrespirator is not suitable for use in
the HCN-containing atmospheres, and either supplied air or SCBA respirators are
used for protection against HCN.
Problem No. 66
CHEMICAL ENGINEERING TOPIC: Mass Transfer, Diffusion Through Solids
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: Chemical protective clothing (CPC) is used as a backup to
engineering controls to protect workers from exposure to toxic materials that may
be encountered. In most cases engineering controls, which are controls that keep
harmful agents out of the workplace environment,make the workplace safewithout
other protection. However, there are inevitably instances in which some protective
clothing is needed, including work at hazardous waste disposal sites.
Typically, CPC is made of some kind of polymer membrane, but all such
materials are permeable to some extent, and selection is often made on the basis
of the permeabiity of a particular polymer membrane to particular chemicals. The
permeability of a polymer depends upon the diffusivity of the chemical in the
polymer matrix and upon the solubility of the chemical in the polymer. The
solubility of the chemical in the polymer is the upper limit of the concentration in
the polymer, so that the solubility becomes the driving force for permeation
through the membrane.
Other properties of importance include the tensile strength, since it is important
that the clothing material resist tearing, and ease with which seams can be made
to resist leakage. Of course, it is also important that the material allow for comfort
and freedom of movement. The selection of appropriate protective materials and
clothing is crucial since there is no backup in the event of failure.
PROBLEM: Methylene chloride is a common ingredient of paint removers, so it
sometimes comes in contact with the skin. Besides being an irritant, it also may be
absorbed through the skin where it may add with the larger potential exposure frcm
inhalation.
Although there is little information available on the dermal dosage that could
have immediate adverse effects, let us assume that we might deduce such a lower
dose limit from airborne exposure limits. If we do this, we find that a dosage of
about 1or 2 g in 2 hr would be a maximum that should be allowed. Obviously, if
one is using this material as a paint remover, and if hand operations are required,
protective gloves should be worn. Consider the following situation:
Butyl rubber gloves, 0.04 cm thick, are being worn by an individualworking with
methylene chloride paint stripper. How long in continuous contact would be
134
SAFETY,HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
required before one received a dosage of 1 g due to permeation through the glove
material. The data needed for this problem are presented below.
Solubility in butyl rubber: 25% by weight (solute-free)
Diffusivity of solute in butyl rubber: 1.95 x 10-10 m21s
Approximate surface area exposed b two hands: 0.08 m2
Density of butyl rubber: 1200 kg/m
Y
Problem No. 67
CHEMICAL ENGINEERING TOPIC: Mass Transfer; Diffusion Through Solids
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: Chemical protective clothing (CPC) is used as a backup to
engineering controls to protect workers from exposure to toxic materials that may
be encountered. In most cases, engineering controls, which are controls that keep
harmful agents out of the workplace environment,make the workplace safewithout
other protection. However, there are inevitably instances in which some protective
clothing is needed, including work at hazardous waste disposal sites.
Typically, CPC is made of some kind of polymer membrane, but all such
materials are permeable to some extent, and selection is often made on the basis
of the permeability of a particular polymer membrane to particular chemicals. The
permeability of a polymer depends on the diffusivity of the chemicalin the polymer
matrix and on the solubility of the chemical in the polymer. The solubility of the
chemical in the polymer is the upper limit of the concentration in the polymer, so
that the solubilitybecomes the driving force for permeation through the membrane.
Other properties of importance include the tensile strength, tear resistance, and
the ease with which seams can be made to resist leakage. It is also important that
the material allow for comfort and freedom of movement. The selection of appropriate protective materials and clothing is crucial since there is no backup in
the event of failure.
PROBLEM: In one type of standard permeability test, the material being tested is
clamped between two chambers, one holding the liquid chemical, the other being
continuously swept with an inert gas. The permeation rate is determined by the
change in concentration of the test chemical in the gas side.
One such test* with tetrachloroethylenepermeating polyethylene showed a rate
of 769.87 mg/cm2min, whereas the rate through a sample of Teflon was 2.3 mg/cm2
min. If the solubility of tetrachloroethylenein polyethylene is 13% (by weight) and
0.02% in Teflon, estimate the diffusivity of tetrachloroethylene in these two
polymers. The membrane thickness in each case was 0.01 cm. Report your results
in m2/sec. The density of Teflon may be taken as 2200 kg/m3 and that of polyethylene as 930 kg/m3.
*Thedata are from Guidelinesfor ChemicalProtective Clothing, 3rd ed., ACGIH,
Cincinnati (Feb 1987).
Problem No. 68
CHEMICAL ENGINEERING TOPIC: Mass Transfer
SAFETY AND HEALTH CONCEPT: Properties of Materials
BACKGROUND: The mention of the term PCB, which is an acronym for
polychlorinated biphenyl, creates an image of extreme hazard in the minds of many
persons. Actually, however, PCB does not present much hazard to humans in
incidental contact.
Many chlorinated organic compounds display excellent properties for certain
uses, and PCB was just such a case. It was widely used as a dielectric fluid in
transformers and capacitorsbefore its manufacture and use were discontinuedand
ultimately banned. It was also used as a heat transfer fluid in high-temperature
service. Most of its uses followed from its exceptional chemical stability and
inertness.
Its harmfulness derivesfrom its tendency to "bioconcentrate." It isvery sparingly
soluble in water and more dense than water, so that a quantity spilled in a waterway
will sink to the bottom and remain to dissolve continuously over the course of many
years. However, marine animals (fish, etc.) accumulate the chemicalin their tissues
at much higher concentration than in the surrounding water because when it is
absorbed by the organism it does not readily metabolize. This property is shared
by a number of inert, high-molecular-weight, sparingly soluble organic chemicals.
Bioaccumulation seems to be a frequent property among chlorinated compounds.
In the case of PCB, the concentration in some fish organs will be several
thousands times the concentration in the water in which the fish resides. At these
levels, a variety of physiological harmful effects may come to the fish. Predators
that feed on the fish will also accumulate the chemical. Such has been the case of
some birds of prey, particularly osprey and eagles.
Transportation accidentshave sometimescaused contamination of water bodies
with bioaccumulating chemicals.
PROBLEM: Consider the following hypothetical accident: 1400 lb of PCB has
just been spilled in a river because a rail car hauling a transformer in for replacement of the dielectric fluid was derailed while crossing a bridge. It appears that
the chemical has collected in large pools on the bottom, over a combined area of
about 150 ft2.
PROBLEM NO.68
137
Estimate the rate at which the chemical will be released into the water, and how
long it would require for 1%of the chemical to be dissolved. The following data
may be assumed:
Solubility of PCB in water: 0.25 mg/L
Mass transfer coefficient, liquid pool to water: 0.5 Ib mole/ft2 hr
PCB is a general term for a number of compounds with similar properties, and
these were normally used as mixtures. Assume the average molecular weight in this
case is 260.
Problem No. 69
CHEMICAL ENGINEERING TOPIC: Fundamentals; Mass Transfer
HEALTH AND SAFETY CONCEPT: Vapor Releases
BACKGROUND: No matter how carefully workers do their jobs, there remains
the possibility of accidents. The more planning and preparation that has gone into
accident anticipation and contingency planning, the better the chance of avoiding
complications, injury, or property damage if and when an accident occurs. In
planning for possible accidents, one of the more likely occurrences might be a
release of a large quantity of toxic or flammable vapors or gases. Methods are
available for estimating the resulting concentrationsfrom such releases; and from
such estimates it is sometimes possible to predict what areas of a plant or of the
area surrounding a plant might have to be evacuated.
As a useful guide in the matter of what concentration of a chemical is safe for
persons to breathe, many people refer to the "Threshold Limit Values" (TLV) as
published by the American Conference of Governmental Industrial Hygienists
(ACGIH). The TLV is considered to be a safe upper limit for persons working in
an industrial environment. However, for persons off site, the limit usually is
considered to be much less for several reasons, among them are the fact that the
affected population may be much more susceptible to injury. For example, young
children, the very old, and expectant mothers are usually considered to be more
susceptible to injury from chemical exposure. Therefore, a frequently used guide
is to limit their exposure to no more than 1%of the TLV. These considerations
are taken into account when making emergency preparedness plans for the community surroundinga chemical processing plant, and estimates of off-site exposure
must be made for accidents that might be anticipated.
Some of the computational methods used for such emission sources as power
plant stacks can also be applied to the estimation of the results of accidental
releases. Such computations estimate the effect of dilution as a plume leaves the
source location. A derivation of any of the methods is beyond the scope of this
problem statement.
One of the simpler models to predict dispersion is called the "Gaussian Plume
Model" and expresses the average concentration at a location downwind of a
continuous source. A Gaussian plume model can be modified to express downwind
concentrations of instantaneous releases as well. Such a model is sometimes
referred to as a "puff' model. The cloud of gas from a puff release will travel
downwind, dispersing somewhat as it travels. Thus, at downwind locations the
PROBLEM NO. 69
139
concentration becomes less as a function of the distance travelled and the degree
of turbulence in the mixing process. An equation for a puff model is given below.
C =Q / [ ( ~ ~ ~ ) ~ ~ C J ~ U ~ ]
where
C = the concentration at a selected point downwind, rng/m3 (time-weighted
average)
s = Diffusion coefficient in thex direction (downwind), m
cry = Diffusion coefficient in they direction (cross-wind), m
a, = Diffusion coefficient in thez direction (vertical), m
Q = Source strength (mass of gas or vapor making up the cloud), mg
This equation is valid for windblown puffs across fairly level ground and is based
on an assumptions of how the components of the puffs would be dispersed. It
predicts the maximum concentration within the cloud, that is, in the center. The
diffusion coefficientsdepend upon the stabilityof the atmosphere and the distance
downwind from the source. The diffusion coefficients are subject to a number of
uncertainties, and disagreements from one author to another. For the present
purposes, the values of axand uymaybe taken as equal, but the value of the vertical
dispersion coefficient (a,) is different. These values may be estimated from the
following abbreviated listing.
Unstable atmosphere (good mixing):
u - 0.14?.~~; uZ = 0 . 5 2 2 . ~ ~
- 0.k0.92. , a, = o.I~xO.~O
Neutral atmosphere (Little mixing):
uy =
Stable atmosphere (mixing suppressed): ay = 0.02-~~.*~;
uZ = 0.05~'.~'
In the above equations, x is the downwind distance from the release in meters,
and the a values are the dispersion coefficients, in meters.
PROBLEM: Emergency plans are being formulated so that rapid action can be
taken in the event of an accident. It is predicted that if aparticular accident occurs,
1.0 kg of chlorine will be instantaneously released. There is a residential area 500
m away from the prospective release location. For a situation when there is a wind
of 2 d s , blowing toward the residential area, estimate the time required for the
gas cloud to arrive at the residences and the maximum concentration that would
occur in the center of the cloud. How does this concentration compare with 1
percent of the TLV? The TLV for chlorine gas is 1ppm (a molar ratio).
Determine the worst case situation, assuming the different stabilities presented
above. Which case should we plan for?
(This problem is based on a problem in the text, Chemical Process Safety:
Fundamentals with Applications, by D.A Crow1 and J.F Louvar, published by
Prentice Hall, Englewood Cliffs, NJ.)
Problem No. 70
CHEMICAL ENGINEERING TOPIC: Mass Transfer; Design
HEALTH AND SAFETY CONCEPT: Pressure Relief Systems
BACKGROUND: There are a number of instances when pressure relief systems
must be utilized to help ensure safe operation of chemical processing equipment
by providing for venting of contents of equipment in the event the pressure exceeds
safe limits. Several things come to mind immediately, such as pressure vessels,
reactors, heaters, and storage tanks. Generally, if it is possible that a piece of
equipment can become overpressured, then some device must be provided to vent
the excessive pressure.
Not so obvious, perhaps, but nevertheless important, is the need to prevent
underpressure, or unwanted vacuum. Generally, tanks, process vessels, and equipment items that are constructed to withstand only modest internal pressure, will
collapse, perhaps with catastrophic consequences, if the pressure is suddenly
reduced to below atmospheric. Devices to relieve under-pressure are generally
called "vacuum breakers," but the principle is essentially the same, that is, venting
is provided to prevent pressure swings of such magnitude that failure will result.
An interesting situation that requires avacuum breaker can arise in a distillation
column. Unless the column is constructed to withstand a considerable internal
pressure, it will be subject to possible collapse in the event that a vacuum is created
within it, since,in most cases, cylindrical vessels will withstand much higher internal
pressures than external pressures. If the steam to the reboiler should suddenly be
interrupted, while the condenser continues to operate, and especially if a subcooled feed continues to enter, a serious drop in pressure can result that might
cause a collapse of the column due to the creation of a vacuum within the column.
There are two general types of devices to relieve higher than desired internal
pressures. The first is a device called a "pressure relief valve," which is so designed
that it will open when a set pressure is reacbed. If the contents of the equipment
are such that additional hazard would follow from a discharge to the atmosphereand this is the usual case- then a piping system of some kind must be provided to
move the vented material to a safe disposal location, a flare, or to a containment
vessel. Sometimes, vent lines are to absorbers, where the venting material can be
absorbed to prevent its loss to the atmosphere.
A second relief device is called a "rupture disk," which is, as its name suggests,
a disk that is designed to burst when a dangerously high pressure is reached, thus
preventing the entire equipment bursting in an uncontrolled release and/or explosion. Of course, the same need arises to prevent undesirable discharges to the
PROBLEM NO. 70
141
atmosphere. Rupture disks may be placed within a vent line in order to facilitate
the containment of the vented material.
Distillation columnswere mentioned above in connection withvacuum breakers.
However, there is perhaps a greater possibility for overpressure in a distillation
column. In the event of loss of coolant to the condenser, a situation can develop
where the reboiler continues to operate and the pressure will rise rapidly. Also, in
the case of a cold liquid feed to a distillation column, the feed may account for a
considerableportion of the cooling that occurs in the equipment. Thus, loss of feed
can also cause overpressure in such a case. A particularly serious problem might
arise when there is loss of coolant and the operator shuts off the feed while
attempting to bring the process back to control. If the feed is a subcooled liquid,
then the pressure rise may be very rapid, indeed.
A situation similar to the loss of coolant mentioned in the paragraph above is
considered in the following problem.
PROBLEM: A distillation column is separating a feed mixture of ethanol and
water. The feed enters as a saturated liquid at an ethanol mole fraction of 0.035.
The feed rate is 50,000 lblhr, and the overhead composition is 0.83 mole fraction
ethanol. Assume that the bottoms is about 0.99 mole fraction water.
The column normally operates at 1atm as a nominal pressure. However, if the
cooling to the condenser is lost, how long will it take for the pressure to rise to 1.5
atm? If the pressure must not rise above 1.5 atm, what venting rate in pounds per
second will be required?
Note that there are a number of missing data items in the problem statement.
These should be supplied by you if they are not supplied by your instructor. You
are to make suitable engineeringjudgments regarding the height and diameter of
the column, and such other items as might be required.
Actually, a rigorous solution to this problem is quite difficult, but some simplifying assumptions may be made. Your instructor may suggest some simplifications,
you may wish to make some yourself. You might assume that there are no heat
lossesfrom the column, which seems quite reasonable. You will need to make some
kind of realistic assumption about the rate of vapor generation that follows the
upset. You could assume that the rate of vapor generation remains the same, and
you could assume that the column temperature remains approximately the same,
even though the pressure increases. Both of these would be what we would call
"conservative" because the real rate of pressure rise would be less than estimated
by using those assumptions. If you do not wish to assume the vaporization rate is
the same, then you will have to estimate what effect the increased pressure (and
consequently the increased boiling temperature) would have on the rate of
vaporization. You can do this, of course, but you will need to know a typical, or
probable value for the heat transfer coefficient in the reboiler. Then as the pressure
increases, the boiling rate will decrease due to the decreased temperature driving
force in the reboiler.
Problem No. 71
CHEMICAL ENGINEERING TOPIC: Mass Transfer
HEALTH AND SAFETY CONCEPT: Vapor Releases
BACKGROUND: Anyone who watches newscasts on television or reads a daily
newspaper is aware of frequent transportation accidents that cause chemicals to
be spilled into the environment. Very often the chemical is one that wiil vaporize
at an appreciable rate, which then causes a vapor cloud to spread downwind of the
spill site.
There are a number of models available to predict the downwind concentration
of the chemicalif the emission rate is available. The better models take into account
the vapor cloud densityand certain aspectsof the terrain. If the cloud is more dense
than the air, it may move down gradient under the influence of gravity, and this
could be against the wind in some cases. For an evaporating pool of chemical, the
rate at which it emits vapor is, of course, the evaporation rate.
To a fairly good approximation, it is possible to predict evaporation rates for
some cases from heat transfer and mass-transfer principles. The process involves
coupled, simultaneous heat and mass transfer, in which the temperature of the
pool is a function of the rate of heating by solar radiation, convective heating (or
possibly cooling) from the soil, and convective heating or cooling by the air
movement across the pool.
The heat input is then related by an energy balance over the surface of the pool
to the heat of vaporization of the chemical that evaporates.
Finally, a simultaneous solution for the surface temperature of the pool, based
on the premise that the heat gain from the surroundings is equal to the loss from
evaporative cooling, will result in a pool surface temperature from which a mass
transfer rate can be evaluated.
Emission rate models for such processes are considered by Hanna, Guidelines
for the Use of Vapor Cloud Dispersion Models, published by the AIChEICCPS. An
emissions from spills model by Wu and Schroy is available from the Chemical
Manufacturers Association.
The student will no doubt note that the process, taken as a whole, may be quite
complicated. However, it is instructive to consider certain aspects of such a
problem and find solutions based on some simplifying assumptions, and on some
realistic assumed conditions.
Our common observations tell us that if a spilled chemical is fairly volatile, as is
the case for a number of organic solvents, the surface temperature of an evaporating pool might be well below the ambient temperature. Thus, we would anticipate
PROBLEM NO.71
143
that a benzene spill, as suggested in the following problem would be at a temperature below the ambient.
PROBLEM: A tank truck hauling benzene has overturned, and a large pool of
benzene has formed on the ground. The terrain is fairly flat, and the benzene has
spread into a pool that is approximately 20 m in diameter. The wind is blowing
across the pool at a velocity of 7 mls.
It is a clear, warm day, with midafternoon temperatures at about 30°C. Because
the soil is still wet from the overnight rain, it is probable that not much of the
benzene will soak into the ground. This is no doubt fortunate, since it is less likely,
therefore, that the benzene will contaminate the groundwater.
Benzene is considered to be a carcinogen, and worker exposure is to be limited
to no more than 1ppm, as a time-weighted average by OSHA regulations. It is
therefore of considerableconcern in the present instance, because some populated
areas lie downwind of this spill. Generally, it is considered that the general
population should not be exposed to the OSHA limit, because the members of the
general population may not be as resistant as workers in good health.
We wish to estimate the rate of evaporation. We will guess that the temperature
of the benzene pool will be about 18°C.
To work this problem, use a flat plate, turbulent boundary layer model for heat
transfer and then estimate the mass transfer coefficient from the Chilton-Colburn
analogy.
Your instructor may elect to provide some of the property data for your use with
this problem.
Problem No. 72
CHEMICAL ENGINEERING TOPIC: Mass Transfer; Design
SAFETY AND HEALTH CONCEPT: Storage, Handling, and Transport
BACKGROUND: Part of any chemical manufacturing operation is to provide an
emergency operations plan that details the procedures to be followed if hazardous
materials are released from confinement. Two of the major chemical properties
that must be accounted for are the toxicity of the chemicals and their flammability.
Some materials are both toxic and flammable, but the concentrationsthat are toxic
when the chemical is released to the atmosphere are usually much lower than the
flammable concentrations. Storage of hazardous materials should be in the minimum quantities required for the process and the transport and storage facilities
should be designed to prevent any leaks. The transport and storage systems should
include spill detection systems and methods to shut down transfer piping if any
leak is detected.
If the chemical is a liquid, one of the first considerations is to determine the rate
at which the chemical will evaporate and enter the atmosphere. Seepage into the
soil and drainage into streams are also potential problems; they are usually more
long-term considerations. If toxic or flammable material enters the atmosphere, it
will be carried outside the plant in a short time. Toxic or flammable materials may
be dangerous to either the workers in the plant or to the neighbors if it enters the
atmosphere.
Consider the case where a hazardous liquid is spilled. For it to be an immediate
threat it must be vaporized into the atmosphere. Its evaporation rate must be
known in order to estimate the potential danger to workers or the public. It is
unusual to be able to specify exactly how fast a liquid will evaporate if it is spilled
because the physical layout of the plant, the size of the pool formed by the leaking
liquid, and the atmospheric conditions cannot be forecast exactly. Most data for
mass transfer have been taken in laboratoriesunder carefully controlled conditions
that do not exactly simulate those found outdoors. However, nontoxic materials
might be used in tests on the plant site. Those test results could then be used to
predict the evaporation rates of toxic or flammable materials. Straightforward
mass transfer correlationsmight be used for the purpose. For example, evaporation
from an open pool could be correlated on the basis of the Sherwood number, the
Reynolds number, and the Schmidt number. (The Sherwood number is frequently
PROBLEM NO. 72
called the Nusselt number for mass transfer.) They are defined as follows:
Re = Lvph
Sh = k c L / D a
and
Sc = p l p D a
where
L = dimension of the pool in the direction parallel to the wind
v = wind velocity
p = air density
p = air viscosity
DAB = didfusivity of vapor in the air
kc = mass transfer coefficient.
Any system of units may be used, but all the units must be consistent so the Re, Sc,
and Sh numbers are dimensionless. A correlation frequently used for determining
mass trmsfer coefficients for calculating evaporation rates from spilled liquids is
but it is based on laboratorydata where the air flow is smooth and the leading edge
of the liquid pool is at the same elevation as the surroundings. In the plant, the
wind will be gusty, and the topography may change the evaporation rates. The
evaporation rates may also depend on the elevation of the liquid below the top of
the impounding space in which it is collected. Data taken at a plant site might be
correlated in the same form as shown above. If such a correlation is attempted, it
is unlikely that the Schmidt number will vary as much as the Reynolds number or
the Sherwood number. In addition, the exponent on the Schmidt number is about
0.33 in a number of correlations, and would be expected to be 0.33 for a first
approximation. Thus, the primary coefficients to be evaluated are the constant
preceding the Reynolds number and the exponent for the Reynolds number.
PROBLEM: Acrolein is to be used as an intermediate material in a process. It is
both flammable and toxic. The acrolein will be stored in tanks, and the possibility
of a leak from the tanks is being considered as part of the emergency operations
planning for the plant. In the event of a large spill from a storage tank, a drainage
systemwillconvey the spilled acrolein to a holding trench. The trench will hold the
full contents of the tank. It will be 100 ft long and 5 ft wide at the surface. Several
tests have been run using water as the evaporating fluid in the trench to determine
what evaporation rates might be expected in the event of a spill. The trench was
filled with water and the rate at which the liquid surface receded was measured.
During the measurements, the pool temperature was recorded, and the tests were
run when the wind direction was across the short dimension of the pool and along
the long dimension of the pool. The air temperature and humidity were also
measured.
The table on the next page includes a summary of the data obtained.
146
SAFETY,HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
Wind speed
(mfir>
Temperatures, O F
Air
Dew Water
L
(ft)
Rate
(lb/hr-ft2)
As the new engineer at the plant, you are asked to correlate the data so that the
correlation can be used to estimate the evaporation rate of acrolein if a spill occurs
at the plant. You have specificallybeen asked to estimate the acrolein evaporation
rate for a wind speed of 5 mph both across the trench and along the trench. The
information is needed for determining the concentrations of acrolein that might
be found downwind of a spill. A separate atmospheric dispersion model will be
used for that purpose. You may assume the air temperature to be 90°F and the
liquid acrolein temperature to be 70°F at the time of the spill.
Problem No. 73
CHEMICAL ENGINEER11gG TOPIC: Mass Transfer
SAFETYAND HEALTH CONCEPT: Hazardous Waste Generation and Disposal
BACKGROUND: Most chemical manufacturing operations involve the generation, use, and disposal of substances that are hazardous because of their toxicity
or flammability.In many cases, the hazardous substance is a chemical that is widely
used and not considered dangerous. For example, common salt is used in many
processes without causing any particular problems with toxicity. However, if salt
is contained in water that is discharged to streams, it may cause irreversible damage
to the environment, including both plant and animal life. Thus, while salt is not a
particularly hazardous material to handle and to use in processing, it must be kept
out of the water that is discharged from a plant. Other substances may be much
more toxic, of course, and they must be kept from the environment as well.
The nature of the substancesand their effects on the environment are sometimes
known quite well, but sometimes it is only known that they have a toxic effect.
Sometimes the toxiceffects are inferred for one material through comparison with
another similar material. For example, if we know that high concentrations of
sodium chloride are toxic to certain plants in the ecosystem,we can infer that high
concentrations of potassium chloride will also be toxic to the same plants. (Of
course, we must keep in mind that the same substances may be required for life.
Sodium and potassium salts are necessary for proper cell growth and reproduction;
and potassium deficiencies in particular are encountered in nature. We frequently
add potassium salts in the form of fertilizers to enhance plant growth.)
We cannot add large quantities of most substances to the ecosystem without
damage. Thus, the kind and amount of materials present in water streams discharged from a plant must be closelymonitored to make certain that the discharged
water will be within the quality standards specified by the Environmental Protection Agency. The standards for the purity of discharged water depend on the
substances in the water and the potential damage to the environment. Specific
concentrations can be obtained from the Environmental Protection Agency.
The specific method used to remove hazardous substances from water depends
on the nature of the hazardous substance and the concentrationsinvolved. Several
different methods can be used. The following problem illustratesone method that
might be used.
PROBLEM: The water used in a chemical processing plant contains a mixture of
organic acids. The acids are not particularly toxic to the environment in small
concentrations, but the concentrationin the process water stream is 0.5% by weight
148
SAFETY, HEALTH, AND LOSS PREVENTION IN CHEMICAL PROCESSES
leaving the process, and that concentration is too large for discharge. The acids
cannot be used in the plant for any other purpose, and there is insufficient acid for
their recovery, purification, and sale. The acids are soluble in hydrocarbons, and
it is decided to extract the acids from the water streams by countercurrent
liquid-liquid extraction, then use the hydrocarbon as a fuel for one of the plant
process heaters. There is no chlorine, nitrogen, or sulfur in the acids, so when they
are burned with the fuel, they will not contribute any additional pollution to the
air. The hydrocarbon used for the extraction is the fuel oil for the process heaters.
Equilibrium data for the acid-water-oil system are given in the table below. Plot
the data on a triangular diagram. Determine how much oil will be required to
reduce the concentration of acid from the 0.5 mass percent in the feed to 0.05 mass
percent, which has been found to be acceptable for discharge. The oil rate used in
the process will be 1.5 times the minimum, and the water to be treated will enter
the extraction system at a rate of 3500 gallonsper day. The oil has a specificgravity
of 0.88. Assume the process is to be performed in a countercurrent liquid-liquid
extractor having an overall efficiency of 20%. How many stages will be required
for the extractor?
Mass percent in water layer
Acid Water Oil
Mass percent in oil layer
Acid Water Oil
Problem No. 74
CHEMICAL ENGINEERING TOPIC: Mass Transfer
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control
BACKGROUND: Some chemicals such as benzene, formaldehyde, and asbestos
have been implicated as carcinogins. Their use in commerce has been more strictly
controlled since their carcinogenic potential was discovered.
As materials are found to be hazardous, precautionsmust be taken to keep them
from entering the workplace and the surroundingneighborhoods. Several processes have been devised to remove hazardous materials from the atmosphere and
water. The procedure best suited to a given use might depend on the rate at which
the hazardous substance must be removed from a process. In the following
problem, you are asked to estimate the rate of discharge for a pollutant that must
diffuse to the surface of a porous wall before it can be removed.
PROBLEM: A refrigerated food warehouse is insulated with a layer of urea-formaldehyde foam 18 in. thick. There is some residual formaldehyde in the foam,
and it has a concentrationof 0.04 lb/ft3of foam when the material is manufactured.
Formaldehyde can diffuse into the air where workers will be exposed to the vapor.
The outside of the foam layer is covered with an impermeable layer to prevent
moisture migration, so the formaldehyde can only diffuse in one direction, which
is into the warehouse. Ventilation for removal of the formaldehyde is not practical
because the refrigeration system cannot handle the cooling load imposed by
bringing in fresh, warm air. There is a system available that will remove 0.1 lb/hr
of formaldehyde from the air during recirculation through the refrigeration system.
It will return the air to the warehouse with essentially no formaldehyde, and you
can assume the concentration of formaldehyde to be zero in the warehouse as long
as the removal system is not overloaded. The warehouse has 24,000 ft2of insulated
surface area and the effective diffusivity of formaldehyde through the foam is
1.5(10-~)cm2/sec at the average temperature in the system. How long will it be
before the formaldehyde removal system will be able to keep the air free of
formaldehyde? The warehouse must be freelyventilated until that time, so it cannot
be used for cold storage. How much formaldehyde must the system be able to
remove if the warehouse is to be used within 1month after the manufacture of the
foam?
Problem No. 75
CHEMICAL ENGINEERING TOPIC: Mass Transfer
SAFETYAND HEALTH CONCEPT: Hazardous Waste Generation and Disposal
BACKGROUND:Many chemical operations utilize hazardous materialswhich are
processed to form products useful to society. Sometimes the hazardous material
is a reactant or an intermediate in the process, and sometimes it is an unwanted
byproduct of a reaction. Hazardous materials are also used as solvents or carriers.
Regardless of the reason, if a hazardous material appears in an operation, it must
be removed from any process stream that is discharged to the environment.
Workers and people who live nearby must also be protected from the hazardous
effects.
The Occupational Safety and Health Administration (OSHA) has promulgated
standards for exposure to many of the substances used in various chemical operations. The Permissible Exposure Limit (PEL) has been defined as the concentration of a hazardous material in the air that can be tolerated on an 8-hrday, 40-hr
week basis during the worker's lifetime. The PEL is based on a time-weighted
average. OSHA has also established ceiling concentrations that cannot be exceeded at any time. These ceiling values are higher than the PEL, and represent a
more dangerous level for exposed people.
Another level, called the IDLH level (Immediately Dangerous to Life or
Health), has been established for many substances as well. It represents the level
of exposure from which a person can escape within 30 min without experiencing
escape-impairingor irreversible health effects. IDLH levels are not specified for
potential human carcinogens.
If toxic substances are used in chemical processing plants, care must be taken
to assure that both workers and the environment are not exposed to adverse effects.
If toxic wastes are generated, they must be removed from waste streams before the
waste is discarded. Wastes maybe either gases, liquids, or solids, and the methods
of treating or disposing of the waste depends not only on the phase in which the
waste exists, but also on the waste itself. Generally, if the waste is a gas or volatile
liquid, any air used in processing must be purified before it can be discharged to
the atmosphere. The problem that follows indicates one way in which the purification might be accomplished.
PROBLEM: Benzene is present in an air stream in a concentration of 0.05 mole
percent. The concentration is to be reduced to 10 ppm (by volume) by absorbing
the benzene in a light oil. The air stream will be fed to the absorption column at
1.6 m3/sec and the system operates at 25OC and 1.0 atm. The superficial velocity of
PROBLEM NO. 75
151
the air stream in the column is not to exceed 0.9 mlsec. The tower packing has a
gas phase mass transfer coefficient (FG) of 2.4(10-~)kmoles/m2-sec and a liquid
phase mass transfer coefficient (FL) of %(lo4) kmoles/m2-sec. The packing area
is 49.2 m2/m3.For the dilute solutions and low pressures involved in the process,
you may assume that Raoult's law applies. The average molecular weight of the oil
is 240, and your column design should be based on an oil rate twice the minimum
required rate. Determine the tower diameter and the depth of packing to be used.
Problem No. 76
CHEMICAL ENGINEERING TOPIC: Mass Transfer
SAFETY AND HEALTH CONCEPT. Process Control
BACKGROUND: Process control is an important part of chemical processing.
With proper control, the quality of product is improved, and production rates will
be more constant. Without proper control, reactions may proceed too fast and
damage equipment. Workers may be injured or killed by contact with toxic
substancesor through explosions and fues. Proper process control requires instruments to measure and control the process variables. The instruments must be
designed for the property being measured. Usually, thermocouples or resistance
temperature detectors will be used for temperature measurements, and pressure
transducers and switches will be used for pressure measurements. Their design
and location need to be considered carefully to assure they will operate properly
throughout the life of the plant.
It is frequently necessary to measure the composition of material in processing
systems. Many different instruments and techniques have been devised to measure
system composition. Some measure composition changes quickly and accurately,
and others require more time for a careful analysis. Composition measurements
may be based on periodic samples or on continuous measurements; but whichever
method is selected, it must be chosen with the idea in mind that the measurement
will be accurate enough for the intended purpose and that the response will be
quick enough for adjustments to be made in control parameters. Composition
measurements are frequently more difficult to make and are more unreliable than
either temperature or pressure measurements.
Composition measurements are based on a number of physical and chemical
properties of the material being measured. One method widely used for the
measurement of concentrationsof flammable gases and vapors in the atmosphere
is to detect the temperature change on the surface of a small bead coated with a
catalyst. The flammable gas or vapor diffuses through a layer of porous material
until it reaches the catalyst surface, where it immediately reacts with oxygen from
the air. The oxidation reaction generates heat, raising the temperature of the bead.
The bead is part of an electrical bridge circuit, and the bridge becomes unbalanced
when the bead is heated. The resulting voltage change is amplified and measured.
The followingproblem is a simplified analysis of some of the design considerations
for such a gas detector.
PROBLEM: An instrument is to be designed to measure the concentration of
methane in air. Methane has a lower flammable limit concentration of 5 mole
PROBLEM NO. 76
153
percent in air, and the instrument is to be designed for a known response at steady
state when the methane concentration is 25% of the lower flammable limit (the
LFL for methane is 5.0 mole percent). Methane and oxygen diffuse through a thin
porous coating to the catalyst surface, where they react immediately, producing
carbon dioxide and water. It has been found that methane must diffuse to the
catalyst surface at a rate of 0.1 g,/hr-cm2to produce the desired heating rate at the
catalyst surface. The porous coating for the catalyst surface has an effective
diffusivity 36.5% of the molecular diffusivity. The surface temperature of the
catalyst reaches 175°C at steady state and the ambient temperature is 25°C. How
thick should the porous coating be to provide the correct diffusion rate? The
reaction at the surface is
Problem No. 77
CHEMICAL ENGINEERING TOPIC: Mass Transfer
SAFETY AND HEALTH CONCEPT. Process Design
BACKGROUND: When hazardous materials are required for chemical processing, they must be very carefully controlled to keep them from entering the environment. Hazardous materials always have one of two general properties: they are
flammable or toxic. Frequently, a hazardous material is both flammable and toxic,
and the toxic concentrationsare usually much lower than the flammable concentrations. Toxicity may further be divided into two other general groups, one including
materials that are acutely toxic and whose effects show up within a short time of
exposure. Other materials have more insidious effects; their damage to health
becomes known only after long exposure. A single material may exhibit both acute
and chronic toxicity, depending on concentrations and effects.
It is sometimes very hard to show that a material exhibits long-term health
effects. Two good examples are the artificial sweeteners saccharin and cyclamates.
Saccharin is generally accepted to be carcinogenic; however, it is still used in a
variety of foods and cosmetics because there is no completely acceptable alternative. Some years ago, cyclamates were widely used as artificial sweeteners; however, some evidence seemed to show that cyclamates were carcinogens, and they
were banned for use in the United States. Data released in the spring of 1989
indicate that cyclamates are not the carcinogens they were once thought to be, and
they may again be allowed to be used for artificial sweeteners.
It is usually much easier to determine a material's flammability characteristics
because no living organism needs to be tested to find the properties. For example,
the lower flammable limit (LFL) is the lowest concentration at which a vapor or
gas will ignite in air. Although the LFL depends on the system temperature and to
some lesser extent pressure, the most obvious need is for data at ordinary ambient
conditions.Thus, definingLFLs at 1atm and room temperature provides sufficient
data to use for estimating danger of ignition. Such properties as the amount of
energy released during combustion are also easily measured. An ignition temperature is more difficult to define and measure. For many materials, an ignition
temperature measured as the temperature at which a stoichiometric mixture of
flammable vapor and air begins a self-sustaining exothermic reaction has been
determined. It is called the autoignition temperature (AIT) and serves as an
approximate limit for determiningwhen ignition will occur.
In order for a fire or deflagration to occur, there must be a fuel, an oxidizer, and
an ignition source such as a spark or a temperature high enough for ignition. The
ignition temperature is low enough for some materials that they will ignite at
PROBLEM NO. 77
155
ambient temperature or below when mixed. Such materials are said to be hypergolic. Even materials that ignite only at temperatures several hundred degrees
above ambient can be ignited by small amounts of energy, frequently less than 1
mJ. Most vapor-air mixtures can be ignited by the spark from a static discharge,
for example. Because of the ease of ignition and the rapid combustion reaction
that ensues, substantial rules and standards have been written to defme conditions
for use of flammable materials. The standards of the National Fire Protection
Association are available in most libraries. They form the basis for many laws
governing use of flammable materials.
Whether a material is toxic or flammable or both, it must be kept well confined
if it is to be used in commerce. The basic process design must account for such
materials and assure they are either used during the processes or destroyed so they
will not harm workers or the environment. Where it is possible, recovery and
recycling of chemicals is the best way to keep them under control. If a material is
recycled, it will never be discharged to become a potential pollutant.
PROBLEM: Benzene is used in a process as a solvent for a solid product, and it is
dried from the solid at the end of the process. Since benzene is quite flammable
(its LFL is 1.3%) and toxic (its permissible exposure limit is 10 ppm), nitrogen is
recycled as a carrier gas during drying. Neither the nitrogen nor the benzene is
ever to be released from the process. In order to recycle both the benzene (as a
liquid solvent) and the nitrogen (as a carrier in the drying process), the benzene
in the nitrogen is stripped out in a tray absorber. The benzene enteringthe absorber
is at a concentration of 7.4 mole percent in nitrogen. It must be reduced to a
concentration of 0.4mole percent in nitrogen, after which the nitrogen stream will
be heated and recycled to dry the product. The benzene will be absorbed in an oil
having a molecular weight of 200. The oil enters the absorber at a rate of 0.5 moles
of oil per mole of pure nitrogen entering the absorber. Raoult's law can be assumed
to apply, and the absorber is designed to operate at 50°C (because the nitrogenbenzene stream entering is hot) and 1.0 atm. The vapor pressure of the oil is
negligible, and the nitrogen can be assumed to be insoluble in the oil. Determine
the mole fraction of benzene in the liquid leaving the absorption tower and the
number of ideal trays required for the process.
Problem No. 78
CHEMICAL ENGINEERING TOPIC: Kinetics; First-Order Reaction
SAFETY AND HEALTH CONCEPT: Toxicology and Industrial Hygiene
BACKGROUND: Toxicology is the study of the effects of toxic materials on
organisms, including humans. It is an area of knowledge that is becoming increasingly important to chemical engineers. One of the most important tenets of the
toxicologist was stated in the sixteenth century by a physician-alchemist named
Phillipus Aureolus Theophrastus Bombastus von Hoenheim (who came to be
known as Paracelsus) when he was defending his use of mercury to treat a disease
with the statement, "What is it that is not poison? AU things are poison and none
are without poison. Only the dose determines that a thing is not poison." Various
translations later, the last statement is, "The dose alone makes a poison."
So it is with toxicology. All things are toxic, but whether there is harm in exposure
depends on the dosage. We are accustomed to thinking of materials as being
u t ~ x i or
~ 7"nontoxic,"
7
and perhaps the difference is in whether or not one is likely
to receive a toxic dose in his or her ordinary conduct in the presence of the material.
In any event, we distinguish between "toxic" and "hazardous," because the two are
not synonymous.
When a potentially toxic material is taken into one's body, there will be natural
processes (((elimination,"such as exhaling the material as a vapor, or by passage
through the kidneys, or "metabolism," which changes the chemical through reaction) that will tend to rid the body of the material. It is only when the defenses are
overwhelmed that toxic effects wiII be seen.
PROBLEM: "Chronic exposure" means continued exposure on a day-to-daybasis
as might be the case for a worker exposed all the time while he is working. In the
case of a particular hypothetical chemical agent, it has been determined that a
human can tolerate a level of 10 mg of the agent per kilogram of body weight, that
the metabolism is a first-order process, and the half-life for the agent in a human
is 3 hr.
If it is assumed that all the agent that is inhaled is adsorbed by the body, what
continuous concentration could a person with a body mass of 68 kg tolerate in the
workroom air? Assume that the average breathing rate is 45 Llmin.
Note: The student should be aware that this is a hypothetical case and it would
be unusual to find a simple model such as this to be adequate. In reality, metabolic
rates and processes are not well known, and defining a safe dosage must frequently
be done in other ways.
Problem No. 79
CHEMICAL ENGINEERING TOPIC: Kinetics
SAFETY AND HEALTH CONCEPT. Explosions: Runaway ReactionTemperature
BACKGROUND: Many common chemical reactions used in industry are exothermic, that is, they produce energy as the reaction proceeds. In most cases, the
reactions are run in reasonably small reactors, and the amount of energy that is
producedis small enough that the reaction canbe kept under control without much
difficulty. However, in some reactions, particularly those run in large reactors, the
rate at which energy is produced can become much larger than the rate at which
the energy can be removed from the reaction vessel. Since the rate of reaction
increases as the temperature increases, the increasing temperature is reinforced.
The reaction rate increases very quickly, sometimes so quickly that the reaction
gets out of control and the reactor is damaged or destroyed if it is not adequately
vented or protected. In extreme cases, an explosion may occur. The rate at which
energy is produced in a reaction can be written as
where
Qg = rate of heat generation, caVs
AHr = heat of reaction, cal/g-mole
ko = frequency factor, sec-I
C = concentration of reactant, g-mole/cm3
V = reactor volume, cm3
E = activation energy, cal/g-mole
R = gas law constant, cal/g-mole K
T = absolute temperature, K
If the reaction is exothermic, heat must be removed from the reactor to keep the
temperature from increasing out of control. The heat transfer rate can be written as
where
Qr = rate of heat removal, caVsec
U = overall heat transfer coefficient, cal/cm2 sec K
A = heat transfer area, cm2
To = coolant temperature, K
158
SAFETY,HEALTH, AND LOSS PREVENTIONIN CHEMICALPROCESSES
If heat can be removed as fast as it is generated by the reaction, the reaction can
be kept under control. Under steady state operating conditions, the heat transfer
rate will equal the energygeneration rate. However, if the heat removal rate is less
than the heat generation rate, a condition that might occur because of failure of a
cooling water pump, for example, the temperature in the reactor will begin to rise.
The net rate of heating of the reactor contents is the difference between Qg and
Qr, or
Qn = Qg
- Qr
If the net heating rate, Qn, is positive, the reaction will have an increasing
temperature. If the rate of increase of Qn is positive, that is, if dQJdT > 0, the
reaction will have the potential to accelerate and become uncontrollable. The rise
in temperature will increase both the heat transfer rate and the reaction rate, but
the heat transfer rate is a linear function of temperature, and the reaction rate is
an exponential functionof temperature. If there is sufficient reactant in the reactor,
the temperature will increase until the resulting pressure causes the reactor to fail.
PROBLEM
(a) Show that the critical temperature above which heat is generated faster than
it can be removed is given by the solution of the equation
where Tc is the temperature at which heat is produced faster than it can be
removed.
(b) Assume that a reaction occurs in a continuously stirred tank reactor with the
inlet and outlet flows controlled at steady and equal rates. Although the reaction
occurs in the liquid phase, the heat of reaction is several times aslarge as the energy
required to heat the reacting mixture to its boiling point and vaporize it. The
reactant is pure as it enters the reactor, and the reaction goes essentially to
completion. The heat exchanger can transfer all the heat from the reactor at a
reactant feed rate that is 25% greater than the nominal or design rate. After a few
years of operation, it is decided to increase the production rate of the product. The
operators know that the product rate can be increased by increasing the reactant
flow rate. In fact, on several occasions they have run the reactor at rates up to 20%
more than the design rate and have experienced no problems. They also know that
the reactor temperature is only a few degrees higher than the cooling water
temperature, so they assume that the reactant can be fed at much higher rates,
thereby increasing the production rates. The engineering team that designed the
reactor is no longer at the plant, and the plant manager is new. He has had no
experience with chemical reactions. The pump for the reactor is replaced with a
PROBLEM NO.79
159
larger pump, and the product pump is also replaced to match. The cooling water
pump is also replacedwith a larger model so that the coolingwater rate is consistent
with the reactant and product rates. The heat exchanger in the reactor is not
changed, so the heat removal rate can be increased onlyby increasing the temperature in the reactor. The reaction has an activation energy of 30,OOO cal/g-mole and
the coolingwater temperature is 20°C.What is the maximum temperature at which
the reaction can operate without having the reaction run away?
(c)
What actions might be taken to assure that the reactor will operate safely?
Problem No. 80
CHEMICAL ENGINEERING TOPIC: Kinetics: First-Order Kinetics
SAFETYAND HEALTH CONCEIYT:Toxicology and Industrial Hygiene: Biological Elimination of Toxic Chemical
BACKGROUND: Many chemicals have a toxic effect on humans. Some of them
have acute toxicity, in which case the effect of a given concentration or exposure
on the body can be sudden. Some have more long-term effects and take years to
cause disease or death. Acommon example of a chemical that has immediate effect
on the body is ethyl alcohol. In small amounts, ethyl alcohol does not harm humans.
However, if the amount of alcohol ingested becomes large, the effects become
noticeable. At first, the effects are primarily a slowing of physical reactions and a
gradual loss of mental capacity. If the amount sf alcohol ingested over a short
period is large, unconsciousness usually occurs. However, if the alcohol is ingested
very rapidly, and in large quantities, it can kill. The effects of alcohol ingestion are
usually reversible within a few hours or days, unless the amount ingested is very
large. Some death of brain cells and some damage to the liver may result from large
quantities ingested over a long period.
Tobacco smoke appears to have minor short-term effects once the body learns
to tolerate the smoke. However, the long-term effects can be severe: smoking
tobacco has been implicated as the a major factor in causing lung cancer in many
countries. It has also been implicated as a factor in heart disease and lung diseases.
The symptoms may take many years to appear.
These are only two examples of materials that might cause immediate or
long-term health effects. In fact, if either tobacco or alcohol were being introduced
into society under today's regulations, it is likely that both would be banned. It is
certain that the short-term impairment of mental and physical abilities of alcohol
would not be tolerated in a chemical plant (and, in fact, drinking on the job is
forbidden). If a chemical is found in the workplace that has the long-term effects
caused by smoking tobacco, it would be strictly regulated or forbidden. Many of
the present rules limiting hazardous chemical exposures are the result of exposures
that cause fewer health effects than tobacco. The chemical engineer must realize
that rules will likely be in effect for most of the materials he or she may encounter
in the workplace.
The elimination of toxic chemicals from the human body is a complicated
procedure that may require the action of several organs. Mathematical modeling
of the detoxification process is rarely simple or straightforward. However, in some
cases simple models can help our understanding of the elimination process. One
such simple model is to consider the rate at which the chemical is eliminated from
PROBLEM NO. 80
161
the body to depend on the concentration in the blood. If that model is assumed to
represent the detoxification process, then
dcldt = -kc
where c is the concentration of toxic chemical in moles per liter, mole fraction, or
other concentration units; t is the time in hours; and k is the "reaction rate
constant," in hr-'
The rate of metabolism of the toxic chemical is thus modeled as a first-order
chemical reaction. Of course, the real elimination process is much more complex
than a first-order rate equation implies, but the equation does offer some insight
into modeling of the elimination process.
PROBLEM: At a party held to celebrate the (successful) completion of a course
in kinetics, a student imbibes too much spirits and becomes inebriated. His blood
alcohol concentration is 0.21%. The student knows he shouldn't drive home until
his blood alcohol concentrationdrops to 0.02%. If his blood alcohol drops to 0.17%
in a half hour and he stops drinking at midnight, at what time can he go home? You
may assume that the elimination of alcohol is a first-order process.
Problem No. 81
CHEMICAL ENGINEERING TOPIC: Kinetics; Heat Transfer
SAFETY AND HEALTH CONCEPT: Process Design
BACKGROUND: The problems involved in designing chemical reactors always
require attention to both the design of the reactor itself and the reaction that is to
be run. All reactions are either exothermic or endothermic, although sometimes
the amount of energy used or released by the reaction is relatively small and is
easily handled. In addition, the reaction rate is sensitive to the temperature of the
system and an exothermic reaction can get out of control if it generates energy
faster than the energy can be removed from the system. In practice, that means
that the kinetics of the reaction must always be taken into account in determining
the heat transfer rates and that the heat transfer rate must be accounted for in
studying the reaction kinetics. In the laboratory, it is frequently quite easy to
maintain a constant temperature during study of a reaction, but in commercial
production, the larger quantities of materials make constant temperature more
difficult to attain. The heat transfer from a reacting system will depend on the
system geometry primarily as an area function, while the reaction rate (and thus
the energy generation rate) will be primarily a function of volume.
If a reaction is to be run, one of the first choices to be made is whether it is to be
a batch process or a continuous process. Continuous processes are preferred if the
quantity of product is large. There are sometimes advantagesin continuous process
reactors in ease of control, as well. However, some processes cannot easily be made
to operate properly under continuous conditions, and batch or semi-batch reactions are usedinstead. Careful control must be used in either case to avoid potential
failure of the reactor. One of the most important things to avoid is a runaway
reaction, in which the energy generated by the reaction is greater than the energy
that can be removed from the reactor. Exothermic reactions may reach a temperature that limits the equilibrium conversion, of course, but that characteristic cannot
be relied on for reaction control if the equilibrium temperature is too high for the
system. For example, in a liquid phase reaction, as the temperature increases, the
vapor pressure of the liquid increases, which may cause the pressure inside the
reactor to become higher than the reactor design pressure. In a gas phase reaction,
the pressure also increases as the temperature increases, and if the reaction
generates more moles than are in the reactants, the pressure will rise even faster.
The following problem illustrates some of the factors that have to be taken into
account for a batch reactor design. Relief valves or rupture discs will be required
for all reactors to provide added safety in the event of runaway reactions.
PROBLEM NO. 81
163
PROBLEM: A reaction is to be carried out in a stirred batch reactor. The reaction
has the stoichiometry
and the reaction is second order and irreversible at any temperature encountered
in practice. The reaction rate equation is
where
- r =~ reaction rate, g-moles of AIL-sec
CA = concentration of A, g-moles/L
CB = concentration of B, g-moles/L
k = reaction rate constant, Llg-mole-sec
The reaction rate constant is a function of the system temperature and is given by
k = ko exp-
(fi)
where
ko = pre-exponential factor, Llg-mole sec
E = activation energy, cal/g-mole
R = gas law constant, cal/g-mole K
T = absolute temperature, K
The heat of reaction is -90 Wg-mole. The reactor to be used is cylindrical with
a diameter of 5 ft and a height of 11ft. It is well insulated on the top and bottom,
and heat transfer through the top and bottom can be neglected. The reactor is
jacketed, with heat transfer occurring through the jacket. The maximum working
volume of the reactor is 1500 gal, and the jacket extends from the bottom of the
reactor to 1.0 ft from the top. The reactants A and B are mixed in the reactor at
75°F to give a concentration of 5 g-moles/L for each at the start of the reaction
cycle. The reactor is then heated to initiate the reaction. The heat is supplied by
saturated steam at 50 psig. Once the reaction has begun, heat must be removed
from the reactor. Heat is removed by cooling with water circulated through the
reactor jacket. The cooling water has an average temperature of 75°F.The reactor
is well-stirred, and you may assume the liquid in the reactor to be at a uniform
temperature and composition at any time (although both will change with time).
The reactor has a maximum working pressure of SO psig, so the temperature in the
reactor must be kept low enough to keep the vapor pressure of the contents below
50 psig. You may assume the reacting mixture to have the vapor pressure of pure
164
SAFETY,HEALTH, AND LOSS PREVENTION IN CHEMICALPROCESSES
water. For simplicity,you may also assume the specific heat of the reacting mixture
to be constant and e ual to 1.0 caVg-OC.The density of the mixture may be assumed
constant at 1.0 g/cm .Heat transfer coefficientsmay also be assumed constant. For
heating, the overall coefficient is 200 ~ t u / h r - f t ~ -and
~ F for cooling, the overall
coefficient is 100 Btulhr-ft2-OF.The activation energy for the reaction is 30 kcallgmole and the pre-exponential factor is l.0(10'~) Llg-mole-sec. You are to do the
following:
9
1. Write the energy balance and kinetic reaction rate equations describing the
system. These will involve both heat transfer and the reaction rate. The result will
be two nonlinear time-dependent differential equations that can be solved simultaneously to give the temperature and concentration of reactants in the reactor as
a function of time. You may assume the reactor contents are uniform throughout
the volume of the reactor.
2. Solve the equations to determine if the reaction can be run to 99% conversion
of A without exceeding the reactor design pressure. The pressure can be estimated
as the vapor pressure of water at the system temperature. You will have to change
the heat transfer temperature for the jacket when the steam heat is turned off and
the cooling water begins to enter. The heating temperature in the jacket will be
constant because you can use condensing steam; you may assume the average
cooling water temperature will be constant. We use this simplifying assumption to
make the solution easier, although in practice the coolant will heat as it passes
through the reactor. In practice, the increase in temperature of the coolant must
be taken into account.
3. You will have to solve the equationsnumerically. Since they willbe reasonably
simple first-order equations, you may use a technique such as the Euler method or
the RungeKutta method. However, be sure your time increments are short
enough to keep the temperature rise during each increment reasonable. A
reasonable temperature rise for each time increment might be about 1.0 K.
4. Prepare a short table showing how the time for 99% conversion is affected
by the temperature of the reaction at which heating is stopped and cooling is
started. At what temperature would you begin cooling?
Problem No. 82
CHEMICAL ENGINEERING TOPIC: Kinetics
SAFETY AND HEALTH CONCEPT: Rupture Discs and Relief Valves
BACKGROUND: Whenever materials are contained in a process vessel, there is
a possibility the vessel may rupture because of overpressure. Sometimes the vessel
rupture will result in only a small amount of leakage of innocuous material, but
sometimes a violent explosion may occur, with substantial destruction. In either
case, the vessel will have to be repaired or replaced and any damage to the
surroundings will have to be repaired. Workers may be killed or injured by an
explosion, or may suffer health effects if the leak contains toxic materials. In order
to reduce the probability of tank failure, relief systems must be supplied.
A relief system may incorporate either a rupture disc or a relief valve. Both have
similar functions, although there are some important design and operational
differences. A rupture disc is a simple device consisting of a thin disc sandwiched
between two flanges. If the pressure on one side of the disc becomes greater than
the disc can withstand, the disc ruptures, relieving the pressure. A rupture disc can
be made to quite close tolerances, and rupture discs have been shown to be reliable
for venting vessels and piping. One of the drawbacks to using rupture discs is that
once the disc is broken, flow cannot be stopped, and the vessel will have its pressure
reduced to the ambient pressure at the end of the discharge line. The rupture disc
must also be carefully selected to assure it does not deteriorate because of
corrosion and that changing temperature will not cause it to fail at the wrong
pressure,
Relief valves are similar in design to ordinary valves except that rather than
opening because a handle is turned, they have a spring-loaded stem that remains
closed until the pressure in the system gets large enough to overcome the load of
the spring. Then the valve will begin to open. A spring-loaded relief valve will
usuallybe fully open when the system pressure reaches about 10 percent above the
set point pressure. Relief valves have the advantage that they are designed to close
once the pressure decreases below the set point. Thus, if the pressure rises again,
the relief valve can open again to relieve the pressure. This intermittent operation
of the relief valve minimizes the quantity of material discharged from the vessel.
Of course, the discharge from the relief valve, like that from the rupture disc, must
be directed to a safe location. If toxic or flammable materials are involved, they
may have to be routed to a recovery system to eliminate the possibility of further
danger.
The choice of what kind of relief device to use and the location for the relief
device is based partly on the properties of the materials in the system and partly
166
SAFEI"Y, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
on the level of venting needed. The choice should be integrated with the designers
of other parts of the system and should be made in accordance with the standards
of the applicable codes and regulations. (One general consideration is that relief
valves are frequently used for process protection and rupture discs are frequently
used for vessel protection.)
The relief valve design depends on the capacity it must have and on the material
being vented. Recent studies sponsoredthrough the Design Institute for Emergency Relief Systems (DIERS) have made substantial progress in providing techniques for specifying relief valves and rupture discs based on the system properties
and the material being vented. The paper by H. K Fauske, "Emergency Relief
System Design for Reactive and Non-Reactive Systems: Extension of the DIERS
Methodology," contained in the July 1988, issue of Plantloperations Progress
provides a review of some of the results of the DIERS work and lists a number of
more detailed references to the work.
In the following problem, you are to consider how the reaction rate in a gaseous
reaction system changes as the pressure increases. The change in reaction rate can
have a substantial effect on relief system design.
PROBLEM: The data contained in the followingtable were taken in the laboratory
using a constant volume batch reactor initially charged with a pure reactant. Use
the data to determine the volume required for a constant temperature, constant
pressure plug flow reactor operated at 500°K and 10 atm absolute pressure. We
wish to react an input stream containing 2.0 kmolelmin of pure reactant to give
95% conversion. You may assume that the reactant and the product mixture are
ideal gases. Discuss qualitatively how an operating pressure higher than that at
which the data were taken might affect the selection of a relief device for the
reactor, for example, the type and size of relief device used. You may have to review
some of your studies on fluid mechanics to complete the discussion. You do not
have to go into detail, but should pick out one or two key points for discussion and
mention their effects on the flow through a relief system. The reaction being studied
goes essentiallyto completion at 500°K.The reaction data from the laboratory are
as follows:
Time,
Reactor Pressure,
seconds
atm abs
1.0
0
1.1
5
1.2
10
1.3
15
20
1.4
1.5
30
45
1.6
1.7
60
At completion
2.0
Problem No. 83
CHEMICAL ENGINEERING TOPIC: Kinetics
HEALTH AND SAFETY CONCEPT: Toxic Exposure Control and Personal
Protective Equipment
BACKGROUND: It is always required that the air qualitywithin a vessel be known
before anyone is allowed to enter it. The fact that the vessel has not been used for
some time, or was not used in service with a hazardous material, is not a sufficient
reason to enter it without testing the air.
One of the potential hazards of closed vessels is that of an oxygen-deficient
atmosphere. Normal air contains approximately 21% oxygen, by volume (or
moles). OSHA regulations specify respiratory protection (a supply of breathing
air) if the level of oxygen is below 19.5%. Oxygen levels below 16% will cause
dizziness, rapid heartbeat, and possible headache. Slightly lower levels will cause
an inability to move about readily, and an apathetic view of the impending death.
Usually the victim will feel no symptoms, will lose consciousness, and will have no
recollection of the incident if rescued.
Oxygen-deficient atmospheres can occur in any closed vessel, such as the holds
of ships, vats, tanks, silos, or mines. The following problem will provide an
opportunity for you to see how an otherwise harmless appearing situation can lead
to serious consequences.
PROBLEM: The interior of a steel tank was taken out of service for subsequent
repair. Before closing, it was thoroughly steam cleaned, and the level of residual
vapors was found to be acceptably low for entry. It was not immediately entered,
however, because of the press of other maintenance work, so it was closed for a
time to await its repair priority.
The tank dimensions are 27 ft diameter and 35 ft high. You may assume that both
the top and bottom are flat. The material of construction was plain carbon steel
(primarily iron).
It is known that the typical corrosion rate for iron (or plain carbon steel) in very
moist air is about 0.005 in. per year, but is about first order with respect to the
oxygen concentration.Assume that to be the case, and assume that the corrosion
reaction is approximated by the reaction:
Estimate how long it will take to reduce the oxygen content in thevessel to 19.5%
and to 16%.
Problem No. 84
CHEMICAL ENGINEERING TOPIC: Process Control
HEALTH AND SAFETY CONCEPT: Process Control, Interlocks and Alarms
BACKGROUND: A complex chemical process requires the extensive use, of
automatic controls. Process controls are used not only to maintain product quality
and production rates, but are widely used to help maintain a safe process. A loss
of production may be serious, but a loss of life, or an injury or explosion will be far
more serious and generally many times more costly. Thus, when instrumentation
is used to maintain process safety, it is important that the devices be adequately
maintained.
Many times it is possible to provide a process control scheme with some
"fail-safe" attributes, that is, if the controller should fail, the process will be
controlled in a safe manner. For example, if a controller is being used to maintain
coolant flow to the heat exchanger on an exothermic reactor, the control scheme
should be such that in the event of a failure of the controller, the coolant control
valve will remain in the open position so that cooling will not be lost. However, the
open valve does not necessarily mean that the coolant will flow, because whatever
caused the failure might also cause a loss of coolant.
Failure can arise because of a number of reasons, one important one being the
loss of utilities, that is, electric power to an electronic controller or compressed air
in the case of a pneumatic controller. Also, one must prepare for loss of the process
stream that is being controlled, as for example, interruption of coolant supply.
There are other modes of control system failure, and these should be anticipated
and provision made to maintain control when one or more of them occur.
When a control scheme cannot be made fail-safe, and if failure would result in
an accident, then it may be appropriate to use backup, or redundant controls.
Studies of failure rates have been made, and of course the rates vary a great deal.
Generally, it may be expected that failures will occur at a frequency of from 0.1 to
2.5 times per year, dependingupon the device and the application. The seriousness
or severity of any particular failure rate depends upon a number of circumstances.
The seriousness of a particular failure depends upon the nature of the process.
A failure of any element of the control system will cause the failure of the system.
In the problem that followswe will examine the consequences of a failure, or partial
failure, of a control valve.
PROBLEM NO. 84
169
PROBLEM: An exothermic reaction occurs in a 1000-gal reactor. Reactor
&
function relating
temperature controlis by cooling water in thejacket. ~ h transfer
reactor temperature to cooling water flow rate is given by:
The linear control valve delivers 480 gaVmin (GPM) of cooling water at 3 psig
and 0 GPM at 15psig.
The controller is a proportional, integral, derivative (PID) controller, tuned by
the Zeigler-Nichols technique.
After several weeks of operation the valve performance deteriorates, so that the
stem "sticks" in a given position. A dead time of 21sec is introduced into the control
loop by the sticky valve. Valve gain remains at 40 GPMIpsi.
What will happen to the reactor temperature control because of the poor valve
performance?
(This problem was submitted by Dr. Jim L. Turpin, Department of Chemical
Engineering, University of Arkansas.)
Problem No. 85
CHEMICAL ENGINEERING TOPIC: Process Control
HEALTH AND SAFETY CONCEPTS: Process Control, Interlocks and Alarms
BACKGROUND: A complex chemical process requires the extensive use of
automatic controls. Process controls are used not only to maintain product quality
and production rates, but are widely used to help maintain a safe process. A loss
of production may be serious, but a loss of life, or an injury or explosion will be far
more serious and generally many times more costly. Thus, when instrumentation
is used to maintain process safety, it is important that the devices be adequately
maintained and managed.
The potential for control element failure is a significant problem in the maintaining of safe operations. A regular schedule of testing and maintenance is vital
to the continuing satisfactory performance of process control equipment.
The operations personnel must also participate in keeping the instrumentation
operating as intended. The instrumentation must be kept tuned to the possible
changes in the process, and all personnel should be advised of any changes in the
process. The following problem illustrates one of the difficulties that may arise
when there is a process change.
PROBLEM: An exothermic reaction occurs in a 1000-gal reactor in which the
temperature is maintained by cooling water in the jacket. The transfer function
relating reactor temperature to cooling water flow is given by:
Each gallon of cooling water removes 167 Btu. The coolant control valve is a
square root valve which delivers 50 GPM wide-open at a controller output of 15
psig and is fully closed at 3 psig controller output.
The steady-state heat removal rate is 450,000 Btu/hr when the reactor is fully
loaded. The controller is a PID, tuned by the Cohen-Coon procedure at the fully
loaded operating condition.
A few days later the reactor throughput is reduced, with a corresponding
reduction in the heat removal rate to 190,000 Btulhr. The controller was not
re-tuned.
What will happen to the process control and why?
(This problem was submitted by Dr. Jim L. Turpin, Department of Chemical
Engineering, University of Arkansas)
Problem No. 86
CHEMICAL ENGINEERING TOPIC: Laboratory
SAFETY AND HEALTH CONCEPT: Toxic Exposure Control and Personal
Protective Equipment; Toxicology and Industrial Hygiene
BACKGROUND: There are a number of areas in and around educational
laboratorieswhere there is a potential for hazardous conditions to exist. Many such
areas, if they exist, can be identified by what industrial hygienists call a "walkthrough survey," which is simply a visual inspection of the facilities with detailed
attention being paid to potential sources of hazard, such as places where toxic
agents may be emitted into the air or locations where there may be a potential fire
hazard.
An industrial hygienist might also conduct some measurements of air flow rates,
etc., but without such measurements, one still cangain muchinformation by careful
observation.
PROBLEM: You are asked to conduct a walk-throughsurvey of your department's
mass transfer laboratory, or that laboratory where students do most of the mass
transfer experiments. While you are making the survey, be especially alert:
See if you can locate all of the emission points. (Emission points will be such
places as addition points, discharge points, sampling locations, and sealing
points. Sealing points include "static seals" such as covers, flanges, etc., and
"dynamic seals," which are shaft seals as on pump shafts, valve stems, etc.)
Are procedures posted where all can see them?
How does the student know what are the safe procedures?
Are material safety data sheets available for the chemicals in use?
Is there any way for the student to determine what the potential hazards are for
the chemicals in use?
Evaluate the ventilation:
Is there any use of local exhaust ventilation?
Where should local exhaust ventilation be provided?
Where or how should hoods be positioned?
Where does the supply air come from?
Where does the exhaust air go?
How many air changes per hour would it provide?
What might be a reasonable emission rate for any toxic material in your laboratory?
Could there be short circuiting of the ventilation air?
What is the probable static pressure with respect to the offices and classrooms?
Who has the responsibility of keeping your laboratory safe?
If you feel that your laboratory is unsafe, what will you do about it?
Problem No. 87
CHEMICAL ENGINEERING TOPIC: Laboratory
SAFETY AND HEALTH CONCEPT: Fire Protection
BACKGROUND: Almost all chemical engineering laboratories stock flammable
materials for various experiments. The flammable materials should be stored in a
cabiinet according to the requirements of National Fire Protection Association
(NFPA) Standard 30, "Flammable Liquids Code." A flammable liquids storage
cabinet is designed to prevent leakage of flammable materials to the outside, to
protect the stored materials from external fires, and to limit access to the materials
where necessary. The amount of material stored should be limited in quantity as
specified by NFPA 30, and the individualcontainers of flammable material should
be tightly closed when stored in the cabinet to prevent leakage or vaporization.
Each container should be labeled accordingto its contents,and the laboratory data
availableto each user of the materials should include a MaterialsSafetyData Sheet
(MSDS) for each material stored in the cabiinet. If larger quantities of material are
required, they should be stored in separate facilities that meet the requirements
of NFPA 30. Personnel who will be using the chemicals should be familiar with
their flammability properties as well as their chemical properties. (If the chemicals
are also toxic, they should also be handled so that there will be no toxicity danger
to the users.)
Experiments should be run in well-ventilated areas, and potential ignition
sources should be removed before experiments are started. Ventilation systems
shouldbe well designed so that the discharge will not be in a location where ignition
might result. In some cases, recovery systems for preventing the vaporized material
from reaching the environment may be required.
Each flammable material has a characteristic temperature range over which it
can be ignited. The lowest temperature at which a flammable liquid can be ignited
is called the flash point. The flash point temperature is the temperature at which
the concentration of vapor in air above the liquid surface is high enough to be
ignited. Tables of data for flash point temperature can be found in NFPA Manual
325M.
There is a minimum concentration of fuel in air that is required for sustained
combustion. It is called the lower flammable limit (LFL), and is measured at
atmospheric pressure and ambient temperature. The LFL is given in volume
percent of vapor or gas in air. If the fuel concentration increases, it will reach a
concentration high enough that there is insufficient oxygen for ignition to occur.
The concentration above which ignition will not occur is called the upper flammable limit (UFL). At any concentration between the LFL and the UFL, a
PROBLEM NO. 87
173
vapor-air mixture can be ignited. The flammable range may be relatively narrow,
or it may be quite wide, For example, n-heptane has an LFL of 1.05% and a UFL
of 6.7%. Carbon disultide has an LFL of 1.3%and a UFL of 50%. Thus carbon
disulfidewillignite over a much wider range of concentrationsthan will n-heptane.
Flammable liquids may be spilled during use in the laboratory, and if they are
spilled, a fire may occur. If a fire does occur, some method should be available for
controlling or extinguishing it. There are a number of methods available for
laboratory fire control, but the method preferred may depend on the fuel being
burned. For example, most small hydrocarbon fires can be extinguished by a dry
chemical fire extinguisher using sodium bicarbonate or potassium bicarbonate
agents. They may also be extinguished or controlled using foam agents. However,
some other materials, for example, carbon disdtide, are not easily extinguished
with dry chemicals. Foam or water may be used to fight carbon disulfide fires under
some circumstances. Foams that are used to control hydrocarbon fires may not be
suitable for use on alcohols and other water-soluble flammable liquids. Special
alcohol foams are available for such chemicals. The MSDS should list suitable fire
control agents. Everyone who is to use flammablematerials should read the MSDS
and become familiar with the techniques of handling the material as well as
methods of preventing, controlling, and extinguishing fires.
If the vapor-air mixture above a flammable liquid is in equilibrium with the
spilled liquid, there is a range of temperatures for which the fuel-air mixture will
be flammable. The temperature at which the LFL concentration is reached will
correspond approximately to the flash point. If the fuel is in a closed system, the
UFL concentration will correspond to a temperature above which the fuel-air
mixture will not ignite. In most cases, there is negligible solubilityof air in the liquid,
so the liquid can be assumed to be pure, and since the systems are at low pressure,
it can be assumed that the vapor-air mixture is ideal. Thus, Raoult's law can be
used to calculate the concentration of fuel in air at any temperature. The calculation can be used to determine the range of temperatures for which the mixture of
vapor and air in equilibriumwith the liquid is flammable. (A practical consequence
of such a calculation is to show that the gasoline-air mixture in the fuel tank of a
car is usually too rich to ignite inside the tank. One of the special considerations
for burning methanol or methanol-gasoline mixtures is that flammable mixtures
may form at ambient temperatures in the fuel tank vapor space. Flame arrestors
might be required to reduce the risk of ignition and explosion of the fuel tank if
methanol is to be used as a motor vehicle fuel.)
PROBLEM. Flammable liquids are to be stored in a flammable liquids storage
cabinet that meets NFPA 30 standards. The laboratorytemperature may vary from
60 to 80°F.Determine which of the following materials might form flammable
vapor-air mixtures if spilled in the storage cabinet and allowed to reach equi-
174
SAFETY, HEALTH, AND LOSS PREVENTIONIN CHEMICAL PROCESSES
librium. Also suggest the type of fire extinguisher(s) that would be appropriate if
a small spill is ignited in the laboratory. The materials are
n-pentane
ethyl ether
carbon disuKde
acetone
benzene
methyl alcohol
n-octane
isopropyl alcohol
p-xylene
\
Note that the calculations you make are only valid if equilibrium is reached. If
equilibriumis not attained, the concentrationswill be lower than those calculated.
Problem No. 88
CHEMICAL ENGINEERING TOPIC: Design
HEALTH AND SAFETY CONCEPT: Toxicology and Industrial Hygiene
BACKGROUND: When toxic materials are used and/or produced by chemical
processes, it is necessary to assure that the workers are not exposed to the
material(s) to such an extent that they receive a harmful dose. Since the most
frequent route of entry for toxic materials is by inhalation, limiting the extent of
exposure potential often takes the form of limiting the concentration of toxic
material in the air that the workers breathe.
Many of the potentially harmful agents are vapors that may be in the air.
Whenever there are volatile materials used in a process, they are likely to escape
into the air through various leaks, called emission points, which include sealing
points, sample withdrawal points, addition points, and so forth.
Sealing points are those places where the processing equipment components
come together. "Static seals" include such seals as flanges and covers, where there
is no relative motion between the components. "Dynamic seals" are those places
where there is relative motion between components. Equipment with rotating and
reciprocatingshafts that transmit mechanical energy through seals include pumps,
compressors, agitators, and valves. Normally it would be expected that dynamic
seals would cause the more serious leakage problems.
In recognition of the exposure potential from inhalation of toxic vapors, the
Occupational Safety and Health Administration (OSHA) has established maximum concentration limits for a large number of agents. OSHA regulations, which
have the effect of law, specify permissible exposure limits (PELS) for worker
exposure as a time-weighted average (TWA) over a work day. In some instances,
there are also maximum, or "ceiling" concentrations which must never be exceeded, even for a short time. Other agencies and groups make recommendations,
especiallythe National Institute for OccupationalHealth and Safety (NOSH) and
the American Conference of Governmental Industrial Hygienists (ACGIH).
NOSH is a Federal government agency responsible for research and training.
ACGIH is an association of technical persons who work in industrial hygiene and
are employed by a governmental agency.
PROBLEM: Your company has been a major supplier of a fast drying ink, available
in various colors. Your market share has been increasing a bit lately, and one of
your competitors has decided to discontinue ink manufacturing. To meet the
anticipated increased demand and to forestall foreign competition, your management has decided to install a new process line, parallel to and essentially the same
176
S
m
, HEALTH,AND LOSS PREVENTION IN CHEMICALPROCESSES
as the existing line, except with double the capacity. The new process equipment
will include 10 pumps along with miscellaneous other equipment in a room that is
48 ft long, 22 ft wide with 14 ft ceiling height. It is planned that the pumps will be
equipped with packing glands. The pump type selected has been used for several
years in the existing line and has given reliable service. Packing glands, however,
always have a finite leak rate because it is necessary to lubricate the packing. As
the pumps operate, normal wear will cause the leak rate to increase. With appropriate periodic maintenance the leakage can be held to a low level, however.
To assess the leak rate on the existing pumps, the plant environmental engineer
conducted a test as follows: A portable plastic enclosure was placed around the
six existing pumps, and 12Llmin of air was drawn through the enclosurewhile the
pumps were running. This was done until a near steady state was achieved as shown
by a constant reading on a hydrocarbon analyzer placed in the exit air stream. The
exit air stream was then sampled for 30 min by using a small air pump to draw a
sample of the air through a charcoal adsorber tube. The charcoal was assumed to
have adsorbed all the organic vapors from the air that passed through it. The
sampling rate was 100 rnllmin, the air temperature was 74°F and the barometric
pressure was 740 mm Hg. The existing process is in a room with a total volume of
17,000 ft3.
The charcoal tubes were taken to the laboratory, desorbed and the vapors
analyzed. The following amounts were found:
Component
Sample Mass
(mg)
Toluene
1,1,1 Trichloroethane
1,lDichloroethane
Acetone
Methyl ethyl ketone
430
118
133
675
216
TLV- TWA
@pm, molar)
100
350
200
750
200
LFL
(mole %)
1.3
6
6
2.6
2
What is the leak rate for these chemicals in grams per hour for each solvent? If
the building has five air changes per hour and there is good mixing, what will be
the concentration of solvents in the building? Are these below the TLV in each
case? Is the mixture concentration below the TLV?
TLV means "Threshold Limit Value" and is a concentration limit established
by the American Conference of Governmental Industrial Hygienists (ACGIH).
The TLV-TWA represents the air concentration to which most workers may be
exposed for a normal 8-hr day, 40-hr work week, without ill effects.
To determine if a mixture is below the TLV when there are no synergisticeffects,
but where the health effectsare similar and perhaps additive, sum the quotients of
each concentration divided by its respective TLV. If the sum is less than 1then the
mixture is below the TLV.
PROBLEM NO. 88
177
LFL is the "Lower Flammable Limit" and represents the lowest concentration
in air that would be flammable.
Further evaluation of the existing process showed that with the general ventilation rate of five air changes per hour, the level of acetone in the exhaust was 83
mg/m3. This is the building with the six pumps. What proportion of the emissions
do the pumps account for? If the pumps in the new process account for the same
proportion of the leakage, what will the expected level of vapors be in the new room
if it also has a ventilation rate of five air changes per hour?
Do you feel that the assumption of complete mixing of the ventilation air is a
good one? What, if any, importancewould there be to a finding that there was only
very poor mixing of the ventilation air in the room? The book, Industrial Ventilation-A Manual of Recommended Practice, 19th ed. (American Conference of
Governmental Industrial Hygienists, Cincinnati, 1986), suggests that the required
air flow for this type of ventilation be larger than a calculated requirement by a
factor of from 5 to 10 to account for poor mixing.
If there is a problem with possible overexposure in the new facility, what should
be done about it? Would you recommerld changing the design? If so, how? Is there
any possibility that the total emission might cause an air pollution problem off site?
HINTS FOR PROBLEM SOLUTION: Determine the fraction of the vent air that
is sampled, then from the length of the sampling time and the amounts collected,
determine the total emission rate in grams per hour.
The dilution air will remove all the contaminants released, but the concentration
in the room air and the dilution air depends on the release rate and the dilution
air flow rate.
Assume that the pumps in the new building account for the same fraction of the
total emissions and that the ratio of acetone to all the other materials is the same
as in the test in the old building.
Problem No. 89
CHEMICAL ENGINEERING TOPIC: I-Ieat Transfer; Design
SAFETY AND HEALTH CONCEPT: Storing, Handling, and Transport
BACKGROUND: Large quantities of hazardous materials must be stored for
industrial and commercial use. As an example, liquid hydrocarbons from methane
to crude petroleum are stored for various periods between production and end
use. Refinery tank farms frequently contain millions of gallons of petroleum
products awaiting processing into final products, being used in processing, and
awaitingfinal distribution to the user. Individual storage tanks are often quite large,
and storage of several hundred thousand barrels of product in a single tank is not
unusual. There are a number of safety and health concerns that must be accounted
for in the design and operation of a large storage tank, beginning with the design
of the tank itself and ending with the maintenance of the tank and the equipment
used to transfer product to and from the tank.
The tank used for storage of product must be designed using materials and
techniques that are suitable for the liquid being stored. Temperatures and pressures under which the product will be stored may vary, and economic considerations may dictate the style of tank used. Products that are liquid at ambient
temperature will be stored in ambient pressure tanks at ambient temperature, but
products that have boiling points below ambient temperature may be stored either
as compressed gases, pressurized liquids, or refrigerated or cryogenic liquids. If
refrigerated liquids are stored, the tanks will have to be insulated to prevent the
liquid from boiling away. Many design factors will have to be considered for each
individual case. The followingproblem considers one aspect of the design of a tank
used for the storage of n-butane.
PROBLEM: Large amounts of n-butane must be stored at an industrial site. An
economicstudy has shown that a 500,000 bbl tank containingn-butane at a pressure
that does not exceed 1.0 psig will be preferred to storage of an equivalent quantity
of n-butane at ambient temperature as a pressurized liquid. At this low pressure,
n-butane must be refrigerated and stored at subambient temperature. The tank
must therefore be insulated. The tank design selected is a double-walled metal tank
with a suspended deck. The figure below shows a schematic diagram of the tank.
The outer wall is the pressure containment vessel. It must be designed for containment of the gas pressure exerted by the vapor pressure of the n-butane inside the
tank. The outer wall and roof of the tank will be in continuous contact with butane
vapor. The inner tank is open to the outer tank at the top, so it must be designed
only for the hydrostatic pressure of the liquid it contains. The annular space
PROBLEM NO.89
SUSPENDED
OUTER
WALL
INNER
TANK
FOAMED =ASS
BLOCK INSULATION
between the walls will be filled with insulation. Expanded perlite is the most
common insulation used for the purpose. The suspended deck above the liquid
insulates the space at the top of the tank. Glass fiber insulation is usually used for
insulation on the suspended deck The tank bottom must be insulated as well.
Foamed glass blocks are usually used for insulating the tank bottom because the
insulation must support the weight of the tank and its contents. The foundation
under the tank must be heated to prevent freezing of the soil beneath the tank. If
the soil freezes, frost heaving may cause tank failure. A boiloff compressor must
be used to withdraw the evaporating vapor from the tank. The vapor may be
processed or re-liquefied and returned to the tank.
There are many other safety and operational aspects of tank design. For example, if the ambient temperature decreases, there will be a point at which the
butane vapor in contact with the tank roof will become cold enough to begin to
condense. The butane that condenses will fall to the suspended deck where it will
flow back into the bulk of the liquid below. However if much butane condenses,
the tank pressure will drop below ambient and the tank will collapse. You are to
calculate the ambient temperature at which butane will begin to condense. That is
more difficult than you might imaginebecause there will be either radiation heating
(during the day) or radiation cooling (during the night). You may assume the night
sky to be equivalent to a blackbody that radiates at -50°F and that solar radiation
during the daytime is 300 ~tu/hr-ft2.If the wind is blowing, the convective coefficient will be different than if the wind is calm. At night, assume the wind is nearly
calm and the convective coefficient is2.7 ~ t u / h r - f t ~ -During
" ~ . the daytime, assume
the wind blows at about 25 mph and the convective coefficient is 4.8 ~ t u / h r - f t ~ - " ~ .
(Your instructor may ask you to show these values are reasonable. If so, you may
wish to review the article by Kumana and Kothari, "Predict Storage Tank Heat
Transfer Precisely," Chemical Engineering, March 22,1982, p. 127.)
Problem No. 90
CHEMICAL ENGINEERING TOPIC: Design
SAFETY AND HEALTH CONCEPT: Storage and Handling
BACKGROUND: Many industrial materials are produced and utilized in large
quantities. They maybe transported from the place of manufacture to the place of
consumption in several ways, and they may be used at the point of manufacture as
an intermediate. Ammonia is one of the most important commercial chemicals. It
is produced and used in large quantities in a number of industries such as plastics,
explosives, and fertilizers. The fertilizer industry is the largest user of ammonia,
and the use is seasonal, so that large quantities must be stored in order to meet the
demand and yet keep the productions levels about constant. Ammonia may be
stored in very large insulated tanks at pressures near ambient; in large spheres at
moderate pressures, but refrigerated to reduce the pressure; and at ambient
temperature but higher pressure, corresponding to the vapor pressure at ambient
temperature. Shipment is usually either by railroad tank car, by tank truck, or by
pipeline, in which cases it is usually shipped at ambient temperature.
The choice of whether to store ammonia as an ambient temperature liquid, a
partially refrigerated liquid, or an ambient pressure liquid depends on economic
considerations. One of the factors that determines the storage method is the
quantity of ammonia to be stored.
PROBLEM: A company produces ammonia in a 1000-todday plant. It sells the
ammonia to customers throughout the year, and the ammonia at the production
plant is stored in large tanks that have a maximum working pressure of 2.5 psig.
When the ammonia is sold, the buyers frequently ask for guidance on the choice
of storage tanks. You are assigned the task of determining an approximate break
point for storing the ammonia as a pressurized liquid, a partially refrigerated liquid,
or a liquid near ambient pressure. Your analysis should include not only the cost
of the tank, but also an analysis of the problems that might arise if the ammonia is
transported at ambient temperature and then cooled at the customer's location
before storage if storage is not at ambient temperature. You should also consider
that the ammonia might have to be warmed before it leaves the production plant
if it is transported at ambient temperature. The customers may need to store
ammonia in quantities from about 30 tons to 30,000 tons, depending on their usage.
The largest pressurized tanks available for ambient temperature storage have a
volume of about 50,000 gal. Be sure to provide some ullage space in the tank to
allow for liquid expansion as the liquid warms. Be sure to include some considerations of any special safety features you might find necessary.