Safety
“Laboratory.
Someone mixes chemicals.
Look! A reaction.”
Screen Reactive
Chemical Hazards Early
in Process Development
— Neil Dickinson (1)
David W. Mosley,
M.I.T
Albert I. Ness and
Dennis C. Hendershot,
Rohm and Haas Co.
T
©Copyright
2000
American Institute
of Chemical Engineers.
All rights reserved.
Copying and
downloading permitted
with restrictions.
Using two tools can lead to
selecting an inherently safer
chemistry, identifying potential
hazards and, thus, mitigating
them, and saving money by
providing safety layers for these
hazards early on, avoiding rework.
he chemical process industries
(CPI), by their very nature, involve chemical reactions and the
handling of reactive chemicals. As
such, there are inherent dangers involved in
the CPI that can lead to catastrophic consequences. Much activity is directed towards
avoiding these types of events.
Safety is not the only driver in process development. Chemical processes are developed
to earn a positive return on investment. The
overall cost of developing, starting up, and
running the process at the projected volume
needs to be minimized. This places a wide variety of demands on a chemical process development program. The reaction must be optimized for both yield and purity, a manufacturing site must be chosen or designed, and raw
materials sources must be found. Additionally,
the entire process development should be
done as quickly as possible. Safety concerns
are addressed within the context of this broader picture of process development.
Developing the knowledge to prevent reactive chemical incidents should not be done independently from, and later than, the other aspects of process development. Using the tools
of hazard analysis early in process development can lead to a more rapid understanding
of the reaction chemistry, aiding in process
optimization studies, as well as appreciation
of safety issues. Exploiting this synergistic effect will foster greater efficiency in process
development, both in terms of safety and economic concerns.
Case histories
Understanding reactive chemical hazards
may seem like a very basic concept. However,
there is no shortage of incidents resulting
from inadequate understanding, or from mishandling reactive chemicals. Reactive chemical incidents can occur in any size, from the
laboratory, through scale-up and pilot-plant
operations, to large-scale production. Some
examples include:
• A laboratory digester exploded when hydrogen peroxide was added to an organic sample. The digestion process called for adding
sulfuric acid to the sample before adding the
peroxide. An operational error or equipment
malfunction caused the sulfuric acid addition
to be skipped. During the incident investigation, a review of “Bretherick’s Handbook of
Reactive Chemical Hazards” (2) indicated that
the explosive decomposition reaction was
known. The referenced literature emphasized
the hazard of undercharging or not charging
sulfuric acid. The laboratory equipment, although automated so it could be run unattend-
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Safety
The CPI have long
recognized the importance
of early understanding
of the hazards of a
chemical process.
ed, had no safeguards to check for
sulfuric acid addition before the hydrogen peroxide charge. Fortunately, damage was limited to the equipment and the hood.
• During scale-up of a process to
a pilot plant, a calculation error led
to an overcharge of caustic to a phenolic compound. In the next step, a
halogenated aromatic was added to
the mixture. The excess caustic
caused an uncontrolled polymerization. The unexpected reaction was
successfully vented through a rupture disk, preventing serious damage. During the investigation, the
chemist explained that the consequence of the overcharge was obvious to any chemist. It was not obvious to the chemical engineer, who
had chosen to scale up the process
in a reactor with a large rupture disk
by chance, not design. The pilotplant reactor had previously been
used for development of a specialty
monomer process, and it had a rupture disk sized to protect against a
runaway polymerization. The final
plant process had engineering and
administrative safeguards to prevent
the undesired reaction, which could
occur if excess caustic were accidentally fed. In addition, a large
emergency relief vent was provided
on the production reactor in case the
safeguards failed.
• In 1995, at a plant in Lodi, NJ,
an explosion occurred during the
blending of two water-reactive
chemicals, aluminum powder, and
sodium hydrosulfite, in a blender
that had water connected to its jack-
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et. Five persons were killed
and most of the plant was destroyed. Hundreds were evacuated in neighboring residences, nearby businesses
suffered damage, and chemicals were released into the environment in the fire-water
runoff. Among the causes
identified by a joint OHSA/
EPA investigation team were
an inadequate process hazard
analysis and the use of blending
equipment inappropriate for water-reactive materials (3). One recommendation was that, “Facilities need to
understand chemical and process
hazards, failure modes and safeguards, deviations from normal and
their consequences… . This is best
achieved through process hazards
analysis, standard operating procedures, and training.”
• A 1999 explosion in an agricultural chemical plant in WuppertalElberfeld, Germany, was reported to
be caused by adding potassium hydroxide to a process, instead of
potassium carbonate. Initial reports
stated 91 persons were injured and
damage was estimated to be “hundreds of millions” of DM (4).
We believe that reactive chemical
incidents such as these can be predicted and prevented with good process hazard analysis (PHA) and reactive chemical evaluation programs. The reactive chemical evaluation is best done early in the development cycle, before the multitude
of decisions involved in process development are made, and before the
process is transferred to personnel
with less process-specific knowledge. Two tools that can form a
valuable part of the PHA program
will be described.
Early understanding of
reactive chemical hazards
The CPI have long recognized
the importance of early understanding of the hazards of a chemical process. Early in product and process
development, the chemist may have
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a great deal of freedom in the selection of synthesis routes and reactions, raw materials, process intermediates, and physical and chemical
purification and isolation operations. Each of the many alternatives
available to the chemist will have
various hazards. These may include
material hazards such as toxicity,
flammability, and stability, and reactivity hazards associated with the
materials and reaction steps. The
process chemist and process engineer must understand all hazards to
make informed determinations of
which process route best meets the
many, often conflicting, demands
imposed on a commercial synthesis.
As the process moves through its
life cycle, it becomes more difficult
to make fundamental changes in the
chemistry. Research and development on a selected route have progressed, and much time might be
lost if it is necessary to go back and
start over again with a different synthesis. Product performance may be
affected by the route, and samples
of a product containing a particular
spectrum of impurities may have already been given to potential customers for evaluation. Changing the
chemical synthesis path may affect
the performance of the product for
customers, forcing them to do additional work (or, they may choose not
to pursue use of the product at all).
For some products that are highly
regulated, such as pharmaceuticals
or agricultural chemicals, it may be
necessary to repeat time-consuming
and expensive toxicity tests if the
process chemistry is changed. In the
face of these obstacles, it is much
more likely that the chemist and
process engineer will adopt a strategy of managing and controlling hazards with the process chemistry that
is already under development.
If some attention is devoted to
understanding all hazards, including
reactivity ones, early in development, it is more likely that an inherently safer route can be developed
from the start. Even if the most-fa-
vorable chemical synthesis route
still involves significant reactivity
hazards, it is best to understand
them as soon as possible. This will
allow plenty of time to develop the
data and risk management strategies
that will be required later in the process life cycle.
Determining possible hazards in
a chemical process early on in its
development has other benefits as
well. Inevitably, questions concerning the reaction chemistry will arise.
At the start of a process, the chemist
typically has only considered the desired reaction pathway. But, hazard
analysis considers the undesired reaction pathways. This is of great
utility in highlighting aspects of the
chemistry that are not fully understood. These unknown facets of the
reaction chemistry will spur the
chemist to develop a far more detailed picture of the reaction, leading to possibilities for further process optimization, as well as hazard
prevention.
There are many references that
describe laboratory and theoretical
methods for understanding and
evaluating chemical reactivity hazards (5, 6, 7). But, how do the
chemist and engineer identify those
that need to be evaluated? In particular, how can this be done early in
process development, when the understanding of reactivity hazards
can have the greatest impact in
defining inherently safer processes
and chemistry? What are the tools
for identifying the right experiments to run using these well-established techniques for understanding the kinetics and thermodynamics of chemical reactions and
decomposition?
The rest of this article will focus
on two techniques that chemists and
engineers can use to ask the right
questions at the start of process development — the interaction matrix
and the chemistry hazard analysis
(CHA). These systems focus on asking questions, leading to literature
searches and laboratory experiments
to understand reactivity hazards. Before describing them, we will provide some background on a chemical process that will be used as an
example to illustrate the application
of both hazard analysis techniques.
Example reaction
Rather than discussing the application of the interaction matrix and
the process chemistry guideword
hazard analysis in abstract terms, we
will illustrate their application to a
real process. We have chosen some
paper chemistry to point out the
types of issues that will be brought
up early in process development
using an interaction matrix and a
process chemistry guideword hazard
analysis.
The procedure is adapted from an
article by Sato et al. (8) that presents a straightforward oxidation
protocol for alcohols. The raw materials are inexpensive and the major
byproduct of the reaction is water,
making the reaction appealing. Indeed, the methodology has been
proposed as a green alternative to
the current methods of adipic acid
production (9). The procedure outlined in the article involves premixing 30% hydrogen peroxide, a
phase-transfer catalyst (PTC), and
sodium tungstate. The alcohol is
added all at once, followed by heating to 90°C.
For our example, we will oxidize
a hypothetical agrochemical intermediate, 1-phenyl-2-propanol. The
procedure is typical of what a
chemist might develop early on in a
process and is shown in Figure 1.
Laboratory procedure
A 500 mL reactor equipped with
an overhead stirrer was charged
with 1.5 mmol (0.50 g) of sodium
tungstate dihydrate and 1.5 mmol
(0.71 g) of methyl(tricapryl)ammonium hydrogen sulfate, the PTC.
Next, 880 mmol (99.9 g) of 30% hydrogen peroxide was slowly charged
to the reactor. The reaction was
stirred for 10 min at 800 rpm and
then 800 mmol (109 g) of alcohol
was slowly charged. The mixture
was heated at 90°C for 4 h, then
cooled to room temperature. The
aqueous phase (containing the tungsten catalyst) was removed, and the
product was washed with aqueous
sodium thiosulfate. The organic
phase was then distilled, to give a
95% yield of 1-phenyl-2-propanol.
The reaction would result in two
output streams, produced by a single
phase cut. One would be composed
of water, excess peroxide, sodium
tungstate, and possible impurities.
The other would be the desired
product, the PTC (presumably), and
process impurities. If desired, the
sodium tungstate catalyst can be
reused — Ref. 8 details reusing the
aqueous phase of this reaction to recycle this catalyst. If the sodium
tungstate catalyst is reused, the PTC
needs to be recharged. The initial
PTC may be contained in the bottoms of the product distillation step,
or it may degrade under the reaction
conditions. The fate of excess peroxide is unclear from the article,
and this might affect the methods
used to recycle the aqueous phase.
Sato et al. (8) briefly discuss the
overall acidity of the reaction medium. This is due to the acidity of the
incoming peroxide solution — typically around a pH of 4. Use of a
more acidic peroxide solution, pH =
2, resulted in a faster reaction rate.
In addition, a 5% H2O2 solution is
reported to increase the initial oxidation rate. The order of addition in
Na2WO4, H2O2
OH
[Me(Oct)3N]HSO4
O
95%
■ Figure 1. Reaction step for the example.
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Safety
the reaction sequence is not the
most desirable. It is unclear whether
or not the H2O2 really needs to be
added prior to the alcohol. If the
peroxide could, instead, be added
slowly to a mixture of the catalyst
and the alcohol, the overall safety of
the process would increase. Changing this order of addition would be a
high priority for the development
chemist.
Table 1. Notes for the example interaction matrix in Figure 2.
Note
No. Content
Disclaimer
This example is presented for the
purpose of illustrating the methodology of the PHA techniques described. It is not intended to represent a complete hazard analysis of
this chemistry, which would take
more space than is available for this
article. The example is intended to
show how these techniques can be
used to understand the hazards associated with a new process. The authors have approached the problem
from the point of view of a development chemist or engineer who must
try to understand the hazards associated with a new chemical reaction.
The hazards described are the results of early literature searches and
previous experience of the authors,
and should not be assumed to include all potential hazards. In an actual process development, the initial
information presented in this article
would be continually updated based
on further literature, and actual laboratory experience and data. Before
working with this chemistry, you
should undertake your own hazard
analysis and make sure that you are
completely familiar with the potential hazards and the appropriate precautions and safeguards.
Interaction matrix
How do you do it?
The interaction matrix is a useful
tool for understanding possible reactions, both intended and unintended, among the various materials
used in a chemical process. The matrix can be applied at any stage in
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1
Describe self-reaction (for example, polymerization) or other instability of the pure Material A.
2
Describe any reaction between Material A and Solvent S; also describe solubility of Material A
in Solvent S.
3
Describe the reaction of Material A and Material B in the absence of solvent, catalyst, and
other materials.
4
Describe the reaction of Material A and Material B in Solvent S with no Catalyst C present.
5
Describe self-reaction (for example, polymerization) or other instability of the pure Material B.
6
Describe any reaction between Material B and Solvent S; also describe solubility of Material B
in Solvent S.
7
Describe any reaction between Material A and Contaminant X with no other materials present.
8
Describe any reaction between Material A and Contaminant X in the presence of Solvent S.
9
Describe any reaction between Material B and Contaminant X.
10
Describe any reaction between Solvent S and Contaminant X.
11
Describe any interaction between Material A and Catalyst C, including any catalytic effects on
self-reaction of Material A or the stability of Material A.
12
Describe any interaction between Material A and Catalyst C in the presence of Solvent S,
including any catalytic effects on self-reaction of Material A or the stability of Material A.
13
Describe any interaction between Material B and Catalyst C, including any catalytic effects on
self-reaction of Material B or the stability of Material B.
14
Consider the impact of Contaminant Z on the thermal stability or self-reaction of Material A.
15
Consider the impact of Contaminant Z on the thermal stability, solubility, and self-reaction of
Material A in the presence of Solvent S.
16
Consider the impact of Contaminant Z on the reaction between Material A and Material B.
17
Describe the thermal stability or self-reaction of Contaminant Z, if it can be concentrated by
some mechanism (for example, by crystallization).
18
Describe the impact of Contaminant Z on Catalyst C (reaction, impact on catalyst performance, etc.).
19
Consider the thermal stability of Material A if exposed to the maximum possible temperature.
20
Consider the thermal stability of Material A if exposed to the maximum possible temperature,
with Solvent S present.
21
Consider the thermal stability of Material B if exposed to the maximum possible temperature.
22
Consider the thermal stability of Solvent S if exposed to the maximum possible temperature.
23
Consider the thermal stability of Catalyst C if exposed to the maximum possible temperature.
24
Tabulate flammability data for Material A.
25
Tabulate flammability data for the Material A/Solvent S mixture.
26
Tabulate flammability data for Material B.
27
Tabulate flammability data for Solvent S.
28
Does iron or rust contamination impact the performance of Catalyst C?
29
Tabulate acute and chronic toxicity data for Material A.
30
Consider the impact of Solvent S on the toxic hazards of Material A (for example, will Material
A be absorbed more rapidly in case of skin contact?).
31
Tabulate acute and chronic toxicity data for Material B.
32
Tabulate acute and chronic toxicity data for Solvent S.
33
Tabulate acute and chronic toxicity data for Contaminant X.
34
Tabulate acute and chronic toxicity data for Catalyst C.
the process life cycle, from early research through commercial plant
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operation, and it can even be used
to help understand the hazards asso-
Raw Material
A
Raw Material
A/Solvent S
Raw Material
B
Solvent S
Contaminant
Z (may be in
RM A)
Catalyst C
Raw Material
A
See Note 1
X
X
X
X
X
Raw Material
A/Solvent S
See Note 2
See Note 2
X
X
X
X
Raw Material
B
See Note 3
See Note 4
See Note 5
X
X
X
Solvent S
See Note 2
See Note 2
See Note 6
No
X
X
Contaminant
X (used in area)
See Note 7
See Note 8
See Note 9
See Note 10
No
X
Catalyst C
See Note 11
See Note 12
See Note 13
No
No
No
Contaminant
Z (may be in RM A)
See Note 14
See Note 15
See Note 16
See Note 15
See Note 17
See Note 18
Glycol Coil
Coolant
No
No
No
No
No
No
150 psig
Steam
See Note 19
See Note 20
See Note 21
See Note 22
No
See Note 23
Air
See Note 24
See Note 25
See Note 26
See Note 27
No
No
Rust
No
No
No
No
No
See Note 28
People
See Note 29
See Note 30
See Note 31
See Note 32
See Note 33
See Note 34
Etc.
.......
Etc.
.......
■ Figure 2. An example of an interaction matrix for a generic chemical process.
ciated with plant decommissioning
and demolition. The matrix is particularly valuable early in the development of a new chemical process
to identify known chemical interactions. In many cases, when the
matrix is employed sooner during
process development, it will raise
more questions than answers. The
chemical interactions may not be
known to the research team, and
the use of the matrix will generate
a list of questions for research, either through literature searching or
actual experiments.
Figure 2 is an example of an interaction matrix for a generic chemical process. The notes for the matrix
are found in Table 1. To create this
arrangement, list all of the materials, materials of construction, likely
contaminants, potential sources of
energy, process utilities (such as
steam, water, nitrogen, compressed
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air, ethylene glycol coolant, and
heat-transfer oil), and other relevant
parameters along each axis. Then,
ask what happens at each interaction
and document the answers. It is a
good idea to also include “people”
on one of the axes, to prompt questions about toxicity and other adverse impacts of materials. Interestingly, one of the entries in “Bretherick’s Handbook of Reactive Chemical Hazards” (2) is for “Workers at
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the next bench” (p. 376, Vol. 2).
The point is that it is important to
understand what else is happening
in the area in the laboratory, and the
potential impacts on your reaction.
The same is true for every scale of
chemical production, including pilot
plants and commercial facilities.
In general, it is not necessary to
list plant utility fluids, such as
steam, cooling-water treatment
chemicals, water, and others, on
both axes. This is why glycol coil
coolant, 150 psig steam, and people
are not listed on the horizontal axis
in Figure 2. This also applies to
plant contaminants such as rust, lubricating oil, dirt, and various
foulants from other processes. When
doing the interaction matrix, the
process chemist and engineer are interested in the potential interactions
among the process materials in a
process under development, and between materials used in that process
and other materials or energy
sources that they may contact. Interactions among other plant contaminants or between plant utilities are a
separate issue from the understanding of the hazards of the process
being evaluated. Hazards from these
interactions usually will not impact
on the process under development.
The matrix is best suited to ask
questions about interactions between
two materials, energy sources, or
other items on its axes. But, many
times, hazards arise from the interaction of three or more components.
How can the researcher use the matrix to understand this? In principle,
the matrix could be expanded to a
three-dimensional form to consider
three-way interactions, or even to n
dimensions to consider n-way interactions, if we were capable of constructing and visualizing such a
shape. However, this is not really
feasible for most real systems, because the number of cells representing possible interactions increases
dramatically as the number of simultaneously interacting components increases. For example, a system with
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10 components has over a thousand
possible combinations.
One approach to addressing the
“combinatorial explosion” for an interaction matrix is to identify likely
combinations of components and list
them as separate entries. For example, one might list “Raw Material
A/Solvent S” (Raw Material A dissolved in Solvent S) in the binary
matrix, and evaluate its interactions
with other matrix components as
shown in the second row and column of Figure 2.
The matrix should go beyond
simple yes/no answers. It will be
much more valuable for future reference if detailed information on the
nature of the interaction can be provided in attached notes. For example, consider the interaction of
methacrylic acid, a vinyl monomer,
and carbon steel pipe. In the interaction matrix, one might simply indicate, “Yes, there is an interaction.”
Or, that this interaction includes the
corrosion of the steel pipe and the
resulting dissolved iron increases
the risk of polymerization of the
methacrylic acid monomer. But, a
more-useful
interaction
matrix
would show that there is an interaction, including corrosion and increased susceptibility of polymerization. It would also reference specific data from company or published sources giving corrosion
rates, and data on the impact of various iron concentrations on stability.
The matrix would also cite specific
references to incidents in which the
interaction caused a problem for the
specific material or a closely related
material. For example, for the
methacrylic acid/iron interaction:
• Warm water was circulated
through a steel coil in a tank containing methacrylic acid (freezing
point ~14–15°C) to prevent freezing. After a number of years, a mechanic was splashed with the acid
while working on the warm-water
supply pipe. Methacrylic acid had
corroded the coils and leaked into
the cooling water piping, which was
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expected to contain only water.
• Reduced stability due to iron
contamination was one of the multiple causes responsible for the explosion of a railroad tank car containing methacrylic acid (10).
• A stainless steel pipe for acrylic
acid (closely related to methacrylic)
repeatedly plugged with polymer
and had to be replaced. After this
happened several times, a carbon
steel fitting was found at the discharge of the pump, and the iron
contamination was sufficient to
cause polymer formation. When the
fitting was replaced with a stainless
steel one, polymer formation immediately stopped.
Obviously, this type of information cannot be displayed in a simple
grid or matrix. Instead, the matrix
should briefly indicate that there is
an interaction, and reference a separate document that will detail the
nature of the interaction(s).
Who does it?
An interaction matrix can be generated as a group activity in a PHA
meeting, but this is not necessary,
and may not be the most efficient
way of using the tool. An alternative
is to have a chemist or chemical engineer generate an initial prototype
matrix, and fill in as many of the interactions as possible. It can then be
circulated for modification to others
who have expertise in chemistry and
plant operations. These changes
might include the addition of other
components, further information on
interactions, and incorporation of
material that the originator of the
matrix did not have. A final meeting
to review the completed form will
allow discussion and encourage interactive understanding of potential
hazards.
What is the result?
What is the product of the matrix? It is a concise documentation
of the known interactions, both hazardous and nonhazardous, that
should be consulted and updated as
Alcohol
Sodium
Tungstate
PTC
Hydrogen
Peroxide,
30%
Sodium
Tungstate,
PTC
Sodium
Tungstate,
PTC, H2O2
Reaction
Product
X
X
X
X
X
X
X
X
Temp. = 90˚C Contaminant
(in RMs)?
Alcohol
1
Sodium Tungstate
2
3
X
X
X
X
X
X
X
PTC
4
5
6
X
X
X
X
X
X
Hydrogen
Peroxide, 30%
7
8
9
10
X
X
X
X
X
Sodium Tungstate,
PTC
11
12
13
14
15
X
X
X
X
Sodium Tungstate,
PTC, H2O2
16
17
18
19
20
21
X
X
X
Reaction Product
22
23
24
25
26
27
28
X
X
Temp. = 90˚C
29
30
31
32
33
34
35
36
X
Contaminant
(in RMs)?
37
38
39
40
41
42
43
44
45
People
46
47
48
49
50
51
52
53
54
1
■ Figure 3. Interaction matrix for the example reaction, an alcohol oxidation. (Numbers refer to the descriptive material notes in the text.)
the process goes through its life
cycle. In most cases, particularly
early in process development, the
matrix may generate a lot of questions. There may be many holes in it
— interactions with consequences
that are not known or well understood. The matrix can then be used
to generate a program to fill in these
gaps, either by searching for additional information or by actual lab
experiments.
Completing the form requires a
more thorough understanding of the
chemistry, and aids not just in hazard analysis, but also in process optimization — for example, increasing yields, decreasing cycle time,
reducing reagent usage, or predict-
ing byproducts. By highlighting
every possible chemical interaction,
the matrix is useful for guiding thorough mechanistic work on the reaction chemistry. This improved understanding benefits the process
chemist’s efforts to make process
improvements, as well as reduces
the chemical hazards.
The authors found, in working
through the example in this article,
that many of the interactions in the
matrix might play a part in determining the catalyst activity and
product purity and yield. Yet, the effects of these secondary interactions
are rarely investigated systematically. Instead, they are often discovered
in a piecemeal fashion as unexpect-
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ed problems arise during scale-up.
What resources will help?
There exists extensive published
literature on chemical reactivity and
interactions that is useful in constructing an interaction matrix.
“Bretherick’s” (2) lists thousands of
reported reactions and chemical incompatibilities, and includes literature citations for more information.
This is probably the best single
source for chemical compatibility
information.
The U.S. Coast Guard maintains
a database and compatibility chart
of chemical combinations known or
believed to be dangerously reactive
if accidentally mixed. This informa-
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tion is updated periodically and is
available in the “CHRIS Manual”
(11). The U.S. National Oceanic and
Atmospheric Administration (NOAA)
has developed a Chemical Reactivity Worksheet (12) that provides information about reactions among
various combinations of chemicals.
This worksheet is offered as a personal computer (PC) program (also
available in Macintosh format) that
can be downloaded from the NOAA
Internet Web site via http://
response.restoration.noaa.gov/chem
aids/react.html.
A PC program, CHEMPAT, is
available from AIChE for documenting chemical interactions (13).
CHEMPAT does not include a
database of known chemical interactions, but, rather, provides a
database shell that can be used to
document the results of interaction
matrix studies. Over time, the user
can build a database of known
chemical interactions for materials a
plant or company frequently uses.
Clark (14), Leggett (15), Gay and
Leggett (16), and CCPS (17) provide additional valuable perspectives on the use of the interaction
matrix for understanding reactive
chemical hazards.
Applying the matrix
to the example
The matrix is a good starting
point for analyzing our example reaction, once the process has passed
initial economic evaluations. The
The matrix should
go beyond simple
yes/no answers,
and attached notes
should offer detailed
information.
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November 2000
matrix will not allow us to consider
the workup procedure specifically,
but study of the product distillation
can be contained within the context
of the matrix. Note that two-, three-,
and four-way combinations have
been included. These multiple-component interactions often lead to
fruitful questions involving both the
hazard analysis and reaction chemistry. In this example, including
multiple interactions leads to some
interesting questions involving the
stability of the product and the active catalyst under the reaction conditions. The interaction matrix for
the alcohol oxidation is shown in
Figure 3. “Bretherick’s” was used as
a first screen for incompatibilities.
The notes for the matrix follow —
the numbers refer to Figure 3:
1. Alcohol/alcohol — The alcohol should be stable. It is peroxidizable upon storage, but this will not
be a problem with this reaction.
2. Sodium tungstate/alcohol —
The sodium tungstate should be
mildly basic; it is the conjugate base
of tungstic acid. The basicity of the
complex should be determined.
Upon mixing with the PTC, it might
be expected to deprotonate a certain
amount of the hydrogen sulfate
anion. Indeed, the active catalyst
may actually have a proton on one
of the oxygens, and be associated
with the ammonium salt, as well.
Acidic aqueous solutions of sodium
tungstate would be expected to form
polymeric anions (18). The interactions might be studied in more detail, since these two components
would be mixed at 90°C.
3. Sodium
tungstate/sodium
tungstate — Sodium tungstate will
be stable, with no self-reactions.
4. PTC/alcohol — The PTC
will be stable when mixed with
the alcohol.
5. PTC/sodium tungstate — The
PTC should be fairly stable in the
presence of the tungstate catalyst.
However, alkyl PTCs are degraded
by bases, either by Hoffmann elimination or nucleophilic substitution.
CEP
There could be some degradation of
the PTC, if the tungstate complex is
particularly basic. It is likely that
the hydrogen sulfate will buffer this
type of degradation pathway. This
might be one of the reasons why hydrogen sulfate is the preferred
anion, instead of a halide. Or, if the
tungsten is closely associated with
the PTC, then some other type of
degradation might occur. This could
be checked with a thermal study.
6. PTC/PTC — The methyl(tricapryl)ammonium hydrogen sulfate
is stable.
7. Hydrogen
peroxide/alcohol
— Mixtures of concentrated H2O2
and alcohols are reported to be explosive. However, below concentrations of 50% H2O2, the explosivity
drops off. Clearly, the precise nature
of this interaction would need to be
explored. To maintain safety, the hydrogen
peroxide
concentration
charged should be demonstrated to
have no possible explosive interactions with the alcohol.
8. Hydrogen
peroxide/sodium
tungstate
—
Again,
sodium
tungstate would be expected to accelerate the rate of H2O2 decomposition, if no alcohol is present. The
degree of this acceleration needs to
be ascertained. It is possible that
this reaction is not noticed on a
small lab scale, but might lead to
uncontrollable self-heating due to
auto-oxidation on a larger scale.
Also, the acidity of the aqueous hydrogen peroxide solution might
form the tungsten-based polymeric
anions referred to earlier. This
would, by no means, be a runaway
exothermic reaction, though.
9. Hydrogen peroxide/PTC —
The effects of a quaternary salt on
hydrogen peroxide are unknown.
This would need to be investigated.
The interaction of concentrated
H2O2 and concentrated sulfuric acid
with an alcohol can result in the formation of peroxymonosulfuric acid,
which is quite unstable (“Bretherick’s” (2)). Can the monoanion of
sulfuric acid also be involved with
this type of chemistry? Would this
be a problem in catalytic quantities?
Does the sodium tungstate introduce
a buffering effect on the acidity of
hydrogen sulfate anion?
10. Hydrogen peroxide/hydrogen
peroxide — Aqueous H2O2 is highly
unstable. It produces oxygen gas
upon decomposition, and significantly enhances combustion of other
substances. In general, solutions are
kept in vented storage systems to
avoid pressure buildup. Fortunately,
the rate of decomposition depends
upon both the peroxide concentration and the contaminants that are
present (19). Both of these need to
be controlled. Any contaminants are
likely to increase the rate of decomposition, possibly leading to a runaway reaction. Iron, brass, copper,
Monel, solder, metal salts, and even
dust are among the things that have
been reported to result in explosive
decomposition
of
concentrated
(> 30%) H2O2 solutions. Deflagration of solutions of hydrogen peroxide and organics has been reported
due to thermal or shock effects.
Spontaneous combustion can also
occur with mixtures of hydrogen
peroxide and organics in the vapor
phase. All of these issues are of concern. Is there a chance of explosive
decomposition in our system?
Typical pHs for a commercial
H2O2 solution are 1.5–4.0. Typical
commercial concentrations available are 35% and 50%. What is our
preferred range for these two variables? In addition, commercial peroxide often has a variety of stabilizers present. Do these help to reduce
the chances of explosive decomposition in our case? Do they add impurities that may interfere with the
process? In addition to these concerns, the materials of construction
and handling in the plant will need
to meet specifications for handling
H2O2 — Does this require a dedicated facility?
11. Tungstate + PTC/alcohol —
The interaction of the sodium
tungstate, methyl(tricapryl) ammo-
nium hydrogen sulfate, and alcohol
is undefined at 90°C. Upon prolonged heating, you might see
degradation of the PTC.
12., 13. Not applicable.
14. Tungstate + PTC/hydrogen
peroxide — The interaction of these
three would be expected to result in
some decomposition of the peroxide. The rate of peroxide decomposition with this catalyst system
needs to be clearly defined. It could
be very concentration-dependent,
with regard to the weight percentage
of aqueous hydrogen peroxide
charged. Regardless, close monitoring of the peroxide concentration
will need to be done at every stage
of the process. The heat of mixing
should be checked. Could PTC
degradation via oxidation be possible, if there is a long wait for alcohol addition?
15. See Note 5.
16. Tungstate + PTC + hydrogen
peroxide/alcohol — The interaction
of all four components results in the
desired reaction. What are the side
products? Can oxidative cleavage
occur? Is benzylic oxidation possible? Is a complete reaction obtained? What is the effect of temperature on the rate of reaction? How
about exothermicity of the reaction?
Does temperature affect byproduct
formation?
17.–20. Not applicable.
21. See Note 14.
22. Product/alcohol — The product could form acetals with the alcohol, if the reaction medium is acidic.
This is reversible, and, eventually,
any alcohol present would be converted to a ketone.
23. Product/sodium tungstate —
The product might form an enolate,
again, assuming some slight basicity
of the catalyst system. This is an unlikely scenario.
24. Product/PTC — The ketone
should be inert to the PTC.
25. Product/hydrogen peroxide —
A variety of ketone peroxides can be
formed through the interaction of
hydrogen peroxide and a ketone.
CEP
These compounds can be explosive,
and sensitive to both heat and shock.
Certainly, the nature of the peroxides resulting from the interaction of
our product ketone and hydrogen
peroxide should be investigated. Are
these impurities being removed during the workup, before the product
is distilled? “Bretherick’s” specifically
mentions
low-molecularweight ketones as a problem, thus,
using acetone for cleaning of the reactors would be undesirable.
26. Product/sodium tungstate +
PTC — Same as Note 23.
27. Product/sodium tungstate +
PTC + hydrogen peroxide — As discussed in Note 25, ketone peroxides
could be formed.
28. Product/product — The ketone is stable by itself. Residual peroxide should definitely be removed
before it is distilled, however.
29. Heat/alcohol — The alcohol
is stable to heat. Its boiling point is
219°C.
30. Heat/sodium tungstate —
Sodium tungstate is stable to heat.
Melting point = ?
31. Heat/PTC — This PTC will
slowly decompose upon excessive
heating at high temperatures. What
is the profile of this decomposition?
32. Heat/hydrogen peroxide —
Heat will accelerate any decomposition processes that might be occurring. The boiling point of the solution is ~108°C (for 35% H2O2).
33. Heat/Sodium tungstate +
PTC — Heating this solution of
PTC and tungstate for too long will
probably result in loss of catalytic
activity. This aging effect should be
studied.
34. Heat/sodium tungstate + PTC
+ peroxide — Poor temperature
control could result in a runaway
decomposition of the peroxide. The
parameters of this definitely need to
be explored. What about possible
pressure buildup in the reactor?
How crucial is temperature control?
35. Heat/product — The ketone
itself should be fine upon heating, if
peroxides are not present. Boiling
November 2000
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59
Safety
point = ? What about flammability?
Is there an explosive region present
in mixtures of hydrogen peroxide,
water, and ketone? What about hydrogen peroxide, water, and alcohol? Are there any azeotropes of hydrogen peroxide, ketone, alcohol, or
water present?
36. Not applicable.
37–45. Contaminants/all components — What are the common contaminants in the raw materials and
products? A contaminant in the sodium tungstate could have drastic effects on the stability of the reaction
mixture, due to hydrogen peroxide
decomposition — a variety of metal
impurities are known to rapidly accelerate H2O2 decomposition. How
rigorous does our quality control
need to be? The same sort of issue is
brought up by possible rust contamination. The reactor system will
probably need to be very clean.
More literature searching is definitely in order to determine what types
of controls will be necessary.
“Bretherick’s” references several reviews on general and specific aspects of hydrogen peroxide handling. These issues are important,
not just from a safety standpoint,
but also an economic one, since
wasting H2O2 through decomposition pathways is poor economics.
Contaminants may also poison the
catalyst, so understanding of what
types of compounds can do this is
also needed.
46. People/alcohol — 1-phenyl2-propanol has a Chemical Abstracts
Services (CAS) No. 14898-87-4.
From the material safety data sheet
(MSDS), we find that the material is
combustible, but toxicity issues are
not mentioned. Who are the possible
suppliers? Impurity profiles? What
are the contact and inhalation hazards, toxicity issues?
47. People/sodium tungstate —
Sodium tungstate dihydrate, CAS
No. 10213-10-2. From the MSDS: it
is toxic, harmful by inhalation and
by contact. Who are the possible
suppliers? Impurity profiles? What
60
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November 2000
are the toxicity issues? The OSHA
permissible exposure limit (PEL)
for tungsten compounds is 1 mg/m3,
as an 8-h time-weighted average
(TWA).
48. People/PTC — Methyl(tricapryl)ammonium hydrogen sulfate.
CAS No.? Look up MSDS. Who are
the possible suppliers? Impurity
profiles? What are the contact hazards, toxicity issues?
49. People/hydrogen peroxide —
Hydrogen peroxide, 30%. CAS No.
7722-84-1. From the MSDS: it
produces contact burns. Who are
the possible suppliers? Impurity
profiles?
50. 51. People/reactants — Do
any of the side reactions produce
highly toxic materials? What about
the catalytically active species?
52. People/product — Basic toxicity data need to be determined.
What types of tests will be necessary to satisfy regulatory issues?
Does the product need a Toxic Substances Control Act (TSCA) listing?
53. People/heat — Thermal contact hazards (with the reactor, reaction media) will exist.
54. The suppliers should note any
particular human health hazards associated with their process impurities in the MSDS. Do any of our impurities have particular hazards?
CHA: How do you do it?
The chemistry hazard analysis
(CHA) is derived from the hazard
and operability (HAZOP) study
methodology (17). HAZOP was
originally developed for hazard analysis of a detailed design or an existing plant, and usually requires a lot
of itemized information about the
plant design and operation. Much of
this will not exist early in process
development, and application of the
complete HAZOP method is not possible. However, the thought process
of a HAZOP and its general methodology can be applied at any stage in
process development.
HAZOP assumes that a process is
safe if operated as designed, and the
CEP
designer must confirm that this is,
indeed, true. HAZOP presupposes
that incidents occur as a result of a
deviation from intended operation.
The HAZOP study team applies a
series of guidewords to the stated
design intention for the plant as an
aid in identifying potential deviations. The team then determines the
potential causes, consequences,
safeguards, and required actions for
each deviation that is recognized.
This concept and process can be
applied to a chemical reaction. We
call this application of the HAZOP
process a CHA to differentiate it
from a HAZOP, which requires
much more plant design information. In a CHA, we also assume that
the chemistry is safe, if the reaction
is done as intended — the chemist
must confirm that this is true. The
HAZOP guidewords are then applied to the chemistry to identify deviations from the intended operation, and the review team must determine the consequences of these
deviations.
The methodology of a CHA can
be shown using a generalized chemical reaction:
A + B → Y +Z
at temperature T and pressure P,
promoted by catalyst C, and occurring in solvent S.
To do the CHA, apply the seven
basic HAZOP guide words to the
chemical reaction: no, less, reverse,
other, more, part of, and as well as.
In a HAZOP study, the team
members then look for specific
causes and consequences for each
deviation. However, in a CHA, we
are more interested in the consequences of deviations from intended
operation and are less concerned
about causes and specific incident
scenarios. Indeed, it may not be possible to identify specific causes and
incident scenarios for a process or
plant in early development. Therefore, the chemist or engineer should
simply assume that the deviation
The results of the CHA can be
documented in a tabular form, with
columns listing the guidewords, deviations, consequences, and comments/actions, as shown in Table 3.
The summary matrix format used
for the example reaction in the next
section can be used to help in construction of a CHA table.
Table 2. CHA guidewords with generalized examples.
Guideword
Examples of possible deviations for consideration
No
No Reactant A; no Reactant B; no Solvent S; no Catalyst C; no reaction; no
agitation; etc.
More
More Reactant A; more Reactant B; more Solvent S; more Catalyst C, More
(higher) temperature; More (higher) pressure; more reaction time; more reaction rate; more (faster) rate of addition of material; etc.
Less
Less Reactant A; less Reactant B; less Solvent S; less Catalyst C; less (lower)
temperature; less (lower) pressure; less reaction time; less reaction rate;
less (slower) rate of addition of material; etc.
Part of
Part of A dissolved in S; part of B dissolved in S; partial reaction; etc.
Reverse
Reverse order of addition of materials; reverse reaction; reverse steps in a
procedure (for example, adding a reactant to the reaction vessel before
cooling the reaction vessel contents instead of after cooling); etc.
As well as
Anticipated contaminants in supplied materials (Reactants A and B, Solvent
S, Catalyst C), as well as intended materials; common industrial contaminants (air, water, rust or iron, oil, lubricants, greases, glycol, brine, or other
heat-transfer fluids), as well as intended materials; etc.
Other
Other (wrong) materials charged (particularly, materials with a similar name
that might be present in the facility where the process will ultimately be run
— for example, acetic anhydride instead of acetic acid); other possible reactions; other forms of materials (for example, solid materials as a powder
instead of a granule or pellet); etc.
identified by the application of the
guideword to the chemical reaction
does occur for some reason, and investigate the consequences.
Table 2 lists the seven CHA
(HAZOP) guidewords, with examples of how they might be applied to
the example general chemical reaction. If the consequences are known,
then the chemist should determine
whether or not they represent a hazard that must be understood and
managed as a part of the continuing
process development, and document
this information for future action or
reference. In many cases, early in
process development, the consequences may not be known, and additional research or experiments
may be needed.
Who does it?
A CHA can be done as a group activity, in the same way that a HAZOP
is normally conducted. However, as
for the interaction matrix, this may
not be the most efficient way to do a
CHA. Instead, it may be better for
the process chemist or process development engineer to create an initial
draft of the CHA, and circulate it to
colleagues for comments and completion. These colleagues should be
selected to provide a good spectrum
of experience with the chemistry and
an understanding of the kinds of upsets that can occur in plant operations. A good chemist who is not familiar with the specific chemistry
being studied is also a good addition
— to bring a new, outside perspective to the chemistry, because it is
not familiar. A short meeting to review the final CHA is also valuable
to allow for discussion and interaction among the participants.
What is the result?
The CHA, like the interaction matrix, generates a list of potentially
Table 3. Examples of deviations and consequences.
Guideword
Deviation
Consequence
Comments/Actions
No
Catalyst C left out
No reaction when Reactants A and B
are mixed; if Catalyst C is added after
the entire charge of Reactants A and B
has been completed, a rapid and violent
reaction can occur.
Develop kinetic and
thermodynamic data
on this reaction.
More
Higher temperature;
greater than 70ºC
Side reactions have been observed
in similar systems above 70ºC, and
may also occur with this chemistry.
Investigate the behavior
of the reaction at
elevated temperature.
As well as
Rust, as well as
normal materials
The effect of contamination with
iron or rust is unknown.
Determine the effect of iron
or rust contamination.
CEP
November 2000
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61
Safety
No
hazardous deviations from the intended chemistry that should be consulted during the ongoing process development. As the project proceeds
through the laboratory, scale-up, and,
finally, to commercial operation, the
CHA can be used as a resource for
the more-detailed process hazard
studies that will be required at these
stages in process development. These
thorough reviews will generate specific potential incident scenarios by
which the deviations uncovered by
the CHA might actually occur in specific laboratory or plant equipment.
The CHA is also likely to generate an extensive list of questions
that must be researched as a part of
the ongoing process development.
Perhaps, early in development, no
one knows the effect of operating
the chemistry at a temperature 15°C
higher than intended. This should be
determined as a part of the research
program, because, eventually, somebody is bound to make a mistake
and do this in a laboratory, pilot
plant, or production unit. If there are
significant hazards associated with
this error, we must understand them
early, so that adequate safeguards
can be provided. Similarly, we must
comprehend the consequences of all
of the deviations from the desired
chemistry that the CHA can identify,
and provide appropriate safeguards
where required.
What resources
are available to help?
The most important expedient for
understanding the deviations from
intended chemistry that a CHA can
identify is the knowledge and understanding of expert chemists. This includes their ability to search the literature and company knowledge
bases to understand the impact of
the reaction being studied under
conditions different from those intended. In addition, many of the resources previously referenced as
useful for the interaction matrix will
also be valuable for the CHA.
62
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November 2000
More
Less
1 1
22
33
44
5
6
7
Sodium
Tungstate
8
9
10
11
12
13
14
PTC
15
16
17
18
19
20
21
Hydrogen
Peroxide, 30%
22
23
24
25
26
27
28
Temp. = 90˚C
29
30
31
32
NA
NA
NA
Rust
NA
33
NA
NA
NA
NA
NA
Agitation
34
35
36
NA
NA
NA
NA
Alcohol
Part of
Reverse As well as
Other
■ Figure 4. Matrix used for generating CHA table for the alcohol oxidation example. (Numbers refer
to the deviation notes in Table 4, which appears on CEP’s Web site.)
Applying CHA to the example
The CHA will help to identify a variety of upset studies, and allow for
early experiments to address any significant deviations before a specific
plant is chosen or designed for production. When an existing plant is to be
used, these results will aid in site selection based on specific issues identified.
Often, the production site is chosen before this type of analysis is performed.
The result can be the need for additional equipment to manage safety concerns, or even a major change of the
process during development, wasting
time and money. The CHA will also
help in identifying candidate scenarios
for emergency-relief vent sizing.
The CHA may be summarized in
the traditional HAZOP format as
shown in Table 4, which, due to space
limitations, appears only at CEP’s
Web site. A helpful tool for constructing the CHA table is the matrix
shown in Figure 4, which graphically
represents each HAZOP guideword
matched with each component that
may deviate. The deviation numbers
in Table 4 correspond to the numbers
CEP
in the Figure 3 matrix for the various
guideword/parameter combinations.
To sum up
Two tools have been described
for identifying and assessing reactivity hazards in a chemical process:
the interaction matrix and CHA. Applying these tools early in product
and process development provides
many advantages:
• Knowledge of the comparative
hazards among competing synthesis
routes can lead to selecting an inherently safer chemistry before you become locked-in to a suboptimal synthesis method.
• Process chemists and engineers
can identify gaps in knowledge of potential hazards and plan experimental
programs to close these gaps. The
greater understanding can improve
the optimization process, as well.
• Process chemists and engineers
can decide how to manage hazards
early in the process design, which
will save money due to rework or
addition of unexpected safety layers
later in process design.
Application of these tools can
begin in any company or laboratory
immediately. The main ingredient
needed is teamwork between the process chemist and engineer. The resources required to use the tools are
readily available through the literature and from existing computer
databases. The expertise of others,
such as production engineers and operators, should also be used. These
tools are flexible enough so that they
can be used in a team setting, or individually, then circulated among those
with the appropriate expertise. These
tools lead to the design of experi-
Literature Cited
1. Chem. & Industry, “Poetry Corner,” No. 6,
p. 243 (Mar. 15, 1999). (This haiku with
the classical 5-7-5 syllable structure —
using the British pronunciation of “laboratory” — was a grand prize winner in the
Chem. & Industry haiku competition.)
2. Urben, P. G., ed., “Bretherick’s Handbook
of Reactive Chemical Hazards,” 5th ed.,
Butterworth-Heinemann, Boston (1995).
3. “EPA/OSHA Joint Chemical Accident Investigation Report — Napp Technologies,
Inc., Lodi, New Jersey,” EPA 550-R-97002, Environmental Protection Agency,
Washington, DC (Oct. 1997).
4. “Explosion Rocks Bayer’s Wuppertal
Plant,” Chem. Week, p. 8 (June 16, 1999);
“Bayer Says Chemicals Mix-Up Caused
Blast,” Chem. Week, p. 8 (June 23, 1999).
5. “Guidelines for Chemical Reactivity
Evaluation and Application to Process
Design,” Center for Chemical Process
Safety, AIChE, New York (1995).
6. Barton, J., and R. Rogers, “Chemical
Reaction Hazards,” 2nd ed., Institution of
Chemical Engineers, Rugby, Warwickshire, U.K. (1997).
7. “Guidelines for Safe Storage and Handling of Reactive Materials,” Center for
Chemical Process Safety, AIChE, New
York (1995).
8. Sato, K., et al., “Organic Solvent- and
Halide-Free Oxidation of Alcohols with
Aqueous Hydrogen Peroxide,” J. Am.
Chem. Soc., 119, p. 12386 (1997).
9. Sato, K., et al., “A ‘Green’ Route to
Adipic Acid: Direct Oxidation of Cyclohexenes with 30 Percent Hydrogen Peroxide,” Science, 281, pp. 1646–1647 (Sept.
11, 1998).
ments that will provide answers to
process safety and reaction chemistry
questions. The final output is concise
documentation of the chemical process hazards that will be an invaluable component of the process safety
information throughout the life of the
CEP
product or process.
Discuss This Article!
To join an online discussion about this article
with the author and other readers, go to the
ProcessCity Discussion Room for CEP articles
at www.processcity.com/cep.
D. W. MOSLEY is a PhD student at
Massachusetts Institute of Technology,
Cambridge, MA ((617) 258-6536; E-mail:
[email protected]). His current
research interests involve molecular design
and nanoscale design, and he is generally
interested in practical applications of
chemistry in industry. Mosley previously
worked in the agricultural chemicals
business of Rohm and Haas as a process
chemist. At Rohm and Haas, he developed
and optimized chemistries for scale-up, and
participated in safety studies on several
projects. He has a MS in synthetic organic
chemistry from Stanford University, and a BS
in chemistry from Texas A&M University.
Acknowledgments
We would like to thank Jim Ackert, Andy
Gross, and Greg Keeports for their review of the manuscript and valuable suggestions to improve it.
10. Anderson, S. E., and R. W. Skloss,
“More Bang for the Buck: Getting the
Most From Accident Investigations,”
Plant/Operations Progress, 11 (3), pp.
151–156 (July 1992).
11. “Chemical Hazards Response Information System: Volume 2, Hazardous
Chemical Data Manual,” (GPO Stock
No. 050-012-00329-7), U.S. Government
Printing Office, Washington, DC (1992).
12. Farr, J. K., et al., “New Program for
Chemical Compatibility,” Chem. Health
and Safety, 5 (6), pp. 33–36 (Nov./Dec.
1998).
13. “CHEMPAT: A Program to Assist Hazard
Evaluation and Management,” AIChE,
New York (1995).
14. Clark, D. G., “Apply These Matrices to
Help Ensure Plant Safety,” Chem. Eng.
Progress, 93 (12), pp. 69–73 (Dec.
1997).
15. Leggett, D. J., “Management of Chemical Plants Using Chemical Compatibility
Information,” Process Safety Progress,
16 (1), pp. 8–13 (Spring 1997).
16. Gay, D. M., and D. J. Leggett, “Enhancing Thermal Hazard Analysis
Awareness with Compatibility Charts,” J.
of Testing and Evaluation, 21 (6), pp.
477–480 (1993).
17. “Guidelines for Hazard Evaluation Procedures, Second Edition, with Worked
Examples,” Center for Chemical Process
Safety, AIChE, New York (1992).
18. Cotton, F. W., et al., “Advanced Inorganic Chemistry,” 6th ed., John Wiley,
New York (1999).
19. Goor, G., et al., “Hydrogen Peroxide,” in
“Ullmann’s Encyclopedia of Industrial
Chemistry,” 5th ed., B. Elvers, ed., VCH
Publishers, New York (1989).
A. I. NESS is a risk analyst with Rohm
and Haas, Bristol, PA ((215) 785-7567;
Fax: (215) 785-7077; E-mail:
[email protected]). He has
been at Rohm and Haas for 24 years as a
research process development engineer, a
process cost analyst and, for the past 12
years, as a risk analyst. Ness has worked
on the CCPS Subcommittee for Reactive
Materials Storage and participated in the
writing of the CCPS “Guidelines for Safe
Storage and Handling of Reactive Materials.”
Currently, he is a member of the Process
Safety Information Database Subcommittee
and is the secretary/treasurer of the Safety
and Health Division of AIChE. He has a
BS in chemical engineering from the
University of Arizona and an MS from the
University of Illinois.
D. C. HENDERSHOT is a senior technical fellow
with Rohm and Haas Co. Engineering
Division, Bristol, PA ((215) 785-7243;
Fax: (725) 785-7077; E-mail:
[email protected]).
His initial work with Rohm and Haas was in
process research and development, plant
design, and startup of new processes. Since
1980, he has been involved with process
safety and risk analysis for new and existing
facilities throughout the company. He is a
past chair of the AIChE Safety and Health
Division, has been active in a number of
CCPS subcommittees, and is currently chair
of the CCPS Risk Assessment Subcommittee,
as well as a member of the CEP Editorial
Advisory Board. He has a BS in chemical
engineering from Lehigh University and an
MS from the University of Pennsylvania.
CEP
November 2000
www.aiche.org/cep/
63
Table 4. Deviations and consequences for alcohol oxidation example in tabular format.
Guideword
Deviation
Consequence
Comments/Actions
No
1. No alcohol
charged
If no alcohol is charged,
the catalyst will degrade the
hydrogen peroxide.
What is the heat of reaction? Need to investigate what happens if the
mixture is heated to 90ºC without alcohol present. Does the interaction
described in Note 8 of the interaction matrix example, peroxide
decomposition, pose a safety hazard?
More
2. More alcohol
charged
Excess alcohol will result in
incomplete reaction. This
will result in a more difficult
isolation of product.
Less
3. Less alcohol
charged
If less alcohol is charged, the catalyst
may degrade the hydrogen peroxide.
If there is excess peroxide, maybe
there will be benzylic oxidation of the
starting material or product.
If excess peroxide causes benzylic oxidation, and it is very rapid and
exothermic, excess peroxide may be a candidate scenario for the
design basis of the emergency pressure-relief system for the reactor.
Part of
4.Poor agitation
(intention is to
have good mixing
for the reaction)
The alcohol is not miscible
with the aqueous catalyst.
Good agitation must be provided on
scale-up for this solid/liquid system.
Reverse
5. Alcohol charged
before peroxide or
other components
The reverse addition of alcohol would
actually be more desirable from a
safety standpoint, if it were feasible
within the scope of the reaction.
Investigate the possibility of charging alcohol before peroxide
with this catalyst system.
As well as
6. Contaminants
Contaminants could have a
deleterious effect.
As contaminants are identified, their effects should be checked.
Other
7. Other chemicals
Charging a base at this point might
lead to rapid peroxide decomposition.
When the manufacturing site is identified, other chemicals that might
be present should be checked for interactions.
No
8. No sodium
tungstate charged
The desired reaction will not occur.
There is a possibility of some sort of
runaway decomposition of peroxide
if improper measures are taken to
“save the batch.”
Upset studies will need to be run in a reaction calorimeter,
the Mettler-Toledo RC-1, to measure the rate of heat evolution.
Investigate what happens if the reaction mixture is heated to 90ºC
without sodium tungstate. Does the peroxide/alcohol
decomposition described in Note 7 of the interaction matrix example
become more likely?
More
9. More sodium
tungstate charged
Excess catalyst will result in a faster
reaction, raising self-heating issues.
What if the catalyst is charged twice?
Investigate effects of double overcharge.
Less
10. Less sodium
tungstate charged
See “No sodium tungstate charged”
(Deviation No. 8).
Part of
11. Part of sodium
tungstate charged
Similar to “No sodium tungstate”
(Deviation No. 8).
Reverse
12. Charge order
Incorrect addition order may affect
the catalyst activity.
Investigate order of addition in lab.
As well as
13. Contaminant in
the catalyst
Contaminants could have a deleterious
effect on the reaction.
Need to study consequences as potential
contaminants are identified.
Other
14. Other materials
Any other metal species accidentally
charged could be catastrophic, leading to
peroxide decomposition.
Same as No. 7. This possibility should be taken quite seriously. There
are several passivation options that can be used to reduce this
possibility. This will need to be detailed in the operating instructions.
Are engineering controls necessary? What is the worst case?
No
15. No PTC charged According to the literature reviewed
so far, the reaction will be significantly
slower without a PTC. However, other
pathways of peroxide decomposition
may be acting. Self-heating could
be an issue.
Investigate if the absence of the PTC makes
the peroxide/alcohol decomposition
described in Note 7 of the interaction matrix
example more likely.
More
16. More PTC
charged
Investigate the impact of a double charge. (Note: This type of error is
surprisingly easy to make in manual pilot plant or plant operations.
For example, at shift changes, an operator gets distracted by other job
activities before he writes it on the batch record, and forgets to tell
the next shift operator.)
The reaction may run faster.
Less
17. Less PTC
charged
See “No PTC charged”
(Deviation No. 15).
Part of
18. Not applicable
Reverse
19. Charge order
May affect catalyst activity.
As well as
20. Contaminants
See Deviation No. 6 above.
Other
21. Other chemicals See Deviation No. 7 above.
No
22. No peroxide
charged
No reaction will occur.
Investigate what happens if peroxide is
added rapidly and late.
More
23. More peroxide
charged
Excess hydrogen peroxide might result in
benzylic oxidation. It also might leave
residual peroxides in the product
streams.
Investigate the benzylic oxidation reaction. If it occurs, is it
rapid and exothermic? This may be a candidate scenario for
the design basis of the emergency pressure-relief system for
the reactor in a pilot or commercial plant. Investigate effect of
excess peroxides in the workup. We may need extra safeguards.
Less
24. Less peroxide
charged
Incomplete reaction will occur.
Investigate how sensitive our reaction is to peroxide concentration,
if the analysis of peroxide content is off.
Part of
25. Poor agitation
(intention is to have
good mixing)
Could cause it to look like more
hydrogen peroxide needs to
added to the reactor.
See Deviations Nos. 4 and 23 above.
Reverse
26. Reverse order of See Deviation No. 5 above.
addition
As well as
27. Contaminants in
the peroxide
Other
28. Other chemicals See Deviation No. 7 above.
No
29. No heating
No heating could be a problem,
if we slowly feed the alcohol into the
reactor. If a significant portion of the
alcohol has been fed, and then heat
is applied, the reaction could take off.
Commercial plant design may need
safeguards against feeding alcohol at
low temperatures.
More
30. More heat
(T > 90°C)
Overheating could create more
byproducts, may cause side reactions,
or result in a faster reaction.
If peroxides get into the vessel overheads,
they should decompose
in the vapor phase (“Bretherick’s”
hints that many peroxides do this).
This is a possible candidate for the
scenario to be used as the reactor
emergency-relief system design basis.
Check on all of the mentioned effects
at higher temperatures.
Less
31. Less heat
(T < 90°C)
See Deviation No. 29 above.
Part of
32. Poor agitation
(intention is to have
good mixing for this
reaction)
Localized heating gradient — more
byproducts?
See Deviation No. 4 above.
More
33. More rust
Rust is probably not compatible
with any part of this reaction.
Discuss materials of construction
alternatives with appropriate specialists.
No
34. No agitation
Essentially, no reaction will occur
without mixing. A large and rapid
exotherm could occur if the agitator
is started in the middle of the
alcohol feed.
This scenario is also a potential candidate
for the design basis scenario for the reactor
emergency-relief system design.
More
35. More agitation
Too much agitation should not be a problem.
Less
36. Less agitation
See Deviation No. 34 above.
If there are contaminants in the
incoming hydrogen peroxide, they must
not be bad actors as far as peroxide
decomposition goes. Could they affect
the catalyst system?
Investigate order of addition and
corresponding catalyst activity in the lab.
Investigate effects of using commercialgrade hydrogen peroxide with different
stabilizers present.