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THE STORAGE, HANDLING AND
PROCESSING OF DANGEROUS SUBSTANCES
ELEMENT
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LEARNING OUTCOMES
On completion of this element, you should
be able to demonstrate understanding
of the content through the application
of knowledge to familiar and unfamiliar
situations and the critical analysis and
evaluation of information presented in
both quantitative and qualitative forms. In
particular you should be able to:
the main physical and chemical
 Outline
characteristics of industrial chemical processes.
the main principles of the design and use
 Outline
of electrical systems and equipment in adverse or
hazardous environments.
the need for emergency planning, the typical
Explain
organisational arrangements needed for emergencies
and relevant regulatory requirements.
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the main principles of the safe storage,
Outline
handling and transport of dangerous substances.
© RRC International Unit C – Element C4: Storage, Handling and Processing of Dangerous Substances
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Contents
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INDUSTRIAL CHEMICAL PROCESSES
Effects of Temperature, Pressure and Catalysts
Heat of Reaction
Examples of Endothermic and Exothermic Reactions
Methods of Control of Temperature and Pressure Revision Questions
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HAZARDOUS ENVIRONMENTS
Principles of Protection
Wet Environments
Selection of Electrical Equipment for Use in Flammable Atmospheres
Classification of Hazardous Areas and Zoning
Use of Permits-To-Work
Principles of Pressurisation and Purging Types of Equipment
Revision Questions
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STORAGE, HANDLING AND TRANSPORT OF DANGEROUS SUBSTANCES
Dangerous Substances and Hazardous Substances
Hazards Presented and Assessment of Risk
Storage Methods and Quantities
Storage of Incompatible Materials, Segregation Requirements and Access
Leakage and Spillage Containment
Handling of Dangerous Substances
Transport of Dangerous Substances
Labelling of Vehicles and Packaging of Substances
Driver Training and The Role of Dangerous Goods Safety Adviser
Revision Questions
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EMERGENCY PLANNING
Need for Emergency Preparedness Within an Organisation
Consequence Minimisation via Emergency Procedures
Development of Emergency Plans
Preparation of Major Accident Hazard Emergency Plans to Meet Regulatory Requirements
Preparation of On-Site and Off-Site Emergency Plans Including Monitoring and Maintenance
Revision Questions
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EXAM SKILLS
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SUMMARY4-44
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| Unit C – Element C4: Storage, Handling and Processing of Dangerous Substances
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Industrial Chemical Processes
KEY INFORMATION
• The rate of a reaction will usually increase with temperature and pressure. A catalyst will affect the rate of a
chemical reaction without being changed itself.
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• Endothermic reactions take place with absorption of heat and require high temperatures for their initiation and
maintenance, e.g. photosynthesis, or reaction of ethanoic acid with sodium carbonate .
• Exothermic reactions are accompanied by the evolution of heat, e.g. combustion, or the reaction of sodium and
chlorine.
• A runaway reaction is an exothermic reaction where the heat generated continues to increase the temperature,
accelerating the reaction out of control.
• The temperature of a chemical process can be controlled by cooling and stirring to ensure even distribution of
materials and no formation of “hot spots”. Over-pressure can be prevented by safety relief valves and/or rupture
discs.
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All chemical reactions involve changes in energy, usually
evident as heat. Reactive chemical hazards invariably
involve the release of energy in a quantity or at a rate too
great to be dissipated by the immediate environment of
the reacting system, so that destructive effects appear. It is
essential for a process designer to understand the nature
of the reactive chemicals involved in a process.
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Careful control of chemical reactions
© RRC International EFFECTS OF TEMPERATURE, PRESSURE AND
CATALYSTS
The rate of a reaction will increase exponentially with
increase in temperature; in practical terms an increase of
10°C roughly doubles the reaction rate in many cases. This
has often been the main contributory factor in cases where
inadequate temperature control had caused exothermic
reactions to run out of control.
Things can be made worse in closed systems at relatively
high pressures and/or temperatures. In high-pressure
autoclaves, for example, the thick vessel walls and generally
heavy construction necessary to withstand the internal
pressures implies high thermal capacity of the equipment.
Rapid cooling of such vessels to attempt to check an
accelerating reaction is impracticable, so bursting discs or
other devices must be fitted as pressure reliefs to highpressure equipment.
Substances which are highly reactive or unstable when
subject to heat, pressure, mechanical force or on contact
with other chemicals represent a potential source of
explosive energy. Acetylene, for example, has a tendency
to decompose exothermically and result in explosions or
detonations. The character and course of the explosion
depend upon many factors. Pressures in excess of 10 times
the initial pressure can result from acetylene explosions
and pressures above 50 times the initial pressure have
arisen with detonations.
GLOSSARY
CATALYST
A catalyst is any agent (usually a substance) which,
when added in very small quantities, notably affects
the rate of a chemical reaction without itself being
consumed or undergoing a chemical change. Most
catalysts accelerate reactions but a few retard them
(negative catalysts or inhibitors).
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The activity of a solid catalyst is often centred on a small
fraction of its surface, so the number of active points
can be increased by adding promoters which increase
the surface area in some way, e.g. by increasing porosity.
Catalytic activity is decreased by substances that act as
poisons which clog and weaken the catalyst surface, e.g.
lead in the catalytic converters used to control exhaust
emissions.
Catalysts can be highly specific in their application, and
are essential in virtually all industrial chemical reactions,
especially in petroleum refining and synthetic organic
chemical manufacturing.
There are many organic catalysts that are vital in the
metabolic processes of living organisms. These are called
enzymes and are essential, e.g. in digestion.
HEAT OF REACTION
Endothermic
• Acetylenic compounds.
• Alkyl metals.
• Azides.
• Boranes (boron hydrides).
• Cyano compounds.
• Dienes.
• Halogen oxides.
• Metal acetylides.
• Metal fulminates.
• Oxides of nitrogen.
Exothermic
An exothermic reaction is a reaction accompanied by the
evolution of heat, such as a combustion reaction.
Exothermic reactions must be carefully controlled and
monitored to ensure there is no failure of the cooling or
stirring systems. Quantities should be kept to a minimum
and suitable screening should be provided. No operation
of this kind should be entrusted to anyone apart from a
highly skilled and competent chemist knowledgeable in
the dangers involved and the precautions to be taken.
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Endothermic reactions take place with absorption of
heat and require high temperatures for their initiation and
maintenance. An example is the production of carbon
monoxide and hydrogen by passing steam over hot coke.
Examples of endothermic compounds are found in the
following groups:
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Light can act as a catalyst (although not a ‘substance’ as
such) in both the visible and ultra-short wavelengths, e.g.
in photosynthesis and other photochemical reactions.
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Most reactions are exothermic. They tend to accelerate
as the reaction proceeds unless the rate of cooling
is sufficient to prevent a rise in temperature. The
exponential temperature effect accelerating the reaction
will exceed the (usually) linear effect of falling reactant
concentration in decelerating the reaction. Where the
exotherm is large and cooling capacity is inadequate, the
resulting accelerating reaction may proceed to the point of
loss of control, and decomposition, fire or explosion may
result. Reactions at high pressure may be exceptionally
hazardous owing to the enhanced kinetic energy content
of the system.
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In the relatively few endothermic reactions, heat is
absorbed into the reaction product(s), which are thus
endothermic (and energy-rich) compounds. These are
thermodynamically unstable, because no energy would be
required to decompose them into their elements, and heat
would, in fact, be released. Most endothermic compounds
possess a tendency towards instability and possibly
explosive decomposition under various circumstances.
The rate of an exothermic chemical reaction determines
the rate of energy release; so factors which affect reaction
kinetics are important so far as possible hazards are
concerned.
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Many, but not all, endothermic compounds have been
involved in violent decompositions, reactions or explosions.
In general, compounds with significantly positive values for
their standard heat of formation can be considered suspect
on stability grounds. Values of thermodynamic constants
for elements and compounds are available, conveniently
tabulated, but also note that endothermicity may change
to exothermicity with increase in temperature.
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Runaway Reactions
Recycling of reactants, products or diluent is common in
continuous reactors. This may be in conjunction with heat
removal (in an external exchanger) which is an important
means of controlling the progress of the reaction. To
minimise hazards associated with chemical reactors it’s
important to have comprehensive data on the following
aspects:
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Reactive hazards can involve the release of energy in
quantities or at rates too high to be absorbed by the
immediate environment of the reacting system. In these
cases material damage occurs. The source of the energy
may be an exothermic, multi-component reaction, or
the exothermic decomposition of a single unstable
(often endothermic) compound. The presence of an
unsuspected contaminant or catalytic impurity may affect
the velocity or change the course of reaction.
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Important factors in preventing such thermal runaway
reactions are mainly related to the control of reaction
velocity and temperature within suitable limits. These
will include considerations such as:
• Adequate heating and particularly cooling capacity in
both liquid and vapour phases of a reaction system.
• Proportions of reactants and rates of addition (allowing
for an induction period).
• Physical.
• Chemical.
• Thermal.
• The effect of corrosion products and impurities.
• Thermal stability.
A dangerous runaway reaction is most likely to occur if all
the reactants are initially mixed together with any catalyst
in a batch reactor, where heat is supplied to start the
reaction.
• Use of solvents as diluents and to reduce viscosity of
the reaction medium.
• Adequate agitation and mixing in the reaction vessel.
• Use of an inert atmosphere.
Reactions may be:
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• Control of reaction or distillation pressure.
• In gas, liquid (neat or in solution, suspension or
emulsion) or solid phase.
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• Catalytic or non-catalytic.
• Exothermic, endothermic or negligible heat loss/gain.
• Reversible or irreversible.
There are two main methods of operation used in reactors:
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• Batch - where each chemical reaction is carried out
separately with fixed quantities of materials and when
the reaction is completed the process stops.
• Continuous - where the reactants flow into a vessel
and products flow out so that the reaction can operate
for long periods of time until the flow of reactants is
stopped.
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Continuous operation tends to predominate in largescale production. It has the advantage of low materials
inventory and less variation of operating variables.
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Some causes of runaway in reactors or storage tanks
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| Unit C – Element C4: Storage, Handling and Processing of Dangerous Substances
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TOPIC FOCUS
The conditions that might give rise to a ‘runaway
reaction’ include:
• A strongly exothermic reaction process.
• Inadequate provision of cooling.
• Catalysis by contaminants.
• Lack of temperature detection and control.
• Excessive quantities of reactants in the reaction
vessel.
• Photosynthesis - the process by which plants convert
carbon dioxide into organic compounds, especially
sugars - requires energy from the sun and is therefore
an endothermic reaction.
carbon dioxide + water +
sunlight energy
→
glucose + oxygen
6CO2 + 6H2O +
sunlight energy
→
C6H12O6 + 6O2
• Ethanoic acid, more commonly known as acetic acid,
reacts with sodium carbonate (washing soda) but
requires the input of energy to do so.
ethanoic acid +
sodium carbonate +
energy
→
sodium ethanoate +
carbon dioxide + water.
2CH3CO2H(aq) +
Na2CO3(s) + energy
→
2CH3CO2Na(aq) + CO2(g) +
H2O(l)
An exothermic reaction is a reaction accompanied by the
evolution of heat, such as a combustion reaction.
• Failure of mixing or agitation.
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EXAMPLES OF ENDOTHERMIC AND
EXOTHERMIC REACTIONS
GLOSSARY
Endothermic reactions require heat or energy to make
them proceed:
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Many chemical processes require equipment designed
to rigid specifications, with sophisticated automatic
control and safety devices. With some reactions, it is
important to provide protection against failure of cooling
media, agitation, control, or safety instrumentation, etc.
The reactor itself must be adequately designed for the
operating conditions, e.g. pressure, temperature, corrosive
environment.
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Chemical reactions are depicted as chemical
equations to explain what is happening.
In a chemical equation the tiny entities
(molecules) that react with each other are shown
as chemical formulae, e.g. H2O which represents
water, CO2 which represents carbon dioxide.
sodium + chlorine
→
salt + heat
Na(s) + 0.5Cl2(g)
→
NaCl(s)
Propane is an important fuel gas and reacts exothermically
with oxygen. It will burn in excess oxygen with the
generation of heat to form water and carbon dioxide.
propane + oxygen
→
carbon dioxide + water
+ heat
C3H8 + 5O2
→
3CO2 + 4H2O
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Sometimes more than one molecule of a substance
reacts with another one in a reaction so this is shown
by a number in front of the formulae, e.g. 6H2O
which represents six molecules of water.
Sodium is a reactive metal and chlorine is a powerful
disinfectant. Together they react so violently that flames
can be seen as the exothermic reaction gives off heat.
However the product of the reaction is common salt.
When carbon dioxide reacts with water in the
photosynthesis reaction we find that six molecules
of each react to form one molecule of glucose
(C6H12O6) and six molecules of oxygen.
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6CO2 + 6H2O + sunlight energy → C6H12O6 + 6O2
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As we have seen, many chemical reactions produce heat,
i.e. they are exothermic. The heat raises the temperature
of the reactants and causes the reaction to accelerate. The
larger the amount of material, the more heat is produced.
This is because heating depends on the volume of material
but cooling depends on the surface area exposed to the
environment. So, one way of controlling the temperature
is to keep the batch size small.
In most batch processes on an industrial scale, the reactor
needs to be cooled and stirred. Some safety devices could
be:
• Internal cooling coils (more responsive than an external
jacket).
• Sensor for rotation of stirrer blades.
• Limited feedpipe size for catalysts to limit possible
over-addition.
• Duplicated thermometers or thermostats.
• Automatic shut-off valves.
• Duplicated dump valves (in case of malfunction).
• Explosion reliefs (bursting discs, blowout panels).
However, over-pressure protection must still be provided,
regardless of the number of lines of defence and
depressuring systems in place. Emergency pressure relief
systems need to be designed for high reliability even
though they should only have to function infrequently, as
they are the last line of defence.
The relief system protecting heat exchangers and other
vessels must be of sufficient capacity to avoid overpressure in cases of internal failure. Most equipment
failures leading to potential over-pressure situations
involve the breakage or rupture of internal tubes and the
failure of valves and regulators.
Runaway temperature and pressure in process vessels can
occur as a result of many factors, including excessive feed
rates or temperatures, loss of cooling, feed or quench
failure, contaminants, catalyst problems and agitation
failure. The major concern is the high rate of energy
release and/or formation of gaseous products, which may
cause a rapid pressure rise in the equipment. In order to
assess these effects properly, the reaction kinetics must be
known or obtained experimentally.
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• Generally fail-safe equipment.
A further safety layer can be provided by using
depressuring or instrumented shutdown of key
equipment to control any over-pressure without activating
pressure relief devices. Relief devices may no longer be
reliable once used, and maintenance of them is often
sporadic, so this redundancy serves to minimise the
probability of such devices failing.
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METHODS OF CONTROL OF TEMPERATURE
AND PRESSURE
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Sometimes it is possible to moderate chemical processes:
• Refluxing: boiling removes heat, so that the boiling
point of a component or added solvent fixes and limits
the temperatures which can be reached.
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• Dilution: a large excess either of an inert additive or of
one component acts as a heat sink.
The most common method of over-pressure protection is
by safety relief valves and/or rupture discs which discharge
into a containment vessel, a disposal system, or directly to
the atmosphere.
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Continuous processing avoids many of the problems
associated with large inventories and runaway reactions.
Frequently the reason for using them is related to poor
mixing between phases, leading to slow processing.
Tubular reactors, perhaps working under intensified
conditions, limit the inventory, are highly reliable and can
be isolated in sections, reducing the consequences of a
malfunction. These reactors lend themselves to vapour
phase reactions which often run much more smoothly.
Chemical process control
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All process designs should aim to produce an inherently
safe facility - i.e. one where a worst-case event cannot
cause injury or damage to people, equipment or the
environment. Safety features that are built-in at design
stage, rather than added on later, together with use of
high-integrity equipment and piping, provide the first
lines of defence against the effects of over-pressure and
subsequent rupture.
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MORE…
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Information on identifying the main hazards of
carrying out chemical reactions and guidance on
how to ensure a safe operation is contained in
INDG254 Chemical reaction hazards and the risk of
thermal runaway.
INDG254(rev1) is available at:
www.hse.gov.uk/pubns/indg254.pdf
© RRC International REVISION QUESTIONS
1. In general terms, how is the rate of a chemical
reaction affected by temperature?
2. What are the two key factors in controlling a
thermal runaway?
(Suggested Answers are at the end.)
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