Reaction / Product testing
This Technical Measures Document covers the identification and control of
reaction hazards and refers to relevant codes of practice and standards.
Related Technical Measures Documents are:
The relevant Level 2 Criteria are 22.214.171.124(35)d,
126.96.36.199 (38) e and 188.8.131.52(50).
Chemical reaction hazards may result from loss of control of an
exothermic chemical synthesis reaction, or an undesired reaction occurring
in the reaction mixture such as the decomposition of a chemical present. So
called 'runaway reactions' are normally associated with batch reactors
however an uncontrolled exotherm may occur in many types of equipment.
Storage vessels, batch distillation units and drying operations are some of
the more common types of process equipment where undesired exothermic
reactions have resulted in severe incidents.
Thermal runaway begins when the heat generated by a reaction exceeds the
rate at which heat is lost to the surroundings. The heat generation rate is
a function of temperature and chemical composition. Whilst the temperature
of the reactants may not directly constitute a major hazard, the pressure
developed as a result of thermal runaway in a vessel or other item of
process equipment can cause catastrophic failure of the equipment.
The operator should be able to demonstrate that it has evaluated the
potential reaction hazards of a process and carried out reaction hazard
studies as part of an overall hazard assessment of a process. This should
involve the following activities:
- Preliminary reviews
As a starting point for a reaction hazard assessment a literature
survey and some theoretical calculations are useful, but are no
substitute for chemical hazard testing. Thermochemical calculations
based on bond energies or heats of formation can be used to calculate a
heat of reaction or a decomposition energy. The structure of individual
molecules can be reviewed to identify potentially reactive groups such
as acetylenic compounds, peroxides and nitro compounds. An oxygen
balance of an organic compound such as propylene oxide can give an
indication of the chemical's propensity to decompose on heating.
- Screening tests
The simplest and most common apparatus used are Differential Scanning
Calorimetry (DSC) and Differential Thermal Analysis (DTA). These
apparatus use a sample size of a few milligrams and are used to for
purposes such as identifying at what temperature materials involved in a
reaction decompose and the possible effects of contaminants on thermal
- Worst case
To define the worst case foreseeable upset conditions consideration
of failure of equipment, instrumentation, utility supplies, etc is
required to identify the scenarios that could result in uncontrolled
process temperature rise and subsequent over pressurisation of process
- Adiabatic tests
Adiabatic calorimeters are used to mimic plant conditions and give
accurate data on rates of heat production and gas evolution under
runaway conditions. Commonly used apparatus are the Dewar Calorimeter,
the Vent Sizing Package, the Phi-Tec Calorimeter and the Reactive
Systems Screening Tool. The data from these tests is used to determine
the Time to Maximum Rate and whether the pressure developed is
sufficient to cause failure of the vessel or relevant process equipment.
The data can be used to calculate a vent size where appropriate. The
Time to Maximum Rate is the time taken from onset of the runaway to its
maximum rate of heat generation.
- Basis for safe operation
Where reaction hazard studies identify that a thermal runaway can
occur, the studies should clearly define the technical measures in place
to ensure the safe operation of the process. For a reaction the
important parameters of the reaction such as temperature, cooling
conditions and time of addition should be defined in order to maintain
the reaction within safe limits. The requirement for protective
measures, such as emergency relief or quench systems, should be
Chemical reactions are widely used in the process industries and are the
process operation commonly associated with thermal runaway. Surveys have
shown that the following types of reaction have been involved in incidents:
- Salt formation
The screening test programme and the adiabatic tests should show whether
the normal reaction or a secondary reaction or decomposition are capable of
over pressurising the reactor. The test work should be used as the basis for
determining whether additional protective measures need to be included in
the plant design such as :
- emergency pressure relief devices
- redundant systems / high integrity controls and trips
- emergency cooling
- quench systems
- reaction inhibition
Typically for a batch or semi batch reactor, failures such as cooling
failure, loss of agitation, addition rate of reactants, and reactant
temperature are considered to determine the worst case scenario. In deciding
whether emergency protective measures are appropriate it is necessary to
taken into account the time available to carry out emergency preventative
measures and rectify the situation. Where emergency relief devices are
employed, the toxic and flammable hazards of the materials in the process
may require additional features to limit the consequences of a release.
Depending on the severity of the hazard these may include liquid catchpots,
scrubbing systems, quench pools or secondary containment vessels.
Other process operations
The situations in which the handling of self-reactive materials may
result in thermal explosion are numerous. This can be due to the
decomposition of an energetic substance or due to contamination causing an
undesired reaction. Some of the situations common to the process industries
are given below:
In a batch distillation process loss of vacuum may result in increased
temperatures that could initiate a decomposition. Additionally batch
distillation residues can be prone to thermal explosion due to the thermal
ageing process that takes place during distillation. In 1993 a severe
incident occurred at a plant in southern Ireland when an operational change
to a batch distillation process resulted in a thermal explosion followed by
a large fire.
The slow decomposition of a reactive material in storage may cause an
increase in temperature over a period of leading to a thermal explosion.
This process can occur in a variety of situations from large scale storage
in vessels to small transport packages. Accidental contamination of
materials in storage has resulted in some significant incidents, notably the
Loss of temperature control in a powder drier may expose a self-reactive
powder to a temperature that causes it to decompose. Similarly when a drier
is shut down the thermal cladding may cause heat to be retained for a long
period and a powder build-up or residues may start to decompose after a
period of time.
Codes Of Practice relating to chemical reactions and reactive materials
- IND(G)254 Chemical reaction hazards and the risk of thermal runaway,
This document gives brief and concise guidance on the subject of
chemical reactors and the potential for thermal explosion. It details
the testing programme that should be implemented and the types of safety
measures that can be used.
- HS(G)131 Energetic and spontaneously
combustible substances : identification and safe handling, HSE,
This guidance note covers self-reactive materials and the circumstances
in which these materials can be hazardous. It deals with the handling of
them in a wide range of situations from batch distillation through to
laboratory storage of materials.
Designing and operating safe chemical reaction processes, HSE, 2000
This guidance is aimed at those directly responsible for the
development, design and operation of chemical plant and processes,
particularly process chemists and process engineers.
- CS21 Storage and handling of organic
peroxides, HSE, 1991.
This document deals specifically with the handling and storage of
organic peroxides and the hazards that can arise. It gives guidance on
the safety measures required for the safe storage of organic peroxides.
- The Fire at Hickson and Welch, HSE, 1994.
Paragraph 110, Lesson 1 states that where the batch distillation of
highly energetic materials is carried out still residues should be
analysed, monitored and removed to prevent possible build-up of unstable
- Recommendations on the Transport of Dangerous Goods, Tests And
Criteria, (2nd Edition) United Nations New York, 1990.
This guidance document details the classification of self reactive
materials, the test methods for determining the self accelerating
decomposition temperature (SADT), and the packaging requirements for
Further reading material
- Chemical reaction hazards, Barton and Rogers, IChemE, 1998.
- Barton and Nolan , 'Incidents in the chemical industry due to
thermal-runaway chemical reactions', Hazards X, IChemE Symposium Series No
- Handbook of Reactive Chemicals, Bretherick, L, Butterworths, London,
- Control Of Exothermic Reactions, Video, IChemE, 1991.
- Guidelines for Chemical Reaction Hazard Evaluation - The Association of
the British Pharmaceutical Industry.
Case studies illustrating the importance of Reaction / Product testing