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:
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:
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.
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 stability.
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 equipment.
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.
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 detailed.
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:
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 :
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.
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 Bhopal incident.
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.
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 115, 1989.
Handbook of Reactive Chemicals, Bretherick, L, Butterworths, London, 1990.
Control Of Exothermic Reactions, Video, IChemE, 1991.
Guidelines for Chemical Reaction Hazard Evaluation - The Association of the British Pharmaceutical Industry.