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A chemical reaction that goes out of control and runs away can create a serious incident with the risk of injury to people and damage to property and the environment. This leaflet:
The advice is aimed at small and medium-sized companies in the chemical manufacturing industry, although the principles apply equally to all firms.
During the manufacture of a chemical, raw materials react together to give the product. Such a chemical process often releases energy, in the form of heat, and the reaction is described as exothermic. A reaction may be exothermic even if you have to heat the reaction mass initially to get the reaction started.
This web-leaflet concentrates on the hazards arising directly from exothermic chemical reactions. There are other hazards associated with chemical manufacturing - those arising from handling toxic or flammable chemicals and general hazards such as noise or working at heights. These are beyond the scope of this leaflet.
An exothermic reaction can lead to thermal runaway, which begins when the heat produced by the reaction exceeds the heat removed. The surplus heat raises the temperature of the reaction mass, which causes the rate of reaction to increase. This in turn accelerates the rate of heat production. An approximate rule of thumb suggests that reaction rate - and hence the rate of heat generation - doubles with every 10°C rise in temperature.
Thermal runaway can occur because, as the temperature increases, the rate at which heat is removed increases linearly but the rate at which heat is produced increases exponentially. Once control of the reaction is lost, temperature can rise rapidly leaving little time for correction. The reaction vessel may be at risk from over-pressurisation due to violent boiling or rapid gas generation. The elevated temperatures may initiate secondary, more hazardous runaways or decompositions.
A runaway exothermic reaction can have a range of results from the boiling over of the reaction mass, to large increases in temperature and pressure that lead to an explosion. Such violence can cause blast and missile damage. If flammable materials are released, fire or a secondary explosion may result. Hot liquors and toxic materials may contaminate the workplace or generate a toxic cloud that may spread off-site.
There can be serious risk of injuries, even death, to plant operators, and the general public and the local environment may be harmed. At best, a runaway causes loss and disruption of production, at worst it has the potential for a major accident, as the incidents at Seveso and Bhopal have shown.
The scale on which you carry out a reaction can have a significant effect on the likelihood of runaway. The heat produced increases with the volume of the reaction mixture, whereas the heat removed depends on the surface area available for heat transfer. As scale, and the ratio of volume to surface area, increases, cooling may become inadequate. This has important implications for scale-up of processes from the laboratory to production. You should also consider it when modifying a process to increase the reaction quantities.
An analysis1 of thermal runaways in the UK has indicated that incidents occur because of:
These are some of the key factors you will want to consider in defining a safe process.
In order to deal with chemical reaction hazards first you need to identify them, then to decide how likely they are to occur and how serious their consequences would be. In other words, you need to carry out a risk assessment of your process. You are required to assess the risks that the process presents and, if you have five or more employees, to record the significant findings2.
A typical assessment will involve:
You should start the assessment as early as possible during the development of the process. The assessment should be sufficient to identify the potential hazards and to investigate their causes. Where possible, hazards should be avoided.
As the process design develops, foreseeable deviations from the normal process, such as equipment failure or operator error, should be considered. You may need to follow a structured method for identifying hazards, such as a hazard and operability study (HAZOP)3, particularly when the plant or processes are highly hazardous, complex or involve new technology.
In order to determine the hazards of a reaction, you need information on the chemistry and thermochemistry of the reaction. This includes:
As it is not safe to test unknown reactions in a full-size reactor, various techniques and tests have been developed to provide predictive data. The main methods are:
There is no standard procedure that can be followed for all reactions - the aim is to obtain the data you need to assess the risk adequately. To avoid undue time and effort, any investigation should reflect the complexity of reaction and the size of the risks involved. Further information on assessing reaction hazards is given in the References at the end of this leaflet.
It is important that the assessment of chemical reaction hazards, the selection of suitable test methods and the interpretation of results is carried out by competent, experienced personnel. It may not be cost-effective for a smaller firm to buy specialised test equipment and you may want to use a test house or consultancy.
HSE has published guidance on selecting a consultancy4. Trade associations and professional bodies may also be able to help. For example, the Institution of Chemical Engineers maintains a register of consultants5. Your insurers may also be able to provide technical advice to help you assess your chemical processes.
Once you know what the risks are, you can select the measures to ensure safe operation. You can ensure safe operation in a number of ways, by using:
Where possible, you should first eliminate or reduce hazards by inherently safer design. For example:
As the examples suggest, inherently safer methods can fundamentally affect the process - it will be easier to use such methods if you consider them in the early stages of process development.
Process control includes the use of sensors, alarms, trips and other control systems that either take automatic action or allow for manual intervention to prevent the conditions for uncontrolled reaction occurring. Specifying such measures requires a thorough understanding of the chemical process involved, especially the limits of safe operation.
Protective measures do not prevent a runaway but reduce the consequences should one occur. They are rarely used on their own as some preventive measures are normally required to reduce the demand upon them. As they operate once a runaway has started, a detailed knowledge of the reaction under runaway conditions is needed for their effective specification. You can:
The basis of safety for a chemical reaction is the combination of measures which are relied upon to ensure safe operation. The measures you choose for a particular case will depend on a number of factors, including:
Whatever methods you choose, they must cater for all cases that can foreseeably occur and reduce the risk of runaway to a level that is as low as reasonably practicable7.
In practice, you may not be able to eliminate all hazards by inherently safer methods and may choose to add control measures to further reduce risk and back these up with protection, such as a vent, to deal with the residual risk. Such a combination of methods is common. As a runaway incident may affect the environment, you should also consider whether your measures are adequate to comply with environmental law.
Your carefully selected safety measures may be ineffective if your operators do not know what to do if an emergency occurs. Safety measures have to be supported by appropriate management systems that deal with factors such as:
It does not need to be time-consuming or expensive to assess the risks of your chemical processes and to implement adequate safety measures. It is essential that you can demonstrate that you have carried out a suitable and sufficient assessment and that the systems in place reduce the risk of runaway to a level that is as low as reasonably practicable.
The effort you take to do this should reflect the complexity of the process and the scale of risks involved. Apart from complying with health and safety law, you can benefit by avoiding the disruption, costs and potential damage and injuries that a runaway may cause.
Have you adequately assessed the risks of your processes and, if appropriate, recorded the significant findings?
Do you consider inherently safer ways of operating when you develop a process?
Do you know the heats of reaction for the chemical reactions that you carry out?
Do you consider the effect of scale on heat transfer when transferring a process from the laboratory to the plant?
Have you assessed the safe operating limits of your process?
Do you know how the protective measures on your reactors have been designed?
Is the basis of safety for each of your reactions adequate to cope with the event, or sequence of events, that could produce the most harmful consequences?
Would you and your employees know what to do in an emergency?
Barton JA and Nolan PF 1989 Incidents in the chemical industry due to thermalrunaway chemical reactions Hazards X: Process Safety in Fine and Speciality Chemical Plants- IChem 115: 3-18
Management of health and safety at work. Management of Health and Safety at Work Regulations 1999. Approved Code of Practice and guidance L21 (Second edition) HSE Books 2000 ISBN 0 7176 2488 9
A Guide to Hazard and Operability Studies CIA 1992 The Chemical Industries Association, London
Need help on health and safety? Guidance for employers on when and how to get advice on health and safety Leaflet INDG322 HSE Books 2000 (single copy free or priced packs of 10 ISBN 0 7176 1790 4) Web version: www.hse.gov.uk/pubns/indg322.pdf
List of Consultants IChemE Books 165-189 Railway Terrace, Rugby, CV21 3HQ
Etchells JC and Wilday J Workbook for chemical reactor relief system sizing HSE Contract Research Report Number 136 ISBN 0 7176 1389 5 HSE Books. To be published in early 1998.
Health and Safety at Work etc Act 1974 The Stationary Office ISBN 0 10 543774 3
Further information on assessing reaction hazards can be found in:
Barton JA and Rogers RL 1995 Chemical Reaction Hazards 2nd
edition I Chem E ISBN 0 85295 341 0
Control of Exothermic Chemical Reactions HSE Video available
from CFL Vision, PO Box 35, Wetherby, Yorks LS23 7EX
Information on hazardous reactions can be found In:
Bretherick L 1995 Handbook of Reactive Chemical Hazards- (fifth edition) Butterworths London ISBN 0 7506 1557 5
Guidance for small firms on improving health and safety management can be found In:
Managing health and safety: Five steps to success Leaflet INDG275 HSE Books 1998 (single copy free or priced packs of 10 ISBN 0 7176 2170 7) Web version: www.hse.gov.uk/pubns/indg275.pdf
Further information may be obtained from your employers' or trade association.
HSE priced and free publications are available by mail order from HSE Books, PO Box 1999, Sudbury, Suffolk CO10 2WA Tel: 01787 881165 Fax: 01787 313995 Website: www.hsebooks.co.uk (HSE priced publications are also available from bookshops and free leaflets can be downloaded from HSE’s website: www.hse.gov.uk.)
For information about health and safety ring HSE's Infoline Tel: 0845 345 0055 Fax: 0845 408 9566 Textphone: 0845 408 9577 e-mail: hse.infoline@natbrit.com or write to HSE Information Services, Caerphilly Business Park, Caerphilly CF83 3GG.
This document contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.
This document is available at: www.hse.gov.uk/pubns/indg254.htm.
© Crown copyright This publication may be freely reproduced, except for advertising, endorsement or commercial purposes. First published 10/97. Please acknowledge the source as HSE.
Published by the Health and Safety Executive
INDG254 C100 10/97
Added to the web Site 07/08/98