제  목 : CHEMICAL REACTION HAZARDS and the risk of THERMAL RUNAWAY
  일  자 : 1998년 08월
  제공처 : Internet

      CHEMICAL REACTION HAZARDS and the risk of THERMAL RUNAWAY
      =========================================================

    ㅇ WHAT ARE CHEMICAL REACTION HAZARDS?
    ㅇ WHAT DO YOU NEED TO DO?
    ㅇ SAFETY MEASURES
    ㅇ SAFETY MANAGEMENT
    ㅇ CONCLUSIONS
    ㅇ KEY QUESTIONS
    ㅇ REFERENCES

  CHEMICAL REACTION HAZARDS and the risk of THERMAL RUNAWAY
  ---------------------------------------------------------

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:

     identifies the main hazards of carrying out chemical reactions;
     provides guidance on how you can ensure a safe operation; and
     identifies some sources of further information and guidance.

The advice is aimed at small and medium-sized companies in the chemical
manufacturing industry, although the principles apply equally to all firms.

  WHAT ARE CHEMICAL REACTION HAZARDS?
  -----------------------------------

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 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.

  Thermal runaway
  ---------------

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.

  Effects of thermal runaway
  --------------------------

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.

  Effect of scale
  ----------------

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.

  Causes of incidents
  -------------------

An analysis1 of thermal runaways in the UK has indicated that incidents occur
because of:

     inadequate understanding of the process chemistry and thermochemistry;
     inadequate design for heat removal;
     inadequate control systems and safety systems; and
     inadequate operational procedures, including training.

These are some of the key factors you will want to consider in defining a safe
process.

  WHAT DO YOU NEED TO DO?
  -----------------------

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.

  Chemical process risk assessment
  --------------------------------

A typical assessment will involve:

     defining the process, operating conditions and plant;
     identifying the hazards;
     evaluating the risks arising from the hazards and deciding whether
     existing precautions are adequate or more should be done;
     selecting and specifying appropriate safety measures; and
     implementing and maintaining the selected safety measures.

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.

  Evaluating reaction hazards
  -------------------------- -

In order to determine the hazards of a reaction, you need information on the
chemistry and thermochemistry of the reaction. This includes:

     the possibility of thermal decomposition of raw materials, intermediates,
     products and by-products;
     whether exothermic runaway can occur; and
     the rate and quantity of heat and gas produced by the reaction.

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:

     literature data and calculation, to give preliminary information;
     basic screening tests, such as differential scanning calorimetry or
     carius tube;
     isothermal calorimetry (mainly to measure reaction kinetics and heats of
     reaction);
     adiabatic calorimetry (mainly to examine runaways); and
     relief vent sizing tests.

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.

  SAFETY MEASURES
  ---------------

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:

     inherently safer methods, which eliminate or reduce the hazard;
     process control, which prevents a runaway reaction occurring; and
     protective measures, which limit the consequences of a runaway.

Inherent safety

Where possible, you should first eliminate or reduce hazards by inherently
safer design. For example:

     Replace hazardous materials with safer ones.
     Have less unreacted material in the reactor, eg using a continuous
     process instead of a batch reactor.
     Use a semi-batch method (in which one of the raw materials is added over
     time) instead of a batch process.
     Use a heating medium which has a maximum temperature that is too low for
     the reaction mixture to decompose.

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

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

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:

     design the plant to contain the maximum pressure; fit emergency relief
     vents and ensure vented material goes to a safe place6;
     crash cool the reaction mixture if it moves outside set limits;
     add a reaction inhibitor to kill the reaction and prevent runaway; or
     dump the reaction into a quenching fluid.

  Selecting the basis of safety

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:

     how easy it is to prevent runaway;
     how applicable the various methods are; and
     how compatible the measures are with plant operation.

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.

  SAFETY MANAGEMENT
  -----------------

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:

     operating and emergency procedures;
     consultation with employees;
     training and supervision of operators;
     maintenance of equipment; and
     control of modifications.

  CONCLUSIONS
  ------------

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.

  KEY QUESTIONS
  -------------

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?

  REFERENCES
  ----------

   1.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, IChemE Symp Ser No 115: 3-18
   2.Management of Health and Safety at Work Regulations 1992: Approved Code
     of Practice L21 1992 HSE Books ISBN 0 7176 0412 8
   3.A Guide to Hazard and Operability Studies CIA 1992 The Chemical
     Industries Association, London
   4.Selecting a health and safety consultant INDG133 1992 Free leaflet HSE
     Books
   5.List of Consultants IChemE Books 165-189 Railway Terrace, Rugby, CV21 3HQ
   6.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.
   7.Health and Safety at Work etc Act 1974 HMSO 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:

Five steps to successful health and safety management INDG132 1992 Free
leaflet HSE Books

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 6FS Tel: 01787 881165; Fax: 01787 313995.

HSE priced publications are also available from good booksellers.

For other enquiries, ring HSE's InfoLine, Tel: 0541 545500, or write to HSE's
Information Centre, Broad Lane. Sheffield S3 7HQ.

HSE home page on the World Wide Web: http:/www.open.gov.uk/hse/hsehome.htm

This publication may be freely reproduced, except for advertising, endorsement
or commercial purposes. The information is current at 10/97. Please
acknowledge the source as HSE.