Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutiveHazard assessment ofchemical reactions56 The primary aim of this publication is to give you an understanding of thephilosophies and stages you should go through when designing a safe chemicalreaction process. It does not cover all aspects of this process but concentrates onthe hazards that may be generated during the chemical reaction.57 The three major chemical reaction hazards you need to consider are:(a) the thermal instability of reactants, reactant mixtures and products (includingany intermediates, potential contaminants and by-products);(b) exothermic reactions which raise the temperature to produce decompositionreactions or violent boiling; and(c) gas evolution.58 The approach taken relies upon the life cycle concept 15 discussed earlier(paragraph 27) and the risk assessment you must carry out to comply with healthand safety regulations.59 A typical safety assessment procedure will include:(a) defining the chemistry, the process operating conditions and the process plant;(b) identifying the hazards;(c) assessing the consequences, in terms of their severity and likelihood;(d) selecting and specifying a basis of safety; and(e) implementing and maintaining the safety measures.60 For new processes and plant, beginning this assessment procedure at theearliest stages of research and development allows you to achieve the highestlevel of inherent safety. At this stage you can also minimise the costs of safetyprecautions. For existing plant and processes, you should consider your riskassessment to ensure you have taken all necessary steps to reduce risks to as lowas is reasonably practicable.61 Chemical reaction hazards need assessing by technically qualified peoplewho have some experience of hazard evaluation and a good knowledge ofprocess chemistry. Ideally they should be part of an independent team to avoidconflicts of interest between production and safety. However, they do needgood communications with plant staff in order to minimise the chance of anymisunderstandings.62 The depth of the hazard assessment depends on the nature and magnitudeof the hazards and on the scale of operation and its complexity. For example, itmay be easy to show for some processes that they are unable to runaway fromknowledge of the process chemistry combined with small scale screening tests.For others, you will need to do more detailed and accurate tests. There may belittle justification for accurately measuring the rate of temperature rise of a runawayreaction if the process is only being operated at the gram scale in the laboratory.Designing and operating safe chemical reaction processes Page 15 of 64