Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutiveInitial assessments63 Research chemists and other research and development professionals playa fundamental role in the development of a safe process. They must make anin-depth investigation into the process chemistry and into the entire processthat may develop based on that chemistry. These professionals have manyopportunities to incorporate inherently safer design concepts. For example,they can choose inherently safer synthesis routes. See Appendix 2 for furtherinformation.Literature surveys64 The initial assessment you can carry out at this early stage involves a desktopstudy of the potential reactants and routes to a given product. A literature search ofthe chemicals to be used and the process chemistry can be useful, if it is thorough.65 Bretherick’s Handbook of reactive chemical hazards 22 and the National FireProtection Association’s Manual of hazardous chemical reactions 23 are useful texts.They give accounts of numerous previous incidents with many surprising andunexpected exothermic runaway reactions. However, the absence of particularinformation does not imply that no hazards exist.66 It is at this early stage that you can consider inherently safer synthesis routes.Examples of synthesis routes considered inherently safer include:(a) Avoiding highly exothermic reactions or thermally unstable reactants andintermediates.(b) Replacement of batch reaction processes with semi-batch or continuousprocesses. This reduces the quantity of reactant present and controlling theaddition rate may stop the reaction in the event of a hazard arising (seeparagraphs 131-135 on controlling reaction rates).(c) Reactions in water as opposed to those which proceed in a more hazardousorganic solvent (provided water is compatible with the reaction mixture).(d) The use of processes which are less sensitive to variations in operatingconditions, especially those defined as critical such as temperature or pressure.(e) Supercritical processing uses relatively non-hazardous materials such as wateror carbon dioxide as solvents, albeit at high temperatures and pressures.(f) The use of catalysts which can lead to less severe operating conditions, allowthe use of a less reactive reagent or the opportunity to reduce or eliminatea hazardous solvent.(g) Use of solvents to act as a heat sink. The addition of a higher boiling pointsolvent may prevent the reaction mass boiling and overpressurising thereactor. Alternatively, in some processes the use of lower boiling point solventsallows the removal of energy from the system in the form of latent heat ofvaporisation. You need to take care when selecting either of these options. Forexample, the solvent boiling point should be below any temperature at which adecomposition reaction can occur; and with low boiling point solvents, the heatcapacity of the reaction mass should be such that evaporation to drynesscannot occur.Designing and operating safe chemical reaction processes Page 16 of 64