Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutiveProtective measures146 Protective measures mitigate the consequences of a hazard. They are rarelyused on their own. Some preventive measures are usually present to reduce thedemand on the protective system. Protective measures include:(a) containment within the reactor system;(b) emergency pressure relief or venting, and dumping;(c) crash cooling;(d) reaction inhibition;(e) drown-out and quenching; and(f) secondary containment.147 All the protective measures, with the exception of containment and venting,normally rely on a control system to operate them. Therefore, the safety integrityof the protective measure and the integrity of the control system 17,18 are bothimportant in its selection.148 As with all other aspects of safe chemical processes, it is important thatthe design of any protective measure is adequate. For this to happen you mustunderstand the runaway reaction process in detail. An undersized vent on a reactorwill not totally protect the reactor against damage. As the protective measure is thelast line of defence, it needs designing to protect against the worst case scenario.Assessing the worst case scenario149 The worst case scenario is the one in which a credible combination ofequipment failures and process maloperations occur which places the highestdemand on the protective system. It therefore depends on the basis of safetychosen.150 For example, if emergency relief is the chosen basis of safety then the worstcase scenario is that which results in the largest vent area being required to protectthe reactor. If the basis of safety is to be containment then the worst case scenariois that which results in the highest final pressure and temperature.151 The information you need to characterise the worst case scenario will varywith the protective measure selected. It may include:(a) the worst rate of heat production at runaway;(b) the worst rate of gas evolution and pressure development at runaway; and(c) the maximum pressure and temperature developed in a closed vessel when arunaway occurs.152 The problems associated with measuring heat generation and heat lossdescribed previously also apply to the worst case scenario. For realistic simulationof the reaction under runaway conditions you need to measure these parametersusing an adiabatic calorimeter with low thermal mass.Containment within the reactor153 In this case the design of the reactor system should withstand the maximumpressure generated by the runaway reaction.Designing and operating safe chemical reaction processes Page 30 of 64