Designing and operating safe chemical reaction processes HSG143

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Health and SafetyExecutiveSelecting and specifyinga basis of safety126 Take the opportunity at this stage to review the process chemistry and plantrequired. Try to use some of the principles discussed earlier to design an inherentlysafer process. You need to try and reduce the number, complexity and cost of‘add-on’ safety features and increase the integrity of any features you do use. Askyourself the following questions:(a) Is it possible to eliminate hazardous raw materials, process intermediates orby-products by using alternative chemistry?(b) Is it possible to substitute less hazardous raw materials?(c) Have all in-process inventories of hazardous materials in storage tanks beenminimised?(d) Has all processing equipment handling hazardous materials been designed tominimise inventory?(e) Is process equipment located to minimise the length of hazardous materialpiping?(f) Is it possible to generate hazardous reactants in situ from less hazardous rawmaterials, rather than to have to store them on site?(g) For equipment containing materials that become unstable at elevatedtemperatures or freeze at low temperatures, is it possible to use heating andcooling fluids that limit the maximum and minimum temperatures attainable?(h) Can reaction conditions be made less severe by using a catalyst?(i)(j)Can the equipment be designed to totally contain the maximum pressuregenerated?Can equipment be designed so that it is difficult or impossible to create apotentially hazardous situation due to operating error?(k) Can process units be located to reduce or eliminate adverse impacts fromother adjacent hazardous installations?(l)Can semi-batch processes be modified to reduce accumulation of reactants?(m) Has the heating effect of the surroundings been considered under failureconditions when working below ambient temperatures or using reactorheating?This list is not exhaustive, but it should give you a flavour of the thinking you canadopt in trying to produce an inherently safer process.127 In many situations it will be necessary to provide additional safety measures.These can be of two types:(a) preventive, ie they reduce the likelihood of a hazardous situation occurring; andDesigning and operating safe chemical reaction processes Page 26 of 64
Health and SafetyExecutive(b) protective, ie they reduce the consequences of a hazardous situation.128 Whatever the basis of safety selected it is important that you consider:(a) all foreseeable hazards; and(b) that there are suitable and sufficient safety systems in place to reduce theoverall risk to a level that is as low as reasonably practicable.Preventive measures129 Preventive measures relate to your defined safe operating envelope. Theseare the means by which you control the desired chemical reaction. They includemeasures to control:(a) Temperature: the safety margin between the temperature at which anundesired exothermic reaction starts and the operating temperature.(b) Reactant addition: to prevent addition of the wrong reactant; or the correctreactant(s) too early, too late, too quickly, too slowly, in too much or too littlequantities, or in the wrong order.(c) Agitation: what happens when the agitator or circulation pump fails, is startedtoo late, is stopped or not turned on, or is operating at the wrong speed?(d) Other variables: used to control reactions including varying pH, conductivity,concentration and pressure, etc.130 Other measures include aspects of the design of the process and plantalready discussed, particularly the inherently safer options. But they also includethe engineering and procedural controls; the safety interlocks and emergencyshutdown systems. 15Controlling rates of reaction131 You can affect the heat balance by how you control the rate of reaction. Ifall the reactants are present in the reactor at the start of the process, and youthen start the reaction (eg by heating), then this is a batch reaction. The maindisadvantage of batch reactions is that because all the reactants are present,if something goes wrong (eg failure of cooling) then nothing can be done tocontrol the reaction. You must then rely on protective measures to mitigate theconsequences.132 If you add one or more reactants gradually during the reaction then theprocess is a semi-batch reaction. The rate of reaction is controllable by a numberof factors, in particular addition rate and temperature.133 In most cases, stopping the addition will not completely stop the reaction.Some temperature rise can be developed in the reaction mass because somereaction of accumulated reactants continues. The effect of this accumulation andits extent depends on the balance between reaction kinetics and addition rate. Itmay be possible to control addition rates and temperatures so that there is only asmall accumulation of unreacted material in the reactor. In this case the reactionmixture cannot reach an unsafe temperature. Isothermal calorimetry can be usefulin determining the optimum conditions for this.Designing and operating safe chemical reaction processes Page 27 of 64
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