Designing and operating safe chemical reaction processes HSG143

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Health and SafetyExecutive89 It is not possible to obtain all the information needed for your chemical reactionassessment from a single simple test. Figure 3 shows a useful assessment strategyflow diagram.90 Some of the screening tests are described briefly below. Further information isavailable. 19,30,31,32,33Differential scanning calorimetry91 You can use DSC to indicate the thermal stability of a reactant, reaction mixtureor product, and the heat of reaction or decomposition.92 The temperature trace of the sample indicates exothermic or endothermicactivity by means of peaks, troughs or discontinuities. The trace gives the totalamount of energy released (ie measuring the area under the trace determines theheat of reaction or heat of decomposition, etc) and an estimate of the rate of therelease (by measuring the slope of the peaks, etc). Both the total energy and themaximum rate of energy release are indicators of the degree of the hazard.Insulated exotherm test93 You can use this test to give early detection of the initial exothermicity. It ispossible for you to estimate thermo-kinetic parameters (eg the activation energyand the adiabatic self-heat rate) and to estimate how the initial temperature for selfsustainingreactions will vary with the quantity of material present.Decomposition pressure test94 You can use the DPT to determine the pressure characteristics and gasgeneration rates where they occur during a decomposition reaction.95 The output from the test is a pressure-time plot that you can use to determinethe volume of gas generated in the decomposition reaction.Carius (or ICI) sealed tube test96 A number of variations of this test exist. An oven temperature is increasedlinearly. Continuous monitoring of the temperature and pressure outputs froma sample tube in the oven provides qualitative information about the thermalcharacteristics of the sample.97 In many cases, the pressure data can also yield valuable information. Anydiscontinuity in a plot of 1n P against 1/T indicates non-condensable gasgeneration (the plot is often essentially a straight line if the pressure increase is duesolely to the vapour pressure).Interpreting screening data98 The selection and use of testing equipment and procedures, and particularlythe interpretation of the results, requires competent people. Some majorcompanies have their own testing facilities, but there are a number of testinghouses and consultancies available that you could use.99 The tests are useful for:(a) initial screening of a large number of samples;(b) testing samples over a wide temperature range;(c) samples only available in laboratory quantities;Designing and operating safe chemical reaction processes Page 20 of 64
Health and SafetyExecutive(d) samples taken from reaction mixtures at various stages of the process, to see ifthere is any change in mixture stability;(e) samples of distillation residues and recycled materials;(f) mixtures containing the reactants in varying proportions, to check the effect ofmischarging reactants; and(g) ageing effects - changes in stability may occur after prolonged periods atelevated temperatures.Note: see reference 30Figure 3 Assessmentstrategy flow diagramDesigning and operating safe chemical reaction processes Page 21 of 64
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