Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutive100 The results of the screening tests will give you a preliminary indication of:(a) the possibility of thermal decomposition;(b) the quantity and rate of heat release;(c) gas evolution;(d) induction time effects (autocatalysis) - ie the development of thermal instabilityafter prolonged storage; and(e) high-rate decompositions (showing that a substance could deflagrate).101 Often the initial deviation in the temperature trace is quoted as the initialexotherm temperature or the onset temperature (see Glossary). You must take carein interpreting these temperatures. A number of factors affect the temperature thatare unrelated to the inherent chemical stability of the test material. These include:(a) the sensitivity of the test device;(b) the sample size;(c) the experimental heating mode and heating rate (eg the onset temperatureincreases as the rate of heating of the sample increases);(d) heat losses from the sample container;(e) reactions with the sample container; and(f) sample vaporisation prior to decomposition.An explosion occurred in a process vessel involving a thermally unstablematerial. Published values of the onset temperature for thermal decomposition inlaboratory tests were in the range 270-300 o C. However, following the incident,further investigation indicated that the material would decompose at around150 o C on the plant scale.102 What you have achieved at this point in the assessment process is a goodintroductory understanding of your chosen chemical process and the hazardsinvolved. You may now have sufficient information to define the safe operatingenvelope for full-scale plant, particularly if you are going to operate at temperatureswell below the onset temperature for runaway or decomposition when related tothe operating conditions.103 However, in a number of cases, you will need to carry out more detailed teststo determine the safe operating envelope.Reaction calorimetry104 It is possible for you to directly measure the instantaneous heat output of areacting system due to chemical or physical processes as a function of the processtime. This quantity shows directly whether and how quickly chemical conversionsoccur in the process phase under consideration. Such an approach can be useful,not only from a safety perspective but also for process design and optimisation.Designing and operating safe chemical reaction processes Page 22 of 64