Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutive52 To additionally illustrate the effect of scale up on the heat balance consider anexothermic reaction where the heat of reaction is -416 kJ kg -1 of reactant mass;the specific heat of the reaction mass is 1.26 kJ kg -1 K -1 ; and the reaction is carriedout over one hour with a jacket-to-contents temperature differential of 80 o C. Thedifferences between laboratory scale and plant scale are:Laboratory scalePlant scale(100 ml) (4500 litres)Heat loss (W kg -1 K -1 ) 2.3 0.04Cooling rate (K hr -1 ) 520 9Adiabatic temperature rise (K) 330 330(See paragraph 79)Result No exotherm seen Possible runawayAn explosion occurred during the manufacture of a phenolic resin by a phenolformaldehydereaction catalysed by caustic soda in a 5.9 m 3 reaction vessel. Theexplosion ruptured the vessel and caused considerable damage to the building.The incident occurred because of insufficient cooling capacity on scaling up theprocess. The reaction process was scaled up from 2.3 m 3 to 5.9 m 3 withouttaking due account of the reduction in vessel cooling capacity. This resulted inan increased reaction temperature that caused a runaway exothermic reaction.The safe operating envelope53 What the designer is trying to achieve is a controlled chemical reaction processthat avoids a hazardous situation from occurring. The safe operating envelopedefines the boundaries that contain the controlled reaction.54 It is inherently safer if you develop processes with wide safe operatingenvelopes. These are less sensitive to variations in critical operating parameters.For example, controlling your process within a very small temperature band toavoid hazardous conditions means that the process has a narrow safe operatingenvelope. A process with a larger temperature band will have a wider safeoperating envelope.55 Discussion about the determination of the safe operating envelope occurs inparagraphs 123-125.Designing and operating safe chemical reaction processes Page 14 of 64