Emergency Relief And Containment For Hazardous Processes

Keith Middle of Chilworth Technology describes work done for FMC on the design of systems for FMC's butyl lithium plant (The Chemical Engineer, 2001, Dec, p32). The process involves producing an activated form of lithium and reacting this with butyl chloride in an exothermic semi-batch process. Although the reaction is dose-controlled, it can stall at lower temperatures leading to significant accumulation. Density differences in the reagents may cause layering in the case of poor agitation or agitation failure. The heat of reaction is such that an adiabatic temp rise of 1000^o would arise in case of an "all in" batch. The onset temp for decomposition was also not far from the reaction temperature and resulted in considerable gas release. It became clear that no emergency relief system would cope with substantial levels of reactant accumulation and therefore the basis of safety for control of potential runaway reactions was essentially prevention - a high integrity protection system was designed to back up the existing process control. The pyrophoric nature of the product is a potential problem in case of a fire from external sources, and previous plants have had a relief system based on discharge to a ground level burning pit, where the pyrophoric materials would burn in a semi-controlled manner. The revised design was a specially-designed quench tank containing mineral oil as the quench fluid. A key component of this unit is the sparger, which may have to cope with an evolving fluid stream, may be 3-phase and be of varying pressure - it also must not block! A schematic representation is shown below.