Control of runaway reactions

Emergency Relief Systems to control Runaway Reactions are designed taking into account the occurrence or a two-phase release. Catastrophic accidents such as those of Bhopal, Seveso, Griesheim and others, proved that emergency relief systems should not only prevent vessels from bursting but also provide a reliable release containment system. A two-phase release of the vessel inventory sprayed over the neighborhood is not acceptable. The size and cost of containment systems is such that one cannot afford to accommodate the worst credible scenario. Runaway reactions may occur in various pieces of equipment: reactors, distillation column boilers, pipes, high inventory storage tanks. Therefore much effort is devoted to the design of safe processes and safe process conditions. These are accomplished by the reduction of runaway scenarios using high integrity interlocks, the prevention of runaway decompositions or polymerizations, and the reduction of inventories of reactive, flammable and toxic chemicals. This results in the optimization of both the process and the size of its emergency relief system. Runaway reaction hazards are investigated through literature searches, thermal stability screening and experimental investigation of runaway reaction scenarios for normal and abnormal process conditions. Thermal stability screening is achieved using DSC/DTA methods. The heat of reaction in normal process conditions and the possible controlling reactant accumulation are obtained from reaction calorimetry. The systematic investigation of runaway reaction scenarios is best achieved using low phi-factor pseudo-adiabatic techniques such as the Vent Sizing Package, where the process conditions / process deviations are reproduced. Possible decompositions are initiated under temperature scan or isothermal exposure conditions. VSP tests provide both information on the severity of the runaway reaction and the data necessary for vent sizing. Various processes / process conditions are compared considering their possible runaway scenarios. The safer process / process conditions may be selected on this basis. A wide range of process deviations may be controlled by high integrity safety interlocks which greatly reduce the probability of occurrence. The residual runaway reaction hazard is controlled using emergency relief systems. (author)