Safe optimization of potentially runaway reactions: From fedbatch to continuous stirred tank type reactor

A great variety of processes of both fine chemical and pharmaceutical industries are commonly carried out in semibatch reactors (SBRs). Most of such syntheses often involve strongly exothermic and very fast reactions where the control of the heat evolution is achieved thanks to the combined effect of a dedicated cooling system and the feeding of a “cold” (mostly, ambient temperature) co-reactant. For these types of process, the well known phenomenon of “thermal runaway” can take place. Accordingly to the ever increasing market requests, the desired goal of whatever enterprise is to increase the productivity of a certain product being sure of maintaining safe operating conditions during all the synthesis steps. This is a very hard task which can be performed by changing the reactor operating mode from semibatch to continuous. A continuous reactor can achieve the same productivity of a discontinuous reactor using significantly lower reaction volumes; this means that the intrinsic safety of the process is increased. In this work, the switch from a semibatch reactor to a series of continuous stirred tank reactors has been investigated using as a case study the nitration of N-(2-Phenoxyphenyl) methane sulphonamide. Particularly, after the determination of the most effective number of reactors in the series and their effective volumes, dynamical simulations have been carried out in order to guarantee that possible thermal instabilities are not generated during the synthesis in both the start-up and the normal exercise phases.