Temperature diagrams for preventing decomposition or side reactions in liquid–liquid semibatch reactors

Abstract

The operation of an indirectly cooled semibatch reactor in which an exothermic reaction occurs is usually considered safe if the characteristic time of the coreactant dosing is much higher than the characteristic times of all the other phenomena involved (chemical reaction and mass transfer), so that the conversion rate is controlled by the coreactant supply itself. Such operating conditions imply a small accumulation of the coreactant in the system and are characterized by a temperature evolution which quickly approaches a target temperature and remains close to it throughout the dosing period, at the end of which the conversion is almost complete. The so-called boundary diagrams are useful tools for identifying safe operating conditions without solving the mathematical model of the reactor. However, avoiding accumulation phenomena can be not sufficient for classifying a set of operating conditions as thermally safe when the maximum temperature reached by the system under normal operation exceeds a maximum allowable temperature (which can be related either to safety problems, when dangerous decomposition reactions can be triggered, or to productivity problems, when side reactions can significantly lower the product yield above a given threshold temperature). In this work the boundary diagrams for the prevention of excessive accumulation conditions in liquid–liquid semibatch reactors are coupled with new diagrams, called temperature diagrams. These new diagrams, involving the same dimensionless parameters used for the representation of the boundary diagrams, allow determining—for a given set of operating conditions—the maximum temperature increase with respect to the initial reactor temperature which can be expected to occur during normal operation. This information can be compared with the maximum allowable temperature for the reacting mixture. Then the operating conditions can be verified through the boundary diagrams in order to reject conditions of excessive coreactant accumulation. Several temperature diagrams are provided for various kinetically or diffusion controlled reactions with different reaction orders and their use together with a general procedure for calculating them is presented.