A Contribution to the Design and Operation of an MSMPR Cascade for Cooling Crystallization

Crystallization is a fundamental unit operation for the isolation of intermediates or as the final product purification step in a chemical process. In the field of continuous crystallization, there exist numerous concepts of apparatuses for the production of crystalline products. One of them is the continuous stirred tank crystallizer, commonly referred to as Mixed Suspension Mixed Product Removal (MSMPR) crystallizer. This type of crystallizer is, in a sense, the “bestseller” among the industrial continuous crystallizers as it is characterized by a simple design which can be easily adapted to existing batch processes. Within the scope of this thesis, a four-stage cascade of MSMPR crystallizers was designed, set up and operated to produce L-alanine crystals aiming at an enhanced control of product quality in terms of crystal size and size distribution. The cascade built is characterized by a local separation of the crystallization phenomena nucleation and crystal growth as well as a unique vessel design in which lateral and vertical overflow tubes were installed to allow a gentle suspension transfer by gravity. For the design of the nucleation and growth zone within the cascade, in a first step fundamental experimental investigations regarding nucleation and growth kinetics were carried out. Controlled induction of nucleation was achieved by gassing with synthetic air. Based on the investigations on crystallization kinetics and the experiences gained from the continuous operation of the cascade, a guideline for the design of an MSMPR cascade was finally developed. This guideline is intended to show scientists and process engineers working in the field of crystallization the necessary steps for an efficient continuous crystallization process development and to facilitate the path towards the final operation.