PROCESS CONFIGURATIONS OF A CRYSTALLIZER AND THEIR INFLUENCE ON CRYSTAL SIZE DISTRIBUTION

Crystal size distribution depends on process configuration of a crystallizer. The latter is the set of constraints imposed on suspension inside of an apparatus and at its withdrawal. The following four internal configurations are distinguished and two-letter English abbreviations are given to them: ideally mixed suspension (MS), fluidised bed (FB), imperfectly mixed suspension (IS) and imperfectly mixed liquid (IL). For the internal configuration MS the following four idealised external configurations are discerned: mixed product removal (MP), fines removal (FR), double draw-off of suspension (DD) and classified product removal (CP). Also combinations of them are used in practice, that is, FRDD, FRCP and FRDDCP. Withdrawal functions for crystals leaving a crystallizer as well as for the crystal product are defined and expressed in terms of the appropriate mean residence times of suspension and crystals. Population balances of crystals for the external configurations of the internal configuration MS, and their combinations, are formulated. For the simplest kinetics of secondary nucleation and crystal growth (obeying MCCABE DELTA-L law) the formulae for population density of crystals inside of the crystallizer and in the product are given. Integration of these distributions yields equations for mean mass weighted crystal size, coefficient of variation of the product as well as density of crystals in the crystallizer and in the product suspension. The last ones are then transformed into design equations. These provide an estimation of the effect. of design or operating variables on crystallizer performance at given productivity and volume when nucleation and crystal growth kinetics is known. Design equations of DD and FRDD configuration are presented for the first time. The use of equations is illustrated by numerical examples reflecting the effect of a separate configuration choice.