Crystallization kinetics in an airlift and a stirred draft draft tube crystallizer; Secondary nucleation models revisited

In this research, a process model has been developed for an airlift and compared with that of a draft tube stirred crystallizer to clarify the crystallization kinetics in this novel type of crystallizer. Recently it has been shown that although secondary nucleation is strongly suppressed in this crystallizer, it is not completely absent and further development and scale up requires a more quantitative prediction of the kinetics in this type of crystallizer. A number of growth and nucleation models were examined in a parameter estimation study in which a dataset of a number of seeded batch experiments has been used to estimate the kinetics in both an airlift and a draft tube stirred crystallizer. It is shown that a kinetic model, consisting of a two-step growth model and two additive nucleation mechanisms, i.e. an attrition based and a surface nucleation mechanism, gives an excellent and statistically acceptable description of all studied experiments with one parameter set each for both type of crystallizers. The main difference in the two types of crystallizers being that the attrition terms by crystal-impeller and crystal-crystal collisions in the airlift crystallizer can be completely neglected. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.