Secondary effects in precipitation processes

Agglomeration of L-glutamic acid in stirred tank reactors

Crystals of sparingly soluble materials can be produced by precipitation, which is the rapid crystallization of a substance induced either by changes of the physical properties in the mother liquid or a chemical reaction in a solution upon mixing of the solution with another fluid.
In crystallization processes particle size and its distribution have a significant influence on the product properties. Thus, predictive and reliable models for the process must be developed. A number of mechanisms influence the particle size distribution: nucleation, growth, agglomeration, breakage and dissolution. The characterization of these mechanisms is a difficult task since several parameters can influence more than one of the mentioned mechanisms and they can be coupled. Therefore, the underlying mechanism must be revealed separately by suitable experiments.
In this study the effect of agglomeration on the final particle size distribution is investigated for batch precipitation processes carried out in stirred tank reactors. Agglomeration kinetics of L-glutamic acid was determined based on seeded batch experiments by combination with a population balance model and an integral parameter estimation technique. Different modeling approaches for the description of agglomeration are applied and assessed. The empirical model only takes into account the influence of supersaturation and stirring rate on the agglomeration process, while it neglects size-dependencies. In the more rigorous modeling approaches the agglomeration kernel is decomposed into a size-dependent collision fre-quency and an agglomeration probability. Computational fluid dynamics (CFD) is used to model the turbulent flow in the stirred reactor and to extract information about the shear rate distribution, which in turn can be used to incorporate the dependence of the agglomeration kernel on the local shear rate. The population balance model accounting for nucleation, growth and agglomeration is used to predict the particle size distribution in precipitation experiments.

Publications

Lindenberg C., J. Schöll, L. Vicum, J. Brozio, M. Mazzotti: L-glutamic acid precipitation: agglomeration effects. Crystal Growth & Design, 8, 224-237, (2008).