(300f) Theoretical and Numerical Analysis of Critical Seed Loading in Batch Crystallization

Doki et al. (2002) introduced the idea of critical seed loading for batch crystallization. The idea is that for a given crystallization process there is a certain seed mass that will effectively suppress nucleation and lead to a product with a large averages crystal size and narrow size distribution. Furthermore, the critical seed loading ratio depends primarily on the average seed size and much less on other process variables. Doki et al. determined the critical seed loading chart experimentally and developed the following empirical expression for critical seed loading:

C_s*=2.17×10^-6L_s²

where  is the critical seed loading (initial seed mass divided by theoretical crystal yield) and L_s is the seed volume mean size in microns. Later authors (Hojjati et al. (2005) and Kubota et al. (2009)) determined critical seed loading ratios experimentally for other solute-solvent systems.

In this work, we show that critical seed loading charts can also be produced by numerical simulation based on kinetic models of crystallization processes. Results based on processes for the crystallization of potassium sulphate and potassium nitrate show good agreement with the correlation suggested by Kubota and coworkers. This further supports the utility of the critical seed loading concept for rapid process development and suggests that the critical seed loading can be determined for other solute-solvent systems using kinetic models already published in the literature.

Furthermore, we show that with certain reasonable assumptions it is possible to develop an implicit analytical expression for the critical seed loading as a function of seed mean size. These results are helpful for understanding the concept of critical seed loading intuitively and using it in process development.

 

References

Doki N, Kubota N, Yokota M, Chianese A. Determination of critical seed loading ratio for the production of crystals of uni-modal size distribution in batch cooling crystallization of potassium alum. J Chem Eng Jpn. Jul 2002;35(7):670-676.

Hojjati H, Rohani S. Cooling and seeding effect on supersaturation and final crystal size distribution (CSD) of ammonium sulphate in a batch crystallizer. Chemical Engineering and Processing 2005;44(9): 949-57.

Kubota N, Onosawa M. Seeded batch crystallization of ammonium aluminum sulfate from aqueous solution. Journal of Crystal Growth 2009;311(20): 4525-29.