Effect of drying method on agglomeration degree of crystalline products

- CSDs of same characteristics may result in more impure product due to higher agglomeration.
- Agglomeration degree distribution enables deeper insight in crystal agglomeration.
- Reduced agglomeration results in broader crystal size distribution.
- Fluid bed drying most effective method to prevent agglomeration.

In crystallization a high effort for optimization and process control is made to produce crystalline batches with required specifications, like purity or crystal size distribution (CSD). However, the final product is affected by solid-liquid separation and drying also so that for an efficient process optimization all unit operations have to be considered. Especially if a high temperature dependency of solubility exists, a special attention should be paid to the drying process. Previous studies show that classical static drying is not the best choice in this case, since long contact time between crystals lead to uncontrolled agglomeration. This event need not necessarily result in different characteristics of the crystal size distribution (CSD), but reduced purity.Therefore we investigate systematically two different drying methods - fluid bed and rotary tube drying - concerning behavior of CSD and agglomeration degree in dependence of drying parameters used. Additionally we show that with the aid of the so-called agglomeration degree distribution, which we developed before, a deeper understanding of crystal agglomeration within the CSD is gained. As model system l-alanine/water is used. The results show that the product quality designed by cooling crystallization cannot be entirely maintained, but the formation of agglomerates is reduced in case of both methods.