Impact of agitated drying on crystal morphology: KCl–water system

Abstract

An experimental procedure was developed to study agitated drying of crystals. The procedure quantifies the impact of drying conditions (temperature, agitation speed and vacuum) on crystal properties (size and shape distribution). KCl was used as a model compound. The morphology of the crystals was determined using light microscopy and image analysis. The analysis of the transient behavior of the crystal size distribution showed that there is a permanent competition between attrition and agglomeration during drying. However, these two phenomena only had a significant effect on the crystal size distribution when the moisture content was below a critical value. In particular, it was found that attrition dominates the drying process when the drying rate is low and/or the shear rate is high. Under these conditions, the drying time is long enough for particles to encounter many particle-impeller and particle–particle collisions that can produce small crystal fragments. For higher drying rates, which are obtained at high temperature and low pressure, the number of ‘collisions’ decreases, stronger agglomerates are formed and thus agglomeration becomes more dominant. Other physical phenomena, which can take place during drying, such as crystal redissolution, had no significant impact on crystal morphology.