The effect of agitated drying on the morphology of L-threonine (needle-like) crystals

Experiments have been carried out to study the behavior of L-threonine (needle-like) crystals during agitated drying. For an L-threonine/water system the morphology of the crystals was monitored using light microscopy and image analysis. Analysis of the transient behavior of the crystal size and shape distribution showed that attrition and agglomeration took place simultaneously during the process. The variation of the operating conditions (temperature, agitation and vacuum) revealed that attrition dominated the drying process when the drying rate was low and/or the shear rate was high. For high drying rates and low shear rates, agglomeration became dominant. This study suggests that crystal redissolution had no significant impact on crystal morphology. It was also found that due to their needle-like shape, large L-threonine crystals were very sensitive to attrition. When attrition controlled the drying process, most of the particle size reduction took place below a critical moisture content of 4%. When agglomeration controlled the drying process, most of the crystal size enlargement occurred below a moisture content of 6%.