The effect of intermittent mixing on the rate of drying of L-threonine in an agitated filter dryer

Intermittent mixing, alternating periods of agitation and no agitation, is often applied during the drying of active pharmaceutical ingredients in agitated filter dryers. Intermittent mixing is used as a strategy to obtain reasonable drying rates without obtaining unacceptable levels of particle attrition. In this work, the impact of intermittent mixing on drying of L-threonine was investigated experimentally. Two process parameters: the agitation ratio (proportion of the total time the agitator is active) and the intermittent cycle number (number of on/off cycles), determine the intensity and distribution of agitation. Increasing the agitation ratio from a static bed, the drying rate initially increased significantly, followed by a gradual increase at higher agitation ratios. Furthermore, the drying rate initially improved with increasing intermittent cycle number, but beyond a critical value reached a plateau. Thus, we observe that additional mixing beyond a critical value provides little benefit in terms of the drying rate but can be detrimental in terms of attrition. These findings provide understanding for development of intermittent mixing protocols.