(388f) Impact of Operating Conditions On Physical Properties in an Agitated Filter Dryer

Drying is often a bottleneck and in an effort to minimize drying times, there is often fiscal pressure to select aggressive drying conditions. Nonetheless, the extreme conditions can have a major impact on the properties, and quality of the material thereby affecting further downstream operations. The study investigates the effect of critical process parameters (wall temperature, impeller speed, pressure, and bed depth) on the drying performance in an agitated filter-dryer with effect on physical properties (particle size, shape, specific surface area, roughness) as the output attributes. The variation of the process parameters in the range investigated revealed that the rate of solvent drying is enhanced with increasing wall temperatures (40 – 80 °C) and decreasing bed depth (28 – 74 mm) due to increased driving force and reduced resistance to heat transfer respectively. The effect of agitation speed (5 – 25 rpm) was variable for both glassbeads and lactose monohydrate suggesting that the influence of operating parameter is dependent on the material properties of the compound under study. This change in the effect of speed was mainly attributed to frictional behavior which was demonstrated from flow dynamics using the velocity profiles. The velocity profiles lactose showed the heap formation leading to formation of a recirculation zone due to high friction coefficients. Because of low friction as in the case of glassbeads, recirculation zones were not created. Although mechanical agitation aids in improving heat transfer, consequences on particle size can be profound. Decrease in the particle size D[4,3] and specific surface area was evident with an increase in impeller speed and decrease in the bed depth due to increased collision frequency and reduction in the fill volume. It was concluded from the particle size distribution studies that attrition dominates the drying process when the loss of solvent drying rate was low and/or the shear rate was high. Under these conditions, the drying time was sufficiently long for particles to encounter countless particle-impeller and particle–particle collisions that can result in small fragments. Particle attrition was much higher towards the end of drying where solvent acting as a lubricant was reduced further. For higher dryer loads, the number of collisions decreased, stronger agglomerates were formed and thus agglomeration becomes more dominant. Particle morphology was greatly affected with fines being generated at higher impeller speeds. Thermograms obtained from conventional DSC scans showed endothermic peaks at 205 – 210 °C demonstrating the melting of α-lactose monohydrate. This was further confirmed by PXRD with specific diffraction peaks at 12.0–12.5°, 16.3–16.6° and 19.10–20.05° two-theta values which were consistent with those of α-lactose monohydrate. The knowledge of how particle properties affect the macroscopic properties is important for an effective operation, scale-up and design.