Direct pelletization in a rotary processor controlled by torque measurements. I. Influence of process variables

The aim of this study was to elucidate the feasibility of using torque measurement to control the end point of a wet pelletization process in a rotary processor at varying levels of friction plate rotation speed, air gap pressure difference, and batch size. A 1:1 mixture of lactose monohydrate (200 mesh) and microcrystalline cellulose (PH-101) was granulated into pellets in an instrumented laboratory scale rotary processor using water as aqueous binder liquid. A full factorial designed study was performed to investigate the influence of the friction plate rotation speed (600 and 1200 rpm), the air gap pressure difference (1 and 3 kPa), the torque increase (0.4 and 0.8 N.m) and the batch size (500 and 1000 g) on the pellet properties. All pellets produced were round and showed a narrow size distribution. The geometric mean diameter varied from 400 to 1900 microns with a good reproducibility. Increasing the batch size and the rotation speed led to smaller pellets, whereas a higher torque produced larger pellets. This study showed that the process can be controlled by means of the torque increase because it was possible to produce pellets of a reproducible size by stopping the liquid addition at a certain torque level.