Roll compaction of a pharmaceutical excipient: Experimental validation of rolling theory for granular solids

Roll compaction is widely used in industry to produce free flowing agglomerates from a fine particulate feed. Two of the main advantages of this process are that it is dry and continuous. Despite being superficially a simple process, a quantitative understanding has proved difficult to develop because of the complex behaviour of particulate materials. Sub-optimal design and operation of the equipment can lead to unsatisfactory products. Johanson (1965, ASME, Journal of Applied Mechanics Series E, 32(4), 842–848) developed a theoretical model that enables the surface pressure, torque and separating force of the rolls to be predicted from the physical characteristics of the powder and the dimensions of the rolls. However, a detailed experimental validation of the theory has yet to be accomplished. The current paper describes such a study using a gravity fed instrumented roll press and a microcrystalline cellulose powder. The measured pressure profiles in the nip region of the roll press were comparable to the calculated values. The theory was also found to predict the effect of material properties on the nip angle and the peak pressure but it was unable to account for the influence of roll speed.