(655d) Experimental and Computational Investigation of Breakage of Pharmaceutical Spherical Particles

By Shivangi Naik^1,a, Bodhisattwa Chaudhuri^1,b,  and Ramesh B. Malla^2,c

¹Department of Pharmaceutical Sciences, University of Connecticut, Storrs, CT 06269-3092

²Department of Civil & Environmental Engineering, University of Connecticut, Storrs, CT 06269-2037

Abstract

Particle size reduction of dry granular material by mechanical means, also known as milling or comminution, is undoubtedly a very important unit operation in pharmaceutical, agricultural, food, mineral and paper industries. As comminution is a stochastic and a non-linear process, an attempt was made to understand this complicated process by conducting parametric studies experimentally and computationally using Discrete Element Method (DEM). Studies were performed lactose spheres to understand the effect of hammer speed (rotational), feed rate, and hammer-wall tolerance in a hammer mill. The size and shape of the resulting progeny of particles were analyzed by sieve analysis and microscope/image analysis techniques respectively. The feed rate determines the hold up of material in sizing chamber and hence energy required for size reduction. Greater size reduction was observed at higher speeds and low feed rates owing to the greater centrifugal force experienced by the particles and longer mean free path lengths respectively. Particle shape analysis revealed fragmentation to be the dominant mechanism of size reduction at higher speeds. Increase in impeller wall tolerance resulted in accumulation of powder bed which was found to be significant at low impeller speeds. To develop an understanding of how grain breakage evolves in a granular system under impact and how it affects the  dynamics of the system, the breakage force for the material of interest viz. lactose non-pareils were determined. To obtain these values, single particle impact studies were performed using Dynamic Mechanical Analyzer. In this method, the granules were subjected to a quasi-static compression process and the force required to break the granule was recorded. The tests were performed on large sample size to obtain statistical significance. The flow and fragmentation of non-pareils were simulated using DEM, an explicit numerical technique scheme, which calculates interaction forces between grains for each grain-grain contact, and the resulting motion of each grain. The fragmentation of each spherical particle was simulated using the Grady’s Algorithm. The diameter of the resultant progeny particle was evaluated base on the fracture toughness of material, propagation velocity of longitudinal elastic waves in the material, and the induced strain rate.