Drying behaviour of two sets of microcrystalline cellulose pellets.

The objective was to study contraction and densification of two sets of microcrystalline cellulose pellets, prepared using water (W) or a 25/75% w/w water/ethanol (W/E) mixture, during drying. The pellets were dried on microscope slides, photographed and weighed at set times. The porosity of the dry pellets was determined by mercury pycnometry. From pellet size, weight and porosity data, contraction and densification of the pellets and the relationship of these to the liquid content of the pellets during drying were calculated. Both types of pellets contracted and densified during drying. The initial porosity was similar for both types, but the final porosity of the dry pellets was higher for the W/E pellets. Thus, the difference in final pellet porosity between the two types was caused by a difference in densification during drying rather than a different degree of densification during the pelletisation procedure. The contraction rate and the relationships between contraction and the volume of removed liquid, and contraction and the degree of liquid saturation differed between the two types of pellet. The difference in drying behaviour between the two types of pellets can be explained by a liquid related change in both contraction driving force and contraction counteracting force or by a different contraction mechanism.