THE USE OF AIR PERMEAMETRY FOR THE ASSESSMENT OF EXTERNAL SURFACE-AREA AND SPHERICITY OF PELLETIZED GRANULES

Permeametry surface areas which compared favourably with surface areas calculated from analysis of granule dimensions by microscopy and ring gap sizing were measured with a steady-state and a transient permeameter for beds of coarse (0.9-1.0 mm), spherically shaped, porous granules. The permeametry surface areas were calculated with an aspect factor pre-determined on beds of mono-dispersed steel balls and with an effective bed voidage calculated from the effective particle density of the granules. The permeametry methods were also found to be capable of giving significantly different surface areas between granule masses of similar granule size but of small differences in granule shape. Hence, the air permeametry principle is suitable for the assessment of the external surface area and the geometrical shape (sphericity) of granules. As test materials, six granule masses (microcrystalline cellulose-lactose mixtures) prepared by extrusion-spheronization were used.