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 permeame ter for beds of coarse (0.9-1.0 mm), spherically shaped, porous granul es. The permeametry surface areas were calculated with an aspect facto r pre-determined on beds of mono-dispersed steel balls and with an eff ective bed voidage calculated from the effective particle density of t he 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 granul es. As test materials, six granule masses (microcrystalline cellulose- lactose mixtures) prepared by extrusion-spheronization were used.