Table I: Differences between foamed and liquid binder addition on granulation.
A study at pilot-scale flow rates (20-40 kg/h) compared foamed-binder addition and direct liquid-injection on granulation
(6). A methylcellulose binder (Dow, Methocel A15 PLV) was used at two concentrations, 6% and 11% (w/w), relative to a-lactose
monohydrate powder. Two screws were tested in the work to produce differing axial compression characteristics (which was mentioned
previously as an important factor for granule growth inside the extruder) with changing flow rate: one with a single pair
of mixing elements producing lower axial compression (LAC) and a second with two pairs in series to provide a more restrictive
flow path and higher axial compression (HAC). Notable differences between the two methods of granulation are summarized in
Table 1. The granule properties from the study showed that comparable sizes and intragranular porosity were achieved by either method,
provided appropriate conditions were used. The reduced requirement for liquid in the process was a comparable finding to that
found with high-shear batch mixers (15).
Wet granulation in twin-screw extrusion machinery has several key advantages over conventional methods, but to advance in
acceptance for GMP production, its operations need to be better understood and challenges regarding process stability need
to be solved. Continuous foam granulation is a new, robust technique that solves the process- surging issues that relate to
poor powder wetting by conventional, liquid-addition methods. The high spreading tendency of foam in granulation, versus the
immediate soaking nature of liquids, produces more uniformly wetted powders and increases the overall lubricity of the process,
which benefits wear behavior of the machine and minimizes dissipative heating of the product. With comparable particle properties
to conventional wet granulation, foam granulation gives formulators greater flexibility in achieving production goals.
Michael R. Thompson is associate professor in the department of chemical engineering at McMaster University, Hamilton, Ontario, L8S 4L7, Canada; Tel 905-525-9140, firstname.lastname@example.org
Articles by Michael R. Thompson