Velocity distribution and shear rate variability resulting from changes in the impeller location in the USP dissolution testing apparatus II

Purpose. The United States Pharmacopoeia (USP) imposes strict requirements on the geometry and operating conditions of the USP Dissolution Testing Apparatus II. A previously validated Computational Fluid Dynamics (CFD) approach was used here to study the hydrodynamics of USP Apparatus II when the impeller was placed at four different locations, all within the limits specified by USP. Method. CFD was used to predict the velocity profiles, energy dissipation rates, and strain rates when the impeller was placed in the reference location (centrally mounted, 25 mm off the vessel bottom), 2 mm off-center, 2 mm higher, and 2 mm lower than the reference location. Results. Small changes in impeller location, especially if associated with loss of symmetry, produced extensive changes in velocity profiles and shear rates. Centrally located impellers, irrespective of their off-bottom clearance, produced non-uniform but nearly symmetric strain rates. The off-center impeller produced a more uniform but slightly asymmetric strain rate distribution. Conclusions. The system hydrodynamics depends strongly on small differences in equipment configurations and operating conditions, which are likely to affect significantly the flow field and shear rate experienced by the oral dosage form being tested, and hence the solid-liquid mass transfer and dissolution rate.