(343d) Velocity Distribution and Shear Rate Variability Resulting From Changes in the Agitation Speed in the USP Dissolution Testing Apparatus II

In the pharmaceutical industry, dissolution testing is a critical step in quality control and a standard method for assessing batch-to-batch consistency of solid oral drug delivery systems, such as tablets. One of the most widely used test devices is the UPS apparatus II (paddle). Even to this day, dissolution testing remains susceptible to significant error and test failures. Previous studies, including our earlier work, indicate that poor reproducibility of dissolution testing data and inconsistency of dissolution results can arise from the complex hydrodynamics present in the unbaffled, hemispherical-bottom, agitated vessel that constitute the UPS apparatus II.

In previous work, our group investigated the system's hydrodynamics under different operating conditions. The objective of this study is to compare those results with those obtained when the USP apparatus II is stirred at different speeds, i.e., at 50 rpm (N_Re=4939), 75 rpm (N_Re=7409) and 100 rpm (N_Re=9878). The velocity distribution profiles in the USP apparatus II dissolution vessel were obtained via Laser Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD), respectively. The κ-ω model was applied in all CFD simulations. A comparison of the numerical predictions against the experiments data is provided. The strain rate was also predicted by CFD at three agitation speeds mentioned above.

The velocity distribution profiles from LDV measurements were found to be in substantial agreement with those from the CFD simulations. The flow patterns did not change appreciably when the agitation speed was increased from 50 rpm, to 75 rpm and 100 rpm. At the same location, the non-dimensional velocity profiles (scaled based on the impeller tip speed) were typically very similar to each other irrespective of the agitation speed, although difference were observed and predicted near the vessel bottom, indicating that increasing the agitation speed does not necessarily produce a proportionally higher velocity near the bottom of the vessel where the tablet is usually located.