Fluid and Particle Laser Doppler Velocity Measurements and Mass Transfer Predictions for the Usp Paddle Method Dissolution Apparatus

Abstract

This research was motivated by the lack of experimental data concerning the complex flow fields produced by the USP Paddle Method Dissolution Test Apparatus, which influence the reproducibility and sensitivity of the resulting dissolution data. A one-component He-Ne fiber optics laser Doppler anemometer and a conditional sampling computer data acquisition provided unique three-dimensional fluid velocity measurements in most regions of the fluid inside the vessel. Tangential velocities which are predominant in magnitude decrease as a function of distance from the paddle to the liquid surface. Low magnitude circulation patterns in the axial direction exist with little Interaction between the fluid above and below the paddle. The radial velocity component also exhibits a low magnitude. In the vicinity of the paddle, a periodic fluid motion produced by the wake formed behind the paddle exists. Close to the bottom of the vessel, tangential fluid velocities were approximately equal to solid body rotation created by the paddle, and make possible predictions of mass transfer rates from a non-disintegrating calibrator resting at the bottom of the vessel. The motion of drug particles from a dissolving tablet was simulated by using 216 /im diameter polystyrene latex spheres. They provided the Stokes number matching an average drug particle diameter of 190 /im. Tangential particle velocity measurements taken at selected locations using two different paddle speeds (50 and 60 RPM) revealed that the particles closely followed the fluid. A comparison between the fluid and the particle tangential velocities at 50 and 60 RPM showed that for 60 RPM the particle to fluid relative velocity above the paddle increased by about 100%, with the mass transfer rate in that region predicted to be enhanced by 37%.