Particle diffusional layer thickness in a USP dissolution apparatus II: A combined function of particle size and paddle speed

This work was to investigate the effects of particle size and paddle speed on the particle diffuisonal layer thickness happ in a USP dissolution apparatus II. After the determination of the powder dissolution rates of five size fractions of fenofibrate, including <20, 20-32, 32-45, 63-75, and 90-106 µm, the present work shows that the dependence of happ on particle size follows different functions in accordance with the paddle speed. At 50 rpm, the function of happ is best described by a linear plot of happ=9.91d−23.31 (R2 = 0.98) throughout the particle diameter, d, from 6.8 to 106 µm. In contrast, at 100 rpm a transitional particle radius, r, of 23.7 µm exists, under which linear relationship happ = 1.59r (R2 = 0.98) occurs, but above which happ becomes a constant of 43.5 µm. Thus, happ changes not only with particle size, but also with the hydrodynamics under standard USP configurations, which has been overlooked in the past. Further, the effects of particle size and paddle speed on happ were combined using dimensionless analysis. Within certain fluid velocity/particle regime, linear correlation of happ/d with the square‐root of Reynolds number (dϖ/υ)1/2, that is, happ/d=1.5207−9.25×10−4(dϖ/ν)1/2 (R2 = 0.9875), was observed.