A NUMERICAL CONVECTIVE-DIFFUSION MODEL FOR DISSOLUTION OF NEUTRAL COMPOUNDS UNDER LAMINAR-FLOW CONDITIONS

The convective-diffusion model reported earlier by Shah and Nelson (J. Pharm. Sci., 64 (1975) 1518-1520) described the dissolution of neutra l compounds under laminar flow conditions. A linear velocity profile o ver the tablet surface was assumed to allow calculation of an analytic al solution. In the present study, dissolution under laminar flow cond itions was modified to include the actual parabolic velocity profile. The modified model was solved numerically using finite difference appr oximations. The numerical solution to the model predicted dissolution rates to be within 10% of experimental values for benzocaine, a neutra l compound, and for benzoic acid and naproxen at suppressed ionization conditions. The modified model predictions were slightly better at hi gh flow rates (100 ml/min) when compared to the earlier model. Overall , however, the Shah-Nelson simplified assumption was found to predict the experimental results as closely as the numerical solution. Using t he same apparatus as described earlier by Shah and Nelson (1975), the flow conditions in the device were characterized over the entire range of flow rates considered (1.10-110 ml/min). Flow was found to be in t he laminar region from Reynolds number calculations and to be fully de veloped, devoid of entrance effects, before reaching the tablet surfac e. As expected, the concentration boundary layer thickness grew with d istance from the leading edge of the tablet, but stayed within the dim ensions of the flow chamber even at the lowest flow rates employed (1. 10 ml/min).