Heat and Mass Exchange During Fluidized-Bed Coating of Tablets

Abstract

In the process of coating tablets in a fluidized-bed reactor, the coating material (typically an aqueous solution) supplied via a pneumatic injector distributes homogeneously over the surface of tablets owing to their intensive stirring. The shell grows in a cyclic regime, whereby a thin liquid film formed on tablets occurring in the injector torch zone is dried in the intermediate layer and then the cycle is repeated. Thus, the shell deposition and drying processes are combined, This process is usually described in terms of the heat and mass balance equations. However, use of these relations frequently leads to errors in determination of the process parameters, violation of the system stability, and the related decrease in the efficiency of apparatuses. The results of preliminary investigations showed that one of the main factors determining stability of the coating process is the kinetics of heat and mass exchange between tablets in the fluidized bed and a hot heat-transfer agent. The kinetics of the external heat and mass exchange is determined by changes in the density of water vapor and the temperature of the heat-transfer agent at the tablet surface. The density gradient creates a vapor flow directed from the surface to the core of the heat-transfer agent flow, while the temperature gradient provides for the heat supply to the tablet surface. A strict theoretical analysis of the external heat and mass exchange is based on the system of differential equations describing the transfer of momentum, energy, mass, and heat. The mutual influence of these process is taken into account by introducing the corresponding kinetic coefficients [1]. However, an analytical solution of the external heat and mass exchange problem with allowance for the mutual influence of various factors is extremely difficult to obtain because of difficulties encountered in the calculation of these kinetic coefficients [2]. For this reason, the analysis of heat and mass exchange processes requires using experimental data. In the course of coating, a tablet initially exhibits cooling because the liquid is evaporated at the expense of accumulated heat. Then, the evaporation of liquid from the surface proceeds at a constant temperature at the expense of heat supplied with the heat-transfer agent, after which the tablet is heated again. The molecules of water occurring on the tablet surface diffuse both to the stream of heat-transfer agent and, partly, inward the tablet. However, the latter diffusion proceeds at a significantly lower rate and can be ignored in comparison to the former process. Taking into account that the thickness of the liquid film formed in every wetting cycle is relatively small, we can assume that the rate of heat and mass exchange is determined entirely by the outdiffusion. Then, the process under consideration is described by the following equation: mc