STABILITY OF THE DYNAMICAL SYSTEM DESCRIBING ISOTHERMAL GAS-PHASE-CONTROLLED DRYING OF TERNARY MIXTURES

ABSTRACT

A stability analysis of the ordinary differential equations describing the process of convective gas-phase-controlled evaporation during drying of ternary mixtures was performed. The case considered was that of a gas flowing parallel to the surface of a flat solid, wetted with a liquid mixture, during drying at constant gas conditions. If evaporation is isothermal, the solutions of a system of ODEs are the trajectories in the phase plane represented by a triangular diagram of compositions. Analysis of the ternary mixture showed that the predicted ternary dynamic azeotropes were unstable nodes and composition trajectories moved away from them. On the other hand, some of the binary azeotropes could be stable nodes. Nodes represented by pure components could be either stable or unstable. Loading the gas with one of the components might give rise to additional critical points and might reverse or change the stability of the previous ones. The present stability analysis permits the prediction of the trajectories and final state in a gas-phase-controlled drying process. It is of great importance due to the critical influence of the composition of the remaining moisture on the quality and properties of the final product.