Stability analysis in multicomponent drying of homogeneous liquid mixtures

A stability analysis of the ordinary differential equations describing the process of convective gas-phase-controlled evaporation during drying is per formed. Isothermal and non-isothermal as well as batch and continuous dryin g processes are considered. For isothermal evaporation of a ternary mixture into pure gas, the solutions of the differential equations are trajectorie s in the phase plane represented by a triangular diagram of compositions. T he predicted ternary dynamic azeotropic points are unstable or saddle. On t he other hand, binary azeotropes are stable when the combination of the sel ectivities of the corresponding components is negative. In addition, pure c omponent singular points are stable when they are contained within their re spective isolated negative selectivity zones. Under non-isothermal conditio ns, stable azeotropes are characterized by presenting maximum temperature v alues. Loading the gas with one or more of the components up to some value leads to a node-saddle bifurcation, where a saddle azeotrope and a stable a zeotrope coalesce and disappear. The continuous drying process yields simil ar results for both flat and annular geometries. The singular points, in th is case, are infinite and represent dynamic equilibrium points whose stabil ity is mainly dependent on the inlet gas-to-liquid flowrate ratio. As this ratio grows to infinity, the phase portrait changes and the process approac hes a batch behaviour so that the stability analysis for that case may be a pplied. The present stability analysis permits the prediction of trajectori es and final state of a system in a gas-phase-controlled drying process. (C ) 1999 Elsevier Science Ltd. All rights reserved.