Thermodynamics of the multicomponent vapor-liquid equilibrium under capillary pressure difference

We discuss the two-phase multicomponent equilibrium, provided that the phas e pressures are different due to the action of capillary forces. We prove t he two general properties of such an equilibrium, which have previously bee n known for a single-component case, however, to the best of our knowledge, not for the multicomponent mixtures. The importance is emphasized on the s pace of the intensive variables P, T and mu (i), where the laws of capillar y equilibrium have a simple geometrical interpretation. We formulate thermo dynamic problems specific to such an equilibrium, and outline changes to be introduced to common algorithms of flash calculations in order to solve th ese problems. Sample calculations show large variation of the capillary pro perties of the mixture in the very neighborhood of the phase envelope and t he restrictive role of the spinodal surface as a boundary for possible equi librium states with different pressures. (C) 2001 Elsevier Science B.V. All rights reserved.