Irreversible thermodynamics - a tool to describe phase transitions far from global equilibrium

In the study of multi-component mass transfer it is common to use the film model, in which all the resistance to mass transfer across a phase boundary is assumed to be localized in two films on both sides of the interface. At the phase boundary one assumes that the temperature and chemical potentials are continuous, i.e. there is equilibrium across the interface. The coupling of a possible heat flux to the mass fluxes across the interface is neglected. Using irreversible thermodynamics for heterogeneous systems, we show bow to write flux equations for heat and mass transfer across these films, but without these assumptions. The system as a whole can be very far from global equilibrium, but there is local equilibrium everywhere, including in the interface. The interface is introduced as a separate thermodynamic system between other phases. Irreversible thermodynamics then gives a consistent and complete description of phase transitions in systems that are in local equilibrium, but are far from global equilibrium. The interface features as an "additional film" between the two films in the phases on both sides of the interface. We present how one can systematically set up the complete description, in which the three "films" sum to one effective film. The practical use of the description is indicated. (C) 2003 Elsevier Ltd. All rights reserved.