Condensation of mixed vapors and thermodynamics



A literal interface between thermodynamics and transport phenomena occurs in the steady-state condensation/vaporization of mixed vapors/liquids in wetted-wall towers. Unfortunately, several prominent transport texts assume that the ratio of molar fluxes is the same as the corresponding ratio of mole fractions at the interface for the phase being formed. This is shown to be true only at particular vertical positions corresponding to the first drop of condensate. The thermodynamics of the condensation/vaporization of mixtures is reviewed to understand why the ratio of pure-component fluxes at the interface differs from that of corresponding equilibrium mole fractions. The dynamics of steady-state condensation in wetted-wall towers is also developed based on the film theory of Colburn and Drew. The temperature/composition diagram is used to trace both transport phenomena and thermodynamic variables at various tower heights. The Ackermann factor is corrected allowing more general use of the design equations when the vapor is nonideal. The usual neglect of liquid-film resistance to heat transfer is examined. Numerical calculations show an unusual rise in the interfacial temperature in a parallel flow tower immediately below the onset of condensation. Possible and impossible sets of temperature and composition profiles are described.