Influence of Heat and Mass Transfer Resistances on the Separation Efficiency In Molecular Distillations

A general theoretical analysis of unsteady evaporation of a binary mixture Into a partial vacuum is presentedwhich accounts for the resistances to heat and mass transfer in the bulk liquid as well as the kinetic constraints on mass exchange at the vapor-liquid Interface. In particular, the coupling between interfacial cooling and surface depletion of the more volatile component is explicitly Included in the analysis. Calculations reveal that separation factors approach thermodynamic and kinetic limits only at low temperatures and correspondingly low distillation rates. At higher temperatures, the separation factor decreases sharply to values which may be nearly half the theoretical maximum. These trends are in complete agreement with the experimental results of numerous Investigators. Computations also reveal that surface depletion Is the dominant factor responsible for this reduction In separation factor with Increasing temperature. Surprisingly, Interfacial cooling partially compensates for the effect of surface depletion. If Interfaclal cooling Is Ignored separation factors may be underestimated by 25% or more. © 1984, American Chemical Society. All rights reserved