Fractionation during condensation of vapor mixtures

Partial condensers of the reflux type are capable of producing substantial fractionation of vapor mixtures. For vapor products having low dew points they may be preferred to conventional equipment for adiabatic distillation plus total condensation. Experimental studies of a condenser used to fractionate mixtures of ethylene dichloride and toluene show that observed mass transfer effects are in agreement with the Colburn‐Drew film theory, which accounts for the influence of the net mass exchange on the relative mass transfer rates of the components. In an adaptation of the transfer‐unit concept to partial condensation, new equations are derived relating the number of transfer units to liquid‐ and gas‐phase resistances, surface area, and amount of condensation. Observed resistances to mass transfer agree qualitatively with values independently measured; the diffusional resistance of the liquid phase has a pronounced effect on the fractionation, which does not conform to the Rayleigh theory of equilibrium differential condensation. Partial condensers appear, to require more interracial area than adiabatic distillation equipment in order to produce the same fractionation at the same heat load; for the same interfacial area condensers require more heat removal. Although only the system ethylene dichloride‐toluene was investigated here, the results of this work are believed pertinent binary liquid mixtures also. Knowledge of the vapor‐liquid equilibrium relationship of the new system and of the liquid and vapor‐phase transfer coefficients is required in order to employ the calculation methods introduced here.