Direct contact heat transfer with change of phase: Evaporation of discrete volatile films from the surface of a stagnant immiscible liquid

Abstract

The transfer efficiency of spray columns and other three phase exchangers, in which volatile matter evaporates (or condenses) while submerged in the non-volatile phase, is adversely affected by the hydrostatic head. This undesirable effect on the temperature driving force may be overcome by utilizing systems in which the evaporating matter is allowed to float on an immiscible non-volatile liquid. This work is a first attempt to test the practicality of such an exchanger. Lower bound values of the heat-transfer coefficients are obtained from a quasi-steady state solution for an evaporating lens-shaped drop placed on a semi-infinite stagnant liquid medium. Experimental work confirms the theoretical prediction that the rate of change of the radius of the lens with time is a Δ T dependent constant. However, the measured evaporation rates, hence the transfer co-efficients, are found larger than those calculated. This is taken as an indication that interfacial convection plays an important role in processes of this type even at low temperature driving forces. The experimental transfer coefficients obtained in this work, compare favourably with those reported in the literature for evaporation of volatile liquids in disperions.