Heat Transfer During Boiling of Mixtures in Free Convection

Abstract

Frequently, in industrial processing procedures, liquid mixtures of two or more components have to be evaporated in order to separate them from one another. Examples are the concentrating of solutions, the thickening of lyes, the recovery of solvents, the distillation of sea water for the production of drinking water, or the separation of substances by evaporation during distillation. Heat and mass transfer are closely connected with one another during the evaporation of multi-component mixtures, and the quantity of vapor produced usually is determined primarily by the mass transfer in contrast to the evaporation of pure substances. It is known from previous experiments that heat transfer coefficients during evaporation of mixtures can be substantially smaller than those of the pure components of the mixture. On the other hand, marked improvements of the heat transfer have been noted, if one of the components of the mixture is surface-active. This leads to a reduction of the surface tension and, thus, as was presented in Chap. 10, to a greater bubble density. Simultaneously, the bubble frequency and, in turn, also the heat transfer increases. Mixtures of organic or inorganic liquids contain, however, surface-active components only in certain cases (soaps, addition of wetting agents), so that one usually must reckon with a decrease of the heat transfer coefficient in comparison with that of the pure component.