Hygroscopic absorption to falling films: The effects of the concentration level

The aim was to investigate analytically the influence of the concentration of the absorbate in a liquid film on the process of vapor absorption into the film. The theoretical approach is an extension of earlier work on isothermal absorption and on non-isothermal absorption under conditions of low concentration and short contact time. Following presentation of a model for isothermal absorption, a model for non-isothermal absorption is developed which considers simultaneous heat and mass transfer. The main assumptions in the model are short exposure time, uniform film thickness and constant physical properties. In contrast to the low-concentration case, the governing diffusion equation is nonlinear with respect to absorbate concentration; an analytical solution for the equation was established; it leads to a pair of algebraic equations for isothermal absorption and to a set of four algebraic equations for non-isothermal absorption. Computer-calculated numerical solutions are presented for both cases. Theories based on the assumption of low concentration can lead to inaccurate mass-transer rate estimation; it was found that the mass-transfer rate increases with concentration, so that boundary layer growth is faster in a high-concentration film than in one with low concentration. Coupling between heat transfer and mass transfer leads to reduced mass transfer in the non-isothermal case.