Nonabsorbable gas effects on heat and mass transfer in falling film absorption.

摘要

Absorption of water vapor in a liquid film is an important process in a proposed solar cooling system. Film absorption involves simultaneous heat and mass transfer in the gas-liquid system. The heat of absorption gives rise to temperature gradients leading to the transfer of heat, while the temperature affects the vapor pressure-composition equilibrium at the interface between the two phases, which in turn influences the mass transfer. The composition of the gas phase, in which a non-absorbable gas is combined with the absorbate, also influences the combined heat and mass transfer. The non-absorbable gas, in conjunction with the absorbate, is transported by convection and diffusion toward the liquid-gas interface. Since the interface is impermeable to the non-absorbable gas, the concentration of the non-absorbable gas at the interface is significantly greater than that in the bulk of the gas resulting in a reduction in absorbate mass transfer. An experimental program has been utilized to study this non-absorbable gas effect on the combined heat and mass transfer in film absorption.;The experimental absorber consists of a vertical stainless steel tube over which an aqueous lithium chloride absorbent flows in a thin film. The film is in contact with a binary mixture of air (non-absorbable gas) and water vapor (absorbate). The heat of absorption is removed by cooling water flowing countercurrently within the vertical tube. The gas and vertical tube are contained within a glass tube in which the total pressure is maintained at approximately 10 Torr. The non-absorbable gas concentration was varied from 0 to 10% by volume while the heat and mass transfer effects of the other control variables were also studied for typical operating conditions.;The experimental results indicate that significant increases in the heat and mass transfer rates can be obtained by reducing the non-absorbable gas concentration to levels approaching 0%. For Re = 60, the mass transfer rate is enhanced 35% by reducing the non-absorbable gas concentration from 5 to 0.5%. The heat and mass transfer coefficients are correlated using the film Reynolds number, Prandtl number, Schmidt number, Lewis number, and air concentration. The effects of other operational parameters, such as the inlet absorbent concentration and temperature, the tube wall temperature, and the absorber total pressure, on the heat and mass transfer rates are also reported.