Limits of mass transfer enhancement in film absorption

A method to calculate the theoretical limits of mass transfer enhancement in a falling film absorber using LiBr aqueous solution has been developed and it provides a means of comparative absorber performance evaluation. During gas absorption, due to the relatively small mass diffusivity in fluids, there exists a very thin but high concentration gradient layer beneath the liquid-vapor interface which hinders the absorption process and contributes most of the mass transfer resistance. Every mass transfer enhancement technique, passive or active, consists of basically disturbing the film and causing mixing of the fluid to break the mass transfer resistant layer at the liquid-vapor interface. The higher the mixing rate, the higher the mass absorption rate. The method solves one-dimensional heat and mass coupled equations to derive an asymptote of the mass transfer rate by increasing the mixing rate. Taking account of all relevant parameters such as temperatures, pressures, concentrations, mass flow rates, and geometry makes the method ideal for making case by case comparisons between existing advanced absorber surfaces. Results showed that for vertical grooved tubes an 70% of the asymptotic mass absorption rate was achieved and the corresponding mixing rate was 53Hz. The model presented in the paper is for vertical tube absorbers in which solution and coolant flow countercurrently. A model for horizontal tube absorbers has also been developed to cope with the cross flow nature.