Passive enhancement of ammonia-water absorption by the addition of surfactants

Abstract

Surface-active agents or surfactants have the potential to substantially enhance heat and mass transfer in ammonia-water absorption by reducing the surface tension of the working fluid. The enhancement is caused by interfacial turbulence at the vapor-liquid interface that results from surface tension gradients. A surfactant selection criterion is developed based on the plateau value of surface tension, critical concentration, and the critical Marangoni number required to initiate interfacial turbulence. Based on this criterion, surface-active agents, and their ideal concentrations for the enhancement of ammonia-water absorption are recommended. The preferred additives are found to be 500 PPM of 1-octanol or 2-ethyl-1-hexanol. A heat and mass transfer model is developed to predict the performance of a falling-film absorber due to the addition of surfactants at conditions representative of an absorption heat pump. The model indicates that the overall conductance of the absorber is improved by ~30% by the addition of surfactants. The results from this work can guide intensification of various coupled heat and mass transfer processes using surfactants.