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

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 similar to 30% by the addition of surfactants. The results from this work can guide intensification of various coupled heat and mass transfer processes using surfactants. (C) 2021 Published by Elsevier Ltd.

Automated summary

Surfactants can enhance heat and mass transfer in ammonia-water absorption by reducing surface tension, with 500 PPM of 1-octanol or 2-ethyl-1-hexanol recommended for significant improvement. Addition of surfactants improves absorber conductance by approximately 30% in a heat pump system, guiding intensification of various heat and mass transfer processes.