Entropy and exergy analysis of a liquid-liquid air-gap hollow fiber membrane contactor

Abstract

A liquid-liquid air-gap hollow fiber membrane contactor consisting of a number of hollow fiber membrane tubes forms a hollow fiber membrane absorption heat pump where the refrigerant (water) and the absorbent (LiCl solution) flow inside the tubes in a cross-flow arrangement. There are defined air-gaps between neighboring tube rows. A lumped parameter model which describes the heat and mass transfer is established by transforming the membrane contactor into a parallel-plate one. The governing equations are solved by the finite difference approach. The model is validated experimentally. The influences of various structural parameters, operation conditions, and membrane parameters on the contactor performances are analyzed. An entropy generation model and an exergy destruction model are established to solve and disclose the thermodynamic properties of the heat and mass transfer process inside the membrane contactor, which is an irreversible process. It has been found that the contactor performance can be improved by using the hollow fiber membranes with larger diffusivity or a smaller diameter of about 1.5 mm. Furthermore, the packing fraction of about 0.349 is optimal for the performance. The maximum mass transfer entropy generation rate usually corresponds to the best performance. The entropy generation minimization method is not suitable for the optimization of this membrane contactor. The optimized regions of the structural parameters, operation conditions, and membrane transport parameters with better performances and smaller entropy generation can be obtained to reduce irreversible loss in future.