Hollow fiber nanoporous membrane contactors for evaporative heat exchange and desalination

Abstract

The paper reports the performance of nanoporous polypropylene membrane contactors in water evaporative transfer processes, depending on membrane packing density, flow, and temperature conditions. The membrane evaporator's heat and mass transfer efficiency were evaluated in a wide range of partial pressure gradients and gas phase Reynolds numbers. A protocol for simple evaluation of evaporative heat flux to the heat exchange flux is introduced to reveal the efficiency of mass-to-heat transfer. It has been shown the evaporation efficiency significantly depends on gas flow conditions, increasing at Re ∼ 30 and exceeding 4.6 kg × m-2×h−1 (3.0 kW × m-2×h−1) at the temperature of inlet water of 60 °C. It is achieved by inducing convective flows in the gas phase, which enable the rise of effective heat transfer coefficient for membrane evaporator ∼250 W × m-2×K−1. The attained efficiency is confirmed to originate from heat extraction from the gaseous phase and the evaporation of water from the external surface of fibers. While exhibiting maximal heat flux the regime requires liquid phase penetration through the pores, leading to degradation of the membrane performance in desalination applications. To avoid penetration of ions thin (∼300 nm) graphene oxide coating was deposited onto the internal surface of the nanoporous evaporator, enabling to avoid liquid penetration through the pores and salts crystallization at the external surface, while exhibiting slightly lower performance of the membrane.