Experimental study of thermal diffusion enhanced vapor transfer performance with perhydropolysilazane-derived silica (PDS) coating membranes in air dehumidification process

Abstract

In membrane-based air dehumidification process, the membrane acts as an intermediate medium separating the humid air and the desiccant solution and be used to simultaneously transfer sensible heat and moisture. However, it produces additional mass transfer resistance reducing the performance of dehumidifier. In order to relieve the additional resistance, breakthroughs need to be found in membrane material and driving force. In this investigation, three different membranes, namely, AAc-modified PTFE with silicon oxynitride coating, neat PTFE with amorphous silica coating and neat PTFE are selected. Large temperature gradient coupled with vapor pressure difference forms the enhancement in driving force and faster desorption rate in the membrane. The composite membrane acts as a temporary solid desiccant rather than a barrier between humid air and desiccant solution. Results show that the water vapor adsorption/desorption performance of neat PTFE has been significantly improved with a thin layer of amorphous silica coating added on. Its permeation activation energy is greatly reduced from 54.8 KJ·mol−1 to 27.2 KJ·mol−1 and intrinsic resistance from 225 s·m−1 to 180 s·m−1. The thermal diffusion enhanced effect improves the latent effectiveness of AAc-modified PTFE membrane from 0.38 to 0.5 and neat PTFE membrane with amorphous silica coating from 0.3 to 0.41. These are mainly due to the decrease of sorption curve constant C and increase of water uptake at lower relative humidity. It can be regarded as the temperature gradient across the membrane applying additional driving force to the water vapor diffusion process. Then better dehumidification performance which originally require larger vapor pressure difference can be achieved, reducing energy consumption in the regeneration process.