Design And Experimental Study Of Casing Type Air Gap Membrane Distillation Module

Air gap width has an important influence on the performance of air gap membrane distillation process.In this study,a novel double-pipe hollow fiber AGMD module consisted of PVDF hollow fiber membrane and heat exchange capillary copper tubes was successfully developed.The even distribution and easy control of the air gap width in the DPHF-AGMD module can be implemented.The concept of the equivalent membrane distillation flux(JAGMD)was firstly introduced in this paper and used to evaluate the comprehensive performance of DP-AGMD-M.Within the experimental range,the maximum membrane distillation flux(J)arrived at 11.41 kg/(m2·h),the maximum gained output ratio(GOR)reached 6.62 and the maximum JAGMD was 29.60 kg/(m2·h).In the AGMD process,the non-condensable gases seriously hinder the vapor diffusion and condensation heat transfer of water vapor.These effects severely hamper the distillate production and heat efficiency of the MD process.In this study,a vacuum assisted air gap membrane distillation process(VA-AGMD)based on the DPHF-AGMD module was designed to weaken the negative influence of non-condensable gases and improve the process performance.It was found that the applied vacuum(within a certain range of vacuum pressure)could remove the non-condensable gases and effectively improve the process performance.It was also found that water vapor cannot condense at the cooling surface in the module when the vacuum pressure was too high.Hence,the concept of optimal vacuum pressure(OVP)was introduced and studied.The OVP was considered a critical factor to ensure that the VA-AGMD process was dominated by AGMD rather than by vacuum membrane distillation.Within the experimental range,compared to AGMD,J and GOR of the VA-AGMD process could be increased by 20.0%?50.0%,and the specific energy consumption did not increase greatly.In this paper,a kind of porous PVDF heat exchange tube(HET)was prepared via NIPS method with DMAc as the solvent.The through hole in the tube wall was eliminated by changing the spinning conditions.This PVDF HET possessed high mechanical strength and excellent airtightness.The tensile strength and porosity the HET were 11.06 MPa and 50.8%,respectively.Since the thermal conductivity of water is higher than that of PVDF,wetting the pores in the tube wall and filling them with water can significantly improve the thermal conductivity of the porous HET.A full plastic double-pipe air gap membrane distillation module was fabricated with the HETs and PVDF hollow fiber membranes.This module was used to explore the feasibility of the porous PVDF HET for the AGMD process and the actual effect of wetting the pores in the tube wall.When the pores were wetted,J and GOR of the AGMD process could be improved by about 30.0%.Within the experimental range,the maximum J and GOR could reach 9.73 kg/(m2·h)and 10.93,respectively.The porous HETs showed good stability even under the condition of high concentration of salt water.