Performance Comparison Of Air Gap And Permeate Gap Membrane Distillation And The Study Of Membrane Fouling

Membrane distillation has become a new energy-saving technology in the seawater desalination and sewage treatment field.It has received sufficient attention and recognition.However,there are still many problems in the process:energy efficiency and membrane fouling.These problems hinder the further promotion of membrane distillation.This paper aims to improve the performance(energy efficiency,permeate flux)of air gap membrane distillation(AGMD)based on the analysis of heat and mass transfer mechanism of process and find that the performance of permeate gap membrane distillation(PGMD)will be better than that of AGMD theoretically.AGMD and PGMD were investigated and compared through self-developed hollow fiber membrane modules with internal energy recovery.The effects of the feed temperature,the coolant temperature,the flow rate,the gap thermal conductivity,and the gap width on the flux and the gained output ratio(GOR)were evaluated experimentally in AGMD and PGMD.The gap between the membrane and the condensing surface was a key factor affecting the flux and the GOR in both AGMD and PGMD.It was found that the improvement of gap thermal conductivity using brass net was useless in improving flux and GOR.The permeate water filled in the gap changed the process of mass and heat transfer in the gap which gave rise to the increase in the flux and thermal recovery.The flux and GOR in PGMD increased by 7.9%and 59.82%in the gap of 0.5 mm compared to the AGMD.The salt rejection of all experiments were greater than 99.8%.Finally,the long-period experiment results of membrane distillation were discussed,and the influence of the pollution produced during membrane distillation on the flux and the GOR was analyzed.In this experiment,the flux decreased from the initial 3.05 L/(h·m~2)to 1.99 L/(h·m~2),the flux decreased by 35%,the GOR decreased from 1.28 to 0.53,and the GOR decreased by 58%.The salt rejection has declined somewhat,but it remains above 99.6%,the membrane fouling is serious.