The Design And Experimental Study Of An Energy-saving Spiral Wound Air-gap Membrane Distillation Module

Membrane distillation is a new separation technology which combines membrane technology with traditional distillation process.It can work with low grade heat source by simple equipment under mild operating conditions.Therefore,it is often coupled with solar energy and waste heat source system and has attracted wide attention in the fields of high salt wastewater treatment and juice concentration.However,the industrial application process of membrane distillation is slow,and one of the key factors restricting the development is the high heat consumption in its process.For evaporation,the recovery of second steam latent heat is an effective means to reduce the energy consumption.Consequently,the design of a new type of heat recovery membrane module has become a hot spot of research.Accordingly,a new type of air gap membrane distillation module which combines hydrophobic flat membrane with heat-exchanger hollow fibers is designed.It combines the advantages of air gap membrane distillation and vacuum membrane distillation,and can work in negative pressure environment.The membrane module can get high flux and thermal efficiency at the same time by improving the vapor pressure difference on both sides of the membrane.The feasibility and economy of the module are fully verified by single stage and multistage membrane distillation experiments.In the experiment of single stage membrane distillation,the effects of the membrane module parameters and operating conditions on the water production of the module were systematically investigated with 3.5%?the mass fraction?Sodium Chloride Solution as the feed.And the long-term stability experiments were carried out.The results show that the permeate flux?J_D?and gained output ratio?GOR?can be improved effectively by increasing the membrane pore size,heat transfer coefficient of the heat-exchanger hollow fibers and the temperature of hot feed.J_D and GOR gets the opposite trend with the change of the effective length of the module,the flow rate of the feed and the temperature of the cold feed.The overall performance of the module will be reduced when the thickness of air gap and permeable side vacuum?P_V?are too large or too small.The result of long-term stability experiment of the module is good.On the basis of single stage membrane distillation experiment,the effects of operating conditions on J_D,GOR and salt rejection rate?R_s?of single stage and multistage membrane distillation unit were compared by the amplification of three stage membrane distillation experiments.The experimental results show that the J_D of the multistage membrane distillation unit is decreased,but the total water production and GOR of the system are greatly improved.The maximum J_D of 16.38 kg·m^-2·h^-1,maximum GOR of 3.77 were obtained respectively in the whole experiment and the R_s was over 99.95%all the time.The membrane distillation system can maintain good heat recovery and good desalination effect while obtaining high J_D.It has important guiding significance for the research and application of energy-saving membrane distillation modules.