Experimental Study Of A Novel Spiral-wound Membrane Distillation Module With Heat Recovery

Membrane distillation(MD),combined with the membrane separation technology and the traditional distillation process,is a new separation technology in seawater desalination and waste water treatment with greate potential.However,the high energy consumption problems in membrane distillation cast a serious limitation in large-scale application until now.One effective way to solve this problem is to reuse the heat from the energy efficient recovery and utilization device in membrane distillation process.Based on this concept,a novel spiral-wound air gap membrane distillation module with heat recovery was designed and tested in this article.This module combined the advantages of flat membrane and heat exchange hollow fiber.The stream produced from the membrane distillation process was designed to heat the low temperature feed in the hollow fiber.Therefore,the recovery and utilization process of latent heat of hot vapor steam inside the module can be realized.The advantages of air gap membrane distillation(AGMD)and vacuum membrane distillation(VMD)were united in this design.A vacuum was introduced into the air gap membrane distillation process.Both single-stage and multi-stage air gap membrane distillation processes with vacuum auxiliary were designed and the corresponding experimental apparatus were built up,and the MD tests were carried out as well.During the process of single-stage membrane distillation,the principle of single variable was applied.The effect of feed inlet temperature,feed inlet flow rate and the vacuum degree on the permeate flux,gained output ratio(GOR),thermal efficiency were considered.Salt rejection rate were studied with 3.5%(mass fracetion)simulated seawater as feed solution.The results show that: The membrane fluxes and the GOR increase with a higher value of feed temperature and vacuum degree.And the rise of feed flow rate is benefit for the permeate flux but not conductive for the GOR.However,the operating parameters such as temperature and vacuum degree had little effect on the salt rejection rate,lower conductivities of less than 10?S/cm were observed in all of the experiments.Based on the single-stage membrane distillation experiments,the corresponding experiments for multi-stage membrane distillation were conducted.Comparison analysis of the operation parameters on membrane flux,gained output ratio(GOR),thermal efficiency and salt rejection rate in single-stage and multi-stage system were carried out.Compare to the single-stage system,the membrane flux in multi-stage system decreased,but the total water output increased more than twice and GOR rised from 2.48 to 3.42.However the salt rejection rates were not change.In conclusion,a novel spiral-wound air gap membrane distillation module with internal heat recovery was proposed.Single-stage and multi-stage membrane distillation experiments were carried out.The results validate the effectiveness of heat recovery with the novel designed membrane module and laid a good foundation for the development of the energy-saving membrane distillation technology.