Process Analysis Of Energy-saving Vacuum Membrane Distillation

MD (membrane distillation) is a desalination technology combining membranewith distillation technology and as a new separation technology in the field of watertreatment it has many comparable advantages. However the development of itslarge-scale application is still restricting. One of the main reasons is the energy savingproblems in MD process.In this paper, the problem of energy recovery in MD process was in-depth anddetailed analysis. First, from a theoretical point application thermodynamic theory inMD to find the real reason of energy loss: the water vapor condensation process. Thenaccording to the results of theoretical analysis and VMD characteristics, we designedthe energy-saving heat recovery vacuum membrane distillation process: multi-stagevacuum membrane distillation process (MS-VMD) and multi-effect vacuummembrane distillation process (ME-VMD).On the basis of previous studies on VMD process, the characteristics has beendeeper discussion: a turning point interval and the transition zone are turned out in thecurves of permeate flux changing with the feed velocity and inlet temperature, andafter the point or the transition period, the characteristics in VMD had some changes;and the nature of the vapor product in VMD process was almost equilibrium with theoperating pressure.Two types of distillation membrane were applicated in MS-VMD experimentalstudy. The results showed that: under the same operating conditions, the recoveryefficiency of high-flux membrane is higher than the low-flux membrane. In a fivestages MS-VMD, GOR of the low-flux membrane was2.14and the heat recovery wasabout51.5%. However, under the same operating conditions, GOR of high-fluxmembrane was3.14and the heat recovery rate was65.5%.Application of high-flux membrane in ME-VMD experimental study, the resultsshowed that: the process of ME-VMD can easily obtain higher efficiency heatrecovery and GOR than MS-VMD. But meanwhile, ME-VMD process has on thenature of the distillation membrane. The GOR in a three effects ME-VMD process canreach2.Two pilot plants were successfully built based on the above studies: a multi-stagevacuum membrane distillation pilot plant and a multi-effect distillation vacuum membrane distillation pilot plant. And these two plants were all conducted apreliminary run and test, in which multi-effect vacuum membrane distillation pilotplant achieved a good result that GOR reached about5.