Study On Process Of Membrane Distillation Separating The Salt Solution

Seawater is an inexhaustible water resources,however,the seawater desalination technology is based on the principle of water vapor evaporation,which has the disadvantages of complex technology,energy consumption and other shortcomings.But membrane distillation technology has the characteristics of high separation efficiency,simple equipment and so on.Therefore,the application of membrane distillation separation in seawater desalination has a broad prospect.In this paper,to carry out the membrane distillation seawater desalination project of Memsino membrane Technology(Beijing)Co.,Ltd.,we applied PVDF hollow fiber membrane treated by hydrophobic coating and a new membrane module designed by corporation to study membrane distillation separation of sodium chloride aqueous solution.Maintaining cooling water sufficient in membrane module,we studied processes of air gap coupled with vacuum membrane distillation(AG+VMD),air gap membrane distillation(AGMD)and vacuum membrane distillation(VMD).The membrane flux and the hot feed temperature difference between the inlet and outlet of membrane module were investigated by changing hot feed temperature,velocity inside the hollow fiber membrane,hot feed salinity.The study results indicated that the membrane distillation flux increased as the temperature of hot feed increased.Under the same operating conditions,the flux of AG+VMD process is 0.10~0.24 L/(m~2.h)less than the total flux of separate AGMD and VMD.And the AG+VMD process has the maximum hot feed temperature difference between the inlet and outlet of membrane module.When the hot feed salt content increased from 3%to 17%,the membrane distillation flux declined 1~2 LMH for three kinds of processes.In order to study the heat recovery efficiency of membrane distillation,we modified the membrane distillation device,employing the untreated salt solution as cooling water for permeation vapour.By changing the flow of salt solution and vacuum degree in permeate side,we studied the changes in membrane flux,heat recovery efficiency and temperature of boundary layer in the processes of AG+VMD,AGMD and VMD.It was found that increase in the degree of vacuum caused the flux of membrane distillation increased,the temperature of boundary layer decreased and heat recovery efficiency increased.With the circulation9.8 L/h salt solution,when the vacuum degree increased from 0.070 MPa to 0.085 MPa,energy recovery efficiency of AG+VMD and VMD increased from 19.7%and 25.1%to25.2%and 32.7%,respectively;As the flow of salt solution increased from 3 L/h to 11.7L/h,the flux increased,the temperature of boundary layer decreased and heat recovery efficiency increased.When the vacuum degree is 0.09 MPa,the energy recovery efficiency of AG+VMD process increased from 20.1%to 24.2%,while the energy recovery efficiency AGMD process increased from 28.8%to 42.7%.In conclusion,when designed coupling AG+VMD device,an additional heat exchanger should be added in order to make full use of the heat carried by the permeate vapour.