Study On The Phenomenon Of Low Water Flux In Forward Osmosis

FO(Forward osmosis)has been widely conecerned by researchers and scholars due to its advantages,such as low energy consumption,and anti-pollution,and so on.However,the low water flux of FO has been the reason of its slowly development.At present,the commercialized FO membrane has been improved and its performance has been greatly increased which doesn’t slove the problem completely.In this paper,the CTA FO membrane of HTI company is used to study the mechanism of FO process,and to explore the reason of low water flux,then give some advice for FO membrane production.First,use the existing external concentration polarization model to describe the concentration distribution of the solution on both sides of the FO membrane.Second,inside of the FO membrane is divided into two part——antive layer and support layer,and use the theory of flow diffusion to explain the "countercurrent diffusion" phenomenon at the membrane pore.Third,the hydrodynamic numerical simulation of fluid in the membrane pore is carried out to determine the flow field distribution at the membrane pore then use the the experimental data to calculate solution solubility at the membrane pore.At last,use the theory of flow diffusion to calculate the concentration distribution of the solution inside the active layer and the support layer in order to obtain the concentration distribution model throughout the membrane unit.Using the established model to analyze the main factors influencing the water flux in the process of FO process – the porosity of active layer,the thinckness of active layer and the bending coefficient of active layer,the conclusions as below:(1)The model is consistent with the measured data and can the mathematical model can be used to describe the Foward osmosis process.(2)The reason for low water flux in FO process is mainly due to the phenomenon of "countercurrent diffusion" at the pores of the active layer.(3)In the process of FO,the pore density of the outer surface of the active layer has a significant effect on the water flux,which is beneficial to improve the water flux.However,the rise of the pore density of the outer surface of the active layer will reduce the difference between the flow rate of the main solution and the membrane pore,which will the increase of the external flow rate,as well as the external concentration polarization.(4)In the PRO mode,the increase of the porosity of active layer,the thinckness of active layer and the bending coefficient of active layer leads to higher water flux.