Study Of The Effect Of Spacer On Forward Osmosis Membrane Separation Process

Forward osmosis membrane technology,as a new membrane separation technology,has shown potential and advantages in wastewater treatment,seawater desalination and power generation.Forward osmosis membrane separation technology is driven by osmotic pressure difference on both sides of semi-permeable membrane.Concentration polarization is unavoidable in the process of membrane separation.Spacer can be added to the spiral wound module to increase the disturbance to the velocity boundary layer and slow down the concentration polarization.Computational fluid dynamics(CFD)is an effective method to simulate the flow and mass transfer in membrane module.It can obtain the pressure distribution,velocity distribution and wall shear stress distribution in membrane module.In this study,computational fluid dynamics software FLUENT 15.0 and meshing software ANSYS Meshing were used.In this paper,cuboid spacer is added to the feed solution channel of the forward osmosis membrane module.The arrangement way of the spacer is cavity,submerged and zigzag.From the two aspects of channel pressure drop and wall shear stress,by comparing the three different arrangement ways,it is concluded that the wall shear stress of submerged spacer is the largest,and its pressure drop is also the largest.Compared with traditional cylindrical spacer,the wall shear stress and pressure drop of cuboid spacer are larger than that of cylindrical spacer in either arrangement way.Comparing the two-dimensional model with the three-dimensional model of submerged spacer,it is found that the two-dimensional model is basically consistent with the three-dimensional model in terms of pressure drop and wall shear stress.The pressure drop and average wall shear stress increase with the increase of inlet velocity in the three arrangement.The range of wall shear stress has not changed,but the peak value and valley value have changed.In order to enhance the mass transfer,two types of spacer are designed.The results show that the b3 model in scheme 2 has the largest wall shear stress which is the most effective in reducing concentration polarization.Finally,the ladder-type spacer is simulated by changing the transverse spacer filament spacing and axial spacer filament spacing,the thickness of spacer filament and the angle between the axial spacer filament and the flow direction.The simulation results show that the average shear stress on the bottom wall first decreases and then increases with the increase of the transverse spacer filament spacing,while the average shear stress on the top wall and the average pressure drop in the flow channel decreases with the increase of the transverse spacer filament spacing.It is not suitable for ladder-type spacer if the thickness of transverse spacer filament spacing is too large.Considering the wall shear stress and pressure drop,the angle between the top spacer filament and the flow direction is 45 degrees,which is the best scheme in the ladder-type spacer.