Study On Quantitative Evaluation And Mitigation Strategy Of Concentration Polarization In Forward Osmosis Process

The concentration polarization(CP) generally appears on both sides of the membrane in the forward osmosis(FO)process,reducing the effective driving force and making the actual water flux much lower than theoretical value.Therefore,it’s significant to use the quantitative analysis method to study and evaluate the influence of CPs under the different conditions,then observing different methods to relieve the adverse effect.This paper first investigated the influence of membrane orientations on the device performance,showing that the water flux and reverse solute flux of PRO mode are always higher than that of FO mode,but decrease sharply with time.Moreover,when the concentration difference keeps constant,the water flux in PRO mode declines seriously with the increase of feed solution(FS)concentration.In addition,under the same concentration,counter-current pattern is more beneficial for the improvement of water flux.Through the quantitatively analyze,it can be found that in the FO mode,as the DS concentration increased from 0.25 M to 1.5M,the proportions for both the dilutive internal CP and dilutive external CP showed a significant increase from 31.43% to 50.59% and 11.42% to 29.96% respectively,while the proportion of effective osmotic pressure decreased from 56.75% to 19.26%;while in the PRO mode,the proportions for both the dilutive internal CP and dilutive external CP showed a significant increase from 18.20% to 46.84% and 12.45% to 23.37%respectively,while the proportion of effective osmotic pressure decreased from69.08% to 29.64%.This indicates that the ICP always occupies a domain position for the osmotic pressure drop,while the influence of the ECP can’t be ignored especially for the desalination of high salinity solutions.Under the FO mode,the influences of flow velocity was studied by using a quantification analysis,then temperature and spacer configuration on the FO performance were respectively investigated to relieve the CPs effect.Results of the study showed that increasing the flow velocity can increase the diffusion and disturbance,then mitigating the adverse effect of CPs.The unilateral strengthen of flow rate in draw solution is more meaningful than that in feed solution flow rate,orbilateral strengthen for both feed solution and draw solution.Using 0.5M NaCl as feed solution and 1.0M NaCl as draw solution,the water flux increased by 52.81%when the operating temperature of both solutions increased from 20℃ to 30℃.Keep the draw solution temperature at 20℃,the water flux got 46.87% increase with only increasing DS temperature to 30℃,higher than that 27.78% with only increasing FS temperature,indicating the effect of draw solution temperature on the improvement of water flux is more significant.Lastly by placing the spacers of different size in the specific location of FS channel and DS channel,the fluid state and mass transfer was changed to relieve the influence of CPs.It is indicating that an optimal configuration of the spacers in the FO channels was obtained with the spacer in contact with the active layer in the feed solution channel and 2.7mm away from the support layer in the draw solution channel.