Study On Mass And Heat Transfer In Forward Osmosis

Forward osmosis（FO）is an osmosis process which water molecules diffuse from feed solution（low osmosis difference）to draw solution（high osmosis difference）.Limited by the fabrication of FO membrane,the feed solutes will transport across the membrane to the bulk draw solution（DS）,and the draw solutes will transport across the membrane to the bulk feed solution（FS）as well.Various concentration polarizations（CPs）caused by the bidirectional mass transfer will take place,which will result in the effective osmotic pressure drop and have a bad influence on the performance of FO.In addition,heat current will transfer from high temperature side to low temperature side when the temperatures of FS and DS are different,which will cause temperature alteration and impact the performance of FO process.Therefore,study on mass and heat transfer in FO is significant.This study described the forward mass transfer of feed solutes,the reverse mass transfer of draw solutes,and the heat transfer between FS and DS in FO process.A FO model was developed with bidirectional mass and heat transfer,which could predict the water flux and the revese draw solute flux accurately.The model was verified by data obtained from existing KCl-NaCl FO experiments.The maximum relative error in water flux between predicted data and experimental data was18.81%.The maximum relative error in reverse draw solute flux between predicted data and experimental data was 7.09%respectively.Analyzed the effective osmosis difference caused by various Cps and found internal concentration polarization is the main cause of the osmotic pressure drop.This study designed the FO cell channel and the FO system,and investigated the influence of temperature and flow velocity on the performance of FO membrane and FO process using NaCl solution as FS and NH₄HCO₃ solution as DS.The water and salt permeability coefficients increased with the increase of temperature,while the membrane structural parameter decreased with the increase of temperature.Increasing temperature,water flux,reverse draw solute flux and forward rejection of feed solute increased as well.The enhance in FO performance among different temperature strengthened operations,increasing the temperatures of both solutions occupied first place,just increasing the temperature of DS came second,and increasing FS temperature alone was the smallest.The extent of concentrative external concentration polarization decreased with the increase of FS flow velocity,the extent of dilutive external concentration polarization decreased with the increase of DS flow velocity,and the dilutive external concentration polarization plays a more influential role on the water flux,the contribution of increasing DS flow velocity to water flux is more significant.