Study On The Transport Characteristics And Physical Mechanism Of Solute Through Forward Osmosis Membrane

Forward osmosis(FO)is a kind of membrane separation technology that utilizes natural osmotic pressure as a driving force to make water pass through the membrane.Compared with large-scale commercialized membrane technologies such as reverse osmosis(RO),FO is favored by the advantages of free of hydraulic pressure,lower membrane fouling potential and ease of membrane cleaning.FO has gained great attention in the industries of seawater desalination,zero liquid discharge,fertigation in agriculture,liquid food processing,etc.The complex reverse solute diffusion transport phenomenon,which is inherently linked with the water flux and membrane fouling,has received growing attention to obtain insight into developing an efficient forward osmosis membrane.The study on the complex mass transfer process in reverse solute diffusion is of great significance for the development of high performance forward osmosis membranes.The comprehension of the transport characteristics of solute through FO membrane depends on understanding the fluidics behaviors and revealing the physical mechanism based on the combination of mass transfer theory.In this thesis,based on the solution-diffusion mechanism,three models are established,including the unidirectional transport model of solutes incorporating the surface charge on solute partitioning and the bidirectional transport model of solutes and the large-scale module mass-transfer model of solutes.The effects of temperature and draw solution concentration on the water and solute flux during the FO process were investigated,and the results and conclusions are obtained as follows:1.The influence of the selection of diffusion coefficient and thevariation of surface charge density on the calculated reverse solute flux by the unidirectional transport model of solute is investigated.By comparing the calculated values of the model based on the mutual diffusion coefficient and the average diffusion coefficient with the experimental values,it is found that the mutual diffusion coefficient is better suitable to simulate the water flux and the reverse solute flux during FO.The effect of surface charge on the reverse solute flux decreases with the temperature increases,which makes the effect of the model based on average diffusion coefficient on the calculated membrane characteristic parameters change from overestimating the solute permeability coefficient to underestimating the structural parameter.By comparing the calculated reverse solute flux for draw solution concentrations at different operating temperatures by the model based on mutual diffusion coefficient,the range of the draw solution concentration at the support layer-active layer interface applicable to the constant surface charge density is found.The concentration beyond which the surface charge density is not constant,decreases with increasing operating temperature.2.The effect of the feed solution solute on the transmembrane transport of the draw solution solute is investigated.By comparing the calculated values of the impact-free model with the experimental values,it is found that the feed solution solute will affect the transport of the draw solution solute,and the effect is mainly manifested in the active layer.The cross solute permeability coefficient produced by the mixture of the feed solution and the draw solution in the active layer appearing as an inhibition of the draw solution solute passing through the active layer,and the cross solute permeability coefficient decreases with the increase of the draw solution concentration and temperature.After the feed solution solute changes from deionized water to Na Cl,the dilutive external concentration polarization and the dilutive internal concentration polarization both decreased,and the decreasing amplitude increased with the increase of temperature.The dilutive external concentration polarization difference between DI and Na Cl decreases with the increase of the draw solution concentration,and the dilutive internal concentration polarization difference has the same trend.3.The effect of surface charge characteristics on the mass transfer of the large-scale module is investigated.The predicted water flux,which is calculated by the large-scale module mass-transfer model incorporating the surface charge on solute partitioning,would decrease with the increase of the membrane chamber length,which is consistent with previous research results.The predicted reverse solute flux,which is affected by the Donnan potential generated by the surface charge and the decrease of the draw solution concentration at the interface between the active layer and the support layer with the increase of the membrane chamber length,cannot be simply summarized by a monotonic decreasing trend as the predecessors did.Based on the influence of the draw solution concentration,the predicted reverse flux has three trends with the change of the membrane chamber length: gradually increasing(0.5M);Increasing first and then decreasing(1M and 1.5M);And decreasing gradually(2M).In the previous study,the effect of Donnan potential on the increase of the reverse solute flux was ignored,which is the main reason for the overestimated rate of the predicted reverse solute flux decreased with the increase of membrane chamber length.With the increase of the draw solution concentration and the membrane chamber length,the predicted reverse solute flux changes from decreasing to increasing by increasing both the flow velocity of the channel of the draw solution and the feed solution.