Hydrophilic Modification Of Pervaporation Membrane And Intensification Of Water Transport Process

Pervaporation shows wide application prospect in organic dehydration due to low energy consumption, small investment and without the limitation of vapor equilibrium. In the present work, the pervaporation separation of the water and acetone mixture was chosen as the investigation object and the goal was to solve the “trade-off” effect by increasing the water solubility and diffusivity in the membrane. In order to enhance the water solubility in membrane, the hydrophilic modification of membrane was adoped by doping hydrophilic inorganic particles and grafting hydrophilic groups to polymer to incerase the hydrophilicity of membrane. Meantime, in order to intensify the water transport process, inorganic channel in the membrane was construted to provide continuous channel for water diffusion. In addition, changing the large water clusters into small water cluster, named “activated water”, could enhance the penetration and diffusion of water molecules to increase the water flux. The details were summarized as follows:Firstly, hydrophilic inorganic particles K+MMT were incorporated into the CS membrane to construct ionized water channel. Taking advantage of the solvent effect of K+ in the intergallery channel of MMT, water could transport through the hydrated layer of K+ which was beneficial to increase the selectivity of water. Meantime, the intergallery of MMT provided a continuous channel to intensify the water transport and increase the water flux. The result showed that CS-K+MMT-10 hybrid membrane exhibited the high performance with flux of 1.56kg/m2 h and separation factor of 2200, at 50 oC for 5 wt% of water in the feed, which was almost 8 times higher than 249 of pristine CS membrane.Secondly, the hydrophobic porous carbon molecular sieve was suifonated to prepare sulfonated carbon molecular sieve(SCMS). SCMS was doped into the CS matrix to construct the hydrophobic channel with a minumal number of hydrophilic sites in membrane. The sulfonic acid groups which were hydrophilic could be used to enhanced the water selectivity, while the hydrophobic channels which lower the energy barrier for water transport could facilitate rapid transport of water. The result showed that when the SCMS content was 2wt.%, an optimum performance with a separation factor of 832 and a permeation flux of 1.81kg/m2 h at 50 oC for 5 wt.% of water in the feed, which was increased by 2 times in separation factor and 1.67 times in permeation flux compared with pure CS membrane.Thirdly, strong electrolyte can be fully ionized and formed more hydrated ion conpared with weak electrolyte. Based on the characteristic of strong electrolyte, weakly alkaine amino groups of CS were modified into strongly basic guanidine groups to prepare CG which was more easily induced polarization to form hydrated layer. The hydrated layer was beneficial to transfer water, and increase the water selectivity. Meantime, due to the “structure breaking” effect of guanidine group, the large water cluster around the guanidine group can be changed into small water cluster which was helpful to enhance the permeability of water molecules and increase the water flux. The result showed the degree of substitution of guanidine group was 8.69%, when reaction temperature was 50 oC, pH was 2.5 and mole ratio of cyanamide to CS was 1:2. The high pervaporation performance was obtained by CS-50-pH2.5-1:2 membrane with flux of 1.9kg/m2 h and separation factor of 3300 at 50 oC for 5 wt.% of water in the feed, which was increased by 1.26 times in permeation flux and 13 times in separation factor. Finally, polyvinylamine(PVAm) with more amino group per unit srtucture was selected for the membrane material and the amino groups of PVAm were modified into guanidine groups to prapare PVG. The result showed that the water contact angle of membrane was decreased and the hydrophilicity was enhanced with increasing the degree of substitution of guanidine group. The degree of substitution of guanidine group was 18.41%, when reaction temperature was 50 oC, pH was 2.5 and mole ratio of cyanamide to CS was 1:2. PVG-50-pH2-1:2 membrane exhibited the high performance with flux of 2.32kg/m2 h and separation factor of 3300 at 50 oC for 5 wt.% of water in the feed, which was higher than the performance of CG.