| Membrane separation technology has been widely used in water treatment, food, medicine, and chemical industry. The membrane technology has some advantages such as energy saving, not involving the phase change, not introducing’other solvents, operating at normal temperature, products without thermal damage, less investment, simple structure and maintenance, products without pollution etc. Spiral nanofiltration membrane has universal application in industry. But due to its narrow channel, poor hydraulic conditions, low velocity, rejected material deposited on the membrane surface easily, serious concentration polarization, and seriously affecting separation performance, the performance of NF is seriously affected. Therefore, it is urgent to establish low-cost, low pollution improvement measures. Techniques shown to be effective to control membrane pollution include feed pretreatment, membrane surface modification, turbulence promoters, corrugated membrane surfaces, pulsatile flow and vortex generation. Induced surface shear is a major strategy to control the pollution phenomena. Surface shear can also be enhanced by two-phase flow.There are many reported data about the use of air in tubular and hollow fibre membranes and channels containing flat sheets for UF and MF, and the enhancement of gas sparing on permeate flux has been experimentally assessed. However, the membrane process applications involve feed flow through nanofiltration membrane and spiral membrane have been rarely reported. This paper aims at to study the enhancement of using gas in nanofiltration process of aqueous solution of inorganic salts. In consideration of nanofiltration membrane rejection properties of divalention, MgSO4aqueous solution is selected to be the research systems.Firstly, the gas-liquid two-phase flow experimental platform has been built. Then the effects of two-phase flow on nanofiltration characteristics were examined under the operating conditions of transmembrane pressure of0.4-0.7MPa, filtration feed cross-flow velocity of1.0-0.4m·s-1, gas superficial velocity of1.0-3.5m·s-1and MgS04aqueous solution concentration of30g·L-1and50g·L-1at34℃. By optimizing operation process, the optimal experimental condition is transmembrane pressure of0.4MPa, filtration feed cross-flow velocity of1.0m·s-1, gas superficial velocity of3.5m·s-1.Under the condition, flux increasing rate is up to47.06%. On the basis of optimal operating parameters, further researches have been made to analyze the gas enhancement at different temperature and concentration of MgSO4.Experimental results show that the gas-liquid two-phase flow can significantly enhance the nanofiltration process and the enhancement effect is related to the transmembrane pressure, feed cross-flow velocity and gas superficial velocity. The flux enhancement of nanofiltration increases with the increase of feed cross-flow velocity and gas superficial velocity and decrease of transmembrane pressure. At the same feed cross-flow velocity, due to the pressure increase, the enhancement effect appears with gas velocity increasing. The effect of MgSO4concentration on gas-liquid two-phase flow enhancement is obvious; when operating temperature reaches30℃effect of temperature on membrane flux tends to stability.Experimental studies aiming at enhancing the nanoiltration process through applying gas-liquid two-phase flow were carried out by injecting air into the high concentration of MgSO4aqueous solution during its nanofiltration. And the experimental studies are reported for the first time at home and abroad. The thesis will be to study the inorganic, inorganic-organic hybrid system solution system and provide a theoretical basis for related industry applications. |
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