Preparation Of Flat Sheet Polysulfone Membranes Via Vapor-induced Phase Separation And Its Direct Contact Membrane Distillation Research

Membrane distillation (MD) is a novel membrane separation technology whichcombined with traditional distillation technology and membrane separationtechnology. It uses hydrophobic microporous membrane as medium, vapor pressuredifference at both sides of the membrane as mass transfer power, volatile componentsof feed permeate through the membrane pores in the form of vapor, so as to achievethe separation purpose. Membrane distillation has the advantages of high separationefficiency, mild operation condition, low power consumption, which can be widelyused in the field of seawater desalination, waste water treatment and the like.Influenced by membrane and others, there are problems such as low flux and lowthermal efficiency in membrane distillation, thereby MD has not been industrializedup to now. So it’s critical to develop the membranes with excellent performance andlow cost. In order to maintain relatively high flux and rejection, a qualified membranenot only has hydrophobic surface, but also has suitable pore size, high porosity andmechanical strength, in addition, to increase thermal efficiency, low thermalconductivity is also necessary for a fine membrane. Based on the above，The paperfocuses mainly on the following three aspects:(1) Preparation of hydrophobic PSf microporous membranes and research on theinfluencing factors of performanceAccording to membrane formation mechanism in vapor-induced phaseseparation (VIPS) process, the study has explored the factors that influence the finalmembrane structure and performance thereof. It was found that the surface andcross-sectional morphology of the PSf membranes and the performance ofdesalination in DCMD were affected by exposure time, polymer content and relativehumidity (RH) of air. An increase in exposure time during the VIPS process led to thereplacement of the bicontinuous top surface with a dense skin, and the permeate fluxin DCMD decreased greatly. The water contact angle, thickness, porosity, mean poreradius, and liquid entering pressure (LEPw) of the membranes changed very little. Thehigh PSf content solution was favorable for finger-like bulk, while the solution withlow PSf content was favorable for symmetrical sponge-like structures. Low RH wasgood for high hydrophobicity. The best PSf membrane fabricated in this study had amean pore radius of0.32μm, a static contact angle of106.4°with water, a LEPwof300kPa, and a total porosity of82.1%. During the DCMD process with a35g/L sodium chloride (NaCl) aqueous solution, the best membrane that was fabricatedproduced a permeating flux of30.0kg m-2h-1and a very low conductivity of distilledwater at a hot-feed and cold-distillate side temperature of73°C and25°C, respectively,This performance is comparable or superior to the performance of most flat-sheetpolymer membranes reported in the literature. The VIPS method explored in thisstudy is a promising method to fabricate porous and hydrophobic hollow-fiber PSfmembranes using a dry/wet spinning process, because the best exposure time waswithin1min, effectively controlling the air-gap length can achieve this goal.(2) Application research of hydrophilic/hydrophobic PSf membrane in DCMDprocessAccording to (1), hydrophilic/hydrophobic PSf membrane with stable structureand performance and actual application value which is used for membrane distillationhas been prepared. The membrane has been used in a long-time DCMD operationprocess, Little wetting was observed after90h of operation, indicating that theflat-sheet PSf membrane fabricated in this study showed good performance in termsof DCMD desalination. Mass transfer and heat transfer mechanism of the membranewith the above said structure in DCMD process has been discussed in this study, thepredicted flux value calculated with the mass transfer mechanism of theKnudsen-molecular transition agreed with the experimental flux value, and the largestdeviation was only11.9%. Therefore, the thickness of air gap measured by weightwas feasible. The effect of the operating conditions, such as the concentration of thefeed solution, the feed temperature and flow velocity on both the hot and cold sides onthe permeate flux were investigated, The results show that, when the concentration ofthe feed solution was relative low, effect of concentration polarization in boundarylayer on flux was negligible, but once the concentration of the feed solution increasedgradually, decline of flux increased correspondingly. The flux increased withincreased feed temperature and flow velocity; besides flux increased faster withincreased flow rate of the hot feed solution than with that of the cold distillate water.the flux increased by118%when the flow rate on the hot side increased from27L/hto162L/h, while it increased by only68%when the flow rate on the cold sideincreased by the same quantity. When the flow rate on the cold side increased from108L/h to162L/h, the flux increased by7%, while it increased by31%for the hotside. This suggests that the effect of the hot feed solution flow rate on permeate fluxwas larger than that of the cold distillate water. This can be deduced from the effect of the flow rate on the Reynolds number, heat transfer coefficient, temperaturedifference and driving force on both sides, which increased more rapidly on the hotside. The flow rate on the hot side had a larger impact on the permeate flux.(3) Research of thermal efficiency in DCMD processHeat and mass transfer model of a single layer membrane in DCMD process hasbeen established, according to experiment and mathematical simulation calculation,effects of membrane parameters (pore size, porosity, thickness, thermal conductivityetc.) on thermal efficiency in membrane distillation process has been studied. Theresults showed that, when thermal conductivity of the membrane decreased, thermalefficiency of DCMD process increased gradually; while the whole pore size ofmembrane increased gradually, diffusion coefficient of water vapor in pores increased,so thermal efficiency of DCMD process increased gradually, Taken flux and rejectioninto consideration, the optimal pore size was controlled between0.3μm-0.45μm;because membrane thickness could affect the temperature of membrane surface, it hadan indirect influence on the thermal efficiency of DCMD, however, membranethickness was inversely proportional with conduction heat and evaporation heat flux,so there was a little effect on the thermal efficiency of DCMD. On the basis, the studyhas preliminarily explored how to prepare membranes with low thermal conductivityby implanting thermal insulating material into membrane.