| Inorganic ceramic microfiltration (MF) membranes are widely used in treating oily wastewater, removing microorganism in biomedicine industry, and separating and purifying effective ingredients in biological fermentation, due to their outstanding advantages, such as high pressure resistance, high endurance in organic solvent, long durability and easy cleaning ability. However, ceramic MF membranes have been prepared by coating inorganic powder onto ceramic substrate, followed by particle sintering method, where high temperature sintering process is needed, which usually leads to a high manufacture cost and restricts the development of ceramic membranes. On the other hand, organic polymeric membranes can be easily fabricated through solution casting method. Besides, membrane structure can be easy-tuned through changing parameters, such as concentration, temperature, and evaporation processes. The fabrication process is quite simple and the achieved membranes possess all the positive aspects, including low cost, high selectivity and desired pore structure. Nevertheless, polymer membranes are unstable in high temperature or pressure, poor stability in organic media. This encourages us to composite inorganic ceramic and organic polymer materials to prepare inorganic-organic membranes, which can combine the merits of two materials together. Casting polymer solution onto a ceramic support is an easy way to prepare inorganic-organic composite membranes. In previous work, however, the pore sizes of ceramic supports focused on 0.1-1.2 μm, namely ceramic MF membrane (not raw substrate), which inevitably brings in a high cost of ceramic-polymer composite membranes.Hence, in our work, a low-cost tubular ceramic-PEKC-CA composite microfiltration membrane was prepared by using 19-channel ceramic substrate (pore size ranging from 5-20 μm) as the support, with cardo polyetherketone (PEKC) as intermediate layer, and cellulose acetate (CA) as separation layer. A convenient decompression coating approach was employed. To enhance the adhesion between the top layers and the bottom substrate, dry-wet phase transformation was used. The wettability and morphology of ceramic-PEKC-CA composite membranes were characterized by static contact angle meter and scanning electron microscope (SEM), respectively. The retention rate of composite membranes to polyethylene oxide (PEO, Mw=600,000) is 45%, and polyethylene oxide (PEO, Mw=1,000,000) is 55%. Finally, the composite membranes were applied to remove insoluble impurities in rice bran fermentation broth, obtaining clarified and transparent permeation solution with merely 0.2% sugar content lost. This approach did reduce the cost of membranes, but the membrane flux is quite lower than that of the commercial ceramic membranes.Therefore, we came up with another project. Low cost tubular ceramic-PVDF/CA composite microfiltration membranes were prepared by a handy compression coating method using 19-channel ceramic support (pore sizes ranging from 5-20 μm) as substrate, and PVDF/CA blend solution as separation layer. It is observed that the concentration of PVDF/CA coating solution appeared to be the crucial factor on the separation performance of composite membranes, and higher membrane concentration would result in a higher rejection rate. To understand the connection between the preparation condition and membrane performance, arotational viscometer and scanning electron microscope (SEM) were used to characterize the viscosity of coating solution and morphology of composite membranes, respectively. The membranes were characterized with regard to water permeability, selectivity, as well as the performance of treating rice bran fermentation. Obtained results showed that with the PVDF/CA concentration in coating solution decreasing from 18 to 8wt%, the pure water flux of composite membranes increases from 200 to 2000 L/m2 h, and the retention rate to polyethylene oxide (Mw= 1,000,000) is much higher than that of ceramic membrane with an average pore diameter of 50 nm, and the ratio of turbidity removal is up to 99% when the composite membranes were applied to treat rice bran fermentation broth. More significantly, the composite membranes showed a high recovery rate of pure water flux after a simple physical cleaning process.Then, an economic evaluation was made to prove the low cost of ceramic-PVDF/CA composite microfiltration membranes. It can be estimated that the total cost for a length of 24 cm composite microfiltration membrane is about 9.70$, while that for the same length standard ceramic membrane (50 nm), the cost reaches up to 24.12$, which is nearly 2.5 times as much as composite membranes. Thus, the result of cost analysis shows that the composite membranes could be recognized as a promising alternative for industrial applications, because they have an advantage over the commercial ceramic membranes. However, the treating capacity of a 24 cm composite membrane is too small to satisfy the demand of real industrial applications. A composite membrane with bigger treating capacity and the same performance is required.Finally, ceramic-PVDF/CA composite microfiltration membranes were fabricated with an enlarged length of ceramic supports of 1016 cm through the compression coating approach via a modified coating setup, and 12wt% PVDF/CA coating solution was used as the separation layer. The composite membrane was applied to treat rice bran fermentation broth, achieving a high turbidity removal rate with 98.9%. In addition, little sugar was lost in the filtration process. Considering the membrane flux and retention ability, the amplified composite membranes can satisfy the demands of real industrial applications. |
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