Asymmetric Microporous Membranes

The mechanism for the formation of symmetric or asymmetric microporous membranes outlined above allows many of the variables of the membrane preparation procedures to be rationalized. 

J. Sasaki, K. Naruo, Kyoichi (Fuji Photo Film). Asymmetric microporous membrane containing a layer of minimum size pores below the surface thereof. US Patent 4933081, June 1990. 

Ultrafiltration Asymmetric microporous membrane, 1 to 10 lA pore radius Hydrostatic pressure difference, 0.5 to 5 bar Sieving mechanism Separation of macromolecular solutions... 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

The production process for (S)-phenylalanine as an intermediate in aspartame perpetuates the principle of reracemization of the nondesired enantiomer (Figure 4.5) in a hollow fiber/ liquid membrane reactor. Asymmetric hydrolysis of the racemic phenylalanine isopropylester at pH 7.5 leads to enantiopure phenylalanine applying subtilisin Carlsberg. The unconverted enantiomer is continuously extracted via a supported liquid membrane [31] that is immobilized in a microporous membrane into an aqueous solution of pH 3.5. The desired hydrolysis product is charged at high pH and cannot, therefore, be extracted into the acidic solution [32]. 

In gas separation applications, polymeric hollow fibers (diameter X 100 fim) are used (e.g. PAN) with a dense skin. In the skin the micropores develop during pyrolyzation. This is also the case in the macroporous material but is not of great importance from gas permeability considerations. Depending on the pyrolysis temperature, the carbon membrane top layer (skin) may or may not be permeable for small molecules. Such a membrane system is activated by oxidation at temperatures of 400-450 C. The process parameters in this step determine the suitability of the asymmetric carbon membrane in a given application (Table 2.8). 

Asymmetric Microporous Nonporous, skinned on microporous substrate Flat-sheet, tubular, hollow fiber Flat-sheet, tubular, hollow fiber Phase-inversion casting or spinning Phase-inversion casting or spinning Microfiltration, ultrafiltration, membrane reactors Reverse osmosis, gas separation, pervaporation, perstraction, membrane reactors... 

Tsai CY, Tam SY, Lu YF, and Brinker CJ. Dual-layer asymmetric microporous silica membranes. J. Membr. Sci. 2000 169 255-268. 

The oldest reported microporous membranes are based on carbon and are obtained by controlled pyrolysis of suitable polymeric precursors. Koresh and Soffer were the first to report properties of these membranes in a series of papers starting in 1980 (see refs, in Ref. [78]. Recently Linkov et al. [79] improved this method and arrived at mesoporous asymmetric hollow-fibre carbon membranes which could be transformed to microporous systems by coating the carbon membrane by e.g. vapour deposition polymerisation of polyimide forming precursors. 

Microporous membrane supports may be symmetric, asymmetric, or composite. They may have a uniform pore size or a distribution of pore sizes. They may be thick, thin, or ultrathin, with or without charges on external and internal surfaces. 

Membrane most UF membranes are polysulfone. Asymmetric microporous with thin skin... 

Membrane symmetric or asymmetric microporous. Ceramic, sintered metals, or polymers with pores 0.2 to 1 pm. Symmetric polymers have a porosity of 60 to 85% asymmetric ceramic membranes, porosity 30 to 40%, are used for high pressure and higher temperature <200°C. 

The neutral, microporous films represent a very simple form of a membrane which closely resembles the conventional fiber filter as far as the mode of separation and the mass transport are concerned. These membranes consist of a solid matrix with defined holes or pores which have diameters ranging from less than 2 nm to more than 20 //m. Separation of the various chemical components is achieved strictly by a sieving mechanism with the pore diameters and the particle sizes being the determining parameters. Microporous membranes can be made from various materials, such as ceramics, graphite, metal or metal oxides, and various polymers. Their structure may be symmetric, i.e., the pore diameters do not vary over the membrane cross section, or they can be asymmetrically structured, i.e., the pore diameters increase from one side of the membrane to the other by a factor of 10 to 1,000. The properties and areas of application of various microporous filters are summarized in Table 1.1. 

Figure 1.14 Scanning electron micrograph of membrane cross sections with typical structures (a) symmetric microporous membrane without a "skin" (b) asymmetric membrane with a "finger"-type structure and a dense skin at the surface (c) asymmetric membrane with a "sponge"-type structure, a dense skin, and pore sizes increasing from the surface to the bottom side (d) symmetric membrane with a sponge structure, a dense skin and a uniform pore size distribution in the substructure.

Figure 1.29 Schematic diagram of an asymmetric composite membrane showing the microporous support structure and the selective skin layer.

C. Tsai, S. Tam, Y. Lu, C. J. Brinker, Dual layer asymmetric microporous silica membranes. Journal of Membrane Science 169 (2000) 255. 

Membrane Most UF membranes are polysulfone asymmetric microporous with thin skin 0.1 to 1 [rm supported on a porous layer 50 to 250 jrm. Pore size 0.001-0.2 [rm. This is too porous for RO. Pore size prevents concentration polarization (limiting RO) but performance is limited by gel polarization with 0.2-0.4. Xgei = 0.25-0.35 for macromolecules = 0.75 for colloids. Need to have membrane life > 1 year. 

Membrane symmetric or asymmetric microporous. ceramic, sintered metals or polymers with pores 0.2-1 gm. Symmetric polymers have a porosity of 60 to 85% asymmetric ceramic membranes, porosity 30 to 40%, are used for high pressure and higher temperature < 200 °C. Pressure 0.03-0.35 MPa. Pressure 0.3-0.5 MPa for ceramic. Hydraulic permeability. A 70 to 10000 g/s m MPa, capacity/unit 0.001-1 L/s. Liquid permeate flux 0.001-0.2 L/s m with the perme-... 

Anisotropic (asymmetric) Defines a particular type of ultrastructure of microporous membranes. The surface of the membrane where separation occurs is more dense than the rest of the membrane body. The pore diameter increases in a direction perpendicular to the membrane surface, with the pore opening near the separation surface being smaller than the pore opening on the bottom of the membrane. This skin layer is typically present in polymeric membranes made by the phase-inversion process. Some asymmetry is also present in many inorganic membranes. 

Hollow fiber membranes with a positively charged nanofiltration selective layer have been fabricated by using asymmetric microporous hollow fibers made from a Torlon PAI type as the porous substrate followed by a post-treatment with poly(ethyleneimine) [79]. The membrane structure and the surface properties can be tailored by adjusting the polymer dope composition, spinning conditions, and the posttreatment parameters. 

For PET track membranes treated in air plasma, a decrease in their thickness and an increase in the effective pore diameter were observed. Additionally, the pores became asymmetric. The permeability increased and depended on the pH of the filtered solution. The membrane surface was no longer smooth, because of the faster etching of the amorphous areas than of the crystalline areas (Dmitriev et al. 2002). The surface of the PET membrane becomes hydrophilic and, in properly chosen conditions, the surface properties are stable (Dmitriev et al. 1995). In polypropylene hollow fiber microporous membranes (PPHFMMs), both the O and the N functionalities were found and numerous cracks could be seen on the surface. Generally, a decrease in the flow rate was observed as a result of faster cake formation and its compaction. The main positive result of the plasma treatment was a significant improvement in the membrane regeneration characteristics (Yu et al. 2008b). 

Schauer et al. (2003) used poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) membranes to separate water-EtOH mixtures by PV. Asymmetric membranes were prepared from solutions containing chloroform as a solvent and 1-butanol as a nonsolvent via the phase inversion technique. Dense membranes were prepared from chloroform solution by evaporation. Nonporous membranes (membranes precipitated from solutions with a small amount of the nonsolvent or prepared by evaporation) were preferentially permeable to water. Microporous membranes (prepared from solutions with a large amount of the nonsolvent) were preferentially permeable to EtOH, provided the membrane was not wetted by the feed solution. 

Qi et al. (2006a) developed PDMS/PAN membranes for sulfur removal from gasoline by PV. PDMS, ethyl orthosilicate, dibutyltin dilaurate, and -heptane were used for the preparation of manbranes, and asymmetric microporous PAN manbranes... 

Commercial laccase was immobilized onto a spiral-wound asymmetric polyethersulfone membrane. The laccase membrane reactor was applied to the biodegradation of a model phenol solution. The feasibility of using a hollow-fiber membrane reactor for the Upase-catalyzed interesterification reaction of triglycerides and fatty acids in a micro aqueous n-hexane system was developed by Basheer et al. In this case they use a stirred-tank reactor as well as a hollow-fiber membrane reactor system. Moreover, in 2004 a new immobilization of lipase into microporous... 

The membranes used in ultrafiltration (UF) are almost exclusively of an asymmetric, microporous construction and available with pore ratings in the range 0.001-0.02 pm. They are manufactured as microporous structures from a range of polymers and ceramics and formed as either flat sheets or tubes for use in one of four basic arrangements (see Figure 1.51 and Table 1.5) ... 

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