Polyolefinic membranes

The most common supported tubes are those with membranes cast in place (Fig. 17). These porous tubes are made of resin-impregnated fiber glass, sintered polyolefins, and similar materials. Typical inside diameters are ca 25 mm. The tubes are most often shrouded to aid in permeate collection and reduce airborne contamination. 

Externally cast membranes are first formed on the iaside of paper, polyester, or polyolefin tubes. These ate then iaserted iato reusable porous stainless-steel support tubes inside diameters ate ca 12 mm. The tubes ate generally shrouded in bundles to aid in permeate collection. 

Stretched Polymers MF membranes may be made by stretching (Fig. 20-68). Semicrystalline polymers, if stretched perpendicular to the axis of crystallite orientation, may fracture in such a way as to make reproducible microchannels. Best known are Goretex produced from Teflon , and Celgard produced from polyolefin. Stretched polymers have unusually large fractions of open space, giving them very high fluxes in the microfiltration of gases, for example. Most such materials are very hydrophobic. 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, poly(tetrafluoroethylene), and cellulose acetate, have been used for porous substrates for supported-liquid membranes. The PVdF coated polyolefin-based microporous membranes used in gel—polymer lithium-ion battery fall into this category. Gel polymer... 

All lithium based batteries use nonaqueous electrolytes because of the reactivity of lithium in aqueous solution and because of the electrolyte s stability at high voltage. The majority of these cells use microporous membranes made of polyolefins. In some cases, nonwovens made of polyolefins are either used alone or with microporous separators. This section will mainly focus on separators used in secondary lithium batteries followed by a brief summary of separators used in lithium primary batteries. 

Microporous polyolefin membranes (see Figure 2) in current use are thin (<30 /[Pg.185]

Prior work related with shutdown separators also involved application of waxes on membranes." " In these cases, the wax or low melting polymers were coated on the polyolefin separator. The disadvantage of this technique is that the coating can block the pores of the separator and thus can affect the performance by increasing separator resistance. Moreover, the coating level has to be very high to get complete shutdown. 

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

The best hope for olefin/paraffin facilitated membrane separations seems to be the solid polymer electrolyte membranes discussed earlier, the results of which are shown in Figures 11.21 and 11.22. If stable membranes with these properties can be produced on an industrial scale, significant applications could develop in treating gases from steam crackers that manufacture ethylene and from polyolefin plants. 

Adopted from the state-of-the-art in RO, TFC membranes have become increasingly interesting for UF as well. One of the first examples of a commercial membrane of this type is composed of a thin barrier layer from regenerated cellulose on a porous polyolefine support [32]. Significant increase in selectivity in protein UF via electrostatic exclusion in addition to size exclusion has been achieved by introducing fixed charges into the barrier layer of a cellulose-based TFC membrane [33]. 

Figure 8.14 Flow diagram showing the use of hydrocarbon-permeable membranes to recover unreacted monomers from a polyolefin plant resin degassing unit. The photograph is of a system installed by MTR in Qatar in 2007.

The problem of the interactions between membrane and absorbent solution interests, for instance, the removal of CO2. Reactive absorption liquids, such as amines, that are used for this type of removal, usually wet polyolefin membranes. Wettability depends on the surface tension of the liquid, membrane material, contact angle, and pore properties of the membrane. Possible solutions to this problem are to employ more resistant membrane materials, to use different absorbent liquids, and to deposit a nonporous layer on the membrane surface that prevents any passage of the liquid through pores. In order to do not increase too much the resistance to the mass transport, the layer has to be thin and highly permeable to the gaseous species. The dense skin can be useful also for avoiding any possible contamination of the feed gas by the absorbent (Figure 38.4). 

Feron PHM and Jansen AE. CO2 separation with polyolefin membrane contactors and dedicated absorption liquids Performances and prospects. Sep. Purif. Technol. 2002 27 231-242. 

OD applications are those fabricated from nonpolar polymers with low surface free energies. The most commonly used OD membrane materials are polyolefins, such as polyethylene and polypropylene, and fluoropolymers, such as polytetrafiuoroethylene (PTFE) and polyvinylidine difiuoride (PVDF). ... 

Polyolefins. Low density polyethylene and polypropylene have been developed as sheet and hollow fiber mlcroporous membranes, respectively, for use In plasmapheresis. Polyethylene Is made porous by stretching the annealed film ( ), while polypropylene la made porous by coextruding hollow fibers with a leachable plasticizer. Neither membrane has been prepared with small pore dimensions suitable for protein rejection. These polyolefin membranes are characterized by good chemical stability, but require special surfactant treatments to make them wettable. Their low deformation temperature precludes the use of steam sterilization. Because they are extruded without the usual antl-oxldants and stabilizers, their stability la lower than Injection molding formulations of the same polymer. 

The thermal process is perhaps the most universally applicable of all the phase inversion processes because it can be utilized over the widest range of both polar and nonpolar polymers. However, its commercial use for membrane applications will probably be restricted to polyolefins, particularly polypropylene. A large number of the substances can function as latent solvents (Table X). They usually consist of one or two hydrocarbon chains terminated by a polar hydrophilic end group. Therefore, they exhibit surface activity which may explain their ability to form the emulsion-like Sol 2 micelles at elevated temperatures. One latent solvent which is worthy of special mention because of its broad applicability is N-Tallowdiethanolamlne (TDEA). 

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