Thin dense skin

Particularly, the nonsolvent immersion, that is, the Loeb-Sourirajan preparation method is an important methodology. In this method, a polymer solution is cast into a film and the polymer precipitated by immersion into water [10,144], The nonsolvent (water) quickly precipitates the polymer on the surface of the cast film, producing an extremely thin, dense-skin layer of the membrane [10,144], The polymer under the skin layer precipitates gradually, ensuing in a more porous polymer sublayer [145], Following polymer precipitation, the membrane is usually annealed in order to improve solute rejection [10,144]. 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

An asymmetric membrane has a very thin dense top layer (or skin) with a thickness of 0.1-0.5 pm. A porous sublayer with a thickness of approximately 50-150 pm supports the dense top layer. The thin dense skin facing the feed solution acts as the selective layer, allowing water passage but rejecting dissolved solids. The resistance to mass transfer across the membrane is also mainly determined by the thin top layer. In asymmetric membranes, the selective top layer and the porous support layer are made of the same polymer material. Asymmetric membranes can be obtained by phase inversion, a technique in which a polymer in solution is transformed in a controlled manner from a liquid into a solid form. The top skin layer and the porous support layer are formed in a single-step process. 

The high value of the water flux parameter DiCi, along with adequately low NaCl transport, motivated the fabrication of p-2221 into asymmetric membranes. The preparation of asymmetric membranes is a multistep process involving solution casting, partial solvent evaporation, and gelation as the most critical operations. The resulting asymmetric structure (a thin dense "skin" supported by a porous substructure 50-100 urn in thickness) combines the high flux of a thin membrane with the mechanical properties of a much thicker film. 

In order to achieve high permeabilities and high separation factors for small molecules, asymmetric membranes have been developed in recent years. They consist of a thin dense skin about 0.1 to 1.0 pm thick formed over a much thicker microp-orous layer that provides support for the skin. 

PEEK-WC is a modified PEEK, as shown in Figure 6.2. Ultra-thin asymmetric gas separation membranes of modified PEEK can be prepared by a dry/wet phase inversion technique [61,62]. Under optimized conditions, membranes with an open cellular morphology and an ultra-thin dense skin of about 50 nm can be obtained. The membranes are prepared by casting a film of a solution of PEEK-WC on a glass plate. The films are then coagulated, dried, and removed from the glass plate. 

The membranes used in GS can be distinguished in two main categories, polymeric and inorganic. Polymeric membranes, specifically used for GS, are generally asymmetric or composite and based on a solution-diffusion transport mechanism. These membranes, made as flat sheet or hollow fibres, have a thin, dense skin layer on a micro-porous support that provides mechanical strength [7]. Typically, polymeric membranes show high, but finite, selectivities with respect to porous inorganic materials due to their low free-volume... 

Membranes for ultrafiltration are in general similar to those for reverse osmosis and are commonly asymmetric and more porous. The membrane consists of a very thin dense skin supported by a relatively porous layer for strength. Membranes are made from aromatic polyamides, cellulose acetate, cellulose nitrate, polycarbonate, polyimides, polysulfone, etc. (M2, P6, Rl). 

Polymeric membranes utilized for nano- and ultrafiltration applications are usually produced by the phase inversion process. Unlike in reverse osmosis, there is no continuous thin dense skin on the membrane surface. The membrane, however, has a tighter pore surface than the bulk of the membrane to allow the desired separation. The bulk of the membrane is much more open, to provide membrane support while limiting the resistance to water flux. Polymers employed in these membranes include polysulfone, poly(ether sulfone), polyacrylonitrile, poly(vinylidene fluoride), aromatic polyamides, sulfonated poly(ether sulfone), and cellulose acetate. 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Numerous asymmetric membranes were prepared under various conditions and their cross-section was examined by SEM. Typical of the results are those shown in Figure 5 for the membrane cast from 70 30 THF-formamide and gelled in IPA. Close inspection of Figure 5 reveals a thin, relatively dense skin supported by a microporous layer. The support layer contains macrovoids, the cause of which is presently under investigation. 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

Kimura and Sourirajan have offered a theory of preferential adsorption of materials at interfaces to describe liquid phase, selective transport processes in portms membranes. Lonsdale et al. have ofiered a simpler explanation of the transport behavior of asymmetric membranes which lack significant porosity in the dense surface layer. Their solution-diffusion model seems to adequately describe the cases for liquid transport considered to date. Similarly gas transport should be de-scribable in terms of a solution-diffusion model in cases where the thin dense membrane skin acts as the transport moderating element. 

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). 

Asymmetric ultrafiltration membranes consist of a thin, dense top layer (the skin), which is responsible for the selective rejection of solute molecules, and a more open, porous substructure that does not affect the membrane performance negatively. 

The reason for this breakthrough resided in the asymmetric structure of the membrane. When the product of the water flux and the total membrane thickness was calculated, the value was 666 times greater than Sourirajan s CA films. The most obvious explanation was that the effective membrane thickness was much less than the total membrane thickness. Loeb postulated the existence of a dense skin less than 1 ju in thickness supported by a relatively porous substrate. Thus, the substrate provided mechanical strength and the thin skin minimized the resistance to hydraulic permeability through the membrane. For the first time in history, it became possible to remove salt from water (95 to 98%) at pressures of 50 to 75 atmospheres with flux values of 10 to 15 gallons of product water per day per square foot of membrane area (GSFD). 

The term asymmetric refers to membranes comprised of a porous spongy wall supporting a very thin dense layer. The thin skin layer, approximately 0.5... 

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