Asymmetric composite membranes

Figure 2.2. Schematic representation of an asymmetric-composite membrane (Keizer and Burg-graaf 1988).

Table 2.2. Asymmetric Composite Membranes Combinations of Substrates...

The types of polymeric membranes that have attracted much interest for analytical applications and are nowadays in common use are characterized as nonporous membranes such as low-density polyethylene (LDPE), dense PP and PDMS silicone rubbers, and asymmetric composite membranes... 

Asymmetric/composite membrane This typically consist of a thin (0.5 to 20 microns) fine-pore layer responsible for separation and a support or substrate with single or multiple layers having progressively larger pores which provide the required mechanical strength. This type of structure maximizes the flux by minimizing the overall hydraulic resistance of the permeate (filtrate) flowing across the membrane structure. 

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

FIGURE 10.1.2 SEM photo (a) and a schematic representation (b) of an asymmetric, composite membrane. 

Conventionally, the production of symmetric macroporous ceramic filter elements, which also double as support structures for asymmetric composite membranes, is achieved through physical, shaping techniques which are designed to shape a suspension or slurry of ceramic particles into a solid membrane green (unsintered) body (Rg. 8.3). The production of the final porous... 

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. 

Membranes with a relatively uniform pore size distribution throughout the thickness of the membrane are referred to as symmetric or homogeneous membranes. Others may be formed with tight skin layers on the top or on both the top and bottom of the membrane surfaces. These are referred to as asymmetric or nonhomogeneous membranes. In addition, membranes can be cast on top of each other to form a composite membrane. 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

In order to gain some understanding of the behavior of an asymmetric membrane, let s consider a composite membrane consisting of two homogeneous membranes laminated together as shown in Figure 3. The same model has been studied recently by Henkens et al. (10). The first layer solution is ... 

The structure of the so-called "composite" membranes used in reverse osmosis is also much more complex than the conventional, simplistic description of the ultrathin semipermeable film deposited on and supported by a porous substrate. Most of these membranes which exhibit high flux and separation are composed of an anisotropic, porous substrate topped by an anisotropic, ultrathin permselective dense layer which is either highly crosslinked, or exhibits a progressively decreased hydrophilicity toward the surface. The basic difference between the conventional anisotropic (asymmetric) membrane and the thin film composite is that the latter might be... 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

NS-300 Membrane. The NS-300 membrane evolved from an effort at North Star to form an interfacial poly(piperazine Isophthala-mide) membrane. Credali and coworkers had demonstrated chlorine-resistant poly(piperazineamide) membranes in the asymmetric form (20). The NS-lOO, NS-200, and PA-300 membranes were all readily attacked by low levels of chlorine in reverse osmosis feedwaters. In the pursuit of a chlorine-resistant, nonbiodegra-dable thin-fiim-composite membrane, our efforts to develop interfaclally formed piperazine isophthalamide and terephthalamide membranes were partially successful in that membranes were made with salt rejections as high as 98 percent in seawater tests. 

Fig. 1. Water flux and NaCl rejection of several membrane types (10), where (D) represents seawater membranes, which operate at 5.5 MPa and 25°C ( ), brackish water membranes, which operate at 1500 mg/L NaCl feed, 1.5 MPa, and 25°C and (SSI) nanofiltration membranes, which operate at 500 mg/L NaCl feed, 0.74 MPa, and 25°C. A represents cellulose acetate—cellulose triacetate B, linear aromatic polyamide C, cross-linked polyether D, cross-linked fully aromatic polyamide E, other thin-film composite membranes F, asymmetric membranes G, BW-30 (FilmTec) H, SU-700 (Toray) I, A-15 (Du Pont) J, NTR-739HF (Nitto-Denko) K, NTR-729HF (Nitto-Denko) L, NTR-7250 (Nitto-Denko) M, NF40 (FilmTec) N, NF40HF (FilmTec) O, UTC-40HF (Toray) P, NF70 (FilmTec) Q, UTC-60 (Toray) R, UTC-20HF (Toray) and S, NF50 (FilmTec). To convert MPa to psi,...

Two different RO membrane types were evaluated in this study. The first was a standard cellulose acetate based asymmetric membrane. The second type, a proprietary cross-linked polyamine thin-film composite membrane supported on polysulfone backing, was selected to represent potentially improved (especially for organic rejection) membranes. Manufacturer specifications for these membranes are provided in Table III. Important considerations in the selection of both membranes were commercial availability, high rejection (sodium chloride), and purported tolerance for levels of chlorine typically found in drinking water supplies. Other membrane types having excellent potential for organic recovery were not evaluated either because they were not commercially... 

The mere preparation of porous membranes is accompanied with a noticeable decrease of permselectivity 11, which is undesirable for reverse osmosis and ultrafiltration, A thin dense layer should be adopted to attain a high permeability with — out the decrease of permselectivity, but this necessarily decreases the mechanical strength. This conflict is largely resolved by the construction of asymmetric or composite membranes as described also in the present review. 

Hollow-tiber membranes are subjected lo extensile studies lor gaseous separation (e.g.. CO-. 11-. CL. Ny. 1LS. CO. CH4). where the capillary configuration has an advantage over the spiral-wound fiat Hint and plate und-lramc devices. Another significant area of development and commercialization is pervaporation. These membranes are dense, rather than porous. structures. Generally asymmetric composite constructions arc employed with the ulirathin membranes on an open support. 

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