Membrane commercial availability

Inorganic membranes commercially available today are dominated by porous membranes, particularly porous ceramic membranes which are essentially the side-products of the earlier technical developments in gaseous diffusion for separating uranium isotopes in the U.S. and France. Summarized in Table 3.1 are the porous inorganic membranes presently available in the market (Hsieh 1988). They vary greatly in pore size, support material and module geometry. 

Release of peptides with ammonia might work only on homemade membranes. Commercially available membranes with amino functional groups often have other than ester bonds between the spacer and cellulose. For such membranes the use of additional linkers might be necessary (e.g., thioester (66), HMB linker (67), Rink linker (68)). 

For the preparation of tubular silica membranes, commercially available mesoporous membranes [17] are used. These tubular supports have a total length of 25 cm and are enamelled at both ends, required for a gas-tight sealing with carbon seals to the reactor, so that an effective porous length of 20 cm remains. The tube consists of 4 layers. Layer 1, 2 and 3 consist of a-alumina with a thickness of 1.5 mm, 40 and 20 im and a pore diameter of 12, 0.9 and 0.2 im respectively. Layer 4 consists of y-alumina with a thickness of 3-4 im a Kelvin radius of 4 nm. A schematic drawing of the cross-section of a mesoporous support tube is provided in Figure 4. 

The trend for market penetration will probably follow a path as shown schematically in Fig. 1.2. The figure does not pretend to give quantitative information but merely shows the relative importance of different application fields in time and illustrates the increasing complexity. Gas separation with microporous membranes will probably only start on a commercial scale if membrane business for liquid filtration has become sufficiently profitable to bear the developments necessary to produce commercial gas separation membranes. Commercial availability should therefore be improved for applications not directly making use of liquid filtration membranes. 

In addition to ceramic membranes commercially available for filtration, a multilayer membrane structure can be found in other applications such as catalytic membrane... 

The earliest commercially available filters were manufactured in two pore sizes 0.45 and 0.8 pm. The 0.45 pm-rated membranes were considered to be stefilizing-grade filters and were successfully used in the sterile filtration of pharmaceuticals and parenterals. The membrane filters were qualified using Serratia marcescens a standard bacterium, having dimensions of 0.6 x 1 pm. However, in the late 1960s it became apparent that the matrix of the 0.45 pm-rated filters could be penetrated by some pseudomonad-like organisms (1). For sterile filtration apphcations in the 1990s, 0.2 pm-rated membranes are the industry standard in the manufacture of sterile parenterals and pharmaceuticals. 

Electroporation. When bacteria are exposed to an electric field a number of physical and biochemical changes occur. The bacterial membrane becomes polarized at low electric field. When the membrane potential reaches a critical value of 200—300 mV, areas of reversible local disorganization and transient breakdown occur resulting in a permeable membrane. This results in both molecular influx and efflux. The nature of the membrane disturbance is not clearly understood but bacteria, yeast, and fungi are capable of DNA uptake (see Yeasts). This method, called electroporation, has been used to transform a variety of bacterial and yeast strains that are recalcitrant to other methods (2). Apparatus for electroporation is commercially available, and constant improvements in the design are being made. 

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. 

Membrane filtration has been used in the laboratory for over a century. The earliest membranes were homogeneous stmctures of purified coUagen or 2ein. The first synthetic membranes were nitrocellulose (collodion) cast from ether in the 1850s. By the early 1900s, standard graded nitrocellulose membranes were commercially available (1). Their utihty was limited to laboratory research because of low transport rates and susceptibiUty to internal plugging. They did, however, serve a useflil role in the separation and purification of coUoids, proteins, blood sera, enzymes, toxins, bacteria, and vimses (2). 

Whey has been used ia some substitute dairy products but aot as a source of proteia. Whey proteias have beea used ia dairy substitutes only siace the commercialisation of ultrafiltration (qv) technology. Membranes are used that retain proteia and permit water, lactose, and some minerals to pass through as permeate. Proteia coaceatrates are available from both acid and sweet whey and ia coaceatratioas of 35—80 wt % proteia. Whey proteia isolates are commercially available having proteia >90 wt%. The cost of these isolates is too high, however, to make them economical for substitute dairy foods. 

Commercially available membranes are usually reinforced with woven, synthetic fabrics to improve the mechanical properties. Several hundred thousand square meters of IX membranes are now produced aimuaHy, and the mechanical and electrochemical properties are varied by the manufacturers to suit the proposed appHcations. The electrochemical properties of most importance for ED are (/) the electrical resistance per unit area of membrane (2) the ion transport number, related to current efficiency (2) the electrical water transport, related to process efficiency and (4) the back-diffusion, also related to process efficiency. 

Hydrogen Hydrogen recovery was the first large commercial membrane gas separation. Polysulfone fiber membranes became available in 1980 at a time when H9 needs were rising, and these novel membranes qiiickly came to dominate the market. Applications include recovery of H9 from ammonia purge gas, and extraction of H9 from petroleum crackiug streams. Hydrogen once diverted to low-quahty fuel use is now recovered to become ammonia, or is used to desulfurize fuel, etc. H9 is the fast gas. 

In Sittt Filter Membranes In situ membranes are being fitted into incinerator flue-gas stacks in an attempt to reduce hydrocarbon emissions. Two types of commercially available gas separation membranes are being stndied (I) flat cellnlose acetate sheets and (2) hoUow-tnbe fiber modules made of polyamides. 

The electrolyte is a perfluorosulfonic acid ionomer, commercially available under the trade name of Nafion . It is in the form of a membrane about 0.17 mm (0.007 in) thick, and the electrodes are bonded directly onto the surface. The elec trodes contain veiy finely divided platinum or platinum alloys supported on carbon powder or fibers. The bipolar plates are made of graphite or metal. 

Commercially available plate- and frame- type ultrafiltration equipment are used for exopolysaccharide concentration. The membranes are polysulphone or polyvinylidine fluoride with molecular weight cut-off between 20-60,000. There is a relatively low eneigy requirement (1-2 kWh m 3) for pumping the fluid through the filtration unit at the desired pressure. Pressure difference across the membrane is of the order 2-14 atmospheres. 

Table l. Commercially available microporous membrane materials used as separators in lithium-ion batteries. 

The discussion of Section 5-1 clearly illustrates that the most important response characteristic of an ISE is selectivity. Depending on the nature of the membrane material used to impart the desired selectivity, ISEs can be divided into three groups glass, liquid, or sohd electrodes. More than two dozen ISEs are commercially available and are widely used (although many more have been reported in the literature). Such electrodes are produced by firms such as Orion Research, Radiometer, Coming Glass, Beckman, Hitachi, or Sensorex. 

The catalysts at the anode can be made less sensitive to CO poisoning by alloying platinum with other metals such as ruthenium, antimony or tin[N.M. Markovic and P.N. Ross, New Flectro catalysts for fuel cells CATTECH 4 (2001) 110]. There is a clear demand for better and cheaper catalysts. Another way to circumvent the CO problem is to use proton-exchange membranes that operate at higher temperatures, where CO desorbs. Such membranes have been developed, but are not at present commercially available. 

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