Membranes material

Since these membranes are polymer-based, their MWCO is highly dependent upon the solvent used. [21] 

TABLE 4.2. Commercially available solvent resistant nanofiltration membranes 

Membrane MWCO Manufacturer Material Solvents Max temp (°C) 

STARMEM240 400 MET polyimide silicon derived Tol, Xyl, EtOAc, C6 common org solvents, limited 60 

Dense membranes are a special type of polymeric membranes. Jacobs et al. published on the use of polydimethylsiloxane (PDMS) dense membranes in the hydrogenation of dimethylitaconate and acetophenone using standard homogeneous catalysts (see Section 4.6.1)[48]. The membranes were homemade from a PDMS solution in hexane, which was cross-linked in a vacuum oven at 100°C. The membranes were able almost completely to retain unmodified Ru-BINAP dissolved in isopropanol. However, as mentioned earlier, these applications will strongly depend on the size, i.e. molecular weight, of the substrate to be converted in order to guarantee a sufficient difference in size of the product and the catalyst to be retained. 

The variety of porous membranes, in terms of both materials and microstructures, makes them popular in different chemical reaction processes. They are applied mostly in a tubular configuration. However, the hollow fiber represents a trend for future development due to its remarkably high area/volume ratio. Table 2.5 summarizes the conventional inorganic porous membranes used in membrane reactors. 

Alumina occupies the dominant position in the research and development of ceramic membranes due to its high chemical and mechanical stability as well as low cost. Anodic alumina membranes are ideally suited 

Vycor glass Mesoporous Low mechanical strength Porous substrate 

AI2O3 Macroporous High chemical stability Porous substrate 

ZrOj Mesoporous High mechanical stability PBMR 

The performance of reverse osmosis is directly dependent on the properties of the membrane material. More specifically, the chemical nature of the membrane polymer and the structure of the membrane are what determines the rejection and flux properties 

Two most common families of RO membranes, based on the type of polymer backbone, are cellulose acetate and polyamide. Membranes made from these polymers differ in many respects, including performance, physical properties, structure and the manner in which they are created. These aspects are discussed below. 

Cellulose acetate (CA) membranes were the first commercially-via-ble RO membranes developed.These membranes were commercially viable because of their relatively high flux due to the extreme thinness of the membrane. High flux was important to reduce the size and cost of an RO system. 

Polyamide membranes were developed in an effort to improve upon the performance of CA membranes. In particular, the higher operating pressure and relatively low salt rejection of CA membranes were holding back RO technology from becoming more commercially viable. 

Neutral surface charge minimizes the potential for fouling with cationic polymer that might carry over from the pretreatment system. 

Being able to tolerate up to 1 ppm free chlorine on continuous basis offers some protection from biological growth on the membrane. This is particularly important because the CA polymer itself suppHes nutrients for microbial populations, which then metabolize the polymer and degrade the membrane. 

The chemical structure of ABS suggests that ABS would allow to reach high permeation fluxes because of its rubbery regions and high separation factors due to the glassy matrix. 

The sorption and the permeation of pure CO2, CH4, N2 and O2 through an ABS membrane (Lustran 246) have been measured (87). 

Since glassy polymers tend to become plasticized by C02, it is important to determine the magnitude to which CO2 effects plasticization. This is achieved in monitoring nonlinear effects of gas permeability of mixtures containing CO2. CO2 shows a higher sorption than expected. This is attributed to the presence of acrylonitrile moieties in the ABS copolymer. 

In the system N2/O2, the ABS membrane shows a high selectivity. This property makes this polymeric material attractive as a gas separation membrane for N2 and O2 at moderate temperatures. 

Poly(ethersulfone) (PES) is widely used for the preparation of membranes, including ultrafiltration, nanofiltration, and reverse osmosis membranes (88). However, PES lacks hydrophilic groups and the membrane material must be therefore modified. 

---

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Validation Considerations. Mechanisms other then size exclusion maybe operative ia the removal of vimses from biological fluids. Thus vims removal must be vaUdated within the parameters set forth for the production process and usiag membrane material representative of the product line of the filter. 

Nonporous Dense Membranes. Nonporous, dense membranes consist of a dense film through which permeants are transported by diffusion under the driving force of a pressure, concentration, or electrical potential gradient. The separation of various components of a solution is related directiy to their relative transport rate within the membrane, which is determined by their diffusivity and solubiUty ia the membrane material. An important property of nonporous, dense membranes is that even permeants of similar size may be separated when their concentration ia the membrane material (ie, their solubiUty) differs significantly. Most gas separation, pervaporation, and reverse osmosis membranes use dense membranes to perform the separation. However, these membranes usually have an asymmetric stmcture to improve the flux. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

A third factor is the ease with which various membrane materials can be fabricated into a particular module design. Almost ah membranes can be formed into plate-and-frame, spiral, and tubular modules, but many membrane materials caimot be fabricated into hollow-fine fibers or capihary fibers. Finahy, the suitabiHty of the module design for high pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can sometimes be important considerations. 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

Permeability P, can be expressed as the product of two terms. One, the diffusion coefficient, reflects the mobility of the individual molecules in the membrane material the other, the Henry s law sorption coefficient, reflects the number of molecules dissolved in the membrane material. Thus equation 9 can also be written as equation 10. 

FoUowiag Monsanto s success, several companies produced membrane systems to treat natural gas streams, particularly the separation of carbon dioxide from methane. The goal is to produce a stream containing less than 2% carbon dioxide to be sent to the national pipeline and a permeate enriched ia carbon dioxide to be flared or reinjected into the ground. CeUulose acetate is the most widely used membrane material for this separation, but because its carbon dioxide—methane selectivity is only 15—20, two-stage systems are often required to achieve a sufficient separation. The membrane process is generally best suited to relatively small streams, but the economics have slowly improved over the years and more than 100 natural gas treatment plants have been installed. 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Developments and advances in both membrane materials and reverse osmosis modules have increased the range of appHcations to which RO can be apphed. Whereas the RO industry has developed around water desalination (9,53,73,74), RO has become a significant cornerstone in other industries. 

The membrane material is a thin-film composite in a 20-cm module. 

Table 1. Simplified Hot Applied Membrane Material Specifications...

Ultrafiltration separations range from ca 1 to 100 nm. Above ca 50 nm, the process is often known as microfiltration. Transport through ultrafiltration and microfiltration membranes is described by pore-flow models. Below ca 2 nm, interactions between the membrane material and the solute and solvent become significant. That process, called reverse osmosis or hyperfiltration, is best described by solution—diffusion mechanisms. 

Fouling is controlled by selection of proper membrane materials, pretreatment of feed and membrane, and operating conditions. Control and removal of fouling films is essential for industrial ultrafiltration processes. 

Cellulose acetate, the earhest reverse osmosis membrane, is still widely used. Asymmetric polyamide and thin-film composites of polyamide and several other polymers have also made gains in recent years whereas polysulfone is the most practical membrane material in ultrafiltration appHcations. 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

Membrane Processes Membrane processes are also used diafiltration is convenient for the removal of small contaminating species such as salts and smaller proteins, and can be combined with subsequent steps to concentrate the protein. Provided that proper membrane materials have been selected to avoid protein-membrane interactions, diafiltration using ultrafiltration membranes is typically straightforward, high-yielding and capital-sparing. These operations can often tolerate the concentration or the desired protein to its solu-bihty limit, maximizing process efficiency. 

Saponification to the sulphonic acid yields the product marketed as Nafion. This material is said to be permselective in that it passes cations but not anions. It is used as a membrane material in electrochemical processes, in for example the manufacture of sodium hypochlorite. 

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

Basically, the spiral-wound configuration consists of a jelly roll-like arrangement of feed transport material, permeate transport material and membrane material. At the heart of the wall is a perforated permeate collector tube. Several rolls are usually placed end to end in a long pressure vessel. 

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

Common types of membrane materials used are listed in Table 3. This gets us into the concept of geometry. There are three types of modules generally used, namely Tubular, Spiral wound, and Hollow fiber. A comparison of the various geometries is given in Table 4. 

Table 3. Summary of Common Membrane materials and Their Characteristics.

Cellulose acetate is a common membrane material, but others include nylon and aromatic polyamides. The mechanism at the membrane surface involves the influent water and impurities attempting to pass through the pressurized side, but only pure water and certain impurities soluble in the membrane emerge from the opposite side. 

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material. 

---