Cast membranes

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

Another type has several flat plates manifolded into a plastic header. The surface of the laminate is suitable for dip-casting membranes, whereas the interior is several orders of magnitude more porous. Permeate collects in the center of the laminate and drains into the header. 

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. 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

TFF module types include plate-and-frame (or cassettes), hollow fibers, tubes, monoliths, spirals, and vortex flow. Figures 20-52 and 20-53 show several common module types and the flow paths within each. Hollow fiber or tubular modules are made by potting the cast membrane fibers or tubes into end caps and enclosing the assembly in a shell. Similar to fibers or tubes, monoliths have their retentive layer coated on the inside of tubular flow channels or lumens with a high-permeability porous structure on the shell side. 

Solution-cast membranes (100—200 /urn in thickness) from sulfonated polymers with an ion exchange... 

The term membrane encompasses a wide range of potential substrates that can be used for the immobihzation of NAs. It includes traditional polymeric cast membranes , such as nitrocellulose, nylon or polypropylene, and also innovations such as ceramic or track-etched materials (alumina membranes). Membranes as substrates in DNA array fabrication can possess advantages over other surfaces their surface area can be much greater (200% more in cast membranes and 500% more in aliuniniiun membranes) than certain other alternatives. This is primarily a consequence of their possession of pores. 

Lamination (used by Patel et al. [18] above) is a fabrication technique that could be considered more widely by researchers in laboratories as an adjunct to screen-printing, particularly for deposition of outer diffusion-limiting membranes. Sensors can be constructed entirely by screen-printing technology [16], but it is difficult to maintain control over membrane thickness, porosity, etc. Deposition of pre-cast membranes by lamination may be a way of controlling these parameters more precisely. 

Possibly coat tubular centrifugal cast membranes with well-developed intermediate layers and perform stability tests. Test on changes in pore-size (permselectivity measurements). 

Williams (2002), Chairman of E I Du Pont de Nemours, describes its new process (solution cast membrane technology) for manufacturing Nafion membranes aimed at PEFC/MEAs. With large production the membrane NR112 can fall to about 80 per m, and NRlll to about 45 per m. Details of the two types are given. 

Kopaciewicz W, Sheer DG, Arnold TE, and Goel V. Cast membrane structure for sample preparation, US Patent 6,048,457, 2000. 

FIGURE 27.32 Fuel cell polarization curve of a 50 cm MEA made with the new 3M cast membrane (30 pm). (From 3M, www.3m.com.)... 

The PEC-1000 membrane of Toray Industries, Inc., has been described by Kurihara et al (21). This membrane was characterized as a thin-film composite type made by an acid catalyzed polymerization on the surface. Membrane performance reported for seawater tests was 99.9 percent TDS rejection at fluxes of 5.0 to 7.4 gfd (8.3 to 12.3 L/sq m/hr) when tested with 3.5 percent synthetic seawater at 800 psi (5516 kPascals). The membrane was stable in 1500-hour tests in spiral-wrap elements and exhibited stability in a temperature range of 25 to 55°C and in a pH range from 1 to 13. High organic rejections were reported for the PEC-1000 membrane rejection of dimethylformamide from a 10 percent solution was 95 percent and similar tests with dimethylsulfoxide showed 96 percent rejection. The composition and conditions for preparation of PEC-1000 membrane is not disclosed in Reference 21. Apparently it is a dip-cast membrane related to compositions described by Kurihara, Watanaba and Inoue in Reference 18. 

These solution-cast membranes are registered as NRE membranes. The effects of the manufacturing processes on membrane properties are compared in Table 10.1 [24, 32],... 

The membranes formed via these two processes demonstrate different performance levels. Typically, the post-doped PEMs are stronger and tougher than directly cast membranes, which require a polymer of a higher inherent viscosity in order to prepare membranes with reasonable strength. In general, the conductivity of the directly cast membrane will be significantly higher than that of post-doped membranes [154]. 

In order to confirm that cyclohexanone is extracted more reluctantly than other solvents, the concentrations of extracted solvents in the gelation bath water were measured with immersion time lapse by using gas chromatography. As illustrated in Figure 12, the extraction rate of cyclohexanone is less than 1/3 of DMS s. In case of well-known acetone-formamlde casting dope for cellulose diacetate RO membrane, formamide is extracted faster than acetone when as cast membrane is immersed into chilled water... 

Dacron pressed paper was found to be highly adequate for permeate transport while possessing the wet strength plastic properties needed in winding operations. Dacron paper was also used in casting membranes directly on support structures. 

Security in delivery is most likely best obtained by dealing with companies of a reasonable size and well introduced in the field of membrane filtration. But also here, the plate-and-frame system has an additional advantage, because membranes from other manufacturers who can cast membranes in sheet form can be used direct in our modules, though this is not something DDS is very much in favour of. 

The six flat cast membranes were shrunk at different temperatures (from 68 to 85°C) prior to loading the membranes into the reverse osmosis test cells. This treatment adjusts the average surface pore size of each membrane so that a range of porosities could be studied. A prepressurization at a pressure of 11 720 kPa for 2 hours was used to stabilize the membranes for subsequent use at pressures of 6900 kPa or lower. (All pressures listed are gauge pressure.)... 

Figure 3.3 Scale of heterogeneity in cast membranes-variation in pore size with rate of precipitation and distance into membrane.

When producing membranes for use in the pharmaceutical industry, the use of depyrogenated water is required. Otherwise, pyrogens will be incorporated in the cast membrane and some will leach out in the filtrate with use. Depyrogeniza-tion may be accomplished by filtering water from the ion exchange columns with a 10,000 MWCO UF membrane. 

Typical results obtained with the membranes, fabricated and utilized as described above, are shown in Figures 7 and 8. It is perhaps instructive to compare the results of the laboratory-cast membranes in Figure 7 to those of the commercial cellulose acetate membranes in Figure 5. 

---