Ceramic membranes stainless steel

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Laboratory-scale bubble columns for ozonation preferably have a reactor liquid phase volume of VL = 2-10 L, with a height-to-diameter-ratio of hid = 5-10. The ozone/oxygen (ozone/air) gas mixture is supplied through a ceramic or stainless steel porous plate fine pore diffuser (porosity 3,10-40 pm hole diameter). PTFE-membranes are a comparatively new alternative for the ozone gas-to-water transfer (Gottschalk et al., 1998). 

Some of those developments at Oak Ridge were believed to spin off in some form at Union Carbide and some aspects of the efforts led to the commercialization of dyiuunically formed membranes primarily for ultrafiltration and hypeiTiltration (reverse osmosis) applications. In these dynamic membranes, a mixture of zirconium hydroxide and polyacrylic acid deposited on a porous support which provides the necessary mechanical strength. The support is mostly made of porous carbon although porous ceramic and stainless steel are also used. These non-sintered membranes, in great contrast to most of the membranes discussed in this book, are formed in situ and require periodic regeneration with new zirconium hydroxide and polyacrylic acid. 

Application at high temperature requires robust and thermostable systems. Both for ceramically and stainless-steel-supported systems the thermostability has been demonstrated. So, in spite of the different thermal expansion coefficients, the asymmetric membrane remains intact. However, there are no data available on the resistance of zeolitic membranes to thermal stresses, as a result of, for example, large sudden changes in temperature. The siainless-steel-supported system seems the most promising configuration... 

In addition to porous ceramic and stainless steel plates and tubes commonly employed as supports of zeolite membranes and films, a wide variety of alternative supports have been used. Among these are steel [250], ceramic [251,252] monoliths... 

Cross-flow MF Pall PSS (2.5 pm limit of separation), Fairey Microfilfrex FM4 (1 pm) and APV Ceraver (1.4 pm) ceramic and stainless steel membranes SpinTek ST IIL centrifugal membrane filtration technology... 

In addition to porous ceramic and stainless steel plates and tubes commonly employed as supports of zeolite membranes and films a wide variety of alternative supports have been reported steel [357] and ceramic monoliths [119,358,359] (see Figure 11.29) (also shaped as wheels or rotors [360]), ceramic hollow fibers [23,57,166,306], stainless steel grids [361,362], porous metal support sheet [363], wire gauze packing [364], glass fibers [365,366], nonporous ceramic [367], and metal plates [368,369], glass and steel beads [370], and zeolite discs to avoid mismatch in expansion coefficient [371]. 

Up to today inorganic membranes are far more expensive than polymeric ones. This is due to the higher cost of the substructure, a sintered ceramic or stainless steel tube, and to the multilayer coating procedure, usually requiring a high-temperature heat treatment between two coating steps. Module assembly with connections between ceramic tubes and the stainless steel of the other module components is complicated and expensive, too. At least partially these... 

The search for a high permeability at low cost have led to the development of membranes composed of an ultrathin Pd-Ag layer (<10 pm) on a ceramic or stainless steel porous supports [16-18]. They combine the infinite selectivity to hydrogen of the dense Pd-Ag film with a mechanical strength of the porous support, which resistance to hydrogen transfer is negligible. These membranes are very promising, since they exhibit remarkably high fluxes [0.5-2.5 mol/(m s). 

Development studies are often performed on samples prepared as flat discs. The disc is clamped in a fixture that provides physical support (in the case of a freestanding sample) and eUminates leak paths around the edge of the sample [lb-18]. Care needs to be taken to avoid interdiffusirMi of the membrane with metallic components of the fixture. Tubular membranes, typically prepared by electroless deposition of the alloy functional layer onto porous ceramic or stainless steel tubes, are also common [19,20]. Sealing of metal tubes in a text fixture is straightforward ceramic tubes require additional care because of their fragility. 

The membrane is usually made from one of several materials. Woven polyester or cotton, the most commonly used and least expensive material, is adequate for temperatures up to 150°C. Siatered plastic is used where a low cost, washable surface is desired. This material is temperature limited by the polymer material to about 60°C and the flow of some powders may cause a static charge build-up on the membrane that could be hazardous ia some operatioas. Wovea fiberglass fabric or porous ceramic block is used for temperatures up to about 425°C. Siatered stainless steel powder or bonded stainless mesh is used for corrosion resistance, and for temperatures up to 530 to 650°C. Additional information can be found ia the Hterature (38,39). 

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. 

Fig. 6. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane disk was prepared by pressing Bao.5Sro.5Coo.8Feo.2O3-s oxide powder in a stainless steel module (17 mm inside diameter) under a pressure of (1.3-1.9) X 109 Pa. The effective area of the membrane disk exposed to the feed gas (CH4) was 1.0 cm2 (72).

Zeolite membranes are generally synthesized as a thin, continuous film about 2-20 xm thick on either metallic or ceramic porous supports (e.g., alumina, zirco-nia, quartz, siHcon, stainless steel) to enhance their mechanical strength. Typical supported membrane synthesis follows one of two common growth methods (i) in situ crystallization or (ii) secondary growth. Figure 10.2 shows the general experimental procedure for both approaches. 

Microfiltration membranes usually have a nominal pore diameter in the range of 0.1-10 pm. However, the membrane specification is not an absolute parameter. The membranes usually present a pore size distribution around the nominal value and the shape of the bioparticles can determine whether they are retained or pass through the membrane. The membranes are manufactured from polymers, such as Teflon, polyester, PVC (polyvinyl chloride), Nylon, polypropylene, polyethersulfone, and cellulose, or from inorganic materials, such as ceramic and sinterized stainless steel. 

Microfiltration units can be configured as plate and frame flat sheet equipment, hollow fiber bundles, or spiral wound modules. The membranes are typically made of synthetic polymers such as Polyethersulfone (PES), Polyamide, Polypropylene, or cellulosic mats. Alternate materials include ceramics, stainless steel, and carbon. Each of these come with its own set of advantages and disadvantages. For instance, ceramic membranes are often recommended for the filtration of larger particles such as cells because of the wider lumen of the channels. However, it has been shown that spiral wound units can also be used for this purpose, provided appropriate spacers are used. 

Mechanical stability - Organic membranes contact and can undergo inelastic deformations under high pressures, leading to lower permeabilities. Ceramic membranes supported on robust materials such as stainless steel or structural ceramics can be expected to withstand very high pressures. 

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