Porous membrane 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. 

The permeation examples that are given here are based on own work and have been published in different articles [61-66]. The membrane used is of the asymmetric type and consists of a 40 pm thick layer of intergrown silicalite-1 crystals on a 3 mm thick layer of highly porous sintered stainless steel. The geometric surface area amounts to 3 cm. Stainless steel has the advantage of easy mounting in all types of equipment which facilitates practical application compared to ceramic supports. 

Some more module configurations are reported for use in MD. Lawson and Lloyd [77] have designed a laboratory-scale MD module as shown in Figure 19.13, where the membrane was sandwiched between the two half-cells, and several hose clamps held the module together. The total area available is 9.7 cm and the smooth transitions at the module entrance as well as exit allow achievement of relatively high Reynolds numbers, whereby conventional boundary layer equations are applicable. The module does not require a support in low pressure-drop applications such as DCMD. Wider permeate channels would require a support for VMD experiments. A porous sintered stainless steel material has been used for the gas permeation experiments. 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

The technology uses a proprietary formed-in-place membrane technique. The membrane is formed on porous sintered stainless steel tubes by depositing microscopic layers of inorganic and polymeric chemicals. The properties of the formed-in-place membrane can be varied by controlling the type of membrane chemicals used, their thickness, and the number of layers. This important feature allows for customization of the membrane system to a wide variety of waste characteristics and clean-up criteria. The formed-in-place membrane can be quickly and economically reformulated in the field to accommodate changes in waste characteristics or treatment requirements. 

Examples of commercial porous inorganic membranes are ceramic membranes (alumina, silica), glass and porous metals (stainless steel and silver). 

In order to reduce the thermal mismatch between the Pd alloy film and the porous substrate, metal porous supports (stainless steel, nickel, etc.) are used with an intermediate layer to reduce the intermetallic diffusion (with consequent poisoning) between the metal porous support and the Pd/Ag layer. The intermediate layer can be a ceramic or a porous Pd/Ag layer prepared by the bi-metal multi-layer deposition technique [17]. The resulting membranes demonstrate high operating temperature (over 500°C) and long-term durability. 

Preferential segregation of certain elements from dense alloy membranes can also result in degradation of the performance of H2 permeation membranes. For example, Pd-Ag films ( 2.4%Ag, 20-26 pm thick) were deposited by sequential electroless plating onto porous tubular stainless steel substrates with AI2O3 oxide layers to modify the substrate pore size and to prevent intermetallic diffusion of the stainless steel components into the Pd-Ag layer (Bosko et al., 2011). Composite membranes annealed at temperatures of 500-600°C were characterized for film structure (XRD), morphology (SEM), bulk and surface component distribution (EDS, XPS), and H2 permeance. Composition measurements within the Pd-Ag layer revealed preferential segregation of the Ag component to the top surface. This result is consistent with the lower surface free energy of Ag. 

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. 

Tubes for dynamic membranes ate usually smaller (ca 6-mm ID). Typically, the tubes ate porous carbon or stainless steel with inorganic membranes (sihca, zirconium oxide, etc) formed in place. 

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. 

Tubular Tubular membranes (Fig. 22-51) are supported by a pressure vessel, iisiiallv perForated or porous. It can be as simple as a wrapped nonw oven Fabric, or as robust as a stainless-steel tube. All rim with tube-side Feed. Thev are rnainlv used For UF, with some RO applications, particularly For Food and daiiw. The primary diameters available are 12 and 25 mm. Tubes are oFten connected in series parallel bundles, gasketed or potted, are also common. 

Ma, Y.H., P.R Mardilovich, and Y. She, Stability of hydrogen flux through Pd/Porous stainless steel composite membranes, Proceedings of International Conference on Inorganic Membranes, ICIM5, Nagoya, 1998. 

Mardilovich, I.P., E. Engwall, and Y.H. Ma, Dependence of hydrogen flux on the pore size and plating surface topology of asymmetric Pd-porous stainless steel membranes, Desalination, 144, 85-89,2002. 

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. 

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). 

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