Support layers tubes

Figure 1 Schematic of a cross-section of a commercial SCT tube used as support. Layers 1, 2 and 3 are made of a-Al203 and have respective thicknesses of (pm) 1500, 40, 20 and average pore sizes of (pm) 12, 0.9, 0.2. Layer 4 (optional) is made of y-Al203 and has a thickness of 3-4 pm and average pore size of 4.5 nm.

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

FIGURE 6.7 Extrusion process for fabricating tubular SOFC support layers, (a) Open-ended die with cathode slurry in it and (b) Mandril insertion into the die, extruding the cathode slurry into a closed-ended hollow tube. 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

Under the above assumptions, the convective mass and heat transfer equations governing the changes of concentrations of species j and temperatures for the feed (tube) side, membrane layer, support layer and permeate (shell) side (i.e.,... 

The above set of equations for the four regions (tube side, membrane layer, support layer and shell side) are to be solved with prescribed inlet and other boundary conditions. They require an inlet condition for each of the J species and the temperature on both the tube and shell sides. Additionally, the following boundary conditions in the r-direction are needed two for each of the J species and the temperature for each of the four regions. 

The models presented so far are quite general with respect to the catalytic activities of the various regions tube side, membrane (and support) layer and shell side. In practice, however, not all the regions are catalytic and almost all inorganic membrane reactor case studies only involve one or two catalytic regions. 

A similar type of membrane reactor has been used and modeled for the ethylbenzene dehydrogenation reaction [Becker et al., 1993]. It is assumed that the reaction rate is the same in the membrane layer as in the catalyst particles on the tube side. Four regions are considered the tube (feed) side which is catalytic, the catalytic membrane layer, the inert support layer and the inert shell (permeate) side. 

A number of researchers have produced capillary tubing and hollow-fibers from materials of sufficient strength to avoid the need for a porous support. These materials are typically melt spun into hair-size fibers having dense walls of sufficient strength to obviate the need for a supporting layer.. While the dense layer offers substantial resistance and limits the permeation flux, the hair-size of the hoUow-fibers enables designs that accommodate high membrane surface area densities (area per unit volume) [32]. 

There are other different coating methods on porous stainless steel support media in the production of carbon membranes supported on tube including bmsh coating spray-coating and nltrasonic deposition of the polymer resin. For example, Shiflett and Foley reported varions approaches to prepare carbon molecular sieve layers on the stainless steel snpportby nltrasonic deposition [31]. 

An electrochemical vapor deposition (EVD) technique has been developed that produces thin layers of refractory oxides that are suitable for the electrolyte and cell interconnection in SOFCs (9). In this technique, the appropriate metal chloride (MeCl ) vapor is introduced on one side of a porous support tube, and H2/H2O gas is introduced on the other side. The gas environments on both sides of the support tube act to form two galvanic couples, ie. 

HoUow fibers are usuaUy on the order of 25 p.m to 2 mm in diameter. They can be made with a homogeneous dense stmcture, or preferably with a microporous stmcture having a dense permselective layer on the outside or inside surface. The dense surface layer can be integral, or separately coated onto a support fiber. The fibers are packed into bundles and potted into tubes to form a membrane module. More than a kilometer of fibers may be requited to... 

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. 

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