Ceramic membranes multilayered

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

Ulhom R.J.R., Keizer K. and Burggraaf A.J., Gas transport and separation with ceramic membranes. Part I. Multilayer diffusion and capillary condensation, J. Membr. ScL 66 259 (1992). 

Supported, multilayered (as5onmetric) - dense oxide or metal - porous ceramic membranes alumina, zirconia, titania, carbon - composite ceramic-metal, ceramic-ceramic layers on porous support tube, disk multilayers on porous support plate, disk, tube, monolith... 

Fig. 2.1. Schematic picture of pore shapes. A and B are single wall, symmetric and asymmetric membranes respectively with straight cylindrical (a) or conical (b) pore shape (c) represents a ceramic asymmetric multilayered membrane with intercormected pores.

Multilayered asymmetric (ceramic) membranes with an architecture as shown schematically in Fig. 2.6 are economically the most important. 

SUPPORTS FOR CERAMIC MEMBRANES 6.2.1 The Multilayer Support System... 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

Kovalevsky, A.V., Yaremchenko, A.A., Kolotygin, V.A. et al. (2011) Oxygen permeability and stability of asymmetric multilayer Ba0.5Sr0.5Co0.8Fe0.2O3 ceramic membranes. Solid State Ionics, 192, 677-681. 

Commercialized ceramic membranes can be of different configurations, from disc/fiat-sheet to multichannel tubes. The membrane microstructure is always very similar, that is, a thin separation layer made of finer particles supported onto a more porous multilayer substrate with a gradient pore structure, as shown in Figure 10.1. The only layer involved in separation is normally the thinnest layer with the finest pore size. The thickest layer, which consists of big particles for low resistance to permeates, provides the mechanical strength, whereas the layers in between are needed for a uniform formation of the top separation layer. Normally, different techniques are used for each individual layer, which results in soaring expenditures with the number of layers involved. 

FIGURE 10.1 Schematic diagram of multilayer structure of ceramic membranes. 

In addition to ceramic membranes commercially available for filtration, a multilayer membrane structure can be found in other applications such as catalytic membrane... 

Ceramic porous membranes can be used as supports for composite membranes using metal active layers. The following sections describe a few examples of multilayer ceramic membranes obtained via sol-gel and CVD techniques. 

In principle, the separation properties of a multilayer porous ceramic membrane, such as permselectivity, should be dependent only on the pore size distribution of the top separation layer. However, they can be compromised if resistances in the intermediate layers and the macroporous support become significant. For transport through macro- and meso-pores, molecular diffusion, Knudsen diffusion and viscous flow all contribute to the total transport, while the activated surface flow of the adsorbed phase will affect microporous transport. Therefore, any theoretical models used in analysing the transport data of gases through a porous ceramic membrane with a distributed pore size must take the following contributions into consideration (1) viscous flow, (2) Knudsen flow, (3) surface flow and (4) molecular sieving... 

Chen, Y., Xiangli, F., Jin, W., Xu, N. (2007). Organic-inorganic composite membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports. J. Membr. Sci., 302, 78-86. 

Most inorganic membrane supports exhibit a tubular shape. This is a well-adapted geometry for cross-flow filtration in which the feed stream is circulated across the surface of the membrane and the permeated flux passes through the membrane in a perpendicular direction. Stainless steel, carbon, and ceramic are the most frequently used materials in the preparation of supports. As shown in Fig. 2, tubes or multichannel substrates can act as membrane supports. A well-designed support must be mechanically strong, and its resistance to fluid flow must be very low. Aiming at enhancing flux performances, multilayered substrates have been prepared that exhibit an asymmetric structure... 

For use in portable electronics, the fuel cell products must be manufactured to fit similar battery form factors. To achieve these mechanical size limitations, fuel cells must be manufactured with materials that permit small features allowing fuel and airflows, electrical connections, and a membrane assembly. In addition, these materials must survive in high temperatures, acidic environments, and remain leak proof. Alumina ceramics have been demonstrated to perform with these requirements using standard multilayer manufacturing techniques employed widely by the semiconductor industry. 

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... 

FIGURE 9.19 Membrane reactor in practice. The application dehydrogenation of ethane, CH3CH3 CH2 = CH2 + H2. The ceramic tube consists of a multilayered composite Pt crystallites. (Data from Moulijn, J.A., Makkee, M., and van Diepen, A., Chemical Process Technology, Wiley, 2001.)... 

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