Ceramic membranes scale

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

The casein micelles are retained by fine-pore filters. Filtration through large-pore ceramic membranes is used to purify and concentrate casein on a laboratory scale. Ultrafiltration (UF) membranes retain both the caseins... 

Much of the impetus for the awakened interest and utilization of inorganic membranes recently came hom a history of about forty or fifty years of some large scale successes of porous ceramic membranes for gaseous diffusion to enrich uranium in the military weapons and nuclear power reactor applications. In the gaseous diffusion literature, the porous membranes are referred to as the porous barriers. For nuclear power generation, uranium enrichment can account for approximately 10% of the operating costs (Charpin and Rigny, 1989]. 

One of the major leasons for the widespread interest of inorganic membranes in recent years is the ability to manufacture ceramic membranes with consistently uniform-sized pores and precursor particles on an industrial scale. Figure 4.3 reveals the surface morphology of a micr iltration grade ceramic membrane. It is an essential requirement in separation applications that a membrane is free of defects or cracks. 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

Originally, multichannel ceramic membranes have been produced at the industrial scale by SCT-Exekia and Orehs in France with commercial elements registered, respectively, as Membralox and Kerasep. The membrane filtration area in this case can reach 0.35 m /element depending on channel diameter and the number of channels per element. FILTANIUM elements, representative of flower-like geometries, have been produced more recently by TAMI industries in France (Figure 6.3). These elements with a cross-section diameter of either 10 or 25 mm exhibit a number of channels that vary from 3 to 39 and a membrane filtration area of 0.5 m for the largest elements. The increase in membrane surface compared to equivalent cylindrical-shaped channels can be estimated at 30%. 

Compared to modules based on cylindrical elements, flat ceramic membrane modules are not developed in a large extent and are limited to date to small liquid volume treatment [27]. Flat ceramic membranes are generally implemented as disks in laboratory scaled cells, offering a limited filtration surface area. Indeed a diameter of 90 mm that is one of the largest available dimensions for these membrane disks results in a filtration surface of -56 cm. Anopore alumina membranes supplied by Whatman or ATZ ceramic membrane disks with zirconia or titania top-layers from Sterlitech are typical examples of these commercially available flat ceramic membranes. Sterlitech ATZ ceramic membrane disks and the corresponding membrane holder are shown in Figure 6.16. 

Koukou MK, Papayannakos N, Markatos NC, Bracht M, Van Veen HM, and Roskam A. Performance of ceramic membranes at elevated pressure and temperature Effect of non-ideal flow conditions in a pilot scale membrane separator. J. Membr. Sci. 1999 155(2) 241-259. 

When diffusional relaxation of a suspension brought out of equilibrium by shearing is slow with respect to the time-scale of the process (De number), the suspension is said to be thixotropic. This behaviour is illustrated in Fig. 6.21. Thixotropy is usually imwanted in ceramic membrane support coatings, but does occur for some suspension formulations. The layer thickness obtained in film-coating with the same suspension but with a different shear history can then differ. 

In this study the feasibility of implementing ceramic membranes on an industrial scale in the styrene production process is treated. Therefore, a model has been set up in the flowsheeting package ASPEN PLUS , which describes a styrene process production plant. Some modelling has been done with different types of membrane reactors in different reactor section configurations to investigate the influence on the performance of the production of styrene. 

Inorganic membrane development is still in progress [57] (see also Section 14.2.2). Microporous silica membranes have been developed at several universities and research institutes. Membrane selectivities of 15 and 20 for the separation of H2 from CO2 have been reported. Even higher selectivities for H2 arid CO, CH4 and N2 have been measured [20,57]. Most measurements reported in the literature have been performed on a laboratory scale. However, it has been shown that it is possible to upscale these microporous ceramic membranes to, at least, bench scale [31,57]. With other membranes such as noble (Pd) metal membranes and dense ceramic membranes very high and almost infinite selectivities for hydrogen are possible [58]. The permeation of these membranes is generally smaller than the permeation of microporous membranes. 

When the membranes are used on an industrial scale, a considerable amoimt of surface area will be necessary to process the gas stream involved. A typical surface area necessary is 1500 m for a 300 MW class power plant. For ceramic membranes this is a rather large surface area. Considering that permselectivity is already good for this application, it seems reasonable to direct research towards enlargement of the permeation or explore module concepts with a high surface area to volume ratio (e.g. monolytic systems) next to selectivity improvement. 

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