Porous ceramic membranes techniques

In this section a short introduction will be given on the synthesis of porous ceramic membranes by sol-gel techniques and anodization, carbon membranes, glass membranes and track-etch membranes. An extensive discussion will be given in Sections 2.3-2.S. 

Hollow glass microbeads Porous ceramic membranes Microbeads coated by Ti02 particles Porous ceramic Ti02 and ZnO membranes prepared by sol-gel technique... 

The in situ membrane growth technique cannot be applied using the zeolite-based ceramic porous membrane as support, under hydrothermal conditions in a solution containing sodium hydroxide. The high pH conditions will cause membrane amorphization and lead to final dissolution. Therefore, we tried to synthesize an aluminophosphate zeolite such as AlP04-5 [105] over a zeolite porous ceramic membrane. For the synthesis of the AlP04-5-zeolite-based porous membrane composite, the in situ membrane growth technique [7,13,22] was chosen. Then, the support, that is, the zeolite-based porous ceramic membrane, was placed in contact with the synthesis mixture and, subsequently, subjected to a hydrothermal synthesis process [18]. The batch preparation was as follows [106] ... 

Yeast can be separated from an ethanol fermentation broth by porous ceramic membranes with backflushing [Matsumoto et al., 1988]. Tubular alumina membranes with a nominal pore diameter of 1.6 pm were demonstrated to be effective for this application with a maximum permeate flux of 1,1(X) IVhr-m with backflushing. The permeate flux increases with increasing feed rale (or crossflow velocity) and TMP and with decreasing yeast concentration. Various backflushing techniques were investigated and the reverse flow of filtrate (instead of air) either by pressure from the permeate side... 

Porous metal membranes are commercially available in stainless steel and some other alloys (e.g.. Inconel, Hastelloy) and they are characterized by a macroporous structure. On the other hand, porous ceramic membranes can be found commercially in various oxides and combination of oxides (e.g., Al203,li02,Zr02, Si02) and pore size families in the mesopore and macropore ranges (e.g., from 1 nm to several microns). Most of the literature studies on three-phase catalytic membrane reactors have been carried out by developing catalytic ceramic membranes. The deposition techniques for the preparation of catalytic ceramic membranes involve methods widely used for the preparation of traditional supported catalysts (Pinna, 1998), and methods specifically developed for the preparation of structured catalysts (Cybulski and Moulijn, 2006). Other methods to introduce a catalytic species on a porous support include the chemical vapour deposition and physical vapour deposition (Daub et al, 2001). The catalyst deposition method has a strong influence on the catalytic membrane reactor performance. 

The dip-coating technique is widely used for the preparation of porous ceramic membranes [1]. Figure 2.7 illustrates the flow sheet of a dip-coating process. The prepared coatings may be adjustable between lOOnm and 100pm in thickness and the pore size covers from micropore to mesopore and part of the macropore range. 

Abstract This chapter discusses the research and development of porous ceramic membranes and their application as membrane reactors (MRs) for both gas and liquid phase reaction and separation. The most commonly used preparation techniques for the synthesis of porous ceramic membranes are introduced first followed by a discussion of the various techniques used to characterise the membrane microstructure, pore network, permeation and separation behaviour. To further understand the structure-property relationships involved, an overview of the relevant gas transport mechanisms is presented here. Studies involving porous ceramic MRs are then reviewed. Of importance here is that while the general mesoporous natnre of these membranes does not allow excellent separation, they are still more than capable of enhancing reaction conversion and selectivity by acting as either a product separator or reactant distributor. The chapter closes by presenting the future research directions and considerations of porous ceramic MRs. 

There are a wide variety of techniques used for the preparation of porous ceramic membranes however, they all share the following common steps ... 

Preparation scheme of a porous ceramic membrane using conventional techniques. CVD, chemical vapour deposition EVD, electrochemical vapour deposition. 

The thickness of porous ceramic membranes prepared in this manner is typically in the range of a few millimetres (Larbot, 1996), although thinner membranes have been realised (Sahibzada e/a/., 2000).Tape casting is a high volume production technique for fiat sheet membranes and is employed on both a research and commercial scale. 

The performance of porous ceramic membranes is typically expressed by the permeate flux (throughput) and the selectivity (separation ability), which in turn are governed by the pore size distribution, porosity and intrinsic membrane surface properties, and as such there are a variety of direct and indirect characterisation techniques used to evaluate the potential of a membrane and predict its performance. In this way new membranes can be efficiently and effectively screened and later optimised without the need for lengthy permeation experiments, reducing overall development time. Similarly, these techniques can be used to understand and/or verify a new membrane s transport and separation mechanisms. 

Despite its widespread usage and relatively well-understood analysis techniques, the use of adsorption/desorption isotherms to characterise porous ceramic membrane materials is not without its drawbacks. Firstly,... 

Porous ceramic membranes are of particular interest in this respect as they exhibit high mechanical, chemical and thermal robustness and are therefore excellent candidates for high temperature and pressure MR applications. Furthermore, the wide variety of cheap, commercially available porous ceramic membrane geometries and large-scale production techniques will ensure that porous ceramic MRs will remain under the research spotlight despite several better performing, but otherwise expensive and temperamental materials that have come to light in the last decade. Indeed, the major drawback with porous ceramic membranes is the low selectivity offered by the, primarily mesoporous, materials for gas-phase separations. This can limit... 

The sol—gel technique has been used mosdy to prepare alumina membranes. Figure 18 shows a cross section of a composite alumina membrane made by sHp coating successive sols with different particle sizes onto a porous ceramic support. SiUca or titanium membranes could also be made by the same principles. Unsupported titanium dioxide membranes with pore sizes of 5 nm or less have been made by the sol—gel process (57). 

In this case study, a zirconia-alumina membrane has been developed using the sol-gel technique with and without support.6-7 The porous ceramic was prepared to fabricate the membrane support. A thin film of aluminum and zirconium were formed on the porous ceramic support. Unsupported membrane was also prepared. The unsupported membrane was not strong enough to hold a high-pressure gradient it was very fragile and not useful... 

The vast increase in the application of membranes has expanded our knowledge of fabrication of various types of membrane, such as organic and inorganic membranes. The inorganic membrane is frequently called a ceramic membrane. To fulfil the need of the market, ceramic membranes represent a distinct class of inorganic membrane. There are a few important parameters involved in ceramic membrane materials, in terms of porous structure, chemical composition and shape of the filter in use. In this research, zirconia-coated y-alumina membranes have been developed using the sol-gel technique. 

Different methods have been used to deposit microporous thin films, including solgel, pyrolysis, and deposition techniques [20], Porous inorganic membranes are made of alumina, silica, carbon, zeolites, and other materials [8], They are generally prepared by the slip coating method, the ceramic technique, or the solgel method (Section 3.7). In addition, dense membranes are prepared with metals, oxides, and other materials (Chapter 2). 

This method has been applied to ceramic membranes (e.g., gamma-alumina membranes) and compared to other methods such as nitrogen adsorption/desorption and thermoporometry (to be discussed next) in Figure 4.13. It can be seen that the mean pore diameter measured by the three methods agrees quite well. The pore size distribution by permporometry, however, appears to be narrower than those by the other two techniques. Similar conclusions have been drawn regarding the comparison between permporometry and nitrogen adsorption/desorption methods applied to porous alumina membranes [Cao et al., 1993]. The broader pore size distribution obtained from nitrogen adsorption/desorption is attributable to the notion that the method includes the contribution of passive pores as well as active pores. Permporometry only accounts for active pores. 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

A combination of characterization techniques for the pore structure of mesoporous membranes is presented. Equilibrium and dynamic methods have been performed for the characterisation of model membranes with well-defined structure while three-dimensional network models, combined with aspects from percolation theory can be employed to obtain structural information on the porous network topology as well as on the pore shape. Furthermore, the application of ceramic membranes in separations of condensable from noncondensable vapors is explored both theoretically and experimentally. 

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