Porous ceramic membranes for membrane reactors

The University of Queensland, Australia, S. LIU, Curtin University, Australia, J. M. SERRA, Universidad Polit6cnica de Valencia, Spain, J. C. DINIZ DA COSTA, The University of Queensland, Australia and A. lULIANELLI and 

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. 

Key words mesoporous ceramic membranes, alumina, titania, zirconia, membrane reactors for dehydrogenation reactions. 

Type Pore diameter (nm) Transport mechanism Applications field 

Mesoporous 2-50 Knudsen diffusion Gas separation, ultrafiltration, nanofiltration 

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Lafaiga D., Santamaria J. and Menendez M., Methane oxidative coupling using porous ceramic membrane reactors. Part I. Reactor development, Chem. Eng. ScL 49 2005 (1994). Sloot H.J., Smolders C.A., van Swaaij W.P.M. and Versteeg G.F., High-temperature membrane reactor for catalytic gas-solid reactions, AIChE J. J5 887 (1992). 

Yu, W., Ohmori, T., Yamamoto, T., Endo, A., Nakaiwa, M., Hayakawa, T., and Itoh, N. Simulation of a porous ceramic membrane reactor for hydrogen production. International Journal of Hydrogen Energy, 2005, 30 (10), 1071. 

The general behavior of product-removal membrane reactors has been well studied. More details on porous ceramic membrane reactors can be found in the series of publications by Mohan and Govind. An analysis of different flow configurations and the limits of each has been provided for dense Pd membrane reactors by Itoh. ... 

Yu W, OhmOTi T, Yamamoto T, Endo E, Nakaiwa T, Hayakawa T, Itoh N (2005) Simulation of porous ceramic membrane reactor for hydrogen production. Int J Hydrogen Energy 30 1071-1079... 

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

Porous ceramic membranes for catalytic reactors - overview and new ideas. Journal of Membrane Science, 181, 3-20. 

IV. Development of porous ceramic membranes for a solar thermal water-splitting reactor, Int. J. Hydrogen Energy, 25 1043-1050 (2000). 

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]. 

The advent of reliable quality ceramic membranes entering the industrial market has heightened the interest for porous inorganic membrane reactors at high temperatures,... 

Julbe A, Farrussseng D, and Guizard C. Porous ceramic membranes for catalytic reactors—overview and new ideas. J. Membr. Sci. 2001 181 3-20. 

Farrusseng D, Julbe A, and Guizard C. Evaluation of porous ceramic membranes as O2 distributors for the partial oxidation of alkanes in inert membrane reactors. Sep Purif Technol 2001 25 137-149. 

The porous ceramic membrane can be used to either separate biologically reacting material in reactors, or carry catalysts, microbes or enzymes to influence the desired reactions. An overview of the Japanese efforts for the establishment of membrane reactors in the "Aqua Renaissance 90 Project" are summarised by Kimura [95] a very recent review was written by Zaman and Chakma [96]. The preparation of microporous membranes (pore diameters smaller than 2 nm) for the application in membrane reactors is described by Keizer et al. [97] and Julbe et al. [98], however without detailing the membrane reactor itself. 

Porous Ceramic Membranes for Catalytic Reactors - Overview and New Ideas. Journal of Membrane Science,... 

Extensive research for substitutes for the expensive Pd membrane has been conducted. Porous membranes that consist of a highly porous metal or ceramic support with a thin top layer, tailored to have the desired selectivity, yield quite a high permeability but a relatively low selectivity. Some of the applications that have been tested on porous silica, vycor, alumina, and other membranes are listed in Saracco and Specchia [19] and Hsieh [20]. Most of the studies focused on selective permeation of products or reactants (mostly H2, in some cases O2) but the selectivity, which is determined by Knudsen diffusion, was very modest. While some improvement may be gained in ceramic membrane reactor when compared to conventional reactors it is often attributed to the dilution effect of the sweep gas [21]. 

The first decision to be made in designing a membrane reactor for a DH reaction is the membrane choice the insufficient selectivities obtained with porous ceramic membranes and the high permeabilities obtained with Pd-composite membranes suggest the latter to be the best choice for a membrane. This indeed, may be an expensive solution, as discussed below, and the quest for other avenues should be pursued. 

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. 

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