CATALYTIC MEMBRANE REACTORS INVOLVING
– A short overview –
Anne JULBE and André AYRAL
Institut Européen des Membranes (ENSCM, UM2, CNRS), Université Montpellier 2 (cc047) Place
Eugène Bataillon, 34095 Montpellier cedex 5 FRANCE
Membrane reactors (MRs) as a concept, dates back to 1960s and a large number of papers have been published on this multidisciplinary vibrant subject at the frontier between catalysis, membrane science and chemical engineering [1-14]. In such an integrated process, the membrane is used as an active participant in a chemical transformation for increasing the reaction rate, selectivity and yield. The membrane does not only play the role as a separator but also as part of the reactor. Membrane reactors, combining in the same unit a conversion effect (catalyst) and a separation effect (membrane), already showed various potential benefits (increased reaction rate, selectivity and yield) for a range of reactions involving the membrane as extractor, distributor or fluid-solid contactor. Due to the generally severe conditions of heterogeneous catalysis, most MR applications use inorganic membranes, which can be dense or porous, inert or catalytically active.
The interest of MRs has been largely demonstrated at the laboratory scale, namely for hydrogenation, dehydrogenation, decomposition and oxidation reactions including partial oxidation and oxidative coupling of methane. Though some small industrial installations already exist, the concept has yet to find widespread industrial applications. One of the important factors hindering further commercial development of MRs are the membrane themselves, their support and the surrounding modules (performance, stability, cost, sealing,..) which still need optimization and new developments. After a rapid overview of the working concepts of MRs, several examples of current research and developments in the field of inorganic membrane reactors will be described. General considerations on inorganic membranes for MRs
Inorganic membranes for MRs can be inert or catalytically active, they can be either dense or porous, made from metals, carbon, glass or ceramics. They can be uniform in composition or composite, with a homogeneous or asymmetric porous structure. Membranes can be supported on porous glass, sintered metal, granular carbon or ceramics such as alumina.
Different membrane shapes can be used such as flat discs, plates, corrugated systems, tubes (dead-end or not), capillaries, hollow fibers, or monolithic multi-channel elements for ceramic membranes, but also foils, spirals or helix for metallic membranes. The shape of the separative element induces a specific surface/volume ratio for the reactor, which needs to be maximized, 23typically above 500 m/m, for industrial applications . Apart from the evident need for low cost, resistant and efficient membranes for the process, highly permeable membranes are required for all 30
applications. This parameter is directly related to the membrane structure which can be dense or porous, and which defines the transport mechanisms.
Dense membranes have been largely and successfully investigated in MRs, for reactions
or O. Indeed these membranes, exclusively permselective to either consuming or generating H22
H  or O[16,17], are generally used either as efficient H extractors or as O distributors. H 22 222
permselective dense membranes include Pd and Pd alloys membranes, which are commercially available (Johnson–Matthey). Thin supported films and new alloyed compositions have been
recently developed in order to reduce membrane cost, sensitivity to sulfur species and embrittlement upon aging. Dense ceramic membranes are also considered for H separation, e.g. with proton 2
conducting membranes based on zirconate and cerate perovskite systems .
O permselective dense membranes include metallic (Ag) or ceramic membranes (e.g. 2
Yttrium-stabilized zirconia (YSZ), BiMeVO, LaNiO or (La-Sr)(Fe-Co)Operovskites and related x24+？3-？
oxides). Gas transport in dense metallic membranes occurs via a solution/ diffusion mechanism. In stabilized zirconia, ionic conduction is involved at high temperature (800-1000?C); an electronic current is needed in such electrochemical reactors (Fig 1a). The mixed ionic and electronic conductivity of perovskites avoids the need of an external electrode for these materials (Fig. 1b). The gradient of partial pressure P on both sides of the membranes is the driving force for ion transport. Composite materials O2
involving a mixture of an ion conducting oxide with an electronic conducting phase (e.g. metal) is also considered (Fig. 1c). One of the common drawbacks to these attractive dense ceramic membranes is their limited permeability and thermo-chemical stability upon aging in operating conditions. Dense membranes performance has been improved namely by decreasing membrane thickness (supported membranes), by increasing the surface roughness and by developing new materials .
a b c
Figure 1. Different
incorporating an oxygen
ion conductor: (a) mixed
conducting oxide, (b) solid
electrolyte cell (oxygen
pump), and (c) dual-phase
Although being less permselective than dense membranes, porous membranes offer a higher permeability and have been extensively used in catalytic reactors. The gas transport mechanisms in porous membranes can be related to the ratio between the pore sizes and the mean free path length of gas molecules . The typical gas transport mechanisms in porous membranes are: molecular diffusion and viscous flow (macropores and mesopores), capillary condensation (mesopores), Knudsen diffusion (mesopores), surface diffusion (mesopores and micropores) and micropore activated diffusion. The contribution of the different mechanisms is dependent on the properties of the membranes and the gases as well on the operating conditions of temperature and pressure. At high temperature, when adsorption is no more effective, capillary condensation and surface diffusion are no more involved. In these conditions, the permselectivity increases when pore sizes decrease and pressure can be used to control the transport through membranes with macropores or big mesopores. As far as the permselectivity is not always a key factor in membrane reactors, membrane research for MRs focus on
both microporous and mesoporous materials, with a large range of porous texture and compositions adaptable to a large number of applications.
The main membrane functions in MRs
The concept of combining membranes and reactors is being explored in various configurations, which can be classified in three groups, related to the role of the membrane in the process . As shown in figure 2, the membrane can act as:
(a) an extractor, where the removal of the product(s) increases the reaction conversion by shifting the reaction equilibrium;
(b) a distributor, where the controlled addition of reactant(s) limits side reactions; and
(c) an active contactor, where the controlled diffusion of reactants to the catalyst can lead to an engineered catalytic reaction zone.
In the first two cases, the membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides.
A, BA, B1-EXTRACTOR1-EXTRACTORCCEquilibriumshiftEquilibriumshift