Catalysis with Inorganic Membranes
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Catalytic inorganic membrane reactors are classified according to the type of membrane (permselective versus nonpermselective) and the location of the catalyst (within the membrane versus external to the membrane). Figure 1 shows four common catalytic membrane types, following the notation of Tsotsis et al.6 The catalytic membrane reactor (CMR) consists of a support layer and a permse-
lective layer (membrane) with a catalytic function. The catalytic nonpermselective membrane reactor (CNMR) consists of a support layer and a nonpermselective catalytic layer. In the packed-bed membrane reactor (PBMR) the catalyst is located external to the supported permselective membrane. In the packed bed catalytic membrane reactor (PBCMR) the catalyst is located external to the permselective membrane which itself has catalytic activity. This review describes the working concepts of several successful applications of catalytic inorganic membranes. We identify the materials requirements of the membrane, discuss how the combination of membrane permeation and catalysis can result in a device with capabilities not achievable with more conventional chemical reactor designs, and describe how certain membrane characteristics affect the performance of the reactor. Reactions Requiring Strict Stoichiometric Feeds A class of catalytic reactions requires precise control over the feed composition. Consider the general reaction given by A + j> B B->i'pP+ vQQ.
(1)
A notable example is the Claus reaction SO 2 + 2 H 2 S 3/8 S s + 2 H 2 O . 1.CMR
2. CNMR
3. PBCMR
4. PBMR
Permselective Active Layer Nonpermselective Active Layer Permselective Layer without Activity Support Catalyst Particle
Figure 1. Four different membrane reactor types: catalytic membrane reactor (CMR), catalytic nonpermselective membrane reactor (CNMR), packed bed catalytic membrane reactor (PBCMR), and packed bed membrane reactor (PBMR).
(2)
O n e could feed a conventional reactor with a stoichiometric ratio of reactants A and B, i.e., l/vB- If there is a change in the supply of either reactant, however, then "slip" of the excess reactant to the reactor effluent stream will occur. Slip of H2S must be avoided in the example reaction because it is a toxic pollutant. A nonpermselective catalytic membrane can effectively minimize reactant slip.73 The idea is to flow, for the general reaction above, reactants A and B on opposite sides of a porous catalytic membrane (Figure 2). The key to the concept is that the feed flow rates of A and B need not be adjusted to match the needs of stoichiometry. Suppose the membrane reactor consists of a porous tube separating the flowing A and B streams. Moreover, suppose the reaction between A and B is instantaneous and irreversible. At a given position along the length of the membrane reactor, A and B are consumed completely at an interface within the membrane. In a nonstoichiometric, steady-state situation, the reaction interface will be located so that the local concentrations of A and B vanish and the supply fluxes of A and B match t
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