Membrane reactor advantages

M. Reif, Tubular inorganic catalytic membrane reactors Advantages and performance in multiphase hydrogenation reactions, Catal. Today 2003, 79-80, 139-149. 

Figure 5.45 reproduced from Centi, G., Dittmeyer, R., Perathoner, S. and Reif, M. Tubular inorganic catalytic membrane reactors advantages and performance in multiphase hydrogenation reactions. Catalysis Today, Vol. 79-80, pp. 139-149,... 

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

Most research reports involve an inert, selective membrane that encloses a PB of catalyst particles, a packed-bed membrane reactor (PBMR). It must be noted that the catalyst bed can also be fluidized or fixed, but types other than PBs are rarely found in literature. The following are the advantages of this type of reactor ... 

In the case of a catalytic membrane reactor (CMR), the membrane is (made) intrinsically catalytically active. This can be done by using the intrinsic catalytic properties of the zeolite or by making the membrane catalytically active. When an active phase is deposited on top of a membrane layer, this is also called a CMR because this becomes part of the composite membrane. In addition to the catalytic activity of the membrane, a catalyst bed can be present (PBCMR). The advantages of a CMR are as follows ... 

Moreover, the membrane could be mounted as an interface between the apolar substrate and the polar oxidant in a membrane reactor, avoiding the use of any solvent. Dilution of the reagents by solvent and competition between solvent and reagents on the active sites can thus be avoided. In the countercurrent membrane reactor, the substrate and the oxidant are circulated at each side of the membrane and alkanes can be oxidized with peroxides without solvents. Of course, the system carries all of the other advantages of membrane reactors continuous operation and easy separation. 

There are certainly quite significant advantages that membrane reactor processes provide as compared to conventional reaction processes. The reactor can be divided by the membrane into two individual compartments. The bulk phases of the various components or process streams are separated. This is of importance for partial oxidation or oxidative dehydrogenation reactions, where undesirable consecutive gas phase reactions leading to total oxidation occur very often. By separating the process stream and the oxidant. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

There are maty other examples of cofactor regeneration reactions and/or of reactions which may be performed in an enzyme membrane reactor. An important example is the regeneration of NADH by formate dehydrogenase (FDH), starting with formate (Wichmaim et al, 1981). The advantage of this reaction is that it is irreversible because carbon dioxide is hberated, while formate is a relatively cheap electron donor. 

Immobilized enzyme and enzyme membrane reactors each have advantages and disadvantages (Table 5.1). 

The utilization of enzyme membrane reactors with soluble, homogeneous enzymes has been reviewed on several occasions (Bommarius, 1992 Kragl, 1992). The principal advantages and disadvantages of an enzyme-membrane reactor operated as a CSTR are listed in Table 5.2. 

Table 5.2 Advantages and disadvantages of enzyme membrane reactors.

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