Membrane reactors advantages/disadvantages

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.

In the preceding section, we analyzed an immobilized enzyme process and calculated some important parameters such as productivity. In this section, we investigate another process configuration for retaining biocatalysts, the membrane reactor. The advantages and disadvantages of immobilization and membrane retention have already been discussed in Chapter 5. As in the case of immobilization, retention of catalyst by a membrane vastly improves biocatalyst productivity, a feature important on a processing scale but usually not on a laboratory scale. 

Membrane Fermentors. Many of the advantages of enzyme membrane reactors are applicable to fermentors. In some cases, a fermentor is simply an enzyme reactor using intracellular enzymes rather than extracellular enzymes. Enzymes are usually more stable within the cell wall and the living cell can regenerate itself along with the enzyme. A possible disadvantage is that the enzyme activity per unit reactor volume may be less in the cells than that of a pure enzyme preparation. 

Table 4.1 Advantages and Disadvantages of Conventional Reactors Versus Packed Bed Membrane Reactors Versus Fluidized Bed Membrane Reactors ...

Dense ceramic membranes allow oxygen or hydrogen permeation in a dissociated or ionized form other than the conventional molecular diffusion, and thus exhibit extremely high selectivity (up to 100%). They can be incorporated into membrane reactors for a variety of oxidation and dehydrogenation reactions where the membrane functions as either a product extractor or a reactant distributor. Three configurations, that is, disc/flat-sheet, tubular and hollow fibre membranes have been applied in membrane reactors. They exhibit respective advantages/disadvantages in terms of the ease and cost of fabrication, the effective membrane area/volume ratio, and... 

It is obvious from the above discussion that porous and dense membranes form two different cases, each with its own advantages and disadvantages. Dense membranes, (permeable only to one component) operating at optimum conditions, can be used to obtain complete conversions. However, because the permeation rate is low, the reaction rate has also to be kept low. Porous membranes (permeable to all components but at different permselectivities) are limited under optimum conditions to a maximum conversion (which is not 100%) due to the permeation of all the components. The permeation rates through porous membranes are, however, much higher than those through dense membranes and consequently higher reaction rates or smaller reactor volumes are possible. 

Table 5.1 Advantages and disadvantages of membrane and immobilization reactors.

The immobilization of the enzyme, the redox catalyst, and sometimes also the cofactor can also take place at a solid support different from the electrode so that the components can be recovered within a solid-bed reactor (a column filled with the enzyme-containing particles) or by a filter plate or membrane. The immobilization of enzymes at solid supports or by the foraiation of cross-linked enzyme crystals can sometimes also enhance the enzyme stability. This concept has the advantage of the ease of separation but the disadvantage of diffusional limitations due to the heterogeneity of the reactions between the enzyme and the substrate and the cofactor or the redox catalyst. Additionally, the number of available redox centers is usually limited. 

The presented overview of configurations of PMRs utilizing pressure driven techniques does not exhaust the subject but only shows the most typical reactor designs. A short summary on advantages and disadvantages of PMRs with suspended photocatalyst utilizing pressure driven membrane techniques is presented in Table 21.2. 

There was no comprehensive comparison smdy to illustrate the advantages and disadvantages of MR systems with respect to TR. Therefore, as a case study, the generation of H2 and O2 via H2O decomposition reaction was investigated in both conventional Z-scheme reactor and Z-scheme MR systems (Lo et al., 2010). In conventional Z-scheme reactor, H2 and O2 gases were evolved simultaneously in the form of a mixture in a single reactor and in the Z-scheme MR, modified Nafion membrane with Fe -form was used to separate the two compartments of a linked twin reactor and the results are presented in Figure 7.11 and Table 7.4. 

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