Membrane reactors catalyst configuration

As one of the two common types of membrane modules, the hollow-fiber membrane module has shown excellent mass transfer performance due to its large surface area per unit volume (about 1000-3000ft2/ft3 for gas separation). In the modeling work, the WGS membrane reactor was configured to be a hollow-fiber membrane module with catalyst particles packed inside the fibers. 

Figure 3.5 Membrane reactor catalyst-in-tube- (a) and catalyst-in-shell (b) configurations...

Fig. 4. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane tube, with an outside diameter of about 6.5 mm and a length of up to about 30 cm and a wall thickness of 0.25-1.20 mm, was prepared from an electronic/ionic conductor powder (Sr-Fe-Co-O) by a plastic extrusion technique. The quartz reactor supports the ceramic membrane tube through hot Pyrex seals. A Rh-containing reforming catalyst was located adjacent to the tube (57).

It is evident, that in any membrane reactor operation mode there are important parameters which determine the performance of the process (Shah, Remmen and Chiang 1970). These are (1) the total and partial pressures on both sides of the membrane, (2) the total and partial pressure differences across the membrane, (3) the diffusion mechanism through the support and the membrane layer (membrane structure), (4) the thickness of the membrane, (5) the reactant configuration (i.e. whether the reactants are supplied from the same or from opposite sides of the membrane, in counter or co-current flow) and (6) the catalyst distribution. 

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. 

Molinari et at. [73] reported a study on various configurations of photocatalytic membrane reactors for the degradation of 4-nitrophenol using Ti02 as catalyst. Two configurations have been studied irradiation of a recirculation tank, with the suspended catalyst confined by means of the membrane, and irradiation of the cell containing the membrane with three sub-cases catalyst deposited on the membrane, catalyst suspended, and catalyst included in a membrane during its preparation. 

Two types of reactors were utilized 1) a conventional (co-feed) fixed bed reactor and 2) a tubular membrane reactor. Both reactors were filled with the catalyst particles. A schematic illustration of the configurations used is shown in Fig. 12.15. 

Zaspalis and Burggraaf [47] have summarized typical membrane reactor configurations, different membrane/ catalyst combinations, and a large number of membrane reactor studies. Their article clearly shows that inorganic membranes prepared by the sol-gel method, with their dual ability in catalysis and separation, have many unique advantages over other product forms. At the same time, it is important to realize that the parameters which affect a membrane s characteristics and the advantages which the sol-gel process offers are similar to what has been presented thus far. 

Another favorable aspect of stirred batch reactors is the fact that they are compatible with most forms of a biocatalyst. The biocatalyst may be soluble, immobilized, or a whole-cell preparation in the latter case a bioconversion might be performed in the same vessel used to culture the organism. Recovery of the biocatalyst is sometimes possible, typically when the enzyme is immobilized or confined within a semi-permeable membrane. The latter configuration is often referred to as a membrane reactor. An example is the hollow fiber reactor where enzymes or whole cells are partitioned within permeable fibers that allow the passage of substrates and products but retain the catalyst. A hollow-fiber reactor can be operated in conjunction with the stirred tank and operated in batch or... 

Ilias and Govind(lO) have reviewed the development of high temperature membranes lor membrane reactor application. Hsieh(4) has summarized the technology in the area of important inorganic membranes, the thermal and mechanical stabilities of these membranes, selective permeabilities, catalyst impregnation, membrane/reaction considerations, reactor configuration, and reaction coupling. 

In a generalized case, both the tube and the shell sides are packed with catalyst beds (e.g., in a reaction coupling situation explained in Chapter 8) and the membrane layer is catalytic either inherently or through impregnation on the pore surfaces. Two commonly occurring membrane reactor configurations will be treated as special cases of this generalized model. 

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

Enhancement of reaction conversion by employing a permselective membrane often has the implication that, for a given conversion, it is possible to run the reaction at a lower temperature in the membrane reactor than in a conventional reactor. Catalyst deactivation due to coke formation generally becomes more severe as the reaction temperature increases. Therefore, the use of a membrane reactor to replace a conventional one should, in principle, reduce the propensity for coke formation because for the same conversion the membrane reactor configuration may be operated at a lower temperature than a conventional reactor. This is particularly true for such reactions as dehydrogenation. 

The different types of membrane reactor configurations can also be classified according to the relative placement of the two most important elements of this technology the membrane and the catalyst. Three main configurations can be considered (Figure 25.13) the catalyst is physically separated from the membrane the catalyst is dispersed in the membrane or the membrane is inherently catalytic. The first configuration is often called the inert membrane reactor (IMR), in contrast to the two other ones, which are catalytic membrane reactors (CMRs).5o... 

As shown in Figure 7.3, two membrane reactor configurations can be proposed one with free catalyst and the other one with catalyst fixed on the membrane. In the first case, the membrane insures rejection of catalyst and keeps it in a restricted part of the system where reaction takes place. In the second case, the catalyst is fixed on the surface or in the pores of the membrane and reaction takes place at crossing. 

Similarly, Mota et al. [210] carried out the selective oxidation of butane to maleic anhydride over VPO mixed oxides-based catalysts enclosed in an MFI membrane. Different feed configurations of the zeohte-membrane reactor were tested to outperform the conventional co-feed configuration. The results achieved were rather similar however, the authors pointed out the possibility to take advantage of the O2 distribution, which limits the flammability problems and allows operation with higher butane concentrations than those used in conventional processes. 

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