Membrane reactors types

The coupling of a permselective membrane with a packed bed of catalyst pellets (Fig. 5b) has been one of the most widely studied membrane reactor setups. Generally, the catalyst fixed bed is enclosed on the tube side of a porous membrane, although several cases can be found in the literature in which permselective tubular membranes have been inserted at regularly spaced intervals into the packed bed of catalyst pellets (e.g.. Ref. 25). The most interesting property of this membrane reactor type is that the amount of catalyst and the membrane surface area can be varied almost independently within wide ranges, so as to optimize the coupling of reaction and separation. 

Most of the comparisons are based on conversion, yield or a similar feature of reactor performance. These have usually been made separately for the two main membrane reactor types. 

Process Catalyst Catalytic properties Reaction conditions Membrane reactor type References... 

Degussa AG uses immobilised acylase to produce a variety of L-amino adds, for example L-methionine (80,000 tonnes per annum). The prindples of the process are the same as those of the Tanabe-process, described above. Degussa uses a new type of reactor, an enzyme membrane reactor, on a pilot plant scale to produce L-methionine, L-phenylalanine and L-valine in an amount of 200 tonnes per annum. 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

Figure 10.4 Meaning of acronyms used to define types of membrane reactors at the reactor level. (After [2]).

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 ... 

Reactor type Chip reactor with thin-film sensors and membrane Catalyst material Pt... 

Reactor type Membrane chip reactor Membrane width 700 pm... 

Reactor type Filament-bed membrane reactor Porosity 0.8... 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Two types of continuous membrane reactors have been applied for oligomer- or polymer-bound homogeneous catalytic conversions and recycling of the catalysts. In the so-called dead-end-filtration reactor the catalyst is compartmentalized in the reactor and is retained by the horizontally situated nanofiltration membrane. Reactants are continuously pumped into the reactor, whereas products and unreacted materials cross the membrane for further processing [57]. 

Table 73. Membrane Reactor Studies on Hydrogenation, Oxidation and Other Reaction Types...

Various reactor types have been used as the foundation for microreactor designs, including coated wall reactors, packed-bed reactors, structured catalyst reactors, and membrane reactors. 

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

Membrane reactors can offer an improvement in performance over conventional reactor configurations for many types of reactions. Heterogeneous catalytic reactions in membrane reactors [1] and the membranes used in them [2,3] have been reviewed recently. One well studied application in this area is to remove a product from the reaction zone of an equilibrium limited reaction to obtain an increase in conversion [4-10]. The present study involves heterogeneous... 

The present study investigates a different approach. The membrane is used to allow the desired intermediate product to escape from the reaction zone before it is consumed by further reaction. This use of a membrane reactor was first suggested by Michaels [15]. The partial oxidation of methane, which is a challenging reaction of the type propos for this application of membrane reactors, has been analyzed herein. There is no thermodynamic limitation for the production of carbon dioxide and water, actually these products are favored. It is desired to remove any partial oxidation product, for example formaldehyde, before it has a chance to be further oxidized. 

In analogy to the enzyme membrane reactors (EMRs) [8], a chemzyme membrane reactor (CMR) is used to retain a polymer-enlarged chemical catalyst of this kind. Tremendous progress could be made in the recycling of polymer-enlarged catalysts (Fig. 3.1.3) by employing different types of catalysts for both the enan-tioselective C-C bond formation and redox reactions. 

Reaction engineering helps in characterization and application of chemical and biological catalysts. Both types of catalyst can be retained in membrane reactors, resulting in a significant reduction of the product-specific catalyst consumption. The application of membrane reactors allows the use of non-immobilized biocatalysts with high volumetric productivities. Biocatalysts can also be immobilized in the aqueous phase of an aqueous-organic two-phase system. Here the choice of the enzyme-solvent combination and the process parameters are crucial for a successful application. 

Figure 8.4 Reactor types used in organic-aqueous biphasic systems (a) Emulsion reactor, (b) Lewis cell, (c) passive membrane reactor, (d) active membrane reactor. E represents enzyme molecules.

Membrane reactors, using semi-permeable membranes, usually of sheet or hollow fiber type... 

While a number of dendritic catalysts have been described, catalyst recyclization in most cases is an unsolved problem. Diaminopropyl-type dendrimers bearing Pd-phosphine complexes have been retained by ultra- or nanofiltration membranes, and the constructs have been used as catalysts for allylic substitution in a continuously operating chemzyme membrane reactor (CMR) (Brinkmann, 1999). Retention rates were found to be higher than 99.9%, resulting in a sixfold increase in the total turnover number (TTN) for the Pd catalyst. 

For example, POPAM dendrimers of 1,3-diaminopropane type have been used in membrane reactors as supports for palladium-phosphine complexes serving as catalysts for allylic substitution in a continuously operated chemical membrane reactor. Good recovery of the dendritic catalyst support is of advantage in the case of expensive catalyst components [9]. It is accomplished here by ultra-or nanofiltration (Fig. 8.2). 

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