Membrane reactor system, oxidation

The use of a membrane reactor for shifting equilibrium controlled dehydrogenation reactions results in increased conversion, lower reaction temperatures and fewer byproducts. Results will be presented on a palladium membrane reactor system for dehydrogenation of 1-butene to butadiene, with oxidation of permeating hydrogen to water on the permeation side. The heat released by the exothermic oxidation reaction is utilized for the endothermic dehydrogenation reaction. 

Some of the many different types of catalysts which have good catalytic properties for the OCM reaction qualify as membrane materials. Membrane reactors for OCM were designed and tested by Nozaki et al. (1992). Three kinds of reactors were developed the first one consisted of a porous membrane covered with a thin film of catalyst (type I) the second one, a dense ionic-conducting membrane (non porous) with catalytic layer (type II) and the third one was a membrane made of perovskite-type mixed oxides which was active for OCM (type III). Figure 11 presents the diagram for the membrane reactor system and table 13 shows the different materials used for supports and coated catalysts. 

The preparation of a novel catalytic membrane system to be used in multiphase H2O2 production has also been discussed in detail by Tennison et al. in 2007. In their review, it was shown that it is possible to produce a membrane system that is potentially suitable for use in both multiphase and gas phase membrane reactor systems based on a 2-layer ceramic substrate. Moreover, the performance is sensitive to the degree of perfection of the support. The carbon membrane deposited within the nanoporous layer of the substrate has the structure and surface area to enable high dispersions of catalyst metals to be achieved when oxidized in carbon dioxide that have shown good performance in the direct synthesis of H2O2. When prepared under nitrogen, despite the simple production route, the carbon membrane shows excellent gas separation characteristics. 

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. 

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... 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

Rautenbach and MeUis [75] describe a process in which a UF-membrane fermentor and a subsequent NF-treatment of the UF-permeate are integrated. The retentate of the NF-step is recycled to the feed of the UF-membrane reactor (Fig. 13.8). This process has been commercialised by Wehrle-Werk AG as the Biomembrat -plus system [76] and is well suited for the treatment of effluents with recalcitrant components. The process also allows for an additional treatment process, like adsorption or chemical oxidation of the NF-retentate, before returning the NF-retentate to the feed of the UF-membrane fermentor. Usually, the efficiency of these treatment processes is increased as the NF-retentate contains higher concentrations of these components. Pilot tests with landfiU leachates [75] and wastewater from cotton textile and tannery industry have been reported [77]. An overview of chemical oxygen demand (COD) reduction and COD concentrations in the permeate are shown in... 

Although the use of membrane reactors for the retention of the enzyme is mostly applied in continuous processes, some authors used a membrane batch reactor in order to reuse the enzyme in consecutive cycles [11, 74]. Flock et al. used a membrane unit coupled to the reactor with recycling of both permeate and retentate streams to the reactor vessel. A valve at the outlet of the membrane maintained pressure within the range fixed by the manufacturer [74]. Pasta et al. operated a reactor with the membrane inside, emptied the reactor content at scheduled times, and thereafter, replenished it with fresh solution of the substrate and the oxidizing system [11]. 

Watercatox Not an established process, but a project of the Fifth Framework Program of the European Union. The purpose was to develop catalytic processes for destroying organic residues in water by wet air oxidation ( WAO).The chosen system used a tubular catalytic membrane reactor for contacting the aqueous solution with air. Several companies and research institutes participated in this project from 2000, and the process was piloted with several real industrial liquid effluents. 

The feasibility of the palladium membrane system with an oxidation reaction on the permeation side and 1-butene dehydrogenation reaction on the reaction side in a membrane reactor has been successfully demonstrated. The palladium and its alloy membrane not only can withstand high temperature but also are selectively permeable to hydrogen... 

Electroenzymatic processes in enzyme membrane reactors may be considered as bridges toward biocatalytic reaction systems, with the ferricinium cation acting as a clean one-electron oxidant (Scheme 3). To give an example, 4-ethylphenol can be oxidized to 4-hydroxyacetophenone according to Scheme 4 [53]. 

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