Selective catalytic membrane process

Selective Catalytic Membrane Process Linked with a Separation... 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

The membrane has the premier function in the process of biogenesis. It allows for individual ownership and retention of biocatalysts, and thereby for up to a million fold increases in catalytic activity. Substrate/enzyme ratios in cells may approach unity and thus enzymes can actually change the equilibrium of some reactions. Clearly, membranes are essential and the hurdle for nascent life is the need for a selectively permeable membrane... that means a membrane that contains, suspended in its lipid layers, the first communication proteins.13,14 The cell must breathe at once if there is to be any future and that again equalizes units from different clones. Is it surprising then that all life forms have membranes Shapeless wafting life is a thing of poor science fiction. Membrane formation is the moment when life became competitive, it... 

In a catalytic membrane reactor the pressure difference between feed and permeate could be adjusted such that high selectivity and high conversion in a once through process is obtained (Fig. 29d). The amount of catalyst necessary and the required residence time of the gas would be less than in conventional fixed bed reactors since the diffusional resistance is overcome by the external pressure gradient. The above advantages are already partly exploited by the use of macro-porous catalyst pellets, mentioned in Section 10.1.2.3 [19, 20]. 

A membrane-induced structure-reactivity trend that may be exploited to achieve selective processes has been recently observed in polymeric catalytic membranes prepared embedding polyoxotungstates, W(VI)-oxygen anionic clusters having interesting properties as photocatalysts, in polymeric membranes [17]. These catalytic membranes have been successfully apphed in the photooxidation of organic substrates in water providing stable and recyclable photocatalytic systems. 

Takeuchi et al. 7 reported a membrane reactor as a reaction system that provides higher productivity and lower separation cost in chemical reaction processes. In this paper, packed bed catalytic membrane reactor with palladium membrane for SMR reaction has been discussed. The numerical model consists of a full set of partial differential equations derived from conservation of mass, momentum, heat, and chemical species, respectively, with chemical kinetics and appropriate boundary conditions for the problem. The solution of this system was obtained by computational fluid dynamics (CFD). To perform CFD calculations, a commercial solver FLUENT has been used, and the selective permeation through the membrane has been modeled by user-defined functions. The CFD simulation results exhibited the flow distribution in the reactor by inserting a membrane protection tube, in addition to the temperature and concentration distribution in the axial and radial directions in the reactor, as reported in the membrane reactor numerical simulation. On the basis of the simulation results, effects of the flow distribution, concentration polarization, and mass transfer in the packed bed have been evaluated to design a membrane reactor system. 

Process. Some biological membranes take up specific chemicals, eliminate wastes and secrete products that Is, they act as a selective catalytic process linked with a separation process. Several attempts have been made to mimic such a blocatalytlc process. 

M. M. Ermilova, N.V. Orekhova, and V.M. Gryaznov, in R. Bredesen, Ed., "Optimization of the Selective Hydrogenation Process by Membrane Catalysts", Proc. Fourth Workshop Optimisation of Catalytic Membrane Reactors Systems, Oslo, Norway, May, 1997, 187. 

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