Equipment Membrane reactors

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... 

For dry reforming, carbon formation is very likely, especially when carried out in a membrane reactor [24]. For this application noble metals are used, which are intrinsically less prone to carbon formation because, unlike nickel, they do not dissolve carbon. Irusta et al. [24] have shown above-equilibrium methane conversion in a reactor equipped with a self-supported Pd-Ag tube. Small amounts of coke were formed on their Rh/La203/Si02 catalyst, but this is reported not to have any effect on activity. 

Pharmaceutical production generally uses multipurpose equipment, and so entrapment behind a membrane would require significant capital expenditure on specialized equipment. In spite of this, the use of membrane reactors in biocatalysis represents an efficient method of enzyme immobilization, given the large molecular weight difference between enzymes (10-150 kDa) and most substrates (300-500 Da). The reader is referred to some recent reviews on the topic. 

Researchers at Degussa AG focused on an alternative means towards commercial application of the Julia-Colonna epoxidation [41]. Successful development was based on design of a continuous process in a chemzyme membrane reactor (CMR reactor). In this the epoxide and unconverted chalcone and oxidation reagent pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The equipment used for this type of continuous epoxidation reaction is shown in Scheme 14.5 [41]. The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which is already used for enzymatic a-amino acid resolution on an industrial scale at a production level of hundreds of tons per year [42]. 

Catalytic membrane reactors are not yet commercial. In fact, this is not surprising. When catalysis is coupled with separation in one vessel, compared to separate pieces of equipment, degrees of freedom are lost. The MECR is in that respect more promising for the short term. Examples are the dehydrogenation of alkanes in order to shift the equilibrium and the methane steam reforming for hydrogen production (29,30). An enzyme-based example is the hydrolysis of fats described in the following. 

The pincer-based carbosilane dendrimers Go-2 and Gi-2 were tested for their degree of retention in a membrane reactor equipped with a SelRO-MPF-50 nanofiltration membrane [35,36]. Their retentions were measured... 

Moreover, this catalytic reaction could be employed in a continuously operated membrane reactor [105,106]. A stirred membrane reactor module equipped with a solvent-stable Koch MPF-50 membrane [107] was operated at 40 atm. After exchange of a few reactor volumes a steady conversion is achieved, e.g., 30% cyclohexene conversion for the example shown in Fig. 9 [32], corresponding to a catalytic activity of 1200 TO h 1. Over 30 exchanged reactor volumes, corresponding to a time of operation of 30 h, a productivity of a total of 29 000 turnovers was observed. 

The use of a membrane reactor in steam reforming has several advantages. Because of the lower temperature operation, the energy consumption of the process is reduced which results in lower emission of C02. The lower temperature also requires less expensive catalyst, tubing and other reactor materials. Since hydrogen of sufficient purity is produced directly from the reformer, the downstream shift conversion can be omitted. Moreover, the dimensions of the C02 removal and final purification units can be reduced. Hence, significant savings in equipment costs can be expected. 

Steam reformers equipped with the Pd membranes were developed and have been tested in Japan to produce pure hydrogen from city gas.3 Because of the working principle of the membrane reactor, the performance of this type of steam reformer directly depends on hydrogen permeability of the membranes. This has led us to develop membranes with higher hydrogen permeability. 

Nourbakhsh et al. [1989] employed a compressible Knudsen flow-type seal for flat membrane reactors. To avoid membrane cracking due to the difference in the thermal expansion coefficients of the membrane and the reactor wall, the membrane reactor is equipped with a pneumatic system which compresses the membrane after the desired operating temperature has been established and all reactor parts have reached steady state temperauires. This type of sealing is also limited to laboratory applications and not practical for production environments. 

In a simple membrane reactor, basically the membrane divides the reactor into two compartments the feed and the permeate sides. The geometries of the membrane and the reaction vessel can vary. The feed may be introduced at the entrance to the reactor or at intermediate locations and the exiting retentate stream, for process economics, may be recycled back to the reactor. Furthermore, the flow directions of the feed and the sweep (including permeate) streams can be co-current or counter-current or some combinations. It is obvious that there are numerous possible process and equipment configurations even for a geometrically simple membrane reactor. 

Thus, expectedly no rigorous mathematical models are available that can accurately describe the detailed flow behavior of the fluid streams in a membrane separation process or membrane reactor process. Recent advances in computational fluid dynamics (CFD), however, have made this type of problem amenable to detailed simulation studies which will assist in efficient design of optimal membrane filtration equipment and membrane reactors. 

A type a example is given in the Mobil patent mentioned above [86], The membrane reactor having the configuration as described before, was equipped with a K-exchanged ZSM-5 membrane having a Si/Al ratio of 220. The membrane was impregnated with chloroplatinic acid to give 0.001 wt% Pt based on total quantity of zeolite of zeolite, and reduced at 500 C in... 

In spite of the potential advantages of the use of a catalytic membrane reactor to perform chemical reactions in SC CO2, very few references are available on this topic. The concept was however demonstrated for the hydrogenation of 1-butene using a fluorous derivative of Wilkinson s catalyst [32]. The reaction was successfully performed in a free catalyst membrane reactor equipped with a silica membrane. 

The OHLM systems, integrating reaction, separation, and concentration functions in one equipment (bioreactor), find a great interest of researchers in the last few years. A bioreactor combines the use of specific biocatalyst for the desired chemical reactions, and repeatedly or continuously application of it under very specific conditions. Such techniques were termed as hybrid membrane reactors. In biotechnology and pharmacology, these applications are termed as hybrid membrane bioreactors or simply bioreactors (see Table 13.11). Experimental setup of the bioreactor system is shown schematically in Figure 13.17. 

The applications mentioned demonstrate the potential of membrane reactors for the recovery and repeated use of homogeneously soluble catalysts. For some of the examples a strong increase in the total turnover number has been achieved. First publications indicate that this technique is also being investigated by industry [58]. Due to the additional steps required for coupling as well as for the equipment necessary it might be applicable in the majority to high value-added products. On... 

Scaling up of the processes to large surface areas (i.e. to obtain asymmetric membrane systems with several layers) as is necessary for large-scale operations has been successfully demonstrated for micro/ultrafiltration and bioseparation processes, but not for other applications such as gas/vapour separation and membrane reactors, for which only small-scale laboratory equipment is available. 

The conversion in the reactor is plotted in Fig. 14.13 against the Da number which can be regarded as a dimensionless residence time. From this plot it follows that the conversion in the membrane reactor equipped with high selective membranes can exceed the values possible with an ordinary plug flow reactor. From the graph it is clear that the conversion increases with increasing... 

SEM. To analyze the unreacted 1-butene and maleic anhydride in the membrane reactor, gas chromatography equipped with FID and Tenax-GC column was used. Also to analyze the CO and CO2, gas chromatography equipped with TCD and molecular sieve 5A and Porapak Q column was used. 

Sometimes reaction rates can be enhanced by using multifunctional reactors, i.e., reactors in which more than one function (or operation) can be performed. Examples of reactors with such multifunctional capability, or combo reactors, are distillation column reactors in which one of the products of a reversible reaction is continuously removed by distillation thus driving the reaction forward extractive reaction biphasing membrane reactors in which separation is accomplished by using a reactor with membrane walls and simulated moving-bed (SMB) reactors in which reaction is combined with adsorption. Typical industrial applications of multifunctional reactors are esterification of acetic acid to methyl acetate in a distillation column reactor, synthesis of methyl-fer-butyl ether (MTBE) in a similar reactor, vitamin K synthesis in a membrane reactor, oxidative coupling of methane to produce ethane and ethylene in a similar reactor, and esterification of acetic acid to ethyl acetate in an SMB reactor. These specialized reactors are increasingly used in industry, mainly because of the obvious reduction in the number of equipment. These reactors are considered by Eair in Chapter 12. 

For synthesis on a preparative scale, repetitive batch processing has proved to be an effective and easy-to-handle method1128. The repeated use of the enzyme is possible after concentration of the solution by means of commercially available ultrafiltration equipment and adding fresh substrate solution. Some of the advantages given for the Enzyme Membrane Reactor (see below) are also valid for the repetitive batch technique. 

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