Zeolite membrane reactors concentration

Similarly, Mota et al. [297] 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 zeolite-membrane reactor were tested in order 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. 

Aguado, S., Coronas, J. and Santamaria, J. (2005) Use of zeolite membrane reactors for the combustion of VOCs present in air at low concentrations. Chemical Engineering Research and Design, 83,295-301. 

Oxidation of ligands can be avoided by the use of purely inorganic catalysts. If the catalysts are insoluble in the medium, as in zeolites or heteropolyacids, the workup is much simpler. If the oxidation can be run in the gas phase or in a melt, no solvent has to be separated. If the oxidant is oxygen, a membrane reactor can sometimes be used to keep the oxygen concentration low and allow the product to be separated continuously, thereby avoiding overoxidation. 

Tanaka et al. (2001) used zeolite T membranes in an ISU membrane reactor (at 343 K). Almost complete conversion was reached. The reaction time courses were well described by a model based on two assumptions (1) the reaction obeyed to a second-order kinetic and (2) the permeation flux of each component was proportional to its concentration. 

Santamaria and coworkers (Aguado cf a/.,2005) preparedPt/ZSM-5 membrane reactors for the combustion of n-hexane present at a low concentration in the air. Experimental results showed that n-hexane combustion was achieved at 210°C. A comparison of the conversion of the hexane obtained using the membrane reactor and the fixed bed reactor evidenced the better performance of the membrane reactor with a light-off temperature lower (about 70°C) than that obtained in the fixed bed reactor as illustrated in Fig. 6.5. Another example of the successful application of a Pt/ZSM-5 membrane in the catalytic combustion of volatile organic compounds (VOCs) (Bottino et al, 2001) showed that a high removal efficiency of toluene can be obtained and that the zeolite membrane with a relatively thick layer (40 pm) is clearly affected by mass transfer limitations. 

Langhendries et al [5.74] analyzed the liquid phase catalytic oxidation of cyclohexane in a PBMR, using a simple tank-in-series approximate model for the PBMR. In their -reactor the liquid hydrocarbon was fed in the tubeside, where a packed bed of a zeolite supported iron-pthalocyanine catalysts was placed. The oxidant (aqueous butyl-hydroperoxide) was fed in the shellside from were it was extracted continuously to the tubeside by a microporous membrane. The simulation results show that the PBMR is more efficient than a co-feed PBR in terms of conversion but only at low space times (shorter reactors). A significant enhancement of the organic peroxide efficiency, defined as the amount of oxidant used for the conversion of cyclohexane to the total oxidant converted, was also observed for the PBMR. It was explained to be the result of the controlled addition of the peroxide, which gives low and nearly uniform concentration along the reactor length. 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

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