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Considering the application of MFI zeolite membrane, several methods for modification of zeolite membranes prior to application in PBMR for WGS reaction are proposed. For instance, Tang et al. modified the pores of MFI zeolite membranes by a so called in-situ catalytic cracking deposition (CCD) of silane precursors [124,152,214]. The formed deposits reduce the effective pore size to below 0.36 nm hindering the entry of CO2 into the pore channels. The idea behind this modification was the need to obtain controlled mass transport of H2 over CO2 since thought the unmodified MFI the transport of H2 and CO2, respectively, is controlled by gaseous diffusion resulting in separation factors slightly below the Knudsen factor. Accordingly, thanks to the the deposition of molecular silica species in the zeolite channels, the access of the slightly bigger CO2 molecules (kinetic diameters of H2 and CO2 are 0.289 and 0.33 nm, respectively) was restrained giving rise to H2/CO2 permselectivity of 68.3 and equimolar mixture separation factors of nearly 38 at 550 ◦C [152]. The modified zeolite MFI membrane packed with a Fe1.82Ce0.18O3 catalyst was tested in WGS reaction at temperatures between 400 and 550 ◦C and near atmospheric pressure. At a reaction temperature of 550 ◦C (WHSV = 60,000 h−1, H2O/CO = 1) the membrane reactor configuration exhibited CO conversion (81.7%) exceeding the equilibrium limit (65%) and the performance of the traditional packed-bed reactor (62.5%). However, decreasing the reaction temperatures below 500 ◦C caused a conversion drop due to kinetic resistance. With regard to the WGS reaction, low H2O/CO ratios are preferred since the hydrogen partial pressure is large and the driving force for permeation as a result, too. The same group evaluated the impact of elevated pressure (2–6 atm) on the efficiency of modified MFI disc membranes in high temperature (400–550 ◦C) WGS reaction and demonstrated its positive effect on the CO conversion. Furthermore, the authors pointed towards to the larger driving force for H2 permeation at increased feed pressure and constant permeate pressure [154]. Figure 5 depicts the influence of reaction pressure and temperature on CO conversion (χco), hydrogen recovery (RH2 ), and permeate side H2 concentration (γH2,p ). As shown in Figure 5 (left) a higher driving force for hydrogen permeation is achieved by increasing the feed pressure leading to higher RH2 . At the applied temperaturestheseparationfactorαH2CO2 isnotinfluencedbyadsorptionandthe gases exhibit ideal gas behavior. The authors ascribed the decrease in γH2,p with increasing RH2 and χco at high pressure to the decreased value of (γH2 /γCO2 )permeate (=αH2CO2/(γH2/γCO2)feed)whenreducing(γH2/γCO2)feed. Furthermore,itisshown (Figure 5 (right)) that an increased feed pressure could be used to overcome the equilibrium CO conversion (χCO,e) using the PBMR. Generally, the findings indicate that even membranes with moderate selectivity could be powerful tool for conversion enhancement. Particularly, it was emphasized that the prepared MFI membrane showed good resistance against H2S and was stable at the applied high temperatures and pressures.Recently, the same authors applied the model of one-dimensional 27PDF Image | Zeolite Catalysis
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