Enhanced gas selectivity

B.D. Reid, V.H.M. Ebron, I.H. Musselman, J.P. Ferraris, K.J. Balkus, Enhanced gas selectivity in thin film composite membranes of poly(3-(2-acetoxyethyl)thiophene), JMembr Sci, 195, 181-192, 2002,... 

P. B. Eamswoith, M. Wu, M. Tacquai d and M. L. Lee, Background correction device for enhanced element-selective gas cltromatograpltic detection by atomic emission spec-ti oscopy , Appl. Spectr. 48 742-746 (1994). 

Ethylene is currently converted to ethylene oxide with a selectivity of more than 80% under commercial conditions. Typical operating conditions are temperatures in the range 470 to 600 K with total pressures of 1 to 3 Mpa. In order to attain high selectivity to ethylene oxide (>80%), alkali promoters (e.g Rb or Cs) are added to the silver catalyst and ppm levels of chlorinated hydrocarbons (moderators) are added to the gas phase. Recently the addition of Re to the metal and of ppm levels of NOx to the gas phase has been found to further enhance the selectivity to ethylene oxide. 

Increasing of temperature and gas flow rate can enhance the selectivity, however the selectivity decrease with increasing impeller rotation speed (Fig. 5 and 6). In general, increasing selectivity was followed by decreasing of conversion, as shown in Fig. 7. 

It may be desirable to operate in semibatch fashion in order to enhance reaction selectivity or to control the rate of energy release by reaction through manipulation of the rate of addition of one reactant. Other situations in which semibatch operation is employed include a variety of biological fermentations where various nutrients may be added at predetermined rates to achieve optimum production capacity and cases where one reactant is a gas of limited solubility that can be fed only as fast as it will dissolve. 

Weakly adsorbed molecules (A in Figure 8. lb), which can exist only under catalytic reaction conditions, play an important role in surface catalytic reactions even if the adsorption of the promoter is very weak or is undetectable at the surface. Surface intermediates (Cat-X in Figure 8. lb) under the ambient gas molecules behave in a different way from those under vacuum, showing rate enhancement and selectivity change of the surface reaction in the presence of ambient gas. 

This concept is in essence a chromatographic effect similar to that observed in gas chromatography (GC), with the conjugated polymer film acting as the stationary phase. It is possible that like in GC and other candidate technologies for explosive detection, these responses could be empirically standardized for expected analytes of interest and sensory devices caHbrated to deconvolute temporal quenching signals to determine which analytes are present. This would further enhance the selectivity of what is already a very selective sensor for TNT and related compounds. 

Results obtained by ES/MS confirm that the stability of calixarene/cation complexes depends upon the medium. The calixarene in solution presents a strong affinity for cesium, whereas in the gas phase, it displays a stronger affinity for sodium. Moreover, the stability of calixarene/Na+ complexation in a solvent phase is increased by the presence of water in the dilution system (up to 40% in acetonitrile), whereas other alkali complexes are destabilized by the presence of water. Finally, affinity for sodium, which is weak in the solution for calixarenes bearing benzo moieties, considerably increases in the gas phase. These results confirm the interpretation of the MD simulations in an aqueous phase, which lead the authors to conclude that cesium-over-sodium selectivity is governed by the hydration of the sodium cation in the complex, and by the higher hydrophobicity of the complexation site leading to an enhancement of selectivity for cesium over sodium 49... 

Selectivity enhancement Gas miscibility Solvent replacement Burk et al. (1995)... 

As discussed in the previous sections any improvements in coke selectivity can be utilized to extend the frontiers of resid processing as several types of poisons in the resid feed, most of all the polar hydrocarbons will increase the coke formed. Also for fuel gas we can observe that non-optimal vaporization and cracking conditions with resid will strongly enhance gas formation. A low coke and gas catalyst is hence a big plus for resid processing. (Table XII.)... 

It is well known in industrial media that a large catalyst to oil circulation ratio (C/0) enhances the gasoline yield and selectivity, as the coke yield is kept constant. This effect cannot be described by kinetics if we consider that is the same for all reactions. In this case the curve yield versus conversion would remain unaffected by the C/0. To demonstrate the effect of coke on selectivity, a second set of experiments was done where the variations of conversion were obtained by variation of the equilibrium catalyst mass in the MAT (initial c=0). The comparison of the two sets shown on the figure 4, demonstrates unambiguously that the coke drops the gasoline and coke selectivitres and enhances the gas selectivity. This is a direct laboratory representation of the industrial effect of C/O. In industrial plants, the final coke content of the catalyst is inversely proportional to the C/O because the coke yield is maintained constant to equilibrate the thermal balance of the plant. 

The Knudsen diffusion gas separation layer can be modified by e.g. sol-gel, cvd, or crystallisation techniques to enhance the selectivity, but this decreases the permeation. Silica is the material mainly used for modification. However, data on reproducibility and stability are still scarce. The large scale use of high selective inorganic membranes and these membranes at high temperatures, up to at least 600°C, will probably last another 5-10 years. On a laboratory scale (maximum membrane surface area of about 50 cm ) these high selective membranes are now available, although stability can be a problem in certain atmospheres. 

Metal oxide semiconductive sensors are not limited to tin oxide only. Many other metal oxides, such as zinc oxide, tungsten oxide, and others can also be used for chemical and gas sensing. It is understandable that an incorporation of a selective catalyst or a dopant may enhance the selectivity of the MOS sensors. Palladium, platinum, and others have been used as catalytic dopants for these sensors. The processes... 

Many problems remain to be solved for these fascinating processes that directly oxidize propene with green oxidants in the gas phase (i) the rapid deactivation of the catalyst, due to the accumulation of heavy compounds, precursors for coke formation (ii) the need for gas-phase promoters, for example, NO, or chlorocarbons, which act to moderate activity and enhance PO selectivity even in the very first patents issued in this field, this was claimed to be a key feature for optimal performance [46] and (iii) the low space-time-yield achieved, due to the low conversion of propene and/ or the low residence time. 

Klyueva et al. have investigated the acidic properties of erionite modified by isomorphous substitution of B , Ga , and Fe " " by Si and Al . The incorporation of these elements in the aluminosilicate framework led to the generation of new acid centers. These acid centers have a lower concentration of aluminum cations than aluminosilicates, leading to s unples with lower acidity. Consequently, the rate of reactions involving hydrogen transfer, like olefin conversion into paraffins, was lower on isomorphous-substituted erionite samples. Table 5 shows that this enhanced the selectivity toward light olefins. The production of aromatics may... 

Discussion at the First European Workshop on Catalytic Methane Conversion also addressed the effect of chloride addition to catalysts. It was stated that the addition of halides significantly enhanced Cj selectivities and that the enhancement could be observed by cofeeding chlorine or methyl chloride. The mechanism of ethylene formation over chloride-containing catalysts has been described in terms of a purely gas-phase dehydrogenation of ethane. ... 

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