Thickness on CO conversion

Figure 7. Effect of catalyst thickness on CO conversion by base metal catalyst...

Figure 8. Effect of catalyst layer thickness on CO conversion for platinum...

A schematic of a monolith catalyst is shown in Fig. 19-18a. In cases where pressure drop is limiting, such as for CO oxidation in cogeneration power plant exhausts, monolith catalyst panels may be stacked to form a thin (3- to 4-in-thick) wall. The other dimensions of the wall can be on the order of 35 x 40 ft. CO conversion is over 90 percent with a pressure drop across the catalyst of 1.5 in of water. Alternatively, the monolith may be used as a catalyst and filter, as is the case for a diesel particulate filter. In this case, monolith channels are blocked and the exhaust gases from a diesel truck are forced through the walls (Fig. 19-18b). The filter is a critical component in a continuous regenerable trap. NO in the exhaust... 

Fig. 28. Site density, pellet size, and eggshell thickness effects on FT synthesis selectivity (Co catalysts Si02, TiOi, and AI2O3 supports 473 K, H2/CO = 2.1, 2000 kPa, 50-62% CO conversion)., Dispersion/support effects , pellet size effects O, eggshell catalysts (Table VII) A, from Refs. 47 and 48.

However, the kinetics of CO and hydrocarbon oxidation over platinum has a negative first order dependence on CO concentration for most of the ranges of temperature and concentration of interest to automotive catalysis. Voltz et ah (I) demonstrated that, under a total pressure of 1 atm. at 400°-800°F, the kinetics depend inversely on CO concentration from 0.2% to 4%. Therefore, when there are diffusion effects, a decline in CO concentration toward the interior of a porous catalytic layer would lead to an increase in reaction rates in the interior. Diffusion then has a beneficial rather than a detrimental effect on overall kinetic rates, and a thick catalytic layer may effect a higher conversion than a thin layer under identical ambient conditions. 

Criscuoli et al. compared Pd membrane reactor with mesoporous membrane reactor and fixed-bed reactor [5]. Figure 6.5 shows the effect of space velocity on the CO conversion for the three reaction systems. As expected both membrane reactors exhibit better CO conversion than traditional reactor. Between the two membrane reactors Pd membrane reactor exhibits much better CO conversion compared to mesoporous membrane reactor. At the highest time factor, Pd membrane reactor exhibits 100% CO conversion. By increasing the Pd membrane thickness, the hydrogen permeation rate decreases and lower conversions of carbon monoxide are achieved. When they compared experimental results with simulation results the model fits well with the experimental points. 

As an example. Fig. 3.13 shows the results of WGS tests carried out at 410°C in a Pd membrane reactor with gas mixtures simulating a syngas produced in IGCC plants (Pinacci et al., 2010). A composite palladium-porous stainless steel membrane, 29 pm thick, obtained by electroless plating on a porous stainless steel support, has been used for this purpose. The reactor was fed with a shift gas mixture with a 7.6% CO concentration and HjO/CO ratio of 2.7 and 3.6,respectively. CO conversions up to the 85.0% and 78%,respectively, have been reached, while operating at a feed pressure of 6 bar. These values can be compared with the corresponding conversions obtained with a... 

WGS tests have been also performed with gas mixtures simulating a syngas produced by autothermal reforming of natural gas (Bi et al., 2009). The performances of two different catalysts (Fe-Cr vs. Pt/Ceo,6Zro402) have been evaluated in a wide range of operating conditions, in a membrane reactor equipped with a 1.4 pm thick Pd membrane on a ceramic support. CO conversion remained above thermodynamic equilibrium up to feed space velocities of 9100 L kg h at 350°C, pressure 12 bar and steam-to-carbon ratio S/C = 3. 

In a first set of experiments, the impact of the sodium hydroxide concentration (0.1, 1.0, 2.0 M) and gas-flow direction (co-current, counter-flow) was analysed (50 ml h liquid flow, 65 pm film thickness) [5]. The higher the base concentration, the higher is the conversion of carbon dioxide. For aU concentrations, complete absorption is achieved, but at different carbon dioxide contents in the gas mixture. The higher the carbon dioxide content, the higher is the gas flow velocity and the larger must be the sodium hydroxide concentration for complete absorption. The gas flow direction had no significant effect on carbon dioxide absorption as the gas velocities were still low, so that no pronounced co- or counter-flow operation was realized. 

The basic Brophy and Ingraham technique was studied by several other authors. Bayer and Trivedi " found that the effectiveness of the technique depended more on the nature of the coating than on its thickness, and that retained moisture in the electroplate was essential for effective conversion. They recommended a current density of 21.5 A/m for 5 minutes to produce a coating thickness of 1.25 to 2.5 tjm. Nishimura and co-workers found that the presence of air or water or both in the conversion gas improved the wear life. Table 9.5 compares the properties of the in situ films with those of burnished and sputtered films, and shows superior wear life for the in situ films. Their friction results were curious, in that they found that the initial films which were formed gave low friction in air or nitrogen but not in vacuum. Low friction in vacuum was obtained when the initial product was heated in vacuum to 400°C. 

A practical disadvantage of the sensitized planar electrodes described in the previous section was their low solar energy conversion efficiencies. An experimental drawback was that characterization was limited to photoelectrochemical or luminescence techniques. Both of these issues arose from the poor light harvesting by monolayers on flat surfaces or inefficient interfacial electron transfer yields from thick films or concentrated solutions. In 1990, Gratzel and co-workers reported experimental studies of dye sensitized colloidal Ti02 films that eliminated both of these problematic issues [112]. The effective surface area for sensitizer binding... 

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