Conversion catalyst thickness

Figure 10.20 Effect of radius ratio on conversion of reactant A at reactor exit with (a) constant throughput and constant catalyst thickness and (b) constant throughput and constant catalyst volume. [Akyurtlu et al., 1988]...

Cybulski et al. [39] have studied the performance of a commercial-scale monolith reactor for liquid-phase methanol synthesis by computer simulations. The authors developed a mathematical model of the monolith reactor and investigated the influence of several design parameters for the actual process. Optimal process conditions were derived for the three-phase methanol synthesis. The optimum catalyst thickness for the monolith was found to be of the same order as the particle size for negligible intraparticle diffusion (50-75 p.m). Recirculation of the solvent with decompression was shown to result in higher CO conversion. It was concluded that the performance of a monolith reactor is fully commensurable with slurry columns, autoclaves, and trickle-bed reactors. 

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

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Catalysts with very low thicknesses such as 50 nm display a different behavior. A high initial selectivity of 56% at about 2% conversion decreases steeply with a change of conversion to 1 %. This is explained, however, as insufficient initial activation of the catalyst (20 vol.-% ethylene, 80 vol.-% oxygen 3 bar 0.23-2 s 250 °C) [43]. 

Both processes - referring to the non-substituted and substituted methanol reactant- utilize elemental silver catalyst by means of oxidative dehydrogenation. Production is carried out in a pan-like reactor with a 2 cm thick catalyst layer placed on a gas-permeable plate. A selectivity of 95% is obtained at nearly complete conversion. This performance is achieved independent of the size of the reactor, so both at laboratory and production scale, with diameters of 5 cm and 7 m respectively. 

Porous alumina tube externally coated with a MgO/PbO dense film (in double pipe configuration), tube thickness 2.5 mm, outer diameter 4 mm, mean pore diameter 50 nm, active film-coated length 30 mm. Feed enters the reactor at shell side, oxygen at tube side. Oxidative methane coupling, PbO/MgO catalyst in thin film form (see previous column). r-750X,Pr ed 1 bar. Conversion of methane <2%. Selectivity to Cj products > 97%. Omata et al. (1989). The methane conversion is not given. Reported results are calculated from permeability data. 

Fig. 2. The selectivity of saturated products (2MP + 3MP + MCP) and benzene produced from n-hexane (total conversion = 100%) as a function of the final hydrogen pressure. Thick full lines represent calculated equilibrium concentrations. Dashed lines denote experimental data with respect to benzene (x ) and saturated Cg products (O). Pulse system, catalyst 1.0 g platinum black, T = 327 3°C ( 600 K) (31).

Ceria, particularly when doped with Gd203 or SmzOs," has received some attention for direct hydrocarbon conversion in SOFC. Dating back to Steele and co-workers,interesting properties have been demonstrated for ceria-based anodes in direct utilization of methane. Later work suggested that the performance of ceria-based anodes in hydrocarbons could be improved by the addition of precious-metal catalysts, at dopant levels,but the performance of these cells was still too low for practical considerations. The problem with doped ceria is likely that its electronic conductivity is not sufficient. In general, the electrode material should have a conductivity greater than 1 S/cm in order to be practical since a conductivity of 1 S/cm would lead to a cell resistance of 0.1 Q cm for an electrode thickness of 1 mm, even... 

The results of XPS combined with Ar" -sputtering on a VlNb imp.-sample are shown in Figure 4. In this figure, sputtering profiles are shown for a fresh catalyst and for one which had been used for 300h at 510 °C (at this temperature 100% oxygen conversion was reached, no deactivation was observed). Since XRF showed that no vanadium was lost from the catalyst, it can be concluded that in the fresh catalyst, vanadium is present in a homogeneous layer which is at least 40-80 A thick. Upon use, sub-surface vanadium has diffused into the bulk of the catalyst, while the surface concentration of vanadium remains the same. The vanadium at the surface was found to be mostly V " in both the fresh and the used catalyst. Because of the low concentrations of vanadium at the surface and interference from the Ols peak, the presence of small amounts of reduced vanadium cannot be excluded. 

A commercial Cu0/Zn0/Al203 catalyst was coated on quartz and fused silica capillaries by Bravo et al. [29] for methanol steam reforming and compared with packed-bed catalysts. The coatings had a thickness of 25 pm and showed 97% conversion and 97% selectivity towards carbon dioxide at 230 °C reaction temperature, a water/methanol molar feed composition of 1.1 and a space velocity of 45 kgcat s moh1 (methanol). 

The wafers were coated with silicon dioxide (400 nm thickness) and silicon nitride by low pressure chemical vapor deposition (LPCVD) alternately. The chips were fabricated by photolithography and etching. The catalyst (for the application Pt) was introduced as a wire (150 pm thickness), which was heated resistively for igniting the reaction. The ignition of the reaction occurred at 100 °C and complete conversion was achieved at a stochiometric ratio of the reacting species generating a thermal power of 72 W (Figure 2.28). 

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