Conversion catalyst layer thickness

In another case study, the thickness of the catalyst layer was increased at constant WHSV by increasing the inlet flow rate. Increasing the catalyst layer thickness from 10 to 60 pm led to a decrease in conversion from 100 to < 70% for both reactions. A faster increase in the axial temperature was achieved for the thin coatings. 

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

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. 

It is well known that though NO conversion is unaffected by the thickness of the monolith wall beyond a small critical value, SO2 conversion increases linearly with increasing wall thickness. This is indicated in Fig. 9 such trends reflect the different influence of internal diffusional resistances on DeNOx reaction and SO2 oxidation, which, as discussed previously, are respectively confined to a superficial layer of the catalyst and active inside the whole wall. Consequently, the design of SCR monoliths should pursue the realization of very thin catalytic walls Fig. 9, for example, shows that reducing the catalyst half-thickness from 0.7 mm to 0.2 mm does not alter the DeNOxing performance but causes a decrease of SO2 oxidation as significant as 78%. 

Tlie conversion and product selectivity is optimized when the gas mixing is improved, which reduces the boundary layer thickness at the catalyst surface and increases the mass transfer coefficient for a given channel dimension. The laminar flow through the honeycomb channels in the extrudate monolith results in a relatively thick boundary layer... 

Owing to the comparatively small size of the pores (up to 100 p.m, compared to a pitch of a few millimeters for the honeycomb channels) and the small thickness of the catalyst layer (a few microns, compared to some tenths of a millimeter for the catalytic wall of the honeycomb channels), both internal and external mass transfer limitations to NO conversion in catalytic filters can easily be neglected. An efficiency factor equal to unity can thus be assumed with confidence for NO reduction, contrary to honeycomb catalysts, for which this parameter is hardly higher than 0.05% at the conventional operating temperatures (320-380 C). 

Equations (47-49) describe the nature of the losses incurred by transport within the catalyst layer. The catalyst surface area (catalyst sites) active in the ORR process of conversion of oxygen flux to protonic current flux (Eq. 49) is distributed linearly along the thickness of the catalyst layer. Consequently, gas supply to a catalyst site removed from the backing layer/catalyst layer interface is limited by the effective permeability of oxygen in the catalyst layer,... 

The thickness of a catalyst layer on the wall of a microchannel is typically approximately several micrometers. However, if the catalyst is only moderately active, a larger amount of catalyst will be necessary to achieve a high conversion while keeping the mean residence time suitable for a MSR, that is, in the millisecond or even microsecond range. 

PV is a promising option to enhance the conversion of reversible condensation reactions in which water is formed as a by-product. Peters et al. (2005) prepared composite catalytic membranes by a dip-coating technique. Composite catalytic membranes have been prepared by applying a zeolite coating on top of ceramic hf silica membranes. In the PV-assisted esterification reaction, the catalytic manbrane was able to couple catalytic activity and water removal. A reactor evaluation proved that the outlet conversion of the catalytic PV-assisted esterification reaction exceeded the conversion of a conventional inert PV membrane reactor, with the same loading of catalyst dispersed in the bulk liquid. Further, the performance of the zeolite-coated PV membranes can be increased by optimization of the catalytic layer thickness or by an increase in catalytic activity. 

Membrane in the MRMembrane Catalyst Palladium thickness/ layer ( rm) H2O/ CH3OH T(°C) p (bar) Conversion (%) H2 recovery (%) H2 permeate purity (%) Reference... 

The thickness of the cathode catalyst layer (CCL) in low-temperature fuel cells varies from ten micrometres in modern PEFCs to hundred of micrometres in direct methanol fuel cells (DMFCs). The CCL in DMFCs operates at a high level of flooding, which reduces ORR efficiency. The considerable thickness of the active layer facilitates electrochemical conversion under these conditions. 

Figure 5.12 Ratio of calculated conversion of methane steam reforming to equilibrium conversion for various space velocities and thicknesses ofthe catalyst layer the gas flow was assumed to take place in a 50-p,m high gap above the catalyst [385].

These dual layer results provide clear evidence of the existence of mass transport limitations. That the conversion for the dual-layer Fe/Cu catalyst (I, J, K) approached that for the Fe (top) layer at sufficiently high temperature indicates that significant transport limitations were present. In fact, the experiment helps to pinpoint the temperature at which the onset of diffusion limitations occurs for an Fe top layer of a prescribed loading (thickness). As the Fe top layer thickness decreases, the temperature at which the dual layer catalyst conversion is within a few percent of the single layer Fe catalyst (sample F) conversion increases. For example, the conversion for the thickest Fe top layer catalyst (sample I) approaches that of the single layer Fe catalyst at about 300 °C. For next thinner top layers (samples J), the temperature increases to 400 °C. Were diffusion limitations not present, the conversion would approach the arithmetic average of the Fe and Cu catalysts, not unlike a mixed layer catalyst. 

The first derivative of the concentration in the y-direction can now be computed by substituting C = in eq (10), provided that the concentration at the reactor wall, C, is known. We have assumed that it is equal to 0 to a first approximation, in other words that the catalyst layer is so thick that complete conversion of CO2 entering it is obtained. This appeared justified on the basis of numerical calculations at the conditions applied in our experiments as well as the calculations reported here. Equation (11) then represents the mass transport into the catalyst bed ... 

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. 

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