Washcoat thickness

To further ensure there will be no contribution of transport within the washcoat, calculations done based on a method of transient analysis for catalytic ignition by Balakotaiah should be done. Fig. 15 shows the results of the calculation in comparison to where washcoat diffusion occurs. By maintaining a nominal washcoat thickness of 7 micron, the catalyst will operate in the kinetic regime for all reaction conditions considered. 

The internal diffusion effects cannot be simply included into the reaction kinetics particularly in the case of parametric studies on the varying washcoat thickness. 

When the internal diffusion effects are considered explicitly, concentration variations in the catalytic washcoat layer are modeled both in the axial (z) and the transverse (radial, r) directions. Simple slab geometry is chosen for the washcoat layer, since the ratio of the washcoat thickness to the channel diameter is low. The layer is characterized by its external surface density a and the mean thickness <5. It can be assumed that there are no temperature gradients in the transverse direction within the washcoat layer and in the wall of the channel because of the sufficiently high heat conductivity, cf., e.g. Wanker et al. 

Washcoat thickness is analogous to particle size in that reactants must penetrate its pore structure and interact with the dispersed active sites. The products produced must diffuse through the structure and out into the bulk gas. This phenomenon differs from that involving a particle in that only the gas-solid washcoat surface is available since the other side is bonded to the wall of the monolith. 

Des is the effective diffusivity of the solute in the solid phase while Dm is the molecular diffusivity in the fluid phase.) The function Ai(e, p) depends on the shape of the channel and the washcoat surrounding it. For the case of a circular channel with a uniform washcoat thickness, it can be shown that... 

The thickness of the catalytic layer deposited on channel walls is very small The average varies typically from 10 to 150 p,m. The first approximation concerning the deposit distribution is that the layer is distributed uniformly around the channel periphery. This may be true if circular channels are considered. The typical cross section of the monolith channels is, however, square. In this case a significant nonuniformity in the washcoat thickness is often encountered (Fig. 6). The reason is that the liquid from which the... 

For the TBR, spherical catalyst particles of uniform size with the catalytically active material either uniformly distributed throughout the catalyst or present in a shell were considered. For the MR, channels of square cross section were assumed to have walls covered by the washcoat distributed in such a way that the comers are approximated by the circle-in-square geometry, while the sides are approximated by a planar slab geometry. The volumetric load of catalytic material was a function of the washcoat thickness... 

Combined with the use of precious metal catalyzed washcoats deposited on the walls, microchannel reactors can realize nearly 10 times reduction in reactor size compared with that of a process that utilizes catalyst particles. The washcoat thickness is usually less than lOOjit and provides greater structural stability. This stability arises from smaller thermal expansion ratios and lower temperature gradients. 

X-ray diffraction studies were performed to verify the complete transformation of boehmite (raw material to prepare the washcoat) to y-alumina. A Pd content of 0.024 wt% (Pd mass/ total mass) was measured by atomic absorption. An average washcoat thickness of 90 pm was estimated by SEM. Besides, a BET surface area of 80 m /gwashc i was measured. 

However, in other cases internal diffusion limitation can be significant even with very thin washcoat thicknesses [127], when temperature is high (> 700°C). This refers, for example, to catalytic combustions, which are extremely fast. Hayes et al. [135] evaluated the extent of intraphase and interphase resistances to the catalytic conversion of low concentrations of carbon monoxide in air in a tube wall reactor (coated with a platinum-alumina deposit). Above 610 K there was strong evidence of both intraphase and interphase resistances to catalytic conversion. In Sections 8.3.2, 8.3.3, and 8.3.4, we provide a systematic analysis for prediction of the extension of external and internal diffusion limitations. 

Catalyst coating design. More efficient calculation of the effectiveness factor in monolith channels is possible for nonuniform washcoat geometries and nonlinear kinetics [94]. Trade-offs in reactor performance arising from catalyst loading and washcoat thickness were also considered [43]. 

Stutz MJ, Poulikakos D. Optimum washcoat thickness of a monolith reactor for syngas production by partial oxidation of methane. Chemical Engineering Science 2008 63 1761-1770. 

A microchannel reactor configuration, in which catalytic endothermic (hydrocarbon SR) and exothermic (hydrocarbon combustion) reactions can be coupled, is shown in Figure 11.8 [ 24]. The reactor is composed of parallel groups of endothermic and exothermic channels which are separated by thin solid walls. The reactive flows are considered to be co-current. Each channel is square shaped, and the inner walls of the channels are wash-coated with a porous supported metal catalyst specific for the reaction type. Washcoat thickness is assumed to be uniform... 

In this paper, we present a study in which combustion catalysts based on silica-coated metal monoliths were prepared. The aim of this study was to prepare washcoated metal monoliths with controlled properties. The properties varied are specific surface area of the washcoat and washcoat thickness or washcoat loading. Furthermore, we discuss how the preparation procedure affects the resulting catalyst properties and related performance. We deposited washcoats based on colloidal silica sols. Colloidal silica sols give porous materials with rather narrow pore size distributions when dried and calcined. This gives us excellent control over the pore size distribution of the washcoat, as will be discussed. The technique presented here, allows deposition of washcoats with controllable thickness in one step, unlike techniques based on pure silica sols, reported elsewhere [7,8]. Washcoats were impregnated with paUadium salts to make active catalysts that were tested in methane combustion. The effects of the preparation procedure of the silica and of the impregnation procedure were studied using particulate catalysts. 

Common cell densities used for heavy-duty diesel applications with vanadia SCR catalysts are 300 cpsi [19, 36] and 400 cpsi [6, 18, 35, 37, 40]. For these cell densities, the wall thicknesses for cordierite substrates range typically from 4 mil (100 pm) to 8 mil (200 pm) [18, 19, 37, 41]. In Fig. 3.12, with wall thickness is here considered the total wall thickness resulting from the substrate including the catalyst washcoat. The washcoat thickness for a coated vanadia SCR catalyst depends on the washcoat loading and could range from 20 to 100 pm [37]. For a washcoated SCR catalyst, the wall thickness could of course be reduced either by decreasing the inert substrate wall thickness or reduce the washcoat thickness as long as this does not effect the catalyst performance. 

The scope of this paragraph is to analyze the impact of internal washcoat diffusion on the performance of zeolite-based catalysts both by experimental and simulation results. In the first part, an experimental study of mass transfer limitations in Fe- and Cu-zeolite catalysts performed by Metkar et al. [40] is presented. The authors investigated catalysts with different washcoat loadings, washcoat thicknesses, and lengths under various SCR reactions in order to identify the presence of diffusion limitations throughout an extended temperature range. In the second part, the flow-through catalyst model, presented in Sect. 13.2, was employed to reproduce the test conditions of the fore-mentioned experiments. 

We first assessed the impact of internal diffusion limitations in the PGM layer. For this purpose a simulation study was performed progressively increasing the PGM washcoat thickness, from 10 pm up to 71 pm. From the analysis of NH3 concentration profiles a dramatic impact of dififiisional limitations was apparent indeed only the surface of this layer is effectively active due to the extremely high reactions rates of the PGM catalyst. For this reason, we developed a Layer -I- Surface Model (LSM) of dual-layer ASC where we treat the PGM layer as a surface, while we retain the rigorous description of coupled reaction/dififusion in the SCR layer, based on the previous ID -I- ID model of SCR monolithic converters [12,25,26]. Indeed, avoiding the description of diffusion phenomena in the PGM layer enables the direct inclusion of the PGM reactivity in the SCR converter model by simply modifying the inner boundary conditions of the species differential mass balances in the SCR layer, i.e., those now at the interface with the PGM phase. Treating the PGM layer as a surface thus enabled a simple extension of the ID -I- ID SCR converter model to simulate dual-layer catalytic systems too. 

Stutz, M. and Poulikakos, D. (2008). Optimum Washcoat Thickness of a Monolith Reactor for Syngas Production by Partial Oxidation of Methane, Chem. Eng. ScL, 63, pp. 1761-1770. 

Figure 28.9. Epoxidation of ethylene calculated effect of the monolith pitch on C2H4 conversion, C2H4O selectivity and molar yield. Washcoat thickness 120 xm support volume fraction 0.2 ks 200 W/(m K) Tcool 250 C. Tube diameter 39.2 mm. Reprinted from Ref. 102. Copyright 2001 with permission from Elsevier.

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