Particulate catalysts

The work of Thiele (1939) and Zeldovich (1939) called attention to the fact that reaction rates can be influenced by diffusion in the pores of particulate catalysts. For industrial, high-performance catalysts, where reaction rates are high, the pore diffusion limitation can reduce both productivity and selectivity. The latter problem emerges because 80% of the processes for the production of basic intermediates are oxidations and hydrogenations. In these processes the reactive intermediates are the valuable products, but because of their reactivity are subject to secondary degradations. In addition both oxidations and hydrogenation are exothermic processes and inside temperature gradients further complicate secondary processes inside the pores. 

For example, sulphur dioxide is highly water soluble and tends to be absorbed in the airways above the larynx. Responses at various concentrations are summarized in Table 5.3. However, in the presence of particulate catalysts and sunlight, conversion to sulphur trioxide occurs and the in itant response is much more severe. 

However, in the presence of particulate catalysts and sunlight, conversion to sulphur trioxide occurs and the irritant response is much more severe. 

High geometrical areas per reactor volume, typically 1.5-4 times higher than in the reactors with particulate catalysts Very high catalytic efficiency, practically 100%, due to very short diffusion paths in thin washcoat layer... 

Screens of precisely calibrated mesh sizes and openings are the principal devices for measuring the distribution of sizes for particulate catalysts. Procedures are discussed in Section II.B.l. 

Particulate catalysts are usually sold by weight but are charged to a reactor by volume. Thus, the density of the support has a strong impact on the economics of the process. Fluidization in moving-bed reactors is also dependent on catalyst density. The density of powders affects the extent to which they can be suspended and eventually settled in slurry-phase reactors. 

Edges or surface irregularities on particulate catalysts used in fluid- or fixed-bed applications are susceptible to attrition and erosion during reaction. The morphology of a typical fluid-bed cracking catalyst is shown in Fig. 9. The surface of spheres, extrudates, and tablets is relatively free of topological features, and thus physical losses are usually not a serious problem. 

The particulate catalyst is composed of 90% Fe and 10% Cr. The Cr minimizes sintering of the active Fe phase. The catalytic reaction is limited by pore diffusion so small particles are used. The exit process gas contains about 2% CO as governed by the thermodynamics and kinetics of the reaction. This reaction is slightly exothermic and thermodynamics favor low temperatures that decrease the reaction rate. It is therefore necessary to further cool the mix to about 200°C where it is fed to a low-temperature shift reactor (LTS)... 

One clear piece of evidence for surface character-property relationships lies in the observation that particulate catalysts with the same composition, but different crystal habit, have different reactivities. This phenomenon, sometimes referred to as a catalytic anisotropy or a structure-sensitive catalytic reaction, has been observed in a variety of Mo, V, and W containing materials [1]. Because the catalytic anisotropy of a-MoOj has been studied in relatively greater depth, this exemplary case is described below [1-10]. 

Particulate catalyst can be arranged in arrays of any geometric configuration. In such arrays, three levels of porosity (TLP) can be distinguished. The fraction of the reaction zone that is free to the gas flow is the first level of porosity. The void fraction within arrays is the second level of porosity. The fraction of pores within the catalyst pellets is referred as the third level of porosity. Parallel-passage and lateral-flow reactors... 

Both catalysts were activated at the optimum conditions determined using TPR. The rates at the maximum selectivity to benzyl alcohol were compared. In the presence of particulate catalyst the rate amounted to 0.0009S mol/(gNi./ min), while for monolithic catalyst the rate was approximately 0.00175 mol/(gNi min), i.e., about two times more. The diffusion length in the nickel monolith is much shorter than in the 3.2effectiveness factor for the nickel pellets, and hence in a lower reaction rate. Selectivity of both catalysts with respect to benzyl alcohol was nearly the same, at least within the precision of analytical methods used 94.9% for pelleted catalyst and 95.1% for monoliAic catalyst. We may therefore conclude that the selectivity is not controlled by internal diffusion but by the surface properties of the catalysts. 

The diffusion length in a particulate catalyst is longer than in a monolithic catalyst, and hence the effectiveness factor is typically lower for the TBR [14]. As a consequence, the conversion per unit volume of catalyst is lower in the TBR. Moreover, the diffusion length can influence the selectivity of the reaction if the rate is affected by diffusion. Two ways to reduce the diffusion length in TBRs are (1) to use smaller catalyst particles, and (2) to use a catalyst with the catalytically active species located near only the outer surface... 

Table 2 Comparison of Packed Beds of Particulate Catalysts with a Monolith Catalyst...

Table 3 lists some data on internally finned monoliths of the types depicted in Fig. 8, with relative instead of absolute values for the dimensions. It can be seen that for the relative wail thickness chosen, the fractional catalyst volumes (in the case of incorporated catalytic monoliths) are of the same order of magnitude (about 0.6) as in conventional packings of particulate catalysts. 

The internally finned monolith as catalyst in a fixed-bed reactor can be designed for similar volumes of active catalyst material per unit of reactor space and similar surface-area-to-volume ratios as common particulate catalysts in fixed beds. 

Giroux, T., Hwang, S., Liu, Y., Ruettinger, W., and Shore, L. Monolithic structures as alternatives to particulate catalysts for the reforming of hydrocarbons for hydrogen generation. Applied Catalysis. B, Environmental, 2005, 56, 185. 

Water gas shift converters in industry are largely unchanged from their original design adiabatic fixed bed reactors with particulate catalysts. There are only two kinds of these reactors in use in industry today HTS and LTS. They differ in the operating temperatures and catalysts which they use. Two reactors are needed to convert the majority of CO, because of the equilibrium limitations of the process (see previous section). The inlet temperature of the HTS reactor is typically around 320°C. The outlet temperature rises to about 400-450° C because of the reaction exotherm. The gases are cooled to about 200°C before entering the LTS reactor, where the final 2-3% CO is partially converted to CO2 and H2. 

The outlet from the secondary reformer contains about 10-14% CO (dry gas) which is fed to a high-temperature water gas shift (WGS) reactor (Fig. 2.2), typically loaded with Fe or Cr particulate catalyst at about 350°C. This further increase the H2 content lowering CO content to about 2% as governed by the thermodynamic and kinetics of the Eq. 2.3, that is an exothermic reaction. Water gas shift reaction equilibrium is sensitive to temperature with the tendency to shift towards products when temperature decreases. 

In the concluding section, consideration will be given to the relationship between studies on single crystals and the behaviour of particulate catalysts, especially with respect to the relevance of one to the other. A very old concept in catalysis is that of the active site . This concept recently has come again to the forefront of research in this area and consideration of the possibility of directly observing the active site will be given in the final section. 

The apparatus consists of an oven surrounding a heat-resistant tube that can be evacuated. A substrate with particulate catalyst dispersed on it is placed inside the tube (Figure 3.17a). Then, a stream of the gaseous reactant mixture is led over... 

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