Reaction rate catalyst particle size effect

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

It is seen from Fig. 8.18 that some of the curves of the change of reaction rate with particle size at different temperatures intersect. This indicates that when the reaction temperature varies, the effect of particle size on reaction rate will also vary. This phenomenon shows that the pore surface utilization ratio does not just vary with the size of particles, but also with the reaction temperature and the so-called catalyst efficiency as defined in equation (8.8). The pore surface utilization ratio (ISUR) for different sizes of particles at different temperatures is shown in Fig. 8.19. It can clearly be seen from Fig. 8.19 that particle size has the greatest effect on 77. The ISUR (77) is significantly lowered with increasing particle size. For example, when the reaction temperature (350°C) and other factors are the same, with the size increased from 0.6-0.9 mm to 4.0-6.7mm, 77 is decreased from 1 to 0.785 and 0.740 at 7.0 MPa and 15.0 MPa respectively. 

The effectiveness factor depends, not only on the reaction rate constant and the effective diffusivity, but also on the size and shape of the catalyst pellets. In the following analysis detailed consideration is given to particles of two regular shapes ... 

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

Where intraparticle diffusion appreciably affects the rate of the reaction, reduction in catalyst particle size would be necessary to increase the effectiveness factor and hence conversion. But this may not be possible due to the pressure drop limitations in conventional packed beds. In such situations, the use of Monoliths would provide the advantage of higher effectiveness factor. 

A rather special possibility to attain information about molecular diffusion is provided by catalytic reactions if they proceed in the range of medium Thiele moduli (i.e. in the transition range between intrinsically and transport controlled reactions) [100]. By analyzing the dependence of the effective reaction rate on the catalyst particle size [101. 102] and/or the intrinsic re-... 

Applying the Broensted Polanyi correlations is sometimes useful for describing the dependence of the reaction rate on the size of the catalyti caUy active component. A huge amount of experimental data have been compiled to date regarding the effect of the particle size of the catalyst active components on the specific catalytic activity, SCA, as well as on the turnover frequency, TOP, of the active center. Both parameters do not relate to the total surface area of the catalyticaUy active phase or to the total number of active centers and, therefore, characterize directly the properties of the active center. There are also some experimental data on the size dependence of the adsorption properties of small metal parti cles, as well as on the selectivity of a few catalytic processes. 

For pore diffusion resistances in reactions having moderate heat evolution, the following phenomena characteristically hold true in industrial ammonia synthesis [212] in the temperature range in which transport limitation is operative, the apparent energy of activation falls to about half its value at low temperatures the apparent activation energy and reaction order, as well as the ammonia production per unit volume of catalyst, decrease with increasing catalyst particle size [211], [213]-[215]. For example at the gas inlet to a TVA converter, the effective rate of formation of ammonia on 5.7-mm particles is only about a quarter of the rate measured on very much smaller grains (Fig. 13) [157]. 

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

Catalyst particles size -35 -i-48 Tyler mesh were used in all tests. Porosity was measured using a mercury porosimeter. A 0.1356 pm pore mean diameter was determined. The Satterfield and Sherwood (7) methodology was used to verify that reaction occurs without any diSusional limitation (internal or external). The effective diffusivity was estimated from the porosity measurements and binary diffusion coefficient and pore tortuosity pubhshed in the hterature, leading to an estimated value of 10 for the generahzed Thiele Modvdus based on the reaction rate. The efi ectiveness factor was then considered as 1.0. 

For working in absence of intraparticle gradients the criterium generally utilised is to decrease the catalyst particle size until no effect in the rate of reaction is obtained. 

As described above, the SCF had an effect not only on the diffusion of the reactant, but also on the desorption of adsorbed species. Figure 4.8-5 shows the influence of catalyst particle size on the alkene content in the supercritical phase reaction. It was found that a change of catalyst particle size had little effect on the alkene content in the product. This suggests that the supercritical fluid had a more obvious effect on the desorption of the produced alkenes. The diffusion rate may have been higher than the adsorption rate of the alkenes, at least under these experimental conditions. 

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