Particle Size of Catalysts

The particle size of catalyst is an important parameter for an industrial catalyst. It not only significantly affects the pressure drop of the reaction gas through the catalyst bed, but also affects the diffusion rate, and thus the macro reaction rate. Reducing the size of catalyst can increase the utilization ratio of the inner surface of catalyst and decrease the influence of intraparticle diffusion, improve the macro reaction rate, and reduce the needed amount of catalyst. Reducing the amount of catalyst can reduce the height of catalyst bed and pressure drop. However, if the particle size of catalysts is reduced, the resistance of unit bed of catalyst to gas flow increases and power consumption for gas transmission also increases. Thus the particle size of catalyst has contradictory effects on a reactor. 

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The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

Table 3.1 Maximum power densities in single cell and average particle sizes of catalysts calculated by XRD.

The contribution of intraparticle diffusion to rate limitation can be seen from dependence of kobsi on the particle size of catalysts. Fig. 10 shows the effect of particle size on Kbsd for iodide displacement reactions (Eq. (4)) with catalysts 34, 35, and 41149). 

Example CD 12-4 Calculating the Resistance s Example CD 12-5 Effect of Particle Size of Catalyst Weight for a Slow Reaction... 

Table 1 also shows the Rh mean particle size determined by Ha adsorption. The Rh mean particle size of catalysts increased in the order acetate hydrate precursor < nitrate precursor < chloride precursor. CO selectivity decreased and methane selectivity increased dramatically in the same order. From these results, it has been suggested that the difference in Rh precursor changed the particle size of Rh, resulting in the change in the product selectivity consequently. 

Therefore, the prepared catalyst should possess the given chemical composition and physical structure and shape, which can meet the requirements of engineering. The geometrical shape and particle size of catalyst are determined by the requirements in industrial process, including the type of the reactor, the operation pressure, flow velocity, permitted pressure drop of the bed, the reaction kinetics, the physicochemical properties of the catalyst, shaping properties and economic factors. 

If p = l pR < 0.2), the discussed system is a dynamically controlled reduction. In this system, the process rate is proportional to the inner surface area, and has nothing to do with pore size, diffusion coefficient and the particle size of catalyst. When pR >15 then this is an inner diffusion controlled process. In this case, the overall rate of process is related with particle size of catalyst, pore radius and the gas diffusion coefficient. When 0.2 < pR < 15, it is a transitional area. 

In the study of dynamics, the integral reactor can be divided into two types — isothermal and adiabatic reactor. Because isothermal integral reactor is simple and cheap as well as the lower demand on the accuracy on analysis, it is always the preferred option. In order to overcome its difficulties to maintain the temperature uniform, there are several actions. The first one is to reduce the diameter to obtain uniform radial temperature as much as possible. When the inner diameter of reactor is 4-6 times bigger than the particle size of catalyst, the effect of reducing the diameter of tube on the distribution of temperature still is the main factor. The second is to use various mediums with high thermal conductivity, to provide heating indirectly through the whole piece of metal or sand bath, which is commonly used presently. The third is to dilute the catalyst bed with inert materials. 

It can be seen from equation (7.15) that the temperature difference is proportional to reaction rate, heat effect and the square of diameter of the reactor, and is inversely proportional to the effective conductivity factor. The temperature difference increases with a decrease in the particle diameter of catalysts because the effective conductivity factor A reduces with the decrease in the size of catalyst particles. When decreasing the particle diameter of catalyst in order to eliminate the effect of inside diffusion on reaction, it also enhances the factor of temperature difference. Therefore, it need to weigh the pros and cons of these factors in order to determine the most appropriate particle size of catalyst and the diameter of reactor. 

Table 7.4 Relationship between the compacted packing density and the particle size of catalyst ZA-5 ...

In ammonia production, the space velocity is often (0.6—2) x 10 h. In axial flow converters, the velocity of gas flow can be up to several m s the effect of external diffusion should be absent, but the effect of intraparticle diffusion cannot be ignored. The gas flow area In a radial flow converter is very large, the gas velocity is very low, and thus both effects of external and intraparticle diffusion cannot be ignored. Among the many factors that influence diffusion, the particle size of catalyst is the most significant and can be easily adjusted. 

The optimum particle size depends on the gas flow and the specific bed characteristics in question. As the size of catalyst particles affects the reactor bed diameter, the particle size of catalysts should be chosen based on the type and diameter of reactor, height of beds and specific plant conditions. 

Axial-flow ammonia converters should use a larger particle size of catalysts. For the multi-bed axial-flow converter with direct heat-exchange between beds (cold-quench) having a diameter of 1600-3200 mm and a production capacity of 1000 t d" or more, and the height of catalyst bed 10-12m, choose large particles with a diameter of 6.7-9.4mm and 9.4-13 mm to minimize the pressure drop. For an axial-converter with a diameter of 800-1300 mm, height of catalyst bed 7-8 m, use 4.7-6.7mm or 9.4 mm particles to keep the low pressure drop. For an axial-converter with a diameter of 500-600 mm, the height of catalyst bed is only about 5 m, 2.2-3.3 mm, 3.3-4.7mm, and smaller particles may be used to increase the ammonia production. 

Note The particle size of catalyst is 2.2-3.3 mm the abrasion ratio is measured in accordance with the standards of HG/T2782-1996. 

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