Choice of catalyst particle size for industrial applications

4 Choice of catalyst particle size for industrial applications 

Since the size of catalyst particles has adverse effects on catalytic activity and bed pressure drop, there is a reasonably economic catalyst particle size for use. Therefore, there is a need in choosing the optimum size of catalyst particle to arrive at the lowest total cost (the cost of the catalyst and the cost of consumption of energy) and the highest productivity. The general principle is to select particle size as small as possible given an allowable pressure drop. 

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. 

For the radial-flow ammonia converter, one generally can use small size catalyst particles, such as 1.5-3 mm, that have relatively high activity. As the cross-sectional area for gas flow is larger and the flow velocity lower, the path of gas flow is short. Even if small size catalysts are used, the pressure drop is small. However, the effect of external diffusion is significant. 

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