Application to Converter Design and Operation

In the previous sections, the fundamental kinetic equations that can be used to correlate experimental data and to calculate ammonia production under given operating conditions have been discussed. Chemical and physical factors, such as mass, heat, and momentum transfer, which could limit the available active surface of the catalyst and the reaction rate in industrial service, have also been mentioned and evaluated. In the following section, we shall consider the basic design criteria 

Other limitations to the industrial operating conditions are imposed by the intrinsic activity of the catalyst and its resistance to thermal sintering. When the catalyst is kept at a temperature higher than its maximum allowable temperature (540 °C for common commercial catalysts when operated on pure synthesis gas) it quickly loses its activity due to a surface reorganization of its structure. The limitations imposed by the materials of construction are a further reason why temperatures above this value must not be allowed in industrial converters. 

The minimum allowable temperature is determined by the minimum industrially acceptable reaction rate. Under normal operating conditions, the exothermic reaction is self-substaining. Whenever the catalyst temperature is lowered, the 

The reaction rate decreases continuously along the ideal temperature profile line, and a continuous decrease in the heat removed from the system as the reaction proceeds should therefore be necessary for following the optimal sequence. Since, in practice, it is impossible to achieve this target precisely, the ideal converter is not realizable. Nevertheless, the best approximation to the T p line should be the aim both in converter design and in converter operation. 

Superimposed on the graphs in Fig. 6.8 is the reaction profile occurring in a typical three-bed converter. The first intermediate cooling is performed by a heat exchanger, in which the gas temperature decreases without any dilution, while the second cooling is performed by direct quench with cold gas. 

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