Reactors for catalytic gas phase processes

Reactors for catalytic gas phase processes The fixed bed reactor 

Calculations describing the performance of adiabatic fixed bed reactors are complicated. One needs differential mass balances of the type given by eq. (7.18), where the rate constant is replaced by the volumetric rate constant of a packed bed reaction, a, see eqs. (5.51) and (7.29). 

An adiabatic (isolated) pilot reactor is constructed, consisting of a number of fixed beds in series. The reactor is filled with the catalyst particles that may be smaller than those that will be used on a technical scale. They are preferable smaller than 0.1 times the tube diameter. Exit temperature and conversions are measured after each fixed bed, for various concentrations, flow rates and entrance temperatures. When the desired conversion cannot be reached, partially converted reaction mixtures may be used as reactor feeds. From these experiments the relation between conversion, temperature and bed length will be found. In another series of adiabatic reactor experiments the influence of the catalyst particle diameter is investigated. From these experiments one finds by which factor the reactor length has to be increased to obtain the same degree of conversion with the larger particles. 

By these mostly empirical means a reactor can be designed for the largest part of the conversion. It may be that the desired very high conversions are not reached in the bench scale reactor. However, for the last part of the conversion the calculations can be very much simplified, since this part of the process can often be considered as isothermal. With the help of the calculations in sections 7223 and 5,43,1 the required length for very high degrees of conversion can be readily estimated. 

When heat of reaction has to be removed through the reactor walls, a shell and tube type construction can be used, very similar to a conunon heat exchanger. In most cases the catalyst is put inside the tubes, and a cooling medium passes on the shell side. Occasionally, the catalyst is put in the shell, and the cooling medium is passed through the tubes. However, the first arrangement appears to have advantages for easy replacement of the catalyst. 

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