Temperature Forcing of Reactors with Catalyst Decay

4 Temperature Forcing of Reactors with Catalyst Decay 

In previous sections we have dealt with nonisothermal effects arising from the thermochemistry of the reactions involved. There is another type of thermal effect that appears in the operation of large-scale reactors such as those used in hydrotreating. These reactors are normally subject to slow catalyst deactivation, and constant conversion operation is required in order not to upset subsequent processing units. Here the reactor temperature is used to cope with the loss of intrinsic catalyst activity and the thermal parameters of the main and deactivation reactions, particularly the activation energies, have a great influence on the operation. Further, it has been common practice in many industrial laboratories for many years to evaluate catalyst activity and activity maintenance for such processes in laboratory experiments which are also conducted under constant-conversion conditions. In this procedure, catalyst deactivation effects are manifested in the rate of temperature increase needed to maintain constant conversion, that is, a temperature increase required (TIR). 

To simulate this type of operation a mixing-cell sequence is quite useful [J.B. Butt and D.M. Rohan, Chem. Eng. Sci., 23,489 (1968)]. Let us start with the familiar case of first-order kinetics for the main reaction and first-order kinetics for the deactivation, where coke deposition occurs via a reactant precursor mechanism [see scheme (XXVII) in Chapter 3). For the main reaction we may write 

The simplifications realized in equation (6-130) arise from the isothermality, hence Ti = T, and so on. If it is wished to retain generality in the model where the mechanism (and kinetics) of deactivation is concerned, then 

Now for the restriction to a constant conversion operation, in addition to the initial condition above, the temperature must satisfy the following requirement. 

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