Using the RTD Versus Needing a Model

This chapter has discussed how to predict conversions in a nonideal reactor. Almost always, we would need to measme or estimate a reactor s RTD. If the reaction is first-order, the conversion can be predicted knowing only the RTD, as shown in Chapter 13. More extensive knowledge of flow pattern characteristics is not required for predicting conversions for first-order reactions. 

If the reaction is not first-order, the RTD can be used to place bounds on the conversion using concepts associated.with micromixing. One bound is obtained treating the reactor fluid as if it were completely segregated the other bound results from considering the fluid as being in a state of maximum mixedness. 

Summary If flhe reaction is not first-order and a more precise estimate of reactor conversion is required than can be obtained from the boimds, a reactor model must be assumed. The choice of a proper model is almost pure art requiring creativity and engineering judgment. The flow pattern of the model must possess the most important characteristics of that in the real reactor. Standard models are available that have been used with some success, and these can be used as starting points. Models of tank reactors usually consist of combinations of PFRs, perfectly mixed CSTRs, and dead spaces in a configuration that matches as well as possible the flow pattern in the reactor. For tubular reactors, the simple dispersion model has proven most popular. 

The parameters in the model, which with rare exception should not exceed two in number, are obtained from the RTD. Once the parameters are evaluated, the conversion in the model, and thus in the real reactor, can be calculated. For typical tank-reactor models, this is the conversion in a series-parallel reactor system. For the dispersion model, the second-order differential equation must be solved, usually numerically. Analytical solutions exist for the first-order situation, but as pointed out previously, no model has to be assumed for the first-order system if the RTD is available. 

Correlations exist for the amoimt of dispersion that might be expected in common packed-bed reactors, so these systems can be designed using the dispersion model without obtaining or estimating the RTD. This situation is perhaps the only one where an RTD is not necessary for designing a nonideal reactor. 

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