Wrong-Way Behavior

The final open-loop reactor issue we discuss is the problem of inverse response or wrong-way behavior as it is called in the reactor engineering literature. The inverse response refers to the temporary increase in the exit temperature in some packed, plug-flow reactors following a decrease in the feed temperature (Fig. 4.12). The wrong-way behavior stems from the difference in propagation speed between concentration 

Inverse response creates control difficulties. Assume, for example, that we wish to control the exit temperature of an adiabatic plug-flow reactor by manipulating the inlet temperature as shown in Fig. 4.13. From a steady-state viewpoint this is a perfectly reasonable thing to consider, since there are no issues of output multiplicity or open-loop instability, assuming the fluid is in perfect plug flow and there is no 

The other parameter groups in the model are defined as follows  

Lewis number (.ratio of thermal time constant to material time constant due to fluid flow)  

Peclet number for mass (ratio of linear flowrate to diffusivity)  

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Pinjala V, Chen YC, Luss D. Wrong-way behavior of packed-bed reactors. II. Impact of thermal dispersion. AIChE J 1988 34 1663-1672. 

A wide class of forced unsteady-state processes have already been realized on the commercial scale using specific dynamic phenomenon, that takes place during performance of an exothermic reaction in a fixed bed of catalyst. This phenomenon is referred to in the literature as wrong-way behavior of a fixed bed reactor [20]. Substantial differences in characteristic times of heat and mass transfer in a packed bed reactor result in a surprising rise of temperature inside the reactor after... 

This means that it takes the fluid stream 29 times longer to cause a temperature change in the reactor than it takes to change the reactor composition. This is a sufficient difference in the propagation speeds to induce wrong-way behavior. However, a large Lewis number is not sufficient to create a problem. Another important requirement is that the reaction should be near completion at the exit of the reactor. This requirement is not met for the vinyl acetate reaction which has only a 36 percent conversion in oxygen. 

ITin, A., and Luss, D. Wrong-Way Behavior of Packed-Bed Reactors Influence of Reactant Adsorption on Support, AIChE -J., 38, 1609-1617 (1992). 

The HDA reactor is unpacked and therefore cannot exhibit the wrongway behavior discussed in Chap. 4. However, it is quite common that gas phase reactions are carried out over a catalyst so it is important to understand the implications of the wrong-way behavior on the control of reactors with feed-effluent heat exchangers. 

To that end we have constructed a simulation of a fictitious system that has a severe inverse response. We show the design in Fig. 5.28 and give the design parameters in Table 5.1. The reactor has a large Lewis number (Le = 25), nearly complete per pass conversion of the reactant, and little axial dispersion. These are all factors necessary for wrong-way behavior. In fact the example plot of wrong-way behavior shown in Chap. 4 was generated from this reactor. 

A related phenomenon is the "wrong-way behavior" of packed-bed reactors, where a sudden reduction in the feed temperature leads to a transient temperature rise. This has been observed (52, 59) and satisfactorily analyzed using a plug-flow pseudohomogeneous model (60). 

Oh and Cavendish [25] noted that a sudden decrease of the inlet temperature may cause a local temperature rise that exceeds the adiabatic temperature rise based on the feed composition. The same result, obtained with other models [13,37], was ascribed to fast diffusion of hydrogen. But contrary to these authors. Oh and Cavendish also find this so-called wrong-way behavior in the absence of hydrogen. Due to decreasing temperatures in the inlet part of the monolith, much reactant may reach the hotter part of the reactor, leading to a very fast reaction and corresponding heat production. The resulting hot spot moves in time to the outlet of the reactor and disappears. 

This behavior is called inverse response, where the output initially goes the opposite direction to the final value. This wrong-way behavior is seen in boilers and distillation columns. 

Figure 7.17 also shows the dynamic temperature simulation for the commercial reactor. The phenomenon called wrong-way behavior was not encountered at the beginning of the reactor as reported for other cases (Mederos et al., 2006). This can be attributed to the fast and high conversion of reactants in the first 25% of the... 

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