Nonideal Batch Reactors

In Chaps. 5 and 6 model-based control and early diagnosis of faults for ideal batch reactors have been considered. A detailed kinetic network and a correspondingly complex rate of heat production have been included in the mathematical model, in order to simulate a realistic application however, the reactor was described by simple ideal mathematical models, as developed in Chap. 2. In fact, real chemical reactors differ from ideal ones because of two main causes of nonideal behavior, namely the nonideal mixing of the reactor contents and the presence of multiphase systems. 

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Nonideal batch reactors may have Nonideal tubular reactors may have... 

First, different typologies of nonideal batch reactors are considered. In particular gas-liquid reactors are discussed, which may be used for different industrial applications (e.g., reactions of oxidation) and are often encountered in the case of gassy reactions (i.e., liquid-phase reactions which do not produce significant thermal effects but in which the production of gaseous products may lead to explosions). 

Fig. 7.1 Nonideal batch reactors liquid-phase batch reactor (a), liquid-phase batch reactor with release of gaseous bubbles (b), semi-batch gas-liquid bubble column (c), and slurry batch reactor (d)...

Fed batch reactors, Qin 0 Nonideal batch reactors may have spatial variations in concentration... 

The classic analysis of reactors involves two idealized flow patterns— plug flow and mixed flow. Though real reactors never fully follow these flow patterns, in many cases, a number of designs approximate these ideals with negligible error. However, deviation from ideality can be considerable. Typically, in a reaction vessel, we can have several immediate cases closer to plug or mixed flow. Of course, nonideal flow concerns all types of reactors used in heterogeneous processes, i.e. fixed beds, fluidized beds, continuous-flow tank reactors, and batch reactors. However, we will focus on fixed beds and batch reactors, which are the common cases. 

This last chapter sketches the extension of the methods developed in the previous chapters to real chemical batch reactors, characterized by nonideal fluid dynamics and by the presence of multiphase systems. 

The modeling of chemical batch reactors has been chosen as the starting point for the roadmap developed in this book. The simplified mathematical models presented in the first sections of the chapter allow us to focus the attention on different aspects of chemical kinetics, whereas the causes of nonideal behavior of chemical batch reactors are faced in the last chapter. 

The flows in PFR and MFR can be precisely deflned by simple mathematical eqnations, and the batch reactor is simply the batch version of the PFR. A reactor is now considered where the flow is between plug and fnlly mixed, i.e., a nonideal reactor. Two common examples of such partially mixed reactors are the recycle reactor and the tanks-in-series reactor. In the recycle reactor, part of the outlet from a reactor is recycled at the inlet, thns establishing some mixing between the downstream and the upstream fluids. In the tanks-in-series reactor, several mixed-flow reactors are operated in series. A single MFR is fully mixed, whereas an inflnite nnmber of MFRs (or a... 

Experimental kinetic data always should be taken in a reactor that behaves as one of tiie tiiree ideal reactors. It is relatively straightforward to analyze the data from an ideal batch reactor, an ideal plug-flow reactor, or an ideal stirred-tank reactor. This is not the case if the reactor is nonideal, e.g., somewhere between a PFR and a CSTR. Characterizing the behavior of nonideal reactors is difficult and imprecise, as we shall see in Chapter 10. This can lead to major uncertainties in the analysis of data taken in nonideal reactors. 

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

The reactors treated in the book thus far—the perfectly mixed batch, the plug-flow tubular, and the perfectly mixed continuous tank reactors—have been modeled as ideal reactors. Unfortunately, in the real world we often observe behavior very different from that expected from the exemplar this behavior is tme of students, engineers, college professors, and chemical reactors. Just as we must learn to work with people who are not perfect, so the reactor analyst must learn to diagnose and handle chemical reactors whose performance deviates from the ideal. Nonideal reactors and the principles behind their analysis form the subject of this chapter and the next. 

As we shall see in Section 5.1.1.1, reactors can be classified as batch and continuous reactors, which in turn can be idealized as stirred-tank and plug-flow reactors. We shall not consider any nonideality of fluid flow behavior, since most industrial reactors exhibit only small deviations from ideality. One object of reactor design and operation is to ensure this. 

However, both batch and semibatch reactors can also be nonideal if the concenlrations and the temperatuie are not spatially uniform at every time. 

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