Reaction-Separation Processes

The use of TMS systems for the telomerization reactions did not lead to efficient reaction/separation processes until now. However, some solvent systems were determined, which could be used for other reactions. Basic principles of the influence of the substrates, products and the catalyst were investigated and the results were applied to further reactions. 

Chemical synthesis can include chlorination, alkylation, nitration, and many other substitution reactions. Separation processes include filtration, decantation, extraction, and centrifugation. Recovery and purification are used to reclaim solvents or excess reactants as well as to purify intermediates and final products. Evaporation and distillation are common recovery and purification processes. Product finishing may involve blending, dilution, pelletizing, packaging, and canning. Examples of production facilities for three groups of pesticides foUow. 

Combes, D. Reaction/Separation Process in Supercritical C03 Using Lipases. In NATO ASI Series. Series E Applied Science, Vol. 317 Malcata, F. X., Ed. 1996, pp. 613-618. 

Equilibrium Theory and Nonlinear Waves for Reaction Separation Processes... 

In this chapter, unifying concepts for analyzing and understanding the dynamics of integrated reaction separation processes with rapid chemical reactions are introduced. The text is based on some recent studies [11-13], and extends the concepts introduced earlier for reactive distillation processes [23] to other integrated reaction separation processes. The class of processes to be considered is rather broad. It includes reaction processes where simultaneous separation is used to enhance a reaction, for example, by shifting inherent equilibrium limitations. Various process examples of this kind are provided in this book. The chapter also includes separation processes with potentially reactive mixtures. In this case, a chemical reaction can be either an unwanted side effect or it can be used directly to achieve a certain separation, which is not possible under nonreactive conditions (see e.g. Ref. [10]). The latter represents a reaction-enhanced separation. 

The outline of this chapter is as follows First, some basic wave phenomena for separation, as well as integrated reaction separation processes, are illustrated. Afterwards, a simple mathematical model is introduced, which applies to a large class of separation as well as integrated reaction separation processes. In the limit of reaction equilibrium the model represents a system of quasilinear first-order partial differential equations. For the prediction of wave solutions of such systems an almost complete theory exists [33, 34, 38], which is summarized in a second step. Subsequently, application of this theory to different integrated reaction separation processes is illustrated. The emphasis is placed on reactive distillation and reactive chromatography, but applications to other reaction separation processes are also... 

Similar patterns of behavior can be observed in reactive distillation columns or other integrated reaction separation processes with fast reversible reactions [11, 23] as illustrated in Fig. 5.3 for a pure rectifying column with a ternary mixture and a reaction of type 2B A + C taking place in the liquid phase. However, due to the reaction equilibrium the profiles consist of a single concentration front, which is clearly different from the nonreactive problem illustrated in Fig. 5.2. 

In the remainder of the chapter, wave dynamics in integrated reaction separation processes will be studied in more detail. The analysis is based on a simple mathematical model, which will be discussed in the following section. 

Fig. 5.4. A single section of an integrated reaction separation process.

Using the transformed variables the reactive problem (Eq. (5)) is completely equivalent to a nonreactive problem (Eq. (4)) of reduced dimension. Hence, in the limit of chemical equilibrium the dynamic behavior of reaction separation processes is equivalent to the dynamic behavior of nonreactive processes. 

On the conceptual level considered here, different processes have similar properties. Methods developed for a specific class of integrated reaction separation processes can also be applied to other processes within the framework outlined above. 

The wave and pulse patterns of nonreactive separation processes, as well as the integrated reaction separation processes illustrated above, can be easily predicted with some simple graphical procedures derived from Eqs. (4) and (5). The behavior crucially depends on the equilibrium function y(x) in the nonreactive case, and on the transformed equilibrium function Y(X) in the reactive case. In addition to phase equilibrium, the latter also includes chemical equilibrium. An explicit calculation of the transformed equilibrium function and its derivatives is only possible in special cases. However, in Ref. [13] a numerical calculation procedure is given, which applies to any number of components, any number of reactions, and any type of phase and reaction equilibrium. 

In this section the methods developed in the previous section will be applied to analyze the dynamic behavior of integrated reaction separation processes. Emphasis is placed on reactive distillation and reactive chromatography. Finally, possible applications to other integrated reaction separation processes including membrane reactors and sorption-enhanced reaction processes will be briefly discussed. More details about reactive distillation processes were provided in Ref. [39]. For chromatographic reactors the reader should refer to Chapter 6 of this book, for sorption-enhanced reaction processes to Chapter 7, and for membrane reactors to Chapter 12. 

The theory presented above also applies to other integrated reaction separation processes which fall into the class of systems illustrated in Fig. 5.1. Typical examples are sorption-enhanced gas phase reactions (as described in Chapter 7) or membrane reactors (as described in Chapter 12). 

Insights from nonlinear wave theory can also be used for designing new control strategies. A major problem in controlling product purities in separation as well as integrated reaction separation processes is often the lack of a cheap, reliable and fast online concentration measurement. This problem can be solved in two different ways (i) through simple inferential control, or (ii) model-based measurement. 

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

In this chapter it was shown that equilibrium theory, which was first developed for nonreactive chromatographic and nonreactive distillation processes is readily extended to many integrated reaction separation processes with fast reversible reactions. The theory provides an easy understanding of the dynamics of these processes and therefore has many useful applications in process control. Further, the theory nicely predicts inherent limitations of integrated reaction separation processes, which may not be obvious from first glance. It emphasizes fundamental features, which are common to many of these processes. 

The analysis presented in this chapter was based on three crucial assumptions. Namely, mass transfer and reaction kinetics were neglected. Further, constant flow rates were assumed. Although these assumptions are valid in many applications, an extension to finite mass transfer and reaction kinetics as well as variable flow rates seems challenging for future research in this field. As indicated in the last section, finite mass transfer and reaction kinetics may affect the feasible products of integrated reaction separation processes quite significantly. Strong impact can therefore also be expected for the dynamic behavior. The same applies to variable convective flow rates due to nonequimolar reactions. While this effect is not too important... 

S. Grtiner, M. Mangold, A. Kienle, Dynamics of reaction separation processes in the limit of chemical equilibrium, 2004. Submitted for publication to AIChE J. 

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