Reactor-Separator-Recycle systems

Reactor-Separator-Recycle System—Benzene Chlorination. Here the elementary reactions are... 

Chapter 10 Synthesis of Reactor Networks and Reactor-Separator-Recycle Systems... 

This chapter discusses the application of MINLP methods in the synthesis of reactor networks with complex reactions and in the synthesis of reactor-separator-recycle systems. 

Synthesis of Reactor-Separator-Recycle Systems 10.3.1 Introduction... 

In most chemical processes reactors are sequenced by systems that separate the desired products out of their outlet reactor streams and recycle the unconverted reactants back to the reactor system. Despite the fact that process synthesis has been developed into a very active research area, very few systematic procedures have been proposed for the synthesis of reactor/separator/recycle systems. The proposed evolutionary approaches are always based upon a large number of heuristic rules to eliminate the wide variety of choices. Many of these heuristics are actually extensions of results obtained by separately studying the synthesis problem of reactor networks or separator systems, and therefore the potential trade-offs resulting from the coupling of the reactors with the separators have not been investigated. 

Floquet et al. (1985) proposed a tree searching algorithm in order to synthesize chemical processes involving reactor/separator/recycle systems interlinked with recycle streams. The reactor network of this approach is restricted to a single isothermal CSTR or PFR unit, and the separation units are considered to be simple distillation columns. The conversion of reactants into products, the temperature of the reactor, as well as the reflux ratio of the distillation columns were treated as parameters. Once the values of the parameters have been specified, the composition of the outlet stream of the reactor can be estimated and application of the tree searching algorithm on the alternative separation tasks provides the less costly distillation sequence. The problem is solved for several values of the parameters and conclusions are drawn for different regions of operation. 

In the following section, we will present the synthesis approach of reactor-separator-recycle systems proposed by Kokossis and Floudas (1991). 

The derivation of a superstructure for a reactor-separator-recycle system is based on combining the individual representations of the reactor network and the separation network. The new additional element is the allocation of the potential recycle streams from the separation system to the inlets of the reactors in the reactor system. 

The different configurations for the reactor/separator/recycle system can be obtained by eliminating the appropriate streams of the proposed superstructure. Thus, elimination of all but streams 1,7, 11, 14, 18, 22, 24, and 27 results in the configuration shown in Figure 10.5(a) where the three CSTRs are connected in series, the separator system includes columns A/BC and B/C and a total recycle of A is fed into the first CSTR. Should all but streams 1, 2, 3, 4, 5, 9, 12, 14, 17, 21, 23, and 26 be eliminated from the superstructure, the configuration of Figure 10.5(b) is obtained where the CSTRs are connected in parallel, the separator network consists of columns AB/C and A/B and the recycle stream from A/B feeds the second CSTR. A different configuration... 

To determine a reactor-separator-recycle system that optimizes the performance criterion (e.g., maximum profit) from the rich set of alternatives postulated in the superstructure, we define variables for... 

This chapter presents an introduction to the key issues of reactor-based and reactor-separator-recycle systems from the mixed-integer nonlinear optimization perspective. Section 10.1 introduces the reader to the synthesis problems of reactor-based systems and provides an outline of the research work for isothermal and nonisothermal operation. Further reading on this subject can be found in the suggested references and the recent review by Hildebrandt and Biegler (1994). 

Section 10.2 describes the MINLP approach of Kokossis and Floudas (1990) for the synthesis of isothermal reactor networks that may exhibit complex reaction mechanisms. Section 10.3 discusses the synthesis of reactor-separator-recycle systems through a mixed-integer nonlinear optimization approach proposed by Kokossis and Floudas (1991). The problem representations are presented and shown to include a very rich set of alternatives, and the mathematical models are presented for two illustrative examples. Further reading material in these topics can be found in the suggested references, while the work of Kokossis and Floudas (1994) presents a mixed-integer optimization approach for nonisothermal reactor networks. 

A. C. Kokossis and C. A. Floudas. Optimal synthesis of isothermal reactor-separator-recycle systems. Chem. Eng. Sci., 46 1361, 1991. 

M. L. Luyben and C. A. Floudas. Analyzing the interaction of design and control, Part 2 Reactor-separator-recycle system. Comp. Chem. Eng., 18( 10) 971,1994b. 

Particularly strong and complex interactions prevail among reaction and separation systems that are generally not at all or not fully exploited as a result of the application of the available synthesis methods for reactor networks and separation systems in isolation. The lack of generality in the synthesis methods is a tribute to the nonlinear process models required to capture the reaction and separation phenomena as well as to the vast number of feasible process design candidates. These complexities even make it difficult to synthesize the decomposed subsystems, which are typically reactor networks, separation systems, reactor-separator-recycle systems, and reactive separation systems. The development of reliable synthesis tools for these sub-systems is still an active research area. 

Bildea, C.S., Dimian, A. C., and Iedema, P. D. (2000). Nonlinear behavior of reactor-separator-recycle systems. Comput. Chem. Eng., 24, 209-214. 

Figure 2.6 Nonlinear phenomena in reactor/separator/recycle systems.

In this section we replace the CSTR by a plug-flow reactor and consider the conventional control structure. Section 4.5 presents the model equations. The energy balance equations can be discarded when the heat of reaction is negligible or when a control loop keeps constant reactor temperature manipulating, for example, the coolant flow rate. The model of the reactor/separation/recycle system can be solved analytically to obtain (the reader is encouraged to prove this) ... 

Note that first inequality of Eq. (4.21) is identical to the feasibility constraint (4.13) characterizing the reactor/separation/recycle system involving a CSTR. 

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