Reactor-separator-recycle system synthesis

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. 

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

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. 

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. 

Kokossis, A. C., and Floudas, C. A. Synthesis of Isothermal Reactor-Separator-Recycle Systems, Chem. Eng. Sci. 46, 1361-1383 (1991). 

Simultaneous synthesis of the reactor/separator/recycle system. The cost of separations and recycle affects the economic trade-offs. The optimal reactor would be somewhere between maximum yield (minimum recycles) and maximum selectivity (minimum separations). 

Recycles are an essential part of most processes involving chemical reactions because it is usually difficult to achieve near-equilibrium conversion in a single pass of reactants through a reactor. It may also be the case that equilibrium conversion is very low, e.g. in ammonia or methanol synthesis (see section 12.4). The overall conversion for the reactor-separator-recycle system can be much closer to 100%. The following example illustrates this. 

This case study is a reactor-separator-recycle system to produce monochlorobenzene. The operating parameters and sizes for one of the synthesis alternatives are optimised using the detailed models and the costing information provided. Each unit has a capital cost, Cc, and an operating cost, Q, which is incorporated into the objective function through a pay out time of 2.5 years. The principal units are a CSTR and two separation columns. The models have been reformulated in terms of component flowrates, Fsj. The reactor is a continuous stirred tank reactor (CSTR) which models the reaction between chlorine and benzene (A) to produce monochlorobenzene ( B) and dichlorobenzene (C) at a constant temperature. The maximum (global) profit is 2081/day. 

Having described the overall process system, its three main interactive components (i.e., the chemical plant, the heat recovery system and the utility system), as well as the three subsystems of the chemical plant (i.e., the reactor system, the separation system, and the recycle system) we can now define the process synthesis problem. 

In both cases, synthesis gas is added to a recycle loop where reactor feed gas is passed through a reactor and heat recovery system. Product is separated and the unconverted gas is recycled to the reactor system. At some point a purge gas is taken in order to prevent accumulation of inerts, excess reactants and/or byproducts in the recycle system. Detailed descriptions of the process layout and discussions concerning energy consumption, etc. are given elsewhere and and will not be repeated here. 

The synthesis of reaction-separation systems. The recycling of material is an essential feature of most chemical processes. The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes... 

The first application of a rhodium-ligand system was realized in the LPO-process (low pressure oxo Fig. 18). Huge stirred tank reactors are used, equipped with internal heat exchangers to control the heat of reaction. The solution of the catalyst recycle is simple but efficient. The catalyst remains in the reactor, products and unconverted propene are stripped by a huge excess of synthesis gas. Because of strong foaming, only a part of the reaction volume is used. After the gas has left the reactor, the products are removed by condensing, the big part of synthesis gas is separated from the liquid products and recycled via compressors. The liquid effluent of the gas-liquid separator... 

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