Superstructure of the Separation System

As a strategy, the synthesis procedure should start with vapor recovery and gas separations, from which some components are sent to liquid separations. For the same reason, the solid-separation system should be placed in the second place. Note that the subsystems of gas and solid separations are largely uncoupled. As a result, the liquid-separation system should is handled the last. 

At this point, we pause the problem of synthesis of subsystems of separations. This will be resumed in Chapter 3, where a systematic methodology based on a task-oriented approach will be described in more detail. 

The hydrodealkylation of side-chain aromatics to nonsubstituted parents is a major process in petrochemistry. A typical example is the conversion of toluene to benzene  

The reaction may be carried out at 30-50 bar, either thermally at 550-700 °C, or over suitable catalyst at somewhat lower temperatures. Higher boiling subproducts form, such as biphenyl and fluorene. In thermal processes the conversion is usually 60-80%, with a selectivity of about 95%. In catalytic processes the selectivity is significantly higher, which can compensate the cost of the catalyst. Sketch the structure of separations by considering as feeds pure toluene and hydrogen with 5% methane. 

Solution Firstly, we solve the problem of the input/output material balance. Besides the main reaction a secondary reaction leading to the subproduct di-phenil takes place  

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Figure 2.10 Superstructure of the separation system (after Douglas [2]).

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... 

The separation system can be described as a superstructure of subsystems [2], as illustrated by Figure 2.10, corresponding to the dominant physical state during processing, respectively as gas and vapor, liquid and solid. Inspection of Figure 2.10 emphasizes the role of the first separation step in generating separation subsystems. The subsystems are interconnected by recycles. Because recycling is... 

The same separation steps (same feed and product compositions) can occur in different sequences. This can be seen in Fig. 1, where the first and the second sequence have the first separation in common. This property is used in a superstructure to reduce the complexity of the multicomponent systems. The state task network [3] is applied. In this superstructure, every possible composition, which can be attained, is called a state. The states represent the feed, possible intermediate products and products of the separation sequence. 

The olefin separation process involves handling a feed stream with a number of hydrocarbon components. The objective of this process is to separate each of these components at minimum cost. We consider a superstructure optimization for the olefin separation system that consists of several technologies for the separation task units and compressors, pumps, valves, heaters, coolers, heat exchangers. We model the major discrete decisions for the separation system as a generalized disjunctive programming (GDP) problem. The objective function is to minimize the annualized investment cost of the separation units and the utility cost. The GDP problem is reformulated as an MINLP problem, which is solved with the Outer Approximation (OA) algorithm that is available in DICOPT++/GAMS. The solution approach for the superstructure optimization is discussed and numerical results of an example are presented. 

The photosyntbetic reaction center is a highly complex array of functional moieties designed and arranged by nature in a superstructure that optimizes the overall efficiency of charge separation. In contrast, the reaction partners in most other electron transfer reactions are chosen by investigators who either emulate nature or pursue the photoreactions of molecules or systems conceived and synthesized by man. 

The systematic development of superstructures for heterogeneous systems is, in principle, a more difficult task. Consider, for instance a process flowsheet that is composed of reaction, separation, and heat integration subsystems. In theory, one could develop a superstructure by combining the superstructures for each subsystem. However, this approach could lead to a very large MINLP optimization problem. 

The principle of the knowledge-based approach for the synthesis of separation systems (Bamicki Fair, 1990, 1992) is displayed in Fig. 7.18. On the top of we place the generation of the superstructure of separations depicted in Fig. 7.17, where three separation subsystems have been identified ... 

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