Synthesis of subsystems

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. 

In the subsequent sections, we will explain the approach in detail illustrated by examples. To learn it properly, it is important that the reader goes through all the steps. Because of the complexity of the overall synthesis problem, some issues regarding the synthesis of subsystems, as chemical reactors, azeotropic distillations, or heat exchangers networks, will be presented in separate chapters. 

The synthesis of subsystems can be described by a logical diagram in Fig. 7.19. The steps are briefly explained below. 

Hierarchical Approach is a simple but powerful methodology for the synthesis of process flowsheets. It consists of a top-down analysis organised as a clearly defined sequence of tasks grouped in levels. Each level solves a fundamental problem as, number of plants, input/output structure, reactor design and recycle structure, separation system, energy integration, environmental analysis, safety and hazard analysis, and plantwide control. At each level, systematic methods can be applied for the synthesis of subsystems, as chemical reaction, separations, or heat exchangers network. 

Synthesis of subsystems. The main objective of this approach so far has been the reduction and optimization of energy, including thermal pinch, distillation sequences, and mass and heat exchange networks, which will be described in a Httle more detail later in the chapter (In addition, see chapter 4). 

We should mention that the modern process-synthesis methods can ensure intrinsically the optimization of subsystems, such as reactions, separations and heat exchange. Therefore, by searching in the first stage the structural optimization of the flowsheet and by performing later the optimization of units should lead quite soon to the true overall optimum in a large number of situations. 

For the synthesis of separation systems the strategy is decomposing the overall problem into subsystems handling essentially gases, liquids and solids, for which systematic methods exist. Environmental protection can also be handled by taking into account the tasks defined early in the above procedure. 

Regarding the strategy, in the first place should be considered the synthesis of the vapour recovery and gas separation systems, from which recovered condensable components are sent to the liquid separation system. For the same reason, the synthesis of the solid recovery and solid separation systems should be placed in the second place. The subsystems for gas and solid separations are largely uncoupled, and can be treated as stand-alone. Thus, the synthesis of liquid separation system is the last, but by far the most difficult. Its decomposition in subsystems is again the best approach. 

The inspection of the superstructure illustrated by the Fig. 7.17 emphasises again the role of the first-separation step. Sharp recovery will minimise the interactions between subsystems. On the other hand, the selection of the separation techniques and the design of units should bring flexibility to prevent bottlenecks. For example, adsorption or membranes should not be used in the first split, but could be certainly considered in the synthesis of the subsystem for gas separations. 

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 ... 

Thus, the overall problem can be decomposed in smaller problems, namely the synthesis of separation subsystems. This activity can be done by means of specialised managers , in this case designated by Gas Split Manager (GSM), Liquid Split Manager (LSM) and Solid Split Manager (SSM). 

The Hierarchical Approach developed in this chapter incorporates a knowledge-based procedure for the synthesis of separations. This consists of dividing the separation section in three subsystems gas vapour, liquid and solid separations. Each subsystem is further managed by selectors, which makes use of unit operations. Split sequencing is based mainly on heuristics, although may include algorithmic or optimisation techniques. This chapter describe in more detail the synthesis of distillation trains for zeotropic distillations, the non-ideal case being left for the Chapter 9. 

Synthesis of heat exchanger network (HEN) for minimum energy requirements and maximum heat recovery. Determine matches in subsystems and generate alternatives. Network optimisation. Reduce redundant elements, as small heat exchangers, or small split streams. Find the trade-off between utility consumption, heat exchange area and number of units. Consider constraints. 

A companion paper ) describes the subgoal-generating and synthesis-discovery subsystems of SYNCHEM in sufficient detail to enable the interested reader to make a critical evaluation of this work. We briefly synopsize these programs here to clarify the results to follow. 

Greater catabolism than synthesis of muscle-fiber proteins, especially catabolism of myofibrillar proteins, which occurs mainly through two subsystems ... 

If we view the whole system from a slow dynamics perspective (that of A molecules), we can think of subsystem 1 as having so fast dynamics that it equilibrates instantaneously with the slow changing variables. At the same time, a second subsystem (labeled 2), in which the effects of the fast subsystem are averaged over the fast time scale, becomes apparent. Subsystem 2 corresponds to the synthesis and degradation of A molecules. 

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