Recycle—reactor system structure flowsheet

Figure 3.2 Recycle—reactor system structure flowsheet for the chlor-alkali process. The energy flow, electricity, is shown with a clashed arrow. This also represents the mercury process separation system structure flowsheet, as it does not need to further concentrate NaOH as the diaphragm process does.

The earliest stage where controllability can be considered is when the recycle structure of the flowsheet has been designed. Design decisions concern the performance of reaction (conversion, selectivity) and separation sections (purity, recovery), as well as recycle policy. These will freeze the most important elements of the mass balance. At this level the reactor modelling, sizing, and control may be considered in more detail. Hence, the plant may be viewed as a Reactor-Separator-Recycle super-system. 

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... 

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. 

The key result of the Hierarchical Approach is the development of the basic flowsheet structure, formed by Reactor-Separations-Recycles. This structure defines the material balance envelope. In this respect of highest importance is the behaviour of the reaction system, which should deliver a realistic image of the reaction mixture. Other constraints regarding the reactor operation, as molar ratio of reactants, or safety requirements, are determinant for the structure of recycles. Optimal conversion represents a complex optimisation problem between the valorisation of raw materials and the cost of reactor, separators and recycles. 

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