Hierarchical separation system

The subgroups were determined based on a hierarchical decomposition strategy for a FCCU (Ramesh et al, 1992). The FCCU was decomposed into four separate units feed.system, catalyst.system, reactor/regenera-tor.system, and separation.system as shown in Fig. 32. Each of these units was further divided into more detailed functional, structural, or behavioral... 

One of the most important factors that determines the column configuration is the formulation (or goals) of the separation task with respect to the total flow sheet. Although a mixture may consist of C components, it does not mean that all C products are necessary. The components contained in streams recycled into the process (e.g., unreacted reactants recycled to the reactor) usually do not have to be separated from each other. Also separation of streams that are later mixed (blended) should be avoided, if possible. The separation system needs to be optimized together with the entire plant, either simultaneously or in a hierarchical approach, as described by Douglas (The Conceptual Design of Chemical Processes, McGraw-ffill, New York, 1988). 

For each plant specify the separation system and estimate its annualised cost. Additional hierarchical refinement reveals seven sub-levels. 

The chemical reactor has a determinant role on both the material balance and the structure of the whole flowsheet. It is important to stress that the downstream levels in the Hierarchical Approach, as the separation system and heat integration, depend entirely on the composition of the reactor exit stream. However, a comprehensive kinetic model of the reaction network is hardly available at an early conceptual stage. To overcome this shortcoming, in a first attempt we may neglect the interaction between the reactor and the rest of the process, and use an analysis based on stoichiometry. A reliable quantitative relationship between the input and the output molar flow rates of components would be sufficient. This information is usually available from laboratory studies on chemistry. Kinetics requires much more effort, which may be justified only after proving that the process is feasible. Note that the detailed description of stoichiometry, taking into account the formation of sub-products and impurities is not a trivial task. The effort is necessary, because otherwise the separation system will be largely underestimated. 

Level 4 of the Hierarchical Approach deals with the synthesis of the Separation System. A separation problem can be formulated as follows ... 

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. 

Application of a Hierarchical Approach for the Synthesis of Biorefineries 49 Table 2.7 Specification for separation system used in Aspen Plus... 

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