Process synthesis design heuristics

The initial aim of the procedure is to generate a reasonable base case design that can be used for preliminary economic evaluation of the process. This can subsequently be optimized and/or compared with any process alternatives that are identified. The complete process is always considered at each decision level, but additional fine detail is added to the structure of the flowsheet at any stage. Established heuristics and equipment selection procedures are used together with new process synthesis insights to guide each flowsheet decision. 

In the hierarchical decomposition method, or, process synthesis one only looks at the input-output structure of the process at the first level. In subsequent levels more detail is added, finally ending with the entire flowsheet. Design decisions are made by using heuristics and models. The amount of detail in the models depend on the level at which the models are applied. 

Presented below is a list of those heuristics most commonly cited in separation process synthesis. This Is certainly not a complete list but a compilation of several proposed in (he literature. If anything, what is needed is a more detailed list of heuristics thei have been tested through calcalalion and experimentation. As more amphasis is placed on process synthesis as a vital pert of process design, a more comprehensive and useful asl of hanristics will evolve. 

Throughout Chapters 1 and 3, the role of die process simulators is deemphasized. Rather, emphasis is placed on the steps in process synthesis and the development of the base-case design. Although from time to time these chapters refer to simulation results, a formal discussion of the role of process simulators during process creation is not attempted until Chapter 4 (and the accompanying multimedia CD-ROM). Chapter 3 introduces the steps in process synthesis without discussing many of the key heuristics used in making decisions. A more formal treatment is reserved for Chapter 5. 

Finally, before leaving this section on preliminary process synthesis, the limitations of the heuristic approaches should not be overlooked. Many algorithmic methods are very effective for the synthesis of alternative flowsheets, their analysis, and optimization. These methods are usually used by design teams in parallel with their work on the development of the base-case design, which is the subject of the next section. The algorithmic methods are easily implemented and are illustrated with many examples in Part Two of this text (Chapters 6-12). 

Recognize the limitations of the heuristics in Table 5.2 and the role of the process simulator in permitting the systematic variation of parameters and the examination of alternative designs. The reader should also recognize that the heuristics listed are a subset of the many rules of thumb that have been applied by design teams in carrying out process synthesis. 

Process synthesis is introduced mostly using heuristics in Part One (Chapters 3 and 5), whereas Part Two provides more detailed algorithmic methods for chemical reactor network synthesis, separation train synthesis, the synthesis of reactor-separator-recycle networks, heat and power integration, mass integration, and the optimal design and sequencing of batch processes. 

Effective evolutionary methods are important for systematic process synthesis. They usually contain either heuristic or algorithmic elements and can be used effectively with computer-aided design packages. 

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