Heuristics hierarchical approach

Heuristic based approaches are more relevant for structured products. The most well known heuristic based approach is the hierarchical decomposition method developed by Douglas (Douglas, 1988). In the first level of this method one only looks at the input-output structure of the process. In subsequent levels more detail is added, finally ending with the entire flowsheet. Design decisions are made by using heuristics and short-cut models. An alternative method is due to Siirola (1996) means end analysis. In this method the properties of the feedstock and the desired products are compared. Tasks are defined to eliminate the property differences between the feedstock and the desired product. 

Process Synthesis by Hierarchical Approach Table 2.1 Heuristics at input/output level. 

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. 

Formulae including mathematical formulae reaction equations potentially clauses of logic programs to capture design heuristics of hierarchical approaches, such as developed in (Douglas 1988). 

One approach reUes heavily on heuristics but allows the engineer to interact during the synthesis procedure through a framework of hierarchical decision levels (58,59). 

The top-down approach, which defines appropriate hierarchical coordination mechanisms between the different decision levels and decision structures at each level. These structures force constraints on lower operating levels and require heuristic decision rules for each task. Although this approach reduces the size and complexity of scheduling problems, it potentially introduces coordination problems. 

Heuristic methods were developed by well-experienced engineers and researchers. The first attempt to develop a systematic heuristic approach for the synthesis of multicomponent separation sequences was made by Siirola and Rudd. Common example is hierarchical heuristic approach.Heuristic rules are applied at five design levels to generate and evaluate the alternatives using economic criteria. The hierarchical heuristic method emphasizes the strategy of decomposition and screening. It allows for a quick location of flowsheet structures that are often near ... 

Moran and coworkers27 29 describe a procedure for control structure development based on the multilayer-multiechelon approach of hierarchical control theory, which is flee of heuristics. The key to the procedure is the effective application of decomposition to produce menaganble subeyslems of the problem. Examples are provided to show how structural controllability and observability can lead to a control system that is consistent with processing objectives. 

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