Process synthesis superstructures

Yeomans, H. and Grossmann, I.E. (1999) A systematic modeling framework of superstructure optimization in process synthesis. Computers el Chemical Engineering, 23, 709. 

The basic idea is deriving a HEN superstructure is to embed all alternative network structures using a graph-theoretical approach similar to the one described in the process synthesis chapter, in which each unit, input, and output is represented as a node in a graph with two-way arcs between each pair of units and one-way arcs from the inputs to the units/outputs and from the units to the outputs. 

Process synthesis by superstructure optimization consists of the identification of the best flowsheet from a superstructure that considers many possible alternatives, including the optimal one. A substantial advantage is that integration and design features may be considered simultaneously. At today s level of software technology the superstructure optimization is still an emerging technique. However, notable success has been achieved in numerous applications. The reference in this field is the book of Biegler et al. [6]. 

Procedures for the actual synthesis of heat-exchanger networks are somewhat less well developed however, all of the standard process synthesis approaches— including systematic generation, evolutionary modification, and superstructure... 

Therefore, in the first place the process synthesis should solve a structural problem, the configuration of the optimal reactor system. Then detailed design and refinement can follow. If there are several candidates close to optimum, these could be assembled in a superstructure and submitted to structural optimisation. 

The main contributions of this chapter are reflected in the process synthesis problem analyzed in the second example. The implementation of the optimization model takes the concept of separation to the limit, by separating the structure of the flow sheet from the equations of the model. This separation is extremely useful for more complex process synthesis problems using superstructures. 

The objective of this paper is applying the multi-objective based analysis for the process synthesis with wastes minimization. A step-by-step systematic approach is proposed to ensure the reduction or elimination of the conflicts with regards to the waste handling and multi-objective synthesis. The pollution prevention alternatives are identified and the superstructure aimed at wastes minimization is formulated for the further mathematical optimisation. A case study, air-based direct oxidation process for the production of ethylene oxide, is presented for the illustration of the proposed approach. 

Within each of the three general approaches toward process synthesis, key decisions are made about the flowsheet design that have a bearing on the operability characteristics of the plant. For example, in a hierarchical procedure (Ref. 6) we will make decisions about whether the plant is batch or continuous, what types of reactors are used, how material is recycled, what methods and sequences of separation are employed, how much energy integration is involved, etc. In a thermodynamic pinch analysis, we typically start with some flowsheet information, but we must then decide what streams or units to include in the analysis, what level of utilities are involved, what thermodynamic targets are used, etc. In an optimization approach, we must decide the scope of the superstructure to use, what physical data to include, what constraints to apply, what disturbances or uncertainties to consider, what objective function to employ, etc (Ref. 7). 

In the second stage, p-strands, p-sheets, and disulfide bridges are identified through a novel superstructure-based mathematical framework originally established for chemical process synthesis problems [121]. Two types of superstructure are introduced, both of which emanate from the principle that hydrophobic interactions drive the formation of p-stmcture. The first one, denoted as hydrophobic residue-based superstructure, encompasses all potential contacts between pairs of hydrophobic residues (i.e., a contact between two hydrophobic residues may or may not exist) that are not contained in helices (except cystines that are allowed to have cystine-cystine contacts even though they may be in hehces). The second one, denoted as -strand-based superstructure, includes all possible p-strand arrangements of interest (i.e., a p-strand may or may not exist) in addition to the potential contacts between hydrophobic residues. The hydrophobic residue-based and p-strand-based superstructures are... 

A.E.S. Konukman, M.C. Camurdan, U. Akman, Simultaneous flexibility targeting and synthesis of minimum-utility heat exchanger networks with superstructure-based MILP formulation, Chem. Eng. Processing 41 (2002) 501-518. 

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