Superstructure optimization reactor network synthesis

Significant recent approaches to chemical reactor network synthesis can be classified into two categories, viz. superstructure optimization and network targeting. In the former, a superstructure is postulated and then an optimal sub-network within it is identified to maximize performance index (Kokossis and Floudas, 1990). 

This paper describes an integrated MINLP synthesis of overall process schemes using a combined synthesis / analysis approach. The synthesis is carried out by a multilevel-hierarchical MINLP optimization of the flexible superstructure, whilst the analysis is performed in an economic attainable region (EAR). The role of the MINLP synthesis step is to obtain a feasible and optimal process structure, and the role of the subsequent EAR analysis step is to verify the MINLP solution and to propose in the feedback loop, any profitable superstructure modifications for the next MINLP. The main objective of the integrated synthesis is to exploit interactions between the reactor network, separator network and the remaining part of the heat/energy integrated process scheme. 

The general reactor network superstructure was first used without intermediate separation and with ad hoc cost for the final separation. The profit was 17.154 M /a with one PFR. Analysis in the EAR (Fig. 2a) shows that the actual PFR trajectory is significantly below the horizontal boundary of the AR which would be obtained when cb is kept zero. This observation indicates that additional profit can be gained if suitable intermediate separation of B can be found. When the MINLP synthesis was performed with intermediate separation (IS) and no cost for IS was charged to the profit, the optimal topology resulted in a set of three reactors, the first was PFR and the next two were recycle PFRs with a profit of 18.773 M /a. The analysis in the EAR (Fig. 2b) shows reactor trajectories at higher molar profit. 

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