Structural constraint reduction in process simulator-based optimisation: Leveraging the P-graph framework

Abstract

In superstructure-based process synthesis, the same superstructure can be modelled with different levels of detail. Models can be characterised into three main categories: high-level aggregate models, shortcut models, and detailed rigorous models. If a detailed modelling level is required, process simulators offer a reliable and rigorous modelling environment. In this context, process simulator-based superstructure optimisation may be performed by postulating the superstructure: (1) in an external optimisation environment, or (2) as a flowsheet in the process simulator itself. This work is focused on the latter option.

To reduce the tedious mathematical writing of the logical constraints required to guarantee the structural coherence of a sequence of unit operations, a P-graph-based framework is proposed in the current work. The developed framework consists of three algorithms. The first algorithm transforms the superstructure flowsheet into a P-graph. The second algorithm gets process sub-flowsheets from the superstructure by searching for active units corresponding to a set of decisions made, for example, by an optimiser. The third one checks structural feasibility by verifying that the resulting process satisfies the five axioms of the original P-graph framework and two additional connectivity tests proposed in this work.

The methodology is tested on two different examples based on Organic Rankine Cycles. The first case study, based on a published article, consists in applying the methodology to build a superstructure implicitly equivalent to the original logic constraints formulation. The second case study is presented to implement the proposed framework into a Combined Heat and Power Cycle optimisation problem.