Superstructure optimization with rigorous models via an exact reformulation

The applicability of superstructure optimization to process synthesis is often limited to simple models and flowsheets. The state operator network (SON) (Smith and Pantelides, 1995) overcomes some limitations via a mixer–splitter network, allowing the use of rigorous unit models. However, setting flowrates to zero for non-selected units can result in numerical issues. Here, the modified state operator network (MSON), a new exact reformulation with modified mixers and splitters, is introduced. When a unit is deselected, a fictitious, strictly positive, mixer inlet flow ensures the unit model is easily solved. A corresponding fictitious splitter outlet counteracts this inlet, resulting in correct flowsheet behaviour. When applied to a toy flowsheet, the MSON outperforms standard formulations. When applied to the synthesis of a reactor–separator network and to a challenging counter-current column synthesis problem, the MSON offers a systematic and robust approach to superstructure optimization.