An extended superstructure formulation for non-isobaric flowsheet synthesis

Flowsheet synthesis is an integral step in process design, entailing the selection of a set of unit operations and their connectivity to convert raw materials to products. Superstructure optimisation represents a promising class of synthesis approaches, allowing for the systematic exploration of the flowsheet design space. Despite this, many superstructure formulations suffer from numerical instabilities, combinatorial explosion, and/or rely on restrictive assumptions on the types of flowsheet alternatives that can be considered. The modified state-operator network (MSON) formalism has recently been proposed to address some of these issues for isobaric flowsheets. The constant-pressure assumption restricts the applicability of the MSON to real process applications as pressure is a key process variable in many unit operations, such as distillation, reaction, and extrusion, and is necessary to elicit flow. In this work, we present the extended MSON (E-MSON) which inherits the numerical stability of MSON, whilst removing the isobaric assumption. This is achieved through the introduction of new constraints that further improve the numerical behaviour of the MSON. The E-MSON is then applied to a simple non-isobaric superstructure optimisation problem, the results of which demonstrate that the E-MSON can serve as a framework for non-isobaric flowsheet synthesis, enabling a broader range of flowsheet alternatives to be considered.