Advances in process synthesis: New robust formulations

We present new modifications to superstructure optimization paradigms to i) enable their robust solution and ii) extend their applicability. Superstructure optimization of chemical process flowsheets on the basis of rigorous and detailed models of the various unit operations, such as in the state operator network (SON) paradigm, is prone to non-convergence. A key challenge in this optimization-based approach is that when process units are deselected from a superstructure flowsheet, the constraints that represent the deselected process unit can be numerically singular (e.g., divide by zero, logarithm of zero and rank-deficient Jacobian). In this paper, we build upon the recently-proposed modified state operator network (MSON) that systematically eliminates singularities due to unit deselection and is equally applicable to the context of both simulation-based and equation-oriented optimization. A key drawback of the MSON is that it is only applicable to the design of isobaric flowsheets at a pressure fixed a priori. In this paper, as a first step towards the synthesis of general flowsheets with variable pressures, we extend the MSON to the synthesis of a gas-liquid absorption column at variable pressure (i.e., the pressure is a degree of freedom that may be optimized). We illustrate the use of the extended MSON on a carbon-capture process. The extended MSON is robust and enables the design of the column on the basis of detailed thermodynamic models and simulation-based optimization.