Relaxation strategy for the structural optimization of process flowsheets

Abstract: "This paper presents an Equality Relaxation variant to the Outer-Approximation algorithm for solving mixed-integer nonlinear programming (MINLP) problems that arise in structural optimization of process flowsheets. The propsed algorithm has the important capability of being able to explicitly handle nonlinear equations within MINLP formulations that have linear integer variables and linear/nonlinear continuous varibales. It is shown that through the explicit treatment of nonlinear equations, the proposed algorithm avoids computational difficulties (e.g. singularities, destruction of sparsity) that are experienced with algebraic or numerical elimination schemes.Also, theoretical properties of the Equality-Relaxation algorithm are discussed, and its performance is demonstrated with a planning problem and a flowsheet synthesis problem. Finally, a simple procedure for structural sensitivity analysis is presented."