Relative Optimality Index for Evaluation of the Approximate Methods for Synthesis of Flexible Processes under Uncertainty

摘要

This contribution introduces a new indicator called Relative Optimality Index (ROI) which measures the extent to which the expected objective value of a flexible process flow sheet obtained by an approximate sequential approach is similar to the optimal solution obtained by more rigorous methods.MINLP flexible process syntheses were performed using various approximate robust approaches and the results were compared to the reference value obtained with the Gaussian quadrature method: a) sequential one-scenario method over randomly selected vertices, b) sequential method in which a two-scenario problem was solved in each iteration simultaneously at the nominal point and randomly selected vertex point, c) a method similar to b) but the most violated point from the previous iteration was used in the subsequent iteration instead of a randomly selected vertex, and d) a novel method which identisses a few vertices critical for flexibility, and solves a few-scenario problem simultaneously in the nominal point and these critical vertices.In this way, better trade-offs between first- and second-stage variables are established.These estimated values are then used as target values during the fine tuning of process structure and design variables that should bring both the flexibility index and the ROI as close as possible to the desired value of 1.