Collocation methods for distillation design.

Abstract: "In this third paper on collocation methods for distillation design, we explore the use of the collocation models for design of simple distillation columns as well as flexible columns. Solvent recovery plants must deal with a wide range of feeds and still return pure solvents. The design problem we address is a single flexible column within the overall solvent recovery plant. We have developed the models and algorithms in the ASCEND system. We discuss the attributes and use of the ASCEND system. With ASCEND we can create complex models with simple building blocks and interactively learn to solve them. We found the collocation model an excellent tool for distillation design, allowing us to develop new concepts in design strategies. We designed a single column as would exist in a flexible solvent recovery plant for an azeotropic system. It was designed to handle three possible feeds, each with a distinct separation task. For each possible feed to a column, we approximate the operation of the column for that feed by creating a quadratic approximation of the reflux ratio as a function of the number of trays and feed location. We optimize the cost of the column over the approximation range, and reapproximate if the minimum is on a bound. We move the approximation range until the local optimum occurs."