Finding Better Limit Cycles of Semicontinuous Distillation. 1. Back Stepping Design Methodology

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Semicontinuous ternary zeotropic distillation is a periodic process that is carried out in a single distillation column and a tightly integrated external middle vessel. In the state-of-the-art design procedure of this process, a continuous distillation process that separates the top and bottom products to the desired purity is used to generate an arbitrary initial state for simulating the dynamics of the semicontinuous distillation process. Although this method is useful in estimating the limit cycle, it was later found that the operation of the process in this limit cycle was economically suboptimal. In this study, a new algorithmic design procedure, called the back-stepping design methodology, is proposed to find better limit cycles for zeotropic ternary semicontinuous distillation using the aspenONE Engineering suite. The proposed methodology was applied to two different case studies using feed mixtures with different chemical components. A comparison with the current design procedure for the two case studies indicates that the new method outperforms the state-of-the-art by finding limit cycles that were 4–16% lower in separating cost, which was the chosen measure of cycle performance.