Optimization of Distillation Sequences with Nonsharp Separation Columns

Xi Wang,Wei Sun,Jianhong Wang,Jingde Wang,Zengzhi Du,Yunlu Zhang

doi:10.3390/pr7060323

Abstract

Nonsharp distillation sequences are widely used in industrial separation processes; however, most current research has not discussed this topic, except in sequences with heat integration under special operating conditions, including complex columns. The sequence with nonsharp separation has the features of general distillation sequences, which are usually optimized by adjusting the separation sequence and the design/operation parameters of each column in the sequence, making the optimization a mixed integer nonlinear programming (MINLP) problem, which is usually hard to solve. With inclusion of nonsharp separation columns, the sequence optimization becomes even more complicated and computationally intensive. This work aimed to optimize the distillation sequence, including nonsharp distillation alongside simple columns and dividing wall columns. Inspired by the dynamic programing method for sharp distillation sequence, a framework for automatic optimization is proposed to decompose the MINLP problem into integer programming (IP) and nonlinear programming (NLP) problems. The optimization processes of sharp and nonsharp distillation sequences are compared and the solution space in terms of the possible number of distillation sequences with nonsharp separation is discussed. Two optimization cases, including an industrial one, are included to validate the proposed framework.

Highlights

In industrial processes [1], the nonsharp distillation sequences are widely used for obtaining nonsharp separated products
Sequences with nonsharp separation have the features of the general distillation sequences, but with more options in terms of the choice of sequence and columns, which make their optimization complicated and computationally intensive
Inspired by the dynamic programing method for sharp distillation sequences, a bi-level distillation sequence optimization algorithm is proposed in this work to decompose the mixed integer nonlinear programming (MINLP) into an integer programming (IP) problem in the upper level and nonlinear programming (NLP) problems in the lower level, by which the optimization of the separation sequence is implemented at the upper level and the single column within the sequence is simulated and optimized at the lower level

Summary

Introduction

In industrial processes [1], the nonsharp distillation sequences are widely used for obtaining nonsharp separated products. Compared to sharp separation sequences, nonsharp sequences are relatively understudied. Sequences with nonsharp separation have the features of the general distillation sequences, but with more options in terms of the choice of sequence and columns, which make their optimization complicated and computationally intensive. The distillation sequence is usually optimized by adjusting the discrete variables of the separation sequence and the continuous design/operation parameters of each column in the sequence. Column-associated equations contribute to the nonlinear part. All these make the problem a nonconvex MINLP problem with a discontinuous feasible region, making it difficult to find the global optimum

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