Optimal start-up strategies for a conventional distillation column using simulated annealing

Abstract

The start-up process for a distillation column presents a challenging control problem in the chemical industry. To address this problem, reducing wasting time and duty consumption during the transient period are crucial issues. In this study, the separation of the dichloromethane, chloroform, and carbon tetrachloride system (CH2Cl2 + CHCl3 + CCl4) was investigated as a start-up example. The single column was used to obtain 83.1% CHCl3 and 99% CCl4. The distillation start-up steps were divided into the following four parts for investigation: feed control, bottom product discharge time, total reflux switch time, and medium temperature heating operation method of the reboiler duty. The feeding time depended on the sump level for safety, and the bottom product was discharged until reaching purity near the specification to avoid producing excess undesired products. Switch total reflux policies were presented using the minimum temperature difference or waiting until the top composition CHCl3 reached 50 wt%. Furthermore, the simulated annealing (SA) algorithm was applied to determine the optimal heating step. The objective function (j) and a set of variables during optimization (d) were subsequently designed, and a flow chart for the SA approach was developed and connected to Aspen Plus Dynamic software. Following the SA calculation, the modified cases with optimal heating steps exhibited better behavior than did the example case. These rules enabled the complicated evaluation process to be simplified for designing a start-up strategy.