Time-Optimal Control of Startup Distillation of Nonideal Mixtures

Abstract

In this work, the time-optimal control of startup distillation columns by iterative programming proposed by Woinaroschy [Ind. Eng. Chem. Res.2008474158] is extended to the case of nonideal mixtures. The minimization of distillation startup time is performed by iterative dynamic programming employing randomly chosen candidates for admissible control. The control variables are the reflux ratio and/or the reboiler heat duty. The dynamic distillation model proposed by the author in the mentioned work is applied with a preliminary computation of suitable correlation relations for equilibrium constants. An illustrative application for separation of a propene−propane mixture at plant scale is presented. Because of the small relative volatility of this mixture and respective high reflux ratio, the decrease of the reboiler heat consumption corresponding to the minimization of the startup time is significant.

Optimal Control for Chemical Reactors with Distributed Parameters Using Genetic Algorithms

“…This control is applicable to industrial processes where tight control is not necessary and where deadtime is negligible . The bang‐bang control is a frequent case in time‐optimal control of startup distillation columns .…”

Time-Optimal Control of Dividing-Wall Distillation Columns

“…This equation is applied for ideal and nonideal mixtures in the liquid phase. For nonideal mixtures, in liquid and/or vapor phase, the temperature variation can be computed with the equation d T j d t = − ∑ i = 1 m normalκ i , j normald x i , j normald t ∑ i = 1 m normald normalκ i , j normald T j Therefore, knowing the liquid composition and temperature on each tray, the vapor composition is obtained by straightforward computation, without iterative calculations.…”

“…In fact, the startup is the most “dynamic” operation stage, involving important economic and safety considerations. Several researchers have studied this subject: Fieg and Wozny, Woinaroschy, − Yasuoka et al, Ruiz et al, Barolo et al, , and Han and Park . None of these papers consider the case of thermally coupled distillation columns.…”

“…The application of TOC is the same as in the previous papers, , using the algorithm proposed by Bojkov and Luus, based on the IDP procedure given by Bojkov and Luus, , which employs randomly chosen candidates for the admissible control . The system of differential equations was numerically integrated by a fourth order Runge−Kutta−Gill method, which automatically chooses the time step to satisfy local error tolerance.…”

“…The system of differential equations was numerically integrated by a fourth order Runge−Kutta−Gill method, which automatically chooses the time step to satisfy local error tolerance. It can be appreciated that the corresponding computer time increase due to the small values of the integration step is justified, thus avoiding the iterative solution of algebraic equations, which are even more computer time consumers. , The computer programs were coded in FORTRAN.…”