Optimal Control of the Extractive Distillation for the Production of Fuel-Grade Ethanol Manuel A. Ramos, Pablo García-Herreros, and Jorge M. Gó mez* Grupo de Diseñ o de Productos y Procesos, Departamento de Ingeniería Química, Universidad de los Andes, Carrera 1 No. 18 a -10, Bogota ́ , Colombia Jean-Michel Reneaume Laboratoire de Thermique, E ́ nerge ́ tique et Proce ́ de ́ s (LaTEP), E ́ cole Nationale Supe ́ rieure en Ge ́ nie de Technologies Industrielles (ENSGTI), Universite ́ des Pays de l’Adour (UPPA), Rue Jules Ferry, BP 7511, 64 075 PAU Cedex, France ABSTRACT: The extractive distillation of ethanol using glycerol as the entrainer was studied to determine its optimal control profiles when the azeotropic feed was subjected to composition disturbances. The process was modeled by a differential-algebraic equation (DAE) system that represents the dynamics of the equilibrium stages in the extraction column. The model equations were solved by discretizing the time domain using orthogonal collocation on finite elements. Initially, the effects of feed disturbances on the product flow rate and quality were analyzed. Subsequently, a profit objective function was formulated, and the optimal profiles of the manipulated variables (reflux ratio and reboiler duty) were determined, subject to quality constraints. The solution was obtained by solving the nonlinear programming (NLP) problem that resulted from the discretization. The problem was solved in GAMS using IPOPT as the nonlinear solver, testing two different linear solvers, the Harwell subroutines MA57 and MA86. The optimal control strategy was compared to a simple PI control scheme. 1. INTRODUCTION Many governments around the world are moving to encourage the production and consumption of liquid fuels from biomass, 1 such as fuel-grade ethanol. 2 Their objectives are to reduce oil dependence, to lessen environmental pollution, and to promote agro-industrial production. These initiatives are turning ethanol produced from renewable resources into the most popular substitute for gasoline. 3 Ethanol is mainly obtained by sugar fermentation, a process that produces a mixture with high contents of water and impurities. Fuel-grade ethanol requires a molar composition of 0.995 to avoid two-phase formation when mixed with gasoline. 4 Among the most popular processes used in ethanol dehydratation are the following: 5,6 heterogeneous azeotropic distillation using solvents such as benzene, pentane, iso-octane, and cyclohexane; extractive distillation with solvents and salts as entrainers; adsorption with molecular sieves; and processes that use pervaporation membranes. A comparison among the main ethanol dehydratation techniques is available elsewhere (see Bastidas et al. 6 ). The purification of ethanol produced by fermentation implies an energetically intensive separation process, even more so when it is successfully accomplished by distillation. Distillation processes not only represent a high percentage of the separation operations used in chemical industries but also have a strong impact on the total energy consumption of the overall process. 5 For example, distillation consumes around 53% of the total energy used in separation processes, which makes it the most energy-consuming unit operation. 7 The separation of ethanol-water mixtures by conventional distillation is limited by the presence of a minimum-boiling azeotrope. To obtain high-purity ethanol by distillation, certain techniques have been developed to alter the relative volatilities of the substances in the mixture and thereby allow the azeotropic composition to be exceeded. Among these techniques, the most commonly used are vacuum distillation, azeotropic distillation, and extractive distillation. 8 Recently, a study presented by Garci ́ a-Herreros et al. 9 proposed a design and operating conditions that maximize a profit function for the extractive distillation of fuel-grade ethanol using glycerol as the entrainer. That process is aimed at offering the greatest economic benefit under stationary conditions. However, to maintain the optimal operating conditions of the process, dynamics ought to be neglected. In practice, distillation processes are subjected to disturbances and/or process transitions. To establish the usefulness of the proposed process for the industrial production of fuel-grade ethanol, it is necessary to analyze its dynamic behavior and controllability. The dynamic stability of extractive distillation might represent a potent advantage over azeotropic distillation with benzene, the traditional method for the production of fuel-grade ethanol. The azeotropic distillation has proven to have a high parametric sensitivity and the presence of multiple steady states, 10-12 which often implies low ethanol recovery. 13 Several authors have studied the dynamic behavior and control of other extractive distillation processes, 11,14-22 as shown in Table 1. The evaluation done by Maciel and Brito 20 Received: July 30, 2012 Revised: May 31, 2013 Accepted: May 31, 2013 Published: May 31, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 8471 dx.doi.org/10.1021/ie4000932 | Ind. Eng. Chem. Res. 2013, 52, 8471-8487