(384d) Numerical Optimization Applied to the Design of An Extractive Distillation System for the Production of Fuel Grade Alcohol

The optimal design of an extractive distillation system is developed for the separation of the ethanol - water azeotropic mixture using ethylene-glycol as entrainer. The research in this field has been motivated by the growing demand of anhydrous ethanol as motor fuel and the necessity for optimizing the process in order to turn it environmentally and economically efficient [1].

The system is made up by an extractive distillation column and a recovery column, each one with total condenser and reboiler. The extractive distillation column is fed with an ethanol ? water mixture which composition is close to the azeotrope and a high purity stream of ethylene-glycol; this column produces as distillate ethanol with purity higher than 99.5% molar and as bottom product a mixture composed mainly by water and ethylene glycol. The recovery column is fed with the bottoms from the extractive distillation column; water with a small content of ethanol is obtained as distillate and high purity ethylene glycol as bottom product.

The purity of ethanol produced is fixed according to the legal parameters established in Colombia [2] for its blend with gasoline and its use in conventional combustion engines. The purity of ethylene glycol recovered from the second column is adjusted according to the optimization criteria, considering the possibility of recycling this product.

The optimization maximizes an economic objective function of profitability that depends on capital cost, energy cost, value of raw materials and value of products; the problem is submitted to the constraints concerning the model that represents both columns and the specifications of the products. The optimization includes the integration of the two columns, which allows exploration of the model variables interactions and lead to a comprehensive optimization of the process [3].

The model is based on a system of non linear equations that represent the equilibrium stages rigorously through the mass balances, energy balances, mole fraction summations and phase equilibriua; this system is known as MESH equations. Furthermore, Murphree tray efficiencies [5] are used to allow a realistic representation of the process.

The thermodynamic equilibrium is calculated with the use of the Non-Random Two Liquids (NRTL) model for the liquid phase and supposes ideal gas behavior for the vapor phase. The validity of the thermodynamic representation is tested by comparison with experimental data [6; 7; 8] to conclude that there is a satisfactory description of the equilibrium.

Prior to optimization, an analysis of the degrees of freedom of each column is made to establish the number of variables that have to be fixed in order to obtain a fully specified system that works as a framework of reference for the evaluation of the optimization variables. The results of this analysis show the influence of each optimization variable and allow establishing the sensitivity of the objective function.

The solution of the optimization problem is achieved through a two-level strategy; the integer variables are considered on a master problem that proposes system configurations to a non-linear programming (NLP) subproblem that optimizes the continuous variables. Continuous variables are determined through an Interior Point [9] deterministic algorithm in a strategy that solves the columns that compose the system simultaneously.

Interaction between the two levels of the problem allows obtaining the optimal design of the equipment used for the separation and their operating conditions. The optimization produces values of the optimization variables that maximize the economic objective function inside the feasibility region determined by constraints of the model. The continuous optimization variables are: solvent rate to the extraction column, reflux ratios, reboiler heat duties and solvent recirculating percentege.

A thorough bibliographic review did not find any similar researches where extractive distillation systems for azeotropic mixtures were optimized by numeric methods analyzing simultaneously the effect of two columns over an economic objective function of profitability. However, the results of other researches concerning the separation of the ethanol ? water mixture by distillation [3; 11; 12; 13] are considered for comparison in order to confirm the importance of approaching optimization in a comprehensive way.

Acknowledgements

This work was supported financially by research grants from Colciencias, by financial support of research project code: 1101-452-21113.

References

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[2] Ministerio de Ambiente y Desarrollo Territorial y Ministerio de Minas y Energía. (2003) Resolución No. 0447 de abril 14 de 2003. Ministerio de Ambiente y Desarrollo Territorial y Ministerio de Minas y Energía. República de Colombia.

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[5] Murphree, E. V. (1925). Rectifying Column Calculations ? With Particular Reference to N Component Mixtures. Industrial and Engineering Chemistry, 17, 747. [6] Ramanujamm, M. & Laddha, G. (1960). Vapor liquid equilibrium for ethanol ? water ? ethylene glycol system. Chemical Engineering Science, 12, 65.

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[11] Meirelles, A.; Weiss, S. & Herfurth, H. (1992). Ethanol dehydration by extractive distillation. Journal of Chemical Technology and Biotechnology, 53, 181.

[12] Gil, I.D.; Uyazán, A.M.; Aguilar, J.L.; Rodríguez, G. & Caicedo, L.A. (2008). Simulation of ethanol extractive distillation with a glycols mixture as entrainer. 2nd Mercosur Congress on Chemical Engineering & 4th Mercosur Congress on Process Systems Engineering.