A Comparison of the Exergetic and Economic Optimization of the Catalytic Distillation, Based On a Rigorous Model. Case Study: ETBE Synthesis

Abstract

With finite natural resources and high levels of consumption, due to the incredible growing speed of global population, it is really important to think about sustainable development [1]. One of the highest consumption resources is energy, not in vain it is the main constraint to reach sustainable development. There are many possible causes of high energy consumption, perhaps the most important one is the lack of industrial processes focused on energy saving [2]. As a result, exergetic optimization arises as an interesting alternative solution to mitigate this problem. Exergy is well-known as the potential to cause change in terms of available work and useful energy [3].

In this work a comparison between an exergetic and economic optimization, of a catalytic distillation (CD) process to obtain Ethyl Tert-butyl Ether (ETBE) is presented. In order to achieve this, a Mixed Integer Non-Linear Programming (MINLP) problem was formulated, aiming to optimize not only continuous variables but integer ones too [4]. The strategy of solution was a combination between the algorithms: Reduced Sequential Quadratic Programming (r-SQP) and Simulated Annealing (SA). The exergetic optimization was carried out through the minimization of the exergetic objective function, defined as the second-law balance around the catalytic distillation column. The main purpose of this exergetic optimization was to reduce the exergy losses by optimizing design and operating variables of the process [5]. In addition an economic optimization was presented and reached by the minimization of the economic objective function, aiming to reduce the costs of the process and increase its profitability.

In order to assure the correct performance of the column, both, exergetic and economic objective functions, were subjected to a group of constraints, composed by: model constraints, operational constraints, and hydraulic constraints. The first group of constraints was defined by mass balances, energy balances, mole fraction summations, and phase equilibrium (MESH equations); the second was established by the purity of the product (ETBE) and the third ones avoids entrainment flooding,  down –flow flooding and weeping – dumpling problems. Finally, a comparison between both, the results obtained with an economic optimization and the results achieved in an exergetic optimization are presented.

Bibliography

[1] J. Arons and H. van der Kooi. Efficiency and Sustainability in the Energy and Chemical Industries. New York: Marcel Dekker. (2004)

[2] Gong Mei. Using Exergy and Optimization Models to Improve Industrial Energy Systems towards Sustainability. Linkoping University. Linkoping, Sweden, pages: 1-10.(2004)

[3] Cerci Yunus. The minimum work requirement for distillation processes. Exergy, an International Journal, 2, pages: 15-23. (2002)

[4] Gómez Jorge, Reneaume Jean-Michel, Roques Michel,Meyer Michel and Meyer Xuan. A “MINLP” formulation for optimal design of a catalytic distillation column based on a generic non equilibrium model. Computer Aided Chemical Engineering, 20, pages:  925-930 (2005)

[5] Kaiser Victor and Gourlia Jean-Paul. The Ideal-Column concept: applying Exergy to Distillation. Chem. Eng, 19, pages: 41-53 (1985)