(42k) Comparison of an Extractive Dividing Wall Column Operating at Vacuum and a Conventional Extractive Distillation Sequence

Bioethanol has been considered one of the most promising substitutes for fossil fuels. In order to be used in engines available in the market, bioethanol needs to have a maximum water content of 1.0 vol % according to the international standard ASTM D 4806, USA. Ethanol-water mixtures are highly nonideal with a binary azeotrope at 95.6 wt. %. In order to achieve the separation, different kinds of technologies have been developed, such as pervaporation, adsorption with molecular sieves, azeotropic distillation, pressure swing distillation and extractive distillation, among others. In the specific case of extractive distillation, a solvent is used to modify the phase equilibrium and allows the separation of ethanol at high purity. The inclusion of an additional component results in higher capital and operation costs due to the need of an extra column for the recovery of the solvent.

Some studies have worked on the design, optimization and control of dividing wall columns for the separation of azeotropic mixtures ethanol-water. They have reported some advantages in terms of energy savings and investment costs (Bravo-Bravo et al., 2010) (Kiss & Suszwalak, 2012) (Segovia-Hernandez et al., 2014) (Segovia-Hernandez et al., 2014) (Kiss & Ignat, 2012). Most of them have compared the results of the dividing wall column with a conventional sequence of distillation columns working at the same pressure of 1 atm and with reboiler temperatures around 200°C using ethylene glycol as solvent. According to (Meirelles, Weiss, & Herfurth, 2007), the recovery column in the real distillation sequence should work at lower temperature (less than 150°C) in order to avoid degradation of the solvent. This is achieved by operating the recovery column at lower pressure, which is usually less than the pressure in the extractive column. Similar requirement was reported by (Garcia-Herreros, Gomez, Gil, & Rodriguez, 2011) with the use of glycerol. The difference of pressures between the columns in the conventional extractive distillation sequence is a limiting factor in the use of dividing wall columns as an equivalent process. In dividing wall columns, once the position of the wall is fixed the design should ensure the same pressure in both sides of the wall.

In this work, a comparison of a conventional sequence of two distillation columns working at different pressure with an equivalent separation using a dividing wall column working at one single pressure is made. In the conventional separation, the first column (extractive) works at 1 atm and the second column (recovery) is designed with the aim to ensure a reboiler temperature lower than 150°C. In the case of the dividing wall column, the extractive zone and the recovery zone are in the same shell, so that the pressure at both sides of the wall is the same. Modeling and simulation are done using Aspen Plus®, where NRTL was selected as thermodynamic model and RadFrac as solution module. In order to achieve the minimal energy consumption for the dividing wall column, the optimal operating vacuum pressure is found using as constraint the degradation temperature of the solvent. This energy consumption is compared with the result obtained for the conventional sequence. In addition, two different solvents, ethylene glycol and glycerol, are used for comparison purposes. Results of this work validate the use of dividing wall columns in systems with common operative constraints.