Computers and Chemical Engineering 100 (2017) 27–37 Contents lists available at ScienceDirect Computers and Chemical Engineering journal homepage: www.elsevier.com/locate/compchemeng Comparison of heterogeneous azeotropic distillation and extractive distillation methods for ternary azeotrope ethanol/toluene/water separation ∗ Lei Zhao, Xinyu Lyu, Wencheng Wang, Jun Shan, Tao Qiu Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, No.1, Gehu Road, Wujin District, Changzhou, Jiangsu 213164, China a r t i c l e i n f o a b s t r a c t Article history: Ethanol and toluene are important organic solvents in chemical industry. Ethanol, toluene and water Received 31 October 2016 present a ternary minimum-boiling azeotrope and three binary azeotropes. In this article, two methods Received in revised form 1 February 2017 are studied to separate the ternary azeotrope: heterogeneous azeotropic distillation using the toluene, Accepted 4 February 2017 which is in the system itself, as entrainer and extractive distillation with glycerol as the only one solvent. Available online 6 February 2017 In the heterogeneous azeotropic distillation, the pressures of the three columns are changed respectively to achieve heat integration. In the extractive distillation, only one solvent and two columns are used Keywords: to separate the ternary mixture. Experiment shows that the partially heat-integrated heterogeneous Ternary azeotrope azeotropic distillation reduces the energy cost and total annual cost by 27.7% and 13.4% respectively Heterogeneous azeotropic distillation compared with the conventional heterogeneous azeotropic distillation. Unexpectedly, the extractive dis- Extractive distillation Ethanol/toluene/water tillation can save 18.8% and 39.3% in the energy cost and total annual cost respectively compared with the partially heat-integrated heterogeneous azeotropic distillation. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction rities. This special HAD method is applied to separate azeotrope or close-boiling mixtures using the entrainer, which is in the sys- Ethanol and toluene are widely used in chemical, pharmaceuti- tem itself, to generate a lower-boiling azeotrope with the one or cal, dye and other fields as good solvents. Ethanol can deliquesce two components in the mixture. The minimum-boiling azeotrope easily and absorb moisture from the air quickly because of the pres- produces two immiscible liquid phases in the decanter. Aque- ence of hydrogen bond. Since ethanol, toluene and water present ous phase and organic phase both flow back to the column with three binary azeotropes and one ternary azeotrope, ordinary dis- optimum reflux ratios and the rest are separated and fed into tillation methods can not separate the mixture effectively. Some different columns to obtain pure products. HAD for azeotropic mix- other kinds of distillation methods should be studied to separate ture is expected to cross the distillation boundary (Doherty and the ethanol/toluene/water mixture. Malone, 2001; Widagdo and Seider, 1996). Kiss used HAD method Many methods can be used to separate azeotropic mixture, for in bioethanol dehydration with n-pentane as entrainer (Kiss et al., example, pressure-swing distillation (Yu et al., 2012), azeotropic 2012). Luyben also used benzene as the entrainer in the separation distillation (Abu-Eishah and Luyben, 1985), extractive distillation of ethanol/water azeotrope by HAD (Luyben, 2006a, 2006b). (Meirelles et al., 1992), adsorption (Garg and Ausikaitis, 1983), per- ED is generally applied to separate binary azeotrope and other vaporation using membrane (Fleming, 1992) and some other new close-boiling mixtures with the relative volatility of light compo- separation techniques. In this article, heterogeneous azeotropic dis- nents and heavy components below 1.1 (Seader and Henley, 1998; tillation (HAD) and extractive distillation (ED) methods are studied Errico et al., 2013a, 2013b; Kossack et al., 2008; Lei et al., 2003; to separate the ethanol/toluene/water mixture. Liang et al., 2014). The solvent added in the mixture improves It is innovative to use toluene, which is in the system itself, as the relative volatility of light components and heavy components entrainer for the HAD, which will not bring in any other impu- because the solvent interacts differently with the light components and heavy components in the mixture. When the relative volatility is improved, the light components are acquired in the top of the ∗ extractive column (EC) while the heavy components and solvent Corresponding author. are acquired from the bottom of the EC for subsequent separation. E-mail address: [email protected] (T. Qiu). http://dx.doi.org/10.1016/j.compchemeng.2017.02.007 0098-1354/© 2017 Elsevier Ltd. All rights reserved. 28 L. Zhao et al. / Computers and Chemical Engineering 100 (2017) 27–37 Fig. 1. Process flow sheet of HAD. Table 1 Azeotropic Temperature and Composition of the System at 1 atm. Temp (K) Type No. Comp. Toluene (mass%) Ethanol (mass%) Water (mass%) 347 Heterogeneous 3 47.49 42.75 9.77 357.68 Heterogeneous 2 80.09 0 19.91 351.3 Homogeneous 2 0 95.62 4.38 350.01 Homogeneous 2 31.92 68.08 0 Solvent and heavy components are separated by a solvent recov- optimization programs are used to get optimal configurations of ery column (SRC) easily. Then, the solvent obtained in the bottom the two distillation methods. of SRC is recycled to the EC (Kossack et al., 2008). Luyben used dimethyl sulfoxide as solvent to separate maximum-boiling ace- 2. Simulation tone/chloroform mixture by ED (Luyben, 2008). Figueroa used ED method with ionic liquids as solvent to produce anhydrous ethanol 2.1. Heterogeneous azeotropic distillation and saved energy a lot (Figueroa et al., 2012). Lei used ED method with solid inorganic salt and ionic liquid as solvents to separate 2.1.1. Process design and ternary diagrams In the separation of the ternary mixture with ethanol/water mixture (Lei et al., 2014). Long retrofitted conven- ethanol/toluene/water, the toluene can also serve as an entrainer tional ED to a thermally coupled distillation scheme and reduced and form a low-boiling ternary azeotrope with ethanol and water. the energy cost significantly (Duc Long and Lee, 2013). So, it does not need to add any other entrainer, which will not Separation of the ethanol/toluene/water mixture has visible bring in any other impurities. The ethanol/toluene/water mixture economic benefit, social benefit and environmental benefit. There presents three binary azeotropes and one ternary azeotrope. One are a lot of papers discussing the separation of binary azeotrope ternary azeotrope (ethanol/toluene/water) is satisfactory because via HAD or ED. However, the articles on the separation of ternary it is heterogeneous and its azeotropic temperature (347 K) is azeotrope using HAD and ED are quite limited. So, it is neces- the lowest in the system. Another three binary azeotropes are sary to study some workable distillation methods to separate the also formed with azeotropic temperatures of 357.68, 351.3 and ethanol/toluene/water ternary azeotrope, with the emphasis on 350.01 K, respectively. Table 1 shows the azeotropic temperature economic optimization. and composition of ethanol/toluene/water mixture at 1 atm. When In order to separate the ethanol/toluene/water ternary the pressure is 2.18 atm or 0.33 atm, the ternary azeotrope is still azeotrope, two special distillation methods are studied in this arti- heterogeneous and its azeotropic temperature is still the lowest cle: HAD and ED methods. According to the total annual cost (TAC), in the system. The NRTL physical property model is used in the simulation for the separation of the ternary azeotrope. L. Zhao et al. / Computers and Chemical Engineering 100 (2017) 27–37 29 Table 2 Goal Functions and Necessary Parameters for Economic Evaluation. Parameters Data Condensers Heat transfer 2 coefficient = 0.852 kW/K m 2 0.65 Capital cost = 7296 × (A, m ) Q Heat transfer area: A = (K×t) Reboilers Heat transfer 2 coefficient = 0.568 kW/K m 2 0.65 Capital cost = 7296 × (A, m ) Q Heat transfer area: A = (K×t) Column vessel Capital 1.066 0.802 cost = 17640 × (D, m) × (L, m) Length: H = 1.2 × 0.61 × (NT − 2) Energy cost LP stream (433 K): $7.78 per GJ MP stream (457 K): $8.22 per GJ HP stream (527 K): $9.88 per GJ Chilled-water (253.15 K): $7.89 per GJ totalcapitalcost TAC paybackperiod + annual energy cost Payback period 3 years The process flow sheet of the HAD using toluene as entrainer is shown in Fig. 1 with three columns and one decanter. The pressures of column 2 (C2) and column 3 (C3) are both reduced to 0.33 atm to ensure that the temperature of the condensers is about 323.75 K. So, cooling water can still be used in the condensers. The pressure of column 1 (C1) is raised to 2.18 atm to ensure that the tempera- ture in the top stage of C1 is about 20 K more than the temperature in the bottom of C2 and C3. So, heat integration can be achieved in the HAD in the next section. The C1 has a decanter and the feed- stock is fed into this column (Stage 1 is the top tray). High-purity ethanol product is obtained from the bottom of C1 while the min- imum boiling ethanol/toluene/water heterogeneous azeotrope is obtained from the top of C1. Then, the overhead vapor stream of C1 is cooled, and two liquid phases are separated in the decanter. Some of both the aqueous and organic phases are refluxed to the top tray. The rest of the aqueous phase is fed into C2 and the rest of the organic phase is fed into C3. High-purity water and toluene products are obtained in the bottom of C2 and C3 respectively while the top products of these two columns are combined and back to C1.