Selection of Ionic Liquids as Entrainers for the Separation of Ethyl

Sep 3, 2009 - This work tries to explore the relation between molecular structures of ionic liquids (ILs) as entrainers and separation performance on ...
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SEPARATIONS Selection of Ionic Liquids as Entrainers for the Separation of Ethyl Acetate and Ethanol Qunsheng Li, Jiguo Zhang, Zhigang Lei,* Jiqin Zhu, Jiujuan Zhu, and Xiaoqiao Huang State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Box 35, Beijing, 100029, China

This work tries to explore the relation between molecular structures of ionic liquids (ILs) as entrainers and separation performance on ethyl acetate (1) and ethanol (2). The vapor-liquid equilibrium (VLE) data have been measured for ethyl acetate (1) + ethanol (2) containing ILs at 101.32 kPa. The ionic liquids investigated were 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]+[BF4]-) and 1-methyl-3-octylimidazolium tetrafluoroborate ([OMIM]+[BF4]-), as well as 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]+[BF4]-) that has been studied in our previous work. All the ILs could give rise to the salting-out effect. It was found that, at low IL concentration, the separation ability of ILs is in the order [EMIM]+[BF4]- > [BMIM]+[BF4]> [OMIM]+[BF4]-, while at high IL concentration (x3 ) 0.10-0.30) the separation ability of ILs is in the order [OMIM]+[BF4]- > [EMIM]+[BF4]- > [BMIM]+[BF4]- due to the difference of polarity of the three ILs and demixing effect. [EMIM]+[triflate]- was included as entrainer and compared with the tetrafluoroboratebased ILs. The measured ternary data were correlated using the NRTL model. 1. Introduction Up to now, extractive distillation is the most widely utilized technology being used to separate azeotropes and other mixtures that have key components with a relative volatility below about 1.1 over an appreciable range of concentration.1-3 However, the entrainers used in extractive distillation such as solid salts and organic substances comprise many disadvantages, which have been the handicaps for further application of extractive distillation. The solid salts may corrode the column and pipeline, and the organic solvents will not only give rise to volatile organic compound (VOC) emission but also demand high energy. As green and potential environmentally friendly solvents, ionic liquids (ILs) have attracted increasing attention for the replacement of solid salts and organic solvents in extractive distillation because of their many unique merits, e.g., no volatility, less corrosion, no flammability, thermal stability, high dissolving ability, and good performance in enhancing the relative volatility of the mixtures to be separated.4-6 The use of ILs as entrainers for extractive distillation was first proposed by Arlt and co-workers.7-12 They investigated the salt effect of various ionic liquids on several azeotropic mixtures by measuring and analyzing the vapor-liquid equilibrium (VLE) data. Besides, many similar works that focused on the phase behavior of the azeotropic mixtures or close boiling mixtures containing ionic liquids were also reported.13-33 It was believed that the work on identifying the relation between molecular structures of ILs as entrainers and separation performance is important to select the best suitable ILs for extractive distillation. Furthermore, the phase equilibrium data are essential for development of VLE models that will be used in the design of the extractive distillation process. However, the VLE data in this field are still scarce, thus making further studies on phase behaviors of IL-containing systems still necessary. * To whom correspondence should be addressed. Tel.: +86 10 64433695. Fax: +86 10 64419619. E-mail: [email protected].

In this work the system of ethyl acetate and ethanol was selected because ethyl acetate is an important solvent in industry and has promising applications. However, the presence of the azeotropic point of this binary mixture at atmospheric pressure makes it difficult to obtain high purity ethyl acetate from the mixtures by conventional distillation or rectification. Many special separation technologies are under development that could provide higher purity ethyl acetates from the mixture than the conventional separation method. Among them are salt extraction combined with azeotropic distillation,34,35 extractive dehydration,36 membrane separation,37,38 solvent extraction,39,40 catalytic distillation,41 reactive distillation,42 and extractive distillation.43-46 Compared to the other methods, extractive distillation with ionic liquids as entrainer will be more simple and effective, and it can deal with large amounts of feedstock, which can meet up with the requirement of scale benefit in the production of ethyl acetate. In our previous work47 we investigated the phase behavior of ethyl acetate (1) + ethanol (2) + [EMIM]+[BF4]- (3). However, the data of the vapor-liquid equilibrium for the system of ethyl acetate (1) + ethanol (2) containing ionic liquids is still inadequate. Therefore, in this work we continued to measure the VLE about the ternary system containing [BMIM]+[BF4]- and [OMIM]+[BF4]- at 101.32 kPa since these data will be essential for the design of extractive distillation process. 2. Materials and Methods Chemicals. The ILs 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]+[BF4]-), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]+[BF4]-), and 1-methyl-3-octylimidazolium tetrafluoroborate ([OMIM]+[BF4]-) were supplied by the Chemical Engineering Institute of the Normal University of HeBei (China), with mass purity >98% checked by liquid chromatography. Moreover, the ILs were dried for 48 h at 363-383 K under a vacuum by a rotary evaporator to remove

10.1021/ie8017127 CCC: $40.75  2009 American Chemical Society Published on Web 09/03/2009

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Figure 1. Isobaric VLE diagram for ethyl acetate (1) + ethanol (2) + [BMIM]+[BF4]- (3) system at 101.32 kPa: b, x3 ) 0 (IL-free); O, x3 ) 0.10; 4, x3 ) 0.20; 0, x3 ) 0.30; 2, x3 ) 0.20 ([EMIM]+[triflate]-); 9, x3 ) 0.30 ([EMIM]+[triflate]-); solid lines, correlated using the NRTL model.

Figure 2. Isobaric VLE diagram for ethyl acetate (1) + ethanol (2) + [OMIM]+[BF4]- (3) system at 101.32 kPa: b, x3 ) 0 (IL-free); O, x3 ) 0.10; 4, x3 ) 0.20; 0, x3 ) 0.30; 2, x3 ) 0.20 ([EMIM]+[triflate]-); 9, x3 ) 0.30 ([EMIM]+[triflate]-); solid lines, correlated using the NRTL model.

volatile impurities and water before experiments. After experiments, the ILs were reused after the volatile byproducts were separated by rotary evaporation. The water mass fraction in ILs determined by Karl Fischer titration was less than 0.5%. Ethyl acetate and ethanol were purchased from Tianjin Chemical Reagents Company (China) with a purity of above 99.8%.The purity of the reagents was checked by gas chromatography (GC 4000A, China), and they were not further purified before experiments. Apparatus and Procedure. The VLE data were measured by a circulation vapor-liquid equilibrium still (a modified Othmer still) at 101.32 kPa. The detailed description of this apparatus is available in our previous publications.47,48 The equilibrium temperature was measured by a precision and calibrated thermometer with an uncertainty of 0.1 K. Each solution was prepared gravimetrically using an electronic balance (Sartorius; the uncertainty was about 0.1 mg). The uncertainty of the mole fraction of the components in the liquid and vapor phases was 0.002. The equilibrium pressure was kept constant by an on-off pressure controller whose standard uncertainty was 0.10 kPa.

101.32 kPa; Pis is the vapor pressure of pure component i at system temperature, which could be calculated by the Antoine equation using the Antoine constants from the literature;49 φi is the fugacity coefficient of component i in the vapor mixture and φis is the fugacity coefficient of pure component i in its saturated state. To simplify, the IL is treated as a nondissociating component, and the assumption of an ideal behavior is adopted for the vapor. The fugacity coefficients φi and φis are equal to unity at low pressure. Therefore, eq 1 could be rewritten as

3. Results and Discussion The influence of ILs on the VLE of ethyl acetate and ethanol at 101.32 kPa was investigated, and the experimental data for the ethyl acetate (1) + ethanol (2) + [BMIM]+[BF4]- and ethyl acetate (1) + ethanol (2) + [OMIM]+[BF4]- systems (at IL mole fractions 10%, 20%, and 30%) are plotted in Figures 1 and 2, respectively, in which x1′ is the mole fraction of ethyl acetate in the liquid phase on an IL-free basis and the IL concentration is given for each curve separately. The effect of IL on the solution nonideality can be expressed by the activity coefficient of component i, γi, which could be calculated by the following equation: γi )

yiφiP xiφisPis

(1)

where yi represents the mole fraction of component i in the vapor phase; xi is the mole fraction of component i in the liquid phase containing IL; P is the total pressure of the equilibrium system,

γi )

yiP xiPis

(2)

It should be noted that IL does not appear in the vapor phase due to its nonvolatility. However, its mole fraction in the liquid phase is considered when calculating activity coefficients of ethyl acetate or ethanol. In addition to the activity coefficient, the relative volatility of ethyl acetate to ethanol is also calculated as follows: R12

y1 /x1 γ1P1s ) ) y2 /x2 γ2P2s

(3)

where x1 and x2 are mole fractions of ethyl acetate and ethanol in the liquid phase containing IL, respectively. The selectivity and solvent capacity at infinite dilution provides a useful index for selection of suitable entrainers and should be considered at the same time, which are defined as S∞12 ) SP )

γ∞1 γ∞2 1 γ∞1

(4)

(5)

It can be seen from Figures 1 and 2 that all the ILs investigated are enable to break the azeotropic point at x1 ) 0.550 for the ethyl acetate + ethanol binary mixture. With the increase of IL concentration, the ethyl acetate concentration in the vapor phase becomes more and more higher until the azeotropic point disappears. The IL [BMIM]+[BF4]- can break

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Figure 3. Relative volatility of ethyl acetate (1) to ethanol (2) containing [BMIM]+[BF4]- (3) at 101.32 kPa: b, x3 ) 0 (IL-free); 0, x3 ) 0.10; 4, x3 ) 0.20; O, x3 ) 0.30; solid lines, correlated using the NRTL model.

Figure 5. Relative volatility of ethyl acetate (1) to ethanol (2) at various ILs: b, IL-free (x3 ) 0); 0, [BMIM]+[BF4]- (x3 ) 0.20); 4, [EMIM]+[BF4](x3 ) 0.20); O, [OMIM]+[BF4]- (x3 ) 0.20); solid lines, correlated using the NRTL model.

Figure 6. Selectivity vs solvent capacity at infinite dilution for all the ILs investigated in this work. The data were derived from the UNIFAC model. 1, [EMIM]+[BF4]-; 2, [BMIM]+[BF4]-; 3, [OMIM]+[BF4]-; 0, 300 K; 4, 360 K. Figure 4. Relative volatility of ethyl acetate (1) to ethanol (2) containing [OMIM]+[BF4]- (3) at 101.32 kPa: b, x3 ) 0 (IL-free); 0, x3 ) 0.10; 4, x3 ) 0.20; O, x3 ) 0.30; solid lines, correlated using the NRTL model.

the azeotropic point at its concentration of about 30%, while the IL [OMIM]+[BF4]- breaks the azeotropic point at its concentration of about 10%. However, it has been reported that the IL [EMIM]+[BF4]- can break the azeotropic point at its concentration of about 20%. Therefore, these three ILs are good candidates to act as entrainers for the separation of ethyl acetate and ethanol. Figures 3 and 4 show the influence of the ILs [BMIM]+[BF4]and [OMIM]+[BF4]- on the relative volatility of ethyl acetate to ethanol, respectively. It can be seen that both ILs can enhance the relative volatility in the whole x, y concentration range. The greater IL concentration, the higher mole fraction of ethyl acetate in the vapor phase. The salting-out effect of [BMIM]+[BF4]- and [OMIM]+[BF4]- follows the order 30% > 20% > 10%. This conclusion also holds for [EMIM]+[BF4]-. As we know, the ILs have greater attractive interaction with ethanol than with ethyl acetate due to their strong polarity, and thus increase the relative volatility of ethyl acetate to ethanol. With the increase of IL concentration in the ternary system, the interaction between ILs and ethanol becomes much stronger, which leads to enhancement of the relative volatility of ethyl acetate to ethanol. On the other hand, with the increase of the

alkyl chain length in the imidazolium ring from C2 to C8, the polarity of ILs decreases. Therefore, the separation ability of ILs should in principle follow the order [EMIM]+[BF4]- > [BMIM]+[BF4]- > [OMIM]+[BF4]-. Figure 5 shows the relative volatility of ethyl acetate to ethanol at various ILs with x3 ) 0.20. However, it seems that [OMIM]+[BF4]- exhibits the best separation ability. The separation ability of ILs is in the order [OMIM]+[BF4]- > [EMIM]+[BF4]- > [BMIM]+[BF4]- under the same condition. We relate this phenomenon to the formation of a liquid-liquid demixing. That is, the ternary systems are not always miscible in the whole concentration range. The selectivity and solvent capacity at infinite dilution were derived from the UNIFAC model,50,51 as shown in Figure 6. It can be seen that [OMIM]+[BF4]- has the highest solvent capacity among all the ILs investigated. The solvent capacity of ILs is in the order [OMIM]+[BF4]- > [BMIM]+[BF4]- > [EMIM]+[BF4]-. Therefore, the IL with high solvent capacity is desirable in selecting the potential entrainer. To further investigate the separation ability of these three ILs, we measured the VLE of the ternary system at low IL concentration in the liquid phase. The experimental data of relative volatilities of ethyl acetate to ethanol at various ILs with the feeding concentration of ethyl acetate x1′ ) 0.60 are given in Table 1. Figure 7 shows the influence of IL concentration on relative volatility. It can be seen that, as the IL

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Table 1. Relative Volatilities of Ethyl Acetate (1) to Ethanol (2) at Various ILs with the Feeding Concentration of Ethyl Acetate x1′ ) 0.60 IL

x3

R12

[EMIM]+[BF4]-

0.010 0.020 0.040 0.060 0.072 0.080 0.010 0.020 0.040 0.060 0.080 0.101 0.010 0.020 0.040 0.060 0.080 0.102

0.956 1.017 1.086 1.129 1.156 1.185 0.938 0.950 0.961 1.026 1.049 1.109 0.900 0.946 1.149 1.239 1.305 1.388

[BMIM]+[BF4]-

[OMIM]+[BF4]-

concentration increases, the relative volatilities also increase. At low IL concentration, the separation ability of ILs is in the order [EMIM]+[BF4]- > [BMIM]+[BF4]- > [OMIM]+[BF4]-. However, at high IL concentration, the separation ability of ILs is in the order [OMIM]+[BF4]- > [EMIM]+[BF4]- > [BMIM]+[BF4]-, which is consistent with the observation from Figure 5. In this case the separation ability of [OMIM]+[BF4]increases the most rapidly from low to high IL concentration. The final separation ability depends on the double actions of polarity (i.e., alkyl chain length in the imidazolium ring) and solvent capacity of ILs. In addition, the comparison of separation ability of [BMIM]+[BF4]- and [OMIM]+[BF4]- with [EMIM]+[triflate](1-ethyl-3-methylimidazolium trifluoromethanesulfonate) as proposed by Orchille´s et al.46 was also made (see Figures 1 and 2). If [EMIM]+[triflate]- is included, the separation ability of four ILs at x3 ) 0.20 and 0.30 is in the order [OMIM]+[BF4]> [EMIM]+[BF4]- > [EMIM]+[triflate]- > [BMIM]+[BF4]-. This indicates that the tetrafluoroborate-based ILs investigated in this work are promising. The T, x, y diagram of the ternary systems containing [BMIM]+[BF4]- and [OMIM]+[BF4]- are shown in Figures 8 and 9, respectively. Figures 8 and 9 demonstrate that the equilibrium temperatures increase when higher mole fractions

Figure 8. T, x, y diagram for the ternary system of ethyl acetate (1) + ethanol (2) containing [BMIM]+[BF4]- (3) at different concentrations of IL: b, x1 ) x1′ (x3 ) 0); O, y1 (x3 ) 0); 2, x1′ (x3 ) 0.10); 4, y1 (x3 ) 0.10); 9, x1′ (x3 ) 0.20); 0, y1 (x3 ) 0.20); 1, x1′ (x3 ) 0.30); 3, y1 (x3 ) 0.30); solid lines, correlated using the NRTL model.

Figure 7. Influence of IL concentration on the relative volatility of ethyl acetate (1) to ethanol (2): 0, [EMIM]+[BF4]-; O, [BMIM]+[BF4]-; 9, [OMIM]+[BF4]-; s, smoothed line.

of the ILs are added into the system, which means that the heat quality of the extractive distillation column will increase. Moreover, the reboiler of the solvent recovery column that is used to recover the entrainer will need extra heat. In addition, for a given purity of the distillate, the reflux ratio of the extractive distillation column can be reduced when the relative volatility of ethyl acetate to ethanol becomes higher. This indicates that the energy demand for the extractive distillation

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Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009 Table 2. Estimated Values of Binary Interaction Parameters ∆gij and ∆gji in the NRTL Model i component

Rij

j component

ethyl acetate (1) ethanol (2) ethyl acetate (1) [BMIM]+[BF4]- (3) ethanol (2) [BMIM]+[BF4]- (3) ethyl acetate (1) [OMIM]+[BF4]- (3) ethanol (2) [OMIM]+[BF4]- (3)

0.300 0.108 0.350 0.108 0.350

∆gij/J mol-1 ∆gji/J mol-1 1638.70 16211.84 9358.14 11432.18 -437.42

971.98 -8821.95 -5055.79 -6489.74 -4392.58

systems can be deduced from binary systems. However, in most cases the NRTL model gives better agreement with the experimental results. Therefore, in this work the binary interaction parameters of the NRTL model were first obtained from the vapor-liquid equilibrium data of the ethyl acetate (1) + ethanol (2) system, and then other binary interaction parameters were obtained from ternary vapor-liquid equilibrium data. The Marquardt method as in Press et al.52 was used for data correlation, and the correlated results are given in Table 2, where the average relative deviation (ARD) is defined as ARD (%) )

1 n

∑ n

|

γexp - γcal i i γexp i

|

× 100

(6)

In the NRTL model, the nonrandomness parameters R are set to be the same as those in ref 46. In this case the ARD is 3.84% for the ethyl acetate (1) + ethanol (2) + [BMIM]+[BF4]- (3) system and 3.12% for the ethyl acetate (1) + ethanol (2) + [OMIM]+[BF4]- (3) system. 4. Conclusions

Figure 9. T, x, y diagram for the ternary system of ethyl acetate (1) + ethanol (2) containing [OMIM]+[BF4]- (3) at different concentrations of IL: b, x1 ) x1′ (x3 ) 0); O, y1 (x3 ) 0); 2, x1′ (x3 ) 0.10); 4, y1 (x3 ) 0.10); 9, x1′ (x3 ) 0.20); 0, y1 (x3 ) 0.20); 1, x1′ (x3 ) 0.30); 3, y1 (x3 ) 0.30); solid lines, correlated using the NRTL model.

process can be saved. This study confirms the capability of ILs as entrainers for the separation of ethyl acetate and ethanol. As suggested in previous works,10,11 the NRTL, Wilson, and UNIQUAC models are commonly used to correlate the vapor-liquid equilibrium data of the systems containing ILs because their binary interaction parameters can be input into some famous simulation programs such as ASPEN PLUS, PROII, etc. and the thermodynamic behavior of multicomponent

Isobaric VLE data for the ethyl acetate (1) + ethanol (2) system containing ILs [BMIM]+[BF4]- and [OMIM]+[BF4]- were measured at 101.32 kPa. All the ILs investigated showed a notable salting-out effect according to the experimental data. At low IL concentration, the separation ability of ILs is in the order [EMIM]+[BF4]- > [BMIM]+[BF4]- > [OMIM]+[BF4]-, which means that in this case a long alkyl chain length on the cation is unfavorable for increasing the relative volatilities. However, at high IL concentration (x3 ) 0.10-0.30), the separation ability of ILs is in the order [OMIM]+[BF4]- > [EMIM]+[BF4]- > [BMIM]+[BF4]due to the difference of polarity of the three ILs and the demixing effect. Therefore, care should be taken to consider the demixing effects at finite concentration in separation processes. On the other hand, if [EMIM]+[triflate]- is included, the separation ability of the four ILs at x3 ) 0.20 and 0.30 is in the order [OMIM]+[BF4]> [EMIM]+[BF4]- > [EMIM]+[triflate]- > [BMIM]+[BF4]-. This implied that the tetrafluoroborate-based ILs investigated in this work are promising additives for the separation of ethyl acetate and ethanol due to their good separation ability and desirable properties, such as nonvolatility, nonflammability, and chemical stability. Moreover, these ILs, i.e., [EMIM]+[BF4]-, [BMIM]+[BF4]-, and [OMIM]+[BF4]-, are much easier to obtain from chemical markets at a lower price and thus the separation of ethyl acetate and ethanol could be implemented in real processes. Acknowledgment This work is financially supported by the National Nature Science Foundation of China (Grants 20821004 and 20706005), the Program for New Century Excellent Talents in University and the Fok Ying Tong Education Foundation (No. 111074).

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ReceiVed for reView November 10, 2008 ReVised manuscript receiVed August 11, 2009 Accepted August 17, 2009 IE8017127