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salt mixtures as entrainer in a (vapor + liquid) system to separate n-heptane from toluene
J. Chem. Thermodynamics 91 (2015) 156–164
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Use of selective ionic liquids and ionic liquid/salt mixtures as entrainer in a (vapor + liquid) system to separate n-heptane from toluene Emilio J. González, Pablo Navarro, Marcos Larriba, Julián García ⇑, Francisco Rodríguez Department of Chemical Engineering, Complutense University of Madrid, E-28040 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 21 July 2015 Accepted 29 July 2015 Available online 3 August 2015 Keywords: Ionic liquids Ionic liquid/salt mixtures Aromatic/aliphatic separation VLE HS-GC
a b s t r a c t During the last years, a large number of studies have evaluated the ability of ionic liquids (ILs) to separate aromatic from aliphatic hydrocarbons by liquid extraction. Nevertheless, in order to design a global process, a post-extraction step based on the aromatic recovery from the extract stream and the regeneration of the IL is required. Taking into account the negligible vapor pressure of the ILs, the use of separation units based on the difference of volatility among the components of the extract could be an appropriate way. However, that requires additional (vapor + liquid) equilibrium (VLE) data, which are scarce today. In this work, the isothermal VLE data for {n-heptane + toluene + 1-ethyl-3-methylimidazolium thiocyanate ([EMim][SCN])} and {n-heptane + toluene + 1-butyl-3-methylimidazolium thiocyanate ([BMim][SCN])} mixtures were experimentally measured at T = (323.2, 343.2 and 363.2) K over the whole composition range within the rich-IL miscibility region. For that, a static headspace gas chromatograph (HS-GC) was used. In addition, the non-random two liquids (NRTL) thermodynamic model was satisfactory applied to correlate the experimental VLE data. Finally, the effect of thiocyanate-based inorganic salts (AgSCN, Co(SCN)2 and CuSCN) on the phase behavior of the above mentioned mixtures were also analyzed through the experimental determination of the isothermal VLE of the pseudo-ternary systems {n-heptane + toluene + [EMim][SCN]/salt mixture}. The obtained results show that the use of pure thiocyanate-based ILs as entrainer increases the n-heptane relative volatility from toluene whereas the addition of inorganic salts has not led to an improvement of these results. Ó 2015 Published by Elsevier Ltd.
1. Introduction One of the most studied applications of the ionic liquids (ILs) is their use as solvent in separation processes, especially in petrochemical fields. The main reason is related to their negligible vapor pressure that clearly facilitates their regeneration by eliminating of the volatile compounds. Numerous works can be found in the literature in which these ionic substances are proposed as solvents to separate different kind of hydrocarbons, ranging from simple aliphatic compounds to more complex substances such as sulfur or nitrogen compounds [1–10]. Most of them are based on the ability of the ILs to extract aromatic hydrocarbons, which is mainly evaluated from phase equilibrium studies like (liquid + liquid) equilibrium (LLE) of mixtures containing ILs and hydrocarbons. In this field, our research group has also published several works about the selective extraction of aromatic from aliphatic hydrocarbons using pure ILs and mixtures thereof as solvents [11–15]. The ⇑ Corresponding author. Tel.: +34 91 394 51 19; fax: +34 91 394 42 43. E-mail address: [email protected] (J. García). http://dx.doi.org/10.1016/j.jct.2015.07.041 0021-9614/Ó 2015 Published by Elsevier Ltd.
obtained results show that, in general, a higher aromatic/aliphatic selectivity values are obtained when sulfolane (one of the most used solvent to extract aromatic hydrocarbons from naphtha and pyrolysis or reformer gasoline) is replaced by ILs, which means that highly pure aromatics hydrocarbons are obtained. While it is also true that a greater amount of solvent is required because the solute distribution ratios are lower, it should not be understood as a problem because ILs could be recovered and reused in the same process. In order to establish a global separation process that involves not only extraction but also the purification of the extracted aromatics and regeneration of the IL, new researches in this field are necessary. Taking into account that ILs are known for their low vapor pressures, a post-extraction scheme that includes the separation of aromatics from ILs by evaporation of the corresponding hydrocarbons could be a promise alternative, as already suggested Anjan [16]. For that, experimental vapor liquid equilibrium (VLE) data are required. To date, a reduced number of articles were found in the literature in which the effect of the ILs on the separation factor in systems containing aliphatic and aromatic hydrocarbons was studied [17–22]. In these works, the suitability of several ILs as entrainer
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E.J. González et al. / J. Chem. Thermodynamics 91 (2015) 156–164 TABLE 1 CAS number, supplier, and purity of the pure components. Compound [EMim][SCN] [BMim][SCN] Heptane Toluene AgSCN CuSCN Co(SCN)2 a b c
CAS number 331717–63-6 334790–87-0 142-82-5 108-88-3 1701-93-5 1111-67-7 3017-80-5
Supplier Iolitec Iolitec Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich
Analysis method a
b
NMR , IC NMRa, ICb GCc GCc -
Purity, in mass fraction >0.98 >0.98 P0.995 P0.997 0.99 0.99 0.999
Nuclear magnetic resonance. Ion chromatography. Gas chromatography.
TABLE 2 Key parameters for Agilent GC 7890 A. Inlet Detector Carrier Gas Column Oven
T = 523.2 K, 100:1 split T = 573.2 K, FID He 3X, supplied by Praxair Agilent HP-5, 30 m 0.32 mm 0.25 lm T = 348.2 K
for several separation problems (such as aliphatic–aromatic, alkane–alkene and alcohol–water) was evaluated from isothermal VLE data by headspace gas chromatography (HS-GC), concluding that the investigated ILs show a high aliphatic relative volatility from aromatics. Since ILs, have a negligible vapor pressure, the HS-GC is a rapid, reliable, efficient and accurate technique for measuring VLE data of mixtures containing hydrocarbons and ILs.
FIGURE 1. VLE data for ternary system {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} at T = 323.2 K. y-x’ diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.95; }, x3 0.93; 4, x3 0.90; h, x3 0.85. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 323.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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As continuation of our previous work focused on the use of HS-GC to experimentally measure isothermal VLE data for systems containing cyano-based ILs [15], in this article, the VLE for {n-heptane + toluene + 1-ethyl-3-methylimidazolium thiocyanate ([EMim][SCN]) and {n-heptane + toluene + 1-butyl-3-methylimidazolium thiocyanate ([BMim][SCN]) mixtures was experimentally measured at T = (323.2, 343.2, and 363.2) K over the whole composition range within the rich-IL miscibility region. The experimental data were satisfactorily correlated using the non-random two liquids (NRTL) thermodynamic model [23]. The thiocyanate-based ILs were chosen due to their high toluene/n-heptane selectivity, which may become three times higher than the sulfolane values, as was previously reported by Larriba et al. [14]. On the other hand, the temperatures were selected because these values are intermediate between the thermal decomposition temperature of both [EMim][SCN] and [BMim][SCN] [24] and the common equilibrium temperature used in the LLE (313.2 K) [12,14]. The use of mixtures containing ILs and inorganic salts as potential separation agents has also emerged in the last years. In 2012, Li et al. [25] found that a selective extraction of 1-hexene/n-hexane
can be performed much better using ILs containing silver salts than pure ILs. More recently, in 2015, Bastos et al. [26] have demonstrated that increasing the ionicity of a pure IL, which can be achieved adding salts, led to a more efficient and selective separation of the azeotropic mixture of (n-heptane + ethanol). In order to confirm if these results can be also applied to the separation of aromatic/aliphatic hydrocarbons, in the second part of this work, the isothermal VLE data for {n-heptane + toluene + IL/salt mixture} systems were also measured using as entrainer a mixture constituted by [EMim][SCN] and thiocyanate-based inorganic salts such as AgSCN, Co(SCN)2 and CuSCN.
2. Materials and methods 2.1. Chemicals The ILs 1-ehyl-3-methylimidazolium thiocyanate, [EMim][SCN], and 1-butyl-3-methylimidazolium thiocyanate, [BMim][SCN], were supplied by Iolitec GmbH (Germany) with a halides content, in mass fraction, below than <2%, and a water content of (960 and
FIGURE 2. VLE data for ternary system {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} at T = 343.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.96; }, x3 0.93; 4, x3 0.90; h, x3 0.84. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 343.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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770) ppm, respectively. The hydrocarbons and the inorganic salts were purchased from Sigma–Aldrich. The CAS number, supplier, and purity of each chemicals used in this work are reported in table 1. All chemicals were used as received without any further treatment. In order to avoid water absorption, chemicals were stored in a desiccator in their original tightly closed bottles and the handling of the ILs was made in a glove box filled with dry nitrogen. 2.2. Apparatus and procedure In the present work, isothermal VLE data have been measured using a static method based on the use of a gas chromatograph (GC) attached to a headspace injector (HS). Specifically, an Agilent GC 7890A equipped with a flame ionization detector (FID) and an HP-5 Agilent column is coupled to an Agilent HS 7697A injector that uses a loop system to extract the vapor sample. Key parameters for Agilent GC 7890 A are included in table 2. This method was selected because both the viscosity of the ILs, which is higher than organic volatile compounds, and the low solubility of hydrocarbons in the studied ILs can hinder the mixing
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process, mandatory in a dynamic isobaric apparatus. Moreover, the static method used in this work considerably reduces the chemical consumption since small quantities of ILs and hydrocarbon are required. Miscible mixtures containing the pure ionic liquid (or [EMim][SCN]/salt mixtures) and the two hydrocarbons (total volume: 1 mL) were prepared by weighting, using a Mettler Toledo X205 balance with a precision of ±105 g. Firstly, a volume of IL (or IL/salt mixture) was introduced into 20.0 mL flat bottom vials, and then, the corresponding volumes of the mixtures {n-heptane + toluene}. It is important to comment that both ionic liquid/salt and {n-heptane + toluene} mixtures were separately prepared to minimize errors. After that, the vials were sealed using aluminum caps with PTFE septum, vigorously stirred using a Labnet Vortex Mixer, and inserted in the thermostatically controlled oven of the HS sampler at the equilibrium temperature. The samples have remained in the oven with agitation (100 rpm) during 2 h. This agitation speed and time were determined from preliminary studies and they are enough to ensure that equilibrium state is reached. After this time, a sample from the vapor phase was withdrawn
FIGURE 3. VLE data for ternary system {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} at T = 363.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.96; }, x3 0.94; 4, x3 0.91; h, x3 0.88. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 363.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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using the HS injector and analyzed by GC, following the same procedure previously published [15]. As ILs have a negligible vapor pressure, these compounds are not present in the vapor phase and only two peaks are observed for the ternary mixtures {n-heptane + toluene + ionic liquid} from the HS-GC analysis. As was commented in our previous paper [15], a GC response factor (RF) was determined to both hydrocarbons involved in the mixtures (n-heptane and toluene) to correct peak areas and calculate the composition of the vapor phase. The total pressure of the vapor phase (p) was calculated as the sum of partial pressures (pi) of both hydrocarbons, which were determined from the following equation [27]:
pi ¼
p0i Ai A0i
molar fraction for each component i of equilibrium liquid phase (xi) was calculated as follows:
xi ¼ P 3
zi F ðpi V G =R TÞ
i¼1 ðzi
F ðpi V G =R TÞÞ
ð2Þ
where zi denotes the mole fraction of the component i in the VLE feed (1 for n-heptane, 2 for toluene, and 3 for ionic liquid), F is the molar amount of the feed, and VG refers to the vapor volume of the vial. In this case, VG is 19.0 mL for all experiments carried out in this work. 3. Result and discussion
ð1Þ
where Ai is the peak area of the hydrocarbon i measured from the {n-heptane + toluene + IL} mixture, A0i is the peak area of the hydrocarbon i when the hydrocarbon alone is maintained at the measurement temperature, and p0i denotes the saturated vapor pressure of the hydrocarbons taken from the literature [28]. Then, the real
3.1. Experimental VLE data for {n-heptane + toluene + IL} mixtures In order to analyze the effect of both the temperature and the alkyl chain length of the imidazolium ring on the phase equilibrium, experimental VLE data for the {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} and {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} mixtures were measured at T = (323.2, 343.2,
FIGURE 4. VLE data for ternary system {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} at T = 323.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.98; }, x3 0.91; 4, x3 0.82; h, x3 0.67. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 323.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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and 363.2) K using the above described HS-GC method. The obtained results are plotted in figures 1–6, and also reported in tables S1 and S2, available as Supporting Information (SI). In these figures, the corresponding y–x0 and p–x0 y diagrams, in which x0 is the molar fraction of n-heptane in the liquid based on an ionic liquid free basis, are plotted for each mixture and temperature studied in this work. VLE data for {n-heptane (1) + toluene (2)} provided by Aspen Plus Simulator Software Database [33], whose data can be found in our previous paper [15], were also included in these figures for comparison purpose. From these figures, it is possible to observe that the presence of [EMim][SCN] or [BMim][SCN] results in a shift of the y–x0 curve towards lower x0 values. In other words, for a constant x0 value, a higher presence of n-heptane in the vapor phase is observed when the ILs are used as entrainer, which improves the separation of the hydrocarbons. This is observed for all the systems and temperatures studied in this work although it is less pronounced at high temperatures. This behavior can be explained from the aromatic nature of the imidazolium cation which leads to a higher ionic liquid/toluene interaction, encouraging the presence of toluene in the liquid phase. When temperature increases, this interaction is lower and
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a higher amount of toluene is presented in the vapor phase. Regarding the p–x0 y diagrams, the presence of ionic liquid in the mixture {n-heptane + toluene} causes a decrease of the total pressure (p), as was expected due to ILs are salts. Moreover, a greater separation of the p–x0 y curves for the liquid and vapor is also observed. From VLE data reported in tables S1 and S2, the relative volatility (a12) of n-heptane (1) from toluene (2) was calculated using the following expression:
a12 ¼
K 1 y1 =x1 ¼ K 2 y2 =x2
ð3Þ
where Ki is the (vapor + liquid) distribution ratio for each volatile compound. This parameter is commonly used to evaluate the performance of entrainers in (vapor + liquid) separations. In this case, the obtained a12 values, which are also included in tables S1 and S2, were used to evaluate both the effect of temperature and the alkyl chain length of the cation on the VLE. Since x1 values for the studied systems are very low, the values a12 values should be interpreted as a range because a small change in x1 leads to an important variation of a12. In any way, the a12 values obtained in this work are
FIGURE 5. VLE data for ternary system {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} at T = 343.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.98; }, x3 0.91; 4, x3 0.83; h, x3 0.69. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 343.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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far higher than those obtained for the same system without ILs, which means that both [EMim][SCN] and [BMim][SCN] could be considered as appropriate entrainers. In order to analyze the effect of the temperature and the structure of the ILs (alkyl chain length of the cation and anion nature) on the VLE, the a1,2 values obtained for the ternary mixtures {n-heptane (1) + toluene (2) + [EMim][SCN], or [BMim][SCN] (3)} at T = 363.2 K were plotted in figure 7 together with those a1,2 values previously published for {n-heptane (1) + toluene (2) + [EMim][DCA] (3)} [15]. For both ILs, a decrease in the value of n-heptane relative volatility from toluene is observed when temperature increases. In addition, the same effect is caused by an increase in the alkyl chain length of the cation (using a butyl instead of an ethyl group). The obtained results also showed that [EMim][SCN] improves the efficiency obtained with [EMim][DCA], while the best results were obtained using high concentrations of [EMim][SCN]. This fact confirms ILs with high aromatic/aliphatic selectivity, as is the case here, are a good alternative to carry out a selective (vapor + liquid) separation of those hydrocarbons previously obtained by (liquid + liquid) extraction. Finally, the well known NRTL thermodynamic model [23] was employed to describe the experimental VLE data. The objective
function (OF) used to minimize the global deviation was defined as:
OF ¼
a
PI PI i¼1 xi;calc xi;exptl þ i¼1 pi;calc pi;exptl N
ð4Þ
where a is the weighting coefficient of mole fraction deviations for minimizing the deviation of the adjustment and N is the number of VLE points used in the adjustment. The best adjustment was achieved for a = 300, balancing the difference of magnitude between x and p. The experimental data were correlated using the Solver tool in the Microsoft Excel spreadsheet software. For the ternary mixtures studied in this work, the value of the nonrandomness parameter, a, in the NRTL model was set to 0.3, which is a value commonly used in the literature [20,21,29–32]. The interaction parameters are listed in table 3, jointly with the deviations of the compositions, Dx, and the pressures, Dp, calculated as:
Dx ¼
Dp ¼
PI xi;calc xi;exptl i¼1
N
;
PI pi;calc pi;exptl i¼1
N
:
ð5Þ
ð6Þ
FIGURE 6. VLE data for ternary system {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} at T = 363.2 K. y-x’ diagram (a) and px0 ,y diagram (b). IL mole fraction (x3): s, x3 0.98; }, x3 0.92; 4, x3 0.84; h, x3 0.70. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 363.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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FIGURE 7. Values of n-heptane relative volatility from toluene (a1,2) under the presence of several ILs in {n-heptane (1) + toluene (2) + IL (3)}: s, [EMim][SCN] with x3 0.96; h, [BMim][SCN] with x3 0.98; 4, [EMim][DCA] with x3 0.98 from reference [15] Full symbols refer to a1,2 values at T = 323.2 K, whereas empty symbols represent those achieve at 363.2 K.
be also obtained from the analysis of figures 1–6, where the NRTL adjustments are included together with the VLE data experimentally obtained.
TABLE 3 NRTL parametersa from the adjustment of VLE of {n-heptane + toluene + [EMim][SCN]} and {n-heptane + toluene + [BMim][SCN]} systems. i–j
a
Dgij/J mol1
Dgji/J mol1
Dx
1–2 1–3 2–3
{n-heptane (1) + toluene (2) + [EMim][SCN] (3)} 1203.6 1310.8 0.005 6107.2 3634.2 5016.8 406.03
1–2 1–3 2–3
{n-heptane (1) + toluene (2) + [BMim][SCN] (3)} 7899.4 8744.7 0.005 3179.2 10616 3159.3 3025.9
Dp/kPa
3.2. Experimental VLE data for {n-heptane + toluene + [EMim][SCN]/ inorganic salt} mixtures
0.2
As second part of this work, several [EMim][SCN]/salt mixtures were tested as entrainers. For that, a binary mixture {n-heptane, 5 mass% + toluene, 95 mass%} and several IL/salt mixtures containing AgSCN, Co(SCN)2 and CuSCN (0.45 m in salt concentration) were prepared by weighting. A salt concentration value of 0.45 m was selected to guarantee a complete dissolution of the salts in the IL. Then, specific volumes of each mixture were mixed to obtain ternary mixtures {n-heptane + toluene + ionic liquid/salt} whose (vapor + liquid) equilibria were measured at T = 323 K. The a12 values obtained using inorganic salts are shown in figure 8 together with that obtained using pure [EMim][SCN] at the same conditions (second row of the table S1).
0.4
a12, a13, and a23 were set in 0.3 for all systems.
From the deviation values reported in table 3, it is possible to conclude that the NRTL model satisfactorily correlates the experimental VLE data obtained in this work. The same conclusion can
[EMim][SCN] + CuSCN 0,45 m
107.6
[EMim][SCN] + Co(SCN)2 0,45 m
83.3
[EMim][SCN]+ AgSCN 0,45 m
107.3
[EMim][SCN]
134.5
0
20
40
60
80
100
120
140
160
α2,1 FIGURE 8. Relative volatility (a12) values from n-heptane/toluene mixtures with 5% of n-heptane, in mass basis at T = 323.2 K. Salt concentration in [EMim][SCN] was 0.45 m in all cases.
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From figure 8, it is possible to conclude that the presence of inorganic salts with thiocyanate anion does not improve the values of n-heptane relative volatilities from toluene achieved using pure [EMim][SCN]. A similar conclusion was also obtained by Larriba et al. [33] studying the (liquid + liquid) extraction of these hydrocarbons using the same ionic liquid and its mixtures with several transition metal salts. Both works suggest that the use of IL/salt mixtures is not the best strategy to separate aliphatic and aromatic compounds. Nevertheless, it is important to comment that these results are different to those obtained by Li et al. [25] or Bastos et al. [26] who have demonstrated that the use of ionic liquid/salts mixtures favors the extraction of 1-hexene/n-hexane and leads to a more efficient and selective separation of the azeotropic mixture (n-heptane + ethanol). Taking into account all these results, everything seems to indicate that the interaction of an IL/salt mixture with other compounds is complex and their effect on a separation process is directly related to the nature of them. In order to get a better understanding of this behavior, new theoretical and experimental works concerning the use of ionic liquid/salt systems are required. 4. Conclusions In this work, a static headspace gas chromatography (HS-GC) technique was used to measure VLE data for the mixtures {n-heptane + toluene + [EMim][SCN], or [BMim][SCN] at T = (323.2, 343.2, and 363.2) K. In all cases, the presence of an ionic liquid has resulted in a considerable enhancement of the relative volatility of n-heptane from toluene. A comparison of these results with those previously published using [EMim][DCA] confirm that ILs containing SCN anion are more selective than dicyanamide-based ILs and therefore, thiocyanate-based ILs improve the efficiency of the (vapor + liquid) separation of hydrocarbons. The experimental VLE data were satisfactory model using the non-random two liquids (NRTL) thermodynamic model. Finally, several [EMim][SCN]/salt mixtures were tested as entrainer in order to evaluate the effect of the presence of inorganic salts on the phase behavior. The obtained results show that [EMim][SCN]/ thiocyanate-based salt mixtures does not get improve the high relative volatility of n-heptane from toluene achieved with pure ILs.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2015.07.041. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Acknowledgments The authors are grateful to Ministerio de Economía y Competitividad (MINECO) of Spain and Comunidad Autónoma de Madrid for financial support of Projects CTQ2011-23533 and S2013/MAE-2800, respectively. Emilio J. González and Pablo Navarro thank MINECO for their Juan de la Cierva Contract (Reference JCI-2012-12005) and FPI grant (Reference BES-2012-052312), respectively. Marcos Larriba thanks Ministerio de Educación, Cultura y Deporte for awarding him an FPU grant (Reference AP-2010-0318).
[29] [30] [31] [32] [33]
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JCT 15-381
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Use of selective ionic liquids and ionic liquid/salt mixtures as entrainer in a (vapor + liquid) system to separate n-heptane from toluene Emilio J. González, Pablo Navarro, Marcos Larriba, Julián García ⇑, Francisco Rodríguez Department of Chemical Engineering, Complutense University of Madrid, E-28040 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 21 July 2015 Accepted 29 July 2015 Available online 3 August 2015 Keywords: Ionic liquids Ionic liquid/salt mixtures Aromatic/aliphatic separation VLE HS-GC
a b s t r a c t During the last years, a large number of studies have evaluated the ability of ionic liquids (ILs) to separate aromatic from aliphatic hydrocarbons by liquid extraction. Nevertheless, in order to design a global process, a post-extraction step based on the aromatic recovery from the extract stream and the regeneration of the IL is required. Taking into account the negligible vapor pressure of the ILs, the use of separation units based on the difference of volatility among the components of the extract could be an appropriate way. However, that requires additional (vapor + liquid) equilibrium (VLE) data, which are scarce today. In this work, the isothermal VLE data for {n-heptane + toluene + 1-ethyl-3-methylimidazolium thiocyanate ([EMim][SCN])} and {n-heptane + toluene + 1-butyl-3-methylimidazolium thiocyanate ([BMim][SCN])} mixtures were experimentally measured at T = (323.2, 343.2 and 363.2) K over the whole composition range within the rich-IL miscibility region. For that, a static headspace gas chromatograph (HS-GC) was used. In addition, the non-random two liquids (NRTL) thermodynamic model was satisfactory applied to correlate the experimental VLE data. Finally, the effect of thiocyanate-based inorganic salts (AgSCN, Co(SCN)2 and CuSCN) on the phase behavior of the above mentioned mixtures were also analyzed through the experimental determination of the isothermal VLE of the pseudo-ternary systems {n-heptane + toluene + [EMim][SCN]/salt mixture}. The obtained results show that the use of pure thiocyanate-based ILs as entrainer increases the n-heptane relative volatility from toluene whereas the addition of inorganic salts has not led to an improvement of these results. Ó 2015 Published by Elsevier Ltd.
1. Introduction One of the most studied applications of the ionic liquids (ILs) is their use as solvent in separation processes, especially in petrochemical fields. The main reason is related to their negligible vapor pressure that clearly facilitates their regeneration by eliminating of the volatile compounds. Numerous works can be found in the literature in which these ionic substances are proposed as solvents to separate different kind of hydrocarbons, ranging from simple aliphatic compounds to more complex substances such as sulfur or nitrogen compounds [1–10]. Most of them are based on the ability of the ILs to extract aromatic hydrocarbons, which is mainly evaluated from phase equilibrium studies like (liquid + liquid) equilibrium (LLE) of mixtures containing ILs and hydrocarbons. In this field, our research group has also published several works about the selective extraction of aromatic from aliphatic hydrocarbons using pure ILs and mixtures thereof as solvents [11–15]. The ⇑ Corresponding author. Tel.: +34 91 394 51 19; fax: +34 91 394 42 43. E-mail address: [email protected] (J. García). http://dx.doi.org/10.1016/j.jct.2015.07.041 0021-9614/Ó 2015 Published by Elsevier Ltd.
obtained results show that, in general, a higher aromatic/aliphatic selectivity values are obtained when sulfolane (one of the most used solvent to extract aromatic hydrocarbons from naphtha and pyrolysis or reformer gasoline) is replaced by ILs, which means that highly pure aromatics hydrocarbons are obtained. While it is also true that a greater amount of solvent is required because the solute distribution ratios are lower, it should not be understood as a problem because ILs could be recovered and reused in the same process. In order to establish a global separation process that involves not only extraction but also the purification of the extracted aromatics and regeneration of the IL, new researches in this field are necessary. Taking into account that ILs are known for their low vapor pressures, a post-extraction scheme that includes the separation of aromatics from ILs by evaporation of the corresponding hydrocarbons could be a promise alternative, as already suggested Anjan [16]. For that, experimental vapor liquid equilibrium (VLE) data are required. To date, a reduced number of articles were found in the literature in which the effect of the ILs on the separation factor in systems containing aliphatic and aromatic hydrocarbons was studied [17–22]. In these works, the suitability of several ILs as entrainer
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E.J. González et al. / J. Chem. Thermodynamics 91 (2015) 156–164 TABLE 1 CAS number, supplier, and purity of the pure components. Compound [EMim][SCN] [BMim][SCN] Heptane Toluene AgSCN CuSCN Co(SCN)2 a b c
CAS number 331717–63-6 334790–87-0 142-82-5 108-88-3 1701-93-5 1111-67-7 3017-80-5
Supplier Iolitec Iolitec Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich
Analysis method a
b
NMR , IC NMRa, ICb GCc GCc -
Purity, in mass fraction >0.98 >0.98 P0.995 P0.997 0.99 0.99 0.999
Nuclear magnetic resonance. Ion chromatography. Gas chromatography.
TABLE 2 Key parameters for Agilent GC 7890 A. Inlet Detector Carrier Gas Column Oven
T = 523.2 K, 100:1 split T = 573.2 K, FID He 3X, supplied by Praxair Agilent HP-5, 30 m 0.32 mm 0.25 lm T = 348.2 K
for several separation problems (such as aliphatic–aromatic, alkane–alkene and alcohol–water) was evaluated from isothermal VLE data by headspace gas chromatography (HS-GC), concluding that the investigated ILs show a high aliphatic relative volatility from aromatics. Since ILs, have a negligible vapor pressure, the HS-GC is a rapid, reliable, efficient and accurate technique for measuring VLE data of mixtures containing hydrocarbons and ILs.
FIGURE 1. VLE data for ternary system {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} at T = 323.2 K. y-x’ diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.95; }, x3 0.93; 4, x3 0.90; h, x3 0.85. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 323.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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As continuation of our previous work focused on the use of HS-GC to experimentally measure isothermal VLE data for systems containing cyano-based ILs [15], in this article, the VLE for {n-heptane + toluene + 1-ethyl-3-methylimidazolium thiocyanate ([EMim][SCN]) and {n-heptane + toluene + 1-butyl-3-methylimidazolium thiocyanate ([BMim][SCN]) mixtures was experimentally measured at T = (323.2, 343.2, and 363.2) K over the whole composition range within the rich-IL miscibility region. The experimental data were satisfactorily correlated using the non-random two liquids (NRTL) thermodynamic model [23]. The thiocyanate-based ILs were chosen due to their high toluene/n-heptane selectivity, which may become three times higher than the sulfolane values, as was previously reported by Larriba et al. [14]. On the other hand, the temperatures were selected because these values are intermediate between the thermal decomposition temperature of both [EMim][SCN] and [BMim][SCN] [24] and the common equilibrium temperature used in the LLE (313.2 K) [12,14]. The use of mixtures containing ILs and inorganic salts as potential separation agents has also emerged in the last years. In 2012, Li et al. [25] found that a selective extraction of 1-hexene/n-hexane
can be performed much better using ILs containing silver salts than pure ILs. More recently, in 2015, Bastos et al. [26] have demonstrated that increasing the ionicity of a pure IL, which can be achieved adding salts, led to a more efficient and selective separation of the azeotropic mixture of (n-heptane + ethanol). In order to confirm if these results can be also applied to the separation of aromatic/aliphatic hydrocarbons, in the second part of this work, the isothermal VLE data for {n-heptane + toluene + IL/salt mixture} systems were also measured using as entrainer a mixture constituted by [EMim][SCN] and thiocyanate-based inorganic salts such as AgSCN, Co(SCN)2 and CuSCN.
2. Materials and methods 2.1. Chemicals The ILs 1-ehyl-3-methylimidazolium thiocyanate, [EMim][SCN], and 1-butyl-3-methylimidazolium thiocyanate, [BMim][SCN], were supplied by Iolitec GmbH (Germany) with a halides content, in mass fraction, below than <2%, and a water content of (960 and
FIGURE 2. VLE data for ternary system {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} at T = 343.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.96; }, x3 0.93; 4, x3 0.90; h, x3 0.84. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 343.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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770) ppm, respectively. The hydrocarbons and the inorganic salts were purchased from Sigma–Aldrich. The CAS number, supplier, and purity of each chemicals used in this work are reported in table 1. All chemicals were used as received without any further treatment. In order to avoid water absorption, chemicals were stored in a desiccator in their original tightly closed bottles and the handling of the ILs was made in a glove box filled with dry nitrogen. 2.2. Apparatus and procedure In the present work, isothermal VLE data have been measured using a static method based on the use of a gas chromatograph (GC) attached to a headspace injector (HS). Specifically, an Agilent GC 7890A equipped with a flame ionization detector (FID) and an HP-5 Agilent column is coupled to an Agilent HS 7697A injector that uses a loop system to extract the vapor sample. Key parameters for Agilent GC 7890 A are included in table 2. This method was selected because both the viscosity of the ILs, which is higher than organic volatile compounds, and the low solubility of hydrocarbons in the studied ILs can hinder the mixing
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process, mandatory in a dynamic isobaric apparatus. Moreover, the static method used in this work considerably reduces the chemical consumption since small quantities of ILs and hydrocarbon are required. Miscible mixtures containing the pure ionic liquid (or [EMim][SCN]/salt mixtures) and the two hydrocarbons (total volume: 1 mL) were prepared by weighting, using a Mettler Toledo X205 balance with a precision of ±105 g. Firstly, a volume of IL (or IL/salt mixture) was introduced into 20.0 mL flat bottom vials, and then, the corresponding volumes of the mixtures {n-heptane + toluene}. It is important to comment that both ionic liquid/salt and {n-heptane + toluene} mixtures were separately prepared to minimize errors. After that, the vials were sealed using aluminum caps with PTFE septum, vigorously stirred using a Labnet Vortex Mixer, and inserted in the thermostatically controlled oven of the HS sampler at the equilibrium temperature. The samples have remained in the oven with agitation (100 rpm) during 2 h. This agitation speed and time were determined from preliminary studies and they are enough to ensure that equilibrium state is reached. After this time, a sample from the vapor phase was withdrawn
FIGURE 3. VLE data for ternary system {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} at T = 363.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.96; }, x3 0.94; 4, x3 0.91; h, x3 0.88. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 363.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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using the HS injector and analyzed by GC, following the same procedure previously published [15]. As ILs have a negligible vapor pressure, these compounds are not present in the vapor phase and only two peaks are observed for the ternary mixtures {n-heptane + toluene + ionic liquid} from the HS-GC analysis. As was commented in our previous paper [15], a GC response factor (RF) was determined to both hydrocarbons involved in the mixtures (n-heptane and toluene) to correct peak areas and calculate the composition of the vapor phase. The total pressure of the vapor phase (p) was calculated as the sum of partial pressures (pi) of both hydrocarbons, which were determined from the following equation [27]:
pi ¼
p0i Ai A0i
molar fraction for each component i of equilibrium liquid phase (xi) was calculated as follows:
xi ¼ P 3
zi F ðpi V G =R TÞ
i¼1 ðzi
F ðpi V G =R TÞÞ
ð2Þ
where zi denotes the mole fraction of the component i in the VLE feed (1 for n-heptane, 2 for toluene, and 3 for ionic liquid), F is the molar amount of the feed, and VG refers to the vapor volume of the vial. In this case, VG is 19.0 mL for all experiments carried out in this work. 3. Result and discussion
ð1Þ
where Ai is the peak area of the hydrocarbon i measured from the {n-heptane + toluene + IL} mixture, A0i is the peak area of the hydrocarbon i when the hydrocarbon alone is maintained at the measurement temperature, and p0i denotes the saturated vapor pressure of the hydrocarbons taken from the literature [28]. Then, the real
3.1. Experimental VLE data for {n-heptane + toluene + IL} mixtures In order to analyze the effect of both the temperature and the alkyl chain length of the imidazolium ring on the phase equilibrium, experimental VLE data for the {n-heptane (1) + toluene (2) + [EMim][SCN] (3)} and {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} mixtures were measured at T = (323.2, 343.2,
FIGURE 4. VLE data for ternary system {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} at T = 323.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.98; }, x3 0.91; 4, x3 0.82; h, x3 0.67. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 323.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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and 363.2) K using the above described HS-GC method. The obtained results are plotted in figures 1–6, and also reported in tables S1 and S2, available as Supporting Information (SI). In these figures, the corresponding y–x0 and p–x0 y diagrams, in which x0 is the molar fraction of n-heptane in the liquid based on an ionic liquid free basis, are plotted for each mixture and temperature studied in this work. VLE data for {n-heptane (1) + toluene (2)} provided by Aspen Plus Simulator Software Database [33], whose data can be found in our previous paper [15], were also included in these figures for comparison purpose. From these figures, it is possible to observe that the presence of [EMim][SCN] or [BMim][SCN] results in a shift of the y–x0 curve towards lower x0 values. In other words, for a constant x0 value, a higher presence of n-heptane in the vapor phase is observed when the ILs are used as entrainer, which improves the separation of the hydrocarbons. This is observed for all the systems and temperatures studied in this work although it is less pronounced at high temperatures. This behavior can be explained from the aromatic nature of the imidazolium cation which leads to a higher ionic liquid/toluene interaction, encouraging the presence of toluene in the liquid phase. When temperature increases, this interaction is lower and
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a higher amount of toluene is presented in the vapor phase. Regarding the p–x0 y diagrams, the presence of ionic liquid in the mixture {n-heptane + toluene} causes a decrease of the total pressure (p), as was expected due to ILs are salts. Moreover, a greater separation of the p–x0 y curves for the liquid and vapor is also observed. From VLE data reported in tables S1 and S2, the relative volatility (a12) of n-heptane (1) from toluene (2) was calculated using the following expression:
a12 ¼
K 1 y1 =x1 ¼ K 2 y2 =x2
ð3Þ
where Ki is the (vapor + liquid) distribution ratio for each volatile compound. This parameter is commonly used to evaluate the performance of entrainers in (vapor + liquid) separations. In this case, the obtained a12 values, which are also included in tables S1 and S2, were used to evaluate both the effect of temperature and the alkyl chain length of the cation on the VLE. Since x1 values for the studied systems are very low, the values a12 values should be interpreted as a range because a small change in x1 leads to an important variation of a12. In any way, the a12 values obtained in this work are
FIGURE 5. VLE data for ternary system {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} at T = 343.2 K. y–x0 diagram (a) and p–x0 ,y diagram (b). IL mole fraction (x3): s, x3 0.98; }, x3 0.91; 4, x3 0.83; h, x3 0.69. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 343.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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far higher than those obtained for the same system without ILs, which means that both [EMim][SCN] and [BMim][SCN] could be considered as appropriate entrainers. In order to analyze the effect of the temperature and the structure of the ILs (alkyl chain length of the cation and anion nature) on the VLE, the a1,2 values obtained for the ternary mixtures {n-heptane (1) + toluene (2) + [EMim][SCN], or [BMim][SCN] (3)} at T = 363.2 K were plotted in figure 7 together with those a1,2 values previously published for {n-heptane (1) + toluene (2) + [EMim][DCA] (3)} [15]. For both ILs, a decrease in the value of n-heptane relative volatility from toluene is observed when temperature increases. In addition, the same effect is caused by an increase in the alkyl chain length of the cation (using a butyl instead of an ethyl group). The obtained results also showed that [EMim][SCN] improves the efficiency obtained with [EMim][DCA], while the best results were obtained using high concentrations of [EMim][SCN]. This fact confirms ILs with high aromatic/aliphatic selectivity, as is the case here, are a good alternative to carry out a selective (vapor + liquid) separation of those hydrocarbons previously obtained by (liquid + liquid) extraction. Finally, the well known NRTL thermodynamic model [23] was employed to describe the experimental VLE data. The objective
function (OF) used to minimize the global deviation was defined as:
OF ¼
a
PI PI i¼1 xi;calc xi;exptl þ i¼1 pi;calc pi;exptl N
ð4Þ
where a is the weighting coefficient of mole fraction deviations for minimizing the deviation of the adjustment and N is the number of VLE points used in the adjustment. The best adjustment was achieved for a = 300, balancing the difference of magnitude between x and p. The experimental data were correlated using the Solver tool in the Microsoft Excel spreadsheet software. For the ternary mixtures studied in this work, the value of the nonrandomness parameter, a, in the NRTL model was set to 0.3, which is a value commonly used in the literature [20,21,29–32]. The interaction parameters are listed in table 3, jointly with the deviations of the compositions, Dx, and the pressures, Dp, calculated as:
Dx ¼
Dp ¼
PI xi;calc xi;exptl i¼1
N
;
PI pi;calc pi;exptl i¼1
N
:
ð5Þ
ð6Þ
FIGURE 6. VLE data for ternary system {n-heptane (1) + toluene (2) + [BMim][SCN] (3)} at T = 363.2 K. y-x’ diagram (a) and px0 ,y diagram (b). IL mole fraction (x3): s, x3 0.98; }, x3 0.92; 4, x3 0.84; h, x3 0.70. Solid lines denote the NRTL adjustment and dashed lines refer to VLE data from Aspen Plus Simulator Software Database at T = 363.2 K for the binary system of {n-heptane (1) + toluene (2)}.
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FIGURE 7. Values of n-heptane relative volatility from toluene (a1,2) under the presence of several ILs in {n-heptane (1) + toluene (2) + IL (3)}: s, [EMim][SCN] with x3 0.96; h, [BMim][SCN] with x3 0.98; 4, [EMim][DCA] with x3 0.98 from reference [15] Full symbols refer to a1,2 values at T = 323.2 K, whereas empty symbols represent those achieve at 363.2 K.
be also obtained from the analysis of figures 1–6, where the NRTL adjustments are included together with the VLE data experimentally obtained.
TABLE 3 NRTL parametersa from the adjustment of VLE of {n-heptane + toluene + [EMim][SCN]} and {n-heptane + toluene + [BMim][SCN]} systems. i–j
a
Dgij/J mol1
Dgji/J mol1
Dx
1–2 1–3 2–3
{n-heptane (1) + toluene (2) + [EMim][SCN] (3)} 1203.6 1310.8 0.005 6107.2 3634.2 5016.8 406.03
1–2 1–3 2–3
{n-heptane (1) + toluene (2) + [BMim][SCN] (3)} 7899.4 8744.7 0.005 3179.2 10616 3159.3 3025.9
Dp/kPa
3.2. Experimental VLE data for {n-heptane + toluene + [EMim][SCN]/ inorganic salt} mixtures
0.2
As second part of this work, several [EMim][SCN]/salt mixtures were tested as entrainers. For that, a binary mixture {n-heptane, 5 mass% + toluene, 95 mass%} and several IL/salt mixtures containing AgSCN, Co(SCN)2 and CuSCN (0.45 m in salt concentration) were prepared by weighting. A salt concentration value of 0.45 m was selected to guarantee a complete dissolution of the salts in the IL. Then, specific volumes of each mixture were mixed to obtain ternary mixtures {n-heptane + toluene + ionic liquid/salt} whose (vapor + liquid) equilibria were measured at T = 323 K. The a12 values obtained using inorganic salts are shown in figure 8 together with that obtained using pure [EMim][SCN] at the same conditions (second row of the table S1).
0.4
a12, a13, and a23 were set in 0.3 for all systems.
From the deviation values reported in table 3, it is possible to conclude that the NRTL model satisfactorily correlates the experimental VLE data obtained in this work. The same conclusion can
[EMim][SCN] + CuSCN 0,45 m
107.6
[EMim][SCN] + Co(SCN)2 0,45 m
83.3
[EMim][SCN]+ AgSCN 0,45 m
107.3
[EMim][SCN]
134.5
0
20
40
60
80
100
120
140
160
α2,1 FIGURE 8. Relative volatility (a12) values from n-heptane/toluene mixtures with 5% of n-heptane, in mass basis at T = 323.2 K. Salt concentration in [EMim][SCN] was 0.45 m in all cases.
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From figure 8, it is possible to conclude that the presence of inorganic salts with thiocyanate anion does not improve the values of n-heptane relative volatilities from toluene achieved using pure [EMim][SCN]. A similar conclusion was also obtained by Larriba et al. [33] studying the (liquid + liquid) extraction of these hydrocarbons using the same ionic liquid and its mixtures with several transition metal salts. Both works suggest that the use of IL/salt mixtures is not the best strategy to separate aliphatic and aromatic compounds. Nevertheless, it is important to comment that these results are different to those obtained by Li et al. [25] or Bastos et al. [26] who have demonstrated that the use of ionic liquid/salts mixtures favors the extraction of 1-hexene/n-hexane and leads to a more efficient and selective separation of the azeotropic mixture (n-heptane + ethanol). Taking into account all these results, everything seems to indicate that the interaction of an IL/salt mixture with other compounds is complex and their effect on a separation process is directly related to the nature of them. In order to get a better understanding of this behavior, new theoretical and experimental works concerning the use of ionic liquid/salt systems are required. 4. Conclusions In this work, a static headspace gas chromatography (HS-GC) technique was used to measure VLE data for the mixtures {n-heptane + toluene + [EMim][SCN], or [BMim][SCN] at T = (323.2, 343.2, and 363.2) K. In all cases, the presence of an ionic liquid has resulted in a considerable enhancement of the relative volatility of n-heptane from toluene. A comparison of these results with those previously published using [EMim][DCA] confirm that ILs containing SCN anion are more selective than dicyanamide-based ILs and therefore, thiocyanate-based ILs improve the efficiency of the (vapor + liquid) separation of hydrocarbons. The experimental VLE data were satisfactory model using the non-random two liquids (NRTL) thermodynamic model. Finally, several [EMim][SCN]/salt mixtures were tested as entrainer in order to evaluate the effect of the presence of inorganic salts on the phase behavior. The obtained results show that [EMim][SCN]/ thiocyanate-based salt mixtures does not get improve the high relative volatility of n-heptane from toluene achieved with pure ILs.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2015.07.041. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Acknowledgments The authors are grateful to Ministerio de Economía y Competitividad (MINECO) of Spain and Comunidad Autónoma de Madrid for financial support of Projects CTQ2011-23533 and S2013/MAE-2800, respectively. Emilio J. González and Pablo Navarro thank MINECO for their Juan de la Cierva Contract (Reference JCI-2012-12005) and FPI grant (Reference BES-2012-052312), respectively. Marcos Larriba thanks Ministerio de Educación, Cultura y Deporte for awarding him an FPU grant (Reference AP-2010-0318).
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JCT 15-381