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Separation of propylbenzene and n-alkanes from their mixtures using 4-methyl-N-butylpyridinium tetrafluoroborate as an ionic solvent at several temperatures
Fluid Phase Equilibria 309 (2011) 102–107
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Separation of propylbenzene and n-alkanes from their mixtures using 4-methyl-N-butylpyridinium tetrafluoroborate as an ionic solvent at several temperatures Khaled H.A.E. Alkhaldi, Mohammad S. Al-Tuwaim, Mohamed S. Fandary, Adel S. Al-Jimaz ∗ Chemical Engineering Department, College of Technological Studies, PAAET, P.O. Box 3242, Salmyiah 22033, Kuwait
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 22 June 2011 Accepted 29 June 2011 Available online 6 July 2011 Keywords: LLE Ionic solvent Aromatics Alkanes UNIQUAC NRTL
a b s t r a c t Liquid–liquid extraction is the most common method for separation of aromatics from their mixtures with n-alkanes hydrocarbons. An ionic liquid (IL), 4-methyl-N-butylpyridinium tetrafluoroborate [(mebupy)(BF4 )], was evaluated as solvent for this separation. Liquid equilibria (LLE) for 2 ternary systems comprising tetradecane, or hexadecane + propylbenzene + [(mebupy)(BF4 )] were measured over a temperature range of 313–333 K and atmospheric pressure. The reliability of the experimental data was evaluated using the Othmer–Tobias correlation. The effect of temperature, n-alkane chain length and solvent to feed ratio upon solubility, selectivity, and distribution coefficient were investigated experimentally. In addition, the experimental results were regressed to estimate the interaction parameters between each of the 3 pairs of components for the UNIQUAC and the NRTL models as a function of temperature. Both models satisfactorily correlate the experimental data, however the UNIQUAC fit was slightly better than that obtained with the NRTL model. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The separation of aromatic and aliphatic compounds from their mixtures is an important goal in chemical operations that produce both types of compounds. On the other hand, smoke point of kerosene, cetane index of diesel, and viscosity index of lubricating oil can be improved by removing aromatic hydrocarbons. Ternary phase equilibrium data are essential to the proper understanding of the solvent extraction processes. Over the past few years, research about ionic liquids (ILs) has increased greatly, mainly in 2 directions: as reaction media and as a solvent for separation processes. Ionic liquids properties, such as a negligible vapour pressure and stability at high temperatures allow them to substitute classic organic solvents with improving performance and less damage to the environment [1–3]. The knowledge of the liquid–liquid equilibria (LLE) for the ternary systems (aliphatic hydrocarbons + aromatic hydrocarbons + ILs) is essential to evaluate the feasibility of using ILs as extractive solvents in the separation of aromatic and aliphatic hydrocarbons [4–16]. The 4-methyl-N-butylpyridinium tetrafluoroborate [(mebupy)(BF4 )] has been one of the few ILs that has shown both higher selectivity and extractive capacity than those of sulfolane
for the extraction of aromatic hydrocarbons [1]. Nevertheless, LLE data are still quite scarce for systems containing ionic liquids. Currently, there are only a few experimental liquid–liquid equilibrium data have been published for [(mebupy)(BF4 )] + aromatic + naliphatic (C5 –C9 ) and hardly any for systems containing carbon number greater than nine for the aliphatic and/or aromatics [1,3,17–19]. This article is a continuation of our study on the liquid–liquid phase equilibria for dearomatization of Kuwait middle distilled fraction [20,21]. Our interest in IL is focused on providing experimental LLE data. The purpose of this work is to study LLE of 2 ternary systems; system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)}, and system-II {hexadecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)}. The LLE data for the studied ternary systems were measured at 313, 323, and 333 K and atmospheric pressure. The reliability of the experimentally measured tie line data was ascertained by the Othmer–Tobias correlation [22], and the distribution coefficient as well as the selectivity was calculated from these data. Finally, the UNIQUAC and the NRTL models [23,24] were used to correlate our experimental data. 2. Experimental 2.1. Chemicals
∗ Corresponding author. E-mail address: a [email protected] (A.S. Al-Jimaz). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.06.036
The [(mebupy)(BF4 )] and propylbenzene were stored under 4 nm molecular sieve. The purities of the n-alkanes and propyl-
K.H.A.E. Alkhaldi et al. / Fluid Phase Equilibria 309 (2011) 102–107
103
Table 1 Details of the chemicals, purities (mass%), UNIQUAC structural parameters, and refractive indicies (nD ) at 20 ◦ C and atmospheric pressure. Compound
Supplier
Purity (mass%)
[(Mebupy)(BF4 )] Propylbenzene Tetradecane Hexadecane
Merck Fluka Aldrich Aldrich
>97 >99 >99 >99
a b c
UNIQUAC structural parameter
nD 20
r
q
Exp
Lit
8.4337 5.4983 9.8950 11.2438
6.5290 4.3560 8.1759 9.2560
1.4509c 1.4915 1.4289 1.4291
1.4517a , c 1.4920b 1.4290b 1.4290b
Ref. [28]. Ref. [29]. nD at 25 ◦ C.
benzene were determined by gas chromatography. All chemicals were used without further purification. The purities and refractive indices of all chemicals used in this study are presented in Table 1. 2.2. Apparatus and procedure The experimental apparatus used for extraction consists of a 60 mL glass cells with a water jacket in order to maintain a constant temperature. The temperature was controlled with an uncertainty of ±0.2 K. The cells were connected to a Haake K15 water bath fitted with a Haake DC1 thermostat. Mixtures, comprising of 20 g of [(mebupy)(BF4 )], 20 g of n-alkanes, and different amounts of propylbenzene were placed in the extraction vessels. The mixtures were vigorously stirred for 1 h, and then left to settle for 4 h. A series of LLE measurements over a temperature range of 313–333 K were performed. 2.3. Measurements of phase compositions Samples were carefully taken by a syringe from the lower and upper layers. Each sample was dissolved in 0.5 mL 1-butanol, to avoid splitting and maintain a homogeneous mixture, and analyzed using a Varian 450 gas chromatography equipped with an autosampler (Varian CP-8400), an on-column injecter, flame ionization detector (FID), and a data processoring system. The column used was Varian VF-5 ms CP8944 (30 m length and 0.25 mm I.D., 0.25 m film thickness). The ionic solvent [(mebupy)(BF4 )], has negligible vapor pressure it cannot be analyzed by GC. In the ternary mixture, only 2 components need to be analyzed; the third one, the ionic liquid, was determined by a mass balance of the measured mass fractions of n-aliphatic and aromatic. In order to avoid inaccuracy of the analysis caused by fouling of the GC by the ionic liquid a pre-column was used to protect the column and collect the ionic solvent in order not to disrupt the analysis. The GC column temperature was programmed for initial temperature of 363 K maintained for 2 min, and a final temperature of 673 K maintained for 5 min. The heating rate was 35 K/min, and the carrier gas (Helium) flow rate was maintained at 3 × 10−5 m3 /min. The injection temperature was 523 K and the detector temperature was 573 K. The temperature was controlled with a precision of ±0.03 K. Each mole fraction was measured repeatedly for four times to reduce experimental uncertainity associated with random errors and the average value was recorded. The compositions in mole fractions were measured with an experimental uncertainity of ±0.0005.
Fig. 1. Experimental and predicted LLE data for system-I: tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3) at 313 K. 䊉: Experimental, dashed line: NRTL, and solid line: UNIQUAC.
Tables 2 and 3 respectively. As shown in these tables, the temperature has no effect on the solubility of [(mebupy)(BF4 )] in n-alkane rich phase, while it has a little effect upon the solubility of n-alkane in ionic solvent rich phase. On the other hand, the concentration of propylbenzene has no effect on the solubility of [(mebupy)(BF4 )] in n-alkane rich phase, while it has a little effect upon the solubility of n-alkane in ionic solvent rich phase. The experimental and the predicted tie lines for the 2 ternary systems at 313 K are shown in Figs. 1 and 2. As can be observed from these figures, an increase in the chain length of the alkane, leads to a slight increase in the size of the immiscibility region. This behavior agree with the results obtained by other workers [3,4]. No detectable concentra-
3. Results and discussion 3.1. Experimental data The measured equilibrium mole fractions over a temperature range 313–333 K for the ternary systems; I and II are reported in
Fig. 2. Experimental and predicted LLE data for system-II: hexadecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3) at 313 K. 䊉: Experimental, dashed line: NRTL, and solid line: UNIQUAC.
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Table 2 Experimental data for the ternary system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at T = 313–333 K and P = 101.3 kPa. T (K)
Tetradecane rich phase
Solvent rich phase
K
S
x1
x2
x1
x2
313
0.8769 0.7792 0.7001 0.6347 0.5798 0.5358 0.4952 0.4606 0.4045 0.3612
0.1231 0.2208 0.2999 0.3653 0.4202 0.4642 0.5048 0.5394 0.5955 0.6388
0.0031 0.0033 0.0034 0.0036 0.0037 0.0039 0.0040 0.0042 0.0043 0.0045
0.0353 0.0655 0.0920 0.1150 0.1352 0.1609 0.1760 0.1917 0.2231 0.2553
0.29 0.30 0.31 0.31 0.32 0.35 0.35 0.36 0.37 0.40
80.07 70.93 62.98 56.14 50.14 47.86 43.06 39.26 34.99 32.23
323
0.8778 0.7807 0.7031 0.6391 0.5838 0.5378 0.4998 0.4652 0.4078 0.3645
0.1222 0.2193 0.2969 0.3609 0.4162 0.4622 0.5002 0.5348 0.5922 0.6355
0.0033 0.0036 0.0040 0.0042 0.0044 0.0045 0.0046 0.0052 0.0055 0.0057
0.0366 0.0682 0.0981 0.1253 0.1459 0.1670 0.1912 0.2089 0.2382 0.2728
0.30 0.31 0.33 0.35 0.35 0.36 0.38 0.39 0.40 0.43
78.97 66.84 58.81 52.57 46.73 43.38 41.44 34.94 29.83 27.45
333
0.8789 0.7821 0.7043 0.6381 0.5844 0.5386 0.5001 0.4682 0.4105 0.3664
0.1211 0.2179 0.2957 0.3619 0.4156 0.4614 0.4999 0.5318 0.5895 0.6336
0.0037 0.0039 0.0043 0.0044 0.0046 0.0047 0.0049 0.0055 0.0059 0.0066
0.0382 0.0706 0.1005 0.1231 0.1475 0.1694 0.1923 0.2192 0.2496 0.2825
0.32 0.32 0.34 0.34 0.35 0.37 0.38 0.41 0.42 0.45
75.57 65.28 56.07 48.89 45.20 42.25 39.18 34.90 29.41 24.87
Table 3 Experimental data for the ternary system-II {hexadecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at T = 313–333 K and P = 101.3 kPa. T (K)
Hexadecane rich phase
Solvent rich phase
K
S
0.0347 0.0638 0.0883 0.1087 0.1319 0.1524 0.1693 0.1874 0.2193 0.2402
0.25 0.26 0.27 0.27 0.29 0.30 0.31 0.33 0.35 0.36
109.81 94.18 82.39 71.60 67.12 62.33 57.41 52.36 47.73 42.37
0.0023 0.0024 0.0025 0.0026 0.0027 0.0029 0.0030 0.0031 0.0032 0.0033
0.0367 0.0671 0.0936 0.1172 0.1432 0.1616 0.1791 0.1930 0.2239 0.2583
0.27 0.28 0.29 0.30 0.32 0.32 0.33 0.34 0.36 0.39
100.23 86.74 76.73 70.10 64.82 56.28 51.38 46.55 41.59 38.79
0.0028 0.0028 0.0029 0.0030 0.0033 0.0034 0.0036 0.0038 0.0040 0.0042
0.0407 0.0744 0.1020 0.1272 0.1513 0.1741 0.1939 0.2095 0.2443 0.2823
0.30 0.31 0.32 0.32 0.34 0.35 0.36 0.37 0.39 0.43
94.89 83.63 73.01 66.81 57.58 52.62 47.21 41.96 37.01 34.01
x1
x2
x1
x2
313
0.8612 0.7542 0.6693 0.6003 0.5456 0.4995 0.4598 0.4264 0.3719 0.3282
0.1388 0.2458 0.3307 0.3997 0.4544 0.5005 0.5402 0.5736 0.6281 0.6718
0.0020 0.0021 0.0022 0.0023 0.0024 0.0024 0.0025 0.0027 0.0027 0.0028
323
0.8627 0.7562 0.6720 0.6040 0.5499 0.5025 0.4626 0.4278 0.3728 0.3314
0.1373 0.2438 0.3280 0.3960 0.4501 0.4975 0.5374 0.5722 0.6272 0.6686
333
0.8658 0.7608 0.6764 0.6085 0.5530 0.5068 0.4671 0.4322 0.3773 0.3360
0.1342 0.2392 0.3236 0.3915 0.4470 0.4932 0.5329 0.5678 0.6227 0.6640
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105
tions of [(mebupy)(BF4 )] were found in the n-alkane rich phase. The complete absence of [(mebupy)(BF4 )] in the n-alkane rich phase is desirable, since it permits to avoid the need of a unit for recovering the solvent from the raffinate in a continuous extraction process. The reliability of the experimentally measured tie line data can be ascertained by applying the Othmer–Tobias correlation [22]:
ln
1 − w3II w3II
= a + b ln
1 − w1I
w1I
(1)
where w3II is the mass fraction of [(mebupy)(BF4 )] (3) in the lower layer (IL-rich phase), w1I is the mass fraction of n-alkane (1) in the upper layer (n-alkane-rich phase), and a and b are the fitting parameters of the Othmer–Tobias correlation. The linearity of the plot indicates the degree of consistency of the data. The parameters of the Othmer–Tobias correlation are given in Table 4. The regression coefficients (R2 ) are very close to unity and the low values of the standard deviation () indicate the degree of consistency of the related data at 313, 323 and 333 K are presented in the same table. 3.2. Distribution coefficient and selectivity Together with the LLE data, Tables 2 and 3 include the corresponding values for the solute distribution ratio (K) and the selectivity (S), which are widely used parameters to characterize the suitability of a solvent in liquid–liquid extraction. The operation of liquid–liquid extraction was significantly affected by variations in the operating temperature and solvent to feed ratio. Thus, the determination of the effects of temperature and solvent to feed ratio on the values of distribution coefficient (K) and selectivity (S) were quite crucial for the liquid–liquid extraction process. The distribution coefficient of propylbenzene (K), which is the measure of the solvent power or capacity of [(mebupy)(BF4 )], is given by: K=
x2II x2I
(2)
where x2I and x2II represent the mole fraction of propylbenzene in the alkane-rich phase and the IL-rich phase respectively. Fig. 3 represents the relationship of the solvent to feed ratio (˛stf ) with the measured distribution coefficients (K), for the ternary system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at temperatures of 313–333 K. The distribution coefficient values increased as the temperature increased and/or solvent to feed ratio (˛stf ) decreased.
Fig. 3. Measured distribution coefficient (K) against solvent to feed ratio (˛stf ) for system-I at: 䊉: 313, : 323, and : 333 K.
Fig. 4. Measured distribution coefficient (K) against solvent to feed ratio ˛stf ) at 313 K for: 䊉: system-I, and : system-II.
To investigate the effect of n-alkane chain length on distribution coefficient a series of experiments using C14 and C16 were carried out. Fig. 4 represents the relationship of the solvent to feed ratio (˛stf ), with the measured distribution coefficients (K), for the two ternary systems at 313 K. As can be seen from Fig. 4, the distribution coefficient values of the two ternary systems decreased as the chain length of n-alkanes increased. The effectiveness of a solvent can be expressed by the selectivity (S), of the solvent. The selectivity of [(mebupy)(BF4 )], which is a measure of the ability of [(mebupy)(BF4 )] to separate propylbenzene from n-alkanes, is given by: S=
x2II x1I x2I x1II
(3)
where x1I and x2I represent the mole fraction of alkane and propylbenzene respectively, in the alkane-rich phase and x1II and x2II represent the mole fraction of alkane and propylbenzene respectively, in the IL-rich phase. From the selectivity data represented in Fig. 5, for the ternary system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at temperatures of 313–333 K, selectivity values show small variation with temperature, and in general; the higher the temperature the lower the selectivity. To investigate the effect of n-alkane chain length on selectivity a series of run was carried out using C14 and C16 at 313 K. Fig. 6 represents the solvent to feed ratio (˛stf ) with the measured selectivity (S) for the ternary systems; I and II at 313 K, the values of selectivity are higher for the systems containing alkanes with larger chain following the rela-
Fig. 5. Measured selectivity (S) against solvent to feed ratio (˛stf ) for system-I at: 䊉: 313, : 323, and : 333 K.
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Table 4 Constants of the Othmer–Tobias correlation, correlation factor (R2 ) and standard deviation () for the two ternary systems at T = 313–333 K and P = 101.3 kPa. T (K)
a
b
R2
313
System-I System-II
−1.7798 −1.8482
0.8376 0.8342
0.9995 0.9995
0.0417 0.0395
323
System-I System-II
−1.6618 −1.7708
0.8644 0.8319
0.9995 0.9993
0.0457 0.0418
333
System-I System-II
−1.6211 −1.6520
0.8674 0.8295
0.9987 0.9994
0.0482 0.0463
Table 5 UNIQUAC and NRTL interaction parameters and root mean square deviation (rmsd) for the two ternary systems at T = 313–333 K and P = 101.3 kPa. T (K)
i
j
313
Tetradecane Tetradecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
122.84 441.82 8.88
−83.01 312.68 106.41
323
Tetradecane Tetradecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
−65.78 534.23 273.76
51.51 −52.50 −108.74
333
Tetradecane Tetradecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
−61.06 588.50 247.85
38.34 −64.61 −100.88
Hexadecane Hexadecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
−29.27 357.27 129.92
32.44 231.25 −3.95
323
Hexadecane Hexadecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
99.52 448.24 −19.86
−52.05 351.47 156.20
333
Hexadecane Hexadecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
109.53 475.09 −67.42
−27.86 403.24 242.27
UNIQUAC aij
313
tion tetradecane < hexadecane as observed in previous study [16]. As shown in Tables 2 and 3, the selectivity values are not constant over whole two-phase region, and it is decreased as the concentration of propylbenzene increased this was observed in several publications [3,4,7], it means the higher the concentration of propylbenzene in the feed the lower the selectivity of [(mebupy)(BF4 )]. The selectivity values in the systems under study are higher than unity, which ensures the feasibility of separation of propylbenzene and n-alkanes from their mixtures. Moreover, the use of [(mebupy)(BF4 )], and of ionic liquids in general, in solvent extraction is favourable because it can be easily recovered and reused.
NRTL aji
aij
aji
rmsd
0.2179
−544.52 1610.00 1479.30
466.53 1177.90 −317.28
0.2061
0.1243
−571.81 1658.90 1534.90
465.13 1192.10 −371.71
0.1901
0.1769
−655.25 1724.30 1540.30
510.40 1187.60 −422.03
0.2442
0.1655
−575.70 1566.50 1527.20
478.27 1244.60 −282.15
0.2091
0.1836
−582.80 1632.30 1545.50
505.54 1266.10 −304.36
0.2303
0.1914
−626.36 1713.20 1550.50
540.04 1262.00 −358.28
0.2726
3.3. Correlations The UNIQUAC model of Abrams and Prausnitz [23] and the NRTL model of Renon and Prausnitz [24] were used to correlate our experimental data. The LLE experimental data were used to determine the optimum UNIQUAC and NRTL binary interaction parameters between [(mebupy)(BF4 )], propylbenzene, and tetradecane, or hexadecane. The UNIQUAC and the NRTL models were fitted to experimental data using an iterative computer program, based on flash calculation method, developed by S∅rensen [25]. The objective function used in this case was determined by minimizing the square of the difference between the mole fractions predicted by the respective method and these experimentally measured over all the tie lines in the ternary systems. For the UNIQUAC correlation the pure component structural parameters (r and q) listed in Table 1, were taken from literature [26] or calculated from the group contribution data [27]. The objective function (OF) used is:
OF = min
Fig. 6. Measured selectivity (S) against solvent to feed ratio (˛stf ) at 313 K for: 䊉: system-I, and : system-II.
rmsd
k
j
i
(xijk,
exp
− xijk,
cal )
2
(4)
where x is mole fraction, subscripts exp, cal, i, j, and k are experimental, calculated, components, phases and tie lines respectively. The NRTL model was fitted with fixed values of the third nonrandomness parameter, ˛ij , for each pair of components. A fixed value of ˛ij = 0.2 between each pair of components was found to be satisfactory.
K.H.A.E. Alkhaldi et al. / Fluid Phase Equilibria 309 (2011) 102–107
The optimization results were judged by calculating the corresponding rmsd values using the following equation:
j i (xijk,
rmsd = 100
k
exp
6n
− xijk,
cal )
2
1/2 (5)
where n is number of tie lines. The values of interaction parameters and the relative mean square deviation (rmsd) for the UNIQUAC and the NRTL models at different temperatures are shown in Table 5. These parameters are used to calculate LLE tie lines for the present systems. The calculation based on both models gave good representation of the tie line data for those systems. However, the UNIQUAC model, fitted to LLE experimental data, is more accurate than the NRTL model, according to the analysis of the average rmsd (the average rmsd was 0.1766 for UNIQUAC as compared to 0.2254 for NRTL). 4. Conclusions An experimental investigation of equilibrium behavior of liquid–liquid, tetradecane, or hexadecane + propylbenzene + [(mebupy)(BF4 )] ternary systems were carried out at temperatures of 313 to 333 K and at atmospheric pressure. While the temperature has no effect on the solubility of [(mebupy)(BF4 )] in n-alkane rich phase increases, it has a little effect upon the solubility of n-alkane in ionic solvent rich phase. The solubility of n-alkane in the ionic solvent rich phase increased as the concentration of propylbenzene increases, but it has no effect upon the solubility of [(mebupy)(BF4 )] in the n-alkane rich phase. A good degree of consistency of the experimental LLE data was ascertained by applying the Othmer–Tobias correlation with an average regression coefficient (R2 ) equal to 0.9994. In addition, the UNIQUAC and the NRTL models satisfactorily correlate the LLE experimental data, however the former model was more suitable for the studied systems (the average rmsd was 0.1766 for UNIQUAC as compared to 0.2254 for NRTL). The effects of temperature, the chain length of n-alkane and solvent to feed ratio on distribution coefficient and selectivity were evaluated. While the distribution coefficient increased as the temperature increased and/or the chain length of n-alkane decreased, the selectivity increased as the temperature decreased and/or the chain length of n-alkane increased. Since the selectivity in all cases is greater than unity [(mebupy)(BF4 )] can be used to separate propylbenzene and tetradecane, or hexadecane from their mixtures.
107
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Separation of propylbenzene and n-alkanes from their mixtures using 4-methyl-N-butylpyridinium tetrafluoroborate as an ionic solvent at several temperatures Khaled H.A.E. Alkhaldi, Mohammad S. Al-Tuwaim, Mohamed S. Fandary, Adel S. Al-Jimaz ∗ Chemical Engineering Department, College of Technological Studies, PAAET, P.O. Box 3242, Salmyiah 22033, Kuwait
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 22 June 2011 Accepted 29 June 2011 Available online 6 July 2011 Keywords: LLE Ionic solvent Aromatics Alkanes UNIQUAC NRTL
a b s t r a c t Liquid–liquid extraction is the most common method for separation of aromatics from their mixtures with n-alkanes hydrocarbons. An ionic liquid (IL), 4-methyl-N-butylpyridinium tetrafluoroborate [(mebupy)(BF4 )], was evaluated as solvent for this separation. Liquid equilibria (LLE) for 2 ternary systems comprising tetradecane, or hexadecane + propylbenzene + [(mebupy)(BF4 )] were measured over a temperature range of 313–333 K and atmospheric pressure. The reliability of the experimental data was evaluated using the Othmer–Tobias correlation. The effect of temperature, n-alkane chain length and solvent to feed ratio upon solubility, selectivity, and distribution coefficient were investigated experimentally. In addition, the experimental results were regressed to estimate the interaction parameters between each of the 3 pairs of components for the UNIQUAC and the NRTL models as a function of temperature. Both models satisfactorily correlate the experimental data, however the UNIQUAC fit was slightly better than that obtained with the NRTL model. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The separation of aromatic and aliphatic compounds from their mixtures is an important goal in chemical operations that produce both types of compounds. On the other hand, smoke point of kerosene, cetane index of diesel, and viscosity index of lubricating oil can be improved by removing aromatic hydrocarbons. Ternary phase equilibrium data are essential to the proper understanding of the solvent extraction processes. Over the past few years, research about ionic liquids (ILs) has increased greatly, mainly in 2 directions: as reaction media and as a solvent for separation processes. Ionic liquids properties, such as a negligible vapour pressure and stability at high temperatures allow them to substitute classic organic solvents with improving performance and less damage to the environment [1–3]. The knowledge of the liquid–liquid equilibria (LLE) for the ternary systems (aliphatic hydrocarbons + aromatic hydrocarbons + ILs) is essential to evaluate the feasibility of using ILs as extractive solvents in the separation of aromatic and aliphatic hydrocarbons [4–16]. The 4-methyl-N-butylpyridinium tetrafluoroborate [(mebupy)(BF4 )] has been one of the few ILs that has shown both higher selectivity and extractive capacity than those of sulfolane
for the extraction of aromatic hydrocarbons [1]. Nevertheless, LLE data are still quite scarce for systems containing ionic liquids. Currently, there are only a few experimental liquid–liquid equilibrium data have been published for [(mebupy)(BF4 )] + aromatic + naliphatic (C5 –C9 ) and hardly any for systems containing carbon number greater than nine for the aliphatic and/or aromatics [1,3,17–19]. This article is a continuation of our study on the liquid–liquid phase equilibria for dearomatization of Kuwait middle distilled fraction [20,21]. Our interest in IL is focused on providing experimental LLE data. The purpose of this work is to study LLE of 2 ternary systems; system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)}, and system-II {hexadecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)}. The LLE data for the studied ternary systems were measured at 313, 323, and 333 K and atmospheric pressure. The reliability of the experimentally measured tie line data was ascertained by the Othmer–Tobias correlation [22], and the distribution coefficient as well as the selectivity was calculated from these data. Finally, the UNIQUAC and the NRTL models [23,24] were used to correlate our experimental data. 2. Experimental 2.1. Chemicals
∗ Corresponding author. E-mail address: a [email protected] (A.S. Al-Jimaz). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.06.036
The [(mebupy)(BF4 )] and propylbenzene were stored under 4 nm molecular sieve. The purities of the n-alkanes and propyl-
K.H.A.E. Alkhaldi et al. / Fluid Phase Equilibria 309 (2011) 102–107
103
Table 1 Details of the chemicals, purities (mass%), UNIQUAC structural parameters, and refractive indicies (nD ) at 20 ◦ C and atmospheric pressure. Compound
Supplier
Purity (mass%)
[(Mebupy)(BF4 )] Propylbenzene Tetradecane Hexadecane
Merck Fluka Aldrich Aldrich
>97 >99 >99 >99
a b c
UNIQUAC structural parameter
nD 20
r
q
Exp
Lit
8.4337 5.4983 9.8950 11.2438
6.5290 4.3560 8.1759 9.2560
1.4509c 1.4915 1.4289 1.4291
1.4517a , c 1.4920b 1.4290b 1.4290b
Ref. [28]. Ref. [29]. nD at 25 ◦ C.
benzene were determined by gas chromatography. All chemicals were used without further purification. The purities and refractive indices of all chemicals used in this study are presented in Table 1. 2.2. Apparatus and procedure The experimental apparatus used for extraction consists of a 60 mL glass cells with a water jacket in order to maintain a constant temperature. The temperature was controlled with an uncertainty of ±0.2 K. The cells were connected to a Haake K15 water bath fitted with a Haake DC1 thermostat. Mixtures, comprising of 20 g of [(mebupy)(BF4 )], 20 g of n-alkanes, and different amounts of propylbenzene were placed in the extraction vessels. The mixtures were vigorously stirred for 1 h, and then left to settle for 4 h. A series of LLE measurements over a temperature range of 313–333 K were performed. 2.3. Measurements of phase compositions Samples were carefully taken by a syringe from the lower and upper layers. Each sample was dissolved in 0.5 mL 1-butanol, to avoid splitting and maintain a homogeneous mixture, and analyzed using a Varian 450 gas chromatography equipped with an autosampler (Varian CP-8400), an on-column injecter, flame ionization detector (FID), and a data processoring system. The column used was Varian VF-5 ms CP8944 (30 m length and 0.25 mm I.D., 0.25 m film thickness). The ionic solvent [(mebupy)(BF4 )], has negligible vapor pressure it cannot be analyzed by GC. In the ternary mixture, only 2 components need to be analyzed; the third one, the ionic liquid, was determined by a mass balance of the measured mass fractions of n-aliphatic and aromatic. In order to avoid inaccuracy of the analysis caused by fouling of the GC by the ionic liquid a pre-column was used to protect the column and collect the ionic solvent in order not to disrupt the analysis. The GC column temperature was programmed for initial temperature of 363 K maintained for 2 min, and a final temperature of 673 K maintained for 5 min. The heating rate was 35 K/min, and the carrier gas (Helium) flow rate was maintained at 3 × 10−5 m3 /min. The injection temperature was 523 K and the detector temperature was 573 K. The temperature was controlled with a precision of ±0.03 K. Each mole fraction was measured repeatedly for four times to reduce experimental uncertainity associated with random errors and the average value was recorded. The compositions in mole fractions were measured with an experimental uncertainity of ±0.0005.
Fig. 1. Experimental and predicted LLE data for system-I: tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3) at 313 K. 䊉: Experimental, dashed line: NRTL, and solid line: UNIQUAC.
Tables 2 and 3 respectively. As shown in these tables, the temperature has no effect on the solubility of [(mebupy)(BF4 )] in n-alkane rich phase, while it has a little effect upon the solubility of n-alkane in ionic solvent rich phase. On the other hand, the concentration of propylbenzene has no effect on the solubility of [(mebupy)(BF4 )] in n-alkane rich phase, while it has a little effect upon the solubility of n-alkane in ionic solvent rich phase. The experimental and the predicted tie lines for the 2 ternary systems at 313 K are shown in Figs. 1 and 2. As can be observed from these figures, an increase in the chain length of the alkane, leads to a slight increase in the size of the immiscibility region. This behavior agree with the results obtained by other workers [3,4]. No detectable concentra-
3. Results and discussion 3.1. Experimental data The measured equilibrium mole fractions over a temperature range 313–333 K for the ternary systems; I and II are reported in
Fig. 2. Experimental and predicted LLE data for system-II: hexadecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3) at 313 K. 䊉: Experimental, dashed line: NRTL, and solid line: UNIQUAC.
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Table 2 Experimental data for the ternary system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at T = 313–333 K and P = 101.3 kPa. T (K)
Tetradecane rich phase
Solvent rich phase
K
S
x1
x2
x1
x2
313
0.8769 0.7792 0.7001 0.6347 0.5798 0.5358 0.4952 0.4606 0.4045 0.3612
0.1231 0.2208 0.2999 0.3653 0.4202 0.4642 0.5048 0.5394 0.5955 0.6388
0.0031 0.0033 0.0034 0.0036 0.0037 0.0039 0.0040 0.0042 0.0043 0.0045
0.0353 0.0655 0.0920 0.1150 0.1352 0.1609 0.1760 0.1917 0.2231 0.2553
0.29 0.30 0.31 0.31 0.32 0.35 0.35 0.36 0.37 0.40
80.07 70.93 62.98 56.14 50.14 47.86 43.06 39.26 34.99 32.23
323
0.8778 0.7807 0.7031 0.6391 0.5838 0.5378 0.4998 0.4652 0.4078 0.3645
0.1222 0.2193 0.2969 0.3609 0.4162 0.4622 0.5002 0.5348 0.5922 0.6355
0.0033 0.0036 0.0040 0.0042 0.0044 0.0045 0.0046 0.0052 0.0055 0.0057
0.0366 0.0682 0.0981 0.1253 0.1459 0.1670 0.1912 0.2089 0.2382 0.2728
0.30 0.31 0.33 0.35 0.35 0.36 0.38 0.39 0.40 0.43
78.97 66.84 58.81 52.57 46.73 43.38 41.44 34.94 29.83 27.45
333
0.8789 0.7821 0.7043 0.6381 0.5844 0.5386 0.5001 0.4682 0.4105 0.3664
0.1211 0.2179 0.2957 0.3619 0.4156 0.4614 0.4999 0.5318 0.5895 0.6336
0.0037 0.0039 0.0043 0.0044 0.0046 0.0047 0.0049 0.0055 0.0059 0.0066
0.0382 0.0706 0.1005 0.1231 0.1475 0.1694 0.1923 0.2192 0.2496 0.2825
0.32 0.32 0.34 0.34 0.35 0.37 0.38 0.41 0.42 0.45
75.57 65.28 56.07 48.89 45.20 42.25 39.18 34.90 29.41 24.87
Table 3 Experimental data for the ternary system-II {hexadecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at T = 313–333 K and P = 101.3 kPa. T (K)
Hexadecane rich phase
Solvent rich phase
K
S
0.0347 0.0638 0.0883 0.1087 0.1319 0.1524 0.1693 0.1874 0.2193 0.2402
0.25 0.26 0.27 0.27 0.29 0.30 0.31 0.33 0.35 0.36
109.81 94.18 82.39 71.60 67.12 62.33 57.41 52.36 47.73 42.37
0.0023 0.0024 0.0025 0.0026 0.0027 0.0029 0.0030 0.0031 0.0032 0.0033
0.0367 0.0671 0.0936 0.1172 0.1432 0.1616 0.1791 0.1930 0.2239 0.2583
0.27 0.28 0.29 0.30 0.32 0.32 0.33 0.34 0.36 0.39
100.23 86.74 76.73 70.10 64.82 56.28 51.38 46.55 41.59 38.79
0.0028 0.0028 0.0029 0.0030 0.0033 0.0034 0.0036 0.0038 0.0040 0.0042
0.0407 0.0744 0.1020 0.1272 0.1513 0.1741 0.1939 0.2095 0.2443 0.2823
0.30 0.31 0.32 0.32 0.34 0.35 0.36 0.37 0.39 0.43
94.89 83.63 73.01 66.81 57.58 52.62 47.21 41.96 37.01 34.01
x1
x2
x1
x2
313
0.8612 0.7542 0.6693 0.6003 0.5456 0.4995 0.4598 0.4264 0.3719 0.3282
0.1388 0.2458 0.3307 0.3997 0.4544 0.5005 0.5402 0.5736 0.6281 0.6718
0.0020 0.0021 0.0022 0.0023 0.0024 0.0024 0.0025 0.0027 0.0027 0.0028
323
0.8627 0.7562 0.6720 0.6040 0.5499 0.5025 0.4626 0.4278 0.3728 0.3314
0.1373 0.2438 0.3280 0.3960 0.4501 0.4975 0.5374 0.5722 0.6272 0.6686
333
0.8658 0.7608 0.6764 0.6085 0.5530 0.5068 0.4671 0.4322 0.3773 0.3360
0.1342 0.2392 0.3236 0.3915 0.4470 0.4932 0.5329 0.5678 0.6227 0.6640
K.H.A.E. Alkhaldi et al. / Fluid Phase Equilibria 309 (2011) 102–107
105
tions of [(mebupy)(BF4 )] were found in the n-alkane rich phase. The complete absence of [(mebupy)(BF4 )] in the n-alkane rich phase is desirable, since it permits to avoid the need of a unit for recovering the solvent from the raffinate in a continuous extraction process. The reliability of the experimentally measured tie line data can be ascertained by applying the Othmer–Tobias correlation [22]:
ln
1 − w3II w3II
= a + b ln
1 − w1I
w1I
(1)
where w3II is the mass fraction of [(mebupy)(BF4 )] (3) in the lower layer (IL-rich phase), w1I is the mass fraction of n-alkane (1) in the upper layer (n-alkane-rich phase), and a and b are the fitting parameters of the Othmer–Tobias correlation. The linearity of the plot indicates the degree of consistency of the data. The parameters of the Othmer–Tobias correlation are given in Table 4. The regression coefficients (R2 ) are very close to unity and the low values of the standard deviation () indicate the degree of consistency of the related data at 313, 323 and 333 K are presented in the same table. 3.2. Distribution coefficient and selectivity Together with the LLE data, Tables 2 and 3 include the corresponding values for the solute distribution ratio (K) and the selectivity (S), which are widely used parameters to characterize the suitability of a solvent in liquid–liquid extraction. The operation of liquid–liquid extraction was significantly affected by variations in the operating temperature and solvent to feed ratio. Thus, the determination of the effects of temperature and solvent to feed ratio on the values of distribution coefficient (K) and selectivity (S) were quite crucial for the liquid–liquid extraction process. The distribution coefficient of propylbenzene (K), which is the measure of the solvent power or capacity of [(mebupy)(BF4 )], is given by: K=
x2II x2I
(2)
where x2I and x2II represent the mole fraction of propylbenzene in the alkane-rich phase and the IL-rich phase respectively. Fig. 3 represents the relationship of the solvent to feed ratio (˛stf ) with the measured distribution coefficients (K), for the ternary system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at temperatures of 313–333 K. The distribution coefficient values increased as the temperature increased and/or solvent to feed ratio (˛stf ) decreased.
Fig. 3. Measured distribution coefficient (K) against solvent to feed ratio (˛stf ) for system-I at: 䊉: 313, : 323, and : 333 K.
Fig. 4. Measured distribution coefficient (K) against solvent to feed ratio ˛stf ) at 313 K for: 䊉: system-I, and : system-II.
To investigate the effect of n-alkane chain length on distribution coefficient a series of experiments using C14 and C16 were carried out. Fig. 4 represents the relationship of the solvent to feed ratio (˛stf ), with the measured distribution coefficients (K), for the two ternary systems at 313 K. As can be seen from Fig. 4, the distribution coefficient values of the two ternary systems decreased as the chain length of n-alkanes increased. The effectiveness of a solvent can be expressed by the selectivity (S), of the solvent. The selectivity of [(mebupy)(BF4 )], which is a measure of the ability of [(mebupy)(BF4 )] to separate propylbenzene from n-alkanes, is given by: S=
x2II x1I x2I x1II
(3)
where x1I and x2I represent the mole fraction of alkane and propylbenzene respectively, in the alkane-rich phase and x1II and x2II represent the mole fraction of alkane and propylbenzene respectively, in the IL-rich phase. From the selectivity data represented in Fig. 5, for the ternary system-I {tetradecane (1) + propylbenzene (2) + [(mebupy)(BF4 )] (3)} at temperatures of 313–333 K, selectivity values show small variation with temperature, and in general; the higher the temperature the lower the selectivity. To investigate the effect of n-alkane chain length on selectivity a series of run was carried out using C14 and C16 at 313 K. Fig. 6 represents the solvent to feed ratio (˛stf ) with the measured selectivity (S) for the ternary systems; I and II at 313 K, the values of selectivity are higher for the systems containing alkanes with larger chain following the rela-
Fig. 5. Measured selectivity (S) against solvent to feed ratio (˛stf ) for system-I at: 䊉: 313, : 323, and : 333 K.
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K.H.A.E. Alkhaldi et al. / Fluid Phase Equilibria 309 (2011) 102–107
Table 4 Constants of the Othmer–Tobias correlation, correlation factor (R2 ) and standard deviation () for the two ternary systems at T = 313–333 K and P = 101.3 kPa. T (K)
a
b
R2
313
System-I System-II
−1.7798 −1.8482
0.8376 0.8342
0.9995 0.9995
0.0417 0.0395
323
System-I System-II
−1.6618 −1.7708
0.8644 0.8319
0.9995 0.9993
0.0457 0.0418
333
System-I System-II
−1.6211 −1.6520
0.8674 0.8295
0.9987 0.9994
0.0482 0.0463
Table 5 UNIQUAC and NRTL interaction parameters and root mean square deviation (rmsd) for the two ternary systems at T = 313–333 K and P = 101.3 kPa. T (K)
i
j
313
Tetradecane Tetradecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
122.84 441.82 8.88
−83.01 312.68 106.41
323
Tetradecane Tetradecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
−65.78 534.23 273.76
51.51 −52.50 −108.74
333
Tetradecane Tetradecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
−61.06 588.50 247.85
38.34 −64.61 −100.88
Hexadecane Hexadecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
−29.27 357.27 129.92
32.44 231.25 −3.95
323
Hexadecane Hexadecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
99.52 448.24 −19.86
−52.05 351.47 156.20
333
Hexadecane Hexadecane Propylbenzene
Propylbenzene (Mebupy)(BF4 ) (Mebupy)(BF4 )
109.53 475.09 −67.42
−27.86 403.24 242.27
UNIQUAC aij
313
tion tetradecane < hexadecane as observed in previous study [16]. As shown in Tables 2 and 3, the selectivity values are not constant over whole two-phase region, and it is decreased as the concentration of propylbenzene increased this was observed in several publications [3,4,7], it means the higher the concentration of propylbenzene in the feed the lower the selectivity of [(mebupy)(BF4 )]. The selectivity values in the systems under study are higher than unity, which ensures the feasibility of separation of propylbenzene and n-alkanes from their mixtures. Moreover, the use of [(mebupy)(BF4 )], and of ionic liquids in general, in solvent extraction is favourable because it can be easily recovered and reused.
NRTL aji
aij
aji
rmsd
0.2179
−544.52 1610.00 1479.30
466.53 1177.90 −317.28
0.2061
0.1243
−571.81 1658.90 1534.90
465.13 1192.10 −371.71
0.1901
0.1769
−655.25 1724.30 1540.30
510.40 1187.60 −422.03
0.2442
0.1655
−575.70 1566.50 1527.20
478.27 1244.60 −282.15
0.2091
0.1836
−582.80 1632.30 1545.50
505.54 1266.10 −304.36
0.2303
0.1914
−626.36 1713.20 1550.50
540.04 1262.00 −358.28
0.2726
3.3. Correlations The UNIQUAC model of Abrams and Prausnitz [23] and the NRTL model of Renon and Prausnitz [24] were used to correlate our experimental data. The LLE experimental data were used to determine the optimum UNIQUAC and NRTL binary interaction parameters between [(mebupy)(BF4 )], propylbenzene, and tetradecane, or hexadecane. The UNIQUAC and the NRTL models were fitted to experimental data using an iterative computer program, based on flash calculation method, developed by S∅rensen [25]. The objective function used in this case was determined by minimizing the square of the difference between the mole fractions predicted by the respective method and these experimentally measured over all the tie lines in the ternary systems. For the UNIQUAC correlation the pure component structural parameters (r and q) listed in Table 1, were taken from literature [26] or calculated from the group contribution data [27]. The objective function (OF) used is:
OF = min
Fig. 6. Measured selectivity (S) against solvent to feed ratio (˛stf ) at 313 K for: 䊉: system-I, and : system-II.
rmsd
k
j
i
(xijk,
exp
− xijk,
cal )
2
(4)
where x is mole fraction, subscripts exp, cal, i, j, and k are experimental, calculated, components, phases and tie lines respectively. The NRTL model was fitted with fixed values of the third nonrandomness parameter, ˛ij , for each pair of components. A fixed value of ˛ij = 0.2 between each pair of components was found to be satisfactory.
K.H.A.E. Alkhaldi et al. / Fluid Phase Equilibria 309 (2011) 102–107
The optimization results were judged by calculating the corresponding rmsd values using the following equation:
j i (xijk,
rmsd = 100
k
exp
6n
− xijk,
cal )
2
1/2 (5)
where n is number of tie lines. The values of interaction parameters and the relative mean square deviation (rmsd) for the UNIQUAC and the NRTL models at different temperatures are shown in Table 5. These parameters are used to calculate LLE tie lines for the present systems. The calculation based on both models gave good representation of the tie line data for those systems. However, the UNIQUAC model, fitted to LLE experimental data, is more accurate than the NRTL model, according to the analysis of the average rmsd (the average rmsd was 0.1766 for UNIQUAC as compared to 0.2254 for NRTL). 4. Conclusions An experimental investigation of equilibrium behavior of liquid–liquid, tetradecane, or hexadecane + propylbenzene + [(mebupy)(BF4 )] ternary systems were carried out at temperatures of 313 to 333 K and at atmospheric pressure. While the temperature has no effect on the solubility of [(mebupy)(BF4 )] in n-alkane rich phase increases, it has a little effect upon the solubility of n-alkane in ionic solvent rich phase. The solubility of n-alkane in the ionic solvent rich phase increased as the concentration of propylbenzene increases, but it has no effect upon the solubility of [(mebupy)(BF4 )] in the n-alkane rich phase. A good degree of consistency of the experimental LLE data was ascertained by applying the Othmer–Tobias correlation with an average regression coefficient (R2 ) equal to 0.9994. In addition, the UNIQUAC and the NRTL models satisfactorily correlate the LLE experimental data, however the former model was more suitable for the studied systems (the average rmsd was 0.1766 for UNIQUAC as compared to 0.2254 for NRTL). The effects of temperature, the chain length of n-alkane and solvent to feed ratio on distribution coefficient and selectivity were evaluated. While the distribution coefficient increased as the temperature increased and/or the chain length of n-alkane decreased, the selectivity increased as the temperature decreased and/or the chain length of n-alkane increased. Since the selectivity in all cases is greater than unity [(mebupy)(BF4 )] can be used to separate propylbenzene and tetradecane, or hexadecane from their mixtures.
107
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