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Study on the separation of propylbenzene from alkanes using two methylsulfate-based ionic liquids at (313 and 333) K
Fluid Phase Equilibria 354 (2013) 29–37
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Study on the separation of propylbenzene from alkanes using two methylsulfate-based ionic liquids at (313 and 333) K Adel S. Al-Jimaz ∗ , Khaled H.A.E. Alkhaldi, Mohsen H. Al-Rashed, Mohamed S. Fandary, Mohammad S. AlTuwaim Chemical Engineering Department, College of Technological Studies, PAAET, P.O. Box 42325, Shuwaikh 70654, Kuwait
a r t i c l e
i n f o
Article history: Received 10 February 2013 Received in revised form 20 May 2013 Accepted 6 June 2013 Available online 15 June 2013 Keywords: LLE Ionic solvent Alkane Propylbenzene UNIQUAC NRTL
a b s t r a c t The separation of propylbenzene from dodecane or tetradecane using different ionic liquids (ILs) based on the methylsulfate anion (CH3 SO4 − ) was studied at two temperatures (313 and 333) K and atmospheric pressure. The ionic liquids, 1, 3-dimethylimidazolium methylsulfate [(mmim)(CH3 SO4 )] and 1-ethyl-3-methylimidazolium methylsulfate [(emim)(CH3 SO4 )], were evaluated as solvents for the extraction process. Four ternary systems were formed by {dodecane or tetradecane + propylbenzene + [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )]}. The degree of quality of the experimental data was ascertained using the Othmer–Tobias correlation. A comparison of the phase behaviour of the systems studied has allowed us to investigate the effect of ILs cation, alkane chain length, temperature and solvent-to-feed ratio upon solubility, percent aromatic removal, distribution ratio, and selectivity. Finally, the experimental LLE data was correlated using the UNIQUAC and the NRTL models. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the petroleum refinery, extraction of aromatics from the middle distillate fuels is a process of major importance. The properties of several petroleum products may be improved by removing the aromatic hydrocarbons from vacuum distillates [1,2]. Due to environmental legislation, the demand of ‘clean’ fuels is increasing and most likely will increase even more towards fuels with almost zero content of certain aromatics, e.g. benzene and toluene. In particular, the concentration of benzene must be reduced to ≤0.1% by weight in carburant fuels. The most common processes use solvents such as dimethylsulfoxide (DMSO), furfural, N-methylpyrrolidone (NMP), dimethylformamide (DMF), and sulfolane for extraction of aromatic and sulfur hydrocarbons from middle distillate fractions [3–8]. Although the processes based on these solvents are well known, there is a need to study environmental friendly solvents with better selectivity and capacity. From activity coefficients at infinite dilution measurements, it has been shown that many ionic liquids have better selectivity and capacity in extraction of aromatics from aromatic + aliphatic mixtures than typical solvents such as sulfolane and NMP [9].
∗ Corresponding author. Tel.: +965 99088996. E-mail address: a [email protected] (A.S. Al-Jimaz). 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.06.010
The design of safe and environmentally friendly separation processes is of increasing importance in the chemical industry. Over the last few years, ionic liquids (ILs) were suggested as an alternative to traditional organic solvents for extraction processes. The ILs show interesting positive properties, such as negligible vapour pressure, stability at high temperatures, and the ability to tailor ILs to feedstock composition. [10–13]. Many researchers focused on liquid–liquid equilibria (LLE) measurements for the ternary systems containing (aliphatic + aromatic + ILs) [9,14–29]. Ionic liquids based on the alkylsulfate anion (R-SO4 − ) have attracted great interest because they are less viscous, more hydrolytically stable, and more environmental friendly than other ILs, as well as easily synthesized at a reasonable cost [30]. Moreover, the alkylsulfate-based ILs show chemical and thermal stability and low melting points [31]. Recent research has explored combining the anion (R-SO4 − ) with the cations-imidazolium or pyridinium [30–33,23,34–38]. As a continuation of our ongoing work on aromatic extraction from middle distillate fractions using ILs [39–43], we analyzed the suitability of the ILs to separate propylbenzene from dodecane or tetradecane in four ternary mixtures: system-I {dodecane (1) + propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)}, system-II {dodecane (1) + propylbenzene (2) + [(emim)(CH3 SO4 )] (3)}, system-III {tetradecane (1) + propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} and system-IV {tetradecane (1) + propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at two temperatures (313 and 333) K and atmospheric pressure. The reliability of
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Table 1 Specifications of chemicals used in this study. Compound
Supplier
mmim (CH3 SO4 ) emim (CH3 SO4 ) Dodecane Tetradecane Proylbenzene
Merk Merk Sigma Aldrich Fluka
a b c d e f
Purity
UNIQUAC structural parameter
Mass fraction
r
q
Exp
Lit
Exp
Lit
>0.98 >0.98 >0.99 >0.99 >0.99
6.9034 7.5774 8.5462 9.8950 5.4983
5.5010 6.0280 7.0960 8.1759 4.3560
1.32899a 1.29205a 0.74523a 0.75921a 0.86313a
1.32725a,d na 0.74518a,e 0.75913a,e 0.86330a,f
1.4817a 1.4741a 1.4217 1.4288 1.4917
1.4827a ,b na 1.4220c 1.4291c 1.4915c
nD 20
At 25 ◦ C. [51]. [52]. [53]. [54]. [55].
the experimentally measured tie-line data was ascertained by the Othmer–Tobias correlation [44]. Additionally, the separation efficiency of ILs was evaluated using the distribution ratio as well as the selectivity. We studied the influence of the temperature, n-alkane structure,and the cations of two methylsulfate-based ionic liquids (1, 3-dimethylimidazolium and 1-ethyl-3-methylimidazolium) on the separation efficiency. The experimental data were correlated by the UNIQUAC and the NRTL models [45,46]. 2. Experimental methods 2.1. Chemicals The [(mmim)(CH3 SO4 )], [(emim)(CH3 SO4 )] and propylbenzene were stored under 4 nm molecular sieve. The water content in IL was regularly measured using Karl Fischer titration method with an average amount of 523 ppm for [(mmim)(CH3 SO4 )] and 435 ppm for [(emim)(CH3 SO4 )], no increase of water content was observed. The purities of dodecane, tetradecane and propylbenzene were determined by gas chromatography. The percentage peak areas of the pure substances are: 99.64%, 99.69% and 99.79% for dodecane, tetradecane, and propylbenzene respectively. All chemicals were used without further purification. The purities (mass fraction), refractive indices and densities of all chemicals used in this study are presented in Table 1.
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 processing system. The column was a Varian VF-5 ms CP8944 (30 m length and 0.25 mm I.D., 0.25 m film thickness). The ionic solvents [(mmim)(CH3 SO4 )] and [(emim)(CH3 SO4 )] have negligible vapor pressure and cannot be analyzed by GC. In the ternary mixtures only two components need to be analyzed; the third one, the ionic liquid, is determined by a mass balance of the of alkane and propylbenzene fractions in the two phases. In order to avoid inaccuracy and disruption of the analysis caused by fouling of the GC with ionic liquid, a pre-column was used to collect the ionic solvent and protect the primary column. The GC column temperature was programmed for an intial temperature of 363 K for 2 min, and a final temperature of 673 K for 5 min. The heating rate was 35 K/min, and the helium carrier gas flow rate was maintained at 3 × 10−6 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. To reduce error, each mole fraction was measured three times and the average value recorded. The experimental uncertainty of mole fraction measurements was ±0.0005. 3. Results and discussion
2.2. Apparatus and procedure
3.1. Experimental data
The experimental apparatus used for extraction consists of six 60 mL glass cells with a water jacket in order to maintain a constant temperature within a toleranece 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 [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )], 20 g of dodecane or tetradecane, 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 were performed over two temperatures, 313 K and 333 K. The density and refractive index of all pure components were measured at 298 K using a precision digital densimeter (model DMA 5000, Anton Paar, Germany) and a thermostatic digital refractometer (ABBE Mark II model 104810, Cambridge Instrument Inc., USA). The uncertainties of refractive index and density were within ±4 × 10−4 and ±5 × 10−3 kg m−3 respectively. The details of measurements and calibration of the instruments are described in our earlier work [47].
The measured equilibrium mole fractions for the feed, alkane rich-phase (raffinate) and IL-rich phase (extract) of the four ternary mixtures at the two temperatures (313 and 333) K and atmospheric pressure are reported in Tables 2–5. These tables also include the corresponding percent removal of the aromatic, expressed as [100 × (amount of aromatic extracted) ÷ (initial amount of aromatic in the feed)]. As shown in these tables, neither the temperature nor the concentration of propylbenzene had any effect on the solubility of IL in the alkane-rich phase. On the other hand, they also had an insignificant effect upon the solubility of alkanes in the ionic solvent-rich phase. The percentage of aromatic removal increased as the temperature and/or the concentration of propylbenzene in the feed increased, was higher using [(mmim)(CH3 SO4 )] than [(emim)(CH3 SO4 )], and increased with the chain length of alkanes. The experimental and the predicted tie lines of the four ternary systems at 313 K and 333 K are shown in Figs. 1 and 2 respectively. Comparing the miscibility of alkanes in ILs, dodecane is more soluble in ILs than tetradecane, i.e., the solubility increases as the chain length of the alkane decreases. This behavior between the alkanes and ionic liquids agrees with data published using other ILs [27–29,38]. No detectable concentrations of IL were found in the alkane-rich phase. The complete absence of IL in the alkane-rich
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
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Table 2 Experimental data for the ternary system {dodecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Dodecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
%Aromatic
K
S
T = 313 K 0.4690 0.4357 0.4068 0.3815 0.3591 0.3393 0.3215 0.3054 0.2910 0.2778 0.2657 T = 333 K
0.0765 0.1421 0.1990 0.2489 0.2929 0.3320 0.3670 0.3986 0.4271 0.4531 0.4768
0.8772 0.7827 0.7073 0.6444 0.5926 0.5489 0.5101 0.4769 0.4490 0.4235 0.4006
0.1228 0.2173 0.2927 0.3556 0.4074 0.4511 0.4899 0.5231 0.5510 0.5765 0.5994
0.0026 0.0031 0.0035 0.0040 0.0044 0.0048 0.0051 0.0054 0.0057 0.0060 0.0063
0.0236 0.0485 0.0734 0.0957 0.1189 0.1420 0.1610 0.1807 0.2035 0.2230 0.2408
16.19 16.51 16.42 16.26 16.75 16.74 17.21 17.50 17.63 17.94 18.30
0.19 0.22 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.40
64.70 56.36 50.25 43.93 39.39 36.21 33.08 30.68 29.24 27.44 25.67
0.4690 0.4357 0.4068 0.3815 0.3591 0.3393 0.3215 0.3054 0.2910 0.2778 0.2657
0.0765 0.1421 0.1990 0.2489 0.2929 0.3320 0.3670 0.3986 0.4271 0.4531 0.4768
0.8796 0.7857 0.7096 0.6470 0.5953 0.5509 0.5131 0.4800 0.4509 0.4254 0.4024
0.1204 0.2143 0.2904 0.3530 0.4047 0.4491 0.4869 0.5200 0.5491 0.5746 0.5976
0.0037 0.0042 0.0045 0.0052 0.0057 0.0061 0.0065 0.0068 0.0071 0.0075 0.0078
0.0268 0.0531 0.0778 0.1014 0.1256 0.1477 0.1699 0.1907 0.2106 0.2305 0.2486
17.59 17.43 18.00 18.28 18.05 18.38 18.22 18.52 18.94 19.09 19.18
0.22 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.38 0.40 0.42
52.89 46.39 41.96 36.08 32.46 29.86 27.66 26.00 24.46 22.84 21.54
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
phase is desirable, since it eliminates the need for a unit to recover the solvent from the raffinate in a continuous extraction process. The reliability of the experimentally measured tie-line data ascertained by applying the Othmer–Tobias correlation [44] is:
ln
1 − w3II w3II
= a + b ln
1 − w1I
(1)
w1I
where w3II is the mass fraction of IL (3) in the lower layer (ILrich phase), w1I is the mass fraction of alkanes (1) in the upper layer (alkanes-rich phase), 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 as well as the regression coefficients (R2 ) and standard deviation () are given in
Table 3 Experimental data for the ternary system {dodecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Dodecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
0.0801 0.1483 0.2071 0.2583 0.3033 0.3431 0.3786 0.4105 0.4393 0.4654 0.4892
0.8747 0.7777 0.7003 0.6376 0.5845 0.5401 0.5031 0.4699 0.4408 0.4139 0.3927
0.1253 0.2223 0.2997 0.3624 0.4155 0.4599 0.4969 0.5301 0.5592 0.5861 0.6073
0.0022 0.0025 0.0028 0.0031 0.0034 0.0037 0.0040 0.0043 0.0046 0.0049 0.0052
0.0801 0.1483 0.2071 0.2583 0.3033 0.3431 0.3786 0.4105 0.4393 0.4654 0.4892
0.8766 0.7800 0.7039 0.6413 0.5889 0.5443 0.5061 0.4725 0.4440 0.4189 0.3974
0.1234 0.2200 0.2961 0.3587 0.4111 0.4557 0.4939 0.5275 0.5560 0.5811 0.6026
0.0027 0.0030 0.0034 0.0037 0.0041 0.0045 0.0049 0.0053 0.0056 0.0059 0.0062
T = 313 K 0.4910 0.4546 0.4232 0.3959 0.3718 0.3506 0.3316 0.3146 0.2992 0.2853 0.2726 T = 333 K 0.4910 0.4546 0.4232 0.3959 0.3718 0.3506 0.3316 0.3146 0.2992 0.2853 0.2726
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
%Aromatic
K
S
0.0225 0.0447 0.0665 0.0892 0.1088 0.1293 0.1526 0.1714 0.1896 0.2023 0.2260
14.37 15.19 15.75 15.79 16.11 16.47 16.39 16.48 16.98 17.18 17.27
0.18 0.20 0.22 0.25 0.26 0.28 0.31 0.32 0.34 0.35 0.37
71.39 62.57 55.48 50.63 45.02 41.05 38.62 35.33 32.48 29.15 28.10
0.0252 0.0487 0.0740 0.0978 0.1207 0.1420 0.1627 0.1814 0.2024 0.2234 0.2468
16.41 16.75 16.78 16.85 17.17 17.28 17.50 17.65 17.77 17.97 18.06
0.20 0.22 0.25 0.27 0.29 0.31 0.33 0.34 0.36 0.38 0.41
67.66 58.53 51.76 47.25 42.17 37.70 34.01 30.67 28.86 27.30 26.25
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Table 4 Experimental data for the ternary system {tetradecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Tetradecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
%Aromatic
K
T = 313 K 0.4357 0.4029 0.3747 0.3503 0.3288 0.3098 0.2928 0.2777 0.2640 0.2516 0.2403
0.0813 0.1504 0.2098 0.2614 0.3068 0.3468 0.3825 0.4145 0.4434 0.4695 0.4933
0.8687 0.7679 0.6875 0.6216 0.5688 0.5243 0.4863 0.4535 0.4259 0.4010 0.3777
0.1313 0.2321 0.3125 0.3784 0.4312 0.4757 0.5137 0.5465 0.5741 0.5990 0.6223
0.0020 0.0023 0.0026 0.0029 0.0032 0.0035 0.0038 0.0041 0.0044 0.0047 0.0050
T = 333 K 0.4357 0.4029 0.3747 0.3503 0.3288 0.3098 0.2928 0.2777 0.2640 0.2516 0.2403
0.0813 0.1504 0.2098 0.2614 0.3068 0.3468 0.3825 0.4145 0.4434 0.4695 0.4933
0.8706 0.7695 0.6895 0.6244 0.5719 0.5259 0.4885 0.4564 0.4280 0.4032 0.3817
0.1294 0.2305 0.3105 0.3756 0.4281 0.4741 0.5115 0.5436 0.5720 0.5968 0.6183
0.0025 0.0028 0.0031 0.0035 0.0038 0.0041 0.0044 0.0048 0.0051 0.0055 0.0058
S
0.0312 0.0607 0.0877 0.1118 0.1380 0.1629 0.1866 0.2091 0.2338 0.2554 0.2722
19.18 19.23 19.08 18.75 19.10 19.34 19.56 19.73 20.25 20.51 20.34
0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.41 0.43 0.44
103.24 87.27 74.18 63.31 56.91 51.28 46.49 42.31 39.41 36.38 33.04
0.0334 0.0630 0.0908 0.1171 0.1445 0.1668 0.1923 0.2174 0.2403 0.2629 0.2858
20.60 20.03 19.85 19.78 20.16 19.92 20.31 20.76 21.02 21.34 21.79
0.26 0.27 0.29 0.31 0.34 0.35 0.38 0.40 0.42 0.44 0.46
89.97 75.11 65.06 55.63 50.80 45.13 41.73 38.02 35.26 32.29 30.41
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
Table 6. The regression coefficients are very close to unity, and the low standard deviations indicate a high degree of consistency. 3.2. Distribution ratio and selectivity Together with the LLE data, Tables 2–5 include the corresponding values for the solute distribution ratio (K) and the selectivity (S) for the four ternary systems, which are widely used parameters
in assessing the feasibility of utilizing the solvent in liquid-liquid extraction. The distribution ratio of propylbenzene, which is the measure of the solvent power or capacity of IL, is given by: K=
x2II
(2)
x2I
Table 5 Experimental data for the ternary system {tetradecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Tetradecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
T = 313 K 0.4574 0.4214 0.3907 0.3642 0.3410 0.3206 0.3025 0.2863 0.2718 0.2587 0.2468
0.0854 0.1573 0.2187 0.2718 0.3182 0.3589 0.3951 0.4275 0.4565 0.4827 0.5065
0.8646 0.7621 0.6807 0.6147 0.5620 0.5166 0.4794 0.4469 0.4183 0.3936 0.3720
0.1354 0.2379 0.3193 0.3853 0.4380 0.4834 0.5206 0.5531 0.5817 0.6064 0.6280
0.0017 0.0020 0.0023 0.0026 0.0029 0.0032 0.0035 0.0038 0.0041 0.0044 0.0047
T = 333 K 0.4574 0.4214 0.3907 0.3642 0.3410 0.3206 0.3025 0.2863 0.2718 0.2587 0.2468
0.0854 0.1573 0.2187 0.2718 0.3182 0.3589 0.3951 0.4275 0.4565 0.4827 0.5065
0.8665 0.7640 0.6847 0.6203 0.5657 0.5214 0.4823 0.4498 0.4220 0.3968 0.3743
0.1335 0.2360 0.3153 0.3797 0.4343 0.4786 0.5177 0.5502 0.5780 0.6032 0.6257
0.0021 0.0024 0.0027 0.0031 0.0033 0.0037 0.0040 0.0043 0.0047 0.0050 0.0053
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
%Aromatic
K
S
0.0293 0.0580 0.0840 0.1080 0.1348 0.1574 0.1830 0.2064 0.2276 0.2498 0.2719
16.19 16.51 16.42 16.26 16.75 16.74 17.21 17.50 17.63 17.94 18.30
0.22 0.24 0.26 0.28 0.31 0.33 0.35 0.37 0.39 0.41 0.43
109.97 92.81 77.88 66.25 59.65 52.55 48.13 43.88 39.91 36.85 34.27
0.0317 0.0610 0.0913 0.1197 0.1437 0.1701 0.1916 0.2157 0.2403 0.2614 0.2811
17.59 17.43 18.00 18.28 18.05 18.38 18.22 18.52 18.94 19.09 19.18
0.24 0.26 0.29 0.32 0.33 0.36 0.37 0.39 0.42 0.43 0.45
98.02 82.22 73.48 63.08 56.73 50.09 44.61 41.02 37.31 34.40 31.73
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Fig. 1. (a) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (b) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (c) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (d) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line).
where the x2 represents mole fraction of propylbenzene, superscripts I and II represent alkanes-rich phase and ILs-rich phase respectively. As shown in Tables 2–5, the distribution ratio increased with the chain length of alkanes and/or temperature and/or concentration of propylbenzene in the feed. Fig. 3 presents the relationship of the solvent-to-feed ratio ˛stf , which is expressed as (amount of solvent) ÷ (amount of aromatic and alkane in the feed), with the measured distribution ratios, for the ternary systems I and II at 313 K. The distribution ratio values decreased with the solvent to feed ratio ˛stf . A comparison of our data with previous studies using other ILs [39,42] is also presented in Fig. 3. As shown in this figure, the values of distribution ratio decrease in the following order; [(bmim)(PF6 )] > [mebupy][BF4 ] ≥ [(mmim) (CH3 SO4 )] > [(emim)(CH3 SO4 )].
The effectiveness of extraction of propylbenzene from alkane using ILs can be expressed by the selectivity of the solvent. The selectivity of IL, which is a measure of the ability of IL to separate propylbenzene from alkanes, is given by: S=
x2II x2I
(3)
x2I x1II
where the x represents mole fraction, subscripts 1 and 2 represent alkanes and propylbenzene respectively, superscripts I and II represent alkanes-rich phase and ILs-rich phase respectively. From the selectivity data presented in Fig. 4, for the ternary systems I and II at 313 K, the selectivity values increased with the solvent-to-feed ratio ˛stf . A comparison of our data with previous studies using other ILs [39,42] is presented in the Fig. 4. As shown
Table 6 Constants of the Othmer–Tobias correlation, correlation factor (R2 ) and standard deviations () of the four ternary systems at T = 313 K and 333 K and P = 101.3 kPa. T/K 313 333 313 333 313 333 313 333
a
b
{dodecadecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} −1.5045 0.7521 −1.4218 0.7344 {dodecadecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} −1.3409 0.7532 −1.2834 0.7474 {tetradecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} −0.9383 0.8695 −0.8396 0.8658 {tetradecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} −0.7201 0.8943 −0.5802 0.8958
R2
˛
0.9968 0.9986
0.0340 0.0222
0.9988 0.9994
0.0206 0.0150
0.9989 0.9983
0.0229 0.0283
0.9980 0.9978
0.0316 0.0332
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A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
Fig. 2. (a) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (b) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (c) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (d) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line).
in this figure, the selectivity values increase in the following order; [(bmim)(PF6 )] < [(mmim)(CH3 SO4 )] < [mebupy][BF4 ] ∼ = [(emim) (CH3 SO4 )]. As shown in Tables 2–5, the selectivity values are not constant over the whole two-phase region, and while it increased with the chain length of alkanes it decreased with temperature and/or concentration of propylbenzene in the feed. The selectivity values
in the systems under study are higher than unity, which ensures the feasibility of separation of propylbenzene and dodecane or tetradecane from their mixtures. Moreover, the use of [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )], and of ionic liquids in general, in solvent extraction is favourable because it can be easily recovered and reused.
Fig. 3. Measured distribution coefficient (K) against solvent to feed ratio (˛stf ) at 313 K for the ternary systems: dodecane + propylbenzene + [(mmim)(CH3 SO4 )] (䊉), or + [(emim)(CH3 SO4 )] (), or + [mebupy][BF4 ] () reference [39], or + [(bmim)(PF6 )] () reference [42].
Fig. 4. Measured selectivity (S) against solvent to feed ratio (˛stf ) at 313 K the ternary systems: dodecane + propylbenzene + [(mmim)(CH3 SO4 )] for (䊉), or + [(emim)(CH3 SO4 )] (), or + [mebupy][BF4 ] () reference [39], or + [(bmim)(PF6 )] () reference [42].
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
35
Table 7 UNIQUAC interaction parameters and root mean square deviation (rmsd) of the four ternary systems at T = 313 and 333 K and P = 101.3 kPa. UNIQUAC
313 K
333 K
i
j
aij
aji
Dodecane Dodecane Propylbenzene Dodecane Dodecane Propylbenzene Tetradecane Tetradecane Propylbenzene Tetradecane Tetradecane Propylbenzene
Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 ) Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 )
−128.92 589.99 301.77 −107.74 506.49 285.36 −111.53 469.38 55.537 −86.909 459.60 −9.9696
46.655 −32.785 −150.51 24.144 −11.644 −132.08 98.029 362.29 27.446 73.36 382.78 101.55
RMSD
0.1140
0.1263
0.2028
0.1791
aij
aji
−124.58 648.33 315.61 −160.980 472.090 107.650 −89.831 527.64 −5.1138 −124.34 436.12 97.98
48.434 −49.582 −155.14 113.230 169.700 −27.000 148.72 435.68 139.88 91.894 284.01 −16.484
RMSD
0.1093
0.2481
0.2250
0.2057
Table 8 NRTL interaction parameters and root mean square deviation (rmsd) of the four ternary systems at T = 313 K and 333 K and P = 101.3 kPa. NRTL (˛ = 0.2)
313 K
333 K
i
j
aij
aji
Dodecane Dodecane Propylbenzene Dodecane Dodecane Propylbenzene Tetradecane Tetradecane Propylbenzene Tetradecane Tetradecane Propylbenzene
Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 ) Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 )
−752.44 1638.3 1464.3 −742.41 1613.5 1481.3 −874.99 1633.2 1447.3 −919.35 1627.3 1440.9
303.6 1130.1 −477.12 302.07 1169.2 −430.56 514.99 1177.7 −523.28 521.2 1186.5 −531.03
NRTL (˛ = 0.3)
RMSD
0.1674
0.1747
0.2013
0.2004
313 K
i
j
Dodecane Dodecane Propylbenzene Dodecane Dodecane Propylbenzene Tetradecane Tetradecane Propylbenzene Tetradecane Tetradecane Propylbenzene
Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 ) Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 )
aij
aji
−744.55 1745.40 1551.50 −852.94 1729.9 1510.7 −951.18 1747.2 1497.2 −889.20 1718.80 1548.60
309.18 1129.60 −498.34 406.44 1190.5 −510.09 569.31 1202.7 −581.05 504.37 1275.30 −551.24
RMSD
0.1677
0.2052
0.2342
0.1932
333 K
aij
aji
−942.98 1577.1 1498 −915.76 1565.80 1505.90 −994.25 1563.3 1499.5 −1057.40 1555.60 1497.80
RMSD
552.79 1466.7 −414.34 589.93 1484.30 −337.26 724.36 1472.1 −454.81 709.89 1486.70 −494.54
0.1500
0.1666
0.1812
0.1751
aij −928.60 1665.20 1582.60 −1024.2 1660.8 1565.7 −1101.4 1651.1 1571.8 111.36 1839 1186.1
aji 574.87 1484.30 −401.94 627.85 1531.6 −450.29 746.43 1522.5 −551.96 1771.2 1075.5 309.33
RMSD
0.1442
0.1839
0.2061
0.2840
3.3. Thermodynamic correlations The UNIQUAC model of Abrams and Prausnitz [45] and the NRTL model of Renon and Prausnitz [46] were used to correlate our experimental data. For the UNIQUAC model, the excess Gibbs energy (GE ) of the UNIQUAC model is
GE = xi ln i + 5 xi qi ln i − xi qi ln RT xi i 3
3
3
i=1
i=1
i=1
where
i
=
xi ri
3
xr i=1 i
⎛ ⎝
n
j=1
⎞ j ji ⎠
(4)
i =
xi qi
3
xq i=1 i i
u −u ij jj
ij = exp −
RT
u −u ji ii
ji = exp −
RT
here q and r represent the UNIQUAC surface area and volume, while i and i represent the area fraction and segment fraction of species i, respectively. The u is the energy of interaction for each binary pair of compounds and is adjustable parameter.
36
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
In the NRTL model, the excess Gibbs energy of mixing (GE ) of the NRTL model is
GE xi = RT 3
n
j=1 ji
n
Gji xj
(5)
G x k=1 ki k
i=1
where ij =
gij − gjj
=
RT
aij T
Gij = exp(−˛ij ij ) where R is the gas constant, T is the absolute temperature, x is mole fraction, g is energy of interaction for each binary pair of compounds, G is binary interaction parameter, is adjustable parameter, ˛ is nonrandomness parameter. The aij and aji are the two interaction parameters for each binary pair that we find from correlation. The LLE experimental data were used to determine the optimum UNIQUAC and NRTL binary interaction parameters between [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )], propylbenzene, and dodecane or tetradecane. The UNIQUAC and the NRTL models were fitted to experimental data using S∅rensen’s iterative computer programme, based on the flash calculation method [48]. The objective function was determined by minimizing the square of the difference between the mole fractions predicted by the respective method and those experimentally measured over all the tie lines in the ternary system. For the UNIQUAC correlation, the pure component structural parameters (r and q) listed in Table 1 were calculated from the group contribution data [49] or taken from literature [50]. The NRTL model was fitted with fixed values of the third non-randomness parameter, ˛ij , for each pair of components (˛ij = 0.2 or 0.3) during calculations, and the correlation with ˛ij = 0.3 gave better results according to relative mean square deviation (RMSD) values. The objective function, OF, is: OF = min
k
j
i
(xijk,exp − xijk,cal )2
(6)
where x is mole fraction, subscripts exp, cal, i, j, and k are experimental, calculated, components, phases and tie lines respectively. The optimization results were assessed by calculating the corresponding RMSD values using the following equation:
RMSD = 100
k
j
i
(xijk,exp − xijk,cal )2 6n
1/2 (7)
where n is number of tie lines. The values of interaction parameters and the RMSD for the UNIQUAC and the NRTL models at different temperatures are shown in Tables 7 and 8, respectively. These parameters are used to calculate LLE tie lines for the present systems. The calculations based on both models gave good representation of the tie line data for those systems and analysis of the mean RMSD indicates they are effectively the same (UNIQUAC x¯ RMSD = 0.1763, NRTL x¯ RMSD = 0.1864). 4. Conclusions The experimental liquid–liquid equilibria investigation of four ternary mixtures {dodecane or tetradecane + propylbenzene + [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )]} was carried out at two temperatures (313 and 333) K and at atmospheric pressure to study the effect of ILs structure and alkane chain length on the equilibrium behavior. While the temperature and/or the concentration of propylbenzene have no effect on the solubility of [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )]
in the alkane-rich phase, they have some effect upon the solubility of alkanes in the ionic solvent-rich phase. The consistency of the experimental LLE data was determined by applying the Othmer–Tobias correlation, producing an average regression coefficient (R2 ) of 0.999. In addition, the UNIQUAC and the NRTL models satisfactorily correlate with the LLE experimental data, and they are effectively the same for the studied systems. Evaluation of the effects of temperature and the solvent-to-feed ratio on distribution ratio and selectivity indicate that a shorter alkyl chain on the imidazolium cation of methylsulfate anion-based ionic liquids is favourable for higher percentage aromatic removal and distribution ratio, but it leads to lower selectivity. While temperature is directly related to the percentage of aromatic removal and the distribution ratio, it is inversely related to selectivity. In all cases, the studied systems provided high values of selectivity, implying that [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )] can be used to extract propylbenzene from alkane mixtures. Acknowledgements The authors thank the Public Authority for Applied Education and Training (PAAET-TS-06-03) for the financial support of this work. References [1] J. Rydberg, M. Cox, C. Musikas, G.R. Choppin, Solvent Extraction Principles and Practice, Revised and Expanded, second ed., Dekker, New York, 2004. [2] R.D. Rogers, K.R. Seddon, Ionic Liquids-Industrial Applications to Green Chemistry, in ACS Symposium Series 818, American Chemical Society, Washington DC, 2002. [3] V. Toteva, L. Topalova, P. Manolova, J. Univ. Chem. Technol. Metal 42 (2007) 17–20. [4] J.M. Bolard, in: A.G Lucas (Ed.), Modern Petroleum Technology, 6th ed., John W. & Sons L, Chichester, U.K, 2000, pp. 367–387. [5] H.G. James, E.H. Glenn, Petroleum Refining Technology and Economics, Marcel Dekker, INC, New York, Basel, 2001. [6] R. Krishna, H. Singh, K. Kishore, G.S. Choudhary, R.S. Kaushik, Revue-de-L’ &TDREFS;Institut Franc¸a&TDREFS;is du Petrole 42 (6) (1987) 827–836. [7] A.A. Gaile, G.D. Zalishchevskii, L.V. Varshavskii, L.V. Semenov, Russ. J. Appl. Chem. 74 (5) (2001) 864–869. [8] J. Mahmoudi, M.N. Lotfollahi, J. Chem. Thermodyn. 42 (2010) 466–471. [9] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, Fluid Phase Equilib. 247 (2006) 158–168. [10] A. Heintz, D.V. Kulikov, S.P. Verevkin, J. Chem. Eng. Data 46 (2001) 1526–1529. [11] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, Fuel Process. Technol. 87 (2005) 59–70. [12] G.W. Meindersma, A.B. de Haan, Chem. Eng. Res. Des. 86 (2008) 745–752. [13] G.W. Meindersma, A.R. Hansmeier, A.B. de Haan, Ind. Eng. Chem. Res. 49 (2010) 7530–7540. [14] A. Marciniak, Fluid Phase Equilib. 294 (2010) 213–233. ´ [15] U. Domans&TDREFS;ka, A. Pobudkowska, M. Król&TDREFS;ikowski, Fluid Phase Equilib. 259 (2007) 173–179. [16] J. García&TDREFS;, S. García&TDREFS;, J.S. Torrecilla, M. Oliet, F. Rodríg&TDREFS;uez, J. Chem. Thermodyn. 42 (2010) 1004–1008. [17] A. Arce, M.J. Early, H. Rodríg&TDREFS;uez, K.R. Seddon, Green Chem. 9 (2007) 70–74. [18] A. Arce, M.J. Early, H. Rodríg&TDREFS;uez, K.R. Seddon, J. Phys. Chem. B 111 (2007) 4732–4736. [19] A. Arce, M.J. Early, H. Rodríg&TDREFS;uez, K.R. Seddon, Green Chem. 10 (2008) 1294–1300. [20] R. Wang, J. Wang, H. Meng, C. Li, Z. Wang, J. Chem. Eng. Data 53 (2008) 1159–1162. [21] R. Wang, J. Wang, H. Meng, C. Li, Z. Wang, J. Chem. Eng. Data 53 (2008) 2170–2174. [22] R.M. Maduro, M. Aznar, Fluid Phase Equilib. 265 (2008) 129–138. [23] E.J. Gonzál&TDREFS;ez, N. Calvar, B. Gonzál&TDREFS;ez, A. Domín&TDREFS;guez, J. Chem. Thermodyn. 41 (2009) 1215–1221. [24] A.B. Pereiro, A. Rodriguez, J. Chem. Thermodyn. 41 (2009) 951–956. [25] A. Arce, M.J. Early, H. Rodríg&TDREFS;uez, K.R. Seddon, A. Soto, Green Chem. 11 (2009) 365–372. [26] J. Han, C. Yu, Y. Wang, X. Xie, Y. Yan, G. Yin, W. Guan, Fluid Phase Equilib. 295 (2010) 98–103. [27] J. García&TDREFS;, S. García&TDREFS;, J.S. Torrecilla, M. Oliet, F. Rodríg&TDREFS;uez, Fluid Phase Equilib. 301 (2011) 62–66. [28] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, J. Chem. Eng. Data 51 (2006) 1814–1819.
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Contents lists available at SciVerse ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Study on the separation of propylbenzene from alkanes using two methylsulfate-based ionic liquids at (313 and 333) K Adel S. Al-Jimaz ∗ , Khaled H.A.E. Alkhaldi, Mohsen H. Al-Rashed, Mohamed S. Fandary, Mohammad S. AlTuwaim Chemical Engineering Department, College of Technological Studies, PAAET, P.O. Box 42325, Shuwaikh 70654, Kuwait
a r t i c l e
i n f o
Article history: Received 10 February 2013 Received in revised form 20 May 2013 Accepted 6 June 2013 Available online 15 June 2013 Keywords: LLE Ionic solvent Alkane Propylbenzene UNIQUAC NRTL
a b s t r a c t The separation of propylbenzene from dodecane or tetradecane using different ionic liquids (ILs) based on the methylsulfate anion (CH3 SO4 − ) was studied at two temperatures (313 and 333) K and atmospheric pressure. The ionic liquids, 1, 3-dimethylimidazolium methylsulfate [(mmim)(CH3 SO4 )] and 1-ethyl-3-methylimidazolium methylsulfate [(emim)(CH3 SO4 )], were evaluated as solvents for the extraction process. Four ternary systems were formed by {dodecane or tetradecane + propylbenzene + [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )]}. The degree of quality of the experimental data was ascertained using the Othmer–Tobias correlation. A comparison of the phase behaviour of the systems studied has allowed us to investigate the effect of ILs cation, alkane chain length, temperature and solvent-to-feed ratio upon solubility, percent aromatic removal, distribution ratio, and selectivity. Finally, the experimental LLE data was correlated using the UNIQUAC and the NRTL models. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the petroleum refinery, extraction of aromatics from the middle distillate fuels is a process of major importance. The properties of several petroleum products may be improved by removing the aromatic hydrocarbons from vacuum distillates [1,2]. Due to environmental legislation, the demand of ‘clean’ fuels is increasing and most likely will increase even more towards fuels with almost zero content of certain aromatics, e.g. benzene and toluene. In particular, the concentration of benzene must be reduced to ≤0.1% by weight in carburant fuels. The most common processes use solvents such as dimethylsulfoxide (DMSO), furfural, N-methylpyrrolidone (NMP), dimethylformamide (DMF), and sulfolane for extraction of aromatic and sulfur hydrocarbons from middle distillate fractions [3–8]. Although the processes based on these solvents are well known, there is a need to study environmental friendly solvents with better selectivity and capacity. From activity coefficients at infinite dilution measurements, it has been shown that many ionic liquids have better selectivity and capacity in extraction of aromatics from aromatic + aliphatic mixtures than typical solvents such as sulfolane and NMP [9].
∗ Corresponding author. Tel.: +965 99088996. E-mail address: a [email protected] (A.S. Al-Jimaz). 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.06.010
The design of safe and environmentally friendly separation processes is of increasing importance in the chemical industry. Over the last few years, ionic liquids (ILs) were suggested as an alternative to traditional organic solvents for extraction processes. The ILs show interesting positive properties, such as negligible vapour pressure, stability at high temperatures, and the ability to tailor ILs to feedstock composition. [10–13]. Many researchers focused on liquid–liquid equilibria (LLE) measurements for the ternary systems containing (aliphatic + aromatic + ILs) [9,14–29]. Ionic liquids based on the alkylsulfate anion (R-SO4 − ) have attracted great interest because they are less viscous, more hydrolytically stable, and more environmental friendly than other ILs, as well as easily synthesized at a reasonable cost [30]. Moreover, the alkylsulfate-based ILs show chemical and thermal stability and low melting points [31]. Recent research has explored combining the anion (R-SO4 − ) with the cations-imidazolium or pyridinium [30–33,23,34–38]. As a continuation of our ongoing work on aromatic extraction from middle distillate fractions using ILs [39–43], we analyzed the suitability of the ILs to separate propylbenzene from dodecane or tetradecane in four ternary mixtures: system-I {dodecane (1) + propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)}, system-II {dodecane (1) + propylbenzene (2) + [(emim)(CH3 SO4 )] (3)}, system-III {tetradecane (1) + propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} and system-IV {tetradecane (1) + propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at two temperatures (313 and 333) K and atmospheric pressure. The reliability of
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A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
Table 1 Specifications of chemicals used in this study. Compound
Supplier
mmim (CH3 SO4 ) emim (CH3 SO4 ) Dodecane Tetradecane Proylbenzene
Merk Merk Sigma Aldrich Fluka
a b c d e f
Purity
UNIQUAC structural parameter
Mass fraction
r
q
Exp
Lit
Exp
Lit
>0.98 >0.98 >0.99 >0.99 >0.99
6.9034 7.5774 8.5462 9.8950 5.4983
5.5010 6.0280 7.0960 8.1759 4.3560
1.32899a 1.29205a 0.74523a 0.75921a 0.86313a
1.32725a,d na 0.74518a,e 0.75913a,e 0.86330a,f
1.4817a 1.4741a 1.4217 1.4288 1.4917
1.4827a ,b na 1.4220c 1.4291c 1.4915c
nD 20
At 25 ◦ C. [51]. [52]. [53]. [54]. [55].
the experimentally measured tie-line data was ascertained by the Othmer–Tobias correlation [44]. Additionally, the separation efficiency of ILs was evaluated using the distribution ratio as well as the selectivity. We studied the influence of the temperature, n-alkane structure,and the cations of two methylsulfate-based ionic liquids (1, 3-dimethylimidazolium and 1-ethyl-3-methylimidazolium) on the separation efficiency. The experimental data were correlated by the UNIQUAC and the NRTL models [45,46]. 2. Experimental methods 2.1. Chemicals The [(mmim)(CH3 SO4 )], [(emim)(CH3 SO4 )] and propylbenzene were stored under 4 nm molecular sieve. The water content in IL was regularly measured using Karl Fischer titration method with an average amount of 523 ppm for [(mmim)(CH3 SO4 )] and 435 ppm for [(emim)(CH3 SO4 )], no increase of water content was observed. The purities of dodecane, tetradecane and propylbenzene were determined by gas chromatography. The percentage peak areas of the pure substances are: 99.64%, 99.69% and 99.79% for dodecane, tetradecane, and propylbenzene respectively. All chemicals were used without further purification. The purities (mass fraction), refractive indices and densities of all chemicals used in this study are presented in Table 1.
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 processing system. The column was a Varian VF-5 ms CP8944 (30 m length and 0.25 mm I.D., 0.25 m film thickness). The ionic solvents [(mmim)(CH3 SO4 )] and [(emim)(CH3 SO4 )] have negligible vapor pressure and cannot be analyzed by GC. In the ternary mixtures only two components need to be analyzed; the third one, the ionic liquid, is determined by a mass balance of the of alkane and propylbenzene fractions in the two phases. In order to avoid inaccuracy and disruption of the analysis caused by fouling of the GC with ionic liquid, a pre-column was used to collect the ionic solvent and protect the primary column. The GC column temperature was programmed for an intial temperature of 363 K for 2 min, and a final temperature of 673 K for 5 min. The heating rate was 35 K/min, and the helium carrier gas flow rate was maintained at 3 × 10−6 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. To reduce error, each mole fraction was measured three times and the average value recorded. The experimental uncertainty of mole fraction measurements was ±0.0005. 3. Results and discussion
2.2. Apparatus and procedure
3.1. Experimental data
The experimental apparatus used for extraction consists of six 60 mL glass cells with a water jacket in order to maintain a constant temperature within a toleranece 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 [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )], 20 g of dodecane or tetradecane, 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 were performed over two temperatures, 313 K and 333 K. The density and refractive index of all pure components were measured at 298 K using a precision digital densimeter (model DMA 5000, Anton Paar, Germany) and a thermostatic digital refractometer (ABBE Mark II model 104810, Cambridge Instrument Inc., USA). The uncertainties of refractive index and density were within ±4 × 10−4 and ±5 × 10−3 kg m−3 respectively. The details of measurements and calibration of the instruments are described in our earlier work [47].
The measured equilibrium mole fractions for the feed, alkane rich-phase (raffinate) and IL-rich phase (extract) of the four ternary mixtures at the two temperatures (313 and 333) K and atmospheric pressure are reported in Tables 2–5. These tables also include the corresponding percent removal of the aromatic, expressed as [100 × (amount of aromatic extracted) ÷ (initial amount of aromatic in the feed)]. As shown in these tables, neither the temperature nor the concentration of propylbenzene had any effect on the solubility of IL in the alkane-rich phase. On the other hand, they also had an insignificant effect upon the solubility of alkanes in the ionic solvent-rich phase. The percentage of aromatic removal increased as the temperature and/or the concentration of propylbenzene in the feed increased, was higher using [(mmim)(CH3 SO4 )] than [(emim)(CH3 SO4 )], and increased with the chain length of alkanes. The experimental and the predicted tie lines of the four ternary systems at 313 K and 333 K are shown in Figs. 1 and 2 respectively. Comparing the miscibility of alkanes in ILs, dodecane is more soluble in ILs than tetradecane, i.e., the solubility increases as the chain length of the alkane decreases. This behavior between the alkanes and ionic liquids agrees with data published using other ILs [27–29,38]. No detectable concentrations of IL were found in the alkane-rich phase. The complete absence of IL in the alkane-rich
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
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
31
Table 2 Experimental data for the ternary system {dodecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Dodecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
%Aromatic
K
S
T = 313 K 0.4690 0.4357 0.4068 0.3815 0.3591 0.3393 0.3215 0.3054 0.2910 0.2778 0.2657 T = 333 K
0.0765 0.1421 0.1990 0.2489 0.2929 0.3320 0.3670 0.3986 0.4271 0.4531 0.4768
0.8772 0.7827 0.7073 0.6444 0.5926 0.5489 0.5101 0.4769 0.4490 0.4235 0.4006
0.1228 0.2173 0.2927 0.3556 0.4074 0.4511 0.4899 0.5231 0.5510 0.5765 0.5994
0.0026 0.0031 0.0035 0.0040 0.0044 0.0048 0.0051 0.0054 0.0057 0.0060 0.0063
0.0236 0.0485 0.0734 0.0957 0.1189 0.1420 0.1610 0.1807 0.2035 0.2230 0.2408
16.19 16.51 16.42 16.26 16.75 16.74 17.21 17.50 17.63 17.94 18.30
0.19 0.22 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39 0.40
64.70 56.36 50.25 43.93 39.39 36.21 33.08 30.68 29.24 27.44 25.67
0.4690 0.4357 0.4068 0.3815 0.3591 0.3393 0.3215 0.3054 0.2910 0.2778 0.2657
0.0765 0.1421 0.1990 0.2489 0.2929 0.3320 0.3670 0.3986 0.4271 0.4531 0.4768
0.8796 0.7857 0.7096 0.6470 0.5953 0.5509 0.5131 0.4800 0.4509 0.4254 0.4024
0.1204 0.2143 0.2904 0.3530 0.4047 0.4491 0.4869 0.5200 0.5491 0.5746 0.5976
0.0037 0.0042 0.0045 0.0052 0.0057 0.0061 0.0065 0.0068 0.0071 0.0075 0.0078
0.0268 0.0531 0.0778 0.1014 0.1256 0.1477 0.1699 0.1907 0.2106 0.2305 0.2486
17.59 17.43 18.00 18.28 18.05 18.38 18.22 18.52 18.94 19.09 19.18
0.22 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.38 0.40 0.42
52.89 46.39 41.96 36.08 32.46 29.86 27.66 26.00 24.46 22.84 21.54
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
phase is desirable, since it eliminates the need for a unit to recover the solvent from the raffinate in a continuous extraction process. The reliability of the experimentally measured tie-line data ascertained by applying the Othmer–Tobias correlation [44] is:
ln
1 − w3II w3II
= a + b ln
1 − w1I
(1)
w1I
where w3II is the mass fraction of IL (3) in the lower layer (ILrich phase), w1I is the mass fraction of alkanes (1) in the upper layer (alkanes-rich phase), 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 as well as the regression coefficients (R2 ) and standard deviation () are given in
Table 3 Experimental data for the ternary system {dodecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Dodecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
0.0801 0.1483 0.2071 0.2583 0.3033 0.3431 0.3786 0.4105 0.4393 0.4654 0.4892
0.8747 0.7777 0.7003 0.6376 0.5845 0.5401 0.5031 0.4699 0.4408 0.4139 0.3927
0.1253 0.2223 0.2997 0.3624 0.4155 0.4599 0.4969 0.5301 0.5592 0.5861 0.6073
0.0022 0.0025 0.0028 0.0031 0.0034 0.0037 0.0040 0.0043 0.0046 0.0049 0.0052
0.0801 0.1483 0.2071 0.2583 0.3033 0.3431 0.3786 0.4105 0.4393 0.4654 0.4892
0.8766 0.7800 0.7039 0.6413 0.5889 0.5443 0.5061 0.4725 0.4440 0.4189 0.3974
0.1234 0.2200 0.2961 0.3587 0.4111 0.4557 0.4939 0.5275 0.5560 0.5811 0.6026
0.0027 0.0030 0.0034 0.0037 0.0041 0.0045 0.0049 0.0053 0.0056 0.0059 0.0062
T = 313 K 0.4910 0.4546 0.4232 0.3959 0.3718 0.3506 0.3316 0.3146 0.2992 0.2853 0.2726 T = 333 K 0.4910 0.4546 0.4232 0.3959 0.3718 0.3506 0.3316 0.3146 0.2992 0.2853 0.2726
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
%Aromatic
K
S
0.0225 0.0447 0.0665 0.0892 0.1088 0.1293 0.1526 0.1714 0.1896 0.2023 0.2260
14.37 15.19 15.75 15.79 16.11 16.47 16.39 16.48 16.98 17.18 17.27
0.18 0.20 0.22 0.25 0.26 0.28 0.31 0.32 0.34 0.35 0.37
71.39 62.57 55.48 50.63 45.02 41.05 38.62 35.33 32.48 29.15 28.10
0.0252 0.0487 0.0740 0.0978 0.1207 0.1420 0.1627 0.1814 0.2024 0.2234 0.2468
16.41 16.75 16.78 16.85 17.17 17.28 17.50 17.65 17.77 17.97 18.06
0.20 0.22 0.25 0.27 0.29 0.31 0.33 0.34 0.36 0.38 0.41
67.66 58.53 51.76 47.25 42.17 37.70 34.01 30.67 28.86 27.30 26.25
32
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
Table 4 Experimental data for the ternary system {tetradecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Tetradecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
%Aromatic
K
T = 313 K 0.4357 0.4029 0.3747 0.3503 0.3288 0.3098 0.2928 0.2777 0.2640 0.2516 0.2403
0.0813 0.1504 0.2098 0.2614 0.3068 0.3468 0.3825 0.4145 0.4434 0.4695 0.4933
0.8687 0.7679 0.6875 0.6216 0.5688 0.5243 0.4863 0.4535 0.4259 0.4010 0.3777
0.1313 0.2321 0.3125 0.3784 0.4312 0.4757 0.5137 0.5465 0.5741 0.5990 0.6223
0.0020 0.0023 0.0026 0.0029 0.0032 0.0035 0.0038 0.0041 0.0044 0.0047 0.0050
T = 333 K 0.4357 0.4029 0.3747 0.3503 0.3288 0.3098 0.2928 0.2777 0.2640 0.2516 0.2403
0.0813 0.1504 0.2098 0.2614 0.3068 0.3468 0.3825 0.4145 0.4434 0.4695 0.4933
0.8706 0.7695 0.6895 0.6244 0.5719 0.5259 0.4885 0.4564 0.4280 0.4032 0.3817
0.1294 0.2305 0.3105 0.3756 0.4281 0.4741 0.5115 0.5436 0.5720 0.5968 0.6183
0.0025 0.0028 0.0031 0.0035 0.0038 0.0041 0.0044 0.0048 0.0051 0.0055 0.0058
S
0.0312 0.0607 0.0877 0.1118 0.1380 0.1629 0.1866 0.2091 0.2338 0.2554 0.2722
19.18 19.23 19.08 18.75 19.10 19.34 19.56 19.73 20.25 20.51 20.34
0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.41 0.43 0.44
103.24 87.27 74.18 63.31 56.91 51.28 46.49 42.31 39.41 36.38 33.04
0.0334 0.0630 0.0908 0.1171 0.1445 0.1668 0.1923 0.2174 0.2403 0.2629 0.2858
20.60 20.03 19.85 19.78 20.16 19.92 20.31 20.76 21.02 21.34 21.79
0.26 0.27 0.29 0.31 0.34 0.35 0.38 0.40 0.42 0.44 0.46
89.97 75.11 65.06 55.63 50.80 45.13 41.73 38.02 35.26 32.29 30.41
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
Table 6. The regression coefficients are very close to unity, and the low standard deviations indicate a high degree of consistency. 3.2. Distribution ratio and selectivity Together with the LLE data, Tables 2–5 include the corresponding values for the solute distribution ratio (K) and the selectivity (S) for the four ternary systems, which are widely used parameters
in assessing the feasibility of utilizing the solvent in liquid-liquid extraction. The distribution ratio of propylbenzene, which is the measure of the solvent power or capacity of IL, is given by: K=
x2II
(2)
x2I
Table 5 Experimental data for the ternary system {tetradecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} at T = 313 K and 333 K and P = 101.3 kPa. Feed (global composition)
Tetradecane rich phase
Solvent rich phase
x1
x2
x1
x2
x1
x2
T = 313 K 0.4574 0.4214 0.3907 0.3642 0.3410 0.3206 0.3025 0.2863 0.2718 0.2587 0.2468
0.0854 0.1573 0.2187 0.2718 0.3182 0.3589 0.3951 0.4275 0.4565 0.4827 0.5065
0.8646 0.7621 0.6807 0.6147 0.5620 0.5166 0.4794 0.4469 0.4183 0.3936 0.3720
0.1354 0.2379 0.3193 0.3853 0.4380 0.4834 0.5206 0.5531 0.5817 0.6064 0.6280
0.0017 0.0020 0.0023 0.0026 0.0029 0.0032 0.0035 0.0038 0.0041 0.0044 0.0047
T = 333 K 0.4574 0.4214 0.3907 0.3642 0.3410 0.3206 0.3025 0.2863 0.2718 0.2587 0.2468
0.0854 0.1573 0.2187 0.2718 0.3182 0.3589 0.3951 0.4275 0.4565 0.4827 0.5065
0.8665 0.7640 0.6847 0.6203 0.5657 0.5214 0.4823 0.4498 0.4220 0.3968 0.3743
0.1335 0.2360 0.3153 0.3797 0.4343 0.4786 0.5177 0.5502 0.5780 0.6032 0.6257
0.0021 0.0024 0.0027 0.0031 0.0033 0.0037 0.0040 0.0043 0.0047 0.0050 0.0053
Uncertainty of mole fraction is ±0.0005. Uncertainty of temperature is ±0.2 K.
%Aromatic
K
S
0.0293 0.0580 0.0840 0.1080 0.1348 0.1574 0.1830 0.2064 0.2276 0.2498 0.2719
16.19 16.51 16.42 16.26 16.75 16.74 17.21 17.50 17.63 17.94 18.30
0.22 0.24 0.26 0.28 0.31 0.33 0.35 0.37 0.39 0.41 0.43
109.97 92.81 77.88 66.25 59.65 52.55 48.13 43.88 39.91 36.85 34.27
0.0317 0.0610 0.0913 0.1197 0.1437 0.1701 0.1916 0.2157 0.2403 0.2614 0.2811
17.59 17.43 18.00 18.28 18.05 18.38 18.22 18.52 18.94 19.09 19.18
0.24 0.26 0.29 0.32 0.33 0.36 0.37 0.39 0.42 0.43 0.45
98.02 82.22 73.48 63.08 56.73 50.09 44.61 41.02 37.31 34.40 31.73
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
33
Fig. 1. (a) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (b) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (c) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (d) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 313 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line).
where the x2 represents mole fraction of propylbenzene, superscripts I and II represent alkanes-rich phase and ILs-rich phase respectively. As shown in Tables 2–5, the distribution ratio increased with the chain length of alkanes and/or temperature and/or concentration of propylbenzene in the feed. Fig. 3 presents the relationship of the solvent-to-feed ratio ˛stf , which is expressed as (amount of solvent) ÷ (amount of aromatic and alkane in the feed), with the measured distribution ratios, for the ternary systems I and II at 313 K. The distribution ratio values decreased with the solvent to feed ratio ˛stf . A comparison of our data with previous studies using other ILs [39,42] is also presented in Fig. 3. As shown in this figure, the values of distribution ratio decrease in the following order; [(bmim)(PF6 )] > [mebupy][BF4 ] ≥ [(mmim) (CH3 SO4 )] > [(emim)(CH3 SO4 )].
The effectiveness of extraction of propylbenzene from alkane using ILs can be expressed by the selectivity of the solvent. The selectivity of IL, which is a measure of the ability of IL to separate propylbenzene from alkanes, is given by: S=
x2II x2I
(3)
x2I x1II
where the x represents mole fraction, subscripts 1 and 2 represent alkanes and propylbenzene respectively, superscripts I and II represent alkanes-rich phase and ILs-rich phase respectively. From the selectivity data presented in Fig. 4, for the ternary systems I and II at 313 K, the selectivity values increased with the solvent-to-feed ratio ˛stf . A comparison of our data with previous studies using other ILs [39,42] is presented in the Fig. 4. As shown
Table 6 Constants of the Othmer–Tobias correlation, correlation factor (R2 ) and standard deviations () of the four ternary systems at T = 313 K and 333 K and P = 101.3 kPa. T/K 313 333 313 333 313 333 313 333
a
b
{dodecadecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} −1.5045 0.7521 −1.4218 0.7344 {dodecadecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} −1.3409 0.7532 −1.2834 0.7474 {tetradecane (1) + propylbenzene (2) + mmim (CH3 SO4 ) (3)} −0.9383 0.8695 −0.8396 0.8658 {tetradecane (1) + propylbenzene (2) + emim (CH3 SO4 ) (3)} −0.7201 0.8943 −0.5802 0.8958
R2
˛
0.9968 0.9986
0.0340 0.0222
0.9988 0.9994
0.0206 0.0150
0.9989 0.9983
0.0229 0.0283
0.9980 0.9978
0.0316 0.0332
34
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
Fig. 2. (a) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (b) Experimental and predicted LLE data for the ternary system {dodecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (c) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(mmim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line). (d) Experimental and predicted LLE data for the ternary system {tetradecane (1) + Propylbenzene (2) + [(emim)(CH3 SO4 )] (3)} at 333 K: experimental (䊉), UNIQUAC (solid line) and NRTL (dashed line).
in this figure, the selectivity values increase in the following order; [(bmim)(PF6 )] < [(mmim)(CH3 SO4 )] < [mebupy][BF4 ] ∼ = [(emim) (CH3 SO4 )]. As shown in Tables 2–5, the selectivity values are not constant over the whole two-phase region, and while it increased with the chain length of alkanes it decreased with temperature and/or concentration of propylbenzene in the feed. The selectivity values
in the systems under study are higher than unity, which ensures the feasibility of separation of propylbenzene and dodecane or tetradecane from their mixtures. Moreover, the use of [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )], and of ionic liquids in general, in solvent extraction is favourable because it can be easily recovered and reused.
Fig. 3. Measured distribution coefficient (K) against solvent to feed ratio (˛stf ) at 313 K for the ternary systems: dodecane + propylbenzene + [(mmim)(CH3 SO4 )] (䊉), or + [(emim)(CH3 SO4 )] (), or + [mebupy][BF4 ] () reference [39], or + [(bmim)(PF6 )] () reference [42].
Fig. 4. Measured selectivity (S) against solvent to feed ratio (˛stf ) at 313 K the ternary systems: dodecane + propylbenzene + [(mmim)(CH3 SO4 )] for (䊉), or + [(emim)(CH3 SO4 )] (), or + [mebupy][BF4 ] () reference [39], or + [(bmim)(PF6 )] () reference [42].
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
35
Table 7 UNIQUAC interaction parameters and root mean square deviation (rmsd) of the four ternary systems at T = 313 and 333 K and P = 101.3 kPa. UNIQUAC
313 K
333 K
i
j
aij
aji
Dodecane Dodecane Propylbenzene Dodecane Dodecane Propylbenzene Tetradecane Tetradecane Propylbenzene Tetradecane Tetradecane Propylbenzene
Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 ) Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 )
−128.92 589.99 301.77 −107.74 506.49 285.36 −111.53 469.38 55.537 −86.909 459.60 −9.9696
46.655 −32.785 −150.51 24.144 −11.644 −132.08 98.029 362.29 27.446 73.36 382.78 101.55
RMSD
0.1140
0.1263
0.2028
0.1791
aij
aji
−124.58 648.33 315.61 −160.980 472.090 107.650 −89.831 527.64 −5.1138 −124.34 436.12 97.98
48.434 −49.582 −155.14 113.230 169.700 −27.000 148.72 435.68 139.88 91.894 284.01 −16.484
RMSD
0.1093
0.2481
0.2250
0.2057
Table 8 NRTL interaction parameters and root mean square deviation (rmsd) of the four ternary systems at T = 313 K and 333 K and P = 101.3 kPa. NRTL (˛ = 0.2)
313 K
333 K
i
j
aij
aji
Dodecane Dodecane Propylbenzene Dodecane Dodecane Propylbenzene Tetradecane Tetradecane Propylbenzene Tetradecane Tetradecane Propylbenzene
Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 ) Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 )
−752.44 1638.3 1464.3 −742.41 1613.5 1481.3 −874.99 1633.2 1447.3 −919.35 1627.3 1440.9
303.6 1130.1 −477.12 302.07 1169.2 −430.56 514.99 1177.7 −523.28 521.2 1186.5 −531.03
NRTL (˛ = 0.3)
RMSD
0.1674
0.1747
0.2013
0.2004
313 K
i
j
Dodecane Dodecane Propylbenzene Dodecane Dodecane Propylbenzene Tetradecane Tetradecane Propylbenzene Tetradecane Tetradecane Propylbenzene
Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 ) Propylbenzene mmim (CH3 SO4 ) mmim (CH3 SO4 ) Propylbenzene emim (CH3 SO4 ) emim (CH3 SO4 )
aij
aji
−744.55 1745.40 1551.50 −852.94 1729.9 1510.7 −951.18 1747.2 1497.2 −889.20 1718.80 1548.60
309.18 1129.60 −498.34 406.44 1190.5 −510.09 569.31 1202.7 −581.05 504.37 1275.30 −551.24
RMSD
0.1677
0.2052
0.2342
0.1932
333 K
aij
aji
−942.98 1577.1 1498 −915.76 1565.80 1505.90 −994.25 1563.3 1499.5 −1057.40 1555.60 1497.80
RMSD
552.79 1466.7 −414.34 589.93 1484.30 −337.26 724.36 1472.1 −454.81 709.89 1486.70 −494.54
0.1500
0.1666
0.1812
0.1751
aij −928.60 1665.20 1582.60 −1024.2 1660.8 1565.7 −1101.4 1651.1 1571.8 111.36 1839 1186.1
aji 574.87 1484.30 −401.94 627.85 1531.6 −450.29 746.43 1522.5 −551.96 1771.2 1075.5 309.33
RMSD
0.1442
0.1839
0.2061
0.2840
3.3. Thermodynamic correlations The UNIQUAC model of Abrams and Prausnitz [45] and the NRTL model of Renon and Prausnitz [46] were used to correlate our experimental data. For the UNIQUAC model, the excess Gibbs energy (GE ) of the UNIQUAC model is
GE = xi ln i + 5 xi qi ln i − xi qi ln RT xi i 3
3
3
i=1
i=1
i=1
where
i
=
xi ri
3
xr i=1 i
⎛ ⎝
n
j=1
⎞ j ji ⎠
(4)
i =
xi qi
3
xq i=1 i i
u −u ij jj
ij = exp −
RT
u −u ji ii
ji = exp −
RT
here q and r represent the UNIQUAC surface area and volume, while i and i represent the area fraction and segment fraction of species i, respectively. The u is the energy of interaction for each binary pair of compounds and is adjustable parameter.
36
A.S. Al-Jimaz et al. / Fluid Phase Equilibria 354 (2013) 29–37
In the NRTL model, the excess Gibbs energy of mixing (GE ) of the NRTL model is
GE xi = RT 3
n
j=1 ji
n
Gji xj
(5)
G x k=1 ki k
i=1
where ij =
gij − gjj
=
RT
aij T
Gij = exp(−˛ij ij ) where R is the gas constant, T is the absolute temperature, x is mole fraction, g is energy of interaction for each binary pair of compounds, G is binary interaction parameter, is adjustable parameter, ˛ is nonrandomness parameter. The aij and aji are the two interaction parameters for each binary pair that we find from correlation. The LLE experimental data were used to determine the optimum UNIQUAC and NRTL binary interaction parameters between [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )], propylbenzene, and dodecane or tetradecane. The UNIQUAC and the NRTL models were fitted to experimental data using S∅rensen’s iterative computer programme, based on the flash calculation method [48]. The objective function was determined by minimizing the square of the difference between the mole fractions predicted by the respective method and those experimentally measured over all the tie lines in the ternary system. For the UNIQUAC correlation, the pure component structural parameters (r and q) listed in Table 1 were calculated from the group contribution data [49] or taken from literature [50]. The NRTL model was fitted with fixed values of the third non-randomness parameter, ˛ij , for each pair of components (˛ij = 0.2 or 0.3) during calculations, and the correlation with ˛ij = 0.3 gave better results according to relative mean square deviation (RMSD) values. The objective function, OF, is: OF = min
k
j
i
(xijk,exp − xijk,cal )2
(6)
where x is mole fraction, subscripts exp, cal, i, j, and k are experimental, calculated, components, phases and tie lines respectively. The optimization results were assessed by calculating the corresponding RMSD values using the following equation:
RMSD = 100
k
j
i
(xijk,exp − xijk,cal )2 6n
1/2 (7)
where n is number of tie lines. The values of interaction parameters and the RMSD for the UNIQUAC and the NRTL models at different temperatures are shown in Tables 7 and 8, respectively. These parameters are used to calculate LLE tie lines for the present systems. The calculations based on both models gave good representation of the tie line data for those systems and analysis of the mean RMSD indicates they are effectively the same (UNIQUAC x¯ RMSD = 0.1763, NRTL x¯ RMSD = 0.1864). 4. Conclusions The experimental liquid–liquid equilibria investigation of four ternary mixtures {dodecane or tetradecane + propylbenzene + [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )]} was carried out at two temperatures (313 and 333) K and at atmospheric pressure to study the effect of ILs structure and alkane chain length on the equilibrium behavior. While the temperature and/or the concentration of propylbenzene have no effect on the solubility of [(mmim)(CH3 SO4 )] or [(emim)(CH3 SO4 )]
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