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Experimental Mutual Solubility Data for Cyclohexane and Water in Aqueous Solutions of Diethanolamine Ismaila Shittu, Priyabrata Pal, and Fawzi Banat* Department of Chemical Engineering, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates
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S Supporting Information *
ABSTRACT: This study presents liquid−liquid equilibrium solubility data of the mutual solubility of cyclohexane and water in aqueous solutions of diethanolamine (DEA). Measurements were taken using a series of jacketed cells connected to a thermostat to maintain the desired solubility temperature. Different concentrations of DEA (15, 25, 35, 45, and 50 wt %) were prepared and mixed with cyclohexane in the measuring cells at temperatures ranging from 298 to 318 K and atmospheric pressure. Once thermodynamic equilibrium had reached, samples from the aqueous phase were analyzed for cyclohexane content using a gas chromatograph-mass spectrometer, while samples from the hydrocarbon-rich phase were analyzed using a water content apparatus. Results showed that the solubility of cyclohexane increased with an increase in DEA concentration and varied directly with the temperature. Similarly, the solubility of water in the organic phase exhibited an increasing trend with an increase in temperature and concentration of DEA. The alkanolamine concentration effect was modeled using an activity coefficient model that was successfully used to estimate the activity coefficient of cyclohexane in the aqueous DEA phase from the measured solubility data.
1. INTRODUCTION Refinery process streams, natural gas, and liquefied petroleum gas contain variable quantities of acid gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S) that must be removed before the gas moves to other downstream processing units or it enters the transmission pipelines. The conventional method for the removal of sulfur compounds and carbon dioxide is the stripping/absorption processes using an aqueous solution of an alkanolamine.1 The primary advantage of using alkanolamine solutions is their high selectivity for acid gases relative to hydrocarbons. Nevertheless, the solubility of hydrocarbons in alkanolamines is greater than zero and ought to be accounted for; but only limited experimental and design data are available in the literature. It is highly essential to know the solubilities of liquid and gaseous hydrocarbons in alkanolamine solutions to quantify hydrocarbon losses in gas cleanup processes. Thus, the utilization of reliable solubility data of hydrocarbons in alkanolamines will result in the effective optimization of these processes.2 Moreover, hydrocarbon losses are significantly higher for lighter hydrocarbons (C1−C4) compared to heavier liquid hydrocarbons (C5−C7). Hence, most of the available literature data are related to the solubility of lighter hydrocarbons and their associated parametric studies.3,4 However, in the petroleum processing industries, the solubility data for heavy liquid hydrocarbons is highly essential to understand amine foaming.1,3 Two alkanolamines, namely, diethanolamine (DEA) and methyldiethanolamine (MDEA), have received much attention © XXXX American Chemical Society
as potential solvents due to their low corrosiveness and relatively low heat of absorption. Furthermore, the solubilities of hydrocarbons in pure water are relatively less than the solubility in aqueous alkanolamine solutions.1,5 Alhseinat et al.3 investigated the solubility of n-hexane in aqueous MDEA of various initial concentrations of 40, 45, and 50 wt % solutions. The solubility of hexane was found to increase with temperature and MDEA concentration in aqueous solutions. Similarly, Danon et al.6 measured mutual solubilities of cyclohexane and water in aqueous MDEA (25, 40, 45, and 50 wt %) solutions under liquid−liquid equilibrium (LLE) conditions and reported an increase in the solubility of cyclohexane with temperature and MDEA concentration in water. A similar trend was observed for the solubility of water in the cyclohexane-rich phase. Earlier, Critchfield et al.7 have reported the solubility of pentane and butane in MDEA and DEA at 1.7 MPa and 313 K under liquid−liquid equilibrium conditions. The experimental data showed increased solubility of C3−C5 hydrocarbons with both concentration and strength of alkanolamine in the following order: monoethanolamine, diethanolamine, diglycolamine, methyldiethanolamine, and diisopropanolamine. However, LLE data of cyclohexane in aqueous solutions of DEA have not yet been reported in the literature. Received: December 9, 2018 Accepted: May 9, 2019
A
DOI: 10.1021/acs.jced.8b01179 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Metrohm). The coulometric cell consists of an anodic and a cathodic compartment, separated by a ceramic diaphragm. A platinum electrode is used for both coulometry and for the generator electrode with a diaphragm. The measurement was performed in a 250 mL glass vessel containing 1.0 mL sample, and the amount of moisture in the hydrocarbon-rich sample was measured using the electrochemical method. 2.3. Thermodynamics of the System. The two liquid phases considered in this work are the denser aqueous DEA phase and the less dense cyclohexane-rich phase. Since the solubility of water in cyclohexane is very low and has a greater affinity for DEA, the equilibrium between the two phases is easily established. The assumptions used to model the system are as follows: the initial amount of water in the cyclohexanerich phase is negligible, and the aqueous phase is mainly composed of DEA and water. These assumptions have also been used in the literature2,3,8,9 and validated experimentally. As such, the fugacity of cyclohexane in the two liquid phases can be written as
In this work, solubility of cyclohexane in aqueous diethanolamine (DEA) solutions as a function of temperature (25−45 °C) and five different DEA solvent concentrations (15, 25, 35, 45, and 50 wt %) is presented. The selected solvent concentration values are typical values encountered in natural gas processing plants.
2. EXPERIMENTAL SECTION 2.1. Materials. Table 1 lists the details of the chemicals used in this study. All of the chemicals used are of analytical grade, and distilled water was used for dilution of the solvents. Table 1. Chemicals Used chemical
source
formula
CAS no.
purity (%)
diethanolamine cyclohexane ethanol
Merck KGaA Fisher chemical Merck KGaA
C4H11NO2 C6H12 C2H5OH
111-42-2 110-82-7 64-17-5
>99 >99.96 >99.9
2.2. Experimental Procedure. 2.2.1. Measurement of Solubility. Solubility experiments were performed in thermostatic LLE glass cells at atmospheric pressure and temperatures ranging from 25 to 45 °C. Five cells were connected in series, and the temperature in the equilibrium cells was kept constant by using water jackets connected to a thermostat with uncertainty deviation of ±0.1 K temperature. Aqueous solutions of DEA (15−50 wt %) were prepared gravimetrically, and 20 mL of DEA solution was loaded into the cells followed by 15 mL of cyclohexane. The mixture was stirred for 5 h and then left undisturbed overnight to achieve thermodynamic equilibrium and clear phase separation. Samples from the dense aqueous phase were collected using a microsyringe and dissolved in ethanol to be injected into gas chromatographmass spectrometer (GC-MS) for DEA concentration analysis. To ensure the repeatability and the reproducibility of the equipment, each experiment was performed thrice. Reliability of the tests was further confirmed by comparing the results with similar systems available in the literature. The standard uncertainty of the mole fraction of cyclohexane based on the reproducibility and repeatability measurements is around 4.4665 × 10−6. The Brucker SCION GC-MS is equipped with a mass spectroscopy detector coupled with a purge and trap unit. The best reproducibility was achieved by adjusting the split injection mode to a split ratio of 2%, which was found to be the ideal value after several adjustments. The injector and oven temperatures were kept at 548 and 523 K, respectively. The GC-MS was fitted with a capillary column with dimensions of 20 m length, 0.18 mm ID, and 1.0 μm film thickness. Helium was used as a carrier gas having a flow rate of 1.0 mL/min. Periodically, pure cyclohexane was injected into the column between measurements to prevent or mitigate hydrocarbon loses due to split mode and adsorption in the column. To minimize cyclohexane accumulation in the GC column and lining, deionized water was injected into the column between sampling procedures and periodic conditioning of the column was carried out. Details of the schematic diagram of the equilibrium cell and GC-MS technique are shown in our earlier publication.6 2.2.2. Determination of Moisture Content. Samples from the cyclohexane-rich less dense phase were collected and analyzed to measure the moisture content of the sample using a Karl Fischer coulometric titrator instrument (851 Titrando-
aq o f ch = f ch
(1)
aq aq aq sat f ch = xch × γchaq × φchsat × Pch = xch ×H
(2)
o o sat sat f ch = xch × γcho × φchsat × Pch ≅ φchsat × Pch
(3)
o aq o where faq ch and fch are the fugacities, γch and γch are the activity sat coefficients, H is Henry’s law constant, Pch is the saturated aq pressure of cyclohexane, φsat ch is the fugacity coefficient, and xch o and xch are the mole fractions of cyclohexane in the aqueous DEA and the organic/hydrocarbon-rich phases, respectively. Henry’s law constant, “H” in eq 2, has been applied to correlate the solubility data. Both the activity coefficient and mole fractions in eq 3 were set to unity since the organic phase is mainly composed of cyclohexane. The equilibrium between the aqueous and the hydrocarbon-rich liquid phases is reduced to the following forms
aq o xch × γchaq = xch × γcho ≅ 1
γchaq =
(4)
1 aq xch
(5)
A simple model proposed by Alhseinat et al.3 can be used to model the activity coefficient of cyclohexane in the aqueous DEA-rich phase as follows i B aq γCh aA′ × expjjjj k T (K)
zyz zz {
(6)
Taking the natural logarithm on both sides of eq 6 yields aq ln γCh = ln A′ +
B B =A+ T (K) T (K)
(7)
where A and B are the model parameters. Solubility of cyclohexane in pure water can be calculated using the correlation developed by Tsonopoulos et al.14 ln xch = A +
B + C ln T T
(8)
where xch is the mole fraction of cyclohexane and T is the temperature (K). The parameters A, B, and C are equal to −209.12, 8325.49 K, and 29.82, respectively. B
DOI: 10.1021/acs.jced.8b01179 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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3. RESULTS AND DISCUSSION 3.1. Cyclohexane Solubility in Aqueous Solutions of Diethanolamine. The solubility data for cyclohexane in aqueous DEA solutions of five different concentrations are presented in Table 2 and Figure 1. The system pressure was
its corresponding solubility in pure water. This phenomenon is referred to as a salting-in effect in some previous literature.10−12 Furthermore, the solubility of cyclohexane increased with an increase in temperature over the temperature ranges used in this study. These trends are consistent with hydrocarbon−aqueous alkanolamine LLE or VLE data reported in the literature.1,3,6,7,13 The equilibrium concentrations of the mixture at 298 K are represented by tie lines in Gibb’s ternary diagram (Figure S1) shown in the Supporting Information (Appendix A). The solubility data for cyclohexane in pure water at different temperatures were calculated using eq 8, and the resulting values are plotted in Figure 1. From eqs 5 and 7, the solubility of cyclohexane in aqueous DEA solutions could be calculated. The activity coefficients of cyclohexane calculated from the measured solubility values are reported in Table 3.
Table 2. Experimental Solubility Data of Cyclohexane (1)/ Water (2)/DEA (3) in the Aqueous DEA Phase (1) at P = 0.1 MPaa DEA conc. (wt %)
temp. (K)
x11 (×10−5)
x21
x31
15
298 303 313 318 298 303 313 318 298 303 313 318 298 303 313 318 298 303 313 318
6.91000 6.89590 7.1492 7.4193 7.55117 7.62678 8.00218 8.40091 8.34723 8.56942 8.81274 9.50624 9.64877 9.69946 9.99884 12.77880 10.34310 11.83470 11.8739 15.7446
0.971010 0.971010 0.971007 0.971003 0.947299 0.947298 0.947293 0.947286 0.918797 0.918794 0.918791 0.918778 0.883886 0.883885 0.883879 0.883849 0.863330 0.863325 0.863322 0.863279
0.028921 0.028921 0.028922 0.028923 0.052625 0.052626 0.052626 0.052629 0.081119 0.081120 0.081121 0.081126 0.116017 0.116018 0.116021 0.116023 0.136566 0.136552 0.136559 0.136564
25
35
45
50
Table 3. Activity Coefficients of Cyclohexane in the Aqueous DEA Phase Calculated Using Equation 5 activity coefficient (γ) of cyclohexane in the aqueous phase T (K)
15 wt %
25 wt %
35 wt %
45 wt %
50 wt %
298.15 303.15 313.15 318.15
14 472 14 501 13 988 13 478
13 243 13 112 12 497 11 903
11 980 11 669 11 347 10 519
10 364 10 309 10 001 7825
9668 8450 8422 6351
Figure 2 depicts the effect of temperature on the activity coefficient of cyclohexane estimated using eq 7. To evaluate
a Standard uncertainties are as follows: u(T) = 0.1 K; u(P) = 1.0 × 10−3 MPa; u(x) = 4.4665 × 10−6. Relative standard uncertainty for cyclohexane mole fraction, ur(x1) = 0.05. x11, x21 and x31 are the mole fractions of cyclohexane, water, and DEA, respectively, in the aqueous DEA phase.
Figure 2. Effect of temperature on the activity coefficient of cyclohexane (the symbols represent experimental data points, and the solid lines correspond to eq 7).
the quality of the correlation, the normalized error, ε, was calculated from eq 9. Model parameters A and B were estimated by regression with experimental data, and the values of the normalized error (ε) at the corresponding DEA weight concentrations of solvents are reported in Table 4.
Figure 1. Solubility of cyclohexane as a function of temperature and DEA concentration.
n
held constant at all temperatures (298−318 K), so as to ensure that two liquid phases were present. As shown in Table 2, the solubility of cyclohexane increases with the DEA concentration in the aqueous solution. The addition of DEA to the binary mixture of water and cyclohexane leads to an increase in cyclohexane solubility in the aqueous DEA solution more than
ε = 100% ×
∑i = 1 |xexp − xcal| n
(9)
where n is the number of data points and the subscripts “exp” and “cal” indicate the experimental and calculated mole fractions of cyclohexane in the aqueous phase. C
DOI: 10.1021/acs.jced.8b01179 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 4. Model Parameters of Cyclohexane for Different DEA Concentrations Using the Correlation of Alhseinat et al.3 DEA conc. (wt %)
A
B (K)
ε
15 25 35 45 50
8.46 7.84 7.57 8.48 2.60
336.43 493.95 544.16 230.63 1958.10
1.02 1.03 1.39 0.23 0.77
3.2. Solubility of Water and DEA in Cyclohexane. Generally, the solubility of water in hydrocarbons increases in a monotonic manner with an increase in temperature. The solubilities of water and diethanolamine in the cyclohexanerich phase are presented in Table 5 and Figure 3. As shown in Figure 3. Solubility of water in the cyclohexane-rich phase as a function of temperature and DEA concentration.
Table 5. Experimental Solubility Data of Cyclohexane (1)/ Water (2)/DEA (3) in the Cyclohexane-Rich Phase at P = 0.1 MPaa DEA conc. (wt %)
temp. (K)
x12
x22 (×10−4)
x32 (×10−4)
15
298 303 313 318 298 303 313 318 298 303 313 318 298 303 313 318 298 303 313 318
0.999421 0.999397 0.999367 0.999128 0.999415 0.999358 0.999284 0.998977 0.999384 0.999346 0.999288 0.998965 0.999405 0.999377 0.999248 0.999096 0.999281 0.999238 0.999186 0.998921
3.78093 4.02088 4.38091 6.77992 3.78090 4.38082 5.10070 8.21881 4.02084 4.44082 5.04072 8.33883 3.78095 4.08092 5.40061 7.02076 4.26056 4.68077 5.28061 8.04035
2.00524 2.01263 1.94481 1.93701 2.07391 2.03572 2.06205 2.01437 2.13906 2.10017 2.07605 2.00912 2.16966 2.14535 2.11720 2.01552 2.93213 2.93845 2.85584 2.75284
25
35
45
50
DEA has been reported earlier and therefore only a qualitative comparison was presented. The qualitative trend of the solubility data with variation in temperature obtained from this work has been compared with the data reported by Mokraoui et al.1 (solubility of n-hexane in 35 wt % DEA) and Danon et al.6 (solubility of cyclohexane in aqueous MDEA). In Figure 4a, the difference between the solubilities of the samples used in the two studies (n-hexane and cyclohexane) is primarily due to their difference in densities. Cyclohexane being a cyclic compound is more soluble in water than its corresponding straight chain compound, n-hexane, as cyclohexane has lower molar volume and thus dissolves more readily in the cavities of the solvent.15 On the other hand, as shown in Figure 4b, the solubility of cyclohexane in MDEA is higher than its solubility in DEA at the same conditions, which can be attributed to the higher solvating power possessed by MDEA. All experimental solubility data reported in the literature showed that solubility of lighter hydrocarbons (ethane and propane) is higher in aqueous MDEA than in aqueous DEA solutions.2 Moreover, the observation is consistent with the ranking of alkanolamine solvating power for hydrocarbons reported in previous studies.1,4,7
a Standard uncertainties are as follows: u(T) = 0.1 K; u(P) = 1.0 × 10−3 MPa; and u(x) = 6.46 × 10−6. Relative standard uncertainty for water solubility ur(x2) = 0.02. x12, x22, and x32 are the mole fractions of cyclohexane, water, and DEA respectively, in the cyclohexane-rich phase.
4. CONCLUSIONS New LLE data for the solubility of cyclohexane in aqueous solutions of diethanolamine as a function of temperature and at atmospheric pressure were reported. The new experimental data showed that the solubility of cyclohexane, in terms of mole fraction, is enhanced in the presence of diethanolamine. Moreover, over the range of temperatures studied, the solvating power of the solvent slightly increases with the increase in temperature. Thus, the solubility of cyclohexane increases with the concentration of DEA and temperature. Similarly, the solubility of water in the organic phase (cyclohexane-rich) increases with temperature and the amount of DEA in aqueous solutions. Furthermore, a useful and simple correlation for solubility, in terms of the activity coefficient of cyclohexane, has been derived from experimental data in weight-based concentration ranges from 15 to 50 wt % DEA. A good prediction of the solubility of cyclohexane was achieved with the model as normalized deviation was less than 1.40%. Finally, an excellent agreement was found between the experimental data from this work and similar works reported in the literature.
Figure 3, the solubility of water increases rapidly with the increase in temperature but moderately with the concentration of DEA in the aqueous solutions. This observation is similar to the trend obtained by Danon et al.6 3.3. Repeatability and Reproducibility Studies. The quality of the experimental data can be determined by investigating its reproducibility and repeatability. Multiple samples were collected and analyzed, and few of the experiments were also repeated in different equilibrium cells. Three different samples were tested to ensure repeatability, and the entire experiment was duplicated at a few selected temperatures. The error bars are provided in Figures 1 and 3 after calculating the standard deviations (σ) for each repeatability and reproducibility study. 3.4. Comparison Study. To the authors’ knowledge, no experimental LLE data of cyclohexane solubility in aqueous D
DOI: 10.1021/acs.jced.8b01179 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 4. Comparison of solubility of cyclohexane in aqueous DEA (a) with n-hexane in aqueous DEA by Mokraoui et al.1 and (b) with cyclohexane in aqueous MDEA by Danon et al.6 at different temperatures.
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(8) Valtz, A.; Coquelet, C.; Richon, D. Solubility data for toluene in various aqueous alkanolamine solutions. J. Chem. Thermodyn. 2007, 39, 426−432. (9) Coquelet, C.; Valtz, A.; Richon, D. Solubility of ethylbenzene and xylene in pure water and aqueous alkanolamine solutions. J. Chem. Thermodyn. 2008, 40, 942−948. (10) Hooper, H. H.; Michel, S.; Prausnitz, J. M. High-temperature mutual solubilities for some binary and ternary aqueous mixtures containing aromatic and chlorinated hydrocarbons. J. Chem. Eng. Data 1988, 33, 502−505. (11) Kalra, A.; Tugcu, N.; Cramer, S. M.; Garde, S. Salting-in and salting-out of hydrophobic solutes in aqueous salt solutions. J. Phys. Chem. B 2001, 105, 6380−6386. (12) Hatcher, N. A.; Jones, C. E.; Weiland, R. H. In Hydrocarbon and fixed gas solubility in amine treating solvents: a generalized model. Laurence Reid Gas Conditioning Conference, 2013. (13) Carroll, J. J.; Maddocks, J.; Mather, A. E. In The solubility of hydrocarbons in amine solutions. Laurance Reid Gas Conditioning Conference, 1998. (14) Tsonopoulos, C.; Wilson, G. High-temperature mutual solubilities of hydrocarbons and water. Part I: Benzene, cyclohexane and n-hexane. AIChE J. 1983, 29, 990−999. (15) Tsonopoulos, C. Thermodynamic analysis of the mutual solubilities of hydrocarbons and water. Fluid Phase Equilib. 2001, 186, 185−206.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01179.
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Experimental tie lines for cyclohexane + water + diethanolamine ternary mixtures at 298.15 K (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Fawzi Banat: 0000-0002-7646-5918 Funding
The authors are grateful to Khalifa University Gas Processing and Materials Science Research Center, Abu Dhabi, for funding the project (GRC006). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Mokraoui, S.; Hadj-Kali, M. K.; Valtz, A.; Richon, D. New Vapor−Liquid−Liquid Equilibrium Solubility Data for iso-Butane, nButane, n-Pentane, and n-Hexane in Alkanolamine Aqueous Solutions. J. Chem. Eng. Data 2014, 59, 1673−1684. (2) Mokraoui, S.; Hadj-Kali, M. K.; Valtz, A.; Richon, D. New vapor−liquid−liquid equilibrium data for ethane and propane in alkanolamine aqueous solutions. J. Chem. Eng. Data 2013, 58, 2100− 2109. (3) Alhseinat, E.; Danon, R.; Peters, C.; Banat, F. Solubility of hexane in aqueous solutions of methyldiethanolamine. J. Chem. Eng. Data 2015, 60, 3101−3105. (4) Jou, F.-Y.; Ng, H.-J.; Critchfield, J.; Mather, A. Solubility of propane in aqueous alkanolamine solutions. Fluid Phase Equilib. 2002, 194, 825−830. (5) Sada, E.; Kumazawa, H.; Butt, M. Solubilities of gases in aqueous solutions of amine. J. Chem. Eng. Data 1977, 22, 277−278. (6) Danon, R.; Kroon, M. C.; Banat, F. Mutual Solubilities of Cyclohexane and Water in Aqueous Methyldiethanolamine/Cyclohexane Liquid−Liquid Equilibria. J. Chem. Eng. Data 2018, 63, 1123− 1131. (7) Critchfield, J.; Holub, P.; Ng, H.-J.; Mather, A.; Jou, F.-Y.; Bacon, T. In Solubility of hydrocarbons in aqueous solutions of gas treating amines. Laurance Reid Gas Conditioning Conference, 2001. E
DOI: 10.1021/acs.jced.8b01179 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
