Article pubs.acs.org/IECR
Vapor−Liquid−Liquid Equilibrium Measurements and Modeling of Ethanethiol + Methane + Water, 1‑Propanethiol + Methane + Water and 1‑Butanethiol + Methane + Water Ternary Systems at 303, 335, and 365 K and Pressure Up to 9 MPa Javeed A. Awan,†,§ Georgios M. Kontogeorgis,†,* Ioannis Tsivintzelis,† and Christophe Coquelet‡ †
Center for Energy Resources Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kongens Lyngby, Denmark ‡ MINES ParisTech, CTPCentre Thermodynamiques et Procédés, 35 Rue Saint Honoré, 77305 Fontainebleau, France § Institute of Chemical Engineering and Technology, University of the Punjab, Lahore, 54590 Punjab, Pakistan ABSTRACT: New vapor−liquid−liquid equilibrium (VLLE) data for ethanethiol + methane + water, 1-propanethiol + methane + water, and 1-butanethiol + methane + water ternary systems have been measured at three temperatures (303, 335, and 365 K) and pressures up to 9 MPa. A “static-analytic” method was used for performing the measurements; the total system pressure was maintained by CH4. The objective of this work is to provide experimental VLLE data for mixtures of mercaptans (thiols) with other natural gas contents at its crude form, for which no data are available in the open literature. Such data will help the industrial modeling of processes relevant to reduction of sulfur emissions. The Cubic-Plus-Association (CPA) equation of state was applied to describe the phase behavior of the investigated systems. It is shown that the CPA EoS satisfactorily describes the solubilities of mercaptans (thiols) in all phases. However, the model underestimates the water content of the vapor phase, especially at low pressures and at the highest investigated temperature, i.e., at 365 K. Only the ethanthiol + methane + water system showed significant cross-association effects
1. INTRODUCTION Thiols, commonly called mercaptans, are organosulfur compounds analogous to alcohols where the −OH group in the molecule has been replaced by an −SH group. They are present in natural gas, synthesis gas, and various refinery process streams. Their concentration in the host gas stream can range from several parts per million to 50% by volume.1 Appropriate gas treatment processes need to be designed in order to remove not only H2S and CO2 but also organosulfur compounds like ethanethiol, 1propanthethiol, 1-butanethiol, and other regulated compounds. This is necessary because environmental protection agencies are forcing the petroleum industry to decrease sulfur emission in the effluent stream. Furthermore, any thiols (RSH), carbonyl sulfide (COS), and carbon disulfide (CS2) not absorbed from the sour gas through the amine purification units, complicate the process scheme for downstream liquid treatment units. The knowledge of the phase equilibrium behavior and thermo physical properties of sulfur species mixtures with hydrocarbons and water is important to both process design, optimization, and to product specifications. The present work is the continuation of our previous work on the phase equilibrium measurements and modeling of the systems containing thiols with water and hydrocarbons.2−5 Herein, we provide new vapor−liquid−liquid equilibrium (VLLE) data for (C2H5SH) + (CH4) + (H2O), (C3H7SH) + (CH4) + (H2O), and (C4H9SH) + (CH4) + (H2O) at three temperatures (304, 335, and 365 K) and pressures up to 9 MPa. The new VLLE data of these ternary systems were modeled with the Cubic-Plus-Association (CPA) equation of state in a © 2013 American Chemical Society
predictive way, i.e., no adjustable parameters were based on the ternary system data.
2. EXPERIMENTAL SECTION Purities and suppliers of materials are presented in Table 1. No further purification of the chemicals was made. The experimental Table 1. CAS Numbers, Purities, and Suppliers of Materials chemical name
CAS no.
mass fraction purity
supplier
ethanethiol (C2H5SH) 1-propanethiol (C3H7SH) 1-butanethiol (C4H9SH) methane (CH4)
75−08−1 107−03−9 109−79−5 74−82−8
>0.99 >0.99 >0.98 0.999
ACROS ACROS ACROS Messer
work has been carried out at CTP, Mines ParisTech, France, where a “static-analytic” technique based apparatus, consisting of an equilibrium cell equipped with one moveable Rapid Online Sampler Injector (ROLSI), was used as shown in Figure 1. The equipment is the same used by Coquelet et al.6 The two liquid and one vapor samples are analyzed using a gas chromatograph (Varian model CP-3800), equipped with a thermal conductivity detector (TCD), and a flame ionization detector (FID). In all experiments at first, thiols (i.e., ethanethiol or 1propanethiol or 1-butanethiol) were loaded into the equilibrium Received: Revised: Accepted: Published: 14698
March 10, 2013 September 1, 2013 September 6, 2013 September 6, 2013 dx.doi.org/10.1021/ie400779m | Ind. Eng. Chem. Res. 2013, 52, 14698−14705
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Figure 1. Flow diagram of the experimental setup based on synthetic-static technique for vapor−liquid−liquid equilibrium studies. Description of various components of the setup as labeled in figure; C: Carrier Gas; EC: Equilibrium Cell; FV: Feeding Valve; LB: Liquid Bath; PP: Platinum Probe; RSH: Mercaptan (Thiol) bottle; PT: Pressure Transducer; CH4: Methane Cylinder; SM: Sampler Monitoring; SW: Sapphire window; TC1 and TC2 Thermo couples; Th: Thermocouple; TR: Temperature Regulator; VSS: Variable Speed Stirring assembly; and VP: Vacuum Pump.
cell followed by water. Each time the loading was performed in the absence of air. Finally, methane was added to reach the desired pressure. The required temperature was obtained by putting the cell into a thermo-regulated oil bath. The temperature is controlled within 0.01 K. Once equilibrium was achieved inside the equilibrium cell, the vapor, the organic (liquid) and the aqueous (liquid) samples were taken using the ROLSI sampling equipment. These samples were directly introduced to the gas chromatograph through an isothermally heated transfer line. The sample transfer lines have special arrangements to ensure that the mixture of the sample and the carrier gas does not absorb, get trapped, or condense inside the line. They are internally coated by inert silica. The sampler has the possibility of controlling the sample size (few μL) by adjusting the aperture opening time. Two 100 Ω platinum probes (Pt100) were used for temperature measurements at the body of the equilibrium cell at two different levels. The platinum probes were connected to an HP data acquisition unit and periodically calibrated against a 25 Ω reference platinum resistance thermometer (TINSLEY precision instruments, France). The resulting accuracy in temperature measurements was estimated to be within u(T, k = 2) = ± 0.07 K. Pressures were measured by means of a Druck pressure transducer 0.1 to 14 MPa, which was maintained at 353.15 K. The pressure transducer was calibrated against a dead-
weight pressure balance (Desgranges & Huot 5202S, CP 0.3−40 MPa, Aubervilliers, France). Accuracies in pressure measurements have been estimated to be within u(P, k = 2) = ± 0.009 MPa. The gas chromatograph detectors were calibrated using chromatographic syringes with maximum mole numbers uncertainties of 2% in the TCD and 1.5% in the FID, thus the maximum uncertainty as mol fraction is umax.(x or y, k = 2)= ± 0.008. The gas chromatograph generated peaks of the individual components in vapor phases [CH4 (TCD), RSH (TCD), H2O (TCD)] in organic phases [CH4 (FID), RSH (TCD), H2O (TCD)] and in aqueous phases [CH4 (FID), RSH (FID), H2O (TCD)] at specific retention time, were recorded by using WiniLab-III through an interface port RS-232. The areas under such peaks correspond to the number of moles of the individual components, which come from the corresponding calibration. Each experimental data point has been analyzed more than 5 times, until we get consistent values; i.e., (standard deviation σ is within ±5%).
3. MODELING SECTION The CPA (Cubic-Plus-Association) equation of state is a model useful for thermodynamic calculations for mixtures of relevance to the petroleum and chemical industries e.g. mixtures of oil and gas with gas hydrate inhibitors (methanol, glycols) and organic 14699
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Table 2. Experimental VLLE Data for the C2H5SH (1) + CH4 (2) + H2O (3) Ternary Mixturea vapor phase
a
organic phase
aqueous phase
T (K)
P (MPa)
yC2H5SH
yH2O
T (K)
P (MPa)
xCH4
xH2O
T (K)
P (MPa)
xCH4
xC2H5SH
304.11 304.11 304.11 304.11 304.11 333.98 333.98 333.98 333.98 333.98 363.94 363.94 363.94 363.94 363.94
1.62 2.71 3.51 4.47 7.36 1.54 2.28 3.51 4.58 6.20 0.83 2.55 3.57 5.27 7.37
0.0934 0.0438 0.0382 0.0369 0.0298 0.2131 0.1472 0.0939 0.0893 0.0753 0.4809 0.2625 0.2101 0.1693 0.1412
0.0023 0.0014 0.0010 0.0008 0.0006 0.0130 0.0075 0.0041 0.0031 0.0023 0.0286 0.0141 0.0102 0.0081 0.0072
304.11 304.11 304.11 304.11 304.11 333.98 333.98 333.98 333.98 333.98 363.94 363.94 363.94 363.94 363.94
1.35 2.81 3.59 4.51 7.48 1.60 2.30 3.51 4.63 6.26 0.84 2.57 3.60 5.32 7.46
0.0240 0.0510 0.0635 0.0849 0.1478 0.0247 0.0368 0.0578 0.0789 0.1081 0.0050 0.0362 0.0537 0.0865 0.1325
0.0014 0.0013 0.0013 0.0014 0.0014 0.0039 0.0039 0.0035 0.0034 0.0033 0.0076 0.0076 0.0076 0.0077 0.0077
304.11 304.11 304.11 304.11 304.11 333.98 333.98 333.98 333.98 333.98 363.94 363.94 363.94 363.94 363.94
1.36 2.84 3.62 4.55 7.57 1.62 2.34 3.59 4.71 6.35 0.84 2.59 3.61 5.40 7.56
0.0004 0.0015 0.0022 0.0032 0.0061 0.0003 0.0004 0.0006 0.0008 0.0011 0.0004 0.0006 0.0008 0.0010 0.0013
0.0020 0.0019 0.0018 0.0017 0.0014 0.0034 0.0027 0.0027 0.0025 0.0027 0.0038 0.0033 0.0032 0.0031 0.0030
y stands for mole fraction in the vapor phase, and x stands for mole fraction in the organic/aqueous phase.
Table 3. Experimental VLLE Data for the C3H7SH (1) +CH4 (2) + H2O (3) Ternary Mixturea vapor phase
a
organic phase
aqueous phase
T (K)
P (MPa)
yC3H7SH
yH2O
T (K)
P (MPa)
xCH4
xH2O
T (K)
P (MPa)
xCH4
xC3H7SH
305.93 305.69 305.30 305.22 305.04 304.00 304.14 335.11 335.10 335.04 335.08 335.10 335.03 335.08 367.63 367.68 367.66 365.50 365.50 365.20
1.06 2.03 3.39 4.19 5.52 6.65 8.42 1.02 1.96 3.93 4.97 6.63 9.68 7.45 7.54 6.08 4.75 3.54 2.10 1.39
0.0614 0.0310 0.0237 0.0202 0.0182 0.0171 0.0173 0.1073 0.0529 0.0331 0.0294 0.0262 0.0269 0.0264 0.0700 0.0681 0.0735 0.0809 0.1200 0.1909
0.0047 0.0028 0.0020 0.0016 0.0013 0.0011 0.0010 0.0260 0.0150 0.0082 0.0061 0.0047 0.0040 0.0044 0.0152 0.0162 0.0218 0.0266 0.0407 0.0630
305.89 305.59 305.27 305.15 305.01 304.13 304.38 335.10 335.10 335.04 335.08 337.10 335.04 335.11 367.49 367.42 364.93 365.50 365.50 365.53
1.15 2.03 3.39 4.20 5.53 6.66 8.42 1.02 1.97 3.32 4.97 6.63 9.70 7.46 7.57 6.07 4.77 3.54 2.10 1.40
0.0309 0.0566 0.0920 0.1140 0.1538 0.1890 0.2352 0.0240 0.0514 0.0843 0.1278 0.1666 0.2326 0.1837 0.1777 0.1412 0.1121 0.0824 0.0497 0.0299
0.0025 0.0026 0.0023 0.0027 0.0028 0.0028 0.0028 0.0079 0.0078 0.0071 0.0069 0.0067 0.0070 0.0069 0.0152 0.0152 0.0152 0.0153 0.0154 0.0155
305.86 305.55 305.26 305.14 303.99 304.18 304.38 335.10 335.10 335.04 335.08 335.10 335.05 335.11 367.49 367.42 364.91 365.50 365.50 365.45
1.15 2.03 3.40 4.20 5.46 6.67 8.43 1.02 1.97 3.32 4.98 6.64 5.29 7.46 7.53 6.07 4.77 3.54 2.20 1.40
0.0005 0.0008 0.0011 0.0015 0.0018 0.0023 0.0029 0.0003 0.0006 0.0009 0.0013 0.0018 0.0014 0.0020 0.0016 0.0013 0.0010 0.0008 0.0005 0.0003
0.0015 0.0015 0.0015 0.0015 0.0014 0.0014 0.0014 0.0009 0.0009 0.0009 0.0009 0.0009 0.0009 0.0009 0.0007 0.0007 0.0007 0.0007 0.0007 0.0008
y stands for mole fraction in the vapor phase, and x stands for mole fraction in the organic/aqueous phase.
different combining rules have been suggested; however, the CR1 and the Elliott Combining Rule (ECR) rules, described below, were found to be successful in previous applications. The association between two molecules of different type is denoted as cross-association (the term solvation will be used in this study). The combining rules define how the energy and the volume parameters are calculated. In the case of CR-1 the expressions are as follows:
acids. Such polar and hydrogen bonding compounds are difficult to handle with conventional models used in oil industry like cubic equations of state and CPA offers a successful alternative in terms of accuracy and simplicity. Moreover, CPA can predict thermodynamic properties over the extensive temperature and pressure ranges encountered in industrial practice. The present work is the continuation of our efforts to extend this model for mercaptan (thiol)-containing systems.2−5 The CPA EoS has been proposed by Kontogeorgis et al.7,8 CPA has an association term which is a simpler form but mathematically identical version of the term used in SAFT. This form was proposed by Michelsen and Hendriks.9 The CPA EoS has been discussed in detail in one of our previous publication,3 therefore it is not repeated here. For extending the CPA EoS to mixtures of cross associating compounds, combining rules for the association energy (εAiBj) and the association volume (βAiBj) are required. Over the years
ε A iBj =
ε A iBi + ε AjBj 2
and
β A iBj =
β A iBi β AjBj
(1)
The expression of the cross-association strength (ΔAiBj) with ECR is as follows: ΔA iBj = 14700
ΔA iBi ΔAjBj
(2)
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Table 4. Experimental VLLE Data for the C4H9SH (1) + CH4 (2) + H2O (3) Ternary Mixturea vapor phase
a
organic phase
aqueous phase
T (K)
P (MPa)
yC4H9SH
yH2O
T (K)
P (MPa)
xCH4
xH2O
T (K)
P (MPa)
xCH4
xC4H9SH
302.48 302.45 303.13 303.26 302.65 304.25 304.06 335.53 335.53 335.19 335.13 335.62 335.51 335.51 365.80 365.80 365.69 365.80 365.72
0.92 1.89 3.44 4.37 5.24 6.32 7.17 1.08 2.08 3.19 4.31 5.35 6.46 7.93 1.30 2.68 5.13 6.45 7.83
0.0330 0.0195 0.0111 0.0109 0.0101 0.0100 0.0099 0.0618 0.0362 0.0223 0.0200 0.0196 0.0202 0.0197 0.0929 0.0492 0.0303 0.0276 0.0266
0.0092 0.0041 0.0022 0.0016 0.0016 0.0016 0.0016 0.0270 0.0105 0.0066 0.0066 0.0063 0.0061 0.0059 0.0615 0.0332 0.0176 0.0166 0.0162
302.50 302.45 303.26 303.26 302.99 304.30 304.09 335.60 335.60 335.60 335.18 335.60 335.60 335.60 365.70 365.70 365.40 365.70 365.72
0.92 1.89 3.45 4.37 5.26 6.32 7.18 1.09 2.08 3.19 4.31 5.36 6.47 7.94 1.30 2.68 5.12 6.46 7.84
0.0276 0.0528 0.0940 0.1166 0.1375 0.1631 0.1844 0.0246 0.0490 0.0777 0.1046 0.1288 0.1527 0.1847 0.0219 0.0523 0.1116 0.1408 0.1720
0.0029 0.0025 0.0023 0.0023 0.0022 0.0021 0.0021 0.0092 0.0082 0.0076 0.0072 0.0069 0.0067 0.0064 0.0176 0.0171 0.0165 0.0169 0.0168
302.66 303.06 303.49 303.29 303.30 304.39 304.15 335.60 335.60 335.60 335.19 335.60 335.60 335.60 365.79 365.33 365.40 365.79 365.72
0.92 1.89 3.45 4.37 5.27 6.34 7.19 1.09 2.08 3.19 4.32 5.36 6.47 7.95 1.30 2.68 5.14 6.46 7.84
0.0004 0.0007 0.0012 0.0015 0.0018 0.0021 0.0023 0.0003 0.0006 0.0009 0.0011 0.0014 0.0017 0.0021 0.0002 0.0006 0.0012 0.0015 0.0018
0.0002 0.0002 0.0001 0.0001 0.0001 0.0001 0.0001 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0001 0.0004 0.0004 0.0004 0.0004 0.0003
y stands for mole fraction in the vapor phase, and x stands for mole fraction in the organic/aqueous phase.
Table 5. Overview of CPA EoS Pure Component Parameters Used in This Work component 3
methanethiol (CH3SH) ethanethiol (C2H5SH)3 1-propanethiol (C3H7SH)3 1-butanethiol (C4H9SH) methane (CH4)12 water (H2O)12 a
Tc (K)
b(L/mol)
Γ (K)
c1
469.95 499.15 536.6 570.1 190.56 647.29
0.0437 0.0586 0.0762 0.0923 0.0291 0.0145
2266.27 2548.92 2671.42 2859.51 959.1 1017.3
0.8007 0.7221 0.8398 0.8916 0.4472 0.6736
εAiBi(bar L/mol) n.a. n.a. n.a. n.a. n.a. 166.55
βAiBi
%AAD in PSat
%AAD in ρliq
association scheme
n.a. n.a. n.a. n.a. n.a. 0.0692
2.74
0.42
2.93 3.46 0.35 0.91
0.29 0.29 1.97 0.98
n.a. n.a. n.a. n.a. n.a. 4C
n.a. stands for non associating fluid.
Table 6. Overview of CPA EoS Binary Interaction Parameters Used in This Work binary systems methanethiol (CH3SH) − water (H2O) methanethiol (CH3SH) − water (H2O)3 ethanethiol (C2H5SH) − water (H2O)3 ethanethiol (C2H5SH) − water (H2O)3 1-propanehiol (C3H7SH) − water (H2O)3 1-butanethiol (C4H9SH) − water (H2O)3 methane (CH4) − water (H2O)13 methanethiol (CH3SH) − methane (CH4)4 ethanethiol (C2H5SH) − methane (CH4) 1-propanethiol (C3H7SH) − methane (CH4) 1-butanethiol (C4H9SH) − methane (CH4) 3
kij
T (K) (range)
εcross/ (bar·L·mol−1)
−0.035 0.0089 −0.044 0.00196 −0.0476 −0.0522 0.0098 0.079 0.025 0.038 0.038
310−588 310−589 310−588 293−588 303−380 303−380 274−344 253−323 303−380
n.a. CR-1: 83.275 n.a. CR-1: 83.275 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
ε
ε A iBi + ε AjBj = 2
and
β
A i Bj
=
β
A i Bi A j Bj
β
0.0246 n.a. 0.01047 n.a. n.a. n.a. n.a. n.a. n.a. n.a.
the covolume parameters in the expression for the crossassociation volume. Concerning solvation, i.e., the cross association between one self-associating and one nonassociating fluid, Folas et.al. used the so-called modified CR-1 rule.10
Assuming that the radial distribution function is equal to 1, as well as the term exp(εAB/RT) − 1 ≅ exp(εAB/RT), it can be shown that the equivalent expressions for the cross-association energy and cross-association volume parameters with ECR in equation 2 are as follows: A i Bj
βcross n.a.
bibj
ε A iBj =
bij
εassociating 2
and
β A iBj = βcross = fitted
(4)
The association scheme employed in this work is, in accordance to previous studies, the 4C association scheme for water. The association scheme in general depends upon the nature of association, details of which are available in Huang and
(3)
Thus CR-1 (eq 1) and Elliott combining rules (eqs 2 or 3) are similar, the only difference is the second term in eq 3 containing 14701
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Radosz.11 In this work, all of the thermodynamic modeling is based on the CPA EoS. Furthermore, mercaptans are considered as non-self-associating fluids.
4. RESULTS AND DISCUSSION In this work, new vapor−liquid−liquid equilibrium (VLLE) data for the ethanethiol + methane +water, 1-propanethiol + methane
Figure 4. Vapor−liquid−liquid equilibrium of C2H5SH + CH4 + H2O ternary system at 365 K; mole fraction of (Δ) C2H5SH in organic phase, (□) C2H5SH in vapor phase, and (○) C2H5SH in aqueous phase. Solid line shows CPA predictions considering solvation of thiol in water; dotted line represents the modeling with CPA without accounting for solvation (lines coincide).
methanethiol − water, ethanethiol − water, 1-propanehiol − water, 1-butanethiol − water, methane − water, methanethiol − methane, ethanethiol − methane are also from our previous published work and are shown in Table 6. The binary interaction parameters (kij) for 1-propanethiol − methane, and 1butanethiol − methane are fixed to a value of 0.038. This assumption has been made because of the unavailability of binary experimental data for such systems and because the kij values for 1-propanethiol − methane, and 1-butanethiol − methane binary systems do not seem to have an important influence on the VLLE predictions for the ternary mixtures under investigation. Two modeling approaches were investigated. First, thiols were modeled as inert compounds (non-association compounds) and second, thiols were modeled assuming cross association interactions with water. The CPA predictions for the ethanethiol content of the three phases in equilibrium, concerning the system ethanethiol − methane − water, are shown in Figures 2, 3, and 4, respectively. In most cases, the CPA performance is very similar for both modeling approaches, i.e., assuming solvation of thiols in
Figure 2. Vapor−liquid−liquid equilibrium of C2H5SH + CH4 + H2O ternary system at 303 K; mole fractions of (Δ) C2H5SH in organic phase, (□) C2H5SH in vapor phase, and (○) C2H5SH in aqueous phase. Solid line shows CPA predictions considering solvation of thiol in water; dotted line represents the modeling with CPA EoS without accounting for solvation (lines coincide).
+ water, and 1-butanethiol + methane + water at different temperatures (303, 330, and 365 K) and pressures up to 9 MPa are reported in Tables 2, 3, and 4, respectively. The CPA EoS has been successfully used to predict the vapor− liquid−liquid equilibrium data of ethanethiol + methane + water, 1-propanethiol + methane + water, 1-butanethiol + methane + water at different temperature and pressure conditions. The CPA (SRK-part) parameters i.e., b, Γ = a0/(Rb) and c1 are taken from our previous published work and they are presented in Table 5. The proposed sets of binary interaction parameters (kij) of
Figure 3. Vapor−liquid−liquid equilibrium of C2H5SH + CH4 + H2O ternary system at 335 K; mole fraction of (Δ) C2H5SH in organic phase, (□) C2H5SH in vapor phase, and (○) C2H5SH in aqueous phase. Solid line shows CPA predictions considering solvation of thiol in water; dotted line represents the modeling with CPA without accounting for solvation. 14702
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Figure 7. Water mole fractions in the organic and the vapor phase of the C2H5SH + CH4 + H2O ternary system at 365 K; (Δ) H2O in organic phase, and (□) H2O in vapor phase. Solid line shows CPA predictions considering the effect of solvation of thiol in water; dotted line represents the CPA predictions without accounting for solvation (lines coincide).
Figure 5. Water mole fractions in the organic and the vapor phase of the C2H5SH + CH4 + H2O ternary system at 303 K; (Δ) H2O in organic phase, and (□) H2O in vapor phase. Solid line shows CPA predictions considering the effect of solvation of thiol in water; dotted line represents the CPA predictions without accounting for solvation (lines coincide).
Figure 8. Methanethiol (CH3SH) and ethanethiol (C2H5SH) content of aqueous phase in CH3SH + CH4 + H2O and C2H5SH + CH4 + H2O ternary mixtures, respectively. (*) Methanethiol (CH3SH) in the aqueous phase at 365 K, (+) methanethiol (CH3SH) in the aqueous phase at 334 K, (−) methanethiol (CH3SH) in the aqueous phase at 304 K, (Δ) ethanethiol (C2H5SH) in the aqueous phase at 365 K, (□) ethanethiol (C2H5SH) in the aqueous phase at 334 K, (○) ethanethiol (C2H5SH) in the aqueous phase at 304 K, dotted lines present CPA predictions for CH3SH + CH4 + H2O ternary mixture, and dash-dotted lines present CPA predictions for C2H5SH + CH4 + H2O ternary mixtures while considering the effect of solvation.
Figure 6. Water mole fractions in the organic and the vapor phase of the C2H5SH + CH4 + H2O ternary system at 335 K; (Δ) H2O in organic phase, and (□) H2O in vapor phase. Solid line shows CPA predictions considering the effect of solvation of thiol in water; dotted line represents the CPA predictions without accounting for solvation.
water and treating thiols as inert compounds. For ethanethiol + methane + water, this is shown in Figures 2−4 for the thiol content of all phases and in Figures 5−7, where the water content of the organic and the vapor phase is plotted against the total system pressure. However, it was observed that CPA slightly more accurately predicts the ethanethiol content of the aqueous phase assuming solvation of ethanethiol in water, an approach that was applied in Figure 8. However, the effect of solvation was not found to be significant for systems containing 1-propanethiol and 1-butanethiol, and the simpler approach (thiols modeled as inert compounds) was applied for the relevant mixtures (Figures 9−15). From Figures 5−7, it is observed that the model satisfactorily predicts the water content of the organic and vapor phase at 303 and 335 K; however, the water content of the vapor phase is overpredicted at 365 K, especially at low pressures. Figures 5−7 show that the mole fraction of water in the vapor phase decreases with the increase in pressure from 1 to 9 MPa and that the mole fraction of water in vapor phase increases with the increase in temperature at constant system pressure. By
comparison, the VLLE methanethiol content of the aqueous phase (in methanethiol + methane + water ternary mixture, experimental data from a previous study)5 and the ethanethiol content of the aqueous phase (in ethanethiol + methane + water ternary mixture) are plotted in Figure 8. For such systems, it is observed that under the same conditions of temperature and pressure, the fraction of methanethiol is higher than that of ethanethiol. The CPA EoS predictions for the 1-propanethiol content of the three phases in equilibrium, concerning the system 1-propanethiol +methane + water, are presented in Figures 9−11. It is observed that the mole fraction of 1-propanethiol in the vapor phase increases at constant pressure with increase in temperature from 305 to 365 K. Similar results are obtained for the 1-butanethiol + methane + water system as shown in Figures 13−15. The solubility of 1-propanethiol in aqueous phase of the 1-propanethiol + methane + water system and the solubility of 114703
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Figure 9. Vapor−liquid−liquid equilibrium of C3H7SH + CH4 + H2O ternary system at 304 K; mole fractions of (Δ) C3H7SH in organic phase, (□) C3H7SH in vapor phase, and (○) C3H7SH in aqueous phase. Solid lines represent CPA EoS predictions without accounting for solvation.
Figure 12. 1-Propanethiol (C3H7SH) and 1-butanethiol (C4H9SH) in C3H7SH + CH4 + H2O and C4H9SH + CH4 + H2O ternary mixtures, respectively ; (*) 1-propanethiol (C3H7SH) in the aqueous phase at 365K, (+) 1-propanethiol (C3H7SH) in the aqueous phase at 334 K, (×)1-propanethiol (C3H7SH) in the aqueous phase at 304 K, (Δ) 1butanethiol (C4H9SH) in the aqueous phase at 365K, (□)1-butanethiol (C4H9SH) in the aqueous phase at 334 K, (○)1-butanethiol (C4H9SH) in the aqueous phase at 304 K, dash lines present CPA predictions for C3H7SH + CH4 + H2O ternary mixture and solid lines present CPA prediction for C4H9SH + CH4 + H2O ternary mixture.
Figure 10. Vapor−liquid−liquid equilibrium of C3H7SH + CH4 + H2O ternary system at 335 K from 1 to 9 MPa pressure; (Δ) C3H7SH in organic phase, (□) C3H7SH in vapor phase, and (○) C3H7SH in aqueous phase. Solid lines represent CPA predictions without accounting for solvation. Figure 13. Vapor−liquid−liquid equilibrium of C4H9SH + CH4+ H2O ternary system at 303 K; mole fractions of (Δ) C4H9SH in organic phase, (□) C4H9SH in vapor phase, and (○) C4H9SH in aqueous phase. Solid lines represent CPA predictions without accounting for solvation.
butanethiol in the aqueous phase of the 1-butanethiol + methane + water ternary mixture are plotted in Figure 12. It is observed that at identical conditions of temperature and pressure, the solubility of 1-propanethiol is higher than 1-butanethiol in the aqueous phase. From Figures 8 and 12, it can be concluded that at VLLE conditions, the thiols content of the aqueous phase shows the following trend x methanethiol > x ethanethiol > x 1propanethiol > x 1-butanethiol. Also, it can be observed from the experimental data that the methane content of the aqueous phase and the organic phase increases with an increase of pressure and decreases with an increase of temperature. However, the thiol content of the aqueous phase and the organic phase decreases slightly with an increase of pressure and increases with increase of temperature.
Figure 11. Vapor−liquid−liquid equilibrium of C3H7SH + CH4 + H2O ternary system at 365 K from 1 to 9 MPa pressure; (Δ) C3H7SH in organic phase, (□) C3H7SH in vapor phase, and (○) C3H7SH in aqueous phase. Solid lines represent CPA predictions without accounting for solvation.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the Danish Council for Independent Research (Technology and Production Sciences) for the Postdoctoral research grant to J.A.A. at CERE, DTU Chemical Engineering. The authors wish also to thank Statoil and Gassco (Norway), BP (UK, U.S.), TOTAL (France), DONG Energy, and Mærsk Oil and Gas (Denmark) for supporting the CPA-EoS part of this work as part of the CHIGP project (Chemicals in Gas Processing). The authors wish also to thank Alain Valtz from CTP, Mines ParisTech for technical assistance.
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Figure 14. Vapor−liquid−liquid equilibrium of C4H9SH + CH4 + H2O ternary system at 335 K; mole fractions of (Δ) C4H9SH in organic phase, (□) C4H9SH in vapor phase, and (○) C4H9SH in aqueous phase. Solid lines represent CPA predictions without accounting for solvation.
Figure 15. Vapor−liquid−liquid equilibrium of C4H9SH + CH4 + H2O ternary system at 365 K; mole fractions of (Δ) C4H9SH in organic phase, (□) C4H9SH in vapor phase, and (○) C4H9SH in aqueous phase. Solid lines represent CPA predictions without accounting for solvation.
5. CONCLUSIONS New vapor−liquid−liquid equilibrium (VLLE) data were reported for C2H5SH + CH4 + H2O, C3H7SH + CH4 + H2O, and C4H9SH + CH4 + H2O ternary systems at 303, 335, and 365 K and pressures up to 9 MPa. The total system pressure was maintained by CH4. A static analytic method was used for performing all of the measurements. The Cubic-Plus-Association (CPA) equation of state has been successfully used to predict the VLLE of the investigated ternary systems. It has been shown that the CPA EoS satisfactorily describes the solubilities of water in the organic phase, as well as the solubilites of mercaptans (thiols) in the aqueous phase. However, it was observed that CPA overpredicts the water content of the vapor phase, especially at at low pressure and high temperature, i.e., at 365 K. Furthermore, no cross association (solvation) was found to be significant in 1propanethiol + methane + water and 1-butanethiol + methane + water ternary systems.
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REFERENCES
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