experimental study on the solubility of natural gas components

Fluid Phase Equilibria 207 (2003) 143–154

Experimental study on the solubility of natural gas componentsin water with or without hydrate inhibitor

Lu-Kun Wang, Guang-Jin Chen, Guang-He Han, Xu-Qiang Guo, Tian-Min Guo∗High Pressure Fluid Phase Behavior & Property Research Laboratory, University of Petroleum,

P.O. Box 902, Beijing 100083, China

Received 30 September 2002; accepted 6 January 2003

Abstract

Solubility of methane, ethane and a (methane+ ethane) gas mixture in pure water/alcohols (methanol/ethyleneglycol) as well as aqueous solutions of methanol/ethylene glycol have been measured systematically in the pressurerange of 2.00–40.00 MPa for methane and the (methane+ ethane) gas mixture, 0.50–4.50 MPa for ethane, and thetemperature range was 283.2–303.2 K.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Solubility; Natural gas component; Methanol; Ethylene glycol

1. Introduction

The solubility of natural gas components methane and ethane in pure water has been studied extensivelyin the past few decades[1–14]. However, due to their extremely low solubility, the reported data are notin good consistency. For hydrate inhibitor containing systems, limited data were reported by Battino[15],Fauser[16], Hong et al.[17] and Schneider[18] for methane–methanol system; Schneider[18], Hayduk[19], Yaacobi and Ben-Naim[20], McDaniel[21], Ma and Kokl[22], Ohgaki et al.[23] and Weber[24]for the ethane–methanol system. Furthermore, similar solubility data in pure ethylene glycol and aqueoussolutions of alcohols as well as the solubility data for gas mixtures were scarcely reported in the literature.

In conventional hydrate formation calculations, the effect of gas solubility in the aqueous phase areoften neglected, however, for accurate prediction of hydrate formation conditions, its effect should notbe overlooked, especially for the inhibitor containing systems.

The major objective of this work is to expand the gas solubility data base for hydrate inhibitor containingsystems which are important for testing/improving hydrate models.

∗ Corresponding author. Tel.:+86-10-62340132; fax:+86-10-62340132.E-mail address: [email protected] (T.-M. Guo).

0378-3812/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0378-3812(03)00009-8

144 L.-K. Wang et al. / Fluid Phase Equilibria 207 (2003) 143–154

Fig. 1. Schematic of the ROP PVT unit: (1) air bath; (2) pressure controller; (3) primary cell; (4) magnetic stirrer; (5) temperaturecontroller; (6) secondary cell.

Fig. 2. Schematic of the RUSKA PVT unit: (1) motor; (2) oil bath; (3) equilibrium cell; (4) sampling system; (5) pressure gauge;(6) temperature controller; (7) vacuum pump; (8) pressure-stabilizing cylinder; (9) electric metering pump; (10) gas booster;(11) ethane cylinder.

L.-K. Wang et al. / Fluid Phase Equilibria 207 (2003) 143–154 145

2. Experimental section

2.1. Materials

Analytical grade methane (99.99 vol.%) and ethane (99.95 vol.%) were supplied by Beifen Gas IndustryCorporation. Analytical grade (99.9 wt.%) methanol and ethylene glycol were purchased from BeijingChemicals Corporation. Distilled water was produced in our laboratory.

The composition of the synthetic gas mixture tested is 90.13 mol% methane and 9.87 mol% ethane.

Fig. 3. Liquid phase sampling system.

Fig. 4. Comparison of the methane solubility (x1) in water measured by authors and reported from literature[8] (T = 324.2 K).


Table 1Methane solubility (x1) in water

T = 283.2 K T = 293.2 K T = 303.2 K

P (MPa) x1 × 103 (mole fraction) P (MPa) x1 × 103 (mole fraction) P (MPa) x1 × 103 (mole fraction)

6.05 1.585 21.00 3.111 40.03 4.0494.04 1.096 18.00 2.988 35.08 3.6592.00 0.563 15.00 2.757 30.11 3.381– – 12.00 2.421 25.15 3.082– – 9.00 2.009 20.16 2.783– – 6.00 1.526 15.18 2.411– – 3.00 0.755 10.22 1.995

2.2. Experimental apparatus/instruments

For accelerating the experimental work, two PVT units were used simultaneously to measure the gassolubility data in this work. The ROP unit (schematically shown inFig. 1) was adopted for methane/gasmixture systems and the RUSKA unit (schematically shown inFig. 2) used for ethane system.

The ROP unit is a computer-controlled, dual cell, mercury-free high-pressure PVT system. The maxi-mum working pressure is 82.7 MPa and the operating temperature range is 253–473 K. The liquid volumecan be measured with an accuracy of±0.001 cm3. The maximum volume of the primary/secondary cellis 500 and 40 cm3, respectively.

Fig. 5. Solubility of ethane in an aqueous solution of ethylene glycol (20 wt.%).


The RUSKA PVT unit consists of an equilibrium cell of 600 cm3, its maximum working temperatureand pressure are 423.2 K and 68.0 MPa, respectively. Constant temperature (±0.1 K) was maintainedby using a silicone oil bath. A pressure-stabilizing cylinder (1500 cm3) was used for keeping constantpressure in the equilibrium cell. The cylinder has a heating jacket, the maximum working tempera-ture and pressure are 423.2 K and 83.0 MPa, respectively. An electric metering pump is connected tothe pressure-stabilizing cylinder and the pressure-transmitting medium is mercury. A motor is used forrolling the equilibrium cell to accelerate the equilibrium process and a gas booster is available for raisingthe gas pressure to the assigned value.

The composition of the tested gas mixture was analyzed by Hewlett-Packard 6890 gas chromato-graph. An electronic balance (0–200 g) was used for measuring the sample weight with a precision of±0.001 g.

Table 2Methane solubility (x1) in the aqueous solutions of methanol

Concentration of methanol in the aqueous phase

P (MPa) 20 wt.% 40 wt.% 60 wt.% 80 wt.% 100 wt.%

x1 × 103 (mole fraction)

T = 283.2 K40.05 6.332 10.40 25.00 69.09 289.435.06 6.133 10.15 23.84 67.01 261.430.06 5.824 9.553 22.55 62.55 234.225.04 5.383 8.919 20.47 57.90 204.620.04 4.755 7.858 18.70 50.45 177.415.05 4.093 6.783 16.33 42.50 148.910.05 3.179 5.334 12.62 31.53 103.85.05 1.846 3.126 7.062 18.84 45.95

T = 293.2 K40.05 6.244 10.14 23.88 67.99 281.635.06 6.014 9.825 23.06 65.35 257.130.06 5.723 9.228 21.53 61.62 226.125.04 5.275 8.586 19.18 57.61 199.220.04 4.597 7.458 17.62 49.42 170.915.05 3.977 6.474 14.65 38.93 137.710.05 3.095 4.987 11.96 28.89 89.475.05 1.690 2.866 6.979 16.03 44.64

T = 303.2 K40.05 6.068 9.803 23.14 67.91 277.535.06 5.848 9.455 22.28 65.11 249.530.06 5.480 8.807 20.90 61.26 219.025.04 5.132 8.259 18.75 56.15 190.020.04 4.571 7.442 16.88 48.80 162.015.05 3.887 6.276 14.22 38.35 126.010.05 3.013 4.889 11.32 27.19 83.135.05 1.521 2.540 6.574 14.50 42.31


2.3. Instruments calibration

For both PVT units, the digital manometers used for pressure measurements were calibrated against astandard RUSKA dead-weight gauge. The precision of pressure measurement is±0.01 MPa. The plat-inum probes for temperature measurements were calibrated against a 25� standard probe connectedto a Hewlett-Packard Model 34401a digital multi-meter. The precision of temperature measurements isestimated at±0.1 K.

2.4. Experimental procedure

The experimental method adopted in this work for ROP and RUSKA PVT units is similar to that reportedby Zheng et al.[25]. For saving space, detailed experimental procedure is given below for ROP unit only.

Table 3Methane solubility (x1) in the aqueous solutions of ethylene glycol

Concentration of ethylene glycol in the aqueous phase

P (MPa) 20 wt.% 40 wt.% 60 wt.% 80 wt.% 100 wt.%


T = 283.2 K40.06 – 4.538 5.600 9.108 21.5335.06 – 4.277 5.199 8.571 21.1030.05 – 4.032 4.917 7.994 20.1325.05 – 3.691 4.441 7.274 18.6820.05 3.389 3.341 3.900 6.469 16.4715.05 2.868 2.946 3.237 5.568 13.2210.05 2.129 2.324 2.291 4.146 9.9545.00 1.319 1.319 1.458 1.896 4.887

T = 293.2 K40.05 4.074 4.626 5.772 9.258 22.6435.06 3.870 4.469 5.484 8.775 21.3730.05 3.578 4.100 5.047 8.348 19.5725.04 3.288 3.817 4.642 7.704 17.9120.04 2.868 3.496 4.060 7.050 15.8815.05 2.479 2.822 3.385 5.855 13.5210.05 1.881 2.214 2.449 4.063 10.315.00 1.275 1.262 1.461 2.359 5.707

T = 303.2 K40.06 4.400 4.761 6.097 10.17 23.1634.95 4.097 4.509 5.774 9.915 22.0230.06 3.726 4.151 5.392 9.100 19.9425.05 3.359 3.910 4.985 8.257 18.0820.04 2.939 3.383 4.498 7.164 15.9215.05 2.575 2.740 3.791 6.113 13.0810.05 2.059 2.131 2.940 5.196 10.085.00 1.306 1.100 1.535 3.500 5.919


Table 4Ethane solubility (x2) in water

T = 283.2 K T = 293.2 K T = 303.2 K

P (MPa) x2 × 103 (mole fraction) P (MPa) x2 × 103 (mole fraction) P (MPa) x2 × 103 (mole fraction)

4.00 – 4.00 – 4.00 0.8643.50 – 3.50 – 3.50 0.8043.10 – 3.00 0.864 3.10 0.7562.50 – 2.50 0.811 2.50 0.6772.00 – 2.00 0.751 2.00 0.6161.50 0.700 1.50 0.651 1.50 0.5321.00 0.538 1.00 0.477 1.00 0.3660.50 0.310 0.50 0.245 0.50 0.119

Table 5Ethane solubility (x2) in the aqueous solutions of methanol

Concentration of methanol in the aqueous phase

P (MPa) 20 wt.% 40 wt.% 60 wt.% 80 wt.% 100 wt.%


T = 283.2 K3.00 1.705 3.476 12.52 49.33 253.72.50 1.517 3.115 11.21 44.10 201.92.00 1.271 2.659 9.247 36.52 157.21.50 0.987 2.019 7.204 26.76 114.61.00 0.688 1.558 5.150 16.62 77.480.50 0.305 0.894 2.561 8.163 35.68

T = 293.2 K3.00 1.599 3.313 11.02 45.13 230.72.50 1.447 3.094 9.980 41.19 187.82.00 1.224 2.574 8.600 34.80 137.31.50 0.949 1.969 6.532 25.08 93.351.00 0.639 1.457 3.762 15.05 63.260.50 0.193 0.828 1.697 7.062 28.41

T = 303.2 K4.00 1.838 3.803 13.52 53.43 280.13.50 1.705 3.459 11.92 49.59 236.53.00 1.558 3.172 10.59 42.22 194.32.50 1.415 2.904 9.277 35.42 146.52.00 1.171 2.432 7.805 29.74 106.51.50 0.908 1.835 5.781 21.97 75.741.00 0.592 1.336 3.257 12.87 50.990.50 0.133 0.699 0.805 4.361 27.03


Table 6Ethane solubility (x2) in the aqueous solutions of ethylene glycol

Concentration of ethylene glycol in the aqueous phase

P (MPa) 20 wt.% 40 wt.% 60 wt.% 80 wt.% 100 wt.%


T = 283.2 K3.00 1.118 1.230 1.618 3.167 11.182.50 1.032 1.116 1.423 2.958 10.012.00 0.894 0.975 1.174 2.594 8.6371.50 0.722 0.779 0.898 2.071 6.7701.00 0.479 0.486 0.576 1.368 4.7080.50 0.231 0.207 0.200 0.459 2.320

T = 293.2 K3.00 1.021 1.230 1.542 3.018 10.372.50 0.946 1.116 1.360 2.811 9.5782.00 0.812 0.975 1.103 2.462 8.0741.50 0.637 0.779 0.794 1.882 6.0861.00 0.409 0.486 0.529 1.234 3.8850.50 0.188 0.207 0.162 0.368 1.731

T = 303.2 K4.00 1.086 1.382 1.851 3.408 11.433.50 1.021 1.304 1.686 3.157 10.643.00 0.940 1.194 1.485 2.912 9.9182.50 0.825 1.069 1.302 2.707 9.0782.00 0.685 0.941 1.022 2.337 7.7071.50 0.528 0.743 0.723 1.795 5.8031.00 0.326 0.447 0.461 1.155 3.4860.50 0.096 0.146 0.117 0.222 1.188

In starting the experiment, the temperature of air bath was raised to the selected value. When thetemperature became stable, the whole system was evacuated. Gas species was charged into the primarycell from a gas cylinder until the gas volume in the primary cell is about 450 cm3 and the pressure reached5.0 MPa. Then about 25 ml liquid solvent was injected into the primary cell and the system pressure wasraised to the maximum value (40.0 MPa). In order to accelerate the equilibrium process under constant

Table 7Solubility of methane (x1) and ethane (x2) in water for a (CH4 + C2H6) gas mixture

T = 275.2 K T = 283.2 K

P (MPa) x1 × 103

(mole fraction)x2 × 103

(mole fraction)P (MPa) x1 × 103


(mole fraction)

1.80 1.032 0.0986 4.00 1.150 0.14471.50 0.913 0.1147 3.00 1.066 0.14121.00 0.711 0.1128 2.00 0.947 0.1451– – – 1.00 0.643 0.1475


pressure, the gas and liquid was circulating between the two cells. When system pressure and total fluidvolume remain unchanged, the system is considered in the equilibrium state. The liquid outlet valve ofthe primary cell was opened slightly and about 20 ml liquid was charged to the sampling system (Fig. 3,kept at ambient conditions) under constant pressure mode. The collected liquid sample flashes in Flask 1,and the flashed gas displaces a portion of the water filled in Flask 2 into Flask 3. The amount of liquid inFlask 1 and water in Flask 3 was weighed, respectively, and the gas solubility was then calculated fromthe measured data. For guarantee the data quality, at least three parallel runs were conducted and themaximum deviation allowed was 5.0%. The above experimental procedure was repeated at a decreasingpressure-step of 5.0 MPa for each selected temperature.

When measuring the solubility data of each component in the synthetic (methane+ethane) gas mixture,the flashed gas sample was injected into a gas chromatograph (Hewlett-Packard 6890) for compositionanalysis, and the composition of the aqueous phase can thus be determined from material balance.

Table 8Solubility of methane (x1) and ethane (x2) in the aqueous solutions of methanol for a (CH4 + C2H6) gas mixture

Methanolconcentration(wt.%)

T = 275.2 K T = 283.2 K

P (MPa) x1 × 103




(mole fraction)

20 4.95 3.276 0.2829 15.00 4.640 0.36064.00 2.861 0.3500 10.00 3.549 0.30233.00 2.416 0.3793 5.00 2.304 0.26662.00 1.959 0.35281.30 1.658 0.3354

40 40.00 11.49 0.6062 40.00 10.68 0.613130.00 11.05 0.6739 30.00 10.33 0.610520.00 9.483 0.6268 20.00 8.736 0.553710.00 7.211 0.6219 10.00 6.083 0.43485.00 4.464 0.7400 5.00 4.135 0.6464

60 40.00 45.83 4.727 40.00 43.16 4.61630.00 40.21 4.158 30.00 37.54 4.32020.00 32.36 3.552 20.00 28.74 3.37910.00 19.78 2.512 10.00 16.51 2.2155.00 9.417 1.393 5.00 8.209 1.881

80 40.00 59.18 5.982 40.00 55.12 5.64630.00 53.37 5.661 30.00 51.01 5.42320.00 45.22 5.412 20.00 42.70 5.23410.00 29.69 6.727 10.00 26.69 4.7945.00 16.26 6.785 5.00 17.08 4.683

100 40.00 202.2 24.59 40.00 169.1 21.0430.00 182.4 22.61 30.00 152.6 22.3020.00 148.9 22.27 20.00 121.5 20.1110.00 82.27 21.93 10.00 65.11 15.495.00 42.80 15.73 5.00 34.03 12.76


3. Experimental results and discussion

For testing the reliability of the ROP/RUSKA PVT unit and the operating procedure adopted, thesolubility of methane in water at 324.2 K has been measured in both units and compared with literaturedata reported by O’Sullivan and Smith[8]. Test results are presented inFig. 4, in which the gas solubility(x1) is expressed in mole fraction.

FromFig. 4, we can see that the agreement between the measured solubility data using ROP/RUSKAunit and the data reported in literature[8] is fairly good.

In this work, the solubility of pure methane/ethane and a gas mixture (90.13 mol% methane+9.87 mol%ethane) in water/pure alcohol (methanol/ethylene glycol)/aqueous solution of methanol/ethylene glycolhave been measured systematically in the pressure ranges of 5.0–40.0 MPa for methane/gas mixture,0.5–4.5 MPa for ethane, and under three temperatures, 283.2, 293.2 and 303.2 K. The measured solubilitydata are listed inTables 1–9, wherex1 andx2 denote respectively the mole fraction of methane and ethanein the aqueous phase. In order to prevent hydrate formation, the operating pressures were carefully

Table 9Solubility of methane (x1) and ethane (x2) in the aqueous solutions of ethylene glycol for a synthetic (CH4 + C2H6) gas mixture

Methanolconcentration(wt.%)

T = 275.2 K T = 283.2 K

P (MPa) x1 × 103




(mole fraction)

20 2.21 0.7987 0.1424 14.00 2.511 0.17601.00 0.5362 0.0995 10.00 2.272 0.17100.50 0.2023 0.0394 5.00 1.633 0.1432

40 15.00 3.465 0.5109 40.00 4.673 0.222210.00 2.915 0.4476 30.00 4.376 0.23285.00 1.824 0.3310 20.00 3.611 0.2313

10.00 2.659 0.26275.00 1.808 0.2258

60 40.00 6.827 0.3525 40.00 6.230 0.374530.00 6.353 0.3633 30.00 5.782 0.348920.00 5.261 0.3239 20.00 4.731 0.286510.00 3.616 0.3468 10.00 3.108 0.23255.00 2.402 0.3189 4.99 2.005 0.2554

80 40.00 8.900 0.6425 40.00 8.176 0.582430.00 8.225 0.6267 30.00 7.653 0.551320.00 6.971 0.5840 20.00 6.331 0.492710.00 4.526 0.5231 10.00 4.033 0.44275.00 2.756 0.4283 4.99 2.313 0.4484

100 40.00 27.45 2.622 40.00 26.10 2.56230.00 25.19 2.437 30.00 23.70 2.39920.00 21.75 2.217 20.00 20.22 2.15710.00 16.85 2.009 10.00 15.39 1.8955.00 12.75 2.189 4.99 12.12 1.876


controlled within the range where hydrate formation can be avoided. As a typical example, the solubilityof ethane in an aqueous solution of ethylene glycol (20 wt.%) is shown inFig. 5.

The measured data show that for each inhibitor concentration, the gas solubility increases with in-creasing pressure and decreases with increasing temperature. However, the temperature dependency forethylene glycol containing systems is less obvious.

At constant pressure and temperature, the gas solubility increases smoothly with the increase of inhibitorconcentration in the aqueous phase up to 80 wt.%. However, the gas solubility curve increases sharplywhen the inhibitor concentration exceeds 80 wt.%. Under the sameT, P conditions the gas solubility inthe aqueous methanol solutions is always greater than in the aqueous ethylene glycol solutions.

4. Conclusions

In this work, the solubilities of methane, ethane and a (90.13 mol% methane+ 9.87 mol% ethane) gasmixture in water and aqueous solutions of methanol/ethylene glycol have been measured using ROP andRUSKA PVT units. The data can be used for testing/improving thermodynamic models. The presence ofinhibitor (methanol/ethylene glycol) affects the gas solubility significantly. The gas solubility increaseswith increasing inhibitor concentration in the aqueous phase.

Acknowledgements

The financial support received from the National Natural Science Foundation of China (Grant Nos.29806009 and 20176028) is gratefully acknowledged.

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experimental study on the solubility of natural gas components

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