J. Chem. Eng.

Data 2008, 53, 683–686 683

Experimental Data and Predictions of Dissociation Conditions for Ethane and

Propane Simple Hydrates in the Presence of Methanol, Ethylene Glycol, and
Triethylene Glycol Aqueous Solutions

Amir H. Mohammadi, Waheed Afzal, and Dominique Richon*

Mines Paris, ParisTech, CEP-TEP, CNRS FRE 2861, 35 Rue Saint Honoré, 77305 Fontainebleau, France

Experimental dissociation data for ethane and propane simple hydrates in the presence of (0.05 and 0.15)
mass fractions of methanol, ethylene glycol, and triethylene glycol aqueous solutions are reported in this
work. The experimental data have been measured using an isochoric method. All the experimental data are
compared with the predictions of a general correlation (HWHYD correlation) and a thermodynamic model
(HWHYD model). The agreements between the experimental and predicted data are generally found
acceptable.

Introduction Table 1. Dissociation Temperature Ranges Studied in this Work for

Ethane and Propane Simple Hydrates in the Presence of Different
Gas hydrates are solid crystalline compounds stabilized by Concentrations of Methanol, Ethylene Glycol, and Triethylene
the inclusion of suitably sized gas molecules inside cavities, of Glycol Aqueous Solutions
different sizes, formed by water molecules through hydrogen mass fraction of
bonding. They resemble ice in appearance, but unlike ice, they hydrate inhibitor in hydrate dissociation
former inhibitor aqueous solution temperature range/K
may form at temperatures well above the ice point.1 Suitable
ethane methanol 0.05 272.2 to 280.5
conditions for gas hydrate formation commonly occur during 0.15 268.2 to 278.9
hydrocarbon production and exploration operations. Gas hydrate ethylene glycol 0.05 272.7 to 281.0
formation can block pipelines and transfer lines and lead to 0.15 269.7 to 279.6
serious economic, operational, and safety problems.1 Thermo- triethylene glycol 0.05 274.1 to 281.5
0.15 274.0 to 281.1
dynamic inhibitors, such as alcohols and glycols, are normally propane methanol 0.05 272.4 to 275.3
used to inhibit gas hydrate formation, which usually reduces 0.15 266.3 to 269.9
the activity of water in the aqueous phase, shifting the hydrate ethylene glycol 0.05 272.9 to 275.8
phase boundaries to high pressures and/or low temperatures.1 0.15 269.8 to 273.7
triethylene glycol 0.05 273.2 to 276.8
To develop and validate thermodynamic models and other tools 0.15 273.7 and 275.0
for predicting hydrate phase boundaries of natural gases, reliable
gas hydrate equilibrium data for the main components of natural Table 2. Purities and Suppliers of Materialsa
gases in the presence and absence of inhibitor aqueous solution chemical supplier purity (volume fraction)
are necessary.1 Although many data have been reported for gas ethane Messer Griesheim 0.99995
hydrates of methane and carbon dioxide in the presence and propane Messer Griesheim 0.99995
absence of methanol, ethylene glycol, and triethylene glycol methanol Aldrich 0.999
aqueous solutions, information for gas hydrates of other ethylene glycol Aldrich 0.99
triethylene glycol Aldrich 0.99
components of natural gases in the presence of the above-
mentioned aqueous solutions is limited.1 a
Deionized water was used in all experiments.
In this communication, we report experimental dissociation
data for ethane and propane simple hydrates in the presence of Experimental Section
(0.05 and 0.15) mass fractions of methanol, ethylene glycol,
and triethylene glycol aqueous solutions. The data have been Purities and suppliers of materials are provided in Table 2.
measured based on our previous experimental work,2 which A detailed description of the experimental setup used in this
takes advantage of an isochoric method.3 Table 1 summarizes study is given elsewhere.2 Briefly, the main part of the apparatus
the experiments carried out in terms of hydrate former, inhibitor, is a cylindrical vessel, which can withstand pressures higher
inhibitor concentration in the aqueous solution, and dissociation than 10 MPa. The vessel has a volume of (57.5 ( 0.5) cm3
temperature ranges. The experimental hydrate dissociation data with two sapphire windows. A magnetic stirrer ensures sufficient
measured in this work are compared with the predictions of a agitation to facilitate reaching equilibrium. The vessel was
general correlation (HWHYD correlation)4 and a thermodynamic immersed inside a temperature-controlled bath to maintain the
temperatures of study. Two platinum probes (Pt100) inserted
model (HWHYD model),5 and acceptable agreements between
into the vessel were used to measure temperature and check
the experimental and the predicted data are found.
for equality of temperatures within temperature measurement
* Corresponding author. E-mail: richon@ensmp.fr. Tel.: +(33) 1 64 69 49 uncertainties, which is estimated to be less than 0.1 K. This
65. Fax: +(33) 1 64 69 49 68. temperature uncertainty estimation comes from careful calibra-
10.1021/je700527d CCC: $40.75  2008 American Chemical Society
Published on Web 02/27/2008
684 Journal of Chemical & Engineering Data, Vol. 53, No. 3, 2008

Table 3. Experimental Dissociation Data for Ethane and Propane

Simple Hydrates in the Presence of Methanol, Ethylene Glycol, and
Triethylene Glycol Aqueous Solutions (w1: Mass Fraction of
Inhibitor in Aqueous Solution)
T/Ka P/MPab
Ethane + Methanol Aqueous Solution (w1 ) 0.05)
272.2 0.50
275.0 0.72
276.4 0.82
279.0 1.19
280.5 1.54
Ethane + Methanol Aqueous Solution (w1 ) 0.15)
268.2 0.61
270.6 0.82
273.8 1.23
276.0 1.75
278.9 2.60
Propane + Methanol Aqueous Solution (w1 ) 0.05)
272.4 0.21 Figure 1. Experimental and predicted hydrate phase boundaries of ethane
273.6 0.28 in the presence of methanol aqueous solutions. Symbols, experimental data:
274.8 0.35 O, ethane + methanol aqueous solution (w1 ) 0.05), this work; ∆, ethane
275.3 0.41 + methanol aqueous solution (w1 ) 0.15), this work; bold solid lines,
Propane + Methanol Aqueous Solution (w1 ) 0.15) predictions of hydrate phase boundaries using general correlation4 for the
266.3 0.20 ethane + methanol aqueous solutions systems; solid lines, predictions of
267.5 0.27 hydrate phase boundaries using the thermodynamic model5 for the ethane
268.8 0.36 + methanol aqueous solution systems; dashed line, prediction of ethane
269.9 0.42 hydrate phase boundary in the presence of distilled water using the
thermodynamic model5 (w1: mass fraction of inhibitor in aqueous solution).
Ethane + Ethylene Glycol Aqueous Solution (w1 ) 0.05) Error band: 0.5 K.
272.7 0.49
274.1 0.61
276.3 0.79
278.8 1.10
281.0 1.45
Ethane + Ethylene Glycol Aqueous Solution (w1 ) 0.15)
269.7 0.50
272.5 0.70
274.4 0.89
276.9 1.19
279.6 1.73
Propane + Ethylene Glycol Aqueous Solution (w1 ) 0.05)
272.9 0.20
274.0 0.27
275.0 0.33
275.8 0.40
Propane + Ethylene Glycol Aqueous Solution (w1 ) 0.15)
269.8 0.20
271.2 0.28
272.6 0.40
273.7 0.47
Ethane + Triethylene Glycol Aqueous Solution (w1 ) 0.05) Figure 2. Experimental and predicted hydrate phase boundaries of propane
274.1 0.58 in the presence of methanol aqueous solutions. Symbols, experimental data:
275.3 0.69 O, propane + methanol aqueous solution (w1 ) 0.05), this work; ∆, propane
277.5 0.90 + methanol aqueous solution (w1 ) 0.15), this work; bold solid lines,
279.3 1.10 predictions of hydrate phase boundaries using the general correlation4 for
281.5 1.45 the propane + methanol aqueous solutions systems; solid lines, predictions
Ethane + Triethylene Glycol Aqueous Solution (w1 ) 0.15) of hydrate phase boundaries using the thermodynamic model5 for the
274.0 0.69 propane + methanol aqueous solution systems; dashed line, prediction of
275.3 0.84 propane hydrate phase boundary in the presence of distilled water using
277.6 1.07 the thermodynamic model5 (w1: mass fraction of inhibitor in aqueous
279.9 1.50 solution). Error band: 0.5 K.
281.1 1.77
Propane + Triethylene Glycol Aqueous Solution (w1 ) 0.05) tion against a 25 Ω reference platinum probe. The pressure in
273.2 0.20 the vessel was measured with a DRUCK pressure transducer
274.3 0.25
275.5 0.35 (Druck, type PTX611 for pressure ranges from (0 to 8) MPa).
276.8 0.45 Pressure measurement accuracies are estimated to be better than
Propane + Triethylene Glycol Aqueous Solution (w1 ) 0.15) 5 kPa. The hydrate dissociation points were measured with an
273.7 0.29 isochoric pressure search procedure.3 The vessel containing the
275.0 0.43 aqueous solution (60 volume % of the vessel was filled by the
a
Uncertainty on temperatures through calibrated platinum probes is
aqueous solution) was immersed into the temperature-controlled
estimated to be less than 0.1 K. b Uncertainty on pressures through bath, and the gas was supplied from a high-pressure cylinder
calibrated pressure transducer is estimated to be less than 5 kPa. through a pressure-regulating valve into the partially evacuated
 Journal of Chemical & Engineering Data, Vol. 53, No. 3, 2008 685

Figure 3. Experimental and predicted hydrate phase boundaries of ethane

in the presence of ethylene glycol aqueous solutions. Symbols, experimental
data: O, ethane + ethylene glycol aqueous solution (w1 ) 0.05), this work;
∆, ethane + ethylene glycol aqueous solution (w1 ) 0.15), this work; bold Figure 5. Experimental and predicted hydrate phase boundaries of ethane
solid lines, predictions of hydrate phase boundaries using the general in the presence of triethylene glycol aqueous solutions. Symbols, experi-
correlation4 for the ethane + ethylene glycol aqueous solution systems; mental data: O, ethane + triethylene glycol aqueous solution (w1 ) 5), this
solid lines, predictions of hydrate phase boundaries using the thermodynamic work; ∆, ethane + triethylene glycol aqueous solution (w1 ) 15), this work;
model5 for the ethane + ethylene glycol aqueous solution systems; dashed bold solid lines, predictions of hydrate phase boundaries using the general
line, Prediction of the ethane hydrate phase boundary in the presence of correlation4 for the ethane + triethylene glycol aqueous solution systems;
distilled water using the thermodynamic model5 (w1: mass fraction of dashed line, prediction of ethane hydrate phase boundary in the presence
inhibitor in aqueous solution). Error band: 0.5 K. of distilled water using the thermodynamic model5 (w1: mass fraction of
inhibitor in aqueous solution). Error band: 0.5 K. The predictions of the
thermodynamic model5 have not been shown in this figure, as this model
was not developed for triethylene glycol containing systems.

Figure 4. Experimental and predicted hydrate phase boundaries of propane

in the presence of ethylene glycol aqueous solutions. Symbols, experimental
data: O, propane + ethylene glycol aqueous solution (w1 ) 0.05), this work;
∆, propane + ethylene glycol aqueous solution (w1 ) 0.15), this work;
Figure 6. Experimental and predicted hydrate phase boundaries of propane
bold solid lines, predictions of hydrate phase boundaries using the general
in the presence of triethylene glycol aqueous solutions. Symbols, experi-
correlation4 for the propane + ethylene glycol aqueous solution systems;
mental data: O, propane + triethylene glycol aqueous solution (w1 ) 5),
solid lines, predictions of hydrate phase boundaries using the thermodynamic
this work; ∆, propane + triethylene glycol aqueous solution (w1 ) 15),
model5 for the propane + ethylene glycol aqueous solution systems; dashed
this work; bold solid lines, predictions of hydrate phase boundaries using
line, prediction of propane hydrate phase boundary in the presence of
the general correlation4 for the propane + triethylene glycol aqueous solution
distilled water using the thermodynamic model5 (w1: mass fraction of
systems; dashed line, prediction of propane hydrate phase boundary in the
inhibitor in aqueous solution). Error band: 0.5 K.
presence of distilled water using the thermodynamic model5 (w1: mass
fraction of inhibitor in aqueous solution). Error band: 0.5 K. The predictions
vessel. After getting temperature and pressure stability (far of the thermodynamic model5 have not been shown in this figure, as this
enough from the hydrate formation region), the valve between model was not developed for triethylene glycol containing systems.
the vessel and the cylinder was closed. Subsequently, the
temperature was slowly decreased to form the hydrate. Hydrate mental run, from which we determined the hydrate dissociation
formation in the vessel was detected by a pressure drop. The point. If the temperature is increased in the hydrate-forming
temperature was then increased with steps of 0.1 K. At every region, hydrate crystals partially dissociate, thereby substantially
temperature step, the temperature was kept constant for 4 h to increasing the pressure. If the temperature is increased outside
achieve a steady equilibrium state in the vessel. In this way, a the hydrate region, only a smaller increase in the pressure is
pressure-temperature diagram was obtained for each experi- observed as a result of the change in the phase equilibria of the
686 Journal of Chemical & Engineering Data, Vol. 53, No. 3, 2008

Table 4. Constants Ci in Equation 2 for Methanol, Ethylene Glycol, and Triethylene Glycol4
inhibitor C1 C2 C3 C4 C5 C6
methanol 0.478 7.17 · 10-3 -1.44 · 10-5 2.947 · 10-2 5.960 · 10-1 3.100 · 10-5
ethylene glycol 38.93 -5.22 · 10-1 1.767 · 10-2 3.503 · 10-4 5.083 · 10-3 2.650 · 10-5
triethylene glycol 0.1964 -5.81 · 10-3 1.393 · 10-4 2.855 · 10-2 8.540 · 10-1 3.240 · 10-5

fluids in the vessel.6 Consequently, the point at which the slope K deviations. It should be noted that the predictions of the
of pressure-temperature data plots changes sharply is consid- thermodynamic model5 have not been shown in Figures 5 and
ered to be the point at which all hydrate crystals have dissociated 6 as this model was not developed for triethylene glycol
and hence as the dissociation point. containing systems.

Results and Discussions Summary

All experimental dissociation points measured in this work Experimental dissociation data for ethane and propane simple
are reported in Table 3 and are plotted in Figures 1 to 6. A hydrates in the presence of (0.05 and 0.15) mass fractions of
semilogarithmic scale has been used in these figures to show methanol, ethylene glycol, and triethylene glycol aqueous
the data consistency, as the logarithm of hydrate dissociation solutions at various temperature ranges were reported in this
pressure versus temperature has approximately linear behavior. work. An isochoric method2,3 was used for performing all the
The figures also show predictions of a general correlation measurements. All the experimental data were compared with
(HWHYD correlation)4 and a thermodynamic model (HWHYD the predictions of a general correlation (HWHYD correlation),4
model)5 for estimating hydrate inhibition effects of methanol, and a thermodynamic model (HWHYD model)5 and acceptable
ethylene glycol, and triethylene glycol aqueous solutions. agreements were found between experimental and predicted
Briefly, the following equation has been used for predicting data.
hydrate dissociation temperature, T, of a fluid in the presence
of inhibitor from hydrate suppression temperature (or suppres- Literature Cited
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ed.; CRC Press, Taylor & Francis Group: Boca Raton, 2007.
T ) T0 - ∆T (1) (2) Afzal, W.; Mohammadi, A. H.; Richon, D. Experimental Measurements
and Predictions of Dissociation Conditions for Carbon Dioxide and
where T0 stands for hydrate dissociation temperature of the same Methane Hydrates in the Presence of Triethylene Glycol Aqueous
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equation, ∆T is calculated using the following equation (HWHYD (3) Tohidi, B.; Burgass, R. W.; Danesh, A.; Østergaard, K. K.; Todd,
A. C. Improving the Accuracy of Gas Hydrate Dissociation Point
correlation)4 Measurements. Ann. N.Y. Acad. Sci. 2000, 912, 924–931.
(4) Østergaard, K. K.; Masoudi, R.; Tohidi, B.; Danesh, A.; Todd, A. C.
∆T ⁄ K ) [C1(w1 · 100) + C2(w1 · 100)2 + A general correlation for predicting the suppression of hydrate
dissociation temperature in the presence of thermodynamic inhibitors.
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[C6((P0 - 1000) ⁄ kPa) + 1] (2) research/hydrate/. (See also: Tohidi, B.; Burgass, R. W.; Danesh, A.;
Todd A. C. Hydrate inhibition effect of produced water, Part 1. Ethane
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the aqueous phase, the pressure of the system, and the Europe 93 Conference; 1993; 255–264).
dissociation pressure of fluid in the presence of pure water at (6) Ohmura, R.; Takeya, S.; Uchida, T.; Ebinuma, T. Clathrate Hydrate
273.15 K. The constants Ci are given in the original manuscript Formed with Methane and 2-Propanol: Confirmation of Structure II
Hydrate Formation. Ind. Eng. Chem. Res. 2004, 43, 4964–4966.
for various inhibitors.4 These constants for methanol, ethylene (7) Avlonitis, D. Thermodynamics of Gas Hydrate Equilibria, Ph.D Thesis,
glycol, and triethylene glycol are reported in Table 4.4 It should Department of Petroleum Engineering, Heriot-Watt University, Ed-
be mentioned that eq 2 has “six empirically determined inburgh, UK, 1992.
(8) Tohidi-Kalorazi, B. Gas Hydrate Equilibria in the Presence of
constants” and should be used with care out of its application Electrolyte Solutions, Ph.D Thesis, Department of Petroleum Engi-
range. neering, Heriot-Watt University, Edinburgh, UK, 1995.
In eq 1, T0 can be calculated at any given pressure by using (9) Valderrama, J. O. A generalized Patel-Teja equation of state for polar
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scenarios in hydrate phase equilibrium calculations. A detailed Equilibria of Mixtures Containing Petroleum Reservoir Fluids and
description of this model is given elsewhere.7,8 The model5 is Methanol With a Cubic EoS. Fluid Phase Equilib. 1994, 94, 181–
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equation of state9 and nondensity dependent mixing rules10 for
modeling the fluid phases, and the van der Waals and Platteeuw Received for review September 13, 2007. Accepted February 6, 2008.
theory11 is used for modeling the hydrate phase. As can be Waheed Afzal wishes to thank the Higher Education Commission of
observed in the figures, the agreements between the experimental Pakistan for financial support.
and predicted data are generally acceptable with less than 0.5 JE700527D