Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),
Edinburgh, Scotland, United Kingdom, July 17-21, 2011.
MEASUREMENT OF EQUILIBRIUM DATA AND ESTIMATION OF
METHANE HYDRATE CONDITIONS IN THE PRESENCE OF
THERMODYNAMIC MIXED-INHIBITORS
Seyed Mojtaba Hoseini Nasab,1 Mohsen Vafaei Sefti 2, Amir Abbas Izadpanah 3
1, 2-Department of chemical engineerin, Tarbiat Modares University, Tehran, Iran
3- Department of chemical engineerin, Persian Gulf University, Bushehr, Iran
ABSTRACT
Obtaining equilibrium precise condition of hydrate formation is very essential for main
component of natural gas in presence of thermodynamic inhibitors in order to evaluating of those
in hydrate formation prevention, determining concentration of required inhibitor, developing and
validating of thermodynamic models and another tools or methods of prediction of hydrate
formation condition and also for designing of processing equipment and operation units in oil and
gas industry. So in this paper, new equilibrium data has been measured for methane hydrate in the
presence of three mixed inhibitors for various concentrations using constant volume method.
Measured equilibrium data has been reported for hydrate methane in the presence of three mixed
inhibitors: NaCl + DEG, NaCl + TEG and MeOH + TEG. Thermodynamic model of Heriot-Watt
University (HWHYD) has been applied for estimating the measured equilibrium data and good
agreement between measured data and predicted data observed.
Keywords: Hydarte Formation Condition, Equilibrium Data, Thermodynamics Mixed Inhibitor,
HWHYD Model
INTRODUCTION
Gas hydrates are a group of nonstoichiometric,
icelike crystalline compounds formed through a
combination of water and suitably sized “guest”
molecules under low temperatures and high
pressures. In the hydrate lattice, water molecules
form hydrogen-bonded cagelike structures,
encapsulating the guest molecules, which
generally consist of low molecular diameter gases
and organic compounds. A concise review of gas
hydrates is given elsewhere [1,2]. Conditions of
high fluid pressure and low temperature as well as
presence of water leading to gas hydrate formation
may be found in oil and gas production,
transportation, and processing facilities. Formation
of gas hydrates can lead to serious operational,
economic, and safety problems in petroleum
industry due to potential blockage of oil and gas
equipment. Organic inhibitors, such as alcohols
and glycols, are normally used to inhibit gas
hydrate formation. The presence of inhibitors
normally reduces activity of water in aqueous
phase, which shifts hydrate phase boundaries to
high pressures/low temperatures [1]. Reliable gas
hydrate equilibrium data for natural gas main
components in the presence/absence of inhibitors
are necessary to develop and validate
thermodynamic models for predicting hydrate
phase boundaries of natural gases in the presence/
absence of inhibitors. In this work, the hydrate
dissociation data have been measured for methane
in the presence of three mixed inhibitors such as
MeOH + TEG , DEG + NaCl and TEG + NaCl
aqueous solutions. The experimental hydrate
dissociation data measured in this work are
compared with the predictions of a general
correlation [3] and a thermodynamic model
(HWHTD Model) [4, 5, 6] and acceptable
agreements between the experimental and the
predicted data are found.
2. Experimental
2.1Experimental apparatus and materials
The experiments were done at a self-designed
hydrate formation system which consists of a
reactor, a shell for heat transfer and a data
acquisition system. A schematic diagram of the
experimental apparatus used in this work is shown
in Figure 1.Hydrate formation reactor was made
by a inch Schedule No. 80, stainless steel 316
pipe. At the two ends of this pipe a pressure
transmitter and a thermometer was installed. The
volume of reactor was 120 cc. In order to heat and
cool the system, the hydrate formation reactor was
converted to shell and tube by a poly ethylene
shell. The cell pressure was measured using a
Druck PTX1400 pressure transmitter (0–10MPa)
with accuracy of about ±0.25% of the scale (i.e. 25
kPa).
temperature bath was turned on, mixing was
started and now data collection starts. The
traditional method of measuring hydrate
equilibrium consists of monitoring the pressure
and temperature in a hydrate bearing system
and identifying equilibrium at the conditions
where the last hydrate crystal in the system
dissociates. This procedure is illustrated in Fig.
2, which shows the typical pressuretemperature trace produced during a hydrate
equilibrium measurement.
Figure 1: Schematic of the experimental apparatus
The temperature was measured using PT100
thermometers with an accuracy of ±0.1 K. The
signals of pressure and temperature were acquired
by a data acquisition system driven by a personal
computer. The cell pressure and temperature data
from acquisition system were saved at preset
sampling intervals on a computer hard disk. The
temperature of the reactor is controlled by the flow
of an ethanol–water solution by an external
circulating temperature bath through the shell. A
Lauda RE 210 temperature bath was used to
control the temperature of the cooling fluid. The
reactor is mounted on a pivot and mixing is
obtained by rocking the cell [7]. The methane
having a high (nominal) purity of 99.999%
supplied by Air products and carbon dioxide with
99.9% was used. Methanol was supplied by Merck
and the Distilled water was also used.
2.2 Experimental procedure
The hydrate equilibrium conditions were
measured with the batch, constant volume
procedure. The hydrate formation reactor was
firstly evacuated. The 30 cc of mercury and 40
cc of distilled water were charged to the
reactor. The reactor was then pressurized to the
experimental
pressure.
The
constant
Figure 2: Determination of the hydrate dissociation point
by the intersection of cooling and heating curves
Following Fig. 2, the experimental procedure can
be explained as follows. Over segment A of Fig. 2
the system was isometrically cooled. This linear
segment was produced from a hydrate-free system.
The system was sub-cooled well past the hydrate
formation conditions. Segment B shows the
characteristic pressure drop associated with
exothermic formation of the hydrate phase.
Subsequently, the temperature was slowly
decreased to form the hydrate. Hydrate formation
in the vessel was detected by a pressure drop.
Once the hydrate phase formed, the system was
heated stepwise along segment C until the
pressure-temperature trace coincided with segment
A, a hydrate free system, and the last hydrate
dissociated. If the temperature is increased in the
hydrate-forming region, hydrate crystals partially
dissociate, thereby substantially increasing the
pressure. If the temperature is increased outside
the hydrate region, only a smaller increase in the
pressure is observed as a result of the change in
the phase equilibria of the fluids in the vessel (see
fig.2) [8].
During the stepwise heating of the system
sufficient time (2-6 hours) should be given at each
temperature step to ensure equilibrium.
Consequently, the point at which the slope of
pressure-temperature data plots changes sharply is
considered to be the point at which all hydrate
crystals have dissociated and hence as the
dissociation point. Therefore, the hydrate
equilibrium point was assumed to occur at the
conditions at which segment C meet segment [9,
10, 11, 12]. For evaluating the hydrate equilibrium
curve under a wide temperature range, this
procedure can be repeated with different initial
pressures.
3. Results and Discussions
To validate the apparatus, equilibrium conditions
were obtained for CO2 hydrates in pure water and
compared against data already available in the
literature. Also, for two data the reproducibility
test were done. The data obtained using our
apparatus along with some selected experimental
data from the literature are plotted in Figure 3.
Experimental dissociation data for methane hydrate in the
presence MeOH5TEG5 aqueous solution
HDT* (k)
270.034
270.305
273.344
275.325
276.482
solution)
Table 1
278.461
279.936
280.346
282.525
---
HDP
(MPa)
5.9206
6.8322
7.1499
9.3615
---
HDT: Hydrate Dissociation Temperature
HDP: Hydrate Dissociation Pressure
Table 2
Experimental dissociation data for methane hydrate in the
presence MeOH5TEG10 aqueous solution
HDT (k)
269.523
269.803
271.367
274.552
276.212
HDP
(MPa)
2.645
2.812
3.153
4.329
5.166
HDT (k)
277.5
278.494
279.68
280.309
280.71
HDP
(MPa)
6.19
6.663
7.321
7.89
8.376
Table 3
Experimental dissociation data for methane hydrate in the
presence Na5DEG5 aqueous solution
270.562
272.375
273.913
276.104
All experimental dissociation points measured in
this work are reported in Table 1 to 2 and are
plotted in Figures 4 to 9. The figures also show
predictions of a HWHYD thermodynamic model
for estimating hydrate inhibition effects of three
thermodynamics mixed inhibitors i.e. as
MeOH5TEG5 (5%w MeOH + 5%wTEG),
MeOH5TEG5 (5%w MeOH + 10%wTEG),
Na5DEG5 (5%w NaCl + 5%w DEG), Na5DEG15
(5%w NaCl + 15%w DEG), Na5TEG5 (5%w NaCl
+ 5%w TEG) and Na5TEG5 (5%w NaCl + 15%w
TEG). (w is mass percent of inhibitor in aqueous
HDT (k)
*
HDT (k)
Figure 3: Experimental equilibrium conditions for carbon
dioxide hydrates in pure water: comparison of data obtained
in this work with those in literature, vdW-P model and
reproducibility test.
HDP
(MPa)
2.578
2.704
3.581
4.605
4.913
HDP
(MPa)
3.13
3.206
3.662
4.524
HDT (k)
278.13
280.32
281.79
---
HDP
(MPa)
5.427
6.918
8.056
---
Table 4
Experimental dissociation data for methane hydrate in the
presence Na5DEG15 aqueous solution
HDT (k)
266.982
268.518
271.125
274.024
HDP
(MPa)
2.294
2.593
3.365
4.581
HDT (k)
276.528
278.136
279.049
---
HDP
(MPa)
6.063
7.485
8.67
---
Table 5
Experimental dissociation data for methane hydrate in the
presence Na5TEG5 aqueous solution
HDT (k)
269.916
275.165
273.26
276.219
HDP
(MPa)
2.31
2.98
3.31
4.61
HDT (k)
278.339
280.572
282.412
---
HDP
(MPa)
5.72
7.15
8.78
---
Table 6
Experimental dissociation data for methane hydrate in the
presence Na5TEG15 aqueous solution
HDT (k)
267.912
271.153
273.508
HDP
(MPa)
2.89
3.54
4.315
HDT (k)
275.815
277.913
279.76
HDP
(MPa)
5.528
6.694
8.63
Figure 7. Experimental (this work) and predicted dissociation
conditions of methane hydrates in the presence of Na5DEG15
aqueous solutions
Figure 4. Experimental (this work) and predicted dissociation
conditions of methane hydrates in the presence of MeOH5TEG5
aqueous solutions
Figure 8. Experimental (this work) and predicted dissociation
conditions of methane hydrates in the presence of Na5TEG5 aqueous
solutions
Figure 5. Experimental (this work) and predicted dissociation
conditions of methane hydrates in the presence of MeOH5TEG15
aqueous solutions
Figure 6. Experimental (this work) and predicted dissociation
conditions of methane hydrates in the presence of Na5DEG5 aqueous
solutions
Figure 9. Experimental (this work) and predicted dissociation
conditions of methane hydrates in the presence of Na5TEG15
aqueous solutions
The HWHYD thermodynamic model used in this
work, which is capable of predicting different
scenarios of hydrate phase equilibrium. Detailed
description of this model is given elsewhere [4, 5]
.The HWHYD model is briefly based on the
equality of fugacity concept, which uses the
Valderrama modification of the Patel-Teja
equation of state [13] and nondensity dependent
mixing rules [14] for modeling the fluid phases,
and the van der Waals and Platteeuw theory [15]
for modeling the hydrate phase. As can be
observed in the figures, the agreements between
the experimental and predicted data are acceptable
with less than 0.5 K deviations.
Conclusion
An experimental setup was constructed and used
for measurement of hydrate equilibrium
conditions. The apparatus was validated with
equilibrium data on CO2 hydrates in pure water.
The acceptable agreements between the
experimental data measured in this work and the
experimental data reported in the literature helped
to confirm the reliability of the experimental
technique and the data generated in this work.
Experimental dissociation data methane simple
hydrates in the presence of three mixed inhibitors:
Nacl + DEG, Nacl + TEG and MeOH + TEG
aqueous solutions at various temperature ranges
were reported in this work. A constant volume
method was used for performing all the
measurements. All the experimental data were
compared with the predictions a thermodynamic
model (HWHYD model) and acceptable
agreements were found between experimental and
predicted data.
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