Fluid Phase Equilibria 337 (2013) 26–31
Contents lists available at SciVerse ScienceDirect
Fluid Phase Equilibria
journal homepage: www.elsevier.com/locate/fluid
A study on thermodynamics effect of [EMIM]-Cl and [OH-C2 MIM]-Cl on methane
hydrate equilibrium line
Behzad Partoon, Nordiyana M.S. Wong, Khalik M. Sabil ∗ , Khashayar Nasrifar, Mohd Riduan Ahmad
Chemical Engineering Department, Universiti Teknologi PETRONAS, 31750 Tronoh, Malaysia
a r t i c l e
i n f o
Article history:
Received 29 June 2012
Received in revised form
13 September 2012
Accepted 18 September 2012
Available online 26 September 2012
Keywords:
Phase behavior modeling
Methane hydrate equilibrium
Ionic liquid
Experimental measurement
a b s t r a c t
In this study, the equilibrium conditions of methane hydrate is measured experimentally in the presence
of 1-ethyl-3-methyl-imidazolium chloride ([EMIM]-Cl) and 1-hydroxylethyl-3-methyl-imidazolium
chloride ([OH-C2 MIM]-Cl) solutions. These two ionic liquids are chosen to study their performances as
low dosage hydrate inhibitors. To study the effect of these ionic liquids on the equilibrium phase boundary of methane hydrate, several experiments are conducted in a pressure range of 4–12 MPa. In addition,
the equilibrium data in [EMIM]-Cl solutions are modeled using an equation that takes into account the
effects of electrolyte on the activity of water. Results show that phase boundary of methane hydrate is
shifted toward lower temperature at constant pressure from 0.1 to 1.5 K in the presence of these ionic
liquids. This temperature shift, however, becomes more significant at pressures higher than 70 MPa.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Gas hydrates are ice-like crystalline compounds. Gas hydrates
are formed by stabilizing water molecules in lattice network using
suitable gases at temperatures normally higher than ice point [1].
When Hammerschmidt published his investigation results on gas
pipeline plugs, which claimed that the main cause of these plugs
is formation of gas hydrate, the issues related to gas hydrates have
become more important for oil and gas companies [2]. Since then,
efforts for finding gas hydrate inhibitors have seriously started.
Nowadays, different types of gas hydrate inhibitors are available
in market. These inhibitors are divided into two main groups:
thermodynamic hydrate inhibitors (THIs) and low dosage hydrate
inhibitors (LDHIs). THIs inhibits hydrate formation in the same way
as anti-freezing agents hinder ice formation, that is, hydrate would
form at lower temperature at the same pressure. Methanol and glycols are two common THIs that have widely been used in oil and
gas industries. THIs are used normally around 20–40 wt%, however,
some times higher dosage is recommended [3]. LDHIs, on the other
hand, are normally used in smaller quantities, i.e. at ppm level. The
LDHI is known to consist of two main categories: kinetics hydrate
inhibitors (KHIs) and anti-agglomerate (AA) inhibitors.
Recently, Xiao and Adidharma [4] introduced ionic liquids as
another class of gas hydrate inhibitors, which are called “dual
∗ Corresponding author. Tel.: +60 5 368 7684; fax: +60 5 365 5670.
E-mail addresses: khalik msabil@petronas.com.my, halik98@yahoo.com
(K.M. Sabil).
0378-3812/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.fluid.2012.09.025
function inhibitors.” Ionic liquids are organic salts that at room or
moderate temperatures are in liquid phase. Xiao and Adidharma
[4] found that some ionic liquids could show thermodynamic inhibition and at the same time delay hydrate formation by slowing
down the hydrate nucleation rate. This is attributed to their strong
electrostatic charges and hydrogen bond with water. This means
that ionic liquids are both thermodynamic and kinetic inhibitor.
The ionic liquids they initially used are imidazolium cation-based
ones. Later, Del Villano and Kelland [5] performed some experiments by two of the previously studied ionic liquids at typical
subsea temperatures and subcooling. They reported that these ionic
liquids are very weak KHIs at 5000–10,000 ppm concentrations.
Xiao et al. [6] continued their investigation with six other
dialkylimidazolium halide ionic liquids. These ionic liquids were
studied with concentrations about 10 wt% for thermodynamic
effects and 1 wt% for kinetics studies. Their results showed a temperature decrease of 0.2–1.2 K in the dissociation temperature of
methane hydrate. Among all the ionic liquids studied, 1-ethyl-3methyl-imidazolium chloride ([EMIM]-Cl) was shown to be the
most effective thermodynamic inhibitor, while 1-butyl-3-methylimidazolium iodide ([BMIM]-I) was the best kinetic inhibitor.
Kim et al. [7] hypothesized that suitable ionic liquid to be used
as hydrate inhibitor must have the two following criteria. Firstly,
the ILs must be hydrophilic. This would enable them to have access
to water molecules. Secondly, functional group introduced to the
base cation must be able to create intermolecular hydrogen bonding with water molecules. Based on these two hypotheses they
have selected N-(2-hydroxyethyl)-N-methylpyrrolidinium tetrafluoroborate ([HEMP]-BF4 ) and N-butyl-N-methylpyrrolidinium
B. Partoon et al. / Fluid Phase Equilibria 337 (2013) 26–31
27
Table 1
Chemical structures and purity of ionic liquids studied in this work.
Symbol
Chemical name
[EMIM]-Cl
1-Ethyl-3-methyl imidazolium chloride
Chemical structure
Purity/supplier
H3C
CH3
+
N
N
Cl-
99.8 wt% Sigma–Aldrich
OH
H3C
[OH-C2 MIM]-Cl
2. Materials and methods
2.1. Material
[EMIM]-Cl and [OH-C2 MIM]-Cl used in this work are purchased
from Sigma–Aldrich. The chemical structures and purity of these
ionic liquids are provided in Table 1. Methane gas with a purity of
99.95 mol% is supplied by Merck. Distilled water is used to dilute
the ionic liquids to the desired concentrations.
2.2. Apparatus and procedure
2.2.1. Apparatus
Hydreval, a motor driven PVT cell, is used in this work [10]. The
schematic diagram of the experimental apparatus is shown in Fig. 1.
The sapphire chamber is closed at one end by a piston and at the
other end by a titanium alloy cell head. The maximum cell capacity is 80 cm3 . Maximum operating pressure of Hydreval is 20 MPa
and temperature ranges from 253 K to 523 K. This equipment is
equipped with magnetic driven stirrer. Temperature, pressure and
volume of reactor are measured and recorded every 2 s with accuracy of ±0.1 K, ±0.01 MPa and ±0.001 cm3 , respectively. These
parameters are regulated using Hydreval software. In addition, a
camera is attached to monitor and record liquid/gas interface inside
the cell. An external pump is used to inject liquid and gas into the
sapphire cell.
N
N
1-Hydroxylethyl-3-methyl-imidazolium chloride
tetrafluoroborate ([BMP]-BF4 ) for synthesis and investigation on
their inhibition effects. Based on their results, these ILs exhibited good thermodynamic inhibitions at 10 wt% concentration. The
results also stated that an optimum combination of anion and
cation could lead to good LDHIs. Recently, Li et al. [8] studied the
equilibrium conditions of some other ionic liquids.
As the literature data on the dual functionality of ionic liquids as hydrate inhibitors is still limited, more investigations are
required. In this work, the thermodynamics of methane hydrate
formation in the presence of [EMIM]-Cl and [OH-C2 MIM]-Cl are
studied. These two ionic liquids are selected for further investigation, as both were reported to have good thermodynamic inhibition
effects in the previous studies by Xiao et al. [6] and Li et al. [8]
at 10 wt% concentration. The present work is an extension of our
previous work [9] on methane hydrate formation at 0.1 wt% and
0.5 wt% concentration of [EMIM]-Cl solution. Our objective is to
study the thermodynamic inhibition effect of these ionic liquids
on methane gas hydrate at 0.1 wt%, 0.5 wt% and 1 wt% concentrations. In addition, the hydrate equilibrium line for methane in
the presence of [EMIM]-Cl is modeled using optimized parameters.
+
Cl-
99.8 wt% Sigma–Aldrich
2.2.2. Hydrate equilibrium point measurement
The equilibrium hydrate formations are measured by isochoric
method [1]. The sapphire cell is washed using distilled water and
vacuumed. Subsequently, the cell is flushed with methane gas to
ensure it is air free. About 40 cm3 of aqueous solution at desired
concentration is injected into sapphire cell using the external
pump. Then, the cell is cooled down to about 2–3 K above hydrate
equilibrium temperature at the desired pressure. The gas is then
supplied into the cell to a desired pressure. After the temperature
and pressure of the system remains constant, the stirrer is switched
on at 600 rpm and the temperature is lowered at a rate of 0.01 K/min
to form hydrate. Once hydrate is formed, the temperature of the
system is increased with a step method as described by Tohidi et al.
[11]. The hydrate formation in the vessel is detected by the T-cycle
method [1] as well as visual observation for verification.
3. Theory
There are few thermodynamic models for prediction of gas
hydrate equilibrium conditions in the presence of electrolyte solutions [12–14]. Most of these models are based on the effect of
additives on water activity. The Maddox et al. [15] model for nonelectrolyte inhibitors was used in this work to model the ionic liquid
effects on methane hydrate formation phase boundary and details
of the derivation of this equation has been described by Pieroen
[16]. According to this model, effect of electrolyte on the gas hydrate
formation temperature can be explained by:
ln(aw ) =
−H d
nH R
1
T
−
1
Tw
(1)
where aw is the water activity in electrolyte solution, Hd is the
enthalpy of hydrate dissociation, nH is the hydration number, T and
Tw are the hydrate formation temperature in electrolyte solution
and pure water, respectively, and R is the universal gas constant.
Hydrate formation temperature for pure water can be predicted by
any hydrate prediction method such as John et al. method [17].
Activity of aqueous electrolyte solutions is predicted using Eqs.
(2)–(5) as described by Pitzer and Mayorga [18].
ln(aw ) =
−vmMw
ϕ
+ −
ϕ
ϕ − 1 = |z z |f + m
f ϕ = −Aϕ
ϕ
(2)
2v+ v−
v
I 1/2
1 + bI 1/2
BMX = ˇ(0) + ˇ(1) exp(−aI 1/2 )
ϕ
BMX
2
+m
2(v+ v− )
v
3/2
Cϕ
(3)
(4)
(5)
28
B. Partoon et al. / Fluid Phase Equilibria 337 (2013) 26–31
Fig. 1. Illustration of flow schematic of Hydreval.
Table 2
Pitzer and Mayorga parameters for selective salts and ionic liquid.
Material
NaCl
KCl
CaCl2
[EMIM]-Cl
ˇ(0)
ˇ(1)
−2
7.650 × 10
4.835 × 10−2
3.159 × 10−1
2.892 × 10−2
Aϕ = 4.0 × 10−6 (T − 273.15)2 + 5.0 × 10−4 (T − 273.15) + 0.3769(6)
The change in the enthalpy of hydrate dissociation because of
adding electrolytes at two different hydrate temperatures, is considered to depend on the amount of electrolyte and formation of
hydrate. Thus, Javanmardi et al. [12] assumed that the enthalpy of
dissociation per water molecule in hydrate crystals (Hd /nH R) can
be predicted by ionic strength (I) and pressure of the system:
(7)
In this equation, the effect of electrolytes is included by ionic
strength while the effect of hydrate formation was considered by
Ref. no.
−3
2.664 × 10
2.122 × 10−1
1.614 × 100
1.5648 × 10−1
In these equations, ϕ is the osmotic coefficient, Mw is water molecular weight, v+ and v− are number of ions in the salt formula and z+
and z− are number of cation and anion charges, respectively. Also
+ + v− , m is the conventional molality and I is the ionic strength
v = v
(0.5 mi zi ). Pitzer and Mayorga [18] recommended a = 2 and b = 1.2
for all electrolytes. ˇ(0) , ˇ(1) and Cϕ are model parameters. These
parameters for selective salts are shown in Table 2. The parameter
Aϕ is the Debye–Hückel coefficient. Javanmardi et al. [12] used a
value of 0.392 for water at 25 ◦ C in their study; however, Aϕ is a
week function of temperature. Thus, in this work, Eq. (6) is used for
calculation of Aϕ . This equation is fitted to Debye–Hückel parameter
of water reported by Zemaitis et al. [19].
H d
q1 I q2
=
nH R
1 + q3 P + q4 ln(P)
Cf
−1
1.270 × 10
−1.800 × 10−3
−3.400 × 10−4
1.013 × 10−2
[18]
[18]
[18]
[21]
including pressure term. They optimized parameters of Eq. (7) for
electrolytes based on hydrate dissociation condition of some pure
gases. These parameters are later optimized by Nasrifar et al. [20]
and the same concept is used by them to predict Hd /nH R in the
presence of alcohols. In this work, following Javanmardi et al. [12]
and Nasrifar et al. [20], these parameters are optimized in the presence of ionic liquids. Optimization is based on the minimization of
average absolute error (AAE) for the model predictions, Eq. (8), and
experimental hydrate data in the presence of 1 wt% [EMIM]-C1. The
optimized parameters are presented in Table 3.
N
AAE =
1
|Texp . − TCal. |i
N
(8)
i=1
Table 3
Parameters of Eq. (7), optimized based on 1 wt% [EMIM]-Cl hydrate equilibrium data
obtained in this work.
Electrolytes
q1
q2
q3
q4
Ionic liquid
Javanmardi et al. [16]
Nasrifar et al. [20]
This work
597.33
−4.090 × 10−2
2.270 × 10−5
−7.510 × 10−2
1000.0
1.237 × 10−2
−1.205 × 10−2
4.073 × 10−2
222.24
−7.796 × 10−2
3.854 × 10−5
2.530 × 10−2
B. Partoon et al. / Fluid Phase Equilibria 337 (2013) 26–31
Fig. 2. Phase boundary for methane + water in the presence of [EMIM]-Cl at various
concentrations. () 1 wt%, () 0.5 wt% [9], () 0.1 wt% [9], (•) pure water [9].
Pitzer and Mayorga [18] parameters for water activity in the presence of [EMIM]-Cl is reported by Zafarani-Moattar and Sarmad [21]
and is also provided in Table 2. For calculation of hydrate formation
temperature in pure water, John et al. [17] method is used. Unfortunately, Pitzer and Mayorga parameters for [OH-C2 MIM]-Cl are not
reported in the literature and thus this model cannot be employed
for this ionic liquid.
4. Results and discussion
4.1. Phase behavior in the hydrate forming region
In our previous work, we showed that the measured hydrate
formation conditions using Hydreval are in good agreement with
the literature data [9]. The phase equilibrium data for [EMIM]Cl and [OH-C2 MIM]-Cl at different concentrations are shown in
Figs. 2 and 3, respectively. In the literature, the reported equilibrium data for these ionic liquids are at high concentration of 10 wt%
whereas in this work the concentration of these ionic liquids is in
the range of 0.1–1 wt% [6,8]. The measured equilibrium data for
these systems are provided in Table 4.
As expected from the behavior of thermodynamic inhibitors the
equilibrium line of methane hydrate is shifted to the lower temperatures in the presence of ionic liquids as shown in Figs. 2 and 3.
Moreover, the inhibition effect of these ionic liquids on the equilibrium line is small at low pressures and it becomes more significant
Fig. 3. Phase boundary for methane + water in the presence of [OH-C2 MIM]-Cl at
various concentrations. () 1 wt%, () 0.5 wt%, () 0.1 wt%, (•) pure water [9].
29
Fig. 4. Comparison of [EMIM]-Cl and [OH-C2 MIM]-Cl effects with normal electrolyte on methane hydrate phase boundaries. () 1 wt% [OH-C2 MIM]-Cl, () 1.0 wt%
[EMIM]-Cl, () 5.0 wt% NaCl [22], (×) 5.0 wt% KCl [22], (+) 5.0 wt% CaCl2 [22], (䊉) pure
water [9].
at higher pressures. The same behavior is exhibited by the data
published by Xiao et al. [6]. In addition, thermodynamic inhibition
effect is increased when the concentration of ionic liquid in the
aqueous solution increases.
Fig. 4 compares the influence of [EMIM]-Cl and [OH-C2 MIM]-Cl
at 1 wt% on methane hydrate equilibrium. [OH-C2 MIM]-Cl shows
better inhibition effects than [EMIM]-Cl. The same behavior is
reported by Li et al. [8]. This may be attributed to the presence
of hydroxyl group in [OH-C2 MIM]-Cl which can form hydrogen
bonding with water molecules and thus it shows better hydrate
inhibiting effect rather than [EMIM]-Cl. However, this effect is
not too significant. In addition to the measured data, the data for
methane hydrate equilibrium line in the presence of 5 wt % salts in
aqueous solutions are presented in Fig. 4. These data are obtained
from Mohammadi et al. [22] and included for comparison. As shown
in Fig. 4, the thermodynamic inhibition effect of these ionic liquids
is slightly lower than that of the salts. This may due to lower concentration of the ionic liquids or higher activity coefficients of these
ionic liquids as shown in Fig. 5. The mean ionic activity coefficient
( ±) of electrolyte presented in Fig. 5 is calculated using Pitzer and
Mayorga method [18]. The higher activity coefficient of [EMIM]Cl at various concentrations shows less non-ideality behavior and
therefore, its inhibition effect should be less than normal electrolytes. The activity coefficient for [OH-C2 MIM]-Cl is not calculated
due to lack of the model parameter of this ionic liquid.
Fig. 5. Comparison of [EMIM]-Cl, KCl, NaCl and CaCl2 activity as a function of concentration. () NaCl, () KCl, (•)CaCl2 , ()[EMIM]-Cl.
30
B. Partoon et al. / Fluid Phase Equilibria 337 (2013) 26–31
Table 4
Methane hydrates equilibrium data in the presence of ionic liquids, obtained in this work.
Ionic liquid
Concentration (wt%)
0.1
[EMIM]-Cl
0.5
1.0
0.1
[OH-C2 MIM]-Cl
0.5
1.0
a
b
Pa (MPa)
Tb (K)
P (MPa)
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
3.88
4.21
4.43
3.89
4.01
4.43
4.20
4.55
4.95
276.8
277.2
278.0
276.9
277.3
278.4
277.1
278.2
279.3
4.79
5.33
5.64
4.81
5.33
5.84
5.68
6.30
7.21
279.3
280.2
281.2
279.3
280.2
281.1
280.3
281.2
282.3
6.47
7.90
8.66
6.89
8.01
8.70
8.10
9.13
10.17
282.2
283.2
284.2
282.2
283.3
284.2
283.1
284.2
285.3
9.25
10.48
11.29
9.49
10.53
11.43
11.04
12.16
285.2
286.3
287.3
285.2
286.2
287.3
286.2
287.3
4.18
4.32
4.88
4.28
4.31
4.88
4.49
4.80
4.83
276.7
277.3
278.5
276.8
277.2
278.0
276.7
277.3
278.5
5.00
5.77
6.29
4.99
5.77
6.29
5.21
6.16
6.84
278.9
280.4
281.2
279.3
280.2
281.4
278.9
280.2
281.2
6.87
7.84
9.00
6.87
8.04
9.00
7.34
7.91
9.21
282.3
283.2
284.4
282.2
283.3
284.2
282.3
282.9
284.4
9.24
10.39
11.26
9.38
10.58
11.60
9.93
11.55
12.33
285.1
286.2
287.1
285.2
286.3
287.3
285.1
286.2
287.1
Pressure uncertainty is ±0.01 MPa.
Temperature uncertainty is ±0.1 K.
Table 5
Prediction results of methane hydrate equilibrium line in the presence of salts and
[EMIM]-Cl.
NaCl
KCl
CaCl2
[EMIM]-Cl
[EMIM]-Cl
[EMIM]-Cl
Concentration
(wt%)
No. data
point
5
5
5
0.1
0.5
1
5
6
6
12
12
11
Temperature
range
274.2–283.6
271.6–283.2
272.0–283.0
276.8–287.3
276.8–287.3
277.1–287.3
AAE (K)
Ref.
0.1
0.5
0.6
0.6
0.3
0.2
[22]
[22]
[22]
[9]
[9]
This work
4.2. Modeling
Hydrate equilibrium conditions in the presence of selective salts
are modeled and compared to the literature data [22] to assess the
applicability of this model. The results are tabulated in Table 5.
As shown in Table 5, the predicted results are in agreement with
reported data with AAE less than 0.6. Therefore, the model is used
for prediction of hydrate equilibrium condition in the presence
of [EMIM]-Cl. The dependency of hydrate dissociation enthalpy to
the ionic strength is calculated by minimizing the AAE, Eq. (8), of
methane hydrate equilibrium temperature at 1 wt% of [EMIM]-Cl.
Then the optimized parameters are used to predict hydrate equilibrium temperatures of methane hydrate in 0.5 wt% and 0.1 wt%
of [EMIM]-Cl solutions. Results of these calculations are also presented in Table 5 and illustrated in Fig. 6.
As shown in Fig. 6, the model prediction is in good agreement
with experimental data. By taking into consideration that the model
parameters are only optimized for methane hydrate in 1 wt% of
[EMIM]-Cl solution, the model is capable to predict hydrate equilibrium condition at other [EMIM]-Cl concentrations with AAE less
than 0.6. In addition, prediction results and experimental measurements show the same trend of the equilibrium behavior with
regard to pressure changes. At low pressure, the equilibrium line of
methane hydrates in the presence of ionic liquids slightly shifted to
lower temperature compare to methane + pure water hydrate equilibrium line, while at higher pressure this transformation becomes
more significant.
5. Conclusion
The equilibrium conditions of methane hydrate in the presence
of [EMIM]-Cl and [OH-C2 MIM]-Cl are measured in this work. The
thermodynamic inhibition effects on methane hydrate equilibrium
in the presence of these ILs are less significant at low pressures.
However, when the pressure is higher than 7 MPa, their thermodynamic inhibition effects are more significant. Furthermore, a
thermodynamic model is optimized for prediction of gas hydrate
equilibrium temperature in the presence of ionic liquids and the
prediction results are in good agreement with data produced in
this work.
List of symbols
Fig. 6. Experimental and predicted methane hydrate formation temperature for
various [EMIM]-Cl solutions. () 1 wt% [EMIM]-Cl, () 0.5 wt% [EMIM]-Cl [9], ()
0.1 wt% [EMIM]-Cl [9], (•) pure water [9], ( · · )1 wt% [EMIM]-Cl prediction, ( · )
) pure water
0.5 wt% [EMIM]-Cl prediction, (· · ·) 0.1 wt% [EMIM]-Cl prediction, (
prediction.
AAE
Aϕ
Cϕ
H
I
Mw
N
P
R
T
average absolute error
Debye–Hückel parameter
parameter of Pitzer and Mayorga model
enthalpy, J
ionic strength, mol
molecular weight of water
total number of data points
pressure, kPa
universal gas constant, 8.314 J/(mol K)
temperature, K
B. Partoon et al. / Fluid Phase Equilibria 337 (2013) 26–31
V
volume, m3
Z
compressibility factor
aw
water activity
A
parameter of Eq. (7) fixed as 2
B
parameter of Eq. (6) fixed as 1.2
M
molality, mol
N
number of moles, mol
nH
hydration number
q1 , q2 , q3 , q4 parameters of Eq. (7)
V
number of ions in salt or ionic liquid formula
Z
charge of ions in salt or ionic liquid formula
Greek letters
ˇ(0) , ˇ(1) parameters of Pitzer and Mayorga model
ϕ
osmotic coefficient
±
mean ionic activity coefficient
Subscripts and superscripts
+
cation
−
anion
Cal.
calculated
dissociation
d
exp.
experimental
f
moles of gas at any time
i
initial moles of gas
MX
mixed
water
w
ϕ
osmotic coefficient related parameter
Acknowledgment
This study was supported by Ministry of Science, Technology
and Innovation of Malaysia under e-science project Grant No. 0602-02-SF0076.
31
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