Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),
Edinburgh, Scotland, United Kingdom, July 17-21, 2011.
EFFECT OF IONIC LIQUID ON HYDRATE FORMATION RATE IN
CARBON DIOXIDE HYDRATES
Takashi Makino
Research Center for Compact Chemical System
National Institute of Advanced Industrial Science and Technology
Sendai, Miyagi, 983-8551
JAPAN
∗
Yuuki Matsumoto, Takeshi Sugahara, Kazunari Ohgaki
Division of Chemical Engineering, Graduate School of Engineering Science
Osaka University
Toyonaka, Osaka, 560-8531
JAPAN
Hiroki Masuda
Department of Applied Chemistry
Kobe City College of Technology
Kobe, Hyogo, 651-2194
JAPAN
ABSTRACT
To investigate the effect of ionic liquids (ILs) on the formation of the CO2 hydrate, the induction
times of hydrate formation were measured for CO2 + IL aqueous solution systems. ILs used in the
present study had a common cation, 1-ethyl-3-methyl imidazolium cation. The thermodynamic
stability boundaries of the CO2 hydrate in the presence of 0.10 mol% IL were not different
significantly from that in the absence of IL at the same temperatures. The induction times of
hydrate formation for the 0.10 mol%-IL solution systems were much shorter than that for the
distilled water system, which indicated that each IL could function as a kinetic hydrate promoter.
Keywords: thermal energy storage, CO2, ionic liquid, kinetic promoter, thermodynamic inhibitor
NOMENCLATURE
p equilibrium pressure [Pa]
T equilibrium temperature [K]
t induction time [min]
INTRODUCTION
Thermal energy storage is a technology that stores
energy in a thermal reservoir and it is effective in
electric-load leveling. Phase change material
(PCM) is often employed as thermal reservoir.
PCM is a substance having high enthalpy of fusion.
∗
It can reserve and supply much larger amount of
heat than materials without phase change such as
chilled water. Consequently, PCM-based thermal
energy storage has been developed and prevailed
for refrigeration and air conditioning [1-3]. Gas
hydrates also act as PCMs [4-6]. The enthalpies of
fusion of some gas hydrates are as high as that of
ice [4]. We have focused on the CO2 hydrate, of
which the enthalpy of fusion is higher than that of
ice [6], because it is inexpensive, environmentfriendly, low-toxic, and non-flammable.
Corresponding author: Phone: +81 22 237 5257 Fax +81 22 232 7002 E-mail: [email protected]
One of the obstacles to industrial applications of
gas hydrates is the long induction time of hydrate
formation, which is more than one day in some
cases. To overcome this difficulty, kinetic
promoters such as surfactants are generally
employed and widely investigated [7-9]. Ionic
liquids (ILs) have been also reported to function as
a promoter for the CO2 hydrate [10]. ILs are a kind
of salts and have melting points at ambient or
close-to-ambient temperatures. Low vapor
pressure, good thermal stability, excellent
solvating property and that are favorable
characters of ILs. Besides, some ILs show higher
CO2 solubilities (mole fraction base) than
conventional molecular solvents [11].
This paper reports the effect of ILs on the
formation of the CO2 hydrate. We have focused on
the induction times for the CO2 hydrate systems in
the presence of hydrophilic ILs. In addition, prior
to the induction time measurements, three-phase
equilibria for the CO2 + IL aqueous solution
systems have been investigated. ILs used in the
present study had a common cation, 1-ethyl-3methylimidazolium cation ([emim]+). The counter
ions were bromide ([Br]-), nitrate ([NO3]-),
tetrafluoroborate ([BF4]-), and trifluoromethane
sulfonate ([TfO]-) anions.
The stability boundaries were investigated by an
ordinary static measurement. IL aqueous solutions
were prepared by mass on an electronic balance
(uncertainty: 0.0001 g, Mettler, AE 200-S) and
introduced into the high-pressure cell. The solution
was sufficiently degassed with a bubbling method
by CO2. The thermostated water was circulated
through the jacket of the high-pressure cell by
using the programming thermocontroller. Next, the
contents were cooled and agitated to generate the
gas hydrate. A mixing bar was moved up and
down for agitation. The phase behavior was
observed straightforwardly through the windows.
To measure the phase equilibrium relation
including equilibrium composition of aqueous
phase accurately, the temperature was increased
very gradually (0.1 K step). The temperature was
changed when an equilibrium state was established
at each step. The temperature where the negligible
amount of gas hydrate existed was determined as a
three-phase equilibrium temperature (equilibrium
composition of aqueous phase was the same as
that of the initial IL solution).
EXPERIMENTAL
Materials
[emim][Br] (purity: ≥ 97.0 mol%), [emim][NO3]
(purity: ≥ 99.0 mol%), and [emim][TfO] (purity:
97 mol%) were purchased from Sigma-Aldrich
Co., Ltd. Toyo Gosei Co., Ltd. supplied
[emim][BF4] (purity: ≥ 99.0 mol%). CO2 (purity:
99.99 mol%) was bought from Takachiho
Chemical Industrial Co., Ltd. Distilled water was
produced by an auto still (Yamato Scientific Co.,
Ltd., WG-25). All Chemicals were used without
further purifications. When IL aqueous solutions
were prepared, freshly bottled ILs were used.
Figure 1. Experimental apparatus for the phase
equilibrium measurement.
Phase Equilibrium Measurement
Figure 1 is an illustration of an experimental
apparatus consisted of the followings: a highpressure cell with sapphire windows, a pressure
gauge (Valcom, VPRT), and a temperature control
unit
(EYELA,
NCB-3100).
Equilibrium
temperature was measured by a Pt resistance
thermometer
(Thermoprobe
Inc.,
TL-1A).
Equilibrium pressure was recorded by the pressure
gauge. The uncertainties of temperature and
pressure were 0.06 K and 0.04 MPa, respectively.
Induction Time Measurement
Figure 2 shows an experimental apparatus for the
induction time measurement. The setup consisted
of a high-pressure cell with sapphire windows, a
pressure gauge (Valcom, VPRT), and a
temperature control unit (Taitec, CL-80R). The
temperatures (both the contents and the water
bath) were measured by Pt resistance
thermometers calibrated with another thermometer
(Thermoprobe Inc., TL-1A). The pressure was
recorded by the pressure gauge. The temperature
and pressure were acquired by using a data logger
(Yokogawa Electric Co., MX100).
First, the IL solution (15 ml) was introduced into
the high-pressure cell. The cell was immersed in
the thermostated water bath, of which the
temperature was 280.65 K. The cell was let stand
for two hours to be thermally equilibrated prior to
the measurement. After that, the IL solution was
sufficiently degassed with a bubbling method by
CO2. Then, the system was quickly pressurized up
to 3.90 MPa with CO2 and the agitation and data
logging were started immediately. A mixing bar
was manipulated in a vertical direction at a
constant agitating speed (107 rpm). When the CO2
hydrate was generated, the temperature of the
contents showed an abrupt increase. The induction
time was defined as the period between the
beginning time of agitation and the time of the
abrupt increase. The cell was washed and dried
sufficiently after each measurement, then, fresh IL
solution was introduced for next measurement.
The measurement was performed five times for
each IL solution system. Besides, the hydrate
generation was confirmed in every experiment.
ILs function as thermodynamic inhibitors similar
to conventional salts like NaCl. The similar
behaviors have been reported in references
[11,14,15].
Figure 3. Thermodynamic stability boundaries
of the CO2 hydrate in the presence of ILs.
Induction Time
The induction times were measured for the
distilled water and 0.10 mol% IL solution systems.
Figure 4 summarizes the mean induction times
with error bars. In the present study, the simple
CO2 hydrate was generated at around 39 min in the
absence of ILs. Whereas, in the presence of ILs, it
took less than 20 min to generate the CO2 hydrate.
The mean induction times of [emim][Br],
[emim][TfO], [emim][BF4], and [emim][NO3]
systems are 17, 16, 13, and 11 min, respectively.
Figure 2. Experimental Apparatus for the
induction time measurement.
RESULTS AND DISCUSSION
Phase Equilibria
Figure 3 presents three-phase equilibria for the
CO2 + IL solution systems. The p-T relation for
the simple CO2 hydrate agrees with those reported
in the references [12,13]. The equilibrium
pressures for the 0.10 mol% solution systems do
not differ significantly from that for the distilled
water system. The deviations of equilibrium
pressures were within the experimental uncertainty.
While, the CO2 hydrates in equilibrium with 1.04
mol% solutions decompose at lower temperatures
than that of the distilled water. This indicates that
Figure 4. The mean induction times of the CO2
hydrate formation for the distilled water and
0.10 mol% IL solution systems.
Each IL obviously can kinetically promote the
generation of the CO2 hydrate, although the ILs
are the thermodynamic inhibitors. It is reported
that the ILs act as both thermodynamic and kinetic
inhibitor for the CH4 hydrate [14,15]. It is true that
the ILs generally have stronger affinity for CO2
than that for CH4 [11,16,17], however, it is unclear
what causes the difference between the formation
of the CO2 and CH4 hydrates at this time.
CONCLUSION
In the present study, we have investigated the IL
effect on the three-phase equilibria and the hydrate
formation. The decomposition pressures of the
CO2 hydrate for the 0.10 mol% IL solutions are
not different significantly, i.e. within experimental
uncertainty, from that for the distilled water.
Whereas, the 1.04 mol% IL solutions inhibit the
generation of the CO2 hydrate thermodynamically.
The induction times for the 0.10 mol% IL solution
systems are less than half that of the distilled water
system. Each IL acts as a kinetic promoter of the
CO2 hydrate.
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ACKNOWLEDGEMENTS
This study was supported by followings: a Grantin-Aid for Young Scientists (B) (22710081), the
Ministry of Education, Culture, Sports, Science
and Technology (MEXT); Research Grant of Kobe
City College of Technology.