Download Methane storage in mixed hydrates with tetrahydrofuran

Indian Journal of Chemical Technology
Vol. 21, March 2014, pp. 114-119
Methane storage in mixed hydrates with tetrahydrofuran
D V S G K Sharma1, Y Sowjanya1, V Dhanunjana Chari1, 2 & P S R Prasad1, *
1
Gas Hydrates Division, CSIR-National Geophysical Research Institute, Hyderabad 500 007, India
2
Department of Physics, Osmania University, Hyderabad 500 007, India
Received 8 November 2012 ; accepted 31 May 2013
A systematic study on the methane hydrate formation and dissociation with aqueous tetrahydrofuran (~ 6.03 mol %) has
been carried out in a stirred reactor. Experiments are conducted with initial methane pressure in the range 0.88 – 8.21 MPa.
Encaging of methane molecules in vacant cages (512) in mixed hydrate is found to be significantly faster compared to pure
methane hydrates. The hydrate formation temperature and methane consumption depend on the initial gas pressure (0.88 –
8.21 MPa) in the reactor and the consumed methane varies from 22.0 (± 1.4) m.mol to 88.0 (± 2.6) m.mol. The nucleation
and dissociation temperature of the mixed hydrates is always higher than ice melting temperature.
Keywords: Methane hydrates, Methane storage capacity, Phase boundary, Tetrahydrofuran hydrates
Gas hydrates are the three dimensional ice-like
crystalline compounds, often found in nature. The
four essential conditions for their formation are
(i) availability of host water molecules, (ii) suitable
guest molecules having molecular diameter matching
with available space within the host cages,
(iii) moderately high pressure and (iv) lower
temperature typically in the vicinity of freezing point
of ice. The guest molecules are usually smaller
hydrocarbons (methane, ethane, propane and
iso-butane) CO2, H2S and so on1,2. Further, smaller
molecules like H2 and He can also form stable
clathrates but require an order of magnitude higher
pressure than other gas hydrates3,4. Permafrost regions
and the ocean bottom sediments around the globe,
where gas hydrates can naturally occur because of the
conditions described above, coexist.
Methane is the most predominant guest molecule in
natural gas hydrates (NGH) but traces of other guest
molecules significantly alter the structure and phase
stability of methane hydrates5,6. Although gas
hydrates are considered as a nuisance when they
occur in oil/gas flow lines, it has been realised that
these are a potential energy resource when present in
large chunks in natural environment1,2,7. The global
estimate on the amount of energy (methane gas)
trapped within natural gas hydrate deposits varies
widely, but the most conservative estimates show that
——————
*Corresponding author.
E-mail: [email protected]
the amount of energy in hydrate deposits would be
twice to that of all fossil fuel reserves available
worldwide8,9. In summary, the gas hydrates have some
useful and devastating properties. Some of the
destructive events like blockades in oil/gas pipe
lines are often managed by using some inhibitors7.
Another major concern like seafloor collapse or
environmental impact of excess methane (due to
hydrate dissociation) is still a topic of intensive
research10,11. Typically 163 m3 of STP equivalent
guest molecules can be stored in a unit volume
of hydrates and hence this has become an
attractive system of fuel gas storage/transportation2.
Additionally, green house gasses like CO2 can also
form stable hydrates and thus the research on gas
hydrates gained momentum for separating CO2 from
flue gases and storing it under ocean floors.
One of the major technical issues on fuel gas storage
in the form of gas hydrates, eventually for storage and
transportation applications, is how to increase the mass
and heat transfer in the fuel gas-water systems in order
to attain a rapid hydrate formation. It has been well
documented in the literature that the tetrahydrofuran
(THF) can form hydrate having the structure II, with a
stiochiometric ratio of THF·17H2O12,13 and also it
works as a help molecule for gas hydrate formation14–16.
Addition of THF largely shifts the hydrate equilibrium
conditions and extends the hydrate stability region thus
it is known as thermodynamic promoter17–21. The
research on gas hydrate formation with THF solution
has been based on thermodynamics data and there is
SHARMA et al.: METHANE STORAGE IN MIXED HYDRATES WITH TETRAHYDROFURAN
little information available on the hydrate formation
kinetics. Recently, THF hydrate based systems have
been proved to be useful for separating and storing the
methane gas in coal mine methane gas22,23. Also pellets
of pure methane hydrates were prepared with THF
hydrates for possible transportation applications24. In
our previous studies, it was shown that the dissociation
pressure and temperature conditions critically depend
on the concentration of THF in aqueous solution20,25. In
this paper, we report the formation kinetics of CH4
hydrates in aqueous solution of THF (~6.0 mol%) at
different pressures of the range 0.88 – 8.21MPa. The
phase stability behaviour of the mixed hydrate system
in this pressure range and the conversion efficiencies of
mixed hydrates are also investigated in a stirred
reactor.
Experimental Procedure
Materials
Aqueous solutions were prepared by following the
gravimetric method using a Metler Toledo (AB104-S)
high accuracy analytical balance. Consequently, the
uncertainties on mole fractions are estimated to be
below 0.01. Doubly distilled and deionised water and
tetrahydrofuran (98%), supplied by Qualigens (Fine
Chemicals, India) were used in this study.
Experimental apparatus and procedure
Detailed description of experimental layout and
procedure has already been described in previous
reports20. Briefly, the main part of the apparatus is a
SS-316 cylindrical vessel of 100 mL, which can
withstand pressures up to 10 MPa. A stirrer with
variable speed was installed in the vessel to agitate the
fluids and hydrate crystals inside it. All the
experiments were conducted with a fixed speed of
500 rpm. Cold fluid (water + glycol mixture) was
circulated around the vessel with the help of Lab
Companion (RW-0525G) circulator, to maintain the
temperature inside it at a desired level. A platinum
resistance thermometer (Pt100) inserted into the
vessel was used to measure temperature and check for
equality of temperature within measurement
uncertainties, which is estimated to be less than 0.2 K.
The pressure in the vessel was measured with a
WIKA pressure transducer (WIKA, type A-10 for
pressure range 0 – 16 MPa).
The vessel
containing aqueous solution
(approximately 30% by volume of the vessel) was
immersed into the temperature controlled bath and the
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gas was supplied from a cylinder to desired level
using Teledyne ISCO Syringe pump (Model 100DX).
The vessel was evacuated before introducing any
aqueous solution and gas. After obtaining temperature
and pressure stability (far above from the hydrate
formation region), the valve inline connecting the
vessel and the ISCO pump/cylinder was closed.
Subsequently, temperature was slowly decreased to
form the hydrate and hydrate formation in the vessel
was detected by pressure drop. The temperature was
then increased in steps of 1.0 K. At each step,
temperature was kept constant with sufficient time
to achieve equilibrium state in the vessel. In this way,
a pressure temperature diagram was obtained for
each experimental run, from which we determined
the hydrate formation and dissociation pattern. If
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 phase equilibria of fluids in
the vessel. Consequently, the point at which the
slope of pressure-temperature data plot changes
sharply is considered to be the point at which all
hydrate crystals have dissociated. Experiments were
conducted at various initial pressure conditions and
dissociation points were determined by isochoric
pressure search method.
In all the experiments the reactor was filled with
30 g aqueous solution of THF and was then
pressurised with methane gas. Temperature of
circulating bath fluid (glycol + water) was set at 288 ±
1.0 K to bring system into the hydrate formation zone
and stirrer was kept at 500 rpm for the entire
experimental run. Certain degree of super cooling
and overpressure conditions (driving force) are
essential to initiate the nucleation of hydrates. It is
evident that methane hydrate formation occurs at
higher temperatures in the presence of promoter
molecules. The molar concentration (∆nH,t) of
methane gas in the hydrate phase during the
experiment at time t, is defined by the following
equation:
∆nH,t = ng,0 – ng,t = (P0V/Z0RT0) – (PtV/ZtRTt) … (1)
where the compressibility factor Z was referred from
Perry’s Chemical Engineers’ Handbook. The gas
volume (V) was assumed as constant during the
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INDIAN J. CHEM. TECHNOL., MARCH 2014
experiments, i.e. the volume changes due to phase
transitions were neglected. ng,0 and ng,t represent the
number of moles of feed (methane) gas at 303 K
taken as zero time and in the gas phase at time t
respectively. Pressure (P) and temperature (T) were
logged at a fixed time interval (60 s) as the hydrate
formation/dissociation progresses.
The hydrate formation could be divided into three
regions for convenience. In region 1, the change in
vapour concentration of gas was almost constant,
mainly due to real gas nature of the methane
molecules and the typical time we allowed for this
stage being around 40 – 50 min. In the second stage
(region 2), the temperature remained constant
under steady state condition for about 9 – 10 h after
hydrate nucleation for its maximum conversion/
growth. There was a decrease in the gas concentration
due to maximum pressure drop in the reactor vessel
for hydrate formation. Thereafter, in region 3, once
the hydrate formation was completed it is similar to
region 1. The plot between n(CH4) vs. time (inset of
Fig.1a) in these three regions provide necessary
information about the rate of hydrate formation and
the total gas adsorbed during hydrate formation.
Results and Discussion
Figure 1 shows the methane content in vapor phase
(m.mol) and temperature (T) variations with time
during initial 900 min (i.e. hydrate formation)
with pure water alone and in aqueous THF solution.
Corresponding n – temperature (n–T) trajectories
(in both hydrate formation and dissociation stages) are
shown as the insets. Since our goal is to get an insight
into the formation kinetics for hydrate forming
systems with co-guest molecules (THF), we
conducted all the experiments with about 6 mol%
THF solutions. It has been reported in the literature
that the resultant structure for the mixed hydrates as
Fig. 1– Observed variations in methane in vapor phase (○) and temperature (●) with time inside the reactor. (a) water + methane system;
and aqueous THF (6.03 mol%) solution + methane system at different initial pressures (b) 8.21 MPa, (c) 6.21 MPa and (d) 1.88 MPa.
Corresponding n(CH4)-temperature trajectory during hydrate formation and dissociation is shown as inset
SHARMA et al.: METHANE STORAGE IN MIXED HYDRATES WITH TETRAHYDROFURAN
sII, with THF molecules occupying 51264 cages and
CH4 molecules in 512 cages13,26. Formation kinetics in
mixed hydrates and the overall methane storage in sII
is compared with pure methane hydrate system under
identical experimental conditions. Further, the stirring
(at 500 rpm) was continuously on throughout the
experimental duration (during hydrate growth and
dissociation) to promote a faster nucleation and
growth of methane hydrates27,28. It is known that the
outside of hydrate formation zone, pressure in the
reactor vessel monotonically decreases with
temperature, however within the hydrate formation
zone an abrupt pressure drop is observed even at
constant temperature. Because of heat of formation
(exothermic) during the hydrate nucleation, we
observed peaks in the temperature vs. time profiles.
The height and duration of the peaks depend on the
amount of formation of heat and the mass and heat
transfer rates of the hydrate nucleation at different
pressures. As can be seen from Fig. 1, the hydrate
nucleation is more sluggish for the water + methane
system. Comparatively for water + methane + THF
system a very sharp nucleation occurs within the first
one hour of cooling and slowly the system attains the
equilibrium temperature i.e. ~ 288 K (Figs 1b – d). It is
clear that the consumption of 90% of methane to
hydrates occurs significantly faster in methane + THF
+ water system. It takes around 425 min for pure
hydrates (Fig. 1a) while it is much faster, i.e. 150, 160
and 175 min respectively for the mixed hydrate
system at three different initial pressure conditions
(8.21, 6.21 and 1.88 MPa at 303 K). It is observed
that the methane intake into THF hydrates at other
pressures also follows a similar trend and which is
significantly faster compared to pure hydrates.
In Fig. 2, we plotted the phase stability conditions
measured in this study and compared these with the
literature data17. The phase stability point is
determined by following the procedure described by
Sloan and Koh1. The data points plotted in figure
are the hydrate dissociation points following the
isochoric conditions. The results are correlated well
with the literature data and the mixed hydrates could
be formed at much lower pressures in the temperature
range 288 – 298 K. Mixed hydrates are formed at
much lower methane pressure i.e. ~ 0.88 MPa and the
formation temperature is around 288 K. It is clear
from Fig.2 that all the mixed hydrates formed at
methane pressures ranging from 0.88 MPa to 8.21 MPa
are stable upto temperatures 286 – 303 K and thus it
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could be advantageous to reform the methane
hydrates in aqueous THF of sII stoichiometry rather
than using lower mole fractions of THF21. However,
lowering the THF contents significantly alters the
hydrate dissociation temperature19–21.
Kida et al.24 had recently reported the dissociation
behaviour of granular samples of pelletized powdered
mixtures of methane or ethane and THF hydrate
through a temperature ramping method and concluded
that presence of THF clathrate hydrates enhances the
restraining effect on the dissociation of pure methane
or ethane hydrates up to temperatures above the
melting point of ice. In our previous studies, we also
noticed a significant variation in dissociation of pure
methane hydrates under the influence of THF
hydrates20. At lower concentrations of THF the
hydrate formation occurs in two steps, namely
hydrates in first stage (~T=295 K) were mostly with
sII structure and in the second stage (~T=275 K) were
with sI structure. There are no sI type hydrates at THF
concentrations more than 0.0556 mole fractions and
the dissociation temperature is also found to be higher
by about 18 K. As already discussed the formation
kinetics are significantly faster for mixed hydrates.
Further concern is about the overall methane gas that
could be stored in mixed hydrate systems. In Fig. 3,
we show the hydrate yield in mixed hydrates and
compared them with pure methane hydrates. As
already reported by Kida et al.24 there is no noticeable
differential increment in the constant volume
pressure vessel due to the dissociation of pure THF
hydrates and the pressure increment/ decrement is
because of methane gas release/ consumption in the
hydrate system. The hydrate yield is computed from
Fig. 2– The phase boundary curve for the mixed hydrates with
THF and methane [(■) present study and (+) data of ref .16]
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the observed methane gas consumption in the
experiments to the expected values with
stiochiometric compositions as described in the
literature20. We assume methane occupancy in small
(512) and large (51262) cages for pure hydrates (sI).
Further in mixed hydrates (sII) the small (512) cages
are occupied by methane molecules while the large
(51264) cages are populated with THF molecules and
therefore unit cell compositions for the two
respectively are 8CH4·46H2O and for mixed hydrates
it is 16CH4·8THF·136H2O.
As shown in Fig.3, hydrate yield is more at higher
driving force (methane pressure measured at 303 K)
and the behaviour could empirically be correlated to
second order polynomial with a constraint that no
hydrate yield at zero driving force. On the other hand,
the methane consumption (hydrate yield) in mixed
hydrates is always higher than in pure methane
hydrates. The expected methane consumption (mass)
in sI stiochiometry is 0.135, while the same for mixed
hydrates with THF in sII stiochiometry is 0.078.
Therefore, it may be advantageous to use THF
hydrates as a medium for storing the methane gas, in
particular; the present study shows that the formation
kinetics is much faster and gives superior methane
consumption compared to pure methane hydrates.
Additional advantage is that the formation and
dissociation of mixed hydrates occur at much
convenient temperatures (~285 – 295 K) even at much
lower methane pressure (~0.9 MPa).
Another concern is about the dissociation p-T
conditions at which significant amount of
methane gas is released from hydrates. Figure 4
shows the methane gas released at different
temperatures. First the mixed hydrates are formed
(at different initial methane pressures) by the
procedure described. The amount of methane
consumed in mixed hydrates is measured as 88 (±2.6),
73 (±1.4), 67 (±2.2), 53 (±2.8), 44 (±1.4) and
22 (±1.4) m.mol at initial methane pressure of 8.21,
6.21, 4.25, 3.16, 1.88 and 0.88 MPa. It is well known
that the hydrate yield is more at higher gas (guest)
pressures1,2. As already explained, insignificant
pressure variation for longer duration indicates
the saturation in hydrate conversion and then
reactor is cooled to about 265 K. Residual methane
gas is completely removed by maintaining the
reactor vessel at constant temperature ~265 K. Now
the temperature is slowly increased at a rate of
0.05°/min and the pressure variations are continuously
logged at 10 s intervals. As shown in Fig. 4,
the total amount of methane gas released from
mixed hydrate system is 88, 73, 68, 52, 43
and 21 m.mols. As expected, higher amount of
methane gas is stored in hydrates formed at
higher pressures. The methane gas is released from
mixed hydrates at a rate ranging from
0.30 m.moles/min to 0.64 m.moles/min and
significant gas release starts always above 275 K
(higher than ice melting temperature), indicating
that the mixed hydrate systems could hold the
methane molecules at much higher temperatures
even at 0.1 MPa pressure. It is well known that the
stability temperature for methane hydrates at this
pressure is normally around 193 K. In other words,
the stability of mixed hydrates is significantly higher
than pure methane hydrates20, 24.
Fig. 3– Hydrate yield in the mixed hydrate (●) and pure methane
hydrate systems (○) at different initial gas pressures. Best fit for
observed data is shown as solid line
Fig. 4– Methane gas recovered upon dissociation of mixed
hydrates
SHARMA et al.: METHANE STORAGE IN MIXED HYDRATES WITH TETRAHYDROFURAN
Conclusion
Mixed hydrate formation and dissociation behavior
has been studied in methane and aqueous THF system.
The formation is found to be much quicker in methane
and aqueous THF as compared to that in pure methane
hydrates. Typically about 90% of methane gas is
consumed in mixed hydrates within 200 min, while
similar amount is consumed in about 500 min in
pure methane hydrates. Experimental observations
show that the overall hydrate yield in mixed hydrates
(sII structure with THF molecules in 51264 cages &
CH4 molecules in 512 cages) is always more than that in
pure methane hydrates (sI structure with CH4
molecules in 512 & 512 64cages). It is also observed that
significant amount of methane gas release occurs from
mixed hydrates at temperatures higher than the ice
melting temperature. Thus, the mixed hydrate system
with aqueous THF of sII stiochiometry could be used
as medium to store and transport the methane gas,
because of its ability to accommodate methane
molecules in vacant small cages even at lower
pressures with fast formation kinetics and higher
dissociation temperature (>275 K).
Acknowledgement
The authors gratefully acknowledge the funding
support from the Department of Science &
Technology, Ministry of Earth Sciences (gas hydrates
programme), Directorate of Hydrocarbons (NGHP)
and DRDO – ASL, Government of India, through
sponsored projects.
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