KINETICS OF CO2 HYDRATE FORMATION BELOW MELTING POINT OF ICE

KINETICS OF CO2 HYDRATE FORMATION BELOW MELTING POINT OF ICE

https://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/47_1_Orlando_03-02_0100.pdf
Takeshi Komai, Taro Kawamura, and Yoshitaka Yamamoto
National Institute for Advanced industrial Science and Technology
16-1 Onogawa, Tsukuba
305-8569 Japan

Introduction
Gas hydrates are very attractive from both resources and environmental aspects. CO2 gas hydrate can be utilized to develop methane gas hydrate as an important energy source in the near future.

When producing methane gas from gas hydrate reservoirs at permafrost area, the injection of CO2 might be used to promote the gas production by replacement, and to maintain the stability of strata
due to the formation of CO2 gas hydrate. In addition, the sequestration of CO2 mass can be performed in the process of gas hydrate production. An experiment on the formation of CO2 gas
hydrates was carried out under low temperature below melting point of ice, using a specially designed cell-type apparatus. To observe the nucleation and formation process, CO2 gas was introduced to the ice crystals in a pressure cell at a condition of high pressure. Raman spectrometry was used to measure the rate of formation based on Raman analysis due to the dynamic behavior of guest molecules. As a result, it was found that the rate of formation of CO2 gas hydrate depends on various conditions, such as temperature, pressure and diameter of ice crystals. The mechanism of the formation of CO2 gas hydrate under low temperature was also discussed, based on the results of Raman analysis.

Experimental
Apparatus. The experimental apparatus was specially designed and
equipped for the formation and replacement of gas hydrates. It can be
used under the various conditions that pressure, temperature and
concentration of gases are precisely controlled. Fig.1 illustrates the
schematic diagram of the apparatus and measuring system used in the
experiment. The pressure cell, made of stainless steel with a 3.2 ml
internal volume, can be used at a pressure condition of up to 20 MPa.
It contains a glass window for observing Raman scattering of
monochromatic lights, a thermoelectric temperature control module,
and some nozzles for the introduction of gas and liquid components.
The pressure cell is installed on a constant temperature plate filled
with a cooling agent methanol, where the temperature can be
controlled with an accuracy of ±0.1 K. It is equipped with transducers
for detecting and controlling pressures with an accuracy of ±0.05
MPa.
Measuring Methods. The observation system of the Raman
spectroscopy and a CCD camera are mounted in the apparatus. The
mechanism and theoretical background of Raman analysis are
described in Fig.2. The figure also shows the molecular model of
CO2 gas hydrate. Macro mode is applied for the measurement of
Raman spectroscopy, in which Ar laser is emitted to the sample. In
this experiment powder-like fine ice crystals were placed in the
pressure cell for the starter of gas hydrate. The average diameter of
the crystals is about 9 micro meters. CO2 gas was introduced into the
cell and the pressure was controlled to a value of a little higher than
the equilibrium pressure. During the nucleation and crystal growth of
CO2 gas hydrate, Raman spectra were measured and analyzed to get
the data of Raman shifts. As the strength of Raman shift is
proportional to the concentration of guest component, the formation
rate can be calculated by the relative strength. In this experiment
temperature condition was changed to the range between –18.5 deg.
C. and 0 deg. C.. The pressure condition in the formation process of
gas hydrate was set to a constant value of 1.6 MPa.
Fig.1 Experimental apparatus and measuring system of Raman
spectroscopy.
Fig. 2 Molecular model of CO2 gas hydrate and the mechanism of
Raman measurement.
Results and Discussion
In-situ measurement of formation process. As a typical result of
measurement, Fig.3 shows the trend of Raman shifts observed in the
formation of CO2 gas hydrate under a condition of –8.7 deg. C. and
1.6 MPa. Four peaks of Raman shift appeared as a result of Raman
analysis in the process of guest molecule trap into the cages. The left
hand peaks reveal the existence of CO2 in large cage. It was found
that the strength of the peaks gradually increased with time, and
especially the rate of increase was quite large in the first stage of the
formation, such as within 10 minutes. In this duration the formation
mechanism is nucleation or transformation from ice to gas hydrate.
Formation rate of CO2 hydrate vs Temperature. The comparison
of time growth of the strength was made for the formation of CO2 gas
hydrate under the temperature condition that was changed between –
18.5 deg C. and 0.5 deg. C.. Fig.4 shows the relative strength of
Raman shifts, in which the effect of temperature on the formation rate
of CO2 gas hydrate from ice crystals was described. From the result
it was found that the rate of formation was very large in a condition
of -0.5 deg C., compared with other conditions below –3.8 deg. C..
Fuel Chemistry Division Preprints 2002, 47(1), 353