ARTICLE pubs.acs.org/JPCA
The Conversion Process of Hydrocarbon Hydrates into CO2 Hydrates and Vice Versa: Thermodynamic Considerations J. M. Schicks,* M. Luzi, and B. Beeskow-Strauch Helmholtz-Centre Potsdam, GFZ German Research Centre for Geosciences, Section 4.2, Telegrafenberg, 14473 Potsdam, Germany ABSTRACT: Microscopy, confocal Raman spectroscopy and powder X-ray diffraction (PXRD) were used for in situ investigations of the CO2-hydrocarbon exchange process in gas hydrates and its driving forces. The study comprises the exposure of simple structure I CH4 hydrate and mixed structure II CH4 C2H6 and CH4 C3H8 hydrates to gaseous CO2 as well as the reverse reaction, i.e., the conversion of CO2-rich structure I hydrate into structure II mixed hydrate. In the case of CH4 C3H8 hydrates, a conversion in the presence of gaseous CO2 from a supposedly more stable structure II hydrate to a less stable structure I CO2-rich hydrate was observed. PXRD data show that the reverse process requires longer initiation times, and structural changes seem to be less complete. Generally, the exchange process can be described as a decomposition and reformation process, in terms of a rearrangement of molecules, and is primarily induced by the chemical potential gradient between hydrate phase and the provided gas phase. The results show furthermore the dependency of the conversion rate on the surface area of the hydrate phase, the thermodynamic stability of the original and resulting hydrate phase, as well as the mobility of guest molecules and formation kinetics of the resulting hydrate phase.
1. INTRODUCTION Gas hydrates are crystalline, ice-like solids composed of water and gas molecules. From a chemical point of view, gas hydrates are assigned to clathrates: the water molecules arrange in a threedimensional, hydrogen-bonded network with defined cavities, which are stabilized by included gas molecules. Depending on the properties of the guest molecule, such as size and shape, different hydrate structures may form by combination of diverse cavity types.1 Natural gas hydrates are usually structure I hydrates and contain predominantly CH4, but they may also contain CO2, H2S or other hydrocarbons. Those hydrates containing other hydrocarbons beside CH4 typically exhibit structure II or structure I. Natural gas hydrates occur at all passive and active continental slopes, deep lakes, or permafrost regions, thus at places permitting elevated pressure, low temperature, and sufficient amounts of gas and water. Their global occurrences and the fact that huge amounts of CH4 are assumed to be bonded in these natural gas hydrates present them as a potential energy source. In addition, the idea of a combination of CH4 production from hydrate-bearing sediments and sequestration of CO2 as gas hydrates at the same time seems to be an elegant way to use natural gas hydrates as an almost CO2 neutral resource, which becomes more and more of interest. CO2 also forms structure I hydrate, which is stable at lower pressures or higher temperatures compared to pure CH4 hydrate. As referred to in the literature, the higher stability of CO2 hydrate compared to CH4 hydrate is supposed to be the driving force for the exchange reaction.2 4 This higher stability corresponds to the higher equilibrium r 2011 American Chemical Society
temperatures of CO2 hydrates at given pressures, assuming that the chemical environment does not change. Several efforts have been done in the past to determine the exchange kinetics and to optimize the swapping process with respect to the recovery rate of CH4. Depending on experimental conditions, the CH4 recovery rate from the hydrate phase induced by the injection of CO2 varies to a high extent. Hirohama et al. poured 31 mol of water (∼ 560 mL) into a pressure vessel (volume 3.2 L), pressurized the cell with CH4, and converted 24 mol of water into a bulk CH4 hydrate phase. Thereafter, the CH4 gas phase was replaced by liquid CO2. They observed the conversion of the remaining free water phase (7 mol) and the transformation of about 15% of the CH4 hydrate into CO2 hydrate after 800 h.5 Lee et al. investigated much smaller hydrate samples prepared from powdered ice (particle size of ice and resulting hydrates 5 50 μm) in porous silica gel (pore size 15 nm). They also added liquid CO2 to the CH4 hydrate phase and observed the establishment of a steady state after 5 h and a CH4 recovery rate of about 50%, which indicates a hydrate phase with a CO2/CH4 ratio of 1.6 Park et al. investigated the effect of additional N2 on the swapping process. They synthesized CH4 hydrate from powdered ice and CH4 gas in a stirred reactor before exposing a small part of the pure CH4 hydrate sample (about 1 mL) to a N2 CO2 gas mixture (80% N2, 20% CO2).2 It should be noted that calculations with CSMGem show that much higher pressures at Received: October 13, 2010 Revised: September 6, 2011 Published: September 19, 2011 13324
dx.doi.org/10.1021/jp109812v | J. Phys. Chem. A 2011, 115, 13324–13331
The Journal of Physical Chemistry A given temperatures are needed to stabilize a N2 CO2 hydrate compared to a pure CH4 hydrate.7 It should also be noted that Dornan et al. calculated the free energy for the reaction where CO2 or a CO2+N2 mixture is used to substitute CH4 in the hydrate.8 They showed that substitution of CH4 with CO2 in the large cavities of structure I has a large negative energy, which should lead to a spontaneous replacement of CH4 with CO2 in the hydrate. For the CO2+N2 mixture the free energy was also negative for the substitution of CH4, whereas the absolute value is decreased due to the a small positive energy that results from the substitution of methane with N2 in the large cavities of structure I.8 Nevertheless, the results of Park et al. indicate that after about 24 h, 85% of CH4 from the hydrate phase was recovered when exposed to a N2 CO2 gas mixture.2 Kvamme et al. investigated the swapping process on hydrates in pores of sandstone cores. After CH4 hydrate formation, the core was exposed to liquid CO2. Under these conditions, up to 60% of CH4 was recovered from the hydrate phase within 300 h.4,9 Although the experimental conditions and results of the experimental studies vary significantly and the swapping process could not be described on a molecular level, some general assumptions may be allowed: • The replacement of CH4 with CO2 in the hydrate phase is a slow process. • The CH4 recovery rate depends on the surface-to-volume ratio of the hydrate phase. • Not only CO2 but also N2 induces a release of CH4 from the hydrate phase, which indicates that the driving force is the gradient of the chemical potential between the hydrate phase and the surrounding phase rather than pressure and temperature conditions describing the stability limits of certain hydrate phases. • Mass and heat transport play a significant role. • Molecular size of guest molecules and potential cage occupancies are also important. Most of the published data focus on the replacement of CH4 with CO2. Only few data are available for the reverse reaction or the swapping process of hydrocarbons with CO2 in structure II mixed hydrates. Lee et al. mentioned in their study that the reaction of CO2 with CH4 hydrate and the reverse reaction of CH4 with CO2 hydrate are completely different. They argue that the replacement of CO2 with CH4 in the large cavities of structure I is hindered because CO2 is the favored guest due to its larger molecular size.6 As natural gas hydrates may contain other hydrocarbons beside CH4, Park et al. investigated the swapping process of structure II CH4 C2H6 mixed hydrates with CO2 and CO2 N2 gas mixtures. The results show that 92 95% of CH4 and 93% of C2H6 could be recovered from the hydrate phase.2 The intention of this study was to investigate the exchange of hydrocarbons with CO2 in simple CH4 hydrates but also in CH4 C2H6 and CH4 C3H8 mixed hydrates. The mixed hydrates were chosen because natural gas hydrates may contain C2H6 or C3H8 beside CH4. The mixed hydrates formed from these gas mixtures (see also Table 1) are stable at higher temperatures and lower pressures compared to pure CH4 hydrate and are as stable as or even more stable than pure CO2 hydrate, provided that each hydrate phase is in equilibrium with its chemical environment (Figure 1). Since the above-mentioned mixed hydrates form structure II, a structural transition to structure I hydrates induced by CO2
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Table 1. The Composition of the Used Gases and Gas Mixturesa composition [vol%] gas mixture
CH4
1
100
2 3 4
CO2
C2H6
C3H8
99.995 100
93 ( 0.14 98 ( 0.04
quality
99.995 7 ( 0.14 2 ( 0.04
99.5 99.95/99.90
a
The gases were purchased from Linde AG and Air Liquide Deutschland GmbH.
Figure 1. Phase boundaries of hydrates formed from pure CO2 (triangles), pure CH4 (stars) as well as mixed CH4 C2H6 (squares) and CH4 C3H8 (diamonds).20 22
injection can easily be documented by X-ray diffraction. Due to the fact that natural gas hydrates are continuously fed by ascending natural gas from deeper sources, the reverse transformation of CO2 hydrate into the mixed hydrocarbon hydrate was also investigated. In the following, we present experimental data from these investigations on simple CH4 and CO2 hydrates as well as mixed CH4 C2H6 and CH4 C3H8 hydrates regarding the hydrocarbon/CO2 exchange and the reverse reaction. The measurements were performed using both in situ confocal Raman spectroscopy and in situ powder X-ray diffraction (PXRD) to study changes in composition and structure during the swapping process and, hence, to understand the reaction on a molecular level.
2. EXPERIMENTAL SETUP AND PROCEDURE The experimental setup for the in situ Raman spectroscopic and the in situ PXRD measurements are similar. The volumes of the sample cells are 0.393 cm3 for the Raman pressure cell and 0.250 cm3 for the PXRD pressure cell, respectively. One important feature of both sample cells was the use of a continuous gas flow of 1 mL/min. The gas flow assures that the offered gas composition will not change during the experiment, even if one component of a gas mixture is enriched in the hydrate phase. It also assures the excess of hydrocarbons or CO2. The gas flow was measured and regulated with a commercial flowmeter F230FA-11-Z from Bronkhorst. A pressure controller (TESCOM ER 3000) regulated the sample pressure with a precision of 2% rel. Care was taken that the incoming gas was cooled down before 13325
dx.doi.org/10.1021/jp109812v |J. Phys. Chem. A 2011, 115, 13324–13331
The Journal of Physical Chemistry A
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Table 2. The Chosen Pressure and Temperature Conditions for the Different PXRD Experimentsa pressure temperature investigated reaction
results
[MPa]
[K]
presented in
ice + CH4 C2H6 gas CH4 C2H6 sII-hydrate + CO2 gas
1.43 1.43
267,15 267,15
Figure 6
CO2-rich sI-hydrate + CH4 C2H6 gas
1.43
267,15
Figure 7
ice + CH4 C3H8 gas
0.94
267,15
CH4 C3H8 sII-hydrate + CO2 gas
1.16
267,15
Figure 6
CO2-rich sI-hydrate + CH4 C3H8 gas
1.16
267,15
Figure 7
ice + CO2 gas
1.16
267,15
CO2 sI-hydrate + CH4 C3H8 gas
1.16
267,15
Figure 7
CO2 sI-hydrate + CH4 C3H8 gas CO2 sI-hydrate stability test
0.94 0.94
267,15 267,15
Figure 8
a
Pressure conditions were chosen 30% above the equilibrium pressure at a given temperature (calculated with CSMGem7). This was done to permit a better comparability between the different experiments with respect to their varying stability condition.
entering the sample cell to avoid temperature effects such as local warming or temperature gradients in order to enable an isothermal experiment. Although a continuous gas flow of 1 mL/ min was used throughout the whole experiment, the chosen pressure was kept constant. Experiments with Raman spectroscopy were performed at 3.2 MPa and 274 K. Pressure conditions for PXRD measurements were chosen 30% above the equilibrium pressure at given temperature. Thus, the experimental pressure conditions at a given temperature were different for each investigated system; the values are given in Table 2. The latter was done to permit a better comparability between the different experiments with respect to their varying stability condition. The system pressure was measured with a P3MB from Hottinger Baldwin Messtechnik with a precision of 0.01% rel. The temperature of the sample cells was kept constant during the experiments and determined with a precision of (0.1 K, using a Pt 100 sensor. More details regarding the pressure cells and the experimental setup can be found elsewhere.10,11 In situ Raman spectra were taken with a confocal Raman spectrometer (LABRAM, HORIBA Jobin Yvon) equipped with an external 100 mW diode-pumped solid-state (DPSS) laser with a wavelength of 532 nm. The Raman instrument was calibrated using silicium (521 cm 1) and diamond (1332 cm 1). A Bruker AXS D8 Discover microdiffractometer with a General Area Detection Diffraction System (GADDS) was employed for X-ray measurements. The GADDS system uses a multi array detection system instead of a Geiger Mueller counter or a Scintillation counter, which enables photographical pictures to be taken of the diffraction cones. The advantages of these system are short detection times and the gain of additional information regarding the crystallinity of the sample. For hydrate synthesis, ice was prepared from deionized water frozen in a liquid nitrogen bath. The ice was milled in a 6750 Freezer Mill (SPEX CertiPrep) for 300 s. Electron-microscopic investigations (SEM Supra 55 VP, Zeiss) show a particle size of the ice particles
