International Conference on Gas Hydrates (ICGH) (6th : 2008)

NMR studies on CH4 + CO2 binary gas hydrates dissociation behavior Rovetto, Laura J.; Dec, Steven F.; Koh, Carolyn A.; Sloan, E. Dendy 2008-07

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  NMR studies on CH4+CO2 binary gas hydrates dissociation behavior   Laura J. Rovetto, Steven F. Dec, Carolyn A. Koh, E. Dendy Sloan Jr.∗ Center for Hydrate Research, Department of Chemical Engineering Colorado School of Mines 1600 Illinois St, Golden, CO, 80401 USA   ABSTRACT The dissociation behavior of the CH4+CO2 binary gas hydrate has been investigated using Nuclear Magnetic Resonance (NMR) spectroscopy. This technique allows us to distinguish the hydrate structure present, as well as to quantify phase concentrations. Single-pulse excitation was used in combination with magic-angle spinning (MAS). Time-resolved in situ decomposition experiments were carried out at different compositions in sealed, pressurized samples. The decomposition profiles of the CH4+CO2 binary gas hydrate system obtained at various compositions suggest that the decomposition rate is a strong function of the fractional cage occupancy and temperature. An unexpected CH4 hydrate reformation was observed during our decomposition experiments when the temperature reached the ice melting point. A decrease on the CO2 content in the hydrate phase was found during the decomposition experiment, as the pressure and temperature of the system increases.  Keywords: methane, carbon dioxide, decomposition, NMR   ∗ Corresponding author: Phone: +1 303 273 3723 Fax: +1 303-273-3730 E-mail: NOMENCLATURE AM,N peak area of M component in N cage/peak Meq equilibrium magnetization M(TR) magnetization at steady-state T1 spin-lattice relaxation time TR pulse repetition rate [s] zM molar fraction of M component α single-pulse excitation tip-angle [º]  INTRODUCTION Natural gas hydrate abundance in geological settings is estimated to reach 1-5 ×1015 m3 at STP [1- 2]. These deep sea and permafrost deposits had been recognized as a potential large energy resource and recently, natural gas from gas hydrates had been successfully recovered [3]. But natural gas hydrates containing mainly methane (an effective greenhouse gas) have been also identified as a possible environmental hazard; therefore their dissociation could have a significant impact on the environment [4- 5].  On the other hand, anthropogenic CO2 emission is constantly increasing and has been identified as the main factor contributing to the greenhouse effect and global climate change [5].  A combined mechanism for CH4 recovery from natural deposits and simultaneous CO2 sequestration in the remaining gas hydrate structures is in fact, an attractive technology [6]. During the last decade the advanced concept of gas exchange based on replacing the CH4 gas trapped in natural gas hydrates by supplying CO2 had been studied. Raman spectroscopy had been mainly used to investigate the guest replacement in the CH4 gas hydrate structure [7- 9], among some other techniques such as NMR [10] and GC [11]. Replacement studies in porous material had been reported applying Magnetic Resonance Imaging [12]. Field studies Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. are limited to pure CO2 gas hydrate formation [13]. Although CO2 pure gas hydrates are thermodynamically more stable than CH4 pure gas hydrates [14], a total replacement has not been achieved yet; a maximum of only 64% was reported [10]. Thermodynamic information [15] alone is not sufficient to evaluate the potential of this combined technology; kinetics studies must also be performed. The challenges and issues of implementing such technology had been recently reviewed [16]. The understanding of the formation and dissociation mechanism of gas hydrates is highly needed to transform this potential technology into a viable process.  Although kinetics studies of gas hydrates processes had been reported [17], information at the microscopic level is limited. Several techniques, such as NMR and Raman spectroscopy and X-ray and neutron diffraction have been used to study the molecular-scale hydrate formation and decomposition processes [18-23].  Hydrate structure and guest distribution in the hydrate cages have been studied using both time-resolved Raman and NMR spectroscopy in hydrate formation experiments. With both CH4 [22] and Xe [24], formation of the 51262 cage of sI hydrate was found to be the rate limiting step. The relative occupancy of the small 512 cage of sI Xe hydrate was found to be higher during the initial formation stage and therefore considered as a precursor of hydrate formation. Gas chromatography, Raman and X-Ray diffraction were used to show that during the formation of the binary CH4+C3H8 gas hydrate, the propane molecules were preferentially encaged in the hydrate crystals over methane, at propane concentrations below 10 %vol [25]. Also, NMR spectroscopy experiments showed that propane- occupied 51262 cages were formed twice as fast as methane-occupied 512 cages [19].  During the formation of the binary CH4+CO2 gas hydrate, the changes in the gas composition were analyzed using gas chromatography and Raman spectroscopy. According to the authors, methane molecules were preferentially crystallized in the early stages of hydrate formation from a water gas system (at CH4 gas concentrations lower than CO2) [11]. On the other hand, at the beginning of the binary CH4+CO2 gas hydrate growth from ice powder, CO2 gas was reported to react three times faster than CH4 under similar excess pressure, by using neutron diffraction techniques [21].  These results for gas hydrate formation show that hydrate cavities may have different formation rates depending on the guest molecule and conditions applied.  NMR decomposition experiments of sI single- guest gas hydrate with hyperpolarized 129Xe [24] and CH4 [26] have shown no preferential rate of dissociation of either cage. These results suggest that that sI hydrate unit cells of these simple gas hydrates decompose as a single entity, at least on the time scale of the NMR measurements.  The decomposition of binary gas hydrates has been reported for the CH4+C2H6 and CH4+CO2 systems [27- 28]. The observed decomposition rates were found to be strongly dependent on the temperature and the composition of the system.  In this work, 13C MAS NMR spectroscopy was used to study the molecular-scale decomposition of binary CH4+CO2 gas hydrate, with the intention of providing useful information for gas recovery and sequestration processes.  EXPERIMENTAL Materials Enriched 13C gases were used for the 13C NMR studies; this technique provides information only on 13C atoms present. 13CH4 and 13CO2 (both 99% purity) were purchased from Cambridge Isotope Laboratory. Deionized water was used without any further purification.  Experimental Procedure Hydrate Sample Preparation Fine ice particles (250-500 µm) were used as the starting material for gas hydrate formation. Approximately 0.1 cm3 of granular ice was placed into a 1.0 cm3 pyrex glass tube and pressurized with the gas mixture (CH4+CO2), between 3.5 to 5 bar above its calculated equilibrium pressure. The tube filled with ice particles plus the gas mixture was then placed in a temperature controlled bath at -7ºC for 2 days. The bath temperature was later increased to 0ºC (usually for 7-14 days) to allow full conversion of ice particles into gas hydrate. After hydrates were formed, the free gas present in the glass tube was removed by using a vacuum pump, keeping the hydrate portion submersed in the bath at approximately -7ºC and then, immediately transferred to a liquid nitrogen bath to avoid the decomposition of the sample. Finally, the bottom of the tube containing the gas hydrate was sealed using a flame torch while immersed in liquid nitrogen giving as a result a closed system glass bulb containing the hydrate sample. The sealing procedure may cause the decomposition of some small amount of hydrate, therefore the bulb was placed in a freezer at - 20ºC for at least two days for a final conditioning. The overall composition of the sample was determined using 13C MAS NMR spectroscopy.  NMR Spectroscopy All 13C MAS NMR spectra were recorded on a Chemagnetics CMX Infinity 400 NMR spectrometer operating at a frequency of 100.5 MHz for 13C.  Proton decoupling fields of 50 kHz and MAS speeds of about 3 kHz were used.  Single-pulse excitation experiments (90o pulses of 5 µs) and various pulse delays, depending on the spin-lattice time (T1) of the components of the mixture, were used to record fully relaxed spectra at various temperatures. Spin-lattice relaxation times (T1) were measured using a standard inversion-recovery pulse sequence [29]. The number of acquisitions varied depending on the amount of hydrate in the sample, which determines the signal intensity of the spectrum. Time-resolved 13C MAS NMR spectra were recorded with single-pulse excitation in the hydrate dissociation experiments. Under the conditions of the time-resolved 13C MAS NMR experiment with an α pulse and pulse repetition rate of TR the magnetization M(TR ) is given by [30] 1 1 )cos(1 1)( T RT T RT e eMTM eqR − − − −= α    (1) Meq is the equilibrium magnetization. M(TR) reached its steady-state value after about five pulse repetitions.  The following conditions were applied to the samples during the decomposition experiments: on each decomposition step of Sample A a total of 80 single-scan spectra were collected over a period of time of 403 s; a 42° excitation pulse of 2.4 µs and a pulse delay of 5 s was the applied, with acquisition time equal to 0.0496 s. Decomposition steps of Sample B were recorded during 451 s each, collecting 300 single-scan spectra with a 68° excitation pulse of 3.8 µs; a pulse delay of 1.5 s was applied, with an acquisition time of 0.0496 s. Samples C and C’ decomposition steps were recorded for 405 and 410 s respectively; 80 single-scan spectra were recorded with a 49º excitation pulse of 2.72 µs; a pulse delay of 5 s was applied with acquisition times of 0.06144 s for sample C and 0.13652 for Sample C’.  The chemical shift of all components was referenced to the methylene carbon resonance line of adamantane, used as an external chemical shift standard with the assigned value of 38.83 ppm [26]. The spectrometer was equipped with Chemagnetics solid-state MAS speed and temperature controllers.  Temperature calibration at the position of the sample has been described elsewhere [26]. Thermal activation of the samples was achieved using a temperature step increase; single-pulse spectra were recorded during this temperature change over a certain period of time. Temperature gradients across the sample are negligible due to the small sample size.  Details regarding each thermally activated decomposition experiment are provided in the text and figure captions of the results.  RESULTS AND DISSCUSION The changes in phase amount and composition for four different samples were determined during the dissociation experiments. The compositions of each sample at -10ºC are listed in Table 1.  Samples A and B are CO2 rich whereas sample C has a higher concentration of CH4 in both phases. Sample C was totally melted during the dissociation experiments and afterward reformed; the resulting reformed sample is sample C’.  Gas comp (%) Hydrate comp (%) Sample CO2 CH4 CO2 CH4 Cage occupancy ratio L/S for CH4 A 56 44 65 35 0.47 B 64 36 83 17 0.34 C 20 80 45 55 0.69 C’ 20 80  65  36 0.62 Table 1. CH4 and CO2 gas and hydrate composition at -10ºC.  CH4+CO2 NMR spectra and composition A typical single pulse 13C MAS spectrum for the binary sI CH4+CO2 gas hydrate is depicted in Figure 1. The 13C resonance lines corresponding to the CH4 region are at -3.7 ppm and -6.0 ppm for small and large cage of sI hydrate, respectively and the gas phase at -10.5 ppm, in accordance with literature values [31]. The 13C resonance signal for CO2 hydrate without MAS has a motionally averaged powder pattern between about 100.2 and 188.5 ppm due to the anisotropic rotation of CO2 molecules entrapped in the asymmetrical 51262 large cavity of sI hydrate [32]. When the 512 small cavities of sI hydrate (with pseudo-spherical symmetry) are occupied by CO2 molecules, their motion gives a relatively sharp peak at 123.1 ppm superimposed on the CO2 large cage powder pattern. Such information cannot be obtained by performing single-pulse 13C MAS experiments because the isotropic resonance lines of CO2 in the large and small cages of sI and the resonance line of the gas phase are not resolved.  In our experiments, as a result of spinning the sample, the 13C resonance lines of CO2 in the hydrate phase are modulated resulting in the appearance of spinning side bands at approximately 187, 156 and 94 ppm (depending on the spinning speed 2 - 3.5 kHz) as well as the isotropic line at 125.3 ppm (see Figure 1), which is superimposed with the resonance line of CO2 in the gas phase at 125.7ppm.   The total integrated area under a resonance peak in the NMR spectrum is proportional to the equilibrium magnetization vector, which is directly related to the number of spins in the sample and hence to the concentration of that particular compound. The experimental area (magnetization) value was corrected according to Eq. (1)).    Figure 1: 13C NMR spectra of binary CH4-CO2 gas hydrate. Sample A at -10ºC, gas phase 56.0±2.0% carbon dioxide; hydrate phase 65±2.0% carbon dioxide.  The global composition of the samples was calculated by normalizing the integrated peak area of every component present in each phase (see Eq. (2) for CH4 hydrate global composition). hydtotalCOSCHLCH SCHLCH CH AAA AA z 244 44 4 ++ + =                  (2) The total relative intensity of the CO2 hydrate phase is obtained by summing the integrated relative intensity of the 13C isotropic resonance line at 125.3 ppm and all its observable spinning side bands as in Eq. (3). ++= 56187 ,222 peakCOpeakCOhydtotalCO AAA   94125 22 peakCOpeakCO AA ++  (3) The CO2 gas phase relative intensity is obtained by deconvolution of the overlapped peaks corresponding to CO2 in the gas phase at 125.3 ppm and the isotropic resonance line of CO2 hydrate at 125.7 ppm.  Decomposition experiments In our dissociation experiments, samples were thermally activated by increasing the temperature (temperature ramp) at time = 0. Time-resolved 13C MAS NMR spectra were recorded during the decomposition experiment; as the decomposition proceeds, the intensity change of each 13C resonance lines reflects the  0 5000 10000 15000 20000 25000 30000 200 150 100  50  0  -50 ppm CO2 gas CH4 gas CO2 hydrate CH4 512 cage In te ns ity  (a .u .) CH4 51262 cage Adamantane changes in composition of the vapor and hydrate phases for all present components. The effect of raising the temperature of the samples yields the expected result; the relative intensities corresponding to CH4 and CO2 in the hydrate phase decrease; simultaneously an increase in the intensities of the gas phase peaks is observed.  The decomposition profiles were obtained by plotting the integrated peak areas of each component in each phase, of the time-resolved 13C MAS NMR spectra, as a function of time.  Sample A and B decomposition results Two decomposition profiles obtained in successive temperature increases of Sample B are shown in Figure 2. The data points represent the integrated peak area values (relative intensity) of each single-scan of the 13C NMR time resolved experiment. During the first 10 to 15 s of each experiment, the sample approached the set temperature (4 and 11ºC respectively) and started to dissociate.  T ramp = -3 to 4 C 0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 Time (sec) R el at iv e In te ns ity  (a .u .) CO2 gas CH4 gas CO2 hydrate CH4 hydrate  T ramp = 4 to 11 C 0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 Time (s) R el at iv e In te ns ity  (a .u .) CO2 gas CH4 gas CO2 hydrate CH4 hydrate  Figure 2: Decomposition profiles obtained for sample B. Top: temperature ramp -3 to 4ºC. Bottom: temperature ramp 4 to 11ºC (total dissociation of gas hydrate). In the case of the temperature ramp from -3 to 4ºC (Figure 2 top), the decomposition profile of Sample B shows that the gas hydrate decomposes at its fastest rate during the first 180 s. After that, the intensity of all 13C NMR resonance lines reached a constant value. Because significant changes were not observed any longer it is assumed that the partial hydrate decomposition process has ended.  The decomposition profile of Sample B obtained with the temperature ramp from 4 to 11ºC is depicted at the bottom of Figure 2. Also, in this case, changes were only observed in a short period of time of approximately 110 s. At that point all the hydrate present in the sample was melted; this is observed by the dramatic drop of the relative peak intensities corresponding to both CH4 and CO2 in the hydrate phase, down to 0.  The decomposition rate is obtained by a linear fit of the data, during the observed hydrate dissociation time (i.e. intensity peak change over time). Table 2 summarizes the rates of CH4 and CO2 gas released during two successive temperature ramps, for samples A and B. As expected, the higher the decomposition temperature, the faster the hydrate dissociation, as shown for the higher temperature ramps.   Gas release rate (dI/dt) (intensity 104 s-1) Temp ramp (ºC) CO2 CH4 -3 to 4 3.38 0.87 Sample A 4 to 11 18.29 7.52 -3 to 4 10.68 2.85 Sample B 4 to 11 23.92 7.71 Table 2. CH4 and CO2 gas release rate during decomposition experiments.  In order to quantify the system composition and phase amounts after each temperature ramp, single pulse 13C MAS spectra were taken before and after each decomposition step. Table 3 summarizes the changes in concentration for samples A and B during the decomposition experiments between three temperature ramps, starting at -10ºC and up to 11ºC.  Gas comp (%) Hydrate comp (%) T (ºC) CO2 CH4  CO2 CH4 Hydrate amount (%) Sample A -10 56 44 65 35 82 -3 56 44 67 33 82 4 65 35 68 32 74 11 66 34 66 34 24 Sample B -10 64 36 83 17 58 -3 65 35 82 18 59 4 74 26 82 18 33 11 76 24 - - 0 * Compositions averaged from a minimum of 2 single pulse experiments. Composition standard deviation = 3 Table 3. Changes in phase composition due to hydrate dissociation.  During the dissociation process (from -10 to 11ºC) the gas phase becomes richer in CO2; this behavior is predictable from a dissociation of CO2 rich hydrate samples. The major change in composition occurred during the temperature ramp from -3 to 4ºC in both cases; during this decomposition step the total hydrate amount dropped from 82 to 74% and from 59 to 33% for samples A and B respectively. On the other hand the relative composition of the hydrate phase remained almost constant during the whole decomposition experiment (within the experimental error). This behavior is expected from a homogeneous binary gas hydrate sample, considering complete decomposition of unit cells.  Sample C decomposition results Sample C (in contrast with samples A and B) was rich in CH4 in both gas and hydrate phases. The decomposition experiment was carried out following the same procedure as above. The changes in composition of Sample C during the decomposition experiment from -20 up to 15ºC are summarized on Figure 3 and Table 4.  Data from Figure 3 shows no appreciable decomposition of the hydrate phase up to approximately -3 ºC; this observation implies that the sample was stable under these conditions and the temperature increase did not induce hydrate decomposition to that point.    0 0.2 0.4 0.6 0.8 1 -25 -15 -5 5 15 25 T (C) R el at iv eI in te ns ity  (a .u .)  . CO2 hydrate CH4 hydrate CO2 gas CH4 gas  Figure 3: Sample C composition change during dissociation experiment.  The temperature ramp from -3 to 4ºC caused a dramatic change on the CO2 content of the gas and hydrate phase. From Figure 3 it can be observed that the gas phase became richer in CO2 as the hydrate dissociated. However, the CH4 concentration did not show a significant change in either phase and remained almost constant during this decomposition step. The hydrate proceeded to decompose very rapidly upon further heating.  A simple interpretation of this behavior will be that a rich CO2 hydrate portion decomposes whereas the CH4 portion of the sample remains invariable. However this interpretation would contradict thermodynamics. Pure CO2 hydrate has a higher equilibrium temperature at the same pressure than pure CH4 gas hydrate; therefore, CO2 hydrate is not expected to decompose before CH4 hydrate does. This peculiar phenomena is explained in the following section   Gas comp (%) Hydrate comp (%)   T (ºC) CO2 CH4 CO2 CH4 Hydrate ratio CO2/ CH4 -10 20 80 45 55 0.82 -3 21 79 45 55 0.82 4 37 63 35 65 0.54 11 39 61 0 0 - * Compositions averaged from a minimum of 2 single pulse experiments. Composition standard deviation= 2 Table 4. Sample C changes in phase composition due to hydrate dissociation.    CH4 gas uptake across the ice point When the temperature of Sample C is increased, the concentration of the hydrate phase does not show any variation in the CH4 content up to temperature values of 4ºC. However, the CO2 content decreases in the hydrate phase when temperature is increased from -3 to 4ºC. The experimental evidence that elucidate this behavior is found in Figure 4.  The decomposition profile (i.e. changes of the components concentration at real time) for sample C during the temperature ramp from -3 to 4ºC is shown in Figure 4. Surprisingly, an abrupt increase on the CH4 hydrate concentration occurred immediately after the temperature was increased, reaching a maximum at approximately 90 s before it started to decompose. In addition, a sharp decrease in the CH4 gas intensity was observed indicating CH4 gas consumption. This unmistakable hydrate formation was a result of the reaction between the gas phase present in this closed system, and the water that becomes available from the partial hydrate dissociation. Also, due to the vicinity to the ice melting point a further water contribution is expected if unreacted ice was present in the sample  While CH4 hydrate formation was observed to occur when the temperature was increased from -3 to 4ºC, no formation of CO2 hydrate was observed. Under these conditions, the CO2 concentration in the gas phase increased smoothly as temperature increased suggesting that some hydrate containing CO2 was in fact decomposing.  0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 Time (s) R el at iv e In te ns ity  (a .u .) CH4 gas CH4 hydrate CO2 gas  Figure 4: Changes in composition during temperature ramp from -3 to 4 ºC  The rapid formation of a CH4 rich hydrate phase from water and CH4+CO2 gas mixture can be explained by the preferential methane-occupied 512 cage formation and subsequent CH4 hydrate growth at the very initial stage of hydrate formation. Similar observations had been reported in the literature [11].  Obviously the CH4 hydrate formation during the first 90 seconds of the experiments balance out the followed decomposition of the binary hydrate; as a result, a net change is not  observed in the CH4 content of the binary hydrate between the temperature values of -3 and 4ºC.  Results reproducibility In order to corroborate the above results, the analysis of sample C was repeated. After the total dissociation of the hydrate present in Sample C, the sealed sample was placed in a temperature controlled bath at 0ºC for a day and at -5ºC for 5 days to allow for hydrate reformation. Finally the sample was stored at - 20ºC for another 5 days before a repeat analysis. The initial gas phase as result of the total dissociation of Sample C was 39% of CO2. Due to the gas consumption during the reformation process, the gas phase composition changed, and the CO2 concentration in the gas phase dropped down to 20% (i.e. gas phase composition was not constant during hydrate formation). This reformed sample will be referred as Sample C’. Although the hydrate was reformed from an initial gas phase with 61% CH4 the resulting hydrate phase was CO2 rich as expected based on thermodynamic predictions.  Table 5 and Figure 5 summarize the data obtained during the decomposition process of Sample C’ (reformed Sample C).  The temperature ramps applied to Sample C’ were adjusted in order to be able to track the changes in composition more precisely.  In Figure 5 the changes in composition between each dissociation step of Sample C’ are illustrated. A minimum near 0ºC is observed for the CH4 concentration in the gas phase, with an increase of the CH4 content in the hydrate phase at this temperature. The changes in CO2 composition on the other hand, followed the expected decomposition trend; the CO2 gas concentration increased with temperature as the decomposition of the hydrate takes place.   Gas comp (%) Hydrate comp (%)  T (ºC) CO2 CH4 CO2 CH4 Cage occupancy ratio L/S for CH4 Hydrate ratio CO2 / CH4 -10 20 80 65 36 0.62 1.82 -5 21 79 63 38 0.65 1.67 -3 22 79 61 40 0.66 1.53 -1 28 72 57 44 0.63 1.30 1 30 71 58 42 0.60 1.38 5 33 68 34 66 0.54 0.52 11 36 65 - - - - 14 35 65 - - - - 16 35 65 - - - - * Compositions averaged from a minimum of 2 single pulse experiments; the maximum standard deviation in composition was 3% and in cage occupancy 0.03. Table 5. Changes in phase composition due to hydrate dissociation. Sample C’ from -10 up to 16ºC.   0 0.2 0.4 0.6 0.8 1 -15 -10 -5 0 5 10 15 20 T (C) R el at iv eI in te ns ity  (a .u .)   . CO2 gas CH4 gas CO2 hydrate CH4 hydrate  Figure 5: Changes in global composition during the decomposition experiment for Sample C’   These results corroborate the observations found during the decomposition of Sample C. A decrease in the CH4 gas content is observed in the vicinity of the ice melting point, corresponding to the increase of CH4 content in the hydrate phase. Again, a preferential CH4 gas uptake and consequent CH4 rich hydrate formation is observed.    Changes on hydrate composition during dissociation of Sample C’ Another interesting observation arises from the detailed analysis of the hydrate composition during the dissociation process. The relative CO2 hydrate composition decreases as the decomposition experiment proceeds. The hydrate concentration ratio, expressed as CO2/CH4 showed a decreasing trend in its value with successive increases in temperature. Such observations are not the most intuitive behavior for a decomposition process of a homogeneous hydrate sample; changes in the composition of the hydrate were not anticipated.  Because our decomposition experiments are carried out in a closed system, the pressure of the sample increases as the hydrate phase decomposes and releases the trapped gas, by effect of the rising temperature; the system conditions change during this dynamic process.  The change in hydrate ratio CO2/CH4 during the decomposition experiments is a phenomenon not expected. However a possible interpretation of these results is based on the fact that by changing the system conditions (pressure and temperature), the chemical potential of the hydrate phase must change. The constant variation of the conditions is the driving force for changes in the chemical potential of the hydrate phase; the change in the hydrate composition is the system response to such perturbation. Our results show that as the temperature and pressure of the system increases the hydrate phase becomes depleted in CO2 (Table 5).  Predictions of the equilibrium phase composition of a binary CH4+CO2 gas hydrate in equilibrium with a gas phase (80% of CH4 and 20% CO2) were done using the in-house software CSMGem [33]. The obtained predictions indicate that by increasing the temperature and pressure of the system, the calculated relative CO2 concentration in the hydrate phase decreases. Therefore, from a thermodynamic point of view, it is expected that in response to the pressure and temperature increase (as the hydrate decomposes) the system varies its concentration towards what the new conditions dictate. This behavior is shown in our results. The higher pressure and temperature favor a hydrate phase with lower CO2 content.  CONCLUSIONS This work reports results on thermally activated decomposition of CH4-CO2 sI hydrates at various concentrations by using time-resolved 13C MAS NMR spectroscopy. Phase concentrations were quantified during the decomposition experiments with single-pulse excitation experiments.  The gas release rate, as a result of hydrate decomposition, was found to increase as the temperature increases.  The free water phase that becomes available in the system when the temperature crosses the ice melting point, rapidly reacts with the CH4+CO2 gas mixture. The hydrate formed, in the time- scale of the experiment, is rich in CH4 (if not pure) as observed by the preferential CH4 gas uptake.  An unexpected decrease on the hydrate concentration ratio CO2/CH4 was found as the decomposition experiment proceeds (i.e. successive increases in temperature and pressure). Further experiments of the CH4+CO2 system and the C2H6+CO2 system are currently under way in order to validate these observations.  ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Science Foundation, research grant CTS- 0419204.  REFERENCES [1] Max MD, Natural gas hydrate in oceanic and permafrost enviroments. 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