J. Phys. Chem. B 2006, 110, 25803-25809
25803
Hydrate Kinetics Study in the Presence of Nonaqueous Liquid by Nuclear Magnetic Resonance Spectroscopy and Imaging Robin Susilo,†,‡ Igor L. Moudrakovski,† John A. Ripmeester,*,† and Peter Englezos‡ Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada, and Department of Chemical & Biological Engineering, UniVersity of British Columbia, VancouVer, British Columbia, Canada ReceiVed: March 30, 2006; In Final Form: July 21, 2006
The dynamics of methane hydrate growth and decomposition were studied by nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI). Three well-known large molecule guest substances (LMGS) were used as structure H hydrate formers: 2,2-dimethylbutane (NH), methylcyclohexane (MCH), tert-butyl methyl ether (TBME). In addition, the impact of a non-hydrate former (n-heptane/nC7) was studied. The methane diffusion and hydrate growth were monitored by recording the 2H NMR spectra at 253 K and ∼4.5 MPa for 20 h. The results revealed that methane diffuses faster in TBME and NH, slower in nC7, and slowest in MCH. The TBME system gives the fastest hydrate formation kinetics followed by NH, MCH, and nC7. The conversion of water into hydrate was also observed. The imaging study showed that TBME has a strong affinity toward ice, which is not the case for the NH and MCH systems. The degree of ice packing was also found to affect the LMGS distribution between ice particles. Highly packed ice increases the mass transfer resistance and hence limits the contact between LMGS and ice. It was also found that “temperature ramping” above the ice point improves the conversion significantly. Finally, hydrates were found to dissociate quickly within the first hour at atmospheric pressure and subsequently at a much slower rate. Methane dissolved in LMGS was also seen. The residual methane in hydrate phase and dissolved in LMGS phase explain the faster kinetics during hydrate re-formation.
Introduction Gas hydrates are crystalline materials that occur naturally or that may form during oil and gas operations. Consequently, gas hydrate occurrences pose safety and operational concerns in the oil and gas industry.1 Natural gas hydrates are also viewed as a possible abundant energy resource for the future2-4 and offer the possibility of developing innovative clean energy technology.5,6 Finally, gas hydrates in the earth have been linked to past global climate changes and may well be involved in future global environmental changes.7-10 Knowledge of the thermodynamics and kinetics of gas hydrates is important for the safe and economic design of hydrate-based processes because they provide the fundamentals on hydrate formation conditions and reaction rates at which the phase transformation occurs. Hydrates may crystallize in three distinct structures: cubic structure I (sI), cubic structure II (sII), and hexagonal structure H (sH). Both cubic sI and sII hydrates have two cages, small (512) and large (51262 or 51264), but only one molecule is enough to stabilize the crystal. However sH hydrate has three different cages: small (512), medium (435663), and large (51268). The large cage is generally occupied by a large molecule but the presence of a “help gas” like methane is required to fill the other two cages and maintain the crystal stability.11 The large molecule is also referred as LMGS. There are more than 20 LMGS identified;12 however, the number keeps on increasing as more molecules are being studied. The latest discovered LMGS are * To whom correspondence should
[email protected]. † National Research Council Canada. ‡ University of British Columbia.
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n-pentane and n-hexane.13 Most LMGS are non-water-soluble compounds under hydrate formation conditions so that a complex system is encountered. A three-phase system that consists of gas, water, and nonaqueous liquid (NAL) phase is typically observed during sH hydrate formation experiments. Hence additional mass transfer resistances have to be considered. The resistances include gas diffusion into the liquid phase and the contact between each phase that may play an important role in hydrate kinetics, especially in a nonstirred system. Gas hydrate kinetics studies have been carried out primarily through gas uptake measurements during which special care must be paid to the hydrodynamic conditions in the hydrate formation vessel.14 Lee et al.14 used methane as guest substance along with neohexane (NH), tert-butyl methyl ether (TBME), and methylcyclohexane (MCH) as the LMGS to form sH hydrate. The results indicated that the rate of hydrate formation depends on the magnitude of the driving force and the type of LMGS. The driving force was defined as the difference between the experimental hydrate formation pressures with the equilibrium pressure at a given temperature. However, in such an experiment no additional information can be gained. It has been recently shown that the conversion to hydrate is quite an inhomogeneous process and that the observation of gradual conversion in bulk samples arises only as a result of averaging over many local environments.15 Consequently, the kinetics through gas uptake measurements is in reality “average kinetics” over the whole sample. The question is how they are related to kinetics obtained with microscopic techniques like NMR measurements corresponding to the bulk environment; more specifically, how the interaction between each molecule in each phase may explain the kinetics results obtained from
10.1021/jp061980y CCC: $33.50 © 2006 American Chemical Society Published on Web 12/07/2006
25804 J. Phys. Chem. B, Vol. 110, No. 51, 2006 bulk measurements. Identification of factors that may influence hydrate kinetics allows better strategy for designing and developing hydrate-based technology. The objective of the present study is to monitor the kinetics of sH methane hydrate formation involving the above-mentioned LMGS at the microscopic level. This will enable a direct comparison with the previously reported macroscopic kinetic data from Lee et al.14 as well as from Mori and co-workers.16,17 It is noted that microscopic techniques to study hydrates have been carried out but were restricted to simpler gas-liquid water or gas-ice systems using Raman or NMR spectroscopy.18-20 Hydrate kinetic studies reported in the literature using NMR spectroscopy have been limited to 129Xe or 13C NMR. Recently 19F NMR was also employed to study the guest dynamics in clathrates hydrate.21 The low sensitivity of xenon, due mainly to long spin-lattice relaxation times, can be overcome by using hyperpolarized xenon.22 Spectroscopy with enriched 13C was used in the kinetic study of methane-propane hydrate.19 In this study, proton (1H) and deuterium (2H) NMR spectra were employed to distinguish the spectral contributions from LMGS and methane. Protons are abundant, have a high gyromagnetic ratio, and usually have short relaxation times so that short acquisition times can be achieved. Due to the small chemical shift range of protons (between -1 and 12 ppm), the identification of components in a multicomponent system is generally difficult unless the signals are sharp so that chemically inequivalent protons can be resolved. Hence deuterated methane was used to separate the signal of methane from that of the nonaqueous liquid. The quantification of the broad contributions from the solid phase and the sharp contributions from the gas/ liquid phase is then achievable without interference from other components. This work will be the first attempt to monitor structure H hydrate growth and decomposition kinetics by use of NMR spectroscopy. In addition, the effect of a nonaqueous liquid (n-heptane) that does not participate in hydrate formation will be studied. Ice is chosen instead of liquid water to significantly reduce the waiting period due to induction time. Hydrate formed from liquid water without mixing requires a very long time for nucleation. The rate of methane hydrate growth, decomposition, and diffusion in NAL were monitored by deuterium (2H) nuclear magnetic resonance (NMR) spectroscopy. The distribution of LMGS between ice particles was also observed by using 1H microimaging NMR. Experimental Section The kinetics experiments were performed on a Bruker DSX 400 NMR instrument at the NMR facility located at the National Research Council of Canada (NRC), Ottawa, Ontario, Canada. A custom-designed static sample 5 mm NMR probe for handling pressures up to ∼350 bar was employed. The probe is connected to a gas feed line equipped with a pressure gauge. The temperature during the experiment was controlled by a Bruker BVT 3000 temperature control unit. The 1H and 2H NMR spectra were calibrated with a measured quantity of a mixture of H2O/D2O of known concentration at 298 K. The chemical shift of water was assigned to be 4.7 ppm. The probe was cooled before the ice and LMGS were loaded. Hydrates were formed from ∼0.15 g of fresh ground ice particles loaded and packed into the NMR cell plus excess LMGS. The zero time of the measurements was set when the cell was pressurized with 99% deuterated methane (CD4). 1H and 2H NMR spectra were recorded every 5 min for 20 h. Spectra were acquired with 16 scans and 3 s delay time.
Susilo et al.
Figure 1. Deuterium (2H) NMR spectra evolution of ice + neohexane + CD4 at 253 K and ∼4.5 MPa for 20 h. (Inset) Acquired spectra at 0, 0.5, 1, 2, 3, 4, 6, 10, 15, and 20 h. The broad peak corresponds to methane in hydrate phase and the sharp peak corresponds to methane in gas and dissolved in nonaqueous liquid phases. The dashed line helps to visualize the hydrate growth by looking at the increased intensity of the broad peak.
Spectra obtained from the gas and liquid phases appeared as sharp Lorentzian lines, whereas those from the solid phase appeared as broad Gaussian lines; hence the amount of methane in the gas/liquid phase and the solid phase can be quantified. Unfortunately, 1H NMR spectra are not able to distinguish the broad peaks from ice and LMGS in the solid hydrate phase as the nuclear dipolar broadening is much larger than the chemical shift range for 1H. However the 2H NMR spectra took the form of broad and sharp peaks, which correspond to methane in the gas/dissolved in LMGS and the hydrate phase. The methane in the gas and dissolved in LMGS appear as two sharp peaks which are very close to each other and hence are considered as one sharp peak. The spectra were then analyzed with dmfit, a program provided by Massiot et al.23 for analysis of solid-state NMR spectra. The distribution of LMGS in between ice particles was also observed by the 1H microimaging NMR technique. This experiment was performed on a Bruker Avance 200 spectrometer. Multislice spin-echo pulse sequences with Gaussian selective pulses were employed. In most experiments, three slices of 500- µm thickness with a separation of 2 mm were acquired simultaneously in planes parallel to the axis of the cell. The 128 × 128 acquisition matrix with eight scans was accumulated to obtain good signal-to-noise ratio. The experiments were carried out in a cell capable of handling pressures up to ∼350 bar connected to a high-pressure handling system. The experimental arrangement is described in detail elsewhere.15 Results and Discussion The evolution of a typical 2H NMR spectrum for the ice + neohexane (NH) + CD4 system during hydrate formation at 253 K and ∼4.5 MPa over a 20 h period is shown in Figure 1. The spectra appear as a sequence of sharp Lorentzian lines after the cell is pressurized at time t ) 0, which corresponds to deuterated methane in the gas phase and dissolved in the
Hydrate Kinetics in Nonaqueous Liquid
Figure 2. Normalized intensity of methane growth in the solid hydrate phase at 253 K and ∼4.5 MPa.
Figure 3. Normalized intensity evolution of methane in the gas and dissolved in nonaqueous liquid phase at 253 K and ∼4.5 MPa.
nonaqueous liquid (NAL) phase. After the induction period, broad lines from methane-d4 trapped in the hydrate cages appear as shoulders on the sharp lines as ice is transformed into the solid hydrate phase. Due to the broad spectral contributions of the solid phase, the amplitude of the hydrate phase signal is much lower than that of the gas/dissolved phase. Hence the induction time and the early stages of hydrate growth are not so clearly visible in Figure 1, but the broad peak becomes evident as more hydrate is formed with time. Ten spectra were selected and shown as the inset in Figure 1 to better visualize the growth of broad peaks due to hydrate formation. The areas that correspond to the quantity of methane in the gas + dissolved phases and the solid phase are then separated and integrated by use of the dmfit program. The integrated intensity is then normalized to the maximum intensity for comparison purposes and is shown in Figure 2 for the hydrate phase and Figure 3 for the gas + dissolved phase. Methane Growth in Hydrate Phase. As seen in Figure 2, the amount of methane in the hydrate phase increases during the 20 h period. An induction time was observed for NH (
