J. Phys. Chem. B 2004, 108, 17591-17595
17591
Methane and Carbon Dioxide Hydrate Formation in Water Droplets: Spatially Resolved Measurements from Magnetic Resonance Microimaging Igor L. Moudrakovski, Graham E. McLaurin, Christopher I. Ratcliffe, and John A. Ripmeester* Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON, Canada K1A 0R6 ReceiVed: June 19, 2004; In Final Form: August 27, 2004
We have used 1H magnetic resonance microimaging to probe both methane and carbon dioxide hydrate formation processes inside dispersed water droplets. When bulk techniques such as gas uptake measurements are used for determining the kinetics of hydrate formation, these show a gradual conversion to hydrate, suggesting a relatively homogeneous process that might be modeled using a set of intrinsic kinetic parameters. The spatially resolved microimaging measurements show that in fact the conversion to hydrate is quite inhomogeneous, some drops converting quickly, others requiring hours or days. This indicates that the observation of gradual conversion in bulk samples only arises as a result of averaging over many local environments. Quantitative measurements of kinetic processes in subvolumes of a larger sample suggests that the smaller the volume observed, the more inhomogeneous the process appears to be. When hydratecoated water droplets in 3,5,5-trimethylpentane are converted to hydrate, there is evidence that nucleation can take place well away from the hydrate coating, with the hydrate sometimes growing in discrete steps before drops are completely converted. The results obtained indicate that in the quiescent systems studied here the definition of intrinsic kinetic parameters will be difficult, if possible at all, because of a stochastic component that competes with more gradual conversion processes.
Introduction Gas hydrates are of considerable interest from the point of view of global issues such as energy supply1 and climate change,2 as their great capacity to store gases presents a means to recover, transport, and store natural gas for energy use3 and a means of sequestering greenhouse gas CO2.4 Improved understanding of hydrate formation and decomposition will contribute to optimizing process control and efficiency, including the reduction of hydrate blockage hazards in pipelines.5 Most information on hydrate formation kinetics has come from measurements such as gas uptake6 and other bulk measurements such as diffraction,7 or direct visual observation of thin film growth8 and crystal morphology,9 and several models that account for the observed gradual conversion processes have been presented. The control of hydrate formation5 has a longstanding history dating back to the observation by Hammerschmidt10 that pipeline blockages were more likely to be caused by hydrate than by ice. More recently, gas hydrate formation, from water and methane or CO2, has received considerable attention for the transportation of natural gas or the sequestration of the green house gas CO2.3,4,11 In each case, water is converted to solid hydrate, and the intriguing question is how this process takes place, as in neither case is one phase soluble enough in the other to allow bulk conversion of water and gas to hydrate. Visual observation has shown that hydrates tend to nucleate at the gas-water interface,8,9,11 where the concentration of both components is sufficiently high, and that hydrate growth proceeds by forming a film at the interface. However, subsequent hydrate growth must involve transport of either water or gas through the film, and instances of both have been reported.8,9 * Author to whom correspondence should be addressed. E-mail:
[email protected].
Many of the details are not well understood, as there are a variety of factors that govern the details of hydrate formation. In this contribution, we present some initial results on the use of magnetic resonance microimaging to follow hydrate formation in dispersed small droplets. So far, there have been few applications of this technique to hydrate formation, although it holds the promise of probing inside the surface film.13 Here, we show that quantitative, spatially resolved kinetic information on hydrate formation becomes available by following the disappearance of the 1H spin density associated with liquid water. We show that there is a considerable randomness to nucleation and growth events that must be considered in developing molecular scale models for kinetics of hydrate formation processes, although this is not apparent from gas uptake data. The NMR microimaging approach should lend itself well to detailed quantitative studies of the many factors that affect hydrate formation. Experimental Section All 1H NMR microimaging experiments were performed on a Bruker DSX400 NMR instrument using a multislice spinecho pulse sequence with Gaussian selective pulses (Slice, Read, and Phase gradients were 43, 35, and 71 G/cm, respectively). In most experiments, three slices of 250-µm thickness with a separation of 1.5 mm were acquired simultaneously in planes perpendicular to the axis of the cell. The 192 × 192 acquisition matrix was extended to 256 × 256 for Fourier transformation. Typically, eight scans were accumulated to obtain good signalto-noise ratio. The experiments were carried out in a cell capable of handling pressures up to ∼350 bar connected to a highpressure system. The experimental arrangement is shown in Figure 1.
10.1021/jp0473220 CCC: $27.50 Published 2004 by the American Chemical Society Published on Web 10/13/2004
17592 J. Phys. Chem. B, Vol. 108, No. 45, 2004
Moudrakovski et al.
Figure 1. Experimental setup, showing (A) the high-pressure system feeding CH4 to the cell inside the magnet and (B) locations of the image slices.
Figure 2. A typical kinetic run on the formation of methane hydrate. The sample temperature and gas pressure over the sample are monitored as a function of time. The bath temperature is ramped from -20 °C up to ∼5 °C to initiate rapid hydrate formation after the cell is loaded with ice particles and pressurized with methane. The ice particles were sieved through standard mesh screens to select the 180-210-µm-size fraction.
Gas uptake measurements were made in a small (20-mL) stainless steel pressure cell fitted with a thermocouple that was immersed in the powdered ice used as starting material and a pressure transducer (Omega Engineering Inc.). After loading with crushed ice (sieved to select a size fraction), the sample cell was immersed in a programmable cold temperature bath at -20 °C and degassed. The experiment was initiated by exposing the ice to a pressure of methane gas at pressures up to 250 bar, and the temperature of the sample was ramped above the ice melting point. During the experiment, a multichannel data acquisition system (Cole-Parmer) was used to monitor the temperature of the sample, the cold bath temperature, and the pressure inside the cell. The degree of water conversion to hydrate was checked by measuring the volume of gas released upon hydrate decomposition. Results and Discussion Figure 2 shows a typical experimental run of hydrate formation where ice particles are exposed to a pressure of gas, followed by a ramping up of the temperature, meanwhile
monitoring the sample temperature and the pressure in the cell. At -20 °C, initial reaction rates are quite slow, and upon ramping the temperature of the cooling bath through the melting point, the ice melts (∼-1 °C, 130 bar), and the formation rate increases as shown by the pressure drop. At the same time, there is evolution of the heat of formation as water is converted to hydrate.5b Depending on the size of the ice particles, and the method of preparation of the ice, both P and T parameters inside the cell reach approximate steady-state values over a period of 12 h or so. Much the same results are observed for the reaction of ice with CO2 as followed with NMR spectroscopy when the signal amplitude of encaged CO2 is followed as a function of time.14 The picture that emerges from bulk or direct visual measurements is one of gradual transformation, that one might be tempted to fit by models involving intrinsic growth kinetics and limitations posed by mass and heat transfer. These, along with particle size and various additives, then define a set of control parameters that can be optimized for process control. Some insight into the details of the conversion process at the molecular level has come from NMR spectroscopy and microimaging. Previously, we have used 129Xe NMR spectroscopy to show15 that the initial contact of ice with gas produces a hydrate film on the surface of the ice particles after an induction time. This reaction proceeds quite rapidly until a hydrate layer forms that is thick enough to impede mass transport. Further conversion to hydrate then takes place quite slowly, with the bulk kinetics describable by a shrinking core model. If a sample of small hydrate-coated ice particles is heated from low temperatures through the ice melting point, 1H NMR microimaging has shown that the sample consists of intact isolated drops of water inside hydrate shells.13a Figure 3 shows the conversion of several approximately spherical ice particles of various sizes to methane hydrate under much the same conditions as those shown in Figure 2. After melting (t ) 0), the image is one of hydrate-coated water droplets that have a well-defined shape that is maintained throughout the entire timespan of the experiment. As time progresses over a 30 h period, areas turning from dark to light correspond to water converting to hydrate with the associated 1H spin density disappearing from the image. Difference images (right) allow clearer examination of the actual changes with time. It is quite remarkable that major changes, as indicated by the light spots in the difference images, are concentrated in small areas, whereas other regions change very slowly, if at all, over the
Hydrate Formation in Small Water Droplets
J. Phys. Chem. B, Vol. 108, No. 45, 2004 17593
Figure 3. 1H images of methane hydrate formation as function of time. Left: images for two slices; right: difference images; in both, white represents hydrate.
same time period. One can interpret such changes as a rather complete conversion of water to hydrate in individual droplets, or parts thereof. Apparently for samples such as these, even though the conversion appears homogeneous from bulk measurements, the reaction is spatially quite inhomogeneous: each drop appears to be a separate miniature reactor. One striking observation is that only in rare cases can we see an increase in the thickness of the hydrate shell coating the droplets. Such an increase in shell thickness would indicate that the reaction is limited by gas diffusion through the hydrate layer, after which the hydrate accumulates on the internal surface of the hydrate shell. In this scenario, the hydrate then would gradually fill all of the interior space inside the particle coating, with a gradual reduction of the conversion rate as the reaction progresses. More commonly, however, we observe quite a different situation, with hydrate forming not on the interior of the particle wall, but rather in the bulk of the enclosed water. The large droplet in slice 2 that converts in the first 3 h shows that there is a ring of water remaining between the original hydrate shell and the hydrate core, showing quite clearly that in this case most of the hydrate has grown from the inside out. It is important to recognize that the images we observe are actually 2D projections of the threedimensional space within selected slices. This reduction in dimensionality can produce certain problems in the interpretation of the experimental images. When a hydrate crystal grows from the edge of a particle (outside the image) through the plane of the image, it will appear as if the hydrate suddenly appears in the center of the particle. However, for some droplets one would expect that growth of a hydrate crystal from the edge through the center would be completely within the plane of the image. To verify this possibility, on several occasions we performed the experiments where the imaging slice was oriented vertically as well as horizontally. We did not see any images that would support the latter mechanism. These observations may well have implications for natural hydrate formation in sediment pores, as water in partially isolated pores may behave in a fashion similar to the hydrate-
Figure 4. 1H images of CO2 hydrate formation as a function of time. The images shown are for one of three slices, with the difference images indicated below. Top, hydrate is white; bottom, hydrate is blue.
coated droplets: as long as gas can diffuse into the pore water, hydrate growth is as likely to take place by renewed nucleation away from the hydrate-water interface as by growing out from a hydrate-water interface. The formation of CO2 hydrate from ice follows a similar pattern, as shown in Figure 4, where we follow hydrate formation over 26 h. Here, the hydrate-coated water droplets have formed from smaller ice particles which were obtained by spraying liquid water over liquid nitrogen and sieving the produced particles to give a reasonably uniform size range. Difference images show that in each time period several droplets convert rather suddenly to hydrate. The spatial inhomogeneity can be quantified by measuring the amount of liquid 1H spin density disappearing in selected portions of the image as a function of time, shown in Figure 5. In square 5 of Figure 5, conversion is very limited, whereas in square 2 it is essentially complete in the same time period. One can see, however, that the conversion rate aVeraged oVer the slice starts to approximate an exponential growth curve. These results show that rates measured for hydrate formation in bulk have little or no meaning in terms of intrinsic growth mechanisms.
17594 J. Phys. Chem. B, Vol. 108, No. 45, 2004
Moudrakovski et al.
Figure 5. Changes in 1H liquid density during the formation of CO2 hydrate in selected parts of the image (squares) with time. On the left, the images at the beginning and end of the run. On the right, the proton density variation with time is shown for each square. The intensities of the integrals are scaled to the integral intensity of the entire slice at the end of the run.
Figure 6. Time dependence (t in minutes) of methane hydrate formation for hydrate-coated water droplets in 3,5,5-trimethylpentaned18 (isooctane) in a single slice (slice 2), P ) 180 bar.
Another experiment was carried out to simulate the reaction more typical of pipeline conditions, especially those found in a “shut-in” pipeline. Figure 6 shows methane reacting with water droplets coated with a hydrocarbon liquidsisooctane, perdeuterated so that it does not give a signalswhich does not participate in hydrate formation. In this case, the methane gas is not in direct contact with the particle surface, and hydrate formation must take place between water and methane dissolved in the hydrocarbon. Again, a hydrate layer keeps the droplets from coalescing into a bulk aqueous phase. In the large drop a in images 1-3 of Figure 6, the hydrate appears to nucleate and converts the drop almost completely over a period of ∼10 h, whereas the large drop b beside it remains completely intact. Droplet a shows two quite distinct locations of the hydrate mass, the larger of which shows what appears to be well-defined crystal faces, suggesting that this may be a small single crystal. Somewhat later (image 4), the hydrate appears rather suddenly in drop b as well and then grows from its nucleation site toward the surface, completing the conversion by image 5. In another drop c, the hydrate again appears in the middle of a drop (image 6) but then shows no changes for almost 6 h before it starts to grow. Eventually, the drop is converted completely to hydrate in images 8 and 9. These images show that at apparently random times new hydrate appears, sometimes followed by a rapid growth spurt. The process inside the droplets must be diffusion-limited in that gas must diffuse into the water through the hydrate skin. A reasonable expectation is that hydrate would continue to grow on the inner surface of the hydrate skin toward the center, thus increasing its thickness. However, clearly this is a very slow process, as seen by the survival of some drops on a scale of a
day or so. The images do show that there is a sudden conversion of droplets at apparently random times, so there must be other mechanisms at work. The random times at which hydrates appear and grow suggest that hydrates nucleate again inside the droplets away from the hydrate-water interface. The subsequent growth at the new nucleation center may convert the droplet in part, or completely. The best explanation is that after gas diffuses through the hydrate skin, the gas dissolves. The gas can diffuse to the hydrate shell and at sufficiently high partial pressure can contribute to growth, or it can nucleate at a new center, growing at that point until the gas has been depleted. It appears that hydrate growth is sufficiently slow so that when the gas concentration builds up in the liquid, nucleation at centers remote from the interface competes quite effectively with slow growth at the hydrate-water interface. The NMR microimaging results reported here show that neither hydrate growth in thin films nor the techniques used to study bulk hydrate conversion give a complete picture of how hydrate formation takes place in dispersed systems. In addition to the mass and heat transfer processes which are observed to occur in thin films, which have been carried over to models of bulk hydrate formation, account must be taken of a stochastic component that depends on gas diffusion-nucleation-growthgas depletion cycles. The concept of hydrate nucleation as a stochastic process, leading to considerable difficulty in quantifying hydrate nucleation and growth, has been discussed previously by several authors.16 The results presented also have implications for the nature of the hydrate formed. For instance, it is usually assumed that hydrates will have a composition that reflects the P, T conditions in the phase diagram at which the sample was prepared. However, this is only true if processes lead to an equilibrium product after nucleation. Since crystallization is a nonequilibrium process,16c growth by nucleation-growth-depletion cycles will not necessarily lead to hydrates with the equilibrium composition. As a matter of fact, there is now considerable evidence that at early stages of hydrate formation the product may include kinetically controlled structures rather than the equilibrium form.15,17 Another factor that needs to be considered is the possible presence of fugacity variations with time inside the droplets, as the fugacity is likely to be different before and after a nucleation-growth cycle inside a droplet. Therefore, laboratory-prepared hydrates may well be different from natural
Hydrate Formation in Small Water Droplets hydrates that have had time to equilibrate for many years after formation. The time scale of conversion of an as-produced hydrate to an equilibrium hydrate remains unknown. Conclusions The results presented on the growth of hydrates in droplets suggest that very different approaches may be used to control hydrate formation or prevention. In strict diffusion-limited growth, increased reaction rates are likely by reducing the surface-to-volume ratio of the particles, and this has indeed been observed.14 In larger drops, more complete conversion will be quite slow by the first process, so there will be advantages to increasing nucleation events inside particles, perhaps by adding suitable hydrate nucleators. However, on observing the time scale of these events in the imaging experiments, it is clear that controlling nucleation could be an alternative strategy to preventing hydrate growth/agglomeration after nucleation.18 Magnetic resonance microimaging is likely to be the tool of choice for evaluating process control additives and their mechanisms of interaction with hydrate. Acknowledgment. We acknowledge partial support of this work by the Institute of Applied Energy (Japan). References and Notes (1) Kleinberg, R.; Brewer, P. Am. Sci. 2001 89, 244-51. (2) Xu, W.; Lowell, R. P.; Peltzer, E. T. J. Geophys. Res. B 2001, 106, 26413-24. Bains, S.; Corfield, R. M.; Norris, R. D. Science 1999, 285, 724-6. (3) Gudmundsson, J.-S.; Parlaktuna, M.; Khokhar, A. A. SPE Prod. Facil. 1994, 9, 69-73. (4) Yamasaki, A.; Wakatsuki, M.; Teng, H.; Yanagisawa, Y.; Yamada, K. Energy-Int. J. 2000, 25, 85-96.
J. Phys. Chem. B, Vol. 108, No. 45, 2004 17595 (5) (a) Sloan, E. D., Jr. Hydrate Engineering; Monograph 21, SPE: Richardson, TX, 2000. (b) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gas, 2nd ed.; Marcel Dekker: New York, 1998. (6) Bishnoi, P. R.; Natarajan, V.; Kalogerakis, N. Ann. N.Y. Acad. Sci. 1994, 715, 311. Lekvam, K.; Ruoff, P. J. Cryst. Growth 1997, 179, 618. Stern, L. A.; Kirby, S. H.; Durham, W. B. Science 1996, 273, 1843. (7) Wang, X.; Schultz, A. J.; Halpern, Y. J. Phys. Chem. A 2002, 106, 7304-09. (8) Mori, Y. H.; Mochizuki, T. Energy ConVers. Manage. 1998, 39, 567. Uchida, T.; Ebinuma, T.; Kawabata, J.; Narita, H. J. Cryst. Growth 1999, 204, 348-356. Freer, E. M.; Selim, M. S.; Sloan, E. D., Jr. Fluid Phase Equilib. 2001, 185, 65-75. Uchida, T.; Ikeda, I. Y.; Takeya, S.; Ebinuma, T.; Nagao, J.; Narita, H. J. Cryst. Growth 2002, 237, 383. Ito, Y.; Kamakura, R.; Obi, S.; Mori, Y. H. Chem. Eng. Sci. 2003, 58, 107114. (9) Servio, P.; Englezos, P. AIChE J. 2003, 49, 269-76. Knight, C. A.; Rider, K. Philos. Mag. A 2002, 82, 1609-33. (10) Hammerschmidt, E. G. Oil Gas J. 1939, 37, 66. (11) Lee, K. M.; Lee, H.; Lee. J.; Kang, J. S. Geophys. Res. Lett. 2002, 29, 21. (12) Henning, R. W.; Schultz, A. J.; Thieu, V.; Halpern, Y. J. Phys. Chem. A 2000, 104, 5066-5071L. (13) (a) Moudrakovski, I. L.; Ratcliffe, C. I.; McLaurin, G. E.; Simard B.; Ripmeester, J. A. J. Phys. Chem. A 1999, 103, 4969-72. (b) Mork, M.; Scei, G.; Larsen, R. Ann. N.Y. Acad. Sci. 2000, 912, 897. (c) Kuwano, K.; Ogawa, K.; Okazaki, K. Ann. N.Y. Acad. Sci. 2000, 912, 246. (d) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Proceedings of the Fourth International Conference on Gas Hydrates; Yokohama, Japan, May 19-23, 2002; p 444. (14) Lee, H.; Seo, Y.; Seo, Y.-T.; Moudrakovski, I. L.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2003, 42, 5048-50. (15) Moudrakovski, I. L.; Sanchez, A. A.; Ratcliffe, C. I.; Ripmeester, J. A. J. Phys. Chem. 2001, 105, 12338-47. (16) (a) Bishnoi, P. R.; Natarajan, V. Fluid Phase Equilib. 1996, 117, 168-177. (b) Zatsepina, O. Y.; Buffett, B. A. Fluid Phase Equilib. 2002, 2, 263-75. (c) Mullin, J. W. Crystallization, 3rd ed.; ButterworthHeineman: Oxford, 1993. (17) Staykova, D. K.; Kuhs, W. F.; Salamantin, A. N.; Hansen, T. J. Phys. Chem. B 2003, 107, 10299-10311. Schicks, J. M.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2004, 43, 3310. (18) Mehta, A, P.; Hebert, P. B.; Cadena, H. R.; Weatherman, J. P. SPE Prod. Facil. 2003, 18, 73.
