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Reply to “Comments on ‘Anomalous Preservation of Pure Methane Hydrate at 1 atm’” L. A. Stern,*,† S. Circone,† S. H. Kirby,† and W. B. Durham‡ U.S. Geological SurVey, 345 Middlefield Rd, MS/ 977, Menlo Park, California 94025, and U.C. Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: June 8, 2001 We are pleased to receive such prompt and considered commentary on our recent findings1 regarding methane hydrate “anomalous preservation”. We agree that future studies of gas hydrate decomposition may well benefit from further consideration of the rich literature available on the kinetics of nonhydrate, endothermic decomposition reactions. Such a review was beyond the scope of our report because of the necessary inclusion of the numerous technical, experimental, apparatus, and measurement details critical for the full understanding and/ or replication of our test results. We therefore appreciate Wilder and Smith providing their succinct review of several select physical chemistry and kinetic effects that may have relevance to certain types of gas hydrate dissociation pathways. They cite experimental systems under vacuum in which substantial thermal gradients exist or are created between the sample interior and its surroundings. Those systems are possibly relevant to the tests in our study in which rapid dissociation occurred, but differ from the nearly isothermal state of “anomalously preserved” methane hydrate. Wilder and Smith also consider certain effects influenced by other factors (such as mass flux within the system or heat transfer to the reaction boundary, for example) that were not discussed in our report. Determination of rate laws that describe specific reaction pathways of methane hydrate dissociation at 1 atm was the initial goal of our research, given the scarcity of previous reports on direct and accurate measurement of dissociation rates and gas evolution from well-characterized gas hydrate test material. The temperature-dependent phenomenon of anomalously reduced rates of dissociation that we described, however, suggests that while the general reaction of methane hydrate dissociating to H2O ice + CH4 gas occurs at all temperatures above 195 K at 1 atm, the specific reaction pathway (and/or mechanisms involved) may in fact change with temperature. As we stated in our report, further work is needed to isolate and identify the reaction mechanism(s) responsible for anomalous preservation. The thermal state of sample interiors and surroundings, however, was carefully and continually monitored throughout all experiments. Here, we briefly review the experimental parameters of our decomposition and preservation experiments to clarify our results regarding the development of thermal gradients (or temperature differences) within certain tests and to compare those findings with the thermally sensitive effects discussed in the Wilder and Smith comment. Such clarification should prove helpful in identifying the physical chemistry and/or kinetic considerations that govern anomalous preservation behavior. Thermal Parameters. Wilder and Smith write that “Work on the Smith-Topley effect may be of direct relevance to the work of Stern et al., in which the large temperature differences † ‡
U.S. Geological Survey. U.C. Lawrence Livermore National Laboratory.
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(up to 30 K) between the sample and surroundings would certainly have resulted in steep thermal gradients in the gas near the hydrate surface.” In fact, appreciable thermal differences between sample and surroundings were only measured in our tests in which methane hydrate dissociated rapidly, that is, at rates >100 mL/min. (For context, test specimens in our study were typically 30 g cylinders of porous, polycrystalline methane hydrate with composition CH4‚5.89H2O. These samples release nearly 6 L of methane at STP conditions; see ref 1 and 2 for further description.) Such tests include those in which methane hydrate was destabilized by either progressive warming above its equilibrium dissociation temperature of 194 K at 1 atm (“temperature ramping” tests as shown in Figure 2 of ref 1) and those in which methane hydrate was destabilized by rapid reduction of pore pressure (“pressure-release” tests) over certain temperature ranges (210-240 K, and >272 K; see Figures 3A and 4 in ref 1). In all such tests, temperature differences of up to 30 K developed when the rate of the (endothermic) dissociation reaction was sufficiently high to depress the sample temperatures well below those maintained by the external fluid bath (see Figure 1A,B). In contrast, pressure-release tests conducted at temperatures just above the 1-atm dissociation temperature (i.e., 195-205 K tests), as well as those conducted within the anomalous preservation regime (245-270 K), released methane at markedly slower rates ranging from 0.07 to 30 mL/min. These samples exhibited no appreciable development of thermal differences between their interior and surroundings (Figure 1C,D). Anomalously preserved samples typically exhibit a brief temperature excursion (adiabatic cooling) immediately following the porepressure venting, but sample temperature then reequilibrates with the external fluid bath temperature within several minutes (Figure 1C,D). A small but rapid pulse of gas is expelled from each sample during this event (Figure 1D and inset). In comparison, nearly complete dissociation of samples occurs over this same time interval in pressure-release tests conducted at just several degrees below the anomalous preservation window (Figure 1B; see also Figure 4 of ref 1). The rapid thermal rebound of “preserved” samples, consequently, is reflective of the highly suppressed rates of dissociation that can be induced by pressure-release treatment in this specific thermal regime. As shown in Figure 4 in ref 1, these rates are orders of magnitude slower than those predicted by extrapolation of rates from neighboring regimes. The “preservation” portion of each experiment was then ended by actively heating the external bath temperature (and hence the sample) through 273 K. During this heating stage, sample temperature remained buffered at the ice point (thus creating a temperature difference) until all dissociation was complete and the ice product fully melted (Figure 1D, inset). Thermal gradients and heat flow rates into the samples are therefore important considerations under conditions that produce active dissociation but are less relevant to anomalous preservation behavior. The Smith-Topley Effect. The systems and conditions required to produce the Smith-Topley effect described in the Comment and in the cited literature do not appear to us to be applicable to the phenomenon of anomalous preservation. While this effect has been observed for the dehydration of salt hydrates and the evaporation of water and ethanol, it is only observed at near vacuum pressures of less than 0.013 MPa. Furthermore, it shows an anomalous increase in dissociation (or evaporation)
This article not subject to U.S. Copyright. Published 2002 by the American Chemical Society Published on Web 12/05/2001
Comments
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Figure 1. Differences between internal sample temperatures (Tint), surrounding fluid bath temperatures (Text), and rates of gas evolution for methane hydrate destabilized by rapid depressurization at 239 K (A, B), compared to 267 K (C, D). The 239 K test shows the high rate of dissociation that attends pressure-release at large thermal oversteps of the methane hydrate equilibrium curve (see also Figure 4 in ref 1). Over 90% of the sample dissociated within several minutes of the pore pressure release (B). An initial thermal gradient between the internal sample temperature (Tint) and the surrounding fluid bath (Text) results from the adiabatic temperature drop accompanying depressurization, which is then augmented by the high rate and extent of the endothermic dissociation reaction (B, inset). Following the main dissociation event and thermal reequilibration of the system to Text, samples are heated through 273 K during which all remaining methane is released. In contrast, samples destabilized by rapid depressurization in the “anomalous preservation” regime (C, D) exhibit only minor and short-lived temperature gradient effects accompanying the pressure-release treatment. Dissociation rates then rapidly decay, resulting in both the preservation of the material and the rapid reequilibration of Tint and Text. (C and D inset). (The small fluctuations seen in the Tint data in panel C are the result of thermal fluctuations in the thermocouple ice-junction bath, caused by its necessary replenishment with ice on a daily basis.) Heating the preserved (or metastable) hydrate through 273 K (shown at about 165 h in panels C and D) then releases the large volume of remaining gas from the sample. As expected, thermal gradients between Tint and Text then develop as the hydrate rapidly dissociates and the resulting ice product fully melts (D, inset).
rates with increasing pressure or temperature, the opposite effect of the reduced rates that we observe in the anomalous preservation regime. Ice Shielding Effects. Our report included considerable discussion of gas hydrate “self” preservation effects observed in other studies (see subsection “Comparisons to Other Reportings of Preservation” in ref 1 and refs 22-28 in ref 1), including the ice-shielding mechanisms discussed by Davidson et al. and reiterated by Wilder and Smith. As we discussed in ref 1, this is likely the primary mechanism for preservation of small amounts of residual hydrate observed in all temperature-ramping tests, as well as in those pressure-release tests conducted at 195240 K. The ice-shielding hypothesis for these specific cases is further substantiated by the observation that warming such samples into the “premelting” zone of ice increases the release rate of the residual gas within them (shown in the Figure 2A inset of ref 1). The upper extent of the “premelting” zone, however, includes the specific temperature range at which we demonstrated the most successful preservation of methane hydrate by pressure release in the “anomalous preservation” regime (compare Figure 2A inset in ref 1 with Figure 4 in ref 1). We therefore speculated that while preservation of the residual hydrate in both the ramping tests and the lowtemperature depressurization tests is related to the progressive ice encapsulation that develops as hydrate grains dissociate along their surface, “anomalous preservation” appears to be caused by a different process or mechanism.
Furthermore, the methane pressure required to stabilize methane hydrate in the warm-temperature anomalous preservation regime is more than double that required to stabilize hydrate in the lower-temperature regime. This fact argues strongly against a simple ice-shielding effect providing the primary preservation mechanism in the anomalous preservation region for several related reasons. First, the small amount of ice that forms in the anomalously preserved samples (95 vol % of the grain is ice surrounding a small hydrate core. It is difficult to conceptualize how such thin skins of ice could sustain the nearly 2.5 MPa of methane pressure needed to stabilize or retain large quantities of methane hydrate at 267-270 K. Second, ordinary hexagonal water ice is mechanically incapable of containing a sufficiently high pressure of free methane gas to stabilize methane hydrate at temperatures within the anomalous preservation regime. The maximum pressure that an ice “pressure vessel” of any wall thickness or configuration can contain is exactly its flow strength,3 which is well below the equilibrium methane pressure for hydrate stability at the upper reaches of the anomalous preservation regime. The progressively increasing release of residual hydrate in temperature-ramping tests during slow heating through 260 to 273 K (Figure 2A inset in ref 1), for instance, may likely be due to crossing the thermal
230 J. Phys. Chem. B, Vol. 106, No. 1, 2002 threshold at which the flow strength of ice becomes insufficient to stabilize methane hydrate, even though the samples contain merely 3-5 vol % hydrate by that stage. It is conceivable, however, that the low solubility of methane in ice is sufficient to maintain a high methane fugacity around hydrate cores if macroscopic volumes of free gas do not exist. Ice is “tougher” (less prone to fracture) at warmer temperature, even though it is stronger at lower temperatures.3 Accordingly, it could be argued that ice may somehow provide more effective shielding capabilities at warmer temperatures. However, on the basis of our own knowledge and experience with ice rheology, we do not believe that the ice barrier theory can readily explain the well-defined but extremely nonlinear temperature dependence of methane hydrate dissociation rates at 0.1 MPa. Pore Pressure Monitoring. In our study, all efforts were made to ensure that any gas evolving from a sample or collecting in the sample’s pore space had free access to the flow meter and gas collection apparatus. We see no likely manner by which local pore pressures could develop within the anomalously preserved test samples and effectively stabilize large amounts of methane hydrate at temperatures approaching 70 degrees above its (nominal) 1-atm dissociation temperature. The possibility of adsorbed methane creating locally elevated pressures that might cause or contribute to anomalous preservation, for instance, was tested for and ruled out (see ref 1 and ref 16 therein). Furthermore, all samples were very porous as synthesized and “preserved” (29% to >40% porosity), and subsequent dissociation simply serves to increase pore volume (and presumably permeability) because of the accompanying volume reduction of the solid H2O phase. Pore pressure communication was also routinely demonstrated by the immediate adiabatic cooling of pore gas measured inside all samples when subjected to the pressure-release treatment. In several preservation tests, samples were periodically isolated from the flow meter by valve closure and then re-opened to check specifically for the expected pulse of gas predicted to evolve from the sample during the interim. In all cases, the predicted mass of gas flowed quickly into the flow meter/gas collection apparatus. We note that our apparatus operates with high precision over at least the range
Comments of 0.07-3000 mL/min with resolution to (0.01 mL/min at the slowest rates and measures total methane gas mass with accuracy better than 1%. (The reader is referred to endnotes 14 and 16 of ref 1 for related discussion and to ref 2 for further description of the flow meter apparatus, experimental setup, and calibrations.) We also note that no evidence for a temporal delay between the initiation of hydrate dissociation (as indicated by thermal anomalies) and gas collection in the flow meter was ever observed in any of the experiments. When heating the samples, the cessation of gas flow from each sample coincided with the sample temperature reaching the ice melting point, indicating that free flow occurs between gas-filled pores through the samples to the flow meter. Finally, we wish to re-emphasize that all samples subjected to rapid depressurization at isothermally held external test temperatures between 195 and 240 K or above 271 K behaved as would be expected, with dissociation rates increasing monotonically with increasing thermal overstepping of the 1-atm dissociation temperature (see Figure 4 in ref 1). Likewise, all samples depressurized in the anomalous preservation regime of 245-270 K exhibited dramatically lower rates of dissociation (see time scale at right in Figure 4 in ref 1). The exceptional reproducibility of all tests in this study, combined with the sharp, systematic, and reproducible transitions at both the lower and upper extent of the anomalous preservation regime, suggest that the changes in dissociation behavior are affected by a thermally controlled mechanism, process, or structural change rather than local pore pressure variations, because all depressurization tests were otherwise set up and conducted identically. References and Notes (1) Stern, L. A.; Circone, S.; Kirby, S. H.; Durham, W. B. Anomalous Preservation of Methane Hydrate at 1 atm. J. Phys. Chem. B 2001, 105 (9), pp 1756-1762. (2) Circone, S.; Kirby, S. H.; Pinkston, J. Pl.; Stern, L. A. Measurement of gas yields and flow rates using a custom flowmeter. ReV. Sci. Instrum. 2001, 72(6), 2709-2716. (3) Durham, W. B.; Stern, L. A. Rheological properties of water ice - Applications to satellites of the outer planets. Annu. ReV. Earth Planet. Sci. 2001, 29, 295-330.
