Self-Preservation at Low Pressures of Methane Hydrates with Various

Metastable self-preservation of methane hydrates at atmospheric pressure has been reported by a number of researchers. It is suggested that this prope...
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Ind. Eng. Chem. Res. 2004, 43, 6616-6621

Self-Preservation at Low Pressures of Methane Hydrates with Various Gas Contents Carlo Giavarini* and Filippo Maccioni Chemical Engineering Department, University of Rome “La Sapienza”, via Eudossiana 18, 00184 Rome, Italy

Metastable self-preservation of methane hydrates at atmospheric pressure has been reported by a number of researchers. It is suggested that this property could be utilized to transport stranded natural gas at higher temperatures and lower pressures compared to conventional liquefied natural gas methods. In previous papers, a number of factors, such as the influence of the preservation pressure (in addition to the atmospheric pressure) and of the gas concentration in the hydrate, have not been considered. The experimental work, carried out with the help of a reaction calorimeter on samples with different gas (and ice) contents, has revealed that the dissociation rate of the hydrates with high gas contents is higher than that of hydrates with less gas. The influence of pressure and temperature in the ranges of 0.1-0.3 MPa and -4 to -1 °C, respectively, has also been studied, together with the effect of previous subcooling. For the practical and economical application of the self-preservation property to gas storage and transportation, the gas hydrates should be prepared at relatively moderate conditions and stored at pressures (e.g., 0.3 MPa) higher than the atmospheric pressure. Under such conditions, a further cooling of the obtained hydrate is not necessary. 1. Background and Motivation Only a small proportion (about 23%) of the world gas production is internationally traded because of the technological challenge of transporting gas. A number of alternative methods have been considered in order to increase the energy density of natural gas and facilitate its storage and transportation (Table 1).1-4 Compressed (CNG) and liquefied (LNG) natural gas are solutions that are used worldwide; gas to liquid (GTL), that is, the chemical conversion into liquids, is an emerging technology; adsorbed natural gas (ANG) is another method recently proposed.4 All of these methods have some disadvantages: cost of compression and wall thickness of containers and pipes (CNG); high investment, cost of liquefaction, and need for special materials (LNG); dependence on the adsorbent characteristics (ANG) and unsuitability for high volume transport (e.g., shipping); economy or lack of a mature technology (GTL). Another possibility suggested in recent years is the storage and transport of natural gas in the form of hydrate.3,5-8 Gas hydrates are crystalline solids formed from the association of water and gas under conditions of relatively high pressure and low temperature;9,10 the most important example is methane hydrate, a globally distributed mineral. Gas hydrates can be formed both from small ice particles11-13 and from water;14 the transport system can be based on hydrate blocks11,13 or on a slurry phase.15 Methane is the main component of natural gas, and it will be considered in this study. In the past decade, a number of authors5-8,11-13,15,16 reported that when methane hydrate is subject to temperatures slightly below 0 °C at 0.1 MPa pressure, it remains metastable outside its stability region and “preserves” in bulk as much as 80 K above its nominal * To whom correspondence should be addressed. Tel.: +390644585565. Fax: +390644585416. E-mail: carlo.giavarini@ uniroma1.it.

Table 1. Conditions and Properties of Different Systems Used for Methane Storage (4)

LNG CNG CNG ANG GAS

temp (K)

pressure (MPa)

density (g/cm3)

relative density

113 298 298 298 298

0.1 20.0 3.5 3.5 0.1

0.4 0.15 0.0234 0.13 0.0065

600 230 36 200 1

equilibrium temperature (193 K at 0.1 MPa) for a certain time. It was assumed that this anomalous or self-preservation effect occurs because thin ice films, impermeable to gas molecules, form on the hydrate surface during depressurization and can interrupt further decomposition of the hydrate.16,17 However, Stern et al.12 suggest that a different preservation mechanism is involved, besides the ice shielding. By utilization of the self-preservation property in addition to its high-density gas-containing property, it seems possible to store and transport stranded natural gas at higher temperatures compared to the conventional LNG and at atmospheric pressure5-8,13 without adding any adsorbent material. The fact, reported by the media (e.g., 2003 Technologyreview.com), that industrial giants such as Mitsui and Mitsubishi are developing pilot plants to master this technology underlines the importance of a successful outcome. However, a number of factors have not been taken into account in many of the previously published works: (a) The influence of the gas hydrate concentration in the sample, which normally contains a certain percentage of ice; in fact, the hypothesis of economically producing a 100% pure hydrate in an industrial unit seems unrealistic. (b) The influence of the storage pressure: all preservation tests were carried out at various subzero temperatures but at a single (atmospheric) pressure; a small pressure increase (0.1-0.2 MPa) could favor the preservation with a minor influence on storage costs.

10.1021/ie040038a CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004

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Figure 1. Schematic of the experimental apparatus (modified RC1 reactor).

(c) The importance of thermal history after the hydrate preparation. (d) The necessity of utilizing a production process that can easily be transferred to the industrial scale, in a practical and economical way. The use of carefully selected (and stored in liquid nitrogen) ice particles13 does not seem practical. In the same way, the application of very high pressures (20.0-25.0 MPa) for hydrate formation could be too expensive. The feasibility of natural gas storage in the form of hydrates depends not only on the phase-equilibrium issue but also on the development of practical means of rapid and economic hydrate formation.14 During the present work the authors conducted laboratory experiments on methane hydrate formation and dissociation in order to add more practical information on the hydrate potential for application to natural gas storage and transportation. Samples with different hydrate contents (about 30-88% with respect to the ideal stoichiometry) were produced at relatively low pressure (about 5.0 MPa). The composition CH4‚ 5.89H2O was considered as ideal stoichiometry;12 this composition was used as the basis for yield calculation. The preservation tests were carried out in the same reactor (with and without further cooling to -8 °C) at

pressures of 0.1, 0.2, and 0.3 MPa and temperatures ranging from -4 to -1 °C. The slurry phase, which is suitable for special applications (e.g., crude oil and gas hydrate combined transportation), was not considered in this paper. 2. Experimental Section 2.1. Apparatus. A schematic view of the experimental apparatus is shown in Figure 1. It consists of a jacketed stainless steel reactor (RC1-Mettler Toledo) having an internal volume of about 2 L, equipped with a stirrer. Temperature control is assured by resistance and by a cooling fluid circulated by a cryostat (OcrasZambelli). Reactor and jacket temperatures are measured by a thermocouple system; the reactor pressure is detected by a transducer (Genspec Instruments). Pressure, temperature, and stirrer revolutions per minute are transmitted into a personal computer through a data acquisition board and are recorded by QNX Mettler-Toledo software at 3 s intervals. Pure methane gas is supplied from a cylinder bottle in the reactor before hydrate formation. 2.2. Hydrate Formation Procedure. Two different procedures are considered to produce the hydrate samples from water and methane.

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Figure 3. Methane hydrate formation from spray water. Typical pressure trend during water injection. Figure 2. Methane hydrate formation from bulk water (500 mL). The upper curve refers to the pressure drop; the lower curve refers to the temperature. The start of stirring is indicated by a sharp P drop, corresponding to hydrate formation at 4.63 MPa.

The first system is used for the production of lowconcentration hydrates (about 30-70% conversion with respect to the ideal stoichiometry). Each experimental run starts by supplying 500 g of water to the evacuated reactor chamber; the reactor is then pressurized with methane to 4.5-5.5 MPa and the temperature lowered to the desired level (1 °C). Stirring is then started at 500-600 rpm. Hydrate formation begins immediately after stirring is started and is shown by a pressure decrease and a small temperature increase18 due to reaction exothermicity (Figure 2). The amount of formed hydrate can be calculated from the pressure drop. Small variations of pressure and of stirring rate produce different hydrate concentrations, in the conversion range 30-70%. The second procedure (water spraying) can be used to produce more concentrated samples (e.g., in the range 50-90%).14 A special nozzle is installed at the top of the reactor chamber to spray water into the guest-gas (methane) phase in the reactor in order to increase the gas/water interfaces and yield a sufficiently high rate of concentrated hydrate formation. An external water circulation loop is attached to the chamber; the rate at which cold water flows through this loop is controlled by a plunger pump. The reactor can contain a steel net (not far from the chamber bottom), on which surface the hydrate is formed; together with the spray nozzle, it contributes to increasing the gas/water interfaces and allows draining of excess water. The water, if any, pools at the bottom of the reactor, from which it can be drained. Hydrate formation starts immediately; its concentration can be regulated by the changing type of nozzle and regulating the addition of water. Samples weighing about 50 g were prepared following the above-described procedures. Figure 3 shows a typical pressure trend during water injection (about 1 g at each step) into the pressurized methane; the injection causes a sudden pressure increase followed by a drop due to hydrate formation. The general pressure trend is decreasing, also because of the “secondary” hydrate formation on the steel net caused by the water not reacted during spraying. The operation is carried out for the time necessary to have a sufficient quantity of sample. The hydrate produced by this procedure is granular, while the previous one is more compact.

2.3. Hydrate Characterization. When the amount of added water is known, the percentage of hydrate in the sample can be calculated from the pressure drop. Modulated differential scanning calorimetry (MDSC) can be used for hydrate characterization.18-20 The MDSC concept involves the imposition of a sinusoidal wave in the normally linear heating ramp so that portions of each cycle are at different heating and cooling rates, although the general overall trend is a linear change in the average temperature.19 The mode of modulation affords the thermal analyst an opportunity to study a physical or chemical change in greater detail. One of the major contributions of this technique and analysis is that the total heat flow rate can be separated into two additional signals. The first one is directly correlated to the heat capacity of the material, while the second is a function of temperature and time characteristic of the tests. These terms can be plotted separately in two curves. One of these curves represents the component that is heating rate dependent (i.e., which is in phase with the modulated heating); it is frequently defined as the “thermodynamic” component and associated with a “reversing heat flow”. The second curve corresponds to the rate of heat flow that depends only on the absolute temperature (i.e., which is out of phase with the modulated heating): it is usually defined as the “kinetic” component and associated with a “nonreversing heat flow”. In the present work, all MDSC tests were performed at atmospheric pressure following the procedure described in previous papers.18,20 A typical MDSC curve of a sample containing about 44% hydrate (with respect to the ideal stoichiometry) is shown in Figure 4. The MDSC reversing (heat capacity) curve of methane hydrates is splitted into two peaks with apexes at 0.25 and 4.47 °C, respectively. The first peak is due to the melting of the ice and the second to the melting and decomposition of the hydrate; therefore, the area of the second peak is proportional to the hydrate concentration.20 2.4. Hydrate Dissociation Tests. To obtain reproducibility, the hydrate dissociation tests are conducted immediately after hydrate formation. The temperature and pressure are set at its nonequilibrium condition, i.e., between -4 and -1 °C, and at pressures ranging from atmospheric (0.1 MPa) to 0.3 MPa. Under these conditions, all samples are composed of hydrate and ice. Some samples are subcooled at -8 °C before the dissociation tests, to check how such a procedure affects the self-preservation.

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Figure 4. Typical MDSC curve (reversing heat flow) of methane hydrate containing about 44% CH4. The first peak is due to ice melting; the second peak is related to hydrate melting and dissociation.20

Figure 5. Typical dissociation curve showing the three temperature plateaux (at -4, -2, and -1 °C, respectively) and the corresponding pressure variations.

The experimental apparatus permits rapid control of temperature and pressure. After the desired temperature (e.g., -4 °C) is reached, the reactor is depressurized at the required pressure; the rate of dissociation is followed by measurement of any pressure increase. After 1 h the temperature is increased to -2 °C and, in a following step, to -1 °C. The typical dissociation curve in Figure 5 shows the three temperature plateaux (at -4, -2, and -1 °C, respectively) considered for the conservation tests. The dissociation rate can be calculated from the pressure curve. A further small temperature increase near the ice melting point causes a rapid pressure increase due to hydrate decomposition. 3. Results and Discussion According to Stern et al.,11,12 methane hydrate dissociates extremely slowly near -5 °C and atmospheric pressure, and the dissociation is about 1/100 of those for -10 and 0 °C. Extremely slow dissociation is confirmed by Shirota et al.13 within a temperature range between -7.5 and -3 °C, and the measured dissociation rate is

Figure 6. Correlation between the dissociation rate and temperature for different gas concentrations in the hydrates (P ) 0.3 MPa).

about 1/10 of that for 0 °C. The Shirota dissociation rate at -5 °C is somewhat larger than the data by Stern et al. In both works, hydrates were formed from ice particles and methane gas at high pressure (27 and 12 MPa, respectively) with high conversion (greater than 92% for Shirota and with no detectable secondary phase for Stern). The dissociation rates at 0.1 MPa measured in this work are in agreement with those of the previously mentioned authors if we consider the less concentrated samples. Both in this paper and in the previous ones,11-13 the dissociation rate is expressed as percent of gas moles lost with respect to the gas moles in the hydrate over a time of 1 s. 3.1. Correlation between Hydrate Dissociation and Concentration. For a given pressure, the dissociation rate at a fixed temperature seems to depend on the hydrate concentration in the sample, as shown in Figure 6 (which reports the dissociation rate vs temperature in a range of hydrate conversions between 30% and 88%, at 0.3 MPa) and in Figure 7, which shows

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Figure 7. Correlation between the dissociation rate and CH4 concentrations in the hydrate at different storing temperatures (P ) 0.3 MPa).

Figure 9. Influence of further cooling to -8 °C, before storage at temperatures between -4 and -1 °C (P ) 0.1, 0.2, and 0.3 MPa). Table 2. Days Required for Complete Dissociation of Some Hydrate Samples (30% Conversion) V (% mol/s) temp (°C) and pressure of not further further storage (MPa) cooled cooled -4 and 0.1 -4 and 0.3 -1 and 0.3

Figure 8. Correlation between the dissociation rate and pressure at different storing temperatures (30% hydrate).

the dependence of the dissociation rate on the gas conversion in the hydrate, at three different temperatures. Hydrates containing low quantities of gas (i.e., richer in ice) are more stable than the more concentrated ones. The fact that the particles of the hydrates produced in this work have different shapes and sizes depending on the production procedure could influence the dissociation rate;13 however, the trend shown in Figures 6 and 7 seems to confirm the fact that the self-preservation effect depends on the ice content in the hydrate particles. More ice (or less CH4) in the sample is responsible for a higher preservation effect. This is a factor that should be considered during storage, eventually optimizing T, P, and concentration parameters in order to decide the preservation conditions economically most favorable. 3.2. Correlation between Hydrate Dissociation and Pressure. The preservation pressure obviously influences the dissociation rate, as shown in Figure 8, which refers to a sample containing 30% hydrate. While the shifting from 0.1 to 0.2 MPa does not dramatically affect the preservation, the pressure of 0.3 MPa greatly reduces the dissociation rate, at least in the temperature range experimented with in this work. Similar behavior is shown by more concentrated samples. It is important to observe that at 0.3 MPa and in the T range between -1 and -4 °C the effect of the conservation temperature is negligible. 3.3. Effect of Thermal History. Some hydrate samples were cooled to -8 °C after preparation and kept at this temperature for 2 h. The temperature was then raised to the test value (at the planned pressure). The

4.3 (10-5) 2.9 (10-5) 7.0 (10-5)

1.0 (10-4) 1.3 (10-5) 1.8 (10-5)

100% dissociation (days) not further cooled

further cooled

3 40 16

12 90 64

dashed lines in Figure 9 are referred to samples further cooled after formation. It is evident that further cooling to -8 °C increases the preservation effect at any temperature, in the pressure range 0.1-0.2 MPa. Further cooling has only a minor effect on the samples stored at 0.3 MPa. Table 2 shows how many days are necessary to fully dissociate some subcooled samples, with respect to nonsubcooled samples. 3.4. Correlation between Hydrate Dissociation and Temperature. Figure 9 also shows the dependence of the dissociation rate on the experimented temperatures, at 0.1 and 0.2 MPa for a sample containing 30% hydrate. It is confirmed that in this pressure range the preservation is better at -4 °C than at -2 or -1 °C. In the range -1 to -4 °C, the temperature does not affect the dissociation if the pressure is kept above 0.3 MPa. 4. Conclusion This study on methane hydrate formation and dissociation was conducted to examine its potential for application to natural gas storage and transportation. From an industrial and economic point of view, it seems unrealistic to prepare concentrated methane hydrates at very high pressures (above 100-120 MPa) and starting from selected ice particles (eventually stored in liquid nitrogen) as experimented by some authors.11,13 It is probable that, by using lower pressures and more conventional methods, the produced hydrate will contain percentages of gas up to 80-90% with respect to the ideal stoichiometric content. Therefore, it is important to know the influence of the gas content on the hydrate preservation. The knowledge of the effect of other preservation pressures, besides the atmospheric pressure, is also important for the definition of the optimum conservation parameters. The experimental data collected during this work have shown that more concentrated hydrates (i.e., containing less entrapped water or ice) are more easily decomposed and that the poorer hydrates (containing less methane) are more stable in the self-preservation

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region. The influence of the gas content on hydrate decomposition is also important during methane recovery from the submarine deposits: natural hydrates contain different quantities of entrapped ice, which will surely influence the hydrate stability and, therefore, the recovery conditions. A further cooling of the hydrate samples helps selfpreservation only at pressures lower than 0.3 MPa. In the explored range (0.1-0.3 MPa), the pressure has an appreciable effect on the hydrate conservation, suggesting the possibility of storing and transporting the gas (in the hydrate form) at pressures higher than 0.1 MPa. Referring to the P and T ranges considered in this work, the best conditions suggested for self-preservation are 0.3 MPa and -4 °C, without any preliminary further cooling of the samples. Under these conditions, the 30% hydrate will completely dissociate in about 40 days (Table 2), while the 88% hydrate will dissociate in about 10 h. Literature Cited (1) Bercy, J.; Fischer, B.; Thomas, C. Potential cost reductions in gas trade. Proceedings of the 16th World Petroleum Congress, Calgary, Alberta, Canada, 2000. (2) Stenning, D.; Eng, P.; Cran, J. A. The coselle CNG carrier the shipment of natural gas by sea in compressed form. Proceedings of the 16th World Petroleum Congress, Calgary, Alberta, Canada, 2000. (3) Badakshaw, A.; Pooladi-Darvish, M. Gas hydrate a new means for natural gas storage and transportation. Proceedings of the 16th World Petroleum Congress, Calgary, Alberta, Canada, 2000. (4) Lonzano Castello, D.; Alganiz Monge, J.; de la Casa Lillo, M. A.; Cazorla Amoros, D.; Linares Solano, A. Advances in the study of methane storage in porous carbonaceous materials. Fuel 2002, 81, 1777. (5) Gudmundsson, J. S. Method for production of gas hydrate for transportation and storage. U.S. Patent 5,536,893, 1993. (6) Gudmundsson, J. S.; Andersson, V.; Levik, O. I. Gas storage and trasport using hydrates. Proceedings of Offshore Mediterranean Conference, Ravenna, Italy, 1997. (7) Gudmundsson, J. S.; Parlaktuna, M.; Kmokhar, A. A. Storing natural gas as frozen hydrate. SPE Prod. Facil. 1994, 69.

(8) Gudmundsson, J. S.; Borremaug A. Frozen hydrate for transport of natural gas. Proceedings of the 2nd International Conference on Natural Gas Hydrates, Tolouse, France, 1996. (9) Sloan, E. D., Jr. Clathrate hydrates of natural gases; Marcel Dekker: New York, 1998. (10) Makogon, Y. F. Hydrates of hydrocarbons; Penwell Books: Tulsa, OK, 1997. (11) Stern, L.; Circone, S.; Kirby, S.; Durham, W. Anomalous preservation of pure methane hydrate at 1 atm. J. Phys. Chem. B 2001, 105, 1756. (12) Stern, L.; Circone, S.; Kirby, S.; Durham, W. New insights into the phenomenon of anomalous “self” preservation of gas hydrates. Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, 2002. (13) Shirota, H.; Aya, I.; Namie, S.; Varam, B.; Turner, D.; Sloan, E. D. Measurement of methane hydrate dissociation for application to natural gas storage and transportation. Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, 2002. (14) Ohmura, R.; Kashiwazaki, S.; Shiota, S.; Tsuji, H.; Mori, Y. M. Structure I and structure H hydrate formation using water spraying. Energy Fuels 2002, 16, 1141. (15) Gudmundsson, J. S.; Andersson, V.; Levik, O. I.; Mork, M. Hydrate technology for capturing stranded gas. In Gas Hydrates: Challengers for the Future; Holder, G. D., Bishnoi, P. R., Eds.; Annals of the New York Academy of Sciences: New York, 2000. (16) Yakushev, V.; Istomin, V. Gas hydrate self-preservation effect. In Physics and Chemistry of Ice; Maeno, N., Hondah, T. E., Eds.; Hokkaido University Press: Sapporo, Japan, 1992. (17) Takeya, S.; Shimada, W.; Kamata, Y.; Ebinuma, T.; Uchida, T.; Nagao, T.; Narita, H. In situ X-ray diffraction measurements of the self-preservation effect of CH4 hydrate. J. Phys. Chem. A 2001, 105, 9756. (18) Giavarini, C.; Maccioni, F.; Santarelli, M. L. Formation kinetics of propane hydrate. Ind. Eng. Chem. Res. 2003, 42, 1517. (19) TA Instruments DSC 2920. Operator’s manual appendix C: modulated DSCTM option, 1998. (20) Giavarini, C.; Maccioni, F.; Santarelli M. L. Characterization of Gas Hydrates Structures by Modulated DSC. Pet. Sci. Technol. 2004, in press.

Received for review January 26, 2004 Revised manuscript received May 4, 2004 Accepted May 13, 2004 IE040038A