J. Ind. Eng. Chem.,Vol. 12, No. 5, (2006) 733-738
A Study of the Kinetic Characteristics of Natural Gas Hydrate
Yong-han Jeon, Nam-Jin Kim*, Won-Gee Chun*, Sang-Hoon Lim**
Chong-Bo Kim, and Byung-Ki Hur***,?
Department of Mechanical Engineering, Inha University, Inchon 402-751, Korea * Department of Nuclear & Energy Engineering, Cheju National University, Jeju, 690-756, Korea
** Department of New & Renewable Energy Research, Korea Institute of Energy Research, Daejeon, 305-343, Korea *** Department of Biological Engineering, Inha University, Inchon 402-751, Korea
Received March 17, 2006; Accepted June 22, 2006
Abstract:When referred to standard conditions, 1 m3 solid hydrate contains up to 200 m3 of natural gas, depending on the pressure and temperature. Such a large volume of natural gas hydrate can be utilized to store and transport a large quantity of natural gas in a stable condition. In the present investigation, experiments were performed to investigate how the formation of natural gas hydrate is governed by such factors as the pressure, temperature, and gas composition. The results indicate that if the subcooling of structure Ⅱ is more than 11 K, a hydrate is rapidly formed. In addition, we found that a pressure increase was more advantageous than a temperature decrease at effecting an increase in the gas consumption.
Keywords:gas hydrate, natural gas, induction time, nucleation, subcooling
Introduction
Hydrates (Figure 1) are solid containers similar to ice, that have crystal structures exhibiting polyhedron cavities consisting hydrogen bonded water molecules their structures are expressed as nm. For example, 51262 means 14 cavities with 12 pentagons and 2 hexagons. In addition, 512, 51262, 51264, 51268, and 435663 systems are so far recognized as cavity types. Structure I (Figure 2) is a combination of 6 polyhedrons (51262) containing 14 facets and 2 polyhedrons (512) containing 12 facets; structure Ⅱ (Figure 3) is a combination of 16 polyhedrons (512) containing 12 facets and 8 polyhedrons (51264) containing 16 facets; structure H (Figure 4) is a combination of 3 polyhedrons (512) containing 12 facets, 2 polyhedrons (435663) containing 16 facets, and 1 polyhedron (5
1268) containing 20 facets [1,2].
Siberian chemical plants in the 1930s had frequent blocking problems in the transportation pipelines of natural gas. The cause of such problems was attributed to the combination of gas and water in the pipe producing hydrates, which adhered to the inner pipe wall. The natural gas hydrates have received great attention since ?To whom all correspondence should be addressed.
(e-mail: biosys@inha.ac.kr)Figure 1. Natural gas hydrate.
this discovery [3]. More than 99 % of naturally produced natural gas hydrate consists of methane, and is widely dispersed in the continental slope and continental shelf of, for example, the Pacific and Atlantic oceans and Antarctica. The reserves of fossil fuels amount to 500 billion carbon tons and the reserves of methane are 360 million carbon tons. The reserves of gas hydrate amount to more than 1 trillion carbon tons, which is twice the amount of fossil fuel [4,5]. Therefore, natural gas hydrate is expected to replace fossil fuels as new gas hydrate energy source for the 21st century.
A 1 m3 hydrate of pure methane can be decomposed to a maximum of 216 m3 of methane under standard conditions and a 1 m3 hydrate of natural gas can be
Yong-han Jeon, Nam-Jin Kim, Won-Gee Chun, Sang-Hoon Lim, Chong-Bo Kim, and Byung-Ki Hur
734
Table 1. Compositions of Natural Gas
Certified Concentration (mol/mol)
Uncertainty
Methane (CH 4)
90.8784 %Ethane (C 2H 6) 5.63 %± 2 %
Propane (C 3H
8) 2.52 %± 2 %iso-Butane (i-C 4H 10)0.493 %± 2 %n-Butane (n-C 4H 10)0.447 %± 2 %iso-Pentane (i-C 5H 12)102 PPM ± 3 %Nitrogen (N 2)214 PPM
± 3 %
Figure 2. Crystalline lattice of natural gas hydrate, structure Ⅰ.
Figure 3. Crystalline lattice of natural gas hydrate, structure Ⅱ.
decomposed to 200 m 3
of natural gas. If these characteristics of hydrates are utilized reversely, natural gas is fixed into water in the form of hydrate solid. Therefore, hydrates are considered to be a great way to transport and store natural gas in large quantities. Especially attractive is the transportation cost, which is known to be 24 % lower than that of liquefied transportation [6].
Figure 4. Crystalline lattice of natural gas hydrate, structure H.
However, when natural gas hydrates are artificially formed for the solid transportation of natural gas, the reaction time may be too long, and the gas consumption in water relatively low, because the reaction rate between water and gas is low. Therefore, for practical applica-tions, this present investigation focused on the rapid production of hydrates.
Experimental
Apparatus
Figure 5 shows a schematic diagram for the experimen-tal apparatus. The 600-mL reactor (1) and 1.5-L supple-mental tanks (24,27) were manufactured using SUS316 to endure a pressure of 30 MPa and salt erosion. Con-sidering the high-pressure operations within the reactor, a check valve (8) was installed at the rear side of the tube connected to the reactor to prevent the back-flow of gas and water. Sapphire glass (2) of 80-mm diameter was installed for visualization on the front and rear sides of the reactor. Tube (7) was 2 m long to ensure full heat transfer between the gas and water entering the reactor. In the case of the MFCs (Bronk-horst Hi-tech Co.), an MFC (21, 0～1000 g/hr) for liquid, an MFC (22, 0～60 L/min) for a large gas quantity, and an MFC (23, 0～1500 mL/min) for a small gas quantity were installed separately. For experimental precision, 97～98 % of the experimental pressure was controlled by the MFC for the large gas quantity and the remaining 2～3 % by the MFC for the small gas quantity to reduce over-pressure, which may be generated by instantaneous inflow of a large quantity. An analog-style Heise manometer (19, 0～350 kgf/cm2, Heise Co.) and a digital gauge (16, Sensys Co.), error range within 0.25 % in pressure measurement, were used. A 1/32-inch T-type heat transmitter (3, OMEGA Co.) and a digital gauge (16, Sensys Co.) were used for temperature measurement. A chiller (11, 228～403 K, Jeio Tech Co.) for control of the reactor temperature, a gas booster (28, 700 kg/cm 2, Schmidt, Kranz & Co
A Study of the Kinetic Characteristics of Natural Gas Hydrate
735
Figure 5. Schematic diagram of the apparatus.
1. Reactor
2. Sapphire glass
3. Thermocouple
4. Double distilled water
5. Stirrer
6. Water injection nozzle
7. Precooling tube
8. Check valve
9. Bath 10. coolant 11. Chiller 12. Magnetic driver 13. Motor 14. MD controller 15. T. digital indicator 16. P. digital indicator 17. PC 18. A/D, D/A converter 19. Heise gauge 20. 3-way valve 21. Liquid MFC 22. Gas MFC (high) 23. Gas MFC (low) 24. Water reservoir tank 25. Liquid pump 26. Water tank 27. Gas reservoir tank 28. Gas booster 29. Air compressor 30. Gas bombe 31. Relief valve 32. Stop valve 33. Coolant inlet 34. coolant outlet 35. Gas vent 36. Water vent 37. Gas & water vent 38. Pressure gauge 39. Back pressure
regulator 40. Regulator 41. Vacuum pump 42. Vacuum gauge
Figure 6. Experimental method.
Gmbh) for high-pressure gas, and a PC (17) for reading and recording such data as the pressure, temperature, and flow rate were installed. The reactants used in most experimentats were secondary distilled water and Indo-
nesian natural gas (47, Rigas Co.). Table 1 shows the composition of the reactant gas.Methods
Equilibrium Measurement
Hydrates are generally stable under high pressure and low temperature. They are readily decomposed to water and gas out of the stable region. Because the formation and decomposition of a hydrate can be confirmed visu-ally, phase equilibrium is performed considering the spe-cialty of experimental apparatus. Model cases are iso-thermal and isobaric experiments as shown in Figure 6. In the isothermal experiment, the hydrate is formed at ar-bitrary temperature B (T equ ) and pressure A (P exp ), which is higher than pressure, D (P equ ). The pressure is gradu-ally decreased at constant temperature in the course, A →B. In the isobaric experiment, the temperature is gradu-ally increased at constant pressure in the course, A →C [8]. After performing two experiments in the present study, the isobaric experiment was found to be more convenient to perform than the isothermal experiment. Because the decomposition of the hydrate can be ob-served visually, the isobaric experiments were performed in the following manner.
Yong-han Jeon, Nam-Jin Kim, Won-Gee Chun, Sang-Hoon Lim, Chong-Bo Kim, and Byung-Ki Hur
736Table 2. Hydrate Crystal Structure According to Composition Ratios Based on Methane Gas
Compositions Ratios (%)Structure
Ethane (C 2H 6)
10Ⅰ(Ⅱ *)Propane (C 3H 8)0.0146Ⅱiso-Butane (i-C 4H 10)0.01Ⅱn-Butane (n-C 4H 10)0.108Ⅱ(Ⅰ *)iso-Pentane (i-C 5
H 12)0.866H n-Pentane (n-C 5H 12)0No formation Nitrogen (N 2)0Ⅰ or ⅡCarbon dioxide (CO 2)
Ⅰ
* : Apparent character
Figure 7. Photograph of nucleation.
Distilled water (300 mL) was poured into the reactor using a liquid phase pump and cooled to 274.15 K. The experimental gas (under 19 MPa) was injected into the reactor at the pressure of 1～15 MPa according to the experimental conditions. Experimental conditions (A) were kept constant for several hours to completely grow the hydrate. After the hydrate had fully grown, the reactor temperature was raised by 0.1 K every 2 h (course A →C). The pressure increase due to the decomposition of hydrate was controlled to maintain a constant flow of discharging (37) gas within the reactor through the 3-way valve (20) and the MFC.
Measurement of Hydrate Nucleation Time
As shown in Figure 6, hydrate formation was acceler-ated at pressures higher than the equilibrium pressure and at temperatures lower than the equilibrium temperature. The subcooling temperature (ΔT subc ) is defined as the difference between the experimental and equilibrium temperatures. Distilled water (300 mL) was poured into the reactor and cooled to the experimental temperature prior to the gas being injected. After reaching the experimental pressure, the hydrate nucleus was observed
Figure 8. Equilibrium points.
for 24 h in the circle, as shown in Figure 7.
Measurement of Gas Consumption on Subcooling
Distilled water (300 mL) was poured into the reactor
and cooled to 274.15 K; the experimental gas was then injected at the experimental pressure. Experiments were performed for 24 h at this temperature. As the ex-perimental gas reacts with distilled water to form the hydrate, the consumed gas was made up by the MFC and the pressure was maintained constant.
Results and Discussion
Structure of Natural Gas Hydrate
Because natural gas is a mixed gas, its hydrate structure depends on its composition. The structure of natural gas investigated by CSMHYD, a program provided by the Colorado School of Mines, is shown in Table 2. The natural gas containing more than 10 % ethane formed structure I, but had the characteristics of structure Ⅱ. The mixed gas containing more than 0.108 % n - butane formed structure Ⅱ but had the characteristics of structure I. On the other hand, the natural gas containing more than 0.0146 % propane and 0.01 % i -butane formsed structure Ⅱ, and the natural gas with more than 0.866 % i -pentane formed structure H. Because the natural gas including n -pentane cannot form any hydrates, n -pentane does not affect the structure of the hydrate. Carbon diox-ide formed either structure I, regardless of the composi-tion ratio, and nitrogen can form either structure I or Ⅱ, depending on the formation temperature or pressure [7]. However, gases of larger molecular weight affect the structures when they form hydrates of mixed gases. The
A Study of the Kinetic Characteristics of Natural Gas Hydrate
737
Table 3. Experimental Equilibrium Points of Natural Gas Hydrate in Pure Water
No.P (MPa)T (K)1 1.1275.122 1.45278.053 2.06281.454 3.06284.955 4.05287.256 5.22289.0577.07291.0589.30292.55911.11293.351013.21294.3511
15.35
295.05
Table 4. Induction Time Data of Methane Hydrate No.P exp (MPa)T exp (K)T equ (K)ΔT sub (K)t induc (min)1 3.00274.43
285.4110.9813.332 3.00274.47285.4110.948.173 2.99274.57285.3810.81 3.83
4 3.00
274.62285.4110.7814.005 3.03275.27285.4910.2241.006 3.04275.37285.5110.1444.177 2.98276.10285.369.2695.338 3.04276.05285.519.4667.499 3.04276.95285.518.56181.1010 2.98277.95285.367.41593.0111 3.04
276.05
285.51
9.46
68.77
natural gas imported into Korea has less than 0.05 % i -butane, which has the largest molecular weight, and its hydrate forms structure Ⅱ, not structure H. In the case of the natural gas hydrates that exist naturally, more than 99 % of the decomposed gas is methane and the rest is carbon dioxide; its hydrate forms structure I. Therefore, reactant natural gas forms the structure-Ⅱ hydrate.Equilibrium Measurement
Figure 7 shows that good agreements exist between the results calculated by the CSMHYD program and the equilibrium points of structure Ⅱ determined from this study. Therefore, the experimental apparatus and meth-ods used in this investigation were adequate; Equation (1) shows the relationship between equilibrium tem-peratures and pressures.
P equil =a +b ×e
cT equil
(1)
a=1.111, b=3.3034E-26, c=2.078E-01
Figure 9. Induction time of hydrate nucleation.
Figure 10. Gas consumption volume for variable degrees of subcooling.
Measurement of Hydrate Nucleation Time
Figure 9 shows the formation time of the hydrate nucle-us. The hydrate formation time increaseed as the system approached the equilibrium conditions. At the same time, if the subcooling temperature increased, the hydrate for-mation time decreased. However, the results were dif-ferent when compared to those of Yousif [9] because of the different experimental gas, formation rate, and exper-imental set-up. We attribute our findings to the difference between the gas components and its composition. However, if the subcooling condition was set above 11 K, the hydrate formed rapidly regardless of the gas com-ponents and its composition during pressurization. The
Yong-han Jeon, Nam-Jin Kim, Won-Gee Chun, Sang-Hoon Lim, Chong-Bo Kim, and Byung-Ki Hur
738Figure 11. Gas consumption volume at the same subcooling below the freezing point.
following equation expresses the nucleation time:
t
induc =Exp (a ×ΔT
subc +b )
(2) a=-1.645, b =20.238
Measurement of Gas Consumption on Subcooling
Figure 10 shows the gas consumption, temperature, and pressure as mean-values between the initial time reached at the experimental pressure and the terminal time. Because subcooling increases as the pressure is raised at a constant temperature, an increase in gas consumption can be observed in this Figure.
Figure 11 shows the gas consumption above and below the freezing point under the same subcooling conditions. More gas was consumed above the freezing point, because the main component of natural gas is methane, which is a non-polar gas that is insoluble in water. This result coincides with Henry’s law, which states that the solubility in a non-polar gas or slightly polar gas is proportional to the pressure. Therefore, a pressure in crease is better than a temperature increase when aiming to increase the gas consumption.
Conclusions
Major natural gas hydrates imported into Korea have structure Ⅱ, the subcooling conditions of which must be above 11 K in order to form the hydrate rapidly, regard-less of the gas composition. Also, because subcooling increases as the pressure is raised at a constant tem-perature, an increase in gas consumption can be ob-served. Because the main component of the natural gas is methane, which is non-polar, a pressure increase is more advantageous than a temperature decrease when attemp-ting to increase the gas consumption.
Acknowledgments
The part of researchers participating in this study is supported by the grant from the 2nd phase BK 21 project.
References
1.N. J. Kim, J. Y. Lee, T. B. Seo, and C. B., KSME Int.
J., 14, 1276 (2000).
2.E. D. Sloan, Clathrate hydrates of natural gases , 2nd
Edn., pp. 1-318, Dekker, New York (1999).
3.E. G. Hammerschmidt, Int. Eng. Chem., 26, 851 (1934).
4.K. A. Kvenvolden, Chem. Geol., 71, 41 (1988).
5.D. G. Chang, N. J. Kim, J. Y. Lee, and C. B. Kim, in Proc. KSME Fall Ann. Meeting , pp. 241-246, Seoul, Korea (2000).
6.J. Gudmundsson and A. Borrehaug, in Proc. 2th
Int. Conf. on NGH , pp. 415-422, Toulouse, France (1996).7.W. F. Kuhs, B. Chazallon, G. Radaelli, and F. Pauer, J. Incl. Phenom., 29, 65 (1997).
8.P, Englezos, N. Kalogerakis, P. D. Dholabhai, and P. R. Bishnoi, Chem. Eng. Sci., 42, 2647 (1987).
9.M. H. Youslf, SPE Int., SPE 28479, 169-177 (1994).