Morphology Study of Structure H Hydrate Formation from Water Droplets Phillip Servio and Peter Englezos* Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 1 61-66
Received September 10, 2002
ABSTRACT: Gas hydrate formation on water droplets immersed in liquid neohexane and exposed to methane gas was visually observed and recorded. The diameter of the water droplet was approximately 5 mm. The temperature was set at 274.6 K, and the pressures ranged from 1800 to 5600 kPa to monitor the effect of driving force. It was found that elevated driving forces gave rise to smaller induction times than those obtained under low driving force. Driving force had no visible effect on the crystal morphology. The crystal growth was affected by the amount of dissolved methane present in the neohexane phase and not the magnitude of the driving force. Introduction Gas hydrate crystals are formed by water and a number of other molecules such as natural gas components at suitable temperature and pressure conditions. Gas hydrates play an important role in hydrocarbon exploration, reservoirs, transportation, and processes. They are also important as a future energy resource and a potential global climate threat.1-3 Crystal morphology studies give valuable information on the mechanistic aspects of gas hydrate crystal nucleation, growth, and decomposition.4-16 The studies focused on carbon dioxide droplets or methane bubbles or fluorocarbons in the liquid or vapor phase in the presence of water as the bulk phase. As seen, the studies concerned structure I or II gas hydrates and two phase systems. In this study, we conduct crystal morphology experiments on a nearly stagnant water droplet immersed in liquid neohexane, which is in contact with methane gas. This three phase, three component system is capable of forming structure H gas hydrate under suitable temperature and pressure conditions. The potential development of natural gas storage and transport technology based on structure H hydrate renders the study practically important. Thus, the objective of this work is to reveal the mechanistic aspects of structure H hydrate nucleation and growth. Experimental Apparatus and Procedure A simplified schematic of the experimental apparatus is given in Figure 1. The apparatus consisted of a stainless steel crystallizer submerged in an ethylene glycol-water cooling bath. Details of the crystallizer were given elsewhere.17 The crystallizer had three windows, one on the top made from Plexiglas (poly(methyl methacrylate)) and two on the sides made out of Lexan (polycarbonate). The window on top of the reactor was fitted with a fiber optic light pipe (41720 series, Cole Palmer, Anjou, Quebec), which delivered 40 000 footcandles of light. The fiber optic light pipe was equipped with an infrared filter to eliminate heat transmission. The cooling bath was an equal mixture of filtered water and ethylene glycol. Digital imaging was carried out by a Nikon SMZ 2000 * To whom correspondence should be addressed. Tel: (604)822-6184. Fax: (604)822-6003. E-mail:
[email protected].
microscope fitted with a 3.34 mega pixel Nikon CoolPix 995 digital camera. The output of the camera was also viewed and recorded by a PC through a Dazzle digital video creator. A simplified schematic of the inside of the crystallizer, not drawn to scale, is given in Figure 2. A Teflon-coated hollow stainless steel cylinder was placed inside the crystallizer. The stainless steel cylinder was open at both ends with a bridge on top to hold the Teflon bar. The Teflon bar was shaped with a concave depression so that a water droplet could be placed inside to mildly agitate (150 rpm) the neohexane liquid without disturbing the water droplet. Two copper-constantan thermocouples were used to report the temperature in the crystallizer to 0.1 K. The pressure in the crystallizer was measured by a Rosemount Smart Pressure transducer (3051CD, Norpac Controls, Vancouver, BC) with a range of 0-13 790 kPa and accuracy of 0.075% of the span. The experimental procedure first involved inserting neohexane into the crystallizer. Immediately following, a water droplet was positioned on the Teflon bar and sat immersed inside the neohexane phase. The contents of the crystallizer were flashed three times at 1000 kPa with methane to remove any residual air in the crystallizer and connecting lines. The system was then pressurized to the experimental pressure and maintained at the experimental temperature. It should be noted that the droplet rested approximately 1 cm below the gas-liquid interface. One reformation experiment was also preformed to observe the crystal morphology. Only one experiment was performed because as the crystal was decomposing the gas rapidly evolved from the liquid neohexane and hydrate. This rapid upward movement of gas caused the droplet to move out of the microscope viewing area. Only one decomposition experiment left the droplet at the proper location to be viewed when all of the hydrates had dissociated. Hydrates were then reformed on this water droplet. To form structure H hydrate, the addition of methane gas was required. The methane must dissolve into the neohexane and then travel toward the liquid-liquid interface between water and neohexane. Hence, the mass transfer between the methane, neohexane, and water phase must be aided by mildly agitating the neohexane phase.
Results and Discussion A list of the number of experiments performed along with the experimental conditions and observed nucleation times is given in Table 1. In addition, the time elapsed after nucleation until the crystal structure became opaque is also given in Table 1. It should be noted that experiments 1-7 were carried out at pres-
10.1021/cg020044e CCC: $25.00 © 2003 American Chemical Society Published on Web 11/05/2002
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Figure 1. Simplified experimental apparatus. Table 1. Experimental Conditions and Nucleation Times with Observed Times of When Crystal Structure Became Opaquea experiment
T (K)
1 2 3 4 5 6d 7c 8d 9d 10d 11 12 13
274.65 274.65 274.65 274.65 274.65 274.65 274.6 274.65 274.65 274.65 274.65 274.65 274.65
size P (kPa) (mm) 5600 5600 5600 4500 4500 4000 5600 1800 1800 2000 2000 2300 2500
5 5 5 5 5 5 5 5 5 5 5 5 5
nucleation time (min)
opaque time (min)b
41 1752 61 890 703
185 39 180 48 56
1
45
3961-4021 2371 1654
0, z ) 0, c1 ) c10
(A3)
z ) ∞, c1 ) c1∞
(A4)
The solution to eq A1 is the following:19
c1 - c10 ) erf(ξ) c1∞ - c10
(A5)
where c10 is the concentration of dissolved gas at the gas-liquid interface, erf is the error function, and
ξ)
z (4Dt)1/2
(A6)
Assuming that the concentration of dissolving gas is zero at an infinite distance in the liquid from the interface, then c1∞ ) 0 and eq A5 reduces to
c1/c10 ) 1 - erf(ξ) ) S
(A7)
where S is defined as the degree of saturation at a given time and depth from the interface. The Wilke-Chang correlation19 is used to determine the diffusivity of a gas into a liquid:
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D)
Servio and Englezos
7.4 × 10-8 (φM2)1/2T
(A8)
µV10.6
where T is the temperature given in Kelvin, M2 is the molar weight of solvent, µ is the viscosity of the solvent, and V1 is the molar volume of the solute at its normal boiling temperature. The factor φ is for solute-solvent interactions and is equal to 2.26 for water, 1.9 for methanol, 1.5 for ethanol, and 1.0 for nonhydrogenbonded solvents. Thus, in our case, the factor may be set to 1.0. The temperature dependence on the viscosity of neohexane can be given by the Andrade equation as follows:
ln(µ) ) A + B/T
(A9)
where T is in Kelvin and µ is in cP. The coefficients A and B are -4.454 and 1010 K, respectively.21 The viscosity of neohexane at 274.6 K can then be calculated.
(
µ ) exp -4.454 +
1010 K 274.6 K
)
µ ) 0.46 cP
(A10)
Now returning to eq A8, the diffusivity of methane gas in stagnant neohexane liquid at 274.6 K is
D)
7.4 × 10-8 (1 × 86.18 g/mol)1/2 274.6 K 0.460 cP × (37.1 cm3/mol)0.6 D ) 4.69E - 5 cm2/s
(A11)
The molar volume of methane at its boiling point temperature21 of 111.6 K was estimated using the Trebble-Bishnoi equation of state.21 It should be noted that this value of the diffusion coefficient is approxiamtely twice as large as the prediction made by the Stokes-Einstein equation. All of the coefficients required to solve eq A7 have now been determined. For this study, two degrees of saturation have arbitrarily been chosen, 99 and 95%. Solving eq A7 at S ) 0.99 for ξ gives
0.5 ) erf(ξ) and ξ ) 0.0089
(A12)
and for S ) 0.95
0.05 ) erf(ξ) and ξ ) 0.0443
(A13)
The final step is to manipulate eq A6 to obtain time as a function of depth. This is given below in eq A14.
t)
z2 4Dξ2
(A14)
References (1) Englezos, P. Ind. Eng. Chem. Res. 1993, 32 (7), 1251-1274. (2) Englezos, P. Energy, Environment, and Naturally Occurring Methane Gas Hydrate: Connections. In Energy and Environment: Technological Challenges for the Future; Mori, Y. H., Ohnishi, K., Eds.; Springer-Verlag: Tokyo, 2001; pp 181-194. (3) Sloan, E. D. Clathrate Hydrates of Natural Gases; Marcel Dekker: New York, 1998. (4) Maini, B. B.; Bishnoi, P. R. Chem. Eng. Sci. 1981, 36, 183189. (5) Nojima, K.; Mori, Y. H. Proc. Int. Heat Transfer Conf., 10th, 1994, 3, 377-382. (6) Sugaya, M.; Mori, Y. H. Chem. Eng. Sci. 1996, 51 (13), 3505-3517. (7) Makogon, Y. F. Hydrates of Hydrocarbons; Pennwell Publishers: Tulsa, OK, 1997. (8) Smelik, E. A.; King, H. E., Jr. Am. Mineral. 1997, 82, 8898. (9) Uchida, T.; Ebinuma, T.; Kawabata, J.; Narita, H. J. Cryst. Growth 1999, 204, 348-356. (10) Ohmura, R.; Shigetomi, T.; Mori, Y. H. J. Cryst. Growth 1999, 196, 164-173. (11) Uchida, T.; Ebinuma, T.; Narita, H. J. Cryst. Growth 2000, 217, 189-200. (12) Kobayashi, I.; Ito, Y.; Mori, Y. H. Chem. Eng. Sci. 2001, 56, 4331-4338. (13) Fukumoto, K.; Tobe, T.; Ohmura, R.; Mori, Y. H. AIChE J. 2001, 47 (8), 1899-1904. (14) Servio, P.; Englezos, P. AICHE J. 2002, in press. (15) Sakaguchi, H.; Ohmura, R.; Mori, Y. H. Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002; Vol. 2, pp 509-515. (16) Ho, C.; Honda, S.; Ohmura, R.; Mori, Y. H. Proceedings of the 4th International Conference on Gas Hydrates; Yokohama, Japan, May 19-23, 2002; Vol. 2, pp 850-855. (17) Servio, P. Kinetic, Equilibrium and Morphology Studies of Hydrate Forming Systems. Ph.D. Thesis, University of British Columbia, Department of Chemical and Biological Engineering, Vancouver, BC, Canada, 2002. (18) Servio, P.; Lagers, F.; Peters, C.; Englezos, P. Fluid Phase Equilib. 1999, 158-160, 795-800. (19) Cussler, E. L. Diffusion Mass Transfer in Fluid Systems; Cambridge University Press: Cambridge, 1997. (20) Hatzikiriakos, S. G.; Englezos, P. The Proceedings of the Fourth 1994 International Offshore and Polar Engineering Conference; Chung, J. S., Natvig, B. J., Das, B. M., Eds.; Osaka, Japan, 10-15 April, 1994, p 337. (21) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (22) Trebble, M. A.; Bishnoi, P. R. Fluid Phase Equilib. 1987, 35, 1-18.
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