Chemistry of hydrocarbon compounds – Hydrate or production thereof
Reexamination Certificate
1997-10-14
2001-01-30
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Hydrate or production thereof
C062S045100, C062S048100, C062S054300, C062S057000, C062S601000
Reexamination Certificate
active
06180843
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus and method for continuous production of gas hydrates. The invention includes the use of a fluidized or expanded bed reactor for contacting the gas and water reactants to produce gas hydrates.
BACKGROUND OF THE INVENTION
Gas hydrates have been known for many years. These hydrates are inclusion compounds wherein various light hydrocarbon gases or other gases, such as natural gas, methane, ethane, propane, butane, carbon dioxide, hydrogen sulfide, nitrogen, and combinations thereof, physically react with water at elevated pressures and low temperatures. The gas becomes included or entrapped within the extended solid water lattice network which includes hydrogen bonded water molecules. The hydrate structure is stable due to weak van der Waals' forces between the gas and water molecules and hydrogen bonding between water molecules within the lattice structure.
At least two different hydrate crystal structures are known, each of which is a clathrate crystalline structure. A clathrate hydrate unit crystal of structure I includes two tetrakaidecahedron cavities and six dodecahedron cavities for every 46 water molecules. A clathrate hydrate unit crystal of structure II contains eight large hexakaidecahedron cavities and 16 dodecahedron cavities for every 136 water molecules. A relatively large volume of gas can be entrapped under pressure in these cavities. For example, it has been determined that natural gas hydrates can contain as much as 180 standard cubic feet of gas per cubic foot of the solid natural gas hydrates.
Early on, gas hydrates were considered an industrial nuisance. Petroleum and natural gas production facilities are often located in cold environments, where the product is located in deep underground or underwater wells. When tapping these wells, all of the necessary ingredients and conditions are present for producing gas hydrates—i.e., light hydrocarbon gases and water are present, the temperature is low, and the pressure is high. Therefore, gas hydrates were often produced spontaneously in the drilling and transmission pipes during oil or natural gas production. Because gas hydrates are solid materials that do not readily flow in concentrated slurries or in solid form, when spontaneously produced during oil or natural gas production, the hydrates tend to clog the pipes, channels, and equipment in the production and transmission systems. These disadvantageous properties of gas hydrates spawned much research into methods for inhibiting hydrate formation and eliminating this nuisance. See, for example, D. Katz, et al.,
Handbook of Natural Gas
, McGraw-Hill, New York (1959) pp. 189-221; E. D. Sloan, Jr.,
Clathrate Hydrates of Natural Gases
, Marcel Dekker, Inc. (1991). These documents are entirely incorporated herein by reference.
But, because of the relatively high volume of gas that potentially can be stored in gas hydrates, eventually researchers began to look at this “nuisance” as a possible method for storing and/or transporting gases. See B. Miller, et al.,
Am. Gas. Assoc. Mon
. Vol. 28, No. 2 (1946), pg. 63. This document is entirely incorporated herein by reference. U.S. Pat. No. 3,514,274 to Cahn, et al., also entirely incorporated herein by reference, describes a process in which a solid hydrate phase is generated in one or more process steps. The hydrate then is conveyed to storage or a marine transport vessel. This process is disadvantageous, however, because it requires conveyance of a concentrated hydrate slurry in a liquid propane carrier.
Hutchinson, et al., U.S. Pat. No. 2,375,559 (which patent is entirely incorporated herein by reference), describe a process for hydrating hydrocarbon gases. In this process, the gas and water components are mixed in a pipe that moves the hydrate product to storage tanks. Because of the poor flowing properties of gas hydrates, as noted above, this device would be subject to clogging.
U.S. Pat. No. 2,904,511 to Donath illustrates a water desalination apparatus that produces desalinated water from salt water by forming gas hydrates. The hydrate forming vessel of Donath is partially filled with water to be purified, and the hydrate-forming gas is introduced into the liquid water to form the hydrate. Because of the presence of the liquid water in the hydrate forming vessel, this apparatus would not be well suited for use on board a ship or oil platform or other areas influenced by waves. This Donath patent is entirely incorporated herein by reference.
Gudmundsson describes various systems for making gas hydrates. See, for example, U.S. Pat. No. 5,536,893; WO Patent Publication No. 93/01153; “Transport of Natural Gas as Frozen Hydrate,” ISOPE Conference Proceedings, V1, The Hague, Netherlands, June 1995; and “Storing Natural Gas as Frozen Hydrate,” SPE Production & Facilities, February 1994. These documents each are entirely incorporated herein by reference. A typical system
100
of Gudmundsson is generally shown in FIG.
1
. In this system, natural gas from a gas source G is compressed (
102
), cooled (
104
), and fed to a continuously stirred tank reactor vessel
106
. Water from a suitable source S is pumped (pump “P”) through a cooler
108
to form a water/ice slurry that is introduced into the tank
106
. The tank
106
is maintained under conditions appropriate to produce a gas hydrate (e.g., 50° F., 720 psig). The gas hydrate slurry produced in the tank
106
is transported via line
110
to a separator
112
where water is removed via line
114
. The separator
112
includes a series of cyclones and a rotary-drum filter. Finally, the purified hydrates are frozen to 5° F. in a freezer
116
, from where the hydrates
118
are transferred to storage or a transport device.
SUMMARY OF THE INVENTION
It is an object of this invention to overcome various disadvantages and problems with known gas hydrate production methods. As objectives, this invention seeks to provide a method and apparatus for producing gas hydrates continuously, simply, efficiently, and cleanly, using a minimal amount of equipment.
In a first aspect, this invention relates to an apparatus for producing gas hydrates. The apparatus includes a reactor vessel which has a fluidized or expanded bed reaction zone. The reactor vessel has an upper portion and a lower portion, wherein a cross-sectional area of the upper portion of the reactor vessel is larger than a cross-sectional area of the lower portion. A means is provided for introducing water into the reactor vessel, preferably into the upper portion of the vessel. Also, a means is provided for introducing a hydrate-forming gas under an elevated pressure into the lower portion of the reactor vessel. Preferably, the means for introducing water and the means for introducing hydrate-forming gas are arranged such that the gas and water flow in a countercurrent manner to the fluidized or expanded bed.
Hydrate production can be controlled in the apparatus of the invention by adjusting the relative diameters of the upper and lower portions of the reactor vessel. As noted above, the cross-sectional area of the upper portion is larger than the cross-sectional area of the lower portion. Therefore, typically, the diameter D
1
of the upper portion of the reactor vessel will be greater than D
2
, the diameter of the lower portion of the vessel. Preferably D
1
/D
2
is greater than about 1 and about 8 or less, advantageously, D
1
/D
2
is about 1.2 or greater and about 6 or less, and even more preferably, D
1
/D
2
is about 2 or greater and about 5 or less.
Additionally, the relative heights of the upper and lower portions can be adjusted to control gas hydrate production. Assuming that the upper and lower portions each independently maintain an essentially constant diameter (D
1
for the upper portion and D
2
for the lower portion), it is preferred that the ratio of the height of the upper portion (H
1
) to the height of the lower portion (H
2
) remains less than about 10, and even more preferably, this ratio is less than about 5. Typically, this rat
Heinemann Robert Frederick
Huang David Da-Teh
Long Jinping
Saeger Roland Bernard
Griffin Walter D.
Mobil Oil Corporation
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