Gas separation: processes – Liquid contacting – And deflection
Reexamination Certificate
1999-08-26
2001-02-06
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And deflection
C095S226000, C095S231000, C096S278000, C096S295000, C096S326000, C096S346000, C096S350000
Reexamination Certificate
active
06183541
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to equipment used to purify natural gas and more particularly concerns dehydrators for removing water from natural gas.
Natural gas is presently dried by use of trayed towers, perhaps 1.5 to 9 feet in diameter and 20 or more feet tall. The gas is passed from a spreader at the bottom of the tower to a small outlet at the top of the tower while liquid desiccant, such as triethylene glycol spurted into the top of the tower against approximately 1,000 psi flows to bottom of the tower as each tray overflows to the next in zig-zag fashion. Each tray interposes an approximately one inch thick lamina of desiccant in the path of the gas. Since there are typically 5 to 9 trays in a tower, every gas molecule is expected to pass through 5 to 9 inches of desiccant. The wet desiccant at the bottom of the tower is then heated to boil off the water and the pure desiccant is pumped to the top of the tower. For example, if triethylene glycol is used, the wet glycol is heated to approximately 375° F. to boil off the water.
There are serious disadvantages in using the trayed tower equipment. A first disadvantage is that the diameter of the dried gas outlet at the top of the tower, typically is ⅙ the diameter of the trays. Since the gas tries to flow along the most direct path to the outlet, the gas flow pattern is generally conical. Thus, while the trays and, therefor, the glycol laminae are of equal diameter, the unused area of each sequentially higher lamina concentrically increases until, at the top of the tower, very little of the glycol is put to use. A second disadvantage related to the first is that, as tower diameter is increased, the inefficiency of the unit is also increased because the unused area is a function of the square of the diameter. Thus, while an 18″ diameter tower might require 3 gallons of glycol to absorb 1 pound of water, a 9′ diameter tower might require 4 gallons of glycol to absorb 1 pound of water. This leads us to a third disadvantage which is that, even for small diameter towers, more glycol must be heated in the boil-off process than was effectively used in the drying process and, for larger diameter towers, energy is wasted at an increasing rate. A fourth disadvantage is that the trayed tower system blows an initial gas molecule through a small hole in the bubble cap associated with the tray. Subsequent gas molecules tend to follow the path cleared by the initial molecule and, therefor, may not be fully contacted by the desiccant. A fifth disadvantage is that, since the tower configuration provides 5 to 9 one inch laminae of desiccant, the residence time of the gas in the desiccant is quite limited, further reducing the efficiency of the equipment. A sixth disadvantage of trayed tower dehydrators is that, although the gas is relatively volatile, the gas flow pattern is so linear that there is too little interaction of the molecules to promote useful contact with desiccant between laminae. A seventh disadvantage of trayed tower dehydrators is that they cannot be used in floating production system operation or FPSO such as a tanker retrofitted with production equipment on the deck. The angle and alignment of the trays must be maintained to assure the uniform flow of desiccant across each tray and overflow between trays. The natural motion of a ship or tanker floating on a body of water prohibits this necessary uniform flow and overflow. Most of the future offshore drilling and production will be in water one to two miles or more deep. This is impossible with fixed leg platforms and, therefore, all such drilling and production will be done using FPSO systems.
It is, therefor, an object of this invention to provide a dehydrator for and method of dehydrating natural gas in which the flow path of the gas is sinusoidal along a horizontal axis into and out of the liquid desiccant. Another object of this invention is to provide a dehydrator for and method of dehydrating natural gas in which the flow path of the gas is sinusoidal between zeniths and nadirs radially distant from the longitudinal axis of the dehydrator. Yet another object of this invention is to provide a dehydrator for and method of dehydrating natural gas which uses the liquid desiccant available for the drying process and the energy required in the boil-off process more efficiently than in presently known dehydrators. Still another object of this invention is to provide a dehydrator for and method of dehydrating natural gas in which a greater percentage of the gas molecules come in full contact with the liquid desiccant while they are in the desiccant pool than in presently known dehydrators. A further object of this invention is to provide a dehydrator for and a method of dehydrating natural gas which afford greater residence time of the gas in the liquid desiccant than presently known dehydrators. Another object of this invention is to provide a dehydrator for and method of dehydrating natural gas which take advantage of the volatile nature of the gas to cause the gas to come into useful contact with liquid desiccant even when the gas is not in the desiccant pool. It is also an object of this invention to provide a dehydrator for and method of dehydrating natural gas which is useful in floater applications.
SUMMARY OF THE INVENTION
In accordance with the invention, a natural gas dehydrator and process for dehydrating natural gas are provided in which a longitudinally horizontal pressure vessel containing a threshold level of liquid desiccant has an inlet for admitting gas at one end and an outlet for discharging gas at an opposite end. A plurality of spaced-apart baffles divides the vessel into a series of pneumatically discrete compartments. Each of the baffles has a plurality of apertures therethrough, the apertures of alternate baffles being disposed in lower and upper portions thereof so as to define a sinusoidal gas flow path into and out of the liquid desiccant from the inlet to the outlet. Each of the baffles also has an opening in the lowermost portion thereof so as to define a linear primary liquid desiccant flow path from the inlet to the outlet. The baffled vessel is fixed within another longitudinally horizontal pressure vessel so as to define a chamber therebetween. Dried gas and wet desiccant are discharged through the inner or baffled vessel outlet into the chamber where they are gravity separated, the dried gas flowing to an outlet on top of the outer vessel and the wet desiccant flowing to a sump at the bottom of the outer vessel.
Preferably, the vessels are concentrically cylindrical and the baffles are substantially circular with the apertures disposed in the upper and lower 40% of the surface area of alternate baffles. The threshold level establishes a pool for liquid desiccant which covers the lower 60% of the surface area of all the baffles. The apertures of each baffle are spaced so as to promote the homogeneous dispersal of the gas in the liquid desiccant.
Preferably, an inlet pipe extending from the inner vessel inlet through an end wall of the outer vessel is turned upwardly to prevent backflow of liquid desiccant toward the natural gas sources. An outlet pipe extending from the inner vessel outlet turns upwardly into the chamber between the vessels to set the threshold level of liquid desiccant. The outlet pipe has a tee configuration so as to discharge laterally in two directions. An annular baffle proximate the inner vessel outlet divides the chamber. The annular baffle has perforations for removing wet desiccant from the dried gas molecules as the gas flows therethrough and has an open lower portion for passing wet liquid desiccant therethrough. Also preferably, a level control, valve and sight glass are provided to facilitate maintenance and observation of the desiccant level in the sump. A mist extractor disposed in the chamber filters the dried gas before discharge through the outer vessel gas outlet.
According to the process for dehydrating natural gas, liquid desiccant is pumped at the pred
Catalano Frank J.
Smith Duane S.
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