Gas separation: processes – Liquid contacting – And degasification of a liquid
Reexamination Certificate
2001-10-22
2004-03-30
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And degasification of a liquid
C095S204000, C095S227000, C095S228000, C095S237000, C062S618000, C062S625000
Reexamination Certificate
active
06712880
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to cryogenic gas processes for separating multi-component gaseous hydrocarbon streams to recover both gaseous and liquid compounds. More particularly, the cryogenic gas processes of this invention utilize a high pressure absorber.
2. Background and Prior Art
In most plants, gas processing capacity is generally limited by the horsepower available for recompression of the pipeline sales gas stream. The feed gas stream is typically supplied at 700-1500 psia and expanded to a lower pressure for separation of the various hydrocarbon compounds. The methane-rich stream produced is typically supplied at about 150-450 psia and is recompressed to pipeline sales gas specifications of 1000 psia or above. This pressure difference accounts for the major portion of the horsepower requirement of a cryogenic gas processing plant. If this pressure difference can be minimized, then more recompression horsepower will be available, thereby allowing increased plant capacity of existing gas processing plants. Also, the process of the invention may offer reduced energy requirements for new plants.
Cryogenic expansion processes produce pipeline sales gas by separating the natural gas liquids from hydrocarbon feed gas streams.
In the prior art cryogenic processes, a pressurized hydrocarbon feed gas stream is separated into constituent methane, ethane (C
2
) compounds and/or propane (C
3
) compounds via a single column or a two-column cryogenic separation schemes. In single column schemes, the feed gas stream is cooled by heat exchange contact with other process streams or external refrigeration. The feed gas stream may also be expanded by isentropic expansion to a lower pressure and thereby further cooled. As the feed stream is cooled, high pressure liquids are condensed to produce a two-phase stream that is separated in one or more cold separators into a high pressure liquid stream and a methane-rich vapor stream in one or more cold separators. These streams are then expanded to the operating pressure of the column and introduced to one or more feed trays of the column to produce a bottom stream containing C
2
compounds and/or C
3
compounds and heavier compounds and an overhead stream containing methane and/or C
2
compounds and lighter compounds. Other single column schemes for separating high pressure hydrocarbon streams are described in U.S. Pat. Nos. 5,881,569; 5,568,737; 5,555,748; 5,275,005 to Campbell et al; U.S. Pat. No. 4,966,612 to Bauer; U.S. Pat. Nos. 4,889,545; 4,869,740 to Campbell; and U.S. Pat. No. 4,251,249 to Gulsby.
Separation of a high pressure hydrocarbon gaseous feed stream may also be accomplished in a two-column separation scheme that includes an absorber column and a fractionation column that are typically operated at very slight positive pressure differential. In the two-column separation scheme for recovery of C
2+
and/or C
3+
natural gas liquids, the high pressure feed is cooled and separated in one or more separators to produce a high pressure vapor stream and a high pressure liquid stream. The high pressure vapor stream is expanded to the operating pressure of the fractionation column. This vapor stream is supplied to the absorber column and separated into an absorber bottom stream and an absorber overhead vapor stream containing methane and/or C
2
compounds along with trace amounts of nitrogen and carbon dioxide. The high pressure liquid stream from the separators and the absorber bottom stream are supplied to a fractionation column. The fractionation column produces a fractionation column bottom stream which contains C
2+
compounds and/or C
3+
compounds and a fractionation column overhead stream which may be condensed and supplied to the absorber column as reflux. The fractionation column is typically operated at a slight positive pressure differential above that of the absorber column so that fractionation column overheads may flow to the absorber column. In many of the two-column systems, upsets occur that cause the fractionation column to pressure up, particularly during startup. Pressuring up of the fractionation column poses safety and environmental threats, particularly if the fractionation column is not designed to handle the higher pressure. Other two-column schemes for separating high pressure hydrocarbon streams are described in U.S. Pat. No. 6,182,469 to Campbell et al.; U.S. Pat. No. 5,799,507 to Wilkinson et at.; U.S. Pat. No. 4,895,584 to Buck et al.; U.S. Pat. No. 4,854,955 to Campbell et al.; U.S. Pat. No. 4,705,549 to Sapper; U.S. Pat. No. 4,690,702 to Paradowski et al.; U.S. Pat. No. 4,617,039 to Buck; and U.S. Pat. No. 3,675,435 to Jackson et al.
U.S. Pat. No. 4,657,571 to Gazzi discloses another two-column separation scheme for separating high pressure hydrocarbon gaseous feed streams. The Gazzi process utilizes an absorber and fractionation column that operate at higher pressures than the two-column schemes discussed above. However, the Gazzi process operates with the absorber pressure significantly greater than the fractionation column pressure, as opposed to most two-column schemes that operate at a slight pressure differential between the two vessels. Gazzi specifically teaches the use of a dephlegmator within the fractionation column to strip the feedstreams of a portion of the heavy constituents to provide a stripping liquid for use in the absorber. Gazzi's tower operating pressures are independent of each other. The separation efficiency of the individual towers is controlled by individually altering the operating pressure of each tower. As a result of operating in this manner, the towers in the Gazzi process must operate at very high pressures in order to achieve the separation efficiency desired in each tower. The higher tower pressures require higher initial capital costs for the vessels and associated equipment since they have to be designed for higher pressures than for the present process.
It is known that the energy efficiency of the single column and two-column separation schemes may be improved by operating such columns at higher pressure, such as in the Gazzi patent. When operating pressures are increased, however, separation efficiency and liquid recovery are reduced, often to unacceptable levels. As column pressures increase, the column temperatures also increase, resulting in lower relative volatilities of the compounds in the columns. This is particularly true of the absorber column where the relative volatility of methane and gaseous impurities, such as carbon dioxide, approach unity at higher column pressure and temperature. Also, the number of theoretical stages in respective columns will have to increase in order to maintain separation efficiency. However, the impact of the residue gas compression costs prevails above other cost components. Therefore, the need exists for a separation scheme that operates at high pressures, such as pressures above about 500 psia, yet maintains high hydrocarbon recoveries at reduced horsepower consumption.
Earlier patents have addressed the problem of reduced separation efficiency and liquid recovery, typically, by introducing and/or recycling ethane-rich streams to the column. U.S. Pat. No. 5,992,175 to Yao discloses a process for improving recovery of C
2+
and C
3+
natural gas liquids in a single column operated at pressures of up to 700 psia. Separation efficiency is improved by introducing to the column a stripping gas rich in C
2
compounds and heavier compounds. The stripping gas is obtained by expanding and heating a liquid condensate stream removed from below the lowest feed tray of the column. The two-phase stream produced is separated with the vapors being compressed and cooled and recycled to the column as a stripping gas. However, this process has unacceptable energy efficiency due to the high recompression duty that is inherent in one-column schemes.
U.S. Pat. No. 6,116,050 to Yao discloses a process for improving the separation efficiency of C
3+
compounds in a
Foglietta Jorge H.
Haddad Hazem
Mowrey Earle Ross
Patel Sanjiv N.
Sangave Ajit
ABB Lummus Global Inc.
Bracewell & Patterson L.L.P.
Smith Duane S.
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