Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-11-12
2001-02-06
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S625000
Reexamination Certificate
active
06182469
ABSTRACT:
Ethylene, ethane, propylene, propane and/or heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. Natural gas usually has a major proportion of methane and ethane, i.e., methane and ethane together comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes and the like, as well as hydrogen, nitrogen, carbon dioxide and other gases.
The present invention is generally concerned with the recovery of ethylene, ethane, propylene, propane and heavier hydrocarbons from such gas streams. A typical analysis of a gas stream to be processed in accordance with this invention would be, in approximate mole percent, 92.12% methane, 3.96% ethane and other C
2
components, 1.05% propane and other C
3
components, 0.15% iso-butane, 0.21% normal butane, 0.11% pentanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present.
The historically cyclic fluctuations in the prices of both natural gas and its natural gas liquid (NGL) constituents have at times reduced the incremental value of ethane, ethylene, propane, propylene, and heavier components as liquid products. Competition for processing rights has forced plant operators to maximize the processing capacity and recovery efficiency of their existing gas processing plants. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, and refrigerated oil absorption. Additionally, cryogenic processes have become popular because of the availability of economical equipment that produces power while simultaneously expanding and extracting heat from the gas being processed. Depending upon the pressure of the gas source, the richness (ethane, ethylene, and heavier hydrocarbons content) of the gas, and the desired end products, each of these processes or a combination thereof may be employed.
The cryogenic expansion process is now generally preferred for natural gas liquids recovery because it provides maximum simplicity with ease of start up, operating flexibility, good efficiency, safety, and good reliability. U.S. Pat. Nos. 4,157,904; 4,171,964; 4,185,978; 4,251,249; 4,278,457; 4,519,824; 4,617,039; 4,687,499; 4,689,063; 4,690,702; 4,854,955; 4,869,740; 4,889,545; 5,275,005; 5,555,748; 5,568,737; 5,771,712; 5,799,507; 5,881,569; 5,890,378; reissue 32141-60/110,502 U.S. Pat. No. 33,408; and co-pending application no. 09/054,802 now U.S. Pat. No. 5,983,664 describe relevant processes (although the description of the present invention in some cases is based on different processing conditions than those described in the cited U.S. patents and patent applications).
In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high-pressure liquids containing some of the desired C
2
+ components. Depending on the richness of the gas and the amount of liquids formed, the high-pressure liquids may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquids results in further cooling of the stream. Under some conditions, pre-cooling the high pressure liquids prior to the expansion may be desirable in order to further lower the temperature resulting from the expansion. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation (demethanizer) column. In the column, the expansion cooled stream(s) is (are) distilled to separate residual methane, nitrogen, and other volatile gases as overhead vapor from the desired C
2
components, C
3
components, and heavier hydrocarbon components as bottom liquid product.
If the feed gas is not totally condensed (typically it is not), at least a portion of the vapor remaining from the partial condensation can be passed through a work expansion machine or engine, or an expansion valve, to a lower pressure at which additional liquids are condensed as a result of further cooling of the stream. The pressure after expansion is essentially the same as the pressure at which the distillation column is operated. The combined vapor-liquid phases resulting from the expansion are supplied as a feed to the column. In recent years, the preferred processes for hydrocarbon separation involve feeding this expanded vapor-liquid stream at a mid-column feed point, with an upper absorber section providing additional rectification of the vapor phase.
The source of the reflux stream for the upper rectification section is typically a portion of the above mentioned vapor remaining after partial condensation of the feed gas, but withdrawn prior to work expansion. An alternate source for the upper reflux stream may be provided by a recycled stream of residue gas supplied under pressure. Regardless of its source, this vapor stream is usually cooled to substantial condensation by heat exchange with other process streams, e.g., the cold fractionation tower overhead. Some or all of the high-pressure liquid resulting from partial condensation of the feed gas may be combined with this vapor stream prior to cooling. The resulting substantially condensed stream is then expanded through an appropriate expansion device, such as an expansion valve, to the pressure at which the demethanizer is operated. During expansion, a portion of the liquid will usually vaporize, resulting in cooling of the total stream. The flash expanded stream is then supplied as top feed to the demethanizer. Typically, the vapor portion of the expanded stream and the demethanizer overhead vapor combine in an upper separator section in the fractionation tower as residual methane product gas. Alternatively, the cooled and expanded stream may be supplied to a separator to provide vapor and liquid streams, so that thereafter the vapor is combined with the tower overhead and the liquid is supplied to the column as a top column feed.
The purpose of this process is to perform a separation that produces a residue gas leaving the process which contains substantially all of the methane in the feed gas with essentially none of the C
2
components and heavier hydrocarbon components, and a bottoms fraction leaving the demethanizer which contains substantially all of the C
2
components and heavier hydrocarbon components with essentially no methane or more volatile components while meeting plant specifications for maximum permissible carbon dioxide content. The present invention provides a means for providing a new plant or modifying an existing processing plant to achieve this separation at significantly lower capital cost by reducing the size of or eliminating the need for a product treating system for removal of carbon dioxide. Alternatively, the present invention, whether applied in a new facility or as a modification to an existing processing plant, can be used to recover more C
2
components and heavier hydrocarbon components in the bottom liquid product for a given carbon dioxide concentration in the feed gas than other processing schemes.
In accordance with the present invention, it has been found that C
2
recoveries in excess of 84 percent can be maintained while maintaining the carbon dioxide content of the bottom liquid product within specifications and providing essentially complete rejection of methane to the residue gas stream. The present invention, although applicable at lower pressures and warmer temperatures, is particularly advantageous when processing feed gases at pressures in the range of 600 to 1000 psia or higher under conditions requiring column overhead temperatures of −120° F. or colder.
The present invention uses a modified rebo
Campbell Roy E.
Campbell Wanda P.
Hudson Hank M.
Pierce Michael C.
Wilkinson John D.
Baker & Botts LLP
Campbell Wanda P.
Capossela Ronald
Elcor Corporation
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