Patent Application
20120324943
1. Field of the Invention
This invention relates to a system and method for separating nitrogen from methane and other components from natural gas streams. The invention also relates to a system and method for integrating natural gas liquids (NGL) extraction with nitrogen removal. The system and method of the invention are particularly suitable for use in recovering and processing feed streams typically in excess of 50 MMSCFD.
2. Description of Related Art
Nitrogen contamination is a frequently encountered problem in the production of natural gas from underground reservoirs. The nitrogen may be naturally occurring or may have been injected into the reservoir as part of an enhanced recovery operation. Transporting pipelines typically do not accept natural gas containing more than 4 mole percent inerts, such as nitrogen. As a result, the natural gas feed stream is generally processed to remove such inerts for sale and transportation of the processed natural gas.
One method for removing nitrogen from natural gas is to process the nitrogen and methane containing stream through a nitrogen rejection unit or NRU. The NRU may be comprised of two cryogenic fractionating columns, such as that described in U.S. Pat. Nos. 4,451,275 and 4,609,390. These two column systems have the advantage of achieving high nitrogen purity in the nitrogen vent stream, but require higher capital expenditures for additional plant equipment, including the second column, and may require higher operating expenditures for refrigeration horsepower and for compression horsepower for the resulting methane stream.
The NRU may also be comprised of a single fractionating column, such as that described in U.S. Pat. Nos. 5,141,544, 5,257,505, and 5,375,422. These single column systems have the advantage of reduced capital expenditures on equipment, including elimination of the second column, and reduced operating expenditures because no external refrigeration equipment is necessary. In addition to capital and operating expenditures, many prior NRU systems have limitations associated with processing NRU feed streams containing high concentrations of carbon dioxide. Nitrogen rejection processes involve cryogenic temperatures, which may result in carbon dioxide freezing in certain stages of the process causing blockage of process flow and process disruption. Carbon dioxide is typically removed by conventional methods from the NRU feed stream, to a maximum of approximately 35 parts per million (ppm) carbon dioxide, to avoid these issues.
The system and method disclosed herein facilitate the economically efficient removal of nitrogen from methane in a two step process. The system and method are particularly suitable for NRU feed gas flow rates in excess of 50 MMSCFD and are capable of processing NRU feed gas flow rates of up to around 750 MMSCFD. The system and method are also capable of processing NRU feed gas containing concentrations of carbon dioxide up to approximately 75 ppm for typical nitrogen levels between 20-50%
According to one embodiment of the invention, a system and method are disclosed for processing an NRU feed gas stream containing primarily nitrogen and methane through two fractionating columns to produce a processed natural gas stream suitable for sale to a transporting pipeline. The first stage column is designed to remove methane and heavier hydrocarbon components from nitrogen, while the second stage column is designed to remove nitrogen from the remaining methane. The overhead stream from the first stage column feeds the second stage column. The NRU feed gas and the first stage overhead stream are not cooled to traditional targeted temperatures of −200 to −245 degrees F. The bottoms streams from the first and second fractionating columns are at varying pressures after further processing and are separately fed to a series of compressors to achieve a processed gas product stream of sufficient pressure for sale, typically at least 615 psia. The higher temperatures in the feeds to the fractionating columns allows the bulk of the methane to be separated from the NRU feed stream while reducing the overall compression required for the process by up to 40% when compared to traditional NRU processes.
According to another embodiment of the invention, a system and method is disclosed for NGL extraction integrated into the two column NRU process downstream from the first stage column. In traditional nitrogen separation systems, the separation of NGL components is more difficult in streams containing more than 5% nitrogen because nitrogen has a stripping effect, absorbing ethane and heavier components. According to this embodiment of the invention, the bulk methane and heavier components are removed from the nitrogen in the first column, allowing the bottoms stream containing less than 4% nitrogen, to be further processed for extraction of NGL.
There are several advantages to the system and method disclosed herein not previously achievable by those of ordinary skill in the art using existing technologies. These advantages include, for example, an ability to process higher flow rate NRU feed streams from around 50 MMSCFD up to around 750 MMSCFD, NRU feed streams containing up to 75 ppm carbon dioxide, reduction in overall compression requirements, and integration of NGL extraction. Although the present system and method has the disadvantage of higher capital costs associated with additional equipment, compared to prior single column NRU processes, the costs of such are sufficiently offset by the savings in operating expenses, such as those from the reduced compression requirements, and the ability to efficiently produce a suitable processed natural gas stream and valuable NGL stream.
It will be appreciated by those of ordinary skill in the art upon reading this disclosure that references to separation of nitrogen and methane used herein refer to processing NRU feed gas to produce various multi-component product streams containing large amounts of the particular desired component, but not pure streams of any particular component. One of those product streams is a nitrogen vent stream, which is primarily comprised of nitrogen but may have small amounts of other components, such as methane and ethane. Another product stream is a processed gas stream, which is primarily comprised of methane but may have small of other components, such as nitrogen, ethane, and propane. A third product stream, according to one embodiment of the invention, is an NGL product stream, which is primarily comprised of ethane, propane, and butane but may contain amounts of other components, such as hexane and pentane.
It will also be appreciated by those of ordinary skill in the art upon reading this disclosure that additional processing sections for removing carbon dioxide, water vapor, and possibly other components or contaminants that are present in the NRU feed stream, can also be included in the system and method of the invention, depending upon factors such as, for example, the origin and intended disposition of the product streams and the amounts of such other gases, impurities or contaminants as are present in the NRU feed stream.
The system and method of the invention are further described and explained in relation to the following drawings wherein:
Referring to
Referring to
Referring to both
Where present, it is generally preferable for purposes of the present invention to remove as much of the water vapor and other contaminants from the NRU feed gas 11 or 211 as is reasonably possible prior to separating the nitrogen and methane. It may also be desirable to remove excess amounts of carbon dioxide prior to separating the nitrogen and methane; however, the method and system are capable of processing NRU feed streams containing up to around 75 ppm carbon dioxide without encountering the freeze-out problems associated with prior systems and methods. Methods for removing water vapor, carbon dioxide, and other contaminants are generally known to those of ordinary skill in the art and are not described herein.
System 10 is depicted in greater detail in
Stream 62 from the bottom of the first stage fractionating column 13 is desirably directed to virtual heat exchanger 64 that receives heat (designated by energy stream Q-10) from heat exchanger 22. Stream 62 is at approximately −123° F. and 617 psia and contains approximately 4.6% nitrogen and 85% methane. Vapor stream 66, at approximately −117° F., is returned to the first stage fractionating column 13 as the ascending stripping vapor that strips nitrogen from the hydrocarbon flowing downward through the column. The first stage fractionating column also receives heat (designated by energy stream Q-14) from heat exchanger 22.
In this example, the NRU feed stream 11 contains no carbon dioxide. However, system 10 is capable of processing NRU feed streams containing up to 75 ppm carbon dioxide. The physical separation characteristics of carbon dioxide are similar to an average of ethane and propane. With these parameters, the carbon dioxide would be separated in the first stage fractionating column 13 into the bottoms stream, along with methane, ethane, propane, and other hydrocarbons. The bottoms stream 62 (and subsequent process streams) of the first stage fractionating column 13 does not feed the second stage fractionating column 15 so the carbon dioxide containing stream does not enter the cryogenic section of the process (processing stage 14). This eliminates freeze-out problems with prior systems and increases the carbon dioxide tolerance of system 10 according to the invention from approximately 35 ppm in prior systems to 75 ppm.
Overhead stream 30, containing approximately 37% nitrogen and 63% methane at −147.5° F., exits the first stage fractionating column 13. It is not necessary to use a reflux stream in the first stage fractionating column 13 according to the invention. The operating parameters allow sufficient separation of nitrogen, methane, and carbon dioxide without reflux; however, a reflux stream and related equipment could be used with the first stage column of system 10 if desired. Overhead stream 30 then passes through heat exchanger 32 and exits as stream 34 at −215° F. This cooling is the result of heat exchange with other process streams 54, 80, 100, and 126. Stream 34 passes through primary JT valve 36 and exits the valve as stream 38 having the same temperature as stream 34 but having a reduction in pressure of almost half. The primary JT valve is capable of cooling by the well known Joule-Thomson effect, but in post-start up, steady state operation the valve provides less actual thermal cooling, but does provide the necessary pressure reduction for stream 38, which feeds the second stage fractionating column 15 at −215° F. and 325 psia.
Stream 86 from the bottom of the second stage fractionating column 15 is directed to virtual reboiler 88 that receives heat (designated as energy stream Q-16) from heat exchanger 32. Stream 86 is at approximately −169° F. and 315 psia and contains approximately 5.4% nitrogen and 94% methane. Vapor stream 90, at approximately −163° F., is returned to the second stage fractionating column 15. The second stage fractionating column also receives heat from heat exchanger 32, designated by energy streams Q-18 and Q-20.
Overhead stream 40, containing approximately 98% nitrogen and 1.7% methane at −247° F., internally feeds a reflux condenser depicted by separator 42 and heat exchanger 118 and then exits the second stage fractionating column 15. Internal stream 40 passes through internal condenser 118 and then on to separation chamber 42. Liquid stream 44 exits the separation chamber 42 and to provide reflux to the second stage fractionating column 15. Vapor stream 46 exits condenser 42 containing approximately 99.2% nitrogen and 0.8% methane and passes through expansion valve 48 to drop the pressure and temperature of exiting stream 50 to approximately 30 psia and −306.5° F. Stream 50 then passes through subcooler 52, exiting as stream 54 at approximately −187° F. and 25 psia. Stream 54 passes through heat exchanger 32 and exits as stream 56, warmed to −152° F. Valve 58 controls stream 56, but exiting stream 60 is at substantially the same temperature and pressure as stream 56. Valve 58 is strategically placed so as to provide another level of refrigeration and made available in the heat exchanger 22. This valve and the associated Joule-Thomson effect allows for further cooling of the process stream 24. Stream 60 then passes through heat exchanger 22 and exits the system as nitrogen vent stream 16. Vent stream 16 contains approximately 99.2% nitrogen, 0.8% methane and a trace amount of ethane at a temperature and pressure of approximately 105° F. and 15 psia. Vent stream 16 may be recycled for supplying enhanced oil and gas recovery efforts.
There are several methane enriched streams produced in processing stages 12 and 14. One such stream is stream 138, which contains approximately 3% nitrogen, 84% methane, and 8% ethane. Stream 138 is essentially the bottoms stream from the first stage fractionating column 13, after being further processed as described below. Bottoms stream 62 enters virtual heat exchanger 64 to produce vapor stream 66 and liquid stream 68. Liquid stream 68 is split in splitter 70 into streams 72 and 132. Under the parameters of the specific example and operating conditions described herein, splitter 70 is set so that 100% of stream 68 is directed to stream 132. However, under other operating conditions and parameters, some of the flow from stream 68 may be directed to stream 72. Stream 132 is pumped by the first stage bottom pump 134 (powered by energy stream Q-12), with stream 136 exiting pump 134 at approximately 865 psia. Stream 136 then passes through heat exchanger 22 and exits as stream 138. One primary benefit of this design configuration is that all vaporized product in stream 138 can be routed directly to sales gas pipeline without typical sales gas compression. The result is a dramatic reduction in the overall compression requirement as compared to other typical processes.
The remaining methane enriched streams 84, 104, and 130 are essentially the bottoms stream from the second stage fractionating column 15, after being further processed as described below. Bottoms stream 86 enters reboiler 88 to produce vapor stream 90 and liquid stream 92. Liquid stream 92 is split by splitter 94 into streams 96 (approximately 15% of the flow), 108 (approximately 26% of the flow), and 126 (approximately 59% of the flow). Streams 92, 96, 108, and 126 are all at approximately −163° F. and 315 psia. Stream 96 is controlled by valve 98, with stream 100 exiting the valve at −200° F. and 125 psia. Stream 100 then passes through heat exchanger 32 to stream 102, then through heat exchanger 22 to stream 104. Stream 104 is approximately 3% nitrogen and 96% methane at 105° F. and 116 psia.
Stream 108 passes through subcooler 52, exiting as stream 110 at approximately −290° F. and 310 psia. Stream 110 passes through secondary JT valve 112, with stream 114 exiting the valve. Stream 114 is approximately the same temperature as stream 110, but the pressure has been reduced to approximately 37 psia. Further pressure drop is achieved as stream 114 flows through a vertical (up) length of pipe, becoming stream 116 at 22 psia. Stream 115 passes through heat exchanger 118, supplied with energy stream Q-22 from condenser 42, and exits as stream 120 warmed to −249° F. Stream 120 flows through a vertical (down) length of pipe, becoming stream 122, although there is a negligible change in temperature and pressure between streams 120 and 122 in this example. Stream 122 then passes through subcooler 52, exiting as stream 124 with a slight drop in temperature and pressure. Stream 124 then passes through mixer 78 where it is combined with stream 76 to form stream 80. Stream 72 from splitter 70 is controlled by valve 74, from which stream 76 exits. In this example, no flow is directed to streams 72 or 76, so stream 80 is the same composition as stream 124. Stream 80 then passes through heat exchanger 32, with stream 82 warmed to −152° F. exiting and passing through heat exchanger 22. Stream 84, containing 3% nitrogen and 96% methane at 105° F. and 17 psia, exits heat exchanger 22 from stream 82.
Stream 126 passes through heat exchanger 32, with stream 128 warmed slightly exiting and passing through heat exchanger 22. Stream 130, containing 3% nitrogen and 96% methane at 95° F. and 307 psia, exits heat exchanger 22 from stream 128. Three of the four methane enriched streams, 84, 104, and 130, are each at different pressures, increasing from the low pressure stream 84 (at 17 psia) to the high pressure stream 130 (at 307 psia). These streams all feed into processing stage 18, where they pass through a series of compressors (described below) to achieve a processed gas stream of sufficient pressure for sale.
Referring to
Acceptable inlet compositions in which this invention may operate satisfactorily are listed in the following Table 1:
The flow rates, temperatures and pressures of various flow streams referred to in connection with the discussion of the system and method of the invention in relation to
It will be appreciated by those of ordinary skill in the art that these values are based on the particular parameters and composition of the feed stream in the above example. The values will differ depending on the parameters and composition of the NRU Feed stream 11.
System 210 is depicted in greater detail in
Referring to
Stream 262 from the bottom of the first stage fractionating column 213 is desirably directed to virtual heat exchanger 264 that receives heat (designated by energy stream Q-210) from heat exchanger 222. Stream 262 is at approximately −127° F. and 617 psia and contains 5.6% nitrogen and 90% methane. Vapor stream 266, at −119° F., is returned to the first stage fractionating column 213 as the ascending stripping vapor that strips nitrogen from the hydrocarbon flowing downward through the column.
In this example, the NRU feed stream 211 contains 25 ppm carbon dioxide. However, system 210 is capable of processing NRU feed streams containing up to 75 ppm carbon dioxide as previously discussed. The bottoms stream 262 (and subsequent process streams) of the first stage fractionating column 213, which contains 29 ppm, does not feed the second stage fractionating column 215 so the carbon dioxide containing stream does not enter the cryogenic section of the process (processing stage 214). The overhead stream 230 (and subsequent process streams 234 and 238), which contains only 4.9 ppm carbon dioxide, feeds the second stage fractionating column; however, this small amount of carbon dioxide does not create significant freeze-out problems. The carbon dioxide tolerance of system 210 according to the invention is increased from a maximum of around 35 ppm in prior systems to a maximum of around 75 ppm for typical nitrogen levels in the NRU feed stream.
Overhead stream 230 exits the first stage fractionating column 213 containing approximately 50% nitrogen and 49.6% methane at −164° F. It is not necessary to use a reflux stream in the first stage fractionating column 213 according to the invention. The operating parameters allow sufficient separation of nitrogen, methane, NGL components, and carbon dioxide without reflux; however, a reflux stream and related equipment could be used with the first stage column of system 210 if desired. Overhead stream 230 then passes through heat exchanger 232 and exits as stream 234 at −225° F. Stream 234 passes through primary JT valve 236 and exits the valve as stream 238 having substantially the same temperature as stream 234 but having a pressure reduction of almost half. The primary JT valve is capable of cooling by the well known Joule-Thomson effect, but in post-start up, steady state operation the valve provides less actual thermal cooling, but does provide the necessary pressure reduction for stream 238, which feeds the second stage fractionating column 215 at −225° F. and 325 psia. Stream 238 enters fractionating column 215 at an intermediate stage of the column.
Stream 286 from the bottom of the second stage fractionating column 215 is directed to virtual reboiler 288 that receives heat (designated as energy stream Q-216) from heat exchanger 232. Stream 286 is at −168° F. and 315 psia and contains 5% nitrogen and 94% methane. Vapor stream 290, at approximately −164° F., is returned to the second stage fractionating column 215. The second stage fractionating column also receives heat from heat exchanger 232, designated by energy streams Q-218 and Q-220.
Overhead stream 240, containing approximately 98% nitrogen and 1.7% methane at −247° F., internally feeds a reflux condenser depicted by separator 242 and heat exchanger 318 and then exits the second stage fractionating column 215. Internal stream 240 passes through internal condenser 318 and then on to separation chamber 242. Liquid stream 244 exits the separation chamber 242 and to provide reflux to the second stage fractionating column 215. Vapor stream 246 exits condenser 242 containing approximately 99.2% nitrogen and 0.8% methane and passes through valve 248 to drop the pressure and temperature of exiting stream 250 to approximately 30 psia and −306.5° F. Stream 250 then passes through subcooler 252, exiting as stream 254 at −258° F. and 25 psia. Stream 254 passes through heat exchanger 232 and exits as stream 256, warmed to −172° F. Stream 256 then passes through heat exchanger 222 and exits the system as nitrogen vent stream 216. Vent stream 216 contains approximately 99% nitrogen, 0.8% methane and a trace amount of ethane at a temperature and pressure of approximately 105° F. and 16 psia. Vent stream 216 may be recycled for supplying enhanced oil and gas recovery efforts.
There are several methane enriched streams produced in processing stages 212 and 214. One such stream is stream 338, which contains approximately 3% nitrogen, 88% methane, and 5% ethane, and 4.3 ppm carbon dioxide. Stream 338 is essentially the bottoms stream from the first stage fractionating column 213, after being further processed as described below. Bottoms stream 262 enters virtual heat exchanger 264 to produce vapor stream 266 and liquid stream 268. Liquid stream 268 is split in splitter 270 into streams 272 and 332. Under the parameters of the specific example and operating conditions described herein, splitter 270 is set so that 100% of stream 268 is directed to stream 332. However, under other operating conditions and parameters, some of the flow from stream 268 may be directed to stream 272. Stream 332 at −119° F. and 617 psia passes through expansion valve 334 exiting as stream 336 at −154° F. and 315 psia. Stream 336 then passes through heat exchanger 222 and exits as stream 338.
The remaining methane enriched streams 284, 304, and 230 are essentially the bottoms stream from the second stage fractionating column 215, after being further processed as described below. Bottoms stream 286 enters reboiler 288 to produce vapor stream 290 and liquid stream 292. Liquid stream 292 is split by splitter 294 into streams 296 (approximately 42% of the flow), 308 (approximately 37% of the flow), and 326 (approximately 21% of the flow). Streams 292, 296, 308, and 326 are all at −164° F. and 315 psia. Stream 296 passes through expansion valve 298, with stream 300 exiting the valve at −200° F. and 125 psia. Stream 300 then passes through heat exchanger 232 to stream 302, then through heat exchanger 222 to stream 304. Stream 304 is approximately 3% nitrogen and 96% methane at 107.5° F. and 116 psia.
Stream 308 passes through subcooler 252, exiting as stream 310 at approximately −285° F. and 310 psia. Stream 310 passes through secondary JT valve 312, with stream 314 exiting the valve. Stream 314 is approximately the same temperature as stream 310, but the pressure has been reduced to approximately 36 psia. Further pressure drop is achieved as stream 314 flows through a vertical (up) length of pipe, becoming stream 316 at 21 psia. Stream 316 passes through condenser or heat exchanger 318, supplied with energy stream Q-222 from condenser 242, and exits as stream 320 warmed to −252° F. Stream 320 flows through a vertical (down) length of pipe, becoming stream 322, although there is a negligible change in temperature and pressure between streams 320 and 322 in this example. Stream 322 then passes through subcooler 252, exiting as stream 324 warmed to −200° F. and with a slight drop pressure. Stream 324 then passes through mixer 278 where it is combined with stream 276 to form stream 280. Stream 272 from splitter 270 is controlled by valve 274, from which stream 276 exits. In this example, no flow is directed to streams 272 or 276, so stream 280 is the same composition as stream 324. However, under other operating conditions and parameters, some of the flow from stream 268 may be directed to stream 272 through slitter 270. Stream 280 then passes through heat exchanger 232, with stream 282 warmed to −169° F. exiting and passing through heat exchanger 222. Stream 284, containing 3% nitrogen and 96% methane at 107.5° F. and 16 psia, exits heat exchanger 222 from stream 282.
Stream 326 is mixed with stream 414 (from
Referring to
Referring to
Acceptable inlet compositions in which this invention may operate satisfactorily are listed in the following Table 4:
The flow rates, temperatures and pressures of various flow streams referred to in connection with the discussion of the system and method of the invention in relation to
Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading this specification in view of the accompanying drawings, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventor is legally entitled.
