Patent Application
20250101329
The present disclosure relates generally to systems and methods for purification of methane and, more particularly, to a system and method for removing nitrogen and natural gas liquids from methane.
Natural gas can contain methane and other hydrocarbons, primarily ethane and propane, along with other components including nitrogen and carbon dioxide. Furthermore, there are similar streams formed in refining that can also contain primarily methane. These methane-containing mixtures are often liquefied to make them easier and more economical to transport when pipelines are not available.
Processes for liquefying natural gas are known in the prior art. Many of these processes include purification steps to remove hydrocarbons heavier than methane, sometimes known as natural gas liquids (NGL). Other processes include nitrogen removal, but processes to produce high-purity methane are not common.
There is a need for high-purity methane. High-purity methane is required for many engines that are designed to function optimally when high-purity methane is used as fuel. High-purity methane is particularly important for space applications where the additional weight of nitrogen, which provides no heating value, can become significant. There is also a need for hydrocarbon streams that contain primarily ethane and propane. These can be recovered as NGL. There is also a need for fuel gases that contain nitrogen. These low-Btu fuel gases can be used in burners, heaters, boilers, and other combustion processes.
U.S. Pat. No. 5,615,561 (Houshmand et al.) discloses a process to produce a methane-enriched product from a natural gas feed. Natural gas is cooled and fed to a liquid fractionation column that separates methane from heavier hydrocarbons that exit the process as a liquid product stream. The overhead stream is warmed, compressed, cooled, and flashed. While the flash separators remove some nitrogen in the vapor stream while keeping the methane liquid, they do not remove enough nitrogen to meet the goals of the invention disclosed herein. The number of stages is determined by the need to supply other processes in the plant, not by the purity requirements of the methane product. The fact that removing a large amount of nitrogen is not a requirement of the process leads to a process that removes the heavy hydrocarbons first because it is the only column in the process, unlike the invention disclosed herein.
U.S. Pat. No. 6,526,777 (Campbell et al.) discloses a process to produce LNG and natural gas liquids. This can remove the heavier hydrocarbons to a sufficient degree but does not sufficiently separate the lighter components, like nitrogen, from the methane product. The process has two columns but both columns remove components heavier than methane while methane and nitrogen leave the top of both columns. The final product leaving the top of the second column is purified methane with nitrogen as the only significant impurity. The only nitrogen removal means in the invention is the LNG storage tank where the vapor venting from the tank is enriched in nitrogen relative to the liquid product. While this process increases the methane concentration of the feed, it does not produce sufficient methane purity to meet the objectives of the invention described herein if there is nitrogen in the feed.
U.S. Pat. No. 9,074,815 (Malsam) discloses a process for nitrogen removal from natural gas liquids with heavy hydrocarbon removal. The process contains two columns, a first column that removes heavy hydrocarbons and a second column that removes nitrogen. The disclosed processes start with a feed that contains 73% methane and produce product streams with a maximum methane concentration of 90%. While there are means to remove both lighter and heavier impurities, this is not done to a sufficient degree to meet the goals of the invention disclosed herein and the order of the separation columns is reversed.
U.S. Patent Application 2018/0087833 (Murino et al.) discloses a process with two columns, one that removes heavy hydrocarbons and another that removes nitrogen. Like the other prior art, the methane product does not meet the purity requirements of the invention disclosed herein. Unlike the invention disclosed herein, the process of the Murino patent application removes heavy hydrocarbons first and does not involve warming, compressing, and reliquefying the methane product.
There are several aspects of the present subject matter which may be embodied separately or together in the devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto.
The disclosure provides systems and processes to produce high-purity methane from natural gas or other gas mixtures containing primarily methane with other hydrocarbons and other gases. The purity of the methane product stream can exceed 99.9% with high methane recovery. Depending on the feed composition, embodiments of the disclosure can also produce an ethane-rich byproduct stream.
In an embodiment, the process starts with a cold liquefied natural gas or other mixture. The mixture can be cooled from ambient temperature by any means, including those typically used in a natural gas liquefier. Nitrogen and other gases lighter than methane are removed in a nitrogen removal column. The methane-rich stream leaving the bottom of the nitrogen removal column is expanded and then fed to a demethanizer in which heavier hydrocarbons and other heavier components are removed as a liquid stream that leaves the bottom of the column. The purified methane vapor leaving the top of the demethanizer is warmed, compressed, cooled, and liquefied to produce a high-purity liquid methane product.
Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures for shared elements or components without additional description in the specification in order to provide context for other features.
In the claims, letters are used to identify claimed steps (e.g., a., b. and c.). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which the claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.
Descriptive terms such as low-pressure, high-pressure, warm, warmer, cool, cooler, cold, and colder are used relative to other similar streams and are not intended to be absolute universal terms.
In a first embodiment of the disclosure, illustrated in
A nitrogen-rich product 8 leaves the top of the nitrogen removal column 101 and passes through a nitrogen product valve 151 that controls the flow leaving the column forming a rejected nitrogen stream 9 that leaves the system. A nitrogen-rich reflux stream 10 also leaves the top of the nitrogen removal column 101, is cooled in a nitrogen removal column condenser 105, and returns to the column as a cooled nitrogen-rich reflux 11. The cooled nitrogen-rich reflux 11 can be separated and only the liquid portion used as reflux or the entire stream can return to the nitrogen removal column. The flow of a nitrogen removal column reboiler feed 12 is controlled by a nitrogen removal column reboiler feed valve 155 to produce a controlled nitrogen removal column reboiler feed 13 that is heated in a nitrogen removal column reboiler 112 to form a nitrogen removal column reboiled feed 14. Similar to the cooled nitrogen-rich reflux 11, the nitrogen removal column reboiled feed 14 can also be separated with only a portion, in this case the vapor portion, returned to the nitrogen removal column 101.
A hydrocarbon product 15 also leaves the bottom of the nitrogen removal column 101 and is split into a first hydrocarbon 16 and a second hydrocarbon 18. The first hydrocarbon 16 passes through a first hydrocarbon control valve 158 to form a controlled first hydrocarbon 17. The second hydrocarbon 18 passes through a second hydrocarbon control valve 160 to form a controlled second hydrocarbon 19. The controlled first hydrocarbon 17 mixes with a demethanizer recycle 20 to form a cold demethanizer feed 21, which is heated in a second methane cooler 104 to form a warm demethanizer feed 22. The warm demethanizer feed 22 combines with the controlled second hydrocarbon 19 to form a demethanizer feed 23, which is fed to a demethanizer 102. Alternatively, the warm demethanizer feed 22 and the controlled second hydrocarbon 19 can be fed to the demethanizer 102 separately.
A warm demethanizer reflux 24 leaves the top of the demethanizer 102 and is cooled in a demethanizer condenser 106 to form a cold demethanizer reflux 25 which returns to the top of the demethanizer 102. A purified methane product 29 also leaves the top of the demethanizer 102 and is heated in a first methane cooler 107 to form a warm methane product 30. Alternatively, the purified methane product 29 can be drawn from the top of a separator (not shown) that removes only the vapor from the cold demethanizer reflux 25. In the unlikely event that the warm methane product 30 contains traces of liquid, these can be removed in a methane separator 132. A methane product vapor 31 leaving the top of the methane separator 132 is compressed in a methane compressor 130 to form warm pressurized methane product 32 and cooled in a methane compressor aftercooler 131 to form a warm methane product feed 33. The methane compressor 130 and aftercooler 131 can consist of one or multiple stages, represented by a single stage in the figure. When the system is operating at low flow, a portion of the warm methane product feed 33 can be recycled to the methane compressor 130 using a methane recycle valve 156. The warm methane product feed 33 is cooled in a first methane cooler 107 to produce a first cooled methane product 34.
A first methane portion 35 of the first cooled methane product 34 is passed through a first methane control valve 159 to form a controlled first methane portion 37 which is cooled in the nitrogen removal column reboiler 112 to form a cooled controlled first methane portion 38. A second methane portion 36 of the first cooled methane product 34 is passed through a second methane control valve 161 to form a controlled second methane portion 39 which is cooled in the second methane cooler 104 to form a cooled controlled second methane portion 40, which mixes with the cooled controlled first methane portion 38 to form a second cooled methane product 41. The second cooled methane product 41 is cooled in the third methane cooler 108 to form a third cooled methane product 42 which is cooled in a fourth methane cooler 109 to produce a fourth cooled methane product 43. The fourth cooled methane product 43 passes through a methane product control valve 166 and leaves as a final methane product 44.
A demethanizer reboiler feed 26 passes through a demethanizer reboiler control valve 171 to produce a controlled demethanizer reboiler feed 27, which is heated in a demethanizer reboiler 120 to produce a demethanizer reboiler return 28 that is fed to the bottom of the demethanizer 102. Heat for the demethanizer reboiler 120 can be provided by an ambient temperature stream (not shown) or other means.
A demethanizer product 45 is pressurized in a demethanizer product pump 141 to produce a pressurized demethanizer product 46, which is split into three hydrocarbon portions. A first hydrocarbon portion 47 passes through a first hydrocarbon portion control valve 168 to produce a controlled first hydrocarbon portion 52. A second hydrocarbon portion 48 passes through a second hydrocarbon portion control valve 169 to produce a controlled second hydrocarbon portion 53, which is heated in a CO2 stripper feed heater 110 to produce a warmed controlled second hydrocarbon portion 54 that mixes with the controlled first hydrocarbon portion 52 to form a CO2 stripper feed 55. Alternatively, the controlled first hydrocarbon portion 52 and the warmed controlled second hydrocarbon portion 54 can be fed to the CO2 stripper 103 separately.
A purified CO2 rejection stream 58 leaves the top of a CO2 stripper 103 and passes through a CO2 control valve 167, producing a CO2 vent stream 59. A CO2 stripper reflux 60 also leaves the top of the CO2 stripper 103 and is cooled in a CO2 stripper condenser 111 to produce a cooled CO2 stripper reflux 61 that returns to the top of the CO2 stripper 103.
A CO2 stripper reboiler feed 62 passes through a CO2 stripper reboiler control valve 170 to produce a controlled CO2 stripper reboiler feed 63, which is heated in a CO2 stripper reboiler 121 to produce a CO2 stripper reboiler return 64 that is fed to the bottom of the CO2 stripper 103. Heat for the CO2 stripper reboiler 121 can be provided by an ambient temperature stream (not shown) or other means.
A CO2 stripper bottoms 65 leaves the bottom of the CO2 stripper 103 and is cooled in the CO2 stripper feed heater 110 to produce a cooled CO2 stripper bottoms 66, which is split into a first cooled CO2 stripper bottoms 67 and a second cooled CO2 stripper bottoms 68. The first cooled CO2 stripper bottoms 67 passes through a demethanizer recycle control valve 173 to produce the demethanizer recycle 20 described above. The second cooled CO2 stripper bottoms 68 passes through a hydrocarbon product control valve 175 to produce a hydrocarbon product 69 that contains heavier hydrocarbons from the liquid natural gas feed 1.
A third hydrocarbon portion 49 passes through a third hydrocarbon portion control valve 174 to produce a controlled third hydrocarbon portion 56, which is heated in the CO2 stripper condenser 111 to produce a warmed controlled third hydrocarbon portion 57 that returns to the demethanizer 102.
Additional refrigeration for the system is provided by a liquid nitrogen feed 70 that is split into a first liquid nitrogen refrigerant 71, a second liquid nitrogen refrigerant 75 and a third liquid nitrogen refrigerant 79.
The first liquid nitrogen refrigerant 71 passes through a first liquid nitrogen expansion valve 165 to produce a depressurized first liquid nitrogen refrigerant 72 that is heated in the fourth methane cooler 109 to produce a warmed first nitrogen refrigerant 73 that passes through a first nitrogen control valve 163 to produce a controlled first nitrogen refrigerant 83. The second liquid nitrogen refrigerant 75 passes through a second liquid nitrogen expansion valve 164 to produce a depressurized second liquid nitrogen refrigerant 76 that is heated in the demethanizer condenser 106 to produce a warmed second nitrogen refrigerant 77 that passes through a second nitrogen control valve 162 to produce a controlled second nitrogen refrigerant 78. The third liquid nitrogen refrigerant 79 passes through a third liquid nitrogen expansion valve 157 to produce a depressurized third liquid nitrogen refrigerant 80 that is heated in the nitrogen removal column condenser 105 to produce a warmed third nitrogen refrigerant 81 that passes through a third nitrogen control valve 154 to produce a controlled third nitrogen refrigerant 82. The controlled first nitrogen refrigerant 83, the controlled second nitrogen refrigerant 78, and the controlled third nitrogen refrigerant 82 combine to form a nitrogen refrigerant vent 74.
Example data for the streams of
There are many alternative methods to practice the technology of the disclosure that come within the scope of the appended claims. A separator can be used instead of the CO2 stripper or CO2 removal can be eliminated entirely depending on the purity requirements of the hydrocarbon product. Several different configurations of heat exchange can be used, including multi-stream and two-stream heat exchangers. A closed refrigeration loop can be used to provide cooling instead of consuming liquid nitrogen. The refrigerant can be nitrogen, another material, or mixtures of different components, such as nitrogen and hydrocarbons. Many of these alternatives involve trading operating and capital costs, so the decision of which to use will often be made based of what alternatives are available at a particular location and the tradeoff between capital and operating costs to identify the optimum for the particular plant being considered.
An alternative embodiment of the system and method of the disclosure is illustrated in
Many of the streams and pieces of equipment in
A nitrogen-rich product 209 leaves the top of the nitrogen removal column 265 and is cooled in a second heat exchanger 263 to produce a cooled nitrogen-rich product 210, which is separated in a nitrogen separator 269 into a rejected nitrogen vapor 211 and a nitrogen removal column reflux liquid 212, which returns to the nitrogen removal column 265. A nitrogen removal column reboiler feed 205 is heated in a nitrogen removal column reboiler 273 and separated to form a nitrogen removal column reboiled feed vapor 206 and a hydrocarbon product liquid 207, the flow of which is controlled by a hydrocarbon product control valve 292 that produces a controlled hydrocarbon product liquid 208, which is warmed in a third heat exchanger 264 to produce a demethanizer feed 213, which is fed to a demethanizer 266.
In an alternative embodiment, stream 208 of
A demethanizer reboiler feed 214 is heated in a demethanizer reboiler 274 and separated to form a demethanizer reboiled feed vapor 215 and a heavy hydrocarbon product liquid 216, which can be fed to a CO2 stripper similar to what is shown in
A purified methane product 217 leaves the top of the demethanizer 266 and is cooled in the second heat exchanger 263 to produce a cooled purified methane product 218, which is separated in a demethanizer reflux separator 270 to produce a demethanizer reflux liquid 219 and a purified methane vapor 220. The purified methane vapor 220 is warmed in the third heat exchanger 264 to produce a warm purified methane stream 221, which is compressed in a methane compressor 271 to produce a warm compressed methane stream 222, which is cooled in a methane compressor aftercooler 272 to produce a purified compressed methane stream 223, which is cooled in the third heat exchanger against the purified methane vapor 220 to produce a cooled purified methane stream 224, which is cooled further in the second heat exchanger 263 to produce a purified liquid methane 225, which exits a liquid methane control valve 293 to produce a purified liquid methane product 226.
A warm, low-pressure refrigerant 231 is compressed in a first refrigerant compressor 275 to produce a first compressed refrigerant 232, which is cooled in a first aftercooler 279 to produce a first cooled compressed refrigerant 233. The first cooled compressed refrigerant 233 is mixed with a warm refrigerant 257 to produce a combined compressed refrigerant 234, which is compressed in a second refrigerant compressor 276 to produce a second compressed refrigerant 235, which is cooled in a second aftercooler 280 to produce a second cooled compressed refrigerant 236. The second cooled compressed refrigerant 236 is compressed in a third refrigerant compressor 277 to produce a third compressed refrigerant 237, which is cooled in a third aftercooler 281 to produce a third cooled compressed refrigerant 238.
The third cooled compressed refrigerant 238 is cooled in the first heat exchanger 262 to produce a first cold compressed refrigerant 239 which is split between a refrigerant expander feed 240 and a second cold compressed refrigerant 243. The refrigerant expander feed 240 is expanded in a refrigerant expander 278 to produce a cold expanded refrigerant 241. The second cold compressed refrigerant 243 is cooled further in the first heat exchanger 262 to produce a third cold compressed refrigerant 244.
The third cold compressed refrigerant 244 is split into a low-pressure expansion valve feed 245 and an intermediate-pressure expansion valve feed 246. The low-pressure expansion valve feed 245 is expanded in a low-pressure expansion valve 294 to produce a cold low-pressure refrigerant 247 that is separated into a low-pressure refrigerant vapor 251 and a low-pressure refrigerant liquid 249 in a low-pressure refrigerant separator 267. The intermediate-pressure expansion valve feed 246 is expanded in an intermediate-pressure expansion valve 295 to produce a cold intermediate-pressure refrigerant 248 that is separated into an intermediate-pressure refrigerant vapor 252 and an intermediate-pressure refrigerant liquid 255 in an intermediate-pressure refrigerant separator 268.
The low-pressure refrigerant liquid 249 provides cooling to the second heat exchanger 263 and exits as a warmed low-pressure refrigerant 250. The low-pressure refrigerant vapor 251 and the intermediate-pressure refrigerant vapor 252 combine to form a combined refrigerant vapor 253, which combines with the warmed low-pressure refrigerant 250 to form a combined refrigerant 254. The intermediate-pressure refrigerant liquid 255 also provides cooling to the second heat exchanger 263 and exits as an intermediate-pressure refrigerant 256.
The intermediate-pressure refrigerant 256 provides cooling to the first heat exchanger 262 and exits as the warm refrigerant 257 that combines with the first cooled compressed refrigerant 233 described earlier. The combined refrigerant 254 exits the first heat exchanger as a warmed combined refrigerant 258 that combines with the cold expanded refrigerant 241 to produce a cold low-pressure refrigerant 242 that provides additional cooling to the first heat exchanger 262 and exits as the warm low-pressure refrigerant 231 that feeds the first refrigerant compressor 275 described earlier.
While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention.
This application claims the benefit of U.S. Provisional Application No. 63/585,268, filed Sep. 26, 2023, the contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63585268 | Sep 2023 | US |
