Patent Application
20240401877
The present disclosure relates generally to systems and methods for removing nitrogen from a natural gas stream. More particularly, the present disclosure relates to a two-column system and method for removing nitrogen from a natural gas or liquid natural gas stream.
It is often necessary to remove nitrogen from a feed stream of natural gas. This may be done due to purification or nitrogen recovery requirements. The nitrogen removed from the feed stream may be used as fuel or in other applications or vented to atmosphere. Use of a nitrogen rejection unit (NRU) for such processing of natural gas or liquid natural gas feed streams is known in the art, but increases in efficiency and reduced power requirements are desirable.
There are several aspects of the present subject matter which may be embodied separately or together in the methods, 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.
In one aspect, a system for removing nitrogen from a natural gas fluid stream includes a feed stream heat exchanger configured to receive and cool the natural gas fluid stream. A first expansion device is configured to receive and expand fluid from the feed stream heat exchanger. A first feed stream separation device is configured to receive expanded fluid from the first expansion device and includes a vapor outlet and a liquid outlet. A first column is in fluid communication with the liquid and vapor outlets of the first feed stream separation device and is configured to operate at a first operating pressure. The first column includes a first column vapor outlet and a first column liquid outlet and is configured to so that a first nitrogen-enriched vapor stream exits the first column vapor outlet and a first methane-enriched liquid stream exits the first column liquid outlet. A first compressor is configured to receive and compress the first nitrogen-enriched vapor stream. A first after-cooler is configured to receive the compressed first nitrogen-enriched vapor stream from the first compressor. A second expansion device is configured to received fluid from the first after-cooler. A second column is in fluid communication with the second expansion device and is configured to operate at a second operating pressure, where the second operating pressure is greater than the first operating pressure. The second column includes a second column vapor outlet and a second column liquid outlet and is configured so that a second nitrogen-enriched vapor stream exits the second column vapor outlet and a second methane-enriched liquid stream exits the second column liquid outlet. The feed stream heat exchanger is in fluid communication with the second column vapor outlet and the first and second column liquid outlets and is configured to warm nitrogen-enriched fluid and methane-enriched fluid to provide refrigeration within the feed stream heat exchanger.
In another aspect, a method for removing nitrogen from a natural gas fluid feed stream includes the steps of directing a feed stream to a first column, wherein the first column operates at a first operating pressure; separating the feed stream into a first nitrogen-enriched vapor and a first methane-enriched liquid in the first column; compressing and cooling the first nitrogen-enriched vapor; directing the compressed and cooled first nitrogen-enriched vapor to a second column, wherein the second column operates at a second operating pressure that is greater than the first operating pressure; separating the first nitrogen-enriched vapor into a second nitrogen-enriched vapor and a second methane-enriched liquid in the second column; venting the second nitrogen-enriched vapor from the second column; and combining the first methane-enriched liquid from the first column and the second methane-enriched liquid from the second column so as to form a product methane-enriched liquid stream.
A more detailed description of the system and method in accordance with the present disclosure is set forth below. It should be understood that the description below of specific systems and methods is intended to be exemplary, and not exhaustive of all possible variations or applications. Thus, the scope of the disclosure is not intended to be limiting and should be understood to encompass variations or embodiments that would occur to persons of ordinary skill.
It should be noted herein that the lines, conduits, piping, passages and similar structures and the corresponding streams are sometimes both referred to by the same element number set out in the figures.
The term “column” as used below means a distillation, fractionation or rectification column including a contacting column or zone wherein countercurrent liquid and vapor phases are contacted to cause separation of a fluid mixture such as by contacting the vapor and liquid phases on a series of vertically spaced plates or trays or packing material positioned within the column.
Also, as used herein, and as known in the art, a heat exchanger is that device or an area in the device wherein indirect heat exchange occurs between two or more streams at different temperatures, or between a stream and the environment. In addition, all heat exchangers referenced herein may be incorporated into one or more heat exchanger devices or may each be individual heat exchanger devices. As used herein, the terms “communication”, “communicating”, and the like generally refer to fluid communication unless otherwise specified. And although two fluids in communication may exchange heat upon mixing, such an exchange would not be considered to be the same as heat exchange in a heat exchanger, although such an exchange can take place in a heat exchanger.
As used herein, the terms, “high”, “middle”, “warm”, “cold” and the like are relative to comparable streams, as is customary in the art.
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.
An embodiment of the nitrogen rejection unit (NRU) system and method of the disclosure, as illustrated in
The feed stream for system 10 may include a natural gas feed. Typically, natural gas contains nitrogen and methane and may also contain a number of other species which normally occur in natural gas reservoirs such as carbon dioxide and higher hydrocarbons having two to four carbon atoms. Generally, this stream will have been pretreated to remove, to the extent practical, higher hydrocarbons and natural gas liquids. As an example only, the feed stream may contain 4-7 mole % nitrogen and is typically available at about 200-290 psig. The NRU system 10 may, as an example only, produce a final gas product with about 2.0-3.0 mole % nitrogen and a nitrogen vapor with about 0.5-2.5 mole % methane, suitable for venting into the atmosphere.
Referring to
In the illustrated embodiment, a portion of the compressed NRU feed may be refrigerated. For example, a small amount of propane-type refrigeration may be used on a portion of the NRU feed. This would allow a reduced NRU feed pressure by opening up the internal temperature pinch of the NRU inlet heat exchange. Refrigerating the NRU feed could further reduce the system CAPEX and OPEX. This would have a particular advantage for systems which already utilize propane-type refrigeration in an upstream cryogenic natural gas liquids (NGL) recovery unit.
Referring to
Vapor in separation vessel 30 exits the vessel through line 38 towards first main heat exchanger 40. The vapor stream is received by a cooling passage 42 in first main heat exchanger 40 where it is cooled by indirect heat exchange with return streams described below. The cooled stream exits first main heat exchanger 40 via line 44 and passes through expansion valve 46 and line 48 towards column 50. Expansion valve 46, and any other expansion valve described below, may be Joule-Thomson (JT) valve, or other type of expansion valve known in the art. In the illustrated embodiment, a portion of the vapor stream in line 38 may travel through line 52 towards heat exchanger 54. The vapor stream may be received by a cooling passage 56 of heat exchanger 54 and exit heat exchanger 54 via line 58. The cooled stream then may be passed through expansion valve 60 and enter column 50 as a mixed phase via line 62.
Methane enriched liquid in separation vessel 30 exits the vessel via line 64 at the bottom of separation vessel 30. The liquid stream passes through valve 66 and is reduced in pressure before traveling through line 68 to enter separation vessel 70.
In the illustrated embodiment, separation vessel 70 may be a similar vessel to separation vessel 30. Vapor in separation vessel 70 exits through the top of the vessel via line 72. The vapor stream then enters column 50 via line 76. Liquid in separation vessel 70 exits the vessel via line 78 at the bottom of separation vessel 70. The liquid in line 78 may be enriched with methane. The liquid stream passes through expansion valve 80 and line 82 to merge with line 208, which is explained in greater detail below.
First column 50 receives streams via lines 48, 62, and 76 at different locations of the column. Column 50 may provide bulk nitrogen separation. In the first column 50, liquid is passed down against up-flowing vapor to produce a top vapor having a nitrogen concentration exceeding that of the feed and a bottoms liquid having a methane concentration exceeding that of the feed. For example, column 50 may produce an overhead vapor product stream 96 with about 12-24 mole % of nitrogen and a bottoms liquid stream 123 with about 0.5-1.5% nitrogen. The operating pressure of column 50 may be about 230-320 psig. By operating column 50 at this pressure, the bottoms liquid may be vaporized at the NRU supply pressure without re-compression.
As shown in
Methane enriched liquid exits the bottom of column 50 via line 123. After passing through expansion valve 125, the stream travels through line 127 to merge with line 204.
Nitrogen enriched vapor exits the top of column 50 via line 96 to be further separated in second column 142. In some embodiments, a portion of the overhead nitrogen enriched vapor in column 50 can be collected for “low BTU” fuel gas so as to reduce the overall system requirements for nitrogen venting.
As shown in
The compressed and cooled stream may be received by cooling passage 118 in heat exchanger 102 where it is cooled by indirect heat exchange with the vapor stream travelling though warming passage 104. The cooled stream exits heat exchanger 102 via line 120 and is directed towards second column 142. Line 120 may diverge into two lines 121a and 121b. Part of the cooled stream travels through line 121a to be received by cooling passage 124 in second main heat exchanger 122. The cooled stream exits second main heat exchanger 122 via line 126, where it then passes through expansion valve 128. After passing through expansion valve 128, the stream enters second column 142 via line 130. Another portion of the cooled stream travels through line 121b to be received by cooling passage 134 in heat exchanger 132. The cooled stream exits heat exchanger 132 via line 136, after which it then passes through expansion valve 138. After passing through expansion valve 138, the stream merges with line 130 via line 140 to enter column 142.
Second column 142 provides the final nitrogen-methane separation. In an embodiment, second column 142 may be similar to column 50. Second column 142 may produce an overhead vapor product via line 174 with about 0.5-2.5 mole % of methane and a bottoms liquid via line 194 with about 0.5-1.5% nitrogen. The operating pressure of second column 142 may be about 300-350 psig. By operating second column 142 at this pressure, the overhead vapor may be produced by the low-pressure vaporization of a portion of the column bottoms liquid such that the balance of the bottoms liquid may be taken and vaporized at the NRU supply pressure without re-compression.
In the illustrated embodiment, second column 142 may be provided with a reboiler including at least one reboiler line and a heat exchanger 132. In the illustrated embodiment, second column 142 may include a second reboiler line carrying a stream that is received by warming passage 160 in heat exchanger 132. The vaporized stream exits heat exchanger 132 via line 162 and travels back into second column 142.
Additionally, the second column 142 may include a vapor cooling line 164 so that a reflux stream is formed. More specifically, a portion of the vapor in second column 142 may pass through cooling line 164 to be received by cooling passage 166 in a product stream heat exchanger. The cooled stream is then returned from cooling passage 166 to second column 142 as a reflux stream.
Vapor from second separation column 142 may be collected or vented into the atmosphere. For example, nitrogen enriched vapor exits second column 142 via line 174 at the top of second column 142. In the illustrated embodiment, the vapor may be passed through a series of heat exchangers to warm the vapor before it is collected or vented. For example, the vapor stream in line 174 may be received by warming passage 178 in the product stream heat exchanger. After being warmed, the vapor stream exits the product stream heat exchanger and travels through line 180 to be received by warming passage 182 in first main heat exchanger 40. The warmed stream then exits via line 184 and is received by warming passage 186 in heat exchanger 20. The warmed vapor stream exits heat exchanger 20 via line 188 and passes through valve 190 to be collected or vented into the atmosphere via line 192.
Methane enriched liquid exiting second column 142 via line 194 may be collected as a final product gas. In the illustrated embodiment, the methane enriched liquid from second column 142 is combined with methane enriched liquid from other parts of the system to be collected as a final product gas. For example, the methane enriched liquid from second column 142, column 50 and separation vessel 70 may all travel towards line 212 to be collected as a final product gas.
More specifically, methane enriched liquid exits second column 142 via line 194. Line 194 may diverge into lines 195a and 195b. Some of the liquid stream travel through line 195a to pass through expansion valve 196. After passing through valve 196, the stream travels through line 198 to be collected as a product gas. In the illustrated embodiment, the stream in line 198 is received by warming passage 202 in second main heat exchanger 122 to provide cooling therein. The warmed stream exits via line 204 and is received by warming passage 206 in first main heat exchanger 40 to provide cooling therein. The warmed stream then exits via line 208 and is received by warming passage 210 in heat exchanger 20 to provide cooling therein. The warmed stream then exits via line 212 and may be collected. The system may include any number of consecutive warmings for the product liquid without departing from the scope of the disclosure.
Because the low-pressure methane may be produced at about 4-10 psig, some of the methane enriched liquid in second column 142 may be compressed and cooled before being collected. For example, the final product gas may be compressed to the NRU feed pressure. As shown in
In an embodiment, one or more of the valves described above may be associated with a distinct programmable controller. In another embodiment, the controllers are associated with a main programmable controller. In yet another embodiment, the valves included in system 10 may be manually opened and closed.
There are several aspects of the present subject matter which may be embodied separately or together in the methods, 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.
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, the scope of which is defined by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 63/505,316, filed May 31, 2023, the contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63505316 | May 2023 | US |
