US20250251189A1

US20250251189A1 - System and Method for Separating Nitrogen from Methane with Ultra-Low Greenhouse Gas Emissions

Info

Publication number: US20250251189A1
Application number: US18/434,401
Authority: US
Inventors: Rayburn C. Butts
Original assignee: BCCK Holding Co
Current assignee: BCCK Holding Co
Priority date: 2024-02-06
Filing date: 2024-02-06
Publication date: 2025-08-07

Abstract

A system and method for removing nitrogen from natural gas using two fractionating columns to achieve an ultra-low greenhouse gas content in a nitrogen vent/product stream, while also producing three sales gas streams at different pressures and with low nitrogen content within pipeline specifications. A portion of a low pressure column overhead stream may be compressed and cooled and recycled back to provide reflux to the low pressure column. A system feed stream is cooled upstream of feed a high pressure column, but preferably not separated into streams with varying compositions. A portion of the high pressure column bottoms stream and the low pressure column bottoms stream provides refrigerant to the high pressure column to produce a reflux stream. An amount of methane in a nitrogen vent/nitrogen product stream may be less than 0.01%.

Description

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

This disclosure relates to systems and methods for separating nitrogen from methane and other components from natural gas streams, and particularly onshore LNG liquefaction feed streams, from 100 to 500 MMSCFD with ultra-low greenhouse gas emissions in a nitrogen vent or product stream compared to prior art systems and methods.

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 sales streams 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. These processes result in a sales gas stream comprising primarily methane with less than 4% nitrogen and a nitrogen stream, which is typically vented to the atmosphere, but may be flared or used as fuel gas depending on the amount of hydrocarbons. The nitrogen vent stream typically comprises 1 to 3% methane.
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. Many single column systems have a single sales gas stream exiting the NRU fractionating column, usually at a lower pressure requiring compression to meet pipeline requirements. For example, in U.S. Pat. No. 5,141,544, an NRU feed stream is first processed to remove water and carbon dioxide (to avoid freezing problems associated in carbon dioxide) and is then split into three portions prior to feeding the single column NRU. A first portion is cooled through heat exchange with an overhead stream from the NRU column, a second portion is cooled through heat exchange with the NRU column bottoms stream, and a third portion is cooled through heat exchange with a side stream withdrawn from and returned to the NRU column in a reboiler for the NRU column. The first, second and third portions of the feed stream are recombined, the recombined stream is further cooled through heat exchange with the NRU column bottoms stream, and then passes through a JT valve prior to feeding into the NRU column as a liquid and vapor mixed phase stream around −215° F. and around 170 psig. The single NRU bottoms stream is a sales gas stream at a pressure around 60 psig in the example in the '544 patent, requiring further compression. The overhead stream from the single column NRU is the nitrogen stream, which is used as fuel gas and would contain around 3 to 5% methane.
Some single column systems also split the NRU column bottoms stream into two streams to allow for additional heat exchange with other process streams and resulting in two sales gas streams at different pressures. For example, in U.S. Pat. No. 5,375,422, an NRU feed stream is first processed to remove water and carbon dioxide and is then split into four portions prior to feeding the single column NRU. A first portion is cooled through heat exchange with an overhead stream from the NRU column; a second portion is cooled through heat exchange with a first portion of the NRU column bottoms stream after passing through the NRU column reboiler, then an internal reflux condenser in the NRU column, and then back through the reboiler; and a third portion is cooled through heat exchange with a second portion of a bottoms stream from the NRU column. The first, second and third portions of the feed stream are recombined and the recombined stream passes through a JT valve prior to feeding into the NRU column as a liquid and vapor mixed phase stream between −60 and −150° F. and around 315 psig. The fourth portion of the feed stream is cooled through two separate heat exchanges, each with a side stream withdrawn from and returned to the NRU column, before passing through a JT valve and feeding into the NRU column as a liquid and vapor mixed stream between −200 and −250° F. and around 315 psig. The fourth portion of the feed stream feeds into the NRU column at a location that is several trays above the recombined first, second, and third portions. The overhead stream from the single column NRU is the nitrogen vent stream, which would contain around 3 to 5% methane. The NRU bottoms stream is split into the first and second portions, each of which is processed differently to achieve the desired heat exchange with other process streams. The different processing of the two portions of the NRU bottoms stream results in two sales gas streams, one at a pressure of around 20 psig and the other at a pressure around 300 psig. Such a single tower system producing only two sales gas streams, the horsepower per inlet MMSCF generally runs around 100 to 110 HP/MMSCF.
Compared to two column systems, these single column systems have the advantage of reduced capital expenditures on equipment, including elimination of the second column. However, they can also result in higher amounts of greenhouse gases being retained in the nitrogen vent stream (or fuel gas stream), such as in the '544 and '422 patents. In contrast, a two column system disclosed in U.S. Pat. No. 4,451,275 achieves very low methane of around 0.09 to 0.23% in the nitrogen vent stream, but has excess nitrogen of around 15% in the sales gas stream, based on system feeds stream having 21 to 76% nitrogen. Still other two column system examples have better results with respect to nitrogen in the sales gas stream, but not as good results for methane in the nitrogen vent stream. For example, U.S. Pat. No. 11,650,009 discloses around 1.57% to 2.23% methane in the nitrogen vent stream and around 1.9 to 3.2% nitrogen in the sales gas streams based on a system feed with 20% nitrogen and U.S. Patent Application Publication No. 2012/0324943 discloses a nitrogen vent stream that has 0.8% methane and sales gas streams with around 3% nitrogen based on a system feed stream having 25% nitrogen. In recent years, concerns have grown over emissions of greenhouse gases, such as methane. Therefore, there is a need for systems and methods that achieve high percentages of methane and low percentages of nitrogen in the sales gas stream, while also minimizing the amount of methane that may be released to the atmosphere with the nitrogen vent stream.
Many prior NRU systems also 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 100 parts per million (ppm) (although some prior art systems have a maximum of only 35 ppm) carbon dioxide, to avoid these issues. There is a need for systems and methods that can process natural gas streams with higher concentrations of CO₂.
There is a need for systems and methods to efficiently separate nitrogen from methane and other components in natural gas streams with reduced greenhouse gas emissions in the nitrogen vent/product stream. There is also a need to reduce greenhouse gas emissions while also maintaining low levels of nitrogen in the sale gas stream(s), reducing energy/horsepower requirements, and/or the capability to process system feed streams with higher concentrations of carbon dioxide.

BRIEF SUMMARY

The systems and methods disclosed herein facilitate the economically efficient removal of nitrogen from methane with substantially reduced greenhouse gases in a nitrogen vent/nitrogen product stream. The systems and methods herein are uniquely well suited for excess nitrogen removal from onshore LNG liquefaction feed streams, but may also be used with other natural gas feed streams. According to some embodiments, a system and method are disclosed for processing a system feed gas stream containing primarily nitrogen and methane through two fractionating columns to produce (1) one or more sales gas streams with nitrogen content within pipeline specifications and (2) a nitrogen vent, or nitrogen product, stream with ultra-low greenhouse gases.
In some embodiments, an amount of methane in a nitrogen vent/nitrogen product stream in preferred embodiments of systems and methods herein may be less than around 100 ppm or around 0.01% and an amount of non-methane hydrocarbons in the nitrogen vent/nitrogen product stream will be zero. In some embodiments, an amount of methane in a nitrogen vent/nitrogen product stream may be less than 0.05%, or may be less than 0.03%. In some embodiments, a nitrogen vent/nitrogen product stream contains no hydrocarbons other than methane. Because a nitrogen stream comprises so little hydrocarbons, it may be used as a nitrogen product stream rather than venting to the atmosphere. In some embodiments, a nitrogen vent/nitrogen product stream may comprise an amount of methane on a mass basis that is around 10 times less than achievable in some prior art systems and around 100 times less than achievable in other prior art systems.
In some embodiments, an amount of nitrogen in the one or more sales gas streams in some embodiments of systems and methods herein may be less than 1.0%. The systems and methods herein are particularly suitable for feed gas flow rates of around 100 MMSCFD or more and having nitrogen contents ranging from 1.5 mol % to 25 mol %.
According to some embodiments, the first stage column is designed as a high pressure NRU column to remove the bulk of the incoming nitrogen from the methane and heavier hydrocarbon components in a system feed stream, while the second stage column is operated at a lower pressure and designed to remove excess methane from the nitrogen. The first column also operates at a higher pressure, which may be around 300 to 500 psig, compared to prior art systems. The second column also operates at a lower pressure, which may be around 75 to 125 psig.
According to some embodiments, a system and method are disclosed with efficient heat exchange between system streams. In preferred embodiments, a system comprises a first, a second, and a third primary heat exchanger. In some embodiments, a system feed stream is cooled in the first heat exchanger through heat exchange with a bottoms stream from the first column and a bottoms stream from the second column. In some embodiments, the system feed is split after passing through the first heat exchanger and a portion is recycled back through the first heat exchanger for further cooling prior to feeding the first column.
In some embodiments, an overhead stream from the first column is cooled in the second heat exchanger through heat exchange with the second column bottoms stream (upstream of the first heat exchanger) and a portion of the second column overhead stream prior to the first column overhead stream feeding into the second column. In some embodiments, an overhead stream from the first column is not split into multiple portions prior to feeding into the second column. In some embodiments, an entirety of the first column overhead stream passes through the second heat exchanger. In some embodiments, the second heat exchanger is the only heat exchanger through which the first column overhead stream passes prior to feeding into the second column. In some embodiments, the first column overhead stream is not separated prior to feeding into the second column.
According to some embodiments, the second heat exchanger also supplies heat to act as a reboiler for the second column. In a preferred embodiment, the second column does not include an internal reboiler.
In some embodiments, a recycled stream is cooled in the third heat exchanger through heat exchange with at least a second portion of the second column overhead stream prior to the recycled stream feeding back into an upper level of the second column as a reflux stream. According to some embodiments, the first, second, and third heat exchangers are plate and fin exchangers, but other types of exchangers may also be used.
According to some embodiments, a system further comprises two additional heat exchangers that act as a reboiler and condenser for the first fractionating column. In some embodiments, a fourth heat exchanger comprises a shell and tube exchanger that acts as a reboiler for the first column. Preferably, a source of heat on a tube side of the fourth heat exchanger is a second portion of the system feed stream, most preferably downstream of the system feed stream passing through the first heat exchanger. A liquid stream from a bottom of the first column feeds into the shell side of the fourth heat exchanger, with a vapor portion returning to the first column and a liquid portion exiting as the first column bottoms stream.
In other embodiments, a fifth heat exchanger comprises a shell and tube exchanger that acts as a condenser for the first column. A stream from a top of the first column feeds into a tube side of the fifth heat exchanger, with a liquid portion returning to the column and a vapor portion exiting the column as the first column overhead stream. Preferably, a source of refrigerant on the shell side for the fifth heat exchanger comprises a first portion of a bottoms stream from the first column upstream of the first portion passing through the first heat exchanger. In another embodiment, the source of refrigerant for the fifth heat exchanger further comprises a bottoms stream for the second column downstream of passing through the second heat exchanger. The first portion of the first column bottoms stream and the second column bottoms stream are preferably mixed together upstream of the fifth heat exchanger. In some embodiments, by controlling an amount of refrigerant that feeds into the shell side of the fifth heat exchanger, effective control of the concentration of nitrogen exiting the first column overhead stream (and subsequently feeding into the second column) is achieved, which in turn aids in controlling the amount of methane exiting the second column overhead stream (which becomes the nitrogen vent, or nitrogen product, stream). In some embodiments, an amount of refrigerant feeding to the shell side of the fifth heat exchanger is controlled by controlling a portion of the first column bottoms stream that is split into the first portion. The effectiveness of the second column largely depends on the nitrogen content feeding the second column and the reflux provided to the second column (discussed further below).
In some embodiments, a fifth heat exchanger comprises a tube side (tube) and a shell side that are independent pieces of equipment configured as a vertical tube, falling film condenser. In some embodiments, a fifth heat exchanger comprises an internal knockback condenser with the first fractionating column.
According to some embodiments, reflux to the second column is provided by a portion of the second column overhead stream that is compressed and cooled and recycled back to the second column. Preferably, at least a portion of the second column overhead stream is warmed in the third heat exchanger through heat exchange with the recycled stream downstream of the recycled stream passing through a series of compressors and coolers. The recycled stream feeds back into an upper level of the second column as a reflux stream after passing through the third heat exchanger. In some embodiments, the recycled stream passes through a valve to expand and further cool the recycled stream downstream of the third heat exchanger and upstream of feeding into the second column. The effectiveness of the second column largely depends on the nitrogen content feeding the second column, with a higher nitrogen content resulting in more reflux provided to the second column, which achieves a “cleaner” second column overhead stream having more nitrogen and less methane. In preferred embodiments herein, the recycled stream feeding into a top level of the second column as a reflux stream comprises at least 99%, or more preferably at least 99.99%, nitrogen to substantially reduce the methane contained in the nitrogen vent stream. The use of a series of compressors and coolers in these embodiments allows for greater flexibility in system feed stream nitrogen content.
According to some embodiments, the feed streams to the first column and a first column overhead stream are not cooled to traditional targeted temperatures of −200 to −245 degrees F. This allows systems and methods of these embodiments to feed the first column at a warmer temperature than prior art systems, which increases CO₂tolerance in the feed stream. According to some embodiments, systems and methods herein are also capable of processing feed gas containing concentrations of carbon dioxide up to approximately 2500 ppm for typical nitrogen levels between 1.5 to 20% or even to 25%. This is significantly higher than prior art systems that can process up to 100 ppm, and in many cases only up to 35 ppm, carbon dioxide. Increased tolerance for carbon dioxide in systems and methods herein may be attributed to added compression and cooling of a recycle stream to provide reflux to the second column, as it provides a source of refrigeration other than solely the expansion of the methane relied upon in prior art systems.
According to some embodiments, systems and methods herein produce three processed sales gas streams, each at a different pressure, which may be further compressed as needed to meet transporting pipeline requirements (typically around 615 psig). Most preferably, one sales gas stream is a high pressure stream having a pressure between 600 to 1300 psig (more preferably between 750 to 1200 psig), a second sales gas stream is an intermediate pressure stream having a pressure between 175 to 275 psig (more preferably between 210 to 230 psig), and a third sales gas stream is a low pressure stream having a pressure between 60 to 150 psig (more preferably between 60 to 125 psig).
According to some embodiments, a bottoms stream from the first column is split into three portions. A first portion is at least part of the low pressure sales gas stream, a second portion is the high pressure sales gas stream, and a third portion is the intermediate pressure sales gas stream. Most preferably, each of the first, second, and third portions are expanded and cooled to varying degrees. According to some embodiments, a low pressure sales gas stream further comprises a bottoms stream from a second fractionating column. In still other embodiments, a second portion of a bottoms stream from the first column is pumped as a liquid stream, prior to vaporizing to become a high pressure sale gas stream.
According to some embodiments, a system feed stream may be cooled in a first heat exchanger prior to feeding a first fractionating column through heat exchange with a first, a second, and a third portions of a first column bottoms stream, a second fractionating column bottoms stream (which is preferably mixed with the first portion of the first column bottoms stream upstream of the first heat exchanger), and a portion of an overhead stream from the second fractionating column. According to another embodiment, the cooled feed stream is split into two portions, a first portion of which is recycled back through the first heat exchanger to be further cooled prior to feeding the first column and a second portion of which also feeds into the first column. According to some embodiments, the first portion of the cooled feed stream feeds into a mid-upper tray level of the first column as a liquid.
According to some embodiments, a system feed stream is not separated in a separator prior to feeding into a first fractionating column. The system feed stream may be split into portions having the same phase and composition as the system feed stream prior to feeding into the first column, but are not separated into streams with different compositions and phases. According to some embodiments, no NGL stream, nor any stream sent for further processing to refine NGL components, is produced in systems and methods herein.
According to some embodiments, a second portion of the cooled feed stream is cooled and provides reboil heat to a reboiler (or the fourth heat exchanger) for the first fractionating column prior to feeding the first fractionating column. In some embodiments, a reboiler for the first fractionating column may comprise a shell and tube heat exchanger. According to some embodiments, the second portion of the cooled feed stream feeds into a lower-level tray of the first column, preferably as a mixed liquid-vapor stream. In still other embodiments, the first portion of the cooled feed stream feeds the first column at a lower temperature than, but a similar pressure as, the second portion of the cooled feed stream.
In still other embodiments, each of the first and second portions of the feed stream are expanded through a valve prior to feeding into the first column. In some embodiments, the valve may be a JT valve.
According to other embodiments, a liquid stream from a lower level or bottom of a second fractionating column is routed through the second heat exchanger, exiting as a mixed liquid-vapor stream that is returned to the second fractionating column, so the second heat exchanger acts as a reboiler for the second column. In some embodiments, a second fractionating column comprises an internal open section that acts as an internal separator or separation chamber that receives the mixed liquid-vapor stream. An internal separator or separation chamber allows the mixed liquid-vapor stream to separate into an ascending vapor stream for the second column and a liquid stream, that preferably exits as a second column bottoms stream.
In another embodiment, a separate separator external to the second column may be used to separate the mixed liquid-vapor stream from the second heat exchanger into an ascending vapor stream returned to the second column and a second column liquid bottoms stream.
According to other embodiments, a liquid stream from a bottom of a second fractionating column is routed through an external separator that receives heat from the second heat exchanger to act as a reboiler for the second column. In some embodiments, the separator is located near grade elevation level to allow for instrumentation critical for optimal operation and for maintenance to be easily accessible. Preferably, an overhead stream from the separator feeds back into the bottom of the second column as an ascending vapor stream. Preferably, a bottoms stream from the separator is a bottoms stream from the second column.
In some embodiments, the second column bottoms stream is warmed in the second heat exchanger through which a portion of the second column overhead stream, the second column liquid stream withdrawn from the second column, and the first column overhead stream also pass. In another embodiment, the second column bottoms stream is warmed in the second heat exchanger prior to being mixed with a third portion of the first column bottoms stream to form what becomes the low pressure sales gas stream. In these embodiments, four streams may pass through the second heat exchanger.
In some embodiments, when an external separator is used as part of a system to act as a reboiler for the second column, a separator bottoms stream is warmed in the second heat exchanger through which a portion of the second column overhead stream, and the first column overhead stream also pass. In these embodiments, three streams may pass through the second heat exchanger, but the second heat exchanger also supplies heat to the separator.
In another embodiment, a first column bottoms stream is preferably split into three portions, none of which pass through the second heat exchanger or provide reboil heat to the second column.
According to other embodiments, a first column overhead stream cooled in the second heat exchanger prior to feeding the second fractionating column. In some embodiments, the cooled first column overhead stream feeds into a mid-lower level tray of the second column. In still other embodiments, the first column overhead stream is not split into separate portions prior to feeding into the second column and the entirety of the first column overhead stream feeds into the second column as a single stream.
According to some embodiments, a second fractionating column overhead stream is split into at least two portions. In some embodiments, a first portion of the second column overhead stream is warmed in the second heat exchanger and then warmed in the first heat exchanger. A nitrogen vent, or nitrogen product, stream comprises the warmed first portion of the second column overhead stream downstream of the first heat exchanger. In some embodiments, a second portion of the second column overhead stream is split again into a third portion and a fourth portion. Preferably, both the third and fourth portions are warmed in a third heat exchanger through heat exchange with a recycled stream. In some embodiments, the nitrogen vent, or nitrogen product, stream further comprises the warmed third portion downstream of the third heat exchanger. Preferably, the first portion and the third portion of the second column overhead stream are mixed together downstream of the first and third heat exchangers, respectively. A warmed fourth portion of the second column overhead stream is compressed and cooled to become the recycled stream that is recycled back through the third heat exchanger.
Nitrogen separation systems and methods herein may achieve substantial reductions in greenhouse gas content in a nitrogen vent stream, which allows more methane to be in the sales gas streams and reduces emissions that may have negative environmental impact. A reduction in greenhouse gas content also allows the second column overhead stream to have such high purity nitrogen that it may be used as a nitrogen product stream rather than venting to the atmosphere. A second column overhead stream in systems and methods herein may comprise at least 99%, or more preferably at least 99.99%, nitrogen. Unlike some prior art systems that achieve ultra-high nitrogen content in the nitrogen vent stream while sacrificing nitrogen removal from the sales gas stream, systems and methods herein can achieve ultra-high nitrogen purity in the second column overhead stream while still maintaining less than 1.5%, less than 1.0%, less than 0.75%, or less than 0.50% nitrogen in the sales gas streams.
According to still other embodiments, no external refrigeration is needed to separate nitrogen from methane to achieve sales gas and nitrogen vent/products streams described herein. Necessary refrigeration is achieved by efficient heat exchange between process streams and compression of a recycled process stream.
Other advantages of systems and methods herein may include reduced energy/horsepower requirements compared to prior art single column systems. The systems and methods have horsepower requirements that are around 50 to 60% of the horsepower requirements for most prior art single column NRU systems with a single sales gas stream. By splitting a bottoms stream from the first column into three separate sales gas streams, each at a different pressure, with the low pressure stream preferably between 45 to 115 psig, preferred embodiments of the system and method can achieve a substantial reduction in energy/horsepower requirements to around 55 to 75 HP/MMSCF of inlet feed. Many single column prior art systems having a single sales gas stream exiting the NRU column or even two sales gas streams have horsepower requirements of around 110 HP/MMSCF of inlet feed. The horsepower requirements are reduced in many prior art conventional two column systems producing a single gas stream to around 80 to 90 HP/MMSCF of inlet feed. The horsepower requirements are similarly reduced in many prior art single column systems that produce three sales gas streams at differing pressures to around 80 to 90 HP/MMSCF of inlet feed. However, a further reduction to around 55 to 75 HP/MMSCF of inlet feed is achievable according to embodiments of the systems and methods herein.
For inlet feed conditions like those in the computer simulation Example 1 described below, a prior art single column design with the NRU bottoms stream split into two streams at different pressures (like in the '422 patent) would require around 100 hp per inlet feed MMSCF of gas. A two column design with NRU column bottoms split into three streams at different pressures (like in Example 2, FIG. 2 of U.S. Pat. No. 11,650,009) can process that inlet gas feed stream using only 85 hp. However, a preferred embodiment as shown in FIG. 1 can process that inlet gas feed stream using only 70 hp—a difference of more than 30 hp compared to the single column or 15 hp compared to the two column design. Thus, embodiments disclosed herein are capable of operating cost savings and significant reductions in greenhouse gas emissions in the nitrogen vent/nitrogen product stream compared to prior art systems and methods.
One of the aspects that results in the lower energy/horsepower requirements is the availability of three sales gas streams, each at a different pressure level, exiting the NRU first column. The pressure levels of the three streams is higher than prior art systems that split the NRU column bottoms stream into two or three sales streams. For example, in U.S. Pat. No. 9,816,752 the NRU column bottoms stream is split into three streams—a low pressure sales stream at around 15 psig, an intermediate pressure sales stream at around 111 to 132 psig, and a high pressure sales stream at around 248 to 271 psig and requires more HP/MMSCF of inlet feed than preferred embodiments of the systems and methods herein where the pressures of the three sales streams (particularly the low pressure sale stream) are higher. For example, a low pressure sales stream according to the disclosure may have a pressure of around 114.5 psig (as in Example 1) compared to around 15 psig in the '752 patent. Although this may not seem like a large pressure difference, there is a significant difference in HP required to compress any given volume with this higher pressure. When multiple sales gas streams are produced at different pressures, they typically undergo multiple stages of compression where a lower pressure stream is compressed in a first stage and then combined with a higher pressure stream, the combined stream is then compressed in a second stage, etc. until all of the sales gas streams are recombined into a single, final sales gas stream at the desired pressure (typically around 800 psig for pipeline requirements). Most preferably, systems and methods according to the disclosure will allow the use of at least one less stage of sales gas compression to achieve the desired end pressure for the final sales gas stream, resulting in a substantial energy/horsepower reduction.
Systems and methods described herein may also have greater flexibility with respect to nitrogen content in a system feed stream. Changing inlet nitrogen may impact a first column by forcing a temperature profile of the first column to accommodate a change in feed temperatures. If the profiles are not modified, then an amount of nitrogen passing from the first column to the second column may change resulting in an increase in methane content in a nitrogen vent stream. Added compression and cooling of a recycle stream to provide reflux to the second column in embodiments herein stabilize systems and methods herein to minimize the impact of changing nitrogen in the system feed to allow ultra-low amounts of methane in a nitrogen vent/nitrogen product stream. It may also significantly reduce an amount of time required to cool down from a warm start to operating conditions by around 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

The systems and methods of the disclosure are further described and explained in relation to the following drawings wherein:

FIG. 1 is a process flow diagram illustrating a preferred embodiment of a methane and nitrogen separation system and method as described herein; and

FIG. 2 is a simplified cross-sectional elevation view of a preferred downflow knockback condenser that may be used with systems and methods herein and in FIG. 1 .

DETAILED DESCRIPTION

Referring to FIG. 1 , a method and system 10 for separating nitrogen from methane from a feed stream 100 according to one preferred embodiment of the disclosure is depicted. Where present, it is generally preferable for purposes of the present disclosure to remove as much of the water vapor and other contaminants from feed stream 100 as is reasonably possible prior to processing feed stream 100 through system 10. It may also be desirable to remove excess amounts of carbon dioxide prior to separating the nitrogen and methane; however, methods and systems disclosed herein are capable of processing NRU feed streams containing up to or in excess of 2500 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 may be used with system feed stream 100 flow rates of up to 500 MSCFD, more preferably 100 to 500 MMSFCD comprising around 1.5 to 25% nitrogen, more preferably around 2 to 10% nitrogen, and 90 to 98% methane. Feed stream 100 may be at a pressure of 300 to 1200 psig, more preferably 750 to 1100 psig. Feed stream 100 may be at a temperature between 50 to 150 F, more preferably between 100 to 130 F before being cooled in a heat exchanger 101, exiting as stream 102. Stream 102 may then be split in splitter 103 into streams 104 and 106. Split vapor stream 104 is recycled back through heat exchanger 101 where it is cooled and condensed exiting as stream 112. Stream 112 then passes through a valve 113 prior to feeding into an upper level of first fractionating column 115 as liquid stream 114. Valve 113 may be a JT valve that reduces a pressure of stream 112 to stream 114 so that stream 114 is within an operating pressure range of first fractionating column 115.
Split vapor stream 106 also undergoes heat exchange in a heat exchanger 123, as further discussed below, where it is cooled and partially condensed exiting as stream 108. Stream 108 passes through valve 109 (most preferably a throttle valve or an expansion valve) that reduces the pressure of exiting stream 110 so that stream 110 is within an operating pressure range of first fractionating column 115. Stream 110 feeds into a lower level of first fractionating column 115 as mixed liquid-vapor stream.
First column 115 is preferably a high pressure column in system 10 operated at pressures ranging from 300 to 500 psig, more preferably from 375 to 425 psig with feed stream (streams 114 and 110) temperatures ranging from −200 to −120 F, more preferably −180 to −140 F. First fractionating column 115 separates streams 114 and 110 into a first column bottoms stream 126 and a first column overhead stream 120.
In one embodiment, a heat exchanger 123 is a single integrated piece of equipment configured as a shell and tube heat exchanger that acts as a reboiler for first fractionating column 115, while also cooling feed stream 106/108/110 prior to feeding into first fractionating column 115. A liquid stream 122 from a bottom of first column 115 passes through a shell side 123 (shell) of heat exchanger 123, with a vapor portion 124 returning to the bottom of column 115 and a liquid portion 126 exiting as a first column bottoms stream. In some embodiments, shell side 123 (shell) of heat exchanger 123 is external to column 115 and stream 122 is withdrawn from a bottom or a lower side draw tray on column 115, with stream 124 returning to column 115 and first column bottoms stream 126 exiting. In other embodiments, shell side 123 (shell) of heat exchanger 123 may be internal to column 115 and stream 122 is not a separate, distinct stream that exits column 115. Vapor stream 106 (split from feed stream 100) passes through a tube side of a reboiler 123 for a first column 115, exiting as stream 108. Heat energy (Q-3) of around 8 to 10 MMBTU/Hr per inlet 100 MMSCFD (of feed stream 100) passes from 123 (tube) (from stream 106) to 123 (shell) (to stream 122). Other heat exchange configurations may also be used in place of heat exchanger 123 to achieve a cooling of feed stream 106/108/110A and heating of stream 122 as will be understood by those of ordinary skill in the art. Bottoms stream 126 preferably comprises less than 1.0% nitrogen.
Bottoms stream 126 may be split into three portions: 128 (first portion), 136 (second portion), and 140 (third portion) in splitter 127. Of the flow in stream 126, around 15 to 35% is split into stream 128, around 45 to 65% is split into stream 136, and around 10 to 30% is split into stream 140. These amounts may be adjusted according to variations in operating parameters based on conditions for feed stream 100. These amounts, particularly an amount split into stream 128, may be adjusted in order to control an amount of refrigerant flow to shell side 117 (shell), which aids in controlling an amount of nitrogen in first column overhead stream 120. First portion 128 preferably passes through a valve 129, exiting as stream 130. Valve 129 may be a JT valve that reduces the pressure and achieves additional cooling of exiting stream 130. Stream 130 is then mixed with stream 156, which is a bottoms stream from a second fractionating column 147, in mixer 131 to form mixed stream 132. Stream 132 is then warmed in a shell side of heat exchanger 117, exiting as stream 134. Warming of stream 132 in 117 (shell) may result in additional vaporization in exiting stream 134. Stream 134 is then further warmed in heat exchanger 101, exiting as vapor stream 190. Stream 190 is a low pressure (“LP”) sales gas stream. Stream 190 comprises a low concentration of nitrogen that may be less than 1.0%. Stream 190 is at a higher pressure than a low pressure stream in prior art systems that produce multiple sales gas streams at different pressures, which reduces compression requirements for pipeline feed.
Third portion 140 preferably passes through an expansion valve 141, to reduce the pressure and temperature of exiting stream 142. After passing through valve 141, stream 142 has been partially vaporized. Stream 142 is then warmed in heat exchanger 101, exiting as vapor stream 192. Stream 192 is an intermediate pressure (“IP”) sales gas stream. Stream 192 comprises a low concentration of nitrogen that may be less than 1.0%.
Second portion 136 is pumped in pump 137, with stream 138 exiting pump 137. Stream 138 preferably remains a liquid stream until it is warmed in heat exchanger 101, exiting as vapor stream 194. Stream 194 is a high pressure sales gas stream. Stream 194 comprises a low concentration of nitrogen that may be less than 1.0%.
Most preferably, high pressure sales gas stream 194 is at a pressure higher than intermediate sales gas stream 192 and higher than low pressure sales gas stream 190. Most preferably, intermediate pressure sales gas stream 192 is at a pressure lower than high sales gas stream 194 and higher than low pressure sales gas stream 190. Most preferably, low pressure sales gas stream 190 is at a pressure lower than intermediate sales gas stream 192 and lower than high pressure sales gas stream 194. Sales gas streams 190, 192, and 194 may be further compressed as needed to meet pipeline requirements. Depending on the requirements of the installation or pipeline specifications, high pressure sales gas stream 194 may not need further compression to enter existing facility equipment or the compression requirements would be significantly reduced when compared with existing nitrogen rejection technologies.
In one embodiment, a heat exchanger 117 is a single integrated piece of equipment configured as a shell and tube heat exchanger that acts as a condenser for first fractionating column 115. In another embodiment, a heat exchanger 117 comprises a tube side 117 (tube) and a shell side 117 (shell) that are independent pieces of equipment configured as a vertical tube, falling film condenser. Heat exchanger 117 (tube) and 117 (shell) may provide a similar function as an internal knockback condenser 14 and shown and described in connection with FIG. 2 and in U.S. Patent Application Publication 2007/0180855, incorporated herein by reference. In still another alternative embodiment, column 115 may be configured with a knockback condenser 14 as further described with respect to FIG. 2 to provide functionality similar to that of heat exchanger 117.
Preferably a vapor stream 116 from a top of first column 115 passes through a tube side 117 (tube) of a heat exchanger 117, where it is partially condensed, with a vapor portion exiting as first fractionating column overhead stream 120 and a liquid portion 118 returning to column 115 as a reflux stream. Although stream 116 is shown in FIG. 1 as exiting a top of first fractionating column 115 to enter into a tube side 117 (tube) of heat exchanger 117, in preferred embodiments, tube side 117 (tube) is internal to first fractionating column 115 and stream 116 is a not a separate, distinct stream that exits column 115. For example, when a condenser 14 is used, stream 116 remains internal to column 115, but may enter into an internal riser 32 and through tube side 123 (tube) of condenser 14. In other embodiments, 117 (tube) may be external to column 115 such that 116 is a separate, distinct stream that is piped to heat exchanger 117. The refrigerant source for heat exchanger 117 is mixed stream 132, which comprises a first expanded portion 130 of first column bottoms stream 126 mixed with second column bottoms stream 156, downstream of heat exchanger 121. Mixed stream 132 is routed to a shell side 117 (shell) of exchanger 117, and the condensed liquid 118 from first column vapor stream 116 is designed to operate on the tube side 117 (tube) of exchanger 117. Heat energy (Q-4) of around 0.25 to 0.75 MMBTU/Hr per inlet 100 MMSCFD (of feed stream 100) passes from tube side of heat exchanger 117 (tube) (from stream 132) to shell side of heat exchanger 117 (shell) (to stream 116).
Some prior art systems have a similar vertical tube, falling film condenser configurations as may be used in heat exchanger 117. For Example, FIG. 1 in U.S. Pat. No. 11,650,009, heat exchanger 82 may be configured as a vertical tube, falling film condenser where the refrigerant source is a portion of the second column bottoms stream that feeds into the heat exchanger/condenser by gravity feed. In such a system, the second column must be in an elevated position relative to the first column to achieve the gravity feed. However, because system 10 herein utilizes a portion of the first column bottoms stream and the second column bottoms stream as refrigerant, it does not rely solely on gravity feed. This allows second fractionating column 147 to be located in any position and is not limited to an elevated position relative to column 115. Heat exchanger 117 may be mounted above (in an elevated position relative to) column 115, similar to an arrangement for knockback condenser 14 in FIG. 2 . However, since second fractionating column 147 can be installed independently of heat exchanger 117 and column 115, there is greater flexibility with respect to the footprint required and overall height required for installation of system 10 compared to some prior art systems. This can result in cost savings for system 10 compared to those prior art systems, as system 10 has more conventional foundation requirements for installation. Additionally, a pump is not necessary to circulate refrigerant stream 132 in system 10, which flow from mixer 131 to heat exchanger 101 via natural pressure drop.
Although it is generally known in the prior art to use a knockback condenser, the configuration of heat exchanger 117 (shell) and 117 (tube) (or the specific knockback condenser 14 and stream flows herein, if used), and the pressures and temperatures used in system 10, are different from the prior art. In the prior art, the knock back condenser had a single purpose, which is to remove heat from the column 115 overhead. In the configuration of heat exchanger 117 (or knockback condenser 14) in system 10, the purpose is to provide reflux to the first column 115 to allow removal of the bulk amount of nitrogen.
The configuration of heat exchanger 117 (shell) and 117 (tube) (or the specific knockback condenser 14 and stream flows herein, if used) may also aid in providing a thermal block for incoming CO₂to prevent it from proceeding to second fractionating column 147. Heat exchanger 117 (shell) and 117 (tube) (or knockback condenser 14) may be operated at a temperature where the small amount of CO₂entering first column 115 would be liquified and exit the first column 115 via a liquid stream (first column bottoms stream 126). Because the general operation temperatures of second fractionating column 147 are much colder than first fractionating column 115, it is preferred to remove the CO₂in the first column 115 to avoid potential freezing issues in the second column 147.
First column overhead stream 120 is cooled and at least partially condensed in a second heat exchanger 121, before feeding into a second fractionating column 147 as stream 146. Most preferably, stream 120 is fully condensed in heat exchanger 121, exiting as stream 144. Stream 144 then passes through an expansion valve 145 to reduce a pressure of exiting stream 146 to an operating pressure range for second fractionating column 147. Valve 145 also provides pressure control for first fractionating column 115. In system 10, preferably only one heat exchanger (heat exchanger 121) is needed to cool first column overhead stream 120 to create a feed stream 146 into second fractionating column 147. This is simplified compared to some prior art systems that require splitting the first column overhead stream and two separate heat exchangers, one for each of the split streams, upstream of feeding the second column. Stream 146, which may be partially vaporized, feeds into a mid-lower level of second fractionating column 147. Stream 146 preferably feeds into second fractionating column 147 slightly above a location from which stream 150 is withdrawn from second fractionating column 147, as discussed further below. In some embodiments, stream 146 may feed in at a tray location that is one level or two levels higher than a tray location from which stream 150 is withdrawn.
Second fractionating column 147 is a low pressure column preferably operated at pressures ranging from 75 to 115 psig, more preferably from 85 to 100 psig. Second fractionating column 147 separates feed stream 146 into a second column bottoms stream 152/154/156 and a second column overhead stream 157.
Second column 147 preferably uses heat from heat exchanger 121 as a source of reboiler heat. An amount of heat energy required to be transferred from heat exchanger 121 to second column 147 reboiler functionality in various embodiments herein will vary with the amount of inlet nitrogen required to be eliminated, as will be understood by those of ordinary skill in the art. A liquid stream 150 from a lower level or bottom of second column 147 is warmed in heat exchanger 121 to produce a mixed liquid-vapor stream 152 that returns to an internal separator or separation chamber of second fractionating column 147. Stream 152 feeds back into second fractionating column 147 at a location lower than where stream 150 was withdrawn. In some embodiments, stream 152 may feed in at a tray location that is one level or two levels below a tray location from which stream 150 is withdrawn.
Stream 152 preferably comprises a liquid hydrocarbon portion and a nitrogen rich vapor portion. Stream 152 is separated in an internal separator or separation chamber into an ascending vapor stream for second fractionating column 147 and a liquid portion. Stream 148 exits second column 147 as a second column bottoms stream 148, preferably comprising a liquid portion from stream 152. In other embodiments, second column bottoms stream consists of a liquid portion from stream 152. Other configurations that allow heat transfer from heat exchanger 121 to second fractionating column 147, or a stream withdrawn from a lower level of second fractionating column 147, including a separator external to second column 147 that receives and separates stream 152, to provide reboiler functionality may also be used as will be understood by those of ordinary skill in the art.
Level valve 153 may be used to maintain a desired liquid level in a bottom of second fractionating column 147, preferably in an internal separator or separation chamber in second fractionating column 147. A desired liquid level is preferably between a liquid outlet of second fractionating column 147 (where stream 148 exits the column) and a tray location at which stream 152 feeds back into second fractionating column 147. Second column bottoms stream 148 passes through valve 153, exiting as stream 154, having been slightly vaporized due to a pressure drop. In some embodiments, a pressure drop across level valve 153 may be 1 to 5 psi. Stream 154 then passes through heat exchanger 121, exiting as stream 156. Stream 154 provides a significant amount of the refrigeration necessary in heat exchanger 121 to sufficiently cool and condense vapor stream 120 to exiting liquid stream 144. Streams 148, 154, and 156 are all at around the same temperature and pressure, with minor differences that may allow slight vaporization in stream 154 and additional vaporization in stream 156. Stream 156, a mixed liquid-vapor stream, is then mixed with a first portion 130 of first column bottoms stream 126 (with stream 130 also a mixed-liquid vapor stream) in mixer 131 to form mixed stream 132 as previously described.
As shown in FIG. 1 , in some embodiments, an overhead stream 157 from second column 147 is split in splitter 158 into a first portion (or first bypass portion) 159 and a second portion (or first reflux portion) 161. Stream 159 bypasses a reflux recycle loop, while stream 161 feeds into a reflux recycle loop preferably comprising splitter 162, valve 164, heat exchanger 166, and compression block 179. Of the flow in stream 157, around 10 to 20% is split into stream 159 and around 80 to 95% is split into stream 161. A ratio of the split between stream 159 and 161 preferably aids in determining the purity of the nitrogen vent/nitrogen product stream 186. A higher ratio to stream 161 results a “cleaner” (less methane) nitrogen vent/nitrogen product stream 186. This is preferably balanced against the added compression costs in compression block 179 with a higher flow rate to stream 161.
Second portion 161 is then split again in another splitter 162 into a third portion (or a second bypass portion) 163 and a fourth portion (or a second reflux portion) 168. Stream 163 passes through heat exchanger 166 (as stream 165) but bypasses compression block 179, while stream 168 passes through heat exchanger 166 and compression block 179. Of the flow in stream 161, around 5 to 20% is split into stream 163 and around 80 to 95% is split into stream 168. A ratio of the split between streams 163 and 168 preferably aids in thermally optimizing the function of heat exchanger 166 and in lowering operating costs associated with compression block 179. Increasing the flow to stream 161 in splitter 158 produces a cleaner nitrogen stream, but may also increase the compression costs in compression block 179 if the entirety of the higher flow rate in stream 161 were direct to compression block 179. Splitter 162 allows a portion of that higher flow rate in stream 161 to be spilt into a second bypass portion 163/165/167 that bypasses compression block 179. Third portion 163 is expanded through valve 164, exiting as stream 165 having been cooled slightly. Both streams 165 and 168 are warmed in heat exchanger 166, exiting as streams 167 and 169, respectively. Stream 167 is then mixed with stream 172 (which is first bypass portion 159 downstream of heat exchanger 121 and heat exchanger 101) in mixer 173 as further discussed below.
According to another embodiment, the stream flows as shown in FIG. 1 may be modified. For example, second column overhead stream 157 may be split into two portions, streams 159 and 161 with both of those streams being warmed in heat exchanger 166. Stream 159 may exit heat exchanger 166 and then pass through heat exchanger 121, exiting as stream 160 and being further processed as shown in FIG. 1 . Stream 161 may exit heat exchanger 166 and then be split into streams 167 and 169, with streams 167 and 169 being further processed as shown in FIG. 1 .
Stream 169 passes through a series of compressors and coolers represented in FIG. 1 as compression block 179, exiting as recycle stream 180. Inclusion of compression block 179 increases capital and operating costs of system 10 compared to systems in U.S. Pat. No. 11,650,009, but contributes to system 10 being able to achieve ultra-low greenhouse gas emissions in nitrogen vent/product stream 186 by providing additional refrigeration. Stream 180 is then recycled back through heat exchanger 166, exiting as stream 182 having been cooled and partially condensed to a mole vapor fraction of around 85 to 95%. By cooling recycle stream 180 in this manner, heat exchanger 166 and compression system 179 act as a partial condenser for second fractionating column 147 to recycle a portion of second overhead stream 157 back to second fractionating column 147 as a reflux stream. Stream 182 is expanded and further condensed through valve 183, exiting as stream 184 having a pressure within an operating pressure range of second fractionating column 147. Stream 184 preferably has a mole vapor fraction of around 80 to 90% and feeds into an upper level of second fractionating column 147.
Stream 159, a first portion of second overhead stream 157 that comprises almost 100% nitrogen, is warmed in heat exchanger 121, exiting as stream 160. Stream 160 is then expanded in valve 170, exiting as stream 171. Stream 171 is then warmed in heat exchanger 101, exiting as stream 172. Stream 172 is then mixed with stream 167 (a third portion of second overhead stream 157) in mixer 173 to form mixed stream 186. Mixed stream 186 may be a nitrogen vent stream or nitrogen product stream, preferably comprising ultra-low quantities of greenhouse gases. Mixed stream 186 preferably comprises 99% or more nitrogen and less than less than 0.5%, and most preferably less than 0.075% methane. These results are significantly better than those disclosed in the '009 patent. The examples for the two systems in the '009 patent had 1.57% and 2.23% methane in the nitrogen vent streams 118 and 318, respectively, and both based on a system feed of 100 MMSCFD with 20% nitrogen.
Acceptable inlet compositions in which system 10 may operate satisfactorily are listed in the following Table 1:

TABLE 1

INLET STREAM COMPOSITIONS

	Inlet Component	Acceptable Inlet Composition Ranges

	Methane	70 to 98%
		Preferably 75 to 98%
	Ethane and Heavier	0 to 20%
	Components
	Carbon Dioxide	0 to 2500 ppm
	Nitrogen	1.5 to 25%
		Preferably 2 to 10% or less
	Feed Flow Rate	100 to 500 MMSCFD

In some embodiments, various streams in system 10 may comprise the amounts of nitrogen and methane, be within the temperature and pressure ranges, and have mole fraction vapor percentages as indicated in Tables 2A-2G below. When two ranges are provided separated by a semicolon, the second range is a more preferred range.

TABLE 2A

Stream Parameter Ranges for System Feed and 1^stColumn Feed

Stream & Property Ranges

		108 (Cooled		112 (Cooled
		2^ndPart of	110 (HP	1^stPart of	114 (HP
100 (System	102 (Cooled	Feed after	Col. Lower	Feed after	Col. Upper
Feed)	System Feed)	123 (Tube)	Level Feed)	Ht. Ex. 101)	Level Feed)

N2 Range	2 to 10;	Same as	Same as	Same as	Same as	Same as
(Mole	3 to 5	stream 100	stream 100	stream 100	stream 100	stream 100
Fraction)
C1 Range	88 to 98;	Same as	Same as	Same as	Same as	Same as
(Mole	90 to 95	stream 100	stream 100	stream 100	stream 100	stream 100
Fraction)
Temperature	50 to 150;	−25 to −100;	−100 to −175;	−125 to −175;	−140 to −190;	−140 to −190;
Range (F.)	100 to 125	−35 to −75	−125 to −150	−140 to −160	−155 to −170	−155 to −170
Pressure	600 to 1200;	Similar to	Similar to	Operating	Similar to	Operating
Range	700 to 1050	stream 100	stream 100	Range of	stream 100	Range of
(psig)				HP Col.		HP Col.
Mole	100	100	0	0 to 15;	0	0 to 5
Fraction				4 to 10
Vapor %

TABLE 2B

Stream Parameter Ranges for 1^stColumn Top Streams & Refrigerant

Stream & Property Ranges

			132
			(Refrigerant,
			Mixed 1^st
			Part of HP	134
116**	118	120	Col. Bottoms	(Stream 132
(HF Col.	(HP Col.	(HF Col.	and LP Col.	after 117
Vapor)	Reflux)	Overhead)	Bottoms)	(shell))

N2 Range	20 to 50;	15 to 45;	45 to 70;	Higher, but	Same as
(Mole	30 to 40	25 to 35	55 to 65	similar to	stream 132
Fraction)				stream 130,
				but with
				lower CO₂
C1 Range	50 to 80;	55 to 85;	30 to 50;	Lower, but	Same as
(Mole	60 to 70	65 to 80	35 to 45	similar to	stream 132
Fraction)				stream 130
Temperature	−135 to −210;	−150 to −230;	−150 to −230;	−160 to −240;	−140 to −220;
Range	−150 to −180	−180 to −205	−180 to −205	−190 to −210	−170 to −190
(F.)
Pressure	Operating	Operating	Operating	70 to 150;	Similar to
Range	Range of HP	Range of HP	Range of HP	90 to 130	stream 132
(psig)	Col.	Col.	Col.
Mole	100	0	100	10 to 50;	90 to 100
Fraction				25 to 45
Vapor %

**In preferred embodiments, stream 116 is internal to first fractionating column 115 and not a separate, distinct stream removed from first fractionating column 115.

TABLE 2C

Stream Parameter Ranges for 1^stColumn Bottom Streams

Stream & Property Ranges

	124 (HP Col.
122 (HP Col.	Vapor from		130 (Expanded	138 (Pumped	142 (Expanded
Bottom Liquid	Stream 122		1^stPart of	2^ndPart of	3^rdPart of
Feed to Ht.	After Ht.	126 (HP Col.	HP Col.	HP Col.	HP Col.
Ex. 123)	Ex. 123)	Bottoms)	Bottoms)	Bottoms)	Bottoms)

N2 Range	Less than 3;	Less than 4;	Less than 1.5;	Same as	Same as	Same as
(Mole	Less than 2	Less than 3	Less than 1.0	stream 126	stream 126	stream 126
Fraction)
C1 Range	Greater	Greater	Greater	Same as	Same as	Same as
(Mole	than 95;	than 96;	than 95;	stream 126	stream 126	stream 126
Fraction)	Greater	Greater	Greater
	than 96	than 97	than 96
Temperature	−125 to −175;	−125 to −175;	−125 to −175;	−155 to −235;	−105 to −155;	−145 to −190;
Range (F.)	−135 to −155	−135 to −155	−135 to − 155	−185 to −210	−115 to −140	−155 to −180
Pressure	Operating	Operating	Operating	70 to 150;	900 to 1300;	180 to 260;
Range	Range of	Range of	Range of	90 to 130	1000 to 1200	200 to 240
(psig)	HP Col.	HP Col.	HP Col.
Mole	0	100	0	10 to 50;	0	10 to 40;
Fraction				25 to 40		15 to 30
Vapor %

TABLE 2D

Stream Parameter Ranges for 2^ndColumn Feed & Top Streams

Stream & Property Ranges

				169
			165	(4^thPart of			184
144			(Expanded	LP Col.		182	(Top
(Liquified	146	157	3^rdPart of	Overhead	180	(Cooled	Feed to
HP Col.	(LP Col.	(LP Col.	LP Col.	after Ht.	(Recycled	Recycled	LP Col/
Overhead)	Feed)	Overhead)	Overhead)	Ex. 166)	Stream)	Stream)	Reflux)

N2 Range	Same as	Same as	Greater	Same as	Same as	Same as	Same as	Same as
(Mole	stream 120	stream 120	than 99;	stream 157	stream 157	stream 157	stream 157	stream 157
Fraction)			Greater
			than 99.5
C1 Range	Same as	Same as	Less	Same as	Same as	Same as	Same as	Same as
(Mole	stream 120	stream 120	than 0.5;	stream 157	stream 157	stream 157	stream 157	stream 157
Fraction)			Less
			than 0.05;
			Less
			than 0.01
Temperature	−200 to −270;	−225 to −300;	−240 to −310;	−260 to −330;	50 to 150;	90 to 150;	−200 to −280;	−240 to −320;
Range (F.)	−225 to −245	−240 to −280	−260 to −290	−290 to −310	90 to 110	110 to 130	−230 to −250	−270 to −290
Pressure	300 to 500;	Operating	Operating	3 to 20;	75 to 125;	300 to 500;	Similar to	Operating
Range	375 to 425	Range of	Range of	5 to 10	90 to 115	350 to 450	stream 180	Range of
(psig)		LP Col.	LP Col.					LP Col.
Mole	0	5 to 30;	100	100	100	100	85 to 95	75 to 95
Fraction		12 to 20
Vapor

TABLE 2E

Stream Parameter Ranges for Nitrogen Vent/Product Related Streams

Stream & Property Ranges

160			167
(Warmed 1^st		172	(3^rdPart of LP
Portion of	171	(Warmed 171	Col. Overhead	186
157 after Ht.	(Expanded	after Ht. Ex.	after Ht. Ex.	(N2 Vent/
Ex. 121)	160)	101)	166)	Product)

N2 Range	Same as	Same as	Same as	Same as	Same as stream
(Mole	stream 157	stream 157	stream 157	stream 157	157;
Fraction)					Preferably greater
					than 99.9;
					Most preferably
					greater than 99.95
C1 Range	Same as	Same as	Same as	Same as	Same as stream
(Mole	stream 157	stream 157	stream 157	stream 157	157;
Fraction)					Preferably less than
					0.075;
					Most preferably less
					than 0.05
Temperature	−170 to −230;	−175 to −250;	90 to 140;	50 to 150;	Similar to stream
Range (F.)	−190 to −210	−200 to −225	105 to 125	90 to 110	172
Pressure	75 to 125;	5 to 20;	0 to 10;	0 to 7;	Similar to stream
Range (psig)	90 to 115	8 to 15	3 to 7	0 to 5	172
Mole	100	100	100	100	100
Fraction
Vapor

TABLE 2F

Stream Parameter Ranges for 2^ndColumn Bottom Streams

Stream & Property Ranges

		152
		(LP Col.		156
148	150	Mixed	154	(Warmed 154
(LP Col.	(LP Col.	Liquid-	(Expanded	after Ht.
Bottoms)	Liquid)	Vapor)	148)	Ex. 232)

N2 Range	2 to 10;	10 to 30;	Same as	Same as	Same as
(Mole	5 to 7	15 to 25	stream 150	stream 148	stream 148
Fraction)			Same as	Same as	Same as
C1 Range	90 to 98;	45 to 75;	stream 150	stream 148	stream 148
(Mole	93 to 96	55 to 65
Fraction)		65 to 95;
		75 to 85
Temperature	−190 to −240;	−215 to −265;	−190 to −230;	−190 to −230;	−180 to −220;
Range (F.)	−210 to −230	−225 to −255	−200 to −220	−200 to −220	−190 to −210
Pressure	Operating	Operating	Operating	85 to 145;	80 to 140;
Range (psig)	Range of LP	Range of LP	Range of LP	100 to 130	95 to 125
	Col.	Col.	Col.
Mole Fraction	0	0	40 to 60	0 to 5	25 to 65;
Vapor					35-55

TABLE 2G

Stream Parameter Ranges for Sales Gas Streams

Stream & Property Ranges

190	192	194
(LP Sales Gas)	(IP Sales Gas)	(HP Sales Gas)

N2 Range	Same as	Same as	Same as
(Mole Fraction)	stream 132	stream 126	stream 126
C1 Range	Same as	Same as	Same as
(Mole Fraction)	stream 132	stream 126	stream 126
Temperature	90 to 135;	90 to 135;	90 to 135;
Range (F.)	105 to 125	105 to 125	105 to 125
Pressure	60 to 160;	175 to 275;	900 to 1300;
Range (psig)	100 to 125	205 to 225	1000 to 1200
Mole Fraction	100	100	100
Vapor

An indication in the tables herein that a stream in system 10 is 0% mole vapor fraction or 100% mole vapor fraction means that such stream is all liquid or all vapor in most embodiments; however, it does not preclude such streams from being mixed liquid-vapor streams in some embodiments. Variations in system feed parameters and operation of system 10 may alter the phase of the streams, as well as composition, temperature, and pressure as will be understood by those of ordinary skill in the art.

Example 1—Computer Simulation for 500 MMSCFD Feed with 4% Nitrogen in System 10

Referring to FIG. 1 , a system 10 and method for processing a 500 MMSCFD NRU feed stream 100, comprising approximately 4 mol % nitrogen and 93.4 mol % methane at 120° F. and 1000 psig based on a computer simulation is shown and described below. Parameters for feed stream 100 in this example are typical for a gas stream feeding into an LNG (liquefaction) process, although system 10 may be used for processing other natural gas streams. Feed stream 100 passes through first heat exchanger 101, which preferably comprises a plate-fin heat exchanger. The feed stream emerges from the heat exchanger as stream 102 having been cooled to −50.5° F. This cooling is the result of heat exchange with other process streams 134, 142, 138, and 171. Stream 104 also passes through heat exchanger 101 and is cooled. The cooled stream 102 is split in splitter 103 into streams 104 and 106. Vapor stream 104 is recycled back through heat exchanger 101 where it is cooled and condensed exiting as liquid stream 112 at a temperature of −162.5 F and a pressure of 990 psig. Stream 112 then passes through a valve 113 prior to feeding into an upper level of first fractionating column 115 as liquid stream 114. Valve 113 may be a JT valve that reduces the pressure of stream 112 to stream 114 so that stream 114 is within an operating pressure range of first fractionating column 115. Stream 114 is at a pressure of 400 when it feeds into first fractionating column 115 in this example. Stream 114 feed first fractionating column 115 at an upper-mid level, around tray 5 in this example.
Vapor stream 106 passes through the tube side of exchanger 123 (tube) in order to provide heat for the heat exchanger or reboiler 123 for first fractionating column 115. Vapor stream 106 exits heat exchanger 123 (tube) as liquid stream 108 at a temperature of −137.42 F and a pressure of 990 psig. Heat energy (Q-3) of around 8 to 10 MM BTU/Hr per inlet 100 MMSCFD (of feed stream 100) passes from tube side of reboiler 123 (tube) (from stream 106) to shell side of reboiler 123 (shell) (to stream 122). Stream 108 passes through valve 109 that reduces the pressure of exiting stream 110 so that stream 110 is within an operating pressure range of first fractionating column 115. Stream 110 is at a pressure of 402 psig in this example. Stream 110 feeds into a lower level of first fractionating column 115, around tray 16 in this example, as mixed liquid-vapor stream at a temperature of −148.46 F.
First fractionating column 115 is preferably a high pressure column upstream of a low pressure second fractionating column 147. Components of feed streams 114 and 110 are separated in first fractionating column 115 into a bottoms stream 126 and an overhead stream 120. Bottoms stream 126 comprises 0.83% nitrogen and 96.45% methane and small quantities of CO₂, C2, and C3. Stream 126 is at a temperature of −142.68 F. Overhead stream 120 comprises 60% nitrogen and 40% methane and negligible quantities of CO₂, C2, and C3. Stream 120 is at a temperature of −190.68 F.
A liquid stream 122 from a bottom of first column 115 at a temperature of −144.41 F and comprising 1.37% nitrogen and 96.64% methane passes through a shell side 123 (shell) of heat exchanger 123. A vapor portion 124 at a temperature of −142.68 F and comprising 2.55% nitrogen and 97.09% methane is returned to the bottom of column 115 from 123 (shell). A liquid portion 126 at a temperature of −142.68 F and comprising 0.83% nitrogen and 96.45% methane exits 123 (shell) as a first column bottoms stream. Vapor stream 106 (split from feed stream 100) passes through a tube side of a reboiler 123 for a first column 115, exiting as stream 108.
First column bottoms stream 126 may be split into three portions: 128 (first portion), 136 (second portion), and 140 (third portion) in splitter 127. Of the flow in stream 126, around 24.82% is split into stream 128, around 55.18% is split into stream 136, and around 20% is split into stream 140. First portion 128 preferably passes through a JT valve 129, exiting as mixed vapor-liquid stream 130 at a temperature of −196.65 F and a pressure of 108.47 psig. Stream 130 has a 33.65% mole vapor fraction. Stream 130 is then mixed with stream 156, which is a bottoms stream from a second fractionating column 147, in mixer 131 to form mixed stream 132. Stream 132 is at a temperature of −197.2 F, a pressure of 108.47 psig, and comprises 1.27% nitrogen and 96.24% methane. Stream 132 is then warmed in a shell side of heat exchanger 117, exiting as stream 134 at a temperature of −176.4 F. Stream 134 is then further warmed in heat exchanger 101, exiting as vapor stream 190 at a temperature of 114.2 F and a pressure of 103.92 psig. Stream 190 is a low pressure sales gas stream.
Third portion 140 preferably passes through an expansion valve 141, exiting as mixed liquid-vapor stream 142 at a temperature of −170.32 F and a pressure of 219.78 psig. Stream 142 is then warmed in heat exchanger 101, exiting as vapor stream 192 at a temperature of 114.21 F and a pressure of 214.78 psig. Stream 192 is an intermediate pressure sales gas stream.
Second portion 136 is pumped in pump 137, with stream 138 exiting pump 137 at a temperature of −126.59 F and a pressure of 1110 psig. Stream 138 preferably remains a liquid stream until it is warmed in heat exchanger 101, exiting as vapor stream 194 at a temperature of 113.85 F and a pressure of 1105 psig. Stream 194 is a high pressure sales gas stream.
Sales gas streams 190, 192, and 194 may be further compressed as needed to meet pipeline requirements. Most preferably high pressure sales gas stream 194 does not require further compression or requires significantly less compression than prior art nitrogen rejection technologies.
A vapor stream 116 from a top of first column 115 passes through a tube side 117 (tube) of a heat exchanger or condenser 117. Stream 116 is at a temperature of −167.79 F and comprises 33.58% nitrogen and 66.42% methane. Stream 116 is partially condensed in 117 (tube), with a vapor portion exiting as first fractionating column overhead stream 120 and a liquid portion 118 returning to column 115 as a reflux stream. Stream 118 is at a temperature of −190.68 F and comprises 27.96% nitrogen and 72.04% methane. The refrigerant source for heat exchanger 117 is mixed stream 132. Heat energy (Q-4) of around 0.25 to 0.75 MMBTU/Hr per inlet 100 MMCCFD (of feed stream 100) passes from tube side of condenser 117 (tube) (from stream 132) to shell side of condenser 117 (shell) (to stream 116). Heat exchanger 117 (or condenser 14) requires less duty to operate than prior art systems.
First column overhead stream 120, at a temperature of −190.68 F and comprising 60% nitrogen and 40% methane, is used as a feed stream source for second fractionating column 147, which operates as a low pressure column. Stream 120 is cooled and fully condensed in a second heat exchanger 121, exiting as stream 144 at a temperature of −235 F and a pressure of 397.1 psig. Stream 144 then passes through an expansion valve 145, exiting as stream 146 at a temperature of −260.88 F and a pressure of 115 psig. Mixed liquid-vapor stream 146 then feeds into a mid-lower level of second fractionating column 147, at tray level 9 in this example, where it is separated into a second column bottoms stream 152 and a second column overhead stream 157.
A liquid stream 150 at a temperature of −238.23 F and comprising 20.22% nitrogen and 79.77% methane from a lower level or bottom of second column 147 is warmed in heat exchanger 121 to produce a mixed liquid-vapor stream 152, that is returned to an internal separation chamber in second fractionating column 147 where it is separated into an ascending vapor stream and a liquid portion. Stream 152 has a temperature of −211.43 F and comprises 20.22% nitrogen and 79.78% methane and is around 50% mole fraction vapor. Stream 148 also at a temperature of −211.43 F and comprising 5.85% nitrogen and 94.15% methane exits second fractionating column 147 as a second column bottoms stream. Stream 148 preferably comprises the liquid portion of stream 152. A flow rate of stream 148 exiting second fractionating column 147 is controlled by level valve 153 to maintain a desired liquid level in second fractionating column 147.
Stream 154 exits valve 153, at a temperature of −211.95 F and a pressure of 113.42 psig, having been slightly vaporized. Stream 153 then passes through heat exchanger 121, exiting as stream 156 at a temperature of −202.34 F and a pressure of 108.42 psig. Stream 156 is then mixed with a first portion 130 of first column bottoms stream 126 in mixer 131 to form mixed stream 132 as previously described.
An overhead stream 157 from second column 147 is split in splitter 158 into a first portion 159 and a second portion 161. Of the flow in stream 157, around 13.08% is split into stream 159 and around 86.66% is split into stream 161. Second portion 161 is then split again in another splitter 162 into a third portion 163 and a fourth portion 168. Of the flow in stream 161, around 12.24% is split into stream 163 and around 87.76% is split into stream 168. Third portion 163 is expanded through valve 164, exiting as stream 165 at a temperature of −299.34 F and a pressure of 8 psig. Both streams 165 and 168 are warmed in heat exchanger 166 through heat exchange with a recycled stream 180. Stream 165 exits heat exchanger 166 as stream 167 at a temperature of 98.94 F and a pressure of 3 psig. Stream 167 is then mixed with stream 172 in mixer 173 as further discussed below.
Stream 168 exits heat exchanger 166 as stream 169 at a temperature of 98.94 F and a pressure of 107.92 psig. Stream 169 passes through a series of compressors and coolers represented in FIG. 1 as compression block 179, exiting as recycle stream 180. Stream 180 has a temperature of 120 F and a pressure of 400.5 psig. Stream 180 is then recycled back through heat exchanger 166, exiting as stream 182 having been cooled to a temperature of −239.56 F and partially condensed to a mole vapor fraction of 92.95%. Stream 182 is expanded and further condensed through valve 183, exiting as stream 184 having a pressure of 115 psig and a temperature of −276.12 F. Stream 184 has a mole vapor fraction of 84.49% and feeds into an upper level of second fractionating column 147, at tray level 1 in this example. Stream 184 acts as a reflux stream of second fractionating column 147.
Stream 159, a first portion of second overhead stream 157, is warmed in heat exchanger 121, exiting as stream 160. Stream 160 is at a temperature of −202.3 F and a pressure of 107.92 psig. Stream 160 is expanded in valve 170, exiting as stream 171 at a temperature of −213.91 F and a pressure of 10.5 psig. Stream 171 is then warmed in heat exchanger 101, exiting as stream 172 at a temperature of 114.2 F and a pressure of 5.5 psig. Stream 172 is then mixed with stream 167 (a third portion of second overhead stream 157) in mixer 173 to form mixed stream 186. Mixed stream 186 is at a temperature of 107.32 F and comprises 99.9893% nitrogen and 0.0107139% methane. Stream 186 may be an ultra-low greenhouse gas nitrogen vent stream or may be recovered as a nitrogen product stream, if desired.
The specific flow rates, temperatures, pressures, and compositions of various flow streams referred to in connection with the above discussion of a computer simulation for a system 10 appear in Table 3 below. Flow rates are provided as a standard vapor volumetric flow in MMSCFD. Vapor % is mole fraction vapor. These values are based on a feed gas stream 100 comprising 4% nitrogen, around 93% methane, and a flow rate of 500 MMSCFD.

TABLE 3

FLOW STREAM PROPERTIES FOR EXAMPLE 1

Mole	Property - Stream No.

Fraction	100	102	104	106	108	110	112

Nitrogen	4.00	4.00	4.00	4.00	4.00	4.00	4.00
CO2	0.10	0.10	0.10	0.10	0.10	0.10	0.10
Methane	93.42	93.42	93.42	93.42	93.42	93.42	93.42
Ethane	2.41	2.41	2.41	2.41	2.41	2.41	2.41
Propane	0.07	0.07	0.07	0.07	0.07	0.07	0.07
i-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	120*	−50.5*	−50.5	−50.5	−137.418*	−148.461	−162.5*
Pressure	1000*	995	995	995	990	402*	990
psig
Vapor %	100	100	100	100	0	6.02170	0
MMSCFD	500*	500	314.739	185.261	185.261	185.261	314.739

Mole	Property - Stream No.

Fraction	114	116	118	120	122	124	126

Nitrogen	4.00	33.58	27.96	60.00	1.37	2.55	0.83
CO2	0.10	0.00	0.00	0.00	0.08	0.03	0.11
Methane	93.42	66.42	72.04	40.00	96.65	97.09	96.45
Ethane	2.41	0.00	0.00	0.00	1.85	0.33	2.54
Propane	0.07	0.00	0.00	0.00	0.05	0.00	0.07
i-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	−164.799	−167.793	−190.682	−190.682	−144.412	−142.679	−142.679
Pressure	400*	399.556	399.556	399.556	401.556	401.556	401.556
psig
Vapor %	0	100	0	100	0	100	0
MMSCFD	314.739	152.763	125.969	26.7943	689.662	216.457	473.206

Mole	Property - Stream No.

Fraction	128	130	132	134	136	138	140

Nitrogen	0.83	0.83	1.27	1.27	0.83	0.83	0.83
CO2	0.11	0.11	0.10	0.10	0.11	0.11	0.11
Methane	96.45	96.45	96.24	96.24	96.45	96.45	96.45
Ethane	2.54	2.54	2.32	2.32	2.54	2.54	2.54
Propane	0.07	0.07	0.07	0.07	0.07	0.07	0.07
i-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	−142.679	−196.640	−197.178	−176.444	−142.679	−126.593	−142.679
Pressure	401.556	108.417*	108.417	106.417	401.556	1110*	401.556
psig
Vapor %	0	33.6530	34.6401	97.4637	0	0	0
MMSCFD	117.447*	117.447	128.830	128.830	261.117	261.117	94.6411

Mole

Property - Stream No.

Fraction	142	144	146	148	150	152	154

Nitrogen	0.83	60.00	60.00	5.85	20.22	20.22	5.85
CO2	0.11	0.00	0.00	0.00	0.00	0.00	0.00
Methane	96.45	40.00	40.00	94.15	79.78	79.78	94.15
Ethane	2.54	0.00	0.00	0.00	0.00	0.00	0.00
Propane	0.07	0.00	0.00	0.00	0.00	0.00	0.00
i-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	−170.319	−235*	−260.876	−211.508	−238.236	−211.432	−211.952
Pressure	219.784	397.056	115*	115.417	115.208	115.208	113.417
psig
Vapor %	21.7659	0	21.9085	0	0	50.0182	0.275811
MMSCFD	94.6411	26.7943	26.7943	11.3827	22.3145	22.3145	11.3827

Mole	Property - Stream No.

Fraction	156	157	159	160	161	163	165

Nitrogen	5.85	99.99	99.99	99.99	99.99	99.99	99.99
CO2	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Methane	94.15	0.01	0.01	0.01	0.01	0.01	0.01
Ethane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Propane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	−202.239	−276.559	−276.556	−202.302*	−276.556	−276.556	−299.341
Pressure	108.417	112.917	112.917	107.917	112.917	112.917	8*
psig
Vapor %	44.6051	100	100	100	100	100	100
MMSCFD	11.3827	64.8847	8.48466	8.48466	56.3759	6.90275	6.90275

Mole	Property - Stream No.

Fraction	167	168	169	171	172	180	182

Nitrogen	99.99	99.99	99.99	99.99	99.99	99.99	99.99
CO2	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Methane	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Ethane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Propane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	98.9405*	−276.556	98.9434	−213.912	114.208*	120	−239.565
Pressure	3	112.917	107.917	10.5	5.5*	400.5	395.5
psig
Vapor %	100	100	100	100	100	100	92.9492*
MMSCFD	6.90275	49.4731	49.4731	8.48466	8.48466	49.4731	49.4731

Mole

Property - Stream No.

Fraction	184	186	190	192	194

Nitrogen	99.99	99.99	1.27	0.83	0.83
CO2	0.00	0.00	0.10	0.11	0.11
Methane	0.01	0.01	96.24	96.45	96.45
Ethane	0.00	0.00	2.32	2.54	2.54
Propane	0.00	0.00	0.07	0.07	0.07
i-Butane	0.00	0.00	0.00	0.00	0.00
n-Butane	0.00	0.00	0.00	0.00	0.00
i-Pentane	0.00	0.00	0.00	0.00	0.00
n-Pentane	0.00	0.00	0.00	0.00	0.00
Hexane	0.00	0.00	0.00	0.00	0.00
Temp. ° F.	−276.132	107.322	114.208*	114.208*	113.852
Pressure psig	115*	3	103.917	214.784	1105
Vapor %	84.4897	100	100	100	100
MMSCFD	49.4731	15.3874	128.830	94.6411	261.117

It will be appreciated by those of ordinary skill in the art that these values in Example 1 are based on the particular parameters and composition of the feed stream in the above computer simulation example. The temperature, pressure, and compositional values will differ depending on the parameters and composition of the NRU Feed stream 100 and specific operating parameters for various pieces of equipment in system 10.
Systems and methods in accordance with embodiments herein, including an embodiment as shown in FIG. 1 , are able to adjust to fluctuating nitrogen content in system feed stream 100 better than prior art systems to achieve a faster cool down to operating conditions and a cleaner nitrogen vent/nitrogen product stream 186.
According to still another embodiment, a downflow, knockback condenser 14, such as shown in FIG. 2 , may also be used to provide heat exchange in heat exchanger 117 in system 10. A downflow, knockback condenser and method of use as disclosed herein that are particularly useful for partially condensing a vapor stream so that a lighter gas fraction can be efficiently removed and separated from the liquid that is condensed from the vapor stream. The term “lighter” refers to the actual density of the vapor constituent as compared to the liquid constituent density that may be present at any point in the knockback condenser. The knockback condenser and method are particularly useful for separating gaseous nitrogen from condensed natural gas liquid.
A principal distinction between a knockback condenser described herein and condensers disclosed in the prior art is the provision and use of a vapor riser to introduce vapor captured from the fractionation section of a tower into a headspace above a tubular heat exchanger section to thereby establish downflow or countercurrent cooling of the vapor within the tubes of the condenser to partially condense it into a condensed liquid fraction from which a remaining uncondensed gaseous fraction is then separated and removed.
According to one preferred embodiment, a knockback condenser is useful for partially condensing vapor in the upper section of the first fractionation column to separate vapor and a lighter gaseous fraction (as an overhead stream from the first fractionation column) from a condensed liquid component (as a reflux stream for the first fractionation column). The knockback condenser preferably comprises a substantially cylindrical shell and a condenser section having upper and lower tube sheets attached transversely to the inside of the shell. The tube sheets support a plurality of spaced-apart, vertically oriented, heat exchange tubes extending between the upper and lower tube sheets to provide fluid communication through the tubes. Refrigerant inlet and outlet ports are preferably and desirably disposed so as to establish a generally upward flow of refrigerant around the heat exchange tubes between the lower and upper tube sheets. A vapor riser provides fluid communication between a space in the fractionation tower disposed below the liquid trap plate and a headspace disposed above the upper tube sheet, thereby establishing an upward flow of vapor through the riser and a downward flow of vapor, condensed liquid and an uncondensed, lighter gaseous fraction through the heat exchange tubes. As a refrigerant stream (such as stream 132) flows through the shell around the tubes, it sufficiently cools the tubes to condense natural gas passing downwardly through the tubes, thereby liquefying the natural gas and separating it from the gaseous nitrogen.
A vapor outlet port is preferably disposed below the lower tube sheet to receive the lighter gaseous fraction and any remaining vapor exiting the lower tube sheet. Liquid collection and recovery apparatus disposed below the lower tube sheet and below the vapor outlet port receive liquid condensed from the vapor.
According to another preferred embodiment, a method for partially condensing a vapor stream from an upper level or zone of the first fractionating column to separate a lighter gaseous fraction from a condensed liquid fraction, comprises the steps of providing a condenser having a substantially cylindrical, vertically oriented shell; upper and lower tube sheets attached transversely to the inside of the shell, the tube sheets supporting a plurality of spaced-apart, vertically oriented, heat exchange tubes extending between the upper and lower tube sheets, and providing fluid communication through the tubes; providing refrigerant inlet and outlet ports disposed in the shell so as to establish a generally upward flow of refrigerant around the heat exchange tubes between the lower and upper tube sheets; providing a vapor riser providing fluid communication between a space in the shell disposed below the lower tube sheet and a headspace disposed above the upper tube sheet; establishing an upward flow of vapor through the riser and a downward flow comprising vapor, condensed liquid fraction and lighter gaseous fraction through the heat exchange tubes, the refrigerant having sufficient cooling capacity to condense a desired liquid fraction from the vapor while passing through the heat exchange tubes; and separately recovering the lighter gaseous fraction from the condensed liquid fraction collected below the heat exchange tubes.
Through use of a knockback condenser and method disclosed herein, one is able to achieve more predictable condenser performance, improved plant flexibility; higher sales gas recoveries, and lower capital costs. Greater predictability in condenser performance is particularly significant for meeting performance guarantees required by gas plant owners, especially for larger plants, where specific component performance plays a significant role in overall plant design.
In the previous condenser designs, such as in Applicant's prior U.S. Pat. Nos. 5,275,505 and 5,375,422 that utilize an internal condenser in a single column nitrogen rejection system, the gas enters at the bottom of the tubes and exits at the top, whereas with the knockback condenser herein, the gas enters at the top of the tubes and exits at the bottom. The performance improvement arises from the fact that some of the gas entering the tubes is condensed, regardless of gas flow direction. In the vapor up-flow models, the liquid must exit by flowing downward or counter-current to the gas flow. While this was anticipated in the design of the prior art condensers, Applicant learned from their use that the “falling” liquid creates a film that negatively affects the heat transfer coefficient, requiring more condenser surface area to be installed with each condenser application and adding complexity to the estimation of condenser performance.
In contrast, a downflow, knockback condenser utilizes a vapor riser to introduce a flow of vapor into a headspace above a vertical tubular heat exchanger, thereby establishing a downflow of condensed liquid and a lighter gaseous fraction through the heat exchange tubes. Referring to FIG. 2 , an upper portion of a first fractionation tower or column 115 is shown in which the upper portion of the column shell 12 contains a preferred embodiment of downflow, knockback condenser 1414 to provide heat exchange. Fractionation column 115 is preferably made of conventional materials capable of operating at the temperatures and pressures needed for a particular application, and has a nominal diameter ranging from about 18 to about 120 inches, depending upon plant size and throughput. Generally speaking, the fractionation section of fractionation column 115 is disposed below section 60, and is broken away to facilitate enlargement of the upper section of the tower in which condenser 14 resides. As shown in the embodiment depicted in FIG. 2 , section 60 of fractionation column 115 is separated by liquid distribution plate 54 from the gas and condensed liquid recovery zones disposed between section 60 and condenser 14. Liquid distribution plate 54 allows rich vapor 116 rising upwardly from a fractionation section to enter a condenser section of first fractionating column 115, and distributes condensed liquid recovered from condenser 14 as further described below to pass downwardly as reflux liquid 118 into the fractionation section of the tower, as indicated by arrow 118, countercurrent to the upwardly rising rich vapor 116.
As used herein, the term “condenser section” collectively refers to Zones A, B and C and shown in FIG. 2 . In Zone A, rich vapor rising upwardly from the fractionation section 60, such as vapor stream 116 from a top portion of column 115, through liquid distribution plate 54 enters riser 32 and is directed upwardly into the headspace designated as Zone B above condenser 14. From Zone B, as indicated by arrows 62, the rich vapor flows downwardly through upper tube sheet 16 into the plurality of substantially vertical heat exchange tubes 20, which are cooled by refrigerant 132 entering shell 12 through refrigerant inlet 24. The source of refrigerant in system 10 when knockback condenser 14 is used is preferably stream 132, which is a first portion of first column bottoms stream 126 mixed with second column bottoms stream 156 downstream of heat exchanger 121. The refrigerant flows around heat exchange tubes 20 through spaces 22 and, as it absorbs heat from tubes 20, eventually rises to a point where it exits outlet 28 as stream 134. Stream 134 and then proceeds to pass through heat exchanger 101 as shown in FIG. 1 .
As condensed liquid and an uncondensed gaseous fraction exit downwardly from tubes 20 through lower tube sheet 18 into Zone C, the gaseous fraction 120 exits shell 12 through outlet 44 as overhead stream 120, and the condensed liquid is collected on liquid trap plate 40. From liquid trap plate 40, the condensed liquid received into Zone C from condenser 14 flows downwardly through opening 50, through reflux liquid return seal leg 48, as shown by arrow 64, where it is discharged from end 53 into reflux seal pan 52 in Zone A. From reflux seal pan 52, the condensed reflux liquid spills over, as shown by arrow 66, onto liquid distribution plate 54, from which it is returned to the fractionation section as reflux stream 118.
The design, structure and general operation of a preferred embodiment of downflow, knockback condenser 14 is further described and explained below in relation to a computer simulation wherein rich vapor containing natural gas (methane) and nitrogen is partially condensed to separate and remove the gaseous nitrogen from the condensed natural gas liquid. The reference numerals used below generally relate to the structures and flows as described above in relation to FIG. 2 .
Zone A contains both vapor and liquid. The vapor enters Zone A from section 60 of the fractionation tower via liquid distribution tray 54 disposed below liquid trap plate 40. The liquid enters Zone A from condenser 14 above via the reflux liquid return seal leg 48. The Zone A vapor component is expected to exist at the temperature, pressure and composition given below, and is at the dew point of the rich vapor, meaning that any reduction in temperature at the same pressure will create liquid condensate. In a computer simulation of fractionation column 115 in system 10 as operated with the downflow knockback condenser 14 of a preferred embodiment, the Zone A vapor and liquid conditions may be as shown in Table 4 in this example:

TABLE 4

ZONE A

Zone A Vapor (Entering)

	Temperature (deg. F.)	−150 to −175
	Pressure (psig)	350 to 450

Component (mole %)

	Nitrogen	25 to 45
	Methane	55 to 75

Zone A Liquid (Entering)

	Temperature (deg. F.)	−185 to −210
	Pressure (psig)	350 to 450

Component (mole %)

	Nitrogen	20 to 40
	Methane	60 to 80

The liquid in Zone A provides the reflux for fractionation column 115 to minimize the amount of methane that is vented with the nitrogen waste gas through outlet 44. The vapor from Zone A proceeds upward through the vapor riser 32 into Zone B. Entrance 34 to vapor riser 32 is preferably cut obliquely on a 60 degree bias to provide greater entrance area to riser 32 and thereby reduce the entrance velocity and associated pressure losses of the rich vapor. Reducing the velocity at entrance 34 allows less liquid, in the form of droplets, to enter riser 32. Some liquid droplets entering riser 32 will not significantly impair the performance of fractionation column 115 or condenser 14, but neither does it help. The entrance of riser 32 is desirably spaced approximately one foot from the underside of liquid trap plate 40 to reduce the vapor velocity at the lower or bottom face of liquid trap plate 40. Lowering this velocity will help minimize the heat transfer across the plate. Heat transfer across liquid trap plate 40 is not desirable because it will reduce the overall effectiveness of condenser 14, and should be minimized. Upper end 36 of vapor riser 32 is desirably extended about six inches above upper tube sheet 16. This extension will help in more evenly distributing the vapor flow across upper tube sheet 16.
The section between upper tube sheet 16 and lower tube sheet 18 is the principal heat exchanger section of condenser 14. A primary point of distinction between this disclosure and some prior art systems and methods is that in this disclosure a flow direction of the vapor to be cooled through the heat exchange section is reversed. In some prior art systems, the gas enters at the bottom of the heat exchange tubes and exits at the top, whereas with the present design, the gas enters at the top of heat exchange tubes 20 and exits at the bottom.
The Zone B vapor conditions are substantially the same as in Zone A but there is no liquid present. In reality, the temperature in Zone B is slightly lower than in Zone A and the computer predicts a slight temperature decrease and a lower pressure due to the vertical elevation difference between Zone A and Zone B. The temperature differences here are insignificant in the overall operation of the unit, but the pressure drop is significant, as is further explained below. Any temperature reduction in riser 32 is beneficial, but a conservative approach plans for minimal temperature decrease and only as predicted by the computer simulations. The Zone B vapor conditions may be as shown in Table 5 in this example:

TABLE 5

ZONE B

Zone B Vapor

	Temperature (deg. F.)	−175 to −200
	Pressure (psig)	375 to 425

Component (mole %)

	Nitrogen	20 to 50
	Methane	50 to 80

Condenser 14 is desirably mounted on the top of fractionation column 115 approximately 70 feet from grade, but the height may vary depending on feed stream 100 flow rate and size of first fractionating column 115. Condenser 14 is preferably a shell and tube heat exchanger configured with substantially vertical tubes 20 supported at the ends by the upper and lower tube sheets 16, 18, respectively. Heat exchange tubes 20 provide the heat transfer surface between the refrigerant, on the shell side, and the process vapor on the tube side. The shell side of the exchanger is isolated from the tube side as a different process fluid is present on that side. The refrigerant used on the shell side of the condenser is preferably LNG created from a tower bottom source. In one preferred embodiment, the refrigerant stream comprises stream 132 (a portion of the first column bottoms stream mixed with the second column bottoms stream downstream of heat exchanger 121). The refrigerant stream desirably enters condenser 14 through a nozzle at inlet 24 in shell 12 and exits shell 12 through a nozzle at outlet 28. The approximate conditions of the refrigerant stream 132 entering inlet 24 of condenser 14 are as previously described. The approximate conditions of the refrigerant stream 134 exiting condenser 14 at outlet 28 are as previously described.
It should be noted that the temperature is slightly higher on the exiting stream, but, and this is of greater significance, that the vapor fraction is much greater on the exiting stream. Because the temperatures of the refrigerant streams entering and exiting the heat exchanger are lower than the vapor inside the vertical tubes 20, heat will be transferred from the process vapor from Zone B into the refrigerant.
The fluid next passes from Zone B into Zone C through condenser 14, where the temperature is reduced. As stated before, the condition of the vapor in Zone B is at the dew point, which means that any reduction in temperature will produce condensate from the entering vapor.
The conditions of the fluid stream entering Zone C from condenser 14 in this example may be as shown in Table 6:

TABLE 6

ZONE C
Zone C Entering Vapor and Liquid Mixture

	Temperature (deg. F.)	−160 to −240
	Pressure (psig)	375 to 425
	Vapor mole fraction	10 to 50

Completing the circuit, the vapor part of the fluid stream exiting from heat exchange tubes 20 at the lower tube sheet exits the unit at vapor fraction outlet 44, from which liquid is preferably shielded by liquid barrier 42, and the condensed liquid component falls to liquid trap plate 40 where it flows by gravity through inlet 50 into reflux liquid return seal leg 48, and from there into reflux seal pan 52. The purpose of the seal leg 48 is to provide a liquid head created by standing liquid in the seal leg to offset the pressure loss in moving the vapor from Zone A into Zone B and eventually into Zone C. The pressure drop through the total circuit is preferably held to approximately 0.70 psi. The standing liquid in seal leg 48 creates this differential by using gravity and the higher density of the liquid component as compared to the same compounds as vapor. Reflux seal pan 52 provides a liquid trapping mechanism to prevent flow of the vapor in Zone A from flowing directly up seal leg 48 and bypassing condenser 14. Under normal operating conditions, the liquid level is anticipated to be approximately 1 foot deep on top of liquid trap plate 40.
It will be appreciated that systems and/or methods of separating nitrogen from methane to produce a nitrogen vent/nitrogen product stream with ultra-low greenhouse gas content, and sales gas stream(s) with nitrogen content within pipeline specifications, disclosed herein may include one or more of the following embodiments:
Embodiment 1. A system for producing a methane product stream and a nitrogen stream from a feed stream comprising nitrogen, methane, and other components, the system comprising: a first fractionating column wherein the feed stream is separated into a first column overhead stream and a first column bottoms stream; a first splitter for splitting the first column bottoms stream into a first portion, a second portion, and a third portion; a second fractionating column wherein the first column overhead stream is separated into a second column overhead stream and a second column bottoms stream; a second splitter for splitting the second column overhead stream into a first portion and a second portion; a first mixer to mix the second column bottoms stream and the first portion of the first column bottoms stream to form a refrigerant stream; a first heat exchanger wherein the feed stream is cooled upstream of the first fractionating column through heat exchange with the refrigerant stream, the second portion of the first column bottoms stream, the third portion of the first column bottoms stream, and the first portion of the second column overhead stream; a second heat exchanger for cooling a vapor stream from an upper fractionation section of the first fractionating column to produce the first column overhead stream and a reflux stream for the first fractionating column through heat exchange with the refrigerant stream prior to the refrigerant stream undergoing heat exchange in the first heat exchanger; wherein the methane product stream comprises the refrigerant stream, the second portion of the first column bottoms stream, and the third portion of the first column bottoms stream each after undergoing heat exchange in the first heat exchanger; and wherein the nitrogen stream comprises the first portion of the second column overhead stream and comprises less than 0.05% methane.
Embodiment 2. The system of embodiment 1 wherein the first fractionating column is operated at a pressure between 300 and 500 psig and the second fractionating column is operated at a pressure between 75 and 125 psig.
Embodiment 3. The system of any one of embodiments 1 or 2 wherein the second portion of the first column bottoms stream is a high pressure sales gas stream having a pressure between 600 and 1300 psig; wherein the third portion of the first column bottoms stream is an intermediate pressure sales gas stream having a pressure between 175 and 275 psig; and wherein the refrigerant stream is a low pressure sales gas stream having a pressure between 60 and 150 psig.
Embodiment 4. The system of any one of embodiments 1-3 further comprising a third splitter for splitting the feed stream into a first portion and a second portion downstream of the feed stream undergoing heat exchange in the first heat exchanger; and wherein the first portion of the feed stream is cooled in the first heat exchanger prior to feeding into a mid-upper level of the first fractionating column.
Embodiment 5. The system of any one of embodiments 1-4 further comprising a third heat exchanger for warming a liquid stream from a bottom section of the first fractionating column to produce the first column bottoms stream and a first column returning vapor stream for the first fractionating column through heat exchange with the second portion of the feed stream prior to the second portion of the feed stream feeding into a lower level of the first fractionating column.
Embodiment 6. The system of any one of embodiments 1-5 further comprising another splitter for splitting the second portion of the second column overhead stream into a third portion and a fourth portion; and a fourth heat exchanger wherein the third portion and the fourth portion of the second column overhead stream are warmed through heat exchange with a recycled stream; a series of one or more compressors and one or more coolers to compress and cool the fourth portion of the second column overhead stream after heat exchange in the fourth heat exchanger to form the recycled stream; and wherein the nitrogen stream further comprises the third portion of the second column overhead stream after undergoing heat exchange in the fourth heat exchanger.
Embodiment 7. The system of embodiment 6 further comprising a first expansion valve to expand and cool the recycled stream after undergoing heat exchange in the fourth heat exchanger; and wherein the recycled stream feeds into the second fractionating column as a reflux stream after passing through the first expansion valve.
Embodiment 8. The system of any one of embodiments 1-7 further comprising a fifth heat exchanger for cooling the first column overhead stream prior to feeding into the second fractionating column through heat exchange with the second column bottoms stream and the first portion of the second column overhead stream.
Embodiment 9. The system of embodiment 8 further comprising a second expansion valve to expand and cool the first column overhead stream after undergoing heat exchange in the fifth heat exchanger and prior to feeding into the second fractionating column.
Embodiment 10. The system of any one of embodiments 8-9 wherein the second fractionating column comprises an internal separation chamber configured to receive heat from the fifth heat exchanger to separate a liquid stream from a lower level of a fractionation section of the second fractionating column into a second column returning vapor stream and the second column bottoms stream prior to the second column bottoms stream undergoing heat exchange in the fifth heat exchanger.
Embodiment 11. The system of any one of embodiments 1-10 further comprising a third expansion valve for expanding and cooling the first portion of the first column bottoms stream upstream of the first mixer.
Embodiment 12. The system of any one of embodiments 1-11 further comprising a pump to pump the second portion of the first column bottoms stream prior to undergoing heat exchange in the first heat exchanger; and a fourth expansion valve to expand and cool the third portion of the first column bottoms stream prior to undergoing heat exchange in the first heat exchanger.
Embodiment 13. The system of any one of embodiments 1-12 wherein the second heat exchanger comprises a shell and tube heat exchanger.
Embodiment 14. The system of embodiment 13 wherein the shell and tube heat exchanger comprises a knockback condenser.
Embodiment 15. The system of any one of embodiments 1-12 wherein the second heat exchanger comprises a knockback condenser.
Embodiment 16. The system of any one of embodiments 14-15 wherein the knockback condenser comprises: a plurality of heat exchange tubes disposed inside a shell space; a headspace zone disposed above and in fluid communication with the plurality of heat exchange tubes; a riser tube configured to allow fluid communication of the vapor stream from the upper fractionation section of the first fractionating column to the headspace zone; and a refrigerant inlet and a refrigerant outlet to allow fluid communication of the refrigerant stream through the shell space.
Embodiment 17. The system of any one of embodiments 1-16 further comprising a second column reflux stream that feeds into an upper level of the second fractionating column and comprises at least 99.5% nitrogen.
Embodiment 18. The system of any one of embodiments 1-17 wherein the nitrogen stream comprises 0.01% or less methane.
Embodiment 19. The system of any one of embodiments 5-18 further comprising a fifth expansion valve to expand and cool the second portion of the feed stream after passing through the third heat exchanger and prior to the second portion of the feed stream feeding into the lower level of the first fractionating column.
Embodiment 20. The system of any one of embodiments 8-19 wherein the first column overhead stream is further cooled in the fifth heat exchanger through heat exchange with a liquid stream withdrawn from the second fractionating column; wherein the liquid stream withdrawn from the second fractionating column is partially vaporized in the fifth heat exchanger and returned to the second fractionating column.
Embodiment 21. A method for producing a methane product stream and a nitrogen stream from a system feed stream comprising nitrogen, methane, and other components, the method comprising: separating one or more first column feed streams comprising the system feed stream in a first fractionating column into a first column overhead stream and a first column bottoms stream; separating one or more second column feed streams comprising the first column overhead stream in a second fractionating column into a second column overhead stream and a second column bottoms stream; splitting the second column overhead stream in a first splitter into a first portion and a second portion; warming the second portion of the second column overhead stream in a first heat exchanger through heat exchange with a recycled stream; compressing and cooling the second portion of the second column overhead stream in a series of one or more compressors and one or more coolers after being warmed in the first heat exchanger, wherein the recycled stream comprises at least part of the second portion of the second overhead stream after compressing and cooling; and feeding the recycled stream after passing through the first heat exchanger into an upper level of the second fractionating column as a second column reflux stream; wherein the methane product stream comprises the first column bottoms stream; and wherein the nitrogen stream comprises the first portion of the second column overhead stream and comprises less than 0.05% methane.
Embodiment 22. The method of embodiment 21 further comprising: splitting the first column bottoms stream into at least a first portion and a second portion; cooling a vapor stream from an upper fractionation section of the first fractionating column in a second heat exchanger to produce the first column overhead stream and a first column reflux stream for the first fractionating column through heat exchange with a refrigerant stream; and mixing the first portion of the first column bottoms stream and the second column bottoms stream to form the refrigerant stream.
Embodiment 23. The method of embodiment 22 wherein the second heat exchanger comprises a knockback condenser.
Embodiment 24. The method of any one of embodiments 22-23 wherein the second heat exchanger comprises a shell and tube heat exchanger and wherein the vapor stream from the upper fractionation section is on a tube side of the second heat exchanger.
Embodiment 25. The method of any one of embodiments 21-24 further comprising: cooling the first column overhead stream in a third heat exchanger to produce a liquified stream through heat exchange with (1) the second column bottoms stream prior to mixing with the first portion of the first column bottoms stream and (2) the first portion of the second column overhead stream; expanding the liquified stream through a first expansion valve to produce an expanded stream that feeds into the second fractionating column as one of the one or more second column feed streams; and expanding the recycled stream after passing through the first heat exchanger in a second expansion valve to produce the second column reflux stream.
Embodiment 26. The method of embodiment 25 wherein the expanded stream feeds into the second fractionating column at a mid to lower tray level.
Embodiment 27. The method of any one of embodiments 25-26 wherein the expanded stream is a mixed liquid-vapor stream.
Embodiment 28. The method of any one of embodiments 25-27 further comprising cooling the system feed stream upstream of the first fractionating column through heat exchange in a fourth heat exchanger with the refrigerant stream after passing through the second heat exchanger, the second portion of the first column bottoms stream, and the first portion of the second column overhead stream after passing through the third heat exchanger.
Embodiment 29. The method of embodiment 28 further comprising splitting the system feed stream after passing through the fourth heat exchanger into a first portion and a second portion in a third splitter; cooling the first portion of the system feed stream in the fourth heat exchanger prior to feeding the first portion of the system feed stream into the first fractionating column as a first of the one or more first column feed streams; and warming a liquid stream from a bottom section of the first fractionating column to produce a first column returning vapor stream and the first column bottoms stream in a fifth heat exchanger through heat exchange with the second portion of the system feed stream prior to the second portion of the system feed stream feeding into the first fractionating column as a second of the one or more first column feed streams.
Embodiment 30. The method of any one of embodiments 25-29 further comprising warming a liquid stream from a bottoms section of the second fractionating column with heat received from the third heat exchanger to produce a returning vapor stream for the second fractionating column and the second column bottoms stream.
Embodiment 31. The method of any one of embodiments 25-29 further comprising warming a liquid stream withdrawn from a bottom section of the second fractionating column in the third heat exchanger to produce a mixed liquid-vapor stream; and returning the mixed liquid-vapor stream to a separation chamber in the second fractionating column to allow the mixed liquid-vapor stream to separate into an ascending vapor stream and the second column bottoms stream; and wherein the separation chamber is disposed lower in the second fractionating column than a level from where the liquid stream was withdrawn from the bottom section.
Embodiment 32. The method of any one of embodiments 21-31 wherein the first fractionating column is operated at a pressure between 300 and 500 psig and the second fractionating column is operated at a pressure between 75 and 125 psig.
Embodiment 33. The method of any one of embodiments 28-32 wherein the second portion of the first column bottoms stream after passing through the fourth heat exchanger is a sales gas stream having a pressure between 175 and 1300 psig; and wherein the refrigerant stream after passing through the fourth heat exchanger is a low pressure sales gas stream having a pressure between 60 and 150 psig.
Embodiment 34. The method of any one of embodiments 28-33 wherein the first column bottoms stream is further split into a third portion; wherein the third portion of the first column bottoms stream is warmed through heat exchange in the fourth heat exchanger; wherein the second portion of the first column bottoms stream after passing through the fourth heat exchanger is a high pressure sales gas stream having a pressure between 600 and 1300 psig; wherein the third portion of the first column bottoms stream after passing through the fourth heat exchanger is an intermediate pressure sales gas stream having a pressure between 175 and 275 psig; and wherein the refrigerant stream after passing through the fourth heat exchanger is a low pressure sales gas stream having a pressure between 60 and 150 psig.
Embodiment 35. The method of any one of embodiments 21-34 wherein the second column reflux stream comprises at least 99.5% nitrogen.
Embodiment 36. The method of any one of embodiments 21-35 wherein the nitrogen stream comprises 0.01% or less methane.
Embodiment 37. The method of any one of embodiments 29-36 further comprising expanding the second portion of the system feed stream after passing through the fifth heat exchanger and prior to the second portion of the system feed stream feeding into the first fractionating column as the second of the one or more first column feed streams.
The source of system feed gas stream 100 is not critical to the systems and methods herein. Where present, it is generally preferable for purposes of the present disclosure to remove as much of the water vapor, carbon dioxide (to within limits described herein, and other contaminants from feed stream 100 prior to processing with system 10. 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.
The specific operating parameters described with examples herein are based on the specific computer modeling and feed stream parameters set forth above. These parameters and the various composition, pressure, and temperature values described above will vary depending on the feed stream parameters as will be understood by those of ordinary skill in the art.
Heat exchangers as described herein and shown in the figures may be a single heat exchanger (single piece of equipment) in which all streams shown in the figures simultaneously pass through so that certain stream(s) are cooled and other stream(s) are warmed through heat exchange between the passing streams. In some embodiments, only the streams shown on the figures pass through any particular heat exchanger and no other streams undergo heat exchange with that set of streams in any particular heat exchanger. Although other heat exchange configurations and multiple heat exchangers may be used to achieve the heat exchange described herein, most preferably the heat exchange is specifically limited as shown in FIG. 1 , with the heat exchange shown being the only heat exchange between given streams prior to or after various processing equipment. For example, streams 100, 104, 134, 142, 138, and 171 are preferably the only streams that pass through heat exchanger 101 and all of these streams preferably pass simultaneously through a single heat exchanger 101. In some embodiments, other heat exchange between process streams or with external refrigeration or external heat sources not shown in FIG. 1 may be used. In other embodiments, other heat exchange between process streams or with external refrigeration or external heat sources not shown in FIG. 1 are excluded. Any change in temperature of a stream while flowing through piping from one piece of equipment to another piece of equipment as a result of a differential between the temperature of the stream and the ambient air temperature surrounding the piping, without more, is not considered heat exchange for purposes of this disclosure.
It will also 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 a system feed gas to produce various multi-component product streams containing large amounts of the particular desired component, but not necessarily 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. Other product streams are processed gas streams, or sales gas streams, which are primarily comprised of methane but may have small amounts of other components, such as nitrogen, ethane, and propane. Amounts of components in the various streams described herein as a percentage are mole fraction percentage.
The use of “a” or “an” is employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one, and the singular also includes the plural unless it is obvious that it is meant otherwise. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, system, piece of equipment, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: (1) A is true (or present), and B is false (or not present), (2) A is false (or not present), and B is true (or present), and (3) both A and B are true (or present).
All numerical values indicated as a percentage being “at least” X means the range of X % to 100% and values indicated as percentage being “less than” X means the range of 0% to X %. All numerical values herein indicated as a range (including as “at least” or “less than” or “greater than” or the like) include each individual value within those ranges and any and all subset combinations and subranges within ranges, including subsets that overlap from one disclosed range to another disclosed range, such as one range to a more preferred range. References to “about” or “around” with respect to numerical values (not expressed as percentages) generally mean +/−10% of the value, more preferably +/−5% of the value. For example, around 95 psig means 80.5 to 104.5 psig, more preferably 85.25 to 99.75 psig. References to “about” or “around” with respect to numerical values expressed as percentages generally mean +/−10% of the value, more preferably +/−5% of the value, up to a limit of 100% or 0%. For example around 95% means 80.5 to 100%, more preferably 85.25 to 99.75%.
Any operating parameter, step, process flow, or equipment indicated as preferred or preferable herein may be used alone or in any combination with other preferred/preferable features. Any component or processing step described herein with respect to any embodiment may be used with any other embodiment, even if not specifically described with such embodiment, unless it is specifically described as excluded for use with such embodiment. Other alterations and modifications of the disclosure 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.

Claims

I claim:

1. A system for producing a methane product stream and a nitrogen stream from a feed stream comprising nitrogen, methane, and other components, the system comprising:

a first fractionating column wherein the feed stream is separated into a first column overhead stream and a first column bottoms stream;

a first splitter for splitting the first column bottoms stream into a first portion, a second portion, and a third portion;

a second fractionating column wherein the first column overhead stream is separated into a second column overhead stream and a second column bottoms stream;

a second splitter for splitting the second column overhead stream into a first portion and a second portion;

a first mixer to mix the second column bottoms stream and the first portion of the first column bottoms stream to form a refrigerant stream;

a first heat exchanger wherein the feed stream is cooled upstream of the first fractionating column through heat exchange with the refrigerant stream, the second portion of the first column bottoms stream, the third portion of the first column bottoms stream, and the first portion of the second column overhead stream;

a second heat exchanger for cooling a vapor stream from an upper fractionation section of the first fractionating column to produce the first column overhead stream and a reflux stream for the first fractionating column through heat exchange with the refrigerant stream prior to the refrigerant stream undergoing heat exchange in the first heat exchanger;

wherein the methane product stream comprises the refrigerant stream, the second portion of the first column bottoms stream, and the third portion of the first column bottoms stream each after undergoing heat exchange in the first heat exchanger; and

wherein the nitrogen stream comprises the first portion of the second column overhead stream and comprises less than 0.05% methane.

2. The system of claim 1 wherein the first fractionating column is operated at a pressure between 300 and 500 psig and the second fractionating column is operated at a pressure between 75 and 125 psig.

3. The system of claim 2 wherein the second portion of the first column bottoms stream is a high pressure sales gas stream having a pressure between 600 and 1300 psig;

wherein the third portion of the first column bottoms stream is an intermediate pressure sales gas stream having a pressure between 175 and 275 psig; and

wherein the refrigerant stream is a low pressure sales gas stream having a pressure between 60 and 150 psig.

4. The system of claim 1 further comprising a third splitter for splitting the feed stream into a first portion and a second portion downstream of the feed stream undergoing heat exchange in the first heat exchanger; and

wherein the first portion of the feed stream is cooled in the first heat exchanger prior to feeding into a mid-upper level of the first fractionating column.

5. The system of claim 4 further comprising a third heat exchanger for warming a liquid stream from a bottom section of the first fractionating column to produce the first column bottoms stream and a first column returning vapor stream for the first fractionating column through heat exchange with the second portion of the feed stream prior to the second portion of the feed stream feeding into a lower level of the first fractionating column.

6. The system of claim 1 further comprising a third splitter for splitting the second portion of the second column overhead stream into a third portion and a fourth portion; and

a third heat exchanger wherein the third portion and the fourth portion of the second column overhead stream are warmed through heat exchange with a recycled stream;

a series of one or more compressors and one or more coolers to compress and cool the fourth portion of the second column overhead stream after heat exchange in the third heat exchanger to form the recycled stream; and

wherein the nitrogen stream further comprises the third portion of the second column overhead stream after undergoing heat exchange in the third heat exchanger.

7. The system of claim 6 further comprising a first expansion valve to expand and cool the recycled stream after undergoing heat exchange in the third heat exchanger; and

wherein the recycled stream feeds into the second fractionating column as a reflux stream after passing through the first expansion valve.

8. The system of claim 7 further comprising a fourth heat exchanger for cooling the first column overhead stream prior to feeding into the second fractionating column through heat exchange with the second column bottoms stream and the first portion of the second column overhead stream.

9. The system of claim 8 further comprising a second expansion valve to expand and cool the first column overhead stream after undergoing heat exchange in the fourth heat exchanger and prior to feeding into the second fractionating column.

10. The system of claim 8 wherein the second fractionating column comprises an internal separation chamber configured to receive heat from the fourth heat exchanger to separate a liquid stream from a lower level of a fractionation section of the second fractionating column into a second column returning vapor stream and the second column bottoms stream prior to the second column bottoms stream undergoing heat exchange in the fourth heat exchanger.

11. The system of claim 1 further comprising a first expansion valve for expanding and cooling the first portion of the first column bottoms stream upstream of the first mixer.

12. The system of claim 1 further comprising a pump to pump the second portion of the first column bottoms stream prior to undergoing heat exchange in the first heat exchanger; and

a first expansion valve to expand and cool the third portion of the first column bottoms stream prior to undergoing heat exchange in the first heat exchanger.

13. The system of claim 1 wherein the second heat exchanger comprises a shell and tube heat exchanger.

14. The system of claim 13 wherein the shell and tube heat exchanger comprises a knockback condenser.

15. The system of claim 1 wherein the second heat exchanger comprises a knockback condenser.

16. The system of claim 15 wherein the knockback condenser comprises:

a plurality of heat exchange tubes disposed inside a shell space;

a headspace zone disposed above and in fluid communication with the plurality of heat exchange tubes;

a riser tube configured to allow fluid communication of the vapor stream from the upper fractionation section of the first fractionating column to the headspace zone; and

a refrigerant inlet and a refrigerant outlet to allow fluid communication of the refrigerant stream through the shell space.

17. The system of claim 1 further comprising a second column reflux stream that feeds into an upper level of the second fractionating column and comprises at least 99.5% nitrogen.

18. The system of claim 1 wherein the nitrogen stream comprises 0.01% or less methane.

19. The system of claim 6 wherein the nitrogen stream comprises 0.01% or less methane.

20. The system of claim 8 wherein the first column overhead stream is further cooled in the fourth heat exchanger through heat exchange with a liquid stream withdrawn from the second fractionating column; wherein the liquid stream withdrawn from the second fractionating column is partially vaporized in the fourth heat exchanger and returned to the second fractionating column.