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

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%.

Claims

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The systems and methods of the disclosure are further described and explained in relation to the following drawings wherein:

[0043] FIG. 1 is a process flow diagram illustrating a preferred embodiment of a methane and nitrogen separation system and method as described herein; and

[0044] 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

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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%.

[0052] 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%.

[0053] 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.

[0054] 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.

[0055] 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).

[0056] 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.

[0057] 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.

[0058] 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 CO.sub.2 to 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 CO.sub.2 entering 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 CO.sub.2 in the first column 115 to avoid potential freezing issues in the second column 147.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Acceptable inlet compositions in which system 10 may operate satisfactorily are listed in the following Table 1:

TABLE-US-00001 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

[0070] 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-US-00002 TABLE 2A Stream Parameter Ranges for System Feed and 1.sup.st Column Feed Stream & Property Ranges 108 (Cooled 112 (Cooled 2.sup.nd Part of 110 (HP 1.sup.st Part 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-US-00003 TABLE 2B Stream Parameter Ranges for 1.sup.st Column Top Streams & Refrigerant Stream & Property Ranges 132 (Refrigerant, Mixed 1.sup.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 CO.sub.2 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-US-00004 TABLE 2C Stream Parameter Ranges for 1.sup.st Column Bottom Streams Stream & Property Ranges 124 (HP Col. 122 (HP Col. Vapor from 130 (Expanded 138 (Pumped 142 (Expanded Bottom Liquid Stream 122 1.sup.st Part of 2.sup.nd Part of 3.sup.rd Part 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-US-00005 TABLE 2D Stream Parameter Ranges for 2.sup.nd Column Feed & Top Streams Stream & Property Ranges 169 165 (4.sup.th Part of 184 144 (Expanded LP Col. 182 (Top (Liquified 146 157 3.sup.rd Part 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-US-00006 TABLE 2E Stream Parameter Ranges for Nitrogen Vent/Product Related Streams Stream & Property Ranges 160 167 (Warmed 1.sup.st 172 (3.sup.rd Part 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-US-00007 TABLE 2F Stream Parameter Ranges for 2.sup.nd Column 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-US-00008 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

[0071] 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 1Computer Simulation for 500 MMSCFD Feed with 4% Nitrogen in System 10

[0072] 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.

[0073] 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.

[0074] 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 CO.sub.2, 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 CO.sub.2, C2, and C3. Stream 120 is at a temperature of 190.68 F.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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-US-00009 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

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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-US-00010 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

[0102] 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.

[0103] 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.

[0104] 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-US-00011 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

[0105] 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.

[0106] 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.

[0107] 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.

[0108] The conditions of the fluid stream entering Zone C from condenser 14 in this example may be as shown in Table 6:

TABLE-US-00012 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

[0109] 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.

[0110] 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:

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] Embodiment 13. The system of any one of embodiments 1-12 wherein the second heat exchanger comprises a shell and tube heat exchanger.

[0124] Embodiment 14. The system of embodiment 13 wherein the shell and tube heat exchanger comprises a knockback condenser.

[0125] Embodiment 15. The system of any one of embodiments 1-12 wherein the second heat exchanger comprises a knockback condenser.

[0126] 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.

[0127] 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.

[0128] Embodiment 18. The system of any one of embodiments 1-17 wherein the nitrogen stream comprises 0.01% or less methane.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] Embodiment 23. The method of embodiment 22 wherein the second heat exchanger comprises a knockback condenser.

[0134] 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.

[0135] 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.

[0136] Embodiment 26. The method of embodiment 25 wherein the expanded stream feeds into the second fractionating column at a mid to lower tray level.

[0137] Embodiment 27. The method of any one of embodiments 25-26 wherein the expanded stream is a mixed liquid-vapor stream.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] Embodiment 35. The method of any one of embodiments 21-34 wherein the second column reflux stream comprises at least 99.5% nitrogen.

[0146] Embodiment 36. The method of any one of embodiments 21-35 wherein the nitrogen stream comprises 0.01% or less methane.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] 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.

[0152] 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).

[0153] 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%.

[0154] 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.