U.S. patent application number 12/241694 was filed with the patent office on 2010-04-01 for hybrid membrane/distillation method and system for removing nitrogen from methane.
Invention is credited to Sarang Gadre, Bao Ha, Edgar S. Sanders, JR..
Application Number | 20100077796 12/241694 |
Document ID | / |
Family ID | 42055963 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100077796 |
Kind Code |
A1 |
Gadre; Sarang ; et
al. |
April 1, 2010 |
Hybrid Membrane/Distillation Method and System for Removing
Nitrogen from Methane
Abstract
A hybrid gas separation membrane/cryogenic distillation method
and system produces high purity gaseous methane from a gas mixture
containing a majority of methane and a minority of nitrogen.
Inventors: |
Gadre; Sarang; (Bear,
DE) ; Ha; Bao; (San Ramon, CA) ; Sanders, JR.;
Edgar S.; (Newark, DE) |
Correspondence
Address: |
AIR LIQUIDE;Intellectual Property
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Family ID: |
42055963 |
Appl. No.: |
12/241694 |
Filed: |
September 30, 2008 |
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
Y02E 50/30 20130101;
F25J 2200/70 20130101; F25J 2200/02 20130101; B01D 2257/504
20130101; B01D 2256/24 20130101; F25J 3/0257 20130101; B01D
2257/304 20130101; F25J 3/0209 20130101; Y02C 10/10 20130101; C10L
3/10 20130101; Y02C 20/40 20200801; B01D 2258/05 20130101; F25J
2205/04 20130101; B01D 53/229 20130101; F25J 2235/60 20130101; F25J
2205/40 20130101; F25J 2205/80 20130101; F25J 3/0233 20130101; Y02E
50/346 20130101; B01D 2257/102 20130101 |
Class at
Publication: |
62/620 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A method of purifying a gas mixture having a majority of methane
and a minority of nitrogen, comprising the steps of: cooling the
gas mixture; feeding the cooled gas mixture to a gas separation
membrane to provide a permeate stream further enriched in methane
and a residue stream further enriched in nitrogen; cooling the
residue stream to form a cooled residue stream; reducing the
pressure of the cooled residue stream to provide a
nitrogen-enriched vapor and a methane-rich liquid; condensing the
nitrogen-enriched vapor; feeding the condensed nitrogen-enriched
vapor and the methane-rich liquid to a distillation column; warming
gaseous nitrogen withdrawn from a top of the distillation column to
provide a gaseous nitrogen product stream; pressurizing liquid
methane withdrawn from a bottom of the distillation column;
vaporizing the pressurized liquid methane to provide a stream of
vaporized methane; warming the stream of vaporized methane;
combining the permeate stream and the stream of warmed vaporized
methane to provide a gaseous methane product stream.
2. The method of claim 1, wherein said step of cooling the gas
mixture, said step of warming gaseous nitrogen and said step of
warming the vaporized liquid methane are performed at a first heat
exchanger.
3. The method of claim 2, wherein the gaseous nitrogen withdrawn
from the top of the distillation column is further warmed at a
second heat exchanger disposed in fluid communication between the
distillation column and the first heat exchanger and said step of
vaporizing liquid methane is performed at the second heat
exchanger.
4. The method of claim 3, further comprising the step of warming
the liquid methane withdrawn from a bottom of the distillation
column at a third heat exchanger before vaporization thereof,
wherein: the gaseous nitrogen withdrawn from the top of the
distillation column is further warmed at the third heat exchanger
before being warmed at the second heat exchanger; and the condensed
nitrogen-enriched vapor and the methane-rich liquid are cooled at
the third heat exchanger before being fed to the distillation
column.
5. The method of claim 1, wherein said step of condensing the
nitrogen-enriched vapor is conducted in a condenser-reboiler
operatively associated with the distillation column.
6. The method of claim 1, wherein the gas mixture is natural gas
obtained from a subterranean formation.
7. The method of claim 6, wherein the natural gas comprises from
about 60 to about 90 mol % methane, up to about 25 mol % nitrogen,
and from about 0 to about 10 mol % carbon dioxide.
8. The method of claim 7, wherein amounts of CO.sub.2 and H.sub.2S
are removed from the natural gas prior to feeding it to the gas
separation membrane.
9. The method of claim 1, wherein the liquid methane from the
distillation column is pressurized with a pump.
10. The method of claim 1, wherein the gas separation membrane is
maintained at a temperature lower than -20.degree. C.
11. The method of claim 1, wherein the gas separation membrane is
maintained at a temperature of -50 to -90.degree. C.
12. The method of claim 1, wherein the gas separation membrane is
made of a material selected from the group consisting of
polypropylene oxide allyl glycidyl ether) and silicone rubber
[poly(dimethyl siloxane).
13. The method of claim 1, wherein the gas separation membrane is
made of a material that has a methane to nitrogen selectivity of at
least 5.
14. The method of claim 1, wherein the gaseous methane product
stream contains less than 6 mol % N.sub.2 and greater than 94 mol %
methane.
15. The method of claim 1, further comprising the step of expanding
the gaseous nitrogen product stream and compressing the methane
product stream with a turbo expander.
16. The method of claim 1, wherein the gas mixture is landfill gas
from a landfill.
17. A system for purifying a gas mixture having a majority of
methane and a minority of nitrogen, comprising: a source of a gas
mixture comprising a majority of methane and a minority of
nitrogen; a first heat exchanger adapted to cool a stream of said
gas mixture; a gas separation membrane having a feed inlet, a
permeate gas outlet, and a residue gas outlet, said feed inlet
being in fluid communication with said source via said first heat
exchanger; a distillation column having a top and a bottom, a
plurality of inlets, a gaseous nitrogen outlet disposed at said
column top, and a liquid methane outlet disposed at said column
bottom, said plurality of column inlets being in fluid
communication with said residue gas outlet; and a second heat
exchanger adapted to cool a stream of residue gas from said residue
gas outlet, warm a stream of gaseous nitrogen withdrawn from said
column top, and vaporize a stream of liquid methane withdrawn from
said column bottom, wherein said first heat exchanger is further
adapted to: further warm the stream of gaseous nitrogen warmed at
said second heat exchanger; warm a stream of gaseous methane
produced by vaporization at said second heat exchanger; and warm a
stream of permeate gas from said permeate gas outlet.
18. The system of claim 17, further comprising: a Joule-Thomson
valve in fluid communication between said residue gas outlet and
said plurality of column inlets; and a phase separator comprising
an inlet in fluid communication with said Joule-Thomson valve, a
vapor outlet, and a liquid outlet, said vapor and liquid outlets
being in fluid communication with said plurality of column inlets,
said phase separator being adapted to separate a stream of residue
gas expanded at said valve into a stream of nitrogen-enriched vapor
and a stream of methane-rich liquid.
19. The system of claim 18, further comprising: a
condenser-reboiler adapted to condense the stream of
nitrogen-enriched vapor from said phase separator vapor outlet and
vaporize a stream of liquid methane from said column bottom.
20. The system of claim 17, further comprising a third heat
exchanger adapted to warm the stream of gaseous nitrogen withdrawn
from said column top before warming at said second heat exchanger
and warm the stream of liquid methane withdrawn from said column
bottom before warming at said second heat exchanger.
21. The system of claim 17, further comprising a gaseous methane
product conduit receiving a stream of the permeate gas warmed at
said first heat exchanger and a stream of gaseous methane warmed at
said first exchanger to provide a stream of gaseous methane
product.
22. The system of claim 21, further comprising a turbo expander
adapted to expand the stream of gaseous nitrogen warmed at said
first heat exchanger and compress the stream of gaseous methane
product.
23. The system of claim 17, wherein said gas mixture is natural gas
and said source is disposed within a subterranean formation.
24. The system of claim 23, further comprising a purification unit
in fluid communication between said source and said gas separation
membrane, said purification unit being adapted to remove at least a
portion of CO.sub.2 and H.sub.2S from a stream of said natural gas
from said source using adsorption and/or membrane purification
techniques.
25. The system of claim 17, further comprising a pump adapted to
pump a stream of liquid methane from said column bottom.
26. The system of claim 17, wherein said gas separation membrane is
made of a material selected from the group consisting of
poly(propylene oxide allyl glycidyl ether) and silicone rubber
[poly(dimethyl siloxane).
27. The system of claim 17, wherein said gas separation membrane is
made of a material that has a methane to nitrogen selectivity of at
least 5.
28. The system of claim 17, wherein said gas mixture is landfill
gas and said source is a landfill.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] None.
BACKGROUND
[0002] Prior art in nitrogen removal from natural gas includes
several references to cryogenic separation. With adequate feed
pressure, the single column process can perform the separation
using no external energy other than power for a liquid pump which
is used to pump liquid methane to the desired product pressure.
Single and dual pressure columns are common practice in cryogenic
applications such as nitrogen rejection from a natural gas
stream.
[0003] U.S. Pat. No. 4,878,932 describes a single column process
wherein the cooled feed is pre-separated in a phase separator into
vapor and liquid portions, the vapor is condensed and at least
partly employed as reflux for the column. This single column
process scheme tends to have good recovery when the N.sub.2 content
in the feed stream is high, typically more than 20%. However, when
the N.sub.2 content decreases, the methane recovery tends to fall
sharply.
[0004] For natural gas streams having a relatively low N.sub.2
content, dual distillation columns operated at different pressures
typically are used to maintain high recovery. U.S. Pat. No.
4,415,345 describes one example of a double column system.
Generally speaking, in dual distillation columns the high pressure
column provides a methane enriched stream which is sent to the low
pressure column for further enrichment. Liquid methane product is
then pumped to the desired product pressure. More particularly, the
double column system is operated such that condenser duty to the
first column provides the reboiler duty of the second column
whereas in the single column process, heat integration is carried
out by using reboiler duty to condense the feed to the distillation
column. In either of the single or double column schemes, the only
significant external energy that is required is in the form of a
liquid pump. For feed gas pressures of 80 bar or higher and methane
product pressures of up to 35 bar, no additional cooling or
compression is typically required. The Joule-Thomson effect between
the feed gas and product streams is sufficient to satisfy plant
refrigeration requirement.
[0005] A typical example of a dual column system is shown in FIG.
1. According to this scheme, a high pressure N.sub.2-containing
natural gas feed 1 (typically at a pressure of about 80 bar) is
cooled at heat exchanger 5 by heat-exchange with methane stream 13,
high pressure N.sub.2 stream 17 and low pressure N.sub.2 stream 21
The cooled feed 9 is then expanded at Joule-Thomson valve 25
yielding lowered pressure feed 29 which is sent to the high
pressure distillation column 33. Distillation column 33
fractionates feed 29 into a methane-rich liquid component carried
in stream 41 and a high pressure N.sub.2-rich vapor component 37.
Condenser-reboiler 82 condenses a portion 38 of the vapor component
37 to provide a liquid stream rich in N.sub.2 53. A portion 17 of
37 can be recovered as high pressure gaseous N2 stream 17. A
portion 55 of 53 is sent back to column 33 as reflux. The remaining
portion 57 is then directed to column 81. Streams 41, 57 are cooled
at heat exchanger 61 through heat exchange with liquid methane
stream 65 and low pressure gaseous N.sub.2 stream 69 before being
directed to the low pressure distillation column 81. The low
pressure column 81 fractionates the methane/N.sub.2 mixture
contained therein into low pressure gaseous N2 stream 69, and high
purity liquid methane stream 78. Stream 76 is directed to the
condenser-reboiler 82 which receives heat from stream 37 and
returns a stream of vaporized or partially vaporized methane 84 to
column 81. A liquid pump 93 receiving high purity liquid methane 90
from the bottom of column 81 pumps high purity liquid methane
stream 65 through heat exchanger 61 whereat it and the high
pressure gaseous N.sub.2 stream 69 are warmed. Liquid methane
stream 13 is then vaporized in exchanger 5 to provide a stream of
high purity methane 95. Low pressure N.sub.2 stream 21 and high
pressure N.sub.2 stream 17 are warmed at heat exchanger 5 to
provide streams of low pressure N.sub.2 99 and high pressure
N.sub.2 97, respectively.
[0006] FIG. 2 shows a typical example of a single column separation
scheme. Here the feed 101 is cooled in the heat exchanger 105
through heat exchange with streams 112, 116. The cooled stream 109
is expanded in an expansion valve (or also called Joule-Thomson
valve) 120 to lower pressure. This reduction of pressure results in
a two-phase stream which is then phase separated into vapor and
liquid streams at phase separator 124. The vapor stream 128 is
condensed at condenser-reboiler 182. The consensed vapor stream 149
is cooled at heat exchanger 161 through heat exchange with liquid
methane stream 165 and high pressure gaseous N.sub.2 stream 169 and
sent to the distillation column 181 as reflux. The liquid stream
142 from the separator 124 is subcooled at heat exchanger 161 (also
through heat exchange with liquid methane stream 165 and high
pressure gaseous N.sub.2 stream 169) and directed to the column
181. A stream of high purity liquid methane 176 is directed to the
condenser-reboiler 182 which receives heat from stream 128 and
returns a stream of vaporized or partially vaporized methane 184 to
column 181. A liquid pump 193 receiving high purity liquid methane
190 from the bottom of column 181 pumps high purity liquid methane
stream 165 through heat exchanger 161 whereat it and the high
pressure gaseous N.sub.2 stream 169 are warmed. Liquid methane
stream 112 is then vaporized in exchanger 105 to provide a stream
of high purity methane 194. High pressure N.sub.2 stream 116 is
warmed at heat exchanger 105 to provide stream of high pressure
N.sub.2 196.
[0007] Membranes have been used in hybrid application such that the
feed is first sent to the membrane, the product of which is then
sent to a distillation column for separation. There is also prior
art available on use of membrane-distillation hybrid system for
natural gas applications, such as U.S. Pat. No. 5,647,227.
[0008] While the above approaches provide sufficient solutions for
purifying many types of N.sub.2-containing natural gas, they often
suffer from one or more disadvantages. For cryogenic separation
units, variation in the feed N.sub.2 content can pose problem to
the operation of a cryogenic separation unit. This is because while
single column distillation systems work well for high N.sub.2
content natural gas, recoveries can fall sharply as the N.sub.2
content is decreased. In such cases, a second column may be
necessary. This adds to the capital cost.
[0009] Thus, it is the object of the current invention to provide a
scheme which can provide sufficient methane recovery for feeds
having variable N.sub.2 contents and requires minimal energy
input.
SUMMARY
[0010] There is provided a method of purifying a gas mixture having
a majority of methane and a minority of nitrogen. It includes the
following steps. The gas mixture is cooled. The cooled gas mixture
is fed to a gas separation membrane to provide a permeate stream
further enriched in methane and a residue stream further enriched
in nitrogen. The residue stream is cooled to form a cooled residue
stream. The pressure of the cooled residue stream is reduced to
provide a nitrogen-enriched vapor and a methane-rich liquid. The
nitrogen-enriched vapor is condensed. The condensed
nitrogen-enriched vapor and the methane-rich liquid are fed to a
distillation column. The gaseous nitrogen withdrawn from a top of
the distillation column is warmed to provide a gaseous nitrogen
product stream. The liquid methane withdrawn from a bottom of the
distillation column is pressurized. The pressurized liquid methane
is vaporized to provide a stream of vaporized methane. The stream
of vaporized methane is warmed. The permeate stream and the stream
of warmed vaporized methane are combined to provide a gaseous
methane product stream.
[0011] The method may include one or more of the following aspects.
[0012] said step of cooling the gas mixture, said step of warming
gaseous nitrogen and said step of warming the vaporized liquid
methane are performed at a first heat exchanger. [0013] the gaseous
nitrogen withdrawn from the top of the distillation column is
further warmed at a second heat exchanger disposed in fluid
communication between the distillation column and the first heat
exchanger and said step of vaporizing liquid methane is performed
at the second heat exchanger. [0014] the method further comprises
the step of warming the liquid methane withdrawn from a bottom of
the distillation column at a third heat exchanger before
vaporization thereof, wherein: [0015] the gaseous nitrogen
withdrawn from the top of the distillation column is further warmed
at the third heat exchanger before being warmed at the second heat
exchanger; and [0016] the condensed nitrogen-enriched vapor and the
methane-rich liquid are cooled at the third heat exchanger before
being fed to the distillation column. [0017] said step of
condensing the nitrogen-enriched vapor is conducted in a
condenser-reboiler operatively associated with the distillation
column. [0018] the gas mixture is natural gas obtained from a
subterranean formation. [0019] the natural gas comprises from about
60 to about 90 mol % methane, up to about 25 mol % nitrogen, and
from about 0 to about 10 mol % carbon dioxide. [0020] amounts of
CO.sub.2 and H.sub.2S are removed from the natural gas prior to
feeding it to the gas separation membrane. [0021] the liquid
methane from the distillation column is pressurized with a pump.
[0022] the gas separation membrane is maintained at a temperature
lower than -20.degree. C. [0023] the gas separation membrane is
maintained at a temperature of -50 to -90.degree. C. [0024] the gas
separation membrane is made of a material selected from the group
consisting of poly(propylene oxide allyl glycidyl ether) and
silicone rubber [poly(dimethyl siloxane). [0025] the gas separation
membrane is made of a material that has a methane to nitrogen
selectivity of at least 5. [0026] the gaseous methane product
stream contains less than 6 mol % N.sub.2 and greater than 94 mol %
methane. [0027] the method further comprises the step of expanding
the gaseous nitrogen product stream and compressing the methane
product stream with a turbo expander.
[0028] There is also provided a system for purifying a gas mixture
having a majority of methane and a minority of nitrogen,
comprising, a source of a gas mixture; a first heat exchanger; a
gas separation membrane; a distillation column; and a second heat
exchanger. The source of a gas mixture comprises a majority of
methane and a minority of nitrogen. The first heat exchanger is
adapted to cool a stream of said gas mixture. The gas separation
membrane has a feed inlet, a permeate gas outlet, and a residue gas
outlet, said feed inlet being in fluid communication with said
source via said first heat exchanger. The distillation column has a
top and a bottom, a plurality of inlets, a gaseous nitrogen outlet
disposed at said column top, and a liquid methane outlet disposed
at said column bottom, said plurality of column inlets being in
fluid communication with said residue gas outlet. The second heat
exchanger is adapted to cool a stream of residue gas from said
residue gas outlet, warm a stream of gaseous nitrogen withdrawn
from said column top, and vaporize a stream of liquid methane
withdrawn from said column bottom. Said first heat exchanger is
further adapted to: further warm the stream of gaseous nitrogen
warmed at said second heat exchanger; warm a stream of gaseous
methane produced by vaporization at said second heat exchanger; and
warm a stream of permeate gas from said permeate gas outlet.
[0029] The system may include one or more of the following aspects:
[0030] the system further comprises [0031] a Joule-Thomson valve in
fluid communication between said residue gas outlet and said
plurality of column inlets; and [0032] a phase separator comprising
an inlet in fluid communication with said Joule-Thomson valve, a
vapor outlet, and a liquid outlet, said vapor and liquid outlets
being in fluid communication with said plurality of column inlets,
said phase separator being adapted to separate a stream of residue
gas expanded at said valve into a stream of nitrogen-enriched vapor
and a stream of methane-rich liquid. [0033] the system further
comprises: [0034] a Joule-Thomson valve in fluid communication
between said residue gas outlet and said plurality of column
inlets; [0035] a phase separator comprising an inlet in fluid
communication with said Joule-Thomson valve, a vapor outlet, and a
liquid outlet, said vapor and liquid outlets being in fluid
communication with said plurality of column inlets, said phase
separator being adapted to separate a stream of residue gas
expanded at said valve into a stream of nitrogen-enriched vapor and
a stream of methane-rich liquid and [0036] a condenser-reboiler
adapted to condense the stream of nitrogen-enriched vapor from said
phase separator vapor outlet and vaporize a stream of liquid
methane from said column bottom. [0037] the system further
comprises a third heat exchanger adapted to warm the stream of
gaseous nitrogen withdrawn from said column top before warming at
said second heat exchanger and warm the stream of liquid methane
withdrawn from said column bottom before warming at said second
heat exchanger. [0038] the system further comprises a gaseous
methane product conduit receiving a stream of the permeate gas
warmed at said first heat exchanger and a stream of gaseous methane
warmed at said first exchanger to provide a stream of gaseous
methane product. [0039] the system further comprises [0040] a
gaseous methane product conduit receiving a stream of the permeate
gas warmed at said first heat exchanger and a stream of gaseous
methane warmed at said first exchanger to provide a stream of
gaseous methane product; and [0041] a turbo expander adapted to
expand the stream of gaseous nitrogen warmed at said first heat
exchanger and compress the stream of gaseous methane product.
[0042] said gas mixture is natural gas and said source is disposed
within a subterranean formation. [0043] the system further
comprises a purification unit in fluid communication between said
source and said gas separation membrane, said purification unit
being adapted to remove at least a portion of CO.sub.2 and H.sub.2S
from a stream of said natural gas from said source by adsorption
and/or membrane purification techniques. [0044] the system further
comprises a pump adapted to pump a stream of liquid methane from
said column bottom. [0045] said gas separation membrane is made of
a material selected from the group consisting of poly(propylene
oxide allyl glycidyl ether) and silicone rubber [poly(dimethyl
siloxane). [0046] said gas separation membrane is made of a
material that has a methane to nitrogen selectivity of at least
5.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] For a further understanding of the nature and objects of the
present invention, reference should be made to the following
detailed description, taken in conjunction with the accompanying
drawings, in which like elements are given the same or analogous
reference numbers and wherein:
[0048] FIG. 1 is a schematic of a prior art double column system
for nitrogen removal from methane.
[0049] FIG. 2 is a schematic of a prior art single column system
for nitrogen removal from methane.
[0050] FIG. 3 is a schematic of the hybrid membrane/cryogenic
distillation system according to the invention for nitrogen removal
from methane.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0051] As best illustrated in FIG. 3, the method and system
according to the invention starts with a feed gas 201 containing a
majority amount of methane and a minority amount of N.sub.2. The
feed gas 201 may be natural gas obtained from a subterranean
formation or a methane-containing landfill gas from a landfill. In
either case, such a methane-based feed gas 201 typically comprises
from about 60 to about 90 mol % methane, from about 0 to about 25
mol % nitrogen, from about 0 to about 10 mol % carbon dioxide (up
to about 50 mol % carbon dioxide in the case of a
methane-containing gas derived from a landfill), and moisture and
other minor substituents. If the feed gas 201 contains undesirable
amounts of impurities such as CO.sub.2 and H.sub.2S, it may be
pre-treated using a conventional purification unit to remove those
impurities and moisture prior to sending it to membrane separation
unit 208 (before or after heat exchanger 205). The purification
unit may employ any number of well-known adsorption and/or
membrane-based purification techniques. A knock-out drum may be
utilized to remove heavier hydrocarbons from the gas mixture. Feed
gas 201, typically at ambient temperature and a pressure in the
range of from about 35 to 80 bar, is cooled to a temperature of
less than -20.degree. C. preferably to a temperature of about -50
to about -90.degree. C. in heat exchanger 205.
[0052] The cooled feed gas stream 206 is directed to membrane
separation unit 208 that includes one or more membrane selectively
permeable to methane over N.sub.2. Methane, being the fast gas,
permeates through the one or more membranes and the result permeate
stream 210 is directed back across heat exchanger 205 thereby
warming it to yield warmed permeate stream 298. Depending upon the
N.sub.2 content in stream 206, a significant portion of the methane
may be separated out in the permeate. For example, at 15% N.sub.2
content, as much as 65% of the feed is permeated through the
membrane. The operating temperature of the gas separation unit is
maintained at or below -20.degree. C. Preferably, it is maintained
at a temperature of about -60 to about -90.degree. C. Typically,
the permeate stream 210 contains from about 90 to about 95 mol %
methane. A back pressure control valve on the permeate side of the
membrane separation unit 208 may be used to control the pressure of
the permeate stream 210 (which should be slightly higher than the
product pressure). This valve is throttled to adjust the permeate
flux and its composition.
[0053] In the cooled feed gas stream 206, the N.sub.2, being the
slow gas, tends to not permeate through the one or more membranes
and thus accumulates in the residue stream 211. Residue stream 211
is cooled to a temperature of about -110.degree. C. at heat
exchanger 214. The cooled residue stream 209 is then flashed at
valve 220 and directed to phase separator 224 where it is separated
into a N.sub.2-enriched vapor stream 228 and a methane-enriched
liquid stream 242. The vapor stream 228 is condensed at
condenser-reboiler 282 and condensed vapor 249 is optionally cooled
at optional heat exchanger 261 and directed in stream 275 as reflux
to distillation column 281. The liquid stream 242 is optionally
subcooled at optional heat exchanger 261 and also directed in
stream 272 to column 281.
[0054] Column 281 produces a gaseous N.sub.2-rich stream 269 and a
liquid methane-rich stream 278. Typically, stream 269 includes
about 5 mol % methane. Typically, stream 278 includes at least
about 95 mol % methane and preferably more than 97 mol % methane.
Stream 269 is warmed at heat exchangers 261 (optionally), 214, 205
to yield gaseous N.sub.2 product stream 296, typically at a
pressure of about 3 to 5 bar.
[0055] Liquid methane-rich stream 276 is directed to
condenser-reboiler 282 utilizing heat from stream 228 to provide a
stream of vaporized or partially vaporized methane 284 to column
281. A liquid methane-rich stream 290 is sent to liquid pump
293.
[0056] The stream of liquid methane pumped by pump 293 is
optionally directed via stream 265 to optional heat exchanger 261
where it is warmed, but is in any case directed via stream 215 to
heat exchanger 214 where it is vaporized and then directed via
stream 212 where it is warmed to provide gaseous methane stream
294. Gaseous methane stream 294 is combined with warmed permeate
stream 296 at a methane product conduit to provide methane product
stream 295. Typically, stream 295 contains less than 6 mol %
N.sub.2 and greater than 94 mol % methane. If desired, a turbo
expander may be utiized to transfer power from expansion of the
N.sub.2 product stream 296 to compression of the methane product
stream 295. Whether or not the turbo expander is utilized, the
methane product stream 295 typically has a pressure of about 36 bar
with a feed gas 201 pressure of about 77 bar.
[0057] The patent and non-patent literature in the field of gas
separation is replete with details on how to construct or where to
procure the membrane separation unit 208, so their details need not
be duplicated herein. The membrane or membranes in membrane
separation unit 208 may be configured in any way known in the field
of gas separation, including a sheet, tube, hollow fiber, etc.
Preferably, the membrane is a spiral flat sheet membrane or hollow
fiber membrane. Generally speaking, the requisite methane/nitrogen
membrane selectivity will depend upon the N.sub.2 content of the
cooled feed gas stream 206. At a temperature of -67.degree. C., a
selectivity of 7 was sufficient for feed gas stream 206 contents of
15-25% N.sub.2. The selectivity may be modified by changing the
temperature of the cooled feed gas stream 206. If a higher
selectivity is desired, the temperature should be lowered. The
membrane is made of a polymeric material such that, when operated
at a temperature of no greater than -20.degree. C., the membrane
has a selectivity to methane over N.sub.2 of at least 5, preferably
of at least 7. Because the feed gas stream 201 is cooled via heat
exchange with streams 294, 296, and 298 at heat exchanger 205, when
it enters the gas separation unit 208 via stream 206, it is already
at a temperature where the desired selectivity is realized. In
other words, greater selectivity is achieved than that realized at
relatively warmer temperatures. Suitable polymeric materials
include Parel [poly(propylene oxide allyl glycidyl ether)] and
silicone rubber [poly(dimethyl siloxane)]. Preferably, it is
silicone rubber.
[0058] The configurations of the heat exchangers 205, 214, 261 may
be any of the known configurations in the field of gas separation,
including the shell and tube-type or the brazed type. The brazed
exchanger in particular is the preferred choice for this type of
process since it can provide an economical configuration for multi
streams exchangers.
[0059] The patent and non-patent literature in the field of gas
separation is replete with details on how to construct or where to
procure the Joule-Thomson valve 220, phase separator 224, column
281, condenser-reboiler 282, and pump 293, and as such, they need
not be duplicated herein.
[0060] Practice of the process yields several advantages.
[0061] The hybrid scheme of the invention can treat varying N.sub.2
contents in the feed gas stream with relatively high methane
recovery (>97%). The membrane acts as a regulator to optimize
the nitrogen content of the feed for distillation by performing a
partial separation of the N.sub.2 and methane upstream of
distillation. This represents a significant advantage over either a
cryogenic-only solution which operates efficiently over a narrow
range of feed nitrogen or a membrane-only solution which might not
achieve the separation with acceptable recovery.
[0062] The hybrid scheme of the invention also lowers capital costs
of a system separating N.sub.2 from methane. In comparison to the
single or double column systems of FIGS. 2 and 1, the size of the
distillation column 281 may be reduced because the membrane reduces
the feed sent to column 281. Also, the gas mixture is separated
into methane and nitrogen utilizing only a single distillation
column.
[0063] The hybrid scheme of the invention also results in lower
operating costs because the energy requirements, in comparison to
the conventional systems, are relatively low. Expansion of
compressed gas provides cryogenic temperatures for the distillation
column. Thus, external energy for cooling is unnecessary.
Cross-exchange of heat of the N.sub.2 product and methane product
components with the membrane feed provides the desirable low
operating temperature in the membrane, again removing the need for
external energy for cooling. Indeed, this process can achieve the
separation with no external energy other than the small amount
needed for pumping the liquid methane. On the other hand, those
skilled in the art will recognize that operating costs are
relatively greater for systems utilizing a compressor for
compression of gaseous methane, because under most conditions
compressing a gas is much more energy-intensive than pumping a
liquid.
[0064] Preferred processes and apparatus for practicing the present
invention have been described. It will be understood and readily
apparent to the skilled artisan that many changes and modifications
may be made to the above-described embodiments without departing
from the spirit and the scope of the present invention. The
foregoing is illustrative only and that other embodiments of the
integrated processes and apparatus may be employed without
departing from the true scope of the invention defined in the
following claims.
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