U.S. patent application number 11/809450 was filed with the patent office on 2007-10-18 for method for producing liquefied natural gas.
Invention is credited to Henry Edward Howard, Minish Mahendra Shah.
Application Number | 20070240449 11/809450 |
Document ID | / |
Family ID | 36143908 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070240449 |
Kind Code |
A1 |
Howard; Henry Edward ; et
al. |
October 18, 2007 |
Method for producing liquefied natural gas
Abstract
A method for producing liquefied natural gas employing two
separate adsorption steps for removing water and carbon dioxide
from natural gas and employing a subambient expansion to produce
regeneration gas for the regeneration of the dehydration adsorption
step in addition to supplying refrigeration for the cooling or
liquefaction of clean natural gas.
Inventors: |
Howard; Henry Edward; (Grand
Island, NY) ; Shah; Minish Mahendra; (East Amherst,
NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
36143908 |
Appl. No.: |
11/809450 |
Filed: |
June 1, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10962666 |
Oct 13, 2004 |
7231784 |
|
|
11809450 |
Jun 1, 2007 |
|
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|
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25J 1/0232 20130101;
F25J 2205/60 20130101; F25J 1/0219 20130101; F25J 2220/66 20130101;
F25J 1/0037 20130101; F25J 1/0202 20130101; F25J 2220/64 20130101;
F25J 1/0208 20130101; F25J 2210/06 20130101; F25J 2290/32 20130101;
F25J 1/0045 20130101; F25J 1/0022 20130101 |
Class at
Publication: |
062/613 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Claims
1. A method for producing liquefied natural gas comprising: (A)
removing water from a first natural gas stream in a first
adsorption unit to produce dehydrated natural gas, cooling the
dehydrated natural gas to a temperature below the critical
temperature of methane to produce cooled dehydrated natural gas,
expanding the cooled dehydrated natural gas in a subambient
expansion to produce depressurized natural gas, warming the
depressurized natural gas, and using a portion of the warmed
depressurized natural gas as regeneration gas in the first
adsorption unit; and (B) removing carbon dioxide and water from a
second natural gas stream in a second adsorption unit to produce
clean natural gas, liquefying a portion of the clean natural gas to
produce liquefied natural gas, and using another portion of the
clean natural gas as regeneration gas in the second temperature
swing adsorption unit.
2. The method of claim 1 further comprising removing carbon dioxide
from the first natural gas stream in the first adsorption unit.
3. The method of claim 1 wherein the warming of the depressurized
natural gas is by indirect heat exchange with the cooling
dehydrated natural gas.
4. The method of claim 1 wherein the warming of the dehydrated
natural gas is by indirect heat exchange with the liquefying clean
natural gas.
5. The method of claim 1 wherein the liquefied natural gas is
subcooled.
6. The method of claim 1 wherein the warmed depressurized natural
gas is recovered as product.
7. The method of claim 1 wherein the clean natural gas which is
used as the regeneration gas in the second adsorption unit is then
passed to the first adsorption unit.
8. The method of claim 1 wherein the clean natural gas which is
used as the regeneration gas in the second adsorption unit is then
combined with the dehydrated natural gas.
9. The method of claim 1 wherein both the first adsorption unit and
the second adsorption unit are temperature swing adsorption
units.
10. The method of claim 1 wherein the subambient expansion is a
Joule-Thomson valve expansion.
11. The method of claim 1 wherein the subambient expansion is a
turboexpansion.
12. The method of claim 1 wherein the depressurized natural gas for
use as regeneration gas for the first adsorption unit is generated
by a subambient Joule-Thomson valve expansion, a subsequent
compression, and then a subambient turboexpansion.
Description
[0001] This application is a continuation of and claims priority
from copending U.S. application Ser. No. 10/962,666, filed Oct. 13,
2004.
TECHNICAL FIELD
[0002] This invention relates generally to the production of
liquefied natural gas and, more particularly, to the production of
liquefied natural gas using cryogenic expansion and the
pretreatment of the natural gas for use in such a process.
BACKGROUND ART
[0003] Typically natural gas transmission pipelines operate at
pressures ranging between 700 and 1500 psia. Natural gas pressure
reduction points are often referred to as let-down stations. Such
stations enable the regional distribution of natural gas (typically
at pressures of 150 to 500 psia). In general, let-down stations are
not designed for the useful recovery of the pressure energy.
Processes which serve to let-down natural gas while producing a
fraction of the inlet gas as liquefied natural gas are often
referred to as expander cycles or expander plants.
[0004] Typically, natural gas is transmitted with residual amounts
of water 5-10 lbs-H.sub.2O/MMscfd and about 2.0 mole % carbon
dioxide or more. In order to operate a cryogenic process (such as
an expander plant) producing liquefied natural gas from a pipeline
gas, it is necessary to remove both the water and the carbon
dioxide to very low levels (<1 and <50 ppm, respectively).
The removal of high boiling contaminants (water, carbon dioxide,
hydrogen sulfide) is often referred to as pre-purification or
pre-treatment. Adsorption systems are often used for the removal of
water, carbon dioxide and hydrogen sulfide from pipeline gas
streams. The regeneration of adsorption systems requires that a
cleaned (contaminant free) stream be passed over the loaded bed in
order to remove the high-boiling contaminants. Typically,
regeneration gas for these systems is derived from the compression
of low-pressure flash gas. This flash gas is generated upon the
depressurization of highly subcooled supercritical pressure natural
gas. Such an approach results in poor liquefaction efficiency and
low liquefied natural gas yield (typically <10% of the feed is
liquefied).
[0005] Accordingly it is an object of this invention to provide an
improved method for producing liquefied natural gas using
subambient expansion.
SUMMARY OF THE INVENTION
[0006] The above and other objects, which will become apparent to
those skilled in the art upon a reading of this disclosure, are
attained by the present invention which is:
[0007] A method for producing liquefied natural gas comprising:
[0008] (A) removing water from a first natural gas stream in a
first adsorption unit to produce dehydrated natural gas, cooling
the dehydrated natural gas to a temperature below the critical
temperature of methane to produce cooled dehydrated natural gas,
expanding the cooled dehydrated natural gas in a subambient
expansion to produce depressurized natural gas, warming the
depressurized natural gas, and using a portion of the warmed
depressurized natural gas as regeneration gas in the first
adsorption unit; and [0009] (B) removing carbon dioxide and water
from a second natural gas stream in a second adsorption unit to
produce clean natural gas, liquefying a portion of the clean
natural gas to produce liquefied natural gas, and using another
portion of the clean natural gas as regeneration gas in the second
temperature swing adsorption unit.
[0010] As used herein the term "adsorption unit" means a system
incorporating at least one vessel, preferably two or more,
containing a solid adsorbent such as silicon dioxide or molecular
sieves, which preferentially adsorbs at least one constituent from
a feed gas. The adsorption unit also comprises necessary valving to
direct both feed and regeneration gases through the bed(s) at
varying time intervals.
[0011] As used herein the term "regeneration gas" means a fluid
that contains substantially less adsorbing contaminant than the
feed stream to an adsorption unit.
[0012] As used herein the term "Joule-Thomson valve expansion"
means expansion employing an isenthalpic pressure reduction device
which typically may be a throttle valve, orifice or capillary
tube.
[0013] As used herein the term "turboexpansion" means an expansion
employing an expansion device which produces shaft work. Such shaft
work is produced by the rotation of a shaft induced by the
depressurization of a fluid through one or more fluid conduits
connected to the shaft, such as a turbine wheel.
[0014] As used herein the term "subambient expansion" means a
Joule-Thomson valve expansion or a turboexpansion which produces a
lower pressure stream having a temperature lower than ambient.
BRIEF DESCRIPTION OF THE DRAWING
[0015] The sole FIGURE is a simplified schematic representation of
one preferred embodiment of the liquefied natural gas production
method of this invention.
DETAILED DESCRIPTION
[0016] The invention is directed to a process employing at least
one expansion exhibiting a subambient temperature exhaust (or
outlet) which serves to depressurize high-pressure natural gas for
subsequent distribution and/or consumption. The invention serves to
produce at least a fraction of the feed gas in a condensed liquid
state. The subambient exhaust expansion may employ a turbine for
the production of work.
[0017] In the practice of this invention a high-pressure natural
gas stream is extracted from a high-pressure pipeline. A portion of
this stream is directed to a first adsorption unit for the removal
of water and possibly carbon dioxide. Warming the exhaust/outlet of
a sub-ambient expansion generates (at least) a portion of the gas
required for regeneration of this first adsorption unit. A second
stream of lower flow relative to the first high-pressure stream is
obtained directly from the pipeline or from the dehydrated outlet
from the first adsorption unit. This stream is directed to a second
adsorption unit, which serves to remove carbon dioxide and water.
The regeneration gas for the second adsorption unit is obtained
from the carbon dioxide free product stream (gas exiting the unit)
or from subsequent down stream sub-ambient temperature processing.
The regeneration gas exiting the second adsorbent unit is then
introduced into either the feed or product stream from the first
adsorbent unit. Preferably, this introduction is made possible by
either expanding the feed or product of the first stream or by
compressing the regeneration gas from second adsorption unit. After
prepurification the product of the first adsorption unit is used to
generate refrigeration for the cooling and condensation of the
product from the second unit.
[0018] The invention will be described in greater detail with
reference to the Drawing. Referring now to the Figure, natural gas
passing through natural gas transmission pipeline 100 is at a
pressure generally within the range of from 600 to 1500 pounds per
square inch absolute (psia). Natural gas stream 101 is taken from
pipeline 100 for passage into regional distribution pipeline 180
which is typically operated at a pressure within the range of from
100 to 300 psia. A typical route to supplying this gas may involve
a direct depressurization of this gas such as through line 102,
valve 200 and heater 201.
[0019] In the practice of this invention at least some and
preferably a substantial portion of natural gas stream 101 is
directed by way of line 103 for the recovery of expansion energy
and the production of liquefied natural gas. A portion 11,
generally comprising from 60 to 85 percent of stream 103, is passed
through valve 110 and passed in stream 12 as a first natural gas
stream to first adsorption unit 120 which is preferably a
temperature swing adsorption unit but may also be a pressure swing
adsorption unit. Adsorption unit 120 will typically employ at least
two adsorption beds and a configuration of valves (not shown) in
order to facilitate periodic bed switching and regeneration.
[0020] Within first adsorption unit 120 the first natural gas
stream undergoes water removal resulting in the production of
dehydrated natural gas which is withdrawn from first adsorption
unit 120 in stream 13. Dehydrated natural gas in stream 13 is
cooled to a temperature below the critical temperature of methane
(-116.5 F) by passage through heat exchangers 140 and 150. The
resulting cooled dehydrated natural gas 14 is depressurized in a
subambient expansion, for example by passage through Joule-Thomson
expansion valve 155. Typically the pressure of the natural gas 15
at the exit of valve 155 will be within the range of from 300 to
550 psia. The subambient expansion will result in the production of
a two phase mixture.
[0021] Two-phase natural gas stream 15 is passed to phase separator
vessel 156 wherein it is phase separated for purposes of
distribution into a common pass of heat exchanger 150. Liquid from
vessel 156 is passed to heat exchanger 150 in stream 16 and vapor
from heat exchanger 156 is passed to heat exchanger 150 in stream
17. Within heat exchanger 150 and subsequently in heat exchanger
140, the depressurized natural gas is warmed and completely
vaporized by indirect heat exchange with the aforedescribed cooling
dehydrated natural gas. The resulting warmed natural gas exits heat
exchanger 140 in a substantially superheated state, generally
within the range of from 30 to 90 F.
[0022] A portion of the warmed depressurized natural gas is used as
regeneration gas in first adsorption unit 120. The embodiment of
the invention illustrated in the Figure is a preferred embodiment
wherein the warmed depressurized natural gas undergoes compression
and a second subambient expansion prior to recovery and use as a
regeneration gas.
[0023] Referring back now to the FIGURE, warmed depressurized
natural gas 18 is withdrawn from heat exchanger 140 and passed to
compressor 160 wherein it is compressed to a pressure generally
within the range of from 600 to 900 psia. Resulting compressed
natural gas stream 19 is cooled in aftercooler 161, generally to a
temperature within the range of from 80 to 100.degree. F. If
desired, a portion 20 of the compressed natural gas may be recycled
back to stream 13. The remainder of the compressed natural gas is
passed to turboexpander 170 wherein it is turboexpanded to a
pressure marginally above the final let-down pressure existing in
regional distribution pipeline 180. Depending upon feed composition
the exit stream 21 of turboexpander 170 may have a marginal amount
of entrained condensate. This stream may be directed to a phase
separation vessel 147 where the liquid and vapor are separated
prior to distribution and warming in heat exchanger 140. After
exiting heat exchanger 140 a portion 22 of the turboexpanded gas
may be warmed in exchanger 125. The heated gas is used as
regeneration gas for adsorption unit 120. The remaining portion 23
may be pressure reduced through valve 126 combined with the exiting
regeneration stream from adsorption unit 120 and directed into
distribution line 180.
[0024] Another portion 24, generally comprising from 15 to 40
percent of stream 103, is passed as a second natural gas stream to
second adsorption unit 130, which is preferably a temperature swing
adsorption unit but may also be a pressure swing adsorption unit.
Carbon dioxide and water are removed from the second natural gas in
second adsorption unit 130 to produce clean natural gas which is
withdrawn from second adsorption unit 130 in stream 40. A portion
25 of clean natural gas 40, typically from 25 to 75 percent, is
warmed by passage through heat exchanger 135 wherein it is heated
to a temperature within the range of from 400 to 600.degree. F. and
then used as the regeneration gas for second adsorption unit 130.
If desired, and as illustrated in the Figure, the resulting
regeneration gas 26 which exits second adsorption unit 130 may then
be passed into stream 12 for processing as was described above.
Alternatively, stream 26 may be passed into product stream 13 from
the first adsorption unit 120.
[0025] The portion 27 of the feed gas subjected to drying and
carbon dioxide removal within adsorption system 130 and not used
for regeneration is directed to exchanger 140 for cooling. This
"liquefaction" stream is cooled to a temperature typically in the
range of from -40 to -80.degree. F. At this temperature, a small
fraction of heavy hydrocarbons may be condensed from this stream 28
and phase separated from the bulk of the stream within phase
separation vessel 145. The heavy hydrocarbon condensate stream 29
may be flashed through pressure reducing valve 146 and passed in
stream 30 into vessel 147 for subsequent vaporization/warming. The
remaining portion 31 of the carbon dioxide free feed stream is
further cooled to below the critical temperature of methane within
heat exchanger 150. This feed stream exits exchanger 150 in
essentially a dense phase/condensed state 32. This pressurized
liquefied natural gas stream may be taken directly as product or
may be further subcooled by additional indirect heat exchange in
heat exchanger 190. This additional subcooling refrigeration
(embodied by general process means 195) may be generated by
numerous systems including but not limited to direct gas expansion
cooling and mixed gas refrigeration. The subcooled pressurized
liquefied natural gas stream exiting exchanger 190 may then be
depressurized to a pressure marginally above ambient through
expansion valve 196. The product liquefied natural gas 33 may be
directed to suitable storage or transport (not shown).
[0026] Adsorbent systems 120 and 130 may employ a range of
adsorbents. Such systems may also be designed to remove trace
amounts of hydrogen sulfide from the transmission pipeline gas. It
may be possible to use a combination of gases for regeneration. In
addition to the use of turboexpansion gas for dehydration
regeneration, a small amount of flash gas may be obtained from cold
end flashing (valve 196) and storage tank heat ingress. This gas
may be used to supplement regeneration gas heating and/or cooling
needs. Such gas may be optionally compressed and/or heated prior to
use. Although regeneration gas heaters 125 and 135 are shown as
indirect heat exchangers it is also possible to use electric
heaters or indirect heating from a fired heater or other waste heat
source.
[0027] An option relative to the operation of the carbon dioxide
adsorption system involves the elimination of valve 110. This can
be accomplished by including a compressor for purposes of
pressurizing the regeneration gas back to the pipeline pressure
prior to introduction to system 120. In this way, the refrigeration
potential of the feed stream is maximized at some incremental power
consumption. An alternative to the use of valve 110 (feed
throttling) involves purifying an increased fraction of the feed
for carbon dioxide. This increased fraction may be cooled by
passage through exchangers 140 and 150 (as shown). At the cold end
of exchanger 150, this additional flow of carbon dioxide free gas
may be throttled and phase separated like the water free gas
directed to valve 155 and separator 156. The resulting stream may
then be warmed to ambient and used to regenerate adsorbent system
130. After adsorbing the carbon dioxide, the regeneration gas may
then be directed into the carbon dioxide laden circuit. As an
example, after warming, the carbon dioxide laden regeneration gas
may be directed into the feed stream to compressor '160.
[0028] The dehydrated feed refrigeration stream may be optionally
phase separated at the exit of exchanger 140 (like that shown for
the liquefaction feed). In this case, the heavies condensation may
also be directed to vessel 147 and subsequent vaporization within
heat exchanger 140.
[0029] An important option relative to the regeneration of water
removal system 120 involves the use of a gas other than the warmed
turboexpansion exhaust gas. For instance, a portion of the moderate
pressure vaporized Joule-Thomson expanded stream derived from
separator 156 may be used as regeneration gas. In this option, the
water laden regeneration gas may then be throttled into the warmed
turboexpansion exhaust. This approach is consistent with the
essence of this invention in that the regeneration gas for
adsorption system 120 is obtained from a subambient expansion. The
subject expansion is defined as a turboexpansion (with work
production) or subambient Joule-Thomson expansion (or a combination
of the two). Although the heavies removed from the liquefaction
stream are shown being reintroduced into the let-down stream
(turbine exhaust), the heavies stream may be subjected to
additional segregation processes for purposes of generating a
separate liquefied petroleum gas or butane product stream.
* * * * *