U.S. patent application number 17/120403 was filed with the patent office on 2021-11-11 for system and method for natural gas and nitrogen liquefaction with direct drive machines for turbines and boosters.
The applicant listed for this patent is James R. Handley, Nathaniel J. Parr, Neil M. Prosser, Jeremiah J. Rauch. Invention is credited to James R. Handley, Nathaniel J. Parr, Neil M. Prosser, Jeremiah J. Rauch.
Application Number | 20210348838 17/120403 |
Document ID | / |
Family ID | 1000005315149 |
Filed Date | 2021-11-11 |
United States Patent
Application |
20210348838 |
Kind Code |
A1 |
Prosser; Neil M. ; et
al. |
November 11, 2021 |
SYSTEM AND METHOD FOR NATURAL GAS AND NITROGEN LIQUEFACTION WITH
DIRECT DRIVE MACHINES FOR TURBINES AND BOOSTERS
Abstract
Liquefier arrangements configured for co-production of both
liquid natural gas (LNG) and liquid nitrogen (LIN) configured to
operate using direct drive motor/generator arrangement for the warm
and/or cold booster compressors and turbines. Alternatively, the
use of a conventional generator with a bull gear in lieu of the
direct drive motor/generator arrangement on the warm turbine and
warm booster compressor coupling is also disclosed.
Inventors: |
Prosser; Neil M.; (Lockport,
NY) ; Rauch; Jeremiah J.; (Clarence, NY) ;
Handley; James R.; (East Amherst, NY) ; Parr;
Nathaniel J.; (Orchard Park, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Prosser; Neil M.
Rauch; Jeremiah J.
Handley; James R.
Parr; Nathaniel J. |
Lockport
Clarence
East Amherst
Orchard Park |
NY
NY
NY
NY |
US
US
US
US |
|
|
Family ID: |
1000005315149 |
Appl. No.: |
17/120403 |
Filed: |
December 14, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
63020042 |
May 5, 2020 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2210/62 20130101;
F25J 1/005 20130101; F25J 2240/40 20130101; F25J 1/004 20130101;
F25J 2230/40 20130101; F25J 2230/42 20130101; F25J 2230/08
20130101; F25J 1/0223 20130101; F25J 1/0015 20130101; F25J 2230/60
20130101; F25J 2235/60 20130101; F25J 1/0229 20130101; F25J 2210/06
20130101; F25J 1/0264 20130101; F25J 1/0022 20130101 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 1/02 20060101 F25J001/02 |
Claims
1. A method of liquefaction to co-produce liquid nitrogen and
liquid natural gas, the method comprising the steps of: (i)
receiving a gaseous nitrogen feed stream; (ii) compressing the
gaseous nitrogen feed stream and one or more gaseous nitrogen
recycle streams in a recycle compressor to produce a gaseous
nitrogen effluent stream; (iii) further compressing a first portion
of the effluent stream in a cold booster compressor to form a part
of a primary nitrogen liquefaction stream and further compressing a
second portion of the effluent stream in a warm booster compressor
to form a warm nitrogen recycle stream, wherein the warm booster
compressor is coupled to a direct drive motor/generator
arrangement; (iv) cooling the primary nitrogen liquefaction stream
in a first heat exchange passage in a multi-pass brazed aluminum
heat exchanger; (v) expanding a first portion of the cooled primary
nitrogen liquefaction stream extracted at a primary intermediate
location of the first heat exchange passage in a cold booster
loaded turbine to produce a cold turbine exhaust; (vi) warming the
cold turbine exhaust in a second heat exchange passage in the
multi-pass brazed aluminum heat exchanger to form a gaseous
nitrogen recycle stream; (vii) cooling the warm nitrogen recycle
stream in a third heat exchange passage in the multi-pass brazed
aluminum heat exchanger; (viii) expanding the cooled stream exiting
the third heat exchange passage in a warm booster loaded turbine to
produce a warm turbine exhaust wherein the warm booster loaded
turbine is also operatively coupled to the direct drive
motor/generator arrangement; (ix) warming the warm turbine exhaust
in the second heat exchange passage to form part of the gaseous
nitrogen recycle stream; (x) subcooling a second portion of the
primary nitrogen liquefaction stream to produce a subcooled liquid
nitrogen stream; (xi) liquefying a natural gas feed stream in a
fifth heat exchange passage of the multi-pass brazed aluminum heat
exchanger against a first portion of the subcooled liquid nitrogen
stream in a fourth heat exchange passage of the multi-pass brazed
aluminum heat exchanger and the gaseous nitrogen recycle stream to
produce the liquid natural gas; and (xii) taking a second portion
of the subcooled liquid nitrogen stream as the liquid nitrogen.
2. The method of claim 1 further comprising the step of compressing
the natural gas feed stream prior to the step of liquefying the
natural gas feed stream in the fifth heat exchange passage of the
multi-pass brazed aluminum heat exchanger.
3. The method of claim 1 further comprising the step of expanding
the second portion of the primary nitrogen liquefaction stream in a
liquid turbine disposed downstream of the multi-pass brazed
aluminum heat exchanger or a throttle valve disposed downstream of
the multi-pass brazed aluminum heat exchanger.
4. The method of claim 1 wherein the cold booster compressor and
the cold booster loaded turbine are operatively coupled to a second
direct drive motor/generator arrangement.
5. A liquefaction system configured to co-produce liquid nitrogen
and liquid natural gas, the liquefaction system comprising: a
natural gas feed stream; a gaseous nitrogen feed stream; a
multi-pass brazed aluminum heat exchanger; a recycle compressor
configured to compress the gaseous nitrogen feed stream and a
gaseous nitrogen recycle stream to produce an effluent stream; a
cold recycle circuit having a cold booster compressor and a booster
loaded cold turbine and configured to further compress a first
portion of the effluent stream to form a primary nitrogen
liquefaction stream; cool the primary nitrogen liquefaction stream
in a first heat exchange passage of the multi-pass brazed aluminum
heat exchanger; expand a first portion of the cooled primary
nitrogen liquefaction stream extracted at a cold intermediate
location of the first heat exchange passage in the booster loaded
cold turbine to produce a cold turbine exhaust; warm the cold
turbine exhaust in a second heat exchange passage of the multi-pass
brazed aluminum heat exchanger; and recycle the warmed stream
exiting the second heat exchange passage as the gaseous nitrogen
recycle stream; a warm recycle circuit having a warm booster
compressor and a booster loaded warm turbine operatively coupled to
a direct drive motor/generator arrangement and configured to
compress a second portion of the effluent stream in the warm
booster compressor to form warm nitrogen recycle stream, cool the
further compressed warm nitrogen recycle stream in a third heat
exchange passage in the multi-pass brazed aluminum heat exchanger;
expand the cooled, warm nitrogen recycle stream in the booster
loaded warm turbine to produce a warm turbine exhaust; warm the
warm turbine exhaust in the second heat exchange passage to form
part of the gaseous nitrogen recycle stream; a subcooler configured
to subcool a second portion of the primary nitrogen liquefaction
stream to produce a subcooled liquid nitrogen stream; the
multi-pass brazed aluminum heat exchanger further having a fourth
heat exchange passage and a fifth heat exchange passage and
configured to liquefy the natural gas feed stream in the fifth heat
exchange passage against a first portion of the subcooled liquid
nitrogen stream in the fourth heat exchange passage and the gaseous
nitrogen recycle stream; wherein the liquid nitrogen product stream
is a second portion of the subcooled liquid nitrogen stream and the
liquid natural gas stream is the liquefied natural gas exiting a
cold end of the fifth heat exchange passage.
6. The liquefaction system of claim 5 further comprising a natural
gas compressor configured to compress the natural gas feed stream
prior to liquefaction of the natural gas feed stream in the fifth
heat exchange passage of the multi-pass brazed aluminum heat
exchanger.
7. The liquefaction system of claim 5 further comprising a liquid
turbine disposed downstream of the multi-pass brazed aluminum heat
exchanger and configured to expand the second portion of the
primary nitrogen liquefaction stream.
8. The liquefaction system of claim 5 further comprising a second
direct drive motor/generator arrangement operatively coupled to the
cold booster compressor and the cold booster loaded turbine.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
provisional patent application Ser. No. 63/020,042 filed May 5,
2020 the disclosure of which is incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to liquefaction, and more
particularly, to a liquefier arrangement capable of producing
liquid natural gas (LNG) and liquid nitrogen (LIN). Still more
particularly, the present system and method relates to a liquefier
arrangement configured for co-production of both LNG and LIN in
using direct drive motor/generators for the warm and/or cold
booster compressors and turbines.
BACKGROUND
[0003] There are various industrial gas business opportunities
where the production of both liquid natural gas (LNG) and liquid
nitrogen (LIN) is required. U.S. provisional patent application
Ser. No. 62/976,049 filed Feb. 13, 2020, the disclosure of which is
incorporated by reference herein, shows examples of liquefier
arrangements capable of a liquefaction cycle that co-produces LNG
and LIN. As disclosed in U.S. provisional patent application Ser.
No. 62/976,049; liquefier arrangements capable of a liquefaction
cycle that co-produce both LNG and LIN require a separate passage
in a conventional nitrogen liquefier that is employed to cool and
liquefy the natural gas. This modification typically requires
changing the brazed aluminum heat exchanger (BAHX) arrangement to
allocate one of the passages to cool the natural gas feed and then
reallocate a portion of the high pressure gaseous nitrogen feed
passages or layers. Since LNG is sufficiently subcooled at about
110 K it is withdrawn from the BAHX at a location corresponding to
a temperature somewhat warmer than the cold end of the BAHX where
the temperature is about 95 K to 100 K required to liquefy the
nitrogen.
[0004] The natural gas feed is preferably pre-purified for removal
of carbon dioxide and other contaminants as well as removal of
minor amounts of moisture prior to entry in the cold box. Other
potential contaminants may include H.sub.2S, mercaptans, mercury
and mercury compounds which also must be removed or reduced to a
satisfactory level. Usually, heavier hydrocarbons are sufficiently
extracted in NGL facilities prior to supply. If this is not the
case, a significant modification in the liquefier design would be
required in order to capture and remove the heavier hydrocarbons at
an intermediate temperature. Also, if the feed natural gas is at a
low pressure, the liquefaction process may optionally require
pre-compression of the natural gas feed, preferably to a pressure
of about 450 psia to enable the use of a modified nitrogen
liquefier design. If the pressure of the natural gas feed is below
about 450 psia, the temperature difference in the natural gas
condensing zone of the heat exchanger may exceed the allowable
limits for many BAHX designs. Alternatively, if the feed natural
gas is supplied at a lower pressure, the liquefier design would
have to be changed so that at least the condensing portion of the
heat exchanger is of a different design, for example, a stainless
steel brazed heat exchanger or a stainless steel spiral wound heat
exchanger. Thus, to avoid the much more expensive heat exchangers
and to achieve improved efficiencies, natural gas pre-compression
is preferred. needed when it is supplied at lower pressures. The
further compressed natural gas feed would optionally be cooled in
an aftercooler to remove the heat of compression.
[0005] During liquefaction of a high pressure natural gas feed
pressures, the refrigeration demand of the warm turbine is greatly
increased. This increased refrigeration demand is because natural
gas liquefaction or pseudo-liquefaction is now taking place at a
temperature above the exhaust temperature of the warm turbine. As a
result, the warm turbine is larger and passes significantly more
flow. The cold turbine refrigeration primarily is providing
refrigeration for liquefaction or pseudo-liquefaction of the
nitrogen while the warm turbine refrigeration primarily provides
refrigeration for natural gas liquefaction or pseudo-liquefaction.
This means that independent variation in the LNG demand and the LIN
demand likely results in independent variation of the demand for
refrigeration from each turbine and the optimal warm turbine to
cold turbine flow ratio will vary significantly, depending on the
output demand for LNG and LIN. The prior art liquefier arrangement
capable of a liquefaction cycle that co-produces both LNG and LIN
disclosed in U.S. provisional patent application Ser. No.
62/976,049 suffers from a disadvantage of not able to adjust the
warm turbine to cold turbine flow ratio to achieve the optimal
ratio when demand for LNG and LIN changes.
[0006] Varying demands of LNG and LIN in co-production natural gas
liquefaction plants is common. For example, small peak shaver LNG
plants are located strategically on natural gas pipelines and
configured to store natural gas as LNG during the months when it is
less expensive, and to return the natural gas to the pipeline when
price and demand peaks, most often during cold winter weather and
hot summer weather. These facilities produce LNG at maximum levels
for part of the year and produce little or no LNG for the rest of
the year. Co-production of LIN in such plants may be beneficial in
strategic locations where demand for merchant LIN or back-up LIN is
required. Of course, the potential for variation in merchant LIN
demand and back-up LIN demand near a given LNG location can lead to
wide changes in demand for LIN production.
[0007] Nitrogen liquefiers are typically capable of efficient
turndown over a very broad range. Turndown to about 20% of capacity
is achievable at reasonably good efficiency. Turndown is
accomplished naturally by keeping the turbine nozzles unchanged. As
the liquefier is turned down, the feed nitrogen flow is reduced and
the pressure levels within the liquefier fall commensurately. As a
result, the volumetric flows through the turbines, their respective
boosters, and the recycle compressor remains unchanged at their
design rates. The pressure ratios across the machines also remain
unchanged. So, while the machines become more unloaded, they each
continue to operate essentially at their ideal design point. This
means that the aerodynamic efficiencies of the rotating machines
remain unchanged. The feed gas compressor is an exception to this,
as it must be turned down with guide vanes or a suction throttle
valve due to its lower flow and discharge pressure, with a constant
supply pressure. The power demand of the recycle compressor is much
larger than that of the feed gas compressor, though. So, it doesn't
have a very large effect. Other than this, the only penalties for
turndown are those associated with the mechanical and motor losses
of the rotating machinery (which increase as a proportion of the
total power consumption at turndown), and a significant
thermodynamic penalty for the lower pressure liquefaction of
nitrogen. This thermodynamic penalty occurs because at lower
pressures, and particularly below its critical point pressure, the
liquefaction of nitrogen results in a more thermodynamically
irreversible temperature profile. The larger temperature invariant
zones at lower nitrogen liquefaction pressures result in both tight
pinch deltaT (.DELTA.T) values and large deltaT (.DELTA.T)
values.
[0008] What is needed, therefore is a liquefier arrangement capable
of co-production of LNG and LIN that is capable of operating
efficiently in normal or full capacity production modes as well as
in selected turn-down operating modes, including operating modes
targeting low or zero LNG production capacity
SUMMARY OF THE INVENTION
[0009] The present invention may be characterized as a liquefaction
system configured to co-produce liquid nitrogen and liquid natural
gas, the liquefaction system comprises: (a) a natural gas feed
stream; (b) a gaseous nitrogen feed stream; (c) a multi-pass brazed
aluminum heat exchanger; (d) a recycle compressor configured to
compress the gaseous nitrogen feed stream and a gaseous nitrogen
recycle stream to produce an effluent stream; (e) a cold recycle
circuit, (f) a warm recycle circuit; and (g) a subcooler configured
to subcool a second portion of the primary nitrogen liquefaction
stream to produce a subcooled liquid nitrogen stream. The
multi-pass brazed aluminum heat exchanger also includes a fourth
heat exchange passage and a fifth heat exchange passage configured
to liquefy the natural gas feed stream in the fifth heat exchange
passage against a first portion of the subcooled liquid nitrogen
stream in the fourth heat exchange passage and the gaseous nitrogen
recycle stream, wherein the liquid nitrogen product stream is a
second portion of the subcooled liquid nitrogen stream and the
liquid natural gas stream is the liquefied natural gas exiting a
cold end of the fifth heat exchange passage.
[0010] The warm recycle circuit comprises a warm booster compressor
and a booster loaded warm turbine operatively coupled to a direct
drive motor/generator arrangement and configured to compress a
second portion of the effluent stream in the warm booster
compressor to form warm nitrogen recycle stream, cool the further
compressed warm nitrogen recycle stream in a third heat exchange
passage in the multi-pass brazed aluminum heat exchanger; expand
the cooled stream in the booster loaded warm turbine to produce a
warm turbine exhaust; warm the warm turbine exhaust in the second
heat exchange passage to form part of the gaseous nitrogen recycle
stream.
[0011] The cold recycle circuit preferably comprises a cold booster
compressor and a booster loaded cold turbine and configured to
further compress a first portion of the effluent stream to form a
primary nitrogen liquefaction stream; cool the primary nitrogen
liquefaction stream in a first heat exchange passage of the
multi-pass brazed aluminum heat exchanger; expand a first portion
of the cooled primary nitrogen liquefaction stream extracted at a
cold intermediate location of the first heat exchange passage in
the booster loaded cold turbine to produce a cold turbine exhaust;
warm the cold turbine exhaust in a second heat exchange passage of
the multi-pass brazed aluminum heat exchanger; and recycle the
warmed stream exiting the second heat exchange passage as the
gaseous nitrogen recycle stream. The cold booster compressor and
the cold booster loaded turbine are operatively coupled to a second
direct drive motor/generator arrangement.
[0012] The present invention may also be characterized as a method
for liquefaction to co-produce liquid nitrogen and liquid natural
gas, the method comprising the steps of: (i) receiving a gaseous
nitrogen feed stream; (ii) compressing the gaseous nitrogen feed
stream and one or more gaseous nitrogen recycle streams in a
recycle compressor to produce a gaseous nitrogen effluent stream;
(iii) further compressing a first portion of the effluent stream in
a cold booster compressor to form a part of a primary nitrogen
liquefaction stream and further compressing a second portion of the
effluent stream in a warm booster compressor to form a warm
nitrogen recycle stream, wherein the warm booster compressor is
coupled to a direct drive motor/generator arrangement; (iv) cooling
the primary nitrogen liquefaction stream in a first heat exchange
passage in a multi-pass brazed aluminum heat exchanger; (v)
expanding a first portion of the cooled primary nitrogen
liquefaction stream extracted at a primary intermediate location of
the first heat exchange passage in a cold booster loaded turbine to
produce a cold turbine exhaust; (vi) warming the cold turbine
exhaust in a second heat exchange passage in the multi-pass brazed
aluminum heat exchanger to form a gaseous nitrogen recycle stream;
(vii) cooling the warm nitrogen recycle stream in a third heat
exchange passage in the multi-pass brazed aluminum heat exchanger;
(viii) expanding the cooled stream exiting the third heat exchange
passage in a warm booster loaded turbine to produce a warm turbine
exhaust wherein the warm booster loaded turbine is also operatively
coupled to the direct drive motor/generator arrangement; (ix)
warming the warm turbine exhaust in the second heat exchange
passage to form part of the gaseous nitrogen recycle stream; (x)
subcooling a second portion of the primary nitrogen liquefaction
stream to produce a subcooled liquid nitrogen stream; (xi)
liquefying a natural gas feed stream in a fifth heat exchange
passage of the multi-pass brazed aluminum heat exchanger against a
first portion of the subcooled liquid nitrogen stream in a fourth
heat exchange passage of the multi-pass brazed aluminum heat
exchanger and the gaseous nitrogen recycle stream to produce the
liquid natural gas; and (xii) taking a second portion of the
subcooled liquid nitrogen stream as the liquid nitrogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] While the present invention concludes with claims distinctly
pointing out the subject matter that Applicants regard as their
invention, it is believed that the invention will be better
understood when taken in connection with the accompanying drawings
in which:
[0014] FIG. 1 is a schematic diagram of a liquefier capable of
co-producing LNG and LIN in accordance with an embodiment of the
present system and method;
[0015] FIG. 2 is a schematic diagram of a liquefier capable of
co-producing LNG and LIN in accordance with another embodiment of
the present system and method; and
[0016] FIG. 3 is a schematic diagram of a liquefier capable of
co-producing LNG and LIN in accordance with yet another embodiment
of the present system and method.
DETAILED DESCRIPTION
[0017] Turning now to the drawings, there are shown three different
embodiments of the present system and method for the liquefaction
of both LNG and LIN where the warm and/or cold booster compressors
and associated turbines are driven by one or more direct drive
motor/generators.
[0018] FIG. 1 shows a first embodiment of the present system and
method in which a feed stream of gaseous nitrogen 12 and a purified
and compressed natural gas feed stream 82 are introduced into the
liquefier arrangement 100. The gaseous nitrogen feed 12 is
preferably originates from distillation columns in a co-located or
closely located air separation unit (not shown). The gaseous
nitrogen feed 12 is compressed in a feed gas compressor 14 and the
compressed nitrogen feed stream 16 is then cooled in aftercooler
18. The compressed nitrogen feed stream 16 is combined with the
recycle stream 15 and further compressed in a recycle compressor 20
and subsequently cooled in aftercooler 21. The further compressed
nitrogen feed stream 22 is split with a first portion 23 of the
cooled compressed nitrogen feed stream directed to a warm booster
compressor 65 where it is compressed to produce a warm booster
discharge stream 67. A second portion 24 of the cooled compressed
nitrogen feed stream is directed to a cold booster compressor 30 to
produce a cold booster discharge stream 25.
[0019] The warm booster discharge stream 67 and the cold booster
discharge stream 25 are subsequently cooled in aftercoolers 66A.
66B to remove the heat of compression generated from the warm
booster compressor 65 and the cold booster compressor 30,
respectively. The cooled, cold booster discharge stream is the
primary nitrogen liquefaction stream 26 that is directed to a first
heat exchange passage 51 in a brazed aluminum heat exchanger (BAHX)
50 for cooling to temperatures suitable for nitrogen liquefaction.
A first portion 27 of the primary nitrogen liquefaction stream in
the first passage 51 of the BAHX 50 is extracted at an intermediate
location of the first heat exchange passage 51 and directed to the
cold turbine 28 where the first extracted portion 27 is expanded to
produce a cold turbine exhaust 29. The cold turbine exhaust 29 is
then directed to the cold end of a second heat exchange passage 52
in the BAHX 50. The cold turbine exhaust 29 is warmed in the BAHX
50 and the warmed exhaust 15 is recycled to the compressed nitrogen
feed stream 19.
[0020] A second portion 31 of the primary nitrogen liquefaction
stream continues through the BAHX 50 to produce a liquid nitrogen
stream 32. The liquid nitrogen stream 32 is optionally diverted to
a generator loaded liquid turbine 33 where it is expanded to
produce a liquid turbine exhaust stream 34. The liquid turbine
exhaust stream 34 is directed to subcooler 35 configured to produce
a subcooled liquid nitrogen stream 36. The use of the generator
loaded liquid turbine 33 shown in in the drawings is optional. Use
of the liquid turbine likely depends on the power savings that the
liquid turbine provides relative to the cost of electricity at a
given installation site. In lieu of using the generator loaded
liquid turbine 33, the liquid nitrogen stream 32 may proceed
directly to subcooler 35 via control valve 37.
[0021] A first portion 38 of the subcooled liquid nitrogen stream
is routed back via valve 39 through another passage of the
subcooler 35 and then to a fourth heat exchange passage 53 of BAHX
50 to provide the requisite cooling for the nitrogen and natural
gas streams. The resulting recycle stream exiting the warm end of
the fourth heat exchange passage 53 is recycled as stream 13 to the
gaseous nitrogen feed stream 12. A second portion of the subcooled
liquid nitrogen stream is the liquid nitrogen product stream 40
preferably directed to a liquid nitrogen product storage tank
42.
[0022] The cooled, warm booster discharge stream 69 is directed to
a third heat exchange passage 55 in the BAHX 50 where it is
partially cooled. The partially cooled refrigerant stream 68 is
extracted from the third heat exchange passage 55 of BAHX 50 at an
intermediate location and directed to the warm turbine 70 where it
is expanded. The exhaust stream 72 from the warm turbine 70 is
returned to an intermediate location of the second heat exchange
passage 52 in the BAHX 50 where it is warmed with the warmed
exhaust stream 15 being recycled to the compressed nitrogen feed
stream 19.
[0023] The purified, natural gas feed stream 82 is received from a
source of natural gas (not shown) and is optionally compressed in
natural gas compressor 84 and optionally cooled in aftercooler 85.
The conditioned natural gas feed 86 is then directed to a fifth
heat exchange passage 54 in BAHX 50 where it is cooled to
temperatures suitable for liquefaction of natural gas. The LNG
stream 44 existing fifth heat exchange passage 54 in BAHX 50 is
sent to LNG storage tank 45.
[0024] As seen in FIG. 1, there are two distinct features of the
illustrated embodiment that differ from a conventional nitrogen
liquefier. The first differentiating feature is the use of a first
direct drive motor/generator 99B operatively coupling the cold
booster compressor 30 and the cold turbine 28 as well as a second
direct drive motor/generator 99A operatively coupling the warm
booster compressor 65 and the warm turbine 70. The direct drive
motor/generators are preferably a high speed motor with active
magnetic bearings. The second differentiating feature is that the
inlet circuit to the cold turbine is supplied solely from the cold
booster compressor while the inlet circuit to the warm turbine is
supplied solely from the warm booster compressor.
[0025] Advantageously, this liquefier arrangement 100 allows each
direct drive motor/generator 99A, 99B to augment or reduce the
power transmitted by the coupled turbine to the associated booster
compressor. To increase the refrigeration production of the warm
turbine 70 the interposed direct drive motor/generator 99A would
add power by increasing its shaft speed. This increase in shaft
speed would increase the outlet pressure of the warm booster
compressor 65 and correspondingly increase the pressure ratio and
refrigeration production of the warm turbine 70. Similarly,
increasing the shaft power to the direct drive motor/generator 99B
would increase the outlet pressure of the cold booster compressor
65 and correspondingly increase the pressure ratio and
refrigeration production of the cold turbine 70. Note that the
turbine boosters are in a "bootstrapped" configuration such that
power addition or power reduction by each direct drive
motor/generator has a multiplicative effect and provides a greater
change than that due to the power change only. In this manner,
coupling a direct drive motor/generator to each booster compressor
and turbine pair, allows the addition or reduction of energy to the
liquefier to more efficiently follow desired changes in LNG
production and/or LIN production.
[0026] Similar to conventional nitrogen liquefiers, the load on
both the warm turbine and the cold turbine in the present
embodiment can be turned down or turned up by adjusting the
nitrogen feed flow which, in turn, effects the overall circuit
pressure levels, and particularly the recycle compressor discharge
pressure which in many ways governs the overall capacity of LIN and
LNG production from the liquefier arrangement. However, the present
embodiments can also be loaded or unloaded with its interposed
direct drive motor/generator, which enables more efficient loading
of the turbines following of independent capacity changes of LIN or
LNG.
[0027] For example, if it were desired to operate the present
embodiments of the liquefier so that LIN production is at or near
40% of full capacity and LNG is at or near 60% of full capacity,
the liquefier arrangement could be adjusted so that the recycle
compressor discharge pressure is set at a level required for about
50% of full capacity of both products. The direct drive
motor/generator on the cold turbine would extract power (i.e.
operate as a generator) so that the cold turbine refrigeration
production is further reduced to better match the load needed for
the desired LIN production of at or near 40% of full capacity. The
direct drive motor/generator on the warm turbine would add power
(i.e. operate as a motor) so that the refrigeration production of
the warm turbine is comparatively increased to better match the
load needed for the desired LNG production of at or near 60% of
full capacity. In this manner, the direct drive motor/generators on
both the cold turbine booster and the warm turbine booster helps
increase the rangeability of the liquefier.
[0028] As many of the components and streams in the embodiment of
FIG. 2 are the same as in the embodiment of FIG. 1, the drawings
use the same reference numerals and the descriptions thereof will
not be repeated. This liquefier arrangement 200 shown in FIG. 2 is
similar to the liquefier arrangement shown in FIG. 1 except that a
direct drive motor/generator 99A is only used on the warm
turbine/warm booster compressor coupling and not on the cold
turbine/cold booster compressor coupling. Such an arrangement is a
lower capital cost that the embodiment of FIG. 1 due to the
avoidance of a second direct drive motor/generator and allows
efficient and independent turndown or turn-up of LNG production
only, as the LNG liquefaction duty is predominately supplied by the
warm turbine.
[0029] Turning now to FIG. 3, there is shown a third embodiment of
the present liquefier arrangement 300. As many components and
streams in the embodiment of FIG. 3 are the same or similar as in
the embodiment of FIG. 2, the drawings use the same reference
numerals and the corresponding descriptions thereof will not be
repeated. The key differences between the embodiment of FIG. 2 and
that of FIG. 3 is the use of a conventional generator 97 with a
bull gear 98 in lieu of the direct drive motor/generator on the
warm turbine/warm booster compressor coupling. The liquefier
arrangement shown in FIG. 3 has a much lower capital cost that the
embodiments of FIG. 1 and FIG. 2 due to the avoidance of a both
direct drive motor/generators yet still allows for efficient and
independent turndown or turn-up of LNG production over a wide
capacity range.
[0030] In the embodiment illustrated in FIG. 3 the warm turbine is
supplied by the warm booster compressor discharge and the cold
turbine is supplied by the cold booster discharge in operational
modes requiring normal or full capacity LNG production. In such
operating modes valve 92 is open and valves 94, 96 are closed such
that the first portion 23 of the cooled compressed nitrogen feed
stream is directed to the warm booster compressor 65 where it is
compressed to produce a warm booster discharge stream 67 that is
routed to the third heat exchange passage 55 in the BAHX 50 where
it is partially cooled. The partially cooled refrigerant stream 68
is extracted from the third heat exchange passage 55 of BAHX 50 at
an intermediate location and directed to the warm turbine 70 where
it is expanded. The exhaust stream 72 from the warm turbine 70 is
returned to an intermediate location of the second heat exchange
passage 52 in the BAHX 50 where it is warmed with the warmed
exhaust stream 15 being recycled to the compressed nitrogen feed
stream 19. In these operating modes, the generator 97 is not
engaged electrically and so it spins in a "free wheel" fashion and
the warm turbine operates in a conventional booster loaded manner
rotating at its natural speed determined by the process conditions
and warm turbine/warm booster compressor characteristics.
[0031] However, during turn-down operation, including low or zero
LNG production operation, there is a bypass circuit around the warm
booster compressor 65 that is employed. In such low or zero LNG
production modes, valve 92 is closed and valves 94 and 96 are open
while the check valve 93 essentially isolates the warm booster
circuit. In this operating mode, the first portion 23 of the cooled
compressed nitrogen feed stream is diverted around the warm booster
compressor 65 via valve 96 directly to the third heat exchange
passage 55 in the BAHX 50 and to the warm turbine 70. In other
words, the warm turbine is fed directly from the recycle compressor
20. As a result, the pressure ratio of the warm turbine 70 is
greatly decreased and its refrigeration production is diminished.
Also, in this low or zero LNG production operating mode, the
generator 97 is engaged electrically and it essentially fixes the
rotating speed of the warm turbine 70 and warm booster compressor
65. The energy that still goes to the warm booster compressor 65
simply recirculates flow in its circuit via valve 94. Ideally, the
generator 97 extracts most of the load from the warm turbine 70 so
that little of the energy is wasted by the warm booster compressor
65.
[0032] While the present invention has been described with
reference to several preferred embodiments, it is understood that
numerous additions, changes and omissions can be made without
departing from the spirit and scope of the present system and
method for natural gas and nitrogen liquefaction as set forth in
the appended claims.
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