Note: Descriptions are shown in the official language in which they were submitted.
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INCREASING EFFICIENCY IN AN LNG PRODUCTION SYSTEM BY PRE-COOLING A
NATURAL GAS FEED STREAM
[0001] <<This paragraph has been intentionally left blank.>>
[0002] <<This paragraph has been intentionally left blank.>>
FIELD OF THE INVENTION
[0003] The invention relates to the liquefaction of natural gas to form
liquefied natural gas
(LNG), and more specifically, to the production of LNG in remote or sensitive
areas where the
to construction and/or maintenance of capital facilities, and/or the
environmental impact of a
conventional LNG plant may be detrimental.
BACKGROUND
[0004] LNG production is a rapidly growing means to supply natural gas
from locations
with an abundant supply of natural gas to distant locations with a strong
demand of natural
is gas. The conventional LNG cycle includes: a) initial treatments of the
natural gas resource to
remove contaminants such as water, sulfur compounds and carbon dioxide; b) the
separation
of some heavier hydrocarbon gases, such as propane, butane, pentane, etc. by a
variety of
possible methods including self-refrigeration, external refrigeration, lean
oil, etc.; c)
refrigeration of the natural gas substantially by external refrigeration to
form LNG at near
20 atmospheric pressure and about -160 C; d) transport of the LNG product
in ships or tankers
designed for this purpose to a market location; e) re-pressurization and re-
gasification of the
LNG to a pressurized natural gas that may distributed to natural gas
consumers. Step (c) of
the conventional LNG cycle usually requires the use of large refrigeration
compressors often
powered by large gas turbine drivers that emit substantial carbon and other
emissions. Large
25 capital investments in the billions of US dollars and extensive
infrastructure are required as
part of the liquefaction plant. Step (e) of the conventional LNG cycle
generally includes re-
pressurizing the LNG to the required pressure using cryogenic pumps and then
re-gasifying
the LNG to pressurized natural gas by exchanging heat through an intermediate
fluid but
ultimately with seawater or by combusting a portion of the natural gas to heat
and vaporize the
30 LNG. Generally, the available exergy of the cryogenic LNG is not
utilized.
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[0005] A cold refrigerant produced at a different location, such as
liquefied nitrogen gas
("LIN"), can be used to liquefy natural gas. A process known as the LNG-LIN
concept relates
to a non-conventional LNG cycle in which at least Step (c) above is replaced
by a natural gas
liquefaction process that substantially uses liquid nitrogen (LIN) as an open
loop source of
refrigeration and in which Step (e) above is modified to utilize the exergy of
the cryogenic LNG
to facilitate the liquefaction of nitrogen gas to form LIN that may then be
transported to the
resource location and used as a source of refrigeration for the production of
LNG. United
States Patent No. 3,400,547 describes shipping liquid nitrogen or liquid air
from a market place
to a field site where it is used to liquefy natural gas. United States Patent
No. 3,878,689
to describes a process to use LIN as the source of refrigeration to produce
LNG. United States
Patent No. 5,139,547 describes the use of LNG as a refrigerant to produce LIN.
[0006] The LNG-LIN concept further includes the transport of LNG in a
ship or tanker from
the resource location to the market location and the reverse transport of LIN
from the market
location to the resource location. The use of the same ship or tanker, and
perhaps the use of
is common onshore tankage, are expected to minimize costs and required
infrastructure. As a
result, some contamination of the LNG with LIN and some contamination of the
LIN with LNG
may be expected. Contamination of the LNG with LIN is likely not to be a major
concern as
natural gas specifications (such as those promulgated by the United States
Federal Energy
Regulatory Commission) for pipelines and similar distribution means allow for
some inert gas
20 to be present, However, since the LIN at the resource location will
ultimately be vented to the
atmosphere, contamination of the LIN with LNG (a greenhouse gas more than 20
times as
impactful as Carbon Dioxide) must be reduced to levels acceptable for such
venting.
Techniques to remove the residual contents of tanks are well known but it may
not be economic
or environmentally acceptable to achieve the needed low level of contamination
to avoid
25 treatment of the LIN or vaporized nitrogen at the resource location
prior to venting the gaseous
nitrogen (GAN).
100071 United States Patent Application Publication No. 2010/0251763
describes a
variation of the LNG liquefaction process using both LIN and liquefied carbon
dioxide (CO2) as
refrigerants. While CO2 is itself a greenhouse gas, it is less likely that
liquefied CO2 will share
30 storage or transport facilities with LNG or other greenhouse gases and
so contamination is
unlikely. However, the LIN may be similarly contaminated as described above
and should be
decontaminated prior to venting of the resulting GAN streams. In addition, the
LNG liquefaction
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system may be supplemented by pre-chilling of the natural gas with a propane,
mixed
component or other closed refrigeration cycle in addition to the once-through
refrigeration
provided by vaporization of the LIN. In these cases, decontamination of the
gaseous nitrogen
may still be required prior to venting the GAN. What is needed is a method of
using LIN as a
coolant to produce LNG, where if the LIN and the LNG use common storage
facilities, any
greenhouse gas present in the LIN can be efficiently removed.
SUMMARY OF THE INVENTION
100081 The invention provides a liquefied natural gas production system
according to
disclosed aspects. A natural gas stream is supplied from a supply of natural
gas. A liquid
to nitrogen stream is supplied from a liquid nitrogen supply. A natural gas
compressor
compresses the natural gas stream to a pressure of at least 135 bara to form a
compressed
natural gas stream. A natural gas cooler cools the compressed natural gas
stream by indirect
heat exchange with an ambient fluid to form a cooled compressed natural gas
stream. A first
heat exchanger exchanges heat between an at least partially vaporized nitrogen
stream and
the cooled compressed natural gas stream to form an additionally cooled
compressed natural
gas stream. A work-producing natural gas expander expands the additionally
cooled
compressed natural gas stream to a pressure less than 200 bara, but no greater
than the
pressure to which the natural gas compressor compresses the natural gas
stream, to thereby
form a chilled natural gas stream. A second heat exchanger at least partially
condenses the
chilled natural gas stream by exchanging heat between the chilled natural gas
stream and the
liquid nitrogen stream to form the at least partially vaporized nitrogen
stream, wherein the at
least partially vaporized nitrogen passes through the first heat exchanger.
[0009] The invention also provides a method of producing LNG according to
disclosed
aspects. A natural gas stream is provided from a supply of natural gas, and a
liquid nitrogen
stream is provided from a liquid nitrogen supply. The natural gas stream is
compressed in a
natural gas compressor to a pressure of at least 135 bara to form a compressed
natural gas
stream. The compressed natural gas stream is cooled, through indirect heat
exchange with
an ambient fluid in a natural gas cooler, to form a cooled compressed natural
gas stream. The
cooled compressed natural gas stream and an at least partially vaporized
nitrogen stream are
passed through a first heat exchanger. The first heat exchanger exchanges heat
between the
at least partially vaporized nitrogen stream and the cooled compressed natural
gas stream to
additionally cool the cooled compressed natural gas stream to form an
additionally cooled
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compressed natural gas stream. The additionally cooled compressed natural gas
stream is
expanded, in a work producing natural gas expander, to a pressure less than
200 bara, but no
greater than the pressure to which the natural gas compressor compresses the
natural gas
stream, to thereby produce a chilled natural gas stream. The chilled natural
gas stream is
liquefied by passing the chilled natural gas stream and the liquefied nitrogen
stream through a
second heat exchanger that exchanges heat therebetween, wherein the liquefied
nitrogen
stream passes through the second heat exchanger to form the at least partially
vaporized
nitrogen stream.
[0010] The invention also provides a method of removing greenhouse gas
contaminants
to in a liquid nitrogen stream used to liquefy a natural gas stream. The
natural gas stream is
compressed in a natural gas compressor to a pressure of at least 135 bara to
form a
compressed natural gas stream. The compressed natural gas stream is cooled in
a natural
gas cooler. The cooled compressed natural gas stream and an at least partially
vaporized
nitrogen stream are passed through a first heat exchanger. The first heat
exchanger
is exchanges heat between the at least partially vaporized nitrogen stream
and the cooled
compressed natural gas stream to additionally cool the cooled compressed
natural gas stream
to form an additionally cooled compressed natural gas stream. The additionally
cooled
compressed natural gas stream is expanded, in a natural gas expander, to a
pressure less
than 200 bara, but no greater than the pressure to which the natural gas
compressor
20 compresses the natural gas stream, to thereby form a chilled natural gas
stream. The cooled
compressed natural gas stream and the liquefied nitrogen stream are passed
through a second
heat exchanger that exchanges heat between the liquefied nitrogen stream and
the chilled
natural gas stream to form the at least partially vaporized nitrogen stream
and to at least
partially liquefy the chilled natural gas stream. The liquefied nitrogen
stream is circulated
25 through the second heat exchanger at least three times. The at least
partially vaporized
nitrogen stream passes through the first heat exchanger. The pressure of the
at least partially
vaporized nitrogen stream may be reduced, preferably using at least one
expander service. A
greenhouse gas removal unit is provided that includes a distillation column
and heat pump
condenser and reboiler system. The pressure and condensing temperature of an
overhead
30 stream of the distillation column is increased. The overhead stream of
the distillation column
overhead stream and a bottoms stream of the distillation column are cross-
exchanged to affect
both the overhead condenser duty and the bottom reboiler duty of the
distillation column. The
pressure of the distillation column overhead stream is reduced after the cross-
exchanging step
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to produce a reduced-pressure distillation column overhead stream. The reduced-
pressure
distillation column overhead stream is separated to produce a first separator
overhead stream
of gaseous nitrogen that exits the greenhouse gas removal unit having
greenhouse gases
removed therefrom. The first separator overhead stream is vented to
atmosphere.
BRIEF DESCRIPTION OF THE FIGURES
[0011] Figure 1 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
[0012] Figure 2 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
[0013] Figure 3 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
[0014] Figure 4 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
[0015] Figure 5 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
[0016] Figure 6 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
[0017] Figure 7 is a schematic diagram of a system to liquefy natural gas
to form LNG
using liquid nitrogen as the sole refrigerant;
zo [0018] Figure 8 is a schematic diagram of a system to liquefy
natural gas to form LNG
using liquid nitrogen as the sole refrigerant;
[0019] Figure 9 is a schematic diagram of a supplemental refrigeration
system;
[0020] Figure 10 is a flowchart of a method of liquefying natural gas to
form LNG;
[0021] Figure 11 is a flowchart of a method of removing greenhouse gas
contaminants in
a liquid nitrogen stream used to liquefy a natural gas stream;
100221 Figure 12 is a schematic diagram of a system to liquefy natural
gas to form LNG
using liquid nitrogen as the sole refrigerant;
100231 Figure 13 is a schematic diagram of a system to liquefy natural
gas to form LNG
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using liquid nitrogen as the sole refrigerant;
[0024] Figure 14 is a flowchart of a method of producing LNG; and
100251 Figure 15 is a flowchart of a method of removing greenhouse gas
contaminants in
a liquid nitrogen stream used to liquefy a natural gas stream.
DETAILED DESCRIPTION
[0026] Various specific embodiments and versions of the present invention
will now be
described, including preferred embodiments and definitions that are adopted
herein. While the
following detailed description gives specific preferred embodiments, those
skilled in the art will
appreciate that these embodiments are exemplary only, and that the present
invention can be
practiced in other ways. Any reference to the "invention' may refer to one or
more, but not
necessarily all, of the embodiments defined by the claims. The use of headings
is for purposes
of convenience only and does not limit the scope of the present invention. For
purposes of
clarity and brevity, similar reference numbers in the several Figures
represent similar items,
steps, or structures and may not be described in detail in every Figure.
[0027] All numerical values within the detailed description and the claims
herein are
modified by "about" or "approximately" the indicated value, and take into
account experimental
error and variations that would be expected by a person having ordinary skill
in the art.
[0028] As used herein, the term "compressor" means a machine that
increases the
pressure of a gas by the application of work. A "compressor" or "refrigerant
compressor"
zo includes any unit, device, or apparatus able to increase the pressure of
a gas stream. This
includes compressors having a single compression process or step, or
compressors having
multi-stage compressions or steps, or more particularly multi-stage
compressors within a
single casing or shell. Evaporated streams to be compressed can be provided to
a compressor
at different pressures. Some stages or steps of a cooling process may involve
two or more
compressors in parallel, series, or both. The present invention is not limited
by the type or
arrangement or layout of the compressor or compressors, particularly in any
refrigerant circuit.
[0029] As used herein, "cooling" broadly refers to lowering and/or
dropping a temperature
and/or internal energy of a substance by any suitable, desired, or required
amount. Cooling
may include a temperature drop of at least about 1 C, at least about 5 C, at
least about 10
C, at least about 15 C, at least about 25 C, at least about 35 "C, or least
about 50 C, or at
least about 75 C, or at least about 85 C, or at least about 95 C, or at
least about 100 C.
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The cooling may use any suitable heat sink, such as steam generation, hot
water heating,
cooling water, air, refrigerant, other process streams (integration), and
combinations thereof.
One or more sources of cooling may be combined and/or cascaded to reach a
desired outlet
temperature. The cooling step may use a cooling unit with any suitable device
and/or
equipment. According to some embodiments, cooling may include indirect heat
exchange,
such as with one or more heat exchangers. In the alternative, the cooling may
use evaporative
(heat of vaporization) cooling and/or direct heat exchange, such as a liquid
sprayed directly
into a process stream.
[0030] As used herein, the term "expansion device" refers to one or more
devices suitable
io for reducing the pressure of a fluid in a line (for example, a liquid
stream, a vapor stream, or a
multiphase stream containing both liquid and vapor). Unless a particular type
of expansion
device is specifically stated, the expansion device may be (1) at least
partially by isenthalpic
means, or (2) may be at least partially by isentropic means, or (3) may be a
combination of
both isentropic means and isenthalpic means. Suitable devices for isenthalpic
expansion of
natural gas are known in the art and generally include, but are not limited
to, manually or
automatically, actuated throttling devices such as, for example, valves,
control valves, Joule-
Thomson (J-T) valves, or venturi devices. Suitable devices for isentropic
expansion of natural
gas are known in the art and generally include equipment such as expanders or
turbo
expanders that extract or derive work from such expansion. Suitable devices
for isentropic
expansion of liquid streams are known in the art and generally include
equipment such as
expanders, hydraulic expanders, liquid turbines, or turbo expanders that
extract or derive work
from such expansion. An example of a combination of both isentropic means and
isenthalpic
means may be a Joule-Thomson valve and a turbo expander in parallel, which
provides the
capability of using either alone or using both the J-T valve and the turbo
expander
simultaneously. lsenthalpic or isentropic expansion can be conducted in the
all-liquid phase,
all-vapor phase, or mixed phases, and can be conducted to facilitate a phase
change from a
vapor stream or liquid stream to a multiphase stream (a stream having both
vapor and liquid
phases) or to a single-phase stream different from its initial phase. In the
description of the
drawings herein, the reference to more than one expansion device in any
drawing does not
necessarily mean that each expansion device is the same type or size.
[0031] The term "gas" is used interchangeably with "vapor," and is
defined as a substance
or mixture of substances in the gaseous state as distinguished from the liquid
or solid state.
Likewise, the term "liquid" means a substance or mixture of substances in the
liquid state as
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distinguished from the gas or solid state.
[0032] A "heat exchanger" broadly means any device capable of
transferring heat energy
or cold energy from one medium to another medium, such as between at least two
distinct
fluids. Heat exchangers include "direct heat exchangers" and "indirect heat
exchangers."
.. Thus, a heat exchanger may be of any suitable design, such as a co-current
or counter-current
heat exchanger, an indirect heat exchanger (e.g. a spiral wound heat exchanger
or a plate-fin
heat exchanger such as a brazed aluminum plate fin type), direct contact heat
exchanger,
shell-and-tube heat exchanger, spiral, hairpin, core, core-and-kettle, printed-
circuit, double-
pipe or any other type of known heat exchanger. "Heat exchanger" may also
refer to any
Ri column, tower, unit or other arrangement adapted to allow the passage of
one or more streams
therethrough, and to affect direct or indirect heat exchange between one or
more lines of
refrigerant, and one or more feed streams. A heat exchanger as disclosed
herein may include
multiple heat exchangers as needed or desired.
[0033] As used herein, the term "indirect heat exchange" means the
bringing of two fluids
is into heat exchange relation without any physical contact or intermixing
of the fluids with each
other. Core-in-kettle heat exchangers and brazed aluminum plate-fin heat
exchangers are
examples of equipment that facilitate indirect heat exchange.
[0034] As used herein, the term "natural gas" refers to a multi-component
gas obtained
from a crude oil well (associated gas) or from a subterranean gas-bearing
formation (non-
20 associated gas). The composition and pressure of natural gas can vary
significantly. A typical
natural gas stream contains methane (CI) as a significant component. The
natural gas stream
may also contain ethane (C2), higher molecular weight hydrocarbons, and one or
more acid
gases. The natural gas may also contain minor amounts of contaminants such as
water,
nitrogen, iron sulfide, wax, and crude oil.
25 [0035] Certain embodiments and features have been described using a
set of numerical
upper limits and a set of numerical lower limits. It should be appreciated
that ranges from any
lower limit to any upper limit are contemplated unless otherwise indicated.
All numerical values
are "about" or "approximately" the indicated value, and take into account
experimental error
and variations that would be expected by a person having ordinary skill in the
art.
30 [0036] <<This paragraph has been intentionally left blank.>>
[0037] Described herein are systems and processes relating to the natural
gas liquefaction
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process using once-through LIN as a primary refrigerant to remove a
substantial portion of
residual LNG contamination of the LIN prior to venting of the gaseous
hydrogen. Specific
embodiments of the invention include those set forth in the following
paragraphs as described
with reference to the Figures. While some features are described with
particular reference to
only one Figure (such as Figure 1, 2, or 3), they may be equally applicable to
the other Figures
and may be used in combination with the other Figures or the foregoing
discussion.
[0038] Figure 1 shows a system 10 to liquefy natural gas to produce LNG
using liquid
nitrogen (LIN) as the sole external refrigerant. System 10 may be termed an
LNG production
system. A LIN stream 12 is received from a LIN supply system 14, which may
comprise one
to or more tankers, tanks, pipelines, or a combination thereof. The LIN
supply system 14 may
be in alternating service between LIN storage and LNG storage. LIN stream 12
may be
contaminated with a greenhouse gas such as methane, ethane, propane or other
alkanes or
alkenes. LIN stream 12 may be contaminated approximately 1% by volume with
greenhouse
gases, although the level of contamination may vary based on the methods used
to empty and
IS purge the LIN supply system before switching between LIN storage and LNG
storage. LIN
stream 12 is supplied at or near atmospheric pressure at a temperature of
about -196 C, which
is near the atmospheric boiling point of nearly pure nitrogen. The LIN stream
12 is sent through
a LIN pump 16, which increases the pressure of the LIN between approximately
20 bara and
200 bara with a preferred pressure of about 90 bara. This pumping process may
increase the
20 temperature of the LIN within the LIN stream 12, but it is expected the
LIN will remain
substantially in liquid form. The pressurized LIN stream 18 then flows through
a series of heat
exchangers and expanders to remove heat from the incoming natural gas supply
20 to
condense the natural gas to LNG. Still referring to Figure 1, the pressurized
LIN stream 18
flows through a first heat exchanger 22 where it cools a natural gas stream
24. The pressurized
25 LIN stream 18 then flows a first time through a second heat exchanger 26
where it again cools
the natural gas stream.
100391 After the LIN passes through the first heat exchanger 22 and the
second heat
exchanger 26, it is expected that the LIN and any greenhouse gas contaminants
will be fully
vaporized to form a contaminated gaseous nitrogen (cGAN) stream 27. As the
gaseous
30 nitrogen is processed as further described, it may not be fully
vaporized even though it is
described herein as gaseous nitrogen or cGAN. For the sake of simplicity any
mixture of
gaseous and partially condensed nitrogen is still noted as cGAN or gaseous
nitrogen.
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[0040] The cGAN stream 27 is directed to a first expander 28. The output
stream of the
first expander 28, which is an expanded cGAN stream 29, is directed a
greenhouse gas
removal unit 30. The pressure of the expanded cGAN stream 29 may range from 5
bara to 30
bara based largely upon the phase envelope of the cGAN mixture, which
typically is a mixture
of nitrogen, methane, ethane, propane and other potential greenhouse gases. In
one aspect,
the pressure of the expanded cGAN stream 29 is between 19 and 20 bara and the
temperature
of the expanded cGAN stream 29 is about -153 degrees Celsius. However, the
pressure of
the expanded cGAN stream may be as low as 1 bara if alternative removal
technologies, such
as adsorption, absorption, or catalytic processes are used.
to [00411 The greenhouse gas removal unit 30 may be required to
produce a GAN stream
with greenhouse gas content of less than 500 ppm, or less than 200 ppm, or
less than 100
ppm, or less than 50 ppm, or less than 20 ppm. The greenhouse gas removal unit
30 may be
required to produce a greenhouse gas product stream with a nitrogen content of
less than
80%, or less than 50%, or less than 20%, or less than 10%, or less than 5%.
is [0042] The greenhouse gas removal unit 30 may include a partially
refluxed and partially
re-boiled distillation column 32. The distillation column 32 separates the
gaseous nitrogen
from the greenhouse gas contaminants based on the differences in vaporization
temperatures
of nitrogen and the greenhouse gases. The outputs of the distillation column
are an overhead
stream 34, which is a decontaminated gaseous nitrogen stream, and a bottoms
product, which
20 is a greenhouse gas product stream 36. Side-re-boilers, side condensers
and intermediate
draws (not shown) may be included to remove products at other locations in the
distillation
column 32.
100431 The greenhouse gas removal unit 30 may include an overhead
condenser
associated with the distillation column 32 and having a cooling duty supplied
by heat exchange
25 with LIN, GAN, cGAN, natural gas or LNG sources from other parts of the
LNG Production
System, or even from a supplemental refrigeration system. Similarly, the
greenhouse gas
removal unit may include a bottoms reboiler associated with the distillation
column 32 and
having a heating duty supplied by heat exchange with LIN, GAN, cGAN, natural
gas or LNG
from other parts of the LNG Production System or another process external to
the LNG
30 Production System. The disadvantage of these types of arrangements is
the adverse impact
of the largely condensing and largely boiling-type heating requirements of the
distillation
column condenser and reboiler on the overall heating and cooling curves to
condense the
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natural gas to LNG. These impacts may result in temperature pinches in the
heat exchangers
that diminish the effectiveness of the available LIN supply. According to the
invention, the
condenser and reboiler cooling and heating duties are cross-exchanged such
that the cold
duty available from the reboiler is used to meet the hot duty required of the
condenser. To
accomplish this, a heat pump condenser and reboiler system is used to increase
the pressure
of the distillation column overhead stream 34 such that the temperature of the
compressed
overhead stream is higher than the temperature of the greenhouse gas product
stream 36.
Specifically, the heat pump condenser and reboiler system comprises an
overhead
compressor 38 that compresses and warms the overhead stream 34, a heat pump
heat
io exchanger 40 that cools the overhead stream and warms the greenhouse gas
product stream,
and a pressure reduction device 42 that reduces the pressure of the cooled
overhead stream
and reduces its pressure. The pressure reduction device 42 may be a Joule-
Thomson valve
or a turbo-expander. At this point the overhead stream has become a partially
condensed
overhead stream 43. If desired, a first separator 44 may be used to separate
the partially
is condensed overhead stream 43 to form an overhead product stream 45 and a
column reflux
stream 46. The overhead product stream 45, being the overhead product of both
the distillation
column 32 and the first separator 44, is comprised of GAN substantially
decontaminated of
greenhouse gases such as methane, ethane, etc., and exits the greenhouse gas
removal unit
30 for further heat exchange operations and venting as will be described
herein. Because the
20 column reflux stream 46 may include some greenhouse gases, the column
reflux stream is
sent back to the distillation column 32 for further separation steps.
[0044] The other portion of the heat pump condenser and reboiler system
may include a
bottoms pump 48 to deliver the greenhouse gas product stream 36 to the heat
pump heat
exchanger 40 at an increased pressure. After being heated in the heat pump
heat exchanger
25 40, the greenhouse gas product stream 36 is now partially vaporized and
may be sent to a
second separator 50, which separates the partially vaporized greenhouse gas
product stream
to form a separated greenhouse gas product stream 54 and a column reboiler
vapor stream
56. A greenhouse gas pump 58 may be used to deliver the separated greenhouse
gas product
stream 54 to another location in system 10 at a required pressure. In the
embodiment shown
30 in Figure 1, the separated greenhouse gas product stream 54 is mixed
with the natural gas
stream 24 after the natural gas stream 24 has passed through the second heat
exchanger 26
to be included in the LNG product stream of system 10. The column reboiler
stream 56, which
may include a portion of GAN, is returned to the distillation column 32 for
further separation
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steps.
[0045] The overhead product stream 45, which is substantially
decontaminated GAN, exits
the greenhouse gas removal unit 30 and passes iteratively through the second
heat
exchanger 26 and second and third expanders 60, 62 to further cool the natural
gas stream
.. 24. In Figure 1 three expanders are shown, which function as a high-
pressure expander (28),
a medium-pressure expander (60), and a low pressure expander (62), each
expander reducing
the pressure of the nitrogen stream respectively passing therethrough. In an
embodiment the
first, second, and third expanders 28, 60, 62 are turbo expanders. The
expanders may be
radial inflow turbines, partial admission axial flow turbines, full admission
axial flow turbines,
io reciprocating engines, helical screw turbines or similar expansion
devices. The expanders
may be separate machines or combined into one or more machines with common
outputs.
The expanders may be designed to drive generators, compressors, pumps, water
brakes or
any similar power- consuming device to remove the energy from the system 10.
The
expanders may be used to directly drive (or drive via gearboxes or other
transmission devices)
Is pumps, compressors and other machines used within the system 10. In an
embodiment, each
expander is an expander service, wherein expansion may be performed by one or
more
individual expander devices acting in parallel or series or a combination of
parallel and series
operation. At least one expander or expander service is required to
economically operate
system 10 and generally at least two expander services are preferred. More
than three
20 expander services may also be used in this system to possibly further
improve the
effectiveness of the refrigeration by the available LIN supply.
[0046] After passing through the third expander 62 and the second heat
exchanger 26 for
the final time, the overhead product stream 45 passes through a third heat
exchanger 64 that
cools the natural gas stream 24 an additional time. The overhead product
stream, which as
25 previously stated is GAN, is vented to the atmosphere at CAN vent 66 or
is otherwise disposed
of. If the CAN is vented, the GAN plume should be sufficiently buoyant to be
widely distributed
and diluted by the atmosphere prior to any significant part of the plume
returning to near ground
level, which may cause a potentially hazardous oxygen deficiency. Since the
GAN is likely to
have essentially zero relative humidity and a specific gravity only slightly
less than the ambient
30 air, embodiments should ensure CAN vent temperatures greater than the
local ambient
temperature to improve buoyancy and promote dispersal of the CAN plume. Those
skilled in
the art of vent and vent stack design are aware of alternatives to temperature
to improve plume
dispersal, including modifying stack height and providing a higher velocity
stack exit that, as
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CA 03053327 2019-08-12
an example, may be provided by a venturi feature as part of the stack design.
100471 The path of natural gas through system 10 will now be described.
The natural gas
supply 20 is received at pressure, or is compressed to a desired pressure, and
then flows
through various heat exchangers in series, parallel or a combination of series
and parallel to
be cooled by the refrigerant or refrigerants. The natural gas pressure
supplied to the system
is typically between 20 bara and 100 bara with the upper pressure generally
limited by the
economic selection of heat exchange equipment. With future advances in heat
exchanger
design, supply pressure of 200 bara or more may be feasible. In a preferred
embodiment, the
natural gas supply pressure is selected at about 90 bara. Those skilled in the
art are aware
u) that increasing the natural gas supply pressure generally improves the
heat transfer
effectiveness within an LNG liquefaction process. As shown in Figure 1,
natural gas from the
natural gas supply 20 first flows through the third heat exchanger 64. The
third heat exchanger
pre-chills the natural gas before entering the second heat exchanger 26, which
is the main
heat exchanger of the system 10. The third heat exchanger also warms the CAN
in the
overhead product stream 45 to near the incoming temperature of the natural gas
stream. The
third heat exchanger 64 may be eliminated from system 10 if desired.
[0048] After exiting the first heat exchanger, the natural gas stream 24
is chilled and
condensed at pressure in the second heat exchanger 26, where the natural gas
stream is
cooled by several passes of the GAN in the overhead product stream 45. The
natural gas
stream 24 is merged with the separated greenhouse gas product stream 54, which
as
previously described is greenhouse gases with substantially all GAN removed
therefrom. The
natural gas stream 24 then passes through the first heat exchanger 22, which
uses UN from
the LIN supply system 14 to cool the natural gas stream 24. The first heat
exchanger 22 may
be eliminated from system 10 if desired. At this point the natural gas in the
natural gas
stream 24 has been substantially completely liquefied to form LNG. The
condensed high
pressure LNG is reduced to near ambient pressure through a pressure reduction
device 68
that may comprise a single-phase or multi-phase hydraulic turbine, Joule-
Thomson valve or a
similar pressure reduction device. Figure 1 shows the use of a hydraulic
turbine. The LNG
stream 70 exiting the pressure reduction device 68 may then be stored in
tankage, delivered
to a land-based or water-borne tanker, delivered to a suitable cryogenic
pipeline or similar
conveyance to ultimately deliver the LNG to a market location.
100491 The distillation column 32 of the greenhouse gas removal unit 30
may be controlled
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to meet required specifications for greenhouse gas content of the overhead
product stream 45
and the nitrogen content of the greenhouse gas product stream 36 and/or the
separated
greenhouse gas product stream 54. Generally, the temperature and fraction
vaporized of the
expanded cGAN stream 29 will affect the relative condenser and reboiler
duties, with higher
fraction vaporized or higher temperatures of the expanded cGAN stream 29
increasing the
condenser duty while decreasing the reboiler duty at the same product
specifications. Lower
fraction vaporized or lower temperatures of the expanded cGAN stream 29 have
the opposite
effects. In addition, an increase (or decrease) of the heat transfer rate
within the heat pump
heat exchanger 40 tends to increase (or decrease) both the condenser and
reboiler duties that
to affect the product specifications. A controller 72 to adjust both the
temperature and/or fraction
vaporized of the expanded cGAN stream 29 and the heat pump heat exchanger 40
heat
transfer rate may be used to both balance the condenser and reboiler duties
(with adjustments
for the extra energy added by the overhead compressor 38) and the product
specifications of
the distillation column 32. In practice, these controls may be realized by
adjusting the inlet
temperature of the first turbo-expander 28 and by controlling the pressure
increase of the
column overhead compressor 38. Alternatively, other components of the system
10 may be
controlled to achieve the same outcome.
[0050] Having described an aspect of the invention, additional aspects
will now be
described. Figure 2 illustrates an LNG production system 200 similar to system
10 of Figure 1.
LNG production system 200 further includes a natural gas compressor 202 and a
natural gas
cooler 204 that are used to pressurize and cool the natural gas to an optimal
pressure and
temperature prior to entering the third, second, and first heat exchangers 64,
26, 22. The
natural gas compressor 202 and the natural gas cooler 204 may be a plurality
of individual
compressors and coolers or a single compressor stage and cooler. The natural
gas
compressor 202 may be selected from compressor types generally known to those
skilled in
the art, including centrifugal, axial, screw and reciprocating type
compressors. The natural
gas cooler 204 may be selected from cooler types generally known to those
skilled in the art,
including air fin, double pipe, shell and tube, plate and frame, spiral wound,
and printed circuit
type heat exchangers. The natural gas supply pressure following the natural
gas compressor
202 and the natural gas cooler 204 should be similar to the range noted
previously (e.g. 20 ¨
100 bara and up to 200 bara or more as heat exchanger design advances).
[0051] Figure 3 illustrates an LNG production system 300 similar to LNG
production
system 200. LNG production system 300 adds a natural gas expander 302
following the
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natural gas compressor 202 and the natural gas cooler 204. The natural gas
expander 302
may be any type of expander, such as a turbo-expander or another type of
pressure reduction
device such as a J-T valve. In LNG production system 300, the discharge
pressure of the
natural gas compressor 202 may be increased above the range indicated by an
economic
selection of heat exchange equipment and the excess pressure reduced through
the natural
gas expander 302. The combination of compression, cooling and expansion
further pre-chills
the natural gas supply prior to entering the third heat exchanger 64 or the
second heat
exchanger 26. For example, the natural gas compressor 202 may compress the
natural gas
supply to a pressure greater than 135 bara and the natural gas expander may
reduce the
pressure of the natural gas to less than 200 bara, but in no event greater
than the pressure to
which the natural gas compresses the natural gas. In an embodiment, the
natural gas stream
is compressed by the natural gas compressor to a pressure greater than 200
bara. In another
embodiment, the natural gas expander expands the natural gas stream to a
pressure less than
135 bara. However, the location of the third heat exchanger 64 downstream of
the natural gas
expander 302 (as shown in Figure 3) significantly lowers the temperature of
the CAN passing
through the third heat exchanger 64. The temperature of the GAN so cooled may
be well
below the local ambient temperature, thereby complicating efforts to safely
and/or efficiently
vent the GAN to the atmosphere.
[0052] Figure 4 illustrates an LNG production system 400 similar to LNG
production
system 300. In LNG production system 400, the third heat exchanger 64 is
located so that
natural gas from the natural gas supply 20 enters the third heat exchanger
before passing
through the natural gas compressor 202. Placing the third heat exchanger 64 as
shown in
Figure 4 reduces the temperature of the natural gas entering the natural gas
compressor 202
and so reduces the pressure and power required by the natural gas compressor
202.
Additionally, the GAN vent 66 temperature is restored to be similar to the
embodiment shown
in Figure 1.
[0053] Figure 5 depicts an LNG production system 500 similar to LNG
production systems
300 and 400. In LNG production system 500, the third heat exchanger 64 is
located between
the natural gas compressor 202 and the natural gas cooler 204. This placement
sacrifices the
potential power reduction of the natural gas compressor 202 provided by LNG
production
system 400 (Figure 4) but results in a large increase to the GAN vent
temperature to
significantly improve GAN plume buoyancy and dispersal. This placement also
reduces the
cooling duty of the natural gas cooler 204 and so reduces the size, capital
cost and operating
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cost of the natural gas cooler 204 and its related support systems (e.g.
cooling water, air-fin
power supply, etc.).
[0054] Figure 6 illustrates an LNG production system 600 similar to LNG
production
system 400. In LNG production system 600, the GAN in the overhead product
stream 45 is
subjected to additional heat pump refrigeration in a heat pump system as the
overhead product
stream circulates through the second heat exchanger 26 and the second and
third expanders
60, 62. As depicted in Figure 6, the heat pump system includes a nitrogen
compressor 602, a
nitrogen cooler 604, and a feed-effluent heat exchanger 606 are added upstream
of the third
expander 62. The addition of this combination of the nitrogen compressor 602,
the nitrogen
lo cooler 604, and the feed-effluent heat exchanger 606 increases the
pressure available at the
inlet of the third expander 62 with only a small increase to the inlet
temperature of the third
expander 62. This combination of the nitrogen compressor 602, the nitrogen
cooler 604, and
feed-effluent heat exchanger 606 increases the power produced by the third
expander 62 and
increases the heat removed from the GAN in the overhead product stream 45
flowing through
this portion of the LNG production system 600. This combination also results
in a lower GAN
temperature re-entering the second heat exchanger 26 compared to Figure 4, and
also results
in an increase of the effectiveness of the available LIN supply in the LNG
production system
600.
[0055] Figure 7 depicts an LNG production system 700, similar to LNG
production system
10, in which an alternative use of the separated greenhouse gas product stream
54 is shown.
Instead of mixing the separated greenhouse gas product stream 54 with the
natural gas stream
24, as shown in Figure 1, the separated greenhouse gas product stream 54 may
be used as a
fuel gas supply 702 after being pumped to the required pressure in the
greenhouse gas pump
58 and re-vaporized through one or more of the heat exchangers. As an example,
Figure 7
shows the separated greenhouse gas product stream 54 passing through the third
heat
exchanger 64. Other uses of the separated greenhouse gas product stream are
possible and
generally known to those skilled in the art.
[0056] Figure 8 depicts an LNG production system 800 similar to LNG
production systems
10, 200, 400, and 600. In LNG production system 800, the very dry composition
of the GAN
in the overhead product stream 45 is used to effect further cooling within the
LNG production
system 800. Psychometric cooling of the GAN in the overhead product stream 45
can reduce
the temperature of that stream to within a few degrees Celsius of the freezing
temperature of
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CA 03053327 2019-08-12
water, or about 2-5 degrees Celsius by the addition and saturation of water
802 to the overhead
product stream 45 after the overhead product stream 45 has passed through the
third heat
exchanger 64 as shown in Figure 8. The now wet or saturated GAN stream 804,
with its lower
temperature, may be re-routed through the third heat exchanger 64 (or other
appropriate heat
exchanger) to further pre-chill the incoming natural gas stream. Those skilled
in the art will
recognize that many techniques are available to effect this psychometric
cooling, including
spraying of water via fogging or other nozzles into the flowing GAN stream, or
passing the
GAN and water over trays, packing material, or other heat and mass transfer
device(s) within
a tower, column or cooling tower-like device. Alternatively, cooling water or
another heat
io transfer fluid may be further chilled via such psychometric cooling by
passing the very dry GAN
through a cooling tower-like device. This further chilled cooling water may
then be used to
pre-chill other streams within the LNG production system 800 to enhance the
effectiveness of
the available LIN supply. Finally, adding water vapor to the otherwise very
dry gaseous
nitrogen reduces the specific gravity of the GAN and improves GAN plume
buoyancy and
dispersal if the GAN is vented at 806.
[0057] The included figures each depict a greenhouse gas removal unit 30
as part of an
LNG production system 10, 200, 300, 400, 500, 600, 700, 800, where the
greenhouse gas
removal unit is depicted as based on distillation technologies and
methodologies. Alternative
systems and methods may be used to remove the greenhouse gas contaminants of
the LIN
supply 14. These alternative methods are not shown in detail but may include:
adsorption
processes including pressure-swing, temperature-swing or a combination of
pressure and
temperature-swing adsorption; bulk adsorption or absorption such as by an
activated carbon
bed; or catalytic processes.
[0058] The heat exchangers in the disclosed embodiments have been
described as being
.. cooled by solely by LIN, GAN, or a combination thereof, sourced from the
LIN supply 14.
However, it is possible to increase the cooling capability of any of the
disclosed heat
exchangers by employing a supplemental refrigeration system having no fluid
connection with
the natural gas or nitrogen in the LNG production system 10. The refrigerant
used by the
supplemental refrigeration system may comprise any suitable hydrocarbon gas
(e.g., alkenes
or alkanes such as methane, ethane, ethylene, propane, etc.), inert gases
(e.g., nitrogen,
helium, argon, etc.), or other refrigerants known to those skilled in the art.
Figure 9 depicts a
supplemental refrigeration system 900 providing additional cooling capability
to the heat pump
heat exchanger 40 of the greenhouse gas removal unit 30 using an argon stream
902 as the
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CA 03053327 2019-08-12
refrigerant. The supplemental refrigeration system 900 includes a supplemental
compressor
904 that compresses the argon stream 902 to a suitable pressure. The argon
stream 902 then
passes through a supplemental heat exchanger, shown in Figure 9 as a cooler
906. The argon
stream 902 then passes through a supplemental pressure reduction device 908
such as a
Joule-Thompson valve or an expander. The argon stream 902 then passes through
the heat
pump heat exchanger 40 to supplement the cooling efforts of the GAN in the
distillation column
overhead stream 34 to cool the greenhouse gases in the greenhouse gas product
stream 36.
The argon stream 902 then recirculates through the supplemental compressor 904
as
previously described.
Iso 100591 A supplemental refrigeration system similar to supplemental
refrigeration system
900 may be used to increase the cooling effectiveness of other heat exchangers
disclosed
herein, such as the first heat exchanger 22, second heat exchanger 26, third
heat exchanger
64, and/or the feed-effluent heat exchanger 606. Further, while the
refrigerant of the
supplemental refrigeration system 900 is not fluidly connected to the LNG
production system
10, in some embodiments the refrigerant may be sourced from natural gas
streams and/or
nitrogen streams of the LNG production system. Further, the supplemental heat
exchanger
904 may exchange heat (or cold) with gaseous streams and/or liquid streams of
the LNG
production system 10, such as the LIN stream 12, natural gas stream 24, cGAN
stream 27, or
the greenhouse gas product stream 36.
zo 100601 Figure 10 illustrates a method 1000 of producing LNG
according to disclosed
aspects. At block 1002 a natural gas stream is provided from a supply of
natural gas. At block
1004 a refrigerant stream, such as a LIN stream, is provided from a supply of
refrigerant. At
block 1006 the natural gas stream and the liquefied nitrogen stream are passed
through a first
heat exchanger that exchanges heat between the refrigerant stream and the
natural gas
stream to at least partially vaporize the refrigerant stream and at least
partially condense the
natural gas stream. At block 1008 the natural gas stream is compressed in a
natural gas
compressor to a pressure of at least 135 bara to form a compressed natural gas
stream. At
block 1010 the compressed natural gas stream is cooled in a natural gas
cooler. After being
cooled by the natural gas cooler, at block 1012 the compressed natural gas
stream is
expanded in a natural gas expander to a pressure less than 200 bara, but no
greater than the
pressure to which the natural gas compressor compresses the natural gas
stream. At block
1014 natural gas from the natural gas cooler is supplied to the at least one
heat exchanger to
be at least partially condensed therein.
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CA 03053327 2019-08-12
[0061] Figure 11 illustrates a method 1100 of removing greenhouse gas
contaminants in a
liquid nitrogen stream used to liquefy a natural gas stream. At block 1102 the
natural gas
stream is compressed in a natural gas compressor to a pressure of at least 135
bara to form
a compressed natural gas stream. At block 1104 the compressed natural gas
stream is cooled
in a natural gas cooler. After being cooled by the natural gas cooler, at
block 1106 the
compressed natural gas stream is expanded in a natural gas expander to a
pressure less than
200 bara, but no greater than the pressure to which the natural gas compressor
compresses
the natural gas stream. At block 1108 the natural gas stream and the liquefied
nitrogen stream
are passed through a first heat exchanger that exchanges heat between the
liquefied nitrogen
io stream and the natural gas stream to at least partially vaporize the
liquefied nitrogen stream
and at least partially condense the natural gas stream. The liquefied nitrogen
stream is
circulated through the first heat exchanger at least one time, and preferably
at least three times.
At block 1110 the pressure of the at least partially vaporized nitrogen stream
may be reduced,
preferably using at least one expander service. At block 1112 a greenhouse gas
removal unit
is provided that includes a distillation column and heat pump condenser and
reboiler system.
At block 1114 the pressure and condensing temperature of an overhead stream of
the
distillation column is increased. At block 1116 the overhead stream of the
distillation column
overhead stream and a bottoms stream of the distillation column are cross-
exchanged to affect
both the overhead condenser duty and the bottom reboiler duty of the
distillation column. At
block 1118 the pressure of the distillation column overhead stream is reduced
after the cross-
exchanging step to produce a reduced-pressure distillation column overhead
stream. At block
1120 the reduced-pressure distillation column overhead stream is separated to
produce a first
separator overhead stream of gaseous nitrogen that exits the greenhouse gas
removal unit
having greenhouse gases removed therefrom. At block 1122 the first separator
overhead
stream is vented to atmosphere.
[0062] Figures 12 and 13 illustrate LNG production systems 1200, 1300
similar to LNG
production system 600 depicted in Figure 6. However, LNG production system
1200 does not
include the greenhouse gas removal unit 30 of system 600. Elements in Figures
12 and 13
having the same reference numbers as elements in previous Figures are the same
or similar
.. to the respective previously described elements. For LNG production system
1200, instead of
the overhead product stream 45 of the greenhouse gas removal unit 30 entering
the second
heat exchanger 26, the expanded cGAN stream 29 is directed through the second
heat
exchanger 26 and performs additional refrigeration as the stream circulates
through the
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CA 03053327 2019-08-12
second heat exchanger 26 and the second and third expanders 60, 62. LNG
production
systems 1200, 1300 further include a natural gas compressor 1202 and a natural
gas cooler
1204 that are used to pressurize and cool the natural gas to an optimal
pressure and
temperature prior to entering the third heat exchanger 64. The natural gas
compressor 1202
and the natural gas cooler 1204 are similar to natural gas compressor 202 and
the natural gas
cooler 204 previously described. The natural gas supply pressure following the
natural gas
compressor 1202 and the natural gas cooler 1204 should be similar to the range
noted
previously (e.g. 20 ¨ 100 bara and up to 200 bara or more as heat exchanger
design
advances). Natural gas exiting the natural gas cooler 1204 enters the third
heat exchanger
to 64, where vaporized, low-pressure nitrogen exiting the second heat
exchanger 26 is used to
cool the natural gas immediately prior to expanding the natural gas stream in
the high pressure
natural gas expander 302. Using the available cold of the vaporized, low
pressure nitrogen to
further cool the natural gas stream increases the amount of pre-cooling
provided by the high
pressure compression and expansion process. For the configurations illustrated
in Figures 12
and 13, the required liquid nitrogen to LNG ratio is reduced by up to 2.5%
compared to the
liquid nitrogen to LNG ratio of a configuration having the natural gas
compressor, natural gas
cooler, and high pressure natural gas expander arranged as shown in Figure 6.
100631 Figure 14 illustrates a method 1400 of producing LNG according to
disclosed
aspects. At block 1402 a natural gas stream is provided from a supply of
natural gas. At block
1404 a liquid nitrogen stream is provided from a liquid nitrogen supply. At
block 1406 the
natural gas stream is compressed in a natural gas compressor to a pressure of
at least 135
bara to form a compressed natural gas stream. At block 1408 the compressed
natural gas
stream is cooled, through indirect heat exchange with an ambient fluid in a
natural gas cooler,
to form a cooled compressed natural gas stream. At block 1410 the cooled
compressed
natural gas stream and an at least partially vaporized nitrogen stream are
passed through a
first heat exchanger. The first heat exchanger exchanges heat between the at
least partially
vaporized nitrogen stream and the cooled compressed natural gas stream to
additionally cool
the cooled compressed natural gas stream to form an additionally cooled
compressed natural
gas stream. At block 1412 the additionally cooled compressed natural gas
stream is
expanded, in a work producing natural gas expander, to a pressure less than
200 bara, but no
greater than the pressure to which the natural gas compressor compresses the
natural gas
stream, to thereby produce a chilled natural gas stream. At block 1414 the
chilled natural gas
stream is liquefied by passing the chilled natural gas stream and the
liquefied nitrogen stream
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CA 03053327 2019-08-12
through a second heat exchanger that exchanges heat therebetween, wherein the
liquefied
nitrogen stream passes through the second heat exchanger to form the at least
partially
vaporized nitrogen stream.
[0064] Figure 15 illustrates a method 1500 of removing greenhouse gas
contaminants in
a liquid nitrogen stream used to liquefy a natural gas stream. At block 1502
the natural gas
stream is compressed in a natural gas compressor to a pressure of at least 135
bara to form
a compressed natural gas stream. At block 1504 the compressed natural gas
stream is cooled
in a natural gas cooler. At block 1506 the cooled compressed natural gas
stream and an at
least partially vaporized nitrogen stream are passed through a first heat
exchanger. The first
heat exchanger exchanges heat between the at least partially vaporized
nitrogen stream and
the cooled compressed natural gas stream to additionally cool the cooled
compressed natural
gas stream to form an additionally cooled compressed natural gas stream. At
block 1508 the
additionally cooled compressed natural gas stream is expanded, in a natural
gas expander, to
a pressure less than 200 bara, but no greater than the pressure to which the
natural gas
is compressor compresses the natural gas stream, to thereby form a chilled
natural gas stream.
At block 1510 the cooled compressed natural gas stream and the liquefied
nitrogen stream are
passed through a second heat exchanger that exchanges heat between the
liquefied nitrogen
stream and the chilled natural gas stream to form the at least partially
vaporized nitrogen
stream and to at least partially liquefy the chilled natural gas stream. The
liquefied nitrogen
stream is circulated through the second heat exchanger at least three times.
The at least
partially vaporized nitrogen stream passes through the first heat exchanger.
At block 1512 the
pressure of the at least partially vaporized nitrogen stream may be reduced,
preferably using
at least one expander service. At block 1514 a greenhouse gas removal unit is
provided that
includes a distillation column and heat pump condenser and reboiler system. At
block 1516
the pressure and condensing temperature of an overhead stream of the
distillation column is
increased. At block 1518 the overhead stream of the distillation column
overhead stream and
a bottoms stream of the distillation column are cross-exchanged to affect both
the overhead
condenser duty and the bottom reboiler duty of the distillation column. At
block 1520 the
pressure of the distillation column overhead stream is reduced after the cross-
exchanging step
to produce a reduced-pressure distillation column overhead stream. At block
1522 the
reduced-pressure distillation column overhead stream is separated to produce a
first separator
overhead stream of gaseous nitrogen that exits the greenhouse gas removal unit
having
greenhouse gases removed therefrom. At block 1524 the first separator overhead
stream is
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CA 03053327 2019-08-12
vented to atmosphere.
[0065] The embodiments and aspects provide an effective method of
removing
greenhouse gas contaminants from an LIN stream used to liquefy natural gas. An
advantage
of the invention is that the heat pump system in the greenhouse gas removal
unit 30 removes
the necessity of external heating or cooling sources to separate the
greenhouse gases from
the nitrogen.
[0066] Another advantage of the efficient removal of greenhouse gases
from LIN is that
LIN storage facilities can more economically be used as LNG storage
facilities, thereby
reducing the areal footprint of natural gas processing facilities.
[0067] Still another advantage is that the gaseous nitrogen may be vented
without the
unwanted release of greenhouse gases into the atmosphere.
[0068] Although exemplary embodiments discussed herein in with respect to
Figures 1-11
are directed to producing LNG using LIN as primary coolant, a person of
ordinary skill in the
art would understand that the principles apply to other cooling methods and
coolants. For
is example, the disclosed methods and systems may be used where there is no
common storage
for LNG and LIN, and it is desired simply to purify a coolant used in LNG or
other liquefaction
methods.
[0069] Embodiments of the invention may include any combinations of the
methods and
systems shown in the following numbered paragraphs. This is not to be
considered a complete
listing of all possible embodiments, as any number of variations can be
envisioned from the
description above.
[0070] Embodiment 1. A liquefied natural gas production system, the
system comprising:
a natural gas stream from a supply of natural gas; a liquid nitrogen stream
from a liquid nitrogen
supply; a natural gas compressor that compresses the natural gas stream to a
pressure of at
least 135 bara to form a compressed natural gas stream; a natural gas cooler
that cools the
compressed natural gas stream by indirect heat exchange with an ambient fluid
to form a
cooled compressed natural gas stream; a first heat exchanger that exchanges
heat between
an at least partially vaporized nitrogen stream and the cooled compressed
natural gas stream
to form an additionally cooled compressed natural gas stream; a work-producing
natural gas
expander that expands the additionally cooled compressed natural gas stream to
a pressure
less than 200 bara, but no greater than the pressure to which the natural gas
compressor
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CA 03053327 2019-08-12
compresses the natural gas stream, to thereby form a chilled natural gas
stream; and a second
heat exchanger that at least partially condenses the chilled natural gas
stream by exchanging
heat between the chilled natural gas stream and the liquid nitrogen stream to
form the at least
partially vaporized nitrogen stream, wherein the at least partially vaporized
nitrogen stream
passes through the first heat exchanger.
[0071] Embodiment 2. The liquefied natural gas production system of
Embodiment 1,
wherein the natural gas compressor compresses the natural gas stream to a
pressure greater
than 200 bara.
[0072] Embodiment 3. The liquefied natural gas production system of
either Embodiments
m 1 or 2, wherein the natural gas expander expands the additionally cooled
compressed natural
gas stream to a pressure less than 135 bara.
[0073] Embodiment 4. The liquefied natural gas production system of
any of Embodiments
1-3, wherein the liquid nitrogen supply is produced at a different location of
the liquefied natural
gas production system and transported as a liquid to the liquefied natural gas
production
system.
[0074] Embodiment 5. The liquefied natural gas production system of
any of Embodiments
1-4, wherein the at least partially vaporized nitrogen stream is released to
environment as a
vapor after passing through the first heat exchanger.
[0075] Embodiment 6. The liquefied natural gas production system of
Embodiment 1,
wherein the first heat exchanger comprises at least one printed circuit heat
exchanger.
[0076] Embodiment 7. The liquefied natural gas production system of
Embodiment 1,
further comprising a greenhouse gas removal unit configured to remove
greenhouse gas from
the at least partially vaporized nitrogen stream, wherein the greenhouse gas
removal unit
comprises a distillation column having a heat pump condenser and reboiler
system, and further
comprising at least one expander service that reduces the pressure of the at
least partially
vaporized nitrogen stream, wherein an inlet stream of the distillation column
is an outlet stream
of a first of the at least one expander service, and further comprising a heat
pump system
through which the at least partially vaporized nitrogen stream flows after
flowing through a first
of the at least one expander service, the heat pump system including a heat
pump compressor,
a heat pump cooler, and a feed-effluent heat exchanger.
[0077] Embodiment 8. The liquefied natural gas production system of
any one of
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CA 03053327 2019-08-12
Embodiments 1-7, further comprising a psychrometric heat exchanger that uses
the at least
partially vaporized nitrogen stream to pre-chill the natural gas stream prior
to the natural gas
stream entering the first heat exchanger.
[0078] Embodiment 9. The liquefied natural gas production system of any
one of
Embodiments 1-8, wherein the natural gas cooler is configured to cool the
compressed natural
gas stream to near ambient temperature after being compressed by the natural
gas
corn pressor.
[0079] Embodiment 10. The liquefied natural gas production system of any
one of
Embodiments 1-9, further comprising a first nitrogen expander that expands the
at least
partially vaporized nitrogen stream to form a first cool nitrogen gas stream;
wherein the first
cool nitrogen gas stream is passed through the second heat exchanger to at
least partially
condense the chilled natural gas stream and form a first warm nitrogen gas
stream.
100801 Embodiment 11. The liquefied natural gas production system of
Embodiment 10,
further comprising a second nitrogen expander that expands the first warm
nitrogen gas stream
to form a second cool nitrogen gas stream; wherein the second cool nitrogen
gas stream is
passed through the second heat exchanger to at least partially condense the
chilled natural
gas stream and form a second warm nitrogen gas stream.
[0081] Embodiment 12. The liquefied natural gas production system of
Embodiment 11,
further comprising: a nitrogen compressor that compresses the second warm
nitrogen gas
stream to form a compressed nitrogen gas stream; a nitrogen cooler that cools
the compressed
nitrogen gas stream by indirect heat exchange with an ambient temperature
fluid to form a cool
compressed nitrogen gas stream; and a third nitrogen expander that expands the
cool
compressed nitrogen gas stream to form a third cool nitrogen gas stream;
wherein the third
cool nitrogen gas stream is passed through the second heat exchanger to at
least partially
condense the chilled natural gas stream and form a third warm nitrogen gas
stream; wherein
the third warm nitrogen gas stream is the at least partially vaporized
nitrogen stream that
exchanges heat with the cooled compressed natural gas stream in the first heat
exchanger to
form the additionally cooled compressed natural gas stream.
[0082] Embodiment 13. The liquefied natural gas production system of any
of
Embodiments 1-12, wherein the first heat exchanger comprises at least two heat
exchangers,
and wherein the second heat exchanger comprises at least two heat exchangers.
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[0083]
Embodiment 14. A method of producing liquefied natural gas (LNG), the method
comprising: providing a natural gas stream from a supply of natural gas;
providing a liquid
nitrogen stream from a liquid nitrogen supply; compressing the natural gas
stream in a natural
gas compressor to a pressure of at least 135 bara to form a compressed natural
gas stream;
cooling, through indirect heat exchange with an ambient fluid in a natural gas
cooler, the
compressed natural gas stream to form a cooled compressed natural gas stream;
passing the
cooled compressed natural gas stream and an at least partially vaporized
nitrogen stream
through a first heat exchanger that exchanges heat between the at least
partially vaporized
nitrogen stream and the cooled compressed natural gas stream to additionally
cool the cooled
lo compressed natural gas stream to form an additionally cooled compressed
natural gas stream;
expanding, in a work producing natural gas expander, the additionally cooled
compressed
natural gas stream to a pressure less than 200 bara, but no greater than the
pressure to which
the natural gas compressor compresses the natural gas stream, to thereby
produce a chilled
natural gas stream; and liquefying the chilled natural gas stream by passing
the chilled natural
gas stream and the liquefied nitrogen stream through a second heat exchanger
that exchanges
heat therebetween, wherein the liquefied nitrogen stream passes through the
second heat
exchanger to form the at least partially vaporized nitrogen stream.
[0084]
Embodiment 15. The method of Embodiment 14, wherein the natural gas
compressor compresses the natural gas stream to a pressure greater than 200
bara, and
wherein the natural gas expander expands the additionally cooled compressed
natural gas
stream to a pressure less than 135 bara.
[0085]
Embodiment 16. The method Embodiment 14 or Embodiment 15, further
comprising: expanding the at least partially vaporized nitrogen stream in a
first nitrogen
expander to form a first cool nitrogen gas stream; passing the first cool
nitrogen gas stream
through the second heat exchanger to at least partially condense the chilled
natural gas stream
and form a first warm nitrogen gas stream; expanding the first warm nitrogen
gas stream in a
second nitrogen expander to form a second cool nitrogen gas stream; passing
the second cool
nitrogen gas stream through the second heat exchanger to at least partially
condense the
chilled natural gas stream and form a second warm nitrogen gas stream;
compressing the
second warm nitrogen gas stream in a nitrogen compressor to form a compressed
natural gas
stream; cooling the compressed nitrogen gas stream by indirect heat exchange
with an
ambient temperature fluid in a nitrogen cooler to form a cool compressed
nitrogen gas stream;
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expanding the cool compressed nitrogen gas stream in a third nitrogen expander
to form a
third cool nitrogen gas stream; and passing the third cool nitrogen gas stream
through the
second heat exchanger to at least partially condense the chilled natural gas
stream and form
a third warm nitrogen gas stream.
[0086] Embodiment 17. The method of Embodiment 16, wherein the third warm
nitrogen
gas stream is the at least partially vaporized nitrogen stream that exchanges
heat with the
cooled compressed natural gas stream in the first heat exchanger to form the
additionally
cooled compressed natural gas stream.
[0087]
Embodiment 18. The method of Embodiment 16, further comprising removing
greenhouse gas from the at least partially vaporized nitrogen stream using a
greenhouse gas
removal unit, wherein the greenhouse gas removal unit comprises a distillation
column and a
heat pump condenser and reboiler system, and further comprising: increasing a
pressure and
condensing temperature of an overhead stream of the distillation column; cross-
exchanging
the overhead stream of the distillation column and a bottoms stream of the
distillation column
to affect both an overhead condenser duty and a bottom reboiler duty of the
distillation column;
reducing a pressure of the distillation column overhead stream after the cross-
exchanging step
to produce a reduced-pressure distillation column overhead stream; and
separating the
reduced-pressure distillation column overhead stream to produce a first
separator overhead
stream, wherein the first separator overhead stream is gaseous nitrogen that
exits the
greenhouse gas removal unit having greenhouse gases removed therefrom; and
flowing the
at least partially vaporized nitrogen stream through a heat pump system after
flowing through
a first of the at least one expander service.
[0088]
Embodiment 19. The method of any one of Embodiments14-18, wherein the natural
gas cooler cools the compressed natural gas stream to near ambient temperature
after being
.. compressed by the natural gas compressor.
[0089]
Embodiment 20. A method of removing greenhouse gas contaminants in a liquid
nitrogen stream used to liquefy a natural gas stream, comprising: compressing
the natural gas
stream in a natural gas compressor to a pressure of at least 135 bara to form
a compressed
natural gas stream; cooling, in a natural gas cooler, the compressed natural
gas stream to form
a cooled compressed natural gas stream; passing the cooled compressed natural
gas stream
and an at least partially vaporized nitrogen stream through a first heat
exchanger that
exchanges heat between the at least partially vaporized nitrogen stream and
the cooled
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compressed natural gas stream to additionally cool the cooled compressed
natural gas stream
to form an additionally cooled compressed natural gas stream; expanding, in a
natural gas
expander, the additionally cooled compressed natural gas stream to a pressure
less than 200
bara, but no greater than the pressure to which the natural gas compressor
compresses the
natural gas stream, to thereby form a chilled natural gas stream; passing the
cooled
compressed natural gas stream and the liquefied nitrogen stream through a
second heat
exchanger that exchanges heat between the liquefied nitrogen stream and the
chilled natural
gas stream to form the at least partially vaporized nitrogen stream and to at
least partially
liquefy the chilled natural gas stream, wherein the liquefied nitrogen stream
is circulated
to through the second heat exchanger at least three times, and wherein the
at least partially
vaporized nitrogen stream passes through the first heat exchanger; reducing a
pressure of the
at least partially vaporized nitrogen stream using at least one expander
service; providing a
greenhouse gas removal unit that includes a distillation column and heat pump
condenser and
reboiler system; increasing a pressure and condensing temperature of an
overhead stream of
the distillation column; cross-exchanging the overhead stream of the
distillation column
overhead stream and a bottoms stream of the distillation column to affect both
an overhead
condenser duty and a bottom reboiler duty of the distillation column; reducing
a pressure of
the distillation column overhead stream after the cross-exchanging step to
produce a reduced-
pressure distillation column overhead stream; separating the reduced-pressure
distillation
zo column overhead stream to produce a first separator overhead stream,
wherein the first
separator overhead stream is gaseous nitrogen that exits the greenhouse gas
removal unit
having greenhouse gases removed therefrom; and venting the first separator
overhead stream
to atmosphere.
[0090] While the foregoing is directed to embodiments of the present
invention, other and
further embodiments of the invention may be devised without departing from the
basic scope
thereof, and the scope thereof is determined by the claims that follow.
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