Note: Descriptions are shown in the official language in which they were submitted.
NATURAL GAS LIQUEFACTION BY A HIGH PRESSURE EXPANSION
PROCESS
BACKGROUND
[0001] Field of Disclosure
[0002] The disclosure relates generally to liquefied natural gas
(LNG) production.
More specifically, the disclosure relates to LNG production at high pressures.
[0003] Description of Related Art
to [0004] This section is intended to introduce various aspects of the
art, which may be
associated with the present disclosure. This discussion is intended to provide
a framework
to facilitate a better understanding of particular aspects of the present
disclosure.
Accordingly, it should be understood that this section should be read in this
light, and not
necessarily as an admission of prior art.
is [0005] Because of its clean burning qualities and convenience,
natural gas has become
widely used in recent years. Many sources of natural gas are located in remote
areas, which
are great distances from any commercial markets for the gas. Sometimes a
pipeline is
available for transporting produced natural gas to a commercial market. When
pipeline
transportation is not feasible, produced natural gas is often processed into
liquefied natural
zo gas (LNG) for transport to market.
[0006] In the design of an LNG plant, one of the most important
considerations is the
process for converting the natural gas feed stream into LNG. Cun-ently, the
most common
liquefaction processes use some form of refrigeration system. Although many
refrigeration
cycles have been used to liquefy natural gas, the three types most commonly
used in LNG
25 plants today are: (1) the "cascade cycle," which uses multiple single
component refrigerants
in heat exchangers arranged progressively to reduce the temperature of the gas
to a liquefaction
temperature; (2) the "multi-component refrigeration cycle," which uses a multi-
component
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refrigerant in specially designed exchangers; and (3) the "expander cycle,"
which expands gas
from feed gas pressure to a low pressure with a corresponding reduction in
temperature. Most
natural gas liquefaction cycles use variations or combinations of these three
basic types.
[0007] The refrigerants used in liquefaction processes may comprise a
mixture of
components such as methane, ethane, propane, butane, and nitrogen in multi-
component
refrigeration cycles. The refrigerants may also be pure substances such as
propane, ethylene,
or nitrogen in "cascade cycles." Substantial volumes of these refrigerants
with close control of
composition are required. Further, such refrigerants may have to be imported
and stored, which
impose logistics requirements, especially for LNG production in remote
locations.
io Alternatively, some of the components of the refrigerant may be
prepared, typically by a
distillation process integrated with the liquefaction process.
[0008] The use of gas expanders to provide the feed gas cooling, thereby
eliminating or
reducing the logistical problems of refrigerant handling, is seen in some
instances as having
advantages over refrigerant-based cooling. The expander system operates on the
principle that
is the refrigerant gas can be allowed to expand through an expansion
turbine, thereby performing
work and reducing the temperature of the gas. The low temperature gas is then
heat exchanged
with the feed gas to provide the refrigeration needed. The power obtained from
cooling
expansions in gas expanders can be used to supply part of the main compression
power used
in the refrigeration cycle. The typical expander cycle for making LNG operates
at the feed gas
20 pressure, typically under about 6,895 kPa (1,000 psia). Supplemental
cooling is typically
needed to fully liquefy the feed gas and this may be provided by additional
refrigerant systems,
such as secondary cooling and/or sub-cooling loops. For example, U.S. Pat. No.
6,412,302 and
U.S. Pat. No. 5,916,260 present expander cycles which describe the use of
nitrogen as
refrigerant in the sub-cooling loop.
25 [0009] Previously proposed expander cycles have all been less efficient
thermodynamically, however, than the current natural gas liquefaction cycles
based on
refrigerant systems. Expander cycles have therefore not offered any installed
cost advantage
to date, and liquefaction cycles involving refrigerants are still the
preferred option for natural
gas liquefaction.
30 [0010] Because expander cycles result in a high recycle gas stream
flow rate and high
inefficiency for the primary cooling (warm) stage, gas expanders have
typically been used to
further cool feed gas after it has been pre-cooled to temperatures well below -
20 C using an
external refrigerant in a closed cycle, for example. Thus, a common factor in
most proposed
expander cycles is the requirement for a second, external refrigeration cycle
to pre-cool the gas
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before the gas enters the expander. Such a combined external refrigeration
cycle and expander
cycle is sometimes referred to as a "hybrid cycle." While such refrigerant-
based pre-cooling
eliminates a major source of inefficiency in the use of expanders, it
significantly reduces the
benefits of the expander cycle, namely the elimination of external
refrigerants.
[0011] U. S. Patent Application US2009/0217701 introduced the concept of
using high
pressure within the primary cooling loop to eliminate the need for external
refrigerant and
improve efficiency, at least comparable to that of refrigerant-based cycles
currently in use. The
high pressure expander process (HPXP), disclosed in U. S. Patent Application
U52009/0217701, is an expander cycle which uses high pressure expanders in a
manner
io distinguishing from other expander cycles. A portion of the feed gas
stream may be extracted
and used as the refrigerant in either an open loop or closed loop
refrigeration cycle to cool the
feed gas stream below its critical temperature. Alternatively, a portion of
LNG boil-off gas may
be extracted and used as the refrigerant in a closed loop refrigeration cycle
to cool the feed gas
stream below its critical temperature. This refrigeration cycle is referred to
as the primary
is cooling loop. The primary cooling loop is followed by a sub-cooling loop
which acts to further
cool the feed gas. Within the primary cooling loop, the refrigerant is
compressed to a pressure
greater than 1,500 psia, or more preferably, to a pressure of approximately
3,000 psia. The
refrigerant is then cooled against an ambient cooling medium (air or water)
prior to being near
isentropically expanded to provide the cold refrigerant needed to liquefy the
feed gas.
20 [0012] Figure 1 depicts an example of a known HPXP liquefaction
process 100, and is
similar to one or more processes disclosed in U. S. Patent Application
US2009/0217701. In
Figure I, an expander loop 102 (i.e., an expander cycle) and a sub-cooling
loop 104 are used.
Feed gas stream 106 enters the HPXP liquefaction process at a pressure less
than about 1,200
psia, or less than about 1,100 psia, or less than about 1,000 psia, or less
than about 900 psia, or
25 less than about 800 psia, or less than about 700 psia, or less than
about 600 psia. Typically,
the pressure of feed gas stream 106 will be about 800 psia. Feed gas stream
106 generally
comprises natural gas that has been treated to remove contaminants using
processes and
equipment that are well known in the art.
[0013] In the expander loop 102, a compression unit 108 compresses a
refrigerant stream
30 109 (which may be a treated gas stream) to a pressure greater than or
equal to about 1,500 psia,
thus providing a compressed refrigerant stream 110. Alternatively, the
refrigerant stream 109
may be compressed to a pressure greater than or equal to about 1,600 psia, or
greater than or
equal to about 1,700 psia, or greater than or equal to about 1,800 psia, or
greater than or equal
to about 1,900 psia, or greater than or equal to about 2,000 psia, or greater
than or equal to
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about 2,500 psia, or greater than or equal to about 3,000 psia, thus providing
compressed
refrigerant stream 110. After exiting compression unit 108, compressed
refrigerant stream 110
is passed to a cooler 112 where it is cooled by indirect heat exchange with a
suitable cooling
fluid to provide a compressed, cooled refrigerant stream 114. Cooler 112 may
be of the type
that provides water or air as the cooling fluid, although any type of cooler
can be used. The
temperature of the compressed, cooled refrigerant stream 114 depends on the
ambient
conditions and the cooling medium used, and is typically from about 35 F. to
about 105 F.
Compressed, cooled refrigerant stream 114 is then passed to an expander 116
where it is
expanded and consequently cooled to form an expanded refrigerant stream 118
Expander 116
is a work-expansion device, such as a gas expander, which produces work that
may be extracted
and used for compression. Expanded refrigerant stream 118 is passed to a first
heat exchanger
120, and provides at least part of the refrigeration duty for first heat
exchanger 120. Upon
exiting first heat exchanger 120, expanded refrigerant stream 118 is fed to a
compression unit
122 for pressurization to form refrigerant stream 109.
[0014] Feed gas stream 106 flows through first heat exchanger 120 where it
is cooled, at
least in part, by indirect heat exchange with expanded refrigerant stream 118.
After exiting
first heat exchanger 120, the feed gas stream 106 is passed to a second heat
exchanger 124.
The principal function of second heat exchanger 124 is to sub-cool the feed
gas stream. Thus,
in second heat exchanger 124 the feed gas stream 106 is sub-cooled by sub-
cooling loop 104
(described below) to produce sub-cooled stream 126. Sub-cooled stream 126 is
then expanded
to a lower pressure in expander 128 to form a liquid fraction and a remaining
vapor fraction.
Expander 128 may be any pressure reducing device, including, but not limited
to a valve,
control valve, Joule Thompson valve, Venturi device, liquid expander,
hydraulic turbine, and
the like. The sub-cooled stream 126, which is now at a lower pressure and
partially liquefied,
is passed to a surge tank 130 where the liquefied fraction 132 is withdrawn
from the process as
an LNG stream 134, which has a temperature corresponding to the bubble point
pressure. The
remaining vapor fraction (flash vapor) stream 136 may be used as fuel to power
the compressor
units.
[0015] In sub-cooling loop 104, an expanded sub-cooling refrigerant stream
138
(preferably comprising nitrogen) is discharged from an expander 140 and drawn
through
second and first heat exchangers 124, 120. Expanded sub-cooling refrigerant
stream 138 is
then sent to a compression unit 142 where it is re-compressed to a higher
pressure and warmed.
After exiting compression unit 142, the re-compressed sub-cooling refrigerant
stream 144 is
cooled in a cooler 146, which can be of the same type as cooler 112, although
any type of cooler
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may be used. After cooling, the re-compressed sub-cooling refrigerant stream
is passed to first
heat exchanger 120 where it is further cooled by indirect heat exchange with
expanded
refrigerant stream 118 and expanded sub-cooling refrigerant stream 138. After
exiting first
heat exchanger 120, the re-compressed and cooled sub-cooling refrigerant
stream is expanded
through expander 140 to provide a cooled stream which is then passed through
second heat
exchanger 124 to sub-cool the portion of the feed gas stream to be finally
expanded to produce
LNG.
[0016] U. S. Patent Application U52010/0107684 disclosed an improvement to
the
performance of the HPXP through the discovery that adding external cooling to
further cool
io the compressed refrigerant to temperatures below ambient conditions
provides significant
advantages which in certain situations justifies the added equipment
associated with external
cooling. The HPXP embodiments described in the aforementioned patent
applications perform
comparably to alternative mixed external refrigerant LNG production processes
such as single
mixed refrigerant processes. However, there remains a need to further improve
the efficiency
is of the HPXP as well as overall train capacity. There remains a
particular need to improve the
efficiency of the HPXP in cases where the feed gas pressure is less than 1,200
psia.
[0017] U. S. Patent Application 2010/0186445 disclosed the incorporation
of feed
compression up to 4,500 psia to the HPXP. Compressing the feed gas prior to
liquefying the
gas in the HPXP's primary cooling loop has the advantage of increasing the
overall process
20 efficiency. For a given production rate, this also has the advantage of
significantly reducing
the required flow rate of the refrigerant within the primary cooling loop
which enables the use
of compact equipment, which is particularly attractive for floating LNG
applications.
Furthermore, feed compression provides a means of increasing the LNG
production of an
HPXP train by more than 30% for a fixed amount of power going to the primary
cooling and
25 sub-cooling loops. This flexibility in production rate is again
particularly attractive for floating
LNG applications where there are more restrictions than land based
applications in matching
the choice of refrigerant loop drivers with desired production rates. Although
liquefying the
feed gas at high pressures has advantages, it was found that for liquefaction
pressures greater
than 1,500 psia the choice of suitable cryogenic heat exchangers for the
primary cooling and
30 sub-cooling loops were limited to options significantly high in cost,
weight and with reduced
fluid processing capabilities. For example, the use of printed circuit heat
exchangers, which
are capable of operating at pressures greater than 4,500 psia, was shown to
significantly
increase project cost compared to the more widely sourced brazed aluminum heat
exchanger
type where proven operating pressures are less than 1,500 psia. This
significant increase in
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cost may limit the practical application of feed compression to up to 1,500
psia. Thus, there
remains a need to further improve the HPXP without requiring feed compression
or feed
compression greater the 1,500 psia. Additionally, there remains an additional
need to allow
the use of significant feed compression with HPXP without requiring the use of
high-cost main
cryogenic heat exchangers such as printed circuit heat exchangers.
SUMMARY
[0018] The present disclosure provides a method for liquefying a feed gas
stream rich in
methane using a system having first and second heat exchanger zones, where the
method
comprises the following steps: providing the feed gas stream at a pressure
less than 1,200 psia;
io providing a compressed refrigerant stream with a pressure greater than
or equal to 1,500 psia;
cooling the compressed refrigerant stream by indirect heat exchange with an
ambient
temperature air or water, to produce a compressed, cooled refrigerant stream;
directing the
compressed, cooled refrigerant stream to the second heat exchanger zone to
additionally cool
the compressed, cooled refrigerant stream below ambient temperature to produce
a
is compressed, additionally cooled refrigerant stream; expanding the
compressed, additionally
cooled refrigerant stream in at least one work producing expander, thereby
producing an
expanded, cooled refrigerant stream; passing the expanded, cooled refrigerant
stream through
the first heat exchanger zone to form a first warm refrigerant stream, wherein
the first warm
refrigerant stream has a temperature that is cooler, by at least 5 F, than
the highest fluid
20 temperature within the first heat exchanger zone; passing the feed gas
stream through the first
heat exchanger zone to cool at least part of the feed gas stream by indirect
heat exchange with
the expanded, cooled refrigerant stream, thereby forming a liquefied gas
stream; directing at
least a portion of the first warm refrigerant stream to the second heat
exchanger zone to cool
by indirect heat exchange the compressed, cooled refrigerant stream, thereby
forming a second
25 warm refrigerant stream; and compressing the second warm refrigerant
stream to produce the
compressed refrigerant stream.
[0019] The present disclosure also provides a system for liquefying a feed
gas stream rich
in methane, the system having first and second heat exchanger zones. A feed
gas stream at a
pressure less than 1,200 psia is provided. A compressed refrigerant stream is
provided with a
30 pressure greater than or equal to 1,500 psia. A cooler is configured to
cool the compressed
refrigerant stream by indirect heat exchange with an ambient temperature air
or water, to
produce a compressed, cooled refrigerant stream. At least one heat exchanger
is provided
within the second heat exchanger zone, the compressed, cooled refrigerant
stream being
directed to the at least one heat exchanger within the second heat exchanger
zone to additionally
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cool the compressed, cooled refrigerant stream below ambient temperature and
thereby
produce a compressed, additionally cooled refrigerant stream. At least one
work producing
expander is arranged to expand the compressed, additionally cooled refrigerant
stream, thereby
producing an expanded, cooled refrigerant stream. At least one heat exchanger
is provided
within the first heat exchanger zone. The expanded, cooled refrigerant stream
passes through
the at least one heat exchanger in the first heat exchanger zone to form a
first warm refrigerant
stream, wherein the first warm refrigerant stream has a temperature that is
cooler, by at least
5 F, than the highest fluid temperature within the first heat exchanger zone.
The feed gas
stream passes through the first heat exchanger zone to cool at least part of
the feed gas stream
to by indirect heat exchange with the expanded, cooled refrigerant stream,
thereby forming a
liquefied gas stream. At least a portion of the first warm refrigerant stream
is directed to the
second heat exchanger zone to cool by indirect heat exchange the compressed,
cooled
refrigerant stream, thereby forming a second warm refrigerant stream. A
compressor is
configured to compress the second warm refrigerant stream to produce the
compressed
is refrigerant stream.
[0020] The present disclosure also provides a method for liquefying a feed
gas stream rich
in methane, where the method comprises the following steps: providing the feed
gas stream at
a pressure less than 1,200 psia; compressing the feed gas stream to a pressure
of at least 1,500
psia to form a compressed gas stream; cooling the compressed gas stream by
indirect heat
20 exchange with an ambient temperature air or water, to form a cooled,
compressed gas stream;
expanding the cooled, compressed gas stream in at least one work producing
expander to a
pressure that is less than 2,000 psia and no greater than the pressure to
which the gas stream
was compressed, to thereby form a chilled gas stream; providing a compressed
refrigerant
stream with a pressure greater than or equal to 1,500 psia: cooling the
compressed refrigerant
25 stream by indirect heat exchange with an ambient temperature air or
water, to produce a
compressed, cooled refrigerant stream; directing the compressed, cooled
refrigerant stream to
a second heat exchanger zone, to additionally cool the compressed, cooled
refrigerant stream
below ambient temperature, to produce a compressed, additionally cooled
refrigerant stream;
expanding the compressed, additionally cooled refrigerant stream in at least
one work
30 producing expander, thereby producing an expanded, cooled refrigerant
stream; passing the
expanded, cooled refrigerant stream through a first heat exchanger zone to
form a first warm
refrigerant stream, whereby the first warm refrigerant stream has a
temperature that is cooler,
by at least 5 F, than the highest fluid temperature within the first heat
exchanger zone; passing
the chilled gas stream through the first heat exchanger zone to cool at least
part of the chilled
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gas stream by indirect heat exchange with the expanded, cooled refrigerant,
thereby forming a
liquefied gas stream; directing the first warm refrigerant stream to the
second heat exchanger
zone to cool by indirect heat exchange the compressed, cooled refrigerant
stream, thereby
forming a second warm refrigerant stream; and compressing the second warm
refrigerant
stream to produce the compressed refrigerant stream.
[0021] The disclosure also provides a system for liquefying a feed gas
stream rich in
methane and having a pressure less than 1,200 psia. The system comprises: a
compressor for
compressing the feed gas stream to a pressure of at least 1,500 psia, to form
a compressed gas
stream; a cooler for cooling the compressed gas stream by indirect heat
exchange with an
io ambient temperature air or water, to form a cooled, compressed gas
stream; at least one work
producing expander for expanding the cooled, compressed gas stream to a
pressure that is less
than 2,000 psia and no greater than the pressure to which the gas stream was
compressed, to
thereby form a chilled gas stream; a compressed refrigerant stream with a
pressure greater than
or equal to 1,500 psia; a refrigerant cooler for cooling the compressed
refrigerant stream by
is .. indirect heat exchange with an ambient temperature air or water, to
produce a compressed,
cooled refrigerant stream; a heat exchanger zone through which the compressed,
cooled
refrigerant stream is directed to be additionally cooled below ambient
temperature, to produce
a compressed, additionally cooled refrigerant stream; an additional work
producing expander
for expanding the compressed, additionally cooled refrigerant stream, thereby
producing an
20 expanded, cooled refrigerant stream; an additional heat exchanger zone
through which the
expanded, cooled refrigerant stream is passed, to thereby form a first warm
refrigerant stream,
whereby the warm refrigerant stream has a temperature that is cooler, by at
least 5 F, than the
highest fluid temperature within the first heat exchanger zone; wherein the
chilled gas stream
is passed through the additional heat exchanger zone to cool at least part of
the chilled gas
25 stream by indirect heat exchange with the expanded, cooled refrigerant,
thereby forming a
liquefied gas stream; wherein the first warm refrigerant stream is directed to
the heat exchanger
zone to cool by indirect heat exchange the compressed, cooled refrigerant
stream, thereby
forming a second warm refrigerant stream; and an additional compressor for
compressing the
second warm refrigerant stream to produce the compressed refrigerant stream.
30 [0022] The foregoing has broadly outlined the features of the
present disclosure so that the
detailed description that follows may be better understood. Additional
features will also be
described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] These and other features, aspects and advantages of the disclosure
will become
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apparent from the following description, appending claims and the accompanying
drawings,
which are briefly described below.
[0024] Figure 1 is a schematic diagram of a system for LNG production
according to
known principles.
[0025] Figure 2 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0026] Figure 3 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0027] Figure 4 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0028] Figure 5 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0029] Figure 6 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0030] Figure 7 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0031] Figure 8 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0032] Figure 9 is a schematic diagram of a system for LNG production
according to
disclosed aspects.
[0033] Figure 10 is a flowchart of a method according to aspects of the
disclosure.
[0034] Figure 11 is a flowchart of a method according to aspects of the
disclosure.
[0035] Figure 12 is a flowchart of a method according to aspects of the
disclosure.
[0036] It should be noted that the figures are merely examples and no
limitations on the
scope of the present disclosure are intended thereby. Further, the figures are
generally not
drawn to scale, but are drafted for purposes of convenience and clarity in
illustrating various
aspects of the disclosure.
DETAILED DESCRIPTION
[0037] To promote an understanding of the principles of the disclosure,
reference will now
be made to the features illustrated in the drawings and specific language will
be used to describe
the same. It will nevertheless be understood that no limitation of the scope
of the disclosure is
thereby intended. Any alterations and further modifications, and any further
applications of
the principles of the disclosure as described herein are contemplated as would
normally occur
to one skilled in the art to which the disclosure relates. For the sake of
clarity, some features
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not relevant to the present disclosure may not be shown in the drawings.
[0038] At the outset, for ease of reference, certain terms used in this
application and their
meanings as used in this context are set forth. To the extent a term used
herein is not defined
below, it should be given the broadest definition persons in the pertinent art
have given that
term as reflected in at least one printed publication or issued patent.
Further, the present
techniques are not limited by the usage of the terms shown below, as all
equivalents, synonyms,
new developments, and terms or techniques that serve the same or a similar
purpose are
considered to be within the scope of the present claims.
[0039] As one of ordinary skill would appreciate, different persons may
refer to the same
io feature or component by different names. This document does not intend to
distinguish
between components or features that differ in name only. The figures are not
necessarily to
scale. Certain features and components herein may be shown exaggerated in
scale or in
schematic form and some details of conventional elements may not be shown in
the interest of
clarity and conciseness. When referring to the figures described herein, the
same reference
is numerals may be referenced in multiple figures for the sake of
simplicity. In the following
description and in the claims, the terms "including- and "comprising" are used
in an open-
ended fashion, and thus, should be interpreted to mean "including, but not
limited to."
[0040] The articles "the," "a" and "an" are not necessarily limited to
mean only one, but
rather are inclusive and open ended so as to include, optionally, multiple
such elements.
20 [0041] As used herein, the terms "approximately," "about,"
"substantially,- and similar
terms are intended to have a broad meaning in harmony with the common and
accepted usage
by those of ordinary skill in the art to which the subject matter of this
disclosure pertains. It
should be understood by those of skill in the art who review this disclosure
that these terms are
intended to allow a description of certain features described and claimed
without restricting the
25 scope of these features to the precise numeral ranges provided.
Accordingly, these terms
should be interpreted as indicating that insubstantial or inconsequential
modifications or
alterations of the subject matter described and are considered to be within
the scope of the
disclosure. The term "near" is intended to mean within 2%, or within 5%, or
within 10%, of a
number or amount.
30 [0042] As used herein, the term "compression unit" means any one
type or combination of
similar or different types of compression equipment, and may include auxiliary
equipment,
known in the art for compressing a substance or mixture of substances. A
"compression unit"
may utilize one or more compression stages. Illustrative compressors may
include, but are not
limited to, positive displacement types, such as reciprocating and rotary
compressors for
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example, and dynamic types, such as centrifugal and axial flow compressors,
for example.
[0043] "Exemplary" is used exclusively herein to mean "serving as an
example, instance,
or illustration." Any embodiment or aspect described herein as "exemplary" is
not to be
construed as preferred or advantageous over other embodiments.
[0044] 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
distinguished from the gas or solid state.
[0045] As used herein, "heat exchange area" means any one type or
combination of similar
to .. or different types of equipment known in the art for facilitating heat
transfer. Thus, a -heat
exchange area" may be contained within a single piece of equipment, or it may
comprise areas
contained in a plurality of equipment pieces. Conversely, multiple heat
exchange areas may
be contained in a single piece of equipment.
[0046] A "hydrocarbon" is an organic compound that primarily includes the
elements
is hydrogen and carbon, although nitrogen, sulfur, oxygen, metals, or any
number of other
elements can be present in small amounts. As used herein, hydrocarbons
generally refer to
components found in natural gas, oil, or chemical processing facilities.
[0047] As used herein, the terms "loop" and "cycle" are used
interchangeably.
[0048] As used herein, -natural gas" means a gaseous feedstock suitable
for manufacturing
20 LNG, where the feedstock is a methane-rich gas containing methane (Cl) as a
major
component. Natural gas may include gas obtained from a crude oil well
(associated gas) or
from a gas well (non-associated gas).
[0049] The disclosure describes a process/method and system for
liquefying natural gas
and other methane-rich gas streams to produce liquefied natural gas (LNG)
and/or other
25 .. liquefied methane-rich gases. In one or more aspects of the disclosure,
the primary cooling loop
is segmented into two heat exchanger zones. Within the first heat exchanger
zone, the primary
cooling loop refrigerant is used to liquefy the feed gas. Within the second
heat exchanger zone,
all or a portion of the primary cooling loop refrigerant is used to cool the
high pressure primary
cooling loop refrigerant prior to expansion of the refrigerant. The first heat
exchanger zone is
30 physically separate from second heat exchanger zone. Additionally, the
heat exchanger type
of the first heat exchanger zone is different from the heat exchanger type of
the second heat
exchanger zone. One advantage of having two separate heat exchanger zones is
that the types
of heat exchangers in the two zones can be different from each other. As a non-
limiting
example, the type of heat exchanger(s) used in the first exchanger zone may
include a brazed
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aluminum heat exchanger, and the type of heat exchanger(s) used in the second
heat exchanger
zone may be include a printed circuit heat exchanger. It is in the first
exchanger zone where
more the 90% of the heat transfer needed to liquefy the feed gas occurs. Using
the less
expensive brazed aluminum heat exchanger here reduces project cost. The
significantly more
expensive printed circuit heat exchanger may be used in the second heat
exchanger zone
because it can operate at the required 3,000 psia pressure of the high
pressure refrigerant. The
use of a printed circuit heat exchanger in the second heat exchanger zone does
not significantly
impact overall project cost since it is a relatively small heat exchanger.
This is because the
heat transfer duty within the second heat exchanger zone is significantly
smaller than that of
it) the first heat exchanger zone. Both heat exchanger zones may comprise
multiple heat
exchangers.
[0050] In an aspect, a method for liquefying a gas stream, particularly
one rich in methane,
includes: (a) providing the gas stream at a pressure less than 1,200 psia; (b)
providing a
compressed refrigerant with a pressure greater than or equal to 1,500 psia;
(c) cooling the
is compressed refrigerant by indirect heat exchange with an ambient
temperature air or water to
produce a compressed, cooled refrigerant; (d) directing the compressed, cooled
refrigerant to a
second heat exchanger zone to additionally cool the compressed, cooled
refrigerant below
ambient temperature to produce a compressed, additionally cooled refrigerant:
(e) expanding
the compressed, additionally cooled refrigerant in at least one work producing
expander
20 thereby producing an expanded, cooled refrigerant; (f) passing the
expanded, cooled refrigerant
through a first heat exchanger zone to form a first warm refrigerant, whereby
the first warm
refrigerant has a temperature that is cooler, by at least 5 F, than the
highest fluid temperature
within the first heat exchanger zone, and whereby the heat exchanger type of
the first heat
exchanger zone is different from the heat exchanger type of the second heat
exchanger zone;
25 (g) passing the gas stream through the first heat exchanger zone to cool
at least part of the gas
stream by indirect heat exchange with the expanded, cooled refrigerant,
thereby forming a
liquefied gas stream; (h) directing at least a portion of the first warm
refrigerant to the second
heat exchanger zone to cool by indirect heat exchange the compressed, cooled
refrigerant
thereby forming a second warm refrigerant; and (i) compressing the second warm
refrigerant
30 to produce the compressed refrigerant.
[0051] In another aspect, a method for liquefying a gas stream includes:
(a) providing the
gas stream at a pressure less than 1,200 psia; (b) compressing the gas stream
to a pressure of at
least 1,500 psia to form a compressed gas stream; (c) cooling the compressed
gas stream by
indirect heat exchange with an ambient temperature air or water to form a
compressed, cooled
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gas stream; (d) expanding the compressed, cooled gas stream in at least one
work producing
expander to a pressure that is less than 2,000 psia and no greater than the
pressure to which the
gas stream was compressed, to thereby form a chilled gas stream; (e) providing
a compressed
refrigerant with a pressure greater than or equal to 1,500 psia (f) cooling
the compressed
refrigerant by indirect heat exchange with an ambient temperature air or water
to produce a
compressed, cooled refrigerant (g) directing the compressed, cooled
refrigerant to a second
heat exchanger zone to additionally cool the compressed, cooled refrigerant
below ambient
temperature to produce a compressed, additionally cooled refrigerant; (h)
expanding the
compressed, additionally cooled refrigerant in at least one work producing
expander thereby
producing an expanded, cooled refrigerant; (i) passing the expanded, cooled
refrigerant through
a first heat exchanger zone to form a first warm refrigerant, whereby the
first warm refrigerant
has a temperature that is cooler, by at least 5 F, than the highest fluid
temperature within the
first heat exchanger zone, and whereby the heat exchanger type of the first
heat exchanger zone
is different from the heat exchanger type of the second heat exchanger zone;
(j) passing the
is chilled gas stream through the first heat exchanger zone to cool at
least part of the chilled gas
stream by indirect heat exchange with the expanded, cooled refrigerant,
thereby forming a
liquefied gas stream; (k) directing the first warm refrigerant to the second
heat exchanger zone
to cool by indirect heat exchange the compressed, cooled refrigerant, thereby
forming a second
warm refrigerant; and (1) compressing the second warm refrigerant to produce
the compressed
refrigerant.
[0052] In another aspect, a method for liquefying a gas stream includes:
(a) providing the
gas stream at a pressure less than 1,200 psia; (b) providing a refrigerant
stream at near the same
pressure of the gas stream; (c) mixing the gas stream with the refrigerant
stream to form a
second gas stream; (d) compressing the second gas stream to a pressure of at
least 1,500 psia
to form a compressed second gas stream; (e) cooling the compressed second gas
stream by
indirect heat exchange with an ambient temperature air or water to form a
compressed, cooled
second gas stream; (f) directing the compressed, cooled second gas stream to a
second heat
exchanger zone to additionally cool the compressed, cooled second gas stream
below ambient
temperature to produce a compressed, additionally cooled second gas stream;
(g) expanding
the compressed, additionally cooled second gas stream in at least one work
producing expander
to a pressure that is less than 2,000 psia and no greater than the pressure to
which the second
gas stream was compressed, to thereby form an expanded, cooled second gas
stream; (h)
separating the expanded, cooled second gas stream into a first expanded
refrigerant and a
chilled gas stream; (i) expanding the first expanded refrigerant in at least
one work producing
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expander, thereby producing a second expanded refrigerant; (j) passing the
second expanded
refrigerant through a first heat exchanger zone to form a first warm
refrigerant, whereby the
first warm refrigerant has a temperature that is cooler, by at least 5 F,
than the highest fluid
temperature within the first heat exchanger zone, and whereby the heat
exchanger type of the
first heat exchanger zone is different from the heat exchanger type of the
second heat exchanger
zone; (k) passing the chilled gas stream through the first heat exchanger zone
to cool at least
part of the chilled gas stream by indirect heat exchange with the second
expanded refrigerant,
thereby forming a liquefied gas stream; (1) directing the first warm
refrigerant to the second
heat exchanger zone to cool by indirect heat exchange the compressed, cooled
second gas
it) stream, thereby forming a second warm refrigerant; and (m) compressing
the second warm
refrigerant to produce the refrigerant stream.
[0053] Aspects of the disclosure may include the additional steps of
compressing the gas
stream to a pressure no greater than 1,600 psia and then cooling the
compressed gas stream by
indirect heat exchange with an ambient temperature air or water prior to
directing the gas
is stream to the first heat exchanger zone. Aspects of the disclosure may
also include the
additional steps of cooling the gas stream to a temperature below the ambient
by indirect heat
exchange within an external cooling unit prior to directing the gas stream to
the first heat
exchanger zone. Aspects of the disclosure may also include the additional
steps of cooling the
compressed, cooled refrigerant to a temperature below the ambient temperature
by indirect heat
20 exchange with an external cooling unit prior to directing the
compressed, cooled refrigerant to
the second heat exchanger zone. These described additional steps may be
employed singularly
or in combination with each other.
[0054] Aspects of the disclosure have several advantages over the known
liquefaction
processes, in which feed compression is required to significantly improve the
efficiency of the
25 HPXP. In contrast, the efficiency of the disclosed aspects is more than
16% greater than the
efficiency for a comparable configuration according to known liquefaction
processes. Aspects
of the disclosure may have the additional advantage of allowing significant
feed compression
(greater than 1,500 psia) without requiring the use of high cost main
cryogenic heat exchangers
for the first heat exchanger zone. Feed compression by the disclosed method
may provide a
30 means of increasing the LNG production of an HPXP train by more than 25%
for a fixed
amount of power going to the primary cooling and sub-cooling loops. Aspects of
the disclosure
may also have the advantage of combining the compression service of the feed
gas and some
of that of the primary cooling loop to reduce equipment count. Such an
embodiment provides
a highly efficient and compact configuration suitable for small scale LNG
applications.
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[0055] Figure 2 is a schematic diagram that illustrates a liquefaction
system 200 according
to an aspect of the disclosure. The liquefaction system 200 includes a primary
cooling loop
202, which may also be called an expander loop. The liquefaction system also
includes a sub-
cooling loop 204, which is a closed refrigeration loop preferably charged with
nitrogen as the
sub-cooling refrigerant. Within the primary cooling loop 202, an expanded,
cooled refrigerant
stream 205 is directed to a first heat exchanger zone 201 where it exchanges
heat with a feed
gas stream 206 to form a first warm refrigerant stream 208. A portion of the
first warm
refrigerant 208 is directed to a second heat exchanger zone 210 where, in one
or more heat
exchangers 210a, it exchanges heat with a compressed, cooled refrigerant
stream 212 to
to additionally cool the compressed, cooled refrigerant stream and form a
second warm refrigerant
stream 209 and a compressed, additionally cooled refrigerant stream 213. The
one or more
heat exchangers 210a may be of a printed circuit heat exchanger type, a shell
and tube heat
exchanger type, or a combination thereof The heat exchanger types within the
second heat
exchanger zone may have a design pressure of greater than 1,500 psia, or more
preferably, a
design pressure of greater than 2,000 psia, or more preferably, a design
pressure of greater than
3,000 psia.
[0056] The portion of the first warm refrigerant stream 208 directed to
the second heat
exchanger zone 210 has a temperature that is cooler by at least 5 F, or more
preferably, cooler
by at least 10 F, or more preferably, cooler by at least 15 F, than the
highest fluid temperature
within the first heat exchanger zone 201. The portion of the first warm
refrigerant stream 208
that may remain within the first heat exchanger zone (as shown by reference
number 208a)
further exchanges heat with the feed gas stream to form a third warm
refrigerant stream 214.
The second warm refrigerant stream 209 from the second heat exchanger zone 210
may be
combined with the third warm refrigerant stream 214 from the first heat
exchanger zone 201 to
produce a fourth warm refrigerant stream 216. The fourth warm refrigerant
stream is
compressed in one or more compression units 218, 220 to a pressure greater
than 1,500 psia,
or more preferably, to a pressure of approximately 3,000 psia, to form a
compressed refrigerant
stream 222. The compressed refrigerant stream 222 is then cooled against an
ambient cooling
medium (air or water) in a cooler 224 to produce the compressed, cooled
refrigerant stream
212. Cooler 224 may be similar to cooler 112 as previously described. The
compressed,
additionally cooled refrigerant stream 213 is near isentropically expanded in
an expander 226
to produce the expanded, cooled refrigerant stream 205. Expander 226 may be a
work-
expansion device, such as a gas expander, which produces work that may be
extracted and used
for compression.
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[0057] The first heat exchanger zone 201 may include a plurality of heat
exchanger devices,
and in the aspects shown in Figure 2, the first heat exchanger zone includes
first and second
main heat exchangers 232, 234, and a sub-cooling heat exchanger 236 exchange
heat with the
expanded, cooled refrigerant 205. These heat exchangers may be of a brazed
aluminum heat
.. exchanger type, a plate fin heat exchanger type, a spiral wound heat
exchanger type, or a
combination thereof Within the sub-cooling loop 204, an expanded sub-cooling
refrigerant
stream 238 (preferably comprising nitrogen) is discharged from an expander 240
and drawn
through sub-cooling heat exchanger 236 and second and first main heat
exchangers 234, 232.
Expanded sub-cooling refrigerant stream 238 is then sent to a compression unit
242 where it is
tti re-compressed to a higher pressure and warmed. After exiting
compression unit 242, the re-
compressed sub-cooling refrigerant stream 244 is cooled in a cooler 246, which
can be of the
same type as cooler 224, although any type of cooler may be used. After
cooling, the re-
compressed sub-cooling refrigerant stream is passed through first and second
main heat
exchangers 232, 234 where it is further cooled by indirect heat exchange with
part or all of the
warm refrigerant stream 208 and expanded sub-cooling refrigerant stream 238.
After exiting
first heat exchange area 201, the re-compressed and cooled sub-cooling
refrigerant stream is
expanded through expander 240 to provide the expanded sub-cooled refrigerant
stream 238
that is re-cycled through the first heat exchanger zone as described herein.
In this manner, the
feed gas stream 206 is cooled, liquefied and sub-cooled in the first heat
exchanger zone 201 to
produce a sub-cooled gas stream 248. Sub-cooled gas stream 248 is then
expanded to a lower
pressure in expander 250 to form a liquid fraction and a remaining vapor
fraction. Expander
250 may be any pressure reducing device, including but not limited to a valve,
control valve,
Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and
the like. The
sub-cooled stream 248, which is now at a lower pressure and partially
liquefied, is passed to a
surge tank 252 where the liquefied fraction 254 is withdrawn from the process
as an LNG
stream 256, which has a temperature corresponding to the bubble point
pressure. The
remaining vapor fraction (flash vapor) stream 258 may be used as fuel to power
the compressor
units.
[0058] Figure 3 is a schematic diagram that illustrates a liquefaction
system 300 according
to another aspect of the disclosure. Liquefaction system 300 is similar to
liquefaction system
200 and for the sake of brevity similarly depicted or numbered components may
not be further
described. Liquefaction system 300 includes a primary cooling loop 302 and a
sub-cooling
loop 304. Liquefaction system 300 also includes first and second heat
exchanger zones 301,
310. In contrast with liquefaction system 200, all of the first waiin
refrigerant 308 is directed
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to the second heat exchanger zone 310 where, in one or more heat exchangers
310a, it
exchanges heat with a compressed, cooled refrigerant stream 312 to form a
second warm
refrigerant 309.
[0059] The first wami refrigerant stream 308 has a temperature that is
cooler by at least
5 F, or more preferably, cooler by at least 10 F, or more preferably, cooler
by at least 15 F,
than the highest fluid temperature within the first heat exchanger zone. The
second warm
refrigerant stream 309 may be compressed in one or more compressors 318, 320
to a pressure
greater than 1,500 psia, or more preferably, to a pressure of approximately
3,000 psia, to
thereby form a compressed refrigerant stream 322. The compressed refrigerant
stream 322 is
then cooled against an ambient cooling medium (air or water) to produce the
compressed,
cooled refrigerant stream 312 that is directed to the second heat exchanger
zone 310. The
compressed, additionally cooled refrigerant stream 313 is near isentropically
expanded in an
expander 326 to produce the expanded, cooled refrigerant stream 305.
[0060] The feed gas stream 306 is directed through the first heat exchange
area 301 that
is includes a main heat exchanger 332 and a sub-cooling heat exchanger 336.
The number of
main heat exchangers in first heat exchanger zone 301 may be reduced since all
of the first
warm refrigerant 308 is directed to the second heat exchanger zone 310. Within
the sub-cooling
loop 304, an expanded sub-cooling refrigerant stream 338 (preferably
comprising nitrogen) is
discharged from an expander 340 and drawn through sub-cooling heat exchanger
336 and main
heat exchanger 332. Expanded sub-cooling refrigerant stream 338 is then sent
to a compression
unit 342 where it is re-compressed to a higher pressure and warmed. After
exiting compression
unit 342, the re-compressed sub-cooling refrigerant stream 344 is cooled in a
cooler 346, which
can be of the same type as cooler 324, although any type of cooler may be
used. After cooling,
the re-compressed sub-cooling refrigerant stream is passed through main heat
exchanger 232
where it is further cooled by indirect heat exchange with part or all of the
expanded, cooled
refrigerant stream 305 and expanded sub-cooling refrigerant stream 338. After
exiting first
heat exchange area 301, the re-compressed and cooled sub-cooling refrigerant
stream is
expanded through expander 340 to provide the expanded sub-cooled refrigerant
stream 338
that is re-cycled through the first heat exchange area as described herein. In
this manner, the
feed gas stream 306 is cooled, liquefied and sub-cooled in the first heat
exchanger zone 301 to
produce a sub-cooled gas stream 348. Sub-cooled gas stream 348 is then
expanded to a lower
pressure in expander 350 to form a liquid fraction and a remaining vapor
fraction. Expander
350 may be any pressure reducing device, including but not limited to a valve,
control valve,
Joule Thompson valve, Venturi device, liquid expander, hydraulic turbine, and
the like. The
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sub-cooled stream 348, which is now at a lower pressure and partially
liquefied, is passed to a
surge tank 352 where the liquefied fraction 354 is withdrawn from the process
as an LNG
stream 356, which has a temperature corresponding to the bubble point
pressure. The
remaining vapor fraction (flash vapor) stream 358 may be used as fuel to power
the compressor
units.
[0061] Figure 4 is a schematic diagram that illustrates a liquefaction
system 400 according
to another aspect of the disclosure. Liquefaction system 400 is similar to
liquefaction system
200, and for the sake of brevity similarly depicted or numbered components may
not be further
described. Liquefaction system 400 includes a primary cooling loop 402 and a
sub-cooling
io loop 404. Liquefaction system 400 also includes first and second heat
exchanger zones 401,
410. In liquefaction system 400, the sub-cooling loop 404 is an open
refrigeration loop where
a portion 449 of the expanded, sub-cooled gas stream 448 is recycled and used
as the sub-
cooling refrigerant stream. Specifically, the portion 449 of the expanded, sub-
cooled gas
stream is directed through the first heat exchanger zone 401 as previously
described before
is being compressed in a compressor 442, cooled in a cooler 446, and re-
inserted into the feed
gas stream 406. This sub-cooling refrigerant stream may be one stream, as
shown, or may
comprise multiple streams at different pressures: for example, a portion of
the expanded, sub-
cooling gas stream - not to exceed 50% thereof¨ may be diverted and pass
through one or more
pressure reduction valves to reduce its pressure to a range of about 30 to 300
psia, to thereby
20 produce one or more reduced pressure gas streams. The reduced pressure
gas streams may
then be passed through the first heat exchanger zone as the sub-cooling
refrigerant. Having
multiple streams improves the efficiency of the sub-cooling process.
Alternatively, this sub-
cooling loop may be configured to be a closed refrigeration loop.
[0062] Figure 5 is a schematic diagram that illustrates a liquefaction
system 500 according
25 to another aspect of the disclosure. Liquefaction system 500 is similar
to liquefaction system
200 and for the sake of brevity similarly depicted or numbered components may
not be further
described. Liquefaction system 500 includes a primary cooling loop 502 and a
sub-cooling
loop 504. Liquefaction system 500 also includes first and second heat
exchanger zones 501,
510. Liquefaction system 500 stream includes the additional steps of
compressing the feed gas
30 stream 506 in a compressor 560 and then, using a cooler 562, cooling the
compressed feed gas
561 with ambient air or water to produce a cooled, compressed feed gas stream
563. Feed gas
compression may be used to improve the overall efficiency of the liquefaction
process and
increase LNG production.
[0063] Figure 6 is a schematic diagram that illustrates a liquefaction
system 600 according
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to still another aspect of the disclosure. Liquefaction system 600 is similar
to liquefaction
system 300 and for the sake of brevity similarly depicted or numbered
components may not be
further described. Liquefaction system 600 includes a primary cooling loop 602
and a sub-
cooling loop 604. Liquefaction system 600 also includes first and second heat
exchanger zones
601, 610. Liquefaction system 600 includes the additional step of chilling, in
an external
cooling unit 665, the feed gas stream 606 to a temperature below the ambient
temperature to
produce a chilled gas stream 667. The chilled gas stream 667 is then directed
to the first heat
exchanger zone 601 as previously described. Chilling the feed gas as shown in
Figure 6 may
be used to improve the overall efficiency of the liquefaction process and
increase LNG
to production.
[0064] Figure 7 is a schematic diagram that illustrates a liquefaction
system 700 according
to another aspect of the disclosure. Liquefaction system 700 is similar to
liquefaction system
200 and for the sake of brevity similarly depicted or numbered components may
not be further
described. Liquefaction system 700 includes a primary cooling loop 702 and a
sub-cooling
loop 704. Liquefaction system 700 also includes first and second heat
exchanger zones 701,
710. Liquefaction system 700 includes the additional step of chilling, using
an external cooling
unit 770, the compressed, cooled refrigerant 712 in the primary cooling loop
702 to a
temperature below the ambient temperature, to thereby produce a compressed,
chilled
refrigerant 772. The compressed, chilled refrigerant 772 is then directed to
the second heat
exchanger zone 710 as previously described. Using an external cooling unit to
further cool the
compressed, cool refrigerant may be used to improve the overall efficiency of
the process and
increase LNG production.
[0065] Figure 8 is a schematic diagram that illustrates a liquefaction
system 800 according
to another aspect of the disclosure. Liquefaction system 800 is similar to
liquefaction system
300 and for the sake of brevity similarly depicted or numbered components may
not be further
described. Liquefaction system 800 includes a primary cooling loop 802 and a
sub-cooling
loop 804. Liquefaction system 800 also includes first and second heat
exchanger zones 801,
810. In liquefaction system 800, the feed gas stream 806 is compressed in a
compressor 860
to a pressure of at least 1,500 psia to form a compressed gas stream 861.
Using an external
cooling unit 862, the compressed gas stream 861 is cooled by indirect heat
exchange with an
ambient temperature air or water to form a compressed, cooled gas stream 863.
The
compressed, cooled gas stream 863 is expanded in at least one work producing
expander 874
to a pressure that is less than 2,000 psia but no greater than the pressure to
which the gas stream
was compressed, to thereby form a chilled gas stream 876. The chilled gas
stream 876 is then
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directed to the first heat exchanger zone 801 where a primary cooling
refrigerant and a sub-
cooling refrigerant are used to liquefy the chilled gas stream as previously
described.
[0066] The sub-cooling loop 804 is a closed refrigeration loop preferably
charged with
nitrogen as the sub-cooling refrigerant stream. Within the primary cooling
loop 802, an
expanded, cooled refrigerant stream 805 is directed to the first heat
exchanger zone 801 where
it exchanges heat with the chilled gas stream 876 to form a first warm
refrigerant stream 808.
The first warm refrigerant stream 808 is directed to the second heat exchanger
zone 810 where
it exchanges heat with a compressed, cooled refrigerant stream 825 to
additionally cool the
compressed, cooled refrigerant stream 825, thereby forming a second warm
refrigerant stream
it) 809 and a compressed, additionally cooled refrigerant stream 813. The
first warm refrigerant
stream 808 has a temperature that is cooler by at least 5 F, or more
preferably, cooler by at
least 10 F, or more preferably, cooler by at least 15 F, than the highest
fluid temperature
within the first heat exchanger zone 801. Using one or more compressors 818,
820, the second
warm refrigerant stream 809 is compressed to a pressure greater than 1,500
psia, or more
is preferably, to a pressure of approximately 3,000 psia, to form a
compressed refrigerant stream
822. The compressed refrigerant stream 822 is then cooled against an ambient
cooling medium
(air or water) in an external cooling unit 824 to produce the compressed,
cooled refrigerant
stream 825. After being directed through the second heat exchanger area 810,
the compressed,
additionally cooled refrigerant stream is near isentropically expanded in an
expander 826 to
20 produce the expanded, cooled refrigerant 805. The chilled gas stream 876
is liquefied and sub-
cooled in the first heat exchanger zone to produce a sub-cooled gas stream
848, which is further
processed as previously disclosed.
[0067] Figure 9 is a schematic diagram that illustrates a liquefaction
system 900 according
to yet another aspect of the disclosure. Liquefaction system 900 contains
similar structure and
25 components with previously disclosed liquefaction systems and for the
sake of brevity similarly
depicted or numbered components may not be further described. Liquefaction
system 900
includes a primary cooling loop 902 and a sub-cooling loop 904. Liquefaction
system 900 also
includes first and second heat exchanger zones 901, 910. In liquefaction
system 900, the feed
gas stream 906 is mixed with a refrigerant stream 907 to produce a second feed
gas stream
30 906a. Using a compressor 960, the second feed gas stream 906a is
compressed to a pressure
greater than 1,500 psia, or more preferably, to a pressure of approximately
3,000 psia, to form
a compressed second gas stream 961. Using an external cooling unit 962, the
compressed
second gas stream 961 is then cooled against an ambient cooling medium (air or
water) to
produce a compressed, cooled second gas stream 963. The compressed, cooled
second gas
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stream 963 is directed to the second heat exchanger zone 910 where it
exchanges heat with a
first warm refrigerant stream 908. to produce a compressed, additionally
cooled second gas
stream 913 and a second warm refrigerant stream 909.
[0068] The compressed, additionally cooled second gas stream 913 is
expanded in at least
.. one work producing expander 926 to a pressure that is less than 2,000 psia,
but no greater than
the pressure to which the second gas stream 906a was compressed, to thereby
form an
expanded, cooled second gas stream 980. The expanded, cooled second gas stream
980 is
separated into a first expanded refrigerant stream 905 and a chilled feed gas
stream 906b. The
first expanded refrigerant stream 905 may be near isentropically expanded
using an expander
to 982 to form a second expanded refrigerant stream 905a. The chilled feed
gas stream 906b is
directed to the first heat exchanger zone 901 where a primary cooling
refrigerant (i.e., the
second expanded refrigerant stream 905a) and a sub-cooling refrigerant (from
the sub-cooling
loop 904) are used to liquefy the chilled gas stream 906b. The sub-cooling
loop 904 may be a
closed refrigeration loop, preferably charged with nitrogen as the sub-cooling
refrigerant.
is Within the primary cooling loop 902, the second expanded refrigerant
stream 905a is directed
to the first heat exchanger zone 901 where it exchanges heat with the chilled
feed gas stream
906b to form the first warm refrigerant stream 908. The first warm refrigerant
stream 908 may
have a temperature that is cooler by at least 5 F, or more preferably, cooler
by at least 10 F,
or more preferably, cooler by at least 15 F, than the highest fluid
temperature within the first
20 heat exchanger zone 901. The second warm refrigerant stream 909 is
compressed in one or
more compressors 918 and then cooled with an ambient cooling medium in an
external cooling
device 924 to produce the refrigerant stream 907. The chilled feed gas stream
906b is liquefied
and sub-cooled in the first heat exchanger zone 901 to produce a sub-cooled
gas stream 948,
which is processed as previously described to form LNG.
25 [0069] Aspects of the disclosure illustrated in Figure 9 demonstrate
that the primary
refrigerant stream may comprise part of the feed gas stream, which in a
preferred aspect may
be primarily or nearly all methane. Indeed, it may be advantageous for the
refrigerant in the
primary cooling loop of all the disclosed aspects (i.e., Figures 2 through 9)
be comprised of at
least 85% methane, or at least 90% methane, or at least 95% methane, or
greater than 95%
30 methane. This is because methane may be readily available in various
parts of the disclosed
processes, and the use of methane may eliminate the need to transport
refrigerants to remote
LNG processing locations. As a non-limiting example, the refrigerant in the
primary cooling
loop 202 in Figure 2 may be taken through line 206a of the feed gas stream 206
if the feed gas
is high enough in methane to meet the compositions as described above.
Alternatively, part or
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all of a boil-off gas stream 259 from an LNG storage tank 257 may be used to
supply refrigerant
for the primary cooling loop 202. Furthermore, if the feed gas stream is
sufficiently low in
nitrogen, part or all of the end flash gas stream 258 (which would then be low
in nitrogen) may
be used to supply refrigerant for the primary cooling loop 202. Lastly, any
combination of line
206a, boil-off gas stream 259, and end flash gas stream 258 may be used to
provide or even
occasionally replenish the refrigerant in the primary cooling loop 202.
[0070] Figure 10 is a flowchart of a method 1000 for liquefying a feed gas
stream rich in
methane using a system having first and second heat exchanger zones, where the
method
comprises the following steps: 1002, providing the feed gas stream at a
pressure less than 1,200
psia; 1004, providing a compressed refrigerant stream with a pressure greater
than or equal to
1,500 psia; 1006, cooling the compressed refrigerant stream by indirect heat
exchange with an
ambient temperature air or water, to produce a compressed, cooled refrigerant
stream; 1008,
directing the compressed, cooled refrigerant stream to the second heat
exchanger zone to
additionally cool the compressed, cooled refrigerant stream below ambient
temperature to
is produce a compressed, additionally cooled refrigerant stream; 1010,
expanding the
compressed, additionally cooled refrigerant stream in at least one work
producing expander,
thereby producing an expanded, cooled refrigerant stream; 1012, passing the
expanded, cooled
refrigerant stream through the first heat exchanger zone to form a first warm
refrigerant stream,
wherein the first warm refrigerant stream has a temperature that is cooler, by
at least 5 F, than
the highest fluid temperature within the first heat exchanger zone; 1014,
passing the feed gas
stream through the first heat exchanger zone to cool at least part of the feed
gas stream by
indirect heat exchange with the expanded, cooled refrigerant stream, thereby
forming a
liquefied gas stream; 1016 directing at least a portion of the first warm
refrigerant stream to the
second heat exchanger zone to cool by indirect heat exchange the compressed,
cooled
refrigerant stream, thereby forming a second warm refrigerant stream; and
1018, compressing
the second warm refrigerant stream to produce the compressed refrigerant
stream.
[0071] Figure 11 is a flowchart of a method 1100 for liquefying a feed gas
stream rich in
methane, where the method comprises the following steps: 1102, providing the
feed gas stream
at a pressure less than 1,200 psia; 1104, compressing the feed gas stream to a
pressure of at
least 1,500 psia to form a compressed gas stream; 1106, cooling the compressed
gas stream by
indirect heat exchange with an ambient temperature air or water, to form a
cooled, compressed
gas stream; 1108, expanding the cooled, compressed gas stream in at least one
work producing
expander to a pressure that is less than 2,000 psia and no greater than the
pressure to which the
gas stream was compressed, to thereby form a chilled gas stream; 1110,
providing a compressed
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WO 2019/067123 PCT/US2018/047955
refrigerant stream with a pressure greater than or equal to 1,500 psia; 1112,
cooling the
compressed refrigerant stream by indirect heat exchange with an ambient
temperature air or
water, to produce a compressed, cooled refrigerant stream; 1114, directing the
compressed,
cooled refrigerant stream to a second heat exchanger zone, to additionally
cool the compressed,
cooled refrigerant stream below ambient temperature, to produce a compressed,
additionally
cooled refrigerant stream; 1116, expanding the compressed, additionally cooled
refrigerant
stream in at least one work producing expander, thereby producing an expanded,
cooled
refrigerant stream; 1118, passing the expanded, cooled refrigerant stream
through a first heat
exchanger zone to form a first warm refrigerant stream, whereby the first warm
refrigerant
stream has a temperature that is cooler, by at least 5 F, than the highest
fluid temperature
within the first heat exchanger zone; 1120, passing the chilled gas stream
through the first heat
exchanger zone to cool at least part of the chilled gas stream by indirect
heat exchange with the
expanded, cooled refrigerant, thereby forming a liquefied gas stream; 1122,
directing the first
warm refrigerant stream to the second heat exchanger zone to cool by indirect
heat exchange
is the compressed, cooled refrigerant stream, thereby forming a second warm
refrigerant stream;
and 1124, compressing the second warm refrigerant stream to produce the
compressed
refrigerant stream.
[0072] Figure 12 is a method 1200 for liquefying a feed gas stream rich in
methane, where
the method comprises the following steps: 1202, providing the feed gas stream
at a pressure
less than 1,200 psia; 1204, providing a refrigerant stream at near the same
pressure of the feed
gas stream; 1206, mixing the feed gas stream with the refrigerant stream to
form a second gas
stream; 1208, compressing the second gas stream to a pressure of at least
1,500 psia to form a
compressed second gas stream; 1210, cooling the compressed second gas stream
by indirect
heat exchange with ambient temperature air or water, to form a compressed,
cooled second gas
stream; 1212, directing the compressed, cooled second gas stream to a second
heat exchanger
zone, to additionally cool the compressed, cooled second gas stream below
ambient
temperature, thereby producing a compressed, additionally cooled second gas
stream; 1214,
expanding the compressed, additionally cooled second gas stream in at least
one work
producing expander to a pressure that is less than 2,000 psia and no greater
than the pressure
to which the second gas stream was compressed, to thereby form an expanded,
cooled second
gas stream; 1216, separating the expanded, cooled second gas stream into a
first expanded
refrigerant stream and a chilled gas stream; 1218, expanding the first
expanded refrigerant
stream in at least one work producing expander, thereby producing a second
expanded
refrigerant stream; 1220, passing the second expanded refrigerant stream
through a first heat
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WO 2019/067123 PCT/US2018/047955
exchanger zone to form a first warm refrigerant stream such that the first
warm refrigerant
stream has a temperature that is cooler, by at least 5 F, than the highest
fluid temperature
within the first heat exchanger zone; 1222, passing the chilled gas stream
through the first heat
exchanger zone to cool at least part of the chilled gas stream by indirect
heat exchange with the
second expanded refrigerant stream, thereby forming a liquefied gas stream;
1224, directing
the first warm refrigerant stream to the second heat exchanger zone to cool by
indirect heat
exchange the compressed, cooled second gas stream, thereby forming a second
warm
refrigerant stream; and 1226, compressing the second warm refrigerant stream
to produce the
refrigerant stream.
to [0073] The steps depicted in Figures 10-12 are provided for
illustrative purposes only and
a particular step may not be required to perform the disclosed methodology.
Moreover, Figures
10-12 may not illustrate all the steps that may be performed. The claims, and
only the claims,
define the disclosed system and methodology.
[0074] The aspects described herein have several advantages over known
technologies.
is For example, the described technology may greatly reduce the size and
cost of systems that
treat sour natural gas.
[0075] It should be understood that the numerous changes, modifications,
and alternatives
to the preceding disclosure can be made without departing from the scope of
the disclosure.
The preceding description, therefore, is not meant to limit the scope of the
disclosure. Rather,
20 the scope of the disclosure is to be determined only by the appended
claims and their
equivalents. It is also contemplated that structures and features in the
present examples can be
altered, rearranged, substituted, deleted, duplicated, combined, or added to
each other.
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