Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONFIGURATIONS AND METHODS OF HEATING VALUE CONTROL IN LNG
LIQUEFACTION PLANT
Field of The Invention
The field of the invention is natural gas liquids (NGL) recovery and liquefied
natural gas (LNG)
liquefaction for heating value control for LNG export, and particularly
integrated plant
configurations of such processes to existing LNG plants.
Background of The Invention
With the rapid increase of LNG regasification facilities in Europe and North
America, LNG traders
are directing their export focus to these countries in addition to various
Asian countries such as
Japan, Korea, and China. While most Asian countries prefer a high Btu content
natural gas, North
American pipeline specification restricts the import to low Btu value content
gases, for emission
control reasons. Hence, in traditional LNG liquefaction plants, NGL removal is
limited to C5 and
heavier hydrocarbons to avoid plugging of the cryogenic exchanger, and most of
the lighter NGL
components are liquefied together with the methane component, resulting in LNG
with a fairly high
Btu content. When such LNG is exported to North America or Europe, deep
removal of the NGL
components is typically necessary prior to LNG liquefaction, in order to meet
the relatively low
heating value specification, ranging from 960 Btu/scf to 1100 Btu/scf.
Alternatively, the rich LNG when imported to the LNG regasification terminals,
can be diluted with
nitrogen, or blended with a leaner natural gas to lower its heating value or
Wobbe Index. However,
there are upper limits on the amount of nitrogen and inerts that can be
introduced to the pipeline gas,
and in most cases, a lean gas source is not readily available. Moreover,
dilution with nitrogen
requires an air separation plant to produce the nitrogen, which is energy
intensive and costly and
produces no environmental benefit.
Therefore, to compete in the LNG export markets, LNG liquefaction plants must
be provided with
the flexibility to produce different heating value LNG for export to different
customers. This means
the LNG liquefaction plants are required to add an NGL recovery unit for the
removal of the lighter
NGL components when exported to Europe or North America. While the cost of
these NGL
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recovery units may be justified for larger LNG plants, it is often not
economical for smaller LNG
plants, particularly when retrofitting existing LNG plants.
There are numerous configurations and methods known in the art for high
recovery of C3+
components from a natural gas feed. However, all these known processes are
complex and
costly. Some of the NGL/LNG integrated examples include the expander processes
described in
U.S. Pat. Nos. 4,157,904 to Campbell et al., 4,251,249 to Gulsby, 4,617,039 to
Buck, 4,690,702
to Paradowski et al., 5,275,005 to Campbell et al., 5,799,507 to Wilkinson et
al., and 5,890,378
to Rambo et al. Other C3+ recovery methods are also known, as exemplified by
U.S. Pat. No.
6,308,531 to Roberts et al, where a side stream from the cryogenic exchanger
is processed in a
scrub column for the removal of the heavier hydrocarbons. Where a definition
or use of a term
in an incorporated reference is inconsistent or contrary to the definition of
that term provided
herein, the definition of that term provided herein applies and the definition
of that term in the
reference does not apply.
While these processes can achieve heating value reduction to at least some
extent, removal of
C3+ components is limited, especially at high pressure (e.g., 700 psig and
greater) where
separation of C3+ components from C2 and lighter components is difficult.
Consequently, when
processing a rich gas with a high C2 content (e.g., 10% and higher), these
processes will often
require excessive refrigeration and may no longer be economical.
In still further known configurations, as described for example in U.S. App.
No. 2007/0157663,
an NGL recovery unit provides a low-temperature and high-pressure overhead
product directly to
the LNG liquefaction unit and feed gas cooling and condensation are performed
using
refrigeration cycles that employ refrigerants other than the demethanizer/ab
sorbet overhead
product. Thus, the cold demethanizer/absorber overhead product is compressed
and delivered to
the liquefaction unit at significantly lower temperature and higher pressure
without net
compression energy expenditure. While such systems and methods provide certain
advantages,
various drawbacks nevertheless remain. Among other things, external
refrigeration may become
cost-prohibitive, and operational flexibility is often not readily
implementable.
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Thus, while numerous plant configurations and methods for NGL recovery and LNG
liquefaction
are known in the art, all or almost all of them, suffer from various
disadvantages. Thus, there is still
a need for improved NGL recovery and LNG liquefaction, and especially plants
in which NGL
recovery and LNG liquefaction are integrated.
Summary of the Invention
The inventor has now discovered that flexible NGL recovery from natural gas
can be readily
implemented in a conceptually simple and economically attractive manner for
both de novo as well
as retrofit plants where a second, C3+ enriched reflux stream is produced.
Most preferably, the
second reflux stream is generated by expansion of a C5+ depleted vapor
fraction of the column
overhead to so minimize external refrigeration.
In one especially preferred aspect, an NGL recovery plant includes a scrub
column that receives a
cooled natural gas stream and a first and a second reflux stream. Most
typically, the scrub column
operates at a pressure of at least 500 psi to thereby produce a C3+ enriched
bottoms product and a
C5+ depleted overhead product. A first cooler cools the C5+ depleted overhead
product and a first
separator separates the cooled C5+ depleted overhead product into a C5+
depleted vapor fraction
and the first reflux stream, while a second cooler cools a first portion of
the C5+ depleted vapor
fraction using refrigeration generated by expansion of a C3+ depleted vapor
stream, and a second
separator separates the so cooled first portion of the C5+ depleted vapor
fraction into the C3+
depleted vapor stream and the second reflux stream. The plant is still further
configured such that
.. the C3+ depleted vapor stream and a second portion of the C5+ depleted
vapor fraction are
combined to form a liquefaction feed stream, which is then fed into a
liquefaction unit.
In particularly preferred aspects, the scrub column operates at a pressure of
at least 700 psi, and an
NGL fractionation unit is fluidly coupled to the scrub column to receive the
C3+ enriched bottoms
product. It is still further generally preferred that a turbo expander and a
compressor are operably
.. coupled to each other to receive and expand the C3+ depleted vapor stream
and to compress the
expanded C3+ depleted vapor stream. In still further preferred aspects, the
second cooler is also
configured to cool the second reflux stream.
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Therefore, a method of recovering NGL from a natural gas may include a step of
cooling the natural
gas and contacting the cooled natural gas in a scrub column with a first and a
second reflux stream
at a pressure of at least 500 psi (and more typically at least 700 psi) to
thereby produce a C3+
enriched bottoms product and a C5+ depleted overhead product. In another step,
the CS+ depleted
overhead product is cooled and separated into a C5+ depleted vapor fraction
and the first reflux
stream. In yet another step, a first portion of the C5+ depleted vapor
fraction is cooled using
refrigeration generated by expansion of a C3+ depleted vapor stream, and the
so cooled first portion
is separated into the C3+ depleted vapor stream and the second reflux. The C3+
depleted vapor
stream and a second portion of the C5+ depleted vapor fraction are then
combined to form a
liquefaction feed stream that is subsequently liquefied.
In such methods, it is generally preferred that a ratio between the first and
second portions of the
C5+ depleted vapor fraction and/or the discharge pressure of the expansion
device that expands the
C3+ depleted vapor is used to control C3 recovery in the bottom product of the
scrub column. It is
still further generally preferred that the second reflux is cooled by
expansion of the C3+ depleted
vapor stream, and/or that cooling of the natural gas and the C5+ depleted
overhead product is
performed using propane refrigeration. As noted before, it is generally
preferred that the C3+
enriched bottoms product of the scrub column is processed in an NGL
fractionation unit.
Viewed from a different perspective, the inventor also contemplates a method
of recovering NGL
from a natural gas having a step of cooling and separating a first portion of
a C5+ depleted vapor
fraction from an overhead product of a scrub column to produce a second reflux
for the scrubbing
column and a C3+ depleted vapor stream. In another step, the C3+ depleted
vapor stream is
expanded in an expansion device to generate refrigeration for the first
portion of the C5+ depleted
vapor fraction, and in still another step, the expanded C3+ depleted vapor
stream is compressed and
combined with a second portion of the C5+ depleted vapor fraction to thereby
form a liquefaction
feed stream, wherein the ratio between the first and second portions of the
C5+ depleted vapor
fraction and/or discharge pressure of the expansion device is used to control
C3 recovery in a
bottom product of the scrub column. Suitable ratios between the first and the
second portions of the
C5+ depleted vapor fraction are typically between 1:1 and 9:1.
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Thus, C3 recovery in the bottom product of the scrub column may be controlled
by using a ratio
between the first and second portions of the C5+ depleted vapor fraction,
and/or by controlling the
discharge pressure of the expansion device. Most preferably, the C3+ enriched
bottoms product is
further processed in an NGL fractionation unit, and/or the liquefaction feed
stream is liquefied in a
downstream liquefaction unit. In further preferred aspects, the scrub column
is operated at a pressure
of at least 700 psi, and/or the expansion device is a turboexpander that is
operably coupled to a
compressor that compresses the expanded C3+ depleted vapor stream.
Various objects, features, aspects and advantages of the present invention
will become more
apparent from the accompanying drawing and the following detailed description
of preferred
embodiments of the invention.
Brief Description of the Drawing
Figure 1 is a schematic of an exemplary plant configuration according to the
inventive subject
matter.
Detailed Description
The inventor has now discovered that flexible C3+ recovery from natural gas
can be readily
achieved in de novo as well as retrofitted liquefaction plants by fluidly
coupling of a turbo-expander
system to a scrub column to produce a second reflux stream to the scrub
column. Recovery of C3+
can be varied from 40% to 80% or higher by adjusting the expander flow as
necessary to meet the
heating value specifications.
Most preferably, the expanded overhead gas from the scrub column in
contemplated methods and
configuration is employed as a refrigerant after partial expansion via heat
exchange with the scrub
column overhead gas to produce a second reflux stream to the scrub column.
Consequently, it
should be appreciated that no external refrigeration is required, avoiding a
costly and hazardous
complex ethane refrigeration system that would be otherwise required for high
NGL recovery.
In most preferred configurations, the feed gas is pre-cooled by propane
refrigeration and is then
fractionated in a scrub column at relatively high pressure, typically at about
700 psig (e.g., +I- 10%).
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The so formed column overhead is further chilled, typically using a low level
propane refrigerant to
generate a first column reflux and a C5+ depleted overhead gas. The scrub
column also produces a
C3+ rich bottoms. At least a portion of the C5+ depleted overhead gas is
chilled by a lower pressure
expanded gas to -55 F or lower to thereby produce a second reflux to the scrub
column and a C3+
depleted overhead vapor stream. The C3+ depleted overhead vapor stream is then
expanded via a
turbo-expander and is then used to cool a portion of the C5+ depleted vapor
fraction. The so heated
low pressure C3+ depleted overhead vapor stream is then recompressed by the
turbo-expander and a
second compressor feeding the LNG liquefaction plant (typically after
combination with another
portion of the C5+ depleted vapor fraction).
Consequently, it should be appreciated that the scrub column is configured to
separately receive a
first and a second reflux stream, wherein the first reflux stream is produced
using propane
refrigeration on the scrub column overhead while the second reflux stream is
produced using
refrigeration produced by turbo-expansion using the scrub column overhead gas
as a cooling
medium. In such configurations, it is generally preferred that a flow control
valve (or other flow
control implement) varies the flow to the turbo-expander system to meet the
desirable overall C3+
recoveries, typically ranging from 40% to 80%.
Therefore, and as described in more detail below, it is generally contemplated
that an NGL recovery
plant will include a scrub column operating at a pressure of at least 500 psi
that receives a cooled
natural gas stream and two separate reflux streams, and producing a C3+
enriched bottoms product
and a C5+ depleted overhead product. A first cooler and a first separator are
typically coupled to the
scrub column and configured to allow production of a C5+ depleted vapor
fraction and the first
reflux stream from the cooled C5+ depleted overhead product. A second cooler
then cools a first
portion of the C5+ depleted vapor fraction using refrigeration generated by
expansion of a C3+
depleted vapor stream, and a second separator then separates the cooled first
portion of the C5+
depleted vapor fraction into the C3+ depleted vapor stream and the second
reflux stream. A
liquefaction feed stream is then formed by combining the C3+ depleted vapor
stream and a second
portion of the C5+ depleted vapor fraction (10), and the liquefaction feed
stream is then fed to a
liquefaction unit to liquefy the liquefaction feed stream.
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As used herein, and unless the context dictates otherwise, the term "coupled
to" is intended to
include both direct coupling (in which two elements that are coupled to each
other contact each
other) and indirect coupling (in which at least one additional element is
located between the two
elements). Therefore, the terms "coupled to" and "coupled with" are used
synonymously. As further
used herein, the term "depleted" in conjunction with a hydrocarbon fraction in
a second stream
means that the quantity of the hydrocarbon fraction in the second stream is
smaller than the quantity
of the same hydrocarbon fraction in first stream from which the second stream
is formed. Likewise,
the term "enriched" in conjunction with a hydrocarbon fraction in a second
stream means that the
quantity of the hydrocarbon fraction in the second stream is larger than the
quantity of the same
hydrocarbon fraction in first stream from which the second stream is formed.
For example, where a
natural gas stream is separated into a C3+ enriched bottom product and a C5+
depleted overhead
product, the bottom product has a higher C3+ fraction than the natural gas
stream and the overhead
product has a lower C5+ fraction than the natural gas stream. As still further
used herein, the term
"C3+" refers to hydrocarbons and isoforms thereof having 3 or more carbon
atoms (e.g., propane
propylene, butane, isobutane, etc.), the tenu "C4+" refers to hydrocarbons and
isoforms thereof
having 4 or more carbon atoms (e.g., butane, isobutane, pentane, etc.), and
the term "C5+" refers to
hydrocarbons and isoforms thereof having 5 or more carbon atoms (e.g.,
pentane, hexane, benzene,
etc. etc.).
In Figure 1, natural gas stream 1 is a feed stream, typically with a heating
value of 1150 Btu/scf,
enters the plant at about 750 psig and 120 F. Unless the context dictates the
contrary, all ranges set
forth herein should be interpreted as being inclusive of their endpoints, and
open-ended ranges
should be interpreted to include commercially practical values. Similarly, all
lists of values should
be considered as inclusive of intermediate values unless the context indicates
the contrary. Water is
removed from the feed gas in molecular sieve unit 51 forming a dried gas
stream 2. The dried gas is
cooled in cooler/exchanger 52, typically using high pressure propane
refrigeration to typically 0 F,
forming stream 3, which is further chilled in cooler/exchanger 53 to -15 F,
typically via medium
pressure propane refrigeration, forming cooled natural gas stream 4. The so
chilled gas is
fractionated in a scrub column 54, using low pressure propane refrigeration
for C5+ depleted
overhead product 6 at the column overhead in reflux exchanger/cooler 55,
producing a chilled two
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phase stream 7, typically at -35 F. The cooled C5+ depleted overhead product 7
is separated in
reflux drum/separator 56, producing an C5+ depleted vapor fraction as stream
10 and a liquid
stream 8. The reflux liquid is pumped by pump 57 forming reflux stream 9 and
is used as the first
reflux to the scrub column. The C3+ enriched bottom product of the scrub
column (stream 5)
containing mostly the C3 and C4+ components is sent to the NGL fractionation
unit 64 which
produces the C3 and C4+ product streams for storage, sale, or export.
Due to the high operating pressure of the scrub column (700 psig and higher),
separation of C3 from
the C2 and light components is difficult due to the low relative volatility
which limits the extent of
C3 recovery. As a result, a significant amount of C3 would be retained in the
overhead gas,
resulting in a fairly high heating value gas feeding the LNG liquefaction
plant. To reduce the heating
value of the overhead gas, a portion (stream 12) of the C5+ depleted vapor
fraction 10 (ranging from
50% to 90%) is chilled in cooler/exchanger 58 to typically -55 F to -75 F
forming stream 13, a
cooled portion of the C5+ depleted vapor fraction. The two phase stream is
separated in separator 59
producing a lean C3+ depleted vapor stream 14 and a C3 rich liquid stream 15.
The liquid stream is
pumped by pump 60 to about 750 psig forming stream 16, heated in exchanger 58
to about -20 F to
0 F, and fed to the scrub column as the second reflux stream 17 to a location
that is at least one tray
below that of stream 9.
The chilled vapor from separator 59, stream 14, is expanded in an expansion
device 61 (typically a
turbo-expander) to a lower pressure at about 600 psig that chills the gas to a
lower temperature,
typically at -70 F to -85 F, forming stream 18. The chilled expanded vapor is
heat exchanged in
exchanger 58 that cools the overhead gas from -35 F to -55 F or lower. The
heated vapor 19 is then
compressed by the compressor 62 driven by the turbo-expander 61 forming stream
20 which is
further compressed by compressor 63. The compressed gas stream 21 is further
chilled with propane
refrigeration in exchanger 70 to about -35 F forming stream 23, mixed with the
second portion of
the C5+ depleted vapor fraction (bypass stream 11) to form liquefaction feed
stream 22 and fed to
the LNG liquefaction plant 65. It should be appreciated that the level of C3
recovery can also be
varied by adjusting the refrigeration levels by varying the expander discharge
pressure (in stream
18). Lowering the expander discharge pressure would lower the discharge
temperature, increasing
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the available refrigeration for a deeper C3+ recovery, which is required when
processing a rich gas
with greater than 10% C2 content.
Consequently, it should be appreciated that a method of recovering NGL from a
natural gas will
include a step of cooling the natural gas and contacting the cooled natural
gas in a scrub column
with a first and a second reflux stream at a pressure of at least 500 psi (and
more typically at least
700 psi) to thereby produce a C3+ enriched bottom product and a C5+ depleted
overhead product.
The so formed C5+ depleted overhead product is then cooled and separated into
a C5+ depleted
vapor fraction and the first reflux stream. As noted before, the C5+ depleted
vapor fraction is split
into two portions, and a first portion of the C5+ depleted vapor fraction is
cooled, preferably using
refrigeration generated by expansion of a C3+ depleted vapor stream. The so
cooled fraction is then
separated into the C3+ depleted vapor stream (that is then expanded) and the
second reflux. After
recompression, the C3+ depleted vapor stream is then combined with a second
portion of the C5+
depleted vapor fraction to so form a liquefaction feed stream, which is
subsequently liquefied in a
liquefaction unit.
Therefore, it should be noted that contemplated methods and plants allow for
significantly
simplified control over C3+ recovery from a natural gas stream without the
requiring additional
external refrigeration. Indeed, it should be recognized that the ratio between
the first and second
portions of the C5+ depleted vapor fraction and/or the discharge pressure of
the expansion device
can be employed to control C3 recovery in the bottom product of the scrub
column. For example,
.. where the amount of stream 12 relative to stream 11 is increased, C3+
recovery at the bottom
product of the scrub column increases. Alternatively, or additionally, it
should be noted that the
turboexpander discharge pressure could be lowered to thereby increase cooling
of the C5+ depleted
vapor fraction, which in turn increases C3+ recovery. Typically, stream 12
will range between 10 %
and 90 % of stream 10, and more typically between 20 % and 80 % of stream 10.
Thus, and viewed
from a different perspective, the ratio between the first and the second
portions of the C5+ depleted
vapor fraction is typically between 1:1 and 9:1. Of course, the C3+ enriched
bottom product may be
used for various purposes, and among other options, it is generally preferred
to use the bottom
product as feed stream to an NGL fractionation unit.
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Alternatively, in less preferred aspects, contemplated configurations and
methods are also
deemed suitable for situations where liquefaction is not desired, but where
upgrading of natural
gas is the objective prior to transmission of the treated gas into a pipeline
system. Thus, the feed
gas need not be limited to raw or pretreated export natural gas, but all
sources of natural gas
.. (including from regasification of LNG) are deemed suitable for use herein.
Additionally, while
propane refrigeration is typically preferred, alternative refrigeration
processes are also
contemplated, and especially include those in which refrigeration content from
LNG form the
liquefaction unit is used (typically, but not necessarily, via an intermediate
heat transfer fluid).
Still further suitable aspects, modifications, and processes are provided in
our U.S. patent
.. application with the publication number US2007/0157663A1.
It should be apparent to those skilled in the art that many more modifications
besides those
already described are possible without departing from the inventive concepts
herein. The
inventive subject matter, therefore, is not to be restricted except in the
scope of the appended
claims. Moreover, in interpreting both the specification and the claims, all
terms should be
interpreted in the broadest possible manner consistent with the context. In
particular, the terms
"comprises" and "comprising" should be interpreted as referring to elements,
components, or
steps in a non-exclusive manner, indicating that the referenced elements,
components, or steps
may be present, or utilized, or combined with other elements, components, or
steps that are not
expressly referenced. Where the specification claims refers to at least one of
something selected
from the group consisting of A, B, C .... and N, the text should be
interpreted as requiring only
one element from the group, not A plus N, or B plus N, etc..
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