Note: Descriptions are shown in the official language in which they were submitted.
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HYDRAULIC TURBINE BETWEEN MIDDLE AND COLD BUNDLES OF
NATURAL GAS LIQUEFACTION HEAT EXCHANGER
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority benefit of United States
Patent Application
62/479,880 filed March 31, 2017 entitled HYDRAULIC TURBINE BETWEEN MIDDLE
AND COLD BUNDLES OF NATURAL GAS LIQUEFACTION HEAT EXCHANGER, the
entirety of which is incorporated by reference herein.
FIELD
[0002] The disclosure relates to the liquefaction of natural gas to form
liquefied natural gas
(LNG), and more specifically, to improving efficiencies in an LNG-producing
heat exchanger.
BACKGROUND
[0003] LNG production is a rapidly growing means to supply natural gas
from locations
with an abundant supply of natural gas to distant markets having a strong
demand for natural
gas. The conventional LNG cycle includes: a) initial treatments of the natural
gas resource to
remove contaminants such as water, sulfur compounds and carbon dioxide; b)
separating some
heavier hydrocarbon gases, such as propane, butane, pentane, etc. by a variety
of possible
methods including self-refrigeration, external refrigeration, lean oil, etc.;
c) refrigerating the
natural gas substantially by external refrigeration to form LNG at near
atmospheric pressure
and about -160 C; d) transporting the LNG product in ships or tankers
designed for this
purpose to a market location; e) re-pressurizing and re-gasifying the LNG to
form a pressurized
natural gas that may distributed in a natural gas distribution system.
[0004] The liquefaction of step c) may be accomplished using indirect
heat exchange with
a refrigerant in a cryogenic heat exchanger. Such a cryogenic heat exchanger
may include
multiple heat exchange bundles to progressively cool a natural gas stream so
the natural gas
stream is eventually liquefied and sub-cooled. Traditionally, Joule-Thomson
(JT) valves have
been used to control pressures and temperatures in the bundles via isenthalpic
pressure
reduction. While inexpensive, JT valves provide a limited cooling effect and
do not recover
power from the process stream. What is needed is a method of increasing the
cooling effect
inside a cryogenic LNG heat exchanger. What is also needed is a method of
increasing
throughput of an LNG process.
[0005] Hydraulic turbines achieve process control objectives
(temperature/pressure), reach
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lower discharge temperatures, and extract power associated with pressure
reduction. The
thermodynamic basis for a hydraulic turbine (hydraulic expander, expander) is
a near-
isentropic expansion of a liquid process fluid, through which the temperature
of the process
fluid is reduced and mechanical shaft work is generated. U.S. Patent No.
4,334,902 to
Paradowski describes a method of sub-cooling a natural gas stream via
expansion in the liquid
condition, with a hydraulic turbine providing mechanical power possibly for
driving a rotary
machine. Others have since employed applications of expander technology to
refrigeration and
liquefaction processes. Design and application of expander technology is
generally well
understood, and considered standard for latest generation process designs.
Typical natural gas
liquefaction processes apply hydraulic turbines in the expansion of the final
LNG condensate
and in the expansion of liquid coolant in the refrigeration cycle. However,
the use of hydraulic
turbine expanders to expand and cool a process gas stream within an LNG
cryogenic heat
exchanger has not been suggested.
SUMMARY
[0006] The disclosed aspects provide a system for liquefying a natural gas
stream. A
liquefaction heat exchanger has at least three cooling bundles and is arranged
such that the
natural gas stream passes sequentially therethrough. A first cooling bundle is
configured to
condense heavy hydrocarbon components in the natural gas stream. A second
cooling bundle
is configured to liquefy the natural gas stream. The second cooling bundle has
an outlet for
passing an LNG stream therethrough. A third cooling bundle has an inlet to
receive the LNG.
The third cooling bundle is configured to sub-cool the LNG stream. A hydraulic
turbine has
an inlet operationally connected to the outlet of the second cooling bundle
and an outlet
operationally connected to the inlet of the third cooling bundle. The
hydraulic turbine is
configured to cool the LNG stream and reduce a pressure of the LNG stream to
form a reduced-
pressure LNG stream.
[0007] The disclosed aspects also provide a method of liquefying a
natural gas stream to
produce liquefied natural gas (LNG). The natural gas stream is sequentially
cooled in first,
second, and third cooling bundles of a liquefaction heat exchanger. The second
cooling bundle
liquefies the natural gas stream to produce an LNG stream. The LNG stream is
cooled and its
pressure is reduced between the second cooling bundle and the third cooling
bundle using a
hydraulic turbine, to thereby produce a reduced-pressure LNG stream. Work
energy is
produced using the hydraulic turbine.
[0008] The disclosed aspects also provide a method of liquefying a
natural gas stream to
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produce liquefied natural gas (LNG). The natural gas stream is sequentially
cooled in a
liquefaction heat exchanger having first, second, and third cooling bundles.
The second cooling
bundle liquefies the natural gas stream to produce an LNG stream. The LNG
stream is cooled
and its pressure is reduced between the second cooling bundle and the third
cooling bundle
using a hydraulic turbine. Work energy is produced using the hydraulic
turbine. Using the
work energy, power is generated using a generator connected to the hydraulic
turbine. The
pressure of the LNG stream exiting the hydraulic turbine is controlled using a
control valve
disposed between the outlet of the hydraulic turbine and an inlet of the third
cooling bundle.
The method adjusts at least one of a speed of the hydraulic turbine, an LNG
inlet rate of the
hydraulic turbine, a position of the control valve, and a speed of the
generator, based on at least
one of a sensed temperature of the LNG stream prior to entering the hydraulic
turbine, a sensed
pressure of the LNG stream prior to entering the hydraulic turbine, a sensed
temperature of the
LNG stream as the LNG stream exits the hydraulic turbine, and a sensed
pressure of the LNG
stream as the LNG stream exits the hydraulic turbine.
BRIEF DESCRIPTION OF THE FIGURES
[0009] Figure 1 is a schematic diagram of an LNG liquefaction process;
[0010] Figure 2 is a simplified plan view of a main cryogenic LNG heat
exchanger
according to known principles;
[0011] Figure 3 is a simplified plan view of a main cryogenic LNG heat
exchanger
according to disclosed aspects;
[0012] Figure 4 is a simplified schematic of a hydraulic turbine
according to disclosed
aspects;
[0013] Figure 5 is a simplified schematic of a hydraulic turbine
according to disclosed
aspects;
[0014] Figure 6 is a simplified schematic of a hydraulic turbine according
to disclosed
aspects;
[0015] Figure 7 is a simplified schematic of a hydraulic turbine
according to disclosed
aspects;
[0016] Figure 8 is a flowchart of a method according to disclosed
aspects; and
[0017] Figure 9 is a flowchart of a method according to disclosed aspects.
DETAILED DESCRIPTION
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[0018] Various specific aspects and versions of the present disclosure
will now be
described, including preferred aspects and definitions that are adopted
herein. While the
following detailed description gives specific preferred aspects, those skilled
in the art will
appreciate that these aspects are exemplary only, and that the present
techniques can be
practiced in other ways. Any reference to the "invention" or "aspect" may
refer to one or more,
but not necessarily all, of the aspects defined by the claims. The use of
headings is for purposes
of convenience only and does not limit the scope of the present techniques.
For purposes of
clarity and brevity, similar reference numbers in the several Figures
represent similar items,
steps, or structures and may not be described in detail in every Figure.
[0019] All numerical values within the detailed description and the claims
herein are
modified by "about" or "approximately" the indicated value, and take into
account
experimental error and variations that would be expected by a person having
ordinary skill in
the art. Certain aspects and features have been described using a set of
numerical upper limits
and a set of numerical lower limits. It should be appreciated that ranges from
any lower limit
to any upper limit are contemplated unless otherwise indicated.
[0020] The term "gas" is used interchangeably with "vapor," and means 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. As used herein, "fluid" is a
generic term that may
include either a gas or liquid.
[0021] A "hydrocarbon" is an organic compound that primarily includes the
elements
hydrogen and carbon although nitrogen, sulfur, oxygen, metals, or any number
of other
elements may be present in small amounts. As used herein, hydrocarbons
generally refer to
organic materials, such as any form of natural gas or oil. A "hydrocarbon
stream" is a stream
enriched in hydrocarbons.
[0022] "Pressure" is the force exerted per unit area by the gas on the
walls of the volume.
Pressure can be shown as pounds per square inch (psi). "Atmospheric pressure"
refers to the
local pressure of the air. "Absolute pressure" (psia) refers to the sum of the
atmospheric
pressure (14.7 psia at standard conditions) plus the gauge pressure (psig).
"Gauge pressure"
(psig) refers to the pressure measured by a gauge, which indicates only the
pressure exceeding
the local atmospheric pressure (i.e., a gauge pressure of 0 psig corresponds
to an absolute
pressure of 14.7 psia).
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[0023] "Substantial" when used in reference to a quantity or amount of a
material, or a
specific characteristic thereof, refers to an amount that is sufficient to
provide an effect that the
material or characteristic was intended to provide. The exact degree of
deviation allowable
may in some cases depend on the specific context.
[0024] "Well" refers to a hole in the subsurface made by drilling or
insertion of a conduit
into the subsurface.
[0025] The term "natural gas" refers to a hydrocarbon gas obtained from a
crude oil well
(associated gas) or from a subterranean gas-bearing formation (non-associated
gas). The
composition and pressure of natural gas can vary significantly. A typical
natural gas stream
contains methane (CO as a significant component. Raw natural gas will also
typically contain
ethane (C2), higher molecular weight hydrocarbons, one or more acid gases
(such as carbon
dioxide, hydrogen sulfide, carbonyl sulfide, carbon disulfide, and
mercaptans), and
contaminants such as water, nitrogen, iron sulfide, mercury, helium, wax, and
crude oil.
[0026] As used herein, the term "compressor" means a machine that
increases the pressure
of a gas by the application of work. A "compressor" includes any unit, device,
or apparatus
able to increase the pressure of a gas stream. This includes compressors
having a single
compression process or step, or compressors having multi-stage compressions or
steps, or more
particularly multi-stage compressors within a single casing or shell. Gaseous
streams to be
compressed can be provided to a compressor at different pressures. Some stages
or steps of a
cooling process may involve two or more compressors in parallel, series, or
both. The disclosed
aspects are not limited by the type or arrangement or layout of the compressor
or compressors,
particularly in any refrigerant circuit.
[0027] As used herein, the term "JT valve" (also known as Joule-Thomson
valve or
throttling valve) means a control valve that substantially decreases the
pressure of a fluid,
including liquids, without the removal of work (approximating an isenthalpic
throttling
process). Ideally during pressure reduction through a JT valve, the fluid is
maintained at
constant enthalpy, which in most cases, is accompanied by a temperature
reduction. A JT valve
is adjustable such that fluid flow rate, pressure or pressure reduction can be
controlled.
[0028] As used herein, the term "hydraulic turbine" (also known as
"liquid expander" or
"dense fluid expander") means a machine that decreases the pressure of a
liquid by the removal
of work (approximating an isentropic process). Ideally during pressure
reduction through a
hydraulic turbine, the liquid is maintained at constant entropy, which in most
cases, is
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accompanied by a temperature reduction. For the same pressure reduction, an
isentropic
process (hydraulic turbine) results in a lower outlet temperature than an
isenthalpic process (JT
valve). A "hydraulic turbine" includes any unit, device, or apparatus able to
decrease the
pressure of a liquid stream and extract work. This includes hydraulic turbines
having a single
pressure reduction process or stage, or hydraulic turbines having multiple
stages, or more
particularly multi-stage hydraulic turbines within a single casing or shell.
Some stages of a
depressurization process may involve two or more hydraulic turbines in
parallel, series, or both.
The disclosed aspects are not limited by the type or arrangement or layout of
the hydraulic
turbine or hydraulic turbines, particularly in any LNG service.
[0029] As used herein, "cooling" broadly refers to lowering and/or dropping
a temperature
and/or internal energy of a substance by any suitable, desired, or required
amount. Cooling
may include a temperature drop of at least about 1 C, at least about 5 C, at
least about 10 C,
at least about 15 C, at least about 25 C, at least about 35 C, or least
about 50 C, or at least
about 75 C, or at least about 85 C, or at least about 95 C, or at least
about 100 C, or at least
about 150 C, or at least about 200 C, or at least about 260 C. The cooling
may use any
suitable heat sink, such as steam generation, hot water heating, cooling
water, air, refrigerant,
other process streams (integration), and combinations thereof One or more
sources of cooling
may be combined and/or cascaded to reach a desired outlet temperature. The
cooling step may
use a cooling unit with any suitable device and/or equipment. According to
some aspects,
cooling may include indirect heat exchange, such as with one or more heat
exchangers. In the
alternative, the cooling may use evaporative (heat of vaporization) cooling
and/or direct heat
exchange, such as a liquid sprayed directly into a process stream.
[0030] A "heat exchanger" broadly means any device capable of
transferring heat energy
from one medium to another medium, such as between at least two distinct
fluids. Heat
exchangers include "direct heat exchangers" and "indirect heat exchangers."
Thus, a heat
exchanger may be of any suitable design, such as a co-current or counter-
current heat
exchanger, an indirect heat exchanger (e.g. a spiral wound heat exchanger or a
plate-fin heat
exchanger such as a brazed aluminum plate fin type), direct contact heat
exchanger, shell-and-
tube heat exchanger, spiral, hairpin, core, core-and-kettle, printed-circuit,
double-pipe or any
other type of known heat exchanger. "Heat exchanger" may also refer to any
column, tower,
unit or other arrangement adapted to allow the passage of one or more streams
therethrough,
and to affect direct or indirect heat exchange between one or more lines of
refrigerant, and one
or more feed streams. A heat exchanger as disclosed herein may include
multiple heat
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exchangers as needed or desired.
[0031] As used herein, the term "indirect heat exchange" means the
bringing of two fluids
into heat exchange relation without any physical contact or intermixing of the
fluids with each
other. Core-in-kettle heat exchangers and brazed aluminum plate-fin heat
exchangers are
examples of equipment that facilitate indirect heat exchange.
[0032] All patents, test procedures, and other documents cited in this
application are fully
incorporated by reference to the extent such disclosure is not inconsistent
with this application
and for all jurisdictions in which such incorporation is permitted.
[0033] Described herein are methods and systems for liquefying a natural
gas stream to
form liquefied natural gas (LNG). The described methods and systems use a
hydraulic turbine
to cool and reduce the pressure of an LNG stream within a liquefaction heat
exchanger. The
hydraulic turbine may be coupled to an electrical generator or a brake. The
brake dissipates
the work, extracted from the liquid, to the environment. The electric
generator uses the work,
extracted from the liquid, to generate electricity. The electricity from an
electric generator may
be processed by a variable speed constant frequency (VSCF) drive or machine
that will allow
the speed of hydraulic turbine to be adjustable. The adjustable speed of the
hydraulic turbine
allows some control over fluid flow rate, pressure or pressure reduction.
[0034] Specific aspects of the disclosure include those set forth in the
following paragraphs
as described with reference to the Figures. While some features are described
with particular
reference to only one Figure, they may be equally applicable to the other
Figures and may be
used in combination with the other Figures or the foregoing discussion.
[0035] Figure 1 is a schematic diagram showing the basic steps in a
typical natural gas
liquefaction process 100. The process 100 is a simplified rendition of a
liquefaction process,
it being understood that an actual liquefaction process may add, subtract, or
replace one or
more steps disclosed herein. The feedstock gas 102 to the process 100
comprises mostly light
hydrocarbons, and may be a raw feed gas directly transported from one or more
wells.
Alternatively, the feedstock may be gas from a pipeline that has been
partially conditioned to
be suitable for such transport. The feedstock gas may contain free liquid,
mercury, acid gases,
such as carbon dioxide and hydrogen sulfide, water, and other sulfur species.
The gas must be
treated to remove these contaminants and thoroughly dried before it can be
converted to LNG.
The process 100 shows typical steps for this treating and dehydration. At
block 104 preliminary
steps such as liquid removal, pressure control, mercury removal, and metering
are performed.
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At block 106 acid gases such as carbon dioxide and hydrogen sulfide are
removed. At block
108 one or more dehydration processes are performed. At this point in process
100 the
feedstock gas has been converted to a dry gas stream 110. At block 112 the dry
gas stream is
pre-cooled to condense heavy hydrocarbons and aromatics, which might freeze in
the
subsequent liquefaction step. Some liquefied petroleum gases (e.g., ethane,
propane, and
butane) are also condensed and separated from the heavy hydrocarbons and
aromatics in a
fractionation unit at block 114. The liquefied petroleum gases 118 are re-
injected into the dry
gas stream to be liquefied in the liquefaction step at block 116, although
some of the liquefied
petroleum gases may be drawn off for refrigerant make-up or sold as LPG
products. The heavy
hydrocarbons and aromatics separated by the fractionation unit 114 form a
condensate product
120 that is not liquefied in the liquefaction step.
[0036] The liquefaction step at block 116 may be performed by a cryogenic
heat exchanger
that exchanges heat between the dry gas stream 110 and a refrigerant 122 so
that the dry gas
stream is liquefied, thereby producing a liquefied natural gas (LNG) stream
124. The
refrigerant may include methane, propane, nitrogen, one or more noble gases,
and/or one or
more fluorocarbons. After liquefying the dry gas stream 110, the refrigerant
122 is refrigerated
and compressed at block 126 and recycled back to the liquefaction step at
block 116 through a
return line 127. At block 128 the LNG is run through a fractionation column or
flash drum,
where excess nitrogen is rejected, to reduce the nitrogen content of the LNG
stream to a desired
level. The nitrogen-rich gas stream 130 is typically used as a fuel stream for
one or more plant
processes. At block 132 the LNG product stream 133, now at near atmospheric
pressure, is
stored for transport or use.
[0037] Figure 2 is a simplified elevation view of an exemplary known
liquefaction heat
exchanger 200, which is commonly referred to as a main cryogenic heat
exchanger.
Liquefaction heat exchanger 200 has three sections of multi-pass heat
exchange: a warm bundle
202, a middle bundle 204, and a cold bundle 206. The lines identified by
reference numbers
208, 210, and 212 follow the heat exchanger cold passes, which cool all the
other passes ¨
termed the warm passes ¨ in the heat exchanger. The liquefaction heat
exchanger 200 may be
designed as a spiral-wound heat exchanger, in which case the warm passes
comprise bundles
of small-bore tubing wound around a central mandrel and the cold pass stream
is sprayed over
the bundles to provide cooling. Alternatively, the liquefaction heat exchanger
could be a plate-
fin heat exchanger, in which case the warm passes and the cold passes are
integrated into a core
exchanger separated by alternating plates. Other types of liquefaction heat
exchangers may be
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used as well, but for ease of explaining herein the disclosed aspects, a
spiral-wound heat
exchanger design will be described.
[0038] Referring to Figure 2, the hydrocarbon gas to be liquefied, which
in disclosed
aspects may be the dry gas stream 110 shown in Figure 1, enters the warm
bundle 202 where
it is pre-cooled to condense heavy components, which might freeze in the
colder sections of
the liquefaction heat exchanger. The warm bundle is analogous to the
precooling step 112
shown in Figure 1. The pre-cooled natural gas stream 214 leaves the
liquefaction heat
exchanger so that condensed components such as heavy hydrocarbons may be
separated out.
After separating out the condensed heavy components, the natural gas stream
returns through
to line 216 and enters the middle bundle 204. The natural gas stream is
condensed in the middle
bundle and leaves the middle bundle as a high-pressure LNG stream through line
218. A
gaseous or two-phase stream of liquefied petroleum gases (LPGs) 220, which may
be generated
by the fractionation step 114 of Figure 1, is also passed through the warm
bundle 202 and the
middle bundle 204 to produce a cooled LPG stream 222. To combine the high
pressure LNG
stream in line 218 with the cooled LPG stream 222, it is necessary to let-down
or reduce the
pressure of the high pressure LNG stream. According to known principles, the
high-pressure
LNG stream in line 218 is let-down or reduced across a Joule-Thomson (J-T)
valve 224. The
J-T valve 224 operates under pressure control to achieve a suitable downstream
pressure to mix
with the cooled LPG stream 222. The combined LNG/LPG stream 226 is then sub-
cooled as
it passes through the cold bundle 206, and leaves the liquefaction heat
exchanger as a medium-
pressure LNG stream 228.
[0039] A light refrigerant stream 230 is cooled successively in the warm
bundle 202, the
middle bundle 204, and the cold bundle 206, and exits the cold bundle through
line 231. The
refrigerant in line 231 may pass through a control valve 233, which may be a J-
T valve,
according to known liquefaction principles, and re-enters the cold bundle via
line 208, where
it provides cooling for the cold bundle 206. A heavy refrigerant in line 232
is cooled
successively in the warm bundle 202 and the middle bundle 204, and exits the
middle bundle
through line 234. The refrigerant in line 234 may pass through a control valve
241, which may
be a J-T valve, according to known liquefaction principles, and re-enters
liquefaction heat
exchanger 200 via line 210, which is combined with the light refrigerant in
line 208. The
combined refrigerant then provides further cooling for the middle bundle 204
and the warm
bundle 202 before leaving the liquefaction heat exchanger 200 through line
212.
[0040] Figure 3 is a simplified elevation view of a liquefaction heat
exchanger 300
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according to aspects of the present disclosure. The liquefaction heat
exchanger is commonly
referred to as a main cryogenic heat exchanger. Liquefaction heat exchanger
300 has three
sections of multi-pass heat exchange: a warm bundle 302, a middle bundle 304,
and a cold
bundle 306. The lines identified by reference numbers 308, 310, and 312 follow
the heat
exchanger cold passes, which cool all the other passes ¨ termed the warm
passes ¨ in the heat
exchanger. The liquefaction heat exchanger 300 may be designed as a spiral-
wound heat
exchanger, in which case the warm passes comprise bundles of small-bore tubing
wound
around a central mandrel and the cold pass stream is sprayed over the bundles
to provide
cooling. Alternatively, the liquefaction heat exchanger could be designed as a
plate-fin heat
to exchanger, in which case the warm passes and the cold passes are
integrated into a core
exchanger separated by alternating plates. Other types of liquefaction heat
exchangers may be
used, but for ease of explaining herein the disclosed aspects, a spiral-wound
heat exchanger
design will be described.
[0041] Referring to Figure 3, the hydrocarbon gas to be liquefied, which
in disclosed
aspects may be the dry gas stream 110 shown in Figure 1, enters the warm
bundle 302 where
it is pre-cooled to condense heavy components, which might freeze in the
colder sections of
the liquefaction heat exchanger. The warm bundle is analogous to the
precooling step 112
shown in Figure 1. The pre-cooled natural gas stream 314 leaves the
liquefaction heat
exchanger so that condensed components such as heavy hydrocarbons may be
separated out.
After separating out the condensed heavy components, the natural gas stream
returns through
line 316 and enters the middle bundle 304. The natural gas stream is condensed
in the middle
bundle and leaves the middle bundle as a high-pressure LNG stream through line
318. A
gaseous or two-phase stream of liquefied petroleum gases (LPGs) 320, which may
be generated
by the fractionation step 114 of Figure 1, is also passed through the warm
bundle 302 and the
middle bundle 304 to produce a cooled LPG stream 322. To combine the high
pressure LNG
stream in line 318 with the cooled LPG stream 322, it is necessary to let-down
or reduce the
pressure of the high pressure LNG stream. According to aspects of the present
disclosure, the
high-pressure LNG stream in line 318 is passed through a hydraulic turbine
323. While the
pressure let-down across a J-T valve is isenthalpic (i.e., no energy removed),
pressure let-down
across the hydraulic turbine 323 extracts energy in the form of work from the
high-pressure
LNG stream 318. In so doing, the hydraulic turbine 323 contributes to the
process of making
the high-pressure LNG stream 318 colder and thereby reduces the cooling duty
of the
liquefaction heat exchanger 300. As the capacity of the liquefaction heat
exchanger 300 is
typically limited by the power of its associated refrigeration compression
unit, the additional
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refrigeration contribution from the hydraulic turbine 323 means that a higher
LNG production
capacity can be achieved by the liquefaction heat exchanger 300, compared to
the liquefaction
heat exchanger 200 which uses only a J-T valve 224. However, in an aspect, a J-
T valve 324
may be disposed to bypass the hydraulic turbine 323. J-T valve 324 provides a
back-up
function to the hydraulic turbine. The J-T valve 324 may also be used for
start-up operation of
the liquefaction heat exchanger 300. Additionally, the J-T valve may be used
in conjunction
with the hydraulic turbine 323 if the flow of the LNG stream in line 318
exceeds the capacity
of the hydraulic turbine.
[0042] A control valve 325 may be disposed downstream of the hydraulic
turbine. The
it) .. purpose of the pressure control provided by the control valve 325 is to
ensure the LNG stream
327 exiting the hydraulic turbine is at a suitable pressure to mix with the
cooled LPG stream
322. The control valve 325 may also help to keep the LNG stream in the liquid
phase and
prevent it from becoming a two-phase stream. The combined LNG/LPG stream 326
is then
sub-cooled as it passes through the cold bundle 306, and leaves the
liquefaction heat exchanger
as a medium-pressure LNG stream 328.
[0043] A light refrigerant stream 330 is cooled successively in the warm
bundle 302, the
middle bundle 304, and the cold bundle 306, and exits the cold bundle through
line 331. The
refrigerant in line 331 may pass through a control valve 333, which may be a J-
T valve,
according to known liquefaction principles, and re-enters the cold bundle via
line 308, where
it provides cooling for the cold bundle 306 through line 308. A heavy
refrigerant in line 332
is cooled successively in the warm bundle 302 and the middle bundle 304, and
exits the middle
bundle through line 334. The refrigerant in line 334 may pass through a
control valve 341,
which may be a J-T valve, according to known liquefaction principles, and re-
enters
liquefaction heat exchanger 300 via line 310, which is combined with the light
refrigerant in
line 308. The combined refrigerant then provides further cooling for the
middle bundle 304
and the warm bundle 302 before leaving the liquefaction heat exchanger 300
through line 312.
[0044] As previously stated, pressure let-down across the hydraulic
turbine 323 extracts
energy in the form of work from the high-pressure LNG stream 318. This work
may be used
to power a generator 340, for example. The generator may provide power to one
or more parts
of the natural gas liquefaction process 100 or may provide power to other
processes, including
an external electrical grid. Figure 4 is a more detailed schematic view of the
hydraulic turbine
323 operationally connected to the generator 340. A first set of one or more
sensors 402 may
be positioned to measure the pressure and/or temperature of the high-pressure
LNG stream 318
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as it exits the middle bundle 304 (Figure 3) or as it enters the hydraulic
turbine 323. A second
set of one or more sensors 404 may be positioned to measure the pressure
and/or temperature
of the LNG stream 327 downstream of the hydraulic turbine 323. The performance
or
functionality of various components depicted in Figure 4 may be adjusted based
on the
pressures and/or temperatures as sensed by the first and/or second sets of one
or more sensors
402, 404, such as the operating speed of the generator 340, the operating
speed of the hydraulic
turbine 323, the operating position of the control valve 325, the operating
position of the J-T
valve 324, and/or the rate at which the high-pressure LNG stream 318 is
admitted into the
hydraulic turbine 323 (through turbine wicket gates 323a, for example).
[0045] Figure 5 is a schematic view of the hydraulic turbine 323 and
generator 340
according to another aspect of the disclosure. A variable-speed constant-
frequency (VSCF)
drive 350 may be disposed between and operationally connected to the generator
340 and a
power system 354, which may comprise an external power grid. The VSCF drive
350 operates
to selectively control the generator operating speed based on an operator-
defined speed set
point. Such action may convert the frequency of electrical output 352 from the
generator to
match the power system frequency. The generator speed set point in the VSCF
drive may be
adjusted based on the pressures and/or temperatures as sensed by the first
and/or second sets
of one or more sensors 402, 404.
[0046] It is possible for other components to be operationally connected
to the hydraulic
turbine 323 in place of or in addition to the generator 340. For example,
Figure 6 is a schematic
view of another aspect of the disclosure in which a mechanical brake 360 is
operationally
connected to the hydraulic turbine 323. The mechanical brake may be adjusted
based on the
pressures and/or temperatures as sensed by the first and/or second sets of one
or more sensors
402, 404. Alternatively or additionally, as shown in Figure 7, a compressor
such as a
centrifugal compressor 370 may be operationally connected to the hydraulic
turbine via, for
example, a shaft 372. The centrifugal compressor 370 may be used to compress
one or more
fluids in the natural gas liquefaction process 100 or in other processes as
desired.
[0047] Aspects of the disclosure may be modified in many ways while
keeping with the
spirit of the disclosure. For example, the generator 340 may also function as
a motor to power
up the hydraulic turbine 323 during a start-up operation. Additionally, more
than one hydraulic
turbine may be used in series and/or in parallel with hydraulic turbine 323.
[0048] Figure 8 is a method 800 of liquefying a natural gas stream to
produce liquefied
natural gas (LNG) according to disclosed aspects. At block 802 the natural gas
stream is
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sequentially cooled in first, second, and third cooling bundles of a
liquefaction heat exchanger.
The second cooling bundle liquefies the natural gas stream to produce an LNG
stream. At
block 804 the LNG stream is cooled and its pressure is reduced between the
second cooling
bundle and the third cooling bundle using a hydraulic turbine, to thereby
produce a reduced-
.. pressure LNG stream. At block 806 work energy is produced using the
hydraulic turbine.
[0049] Figure 9 is a method 900 of liquefying a natural gas stream to
produce liquefied
natural gas (LNG). At block 902 the natural gas stream is sequentially cooled
in a liquefaction
heat exchanger having first, second, and third cooling bundles. The second
cooling bundle
liquefies the natural gas stream to produce an LNG stream. At block 904 the
LNG stream is
cooled and its pressure is reduced between the second cooling bundle and the
third cooling
bundle using a hydraulic turbine. At block 906 work energy is produced using
the hydraulic
turbine. At block 908, using the work energy, power is generated using a
generator connected
to the hydraulic turbine. At block 910 the pressure of the LNG stream exiting
the hydraulic
turbine is controlled using a control valve disposed between the outlet of the
hydraulic turbine
and an inlet of the third cooling bundle. At block 912 at least one of a speed
of the hydraulic
turbine, an LNG inlet rate of the hydraulic turbine, a position of the bypass
valve, a position of
the control valve, and a speed of the generator, are adjusted based on at
least one of a sensed
temperature of the LNG stream prior to entering the hydraulic turbine, a
sensed pressure of the
LNG stream prior to entering the hydraulic turbine, a sensed temperature of
the LNG stream
as the LNG stream exits the hydraulic turbine, and a sensed pressure of the
LNG stream as the
LNG stream exits the hydraulic turbine.
[0050] The aspects disclosed herein provide a method of expanding and
cooling a natural
gas stream in a liquefaction heat exchanger. This method is applicable in
cryogenic heat
exchangers used to generate LNG, but may also be used in other cryogenic heat
exchangers.
The method and system may be retrofitted into an existing LNG producing
facility, or may be
designed into a new facility. An advantage of the disclosed aspects is that
work energy can be
extracted from the LNG within a liquefaction heat exchanger. This work energy
can be used
advantageously in many ways, such as by powering a generator, a mechanical
brake, and/or a
compressor. Another advantage is that the temperature of the LNG stream is
lowered by
.. passing through the hydraulic turbine. This reduces the cooling duty of the
liquefaction heat
exchanger, and as a result the capacity of the liquefaction heat exchanger can
be increased.
[0051] Aspects of the disclosure may include any combinations of the
methods and systems
shown in the following numbered paragraphs. This is not to be considered a
complete listing
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of all possible aspects, as any number of variations can be envisioned from
the description
above.
1. A system for liquefying a natural gas stream, comprising:
a liquefaction heat exchanger having at least three cooling bundles and
arranged such
that the natural gas stream passes sequentially therethrough, including
a first cooling bundle configured to condense heavy hydrocarbon components in
the
natural gas stream,
a second cooling bundle configured to liquefy the natural gas stream, the
second cooling
bundle having an outlet for passing an LNG stream therethrough, and
a third cooling bundle having an inlet to receive the LNG, the third cooling
bundle
configured to sub-cool the LNG stream; and
a hydraulic turbine having an inlet operationally connected to the outlet of
the second
cooling bundle and an outlet operationally connected to the inlet of the third
cooling bundle,
the hydraulic turbine configured to cool the LNG stream and reduce a pressure
of the LNG
stream to form a reduced-pressure LNG stream.
2. The system of paragraph 1, further comprising:
a first set of one or more sensors situated to sense at least one of a
pressure and a
temperature of the LNG stream prior to entering the hydraulic turbine; and
a second set of one or more sensors situated to sense at least one of a
pressure and a
temperature of the LNG stream as the LNG stream exits the hydraulic turbine.
3. The system of paragraph 2, wherein at least one of a) a speed of the
hydraulic
turbine and b) an LNG inlet flow rate to the hydraulic turbine is adjusted
based on at least one
of the sensed temperature of the LNG stream prior to entering the hydraulic
turbine, the sensed
pressure of the LNG stream prior to entering the hydraulic turbine, the sensed
temperature of
the LNG stream as the LNG stream exits the hydraulic turbine, and the sensed
pressure of the
LNG stream as the LNG stream exits the hydraulic turbine.
4. The system of paragraph 2, further comprising a bypass valve
operationally
connecting the outlet of the second cooling bundle and the inlet of the third
cooling bundle
such that, when open, at least a portion of the LNG stream bypasses the
hydraulic turbine.
5. The system of paragraph 4, wherein the bypass valve is selectively
controlled
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based on at least one of the sensed temperature of the LNG stream prior to
entering the
hydraulic turbine, the sensed pressure of the LNG stream prior to entering the
hydraulic turbine,
the sensed temperature of the LNG stream as the LNG stream exits the hydraulic
turbine, and
the sensed pressure of the LNG stream as the LNG stream exits the hydraulic
turbine.
6. The system
of any of paragraphs 1-5, further comprising a control valve
disposed between the outlet of the hydraulic turbine and the inlet of the
third cooling bundle,
wherein the control valve is selectively controlled based at least in part on
one or more of a
sensed temperature of the LNG stream prior to entering the hydraulic turbine,
a sensed pressure
of the LNG stream prior to entering the hydraulic turbine, a sensed
temperature of the LNG
stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of
the LNG stream
as the LNG stream exits the hydraulic turbine.
7. The
system of any of paragraphs 1-6, further comprising a generator connected
to the hydraulic turbine and configured to generate power based on the work
energy generated
by the hydraulic turbine.
8. The system of paragraph 7, further comprising:
a first set of one or more sensors situated to sense at least one of a
pressure and a
temperature of the LNG stream prior to entering the hydraulic turbine, and
a second set of one or more sensors situated to sense at least one of a
pressure and a
temperature of the LNG stream as the LNG stream exits the hydraulic turbine;
wherein a speed of the generator is adjusted based on at least one of the
sensed
temperature of the LNG stream prior to entering the hydraulic turbine, the
sensed pressure of
the LNG stream prior to entering the hydraulic turbine, the sensed temperature
of the LNG
stream as the LNG stream exits the hydraulic turbine, and the sensed pressure
of the LNG
stream as the LNG stream exits the hydraulic turbine.
9. The system of
paragraph 7, further comprising a variable-speed constant-
frequency (VSCF) drive situated between the generator and a power system,
wherein the VSCF
drive is selectively controlled based at least in part on one or more of the
sensed temperature
of the LNG stream prior to entering the hydraulic turbine, the sensed pressure
of the LNG
stream prior to entering the hydraulic turbine, the sensed temperature of the
LNG stream as the
LNG stream exits the hydraulic turbine, the sensed pressure of the LNG stream
as the LNG
stream exits the hydraulic turbine and the power system frequency.
10. The
system of any of paragraphs 1-9, further comprising at least one of a
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mechanical brake and a compressor operationally connected to the hydraulic
turbine.
11. The system of paragraph 10, wherein the brake is selectively controlled
based
at least in part on one or more of a sensed temperature of the LNG stream
prior to entering the
hydraulic turbine, a sensed pressure of the LNG stream prior to entering the
hydraulic turbine,
a sensed temperature of the LNG stream as the LNG stream exits the hydraulic
turbine, and a
sensed pressure of the LNG stream as the LNG stream exits the hydraulic
turbine.
12. The system of any of paragraphs 1-11, further comprising:
a liquefied petroleum gas (LPG) stream configured to pass through the first
cooling
bundle and the second cooling bundle, the reduced-pressure LNG stream being at
a pressure so
to as to be combined with the LPG stream after the LPG stream has passed
through the second
cooling bundle.
13. A method of liquefying a natural gas stream to produce liquefied
natural gas
(LNG), comprising:
sequentially cooling the natural gas stream in first, second, and third
cooling bundles
of a liquefaction heat exchanger, wherein the second cooling bundle liquefies
the natural gas
stream to produce an LNG stream;
cooling and reducing the pressure of the LNG stream between the second cooling
bundle and the third cooling bundle using a hydraulic turbine, to thereby
produce a reduced-
pressure LNG stream; and
producing work energy using the hydraulic turbine.
14. The method of paragraph 13, further comprising:
adjusting at least one of a) a speed of the hydraulic turbine and b) an LNG
inlet rate of
the hydraulic turbine based on at least one of a sensed temperature of the LNG
stream prior to
entering the hydraulic turbine, a sensed pressure of the LNG stream prior to
entering the
hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream
exits the
hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream
exits the
hydraulic turbine.
15. The method of paragraph 13 or paragraph 14, further comprising:
selectively directing at least a portion of the LNG stream exiting the
hydraulic turbine
through a bypass valve that operationally connects an outlet of the second
cooling bundle and
an inlet of the third cooling bundle; and
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selectively controlling the bypass valve based on at least one of a sensed
temperature
of the LNG stream prior to entering the hydraulic turbine, a sensed pressure
of the LNG stream
prior to entering the hydraulic turbine, a sensed temperature of the LNG
stream as the LNG
stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as
the LNG stream
exits the hydraulic turbine.
16. The method of any of paragraphs 13-15, further comprising controlling a
pressure of the LNG stream exiting the hydraulic turbine by disposing a
control valve between
an outlet of the hydraulic turbine and an inlet of the third cooling bundle,
wherein the control
valve is selectively controlled based at least in part on one or more of a
sensed temperature of
the LNG stream prior to entering the hydraulic turbine, a sensed pressure of
the LNG stream
prior to entering the hydraulic turbine, a sensed temperature of the LNG
stream as the LNG
stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as
the LNG stream
exits the hydraulic turbine.
17. The method of any of paragraphs 13-16, further comprising:
connecting a generator to the hydraulic turbine; and
generating power using the generator based on the work energy generated by the
hydraulic turbine.
18. The method of paragraph 17, further comprising:
adjusting a speed of the generator based on at least one of a sensed
temperature of the
LNG stream prior to entering the hydraulic turbine, a sensed pressure of the
LNG stream prior
to entering the hydraulic turbine, a sensed temperature of the LNG stream as
the LNG stream
exits the hydraulic turbine, and a sensed pressure of the LNG stream as the
LNG stream exits
the hydraulic turbine.
19. The method of paragraph 17, further comprising:
controlling an electrical output of the generator using a variable-speed
constant-
frequency drive situated between the hydraulic turbine and the generator.
20. The method of any of paragraphs 13-19, further comprising:
operationally connecting at least one of a mechanical brake and a compressor
to the
hydraulic turbine.
21. The method of any of paragraphs 13-19, further comprising:
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obtaining a liquefied petroleum gas (LPG) stream from a fractionation process
that
occurs prior to the natural gas stream being sequentially cooled in the
liquefaction heat
exchanger;
cooling the LPG stream in the first cooling bundle and the second cooling
bundle, the
reduced-pressure LNG stream being at a pressure so as to be combined with the
LPG stream
after the LPG stream has passed through the second cooling bundle.
22. The method of paragraph 21, wherein the liquefaction heat exchanger is
part of
an operating LNG process, and further comprising:
retrofitting the hydraulic turbine between the second cooling bundle and the
third
cooling bundle.
23. A method of liquefying a natural gas stream to produce liquefied
natural gas
(LNG), comprising:
sequentially cooling the natural gas stream in a liquefaction heat exchanger
having first,
second, and third cooling bundles, wherein the second cooling bundle liquefies
the natural gas
stream to produce an LNG stream;
cooling and reducing the pressure of the LNG stream between the second cooling
bundle and the third cooling bundle using a hydraulic turbine;
producing work energy using the hydraulic turbine;
using the work energy, generating power using a generator connected to the
hydraulic
turbine;
controlling a pressure of the LNG stream exiting the hydraulic turbine using a
control
valve disposed between the outlet of the hydraulic turbine and an inlet of the
third cooling
bundle; and
adjusting at least one of
a speed of the hydraulic turbine,
an LNG inlet rate of the hydraulic turbine,
a position of the control valve, and
a speed of the generator,
based on at least one of a sensed temperature of the LNG stream prior to
entering the hydraulic
turbine, a sensed pressure of the LNG stream prior to entering the hydraulic
turbine, a sensed
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temperature of the LNG stream as the LNG stream exits the hydraulic turbine,
and a sensed
pressure of the LNG stream as the LNG stream exits the hydraulic turbine.
24. The method of paragraph 23, further comprising:
when the hydraulic turbine is desired to be bypassed, selectively directing at
least a
portion of the LNG stream exiting the middle bundle through a bypass valve
that operationally
connects an outlet of the second cooling bundle and an inlet of the third
cooling bundle; and
adjusting a position of the bypass valve based on at least one of the sensed
temperature
of the LNG stream prior to entering the hydraulic turbine, the sensed pressure
of the LNG
stream prior to entering the hydraulic turbine, the sensed temperature of the
LNG stream as the
LNG stream exits the hydraulic turbine, and the sensed pressure of the LNG
stream as the LNG
stream exits the hydraulic turbine.
[0052] While the foregoing is directed to aspects of the present
disclosure, other and further
aspects of the disclosure may be devised without departing from the basic
scope thereof, and
the scope thereof is determined by the claims that follow.
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