Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYSTEM AND METHOD TO PRODUCE LIQUEFIED NATURAL GAS
USING TWO DISTINCT REFRIGERATION CYCLES
WITH AN INTEGRAL GEAR MACHINE
Technical Field
(0001) The present invention relates to production of liquefied natural gas
(LNG), and
more particularly, to a small or mid-scale liquefied natural gas production
system that
employs at least two distinct refrigeration cycles with a single integral gear
machine.
Background
(0002) Demand for both liquified natural gas production and liquified natural
gas
applications within the energy, transportation, heating, power generation and
utility
sectors is rapidly increasing. The use of liquified natural gas as a lower
cost, alternative
fuel also allows for a potential reduction in carbon emissions and other
harmful
emissions such as nitrogen oxides (N0x), sulphur oxides (S0x), and particulate
matter
which are generally recognized as detrimental to air quality.
(0003) In areas where there is little to no access to natural gas pipeline
distribution
networks, a trend has emerged for small-scale or mid-scale liquified natural
gas
production which involves construction and operation of lower capacity
liquified
natural gas production systems built in regions where attractive sources of
low cost
natural gas or methane derived from biogas sources are available and where
there is a
current demand for liquified natural gas or the demand is expected to grow
over time.
With such small-scale liquified natural gas production, stranded gas resource
owners
can monetize their natural gas assets which could not be connected to a
natural gas
pipeline network.
(0004) Small-scale to mid-scale liquified natural gas opportunities include
various
energy applications such as oil well seeding or boil-off gas re-liquefaction,
integrated
CO2 extraction and natural gas liquefaction, utility sector applications such
as peak-
shaving or emergency reserves, liquified natural gas supply at compressed
natural gas
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filling stations, and transportation applications including marine
transportation
applications, off-road transportation applications, and even on-road fleet
transportation
uses. Other small-scale or mid-scale liquified natural gas opportunities might
include
liquified natural gas production from biogas sources such as landfills, farms,
industrial/municipal waste and wastewater operations.
(0005) Most conventional small-scale or mid-scale liquified natural gas
production
systems target a production of between 100 mtpd and 500 mtpd of liquified
natural gas
(e.g. small-scale plants) and higher, up to about 5000 mtpd of liquified
natural gas for
mid-scale plant operations. Many of these liquefaction systems employ
mechanical
refrigeration or a nitrogen-based gas expansion refrigeration cycle to cool to
the natural
gas feed to subzero temperatures required for natural gas liquefaction. Use of
a
nitrogen-based gas expansion refrigeration cycle is quickly becoming preferred
technology due to its simplicity, safety and ease of operation and maintenance
as well
as good turn-down characteristics.
(0006) A generic example of a conventional natural gas liquefaction system
employing
nitrogen-based gas expansion refrigeration cycle with dual expansion is
schematically
shown in Figs 1A and 1B. Such systems have been in use for many years and are
well
known in the art. For example, Air Products and Chemicals, Inc. offers
multiple
variants of liquefaction systems including: a single expander and dual
expander
nitrogen recycle liquefaction system (AP-N-1'); a single mixed refrigerant
liquefaction
systems (AP-SMR'); and a methane expander based liquefaction systems (AP-C1').
Another natural gas liquefaction system that discloses a three turbine natural
gas
liquefaction cycle is disclosed in United States Patent No. 5,768,912 (Dubar),
specifically employs three booster loaded nitrogen expanders disposed in
series.
(0007) While the overall production and use of liquified natural gas is
increasing and
the need for small-scale or mid-scale liquified natural gas plants is
continuing to rise, it
is the efficiencies of the conventional liquefaction systems and cycles that
is less than
ideal resulting in increased operating costs. When designing natural gas
liquefaction
cycles and liquefaction systems, trade-offs between capital costs and
operational
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efficiencies must often be made. Such decisions are highly dependent on site-
specific
variables, including quality of the natural gas feed as well as the intended
applications
and transport of the liquified natural gas product.
(0008) What is needed, therefore are improvements in the design philosophy and
overall performance of such natural gas liquefaction processes and systems
with the
objective of minimizing the heat exchange liquefaction inefficiencies and
power
consumption while facilitating turbo-machinery design. This goal of minimizing
the
heat exchange liquefaction inefficiencies is critical to achieving meaningful
performance improvements.
Summary of the Invention
(0009) Features and advantages of the present system and method to produce
liquefied
natural gas include: (i) a liquefaction cycle that uses two distinct
refrigeration circuits
having compositionally different working fluids operating at different
temperature
levels, (ii) conditioning of the natural gas feed to produce purified,
compressed natural
gas stream at a pressure equal to or above the critical pressure of natural
gas and
substantially free of heavy hydrocarbons and other impurities; and (iii) use
of a mixed
service integral gear machine having at least three pinions and configured for
driving
the one or more recycle compression stages of the refrigeration circuits while
also
receiving work produced by one or more high efficiency radial inflow turbines
of the
refrigeration circuits, with the pairings of turbomachinery on the different
pinions
optimized to reduce or minimize the heat exchange liquefaction inefficiencies
to
improve the production capacity of the small-scale or mid-scale system while
reducing
the unit power consumption for liquefied natural gas production.
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Brief Description of the Drawings
(00010) It is believed that the claimed invention will be
better understood when
taken in connection with the accompanying drawings in which:
(00011) Fig. lA shows a generalized schematic of the process
flow diagram for a
conventional natural gas liquefaction process known in the prior art;
(00012) Fig. 1B shows a generalized schematic illustration
of a conventional
integral gear machine with three pinions and coupled to two turbines;
(00013) Fig. 2A shows a schematic of the process flow
diagram for the present
system and method for liquefied natural gas production using two distinct
refrigeration
circuits and an integral gear machine with three pinions and three turbines;
(00014) Fig. 2B shows a schematic illustration of the
integral gear machine with
three pinions of Fig. 2A depicting the optimized pairing of turbomachines;
(00015) Fig. 3A shows a more detailed schematic of the
process flow diagram
for an alternate embodiment of the present system and method for liquefied
natural gas
production using two distinct refrigeration circuits and a smaller frame
integral gear
machine with three pinions and including three turbines;
(00016) Fig. 3B shows a schematic illustration of the
integral gear machine with
three pinions of Fig. 3A depicting the optimized pairing of turbomachines;
(00017) Fig. 4A shows a generalized schematic of the process
flow diagram for
the present system and method for liquefied natural gas production using two
distinct
refrigeration circuits and an integral gear machine with four pinions;
(00018) Fig. 4B shows a schematic illustration of the
integral gear machine with
four pinions of Fig. 4A depicting the optimized pairing of turbomachines;
(00019) Fig. 5A shows a generalized schematic of the process
flow diagram for
the present system and method for liquefied natural gas production showing an
alternative embodiment using two distinct refrigeration circuits and an
integral gear
machine with four pinions;
(00020) Fig. 5B shows a schematic illustration of the
integral gear machine with
four pinions of Fig. 5A;
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(00021) Fig. 6A shows a generalized schematic of the process
flow diagram for
the present system and method for liquefied natural gas production using two
distinct
refrigeration circuits and an integral gear machine with three pinions and
including two
turbines;
(00022) Fig. 6B shows a schematic illustration of the
integral gear machine with
three pinions of Fig. 6A depicting the optimized pairing of turbomachines;
(00023) Fig. 7A shows a generalized schematic of the process
flow diagram for
the present system and method for liquefied natural gas production using two
distinct
refrigeration circuits and an integral gear machine with three pinions and a
separate
high speed, high efficiency booster loaded turbine driving a natural gas
compression
stage; and
(00024) Fig. 7B shows a schematic illustration of the
integral gear machine with
three pinions of Fig. 7A depicting the optimized pairing of turbomachines.
Detailed Description
(00025) The design of high efficiency liquefaction processes
is the result of a
simultaneous considerations of heat transfer and turbomachinery. The
minimization of
heat transfer irreversibility is achieved when the divergence of the warming
and cooling
composite curves (e.g. energy vs temperature) is minimized. Process definition
of
flows, pressures and temperatures largely control the resulting composite
curves.
Turbomachinery efficiency is maximized when the head and flow characteristics
of the
process are consistent with experience-based optimums. These optimal designs
are
often characterized by established ratios of geometry, flow and head (Ns, Ds).
Such
considerations resulting from dimensional similarity are well known to the art
of gas
processing. See, for example, the publication entitled 'How to Select
Turbomachinery
for your Application' by Kenneth E. Nichols. These optimal turbomachinery
conditions are a function of the type of machine under consideration.
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(00026) In the present system and method, the use of
centrifugal turbomachines,
and in particular several radial inflow turbines, find particular application.
Satisfying
the characterizing dimensionless ratios is critical to maximizing turbine and
compressor
efficiency. The subject invention addresses the issue of accomplishing both of
these
objectives simultaneously. The introduction of a second working fluid normally
would
require a separate expansion-compression train. For modest scale liquefied
natural gas
production, the capital expense of such additional machinery is prohibitive.
The
integration of a second working fluid into a single common integral gear
compression
system or machine presents numerous challenges. In addition to those
highlighted
above, the work imparted to any particular pinion of such a machine is often
limited to
about 35% to 50% of the total power draw.
(00027) As indicated above, one of the distinct features of
the present system and
method to produce liquefied natural gas is that the liquefaction cycle that
uses two
distinct refrigeration circuits having compositionally different working
fluids operating
at different temperature levels. Details of this feature and the advantages it
provides are
discussed later in this application.
(00028) Another of the advancements disclosed herein is the
conditioning of the
natural gas containing feed to produce purified, compressed natural gas stream
at a
pressure equal to or above the critical pressure of natural gas and
substantially free of
heavy hydrocarbons and other impurities Specifically, a conditioning circuit
is
employed that receives a natural gas containing feed stream, such as natural
gas derived
from a biogas source, and produces a purified, compressed natural gas stream
at a
pressure equal to or above the critical pressure of natural gas. The preferred
conditioning circuit includes a natural gas compression stage and optionally a
phase
separator and/or scrubbing column configured to remove impurities such as
heavy
hydrocarbons from the natural gas feed stream. The scrubbing column may employ
bypass vapor feed or indirect heating as a means of generating stripping
vapor. Indirect
heating may be accomplished by cooling any one of the warm constituent fluids
(e.g.
compressed nitrogen or natural gas). In addition, water and carbon dioxide may
be also
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removed within the conditioning circuit, preferably upstream of the phase
separator or
scrubbing column through the use of an adsorbent-based temperature swing
adsorption
(TSA) unit. For example, to remove the heavy hydrocarbons, the natural gas
feed
stream may be cooled and then directed to a scrubbing column or phase
separator
configured to strip out impurities and produce an overhead stream of purified
natural
gas vapor and an impure bottoms liquid stream. The overhead stream of purified
natural
gas vapor is then directed to a natural gas compression stage.
(00029) The present system and method details an approach
where the natural
gas feed stream is first pretreated by way of partial condensation, phase
separation
and/or rectification (i.e. scrubbing) before the natural gas feed stream is
compressed.
Such pre-treatment operations naturally must be conducted at conditions that
are
substantially removed from the critical point of the natural gas mixture. In
general,
direct phase separation becomes impractical at pressures greater than about
75% of
critical pressure. This fact creates a heat transfer inefficiency in
conventional natural
gas liquefaction plants. The subsequent and direct liquefaction of a sub-
critical gas
stream results in a composite curve divergence near the dewpoint of the
mixture.
Furthermore, the lower pressure of liquefaction generally results in a colder
level of
warm turbine operation. The colder operation of the primary refrigeration
turbine
creates a meaningful penalty in terms of unit power consumption.
(00030) Yet another advantageous feature of the present
system and method to
produce liquefied natural gas is the use of a mixed service integral gear
machine having
at least three pinions and configured for driving the one or more recycle
compression
stages of the refrigeration circuits while also receiving work produced by at
least one of
the one or more high efficiency radial inflow turbines of the refrigeration
circuits. An
important aspect of this advantageous feature relates to the pairings of
turbomachinery
on the different pinions in a manner that optimizes the performance of the
present
system and method.
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(00031) The optimization of the turbomachinery starts with a
consideration of
turbine efficiency. Any given process definition (e.g. Pressures,
Temperatures, and
Flows) that results in a feasible heat transfer (liquefaction) design also
provides the
necessary input, such as flow and head characteristics, that are necessary to
define the
non-dimensional characteristics (Ns, Ds) required to specify component turbine
rotational speed and diameter. It is well established that radial inflow
turbines reach
peak efficiency with U/Co (i.e. Rotor Tip Speed/Isentropic Spouting Velocity)
values
near 0.70. This ratio is also defined by the following equation [U/Co] =
[NsDs]/154. As
such, effective process definition will dictate the speed and diameter
necessary for the
turbine to operate at peak efficiency. In the context of the present
invention, this
optimal turbine speed is then applied to the association compression stage. In
general,
optimal centrifugal compression stage efficiency can be attained for specific
speed (Ns)
values ranging from about 80 to about 130. With respect to gas compression,
process
definition dictates compression stage head and the associated turbine on the
same
pinion dictates rotational speed which in turn results in a specific speed.
The above
calculation form one part of the overall process optimization. More
specifically, the
optimization is an iterative process involving process definition,
turbomachine pairing
based upon the above calculation and finally a consideration of the integral
gear
machine pinion power and overall input power limitations.
(00032) A conventional two-turbine nitrogen expansion-based
liquefier can
follow a more or less sequential design approach. In contrast, the present
system and
method was developed by approaching this problem from the standpoint that high
efficiency liquefaction must be maintained (i.e. the process definition
minimizes heat
transfer irreversibility). The use of a mixed service integral gear 'bridge'
machine
servicing dual refrigeration circuits, each having gas compression stages and
gas
expansion is critical to that end. the'
turbomachinery is then defined so as to satisfy the
conditions for optimal turbine performance (outlined above) as well as the
constraints
imparted by the need to consolidate compression-expansion service into a
single
integral gear 'bridge' machine. The hardware constraints and limitations of
the bridge
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machine are typically a function of bull gear and primary driver size. In
general, the
'bridge' machine drivers pertinent for the present system and method spans the
range of
about 4 MW to 20 MW with associated maximum pinion speeds in the range of
20,000
to 50,000 rpm. Furthermore, the maximum power imparted to any given pinion or
any
given turbine-compression stage pairing is generally limited to less than 50%
and in
some cases to about 35% (of the total bridge machine driver power).
(00033) Conventional small-scale and medium-scale liquified
natural gas plants
(i.e. <1000 mtpd) that use a nitrogen-based gas expansion as the primary
source of
refrigeration typically employ centrifugal recycle compression stages for the
refrigerant
that are typically driven by a single service integral gear 'bridge' machine
contained
within a common housing that includes a large diameter bull gear with several
meshing
pinions upon the ends of which the various compression impellers are mounted
forming
the plurality of refrigerant compression stages. The pinions may have
differing
diameters to best match the speed requirements of the coupled compression
impellers.
Each of the multiple compression impellers and radial turbines are typically
contained
within their own respective housings and collectively provide several stages
of recycle
compression, as desired.
(00034) Linde Inc. has also developed a portfolio of
integral gear machines
combining compression stages and high efficiency radial inflow expanders on a
single
machine having up to four pinions in what is referred to as an integral gear
'bridge'
machine. Linde's bridge machines are conventionally used in hydrogen/syngas
plants
as well as air separation plants and typically come in different frame sizes.
The Linde
'bridge' machines can be used to operatively couple a plurality of radially
inflow
turbines and centrifugal compression stages. The Linde 'bridge' machines come
fully
packaged or integrated with appropriate PLC controllers, control valves,
safety valves,
intercoolers, aftercoolers, oil system, etc.
(00035) Modification of the conventional single service
integral gear
compression machines or the Linde 'bridge' machine to handle mixed gas service
could
involve additional capital costs estimated to be about 5% to 10% of the total
machine.
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The additional capital costs would be targeted for retrofitting the machine
controls and
provide dry gas sealing for the natural gas service. However, these additional
capital
costs are more than offset by the improvement in liquefaction efficiency and
the unit
power cost reduction of the liquefaction process.
(00036)
The closest prior art reference disclosing a liquefaction cycle that uses
both natural gas and a nitrogen-based refrigerant as the two distinct
refrigeration
circuits having compositionally different working fluids operating at
different
temperature levels is United States Patent No. 6,412,302 issued in the name of
Foglietta. One of the key differences between the Foglietta reference and the
present
system and method is that the disclosed Foglietta system and process requires
at least
two stages of natural gas recompression (i.e. centrifugal compression) to
achieve the
disclosed compression ratio of 2.5 to 7.0, which could require a minimum of
two or
perhaps three pinions on the integral gear machine to service the natural gas.
Similarly,
the nitrogen expander in the disclosed Foglietta system and process also
requires at
least two stages of nitrogen compression requiring two additional pinions, for
a
minimum of four pinions on the integral gear machine in the disclosed
Foglietta
system. The process and would likely require use of a larger frame bull gear.
(00037)
The Foglietta reference also discloses a closed loop hydrocarbon based
refrigerant circuit. With the methane in the refrigeration loop, the expander
exhausts at
about 200 psia and -119 F and subsequently compressed in at least two or more
stages
of recompression up to 1400 psia. In contrast, the natural gas feed in
Foglietta is
delivered to the heat exchanger at about 900 psia, which admittedly is above
the critical
pressure but would require either a different machine to drive the compression
stages of
the natural gas feed or yet additional pinions on the single mixed service
machine.
Unlike, the present system and method, there is no disclosure of any
integration of the
conditioning circuit to remove the heavy hydrocarbons from the natural gas
feed
stream, nor is the feed split into a first portion to be liquefied and a
second portion to be
directed to the refrigeration circuit. In short, the Foglietta reference
simply does not
disclose, suggest or even contemplate a mixed service integral gear machine.
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LNG Production with 3-Pinion and 3 Turbine Integral Gear Machine
(00038) Turning to Figs. 2A and 3A, schematics of the high-
level process flow
diagram for similar embodiments of the present system and method for liquefied
natural gas production using two distinct refrigeration circuits and an
integral gear
machine are shown. As seen therein, a natural gas vapor feed 200, at a nominal
feed
pressure of between about 20 bar(a) and 40 bar(a), and by way of example at a
pressure
of about 34 bar(a), is received and thereafter conditioned in a conditioning
circuit to
remove the heavy hydrocarbons and other impurities from the feed stream and
pressurize the purified natural gas containing stream to a pressure equal to
or above the
critical pressure of natural gas.
(00039) As seen in the figures, the conditioning circuit
preferably includes partial
cooling of the natural gas feed 200A in the heat exchanger E4 and then
purifying the
cooled natural gas feed 201 and/or natural gas vapor stream 200B in a
scrubbing column
D1 to remove the heavy hydrocarbons and other impurities from the natural gas
feed
stream. An overhead vapor stream 202 of purified natural gas exits the top of
the
scrubbing column D1 while a liquid bottoms stream 220 containing the heavy
hydrocarbons and impurities is removed from the column. Alternatively, the
conditioning circuit may use a phase separator or both a phase separator and a
scrubbing
column to strip out the heavy hydrocarbons and other impurities from the
natural gas
feed stream. In addition, although not shown, the purification of the natural
gas feed
stream may also include removal of water and carbon dioxide via purification
techniques
well known in the art, such additional purification techniques preferably
conducted
upstream of the scrubbing column. The purification techniques may include
solvent
based absorption systems, adsorptive purification as well as adsorptive
gettering.
(00040) The purified natural gas vapor stream 202 is
directed to a natural gas
compression stage C3 operatively coupled to the integral gear machine (see
Fig. 2B),
preferably a Linde-type 'bridge' machine, where it is further compressed to a
pressure
equal to or above the critical pressure of natural gas, or above 46 bar(a). In
the
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presently illustrated systems, the purified natural gas containing stream is
further
compressed to a pressure preferably between about 50 bar(a) and 80 bar(a), and
more
preferably to a pressure between about 60 bar(a) and 75 bar(a) and then cooled
in
aftercooler E3.
(00041)
A first portion of the purified, further compressed super-critical natural
gas stream 204 is directed to the cooling passages in the heat exchanger(s) E4
where it
is liquefied and subcooled via indirect heat exchange with two or more
different
refrigerant streams traversing the warming passages of the heat exchanger(s)
E4. A
second portion of the purified, further compressed super-critical natural gas
stream 210
is partially cooled in heat exchanger E4 and the partially cooled stream 211
is then
expanded in a natural gas expander T3 to produce a natural gas exhaust stream
212
having a pressure less than or equal to the pressure of the natural gas feed
stream 200.
Preferable, the flow of second portion of the purified, compressed natural gas
stream
210 is at least 2.0 times greater, and more preferably greater than 2.5 times
greater, than
the flow of first portion of the purified, compressed natural gas stream 204.
After
expansion, the natural gas exhaust stream 212 is directed to heat exchanger(s)
E4 to
cool the first portion of the purified, compressed natural gas stream 204 or
other natural
gas streams and is then recycled back to the natural gas compression stage
together
with the purified natural gas stream 202 as recycle stream 203.
(00042)
The natural gas expander T3 is preferably a high speed, high efficiency
radial inflow turbo-expander operatively coupled to the integral gear machine
and
configured with an expansion ratio approximately equal to or comparable to a
compression ratio of the natural gas compression stage C3, which is typically
below
about 3Ø In the embodiment shown in Figs. 3A and 3B, the high speed, high
efficiency radial inflow turbo-expander is also operatively coupled to the
same pinion
of the integral gear machine as the natural gas compression stage. Exactly
what
constitutes a high-speed expander very much depends on the size and capacity
of the
integral gear machine. For example, one skilled in the art would characterize
a natural
gas expander configured to operate at about 50,000 rpm when associated with a
small
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integral gear machine frame (2-4 MW of absorbed power) as high speed whereas a
natural gas expander configured to operate at about 30,000 rpm would be
considered a
high speed expander if associated with a large integral gear machine frame.
(00043) As indicated above, the first portion of the
purified, further compressed
super-critical natural gas stream 204 is cooled within the heat exchanger(s)
E4 via
indirect heat exchange against the combined recycle stream 202, 212, 203 as
well as a
primary nitrogen-based refrigerant streams 104, 105 and yields a subcooled
liquified
natural gas stream 205. A portion of the subcooled liquified natural gas
stream 209
may optionally be directed as a reflux stream to the scrubbing column as
depicted in
Figs. 2A, 4A, and 5A. The remaining portion of subcooled liquified natural gas
stream
or the entire subcooled liquified natural gas stream is thereafter reduced in
pressure via
a valve 208 or a liquid turbine and phase separated in a phase separator D2
yielding a
vapor stream 207 and liquid natural gas stream 206 constituting the liquefied
natural
gas product. It should be noted that in some instances it may be advantageous
to
employ a small portion of the liquefied natural gas as a recycle and reflux
stream to the
scrubbing column.
(00044) The primary refrigeration used in the illustrated
liquefied natural gas
production system that uses two distinct refrigeration circuits and an
integral gear
machine is preferably a nitrogen-based gas expansion refrigeration circuit. In
such
illustrated primary refrigeration circuit, the primary refrigerant 106, 107 is
compressed
in a plurality of serially arranged compression stages Cl, C2 with appropriate
intercooling and aftercooling by aftercoolers El and E2 used to remove the
heat of
compression. Such aftercooling may be accomplished by way of indirect contact
with
air, cooling water, chilled water or other refrigerating medium or
combinations thereof
The compressed primary refrigerant 100 is then further cooled in the heat
exchanger(s)
E4 and directed to one or more turbines Ti, T2 configured to expand the
compressed
refrigerant streams to generate refrigeration.
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(00045)
Specifically, the compressed primary refrigerant stream 100 is partially
cooled in the heat exchanger E4 and the resulting cooled stream 101 is split.
A first
portion of the cooled, compressed refrigerant stream 100 is directed to a warm
turbine
Ti while a second portion of the cooled, compressed primary refrigerant stream
102 is
further cooled in the heat exchanger E4 to produce a cold stream portion 103
which is
then directed to a cold turbine T2. The cold turbine T2 is configured to
expand the cold
stream portion 103 of the primary refrigerant stream to produce a cold turbine
exhaust
stream 104 that is recycled back to the primary refrigerant compression stages
as
recycle stream 105 via one or more of the plurality of warming passages in the
heat
exchanger(s) E4. The partially cooled first portion is a warm stream portion
110 of the
compressed primary refrigerant stream that exits the heat exchanger E4 at a
location
and temperature that is warmer than the cold portion. The warm stream portion
110 of
the compressed refrigerant stream is then expanded in the warm turbine Ti to
produce a
warm turbine exhaust stream 111 that is also recycled to the one or more
primary
refrigerant compression stages as recycle stream 105, 106 via one or more of
the
plurality of warming passages in the heat exchanger(s). Although not
preferred, the
primary refrigerant streams may be warmed in independent passages and
conceivably at
independent pressures. The warmed primary refrigerant streams could be
directed to
differing introduction points in the recycle compression train. More
generally, it is
recognized that the design of multi-pass brazed aluminum heat exchangers are
capable
of processing multiple stream wherein internal redistribution point may be
configured.
Such an option can be employed with the subject invention. For instance, the
first
portion of the conditioned natural gas stream may be subjected to
redistribution into
increasing numbers of passages as the fluid cools. Similarly, the cold turbine
exhaust
stream from the primary refrigeration circuit may be extracted at an
intermediary point
and combined with the warm turbine exhaust stream before or after partial
warming
within the multi-pass heat exchanger.
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(00046) Both the warm turbine Ti and the cold turbine T2 as
well as the serially
arranged compression stages Cl and C2 are operatively coupled to the integral
gear
machine (See Figs 2B and 3B). In particular, one of the primary refrigerant
compression stages C2 and the cold turbine T2 are operatively coupled to the
same
pinion of the integral gear compressor machine. Likewise, the other primary
refrigerant
compression stage Cl and the warm turbine Ti are operatively coupled to the
same
pinion of the integral gear compressor machine.
(00047) Turning now to Figs. 2B and 3B as well as Tables 1A,
1B, and IC,
embodiments of the three pinion and three turbine integral gear machine is
schematically depicted in Figs. 2B and 3B showing a bull gear driven by a
motor and
comprised of a plurality of compression stages and turbines. In Tables 1A, 1B,
and IC,
the power consumption of the three pinion and three turbine integral gear
machine has
been normalized to the nominal liquefied natural gas product flow. In this
example, the
bull gear accommodates three pinions and is sized to deliver roughly 280
metric tonnes
per day (mptd) to about 320 mptd of liquefied natural gas. The first pinion
couples the
bull gear to a first recycle compression stage and the warm turbine and
absorbs about
35% of the input power to the integral gear machine. The second pinion
operatively
couples the bull gear to the second recycle compression stage and the cold
turbine and
absorbs about 42% of the integral gear machine power. In this configuration,
the second
pinion operates near the maximum fractional power for any given pinion
relative to
total integral gear machine absorbed power. Note that the warm turbine
provides more
than 4 times the power than that of the cold turbine, the warm turbine
provides the
largest source of refrigeration, and more particularly in this example about
4.5 times
more power than the cold turbine. In this embodiment, the third pinion
arrangement is
dedicated to the natural gas service, namely the natural gas compression stage
requiring
and natural gas turbine expansion and absorbs the remaining 23% of the
integral gear
machine power.
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(00048) Table 1B compares the simulated performance of the
baseline liquefied
natural gas system and process generically depicted in Fig. 1A with the three-
pinion,
three-turbine arrangement shown in Figs. 2A and 3A using the above-described
arrangement of the turbines and compression stages on the three pinions of the
integral
gear machine. As seen therein, the energy usage per metric tonne of liquefied
natural
gas produced is about 10 percent lower. Given the lower unit power and
distributed
power consumption of the three pinion design any given machine frame size will
likely
deliver a liquefied natural gas product flow increase of about 12% to 15%. The
increased liquefied natural gas production rate resulting from the present
system and
method is dependent upon the maximum absorbable pinion power and the total
potential power consumption of the integral gear machine.
(00049) The process and configurations detailed in Figs. 2A,
2B, 3A and 3B can
be effectively applied over a broad range of liquefied natural gas production
rates by
simply changing the frame size of the integral gear 'bridge' machine and
relative sizes
of the associated turbomachinery. In general, such a process will find utility
with
commercially available bull gears in a liquified natural gas production
capacity range of
between about 150 mtpd and about 1000 mtpd. The relative distribution of power
across the three pinions will vary depending upon the pinion speed and the
power
limitations or constraints imposed by any particular bull gear, with the
approximate
normalized range of total adsorbed power for each pinion shown in Table 1C.
The
target pinion speed per unit of liquified natural gas mass flow will also vary
to the
reciprocal of liquified natural gas mass flow raised to roughly the 3/2 power.
0.213
W313aMiaiaiAiAiifiiWit4.04.4044kagaigniginaggiiMiAN%04.4C%06iNOAM:***44004).5
Pinion #1 N2 Comp #1 Cl 0.279 N2 Warm T1 -0.132
138.1 0.148
Pinion #2 N2 Comp #2 C2 0.231 N2 Cold T2 -0.029
182.6 0.201
Pinion #3 NG Comp #3 C3 0.118 NG Warm T3 -0.043
217.1 0.074
Table 1A
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:iEmhodinientMAWNZEMiiiNOxotooto pINO:t4.4.00.14.00.04g ntGATVW#W N*0000
Pressure Pressure : Pressure Pressure
............. kw-hv/rnt Usate (%)
KIIROMEM
iiiiMISEMOliaiijEMMIUGSMAGNELaigin
Fig. 1 63.0 10.3 34.0 39.0 471
Baseline
Figs. 2A, 3A 51.5 12.0 34.0 69.0 423
-10.2%
Table 1B
Pinion Minimum Mairnum
AMA
Pinion #1 (C1-T1) Warm Turbine 0.10 ().2()
Pinion #2 (C2-T2) Cold Turbine 0.15 0.25
Pinion #3 (C3-T3) NG Warm Expander 0.05 0.20
Table 1C
LNG Production with 4-Pinion and 3 Turbine Integral Gear Machine
(00050) The process flow diagram depicted in Figs. 4A and 5A
are very similar
to the process flow diagrams of Fig. 3A described above and for sake of
brevity, much
of the descriptions of the detailed arrangements will not be repeated. Rather,
the
following discussion will focus on the differences in the process flow diagram
depicted
in Figs. 4A and 5A when compared to the process flow diagram depicted in Fig.
3A.
(00051) In the process flow diagram of 4A, the main
difference is the presence of
a third compression stage C2B in the primary refrigeration circuit. This third
primary
refrigerant compression stage C2B is arranged in a parallel arrangement with
the second
primary refrigerant compression stage C2A where both the second and third
primary
refrigerant compression stages are disposed downstream of the first primary
refrigerant
compression stage Cl. The third primary refrigerant compression stage C2B is
also
operatively coupled to the integral gear machine by a fourth pinion (see Fig.
4B). Note
reference numerals 400, 400A, 400B 401, 402, 403, 404, 405, 406, 407, 410,
411, 412,
and 420 in Fig. 4A generally correspond to the same streams 300, 300A, 300B,
301,
302, 303, 304, 305, 306, 307, 310, 311, 312, and 320 in Fig. 3A, respectively.
Likewise, the reference numerals 450, 451, 452, 453, 454, 455, 456, 457 and
460, in
Fig. 4A generally correspond to the same streams 100, 101, 102, 103, 104, 105,
106,
107 and 110, in Fig. 3A, respectively.
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(00052) In the process flow diagram of 5A, the main
difference is the presence of
a third compression stage in the primary refrigeration circuit and a liquid
turbine LT
disposed downstream of the heat exchanger(s) configured to expand the
subcooled,
liquified natural gas stream 505 to produce stream 505B. Similar to the
embodiment
shown in Fig. 4A, the third primary refrigerant compression stage C2B is
arranged in a
parallel arrangement with the second primary refrigerant compression stage
C2Awhere
both the second and third primary refrigerant compression stages C2A and C2B
are
disposed downstream of the first primary refrigerant compression stage Cl. The
third
primary refrigerant compression stage C2B is operatively coupled to the
integral gear
machine by means of a fourth pinion (see Fig. 5B). Note reference numerals
500, 500A,
500B 501, 502, 503, 504, 505, 506, 507, 510, 511, 512, and 520 in Fig. 5A
generally
correspond to the same streams 300, 300A, 300B, 301, 302, 303, 304, 305, 306,
307,
310, 311, 312, and 320 in Fig. 3A, respectively. Likewise, the reference
numerals 550,
551, 552, 553, 554, 555, 556, 557 and 560, in Fig. 5A generally correspond to
the same
streams 100, 101, 102, 103, 104, 105, 106, 107 and 110, in Fig. 3A,
respectively.
(00053) In general, the cold turbine supplies only about 10%
to 20% of the total
refrigeration required for the liquefaction of supercritical natural gas. In
contrast, the
nitrogen-based warm turbine may provide in excess of 50% of the required
refrigeration. Although effective pairing of the cold turbine is possible with
respect to
the nitrogen-based recompression train, the associated pinion will consume a
disproportionate amount of power relative to the pinion associated with the
warm
turbine. As a consequence, it has been found that it is the pinion associated
with the
cold turbine that is most likely to define or limit the capacity for a three
pinion integral
gear machine design. To alleviate this constraint, the power associated with
the cold
turbine pinion (compression stage) may be partially displaced (or shared) via
an
additional fourth pinion. The purpose of this additional pinion is to reduce
the power
consumed by the booster compression stage associated with the cold turbine. By
diverting about 40% to 60% of the work toward a separate compression stage on
a
separate pinion (i.e. the fourth pinion), the utilization of the integral gear
machine can
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be maximized (from the perspective of total power consumption). By fully
utilizing the
potential power consumption of the integral gear machine the quantity of
liquefied
natural gas produced from a fixed machine frame size is maximized. This is
advantageous from the standpoint of capital utilization.
(00054) The degree to which the high pressure natural gas is
subcooled at the
cold end of the liquefaction heat exchanger will dictate the quantity of gas
that is
ultimately flashed off (i.e. that liquid which is converted to gas upon
depressurization).
A simple isenthalpic expansion via a valve is less efficient than a dense
phase expander
or liquid turbine. Natural gas that is not maintained as a liquid represents a
loss or
inefficiency of the liquefaction process. By extracting mechanical energy from
the fluid,
the amount of flash gas generated at a common inlet pressure and temperature
will be
reduced. This added refrigerating effect becomes more pronounced as the
pressure of
the liquefied natural gas climbs. Since enhanced compression of natural gas
prior to
liquefaction is one of the objectives of the subject invention, the synergy
afforded to the
process by way of liquid turbine is accentuated. It has been found that the
unit power
consumption of the process can be further reduced by about 5% through the
addition of
a dense phase LNG expander. As noted, the total power draw of the integral
gear
machine is often the limiting aspect for small-scale and mid-scale liquefied
natural gas
production systems. Since the introduction of a dense phase LNG expander or
liquid
turbine reduces the net unit power consumption, additional throughput can be
achieved
given the same integral gear machine frame-driver size. A further, secondary
benefit of
the dense phase LNG expander or liquid turbine is the capture of mechanical
power
generated by the dense phase expansion by way of a generator. This secondary
benefit
can further reduce total cycle power consumption by about 0.5% to 1.0 %.
(00055) The embodiments of the three pinion and three
turbine integral gear
'bridge' machine schematically depicted in Figs. 4B and 5B are also compared
to the
baseline integral gear machine of Fig.1B in Tables 2A, 2B, and 2C. As with
Tables 1A,
1B, and 1C, the power consumption and speed values in Tables 2A, 2B, and 2C,
have
been normalized to the nominal liquefied natural gas product flow. In this
instance, a
19
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larger plant is assumed generally in the range of 450 to 475 mtpd of liquified
natural
gas. The first pinion couples the bull gear to first recycle compression stage
and the
warm turbine and preferably absorbs between about 0.10 and 0.2 kw*hr per kg of
liquified natural gas, and in the example depicted in Table 2A about 0.125
kw*hr per
kg of liquified natural gas while the fourth pinion arrangement is dedicated
to the
natural gas service and absorbs between about 0.05 and 0.20 kw*hr per kg of
liquified
natural gas, and in the example depicted in Table 2A about 0.072 kw*hr per kg
of
liquified natural gas which is roughly half of the power adsorbed by the first
pinion.
(00056) The remaining power from the integral gear 'bridge'
machine is to be
adsorbed by the second pinion and third pinion. The second pinion operatively
couples
the bull gear to the cold turbine and a first of two recycle split compression
stages
arranged in parallel while the third pinion operatively couples the bull gear
to the
second of two recycle split compression stages arranged in parallel. By
splitting the
second recycle compression stage into two split recycle compression stages
arranged in
parallel and on two different pinions, neither the second pinion or third
pinion operate
near the maximum fractional power limitations and constraints imposed by the
integral
gear 'bridge' machine. It should be noted that a serial splitting of the cold
turbine
pinion power is possible but is less advantageous than the configuration
shown.
(00057) Table 2B compares the simulated performance of the
baseline liquefied
natural gas system and process generically depicted in Fig lA with the three-
pinion,
three-turbine arrangement shown in Figs. 4A and 5A using the above-described
arrangement of the turbines and compression states on the three pinions of the
integral
gear machine. As seen therein, the reduction in energy usage per metric tonne
of
liquefied natural gas produced in the embodiment depicted in Fig. 4A compared
to the
baseline configuration is 10.2% while the reduction in energy usage per metric
tonne of
liquefied natural gas produced in the embodiment depicted in Fig. 5A compared
to the
baseline configuration is 14.8% percent.
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(00058)
Similar to the embodiments described above with reference to Figs. 2A
and 3A, the embodiments of Figs. 4A and 5A can also be effectively applied
over a
broad range of liquefied natural gas production rates from about 150 mtpd to
over 1000
mtpd by simply changing the frame size of the integral gear 'bridge' machine
and
relative sizes of the associated turbomachinery. However, to spatially
accommodate the
four pinions, it would be advantageous to employ generally larger frame sizes
suitable
for larger production rates, for example, greater than 300 mtpd of liquified
natural gas
capacity.
(00059) The relative distribution of power across the four
pinions will vary
depending upon the pinion speed and the power limitations or constraints
imposed by
any particular frame size of the integral gear 'bridge' machine, with the
approximate
normalized range of total adsorbed power for each pinion shown in Table 2C.
Again,
similar to the earlier described example, the target pinion speed per unit of
liquified
natural gas mass flow will also vary to the reciprocal of liquified natural
gas mass flow
raised to roughly the 3/2 power.
W140$077.g$064komg.g.m.g.gowogg.7m$0Ø40.#cm5.g.g.g.powieg
g.$.$6Ø0.4g.NNalgowoof
MitW4i.W405:
Pinion #1 N2 Comp #C1 0.258 N2 Warm T1 -0.133
70.6 0.125
Pinion #2 N2 Comp #C2A 0.142 N2 Cold T2 0.029
93.5 0.113
Pinion #3 N2 Comp #C2B 0.113
114.5 0.113
Pinion #4 NG Comp #C3 0.115 NG Warm T3 -0.042
114.4 0.072
Table 2A
EmhodirnentMI.4kNtM MIONIEMNOMe
mminimmisimPressure Pressure Pressure Pressure Is-
hthnt Usaie (%)
EIMEEMaag
Fig. 1 63.0 10.3 34.0 39.0 471
Baseline
Figs. 4A, 4B 51.5 12.0 34.0 69.0 423
-10.2%
Figs. 5A, 5B 60.7 14.8 34.0 69.0 401
-14.9%
Table 2B
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Pinion #1 (#C1-T1) Warm Turbine 0.10 0.20
Pinion 1t2 (ItC2A-T2) Cold Turbine 0.05 0.15
Pinion #3 (#C3-T3) NG Expander 0.05 0.20
Pinion #4 (#C2B) 0.05 0.15
Table 2C
LNG Production with 3-1'inion and 2 Turbine Integral Gear Machine
(00060) The process flow diagram depicted in Fig. 6A is in
many regards similar
to the process flow diagrams described above and for sake of brevity, much of
the
following discussion will focus on the differences in the process flow diagram
depicted
in Fig. 6A when compared to the process flow diagram depicted in Fig. 2A. The
main
differences can be seen in Fig 6A and summarized as follows: (1) both natural
gas
compression and nitrogen-based refrigerant compression are done using a series
of
compression stages, with many of the compression stages operatively coupled to
the
integral gear machine via the three pinions, as detailed in Fig. 6B; (2) only
a single
warm turbine/expander that is a natural gas expander operatively coupled to
one of the
natural gas compression stages on one of the three pinions of the integral
gear machine;
and (3) the cold turbine/expander is configured to expand a cold portion of
the nitrogen-
based refrigerant, however, the cold turbine is further configured as a
separate booster
loaded turbine coupled to one of the nitrogen-based refrigerant compression
stages and
not integrated into the integral gear machine.
(00061) Turning to the simplified depiction in Fig 6B as
well as Tables 3A and
3B, the integral gear machine is a 'bridge' type machine with a bull gear
driven by
motor is and a plurality of compression stages and turbines. The bull gear
size in this
example is again the medium size machine and includes three pinions. The first
pinion
arrangement couples the bull gear to first recycle compression stage. In this
embodiment and example, the second pinion arrangement and third pinion
arrangement
are dedicated to the natural gas service. The second pinion arrangement
couples the bull
gear to the first natural gas compression stage and the second natural gas
compression
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stage for a net power requirement which is near the maximum power limit for
any
pinion arrangement on the integral gear machine. The third pinion arrangement
couples
the bull gear to the third natural gas compression stage and the natural gas
expansion.
The cold turbine is a booster loaded turbine that drives the third recycle
compression
stage. Note that the warm turbine provides about 2.8 times the work than that
of the
cold turbine suggesting the warm turbine is again providing the largest
refrigeration
source. For sake of clarity, the reference labels in Figs. 6A, 6B, 7A, and 7B
are as
follows: M= Motor; CT= Cold Turbine; WT= Warm Turbine; CB=Nitrogen
Compression Stage(s) and WB= Natural Gas Compression Stage(s).
(00062) Tables 3A and 3B compare the simulated performance
of the baseline or
conventional liquefied natural gas system and process generically depicted in
Fig. 1
with the three-pinion arrangement shown in Fig. 6A using an integral gear
machine
having a medium frame size. As seen therein, the energy usage per metric tonne
of
liquefied natural gas produced is about 13.8 percent lower.
wimptiosmimmmsimeiimm.mpijw6tmmEsioaoi#uojiwoqmt*ktpiiooii]'I
M.0A10144M;
Pinion #1 N2 Comp CB1 0.088 N2 Comp CB2 0.088
0.175
Pinion #2 NG Comp WB1 0.115 NG Comp WB2 0.115
0.231
Pinion #3 NG Comp WB3 0.212 NGTurbine WT -0.134
0.078
Aux-Booster N2 Comp CB3 0.048 N2 Turbine CT -0.048
LoadedTurbine
Table 3A
66iia
iiiMEMMEEM Mit().$04)fie iM(0.0*).gi!ingltiØ0)ffit M(biiiir ggEffignea
gaggEgna
Fig. I 10.3 34.0 39.0 471
Baseline
Figs. 6A 87.0 12.0 34.0 60.0 406 -
13.8%
Table 3B
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LNG Production with 3-Pinion Integral Gear Machine and Separate NG Compression
(00063) The process flow diagram depicted in Fig. 7A is in
many regards similar
to the process flow diagrams described above and for sake of brevity, much of
the
following discussion will focus on the differences in the process flow diagram
depicted
in Fig. 7A when compared to the process flow diagram depicted in Fig. 2A. The
main
differences can be seen in Fig 7A and summarized as follows: (1) the nitrogen-
based
refrigerant compression are done using a series of compression stages, with
all of the
compression stages operatively coupled to the integral gear machine via the
three
pinions, as detailed in Fig. 7B; (2) there are two warm turbines/expanders
with the first
warm turbine/expander configured to expand a warm portion of the nitrogen-
based
refrigerant and operatively coupled to one of the nitrogen-based refrigerant
recycle
compression stages on one of the three pinions of the integral gear machine,
and (3) the
second warm turbine/expander is configured to expand another warm portion of
the
nitrogen-based refrigerant, however, the second warm turbine/expander is
configured as
a separate booster loaded turbine coupled to the natural gas compression stage
and not
integrated into the integral gear machine.
(00064) Turning now to Fig. 7B as well as Tables 4A and 4B,
the integral gear
machine includes a bull gear driven by a motor and includes a plurality of
compression
stages and turbines/expanders coupled thereto. The bull gear size in this
example is
again a medium size and includes three pinions. The first pinion couples the
bull gear
to first recycle compression stage (CBI) and the cold turbine (CT) while the
second
pinion operatively couples the bull gear to the second recycle compression
stage (CB2)
and the warm turbine (WT2). In this embodiment, the third pinion arrangement
operatively couples the bull gear to the third recycle compression stage
(CB3). In this
embodiment, the natural gas compression stage is driven by an auxiliary
booster loaded
warm turbine (WT1) and drives a natural gas compression stage (WB3).
(00065) Tables 4A and 4B also compares the simulated
performance of the
baseline or conventional liquefied natural gas system and process generically
depicted
in Fig. 1 with the three-pinion, three-turbine arrangement shown in Fig. 7A
using an
24
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integral gear machine having a medium frame size. As seen therein, the energy
usage
per metric tonne of liquefied natural gas produced is about 11.7% lower but
generally
produces more liquefied natural gas product than the baseline system in a
comparable
frame size. It should be noted that the relative power savings associated with
this
embodiment, expressed as kw*hr per kg of liquified natural gas produced is
partially
offset by the additional capital costs associated with the high speed, booster
loaded
natural gas expander driving the natural gas compression stage.
aitV&Air
Pinion #1 N2 Comp CB1 0.139 N2 Cold CT -0.054
0.085
Pinion #2 N2 Comp CB2 0.248 N2 Warm WT1 -0.127
0.120
Pinion #3 N2 Comp CB3 0.210
0.210
Aux-RI .T NG Comp NG 0.028 Aux Warm WT2 -0.028
0.000
Table 4A
=P.iii:60MMAIvikkiiie
mo.a4iiimiip
Fig. 1 610 10.3 34.0 39.0 471
Baseline
Figs. 7A 80.8 10.7 34.0 64.0 416
-11.7%
Table 4B
Industrial Applicability
(00066) Given the similarities of the integral gear machine
configurations in the above-
described embodiments, a possible strategy to reduce the capital costs for the
present system
and method involves standardizing a portion of the integral gear machine. For
example, many
of the embodiments can be modified to standardize the first and second pinion
arrangements
of integral gear machine while allowing customization of the third and
optionally fourth
pinion arrangements. By dedicating the first and second pinions of integral
gear machine to
adsorb the energy on each pinion required for the base refrigeration circuit,
namely the
nitrogen-based gas expansion refrigeration, one can design an LNG platform and
potentially
reduce the capital costs required for such solutions. Using the same LNG
platform the design
of the third (and optional fourth) pinion arrangements would be customizable
to meet the
natural gas refrigeration service requirements or auxiliary refrigeration
requirements for any
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given application or customer. The third and fourth pinion arrangements would
also
accommodate other liquefaction process customizations, such as distributing
compression
power.
(00067) Such platform customizations would foreseeably be
tailored to specific
liquefaction applications, the quality (e.g. rich or lean) and pressure of the
natural gas feed
stream, the availability of auxiliary refrigerants, etc. As presently
envisioned, the third and
fourth pinion arrangements are preferably dedicated to natural gas compression
and
expansion and/or other warm level refrigeration (i.e. >-50 C). Simply put,
this LNG
platform approach using a mixed service integral gear machine provides more
design
flexibility and more options for liquefied natural gas production,
particularly for small to
medium-scale liquefied natural gas production applications.
(00068) While the present invention has been described with
reference to a
preferred embodiment or embodiments, it is understood that numerous additions,
changes
and omissions can be made without departing from the spirit and scope of the
present
invention as set forth in the appended claims.
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