Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
IMPROVED METHOD AND SYSTEM FOR COOLING A HYDROCARBON STREAM
BACKGROUND
[0001] Liquefaction systems for cooling, liquefying, and optionally
subcooling natural gas
are well known in the art, such as the single mixed refrigerant (SMR) cycle,
the propane pre-
cooled mixed refrigerant (C3MR) cycle, the dual mixed refrigerant (DMR) cycle,
C3MR-Nitrogen
hybrid (such as APXTM) cycles, the gas phase expansion process (such as
nitrogen or methane
expander cycle), and cascade cycles. Typically, in such systems, natural gas
is cooled,
liquefied, and optionally sub-cooled by indirect heat exchange with one or
more refrigerants. A
variety of refrigerants might be employed, such as mixed refrigerants, pure
components, two-
phase refrigerants, gas phase refrigerants, etc. Some examples of pure
component two-phase
refrigerants are propane, carbon dioxide, hydrofluorocarbons (HFC), ethane,
ethylene, and
others. Some of these are especially suitable for precooling service.
[0002] Mixed refrigerants (MR), which are a mixture of nitrogen, methane,
ethane/ethylene,
propane, butanes, and pentanes, have been used in many base-load liquefied
natural gas
(LNG) plants. The composition of the MR stream is typically optimized based on
the feed gas
composition and operating conditions.
[0003] The refrigerant is circulated in a refrigerant circuit that includes
one or more heat
exchangers and one or more refrigerant compression systems. The refrigerant
circuit may be
closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled
by indirect heat
exchange against the refrigerants in the heat exchangers.
[0004] Boiling heat transfer is a commonly used heat transfer mode, wherein
the refrigerant
boils at one or more pressure levels to provide the cooling duty required.
Critical point is the
point on a pressure-enthalpy (P-H) diagram at which the saturated liquid and
saturated vapor
lines of the fluid meet. Critical temperature is a thermodynamic property of a
fluid and is the
temperature at the critical point. There are two types of refrigerant
operation ¨ subcritical
operation, wherein all steps in the process take place always below the
critical point, and
transcritical operation, wherein at least one step in the process occurs above
the critical point
while at least one step in the process occurs below the critical point.
[0005] FIG. 1A shows a P-H diagram for subcritical operation for a single
pressure cooling
process. The refrigerant vapor (A) is at a pressure of P1 and temperature of
Ti and is
compressed to pressure P2 and temperature T2 (B). The compressed vapor is then
de-
superheated to the dew point (C), condensed to the bubble point (D), and
subcooled to
CA 2996938 2996938 2018-02-28
produced subcooled liquid (E). The temperature at E is the aftercooler outlet
temperature, also
referred to as TAc and shown with an isotherm in FIG. 1A. The subcooled liquid
is then let down
in pressure to the original pressure P1 (F). The liquid component of the
refrigerant at point F is
vaporized to complete the cycle and return to vapor phase (A). During step B-
E, the process
rejects heat to ambient air or cooling water and during step F-A, the process
provides cooling
duty to a process stream, such as the natural gas feed stream and/or another
refrigerant.
[0006]
FIG. 1B shows the P-H diagram for transcritical operation for a single
pressure
cooling process. The cycle diagram is like that in FIG. 1A, however, the heat
rejection step B-E
occurs above the critical point. The critical temperature, TcRIT, is shown
with an isotherm. The
process starts with refrigerant vapor (A) at pressure P1 and temperature Ti
below the critical
temperature. It is then compressed to pressure P2 and temperature T2 (B),
which is above the
critical temperature. Above the critical point, a fluid does not possess
distinct vapor and liquid
phases. Therefore, when it is cooled from point B to point E, it does not
condense. The fluid
exhibits vapor-like properties at point B and liquid-like properties at point
E. However, unlike the
subcritical condensing process, where temperature stays constant during the
condensation
process (C-D), the temperature reduces continually during the transcritical
heat rejection step.
The heat rejection step for transcritical processes may have lower efficiency
than that for
subcritical processes, which is a drawback of transcritical processes.
[0007]
The temperature at E after heat rejection, for both subcritical and
transcritical
operation, is set by the ambient temperature plus a heat exchanger approach
temperature. Due
to the vertical nature of the isotherms (constant temperature lines) above the
critical point, E is
in the central portion of the graph, for transcritical operation. Therefore,
when refrigerant is
letdown in pressure from E to F, a two-phase stream with large amounts of
vapor is produced.
Therefore, the refrigerant at F has a higher vapor fraction in a transcritical
process than in a
subcritical process. It is the liquid component of the refrigerant at F that
vaporizes to provide
the cooling duty required. Therefore, due to the high vapor fraction at F,
transcritical processes
inherently have lower process efficiency than subcritical processes.
[0008]
The temperature at E, which is the ambient cooler outlet temperature, is given
by the
ambient temperature plus any approach to ambient, and is a critical factor in
determining
whether subcritical or transcritical operation takes place.
If the ambient cooler outlet
temperature is lower than the critical temperature, as in FIG. 1A, subcritical
operation takes
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place.
If the ambient cooler outlet temperature is greater than or equal to the
critical
temperature, as in FIG. 1B, transcritical operation takes place.
[0009]
Refrigerants such as propane and mixed refrigerant have critical temperatures
that
are well above typical ambient cooler outlet temperatures, even for hot
ambient conditions, and
therefore have subcritical operation. Carbon dioxide and ethane have critical
temperatures of
about 31 degrees Celsius. Ethylene has a critical temperature of about 10
degrees Celsius.
Depending on the ambient temperature, carbon dioxide, ethane, and ethylene,
will have
transcritical operation for typical hot and average ambient conditions, and
will therefore have
low process efficiency. This is a significant drawback of transcritical
operation.
[0010]
Another problem with transcritical operation is refrigerant inventory
management with
ambient temperature swings. For transcritical operation, the heat rejection
step B-E takes place
above the critical point and there is no condensation. As the refrigerant
cools, its temperature
continually reduces and its density increases. The refrigerant at E has liquid-
like density but it is
not a liquid. Accordingly, inventory management procedures are preferably
based on pressure,
in a manner similar to how a vapor-phase refrigerant inventory would be
managed. As the
ambient temperature reduces, the ambient cooler outlet temperature is now
lower than the
critical temperature and the operation switches to subcritical. The
refrigerant is fully condensed
and subcooled at E. Therefore, inventory management procedures would
preferably be based
on those for a liquid refrigerant, using liquid level control. In other words,
as operation switches
from transcritical to subcritical with ambient temperature swings, inventory
management
methods may need to change as well. This is an operational challenge
associated with
transcritical refrigerants.
[0011]
Carbon dioxide, for example, is non-flammable and has benefits in floating LNG
(FLNG) applications. It has a high density, which enables a low volumetric
flowrate of
refrigerant, as well as low piping sizes. However, due to the problems stated
herein for
transcritical operation, it has not been preferred for natural gas
liquefaction applications.
[0012]
Therefore, there is an unmet need for an efficient method and system for
solving the
problems associated with transcritical operation and enabling the use of
transcritical refrigerants
for LNG service.
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SUMMARY
[0013]
This Summary is provided to introduce a selection of concepts in a simplified
form
that are further described below in the Detailed Description. This Summary is
not intended to
identify key features or essential features of the claimed subject matter, nor
is it intended to be
used to limit the scope of the claimed subject matter.
[0014]
Some embodiments, as described below and as defined by the claims which
follow,
comprise improvements to cooling and liquefaction systems used as part of an
LNG liquefaction
processes. Some embodiments satisfy the need in the art by using a hybrid
cooling process,
thereby enabling the use of otherwise transcritical refrigerants for LNG
service.
[0015]
In addition, several specific aspects of the systems and methods are outlined
below.
[0016]
Aspect 1: A method for cooling a hydrocarbon feed stream against a first
refrigerant
to produce a cooled hydrocarbon stream, the first refrigerant having a
critical temperature, the
method comprising:
(a)
compressing the first refrigerant in one or more compression stages to produce
a
compressed first refrigerant;
(b)
cooling the compressed first refrigerant against ambient fluid in one or more
ambient heat exchangers to produce a cooled first refrigerant at a first
temperature;
(c)
cooling a fluid stream in each of at least one cooling circuit located in
downstream fluid flow communication from the one or more ambient heat
exchangers, each of
the at least one cooling circuit having at least one evaporation stage, each
of the following steps
being performed in each evaporation stage:
(i) reducing the pressure of the first refrigerant;
(ii) cooling the fluid stream against the reduced pressure first
refrigerant in an
evaporator, resulting in vaporization of at least a portion of the reduced
pressure first
refrigerant; and
(iii) flowing at least a portion of the vaporized reduced pressure first
refrigerant into one of the at least one compression stages;
wherein at least one fluid stream being cooled in the at least one cooling
circuit
comprises the hydrocarbon feed stream and step (c) produces a cooled
hydrocarbon stream;
(d)
after step (b) and before step (c), further cooling the cooled first
refrigerant in at
least one auxiliary heat exchanger against an auxiliary refrigerant to produce
a further cooled
first refrigerant at a second temperature if the first temperature is greater
than or equal to the
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critical temperature of the first refrigerant, the second temperature being
less than the critical
temperature of the first refrigerant; and
(e) after step (b) and before step (c), bypassing the at least one
auxiliary heat
exchanger if the first temperature is less than the critical temperature of
the first refrigerant.
[0017] Aspect 2: The method of Aspect 1, wherein the at least one auxiliary
heat exchanger
comprises an economizer and the auxiliary refrigerant comprises the first
refrigerant.
[0018] Aspect 3: The method of any of Aspects 1-2, wherein the auxiliary
refrigerant is at
least a portion of the hydrocarbon feed stream.
[00191 Aspect 4: The method of any of Aspects 1-3, wherein the at least one
auxiliary heat
exchanger is a part of a closed loop vapor compression system.
[0020] Aspect 5: The method of Aspect 4, wherein the auxiliary refrigerant
is a
hydrofluorocarbon or propane.
[0021] Aspect 6: The method of any of Aspects 1-5, further comprising:
(f) further cooling and liquefying the cooled hydrocarbon stream in at
least one
liquefaction heat exchanger against a second refrigerant stream to produce a
liquefied natural
gas stream.
[0022] Aspect 7: The method of Aspect 6, wherein the at least one fluid
stream being cooled
in the at least one cooling circuit comprises the second refrigerant.
[0023] Aspect 8: The method of any of Aspects 1-7, wherein the first
refrigerant comprises
ethane, carbon-dioxide, or ethylene.
[0024] Aspect 9: The method of any of Aspects 1-8, wherein step (a) further
comprises:
(a) cornpressing the first refrigerant in a plurality of compression
stages to produce a
compressed first refrigerant.
[0025] Aspect 10: The method of Aspect 9, wherein step (c) further
comprises cooling at
least one fluid stream in a plurality of evaporation stages located downstream
from the
economizer, wherein the steps (c)(i) through (c)(iii) are performed in each of
the plurality of
evaporation stages.
[0026] Aspect 11: An apparatus for cooling a hydrocarbon feed stream, the
apparatus
comprising:
at least one compression stage operationally configured to compress a first
refrigerant;
at least one ambient heat exchanger in downstream fluid flow communication
with the at
least one compression stage, the at least one ambient heat exchanger being
operationally
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configured to cool the first refrigerant to a first temperature by indirect
heat exchange against an
ambient fluid;
at least one auxiliary heat exchanger in downstream fluid flow communication
with the at
least one ambient heat exchanger, the auxiliary heat exchanger being
operationally configured
to further cool the first refrigerant to a second temperature that is below
the critical temperature
of the first refrigerant;
at least one cooling circuit located in downstream fluid flow communication
from the at
least one auxiliary heat exchanger, each of the at least one cooling circuit
having at least one
evaporation stage, each of the evaporation stages comprising an expansion
valve in upstream
fluid flow communication with an evaporator, the evaporator operationally
configured to cool a
fluid stream against the first refrigerant and to create a vaporized first
refrigerant stream and a
cooled fluid stream, each of the evaporation stages further comprising a
vaporized first
refrigerant circuit in fluid flow communication with one of the at least one
compression stages;
a bypass system comprising a controller, at least one temperature sensor, a
plurality of
valves, and at least one bypass circuit in fluid flow communication with the
at least one ambient
heat exchanger and the at least one cooling circuit, the bypass system
operationally configured
to (1) prevent flow of the first refrigerant through the at least one bypass
circuit and allow flow of
the first refrigerant through the at least one auxiliary heat exchanger when
the first temperature
is greater than or equal to the critical temperature of the first refrigerant
and (2) allow flow of the
first refrigerant through the at least one bypass circuit and prevent flow of
the first refrigerant
through the at least one auxiliary heat exchanger when the first temperature
is less than the
critical temperature of the first refrigerant;
wherein the fluid stream of at least one of the at least one cooling circuit
comprises the
hydrocarbon feed stream.
[0027] Aspect 12: The apparatus of Aspect 11, wherein the at least one
auxiliary heat
exchanger comprises an economizer.
[0028] Aspect 13: The apparatus of Aspect 11 or 12, wherein the at least
one auxiliary heat
exchanger is part of a closed loop vapor compression system.
[0029] Aspect 14: The apparatus of Aspect 13, wherein the auxiliary
refrigerant comprises a
hydrofluorocarbon or propane.
[0030] Aspect 15: The apparatus of any of Aspects 11-14, further comprising
a liquefaction
heat exchanger operationally configured to further cool and liquefy the
hydrocarbon stream in at
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least one liquefaction heat exchanger against a second refrigerant stream to
produce a liquefied
natural gas stream.
[0031]
Aspect 16: A method for cooling a hydrocarbon feed stream against a first
refrigerant
to produce a cooled hydrocarbon stream, the first refrigerant having a
critical temperature,
wherein the method comprises:
(a)
compressing the first refrigerant in at least one compression stage to produce
a
compressed first refrigerant;
(b)
cooling the compressed first refrigerant against an ambient fluid in at least
one
ambient heat exchanger to produce a cooled first refrigerant at a first
temperature that is greater
than or equal to the critical temperature of the first refrigerant;
(c)
cooling a fluid stream in each of at least one cooling circuit located in
downstream fluid flow communication from the ambient heat exchanger, each of
the at least one
cooling circuit having at least one evaporation stage, each of the following
steps being
performed in each evaporation stage:
(i) reducing the pressure of the first refrigerant;
(ii) cooling the fluid stream against the reduced pressure first
refrigerant in an
evaporator, resulting in vaporization of at least a portion of the reduced
pressure first
refrigerant; and
(iii) flowing at least a portion of the vaporized reduced pressure first
refrigerant into one of the at least one compression stages;
wherein the at least one evaporation stage of each of the at least one cooling
circuit comprises a first evaporation stage that is located at an upstream end
of the at least one
cooling circuit, wherein step (c)(i) comprises the following step in each
first evaporation stage:
(c)(i) reducing the pressure of the first portion of the first refrigerant
using an
isentropic expansion device to produce a first reduced pressure first
refrigerant having a vapor
fraction of no less than 0.2 and no more than 0.6.
wherein at least one fluid stream being cooled in the at least one cooling
circuit is
selected from the group of: the hydrocarbon stream and a second refrigerant
stream.
[0032] Aspect 17: The method of Aspect 16, further comprising:
(d)
further cooling and liquefying the cooled hydrocarbon stream in at least one
liquefaction heat exchanger against a second refrigerant stream to produce a
liquefied natural
gas stream.
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[0033] Aspect 18: The method of Aspect 17, wherein at least one fluid
stream being cooled
in the at least one cooling circuit comprises the second refrigerant.
[0034] Aspect 19: The method of any of Aspects 16-18, wherein the first
refrigerant is
ethane, carbon-dioxide, or ethylene.
[0035] Aspect 20: The method of any of Aspects 16-19, wherein step (a)
further comprises:
(a) compressing the first refrigerant in a plurality of
compression stages to
produce a compressed first refrigerant.
BRIEF DESCRIPTION OF DRAWINGS
[0036] FIG. 1A is a pressure versus enthalpy (P-H) diagram for a
subcritical cooling process
in accordance with the prior art;
[0037] FIG. 1B is a pressure versus enthalpy (P-H) diagram for a
transcritical cooling
process in accordance with the prior art;
[0038] FIG. 2 is a schematic flow diagram of a precooled-gas phase
expansion system in
accordance with the prior art;
[0039] FIG. 3 is a schematic flow diagram of a precooled-MR system in
accordance with the
prior art;
[0040] FIG. 4 is a schematic flow diagram of a cooling system in accordance
with the prior
art;
[0041] FIG. 5 is a schematic flow diagram of a cooling system in accordance
with a first
embodiment;
[0042] FIG. 6 is a schematic flow diagram of a cooling system in accordance
with a second
embodiment;
[0043] FIG. 7 is a schematic flow diagram of a cooling system in accordance
with a third
embodiment;
[0044] FIG. 8 is a schematic flow diagram of a cooling system in accordance
with a fourth
embodiment;
[0045] FIG. 9 is a schematic flow diagram of a first embodiment of an
auxiliary refrigerant
system in accordance with the third and fourth embodiments;
[0046] FIG. 10 is a schematic flow diagram of a second embodiment of the
auxiliary
refrigerant system in accordance with the third and fourth embodiments;
CA 2996938 2996938 2018-02-28
[0047] FIG. 11 is a schematic flow diagram of a third embodiment of the
auxiliary refrigerant
system in accordance with the third and fourth embodiments;
[0048] FIG. 12A is a pressure versus enthalpy (P-H) diagram for a
transcritical cooling
process with isentropic expansion; and
[0049] FIG. 12B is a schematic flow diagram of a cooling system in
accordance with a fifth
embodiment.
DETAILED DESCRIPTION
[0050] The ensuing detailed description provides preferred exemplary
embodiments only,
and is not intended to limit the scope, applicability, or configuration.
Rather, the ensuing
detailed description of the preferred exemplary embodiments will provide those
skilled in the art
with an enabling description for implementing the preferred exemplary
embodiments. Various
changes may be made in the function and arrangement of elements without
departing from their
spirit and scope.
[0051] Reference numerals that are introduced in the specification in
association with a
drawing figure may be repeated in one or more subsequent figures without
additional
description in the specification in order to provide context for other
features.
[0052] In the claims, letters are used to identify claimed steps (e.g. (a),
(b), and (c)). These
letters are used to aid in referring to the method steps and are not intended
to indicate the order
in which claimed steps are performed, unless and only to the extent that such
order is
specifically recited in the claims.
[0053] Directional terms may be used in the specification and claims to
describe portions of
the disclosed embodiments (e.g., upper, lower, left, right, etc.). These
directional terms are
merely intended to assist in describing exemplary embodiments, and are not
intended to limit
the scope of the claims. As used herein, the term "upstream" is intended to
mean in a direction
that is opposite the direction of flow of a fluid in a conduit from a point of
reference. Similarly,
the term "downstream" is intended to mean in a direction that is the same as
the direction of
flow of a fluid in a conduit from a point of reference.
[0054] Unless otherwise stated herein, any and all percentages identified
in the
specification, drawings and claims should be understood to be on a weight
percentage basis.
Unless otherwise stated herein, any and all pressures identified in the
specification, drawings
and claims should be understood to mean gauge pressure.
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[0055] The term "fluid flow communication," as used in the specification
and claims, refers
to the nature of connectivity between two or more components that enables
liquids, vapors,
and/or two-phase mixtures to be transported between the components in a
controlled fashion
(i.e., without leakage) either directly or indirectly. Coupling two or more
components such that
they are in fluid flow communication with each other can involve any suitable
method known in
the art, such as with the use of welds, flanged conduits, gaskets, and bolts.
Two or more
components may also be coupled together via other components of the system
that may
separate them, for example, valves, gates, or other devices that may
selectively restrict or direct
fluid flow.
[0056] The term "conduit," as used in the specification and claims, refers
to one or more
structures through which fluids can be transported between two or more
components of a
system. For example, conduits include, but are not limited to, pipes, ducts,
passageways, and
combinations thereof that transport liquids, vapors, and/or gases.
[0057] The term "natural gas", as used in the specification and claims,
means a
hydrocarbon gas mixture consisting primarily of methane.
[0058] The terms "hydrocarbon gas" or "hydrocarbon fluid", as used in the
specification and
claims, means a gas/fluid comprising at least one hydrocarbon and for which
hydrocarbons
comprise at least 80%, and, more preferably, at least 90% of the overall
composition of the
gas/fluid.
[0059] The term "mixed refrigerant" (abbreviated as "MR"), as used in the
specification and
claims, means a fluid comprising at least two hydrocarbons and for which
hydrocarbons
comprise at least 80% of the overall composition of the refrigerant.
[0060] The terms "bundle" and "tube bundle" are used interchangeably within
this
application and are intended to be synonymous.
[0061] The term "ambient fluid", as used in the specification and claims,
means a fluid that
is provided to the system at or near ambient pressure and temperature.
[0062] The term "compression circuit" is used herein to refer to the
components and
conduits in fluid communication with one another and arranged in series
(hereinafter "series
fluid flow communication"), beginning upstream from the first compressor or
compressor stage
and ending downstream from the last compressor or compressor stage. The term
"compression
sequence" is intended to refer to the steps performed by the components and
conduits that
comprise the associated compression circuit.
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L0063] As used in the specification and claims, the terms "high-high",
"high", "medium",
"low", and "low-low" are intended to express relative values for a property of
the elements with
which these terms are used. For example, a high-high pressure stream is
intended to indicate a
stream having a higher pressure than the corresponding high pressure stream or
medium
pressure stream or low pressure stream described or claimed in this
application. Similarly, a
high pressure stream is intended to indicate a stream having a higher pressure
than the
corresponding medium pressure stream or low pressure stream described in the
specification or
claims, but lower than the corresponding high-high pressure stream described
or claimed in this
application. Similarly, a medium pressure stream is intended to indicate a
stream having a
higher pressure than the corresponding low pressure stream described in the
specification or
claims, but lower than the corresponding high pressure stream described or
claimed in this
application.
[0064] As used herein, the term "cryogen" or "cryogenic fluid" is intended
to mean a liquid,
gas, or mixed phase fluid having a temperature less than -70 degrees Celsius.
Examples of
cryogens include liquid nitrogen (LIN), liquefied natural gas (LNG), liquid
helium, liquid carbon
dioxide and pressurized, mixed phase cryogens (e.g., a mixture of LIN and
gaseous nitrogen).
As used herein, the term "cryogenic temperature" is intended to mean a
temperature below -70
degrees Celsius.
[0065] As used herein, the term "compressor" in intended to mean a device
having at least
one compressor stage contained within a casing and that increases the pressure
of a fluid
stream.
[0066] As used herein, the term "critical point" of a fluid is the point on
the fluid's P-H
diagram where the saturated liquid and saturated vapor lines meet.
[0067] As used herein, the term "subcritical" is intended to refer to a
process that occurs
below the critical point of the refrigerant.
[0068] As used herein, the term "transcritical" is intended to refer to a
process comprising
one or more steps that occur below the critical point of the refrigerant and
one or more steps
that occur above the critical point of the refrigerant.
[0069] As used herein, the term "isotherm" is intended to refer to a
constant temperature
line.
[0070] As used herein, the term "vapor compression cycle" is intended to
refer to a
refrigeration cycle in which the refrigerant undergoes phase change during the
refrigeration
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cycle. For instance, a vapor refrigerant is compressed, cooled and at least
partially condensed,
then reduced in pressure, and at least partially vaporized to provide
refrigeration duty.
[0071] As used herein, the term "vapor expansion cycle" is intended to
refer to a
refrigeration cycle in which the refrigerant is in the vapor phase and does
not undergo phase
change during the cycle. For instance, a vapor refrigerant is compressed,
cooled without phase
change, then reduced in pressure and warmed to provide refrigerant duty.
[0072] As used herein, the term "closed loop vapor compression cycle" is
intended to refer
to a vapor compression cycle in which no refrigerant is added or removed from
the cycle (with
the possible exception of leakage and refrigerant make-up) during steady-state
operation. In all
the embodiments disclosed herein, the precooling refrigeration cycle is a
closed loop vapor
compression cycle.
[0073] ,As used herein, the term "economizer" as used herein, is intended
to mean a heat
exchanger that is operationally configured to provide an indirect heat
exchange between a fluid
stream and at least a portion of the same at a different temperature.
[0074] Table 1 defines a list of acronyms employed throughout the
specification and
drawings as an aid to understanding the described embodiments.
Table 1
Main Cryogenic Heat
SMR Single Mixed Refrigerant MCHE
Exchanger
DMR Dual Mixed Refrigerant MR Mixed Refrigerant
Propane-precooled Mixed
C3MR MRL Mixed Refrigerant Liquid
Refrigerant
LNG Liquid Natural Gas MRV Mixed Refrigerant Vapor
LLP Low-Low Pressure HHP High-High Pressure
LP Low Pressure MP Medium Pressure
Million Metric Tonnes Per
HP High Pressure MTPA
Annum
HFC Hydrofluorocarbon LIN Liquid Nitrogen
CO2 Carbon dioxide LiBr Lithium Bromide
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[0075] The described embodiments provide an efficient process for the
liquefaction of a
hydrocarbon fluid and are particularly applicable to the liquefaction of
natural gas.
[0076] Referring to FIG. 2, a typical precooled-gas phase expansion process
of the prior art
is shown. In this arrangement, the precooling duty is provided by boiling heat
transfer using a
two-phase refrigerant and the liquefaction and subcooling duty is provided by
sensible heat
transfer using a gas phase refrigerant. Some examples of the gas refrigerant
include nitrogen,
methane, and combinations thereof.
[0077] A feed stream 200, which is preferably natural gas, is cleaned and
dried by known
methods in a pre-treatment section 290 to remove water, acid gases such as CO2
and H2S, and
other contaminants such as mercury, resulting in a pre-treated feed stream
201. The pre-
treated feed stream 201, which is essentially water free, is pre-cooled in a
precooling system
218 to produce a pre-cooled natural gas stream 205 and further cooled,
liquefied, and/or sub-
cooled in a main cryogenic heat exchanger (MCHE) 208 (also referred to as a
main heat
exchanger) to produce LNG stream 206. The LNG stream 206 is preferably let
down in
pressure by passing it through a valve or a turbine (not shown) and is then
sent to LNG storage
tank 209. Any flash vapor produced during the pressure letdown and/or boil-off
in the tank is
represented by stream 207, which may be used as fuel in the plant, recycled to
feed, or vented.
[0078] The term "essentially water free" means that any residual water in
the pre-treated
feed stream 201 is present at a sufficiently low concentration to prevent
operational issues
associated with water freeze-out in the downstream cooling and liquefaction
process. In the
embodiments described herein, water concentration is preferably not more than
1.0 ppm and,
more preferably between 0.1 ppm and 0.5 ppm.
[0079] The pre-treated feed stream 201 is pre-cooled to a temperature
preferably below 10
degrees Celsius, more preferably below about 0 degrees Celsius, and most
preferably about -
30 degrees Celsius. The pre-cooled natural gas stream 205 is liquefied to a
temperature
preferably between about -150 degrees Celsius and about -70 degrees Celsius,
more preferably
between about -145 degrees Celsius and about -100 degrees Celsius, and
subsequently sub-
cooled to a temperature preferably between about -170 degrees Celsius and
about -120
degrees Celsius, more preferably between about -170 degrees Celsius and about -
140 degrees
Celsius. MCHE 208 may be any type of heat exchanger such as a coil wound heat
exchanger
with one or more bundles, a plate and fin heat exchanger, a core-in-kettle
heat exchanger, a
shell and tube heat exchanger, and any other type of heat exchanger suitable
for the
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CA 2996938 2018-02-28
liquefaction of subcooling of natural gas. Further, one or more heat
exchangers in series of
parallel may be used. In some cases, an economizer heat exchanger may also be
used.
[0080] As illustrated in FIG. 2, a cooled precooling refrigerant 210 is
warmed against at
least the pre-treated feed stream 201 to produce a warm low pressure
precooling refrigerant
214. The warm low pressure precooling refrigerant 214 is compressed in one or
more
precooling refrigerant compressor(s) 216 that may comprise four compressor
stages 216A,
216B, 216C, 216D. Three side streams 211, 212, and 213 at intermediate
pressure levels enter
the precooling refrigerant compressor 216 at the suction of the final 216D,
third 216C, and
second 216B stages of the precooling refrigerant compressor 216 respectively.
The
compressed precooling refrigerant 215 is cooled in one or more heat
exchangers, such as
desuperheater, condenser, and/or subcooler heat exchangers, depicted as
precooling
refrigerant condenser 217, to produce the cooled precooling refrigerant 210
that provides the
precooling duty required.
[0081] The precooling refrigerant condenser 217 preferably exchanges heat
against an
ambient fluid such as air or water. Although FIG. 2 shows four stages of
precooling refrigerant
compression, any number of compressor stages may be employed. It should be
understood
that when multiple compressor stages are described or claimed, such multiple
compressor
stages could comprise a single multi-stage compressor, multiple compressors,
or a combination
thereof. The compressors could be in a single casing or multiple casings. The
process of
compressing the precooling refrigerant is generally referred to herein as the
precooling
compression sequence, and is described in detail in FIG. 4. Some examples of
the precooling
refrigerant include propane, MR, carbon dioxide, HFC, ethane, ethylene, and
others.
[0082] A warm liquefaction refrigerant 230 is withdrawn from MCHE 208 and
compressed in
a high pressure (HP) compressor 257 to produce a compressed liquefaction
refrigerant 238.
One or more refrigerant compressors, compression stages may be used with
optional inter-
cooling. The compressed liquefaction refrigerant 238 is cooled against ambient
air or water in a
high pressure aftercooler 258 to produce a cooled liquefaction refrigerant 239
in gas phase.
One or more heat exchangers may be used. The high pressure aftercooler 258 may
be of any
type, such as a plate and fin or shell and tube heat exchanger. The cooled
liquefaction
refrigerant 239 is precooled against the precooling refrigerant in the
precooling system 218 to
produce a precooled liquefaction refrigerant 240. The precooled liquefaction
refrigerant 240
may be expanded in one or more gas phase expanders 248 to produce an expanded
gas phase
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CA 2996938 2018-02-28
refrigerant 249, which is sent to the MCHE 208 to provide the liquefaction and
subcooling duty
required.
[0083] The liquefaction and subcooling system of FIG. 2 may use nitrogen,
methane, or a
combination thereof. It could use feed gas or flash gas from the process, in
an open or closed
loop system. It may also comprise one or more cooling systems in series or
parallel using
independent gas phase refrigerant systems. Further, it could employ one or
more gas phase
expanders, compressor-expander assemblies (companders), economizer heat
exchangers, and
other variations.
[0084] Referring to FIG. 3, a typical precooled-MR process of the prior art
is shown. A feed
stream 300, which is preferably natural gas, is cleaned and dried by known
methods in a pre-
treatment section 390 to remove water, acid gases such as CO2 and H2S, and
other
contaminants such as mercury, resulting in a pre-treated feed stream 301. The
pre-treated feed
stream 301, which is essentially water free, is pre-cooled in a precooling
system 318 to produce
a pre-cooled natural gas stream 305 and further cooled, liquefied, and/or sub-
cooled in a main
cryogenic heat exchanger (MCHE) 308 (also referred to as a main heat
exchanger) to produce
LNG stream 306. The LNG stream 306 is preferably let down in pressure by
passing it through
a valve or a turbine (not shown) and is then sent to LNG storage tank 309. Any
flash vapor
produced during the pressure letdown and/or boil-off in the tank is
represented by stream 307,
which may be used as fuel in the plant, recycled to feed, or vented.
[0085] The pre-treated feed stream 301 is pre-cooled to a temperature
preferably below 10
degrees Celsius, more preferably below about 0 degrees Celsius, and most
preferably about -
30 degrees Celsius. The pre-cooled natural gas stream 305 is liquefied to a
temperature
preferably between about -150 degrees Celsius and about -70 degrees Celsius,
more preferably
between about -145 degrees Celsius and about -100 degrees Celsius, and
subsequently sub-
cooled to a temperature preferably between about -170 degrees Celsius and
about -120
degrees Celsius, more preferably between about -170 degrees Celsius and about -
140 degrees
Celsius. MCHE 308 shown in FIG. 3 is a coil wound heat exchanger with three
bundles.
However, any number of bundles and any exchanger type(s) may be utilized.
[0086] The term "essentially water free" means that any residual water in
the pre-treated
feed stream 301 is present at a sufficiently low concentration to prevent
operational issues
associated with water freeze-out in the downstream cooling and liquefaction
process. In the
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CA 2996938 2018-02-28
embodiments described in herein, water concentration is preferably not more
than 1.0 ppm and,
more preferably between 0.1 ppm and 0.5 ppm.
[0087]
As illustrated in FIG. 3, a cooled precooling refrigerant 310 is warmed
against at
least the pre-treated feed stream 301 to produce a warm low pressure
precooling refrigerant
314. The warm low pressure precooling refrigerant 314 is compressed in one or
more
precooling refrigerant compressor(s) 316 that may comprise four compressor
stages 316A,
316B, 316C, 316D. Three side streams 311, 312, and 313 at intermediate
pressure levels enter
the precooling refrigerant compressor 316 at the suction of the final 316D,
third 316C, and
second 316B stages of the precooling refrigerant compressor 316 respectively.
The
compressed precooling refrigerant 315 is cooled in one or more heat
exchangers, shown on
FIG. 3 with precooling refrigerant condenser 317, to produce the cooled
precooling refrigerant
310 that provides the cooling duty required.
[0088]
The precooling refrigerant liquid evaporates to produce the warm low pressure
precooling refrigerant 314. The precooling refrigerant condenser 317
preferably exchanges
heat against an ambient fluid including, but not limited to, air or water.
Although the figure
shows four stages of precooling refrigerant compression, any number of
compressor stages
may be employed. It should be understood that when multiple compressor stages
are
described or claimed, such multiple compressor stages could comprise a single
multi-stage
compressor, multiple compressors, or a combination thereof. The compressors
could be in a
single casing or multiple casings. The process of compressing the precooling
refrigerant is
generally referred to herein as the precooling compression sequence, and is
described in detail
in FIG. 4.
[0089]
A warm liquefaction refrigerant 330 is withdrawn from the MCHE 308 and in case
of
a coil wound heat exchanger, it would be withdrawn from the bottom of the
shell side of the
MCHE 308. The warm liquefaction refrigerant 330 is sent through a low pressure
suction drum
350 to separate out any liquids and the vapor stream 331 is compressed in a
low pressure (LP)
compressor 351 to produce medium pressure MR stream 332. The warm liquefaction
refrigerant 330 is preferably withdrawn at a temperature at or near precooling
refrigerant
precooling temperature and more preferably about -30 degree Celsius and at a
pressure of less
than 10 bar (145 psia). The medium pressure MR stream 332 is cooled in a low
pressure
aftercooler 352 to produce a cooled medium pressure MR stream 333 from which
any liquids
are drained in medium pressure suction drum 353 to produce medium pressure
vapor stream
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CA 2996938 2018-02-28
334 that is further compressed in medium pressure (MP) compressor 354. The
resulting high
pressure MR stream 335 is cooled in a medium pressure aftercooler 355 to
produce a cooled
high pressure MR stream 336. The cooled high pressure MR stream 336 is sent to
a high
pressure suction drum 356 where any liquids are drained. The resulting high
pressure vapor
stream 337 is further compressed in a high pressure (HP) compressor 357 to
produce
compressed liquefaction refrigerant 338 that is cooled in high pressure
aftercooler 358 to
produce a cooled high-high pressure (HHP) MR stream 339. The cooled HHP MR
stream 339
is then cooled against evaporating precooling refrigerant in precooling system
318 to produce a
precooled liquefaction refrigerant 340 that is then sent to a vapor-liquid
separator 359 from
which an MRL stream 341 and a MRV stream 343 are obtained, which are sent back
to MCHE
308 to be further cooled. Liquid streams leaving phase separators are referred
to in the industry
as MRL and vapor streams leaving phase separators are referred to in the
industry as MRV,
even after they are subsequently liquefied. The process of compressing and
cooling the MR
after it is withdrawn from the bottom of the MCHE 308, then returned to the
tube side of the
MCHE 308 as multiple streams, is generally referred to herein as the MR
compression
sequence.
[0090] Both the MRL stream 341 and MRV stream 343 are cooled, in two
separate circuits
of the MCHE 308. The MRL stream 341 is cooled in the first two bundles of the
MCHE 308,
resulting in a cold stream that is let down in pressure to produce a cold MRL
stream 342 that is
sent back to the shell-side of MCHE 308 to provide refrigeration required in
the first two bundles
of the MCHE. The MRV stream 343 is cooled in the first, second, and third
bundles of MCHE
308, reduced in pressure across a cold high pressure letdown valve, and
introduced to the
MCHE 308 as cold MRV stream 344 to provide refrigeration in the subcooling,
liquefaction, and
cooling steps. MCHE 308 can be any exchanger suitable for natural gas
liquefaction including,
but not limited to, a coil wound heat exchanger, a plate and fin heat
exchanger or a shell and
tube heat exchanger. Coil wound heat exchangers are the state of the art
exchangers for
natural gas liquefaction and include at least one tube bundle comprising a
plurality of spiral
wound tubes for flowing process and warm refrigerants and a shell space for
flowing a cold
refrigerant.
[0091] FIG. 4 illustrates an exemplary arrangement of the precooling system
418 and the
precooling compression sequence depicted in FIGS. 2 and 3. The following
arrangement
shows a four pressure level precooling system, however, any number of pressure
levels may be
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CA 2996938 2018-02-28
utilized. The pre-treated feed stream 401, is cooled by indirect heat exchange
in HP feed
evaporator 481 to produce a first intermediate feed stream 402, which is then
cooled in a MP
feed evaporator 482 to produce a second intermediate feed stream 403, followed
by a LP feed
evaporator 483 to produce a third intermediate feed stream 404, and finally a
low-low pressure
(LLP) feed evaporator 484 to produce the pre-cooled natural gas stream 405.
[0092] Each pressure level is also referred to herein as an evaporation
stage. Using the
highest pressure evaporation stage of the cooling circuit for the pre-treated
feed stream 401 as
an example, each evaporation stage includes a pressure letdown valve 473, an
evaporator 481,
an outlet conduit for vaporized precooling refrigerant 421, and a separator
492 (which may be
shared with a corresponding evaporator 485 in another cooling circuit). The
pressure letdown
valve 473 is located upstream from the evaporator 481, on a conduit through
which the
precooling refrigerant 420 flows. Each evaporation stage provides a reduction
in pressure for
the pre-cooling refrigerant, heat transfer between the precooling refrigerant
and the stream
being cooled, and conduits to allow a vaporized portion of the precooling
refrigerant to flow to
the compressor 416 and (in all but the last evaporation stage) a liquid
portion of the precooling
refrigerant to flow to the next evaporation stage. Each cooling circuit
comprises all of the
evaporation stages that provide cooling for each fluid stream being cooled by
the precooling
refrigerant ¨ in this embodiment, the pre-treated feed stream 401 and the
cooled liquefaction
refrigerant stream 439. For example, the four evaporation stages associated
with feed
evaporators 481-484 form a feed cooling circuit.
[0093] The cooled liquefaction refrigerant stream 439 is further cooled by
indirect heat
exchange in an HP liquefaction refrigerant evaporator 485 to produce a first
intermediate
liquefaction refrigerant 445, which is then cooled in an MP liquefaction
refrigerant evaporator
486 to produce a second intermediate liquefaction refrigerant 446, followed by
an LP
liquefaction refrigerant evaporator 487 to produce a third intermediate
liquefaction refrigerant
447, and finally an LLP liquefaction refrigerant evaporator 488 to produce the
pre-cooled
liquefaction refrigerant 440. The four evaporation stages associated with
liquefaction refrigerant
evaporators 485-488 form a liquefaction refrigerant circuit.
[0094] Warm low pressure precooling refrigerant 414 is compressed in
precooling
refrigerant compressor 416 to produce compressed precooling refrigerant 415.
The precooling
refrigerant compressor 416 is shown as a four stage compressor with an LLP
compression
stage 416A, an LP compression stage 416B, an MP compression stage 416C, and an
HP
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CA 2996938 2018-02-28
compression stage 416D. An LP side stream 413, MP side stream 412, and HP side
stream
411 are introduced to the precooling refrigerant compressor 416 at
intermediate locations.
[0095] The compressed precooling refrigerant 415 is preferably cooled by
indirect heat
exchange against ambient air or water in one or more heat exchangers, depicted
by precooling
refrigerant condenser 417 to produce the cooled precooling refrigerant 410.
The cooled
precooling refrigerant 410 is then preferably divided into two portions, a
first portion 419 to
provide cooling duty to the pre-treated feed stream 401, and a second portion
461 to provide
cooling duty to the cooled liquefaction refrigerant stream 439.
[0096] The first portion 419 of the cooled precooling refrigerant may be
let down in pressure
in a first pressure letdown valve 473 to produce a first HP precooling
refrigerant 420. The liquid
fraction of the first HP precooling refrigerant 420 is partially vaporized in
the HP feed evaporator
481 to produce a first HP vapor precooling refrigerant 421 and a first HP
liquid precooling
refrigerant 422. The first HP vapor precooling refrigerant 421 is sent to an
HP precooling
refrigerant separator 492, and subsequently to the suction of the HP
compression stage 416D
as a part of the HP side stream 411.
[0097] The first HP liquid precooling refrigerant 422 is let down in
pressure in a second
pressure letdown valve 474 to produce a first MP precooling refrigerant 423.
The liquid fraction
of the first MP precooling refrigerant 423 is partially vaporized in the MP
feed evaporator 482 to
produce a first MP vapor precooling refrigerant 424 and a first MP liquid
precooling refrigerant
425. The first MP vapor precooling refrigerant 424 is sent to an MP precooling
refrigerant
separator 493, and subsequently to the suction of the MP compression stage
416C as a part of
the MP side stream 412.
[0098] The first MP liquid precooling refrigerant 425 is let down in
pressure in a third
pressure letdown valve 475 to produce a first LP precooling refrigerant 426.
The liquid fraction
of the first LP precooling refrigerant 426 is partially vaporized in the LP
feed evaporator 483 to
produce a first LP vapor precooling refrigerant 427 and a first LP liquid
precooling refrigerant
428. The first LP vapor precooling refrigerant 427 is sent to an LP precooling
refrigerant
separator 494, and subsequently to the suction of the LP compression stage
416B as a part of
the LP side stream 413.
[0099] The first LP liquid precooling refrigerant 428 is let down in
pressure in a fourth
pressure letdown valve 476 to produce a first LLP precooling refrigerant 429.
The liquid fraction
of the first LLP precooling refrigerant 429 is completely vaporized in the LLP
feed evaporator
-19-
CA 2996938 2018-02-28
484 to produce a first LLP vapor precooling refrigerant 460. In this context,
"completely
vaporized" means that at least 95% by weight of the liquid fraction is
vaporized. The first LLP
vapor precooling refrigerant 460 is sent to an LLP precooling refrigerant
separator 495, and
subsequently to the suction of the LLP compression stage 416A as a part of the
warm low
pressure precooling refrigerant 414.
[00100] The second portion 461 of the cooled precooling refrigerant may be let
down in
pressure in a fifth pressure letdown valve 477 to produce a second HP
precooling refrigerant
462. The liquid fraction of the second HP precooling refrigerant 462 is
partially vaporized in the
HP liquefaction refrigerant evaporator 485 to produce a second HP vapor
precooling refrigerant
463 and a second HP liquid precooling refrigerant 464. The second HP vapor
precooling
refrigerant 463 is sent to the HP precooling refrigerant separator 492, and
subsequently to the
suction of the HP compression stage 416D as a part of the HP side stream 411.
[00101] The second HP liquid precooling refrigerant 464 is let down in
pressure in a sixth
pressure letdown valve 478 to produce a second MP precooling refrigerant 465.
The liquid
fraction of the second MP precooling refrigerant 465 is partially vaporized in
the MP liquefaction
refrigerant evaporator 486 to produce a second MP vapor precooling refrigerant
466 and a
second MP liquid precooling refrigerant 467. The second MP vapor precooling
refrigerant 466
is sent to the MP precooling refrigerant separator 493, and subsequently to
the suction of the
MP compression stage 416C as a part of the MP side stream 412.
[00102] The second MP liquid precooling refrigerant 467 is let down in
pressure in a seventh
pressure letdown valve 479 to produce a second LP precooling refrigerant 468.
The liquid
fraction of the second LP precooling refrigerant 468 is partially vaporized in
the LP liquefaction
refrigerant evaporator 487 to produce a second LP vapor precooling refrigerant
469 and a
second LP liquid precooling refrigerant 470. The second LP vapor precooling
refrigerant 469 is
sent to the LP precooling refrigerant separator 494, and subsequently to the
suction of the LP
compression stage 416B as a part of the LP side stream 413.
[00103] The second LP liquid precooling refrigerant 470 is let down in
pressure in an eighth
pressure letdown valve 480 to produce a second LLP precooling refrigerant 471.
The liquid
fraction of the second LLP precooling refrigerant 471 is completely vaporized
in the LLP
liquefaction refrigerant evaporator 488 to produce a second LLP vapor
precooling refrigerant
472. The second LLP vapor precooling refrigerant 472 is sent to the LLP
precooling refrigerant
-20-
CA 2996938 2018-02-28
separator 495, and subsequently to the suction of the LLP compression stage
416A as a part of
the warm low pressure precooling refrigerant 414.
[00104] In a preferred arrangement, using a precooling refrigerant of
carbon dioxide, the
pressure of the warm low pressure precooling refrigerant 414 is between about
5 bara and 30
bara, and the pressure of the compressed precooling refrigerant 415 is between
about 50 bara
and 120 bara.
[00105] In an alternate arrangement, the feed and liquefaction refrigerants
may be cooled in
the same heat exchangers against the precooling refrigerant. In such an
arrangement, the
cooled precooling refrigerant 410 is not divided into a first and second
portion and separate
precooling evaporators for a second cooling circuit are not required. Some
examples of
precooling refrigerants include propane, propylene, ethane, ethylene, ammonia,
carbon dioxide,
MR, hydrofluorocarbons such as R-410A, R22, or any other suitable refrigerant.
[00106] The temperature of the cooled precooling refrigerant 410 varies with
ambient
temperature and the approach temperature of the precooling refrigerant
condenser 417. For
typical hot ambient temperatures, the temperature of the cooled precooling
refrigerant 410 is
between about 30 degrees Celsius and about 60 degrees Celsius. Depending on
the critical
temperature of the precooling refrigerant, the precooling process will either
be subcritical or
transcritical. If the temperature of the cooled precooling refrigerant 410 is
lower than the critical
temperature, then the process will be subcritical. However, if the temperature
of the cooled
precooling refrigerant 410 is greater than or equal to the critical
temperature, then the process
will be transcritical, and will have lower process efficiency than a
subcritical operation.
[00107] FIG. 5 shows a first exemplary embodiment. Referring to FIG. 5, the
compressed
precooling refrigerant 515 is cooled in one or more heat exchangers, such as
desuperheater,
condenser, and/or subcooler heat exchangers, depicted as precooling
refrigerant condenser
517, to produce a cooled precooling refrigerant 510 that provides the
precooling duty required.
The cooled precooling refrigerant 510 is further cooled in an economizer heat
exchanger 525A
to produce a further cooled precooling refrigerant 597. The temperature of the
cooled
precooling refrigerant 510 is at ambient temperature plus the approach
temperature of the
precooling refrigerant condenser 517 also referred to herein as the subcooler
heat exchanger
approach temperature. The subcooler heat exchanger approach temperature is
preferably
between about 5 to 40 degrees Celsius and more preferably between about 10 and
30 degrees
Celsius. The cooled precooling refrigerant 510 is preferably more than 0
degrees Celsius
-21-
CA 2996938 2018-02-28
warmer than the critical temperature, more preferably, more than 10 degrees
Celsius warmer
than the critical temperature or, most preferably, more than 20 degrees warmer
than the critical
temperature. The precooling refrigeration process without the economizer heat
exchanger is
transcritical in nature. The temperature of the further cooled precooling
refrigerant 597 is below
the critical temperature. As a non-limiting example, the further cooled
precooling refrigerant 597
preferably may be more than 0 degrees Celsius colder than the critical
temperature or, more
preferably, more than 2 degrees colder than the critical temperature.
[00108]
The further cooled precooling refrigerant 597 is then divided into the first
portion of
the cooled precooling refrigerant 519 and the second portion of the cooled
precooling refrigerant
561, which are used to provide cooling duty to the pre-treated feed stream 501
and the cooled
liquefaction refrigerant 539 respectively. In a preferred embodiment, the
further cooled
precooling refrigerant 597 is at a temperature preferably ranging from about -
20 degrees
Celsius to about 25 degrees Celsius, and more preferably from about 0 degrees
Celsius to
about 15 degrees Celsius.
[00109] A third portion 519A of the cooled precooling refrigerant is withdrawn
from the further
cooled precooling refrigerant 597 and is letdown in pressure in a ninth
pressure letdown valve
573A to produce a third high pressure precooling refrigerant 520A, which is
used to provide the
cooling duty in the economizer heat exchanger 525A. The third high pressure
precooling
refrigerant 520A may be two-phase and is at least partially vaporized and
preferably fully
vaporized in the economizer heat exchanger 525A to produce third high pressure
vapor
precooling refrigerant 521A. The third high pressure vapor precooling
refrigerant 521A is sent
to the HP precooling refrigerant separator 592, and subsequently to the
suction of the fourth
precooling compression stage 516D as a part of the HP side stream 511. In an
alternate
embodiment, the economizer heat exchanger 525A may be bypassed during average
and cold
ambient conditions when the cooled precooling refrigerant 510 is below the
critical temperature
and the process is already subcritical.
[00110] The pressure of the third high pressure precooling refrigerant 520A
may optionally be
higher than that of the first HP precooling refrigerant 520. In this case, the
third high pressure
vapor precooling refrigerant 521A may be reduced in pressure in a back-
pressure valve or
throttling valve (not shown), prior to introduction into the HP precooling
refrigerant separator
592. Alternatively, the third high pressure vapor precooling refrigerant 521A
may be introduced
into the precooling refrigerant compressor(s) 516 at a higher pressure
location than the suction
-22-
CA 2996938 2018-02-28
of the fourth precooling compression stage 516D, such as at the suction of a
fifth precooling
compression stage 516E (not shown).
[00111] The amount of flow that is used to provide the cooling duty for the
economizer heat
exchanger 525A via the third portion 519A of the cooled precooling refrigerant
will depend upon
the composition of the precooling refrigerant. In the embodiment shown in FIG.
5, 3-20% of the
flow is preferably directed to the third portion 519A (more preferably 5-15%),
15-45% is
preferably directed to the first portion 519, and 45-85% is preferably
directed to the second
portion 561. Any suitable flow regulation devices, such as proportional valves
(not shown) could
be used to regulate the desired flow spit.
[00112] A benefit of the embodiment shown in FIG. 5 is that it converts a
transcritical process
into a subcritical process. By further cooling the cooled precooling
refrigerant 510 in the
economizer heat exchanger 525A, the further cooled precooling refrigerant 597
becomes the
"effective" subcooler outlet temperature. Therefore, to determine whether the
operation is
subcritical or transcritical, the temperature of the further cooled precooling
refrigerant 597 would
need to be compared to the critical temperature of the refrigerant. Since the
further cooled
precooling refrigerant 597 is colder than the cooled precooling refrigerant
510, it increases the
likelihood of a subcritical cycle. As non-limiting examples, CO2 and ethane
have critical
temperatures of about 30 degrees Celsius, much lower than the temperature of
the cooled
precooling refrigerant 510 for typical average and hot ambient conditions. For
a process of the
prior art, this would lead to transcritical operation with significantly lower
the process efficiency,
due to higher vapor fraction. For transcritical operation, the vapor fraction
of the first HP
precooling refrigerant 420 is preferably between about 0.1 and 0.7.
Additionally, for a prior art
transcritical operation, there would be: no phase change in the heat rejection
(to ambient) step;
complicated inventory management with ambient temperature swings; a lack of
references for
baseload LNG facilities as well as other operational challenges.
However, using the
embodiment described in FIG. 5, the critical temperature of 30 degrees Celsius
is preferably
greater than the further cooled precooling refrigerant 597, even for hot
ambient conditions. As a
non-limiting example, using the embodiment of FIG. 5, the further cooled
precooling refrigerant
597 may be at a temperature of about 20 degrees Celsius for hot ambient
temperature. As a
result, the process of FIG. 5 will be subcritical in nature and therefore,
have a higher process
efficiency than the prior art embodiment of FIG. 4, preferably between 5% and
30% higher
efficiency than transcritical prior art processes. The vapor fraction of the
first HP precooling
-23-
CA 2996938 2018-02-28
refrigerant 520 is preferably between about 0 and 0.5, and more preferably
between about 0
and 0.3. The embodiment of FIG. 5 would also not have the challenges with
changes in
inventory management with ambient temperature swings, as described earlier.
[00113] A further benefit of this embodiment is that due to the colder
effective subcooler
outlet, the pressure of the compressed precooling refrigerant 515 can be
lower, which reduces
the compression load on the system. In a preferred embodiment, the pressure of
the
compressed precooling refrigerant 515 is between about 20 bara and 80 bara.
Further, the
lower pressure reduces the specific heat ratio of the precooling refrigerant.
The specific heat
ratio is the ratio of the constant pressure specific heat capacity to the
constant volume specific
heat capacity. As the specific heat ratio reduces, the temperature of the
refrigerant after
compression reduces, which implies lower lost work and therefore higher
process efficiency.
[00114] FIG. 6 shows a second exemplary embodiment and a variation of FIG. 5.
The further
cooled precooling refrigerant 697 is divided into the first portion of the
cooled precooling
refrigerant 619 and the second portion of the cooled precooling refrigerant
661. The first portion
of the cooled precooling refrigerant 619 is letdown in pressure in a ninth
pressure letdown valve
673A to produce a third high pressure precooling refrigerant 620A, which is
used to provide
cooling duty to the economizer heat exchanger 625A. The third high pressure
precooling
refrigerant 620A is partially vaporized in the economizer heat exchanger 625A
and phase
separated to produce a third high pressure vapor precooling refrigerant 621A
and a third high
pressure liquid precooling refrigerant 622A. The phase separation step may
occur within the
economizer heat exchanger 625A or in a separate phase separator (not shown).
The third high
pressure vapor precooling refrigerant 621A is sent to the HP precooling
refrigerant separator
692, and subsequently to the suction of the fourth precooling compression
stage 616D as a part
of the HP side stream 611. The third high pressure liquid precooling
refrigerant 622A is letdown
in pressure in the first pressure letdown device 673 to produce the first high
pressure precooling
refrigerant 620, which is used to provide cooling duty to the pre-treated feed
stream 601, while
the second portion of the cooled precooling refrigerant 661 is used to provide
cooling duty to the
cooled liquefaction refrigerant 639.
[00115] The pressure of the third high pressure precooling refrigerant 620A is
higher than
that of the first HP precooling refrigerant 620. Therefore, the third high
pressure vapor
precooling refrigerant 621A needs to be reduced in pressure in a back-pressure
valve or
throttling valve 621B to produce a reduced pressure third high pressure vapor
precooling
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CA 2996938 2018-02-28
refrigerant 621C, prior to introduction into the HP precooling refrigerant
separator 692.
Alternatively, the third high pressure vapor precooling refrigerant 621A may
be introduced into
the precooling refrigerant compressor(s) 616 at a higher pressure location
than the suction of
the fourth precooling compression stage 616D, such as at the suction of a
fifth precooling
compression stage 616E (not shown).
[00116] In an alternate embodiment, the economizer heat exchanger 625A may be
bypassed
during average and cold ambient conditions when the cooled precooling
refrigerant 610 is below
the critical temperature and the process is already subcritical. FIG. 6 has
all the benefits of the
embodiment shown in FIG. 5.
[00117]
FIG. 7 shows a third exemplary embodiment. Referring to FIG. 7, during a first
period of time, the cooled precooling refrigerant 710 is further cooled in an
auxiliary refrigerant
system 796 to produce a further cooled precooling refrigerant 797. The
temperature of the
cooled precooling refrigerant 710 is at ambient temperature plus subcooler
heat exchanger
temperature approach to ambient temperature. The subcooler heat exchanger
approach
temperature is preferably between about 5 to 40 degrees Celsius and more
preferably between
about 10 and 30 degrees Celsius. The first period of time is defined as a
period of time wherein
the cooled precooling refrigerant 710, referred to herein as the "subcooler
outlet temperature", is
greater than or equal to the critical temperature of the precooling
refrigerant. In other words,
during the first period of time, the temperature of the cooled precooling
refrigerant 710 is greater
than or equal to the critical temperature. As a non-limiting example, the
cooled precooling
refrigerant 710 may be more than 0 degrees Celsius warmer than the critical
temperature or
more than 10 degrees Celsius warmer than the critical temperature or more than
20 degrees
warmer than the critical temperature. Therefore, during the first period of
time, the precooling
refrigeration process without the auxiliary refrigerant system, is
transcritical in nature. As a non-
limiting example, the first period of time may take place during hot and
average ambient
conditions, including, but not limited to, summer months and/or warm days. The
temperature of
the further cooled precooling refrigerant 797 is below the critical
temperature. As a non-limiting
example, the further cooled precooling refrigerant 797 preferably may be more
than 0 degrees
Celsius colder than the critical temperature, more preferably more than 2
degrees colder than
the critical temperature or, most preferably, more than 5 degrees colder than
the critical
temperature.
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CA 2996938 2018-02-28
[00118] The further cooled precooling refrigerant 797 is then divided into
the first portion of
the cooled precooling refrigerant 719 and the second portion of the cooled
precooling refrigerant
761, which are used to provide cooling duty to the pre-treated feed stream 701
and the cooled
liquefaction refrigerant 739 respectively. In a preferred embodiment, the
further cooled
precooling refrigerant 797 is at a temperature preferably ranging from about -
20 degrees
Celsius to about 25 degrees Celsius, and more preferably from about 0 degrees
Celsius to
about 15 degrees Celsius. During the first period of time, the precooling
refrigeration process
with the auxiliary refrigerant system, is subcritical in nature.
[00119] During a second period of time, the cooled precooling refrigerant
710 optionally
bypasses the auxiliary refrigerant system 796 via the optional bypass
precooling refrigerant
710A, which is then divided into the first portion of the cooled precooling
refrigerant 719 and the
second portion of the cooled precooling refrigerant 761. The second period of
time is defined as
a period of time wherein the subcooler outlet temperature is lower than the
critical temperature
of the precooling refrigerant. In other words, during the second period of
time, the temperature
of the cooled precooling refrigerant 710 is lower than the critical
temperature. Therefore, during
the second period of time, the precooling refrigeration process without the
auxiliary refrigerant
system, is subcritical in nature. As a non-limiting example, the second period
of time may take
place during cold ambient conditions, such as winter months and/or cold
nights. As a non-
limiting example, the cooled precooling refrigerant 710 preferably may be more
than 10 degrees
Celsius colder than the critical temperature, more preferably, more than 15
degrees colder than
the critical temperature.
[00120] The auxiliary refrigerant system may utilize any heat transfer method,
such as boiling
heat transfer where the refrigerant evaporates to provide the cooling duty, or
sensible heat
transfer where the refrigerant warms up without changing phase to provide the
cooling duty or a
combination of both. The heat transfer method may also be absorption heat
transfer where the
refrigerant evaporates to provide the cooling duty but the compression step is
replaced by
additional equipment. Further, the auxiliary refrigerant system could use any
number of heat
exchangers. As a non-limiting example, the auxiliary refrigerant may be
propane or a mixed
refrigerant or a gas phase refrigeration process using feed gas. The auxiliary
refrigerant may
also be any suitable absorptive refrigerant.
[00121] Any suitable system could be used to monitor the temperature of the
cooled
precooling refrigerant 710 and control flow through the bypass 710A and the
auxiliary refrigerant
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CA 2996938 2018-02-28
system 796. For example, a controller 700 could be used to control valves 710B
and 710C
based on temperature sensed by a sensor 710D. When the sensor 710D senses that
the
cooled precooling refrigerant 710 is greater than or equal to the critical
temperature, the
controller 700 closes valve 710B and opens valve 710C. Conversely, when the
sensor 710D
senses that the cooled precooling refrigerant 710 is below the critical
temperature, the controller
700 opens valve 710B and closes valve 710C.
[00122] A benefit of the embodiment shown in FIG. 7 is that it converts a
transcritical process
into a subcritical process by further cooling the cooled precooling
refrigerant 710 in the auxiliary
refrigerant system 796. The further cooled precooling refrigerant 797 becomes
the "effective"
subcooler outlet temperature. Therefore, to determine whether the operation is
subcritical or
transcritical, the temperature of the further cooled precooling refrigerant
797 would need to be
compared to the critical temperature of the refrigerant. Since the further
cooled precooling
refrigerant 797 is much colder than the cooled precooling refrigerant 710, it
increases the
likelihood of a subcritical cycle. As a non-limiting example, CO2 and ethane
have critical
temperatures of about 30 degrees Celsius, much lower than the temperature of
the cooled
precooling refrigerant 710 for typical average and hot ambient conditions. For
a process of the
prior art, this would lead to transcritical operation with significantly lower
process efficiency, due
to higher vapor fraction. For transcritical operation, the vapor fraction of
the first HP precooling
refrigerant 420 is preferably between about 0.1 and 0.7. Additionally, for a
prior art transcritical
operation, there would be no phase change in the heat rejection (to ambient)
step, complicated
inventory management with ambient temperature swings, lack of references for
baseload LNG
facilities, as well as other operational challenges. However, using the
embodiment described in
FIG. 7, the critical temperature of 30 degrees Celsius is preferably greater
than the further
cooled precooling refrigerant 797, even for hot ambient conditions. As a non-
limiting example,
using the embodiment of FIG. 7, the further cooled precooling refrigerant 797
may be at a
temperature of about 10 degrees Celsius for hot ambient temperature. As a
result, the process
of FIG. 7 will be subcritical in nature and therefore, have a much higher
process efficiency than
the prior art embodiment of FIG. 4. Preferably, between 10% and 30% higher
efficiency than
transcritical prior art processes is obtained. Further, the embodiments, when
applied to an
transcritical process, will have significantly higher benefit than when
applied to an already
subcritical process, where the benefit is about 5 to 15%. The vapor fraction
of the first HP
precooling refrigerant 720 is preferably between about 0 and 0.5, and more
preferably between
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CA 2996938 2018-02-28
about 0 and 0.3. The embodiment of FIG. 7 also does not have the challenges of
changes in
inventory management with ambient temperature swings, as described earlier.
[00123] A further benefit of this embodiment is that due to the colder
effective subcooler
outlet, the pressure of the compressed precooling refrigerant 715 can be
lower, which reduces
the compression load on the system. In a preferred embodiment, the pressure of
the
compressed precooling refrigerant 715 is between about 20 bara and 80 bara.
Further, the
lower pressure reduces the specific heat ratio of the precooling refrigerant.
The specific heat
ratio is the ratio of the constant pressure specific heat capacity to the
constant volume specific
heat capacity. As the specific heat ratio reduces, the temperature of the
refrigerant after
compression reduces, which implies lower lost work and therefore higher
process efficiency.
[00124] The higher process efficiency of the embodiment of FIG. 7 makes it
optimal to shift
more load into the precooling system by reducing the precooling temperature,
and lowering the
load on the liquefaction system. As a non-limiting example, the temperature of
the pre-cooled
natural gas stream 705 may be between about -30 degrees Celsius to about -60
degrees
Celsius, whereas the temperature of the pre-cooled natural gas stream 405 may
be between
about -10 degrees Celsius to about -40 degrees Celsius.
[00125] In the embodiment shown in FIG. 7 the auxiliary refrigerant system
cools the
precooling refrigerant, however it may also be used to cool the liquefaction
refrigerant. This is
also applicable to an embodiment where there is no dedicated precooling
refrigerant and the
auxiliary refrigerant system cools the liquefaction refrigerant.
[00126] In a preferred embodiment, the liquefaction refrigerant is MR and
the precooling
refrigerant is ethane or CO2. In another preferred embodiment, the
liquefaction refrigerant is
gas phase N2 and the precooling refrigerant is ethane or CO2. In yet another
preferred
embodiment, the liquefaction refrigerant is methane and the precooling
refrigerant is ethane or
CO2. The benefit of using CO2 as the precooling refrigerant is that it is non-
flammable, easily
available, and has a high density. Its high density leads to a lower
volumetric flowrate of
precooling refrigerant required for the same mass of refrigerant. The higher
density also
reduces precooling system piping and equipment sizes. In a further preferred
embodiment
using CO2 as the precooling refrigerant, the CO2 is produced in the LNG
facility in the acid gas
removal unit (AGRU).
[00127] In an alternative embodiment, during the first period of time,
ambient air or water is
cooled against the auxiliary refrigerant in the auxiliary heat exchanger to
produce a cooled
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CA 2996938 2018-02-28
ambient stream. During the second period of time, the auxiliary refrigerant
system is optionally
bypassed. In such an arrangement, the precooling refrigerant is cooled against
the cooled
ambient stream instead of the auxiliary refrigerant.
[00128] FIG. 8 shows a fourth embodiment, which is a variation of the
embodiment shown in
FIG. 7. During the first period of time, the cooled precooling refrigerant 810
is further cooled in
an auxiliary refrigerant system 896 to produce a further cooled precooling
refrigerant 897.
Further, the pre-treated feed stream 801 is cooled in the auxiliary
refrigerant system 896 to
produce a further cooled feed stream 898, which is then sent to the HP feed
evaporator 881 to
be precooled. The cooled liquefaction refrigerant 839 is cooled in the
auxiliary refrigerant
system 896 to produce a further cooled MR stream 899, which is then sent to
the HP
Liquefaction refrigerant evaporator 885 to be precooled.
[00129] During the second period of time, the auxiliary refrigerant system
is optionally
bypassed via an optional bypass precooling refrigerant 810A, an optional
bypass feed stream
801A, and an optional bypass liquefaction refrigerant 839A.
[00130] In a preferred embodiment, the further cooled precooling
refrigerant 897, the further
cooled feed stream 898, and the further cooled MR stream 899 are at a
temperature preferably
ranging from about -20 degrees Celsius to about 25 degrees Celsius, more
preferably from
about 0 degrees Celsius to about 15 degrees Celsius.
[00131] This embodiment has all the benefits of FIG. 7. Additionally, since
the feed and MR
streams are also cooled in the auxiliary refrigerant system 896 during the
first period of time, the
process efficiency for FIG. 8 is higher than that for FIG. 7, for a minimal
increase in capital cost.
[00132] In an alternative embodiment, an intermediate compressed stream from
the
precooling refrigerant system or the liquefaction refrigerant system is
withdrawn and cooled
against the auxiliary refrigerant system 896 prior to being further
compressed.
[00133] FIG. 9 shows an exemplary embodiment of the auxiliary refrigerant
system 996, as
applied to FIG. 8. The cooled precooling refrigerant 910 is further cooled in
an auxiliary heat
exchanger 989 to produce a further cooled precooling refrigerant 997. The pre-
treated feed
stream 901 is cooled in the auxiliary heat exchanger 989 to produce the
further cooled feed
stream 998. The cooled liquefaction refrigerant 939 is cooled in the auxiliary
heat exchanger
989 to produce the further cooled MR stream 999.
[00134] The auxiliary refrigerant system is based on boiling heat transfer. A
vapor auxiliary
refrigerant 954A is withdrawn from the warm end of the auxiliary heat
exchanger 989 and is
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CA 2996938 2018-02-28
compressed in auxiliary refrigerant compressor 945A to produce a high pressure
vapor auxiliary
refrigerant 957A. The high pressure vapor auxiliary refrigerant 957A is cooled
in one or more
heat exchangers, represented by an auxiliary refrigerant condenser 952A to
produce a cooled
auxiliary refrigerant 959A. The cooled auxiliary refrigerant 959A is letdown
in pressure in an
auxiliary refrigerant letdown valve 953A to produce a low pressure auxiliary
refrigerant 944A.
The liquid component of the low pressure auxiliary refrigerant 944A is
evaporated in the
auxiliary heat exchanger 989 to provide the auxiliary cooling duty required
and to produce the
vapor auxiliary refrigerant 954A.
[00135] In an alternative exemplary embodiment of FIG. 9, as applied to
FIG. 7, only the
cooled precooling refrigerant 910 is further cooled in the auxiliary heat
exchanger 989 to
produce the further cooled precooling refrigerant 997.
[00136] In a preferred embodiment, the auxiliary refrigerant is an HFC
refrigerant, including,
but not limited to, R-410A or R-22. In another preferred embodiment, the
auxiliary refrigerant is
propane or ammonia or any other two-phase refrigerant.
[00137] FIG. 10 shows another exemplary embodiment of the auxiliary
refrigerant system
1096, as applied to FIG. 8. The cooled precooling refrigerant 1010 is further
cooled in an
auxiliary heat exchanger 1089 to produce a further cooled precooling
refrigerant 1097. The pre-
treated feed stream 1001 is cooled in the auxiliary heat exchanger 1089 to
produce the further
cooled feed stream 1098. The cooled liquefaction refrigerant 1039 is cooled in
the auxiliary
heat exchanger 1089 to produce the further cooled MR stream 1099.
[00138] The auxiliary refrigerant is a portion of the liquefaction
refrigerant. .. In one
embodiment wherein the liquefaction refrigerant uses boiling heat transfer, as
shown in FIG. 3,
a portion of the MRL stream 341 is removed as the cooled auxiliary refrigerant
1059A. The
cooled auxiliary refrigerant 1059A is letdown in pressure in an auxiliary
refrigerant letdown valve
1053A to produce a low pressure auxiliary refrigerant 1044A. The liquid
component of the low
pressure auxiliary refrigerant 1044A is evaporated in the auxiliary heat
exchanger 1089 to
provide the auxiliary cooling duty required and to produce the vapor auxiliary
refrigerant 1054A.
The vapor auxiliary refrigerant 1054A may be returned to the liquefaction
refrigerant
compression system, by introducing into the medium pressure suction drum 353
or any other
suitable location.
[00139] In an alternative embodiment, the cooled auxiliary refrigerant
1059A may be
obtained from any other location of the liquefaction process, such that it may
not be condensed
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CA 2996938 2018-02-28
and the vapor auxiliary refrigerant 1054A may be returned to any location of
the liquefaction
process.
[00140] In another embodiment, wherein the liquefaction refrigerant uses
sensible heat
transfer, as shown in FIG. 2, a portion of precooled liquefaction refrigerant
240 is removed as
the cooled auxiliary refrigerant 1059A. The cooled auxiliary refrigerant 1059A
is letdown in
pressure in an auxiliary refrigerant letdown valve 1053A, which may be an
expander, to produce
a low pressure auxiliary refrigerant 1044A. The low pressure auxiliary
refrigerant 1044A is
warmed in the auxiliary heat exchanger 1089 to provide the auxiliary cooling
duty required and
to produce the vapor auxiliary refrigerant 1054A. The vapor auxiliary
refrigerant 1054A may be
returned to the liquefaction refrigerant compression system, by introducing
into the HP
compressor 257 or any other suitable location. The vapor auxiliary refrigerant
1054A may also
be compressed prior to returning to the liquefaction refrigerant system.
[00141] In an alternative exemplary embodiment of FIG. 10, as applied to
FIG. 7, only the
cooled precooling refrigerant 1010 is further cooled in the auxiliary heat
exchanger 1089 to
produce the further cooled precooling refrigerant 1097.
[00142] In preferred embodiments, the auxiliary refrigerant is mixed
refrigerant (MR) or
nitrogen.
[00143] In a further alternative embodiment, the auxiliary refrigerant is
comprised of a portion
of the pretreated feed stream 1001 instead of the liquefaction refrigerant of
FIG. 2. The vapor
auxiliary refrigerant 1054A may be returned to an upstream location in the
facility, such as
upstream of a feed compressor, or may be used as fuel in the facility.
[00144] FIG. 11 shows another exemplary embodiment of the auxiliary
refrigerant system
1196, as applied to FIG. 8, using an absorption based process. The cooled
precooling
refrigerant 1110 is further cooled in an auxiliary heat exchanger 1189 to
produce a further
cooled precooling refrigerant 1197. The pre-treated feed stream 1101 is cooled
in the auxiliary
heat exchanger 1189 to produce the further cooled feed stream 1198. The cooled
liquefaction
refrigerant 1139 is cooled in the auxiliary heat exchanger 1189 to produce the
further cooled MR
stream 1199.
[00145] A vapor auxiliary refrigerant 1154A is withdrawn from the warm end of
the auxiliary
heat exchanger 1189 and is sent to an auxiliary refrigerant absorber 1191,
where the vapor
auxiliary refrigerant 1154A is absorbed into an auxiliary refrigerant solvent
1158A to produce a
low pressure liquid auxiliary refrigerant 1155A. The low pressure liquid
auxiliary refrigerant
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CA 2996938 2018-02-28
1155A is pumped in an auxiliary refrigerant pump 1151A to produce a high
pressure liquid
auxiliary refrigerant 1156A, which is sent to an auxiliary refrigerant
generator 1150A, where heat
is provided to separate a high pressure vapor auxiliary refrigerant 1157A from
the auxiliary
refrigerant solvent 1158A, which is sent to the auxiliary refrigerant absorber
1191. The high
pressure vapor auxiliary refrigerant 1157A is cooled in one or more heat
exchangers, depicted
by an auxiliary refrigerant condenser 1152A to produce a cooled auxiliary
refrigerant 1159A.
The cooled auxiliary refrigerant 1159A is letdown in pressure in an auxiliary
refrigerant letdown
valve 1153A to produce a low pressure vapor auxiliary refrigerant 1144A. The
low pressure
vapor auxiliary refrigerant 1144A is evaporated in the auxiliary heat
exchanger 1189 to provide
the auxiliary cooling duty required.
[00146] In one embodiment, the heat provided to the auxiliary refrigerant
generator 1150A is
obtained from waste heat generated in the natural gas liquefaction facility.
In another
embodiment, waste heat generated from liquefaction and precooling gas turbines
driving
liquefaction and precooling compressors, is utilized in the auxiliary
refrigerant generator 1150A.
[00147] In an alternative exemplary embodiment of FIG. 11, as applied to
FIG. 7, only the
cooled precooling refrigerant 1110 is further cooled in the auxiliary heat
exchanger 1189 to
produce the further cooled precooling refrigerant 1197. In one embodiment, the
auxiliary
refrigerant is an aqueous LiBr solution.
[00148] Although the embodiments described here suggest use of the auxiliary
refrigerant in
the precooling system, it may also be used for the liquefaction, subcooling,
or any step of the
process.
[00149] Typical pressure letdown valves, such as Joule-Thomson (JT) valves,
are isenthalpic
in nature. A representation of an isenthalpic pressure letdown step in a
transcritical process in
shown on a P-H diagram in FIG. 1B. Line E-F represents the isenthalpic
pressure letdown step
and due to the vertical nature of the line, results in a high vapor fraction
at point F. This results
in low process efficiency. FIGS. 5-11 discuss embodiments for converting a
transcritical
process to a subcritical one and therefore, improve the process efficiency. An
alternative way to
improve the process efficiency is to move point F to the left by performing
step E-F in an
isentropic manner, as shown in FIG. 12A. Due to the shape of isentropic
(constant entropy)
lines in a P-H diagram, without moving point E, it is possible for point F to
have a lower vapor
fraction. FIG. 12B shows a fifth embodiment, using isentropic expansion.
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CA 2996938 2018-02-28
[00150] Referring to FIG. 12B, the compressed precooling refrigerant 1215
is cooled by
indirect heat exchange against ambient air or water in one or more heat
exchangers, depicted
by precooling refrigerant condenser 1217 to produce the cooled precooling
refrigerant 1210.
The cooled precooling refrigerant 1210 is then divided into two portions, a
first portion 1219 to
provide cooling duty to the pre-treated feed stream 1201, and a second portion
1261 to provide
cooling duty to the cooled liquefaction refrigerant 1239.
[00151] The first portion of the cooled precooling refrigerant 1219 is let
down in pressure in a
first dual phase expander 1248A to produce a first HP precooling refrigerant
1220. The liquid
fraction of the first HP precooling refrigerant 1220 is partially vaporized in
the HP feed
evaporator 1281 to produce a first HP vapor precooling refrigerant 1221 and a
first HP liquid
precooling refrigerant 1222. The first HP vapor precooling refrigerant 1221 is
sent to an HP
precooling refrigerant separator 1292, and subsequently to the suction of the
fourth precooling
compression stage 1216D as a part of the HP side stream 1211.
[00152] The second portion of the cooled precooling refrigerant 1261 may be
let down in
pressure in a second dual phase expander 1249A to produce a second HP
precooling
refrigerant 1262. The liquid fraction of the second HP precooling refrigerant
1262 is partially
vaporized in the HP liquefaction refrigerant evaporator 1285 to produce a
second HP vapor
precooling refrigerant 1263 and a second HP liquid precooling refrigerant
1264. The second HP
vapor precooling refrigerant 1264 is sent to the HP precooling refrigerant
separator 1292, and
subsequently to the suction of the fourth precooling compression stage 1216D
as a part of the
HP side stream 1211. The vapor fraction of the first HP precooling refrigerant
1220 and the
second HP precooling refrigerant 1262 is preferably between about 0.2 and 0.6,
and more
preferably between about 0.2 and 0.4. In contrast, the vapor fraction of the
first HP precooling
refrigerant 420 of the prior art is preferably between about 0.1 and 0.7.
[00153] A benefit of the embodiment of FIG. 12B is that the process efficiency
can be
improved at low capital cost, plot space, and complexity. Another benefit of
using an expander
is that useful work can be extracted from it, leading to lower power
requirement. Since this
embodiment does not convert a transcritical process to a subcritical one, the
inventory
management issues remain. To solve this issue, the embodiment of FIG. 12B may
be
combined with any of the embodiments described previously, such as the
embodiments shown
in FIGS. 5-11. In one embodiment, the cooled precooling refrigerant 1210 may
be further
cooled in the economizer heat exchanger 525A of FIG. 5 to produce a further
cooled precooling
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CA 2996938 2018-02-28
refrigerant 597 prior to performing the isentropic pressure letdown step.
In another
embodiment, the cooled precooling refrigerant 1210 may be further cooled in
the auxiliary
refrigerant system 796 to produce a further cooled precooling refrigerant 797
prior to performing
the isentropic pressure letdown step. Combining the features of FIG. 12B with
the previous
embodiments allows for improving the efficiency of the process and at the same
time converting
a transcritical process to a subcritical one, which further improves process
efficiency and
resolves refrigerant inventory management issues.
EXAMPLE 1
[00154] The following is an example of an exemplary embodiment. The example
process
and data are based on simulations of a precooling and liquefaction process at
a plant that
produces nominally 5 million metric tonnes per annum (MTPA) of LNG. The
precooling
refrigerant for this example is either ethane or carbon dioxide and the
liquefaction refrigerant
may be either MR or N2. This example specifically refers to the embodiment
shown in FIG. 5
but is also applicable to FIG. 6 and other related embodiments. The ambient
temperature is of
77 degrees Fahrenheit (25 degrees Celsius). The critical temperature of ethane
and carbon
dioxide is about 30 degrees Celsius.
[00155]
Referring to FIG. 5, the cooled precooling refrigerant 510 is further cooled
in an
economizer heat exchanger 525A to produce a further cooled precooling
refrigerant 597. The
cooled precooling refrigerant 510 is at psia (85 bara), 90 degrees Fahrenheit
(32 degrees
Celsius) and supercritical. The further cooled precooling refrigerant 597 is
at 81 degrees
Fahrenheit (27 degrees Celsius) and liquid phase. The third portion of the
cooled precooling
refrigerant 519A is 15 mole % of the further cooled precooling refrigerant
597. The process
efficiency of this embodiment is about 4% higher than the prior art.
EXAMPLE 2
[00156] The following is an example of an exemplary embodiment. The example
process
and data are based on simulations of a precooling and liquefaction process at
a plant that
produces nominally 5 MTPA of LNG. The precooling refrigerant for this example
is either
ethane or carbon dioxide and the liquefaction refrigerant may be either MR or
N2. This example
specifically refers to the embodiment shown in FIG. 7 but is also applicable
to other
embodiments. The first period of time occurs during average ambient
temperature of 77
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CA 2996938 2018-02-28
degrees Fahrenheit (25 degrees Celsius) and the second period of time occurs
during cold
ambient temperature of 52 degrees Fahrenheit (11 degrees Celsius). To simplify
the
description of this example, elements and reference numerals described with
respect to the
embodiment shown in FIG. 7 will be used. Reference numerals described with
respect to the
embodiment shown in FIG. 4 (prior art) will also be used for comparison.
[00157] During the first period of time, a pre-treated feed stream 701 at a
temperature of 70
degrees Fahrenheit (21 degrees Celsius), pressure of 834 psia (57.5 bara), and
82,000 Ibmol/hr
(37,196 kgmol/hr) is cooled by indirect heat exchange in an HP feed evaporator
781 to produce
a first intermediate feed stream 702 at temperature of 35 degrees Fahrenheit
(2 degrees
Celsius), which is then cooled in an MP feed evaporator 782 to produce a
second intermediate
feed stream 703 at a temperature of 8 degrees Fahrenheit (-14 degrees
Celsius), followed by a
LP feed evaporator 783 to produce a third intermediate feed stream 704 at a
temperature of -21
degrees Fahrenheit (-29 degrees Celsius), and finally an LLP feed evaporator
784 to produce a
pre-cooled natural gas stream 705 at a temperature of -45 degrees Fahrenheit (-
43 degrees
Celsius). The cooled liquefaction refrigerant 739 is cooled to similar
temperatures in the HP
Liquefaction refrigerant evaporator 785, MP Liquefaction refrigerant
evaporator 786, LP
Liquefaction refrigerant evaporator 787, and the LLP Liquefaction refrigerant
evaporator 788.
[00158] The warm low pressure precooling refrigerant 714 at temperature of -50
degrees
Fahrenheit (-46 degrees Celsius), pressure of 108 psia (7 bara), and flowrate
of 21,450 lbmol/hr
(9,730 kgmol/hr) is compressed in a four stage precooling refrigerant
compressor 716 to
produce compressed precooling refrigerant 715 at temperature of 122 degrees
Fahrenheit (50
degrees Celsius) and pressure of 722 psia (50 bara).
[00159] An LP side stream 713 at temperature of -27 degrees Fahrenheit (-33
degrees
Celsius) and pressure of 188 psia (13 bara), MP side stream 712 at temperature
of 1 degrees
Fahrenheit (-17 degrees Celsius) and pressure of 313 psia (22 bara), and HP
side stream 711
at temperature of 29 degrees Fahrenheit (-2 degrees Celsius) and pressure of
780 psia (32
bara) are introduced to the precooling refrigerant compressor 716 at
intermediate locations.
[00160] The compressed precooling refrigerant 715 is cooled by indirect heat
exchange
against ambient air in three heat exchangers, depicted by precooling
refrigerant condenser 717
to produce the cooled precooling refrigerant 710 at a temperature of 90
degrees Fahrenheit (32
degrees Celsius). The cooled precooling refrigerant 710 is further cooled in
an auxiliary
refrigerant system 796 to produce a further cooled precooling refrigerant 797
at a temperature
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of 50 degrees Fahrenheit (10 degrees Celsius). The further cooled precooling
refrigerant 797 is
then divided into the first portion of the cooled precooling refrigerant 719
and the second portion
of the cooled precooling refrigerant 761, which are used to provide cooling
duty to the pre-
treated feed stream 701 and the cooled liquefaction refrigerant 739
respectively. The first
portion of the cooled precooling refrigerant 719 is about 20 mole percent of
the cooled
precooling refrigerant 710.
[00161] The first portion of the cooled precooling refrigerant 719 is let
down in pressure in a
first pressure letdown valve 773 to produce a first HP precooling refrigerant
720 at a
temperature of 29 degrees Fahrenheit (-1 degrees Celsius), pressure of 486
psia (33 bara), and
vapor fraction of 0.12. The second portion of the cooled precooling
refrigerant 761 is letdown to
similar conditions.
[00162] During the second period of time, the auxiliary refrigerant system
796 is optionally
bypassed via the bypass precooling refrigerant 710A, which is at 64 degrees
Fahrenheit (18
degrees Celsius).
[00163] In contrast, now referring to FIG. 4 of the prior art, the first HP
precooling refrigerant
420 is at a temperature of 62 degrees Fahrenheit (17 degrees Celsius),
pressure of 766 psia
(53 bara), and vapor fraction of 0.28. Also, the compressed precooling
refrigerant 415 is at a
temperature of 160 degrees Fahrenheit (71 degrees Celsius) and pressure of
1228 psia (85
bar). Further, the cooled precooling refrigerant 410 is at a temperature of 90
degrees
Fahrenheit (32 degrees Celsius).
[00164] Since the critical temperature of ethane and carbon dioxide is about
30 degrees
Celsius, the process of the prior art would have transcritical operation at
average ambient
temperature, which is the cause for the higher vapor fraction of the first HP
precooling
refrigerant 420. The embodiments, however would have subcritical operation,
given that the
temperature of the further cooled precooling refrigerant 797 is lower than the
critical
temperature. This is the reason for the lower vapor fraction of the first HP
precooling refrigerant
720. By reducing the vapor fraction of first HP precooling refrigerant 720,
the embodiments
significantly improve the process efficiency.
[00165] Further, by lowering the pressure of the compressed precooling
refrigerant 715, the
embodiments reduce the compression power requirement and the specific heat
ratio of the
precooling refrigerant. Lower specific heat ratio also increases the process
efficiency. Overall,
up to about 20% improvement in process efficiency was observed for FIG. 7 as
compared to
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CA 2996938 2018-02-28
FIG. 4, during the first period of time. Additionally, refrigerant inventory
management issues
associated with ambient temperature swings are also eliminated by the
embodiments. Overall,
the embodiments solve the challenges presented by transcritical refrigerants.
EXAMPLE 3
[00166] The following is an example of an exemplary embodiment. The example
process
and data are based on simulations of a precooling and liquefaction process at
a plant that
produces nominally 5 MTPA of LNG. The precooling refrigerant for this example
is either
ethane or carbon dioxide and the liquefaction refrigerant may be either MR or
N2. This example
specifically refers to the embodiment shown in FIG. 12B.
[00167] The cooled precooling refrigerant 1210 is at 89.6 degrees Fahrenheit
(32 degrees
Celsius), 120 psia (84 bara), and vapor fraction of 1. The cooled precooling
refrigerant 1210 is
then divided into two portions, a first portion 1219 to provide cooling duty
to the pre-treated feed
stream 1201, and a second portion 1261 to provide cooling duty to the cooled
liquefaction
refrigerant 1239. The first portion of the cooled precooling refrigerant 1219
is let down in
pressure in a first dual phase expander 1248A to produce a first HP precooling
refrigerant 1220
at 59 degrees Fahrenheit (15 degrees Celsius), 735 psia (51 bara), and a vapor
fraction of 0.25.
In case a JT valve (isenthalpic) instead of the dual phase expander valve
(isentropic) would
have been used, the vapor fraction of the first HP precooling refrigerant 1220
would have been
0.3. The embodiment of FIG. 12B improves the process efficiency of the prior
art by about 3%.
[00168] An invention has been disclosed in terms of preferred embodiments and
alternate
embodiments thereof. Of course, various changes, modifications, and
alterations from the
teachings of the present invention may be contemplated by those skilled in the
art without
departing from the intended spirit and scope thereof. It is intended that the
present invention
only be limited by the terms of the appended claims.
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