U.S. patent application number 15/830330 was filed with the patent office on 2019-05-30 for method and system for cooling a hydrocarbon stream.
This patent application is currently assigned to Air Products and Chemicals, Inc.. The applicant listed for this patent is Air Products and Chemicals, Inc.. Invention is credited to Gowri Krishnamurthy, Mark Julian Roberts.
Application Number | 20190162469 15/830330 |
Document ID | / |
Family ID | 63917707 |
Filed Date | 2019-05-30 |
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United States Patent
Application |
20190162469 |
Kind Code |
A1 |
Krishnamurthy; Gowri ; et
al. |
May 30, 2019 |
METHOD AND SYSTEM FOR COOLING A HYDROCARBON STREAM
Abstract
A system and method for increasing the efficiency of natural gas
liquefaction processes by using a hybrid cooling system and method.
More specifically, a system and method for converting a
transcritical precooling refrigeration process to a subcritical
process. In one embodiment, the refrigerant is cooled to
sub-critical temperature using an economizer. In another
embodiment, the refrigerant is cooled to a sub-critical temperature
using an auxiliary heat exchanger. Optionally, the economizer or
auxiliary heat exchanger can be bypassed when ambient temperatures
are sufficiently low to cool the refrigerant to a sub-critical
temperature. In another embodiment, the refrigerant is
isentropically expanded.
Inventors: |
Krishnamurthy; Gowri;
(Sellersville, PA) ; Roberts; Mark Julian;
(Kempton, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Air Products and Chemicals, Inc. |
Allentown |
PA |
US |
|
|
Assignee: |
Air Products and Chemicals,
Inc.
Allentown
PA
|
Family ID: |
63917707 |
Appl. No.: |
15/830330 |
Filed: |
December 4, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15822713 |
Nov 27, 2017 |
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15830330 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2210/06 20130101;
F25J 2270/906 20130101; F25J 1/0022 20130101; F25J 1/0207 20130101;
F25J 1/0052 20130101; F25J 1/0085 20130101; F25J 1/0215 20130101;
F25J 2245/02 20130101; F25J 2270/12 20130101; F25B 9/008 20130101;
F25B 2600/2501 20130101; F25J 1/0268 20130101; F25J 1/0245
20130101; F25B 1/10 20130101; F25B 6/04 20130101; F25B 2400/0403
20130101; F25J 1/0218 20130101; F25J 1/0205 20130101; F25J 1/0262
20130101; F25J 1/0265 20130101; F25J 1/0095 20130101; F25J 1/0072
20130101; F25J 2270/90 20130101; F25J 2290/50 20130101; F25B
2400/13 20130101; F25B 5/04 20130101; F25B 9/00 20130101; F25J
1/0057 20130101; F25J 1/0227 20130101; F25B 2341/0662 20130101;
F25J 2270/60 20130101; F25B 2309/061 20130101; F25J 2270/902
20130101 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 1/02 20060101 F25J001/02 |
Claims
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 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.
2. The method of claim 1, wherein the at least one auxiliary heat
exchanger comprises an economizer and the auxiliary refrigerant
comprises the first refrigerant.
3. The method of claim 1, wherein the auxiliary refrigerant is at
least a portion of the hydrocarbon feed stream.
4. The method of claim 1, wherein the at least one auxiliary heat
exchanger is a part of a closed loop vapor compression system.
5. The method of claim 4, wherein the auxiliary refrigerant is a
hydrofluorocarbon or propane.
6. The method of claim 1, 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.
7. The method of claim 6, wherein at least one fluid stream being
cooled in the at least one cooling circuit comprises the second
refrigerant.
8. The method of claim 1, wherein the first refrigerant comprises
ethane, carbon-dioxide, or ethylene.
9. The method of claim 1, wherein step (a) further comprises: (a)
compressing the first refrigerant in a plurality of compression
stages to produce a compressed first refrigerant.
10. The method of claim 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.
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 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.
12. The apparatus of claim 11, wherein the at least one auxiliary
heat exchanger comprises an economizer.
13. The apparatus of claim 11, wherein the at least one auxiliary
heat exchanger is part of a closed loop vapor compression
system.
14. 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.
15. The method of claim 14, 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.
16. The method of claim 15, wherein at least one fluid stream being
cooled in the at least one cooling circuit comprises the second
refrigerant.
17. The method of claim 14, wherein the first refrigerant is
ethane, carbon-dioxide, or ethylene.
18. The method of claim 14, wherein step (a) further comprises: (a)
compressing the first refrigerant in a plurality of compression
stages to produce a compressed first refrigerant.
Description
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 AP-X.TM.) 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 T1 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 produced subcooled liquid (E). The
temperature at E is the aftercooler outlet temperature, also
referred to as T.sub.AC 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, T.sub.CRIT, is shown
with an isotherm. The process starts with refrigerant vapor (A) at
pressure P1 and temperature T1 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 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.
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:
[0017] (a) compressing the first refrigerant in one or more
compression stages to produce a compressed first refrigerant;
[0018] (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;
[0019] (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: [0020]
(i) reducing the pressure of the first refrigerant; [0021] (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 [0022]
(iii) flowing at least a portion of the vaporized reduced pressure
first refrigerant into one of the at least one compression stages;
[0023] 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;
[0024] (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 critical temperature of the first
refrigerant, the second temperature being less than the critical
temperature of the first refrigerant; and
[0025] (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.
[0026] 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.
[0027] Aspect 3: The method of any of Aspects 1-2, wherein the
auxiliary refrigerant is at least a portion of the hydrocarbon feed
stream.
[0028] 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.
[0029] Aspect 5: The method of Aspect 4, wherein the auxiliary
refrigerant is a hydrofluorocarbon or propane.
[0030] Aspect 6: The method of any of Aspects 1-5, further
comprising:
[0031] (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.
[0032] 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.
[0033] Aspect 8: The method of any of Aspects 1-7, wherein the
first refrigerant comprises ethane, carbon-dioxide, or
ethylene.
[0034] Aspect 9: The method of any of Aspects 1-8, wherein step (a)
further comprises:
[0035] (a) compressing the first refrigerant in a plurality of
compression stages to produce a compressed first refrigerant.
[0036] 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.
[0037] Aspect 11: An apparatus for cooling a hydrocarbon feed
stream, the apparatus comprising:
[0038] at least one compression stage operationally configured to
compress a first refrigerant;
[0039] 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 configured to cool
the first refrigerant to a first temperature by indirect heat
exchange against an ambient fluid;
[0040] 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;
[0041] 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;
[0042] 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;
[0043] wherein the fluid stream of at least one of the at least one
cooling circuit comprises the hydrocarbon feed stream.
[0044] Aspect 12: The apparatus of Aspect 11, wherein the at least
one auxiliary heat exchanger comprises an economizer.
[0045] 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.
[0046] Aspect 14: The apparatus of Aspect 13, wherein the auxiliary
refrigerant comprises a hydrofluorocarbon or propane.
[0047] 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 least one
liquefaction heat exchanger against a second refrigerant stream to
produce a liquefied natural gas stream.
[0048] 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:
[0049] (a) compressing the first refrigerant in at least one
compression stage to produce a compressed first refrigerant;
[0050] (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;
[0051] (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: [0052] (i) reducing the
pressure of the first refrigerant; [0053] (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 [0054] (iii) flowing at
least a portion of the vaporized reduced pressure first refrigerant
into one of the at least one compression stages; [0055] 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: [0056] (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.
[0057] 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.
[0058] Aspect 17: The method of Aspect 16, further comprising:
[0059] (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.
[0060] 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.
[0061] Aspect 19: The method of any of Aspects 16-18, wherein the
first refrigerant is ethane, carbon-dioxide, or ethylene.
[0062] Aspect 20: The method of any of Aspects 16-19, wherein step
(a) further comprises: [0063] (a) compressing the first refrigerant
in a plurality of compression stages to produce a compressed first
refrigerant.
BRIEF DESCRIPTION OF DRAWINGS
[0064] FIG. 1A is a pressure versus enthalpy (P-H) diagram for a
subcritical cooling process in accordance with the prior art;
[0065] FIG. 1B is a pressure versus enthalpy (P-H) diagram for a
transcritical cooling process in accordance with the prior art;
[0066] FIG. 2 is a schematic flow diagram of a precooled-gas phase
expansion system in accordance with the prior art;
[0067] FIG. 3 is a schematic flow diagram of a precooled-MR system
in accordance with the prior art;
[0068] FIG. 4 is a schematic flow diagram of a cooling system in
accordance with the prior art;
[0069] FIG. 5 is a schematic flow diagram of a cooling system in
accordance with a first embodiment;
[0070] FIG. 6 is a schematic flow diagram of a cooling system in
accordance with a second embodiment;
[0071] FIG. 7 is a schematic flow diagram of a cooling system in
accordance with a third embodiment;
[0072] FIG. 8 is a schematic flow diagram of a cooling system in
accordance with a fourth embodiment;
[0073] FIG. 9 is a schematic flow diagram of a first embodiment of
an auxiliary refrigerant system in accordance with the third and
fourth embodiments;
[0074] FIG. 10 is a schematic flow diagram of a second embodiment
of the auxiliary refrigerant system in accordance with the third
and fourth embodiments;
[0075] FIG. 11 is a schematic flow diagram of a third embodiment of
the auxiliary refrigerant system in accordance with the third and
fourth embodiments;
[0076] FIG. 12A is a pressure versus enthalpy (P-H) diagram for a
transcritical cooling process with isentropic expansion; and
[0077] FIG. 12B is a schematic flow diagram of a cooling system in
accordance with a fifth embodiment.
DETAILED DESCRIPTION
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] The term "natural gas", as used in the specification and
claims, means a hydrocarbon gas mixture consisting primarily of
methane.
[0086] 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.
[0087] 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.
[0088] The terms "bundle" and "tube bundle" are used
interchangeably within this application and are intended to be
synonymous.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.
[0094] 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.
[0095] As used herein, the term "subcritical" is intended to refer
to a process that occurs below the critical point of the
refrigerant.
[0096] 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.
[0097] As used herein, the term "isotherm" is intended to refer to
a constant temperature line.
[0098] 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 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.
[0099] 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.
[0100] 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.
[0101] 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.
[0102] Table 1 defines a list of acronyms employed throughout the
specification and drawings as an aid to understanding the described
embodiments.
TABLE-US-00001 TABLE 1 SMR Single Mixed Refrigerant MCHE Main
Cryogenic Heat Exchanger DMR Dual Mixed Refrigerant MR Mixed
Refrigerant C3MR Propane-precooled MRL Mixed Refrigerant Liquid
Mixed Refrigerant LNG Liquid Natural Gas MRV Mixed Refrigerant
Vapor LLP Low-Low Pressure HHP High-High Pressure LP Low Pressure
MP Medium Pressure HP High Pressure MTPA Million Metric Tonnes Per
Annum HFC Hydrofluorocarbon LIN Liquid Nitrogen CO2 Carbon dioxide
LiBr Lithium Bromide
[0103] The described embodiments provide an efficient process for
the liquefaction of a hydrocarbon fluid and are particularly
applicable to the liquefaction of natural gas.
[0104] 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.
[0105] 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 CO.sub.2 and H.sub.2S, 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.
[0106] 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.
[0107] 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 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.
[0108] 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 2168 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.
[0109] 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.
[0110] 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 refrigerant 249, which is sent to the MCHE 208
to provide the liquefaction and subcooling duty required.
[0111] 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.
[0112] 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 CO.sub.2 and
H.sub.2S, 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.
[0113] 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.
[0114] 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 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.
[0115] 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 3168 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.
[0116] 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.
[0117] 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 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.
[0118] 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.
[0119] 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 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.
[0120] 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.
[0121] 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.
[0122] 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 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.
[0123] 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.
[0124] 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.
[0125] 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.
[0126] 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.
[0127] 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 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.
[0128] 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.
[0129] 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.
[0130] 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.
[0131] 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 separator 495, and subsequently to the suction of the
LLP compression stage 416A as a part of the warm low pressure
precooling refrigerant 414.
[0132] 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.
[0133] 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.
[0134] 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.
[0135] 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 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.
[0136] 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.
[0137] 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.
[0138] 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 of
the fourth precooling compression stage 516D, such as at the
suction of a fifth precooling compression stage 516E (not
shown).
[0139] 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.
[0140] 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
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.
[0141] 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.
[0142] 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.
[0143] 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 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).
[0144] 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.
[0145] 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.
[0146] 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.
[0147] 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.
[0148] 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.
[0149] 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 system 796. For
example, a controller 700 could be used to control valves 7108 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.
[0150] 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 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.
[0151] 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.
[0152] 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.
[0153] 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.
[0154] 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).
[0155] 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
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.
[0156] 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.
[0157] 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.
[0158] 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.
[0159] 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.
[0160] 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.
[0161] 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.
[0162] 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 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.
[0163] 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.
[0164] 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.
[0165] 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.
[0166] 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.
[0167] 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 and the
vapor auxiliary refrigerant 1054A may be returned to any location
of the liquefaction process.
[0168] 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.
[0169] 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.
[0170] In preferred embodiments, the auxiliary refrigerant is mixed
refrigerant (MR) or nitrogen.
[0171] 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.
[0172] 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.
[0173] 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 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.
[0174] 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.
[0175] 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.
[0176] 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.
[0177] 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.
[0178] 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.
[0179] 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.
[0180] 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.
[0181] 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 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
[0182] 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.
[0183] 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
[0184] 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 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.
[0185] 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 lbmol/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.
[0186] 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).
[0187] 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.
[0188] 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 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.
[0189] 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.
[0190] 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).
[0191] 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).
[0192] 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.
[0193] 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 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
[0194] 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.
[0195] 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%.
[0196] 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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