U.S. patent application number 14/378073 was filed with the patent office on 2015-01-15 for lng formation.
The applicant listed for this patent is Russell H. Oelfke. Invention is credited to Russell H. Oelfke.
Application Number | 20150013379 14/378073 |
Document ID | / |
Family ID | 49260995 |
Filed Date | 2015-01-15 |
United States Patent
Application |
20150013379 |
Kind Code |
A1 |
Oelfke; Russell H. |
January 15, 2015 |
LNG Formation
Abstract
Systems and a method for the formation of a liquefied natural
gas (LNG) are disclosed herein. The system includes a refrigeration
system configured to chill a natural gas using a refrigerant
mixture including a noble gas. The system also includes an
autorefrigeration system configured to use the natural g
self-refrigerant to form the LNG from the natural gas.
Inventors: |
Oelfke; Russell H.;
(Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Oelfke; Russell H. |
Houston |
TX |
US |
|
|
Family ID: |
49260995 |
Appl. No.: |
14/378073 |
Filed: |
March 4, 2013 |
PCT Filed: |
March 4, 2013 |
PCT NO: |
PCT/US13/28906 |
371 Date: |
August 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61618290 |
Mar 30, 2012 |
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61695592 |
Aug 31, 2012 |
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Current U.S.
Class: |
62/612 ; 62/613;
62/614 |
Current CPC
Class: |
F25J 1/0062 20130101;
F25J 1/0217 20130101; F25J 1/0097 20130101; F25J 1/021 20130101;
F25J 1/0219 20130101; F25J 2220/64 20130101; F25J 1/0087 20130101;
F25J 1/0268 20130101; F25J 2220/62 20130101; F25J 1/0035 20130101;
F25J 1/0022 20130101; F25J 1/0214 20130101; F25J 1/0052 20130101;
F25J 1/0077 20130101; F25J 1/0042 20130101; F25J 1/004
20130101 |
Class at
Publication: |
62/612 ; 62/614;
62/613 |
International
Class: |
F25J 1/00 20060101
F25J001/00; F25J 1/02 20060101 F25J001/02 |
Claims
1. A system for formation of a liquefied natural gas (LNG),
comprising: a refrigeration system configured to chill a natural
gas using a refrigerant mixture comprising a noble gas; and an
autorefrigeration system configured to use the natural gas as a
self-refrigerant to form the LNG from the natural gas.
2. The system of claim 1, comprising a first refrigeration system
configured to cool the natural gas using a non-hydrocarbon
refrigerant prior to flowing the natural gas into the refrigeration
system.
3. The system of claim 1, comprising a nitrogen recovery unit
upstream of the autorefrigeration system.
4. The system of claim 1, wherein the system is configured to chill
the natural gas for hydrocarbon dew point control.
5. The system of claim 1, wherein the system is configured to chill
the natural gas for natural gas liquid (NGL) extraction.
6. The system of claim 1, wherein the system is configured to
separate methane and lighter gases from carbon dioxide and heavier
gases.
7. The system of claim 1, wherein the system is configured to
prepare hydrocarbons for liquefied petroleum gas (LPG) production
storage.
8. The system of claim 1, wherein the system is configured to
condense a reflux stream.
9. The system of claim 1, wherein the refrigerant mixture comprises
xenon or krypton, or any combination thereof.
10. The system of claim 1, wherein the refrigerant mixture
comprises xenon, krypton, argon, or nitrogen, or any combinations
thereof.
11. The system of claim 1, wherein the refrigeration system
comprises a mechanical refrigeration system, valve expansion
system, or turbine expansion system, or any combinations
thereof.
12. The system of claim 1, wherein the refrigerant mixture
comprises a hydrocarbon, and wherein the hydrocarbon comprises
methane, ethane, propane, or butane, or any combinations
thereof.
13. The system of claim 1, wherein the refrigeration system
comprises multiple cooling cycles.
14. The system of claim 1, wherein the refrigeration system
comprises multiple cooling cycles, comprising: one or more
pre-cooling stages, wherein the refrigerant mixture comprises a
noble gas, nitrogen, or a hydrocarbon, or any combinations thereof,
and one or more deep cooling cycles, wherein the refrigerant
mixture comprises a noble gas, nitrogen, or a hydrocarbon, or any
combinations thereof.
15. The system of claim 1, wherein the refrigerant mixture
comprising the noble gas is utilized in one or more cooling stages
to achieve deeper cooling than provided by hydrocarbon
refrigerants.
16. The system of claim 1, comprising a nitrogen rejection unit,
wherein a liquid feed from the bottom of the nitrogen rejection
unit is used to provide cooling to a reflux condenser at the top of
the nitrogen rejection unit.
17. The system of claim 1, wherein the refrigerant mixture
comprises a pure component refrigerant.
18. A method for formation of a liquefied natural gas (LNG),
comprising: chilling a natural gas in a refrigeration system,
wherein the refrigeration system uses a refrigerant mixture
comprising a noble gas; and liquefying the natural gas to form the
LNG in an autorefrigeration system.
19. The method of claim 18, comprising cooling the natural gas in a
first refrigeration system prior to chilling the natural gas in the
refrigeration system, wherein the first refrigeration system uses a
non-hydrocarbon refrigerant.
20. The method of claim 18, wherein chilling the natural gas in the
refrigeration system comprises: compressing the refrigerant mixture
to provide a compressed refrigerant mixture; optionally cooling the
compressed refrigerant mixture by indirect heat exchange with a
cooling fluid; expanding the compressed refrigerant mixture to cool
the compressed refrigerant mixture, thereby producing an expanded,
cooled refrigerant mixture; passing said expanded, cooled
refrigerant mixture to a first heat exchange area; optionally
compressing the natural gas; optionally cooling said the natural
gas by indirect heat exchange with an external cooling fluid; and
heat exchanging the natural gas with the expanded, cooled
refrigerant mixture.
21. The method of claim 18, wherein the noble gas comprises xenon
or krypton.
22. The method of claim 18, wherein the refrigerant mixture
comprises nitrogen or a hydrocarbon, or any combination
thereof.
23. The method of claim 18, comprising liquefying the natural gas
to form the LNG via a plurality of expansion valves or hydraulic
expansion turbines and flash drums.
24. The method of claim 18, comprising: chilling the natural gas
via one or more pre-cooling steps using a first refrigerant
mixture, wherein the first refrigerant mixture comprises a noble
gas, nitrogen, or a hydrocarbon, or any combinations thereof, and
chilling the natural gas via one or more deep cooling steps using a
second refrigerant mixture, wherein the second refrigerant mixture
comprises a noble gas, nitrogen, or a hydrocarbon, or any
combinations thereof.
25. The method of claim 18, comprising using the refrigerant
mixture comprising the noble gas in one or more cooling stages to
achieve deeper cooling than provided by hydrocarbon
refrigerants.
26. A cascade cooling system for formation of a liquefied natural
gas (LNG), comprising: a first refrigeration system configured to
cool the natural gas using a non-hydrocarbon refrigerant, wherein
the first refrigeration system comprises a plurality of first
chillers configured to allow for cooling of the natural gas via an
indirect exchange of heat between the natural gas and the
non-hydrocarbon refrigerant; a second refrigeration system
configured to chill the natural gas using a refrigerant mixture
comprising a noble gas, wherein the second refrigeration system
comprises a plurality of second chillers configured to allow for
cooling of the natural gas via an indirect exchange of heat between
the natural gas and the refrigerant mixture; and an
autorefrigeration system configured to form the LNG from the
natural gas, wherein the autorefrigeration system comprises a
plurality of expansion valves or hydraulic expansion turbines, or
any combination thereof, and flash drums.
27. The cascade cooling system of claim 26, wherein the first
refrigeration system comprises a compressor that is configured to
compress the non-hydrocarbon refrigerant and a condenser that is
configured to cool the non-hydrocarbon refrigerant.
28. The cascade cooling system of claim 26, wherein the second
refrigeration system comprises a compressor that is configured to
compress the refrigerant mixture and a condenser that is configured
to cool the refrigerant mixture.
29. The cascade cooling system of claim 26, wherein the plurality
of first chillers comprise evaporators configured to cool the
natural gas by at least partially vaporizing the non-hydrocarbon
refrigerant via a transfer of heat from the natural gas to the
non-hydrocarbon refrigerant.
30. The cascade cooling system of claim 26, wherein the plurality
of second chillers comprise evaporators configured to chill the
natural gas by vaporizing the refrigerant mixture via a transfer of
heat from the natural gas to the refrigerant mixture.
31. The cascade cooling system of claim 26, wherein the LNG
comprises a liquid fraction and a residual vapor fraction, and
wherein the cascade cooling system comprises a liquid separation
vessel configured to separate the residual vapor fraction from the
liquid fraction.
32. The cascade cooling system of claim 26, comprising a nitrogen
rejection unit upstream of the autorefrigeration system.
33. The cascade cooling system of claim 26, wherein the refrigerant
mixture comprises a pure component refrigerant.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application Nos. 61/695,592 filed Aug. 31, 2012 entitled LNG
FORMATION, and 61/618,290 entitled USE OF NOBLE GASES IN LOW
TEMPERATURE HYDROCARBON PROCESSING SYSTEMS, APPARATUS, AND METHODS,
filed on Mar. 30, 2012, the entirety of each of which is
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present techniques relate generally to the field of
hydrocarbon recovery and treatment processes and, more
particularly, to systems and methods that form liquefied natural
gas (LNG) via a refrigeration process. Specifically, provided are
systems and methods for forming LNG from natural gas using
refrigerants that include one or more noble gases.
BACKGROUND
[0003] This section is intended to introduce various aspects of the
art, which may be associated with exemplary embodiments of the
present techniques. This discussion is believed to assist in
providing a framework to facilitate a better understanding of
particular aspects of the present techniques. Accordingly, it
should be understood that this section should be read in this
light, and not necessarily as admissions of prior art.
[0004] Many low temperature refrigeration systems that are used for
natural gas processing and liquefaction rely on the use of
refrigerants including hydrocarbon components and nitrogen to
provide external refrigeration. Such hydrocarbon components may
include methane, ethane, ethylene, propane, and the like. However,
the use of refrigerants including hydrocarbon components and
nitrogen may not be very efficient, since a large heat transfer
area may be required to provide proper refrigeration of the natural
gas. In addition, the flammability of the hydrocarbon components
within the refrigerants may increase the risks associated with the
refrigeration process.
[0005] Low temperature refrigeration systems that are used for
natural gas processing and liquefaction often use synthetic
refrigerants, such as R-404A or R-410A, as substitutes for the
refrigerants including the hydrocarbon components and the nitrogen.
However, such synthetic refrigerants are only suitable for levels
of refrigeration that are above around -100.degree. F. In some
instances, lower levels of refrigeration may be desirable.
[0006] International Patent Application Publication WO/2005/072404,
by Flynn, et al., describes a cooling system that includes a first
refrigerant cycle including a first refrigerant and a second
refrigerant cycle including a second refrigerant that is a mixture
of cryogenic components. The disclosure is also directed to a
cooling system that includes a first refrigerant cycle including a
first refrigerant and a second refrigerant cycle including a second
refrigerant that is a non-reactive component. The second
refrigerant is free of fluorocarbons, chlorofluorocarbons, and
hydrocarbons. At least a portion of the second refrigerant is
condensed in the second refrigerant cycle. However, the disclosure
is not directed to a cooling system that includes any type of
autorefrigeration cycle.
[0007] Related information may be found in U.S. Pat. Nos.
4,533,372, 4,923,493, 5,265,428, 5,062,270, 5,120,338, 6,053,007,
and 5,956,971; U.S. Patent Application Publication Nos.
2002/0088249, 2003/0177785, 2007/0193303, 2007/0227185,
2008/0034789, 2008/0087041, 2009/0217701, 2009/0266107,
2010/0018248, 2010/0107684, 2010/0186445, 2012/0031144,
2012/0079852, and 2012/0125043; and International Patent
Publication No. WO/2012/015554. Other potentially related
information may be found in International Patent Publication No.
WO2007/021351; Foglietta, J. H., et al., "Consider Dual Independent
Expander Refrigeration for LNG Production New Methodology May
Enable Reducing Cost to Produce Stranded Gas," Hydrocarbon
Processing, Gulf Publishing Co., vol. 83, no. 1, pp. 39-44 (January
2004); U.S. Patent Application Publication No. US2003/089125; U.S.
Pat. No. 6,412,302; U.S. Pat. No. 3,162,519; U.S. Pat. No.
3,323,315; German Patent No. DE19517116, and J. M. Campbell, "Gas
Conditioning and Processing, Vol. 2: The Equipment Modules",
8.sup.th edition, John M. Campbell & Company, 2001.
SUMMARY
[0008] An embodiment provides a system for the formation of a
liquefied natural gas (LNG). The system includes a refrigeration
system configured to chill a natural gas using a refrigerant
mixture including a noble gas. The system also includes an
autorefrigeration system configured to use the natural gas as a
self-refrigerant to form the LNG from the natural gas.
[0009] Another embodiment provides a method for the formation of
LNG. The method includes chilling a natural gas in a refrigeration
system, wherein the refrigeration system uses a refrigerant mixture
includes a noble gas. The method also includes liquefying the
natural gas to form the LNG in an autorefrigeration system.
[0010] Another embodiment provides a cascade cooling system for
formation of LNG. The cascade cooling system includes a first
refrigeration system configured to cool the natural gas using a
non-hydrocarbon refrigerant, wherein the first refrigeration system
includes a number of first chillers configured to allow for cooling
of the natural gas via an indirect exchange of heat between the
natural gas and the non-hydrocarbon refrigerant. The cascade
cooling system also includes a second refrigeration system
configured to chill the natural gas using a refrigerant mixture
including a noble gas, wherein the second refrigeration system
includes a number of second chillers configured to allow for
cooling of the natural gas via an indirect exchange of heat between
the natural gas and the refrigerant mixture. The cascade cooling
system further includes an autorefrigeration system configured to
form the LNG from the natural gas, wherein the autorefrigeration
system includes a number of expansion valves or hydraulic expansion
turbines, or any combination thereof, and flash drums.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The advantages of the present techniques are better
understood by referring to the following detailed description and
the attached drawings, in which:
[0012] FIG. 1 is a process flow diagram of a single stage
refrigeration system;
[0013] FIG. 2 is a process flow diagram of a two stage
refrigeration system including an economizer;
[0014] FIG. 3 is a process flow diagram of a single stage
refrigeration system including a heat exchanger economizer;
[0015] FIG. 4 is a process flow diagram of a cascade cooling system
including a first refrigeration system and a second refrigeration
system;
[0016] FIG. 5 is process flow diagram of an expansion refrigeration
system for hydrocarbon dew point control;
[0017] FIG. 6 is a process flow diagram of an expansion
refrigeration system for NGL extraction;
[0018] FIG. 7 is a process flow diagram of an LNG production
system;
[0019] FIG. 8 is a simplified process flow diagram of a cascade
cooling system;
[0020] FIGS. 9A-B are a more detailed process flow diagram of a
cascade cooling system;
[0021] FIG. 10 is a more detailed process flow diagram of an
autorefrigeration system;
[0022] FIG. 11 is a schematic of a methane pressure-enthalpy (P-H)
diagram; and
[0023] FIG. 12 is a process flow diagram of a method for the
formation of LNG.
DETAILED DESCRIPTION OF THE DRAWINGS
[0024] In the following detailed description section, specific
embodiments of the present techniques are described. However, to
the extent that the following description is specific to a
particular embodiment or a particular use of the present
techniques, this is intended to be for exemplary purposes only and
simply provides a description of the exemplary embodiments.
Accordingly, the techniques are not limited to the specific
embodiments described below, but rather, include all alternatives,
modifications, and equivalents falling within the spirit and scope
of the appended claims.
[0025] At the outset, for ease of reference, certain terms used in
this application and their meanings as used in this context are set
forth. To the extent a term used herein is not defined below, it
should be given the broadest definition persons in the pertinent
art have given that term as reflected in at least one printed
publication or issued patent. Further, the present techniques are
not limited by the usage of the terms shown below, as all
equivalents, synonyms, new developments, and terms or techniques
that serve the same or a similar purpose are considered to be
within the scope of the present claims.
[0026] "Acid gases" are contaminants that are often encountered in
natural gas streams. Typically, these gases include carbon dioxide
(CO.sub.2) and hydrogen sulfide (H.sub.2S), although any number of
other contaminants may also form acids. Acid gases are commonly
removed by contacting the gas stream with an absorbent, such as an
amine, which may react with the acid gas. When the absorbent
becomes acid-gas "rich," a desorption step can be used to separate
the acid gases from the absorbent. The "lean" absorbent is then
typically recycled for further absorption. As used herein a "liquid
acid gas stream" is a stream of acid gases that are condensed into
the liquid phase, for example, including CO.sub.2 dissolved in
H.sub.2S and vice-versa.
[0027] As used herein, "autorefrigeration" refers to a process
whereby a fluid is cooled via a reduction in pressure. In the case
of liquids, autorefrigeration refers to the cooling of the liquid
by evaporation, which corresponds to a reduction in pressure. More
specifically, a portion of the liquid is flashed into vapor as it
undergoes a reduction in pressure while passing through a
throttling device. As a result, both the vapor and the residual
liquid are cooled to the saturation temperature of the liquid at
the reduced pressure. For example, according to embodiments
described herein, autorefrigeration of a natural gas may be
performed by maintaining the natural gas at its boiling point so
that the natural gas is cooled as heat is lost during boil off.
This process may also be referred to as a "flash evaporation."
[0028] As used herein, a "cascade cycle" refers to a system with
two or more refrigerants, where a cold second refrigerant is
condensed by a warmer first refrigerant. Thus, low temperatures may
be "cascaded" down from one refrigerant to another. Each
refrigerant in a cascade may have multiple levels of chilling based
on staged evaporating pressures within economizers. Cascade cycles
are considered to be beneficial for the production of LNG as
compared to single refrigerant systems, since lower temperatures
may be achieved within cascade cycles than single refrigerant
systems.
[0029] A "closed-loop refrigeration cycle" refers to a
refrigeration cycle wherein substantially no refrigerant enters or
exits the cycle during normal operation.
[0030] A "closed-loop refrigeration system" refers to a
refrigeration system comprising compression, heat exchange, and
pressure reduction means in which a refrigerant is recirculated
without continuous deliberate refrigerant withdrawal. A small
amount of refrigerant makeup typically is required because of small
leakage losses from the system.
[0031] A "compressor" or "refrigerant compressor" includes any
unit, device, or apparatus able to increase the pressure of a
refrigerant stream. This includes refrigerant compressors having a
single compression process or step, or refrigerant compressors
having multi-stage compressions or steps, more particularly
multi-stage refrigerant compressors within a single casing or
shell. Evaporated refrigerant streams to be compressed can be
provided to a refrigerant compressor at different pressures. Some
stages or steps of a hydrocarbon cooling process may involve two or
more refrigerant compressors in parallel, series, or both. The
present invention is not limited by the type or arrangement or
layout of the refrigerant compressor or refrigerant compressors,
particularly in any refrigerant circuit.
[0032] A "Controlled-Freeze-Zone" (CFZ) process is a process that
has been proposed to take advantage of the freezing potential of
carbon dioxide in cryogenic distillation, rather than avoiding
solid carbon dioxide. In the CFZ process, acid gas components are
separated by cryogenic distillation through the controlled freezing
and melting of carbon dioxide in a single column, without the use
of freeze-suppression additives. The CFZ process uses a cryogenic
distillation column with a special internal section, e.g., CFZ
section, to handle the solidification and melting of carbon
dioxide. This CFZ section does not contain packing or trays like
conventional distillation columns. Instead, the CFZ section
contains one or more spray nozzles and a melting tray. Solid carbon
dioxide forms in the vapor space in the distillation column and
falls into the liquid on the melting tray. Substantially all of the
solids that form are confined to the CFZ section. The portions of
the distillation column above and below the CFZ section of the
column are similar to conventional cryogenic demethanizer columns.
A more detailed description of the CFZ process is disclosed in U.S.
Pat. Nos. 4,533,372; 4,923,493; 5,120,338; and 5,265,428.
[0033] As used herein, "cooling" broadly refers to lowering and/or
dropping a temperature and/or internal energy of a substance, such
as by any suitable amount. Cooling may include a temperature drop
of at least about 1 degree Celsius, at least about 5 degrees
Celsius, at least about 10 degrees Celsius, at least about 15
degrees Celsius, at least about 25 degrees Celsius, at least about
50 degrees Celsius, at least about 100 degrees Celsius, and/or the
like. The cooling may use any suitable heat sink, such as steam
generation, hot water heating, cooling water, air, refrigerant,
other process streams (integration), and combinations thereof. One
or more sources of cooling may be combined and/or cascaded to reach
a desired outlet temperature. The cooling step may use a cooling
unit with any suitable device and/or equipment. According to one
embodiment, cooling may include indirect heat exchange, such as
with one or more heat exchangers. Heat exchangers may include any
suitable design, such as shell and tube, plate and frame, counter
current, concurrent, extended surface, and/or the like. In the
alternative, the cooling may use evaporative (heat of vaporization)
cooling and/or direct heat exchange, such as a liquid sprayed
directly into a process stream.
[0034] "Cryogenic temperature" refers to a temperature that is
about -50.degree. C. or below.
[0035] As used herein, the terms "deethanizer" and "demethanizer"
refer to distillation columns or towers that may be used to
separate components within a natural gas stream. For example, a
demethanizer is used to separate methane and other volatile
components from ethane and heavier components. The methane fraction
is typically recovered as purified gas that contains small amounts
of inert gases such as nitrogen, CO.sub.2, or the like.
[0036] The term "gas" is used interchangeably with "vapor," and is
defined as a substance or mixture of substances in the gaseous
state as distinguished from the liquid or solid state. Likewise,
the term "liquid" means a substance or mixture of substances in the
liquid state as distinguished from the gas or solid state.
[0037] A "heat exchanger" broadly means any device capable of
transferring heat from one media to another media, including
particularly any structure, e.g., device commonly referred to as a
heat exchanger. Heat exchangers include "direct heat exchangers"
and "indirect heat exchangers." Thus, a heat exchanger may be a
plate-and-frame, shell-and-tube, spiral, hairpin, core,
core-and-kettle, double-pipe or any other type of known heat
exchanger. "Heat exchanger" may also refer to any column, tower,
unit or other arrangement adapted to allow the passage of one or
more streams therethrough, and to affect direct or indirect heat
exchange between one or more lines of refrigerant, and one or more
feed streams.
[0038] A "hydrocarbon" is an organic compound that primarily
includes the elements hydrogen and carbon, although nitrogen,
sulfur, oxygen, metals, or any number of other elements may be
present in small amounts. As used herein, hydrocarbons generally
refer to components found in natural gas, oil, or chemical
processing facilities.
[0039] "Hydrofluorocarbons" or HFCs are molecules including H, F,
and C atoms. Hydrofluorocarbons have H--C and F--C bonds and,
depending on the number of carbon atoms in the species, C--C bonds.
Some examples of hydrofluorocarbons include fluoroform (CHF.sub.3),
pentafluoroethane (C.sub.2HF.sub.5), tetrafluoroethane
(C.sub.2H.sub.2F.sub.4), heptafluoropropane (C.sub.3HF.sub.7),
hexafluoropropane (C.sub.3H.sub.2F.sub.6), pentafluoropropane
(C.sub.3H.sub.3F.sub.5), and tetrafluoropropane
(C.sub.3H.sub.4F.sub.4), among other compounds of similar chemical
structure.
[0040] "Liquefied natural gas" or "LNG" is natural gas generally
known to include a high percentage of methane. However, LNG may
also include trace amounts of other compounds. The other elements
or compounds may include, but are not limited to, ethane, propane,
butane, carbon dioxide, nitrogen, helium, hydrogen sulfide, or
combinations thereof, that have been processed to remove one or
more components (for instance, helium) or impurities (for instance,
water and/or heavy hydrocarbons) and then condensed into a liquid
at almost atmospheric pressure by cooling.
[0041] "Mixed refrigerant processes" may include, but are not
limited to, a single refrigeration system using a mixed
refrigerant, i.e., a refrigerant with more than one chemical
component, a hydrocarbon pre-cooled mixed refrigerant system, and a
dual mixed refrigerant system. In general, mixed refrigerants can
include hydrocarbon and/or non-hydrocarbon components. Examples of
suitable hydrocarbon components typically employed in mixed
refrigerants can include, but are not limited to, methane, ethane,
ethylene, propane, propylene, as well as butane and butylene
isomers. Non-hydrocarbon components generally employed in mixed
refrigerants can include carbon dioxide and nitrogen. Mixed
refrigerant processes employ at least one mixed component
refrigerant, but can additionally employ one or more pure-component
refrigerants as well.
[0042] "Natural gas" refers to a multi-component gas obtained from
a crude oil well or from a subterranean gas-bearing formation. The
composition and pressure of natural gas can vary significantly. A
typical natural gas stream contains methane (CH.sub.4) as a major
component, i.e., greater than 50 mol % of the natural gas stream is
methane. The natural gas stream can also contain ethane
(C.sub.2H.sub.6), higher molecular weight hydrocarbons (e.g.,
C.sub.3-C.sub.20 hydrocarbons), one or more acid gases (e.g.,
carbon dioxide or hydrogen sulfide), or any combinations thereof.
The natural gas can also contain minor amounts of contaminants such
as water, nitrogen, iron sulfide, wax, crude oil, or any
combinations thereof. The natural gas stream may be substantially
purified prior to use in embodiments, so as to remove compounds
that may act as poisons.
[0043] As used herein, "natural gas liquids" (NGL) refer to
mixtures of hydrocarbons whose components are, for example,
typically heavier than ethane. Some examples of hydrocarbon
components of NGL streams include propane, butane, and pentane
isomers, benzene, toluene, and other aromatic compounds.
[0044] "Noble gas" refers to any of the chemical elements belonging
to group 18 of the periodic table. More specifically, the noble
gases include helium (He), neon (Ne), argon (Ar), krypton (Kr),
xenon (Xe), and radon (Rn). The noble gases are characterized by
very low chemical reactivity.
[0045] An "open-loop refrigeration cycle" refers to a refrigeration
cycle wherein at least a portion of the refrigerant employed during
normal operation originates from the fluid being cooled by the
refrigeration cycle.
[0046] An "open-loop refrigeration system" is a refrigeration
system comprising compression, heat exchange, and pressure
reduction means in which a refrigerant is recirculated, a portion
of the refrigerant is continuously withdrawn from the recirculation
loop, and additional refrigerant is continuously introduced into
the recirculation loop.
[0047] A "refrigerant component," in a refrigeration system, will
absorb heat at a lower temperature and pressure through evaporation
and will reject heat at a higher temperature and pressure through
condensation. Illustrative refrigerant components may include, but
are not limited to, alkanes, alkenes, and alkynes having one to
five carbon atoms, nitrogen, chlorinated hydrocarbons, fluorinated
hydrocarbons, other halogenated hydrocarbons, noble gases, and
mixtures or combinations thereof.
[0048] "Substantial" when used in reference to a quantity or amount
of a material, or a specific characteristic thereof, refers to an
amount that is sufficient to provide an effect that the material or
characteristic was intended to provide. The exact degree of
deviation allowable may depend, in some cases, on the specific
context.
[0049] Overview
[0050] Embodiments described herein provide a hydrocarbon
processing system and method. Such a hydrocarbon processing system
may include or utilize a refrigeration system, such as a cascade
cooling system. Further, according to embodiments described herein,
the refrigeration system utilizes a refrigerant mixture including a
noble gas.
[0051] Hydrocarbon processing systems include the conventional
systems known to those skilled in the art. Hydrocarbon production
and treatment processes include, but are not limited to, chilling
natural gas for NGL extraction, chilling natural gas for
hydrocarbon dew point control, chilling natural gas for CO.sub.2
removal, liquefied petroleum gas (LPG) production storage,
condensation of reflux in deethanizers/demethanizers, and natural
gas liquefaction to produce LNG.
[0052] Although many refrigeration cycles have been used to process
hydrocarbons, one cycle that is used in LNG liquefaction plants is
the cascade cycle, which uses multiple single component
refrigerants in heat exchangers arranged progressively to reduce
the temperature of the gas to a liquefaction temperature. Another
cycle that is used in LNG liquefactions plants is the
multi-component refrigeration cycle, which uses a multi-component
refrigerant in specially designed exchangers. In addition, another
cycle that is used in LNG liquefaction plants is the expander
cycle, which expands gas from feed gas pressure to a low pressure
with a corresponding reduction in temperature. Natural gas
liquefaction cycles may also use variations or combinations of
these three cycles.
[0053] LNG is prepared from a feed gas by refrigeration and
liquefaction technologies. Optional steps include condensate
removal, CO.sub.2 removal, dehydration, mercury removal, nitrogen
stripping, H.sub.2S removal, and the like. After liquefaction, LNG
may be stored or fed to a gas pipeline for sale or use.
Conventional liquefaction processes can include: APCI Propane
pre-cooled mixed refrigerant; C3MR; DUAL MR; Phillips Optimized
Cascade; Prico single mixed refrigerant; TEAL dual pressure mixed
refrigerant; Linde/Statoil multi fluid cascade; Axens dual mixed
refrigerant, DMR; and the Shell processes C3MR and DMR.
[0054] Carbon dioxide removal, i.e., separation of methane and
lighter gases from CO.sub.2 and heavier gases, may be achieved with
cryogenic processes, such as the Controlled Freeze Zone technology
available from ExxonMobil Corporation.
[0055] While the method and systems described herein are discussed
with respect to the formation of LNG from natural gas, the method
and systems may also be used for a variety of other purposes. For
example, the method and systems described herein may be used to
chill natural gas for hydrocarbon dew point control, perform
natural gas liquid (NGL) extraction, separate methane and lighter
gases from carbon dioxide and heavier gases, prepare hydrocarbons
for LPG production, or condense a reflux stream in deethanizers
and/or demethanizers, among others.
[0056] Refrigerants
[0057] The refrigerants that are utilized according to embodiments
described herein may be one or more single component refrigerants,
or refrigerant mixtures including multiple components. Refrigerants
may include methane, ethane, ethylene, propane, butane, and
nitrogen, or combinations thereof. In embodiments described herein,
refrigerants in one or more refrigeration stages use non-flammable
materials that include noble gases and mixtures of noble gases.
Refrigerants may be imported and stored on-site or, alternatively,
some of the components of the refrigerant may be prepared on-site,
typically by a distillation process integrated with the hydrocarbon
processing system. Exemplary mixed refrigerants are disclosed in
U.S. Pat. No. 6,530,240.
[0058] Commercially available refrigerants including fluorocarbons
(FCs) or hydrofluorocarbons (HFCs) are used in various
applications, as are refrigerants including ammonia, sulfur
dioxide, or halogenated hydrocarbons. Exemplary refrigerants are
commercially available from DuPont Corporation, including the
ISCEON.RTM. family of refrigerants, the SUVA.RTM. family of
refrigerants, the OPTEON.RTM. family of refrigerants, and the
FREON.RTM. family of refrigerants.
[0059] Multicomponent refrigerants are commercially available. For
example, R-401A is a HCFC blend of R-32, R-152a, and R-124. R-404A
is a HFC blend of 52 wt. % R-143a, 44 wt. % R-125, and 4 wt. %
R-134a. R-406A is a blend of 55 wt. % R-22, 4 wt. % R-600a, and 41
wt. % R-142b. R-407A is a HFC blend of 20 wt. % R-32, 40 wt. %
R-125, and 40 wt. % R-134a. R-407C is a hydrofluorocarbon blend of
R-32, R-125, and R-134a. R-408A is a HCFC blend of R-22, R-125, and
R-143a. R-409A is a HCFC blend of R-22, R-124, and R-142b. R-410A
is a blend of R-32 and R-125. R-500 is a blend of 73.8 wt. % R-12
and 26.2 wt. % of R-152a. R-502 is a blend of R-22 and R-115.
[0060] In embodiments discussed herein, refrigerants in one or more
refrigeration stages may also include a noble gas or a noble gas
mixture. The six naturally occurring noble gases are helium (He),
neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). A
noble gas can be used alone or in combination with other noble
gases, or in combination with other refrigerant components. In some
embodiments, the noble gas used as a refrigerant is xenon, krypton,
argon, or combinations thereof.
[0061] Because noble gases are non-flammable, they reduce the risk
of handling refrigerants. In addition, because noble gases exist in
the atmosphere and are readily collected, any noble gas refrigerant
that escapes the refrigeration system can be recycled. Further, if
released into the environment, noble gases do not have any ozone
depleting potential or greenhouse warming potential.
[0062] Noble gas refrigerants may provide cooling below about
-50.degree. F., or below about -100.degree. F., or below about
-120.degree. F., or from about -50.degree. F. to about -162.degree.
F., or from about -50.degree. F. to about -244.degree. F., or from
about -50.degree. F. to about -303.degree. F. In multistage
refrigeration systems, noble gas refrigerants may be utilized in
later stages to achieve deeper cooling than provided by hydrocarbon
refrigerants, such as below about -50.degree. F., or below about
-100.degree. F., or below about -120.degree. F., or from about
-50.degree. F. to about -162.degree. F., or from about -90.degree.
F. to about -162.degree. F., or from about -100.degree. F. to about
-162.degree. F., or from about -120.degree. F. to about
-162.degree. F., or from about -50.degree. F. to about -244.degree.
F., or from about -90.degree. F. to about -244.degree. F., or from
about -100.degree. F. to about -244.degree. F., or from about
-120.degree. F. to about -244.degree. F., or from about -50.degree.
F. to about -303.degree. F., or from about -90.degree. F. to about
-303.degree. F., or from about -100.degree. F. to about
-303.degree. F., or from about -120.degree. F. to about
-303.degree. F.
[0063] In various embodiments, any of a number of different types
of hydrocarbon processing systems can be used with any of the
refrigeration systems described herein. In addition, the
refrigeration systems described herein may utilize any of the
refrigerants described above.
[0064] Refrigeration Systems
[0065] Hydrocarbon systems and methods often include refrigeration
systems that utilize mechanical refrigeration, valve expansion,
turbine expansion, or the like. Mechanical refrigeration typically
includes compression systems and absorption systems, such as
ammonia absorption systems. Compression systems are used in the gas
processing industry for a variety of processes. For example,
compression systems may be used for chilling natural gas for NGL
extraction, chilling natural gas for hydrocarbon dew point control,
LPG production storage, condensation of reflux in deethanizers or
demethanizers, natural gas liquefaction to produce LNG, or the
like. Further, other commercial processes that utilize
refrigeration may take advantage of the decreased flammability
inherent in the noble gases to replace other refrigerants, such as
ammonia.
[0066] FIG. 1 is a process flow diagram of a single stage
refrigeration system 100. In various embodiments, the single stage
refrigeration system 100 utilizes a refrigerant mixture including a
noble gas. The single stage refrigeration system 100 includes an
expansion valve 102, a chiller 104, a compressor 106, a condenser
108, and an accumulator 110. A saturated liquid refrigerant 112 may
flow from the accumulator 110 to the expansion valve 102, and may
expand across the expansion valve 102 isenthalpically. On
expansion, some vaporization occurs, creating a chilled refrigerant
mixture 114 that includes both vapor and liquid. The refrigerant
mixture 114 may enter the chiller 104, also known as the
evaporator, at a temperature lower than the temperature to which a
process stream 116, such as a natural gas, is to be cooled. The
process stream 116 flows through the chiller 104 and exchanges heat
with the refrigerant mixture 114. As the process stream 116
exchanges heat with the refrigerant mixture 114, the process stream
116 is cooled, while the refrigerant mixture 114 may at least
partially vaporize, creating a saturated vapor refrigerant 118.
[0067] After leaving the chiller 104, the saturated vapor
refrigerant 118, as well as any remaining liquid refrigerant, is
compressed within the compressor 106, and is then flowed into the
condenser 108. Within the condenser 108, the saturated vapor
refrigerant 118 is converted to a saturated, or slightly
sub-cooled, liquid refrigerant 120. The liquid refrigerant 120 may
then be flowed from the condenser 108 to the accumulator 110. The
accumulator 110, which is also known as a surge tank or receiver,
may serve as a reservoir for the liquid refrigerant 120. The liquid
refrigerant 120 may be stored within the accumulator 110 before
being expanded across the expansion valve 102 as the saturated
liquid refrigerant 112.
[0068] It is to be understood that the process flow diagram of FIG.
1 is not intended to indicate that the single stage refrigeration
system 100 is to include all the components shown in FIG. 1.
Further, the single stage refrigeration system 100 may include any
number of additional components not shown in FIG. 1, depending on
the details of the specific implementation. For example, in some
embodiments, a refrigeration system can include two or more
compression stages. In addition, the refrigeration system 100 may
include an economizer, as discussed further with respect to FIG.
2.
[0069] FIG. 2 is a process flow diagram of a two stage
refrigeration system 200 including an economizer 202. Like numbered
items are as described with respect to FIG. 1. The economizer 202
may be any device or process modification that decreases the
compressor power usage for a given chiller duty. Conventional
economizers 202 include, for example, flash tanks and heat exchange
economizers.
[0070] As shown in FIG. 2, the saturated liquid refrigerant 112
leaving the accumulator 110 may be expanded across the expansion
valve 102 to an intermediate pressure at which vapor and liquid may
be separated. The expansion valve 102 may be used to control the
downstream temperature and pressure of the saturated liquid
refrigerant 112. For example, as the saturated liquid refrigerant
112 flashes across the expansion valve 102, a vapor refrigerant 204
and liquid refrigerant 206 are produced at a lower pressure and
temperature than the saturated liquid refrigerant 112. The vapor
refrigerant 204 and the liquid refrigerant 206 may then be flowed
into the economizer 202. In various embodiments, the economizer 202
is a flash tank that effects the separation of the vapor
refrigerant 204 and the liquid refrigerant 206. The vapor
refrigerant 204 may be flowed to an intermediate pressure
compressor stage, at which the vapor refrigerant 204 may be
combined with saturated vapor refrigerant 118 exiting a first
compressor 210, creating a mixed saturated vapor refrigerant 208.
The mixed saturated vapor refrigerant 208 may then be flowed into a
second compressor 212.
[0071] From the economizer 202, the liquid refrigerant 206 may be
isenthalpically expanded across a second expansion valve 214. On
expansion, some vaporization may occur, creating a refrigerant
mixture 216 that includes both vapor and liquid, lowering the
temperature and pressure. The refrigerant mixture 216 may have a
higher liquid content than refrigerant mixtures in systems without
economizers. The higher liquid content may reduce the refrigerant
circulation rate and/or reduce the power usage of the first
compressor 210.
[0072] The refrigerant mixture 216 enters the chiller 104, also
known as the evaporator, at a temperature lower than the
temperature to which the process stream 116 is to be cooled. The
process stream 116 is cooled within the chiller 104, as discussed
above with respect to FIG. 1. In addition, the saturated vapor
refrigerant 118 is flowed through the compressors 210 and 212 and
the condenser 108, and the resulting liquid refrigerant 120 is
stored within the accumulator 110, as discussed above with respect
to FIG. 1.
[0073] It is to be understood that the process flow diagram of FIG.
2 is not intended to indicate that the two stage refrigeration
system 200 is to include all the components shown in FIG. 2.
Further, the two stage refrigeration system 200 may include any
number of additional components not shown in FIG. 2, depending on
the details of the specific implementation. For example, the two
stage refrigeration system 200 may include any number of additional
economizers or other types of equipment not shown in FIG. 2. In
addition, the economizer 202 may be a heat exchange economizer
rather than a flash tank. The heat exchange economizer may also be
used to decrease refrigeration circulation rate and reduce
compressor power usage.
[0074] In some embodiments, the two stage refrigeration system 200
includes more than one economizer 202, as well as more than two
compressors 210 and 212. For example, the two stage refrigeration
system 200 may include two economizers and three compressors. In
general, if the refrigeration system 200 includes X number of
economizers, the refrigeration system 200 will include X+1 number
of compressors. Such a refrigeration system 200 with multiple
economizers may form part of a cascade refrigeration system.
[0075] FIG. 3 is a process flow diagram of a single stage
refrigeration system 300 including a heat exchanger economizer 302.
Like numbered items are as described with respect to FIG. 1. As
shown in FIG. 3, the saturated liquid refrigerant 112 leaving the
accumulator 110 may be expanded across the expansion valve 102 to
an intermediate pressure at which vapor and liquid may be
separated, producing the refrigerant mixture 114. The refrigerant
mixture 114 may be flowed into the chiller 104 at a temperature
lower than the temperature to which the process stream 116 is to be
cooled. The process stream 116 may be cooled within the chiller
104, as discussed above with respect to FIG. 1.
[0076] From the chiller 104, the saturated vapor refrigerant 118
may be flowed through the heat exchanger economizer 302. The cold,
low-pressure saturated vapor refrigerant 118 may be used to subcool
the saturated liquid refrigerant 112 within the heat exchanger
economizer 302. The superheated vapor refrigerant 304 exiting the
heat exchanger economizer 302 may then be flowed through the
compressor 106 and the condenser 108, and the resulting liquid
refrigerant 120 may be stored within the accumulator 110, as
discussed above with respect to FIG. 1.
[0077] It is to be understood that the process flow diagram of FIG.
3 is not intended to indicate that the single stage refrigeration
system 300 is to include all the components shown in FIG. 3.
Further, the single stage refrigeration system 300 may include any
number of additional components not shown in FIG. 3, depending on
the details of the specific implementation.
[0078] FIG. 4 is a process flow diagram of a cascade cooling system
400 including a first refrigeration system 402 and a second
refrigeration system 404. In various embodiments, the first
refrigeration system 402 utilizes a refrigerant including a noble
gas, such as xenon or krypton, while the second refrigeration
system 404 may utilize a different noble gas refrigerant, a
fluorocarbon refrigerant, or a hydrocarbon refrigerant. The
refrigerants in either refrigeration system 402 or 404 may include
mixtures. The cascade cooling system 400 may be used for instances
in which a higher degree of cooling than that provided by the
refrigeration systems 100, 200, or 300 is desired. The cascade
cooling system 400 may provide cooling at very low temperatures,
e.g., below -40.degree. C.
[0079] Within the first refrigeration system 402, a liquid
refrigerant stream 406 may be flowed from an accumulator 408
through a first expansion valve 410 and a first heat exchanger 412,
which chills a product stream 413. The resulting vapor/liquid
stream is separated in a first flash drum 414. A portion of the
liquid refrigerant stream 406 may be flowed directly into the first
flash drum 414 via a bypass valve 416, which can be used to control
the temperature of the liquid in the first flash drum 414, as well
as the amount of cooling in the first heat exchanger 412.
[0080] From the first flash drum 414, a liquid refrigerant stream
418 may be flowed through a second expansion valve 420, and flashed
into a second heat exchanger 422, which may be used to further
chill the product stream 413. A gas accumulator 424 feeds the
resulting vapor refrigerant stream 426 to a first stage compressor
428. The resulting medium pressure vapor refrigerant stream 430 is
combined with the vapor refrigerant stream 432 from the first flash
drum 414, and the combined stream is fed to a second stage
compressor 434. The high pressure vapor stream 436 from the second
stage compressor 434 is passed through a condenser 438, which may
use cooling from the second refrigeration system 404. Specifically,
the condenser 438 may cool the high pressure vapor stream 436 to
produce a liquid refrigerant stream 406 using a low temperature
refrigerant stream 440 from the second refrigeration system 404.
The liquid refrigerant stream 406 from the condenser 438 is then
stored in the accumulator 408. A control valve 442 may be used to
control the flow of the low temperature refrigerant stream 440
through the condenser 438. From the condenser 438, the resulting
vapor refrigerant stream 444 back to the second refrigeration
system 404.
[0081] Within the second refrigeration system 404, a liquid
refrigerant stream 448 may be flowed from an accumulator 450
through a heat exchanger 452 that is configured to cool the liquid
refrigerant stream 448 via a chilling system 454. The resulting low
temperature refrigerant stream 456 may be flowed through a first
expansion valve 458 and a first heat exchanger 460, which chills
the product stream 413. The resulting vapor/liquid refrigerant
stream is separated in a first flash drum 462. A portion of the low
temperature refrigerant stream 456 may be flowed directly into the
first flash drum 462 via a bypass valve 464, which can be used to
control the temperature of the liquid in the first flash drum 462,
as well as the amount of cooling in the first heat exchanger
460.
[0082] From the first flash drum 462, a liquid refrigerant stream
466 may be flowed through a second expansion valve 468, and flashed
into a second heat exchanger 470, which may be used to further
chill the product stream 413. The resulting vapor/liquid
refrigerant stream is separated in a second flash drum 472. A
portion of the liquid refrigerant stream 466 may be flowed directly
into the second flash drum 472 via a bypass valve 474, which can be
used to control the temperature of the liquid in the second flash
drum 472, as well as the amount of cooling in the second heat
exchanger 470.
[0083] From the second flash drum 472, a liquid refrigerant stream
476 may be flowed through a third expansion valve 478, and flashed
into a third heat exchanger 480, which may be used to further chill
the product stream 413. A gas accumulator 482 feeds the resulting
vapor refrigerant stream 484 to a first stage compressor 486. The
resulting medium pressure vapor refrigerant stream 488 is combined
with the vapor refrigerant stream 490 from the second flash drum
472, and the combined stream is fed to a second stage compressor
492. The resulting high pressure vapor refrigerant stream 494 is
combined with the vapor refrigerant mixture 496 from the first
flash drum 462, and the combined stream is fed to a third stage
compressor 497. The resulting high pressure vapor refrigerant
stream 498 is flowed through a heat exchanger 499, in which it may
be further cooled through indirect heat exchange with cooling
water. The resulting liquid refrigerant stream 448 may then be
flowed into the accumulator 450.
[0084] It is to be understood that the process flow diagram of FIG.
4 is not intended to indicate that the cascade cooling system 400
is to include all the components shown in FIG. 4. Further, the
cascade cooling system 400 may include any number of additional
components not shown in FIG. 4, depending on the details of the
specific implementation.
[0085] FIG. 5 is process flow diagram of an expansion refrigeration
system 500 for hydrocarbon dew point control. Condensation of heavy
hydrocarbons, e.g., C.sub.3-C.sub.6, in natural gas within pipes
may result in an increase in pressure within the pipes, as well as
an increase in the power usage of handling facilities. Therefore,
the hydrocarbon dew point may be reduced using the expansion
refrigeration system 500 in order to prevent such condensation.
[0086] As shown in FIG. 5, a dehydrated natural gas feed stream 502
may be flowed into a gas/gas heat exchanger 504. Within the gas/gas
heat exchanger 504, the dehydrated natural gas feed stream 502 may
be cooled through indirect heat exchange with a low temperature
natural gas stream 506. The resulting natural gas stream 508 may be
flowed into a first separator 510, which may remove some amount of
heavy hydrocarbons 512 from the natural gas stream 508. In various
embodiments, removing the heavy hydrocarbons 512 from the natural
gas stream 508 decreases the dew point of the natural gas stream
508. The removed heavy hydrocarbons 512 may be flowed out of the
expansion refrigeration system 500 through a first outlet valve
514. For example, the heavy hydrocarbons 512 may be flowed from the
expansion refrigeration system 500 to a stabilizer (not shown).
[0087] The natural gas stream 508 may then be flowed into an
expander 516. In various embodiments, the expander 516 is a
turbo-expander, which is a centrifugal or axial flow turbine. The
expansion of the natural gas stream 508 within the expander 516 may
provide energy for driving a compressor 518, which is coupled to
the expander 516 via a shaft 520.
[0088] From the expander 516, the resulting low temperature natural
gas stream 506 may be flowed into a second separator 522, which may
remove any remaining heavy hydrocarbons 512 from the low
temperature natural gas stream 506. In various embodiments,
removing the heavy hydrocarbons 512 from the low temperature
natural gas stream 506 further decreases the dew point of the low
temperature natural gas stream 506. The removed heavy hydrocarbons
512 may then be flowed out of the expansion refrigeration system
500 through a second outlet valve 524.
[0089] The low temperature natural gas stream 506 may be flowed
from the second separator 522 to the gas/gas heat exchanger 504,
which may increase the temperature of the low temperature natural
gas stream 506, producing a high temperature natural gas stream
526. The high temperature natural gas stream 526 may then be flowed
through the compressor 518, which may return the pressure of the
natural gas stream 526 to acceptable sales gas pressure. The final,
decreased dew point natural gas stream 528 may then be flowed out
of the expansion refrigeration system 500.
[0090] In an embodiment, a cooling system, for example, using a
noble gas refrigerant may be used to add further cooling to the
process. This cooling may be implemented by placing a heat
exchanger 530 in the low temperature natural gas stream 506,
upstream of the second separator 522. A refrigerant liquid 532 may
be flashed across an expansion valve 534, through the chiller 530.
The resulting refrigerant vapor 536 can then be returned to the
refrigerant system. The chilling may allow for the removal of a
much higher amount of condensable hydrocarbons, such as C.sub.3s
and higher. Further, in some embodiments, the heat exchanger 530 is
placed upstream of the expander 516, with a separator located
between the heat exchanger 530 and the expander 516 to prevent
liquids from flowing into the expander 516.
[0091] It is to be understood that the process flow diagram of FIG.
5 is not intended to indicate that the expansion refrigeration
system 500 is to include all the components shown in FIG. 5.
Further, the expansion refrigeration system 500 may include any
number of additional components not shown in FIG. 5, depending on
the details of the specific implementation.
[0092] FIG. 6 is a process flow diagram of an expansion
refrigeration system 600 for NGL extraction. In various
embodiments, NGL extraction may be performed to recover NGLs, which
include any number of different heavy hydrocarbons, from a natural
gas stream. NGL extraction may be desirable due to the fact that
NGLs are often of greater value for purposes other than as a
gaseous heating fuel.
[0093] A dry natural gas feed stream 602 may be flowed into a
gas/gas heat exchanger 604 from a dehydration system. Within the
gas/gas heat exchanger 604, the dry natural gas feed stream 602 may
be cooled through indirect heat exchange with a low temperature
natural gas stream 606. The resulting natural gas stream 608 may be
flowed into a separator 610, which may remove a portion of NGLs 612
from the natural gas stream 608. The removed NGLs 612 may be flowed
from the separator 610 to a deethanizer or demethanizer 614.
[0094] The natural gas stream 608 may then be flowed into an
expander 616. In various embodiments, the expander 616 is a
turbo-expander. The expansion of the natural gas stream 608 within
the expander 616 may provide energy for driving a compressor 618,
which is coupled to the expander 616 via a shaft 620. In addition,
the temperature of the natural gas stream 608 may be reduced via
adiabatic expansion across a Joule-Thomson valve 622.
[0095] From the expander 616, the resulting low temperature natural
gas stream 606 may be flowed into the deethanizer or demethanizer
614. Within the deethanizer or demethanizer 614, NGLs may be
separated from the natural gas stream 606 and may be flowed out of
the deethanizer or demethanizer 614 as an NGL product stream 624.
The NGL product stream 624 may then be pumped out of the expansion
refrigeration system 600 via a pump 626.
[0096] The deethanizer or demethanizer 614 may be coupled to a heat
exchanger 628. In some embodiments, the heat exchanger 628 is a
reboiler 628 that may be used to heat a portion of a bottoms stream
630 from the deethanizer or demethanizer 614 via indirect heat
exchange within a high temperature fluid 632. The heated bottoms
stream 630 may then be reinjected into the deethanizer or
demethanizer 614.
[0097] The separation of the NGL product stream 624 from the
natural gas stream 606 within the deethanizer or demethanizer 614
may result in the production of a low temperature natural gas
stream that may be flowed out of the deethanizer or demethanizer
614 as an overhead stream 634. The overhead stream 634 may be
flowed into a heat exchanger 636, which may decrease the
temperature of the overhead stream 634 through indirect heat
exchange with a refrigerant mixture 638 including a noble gas. The
decrease in temperature can lead to condensation of some of the
vapors. The overhead stream 634 may then be separated within a
separation vessel 640 to produce the low temperature natural gas
stream 606 and a liquid bottoms stream 642. The bottoms stream 642
may be pumped back into the deethanizer or demethanizer 614, via a
pump 644, forming a recycle stream.
[0098] The low temperature natural gas stream 606 may then be
flowed through the gas/gas heat exchanger 604. The temperature of
the low temperature natural gas stream 506 may be increased within
the gas/gas heat exchanger 604, producing a high temperature
natural gas stream 646. The high temperature natural gas stream 646
may then be flowed through the compressor 618, which may increase
the pressure of the natural gas stream 646. In some embodiments,
the high temperature natural gas stream 646 is also flowed through
a second compressor 648, which may increase the pressure of the
natural gas stream 646 to acceptable sales gas pressure. The
natural gas product stream 650 may then be flowed out of the
expansion refrigeration system 600.
[0099] It is to be understood that the process flow diagram of FIG.
6 is not intended to indicate that the expansion refrigeration
system 600 is to include all the components shown in FIG. 6.
Further, the expansion refrigeration system 600 may include any
number of additional components not shown in FIG. 6, depending on
the details of the specific implementation.
[0100] FIG. 7 is a process flow diagram of an LNG production system
700. As shown in FIG. 7, LNG 702 may be produced from a natural gas
stream 704 using a number of different refrigeration systems. As
shown in FIG. 7, a portion of the natural gas stream 704 may be
separated from the natural gas stream 704 prior to entry into the
LNG production system 700, and may be used as a fuel gas stream
706. The remaining natural gas stream 704 may be flowed into an
initial natural gas processing system 708. Within the natural gas
processing system 708, the natural gas stream 704 may be purified
and cooled. For example, the natural gas stream 704 may be cooled
using noble gas refrigerants, e.g., refrigerant mixtures including
one or more noble gases. For example, heavy hydrocarbons 710 may be
removed from the natural gas stream 706, and may be used to produce
gasoline 712 within a heavy hydrocarbon processing system 714. In
addition, any residual natural gas 716 that is separated from the
heavy hydrocarbons 710 during the production of the gasoline 712
may be returned to the natural gas stream 704.
[0101] The natural gas stream 704 may be converted into the LNG 702
within a cryogenic heat exchanger 718. In some embodiments, a mixed
refrigerant stream 720 from a mixed refrigeration system 722 is
used to cool the natural gas stream 704 within the cryogenic heat
exchanger 718. According to embodiments described herein, the mixed
refrigerant stream 720 is a refrigerant mixture including one or
more noble gases. In other embodiments, a hydrocarbon refrigerant
stream (not shown) from a hydrocarbon refrigeration system 724 is
used to cool the natural gas stream 704 within the cryogenic heat
exchanger 718 to produce the LNG 702.
[0102] It is to be understood that the process flow diagram of FIG.
7 is not intended to indicate that the LNG production system 700 is
to include all the components shown in FIG. 7. Further, the LNG
production system 700 may include any number of additional
components not shown in FIG. 7, depending on the details of the
specific implementation. For example, any number of alternative
refrigeration systems may also be used to produce the LNG 702 from
the natural gas stream 704. In addition, any number of different
refrigeration systems may be used in combination to produce the LNG
702.
[0103] Cascade Cooling Systems for the Production of Liquefied
Natural Gas
[0104] FIG. 8 is a simplified process flow diagram of a cascade
cooling system 800. The cascade cooling system 800 may be used to
produce LNG 802 from a raw natural gas 804. The raw natural gas 804
may be flowed into an inlet scrubber 806 within the cascade cooling
system 800. The inlet scrubber 806 may remove unwanted particulates
from the raw natural gas 804. An inlet meter 808 may monitor the
amount and characteristics of the natural gas as it enters the
cascade cooling system 800. The natural gas may be passed through
an amine treater 810, which can remove hydrogen sulfide, carbon
dioxide, and other unwanted gases from the natural gas, and may be
chilled within a heat exchanger 812 via indirect heat exchange with
propane or any other suitable coolant.
[0105] The natural gas may be flowed through a first dehydrator
814, which may remove water 816 from the natural gas via a gravity
separation process. The removed water 816 may be output from the
cascade cooling system 800. The natural gas may then be flowed to a
second dehydrator 818, which may remove any remaining water from
the natural gas. The second dehydrator 818 may be, for example, a
molecular sieve bed or a zeolite bed.
[0106] A mercury removal system 820, which may include a molecular
sieve bed, may remove mercury from the natural gas. In addition, a
dry gas filter 822, such as a pleated paper filter, may remove any
residual particulates from the natural gas.
[0107] From the dry gas filter 822, purified natural gas 823 may be
sent to a first cold box 824 within a refrigeration system 826. In
this example, the first cold box 824 may function as both a heat
exchanger and a flash drum. However, in other implementations, a
separate flash drum, such as the economizer 202 discussed with
respect to FIG. 2, may be used. Thus, the first cold box 824 may
cool the natural gas via indirect heat exchange with a first
refrigerant mixture 828. The first refrigerant mixture 828 may be a
conventional refrigerant, such as a HFC or propane. In addition,
the first cold box 824 may act as a vapor-liquid separator,
separating the first refrigerant mixture into a vapor refrigerant
mixture 830 and a liquid refrigerant mixture. The vapor refrigerant
mixture 830 may be generated via flash evaporation of the first
refrigerant mixture 828 across an expansion valve 832. The
expansion valve 832 may throttle the first refrigerant mixture 828
to decrease the pressure and temperature of the first refrigerant
mixture 828, resulting in the flash evaporation of the first
refrigerant mixture 828. In some embodiments, the first refrigerant
mixture 830 may be entirely vaporized and, thus, no liquid
refrigerant mixture may be present within the first cold box
824.
[0108] The first refrigerant mixture 828 may be continuously
recirculated and reused within the refrigeration system 826. For
example, after the first refrigerant mixture 828 passes through the
first cold box 824, the resulting vapor refrigerant mixture 830 is
compressed within a high pressure compressor 834 that can be
powered by a first gas turbine 836. The high pressure compressor
834 may be powered by a single gas turbine, for example, by being
placed on a common or coupled shaft, or may be powered by electric
motors. The vapor refrigerant mixture 830 is then condensed into
the liquid refrigerant mixture 828 within a first condenser 838.
The liquid refrigerant mixture 828 may then be stored within a
surge tank 840, from which it may be flowed back into the first
cold box 824 to close the cooling cycle.
[0109] A second refrigerant mixture 842 can also be used to further
cool the purified natural gas 823 within a second cold box 844. In
this example, the second cold box 834 further cools the purified
natural gas 823 via indirect heat exchange with the second
refrigerant mixture 842, which includes at least one noble gas. In
addition, the second cold box 844 may act as a vapor-liquid
separator, separating the second refrigerant mixture 842 into a
vapor refrigerant mixture 846 and a liquid refrigerant mixture. The
vapor refrigerant mixture 846 may be generated via flash
evaporation of the second refrigerant mixture 842 across an
expansion valve 848. The expansion valve 848 may throttle the
second refrigerant mixture 842 to decrease the pressure and
temperature of the second refrigerant mixture 842, resulting in the
flash evaporation of the second refrigerant mixture 842. In some
embodiments, the second refrigerant mixture 842 may be entirely
vaporized and, thus, no liquid refrigerant mixture may be present
within the second cold box 844.
[0110] The resulting vapor refrigerant mixture 846 exiting the
second cold box 844 may be compressed within a low pressure
compressor 850 that is powered by a second gas turbine 852,
producing a compressed refrigerant mixture 854. The low pressure
compressor 850 may be powered by a single gas turbine, for example,
by being placed on a common or coupled shaft, or may be powered by
electric motors. The compressed refrigerant mixture 854 may then be
condensed within a sub-ambient condenser 856, such as an ammonia
chiller, to produce the second refrigerant mixture 842. The second
refrigerant mixture 842 may be stored within a surge tank 858, from
which it may be flowed back into the second cold box 844 to close
the cooling cycle.
[0111] After the natural gas 823 has been cooled within the cold
boxes 824 and 844, the natural gas 823 may be further cooled and
liquefied within an autorefrigeration system 860, producing the LNG
802. In some embodiments, the autorefrigeration system 860 includes
a series of expansion valves (not shown) and flash drums (not
shown) that progressively lower the temperature and pressure of the
natural gas until it reaches a liquid state at, or near,
atmospheric pressure. In addition, prior to being flowed into the
autorefrigeration system 860, the natural gas 823 may be flowed
through a high pressure nitrogen rejection unit (NRU) (not shown).
The NRU may remove some portion of the nitrogen from the natural
gas 823 and, thus, may allow for the use of a gas containing a high
percentage of nitrogen.
[0112] The autorefrigeration system 860 may also produce natural
gas vapor, which may be used as fuel 862. The fuel 862 may be
compressed within a compressor 864 that is powered by a third gas
turbine 866 before being flowed out of the cascade cooling system
800. Depending on demand for fuel 862, a large portion of the
natural gas vapor may be recombined with the initial purified
natural gas 823, and returned to the system for further
processing.
[0113] The produced LNG 802 may be stored within an LNG tank 868
prior to being sent out of the cascade cooling system 800. Gases
may be vented out of the LNG tank 868 and pumped back into the
autorefrigeration system 860 via a first pump 870. In addition, gas
872 that is separated from the LNG 802 during loading of the LNG
802 at a loading facility, for example, may be pumped back into the
autorefrigeration system 860 via a second pump 874.
[0114] It is to be understood that the process flow diagram of FIG.
8 is not intended to indicate that the cascade cooling system 800
is to include all the components shown in FIG. 8. Further, the
cascade cooling system 800 may include any number of additional
components not shown in FIG. 8, depending on the details of the
specific implementation.
[0115] FIGS. 9A-C are a more detailed process flow diagram of a
cascade cooling system 900. The cascade cooling system 900 may be a
cascade, open-loop liquefaction system for the production of LNG.
The cascade cooling system 900 may operate at low temperatures,
e.g., below about 0.degree. F., or below about -20.degree. F., or
below about -40.degree. F. In addition, the cascade cooling system
900 may employ more than one refrigerant and provide refrigeration
at multiple temperatures.
[0116] The cascade cooling system 900 may include a first
refrigeration system 902, as shown in FIG. 9A, which may utilize a
non-hydrocarbon refrigerant such as a hydrofluorocarbon, e.g.,
R-404A or R-410a. The cascade cooling system 900 may also include a
second refrigeration system 904, as shown in FIG. 9B, which may
utilize a refrigerant mixture including at least one noble gas,
such as xenon, krypton, argon, or combinations thereof
[0117] FIG. 10 is a more detailed process flow diagram of an
autorefrigeration system 1000. The autorefrigeration system 1000
may be located downstream of the cascade cooling system 900, as
discussed further below.
[0118] A natural gas stream 908 may be flowed into a pipe joint 910
within the cascade cooling system 900. The pipe joint 910 may be
configured to split the natural gas stream 908 into two separate
natural gas streams. One natural gas stream 914 may be flowed into
another pipe joint 912, while the other natural gas stream 916 may
be flowed into the autorefrigeration system 1000.
[0119] Within the pipe joint 912, the natural gas stream 914 may be
combined with a natural gas vapor stream 1066 from the
autorefrigeration system 1000. The resulting natural gas stream 918
may then be flowed into the first refrigeration system 902 in
preparation for cooling of the natural gas stream 918. The natural
gas stream 918 may be cooled by being passed through a series of
heat exchangers 920, 922, 924, and 926 within the first
refrigeration system 902. The heat exchangers 920, 922, 924, and
926 may also be referred to as evaporators, chillers, or cold
boxes. The natural gas stream 918 may be cooled within each of the
heat exchangers 920, 922, 924, and 926 through indirect heat
exchange with a circulating non-hydrocarbon refrigerant. The
non-hydrocarbon refrigerant may be a hydrofluorocarbon, such as
R-404A or R-410A, or any other suitable type of non-hydrocarbon
refrigerant.
[0120] The non-hydrocarbon refrigerant may be continuously
circulated through the first refrigeration system 902, which may
continuously prepare the non-hydrocarbon refrigerant for entry into
each of the heat exchangers 920, 922, 924, and 926. The
non-hydrocarbon refrigerant may exit the first heat exchanger 920
via line 928 as a vapor non-hydrocarbon refrigerant. The vapor
non-hydrocarbon refrigerant can be combined with additional vapor
non-hydrocarbon refrigerant within a pipe joint 930. The vapor
non-hydrocarbon refrigerant is then flowed through a compressor 932
to increase the pressure of the vapor non-hydrocarbon refrigerant,
producing a superheated vapor non-hydrocarbon refrigerant. The
superheated vapor non-hydrocarbon refrigerant is flowed through a
condenser 934, which may cool and condense the superheated vapor
non-hydrocarbon refrigerant, producing a liquid non-hydrocarbon
refrigerant.
[0121] The liquid non-hydrocarbon refrigerant may be flowed through
an expansion valve 935, which lowers the temperature and pressure
of the liquid non-hydrocarbon refrigerant. This may result in the
flash evaporation of the liquid non-hydrocarbon refrigerant,
producing a mixture of the liquid non-hydrocarbon refrigerant and a
vapor non-hydrocarbon refrigerant. The liquid non-hydrocarbon
refrigerant and the vapor non-hydrocarbon refrigerant may be flowed
into a first flash drum 936 via line 938. Within the first flash
drum 936, the liquid non-hydrocarbon refrigerant may be separated
from the vapor non-hydrocarbon refrigerant.
[0122] The vapor non-hydrocarbon refrigerant may be flowed from the
first flash drum 936 to the pipe joint 930 via line 940. The liquid
non-hydrocarbon refrigerant may be flowed into a pipe joint 942,
which may split the liquid non-hydrocarbon refrigerant into two
separate liquid non-hydrocarbon refrigerant streams. One liquid
non-hydrocarbon refrigerant stream may be flowed through the first
heat exchanger 920, partly or completely flashed to vapor, and
returned to the pipe joint 930 via line 928. The other liquid
non-hydrocarbon refrigerant stream may be flowed to a second flash
drum 944 via line 946. The line 946 may also include an expansion
valve 948 that throttles the liquid non-hydrocarbon refrigerant
stream to control the flow of the liquid non-hydrocarbon
refrigerant stream into the second flash drum 944. The throttling
of the liquid non-hydrocarbon refrigerant stream within the
expansion valve 948 may result in the flash evaporation of the
liquid non-hydrocarbon refrigerant stream, producing a mixture of
both vapor and liquid non-hydrocarbon refrigerant.
[0123] The second flash drum 944 may separate the vapor
non-hydrocarbon refrigerant from the liquid non-hydrocarbon
refrigerant. The vapor non-hydrocarbon refrigerant may be flowed
into a pipe joint 950 via line 952. The pipe joint 950 may combine
the vapor non-hydrocarbon refrigerant with vapor non-hydrocarbon
refrigerant recovered from the second and third heat exchangers 922
and 924. The combined vapor non-hydrocarbon refrigerant may be
compressed within a compressor 954 and flowed into the pipe joint
930 via line 956 to be combined with the vapor from flash drum 936
and heat exchanger 920.
[0124] The liquid non-hydrocarbon refrigerant may be flowed from
the second flash drum 944 to a pipe joint 958, which may split the
liquid non-hydrocarbon refrigerant into two separate liquid
non-hydrocarbon refrigerant streams. One liquid non-hydrocarbon
refrigerant stream is flowed through the second heat exchanger 922
and returned to the pipe joint 950 via line 960. The other liquid
non-hydrocarbon refrigerant stream is flowed to a third flash drum
962 via line 964. The line 964 also includes an expansion valve 966
that controls the flow of the liquid non-hydrocarbon refrigerant
stream into the third flash drum 962. The expansion valve 966 may
result in the flash evaporation of the liquid non-hydrocarbon
refrigerant stream, producing a mixture of both vapor and liquid
non-hydrocarbon refrigerant. Flashing across the valve will reduce
the temperature and pressure of the liquid non-hydrocarbon
refrigerant stream.
[0125] The mixture of the vapor and liquid non-hydrocarbon
refrigerant may be flashed into the third flash drum 962, further
reducing the temperature and pressure. The third flash drum 962 may
separate the vapor non-hydrocarbon refrigerant from the liquid
non-hydrocarbon refrigerant. The vapor non-hydrocarbon refrigerant
may be flowed into a pipe joint 968 via line 970. The pipe joint
968 may combine the vapor non-hydrocarbon refrigerant with vapor
non-hydrocarbon refrigerant recovered from the third and fourth
heat exchangers 924 and 926. The combined vapor non-hydrocarbon
refrigerant may be compressed within a compressor 972 and flowed
into the pipe joint 950 via line 974.
[0126] The liquid non-hydrocarbon refrigerant may be flowed from
the third flash drum 962 to a pipe joint 976, which may split the
liquid non-hydrocarbon refrigerant into two separate liquid
non-hydrocarbon refrigerant streams. One liquid non-hydrocarbon
refrigerant stream may be flowed through the third heat exchanger
924 and returned to the pipe joint 968 via line 978. The other
liquid non-hydrocarbon refrigerant stream may be flowed through the
fourth heat exchanger 926 via line 980. The line 980 may also
include an expansion valve 982 that allows the liquid
non-hydrocarbon refrigerant to flash, and, thus, lowers the
pressure and temperature, of the liquid non-hydrocarbon refrigerant
stream as it flows into the fourth heat exchanger 926. From the
fourth heat exchanger 926, the liquid non-hydrocarbon refrigerant
stream may be compressed within a compressor 984 and sent to the
pipe joint 968 via line 986.
[0127] In one embodiment, a refrigerant mixture including a noble
gas is precooled by being flowed through each of the heat
exchangers 920, 922, 924, and 926. The refrigerant mixture may be
flowed from the second refrigeration system 904 to the heat
exchangers 920, 922, 924, and 926 within the first refrigeration
system 902 via line 988, as discussed further below.
[0128] After the natural gas stream has been progressively chilled
within each of the heat exchangers 920, 922, 924, and 926, it is
flowed into the second refrigeration system 904, shown in FIG. 9B,
via line 990. The second refrigeration system 904 may include a
fifth heat exchanger 992 and a sixth heat exchanger 994, which may
be used to further cool the natural gas stream. The fifth heat
exchanger 992 and the sixth heat exchanger 994 may utilize a
refrigerant mixture including one or more noble gases, such as
xenon or krypton, to cool the natural gas stream.
[0129] The refrigerant mixture may be continuously circulated
through the second refrigeration system 904, which prepares the
refrigerant mixture for entry into each of the heat exchangers 992
and 994. The refrigerant mixture may exit the fifth heat exchanger
992 via line 996 as a vapor refrigerant mixture. The vapor
refrigerant mixture may be combined with additional vapor
refrigerant mixture within a pipe joint 998. The vapor refrigerant
mixture may then be flowed through a compressor 1000, which may
increase the pressure of the vapor refrigerant mixture, producing a
superheated vapor refrigerant mixture. The superheated vapor
refrigerant mixture may be flowed through a gas cooler 1002, which
may cool the superheated vapor refrigerant mixture, producing a
liquid refrigerant mixture. In some cases, if the vapor refrigerant
mixture is below ambient temperature, the vapor refrigerant mixture
may not be flowed through the gas cooler 1002. The liquid
refrigerant mixture may then be flowed through the heat exchangers
920, 922, 924, and 926 within the first refrigeration system 902
via line 988, as discussed above.
[0130] Once the refrigerant mixture has passed through the heat
exchangers 920, 922, 924, and 926, the refrigerant mixture may
enter a fourth flash drum 1004 within the second refrigeration
system 904 via line 1006. Line 1006 may include an expansion valve
1008 that controls the flow of the refrigerant mixture into the
fourth flash drum 1004. The expansion valve 1008 may reduce the
temperature and pressure of the refrigerant mixture, resulting in
the flash evaporation of the refrigerant mixture into both a vapor
refrigerant mixture and a liquid refrigerant mixture.
[0131] The vapor refrigerant mixture and the liquid refrigerant
mixture may be flashed into the fourth flash drum 1004, which may
separate the vapor refrigerant mixture from the liquid refrigerant
mixture. The vapor refrigerant mixture may be flowed into the pipe
joint 998 via line 1010. The liquid refrigerant mixture may be
flowed from the fourth flash drum 1004 to a pipe joint 1012, which
may split the liquid refrigerant mixture into two separate liquid
refrigerant mixture streams. One liquid refrigerant mixture stream
may be flowed through the fifth heat exchanger 992 and returned to
the pipe joint 998 via line 996. The other liquid refrigerant
mixture stream may be flowed through the sixth heat exchanger 994
via line 1014. The line 1014 may also include an expansion valve
1016 that controls the flow of the liquid refrigerant mixture
stream into the sixth heat exchanger 994, e.g., by allowing the
refrigerant mixture to flash, lowering the temperature and creating
a vapor refrigerant mixture and a liquid refrigerant mixture. From
the sixth heat exchanger 994, the resulting vapor refrigerant
mixture may be compressed within a compressor 1018 and then flowed
into the pipe joint 998 to be recirculated.
[0132] After the natural gas stream has been cooled within the heat
exchangers 992 and 994 through indirect heat exchange with the
refrigerant mixture including one or more noble gases, the natural
gas stream may be flowed into the autorefrigeration system 1000,
shown in FIG. 10, via line 1020. The autorefrigeration system 1000
may include various components that are used to liquefy the natural
gas, producing LNG.
[0133] The natural gas stream may be flowed into a pipe joint 1022,
which may combine the natural gas stream from line 1020 with a
portion of the natural gas stream 916. Initial cooling of the
natural gas may be performed within a heat exchanger 1024 prior to
flowing the natural gas into the pipe joint 1022 via line 1026.
[0134] From the pipe joint 1022, the natural gas may be flowed into
a reboiler 1028, which may decrease the temperature of the natural
gas. The cooled natural gas may be expanded within a hydraulic
expansion turbine 1030 and then flowed into a NRU system 1032 via
line 1034 to remove excess nitrogen from the natural gas. In
various embodiments, the natural gas is flowed into a cryogenic
fractionation column 1036, such as a NRU tower, within the NRU
system 1032. In addition, heat may be transferred to the cryogenic
fractionation column 1036 from the reboiler 1028 via line 1037.
[0135] The cryogenic fractionation column 1036 may separate
nitrogen from the natural gas via a cryogenic distillation process.
An overhead stream may be flowed out of the cryogenic fractionation
column 1036 via line 1038. The overhead stream may include
primarily methane and low boiling point or non-condensable gases,
such as nitrogen and helium, which have been separated from the
natural gas. The overhead stream may be flowed into an overhead
condenser 1040, which may separate any liquid within the overhead
stream and return it to the cryogenic fractionation column 1036 as
reflux. This may result in the production of one vapor stream, a
fuel stream including primarily methane and another vapor stream
including primarily low boiling point gases. The fuel stream may be
flowed through the heat exchanger 1024 via line 1042. Within the
heat exchanger 1024, the temperature of the vapor fuel stream may
be increased via indirect heat exchange with the natural gas stream
916, producing a vapor fuel stream. The vapor fuel stream may then
be compressed within a compressor 1044 and flowed out of the
cascade cooling system 900 as fuel 1046 via line 1048. A liquid
stream from the overhead condenser 1040 can be returned to the
cryogenic fractionation column 1036 as a reflux stream.
[0136] The bottoms stream that is produced within the cryogenic
fractionation column 1036 includes primarily natural gas with
traces of nitrogen. The bottoms stream, as well as the vapor stream
from the overhead condenser 1040, may be flowed into a fifth flash
drum 1049 via lines 1050 and 1052, respectively. Line 1050 may also
include an expansion valve 1054 that controls the flow of the
bottoms stream into the fifth flash drum 1049, allowing a portion
of the liquid from the bottoms stream to flash, creating a mixed
phase stream that is flowed into the fifth flash drum 1049.
[0137] In addition, some portion of the bottoms stream may be
flowed through the overhead condenser 1040 via line 1055. Line 1055
may also include an expansion valve 1056 that controls the flow of
the bottoms stream into the overhead condenser 1040. The bottoms
stream may be used as refrigerant for the overhead condenser 1040.
The resulting vapor exiting the overhead condenser 1040 may be
returned to the fifth flash drum 1049 via the line 1052.
[0138] The fifth flash drum 1049 may separate the mixed phase
stream into a vapor stream that includes primarily natural gas and
an LNG stream. The vapor stream may be flowed into a pipe joint
1058 via line 1060. The pipe joint 1058 may combine the vapor
stream with another vapor stream recovered from a sixth flash drum
1062. The combined vapor streams may be compressed within a
compressor 1064 and flowed into the pipe joint 912 within the first
refrigeration system 902 via line 1066.
[0139] The LNG stream may be flowed into the sixth flash drum 1062
via line 1068. The line 1068 may include an expansion valve 1070
that controls the flow of the LNG stream into the sixth flash drum
1062, allowing a portion of the liquid from the LNG stream to
flash, creating a mixed phase system that is flowed into the sixth
flash drum 1062.
[0140] The sixth flash drum 1062 may separate the mixed phase
stream into LNG and a vapor stream that includes natural gas. The
vapor stream may be flowed into a pipe joint 1072 via line 1074.
The pipe joint 1072 may combine the vapor stream with another vapor
stream recovered from a seventh flash drum 1076. The combined vapor
streams may be compressed within a compressor 1078 and flowed into
the pipe joint 1058.
[0141] The LNG stream may then be flowed into the seventh flash
drum 1076 via line 1080. The line 1080 may include an expansion
valve 1082 that controls the flow of the LNG stream into the
seventh flash drum 1076, allowing a portion of the liquid from the
LNG to flash. The seventh flash drum 1076 may further reduce the
temperature and pressure of the LNG stream such that the LNG stream
approaches an equilibrium temperature and pressure, as discussed
below with respect to FIG. 11. The produced vapor stream may be
flowed into a pipe joint 1084, which may combine the vapor stream
with boil-off gas recovered from an LNG tank 1086. The combined
vapor streams may be compressed within a compressor 1088 and flowed
into the pipe joint 1072.
[0142] The LNG tank 1086 may store the LNG stream for any period of
time. Boil-off gas generated within the LNG tank 1086 may be flowed
to the pipe joint 1084 via line 1090. At any point in time, the LNG
stream may be transported to an LNG tanker 1092 using a pump 1094,
for transport to markets. The additional boil-off gas 1098
generated while loading LNG stream into the LNG tanker 1092, may be
recovered in the cascade cooling system 900 by adding it to the
pipe joint 1084.
[0143] It is to be understood that the process flow diagrams of
FIGS. 9A, 9B, and 10 are not intended to indicate that the cascade
cooling system 900 and the autorefrigeration system 1000 are to
include all the components shown in FIGS. 9A, 9B, and 10. Further,
the cascade cooling system 900 and/or the autorefrigeration system
1000 may include any number of additional components not shown in
FIGS. 9A, 9B, and 10, depending on the details of the specific
implementation. For example, in some embodiments, the cascade
cooling system 900 includes one or more refrigeration systems that
utilize a single mixed refrigerant including at least one noble
gas. However, the cascade cooling system 900 and/or the
autorefrigeration system 1000 may also include any other types or
combinations of refrigeration systems.
[0144] FIG. 11 is a schematic of a methane pressure-enthalpy (P-H)
diagram 1100. The P-H diagram 1100 shows corresponding pressures
1102 and enthalpies 1104 at various temperatures. Like numbered
items are as described with respect to FIG. 9. The P-H diagram 1100
includes an equilibrium curve 1106. A left side 1108 of the
equilibrium curve 1106 represents a pure liquid, while a right side
of the equilibrium curve 1106 represents a pure gas 1110. In
addition, if the pressure 1102 and enthalpy 1104 of the methane is
within the equilibrium curve 1106, the methane exists as an
equilibrium mixture of liquid and gas. If the pressure 1102 and
enthalpy 1104 of methane is above the equilibrium curve 1106, the
methane is in a critical state.
[0145] According to the autorefrigeration process described herein,
it is desirable to reduce the temperature and pressure 1102 of
methane such that the methane exists as a liquid near atmospheric
pressure. Each flash evaporation process within the expansion
valves 1056, 1070, and 1080 and the flash drums 1049, 1062, and
1076 isenthalpically reduces the temperature and the pressure of
the methane. For example, prior to expansion across the hydraulic
expansion turbine 1030, the methane may be in a critical state
1112. In many cases, it is difficult to reach such a critical state
with typical hydrocarbon refrigerants such as methane. Therefore,
xenon may be used for the autorefrigeration process instead of
methane in some cases.
[0146] The hydraulic expansion turbine 1030 may isentropically
reduce the temperature and the pressure 1102 of the methane to a
first equilibrium state 1114. A NRU may operate at the first
equilibrium state 1114 or at a slightly higher pressure. The first
equilibrium state 1114 may include a large liquid proportion 1116
and a small gas proportion 1118. The gas may be vented out of the
fifth flash drum 1049 such that the methane is in a first pure
liquid state 1120. However, the first pure liquid state 1120 may be
at a pressure 1102 that is substantially higher than atmospheric
pressure. Thus, the methane may be flowed through the expansion
valve 1070 and into the sixth flash drum 1062.
[0147] The expansion valve 1070 may isenthalpically reduce the
temperature and the pressure 1102 of the methane to a second
equilibrium state 1122. Similarly to the first equilibrium state
1118, the second equilibrium state 1122 may include a large liquid
proportion and a small gas proportion. The gas may be vented out of
the sixth flash drum 1062 such that the methane is in a second pure
liquid state 1124. However, the second pure liquid state 1124 may
still be at a pressure 1102 that is substantially higher than
atmospheric pressure. Therefore, the methane may be flowed through
the expansion valve 1080 and into the seventh flash drum 1076.
[0148] The expansion valve 1082 may isenthalpically reduce the
temperature and the pressure 1102 of the methane to a third
equilibrium state 1126. The third equilibrium state 1126 may
include a large liquid proportion and a small gas proportion. The
gas may be vented out of the seventh flash drum 1076 such that the
methane is in a third pure liquid state 1128. In various
embodiments, the pressure 1102 of the third pure liquid state 1128
may be near atmospheric pressure. Therefore, the methane may be in
the final product form, and may be exported as LNG.
[0149] Method for LNG Formation
[0150] FIG. 12 is a process flow diagram of a method 1200 for the
formation of LNG. In various embodiments, the method 1200 is
implemented within any of the systems 800, 900, or 1000 described
above with respect to FIG. 8, 9, or 10, respectively.
[0151] The method 1200 begins at block 1202, at which the natural
gas is chilled in a refrigeration system. The refrigeration system
may be a mechanical refrigeration system, valve expansion system,
turbine expansion system, or the like. The refrigeration system
uses a refrigerant mixture including a noble gas. The noble gas may
include xenon, krypton, argon, or any combinations thereof. In
addition, the refrigerant mixture may include nitrogen or a
hydrocarbon, such as methane, ethane, propane, or butane. According
to embodiments described herein, the refrigerant mixture including
the noble gas is used in any number of cooling stages to achieve
deeper cooling than provided by hydrocarbon refrigerants.
[0152] In various embodiments, the refrigerant mixture is
compressed to provide a compressed refrigerant mixture, and the
compressed refrigerant mixture is cooled by indirect heat exchange
with a cooling fluid. The compressed refrigerant mixture may be
expanded to cool the compressed refrigerant mixture, thereby
producing an expanded, cooled refrigerant mixture. The expanded,
cooled refrigerant mixture may be passed to a heat exchange area,
which may include, for example, a chiller or evaporator. In
addition, the natural gas may be compressed and cooled by indirect
heat exchange with an external cooling fluid. The natural gas may
then be chilled within the heat exchange area using the expanded,
cooled refrigerant mixture.
[0153] The natural gas may be chilled via one or more pre-cooling
steps using a first refrigerant mixture. The first refrigerant
mixture may include a noble gas, nitrogen, or a hydrocarbon, or any
combinations thereof. The natural gas may also be chilled via one
or more deep cooling steps using a second refrigerant mixture. The
second refrigerant mixture may include a noble gas, nitrogen, or a
hydrocarbon, or any combinations thereof.
[0154] At block 1204, the natural gas is liquefied to form LNG in
an autorefrigeration system. In various embodiments, the
autorefrigeration system includes a number of expansion valves and
flash drums that are used to cool and liquefy the natural gas. The
natural gas may be flashed across an expansion valve, lowering the
pressure and temperature of the natural gas and producing a vapor
fraction and a liquid fraction. The vapor fraction and the liquid
fraction may be flashed into a flash drum, which may separate the
vapor fraction from the liquid fraction. This process may be
repeated within any number of expansion valves and flash drums
until a suitable amount of the natural gas has been converted to
LNG.
[0155] It is to be understood that the process flow diagram of FIG.
12 is not intended to indicate that the steps of the method 1200
are to be executed in any particular order, or that all of the
steps are to be included in every case. Further, any number of
additional steps may be included within the method 1200, depending
on the details of the specific implementation. For example, the
natural gas may be cooled in a first refrigeration system prior to
chilling the natural gas in the refrigeration system. In various
embodiments, the first refrigeration system uses a non-hydrocarbon
refrigerant.
Embodiments
[0156] Embodiments of the invention may include any combinations of
the methods and systems shown in the following numbered paragraphs.
This is not to be considered a complete listing of all possible
embodiments, as any number of variations can be envisioned from the
description above. [0157] 1. A system for formation of a liquefied
natural gas (LNG), including: [0158] a refrigeration system
configured to chill a natural gas using a refrigerant mixture
including a noble gas; and [0159] an autorefrigeration system
configured to use the natural gas as a self-refrigerant to form the
LNG from the natural gas. [0160] 2. The system paragraph 1,
including a first refrigeration system configured to cool the
natural gas using a non-hydrocarbon refrigerant prior to flowing
the natural gas into the refrigeration system. [0161] 3. The system
of any of paragraphs 1 or 2, including a nitrogen rejection unit
upstream of the autorefrigeration system. [0162] 4. The system of
any of paragraphs 1-3, wherein the system is configured to chill
the natural gas for hydrocarbon dew point control. [0163] 5. The
system of any of paragraphs 1-4, wherein the system is configured
to chill the natural gas for natural gas liquid (NGL) extraction.
[0164] 6. The system of any of paragraphs 1-5, wherein the system
is configured to separate methane and lighter gases from carbon
dioxide and heavier gases. [0165] 7. The system of any of
paragraphs 1-6, wherein the system is configured to prepare
hydrocarbons for liquefied petroleum gas (LPG) production storage.
[0166] 8. The system of any of paragraphs 1-7, wherein the system
is configured to condense a reflux stream. [0167] 9. The system of
any of paragraphs 1-8, wherein the refrigerant mixture includes
xenon or krypton, or any combination thereof [0168] 10. The system
of any of paragraphs 1-9, wherein the refrigerant mixture includes
xenon, krypton, argon, or nitrogen, or any combinations thereof
[0169] 11. The system of any of paragraphs 1-10, wherein the
refrigeration system includes a mechanical refrigeration system,
valve expansion system, or turbine expansion system, or any
combinations thereof [0170] 12. The system of any of paragraphs
1-11, wherein the refrigerant mixture includes a hydrocarbon, and
wherein the hydrocarbon includes methane, ethane, propane, or
butane, or any combinations thereof [0171] 13. The system of any of
paragraphs 1-12, wherein the refrigeration system includes multiple
cooling cycles. [0172] 14. The system of any of paragraphs 1-13,
wherein the refrigeration system includes multiple cooling cycles,
including: [0173] one or more pre-cooling stages, wherein the
refrigerant mixture includes a noble gas, nitrogen, or a
hydrocarbon, or any combinations thereof, and [0174] one or more
deep cooling cycles, wherein the refrigerant mixture includes a
noble gas, nitrogen, or a hydrocarbon, or any combinations thereof
[0175] 15. The system of any of paragraphs 1-14, wherein the
refrigerant mixture including the noble gas is utilized in one or
more cooling stages to achieve deeper cooling than provided by
hydrocarbon refrigerants. [0176] 16. The system of any of
paragraphs 1-15, including a nitrogen rejection unit, wherein a
liquid feed from the bottom of the nitrogen rejection unit is used
to provide cooling to a reflux condenser at the top of the nitrogen
rejection unit. [0177] 17. The system of any of paragraphs 1-16,
wherein the refrigerant mixture comprises a pure component
refrigerant. [0178] 18. A method for formation of a liquefied
natural gas (LNG), including: [0179] chilling a natural gas in a
refrigeration system, wherein the refrigeration system uses a
refrigerant mixture including a noble gas; and [0180] liquefying
the natural gas to form the LNG in an autorefrigeration system.
[0181] 19. The method of paragraph 18, including cooling the
natural gas in a first refrigeration system prior to chilling the
natural gas in the refrigeration system, wherein the first
refrigeration system uses a non-hydrocarbon refrigerant. [0182] 20.
The method of any of paragraphs 18 or 19, wherein chilling the
natural gas in the refrigeration system includes: [0183]
compressing the refrigerant mixture to provide a compressed
refrigerant mixture; [0184] optionally cooling the compressed
refrigerant mixture by indirect heat exchange with a cooling fluid;
[0185] expanding the compressed refrigerant mixture to cool the
compressed refrigerant mixture, thereby producing an expanded,
cooled refrigerant mixture; [0186] passing said expanded, cooled
refrigerant mixture to a first heat exchange area; [0187]
optionally compressing the natural gas; [0188] optionally cooling
said the natural gas by indirect heat exchange with an external
cooling fluid; and [0189] heat exchanging the natural gas with the
expanded, cooled refrigerant mixture. [0190] 21. The method of any
of paragraphs 18-20, wherein the noble gas includes xenon or
krypton. [0191] 22. The method of any of paragraphs 18-21, wherein
the refrigerant mixture includes nitrogen or a hydrocarbon, or any
combination thereof [0192] 23. The method of any of paragraphs
18-22, including liquefying the natural gas to form the LNG via a
number of expansion valves or hydraulic expansion turbines and
flash drums. [0193] 24. The method of any of paragraphs 18-23,
including: [0194] chilling the natural gas via one or more
pre-cooling steps using a first refrigerant mixture, wherein the
first refrigerant mixture includes a noble gas, nitrogen, or a
hydrocarbon, or any combinations thereof, and [0195] chilling the
natural gas via one or more deep cooling steps using a second
refrigerant mixture, wherein the second refrigerant mixture
includes a noble gas, nitrogen, or a hydrocarbon, or any
combinations thereof [0196] 25. The method of any of paragraphs
18-24, including using the refrigerant mixture including the noble
gas in one or more cooling stages to achieve deeper cooling than
provided by hydrocarbon refrigerants. [0197] 26. A cascade cooling
system for formation of a liquefied natural gas (LNG), including:
[0198] a first refrigeration system configured to cool the natural
gas using a non-hydrocarbon refrigerant, wherein the first
refrigeration system includes a number of first chillers configured
to allow for cooling of the natural gas via an indirect exchange of
heat between the natural gas and the non-hydrocarbon refrigerant;
[0199] a second refrigeration system configured to chill the
natural gas using a refrigerant mixture including a noble gas,
wherein the second refrigeration system includes a number of second
chillers configured to allow for cooling of the natural gas via an
indirect exchange of heat between the natural gas and the
refrigerant mixture; and [0200] an autorefrigeration system
configured to form the LNG from the natural gas, wherein the
autorefrigeration system includes a number of expansion valves or
hydraulic expansion turbines, or any combination thereof, and flash
drums. [0201] 27. The cascade cooling system of paragraph 26,
wherein the first refrigeration system includes a compressor that
is configured to compress the non-hydrocarbon refrigerant and a
condenser that is configured to cool the non-hydrocarbon
refrigerant. [0202] 28. The cascade cooling system of any of
paragraphs 26 or 27, wherein the second refrigeration system
includes a compressor that is configured to compress the
refrigerant mixture and a condenser that is configured to cool the
refrigerant mixture. [0203] 29. The cascade cooling system of any
of paragraphs 26-28, wherein the number of first chillers include
evaporators configured to cool the natural gas by at least
partially vaporizing the non-hydrocarbon refrigerant via a transfer
of heat from the natural gas to the non-hydrocarbon refrigerant.
[0204] 30. The cascade cooling system of any of paragraphs 26-29,
wherein the number of second chillers include evaporators
configured to chill the natural gas by vaporizing the refrigerant
mixture via a transfer of heat from the natural gas to the
refrigerant mixture. [0205] 31. The cascade cooling system of any
of paragraphs 26-30, wherein the LNG includes a liquid fraction and
a residual vapor fraction, and wherein the cascade cooling system
includes a liquid separation vessel configured to separate the
residual vapor fraction from the liquid fraction. [0206] 32. The
cascade cooling system of any of paragraphs 26-31, including a
nitrogen rejection unit upstream of the autorefrigeration system.
[0207] 33. The cascade cooling system of any of paragraphs 26-32,
wherein the refrigerant mixture comprises a pure component
refrigerant.
[0208] While the present techniques may be susceptible to various
modifications and alternative forms, the embodiments discussed
above have been shown only by way of example. However, it should
again be understood that the techniques is not intended to be
limited to the particular embodiments disclosed herein. Indeed, the
present techniques include all alternatives, modifications, and
equivalents falling within the true spirit and scope of the
appended claims.
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