U.S. patent application number 11/422981 was filed with the patent office on 2007-12-13 for lng system with optimized heat exchanger configuration.
Invention is credited to Kevin H. Hulsey, Bobby D. Martinez, Weldon L. Ransbarger.
Application Number | 20070283718 11/422981 |
Document ID | / |
Family ID | 38801769 |
Filed Date | 2007-12-13 |
United States Patent
Application |
20070283718 |
Kind Code |
A1 |
Hulsey; Kevin H. ; et
al. |
December 13, 2007 |
LNG SYSTEM WITH OPTIMIZED HEAT EXCHANGER CONFIGURATION
Abstract
The current invention provides a methodology and apparatus for
the liquefaction of normally gaseous material, most notably natural
gas, which reduces the number of process vessels required and/or
reduces space requirements over convention apparatus.
Inventors: |
Hulsey; Kevin H.; (Houston,
TX) ; Ransbarger; Weldon L.; (Houston, TX) ;
Martinez; Bobby D.; (Missouri City, TX) |
Correspondence
Address: |
ConocoPhilips Company - I.P. Legal
PO BOX 2443
BARTLESVILLE
OK
74005
US
|
Family ID: |
38801769 |
Appl. No.: |
11/422981 |
Filed: |
June 8, 2006 |
Current U.S.
Class: |
62/612 |
Current CPC
Class: |
F25J 1/0085 20130101;
F25J 1/0265 20130101; F25J 2220/64 20130101; F25J 1/021 20130101;
F25J 1/004 20130101; F25J 1/0045 20130101; F25J 1/0262 20130101;
F25J 1/0052 20130101; F25J 2290/62 20130101; F25J 2245/90 20130101;
F25J 1/0022 20130101 |
Class at
Publication: |
62/612 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Claims
1. A process for liquefying natural gas, said process comprising:
(a) cooling a natural gas stream in a first refrigeration cycle via
indirect heat exchange with a first refrigerant; and (b) downstream
of step (a), cooling said natural gas stream in a second
refrigeration cycle via indirect heat exchange with a second
refrigerant, wherein at least one of said first and second
refrigerants is a pure component refrigerant, wherein less than
about 10 percent of the natural gas mechanical cooling duty of at
least one of said first and second refrigeration cycles is provided
by core-in-kettle heat exchangers.
2. The process of claim 1 wherein less than about 5 percent of the
natural gas mechanical cooling duty of said first and second
refrigeration cycles is provided by core-in-kettle heat
exchangers.
3. The process of claim 1 wherein at least about 90 percent of the
natural gas mechanical cooling duty of said first and second
refrigeration cycles is provided by one or more plate-fin heat
exchangers.
4. The process of claim 3 wherein said plate-fin heat exchangers
are brazed aluminum plate-fin heat exchangers.
5. The process of claim 1 wherein said second refrigerant is a pure
component refrigerant.
6. The process of claim 1 wherein said first and second
refrigerants are both pure component refrigerants.
7. The process of claim 1 wherein said first refrigerant comprises
predominately propane and said second refrigerant comprises
predominately ethylene.
8. The process of claim 1 further comprising, downstream of step
(b), cooling said natural gas stream in a third refrigeration cycle
via indirect heat exchange with a third refrigerant.
9. The process of claim 8 wherein said third refrigerant is a pure
component refrigerant.
10. The process of claim 8 wherein said third refrigeration cycle
is an open refrigeration cycle employing a portion of said natural
gas stream as said third refrigerant.
11. The process of claim 8 wherein said first, second, and third
refrigerants are all pure component refrigerants.
12. A process for liquefying natural gas comprising: (a) cooling a
natural gas stream in a first refrigeration cycle employing a first
refrigerant; (b) downstream of step (a), cooling the natural gas
stream in a second refrigeration cycle employing a second
refrigerant; (c) downstream of step (b), cooling the natural gas
stream in a third refrigeration cycle employing a third
refrigerant; wherein said third refrigeration cycle is an open
refrigeration cycle that uses a portion of said natural gas stream
as said third refrigerant, wherein at least about 90 percent of the
combined natural gas mechanical cooling duty of said first, second,
and third refrigeration cycles is provided by plate-fin heat
exchangers.
13. The process of claim 12 wherein said cooling of steps (a), (b),
and (c) is carried out without the use of any core-in-kettle heat
or spiral wound exchangers.
14. The process of claim 12 wherein all of said cooling of steps
(a), (b), and (c) is carried in plate-fin heat exchangers.
15. The process of claim 12 further comprising, cooling said
natural gas stream via expansion in said third refrigeration
cycle.
16. The process of claim 15 wherein the pressure of said natural
gas stream is reduced by at least about 100 psi in said third
refrigeration cycle.
17. The process of claim 15 wherein said natural gas stream enters
said third refrigeration cycle at a pressure of at least about 400
psia, wherein the pressure of said natural gas stream is reduced by
at least 250 psi in said third refrigeration cycle.
18. The process of claim 12 wherein said first refrigerant
comprises predominately propane and/or propylene, said second
refrigerant comprises predominately ethane and/or ethylene, and
said third refrigerant comprises predominately methane.
19. A process for liquefying natural gas comprising: (a) cooling a
natural gas stream in a first methane heat exchanger via indirect
heat exchange with at least one predominately-methane first
refrigerant stream to thereby produce a first cooled natural gas
stream; (b) dividing said first cooled natural gas stream into a
first refrigerant portion and a first product portion; (c)
expanding said first refrigerant portion to thereby produce a first
expanded refrigerant portion; and (d) using said first expanded
refrigerant portion as at least a portion of said first refrigerant
stream in said first methane heat exchanger.
20. The process of claim 19 wherein said first expanded refrigerant
portion is not subjected to phase separation prior to being used as
said first refrigerant stream in said first methane heat
exchanger.
21. The process of claim 19 further comprising, cooling said first
product portion in a second methane heat exchanger via indirect
heat exchange with at least one predominately-methane second
refrigerant stream to thereby produce a second cooled natural gas
stream.
22. The process of claim 21 further comprising, dividing said
second cooled natural gas stream into a second refrigerant portion
and a second product portion.
23. The process of claim 22 further comprising, expanding said
second refrigerant portion to thereby produce a second expanded
refrigerant portion and using said second expanded refrigerant
portion as at least a portion of said second refrigerant stream in
said second methane heat exchanger.
24. The process of claim 23 wherein said second expanded
refrigerant portion is not subjected to phase separation prior to
being used as said second refrigerant stream in said second methane
heat exchanger.
25. The process of claim 23 wherein said second expanded
refrigerant portion is heated in said second methane heat exchanger
to thereby produce a warmed second expanded refrigerant portion,
wherein said process further comprises employing said warmed second
expanded refrigerant portion as at least a portion of said first
refrigerant stream in said first methane heat exchanger.
26. The process of claim 22 further comprising, expanding said
second product portion to thereby produce a two-phase stream
comprising a vapor third refrigerant portion and a liquid third
product portion and using said third refrigerant portion as at
least a portion of said second refrigerant stream in said second
methane heat exchanger.
27. The process of claim 26 wherein said third refrigerant portion
is heated in said second methane heat exchanger to thereby produce
a warmed third refrigerant portion, wherein said process further
comprises using said warmed third refrigerant portion as at least a
portion of said first refrigerant stream in said first methane heat
exchanger.
28. The process of claim 26 further comprising, expanding said
third product portion to thereby produce an expanded final product
stream comprising liquified natural gas and introducing at least a
portion of said expanded final product stream into an LNG storage
tank.
29. The process of claim 28 further comprising, removing vapors
from said LNG storage tank and employing said vapors as at least a
portion of said first and/or second refrigerant streams.
30. The process of claim 19 further comprising, prior to step (a)
cooling said natural gas stream via indirect heat exchange with an
initial refrigerant stream, wherein said initial refrigerant stream
comprises predominately propane, propylene, ethane, and/or
ethylene.
31. The process of claim 30 wherein said initial refrigerant stream
comprises predominately propane.
32. The process of claim 31 further comprising, prior to step (a)
cooling said natural gas stream via indirect heat exchange with
intermediate refrigerant stream, wherein said intermediate
refrigerant stream comprises predominately ethane and/or
ethylene.
33. A process comprising: gasifying LNG produced by the process of
claim 1.
34. A process comprising: gasifying LNG produced by the process of
claim 12.
35. A process comprising: gasifying LNG produced by the process of
claim 19.
36. A computer simulation process comprising: using a computer to
simulate the process of claim 1.
37. A computer simulation process comprising: using a computer to
simulate the process of claim 12.
38. A computer simulation process comprising: using a computer to
simulate the process of claim 19.
39. A facility for processing natural gas into producing LNG, said
facility comprising: a first refrigeration cycle for cooling
natural gas with a first refrigerant; and a second refrigeration
cycle for cooling said natural gas with a second refrigerant,
wherein at least one of said first and second refrigerants is a
pure component refrigerant, wherein at least one of said first and
second refrigeration cycles does not comprise any core-in-kettle
heat exchangers that are operable to significantly cool said
natural gas.
40. The facility of claim 39 wherein said first and second
refrigeration cycles do not comprise any core-in-kettle or
spiral-wound heat exchangers that are operable to cool said natural
gas.
41. The facility of claim 39 wherein said first and second
refrigeration cycles include a plurality of plate-fin heat
exchangers that are operable to cool said natural gas.
42. The facility of claim 39 wherein said first and second
refrigerants are both pure component refrigerants.
43. The facility of claim 39 wherein said first refrigerant
comprises predominately propane and/or propylene and said second
refrigerant comprises predominately ethane and/or ethylene.
44. The facility of claim 39 further comprising, a third
refrigeration cycle for cooling said natural gas with a third
refrigerant.
45. The facility of claim 44 wherein said third refrigerant
comprises predominately methane.
46. The facility of claim 44 wherein said third refrigeration cycle
is an open refrigeration cycle.
47. The facility of claim 39 wherein said third refrigeration cycle
includes a plurality of expansion devices for sequentially lowering
the pressure of said natural gas, wherein said third refrigeration
cycle employs less than three vapor/liquid separation vessels.
48. The facility of claim 47 wherein said third refrigeration cycle
employs less than two vapor/liquid separation vessels.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The inventive methodology and associated apparatus disclosed
herein relates to the liquefaction of a normally gaseous material,
most notably natural gas. In one aspect, the invention concerns a
liquified natural gas (LNG) production system that operates with a
reduced number of process vessels and in a smaller space than
conventional LNG production systems.
[0003] 2. Description of the Prior Art
[0004] It is common practice to cryogenically treat natural gas to
liquefy the same for transport and storage. The primary reason for
the liquefaction of natural gas is that liquefaction results in a
volume reduction of about 1/600, thereby making it possible to
store and transport the liquefied gas in containers of more
economical and practical design. For example, when gas is
transported by pipeline from a supply source to a distant market,
it is desirable to operate the pipeline under a substantially
constant and high load factor. Often the deliverability or capacity
of the pipeline will exceed demand, while at other times the demand
may exceed the deliverability of the pipeline. In order to shave
off the peaks where demand exceeds supply, it is desirable to store
the excess gas in such a manner that it can be delivered when the
supply exceeds demand, thereby enabling future peaks in demand to
be met with material from storage. One practical means for doing
this is to convert the gas to a liquefied state for storage and to
then vaporize the liquid as demand requires.
[0005] Liquefaction of natural gas is of even greater importance in
making possible the transport of gas from a supply source to market
when the source and market are separated by great distances and a
pipeline is not available or is not practical. This is particularly
true where transport must be made by ocean-going vessels. Ship
transportation in the gaseous state is generally not practical
because appreciable pressurization is required to significantly
reduce the specific volume of the gas which in turn requires the
use of more expensive storage containers.
[0006] In order to store and transport natural gas in the liquid
state, the natural gas is preferably cooled to -240.degree. F. to
-260.degree. F. where it possesses a near-atmospheric vapor
pressure. Numerous systems exist in the prior art for the
liquefaction of natural gas in which the gas is liquefied by
sequentially passing the gas at an elevated pressure through a
plurality of cooling stages whereupon the gas is cooled to
successively lower temperatures until the liquefaction temperature
is reached. Cooling is generally accomplished by heat exchange with
one or more refrigerants such as propane, propylene, ethane,
ethylene, methane, or a combination of one or more of the
preceding. The refrigerants are sometimes arranged in a cascaded
manner. Further cooling of the liquid is possible by expanding the
liquefied natural gas to atmospheric pressure in one or more
expansion stages. In each stage, the liquefied gas is flashed to a
lower pressure thereby producing a two-phase, gas-liquid mixture at
a significantly lower temperature. The liquid is recovered and may
again be flashed. In this manner, the liquefied gas is further
cooled to a temperature suitable for liquefied gas storage at
near-atmospheric pressure.
[0007] As previously noted, the present invention concerns the
arrangement/selection of apparatus and associated process
methodologies whereby the number of process vessels in the overall
system is reduced. This reduction in the number of process vessels
also reduces space requirements.
OBJECTS AND SUMMARY OF THE INVENTION
[0008] It is an object of this invention to reduce the number of
process vessels required for liquefying natural gas.
[0009] It is another object of this invention to reduce the space
requirements of a process for liquefying natural gas.
[0010] It is still yet another object of this invention to develop
a process methodology and associated apparatus for liquefying
natural gas which is less capital intensive than alternative
liquefaction methodologies.
[0011] One embodiment of the present invention concerns a process
for liquefying natural gas that includes the following steps: (a)
cooling a natural gas stream in a first refrigeration cycle via
indirect heat exchange with a first refrigerant; and (b) downstream
of the first refrigeration cycle, cooling the natural gas stream in
a second refrigeration cycle via indirect heat exchange with a
second refrigerant. At least one of the first and second
refrigerants is a pure component refrigerant, and less than about
10 percent of the natural gas mechanical cooling duty of at least
one of the first and second refrigeration cycles is provided by
core-in-kettle heat exchangers.
[0012] Another embodiment of the present invention concerns a
process for liquefying natural gas that includes the following
steps: (a) cooling a natural gas stream in a first refrigeration
cycle employing a first refrigerant; (b) downstream of the first
refrigeration cycle, cooling the natural gas stream in a second
refrigeration cycle employing a second refrigerant; (c) downstream
of the second refrigeration cycle, cooling the natural gas stream
in a third refrigeration cycle employing a third refrigerant. The
third refrigeration cycle is an open refrigeration cycle that uses
a portion of the natural gas stream as the third refrigerant, and
at least about 90 percent of the combined natural gas mechanical
cooling duty of the first, second, and third refrigeration cycles
is provided by plate-fin heat exchangers.
[0013] Still another embodiment of the present invention concerns a
process for liquefying natural gas comprising the following steps:
(a) cooling a natural gas stream in a first methane heat exchanger
via indirect heat exchange with at least one predominately-methane
first refrigerant stream to thereby produce a first cooled natural
gas stream; (b) dividing the first cooled natural gas stream into a
first refrigerant portion and a first product portion; (c)
expanding the first refrigerant portion to thereby produce a first
expanded refrigerant portion; and (d) using the first expanded
refrigerant portion as at least a portion of the first refrigerant
stream in the first methane heat exchanger.
[0014] Yet another embodiment of the present invention concerns a
facility for producing LNG. The facility includes the following
components: (a) a first refrigeration cycle for cooling natural gas
with a first refrigerant; and (b) a second refrigeration cycle for
cooling the natural gas with a second refrigerant. At least one of
the first and second refrigerants is a pure component refrigerant,
and at least one of the first and second refrigeration cycles does
not include any core-in-kettle heat exchangers that are operable to
significantly cool the natural gas.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0015] FIG. 1 is a simplified flow diagram of a cryogenic LNG
production process which illustrates one embodiment of the
methodology and apparatus of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0016] As used herein, the term "natural gas" or "natural gas
stream" shall denote any stream principally comprised of methane,
which originates in major portion from a natural gas feed stream. A
natural gas stream typically contains at least 85 mole percent
methane, with the balance being ethane, higher hydrocarbons,
nitrogen, carbon dioxide, and minor amounts of other contaminates
such as, for example, mercury, hydrogen sulfide, and mercaptans. As
used herein, the terms "principally," "predominately," "primarily,"
and "in major portion," when used to describe the presence of a
particular component of a fluid stream, shall mean that the fluid
stream contains at least 50 mole percent of the stated component.
For example, a "predominately" methane stream, a "primarily"
methane stream, a stream "principally" comprised of methane, or a
stream comprised "in major portion" of methane, each denote a
stream containing at least 50 mole percent methane. As used herein,
the terms "upstream" and "downstream" shall be used to describe the
relative positions of various components of a natural gas
liquefaction plant along the main flow path of natural gas through
the plant.
[0017] One of the most efficient and effective methodologies for
natural gas liquefaction is a cascade-type operation in combination
with expansion-type cooling. Cascaded processes utilize one or more
refrigerants to transfer heat energy from the natural gas stream to
the refrigerant and ultimately to the environment. In essence, the
refrigeration system functions as a heat pump by removing thermal
energy from the natural gas stream as the stream is progressively
cooled to lower and lower temperatures. In so doing, the thermal
energy removed from the natural gas stream is ultimately rejected
(pumped) to the environment via energy exchange with one or more
refrigerants.
[0018] In a preferred embodiment, the present invention employs a
cascaded refrigerant system that cools the natural gas stream at an
elevated pressure (e.g., about 650 psia), by sequentially passing
the natural gas stream through an initial refrigeration cycle, an
intermediate refrigeration cycle, and a final refrigeration cycle.
In a preferred embodiment of the invention, the initial and
intermediate refrigeration cycles are closed refrigeration cycles,
while the final refrigeration cycle is an open refrigeration cycle
that utilizes a portion of the feed gas as a source of refrigerant
and which includes therein a multi-stage expansion cycle to further
cool the feed gas and reduce its pressure to near-atmospheric
pressure.
[0019] The refrigerants employed in the initial, intermediate, and
final refrigeration cycles preferably have their own distinct
compositions. In other words, it is preferred for pure component
refrigerants, rather than mixed refrigerants, to be employed in the
initial, intermediate, and final refrigeration cycles of the
present invention. As used herein, the term "mixed refrigerant"
denotes a refrigerant that does not contain more than 80 mole
percent of any single refrigerant component. As used herein, the
term "pure component refrigerant" denotes a refrigerant that is not
a mixed refrigerant. Preferably, a pure component refrigerant
comprises at least about 80 mole percent of a single refrigerant
component, more preferably at least about 90 mole percent of a
single hydrocarbon refrigerant component, and most preferably at
least 95 mole percent of a single hydrocarbon refrigerant
component. In the system of the present invention, it is preferred
for the refrigerant having the highest boiling point to be utilized
in the initial refrigeration cycle, followed by a refrigerant
having an intermediate boiling point employed in the intermediate
refrigeration cycle, and finally a refrigerant having the lowest
boiling point is employed in the final refrigeration cycle.
[0020] In a preferred embodiment of the present invention, the
initial refrigerant employed in the initial refrigeration cycle
contains primarily propane, propylene, and/or carbon dioxide. More
preferably, the initial refrigerant comprises predominately
propane, most preferably, the initial refrigerant consists
essentially of propane. The intermediate refrigerant preferably
comprises predominately ethane and/or ethylene. More preferably,
the intermediate refrigerant comprises predominately ethylene. Most
preferably, the intermediate refrigerant consists essentially of
ethylene. The final refrigerant preferably comprises predominately
methane. Most preferably, the final refrigerant consists
essentially of methane.
[0021] Preferably, each of the initial, intermediate, and final
refrigeration cycles employs a plurality of distinct cooling steps
carried out in one or more heat exchangers. In a preferred
embodiment of the present invention, less than about 10 percent of
the natural gas mechanical cooling duty of the initial,
intermediate, and/or final refrigeration cycles is provided by
core-in-kettle and/or spiral-wound heat exchangers, more preferably
less than about 5 percent of the natural gas mechanical cooling
duty of the initial, intermediate, and/or final refrigeration
cycles is provided by core-in-kettle and/or spiral-wound heat
exchangers, still more preferably less than 2 percent of the
natural gas mechanical cooling duty of the initial, intermediate,
and/or final refrigeration cycles is provided by core-in-kettle
and/or spiral-wound heat exchangers. Most preferably, none of the
natural gas-cooling heat exchangers employed in the initial,
intermediate, and final refrigeration cycles are core-in-kettle
heat exchangers and/or spiral-wound heat exchangers. Rather, it is
preferred that at least about 90 percent of the natural gas
mechanical cooling duty of the initial, intermediate, and/or final
refrigeration cycles is provided by plate-fin heat exchangers, more
preferably at least about 95 percent of the natural gas mechanical
cooling duty of the initial, intermediate, and/or final
refrigeration cycles is provided by plate-fin heat exchangers,
still more preferably at least 98 percent of the natural gas
mechanical cooling duty of the initial, intermediate, and/or final
refrigeration cycles is provided by plate-fin heat exchangers. Most
preferably, all of the natural gas-cooling heat exchangers employed
in the initial, intermediate, and final refrigeration cycles are
plate-fin heat exchangers. It is particularly preferred for the
plate-fin heat exchangers to be brazed aluminum plate-fin heat
exchangers.
[0022] As used herein, the term "natural gas mechanical cooling
duty" denotes a responsibility for extracting heat from natural gas
via indirect heat exchange, expressed in terms of energy per units
of time (e.g., BTU/hr). As used herein, the term "core-in-kettle
heat exchanger" denotes a heat exchange device comprising an outer
vessel shell and an inner core disposed in the vessel shell. A
core-in-kettle heat exchanger facilitates indirect heat transfer
between a first fluid contained in the vessel shell and a second
fluid flowing through the core while the core is at least partly
submerged in the first fluid. As used here, the term "spiral-wound
heat exchanger" denotes a heat exchange device comprising an outer
vessel shell and an inner core of wound tubes disposed in the
shell. As used herein, the term "plate-fin heat exchanger" denotes
a device that defines a plurality of distinct fluid passageways
separated by plates. A plate-fin heat exchanger facilitates
indirect heat transfer between a first fluid flowing through a
first group of fluid passageways and a second fluid flowing through
a second group of fluid passageways. Heat is transferred between
the first and second fluids via heat flux through the plates. Thus,
plate-fin heat exchangers do not require the use of large
containment vessels because the first and second fluids are
contained in the fluid passageways during heat transfer. As used
herein, the term "brazed aluminum plate-fin heat exchanger" denotes
a plate-fin heat exchanger constructed of multiple aluminum plates
brazed to one another.
[0023] In a preferred embodiment of the present invention, the
natural gas stream is delivered to the initial refrigeration cycle
at an elevated pressure or is compressed to an elevated pressure,
that being a pressure greater than about 500 psia, preferably about
500 to about 900 psia, still more preferably about 550 to about 675
psia, still yet more preferably about 575 to about 650 psia, and
most preferably about 600 psia. The stream temperature is typically
near ambient to slightly above ambient. A representative
temperature range being about 60.degree. F. to about 120.degree.
F.
[0024] Generally, the natural gas feed stream will contain such
quantities of C.sub.2+ components so as to result in the formation
of a C.sub.2+ rich liquid in one or more of the cooling stages of
the initial and/or intermediate refrigeration cycles. This liquid
is removed via gas/liquid separation means, preferably one or more
conventional gas/liquid separators. Generally, the sequential
cooling of the natural gas in each stage of the initial and/or
intermediate refrigeration cycles is controlled so as to remove as
much as possible of the C.sub.2 and higher molecular weight
hydrocarbons from the gas to produce a first gas stream
predominating in methane and a second liquid stream containing
significant amounts of ethane and heavier components. An effective
number of gas/liquid separation means are located at strategic
locations downstream of the cooling stages for the removal of
liquids streams rich in C.sub.2+ components. The exact locations
and number of gas/liquid separation means will be dependent on a
number of operating parameters, such as the C.sub.2+ composition of
the natural gas feed stream, the desired BTU content of the final
product, the value of the C.sub.2+ components for other
applications, and other factors routinely considered by those
skilled in the art of LNG plant and gas plant operation. The
C.sub.2 hydrocarbon stream or streams may be demethanized via a
single stage flash or a fractionation column. In the former case,
the methane-rich stream can be repressurized and recycled or can be
used as fuel gas. In the latter case, the methane-rich stream can
be directly returned at pressure to the liquefaction process. The
C.sub.2+ hydrocarbon stream or streams or the demethanized C.sub.2+
hydrocarbon stream may be used as fuel or may be further processed
such as by fractionation in one or more fractionation zones to
produce individual streams rich in specific chemical constituents
(e.g., C.sub.2, C.sub.3, C.sub.4 and C.sub.5+)
[0025] In the last cooling stage of the intermediate refrigeration
cycle, the processed natural gas stream, which is predominantly
methane (typically greater than 95 mole percent methane and more
typically greater than 97 mole percent methane), is condensed
(i.e., liquefied) in major portion, preferably in its entirety. The
cooled and condensed natural gas stream exiting the intermediate
refrigeration cycle is then further cooled in the final
refrigeration cycle via indirect heat exchange with the final
refrigerant. In a preferred embodiment of the present invention,
the final refrigeration cycle is an open methane refrigeration
cycle employing a predominantly-methane refrigerant that originates
from the natural gas feed stream.
[0026] The liquefied gas entering the final refrigeration cycle
preferably has a pressure of at least about 250 psia, more
preferably at least about 400 psia, and most preferably in the
range of from 500 to 800 psia. It is preferred that the expansion
section of the final refrigeration cycle is operable to reduce the
pressure of the liquefied gas stream by at least about 100 psi,
more preferably at least about 250 psi, and most preferably at
least 400 psi. The pressure reduction in the expansion section of
the final refrigeration cycle is preferably accomplished via a
plurality of sequential expansion steps carried out in a plurality
of expansion devices. Each expansion device can be a Joule-Thomson
expansion valve or a hydraulic expander. As used herein, the term
"hydraulic expander" is not limited to an expander which receives
and produces liquid streams but is inclusive of expanders which
receive a predominantly liquid-phase stream and produce a two-phase
(gas/liquid) stream. When a hydraulic expander is employed and
properly operated, the greater efficiencies associated with the
recovery of power, a greater reduction in stream temperature, and
the production of less vapor during the expansion step will
frequently be cost-effective even in light of increased capital and
operating costs associated with the expander. The pressure of the
liquid product entering the final refrigeration cycle is preferably
reduced to near atmospheric pressure so that the final LNG product
has a near-atmospheric pressure and a temperature of -240.degree.
F. to -260.degree. F.
[0027] One embodiment of the present invention provides a final
refrigeration cycle having a reduced number of process vessels
compare to similar refrigeration cycles employing multi-step
expansion cooling of the liquefied gas stream. In particular, in
one embodiment of the present invention, the expansion section of
the final refrigeration cycle employs less than three vapor/liquid
separation vessels (e.g., flash drums), most preferably less than
two vapor/liquid separation vessels. FIG. 1 illustrates one
configuration of a final refrigeration cycle that reduces the
number of process vessels (e.g., flash drums) relative to similar
conventional expansion-type refrigeration cycles.
[0028] The flow schematic and apparatus set forth in FIG. 1
represents a preferred embodiment of the invention employed in an
open-cycle cascaded liquefaction process. Those skilled in the art
will also recognized that FIG. 1 is a schematic representation and,
therefore, many items of equipment that would be needed in a
commercial plant for successful operation have been omitted for the
sake of clarity. Such items might include, for example, compressor
controls, flow and level measurements and corresponding
controllers, temperature and pressure controls, pumps, motors,
filters, additional heat exchangers, valves, etc. These items would
be provided in accordance with standard engineering practice.
[0029] To facilitate an understanding of FIG. 1, items numbered 1
through 99 generally correspond to process vessels and equipment
directly associated with the liquefaction process. Items numbered
100 through 199 correspond to flow lines or conduits which contain
methane in major portion. Items numbered 200 through 299 correspond
to flow lines or conduits which contain the refrigerant ethylene or
optionally, ethane. Items numbered 300 through 399 correspond to
flow lines or conduits which contain the refrigerant propane.
[0030] Referring to FIG. 1, gaseous propane is compressed in
multistage compressor 18 driven by a gas turbine driver which is
not illustrated. The three stages of compression preferably exist
in a single unit although each stage of compression may be a
separate unit and the units mechanically coupled to be driven by a
single driver. Upon compression, the compressed propane is passed
through conduit 300 to cooler 16 where it is liquefied. A
representative pressure and temperature of this liquefied propane
stream exiting cooler 16 is about 100.degree. F. and about 190
psia. Although not illustrated in FIG. 1, it is preferable that a
separation vessel be located downstream of cooler 16 and upstream
of the high-stage propane brazed aluminum plate-fin heat exchanger
2, for the removal of residual light components from the liquefied
propane and to provide surge control for the system. The
refrigerant stream from this vessel or the stream from cooler 16,
as the case may be, is passed through conduit 302 to a high-stage
propane brazed aluminum plate-fin heat exchanger 2, wherein the
stream flows through core passages 10 and is cooled by indirect
heat exchange. The cooled or second propane refrigerant stream is
produced via conduit 303. This stream is then split via a splitting
or separation means (illustrated but not numbered) into two
portions, third and fourth propane refrigerant streams, and
produced via conduits 304 and 307. The third propane refrigerant
stream flows via conduit 304 to a pressure reduction means,
illustrated as expansion valve 14, wherein the pressure of the
liquefied propane is reduced, thereby evaporating or flashing a
portion thereof and producing a high-stage refrigeration stream.
This stream then flows through conduit 305 and through core
passages 12, wherein the stream flows countercurrent to the stream
in passage 10 and the yet to be described streams in passages 4, 6,
and 8 and wherein indirect heat exchange occurs. The resulting
stream, the high-stage propane recycle stream, is routed via
conduit 306 to the high-stage inlet port of propane compressor 18.
In the course of such routing, the stream will generally pass
through a suction scrubber.
[0031] Also fed to plate-fin heat exchanger 2 are the natural gas
stream via conduit 100, a gaseous ethylene stream via conduit 202,
and a methane-rich stream via conduit 152. These streams in flow
passages 6, 8, and 4 and the propane refrigerant stream in passage
10 flow countercurrent, more preferably counterflow, to the propane
stream in passage 12. Indirect heat exchange occurs between such
streams. The streams respectively flowing in passages 4, 6, and 8
are produced via conduits 154, 102, and 204. The stream in conduit
204 will be referred to as a first cooled ethylene stream.
[0032] The cooled natural gas stream in conduit 102, the first
cooled ethylene stream in conduit 204, and the fourth propane
refrigerant stream in conduit 307 respectively flow through
passages 22, 24, and 25 in brazed aluminum plate-fin heat exchanger
20 countercurrent, more preferably counterflow, to a yet to be
identified refrigeration stream thereby producing a further cooled
natural gas stream, a second cooled ethylene stream, and a fifth
propane refrigerant stream which are produced via conduits 110,
206, and 308. The fifth propane refrigerant stream is then split
via a splitting or separation means (illustrated but not numbered)
into two portions, the sixth and seventh propane refrigerant
streams, and respectively produced via conduits 309 and 312. The
sixth propane refrigerant stream flows via conduit 309 to a
pressure reduction means, illustrated as expansion valve 27. In
expansion valve 27, the pressure of the liquefied propane is
reduced, thereby evaporating or flashing a portion thereof and
producing a intermediate-stage propane refrigeration stream. This
stream then flows through conduit 310 and through core passage 26
wherein said stream flows countercurrent to the streams in passages
22, 24, and 25 and wherein indirect heat exchange occurs. The
resulting stream is produced as an intermediate-stage propane
recycle stream via conduit 311. This stream is returned to the
intermediate-stage inlet port of propane compressor 18, again
preferably after passing through a suction scrubber.
[0033] The further cooled natural gas stream and the second cooled
ethylene stream are respectively routed via conduits 110 and 206 to
respective passages 36 and 38 in brazed aluminum plate-fin heat
exchanger 34 wherein the natural gas stream is yet further cooled.
The natural gas and ethylene streams are produced from plate-fin
heat exchanger 34 via conduits 112 and 208, respectively.
[0034] The seventh propane refrigerant stream in conduit 312 is
connected to brazed aluminum plate-fin heat exchanger 28 wherein
the stream flows via passage 29 countercurrent, more preferably
counterflow, to and in indirect heat exchange with a low-stage
propane refrigerant flowing via passage 30 thereby producing an
eighth propane refrigerant stream via conduit 314. The eighth
propane refrigerant flows via conduit 314 to a pressure reduction
means, illustrated as expansion valve 32, wherein the pressure of
the liquefied propane is reduced thereby evaporating or flashing a
portion thereof and producing a two-phase refrigerant stream. The
expanded refrigerant stream is carried to brazed aluminum plate-fin
heat exchanger 34 where it is employed as a cooling agent in
passage 37. A low-stage propane refrigeration stream is removed
from heat exchanger 34 via conduit 318. This conduit is connected
to passage 30 in heat exchanger 28 wherein said stream flows
countercurrent and is in indirect heat exchange with the seventh
propane refrigerant stream in passage 29 thereby producing a
low-stage propane recycle stream. The low-stage propane recycle
stream is then returned to the low-stage inlet port of compressor
18, preferably after flow through a suction scrubber, via conduit
320. In compressor 18, the low-stage propane recycle stream is
compressed, combined with the intermediate-stage propane recycle
stream, and compressed to form a compressed intermediate-stage
recycle stream. This stream is then combined with the high-stage
propane recycle stream to form a combined high-stage propane
recycle stream which is compressed to form the compressed propane
refrigerant stream produced via conduit 300.
[0035] In one embodiment of the invention, the brazed aluminum
plate-fin heat exchangers 2, 20, 28, and 34 of the initial
(propane) refrigeration cycle are separate heat exchangers. In
another embodiment, the heat exchangers are combined into one or
more exchangers. Although resulting in a more complex heat
exchanger which possesses intermediate headers, combined approach
can offer advantages from a lay-out and cost perspective.
[0036] In the intermediate refrigeration cycle depicted in FIG. 1,
the natural gas stream is cooled via indirect heat exchange with an
ethylene refrigeration stream until it is substantially condensed.
As illustrated in FIG. 1, a low-stage ethylene recycle stream
delivered via conduit 232 is compressed in compressor 40 and the
resulting compressed low-stage ethylene recycle stream is
preferably removed from compressor 40 via conduit 234, cooled via
inter-stage cooler 71, returned to the compressor via conduit 236
and combined with a high-stage ethylene recycle stream delivered
via conduit 216 whereupon the combined stream is compressed to
thereby producing a compressed ethylene refrigerant stream via
conduit 200. A preferred pressure for this compressed ethylene
refrigerant stream is approximately 300 psia. Preferably, the two
compressor stages of compressor 40 are a single module although
they may each be a separate module and the modules mechanically
coupled to a common driver. The compressed ethylene refrigerant
stream is routed from the compressor 40 to the downstream cooler 72
via conduit 200. The product from cooler 72 flows via conduit 202
and is introduced, as previously discussed, to the initial
refrigeration cycle wherein the stream is further cooled and
liquefied via heat exchange passages 8, 24, and 38 and then
returned to the intermediate refrigeration cycle via conduit 208.
This stream in conduit 208 preferably flows to a separation vessel
41 which provides for the removal of residual light components from
the liquefied stream and which also provides surge volume for the
refrigeration system. A refrigerant stream, referred to herein with
regard to the intermediate refrigeration cycle as a first ethylene
refrigerant stream, is produced from vessel 41 via conduit 209.
[0037] The cooled natural gas stream produced from the initial
refrigeration cycle via conduit 112 is combined with a yet to be
described methane-rich stream provided via conduit 156. This
combined stream in conduit 114 and the first refrigerant ethylene
stream in conduit 209 are routed to a brazed aluminum plate
fin-heat exchanger 42 wherein these streams flow through core
passages 44 and 46 countercurrent, more preferably counterflow, to
and in indirect heat exchange with a yet to be described high-stage
ethylene refrigerant stream and optionally, a low-stage ethylene
refrigerant stream respectively flowing in passages 48 and 50. A
cooled stream referred to herein as a second ethylene refrigerant
stream is produced from passage 46 via conduit 210. This stream is
then split via a splitting or separation means (illustrated but not
numbered) into two portions, third and fourth ethylene refrigerant
streams, and produced via conduits 212 and 218. The third ethylene
refrigerant stream flows via conduit 212 to a pressure reduction
means, illustrated as expansion valve 52, wherein the pressure of
the liquefied ethylene is reduced thereby evaporating or flashing a
portion thereof and producing a high-stage ethylene refrigeration
stream. This stream then flows through conduit 214 and through core
passage 48 thereby producing a high-stage ethylene recycle stream
which is transported via conduit 216 to the high-stage inlet port
of compressor 40.
[0038] A further cooled natural gas stream is produced from passage
44 via conduit 116 and is optionally combined with a methane-rich
recycle stream delivered via conduit 158. The resulting stream is
routed via conduit 120 to a passage 59 of a brazed aluminum
plate-fin heat exchanger 58 wherein the stream is cooled and
liquefied in major portion and the resulting stream is produced via
conduit 122.
[0039] The fourth ethylene refrigerant stream is transported via
conduit 218 to a passage 54 in a brazed aluminum plate-fin heat
exchanger 53. The fourth ethylene refrigerant stream flows
countercurrent, more preferably counterflow, to and is in indirect
heat exchange with a low-stage ethylene refrigerant flowing via
passage 55 in heat exchanger 53, thereby producing a fifth ethylene
refrigerant stream via conduit 220. The fifth ethylene refrigerant
stream flows via conduit 220 to a pressure reduction means,
illustrated as expansion valve 56, wherein the pressure of the
liquefied ethylene is reduced, thereby evaporating or flashing a
portion thereof and producing a two-phase ethylene refrigerant
stream. The resulting two-phase ethylene refrigerant stream is
carried via conduit 226 to heat exchanger 58 wherein the stream is
employed as a cooling agent in passage 57. A low-stage ethylene
refrigeration stream is removed from heat exchanger 58 via conduit
228. Conduit 228 is connected to passage 55 in heat exchanger 53
wherein said stream flows countercurrent and is in indirect heat
exchange with the fluid in passage 54 thereby producing a low-stage
ethylene recycle stream. This stream is returned to the low-stage
inlet port of compressor 40 via conduit 232. Optionally, and as
depicted in FIG. 1, this stream may also flow to plate-fin heat
exchanger 42 via conduit 230 and through passage 50 wherein said
stream flows countercurrent, more preferably counterflow, to the
fluids in passages 44 and 46 and is further warmed prior to flow to
the compressor 40 via conduit 232.
[0040] In one embodiment of the invention, brazed aluminum
plate-fin heat exchangers 42, 53, and 58 of the intermediate
refrigeration cycle are separate heat exchangers. In another
embodiment, the heat exchangers are combined into a single
exchanger.
[0041] The liquefied natural gas stream produced from plate-fin
heat exchanger 58 via conduit 122 is generally at a temperature of
about -125.degree. F. and a pressure of about 600 psi. The
liquefied stream in conduit 122 is introduced into the final
refrigeration cycle where it undergoes cooling by indirect heat
exchange with a methane refrigerant and by expansion. The stream in
conduit 122 is initially cooled in a main methane economizer 74 via
indirect heat exchange with methane refrigerant streams in passages
82, 95, and 96. In a preferred embodiment of the present invention,
the methane refrigerant employed in the final refrigeration cycle
is derived from the processed natural gas stream, thereby making
the final refrigeration cycle an open methane refrigeration cycle.
Main methane economizer 74 is preferably a plate-fin heat
exchanger, most preferably a brazed aluminum plate-fin heat
exchanger. The liquefied natural gas stream introduced into main
methane economizer 74 via conduit 122 is cooled in passage 76 and
then exits main methane economizer 74 via conduit 124. The cooled
stream in conduit 124 is subsequently divided into a first
refrigerant portion carried in conduit 125 and a first product
portion carried in conduit 126. The first refrigerant portion in
conduit 125 is transported to an expansion means (illustrated as
expansion valve 78), wherein the stream is reduced in pressure to
thereby produce a first expanded refrigerant portion in conduit
127. The first expanded refrigerant portion in conduit 127 is then
introduced into passage 82 of main methane economizer 74 wherein it
is employed as a refrigerant to cool the natural gas stream in
passage 76. The warmed first refrigerant stream exits passage 82
and methane economizer 74 via conduit 128, and is introduced into
the high-stage inlet port of methane compressor 83.
[0042] The first product portion in conduit 126 is carried to a
second methane economizer 87 for further cooling. The second
methane economizer 87 is preferably a plate-fin heat exchanger,
most preferably a brazed aluminum plate-fin heat exchanger. In
second methane economizer 87, the first product portion is cooled
as it passes through passage 88 and indirectly exchanges heat with
the refrigerant streams passing through passages 89 and 90,
described in more detail below. A second cooled natural gas stream
is produced from second methane economizer 87 via conduit 129. The
second cooled natural gas stream in conduit 129 is subsequently
divided into a second refrigerant portion carried in conduit 130
and a second product portion carried in conduit 131. The second
refrigerant portion carried in conduit 130 is subsequently expanded
in expansion valve 91 to thereby produce a second expanded
refrigerant portion. The second product portion in conduit 131 is
subsequently expanded in expansion valve 92 to thereby produce a
two-phase second stream that is subsequently carried to a phase
separator 93 via conduit 132. The second expanded refrigerant
portion expander 91 is transported via conduit 133 to second
methane economizer 87 wherein the second expanded refrigeration
portion is employed as a refrigerant in passage 89 to cool the
stream flowing in passage 88. After being employed as a cooling
agent in passage 89, the warmed second refrigerant portion is
removed from second methane economizer 87 via conduit 134 and
subsequently introduced into a passage 95 of main methane
economizer 74 wherein the warmed second refrigerant portion is used
to cool the stream in passage 76. The further warmed second
refrigerant portion exits methane economizer 74 via conduit 135 and
is subsequently introduced into the intermediate-stage inlet port
of methane compressor 83.
[0043] The two-phase in conduit 132 is separated in vapor/liquid
separator 93 to thereby produce a gaseous third refrigerant portion
via conduit 136 and a liquid third product portion via conduit 142.
The gaseous third refrigerant stream in conduit 136 is combined
with the compressed stream in conduit 138, described in further
detail below. The resulting combined stream flows via conduit 139
to second methane economizer 87 wherein the combined stream is
employed as a refrigerant in passage 90 to cool the stream in
passage 88. The warmed third portion exits passage 90 of second
methane economizer 87 via conduit 140 and is carried to passage 96
of main methane economizer 74 wherein the refrigerant stream is
used to cool the stream in passage 76. The further warmed third
refrigerant portion exits passage 96 and main methane economizer 74
via conduit 141 and is passed to the low-stage inlet port of
methane compressor 83.
[0044] The liquid third product portion that exits separator 93 via
conduit 142 is expanded in expansion valve 94 to thereby produce a
two-phase expanded third product stream which is carried to LNG
storage tank 99 via conduit 143. The vapor portion of the stream
introduced in to LNG storage tank 99 and any boil-off vapors
generated in tank 99 are removed from tank 99 via conduit 144. This
vapor stream in conduit 144 is compressed in compressor 96 to
produce the compressed gas stream in conduit 138 that is
subsequently combined with the separated vapor stream in conduit
136 before being employed as a refrigerant in the second methane
economizer 87 and the main methane economizer 74. The LNG in tank
99 can be stored and subsequently transported to a distant market
where it is gasified for use as an energy source.
[0045] As shown in FIG. 1, the three stages of compression provided
by methane compressor 83 are preferably contained in a single unit.
However, each compression stage may exist as a separate unit where
the units are mechanically coupled together to be driven by a
single driver. The compressed gas from the low-stage section of
compressor 83 preferably passes through an inter-stage cooler 85
and is combined with the intermediate pressure gas in conduit 140
prior to the second-stage of compression. The compressed gas from
the intermediate stage of compressor 83 is preferably passed
through an inter-stage cooler 84 and is combined with the high
pressure gas in conduit 140 prior to the third stage of
compression. The compressed gas is discharged from the high-stage
methane compressor through conduit 150, is cooled in cooler 86 and
is routed to the high-stage propane chiller 2 via conduit 152. The
methane-rich stream exiting chiller 2 via conduit 154 is fed to
main methane economizer 74 wherein the stream is cooled via
indirect heat exchange with one or more of the streams in passages
82, 94, and/or 96. In one embodiment and as illustrated in FIG. 1,
the stream delivered via conduit 154 is cooled in the main methane
economizer 74 via indirect heat exchange means 97, a portion
removed via conduit 156 and the remaining stream further cooled via
indirect heat exchange means 98 and produced via conduit 158. This
is a preferred embodiment. In this split stream embodiment, a
portion of the compressed methane recycle stream delivered via
conduit 156 is combined with the natural gas stream via conduit 112
immediately upstream of the intermediate refrigeration cycle and
the remaining portion delivered via conduit 158 combined with the
stream in conduit 116 immediately upstream of brazed aluminum
plate-fin heat exchanger 58 wherein the majority of liquefaction of
the natural gas stream occurs. In a simpler embodiment (i.e., less
preferred from a process efficiency perspective), the methane
recycle stream is cooled in its entirety in the main methane
economizer 74 and combined via conduit 158 with the natural gas
stream in conduit 112 immediately upstream of the second cycle.
[0046] With regard to the compressor/driver units employed in the
process, FIG. 1 depicts individual compressor/driver units (i.e., a
single compression train) for the propane, ethylene and open-cycle
methane compression stages. However in a preferred embodiment for
any cascaded process, process reliability can be improved
significantly by employing a multiple compression train comprising
two or more compressor/driver combinations in parallel in lieu of
the depicted single compressor/driver units. In the event that a
compressor/driver unit becomes unavailable, the process can still
be operated at a reduced capacity.
[0047] In one embodiment of the present invention, the LNG
production system of FIG. 1 is simulated on a computer using
conventional process simulation software. Examples of suitable
simulation software include HYSYS.TM. from Hyprotech, Aspen
Plus.RTM. from Aspen Technology Inc., and PRO\II.RTM. from
Simulation Sciences Inc.
[0048] While specific cryogenic methods, materials, items of
equipment and control instruments are referred to herein, it is to
be understood that such specific recitals are not to be considered
limiting but are included by way of illustration and to set forth
the best mode in accordance with the present invention.
* * * * *