U.S. patent application number 10/869598 was filed with the patent office on 2005-12-22 for lng system with enhanced turboexpander configuration.
Invention is credited to Cook, Dennis, Eaton, Anthony P., Yao, Jame.
Application Number | 20050279132 10/869598 |
Document ID | / |
Family ID | 35479170 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050279132 |
Kind Code |
A1 |
Eaton, Anthony P. ; et
al. |
December 22, 2005 |
LNG system with enhanced turboexpander configuration
Abstract
Natural gas liquefaction system employing a turboexpander to
convert excess pressure within a predominantly methane stream into
work useable in another location within the liquefaction system.
Primarily, the turboexpander is used to compress a refrigerant used
in at least one of the refrigeration cycles within the liquefaction
system.
Inventors: |
Eaton, Anthony P.; (Sugar
Land, TX) ; Yao, Jame; (Sugar Land, TX) ;
Cook, Dennis; (Kingwood, TX) |
Correspondence
Address: |
ConocoPhilips Company - I.P. Legal
PO BOX 2443
BARTLESVILLE
OK
74005
US
|
Family ID: |
35479170 |
Appl. No.: |
10/869598 |
Filed: |
June 16, 2004 |
Current U.S.
Class: |
62/613 ; 62/611;
62/620 |
Current CPC
Class: |
F25J 3/0233 20130101;
F25J 1/0045 20130101; F25J 1/0294 20130101; F25J 2270/60 20130101;
F25J 3/0209 20130101; F25J 2245/02 20130101; F25J 1/021 20130101;
F25J 1/0052 20130101; F25J 2200/70 20130101; F25J 1/0085 20130101;
F25J 2220/64 20130101; F25J 1/0022 20130101; F25J 1/0285 20130101;
F25J 2270/12 20130101; F25J 2210/06 20130101; F25J 1/004 20130101;
F25J 1/0265 20130101; F25J 3/0238 20130101; F25J 2240/02 20130101;
F25J 2270/02 20130101; F25J 1/0035 20130101; F25J 2205/02 20130101;
F25J 2205/04 20130101; F25J 2230/20 20130101 |
Class at
Publication: |
062/613 ;
062/611; 062/620 |
International
Class: |
F25J 001/00; F25J
003/00 |
Claims
What is claimed is:
1. A method of liquefying a natural gas stream, said method
comprising the steps of: (a) cooling a pressurized, predominantly
methane stream in a first refrigerant chiller; (b) separating at
least a portion of the cooled predominantly methane stream into a
primarily liquid stream and a primarily gaseous stream; (c) passing
at least a portion of the primarily gaseous stream through a
turboexpander to thereby generate work and provide a
reduced-pressure predominantly methane stream; and (d) directing at
least a portion of the reduced-pressure predominantly methane
stream to a heavies removal column for removal of heavy hydrocarbon
components.
2. The method according to claim 1; and (e) passing at least a
portion of the primarily liquid stream to the heavies removal
column.
3. The method according to claim 1, said heavies removal column
being a stripping column having internal packing.
4. The method according to claim 3; and (f) using a portion of the
cooled predominantly methane stream as a stripping gas in the
heavies removal column.
5. The method according to claim 4, said at least a portion of the
reduced-pressure predominantly methane stream entering the heavies
removal column at a location above the internal packing, said
stripping gas entering the heavies removal column at a location
below the internal packing.
6. The method according to claim 5; and (g) introducing at least a
portion of the primarily liquid stream into the heavies removal
column at a location below the internal packing.
7. The method according to claim 6; and (h) separating in the
heavies removal column the at least a portion of the primarily
liquid stream, the stripping gas, and the at least a portion of the
reduced-pressure predominantly methane stream into a heavies
depleted stream and a heavies rich stream.
8. The method according to claim 7; and (i) cooling said heavies
depleted stream in an ethylene refrigerant chiller.
9. The method according to claim 7; and (j) directing said heavies
depleted stream to an open-methane refrigeration cycle.
10. The method according to claim 9, said open-methane
refrigeration cycle comprising a plurality of expansion-cooling
steps.
11. The method according to claim 1; and (k) measuring the pressure
of the primarily gaseous stream prior to step (c), and based on the
pressure measurement, selectively opening or closing a by-pass
valve capable of diverting at least part of the primarily gaseous
stream around the turboexpander.
12. The method according to claim 1, said first refrigerant chiller
comprising a propane refrigerant chiller.
13. The method according to claim 1, said first refrigerant chiller
comprising an ethylene refrigerant chiller.
14. The method according to claim 1; and (l) using at least a
portion of the work generated by the turboexpander to power a
compressor.
15. The method according to claim 14; and (m) directing the
reduced-pressure predominantly methane stream to a second
refrigerant chiller prior to step (d).
16. The method according to claim 15, said compressor being used to
compress a refrigerant employed in the operation of at least one of
said first or second refrigerant chillers.
17. The method according to claim 15, said second refrigerant
chiller comprising an ethylene refrigerant chiller.
18. The method according to claim 1, said predominantly methane
stream having a pressure of at least about 638 psia immediately
prior to step (b).
19. The method according to claim 18, said reduced-pressure
predominantly methane stream having a pressure that is less than
about 95% of the pressure of the predominantly methane stream
immediately prior to step (b).
20. The method according to claim 1, steps (a), (b), (c), and (d)
being carried out in a cascade-type LNG facility having at least
three sequential cooling cycles, each employing a different
refrigerant.
21. The method according to claim 20, said cascade-type LNG
facility employing an open-methane refrigeration cycle.
22. The method according to claim 1; and (n) vaporizing liquefied
natural gas produced via steps (a)-(d).
23. A computer simulation process comprising the step of using a
computer to simulate the method of claim 1.
24. A liquefied natural gas product produced by the method of claim
1.
25. A method of liquefying a natural gas stream, said method
comprising the steps of: (a) cooling a pressurized, predominantly
methane stream in a first refrigerant chiller; (b) separating at
least a portion of the cooled predominantly methane stream into a
primarily liquid stream and a primarily gaseous stream; (c) passing
at least a portion of the primarily gaseous stream through a
turboexpander to thereby generate work and provide a
reduced-pressure predominantly methane stream; (d) directing at
least a portion of the reduced-pressure predominantly methane
stream to a second refrigerant chiller to provide a cooled,
reduced-pressure predominantly methane stream; (e) passing at least
a portion of the cooled, reduced-pressure predominantly methane
stream to a heavies removal column for removal of heavy hydrocarbon
components; (e) passing at least a portion of the primarily liquid
stream to the heavies removal column; and (f) using a portion of
the cooled predominantly methane stream as a stripping gas in the
heavies removal column.
26. The method according to claim 25, said heavies removal column
being a stripping column having internal packing.
27. The method according to claim 26; and (g) introducing at least
a portion of the primarily liquid stream into the heavies removal
column at a location below the internal packing.
28. The method according to claim 25; and (h) measuring the
pressure of the primarily gaseous stream prior to step (c), and
based on the pressure measurement, selectively opening or closing a
by-pass valve capable of diverting at least part of the primarily
gaseous stream around the turboexpander.
29. The method according to claim 25, said first refrigerant
chiller comprising a propane refrigerant chiller.
30. The method of according to claim 25, said first refrigerant
chiller comprising an ethylene refrigerant chiller.
31. The method according to claim 25, said second refrigerant
chiller comprising an ethylene refrigerant chiller.
32. The method according to claim 25, (i) using at least a portion
of the work generated by the turboexpander to power a
compressor.
33. The method according to claim 32, said compressor being used to
compress a refrigerant employed in the operation of at least one of
the first or second refrigerant chillers.
34. The method according to claim 25, said predominantly methane
stream having a pressure of at least about 638 psia immediately
prior to step (b).
35. The method according to claim 34, said reduced-pressure
predominantly methane stream having a pressure that is less than
about 95% of the pressure of the predominantly methane stream
immediately prior to step (b).
36. The method according to claim 25; and (j) separating in the
heavies removal column the at least a portion of the primarily
liquid stream, the stripping gas, and the at least a portion of the
reduced-pressure predominantly methane stream into a heavies
depleted stream and a heavies rich stream.
37. The method according to claim 36; and (k) cooling said heavies
depleted stream in an ethylene refrigerant chiller.
38. The method according to claim 36; and (l) directing said
heavies depleted stream to an open-methane refrigeration cycle.
39. The method according to claim 38, said open-methane
refrigeration cycle comprising a plurality of expansion-cooling
steps.
40. The method according to claim 25, steps (a)-(f) being carried
out in a cascade-type LNG facility having at least three sequential
cooling cycles, each employing a different refrigerant.
41. The method according to claim 40, said cascade-type LNG
facility employing an open-methane refrigeration cycle.
42. The method according to claim 25; and (m) vaporizing liquefied
natural gas produced via steps (a)-(f).
43. A computer simulation process comprising the step of using a
computer to simulate the method of claim 25.
44. A liquefied natural gas product produced by the method of claim
25.
45. An apparatus for liquefying a predominantly methane stream,
said apparatus comprising: (a) a first refrigerant chiller operable
to cool at least a portion of the predominantly methane stream; (b)
a separation vessel located downstream of said first refrigerant
chiller for separating at least a portion of the cooled
predominantly methane stream into a primarily gaseous stream and a
primarily liquid stream; (c) a turboexpander located downstream of
said separation vessel and capable of generating work as the
primarily gaseous stream passes therethrough thereby forming a
reduced-pressure predominantly methane stream; and (d) a heavies
removal column located downstream of said separation vessel for
receiving at least a portion of the primarily liquid stream.
46. The apparatus according to claim 45; and (e) a second
refrigerant chiller located downstream of said turboexpander
operable to cool at least a portion of the reduced-pressure
predominantly methane stream.
47. The apparatus according to claim 46, said second refrigerant
chiller comprising an ethylene refrigerant chiller
48. The apparatus according to claim 46; and (f) a compressor
operable to compress the refrigerant used in at least one of said
first or second refrigerant chillers, said compressor being powered
at least in part by the work produced from said turboexpander.
49. The apparatus according to claim 45, said heavies removal
column comprising a stripping column having internal packing.
50. The apparatus according to claim 45; and (g) a bypass line
including a bypass valve capable of diverting at least a portion of
the primarily gaseous stream around said turboexpander.
51. The apparatus according to claim 45, said first refrigerant
chiller comprising a propane refrigerant chiller.
52. The apparatus according to claim 45, said first refrigerant
chiller comprising an ethylene refrigerant chiller.
53. The apparatus according to claim 45; and (h) an open-methane
refrigeration cycle located downstream from said heavies removal
column.
54. An apparatus for liquefying a predominantly methane stream,
said apparatus comprising: (a) a first refrigerant chiller operable
to cool at least a portion of the predominantly methane stream; (b)
a separation vessel located downstream of said first refrigerant
chiller for separating at least a portion of the cooled
predominantly methane stream into a primarily gaseous stream and a
primarily liquid stream; (c) a turboexpander located downstream of
said separation vessel and capable of generating work as the
primarily gaseous stream passes therethrough thereby forming a
reduced-pressure predominantly methane stream; (d) a heavies
removal column located downstream of said separation vessel for
receiving at least a portion of the primarily liquid stream; (e) a
second refrigerant chiller located downstream of said turboexpander
operable to cool at least a portion of the reduced-pressure
predominantly methane stream; and (f) a compressor operable to
compress the refrigerant used in at least one of said first or
second refrigerant chillers, said compressor being powered at least
in part by the work produced from said turboexpander.
55. The apparatus according to claim 54, said heavies removal
column comprising a stripping column having internal packing.
56. The apparatus according to claim 54; and (g) a by-pass line
including a by-pass valve capable of diverting at least a portion
of the primarily gaseous stream around said turboexpander.
57. The apparatus according to claim 54, said first refrigerant
chiller comprising a propane refrigerant chiller.
58. The apparatus according to claim 54, said first refrigerant
chiller comprising an ethylene refrigerant chiller.
59. The apparatus according to claim 54; and (h) an ethylene
refrigerant chiller located downstream from said heavies removal
column.
60. The apparatus according to claim 54; and (i) an open-methane
refrigeration cycle located downstream from said heavies removal
column.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method and apparatus for
liquefying natural gas. In another aspect, the invention concerns
an improved liquefied natural gas (LNG) facility employing a
turboexpander for converting excess pressure in a predominantly
methane stream into work useful in other areas of the process.
[0003] 2. Description of the Prior Art
[0004] The cryogenic liquefaction of natural gas is routinely
practiced as a means of converting natural gas into a more
convenient form for transportation and storage. Such liquefaction
reduces the volume of the natural gas by about 600-fold and results
in a product which can be stored and transported at near
atmospheric pressure.
[0005] Natural gas is frequently transported by pipeline from the
supply source to a distant market. It is desirable to operate the
pipeline under a substantially constant and high load factor but
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 or the valleys when supply exceeds
demand, it is desirable to store the excess gas in such a manner
that it can be delivered when demand exceeds supply. Such practice
allows future demand peaks 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.
[0006] The liquefaction of natural gas is of even greater
importance when transporting gas from a supply source which is
separated by great distances from the candidate market and a
pipeline either is not available or is impractical. 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. Such
pressurization requires the use of more expensive storage
containers.
[0007] 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 the liquefied natural gas (LNG) 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
indirect heat exchange with one or more refrigerants such as
propane, propylene, ethane, ethylene, methane, nitrogen, carbon
dioxide, or combinations of the preceding refrigerants (e.g., mixed
refrigerant systems). A liquefaction methodology which is
particularly applicable to the current invention employs an open
methane cycle for the final refrigeration cycle wherein a
pressurized LNG-bearing stream is flashed and the flash vapors
(i.e., the flash gas stream(s)) are subsequently employed as
cooling agents, recompressed, cooled, combined with the processed
natural gas feed stream and liquefied thereby producing the
pressurized LNG-bearing stream.
OBJECTS AND SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide a novel
natural gas liquefaction system that employs a turboexpander to
utilize excess pressure in a predominantly methane stream to
generate work to be used elsewhere in the system.
[0009] A further object of the invention is to provide a process of
liquefying a natural gas stream that utilizes a turboexpander to
power a refrigerant compressor employed in a closed refrigeration
cycle of the process.
[0010] It should be understood that the above objects are exemplary
and need not all be accomplished by the invention claimed herein.
Other objects and advantages of the invention will be apparent from
the written description and drawings.
[0011] Accordingly, one aspect of the present invention concerns a
method of liquefying a natural gas stream comprising the steps of:
(a) cooling a pressurized, predominantly methane stream in a first
refrigerant chiller; (b) separating at least a portion of the
cooled predominantly methane stream into a primarily liquid stream
and a primarily gaseous stream; (c) passing at least a portion of
the primarily gaseous stream through a turboexpander to thereby
generate work and provide a reduced-pressure predominantly methane
stream; and (d) directing at least a portion of the
reduced-pressure predominantly methane stream to a heavies removal
column for removal of heavy hydrocarbon components.
[0012] Another aspect of the present invention concerns a method of
liquefying a natural gas stream comprising the steps of: (a)
cooling a pressurized, predominantly methane stream in a first
refrigerant chiller; (b) separating at least a portion of the
cooled predominantly methane stream into a primarily liquid stream
and a primarily gaseous stream; (c) passing at least a portion of
the primarily gaseous stream through a turboexpander to thereby
generate work and provide a reduced-pressure predominantly methane
stream; (d) directing at least a portion of the reduced-pressure
predominantly methane stream to a second refrigerant chiller to
provide a cooled, reduced-pressure predominantly methane stream;
(e) passing at least a portion of the cooled, reduced-pressure
predominantly methane stream to a heavies removal column for
removal of heavy hydrocarbon components; (e) passing at least a
portion of the primarily liquid stream to the heavies removal
column; and (f) using a portion of the cooled predominantly methane
stream as a stripping gas in the heavies removal column.
[0013] A further aspect of the present invention concerns an
apparatus for liquefying a predominantly methane stream comprising:
(a) a first refrigerant chiller operable to cool at least a portion
of the predominantly methane stream; (b) a separation vessel
located downstream of said first refrigerant chiller for separating
at least a portion of the cooled predominantly methane stream into
a primarily gaseous stream and a primarily liquid stream; (c) a
turboexpander located downstream of said separation vessel and
capable of generating work as the primarily gaseous stream passes
therethrough thereby forming a reduced-pressure predominantly
methane stream; and (d) a heavies removal column located downstream
of said separation vessel for receiving at least a portion of the
primarily liquid stream.
[0014] Still another aspect of the present invention concerns an
apparatus for liquefying a predominantly methane stream comprising:
(a) a first refrigerant chiller operable to cool at least a portion
of the predominantly methane stream; (b) a separation vessel
located downstream of said first refrigerant chiller for separating
at least a portion of the cooled predominantly methane stream into
a primarily gaseous stream and a primarily liquid stream; (c) a
turboexpander located downstream of said separation vessel and
capable of generating work as the primarily gaseous stream passes
therethrough thereby forming a reduced-pressure predominantly
methane stream; (d) a heavies removal column located downstream of
said separation vessel for receiving at least a portion of the
primarily liquid stream; (e) a second refrigerant chiller located
downstream of said turboexpander operable to cool at least a
portion of the reduced-pressure predominantly methane stream; and
(f) a compressor operable to compress the refrigerant used in at
least one of said first or second refrigerant chillers, said
compressor being powered at least in part by the work produced from
said turboexpander.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0015] A preferred embodiment of the present invention is described
in detail below with reference to the attached drawing figures,
wherein:
[0016] FIG. 1 is a simplified flow diagram of a cascaded
refrigeration process for LNG production which employs a
turboexpander to reduce the pressure in a methane rich stream
exiting a low-stage propane refrigerant chiller/condenser; and
[0017] FIG. 2 is a simplified flow diagram of a cascaded
refrigeration process for LNG production which employs a
turboexpander to reduce the pressure in a methane rich stream
exiting a high-stage ethylene refrigerant chiller.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0018] A cascaded refrigeration process uses one or more
refrigerants for transferring heat energy from the natural gas
stream to the refrigerant and ultimately transferring said heat
energy to the environment. In essence, the overall refrigeration
system functions as a heat pump by removing heat energy from the
natural gas stream as the stream is progressively cooled to lower
and lower temperatures. The design of a cascaded refrigeration
process involves a balancing of thermodynamic efficiencies and
capital costs. In heat transfer processes, thermodynamic
irreversibilities are reduced as the temperature gradients between
heating and cooling fluids become smaller, but obtaining such small
temperature gradients generally requires significant increases in
the amount of heat transfer area, major modifications to various
process equipment, and the proper selection of flow rates through
such equipment so as to ensure that both flow rates and approach
and outlet temperatures are compatible with the required
heating/cooling duty.
[0019] As used herein, the term "open-cycle cascaded refrigeration
process" refers to a cascaded refrigeration process comprising at
least one closed refrigeration cycle and one open refrigeration
cycle where the boiling point of the refrigerant/cooling agent
employed in the open cycle is less than the boiling point of the
refrigerating agent or agents employed in the closed cycle(s) and a
portion of the cooling duty to condense the compressed open-cycle
refrigerant/cooling agent is provided by one or more of the closed
cycles. In the current invention, a predominately methane stream is
employed as the refrigerant/cooling agent in the open cycle. This
predominantly methane stream originates from the processed natural
gas feed stream and can include the compressed open methane cycle
gas streams. As used herein, the terms "predominantly",
"primarily", "principally", and "in major portion", when used to
describe the presence of a particular component of a fluid stream,
shall mean that the fluid stream comprises at least 50 mole percent
of the stated component. For example, a "predominantly" 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 comprising at least 50 mole percent
methane.
[0020] As used herein, "refrigerant chiller" refers to a device
that cools a feed stream via indirect heat exchange with a
refrigerant. "Propane refrigerant chiller" refers to a refrigerant
chiller that employs a predominantly propane refrigerant or a
refrigerant having a boiling point within 20.degree. C. of propane.
"Ethylene refrigerant chiller" refers to a refrigerant chiller that
employs a predominantly ethylene refrigerant or a refrigerant
having a boiling point within 20.degree. C. of ethylene.
[0021] One of the most efficient and effective means of liquefying
natural gas is via an optimized cascade-type operation in
combination with expansion-type cooling. Such a liquefaction
process involves the cascade-type cooling of a natural gas stream
at an elevated pressure, (e.g., about 650 psia) by sequentially
cooling the gas stream via passage through a multistage propane
cycle, a multistage ethane or ethylene cycle, and an open-end
methane cycle which utilizes a portion of the feed gas as a source
of methane and which includes therein a multistage expansion cycle
to further cool the same and reduce the pressure to
near-atmospheric pressure. In the sequence of cooling cycles, the
refrigerant having the highest boiling point is utilized first
followed by a refrigerant having an intermediate boiling point and
finally by a refrigerant having the lowest boiling point. 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.
[0022] Various pretreatment steps provide a means for removing
certain undesirable components, such as acid gases, mercaptan,
mercury, and moisture from the natural gas feed stream delivered to
the LNG facility. The composition of this gas stream may vary
significantly. As used herein, a natural gas stream is any stream
principally comprised of methane which originates in major portion
from a natural gas feed stream, such feed stream for example
containing at least 85 mole percent methane, with the balance being
ethane, higher hydrocarbons, nitrogen, carbon dioxide, and a minor
amount of other contaminants such as mercury, hydrogen sulfide, and
mercaptan. The pretreatment steps may be separate steps located
either upstream of the cooling cycles or located downstream of one
of the early stages of cooling in the initial cycle. The following
is a non-inclusive listing of some of the available means which are
readily known to one skilled in the art. Acid gases and to a lesser
extent mercaptan are routinely removed via a chemical reaction
process employing an aqueous amine-bearing solution. This treatment
step is generally performed upstream of the cooling stages in the
initial cycle. A major portion of the water is routinely removed as
a liquid via two-phase gas-liquid separation following gas
compression and cooling upstream of the initial cooling cycle and
also downstream of the first cooling stage in the initial cooling
cycle. Mercury is routinely removed via mercury sorbent beds.
Residual amounts of water and acid gases are routinely removed via
the use of properly selected sorbent beds such as regenerable
molecular sieves.
[0023] The pretreated natural gas feed stream is generally
delivered to the liquefaction process at an elevated pressure or is
compressed to an elevated pressure generally greater than 500 psia,
preferably about 500 psia to about 3000 psia, still more preferably
about 500 psia to about 1000 psia, still yet more preferably about
600 psia to about 800 psia. The feed stream temperature is
typically near ambient to slightly above ambient. A representative
temperature range being 60.degree. F. to 150.degree. F.
[0024] As previously noted, the natural gas feed stream is cooled
in a plurality of multistage cycles or steps (preferably three) by
indirect heat exchange with a plurality of different refrigerants
(preferably three). The overall cooling efficiency for a given
cycle improves as the number of stages increases but this increase
in efficiency is accompanied by corresponding increases in net
capital cost and process complexity. The feed gas is preferably
passed through an effective number of refrigeration stages,
nominally two, preferably two to four, and more preferably three
stages, in a first closed refrigeration cycle in indirect heat
exchange with a relatively high boiling refrigerant. Such
relatively high boiling point refrigerant is preferably comprised
in major portion of propane, propylene, or mixtures thereof, more
preferably the refrigerant comprises at least about 75 mole percent
propane, even more preferably at least 90 mole percent propane, and
most preferably the refrigerant consists essentially of propane.
Thereafter, the processed feed gas flows through an effective
number of stages, nominally two, preferably two to four, and more
preferably two or three, in a second closed refrigeration cycle in
indirect heat exchange with a refrigerant having a lower boiling
point. Such lower boiling point refrigerant is preferably comprised
in major portion of ethane, ethylene, or mixtures thereof, more
preferably the refrigerant comprises at least about 75 mole percent
ethylene, even more preferably at least 90 mole percent ethylene,
and most preferably the refrigerant consists essentially of
ethylene. Each cooling stage comprises a separate cooling zone. As
previously noted, the processed natural gas feed stream is
preferably combined with one or more recycle streams (i.e.,
compressed open methane cycle gas streams) at various locations in
the second cycle thereby producing a liquefaction stream. In the
last stage of the second cooling cycle, the liquefaction stream is
condensed (i.e., liquefied) in major portion, preferably in its
entirety, thereby producing a pressurized LNG-bearing stream.
Generally, the process pressure at this location is only slightly
lower than the pressure of the pretreated feed gas to the first
stage of the first cycle.
[0025] 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.
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 is controlled
so as to remove as much of the C.sub.2 and higher molecular weight
hydrocarbons as possible from the gas to produce a gas stream
predominating in methane and a 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 zones for the removal of liquids streams
rich in C.sub.2+ components. The exact locations and number of
gas/liquid separation means, preferably conventional gas/liquid
separators, will be dependant 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 LNG 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 latter case, the resulting methane-rich stream can be
directly returned at pressure to the liquefaction process. In the
former case, this methane-rich stream can be repressurized and
recycle or can be used as fuel gas. 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+).
[0026] The pressurized LNG-bearing stream is then further cooled in
a third cycle or step referred to as the open methane cycle via
contact in a main methane economizer with flash gases (i.e., flash
gas streams) generated in this third cycle in a manner to be
described later and via sequential expansion of the pressurized
LNG-bearing stream to near atmospheric pressure. The flash gasses
used as a refrigerant in the third refrigeration cycle are
preferably comprised in major portion of methane, more preferably
the flash gas refrigerant comprises at least 75 mole percent
methane, still more preferably at least 90 mole percent methane,
and most preferably the refrigerant consists essentially of
methane. During expansion of the pressurized LNG-bearing stream to
near atmospheric pressure, the pressurized LNG-bearing stream is
cooled via at least one, preferably two to four, and more
preferably three expansions where each expansion employs an
expander as a pressure reduction means. Suitable expanders include,
for example, either Joule-Thomson expansion valves or hydraulic
expanders. The expansion is followed by a separation of the
gas-liquid product with a separator. 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 flash
expansion step will frequently more than off-set the higher capital
and operating costs associated with the expander. In one
embodiment, additional cooling of the pressurized LNG-bearing
stream prior to flashing is made possible by first flashing a
portion of this stream via one or more hydraulic expanders and then
via indirect heat exchange means employing said flash gas stream to
cool the remaining portion of the pressurized LNG-bearing stream
prior to flashing. The warmed flash gas stream is then recycled via
return to an appropriate location, based on temperature and
pressure considerations, in the open methane cycle and will be
recompressed.
[0027] The liquefaction process described herein may use one of
several types of cooling which include but are not limited to (a)
indirect heat exchange, (b) vaporization, and (c) expansion or
pressure reduction. Indirect heat exchange, as used herein, refers
to a process wherein the refrigerant cools the substance to be
cooled without actual physical contact between the refrigerating
agent and the substance to be cooled. Specific examples of indirect
heat exchange means include heat exchange undergone in a
shell-and-tube heat exchanger, a core-in-kettle heat exchanger, and
a brazed aluminum plate-fin heat exchanger. The physical state of
the refrigerant and substance to be cooled can vary depending on
the demands of the system and the type of heat exchanger chosen.
Thus, a shell-and-tube heat exchanger will typically be utilized
where the refrigerating agent is in a liquid state and the
substance to be cooled is in a liquid or gaseous state or when one
of the substances undergoes a phase change and process conditions
do not favor the use of a core-in-kettle heat exchanger. As an
example, aluminum and aluminum alloys are preferred materials of
construction for the core but such materials may not be suitable
for use at the designated process conditions. A plate-fin heat
exchanger will typically be utilized where the refrigerant is in a
gaseous state and the substance to be cooled is in a liquid or
gaseous state. Finally, the core-in-kettle heat exchanger will
typically be utilized where the substance to be cooled is liquid or
gas and the refrigerant undergoes a phase change from a liquid
state to a gaseous state during the heat exchange.
[0028] Vaporization cooling refers to the cooling of a substance by
the evaporation or vaporization of a portion of the substance with
the system maintained at a constant pressure. Thus, during the
vaporization, the portion of the substance which evaporates absorbs
heat from the portion of the substance which remains in a liquid
state and hence, cools the liquid portion. Finally, expansion or
pressure reduction cooling refers to cooling which occurs when the
pressure of a gas, liquid or a two-phase system is decreased by
passing through a pressure reduction means. In one embodiment, this
expansion means is a Joule-Thomson expansion valve. In another
embodiment, the expansion means is either a hydraulic or gas
expander. Because expanders recover work energy from the expansion
process, lower process stream temperatures are possible upon
expansion.
[0029] The flow schematic and apparatuses set forth in FIGS. 1 and
2 represent preferred embodiments of the inventive LNG facility
employing systems for extracting work form the feed gas. FIG. 2
represents an alternate embodiment of the enhanced nitrogen removal
system. Those skilled in the art will recognized that FIGS. 1 and 2
are schematics only 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, and valves,
etc. These items would be provided in accordance with standard
engineering practice.
[0030] To facilitate an understanding of FIGS. 1 and 2, the
following numbering nomenclature was employed. Items numbered 1
through 99 are process vessels and equipment which are directly
associated with the liquefaction process. Items numbered 100
through 199 correspond to flow lines or conduits which contain
predominantly methane streams. Items numbered 200 through 299
correspond to flow lines or conduits which contain predominantly
ethylene streams. Items numbered 300 through 399 correspond to flow
lines or conduits which contain predominantly propane streams.
[0031] Referring to FIG. 1, gaseous propane is compressed in a
multistage (preferably three-stage) compressor 18 driven by a gas
turbine driver (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 or combination of drivers.
Upon compression, the compressed propane is passed through conduit
300 to a cooler 20 where it is cooled and liquefied. A
representative pressure and temperature of the liquefied propane
refrigerant prior to flashing is about 100.degree. F. and about 190
psia. The stream from cooler 20 is passed through conduit 302 to a
pressure reduction means, illustrated as expansion valve 12,
wherein the pressure of the liquefied propane is reduced, thereby
evaporating or flashing a portion thereof. The resulting two-phase
product then flows through conduit 304 into a high-stage propane
refrigerant chiller 2 wherein gaseous methane refrigerant
introduced via conduit 152, natural gas feed introduced via conduit
100, and gaseous ethylene refrigerant introduced via conduit 202
are respectively cooled via indirect heat exchange means 4, 6, and
8, thereby producing cooled gas streams respectively produced via
conduits 154, 102, and 204. The gas in conduit 154 is fed to a main
methane economizer 74 which will be discussed in greater detail in
a subsequent section and wherein the stream is cooled via indirect
heat exchange means 98. The resulting cooled compressed methane
recycle stream produced via conduit 158 is then combined in conduit
120 with the heavies depleted (i.e., light-hydrocarbon rich) vapor
stream from a heavies removal column 60 and fed to an ethylene
refrigerant chiller 68.
[0032] The propane gas from chiller 2 is returned to compressor 18
through conduit 306. This gas is fed to the high-stage inlet port
of compressor 18. The remaining liquid propane is passed through
conduit 308, the pressure further reduced by passage through a
pressure reduction means, illustrated as expansion valve 14,
whereupon an additional portion of the liquefied propane is
flashed. The resulting two-phase stream is then fed to an
intermediate stage propane refrigerant chiller 22 through conduit
310, thereby providing a coolant for chiller 22. The cooled feed
gas stream from chiller 2 flows via conduit 102 to separation
equipment 10 wherein gas and liquid phases are separated. The
liquid phase, which can be rich in C.sub.3+ components, is removed
via conduit 103. The gaseous phase is removed via conduit 104 and
then split into two separate streams which are conveyed via
conduits 106 and 108. The stream in conduit 106 is fed to propane
chiller 22. The stream in conduit 108 becomes the feed to heat
exchanger 62 and ultimately becomes the stripping gas to heavies
removal column 60, which is preferably a stripping column
containing internal packing and is discussed in more detail below.
Ethylene refrigerant from chiller 2 is introduced to chiller 22 via
conduit 204. In chiller 22, the feed gas stream, also referred to
herein as a methane-rich stream, and the ethylene refrigerant
streams are respectively cooled via indirect heat transfer means 24
and 26, thereby producing cooled methane-rich and ethylene
refrigerant streams via conduits 110 and 206. The thus evaporated
portion of the propane refrigerant is separated and passed through
conduit 311 to the intermediate-stage inlet of compressor 18.
Liquid propane refrigerant from chiller 22 is removed via conduit
314, flashed across a pressure reduction means, illustrated as
expansion valve 16, and then fed to a low-stage propane refrigerant
chiller/condenser 28 via conduit 316.
[0033] As illustrated in FIG. 1, the methane-rich stream flows from
intermediate-stage propane refrigerant chiller 22 to the low-stage
propane refrigerant chiller 28 via conduit 110. In chiller 28, the
stream is cooled via indirect heat exchange means 30. In a like
manner, the ethylene refrigerant stream flows from the
intermediate-stage propane refrigerant chiller 22 to low-stage
propane refrigerant chiller 28 via conduit 206. In the latter, the
ethylene refrigerant is totally condensed or condensed in nearly
its entirety via indirect heat exchange means 32. The vaporized
propane is removed from low-stage propane refrigerant chiller 28
and returned to the low-stage inlet of compressor 18 via conduit
320.
[0034] As illustrated in FIG. 1, the methane-rich stream exiting
low-stage propane refrigerant chiller 28 is introduced into a
separator vessel 29 via conduit 112 where the stream is separated
into a primarily gaseous, methane-rich stream and a primarily
liquid stream. The primarily gaseous, methane-rich stream is
delivered to turboexpander 31 via conduit 113. The methane-rich
stream passes through turboexpander 31 causing an impeller within
the turboexpander to rotate and turn a shaft thereby generating
useable work. The extraction of work by the expander improves the
efficiency of the process and produces a lower temperature of the
methane rich stream than would have been produced if the pressure
reduction had been through a valve or similar means. Then, the
reduced-pressure predominantly methane stream is carried away
toward high-stage ethylene refrigerant chiller 42 via conduit
115.
[0035] Preferably, the pressure of the fluid in conduit 112 is at
least about 638 psia, and more preferably between about 700-1000
psia. After separation in separator vessel 29, the pressure of the
primarily gaseous, methane-rich stream in conduit 113 is measured,
and based on that measurement, by-pass valve 33 is selectively
opened or closed. For example, if the pressure in conduit 113
exceeds the maximum operating pressure for turboexpander 31,
by-pass valve 33 opens allowing at least a portion of the
methane-rich stream contained in conduit 113 to flow around
turboexpander 31 via by-pass conduits 107, 111 and into conduit
115. Preferably, the pressure of the fluid in conduit 115 is less
than 95% of the pressure of the fluid in conduit 113, more
preferably less than 90%, and most preferably less than 85%. The
primarily liquid stream from flash drum 29 is fed via conduit 125
to heavies removal column 60. The primarily liquid stream
preferably enters heavies removal column at a location below the
column's internal packing 61.
[0036] Ethylene refrigerant exits low-stage propane refrigerant
chiller 28 via conduit 208 and is preferably fed to a separation
vessel 37 wherein light components are removed via conduit 209 and
condensed ethylene is removed via conduit 210. The ethylene
refrigerant at this location in the process is generally at a
temperature of about -24.degree. F. and a pressure of about 285
psia. The ethylene refrigerant then flows to an ethylene economizer
34 wherein it is cooled via indirect heat exchange means 38,
removed via conduit 211, and passed to a pressure reduction means,
illustrated as an expansion valve 40, whereupon the refrigerant is
flashed to a preselected temperature and pressure and fed to
high-stage ethylene refrigerant chiller 42 via conduit 212. Vapor
is removed from chiller 42 via conduit 214 and routed to ethylene
economizer 34 wherein the vapor functions as a coolant via indirect
heat exchange means 46. The ethylene vapor is then removed from
ethylene economizer 34 via conduit 216 and fed to booster
compressor 47. As indicated by dashed line P, booster compressor 47
is powered by turboexpander 31. Upon exiting booster compressor 47,
the ethylene refrigerant is fed into the high-stage inlet of
ethylene compressor 48 via conduit 217. The ethylene refrigerant
which is not vaporized in high-stage ethylene refrigerant chiller
42 is removed via conduit 218 and returned to ethylene economizer
34 for further cooling via indirect heat exchange means 50, removed
from ethylene economizer via conduit 220, and flashed in a pressure
reduction means, illustrated as expansion valve 52, whereupon the
resulting two-phase product is divided into two portions. One
portion is introduced into a low-stage ethylene refrigerant chiller
54 via conduit 222. The other portion is introduced into another
low-stage ethylene chiller 68 via conduit 226.
[0037] After cooling in indirect heat exchange means 44, the
methane-rich stream is removed from high-stage ethylene refrigerant
chiller 42 via conduit 116. As shown in the alternate embodiment of
FIG. 2, turboexpander 31a may be employed to reduce the pressure of
the stream in conduit 116. The overall process is very similar to
that shown in FIG. 1. The methane-rich stream is introduced into a
separator vessel 29a via conduit 116 where the stream is separated
into a primarily gaseous, methane-rich stream and a primarily
liquid stream. The primarily gaseous, methane-rich stream is
delivered to turboexpander 31a via conduit 113a. The methane-rich
stream passes through turboexpander 31a causing an impeller within
the turboexpander to rotate and turn a shaft thereby generating
useable mechanical work. Then, the methane-rich stream is carried
away toward low-stage ethylene refrigerant chiller 54 via conduit
115a.
[0038] Preferably, the pressure of the fluid in conduit 116 is at
least about 638 psia, and more preferably between about 700-1000
psia. After separation in flash drum 29a, the pressure of the
primarily gaseous, methane-rich stream in conduit 113a is measured,
and based on that measurement, by-pass valve 33a is selectively
opened or closed. For example, if the pressure in conduit 113a
exceeds the maximum operating pressure for turboexpander 31a,
by-pass valve 33a opens allowing at least a portion of the
methane-rich stream contained in conduit 113a to flow around
turboexpander 31a via by-pass conduits 107a, 111a and into conduit
115a. Preferably, the pressure of the fluid in conduit 115a is less
than 95% of the pressure of the fluid in conduit 113a, more
preferably less than 90%, and most preferably less than 85%. The
primarily liquid stream from separator vessel 29a is fed via
conduit 125a to heavies removal column 60.
[0039] The methane-rich stream is then condensed in part via
cooling provided by indirect heat exchange means 56 in low-stage
ethylene refrigerant chiller 54, thereby producing a two-phase
stream which flows via conduit 118 to heavies removal column 60.
The methane-rich stream preferably enters column 60 at a location
above the internal packing 61. As previously noted, the
methane-rich stream in line 104 was split so as to flow via
conduits 106 and 108. The contents of conduit 108, which is
referred to herein as the stripping gas, is first fed to heat
exchanger 62 wherein this stream is cooled via indirect heat
exchange means 66 thereby becoming a cooled stripping gas stream
which then flows via conduit 109 to heavies removal column 60. The
stripping gas enters heavies removal column at a location below the
internal packing 61. A heavies-rich liquid stream containing a
significant concentration of C.sub.4+ hydrocarbons, such as
benzene, cyclohexane, other aromatics, and/or heavier hydrocarbon
components, is removed from heavies removal column 60 via conduit
114, preferably flashed via a flow control means 97, preferably a
control valve which can also function as a pressure reduction, and
transported to heat exchanger 62 via conduit 117. Preferably, the
stream flashed via flow control means 97 is flashed to a pressure
about or greater than the pressure at the high stage inlet port to
methane compressor 83. Flashing also imparts greater cooling
capacity to the stream. In heat exchanger 62, the stream delivered
by conduit 117 provides cooling capabilities via indirect heat
exchange means 64 and exits heat exchanger 62 via conduit 119. In
heavies removal column 60, the two-phase stream introduced via
conduit 118 is contacted with the cooled stripping gas stream
introduced via conduit 109 in a countercurrent manner thereby
producing a heavies-depleted vapor stream via conduit 120 and a
heavies-rich liquid stream via conduit 114.
[0040] The heavies-rich stream in conduit 119 is subsequently
separated into liquid and vapor portions or preferably is flashed
or fractionated in vessel 67. In either case, a heavies-rich liquid
stream is produced via conduit 123 and a second methane-rich vapor
stream is produced via conduit 121. In the preferred embodiment,
which is illustrated in FIG. 1, the stream in conduit 121 is
subsequently combined with a second stream delivered via conduit
128, and the combined stream fed to the high-stage inlet port of
the methane compressor 83.
[0041] As previously noted, the gas in conduit 154 is fed to main
methane economizer 74 wherein the stream is cooled via indirect
heat exchange means 98. The resulting cooled compressed methane
recycle or refrigerant stream in conduit 158 is combined in the
preferred embodiment with the heavies-depleted vapor stream from
heavies removal column 60, delivered via conduit 120, and fed to a
low-stage ethylene refrigerant chiller 68. In low-stage ethylene
refrigerant chiller 68, this stream is cooled and condensed via
indirect heat exchange means 70 with the liquid effluent from valve
52 which is routed to low-stage ethylene refrigerant chiller 68 via
conduit 226. The condensed methane-rich product from low-stage
condenser 68 is produced via conduit 122. The vapor from low-stage
ethylene refrigerant chiller 54, withdrawn via conduit 224, and the
vapor from low-stage ethylene refrigerant chiller 68, withdrawn via
conduit 228, are combined and routed, via conduit 230, to ethylene
economizer 34 wherein the vapors function as a coolant via indirect
heat exchange means 58. The stream is then routed via conduit 232
from ethylene economizer 34 to the low-stage inlet of ethylene
compressor 48. It is also within the scope of the invention to
locate booster compressor 47 in conduit 232 instead of between
conduits 216 and 217 as shown in FIGS. 1 and 2. In this manner,
booster compressor compresses the ethylene in conduit 232 prior to
introduction into the low-stage inlet of ethylene compressor 48.
Similarly, booster compressor 47 may be placed in conduits 306,
311, or 320 so as to "pre-compress" a particular propane stream
prior to introduction of the propane stream into the respective
high, intermediate, or low-stage inlets of compressor 18.
[0042] As shown in FIG. 1, the compressor effluent from vapor
introduced via the low-stage side of ethylene compressor 48 is
removed via conduit 234, cooled via inter-stage cooler 71, and
returned to compressor 48 via conduit 236 for injection with the
high-stage stream present in conduit 216. Preferably, the
two-stages are a single module although they may each be a separate
module and the modules mechanically coupled to a single driver or
combination of drivers. The compressed ethylene product from
compressor 48 is routed to a downstream cooler 72 via conduit 200.
The product from cooler 72 flows via conduit 202 and is introduced,
as previously discussed, to high-stage propane refrigerant chiller
2.
[0043] The pressurized LNG-bearing stream, preferably a liquid
stream in its entirety, in conduit 122 is preferably at a
temperature in the range of from about -200 to about -50.degree.
F., more preferably in the range of from about -175 to about
-100.degree. F., most preferably in the range of from -150 to
-125.degree. F. The pressure of the stream in conduit 122 is
preferably in the range of from about 500 to about 700 psia, most
preferably in the range of from 550 to 625 psia.
[0044] The stream in conduit 122 is directed to a main methane
economizer 74 wherein the stream is further cooled by indirect heat
exchange means/heat exchanger pass 76 as hereinafter explained. It
is preferred for main methane economizer 74 to include a plurality
of heat exchanger passes which provide for the indirect exchange of
heat between various predominantly methane streams in the
economizer 74. Preferably, methane economizer 74 comprises one or
more plate-fin heat exchangers. The cooled stream from heat
exchanger pass 76 exits methane economizer 74 via conduit 124. It
is preferred for the temperature of the stream in conduit 124 to be
at least about 10.degree. F. less than the temperature of the
stream in conduit 122, more preferably at least about 25.degree. F.
less than the temperature of the stream in conduit 122. Most
preferably, the temperature of the stream in conduit 124 is in the
range of from about -200 to about -160.degree. F. The pressure of
the stream in conduit 124 is then reduced by a pressure reduction
means, illustrated as expansion valve 78, which evaporates or
flashes a portion of the liquid stream thereby generating a
two-phase stream. The two-phase stream from expansion valve 78 is
then passed to high-stage methane flash drum 80 where it is
separated into a flash gas stream discharged through conduit 126
and a liquid phase stream (i.e., pressurized LNG-bearing stream)
discharged through conduit 130. The flash gas stream is then
transferred to main methane economizer 74 via conduit 126 wherein
the stream functions as a coolant in heat exchanger pass 82 and
aids in the cooling of the stream in heat exchanger pass 76. Thus,
the predominantly methane stream in heat exchanger pass 82 is
warmed, at least in part, by indirect heat exchange with the
predominantly methane stream in heat exchanger pass 76. The warmed
stream exits heat exchanger pass 82 and methane economizer 74 via
conduit 128. It is preferred for the temperature of the warmed
predominantly methane stream exiting heat exchanger pass 82 via
conduit 128 to be at least about 10.degree. F. greater than the
temperature of the stream in conduit 124, more preferably at least
about 25.degree. F. greater than the temperature of the stream in
conduit 124. The temperature of the stream exiting heat exchanger
pass 82 via conduit 128 is preferably warmer than about -50.degree.
F., more preferably warmer than about 0.degree. F., still more
preferably warmer than about 25.degree. F., and most preferably in
the range of from 40 to 100.degree. F.
[0045] The liquid-phase stream exiting high-stage flash drum 80 via
conduit 130 is passed through a second methane economizer 87
wherein the liquid is further cooled by downstream flash vapors via
indirect heat exchange means 88. The cooled liquid exits second
methane economizer 87 via conduit 132 and is expanded or flashed
via pressure reduction means, illustrated as expansion valve 91, to
further reduce the pressure and, at the same time, vaporize a
second portion thereof. This two-phase stream is then passed to an
intermediate-stage methane flash drum 92 where the stream is
separated into a gas phase passing through conduit 136 and a liquid
phase passing through conduit 134. The gas phase flows through
conduit 136 to second methane economizer 87 wherein the vapor cools
the liquid introduced to economizer 87 via conduit 130 via indirect
heat exchanger means 89. Conduit 138 serves as a flow conduit
between indirect heat exchange means 89 in second methane
economizer 87 and heat exchanger pass 95 in main methane economizer
74. The warmed vapor stream from heat exchanger pass 95 exits main
methane economizer 74 via conduit 140 and is conducted to the
intermediate-stage inlet of methane compressor 83.
[0046] The liquid phase exiting intermediate-stage flash drum 92
via conduit 134 is further reduced in pressure by passage through a
pressure reduction means, illustrated as a expansion valve 93.
Again, a third portion of the liquid is evaporated or flashed. The
two-phase stream from expansion valve 93 is passed to a final or
low-stage flash drum 94. In flash drum 94, a vapor phase is
separated and passed through conduit 144 to second methane
economizer 87 wherein the vapor functions as a coolant via indirect
heat exchange means 90, exits second methane economizer 87 via
conduit 146, which is connected to the first methane economizer 74
wherein the vapor functions as a coolant via heat exchanger pass
96. The warmed vapor stream from heat exchanger pass 96 exits main
methane economizer 74 via conduit 148 and is conducted to the
low-stage inlet of compressor 83.
[0047] The liquefied natural gas product from low-stage flash drum
94, which is at approximately atmospheric pressure, is passed
through conduit 142 to a LNG storage tank 99. In accordance with
conventional practice, the liquefied natural gas in storage tank 99
can be transported to a desired location (typically via an
ocean-going LNG tanker). The LNG can then be vaporized at an
onshore LNG terminal for transport in the gaseous state via
conventional natural gas pipelines.
[0048] As shown in FIG. 1, the high, intermediate, and low stages
of compressor 83 are preferably combined as single unit. However,
each stage may exist as a separate unit. The compressed gas from
the low-stage section 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 passed through an
inter-stage cooler 84 and is combined with the high pressure gas
provided via conduits 121 and 128 prior to the third-stage of
compression. The compressed gas (i.e., compressed open methane
cycle gas stream) is discharged from high stage methane compressor
through conduit 150, is cooled in cooler 86, and is routed to the
high pressure propane refrigerant chiller 2 via conduit 152 as
previously discussed. The stream is cooled in chiller 2 via
indirect heat exchange means 4 and flows to main methane economizer
74 via conduit 154. The compressed open methane cycle gas stream
from chiller 2 which enters the main methane economizer 74
undergoes cooling in its entirety via flow through indirect heat
exchange means 98. This cooled stream is then removed via conduit
158 and combined with the processed natural gas feed stream
upstream of the low-stage ethylene chiller 68.
[0049] In one embodiment of the present invention, the LNG
production systems illustrated in FIGS. 1 and 2 are 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.
[0050] The preferred forms of the invention described above are to
be used as illustration only, and should not be used in a limiting
sense to interpret the scope of the present invention. Obvious
modifications to the exemplary embodiments, set forth above, could
be readily made by those skilled in the art without departing from
the spirit of the present invention.
[0051] The inventors hereby state their intent to rely on the
Doctrine of Equivalents to determine and assess the reasonably fair
scope of the present invention as pertains to any apparatus not
materially departing from but outside the literal scope of the
invention as set forth in the following claims.
* * * * *