U.S. patent application number 11/273521 was filed with the patent office on 2007-05-17 for lng system with high pressure pre-cooling cycle.
Invention is credited to Weldon L. Ransbarger.
Application Number | 20070107464 11/273521 |
Document ID | / |
Family ID | 38039354 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070107464 |
Kind Code |
A1 |
Ransbarger; Weldon L. |
May 17, 2007 |
LNG system with high pressure pre-cooling cycle
Abstract
A natural gas liquefaction system employing a high pressure
pre-cooling refrigeration cycle.
Inventors: |
Ransbarger; Weldon L.;
(Houston, TX) |
Correspondence
Address: |
ConocoPhilips Company - I.P. Legal
PO BOX 2443
BARTLESVILLE
OK
74005
US
|
Family ID: |
38039354 |
Appl. No.: |
11/273521 |
Filed: |
November 14, 2005 |
Current U.S.
Class: |
62/612 |
Current CPC
Class: |
F25J 1/021 20130101;
F25J 1/0087 20130101; F25J 1/0052 20130101; F25J 2245/02 20130101;
F25J 1/0292 20130101; F25J 1/0085 20130101; F25J 1/0022 20130101;
F25J 2220/64 20130101; F25J 1/004 20130101; F25J 1/0254
20130101 |
Class at
Publication: |
062/612 |
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 high pressure pre-cooling
cycle via indirect heat exchange with a pre-cooling refrigerant,
said pre-cooling cycle employing a pre-cooling compressor that
discharges said pre-cooling refrigerant at a discharge pressure of
at least 225 pounds per square inch atmospheric (psia), said
pre-cooling refrigerant having a boiling point temperature lower
than -45.degree. F. at one atmosphere; and (b) further cooling and
at least partly condensing at least a portion of said natural gas
stream in a subsequent cooling cycle via indirect heat exchange
with a subsequent refrigerant having a lower boiling point
temperature than said pre-cooling refrigerant.
2. The process of claim 1, said pre-cooling refrigerant having a
latent heat of vaporization in the range of from about 75 to about
205 British thermal units per pound (btu/lb) at one atmosphere and
boiling point temperature.
3. The process of claim 1, said pre-cooling refrigerant having a
boiling point temperature at least about 10 percent lower than the
boiling point temperature of propane at one atmosphere, on a
Fahrenheit temperature scale.
4. The process of claim 1, said pre-cooling refrigerant having a
vapor pressure in the range of from about 130 to about 180 psia at
68.degree. F.
5. The process of claim 1, said pre-cooling refrigerant having a
boiling point temperature at one atmosphere within 150.degree. F.
of the boiling point temperature at one atmosphere of said
subsequent refrigerant.
6. The process of claim 1, said discharge pressure being at least
250 psia.
7. The process of claim 1, said pre-cooling refrigerant having a
boiling point temperature in the range of from about -65.degree. to
about -50.degree. F. at one atmosphere, said pre-cooling
refrigerant having a latent heat of vaporization in the range of
from about 180 to about 195 btu/lb at one atmosphere and boiling
point temperature.
8. The process of claim 7, said pre-cooling refrigerant having a
vapor pressure in the range of from about 140 to about 170 psia at
68.degree. F.
9. The process of claim 7, said pre-cooling refrigerant having a
boiling point temperature at one atmosphere within 110.degree. F.
of the boiling point temperature at one atmosphere of said
subsequent refrigerant.
10. The process of claim 1, said subsequent refrigerant comprising
ethane and/or ethylene.
11. The process of claim 1, said pre-cooling refrigerant comprising
predominately propylene.
12. The process of claim 11, said subsequent refrigerant comprising
predominately ethylene.
13. The process of claim 1, said pre-cooling compressor providing a
maximum inlet-to-discharge pressure increase in the range of from
about 200 to about 350 psi.
14. The process of claim 1, said pre-cooling compressor comprising
low-stage, intermediate-stage, and high-stage inlets each receiving
at least a portion of said pre-cooling refrigerant.
15. The process of claim 14, said pre-cooling compressor operating
at a low-stage inlet pressure of at least about 15 psia, an
intermediate-stage inlet pressure of at least about 40 psia, and a
high-stage inlet pressure of at least about 80 psia.
16. The process of claim 15, said pre-cooling refrigerant having a
temperature in the range of from about -250.degree. to about
-75.degree. F. at said low-stage inlet, said pre-cooling
refrigerant having a temperature in the range of from about
-50.degree. to about 100.degree. F. at said intermediate-stage
inlet, said pre-cooling refrigerant having a temperature in the
range of from about 0.degree. to about 200.degree. F. at said
high-stage inlet.
17. The process of claim 14, said pre-cooling refrigerant having a
density of at least about 0.18 pounds per cubic foot (lb/ft.sup.3)
at said low-stage inlet, said pre-cooling refrigerant having a
density of at least about 0.5 lb/ft.sup.3 at said
intermediate-stage inlet, said pre-cooling refrigerant having a
density of at least about 0.9 lb/ft.sup.3 at said high-stage
inlet.
18. The process of claim 1; and (c) cooling at least a portion of
said subsequent refrigerant via indirect heat exchange with said
pre-cooling refrigerant.
19. The process of claim 1; and (d) further cooling at least a
portion of said natural gas stream in a final cooling cycle via
indirect heat exchange with a final refrigerant having a lower
boiling point temperature than said subsequent refrigerant.
20. The process of claim 19; and (e) separating a portion of said
natural gas stream and employing the separated portion as said
final refrigerant.
21. The process of claim 19, said final refrigerant comprising
predominately methane.
22. The process of claim 19; and (f) cooling at least a portion of
said final refrigerant via indirect heat exchange with said
pre-cooling refrigerant.
23. An apparatus for liquefying a natural gas stream, said
apparatus comprising: a pre-cooling refrigeration cycle for cooling
said natural gas stream, said pre-cooling refrigeration cycle
including a pre-cooling compressor, a pre-cooling chiller, and a
pre-cooling refrigerant circulating through said pre-cooling
compressor and pre-cooling chiller, said pre-cooling compressor
being configured to discharge said pre-cooling refrigerant at a
discharge pressure of at least 225 pounds per square inch
atmospheric (psia), said pre-cooling refrigerant having a boiling
point temperature lower than -45.degree. F. at one atmosphere; and
a subsequent refrigeration cycle for cooling at least a portion of
said natural gas stream downstream of said pre-cooling
refrigeration cycle, said subsequent refrigeration cycle including
a subsequent compressor, a subsequent chiller, and a subsequent
refrigerant circulating through said subsequent compressor and
subsequent chiller, said subsequent refrigerant having a lower
boiling point temperature than said pre-cooling refrigerant.
24. The apparatus of claim 23, said pre-cooling refrigerant having
a latent heat of vaporization in the range of from about 75 to
about 205 British thermal units per pound (btu/lb) at one
atmosphere and boiling point temperature.
25. The apparatus of claim 23, said pre-cooling refrigerant having
a boiling point temperature at least about 10 percent greater than
the boiling point temperature of propane at one atmosphere, on a
Fahrenheit temperature scale.
26. The apparatus of claim 23, said pre-cooling refrigerant having
a vapor pressure in a range of from about 130 to about 180 psia at
68.degree. F.
27. The apparatus of claim 23, said pre-cooling refrigerant having
a boiling point temperature at one atmosphere within 150.degree. F.
of the boiling point temperature at one atmosphere of said
subsequent refrigerant.
28. The apparatus of claim 23, said discharge pressure being at
least 250 psia.
29. The apparatus of claim 23, said pre-cooling refrigerant having
a boiling point temperature in the range of from about -65.degree.
to about -50.degree. F. at one atmosphere, said pre-cooling
refrigerant having a latent heat of vaporization in the range of
from about 180 to about 195 btu/lb at one atmosphere and boiling
point temperature.
30. The apparatus of claim 23, said pre-cooling refrigerant
comprising predominately propylene.
31. The apparatus of claim 30, said subsequent refrigerant
comprising predominately ethylene.
32. The apparatus of claim 23; and a final refrigeration cycle for
cooling at least a portion of said natural gas stream downstream of
said subsequent refrigeration cycle, said final refrigeration cycle
including a final compressor, a final chiller, and a final
refrigerant circulating through said final compressor and final
chiller, said final refrigerant having a lower boiling point than
said subsequent refrigerant.
33. The apparatus of claim 32, said final refrigerant comprising
predominately methane.
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 liquified natural gas (LNG) facility employing a high
pressure pre-cooling cycle.
[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. 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 a series of single-component
refrigerants or one multi-component mixed refrigerant. 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.
[0008] Many LNG facilities employ a propane refrigeration cycle to
pre-cool the natural gas stream prior to cooling and condensing the
natural gas in the main downstream refrigeration cycle(s).
Typically, propane pre-cooling cycles employ a propane compressor,
propane cooler, one or more propane chillers, and associated piping
for routing the propane refrigerant from the compressor, to the
cooler, to the chiller(s), and back to the compressor. The propane
compressor receives propane vapors and compresses the propane in
one or more stages of compression. The compressed propane
refrigerant discharged from the compressor is then cooled and
condensed in the propane cooler via indirect heat exchange with
ambient air or water. The pressure of the liquid propane
refrigerant can then be let down to further cool the refrigerant.
The resulting propane refrigerant can then be employed in the
propane chiller(s) to cool the natural gas stream and, optionally,
to cool other downstream refrigerants. As heat is transferred from
the natural gas stream to the propane refrigerant in the propane
chiller(s), at least a portion of the propane refrigerant
vaporizes. The resulting propane vapor is returned to the propane
compressor for compression.
[0009] There is a definite trend in the LNG industry towards
increasing the capacity of existing LNG facilities and constructing
new LNG facilities of higher capacity. As the demand for high
capacity LNG facilities increases, conventional propane pre-cooling
systems are becoming inadequate. High capacity LNG facilities
require more refrigeration from the pre-cooling cycle than
conventional LNG facilities. This increased refrigeration
requirement necessitates an increase the flow rate of propane
through the pre-cooling cycle. However, a number of disadvantages
are associated with increasing the cooling capacity of a propane
pre-cooling cycle by simply increasing the flow rate of propane
through though the system. For example, increasing the flow rate of
the propane refrigerant in the pre-cooling cycle requires larger
compressors and larger piping. Obviously, the use of larger
equipment and piping increases the cost ofthe facility. Further,
larger equipment and piping takes up more space--an undesirable
feature when plot space is limited, such as for offshore LNG
facilities. In addition, the capacity of conventional propane
refrigerant compressors is already at or near the limits of known
technology. Thus, procuring a propane compressor of significantly
higher capacity than currently available propane compressors would
be very expensive, if not impossible.
OBJECTS AND SUMMARY OF THE INVENTION
[0010] It is, therefore, an object of the present invention to
provide a novel natural gas liquefaction system employing a
pre-cooling cycle that provides more refrigeration duty than
conventional propane pre-cooling cycles.
[0011] A further object of the invention is to provide a high
capacity pre-cooling cycle for LNG facilities that does not require
significantly larger equipment and/or piping than conventional
propane pre-cooling cycles.
[0012] Another object ofthe invention is to provide a high capacity
pre-cooling cycle for LNG facilities that does not require a
refrigerant compressor of significantly larger capacity than
conventional compressors employed in propane pre-cooling
cycles.
[0013] 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.
[0014] Accordingly, one aspect of the present invention concerns a
process for liquefying natural gas comprising: (a) cooling a
natural gas stream in a high pressure pre-cooling cycle via
indirect heat exchange with a pre-cooling refrigerant, where the
pre-cooling cycle employs a pre-cooling compressor that discharges
the pre-cooling refrigerant at a discharge pressure of at least 225
pounds per square inch atmospheric (psia) and the pre-cooling
refrigerant has a boiling point temperature lower than -45.degree.
F. at one atmosphere; and (b) further cooling and at least partly
condensing at least a portion of the natural gas stream in a
subsequent cooling cycle via indirect heat exchange with a
subsequent refrigerant having a lower boiling point temperature
than the pre-cooling refrigerant.
[0015] Another aspect of the present invention concerns an
apparatus for liquefying a natural gas stream. The apparatus
comprises a pre-cooling refrigeration cycle and a subsequent
refrigeration cycle located downstream of the pre-cooling
refrigeration cycle. The pre-cooling refrigeration cycle includes a
pre-cooling compressor, a pre-cooling chiller, and a pre-cooling
refrigerant circulating through the pre-cooling compressor and
pre-cooling chiller. The pre-cooling compressor is configured to
discharge the pre-cooling refrigerant at a discharge pressure of at
least 225 psia. The pre-cooling refrigerant has a boiling point
temperature lower than -45.degree. F. at one atmosphere. The
subsequent refrigeration cycle includes a subsequent compressor, a
subsequent chiller, and a subsequent refrigerant circulating
through the subsequent compressor and subsequent chiller. The
subsequent refrigerant has a lower boiling point temperature than
the pre-cooling refrigerant.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0016] FIG. 1 is a simplified flow diagram of a cascaded
refrigeration process for LNG production that employs a high
pressure pre-cooling cycle.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] A cascaded refrigeration process uses one or more
refrigerants to transfer heat from a natural gas stream to a
refrigerant, and ultimately transferring the heat from the
refrigerant 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.
[0018] 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
ofthe 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.
[0019] 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 multi-stage
pre-cooling cycle, a multi-stage 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 multi-stage
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 flow path of natural gas through
the plant.
[0020] 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 maybe 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 pre-cooling stages. 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 pre-cooling cycle and also downstream of
the first cooling stage in the pre-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.
[0021] 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
pounds per square inch atmospheric (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.
[0022] As previously noted, the natural gas feed stream is cooled
in a plurality of multi-stage 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.
[0023] It is preferred for the feed gas to be initially cooled in a
closed, high pressure, pre-cooling refrigeration cycle via an
effective number of refrigeration stages (nominally two, preferably
two to four, and more preferably three stages), with each cooling
stage defining a separate cooling zone. The pre-cooling
refrigeration cycle employs a pre-cooling refrigerant having a
relatively low boiling point temperature. In the high pressure
pre-cooling refrigeration cycle of the present invention, it is
preferred for the pre-cooling refrigerant to have a boiling point
temperature that is at least about 10 percent lower than the
boiling point temperature of propane, on a Fahrenheit temperature
scale. Preferably, the pre-cooling refrigerant has a boiling point
temperature at one atmosphere of pressure in the range of from
about -75.degree. to about -45.degree. F., more preferably in the
range of from about -65.degree. to about -50.degree. F., and most
preferably in the range of from -60.degree. to -52.degree. F. The
pre-cooling refrigerant preferably has a latent heat of
vaporization at one atmosphere of pressure and boiling point
temperature in the range of from about 175 to about 205 British
thermal units per pound (btu/lb), more preferably in the range of
from about 180 to about 195 btu/lb, and most preferably in the
range of from 185 to 192 btu/lb. The pre-cooling refrigerant
preferably has a vapor pressure at 68.degree. F. in the range of
from about 130 to about 180 psia, more preferably in the range of
from about 140 to about 170 psia, and most preferably in the range
of from 145 to 160 psia. The pre-cooling refrigerant preferably has
a liquid density at one atmosphere of pressure and boiling point
temperature in the range of from about 25 to about 50 pounds per
cubic foot (lb/ft.sup.3), more preferably in the range of from
about 30 to about 45 lb/ft.sup.3, and most preferably in the range
of 37 to 43 lb/ft.sup.3. The pre-cooling refrigerant preferably has
a gas density at one atmosphere of pressure and boiling point
temperature in the range of from about 0.1 to about 0.2
lb/ft.sup.3, more preferably in the range of from about 0.125 to
about 0.175 lb/ft.sup.3, and most preferably in the range of from
0.140 to 0.155 lb/ft.sup.3. In one embodiment of the present
invention, the pre-cooling refrigerant is comprised predominately
of propylene. In another embodiment of the present invention, the
pre-cooling refrigerant consists essentially of propylene.
[0024] After pre-cooling, the processed feed gas is further cooled
in a closed subsequent refrigeration cycle via an effective number
of refrigeration stages (nominally two, preferably two to four, and
more preferably two or three). The subsequent refrigeration cycle
preferably employs a subsequent refrigerant having a lower boiling
point than the pre-cooling refrigerant. However, it is preferred
for the boiling point temperatures at one atmosphere of the
pre-cooling refrigerant and the subsequent refrigerant to be within
about 150.degree. F. of one another, more preferably within about
110.degree. F. of one another, and most preferably within 90 to
105.degree. F. of one another. The subsequent 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.
[0025] 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 subsequent
refrigeration cycle thereby producing a liquefaction stream. In the
last stage of the subsequent 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 pre-cooling refrigeration cycle.
[0026] The pressurized LNG-bearing stream resulting from the final
stage of the subsequent refrigeration cycle is then further cooled
in a final refrigeration 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 final 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 final 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] 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
recycled 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+).
[0028] 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.
[0029] 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.
[0030] The flow schematic and apparatus set forth in FIG. 1
represents a preferred embodiment of the inventive LNG facility
employing a high pressure pre-cooling refrigeration cycle. Those
skilled in the art will recognize that FIG. 1 is a schematic 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.
[0031] To facilitate an understanding of FIG. 1, the following
numbering nomenclature was employed. Items numbered 1 through 99l
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 the pre-cooling refrigerant.
[0032] Referring to FIG. 1, natural gas enters the LNG facility via
conduit 100 and is pre-cooled in high-stage, intermediate-stage,
and low-stage pre-cooling chillers 2, 22, 28. In pre-cooling
chillers 2, 22, 28, the natural gas stream is cooled via indirect
heat exchange with the pre-cooling refrigerant. The pre-cooling
refrigeration cycle employs a pre-cooling compressor 18 to compress
and circulate the pre-cooling refrigerant through pre-cooling
chillers 2, 22, 28. Pre-cooling compressor 18 is preferably a
multi-stage (preferably three-stage) compressor which is driven by
a gas turbine (not illustrated). The three stages of compression
associated with pre-cooling compressor 18 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.
[0033] The compressed pre-cooling refrigerant is discharged from
compressor 18 via conduit 300. The discharged pre-cooling
refrigerant in conduit 300 preferably has a pressure of at least
about 225 psia, more preferably at least about 250 psia, and most
preferably in the range from 275 to 350 psia. The discharged
pre-cooling refrigerant in conduit 300 preferably has a temperature
in the range of from about -300.degree. to about -50.degree. F.,
more preferably in the range of from about -250.degree. to about
-100.degree. F., and most preferably in the range of from
-200.degree. to -150.degree. F. It is preferred for pre-cooling
compressor 18 to be capable of providing a maximum
inlet-to-discharge pressure increase in the range of from about 200
to about 350 psi, more preferably in the range of from 240 to 280
psi.
[0034] The compressed pre-cooling refrigerant in conduit 300 is
passed to a cooler 20 where it is cooled and liquefied. In a
preferred embodiment of the present invention, cooler 20 employs
ambient air and/or water as the cooling medium for removing heat
from the compressed pre-cooling refrigerant. The pre-cooling
refrigerant stream discharged from cooler 20 is passed through
conduit 302 to a pressure reduction means, illustrated as expansion
valve 12, wherein the pressure of the liquefied pre-cooling
refrigerant is reduced, thereby evaporating or flashing a portion
thereof. The resulting two-phase product then flows through conduit
304 into high-stage pre-cooling 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.
[0035] When heat is transferred from the natural gas stream to the
pre-cooling refrigerant in high-stage pre-cooling chiller 2, at
least a portion of the pre-cooling refrigerant vaporizes. The
vaporized portion of the pre-cooling refrigerant from chiller 2 is
returned to a high-stage inlet port of compressor 18 via conduit
306. Preferably, the gaseous pre-cooling refrigerant entering the
high-stage inlet port of compressor 18 via conduit 306 has a
pressure of at least about 15 psia, more preferably at least about
20 psia, and most preferably in the range of from 25 to 35 psia.
The gaseous pre-cooling refrigerant in conduit 306 preferably has
temperature in the range of from about -250.degree. to about
-75.degree. F., more preferably in the range of from about
-225.degree. to about -100.degree. F., and most preferably in the
range of from -200.degree. to -150.degree. F. The pre-cooling
refrigerant in conduit 306 preferably has a density of at least
about 0.18 lb/ft.sup.3, more preferably at least about 0.2
lb/ft.sup.3, and most preferably in the range of from 0.225 to 0.3
lb/ft.sup.3.
[0036] The liquid portion of the pre-cooling refrigerant that is
not vaporized in high-stage pre-cooling chiller 2 exits pre-cooling
chiller 2 via conduit 308 and its pressure further reduced by
passage through a pressure reduction means, illustrated as
expansion valve 14, whereupon an additional portion of the liquid
pre-cooling refrigerant is flashed. The resulting two-phase stream
is then fed to intermediate-stage pre-cooling 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
intermediate-stage pre-cooling 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, 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
evaporated portion of the pre-cooling refrigerant is separated and
passed through conduit 311 to the intermediate-stage inlet port of
compressor 18. The gaseous pre-cooling refrigerant introduced into
the intermediate-stage inlet port of compressor 18 via conduit 311
preferably has a pressure of at least about 40 psia, more
preferably at least about 60 psia, and most preferably in the range
of from 70 to 100 psia. The vaporized pre-cooling refrigerant in
conduit 311 preferably has a temperature in the range of from about
-50.degree. to about 100.degree. F., more preferably in the range
of from about 0.degree. F. to about 75.degree. F., and most
preferably in the range of from 10.degree. to 50.degree. F. The
pre-cooling refrigerant in conduit 311 preferably has a density
greater than about 0.5 lb/ft.sup.3, more preferably in the range of
from 0.55 to 0.75 lb/ft.sup.3. The unvaporized liquid pre-cooling
refrigerant in chiller 22 is removed via conduit 314, flashed
across a pressure reduction means, illustrated as expansion valve
16, and then fed to low-stage pre-cooling chiller 28 via conduit
316.
[0037] As illustrated in FIG. 1, the methane-rich stream flows from
intermediate-stage pre-cooling chiller 22 to the low-stage
pre-cooling 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
pre-cooling chiller 22 to low-stage pre-cooling 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 pre-cooling refrigerant is removed
from low-stage pre-cooling chiller 28 and returned to the low-stage
inlet port of compressor 18 via conduit 320. The gaseous
pre-cooling refrigerant introduced into the low-stage inlet port of
compressor 18 via conduit 320 preferably has a pressure of at least
about 80 psia, more preferably at least about 110 psia, and most
preferably in the range of from 120 to 175 psia. The temperature of
the pre-cooling refrigerant in conduit 320 is preferably in the
range of from about 0.degree. to about 200.degree. F., more
preferably in the range of from about 25.degree. to about
150.degree. F., and most preferably in the range of from 50.degree.
to 100.degree. F. The gas density of the vaporized pre-cooling
refrigerant in conduit 320 is preferably at least about 0.9
lb/ft.sup.3, more preferably in the range of from 0.95 to 1.25
lb/ft.sup.3.
[0038] After cooling the natural gas stream in chillers 2, 22, 28
of the pre-cooling refrigeration cycle, the pre-cooled methane-rich
stream is then further cooled in a subsequent refrigeration cycle
employing a predominately ethylene refrigerant. The subsequent
refrigeration cycle employs high-stage, intermediate-stage, and
low-stage ethylene chillers 42, 54, and 68 to sequentially cool the
methane-rich stream. The methane-rich stream exiting low-stage
pre-cooling chiller 28 is introduced to high-stage ethylene chiller
42 via conduit 112. Ethylene refrigerant exits low-stage
pre-cooling 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 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 the high-stage
inlet port of ethylene compressor 48. The ethylene refrigerant
which is not vaporized in high-stage ethylene 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 introduced into a low-stage ethylene chiller
54 via conduit 222.
[0039] After cooling in indirect heat exchange means 44, the
methane-rich stream is removed from high-stage ethylene chiller 42
via conduit 116. This stream is then condensed in part via cooling
provided by indirect heat exchange means 56 of intermediate-stage
ethylene chiller 54, thereby producing a two-phase stream which
flows via conduit 118 to heavies removal column 60. 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. 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
low-stage ethylene chiller 68. In low-stage ethylene chiller 68,
this stream is cooled and condensed via indirect heat exchange
means 70 with the liquid effluent from conduit 222 which is routed
to low-stage ethylene 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 chiller 54,
withdrawn via conduit 224, and low-stage ethylene 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 port of ethylene compressor 48.
[0042] As noted 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 common driver. 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 pre-cooling 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.degree. to about
-50.degree. F., more preferably in the range of from about
-175.degree. to about -100.degree. F., most preferably in the range
of from -150.degree. 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 725
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 gas 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, and
into the high-stage inlet of methane compressor 83. 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.degree. 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 introduced into the
intermediate-stage inlet port 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 liquefied gas is evaporated or
flashed. The two-phase stream from expansion valve 93 are 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 introduced into the
low-stage inlet port 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 where the units are
mechanically coupled together to be driven by a single driver. 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-stage pre-cooled chiller 2 via
conduit 152 as previously discussed. The stream is cooled in
pre-cooling 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 first stage of ethylene cooling.
[0049] In one embodiment of the present invention, the LNG
production system illustrated in 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.
[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 inventor hereby states his 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.
* * * * *