U.S. patent number 3,581,511 [Application Number 04/841,799] was granted by the patent office on 1971-06-01 for liquefaction of natural gas using separated pure components as refrigerants.
This patent grant is currently assigned to Institute of Gas Technology. Invention is credited to Ralph E. Peck.
United States Patent |
3,581,511 |
Peck |
June 1, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
LIQUEFACTION OF NATURAL GAS USING SEPARATED PURE COMPONENTS AS
REFRIGERANTS
Abstract
A gas liquefaction system wherein the refrigerant components are
mixed with one another and, thereafter, separated into individual
streams which enter the system as liquid streams. Cooling is
produced by evaporating the cooled liquid component streams into a
gas stream, which after all the individual cooled liquid component
streams have been evaporated into it, forms the mixed refrigerant
component stream returned to the compressor.
Inventors: |
Peck; Ralph E. (Chicago,
IL) |
Assignee: |
Institute of Gas Technology
(Chicago, IL)
|
Family
ID: |
25285713 |
Appl.
No.: |
04/841,799 |
Filed: |
July 15, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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770922 |
Oct 28, 1968 |
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Current U.S.
Class: |
62/612 |
Current CPC
Class: |
F25J
1/0291 (20130101); F25J 3/0209 (20130101); F25J
1/0022 (20130101); F25J 3/029 (20130101); F25J
1/0055 (20130101); F25J 1/004 (20130101); F25J
1/025 (20130101); F25J 1/0202 (20130101); F25J
1/0045 (20130101); F25J 3/0233 (20130101); F25J
1/0212 (20130101); F25J 1/0279 (20130101); F25J
2220/64 (20130101); F25J 2205/20 (20130101); F25J
2245/90 (20130101); F25J 2290/62 (20130101); F25J
2215/04 (20130101); F25J 2220/62 (20130101); F25J
2205/50 (20130101) |
Current International
Class: |
F25J
1/00 (20060101); F25J 1/02 (20060101); F25j
001/00 (); F25j 003/00 (); F25j 003/02 () |
Field of
Search: |
;62/9,23,26,28,40 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Pergamon Press-Progress in Refrigeration Science and Technology,
"One Flow Cascade Cycle" pp. 34--39.
|
Primary Examiner: Bascomb, Jr.; Wilbur L.
Assistant Examiner: Purcell; Arthur F.
Parent Case Text
This application is a continuation-in-part of application Ser. No.
770,922, filed Oct. 28, 1968.
Claims
I claim:
1. The method of liquifying a gas comprising the steps of:
supplying the gas in a process stream at an elevated pressure,
supplying a mixed refrigerant component stream at an elevated
pressure, separating the mixed refrigerant component stream to
provide a plurality of individual liquid refrigerant component
streams by fractional distillation using hydrocarbons as an overall
cycle reflux with the removal of heat in a reflux condenser of a
column and the supply of heat in a reboiler of the column, cooling
each of said plurality of individual liquid refrigerant component
streams, evaporating each of said plurality of individual liquid
refrigerant component streams into a single stream to provide a
refrigerant stream which is at approximately the same temperature
as the process stream when the latter is liquified and which is in
heat exchange relationship with the process stream to refrigerate
the gas in the process stream to a liquified state, compressing and
cooling said refrigerant stream after said liquid refrigerant
component streams have been evaporated into it to provide said
mixed refrigerant component stream.
2. The method of claim 1, wherein the mixed refrigerant component
stream is separated to provide a plurality of individual
essentially pure single component liquid refrigerant component
streams.
3. The method of claim 1, wherein the mixed refrigerant component
stream is separated to provide a plurality of individual controlled
composition liquid mixture refrigerant component streams.
4. The method of claim 1, wherein the cooling of each of said
plurality of individual liquid refrigerant component streams and
the refrigeration of the gas in the process stream to liquify the
latter both are provided by evaporating said plurality of
individual liquid refrigerant component streams into said
refrigerant stream.
5. The method of claim 2, further including the steps of mixing
together said gas in said process stream and said mixed refrigerant
gas stream, and thereafter separating said mixture to provide said
plurality of individual essentially pure single component liquid
refrigerant component streams and a feed stream which is thereafter
liquified.
6. The method of claim 2, further including the steps of passing
said gas in said process stream through a plurality of heat
exchangers to refrigerate said gas in said process stream to a
liquified state, expanding the liquified gas in the process stream
to a lower pressure to provide a liquified gas stream for storage
or transportation and a cold gaseous stream resulting from a
portion of the liquified gas in the process stream being flashed
off upon expansion, said cold gaseous stream providing said single
stream forming said refrigerant stream, passing each of said
individual essentially pure single component liquid refrigerant
component streams through predetermined ones of the same plurality
of heat exchangers through which said gas in said process stream is
passed to cool said individual essentially pure single component
liquid refrigerant component streams, expanding each of said
individual essentially pure single component liquid refrigerant
component streams to a lower pressure and evaporating them into
said refrigerant stream in heat exchange relationship with the
process stream to provide the refrigeration for both liquifying
said gas in said process stream and for cooling said individual
essentially pure single component liquid refrigerant component
streams.
7. The method of claim 1, further including the steps of including
nitrogen and helium in the composition of the mixed refrigerant
component stream, expanding the gas in the process stream at a
predetermined intermediate point in the refrigeration cycle to a
reduced pressure, separating nitrogen and helium from said gas in
said process stream after reducing the pressure of said process
stream, cooling the gas in the process stream to a lower
temperature, expanding the further cooled gas in the process stream
to a reduced pressure and passing it into a storage tank, cooling
the nitrogen and helium separated from said gas in said process
stream, expanding this further cooled nitrogen and helium to a
reduced pressure, joining subcooled liquid gas from said storage
tank with said expanded nitrogen and helium, evaporating the
resulting mixture with said refrigeration stream in heat exchange
relationship with both the nitrogen and helium separated from the
gas in the process stream and the latter after said nitrogen and
helium has been separated from it.
8. The method of claim 3, further including the steps of separating
by fractional distillation said mixed refrigerant stream to provide
a first and a second controlled composition liquid mixture stream
at a high pressure, cooling said first controlled composition
liquid mixture stream and thereafter expanding it to a lower
pressure to provide a cold gaseous stream, said cold gaseous stream
providing said single stream forming said refrigerant stream, said
second controlled composition liquid mixture stream being further
separated to provide a plurality of individual controlled component
liquid mixture refrigerant component streams, each of said
individual controlled component liquid mixture refrigerant
component streams being cooled and expanded to a lower pressure and
evaporated into said refrigerant stream in heat exchange
relationship with the process stream to provide the refrigeration
for both liquifying said gas in said process stream and for cooling
said individual controlled composition liquid mixture refrigerant
component streams.
9. The method of claim 1, further including the steps of passing
said gas in said process stream through a plurality of heat
exchangers to refrigerate said gas in said process stream to a
liquified state, expanding the liquified gas in the process stream
to a lower pressure to provide a component gas stream for storage
or transportation, providing a controlled composition liquid
mixture stream at a high pressure, expanding said controlled
composition liquid mixture stream to a lower pressure to provide a
cold gaseous stream, said cold gaseous stream providing said single
stream forming said refrigerant stream, passing each of said
individual controlled composition liquid mixture refrigerant
component stream through predetermined ones of the same plurality
of heat exchangers through which said gas in said process stream is
passed to cool said individual controlled composition mixture
liquid refrigerant component streams, expanding each of said
individual controlled composition liquid mixture refrigerant
component streams to a lower pressure and evaporating them into
said refrigerant stream in heat exchange relationship with the
process stream to provide the refrigeration for both liquifying
said gas in said process stream and for cooling said individual
controlled composition liquid mixture refrigerant component
streams.
10. The method of claim 1 wherein the liquid portion of the
compressed and cooled refrigerant stream is separated from the
gaseous portion thereof between the compressor stages, and is
pumped into the discharge of the following stage of the compressor
causing further condensation of the interstage gas portion.
Description
This invention relates to improved methods for liquifying
gases.
The method of the present invention is particularly adapted to
liquify natural gas which is composed mostly of methane but which
may contain heavier hydrocarbons such as, for example, ethane,
propane, butane and the like. However, it will be apparent from the
description below that the method can be used to liquify a wide
range of normally gaseous fluids, merely by properly selecting the
refrigerant components used in this system.
It is generally well known that natural gas when reduced from a
gaseous state to a liquified state is reduced approximately one
six-hundredth in volume, and that the natural gas, when in a
liquified state, can be more easily transported and stored. For
this reason, considerable effort has been made to improve the
existing methods of liquifying natural gas.
Presently, gas liquefaction systems generally are of two types. One
is based on an expansion cycle wherein gas at high pressure is
allowed to expand, preferably with work, with a corresponding
reduction in temperature until a wet gas is produced wherein the
condensed portion can be removed from the dry gas residue. The
latter can be recompressed and recycled with makeup gas through the
expansion system. The compressed gas is usually passed in heat
exchange relation with a refrigerant to remove heat of compression
and to cool the gas under pressure, whereby a higher percentage of
wet gas is produced during the subsequent expansion step or
steps.
The other main type of gas liquefaction system is the cascade
system which is based upon the use of heat exchangers arranged
progressively to reduce the temperature of the gas to a
liquefaction temperature, at which point all of the gas is
converted to a liquified state. Heat exchange for refrigeration is
usually carried out while the gas is at some higher pressure above
atmospheric pressure, whereby the phase conversion is at a higher
and more accessible temperature level. The compressed and liquified
gas is then let down to a lowered pressure for storage and
transportation. During the reduction in pressure, some of the
liquid will be flashed off as a gas with accompanying reduction in
temperature.
The selection of a cascade cycle or an expansion cycle for
liquefaction depends upon a number of factors, including the
composition of the gas, the power requirements, the space available
for the equipment, the cost of raw materials, the type of
equipment, and the like.
The method of the present invention is, generally, a cascade
system, however, substantial improvements over the classical
cascade cycle, or the newer so-called incorporated cascade cycle
which uses only one compressor, are provided. In this latter
system, the refrigerant components such as propane, ethane and
butane are maintained separated as individual components or
streams, in both the refrigeration cycles of the system and in
storage. Accordingly, as a result, this system requires a
substantial amount of power to condense these refrigerant
components, to maintain the efficiency of the systems. In the
system of the present invention, the refrigerant components are
mixed with one another and, after being compressed, are mixed, in
one embodiment of the invention, with the natural gas in the
process stream. Thereafter, the refrigerant components are
separated into individual streams of essentially pure components
which enter the system as liquid streams. In a second embodiment of
the invention, the refrigerant components are separated into
individual streams of different mixtures of the components. Cooling
is produced by evaporating the cooled liquid component streams
progressively into a gas stream which, after all of the individual
cooled liquid components streams (pure or mixtures) have been
evaporated into it, forms the mixed refrigerant component stream
returned to the compressor. With this arrangement, only a single
multistage compressor is required and the compression energy
required is approximately 40 percent less than the compression
energy required for a classical cascade cycle.
The method of the present invention can be employed both for the
initial liquefaction of a natural gas at the source of supply, for
storage or transportation, and to reliquify natural gas vapors
given off during storage. Generally, in the cascade and the
incorporated cascade systems, the boil-off and flash gas either are
vented to the atmosphere, compressed in a second compressor or used
as a fuel. If vented to the atmosphere, or used as a fuel, the
stored gas may have excess heavy hydrocarbons and may not be
satisfactory for general distribution. If compressed in a second
compressor and returned to storage, equipment costs are increased
as are power requirements. With the system of the present
invention, the boil-off and flash gas can be returned and combined
with the feed stream and are redelivered to the storage tanks so
that no additional equipment is required and the concentration of
the stored gas is maintained relatively constant.
Accordingly, it is an object of the present invention to provide an
improved liquefaction system for gaseous materials. In particular,
it is an object to provide improved liquefaction system for the
liquefaction or reliquifaction of natural gas.
Another object is to provide improved liquefaction system which is
simple in construction and economical and efficient in
operation.
A still further object is to provide an improved liquefaction
system wherein the refrigerant components are delivered to a
compressor as a mixed stream which, after compression is separated
into individual streams which enter the system as liquid
streams.
Still another object is to provide an improved liquefaction system
wherein substantially less compression energy is required than in
similar systems.
Other objects of the invention will in part be obvious and will in
part appear hereinafter.
The invention accordingly comprises the several steps and the
relation of one or more of such steps with respect to each of the
others thereof, which will be exemplified in the method hereinafter
disclosed, and the scope of the invention will be indicated in the
claims.
For a fuller understanding of the nature and objects of the
invention, reference should be had to the following detailed
description taken in connection with the accompanying drawings in
which:
FIG. 1 is a flow sheet illustrating a first embodiment of the
invention;
FIGS. 2 and 3 are partial flow sheets illustrating modified forms
of the invention;
FIG. 4 is a flow sheet illustrating a second embodiment of the
invention; and
FIG. 5 is a partial flow sheet illustrating a modified form of the
embodiment of FIG. 4.
Referring now to the drawings, particularly FIG. 1 thereof, the
natural gas stream 36 is delivered to the system, at a high
pressure and at a temperature achievable by cooling water.
Preferably this natural gas stream is fed to the demethanizer 21 of
the stripper 20, wherein it is joined with the condensed mixed
refrigerant stream 55, fully described below, from the multistage
compressor 30. Alternatively, however, the natural gas stream 36,
if it is a substantially pure methane stream, can be delivered
directly to the inlet of the heat exchanger 10, as indicated by the
stream 26. In the illustrated example, both the natural gas stream
36 and the condensed mixed refrigerant stream 55 are assumed to be
at a pressure of 600 p.s.i.a., and at a temperature of 70.degree.
F., however, different pressures and temperatures can be used, if
desired, and the system appropriately modified accordingly.
The stripper 20 can be a conventional stripper of a type generally
used for fractional distillation, using heavy hydrocarbons as an
overall cycle reflux with the removal of heat in the reflux
condenser of each column and the supply of heat in the reboiler of
each column. In the illustrated example, the different columns of
the stripper 20 are a demethanizer 21, a deethanizer 22, a
depropanizer 23, a debutanizer 24 and a depentanizer 25, and each
includes a reflux condenser 68 and a reboiler 69.
The demehtanizer 21 essentially strips the methane from the
condensed mixed refrigerant stream 55, and at the same time
conditions the natural gas stream 36 so as to deliver an
essentially pure liquid methane stream 40 to the liquefaction
cycle. Most of the heavy ends in the natural gas stream will absorb
in the recycle heavy end stream 62, thereby reducing the
refrigeration required for condensation of the essentially pure
liquid methane stream 40 in the liquefaction cycle.
The methane stream 40 is progressively cooled in the liquefaction
cycle, in the heat exchangers 10--17 in a fashion such that it is
at approximately 600 p.s.i.a. and -250.degree. F, when it emerges
from the heat exchanger 17, in a manner fully described below. It
can then be delivered under pressure into a storage tank 18,
however, preferably it is expanded through the throttling valve 41
to a storage pressure, usually near atmospheric and in the
illustrated example assumed to be 15 p.s.i.a., and separated into a
liquid stream 42 which is delivered into a storage tank 18 or the
like and a gaseous methane stream 43, both of which are at
approximately -260.degree. F.
The gaseous methane stream 43 is joined with the hereinafter
described liquid ethane stream 58, and both of these streams are
joined with the stream 50 returned, through each of the heat
exchangers 10--17, to the multistage compressor 30. Ultimately, all
of the individual refrigerant components are joined with the stream
50 before it is returned to the multistage compressor 30, hence the
stream 50 hereinafter is referred to as the mixed refrigerant
stream 50. The gaseous methane stream 43 functions as the carrier
for the individual refrigerant components which are absorbed as
they evaporate into the mixed refrigerant stream 50 and,
furthermore, provides a gaseous stream which will result in
efficient operation of the heat exchangers of the system.
Accordingly, for proper operation of the system, it is essential
that a gas methane stream be coupled to and joined with the mixed
refrigerant stream 50. This gaseous methane stream can be provided
in the above-described manner or, alternatively, it can be provided
by, for example, coupling the boil-off from the storage tank 18 to
the mixed refrigerant stream. It will be obvious to those skilled
in the art that other gases or mixtures of gases can be substituted
for the methane in the gaseous stream 43, if desired.
Liquefaction of the liquid methane stream 40 in the heat exchangers
10--17 is accomplished by four refrigeration cycles, including an
ethane cycle, a propane cycle, a butane cycle and a pentane cycle.
Each of these refrigeration cycles is provided an essentially pure
liquid refrigeration component, from the stripper 20 and, from the
description below, it will be apparent that each of these cycles is
adapted to achieve a reduction in temperature of the liquid methane
stream 40 in the area where each of them can be operated with the
greatest efficiency.
More particularly, the ethane cycle is provided by the liquid
ethane stream 58 which is delivered to and passed through each of
the heat exchangers 10--17, from the deethanizer 22 of the stripper
20. In passing through these heat exchangers, the pressure of the
ethane stream 58 is maintained substantially constant at 600
p.s.i.a. and it is progressively cooled in respective ones of these
heat exchangers to 0.degree. F, -50.degree. F, -70.degree. E,
-100.degree. F, -140.degree. F, -160.degree. F, -180.degree. F and
-250.degree. F. Upon emerging from the heat exchanger 17, the
cooled liquid ethane stream is expanded through the throttling
valve 67 to 15 p.s.i.a. and -255.degree. F, and is joined with the
gaseous methane stream 43. As indicated above, both of these
streams are joined with the mixed refrigerant stream 50, prior to
entering the heat exchanger 17. The heat of vaporization of the
cooled liquid ethane is absorbed as it evaporates into the mixed
refrigerant stream 50, as the latter is warmed from -255.degree. F
to -180.degree. F in the heat exchanger 17, and provides the
refrigeration in the heat exchanger 17. It may be noted that a
portion of the cold liquid ethane stream 58, at the outlets of the
heat exchangers 14, 15 and 16, also is coupled to and joined with
the mixed refrigerant system 50. The cold liquid ethane stream 58
at the outlet of the heat exchanger 14 is at 600 p.s.i.a. and
-140.degree. F, and the portion thereof which is coupled to the
mixed refrigerant stream 50 is expanded through the throttling
valve 70 to 15 p.s.i.a. This cold liquid ethane stream joined with
the mixed refrigerant stream 50 provides the refrigeration for the
heat exchanger 14, as it evaporates into the mixed refrigerant
stream and warms from -140.degree. F to -100.degree. F. Similarly,
the portions of the cold liquid ethane stream removed at the
outlets of the heat exchangers 15 and 16 are expanded through the
throttling valves 71 and 72, respectively, to 15 p.s.i.a. before
being joined with the mixed refrigerant stream 50. The
refrigeration for these heat exchangers 15 and 16 is provided by
the evaporation of the expanded cold liquid ethane stream, as it
warms from -160.degree. F to -140.degree. and from -180.degree. F
to -160.degree. F in these heat exchangers 15 and 16,
respectively.
The propane cycle is provided by the liquid propane stream 59 which
is delivered to and passed through only the heat exchangers 10--13,
from the depropanizer 23 of the stripper 20. The liquid propane
stream 59 is progressively cooled in respective ones of these heat
exchangers 10--13 to 0.degree. F, -50.degree. F, -70.degree. of and
-100.degree. F, while the pressure thereof is maintained
substantially constant at 600 p.s.i.a. or less. This cooled liquid
propane stream 59 at -100.degree. F, is expanded through the
throttling valve 74 to 15 p.s.i.a. and is joined with the mixed
refrigerant stream 50. As in the case of the ethane stream 58, this
cooled liquid propane stream 59 provides the refrigeration in the
heat exchanger 13, as it evaporates into the mixed refrigerant
stream 50 and warms from -100.degree. F to -70.degree. F.
The butane cycle is provided by the liquid butane stream 60 which
is delivered to and passed through only the first three heat
exchangers 10--12, from the debutanizer 24. This liquid butane
stream is progressively cooled in respective ones of the heat
exchangers 10--12 to 0.degree. F, -50.degree. F and -70.degree. F.
The pressure again is maintained substantially constant at 500
p.s.i.a. or less. A portion of the butane stream 60, at the outlet
of the heat exchanger 11, is joined with the mixed refrigerant
stream 50, after being expanded through throttling valve 75 to 15
p.s.i.a., and the rest of the butane stream 60 is joined with the
mixed refrigerant stream, after passing through the heat exchanger
12 and being expanded through the throttling valve 76 to 15
p.s.i.a. The refrigeration for the heat exchangers 11 and 12 is
provided by this expanded butane stream, as it evaporates into the
mixed refrigerant stream 50 and warms from -50.degree. F to
0.degree. F and from -70.degree. F to -50.degree. F in the heat
exchangers 11 and 12, respectively.
The remaining refrigeration cycle is the pentane cycle and, in this
case, an essentially pure liquid pentane stream 61, at 400 p.s.i.a.
or less and 70.degree. F, is delivered to and passed through the
heat exchanger 10, from the depentanizer 25. In passing through the
heat exchanger 10, the liquid pentane stream 61 is cooled to
0.degree. F and is then expanded through the throttling valve 77 to
15 p.s.i.a. and joined with the mixed refrigerant stream 50. This
cooled pentane stream is evaporated into the mixed refrigerant
stream 50 and provides the refrigeration for the heat exchanger 10,
as it warms from 0.degree. F to 70.degree. F.
The depentanizer 25 can be adapted to also produce a side stream of
hexanes, if desired. A net product stream of heavy ends from the
bottom thereof is delivered to the recycle heavy end stream 62.
These heavy ends are pumped by the pump 63 to the reboiler product
cooler 64, and from the latter they flow through the pipe 65 to the
top of the first column or demethanizer 21 of the stripper 20.
These heavy ends join with the natural gas stream 36 and the mixed
refrigerant stream 55 from the multistage compressor 30, and all of
these streams are again separated in the stripper, in the
above-described manner.
It can be seen from the above description of the refrigeration
cycle of the liquefaction system that cooling is produced by
evaporating the cooled individual liquid refrigerant streams 58,
59, 60 and 61 progressively into the mixed refrigerant stream 50
which is a gas stream and which initially is essentially free of
these individual components. The result is a much more efficient
refrigeration effect, which provides a substantial reduction, up to
40 percent in comparison to the classical cascade cycle, in the
required compression energy.
The mixed refrigerant stream 50 which, as indicated above, is a gas
stream, emerges from the heat exchanger 10 at a pressure of 15
p.s.i.a. and a temperature of 70.degree. F. The mixed refrigerant
stream 50 is compressed in the multistage compressor 30 including
the compressor stages 31--34 to 600 p.s.i.a., and is cooled in
water cooled condensers 35 included between each of the compressor
stages. The compressed mixed refrigerant stream 55 from the
multistage compressor 30, is delivered to the top column or
demethanizer 21 of the stripper 20, for reseparation, in the manner
described above.
The composition of the liquid methane stream 42 fed to the storage
tank 18 can be regulated by removing the quantities of heavy
components which are condensed in heat exchangers 10, 11 and 12.
These separated liquids are coupled from the methane stream 40
through pipes 45--47 to the mixed refrigerant stream 50. Throttling
valves 51 and 52 are included in the pipes 45 and 46 (the stream in
pipe 47 is expanded through the throttling valve 76) for throttling
these heavy liquid components to approximately 15 p.s.i.a. These
three streams will essentially remove all of the propane, butane,
pentane and heavier hydrocarbons from the methane stream 40,
although the total quantity of these heavy ends is usually quite
small.
If it is desired to reduce the ethane content of the liquid methane
stream 42 below the level in the methane stream 40, the heat
exchanger 14 is split into two separate heat exchangers 14 and 14',
as illustrated in FIG. 2. The first heat exchanger 14 cools the
methane stream 40 from -100.degree. F to -115.degree. F, where an
ethane rich liquid stream 27 can be separated, and the methane
stream 40 is then cooled to -140.degree. F in the second heat
exchanger 14'. This ethane rich liquid stream 27 also can be
combined with the ethane stream 58 prior to the latter entering the
heat exchanger 15, to help provide the lowest temperature
cooling.
If it is desired to recover helium from the methane stream 40, a
high pressure separator (not shown) is provided between the outlet
of the heat exchanger 17 and the throttling valve 41. The gas
separated in this heat exchanger will essentially contain all of
the helium present in the natural gas stream 36, which can be a
most valuable byproduct obtainable only with this type of
cycle.
If the natural gas stream 36 is rich in heavy hydrocarbons, such as
ethane, propane, butanes and the like, a net product stream of
these valuable liquids can be withdrawn from the individual
refrigerant component stream 58--61 as the inventory of each
component exceeds the need of the liquefaction system.
Similarly, variations in the composition of the natural gas stream
36 can vary the required amount of each of the individual
refrigerant components in the refrigeration cycle, and these
variations can be made internally within the cycle by the recovery
of the component from the natural gas stream.
As indicated above, the boil-off from the storage tank 18 or the
like can be returned to the storage tank, after being passed
through the liquefaction system. The boil-off stream 28 is joined
with the gaseous methane stream 43, and is subsequently joined with
the mixed refrigerant stream 50 and is passed through the
liquefaction cycle, in the manner described above. Accordingly, the
need for additional compressors is eliminated, and the
concentration of the stored gas is maintained relatively
constant.
In FIG. 3, there is illustrated still another modification which
can be made in the above-described basic liquefaction cycle to
permit production of subcooled liquid natural gas, or slush natural
gas, or even solid natural gas, if it is desired. This modification
includes the addition of a heat exchanger 19 and separators. The
refrigerant composition is also changed to include nitrogen, helium
and/or hydrogen, as well as hydrocarbons. The condensed feed stream
40 from heat exchanger 17 emerges as a stream 80 which is a liquid
and gas mixture, and this stream 80, in this case, is coupled to a
separator 81. The gas stream 80, in this case, is coupled to a
separator 81. The gas stream 82 from the separator 81 contains most
of the nitrogen and helium from the feed stream 40, and is further
cooled in heat exchanger 19. The liquid stream from the separator
81 is further cooled in the heat exchanger 19, then throttled by
means of the throttling valve 84 to storage pressure and coupled
into the storage tank 18. The liquid stream 85 can be either
subcooled liquid, or a mixture of solid and liquid natural gas
known in the industry as slush. Any dissolved gas liberated in the
final throttling operation can be used to maintain pressure over
the storage liquid, and excess gas 86 is passed back through the
heat exchanger 19 where it then joins the mixed refrigerant stream
50.
The cold refrigerant gas stream 82 from the heat exchanger 19, is
throttled by means of the throttling valve 88 to a pressure of 15
p.s.i.a., and thereafter subcooled liquid methane 89, from the
storage tank 18 is added to it. The vaporization of this subcooled
liquid methane 89 into the nitrogen-helium, or nitrogen-hydrogen
gas stream 82 provides the refrigeration in the heat exchanger 19.
The remainder of the above-described liquefaction cycle is a
previously described.
The nitrogen and helium, or nitrogen and hydrogen, will recycle in
the feed stream 40, and again be separated in the separator 81. The
selection of the separation temperature depends on the particular
mixture being used and processed, and relies for its success on a
phenomenon known as reverse solubility which is exhibited by
hydrogen and helium at low temperatures. As the methane feed stream
40 approaches its solid point, the amount of helium or hydrogen
which can be dissolved becomes less, instead of greater as is
normal behavior, and it is possible to obtain almost complete
separations, especially at temperatures below -250.degree. F.
The cycle illustrated in FIG. 3 also can be modified by adapting
the throttling valve 84 to throttle the stream to a selected lower
pressure, say 300 p.s.i.a., and by adding a bleed line 100 and
another throttling valve 101. The bleed line 100 is coupled to the
outlet of the throttling valve 84, before the throttling valve 101,
to permit nitrogen, hydrogen, or helium and methane gases at the
lower pressure to be bled off for other uses, if desired. The
throttling valve 101 then throttles the stream to storage pressure,
prior to its being coupled into the storage tank 18.
Referring now to FIG. 4, there is illustrated the apparatus used to
liquify natural gas in accordance with another, and preferred
embodiment of the invention. The system disclosed here requires
substantially the same refrigeration horsepower as the system
described above, however, its simplified and reduced fractionation
equipment requirement provides substantial economic savings.
In this system, the natural gas stream 125 preferably is a
substantially pure methane stream and is delivered to the system at
a high pressure and at a temperature achievable by cooling water,
such as, for example, 600 p.s.i.a. and 70.degree. F. This natural
gas stream is progressively cooled in the heat exchangers 130--135,
in a fashion such that it is at approximately 600 p.s.i.a and
-250.degree. F, when it emerges from the heat exchanger 135, in a
manner fully described below. It can then be delivered under
pressure into a storage tank 136, however, preferably it is
expanded through the throttling valve 137 to a storage pressure,
usually near atmospheric and again, in the illustrated example,
assumed to be 15 p.s.i.a., and also separated into a liquid stream
138 which is delivered into the storage tank 136 or the like and a
gaseous stream 139, both of which are at approximately -260.degree.
F. This gaseous stream 139 is joined with an exhaust stream 140
which is coupled to and used as fuel to drive the turbines (not
shown) for the multistage compressor 141.
The fractionation equipment in this system is substantially
simplified, in comparison to the stripper 20 used in the system
disclosed in FIG. 1, as will be apparent from the description
below. The mixed refrigerant stream 160, described more fully
below, is delivered from the multistage compressor 141 to the still
142, at a pressure of 600 p.s.i.a. At the top of the still 142, a
vapor stream 161 consisting primarily of methane and ethane is
delivered, at a pressure of 600 p.s.i.a. and at a temperature of
approximately 20.degree. F, to the heat exchanger 131. Upon
emerging from the heat exchanger 131, the stream 161 is at
approximately 6.degree. F, and a stream 162 of methane and ethane,
at a ratio of approximately 3:1, is stripped off and pumped by
means of a pump 163 back to the top of the still 142, to cool the
top thereof. The remaining portion of the stream 161 which now
consists of equal proportions of methane and ethane passes through
each of the heat exchangers 132--135 and, when it emerges from the
heat exchanger 135, it is still at substantially the same pressure
of 600 p.s.i.a., however, it is cooled to a temperature of
-250.degree. F. The stream 161 then is expanded through the
throttling valve 164 to 15 p.s.i.a. and -260.degree. F, and merged
with the refrigerant stream 160 which is passed through the heat
exchangers 135--130 to the multistage compressor 141. The stream
161 is a gaseous stream consisting of substantially equal amounts
of methane and ethane, and functions, as the above-described
gaseous methane stream 43, as the carrier for the individual mixed
refrigerant stream 160, also described below. This gaseous stream
also provides a gaseous stream which will result in efficient
operation of the heat exchangers of the system.
The bottom product of the still 142 is primarily ethane and it is
passed as a stream 166 through a heater 147. Upon emerging from the
heater 147, a portion of the stream 166 is refluxed via the stream
167 back into the still 142. The remainder portion of the stream
166 is principally ethane and butane, and this gaseous mixture,
also at a pressure of approximately 600 p.s.i.a., is coupled as
stream 168 and is throttled to a pressure less than 500 p.s.i.a.
before entering still 143.
The vapors leaving the top of the still 143 are coupled in a stream
169 to a cooler 150 wherein a portion of the stream is condensed
and refluxed back into the still 143, as stream 170. The remaining
portion of the stream is primarily ethane, and this is coupled as
stream 171 to and through the heat exchangers 130--134 wherein it
is cooled so that upon emerging from the heat exchanger 134 it is
at approximately -140.degree. F. This cooled now liquid stream is
expanded through throttling valve 189 to 15 p.s.i.a., and joined
with the mixed refrigerant stream 160. The heat of vaporization of
the cooled liquid stream 171 is absorbed as it evaporates into the
mixed refrigerant stream 160, as the latter is warmed from
-140.degree. F to -80.degree. F in the heat exchanger 134, and
provides the refrigeration in the heat exchanger 134.
The product at the bottom of the still 143 consists primarily of
pentane and butane, and is coupled as stream 175 to and through a
heater 148. A portion of the stream 175 is recycled to the still
143, via stream 176, and the remaining portion thereof is expanded
through throttling valve 178 to 70 p.s.i.a. and 155.degree. F. The
liquid portion of this expanded stream is separated and coupled as
stream 179 through a cooler 151 which cools it to approximately
80.degree. F. From the cooler 151, it is coupled to and through the
heat exchanger 130. This liquid stream is cooled to 20.degree. F on
passing through the heat exchanger 130, and then is expanded
through the throttling valve 180 to 15 p.s.i.a. Upon being
expanded, the liquid stream is joined with the mixed refrigerant
stream 160. The heat of vaporization of the cooled liquid stream is
absorbed as it evaporates into the mixed refrigerant stream 160, as
the latter is warmed from 20.degree. F to 0.degree. F in the heat
exchanger 130, and provides the refrigeration in the heat exchanger
130.
The remaining gaseous portion of the stream 175 is passed
successively through the coolers 144--146. In the cooler 144, the
stream is cooled to approximately 142.degree. F and, upon emerging
therefrom, another liquid stream 181 which consists primarily of
butane is separated and passed through a cooler 152 which cools it
to approximately 80.degree. F. The liquid stream 181 then is
coupled to and through the heat exchangers 130 and 131. Upon
emerging from the heat exchanger 131, the liquid stream 181 is at
approximately 6.degree. F. This cooled liquid stream 181 then is
expanded through throttling valve 182, and joins with the mixed
refrigerant stream 160. Again, the heat of vaporization of the
cooled stream is absorbed as it evaporates into the mixed
refrigerant stream 160, as the latter is warmed from 6.degree. F to
20.degree. F in the heat exchanger 131, and provides the
refrigeration in the heat exchanger 131.
Still another liquid stream 184 is separated from the stream 175
after the latter has passed through the cooler 145 and cooled to
approximately 120.degree. F. This liquid stream 184 also consists
primarily of butane but has some pentane and a small amount of
propane and hexane in it. The liquid stream 184 likewise is cooled
to approximately 80.degree. F in passing through a cooler 153,
before being coupled to and through the heat exchangers 130--132.
In passing through these heat exchangers, the liquid stream 184 is
cooled to approximately -40.degree. F. Thereafter, it is expanded
through the throttling valve 185 to 15 p.s.i.a. and joined with the
mixed refrigerant stream 160. In this case also, the heat of
vaporization of the cooled liquid stream is absorbed as it
evaporates into the mixed refrigerant stream 160, as the latter is
warmed from -40.degree. F to 6.degree. F in the heat exchanger 132,
and provides the refrigeration for the heat exchanger 132.
The remaining vapor portion of stream 175 upon passing through the
cooler 146 is cooled to approximately 80.degree. F, and is coupled
to and through the heat exchangers 130--133 as stream 186. This
stream 186 also consists primarily of butane but has a fair amount
of propane and smaller amounts of ethane, pentane and hexane in it.
Upon passing through the heat exchangers 130--133, the stream 186
is cooled to approximately -80.degree. F, and then is expanded
through throttling valve 187 to 15 p.s.i.a. and joined with the
mixed refrigerant stream 160. As in the case of the streams 171,
179, 181 and 184, the heat of vaporization of this cooled liquid
stream is absorbed as it evaporates into the mixed refrigerant
stream 160, as the latter is warmed from -80.degree. F to
-40.degree. F in the heat exchanger 133, and provides the
refrigeration in the heat exchanger 133.
The mixed refrigeration stream 160 upon emerging from the heat
exchanger 130 is at a pressure of 15 p.s.i.a. and a temperature of
approximately 70.degree. F. The mixed refrigerant stream then is
compressed in the multistage compressor 141 including the
compressor stages 190--193 to 600 p.s.i.a. and is condensed in
water cooled condensers 194 included between each of the compressor
stages. The compressed mixed refrigerant stream 160 from the
multistage compressor 141 is delivered to the still 142 for
reseparation, in the manner described above.
A bypass 196 is provided to strip off liquid forming in the
compressor stages 190 and 191, to prevent damage to the compressor
stage 192. Similarly, a bypass 197 is provided to strip off liquid
forming in the mixed refrigerant stream, to prevent damage to the
compressor stage 193. Stripping off the liquid in this manner and
pumping it into the discharge of the succeeding stage, the horse
power required to compress the mixed refrigerant stream is
substantially reduced.
Referring now again to the natural gas stream 125, it can be seen
that it is successively cooled from 70.degree. F to 20.degree. F,
6.degree. F, -40.degree. F, -80.degree. F, -140 F and -250.degree.
F as it passes through the heat exchangers 130--135, respectively,
in heat exchange relationship with the mixed refrigerant stream
160. As this natural gas stream is cooled, some of the heavier
components such as hexane, pentane and butane are stripped off of
it, as indicated by the streams 198--200, so that substantially
pure methane is delivered into the storage tank 136. As indicated
above, upon emerging from the heat exchanger 135, the stream 125
preferably is expanded through throttling valve 137, to
approximately atmospheric pressure for storage. Boil-off from the
storage tank 136 can be joined with the gaseous methane stream 139
which is subsequently joined with the exhaust stream 140, if
desired.
The mixed refrigerant stream 160 consists of a mixture of methane,
ethane, propane, butane, pentane and hexane, however, these
components rather than being joined with it as individual
substantially pure component streams as in the case of the system
of FIG. 1, are joined with it in the form of mixed component
streams 171, 179, 181, 184 and 186. By using mixed component
streams rather than pure component streams, and coupling them into
the mixed refrigerant stream where each stream can be operated with
the greatest efficiency to achieve the desired cooling, substantial
economic savings are realized in view of the simplified
fractionation equipment which is required in the system.
Helium also can be recovered as a byproduct, with the system of
FIG. 3, in the manner shown in FIG. 4. It can be seen that in this
system, the helium can be recovered simply by adding an additional
throttling valve 202 in series with the throttling valve 137 and
coupling a bleedoff line 203 to the stream 125, between the two
throttling valves 137 and 202, as illustrated in FIG. 5.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently
attained and certain changes may be made in carrying out the above
method. Accordingly, it is intended that all matter contained in
the above description shall be interpreted as illustrative and not
in a limiting sense.
Now that the invention has been described, what is claimed as new
and desired to be secured by Letters Patent is:
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