U.S. patent number 3,808,826 [Application Number 05/075,992] was granted by the patent office on 1974-05-07 for refrigeration process.
This patent grant is currently assigned to Phillips Petroleum Company. Invention is credited to Dunn M. Bailey, Ernest A. Harper.
United States Patent |
3,808,826 |
Harper , et al. |
May 7, 1974 |
REFRIGERATION PROCESS
Abstract
A refrigeration process employing a single- or plural-stage
flashing of a refrigerant liquid to obtain high and/or intermediate
temperature cooling, and condensation of refrigerant vapor and
subsequent flashing thereof to obtain low temperature cooling, of a
heat exchange zone so as to remove heat from said zone. In one
embodiment, a gas is passed through said heat exchange zone in heat
exchange relationship with said refrigerants and at least partially
liquefied.
Inventors: |
Harper; Ernest A.
(Bartlesville, OK), Bailey; Dunn M. (Bartlesville, OK) |
Assignee: |
Phillips Petroleum Company
(Bartlesville, OK)
|
Family
ID: |
22129236 |
Appl.
No.: |
05/075,992 |
Filed: |
September 28, 1970 |
Current U.S.
Class: |
62/612 |
Current CPC
Class: |
C07C
9/04 (20130101); F25J 1/0258 (20130101); F25J
1/0262 (20130101); F25J 3/0233 (20130101); F25J
1/0212 (20130101); F25J 1/0292 (20130101); F25J
1/0249 (20130101); F25J 1/0229 (20130101); F25J
1/0055 (20130101); F25J 1/0045 (20130101); F25J
3/0209 (20130101); F25J 3/0242 (20130101); C07C
7/005 (20130101); F25J 1/004 (20130101); F25J
1/0244 (20130101); F25J 1/0022 (20130101); C07C
7/005 (20130101); F25J 2200/02 (20130101); F25J
2200/30 (20130101); F25J 2290/40 (20130101); F25J
2245/02 (20130101); F25J 2210/06 (20130101); F25J
2200/74 (20130101); F25J 2205/04 (20130101); F25J
2290/32 (20130101) |
Current International
Class: |
C07C
7/00 (20060101); F25J 3/06 (20060101); F25J
1/00 (20060101); F25J 3/02 (20060101); F25J
1/02 (20060101); F25j 001/00 (); F25j 001/02 ();
F25j 003/02 () |
Field of
Search: |
;62/9,11,23,24,27,28,29,30,40 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Purcell; Arthur F.
Claims
We claim:
1. A method for removing heat from a heat exchange zone and the
contents thereof, utilizing refrigerant vapor and refrigerant
liquid streams obtained from a compressed and partially liquefied
refrigerant, which method comprises:
passing a stream of said refrigerant vapor through a first region
of said heat exchange zone in indirect heat exchange relationship
with at least one expanded liquid stream so as to cool said
refrigerant vapor, said expanded liquid stream derived from said
refrigerant liquid expanded to a first pressure and separated into
expanded liquid and expanded vapor producing said expanded liquid
having a temperature less than the temperature of said refrigerant
vapor;
passing said cooled refrigerant vapor through a succeeding
downstream region of said heat exchange zone in indirect heat
exchange relationship with another stream of refrigerant, obtained
as described hereinafter, said another stream of refrigerant at a
temperature less than the temperature of said refrigerant liquid,
so as to further cool and liquefy said refrigerant vapor;
flashing said liquefied refrigerant vapor to a pressure lower than
the pressure to which said one stream of said refrigeration liquid
is expanded so as to further cool same and to obtain said another
refrigerant stream of mixed vapor and liquid; and
removing from said heat exchange zone vapors resulting from the
vaporization of said refrigerants so as to remove heat from said
zone.
2. A method in accordance with claim 1 wherein said refrigerant
from which said refrigerant vapor and said refrigerant liquid are
obtained is a multicomponent refrigerant.
3. A method in accordance with claim 2 wherein:
said vapors resulting from the vaporization of said refrigerants
are compressed, cooled to partially liquefy same, and passed to a
storage zone; and
said first-mentioned refrigerant vapor and first-mentioned
refrigerant liquid are withdrawn from said storage zone for use in
said heat exchange zone.
4. A method in accordance with claim 2 wherein said method
comprises, in combination, the steps of:
a. passing a stream of said refrigerant vapor through said heat
exchange zone;
b. introducing a first stream of said refrigerant liquid at a
temperature less than the temperature of said stream of refrigerant
vapor of step (a) into a first region of said heat exchange zone
and passing same in indirect heat exchange relationship with said
stream of refrigerant vapor of step (a) so as to vaporize said
refrigerant liquid and cool said stream of refrigerant vapor;
c. introducing a second stream of said refrigerant liquid at a
temperature less than the temperature of said first stream of
refrigerant liquid of step (b) into a second region of said heat
exchange zone, downstream from said first region with respect to
the flow of said refrigerant vapor, and passing same in indirect
heat exchange relationship with said stream of refrigerant vapor so
as to vaporize said refrigerant liquid and further cool said stream
of refrigerant vapor;
d. introducing a mixed stream of refrigerant vapor and refrigerant
liquid, obtained as described hereinafter, at a temperature less
than the temperature of said second stream of refrigerant liquid of
step (c) into a third region of said heat exchange zone, downstream
from said second region with respect to the flow of said
refrigerant vapor, and passing same in indirect heat exchange
relationship with said stream of refrigerant vapor so as to
vaporize said refrigerant liquid and further cool and liquefy said
refrigerant vapor;
e. withdrawing a stream of said liquefied refrigerant vapor from
said heat exchange zone;
f. flashing said liquefied stream of step (d) to further cool same
and form refrigerant vapor and refrigerant liquid; and
g. introducing a mixture of said refrigerant vapor and said
refrigerant liquid obtained in said step (f) into said third region
of said heat exchange zone as said mixed stream of refrigerant
vapor and refrigerant liquid of said step (d).
5. A method in accordance with claim 4 wherein:
said refrigerant vapor of step (a) is withdrawn from a refrigerant
storage zone;
refrigerant liquid is withdrawn from said storage zone and flashed
a first time to reduce the temperature and pressure thereof;
a first portion of said flashed refrigerant liquid is introduced
into said heat exchange zone in step (b) as said first refrigerant
liquid; and
a second portion of said flashed refrigerant liquid is flashed a
second time and the resulting twice flashed refrigerant liquid is
introduced into said heat exchange zone in step (c) as said second
refrigerant liquid.
6. A method in accordance with claim 5 wherein:
vapors resulting from said step (b) are withdrawn from said heat
exchange zone and compressed;
vapors resulting from said step (c) are withdrawn from said heat
exchange zone and compressed;
vapors resulting from said step (d) are withdrawn from said heat
exchange zone and compressed; and
said compressed vapors are combined, cooled to partially liquefy
same, and the resulting mixture is passed to said storage zone.
7. A method in accordance with claim 6 wherein:
vapor and liquid from said first flashing of said refrigerant
liquid are passed to a first phase separation zone, a first portion
of the resulting separated liquid is withdrawn from said first
separation zone and utilized in said step (b), and vapors withdrawn
from said first separation zone are combined with said step (b)
vapors withdrawn from said heat exchange zone, prior to said
compression; and
vapor and liquid from said second flashing of said refrigerant
liquid are passed to a second phase separation zone, the resulting
separated liquid is withdrawn from said second separation zone and
utilized in said step (c), and vapors withdrawn from said second
separation zone are combined with said step (c) vapors withdrawn
from said heat exchange zone, prior to said compression.
8. A method in accordance with claim 6 wherein:
said step (d) vapors are compressed in a first stage of a
three-stage compression zone;
said step (c) vapors and said vapors from said second separation
zone are combined with the compressed vapors from said first stage
and the resulting mixture compressed in a second stage of said
compression zone;
said step (b) vapors and said vapors from said first separation
zone are combined with the compressed vapors from said second stage
and the resulting mixture compressed in a third stage of said
compression zone; and
the compressed vapors from said third stage are cooled to partially
liquefy same and the resulting mixture is passed to said storage
zone.
9. A method in accordance with claim 4 wherein:
said refrigerant vapor of step (a) is withdrawn from a refrigerant
storage zone;
a first portion of refrigerant liquid withdrawn from said storage
zone is flashed to reduce the temperature and pressure thereof;
said flashed first portion of refrigerant liquid is introduced into
said heat exchange zone in step (b) as said first refrigerant
liquid;
a second portion of refrigerant liquid withdrawn from said storage
zone is flashed a first time to reduce the temperature and pressure
thereof, and liquid refrigerant resulting from said first flashing
is flashed a second time to further reduce the temperature and
pressure thereof; and
said twice flashed second portion of refrigerant liquid is
introduced into said heat exchange zone in step (c) as said second
refrigerant liquid.
10. A method in accordance with claim 6 wherein:
said vapors resulting from step (c) are withdrawn from said heat
exchange zone after passage through said second region thereof;
and
said vapors resulting from step (d) are withdrawn from said heat
exchange zone after passage through said third region thereof.
11. A method in accordance with claim 6 wherein:
said vapors resulting from step (c) are withdrawn from said heat
exchange zone after passage through said second region and said
first region of said heat exchange zone.
12. A method in accordance with claim 6 wherein:
said vapors resulting from step (d) are withdrawn from said heat
exchange zone after passage through said third region, said second
region, and said first region of said heat exchange zone.
13. A method in accordance with claim 6 wherein:
said vapors resulting from step (c) are withdrawn from said heat
exchange zone after passage through said second region and said
first region of heat exchange zone; and
said vapors resulting from step (d) are withdrawn from said heat
exchange zone after passage through said third region, said second
region, and said first region of said heat exchange zone.
14. A method according to claim 4 comprising, in further
combination, the steps of:
h. passing a stream of gas through said heat exchange zone in
indirect heat exchange relationship with said streams of
refrigerant vapor and refrigerant liquids, and at least partially
liquefying said stream of gas.
15. A method according to claim 14 wherein said stream of gas in
step (h) is a natural gas, and said method comprises, in further
combination, the steps of:
i. withdrawing a partially liquefied stream of said natural gas
from said heat exchange zone and separating same into a vapor
stream comprising methane and a liquid stream comprising ethane and
higher boiling hydrocarbons;
j. recovering said vapor stream comprising methane of step (i) as
one product of the process; and
k. recovering a stream comprising propane and higher boiling
hydrocarbons from said liquid stream of step (i) as another product
of the process.
16. A method according to claim 15 wherein:
the temperature of said mixed stream of refrigerant vapor and
liquid introduced into said heat exchange zone in step (d) is
adjusted by controlling the amount of said liquefied refrigerant
vapor flashed in step (f) in accordance with the temperature of
said partially liquefied stream of natural gas withdrawn from said
heat exchange zone in step (i) so as to maintain the final
temperature on said partially liquefied stream of natural gas at a
predetermined value;
the amount of said liquefied refrigerant vapor flashed in step (f)
is further controlled in accordance with the flow rate of said
refrigerant vapor introduced into said heat exchange zone in step
(a); and
the ratio of the amount of vapor withdrawn from said first phase
separation zone to the amount of said step (b) vapors withdrawn
from said heat exchange zone is maintained at a predetermined
value.
17. A method according to claim 15 comprising, in further
combination, the steps of:
l. passing said stream comprising methane, prior to recovery in
step (j), through said heat exchange zone;
m. in step (k), flashing said liquid stream of step (i) comprising
ethane and higher boiling hydrocarbons to form a vapor and a
liquid;
n. recombining said last-mentioned vapor and liquid, passing said
recombined vapor and liquid through a portion of said heat exchange
zone and then introducing same as feedstock into a deethanizing
zone; and
o. recovering said stream comprising propane and higher boiling
hydrocarbons of step (k) from said deethanizing zone as another
product of the process.
18. A method according to claim 17 wherein said liquid stream of
step (i) also contains some dissolved methane, and said process
comprises, in further combination, the steps of:
p. in said deethanizing zone of step (n), fractionating said
feedstock to recover an overhead stream comprising methane, ethane,
and propane, and a bottoms stream comprising propane and higher
boiling hydrocarbons;
q. partially condensing said overhead stream from step (p);
r. returning a first portion of said condensed overhead of step (q)
to said deethanizing zone as reflux thereto;
s. passing a second portion of said condensed overhead of step (q)
to a demethanizing zone as feedstock thereto; and
t. in said demethanizing zone, fractionating said feedstock to
recover an overhead stream comprising methane, and a bottoms stream
comprising ethane and propane as another product of the
process.
19. A method according to claim 18 wherein:
said overhead stream from said demethanizing zone is combined with
said overhead stream from said deethanizing zone prior to said
partial condensation of step (q) and the resulting condensate is
passed to a common accumulator zone from which said portions of
condensate of steps (r) and (s) are withdrawn;
a stream comprising methane is removed from said common accumulator
zone;
said bottoms stream from said deethanizer zone is removed from a
reboiler region therein;
said bottoms product comprising ethane and propane from said
demethanizing zone is removed from a reboiler region therein;
and
said reboiler region of said demethanizing zone is heated by
indirect heat exchange with warm liquid from said reboiler region
of said deethanizing zone.
20. A method according to claim 19 wherein:
the amount of said first portion of condensed overhead returned to
said deethanizing zone is set at a predetermined amount;
the amount of said second portion of condensed overhead fed to said
demethanizing zone is controlled in accordance with the liquid
level in said reboiler region therein;
the amount of warm liquid from said reboiler region of said
deethanizer zone used to heat said reboiler region of said
demethanizing zone is controlled in accordance with the temperature
in said reboiler region of said demethanizing zone; and
the amount of heat supplied to said reboiler region of said
deethanizing zone is controlled in accordance with the level in
said common accumulator zone so as to maintain a desired level in
said common accumulator zone.
Description
This invention relates to a refrigeration process.
Various refrigeration processes or cycles are known in the art. In
many instances, these refrigeration processes or cycles have been
developed for particular purposes. In all instances, it is of the
utmost importance to obtain maximum efficiency in the refrigeration
cycle so as to keep investment and operating costs at a minimum.
This is particularly important, and difficult, when one is cooling
and at least partially liquefying a gas such as natural gas.
A typical natural gas, while composed predominantly of methane,
will contain significant amounts of higher boiling hydrocarbons.
This complicates the problem of cooling and liquefying the natural
gas because heat must be removed from the gas over a wide
temperature range. Consequently, many of the prior art processes
are inefficient and thus uneconomical from the standpoint of
investment cost and/or operating cost. One prior art method uses
the residue gaseous methane stream as the refrigerant. Said residue
methane stream is passed through an expansion turbine, thereby
lowering its pressure and temperature sufficiently that it can be
employed to cool the incoming feed gas stream in countercurrent
heat exchangers. In other prior art methods, external refrigeration
systems are employed. Depending upon the temperature reduction
desired, more than one refrigerant is frequently employed in the
well known cascade systems. Such a system requires separate
compressors for each refrigerant.
The present invention provides a more efficient external
refrigeration system which is capable of cooling natural gas to a
very low temperature without the need for separate refrigerants and
separate compressors. Thus, the refrigerant employed in the
practice of the invention can be a single-component refrigerant,
for example, as essentially pure hydrocarbon such as ethane or
propane, a freon, etc. However, multicomponent refrigerants are
preferred for use in the refrigeration system of the invention
because they are capable of being employed to effect cooling of a
material over a wider range of temperature than is possible with
single-component refrigerants. This is possible with multicomponent
refrigerants because the composition of such refrigerants, as well
as the pressure, is changed during the refrigeration cycle in order
to obtain the desired refrigeration temperature (boiling point of
the refrigerant). One presently preferred refrigerant which can be
employed in the practice of the invention comprises a mixture of
ethane and propane. However, depending upon the refrigeration
temperature desired, it is within the scope of the invention for
said refrigerant to contain methane, nitrogen, helium, or other
gases more volatile than ethane.
An object of this invention is to provide an improved refrigeration
cycle or process. Another object of this invention is to provide an
improved refrigeration cycle or process wherein both a refrigerant
vapor and a refrigerant liquid are employed for more efficient
refrigeration in the cooling of a heat exchange zone for the
removal of heat therefrom. Another object of this invention is to
provide an improved method for cooling and/or liquefying a natural
gas. Another object of this invention is to provide an improved
method for recovering higher boiling components from a natural gas.
Another object of this invention is to provide an improved method
for recovering a refrigerant mixture from a natural gas. Other
aspects, objects, and advantages of the invention will be apparent
to those skilled in the art in view of this disclosure.
Thus, according to the invention, there is provided a method for
removing heat from a heat exchange zone and the contents thereof,
utilizing refrigerant vapor and refrigerant liquid streams obtained
from a compressed and partially liquefied refrigerant, which method
comprises: passing a stream of said refrigerant vapor through a
first region of said heat exchange zone in indirect heat exchange
relationship with at least one stream of sai refrigerant liquid
having a temperature less than the temperature of said refrigerant
vapor so as to cool said vapor; passing said cooled refrigerant
vapor through a succeeding downstream region of said heat exchange
zone in indirect heat exchange relationship with another stream of
refrigerant, obtained as described hereinafter, and at a
temperature less than the temperature of said refrigerant liquid,
so as to further cool and liquefy said refrigerant vapor; flashing
said liquefied refrigerant vapor so as to further cool same and to
obtain said another refrigerant; and removing from said heat
exchange zone vapors resulting from the vaporization of said
refrigerants so as to remove heat from said zone.
Further according to the invention, there is provided a process for
recovering a mixture comprising ethane and propane from natural
gas, which process comprises, in combination, the steps of: (a)
cooling said natural gas under pressure and temperature conditions
sufficient to partially liquefy same; (b) fractionating said
partially liquefied natural gas from step (a) in a first
fractionation zone to recover an overhead stream comprising
methane, ethane, and propane, and a bottoms stream comprising some
propane and higher boiling hydrocarbons; (c) partially condensing
said overhead stream from step (b); (d) returning a first portion
of said condensed overhead from step (c) to said first
fractionation zone as reflux thereto; (e) passing a second portion
of said condensed overhead from step (c) to a second fractionation
zone as feed thereto; and (f) in said second fractionation zone,
fractionating said feed to recover an overhead stream comprising
methane, and a bottoms stream comprising ethane and propane as one
product of the process.
Any suitable refrigerant, either a single-component refrigerant or
a multicomponent refrigerant, can be employed in the practice of
the invention. However, multicomponent refrigerants are usually
preferred. One presently preferred refrigerant is a multicomponent
refrigerant comprising a mixture of ethane and propane containing
from about 5 percent to about 95 percent ethane, and from about 95
percent to about 5 percent propane. This refrigerant finds wide
application in the recovery of ethane and heavier hydrocarbons from
natural gas. Said preferred refrigerant can contain components
higher boiling than propane, e.g., butane and/or pentanes, usually
present in amounts of from about 0.1 to 5 per cent. However, the
present of said higher boiling components is usually not preferred.
Depending upon the degree of cooling desired, e.g., whether or not
it is desired to completely liquefy a natural gas, said preferred
refrigerant can contain from 10 to 50 per cent of a component more
volatile than ethane, such as methane, helium, nitrogen, etc.
FIG. 1 is a schematic flow sheet diagrammatically illustrating
several embodiments of the invention.
FIG. 2 is a schematic flow sheet diagrammatically illustrating
another embodiment of the invention.
Referring now to the drawings, wherein like reference numerals have
been employed to denote like elements, the invention will be more
fully explained. It will be understood that many valves, pumps,
control instruments, and other conventional equipment not necessary
for explaining the invention have been omitted for the sake of
brevity. For convenience, and not by way of limitation, the
invention will be described with reference to the partial
liquefaction of a stream of natural gas so as to be able to
separate ethane and higher molecular weight components from methane
and other components more volatile than ethane contained in said
natural gas. The ethane and heavier components of natural gas are
oftentimes referred to as natural gas liquids. These "liquids"
include hydrocarbons such as ethane, propane, butanes, pentanes,
and sometimes higher molecular weight components, which are
valuable as raw materials for preparing various petrochemicals. The
more volatile components referred to above include, in addition to
methane, such materials as hydrogen, nitrogen, helium, and the
like. The natural gas feed stream may sometimes be obtained as the
effluent from a natural gasoline plant and will have had at least a
portion of the heavier hydrocarbons, water, and carbon dioxide
removed. In the calculated example used hereinafter in connection
with the description of the drawings, the natural gas stream has
been dehydrated to a -100.degree. F. dew point by conventional
means not shown, and contains less than 0.02 mol per cent carbon
dioxide. While not shown in the drawings, it will be understood
that if a wet gas stream is to be refrigerated for the liquefaction
thereof, and/or the recovery of heavy hydrocarbons therefrom,
provision should be made to dehydrate the gas for water removal and
to withdraw compounds such as benzene and carbon dioxide which
would solidify at the low temperatures employed. Such materials in
liquid state can be tapped off from the heat exchangers at
appropriate points of temperature and pressure. It should also be
understood that the various operating conditions given herein in
connection with the description of the drawing are not to be
construed as limiting on the invention. Said conditions can be
varied depending upon the gas or other material being cooled, the
amount of cooling desired, and in the case of fractionators the
separation to be effected.
The drawings will be described with reference to a calculated
example using a typical natural gas having the composition:
Mol Percent Carbon Dioxide 0.40 Nitrogen 0.91 Methane 85.32 Ethane
6.78 Propane 3.50 Isobutane 0.57 n-butane 1.33 Isopentane 0.39
n-pentane 0.47 Hexanes + 0.33 Total: 100.00
In one embodiment of the invention, a natural gas stream at a
temperature of about 90.degree. F. and a pressure of about 550 psig
is passed via conduit 10 into heat exchanger 12 wherein it is
cooled to a temperature of about -75.degree. F. by indirect heat
exchange with the cold refrigerants and employing the refrigeration
cycle of the invention as described hereinafter. Said heat
exchanger 12 can be any suitable type of heat exchanger. As here
illustrated diagrammatically, said heat exchanger comprises a
plate-type exchanger wherein the various segments of the exchanger
are all housed in one shell. Such exchangers are conventional and
well known in the art. However, it is within the scope of the
invention to employ three or more individual exchangers instead of
a unitary exchanger as illustrated. It is also within the scope of
the invention to employ tube and shell-type exchangers instead of a
plate-type exchanger.
The reduction in temperature of said natural gas stream during its
passage through heat exchanger 12 causes about 20 mol per cent of
the gas to liquefy. About 50 per cent of the ethane, about 80 per
cent of the propane, and all of the heavier hydrocarbons will be
liquefied under the above-described conditions. The partially
liquefied stream is withdrawn from heat exchanger 12 via conduit 14
and passed into phase separator 16. The noncondensed gas comprising
methane is returned to said exchanger 12 via conduit 18 for
countercurrent heat exchanger with the incoming gas stream in
conduit 10. The now warmed residue gas exits from said heat
exchanger via conduit 20 for delivery to a pipeline or other use.
If desired, said residue gas can be withdrawn from the system via
conduit 22 instead of being returned to said heat exchangers 12. If
desired, the liquefied hydrocarbons in separator 16 can be
withdrawn therefrom via conduit 24. Preferably, said liquefied
hydrocarbons are passed through expansion valve 26 for flash
vaporization of a portion thereof by reducing the pressure to about
410 psig and the temperature to about -81.degree. F. The resulting
mixture of liquid and vapor is passed into phase separator 28
wherein the two phases are separated as indicated in the drawing.
Said two phases are withdrawn from separator 28 separately and,
preferably, are recombined prior to being returned to heat
exchanger 12 for passage through at least a portion thereof. The
use of a separator such as separator 28 is preferred in order to
distribute the liquid phase uniformly with the gas phase when two
phases are being passed through the heat exchanger. Here, and
elsewhere, where a liquid and vapor are both introduced into a heat
exchanger it may be desirable to employ a liquid-vapor distributor
such as described in U.S. Pat. No. 3,158,010, issued Nov. 24,
1964.
The partially warmed, recombined stream is withdrawn from heat
exchanger 12 via conduit 30 and passed into deethanizer 32. A
stream comprising essentially all the methane and ethane is removed
overhead from said deethanizer and, preferably, is returned to said
heat exchanger 12 via conduit 34 for additional heat exchange with
the incoming gas stream in conduit 10. Said residue gas in conduit
34 can be removed from exchanger 12 via conduit 36, recompressed to
pipeline pressure by compressor 38, and combined with the residue
gas in conduit 20. If desired, said residue gas can be removed from
the system through conduit 40 prior to compressor 38 or through
conduit 42 after compressor 38, instead of passing same into the
pipline. The bottoms from deethanizer 32 comprises propane and
heavier hydrocarbons and are recovered as one product of the system
via conduit 44.
In the presently preferred refrigeration cycle employed to
refrigerate the natural gas stream just described, a multicomponent
refrigerant comprising a mixture of ethane and propane is
advantageously employed. In said preferred refrigeration cycle,
there is employed two-stage flashing of a refrigerant liquid to
obtain high and intermediate temperature cooling and single-stage
condensation of refrigerant vapor to obtain low temperature
cooling. Said mixture of ethane and propane is compressed in the
compressor system illustrated to a pressure of about 220 psig,
cooled to a temperature of about 95.degree. F. in water cooled
exchanger 46 and passed via conduit 48 into surge or storage tank
50 wherein it separates into a liquid phase and a gaseous phase.
The liquid phase contains about 13 mol per cent ethane, the
remainder being propane. The vapor phase contains about 31 mol per
cent ethane, with the remainder being propane. Thus, two
refrigerant streams are made available by partial liquefaction of a
single, compressed refrigerant stream. The two refrigerant streams
have different boiling points (at the same pressure) by virtue of
having different compositions and therefore are advantageously used
at different temperature levels in the refrigeration cycle.
Refrigerant vapor is withdrawn via conduit 52 and introduced into
heat exchanger 12 wherein it is cooled to a temperature of about
-75.degree. F. and completely liquefied by heat exchange with
colder refrigerant streams as described hereinafter.
Refrigerant liquid is withdrawn from storage or surge tank 50 via
conduit 54, passed through expansion valve 56 wherein it is
expanded or flashed to form a liquid and vapor, and introduced into
separator 58 for phase separation. In passing through said
expansion valve 56 the pressure on said refrigerant liquid is
reduced to about 91 psig and the temperature is reduced to about
45.degree. F.
Refrigerant liquid is withdrawn from separator 58 via conduit 60
and a portion thereof is passed via conduit 62 into a first region
(b) of said heat exchanger 12 wherein it is passed to indirect heat
exchange relationship with said stream of refrigerant vapor
introduced via conduit 52 and said natural gas introduced via
conduit 10. The resulting vaporized refrigerant is withdrawn from
heat exchanger 12 via conduit 64, passed through conduit 65 to
knockout drum 66, compressed in the high stage 68 of the
three-stage compressor system illustrated, then through cooler 46
wherein it is partially liquefied, and then passed via conduit 48
into surge or storage tank 50, previously described. Refrigerant
vapor in separator 58 is withdrawn therefrom via conduit 70 and
introduced into conduit 65 for passage through said high stage
compression zone 68, previously described, to complete the cycle
for both the liquid and vapor from separator 58. It is desirable to
maintain a substantially constant ratio between the refrigerant
vapors flowing in conduits 64 and 52 because the refrigerant in
conduit 64 (vaporized refrigerant from conduit 62) contributes
materially to the cooling and condensation of the refrigerant vapor
in conduit 52. Thus, should the flow in conduit 52 increase, the
flow in conduit 64 should be proportionally increased. The desired
ratio control of said two streams can be conveniently accomplished
by employing ratio controller 74 to adjust or regulate valve 72 in
conduit 70 responsive to the flow in said conduits 52 and 64. For
example, if the flow in conduit 64 drops below a specified ratio of
that in conduit 52, said valve 72 will be closed somewhat, thus
increasing the pressure in conduit 62 with resulting increase in
flow in conduit 64 until the desired ratio is reestablished.
A second portion or stream of said refrigerant liquid from
separator 58 is passed from conduit 60 through expansion valve 76
wherein it is expanded or flashed to reduce its pressure to about
24 psig and its temperature to about -7.degree. F., and then passed
into phase separator 78 wherein a phase separation of the partially
vaporized refrigerant is effected. Liquid refrigerant in separator
78 is withdrawn therefrom via conduit 80 and introduced into a
second region (c) of said heat exchanger 12, downstream from said
first region (b) with respect to the flow of said refrigerant vapor
through conduit 52. Said refrigerant liquid from conduit 80 is
passed in indirect heat exchange relationship in exchanger 12 with
said stream of refrigerant vapor in conduit 52 and said stream of
gas introduced via conduit 10 so as to vaporize said liquid
refrigerant and further cool said stream of refrigerant vapor and
said stream of gas. Vapors resulting from the vaporization of the
refrigerant in conduit 80 are withdrawn from said heat exchanger 12
via conduit 82 and passed via conduit 84 and knockout drum 86 into
the intermediate stage 88 of the compressor system shown. Effluent
from compression stage 88 is passed through cooler 80 for partial
cooling and then combined with the refrigerant vapors in conduit 65
for further compression therewith in compression stage 68 and
return to said storage zone or surge tank 50 as previously
described. Refrigerant vapor in separator 78 is withdrawn therefrom
via conduit 92 and combined in conduit 84 with the vapors from
conduit 82 for compression, as previously described, to complete
the cycle for both the liquid and vapor from separator 58.
Returning now to the stream of refrigerant vapor introduced into
heat exchanger 12 via conduit 52, said stream is completely
liquefied during its passage through heat exchanger 12, is
withdrawn from said heat exchanger via conduit 94, passed through
expansion valve 96 wherein its pressure is reduced to about 4 psig
and its temperature is reduced to about -80.degree. F., and then
introduced into phase separator 98 wherein a phase separation
between the two phases is effected. The liquid and vapor phases in
separator 98 are preferably withdrawn therefrom separately and
recombined as shown, and then introduced into said heat exchanger
12 via conduit 100 for passage through a third region (d) of said
heat exchanger, downstream from said second region (c) with respect
to the flow of said refrigerant vapor in conduit 52 and the natural
gas in conduit 10, and passing said refrigerant in indirect heat
exchange relationship with said streams in conduits 52 and 10 so as
to vaporize said refrigerant in conduit 100, further cool and
liquefy the refrigerant vapor in conduit 52, and further cool and
partially liquefy the natural gas in said conduit 10. Refrigerant
vapor is withdrawn from segment or region (b) of said heat
exchanger 12 via conduit 102, passed through knockout drum 104, and
compressed in low stage 106 of the compressor system shown. The
effluent from low stage compression 106 is combined with the feed
to intermediate stage compression 88 for further compression,
cooling, and return to said storage tank or surge tank 50, as
previously described to complete the cycle.
It is important that the refrigerant mixture in separator 98 be at
the lowest temperature in the system, for example about -80.degree.
F., in order to cool the gas stream in conduit 10 to about
-75.degree. F. In order to control the refrigerant temperature in
said separator 98 at said desired -80.degree. F., said expansion
valve 96 is adjusted or regulated by flow recorder controller 108
responsive to the gas flow in conduit 52 as measured by flow meter
110 and transmitted via transmitter 112. The set point of said flow
recorder controller 108 is in turn reset or regulated by
temperature recorder controller 114 responsive to the measured
temperature of the stream in conduit 14 as transmitted by
transmitter 116. Thus, the desired temperature of the refrigerant
in separator 16 is obtained by applying that temperature as the set
point to temperature recorder controller 114.
The above-described refrigeration cycle employing two-stage
flashing of a refrigerant liquid to obtain high and intermediate
temperature cooling, and condensation of refrigerant vapor with
subsequent flashing thereof to obtain low temperature cooling,
represents one presently preferred embodiment of the invention.
However, it is within the scope of the invention to employ only
one-stage flashing, or three-, four-, or five-stage flashing, or
more, of said refrigerant liquid. Thus, in some situations where a
gas feed stream, or other material, is to be cooled over a more
narrow temperature range, the liquid refrigerant from storage or
surge tank 50 is flashed only once. For example, only the first
stage flashing through expansion valve 56 is used, and the second
stage flashing through expansion valve 76 is omitted. In such
instances, heat exchange zone 12 would comprise only two regions,
e.g., regions (b) and (d) (extended if desired or necessary to
cover the entire zone), and region (c) would be omitted.
Appropriate changes in the compression portion of the cycle can be
made in view of the disclosure herein.
In other instances where it is desired to cool a gas stream or
other material over a more extended temperature range, the liquid
refrigerant from storage or surge tank 50 can be serially flashed
three or more times to provide refrigeration at three or more
temperature levels prior to the lowest temperature level, e.g.,
region (d). In such instances, heat exchange zone 12 would comprise
more than three regions, e.g., regions (b), (c-1), (c-2), and (d),
etc. Again, appropriate changes can be made in the compression
portion of the cycle.
In still other embodiments of the invention, the refrigerant vapors
resulting from the liquid refrigerant introduced into segment or
region (c) of heat exchanger 12 via conduit 80, instead of being
withdrawn from said heat exchanger via conduit 82, can be withdrawn
from said heat exchanger via conduit 118 and introduced into
conduit 84 for compression and further handling to complete the
cycle as previously described. Similarly, if desired, vapors
resulting from the refrigerant introduced into said heat exchanger
12 via conduit 100, instead of being withdrawn from said heat
exchanger via conduit 102, can be withdrawn from said heat
exchanger via conduit 120 and then introduced into conduit 102 for
compression and further handling to complete the cycle as
previously described.
In still another embodiment of the invention, the refrigerant
liquid withdrawn from storage or surge tank 50 via conduit 54 is
divided into two portions. A first portion is passed through
expansion valve 56 into separator 58. The refrigerant liquid in
separator 58 is withdrawn therefrom via conduit 60 and passed via
conduit 62 into segment or region (b) of heat exchanger 12, as
previously described. The second portion of the refrigerant liquid
in conduit 54 is passed via conduit 55, expansion valve 57 wherein
it is expanded or flashed and its temperature reduced to about
50.degree. F. and its pressure reduced to about 95 psig, and then
introduced into phase separator 59. Refrigerant vapor from
separator 59 is withdrawn therefrom and passed via conduit 61 into
conduit 70 for further handling as previously described.
Refrigerant liquid from said separator 59 is passed via conduit 63,
expansion valve 76 wherein its pressure is lowered to about 24 psig
and its temperature is lowered to about -7.degree. F., and then
introduced into phase separator 78. The refrigerant vapor and
refrigerant liquid in said phase separator 78 are handled as
previously described.
Make-up refrigerant can be introduced into the system via conduit
150 downstream from expansion valve 56.
It is a feature of the invention that a single refrigerant of
multicomponent composition is employed at a plurality of
temperature or pressure levels for efficient cooling of a heat
exchange zone and materials passed through said heat exchange zone.
Thus, in a presently preferred embodiment, a refrigerant liquid is
employed at its highest refrigerating temperature in segment or
region (b) of heat exchanger 12 where, because of its higher
pressure and highest propane content (about 91 percent), it effects
the first cooling of the warm incoming streams in said conduits 10
and 52. To obtain a lower temperature refrigerant for additional
cooling of said incoming streams, a second stream of the
refrigerant liquid employed in region (b) of said heat exchanger 12
is expanded or flashed through expansion valve 76 to thereby obtain
a lower boiling mixture, which by virtue of its reduced pressure is
then employed in said second region or section (c) of said heat
exchanger 12. In order to obtain a still lower temperature
refrigerant for use in cooling said incoming streams to their
lowest desired temperature, the gaseous refrigerant from storage
tank or surge tank 50 is employed which, because of its increased
ethane content (about 31 percent) boils at the lowest temperature
after liquefaction by passage through said heat exchanger 12 and
flashing or expanding same through said expansion valve 96.
Said presently preferred embodiment of the invention thus provides
three temperature levels of refrigeration by employing two-stage
flashing of a refrigerant liquid to obtain the high temperature and
ntermediate temperature levels of cooling, and employing a
single-stage condensation of refrigerant vapor, which refrigerant
vapor after liquefaction is expanded to obtain the lowest stage of
cooling.
Referring now to FIG. 2, there is illustrated another embodiment of
the invention which can be employed for recovering a mixture of
ethane and propane from a mixture of methane, ethane, and heavier
hydrocarbons extracted from a natural gas stream by cooling and
partially liquefying said natural gas stream. Said mixture of
ethane and propane can conveniently be employed as the
multicomponent refrigerant employed in the refrigeration system
illustrated in FIG. 1.
A mixture of partially liquefied hydrocarbons comprising methane,
ehtane, and heavier hydrocarbons, having been recovered from a
natural gas stream as described in connection with FIG. 1, is
passed via conduit 122 into a first fractionation zone or
deethanizer 124 as feedstock thereto. In said deethanizer, said
feedstock is fractionated to recover an overhead stream comprising
methane, ethane, and some propane. Typical operating conditions in
fractionator 124 would include a top tower temperature of about
-11.degree. F., a bottom tower temperature of about 219.degree. F.,
and a pressure of 412 psig. Said overhead stream is removed via
conduit 126, partially condensed in condenser 128, and passed into
accumulator 130. A vapor stream comprising methane is removed from
said accumulator 130 via conduit 132. A first portion of the
condensed overhead is removed from accumulator 130 and passed via
conduit 134 into said fractionator 124 near the top thereof, as
reflux liquid. Preferably, the amount of said reflux passed via
conduit 134 is set at a predetermined amount and controlled by rate
of flow controller 136. A second portion of said condensed overhead
is removed from said accumulator 130 and passed via conduit 138 to
a second fractionator or demethanizer 140 as feedstock thereto.
Typical operating conditions in fractionator 140 would include a
top tower temperature of about -16.degree. F., a bottom tower
temperature of about 46.degree. F., and a pressure of about 412
psig. An overhead stream is withdrawn from fractionator 140 via
conduit 142, passed into conduit 126 wherein it s combined with the
overhead from said first fractionator 124, and the total stream
passed through said cooler 128 wherein it is cooled to a
temperature of about -35.degree. F. and thereby partially condensed
or liquefied. Any suitable refrigerant can be employed in said
cooler 128. If the embodiment of the invention illustrated in FIG.
2 is being employed in combination with the embodiment of the
invention illustrated in FIG. 1, said refrigerant can conveniently
be a portion of the refrigerant in conduit 80 in said FIG. 1. Said
accumulator 130 thus serves as a common accumulator for both said
fractionator 124 and said fractionator 140. Thus, the same liquid
is used as reflux in fractionator 124 and as feedstock in
fractionator 140. Preferably, the amount of said feedstock passed
to fractionator 140 is controlled in accordance with the liquid
level in reboiler 144. Flow of said feedstock can be regulated by
means of valve 146 responsive to a signal from liquid level
controller 148. A product mixture comprising ethane and propane is
withdrawn from reboiler region 144 of fractionator 140 by means of
conduit 150. If desired, the rate of withdrawal of said product
stream can be controlled by means of a rate of flow controller, not
shown. A second product of the process is withdrawn as bottoms
product from fractionator 124, being withdrawn from reboiler region
152 via conduit 154. The rate of withdrawal of said second product
can be controlled by valve 156 responsive to a signal from liquid
level controller 158.
Said fractionator 124 can be conveniently reboiled by means of a
hot heat exchange medium supplied by pipe or conduit 160 to
reboiler region 152. The flow rate of said hot heat exchange medium
can be controlled by valve 162 which can be adjusted or regulated
by temperature recorder controller 164 responsive to a temperature
measurement in the lower portion of fractionator 124, which is
transmitted by transmitter 166. The set point of temperature
recorder controller 164 can be in turn regulated responsive to a
signal from liquid level controller 168 on phase separator 130.
Thus, said fractionator 124 will be reboiled sufficiently to
maintain a desired liquid level in phase separator 130, i.e., to
maintain sufficient liquid to supply reflux to said fractionator
124 and feedstock to said fractionator 140.
Said fractionator 140 can be advantageously reboiled by passing a
portion of the warm bottoms stream (about 218.degree. F.) from
reboiler region 152 via conduits 154 and 170 with the flow rate
thereof being controlled by valve 172 which is regulated by
temperature recorder controller 174 responsive to the measured
temperature (about 46.degree. F.) in reboiler region 144. Effluent
heating fluid from reboiler region 144 is returned to conduit 154
via conduit 176 and removed from the system with the product in
said conduit 154. The ethane-propane refrigerant mixture recovered
as product via conduit 150 will typically comprise about 60 percent
ethane and about 40 percent propane. However, it is within the
scope of the invention to recover mixtures having different
proportions of said components. For example, the composition of
said refrigerant mixture can vary from about 25 mol percent to
about 95 mol percent ethane and from about 75 percent to about 5
percent propane.
The embodiment of the invention illustrated in FIG. 2 thus provides
a process for recovering a desired mixture of ethane and propane
from the overhead of a deethanizer in a cryogenic natural gas
processing plant. The recovery of said refrigerant mixture is
effected in a demethanizing fractionator which is refluxed and fed
with part of the condensed overhead stream from said deethanizer.
Further, the demethanizing fractionator producing the desired
refrigerant mixture is reboiled with bottoms from said deethanizer,
thereby saving reboiling costs. Thus, the fractionation method by
which said refrigerant mixture is produced requires neither
separate reflux facilities nor external heat for reboiling.
While the invention has been described with particular reference to
the cooling of a natural gas in a heat exchange zone, the invention
is not limited thereto. The refrigeration system of the invention
can be employed to cool other materials contained in said zone. For
example, the three regions of the heat exchange zone could comprise
three refrigeration zones for storing different materials at three
different temperatures, e.g., three different refrigeration rooms
in a large cold storage plant.
While certain embodiments of the invention have been described for
illustrative purposes, the invention is not limited thereto.
Various other modifications or embodiments of the invention will be
apparent to those skilled in the art in view of this disclosure.
Such modifications or embodiments are within the spirit and scope
of the disclosure.
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