U.S. patent number RE33,408 [Application Number 06/809,666] was granted by the patent office on 1990-10-30 for process for lpg recovery.
This patent grant is currently assigned to Exxon Production Research Company. Invention is credited to James Haliburton, Shuaib A. Khan.
United States Patent |
RE33,408 |
Khan , et al. |
October 30, 1990 |
Process for LPG recovery
Abstract
An improved process is described for the separation and recovery
of substantially all the propane and heavier hydrocarbon components
in a hydrocarbon gaseous feedstream. In this process, the vapor
stream from a deethanizer is cooled to liquefaction and contacted
with a vapor phase from the hydrocarbon gaseous feedstream. The
contact takes place within a direct heat exchanger, and the
resulting vapor fraction, which is essentially ethane and methane,
is the gaseous product of the process.
Inventors: |
Khan; Shuaib A. (Calgary,
CA), Haliburton; James (Calgary, CA) |
Assignee: |
Exxon Production Research
Company (Houston, TX)
|
Family
ID: |
27065378 |
Appl.
No.: |
06/809,666 |
Filed: |
December 16, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
537127 |
Sep 29, 1983 |
04507133 |
Mar 26, 1985 |
|
|
Current U.S.
Class: |
62/621 |
Current CPC
Class: |
C07C
7/09 (20130101); F25J 3/0209 (20130101); F25J
3/0219 (20130101); F25J 3/0233 (20130101); F25J
3/0242 (20130101); F25J 2200/04 (20130101); F25J
2200/74 (20130101); F25J 2200/78 (20130101); F25J
2205/02 (20130101); F25J 2205/04 (20130101); F25J
2210/12 (20130101); F25J 2235/60 (20130101); F25J
2240/02 (20130101); F25J 2270/60 (20130101); F25J
2270/90 (20130101); F25J 2290/40 (20130101) |
Current International
Class: |
C07C
7/09 (20060101); C07C 7/00 (20060101); F25J
3/02 (20060101); F25J 003/02 () |
Field of
Search: |
;62/23,24,27,28,29,30,32,31,34,36,38,39,42,43,44,9,11 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Drawing 83-4306-020-001; Rev O; Imperial Oil Limited; 1974. .
Drawing 4-003 c; Rev O; The Fluor Corporation; 5/16/73..
|
Primary Examiner: Sever; Frank
Attorney, Agent or Firm: Riley; James H. Phillips; Richard
F.
Claims
We claim:
1. In a process for separating propane and heavier hydrocarbons
from a gaseous feedstream containing hydrocarbon components of
different boiling points wherein said feedstream is cooled and
separated into a first vapor fraction and a first liquid fraction
and said first liquid fraction is distilled in a deethanizer to
form a second vapor fraction and a second liquid fraction, the
improvement which comprises .[.expanding and.]. transferring said
first vapor fraction to the lower portion of a direct heat
exchanger, cooling .Iadd.at least a portion of .Iaddend.said second
vapor fraction .Iadd.by passing it through an indirect heat
exchanger .Iaddend.to form a substantially liquefied stream,
.Iadd.partially flashing at least a portion of said liquefied
stream and transferring .[.at least a portion of said liquefied
stream.]. .Iadd.it .Iaddend.to the upper portion of said direct
heat exchanger whereby said liquefied stream contacts said first
vapor fraction to form a third vapor fraction and a third liquid
fraction, .[.returning.]. .Iadd.transferring .Iaddend.said third
liquid fraction to said deethanizer, .[.and.]. removing said third
vapor fraction from said direct heat exchanger .Iadd.and passing
said third vapor fraction through said indirect heat exchanger.
.Iaddend.
2. A process as described in claim 1 wherein said direct heat
exchanger is a packed column.
3. A process as described in claim 1 wherein said direct heat
exchanger is a tray column.
4. A process as described in claim 1, .[.wherein.]. .Iadd.further
comprising expanding .Iaddend.said first vapor fraction .[.is
expanded.]. in a turboexpander .Iadd.prior to transferring it to
the lower portion of said direct heat exchanger. .Iaddend. .[.5. A
process as described in claim 1 further comprising partially
flashing said liquefied stream prior
to transferring it to said direct heat exchanger..]. 6. A process
for separating propane and heavier hydrocarbons from a gaseous
feedstream containing hydrocarbon components of different boiling
points which comprises:
(a) cooling and separating said feedstream into a first vapor
fraction and a first liquid fraction;
(b) distilling said first liquid fraction .[.is.]. .Iadd.in
.Iaddend.a deethanizer to form a second vapor fraction and a second
liquid fraction;
(c) removing said second liquid fraction from said deethanizer as a
liquid product;
(d) .[.expanding said first vapor fraction and.]. transferring
.[.it.]. .Iadd.said first vapor fraction .Iaddend.to the lower
portion of a direct heat exchanger;
(e) cooling said second vapor fraction to form a third vapor
fraction and a third liquid fraction;
(f) returning at least a portion of said third liquid fraction to
said deethanizer as reflux;
(g) .[.further.]. cooling said third vapor fraction to form a
.[.substantially liquefied.]. stream .Iadd.which is at least
partially liquefied.Iaddend.;
(h) .[.transferring.]. .Iadd.partially flashing at least a portion
of .Iaddend.said liquefied stream .Iadd.and transferring it
.Iaddend.to the upper portion of said direct heat exchanger whereby
said liquefied stream contacts said first vapor fraction to form a
fourth vapor fraction and a fourth liquid fraction;
(i) removing said fourth vapor fraction from said direct heat
exchanger .[.as a gaseous product.].; and
(j) .[.returning.]. .Iadd.transferring .Iaddend.said fourth liquid
fraction
to said deethanizer. 7. A process as described in claim 6 wherein
said
direct heat exchanger is a packed column. 8. A process as described
in
claim 6 wherein said direct heat exchanger is a tray column. 9. A
process as described in claim 6 .[.wherein.]. .Iadd.further
comprising expanding .Iaddend.said first vapor fraction .[.is
expanded.]. in a turboexpander .Iadd.prior to transferring it to
the lower portion of said direct heat exchanger.Iaddend.. .[.10. A
process as described in claim 6 further comprising partially
flashing said liquefied stream prior to transferring
it to said direct heat exchanger..]. 11. A process as described in
claim 6 further comprising combining at least a portion of said
third liquid fraction with said third vapor fraction prior to
.[.further.]. cooling
said third vapor fraction. 12. A process as described in claim 6
further comprising transferring at least a portion of said third
liquid fraction
to said direct heat exchanger. 13. A process for separating propane
and heavier hydrocarbons from a gaseous feedstream containing
hydrocarbon components of different boiling points which
comprises:
(a) cooling and separating said feedstream into a first vapor
fraction and a first liquid fraction;
(b) distilling said first liquid fraction in a deethanizer to form
a second vapor fraction and a second liquid fraction;
(c) removing said second liquid fraction from said deethanizer as a
liquid product;
(d) .[.expanding said first vapor fraction and.]. transferring
.[.it.]. .Iadd.said first vapor fraction .Iaddend.to the lower
portion of a direct heat exchanger;
(e) cooling said second vapor fraction to form a third vapor
fraction and a third liquid fraction;
(f) removing said third vapor fraction as a gaseous product;
(g) returning 171 at least.]. a .Iadd.first .Iaddend.portion of
said third liquid fraction to said deethanizer as reflux;
(h) .[.transferring at least.]. .Iadd.partially flashing .Iaddend.a
.Iadd.second .Iaddend.portion of said third liquid fraction
.Iadd.and transferring it .Iaddend.to the upper portion of said
direct heat exchanger whereby said .Iadd.second portion of said
.Iaddend.third liquid fraction contacts said first vapor fraction
to form a fourth vapor fraction and a fourth liquid fraction;
(i) removing said fourth vapor fraction from said direct heat
exchanger .[.as a gaseous product.].; and
(j) transferring said fourth liquid fraction to said deethanizer.
14. A process as described in claim 13 wherein said direct heat
exchanger is a
packed column. 15. A process as described in claim 13 wherein said
direct
heat exchanger is a tray column. 16. A process as described in
claim 13 .[.wherein.]. .Iadd.further comprising expanding
.Iaddend.said first vapor fraction .[.is expanded.]. in a
turboexpander .Iadd.prior to transferring it to the lower portion
of said direct heat exchanger.Iaddend.. .[.17. A process as
described in claim 13 further comprising partially flashing said
third liquid fraction prior to transferring it to said direct heat
exchanger..]. .Iadd.18. The process as set forth in claim 1 wherein
said second vapor fraction is cooled and separated into a fourth
vapor fraction and a fourth liquid fraction upon removal from said
distilling unit, said fourth vapor fraction constituting said
portion of said second vapor
fraction. .Iaddend. .Iadd.19. The process as set forth in claim 1
wherein said distilling unit is maintained at a substantially
higher pressure than said direct heat exchanger. .Iaddend.
.Iadd.20. The process as set forth in claim 18 wherein said
distilling unit is maintained at a substantially higher pressure
than said direct heat exchanger. .Iaddend. .Iadd.21. The process as
set forth in claim 1 wherein said first vapor fraction and said
first liquid fraction are passed through a separator prior to being
transferred to said direct heat exchanger and said deethanizer,
respectively. .Iaddend. .Iadd.22. The process as set forth in claim
6 wherein said deethanizer is maintained at a substantially higher
pressure than said direct heat exchanger. .Iaddend. .Iadd.23. The
process as set forth in claim 23 wherein said first vapor fraction
and said first liquid fraction are passed through a separator prior
to being transferred to said direct heat exchanger and said
deethanizer, respectively. .Iaddend. .Iadd.24. The process as set
forth in claim 22 wherein the step of cooling said third vapor
fraction includes transferring heat from said third vapor
fraction to said fourth liquid fraction. .Iaddend. .Iadd.25. The
process as set forth in claim 13 wherein said deethanizer is
maintained at a substantially higher pressure than said direct heat
exchanger. .Iaddend. .Iadd.26. The process as set forth in claim 25
wherein said first vapor fraction and said first liquid fraction
are passed through a separator prior to being transferred to said
direct heat exchanger and said deethanizer, respectively. .Iaddend.
.Iadd.27. A process for separating methane and ethane from the
heavier components of a hydrocarbon feedstream, comprising the
steps of:
(a) separating said feedstream into a first vapor fraction and a
first liquid fraction;
(b) transferring said first vapor fraction to a direct heat
exchanger;
(c) transferring said first liquid fraction to a distilling
unit;
(d) distilling the fluids introduced into said distilling unit to
yield a second vapor fraction and a second liquid fraction;
(e) cooling and separating said second vapor fraction into a third
layer fraction and a third liquid fraction;
(f) returning at least a portion of said third liquid fraction to
said distilling unit;
(g) passing said third vapor fraction through an indirect heat
exchanger to liquefy at least a portion of said third vapor
fraction;
(h) decreasing the pressure of said at least partially liquefied
third vapor fraction, whereby at least a portion of the liquid
phase flashes;
(i) transferring said partially liquefied third vapor fraction to
said direct heat exchanger whereby said partially liquefied third
vapor fraction mixes with said first vapor fraction, yielding a
fourth vapor fraction and fourth liquid fraction;
(j) transferring said fourth liquid fraction to said distilling
unit whereby said fourth liquid fraction is distilled with said
first liquid fraction; and,
(k) removing said fourth vapor fraction from said direct heat
exchanger and passing it through said indirect heat exchanger to
transfer heat from said
third vapor fraction to said fourth vapor fraction. .Iaddend.
.Iadd.28. The process as set forth in claim 27 wherein said
distilling unit is maintained at a substantially higher pressure
than said indirect heat exchanger. .Iaddend. .Iadd.29. The process
as set forth in claim 28 wherein said first vapor fraction is
expanded in a turboexpander prior to transfer to said direct heat
exchanger. .Iaddend. .Iadd.30. The process as set forth in claim 27
further comprising the step of transferring heat from said third
vapor fraction to said fourth liquid fraction prior to
step (g). .Iaddend. .Iadd.31. The process as set forth in claim 27
wherein said distilling unit is a deethanizer. .Iaddend. .Iadd.32.
The process as set forth in claim 27 wherein said first vapor
fraction and said first liquid fraction are passed through a
separator prior to being transferred to said direct heat exchanger
and said distilling unit, respectively. .Iaddend. .Iadd.33. A
process for separating methane and ethane from the heavier
components of a hydrocarbon feedstream, comprising the steps
of:
(a) separating said feedstream into a first vapor fraction and a
first liquid fraction;
(b) transferring said first vapor fraction to a direct heat
exchanger;
(c) transferring said first liquid fraction to a distilling
unit;
(d) distilling the fluids introduced into said distilling unit to
yield a second vapor fraction and a second liquid fraction;
(e) passing said second vapor fraction through a first indirect
heat exchanger to cool it;
(f) separating said cooled second vapor fraction into a third vapor
fraction and a third liquid fraction;
(g) returning at least a first portion of said third liquid
fraction to said distilling unit;
(h) partially flashing a second portion of said third liquid
fraction and transferring it to said direct heat exchanger whereby
said partially flashed second portion of said third liquid fraction
contacts said first vapor fraction to establish a fourth vapor
fraction and a fourth liquid fraction;
(i) transferring said fourth liquid fraction to said distilling
unit whereby said fourth liquid fraction is distilled with said
first liquid fraction; and,
(j) removing said fourth vapor fraction from said direct heat
exchanger and passing it through said indirect heat exchanger to
transfer heat from said
second vapor fraction to said fourth vapor fraction. .Iaddend.
.Iadd.34. The process as set forth in claim 33 wherein said
distilling unit is maintained at a substantially higher pressure
than said indirect heat exchanger. .Iaddend. .Iadd.35. The process
as set forth in claim 34 wherein said first vapor fraction is
expanded in a turboexpander prior to transfer to said direct heat
exchanger. .Iaddend. .Iadd.36. The process as set forth in claim 33
wherein step (e) includes transferring heat from said second vapor
fraction to said fourth liquid fraction. .Iaddend. .Iadd.37. The
process as set forth in claim 33 wherein said distilling
unit is a deethanizer. .Iaddend. .Iadd.38. The process as set forth
in claim 33 wherein said third and fourth vapor fractions are
removed as the gas product of the process and said second liquid
fraction is removed as the liquid product of the process, said gas
product being substantially free from propane and heavier
hydrocarbons and said liquid product being substantially free from
methane and ethane. .Iaddend. .Iadd.39. The process as set forth in
claim 33 wherein said first vapor fraction and said first liquid
fraction are passed through a separator prior to being transferred
to said direct heat exchanger and said distilling unit,
respectively. .Iaddend. .Iadd.40. A process for separating a
hydrocarbon feedstream into a first portion predominantly composed
of methane and ethane, and a second portion predominantly composed
of propane and heavier hydrocarbons, said process comprising the
steps of:
(a) cooling and separating said feedstream into a first vapor
fraction and a first liquid fraction;
(b) expanding said first vapor fraction and transferring it to a
direct heat exchanger;
(c) transferring said first liquid fraction to a deethanizer, said
deethanizer being effective to convert the fluids introduced
therein into a second vapor fraction and a second liquid fraction,
said deethanizer being maintained at a higher pressure than said
direct heat exchanger;
(d) removing said second liquid fraction as a product predominantly
composed of propane and heavier hydrocarbons;
(e) removing and cooling said second vapor fraction to yield a
third vapor fraction and a third liquid fraction;
(f) returning at least a portion of said third liquid fraction to
said deethanizer;
(g) passing said third vapor fraction through an indirect heat
exchanger to remove heat from and liquefy at least a portion of
said third vapor fraction;
(h) decreasing the pressure of said third vapor fraction to flash
at least a portion of the liquefied gases within said third vapor
fraction whereby said third vapor fraction is further cooled;
(i) introducing said third vapor fraction into said direct heat
exchanger, whereby said third vapor fraction and said first vapor
fraction come into contact to form a fourth vapor fraction and a
fourth liquid fraction;
(j) removing said fourth vapor fraction as a product predominantly
composed of methane and ethane;
(k) passing said fourth vapor fraction through said indirect heat
exchanger to transfer heat from said third vapor fraction to said
fourth vapor fraction; and
(l) transferring said fourth liquid fraction to said deethanizer.
.Iaddend.
.Iadd.41. The process as set forth in claim 40 wherein the
expansion of
step (b) is conducted in a turboexpander. .Iaddend. .Iadd.42. The
process as set forth in claim 40 further including the step of
passing said third vapor fraction and said fourth liquid fraction
through a second indirect heat exchanger to transfer heat from said
third vapor fraction to said fourth liquid fraction. .Iaddend.
.Iadd.43. The process as set forth in claim 40 wherein said first
vapor fraction and said first liquid fraction are passed through a
separator prior to being transferred to said direct heat exchanger
and said deethanizer, respectively. .Iaddend. .Iadd.44. The process
as set forth in claim 40 wherein at least a portion of said third
liquid fraction is added to said third vapor fraction prior to
passing said third vapor fraction through said indirect heat
exchanger.
Description
FIELD OF THE INVENTION
This invention relates to a process for treating a gaseous
hydrocarbon containing feedstream such as natural gas, crude oil
solution gas or refinery gas to separate and recover propane and
heavier hydrocarbon components.
BACKGROUND OF THE INVENTION
Gaseous streams containing methane and ethane occur naturally, such
as in natural gas and crude oil solution gas, and also as
byproducts of a variety of refinery processes. In addition to
methane and ethane, these gases often contain a substantial
quantity of hydrocarbons of higher molecular weight, e.g., propane,
butane, pentane and their unsaturated analogs.
Recent substantial increases in the market for the propane and
heavier hydrocarbon components of natural gas have provided demand
for process yielding higher recovery levels of these products.
Available processes for separating these materials include those
based upon cooling and refrigeration of gas, oil absorption,
refrigerated oil absorption, and the more recent cryogenic process
utilizing the principle of gas expansion through a mechanical
device to produce power while simultaneously extracting heat from
the system. Depending upon the pressure of the gas source, the
richness (propane and heavier hydrocarbon content) of the gas and
the desired end results, each of these prior art processes or a
combination thereof may be employed.
Prior to the advent of the cryogenic expansion process, propane and
the heavier component hydrocarbons were frequently separated by
liquefaction and treatment with an absorption medium. The natural
gas streams were contacted with an absorption oil (usually
heptane), and the propane and the heavier hydrocarbon components
were absorbed and thereafter desorbed and recovered.
In most present day refining processes, propane and the higher
molecular weight components of natural gas and refinery gas are
separated and recovered by liquefaction and cryogenic distillation
at temperatures below 0.degree. F. Refrigeration for separation is
supplied totally or partially by expansion of the gaseous stream in
a turboexpander which produces power that may be used for example
in driving a compressor.
In a typical cryogenic expansion-type recovery process, a
feedstream gas under pressure is cooled by heat exchange with other
streams of the process and/or external sources of cooling such as a
propane compression refrigeration system. As the gas is cooled,
liquids are condensed and are collected in one or more separators
as a high pressure liquid feed containing most of the desired
propane and heavier hydrocarbons. The high pressure liquid feed is
transferred to a deethanizer column after its pressure is adjusted
to the operating pressure of the deethanizer. The deethanizer is a
fractionating column in which the liquid feed is fractionated to
separate residual methane and ethane from the desired produces of
propane and heavier hydrocarbon components.
If the feedstream is not totally condensed (typically it is not),
the vapor remaining from this partial condensation is expanded in a
turboexpander to a lower pressure. Additional liquids are condensed
as a result of the further cooling of the stream during expansion.
The pressure after the expansion is usually the same pressure at
which the deethanizer is operated. Liquids thus obtained are also
supplied as a feed to the deethanizer. Typically, remaining vapor
and deethanizer overhead vapor are combined as a residual
methane/ethane product gas.
In the ideal operation of such a separation process, the vapors
leaving the process will contain substantially all the methane and
ethane found in the feed gas to the recovery plant and
substantially no propane or heavier hydrocarbon components. The
bottoms fraction leaving the deethanizer will contain substantially
all the propane and heavier hydrocarbon components and essentially
no methane or ethane. In practice, this ideal situation is not
obtained because the conventional deethanizer is operated largely
as a stripping column. Therefore, the methane and ethane vapors
leaving the top fractionation stage of the column will contain
vapors not subjected to any rectification step. Substantial losses
of propane and heavier hydrocarbons occur because the vapors
discharged from the low temperature separation steps contain
propane and heavier hydrocarbon components which could be recovered
if those vapors were brought to lower temperature, or if they were
contacted with a significant quantity of a relatively heavy
hydrocarbon, e.g. heptane, capable of absorbing the propane.
U.S. Pat. No. 4,272,269 which issued to Hammond, et al on June 6,
1981 described one such process that combines both the cryogenic
expansion step and the absorption process to increase the recovery
percentage of the propane and hydrocarbon components. The
disadvantage with using an absorption oil is that additional
refining steps are needed to desorb the propane and prepare the
absorption oil for reuse.
The problem associated with all types of propane recovery
operations is one of efficiency. The main objective is to recover
as much of the propane and heavier hydrocarbon components as is
economically possible. The conventional systems in operation today
are capable of economically recovering, at most, about 95% of the
propane in a feedstream. Because of the large volume of gas that is
processed, there is a definite need to find efficient methods to
recover more of the propane and heavier hydrocarbons in a gaseous
feedstream.
SUMMARY OF THE INVENTION
The present invention relates to a process for treating a gaseous
hydrocarbon-containing feedstream where it is desirable to leave
substantially all of the methane and ethane in the hydrocarbon
gaseous stream and to separate and recover substantially all of the
propane and heavier hydrocarbon components. It has been found that
increased percentage of propane and heavier hydrocarbon components
can be economically recovered by contacting the expanded vapor from
a gaseous feedstream with at least a portion of the liquefaction
overhead from a deethanizer.
The process of the present invention comprises cooling a gaseous
hydrocarbon-containing feedstream to form a vapor stream and a
liquid stream. The liquid stream is transferred to a deethanizer
while the vapor stream is expanded transferred to the bottom of a
direct heat exchanger column. The deethanizer overhead, which
consists mainly of methane and ethane, is cooled liquefaction and
fed to the upper portion of the direct heat exchanger column. The
liquid methane and ethane flow downward within the direct heat
exchanger column and contact gaseous propane and heavier
hydrocarbons that flow upward. The methane and ethane vaporize by
absorbing heat from the gaseous propane and heavier hydrocarbons
which causes the propane and heavier hydrocarbons to condense at
the bottom of the direct heat exchanger column. The gaseous methane
and ethane within the direct heat exchanger column are removed from
the overhead as a product of the process. The liquid at the bottom
of the direct heat exchanger column is removed and fed to the lower
portion of the deethanizer. The liquid at the bottom of the
deethanizer is removed as a product of the process, and at least a
portion of the gaseous overhead is cooled and returned to the top
of the direct heat exchanger column.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic flow diagram illustrating the present
invention.
FIG. 2 is a schematic flow diagram illustrating a preferred
embodiment of the present invention.
FIG. 3 is a schematic flow diagram illustrating a variation of the
preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a gaseous feedstream of natural
gas, solution gas or refinery gas, which typically contains
hydrocarbons ranging from methane to hexane, is processed to
separate and recover the propane and heavier hydrocarbon
components.
As illustrated in FIG. 1, the gaseous feedstream comes in through
line 10. The natural gas feed to a natural gas plant will generally
be at about atmospheric temperature and at an elevated pressure
substantially above atmospheric pressure. Prior to the initial
cooling step, the gaseous feedstream enters an inlet separator 12
for removal of liquid hydrocarbons. Additionally, water vapor in
the feedstream is removed in a dehydration unit 16 to avoid the
formation of ice throughout the process. These preparatory steps
are known to those skilled in the art and, depending on the
composition of the gaseous feedstream, may not be necessary.
The gaseous feedstream flows through line 18 and is initially
cooled to a temperature of about -10.degree. F. One method of
cooling the feedstream, as shown in FIG. 1, involves contact with
indirect heat exchangers 20 and 24 and a propane refrigerant 22.
Other methods are available and known to those skilled in the art.
After the gaseous feedstream is sufficiently cooled, it enters an
inlet vessel 28 where it is separated into a vapor stream and a
liquid stream. The vapor stream is expanded and passes through line
94 into the lower portion of a direct heat exchanger column 74
which is a mixing chamber. Direct heat exchanger column 74 may have
a variety of configurations, but its purpose is to allow direct
contact between vapor and liquid phases. A packed column is one
configuration that can function as a direct heat exchanger. A
preferred configuration is similar to a tray-type absorber where
liquid enters the top and flows down a series of trays, contacting
gas which is flowing upward from the bottom.
FIG. 2 represents a preferred embodiment where the vapor stream
from inlet vessel 28 is work expanded in turboexpander 96, cooled
to a temperature of about -50.degree. F. and partially condensed.
This cooled and partially condensed stream flows through line 98 to
the upper portion of a separator 100. The liquid from the inlet
vessel 28 flowing through line 29 passes through an expansion valve
30 and enters the lower portion of separator 100. The liquid from
the bottom of the separator 100 combines with the liquid from the
direct heat exchanger column 74 in line 36. The vapor from
separator 100 is fed to the lower portion of the direct heat
exchanger column 74.
This preferred embodiment is most effective on gaseous feedstreams
rich in propane and heavier hydrocarbon components. The additional
steps separate many of these components early in the process, thus
increasing the efficiency of the subsequent separation steps.
Referring to FIG. 2, the flow from line 102 into the lower portion
of the direct heat exchanger column 74 contains gaseous propane and
heavier hydrocarbon components. These gases flow upward, contacting
downward flowing liquid methane and ethane which enter the upper
portion of the direct heat exchanger column through line 72
(described in more detail later). The liquid methane and ethane
descend from tray to tray in the direct heat exchanger column and
evaporate. The energy for the evaporization is supplied by the
condensation of gaseous propane and heavier hydrocarbons ascending
from the bottom of the direct heat exchanger column.
A vapor stream consisting essentially of pure methane and ethane is
formed within the direct heat exchanger column 74. The overhead
from the direct heat exchanger column flows through line 80, passes
through a series of indirect heat exchangers 66 and 24, and enters
the compressor side 86 of the turboexpander. After this compression
step, the gas is further compressed in a residual gas compressor 90
and is removed as a gaseous product of the process.
The liquid propane and heavier hydrocarbons are removed from the
bottom of the direct heat exchanger column 74 through line 76 and
combined with the liquid stream from separator 100. These streams
are heated through a series of indirect heat exchangers and can
provide some or all of the cooling requirement for gas entering
inlet vessel 28. The warmed liquid stream is fed to the deethanizer
444 which is essentially a fractionating column. Liquid from the
bottom of the deethanizer 44 is removed through line 46 as a liquid
product of the process. The liquid product consists essentially of
propane and heavier hydrocarbon components.
The gaseous overhead from the deethanizer is cooled and fed to the
direct heat exchanger column 75. FIG. 2 shows one embodiment where
the deethanizer overhead, which consists essentially of methane and
ethane, is cooled by propane refrigeration 50 and fed to separator
54. The liquid portion flows out line 56. Part of the liquid is
returned through line 60 to the deethanizer as reflux, and the rest
of it flows through line 62 into line 64. Alternatively, line 62
can flow into the direct heat exchanger column 74. In either case,
vapor in line 64 is further cooled by indirect heat exchangers 38
and 66 to about -50.degree. F. at which point the vapor liquefies.
The liquid flows through line 68 to an expansion valve 70 where the
methane and ethane is partially flashed, further reducing the
temperature to about -70.degree. F. After the pressure reduction,
the cold liquid and gas flow through line 72 into the upper portion
of the direct heat exchanger column 74. The cycle is complete with
the liquid methane and ethane descending from tray to tray
condensing gaseous propane and heavier hydrocarbons and the gaseous
methane and ethane flowing out the top of the direct heat exchanger
column through line 80.
FIG. 3 shows an embodiment where some of the deethanizer overhead
is removed from the process. In this embodiment, the deethanizer
overhead is cooled by propane refrigerant 50 and indirect heat
exchangers 38 and 66 prior to entering separator 54. The gaseous
overhead from separator 54 has little, if any, propane and heavier
hydrocarbons. The overhead is transferred to line 88 through line
64, and the liquid flows out through line 56. Part of the liquid is
returned through line 60 to the deethanizer as reflux, and the rest
flows through expansion valve 70, partially flashing the methane
and ethane. As previously stated, the flow from line 72 is fed to
the upper portion of the direct heat exchanger unit, and the liquid
methane and ethane descend from tray to tray condensing gaseous
propane and heavier hydrocarbons.
EXAMPLE
The process of this invention will be further understood by
reference to a specific example. For illustrative purposes, a
gaseous feedstream having the following composition will be
used:
______________________________________ Component Mole %
______________________________________ Carbon Dioxide .900 Nitrogen
3.540 Hydrogen Sulfide 0.000 Methane 65.043 Ethane 19.353 Propane
7.376 I-Butane 0.835 Butane 2.121 I-Pentane 0.321 Pentane 0.320
Hexane+ 0.191 ______________________________________
By way of illustration, the gaseous feedstream in this instance
would be at a temperature of about 70.degree. F. and a pressure of
about 515 psia.
The following table illustrates the calculated temperatures and
pressures at major points as the feedstream passes through the
system shown in FIG. 2.
______________________________________ Line or Unit Temperature
Pressure Number .degree.F. psia
______________________________________ 26 -10.00 500 34 -26.21 300
102 -41.58 300 80 -71.00 300 76 -55.00 300 42 48.00 490 48 47.10
490 64 24.04 485 60 24.04 490 62 24.04 490 72 -73.83 300 84 53.00
285 46 215.00 490 ______________________________________
The composition of the gaseous product of the process from the
direct heat exchanger column 74 passing through line 80 and the
liquid product of the process from the deethanizer passing through
line 46 would be as follows:
______________________________________ Component Mole %
______________________________________ Gaseous Product Composition
(80) Carbon Dioxide 1.014 Nitrogen 3.987 Hydrogen Sulfide 0.000
Methane 73.269 Ethane 21.595 Propane 0.133 I-Butane 0.000 Butane
0.000 I-Pentane 0.000 Pentane 0.000 Hexane+ 0.000 Liquid Product
Composition (46) Carbon Dioxide 0.000 Nitrogen 0.000 Hydrogen
Sulfide 0.000 Methane 0.000 Ethane 1.627 Propane 64.639 I-Butane
7.437 Butane 18.890 I-Pentane 2.857 Pentane 2.845 Hexane+ 1.702
______________________________________
By way of comparison, the recovery of propane in conventional
systems is about 95 mol % of feedstream, whereas this process
separates and recovers about 98 to 99 mol % of the propane in the
feedstream.
The principle of the invention and the best mode contemplated for
applying that principle have been described. It is to be understood
that the foregoing is illustrative only and that other means and
techniques can be employed without departing from the true scope of
the invention defined in the following claims.
* * * * *