Note: Descriptions are shown in the official language in which they were submitted.
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METHODS AND SYSTEMS FOR RECOVERING LIQUIFIED
PETROLEUM GAS FROM NATURAL GAS
RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent Application No.
61/360,753, filed July 1, 2010.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention is directed toward processes and systems for recovering
liquefied petroleum gas (LPG) from a hydrocarbon gas stream, especially a
natural gas stream
or a refinery gas stream. Particularly, the processes and systems described
herein may be
utilized to enhance LPG recovery, particularly when processing higher pressure
or leaner feed
streams thereby providing broader applicability compared to previous
processes.
Description of the Prior Art
Natural gas comprises primarily methane, but can also include varying amounts
of
heavy hydrocarbon components such as ethane, propane, butane, and pentane, for
example.
It is well known that natural gas streams can be separated into their
respective component
parts. Such processes involve a combination of chilling, expansion,
distillation and/or like
operations to separate methane and ethane from C3 and heavier hydrocarbon
components.
Typically the separation made is of methane and ethane from propane and
heavier compo-
nents. If economically desirable, the ethane could also be recovered and
similarly, it is
desirable in many instances to further fractionate the recovered C3 (or
alternatively C2) and
heavier components.
One process that has been devised for separating a natural gas stream into
light and
heavy component streams is shown in U.S. Pat. No. 5,771,712. The '712 Patent
demon-
strates a typical process wherein an overhead stream from a deethanizer is
passed into heat
exchange with an exit stream from an absorber to cool the overhead stream from
the
deethanizer to a temperature at which it is partially liquefied. This
partially liquefied stream
is then introduced into the absorber wherein the liquid portion of the stream
passes down-
wardly through the absorber to contact a gaseous
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stream passing upwardly through the absorber. While this processing system has
been
effective to separate C2 and lighter components from C3 and heavier
components, it is
relatively inefficient when processing lower pressure feed gas streams. It is
also relatively
inefficient when processing rich feed gas streams with respect to their C3 and
heavier content.
It is particularly ineffective when large amounts of very light gases, such as
hydrogen, may be
present in the feed gas stream charged to the process. Hydrogen in gaseous
streams recovered
from refinery operations, which may be desirably separated in such processes,
is not
uncommon. While the occurrence of hydrogen in significant quantities in
natural gas is rare,
the presence of hydrogen in similar streams from refinery operations is
common.
U.S. Patent No. 6,405,561 discloses a process for recovering C3 and heavier
compo-
nents from low-pressure natural gas or refinery gas streams. The '561 patent
teaches the
improvement of cooling and partially condensing a deethanizer overhead gas
stream to
produce a deethanizer liquid stream that is further cooled and directed into
an upper portion
of a separator/absorber, which separates the inlet feed stream into a liquid
bottoms stream
comprising primarily C3 and heavier components and an overhead gas stream
comprising
primarily C2 and lighter components. The process of the '561 patent is
particularly
effective for treatment of feed gas streams at lower pressure that contain
substantial amounts
of very light components, including hydrogen that is often found in refinery
applications.
The process of the '561 patent is also effective for treatment of feed gas
streams rich with
respect to recoverable C3 and heavier components.
However, as feed gas pressure increases, or if feed gas streams with higher
quantities
of C2 and lighter components are used, the process of the '561 patent becomes
less effective
due to co-adsorption of these lighter components in the separator/absorber
bottoms stream.
As a result, these lighter components tend to reduce the temperature required
to partially
condense the deethanizer overhead gas stream. Thus, the refrigerant medium
used in this
condensation operation must be changed from propane to a colder, more horsepow-
er-intensive refrigeration media. As a result, the investment in equipment and
operating cost
is increased substantially.
SUMMARY OF THE INVENTION
In one embodiment of the present invention, there is provided a process for
separating
a feed gas stream containing methane, at least one C2 component, and at least
one C3
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component into a volatile gas stream containing a major portion of the methane
and at least
one C2 component and a less volatile stream containing a major portion of the
at least one C3
component. The process comprises first cooling the feed gas stream to a
temperature
sufficient to condense the majority of the at least one C3 component in the
feed gas stream to
produce a cooled feed stream. The cooled feed stream is introduced into a
separator vessel to
separate the cooled feed stream into a separator gas stream and a separator
liquid stream. At
least a portion of both of the separator gas and liquid streams from the
separator vessel is
introduced into a fractionation column to produce a fractionation column
bottoms product and
a fractionation column overhead residue gas stream. The fractionation column
bottoms
product is introduced into a deethanizer tower which produces a deethanizer
bottoms stream
comprising a majority of the at least one C3 component and a deethanizer
overhead gas
stream.
In another embodiment of the present invention, there is provided a process
for
separating a feed gas stream containing methane, at least one C2 component,
and at least one
C3 component into a volatile gas stream containing a major portion of the
methane and at
least one C2 component and a less volatile stream containing a major portion
of the at least
one C3 component. The process comprises cooling the feed gas stream to a
temperature
sufficient to condense the majority of the at least one C3 component therein
to produce a
cooled feed stream. The cooled feed stream is passed to a fractionation column
to produce a
liquid fractionation column bottoms product and a fractionation column
overhead residue gas
stream. The fractionation column including a rcboiler operable to vaporize at
least a portion
of the fractionation column liquid which is taken from the bottom or near the
bottom of the
column. The vaporized portion is then reintroduced into the fractionation
column. The
fractionation column bottoms product is introduced into a deethanizer tower
which produces
a deethanizer bottoms stream comprising a majority of the at least one C3
component and a
deethanizer overhead gas stream. The deethanizer overhead gas stream is cooled
and at least
partially condensed thereby producing a deethanizer liquid reflux stream and a
deethanizer
residue gas stream. Optionally, the deethanizer residue gas stream is combined
with at least
a portion of the overhead residue gas stream to form a combined residue gas
stream. At least
a portion of the combined residue gas stream is compressed and cooled to
produce a residue
gas reflux stream. The residue gas reflux stream is introduced into the
fractionation column.
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In a further embodiment of the present invention, there is provided a process
for sepa-
rating a feed gas stream containing methane, at least one C2 component, and at
least one C3
component into a volatile gas stream containing a major portion of the methane
and at least
one C2 component and a less volatile stream containing a major portion of the
at least one C3
component. The process comprises cooling the feed gas stream to a temperature
sufficient to
condense the majority of the at least one C3 component in the feed gas stream
to produce a
cooled feed stream. The cooled feed stream is passed to a fractionation column
to produce a
liquid fractionation column bottoms product and a fractionation column
overhead residue gas
stream. The fractionation column bottoms product is introduced into a
deethanizer tower,
which produces a deethanizer bottoms stream comprising a majority of the at
least one C3
component and a deethanizer overhead gas stream. -Fhe deethanizer overhead gas
stream is
cooled and at least partially condensed thereby producing a deethanizer liquid
reflux stream
and a deethanizer residue gas stream. At least a portion of the fractionation
column overhead
residue gas stream is compressed and cooled to produce a residue gas reflux
stream. The
residue gas reflux stream is then introduced into the fractionation column.
In yet another embodiment of the present invention, there is provided a system
for
separating a feed gas stream containing methane, at least one C2 component,
and at least one
C3 component into a volatile gas stream containing a major portion of the
methane and at
least one C2 component and a less volatile stream containing a major portion
of the at least
one C3 component. The system comprises a feed stream heat exchanger configured
to cool
the feed gas stream to a temperature sufficient to condense the majority of
the at least one C3
component in the feed gas stream to produce a cooled feed stream. A separator
vessel is
located downstream from the first heat exchanger and configured to separate
the cooled feed
stream into a separator gas stream and a separator liquid stream. A
fractionation column is
located downstream from the separator vessel and configured to receive at
least a portion of
both the separator gas and liquid streams and produce a fractionation column
bottoms product
and a fractionation column overhead residue gas stream. A deethanizer tower is
located
downstream from the separator vessel and configured to receive at least a
portion of the
fractionation column bottoms product and to produce a deethanizer bottoms
stream compris-
ing a majority of the at least one C3 component and a deethanizer overhead gas
stream.
In still another embodiment of the present invention, there is provided a
system for
separating a feed gas stream containing methane, at least one C2 component,
and at least one
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C3 component into a volatile gas stream containing a major portion of the
methane and at
least one C2 component and a less volatile stream containing a major portion
of the at least
one C3 component. The system comprises a feed stream heat exchanger configured
to cool
the feed gas stream to a temperature sufficient to condense the majority of
the at least one C3
conwonent therein to produce a cooled feed stream. A fractionation column is
located
downstream from the feed stream heat exchanger and is configured to receive
the cooled feed
stream and produce a fractionation column bottoms product and a fractionation
column
overhead residue gas stream. The fractionation column includes a reboiler
configured to
vaporize at least a portion of the fractionation column liquid and reintroduce
the vaporized
fractionation column liquid back into the fractionation column. A deethanizer
tower is
located downstream from the fractionation column and configured to receive at
least another
portion of the fractionation column bottoms product and produce a deethanizer
bottoms
stream comprising a majority of the at least one C3 component and a
deethanizer overhead
gas stream. A deethanizer heat exchanger is provided and configured to receive
and cool the
deethanizer overhead gas stream. A deethanizer separation vessel is located
downstream
from the deethanizer heat exchanger and is configured to separate the cooled
deethanizer
overhead gas stream into a deethanizer liquid reflux stream and a deethanizer
residue gas
stream. Optionally, the system further includes a conduit configured to merge
at least a
portion of the deethanizer residue gas stream with at least a portion of the
fractionation
column overhead residue gas stream to form a combined residue gas stream. A
residue gas
heat exchanger is provided and configured to condense at least a portion of
the combined
residue stream to form a residue gas reflux stream. Conduit is configured to
deliver at least a
portion of the residue gas reflux stream from the gas condensation unit to the
fractionation
column.
In even a further embodiment, there is provided a system for separating a feed
gas
stream containing methane, at least one C2 component, and at least one C3
component into a
volatile gas stream containing a major portion of the methane and at least one
C2 component
and a less volatile stream containing a major portion of the at least one C3
component. The
system comprises a feed stream heat exchanger configured to cool the feed gas
stream to a
temperature sufficient to condense the majority of the at least one C3
component in the feed
gas stream to produce a cooled feed stream. A fractionation column is located
downstream
from the heat exchanger and is configured to receive the cooled feed stream
and produce a
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fractionation column bottoms product and a fractionation column overhead
residue gas
stream. A deethanizer tower is located downstream from the fractionation
column and
configured to receive at least a portion of the fractionation column bottoms
product and
produce a deethanizer bottoms stream comprising a majority of the at least one
C3 component
and a deethanizer overhead gas stream. A deethanizer heat exchanger is
provided and
configured to receive and cool the deethanizer overhead gas stream. A
deethanizer separa-
tion vessel is provided and configured to separate the cooled deethanizer
overhead gas stream
into a deethanizer liquid reflux stream and a deethanizer residue gas stream.
Conduit is
provided and configured to deliver at least a portion of the deethanizer
liquid reflux stream to
the fractionation column. A residue gas heat exchanger is provided and
configured to
condense at least a portion of the fractionation column overhead residue gas
stream. Conduit
is provided and configured to deliver at least a portion of the condensed
fractionation column
overhead residue gas stream to the fractionation column.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of a process according to one embodiment of
the
present invention; and
Fig. 2 is a schematic diagram of a process according to another embodiment of
the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning to Fig. 1, an embodiment of the present invention is shown that is
particularly
adapted for the recovery of C3 and heavier components from a hydrocarbon-
containing gas
stream, such as a natural gas or refinery gas stream. In particular
embodiments, the inlet feed
gas stream 10 comprises methane, at least one C2 component, at least one C3
component, and
optionally heavier components. In still other embodiments, inlet feed gas
stream 10
comprises methane as the predominant component, with C2, C3, and heavier
components
being present in lesser amounts. In
refinery applications, the feed may also contain
significant quantities of lighter components such as hydrogen. Particularly,
in certain
applications, the feed stream may comprise as much as 10%, or even as much as
50%,
hydrogen.
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The present invention exhibits the flexibility to accommodate a wide variety
of feed
pressures. In one embodiment, the feed stream 10 can be supplied at a pressure
of at least
300 psi, or particularly, between about 350 psi to about 700 psi. Typically,
feed stream 10
will be supplied at a temperature that is above the condensation point for the
C3 components
present therein; thercfore, feed stream will need to be cooled in order to
condense these
components. In this embodiment, feed gas stream 10 is passed through a heat
exchanger 12
where it is cooled to a temperature sufficient to condense the majority of the
at least one C3
component in the feed gas stream to produce a cooled gas feed stream. Note,
the use of the
word "gas" in the term "cooled gas feed stream" should not be taken as
implying that the
entirety of the stream is present in the gaseous state. Certain components,
particularly the
heavier components may be present as liquids. The cooling streams used in heat
exchanger
12 are discussed in greater detail below. It will be understood that the heat
exchange
function shown schematically in heat exchanger 12 may be accomplished in a
single or a
plurality of heat exchange vessels.
The cooled inlet gas is passed via a line or conduit 14 to a separation vessel
16 where
it is separated into a vapor stream 18 and a bottoms stream 20. Vapor stream
18 is directed
toward an expander 22 to reduce the pressure of and further cool the stream.
The expanded
vapor stream is passed via a line 24 to a fractionation column 26 containing
one or more
theoretical stages of mass transfer. In certain embodiments, the fractionation
column 26 is a
conventional distillation column containing a plurality of vertically spaced
trays, one or more
packed beds. or some combination of trays and packing.
The bottoms stream 20 recovered from separator vessel 16 contains primarily C3
and
heavier components, although the bottoms stream 20 will also contain
quantities of lighter
materials. As explained further below, ultimately these lighter components
will be separated
from the C3 and heavier components in subsequent processing steps. In order to
maximize
the efficiency of those subsequent processing steps, the present embodiment
seeks to control
the levels of C2 and lighter components contained in the liquid, predominantly
C3 stream that
will be further processed. Thus, bottoms stream 20 is also passed to
fractionation column 26.
Generally, bottoms stream 20 is introduced into fractionation column 26 below
the introduc-
tion point for the expanded vapor stream carried by line 24, although the
arrangement of the
introduction points for the various streams fed to fractionation column 26 can
be varied as
deemed appropriate. This step of introducing bottoms stream 20 into
fractionation column
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26 provides an opportunity for the lighter materials co-absorbed in liquid
bottoms stream 20
to be separated therefrom. Fractionation column 26 is equipped with an
optional reboiler 28
to assist with separation of the C2 and lighter components from the bottoms of
the fractiona-
tion column. A portion of the fractionation column liquids taken from the
bottom or near
the bottom of column 26 are directed to reboiler 28 and at least partially
vaporized and then
reintroduced into the fractionation column 26. Accordingly, as the liquid
stream exiting
fractionation column 26 contains fewer C2 or lighter components, it has higher
condensation
temperature than the stream 20. This permits a propane refrigerant, or similar
refrigerant, to
be used to condense the overhead stream ti-om a deethanizer 36, which is
discussed in greater
detail below. Otherwise, if the bottoms product from fractionation column 26
contained a
higher level of C2 or lighter components, a colder and therefore more
expensive refrigeration
system would need to be employed.
In fractionation column 26, a liquid bottoms stream comprising primarily Cl
and
heavier components plus some light components is recovered via a line 30 and a
pump 32 and
pumped via a line 34 to heat exchanger 12 where it is used to cool the inlet
gas stream in line
10. The stream in line 34 is then passed to a deethanizer 36. In deethanizer
36 the stream
from line 34 is separated by conventional distillation techniques as well
known to the art for
deethanizers into an overhead vapor stream 38 and a bottoms stream 40.
Deethanizer 36 also
comprises a conventional reboiler 42. The stream recovered from deethanizer 36
through
line 40 comprises primarily C3 and heavier components. An overhead stream is
recovered
from the deethanizer via line 38, which is rich in C2 and lighter components
and is passed to a
heat exchanger 44 where it is partially condensed and then through a line 46
to a separator 48.
From separator 48, a liquid stream is withdrawn via a line 50 and passed to a
pump 52 from
which a portion of the liquid stream is passed via a line 54 into an upper
portion of
deethanizer 36 as a reflux. The vapor stream recovered from separator 48 is
passed via a line
56.
Deethanizer 36 is maintained at a higher pressure than fractionation column
26. The
increased pressure for deethanizer 36 is supplied by pump 32 and maintained by
a valve 57
disposed in line 56. In certain embodiments, the pressure in deethanizer 36 is
at least 25 psi,
or at least 100 psi, or at least 200 psi greater than the pressure in
fractionation column 26.
A second portion of the liquid stream from separator 48 is passed via a line
58,
through a heat exchanger 60, and into an upper portion of fractionation column
26, An
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overhead vapor stream recovered from the upper portion of fractionation column
26 is passed
via a line 62, through heat exchanger 66 and then combined with the stream in
line 56. It is
noted that the stream carried by line 56 is flashed across valve 57. The
combined stream
contains a residue gas that comprises a major portion of the C., and lighter
components from
the inlet gas feed stream. This stream is passed via line 64 through heat
exchanger 12 so as
to provide cooling for feed stream 10. Alternatively, stream 56 and stream 62
can be passed
separately through heat exchanger 12 such that stream 56, which contains a
significant
quantity of C7 components, would be available for internal use thus reducing
the C2 content
of the residue gas.
The cooling to heat exchanger 12 provided by the materials carried by lines 34
and 64
can be supplemented by a refrigerant, such as propane, supplied to heat
exchanger 12 by line
76. Next, the residue gas carried by line 64 is passed through a compressor
66. The residue
gas exits compressor 66 via line 68. Optionally, a portion of the residue gas
carried by line
68 is passed via a line 70 to a heat exchanger 72 where it is cooled and
condensed. In the
embodiment illustrated, the chilled portion of residue gas exiting heat
exchanger 72 is
refluxed to the top of fractionation column 26. The other portion of residue
gas from line 68
is withdrawn from the system via line 74. In those embodiments in which
streams 56 and 62
are not combined and an additional reflux is desired for column 26, a portion
of the contents
of stream 62 are compressed, condensed and refluxed to the column.
In an illustrative embodiment of the process shown in Fig. 1, a dehydrated gas
stream
10 is charged to the process at 340 psia and 114F. The gas stream is cooled in
heat
exchanger 12 to a temperature of -66F and 330 psia and charged to separation
vessel 16. In
separation vessel 16, gaseous overhead stream 18 is produced and passed
through expander
22 and is carried by line 24 at -99F and 150 psia to fractionation column 26.
The liquid
stream recovered via line 20 at -1.5F and 145 psia and directed through pump
32 where its
pressure is increased to 360 psia. The stream carried by line 34 is used to
provide refrigera-
tion to heat exchanger 12 and then directed to deethanizer 36 at a temperature
of 74F and 355
psia.
In deethanizer 36, a bottoms liquid stream composed primarily of C3 and
heavier
components is recovered via a line 40 at a temperature of 173F at 350 psia.
The vapor
stream recovered via line 56 is at a temperature of 24F at 335 psia. In the
current simulation,
the vapor stream recovered via line 56 was withdrawn from the system and used
as fuel gas.
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However, as illustrated in Fig. 1, this stream can be flashed across valve 57
and combined
with the gas carried by line 62. A liquid reflux stream carried by line 58 is
withdrawn from
the deethanizer at a temperature of 24F and 335 psia. This stream is cooled,
expanded, and
refluxed to fractionation column 26 at -11rF and 145 psia.
The overhead vapor from fractionation column 26 carried by line 62 is at a
tempera-
ture of -117F and a pressure of 140 psia and is heat exchanged with the stream
carried by line
58 and emerges from heat exchanger 60 at -99F and 135 psia and directed to
heat exchanger
12 via line 64. This residue gas stream exits heat exchanger 12 at 95F and 125
psia and is
directed toward compressor 66 (in this simulation a series of compressor
stages with
intercooling) where it is boosted to 1265 psia and its temperature raised to
115F. A portion
of this compressed stream is withdrawn via line 70, cooled and condensed by
heat exchanger
72 and refluxed to fractionation column 26 at a temperature of -112F and
pressure of 1255
psia.
While specific temperatures have been referred to in connection with the
embodiment
illustrated in Fig. 1, it should be understood that a wide range of
temperature and pressure
variations are possible within the scope of the present invention. Such
temperature and
pressure variations are readily determined by those skilled in the art based
upon the composi-
tion of the specific feed streams, the desired recoveries and the like within
the scope of the
processes disclosed above.
Figure 2 illustrates another embodiment of a process in accordance with the
present
invention. Note, when applicable, the same reference numerals used in the
description of
Fig. 1 have been used to identify comparable lines or equipment. In the
process of Fig. 2, the
inlet gas stream is charged to the process via a line 10. The inlet feed gas
is cooled in a heat
exchanger 12 and thereafter passed via a line 14 to a heat exchanger 15 where
it is further
cooled to a selected temperature and passed via line 17 to a fractionation
column 26
containing one or more theoretical stages of mass transfer. Fractionation
column 26 is
equipped with a reboiler 28 to assist with separation of the C2 and lighter
components from
the bottoms of the fractionation column. A portion of the tower liquid from
fractionation
column 26 is directed to reboiler 28 and at least partially vaporized and then
reintroduced into
the bottom of fractionation column 26.
In fractionation column 26, a liquid bottoms product comprising primarily C3
and
heavier components plus some light components is recovered via a line 30 and a
pump 32 and
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pumped via a line 34 to heat exchanger 12 where it is used to cool the inlet
gas stream in line
10. The stream in line 34 is then passed via to a deethanizer 36. In
deethanizer 32 the
stream from line 34 is separated by conventional distillation techniques into
an overhead
vapor stream 38 and a bottoms stream 40. A conventional reboiler 42 is shown
for with-
drawing a portion of the deethanizer tower liquid, at least partially
vaporizing the withdrawn
portion, and returning the at least partially vaporized stream back to
deethanizer 36. The
stream recovered from deethanizer 36 through line 40 comprises primarily C3
and heavier
components. An overhead stream is recovered from the deethanizer via line 38,
which is
rich in C2 and lighter components and is passed to a heat exchanger 44 where
it is at least
partially condensed and then through a line 46 to a separator 48. From
separator 48, a liquid
stream is withdrawn via a line 50 and passed to a pump 52 from which a portion
of the liquid
stream is passed via a line 54 into an upper portion of deethanizer 36 as a
reflux. The vapor
stream recovered from separator 48 is passed via a line 56 and through an
expansion valve 57.
The vapor stream is then combined with the residue gas from line 62 and
directed toward a
compressor 66 via line 64.
A second portion of the liquid stream from separator 48 is passed via a line
58 and a
heat exchanger 60 into an upper portion of fractionation column 26. An
overhead vapor
stream recovered from the upper portion of fractionation column 26 is passed
via a line 62
through heat exchanger 60 to combination with the stream in line 26. The
combined stream
carried by line 64 contains a major portion of the C2 and lighter components
from the inlet
gas feed stream. As noted above, the stream in line 64 is compressed by
compressor 66 arid
passed into line 68. A portion of the compressed residue gas carried by line
68 is passed via
a line 70 to a heat exchanger 72 where it is cooled and condensed. In the
embodiment
illustrated, the condensed portion of residue gas exiting heat exchanger 72 is
refluxed to the
top of fractionation column 26. The other portion of residue gas from line 68
is withdrawn
from the system via line 74.
It is noted that, as discussed above with respect to Fig. 1, in certain
embodiments,
streams 56 and 62 may be kept separate. When an additional reflux is desired
for column 26,
a slip stream of the material carried by line 62 can be compressed, condensed,
and refluxed to
the column. It is also noted that for any embodiment discussed above, it is
within the scope
of the present invention for the residue gas reflux carried by line 70 to be
used without
equipping fractionation column 26 with a reboiler 28.
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While the present invention has been described by reference to certain of its
preferred
embodiments, it is respectfully pointed out that the embodiments described are
illustrative
rather than limiting in nature and that many variations and modifications are
possible within
the scope of the present invention.
