U.S. patent number RE32,600 [Application Number 06/816,000] was granted by the patent office on 1988-02-16 for distillative separation employing bottom additives.
This patent grant is currently assigned to Koch Process Systems, Inc.. Invention is credited to John V. O'Brien, James M. Ryan.
United States Patent |
RE32,600 |
Ryan , et al. |
February 16, 1988 |
Distillative separation employing bottom additives
Abstract
An improved method for the distillation of a feed stream
containing hydrocarbon components, which method is directed toward
the production of a bottom product stream and an overhead product
stream, both with desired specifications, which method comprises
recycling a minor portion of the bottom product stream typically,
but not necessarily, derived from said separation directly to a
reflux condenser for the overhead product stream of said method, in
order to effect a savings in energy in said distillative method,
such as by adjusting the column operating conditions, such as the
column operating pressure or the top or bottom operating
temperatures of said column.
Inventors: |
Ryan; James M. (Weston, MA),
O'Brien; John V. (Shrewsbury, MA) |
Assignee: |
Koch Process Systems, Inc.
(Westboro, MA)
|
Family
ID: |
27038838 |
Appl.
No.: |
06/816,000 |
Filed: |
January 3, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
704180 |
Feb 22, 1985 |
RE032068 |
Jan 21, 1986 |
|
Reissue of: |
458047 |
Jan 14, 1983 |
04428759 |
Jan 31, 1984 |
|
|
Current U.S.
Class: |
62/635;
62/929 |
Current CPC
Class: |
B01D
3/146 (20130101); C07C 7/04 (20130101); F25J
3/0266 (20130101); F25J 3/0219 (20130101); F25J
3/0257 (20130101); F25J 3/0209 (20130101); F25J
3/0238 (20130101); F25J 3/0233 (20130101); F25J
3/0242 (20130101); F25J 3/0247 (20130101); Y02P
20/151 (20151101); F25J 2205/50 (20130101); F25J
2215/64 (20130101); Y02P 20/152 (20151101); Y02C
20/40 (20200801); F25J 2210/12 (20130101); F25J
2220/66 (20130101); Y02C 10/12 (20130101) |
Current International
Class: |
B01D
3/14 (20060101); C07C 7/00 (20060101); C07C
7/04 (20060101); F25J 3/02 (20060101); F25J
003/04 () |
Field of
Search: |
;62/17,20,23-28
;55/68 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sever; Frank
Attorney, Agent or Firm: Crowley; Richard P.
Parent Case Text
.Badd.REFERENCE TO PRIOR APPLICATION
.Badd.This application is a reissue of U.S. patent application Ser.
No. 704,180, filed Feb. 22, 1985, now Re. 32,068, issued Jan. 21,
1986, which is a reissue of U.S. patent application Ser. No.
458,047, filed Jan. 14, 1983, now U.S. Pat. No. 4,428,759, issued
Jan. 31, 1984. .Baddend.
Claims
What is claimed is:
1. In a method for the distillative separation, in a distillative
column containing vapor-liquid contact devices, of a
hydrocarbon-containing feed stream, which method comprises:
(a) introducing the feed stream into a distillative column
operating under defined conditions of pressure, temperature and
feed composition;
(b) withdrawing an overhead product stream .[..[.encircled.]..].
.Badd.enriched .Baddend.in at least one component to be removed
from the feed stream;
(c) condensing at least a portion of the overhead product stream in
a condenser and recycling a portion of the condensed overhead
stream to the top portion of the said distillative column;
(d) recovering a bottom product stream with defined specifications
enriched with at least one defined component of the feed stream for
which recovery is sought; and
(e) reboiling at least a portion of the withdrawn bottom product
stream and recycling a portion of the bottom stream to the bottom
portion of the column, the improvement which comprises introducing
from about 1 to 30 mols per 100 mols of feed stream of a recycled
liquid bottom product stream comprising .[.C.sub.3 -C.sub.6 .].
alkanes .Iadd.having at least three carbon atoms .Iaddend.into an
overhead condenser zone of the distillation column, the liquid
bottom product stream introduced into the condenser zone
concurrently and admixed with the overhead product stream in the
condenser zone, the overhead product stream not being subject to
substantial contamination by the recycled bottom product stream in
the condenser zone, and the amount of the recycled bottom product
stream sufficient, to provide adjusting of the operation condition
of the distillation zone to effect a saving in the distillation
column operating energy by:
(a) increasing the temperature of the condenser zone of the
distillation column by about 10.degree. F. or more; or
(b) decreasing the temperature of the bottom of the distillation
column by about 5.degree. F. or more; or
(c) reducing the distillation column operating pressure by about 20
psi or more.
2. The method of claim 1 wherein the amount of recycled liquid
bottom product stream ranges from about 1 to 10 mols per 100 mols
of feed stream. 3. The method of claim 1 which includes introducing
sufficient, recycled, liquid bottom product stream into the
overhead condenser zone, to increase the temperature of the
overhead condenser zone from about 10.degree. F. to 60.degree. F.
from the temperature of the overhead condenser zone, without the
introduction of the bottom product stream. 4. The method of claim 1
which includes introducing sufficient, recycled, liquid bottom
product stream into the overhead condenser zone, to permit the
reduction in the distillative column operating pressure from the
operating pressure employed, without the introduction of the bottom
product stream. 5. The method of claim 4 which includes introducing
sufficient bottom product stream, to reduce the operating pressure
of the distillation column from about 30 to 150 psi. 6. The method
of claim 1 wherein the recycled liquid bottom product stream
comprises a majority of C.sub.4 + alkanes. 7. The method of claim
1, which includes removing heat from the overhead condenser zone to
which the bottom product stream has been added, by employing
cooling water or a refrigerating system. 8. The method of claim 1
wherein the recycled liquid bottom product stream is recycled to
the overhead condenser zone of the same distillative column from
which the bottom product stream is recovered. 9. The method of
claim 8 wherein the feed stream to the distillative column
comprises a cool feed stream of H.sub.2 S and C.sub.2 + alkanes and
is essentially free of carbon dioxide, and the overhead stream is
enriched in H.sub.2 S, C.sub.2 and C.sub.3 and the bottom product
stream is enriched in C.sub.4 + alkanes. 10. The method of claim 1
wherein the recycled bottom product stream has less than about 0.5%
mols of C.sub.3 and a maximum of about 10 ppm of H.sub.2 S. 11. The
method of claim 1 wherein the feed stream comprises CO.sub.2,
H.sub.2 S and C.sub.2 -C.sub.6 alkanes, and the bottom product
stream comprises primarily C.sub.4 -C.sub.6 alkanes. 12. The method
of claim 1 wherein the feed stream is essentially free of acid gas
components. 13. The method of claim 1 which includes introducing
the liquid bottom product stream into the overhead condenser zone
by sparging the liquid bottom stream into the vapor overhead
product stream. 14. The method of claim 1 which includes spraying
the liquid bottom stream concurrently into the vapor overhead
product stream introduced into the overhead condensing zone. 15.
The method of claim 1 for the distillative separation of carbon
dioxide from a gaseous hydrocarbon feed stream containing carbon
dioxide, which method comprises:
(a) introducing the feed stream into a first distillative
column;
(b) introducing a nonpolar liquid C.sub.3 -C.sub.6 alkane additive
agent into the upper section of the column above the point of
introduction of the feed stream;
(c) withdrawing a first overhead product stream from the top of the
first column enriched in carbon dioxide;
(d) withdrawing a first bottom product stream from the bottom of
the first column containing the liquid additive agent;
(e) introducing the first bottom product stream into a second
distillative column as a feed stream for the second column;
(f) withdrawing from the top of the second column a second overhead
product stream comprising primarily C.sub.2 and C.sub.3
alkanes;
(g) withdrawing from the bottom of the second column a second
bottom product stream comprising primarily C.sub.4 + alkanes;
(h) condensing at least a portion of the second overhead product
stream in an overhead condenser and recycling a portion of the
second overhead product stream to the top of the second column;
(i) reboiling at least a portion of the second bottom product
stream in a reboiler and recycling a portion of the reboiled bottom
product stream to the bottom of the second column;
(j) recovering a reboiled second bottom product stream comprising
C.sub.4 +; and
(k) recycling a portion of from about 1 to 30 mols of the second
bottom product C.sub.4 + stream, based on 100 mols of the feed
stream in the second column, into the second overhead condenser and
introducing the recycled C.sub.4 + bottom stream concurrently with
the second overhead product stream into the second overhead
condenser, to increase the temperature of the second overhead
condenser or to reduce the operating
pressure of the first column. 16. The method of claim 15 which
includes recycling a portion of the reboiled bottom product stream
as at least a portion of the liquid additive stream introduced into
the first column. 17. The method of claim 1 for the distillative
separation of carbon dioxide from a gaseous hydrocarbon feed stream
containing the carbon dioxide and C.sub.2, C.sub.3 and C.sub.4 +,
which method comprises:
(a) introducing the feed stream into a first distillative
column;
(b) withdrawing a first overhead product stream enriched in carbon
dioxide and C.sub.2 from the top of the first column;
(c) withdrawing a first bottom product stream enriched in C.sub.3
from the bottom of the first column;
(d) condensing at least a portion of the first overhead product
stream in a first overhead condenser and recycling a portion of the
first overhead product stream to the top of the first column;
(e) reboiling at least a portion of the first bottom product stream
in a reboiler and recycling a portion of the reboiled bottom
product stream to the bottom of the first column;
(f) introducing a first bottom product stream as the feed stream
into a second distillative column containing a plurality of
distillation trays;
(g) withdrawing from the top of the second column a second overhead
product stream comprising primarily C.sub.3 ;
(h) withdrawing from the bottom of the second column a second
bottom product stream comprising primarily C.sub.4 +;
(i) recovering a reboiled second bottom product stream; and
(j) recycling a portion of from about 1 to 10 mols of the second
bottom C.sub.4 + product stream, based on 100 mols of the feed
stream in the first column, into the inlet of the first overhead
condenser concurrently with the first overhead product stream, to
admix uniformly the bottom stream and overhead product stream in
the condenser, or to the upper ten or less trays of the first
column, to reduce the heat duty in the operation of the first
column. 18. The method of claim 1 for the distillative separation
of nitrogen and methane from a gaseous feed stream comprising
nitrogen, methane, carbon dioxide and C.sub.2 + alkanes, which
method comprises:
(a) introducing the feed stream into a first distillative
column;
(b) withdrawing from the top of the first column a first overhead
product stream enriched in nitrogen;
(c) withdrawing from the bottom of the first column a first bottom
product stream containing CH.sub.4, CO.sub.2 and C.sub.2 +;
(d) condensing at least a portion of the first overhead product
stream in a first overhead condenser and recycling a portion of the
condensed first overhead product stream to the top of the first
column;
(e) introducing the first bottom product stream as a second feed
stream into a second distillative column;
(f) withdrawing from the top of the second column a second overhead
product stream enriched in methane;
(g) withdrawing from the bottom of the second column a second
bottom product stream containing CO.sub.2 and C.sub.2 +
products;
(h) introducing into the upper section of the second column, above
the point of introduction of the second feed stream, a nonpolar,
liquid, C.sub.3 -C.sub.6 alkane additive agent;
(i) condensing at least a portion of the second overhead product
stream in a second overhead condenser and recycling a portion of
the condensed second overhead stream to the top of the second
column;
(j) introducing the second bottom product stream as the third feed
stream into a third distillative column;
(k) withdrawing from the top of the third column a third overhead
product stream containing CO.sub.2, C.sub.2 and C.sub.3 ;
(l) withdrawing and recovering a third bottom produce stream from
the bottom of the third column composed primarily of C.sub.4 +
alkanes; and
(m) recycling at least a portion of the third bottom product stream
to the first overhead condenser concurrently with the first
overhead product stream, to admix uniformly the bottom stream and
overhead product stream in the condenser, to provide for an
increase in the temperature of the overhead condenser or a
reduction in the operating pressure of the first column. 19. The
method of claim 18 which includes recycling a portion of the third
bottom product stream for introduction as at least a part of the
liquid additive agent into the second column. 20. The method of
claim 18 wherein the first overhead product stream comprises
essentially nitrogen and methane, and the second overhead product
stream comprises essentially pure methane. 21. The method of claim
1 which includes introducing recycled liquid bottom product stream
into the overhead condenser zone in an amount sufficient to reduce
the operating temperature of the bottom of the column from about
5.degree. to 50.degree. F. from the temperature without the
introduction of the recycled liquid bottom product stream.
The method of claim 1 which comprises the distillative separation
in the distillation column of a carbon dioxide and C.sub.2 overhead
stream and a C.sub.3 + bottom stream which includes recycling a
minor amount of a bottom product stream into the uppermost tray
section of the distillative column separating carbon dioxide and
C.sub.2 overhead stream from the C.sub.3 + and bottom stream. 23.
The method of claim 22 wherein the bottom product stream comprises
a C.sub.4 + stream and is introduced into at least one of the top
ten trays of the distillation column. 24. The method of claim 1
which comprises the distillative separation of nitrogen and methane
from a feed stream in a CH.sub.4 -N.sub.2 distillative separation
column and which includes introducing a liquid bottom product
stream from the distillative separation of CO.sub.2, C.sub.2 and
C.sub.3 as an overhead stream and the C.sub.4 + additive bottom
product stream into the overhead condenser zone of the CH.sub.4
-N.sub.2 distillation column for the separation of CH.sub.4 and
N.sub.2 to increase the column operating temperature of the
CH.sub.4 -N.sub.2 overhead condenser zone. 25. The method of claim
1 which comprises the separation of a hydrogen sulfide, C.sub.2 and
C.sub.3 stream as an overhead stream from a C.sub.4 -C.sub.6 +
liquid bottom product stream in a distillation column which
includes recycling the C.sub.4 -C.sub.6 + liquid bottom product
stream into the overhead condenser zone of the distillation column
to increase the
temperature of the overhead condenser zone. 26. The method of claim
25 wherein from about 1 to 8 mols of the bottom product stream is
recycled per 100 mols of feed stream to the CO.sub.2 -C.sub.2
distillation column. 27. The method of claim 1 which comprises the
distillative separation of carbon dioxide and ethane stream from
propane stream which includes distillatively separating CO.sub.2
and C.sub.2 as an overhead stream in a distillation column and
recovering a C.sub.3 + bottom stream as a feed stream for
distillatively separating in another distillation column of C.sub.3
as an overhead product stream and C.sub.4 + as a liquid bottom
additive stream and recycling a portion of the liquid bottom
product stream to the overhead condenser of the distillative column
for the CO.sub.2 -C.sub.2 separation from C.sub.3 + to increase the
operating temperature of the overhead condenser zone of the
CO.sub.2 -C.sub.2 distillation column. 28. The method of claim 1
wherein the bottom product stream is admixed with the overhead
product stream upstream of the heat exchange area in the condenser
zone so that the admixed overhead and the bottom product stream are
generally uniformly distributed through the vapor portion of the
heat exchange area of the condenser zone. 29. The method of claim 1
wherein the overhead condensing zone is maintained by the recycling
of the bottom product stream at a temperature of about -125.degree.
F. or more. 30. The method of claim 1 which includes recycling the
liquid bottom product stream from a different distillation column
into the condenser zone of the distillation overhead column.
.Iadd. 1. The method of claim 1 wherein the recycled liquid bottom
product stream comprises C.sub.3 -C.sub.7 alkanes. .Iaddend.
.Badd.32. A method for the distillative separation of carbon
dioxide from a gaseous hydrocarbon feed stream containing the
carbon dioxide and C.sub.2, C.sub.3 and C.sub.4.sup.+, which method
comprises:
(a) introducing the feed stream into a first distillative column
containing a plurality of distillation trays;
(b) withdrawing a first overhead product stream enriched in carbon
dioxide and C.sub.2 from the top of the first column;
(c) withdrawing a first bottom product stream enriched in C.sub.3
from the bottom of the first column;
(d) condensing at least a portion of the first overhead product
stream in a first overhead condenser and recycling a portion of the
first overhead product stream to the top of the first column;
(e) reboiling at least a portion of the first bottom product stream
in a reboiler and recycling a portion of the reboiled bottom
product stream to the bottom of the first column;
(f) introducing a first bottom product stream as the feed stream
into a second distillative column containing a plurality of
distillation trays;
(g) withdrawing from the top of the second column a second overhead
product stream comprising primarily C.sub.3 ;
(h) withdrawing from the bottom of the second column a second
bottom product stream comprising primarily C.sub.4.sup.+ ;
(i) recovering a reboiled second bottom product stream; and
(j) recycling a minor portion of the second bottom C.sub.4.sup.+
product stream into the inlet of the first overhead condenser
concurrently with the first overhead product stream, to admix
uniformly the bottom stream and overhead product stream in the
condenser, or to the uppermost tray section above the feed tray of
the first column to reduce the head duty in the operation of the
first column. .Baddend. .Badd.33. The method of claim 32 wherein
the minor portion of the recycled second bottom C.sub.4.sup.+
product stream comprises from about 1 to 10 mols of the second
bottom C.sub.4.sup.+ product stream, based on 100 mols of the feed
stream, into the first column. .Baddend. .Badd.34. The method of
claim 32 which includes recycling a minor portion of the second
bottom C.sub.4.sup.+ product stream into the upper ten or less
trays of the first column. .Baddend. .Badd.35. The method of claim
32 wherein the recycled second bottom C.sub.4.sup.+ product stream
comprises primarily a C.sub.4 -C.sub.6 alkane. .Baddend. .Badd.36.
The method of claim 32 wherein the feed stream into the first
distillative column consists essentially of CO.sub.2, ethane and
C.sub.3.sup.+. .Baddend. .Badd.37. The method of claim 32 which
includes recycling from about 1 to 10 mols of the second bottom
C.sub.4.sup.+ product stream, based on 100 mols of the feed, to the
first column into the uppermost ten or less trays of the first
column. .Baddend.
Description
BACKGROUND OF THE INVENTION
It is desirable to operate distillative processes at minimum
energy, to effect separation of the feed stream into a desired
overhead product stream and a bottom product stream in a
distillative column having vapor-liquid contacting devices, such as
distillation trays, packing devices or a combination thereof. In
typical distillative processes, the overhead product stream is at
least partially condensed and a small portion recycled to the top
of the distillative column, while the bottom product stream is
withdrawn and reboiled and at least a portion recycled to the
bottom of the column, to provide desirable column operating
conditions. Some distillative columns operate under such
conditions, so as to obtain the desired overhead product stream of
defined specifications enriched in a desirable component, or
conversely to obtain a bottom product stream of defined
sepcifications enriched in a particular component, or both, so as
to obtain purified streams for further separation or recovery or
use in a chemical, refinery or petrochemical operation. In any
event, such distillative techniques should be carried out at the
most desirable column operating conditions, wherein optimum energy
savings can be effected.
It is known that the separation of a feed stream in a distillative
column, particularly a gaseous hydrocarbon feed stream comprising
methane and an acid gas component, such as carbon dioxide, may be
separated efficiently through the use of a nonpolar additive agent,
such as a liquid additive agent; for example, a C.sub.3 -C.sub.6
alkane, particularly butane-plus, introduced into the upper portion
of said distillative column in an amount to prevent the formation
of solids in the cryogenic distillation of methane from carbon
dioxide, such as, for example, as more particularly set forth in
U.S. Pat. No. 4,318,723, issued Mar. 9, 1982 (hereby incorporated
by reference).
Also, it has been known that, in the prevention of azeotropic
formation between ethane and carbon dioxide and ethylene and
hydrogen sulfide and other components, the introduction of a
nonpolar additive agent, such as a liquid hydrocarbon additive
agent, such as a C.sub.3 -C.sub.6 alkane, particularly butane-plus,
prevents or inhibits the formation of azeotropes and enables the
separation to provide an overhead product stream more enriched in a
desirable component and a bottom product stream more enriched in a
bottom product component, through the alteration of the azeotropic
formation, such as, for example, as set forth and described more
particularly in U.S. Pat. No. 4,350,511, issued Sept. 21, 1982
(hereby incorporated by reference).
Also, it is known to change the relative volatility of acid gas
components, such as carbon dioxide to hydrogen sulfide, through the
use of an additive agent, such as a nonpolar liquid additive agent,
such as hydrocarbon; for example, a C.sub.3 -C.sub.6 alkane,
particularly butane-plus, in order to enhance the relative
volatility of the carbon dioxide and hydrogen sulfide and,
therefore, to increase the efficiency of separation, as set forth
in U.S. Pat. No. 4,293,322, issued Oct. 6, 1981, now U.S. Pat. No.
4,383,842, issued May 17, 1983.
U.S. patent application Ser. No. 307,672, filed Oct. 1, 1981
(hereby incorporated by reference) relates to an improvement in the
effective separation of methane from carbon dioxide in a
distillative column, wherein the upper portion of the column is
operated at temperatures above the triple point of carbon dioxide;
that is, -70.degree. F., by increasing the amount of nonpolar
additive agent added to the reflux condenser, to maintain the
reflux condenser and all portions of the column above the
triplepoint temperature.
In all of the prior operations, the resulting bottom product stream
contains, in addition to the usual bottom product stream
components, a liquid nonpolar additive, particularly the liquid
hydrocarbon additive added to change the operating conditions in
the column. The liquid additive agent may be recycled with the
bottom product stream or may be separated and recycled for use in
any one or all of the aforementioned uses of an additive agent,
particularly where such distillative processes are employed in
one-, two-, three- or multiple-column operations for the separation
of a natural gas stream or petro-chemical stream into the desired
components. The feed streams employed in such distillation process
include those streams which have major amounts of an acid gas
component desired to be removed and those streams containing minor
amounts of or even no acid gas components. It would be desirable,
in such distillative separations or a combination of operations and
other distillative operations where additives are not used, to
reduce the energy requirements of such distillation techniques.
SUMMARY OF THE INVENTION
The invention relates to a distillative technique, wherein a
product stream is employed to adjust distillative column operating
conditions and to save energy. In particular, the invention
concerns a distillative technique, wherein a small portion of a
bottom product stream of defined specifications, typically a
C.sub.3 -C.sub.6 stream, is recycled to the condenser of a
distillative column, wherein the overhead stream is not
significantly contaminated by the recycled bottom product stream,
to adjust column operating conditions and effect energy
savings.
It has been discovered that the recycling of a portion of a column
bottom product stream; for example, a minor amount, typically such
as less than 30%; for example, 20%, and more typically about 1.0 to
10 mols of recycled product per 100 moles of feed stream, and
introducing the recycled bottom product stream into the condenser
employed for the column overhead stream of that or another column,
or a plurality of distillative columns, permit the column operating
conditions to be adjusted with a considerable savings of energy. It
has been found that the recycling of the column bottom product
stream to the reflux condenser of a distillation system permits the
adjustment of column operating conditions, such as the raising of
the top temperature of the column, the lowering of the bottom
temperature of the column, or the lowering of the pressure of the
column or lowering of the heating and cooling loads or a
combination thereof, in order to permit the savings of energy.
In order to be effective in the practice of the invention, the
introduction of the liquid additive agent into the condenser should
be carried out, so as to provide that the additive agent is
uniformly mixed with the overhead product stream from the
distillative column entering the condenser, and typically to flow
concurrently through the condenser with the overhead stream. It
would be ineffectual to introduce the liquid additive agent to the
condenser outlet and of considerably reduced efficiency to
introduce the liquid additive poorly distributed into the inlet of
the condenser. Therefore, the liquid additive should be introduced
in and admixed with the incoming overhead product vapor stream and
flow concurrently therewith, so that the liquid additive is
generally uniformly distributed at least throughout the major
portion of the heat-exchange surface or area of the condenser. One
method of introducing and admixing is to employ a sparger adjacent
the inlet of the condenser or adjacent or directly upstream of the
heat-exchange surface of the condenser. Another suitable method of
introducing and admixing comprises spraying the liquid additive
concurrently into the incoming vapor overhead product stream.
One method of the invention is directed to those additive-recovery
distillative techniques employing distillative columns, wherein the
technique is directed toward mainly bottom-product-stream end
specifications, and wherein the overhead product stream removes one
or more contaminants or impurities from the feed stream. Thus,
where the introduction of a bottom product stream; for example, a
butane-plus stream, into a condenser, containing an overhead
product stream, does not affect the operating specifications of the
particular distillation technique, then the recycling of the bottom
product stream into said condenser will permit the advantageous
adjustment of the operating conditions of the column.
The invention is particularly useful wherein an additive agent is
introduced into a column, to prevent the formation of a solids
zone, to enhance the relative volatility of particular components,
or to prevent azeotropic formation, such as those distillative
techniques as described in the prior art in the Background of the
Invention. The bottom product stream from the additive-recovery
column of such operation can be recycled to the overhead condenser
of the additive-recovery column or other column in the system to
save energy.
The method of the invention is also of use wherein the feed stream
comprises a hydrocarbon-containing stream, such as a petroleum or
natural or synthetic gas stream high in hydrocarbons and which has
a low amount of acid gas components or essentially few acid gas
components. The invention is directed toward those bottom product
streams, wherein the bottom product stream is typically enriched in
higher alkanes and more particularly C.sub.4 -C.sub.6 alkanes, such
as iC.sub.4, nC.sub.4, iC.sub.5, nC.sub.5 and heavier hydrocarbon
components. The recycling of the heavier hydrocarbon components of
C.sub.4 + directly from the bottom product stream into the overhead
condenser of the distillation system provides for the overhead
condenser to operate at a higher temperature, or alternatively the
distillative column can be operated at a lower pressure, where the
heavier components are present in the overhead product.
For example, in a feed stream which is low in acid gas components
or contains essentially no acid gas components and wherein the
bottom product stream is the desired stream of defined
specifications and the overhead product stream is a stream
containing undesired contaminants, the recycling of the bottom
product stream directly from the bottom product stream, without
separation into the overhead condenser employed for the overhead
product stream, permits the temperature at the top of the column
and of the condenser to increase, for example, from 10.degree. F.
to 60.degree. F.; for example, 15.degree. F. to 50.degree. F.,
while providing for a reduction in temperature of the bottom of the
column; for example, from 5.degree. F. to 50.degree. F. The column
operating temperatures may be maintained and the overall column
pressure reduced, all with the effective savings in energy, without
affecting, or, in fact, in some cases increasing, the efficiency of
separation of the bottom product stream. Also, the amount of the
bottom product stream recycled may vary, to effect a reduction in
column operating pressure of greater than 20 psi; for example, 30
psi, such as from 30 to 150 psi.
The method of the invention is advantageous where the feed stream
comprises a butane-plus additive agent from a prior separation, and
wherein the condenser of the distillative column is refrigerated by
propane, ammonia or Freon (a trademark of du Pont Co.), and the
bottom product additive is fed into the condenser, to raise the
condenser temperature. The increase in temperature of the condenser
permits the overhead product stream to be cooled or condensed
employing a higher temperature; for example, the use of cooling
water or any equivalent, cheap heat sink, providing increasing
energy efficiency of the column operation.
For example, in one embodiment of the method of the invention, it
is desirable to recover a bottom product stream containing C.sub.4
+ hydrocarbons; that is, the nonpolar liquid additive agent from a
prior operation, and not to separate such hydrocarbons, with the
hydrocarbon bottoms having a hydrogen sulfide concentration kept
below a given level; for example, 10 parts per million, and with
C.sub.3 removed to below a given level; for example, 0.5% by
volume, and then to use the bottom product stream as at least a
portion of a liquid additive stream in a cryogenic separation in
another column. It has been found that the overhead stream is
desirably cooled against water or an equivalent energy heat sink,
by recycling a portion of the bottom product stream through the
overhead condenser, so that the overhead condenser will operate at
a higher temperature or, alternatively and preferably, the column
can be operated at a lower pressure, if a sufficient amount of a
heavier C.sub.4 + hydrocarbons are in the overhead product
stream.
In a further embodiment of the invention, such as in the separation
of a substantially pure methane product stream from a natural gas
feed stream containing nitrogen, such as from about 5% to 30% by
volume or more nitrogen, the method provides for a residual gas
stream of low carbon dioxide or hydrogen sulfide content, a fuel
gas stream of low carbon dioxide, hydrogen sulfide and nitrogen
content and a sour liquefied petroleum gas stream wiht a high
ethane-plus recovery containing the bulk of the carbon dioxide and
the hydrogen sulfide.
In this method, a natural gas feed stream containing nitrogen is
introduced into a refrigerated distillative column, wherein a
liquid additive is fed through the condenser to maintain
ethylene-level refrigeration temperatures, and to wash the methane
to the base of the column, with the overhead product stream being
enriched in nitrogen. An additional refrigerated distillative
column employing a liquid additive, such as in U.S. Pat. No.
4,318,723, issued Mar. 9, 1982, is employed to provide for the
separation of a methane and carbon dioxide, with the carbon dioxide
in the bottom product stream, together with the liquid additive of
C.sub.4 +, and the methane removed from the overhead product
stream, the formation of a solids zone prevented by the
introduction of the liquid additive agent to the upper portion of
the column. A third column for additive recovery is then employed,
wherein the bottom product stream, containing the liquid additive,
is employed as a refrigerated feed stream into the first and second
distillative columns.
In a further embodiment, the separation of carbon dioxide and
C.sub.2 from C.sub.3 + in a distillative column has been found to
be enhanced significantly and considerable heat energy saved by the
recycling of a minor amount of the bottom stream, such as the
C.sub.4 + bottom stream, from an additive-recovery column to the
condenser or to the upper section of the column separating CO.sub.2
and C.sub.3. In order to obtain improved recovery of propane or
reduction in energy, it is necessary that the nonpolar liquid
C.sub.4 + additive bottom agent be introduced a sufficient number
of trays above the feed tray. The C.sub.4 + bottom recycled stream
may be added to the overhead condenser; however, some C.sub.4 +
stream will be produced or be present in the CO.sub.2 -C.sub.2
overhead stream. If desired, to reduce the content of C.sub.4 +
stream in the CO.sub.2 -C.sub.2 overhead stream, some or all of the
C.sub.4 + recycled bottom stream may be introduced into the
uppermost tray section of the distillative column, so that the
trays above will reduce the C.sub.4 + content in the overhead
stream. Typically, if the C.sub.4 + recycled bottom stream is
introduced into the upper section of the column, such introduction
occurs in the first top ten trays of the column or less, such as
the first top five or less trays. The reduction in heat removal is
due to the improved relative volatility of the CO.sub.2 to C.sub.3
with the increased concentration of the C.sub.4 + recycled bottom
stream in the liquid in the column. This savings in heat energy
generally occurs without significant changes in the overhead
temperature and column operating pressure, due to the improved
volatility ratio of the components.
This invention will be described for the purpose of illustration
only in connection with certain specific embodiments; however, it
is recognized that those persons skilled in the art may make
various changes or modifications to such illustrated embodiments,
all without departing from the spirit and scope of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of an application of the
invention to additive recovery;
FIG. 2 is a schematic illustration of an application to a nitrogen
separation method employing the invention; and
FIG. 3 is a schematic illustration of an application of CO.sub.2
-propane separation employing the invention.
DESCRIPTION OF THE EMBODIMENTS
FIG. 1 is a schematic illustration of a distillative separation
method employing the present invention of a nonpolar-liquid-agent
additive-recovery application, wherein the feed stream is derived
from one or more prior separation processes, and wherein the feed
stream comprises hydrogen sulfide, carbon dioxide and C.sub.2 +
hydrocarbons introduced through line 12 into a distillative column
14 with vapor-liquid contact devices therein, such as distillation
trays, and with a stream removed overhead through line 16
introduced into a reflux condenser 18 and a liquefied portion of
the overhead stream recycled to the top of the column through line
22, while an enriched carbon-dioxide overhead product stream is
removed overhead through line 20. In the operation of the column, a
liquid additive, such as an alkane mixture; for example, C.sub.3
-C.sub.6, comprising a major amount of butane-plus, is introduced
into the upper section of the column 14 through line 24, to prevent
or to modify azeotropic formations within the column 14. A column
bottom stream is removed through line 26 and is introduced into a
reboiler 28, and a portion is recycled through line 30 to the
bottom of the column, while the bottom product stream, containing
C.sub.2 + and the C.sub.3 -C.sub.6 additive and hydrogen sulfide,
is withdrawn through line 32, is cooled in a heat exchanger 34 and
is then introduced through line 36 as a feed stream into an
additive-recovery distillative column 38 containing distillation
trays.
An overhead product stream is removed through line 40 of column 38
and is introduced into a condenser 42, and a portion of the
liquefied overhead stream is recycled to the top of the column
through line 46, while an overhead product stream, containing
C.sub.2, C.sub.3 and perhaps some very minor amounts of C.sub.4 +
and contaminants, such as hydrogen sulfide, is removed. A bottom
product stream containing the C.sub.3 -C.sub.6 additive agent is
removed through line 48 into a reboiler 50, where a portion is
recycled through line 52 to the bottom of the column, while another
portion is removed by line 60 to line 56 through cooler 62 and is
recycled and introduced concurrently directly by line 58 and line
40 into the inlet of the overhead reflux condenser 42 of column 38,
to increase the operating temperature of the reflux condenser 42.
The remaining portion of the defined bottom product stream
containing C.sub.4 + is removed through line 54. Optionally as
illustrated, a further portion of the bottom stream is removed by
line 24 from line 56 and is introduced as all or a portion of the
additive agent introduced into column 14. Compositionally, it is
not desired to separate the C.sub.3 -C.sub.6 bottom stream, such as
the C.sub.4 + bottom stream or heavier components, and this stream
may be recycled for use as an additive stream in one or more
columns or merely recovered. The bottom product-recovery stream in
the defined method is a specified stream having a hydrogen sulfide
concentration kept below a defined specification level; for
example, about 10 ppm or less of H.sub.2 S, and with C.sub.3
removed to below a given specification level; for example, about
0.5% by volume or less. If operation of the column is desired with
the same overhead condenser temperature, then the operating
pressure of the column could be lowered.
In the method described in FIG. 1, the overhead reflux condenser 42
of the additive-recovery column 38 will operate at a higher
temperature, or, alternatively, the distillative column 38 may be
operated at a lower pressure, while maintaining the same overhead
reflux condenser temperature. In the operation of column 38, the
specifications are directed to the recovery of the additive agent
as bottoms, so that the recycle of the additive bottom stream to
the reflux condenser of the column does not contaminate the
overhead product stream. The higher temperature of the overhead
reflux condenser permits employing a less expensive cooling source,
such as cooling water, air or a more moderate refrigerant
source.
A number of computer simulations of the method were run, employing
a plate-to-plate column calculator program, to simulate column
condition and operation. The software computer-program simulation
employed was the Process SM Simulation Program of Simulation
Sciences, Inc. of Fullerton, Calif., Version 0881. The composition
of the feed, bottom, overhead and recycle additive agent streams in
a representative computer simulation of the operation of an
additive-recovery column, with recycle to the condenser of the
additive-recovery column, such as column 38 of FIG. 1, is set forth
in Table I for 100 psia operation.
The operation of the additive-recovery column at 100 psia, without
any recycle of the additive agent to the reflux condenser 42 of the
column 38, is illustrated by the data of Table II.
TABLE I ______________________________________ FLOW RATES - LB
MOLS/HR Recycle Overhead Bottom Feed Liquid Components Stream (44)
Stream (60) Stream (36) Stream (58)
______________________________________ H.sub.2 S 8.80 0.01 8.81
0.00 N.sub.2 0.00 0.00 0.00 0.00 CO.sub.2 17.6 0.00 17.6 0.00
C.sub.1 0.00 0.00 0.00 0.00 C.sub.2 123.13 0.09 123.22 0.00 C.sub.3
174.79 29.38 201.67 2.50 iC.sub.4 28.76 54.12 78.28 4.60 nC.sub.4
88.52 411.27 464.47 35.32 iC.sub.5 16.70 1263.12 1172.26 107.56
nC.sub.5 15.75 1868.65 1725.44 158.97 nC.sub.6 3.73 1673.61 1535.24
142.10 nC.sub.7 0.44 576.26 527.78 48.92 Totals 478.22 5876.53
5854.75 500.00 Temperature 109.00 232.80 210.00 232.70 Deg. F.
______________________________________
TABLE II ______________________________________ FLOW RATES - LB
MOLS/HR Temp. Products, Duties mm Tray Deg. F. Liquid Vapor Feeds
BTU/hr ______________________________________ 1 93.3 1146 0
(recycle feed) 11.0 462 (overhead net (condenser) product) 2 139.0
1143 1608 3 161.1 1137 1605 4 173.4 1130 1599 5 181.1 1116 1592 6
187.4 1087 1578 7 195.2 1030 1549 8 208.0 6810 1492 5855 (feed) 9
213.0 6955 1417 10 216.5 7054 1563 11 219.3 7127 1661 12 221.9 7190
1735 13 224.7 7249 1797 14 228.6 7293 1856 15 236.9 1900 5392
(bottom net 19.0 product) (reboiler)
______________________________________
The trays shown in Table II are theoretical or equivalent, perfect
equilibrium trays, with the reflux condenser 42 as tray 1, the
reboiler 50 as tray 15 and the feed stream introduced at tray
7.
The operation of the additive-recovery column at the same pressure,
but with the recycle of the liquid additive stream to the reflux
condenser in accordance with the invention, is illustrated by the
data of Table III.
TABLE III ______________________________________ FLOW RATES - LB
MOLS/HR Temp. Products, Duties mm Tray Deg. F. Liquid Vapor Feeds
BTU/hr ______________________________________ 1 109.0 1738 500
(recycle feed) 13.9 478 (overhead net (condenser) product) 2 141.2
1818 1716 3 156.8 1839 1797 4 167.1 1839 1818 5 175.6 1831 1818 6
183.5 1814 1810 7 192.3 1772 1792 8 204.9 7490 1750 5854 (feed) 9
209.4 7635 1614 10 212.4 7729 1759 11 214.9 7798 1853 12 217.2 7856
1921 13 219.8 7911 1980 14 223.8 7953 2034 15 232.8 2076 5876
(bottom net 21.7 product) (reboiler)
______________________________________
The effect of recycling 500-lb moles/hour of the C.sub.4 + additive
agent from the reboiler to the reflux condenser of the recovery
column provides for an increase of the reflux condenser temperature
from 93.3.degree. F. without additive to 109.0.degree. F. with the
additive, and a decrease in the reboiler temperature from
236.9.degree. F. to 232.8.degree. F. This adjustment of the reflux
condenser permits the overhead product stream to be cooled using
cooling water or an equivalent, inexpensive heat sink, while the
recycled additive agent does not contaminate the overhead stream,
since the desired specification stream from the column is the
C.sub.4 + bottom product stream. If desired, the distillation could
be operated at a lower pressure of 70 psia, if the reflux condenser
is desired to be maintained at the same temperature as without
additive. Thus, the data illustrate the significant advantage of
saving energy by the recycling of the C.sub.4 + liquid additive
agent stream from the reboiler to the reflux condenser in the
illustrated method of FIG. 1.
FIG. 2 is a schematic illustration of another distillative
separation employing the present invention, wherein a C.sub.3
-C.sub.6 bottom product stream is recycled to the overhead of a
column different from the column from which the bottom product
stream is removed. In this embodiment, a feed stream, typically
comprising a natural gas stream of a major amount of methane, some
C.sub.2 +, nitrogen; for example, 9% to 25% nitrogen, and CO.sub.2,
is introduced by line 126 into a distillative column 128, and an
overhead product stream is removed through line 130 and introduced
into a condenser 132, and a portion of the liquefied, condensed
stream is recycled through line 134 to the top of the column 128,
while a mixture of essentially all nitrogen and part of the methane
is removed as an overhead stream through line 136. A bottom product
stream is removed through line 138 and is introduced into a
reboiler 140, and a portion is recycled through line 66 to the
bottom of the column 128. The bottom product stream, comprising
part of the methane and practically all of the C.sub.2 + and carbon
dioxide, flows through line 68 into a heat exchanger 70, to cool
the stream, which cool stream is introduced through line 72 as the
feed stream into the next distillative column 74.
An overhead product stream is removed through line 76 and is
introduced into a condenser 78, and a condensed portion is recycled
to the top of the column 74 through line 80, while the overhead
product stream of enriched, substantially pure methane is removed
through line 82. In the operation of the column, a liquid additive
C.sub.3 -C.sub.6 alkane agent is added to the top of the column
through line 84, to prevent the formation of a carbon-dioxide
solids zone, as in U.S. Pat. No. 4,318,723. A bottom product stream
is removed through line 86 and is introduced into a reboiler 88,
and a portion is recycled through line 90 to the bottom of the
column 74, and the bottom product stream from the reboiler,
comprising carbon dioxide and C.sub.2 + (with the additive agent),
is removed through line 100 and is passed through heat exchanger
102, to cool the stream, and is introduced as a feed stream through
line 104 into an additive-recovery distillative column 106. This
column is operated as in FIG. 1, in order to provide a specified
bottom product of essentially the liquid additive agent and to
remove the carbon dioxide as an impurity in the overhead product
stream.
An overhead product vapor stream, comprising carbon dioxide,
C.sub.2 and C.sub.3, is removed from the top of the column through
line 108 and is introduced into a reflux condenser 110, and a
condensed portion is recycled through line 112 into the top of the
column, while carbon dioxide, C.sub.2 and C.sub.3 are removed as an
overhead stream from line 114. A bottom product stream, comprising
primary C.sub.4 +, is removed from the bottom of the column through
line 116 and is introduced into a reboiler 118, and a portion is
recycled through line 120 to the bottom of the column 106. A
specified bottom product stream, with defined specifications as in
FIG. 1, is removed through line 122, while a portion thereof; for
example, 0.5% to 20% or more; for example, 1.0% to 5% by moles
relative to column feed, is continuously recycled through line 124
and cooler 142 and is introduced into condenser 132 of the
distillative column 128, and a portion optionally may be
introduced, via line 124 or 126 as shown, into condenser 78 (as
illustrated by dotted lines), to increase the operating temperature
of the condensers 132 and 78, thereby permitting a reduction in the
operating pressure of columns 128 and 74 or a savings in heat
energy. This operation permits the recovery of essentially all of
the C.sub.2, while CO.sub.2 removal prior to the N.sub.2 /CH.sub.4
separation is not required. In the process, the overhead
temperature of the distillative column 128 can be raised
independently of the overhead N.sub.2 /CH.sub.4 content. The reflux
condenser 132 temperature can be controlled and adjusted by the
rate of the recycled additive addition to the condenser. In view of
the very low boiling point of nitrogen removed in the overhead
stream in column 128, the recycled additive rate may be set to
require only an ethylene refrigeration system operating at about
-125.degree. F.
In a computer simulated example of the operation of distillative
column 128 at 572 psia for the separation of N.sub.2 and CH.sub.4,
the compositions of the streams are set forth in Table IV, while
the column operating conditions are set forth in Table V.
TABLE IV ______________________________________ FLOW RATES - LB
MOLS/HR Recycle Bottom Overhead Bottom Feed Stream Stream Stream
Stream Components (126) (124) (136) (68)
______________________________________ N.sub.2 336.38 0.00 334.8
1.52 CO.sub.2 266.91 0.00 2.47 264.42 CO 6.21 0.00 6.08 .13 COS .65
0.00 .00 .65 H.sub.2 S 131.63 0.00 .00 131.62 C.sub.1 2427.04 0.00
1637.02 790.07 C.sub.2 161.32 0.00 .00 161.31 C.sub.3 126.87 27.38
.44 153.81 iC.sub.4 50.01 130.37 .59 179.78 nC.sub.4 75.39 196.65
.49 271.54 iC.sub.5 40.25 105.17 .07 145.35 nC.sub.5 7.60 19.72 .00
27.31 nC.sub.6 15.21 39.98 .00 55.19 nC.sub.7 10.85 28.48 .00 39.34
Totals 3656.40 547.78 1982.07 2222.11 Temperature -80. -120. -125.
-48. Deg. F. ______________________________________
TABLE V ______________________________________ FLOW RATES - LB
MOLS/HR Temp. Products, Duties mm Tray Deg. F. Liquid Vapor Feeds
BTU/hr ______________________________________ 1 -125 1854 548
(recycle feed) 2.9 1982 (overhead (condenser) net product) 2 -119
1870 3288 3 -116 1802 3305 4 -112 1700 3236 5 -106 1580 3135 6 -97
1466 3015 7 -86 1437 2901 2481 (vapor feed) 8 -81 2659 390 1175
(liquid feed) 9 -78 2701 437 10 -76 2730 479 11 -73 2723 508 12 -48
501 2222 2.4 (reboiler) ______________________________________
The addition of 548-lb moles/hour of the liquid bottom product from
reboiler 118 to reflux condenser 132 in the separation of N.sub.2
and CH.sub.4 from the feed stream provided the recovery of 790-lb
moles/hr of CH.sub.4 with the bottom stream from column 128, which
CH.sub.4 is recovered as an essentially pure CH.sub.4 overhead
stream in column 74. The overhead product stream of column 128 is
essentially over 99% a mixture of N.sub.2 and CH.sub.4. Without the
introduction of the recycled C.sub.4 + bottom additive stream, the
reflux condenser temperature would be well below the limit (about
-150.degree. F.) of an ethylene refrigeration system, while, with
the recycle of the bottom stream, the overhead temperature is
-125.degree. F., effecting a savings in heat energy in the
operation of the column.
FIG. 3 is a schematic illustration of a process of the invention,
wherein the separation of CO.sub.2 and ethane from propane by a
distillative technique was discovered to be enhanced significantly
by the introduction of small amounts of a liquid C.sub.4 + additive
recycled bottom stream to the condenser or to be uppermost tray
section of the distillative separation column. The introduction of
small amounts; for example, 1 to 8 mols, of a C.sub.4 + bottom
stream from the additive-recovery or other distillative column
source per 100 mols of feed improves the relative volatility of the
CO.sub.2 to the propane with increased C.sub.4 + fraction, and
considerably reduces the heat duty on the reflux condenser and
reboiler by over 60%.
In the embodiment shown in FIG. 3, a feed stream, consisting
essentially of CO.sub.2, ethane and C.sub.3 +, is introduced by
line 152 in a distillative column 150 containing a plurality of
distillation trays, and an overhead vapor stream is removed by line
154 and is introduced in a reflux condenser 156, and a portion of
the liquid condensed stream is recycled through line 158 to the top
of column 150. A mixture, composed essentially of CO.sub.2 and
ethane, is removed as an overhead product stream by line 160. A
bottom product stream is removed by line 162 to a reboiler 164, and
a portion is recycled through line 166 to the bottom of column 150.
The bottom product stream, comprising primary propane, together
with any recycled liquid additive agent introduced into the
condenser or upper part of the column 150, is withdrawn by line 168
as a bottom product stream and is introduced into a heat exchanger
170 to cool the bottom product stream.
The cool bottom product stream is introduced by line 172 into the
C.sub.4 + additive-recovery column 174. An overhead product stream
is removed by line 176 and is introduced into a condenser 178, and
a portion from the condenser is recycled by line 180 into the top
of the column 174. The overhead product stream, comprising
primarily propane, is removed by line 182. A bottom product stream,
comprising C.sub.4 + additive agent, is removed by line 184 into
reboiler 186, where a portion is recycled to the bottom of column
174 by line 188, and the C.sub.4 + additive fraction is recovered
by line 192 for recycle, recovery or for other use. A small portion
of the recovered additive bottom product stream from column 174;
for example, 1% to 5% by mol of the feed stream to separation
column 150, is removed from the reboiler 186 by line 190 and is
recycled through cooler 196 to the condenser 156 of separation
column 150, and optionally, if desired, the liquid C.sub.4 +
additive bottom product stream also may be introduced into the
uppermost tray section of the column 150 by line 194 shown as a
dotted line.
In the operation of the separation column 150, Table VI shows a
computer simulated data of the composition of the column feed
stream comprising a major amount of CO.sub.2, ethane and propane.
The recycled additive-recovery column bottom additive stream from
column 174, comprising primarily C.sub.4 -C.sub.6 alkane liquid
additive, is introduced in small amounts (less than about 4%; for
example, 3%, by moles relative to the feed stream) by line 190 to
the condenser 156 (tray 1), while the feed stream is introduced by
line 152 to tray 7 of column 150. The overhead stream removes
essentially all of the CO.sub.2, C.sub.1 and C.sub.2 of the feed
stream, while the bottom product stream is rich in the C.sub.3 +
components; that is, the separated C.sub.3 and the recycled C.sub.4
+ additive bottom stream.
Table VII is the same as Table VI, except illustrating the feed,
overhead and bottom stream compositions, wherein a recycled
additive bottom stream is not employed in separation column
150.
TABLE VI ______________________________________ FLOW RATES - LB
MOLS/HR Recycled Bottom Overhead Column Bottoms Feed Additive
Stream Stream Components (152) (190) (160) (168)
______________________________________ N.sub.2 0.59 0.0 0.59 0.00
O.sub.2 0.06 0.0 0.06 0.00 C.sub.1 60.70 0.0 60.70 0.00 CO.sub.2
247.04 0.0 246.97 0.06 C.sub.2 8.13 0.0 8.13 0.00 C.sub.3 7.04 0.0
0.71 6.33 iC.sub.4 0.75 1.30 0.20 1.85 nC.sub.4 2.00 3.46 0.32 5.14
iC.sub.5 0.39 0.68 0.02 1.05 nC.sub.5 0.36 0.62 0.01 0.96 nC.sub.6
2.27 3.92 0.05 6.14 Totals 329.39 10.00 317.81 21.57 Temperature 90
13 13 303 Deg. F. ______________________________________
TABLE VII ______________________________________ FLOW RATES - LB
MOLS/HR Feed Stream Overhead Stream Bottom Stream Components (152)
(160) (168) ______________________________________ N.sub.2 0.59
0.59 0.00 O.sub.2 0.06 0.06 0.00 C.sub.1 60.70 60.70 0.00 CO.sub.2
247.04 246.97 0.06 C.sub.2 8.13 8.13 0.00 C.sub.3 7.04 0.70 6.34
iC.sub.4 0.75 0.00 0.75 nC.sub.4 2.00 0.00 2.00 iC.sub.5 0.39 0.00
0.39 nC.sub.5 0.36 0.00 0.36 nC.sub.6 2.27 0.00 2.27 Totals 329.39
317.18 12.20 Temperature 90 12 255 Deg. F.
______________________________________
Tables VIII and IX illustrate the operation of the CO.sub.2
+C.sub.2 /C.sub.3 separation column 150 at 500 psia, both with and
without the employment of and illustrating more particularly the
change in the heat duty of the reboiler and condenser by the
addition of the C.sub.4 + recycled bottom additive stream to the
condenser.
TABLE VIII ______________________________________ FLOW RATES - LB
MOLS/HR Temp. Products, Duties mm Tray Deg. F. Liquid Vapor Feeds
BTU/hr ______________________________________ 1 12.0 2516 0
(recycle feed) 11.6 317 (overhead net (condenser) product) 2 26.0
2612 2834 3 29.0 2628 2929 4 29.0 2631 2945 5 29.0 2634 2948 6 29.0
2639 2952 7 30.0 2645 2956 8 30.0 2588 2962 329 (feed) 9 32.0 2609
2576 10 33.0 2622 2597 11 33.0 2641 2610 12 34.0 2669 2629 13 35.0
2704 2657 14 40.0 2644 2693 15 61.0 2288 2631 16 116.0 2292 2276 17
164.0 2643 2280 18 193.0 2709 2631 19 219.0 2406 2697 20 255.0 2394
12.20 (bottom 11.3 net product) (reboiler)
______________________________________
TABLE IX ______________________________________ FLOW RATES - LB
MOLS/HR Temp. Products, Duties mm Tray Deg. F. Liquid Vapor Feeds
BTU/hr ______________________________________ 1 13.0 1081 10
(recycle feed) 4.9 318 (overhead net (condenser) product) 2 25.0
1116 1389 3 27.0 1122 1424 4 28.0 1123 1427 5 28.0 1125 1431 6 28.0
1127 1433 7 28.0 1129 1435 8 29.0 1066 1436 329 (feed) 9 32.0 1080
1044 10 33.0 1086 1053 11 34.0 1093 1064 12 35.0 1102 1071 13 37.0
1102 1081 14 47.0 998 1080 15 89.0 860 976 16 154.0 953 838 17
198.0 1045 931 18 229.0 1016 1023 19 263.0 943 995 20 303.4 922
21.6 (bottom 4.7 net product) (reboiler)
______________________________________
As illustrated, a small amount of recycled C.sub.4 + additive
bottom stream markedly reduces the heat load on the condenser and
reboiler, while significantly enhancing the separation of CO.sub.2
from C.sub.3, resulting not only in enhanced separation efficiency
by a change in relative volatility, but accompanied by a savings in
energy in the column operation.
The advantages of recycling the higher alkane bottom product stream
of defined specifications and into an overhead condenser, wherein
the bottom product stream does not constitute or serve as a
contaminant for the overhead product stream, provide for energy
savings.
* * * * *