U.S. patent application number 12/518863 was filed with the patent office on 2010-01-28 for controlled freeze zone tower.
Invention is credited to Eleanor R Fieler, Edward J. Grave, Paul Scott Northrop, Norman K. Yeh.
Application Number | 20100018248 12/518863 |
Document ID | / |
Family ID | 38139480 |
Filed Date | 2010-01-28 |
United States Patent
Application |
20100018248 |
Kind Code |
A1 |
Fieler; Eleanor R ; et
al. |
January 28, 2010 |
Controlled Freeze Zone Tower
Abstract
A cryogenic distillation tower is provided for the separation of
a fluid stream containing at least methane and carbon dioxide. The
cryogenic distillation tower has a lower stripping section, an
upper rectification section, and an intermediate spray section. The
intermediate spray section includes a plurality of spray nozzles
that inject a liquid freeze zone stream. The nozzles are configured
such that substantial liquid coverage is provided across the inner
diameter of the intermediate spray section. The liquid freeze zone
stream generally includes methane at a temperature and pressure
whereby both solid carbon dioxide particles and a methane-enriched
vapor stream are formed. The tower may further include one or more
baffles below the nozzles to create frictional resistance to the
gravitational flow of the liquid freeze zone stream. This aids in
the breakout and recovery of methane gas. Additional internal
components are provided to improve heat transfer and to facilitate
the breakout of methane gas.
Inventors: |
Fieler; Eleanor R; (Humble,
TX) ; Grave; Edward J.; (Spring, TX) ;
Northrop; Paul Scott; (Spring, TX) ; Yeh; Norman
K.; (Houston, TX) |
Correspondence
Address: |
Exxon Mobil Upstream;Research Company
P.O. Box 2189, (CORP-URC-SW 359)
Houston
TX
77252-2189
US
|
Family ID: |
38139480 |
Appl. No.: |
12/518863 |
Filed: |
October 20, 2007 |
PCT Filed: |
October 20, 2007 |
PCT NO: |
PCT/US2007/024216 |
371 Date: |
June 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60881395 |
Jan 19, 2007 |
|
|
|
Current U.S.
Class: |
62/617 |
Current CPC
Class: |
F25J 3/0209 20130101;
F25J 2240/40 20130101; Y02C 10/12 20130101; F25J 2200/90 20130101;
F25J 2240/02 20130101; B01D 3/26 20130101; F25J 2280/40 20130101;
F25J 2290/40 20130101; F25J 3/0266 20130101; Y02C 20/40 20200801;
F25J 2270/12 20130101; F25J 2270/60 20130101; F25J 2205/04
20130101; F25J 2220/66 20130101; F25J 2260/80 20130101; F25J
2200/74 20130101; B01D 3/166 20130101; B01D 3/24 20130101; F25J
2205/20 20130101; F25J 2290/72 20130101; F25J 2215/04 20130101;
F25J 3/0233 20130101; F25J 2200/02 20130101 |
Class at
Publication: |
62/617 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A cryogenic distillation tower for the separation of a fluid
stream containing at least methane and carbon dioxide, the
cryogenic distillation tower defining an inner diameter that houses
internal components, comprising: a lower stripping section
comprising a melt tray and at least one mass transfer device below
the melt tray, the lower stripping section being configured to
operate at a temperature and pressure at which substantially no
carbon dioxide solids are formed, and vaporized methane is
released; an intermediate spray section comprising a plurality of
spray nozzles configured to inject a liquid freeze zone stream such
that substantial liquid coverage of the intermediate spray section
across the inner diameter is provided, the liquid freeze zone
stream substantially comprising methane at a temperature and
pressure whereby both solid carbon dioxide particles and a
methane-enriched vapor stream are formed upon injection; and an
upper rectification section comprising a collector tray, and at
least one mass transfer device above the collector tray, the upper
rectification section being configured to receive the vaporized
methane from the lower stripping section and the methane-enriched
vapor stream from the intermediate spray section.
2. The cryogenic distillation tower of claim 1, wherein the
plurality of spray nozzles is disposed within at least one spray
header.
3. The cryogenic distillation tower of claim 2, wherein the
plurality of spray nozzles is positioned in a first spray header
having a first plurality of nozzles, and a second spray header
below the first spray head also having a second plurality of
nozzles.
4. The cryogenic distillation tower of claim 1, wherein the
plurality of nozzles are positioned at different levels within the
upper rectification section.
5. The cryogenic distillation tower of claim 5, wherein the
plurality of spray nozzles in the first spray header forms a
substantially linear arrangement, and the plurality of spray
nozzles in the second spray header forms a substantially linear
arrangement essentially transverse to and below the first spray
header.
6. The cryogenic distillation tower of claim 5, wherein the
plurality of spray nozzles is positioned in: a first substantially
linear spray header having a first plurality of nozzles, a second
substantially linear spray header below the first spray header also
having a second plurality of nozzles, and a third substantially
linear spray header below the second spray header also having a
third plurality of nozzles, with the first spray header, second
spray head and third spray header being offset relative to one
another.
7. The cryogenic distillation tower of claim 1, wherein the melt
tray comprises: a base; and a plurality of chimneys extending
upward from the base.
8. The cryogenic distillation tower of claim 7, wherein the melt
tray further comprises a cap over each of the plurality of
chimneys.
9. The cryogenic distillation tower of claim 7, wherein: the base
of the melt tray defines a generally radial base extending across
the inner diameter of the cryogenic distillation tower, the base
having a first side, and a second side opposite the first side,
wherein the first side and second side are sloped inwardly towards
an intermediate portion of the base; and the base further comprises
a sump disposed within the intermediate portion of the base to
receive and direct liquids.
10. The cryogenic distillation tower of claim 9, wherein the base
further comprises a draw-off nozzle for receiving internal reflux
fluids collected in the sump.
11. The cryogenic distillation tower of claim 7, wherein the base
of the melt tray has a generally sinusoidal profile.
12. The cryogenic distillation tower of claim 7, wherein the base
of the melt tray has a generally corrugated profile.
13. The cryogenic distillation tower of claim 7, wherein the at
least one mass transfer device below the melt tray comprises: a
base; a plurality of openings in the base; and a plurality of tabs,
wherein each of the plurality of tabs extend over one of the
plurality of openings in the base to substantially interfere with
the gravitational flow of liquids downward through the one of the
plurality of openings.
14. The cryogenic distillation tower of claim 7, wherein the at
least one mass transfer device below the melt tray comprises: a
packing that creates a plurality of channels through which fluids
gravitationally flow.
15. The cryogenic distillation tower of claim 1, wherein the
collector tray comprises: a base; and a plurality of chimneys
extending upward from the base.
16. The cryogenic distillation tower of claim 15, wherein the
collector tray further comprises a cap over each of the plurality
of chimneys.
17. The cryogenic distillation tower of claim 1, wherein the at
least one mass transfer devices in the lower stripping section is
fabricated from or coated with a fouling-resistant material.
18. The cryogenic distillation tower of claim 1, wherein the at
least one mass transfer device above the collector tray comprises:
a base; a plurality of openings in the base; and a plurality of
tabs, wherein each of the plurality of tabs extend over one of the
plurality of openings in the base to substantially interfere with
the gravitational flow of liquids downward through the one of the
plurality of openings.
19. The cryogenic distillation tower of claim 1, wherein the at
least one mass transfer device above the collector tray comprises:
a packing that creates a plurality of channels through which fluids
gravitationally flow.
20. The cryogenic distillation tower of claim 1, wherein the at
least one mass transfer device in the lower stripping section or in
the upper rectification section above the rectification tray is
fabricated from or coated with a fouling-resistant material.
21. The cryogenic distillation tower of claim 4, further comprising
a baffle or grid packing below at least one of the plurality of
spray headers for creating a frictional fluid path for liquids
moving down the intermediate spray section.
22. The cryogenic distillation tower of claim 1, wherein the at
least one stripping tray comprises a plurality of stripping trays
arranged to create a cascading liquid flow.
23. The cryogenic distillation tower of claim 1, further
comprising: an exit line which receives an exit fluid stream from
the lower stripping section comprised primarily of carbon dioxide;
a reboiler wherein the exit fluids are warmed; and a heater line
for capturing gas vaporized by the reboiler, the vapor line
carrying warmed vapor to the melt tray to heat the melt tray.
24. The cryogenic distillation tower of claim 1, wherein the melt
tray comprises a plurality of bubble caps to increase the surface
area.
25. The cryogenic distillation tower of claim 2, wherein the
plurality of spray nozzles is arranged in: a first spray header to
distribute a portion of the liquid freeze zone stream at a first
temperature; and a second spray header positioned at a different
level in the intermediate spray section to distribute a portion of
the liquid freeze zone stream at a second temperature that is
higher than the first temperature.
26. The cryogenic distillation tower of claim 25, wherein the first
spray header is positioned above the second spray header.
27. The cryogenic distillation tower of claim 1, further comprising
an internal downcomer on the melt tray to provide reflux to the
stripping section.
28. The cryogenic distillation tower of claim 1, wherein the fluid
stream is injected into the cryogenic distillation tower through at
least one spray header having a plurality of nozzles.
29. The cryogenic distillation tower of claim 28, wherein the at
least one spray header comprises a first spray header, and a second
spray header below the first spray header.
30. A method for producing hydrocarbon gases, comprising: receiving
an initial fluid stream containing at least methane and carbon
dioxide; chilling the initial fluid stream to a substantially
liquefied phase; introducing the liquefied fluid stream into a
cryogenic distillation tower to separate the carbon dioxide from
the methane, the distillation tower defining an inner diameter
which houses internal components comprising: a lower stripping
section comprising a melt tray and at least one mass transfer
device below the melt tray, the lower stripping section being
configured to operate at a temperature and pressure at which
substantially no carbon dioxide solids are formed and vaporized
methane is released; an intermediate spray section comprising a
plurality of spray nozzles configured to inject a spray stream such
that substantial liquid coverage of the intermediate spray section
across the inner diameter is provided, the spray stream
substantially comprising methane at a temperature and pressure
whereby both solid carbon dioxide particles and a methane-enriched
vapor stream are formed upon injection; and an upper rectification
section comprising a collector tray, the upper rectification
section being configured to receive vaporized methane from the
lower stripping section and the methane-enriched vapor stream from
the intermediate spray section; and recovering vaporized methane
from the upper rectification section.
31. The method of claim 30, wherein the plurality of spray nozzles
is disposed within at least one spray header.
32. The method of claim 31, wherein the cryogenic distillation
tower further comprises a baffle below at least one of the spray
headers for creating a fluid path for liquids moving down the
intermediate spray section.
33. The method of claim 30, wherein the plurality of spray nozzles
is positioned in a first spray head having a first plurality of
nozzles, and a second spray header below the first spray header
also having a second plurality of nozzles.
34. The method of claim 33, wherein the plurality of spray nozzles
in the first spray header forms a substantially linear arrangement,
and the plurality of spray nozzles in the second spray header forms
a substantially linear arrangement essentially transverse to and
below the first spray header.
35. The method of claim 33, wherein the plurality of spray nozzles
is positioned in: a first substantially linear spray header having
a first plurality of nozzles, a second substantially linear spray
header below the first spray header also having a second plurality
of nozzles, and a third substantially linear spray header below the
second spray header also having a third plurality of nozzles, with
the first spray header, second spray header and third spray header
being offset relative to one another.
36. The method of claim 30, wherein the spray nozzles are each
capable of providing about a 70.degree. to 140.degree. spray
distribution.
37. The method of claim 30, wherein the spray header defines at
least two intersecting fluid lines supporting nozzles.
38. The method of claim 30, wherein the melt tray comprises: a
base; and a plurality of chimneys extending upward from the
base.
39. The method of claim 38 wherein the melt tray further comprises
a cap over each chimney.
40. The method of claim 30, wherein the at least one mass transfer
device below the melt tray comprises: a base; a plurality of
openings in the base; and tabs extending over the respective
openings in the base to substantially interfere with the
gravitational flow of liquids downward through the openings.
41. The method of claim 40, wherein the melt tray is fabricated
from or coated with a fouling-resistant material.
42. The method of claim 30, wherein the at least one mass transfer
device below the melt tray comprises a plurality of stripping trays
arranged to create a cascading liquid flow.
43. The method of claim 30, wherein the cryogenic distillation
tower further comprises: an exit line which receives exit fluids
from the stripping section comprised primarily of carbon dioxide;
and a reboiler wherein exit fluids are warmed.
44. The method of claim 43, wherein the cryogenic distillation
tower further comprises a heater line for capturing gas vaporized
by the reboiler, the vapor line carrying warmed vapor to the melt
tray to heat the melt tray.
45. The method of claim 43, further comprising: collecting the
carbon dioxide for use as a miscible agent in an enhanced oil
recovery operation.
46. The method claim 30, wherein the melt tray comprises a
plurality of bubble caps to increase the surface area.
47. The method of claim 30, wherein the spray nozzles are arranged
in: a first spray header to distribute a portion of the liquid
stream at a first temperature; and a second spray header positioned
at a different level in the intermediate spray section to
distribute a portion of the liquid stream at a second temperature
that is higher than the first temperature.
48. The method of claim 47, wherein the first spray header is
positioned above the second spray header.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/881,395, filed 19 Jan. 2007.
[0002] This application is related to U.S. Provisional No.
60/881,391, entitled Integrated Controlled Freeze Zone (CFZ) Tower
and Dividing Wall (DWC) for Enhanced Hydrocarbon Recovery, filed 19
Jan. 2007, by Vikram Singh, Edward J. Grave, Paul Scott Northrop,
and Narasimhan Sundaram.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to the field of fluid
separation. More specifically, the present invention relates to the
cryogenic separation of carbon dioxide from a hydrocarbon fluid
stream.
[0005] 2. Background of the Invention
[0006] The production of hydrocarbons from a reservoir oftentimes
carries with it the incidental production of non-hydrocarbon gases.
Such gases include contaminants, such as hydrogen sulfide
(H.sub.2S) and carbon dioxide (CO.sub.2). When H.sub.2S and
CO.sub.2 are produced along with a hydrocarbon gas stream (such as
methane or ethane), the gas stream is sometimes referred to as
"sour gas."
[0007] Sour gas is usually treated to remove CO.sub.2, H.sub.2S,
and other contaminants before it is sent downstream for further
processing or sale. The separation process creates an issue as to
the disposal of the separated contaminants. In some cases, the
concentrated acid gas (consisting primarily of H.sub.2S and
CO.sub.2) is sent to a sulfur recovery unit ("SRU"). The SRU
converts the H.sub.2S into benign elemental sulfur. However, in
some areas (such as the Caspian Sea region), additional elemental
sulfur production is undesirable because there is a limited market.
Consequently, millions of tons of sulfur have been stored in large,
above-ground blocks in some areas of the world, most notably Canada
and Kazakhstan.
[0008] As for the carbon dioxide gas, it is oftentimes vented to
the atmosphere. However, the practice of venting CO.sub.2 is coming
under greater scrutiny, particularly in countries that have
ratified the Kyoto protocol which requires the reduction of
CO.sub.2 emissions. Further, CO.sub.2 gas can and is being sold as
a miscible flood agent for enhanced oil recovery. That is, it may
not be a disposal prospect, but is potentially a business
opportunity to get additional revenue from CO.sub.2 sales and/or
additional production from its use.
[0009] For these reasons, acid gas injection ("AGI") may be a
preferred alternative to sulfur recovery and CO.sub.2 venting. AGI
means that unwanted sour gases are reinjected into a subterranean
formation and sequestered for potential later use.
[0010] For "highly sour" streams, that is, production streams
containing greater than about 15% to about 20% CO.sub.2 and
H.sub.2S, it can be particularly challenging to design, construct,
and operate a process that can economically separate these
contaminants from the desired hydrocarbons. Cryogenic gas
processing (i.e., distillation) is a known procedure for sour gas
separation. Cryogenic gas separation avoids the use of solvents,
minimizes acid gas removal equipment, and generates a cooled and
liquefied acid gas stream at moderate pressures (e.g., 350-500
pounds per square inch gauge (psig)). The cold, liquefied stream is
particularly suitable for injection into a subterranean reservoir
through AGI. Because the liquefied acid gas has a relatively high
density, a hydrostatic head can be used in an AGI well. That is,
the energy required to pump the liquefied acid gas is lower than
the energy required to compress low-pressure acid gases to
reservoir pressure. In addition, there are fewer stages involving
compressors and pumps, so there is less equipment utilized for the
acid gas stream.
[0011] One problem which has been encountered in the practice of
cryogenic gas processing relates to the relatively low temperature
at which CO.sub.2 phase changes into a solid. If CO.sub.2 is
present at concentrations greater than about 5% in the gas to be
processed, it freezes out as a solid in a standard cryogenic
distillation unit. The formation of CO.sub.2 as a solid interrupts
the cryogenic distillation process. To circumvent this problem, a
"Controlled Freeze Zone" (CFZ) process was developed to anticipate
the formation of solid CO.sub.2 and captures the frozen CO.sub.2
particles at the bottom of a specially-designed distillation tower.
As a result, a clean methane stream (along with any nitrogen or
helium present in the raw gas) is generated overhead, while a
liquid CO.sub.2/H.sub.2S stream at 30.degree. to 40.degree. F.
(Fahrenheit) is generated at the bottom of the tower. The liquid
sour gas stream is thus synergetic with AGI. Certain aspects of the
CFZ process, associated equipment and technology are described in
U.S. Pat. No. 4,533,372; U.S. Pat. No. 4,923,493; U.S. Pat. No.
5,062,270; U.S. Pat. No. 5,120,338; and U.S. Pat. No. 6,053,007.
Each of these patents is incorporated herein by reference in its
respective entirety.
[0012] The cryogenic distillation column employing the CFZ
technology provides a spray zone in the middle of the column. The
spray zone (or CFZ section) serves as a freezing section that
causes gaseous CO.sub.2 to freeze out of methane (or other light
hydrocarbons) within the column, and gravitationally fall. A cold
liquid (primarily methane) is injected into the intermediate spray
section to provide a liquid spray. The cold liquid within the spray
zone contacts gas as it rises in the column, causing the CO.sub.2
to solidify and fall down.
[0013] Below the spray zone is a stripping section of the
distillation column. The stripping section receives the raw feed
stream (gas and liquid) for initial stripping of CO.sub.2 from the
gas stream. The bottom of the stripping section is warmer than the
feed temperature. Upon entering the column, the liquid portion of
the feed stream falls through the stripping section. As it moves
downward, dissolved methane vaporizes and rises through the spray
section. In the spray zone, entrained CO.sub.2 vapor contacts the
cold liquid spray, which causes the CO.sub.2 to freeze out of the
gas phase. In some embodiments, the feed stream can be introduced
directly into the intermediate spray section.
[0014] Above the spray zone is a rectification section of the
column. In the rectification section, an overhead gas is captured
and made available for fuel or piped away for sales. The overhead
gas may be partially liquefied by cooling, and the liquid returned
to the column as "reflux." The reflux liquid is injected as the
cold spray into the spray section.
[0015] There is a need for improved internal features and
components of the CFZ distillation column. More specifically, there
is a need for improvements to the CFZ cryogenic distillation tower
to facilitate heat and mass transfer within the column. Further,
there is a need for a cryogenic distillation tower offering
improved internal features above and below the controlled freeze
zone section to facilitate the extraction of CO.sub.2 from a gas
stream. Still further, there is a need for an improved sour gas
removal chamber producing a liquefied acid gas stream, thereby
facilitating the injection and sequestration of H.sub.2S and
CO.sub.2 at low wellhead pressure.
SUMMARY OF THE INVENTION
[0016] A cryogenic distillation tower is provided for the
separation of sour gasses in a fluid stream. The fluid stream
contains at least methane and carbon dioxide. The distillation
tower has an inner diameter that houses internal components that
facilitate the separation process. The tower generally includes a
lower stripping section, an intermediate spray section, and an
upper rectification section.
[0017] The lower stripping section comprises a melt tray, and at
least one stripping tray or packed bed below the melt tray. The
stripping section is configured to operate at a temperature and
pressure at which substantially no carbon dioxide solids are
formed, but methane vapor is released.
[0018] The intermediate spray section comprises one or more spray
nozzles configured to inject a liquid freeze zone stream. The
nozzles are uniquely configured such that substantial liquid
coverage of the intermediate spray section across the inner
diameter is provided. The liquid sprayed into the freeze zone
substantially comprises methane at a temperature and pressure
whereby both solid carbon dioxide particles and a methane-enriched
vapor stream are formed upon injection.
[0019] The upper rectification section comprises a rectification
tray and, preferably, one or more additional trays, or packing. The
upper rectification section is configured to receive vaporized
methane from the lower stripping section and the methane-enriched
vapor stream from the intermediate spray section. The rectification
section may also receive a return liquefied stream of condensed
vapor for further capture of residual CO.sub.2 and H.sub.2S.
Unvaporized fluids are collected in the bottom rectification tray,
released from the rectification section, optionally further
chilled, and then reinjected into the spray section as the liquid
freeze zone stream.
[0020] As noted, the nozzles in the intermediate spray section are
uniquely configured such that substantial liquid coverage of the
spray section across the inner diameter is provided. Preferably,
the spray nozzles are disposed within one or more spray heads. In
one aspect, the one or more spray heads define one or more spray
nozzles, each having one or more nozzles and each being positioned
at different levels in the spray section. In another aspect, a
first spray header forms a substantially linear arrangement of
spray nozzles, and a second spray header forms a substantially
linear arrangement of spray nozzles transverse to and below the
first spray header. In yet another aspect, the spray nozzles are
positioned in (i) a first substantially linear spray head having a
plurality of nozzles, (ii) a second substantially linear spray
header below the first spray header also having a plurality of
nozzles, and (iii) a third substantially linear spray head below
the second spray header also having a plurality of nozzles, with
the first, second and third spray headers being offset from one
another
[0021] Preferably, the melt tray in the lower stripping section
includes a base, and a plurality of chimneys extending upward from
the base. The melt tray preferably also includes a cap over each
chimney. In one aspect, the base defines a generally radial base
extending across the inner diameter of the distillation tower, with
the base having first and second opposite sides sloped inwardly
towards an intermediate portion of the base. A downcomer is then
disposed within the intermediate portion of the base to receive
liquids collected by the melt tray. In another aspect, the base has
a generally sinusoidal or other corrugated profile to increase its
surface area.
[0022] In similar fashion, the rectification tray preferably
includes a base and a plurality of chimneys extending upward from
the base. Preferably again, a cap is disposed over each
chimney.
[0023] Any commercially-available mass transfer device may be
employed in either the stripping section or the rectification
section. Such mass transfer devices may be fabricated from an
anti-sticking material, such as Teflon, or from a fouling-resistant
material.
[0024] In another embodiment, the spray section further includes
one or more baffles or grid-type packing. In one aspect, a baffle
is disposed below at least one of the spray headers for creating a
fluid path for liquids moving down the spray section.
[0025] A method for producing hydrocarbon gases is also provided.
In one embodiment, the method includes the steps of receiving a
fluid stream containing at least methane and carbon dioxide, and
then chilling the fluid stream to substantially liquefied phase.
The fluid stream is introduced into a cryogenic distillation tower,
such as any of the towers described above. Low molecular weight
gases are recovered from the upper rectification section.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] So that the manner in which the features of the present
invention can be better understood, certain drawings, charts and
flow charts are appended hereto. It is to be noted, however, that
the drawings illustrate only selected embodiments of the inventions
and are therefore not to be considered limiting of scope, for the
inventions may admit to other equally effective embodiments and
applications.
[0027] FIG. 1 is a side view of a controlled freeze zone
distillation tower of the present invention, in one embodiment. The
initial feed stream is seen being injected into the intermediate
spray section of the tower.
[0028] FIG. 2A is a plan view of the melt tray, in one
embodiment.
[0029] FIG. 2B is a cross-sectional view of the melt tray of FIG.
2A taken across line 2B-2B.
[0030] FIG. 2C is a cross-sectional view of the melt tray of FIG.
2A taken across line 2C-2C.
[0031] FIG. 3 is an enlarged side view of the stripping trays in
the stripping section of the distillation tower, in one
embodiment.
[0032] FIG. 4A is perspective view of a jet tray as may be used in
either the stripping section or rectification section of the
distillation tower, in one embodiment.
[0033] FIG. 4B is a side view of one of the openings in the jet
tray of FIG. 4A.
[0034] FIG. 5 is a side view of the intermediate spray section of
the distillation tower of FIG. 1. In this view, two baffles have
been added to the spray section.
[0035] FIGS. 6A and 6B are nozzle arrangements, in other
embodiments.
[0036] FIG. 6A presents a substantially linear spray head
supporting three spray nozzles. The spray head may be used in the
intermediate spray section of the distillation tower, either to
inject the initial feed stream or to inject the liquid freeze zone
stream, or both. More than one such spray head arrangement may be
placed at different levels in the spray section.
[0037] FIG. 6B presents a pair of transverse, substantially linear
spray heads supporting multiple spray nozzles. The spray heads may
be used in the intermediate spray section of the distillation tower
to inject the initial feed stream. More than one such spray head
arrangement may be placed at different levels in the spray
section.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
Definitions
[0038] As used herein, the term "mass transfer device" refers to
any object that receives fluid and passes it to another object,
such as through gravitational flow. One non-limiting example is a
tray for stripping out certain fluids. A grid packing is another
example.
[0039] The term "baffle" means any object that interferes with the
direct gravitational flow of liquid.
Description of Specific Embodiments
[0040] FIG. 1 presents a schematic view of a cryogenic distillation
tower of the present invention, in one embodiment. The cryogenic
distillation tower is referred to generally as 100. The tower 100
may be interchangeably referred to herein as a "cryogenic
distillation tower," a "column," a "CFZ column," or a "splitter
tower."
[0041] The cryogenic distillation tower 100 of FIG. 1 receives an
initial fluid stream 10. The fluid stream 10 is comprised of
production gasses. Typically, the fluid stream represents a dried
gas stream from a wellhead (not shown), and contains about 65% to
about 95% methane. Some ethane may be present as a light
hydrocarbon, along with certain contaminants, such as CO.sub.2 and
H.sub.2S. Not uncommonly, helium and nitrogen gasses may also be
present. The initial fluid stream 10 is at a post-production
pressure of approximately 600 pounds per square inch (psi). In some
instances, the pressure of initial fluid stream 10 may be up to
1,000 psi.
[0042] The fluid stream 10 is typically chilled before entering the
distillation tower 100. A refrigeration unit 150, such as a propane
refrigerator, is provided for the initial fluid stream 10, which
may be a gas feed stream. The refrigeration unit 150 is used to
bring the temperature of the initial fluid stream 10 down to about
-30.degree. to about -40.degree. F. The chilled fluid stream may
then be moved through an expansion device 152, such as a
Joule-Thompson ("J-T") valve. The expansion device 152 serves as an
expander to obtain subcooling and preferably, partial liquification
of the feed gas. A J-T valve is preferred for gas feed streams that
are prone to forming solids. The expansion device 152 is preferably
mounted close to the cryogenic distillation tower 100 to minimize
heat loss in the piping.
[0043] As an alternative to a J-T valve, the expander device 152
may be a turbo expander. A turbo expander provides greater cooling
and creates a source of shaft work for processes like the
refrigeration unit 150. In this manner, the operator may minimize
the overall energy requirements for the distillation process.
However, because the cryogenic distillation tower 100 is normally
used in fields where the operator anticipates a high CO.sub.2
stream, such that solids formation is likely, solids formation may
become a matter of concern within not only the cryogenic
distillation tower 100, but also within a turbo expander. Thus, the
J-T valve is likely the preferred expander device 152.
[0044] In either instance, the refrigeration unit 150 and expander
device 152 convert the raw gas in the initial fluid stream 10 into
a chilled fluid stream 12 comprised primarily of liquids.
Preferably, the temperature of the chilled fluid stream 12 is
around -30.degree. to -70.degree. F. In one aspect, the cryogenic
distillation tower 100 is operated at a pressure of 550 psi, and
the chilled fluid stream 12 is at approximately -62.degree. F. At
these conditions, the chilled fluid stream 12 is in a substantially
liquid phase, although some vapor phase may inevitably be entrained
into the chilled fluid stream 12. Most likely, no solids formation
has arisen from the presence of CO.sub.2.
[0045] The cryogenic distillation tower 100 is divided into three
primary sections. These are a stripping section 106, an
intermediate spray section or "freeze zone" 108, and a
rectification section 110. In the tower arrangement of FIG. 1, the
chilled fluid stream 12 is introduced into the distillation tower
100 near the top of the stripping section 106. However, the chilled
fluid stream 12 may alternatively be introduced into the spray
section 108.
[0046] It is noted in the arrangement of FIG. 1 that the stripping
section 106, the intermediate spray section 108, the rectification
section 110, and all the components are housed within a single
vessel 100. This is believed to be the most economical design.
However, for offshore applications in which height of the tower 100
and motion considerations may need to be considered, or for remote
locations in which transportation limitations are an issue, the
tower 110 may optionally be split into two separate pressure
vessels (not shown). For example, the rectification section 110 and
the freeze zone 108 may be located in one vessel, while the
stripping section 106 is in another section. External piping would
then be used to interconnect the two vessels portions.
[0047] In either embodiment, the temperature of the stripping
section 106 is higher than the feed temperature of the chilled
fluid stream 12. The temperature of the stripping section 106 is
designed to be above the vapor point of the methane in the chilled
fluid stream 12. In this manner, methane is preferentially stripped
from the heavier hydrocarbon and acid liquid components. Of course,
those of ordinary skill in the art will understand that the liquid
within the distillation tower 100 is a mixture, meaning that the
liquid boils at some intermediate temperature between methane and
CO.sub.2. Further, in the event that there are heavier hydrocarbons
present in the mixture, this increases the boiling temperature of
the mixture. These factors become design considerations for the
operating temperatures within the distillation tower 100.
[0048] In the stripping section 106, the CO.sub.2 and any other
liquid-phase fluids gravitationally fall towards the bottom of the
cryogenic distillation tower 100. At the same time, methane and
other vapor-phase fluids break out and rise upwards towards the top
of the tower 100. This separation is accomplished primarily through
the density differential between the gas and liquid phases.
However, the separation process is aided by internal components
within the distillation tower 100. As described below, these
include a melt tray 130, a plurality of advantageously-configured
mass transfer devices 126, and an optional heater line 25.
[0049] Referring again to FIG. 1, the chilled fluid stream 12 may
be introduced into the column 100 near the top section of the
stripping section 106. Alternatively, it may be desirable to
introduce the feed stream 12 into the intermediate spray section
108 above the melt tray 130. Again, the point of injection of the
chilled fluid stream 12 is a design issue dictated by the
composition of the initial fluid stream 10.
[0050] As with most distillation columns, the feed stream 12 is put
onto a tray or region of the column 100 that has similar
composition to the feed composition. Thus, where the CO.sub.2
content of the feed stream 12 is high enough (such as greater than
60%) such that solids are not expected, it may be preferable to
inject the feed stream 12 directly into the stripping section 106
through a two phase flashbox type device (or vapor distributor) 124
in the column 100. In this case, the expander device 152 could be a
turbo expander or a J-T valve or other device. A turbo expander
would be preferred as it would extract greater thermal energy from
the feed stream 12, as well as recover some work for use elsewhere
in the process. If the likelihood of solids formation is
significant, the expander device 152 of choice would be a J-T
valve. In this case, stream 12 is separated in a two phase vessel
173 to minimize the possibility of solids plugging the inlet line
and internals. The vapor leaving the inlet separator 11 is
introduced into the column 100 through the vapor distributor 124.
The liquid/solid slurry 13 is discharged through an inlet
distributor 121 such that it would not be prone to plugging by
solids. Inlet feed 13 can be fed to the tower by gravity or by pump
175. Thus, where the initial fluid stream 10 has a sufficiently
high CO.sub.2 content such that early solids formation is
contemplated, it may be desirable to separate the chilled fluid
stream 10 in a separator instead of discharging directly into the
lower portion of spray section 108 to avoid vapor maldistribution
into the freeze zone 108.
[0051] In the arrangement of FIG. 1, the chilled fluid stream 12
may be distributed directly into the cryogenic distillation tower
100 through one or more two-phase flashboxes 124, as long as
significant solids or hydrates are not expected. The feed
distributor is slotted such that the two-phase fluid impinges
against baffles in the flashbox 124. The use of a flashbox 124
serves to separate the two-phase vapor-liquid mixture in the
chilled fluid stream 12. If solids are anticipated, the feed may
need to be separated in a vessel 173 prior to feeding the column
100 as described above.
[0052] The chilled fluid stream 12 (or 11) exits the flashbox 124.
The location of the feed inlet in section 106 is dependent on feed
composition. The liquid leaving the flashbox travels down the
stripping trays. Because of the temperature and composition of the
chilled fluid stream 12, carbon dioxide primarily exists in liquid
form. The vapor leaving the flashbox 124 travels through one or
more trays, depending on the composition to maximize mass transfer.
The vapor proceeds to travel through the risers of the melt tray
130 and into the freeze zone 108. The melt tray risers act as a
vapor distributor for uniform distribution through this zone 108.
The vapor contacts the cold liquid from spray headers 120 to
"freeze out" the CO.sub.2. Stated another way, the solid CO.sub.2
"snow" onto the melt tray 130 and then gravitationally flow in
liquid form down the melt tray 130 and through the stripping
section 106 there below. As will be discussed more fully below, the
intermediate spray section 108 is an intermediate freeze zone of
the cryogenic distillation tower 100. With the alternate
configuration in which feed stream 12 is separated in vessel 173
prior to entering the tower 100, the separated cooled fluid stream
13 is introduced into the tower 100 above the melt tray 130. Thus,
a liquid-solid mixture of sour gas and heavier hydrocarbon
components flows from the distributor 121 with liquids falling down
onto the melt tray 130.
[0053] The melt tray 130 is configured to gravitationally receive
liquid and solid materials, primarily CO.sub.2 and H.sub.2S, from
the intermediate spray section 108. The melt tray 130 is referred
to as a "melt tray" as it serves to warm the liquid and solid
materials and direct them downward through the stripping section
106 in liquid form for further purification. The melt tray 130
collects and warms the solid-liquid mixture from the intermediate
spray section 108 in a pool of liquid. The melt tray 130 is
designed to release vapor flow back to the intermediate spray
section 108, to provide adequate heat transfer to melt the solid
CO.sub.2, and to facilitate liquid/slurry drainage to the lower
distillation or stripping section 106 of the column 100 below the
melt tray 130.
[0054] FIG. 2A provides a plan view of the melt tray 130, in one
embodiment. FIG. 2B provides a cross-sectional view of the melt
tray 130, taken across line 2B-2B of FIG. 2A. FIG. 2C shows a
cross-sectional view of the melt tray 130, taken across line 2C-2C.
The melt tray 130 will be described with reference to these three
drawings collectively.
[0055] First, the melt tray 130 includes a base 134. The base may
be a substantially planar body. However, in the preferred
embodiment shown in FIGS. 2A, 2B and 2C, the base 134 employs a
substantially non-planar profile. The non-planar base configuration
provides an increased surface area for contacting liquids and
solids landing on the melt tray 130 from the intermediate spray
section 108. This serves to increase heat transfer from the vapors
passing up from the stripping section 106 of the column 100 to the
liquids and thawing solids. In one embodiment, the base 134 is
corrugated. In another embodiment, the base 134 is substantially
sinusoidal. The corrugated embodiment of the tray design is shown
in FIG. 2B. It is understood that other non-planar geometries may
be used to increase the heat transfer area of the melt tray
130.
[0056] The melt tray base 134 is preferably inclined. The incline
is demonstrated in the side view of FIG. 2C. Although most solids
should be melted, the incline serves to ensure that any unmelted
solids in the liquid mixture drain off of the melt tray 130 and
into the stripping section 106 there below.
[0057] In the view of FIG. 2C, a sump or "downcomer" 138 is seen
central to the melt tray 130. The melt tray base 134 slopes
inwardly towards the downcomer 138 to deliver the solid-liquid
mixture. The base 134 may be sloped in any manner to facilitate
liquid draw-off.
[0058] As described in U.S. Pat. No. 4,533,372, the melt tray was
referred to as a "chimney tray." This was due to the presence of a
single venting chimney. The chimney provided an opening through
which vapors may move upward through the chimney tray. However, the
presence of a single chimney meant that all gasses moving upward
through the chimney tray had to egress through the single opening.
On the other hand, in the melt tray 130 of FIGS. 2A, 2B and 2C, a
plurality of chimneys 131 (or "risers") is provided. The use of
multiple chimneys 131 provides improved vapor distribution. This
contributes to better heat/mass transfer in the intermediate spray
section 108.
[0059] Referring again to FIG. 1, the chimneys 131 may be of any
profile. For instance, the chimneys 131 may be round, rectangular,
or any other shape that allows vapor to pass through the melt tray
130. The chimneys 131 may also be narrow and extend upwards into
the intermediate spray section 108. This enables a beneficial
pressure drop to distribute the vapor evenly as it rises into the
CFZ intermediate spray section 108. The chimneys 131 are preferably
located on peaks of the corrugated base 134 to provide additional
heat transfer area.
[0060] Optionally, a heater may be placed below or just above the
melt tray base 134 to facilitate thawing of the solid. A heater
line 25 is shown and will be discussed further below. The heater
line 25 utilizes thermal energy already available from a bottom
reboiler 160.
[0061] The top openings of the chimneys 131 are preferably covered
with hats or caps 132. This minimizes interaction between the vapor
flowing upwards out of the chimneys 131 and the liquid/solid
mixture falling onto the melt tray 130. In FIGS. 2A, 2B and 2C,
caps 132 are seen above each of the chimneys 131.
[0062] The melt tray 130 may also be designed with bubble caps. The
bubble caps define convex indentations in the base 134 rising from
underneath the melt tray 130. The bubble caps further increase
surface area in the melt tray 130 to provide additional heat
transfer to the CO.sub.2-rich liquid. With this design, a suitable
liquid draw off, such as an increase incline angle, should be
provided to ensure clear liquid is directed to the stripping trays
126 below.
[0063] The melt tray 130 may also be designed with an internal
reflux system. The reflux system serves to ensure that all liquid
is substantially free of solids and that sufficient heat transfer
has been provided. The reflux system first includes a draw-off
nozzle 136. In one embodiment, the draw-off nozzle 136 resides
within the draw-off sump 138. Fluids collected in draw-off sump 136
are delivered to a reflux line 135. Flow through reflux line 135 is
controlled by a control valve 137 and a level controller "LC."
Fluids are returned to the stripping section 106 via the reflux
line 135. If the liquid level is too high, the control valve 137
opens; if the level is too low, the control valve 137 closes. If
the operator chooses not to employ the reflux system in the
stripping section 106, then the control valve 137 is closed and
fluids are directed immediately to the mass transfer devices, or
"stripping trays" 126 below the melt tray 130 for stripping via an
overflow downcomer 139.
[0064] Whether or not an internal reflux system is used, solid
CO.sub.2 is warmed on the melt tray 130 and converted to a
CO.sub.2-rich liquid. The melt tray 130 is heated from below by
vapors from the stripping section 106. Supplemental heat may
optionally be added to the melt tray 130 by various means such as
heater line 25. The CO.sub.2-rich liquid is drawn off from the melt
tray 130 under liquid level control and gravitationally introduced
to the stripping section 106.
[0065] As noted, a plurality of stripping trays 126 are provided in
the stripping section 106 below the melt tray 130. The stripping
trays 126 are preferably in a substantially parallel relation, one
above the other. Each of the stripping trays 126 may optionally be
positioned at a very slight incline, with a weir such that a liquid
level is maintained on the tray. Fluids gravitationally flow along
each tray, over the weir, and then flow down onto the next tray via
a downcomer.
[0066] The stripping trays 126 may be in a variety of arrangements.
The stripping trays 126 may be arranged in generally horizontal
relation to form a sinusoidal, cascading liquid flow. However, it
is preferred that the stripping trays 126 be arranged to create a
cascading liquid flow that is divided by separate stripping trays
substantially along the same horizontal plane. This is shown in the
arrangement of FIG. 3, where the liquid flow is split at least once
so that liquid falls over two opposing downcomers 129.
[0067] FIG. 3 provides a side view of a stripping tray 126
arrangement, in one embodiment. Each of the stripping trays 126
receives and collects fluids from above. Each stripping tray 126
preferably has a weir 128 that serves as a dam to enable the
collection of a small pool of fluid on each of the stripping trays
126. The fluid is contacted with upcoming vapor rich in lighter
hydrocarbons that strip out the methane from the cross flowing
liquid in this "contact area" of the trays 126. The weirs 128 serve
to dynamically seal the downcomers 129 to prevent vapor bypassing
through the downcomers 129 and to further facilitate the breakout
of hydrocarbon gasses.
[0068] The percent of methane becomes increasingly small as the
fluids move downward in the stripping section 106. In the upper
part of the stripping section 106, the methane content of the
liquid may be as high as 25 mol %, while at the bottom stripping
tray the methane content may be as low as 0.04 mol %. The methane
content flashes out quickly along the stripping trays or packing
126. The number of mass transfer devices 126 used in the stripping
section 106 is a matter of composition. However, only a few levels
of stripping trays 126 may be utilized to remove methane to a
desired level of 1% or less in the liquefied acid gas, for example.
The level of purity depends on the number of equilibrium stages in
the stripping section.
[0069] Various individual stripping tray 126 configurations that
facilitate methane breakout may be employed. The stripping tray 126
may simply be a panel with sieve holes, valves or bubble caps.
Sieve holes, valves and bubble caps can be a variety of sizes and
open area, depending on hydraulics. However, to provide further
heat transfer to the fluid and to prevent unwanted blockage due
solids, so called "jet trays" may be employed below the melt tray.
In lieu of trays, random or structured packing may also be
employed.
[0070] FIG. 4A provides a plan view of an illustrative jet tray
426, in one embodiment. FIG. 4B provides a cross-sectional view of
a jet tab 422 from the jet tray 426. As shown, each jet tray 426
has a body 424, with a plurality of jet tabs 422 formed within the
body 424. Each jet tab 422 includes an inclined tab member 428
covering an opening 425. Thus, a jet tray 426 has a plurality of
small openings 425.
[0071] In operation, one or more jet trays 426 may be located in
the stripping 106 or rectification 110 sections of the tower 100
where solids accumulation is possible. The trays 426 may be
arranged with multiple passes such as the pattern of stripping
trays 126 in FIG. 3. However, any tray or packing arrangement may
be utilized that facilitates the breakout of methane gas. Fluid
cascades down upon each jet tray 426. The fluids then flow along
the body 424. The fluid is then contacted with the vapor exiting
the openings 425. The tabs 422 are optimally oriented to move the
fluid quickly and efficiently across the tray 426. An adjoined
downcomer (not shown) is provided to move the liquid to the
subsequent tray 426. The openings 425 also permit gas vapors
released during the fluid movement process in the stripping section
106 to travel upwards more efficiently to the melt tray 130 and
through the chimneys 131.
[0072] In one aspect, the trays 126 or 426 may be fabricated from
fouling-resistant materials, that is, materials that prevent
solids-buildup. Fouling-resistant materials are utilized in some
processing equipment to prevent the buildup of corrosive metal
particles, polymers, salts, hydrates, catalyst fines, or other
chemical solids compounds. In the case of the cryogenic
distillation tower 100, fouling resistant materials may be used in
the trays 126 or 426 to limit sticking of CO.sub.2 solids. For
example, a Teflon coating may be applied to the surface of the
trays 126 or 426.
[0073] Alternatively, a physical design may be provided to ensure
that the CO.sub.2 does not start to build up in solid form along
the inner diameter of the column 100. In this respect, the jet tabs
422 may be oriented to push liquid along the wall of the column
100, thereby preventing solids accumulation along the wall of the
column 100 and ensuring good vapor liquid contact.
[0074] In any of the tray arrangements, as the fluids hit the
stripping trays 126, separation of materials occurs. Methane gasses
break out of solution and move upward in vapor form. The CO.sub.2,
however, is cold enough and in high enough concentration that it
remains in its liquid form and travels down to the bottom of the
stripping section 106. The liquid is then moved out of the
cryogenic distillation tower 100 in an exit line as an exit fluid
stream 22.
[0075] Upon exiting the distillation tower 100, the exit fluid
stream 22 enters a reboiler 160. In FIG. 1, the reboiler 160 is a
kettle type that provides reboiled vapor to the bottom of the
stripping trays. A heater line 25 from the reboiler vapor stream
may be used to provide supplemental heat to the melt tray 130. The
supplemental heat is controlled through a valve 165 and temperature
controller TC. Alternately, a thermosyphon heat exchanger may be
used for the initial fluid stream 10 to economize energy. In this
respect, the liquids entering the reboiler 160 remain at a
relatively low temperature, for example, about 30.degree. to
40.degree. F. By heat integrating with the initial fluid stream 10,
the operator may warm the cold exit fluid stream 22 from the
distillation tower 100 while cooling the production fluid stream
10. For this case, the fluid providing supplemental heat through
line 25 is a mixed phase return from the reboiler 160.
[0076] It is contemplated that under some conditions, the melt tray
130 may operate without a heater. In these instances, the melt tray
130 may be designed with a heating feature. The heater may be
electric. The heating medium may reside external to the cryogenic
distillation tower 100. However, it is preferred that a heater be
offered that employs the heat energy available in exit fluid stream
22. The vapor stream 25 captured from exit fluid stream 22 after
exiting reboiler 160 exists in one aspect at 30.degree. to
40.degree. F., so they contain relative heat energy. Thus, in FIG.
1, vapor stream 25 is shown being directed to the melt tray 130
through a heating coil on the melt tray 130 (not shown). As the
supplemental heat stream passes through and warms the melt tray
130, it is return to the tower 100 bottom liquid pool through line
29.
[0077] In operation, reboiled vapor stream 27 is introduced at the
bottom of the column, above the bottom liquid level and below the
last stripping tray 126. As the reboiled vapor passes through each
tray, residual methane is stripped out of the liquid. This vapor
cools off as it travels up the tower. By the time the stripping
vapors of stream 27 reach the corrugated melt tray 130, the
temperature may have dropped to about 0.degree. F. to -20.degree.
F. However, this remains quite warm compared to the melting solid
on the melt tray 130, which may be around -50.degree. F. to
-80.degree. F. The vapor still has enough enthalpy to melt the
solids CO.sub.2 as it comes in contact with the melt tray 130 to
warm the melt tray 130.
[0078] Referring back to reboiler 160, fluids in a bottom stream 24
that remain in liquid form may optionally enter a valve 162. The
valve 162 reduces the pressure of the bottom liquid product,
effectively providing a refrigeration effect. Thus, a chilled
bottom stream 26 is provided. The bottom stream 24 may be
maintained at pressure for direct feed to a pump, then to an acid
gas injection (AGI) well.
[0079] The chilled bottom stream 26 may be reinjected in its liquid
phase into a geologic reservoir through an AGI well (seen
schematically at 250 in FIG. 1). The liquid exiting the reboiler
160 is pumped downhole. In some situations, the liquid CO.sub.2 may
be pumped into a partially recovered oil reservoir as part of an
enhanced oil recovery process. Thus, the CO.sub.2 could be a
miscible injectant. As an alternative, the CO.sub.2 may be used as
a miscible flood agent for enhanced oil recovery.
[0080] Some or all of the chilled bottom stream 26 may optionally
be moved through a heat exchanger 164. The heat exchanger 164
recovers additional cold from the liquid stream before the CO.sub.2
is warmed into its gas phase for energy recovery. This final stage
is shown at stream 28. However, this step is optional, and it is
preferred that the CO.sub.2 be reinjected through AGI well 250 in
its liquid phase to avoid CO.sub.2 venting.
[0081] Referring again to the stripping section 106 of the column
100, gas moves up through the stripping section 106, through the
chimneys 131 in the melt tray 130, and into the intermediate spray
section 108. The spray section 108 is an open chamber having a
plurality of spray nozzles 122. As the vapor moves through the
spray section 108, the temperature in the intermediate spray
section 108 becomes colder. As the vapor moves upward, the light
gas products are contacted by liquid methane coming from the spray
nozzles 122. This liquid methane is colder than the vaporized
methane moving upward, having been chilled through a refrigerator
170. In one arrangement, the liquid methane exits from spray
nozzles 122 at a temperature of about -120.degree. F. to
-130.degree. F.
[0082] As the methane vapors move further up the cryogenic
distillation tower 100, they leave the intermediate spray section
108 and enter the rectification section 110. The vapors continue to
move upward along with other light gasses broken out from the
original chilled fluid stream 12. The combined hydrocarbon vapors
move out of the top of the cryogenic distillation tower 100,
becoming a condenser exit stream 14, or "condenser feed."
[0083] The hydrocarbon gas in condenser exit stream 14 is moved
into the external refrigeration unit 170. In one aspect, the
refrigeration unit 170 uses an ethylene refrigerant or other
refrigerant capable of chilling the condenser exit stream 14 down
to about -138.degree. to -142.degree. F. This serves to at least
partially liquefy the condenser exit stream 14. The liquefied
condenser exit stream 14 is then moved to a separation chamber
172.
[0084] In another embodiment, the condenser exit stream 14 is taken
through an open-loop refrigeration system (not shown). In this
alternative arrangement, the condenser exit stream 14 is actually
used as a refrigerant. The condenser exit stream 14 is pressurized
to about 1,000 psi to 1,400 psi, then cooled using ambient air, and
possibly an external propane refrigerant. The chilled and
pressurized gas stream is then directed through an expander for
further cooling. At this point, a turbo expander could be used to
recover even more liquid as well as some shaft work. It is
understood here that the present inventions are not limited by the
cooling method for the condenser exit stream 14.
[0085] It is also noted that the degree of cooling between
refrigerator 170 and the refrigeration unit 150 may be varied. In
some instances, it may be desirable to operate the refrigeration
unit 150 at a higher temperature, but then be more aggressive with
cooling the overhead stream 14 in refrigerator 170. Again, the
present inventions are not limited to these types of design
choices.
[0086] As noted above, the chilled gas stream exiting refrigerator
170 is directed to a reflux vessel or separation chamber 172.
Separation chamber 172 is used to separate gas from liquid. A pump
may be used to move liquid back into the column 100 if the
separation chamber 172 is not mounted above the column 100 to
provide gravity feed of liquid. A portion of the condenser exit
stream 14 entering the refrigerator 170 is not condensed; instead,
this portion remains in the vapor phase. This portion represents
the lighter hydrocarbon gasses, primarily methane. The methane is,
of course, the "product" ultimately sought to be captured and sold
commercially, along with any ethane. The methane and, perhaps, some
ethane, is captured as represented in FIG. 1 by product stream 16.
Nitrogen and helium may also be present in product stream 16.
[0087] The remaining liquefied gasses primarily represent residual
sour gasses dissolved in methane. The sour gases are liquefied by
refrigerator 170, and are directed back to the cryogenic
distillation tower 100 for further processing. Optionally,
additional heat exchanging may take place to capture cold energy
from the condenser exit stream 14.
[0088] It should be noted here that the refrigerator 170 may be
referred to as a "reflux condenser." A reflux condenser is a heat
exchanger that causes condensation. This means that a portion of
the condenser exit stream 14 coming from the top of the
rectification section 110 is condensed, and then returned to the
rectification section 110. The condensed gas, or "reflux," is
represented as reflux fluid stream 18. Most of the reflux fluid
stream 18 is methane, typically 95% or more, with traces of
nitrogen, carbon dioxide and hydrogen sulfide. Sufficient reflux
fluid is generated such that the desired CO.sub.2 or H.sub.2S
specification is met at the top of the column 100.
[0089] The reflux fluid stream 18 is returned into the
rectification section 110. The reflux fluid stream 18 is then
gravitationally carried through one or more mass transfer devices
116 in the rectification section 110. In one embodiment, the mass
transfer devices 116 are rectification trays that provide a
cascading series of weirs 118 and downcomers 119.
[0090] As fluids from reflux fluid stream 18 move downward through
the rectification trays 116, additional methane vaporizes out of
the rectification section 110. The methane gasses rejoin the
condenser exit stream 14 to become product stream 16. However, the
remaining liquid phase of reflux fluid stream 18 falls onto a
collector tray 140. As it does so, the reflux fluid stream 18
unavoidably picks up a small percentage of hydrocarbon and residual
acid gasses moving upward from the stripping section 106. The
liquid mixture of methane and carbon dioxide is collected at
collector tray 140.
[0091] The collector tray 140 preferably defines a substantially
planar body for collecting liquids. However, as with melt tray 130,
collector tray 140 also has one, and preferably a plurality of
chimneys for venting gasses coming up from the spray section 108. A
chimney and cap arrangement such as that presented by components
131 and 132 in FIG. 2B may be used. Chimneys 141 and caps 142 for
collector tray 140 are shown in the enlarged view of FIG. 5,
discussed further below.
[0092] It is noted here that in the rectification section 110, any
H.sub.2S present has a slight preference towards being a liquid
versus the gas at the processing temperature. In this respect, the
H.sub.2S has a comparatively low relative volatility. By contacting
the remaining vapor with more liquid, the cryogenic distillation
tower 100 drives the H.sub.2S concentration down to within the
desired parts-per-million (ppm) limit, such as a 4 ppm
specification. As fluid moves through the mass transfer devices
116, the H.sub.2S contacts the liquid methane and is pulled out of
the vapor phase and becomes a part of a chamber liquid stream 20.
From there, the H.sub.2S moves in liquid form downward through the
stripping section 106 and ultimately exits the cryogenic
distillation tower 100 as the liquefied acid gas stream 22.
[0093] In cryogenic distillation tower 100, the liquid captured at
collector tray 140 is drawn off of the rectification section 110 as
a liquid stream 20. The liquid stream 20 is comprised primarily of
methane. In one aspect, the liquid stream 20 is comprised of about
93% methane, 3% CO.sub.2, 0.5% H.sub.2S, and 3.5% N.sub.2, At this
point, liquid stream 20 is at about -125.degree. F. to -130.degree.
F. This is only slightly warmer than the reflux fluid stream 18.
The liquid stream 20 is directed into a reflux drum 174. The
purpose of the reflux drum 174 is to provide surge capacity for a
pump 176. Upon exiting the reflux drum 174, a spray stream 21 is
created. Spray stream 21 is pressurized in a pump 176 for a second
reintroduction into the cryogenic distillation tower 100. In this
instance, the spray stream 21 is pumped into the intermediate spray
section 108 and emitted through nozzles 122.
[0094] Some portion of the spray stream 21, particularly the
methane, vaporizes upon exiting the nozzles 122. From there, the
methane rises through the intermediate spray section 108, through
the chimneys in the collector tray 140, through the rectification
mass transfer devices 116 in the rectification section 110, and
ultimately become commercial product in product stream 16. However,
another portion of the liquid from nozzles 122 vaporizes as it
cools upflowing feed gas, causing carbon dioxide to desublime from
the gas phase. The CO.sub.2 "snow" falls upon the melt tray 130,
and melts into the liquid phase. From there, the CO.sub.2-rich
liquid cascades down mass transfer devices 126 in the stripping
section 106 along with liquid CO.sub.2 from the chilled fluid
stream 12 described above. At that point, any remaining
hydrocarbons from the spray stream 21 of the nozzles 122 should
quickly break out into vapor. These vapors move upwards in the
cryogenic distillation tower 100 and re-enter the rectification
section 110.
[0095] In accordance with one of the many aspects of the present
invention, it has been determined that it is desirable to provide
some additional form of resistance to fluid flow in the spray
section 108 below the nozzles 122. The fluid flow resistance
interferes with the flow of the liquid as it moves down the
intermediate spray section 108 and onto the melt tray 130. Thus,
the cryogenic distillation tower 100 may optionally include baffles
or grid packing.
[0096] FIG. 5 provides an enlarged view of the cryogenic
distillation tower 100. In this view, the intermediate spray
section 108 is primarily seen. Two sets of nozzles 122 are also
seen. Further, a pair of illustrative baffles or grid packing 510
is shown disposed within the spray section 108. The baffles or grid
packing 510 preferably traverse the diameter of the column 100. In
the arrangement of FIG. 5, one of the baffles or grid packing 510
is placed below each of the respective two sets of nozzles 122. As
methane-rich liquid is injected from the nozzles 122 into the spray
section 108, the fluid contacts the baffles or grid packing 510.
This, in turn, facilitates the breakout of methane from
solution.
[0097] The baffles or grid packing 510 may take one of any number
of shapes and forms. The baffles or grid packing 510 are configured
to create a frictional flow path for liquids and snow as the
material gravitationally travels down through the intermediate
spray section 108. As such, the baffles or grid packing 510 may
offer angles, grids, perforations or other types of diversion
surfaces. In general, the baffles or grid packing 510 should be
made to be resistant to fouling as well.
[0098] The use of baffles or grid packing 510 in the intermediate
spray section 108 improves the efficiency of fluid separation and
heat transfer from the spray droplets to the upflowing gas. Baffles
or grid packing 510 further reduce back-mixing of CO.sub.2. In this
respect, the frictional flow path created by the baffles or grid
packing 510 prevent the fine, low mass CO.sub.2 particles from
moving back up the column 100 and re-entering the rectification
section 110. These particles would undesirably remix with methane
and re-enter the condenser exit stream 14, only to be recycled
again.
[0099] Another feature of the column 100 that can improve the
efficiency of fluid separation in the intermediate spray section
108 pertains to the configuration of the nozzles 122. Rather than
employing a single spray source at one or more levels in a reflux
fluid stream, the present disclosure offers spray headers
optionally designed with multiple spray nozzles 122 incorporated
therein.
[0100] The configuration of the spray nozzles 122 has an impact on
the mass transfer taking place within the intermediate spray
section 108. During the separation process, a chilled liquid stream
is injected through the nozzles 122 and into the intermediate spray
section 108. As the liquid enters the intermediate spray section
108 and begins to fall within the cryogenic distillation tower 100,
liquefied methane is evaporated. Some CO.sub.2 momentarily enters
the gas phase and moves upward with the methane. However, because
of the cold temperature within the intermediate spray section 108,
the vaporized carbon dioxide quickly turns into a solid phase and
begins to "snow." This phenomenon is referred to as desublimation.
In this way, some CO.sub.2 never re-enters the liquid phase until
it hits the melt tray 130. The CO.sub.2 particles move down the
column 100 in snow form.
[0101] FIGS. 6A and 6B demonstrate various nozzle 122 arrangements
in perspective views. In each of FIGS. 6A and 6B, a spray head
incorporating multiple spray nozzles 122 is provided. In each
Figure, injection line, which is the exit fluid stream 22, is shown
introducing the injectant fluid through a wall 111 of the column
100.
[0102] FIG. 6A is an enlarged view of the nozzle arrangement from
FIG. 1. Here, a spray header 120 supports three nozzles 122 in
series. In FIG. 1, two reflux spray headers are provided, each
supporting three nozzles 122. Thus, a 3.times.2 array is formed.
Preferably, each nozzle is capable of providing 70.degree. to
140.degree. spray distribution. These multiple spray headers can be
optimized by using a variety of nozzle sizes for turndown
operation.
[0103] FIG. 6B presents an alternate spray header 120' in which two
transverse lines support nozzles 122. Alternatively, three spray
headers 120 at different levels in the intermediate spray section
108 may be disposed at 120.degree. (or other) relative angles.
Alternatively still, the spray nozzles 122 may be supported by a
first substantially linear spray header having a plurality of
nozzles 122, and a second substantially linear spray header
transverse to and below the first spray header, also having a
plurality of nozzles. These arrangements are not specifically shown
but are readily understood from the views of FIG. 1 and FIG. 6B,
together. Other spray patterns are possible.
[0104] It is desirable to have chilled liquid contacting as much of
the gas that is moving up the column as possible. If vapor bypasses
the sprays, higher levels of CO.sub.2 could reach the rectification
section 110 of the tower 100. The unique spray header arrangements
120, 120' (and others described above) avoid dry areas where vapor
could bypass the spray. It is preferred that the individual nozzles
122 provide a more limited spray distribution, e.g., 100.degree. to
120.degree., but that multiple nozzles 122 be used to achieve full
distribution. The use of multiple spray nozzles 122 at a level of
the intermediate spray section 108 provides fuller, overlapping
liquid coverage of the intermediate spray section 108. This serves
to ensure 360.degree. coverage within the spray section 108 and
provide good vapor-liquid contact and heat/mass transfer. This, in
turn, more effectively chills any gaseous carbon dioxide moving
upward through the cryogenic distillation tower 100.
[0105] The use of an overlapping nozzle 122 arrangement for
complete coverage minimizes back-mixing as well. In this respect,
complete coverage prevents the fine, low mass CO.sub.2 particles
from moving back up the column and re-entering the rectification
section 110. These particles would then remix with methane and
re-enter the condenser exit stream 14, only to be recycled
again.
[0106] In one aspect, individual nozzles 122 may be configured to
offer a particularly fine or atomized spray distribution. Finer
distribution aids hydrocarbons in breaking out of solution as the
liquid travels towards the stripping section 106.
[0107] In yet an additional embodiment, separate spray heads 120
may be used to spray reflux liquid at different temperatures. In
this instance, the reflux stream fed into the uppermost spray head
is chilled to a temperature below that of the reflux stream fed
into the lowermost spray head.
[0108] It can be seen that the process of cycling vapors through
the cryogenic distillation tower 100 ultimately produces a
hydrocarbon product comprised of methane and ethane gas in product
stream 16. The product is sent down a pipeline or processed to LNG.
At the same time, sour gasses are removed through exit fluid stream
22.
[0109] The above-described internal features are expected to
improve mass transfer, heat transfer, and solids handling in the
cryogenic distillation tower 100. They are also expected to improve
column operation during upset conditions, when CO.sub.2 levels are
not yet stabilized in the cryogenic distillation tower 100. While
it will be apparent that the invention herein described is well
calculated to achieve the benefits and advantages set forth above,
it will be appreciated that the invention is susceptible to
modification, variation and change without departing from the
spirit thereof. For example, adjustments may be made to the
operational temperatures within the tower 100 to maximize
distillation and fluid separation. The temperature ranges disclosed
herein are merely exemplary, and it is understood that temperatures
could fall outside of these ranges such as during transients or
upsets.
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