U.S. patent application number 13/248344 was filed with the patent office on 2012-09-20 for method for the removal of heat stable amine salts from an amine absorbent.
This patent application is currently assigned to DOW GLOBAL TECHNOLOGIES LLC. Invention is credited to Barath BABURAO, Stephen A. BEDELL, Ross DUGAS, Nareshkumar B. HANDAGAMA, Jonathan W. LEISTER, Frederic VITSE.
Application Number | 20120235087 13/248344 |
Document ID | / |
Family ID | 46827747 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120235087 |
Kind Code |
A1 |
HANDAGAMA; Nareshkumar B. ;
et al. |
September 20, 2012 |
METHOD FOR THE REMOVAL OF HEAT STABLE AMINE SALTS FROM AN AMINE
ABSORBENT
Abstract
The proposed invention relates to a method and a system for the
removal of heat stable amine salts from an amine absorbent used in
a carbon dioxide (CO.sub.2) capture process, the method comprising:
withdrawing amine absorbent containing heat stable amine salts from
the CO.sub.2 capture process; subjecting the withdrawn amine
absorbent containing heat stable amine salts to a residual CO.sub.2
removal step; subjecting the amine absorbent from the residual
CO.sub.2 removal step to a separation step to separate heat stable
amine salts from the amine absorbent; and returning the amine
absorbent having a reduced concentration of heat stable amine salts
to the CO.sub.2 capture process.
Inventors: |
HANDAGAMA; Nareshkumar B.;
(Knoxville, TN) ; BABURAO; Barath; (Knoxville,
TN) ; VITSE; Frederic; (Knoxville, TN) ;
BEDELL; Stephen A.; (Knoxville, TN) ; LEISTER;
Jonathan W.; (Manvel, TX) ; DUGAS; Ross;
(Pearland, TX) |
Assignee: |
DOW GLOBAL TECHNOLOGIES LLC
Midland
MI
ALSTOM TECHNOLOGY LTD.
Baden
|
Family ID: |
46827747 |
Appl. No.: |
13/248344 |
Filed: |
September 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61454079 |
Mar 18, 2011 |
|
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|
Current U.S.
Class: |
252/190 ;
204/544; 422/186.04; 422/187 |
Current CPC
Class: |
B01D 2257/504 20130101;
B01D 2259/4009 20130101; Y02A 50/20 20180101; Y02A 50/2342
20180101; C07C 209/84 20130101; B01J 47/00 20130101; B01D
2252/20484 20130101; B01D 2252/20489 20130101; Y02C 20/40 20200801;
B01D 61/422 20130101; Y02C 10/04 20130101; B01D 2258/0283 20130101;
Y02C 10/08 20130101; B01D 53/1425 20130101; B01D 53/1475 20130101;
Y02C 10/06 20130101; B01D 53/1493 20130101; B01D 61/42 20130101;
B01D 2252/204 20130101 |
Class at
Publication: |
252/190 ;
422/187; 422/186.04; 204/544 |
International
Class: |
C09K 3/00 20060101
C09K003/00; B01J 19/08 20060101 B01J019/08; C25B 7/00 20060101
C25B007/00; B01J 19/00 20060101 B01J019/00 |
Claims
1. A method for the removal of heat stable amine salts from an
amine absorbent used in a carbon dioxide (CO.sub.2) capture
process, comprising: withdrawing amine absorbent containing heat
stable amine salts from the CO.sub.2 capture process; subjecting
the withdrawn amine absorbent containing heat stable amine salts to
a residual CO.sub.2 removal step; subjecting the amine absorbent
from the residual CO.sub.2 removal step to a separation step to
separate heat stable amine salts from the amine absorbent; and
returning the amine absorbent having a reduced concentration of
heat stable amine salts to the CO.sub.2 capture process.
2. A method according to claim 1, wherein the residual CO.sub.2
removal step comprises removing residual CO.sub.2 from withdrawn
amine absorbent by a method selected from the group consisting of
stripping CO.sub.2, flashing CO.sub.2 and combinations thereof.
3. A method according to claim 2, wherein the residual CO.sub.2
removal step comprises stripping the withdrawn amine absorbent to
remove residual CO.sub.2.
4. A method according to claim 2, wherein the residual CO.sub.2
removal step comprises flashing the withdrawn amine absorbent to
remove residual CO.sub.2.
5. A method according to claim 2, wherein the residual CO.sub.2
removal step comprises stripping and then flashing the withdrawn
amine absorbent to remove residual CO.sub.2.
6. A method according to claim 2, wherein the flashing is performed
under near vacuum conditions.
7. A method according to claim 1, wherein the CO.sub.2 capture
process comprises: scrubbing a gas stream comprising CO.sub.2 with
an amine absorbent such that a CO.sub.2 rich amine absorbent is
formed; regenerating the CO.sub.2 rich amine absorbent by heating
it to separate CO.sub.2 from the amine absorbent, such that a
CO.sub.2 lean amine absorbent is formed; and recycling regenerated
CO.sub.2 lean amine absorbent to the scrubbing step.
8. A method according to claim 7, wherein the withdrawn amine
absorbent containing heat stable amine salts is regenerated
CO.sub.2 lean amine absorbent.
9. A method according to claim 8, wherein the regenerated CO.sub.2
lean amine absorbent has a temperature of at least 100.degree. C.,
such as at least 120.degree. C.
10. A method according to claim 1, wherein the separation step
comprises subjecting the amine absorbent from the residual CO.sub.2
removal step to electrodialysis and/or ion exchange.
11. A method according to claim 10, wherein the separation step
comprises subjecting the amine absorbent from the residual CO.sub.2
removal step to electrodialysis.
12. A method according to claim 10, wherein the separation step
comprises subjecting the amine absorbent from the residual CO.sub.2
removal step to ion exchange.
13. A method according to claim 1, further comprising cooling the
amine absorbent from the residual CO.sub.2 removal step before
subjecting it to the separation step.
14. A method according to claim 13, further comprising subjecting
the amine absorbent from the residual CO.sub.2 removal step to
indirect heat exchange with the amine absorbent coming from the
separation step.
15. A carbon dioxide (CO.sub.2) capture system using an amine
absorbent for absorption of CO.sub.2 from a gas stream, having a
subsystem for the removal of heat stable amine salts from an amine
absorbent, said subsystem comprising: a residual CO.sub.2 removal
unit in liquid connection with, and configured to receive, an amine
absorbent stream containing heat stable amine salts from the
CO.sub.2 capture system, and operative for separating residual
CO.sub.2 from said amine absorbent stream; and an amine reclaimer
in liquid connection with, and configured to receive, an amine
absorbent stream containing heat stable amine salts and having a
reduced concentration of CO.sub.2 from the residual CO.sub.2
removal unit, and operative for separating heat stable amine salts
from said amine absorbent stream.
16. A carbon dioxide (CO.sub.2) capture system according to claim
15, wherein the residual CO.sub.2 removal unit is selected from the
group consisting of a stripper, a flash drum and combinations
thereof.
17. A carbon dioxide (CO.sub.2) capture system according to claim
16, wherein the residual CO.sub.2 removal unit comprises a
stripper.
18. A carbon dioxide (CO.sub.2) capture system according to claim
16, wherein the residual CO.sub.2 removal unit comprises a flash
drum.
19. A carbon dioxide (CO.sub.2) capture system according to claim
16, wherein the residual CO.sub.2 removal unit comprises a stripper
and a flash drum arranged in series.
20. A carbon dioxide (CO.sub.2) capture system according to claim
15, comprising: a CO.sub.2 absorber operative for scrubbing a gas
stream comprising CO.sub.2 with an amine absorbent such that a
CO.sub.2 rich amine absorbent is formed; and a regenerator
operative for regenerating CO.sub.2 rich amine absorbent by heating
it to separate CO.sub.2 from the amine absorbent, such that a
CO.sub.2 lean amine absorbent is formed.
21. A system according to claim 20, wherein the residual CO.sub.2
removal unit is in liquid connection with, and configured to
receive, an amine absorbent stream from the regenerator, and
operative for separating residual CO.sub.2 from the CO.sub.2 lean
amine absorbent.
22. A carbon dioxide (CO.sub.2) capture system according to claim
15, wherein the amine reclaimer is selected from the group
consisting of an electrodialysis unit and an ion exchange unit.
23. A carbon dioxide (CO.sub.2) capture system according to claim
22, wherein the amine reclaimer comprises an electrodialysis
unit.
24. A carbon dioxide (CO.sub.2) capture system according to claim
22, wherein the amine reclaimer comprises an ion exchange unit.
25. A carbon dioxide (CO.sub.2) capture system according to claim
15, further comprising an amine absorbent cooler arranged between
the residual CO.sub.2 removal unit and the reclaimer and operative
for cooling the amine absorbent from the residual CO.sub.2 removal
unit before it enters the reclaimer.
26. A carbon dioxide (CO.sub.2) capture system according to claim
25, further comprising an indirect heat exchanger operative for
subjecting the amine absorbent from the residual CO.sub.2 removal
unit to indirect heat exchange with the amine absorbent coming from
the reclaimer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit under 35 U.S.C.
.sctn.119(e) of Provisional Patent Application Ser. No. 61/454,079
entitled "METHOD FOR THE REMOVAL OF HEAT STABLE AMINE SALTS FROM AN
AMINE ABSORBENT" filed Mar. 18, 2011, the disclosure of which is
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The proposed invention relates to a system and a method for
the removal of heat stable amine salts from an amine absorbent used
in a carbon dioxide (CO.sub.2) capture process.
BACKGROUND
[0003] In the combustion of a fuel, such as coal, oil, natural gas,
peat, waste, etc., in a combustion plant, such as those associated
with boiler systems for providing steam to a power plant, a hot
process gas (or flue gas) is generated. Such a flue gas will often
contain, among other things, carbon dioxide (CO.sub.2). The
negative environmental effects of releasing carbon dioxide to the
atmosphere have been widely recognized, and have resulted in the
development of processes adapted for removing carbon dioxide from
the hot process gas generated in the combustion of the above
mentioned fuels.
[0004] In processes used for industrial separation of CO.sub.2,
liquid solutions comprising amine compounds are commonly used as an
absorbent. Examples of amine compounds commonly used in absorption
of CO.sub.2 from gas streams include monoethanolamine (MEA),
diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine
(DIPA) and aminoethoxyethanol (diglycolamine) (DGA). The most
commonly used amines compounds in industrial plants are the
alkanolamines MEA, DEA, and MDEA.
[0005] CO.sub.2 in the gas stream is captured in the liquid
absorbent solution in an absorption process. A CO.sub.2 absorber is
employed to establish suitable conditions (temperature, pressure,
turbulence, etc.) for chemical absorption of CO.sub.2 into the
amine absorbent from a mixed gas stream.
[0006] The amine absorbent containing absorbed CO.sub.2 is
subsequently regenerated, whereby absorbed CO.sub.2 is separated
from the absorbent, and the regenerated absorbent is then reused in
the CO.sub.2 absorption process. Thus, a circulating absorbent
stream is formed. Regeneration is generally achieved by heating the
amine absorbent in a stripper reboiler to a temperature at which
CO.sub.2 is released from the absorbent.
[0007] In the regenerator reboiler the absorbent is subjected to
high temperature (generally about 115.degree. C. or higher),
whereas in the absorber the absorbent is exposed to a higher
O.sub.2 environment. As a result of the exposure to high
temperature and/or the presence of O.sub.2, the amine solvent(s) of
the absorbent may undergo degradation, whereby undesired
degradation products are formed in the liquid phase. These
degradation products, known as heat stable salts or heat stable
amine salts (HSS), may accumulate in the circulating absorbent
stream. The HSS reduce the CO.sub.2 removal potency of the
absorbent and may therefore preferably be removed from the
absorbent stream. A common method of HSS removal is to take a
slipstream from the circulating absorbent, separate the bulk
absorbent from the HSS in a reclaimer and recycle the separated
amine back to the circulating absorbent loop as reclaimed
absorbent. A relaimer can consist of a distillation, ion exchange,
or electrodialysis unit.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide an
improved system and process for removal of heat stable salts (HSS)
from an amine absorbent stream used in a carbon dioxide (CO.sub.2)
capture process.
[0009] In amine based CO.sub.2 capture systems, a separation step,
e.g. electrodialysis (ED), is often employed for separating amine
absorbent from the undesired HSS in order to recycle the absorbent
in the capture process. It has been found, however, that CO.sub.2
in the amine absorbent can be detrimental to the separation
process, such as electrodialysis.
[0010] Amine based CO.sub.2 capture systems are sometimes operated
in a way such that a relatively high CO.sub.2 loading is observed
in the lean solvent leaving the stripper. It has been found that
when a slip stream of the lean solvent is sent to the reclamation
unit, either electrodialysis or ion exchange, the relatively high
lean loading fed to the reclamation unit results in significant
quantities of amine lost through the reclamation waste stream.
Reduced amine losses can significantly reduce amine make-up of the
system and provide an economic advantage.
[0011] As a solution to this problem, there is provided a method
and a system, wherein the amine absorbent containing the heat
stable salts to be removed is first subjected to stripping and/or
flashing, e.g. in a stripper or flash drum respectively, to remove
residual CO.sub.2, before being forwarded to the amine reclaimer
for separation of amine absorbent from the HSS. The
stripping/flashing step is simple and reliable, involves low
additional investment and operational costs, and is easy to
integrate into existing systems. Stripping is performed in a
stripper, wherein the incoming amine absorbent is heated, e.g. by
steam or electricity, to a temperature at which more volatile
components, such as CO.sub.2 in this case, are at least partly
evaporated and leave the stripper via a gas/vapor exit. Stripping
may be performed at atmospheric pressure or at increased or reduced
pressure as necessary. Less volatile components, such as the bulk
amine absorbent in this case, remain in liquid form and leave the
stripper via a liquid exit. Flashing is generally performed in a
flash drum, wherein the incoming amine absorbent undergoes a
reduction in pressure, e.g. by passing through a throttling valve
or other throttling device. More volatile components, such as
CO.sub.2, are at least partly evaporated and leave the flash drum
via a gas/vapor exit. Less volatile components, such as the bulk
amine absorbent in this case, remain in liquid form and leave the
flash drum via a liquid exit. The stripping/flashing step should
result in a reduction of the amount of CO.sub.2 in the amine
absorbent.
[0012] According to aspects illustrated herein, there is provided a
method for the removal of heat stable amine salts from an amine
absorbent used in a carbon dioxide (CO.sub.2) capture process,
comprising:
[0013] withdrawing amine absorbent containing heat stable amine
salts from the CO.sub.2 capture process;
[0014] subjecting the withdrawn amine absorbent containing heat
stable amine salts to a residual CO.sub.2 removal step;
[0015] subjecting the amine absorbent from the residual CO.sub.2
removal step to a separation step to separate heat stable amine
salts from the amine absorbent; and
[0016] returning the amine absorbent having a reduced concentration
of heat stable amine salts to the CO.sub.2 capture process.
[0017] According to embodiments, the residual CO.sub.2 removal step
comprises stripping and/or flashing the withdrawn amine absorbent
to remove residual CO.sub.2.
[0018] According to embodiments, the residual CO.sub.2 removal step
comprises stripping the withdrawn amine absorbent to remove
residual CO.sub.2.
[0019] According to embodiments, the residual CO.sub.2 removal step
comprises flashing the withdrawn amine absorbent to remove residual
CO.sub.2.
[0020] According to embodiments, the residual CO.sub.2 removal step
comprises stripping and then flashing the withdrawn amine absorbent
to remove residual CO.sub.2.
[0021] According to embodiments, the flashing is performed under
near vacuum conditions. By performing flashing at near vacuum
conditions the absorbent may be kept at relatively low temperature.
In addition to saving energy required for heating the absorbent,
this also reduces the exposure of the absorbent to higher
temperatures which could cause further degradation of the
absorbent. The flashing may for example be performed at a pressure
in the range of 0-2 bar gauge.
[0022] The method for the removal of heat stable amine salts from
an amine absorbent is useful in a carbon dioxide (CO.sub.2) capture
process comprising regeneration of the amine absorbent at elevated
temperatures. When performed in such a process, the method for the
removal of heat stable amine salts can be operated with little
additional energy requirement, by withdrawing the slipstream of
amine absorbent from a point in the process where the amine
absorbent has a low CO.sub.2 loading.
[0023] Thus, according to embodiments, the CO.sub.2 capture process
comprises: scrubbing a gas stream comprising CO.sub.2 with an amine
absorbent such that a CO.sub.2 rich amine absorbent is formed;
[0024] regenerating the CO.sub.2 rich amine absorbent by heating it
to separate CO.sub.2 from the amine absorbent, such that a CO.sub.2
lean amine absorbent is formed; and
[0025] recycling regenerated CO.sub.2 lean amine absorbent to the
scrubbing step.
[0026] It has been found that for the purposes of the present
method for the removal of heat stable amine salts, the slipstream
of amine absorbent containing HSS may advantageously be withdrawn
from the lean amine absorbent from the regenerator. More
particularly, the slipstream of amine absorbent may be withdrawn
from the regenerator or from the liquid conduit between the
regenerator and a lean absorbent/rich absorbent heat exchanger. The
lean amine absorbent from the regenerator generally has a
temperature of 100.degree. C. or higher. This allows the thermal
energy provided to the lean amine absorbent in the regenerator to
be utilized in the stripping and/or flashing step. If necessary,
the slipstream of lean amine absorbent containing HSS may also be
withdrawn from the lean absorbent/rich absorbent heat exchanger or
from the liquid conduit between the lean absorbent/rich absorbent
heat exchanger and the CO.sub.2 absorber performing the scrubbing
step. When this is the case, the temperature of the slipstream of
lean amine absorbent containing HSS may have a temperature of less
than 100.degree. C.
[0027] According to embodiments, the withdrawn amine absorbent
containing heat stable amine salts is regenerated CO.sub.2 lean
amine absorbent.
[0028] According to embodiments, the regenerated CO.sub.2 lean
amine absorbent has a temperature of at least 100.degree. C., such
as at least 120.degree. C.
[0029] According to embodiments, the separation step comprises
subjecting the amine absorbent from the residual CO.sub.2 removal
step to electrodialysis and/or ion exchange.
[0030] According to embodiments, the separation step comprises
subjecting the amine absorbent from the residual CO.sub.2 removal
step to electrodialysis.
[0031] According to embodiments, the separation step comprises
subjecting the amine absorbent from the residual CO.sub.2 removal
step to ion exchange.
[0032] According to embodiments, the method further comprises
cooling the amine absorbent from the residual CO.sub.2 removal step
before subjecting it to the separation step.
[0033] According to embodiments, the method further comprises
subjecting the amine absorbent from the residual CO.sub.2 removal
step to indirect heat exchange with the amine absorbent coming from
the separation step.
[0034] According to other aspects illustrated herein, there is
provided a carbon dioxide (CO.sub.2) capture system using an amine
absorbent for absorption of CO.sub.2 from a gas stream, having a
subsystem for the removal of heat stable amine salts from an amine
absorbent, said subsystem comprising:
[0035] a residual CO.sub.2 removal unit in liquid connection with,
and configured to receive, an amine absorbent stream containing
heat stable amine salts from the CO.sub.2 capture system, and
operative for separating residual CO.sub.2 from said amine
absorbent stream; and
[0036] an amine reclaimer in liquid connection with, and configured
to receive, an amine absorbent stream containing heat stable amine
salts and having a reduced concentration of CO.sub.2 from the
residual CO.sub.2 removal unit, and operative for separating heat
stable amine salts from said amine absorbent stream.
[0037] According to embodiments, the residual CO.sub.2 removal unit
comprises a stripper and/or a flash drum.
[0038] According to embodiments, the residual CO.sub.2 removal unit
comprises a stripper.
[0039] According to embodiments, the residual CO.sub.2 removal unit
comprises a flash drum.
[0040] According to embodiments, the residual CO.sub.2 removal unit
comprises a stripper and a flash drum arranged in series.
[0041] The stripper or flash drum provides for inexpensive,
efficient and reliable removal of residual CO.sub.2 from amine
absorbent containing HSS before the absorbent is fed to the amine
reclaimer for separation of heat stable amine salts.
[0042] The use of a stripper or flash drum for removal of residual
CO.sub.2 from an amine absorbent containing HSS is useful in a
carbon dioxide (CO.sub.2) capture system which regenerates the
amine absorbent at elevated temperatures. When used in such a
system, the stripper or flash drum can be operated with little
additional energy requirement, by withdrawing the slipstream of
amine absorbent from a point in the process where the amine
absorbent has a low CO.sub.2 loading.
[0043] According to embodiments, the carbon dioxide (CO.sub.2)
capture system comprises: a CO.sub.2 absorber operative for
scrubbing a gas stream comprising CO.sub.2 with an amine absorbent
such that a CO.sub.2 rich amine absorbent is formed;
[0044] a regenerator operative for regenerating CO.sub.2 rich amine
absorbent by heating it to separate CO.sub.2 from the amine
absorbent, such that a CO.sub.2 lean amine absorbent is formed.
[0045] It has been found that for the purposes of the present
method for the removal of heat stable amine salts, the slipstream
of amine absorbent containing HSS may advantageously be withdrawn
from the lean amine absorbent from the regenerator. More
particularly, the slipstream of amine absorbent may be withdrawn
from the regenerator or from the liquid conduit between the
regenerator and a lean absorbent/rich absorbent heat exchanger. The
lean amine absorbent from the regenerator generally has a
temperature of 100.degree. C. or higher, such as 120.degree. C. or
higher. This allows the thermal energy provided to the lean amine
absorbent in the regenerator to be utilized in the stripping and/or
flashing step. If necessary, the slipstream of lean amine absorbent
containing HSS may also be withdrawn from the lean absorbent/rich
absorbent heat exchanger or from the liquid conduit between the
lean absorbent/rich absorbent heat exchanger and the CO.sub.2
absorber performing the scrubbing step.
[0046] According to embodiments, the residual CO.sub.2 removal unit
is in liquid connection with, and configured to receive, an amine
absorbent stream from the regenerator, and operative for separating
residual CO.sub.2 from the CO.sub.2 lean amine absorbent.
[0047] According to embodiments, the amine reclaimer comprises an
electrodialysis unit or an ion exchange unit.
[0048] According to embodiments, the amine reclaimer comprises an
electrodialysis unit.
[0049] According to embodiments, the amine reclaimer comprises an
ion exchange unit.
[0050] According to embodiments, the subsystem for removal of heat
stable amine salts further comprises an amine absorbent cooler
arranged between the residual CO.sub.2 removal unit and the
reclaimer and operative for cooling the amine absorbent from the
residual CO.sub.2 removal unit before it enters the reclaimer.
[0051] According to embodiments, the subsystem for removal of heat
stable amine salts further comprises an indirect heat exchanger
operative for subjecting the amine absorbent from the residual
CO.sub.2 removal unit to indirect heat exchange with the amine
absorbent coming from the reclaimer.
[0052] The above described and other features are exemplified by
the following figures and detailed description. Further objects and
features of the present invention will be apparent from the
description and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] Referring now to the figures, which are exemplary
embodiments, and wherein the like elements are numbered alike:
[0054] FIG. 1 is a diagram generally depicting an amine based gas
purification system comprising an amine absorbent reclaimer
circuit.
[0055] FIG. 2 is a diagram generally depicting an embodiment of an
amine based gas purification system comprising an amine absorbent
reclaimer circuit.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0056] The term "amine absorbent" or simply "absorbent", as used
herein, refers to a liquid composition comprising at least one
amine compound useful in absorption of CO.sub.2 from gas streams.
Such compositions and suitable amine compounds are well known to a
person skilled in the art. Examples of amine compounds commonly
used in absorption of CO.sub.2 from gas streams include, but are
not limited to, monoethanolamine (MEA), diethanolamine (DEA),
methyldiethanolamine (MDEA), diisopropylamine (DIPA) and
aminoethoxyethanol (diglycolamine) (DGA). The most commonly used
amine compounds in industrial plants are the alkanolamines MEA,
DEA, and MDEA. The absorbent may comprise a single amine compound
or a mixture of two or more amine compounds. In addition, the
absorbent may comprise up to about 90% by volume of water, for
example from about 50 to about 90% by volume of water. The
absorbent may also comprise varying amounts of absorbed CO.sub.2.
Absorbent containing none or only a low concentration of absorbed
CO.sub.2, e.g. following regeneration, is referred to as "CO.sub.2
lean" or simply "lean" absorbent, whereas absorbent containing
higher concentrations of absorbed CO.sub.2, e.g. following
absorption, is referred to as "CO.sub.2 rich" or simply "rich"
absorbent.
[0057] FIG. 1 is a schematic representation of an amine based
carbon dioxide (CO.sub.2) capture system (100). The system
comprises an absorption unit (101) arranged to allow contact
between a gas stream to be purified and one or more wash liquids.
The absorption unit represented in FIG. 1 comprises a CO.sub.2
absorption section (102) and a water wash section (103). Flue gas,
from which CO.sub.2 is to be removed, is fed to the absorption unit
(101) via line (104). In the CO.sub.2 absorption section (102), the
flue gas is contacted with a first wash liquid comprising an amine
compound, e.g. by bubbling the flue gas through said first wash
liquid or by spraying the first wash liquid into the flue gas. The
first wash liquid is fed to the absorption unit (101) via line
(105). In the CO.sub.2 absorption section (102) CO.sub.2 from the
flue gas is absorbed in the first wash liquid. Flue gas depleted of
CO.sub.2 in the CO.sub.2 absorption section then enters the water
wash section (103) of the absorption unit. The water wash section
(103) is arranged to allow contact between the flue gas depleted of
CO.sub.2 from the CO.sub.2 absorption section (102) and a second
wash liquid, which is generally water. The second wash liquid is
fed to the absorption unit via line (106). In the water wash
section, contaminants remaining in the flue gas when it leaves the
CO.sub.2 absorption section are absorbed in the second wash liquid.
Flue gas depleted of CO.sub.2 and contaminants leaves the
absorption unit via line (107). The used first and second wash
liquid containing absorbed CO.sub.2 and contaminants leave the
absorption unit via line (108). The used first and second wash
liquid may be recycled via a regenerator unit (109), wherein
contaminants and CO.sub.2 are separated from the wash water. The
separated CO.sub.2 leaves the system via line (110).
[0058] The used first and second wash liquid to be regenerated
enters the regenerator (109) via line (111). In the regenerator,
the used wash liquids are heated, generally using steam, in a
reboiler (112). The heating causes desorption of absorbed CO.sub.2
from the wash liquids. The desorbed CO.sub.2 then exits the
regenerator via line (113) together with some water vapor also
formed during heating. Regenerated wash liquid, containing a
reduced concentration of CO.sub.2, leaves the regenerator (109) via
line (114). The regenerated wash liquid is also referred to herein
as "CO.sub.2 lean amine absorbent" or simply "lean amine
absorbent". The lean amine absorbent may also contain heat stable
salts (HSS) formed as degradation products in the regenerator as a
result of the exposure to high temperature and/or the presence of
O.sub.2 (absorbed by the absorbent in the absorption unit). The
lean amine absorbent leaving the regenerator may be directed to a
lean absorbent/rich absorbent heat exchanger (123) where it is used
for pre-heating rich amine absorbent from line (108) directed
towards the regenerator (109).
[0059] The amine based carbon dioxide (CO.sub.2) capture system
(100) may further comprise an amine absorbent reclaimer circuit
(115) operative for at least partial removal of HSS from the
circulating amine absorbent, so as to prevent accumulation of HSS
and the problems associated therewith. The amine absorbent
reclaimer circuit (115) is generally configured to withdraw a
slipstream of the main amine absorbent flow. The amine absorbent
reclaimer circuit (115) may preferably be configured to withdraw
the slipstream of lean amine absorbent from a point in the process
where the amine absorbent has a low CO.sub.2 loading, i.e. lean
amine absorbent. More particularly, the slipstream of amine
absorbent may be withdrawn from the regenerator (109) or from the
liquid conduit (114) between the regenerator (109) and a lean
absorbent/rich absorbent heat exchanger (123). The lean amine
absorbent from the regenerator generally has a temperature of
100.degree. C. or higher, such as 120.degree. C. or higher. This
allows the thermal energy provided to the lean amine absorbent in
the regenerator to be utilized in the stripping and/or flashing
step. If necessary, the slipstream of lean amine absorbent
containing HSS may also be withdrawn from the lean absorbent/rich
absorbent heat exchanger (123) or from the liquid conduit (105)
between the lean absorbent/rich absorbent heat exchanger (123) and
the CO.sub.2 absorber (101) performing the scrubbing step. The
slipstream may generally comprise in the range of 0.001-50% by
volume of the main amine absorbent flow, such as in the range of
0.01-10% by volume of the main amine absorbent flow.
[0060] FIG. 2 represents an amine based carbon dioxide (CO.sub.2)
capture system according to the invention, comprising an amine
absorbent reclaimer circuit (115). The amine absorbent reclaimer
circuit (115) is connected to the regenerator side of an amine
based carbon dioxide (CO.sub.2) capture system, e.g. as described
above with reference to FIG. 1.
[0061] The amine absorbent reclaimer circuit (115) comprises an
amine reclaimer (116) for separating heat stable salts from the
amine absorbent. In this embodiment, the amine reclaimer (116) is
an electrodialysis (ED) unit.
[0062] The ED unit is used to transport salt ions, e.g. HSS, from
the amine absorbent through ion-exchange membranes to another
solution under the influence of an applied electric potential
difference. This is done in a configuration called an
electrodialysis cell. The cell consists of a feed (diluate)
compartment and a concentrate (e.g. brine) compartment formed by an
anion exchange membrane and a cation exchange membrane placed
between two electrodes. Multiple electrodialysis cells may be
arranged into a configuration called an electrodialysis stack, with
alternating anion and cation exchange membranes forming the
multiple electrodialysis cells. The ED process results in a
reduction of HSS in the amine absorbent as HSS ions are
concentrated in the concentrate solution.
[0063] In an alternative embodiment, the amine reclaimer (116) is
an ion exchange unit comprising an ion exchange resin suitable for
the removal of HSS ions from the amine absorbent.
[0064] The amine absorbent reclaimer circuit (115) further
comprises a residual CO.sub.2 removal unit (117) arranged upstream
of the amine reclaimer (116) with reference to the lean amine
absorbent stream. In the embodiment of FIG. 2 the residual CO.sub.2
removal unit (117) is a flash drum. Flash (or partial) evaporation
is the partial vaporization that occurs when a saturated liquid
stream undergoes a reduction in pressure by passing through a
throttling valve or other throttling device. If the throttling
valve or device is located at the entry into a pressure vessel so
that the flash evaporation occurs within the vessel, then the
vessel is often referred to as a flash drum.
[0065] The flash drum (117) comprises a pressure vessel having a
feed inlet, a gas outlet and a liquid outlet. The feed inlet is
equipped with a throttling device configured to decrease the
pressure of the feed stream before it enters the pressure vessel.
The exact configuration of flash drums suitable for use in the
system described herein will be readily recognized by a person
skilled in the art.
[0066] The lean amine absorbent enters the flash drum (117) via a
feed line (118). The temperature and pressure of the lean amine
absorbent is determined by the temperature and pressure of the lean
amine absorbent in, or leaving, the regeneration unit (109). The
pressure of the lean amine absorbent may optionally be decreased by
means of a throttling valve or device arranged in the feed inlet of
the flash drum. In the flash drum (117), the pressure is then
reduced, such that more volatile components, e.g. residual
CO.sub.2, at least partially evaporate, while less volatile
components, e.g. amine absorbent and water, remain in liquid phase.
The pressure inside of the flash drum may preferably be low, such
as in the range of 0-2 bar gauge. Evaporated components, e.g.
residual CO.sub.2, leave the flash drum (117) through a gas outlet
via line (119), while liquid components, e.g. amine absorbent and
water, leave the flash drum (117) through a liquid outlet via line
(120).
[0067] In an alternative embodiment, the residual CO.sub.2 removal
unit (117) is a stripper. The stripper may, for example, comprise a
generally cylindrical steel vessel configured to operate within a
pre-determined pressure range. The stripper is preferably equipped
with one or more suitable mass transfer devices, such as valve
trays, sieve trays, structured packing, random packing or other
suitable packing materials, or a combination thereof. A heating
system/device may be provided in the stripper for heating the amine
absorbent. The stripper is preferably configured to provide
sufficient heat to the amine absorbent so that low boiling point
components, for example CO.sub.2, are transferred to a gas phase,
while high boiling point components, for example water and amine,
are collected in a liquid phase at the bottom of the stripper. The
amine absorbent may be heated up appropriately via, for example, a
reboiler. The reboiler may be heated using, for example,
electrically generated heat or steam. The stripper is configured to
discharge the gas phase, containing CO.sub.2, via a gas exit, and
the liquid phase, containing water and amine, via a liquid
exit.
[0068] In yet another alternative embodiment, the residual CO.sub.2
removal unit (117) comprises a stripper and a flash drum arranged
in series, such that a first portion of residual CO.sub.2 may be
removed in the stripper, and a second portion of residual CO.sub.2
may be removed in the flash drum. The stripper and flash drum may
be as described above. The lean amine absorbent first enters the
stripper, where it is heated to a temperature sufficient to
transfer low boiling point components, for example CO.sub.2, to a
gas phase, while high boiling point components, for example water
and amine, are collected in a liquid phase at the bottom of the
stripper. The liquid phase is then forwarded to the flash drum,
where the pressure is reduced so that more volatile components,
e.g. residual CO.sub.2, at least partially evaporate, while less
volatile components, e.g. amine absorbent and water, remain in
liquid phase. The liquid components, e.g. amine absorbent and
water, leave the flash drum through a liquid outlet and is
forwarded to the reclaimer.
[0069] Referring now to FIG. 2, the lean amine absorbent, from
which residual CO.sub.2 has been at least partially removed, is
forwarded via line (120) to the amine reclaimer (116), wherein heat
stable salts are at least partially separated from the amine
absorbent to produce a lean amine absorbent depleted in HSS.
[0070] Optionally, the amine absorbent reclaimer circuit (115)
further comprises a cooler (121) arranged between the residual
CO.sub.2 removal unit (117) and amine reclaimer, and configured to
adjust the temperature of the lean amine absorbent from the
residual CO.sub.2 removal unit before it enters the amine reclaimer
(116).
[0071] Furthermore, an amine absorbent reclaimer circuit (115)
comprising a cooler (121), may optionally further comprise an
indirect heat exchanger (not shown) arranged between the residual
CO.sub.2 removal unit (117) and the cooler (121) and configured to
cool the lean amine absorbent from the residual CO.sub.2 removal
unit (117) using the lean amine absorbent depleted in HSS leaving
the amine reclaimer (116). The indirect heat exchanger may for
example be a conventional plate or shell and tube type heat
exchanger.
[0072] The lean amine absorbent depleted in HSS leaves the amine
reclaimer (116) and is forwarded via return line (122) back to the
CO.sub.2 capture system (100). The lean amine absorbent depleted in
HSS may, for example be reintroduced into the regenerator (109),
absorber (101), or into a suitable liquid conduit connecting the
regenerator (109) and absorber (101). The position for
reintroduction of the lean amine absorbent from the amine reclaimer
circuit may be selected depending on the specific temperature and
pressure of the absorbent. One suitable position for
reintroduction, as shown in FIG. 2, would be into line (114),
either upstream or downstream of a lean absorbent/rich absorbent
heat exchanger (123). The separated heat stable salts leave the
amine reclaimer via line (124).
Example
Amine Loss into the Waste Brine Stream of the Electrodialysis
Unit
[0073] Amine losses from a lean amine absorbent into the waste
brine of a 3-loop ElectroSep electrodialysis unit (ElectroSep Inc.,
USA) was evaluated with various CO.sub.2 loadings in the lean amine
absorbent. 1.2 and 1.4 wt % amine was observed in the waste brine
stream when the lean amine absorbent had CO.sub.2 loadings of 2.2
and 2.9 wt % CO2, respectively. Only 0.3 wt % amine was observed in
the waste brine stream with 0.01 wt % CO.sub.2 in the lean amine
absorbent. This represents a 75-80% reduction in amine losses
compared to the higher lean loadings. This example shows that a
significant reduction of amine losses can be achieved by reduction
of the CO.sub.2 loading of the lean amine absorbent, e.g. by
stipping or flashing, prior to feeding it to a reclaimer unit, such
as a electrodialysis unit.
[0074] While the invention has been described with reference to a
number of preferred embodiments, it will be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted for elements thereof without departing from the
scope of the invention. In addition, many modifications may be made
to adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiments disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended claims.
Moreover, the use of the terms first, second, etc. do not denote
any order or importance, but rather the terms first, second, etc.
are used to distinguish one element from another.
* * * * *