Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED PROCESS FOR THE RECOVERY
OF CO2 FROM FLUE C.ASES
The supply of carbon dioxide from natural
sources, by-product CO2 from ammonia manufacture,
and hydrogen purification, is not sufficient for
pr~sent and future industrial requirements.
S The potential supply of CO2 from power
plant flue gas could furnish the re~uired amount,
providing it could be economically recovered. Flue
gas normally is at or near atmospheric pressure and
contains about 6-10 percent CO2 and about 2-5 percent
oxygen. Sulfur dioxide may be an additional con-
taminant if the fuel source is coal instead of "sweet"
or commercial natural gas.
Most known solvents that can recover CO2
under these conditions will under~o severe solution
o~idative degradation and cause corrosion, thus
rendering the process uneconomical.
The removal of carbon dioxide from flue
gas was practiced in the 1950's and early 1960's by
extracting the carbon dioxide from the combustion
products resulting from burning a fuel. Inert
~ .~
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atmospheres fox large annealing furnaces were pro-
duced in a like manner.
The principal solvent used in the removal of
CO2 from flue gas during this same period employed an
aqueous monoethanolamine (MEA) solution in the concen-
tration range of 5~12 percent. The system was operated
until oxidative degradation products and corrosion became
sufficiently severe as to warrant discarding the solu-
tion wi.th plant cleanup and recharging with fresh
10 solution.
Some processes were improved, vis-a-vis
using only th~ dilute ~EA solution until it went bad
by operating a side stream reclamation still.
Such still removed some of the oxidative degradation
products as a bottom product while taking substan-
tially the MEA and water as an overhead product for
recycle. The side stream still operated on a 2-3
percent side stream. This approach was not partic-
ularly successful because the degradation products
were removed only to a limited extent in the side
stream reclaimer. In addition, degradation products
continued to be produced at a higher rate due to the
high temperatures necessary for operating the
reclaimer still.
Another mode of operation of the dilute
solution process was the utilization of a 5-8 percent
aqueous MEA solution with a 4-8 percent concentration
of sodium carhonate. Sodium carbonate neutralized
degradation products that were acidic in nature
Iformic acid is the prime oxidation product in this
environment). This mode o operation was somewhat
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successful but like the other two mentioned systems,
was unpredictable in the length of time the system
would operate before losing capacity to recover 2'
All the processes mentioned above were
extremely energy intensive due to the lextremely
high circula-tion rates necessitated by the low
concentration of MEA and the very low loadings of
Co2 that were permitted in order to minimize
corrosion.
The recovery of CO2 from a flue gas using
a combustion zone to lower residual oxygen is des-
cxibed in U.S. Patent 4,364,915, dated 1982 December 21.
~ m~de of operation utili~ing copper salts
as an inhibitor is disclosed in U.S. Patent 2,377,966,
dated 1945 June 12. This method was used in the abo~e
mentioned systems that did not include the use of a
reclaimer in the operation. Copper was only moderately
successful as a corrosion inhibitor even at the low CO2
loadings and low concentrations of alkanolamine.
Precipitation of elemental copper was a serious limita-
tion of this pro~.ess and resulted in enhanced corrosion
due to galvanic attack in the peripheral area o~ the
deposited copper metal. This system was operated much
the same way that the uninhibited aqueous MEA solution
first mentioned was utilized, in that when the system
became sufficiently degraded the entire solution was
dumped, the internals of the plant cleaned, fresh
alkanolamine charged back to the system, and the system
put back in service. The length of time the system
remained on stream was again unpredictable.
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The use of activated carbon or ion exchange
resin to remove contaminates from aqueous alkanolamine
solutions is known from U.S. Patents 1,944,122;
2,797,188; 3,568,405; and 4,287,161. However, the~e
patents do not suggest the sup.rising results obtained
herein using an efective amount of copper salts in the
alkanolamine solution in conjunction with the use of
activated carbon or ion exchange resin.
In accordance with the present invention, gas
containing carbon dioxide and oxygen is contacted in
the conventional manner in a suitable gas-liquid
contactor with an alkanolamine solution. The above
alkanolamine solution contains an amount of coppex
effective to inhibit corrosion. The actual amount of
copper used can be any amount of copper greater than
about 5 parts of copper per million parts of solution
wherein the carbon dioxide and, if present, sulfur
containing acid gases (e.g. SO2 with trace amounts of
oth~r sulfur compounds, H2S, COS, and the like) are
absorbed.
Conventionally liquid effluent (rich solvent)
is withdrawn from the bottom of the contactor and cross
exchanged with solvent which has been heated to release
the carbon dioxide and sulfur containing gases (lean
solvent). The rich solvent after heat exchange wikh
the lean solvent is delivered to a stripper wherein the
rich solvent is contacted with rising vapors rom the
lower end of the stripper. The liguid in the lower end
of the stripper is circulated through a reboiler wherein
conventionally it is heated to about 240 to 260F
(115 to 126.5C) and returned to the lower portion of
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the stripper or reboiler surge tank. A portion of the
bottoms drawn off the stripper or reboiler surge tank
is then returned ~o the absorption column.
The present invention involves treating all
or a portion of the alkanolamine solution at any tempera-
ture, however it typically consists of passing a solution,
cool rich or cool lean (conveniently the lean solution
after heat exchange with the rich solution from the
contactor), into and through a mechanical filter, into
and through activated carbon, and into and through a
second mechanical filter. Following this treatment,
the carbon/filtered treated solution can be passed
throu~h an ion exchange resin bed thence to the top of
t.he contactor.
The above procedure surprisingly effectively
removes ionic iron and solvent degradation products.
This allows sufficient ionic copper in solution to
abate corrosion, minimizes the formation of degradation
products, and maintains substantially the efficiency of
the alkanolamine solution.
It is to be understood that while the above
preferred mode of operation includes the activated
carbon treatment, mechanical filtration and ion exchange
treatment, some improvement, eOg. lower corrosivity
and/or degradative quali y of solvent, can be achieved
if only one of the unit operations is employed in
treating the solvent. Thus, under certain operating
conditions, activated carbon treatment can remove
certain of the degxadation products both by adsorption
and/or absorption and its inherent filtering effects
such as mechanical removal of particulate material to
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obtain some improvement. It however has been found
advantageous to couple mechanical filtration both
before and after activated carbon treatment to extend
the life of the carbon bed and collect the insoluble
S iron. Ion exchange treatment may also be employed to
remove some of the degradation products, with or without
either mechanical filtrations or activated carbon
treatment, but the bed must be cleaned more often to
avoid plugging with insoluble iron or other solid
degradation products. Here again, mechanical fil-
tration is preferred to keep at a low level the
insoluble iron and/or solid degradation products from
plugging the bed. Likewise, the use of one or both
filtration mediums as the only treatment will improve
the operation of the process but not to the same degree
as operating on the three unit operations, i.e.
mechanical filtration, activated carbon treatment, and
ion exchange.
The present invention is illustrated by
Figure 1 which is a schematic diagram of a typical
co~nercial operation showing the association of the
contactor 16 with the stripper 74, the acti~ated carbon
bed 48, mechanical filters 42 and 44, and ion e~change
bed 34.
In Figure 1 of the drawings lO represents an
inlet line for the flue gas to be treated. A knock-out
drum 12 with drain line 32 is provided to collect
liquid condensates. From the drum 12, line 14 leads
the flue gases into absorber 16 which has a plurality
of trays 18 and a demister 20.
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The effluent gases from the absorber 16 are
led by line 22 optionally to a condensor ~4 and to
outlet line 26.
Recirculating alkanolamine solution is led by
line 36 into the absorber 16 and the rich amine solution
i.e., amine containing absorbed CO2, leaves the absorber
by exit line 30 and thence to the inlet of pump 68.
From the pump outlet 66, the rich amine solution flows
through the crossexchanger 58 and line 64 to the inlet
of the stripper 74 wherein the rich amine is heated and
stripped of carbon dioxide. The CO2 is removed by
outlet 78 where it flows through a condensor 80 and
then by line 82 to a condensate collector 86. The pur
C0z gas is removed by line 84 and the condensate is
removed by line 88 for r~use by passing it through a
pump 90 and line 94 back to the stripper 74.
At the bottom of the stipper 74, there is
provided an outlet line 112 which leads the alkanol-
amine solution to the inlet of the reboiler 100. The
heated solution recirculates back to the stipper by
line 98. Steam (here described but other sources of
heat may be used) lines 102 and 104 provide an inlet
and an outlet for the steam to heat the reboiler 100.
The heated lean alkanolamine solution leaves
the reboiler 100 by line 108 where the solution is
recirculated by pump 70 and the associated lines 120
and 72 to the heat exchanger 58. A portion of the lean
alkanolamine solution can be withdrawn by line 114 and
oxidized with an oxygen containing gas such as air in
the oxidizing unit 116. Line 118 is provided to return
the oxidized solution back to the main line 120.
52~
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Oxidizing gases are provided by inlet 92 and the used
gases are removed by an outlet (not shown).
From the heat exchanger 58, the alkanolamine
solution flows by line 56 to an amine cooler 54 and
thence by line 40 to a cartridge filter 42 for removal
of fine particulates. From the filter 42 the solution
goes by line 46 to an activated carbon bed 48 and
thence by line 50 to a second cartridge filter 44 for
the removal of carbon fines.
Line 52 is provided to lead the solution back
to the absorber by line 36. If desired, part or all of
the solution can be passed by line 33 to a ion exchange
bed 34 for further purification of the solution prior
to reuse. It i5 to be understood that in the above
description the necessary valves and controls have not
been illustrated in order to clearly point out the
invention. It is also understood that some of the
solution will by-pass the filtration/purification
section through line 38.
With a brief description of the unit opera-
tions which constitute the present invention, the
limits of oparating parameters are now set forth.
Inhibitor -- The inhibitor of choice for this
particular system is ionic copper introduced as any
salk soluble in the alkanolamine solution in a concen-
tration greater than about 5 ppm by weight based on the
total solution. The preferred soluble salt is copper
carbonate. The preferred range is betwelen about 50 ppm
and about 750 ppm and the most preferred range is
5ZZ
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between about 100 ppm and about 500 ppm copper, however
-it is not implied that greater concentrations of copper
are not effective since concentrations in excess of
2000 ppm have been successfully used. It has been
show~ by both laboratory data and pilot plant data that
passivation of the corrosion process can be achieved
and maintained even in concentrations less than 50 ppm.
Likewise, it has been established that between about
5-80 percent concentration of MEA can be effectively
inhibited against corrosion by maintaining a proper
level of copper in the treating solution.
Alkanolamine concentration -- from about S to
about 80 percent solutions o alkanolamine may be
employed with reduced corrosion and reduced solvent
degradation resulting in improved life of solvent, that
is, longer periods between turnarounds or unscheduled
down times to replace the solvent. Primary, secondary,
and tertiary alkanolamines, or mixtures thereof, may be
employed. The preferred alkanolamine being monoethanol-
amine from about 25 to about 50 percent by weight. Ithas been found from pilot plant data that the incorpora-
tion of the present inv~ntion results in little or no
downtime occasioned by corrosion and/or necessity to
replace the solvent.
Temperature control -~ It has been found that
the reduction of active copper ion content, in for
example, monoethanolamine, is greatly accelerated above
150F (65.6C) and that reboiler bulk temperatures of
from 240F (116C) to 260F ~127C) and above are
conducive to excessive reduction of copper particularly
increased residence times. It is preferable to
maintain the reboiler bulk temperature at or below
L2522
-10-
240F (116C) to 260F (127C). Also it is desirable
to employ a maximum heat transfer flux of less than
about 10,000 BTU per square foot hour (31.5 kW/m ) and
preferably less than about 6,000 BTU per square foot
hour (18.9 kW/m ). Higher heat flux and/or residence
times will, of course, function but will contribute
to a higher rate of copper depletion and thus loss of
operability of the overall system.
Contact Pressure -- In accordance with the
present invention flue gas will be contacted with the
alkanolamine at about atmospheric pressure. ~owever,
the invention is applicable to higher pressures, limited
only by the condensation pressure of the gas mixture
being processed.
Mechanical Filter/Activated Carbon Treater --
The judicious use of activated carbon coupled with
mechanical filtration will remove harmful contaminants
resulting fro~ thermal oxidation of alkanolamine,
auto-oxidation of alkanolamine, and corrosion of the
plant equipment. The activated carbon treaters in
conjunction with mechanical filters are utilized for
the passage of alkanolamine solution through first a
mechanical filter operating in for example the 10-75
micrometer range, preferably in a~out the 25-50 micro-
meter range for protection of the activated carbontreater which is located immediately downstream. The
activated carbon treater will operate to some extent
on any of a variety of activated carbons, however, it
has be0n found that the most efficient removal for a
broad range of degradation products and capacity
coupled with longevity of the activated carbon rests
with the coal based activated carbons. Allowable bed
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pressure drop usually determines carbon particle size.
A preferred size is in the 12-40 mesh range ~openings =
A 1.68 mm - 0.420 mm) such as Calgon F-400 or its
equivalent.
The carbon treatment removes certain of the
degradative products of the alkanolamine which are
suspected to be strong iron chelators. Examples of
these products are higher molecular weight organic
acids. It is reported that these acids are produced
from formic acid, generated as a degradation product of
the alkanolamines, and oxalic acid which is the further
degradation product of formic acid and formates. The
primary function of the mechanical filter down stream
o the activated carbon bed is to recover insoluble
lS iron and other particulate material that may be
released during the activated carbon funckion. The
pore openings may range from 1 to 50 micrometers
with the preferred range being between 5 to 25
micrometers. A secondary function is to collect
activated carbon fines thus protecting downstream
equipment.
To illustrate the significance of adequate
solution filtration a pilot plant was operated with and
without filtration whlle measuring -the amount of copper
and iron in solution. At temperatures sufficient to
strip the solution of C02 and while the solution was
being filtered, the concentration of soluble iron was
maintained at low enough concentrations to pxevent
rapid redox with the copper in solution. When the
solution was not filtered or when the filter medium,
activated carbon, was spent, the soluble iron concen-
tration increased and the soluble copper concentration
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rapidly decreased until no copper remained in the
solution which was followed by the occurrance of
corrosion. In the absence of mechanical filtration the
carbon itself caught particulate matter and insoluble
iron salts which diminished the number of active sites
and reduced the overall efficiency of the filtration
process. In addition, insoluble iron which was not
removed from the system accelerated the rate of soluble
iron buildup as the activated carbon began to loose
efficiency or become spe~t. This experiment established
the practical necessity, in an economically compeditive
operation, to carbon filter the solution in order to
maintain low iron levels and to mechanical filter the
solution in order to increase carbon life and minimize
the potential for rapid copper redox as the carbon
began to loose efficiency.
The solvent stream is activated carbon
treated and filtered full flow or as a partial side
stream utilizing 0.025 bed volume per minute to 1 bed
volume per minute. The preferred rate is 0.1-0.2 bed
volume per minute. The present invention likewise
has been surprisingly improved by minimizing both
activated carbon bed and solvent temperatures to a
150F (65.6C) maximum. Operation in this mode
improves the capacity and improves the selectivity
for particular degradation species. Due to the
relatively low temperature re~uirements for most
efficient operation, it is advantageous to place
the activated carbon treater and mechanical filters
downstream of the amine cooler just prior to intro-
duction of the lean solution to the absorber.
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~2~:~522
Ion Exchange -- Heat stable salts of a
number of varieties and from a number of sources are
continually produced and/or inadvertently added to
alkanolamine systems, especially those processing
oxygen containing gas streams. The majority of these
salts such as, for example, sodium chloride, amine
oxalate, and sodium nitrate are of a t~pe which are not
effectively removed by activated carbon and/or mechanical
filtration. However, the fact that these salts promote
both solvent degradation and inhibitor reduction makes
it necessary to remove them fxom solution. There are
two methods of doing this. The known method is solvent
reclamation by distillation. This method is not
recommended as it depletes the inhibitor level (Cu is
not carried over in the distillation process) and
unless controlled very carefully can cause increased
solvent degradation. The present invention preferably
utilizes ion exchange to remove the anionic portion of
the heat stable salt. This is accomplished by passage
of the contaminated sol~ent through any of the number
of strong base anion exchange resins of the styrene-
divinylbenzene type which have a quaternary amine as
their functional group, i.e. DOWEX* 1, DOWEX* 2, DOWEX*
MSA-l, DOWEX* MSA-2 (*Trademark of The Dow Chemical
Company). The anions present in solution displace the
hydroxide groups present on the resin and are removed
from solution. After the resin is spent ~its exchange
capacity fully utilized) the resin may be discarded or
regenerated with a sodium hydroxide solution of essen-
tially any concentrat~on. The preferred concentrationbeing 2-5N. The regeneration effluent, containing the
unwanted salts, is then discarded and the resin ready
for reuse.
~ -13-
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Exemplary of such ion exchange treatment was
the treatment of 100 ml. of a foul 30 percent MEA
solution from the plant which had 300 ppm copper
inhibitor and which was carbon treated. The solution
was treated by passing it downflow through a 25 ml
packed column of DOWEX*1 (OH form) (*Trademark of
The Vow Chemical Company) at 5 cm3 and 78F (25.6C~.
After discarding the hold-up volume of water, the
alkanolamine solution was collected and a sample of
both the starting material and resin bed effluent were
analyzed for heat stable salt content.
Sample ~ Heat Stable Salt
Starting Solution 2.4
Resin Effluent 1.8
Net one pass removal 25%
There was substantially no loss of copper as
a result of the ion exchange treatment.
Inhibitor Regeneration -- Regeneration of
inhibitor is not normally required as long as the
conditions taught by this invention are followed
specifically. However, if by improper plant design or
non-adherance to the conditions set forth herein,
copper metal or copper compounds are formed by the
reduction of the copper, this inhibitor exhibits the
surpising capability of regenerability. There can be
provided a ~idestream withdrawal of a portion of the
solution of the bottom of the reboilPr, going through
an external cooler to drop the temperature of the hot
lean alkanolamine containing particulate matter (which
contains the reduced inhibitor) down to a temperature
less than 150F, preferably 130F or less into a tank
~ -14-
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or suitable vessel as shown in Figure 1 in which the
solution is aerated with an oxygen-containing gas by a
variety of means common to those skilled in the art.
The lean solution thus cooled and with the inhibitor
regenerated it may be returned back to the lean solution
downstream of the heat exchanger or any other advan-
tageous spot in the lean circult. -
~ 15-
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