Note: Descriptions are shown in the official language in which they were submitted.
CA 02831463 2013-09-26
1
RETENTION OF AMINES IN THE REMOVAL OF ACID GASES BY MEANS OF AMINE
ABSORBENTS
Description
The present invention relates to a method for removing acid gases from a fluid
stream, e.g.
for removing carbon dioxide from flue gases.
In numerous processes in the chemical industry, fluid streams occur which
contain acid
gases such as, e.g., CO2, H2S, S02, CS2, HCN, COS or mercaptans. These fluid
streams
can be, for example, gas streams such as natural gas, refinery gas, synthesis
gas, flue
gases, or reaction gases formed in the composting of waste materials
comprising organic
substances. The removal of the acid gases from these fluid streams is
desirable for various
reasons.
The removal of carbon dioxide from flue gases serves, in particular, to reduce
the emission
of carbon dioxide, which is considered to be the main cause of what is termed
the
greenhouse effect.
Synthesis gas comprises substantially carbon monoxide and hydrogen. Synthesis
gas is
generally produced by partial oxidation or steam reforming of hydrocarbons.
The crude
synthesis gas comprises acid gases such as carbon dioxide, hydrogen sulfide,
or carbonyl
sulfide, which must be removed.
The content of acid gases in natural gas is reduced by suitable processing
measures directly
at the natural gas well, since these acid gases, in the water frequently
entrained by the
natural gas, form acids which are corrosive.
On the industrial scale, aqueous solutions of organic bases, e.g. amines, such
as, in
particular, alkanolamines, are frequently used as absorption medium for
removing acid
gases, such as carbon dioxide, from fluid streams. On the dissolution of acid
gases, ionic
products are formed from the base and the acid gas components. The absorption
medium
can be regenerated by warming, expanding to a lower pressure, or stripping,
wherein the
ionic products react back to form acid gases and/or the acid gases are
stripped off by means
of steam. After the regeneration process, the absorption medium can be reused.
CA 02831463 2013-09-26
2
However, the amines have a vapor pressure which is not negligible. Therefore,
the fluid
stream freed from acid gases comprises traces of amines. Contamination of the
treated fluid
stream is undesirable for various reasons. For instance, it is disadvantageous
when traces of
amines escape into the environment together with the treated flue gas.
Synthesis gas is the starting material of further catalytic reactions. Amine
traces can act here
as catalyst poison.
The content of amines in natural gas or liquefied petroleum gas (LPG) produced
therefrom
by liquefaction can likewise be subject to restrictions.
It has been proposed in the prior art to scrub the treated fluid stream with
an aqueous phase
in order to transfer entrained amine at least in part into the aqueous phase.
EP 0 798 029 A2 discloses a method in which a gas is treated with a basic
amine compound
for absorbing carbon dioxide, and the treated gas is then contacted at 20 to
60 C with an
aqueous phase in order to transfer entrained basic amine at least in part into
the aqueous
phase. The aqueous phase should preferably be condensate which is condensed
out of the
carbon dioxide which is released in the regeneration tower.
US 2008/0159937 comprises a method for removing carbon dioxide from a gas
stream, in
which the gas stream that is depleted in carbon dioxide is scrubbed with water
in a packed
section of the absorption tower. The water can be condensate from the top of
the
regeneration column, or fresh water for making up lost amounts.
The use of condensate which is condensed out of the carbon dioxide which is
released in the
regeneration tower as scrubbing water has the advantage that the water balance
of the
absorption medium circuit is not impaired. On the other hand, the condensate
is only
available in a limited amount. Fresh water can be used as scrubbing water only
in a
restricted amount in order not to dilute the absorption medium and accumulate
water in the
method. The amount of fresh water to be added results from the difference
between the
water content of the incoming fluid stream and the exiting streams. For the
configuration
shown in fig. 1, for example, the incoming stream 1 and the two exiting
streams 21 and 25
enter into the water balance. The fresh water required to make up lost amounts
is termed
make-up water (stream 11 in fig. 1). The exiting streams are saturated with
water vapor. The
CA 02831463 2013-09-26
3
water content of the incoming fluid stream depends on various conditions. For
a water-
saturated incoming fluid stream, the makeup stream available decreases with
decreasing
pressure difference between the absorption and the regeneration. Consequently,
in particular
in the case of flue gas scrubbers in which the absorption pressure is close to
atmospheric
pressure, the amount of make-up water available is restricted.
In order, in the case of a restricted amount of scrubbing water, nevertheless
to achieve an
adequate scrubbing action, it has been proposed to conduct the scrubbing water
through the
scrubbing zone not in a single pass, but to circulate the scrubbing water by
pumping, or to
recycle it, i.e. to collect it below the scrubbing zone and reapply it above
the scrubbing zone.
Optionally, the scrubbing water in this case can be conducted via an
additional cooler. By
means of the cooling, water condenses out of the treated fluid stream. In
order to avoid
accumulation in the scrubbing water circuit of absorption medium components
that have
been scrubbed out, a subquantity of the scrubbing water is discharged and
replaced by
make-up water. The water discharged from the scrubbing water circuit is
customarily passed
into the absorption medium circuit.
In Satish Reddy et al., Fluor's Econamine FG Plussm Technology, presented at
the Second
National Conference on Carbon Sequestration, National Energy Technology
Department of
Energy, Alexandria VA, USA, May 5-8, 2003, a typical embodiment of a gas
scrubbing
process having a scrubbing zone with scrubbing water circulated by pumping is
described.
By means of the recycling and optional cooling of the scrubbing water, the
scrubbing action
can be increased. Customary volume ratios of recycled scrubbing water and make-
up water
are between 10 and 500. However, owing to the recycling, a backmixing of the
scrubbing
water occurs. At very high volume ratios of recycled scrubbing water and make-
up water, in
the scrubbing zone only the action of at most one theoretical separation plate
can be
achieved, independently of the length of the contact section in the scrubbing
zone.
WO 2010/102877 describes a method in which the gas stream that is depleted in
carbon
dioxide is scrubbed with an acidic aqueous solution in order to decrease the
amount of
amines and basic degradation products present therein. Between the carbon
dioxide
absorption zone and the acidic scrubbing, a scrubbing with water can be
provided.
CA 02831463 2013-09-26
4
The object of the present invention is to provide a method for removing acid
gases from fluid
streams, in particular for removing carbon dioxide from flue gases, which
permits a more
efficient retention of amines from the treated fluid streams.
The invention provides a method for removing acid gases from a fluid stream,
which
comprises
a) treating the fluid stream in an absorption zone with an absorption
medium which
comprises an aqueous solution of at least one amine,
b) conducting the treated fluid stream through at least two scrubbing zones
and treating it
with a non-acidic aqueous phase in order to transfer entrained amine and/or
entrained
amine decomposition products at least in part to the aqueous phase, wherein
aqueous
phase is recycled via at least one scrubbing zone and aqueous phase is
conducted
through at least one scrubbing zone without recycling.
The treated fluid stream is treated with a non-acidic liquid aqueous phase in
order to transfer
entrained amine and/or entrained amine decomposition products at least in part
into the
aqueous phase. In other words, the amine which is passed over into the fluid
stream is
scrubbed out again therefrom. During the scrubbing, amine decomposition
products such as
nitrosamines or ammonia are also scrubbed out of the treated fluid stream.
As non-acidic aqueous phase, in particular water itself is suitable. Of
course, when the
method is carried out, other components, such as amine and amine decomposition
products,
can pass over into the aqueous phase, and so the non-acidic aqueous phase
generally
comprises components that are more or less different from water. The non-
acidic aqueous
phase is neutral or, owing to entrained amine components, slightly basic.
Generally, the pH
of the non-acidic aqueous phase is 7 to 11, preferably 8 to 10.
In the scrubbing zones, the non-acidic aqueous phase is conducted in
counterflow to the
treated fluid stream. Preferably, the scrubbing zones have packed beds,
ordered packings
and/or trays, in order to intensify the contact between the fluid stream and
the non-acidic
aqueous phase. The non-acidic aqueous phase can be distributed over the cross
section of
the scrubbing zone above the scrubbing zone by suitable liquid distributors.
CA 02831463 2013-09-26
The absorption zone is considered to be the section of an absorption column in
which the
fluid stream comes into mass-transfer contact with the absorption medium.
In preferred embodiments, at least one scrubbing zone is formed as a section
of the
5 absorption column arranged above the absorption zone. Preferably, all the
scrubbing zones
are constructed as sections of the absorption column arranged above the
absorption zone for
treatment with the non-acidic aqueous phase. The scrubbing zones for this
purpose are a
section of the absorption column above the feed of the absorption medium
constructed as a
back-wash section or enrichment part.
In other embodiments, at least one scrubbing zone is arranged in a scrubbing
column
different from the absorption column, e.g. a packed-bed column, ordered
packing column,
and tray column, in which the treated fluid stream is scrubbed with the non-
acidic aqueous
phase.
According to the invention a non-acidic aqueous phase is recycled via at least
one scrubbing
zone. The non-acidic aqueous phase for this purpose is collected beneath the
scrubbing
zone, e.g. by means of a suitable collecting tray, and pumped via a pump to
the upper end of
the scrubbing zone. In preferred embodiments, the recycled non-acidic aqueous
phase is
cooled, preferably to a temperature of 20 to 70 C, in particular 30 to 60 C.
For this purpose
the non-acidic aqueous phase is expediently pumped in circulation via a
cooler. In order to
prevent accumulation in the scrubbing water circuit of absorption medium
components that
have been scrubbed out, expediently a substream of the non-acidic aqueous
phase is
discharged and replaced by feed water.
Non-acidic aqueous phase is conducted through at least one scrubbing zone
without
recycling, i.e. the non-acidic aqueous phase passes through the scrubbing zone
in a single
passage in counterflow to the treated fluid stream.
In a preferred embodiment, a first scrubbing zone is arranged above the
absorption zone and
a second scrubbing zone is arranged above the first scrubbing zone, wherein
aqueous phase
is recycled via the second scrubbing zone, a substream of the non-acidic
aqueous phase
recycled via the second scrubbing zone is conducted through the first
scrubbing zone and
non-acidic aqueous phase draining from the first scrubbing zone is combined
with the
absorption medium in the absorption zone. In preferred embodiments, the non-
acidic
aqueous phase recycled via the second scrubbing zone is cooled, preferably to
a
CA 02831463 2013-09-26
6
temperature of 20 to 70 C, in particular 30 to 60 C. For this purpose, the non-
acidic aqueous
phase is expediently pumped in circulation via a cooler. Expediently, feed
water is fed into
the second scrubbing zone in order to replace the volume of the substream.
In another embodiment, a first scrubbing zone is arranged above the absorption
zone and a
second scrubbing zone is arranged above the first scrubbing zone, wherein non-
acidic
aqueous phase is recycled via the first scrubbing zone, feed water is applied
to the second
scrubbing zone, the feed water is conducted through the second scrubbing zone
and
combined with the non-acidic aqueous phase in the first scrubbing zone.
Expediently, a
substream of the recycled non-acidic aqueous phase is removed and the
substream is
preferably passed into the absorption medium circuit. This embodiment is
generally less
preferred, since the second scrubbing zone is operated exclusively with feed
water and the
amount of aqueous phase which is hydraulically necessary can fall below that
which is
required for a functioning scrubbing, e.g. for wetting the internals in the
scrubbing zone.
The feed water can comprise at least in part condensate which is condensed out
of the acid
gases released in the regeneration, e.g. in a stripper. The use of this
condensate as feed
water has the advantage that the water balance of the absorption medium
circuit is not
impaired. On the other hand, the condensate, depending on the conditions in
the stripper,
can comprise amines from the absorption medium, and so the scrubbing action of
the
condensate is restricted and very low amine concentrations cannot be achieved
in the
treated fluid stream. The (exclusive) use of the condensate as feed water is
therefore not
generally preferred.
Preferably, the feed water comprises, at least in part, fresh water. Fresh
water is considered
to be water, e.g. superheated steam condensate, which comprises no significant
amounts of
amine, amine decomposition products or other absorption medium components.
Preferably,
the amount of fresh water substantially corresponds to the amount of water
lost from the
absorption medium circuit (make-up water) in order not to impair the water
balance of the
absorption medium circuit and to prevent accumulation of water.
The scrubbing according to the invention of the treated fluid stream permits
the removal of
the majority of the entrained amine and/or entrained amine decomposition
products. A more
substantial purification of the treated fluid stream for removing the last
traces of entrained
amine and/or for removing basic amine decomposition products having a high
vapor
pressure succeeds in one embodiment of the method according to the invention
in which the
CA 02831463 2013-09-26
7
treated fluid stream is then scrubbed with an acidic aqueous solution. For
this purpose, the
treated fluid stream can be conducted through a scrubbing zone, preferably in
a scrubbing
column, e.g. a packed-bed, ordered packing and tray column, via which the
acidic aqueous
solution is recycled. The acid protonates the entrained amine traces or amine
decomposition
products and in this manner drastically decreases the vapor pressure thereof.
The
protonated compounds, owing to the salt-like character thereof, are readily
transferred into
the aqueous phase.
Suitable acids are inorganic or organic acids, such as sulfuric acid,
sulfurous acid,
phosphoric acid, nitric acid, acetic acid, formic acid, carbonic acid, citric
acid and the like.
Preferably, the acid used has a pKs from -4 to 7. The preferred pH of the
acidic aqueous
solution is 3 to 7, in particular 4 to 6.
As acidic aqueous solution, in particular acidic process waters are suitable.
Such acidic
process waters occur, in particular, in the treatment of sulfur-dioxide-
comprising gases, e.g.
in an S02-prepurification stage. For instance, in the cooling or prescrubbing
of sulfur-dioxide-
comprising gases, an acidic condensate is obtained which can be used as acidic
process
water.
By means of the absorption of amines and/or amine decomposition products into
the acidic
aqueous solution, the concentration of the amines and/or amine decomposition
products in
the acidic aqueous solution increases. In order to avoid excessive
concentrations of
dissolved salts in the acidic aqueous solution, expediently a substream of the
acidic aqueous
solution is discharged and this is replaced by fresh acidic aqueous solution.
The amines or
amine decomposition products, such as ammonia, can be at least partially
recovered from
the discharged substream. For this purpose, the discharged aqueous solution is
treated with
an alkali, e.g. sodium hydroxide, wherein the amines or amine decomposition
products are
released.
Alternatively, the discharged aqueous solution can be discarded or fed to a
waste water
treatment.
Before the treatment with the absorption medium, the fluid stream, e.g. flue
gas, is subjected,
preferably to a scrubbing with an aqueous liquid, in particular with water, in
order to cool the
fluid stream and to moisten it (quench). During the scrubbing, dusts or
gaseous impurities
such as sulfur dioxide can also be removed. In the treatment of sulfur-dioxide-
comprising
CA 02831463 2013-09-26
8
gases, an acidic process water is thus obtained, which can be used as the
above described
acidic aqueous solution.
The fluid stream is treated with the absorption medium in an absorption tower
or absorption
column, e.g. packed-bed, ordered packing and tray column. The fluid stream is
treated with
the absorption medium, preferably in an absorption column in counterflow. The
fluid stream
in this case is generally fed into a lower region, and the absorption medium
into an upper
region, of the column.
The temperature of the absorption medium in the absorption step is generally
about 20 to
90 C, when a column is used, for example 20 to 60 C at the top of the column
and 30 to
90 C at the bottom of the column. A fluid stream low in acid gas components,
i.e. a fluid
stream depleted in these components, is formed, and also an absorption medium
that is
loaded with acid gas components.
Carbon dioxide and other acid gases can be released in a regeneration step
from the
absorption medium that is loaded with the acid gas components, wherein a
regenerated
absorption medium is obtained. In the regeneration step, the loading of the
absorption
medium is decreased and the resultant regenerated absorption liquid is
preferably then
recirculated to the absorption step.
Generally, the loaded absorption liquid is regenerated by warming, e.g. to 70
to 130 C,
expansion, stripping with an inert fluid or a combination of two or all of
these measures.
Preferably, the loaded absorption liquid is regenerated in a stripper. The
stripping gas
required for the stripping is generated by partial evaporation of the
absorption liquid in the
bottom-phase of the stripper.
Before the regenerated absorption medium is reintroduced into the absorption
tower, it is
cooled to a suitable absorption temperature. In order to utilize the energy
comprised in the
hot regenerated absorption medium, it is preferred to preheat the loaded
absorption medium
from the absorber by indirect heat exchange with the hot regenerated
absorption medium. By
means of the heat exchange, the loaded absorption medium is brought to a
higher
temperature, and so in the regeneration step a lower energy usage is required.
By means of
the heat exchange, also, a partial regeneration of the loaded absorption
medium can already
proceed with release of carbon dioxide.
CA 02831463 2013-09-26
9
The absorption medium comprises at least one amine. Preferably, the amine
comprises at
least one primary or secondary amine.
Preferred amines are the following:
(i) amines of the formula I:
NR1(R2)2 (I)
where R1 is selected from C2-C6 hydroxyalkyl groups, C1-C6 alkoxy-C2-C6 alkyl
groups,
hydroxy C1-C6 alkoxy-C2-C6 alkyl groups and 1-piperazinyl-C2-C6 alkyl groups,
and R2 is
selected independently from H, C1-C6 alkyl groups and C2-C6 hydroxyalkyl
groups;
(ii) amines of the formula II:
R3R4N-X-NR6R6 (II)
where R3, R4, R6 and R6 independently of one another are selected from H, C1-
C6 alkyl
groups, C2-C6 hydroxyalkyl groups, C1-C6 alkoxy-C2-C6 alkyl groups and C2-C6-
aminoalkyl
groups and X is a C2-C6 alkylene group, -X1-NR7-X2- or -X1-0-X2-, where X1 and
X2
independently of one another are C2-C6 alkylene groups and R7 is H, a C1-C6
alkyl group, C2-
C6 hydroxyalkyl group or C2-C6 aminoalkyl group;
(iii) 5- to 7-membered saturated heterocycles having at least one nitrogen
atom in the ring,
which can comprise one or two further hetero atoms in the ring selected from
nitrogen and
oxygen, and
(iv) mixtures thereof.
Specific examples are:
(i) 2-aminoethanol (monoethanolamine), 2-(methylamino)ethanol, 2-
(ethylamino)ethanol,
2-(n-butylamino)ethanol, 2-am ino-2-methylpropanol, N-(2-
aminoethyl)piperazine,
methyldiethanolamine, ethyldiethanolamine, dimethylaminopropanol,
t-butylaminoethoxyethanol, 2-aminomethylpropanol;
CA 02831463 2013-09-26
(ii) 3-methylaminopropylamine, ethylenediamine, diethylenetriamine,
triethylenetetramine,
2,2-dimethy1-1,3-diaminopropane, hexamethylenediamine, 1,4-diaminobutane, 3,3-
iminobispropylamine, tris(2-aminoethyl)amine, bis(3-dimethylaminopropyl)amine,
tetramethylhexamethylenediamine;
5
(iii) piperazine, 2-methylpiperazine, N-methylpiperazine, 1-
hydroxyethylpiperazine, 1,4-bis-
hydroxyethylpiperazine, 4-hydroxyethylpiperidine, homopiperazine, piperidine,
2-
hydroxyethylpiperidine and morpholine; and
10 (iv) mixtures thereof.
Thereof, monoethanolamine, piperazine, methylaminopropylamine, diethanolamine,
1-hydroxyethylpiperazine are particularly preferred.
Generally, the absorption medium comprises 10 to 60% by weight amine.
The absorption liquid can also comprise additives, such as corrosion
inhibitors, enzymes etc.
Generally, the amount of such additives is in the range of about 0.01-3% by
weight of the
absorption liquid.
The method according to the invention is suitable for treating fluid streams,
in particular gas
streams of all types. The acid gases are, in particular, CO2, H2S, COS and
mercaptans.
Furthermore, S03, S02, CS2 and HCN can also be removed. Generally, the acid
gases
comprise at least CO2 or consist solely of CO2.
Fluids which comprise the acid gases are, firstly, gases, such as natural gas,
synthesis gas,
coke oven gas, cracked gas, coal gasification gas, recycled gas, landfill
gases and
combustion gases, and secondly liquids which are substantially immiscible with
the
absorption medium such as Liquefied Petroleum Gas (LPG) or Natural Gas Liquids
(NGL).
In preferred embodiments, the fluid stream is a
(i) hydrogen-comprising fluid stream comprising fluid stream; these
include synthesis
gases which can be produced, e.g. by coal gasification or steam reforming, and
are
optionally subjected to a water gas shift reaction; the synthesis gases are
used, e.g. for
producing ammonia, methanol, formaldehyde, acetic acid, urea, for the Fischer-
CA 02831463 2013-09-26
11
Tropsch synthesis or for energy production in an Integrated Gasification
Combined
Cycle (IGCC) process;
(ii) hydrocarbon-comprising fluid stream; these include natural gas,
exhaust gases of
various refinery processes, such as the Tailgas Unit (TGU), of a Visbreaker
(VDU), of a
Catalytic Cracker (LRCUU/FCC), of a Hydrocracker (HCU), of a Hydrotreater
(HDS/HTU), of a Coker (DCU), of an Atmospheric Distillation (CDU) or of a
Liquid
Treater (e.g. LPG).
The method or absorption medium according to the invention is suitable for
treating oxygen-
comprising fluid streams, such as flue gases.
In preferred embodiments, the oxygen-comprising fluid stream originates from
a) the oxidation of organic substances,
b) the composting or storage of waste materials comprising organic
substances, or
c) the bacterial decomposition of organic substances.
In some embodiments, the partial pressure of carbon dioxide in the fluid
stream is less than
500 mbar, e.g. 30 to 150 mbar.
The oxidation can be carried out with appearance of flame, i.e. as
conventional combustion,
or as oxidation without appearance of flame, e.g. in the form of a catalytic
oxidation or partial
oxidation. Organic substances which are subjected to the combustion are
customarily fossil
fuels such as coal, natural gas, crude oil, petroleum, diesel, raffinates or
kerosine, biodiesel
or waste materials having a content of organic substances. Starting materials
of the catalytic
(partial) oxidation are, e.g. methanol or methane which can be reacted to form
formic acid or
formaldehyde.
Waste materials which are subjected to oxidation, composting or storage, are
typically
domestic refuse, plastic wastes or packaging refuse.
The combustion of the organic substances proceeds usually in customary
combustion plants
with air. Composting and storage of waste materials comprising organic
substances
proceeds generally in refuse landfills. The exhaust gas or the exhaust air of
such plants can
advantageously be treated by the method according to the invention.
CA 02831463 2013-09-26
12
As organic substances for bacterial decomposition, customarily stable manure,
straw, liquid
manure, sewage sludge, fermentation residues, silage and the like are used.
Bacterial
decomposition proceeds, e.g. in customary biogas plants. The exhaust air of
such plants can
advantageously be treated by the method according to the invention.
The method is also suitable for treating the exhaust gases of fuel cells or
chemical synthesis
plants which make use of a (partial) oxidation of organic substances.
The fluid streams of the above origins a), b) or c) can have, for example,
either the pressure
which roughly corresponds to the pressure of the ambient air, that is to say,
e.g. atmospheric
pressure or a pressure which deviates from atmospheric pressure by up to 1
bar.
The invention is illustrated in more detail by the accompanying drawing and
the examples
hereinafter.
Figure 1 shows a plant for removing acid gases from a gas stream having a
scrubbing zone
for scrubbing out entrained absorption liquid from the treated gas according
to the prior art.
Figure 2 shows a plant for removing acid gases from a gas stream, which plant
is suitable for
carrying out the method according to the invention.
Figure 3 shows a plant for removing acid gases from a gas stream, which plant
is suitable for
carrying out a further embodiment of the method according to the invention.
According to figure 1, a gas stream 1 is passed into the lower part of an
absorption column 2.
The absorption column 2 has absorption zones 3, 4 and a scrubbing zone 5. In
the
absorption zones 3, 4, the gas is brought into contact in counterflow with an
absorption
medium which is introduced into the absorption column 2 via the line 7 above
the absorption
zones. The gas depleted in acid gases is scrubbed in the scrubbing zone 5 with
an aqueous
phase which is recycled via the pump 8, the cooler 9 and line 10. Via line 11,
fresh water is
applied to the scrubbing zone 5 to compensate for amounts lost (make-up
water). In the
scrubbing zone 5, the water-vapor-saturated treated gas is simultaneously
cooled, as a result
of which water is condensed out. By means of the water that is condensed out
and the make-
up water, liquid is fed to the scrubbing circuit. Excess liquid phase from the
scrubbing circuit
is passed via the line 12 into the absorption medium circuit. An accumulation
in the scrubbing
CA 02831463 2013-09-26
13
water circuit of absorption medium components that have been scrubbed out of
the treated
flue gas is thereby prevented. The treated gas stream leaves the absorption
column 2 via the
line 25.
The absorption medium loaded with carbon dioxide is taken off at the bottom of
the
absorption column 2 and conducted via the pump 13, heat exchanger 14 and line
15 into the
stripper 16. In the lower part of the stripper 16, the loaded absorption
medium is warmed and
partially evaporated by the evaporator 17. By means of the temperature
elevation, some of
the absorbed carbon dioxide is transferred back into the gas phase. The gas
phase 18 is
taken off at the top of the stripper 16 and fed to the condenser 19.
Condensate is collected in
the phase separation vessel 20 and recirculated to the stripper 16. The
gaseous carbon
dioxide is taken off as stream 21. The regenerated absorption medium 22 is
recirculated
back to the absorption column 2 via the heat exchanger 14, pump 23, the cooler
24 and line
7.
In figure 2, the same reference signs have the same meaning as in figure 1.
The absorption
column 2 has two scrubbing zones 5, 6. The gas depleted in acid gas is
scrubbed in the
scrubbing zone 6 with an aqueous phase which is applied via the line 25 above
the
scrubbing zone 6. The treated gas then passes through the scrubbing zone 5 and
is
scrubbed with an aqueous phase which is recycled via the pump 9, the cooler 9
and line 10.
Via line 11, fresh water is applied to the scrubbing zone 5 for compensating
for amounts lost
(make-up water). Excess liquid phase from the scrubbing water circuit is not
passed directly
into the absorption medium circuit, but is applied via the line 26 above the
scrubbing zone 6.
The aqueous phase draining out of the scrubbing zone 6 runs into the
absorption zone 3 in
the interior of the absorption column 2.
Figure 3 shows a further embodiment of the invention. In figure 3, the same
reference signs
have the same meaning as in figure 2. The treated gas leaving the absorption
column 2 is
fed via the line 33 to a scrubbing column 27 and scrubbed with an acidic
aqueous solution
which is pumped in circulation via the pump 28 and line 29. Exhausted acidic
aqueous
solution is removed via line 32. Fresh acidic aqueous solution is introduced
via line 30. The
treated gas leaves the plant via line 31.
CA 02831463 2013-09-26
14
Comparative example 1:
A plant according to figure 1 was used. The absorption column had a diameter
of 600 mm;
as internals, a structured packing having a specific surface area of 250 m2/m3
was used.
Above the feed of the absorption medium, a scrubbing zone for water scrubbing
was
installed having recycling and cooling. The bed height of the scrubbing zone
was 3 m.
A 32% strength by weight aqueous monoethanolamine solution was used as a basic
absorption medium which flowed in at 40 C. Flue gas (1563 kg/h) was fed to the
absorption
column at a temperature of 40 C and a water content of 3.6% by volume and a
CO2 content
of 14.0% by volume. In the plant, 90% of the fed CO2 was separated off. The
amount of
scrubbing water pumped in circulation was 5500 kg/h and the scrubbing water
was cooled to
41 C. 34 kg/h of make-up water were fed to the plant. The temperature of the
gas above the
absorption section was 67 C and the gas was cooled via the water scrubbing to
41 C.
The content of monoethanolamine in the scrubbing water was determined using a
gas
chromatograph and was 0.8% by weight.
Example 2:
A plant according to figure 2 was used. Above the feed of the absorption
medium, a first
scrubbing zone (without recycling) was installed at a height of 4.5 m, above
the first
scrubbing zone, a second scrubbing zone was installed at a height of 3 m for
water
scrubbing with recycling and cooling.
The flue gas rate was 1540 kg/h with a water content of 3.6% by volume and a
CO2 content
of 14% by volume, of which 90% were separated off as in comparative example 1.
The
content of monoethanolamine in the absorption medium was 27% by weight. In the
second
scrubbing zone, 5000 kg/h of scrubbing water were circulated and cooled in the
course of
this to 39 C. 28 kg/h of make-up water were fed to the second scrubbing zone.
By means of
the cooling of the gas in the second scrubbing zone (from 63.5 C to 39 C),
water was
additionally condensed out. A volumetric flow rate of 188 kg/h of water was
passed into the
first scrubbing zone.
No monoethanolamine could be detected by means of a gas chromatograph in the
recycled
scrubbing water using this configuration. The limit of detection was 0.01% by
weight.
CA 02831463 2013-09-26
The differing contents of monoethanolamine in the recycled scrubbing water
correlate with
corresponding contents of monoethanolamine in the treated fluid stream, i.e.
the treated fluid
stream that is leaving the absorption column, in example 2, comprised
significantly less
5 monoethanolamine than in comparative example 1.
By means of a simulation model, calculations were carried out for both
configurations
(comparative example 1 and example 2). The basis of the simulation model is a
thermodynamic model based on the electrolyte-NRTL approach of Chen et al.
(Chen, C.C;
10 Evans, L.B.: A local Composition Model for the Excess Gibbs Energy of
Aqueous Electrolyte
Solutions, AlChE J. (1986) 32(3), 444), by means of which the phase
equilibrium for this
system can be described. The simulation of the absorption processes is
described by means
of a mass-transfer-based approach; details in this respect are described in
Asprion (Asprion,
N.: Nonequilibrium Rate-Based Simulation of Reactive Systems: Simulation
Model, Heat
15 Transfer, and Influence of Film Discretization; Ind. Eng. Chem. Res.
(2006) 45(6), 2054-
2069).
Comparative example 3:
This example is based on the plant configuration of comparative example 1. The
absorption
column is equipped with a structured packing having a geometric surface area
of 250 m2/m3
and has a diameter of 600 mm. The scrubbing zone with recycling and cooling
has a packing
height of 3 m.
The simulation is based on the following values: 1540 kg/h of flue gas having
a composition
of 14% by volume CO2 and 3.6% by volume of water, 5.5% by volume oxygen and
76.9% by
volume nitrogen is passed into the absorption column at 40 C. There, 90% of
the carbon
dioxide is separated off in counterflow with a 29.3% by weight aqueous
monoethanolamine
solution. The regenerated absorption medium flows in at a temperature of 40 C.
The
recycled scrubbing water is cooled to 40 C. 38 kg/h of make-up water are
required. A
residual content of 5 v-ppm of MEA is calculated at the exit of the water
scrubber using this
configuration.
CA 02831463 2013-09-26
16
Example 4:
This example is based on the plant configuration of example 2. Above the feed
of the
absorption medium, there was installed a first scrubbing zone (without
recycling) of a height
of 4.5 m, above the first scrubbing zone a second scrubbing zone was installed
of a height of
3 m for water scrubbing with recycling and cooling.
In counterflow to the gas, 209 kg/h of scrubbing water was applied to the
first scrubbing
zone, which scrubbing water amount corresponds to the amount of the make-up
water
applied in the second scrubbing zone and the water condensed out by the
cooling of the gas.
A monoethanolamine content in the exiting gas of 40 v-ppb was achieved
thereby. The liquid
loading in the first scrubbing zone is 0.7 m3/(m2h) and is thus above the
dewetting limit of the
structured packing (manufacturer's details: 0.2 m3/(m2h); source: Sulzer
brochure).
Example 5:
In the plant configuration of example 4, the scrubbing zones are exchanged.
Above the feed
of the absorption medium there is situated a first scrubbing zone having
recycling and
cooling of a height of 3 m, and above the first scrubbing zone, a second
scrubbing zone, of a
height of 3 m (without recycling). The low-0O2 gas is first passed through the
first scrubbing
zone and then in counterflow to the make-up water through the second scrubbing
zone. For
the second scrubbing zone, this produces a liquid loading of only 0.1
m3/(m2h). This is below
the dewetting limit of the packing used, and a sufficient or uniform wetting
and thus
separation efficiency cannot be ensured thereby.
