Note: Descriptions are shown in the official language in which they were submitted.
~3~ 3,5
SELECTIVE ABSORBTION OF SULFUR DIOXIDE
FROM GASES CONTAINING SULFUR DIOXIDE
AND CARBON DIOXIDE
The present invention concerns a process for
selectively removing sulfur dioxide gas from a gas
stream in the presence of carbon dioxide. The absor-
bent, which removes the sulfur dioxide, is regenerated
thereby enabling its reuse and a continuous operation of
the process.
Numerous patents and literature describe
techniques for removing sulfur compounds from a gas
stream containing the sulfur compounds. By far the
most common technique is that used to treat natural gas
having one or more of the acid gases, hydrogen sulfide
(H2S), sulfur dioxide (SO2), carbonyl sulfide (COS~,
and carbon dioxide (CO2) with an aqueous liquid lean
(with respect to the acid gases) absorbent to produce a
rich absorbent stream and regenerate the rich absorbent
stream by thermal treatment to produce a recycleable
lean stream. Many compounds have been suggested and
used as the absorbent, some to selectively remove H2S
32,114-F -1-
-2- ~235~5
or CO2 and other absorbents more general in nature to
remove as much of each of the acid gases present as is
possible.
Now, with the renewed interest in coal fired
boilers and the like, coupled with the greater concern
for the environment, there is a need to provide a low
pressure (at or below atmospheric), low temperature
selective process to remove sulfur dioxide from the
` flue gases emitted from such plants without removal of
any major portion of the carbon dioxide. A commercially
desirable feature for the absorbent would be its regen-
eration from the absorbed gases enabling its reuse.
One process for removing the SOz widely in
use today is the limestone scrubbing process. The
disadvantage of this process is the formation of a
large volume of solid waste, calcium sulfite-sulfate,
often contaminated with fly ash, which re~uires disposal.
In areas of the country where paper pulp operations
are being carried out, the waste is oftentimes usable,
but such situations are not widespread.
Another process recently in the forefront is
the use of potassium or sodium citrate as disclosed in
.S. Patent No. 4,366,134. While the absorbent is
regenerated and recycled, the make-up costs can be high
due to thermally stable salts being formed. In addition,
it is necessary to employ stainless steel for the
entire plant to prevent excessive corrosion of the
metals.
It would be advantageous to have a process
which: (a) selectively absorbs sulfur dioxide to -the
32,11~-F -2-
--3~
~35~8~
exclusion of the other acid gases, particularly carbon
dioxide; (b) has low chemical make-up cost; (c) has
reduced operating costs; and (d) permits economical
construc-tion of equipment, to process low pressure,
high volume, gas streams, such as flue gas, which
results in the reduction or elimination of the sulfur
dioxide content of such gases.
Surprisingly, the present process meets the
above stated purposes. The process selectively removes
sulfur dioxide from a gas stream containing sulfur
dioxide and carbon dioxide by contacting the gas with
an aqueous solution of a compound of the general
formula
~ N ~ O (I)
R
wherein X is an oxygen atom or NR'; R' is a hydrogen
atom or C1-C5 alkyl; and R is a hydrogen atom or C1-C5
alkyl.
The gas stream may also contain one or more
of the other acid gases (for example, H2S or COS) com-
monly associated with hydrocarbon, natural or synthetic,
and/or combustion gases (flue gas). Also, for the pur-
poses of this invention, the gas stream need not containcarbon dioxide. However, if carbon dioxide is present
in the gas, the present process enables the selective
removal of sulfur dioxide. The process employs a lean
32,114-F -3-
~35~38~
aqueous absorbent solution of a compound of Formula (I),
preferably at a concentration of 0.1 molar to the
saturation point. The rich absorben-t, containing most
of the SO2 and little of the CO2, is removed from the
absorber (contactor) and thermally regenerated to
produce a lean absorbent solution for recycle to the
absorber.
The absorber is preferably operated at from
5 to 95C under about atmospheric pressure conditions.
Higher temperatures and pressures do not materially
effect the process although equipment design may require
modification to handle the higher temperatures and
pressures.
The concentration of the sulfur dioxide in
the gas streams may vary from about 10 ppm to about 45
percent by volume of the gas stream being treated.
The process for regeneration may be one of
the conventional methods employed in conventional gas
sweetening units as well as by steam stripping.
BRIFF DESCRIPTION OF THE DRAWINGS
Figure :I represents a schematic diagram of
the essential components of a process used to treat
gases in accordance with the present invention.
An integrated àbsorber-stripper (contactor-
regenerator), as illustrated in Figure 1, was construc-
ted by piping a ten tray Oldershaw column, 10, having a
one inch (2.54 cm~ internal diameter and 1 1/4 inch
(3.17 cm~ tray spacings in a manner to receive a lean
absorbent solution at its upper end, 11, and a con-
32,114-F -4-
--5--
~235~8~ii
taminated gas stream at its lower end, 12. The top,
13, and bottom, 14, were each independently piped to
collect the treated gas at the top and the rich absor-
bent at the bottom, respectively. The rich absorbent
was piped to a shell and tube cooler, 15, which passed
the hot lean absorbent on the shell side and the cool
rich absorbent on the tube side. The rich absorbent
was then delivered to the upper end, 16, of a stripper,
17. The stripper, 17, was a two-foot one-inch (0.635
m) internal diameter column packed with 1/4 inch (6.35
mm) Berl saddles. The sulfur dioxide exited the top,
18, with some water vapor and was sent to a condenser,
19, wherein the water vapor was condensed and the
condensate and sulfur dioxide sent to a degasifier, 20,
from which the sulfur dioxide was vented and the con-
densate returned via pump, 20A, to the top, 18, of the
stripper, 17, as reflux. The liquid collecting in the
~ottom, 21, of the stripper, 17, was substantially lean
absorbent, a part of which was passed through a reboiler,
22, and back into the stripper below the packed section.
The remainder of the lean absorbent collec-ting in the
bottom, 21, was piped to the cooler, 15, wherein it
gave up most of its heat to the rich absorbent. The
cool absorbent was drawn to the intake side of a pump,
23, passed through another cooler, 24, and then to the
; l~an feed point of the absorber, 10.
To illustrate the present process, the fol-
lowing examples are provided.
Example 1
The data collected from several runs is set
forth in the table below. The alphabetic headings
refer to like alphabetically numerated streams in
Figure 1.
32,114-F -5-
. . ~ .
~;~3~F~8~
.~ d1 Ln
o .~ .~
o o o
~a . .
.~ O O O
~ ~ Ln
a ~ ~
C~~ . .
a O O
c)
h
a~
r~ ~ In ~ $
~D O ~D
V ~ . . .
O O O
~ L~ O
a a ~ O ~ ~0 ~
~ .~ ~ O O ~ .
1:~ O
a~
H LO Ll'~ P4
~ ~ ) r~ ~
V r~ ~ OO ~ r~
m ~~ ,, o . .. .
~i ~ O O O O
U~
~ LO .
a a ~ o~ ~ ~a
~ ~ ~O OO ~ ~ I
C~ ~ o O OO O
D rd ,1
~a 'O ~ ~
a~ oo ~ .~ ~q O
,~ jo $ ~ '
E~ o o~ ~ o ~ o
N
In K t N
(~ 00 L~ ~
C~ O O ~ U~ ~ ~
C 'C1~I t` O ~
(U . . . ~ O ~1
o o o o o ~ Q~
h
3 O O '-I O OO r l 3
-l ao ~ .c~ 1 o u~ .Ei
o~I d~ N ~ ~ ~ h
X K
* ~ Z
O ~ Z Z
o~ O ~ ~ O0 ~7; !C *
~ ~ ~ O u~ cn zi
32 ,114-F -6-
35~
Example 2
A serie of tests were run to screen the
efficiency of various compounds known to absorb SO2
with respect to their absorbent characteristic for CO2.
The equipment, a steel bomb filled with glass balls,
was fit-ted with a valve at one end through which CO2
and absorbent could be added. The bomb was also fitted
with a pressure sensing instrument. The bomb was
pressurized to 760 mm ~Ig with CO2 and filled with a
measured quan-tity of a 1 molar solution of a specific
absorbent. The bomb was then left at ambient temper-
ature (about 24C~ or heated as indicated in the table
below, and the pressure drop measured over a 10 minute
period for each condition. The results were as follows:
Table II
Cell Mols CO2/
Absorbant Tem~. C Mol Absorbant
.
Water 24 0.046
1 M triethanolamine 24 0.27
none~
1 M 1,4-dimethylpiperazinone 23 0.08
none
1 M triethylene glycol 24 none2
1 M neutralized citric acid 24 none3
1 M DETA'~ 24 1.33
0.95
74 0.76
1 M Na2SO3 24 0.17
44 0.16
7~ 0.1
l High losses due to high vapor pressure.
2 Degrades in presence of oxygen.
3 Solvent used in U.S. 4,366,134, corrosive.
4 Diethylenetriamine
32,114-F -7-
Example 3
Using the ten tray Oldershaw column described
previously various compounds were tested for CO2 and
SO2 absorption characteristics. A synthetic N2/CO2/SO2
S gas mixture of the composition set forth in the following
table was fed to the bottorn of the column at 55C and
4/5 liters/minute. The liquid flow at the top was
about lO cc/minute. The analysis of the gas in and out
was obtained and weight percent CO2 and/or SO2 absorbed
calculated. The results are set forth below.
32,114-F -8-
38~5
o .ooooo~ ~ . o
u~ o~ o o o o o ~ ~ r~ o
o~ ,,
U~
CO I I U~ I I ~ I I ~D I
o . I , ., I . I I . I
V ~ I , o I I ~ , I
I I ~ I I I I
Ln I I I I I
o . I
U~ ~
~ ~ ~ ,
o
U~ t` ~1 ~1 ~` t` ~ O ~ ~ Ln ~
N dl ~1 ~1 ~I r~ ~ N N C5~ 0
V ~ O ~ ~ O ~ ~ O O ~ ~1
r-1 ~J ~ r^l N ~I r~ N N ~1 ~1
~n ~
~; ~o ~ a~ ~ ~ ~ ~ ~ co d~
H /~J (~ ) ~ 00 CO N ~ t` t`
1~ z a~ a~ o ~ ~ o co a~
HU~
HO
~ H
F:~ ~) N 0 ~ N -1 ~ CO 10 0 ~1
N ~ C~
U~ U~ ~ ~1 0 00 t`` ~ ~1
H L)'l r` O d~ dl ~ /:5~~1 ~ LO
V~ O .
~ V ~ cn ~ r` ~ ~D O ~ O O
C~ r1 ~1 ~1 ~1 ~I r l ~1 ~I N N
. . co
E-l (~J r` O N 0 d~ N 0O ~I N
~: Z ,
d1 ~ 0~ CO CO~
~, a) a) ~1 o
N S~ 1 +
~ ~a ~ o ~ ~ ~ ~
O P; E~ ~ O r~
O ~ O
3 Z ~ ~ ~: Z
32 ,114-F -9-
--10--
~3~38~5
Table IV
So2 ABSORPTION; 20 WT % NNDP; HIGH So2 LOADING
-
5 cc/min Liquld Feed; 781 mm Hg Pressure Absolute, ca
0.195 ft3/min gas in and 0.184 ft3/min gas out
5 Temperature C
Absorbent Feed In 56 56 57
Gas Feed In 25 25 25
Wt. % Gas In
N2 77.17 75.18 73.27
CO2 lg.65 19.70 19.00
SO2 3.18 5.12 7.73
Wt. % Gas Out
N2 79.78 80.35 78.97
CO2 20.52 19.65 20.76
SO2 .001 .001 .269
ppm SOzby Drager 10 10 too high to
measure
This run established that NNDP will absorb in
excess of one mol of SO2 per mol of NNDP.
STRIPPER
4 cc/min Liquid Feed; 761 mm Hg Pressure Absolute
Wt % SO2
Liq (in) 7.5 7.57.5 7.5
Liq (out) 2.38 2.29 2.14 2.03
25 Temperatures, C
Feed In 81 81 82 82
Bottoms 104 104 104 103
32,114-F -10-
~2~5~
Table V
SO? ABSORPTION; 20 WT _NNDP
5 cc/min Liquld Feed; 781 mm Hg Pressure
Cu ft/min Gas
In 0.173 0.176 0.176 0.177
Out 0.169 0.175 0.175 0.176
Temperatures C
Liquid Feed
At Inlet 85 81 80 80
Top of Column 56 55 54 55
Gas In
Bottom of Column 25 24 21 22
Wt. % Gas In
N2 77.83 78.11 77.85 77.68
Co2 20.56 20.27 20.66 20.75
SO2 1.61 1.62 1.50 1.58
Wt. % Gas Out
N2 79.10 79.40 79.03 78.92
CO2 20.90 20.60 20.97 21.08
SO2 too low to measure
ppm SO2 by Drager 8 8 10 2
.
STRIPPER
761 mm Hg Pressure
cc/min Liquid Feed 4 4 5 3
Wt % SO2
Liq (in) 3.12 3.09 1.80 1.87
Liq (out) 1.52 1.67 1.44 1.42
Temperatures, C
Feed In 84 84 80 90
Bottoms 100 102 102 101
32,114-F -11-
-12~
~23~ ,5
Table VI
SO~ ABSORPTION; 5 WT % NNDP
5 cc/min Liquid Feed; 781 mm Hg Pressure
Cu ft/min Gas
In 0.195 0.195 0.195 0.195
Out 0.184 0.1~34 0.1~5 0.184
Temperatures 3C
Liguid Feed
At Inlet 80 80 80 80
At Top of Column 55 55 55 55
Gas
Bottom of Column 21 22 22 21
Wt. % Gas In
N2 78.41 78.93 77.27 78.41
Co2 19.97 19.55 21.11 19.97
SO2 1.62 1.51 1.61 1.62
Wt. % Gas Out
N2 78.53 80.15 78.54 79.70
. CO2 21.47 19.85 21.46 20.30
SO2 .0003 .0008 .0003 .0003
ppm SO2 by Drager 3 8 3 3
STRIPPER
.
4 cc/min Liguid Feed; 761 mm ~Ig Pressure
Wt % SO2
Liq (in) 2.94 2.72 2.74 2.74
Lig (out) 0.72 0.72 0.82 0.72
Temperatures, C
Feed In 80 86 87 87
~ottoms 102 103 102 102
These two runs establish that SO2 will be
absorbed selectively vis-a-vis CO2 at tempera-tures
above 50C, the normal water/gas wash temperature, at
5 percent concentration as well as 20 percent concen-
tration.
32,114-F ~12-
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35~
Example 4
In another series of test runs, a 10 percent
by weight aqueous solution of l-methyl-2-morpholinone
in deionized water was passed down a 1/2" x 2 ft.
(1.27 cm x 0.61 m) good (low) packed absorber column
at a nominal rate of -5 cc per minute while a mixture
of 3.17 liters per minute of N2, 1.2 t/min. of air,
750 cc/min of C02 and 70 cc/min of S02 representing
1 percent by weight of SO2 were fed to the bottom of
the column. The liquid withdrawn from the bottom of
the column was fed to a 1/2 " x 2 ft~ (1.27 cm x
0.61 m) ss stripper column packed with 1/4" (0.6 cm)
Berl saddles with a heated bath reboiler.
The off gas from the absorber contained
0.45 percent by weight S02 and the absorber liquid
contained 0.63 percent by weight S02 of which 96.8
percent was stripped from the absorbent in the
stripper column. This represents a pick-up of 1/2
mole per mole of l-methyl-2-morpholinone vs a 10
percent solution, and the ability to regenerate the
absorbent for recycle.
32,114-F -13-
