CA1235885A - Selective absorbtion of so.sub.2 from gases containing the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
There is disclosed a process for selectively removing and recovering sulfur dioxide from a gas stream containing the same by contacting the gas with an absorbent (an aqueous solution of a piperazinone or a morpholinone) and thermally regenerating the absorbent (i.e. releasing the sulfur dioxide from the absorbent) for reuse in the contacting step.

Description

~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-~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-~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 ~12-35~
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-

Claims (9)

WHAT WE CLAIM IS:

1. A process for selectively removing sulfur dioxide from a gas stream containing sulfur dioxide and carbon dioxide which comprises contacting the gas with an aqueous solution of a compound of the general formula (I) 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.

2. A process of Claim 1 wherein X is oxygen.

3. A process of Claim 1 wherein X is NR'.

4. A process of Claim 3 wherein R and R' are both C1-C5 alkyl.

5. The process of Claim 1 or 4 wherein the compound of Formula (I) is 1,4-dimethylpiperazinone.

6. The process of Claim 1 wherein the compound of Formula I is 1-methyl-2-morpholinone.

7. A process of Claim 1 wherein the process is continuous.

8. A process of Claim 1 or 7 which comprise the further step of reyeneration of the aqueous solution of a compound of Formula I.

9. A process of Claim 1 or 7 which comprise the further step of recovering the gas, substantially free of sulfur dioxide, from the aqueous solution.