Process for the removal of SO.sub.2 from gas streams

The removal of SO.sub.2 from two or more gas streams of differing SO.sub.2 content is accomplished by feeding each of the streams to an absorbent so that the more dilute SO.sub.2 -containing stream or streams is supplied to the absorbent upstream from the point at which the more concentrated stream or streams is supplied. By supplying the gas streams to the absorbent flow in ascending order of SO.sub.2 concentration, the amount of SO.sub.2 absorbed by the absorbent flow is increased.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for the removal of SO.sub.2 from 
two or more gas streams. 
2. Description of the Prior Art 
It is known to remove SO.sub.2 gas from a gas stream by absorbing the 
SO.sub.2 in a suitable absorbent. Examples of some prior art patents 
showing such a technique are: U.S. Pat. Nos. 2,563,437 and 3,047,364. 
In certain situations, two or more gas streams containing differing 
SO.sub.2 levels will require treatment to remove the SO.sub.2 from each 
stream. It has been commonplace to combine such gas streams into a single 
stream prior to contact with the absorbent. In order to recover the 
SO.sub.2 values from such a resulting stream, large quantities of 
absorbent solution must be circulated due to the limitations of SO.sub.2 
equilibrium solubility. Hence, a need exists for a more efficient way to 
absorb SO.sub.2 from two or more gas streams which contain varying 
SO.sub.2 levels. 
In U.S. Pat. No. 4,123,507 to R. H. Hass two SO.sub.2 streams, presumably 
of differing SO.sub.2 content, are not combined and are supplied to an 
absorber at two differing points. This patent, however, fails to indicate 
the degree of difference of the SO.sub.2 content of the streams, fails to 
indicate which stream is the more concentrated, and fails to indicate any 
advantage for not combining the gas streams into a single stream prior to 
contact with the absorber. The patent illustrates recirculation of 
SO.sub.2 -rich absorbent through the absorber which would tend to equalize 
the concentration of SO.sub.2 gas in the absorber. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is a process for the removal of SO.sub.2 from two or 
more gas streams each having a differing SO.sub.2 content. The process 
comprises providing a flow of liquid absorbent for the SO.sub.2 
-containing gas streams through at least one absorbent zone and supplying 
the gas streams to the absorbent flow in ascending order of SO.sub.2 
concentration so that the streams are supplied to the flow such that the 
more SO.sub.2 dilute gas stream, or streams, is supplied to the flow 
upstream from the more SO.sub.2 -concentrated gas stream, or streams, in 
order to increase the amount of SO.sub.2 absorbed in the absorbent zone, 
or zones, by the absorbent flow. 
In the present process, advantage is taken of the equilibrium solubility of 
SO.sub.2 to minimize absorbent circulation and thus minimize capital and 
operating costs. By supplying the more concentrated SO.sub.2 stream, or 
streams, to the absorbent flow without being diluted by the more dilute 
SO.sub.2 stream while also contacting the more dilute SO.sub.2 stream with 
the more lean absorbent, higher SO.sub.2 loadings are achieved for the 
present process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the present invention is illustrated in FIG. 1 wherein 
the process is used to remove SO.sub.2 from a tail gas source containing a 
relatively low (e.g., less than 2 vol. %) SO.sub.2 content and from a 
second, somewhat higher (e.g., over 2 vol. %) SO.sub.2 -containing source 
from the incineration of excess H.sub.2 S from a Claus reactor, e.g., an 
aqueous phase Claus reactor. 
The tail gas source can be a Claus plant incinerator, steam boiler, or the 
like, with the SO.sub.2 -containing gas being fed via line 10 to an 
intermediate portion of absorber 11. Lean absorbent solution is fed to the 
top of the absorber via line 12 and can be any SO.sub.2 absorbent known to 
persons of ordinary skill in the art. Exemplary absorbents include alkali 
metal phosphate buffered or unbuffered aqueous solutions, alkali metal 
citrate buffered or unbuffered aqueous solutions, and the like. 
The second, more concentrated SO.sub.2 source results from the incineration 
of excess H.sub.2 S from an aqueous regeneration/aqueous phase Claus 
reactor 13. This incinerated SO.sub.2 is fed via line 14 to a point in the 
bottom portion of absorber 11 which is downstream from the lower 
concentration SO.sub.2 stream fed via line 10. 
The absorbent, which becomes enriched in SO.sub.2, is removed from absorber 
11 by means of line 15 and is sent to the Claus reactor 13 where the 
SO.sub.2 contained in the absorbent reacts with a supply of H.sub.2 S 
forming sulfur in accordance with the classical Claus reaction carried out 
in an aqueous phase which is sent via line 16 to sulfur removal apparatus. 
The lean absorbent solution is then recycled to the absorber 11 by line 
12. 
FIG. 2 illustrates another embodiment of the present invention in which 
three streams of varying SO.sub.2 content are each treated in three 
absorbers connected in series. 
The stream 21 is lowest in SO.sub.2 content (e.g., from the tail gas of a 
steam boiler) and is fed to absorber 22 with absorbent being supplied via 
line 23. The absorbent containing absorbed SO.sub.2 is then fed via line 
24 to second absorber 25. A second stream 26, somewhat more concentrated 
in SO.sub.2 content (e.g., the incinerated tail gas from a Claus reactor), 
is fed to absorber 25 with the absorbent containing additionally absorbed 
SO.sub.2 fed via line 27 to absorber 28. The highest SO.sub.2 -containing 
stream (e.g., from incineration of excess H.sub.2 S from an aqueous Claus 
reactor 29) is fed via line 30 to the third absorber 28 which is 
downstream from absorbers 22 and 25. 
Absorbent effluent from absorber 28 is relatively rich in absorbed SO.sub.2 
and is fed to absorbent/regeneration reactor 29 for combination with 
H.sub.2 S. The resulting Claus reaction yields a liquid stream at 31 which 
is passed to appropriate sulfur removal apparatus with the lean absorbent 
solution being recycled via lines 32 and 23 to the first absorber 22 for 
contact with the most dilute SO.sub.2 -containing stream 21. 
Depending upon the amount of SO.sub.2 removal required, the gaseous 
effluents from each absorber can either be vented as shown in FIG. 2 or 
can be further treated by feeding them into the next absorber (e.g., the 
gaseous vent on absorber 28 can be fed to absorber 25 for further 
treatment or the gaseous vent on absorber 25 can be fed to absorber 22 for 
further treatment). 
The process of the present invention, by insuring that the gas streams are 
supplied to the lean absorbent in ascending order of SO.sub.2 
concentrations, results in higher SO.sub.2 equilibrium concentrations in 
the absorbent thereby allowing for a concomitant reduction in the 
absorbent solution circulation rate. 
The process conditions used to absorb SO.sub.2 in the present process are 
the conventional conditions well known to persons of ordinary skill in the 
art. Generally speaking, the absorption can take place at temperatures 
ranging from about 35.degree. C. to about 80.degree. C. with the pH of the 
absorbent preferably in the range of from about 2 to 6.5. Similarly, the 
Claus reaction, and incineration and sulfur removal steps illustrated in 
connection with the present invention are in accordance with conventional 
practice. 
The following Examples illustrate the equilibrium solubility 
characteristics of SO.sub.2 under various conditions. 
EXAMPLE 1 
A series of experiments were conducted in absorbing SO.sub.2 gas into a 
phosphate absorbent of the type described in U.S. Pat. No. 3,911,093 to F. 
G. Sherif et al. The loading values were obtained by circulating the 
absorbent solution at a fixed temperature and at a constant SO.sub.2 gas 
inlet composition until the SO.sub.2 gas content in and out of the 
absorbent zone was the same. The gas was saturated with water at the 
column operating temperature in order to maintain the water balance. 
Listed below were the results obtained at 74.degree. C..+-.1.degree. C. 
using an absorbent having the following characteristics: SO.sub.4 : 57.5 
gm/l.; 1.57M Na; 1.0M P; 0.26 gm/l. S.sub.x O.sub.6 ; pH: 4.4.+-.0.2; and 
SO.sub.2 O.sub.3 : about 11.5 gm./l. 
______________________________________ 
SO.sub.2 pH at 
Sample SO.sub.2 in 
Loading S.sub.2 O.sub.3 
Equi- 
No. Gas (%) (gm/l.) (gm/l.) 
librium 
______________________________________ 
1 2.0 5.50 13.4 3.22 
2 1.66 5.53 11.1 3.07 
3 1.38 4.86 11.2 3.18 
4 1.16 4.41 11.2 3.18 
5 0.82 3.77 12.9 3.28 
6 0.61 3.20 13.4 3.40 
7 0.32 2.56 13.4 3.50 
8 0.25 2.11 10.8 3.80 
9 0.15 1.31 10.8 3.84 
10 0.10 1.28 11.2 3.86 
11 0.05 0.96 11.2 4.00 
12 0.04 0.57 11.2 4.12 
13 0.03 0.57 11.1 4.17 
14 0.02 0.38 11.6 4.18 
15 0.01 0.24 11.2 4.12 
______________________________________ 
These data illustrate the general trend that the solubility of SO.sub.2 in 
aqueous solutions is increased as the percent SO.sub.2 in the inlet gas is 
increased. 
EXAMPLE 2 
The data presented below illustrate the SO.sub.2 loading values in the 
phosphate absorbent used in Example 1 for relatively dilute SO.sub.2 
-containing gas streams at two differing temperatures: 
______________________________________ 
SO.sub.2 
Sample Temp. SO.sub.2 in Loading 
S.sub.2 O.sub.3 
No. (.degree.C.) 
Gas (%) (gm/l.) 
(gm/l.) 
______________________________________ 
1 53 0.011 0.84 12.32 
2 53 0.035 1.52 12.28 
3 53 0.070 2.06 11.64 
4 74 0.011 0.23 14.10 
5 74 0.035 0.91 11.64 
6 74 0.072 1.30 12.50 
______________________________________ 
These data show that the SO.sub.2 loading increases with increasing 
SO.sub.2 content in the inlet gas and is also increased by use of lower 
temperatures. 
EXAMPLE 3 
The data presented below illustrate the SO.sub.2 loading characteristics 
for gas streams containing a higher SO.sub.2 content than the streams 
tested in Example 2: 
______________________________________ 
SO.sub.2 
Sample Temp. SO.sub.2 in Loading 
SO.sub.2 O.sub.3 
No. (.degree.C.) 
Gas (%) (gm/l.) 
(gm/l.) 
______________________________________ 
1 53 5.5 9.4 11.40 
2 53 10.5 11.3 11.20 
3 74 5.5 6.7 11.60 
4 74 10.5 8.2 11.20 
______________________________________ 
The same general trends noted for Example 2 also pertain. 
EXAMPLE 4 
Listed below are the SO.sub.2 loading data using a somewhat higher 
temperature (i.e., 80.degree. C..+-.1.degree. C.) than used in any of 
Examples 1-3: 
______________________________________ 
SO.sub.2 
SO.sub.2 in Loading S.sub.2 O.sub.3 
Sample No. 
Gas (%) (gm/l.) (gm/l.) 
______________________________________ 
1 1.64 5.03 11.60 
2 1.12 4.84 11.82 
3 0.62 3.77 11.82 
4 0.225 2.32 12.28 
______________________________________ 
The increased SO.sub.2 content in the gas results in an increased SO.sub.2 
loading. 
EXAMPLE 5 
This shows the SO.sub.2 loading values at 53.degree. C..+-.1.degree. C. for 
relatively low PO.sub.4.sup.-3 concentration absorbents (S.sub.2 
O.sub.3.sup.-2 : 11.7.+-.0.4 gm./l. at P=1.0.+-.0.2M and S.sub.2 
O.sub.3.sup.-2 : 6.2.+-.0.1 gm./l. at P=0.53.+-.0.1M): 
______________________________________ 
% SO.sub.2 in Gas SO.sub.2 Loading (gm./l.) 
P = 1.0 P = 0.53 P = 1.0 P = 0.53 
______________________________________ 
1.54 1.54 8.1 5.5 
1.36 1.34 7.7 5.7 
1.12 1.12 7.4 5.4 
0.90 0.82 6.7 4.6 
0.62 0.62 5.7 4.0 
0.375 0.375 4.7 3.4 
-- 0.275 -- 2.7 
0.140 0.140 3.2 2.1 
______________________________________ 
These data also illustrate the general trend that an increased SO.sub.2 
content in the inlet gas produces an increased SO.sub.2 loading in the 
absorbent with a greater loading occurring as the phosphate concentration 
is increased for the relatively dilute PO.sub.4.sup.-3 solutions used. 
The foregoing Examples should not be construed in a limiting sense. The 
scope of protection desired is set forth in the claims which follow.