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Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979
Recovery of Sulfur from SO2 by Reaction with Alkali or Alkaline Earth Sulfides F. P. McCandless,’ Terry L. Parker, and James T. Nichols Department of Chemical Engineering, Montana State University, Bozeman, Montana 597 17
A modified “wet Thiogen” process for removing SO2 from a dilute gas mixture and reducing it to elemental sulfur and/or H2S using a solution or slurry of a metal sulfide has been investigated. In this process the gas mixture is contacted with a slurry of an alkaline earth sulfide or a solution of an alkali sulfide which converts the SO2 to H2S (water slurry or solutions) or elemental sulfur (methanol or certain other anhydrous solvents). A number of other metal sulfides were investigated but none was as effective as the alkaline sulfides. The effects of some of the variables on the absorption and conversion of SO2 is reported and reaction mechanisms in water are proposed.
Introduction and Background Sulfur oxides are among the most common and unhealthful air pollutants. A variety of methods are available for removing SO2from the gases (Slack, 1973; Andro and Laseke, 1977), but improved methods in which the absorbent can be regenerated and/or in which elemental sulfur is produced would be very desirable. The literature of 60 years ago discusses the “Thiogen” processes for the recovery of sulfur from SOz (Fulton, 1915; Young, 1915; Wells, 1917). The dry Thiogen process is based on the reactions reduction: 4CaS + 6S02 CaS03 + 6 s regeneration: 4CaS03 + 3CH4 4CaS + 3 c 0 2 6 H 2 0 overall: 6S02 + 3CH4 6 s 3 c 0 2 6H20
--
This paper presents the results of a preliminary investigation of the use of alkali or alkaline earth sulfides to remove SO2from a dilute gas stream and simultaneously converting it to elemental sulfur and/or H2S in a somewhat modified wet thiogen process in which the SO2containing gas is bubbled through a slurry or solution of the sulfide. When water is used as the solvent large amounts of H2S are formed even if C02 is absent from the gas stream while elemental sulfur is formed when certain anhydrous solvents are used as the slurry medium. Experimental Section A simple semi-batch reactor system (continuous flow of the test gas to the reactor but batch-wise sulfide slurry) was used for many of the preliminary tests. A schematic diagram of this reactor is shown in Figure 1. The test gas mixture containing SOz was fed from a cylinder through a calibrated rotameter to the reaction vessel which was a standard 250-mL gas washing bottle with a fritted glass sparger. A Teflon-coated magnetic stirrer at the bottom kept the sulfide slurry continuously agitated. A cold trap was installed in the exit line to remove most of the solvent which was evaporated because of the exothermic absorption reactions. A swagelock tee with a gas sampling septum provided a means of sampling the exhaust gases. For the semi-batch tests a typical run was made using 10 to 25 g of powdered sulfide in about 200 mL of solvent in the washing bottle. The slurry was agitated with the magnetic stirrer while the test gas was introduced through the sparger at a constant rate. Samples of the exhaust gas were taken periodically with a gas-tight syringe and analyzed by gas chromatography. The run was continued until the absorbent was spent as indicated by the exit gas composition approaching that of the feed. The spent slurry was analyzed by the scheme shown in Figure 2. The slurry was filtered and solid residue dried and analyzed for free sulfur by extraction with carbon disulfide in a Soxhlet apparatus. The sulfur-free material was then analyzed for sulfate by barium precipitation. Sulfide and sulfite analyses were attempted by a gas chromatography technique (Birk et al., 1970) but the apparent presence of thionates, thiosulfates, dithionates, etc., clouded these results. After reaction, the solvent contained a considerable amount of soluble material and so the solvent was removed from the filtrate by evaporation and the soluble material analyzed by the same scheme. Preliminary results showed that elemental sulfur was formed when certain anhydrous solvents such as dimethylformamide or methanol were used but that H2S was
-
+
+
+
In the dry process the Cas can be viewed as playing a catalytic role in the reduction of SO2 with natural gas. However, at SO2 concentrations below 8% the reaction is incomplete and O2 must be excluded before the reaction will approach completion. Another disadvantage is that the reaction occurs only at temperatures above 750 OC. Early plant tests of the dry process were apparently failures. In the “wet Thiogen” process the SO2is first absorbed in water in absorption towers and the aqueous solution containing the SO2is reacted with an alkaline earth sulfide either in a finely divided water suspension or solution where the reactions 2(Ca or Ba)S + 3S02 2(Ca or Ba)S03 + 3 s 2(Ca or Ba)S + 3S02 2(Ca or Ba)S203+ S
--
etc. take place. Apparently some dithionates, trithionates, tetrathionates, bisulfites, etc., are also formed in addition to the sulfites and thiosulfates. Most of the work on the wet thiogen process used Bas because it minimized the formation of the other salts and because the products of reaction were less soluble in water. However, the presence of any COz in the feed gas resulted in the formation of H2S and for this reason the SO2was first absorbed out of the gas in water and treated with the powdered sulfide to form elemental sulfur. The resulting slurry is filtered, the sulfur distilled off, and the sulfite mixture subsequently reduced and returned to the system. Apparently more success was met with the wet process in recovering sulfur from SO2, but it was never commercialized (Bienstock et al., 19581, and no further work was done on the process after an extensive Bureau of Mines test about 1915. 0019-788217911118-0494$01.00/0
0
1979
American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979 VPlT
STIRRED RE ACTOR
TEST G A S MIXTURE
C O L D TRAP
Figure 1. Schematic diagram of semi-batch reactor used for preliminary tests. F R E E SULFUR
y
495
of H2S with SO2. Gas and spent slurry analysis were by the same methods used in the semi-batch tests. All runs were made at room temperature.
Results of Semi-Batch Tests Most of the preliminary batch tests were carried out using calcium sulfide because of its availability and potential abundance. A test gas typical of what might be expected in a nonferrous smelter (6.6% SO2, 1.3% COz, 13.4% 02,78.7% N2) was used for the first tests. A typical breakthrough curve when water was used as the solvent is shown in Figure 4. During the first part of the run very little H2S was produced but then as the run progressed the H2S concentration increased rapidly to a high level and then sharply fell to zero. At this time SOz began to come through the reactor, slowly increasing to the inlet concentration. During the course of the water runs the slurry changed color from a light gray-brown taking on a deepening green hue as the H2Slevel built up but rapidly changing color to white as the H2S production fell to zero. Analysis of the spent slurry from the water runs typically indicated only about 0.7% free sulfur and 2% CaS04with the rest probably being mostly sulfite although a satisfactory material balance was never obtained using the limited analytical techniques available. Apparently considerable thiosulfate, thionates, etc., were also formed. In an effort to reduce the SO2to elemental sulfur in one step, a run was made replacing the water solvent with a mixture of ethylene glycol monoethyl ether, water, and dibutylamine (Diah et al., 19721, as in a modified Claus process, the idea being that H2S being generated in the slurry would react with unreacted SOz in the feed to form elemental sulfur. This attempt proved unsuccessful, however, failing to increase the amount of free sulfur in the spent slurry. However, it was discovered that when certain anhydrous solvents were used elemental sulfur was formed directly to the exclusion of H2S. The first anhydrous solvent test with CaS utilized dimethylformamide because it has been reported to act as a catalyst for the Claus reaction (Deal et al., 1966). This resulted in complete removal of the SOz with the formation of elemental sulfur. Other effective solvents tested with Cas included dimethyl acetamide, dimethyl sulfoxide, ethanolamine, propylene glycol, methanol, and ethanol. Dimethyl sulfoxide and ethanolamine resulted in very
y + SPENT SWRRY
,-kc%
C S p PHASE
C%EXTRerTDN
SOLVEKT
SOL'DS
EVAPORATE
SOLUBLE =IDS
Figure 2. Scheme for analyzing spent slurry.
produced if there was any water present even if COPwas excluded from the test gas. Although the results of the anhydrous tests were interesting, probably the only practical solvent would be water. As a result, later tests were carried out using a two-reactor system in which H2S was continuously generated at a steady-state composition and this HzS stream was then mixed with unreacted feed gas to produce sulfur via the Claus reaction. For these experiments the apparatus shown schematically in Figure 3 was used. In this system HzS was continuously generated by pumping the sulfide-water slurry (or solution) into the first reactor using a tubing finger pump while part of the SOz containing feed gas was bubbled into the slurry through the sparger. This HzS generator was also a 250-mL gas washing bottle with a fritted glass sparger which was modified with inlet and outlet connections for the slurry feed and discharge. This reactor also had a tapered ground-glass fitting so that a pH electrode could be inserted into the reacting slurry. Slurry agitation again was with a Teflon-coated magnetic stirrer. The H2S containing gas from the slurry reactor was then sent to a series of two 250-mL gas washing bottles containing water or other solvents where it was mixed with unreacted feed gas. In these reactors sulfur was formed by the reaction ROTA METERS
r
1
SAMPLE
SPENT SLURRY FZSERVOL R
Figure 3. Schematic diagram of continuous reactor system.
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Ind. Eng. Cham. Process Des. Dev., Vol. 18, No. 3, 1979
T I ME .MINUTES
Figure 4. Exit gas composition as a function of time for a typical CaS-H20 batch run. Table 1. SO, Removal Using CaS in Methanol'
viscous spent slurries which were very difficult to filter. All the effective anhydrous solvents gave essentially the same results, resulting in a spent slurry (solvent free basis) containing about 20% free sulfur. The data from a typical run using methanol as the solvent is shown in Table I. Just as with the spent slurry from the water system, a satisfactory material balance was never obtained, but tests showed there was considerable sulfite present with apparently some of the other sulfur-containing salts also present. Other anhydrous solvents which were tested but which resulted in little or no SO2 removal and/or free sulfur formation included 1-propanol, 1-butanol, acetone, p xylene, benzyl alcohol, and furfural alcohol. These tests were limited to materials on hand and were by no means an exhaustive investigation of all possible solvents. The filtered solids after extraction with CS2 for sulfur removal were regenerated by reaction with CO under conditions previously used to reduce CaS04 to CaS (Zadick et al., 1972). Five percent FezO3(catalyst) was mixed with the solids and reacted with CO at 650 "C while the exit gas was analyzed for COP The regenerated material was then
analyses spent solids MeOH soluble (filter cake) solids weight: free sulfur: sulfate:
51.9 g 23.4% 6.5%
21.5 g 12.4% 24.2%
' Slurry charge, 25 g of Cas, 200 m L of MeOH; gas rate, 1 3 L / h ; 15% SO,, 1 5 % 0,, 70% N,; time for SO, breakthrough, 6.25 1.1. Table 11. SO, Removal Using Regenerated Solids' analyses spent solids MeOH soluble (filter cake) solids weight: free sulfur:
21.5 g 21.3%
4.4 g
a Slurry charge, 13.8 g of regenerated material, 1 0 5 m L of MeOH; gas rate, 6.5 L i h ; 15% SO,, 15% 0,, 70% N,; time for SO, breakthrough, 5.0 h.
Table 111. SO, Removal Using Various Sulfides % of sulfur
sulfide CaS CaS Cas CaS Bas
+
K2S
Mn S Na,S Sr S Cas Cas CaS c us cu,s Cd S MoS, PbS ZnS
Fe,03
time for SO, breakthrough, h
solvent
sulfide, g
solvent, m L
MeOH acetone H2 0 H2O MeOH MeOH MeOH MeOH MeOH MeOH 1-propanol furfural alcohol 1-butanol MeOH MeOH MeOH MeOH MeOH MeOH
10 10 15 15 16.3 17 10 14 10 15 10 10
250 250 200 200 100 100 250 100 250 200 250 250
20.2 8.5 30.2 35.5 20.0 10.8 24.8 20.5 34.5 21.9 5.9 5.5
10 15 15 15 15 15 15
250 250 250 250 250 250 250
1.3 4.0 5.8 3.3 7.9 2.6 5.3
in SO, recovered as free sulfur
100 29.6 11.2 13.7 6.8 0 26.8 0.3 1.2 70.3
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Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979
497
TIME.MINUTES
Figure 5. Exit gas composition and pH of reacting solution as a function of time, 0.77 M Na2S in H20-batch
run.
T I M E ,MINUTES
Figure 6. Exit gas composition and pH of reacting slurry as a function of time, 0.77 M Cas slurry in H,O-batch
tested as before with the results shown in Table 11. Several other sulfides were also tested for SOz removal with the results shown in Table 111. The run mixing Fe203 with calcium sulfide in water slurry was made in another attempt to produce sulfur in one step since Fez03is also a catalyst for the Claus reaction. However, this did not result in increased sulfur formation. As can be seen from the data in Table 111, of the sulfide-solvent combinations tested only CaS and SrS in methanol resulted in significant amounts of free sulfur being formed. With Cas, conversion of the sulfur in SOz appeared to be 100% under the conditions of the test. Continuous Reaction with Sulfide in Water Since the use of organic slurries for SO2scrubbing would probably be impractical, the water-sulfide system was further investigated in the continuous reaction system. For purposes of this study the sulfur formation step was in-
run.
cluded only out of curiosity since the Claus reaction has been investigated in detail as a means of SO2 emission control (Struck et al., 1971). The important part of this investigation was to study the steady-state formation of HzS by reacting the SOz with a slurry or solution which was continuously added to the stirred reactor, and to study the possibility of using pH as a means of controlling the H2S production rate. The results of two batch runs using a water solution of Na2S and a slurry of CaS which were made to obtain a pH vs. time curve are shown in Figures 5 and 6. As can be seen in Figure 5, the original NazS solution had a pH of about 12 but as the SO2-containinggas stream was passed through it the pH dropped and leveled off at about 10.2 and remained at that value until there was an abrupt decrease to pH 7 when the H2S production started. The pH was then relatively constant at about 7 during the H2S production stage and then again dropped off when H2S
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Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979 20 7 7 M Cas SWRRY 4-
6 -
28-
re -
10
/
10
-
I
1
I
I
I
I
B%6
-
T I M E , MINUTES
Figure 7. H2S in generator outlet for continuous run with CaS slurry. 20 7 7 M Ha$
SOLUTION
4 -
-
6 -
%6-
v1
1"s2
4-
0 0 2-
I
OO
130
p%*u
"
I
200
0I
I
3 00
4to T I M E , MINUTES
-
"U 1
I
500
600
Figure 8. H2S in generator outlet for continuous run, NazS solution
production ceased. After about 10 h there was SOz breakthrough (pH -3.0). Significantly, there was no H2S formation for the first 200 min of the run before the abrupt pH change. The results of the batch run using a Cas slurry are shown in Figure 6. The curve is similar to the one presented in Figure 4 except that three distinct steps of H2S production are evident in this run, probably resulting because of the use of a different slurry concentration. Here the pH of the original slurry was about 11.2 and steadily decreased with SO2 addition with no spectacular pH changes as in the NazS system although there was a noticeable increase in the rate of pH change at about 250 min where the large step in HzS production started, and another increase when the H2S production stopped. Figure 7 presents the results of a run attempting to produce H2S at a steady rate using the Cas slurry. After the initial charge was added to the reactor (about 200 mL)
the reactor was run batchwise for about 250 min. Slurry was introduced when the pH reached about 8.5 to try to stabilize the H2S production at about 5 to 6% H2S in the exit gas stream. However, because of difficulties in accurately controlling the addition of slurry using the finger pump, slurry was added in increments. This resulted in a fluctuation in the pH and H2S production. At about 500 min the H2S production began to drop off, probably because too much slurry was added. Slurry addition was then stopped and the HzS production again started to approach the previous "steady-state'' value. Figure 8 presents the results of a run using the 0.77 M NazS solution. Again, the reactor was charged with about 200 mL of fresh solution and the generator run batch wise for about 240 min before starting the addition of fresh solution at a rate of about 1.2 mL/min. For this test a gas rate of 0.9 mol/h was maintained to reduce the total time required to reach a point at which H2S was produced.
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979 499
s
cas
C2.5'
S:.MZ~=SH-.OH
i s c j +SOj.
532. 502. H 2 3
eH 2 S 0 3 eH S 0 j . H ' S H - 5 ti25
4 ' . S', t
