INDUSTRIAL GASES treating flue gases from a power plant, restricted considerably the range of conditions that could be investigated profitably in the laboratory. The tempera. ture of the flue gas is about 150" C. Its dew point, about 55' C., must be considered when cooling the gas by adiabatic evaporation of water. Wet scrubbing systems should be operated preferably a t temperatures below the boiling point of the scrubbing solution. In this study, scrubbers were operated a t temperatures ranging from 55' to 75' C. Data in Table I confirm reports that maximum concentration of sulfuric acid obtainable by the process is about 40%. Sulfur dioxide recovery and ozone efficiency were higher when more dilute acid was used as scrubbing liquor. Ozone efficiency increased with decrease in proportion of ozone and with increase in retention time. Variations in manganese content of the scrubbing liquor and the gas-to-liquor ratio, within the limits tested, had no pronounced effect on the efficiency of the process. Other tests showed that variation in temperature within the range, 55' to 75' C., had no measurable effect on the process, and that ozone efficiency increased with relatively large increases in sulfur dioxide content of the gas. Retention time required for high recovery was also greater for more concentrated sulfur dioxide. Dilute sulfuric acid made by the direct manganese-ozone processes would have low sale value; it would need concent trating to obtain a commercial-grade product. Since stack gas must be cooled before scrubbing, its sensible heat might be used to concentrate the acid. Maximum concentration of acid that can be made by evaporating water with such sensible heat is governed by the water vapor content of the gas and its temperature. Flue gas contains about 12.2% (92.7 mm. of mercury) of moisture and its temperature is about 150' C. This would equilibrate with 81% sulfuric acid. If sensible heat in flue gas between temperatures of 15O'and 55' C., were used in concentrating scrubber acid, results calculated as follows show that to obtain 78% acid (60' Be.), the solution would have to contain 15% or more of sulfuric acid. Interfering Substances in Flue Gases. Kashtanov and Ryzhov (78) reported
that phenol, sometimes present in coal combustion gases, poisons the manganese catalyst. Johnstone (72) noted that the catalytic effect of manganese was much less for combustion gases than for:simulated flue gas. Preliminary tests in this work showed that less than 0.1% of phenol vapor in simulated flue gas almost completely prevented recovery of sulfur dioxide by the manganese-ozone process. Also, in tests with flue gases from a power-plant and a pilot-plant coal burner, recovery of sulfur dioxide was much less efficient than from simulated flue gas. Thorough removal of suspended matter did not improve recovery; it was concluded that substances interfering with the manganese-ozone catalytic process are gaseous. They were not identified, however, and no way was found to counteract their adverse effects on the process. Data for combustion gases are shown in Table 11. The scrubber solution, 10% sulfuric acid, contained 0.3 gram of manganese per 100 grams of water; the temperature was 75' C. and the gasto-liquor ratio ranged from 25 to 100. The power plant gas contained0.19% sulfur dioxide, 5.5% oxygen, 11.7% carbon dioxide, and 12.2% water. The pilot plant gas contained 0.25% sulfur dioxide and 2.4% oxygen. The results showed that recovering a high percentage of sulfur dioxide from flue gas would require a retention time of the order of 90 seconds-an exceptionally long period for such a n operation. The efficiency of ozone utilization, even with a retention time of 90 seconds, was very low. The economic feasibility of the manganese-ozone process depends largely on
Table
21.6 35.6 43.6 56.7
15 15.1 15.5
77.8 81.0 93.4
Sulfur Dioxide Recovery from Flue Gases
Ozone in Retention SOa Input Gas, Time, Recovery, P.P.M. Sec. % Power Plant Gas 40 60 64 72 104 120
Calculated Acid Concentrations Using Sensible Heat H804, % HZSO4, % Scrubber Concd. Scrubber Concd. acid product acid product 10 12 13 14
II.
260
45 44 88 88 88
44 45 88 44 44 88 88 88
Ozone Efficiency5
35 60 70
16 19 21
80
21
75 65 65 75 65 70 90 90
20
95
13
10 13 10
11 15 15 8
Pilot Plant Gas 61 1OOb 134b 206*
'
66 101 105 110
41 64 65 61
19 16 14 8
Moles SO2 recovered per mole 0:added. Averages of 3 to 7 determinations.
size of equipment required, cost of operation, and on ozone efficiency. For a reliable estimate of the cost of the physical plant and its operation, a pilot plant study of the process is needed. Hann (7) estimated that ozone can be made for about 10.5 cents per pound. At this rate, its cost, in dollars per ton of sulfuric acid recovered, would be 103/E, where E is the efficiency of ozone utilization in moles of sulfur dioxide recovered per mole of ozone added. The highest efficiency of ozone utilization obtained in the studies with simulated flue gas (36 seconds' retention time) was 200; this corresponds to a n ozone cost of $0.52 per ton of sulfuric acid recovered. With combustion gases, however, the highest efficiency was 21 ; this corresponds to a cost of about $5.00 per ton of sulfuric acid recovered. Through further study, the substances in coal-combustion gases that interfere with the manganese-ozone process may be identified, and a method for their removal may be developed. Until these problems are solved, the process is probably not economically practical for recovering sulfur dioxide from flue gases of sulfur dioxide. I t might be useful, however, for recovery from gases not derived from coal combustion-e.g., exhaust gases from sulfuric acid plants and sulfide-ore roasters.
Direct Ammonium Sulfate Process This process is similar to the direct sulfuric acid process-the reactions are catalyzed by a combination of manganese in the scrubber solution and ozone in the gas. Absorption equipment and operating procedure are essentially the same. But it differs in that sulfuric acid formed in the scrubber solution is neutralized with ammonia to form a concentrated solution of ammonium sulfate. The scrubber solution is maintained slightly acidic to prevent absorption of carbon dioxide and loss of ammonia. Solid ammonium sulfate ma) be recovered by a standard process; part of the heat required might be recovered from flue gas. I n tests with simulated flue gas to which about 30 p.p.m, of ozone was added, scrubber solutions containing either 20 or 3oy0 ammonium sulfate and 0.3 gram of manganese per 100 grams of water recovered from 7 5 to 1 0 0 ~ of o the sulfur dioxide as the sulfate, when the retention time was 18 to 36 seconds. Corresponding ozone efficiencies ranged from 83 to 146. Recovery of sulfur dioxide and utilization of ozone were improved by holding the scrubber liquor for 2 hours in a n open reservoir before returning it to the scrubbers. The adverse effect of phenol additions to the VOL. 49, NO. 3
MARCH 1957
393
simulated flue gas was much less in the ammonium sulfate process than in the direct sulfuric acid process. I n tests with combustion gas from a pilot-plant coal burner, under stead>-state conditions? 64y0 of the sulfur dioxide was recovered as ammonium sulfate with a retention time of 8.4 seconds. Ozone content of the flue g a s \\.as 20 p.p.m. and ozone efficienck- kvas 70. This corresponds to a COS[ of $1.03 (76.3/E) per ton of ammonium siilfate. This would be a minor item in total cost of ammonium sulfate. Results obtained in the study of the direct ammonium sulfate procrss are preliminary; they cannot be used for a complete evaluation of the procrss. T h e data indicate. however, rhat the process is technically feasible for rccovering sulfur dioxide from powcr-plant flue gases. It might be suitable also for recovering sulfur dioxide froin other gases, such as exhaust gases from sulfuric. acid plants and sulfide-ore roasters. Its economic feasibility depends largely on the relationship betLvren cost of ammonia and market value of ammoiiiu~n sulfate. The price of the ammonium sulfate is uncertain: because the quantity produced probably \vould be l a r ~ r and . new markets might have to be developrd.
Reaction with Rock Phosphate to Produce a Fertilizer Material
Ctilizing the chemical potential ur acidic value of sulfur dioside in flue gas is appealing. Successful use of the sulfur dioxide to convert rock phosphate [o a fertilizer material, for example. ;~.ould give substance to this possibility. Hughes and Cameron ( 7 0 ) found that fumes from boiling sulfuric acid (sulfur trioxide and water), a t ele\-ated temperatures, converted the phosphorus of rock phosphate into a citrate-soluble form suitable for fertilizer. A maximum conversion of 31YG \vas obtained a t 600' C. They found sulfur dioxide much less effective. Tests were madc in this study to dctermine whether sulfur dioxide i n flue gas could be oxidized to trioside? Ivhicli would then react with rock phosphate 10 produce a fertilizer material. I t \vas anticipated that a catalyst \vould be needed LO accelerate oxidation. Sitrogen dioxide and \\rater vapor were added to gas mixtures containing sulfur dioxide and oxygen; the resulting mixtures were passed through a bed of granular rock phosphate. Several temperatures were tested. Best results xvere obtained a t 550' C., but reactions that produced citrate-soluble phosphate \\'ere slow and incomplete. .A feasible process was not indicated. Hughes and Cameron (10) also found that rock phosphate can be dissolved in a saturated solution of sulfur dioxide in
394
water at 25" and 100' C., under pressures of 3 to 10 atm. The reaction betiveen a slurr). of Florida pebble rock phosphate and sulfur dioxide a t atmospheric temperature and pressure proved in this \vork to be slow and ipcomplete. \2'hen manganese ion (about 0.3 gram of manganese per 100 grams of Jvater) \vas added to the slurry, absorption of sulfur dioxide \vas increased greatly. Recovery of sulfur dioxide decreased, however, with rime. \Vhen an apparent steady state was established Jvith a retention time of 32 seconds, about 50%; of sulfur dioxide in simulated flue gas \vas absorbed. Similar resulrs \\-ere obtained in tests wi[h combustion gases. Recovery of sulfur dioxide was improvtd rapid r somewhat b ~more . slurry through tlie toiver and by other changes in the procedure. Results indicated. ho\vevcr, that practical chanqes in operational procedure \tould not, to a great extent. improve the process. Recovery of sulfur dioxidr from flue gases by waction Fvith rock phospliatr, therefore: probably is not feasible.
Regenerated Manganese Oxide Cyclic Process
Sulf~rr dioxide reacts readily \tirh Ivater suspensions of nianganrse dioxide to form a solution of manganese sulfate (AInO? SO? = XInSO,). \\'hen gases containing both siilfiir dioxide and oxygen are scrubbed \vith a manganese dioxide slurry, manganese dithionate and sulfuric acid are also fornicd. \Vhen slurries of manganrse oxide ores are used, the acid extracts orher cornponents of the ores. such as iron oxide, along with manganese. These impurities retard the reaction bet\\.een sulfui. dioxide and nianSanese dioxide, and with efficient recovery of sulfur dioxide, only relatively dilute solutions (about 1 Of;) of manganese sulfate can he madfa from loi\.-grade ore and flue gases. I n practice, the extraction solution ~vould be purified by decomposing the dirhionare and removing sulfuric acid, iron, and possibly other impurities by reaction with lime or some other basic material. The manganesc sulfa1e thtxn \vould be recovered by evaiiora~ion 01' b!- heating the solution under pressure. The latter is possible because solubilit!of manganese sulfate varies inversely \\-iih the temperature. By lieating a t about 1000' C. manganese sulfate \vould be decomposed to produce a high-qradc manganese oxide sinrer and sulfur dioxide. This process has been studied for the purpose of recovering manganese from lo\v-grade ores (7, 25). Gases containing more than 170 sulfur dioxidc \Yere used. I n the present study it was assumed that the process might work satisfactorily with flue gases which contain less than 0.4% sulfiir dioxide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
11survey of the potential sources of manganese ores revealed that the quantity of useful ore within reasonable shipping distances of the Tennessee Valley is relatively small compared to that needed for removing sulfur dioxide from the flue gases of a large steam plant. Since rhe problem \vas to remove sulfur dioxide from Rue gas, rathcr than recover manganese from loiv-grade oies, a Iirocess in which the inangancse \\-odd tie rccycled \vas studied. In preliininar!. tests, regenci,ated inangaiiese oxide (calcinrd sulfate) reacted more readill. with sulfur diosidr in simulated flue gas than did muiiganrse (pyrolusite) ores. Cnder coriil)arablc conditions, the p!.rolusite-ore slurr) absorbed only 60':, of thc sulfur dioxide, Ivhercas a slurry of rcgcneratcd oxide absorbed 907;. Several batches of oxide \\-ere prepared by calcining triaiiganc:se sulfatc a t I ooo3 to 1 l o o o c:. ' l l l t ~ ["'"duct, cooled in c m t a c t \\.itti tlr! air, probably consisted of a inixture of manganese oxides, YlnO?: L f n 2 ( l 3 , LIn301, and XInO. The initial \vdter slurr)- of manganese osides conrained about 1 Cl(l,o solids. Simulated flue gas \cas scrubbed \\-ith tlie slurry in ii stainless stecl cascadc loiver (5) ar 70" c:. Retention tirnc of qas in the t o u w \vas about 3 0 scconds. Essentially all sulfur dioxide \vas r e moved from tlie q d s u n t i l alinost ;ill the solid had reacted. \ V h r n absorption of sulfur dioxide declinrd. i.cgericratcd inanganese oxide \vas added to the sIiiIr~., and complete recovrry of sulfur dioxide !vas re-established. :\t the rnd or this process, continurd for 224 olxratinq hours. cfficienc!. of sulfur dioxidc recovcry had not noticeably decrcased. Some unrcacted loiver osides of inanqanese accumdated in rhc systcrii. 'l'hr clear solution, only slightly acidic, contained manganrse equildent to 31';; anh! drous manganese sulfate. Solubility of n i a n q n e s e sulfate i n l a t e r a t 70' C. is 33.3f&, About 7 ; ;of siillur in the solution was presenr a s dichivnate. Sulfuric acid initially formed i n the scrubbing solution was Iirutralized by lo\ver osides of mangancse in the slurry as follows : LlnO
+ H2SO4 :=
Lfn304 4 2H$C),
=
XlnSOd t H 4 ) 2L11nS04 IlnO:!
LInnOs
+
H&04
= LTnSOr
+
+ 2H,O
A
1lnOa
+ H,O
Vedensky (2.5) used manqanese oxide sinter to neutralize sulfuric acid produced in the extraction step of his process, in \L.hich ra\v manganese dioxide ore was employed. It'hen slurries of Cartersville, Ga., pyrolusite ore and reagentgrade manganese dioxide were treated with simulated flue gas in the present work, a large part of the sulfur dioxide was converted to sulfuric acid Lvhich corrodrd
I N D U S T R I A L OASES the stainless steel scrubber. Other tests showed that the manganese oxide regenerating procedure can be adjusted to give the desired proportion of lower oxides just sufficient to neutralize the sulfuric acid. The scrubbing step of the cyclic regenerated manganese oxide process was studied under practical conditions. Uncleaned flue gas from the pilot-plant coal burner was scrubbed in a stainless steel cascade tower at 60” C. with a slurry of regenerated manganese oxide. Typical composition of the flue gas in volume per cent, dry basis, was sulfur dioxide, 0.26; oxygen, 2.4; carbon dioxide, 15.6; and nitrogen, 81.7. With a retention time of 4 seconds, about 80% of *e sulfur dioxide in the flue gas was absorbed. With retention times of 7.5 seconds or more, essentially all the sulfur dioxide was absorbed. -4dditions of regenerated manganese oxide were made periodically as the test was continued for about 72 hours. During this time, absorption efficiency for sulfur dioxide was essentially constant. At the end of the test, the scrubber solution contained manganese equivalent to 29y0 anhydrous manganese-sulfate. About 10.570 of the sulfur was in the form of dithionate; only a trace of sulfuric acid was present. During this 72-hour test, a considerable amount of fly ash accumulated in the slurry. Although the solid residue separated from the slurry contained iron equivalent to 7.7% ferric oxide, no iron d a s found in the scrubber solution. Vedensky (25) separated a mixture of manganese sulfate and manganese dithionate from his scrubbing solution by evaporation. The solution was first treated with product from the roasting 2401
MnSo4, ?4
Figure 1. Solubility of manganese sulfate in water as a function of temperature 0 (2)
A (11 )
step to neutralize the free sulfuric acid. Allen (7) treated his extraction slurry with air under pressure a t elevated temperature to oxidize dithionate and convert iron, magnesium, and aluminum compounds to relatively insoluble forms. Manganese sulfate was recovered by removing water in multiple-effect vacuum evaporators. Walthall and Beck (26) used the inverse temperature-solubility relationship of manganous sulfate [(2, 77) Figure 11 to recover manganese sulfate extracted from pyrolusite ores. The solution was heated under pressure to about 195’ C. Under these conditions the dithionate is decomposed and manganese sulfate precipitates. Some preliminary tests in the present work indicated that manganese sulfate in a concentrated solution can be crystallized by heating to the boiling point. Much more salt would be recovered per cycle from the solution if it were autoclaved at 195’ C. Allen’s work (7) on decomposition of manganese sulfate by sintering indicates that this step in the over-all process should be straight-forward. Pechkovski1 (22) presented evidence that under optimum conditions for decomposing manganese sulfate to manganese oxides, a mixture containing from 5 to 10% carbon is heated a t about 900’ C. in a gas that is low in oxygen. Under these conditions, essentially all the sulfate is decomposed and over 99% of the evolved sulfur is in the form of the dioxide. When the sulfate is roasted in the absence of carbon a t 1000” C., about 10% of evolved sulfur is present as the trioxide. The calcination step was not investigated in this study. The final step of the process probably is to convert.. sulfur dioxide from the roasting step to sulfuric acid. This step can be carried out in a contact acid plant, and would yield a high-grade product. The estimated cost of making contact sulfuric acid from sulfur dioxide in power-plant flue gases by the regenerated manganese oxide cyclic process compared favorably with that of making sulfuric acid from native sulfur shipped into the Tennessee Valley. Additional data are needed for devel6ping more realistic cost information. It is concluded, however, that the regenerated manganese oxide cyclic process is technically feasible. Economic feasibility depends in part on development of new market outlets for a relatively large amount of sulfuric acid. Acknowledgment
K. L. Elmore, Chief of the Research Branch, TVA, contributed encourage-. ment and helpful technical advice during the study. F. A. Lenfesty assisted in, planning and performing tysu with
power plant gases. J. D. Levine, Jr., collected data pertinent to material balances in connection with the power plant studies. Cost estimates were prepared by L.B. Hein and Z. A. Stanfield. literature Cited
(1) Allen, L. N., J r . , Chem. Ens. Progr. 50,9 (1954). (2) Beck, R. H., Boarts, R. M., Elliot, D. Z., unpublished TVA data, 1938. (3) Bretsznajder, St., Prremysl. Chem. 8, 276 (1952). (4) Copson, R. L., Payne, J. W., IND. ENC.CHEM.25,909 (1933). ( 5 ) Driskell, J. C., Chem. Ens. 63, No. 12, 224 (1956). (6) GrodzovskiY, M. K., J. Phys. Chem. (U.S.S.R.) 6, 496 (1935). (7) Hann, V., Chem. Inds. 67, 386, 515 (1950). (8) Hann, V. A., Manley, T. C., “Encyclopedia of Chemical Technology” (R. E. Kirk and D. F. Othmer, editors), vol. 9, pp. 744-9, Interscience, New York, 1952. (9) Hein, L. B., Phillips, .4.B., Young, R. D., “Problems and Control of Air Pollution” (F. S. Mallette, editor), pp. 155-69, Reinhold, New York, 1955. (10) Hughes, A. E., Cameron, F. K., IND.ENC.CHEM.23,1262 (1931). (11 ) “International Critical Tables,” vol. 4, pp. 224, 246, McGraw-Hill, New York, 1928. (12) Johnstone, H. F., IND.END.CHEM. 23, 559 (1931). (13) Kashtanov, L. I., J . Chem. Ind. (U.S.S.R.) 14, 365 (1937). (14) Kashtanov, L. I., Gulyanskaya, Tz. A., J . Gen. Chem. (U.S.S.R.) 6 , 227 (1936). (15) Kashtanov, L. I., Oleshchuk, 0. N., Ibid.,7, 1413 (1937). (16) Ibid.,8,182 (1938). (17) Ibzd., p. 341. (18) Kashtanov, L. I., Ryzhov, V. P., Izvest. Teplotekh. Inst. 1935, No. 7, p. 37. 19) Ibid., No. 8, p. 43. 20) Kashtanov, L. I., Ryzhov, V. P., J . Gcn. Chem. (U.S.S.R.) 6, 732 (1936). (21) Ibid., 8,746 (1938). (22) Pechkovskiy, V. V., J . Applied Chem. (U.S.S.R.) 28, 237 (1955); 28, 217 (1955) (Engl. translation). (23) President’s Materials Policy Commission, “Resources for Freedom,” vol. 3, Supt. of Documents, Washington 25, D.C., 1952. (24) Vasll’ev, S. S., Frolov, M. V., Kashtanov, L. I., Kastorskaya, T. L., J. Gen. Chem. (U.S.S.R.) 5 , 149 (1935). Vedensky, D. N., Eng. Mining J . 147, No. 7,58 (1946). Walthall, J. H., Beck, R. H., unpublished TVA data, 1938-39. Walthall, J . H., Miller, P., Striplin, M. M., Jr., Trans, Am. Inst. Chem. Engrs. 41, 110 (1945). Zalogin, N. G., Shukher, S. M, “Purification of Stack Gases,” Gosenergoizdat, Moscow, 1948
I
RECEIVED for review September 16, 1956 ACCEPTEDJanuary 2, 1957 Division of Gas and Fuel Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956. VOL. 49, NO. 3
MARCH 1957
395
6
