CA1080441A - Catalytic process for removing sulfur dioxide from gas streams - Google Patents

Abstract

ABSTRACT
A process for the catalytic reduction of sulfur dioxide in gas streams containing sulfur dioxide to elemental sulfur using a reducing gas such as hydrogen or, preferably, carbon monoxide, and a material of the formula xLa2O3 . yCo2o3, where x and y are varied independently from 1-3 (including non-integers). There are also novel compositions where x ? y.
In a further aspect of this invention, a gas stream containing hydro-gen sulfide or carbonyl sulfide and sulfur dioxide is passed through a reac-tion chamber containing xLa2O3 . yCo2O3, where x and y are as set forth above, to catalytically produce at a sufficiently elevated temperature elemental sulfur with concomitant reduction of the undesired hydrogen sulfide or car-bonyl sulfide and sulfur dioxide.

Description

This invention relates to the removal of sulfur diox;de from gas streams containing sulfur dioxide. More particularly, this invention relates to the catalytic reduction of sùlfur dioxide with a redùcing gas, preferably carbon monoxide, to elemental sulfur in gas streams containing sulfur dioxide, such as flue or stack gases, gases resulting from oil or coal gasification which contain sulfur dioxide, smelter gases, etc.
Sulfur dioxide is a constituent o~ many waste gases, such as, for example, smelter gases, flue gases, off gases from coal- and oil-burning furnaces and boilers. Contamination of the atmosphere by sulfur dioxide, whether present in dilute concentrations of 0.05 to 0.3 volume perçent as in power plant flue gases or in higher amounts of 5 to 10 percent as in ore roaster gases, has been a publlc health problem for many years due to its irritating effect on the respiratory system, its adverse affect on plant life, and its corrosive attack on many metals, fabrics and building materials. Millions of tons of sulfur dioxide are emitted into the atmosphere each year in the United States due to combustion of fuel oil and coal; a maJor amount of such sulfur dioxide being produced in the generation of electric power.
Since the reduction of the sulfur dioxide content of stack gases is the key to the production of useful energy from our abundant fuels (coal and high sulfur oil) in an environmentally acceptable manner, many methods have been proposed, and are presently under study, for the removal of sulfur dioxide fron such gases. It is estimated that there are close to 50 sulfur dioxide removal processes presently under investigation in the United States.
While the processes appear technically feasible, the expense of the sulfur dioxide removal is substantial. Some of the more common processes involve ~ scrubbing of the stack gas and precipitation of the sulfur dioxide with lime-- stone as calcium s~lfite Gr, follownng oxidation, as calcium sulfate. Scrub-bing of the very large effluent gas quantities, as well as collection and disposal of the solid precipitate fron the scrubbing liquid, is expensive.
An inherently less expensive method for removing the sulfur dioxide is based on the catalytic reduction thereof with carbon monoxide or some other reductant. Neither scrubbing of a gas by a liguid nor the separation ~ 1 _ ~ .. , .; . - .... .

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, of a solid from a liquid are required in this method. This method has been tried with many different catalysts but, to date, to the best of Applicant's knowledge, such methods have one or more of three major difficulties. Ini-tially, burners, such as those operated by electrical power generation, run on fuel mixes with excess air or "lean fuel mixes". This is done to prevent the formation of explosive carbon dust and to derive more energy from the fuel. As a result of the use of the lean fuel mix, the stack gas is rich in oxygen. This oxygen poisoned many of the catalysts tried in the past, thus killing the catalytic activity thereof and reducing the overall effective-ness of the reduction process. Secondly, certain of the catalysts utilizedcatalyzed the reduction of water by carbon monoxide to fo~m carbon dioxide and hydrogen, or catalyzed the reaction o~ water and sulfur to hydrogen sul-fide and oxygen. Hydrogen reacts with sulfur to form hydrogen sulfide at temperatures as low as about 200C, and thusly, in either case, the sulfur dioxide is converted into another toxic material. Thirdly, certain of the non--specific catalysts catalized the oxidation of carbon monoxide by sulfur to form carbonyl sulfide, another highly toxic gas. These difficulties arise because of the non-specific nature of the catalytic material.
It is, therefore, the primary object of this invention to define a novel process for the renoval o~ sulfur dioxide from gas streams containing sulfur dioxide.
It is a further object of this invention ~o provide a process for the catalytic reduction of sulfur dioxide to elemental sulfur.
It is a further object of this invention to provide a process for the catalytic reduction of sulfur dioxide in gas streams containing sulfur dioxide to elemental sulfur using specific catalytic compositions which are not sub-ject to the poisoning by oxygen or water, and are less subject to ~he afore-mentioned deficiencies.
- It is a further object of this invention to provide a process for the catalytic reduction of sulfur dioxide by a reducing gas to el~mental sulfur, said process being sufficiently specific to operate with lean fuel mixtures while forming environmentally acceptable levels ~f sulfur dioxide, hydrogen ~8~3~4~L

sulf;de, or carbonyl sulf;de.
It is a further object of this invention to provide a process for the catalytic reaction of hydrogen sulfide or carbonyl sulfide and sulfur dioxide to form elemental sulfur using specific catalytic materials which are not subject to poisoning by oxygen.
Yet a still further object of this invention is to provide novel cataly-tic compositions suitable for use in the processes of this invention.
These and still further objects, advantages and features of the present invention will become apparent upon consideration of the follow;ng detailed disclosure-Accordingly, the present invention provides a composition represented by the formula xLa203 . yCo203, where x and y are independently varied from 1 to 3, inclusive (including non-integers), as the catalyst in the catalytic reduction of sulfur dioxide with a reducing gas, such as hydrogen or, prefer-ably, carbon monoxide, in sulfur-dioxide containing gas streams to ele~ental sulfur. The values of x and y set forth above should be understood to relate to the ratio of the oxides to each other, rather than being absolute values.
LaCoO3, which has a perovskite structure, is a known oxidation/reduction catalyst which has been used to reduce cis-2-butene in the presence of hydro-gen (see Libby, SCIENCE, Vol. 171, pages 499-500, February 5, 1971; and Pedersen and Libby, SCIENCE, Vol 176, Pages 1355-1356, June 23, 1972), and to oxidize carbon monoxide in the presence of oxygen to carbon dioxide (see Voorhoeve et al, SCIENCE, Vol. 177, pages 353-3549 July 28, 1972). It was suggested in each of the aforementioned articles that LaCoO3 should be tested as a potential auto exhaust catalyst. However, to the best o~ the Applicant's knowledge, it has never been suggested to use LaCoO3 as a catalytic material for the reduction of sulfur dioxide to elemental sulfur where a reducing gas such as carbon monoxide is added to the sulfur dioxide-containing gas stream. Indeed it has been reported that LaCoO3 is poisoned by sulfur dioxide ~j 30 when LaCo~3 is used as a catalyst for the conversion o~ CO to C02. See Yung-; Fang Yu Yao~ "The oxidation o~ Hydrocarbons and 00 over Metal Oxides, IV.
Perovskite-Type Oxides", a paper presented at the 76th Annual Meeting of . .

the American Ceramic Society, Paper No. 41 E 74~ April 30, 1974~ Chicago, Illinois. Furthermore, once again to the best of Applicant's knowledge, it has never been suggested to use the catalytic materials set forth above as a catalytic material for the removal of sulfur dioxide in a Claus-type reaction or for the reduction in the concentration of COS via reaction with sulfur dioxide as set forth below.
Although LaCoO3 is a known catalytic material and corresponds to a formu-lation of xLa203 . yCo~03 where x = y, the other catalytic compositions (i.e.3 where x ~ y) are presently considered to be novel preparations. Such prepara-tions can be considered to be a LaCoO3 phase in combination with one or morephases of excess La203 and/or oxides of cobalt, and/or other unidentified materials. It should be noted that the cobalt oxide used in the preparation of these materials actually exists as a mixture of cobalt oxides, but this reagent grade cobalt oxide has a cobalt assay which corresponds to 101%
Co203; accordingly, in this application the cobalt oxide will be considered to be Co203. Tests with such preparations have shown that the catalytic activity thereof compares favorably with LaCoO3 notwithstanding the presence of excess La203 or cobalt oxides. This was surprising and quite unexpected since preparations where x = 0 (Co203) and y = 0 (La203) were also tested and found to be much less active than, for example, LaCoO3. Since a dilution of the catalytic activity was expected because of the presence of the excess L~203 or cobalt oxides phases (which had a lesser catalytic activity), the comparable catalytic activity actually found for the mixed phase preparations is considered to be surprising indeed. Since the Applicant believes these mixed phase preparations to be novel per se~ it follows that, to the best of Applicant's knowledge, they have never been suggested for use as catalysts for the reduction of sulfur dioxide to elemental sulfur, especially where a reducing gas, such as carbon monoxide, is added to, or present in sufficient quantities in, the sulfur dioxide-containing gas stream, nor have been used in the catalytic reaction of hydrogen sulfide or carbonyl sulfide with sulfur dioxide to fonm elemental sulfur in the absence, or presence, of oxygen.
Also included with the scope of the applicable catalytic materials are the derivatives of the materials of the above formula resulting from (a) prere-duction thereof w;th carbon monoxide or hydrogen (see below), (b) exposure to the initial or succeeding feed s~reams (for example in a fir~t or second, etc. catalytic converter) or (c) prereduction with carbon monoxide or hydrogen and exposure to the respective feed stream(s). Identified derivatives include CoS (such as Co4S3, CoS2, Co3S4 and CogSg)~ La202Sx (such a 2 2 La202S2), La202S04 and La2Co)4, principally CoS2 and La202S. These deriva-tives can be added as mixtures of such materials, suoh asg for example a

2 and La202S, (e.g., in a CoS2; La202S molar ratio of about 2:1) or as preformed material represented by the formula xLa203 . yCo203 set forth above. To date it does not appear that the individual derivatives are adequate catalytic materials. For example CoSx when used alone sinters rapidly at the elevated temperatures encountered, and in addition leads to the fonmation of excessive carbonyl sulfide. The La202Sx does not have the desired selectivity or efficiency. It is preferred, however, to preform $he catalytic material since a more unifonm dispersion of the lanthanum oxide and cobalt oxides, or derivatives thereof, with each other, is obtained, and this appears to enhance the ~esired catalytic activity and thereby pre-vents excessive sintering and subsequent deactivation of the catalyst.
In its broadest aspects, the process of the present invention is directed to the removal of sulfur dioxide from any sulfur dioxide-containing gas stream where the above-identified catalyst is used and a reducing gas, such as hydro-gen or, preferably, carbon monoxide, is added to, or present in su~ficient quantities in, the sulfur dioxide-containing gas stream to within about + 15%, generally about ~ , of the stoichiometric amount required for complete reduction of all sulfur dioxide present to elemental sulfur. If the amount of reducing gas in the stream is sufficient, no further amount need be added thereto. However, quantities of the reducing gas can be added, or generated in situ, as necessary to provide the desired amount of reductants, relative to oxidants, in the gas stream.
The first, and presently considered to be the most important aspect of the present invention is a process directed to the removal of sulfur dioxide from sulfur dioxide-containing flue or stack gases, especially those resulting -fron coal-burning processes, oil burning processes, or any other process which produces sulfur dioxide in the tail gas. Of special interest is the particularly severe case of a stack gas resulting from a coal-burning opera-tion where the stack gas contains fly ash ~to the extent not removed by precipi-tation) and generally has a composition of about 0.32% S02, 3.2% 2~ 15%
C~2, 7 6% H20~ 0.12% nitrogen oxide, balance nitrogen, i.e., where the 02/S02 ratio is about 10:1 and the H20 content is very high (which could lead to H~S formation), to which is added about 7.2X C0. Since the fly ash that remains and other components (including oxygen) of the gas stream do not l'poison" the catalytic material of this proeess9 it is Rfective to remove the sulfur dioxide as desired. It is contemplated that the catalyst will work even better with gas streams, such as those from oil burning operations, where the 02/S02 ratio is more favorable and the level of fly ash is much lower.
- In further aspects of the invention, the process of the present invention is considered applicable to other applications where the gas stream has a higher S02 content and a lower 2 content, such as those gas streams result-ing from ore roasting, coal processing plants where coal is converted to gas and/or oil, or scrubbing systems where absorbed sulfide is oxidized to S02 to give a concentrated S02-containing gas stream, etc. Typical gas concen-trations contemplated here would be about 3-20~ S0~ 5% 2~ a few % H20, with the balance N2. The S02 in such a gas stream would be catalytically reduced, as taught herein with addition of a reductant, to elemental sulfur and any H2S fonmed, even in appreciable amounts, could be recycled through a scrubber or the product stream, after sulfur removal, can be sent to a second catalytic reactor charged with xLa203 . yCo203 where unconverted S02 and residual H2S are catalytically reacted to fonm elemental sulfur. Such H2S formation would not be prnhibitive since the bulk of the high concentra-tion of the sulfur dioxide would be removed from the stream.
Reduction to elemental sulfur proceeds inter alia, according to the known reactions:
S2 + 2C0 cat. ~S2 + 2C2 ~I) 52 ~ 2H2 cat. ~S2 + 2H2 ~C~ 8~3~g~L

Other forms of sulfur, such as S~ or S8, may also be formed. The important considerations in such processes relate to the reduction (and continued reduc-tion) of the sulfur d;oxide although oxygen, nitrogen oxides and other reduc-ible components are present, the possible reduction of sulfur diox;de to hydrogen sulfide in the presence of water, the possible reduction to carbonyl sulfide by direct reaction between carbon monoxide and the sulfur diox;de, and the fonmation of hydrogen sulfide and carbon oxysulfide by reaction of the gaseous sulfur, produced in ~he principal reduction step, with other components present in the gas stream. In tests conducted to date with gas s~reams which have high S02 levels to which have been added or generated in situ carbon monoxide to increase the concentrat;on thereof to not greater than the stoichiometric amount required to reduce all of the oxygen and sulfur present, it has been detenmined tha~ the reduction of oxygen is favored over the reduction of sulfur dioxide (in the presence of oxygen), but the sulfur dioxide reduction is not excluded ~hile oxygen is present; thus, in the presence; or absence of oxygen, substantially complete reduction o~ the sulfur dioxide to elemental sulfur can be effected at temperatures below 700C, generally between 450C and 650C; the presence of water at the elevated reaction i temperatures does not lead to the formation of unacceptable levels of hydro-gen sulfide; and carbonyl sulfide is not fonmed in appreciable amounts in the reduction process (unless the feed gas contains carbon monoxide in concen-trations greater than the stoichio~etric amount required to reduce all of the oxygen and sulfur dioxide). In addition, in the presence of water, the formation of carbonyl sulfide is further inhibited. The present process, therefore~ as it pertains to gas streams having high 52 levels3 affords distinct advantages over known processes of which Applicant is aware since, in a single stage (though multiple stages are contemplated), with a tempera-ture requirement of less than 700C, the sulfur dioxide is converted to ele-mental sulfur with a conversion efficiency greater than about 90% while forming not greater than minimal quantities of carbonyl sulfide and~ quite unexpectedly, producing only low levels of hydrogen sulfide under present operating conditions. This in itself~ is quite surprising since thenmody-~8~

namic calculations of the equ;libria for the reactions involved predict thatless reduction to elemental sulfur will occur. Therefore, the results, as set forth aboYe~ would not have been anticipated or expectedO
Sor.e hydrogen sulfide and/or carbonyl sulfide is fonmed with gas streams having low S02 and high water (less than 6%) concen~rations, such as gas streams obtained w;th coal- or oil-burning processes. However, the formation of such materials is within acceptable limits (considered to be much less than produced by other ca~alysts used for this purpose). The level of any carbonyl sulfide fonmed may be reduced in a further catalytic converter charged with xLaz03 . yCo203. In addition, activity of the catalyst is main-ta;ned for long periods of time, and the catalyst is resistant to poisoning by oxygen and functions in the presence of water vapor, thereby affording distinct advantages over other known catalysts used for the catalytic reduc-tion of sulfur dioxide with a reducing gas.
In the essential aspects of the process of the present invention, the sulfur diox;de-conta;n;ng gas stream ;s heated, if necessary, from the delivery temperature to a temperature ;n the range from about 450C to about 700C, or higher, if desired, and then, if necessary~ mixed with additional carbon monoxide or hydrogen to provide a gaseous react;on mixture hav;ng the proper (or desîred) sto;chiometric balance between the reducing gas and the sulfur dioxide (and other reducible materials). Carbon monox;de in extreme excess (i.e., less than 10% over the stoichiometrically required amount) is to be avoided since it leads to the undesirable formation of carbonyl sulfide.
The sulfur dioxide/reducing gas gaseous stream is contacted with the catalyst of the present invention in a first converter wherein the sulfur dioxide is converted to elemental sulfur and the carbon monoxide is oxidized to carbon dioxide and/or the hydrogen is oxidized to water. The elemental gaseous sulfur which is formed is then condensed from the gas stream as the gases are cooled. If desired,llthe gas stream can be contacted with a second batch of catalyst in one or more further converters, after cooling to remove elemental sulfur (between each converter), to further increase the conversion efficiency of the processing system. Process parameters, materials of con-~LC~ 3~4~L

struction and type and size of necessary process equipment can be determined by application of those chemical and process engineering pr~nciples well known in this field.
The catalyst is preferably treated with carbon monoxide or hydrogen, preferably carbon monoxide, at about 500 to about 700C, generally 700C, for about 15-45 minutes, generally about 30 minutes, at the desired flow rates of nitrogen and carbon monoxide or hydrogen. This preferred step, which can be~ and generally is, conducted with the catalyst in place ;n the catalytic reactor(s), has been found to raise the sulfur removal efficiency of the catalyst to its desired maximum prlor to the time when it is first contacted by the sulfur dioxide containing gas stream. This ensures that the efficiency of sulfur dioxide removal will be at its highest even during the first few hours of contact, whereas, in contrast, without such a prereduc-tion step, there is a definite time interval, on the order of minutes or hours, depending upon the material, for the material to reach maximum removal efficiency for the given set of operating conditions. Thus, the prereduc-tion step is desirable to ensure maximum renoval of sulfur dioxide at all times.
Satisfaetory conversion rates have been obtained with space velocities .
through the catalytic reactor(s) on the order of 2,000--36,000 volumes of gas/volume of catalyst/hour~ though both higher and lower space velocities, depending on the composition of the gas stream, are contemplated.
A particular advantage of the catalyst and process of this invention is that, upon temperature cycling from the desired operating temperature to a lower t2mperature followed by return to the desired operating tempera-ture, the catalytic conversion returns to substantially the original conver-sion rate. Thus, if there is an emergency shut-down of the system or cataly-- tic reactor(s), or other lowering of the tenpera~ure of the catalytic reactor(s), ; it does not become necessary to replace the catalytic material. Instead~
- 30 when ready, the catalytic reactor(s) can be returned to the desired operating tempera~ure and the catalytic material will perfonm substantially as well as before the tEmperature drop.
The catalyst of this invention can be pelletized by known techniques, 1~8V~
such as by preparing an aqueous slurry, casting ;n the form of a thin sheet (1/8" thick) on an inert material, followed by drying and sintering at ele-vated temperatures. The s;ntered sheet is then broken into small pellets approximately 1/8" on an eldge.
The c~talyst of this invention can also be supported by known techniques as for example, by impregnatin3 a suitable carrier material with an aqueous solution thereof, and subsequently drying and calcining the impregnated material.
Alternatively, the carrier material can be su;tably loaded with the catalyst according to known dry impregnation techniques. Suitable carrier materials include, for example, zirconia, thoria, magnesia, alumina, silica-alumina, and the like~ especially those having extended surface areas. A~ter catalyst impregnation, the catalyst/support has more active sites per unit volume which promote sulfur dioxide reduction.
In an exemplary procedure, the carr;er materials are sieved to -30/~60 mesh, an~ impregnated with aqueous solutions of lanthanum nitrate and cobalt nitrate, or other salts, such as, for example, acetates, oxalates, and carbo-'~ nates, to form, upon firing~ a carrier impregnated with about 5.5% LaCoO3.
In a further exemplary procedure, unstabilized zirconia powders or yttrium oxide-stabili2ed zirconia powders are mixed with lanthanum and cobalt nitrates to prepare agueous suspensions. ~he suspensions are extruded as 1/8 inch di~meter pellets, dried and then fired at temperatures between about 900C
and 1100C, preferably at about 900nc to about 1000C, to yield fired pellets having a nominal 5 wt. % LaCoO3 composition. Auxiliary agents, such as binders,e.g., camphor, lubricating and wetting agents, etc., can be added to the suspension to improve the extrusion or pellet forming process.
~ anthanum cobalt oxide having high surface area has been prepared using a freeze drying technique. In this procedure. a stoichiometric mixture of solutions of soluble salts of lanthanum and cobalt are frozen7 evaporated to remove the ~ater and vacuum decomposed to produce lanthanum and cobalt oxides. This mixture can then be fired in air to produce the desired material.
Similar techniques have been used to produce lanthanum cobalt ox;des supported in zirconia.
Since the pressure drop across a pellet type fixed catalyst bed can ....

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be high and, therefore, will raise the operatîng cost of a catalytic reactor, honeycomb structures, such as cordierite honeycombs7 can be used as supports for the catalytic material in the present invention as pressure drops there-through are usually lower than with pellet type structures.
The sole FIGURE is a schematic flow diagram for the desulfuri~ation of fluè gases from a coal-burning power plant according to this invention.
Referring to the FIGURE there is shown a main power plant 10 wherein high sulfur content fuel is burned in the presence of air. A high tempera-ture ash precipitator 12, for example an electrostatic precipitator~ and, if necessary, other filtering means 14, are used to remove as much as possible (preferably all) of the particulate matter from the flue gas stream. If the flue gas stream contains excess hydrogen other than that limit considered desirable, a sacrificial catalyst can be utilized in catalytic reactor 16 to remove such hydrogen to prevent (or at least limit) the subsequent forma-tion of hydrogen sulfide. A carbon monoxide generator 18, such as a coal or oil gasifier that may be as large as about 10~ of the capacity of main power plant 10, is used to furnish the carbon monoxide needed to reduce the sulfur dioxide and oxygen. Generator 10 is connected via line 20 to the flue gas stream 22 exiting from catalytic reactor 16 or, if reactor 16 is unnecessary, to the flue gas stream exiting from filter means 14. The catalytic reactor, containing the catalytic material of this invention, may be in a single stage or in multiple stages if interstage cooling is required or where a second stage is required to improve the overall efficiency of the sulfur removal process. As shown, flue gas stream 24 containing sulfur dioxide, oxygen and carbon monoxide enters interstage cooler 26 and flows countercurrently to the gas stream exiting from first stage catalytic reactor 28. After the gas stream has passed through cooler 26, catalytic reactor 28 and then cooler 26 again, the sulfur formed in reactor 28 is removed (as at 30) from the flowing stream before the gas stream enters second stage catalytic reactor 32. Since the carbon monoxide reacts exothenmically with at least a part of the oxygen presen~, if any, it is advantageous to recover this heat in heat removal unit 34. The sulfur collected fro~ the resultant gas stream 36 in sulfur recovery unit 38 is combined with the sulfur removed at 30 and used as a valuable by-product of this process. After the resultant gas stream pases through precipitator 40 and compressor 42~ it is exhausted through stack 44. By-pass line 46 allows the gas stream to be directly exited via stack 44 to allow, for example, for catalyst replacement, emergency shutdown of the reactor system, etc.¦
The effluent from the f;rst stage ca~alytic reactor ;ncludes unconverted sulfur dioxide, hydrogen sulfide and carbonyl sulfide which are formed in the first stage reactor, and elemental sulfur which is subsequently removed.
It has been found that passage of the gas stream containing hydrogen sulfide and sulfur diox;de (after sulfur removal) through the second stage catalytic reactor, also charged with xLa303 .yCo203 (where x and y are as defined above) results in the catalytic oxidation of the hydrogen sulfide and the catalytic reduction of the sulfur dioxide to elemental sulfur with concomitant reduc-tion of the concentration of the hydrogen sul~ide and sulfur diox;de or;ginallypresent ;n the gas s~ream. This elimination of two undesired materials proceedsaccording to the well known Claus reaction:
2 H2S ~ S02 cat. 3/2S2 2 H20 (III) wh7ch calls for a definite ratio of hydrogen sulfide to sulfur dioxide of 2 to 1. Known Claus catalysts are bauxite, var;ous aluminates and iron ox;de. Adjustment ;n the hydrogen sulfidelsuifur dioxide ratio, if necessary, has been accomplished by burning the hydrogen sulfide with a careful~y controlled amount of air or oxygen in a waste-heat boiler to increase the concentration of the sulfur dioxide in the gas stream. This additional step is not neces-sary since the material (i.e.~ xLa203 . yCo203) of this invention which is charged to the catalytic reactor is not poisoned by oxygen. Thus air or oxygen can be added directly to the catalytic reactor to adjust the hydrogen sulfide/sulfur dioxide ratio if desired. This is believed to be a significant advantage of this aspect of this invention since it affords greater flexibility in the treatment of the streams being processed. If the concentration of hydrogen sulfide is insuff;cient, additional hydrogen sulfide can be added or some of the sulfur dioxide in the stream can be reduced with other reduc-tants to give the proper molar balance. In addition~ any carbonyl sulfide initially present in the feed stream is believed to be eliminated, at least in part, by the reaction:
2 COS ~ S02 cat. 2 C02 ~ 3/2S2 (IY) ;^
However, limited amounts of COS may later be formed, part;cularly if carbon monoxide is present in ~he feed stream. This aspect of this invention is also conducted at elevated temperatures, generally in the range from about 450C to about 700C, or higher; however, it has been deter~ined that these reactions can also be conduc~ed at ~emperatures in the range between about 17~C and 450C where substan~ial convers;ons ~o elemental sulfur have been effected. The actual lower t~mperature limit is determined, at least in part, for a given set of conditions (e.g., composition of feed or product streams by the vapor pressure of the sulfur formed in each of the catalytic stages. This can be determined by routine experiment once the other condi-tions are established, and may be lower than 175C under specific conditions.
Depending upon the nature and composition of the initial feed stream and the conversion(s) desired, it may be advantageous or desirable to rearrange the sequence of reactions taking place. For example, if the initial feed stream contains both hydrogen sulfide and sulfur dioxide, as may be the case - with the em;ssion from a coal gasifier, it may be desirable to ~irst oxidize hydrogen sulfide to sulfur, followed first by sulfur dioxide to sulfur, with further sulfur removal. Alternatively, a hydrogen sulfide/sulfur dioxide-containing gas stream can be processed in a single catalytic reactor withreduction in the hydrogen sulfide and sulfur dioxide concentrations as des-cribed herein. Other process variations, considered to be within the scope of this invention, will be apparent to those skilled in this art in view of this disclosure.
; The following Examples are given to enable those skilled in this are to more clearly understand and practice the present invention. They should not be considered as a limitation upon the scope of the invention, but merely as being illustra$ive and representative thereof.
EXAMPLE I:
6.517 Grams of La203 and 3.3.7 grams of Co203 which represents an excess over the 1:1 stoichiometric requirement of 0.057 9. of Co~03 were dry blended for 3 hours in a ball mill at room t~mperature and placed into an u ncovered o~

platinum crucible. The sample was placed in a furnace at 200C, the tenpera-ture raised to 500C, and held there for 30 minutes. Then the temperature was raised to 1100C and held there for two hours in air. The sample was allowed to cool to roon temperature, ground with mortar pestle, placed back in a platinum crucible, and re-fired in air at 1100C for an addi~ional two hours. The sample was again allowed to cool to roam temperatu~e, rem~ved from the furnace, ground with mortar and pestle and sieved through a 325 mesh screen to ~leld 8.7 grams o~ perovskite LaCoO3 with excess Co203.
EXAMPLE II:
6.517 6rams of La203 and 3.26 grams oF Co203 were dry ground and blended using a mortar and pestle, and fired in air at 1100C for 4 hours in an uncovered platinum crucible. After the sample had been cooled to room temperature, it was removed from the furnace, re-ground with mortar and pestle, and re-fired at 1100C for an additional 4 hours. After the second firing the sample was cooled, re-ground and sieved through a 325 mesh screen to afford perov-skite LaCoO3 (i.e., a 1:1 formulat;onlof La203 . Co203).
EXAMPLE III.
The procedure of Example II is repeated using 4.562 grams of La203 and 4.564 grams of Co203 to prepare La203 . 2Co203.
_XAMPLE IV:
The procedure of Example II is repeated using 3.259 grams of La203 and 4.89 grams of Co203 to prepare La203 . 3Co203.
EXAMPLE V:
The procedure of Example II is repeated using 6.517 grams oF La203 and 1.63 grams of Co~03 to prepare 2La203 . Co203.
EXAMPLE VI:
The procedure of Example II is repeated using 5.865 grams of La203 and 0.978 grams of 23 to pre~are 3La203 . Co203.
EXAMPLE VII:
-400 Mesh pre-synthesized LaCoO3 (for example, as prepared in a manner similar to that of Example II) is mixed with deionized water in the ratio of about 10 9. of powder to 150 ml. of water. The resultant slurry is slip-cast onto ashless filter paper which is saturated with water. A vacuum is ~ -14-J

drawn on the opposite side of the filter paper to remove the supernatant liquid. After the excess water is removed, the cast cake is dried at 70C
for 17 hours while still on the filter paper. After dry;ng, the cake/filter paper combination is fired in air at 900C for 4 hours~ followed by 100C
for 1 hour. The resultant body, 0.3 cm. thick, is diced into 0.3 cm. x 0.5 cm. rectangular pellets of pure LaCoO3. In this example, the catalyst composi-tion is in ~he configuration of a support where the catalyst material per se is both catalyst and carrier.
EXAMPLE VIII:
One 9. of -400 mesh pre-synthesized LaOoO3 (for example, as prepared in a manner similar to that of Example II) is dry blended with 19 9. of yttrium oxide (Y203; 6%) - stabilized zirconia (Union Carbide Corp., New York, N. Y.).
The mixture is combined with 8 ml. of deionized water to produce a paste which is estruded through a 0.32 cm. diameter orifice. The resultant extru-date is dried at 35C for 17 hours, sliced to approximately 0.6 cm. lengths and fired at 900C for 4 hours in air to provide yttrium oxide-stabilized zirconia pellets having nominal 5 wt. % LaCoO3 are prepared.
FXAMPLE IX:
A solution of 9.80 9. of La(N03)2 and 6.59 g. of Co (N03)2 in 34 ml.
of deionized water is added to 94.4 9. of Y203 (6~) - stabilized zirconia (sieved to -30/+60 mesh) to fonm a paste. The paste is extruded, dried, sliced and fired as set forth in Example VIII to provide Y203 - stabilized zirconia pellets having about 5.5% LaCoO3.
The procedure of the preceding paragraph is repeated with unstabilized 7ircon;a, magnesia, alumina, and alumina-sil;ca, respectively, to form catalyticpellets having about 5.5% LaCoO3.
EXAMPLE X:
In this example, which illustrates the catalytic activity of LaCoO3 in reducing sulfur dioxide to elamental sulfur, six gases (N2 saturated with H20, N2, C0~ 52~ 2 and H2) are fed to a stainless steel ~anifold. From the manifold the gases pass through a mixing chamber9 a 1" diameter, 18"
long stainless steel tube filled with 1/4" di~meter glass balls, through a preheating zone where the temperature of the gas stream is raised to approxi-~L638 ~ ~4~L

mately that of the test reactor, and then to the test reac~or, a ~" outer dia~eter tube furnace surrounding a 1/2" diameter, 18" long quartz tube having fitted joints at both ends. The catalyst, in this case the LaCoO3 of Example I, sits in the reactor, 3" above the bottom of the furnace and is suppor~ed by a small amount of f;berfrax wool. The amount of catalyst used is 0.75 grams. The effluent from the test reactor goes into a sulfur collector, a 250 ml., 2 neck heated flask. Samples of the effluent are taken from the flask for analysis with a gas chromatograph.
At a furnace tempera~ure of 600C-720C, with a flow of 12 ml./min.
of S02 and 24 ml./min. of C0, the conversion efficiency to elemental sulfur is greater than 90X. Upon addition of 84 ml./min. or 298 ml./min. of N2 to the gas stream at 680C, the conversion efficiency remains about 90%.
At a furnace temperature of 700C with a flow of 12 ml.lmin. of S02, 46 ml./min. of C0, 190 ml./min. of N2 and 9 ml./min. of 2~ or 12 ml./min f S02~ 54 ml./min. of C0, 180 ml./min~ of N2 and 9 ml./min. of 0~, with a contact time of about 0.2 second in each case, the conversion efficiency was about lOO~o.
At furnace temperature of 700C, after the catalyst had been on stream for 960 hours, with a flow of 12 ml./min. of S02, 24 ml./min. of C0 9 and 2i4 ml./min. of N2, with a contact time of 0.2 second, conversion efficiency remained at about lOO~o~
The above conditions were continued, altering among the above and other conditions, up to a total of 3700 hours, at which time the testing was discon-tinued. Catalytic activity had not notably decreased throughout that time.
EXAMPLES XI - XXIV
In the following Examples, XI - XXIV, a screening reactor syst~m (described below) has been utilized to test the relative catalytic activity of the materials embraced by this invention. The system has been set to give a conversion efficiency of about 60~o (instead of lOO~o) with LaCoO3~ thereby enabling the detection o~ still more effective catalyst compositions.
Three gases (N2, C0~ and S02) are fed to a stainless steel manifold.
From the manifold the gases pass through a 3/8" diameter, 12" long~ 21 element stainless steel static mixer (Kenics Oorp., Danvers, Mass.) then to a reactor 16~

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which consists o~ a 15" tube ~urnace surrounding a 1/2" diameter, 18" ~ong quart~ tube hav;ng fitted jo;nts at both ends. The catalyst sits in the reactor 4" above the bottom of the furnace and is supported by small amount of fiberfrax wool. The amount of catalyst used is O.S grams. The effluent from the reactor system goes into a sulfur collector, a 1/2" diameterj 8"
long pyrex tube with fitted joints at both ends and a 1~4" tube in the center which leads to a 1/4" sta;nless steel millipore filter. From the filter, the effluent passes to a Carle Automatic Sampling Valve and timer which injects samples every 10 minutes into a gas chromatograph.
The data for various catalytic compositions embraced by this ;nvention, with flow rates of 12 ml~/min. of S02, 24 ml./min. of C0, and 84 ml./min.
of N2 (catalyst volume = 0.5g cm3~ contact time = 0.29 second) are tabulated in Table I below.
By way of comparison, one sample of Co203 removed 6~o S02 at 700C, but the conversion rate decreased rapidly with decreasing temperature, e.g., at 690C the conversion had decreased to 20%. A second sample removed only 27% at 700C, while a third sample removed only 5~ at 700C, and a sample of La203 removed only 43% at the same temperature. Thus~ from this data, one would not expect that combinations of LaCoO3 with either excess Co203 2U or La203 would be as effective as they unexpectedly are, as shown above.
EXAMPLES XXV ~ XXXII: -In the following Examples XXV - XXXII, a more elaborate reactor system (which permits the synthesis of gas compositions which closely simulate stack gases) is utilized. Eight gases (N~, C0, S02 in N2, N0 in N2, 2~ H2, C2 and CH4~ are fed into a stainless steel manifold. From the manifold the gases pass through a 1" diameter, 18" long stainless steel tube, filled with 1/4" diameter glass balls, which serves as a mixing chamber~ then to a water injection zone consisting of a Sage Model 341 syringe pump having a 10 ml.
plast;c syringe connected to the system via a 1/8" polyethylene line. After the water injection 7One, the gases flow through an inverted, heated, U-shaped glass tube which leads directly to the reactor, a 15" tube furnace surrounding a 1/2" diameter, 18" long quartz tube having fi~tted joints at both ends.

~L~8~3~ L

The catalyst sits in the reac~or 4" above the bottom of the furnace and is supported by a small amount of fiberfrax wool. The amount of catalyst used is 0.5 grams of unsupported material or 1.5 grams of supported material.
The effluent are taken from the flask by syringe for analysis with a gas chromatograph.
Reaction conditions are not optimal, but were chosen such that small changes in the inccming gases or catalytic act;vity and select;v;ty would effect large changes in the products. Flow rates are set such that the reac-tor is not able to effect maximum conversion. This affords greater flexi-bility in determining the effect and different reaction parameters, as varied,upon the product stream.
The 02/S02 ratio is maintained constant at 8.0/0.8 = 10. Total reductants (H2, C0, H2S, and CH4) to oxidants (2 and 52) is maintained constant at 18/8.8 = 2.04, or just above stoichiometric requirements. For example not containing H20 in the feed strean, the N2 flow rate is increased by 18 ml./min.
to keep the overall feed flow rate constant. No adjustment in the overall feed flow rate is made when 0.3 ml.min. of N0 is added to the feed stream.
EXAMPLE XXV:
The catalyst is pelletized LaCoO3 prepared according to the procedure of Example VII except the dried cake is fired at 1100C for one hour before being diced into pellets. The feed is 50 ml./min. of 1.6% S02 (about 3243 ppm), 114 ml./min. of N2, 8.0 ml./min of 2~ 38 ml./min. of C02 (about 15%), 18.7 ml./min. of H20 (about 7.6%) and 18 ml./min. of C0 (about7.2%). Reactor temperature is 700C. After passage through the reactor, 400 ppm S02, o.2%
and o.25% C0 remain, and 850 ppm H2S and 400 ppm COS are formed. This corres-ponds to an S02 removal efficiency of about 88%; decreasing the flow rate (equivalent to increasing the contact time) to 227 ml./min. by decreasing the N2 f7OW rate increases the conversion to 91%.
EXAMPLE XXVI
-

3~ Example XXI is repeated except no water is added and the nitrogen flow rate is increased to 132 ml./min. After passage through the reactor, 380 ppm 52' o.08% 2 and 0.1% C0 remain, and less than 20 ppm H2S and only a trace of COS is formed.

EXAMPLE XXVII;
Example XXV is repeated except that the C0 flow rate is decreased to 9 ml./min., the N2 flow rate is decreased by 57 ml./min.9 and 9.0 ml./m;n.
of CH4 and 57 ml./min. of 0.53% N0 in N2 (equivalent to 0.3 ml./min. of N0;
1222 ppm N0) are added to the feed stream. After passage throuyh the reactor, 100 ppm S02, 2.1% CH~, 0.06% 2~ and a trace of C0 remain, and 100 ppm H2S
and 42 ppm COS are formed. S02 removal efficiency is about 97%.
EXAMPLE XXVIII:
Example XXV is repeated except the yttrium oxide stabilized zirconia -LaCoO3 pellets of Example VIII are utilized as the catalyst material. After passage through the reactor, 366 ppm S02, 0.08% 2 and 0.25X C0 remain, and 1300 ppm H2S and 190 ppm COS are formed.
EXAMPLE XXIX;
Example XXVIII is repeated except 0.3 ml./min. ( 1200 ppm) of N0 are added to the feed stream. After passage through the reactor, 250 ppm S02, 0.10% 2 and 0.25% C0 remain, and 1300 ppm H2S and 160 ppm COS are formed.
This Example and Examples XVII and XXXI show that the addition of N0 assists in the reduction o~ S02.
EXAMPLE XXX:
Example XXVIII is repea~ed except the flow of C0 is decreased to 9 ml./min., and 9 ml./min. of H2 are added to the feed stream. After passage through the reactor, 190 ppm S02 and 0.10% 2 remain, and 1300 ppm H2S and 100 ppm COS are formed. This run shows that H20 hinders the formation of COS.
EXAMPLE XXXI:
xample XXX is repeated except 0.3 ml./min. (1200 ppm) of N0 are added to the feed stream. After passage through the reactor, 94 ppm S02, 0.08%
2 and 0.45% C0 remains, and 1200 ppm H2S and 120 ppm COS are formed. This Example and Examples XXVII ànd XXIX show that the addition of N0 assists in the reduction of S02.
EXAMPLE XXXII:
Example XXYIII is repea~ed except no water is added and the nitrogen flow rate is increased to 132 ml./min. After passage through the reactor, ... ....

165 ppm S02 and O.lOX remain, and 1200 ppm H~S and 300 ppm COS are formed.
In comparison with Example XXX~ this run shows that the presence of water hinders the formation of carbonyl sulfide. The run also shows that H2 is an effective reductant, but also causes the formation of H2S with this sup-ported catalyst.
EXAMPLE XXXIII:
In this Example, a reactor system is utilized in conjuction with a coal-burning stove to test the effect of the fly ash from the coal on the catalysts.
A portion of the flue gases from the stove were drawn off through a 1" dia-meter stainless steel line to which the reductant (C0) was intruduced. Inaddition, since the S02 produced from the burning of the coal was below the amount normally expected~ additional S02 was also injected at this point.
The gas stream is then passed through an ash filter to remove a portion of the particulate matter, and then through a stainless steel mixing tube to ensure a homogeneous gas mixture. The catalyst is placed in a reactor tube heated by a 20" Lindberg tube type furnace which maintains the catalyst bed at the desired temperature. The sulfur which is produced in the catalytic reactor is condensed out in a trap just below the furnaee. The system also has gas sampl;ng ports before and after the reactor, and after the sulfur trap, for obtaining gas samples for analysis. Four catalyst compositions (LaCoO3 on W-A1203, LaCoO3 on Norton SA-3232(A1203 + SiO2), and pelletized LaCoO3 (2 different samples)) were tested for catalytic conversion in the reactor system before passing flue gases over the catalyst to detenmine initial activity. The coal stove was then run for one week, and the conver-sion efficiency tested again. Finally, samples of fly ash from the flue are ground together with a small portion of stove ash and unburned coal, and added directly to the top of the catalyst bed, and the conversion effi-ciency tested ~gain. In all cases, the reduction of S02 was greater than 90% both before and after the poisoning attempts, and no loss in catalytic efficiency was obtained. Large amounts of H2S and COS were formed with th2 LaCoO3 on the A1203 ~ SiO2 suppor~, probably because of the formation of CoA1204 on the catalyst surface by reaction with the support material. For ; -20-~ .

this reason, A1203 ~ SiO~ is not a preferred support. Flow rates for these tests were 1538 ml./min. of N2, 17.6 ml./min. or 24.5 ml./min. of S02 and CO as required for stoichiometric balance with 2 and S02. Sone H20 is always present, usually about 0.5-1.0~O.
EXA~PLE XXXIV:
A gas stream was synthesized to contain 15% C02, 3.6% H20 (gas), 1.0%
H2S, lX 52~ 0-5% 2 and 79-9~ N2. This gas stream is representative of the emission from a first stage catalytic reactor where the sulfur dioxide-containing feed stream contains 14% S02, 3% 2 and 83% N2 and the reducing gas contains 7.6% C02, 7.1% H20 (gas), 17-5% H2~ 19.2% C0 and 48.6~ N2, the composite stream having 3.6% C02, 3.4X H20 (gas)~ 8.4~o H2, 9.2% C0, 7.3%
S2' 1 5Z 2 and 66.6~ N2.
Using the reactor of Example X at a temperature of 660C with a gas hourly space velocity of 2000 over a material represented by the formula La203 . 1.02 Co203, the H2S ~as reduced by more than 94% to 0.055% and the S2 was reduced by 24~ to 0.76%.
~~ After running 24 hours under the above conditions, the 2 was removed from the feed stream so the S02 was the only oxidant present. The exit stream ; then contained 0.089% H~S (91% removal) and 0.57% S02 (43% removal). These values indica~e that reduction of the S02 via the Claus reaction in the pre-vious case is partially offset by some oxidation of the H2S to S02 by 2 More importantly, however~ these values indicate that the material charged to the reactor is not poisoned by air or oxygen in the feed stream.
The H2S in the feed stream was then increased to 2% (2 still at 0~O).
Under these conditions, the ex;t stream contained 0.14% ~2S (93% removal) and 0.23~ 52 (77% removal).
0.5% was then added to the feed stream and under these conditions the exit stream contained 0.11~ H2S (~5% removal) and 0-59% 52 (41% removal).
Finally, the H2S in the feed stream was increased to 3%. Under these condi~ions, the exit stream contained 0.66% H2S (78% removal) and 0.43~ S02 (57% removal).

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... .. ..

~Lt~8(3~ L

This Example illustrates the various ways, contemplated by th;s inven-tion, of processing a gas stream containing S02 and H2S to convert both to elemental sulfur, thereby reducing the concentration of each in the gas stream.
As can be seen from this Example, the system parameters are such that many options are available, depending upon the products, and the concentrations thereof, which are desired to be ;n the exit or final product stream. Thus, for example, by passing an initial feed stream having a S02 content of 14%
through two separate catalyt;c converters first with a reducing gas (first converter) and then with the addition of oxygen if necessary (second converter),with sulfur removal in between, more than 90% of the S02 is converted to elemental sulfur with the generation of less than 0.1% H25. This is achieved notwithstanding the presence of minor anounts o~ oxygen in the feed streams which do not, as set forth above, poison the material ch`arged to the catalytic converters.
EXAMPLE XXXV:
Example XXXIV is repeated with a feed stream additionally containing a minor amount (ca. 1-1.5%) of methane with similar results.
EXAMPLE XXXVI
A gas stream containing 15% C0, 3.5X H20 (9?. 7-0~ H2S, 3~G~o 52~ 0-5%
2 and 71% N2 (all volume %) is passed through the reactor of Example X charged with 0.7 cc. of La203 . 3Co203 of Example IV at a temperature of 700C and a gas hourly space velocity of about 2000 v/v/hour (corresponding to a flow rate of 23.3 ml./min. and a residence time of 1.8 sec.), the H2S is reduced by 66% to 2.40%, the S02 is reduced by 81~ to 0.58%, and only 0.070~ of COS
is formed. By difference from 10%, the overall sulfur removal efficiency in this single pass is about 70%.
EXAMPLE XXXVII:
Example XXXVI is repeated using 0.7 cc. of 3La203 . Co203 of Example VI. The H2S is reduced by 63% to 2.60X~ the S0~ is reduced by 87% of 0.40~, and only 0.100% COS is ~onmed. By difference from 10%, the overall sulfur r~moval efficiency in this single pass is about 69~.
.

EXAMPLE XXXVIII:
491 G. of La203 and 237.7 9. of Co203 are ball milled for 2 hours then fired for 24 hours at 1100C. After cooling to room temperature, the mixture is ball milled for 4 hours and fired at 1100C for an additional 24 hours.
The material is cooled to 700C, held there for 16 hours, then cooled to room temperature. 669 G. of the resultant LaCoO3 (actually La203 . 1.02 Co203) is m;xed with 26.76 9. (4%) of methocel ~inder and suffic;ent water is added to make a thick paste. Pellets are made by casting this paste into 1/4" diameter, 1/4" deep holes in an aluminum block~ drying the paste and punching out the pellets. The pellets are then annealed for 15 hours at 700C, then gently ground and a portion tha~ passed a 20 mesh screen but not a 60 mesh screen used as described below.
Example XXXVI is repeated using 0.7 cc. of the material prepared in the preceeding paragraph. The H2S is reduced by 80% to 1.4G%, the S02 is reduced 83% to 0.50%, and only 0.024% COS is formed. By difference from 1O%J the overall sulfur removal efficiency in this single pass is about 8.%.
EXAMPLE XXXIX:
A gas stream containing 7.3% S02, 1.2% CH49 8.4% H2, 3.~% H20 (9), 3.6%
C02~ 1-5% 2~ 9.2% C0, and 65.4% N2 ~all volume %) is passed through the reactor of Example X charged with powdered LaCoO3 at a temperature of 700C, a gas hourly space velocity of 2000 v/v/hour, and a reductant (C0 ~ H2) to oxidant (S02 + 2) ratio of 1.04. The S02 is reduced 85% to 1.06X, while only 1.02% of H2S and 0.080% of COS is formed.
EXAMPLE XXXX:
In this Example, which illustrates the catalytic activity of commercial size pellets, 600 grams of the pellets of Example XXXVIII are placed on a perforated silica platfonm inside a silica reactor tube that is 1.78" in diameter and 36" high. The resultant catalyst bed is about 5" deep. Two gas mixtures, one conta;ning 2 and N2, and the second containing C0, H2, CH4, C02 and N2, are combined with S02 and H20 to form a mixture having the composition as in Example XXXIX. These gases are passed into a si1ica mixing chamber, and then into a preheating zone o~ the silica reactor tube which ,, .

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contains short sections of silica tubing as a heat transfer medium. The gases next pass through the catalyst bed and into a sulfur collector. The S2 is reduced by 93% to 0.50YO, while only 1.30% of H2S and 0.070% of COS
are formed.
EXAMPLE XXXXI:
Example XXXIX is repeated using 10 1.~" diameter x 1" long Torex honey-combs coated with a total of 130 9. of LaCoO3 in the reactor of Example XXXX.
The S02 is reduced by 88% to 0.87%, while only 1.30% of H2S and 0.077% of COS are formed.
EXAMPLE XXXXII:
The material of Example XXXYIII is charged into two reactors as in Example X, which are placed in series and operated at 700C. Using a space velocity of 2000 v/v/hr., with sulfur condensation in between and adjusting the gas compositions of Example XXXIX so that the reducing gas is 4% in excess over that required to reduce both the oxygen and dulfur dioxide, the exiting gases contain 1.28% H2S, 0.070% COS and 0.76% S02 after the first reactor9 and 0.7~% H2S, 0!044% COS and 0.20% S02 after the second reactor.
EXAMPLE XXXXIII:
33.68 9 of lanthanum acetate La(C2H302)3 . nH20 (assay 41.24% La) and 24.91 9 of cobalt acetate Co (C2H302)2 . 4H20 were dry blended for 4 hours in a ball mill at room temperature, transferred to a porcelain crucible and dried at 150C for 4 hrs. It was then placed in a furnace and the temperature raised to 200C and held there for 2 hours. The sample was then fired at 350C for 4 hours.
A gas stream was synthesized to contain 15% C02, 7% H25, 3.0~ S02, 0 5X 2 and 74.5% N2. Using the reactor of Examples XXV-XXXII with a gas hourly space velocity of 2000 over the material of this Example (i.e., La203 . Co203), the existing components3 as a function of reactor temperature - as measured just above the catalyst bed, were found to be:

... . .. ..

38~34~4~L
S % (by Overall Temperature (C) H2S % COS X S02 % difference conversion from 10%) to S
:
485 0.62 0.015 1.65 7.715 77.2~
440 0.15 0.015 1.44 ~.395 ~4.0%
400 0.16 0.014 1.42 8.406 84.1%
330 0.15 not de- 0.68 9.170 91.7%
tected 250 0.08 not d~ 0.73 9.190 91.9 tected On the basis of the in;tial concentration of sulfur-containing components, the results at 485C amount to an overall conversion to elenental sulfur of 77.2% and the results at 250C amount to an overall conversion to elemental sulfur o~ 91.9X. The occasional apparent inconsistency in the trends of the H2S and S02 concentra~ions w~th tenperature i5 due to the difflculties in the control of the balance of these components with the flow meters used for this particular system.
EXAMPLE XXXXIV:
Example XXXXIII is repeated except that instead of ~iring at 350C, the sample was fired twice for 2 hours at 700C with mixing in a mortar and pestle between these last two firings.
A gas stream was synthesized to contain 15% C02, 2% H2S , 1% S02, 3.5X H20 (9) and 78.5% N2. The reactor has two furnaces (of the configura-tion described in Examples XXV-XXXII) in series with a sulfur condenser in between. The components exiting from each stage, as a ~unction of temperature as measured in the catalyst beds, and gas hourly space velocity, were found to be:

. S % (by Overall Tempe~ature difference conversion (C ) Stage ghsv H2S % S02 % from 3X) to S

AFTER 1 HOUR FROM START OF RUN:
384 1 6000 0.40 0.23 2.37 79%
340 2 6000 0.~6 0.20 2.74 . 91.3%
3~0 1 3698 0.42 0.18 2.40 80.0%
350 2 3698 0.07 0.10 2.93 97.7%

~Lq~8~9~4~L
AFTER 24 HOURS FROM START OF RUN:
300 1 2000 0.25 0.12 2.63 87.7%
300 ? 2000 0.10 0.07 2.83 94.3%
280 1 2000 0.24 0.10 2.66 88.7 260 2 2000 0.03 Q.02 2.95 ~8.3 250 2 2000 0.02 0.025 2.955 g8.5%
240 2 2000 0.02 0.02 2.g6 98.7%
~14 2 2000 0.02 0.015 2.965 98.8%
AFTER 48 H~URS FROM START OF RUN:
300 1 3200 0.18 0.09 2.71 90.3~
214 2 3200 0.009 0.040 ~.g51 98.~%

No COSIwas detected under any of the aboYe conditions. The ~ina; result, based on the initial 3X of sul~ur components, represents a 90.3% conversion to sulfur in one stage and a 98.4% conversion to sulfur a~ter a second stage.
In certain instances whére the gas stream has a composition different from that set forth above or used in the above Examples, the catalytic conversion efficiency may be on the order or 80~ or so. However, under appropriate conditions and with properly constituted gas streams, conversion efficiencies on the order o~ 90~0 can be obtained.
While the present ~invention has been described with reference to specific embodiments thereof, it should be understood by those skilled in this art that various changes may be made and equivalents may be substi-tuted without departing from the true spirit and scope of the invention.
In addition, many modifications can be made to adapt a particular situation, material or composition of matter, process, process step or steps, or then-present objective to the spirit of this invention without departing from its essential teachings. ~.
~ .
.

Claims (39)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for removing sulfur dioxide from a gas stream comprising passing a gas stream containing sulfur dioxide and a reducing gas selected from the group consisting of carbon monoxide, hydrogen, hydrogen sulfide, carbonyl sulfide, and mixtures thereof through a reaction chamber containing a catalyst composition comprising:
a) xLa2O3 . yCo2O3, where x and y are varied independently from 1 to 3 inclusive, including non-integral values, or b) derivatives of xLa2O3 . yCo2O3, wherein x and y are defined as above, formed by pretreatment thereof with said gas stream containing sulfur dioxide and a reducing gas or pretreatment with said reducing gas, to catalytically produce at a sufficiently elevated temperature a product stream containing elemental sulfur, and thereafter removing said elemental sulfur from said product stream.

2. A process for removing hydrogen sulfide and sulfur dioxide from a gas stream comprising passing a gas stream containing hydrogen sulfide and sulfur dioxide through a reaction chamber containing xLa2O3 . yCo2O3, where x and y are varied independently from 1 to 3, inclusive, to catalytically produce at a sufficiently elevated temperature a product stream containing elemental sulfur and water, and thereafter removing said elemental sulfur from said product stream.

3. The process of claim 2 wherein x - y.

4. The process of claim 2 wherein x ? y.

5. The process of claim 2 wherein said gas stream contains oxygen.

6. The process of claim 2 wherein said xLa2O3 ? y Co2O3 is supported on a magnesia or zirconia carrier.

7. The process of claim 2 further including pretreating in situ said xLa2O3 ? yCo2O3 with carbon monoxide or hydrogen at an elevated temperature prior to contact with said gas stream.

8. The process of claim 2 further including pretreating said xLa2O3 ? yCo2O3 with carbon monoxide or hydrogen at an elevated temperature in a furnace different from said reaction chamber, and charging said pretreated material to said reaction chamber.

9. The process of claim 2 further including pretreating said xLa2O3 ? yCo2O3 with a hydrogen sulfide or sulfur dioxide containing gas stream at an elevated temperature in a furnace different from said reaction chamber, and charging said pretreated material to said reaction chamber.

10. The process of claim 2 further including pretreating said xLa2O3 ? yCo2O3 with carbon monoxide or hydrogen and sulfur dioxide or hydrogen sulfide at least one elevated temperature in one or more furnaces different from said reaction chamber and charging said pretreated material to said reaction chamber.

11. The process of claim 2 wherein said elevated temperature is in the range from about 175°C to about 700°C.

12. A process for removing hydrogen sulfide and sulfur dioxide from a gas stream comprising heating a gas stream containing hydrogen sulfide and sulfur dioxide to a temperature from about 450°C to about 700°C, passing said heated gas stream through a reaction chamber containing xLa2O3 ? yCo2O3, where x and y are varied independently from 1 to 3 inclusive, to catalytically produce a product stream containing elemental sulfur and water, and thereafter removing said elemental.
sulfur from said product stream.

13. The process of claim 12 wherein x = y.

14. The process of claim 12 wherein x ? y.

15. The process of claim 12 further including the step of adding air or oxygen to said gas stream to oxidize a portion of said hydrogen sulfide to sulfur dioxide whereby the ratio of hydrogen sulfide to sulfur dioxide in said gas stream is adjusted to about 2:1.

16. The process of claim 12 wherein said xLa2O3 ? yCo2O3 is supported on a magnesia or zirconia carrier.

17. A process for removing hydrogen sulfide and sulfur dioxide from a gas stream comprising passing a gas stream containing hydrogen sulfide and sulfur dioxide through a reaction chamber containing derivatives of xLa2O3 ? yCo2O3 after exposure to said gas stream, derivatives of xLa2O3 ? yCo2O3 after prereduction with carbon monoxide or hydrogen, or derivatives of xLa2O3 ? yCo2O3 after prereduction with carbon monoxide or hydrogen and exposure to said gas stream, where x and y are varied independently from 1 to 3, inclusive, to catalytically produce at a sufficiently elevated temperature a product stream containing elemental sulfur and water, and thereafter removing said elemental sulfur from said product stream.

18. The process of claim 17 wherein x = y.

19. The process of claim 17 wherein x ? y.

20. The process of claim 17 wherein the temperature of said gas stream in said reaction chamber is from about 450°C to about 700°C.

21. The process of claim 17 further including the step of adding air or oxygen to said gas stream to oxidize a portion of said hydrogen sulfide to sulfur dioxide whereby the ratio of hydrogen sulfide to sulfur dioxide in said gas stream is adjusted to about 2:1.

22. The process of claim 17 wherein said derivatives are supported on a magnesia or zirconia carrier.

23. The process of claim 17 wherein said derivatives include CoS2 and La2O2S.

24. The process of claim 17 wherein said derivatives include CoS2 and La2O2S in intimate mixture.

25. The process of claim 17 wherein the temperature of said gas stream in said reaction chamber is from about 175°C
to about 700°C.

26. A process for removing sulfur dioxide from a gas stream containing sulfur dioxide comprising passing a gas stream containing sulfur dioxide and carbon monoxide or hydrogen through a reaction chamber containing derivatives of xLa2O3 ? yCo2O3 after prereduction by carbon monoxide or hydrogen, derivatives of xLa2O3 ? yCo2O3 after exposure to said gas stream or derivatives of xLa2O3 ? yCo2O3 after prereduction by carbon monoxide or hydrogen and exposure to said gas stream, where x and y are varied independently from 1 to 3, inclusive, to catalytically produce at a sufficiently elevated temperature a product stream containing elemental sulfur and carbon dioxide or water, and thereafter removing said elemental sulfur from said product stream.

27. The process of claim 26 wherein x = y.

28. The process of claim 26 wherein x ? y.

29. The process of claim 26 wherein said gas stream contains oxygen.

30. The process of claim 26 wherein said carbon monoxide or said hydrogen present in said gas stream is within +15% of the stoichiometric amount required for the complete reduction of all oxidants in said gas stream.

31. The process of claim 26 wherein the temperature of said gas stream in said reaction chamber is from about 450°C to about 700°C.

32. The process of claim 26 wherein said derivatives include CoS2 and La2O2S.

33. The process of claim 26 wherein said derivatives include CoS2 and La2O2S. in intimate mixture.

34. The process of claim 26 wherein said derivatives are supported on a magnesia or zirconia carrier.

35. A process for reducing the concentration of carbonyl sulfide and sulfur dioxide in a gas stream comprising passing a gas stream containing carbonyl sulfide and sulfur dioxide through a reaction chamber containing a material represented by the formula xLa2O3 ? yCo2O3, where x and y are varied independently from 1 to 3, inclusive, derivatives of said xLa2O3 ? yCo2O3 after exposure to said gas stream, derivatives of said xLa2O3 ? yCo2O3 after prereduction with carbon monoxide or hydrogen, or derivatives of said xLa2O3 ? yCo2O3 after prereduction with carbon monoxide or hydrogen and exposure to said gas stream, to catalytically produce at a sufficiently elevated temperature a product stream containing elemental sulfur, and thereafter removing said elemental sulfur from said product stream.

36. The process of claim 35 wherein the temperature of said gas stream in said reaction chamber is from about 450°C to about 700°C.

37. The process of claim 35 wherein x = y.

38. The process of claim 35 wherein x ? y.

39. The process of claim 35 wherein said material or said derivatives are supported on a magnesia or zirconia carrier.