Selective oxidation of hydrogen sulfide in the presence of iron-based catalysts

Abstract

The present invention discloses a method of selectively oxidizing hydrogen sulfide to elemental sulfur, in which a H.sub.2 S-containing gas mixture contacts with an oxygen-containing gas at 50.degree.-500.degree. C. in the presence of an iron-based catalyst. The reaction product mixture contains substantially no sulfur dioxide. The iron-based catalyst, in addition to iron atom, further contains a promoter of cerium, tin or antimony.

Description

FIELD OF THE INVENTION

The present invention relates to a process for recovering elemental sulfur by selectively oxidizing hydrogen sulfide to elemental sulfur, and in particular to a novel catalyst used therein.

BACKGROUND OF THE INVENTION

In the petroleum refinery processes for producing various fuel oils such as gasoline, diesel and kerosene, etc., the sulfur contained in the crude oils is removed as hydrogen sulfide gas by hydrodesulfurization process. The highly toxic hydrogen sulfide gas is then converted to elemental sulfur in sulfur-recovery plants or so-called Claus plants. The Claus plants can be blamed for part of the hydrogen sulfide emissions, as the sulfur-recovery rate is 90-98% depending on the number of reactors used therein. During the last two decades a great number of Claus tail-gas treating (TGT) processes have been developed to increase the total sulfur-recovery efficiency. Conventional Claus TGT processes involve a hydrogen sulfide absorption step, in which a tail gas containing unreacted hydrogen sulfide is introduced into an alkaline bath. Removing the last percentages of sulfur by means of these conventional Claus TGT processes is expensive, both in terms of capital investment cost and energy consumption. In order to avoid the shortcoming of these solution-absorption type Claus TGT processes, a dry type Claus TGT process has been developed in Netherlands which comprises recovering elemental sulfur from the Claus tail gas by selective oxidation of hydrogen sulfide in the presence of a catalytically active mixture of iron and chromium oxides deposited on an alpha-alumina support �Oil and Gas Journal, 86(41), p. 68-71, (Oct. 10,1988); Catalysis Today, 16, p. 263-271, 1993!. The dry type Claus TGT process is simple and economical; however, the chromium atom contained in the catalyst is a toxic substance.

The primary objective of the present process is to provide a process for recovering elemental sulfur from a gas mixture containing hydrogen sulfide by selective oxidation of hydrogen sulfide in the presence of a iron-based catalyst.

DETAILED DESCRIPTION OF THE INVENTION

The possible reactions between hydrogen sulfide and oxygen are as follows:

H.sub.2 S+1/2 O.sub.2 .fwdarw.1/n S.sub.n +H.sub.2 O (n=6.about.8) (1) 1/n S.sub.n +O.sub.2 .fwdarw.SO.sub.2 (2) H.sub.2 S+1/2 O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O (3) 3/n S.sub.n +2H.sub.2 O.fwdarw.2H.sub.2 S+SO.sub.2 (4)

In the presence of an appropriate catalyst, such as the iron/chromium oxides used in the prior art �Oil and Gas Journal, 86(41), p. 68-71, (Oct. 10,1988); Catalysis Today, 16, p. 263-271, 1993!, the reaction (1) will take place while the reactions (2) to (4) are inhibited, i.e. the hydrogen sulfide is selectively oxidized to elemental sulfur, wherein the hydrogen sulfide is converted with a high selectivity into elemental sulfur, while the formation of sulfur dioxide (SO.sub.2) is very small even in the presence of an overstoichmetric amount of oxygen.

The present invention discloses a novel catalyst suitable for selective oxidation of hydrogen sulfide to elemental sulfur, which is an iron-based catalyst comprisings iron atom and a promoter selected from the group consisting of cerium atom, tin atom, antimony atom and molybdenum atom.

The present invention also discloses a process for recovering elemental sulfur from a gas mixture containing hydrogen sulfide, which comprises contacting said gas mixture with an oxygen-containing gas at an elevated temperature in the present of an iron-based catalyst, wherein the improvement comprises said iron-based catalyst comprising iron atom and a promoter selected from the group consisting of cerium, tin, antimony and molybdenum.

Preferably, the present iron-based catalyst has a molar ratio of iron atom to the promoter ranges from 1:100 to 100:1, preferably 1:10 to 10:1.

The iron-based catalyst used in the present process may be a metal, metal oxide, metal sulfide, or metal salt, which can be in the form of monolith, particle or pellet, or deposited on a porous carrier selected from the group consisting of alumina, silica and zeolite.

Said elevated temperature of the present process ranges from 50.degree. C. to 400.degree. C., preferably from 100.degree. C. to 350.degree. C.

Pressure has no significant effect on the present process. A suitable pressure for contacting said gas mixture with said oxygen-containing gas in the present process is about 1-10 atm.

Said oxygen-containing gas used in the present process includes (but not limited to) air, oxygen-enriched air, and pure oxygen. Preferably air is used as the oxygen-containing gas.

Said gas mixture containing hydrogen sulfide used in the present process has no limitation on the hydrogen sulfide concentration. However, in the SuperClaus-99 process the hydrogen sulfide concentration contained in the tail gas is controlled at 0.8-3 vol % �Oil and Gas Journal, 86(41), p. 68-71, (Oct. 10,1988); Catalysis Today, 16, p. 263-271, 1993!.

The stoichmetric ratio of hydrogen sulfide to oxygen in the reaction (1) is 2:1. Preferably, in the present process said gas mixture is contacted with said oxygen-containing gas with a molar ratio of hydrogen sulfide to oxygen less than 2, i.e. with an overstoichmetric amount of oxygen.

The invention will be further illustrated by the following examples. The following examples are only meant to illustrate the invention, but not to limit it.

EXAMPLES

Preparation of Catalysts

Control Example 1: Iron (III) Oxide Catalyst Deposited on .alpha.-Alumina

0.48 g ammonium iron (III) citrate �purchased from Riedel-de Haen, Germany! was dissolved in 4 ml distilled water. To the resulting solution 4 g .alpha.-alumina carrier (NGC H-218B, manufactured by Nissan Girdler Catalyst Co., Japan) was added for impregnation. The impregnated .alpha.-alumina carrier was dried in room for 24 hours with good circulation of air, and was calcined at 500.degree. C. for five hours to obtain an iron (III) oxide catalyst deposited on .alpha.-alumina carrier.

Control Example 2: Cerium (III) Oxide Catalyst Deposited on .alpha.-Alumina

0.6 g cerium (III) nitrate hexahydrate �Ce(NO.sub.3).sub.3.H.sub.2 O available from STREM Chemicals, U.S.! was dissolved in 4 ml distilled water. To the resulting solution 4 g .alpha.-alumina carrier (NGC H-218B) was added for impregnation. The impregnated .alpha.-alumina carrier was dried in room for 24 hours with good circulation of air, and was calcined at 500.degree. C. for five hours to obtain a cerium (III) oxide catalyst deposited on .alpha.-alumina carrier.

Control Example 3: Tin Oxide Catalyst Deposited on .alpha.-Alumina

0.3 g SnCl.sub.2.H.sub.2 O �available from Riedel-de Haen, Germany! was dissolved in 4 ml distilled water. To the resulting solution 4 g .alpha.-alumina carrier (NGC H-218B) was added for impregnation. The impregnated .alpha.-alumina carrier was dried in room for 24 hours with good circulation of air, and was calcined at 500.degree. C. for five hours to obtain a tin oxide catalyst deposited on .alpha.-alumina carrier.

Control Example 4: Iron (III) Oxide Catalyst

2 g Fe.sub.2 O.sub.3 powder (manufactured by Showa Chemicals, Inc., Japan) was mixed with 0.4 g bentonite adhesive (purchased from Fisher Scientific Co., U.S.), and the resulting mixture was hot pressed into tablets. The tablets were calcined at 500.degree. C. for 5 hours in air, the calcined tablets were broken and then screened to obtain particles of 10-20 mesh number.

Control Example 5: Antimony Oxide Catalyst

2 g antimony oxide powder (manufactured by Strem Chemicals, U.S.) was mixed with 0.4 g bentonite adhesive (purchased from Fisher Scientific Co., U.S.), and the resulting mixture was hot pressed into tablets. The tablets were calcined at 500.degree. C. for 5 hours in air, the calcined tablets were broken and then screened to obtain particles of 10-20 mesh number.

Control Example 6: Molybdenum Oxide Catalyst

To 25 ml of distilled water 12.35 g (0.01 mole) of ammonium molybdate �(NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, manufactured by Riedel-de Haen, Germany!, and then nitric acid was added to the resulting solution to maintain the pH value thereof at 1.5 and to yield a Mo metal salt precipitation therein. The solution/precipitation mixture was kept in room for 24 hours, and the precipitation was removed by filtration, and dried at 150.degree. C. for 4 hours. Finally the dried precipitation was calcined at 400.degree. C. for 8 hours to obtain molybdenum oxide catalyst.

Example 1: Iron (III) and Cerium (III) Mixed-Oxide Catalyst Deposited on .alpha.-Alumina

(a) The procedures of Control Example 1 were repeated to obtain an iron (Ill) oxide catalyst deposited on .alpha.-alumina carrier. (b) 0.6 g cerium (III) nitrate hexahydrate �available from STREM Chemicals, U.S.! was dissolved in 4 ml distilled water. To the resulting solution the iron (III) oxide catalyst deposited on .alpha.-alumina carrier prepared in step (a) was added for impregnation. The impregnated solid was dried in room for 24 hours with good circulation of air, and was calcined at 500.degree. C. for five hours to obtain an iron (III) and cerium (III) mixed-oxide catalyst deposited on .alpha.-alumina carrier.

Example 2: Iron (III) and Tin Mixed-Oxide Catalyst Deposited on .alpha.-Alumina

(a) The procedures of Control Example 1 were repeated to obtain an iron (III) oxide catalyst deposited on .alpha.-alumina carrier.

(b) 0.3 g of SnCl.sub.2.2H.sub.2 O �purchased from Riedel-de Haen, Germany! was dissolved in 4 ml distilled water. To the resulting solution the iron (III) oxide catalyst deposited on .alpha.-alumina carrier prepared in step (a) was added for impregnation. The impregnated solid was dried in room for 24 hours with good circulation of air, and was calcined at 500.degree. C. for five hours to obtain an iron (III) and tin mixed-oxide catalyst deposited on a .alpha. .alpha.-alumina carrier.

Example 3: Iron (III) and Antimony Mixed-Oxide Catalyst

(a) 5.46 g of FeCl.sub.3.6H.sub.2 O �purchased from Showa Chemicals, Inc., Japan! was dissolved in 10 ml deionized water to obtain an aqueous solution containing iron (III) ions.

(b) 7.36 g of SbCl.sub.3 �purchased from Merck, Germany! was added to the aqueous solution of step (a) and dissolved therein.

(c) 5.6 g ammonium carbonate �(NH.sub.4).sub.2 CO.sub.3, purchased from Fisher Scientific Co., U.S.! was dissolved in 30 ml deionized water, and the resulting solution was added dropwise through a titration tube to the aqueous solution prepared in step (b) to form a co-precipitation of iron and antimony salts. The precipitate was removed by filtration, washed with 100 ml deionized water, dried and calcined at 500.degree. C. for five hours to yield a mixture of iron (III) and antimony oxides.

(d) 2 g of the mixture of iron (Ill) and antimony oxides prepared in step (c) was mixed with 0.4 g bentonite adhesive (purchased from Fisher Scientific Co., U.S.), and the resulting mixture was hot pressed into tablets. The tablets were calcined at 500.degree. C. for 5 hours in air, the calcined tablets were broken and then screened to obtain particles of 10-20 mesh number.

Example 4: Iron (III) and Molybdenum Mixed-Oxide Catalyst

(a) 2.731 g of FeCl.sub.3.6H.sub.2 O �purchased from Showa Chemicals, Inc., Japan! was dissolved in 20 ml deionized water to obtain an aqueous solution containing iron (Ill) ions.

(b) 1.7838 g of ammonium molybdate �(NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, purchased from Riedel-de Haen, Germany! was dissolved in 20 ml deionized water to give an aqueous solution containing molybdenum ions.

(c) The two aqueous solutions prepared in steps (a) and (b) were mixed, and the resulting mixed solution was stirred and heated to a temperature of 70.degree. C.

(d) 1.1354 g ammonium carbonate �(NH.sub.4).sub.2 CO.sub.3, purchased from Fisher Scientific Co., U.S.! was dissolved in 30 ml deionized water, and the resulting solution was added dropwise through a titration tube to the mixed solution prepared in step (c) to form a co-precipitation of iron and molybdenum salts. The precipitate was removed by filtration, washed with deionized water to remove the residue ions thereon, dried at 70.degree. C. for eight hours, and calcined at 400.degree. C. for five hours to yield a mixture of iron (III) and molybdenum oxides.

(e) 2 g of the mixture of iron (III) and molybdenum oxides prepared in step (d) was mixed with 0.4 g bentonite adhesive (purchased from Fisher Scientific Co., U.S.), and the resulting mixture was hot pressed into tablets. The tablets were calcined at 500.degree. C. for 5 hours in air, the calcined tablets were broken and then screened to obtain particles of 10-20 mesh number.

Selective oxidation of hydrogen sulfide

Example 5

A continuous fixed bed reactor having an inner diameter of 7 mm, an outer diameter of 9 mm and a length of 80 cm was used in this example. 1 g catalyst in the form of 10-20 mesh number particles was packed into the reactor, in which quartz sand was filled at the feed end of the catalyst bed to form a preheating zone. The reaction temperature was controlled by electrical heaters installed around the reactor. The catalyst bed was pre-sulfurized at 240.degree. C. for 20 hours or longer by introducing a high concentration hydrogen sulfide gas mixture (10 vol % H.sub.2 S) into the reactor at an elevated temperature until a gaseous reaction product leaving the reactor had a stable hydrogen sulfide concentration. After the presulfurization step, a gaseous feed stream consisting of 1 vol % hydrogen sulfide, 5 vol % oxygen and 94 vol % nitrogen was then introduced into the reactor at 100 ml/min.. The reaction product exiting the reactor was introduced into a gas-solid separator which was maintained at 25.degree. C., in which the reaction product was separated into a solid product and a gaseous product. The composition of the gaseous product was analyzed by a China Chromatography 8900 gas chromatography with a 9 feet long Porapak Q80/100 mesh S.S. coiled column.

The reaction conditions and results are listed in Tables 1-10, in which the conversion (%) is defined as the mole of hydrogen sulfide reacted per mole of hydrogen sulfide in the feed stream, and the selectivity is defined as the mole of elemental sulfur formed per mole of the reacted hydrogen sulfide, which can be calculated as follows: ##EQU1## The elemental sulfur formation rate (mole/min.) is equal to the hydrogen sulfide feeding rate (mole/min.) subtracts the exiting rate (mole/min.) of hydrogen sulfide and subtracts the exiting rate (mole/min.) of sulfur dioxide in the gas product.

TABLE 1______________________________________Iron (III) Oxide Catalyst Deposited on .alpha.-Alumina Prepared inControl Example 1Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________180 47 100200 56 100220 61 100230 64 100240 70 100250 74 100260 88 89280 96 37.5______________________________________ TABLE 2______________________________________Cerium (III) Oxide Catalyst Deposited on .alpha.-Alumina Prepared inControl Example 2Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________200 11 100230 14 100250 17 100280 20 100300 21 99320 29 85______________________________________ TABLE 3______________________________________Tin Oxide Catalyst Prepared in the Control Example 3Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________180 21.9 100230 24.3 100______________________________________ TABLE 4______________________________________Iron (III) Oxide Catalyst Prepared in the Control Example 4Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________200 50.7 100210 59.6 100220 67.9 98.9240 81.3 95.6260 90.7 90.8280 96 61.9300 98.4 7.2______________________________________ TABLE 5______________________________________Antimony Oxide Catalyst Prepared in the Control Example 5Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________200 13 100250 17.4 100280 25.5 100300 32.9 100______________________________________ TABLE 6______________________________________Molybdenum Oxide Catalyst Prepared in the Control Example 6(amount of catalyst used: 2 g)Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________190 9.6 100200 10.4 100210 11.6 100220 17.1 100240 23.0 100250 28.4 100260 38.5 83.6270 61.5 55______________________________________ TABLE 7______________________________________Iron (III) and Cirium (III) Mixed-Oxide Catalyst Deposited on .alpha..alpha.-Alumina Prepared in Example 1Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________200 79 100230 91 100250 94 100280 100 98300 100 92______________________________________

Comparing the data of Table 7 with Table 1 and Table 2, it can be seen that the iron (III) and cerium (III) mixed-oxide catalyst deposited on .alpha.-alumina has significantly higher conversion and selectivity for the selective oxidation reaction of H.sub.2 S than .alpha.-alumina supported iron (III) single oxide catalyst and .alpha.-alumina supported cerium (III) single oxide catalyst. This indicates that cerium atom is an effective promoter to iron (III) oxide catalyst for the selective oxidation reaction of H.sub.2 S to elemental sulfur.

TABLE 8______________________________________Iron (III) and Tin Mixed-Oxide Catalyst Deposited on .alpha.-AluminaPrepared in Example 2Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________180 28 100200 48 100220 72 100240 79 100260 84 100280 87 100300 90 100320 92 98340 96 95______________________________________

The date in Table 1, 3, and 8 show that tin atom is also an effective promoter to iron (III) oxide catalyst for the selective oxidation reaction of H.sub.2 S to elemental sulfur in terms of higher conversion and selectivity.

TABLE 9______________________________________Iron (III) and Antimony Mixed-Oxide Catalyst Prepared in Example 3Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________200 92.7 100210 94.9 95.6220 95.8 72.8240 97.9 71.3______________________________________

The data in Tables 4, 5 and 9 show that antimony atom is also an effective promoter to iron (III) oxide catalyst for the selective oxidation reaction of H.sub.2 S to elemental sulfur in terms of higher conversion and selectivity.

TABLE 10______________________________________Iron (III) and Molybdenum Mixed-Oxide Catalyst Prepared in Example 4Temperature (.degree.C.) Conversion (%) Selectivity (%)______________________________________180 39.06 100190 46.24 100200 55.92 100210 67.35 100220 80.95 100230 88.85 96.95240 94.81 90.27______________________________________

It can be seen from Tables 4, 6 and 10 that molybdenum atom is also an effective promoter to iron (III) oxide catalyst for the selective oxidation reaction of H.sub.2 S to elemental sulfur in terms of higher conversion and selectivity.

Claims

1. A process for recovering elemental sulfur from a gas mixture containing hydrogen sulfide consisting essentially of contacting said gas mixture with an oxygen-containing gas to result in a gas having a molar ratio of hydrogen sulfide to oxygen that is less than 2 in an temperature range of from about 50.degree. C. to about 400.degree. C. in the presence of a supported iron (III) oxide catalyst, and a promoter for the selective oxidation of hydrogen sulfide to elemental sulfur, said promoter is a cerium component, a tin component, or an antimony component.

2. A process according to claim 1 wherein said promoter is a cerium component.

3. A process according to claim 1 wherein said promoter is a tin component.

4. A process according to claim 1 wherein said promoter is an antimony component.

5. A process according to claim 1 wherein said iron-(III) oxide catalyst has a molar ratio of iron atom to said promoter ranging from 1:100 to 100:1.

6. A process according to claim 5 wherein said iron-(III) oxide catalyst has a molar ratio of iron atom to said promoter ranging from 1:10 to 10:1.

7. A process according to claim 1 wherein said temperature ranges from about 100.degree. C. to about 350.degree. C.

8. A process according to claim 1 wherein said iron-(III) oxide catalyst is in the form of monolith, particle or pellet.

9. A process according to claim 1 wherein said iron-(III) oxide catalyst is supported on a porous carrier selected from the group consisting of alumina, silica and zeolite.

10. a process according to claim 1 wherein said oxygen-containing gas is air.

11. A process according to claim 1 wherein said gas mixture contains 0.8-3 vol % of hydrogen sulfide.