CA1069273A

CA1069273A - Purification of sulfur-containing waste gases with hydrogen peroxide

Info

Publication number: CA1069273A
Application number: CA224,491A
Authority: CA
Inventors: Paul R. Mucenieks
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1974-05-23
Filing date: 1975-04-14
Publication date: 1980-01-08
Also published as: IT1037847B; NL7505539A; FR2271862B1; ES437900A1; CH598857A5; BE829372A; AU8022575A; SE7505913L; FR2271862A1; GB1499536A; NO751818L; DE2522279A1

Abstract

ABSTRACT:
A process for simultaneously absorbing and oxidizing sulfur-containing gases present in a waste gas stream by contacting the sulfur-containing waste gas with an aqueous hydrogen peroxide solu-tion having a pH above 7.0 at a temperature above the freezing point but below the boiling point of the solution.

Description

1~69;~:73 This invention relates to the removal of sulfur-containing gases present in a waste gas stream before the gases are released into the atmosphere. More particularly, this invention relates to a process for simultaneously absorbing and oxidizing sulfur-containing gases present in a waste gas stream in a simple and convenient manner.
Sulfur-containing waste gases are noxious, often ; toxic, and are produced as by-products in many industrial operations. For example, sulfur-containing gases are present in effluent gas streams from flue gases, smelter gases, off-gases from chemical and petroleum processes, and stack gases produced from the combustion of sulfur-containing hydrocarbon fuels. These gases contain hydrogen sulfide, sulfur dioxide, aliphatic thiols, and organic sulfide compounds including sulfides, disulfides, polysulfides, and thiophenes and mixtures thereof. The term "organic sulfide compounds" refers to organic -compounds containing a divalent sulfur atom which is not bonded to a hydrogen atom. Pollution of the environment by such gases has been offensive to communities surrounding the pollution source because of their noxious presence in the atmosphere and because of their harmful effect on natural habitat.
Many processes have been proposed for removing sulfur-containing gases from gaseous effluents. One of the earliest methods was the incineration method. In this method, toxic hydrogen sulfide and organic sulfides were converted to less toxic and less offensive sulfur dioxide and sulfur trioxide by air oxidation at high . ' - ' .
-1- ~ ''". ,.

. ' "' .

temperatures. While this process converted toxic substances into less toxic substances, the less toxic substances were still noxious and potentially dangerous to the environment.
To avoid the problems associated with the incinera-tion method, numerous chemical processes have been suggested. United States Patent 3,716,620 discloses the oxidation of hydrogen sulfide and thiols with iodine in the presence of an organic solvent. While this process is technically effective in oxidizing these specific gases, the process is not commercially feasible because the compounds used are expensive and even small losses of these compounds make the process commercially uneconomical. United States Patent 3,475~122 discloses a process for recovering sulfur dioxide from a gas stream by passing the gas stream through an aqueous basic solution such as potassium hydroxide to form a bisulfite solution. The bisulfite solution is treated to recover sulfur dioxide therefrom and is then recycled to recover further sulfur dioxide. This process, however, is specific for sulfur dioxide recovery, and does not avoid the pollution problems associated with the discharge of the recovered sulfur dioxide. British Patent 421,970 discloses a four stage process for oxidizing hydrogen sulfide with hydrogen peroxide. In the first stage, hydrogen sulfide is absorbed in an alkaline solution.
In the second stage, the solution is acidified by treatment with carbon dioxide. In the third stage, the solution ; is boiled to expel most of the absorbed hydrogen sulfide.
In the forth stage, the solution is treated with an

-2--.

': . .' '~L06~Z73 oxidizing agent to oxidize the remaining hydrogen sulfide.
While the patentee states that a ten-fold reduction of hydrogen sul~ide in the scrubber e~luent is achieved in fifteen minutes, this process is not a commercially feasible process, primarily because of the time necessary to perform the complete process.
It is apparent from these processes that there has been a long felt need for a commercially effective process capable of rapidly removing a multitude of dif~erent sulfur-containing gases present in a waste gas stream in a simple and convenient manner without the ~ormation of by-product pollutants.
In accordance with the present invention there is provided a process for simultaneously absorbing and oxi~izing sulfur-containing gases present in a waste gas stream wherein the sulfur-containing gas is hydrogen :
sulfide, sulfur dioxide, or aliphatic thiols, or mixtures thereof and may also contain oxidizable gases such as organic sulfides, thiophenes and the like by contacting the waste gas stream with an aqueous hydrogen peroxide solution having a pH above 7.0 at a temperature above the freezing point but below the boiling point of the solution for a su~ficient time to simultaneously absorb and oxidize the sulfur-", :, containing gases. ~
- The process of this invention permits the removal of ;:
essentially all of the sulfur-containing gases present in a waste gas stream to below levels detectable by conventional equipment within a matter of a few seconds. Furthermore, the sulfur-containing gases are oxidized to non-polluting alkali sulfates and sulfonates. These substances may be ~, ,

-3-. ~ . . ... .. .

discharged directly into natural waterways without harm to natural fauna or flora.
The sulfur-containing gases that are removed from a waste gas stream according to the process of this invention are hydrogen sulfide; sulfur dioxide; and aliphatic thiols (mercaptans) containing 1 to 12 carbon atoms, such as methanethiol, ethanethiol, propanethiol, and butanethiol.
These sulfur-containing gases are the gases which make up the majority of the sulfur-containing gas content present in most waste gas streams. In addition to the above sulfur-containing gases, organic sulfide compounds includ-ing organic disulfides, polysulfides, thiophenes and the like may also be present in waste gas streams. These compounds include organic sulfides, such as dimethyl sulfide, diethyl sulfide, dibutyl sulfide, and methyl ethyl sulfide; organic disulfides~ such as dimethyl disulfide, diethyl sulfide; organic polysulfides, such as dimethyl disulfide; thiophene and substituted thiophenes. The organic sulfide compounds are not pH dependent even though ` 20 they are absorbed and oxidized by the aqueous hydrogen peroxide solution to non-polluting compounds. Accordingly, the organic sulfide compounds are processed simultaneously -with the other sulfur-containing gases, namely hydrogen sul~ide, sulfur dioxide, and aliphatic thiols, and thus avoid costly and difficult separation procedures and subsequent processing steps.
- The concentrations of sulfur-containing gases that .
are treated can vary widely. Generally, the sulfur- ~

containing gas concentratlon is source dependent and --varies from a few mg/l to several percent, such as 5% by ~.

-4-~::
' ' ' :--~6~;~73 weight. The process is most economical if the sulfur-containing gas concentration in the waste gas stream is maintained at less than 2% by weight. By decreasing the sulfur-containing gas concentration to less than 2%, such as by diluting the gas with air, the quantity of hydrogen peroxide necessary to oxidize a unit quantity of the sulfur-containing gas is substantially reduced. Dilu-tion with air, prevents hydrogen sulfide gas mixtures from containing more than 4.3% hydrogen sulfide~ which mixtures 10 are explosive. ~:
In order ~or the sulrur-containing gases to be simul-- taneously absorbed and oxidized by the aqueous hydrogen peroxide solution, the aqueous solution must have a pH
above 7.0, and pre~erably above 7.0 to about 13.5. The desired pH is obtained by adding an alkali to the aqueous hydrogen peroxide solution. The preferred alkali is sodium hydroxide which may be replaced in whole or in part by potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate and magnesium carbonate. If the pH
of the aqueous solution drops below 7.0 during the reaction, additional alkali is added to raise the pH of the solution , above 7Ø Maintenance o~ the pH above 7.0 is essential in order to neutralize the sul~uric and sul~onic acids ` produced during the course of the reaction. This procedure I

prevents the need for subsequent pH ad~ustments prior to the discharge of the aqueous solution. Monitoring the pH
o~ the reaction mixture is achieved by conventional means according to well known procedures.
The pH of the aqueous hydrogen peroxide solution _5_ , , . . .. .. . ,. - ... - ~

~3692~3 may be further adjusted within the above pH ranges to achieve optimum absorption and oxidation of specific sulfur-containing gases present in specific waste gas streams. For example, when hydrogen sulfide i9 the sulfur-containing waste gas, the pH is preferably between about 8.o and about 13.5, and most preferably between about 11.0 and about 13Ø Within the preferred pH range, hydrogen sulfide is rapidly absorbed and oxidized.
Absorption and oxidation rates are significantly improved at the higher pH's. When sulfur dioxide is the sulfur-containing waste gas, the pH is preferably above 7.0 to about 12Ø In this pH range, the absorption rates are significantly improved. When the pH is above about 12.0 the oxidation rate of sulfur dioxide is too slow for commercial operation. When aliphatic thiols are the sulfur-containing waste gas, the pH is preferably above 7.0 to about 13.5. When the waste gas streams contain a mixture of the foregoing sulfur-containing gases, the absorption and oxidation rate of all of the sulfur-containing gases is optimal at the preferred pH rangebetween 8.o and about 12Ø The oxidation rates of the organic sulfide compounds, are not pH dependent and consequently any pH may be employed to oxidize these gases.
Any available grade of aqueous hydrogen peroxide can be employed, with 50% technical grade being preferred.
The exact quantity of hydrogen peroxide in the aqueous - -~
solution depends upon the concentration of the sulfur-contalning l~ases present in the waste gas stream and the extent to which these gases are to be removed. The 30 aqueous hydrogen peroxide solution can be prepared with -. . ~

-; ~ , ,.

~LC3 69~7;~
deionized, distilled or tap water.
To reduce the sulfur-containing gas content present in a waste gas stream to non-detectable limits, hydrogen peroxide is employed in concentrations of about 0.01% to 50% by weight. The specific amount of hydrogen peroxide to be employed to oxidize a specific sulfur-containing gas is easily determined from the stoichiometry of the reaction.
For example, four moles of hydrogen peroxide are needed to completely oxidize one mole of hydrogen sulfide. One mole of hydrogen peroxide is needed to completely oxidize one mole of sulfur dioxide. However, amounts of hydrogen peroxide slightly above the stoichiometric amount may be employed to oxidize either hydrogen sulfide or sulfur dioxide. The ~ gaseous organic sulfur compounds require an excess of - hydrogen peroxide over the stoichiometric amount with a maximum concentration of 10% hydrogen peroxide being preferred.
The term "gaseous organic sulfur compounds" refers to both the aliphatic thiols and the organic sulfide compound.
The use of hydrogen peroxide under alkaline conditions to s~multaneously absorb and oxidize sulfur-containing waste ~ ~-gases is completely unexpected because hydrogen peroxide decomposes under alkaline conditions. It has been discovered, however, that the oxidation rate of hydrogen sulfide and ;~
sulfur dioxide is significantly faster than the hydrogen peroxide decomposition rate when hydrogen peroxide is employed in stoichiometric amounts or in amounts slightly above the stoichiometric amount. It has also been discovered that hydrogen peroxide :
` 30 ' : ; ' , ':
'' ~

~ ; -7- ~

.. . . ~ ~ , - , . . , . ~ - .

1~69273 decomposition is kept to a nominal extent when oxidizing any of the gaseous organic sulfur compounds even when greater than stoichiometric amounts of hydrogen peroxide are employed, by maintaining the pH of the solution above 7.0 to about 12Ø
The time necessary to contact the sulfur-containing waste gas must be sufficient to simultaneously absorb and oxidize the sulfur-containing gases. Contact times of 1 second or less are sufficient to completely absorb and oxidize hydrogen sulfide and sulfur dioxide. Longer contact times are necessary to absorb and oxidize the `
gaseous organic sulfur compounds. These times range from 1 to 60 seconds depending upon the specific gaseous organic sulfur compound. To limit hydrogen peroxide decomposition during the longer contact times, the aqueous hydrogen peroxide solution may be optionally stabilized -.
by conventional methods, such as by employing magnesium oxide or other stabillzers in the aqueous hydrogen peroxide solution. Likewise, a conventional metal catalyst may also be employed to assist in the oxidation reaction.

. :
These catalytst include salts of iron, cobalt, nickel, copper, manganese, molybdenum, vanadium, platinum, . ¦ ~'! ' - " ' palladium and silver. If a catalyst is employed, the first four catalytic salts are preferred. The catalysts can be employed with or without conventional complexing agents such as gluconic acid, and citric acid. The use of hydrogen peroxide stabilizers and metal catalysts ~ may also be employed during the absorption and oxidation -of hydrogen sulfid0 and sulfur dioxide even thoueh they ~ ; -are not necessary for the reaction.
,.

-8- ;

... . ' ' ' .

~q~69Z73 The reaction temperature is critical only to the extent that it must be above the freezing point but below the boiling point of the aqueous solution. The reaction is preferably carried out between 25 and 85C, and most preferably between 45 and 65C, which are the normal ; temperatures of waste gas streams. When oxidizing any of the gaseous organic sulfur compounds, temperatures between about 60 and 70C are preferred. At these temperatures, the gaseous organic sulfur compounds are -rapidly oxidized at substantially increased rates. This rapid oxidiation permits the use of only stoichiometric amounts of hydrogen peroxide instead of requiring excess - hydrogen peroxide to completely oxidize all of the gaseous organic sulfur compounds present in the waste gas stream.
` The waste gas stream is contacted with the aqueous hydrogen peroxide solution in any conventional contacting -device. The preferred contacting device is a packed column such as a packed bed or tower. The waste gas stream ;
and contacting solution may be fed into the contactor either counter-currently, cross-currently or co-currently.
The treated waste gas and spent aqueous hydrogen peroxide solution are then discharged directly into the environment.
When contactine waste gases which require only ; stoichiometric amounts of hydrogen peroxide to oxidize ;~ the sulfur-containing gases, it is preferred to pass the waste gas stream and aqueous hydrogen peroxide solution through the contactor only once. When contacting waste `
gases which require an excess of hydrogen peroxide over the stoichiometric~amount, it is preferred to pass the ~ 30 waste gas skream and aqueous hydrogen peroxide solution '`; 9 :
. ` ~
., .. , -. .

through the contactor, separate the spent aqueous solution, and reactivate the spent aqueous solution by adding fresh hydrogen peroxide to the solution. This reactivated solution is then recycled to the contactor. By employing this procedure, excess hydrogen peroxide is continuously provided in the contactor in an efficient and economic way.
Commercially available gas analyzers are useA to analyze the sulfur-containing gas content present in both the waste gas stream and in the effluent gas stream. If the sulfur-containing gas concentration in the waste gas stream changes, the required amount of aqueous hydrogen peroxide solution added to the contactor can be added either manually or automatically. Furthermore, the pH
of the spent aqueous hydrogen peroxide solution removed from the contactor is analyzed by conventional means in order to keep the pH of the aqueous hydrogen peroxide j; . .. ,. : .
` solution during the reaction above 7Ø It has been found that if the pH of the aqueous hydrogen peroxide solution fed into the contactor is between 8.o and 12.0, the pH of the removed aqueous solution will be above 7Ø
The following examples further illustrate the invention.
All percentages given are based upon weight unless other- -wise indicated.
Example 1 A gas stream containing 1% H2S by volume in air was passed at a velocity of 56 cm/sec through a contactor con- ~ -sisting of a 5.08 cm (2 inch) diameter heat and chemically :~
resistant glass (PyrexTM) pipe containing a 35.56 cm (14 inch) column of o.63 cm (1/4 inch) chemically resistant ceramic pacl~ing (IntaloxTM saddles). The total gas flow -10- .

.

1~69;~73 was 50 l/min. An aqueous solution containing 10 g/l of NaOH and 4.3 g/l of H202 having a pH of 13.0 was prepared with deionized water and passed through the column counter-current to the gas ~low at a solution ~low rate of 0.45 1/min. The temperature o~ the aqueous solution was 25C. The residence time of the gas stream in the contactor was 0.66 seconds. The process was carried out continuously for one hour. The effluent gas stream contained less than 0.001 ppm (parts per million) H2S.
The effluent solution had a pH of 12.5 and contained 0.5 mg/l unoxidized sulfide values (H2S,NaHS, and Na2S).
Example 2 The procedure o~ Example 1 was repeated, except that the gas stream contained 0.1% H2S by volume in air and the aqueous solution contained 0.165 g/l NaOH and 0.28 g/l H202 and had a pH of 11Ø The effluent gas stream contained less than 0.001 ppm H2S. The effluent solution had a pH of 10.4 and contained 7 mg/l unoxidized sulfide values.
2Q Example 3 ;
The procedure of Example 1 was repeated except that ~'~ the gas stream contalned 0.006% H2S by volume in air and the aqueous solution contained 0.01 g/l NaOH and -0.02 g/l H202 and had a pH of 9.5. The effluent gas ~; stream contained less than 0.001 ppm H2S. The effluent solution hacl a pH of 9.3 and contained 1.7 mg/l unoxidized ~, sul~ide values.
Example 4 The procedure Or Example 1 was repeated except that the gas stream contained 0.1% SO2 by volume in air instead '~ '. .
-11 - .

1~69Z~3 of HzS, and the aqueous solution contained 0.2 g/1 NaOH
and 0.16 g/l H20z and had a pH of 11.2. The e~fluent gas stream contained less ~han 1 ppm SO2. The effluent liquid had a pH of 9.0 and contained 1 mg/l sulfite values (Na2~03, NaHSO3).
Example 5 A gas stream containing 1000 ppm methanethiol by volume in air was passed at a velocity of approximately 35 cm/sec through a contactor consisting of a 5.08 cm (2 inch) diameter Pyrex M pipe containing a 71.12 cm (28 inch) column of 0.63 cm (1/4 inch) IntaloxTM saddles.
The total gas flow was 15 l/min. An aqueous solution con-taining 1.0 g/l NaOH and 1.0 g/l H2O2 having a pH of 11.9 was prepared with deionized water and passed through the column counter-current to the gas flow at a solution flow rate of 1.35 l/min. The temperature of the aqueous solu-.
tion was 25C. The~residence time of the gas stream in thecontactor was 4.4 sec. The process was carried out continuously for one hour. The effluent gas stream con-tained 4 ppm methanethiol. The effluent solution had apH of 11.0 and the content of unoxidized sulfur compounds , . .
was below detectable limits. -Example 6 ;
:
The procedure of Example 5 was repeated except that ~ `
the gas stream contained 8000 ppm H2S by volume and 200 ppm methane~hiol by volume in air. The effluent gas stream contained no detectable H2S and 2 ppm methanethiol by volume. The e~luent solution had a pH of 11.2 and ; ~ the content of unoxidized sulfur compounds was below ~ 30 detectable limits. ~ ;

` . .
~ -12- ~ -~ ' ~ ' :' i9~:73 Example 7 The procedure of Example 5 was repeated except that the gas stream containing 1000 ppm ethanethiol by volume, 1000 ppm dimethylsulfide by volume, and 100 ppm thiophene.
The effluent gas stream contained no detectable sulfur compounds. The effluent solution had a pH of 11.9 and contained approximately 5 mg/l diethyldisulfide.

. .

: . . :

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for simultaneously absorbing and oxidizing sulfur-containing gases, present in a waste gas stream in con-centrations up to about 5% by weight of the total gas stream, to form alkali sulfates and sulfonates, wherein the sulfur-containing gas is sulfur dioxide or aliphatic thiols containing 1 to 12 carbon atoms and mixtures thereof and which waste gas stream may also contain hydrogen sulfide, organic sulfides and thiophenes which comprises:
contacting the waste gas stream with an aqueous hydrogen peroxide solution having a pH above 7.0 and a hydrogen peroxide concentration in the range of 0.01% to 50% by weight at a temperature above the freezing point but below the boiling point of the solution for a sufficient time to simultaneously absorb and oxidize sulfur dioxide or aliphatic thiols or mixtures thereof alone or with hydrogen sulfide, organic sulfides and thiophenes and thereby form alkali sulfates and sulfonates.

2. The process of claim 1 wherein the pH of the solution is above 7.0 to about 13.5.

3. The process of claim 1, wherein sulfur dioxide is the sulfur-containing gas and the aqueous hydrogen peroxide solution has a pH about 7.0 to about 12Ø

4. The process of claim 1 wherein the temperature of the aqueous solution is between 25 and 85°C.

5. The process of claim 1 wherein the temperature of the aqueous solution is between 45° and 65°C.

6. The process of claim 1 wherein the aliphatic thiols are selected from the group consisting of methanethiol, ethane-thiol, propanethiol and butanethiol.

7. The process of claim 1 wherein the waste gas stream contains organic sulfur compounds selected from the group consisting of organic sulfides, organic disulfides, organic polysulfides and thiophenes.

8. A process for simultaneously absorbing and oxidizing sulfur-containing gases, present in a waste gas stream in concentrations up to about 5% by weight of the total gas stream, to form alkali sulfates and sulfonates, wherein the sulfur-containing gas is selected from the group consisting of sulfur dioxide, and aliphatic thiols containing 1 to 12 carbon atoms, which comprises:
contacting the waste gas stream with an aqueous hydrogen peroxide solution having a hydrogen peroxide concentration in the range of 0.01% to 50% by weight and containing a sufficient amount of alkali to adjust the pH of the solution above 7.0 to about 13.5 at a temperature between 25°C and 85°C for sufficient time to simultaneously absorb and oxidize the sulfur-containing gases and thereby form alkali sulfates and sulfonates.