Note: Descriptions are shown in the official language in which they were submitted.
FMC 1613
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
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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
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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
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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
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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
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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 -
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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
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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,
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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.
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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
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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
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` 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
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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
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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. ~ ;
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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.
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