Note: Descriptions are shown in the official language in which they were submitted.
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Title: Method for desulfurizing off-gases
This invention relates to a method for desulfurizing
off-gases which contain a high water vapor content. More
specifically, the invention comprises a method for reducing
the total sulfur content of off-,gases from sulfur recovery
plants.
The preparation of elemental sulfur from hydrogen
sulfide (HZS) by partial oxidation thereof by means of oxygen
or an oxygen-containing gas such as air, followed by reaction
of the sulfur dioxide (SOZ) formed from the hydrogen sulfide,
with the residual part of the hydrogen sulfide, in the
presence of a catalyst, is known as the Claus process. This
process is frequently employed both in refineries and for the
processing of hydrogen sulfide recovered from natural gas. A
conventional Claus plant consists of a burner with a
combustion chamber, the so-called thermal stage, followed by
a number of - generally two or three - reactors which are
filled with a catalyst. These last stages constitute the so-
called catalytic stages. In the combustion chamber, the
incoming, HZS-rich gas stream is combusted with an amount of
air at a temperature of about 1200°C. The amount of air is
set such that one-third of the HZS is combusted to S02
according to the reaction:
2HZS + 302 --~ 2Hz0 + 2S02 ( 1 )
After this partial combustion of HZS, the unreacted part
of the HZS (i.e. about two-thirds of the amount presented)
and the SOZ formed react further for a considerable part
according to the Claus reaction:
4H2S + 2502 H 4H20 + 3S2 (2)
Thus, in the thermal stage, about 60% of the HZS is
converted to elemental sulfur. The gases coming from the
combustion chamber are cooled to about 160°C in a sulfur
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2
condenser, in which the sulfur formed condenses, which
subsequently flows via a siphon into a sulfur sink. The
uncondensed gases, in which the molar ratio of HzS to SOz is
still 2 . 1, are subsequently heated to about 250°C and
passed through a first catalytic reactor, in which again the
6
equilibrium 4HzS + 2S02 t-~ 4H20 + - Sn is established.
n
The gases coming from this catalytic reactor are
subsequently cooled again in a sulfur condenser, whereafter
the liquid sulfur formed is recovered and the residual gases,
after re-heating, are passed to a second catalytic reactor.
Depending on the number of catalytic stages, the sulfur
recovery percentage in a conventional Claus plant amounts to
94-97%. Accordingly, an amount of HZS and SOz is left.
One of the important limitations of the Claus process is
the increase of the water content in the process gas as the
conversion of HzS to sulfur proceeds.
The Claus reaction is thermodynamically limited by this
increase of the water vapor content and simultaneously by the
decrease of the HzS and SOz concentration, with the result
that the equilibrium of the Claus reaction (2) shifts to the
left. Condensation of the water vapor in the process gas
would be desirable to remove this limitation as much as
possible. However, since the water dew point lies far below
the solidification point of sulfur, condensation of water
vapor in the Claus process meets with insurmountable
problems, such as clogging due to the solidification of
sulfur and corrosion due to the formation of sulfurous acid.
In the past, off-gas of the Claus process was burnt in
an afterburner. However, in view of the increasingly more
stringent environmental requirements, this is no longer
permitted.
This has led to improvements of the Claus process and
the development of Claus off-gas removal processes. One
improvement of the Claus process is known as the.SUPERCLAUS~
process, whereby the efficiency of the Claus process is
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3
increased from 94-97% to more than 99%. The SUPERCLAUS~
process is described in "SUPERCLAUS~, the answer to Claus
plant limitations", publ. 38th Canadian Chem. Eng.
Conference, October 25th, 1988, Edmonton, Alberta, Canada.
In the SUPERCLAUSO-99 process, reaction (2) in the
thermal stage and in the Claus reactors is operated with
excess H2S, so. that in the gas from the last Claus reactor
the HZS content is approximately 1 vol.% and the SOz content
approximately 0.02 vol.%. In a next reactor stage, the H2S is
selectively oxidized to elemental sulfur according to the
reaction:
2
2HzS + 02 -~ 2Hz0 + - Sn (3)
n
in the presence of a special selective oxidation
catalyst. These catalysts are described, for instance, in
European patents 0242920 and 0409353.
As stated, increasingly more stringent environmental
requirements have led not only to improvements of the Claus
process but also to developments of Claus tailgas processes,
for the further desulfurization of off-gas from sulfur
recovery plants.
Most Claus tailgas processes utilize a hydrogenation
reactor, also referred to as reduction reactor, in which SOz,
carbonyl sulfide (COS), carbon disulfide (CSZ), sulfur vapor
and any entrained sulfur droplets (sulfur mist) are converted
with hydrogen (Hz) or a reducing gas, which contains, for
instance, hydrogen and carbon monoxide, to hydrogen sulfide.
The hydrogen sulfide is then removed by absorption in a
solution or by conversion in the gas phase to elemental
sulfur, using a catalyst.
Only a few tailgas processes have been developed which,
after the combustion of Claus tailgas, absorb SOz from
chimney gas. These processes are not further discussed. Most
well-known among the Claus tailgas processes which, after
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hydrogenation, absorb the resultant HzS in a solution are
SCOT, BSR-Stretford, BSR-MDEA, Trencor-M and Sulften. These
processes are described in a publication by B.G. Goar: "Tail
Gas Clean-Up Processes, a review", presented at the 33ra
Annual Gas Conditioning Conference, Norman, Oklahoma, March
7-9, 1983 and in Hydrocarbon Processing, February 1986.
The most well-known, and to date most effective, process
for desulfurizing tailgas is the SCOT process described in
Maddox "Gas and liquid sweetening" (1977). The SCOT process
achieves a sulfur recovery of 99.8 to 99.9%.
Of the tailgas processes which, after hydrogenation,
convert the resultant HzS in the gas phase using a catalyst,
only a few processes have been built and become known, such
as MODOP, CLINSULF, BSR-Selectox, Sulfreen, SUPERCLAUS-99.5.
These processes are described in the above-mentioned
publication by B.G. Goar, in the journal C&EN of 11 May 1987,
the journal Sulphur Jan/Feb 1995, and in DE-A 2648190.
In all these Claus tailgas processes, after the
hydrogenation, the water formed in the Claus reaction (2) and
in the selective oxidation reaction (3) is condensed, because
the presence of water has an adverse effect on the subsequent
H2S removal in an absorption liquid or in the catalytic
conversion of HZS to elemental sulfur. The absorption liquids
used in the above-mentioned processes are secondary or
tertiary alkanolamine solutions such as Diisopropanolamine
(DIPA) or Methyldiethanolamine (MDEA) or complex Redox
solutions. Without removal of water, the absorption process
would be thoroughly disturbed, viz., either by the too high
temperatures at which no or only very slight absorption
occurs, or in that the water condenses in the absorber during
the absorption and the circulating solution is continuously
diluted, so that no absorption can take place anymore.
In HzS conversion in the gas phase using a catalyst,
without water removal the thermodynamic conversion of HzS
according to the Claus reaction (2) is strongly reduced and a
situation is obtained comparable to that in the last reactor
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stage in the Claus process, so that a total sulfur recovery
efficiency of more than 99.5% is impossible to achieve.
Although the use of a selective oxidation catalyst such
as used in the SUPERCLAUS process gives a higher efficiency,
5 with SUPERCLAUS-99.5 too, it has been found impossible in
practice to achieve a sulfur recovery efficiency of more than
99.5%.
In general, it can be stated that the disadvantage of
Claus tailgas processes, in which, after hydrogenation, the
HzS in the gas phase is converted to elemental sulfur using a
catalyst, is that the current requirements of a total sulfur
recovery efficiency of more than 99.90% cannot be met.
Claus tailgas processes with hydrogenation followed by
condensation of water whereafter the HZS is absorbed in an
absorption liquid such as, for instance, in the SCOT process,
can achieve total sulfur recovery efficiencies of more than
99.90%, but have as a major disadvantage that the investment
costs and the energy costs are tremendously high. Newer
versions of the SCOT process, such as SUPERSCOT and LS-SCOT,
achieve a total sulfur recovery efficiency of 99.95%, but are
even more expensive.
Another disadvantage of these processes is that acidic
hydrogen sulfide-containing condensate must be discharged and
treated, for instance in a Sour Water Stripper, whereby the
dissolved acid gas is separated with steam. This, too, is
costly.
The environmental requirements have had an influence not
only on the development of Claus and Claus tailgas processes,
but also on the development of chimney gas processes, also
referred to as flue gas processes, for power plants. Various
processes for 'flue gas desulphurization' (FGD) are known, in
which SOZ is converted with lime milk to gypsum (Ca2S04) .
Because a surplus of gypsum has formed, processes have been
searched for, in which S02 can be converted to elemental
sulfur. The Wellman Lord process, described in Gas
Purification, fourth edition 1985, A.L. Kohl, F.C.
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Riesenfeld, pp. 351-356, is an example, where SOZ is
eventually released as concentrated gas. After two-thirds of
the SOz are converted to HzS in a hydrogenation step, the H2S
and SOZ gas can be converted to elemental sulfur in a Claus
plant. This process route, too, is costly. Another
development in this field is the biological desulfurization
of flue gases.
Biological desulfurization of flue gases is described in
the journal Lucht, number 4, December 1994. The BIO-FGD
process described therein is for removing S02 from chimney
gas from power stations and consists of an absorber where SOz
is dissolved in a diluted sodium hydroxide solution according
to the reaction
SOZ + NaOH -~ NaHS03 ( 4 )
This solution is subsequently treated in two biological
reactor stages.
In the first biological step, in an anaerobic reactor,
the sodium bisulfite (NaHS03) formed is converted with an
electron donor to sodium sulfide (NaHS).
NaHSOj + 3H2 -~ NaHS + 3H20 ( 5 )
Suitable electron donors are, e.g., hydrogen, ethanol,
hydrogen and glucose. In the second step, in an aerobic
reactor, the sodium sulfide is oxidized to elemental sulfur,
which is separated.
NaHS + ~O2 -~ NaOH + S ( 6 )
Chimney gases contain, after combustion of coal or fuel
oil, a slight amount of water vapor. The water content is
typically between 2-15 vol.%, which corresponds to a water
dew point of 20-55°C.
If the BIO-FGD process were used for desulfurization of
Claus off-gas which has been afterburnt and whereby all
sulfur components have been converted to S02, the gas must be
cooled because of the high water vapor content of the Claus
off-gas. This is done to prevent the water vapor from
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condensing in the sodium hydroxide solution, as a result of
which a part of the sodium hydroxide solution would
constantly have to be discharged.
Claus off-gas must therefore be cooled, whereby sour
condensate is formed and must be discharged.
In desulfurizing off-gas from a coal- or oil-fired power
plant, this problem does not occur because the water dew
point lies under the operating temperature in the absorber.
Cooling of this off-gas can therefore be done in a simple
manner without the occurrence of condensation of water.
A first object of the invention is to provide a method
for desulfurizing off-gases with a high water vapor content
of 20 to 40 vol.% and in which condensation of this water is
not necessary, thereby preventing the formation of acidic
hydrogen sulfide-containing condensate which must then be
discharged.
A second object of the invention is to provide a method
in which the HzS formed upon hydrogenation can be absorbed in
an absorption liquid at a temperature above the dew point of
water in the gas, so that also during the absorption of HzS
no condensation of water occurs.
A next object of the invention is to provide a method
whereby a total sulfur recovery efficiency of more than
99.90% is achieved without the above-mentioned disadvantages
occurring.
The invention is based on the surprising insight that it
is possible to absorb H2S from such a gas with a water
content of 20 to 40 vol.% at a temperature above the water
dew point, in an alkaline solution, whereafter the sulfide-
containing solution formed is subjected to an aerobic
biological oxidation.
The invention accordingly relates to a method for
removing H2S from off-gases which contain at least 20 vol.%
of water vapor, comprising treating the off-gases at a
temperature above the water dew point of the off-gases with
an aqueous, alkaline solution, under absorption of the HzS,
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followed by subjecting the sulfide-containing solution formed
to a biological oxidation of the sulfide.
Surprisingly, it has now been found that the HzS
dissolved in the alkaline solution, preferably a sodium
hydroxide solution, can be oxidized to elemental sulfur with
air in a biological aerobic reactor at a temperature which is
preferably the.same as that at which the absorption has taken
place.
Such gases with a water content of 20-40 vol.% have a
water dew point of 60-80°C, which means that in practice the
biological oxidation will occur at a temperature of at least
65°C, more specifically at a temperature of 70 to 90°C. It is
particularly surprising that it is possible to carry out an
efficient and proper biological oxidation at such high
temperatures.
In the method according to the invention, the total
sulfur content of off-gases is reduced by first raising these
off-gases in temperature to a temperature above 200°C and
subsequently passing them together with a hydrogen and/or
carbon monoxide-containing gas over a sulfided group VI/group
VIII metal catalyst on an inorganic oxidic support, whereby
sulfur components such as SO2, sulfur vapor and sulfur mist
are converted with hydrogen or another reducing gas which
contains, for instance, hydrogen and carbon monoxide, to
hydrogen sulfide, according to the reactions:
SOZ + 3H2 ~ HZS + H20 ( 7 )
S + H2 -~ HzS (g)
If oxygen is present in the off-gases, a catalyst from
the above group is used which further has the property of
hydrogenating oxygen according to the reaction
Oz + 2H2 ~ 2H20 (9)
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g _
Preferably, a catalyst from the above group is used
which further has the property of hydrolyzing COS and CSZ
according to the reactions
COS + H20 ~ H2S + CO2 ( 10 )
CSz + 2H20 ~ 2HZS + COZ ( 11 )
In the method according to the invention, the off-gases
from the hydrogenation reactor are cooled to just above the
dew point of the water vapor present in the gas, such that no
condensation occurs. Preferably, cooling proceeds to 3 to 5°C
above the dew point.
Off-gases, specifically off-gases from a Claus recovery
plant, with a water vapor content of 20 to 40 vol.%, have a
dew point between 60-80°C.
In an absorber, these off-gases are subsequently
contacted directly with a diluted alkaline solution,
preferably sodium hydroxide solution, with a pH between 8 and
9, whereby the HzS present in the gas is dissolved according
to the reaction:
HZS + NaOH ~ NaHS + H20 ( 12 )
The non-absorbed part of the off-gases mentioned is,
optionally after combustion, discharged to the air.
Because the regenerated alkaline solution contains no
HzS, the HZS present in the off-gases is completely absorbed
and in this manner a total sulfur recovery efficiency of more
than 99.90 can be achieved. In the method according to the
invention, the solution is passed to the biological aerobic
reactor at the same temperature, preferably at the same
temperature as that at which absorption has taken place, so
that no heat needs to be removed or supplied. In the aerobic
reactor an amount of air is supplied, such that the dissolved
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HzS is partially oxidized with oxygen from the air, to form
elemental sulfur according to the reaction:
HzS + ~ OZ -~ S + Hz0 ( 13 )
5
Subsequently, in a sulfur separator, preferably again at
the same temperature, the sulfur is separated from the sodium
hydroxide solution, whereafter the solution is recirculated
to the absorber. It is possible to cool the sodium hydroxide
10 solution having the HzS absorbed therein before it is fed to
the biological aerobic reactor. After the sulfur separation,
however, the solution is then heated again before it is
supplied to the absorber.
The invention will now be elucidated with reference to
two figures, in which the method according to the invention
is described in the form of block diagrams.
In Fig. 1 a general process diagram is represented. The
off-gas of a sulfur recovery plant, not shown, is passed via
line 1, with addition of hydrogen or another reducing gas via
line 2, and adjusted to the desired hydrogenation temperature
with heater 3, before being passed via line 4 into the
hydrogenation reactor 5.
In the hydrogenation reactor 5, the sulfur dioxide,
sulfur vapor and organic sulfur compounds present in the gas
are converted with H2 to HZS. If oxygen is present in the
gas, it is converted to H20. COS and CS2, if present, are
converted with the water vapor present, to H2S and C02.
The gas from the hydrogenation reactor 5 is adjusted via
line 6 to the desired absorption temperature with cooler 7,
before being passed via line 8 into the absorber 9 of a
bioplant. In the absorber, H2S is washed from the gas with a
diluted sodium hydroxide solution, which is subsequently
passed via line 10 to an aerobic biological reactor 11, in
which HZS, with addition of oxygen from the air supplied via
line 12, is converted to elemental sulfur. Via line 13 the
sodium hydroxide solution is passed into a sulfur separator
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14, from which the sulfur formed is discharged via line 15.
The solution is recirculated via line 16 to the absorber. The
gas from the absorber, which now contains only a very low
content of HzS, is passed via line 17 to the afterburner 18
before the gas is discharged via the chimney 19.
In Fig. 2 a diagram is given for a plant according to
the invention, in which off-gas from a Claus plant with a
high HZS/SOZ ratio is absorbed directly, without intermediate
hydrogenation.
Off-gas coming from a three-stage Claus plant 100 is
added via line 101 to absorber 102. The Claus plant 100 is
operated such that the molar HZS/SOz ratio is at least 100.
In the absorber 102, HZS is washed from the gas with a
diluted sodium hydroxide solution, which is subsequently
passed via line 103 to an aerobic biological reactor 104, in
which HZS, with addition of oxygen from the air supplied via
line 105, is converted to elemental sulfur. Via line 106,
pump 107 and line 108, a portion of the sodium hydroxide
solution is passed into a sulfur separator 109, from which
the sulfur formed is discharged via line 110. The solution is
recirculated via lines 111 and 112 to the absorber, with a
small discharge via line 113. The gas from the absorber,
which now contains only a very low content of HzS, is passed
via line 114 to an afterburner, not drawn, before the gas is
discharged via a chimney, also not drawn.
EXAMPLE 1
An amount of sour gas of 9700 Nm3/h coming from a gas
purification plant had the following composition at 45°C and
1.6 bar abs
60 . 0 Vol . HZS
%
3.0 Vol.% NH3
3 0 . Vol . COZ
0 %
5 . Vol . H20
0 %
2.0 Vol.% CH4
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This sour gas was fed to a Claus plant with two Claus
reactors. The sulfur formed in the sulfur recovery plant was,
after the thermal stage and the catalytic reactor stages,
condensed and discharged. The amount of sulfur was 7768 kg/h.
The sulfur recovery efficiency of the Claus plant, based on
the sour gas, was 93.3%.
The amount of off-gas of 29749 Nm3/h coming from the
Claus plant had the following composition at 164°C and a
pressure of 1.14 bar abs.
0 . 47 Vol . HZS
%
0.24 Vol.% S02
0.03 Vol.% COS
0.04 Vol.% CSz
0.01 Vol.% S6
0.04 Vol.% Se
1.38 Vol.% CO
1.53 Vol.% Hz
11.37 Vol.% COZ
55.96 Vol.% NZ
0.66 Vol.% Ar
2 8 . 2 Vol . H20
7 %
This off-gas was supplied with 103 Nm3/h of hydrogen as
reducing gas and then heated to 280°C to hydrogenate all
sulfur dioxide (S02) and sulfur vapor (S6, Se) present to HzS,
and further to hydrolyze carbonyl sulfide (COS) and carbon
sulfide (CS2) to H2S in the hydrogenation reactor which
contains a sulfided group 6 and/or group 8 metal catalyst, in
this case a Co-Mo catalyst.
The amount of off-gas from the hydrogenation reactor was
31574 Nmj/h and had the following composition at 317°C and
1.10 bar abs.
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1 .24 Vol. % HzS
28 ppm COS
2 ppm CS2
2.02 Vol.% HZ
12.64 Vol.% C02
56.62 Vol.% N2
0.67 Vol.% Ar
26 . 80 Vol . % H20
The off-gas was then cooled to 72°C, a temperature which
is 3°C above the dew point of the water vapor present in the
of f -gas .
Then the cooled off-gas was treated in a bioplant at
72°C, with no water condensation from the off-gas taking
place. In the absorber of the bioplant, HzS is washed from
the off-gas with diluted sodium hydroxide solution,
whereafter the solution with the absorbed HZS was passed to
an aerobic biological reactor in which the HzS was converted
to elemental sulfur.
In the bioplant no heat is supplied or removed, so that
the absorption of HZS and the conversion to elemental sulfur
occurred at the same temperature of 72°C.
To the aerobic reactor an amount of 945 Nm3/h of air was
supplied for the selective oxidation of H2S to sulfur. The
amount of gas from the absorber was 31189 Nm3/h and had the
following composition at 72°C and 1.05 bar abs.
2 5 0 ppm HZ
S
28 ppm COS
3 0 2 ppm CSZ
2.04 Vol.% Hz
12.80 Vol.% COZ
57.32 Vol.% N2
0.68 Vol.% Ar
3 5 2 7 Vol . H20
. 13 %
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Via an afterburning, this gas was passed to the chimney.
The amount of sulfur formed in the bioplant was 551 kg/h. The
total amount of sulfur produced in the sulfur recovery plant
and the bioplant was 8319 kg/h, which raised the total
desulfurization efficiency, based on the original sour gas,
to 99.87%.
EXAMPLE 2
An amount of sour gas of 6481 Nm'/h coming from a gas
purification plant had the following composition at 45°C and
1.6 bar abs
90 . 0 Vol . % HZS
3.0 Vol.% NH3
5 . 0 Vol . % H20
2.0 Vol.% CH4
This sour gas was supplied to a SUPERCLAUS~ plant with
two Claus reactors and a selective oxidation reactor. The
sulfur formed in the sulfur recovery plant was, after the
thermal stage and the catalytic reactor stages, condensed and
discharged. The amount of sulfur was 8227 kg/h. The sulfur
recovery efficiency of the Claus plant, based on the sour
gas, was 98.5%.
The amount of off-gas of 21279 Nm3/h coming from the
Claus plant had the following composition at 129°C and a
pressure of 1.14 bar abs
0.03 Vol . HZS
%
0.20 Vol.% SOZ
20 ppm COS
3 0 ppm CSz
10 ppm S6
0.01 Vol.% SB
0.15 Vol.% CO
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1.72 Vol. % Hz
1 . 14 Vol . % COZ
62.45 Vol.% NZ
0.74 Vol.% Ar
5 33 . 05 Vol . % H20
0.50 Vol.% 02
This off-gas was supplied with 133 Nm3/h of hydrogen as
reducing gas and then heated to 280°C to hydrogenate all
10 sulfur dioxide (SOZ) , sulfur vapor (S6, S8) present to HzS and
HzO, and further to hydrolyze the carbonyl sulfide (COS) and
carbon sulfide (CS2) to H2S in the hydrogenation reactor
which contains a sulfided group 6 and/or group 8 metal
catalyst, in this case a Co-Mo catalyst.
15 The amount of off-gas from the hydrogenation reactor was
22863 Nm3/h and had the following composition at 367°C and
1.10 bar abs.
0 . 37 Vol . % HzS
2 ppm COS
0.82 Vol . % HZ
1.90 Vol.% COZ
62.89 Vol.% N2
0.75 Vol.% Ar
33.27 Vol. % Hz0
The off-gas was then cooled to 76°C, a temperature which
is 3°C above the dew point of the water vapor present in the
of f -gas .
Then the cooled off-gas was treated in a bioplant at
76°C, with no water condensation from the off-gas taking
place. In the absorber of the bioplant, H2S is washed from
the off-gas with a diluted sodium hydroxide solution,
whereafter the solution with the absorbed HZS was passed to
an aerobic biological reactor in which the HZS was converted
to elemental sulfur.
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In the bioplant, no heat is supplied or removed, so that
the absorption of HzS and the conversion to elemental sulfur
occurred at the same temperature of 76°C. The aerobic reactor
was supplied with an amount of 205 Nm3/h of air for the
partial oxidation of H2S to sulfur. The gas from the absorber
was 22780 Nm'/h and had the following composition at 76°C and
1.05 bar abs.
7 5 ppm H2 S
2 ppm COS
0.82 Vol.% HZ
1.91 Vol.% COz
63.12 Vol.% NZ
0.75 Vol.% Ar
3 3 Vol H20
. 3 9 . %
Via an afterburning, this gas was passed to the chimney.
The amount of sulfur formed in the bioplant was 119 kg/h. The
total amount of sulfur produced in the sulfur recovery plant
and the bioplant was 8346 kg/h, which raised the total
desulfurization efficiency, based on the original sour gas,
to 99.97%.
EXAMPLE 3
An amount of sour gas of 3500 Nm3/h coming from a gas
purification plant had the following composition at 40°C and
1.7 bar abs.
88 . 0 Vol . HZS
%
6.1 Vol.% COZ
1.5 Vol.% CH4
4 . 4 Vol . % H20
This sour gas was supplied to a Claus plant with three
Claus reactors.
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The air supply to this Claus plant was set such that the
reaction (2) the thermal stage and in the Claus reactors
in
was opera ted
with
excess
HZS,
so
that
the
HZS:SOZ
content
after the third reactor stage is greater than 100 to 1, so
A, 5 that the SOZ tent became less than 0.009 vol.%.
con
The sulfur formed in the sulfur recovery plant was,
after the thermal
stage
and
the
catalytic
reactor
stages,
condensed and
discharged.
The
amount
of
sulfur
was
4239
kg/h.
The sulfur
recovery
efficiency
of the
Claus
plant,
based
on
the sour gas,
was
96.4%.
The
amount
of
off-gas
of
10001
Nm3/h
coming Claus plant had the following composition
from the at
130C and a pressure
of
1.15
bar
abs.
0. 93 Vol. HZS
%
0.009 Vol.% SO2
0.04 Vol.% COS
0.04 Vol.% CSZ
0.001 Vol.% S6
0.01 Vol.% Se
0.36 Vol.% CO
1.83 Vol.% HZ
2.79 Vol.% C02
59.68 Vol.% NZ
0.60 Vol.% Ar
33 . 71 Vol Hz0
. %
The off-gas was then cooled to 78°C, a temperature which
is 3°C above the dew point of the water vapor present in the
off-gas. Then the cooled off-gas was treated in a bioplant at
73°C, with no water condensation from the off-gas taking
place. In the absorber of the bioplant, HzS is washed from
the off-gas with diluted sodium hydroxide solution,
whereafter the solution with the absorbed HZS was passed to
an aerobic biological reactor in which the HzS was converted
to elemental sulfur. In the bioplant, no heat is supplied or
CA 02295443 1999-12-16
WO 98/57731 PCT/NL98/00342
18
removed, so that the absorption of HZS and conversion to
elemental sulfur occurred at the same temperature of 73°C.
To the aerobic reactor an amount of 320 Nm3/h of air was
supplied for the selective oxidation of HZS to sulfur. The
amount of gas from the absorber was 9901 Nm3/h and had the
following composition at 73°C and 1.05 bar abs.
190 ppm HZS
7 ppm COS
9 ppm CSz
1.85 Vol.% HZ
0.36 Vol.% CO
2.82 Vol.% COZ
60.28 Vol.% Nz
0.61 Vol.% Ar
34 . 06 Vol . %
Hz0
Via an afterburning, this gas was passed to the chimney.
The amount of sulfur formed in the bioplant was 156 kg/h. The
total amount of sulfur produced in the sulfur recovery plant
and the bioplant was 4395 kg/h, which raised the total
desulfurization efficiency, based on the original sour gas,
to 99.93%.
The small amount of SOz was converted to sulfate in
the lye solution. In order not to obtain any build-up of
sulfates, a small amount of 85 kg/h of the lye solution was
discharged and replaced with a corresponding amount.
