Note: Descriptions are shown in the official language in which they were submitted.
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Title: Process for the recovery of sulfur from S~2
containing gases
In a number of processes, such as the refining of
petroleum, the purification of natural gas, and the
production of synthesis gas from coal or from oil residue,
sulfur-containing gas is released, in particular H2S. This
H2S is to be removed before the above-mentioned gases can be
used. The most important reason for H2S removal is the
prevention of SO2 emission through combustion of H2S. Also,
it is well known that H2S is a very toxic gas and has a
nasty smell.
The most common method in the industry is to
remove H2S from gases through a liquid absorption agent,
whereby the H2S is brought into concentrated form,
whereafter the regenerated H2S gas is converted to elemental
sulfur, which is harmless.
Also, in a number of cases it is possible to skip
the first step, that is ! bringing H2S into concentrated
form, and to convert the H2S directly into elemental sulfur.
One of the most well-known and widely used methods
for converting H2S to elemental sulfur is the so-called
Claus process. The Claus process is carried out in dlfferent
ways, depending on the H2S content in the feed gas.
According to the most conventional embodiment, a part of the
H2S is burned to SO2, which then proceeds to react further
with the remaining H2S to form elemental sulfur.
A detailed description of the Claus process is
found in R.N. Maddox "Gas and Liquid Sweetening"; Campbell
Petroleum Series (1977) pp. 239 - 243 and in H.G. Paskall
"Capabilities of the Modified Claus Process", publ. Western
Research & Development, Calgarv, Alberta, Canada (1979).
.
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The Claus process is based on the following
reactions:
2 H2S ~ 3 ~2 ~> 2 H2O + 2 SO2 (l)
4 H2S + 2 SO2 <-> 4 H2O + 6/n Sn (2)
Reactions (l) and (2) result in the overall
reaction
2 H2S + ~2 <~> 2 H2O + 2/n Sn (3)
A conventional Claus plant suitable for processing
gases with a H2S content between 50 and 100% consists of a
thermal stage (burner, combustion chamber, tail gas vessel
and sulfur condenser) followed by a number, generally two or
three, of reactor stages (gas heating, reactor filled with
catalyst and sulfur condenser) In the thermal stage
reactions (l) and (2) occur, in the reactor stages only
reaction (2) known as the Claus reaction. In the Claus
process, however, the H2S is not completely converted to
elemental sulfur, mainly as a result of the fact that the
Claus equilibrium reaction (2) does not go to completion.
So a certain amount of H2S and SO2 remain. Burning
this residual gas is no longer is no longer permitted in
view of the stricter environmental requirements. This so-
called tail gas must be further desulfurized. Tail gas
processes are known to those skilled in the art and are
described, for instance, in B.G. Goar, Tail Gas Clean-up
Processes, a review, paper at the 33rd Annual Gas
Conditioning Conference, Norman, Oklahoma, March 7-9, l983.
The most well-known and to date most effective
process for desulfurizing tail gas is the SCOT process
described in Maddox "Gas and liquid sweetening" (1977). The
SCOT process achieves a sulfur recover~ of 99. 8 tO 99 . 9~ .
Drawbacks of the SCOT process are the high investment costs
and the high energy consumption.
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Another process for increasing the efficiency of
the Claus process is the SUPERCLAUS process. With this
process the efficiency of the Claus process is 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, Oct. 25, 1988,
Edmonton, Alberta, Canada.
The SUPERCLAUS process is cheaper than other known
tail gas treating processes. In the SUPERCLAUS process,
reaction (2) in the thermal stage and in the Claus reactor
stages is operated at excess H2S, so that in the gas from
the last Claus reactor stage the H2S content is about 1% by
volume and the SO2 content ahout 0.02% by volume. In the
downstream reactor stage connected to it, the H2S is
selectively oxidized to elemental sulfur according to the
reaction
2 H2S + ~2 ~> 2 H2O + 2/n Sn (4)
over a special selective oxidation catalyst.
These catalysts are described in European patents
0242920 and 0 409 353.
The tail gas from the SUPERCLAUS reactor stage
then still contains an H2S content of 0.02% by volume and a
S~2 content of about 0.2% by volume and an ~2 content of
0.2-0.5~ by volume.
Another Claus process is described in U.S. Patent
4,280,990 to Jagodzinski et al, where Claus reaction (2)
occurs in li~uid sulfur in the presence of standard Claus
catalyst at elevated pressure, without condensation of
water.
In this process the thermal stage is operated at
pressures of 5 to 50 bar, whereafter the exiting gases are
passed at the same pressure .into a reactor, which is filled
with a catalyst. The reaction between H2S and SO2 therefore
occurs at pressures between 5 en 50 bar, whereby the sulfur
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condenses on the catalyst. Liquid sulfur is circulated over
the catalyst beds to dissipate the reaction heat. The gas
from the thermal stage contains about 7.9% by volume H2S and
3.95% by volume SO2, so that the ratio H2S : SO2 = 2 : l. The
reactor temperature in the first bed is set such that the
exit temperature is 275~C. In a second bed the exit
temperature is set at l9S~C. From the examples of this
method it can be derived that the conversion of these high
percentages of H2S and SO2 proceeds better with increasing
pressure. Alternatively, the same method is proposed for the
desulfurization of Claus tail gas. In this case, Claus tail
gas is brought to a considerable pressure.
A drawback of this process for desulfurization of
both Claus process ~as and Claus tail gas are, respectively,
the high costs for compressors of H2S gas (claus feed gas)
and air, and the high costs of a tail gas compressor, the
high energy consumption of these compressors, the danger of
leakages of toxic H2S gas in these compressors and in other
apparatus in de plant, and the operational reliability of
these compressors.
That is the reason why this process so far has
never found any commercial application. In the method
described in U.S. Patent 4,280,990, use was made of a
standard Claus catalyst. At the time of the above-mentioned
patent, activated aluminas were used as Claus catalyst with
a surface area of about 300 m2/gr with an average pore
diameter of about 50 Angstrom. Such a catalyst is also
described in U.S. Patent 4,280,990.
In the years that this process was developed, it
was customary for a standard alumina catalyst to be
installed in Claus reactors. It is therefore plausible that
no further research was performed into other types of
catalysts or that they were not available, or had not been
developed yet. Nor was any research done on the required
working pressure depending on the ~2S and SO2 concentration.
Most experiments described in U.S. Patent 4,280,990 are
carried out with 2.5~ by volume H2S and 1.2% by volume SO2.
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U.S. Patent 3,447,903 discloses another process,
which is also based on the application of the Claus process
in liquid sulfur. According to this method, the reaction is
catalyzed by the presence of a slight amount of a basic
nitrogen compound. It appears from the examples that amounts
of about l to 50 ppm of this compound were used. This
process has never been applied commercially either.
It is an object of the invention to provide an
improved method for recovering sulfur from tail gases,
whereby SO2 and H2S are removed as much as possible. More
particularly, it is an object of the invention to provide a
method whereby the conventional sulfur recovery methods are
improved in such a manner that an industrial-scale recovery
efficiency of more than 99.5% is achieved.
The invention provides a process for recovering
sulfur from an SO2 containing gas stream through catalytic
conversion thereof to elemental sulfur, comprising
converting SO2 and H2S in the presence of liquid sulfur and a
catalyst system based on a heterogeneous catalyst which
catalyzes the Claus reaction, while as promoter for the
Claus reaction a basic nitrogen compound is present in the
liquid sulfur.
Surprisingly, it has been found that with the
process according to the invention, utilizing the specific
promoter for the heterogeneous catalyst, a clearly improved
conversion efficiency to elemental sulfur is achieved. As
such, the use of liquid sulfur as a medium for the reaction
had long been known. Only with the process according to the
invention, however, has it become possible to carry out this
method at low pressures, that is, at atmospheric pressure or
~ slightly above it.
The process can be carried out in .~ number of
ways. Essential is that the catalyst is in direct contact
with liquid sulfur which has been supplied from an external
source. It is preferred that this liquid sulfur already
contains an amount of the H2S to be converted, since the
conversion efficiency is then clearly higher. So, it is
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possible to supply both H2S and S02 from the gas phase, but
this yields a lower efficiency.
In the process accord~ng to the invention, the
reaction between H2S and S02 to sulfur and water, in the
ratio of H2S : S02 = 2 : l, is carried out in the presence
of liquid sulfur with a suitable catalyst, with the pressure
being preferably between l and 5 bar and the temperature
being preferably between 120 and 250~C.
In the process according to the invention,
suitable catalysts have a structure with large macropores.
These include those activated aluminas that have a small
micropore structure and a large volume of meso and
macropores. These activated aluminas have a meso, macro and
ultrastructure which contain more than 65% of the total pore
volume. It is also possible to use catalysts which have
these properties as support material, this support material
being impregnated with an active material, e.g. a metal
oxide. These catalysts are often referred to as "promoted
catalysts".
In general, it can be stated that those catalysts
are useful that catalyze the Claus reaction. In addition to
the activated alumlnum oxide catalysts already discussed,
the other catalysts known for this reaction are also
suitable, such as titanium dioxide, and metal oxides on
support.
It was found that when water vapor is fed to the
gas to be treated at pressures lower than 5 bar or when
water vapor is present in the gas, this promotes the
reaction between H2S and SO2 to sulfur and water. Also,
through an appropriate choice of the residence time, the
efficiency can be influenced considerably.
It was also established that at pressures lower
than 5 bar, when polysulfides are present in the sulfur,
these react in the same manner wit.h S02 to form sulfur and
water as does H2S. It was found that when the gas contains
oxygen, this oxygen hardly reacts, if at all, with the H2S
or sulfur present to form S02.
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A major advantage of the process according to the
invention is the reaction at the low pressure, as a result
of which all drawbacks of the process according to U.S.
Patent 4,280,990 are removed.
In the process according to the invention, it is
also possible to treat SO2 containing gases by adding H2S
gas to these gases or by priorly disso~ving H2S in the
liquid sulfur.
In the process according to the invention, it was
established that when H2S was priorly dissolved in the
liquid sulfur, this yields a higher conversion with regard
to the SO2 and provides the advantage that the control of
the required H2S to convert the SO2 can be simplified
considerably, because the dissolved, unused H2S remains
behind in the sulfur, whereafter the sulfur can be loaded
with H2S again.
Surprisingly, it was found that when in the
process according to the invention a small amount of a basic
nitrogen compound is present in the sulfur, the efficiency
of the conversion of H2S and SO2 to sulfur and water is
improved considerably, even to the point where a practically
complete equilibrium is achieved at the temperature set.
Suitable basic nitrogen compounds are amines (such
as alkyl amines), alkanol amines (such as MEA, DGA, DEA,
DIPA, MDEA, TFA), ammonia, ammonium salts, aromatic nitrogen
compounds (such as quinoline, morpholine).
Preferably, tertiary alkanol amines are used,
because they do not form sulfamate, have a high boiling
point, and because these amines are relatively cheap.
The invention will now be further clarified with
reference to the drawing. In Fig. l H2S- and SO2-containing
gas is supplied via line l to a reactor 2 in which a
catalyst 3 is present.
Liquid sulfur is supplied ~iia line ~ and, together
with the entrant gas, passed over the catalyst. Liquid
sulfur is produced in the catalyst bed from the reaction
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between H2S and SO2. The exiting gas, after reaction between
H2S and SO2, is discharged via line 5.
The liquid sulfur is passed via line 6 from the
reactor to a cooler 7, where the reaction heat is
dissipated. With the aid of pump 8, the sulfur is
recirculated to the reactor 2 via line 4. The sulfur formed
is discharged via line 9.
In Fig. 2 H2S-containing gas containing more than
90% by volume H2S is supplied via line 1 to a Claus plant
10, consisting of a thermal stage followed by two catalytic
reactor stages.
The air required for the Claus reaction is
supplied via line 11. The sulfur formed in the thermal stage
and reactor stages ls discharged via line 12. The tail gas
from the second catalytic reactor stage, which still
contains H2S and SO2, is supplied via line 13 to a reactor 2
in which a catalyst 3 is present. Over the catalyst bed,
liquid sulfur is supplied via line 4. After H2S and SO2 have
reacted in the catalyst bed to form sulfur, the tail gas
leaves the reactor via line 5. The liquid sulfur leaves the
reactor via line 6 and, via a cooler 7, is recirculated to
the reactor 2. The sulfur formed is discharged via line 9.
Alternatively, a basic nitrogen compound can be added via
line 14.
In Fig. 3 a preferred embodiment of the process
according to the invention is described, where via line 1
H2S-containing gas is supplied to a Claus plant 10,
consisting of a thermal stage followed by two catalytic
reactor stages.
The air required for the Claus reaction is
supplied via line 11. The sulfur formed in the thermal stage
and reactor stages is discharged via line 12. The tail gas
from the second catalytic reactor stage, which still
contains H2S and SO2, is supplied via line 13 to a
SUPERCLAUS plant 15.
Via line 16 air for the selective oxidation is
supplied, while via line 17 liquid sulfur is discharged. The
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tail gas is supplied via line 13 to reactor 2, in which a
catalyst 3 is present. Over the catalyst bed, liquid sulfur
is supplied via line 4.
This liquid sulfur comes from column 18, in which
the sulfur has been contacted with the H2S-containing gas
which was supplied to the Claus plant via line 1. In the
column 18 the liquid sulfur has incorporated a part of the
H2S from the gas. After H2S, dissolved in the liquid sulfur,
and S02 have reacted to sulfur in the catalyst bed, the tail
gas leaves the reactor via line 5. The liquid sulfur leaves
the reactor 2 via line 6 and is recirculated with the aid of
a pump 8 via line 19 to the column 18. The sulfur formed is
discharged via line 9.
In the column, the sulfur takes up H2S again and is
lS supplied to reactor 2 again via line 20, pump 21, cooler 22
and line 4. If desired, via line 14 a basic nitrogen
compound-can be supplied to the liquid sulfur.
The invention is further explained in and by the
following examples.
EXAMPLE
Using the plant as described in Fig. 2, in a Claus
plant with two catalytic stages, the Cl.aus reaction is
carried out. To the thermal stage is supplied a Claus gas
containing 90.0% by volume H2S, corresponding with 36.0
kmol/h, 3.5% by volume C02, 2.0% by volume hydrocarbons and
4.5% by volume H20 and 19.5 kmol/h ~2 as air oxygen. The H2S
percentage by volume in the tail gas after the second
catalytic stage is 0.58% by volume, while the S02 content
therein is 0.29~ by volume and the water content therein is
33.2~ by volume. The sulfur recovery efficiency of the Claus
plant is 94~.
The tail gas in an amount of 120 kmol/h with a
temperature of 150~C, and a pressure of 1.13 bar, is
supplied to the catalyst bed outlined in Fig. 2. The
catalyst 3 is an activated alumina with a high meso and
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macropore structure. Over the bed, liquid sulfur is
circulated in an amount of 50 m3/h at a temperature of
150~C. The temperature of the circul~ating sulfur is kept
constant by dissipating the evolved reaction heat of the
process in a cooler. In order not to cause the sulfur level
in the reactor to rise too far, from time to time some
sulfur is drained from the system. The H2S percentage by
volume in the gas after the catalyst bed is 0.188%, while
the SO2 percentage by volume therein is 0.088%. The
conversion of H2S to sulfur in the reactor is therefore 68%
and that of SO2 is 70%.
The total sulfur recovery efficiency of the Claus
plant followed by this reactor stage in which the reaction
between H2S and SO2 occurs in liquid sulfur is thereafter
more than g7.7%.
EXAMPLE 2
In the same plant as described in Fig. 2, an
aromatic amine (quinoline) is added to the circulating
sulfur via line 14. The amount of quinoline supplied is such
that the concentration in the sulfur stream to the reactor
is 500 ppm by weight.
The Claus gas to the thermal stage is the same as
described in Example l, but now 19.85 kmol/h ~2 as air
oxygen is supplied in order to obtain as much SO2 as H2S in
the tail gas after the second catalytic stage. The H2S and
S~2 percentages by volume in the tail gas are then 0.46%
each and the water content therein is 33.0% by volume. The
H2S percentage by volume in the tail gas after the catalyst
bed is 0.046%, while the SO2 percentage by volume therein is
0.018%. The conversion of H2S to sulfur in the reactor is
therefore 90% and that of SO2 is g6%.
The total sulfur recovery efficiency of the Claus
plant followed by this reactor stage in which the reaction
between H2S and SO2 occurs in liquid sulfur is thereafter
more than 99.0~.
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EXAMPLE 3
In the plant as described.in Fig. 3, a SUPERCLAUS
reactor stage is arranged after the second catalytic stage
of the Claus plant to allow selective oxidation of H2S to
sulfur in the gas from the second catalytic stage. The tail
gas from the SUPERCLAUS stage is supplied to catalyst bed as
outlined in Fig. 3. The Claus gas is first contacted
countercurrently with a sulfur stream in a contacting vessel
before the gas is passed to the thermal stage. The Claus
feed gas which flows to this contacting vessel is the same
as in Example 1. In the contacting vessel, 0.193 kmol/h H2S
is dissolved in the sulfur and hence withdrawn from the
Claus feed gas that is passed to the thermal stage. To the
thermal stage, 18.87 kmol/h ~2 as air oxygen is supplied. To
the SUPERCLAUS stage, another 1.40 kmol/h ~2 as air oxygen
is supplied. The H2S percentage by volume in the tail gas
after the SUPERCLAUS stage is 0.032%, while the SO2 content
therein is 0.189% by volume and the ~2 content therein is
0.50% by volume. The tail gas from the SUPERCLAUS stage in
an amount of 122 kmol/h, with a temperature of 130~C and a
pressure of 1.13 bar absolute is supplied to the catalyst
bed outlined in Fig. 3. Over the bed is passed the liquid
sulfur coming from the contacting vessel. To the liquid
2S sulfur a tertiary alkanol amine (TEA) is added.
The sulfur is thereafter returned to the
contacting vessel. The magnitude of the circulation stream
is set such that sufficient H2S with respect to the SO2 is
supplied to the catalytic bed, so that the H2S : SO2 ratio
is minimally 1 : 1.
The H2S concentration in the exiting gas after the
catalyst bed is 0.015% by volume, while the SO2 percentage
by volume therein is 0.011% by volume. The conversion of H2S
to sulfur in the reactor is therefore 92% and that of SO2 is
94%
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The total sulfur recovery efficiency of the Claus plant
with SUPERCLAUS reactor stage followed by this reactor stage
in which the reaction between H2S and S02 proceeds in liquid
sulfur, is thereupon more than 99.5~.
