CA2259946A1 - Process for the recovery of sulfur from so2 containing gases - Google Patents

Abstract

The invention relates to 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.

Description

CA 022~9946 lgg9-ol-o7 WO9B/01387 . PCT ~ 97/00392 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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CA 022~9946 1999-01-07 WO98/01387 PCT~L97/00392 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.

CA 022~9946 1999-01-07 WO98/01387 PCT~L97/00392 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 CA 022~9946 1999-01-07 WO98/01387 PCT~L97J00392 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.

CA 022~9946 1999-01-07 WO98/01387 PCT~L97/00392 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 CA 022~9946 1999-01-07 WO98/01387 PCT~L97/00392 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.

CA 022~9946 1999-01-07 WO98/01387 PCT ~ 97/00392 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 CA 022~9946 1999-01-07 WO 98/01387 PCTtNL97/00392 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 CA 022~9946 1999-01-07 W098/01387 PCTn~L97tO0392 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 CA 022~9946 1999-01-07 WO98/01387 PCTnNL97/00392 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%.

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~.

CA 022~9946 lggg-ol-o7 WO98/01387 PCT~L97/00392 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%

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~.

Claims (14)

Claim

1. 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 and a promoter therefor, which heterogeneous catalyst catalyzes the Claus reaction, while as promoter for the Claus reaction a basic nitrogen compound is used, which compound is present in the liquid sulfur.

13a

2. A process according to claim 1, wherein the promoter is selected from the group consisting of amines, alkyl amines, alkanol amines, ammonia, ammonium salts and aromatic nitrogen compounds.

3. A process according to claim 2, wherein the promoter is selected from the group consisting of monoethanolamine, diethanolamine, DGA, DIPA, MDEA and triethanolamine.

4. A process according to claim 2 or 3, wherein a tertiary amine is used.

5. A process according to claims 1-4, wherein as Claus-active heterogeneous catalyst a porous alumina, or a porous alumina with a metal oxide provided thereon, is used.

6. A process according to claim 5, wherein the alumina has a surface area of at least 150 m2/g.

7. A process according to claim 6, wherein of the pore volume, measured with nitrogen, not more than 35% by volume is present in pores of a diameter of 5 nm or less.

8. A process according to claims 1-7, which is carried out at a pressure of 1-5 bar.

9. A process according to claims 1-8, which is carried out at a temperature of 120 to 250°C.

10. A process according to claims 1-9, wherein H2S is dissolved in liquid sulfur, which is thereafter contacted with SO2.

11. A process according to claim 10, wherein gas having a H2S content of at least 0. 5% by volume is contacted with liquid sulfur, whereby a part of the H2S dissolves in the sulfur, thereafter the H2S-containing gas stream is supplied to a Claus plant, whereby a part of the H2S is thermally converted to SO2, whereafter in one or more stages sulfur is formed in a catalytic Claus plant, the gas mixture thereby obtained, after separation of sulfur, is converted directly or, if desired, after a selective oxidation step, in the presence of the liquid sulfur which contains dissolved H2S.

12. A process according to claims 1-10, wherein tail gas of a catalytic stage of a Claus plant, having an H2S
content of at least 0.25% by volume is contacted with liquid sulfur, whereby at least a part of the H2S dissolves in the liquid sulfur, whereafter said H2S-containing liquid sulfur is contacted with the SO2-containing gas, in the presence of the 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.

13. A process according to claims 1-12, wherein the amount of promoter, based on the weight of the liquid sulfur, is between 1 and 1000, preferably between 1 and 50 ppm.

14. A process according to claims 1-14, wherein the reaction is carried out in a fixed bed of catalyst particles or other bodies on which catalyst has been provided, and wherein these particles or bodies are irrigated with liquid sulfur.