WO2012152919A1

WO2012152919A1 - A method for treating a sulfur-containing gas stream

Info

Publication number: WO2012152919A1
Application number: PCT/EP2012/058768
Authority: WO
Inventors: Wei Wang; Xiping Song
Original assignee: Hofung Sulphur Technology Limited; SCHULTE NORDHOLT, Hendrik
Priority date: 2011-05-11
Filing date: 2012-05-11
Publication date: 2012-11-15
Also published as: NL2008801C2; CN102198365B; NL2008801A; CN102198365A

Abstract

The invention pertains to a method for treating a sulfur-containing gas stream, comprising the steps of (1) providing a gas stream comprising H₂S and SO₂ to a Claus reactor where it is contacted under reaction conditions with a catalyst, to form a product comprising elemental sulfur, water, and residual H₂S and SO₂, (2) contacting a tail gas from a Claus reactor which comprises H₂S and SO₂ with oxygen in an oxidation step under oxidising conditions to obtain a gas stream comprising SO₂, (3) contacting the SO₂-containing gas stream in an SO₂conversion step with a catalyst to form a gas stream comprising SO₃, (4) contacting the gas stream comprising SO₃ with water under hydration conditions to form sulfuric acid. The present invention provides a process in which a sulfur-containing gas stream is efficiently converted to elemental sulfur and, if so desired, sulfuric acid while the sulfur content of the flue gas is so low that direct provision to the atmosphere is possible, while at the same time efficient use is made of energy and resources.

Description

A method for treating a sulfur-containing gas stream The present invention relates to a method for treating a sulfur-containing gas stream, in particular a gas stream containing ¾S and/or SO₂.

While various processes for converting gas streams containing ¾S and S0₂ are known in the art, it has been found that there is still need for improved processes. More in particular, it has been found that there is a need for a process which converts H₂S and SO₂ in an efficient manner to high value products such as elemental sulfur, while producing gas streams which do not require purification through processes such as solvent adsorption or caustic washing treatment, and which at the same time make efficient use of energy and resources through appropriate use of recycle streams. That a caustic washing treatment can be dispensed with is particularly attractive due to the following: Normally, the caustic solvent is a Ca (OH) ₂ solution at a certain concentration, NaOH, or ammonia. The investment of the above methods is big and the process is complicated, while the desulfurization efficiency is low . In industrial applications, the Ca (OH) ₂ solution is made by several sequences, such as CaO grinding, manufacturing of a solution, multistage filtration and dilution. The calcium sulfite hyperoxidation needs air, and the atomizing nozzle is easily plugged during operation. The NaOH solution is costly and the washing process discharges a large waste water stream.

The ammonia absorption process needs an oxidizing process to obtain (NH₄)₂ S0₄, and the tower circulation flow is very big, causing the need for a large column diameter, and in order to achieve (NH₄ ) ₂ S0₄, it is necessary to blow large amounts of air into the column to oxidize the sulfite and hydrosulfite .

This oxidizing reaction is very slow and meanwhile the ammonia can easily escape from the solvent.

The present invention provides a process in which a sulfur-containing gas stream is efficiently converted to elemental sulfur and, if so desired, sulfuric acid while the sulfur content of the flue gas is so low that direct provision to the atmosphere is possible, while at the same time efficient use is made of energy and resources.

The present invention pertains to a method for treating a sulfur-containing gas stream, comprising the steps of (1) providing a gas stream comprising H₂S and SO₂ to a Claus reactor where it is contacted under reaction conditions with a catalyst, to form a product comprising elemental sulfur, water, and residual H₂S and S€½,

(2) contacting a tail gas from a Claus reactor which comprises H₂S and S0₂ with oxygen in an oxidation step under oxidising conditions to obtain a gas stream comprising S0₂,

(3) contacting the S0₂-containing gas stream in an S0₂ conversion step with a catalyst to form a gas stream comprising S0₃,

(4) contacting the gas stream comprising S0₃ with water under hydration conditions to form sulfuric acid.

It has been found that the integrated method according to the invention combined a high yield of elemental sulfur with efficient removal of remaining sulfur compounds from the tail gas, and formation of sulfuric acid which can be recycled back into the process to form elemental sulfur, and/or be recovered separately. More specifically, it has been found that the process according to the invention can simultaneously fulfill both the highest sulfur recovery requirements and the lowest SO₂ emission requirements known in the industry today. This is a unique feature of the process according to the invention. Additional embodiments and advantages of the present invention will become clear from the further specification.

The process according to the invention starts out with a gas stream comprising ¾S and S0₂, which is provided to a catalytic Claus reactor.

The gas stream provided to the catalytic Claus reactor generally contains between 0.5 and 8 vol . % of H₂S , in particular between 2.0 and 6 vol . % of H₂S . The gas stream generally contains between 0.3 and 5 vol . % of S0₂, in particular between 1.0 and 3 vol. ! of S0₂.

The gas stream comprising H₂S and S0₂ may be obtained in various manners, depending on the concentration of the H2S stream that is available.

In a first embodiment, the gas stream comprising H₂S and S0₂ is obtained through partial combustion of a gas stream containing H₂S . The gas stream subjected to partial combustion may contain, e.g., at least 15 vol . % of H₂S . It is preferred, however, for the gas stream to contain a relatively high concentration of H₂S , e.g., at least 30 vol . % , more in particular at least 35 vol . % . The gas stream may contain up to 100 vol . % of H₂S . In one embodiment the gas stream contains at least 55 vol . % of H₂S, in particular at least 60 vol . % of H₂S . In another embodiment the gas stream contains between 15 and up to 55 vol . % of H₂S .

In the combustion step the gas stream containing H₂S is partially combusted to form a gas stream comprising H₂S and S0₂. The combustion step is carried out in the presence of oxygen, which can be provided using an oxygen-containing gas such as oxygen, air, or air to which additional oxygen has been added. Where necessary, fuel gas may be added to the combustion step to raise the temperature. The fuel gas for the combustion step, if present, may be any kind of combustible gas, solid, or liguid.

The combustion temperature generally is in the range of 350-1400°C, preferably in the range of 1G00-1350°C. During the combustion step, a burner is generally used, with or without atomizing fluid . It is preferred for the burner to atomize air or steam. The use of a burner which atomizes steam is particularly preferred, because steam is the most efficient atomizing fluid, and is readily available in the maj ority of cases .

If so desired, the gas to be provided to the combustion step is preheated to a temperature of 80™400°C, in particular 80-220°C.

The gas resulting from the combustion reaction in the burner comprises ¾5 and S0₂. In general , the gas contains between 1 and 60 vol . % of H₂S, in particular between 10 and 30 vol . % of H₂S. In general, the gas contains between 0.5 and 30 vol . % of S0₂, in particular between 5 and 15 vol . % of SO₂. The molar ratio between H₂S and S0₂ generally is between 4 : 1 and 0.5:1, in particular between 3 : 1 and 1.5:1, still more in particular between 2.1:1 and 1.9:1.

Where the gas resulting from the combustion reaction has a relatively high concentration, e.g., in the particular ranges indicated above, the ¾S and the S0₂ will react in the combustion chamber, at the prevailing high temperature to form elemental sulfur according to the following equilibrium reaction: 2H₂S + S0₂ = 2H₂0 + 3S

In general after the above mentioned reaction the gas contains between 0.5 and 8 vol . % of H₂S, in particular between 2.0 and 6 vol . % of H₂S . In general after the above mentioned reaction the gas contains between 0.3 and 5 vol. % of S0_2r in particular between 1.0 and 3 vol . % of SO₂.

After removing the elemental sulfur by a low temperature condensing step this gas is suitable for further processing in the catalytic Claus reactor.

The composition of the gas from the combustion chamber (also sometimes indicated as combustion reactor or thermal reactor) is dependent on the composition of the gas provided to the combustion step, on the combustion conditions, and on the amount of oxygen provided during the combustion step. It is within the scope of the skilled person to select the parameters in such a manner that a gas with a suitable composition is obtained .

The gas resulting from the combustion step may be provided directly to the Claus reactor, or may be subjected to intermediate processing steps such as heating up, cooling down, Heating up by of the gas to the required Claus reactor inlet temperature may be necessary by means of direct heating or a heat exchanger. The heating medium can be steam, hot oil, hot gas, hot gas derived from fuel gas combustion with an oxygen containing gas stream, or an electric type heat exchanger using an electric power source.

It is preferred for the gas obtained from the combustion step to be provided directly to the Claus reactor, whether or not after an intermediate cooling of heating step, with an intermediate cooling step being more often required.

In another embodiment, the starting gas stream comprising ¾S and S0₂ is not derived from a partial combustion step as described above, but from other sources. In one embodiment, the starting gas stream containing ¾S and SO₂ is obtained by adding 5(¾ to a gas stream containing H₂S in an amount of, say, 1-15 vol . % , in particular 1 to 8 vol . % . The amount of S0₂ to be added is governed by the desired composition of the gas to be provided to the catalytic Claus reactor, as has been discussed in more detail above. By adding SO₂ to a ¾S containing gas stream, the process according to the invention is a suitable outlet for relatively lean H₂S-containing gas streams. This is of particular interest, as the other currently available processes to treat these gases to form elemental sulfur are troublesome to operate, have low recovery efficiency, and have high investment costs. Further, they often yield a sulfur product with low guality.

The S02 used in this embodiment can, e.g., be obtained by combustion of elemental sulfur with oxygen in a combustion chamber, and subsequent cooling in a heat exchanger, if this is required.

For gas streams containing an intermediate amount of H₂S, e.g. , in the range of 5-30 vol . % , more in particular 8-15 vol . % , a further approach may be followed. This gas has a H₂S content which is less preferred for direct provision to the catalytic Claus reactor. On the other hand, if this gas is subjected to the partial combustion step as described above, the ¾S concentration will be undesirably low. Further, this process is difficult to carry out through dependable methods.

It has been found that these cases can be converted to a gas stream containing H₂S and S0₂ in the desired range by providing the H₂S containing gas stream to a combustion chamber which also contains a suitable amount of elemental sulfur and oxygen The elemental sulfur is converted to S0₂, which results in the formation of a gas stream with a relatively high concentration of H₂S and S0₂. At the prevailing high temperature this results in the formation of elemental sulfur through the thermal Claus reaction, leaving a gas stream with a containing H₂S and S0₂ in the ranges specified above. This stream is suitable for provision to the catalytic Claus reactor. An attractive feature of this process is that the amount of sulfur added to the reactor is not very critical, as elemental sulfur will be consumed by the combustion and generated through the thermal Claus reaction. The amount of oxygen should be resulted to ensure that the desired amount of SO₂ is generated. The combustion step can be carried out as described above for the combustion of more concentrated H₂S streams.

The gas stream comprising ¾S and SO₂ that may, e.g., be obtained through any of the processes described above is provided in step (1) of the process according to the invention to a Claus reactor where it is contacted under reaction conditions with a catalyst, to form a product comprising elemental sulfur, water, and residual H₂S and S0₂, in accordance with the following formula:

2H₂S + S0₂ = 2H₂0 + 3Sj

As indicated above, the starting gas stream generally comprises between 0.5 and 8 vol . % of ¾S, in particular between 2.0 and 6 vol . % of H S . The gas stream generally comprises between 0.3 and 5 vol . % of S0½, in particular between 1.0 and 3 vol . % of SO₂. The molar ratio between H₂S and S0₂ generally is between 4 : 1 and 0.5:1, in particular between 3 : 1 and 1.5:1, still more in particular between 2.1:1 and 1.9:1.

The gas stream may also comprise one or more of the following components, e.g., water, e.g., in an amount of 10-30 wt . % .

In the Claus reactor the gas comprising H₂S and S0₂ is contacted under reaction conditions with a catalyst, to form a gas product comprising elemental sulfur, water, and a tail gas containing residual H₂S and S0₂.

Claus reactors, catalysts, and reaction conditions are known in the art. Reaction conditions include, e.g., a temperature in the range of 200 to 350°C, in particular 220 to 300°C and a pressure of , e.g., 0.1 bar gauge to 3 bar gauge, in particular 0.3 bar gauge to 1.5 bar gauge, still more in particular 0.6 bar gauge to 0.8 bar gauge. Suitable catalysts include those known in the art such catalysts containing alumina and/or titania .

The gas product comprising elemental sulfur, water, and a tail gas containing residual H₂S and SG½ is provided to a condenser to condense the elemental sulfur . The tail gas may if so desired be provided to a further Claus reactor followed by a further condenser, which again may be followed by further reactor condensor sets. The number of Claus reactor / condenser assemblies used in the process according to the invention is not critical, it generally ranges from 2 to 4 ,

The tail gas from the final Claus reactor generally has to following composition: In general, the gas contains between 0.2 and 10 vol . % of ¾S, in particular between 0.5 and 3 vol . % of H₂S . In general, the gas contains between 0.1 and 5 vol . % of S0₂, in particular between 0.2 and 1.5 vol . % of S0₂. The molar ratio between H₂S and SC½ generally is between 3:1 and 1 : 1 , more in particular between 2.1:1 and 1.5:1. The tail gas may contain COS, e.g., in an amount of 0 to 0.2 vol . % , and/or CS₂, in an amount of 0 to 0.2 vol . % .

The gas is provided to an oxidation step, where it is contacted with oxygen to convert the sulfur compounds to S0₂.

The main reactions are:

2H₂S + 30₂ = 2S0₂ + 2H₂0

2COS t 30₂ = 2S0₂ + 2C0₂

CS₂+ 30₂ = 2S0₂ +C0₂

If present, small amounts of elemental sulfur may also be converted to S0₂. It may be needed to add oxygen containing gas to the tail gas. This depends on the amount of oxygen already present in the gas as it leaves the Claus reactor. Oxygen may or may not be present in this gas. Whether or not oxygen has to be added and if so how much also depends on the concentration of components that will react with oxygen, such as ¾S, COS, CS₂, vapor-phase elemental sulfur, but also CO and H₂. It is within the scope of the skilled person to calculate the required amount of oxygen. After the addition, the oxygen content is generally in the range of 3 - 15 vol%. The oxygen may be provided as normal air, or other oxygen-containing gases, such as air to which additional oxygen has been added. The reaction temperature in this step generally is 300-800°C, best between 400-700°C. Where necessary, fuel gas may be added to the oxidation step to raise the temperature. The fuel gas for the oxidation step, if present, may be any kind of combustible gas, solid, or liquid .

The product from the oxidation step is a gas stream wherein at least 95% of the sulfur is present, in the form of S0₂, more in particular at least 97% of the sulfur is present in the form of S0₂, still more in particular at least 99%. More in particular, at least 99.9% of the sulfur is present in the form of S0₂-

The gas stream generally contains between 0.1 and 5 vol . % of S0₂, in particular between 0.3 and 3 vol . % of S0₂. The gas stream may further contain other components, e.g., water, generally in an amount of 10-30 vol . %, nitrogen, e.g., in an amount of 50-70 vol . % , C0₂, e.g., in an amount of 1- 0 vol . % , NOx, e.g., in an amount of 0 to 1000 ppm, and oxygen, e.g., in an amount of 2-8 vol . % .

The H₂S content preferably is very low, e.g., in the range of 0-20 ppm, in particular 0-10 ppm. CO and hydrogen may, e.g., be present in an amount of 0-3000 ppm, in particular 0-1000 ppm, more in particular 0-600 ppm for each compound. The gas may also contain minor amounts of COS and CS₂, e.g. for each compound in an amount of 0-20 ppm, in particular 0-10 ppm.

The SO₂-containing gas stream is provided to a reactor where the S0₂ is converted at least in part to SO₃. The oxidation reaction is generally carried out in the presence of a catalyst, which can be any S0₂/S0₃ conversion catalysts for, such as vanadium based catalyst ( anadic oxide and its carrier), or platinum based catalyst.

The S0½ to SO₃ conversion is an exothermic reaction, and it is necessary to cool the process gas gradually in the heat exchanger. The best option for the cooling medium is air (but it can also be considered to use oil, water or steam) . In one embodiment, the cooling air from the SCh to SO₃ conversion unit is returned in whole or in part to one or more of the combustion step, the oxidation step, or other section as oxygen supply.

In the effluent from the S0₂ to SO₃ conversion step generally at least 95 % of the sulfur is present in the form of S0₃, in particular at least 97 %, more in particular at least 99%.

The effluent from the S0₃ formation step is hydrated and cooled by contacting it with water to form sulfuric acid. The concentration of sulfuric acid in the resulting product is 10-98 wt . %, preferably 50 to 90 wt . %, more preferably 75 to 85 wt . % . The temperature generally is 30-300 °C, preferably 200 to 250°C.

The sulfuric acid may sometimes contain trace amounts of sulfinic acid, formed as a result of the presence of trace S02. If present, the sulfinic acid is present in an amount of 0.01 to 0.5 wt . % , calculated on sulfuric acid. The tail gas from the hydration step has a very low SC½ content . For example, the amount of SC½ in the tail gas or flue gas is in the range of 0 to 1 mol.% more specifically in the range of 0 to 0.5 mol.%. It is possible to obtain values in the range of 0 to 0.2 mol.%, or even lower, e.g., in the range of 0 to 0.1 mol.%, or below 0.05 mol.%, or even below 0.01 mol . % .

The H₂S content of the tail gas from the hydration unit preferably is very low, e.g., in the range of 0-20 ppm, in particular 0-10 ppm, or even in the range of 0-5 ppm. The content of COS and CS₂, if present at all is preferably also very low, e.g., in an amount of 0-20 ppm, in particular 0-10 ppm. for each compound. The same goes for the S03 content, which, if present is preferably in the range of 0 to 20 ppm, in particular 0 to 10 ppm. CO and hydrogen may, e.g., be present in an amount of 0-3000 ppm, in particular 0-1000 ppm, more in particular 0-600 ppm for each compound.

The tail gas can be sent to the downstream unit for further treatment and released to the atmosphere. It is a particular feature of the invention that the tail gas may in some embodiments be directly released into the atmosphere as it may meet all presently known environmental requirements.

The sulfuric acid and, if present, sulfinic acid can be discharged directly. It should be noted that the sulfuric acid has a purity which is sufficiently high for it to be used as starting material in other processes.

The sulfuric acid may also be recycled in whole or in part. E.g. it can be returned back to the H₂S combustion step described above, if present. It can also be provided to a step where S02 is manufactured by combustion of elemental sulfur, e.g., in the case where gases with a low H2S content are processed .

At a high temperature of 1000-1350 °C, the sulfuric acid can react with H₂S in the acid gas to obtain S0₂ and elemental sulfur, in accordance with the following formula:

H2SO4 = S0₂ + O.SO₂ + H₂0

H₂S + H2SO4 = S + S0₂ + 2H₂0

The trace amount of sulfinic acid, if present, will decompose in the combustion step to SO₂, which can react with H₂S in the acid gas to obtain elemental sulfur.

A particular feature of the present invention is that it allows reuse of various gas streams.

In a first embodiment, the oxygen-containing hot gas withdrawn from the hydration unit is provided in whole or in part to one or more of the combustion step, if present , the oxidation unit , the S02 conversion unit, or a unit where elemental sulfur is combusted to form S02 , if present .

In a second embodiment cooling air from the S02 to S03 conversion unit is returned in whole or in part to one or more of the combustion step, the oxidation step or other section as oxygen supply . Alternatively cooling can be achieved by raising valuable steam and/or heating boiler feed water and/or heating hot oil for general use in the plant or other facilities .

The present invention provides a process wherein ¾S containing gas streams can efficiently be converted to elemental sulfur and sulfuric acid, which are both suitable for further processing as desired. Due to the integration of the various process steps optimum use may be made of the various recycle possibilities. Further, a particular advantage of the present invention is that not only elemental sulfur can be recovered after treating, but the tail gas is also able to meet emission requirements . The air from the heat exchanger mentioned above can be the source of oxygen, so reaction heat can be recovered and resources recycled.

The process according to the invention makes it possible to achieve an overall achievable sulfur recovery figure of 99.99 %, and this is particularly attractive in combination with the low S02 emissions. The process according to the invention can simultaneously fulfill both the highest sulfur recovery requirements and the lowest SO₂ emission requirements known in the industry today. This is a unique feature of the process according to the invention.

The process according to the invention dispenses with the need to use caustic washing processes, which lead to loss of elemental sulfur which put additional loads on the downstream unit. The sulfur content in the discharged gas from this invented process is very low, achieving higher sulfur recovery efficiency. The process in this invention is easy to operate, effectively purifies the acid gas and adopts a simplified acid gas treating process scheme, reducing operating costs and difficulties and making it suitable for use in many different applications .

Exam] The following examples illustrate the process of this invention, but the invention is not limited thereto or thereby .

In Example 1, reference is made to Figure 1, which illustrates the invention, without limitation. In this figure, the signs have the following meaning:

11 is the combustion reactor, followed by a cooling step 12, which is e.g. a condensor

21 , 22 are Claus reactors

31 and 32 are sulfur condensors , where elemental sulfur is removed from the gas stream

4 is the oxidation chamber where sulfur compounds are reacted with oxygen to form S0₂

5 is the S0₂ converter, where S0₂ is converted into S0₃ 6 is the hydration unit where SO₃ is reacted with water to form sulfuric acid and optionally trace amounts of sulfinic acid

Exam] With reference to figure 1 : Acid gas G comprising H₂S with a temperature of about 40°C is provided to a combustion reactor 11 together with oxygen containing gas provided through a line from the condenser 6 (air, pure oxygen, or other oxygen-containing gas) (and if necessary additional air through feeds not shown) . In combustion reactor 11 vapor phase elemental sulfur is obtained and a gas mixture containing H₂S and SO½. The gas mixture is provided to a condenser/cooler 12, where the elemental sulfur is removed (arrow SI) . The gas mixture containing H₂S and S0₂ is provided to Claus reactor 21, where H₂S and S0₂ react to form elemental sulfur. The effluent from the Claus reactor is provided, to a condenser 31, where elemental sulfur is removed. The Claus tail gas from which the elemental sulfur is removed is provided to a second Claus reactor 22, which is followed by a second condenser 32. Further Claus reactors/condensers may be present. The effluent from the last condenser is combined with hot air from the hydration unit 6 are preheated to 350 °C and then thermally incinerated in the oxidation chamber 4 to convert sulfur compounds into S0₂ under a temperature between 300-800 °C . The gas containing S0₂ is mixed with the hot air from 11 and enters the S0₂ converter 5 to obtain S0₃, The required quantity of air is calculated by measuring the flue gas flow and the 0₂ and S0₂ contents in the incinerator flue gas to get the theoretical air demand. The actual air demand is 12% higher than the theoretical air demand. The converter catalyst is a vanadium based catalyst (such as vanadic oxide) , and the reaction is carried out at a space velocity of 3000 h^{- 1} . The temperature is, e.g., 500°C.

The SO3 containing gas from convertor 5 enters hydration unit 6, also indicated as condenser. The cooling medium is oxygen enriched air 10, entering through line A. The cooling medium enters the condenser to cool the SO₃ containing gas and heated air 1 is emitted through line B . The S0₃ containing gas is cooled to about 100 °C, meanwhile sulfuric acid and, if formed, trace amounts of sulfinic acid are obtained in the bottom of the condenser 6, and withdrawn through line SA. The sulfuric acid and, where present , sulfinic acid may be returned in whole or in part to the combustion step 1 through return line SA2 to be converted into elemental sulfur , The sulfuric acid and, where present , sulfinic acid also be removed from the system for further uses through removal line SA1. The tail gas (or flue gas ) stream from condenser 6 contains hardly any sulfur compounds, and may be emitted to the atmosphere through line 0 or otherwise processed further .

The equipment to be used in this case , such as incinerator, Claus reactor, and thermal stage, are all general equipment widely known and used by the technical people and operators in this field .

Acid gas feed composition for this example

¾ 1.1066 2.2132 0.8478 0.0415 co₂ 88.0374 3373.6445 67.4468 72.6824

N₂ 2.3984 67.1545 1.8374 1.2600

H₂0 0.7939 0.6082 0.2681

CH₄ 0.5375 8.6000 0.4118 0.1614

C2H6 0.5283 15.8483^~1 0.4047 0.2974

C₃H₈ 1.7597 77.4279 1.3482 1.4528

C H10 2. ° 05 173.4505 2.2911 3.2545

CH₃OH 0.1504 4.8112 0.1152 0.0903

Total 130.5287 5329.5526 100.0000 100.0000

Proc ^" ί condenser 12 provided to the Claus reactor

22

Claus tail gas , provided to oxidation unit 4

co₂ 89.6695 3945.4558 46.1934 67.8787

N₂ 26.8241 751.0734 13.8185 12.9217

Ar 0.2861 11. 40 0.1474 0.1969

H₂ 53.6100 107. ^199 27.6173 1.8446

Total 194.1173 5812.5075 100.0000

Fuel gas to oxidation unit 4

Combustion air to oxidation unit 4

Flue gas from oxidation unit 4 _r provided to SOg converter 5

N₂ 110.3558 3089.9631 37.5442 32.6398

Ar 1.2806 51.2250 0.4357 0.5411

H₂0 62.2459 11?0. 262 21.1767 11.8353

Total 293.9358 9466.8549 100.0000 100.0000

Proc i the S02 convertor 5 provided to hydration unit 6

Tail gas from hydration unit 6

In the process of this Example, the acid feed gas flow is 5333.25kg/h . After a series of process treatments, the S02 content in the tail gas of the hydration unit is less than 106mg/Nm³, which makes it suitable for direct emission to the atmosphere. The process of this example shows a sulfur recovery yield of more than 93.9%, specifically 99.96%, with an SC½ content in the hydration unit tail gas of 0.0037 mol . % .

Example 2

In this example, the treatment of an acid gas 'with a H₂S content of 1-8 mol. % will be described in more detail. It should be noted that the description in this example is applicable in combination with the entire description above.

Acid gas with a H₂S content of 1-8 mol. % and a temperature of about 40 °C is mixed with a S0₂ containing gas such that in the mixture the molar ratio between H₂S and SO₂ is between 3 : 1 and 0.5 : 1, more particular between 2.1 :1 and 1.9 : 1. In most cases the S0₂ containing gas is not available in adequate quantity and composition . Therefore the gas is generated by the combustion of elemental sulfur, as produced in the downstream Claus reactors and sulfur condensers or a sulfur species containing gas. For the combustion of sulfur an oxygen containing gas , pure oxygen, air enriched with oxygen or normal air can be used . The combustion of sulfur takes place in a suitable burner and combustion chamber with or without an atomizing fluid and may be assisted by co- firing fuel gas , any kind of combustible gas , solid or liquid . The combustion temperature in the combustion chamber is generally in the range of 700 - 1500 °C, preferably in the range of 1000-1300 °C . The SO₂ containing gas is cooled in a heat exchanger by means of a suitable cooling fluid such as steam, water, oil or air . Preferably a fire-tube waste heat boiler type heat exchanger is used whereby steam is produced from boiler feed water generally at a pressure in the range of 15- 50 Bar(g) , more particular in the range of 25-45 Bar(g) . The temperature of the SO₂ containing gas from the heat exchanger is adjusted such that after mixing with the H₂S containing acid gas the required inlet temperature of the Claus reactor is obtained which generally is in the range of 180-260 °C, in particular in the range of 210-240 °C . After the cooling step a heat exchanger may be included to slightly heat up the SO₂ containing gas in order to obtain a more precise inlet temperature of the Claus reactor . This heat exchanger can be provided with a heating fluid such as steam, hot water o hot oil , or the heater can be of the electric type . The mixture of H₂S containing acid gas and the S0₂ containing gas is introduced into the catalytic Claus reactor and further processed according to the invention as described in Example 1. If so desired, sulfuric acid can be recycled to the SO₂ generation step and decomposed .

Example 3

In this example , the treatment of an acid gas with a H₂S content of 8-25 mol . % will be described in more detail . It should be noted that the description in this example is applicable in combination with the entire description above .

A variation to the above principle which is suitable for acid gas with a H₂S content between approximately 8 and 15% is as follows :

The acid gas with a H₂S content of 8-15 mol . % is introduced in the combustion chamber of the S0₂ generation step . The S0₂ containing gas is produced as described above . The temperature in the combustion chamber is maintained in the desired range by co-firing fuel gas , any kind of combustible gas , solid or liquid . The combustion chamber has adequate dimensions to allow for the combustion of the elemental sulfur and/or a sulfur species containing gas, the mixing with the acid gas containing H₂S, and the equilibrium reaction between H₂S and S0₂ according to the formula: 2H₂S+S0₂=2H₂0+3S . The cooling of the gas takes place in a heat exchanger as described above followed by a sulfur condenser whereby sulfur is condensed and separated from, the gas stream. The resulting gas stream contains H₂S in the range of 0.5- 8 mol . % and S0₂ in the range of 0.2-5 mol % suitable for treatment in a Claus reactor. The gas is heated up to the required inlet temperature of the Claus reactor as described above and further processed according to the invention.

If so desired sulfuric acid can be recycled to the S0₂ generation step and decomposed.

Although the above explanations offer a general introduction and more detailed descriptions about the use of the invented process, modifications and improvements to the basis of the invented process which are obvious to experts in the field. Therefore, any such modifications and/or improvements that do not deviate from the spirit of this invention shall all be considered as part of the invention and within the scope for protection.

Claims

1. Method for treating a sulfur-containing gas stream, comprising the steps of

(1) providing a gas stream comprising H₂S and S0₂ to a Claus reactor where it is contacted under reaction conditions with a catalyst, to form a product comprising elemental sulfur, water, and residual H₂S and S0₂,

(2) contacting a tail gas from a Claus reactor which comprises H₂S and S0₂ with oxygen in an oxidation step under oxidising conditions to obtain a gas stream comprising SO_?., (3) contacting the S0₂-containing gas stream in an S0₂ conversion step with a catalyst to form a gas stream comprising so₃,

(4) contacting the gas stream comprising SCg with water under hydration conditions to form sulfuric acid.

2. Process according to claim 1, wherein the gas stream comprising H₂S and S0₂ used in step ( 1 ) is obtained by providing a gas steam comprising H₂S to a combustion step, 'where a gas stream containing H₂S and S0₂ is generated.

3. Process according to claim. 1, wherein the gas stream comprising H₂S and S0₂ used in step ( 1 ) is obtained by adding 50>2 to a H₂S containing gas stream, wherein the S0₂ may optionally be formed by combustion of elemental sulfur.

4. Process according to claim 1, wherein the gas stream comprising H₂S and SC used in step (1) is obtained by providing a gas steam comprising H₂S to a combustion chamber which also contains elemental sulfur, at least part of which is combusted to form S0₂.

5. Process according to any one of the preceding claims , wherein the sulfuric acid, and any sulfinic acid present therein, is at least partially recovered.

6. Process according to claim 2, wherein the sulfuric acid, and any sulfinic acid present therein, is at least partially recycled to the combustion step.

7. Process according to claim 3 or 4, wherein the sulfuric acid, and any sulfinic acid present therein, is at least partially recycled to the S0₂ formation step.

8. Process according to any one of the preceding claims, wherein the tail gas of hydration step 4 is released into the atmosphere .

9. Process according to any one of the preceding claims, wherein oxygen-containing hot gas withdrawn from the hydration _unit_ in step ( 4 ) is provided in whole or in part to one or more of the combustion step, if present, the oxidation step, the S02 conversion unit, or a unit where elemental sulfur is combusted to form S02, if present.

10. Process according to any one of the preceding claims, wherein cooling air from the S0₂ conversion step is returned in whole or in part to one or more of the combustion step, if present , the oxidation step, or another section as oxygen supply .

11. Process according to any one of the preceding claims, wherein the tail gas of a first Claus reactor in step (1) is provided to a second Claus reactor, the tail gas of which is optionally provided to a further Claus reactor, while the tail gas of the final Claus reactor is provided to the oxidation step (2) .

12. Process according to claim 11 , wherein in total at least 2 Claus reactors are applied, in particular 2 to 4 Claus reactors .