The present invention is related to a process for conversion of H2S to elemental sulfur and sulfuric acid, optionally with an adjustable ratio between sulfur and sulfuric acid. Sulfide abatement may take place via several processes, including the Claus process and the sulfuric acid process.

In the Claus process H2S is partly converted to SO2 which in a number of reaction and condensation steps reacts with the remaining H2S to form elemental sulfur. Elemental sulfur can be solidified and stored over long time. A Claus sulfur plant and catalyst is of moderate investment, but a rather expensive tail gas cleaning is required to meet today's environmental requirements.

A common alternative to the Claus process is the conversion of H2S to sulfuric acid, e.g. by the so-called Wet Sulfuric Acid (WSA) process. The sulfuric acid produced may be used in other chemical processes in the plant. A WSA process may also constitute the tail gas cleaning of a Claus process plant. A similar dry sulfuric acid process may also find use in this relation.

The conversion of H2S to elemental sulfur may be preferred, as storage and transport of elemental sulfur is simpler, but the requirements for sulfuric acid and elemental sulfur may vary over time, such that it is desirable to be able to adjust the relative produc- tion of sulfuric acid and elemental sulfur, for either direct use in the plant or for storage.

A process for conversion of H2S to elemental sulfur and sulfuric acid with increased efficiency has now been developed, in which a Claus process is combined with a sulfuric acid process. According to the process an amount of H2S is transferred to a combustor, in which sulfur in all forms is converted to form SO2. One fraction of the produced SO2 is combined with additional H2S and directed to a Claus process for production of elemental sulfur and the remaining fraction of the produced SO2 is, by contact with a cata- lytically active material, converted to SO3, which is withdrawn as H2SO4 either after hydration and condensation or after absorption in sulfuric acid. By this method, an amount of sulfuric acid required in the process may be provided, while excess sulfur may be transferred to other processes or sold, in the most effective way, while only requiring a single combustor. By controlling the amount of H2S directed to the combustor and/or directing either elemental sulfur or sulfuric acid to the combustor the ratio between the two products may in principle be varied from pure production of elemental sulfur to pure production of sulfuric acid, but the present disclosure will have its main use where it is desired to produce elemental sulfur and sulfuric acid in combination as effectively as possible.

In a broad aspect the present invention relates to a process for production of sulfur and sulfuric acid from a feedstock gas comprising 20-100% hbS and a recycled process gas comprising at least 5% or 10% SO2 involving the steps of

a. providing a Claus converter feed gas,

b. directing said Claus converter feed gas to contact a material catalytically active in the Claus reaction,

c. withdrawing a Claus tail gas and elementary sulfur, optionally by cooling the effluent from said material catalytically active in the Claus reaction,

d. directing a stream comprising said Claus tail gas and oxygen as a second stage feedstock gas to a means for oxidation providing an SO2 rich process gas, e. splitting said SO2 rich process gas in said recycled process gas comprising SO2 and an SO2 converter feed gas,

f. directing said SO2 converter feed gas to contact a material catalytically active in

502 oxidation to SO3, providing an SO3 rich gas,

g. converting said SO3 rich gas to concentrated sulfuric acid, either by absorption of

503 in sulfuric acid or by hydration of SO3, cooling and condensation of sulfuric acid,

with the associated benefit of an integrated process for variable conversion of H2S to elemental sulfur and concentrated sulfuric acid, requiring less equipment than two separate processes.

In a further embodiment said Claus converter feed gas is provided by combining said feedstock gas comprising and said recycled process gas and optionally directing either the combined gas or said recycled process gas to contact a material catalytically active in oxidation of H2S to SO2, with the associated benefit of the ability to design the Claus converter feed gas according to process needs.

In a further embodiment said Claus converter feed gas has a H2S:SC>2 ratio above 2:1 or 2.1 :1 , with the associated benefit of such a feed gas providing a favorable amount of elemental sulfur as well as an amount of excess SO2 for producing sulfuric acid. In a further embodiment said Claus converter feed gas has a H2S:SC>2 ratio below 3:1 , 2.5:1 or 2.2:1 , with the associated benefit of such a feed gas providing a favorable amount of elemental sulfur as well as an amount of excess hbS for releasing heat in the means for oxidation, which may reduce the requirement for adding a support fuel. In a further embodiment the process comprises a process step after step a and before step b, involving oxidation of the Claus converter feed gas at high temperature, and optionally condensation of elemental sulfur, with the associated benefit of decomposing ammonia and hydrocarbons, in combination with homogeneous Claus reaction.

In a further embodiment steps b and c are carried out sequentially 2-5 times, with the associated benefit of enabling a higher conversion in the process.

In a further embodiment the means for oxidation is a material catalytically active in oxidation of H2S to SO2, with the associated benefit of a low temperature process, especially suited for oxidation of Claus tail gases comprising less than 10% H2S.

In a further embodiment the means for oxidation is a combustor, and step (d) optionally includes directing a fuel to said combustor, with the associated benefit of a process not having a cost associated with catalytically active material, especially suited for oxidation of gases comprising at least 30% H2S, and for being robust for feeds with varying amounts of H2S, and thus varying degrees of heat release, and furthermore having the benefit related to the addition of a fuel of enabling incineration of waste sulfuric acid or weak H2S streams.

In a further embodiment elemental sulfur and/or sulfuric acid is directed to said combustor, with the associated benefit of freely defining the process output as any combination of elemental sulfur and sulfuric acid.

In a further embodiment the material catalytically active in the Claus reaction comprises activated aluminum(lll) or titanium(IV) oxide, with the associated benefit of such a material providing an efficient process for production of elemental sulfur.

In a further embodiment step (b) is carried out under a pressure of 50mbar g to 200 mbar g, a temperature of 200°C to 350°C and a space velocity of 800 Nm ³ /h/m ³ to 3000 Nm ³ /h/m ³ , with the associated benefit of such conditions being efficient for the production of elemental sulfur.

In a further embodiment step (b) is carried out at a temperature of 100°C to 150°C and step (c) involves the step of periodically heating said material catalytically active in the Claus reaction to allow withdrawal of condensed elementary sulfur in a liquid or gas phase, with the associated benefit of the low temperature being beneficial for the exothermic reaction such conditions being efficient for the production of elemental sulfur.

In a further embodiment said material catalytically active in conversion of SO2 to SO3 comprises vanadium, with the associated benefit of such a material providing an efficient process for production of sulfuric acid.

In a further embodiment step (f) is carried out under a pressure of 50mbar g to 200 mbar g, a temperature of 380°C to 520°C and a space velocity of 800 Nm ³ /h/m ³ to 1500 Nm ³ /h/m ³ , per catalyst bed, with the associated benefit of such conditions being efficient for the oxidation of SO2 to form SO3.

In a further embodiment at least one of said catalytically active materials and/or at least one product withdrawn from one of said catalytically active materials are cooled by heat exchange, such as interbed heat exchange or an internally cooled catalytic reactor, with the associated benefit of enabling active control of the temperature of the highly exothermic processes by interbed heat exchange or an internally cooled catalytic reactor such as a boiling water reactor, having a tubular or a thermoplate cooling circuit.

In a further embodiment an amount of gas in the process is cooled and directed to an upstream position for controlling the process temperature, with the associated benefit of enabling active control of the temperature of the highly exothermic processes.

A further aspect of the present disclosure relates to a process plant comprising a Claus conversion section, a means for sulfur oxidation and a sulfuric acid section, wherein the Claus conversion section has a gas inlet, a gas outlet and an elemental sulfur outlet, the means for sulfur oxidation has an inlet and an outlet and the sulfuric acid section has a gas inlet, a gas outlet and a sulfuric acid outlet, and wherein the gas inlet of the Claus conversion section is configured for receiving a feedstock gas, the means for sulfur oxidation inlet is configured for being in fluid connection with the outlet of said Claus conversion section gas outlet, the means for sulfur oxidation outlet is configured for being in fluid connection with the inlet of the sulfuric acid section, characterized further in the outlet of the means for sulfur oxidation being in fluid connection with the gas inlet of said Claus conversion section, with the associated benefit of such a process plant being reduced need of equipment, compared to a plant without the same level of integration. H2S is common side product in many processes, including hydrodesulfurization of refinery streams and production of viscose. It is desirable to convert H2S prior to emission to the atmosphere as H2S is highly odorous and an environmental challenge.

The conversion of H2S to elemental sulfur by the Claus process has been known for a long time. The Claus process proceeds by sub-stoichiometric combustion of H2S producing SO2 in a Claus combustor: For maximum conversion to elemental sulfur 1/3 of the H2S must be converted to SO2.

H ₂ S + 1 .5 0 ₂ - > S0 ₂ + H ₂ 0

The partially oxidized Claus feed gas is the converted to elemental sulfur by the follow- ing reactions at a temperature above 200°C in the presence of a catalytically active material, such as activated aluminum(lll) or titanium(IV) oxide.

Often 3-4 Claus converters are operated in series, to increase the conversion to a maximum, which will increase the cost of a Claus plant.

The control of temperature in the Claus process is important to ensure that elemental sulfur formed in the initial reaction remains gaseous, such that it is condensed in the desired process position. As the Claus reaction is exothermic, a further restriction is related to the fact that as the Claus process is exothermic it is beneficial to operate at low temperatures, and furthermore the typical Claus catalyst may become unstable above 350°C, which may be obtained by presence of non-reacting gases, e.g. by one of the reactants being present in sub-stoichiometric amounts.

An alternative to the above process is the so-called sub-dewpoint Claus process, in which the material catalytically active operates at temperatures where elemental sulfur is not on the gas phase. Such a sub-dewpoint Claus process will require an appropriate scheme for withdrawal of condensed sulfur, e.g. by pulsing of the temperature and purging of elementary sulfur by an inert gas.

The produced elemental sulfur, does typically not have a direct use in the plants producing sulfide waste, but elemental sulfur is simple to transport to other sites.

Other well-established processes for abatement of sulfide are sulfuric acid processes. The sulfuric acid processes oxidize SO2 into SO3 and subsequently hydrate SO3 into sulfuric acid, either by reaction with water in the gas phase in the so-called wet sulfuric acid process (WSA ^® process) or by absorption in weak sulfuric acid in the so-called contact process or dry process. The reaction temperature during oxidation will be in the range 400-500°C, in the presence of a catalytically active material, typically comprising vanadium. Typically the wet sulfuric acid processes produce sulfuric acid having a con- centration in the range 92%-98%, whereas dry sulfuric acid processes may also produce sulfuric acid having a concentration in excess of 98%.

Sulfuric acid processes are attractive as concentrated sulfuric acid is used in many chemical processes, including production of viscose and alkylation processes in refineries, from both of which sulfide may need to be abated.

In addition it may also be attractive to collect high pressure steam in the range from 20 barg to 100 barg from the exothermic sulfuric acid processes, whereas the Claus process will only provide low pressure steam.

Production of excess sulfuric acid may, however, be less attractive, even though sulfuric is traded commercially, as transport of sulfuric is complex and regulated.

A process which effectively can produce the amount of sulfuric acid required by a process plant and convert excess sulfur to elemental sulfur which may be transported to other sites is therefore attractive.

According to the present disclosure such a process may be designed with a simple Claus plant design, e.g. only a single Claus converter. Furthermore, the integration of two processes may remove the need for a Claus combustor as long as a sulfide combustor or combustor is available in the WSA process. The Claus feed gas may then be provided by combination of h S with SO2, in an appropriate ratio.

A H2S:SC>2 ratio of 2:1 will result in pure elemental sulfur production, and a H2S:SC>2 ratio of 0:1 will result in pure sulfuric acid product, and ratios between these two ex- tremes will provide optimal balance between such production. If the H2S:SC>2 ratio is above 2, excess H2S will be released from the Claus process, but this will be directed to be oxidized in the sulfuric acid plant, and thus not a problem, except for an excess process volume in the Claus process.

Figures:

Figure 1 shows a process according to the present disclosure

Figure 2 shows a process according to the prior art In Figure 1 a process according to the present disclosure is shown. A gas rich in H2S 2 is combined with a gas rich in SO2 36 and as a Claus feed gas 4 is directed to a reactor 8, which, especially if the gas rich in SO2 36 contains O2, may contain an optional material catalytically active in H2S oxidation for converting O2 and H2S into SO2 and H2O, forming an O2 free Claus feed gas. The O2 free Claus feed gas is directed to contact a material catalytically active in the Claus process 10 in the same or a further reactor providing a Claus process product 14. The Claus process product 14 is directed to a sulfur condensation unit 16, providing condensed sulfur 18 and a wet Claus tail gas 20. The wet Claus tail gas 18 may optionally be further reacted in the presence of addi- tional material catalytically active in the Claus process followed by further condensation of sulfur, in one to four further Claus stages (not shown here), to provide a final wet Claus tail gas. An aqueous phase 24 may optionally be separated from the wet Claus tail gas 20 in a separator 22, providing a dry Claus tail gas 26. An amount of the dry Claus tail gas comprising H2S 28 is, optionally together with an amount of sulfuric acid 60, directed to a combustor 32, providing a process gas rich in SO2 34, which is split in a recycled process gas comprising SO2 36 and an SO2 converter feed gas 38. An amount of either the wet Claus tail gas 20 or of the dry Claus tail gas comprising H2S 28 may be directed as a recycled dry Claus tail gas 30, to enhance the temperature control by dilution of the exothermic reaction mixture. The SO2 converter feed gas 38 is directed to an SO2 converter 40, containing one or more beds of catalytically active material 42, 44, 46 optionally with interbed cooling, from which an SO3 rich gas 48 is withdrawn. As the SO3 rich gas contains water, the SO3 may hydrate to form H2SO4. H2SO4 is condensed as concentrated sulfuric acid 52 in a sulfuric acid condenser 50. If the amount of water is insufficient for full hydration of SO3, addition of steam in a position upstream may be preferred. From the sulfuric acid condenser 50 a substantially pure gas 62 may be withdrawn and directed to stack 64. If excess sulfuric acid is produced, an amount 56 may be directed to the combustor 32 for excess SO2 36 being directed to production of elemental sulfur, whereas if the sulfuric acid is required in the process, all sulfuric acid may be withdrawn to other use in the plant.

In a further embodiment the conversion and condensation of sulfuric acid may be made in two stages, where remaining SO2 is oxidized, hydrated and condensed, with the associated benefit of providing increased sulfur removal. In a further embodiment the SO2 converter feed gas 38 may be dried, such that the SO3 rich gas 48 will contain little or no water. In that case the condenser 50 may be replaced with an absorber, in which SO3 may be absorbed in sulfuric acid, to provide concentrated sulfuric acid, by a dry sulfuric acid process.

In a further embodiment an amount of elemental sulfur may also be transferred to the combustor 32, which will have the effect of providing SO2 to the sulfuric acid process without introduction of water, which may be beneficial if it is desired to increase the SO3 concentration, which may be beneficial in a dry sulfuric acid process.

In a further embodiment, an amount of the gas rich in H2S may also be split in an amount directed to the reactor of the Claus process 8 and an amount directed to the combustor 32, for oxidation.

In Figure 2 a process for production of sulfur and sulfuric acid according to the prior art is shown. Here a gas rich in H2S 2 is directed to a Claus process, from which the tail gas 26 is directed to a sulfuric acid process. The gas rich in H2S 2 is directed to a com- bustor 66 converting an amount of the of H2S to SO2, to form a Claus feed gas 4 having a ratio between H2S and SO2 of 2:1 . The Claus feed gas 4 is directed to a reactor 8 containing a material catalytically active in the Claus process 12, providing a Claus process product 14. The Claus process product 14 is directed to a sulfur condensation unit 16, providing condensed sulfur 18 and a wet Claus tail gas 20. The wet Claus tail gas 20 is typically further reacted in the presence of additional material catalytically active in the Claus process followed by further condensation of sulfur, in one to four further Claus stages (not shown here), to provide a final wet Claus tail gas. An aqueous phase 24 may optionally be separated from the wet Claus tail gas 20 in a separator 22, providing a dry Claus tail gas 26 which is directed to a combustor 32, providing a SO2 converter feed gas 34. The SO2 converter feed gas 34 is directed to an SO2 converter 40, containing one or more beds of catalytically active material 42, 44, 46 optionally with interbed cooling, from which an SO3 rich gas 48 is withdrawn. As the SO3 rich gas contains water, the SO3 may hydrate to form H2SO4. H2SO4 is condensed as concentrated sulfuric acid 52 in a sulfuric acid condenser 50. From the sulfuric acid condenser 50 a substantially pure gas 62 may be withdrawn and directed to stack 64. Examples:

Three examples have been investigated by process modelling of a typical Claus feed, which includes hydrocarbons, of no relevance to the present invention. Sulfur is reported as a sum of species, presented under the assumption of Ss, in the range from S2 - Ss depending on the actual temperature. Hydrocarbons and CO2 will collectively be reported as C _x . Similarly N2 and Ar grouped and reported as "Inerts". Other constituents present in less than 0.1 % will not be reported. The reported concentrations are reported as molar %.

Example 1 relates to a process according to the present disclosure as illustrated in Fig- ure 1 , in which it is desired to convert 70% of the H2S to elemental sulfur and the remaining 30% to sulfuric acid. This example will require only a single H2S combustor, and the volume of gas treated in the Claus section will be 67% of volume of gas treated in the the sulfuric acid section.

Example 2 relates to a process according to the present disclosure as illustrated in Fig- ure 1 , in which it is desired to convert 100% of the H2S to elemental sulfur by recycle of all sulfuric acid produced. This example will also require only a single H2S combustor, and the volume of gas treated in the Claus section will be 67% of volume of gas treated in the the sulfuric acid section.

Example 3 relates to a process according to the prior art as illustrated in Figure 2, in which it is desired to convert 70% of the H2S to elemental sulfur and the remaining 30% to sulfuric acid. Such process may be configured with a single Claus stage, but will require a Claus combustor as well as a WSA combustor.

It is clear from the above examples, that integration of the Claus process and the WSA process, significant equipment cost savings are possible. The integration may avoid the requirement of a combustor, and in addition the number of Claus stages may also be reduced. Table 1 :

Stream 2 4 14 18 28 30 36 38 48 54 62

Flow 2350 8487 8487 1453 5539 682 5456 12585 12585 1970 10616 [kg/h]

H ₂ S 91.6 24.6 12.9 0.0 15.9 15.9 0.0 0.0 0.0 0.0 0.0 o ₂ 0.0 2.0 0.0 0.0 0.0 0.0 3.0 7.7 5.6 0.0 6.1

S0 ₂ 0.0 4.1 0.1 0.0 0.1 0.1 6.2 4.5 0.0 0.0 0.0

S0 ₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 2.7 0.0 0.0

H ₂ 0 3.7 9.4 22.4 0.0 7.1 7.1 11.9 9.6 8.0 12.3 5.0

Inerts 0.0 57.8 60.0 0.0 74.2 74.2 77.9 77.5 80.8 0.0 88.0

Cx 2.8 1.5 1.6 0.0 2.0 2.0 1.0 0.7 0.8 0.0 0.8

H ₂ 1.9 0.5 0.6 0.0 0.7 0.7 0.0 0.0 0.0 0.0 0.0

H2SO4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 87.8 0.0

Sx 0.0 0.0 2.4 100.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Table 2:

Stream 2 4 14 18 28 30 36 38 48 54 62

Flow 2350 1516 1516 2079 8640 3062 9757 11904 11904 0 10545 [kg/h] 8 8

H ₂ S 91.6 13.8 3.9 0.0 4.8 4.8 0.0 0.0 0.0 0.0 0.0

0 ₂ 0.0 2.0 0.0 0.0 0.0 0.0 3.0 6.6 6.6 0.0 7.0

S0 ₂ 0.0 2.8 0.2 0.0 0.2 0.2 4.2 0.3 0.0 0.0 0.0

S0 ₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.5 3.1 1.8 0.0 0.0

H ₂ 0 3.7 10.5 21.4 0.0 7.1 7.1 13.5 10.7 9.4 0.0 7.4

Inerts 0.0 65.9 67.8 0.0 81.9 81.9 74.5 76.1 77.4 0.0 82.1

Cx 2.8 4.4 4.5 0.0 5.5 5.5 4.4 3.2 3.2 0.0 3.4

H ₂ 1.9 0.4 0.4 0.0 0.5 0.5 0.0 0.0 0.0 0.0 0.0

H2SO4 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 1.6 0.0 0.0

Sx 0.0 0.0 1.8 100.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Table 3:

Stream 2 4 14 18 28 30 36 38 48 54 62

Flow 2350 6178 6178 1461 3859 0 0 12489 12489 1946 10543 [kg/h]

H ₂ S 91.6 24.8 9.6 0.0 13.5 0.0 0.0 0.0 0.0 0.0 0.0 o ₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.7 5.7 0.0 6.2

S0 ₂ 0.0 8.4 0.7 0.0 0.9 0.0 0.0 4.5 0.0 0.0 0.0

S0 ₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 2.8 0.0 0.0

H ₂ 0 3.7 13.5 30.3 0.0 7.1 0.0 0.0 9.1 7.5 12.3 4.5

Inerts 0.0 52.1 54.4 0.0 76.5 0.0 0.0 77.9 81.3 0.0 88.5

Cx 2.8 1.3 1.3 0.0 1.9 0.0 0.0 0.7 0.8 0.0 0.8

H ₂ 1.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

H2SO4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.9 87.8 0.0

Sx 0.0 0.0 3.8 100.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0