GB2114106A

GB2114106A - Process for the production of elemental sulphur

Info

Publication number: GB2114106A
Application number: GB08202988A
Authority: GB
Inventors: Roelof Cornelisse
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1982-02-02
Filing date: 1982-02-02
Publication date: 1983-08-17
Also published as: GB2114106B

Abstract

Production of elemental sulphur from H2S-containing gases according to a Claus-type process having a thermal zone in which these gases are combusted with a free oxygen- containing gas, followed by one or more catalytic zones in which H2S and SO2 react together to form elemental sulphur, in which process at least a portion of the free-oxygen-containing gas and/or of the H2S-containing gases is pre-heated by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds- containing waste gases.

Description

SPECIFICATION Process for the production of elemental sulphur The invention relates to a process for the production of elemental sulphur from hydrogen sulphide-containing gases according to a Claus-type process having at least a thermal zone in which a hydrogen sulphide-containing gas is combusted with a first free oxygen-containing gas, followed by one or more catalytic zones in which hydrogen sulphide and sulphur dioxide react together to form elemental sulphur and water.

H2S-containing gases may originate from, for example, thermally or catalytically processing crude mineral oils or products derived from such oils, or from processing coke-oven gas. Examples of such catalytic processes are desulphurization processes carried out in a petroleum refinery. Natural gas may also contain H2S. H2S may be removed from H2S-containing gases by means of absorption in a regenerable absorbent. Regeneration of the absorbent yields a gas having a higher H2S content than the H2S-containing starting gas and usually also containing CO2.

H2S ma also be present in ammonia(NH3)-containing waste gases. The above thermal and catalytic processes and processing of coke-oven gas may also give rise to the formation of NH3containing waste gases. Such processes yield liquid or gaseous product streams containing ammonia, which must be removed therefrom. Ammonia may be removed from such streams by washing with water at, for example, elevated pressure and reduced temperature. Washing is mostly carried out with an abundant quantity of water, so that dilute, ammonia-containing solutions are formed. Steam stripping of ammonia-containing solutions yields water suitable for discharge into open surface water and an ammonia- and water vapour-containing waste gas. Such waste gases may also contain H2S.

In a Claus-type process the following reaction takes place with regard to sulphur formation: 6H2S+302=6S+6H20 (1) which can be considered as occurring in two steps: 2H2S+302=2S02+2H20 (2) 4H2S+2SO2=6S+4H20 (3) In Claus-type processes an H2S-containing gas is partially or completely combusted with a free oxygen-containing gas in a furnace, also referred to herein as "thermal zone", followed by one or more catalytic zones in which reaction (3) is carried out. The gases formed in the thermal zone are suitably cooled in a waste heat boiler prior to their introduction into the first catalytic zone. Thus, heat is recovered by producing steam; it is desirable to recover as much steam as possible.The amount of the steam generated in the waste heat boiler will, i.a., depend on the temperature of the gases formed in the thermal zone. This temperature, in turn, will be determined by the temperature and composition (H2S content and content of hydrocarbons and ammonia, if any) of the H2S-containing feed gases and of the temperature and composition of the oxygen-containing gas supplied to the thermal zone.

If the H2S content of the H2S-containing gas is low, for example less than 20% by volume, and the balance consists of non-combustible gases, for example carbon dioxide (CO2), then the requisite temperature for the combustion of the H2S cannot be attained in the thermal zone. In this case the thermal zone is difficult to operate, the combustion not being sufficiently stable and ammonia-which is often present in H2S-containing gases-is only partiy burned. Moreover, the amount of steam produced in the waste heat boiler is low. These difficulties particularly hold when the free oxygencontaining gas has a low temperature, for example below OOC and, particularly, below --200C, as may occur in areas with a very cold season.These difficulties may be avoided by applying special measures, such as combusting the H2S-containing gas with oxygen-enriched air or pure oxygen, but these special measures are expensive.

In one version of the Claus-type process, referred to as "straight-through operation", sufficient free oxygen-containing gas is used to convert the H2S to elemental sulphur in accordance with reaction (1). This implies that the quantity of free oxygen-containing gas is such that only one third of all of the H2S can be oxidized to SO2 in accordance with reaction (2), the oxygen contained in the free oxygen containing gas being consumed according to reaction (2). As long as the H2S-containing gas has a high H2S content (more than, for example, 40% by volume) and a low content of non-combustible gases such as CO2, they can be readily combusted with production of elemental sulphur in the thermal zone according to reaction (3). Reaction (3) is also referred to as "Claus reaction".Thus, the effluent gas from the thermal zone finally contains H2S, SO2, sulphur vapour, nitrogen, water vapour, CO2, CO, COS, CS2 and hydrogen. CO2 and CO are present, because they are formed by combustion of hydrocarbons which are present in most of the H2S-containing feed gases and part of the CO2 is present, because most of the H2S-containing feed gases also contain CO2.

The effluent gas from the thermal zone usually has a temperature in the range of from 6000C to 1 6500C and is suitably first cooled in a waste heat boiler and then in a condenser to a temperature which is sufficiently low to condense almost all of the sulphur vapour present. The latter temperature is suitably in the range of from 1 700C to 2000 C. Alternatively, the effluent gas from the thermal zone is cooled in one step to a temperature in the range of from 1 700C to 2000C with production of steam.

After cooling of the effluent gas from the thermal zone to a temperature in the range of from 1 700C to 2000C the gases from which almost all of the sulphur has been removed are re-heated to a temperature in the range of from 2300C to 2800C and introduced into a first catalytic zone (a catalytic zone is hereinafter also referred to as "Claus stage"), where H2S reacts with SO2 to produce more elemental sulphur in accordance with reaction (3). This production of elemental sulphur is increased by the prior condensation of elemental sulphur. It is common use to cool the effluent of the first Claus stage to a temperature in the range of from 1 800C to 1 70 C in order to condense elemental sulphur which is withdrawn.Then, the gas stream freed from elemental sulphur is heated to the reaction temperature required for the next Claus stage, usually to a temperature in the range of from 2000C to 2800C. The effluent of the second Claus stage is cooled to a temperature in the range of from 1 600C to 1 700C in order to condense elemental sulphur which is withdrawn. The second Claus stage may be followed by a third or fourth Claus stage in order to increase the sulphur production. Sulphur production is further increased by a two-step indirect cooling of the effluent of the last Claus stage with water, in the first and second step producing steam of a pressure of, for example, 3.5 and 2 bar, respectively.

Catalysts suitable to cataiyze reaction (3) are well known. Preferably, activated alumina or bauxite is used, but any other catalyst promoting this reaction may be used as well.

In another version of the Claus-type process, referred to as "split-flow operation", the H2S containing gas is split up into a first and a second portion, representing about one third and about two thirds, respectively, of the total H2S-containing gas. The first portion is combusted with a free oxygen containing gas in an amount stoichiometric with respect to reaction (2), all or almost all of the H2S being oxidized to SO2. In this case a negligible amount of elemental sulphur, if any, is formed. Thus, the effluent gas from the thermal zone essentially contains SO2, nitrogen, water vapour, CO2, CO and hydrogen.

In split-flow operation, the effluent gas from the thermal zone is suitably cooled in a waste heat boiler and then combined with the said second portion of the H2S-containing gas. The gas mixture thus obtained is introduced at a temperature usually in the range of from 2300C to 2800C into the first Claus stage. The operation is then continued as described above for the straight-through operation.

Split-flow operation is particularly suitable when gases having a relatively low H25 content, for example less than 40% by volume, have to be combusted.

If acceptable, the gases which have left the cooler after the final Claus stage in straight-through or split-flow operation may be incinerated in the presence of fuel gas and the combustion gases thus obtained released into the atmosphere. This procedure, however, has as a disadvantage that the heat present in these combustion gases is lost.

It is an object of the present invention to recover heat from the combustion gases mentioned above.

It is a second object of the present invention to provide a process which allows a high recovery of steam from the heat present in the gases formed in the thermal zone of a Claus-type process.

It is a third object of the present invention to provide a process which enables gases having a low H25 content to be combusted without being forced to apply oxygen-enriched air or pure oxygen.

A further object of the present invention is to provide a process which can be carried out with the aid of free oxygen-containing gases having a low temperature.

A fourth object of the present invention is to provide a process which allows a higher ammonia conversion to nitrogen and water in the thermal zone, thus providing an environmental clean process.

Accordingly, the invention provides a process for the production of elemental sulphur from hydrogen sulphide-containing gases according to a Claustype process having at least a thermal zone in which a hydrogen sulphide-containing gas is combusted with a first free oxygen-containing gas, followed by one or more catalytic zones in which hydrogen sulphide and sulphur dioxide react together to form elemental sulphur and water, in which process at least a portion of the first free oxygencontaining gas and/or of the hydrogen sulphide-containing gases is preheated by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds-containing waste gases with a second free oxygen-containing gas.

The gases obtained by incineration of combustible sulphur compounds-containing waste gases with a free oxygen-containing gas are below also referred to as "incinerator off-gases". The H2Scontaining gases may be introduced into the thermal zone as one stream or as two or more different streams. Any of these streams may contain NH3.

The pre-heating of at least a portion of the H2S-containing gases and/or of the first free oxygencontaining gas by means of indirect heat exchange with incinerator off-gases is suitably carried out in such a manner so as to maximize heat recovery from the incinerator off-gases for the production of high pressure steam in the waste heat boiler.

Any portion of the first free oxygen-containing gas and of the H2S-containing gases which is not preheated by means of indirect heat exchange with incinerator off-gases may, if desired, be preheated by any other suitable heat source. This heat source may be available inside or outside the Claus-type process. Preferably, all of the first free oxygen-containing gas and all of the H2S-containing gas are preheated by means of indirect heat exchange with incinerator off-gases. In the case of split-flow operation, the said pre-heating may take place before splitting up the H2S-containing gas into a first and second portion or each of the two portions may be separately pre-heated after splitting up.

The first free oxygen-containing gas and the H2S-containing gases may be pre-heated by means of indirect heat exchange with incinerator off-gases starting at a temperature at which they are available, the former gas at a temperature below, for example, OOC and the latter gas at a temperature in the range of from, for example, 100C to 500 C. The use of such temperatures may cause cold spots in the material of the heat exchanger being in contact with incinerator off-gases. As incinerator off-gases contain water vapour and sulphur trioxide (S03) these cold spots may give rise to corrosion of this material.This corrosion is avoided when, according to a preferred embodiment of the invention, a gaseous effluent is withdrawn from the catalytic zone or, if more than one catalytic zone is applied, from a catalytic zone which is final with respect to sulphur formation (the "final catalytic zone") and the withdrawn effluent is cooled by means of indirect heat exchange with water, to condense elemental sulphur and generate steam, which steam is used for indirectly warming up the first free oxygencontaining gas and/or the hydrogen sulphide-containing gases, prior to the pre-heating by means of indirect heating with incinerator off-gases.

Water to be used for indirect cooling of the gases formed in the thermal zone with simultaneous generation of steam may be pre-heated in any desired manner and suitably with the aid of incinerator off-gases. These incinerator offgases and those to be used for pre-heating at least a portion of the H2Scontaining gases and/or of the first free oxygen-containing gas may originate from the same or from different incinerators. According to a preferred embodiment of the invention incinerator off-gases are first used for pre-heating the water and then for pre-heating at least a portion of the first free oxygencontaining gas and/or of the hydrogen sulphide-containing gases.Alternatively, the incinerator offgases are first used for pre-heating at least a portion of the first free oxygen-containing gas and/or of the hydrogen sulphide-containing gases and then for pre-heating the water.

The first free oxygen-containing gas, to be supplied to the thermal zone, is preferaby air. Oxygenenriched air or pure oxygen may be used if the H2S-containing gas contains so little H2S that sustaining the combustion becomes necessary.

The second free oxygen-containing gas, required for the incineration of the combustible sulphur compounds-containing waste gases, may be heated before it is used for the incineration of the combustible sulphur compounds-containing waste gases. This heating may be carried out in any suitable manner, for example by means of indirect heat exchange with steam; this heating is very suitably carried out by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds-containing waste gases.According to a preferred embodiment of the present invention a stream of free oxygen-containing gas, heated by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds-containing waste gases, is split up into the first stream which is supplied to the thermal zone, and the second stream which provides free oxygen required for the incineration of the combustible sulphur compounds-containing waste gases.

If desired, a portion of the incinerator off-gases may be mixed with the second free oxygencontaining gas, required for the incineration; the mixture thus obtained can be incinerated.

The process according to the invention is particularly suitable for split-flow operation, because the application thereof in combination with high recovery of heat from incinerator off-gases allows a higher temperature in the thermal zone and therefore a more favourable combustion of gases having a low H2S content, a higher conversion of NH3, if present, to nitrogen and water, and a higher production of steam.In split-flow operation, all of the H2S introduced into the thermal zone is combusted to SO2 and a hydrogen sulphide-containing gas is supplied to the first catalytic zone; the H2S-containing gas to be supplied to the thermal zone and the H2S-containing gas to be supplied to the first catalytic zone may originate from different sources and have different compositions, but suitably originate from the same source and have the same composition.According to a preferred embodiment of the present invention, an H2S-containing gas is split up into a first and a second portion, all of the H2S present in the first portion being combusted to SO2, and the combustion gases thus formed being cooled, after which the cooled combustion gases are mixed with the second portion, the mixture thus obtained being introduced into the first catalytic zone. The said first and second portion are suitably about one third and two thirds, respectively, of the starting H2S-containing gas.

The steam generated in the waste heat boiler by cooling the gases formed in the thermal zone may be used for any suitable purpose. Preferably, H2S- and SO2-containing gases which have been cooled to condense elemental sulphur, are re-heated, prior to introduction into a catalytic zone, by means of indirect heat exchange with steam generated by indirect cooling of the gases formed in the thermal zone.

Incinerating combustible sulphur compounds-containing waste gases involves conversion of the combustible sulphur compounds to SO2. This incineration may be thermal. However, thermal incineration requires heating of the combustible sulphur compounds-containing waste gases to a temperature in the range of from about 5000C to about 6000C, which is costly because of the necessary heat input by, for example, combustion of fuel. Furthermore, concomitant presence of water and formation of relatively large amounts of sulphur trioxide (S03) is a problem for waste heat recovery with regard to the selection of materials and the reliability of the equipment used. Therefore, the incineration is preferably carried out in the presence of incineration catalyst; this allows incineration temperatures much lower than 5000C.In this case less fuel, if any, need to be burned, thus reducing the amount of S03 formed thereby and alleviating the said problem.

According to a preferred embodiment of the present invention the incineration is carried out at a temperature in the range of from 1 500C to 4500C and in the presence of an incineration catalyst comprising as the catalytically active components bismuth in an amount in the range of from 0.6 to 10% by weight and copper in an amount in the range of from 0.5 to 5% by weight, calculated on the total weight of the catalyst. This method provides substantial conversion of the H2S, COS, CS2 and elemental sulphur, and concomitantly provides low S03 formation. The method is economical in that the necessary heat'input is low, as evidenced by relatively low incineration temperatures. Moreover, this incineration catalyst shows an activity which remains constant over a long period.

While the temperatures at which the catalytic incineration is carried out are not critical, it is an advantage that lower or more moderate temperatures may be employed. Temperatures of 1 500C to 4500C are quite satisfactory, while temperatures of 2500C to 4200C are preferred.

The amount of free oxygen supplied to the incinerator is important, in that a stoichiometric excess, preferably a large excess, of free oxygen is desired in order to react all the H28 and any COS and C82 present. In generai, at least twice, and normally up to five times the stoichiometric amount of free oxygen required for the reaction may be supplied. Preferably, an excess of about 20 to about 280% of the stoichiometric amount of free oxygen, based upon all total combustibles, will be supplied.

Amounts as high as 100 or even 200 times the stoichiometric amount of free oxygen may be supplied, if desired.

Methods of preparation and the most suitable composition of this incineration catalyst, suitable sizes of catalyst particles, methods for imparting a surface area to the catalyst and the conditions at which the incineration may be carried out, such as the pressure and the gas hourly space velocity are found in British patent specification No. 1 ,558,656, which is incorporated herein by reference.

The free oxygen in the second free oxygen-containing gas may be supplied as air, oxygenenriched air or pure oxygen or from other gaseous streams containing significant quantities of free oxygen and other components which do not interfere significantly with the incineration.

Any suitable combustible sulphur compounds-containing waste gas may be used in the process according to the present invention; H2S-containing waste gases, for example Claus tail gases, are very suitable. By "Claus tail gases" are meant the remaining gases as obtained after condensation of elemental sulphur from the gases leaving the final Claus stage. As Claus tail gases are available in large quantities, the off-gases obtained by incineration of Claus tail gases are alao available in large quantities. From this point of view and as these off-gases have a relatively high temperature, these gases are very suitable for indirect heat exchange with at least a portion of the H2S-containing gases and/or the first free oxygen-containing gas according to the invention.

Claus tail gases still contain sulphur compounds and elemental sulphur. A typical Claus tail gas may have the following composition: H2S 0.1-2% vol. S 2 0.05-1% vol.

COS 0.01-0.2% vol. C82 0.010.2% vol.

Element 0.01 .2% vol. H2 0--5% vol.

CO 0--30 vol. CO2 250% vol.

H20 25 40% vol. N2 balance In order to reduce air pollution, various procedures have been developed to remove sulphur compounds and elemental sulphur from Claus tail gases, and even recover, if possible, H28 or reaction products of H28 contained therein. Such procedures considerably reduce the amounts of 802, S03 and water in the incinerator off-gases. In this light the combustible sulphur compounds-containing waste gases are preferably off-gases of a process for the removal of sulphur compounds and elemental sulphur from the tail gases obtained by cooling the gases leaving the final catalytic zone and separating element sulphur from the cooled tail gases.Such a process for the removal of sulphur compounds and elemental sulphur from Claus tail gases may comprise the steps of reducing the sulphur compounds and elemental sulphur under suitable conditions in the presence of a catalyst, absorbing the H28 thus formed, followed by desorption of the H28 and, if desired, introduction of the desorbed H28 into the Claus plant. Thus, the H2S-containing waste gases to be incinerated contain quite minor amounts of H2S.

According to a preferred embodiment of the present invention, the process for the removal of sulphur compounds and elemental sulphur from Claus tail gases comprises passing the tail gases at a temperature above 1 750C, together with a free hydrogen- and/or carbon monoxide-containing gas, over a sulphided Group VI and/or Group VIII metal catalyst supported on an inorganic oxidic carrier and passing the reduced tail gases thus obtained through a liquid and regenerable absorbent for hydrogen sulphide. Group VI and Group VIII refer to the Periodic Table of the Elements as shown on the inside cover of "Handbook of Chemistry and Physics", 59th edition (1978-1979).

After having passed the final catalytic zone and the relevant condenser for the recovery of elemental sulphur, the Claus tail gases normally have a temperature in the range of from 1 250C to 170"C. The said reduction of the Claus tail gases is preferably carried out at a temperature in the range of from 1 750C to 4500C and more preferably of from 2600C to 4000C.

Before being contacted with the sulphided Group VI and/or Group VIII metal catalyst, the temperature of the Claus tail gases has to be increased to a value above 1 700C. This may be effected in any desired manner, for example by means of in-iine burning, i.e. of thermal combustion of a fuel in the Claus tail gases. In-line burning, however, has the disadvantages of requiring burners, blowers for the supply of air, brick wall linings and fuel to be burned. Moreover, larger gas volumes have to be passed over the metal catalyst and soot might be formed, causing fouling of the metal catalyst.

Furthermore S03 may be formed when burning gases containing sulphur compounds and special precautions have to be taken.

The invention, however, allows obviating these disadvantages as a result of the high production of high pressure steam, which, in turn, results from the high heat input in the thermal zone. So, in accordance with a preferred embodiment of the present invention, the Claus tail gases are indirectly heated by means of high pressure steam, generated by cooling the gases formed in the thermal zone.

As the invention allows the production of a large amount of high pressure steam, the Claus tail gases can thus be heated to a relatively high temperature; sometimes a small further increase of temperature is desired before they are contacted with the Group VI and/or Group VIII metal catalyst. This small further increase may be obtained in any suitable manner, but, according to a further aspect of the invention, is preferably carried out by means of indirect heat exchange with the reduced Claus tail gases which have been passed over the sulphided Group VI and/or Group VIII metal catalyst. This indirect heat exchange causes only a small decrease of the temperature of the latter gases and may sometimes be omitted.

The said reduction in the presence of the Group VI and/or Group VIII metal catalyst is preferably carried out in the presence of at least the stoichiometric quantity of H2 and/or CO required for complete conversion of SO2 and elemental sulphur into H2S. Generally, 1.3-2.0 times the required stoichiometric quantity is used.

Methods of preparation and the most suitable composition of the sulphided catalyst and of the carrier and the conditions at which the reduction may be carried out, such as the pressure, the gas hourly space velocity and the composition of the free hydrogen and/or carbon monoxide-containing gas are found in British patent specification No. 1 ,356,289, which is incorporated herein by reference.

Before being contacted with the liquid and regenerable absorbent for H2S, the reduced Claus tail gases are first cooled, in accordance with common practice, to a temperature below the dew point of water. It is preferably cooled to a temperature in the range of from 6 to 600C. This cooling may be carried out in two stages, for example by indirect heat exchange with water, thus producing low pressure steam and then by direct heat exchange with water. In the latter step most of the water vapour present in the reduced Claus tail gases is condensed.

The off-gases withdrawn from the liquid and regenerable absorbent must be heated before incineration, in the case of catalytic incineration to a temperature of at least 1 500 C. This heating may be effected in any desired manner, for example by means of in-line burning. In-line burning, however, has the same disadvantages as mentioned above for in-line burning Claus tail gases, but to a lesser extent since less sulphur and less water vapour are present.

The invention, however, allows obviating these disadvantages. The reduced Claus tail gases, after indirect heat exchange with the Claus tail gases to be reduced, if this indirect heat exchange is carried out at all, still have a relatively high temperature; the heat thereof may be used for any suitable purpose. According to a feature of the present invention, the off-gases which have left the liquid and regenerabie absorbent are heated, prior to their incineration, by means of indirect heat exchange with the reduced tail gases passed over the sulphided Group VI and/or Group Vlil metal catalyst, preferably after the above-mentioned heat exchange of the latter gases with the Claus tail gases.

After cooling, the reduced Claus tail gases are passed through a liquid and regenerable absorbent for H2S. Preferably, this absorbent is an aqueous solution of an amine or a substituted amine.

Absorbents of this type are well known in the art, such as for example an alkali metal salt of dialkylsubstituted amino-acids, for example potassium dimethylaminoacetate, and alkanolamines. More preferably, a polyalkanolamine, such as diethanolamine, triethanolamine, diisopropanolamine, methyldiethanolamine or ethanoldiethylamine is used. Very good results are obtained when using diisopropanolamine or methyldiethanolamine as absorbent. The alkanolamines are preferably used in aqueous solutions in a molar concentration of 0.5 to 5 and preferably 1 to 4, relative to the said alkanolamines. After absorption, the H2S-enriched absorbent is regenerated by heating and/or stripping, which produces an H2S-enriched gas mixture and a regenerated absorbent which may be reused.Reduced Claus tail gases having a high CO2 content are preferably contacted with the H2S absorbent under such conditions that the H28 is selectively absorbed, thus minimizing the recycle of CO2 to the Claus plant. These conditions include a low temperature and high gas velocities, the said contacting taking place in an absorption column having at most 20 or, and preferably, less than 20 contacting trays. More preferably, the absorption column has 4 to 1 5 contacting trays. The gas velocity to be used is at least 1.5 m/sec, and more preferably 2 to 4 m/sec. These gas velocities are based on the "active" or aerated tray surface. A low absorbent temperature enhances the selectivity of the H2S/CO2 separation.The temperature is preferably lower than 400 C; most satisfactory results are obtained at temperatures in the range of from 50C to 300 C. The reduced Claus tail gases are contacted with the aqueous alkanolamine solution at atmospheric or substantially atmospheric pressure.

Contacting is preferably effected counter currently.

The unabsorbed part of the reduced Claus tail gases mainly consists of nitrogen and CO2, has a very small content of hydrogen and contains traces of H2S, COS and CS2, for example 100--1000 parts per million (vol.). Hence, it is acceptable to release the off-gases, obtained by incineration of this unabsorbed part, through a stack.

The H2S-enriched gas mixture liberated in the regeneration is preferably recycled to the Claustype process.

The invention will now be illustrated with the aid of the following Examples and the Figure.

The Figure shows a schematic representation of a Claus plant using split-flow operation, a process for the removal of sulphur compounds and elemental sulphur, and an incinerator for the combustion of the off-gases of the latter process.

Example 1 Air (2055 kg/h, temperature -300C) is sucked in via a line 1, a pre-heater 2 and a line 3 by a blower 4. In the pre-heater 2 the air is warmed up by means of steam generated in a second sulphur condenser 5, being the final sulphur condenser. The air discharged by the blower 4 is conducted via a line 6 (temperature 1 1 OOC) to a pre-heater 7 in which it is pre-heated to a temperature 3000 C.

Alternatively, the pre-heater 2 may be situated between the blower 4 and the pre-heater 7. The air leaves the pre-heater 7 via a line 8 and is divided into a stream (1795 kg/h, the first oxygen-containing as mentioned hereinbefore), conducted via a line 9 to a thermal zone 10 and a stream (260 kg/h, the second oxygen-containing gas mentioned hereinbefore) conducted via a line 11 to an incinerator 12.

An H2S-containing gas (4305.8 kg/h, temperature 240C, CO2 content 70.6% v, H28 content 22.9% v, H20 content 4.6% v, hydrocarbon content 1.9% v) is supplied via a line 13, a pre-heater 14, a line 15, a pre-heater 1 6 and a line 1 7. In the pre-heater 14 the H2S-containing gas is pre-heated to a temperature of 11 00C by means of steam generated in the final sulphur condenser 5 and in the preheater 1 6 by means of incinerator off-gases to a temperature of 2600C.The incinerator off-gases supplied via the line 1 8 to and those withdrawn from the pre-heater 1 6 via a line 1 9 have a temperature of 3440C and 1400 C, respectively. The H2S-containing gas conducted via the line 17 is split up into a first stream (one third or 1435.3 kg/h) conducted via a line 20 to the thermal zone 10 and a second stream (two thirds or 2870.5 kg/h) by-passing the thermal zone 10 and conducted via a line 21. Furthermore, an ammonia-containing gas (19.2 kg/h, temperature 900C, H2S content 22.2% v NH3 content 22.2% v, H20 content 55.6% v) issupplied to the thermal zone 10 via a line 22. In this example, the NH3-containing gas is not preheated.However, it may be preheated by means of indirect heat exchange with incinerator off-gases. All of the H28 thus introduced into the thermal zone 10 is combusted to SO2 and the NH3 is converted to N2 and H20.

The effluent from the thermal zone 10 is conducted via a waste heat boiler 23 and a line 24 to a first Claus stage 26. The H2S-containing gas supplied via the line 21 is also conducted via the line 24 to the first Claus stage 26 where an additional amount of sulphur is formed. Water (temperature 148"C) is supplied via a line 27, a heat exchanger 28 and a line 29 to the waste heat boiler 23. In the heat exchanger 28 the water is heated to a temperature of 1 660C by incinerator off-gases supplied via a line 30 (temperature 3800C) and discharged via the line 18 (temperature 3440C). A part of the incinerator off-gases (temperature 3440 C) from the line 18 is conducted to the pre-heater 7 via a line 31, withdrawn from this pre-heater via a line 32 (temperature 1400 C) and discharged into the atmosphere.The steam generated in the waste heat boiler 23 (temperature 2600C) is discharged via a line 33 and part thereof (2385 kg/h) via a line 34 to a destination outside the process.

The gases withdrawn from the first Claus stage 26 are conducted via a line 35 to a first sulphur condenser 36 in which they are cooled by means of indirect heat exchange with water to a temperature of 1 700C. Sulphur is withdrawn from the first sulphur condenser 36 via a line 44. The gases leaving the first sulphur condenser 36 are conducted via a line 37 to a re-heater 38 in which they are re-heated to a temperature of 21 00C. The re-heated gases leaving the re-heater 38 are conducted via a line 39 to a second Claus stage 40, where an additional amount of sulphur is formed. In the Claus stages 26 and 40 the gases are contacted with aluminium oxide. The gases leaving the second Claus stage 40 are conducted via a line 41 to the second sulphur condenser 5 in which they are cooled by means of indirect heat exchange with water to a temperature of 1270C. Sulphur is withdrawn from the second sulphur condenser 5 via a line 48. The gases from the second sulphur condenser 5 are conducted via a line 42 to a re-heater 43 in which they are re-heated to a temperature of 2500C.

Steam (temperature 1 480C) is produced in the first sulphur condenser 36. If desired, this steam may be used for pre-heating the air and/or the H2S-containing gas to be conducted to the thermal zone 1 0.

Re-heating in the re-heater 38 takes place by means of steam supplied via the line 33, a line 45 and a line 46 and re-heating in the re-heater 43 by means of steam supplied via the line 45. Steam (temperature 11 80C) is produced in the second sulphur condenser 5; a portion of this steam is used for pre-heating air in the pre-heater 2, another portion for pre-heating the H2S-containing gas in the preheater 1 4 and the balance is conducted to a destination outside the process (the three lines required are not shown).

The Claus tail gases leaving the re-heater 43 are conducted via a line 49, a heat exchanger 50 in which they are heated to a temperature of 2800C and a line 51 to a reactor 52, to which hydrogen is supplied via a line 53 and the line 51. The reactor 52 is charged with a sulphided Co/Mo/Al203 catalyst (3.2 parts by weight of Co, 1 3.4 parts by weight of Mo, 1 00 parts by weight of Al2O3). In the present case upflow is applied in the reactor 52; alternatively, downflow may be applied.

The reduced Claus tail gases (temperature 310by) leaving the reactor 52 are conducted via a line 54, the heat exchanger 50 in which they are cooled to a temperature of 2800 C, a line 55, a heat exchanger 56 in which they are cooled to a temperature of 1 200C and a line 57 to a quench column 58 in which they are quenched by direct contact with water supplied via a line 59; spent water is discharged via a line 60. The quenched gases (temperature 400 C) are withdrawn from the quench column 58 via a line 61 and introduced into an absorption column 62. An aqueous solution of diisopropanolamine is introduced at the top of the absorption column 62 via a line 63.The off-gases from the absorption column 62 (temperature 400 C) are conducted via a line 64, the heat exchanger 56 in which they are heated to a temperature of 2260C and a line 65 to the incinerator 12. Fuel gas (15 kg/h) is supplied to the incenerator 1 2 via a line 66. This fuel gas is preheated to a temperature of 370C by means of indirect heat exchange with air from the line 11 (this pre-heating is not shown). A very small amount of fuel gas has been burned separately to increase the temperature of the off-gases in the line 65 to a temperature of 3800C. The incinerator 1 2 is charged with an incineration catalyst consisting of 1% Cu and 3% Bi on Al2O3, calculated as % weight on total catalyst.

The enriched absorbent leaves the absorption column 62 via a line 67, is heated in a heat exchanger 68 and conducted via a line 69 to a desorption column 70. If desired, part of this enriched absorbent may be used as fresh solvent for contacting an H2S-rich stream and the resulting highly loaded absorbent be regenerated in the desorption column 70, as described in British patent specification 1,461,070. An aqueous solution of diisopropanolamine enriched with H2S and CO2 is supplied from a source outside the process via a line 71 and the line 67 and is simultaneously regenerated in the desorption column 70. The absorbent in the column 70 is regenerated at elevated temperature by heating with steam (not shown).The regenerated absorbent is withdrawn from the desorption column 70 via a line 72 and conducted via the heat exchanger 68, a line 73, a cooler 74 and the line 63 to the absorption column 62. Part of the regenerated absorbent may be conducted, for example, from the line 63, to a destination outside the process and, after being loaded with H28 and CO2, returned via the line 71.

The H2S-containing gas is discharged from the desorption column 70 via the line 13 and is cooled in a cooler (not shown) in order to condense water vapour; the condensed water is returned to the desorption column 70.

Example 2 An experiment is carried out which differs from the described in Example 1 in these respects that in the line-up of the process flow scheme as shown in the Figure the pre-heaters 2, 7, 14 and 1 6 and the heat exchanger 28 are not present. Instead, a steam heater is used for indirect heating of the gases in line 25 to the same temperature as in Example 1. In this case 1 51 5 kg/h of steam is conducted via the line 34 to a destination outside the process; this amount is only 63% of the corresponding amount produced in Example 1.

Claims

1. A process for the production of elemental sulphur from hydrogen sulphide-containing gases according to a Claus-type process having at least a thermal zone in which a hydrogen sulphidecontaining gas is combusted with a first free oxygen-containing gas, followed by one or more catalytic zones in which hydrogen sulphide and sulphur dioxide react together to form elemental sulphur and water, in which process at least a portion of the first free oxygen-containing gas and/or of the hydrogen sulphide-containing gases is pre-heated by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds-containing waste gases with a second free oxygencontaining gas.

2. A process as claimed in claim 1, in which all of the first free oxygen-containing gas and all of the hydrogen sulphide-containing gases are pre-heated by means of indirect heat exchange with the off-gases.

3. A process as claimed in claim 1, in which a gaseous effluent is withdrawn from the catalytic zone or, if more than one catalytic zone is applied, from a catalytic zone which is final with respect to sulphur formation and the withdrawn effluent is cooled by means of indirect heat exchange with water, to condense elemental sulphur and generate steam, which steam is used for indirectly warming up the first free oxygen-containing gas and/or the hydrogen sulphide-containing gas, prior to the pre-heating by means of indirect heating with the said off-gases.

4. A process as claimed in claim 1, in which water is preheated by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds-containing waste gases with the second free oxygen-containing gas and the water thus pre-heated is converted into steam by using it for indirect cooling of the gases formed in the thermal zone.

5. A process as claimed in claim 4, in which the said off-gases are first used for pre-heating the water and then for pre-heating at least a portion of the free oxygen-containing gas and/or of the hydrogen sulphide-containing gases.

6. A process as claimed in any one of claims 1, 2 or 3, in which a stream of free oxygencontaining gas, heated by means of indirect heat exchange with off-gases obtained by incineration of combustible sulphur compounds-containing waste gases, is split up into the first stream which is supplied to the thermal zone, and the second stream which provides free oxygen required for the incineration of the combustible sulphur compounds-containing waste gases.

7. A process as claimed in claim 1, in which all of the hydrogen sulphide introduced into the thermal zone is combusted to sulphur dioxide and a hydrogen sulphide-containing gas is supplied to the catalytic zone or, if more than one catalytic zone is applied, to a catalytic zone which is first with respect to sulphur formation.

8. A process as claimed in claim 7, in which a hydrogen sulphide-containing gas is split up into a first and a second portion, all of the hydrogen sulphide present in the first portion being combusted to sulphur dioxide and the combustion gases thus formed being cooled, after which the cooled combustion gases are mixed with the second portion, the mixture thus obtained being introduced into the first catalytic zone.

9. A process as claimed in any one of claims 1,2 or 3, in which hydrogen sulphide- and sulphur dioxide-containing gases which have been cooled to condense elemental sulphur, are re-heated, prior to introduction into a catalytic zone, by means of indirect heat exchange with steam generated by indirect cooling of the gases formed in the thermal zone.

10. A process as claimed in claim 1, in which the incineration is carried out in the presence of an incineration catalyst.

11.A process as claimed in claim 10, in which the incineration is carried out at a temperature in the range of from 150 to 4500C and in the presence of an incineration catalyst comprising as the catalytically active compounds bismuth in an amount in the range of from 0.6 to 10% by weight and copper in an amount in the range of from 0.5 to 5% by weight, calculated on the total weight of the catalyst.

12. A process as claimed in claim 1, in which the combustible sulphur compounds-containing waste gases are off-gases of a process for the removal of sulphur compounds and elemental sulphur from the tail gases obtained by cooling the gases which have left a catalytic zone which is final with respect to sulphur formation and separating elemental sulphur from the cooled gases.

13. A process as claimed in claim 12, in which the process for the removal of sulphur compounds and elemental sulphur comprises passing the tail gases at a temperature above 1 750C, together with a free hydrogen- and/or carbon monoxide-containing gas, over a sulphided Group VI and/or Group VIII metal catalyst supported on an inorganic oxidic carrier and passing the reduced tail gases thus obtained through a liquid and regenerable absorbent for hydrogen sulphide.

14. A process as claimed in claim 13, in which steam, generated by cooling the gases formed in the thermal zone, is used for heating the tail gases obtained by cooling the gases which have left the final catalytic zone, before they are passed over the sulphided Group VI and/or Group VIII metal catalyst.

1 5. A process as claimed in claim 13, in which tail gases obtained by cooling the gases which have left the final catalytic zone are heated by means of indirect heat exchange with the reduced tail gases passed over the sulphided Group VI and/or Group VIII metal catalyst.

16. A process as claimed in claim 1 5, in which the off-gases which have left the liquid and regenerable absorbent are heated, prior to their incineration, by means of indirect heat exchange with the reduced tail gases passed over the sulphided Group VI and/or Group VIII metal catalyst.

17. A process as claimed in claim 13 or 14, in which the hydrogen sulphide-enriched absorbent is regenerated, the regenerated absorbent used again for further absorption of hydrogen sulphide and the hydrogen sulphide-rich gas mixture liberated in the regeneration is used as hydrogen sulphidecontaining gas in claim 1.

18. A process for the production of elemental sulphur from hydrogen sulphide-containing gases according to a Claus-type process having at least a thermal zone in which a hydrogen sulphidecontaining gas is combusted with a free oxygen-containing gas, followed by one or more catalytic zones in which hydrogen sulphide and sulphur dioxide react together to form elemental sulphur and water, which process comprises the following additional steps:: a) passing the tail gases obtained by cooling the gases which have left the catalytic zone or, if more than one catalytic zone is applied, a catalytic zone which is final with respect to sulphur formation, at a temperature above 1 700 C, together with a free hydrogen- and/or carbon monoxidecontaining gas, over a sulphided Group VI and/or Group VIII metal catalyst supported on an inorganic oxidic carrier; b) separating reduced tail gases from the catalyst used in step a); c) contacting the reduced tail gases separated in step b) with a liquid and regenerable absorbent for hydrogen sulphide; d) separating off-gases from a hydrogen sulphide-enriched absorbent formed in step c); e) heating the off-gases separated in step d) by means of indirect heat exchange with the reduced tail gases separated in step b);; f) incinerating the off-gases heated in step e) with formation of incinerator off-gases, and g) discharging the incinerator off-gases formed in step f) into the atmosphere.

1 9. A process as claimed in claim 18, in which the incineration is carried out in the presence of an incineration catalyst.

20. A process as claimed in claim 19, in which the incineration is carried out at a temperature in the range of from 1 50 to 450 C and in the presence of an incineration catalyst comprising as the catalytically active components bismuth in an amount in the range of from 0.6 to 10% by weight and copper in an amount in the range of from 0.5 to 5% by weight, calculated on the total weight of the catalyst.

21. A process as claimed in claim 18, 19 or 20, in which the hydrogen sulphide-enriched absorbent separated in step d) is regenerated, the regnerated absorbent used again for further absorption of hydrogen sulphide and the hydrogen sulphide-rich gas mixture liberated in the regeneration is recycled to the Ciaus-type process.

22. A process as claimed in claim 18, in which the said tail gases, prior to step a), are heated by means of indirect heat exchange with steam generated by cooling the gases formed in the thermal zone.

23. A process as claimed in claim 18, in which the said tail gases, prior to step a), are heated by means of indirect heat exchange with the reduced tail gases separated in step b).

24. A process for the production of elemental sulphur from hydrogen sulphide-containing gases according to a Claus-type process having at least a thermal zone in which a hydrogen sulphidecontaining gas is combusted with a free oxygen-containing gas, followed by one or more catalytic zones in which hydrogen sulphide and sulphur dioxide react together to form elemental sulphur and water, which process comprises the following additional steps:: a) heating the tail gases obtained by cooling the gases which have left the catalytic zone or, if more than one catalytic zone is applied, a catalytic zone which is final with respect to sulphur formation, by means of indirect heat exchange with steam generated by cooling the gases formed in the thermal zone; b) passing the tail gases heated in step a), at a temperature above 1700 C, together with a free hydrogen- and/or carbon monoxide-containing gas, over a sulphided Group VI and/or Group VIII metal catalyst supported on an inorganic oxidic carrier; c) separating reduced tail gases from the catalyst used in step b); d) contacting the reduced tail gases separated in step c) with a liquid and regenerable absorbent for hydrogen sulphide; e) separating off-gases from a hydrogen sulphide-enriched absorbent formed in step d);; f) incinerating the off-gases separated in step e) with formation of incinerator off-gases, and g) discharging the incinerator off-gases formed in step f) into the atmosphere.

25. A process as claimed in claim 24, in which the said tail gases, subsequent to step a) and prior to step b) are heated by means of indirect heat exchange with reduced tail gases separated in step c).

26. A process as claimed in claim 24 or 25, in which the hydrogen sulphide-enriched absorbent separated in step e) is regenerated, the regenerated absorbent used again for further absorption of hydrogen sulphide and the hydrogen sulphide-rich gas mixture liberated in the regeneration is recycled to the Ciaus-type process.

27. A process as claimed in claim 24, in which the incineration is carried out in the presence of an incineration catalyst.

28. A process as claimed in claim 27, in which the incineration is carried out at a temperature in the range of from 1 500C to 4500C and in the presence of an incineration catalyst comprising as the catalytically active components bismuth in an amount in the range of from 0.6 to 10% by weight and copper in an amount in the range of from 0.5 to 5% by weight, calculated on the total weight of the catalyst.

29. A process for the production of elemental sulphur from hydrogen sulphide-containing gases according to a Claus-type process having at least a thermal zone in which a hydrogen sulphidecontaining gas is combusted with a free oxygen-containing gas, followed by one or more catalytic zones in which hydrogen sulphide and sulphur dioxide react together to form elemental sulphur and water, which process comprises the following additional steps:: a) passing the tail gases obtained by cooling the gases which have left the catalytic zone or, if more than one catalytic zone is applied, a catalytic zone which is final with respect to sulphur formation, at a temperature above 1 700 C, together with a free hydrogen- and/or carbon monoxidecontaining gas, over a sulphided Group VI and/or Group VIII metal catalyst supported on an inorganic oxidic carrier; b) separating reduced tail gases from the catalyst used in step a); c) heating the said tail gases prior to step a) by means of indirect heat exchange with the reduced tail gases separated in step b); d) contacting the reduced tail gases subsequent to step c) with a liquid and regenerable absorbent for hydrogen sulphide; e) separating off-gases from a hydrogen sulphide-enriched absorbent formed in step d);; f) incinerating the off-gases separated in step e) with formation of incinerator off-gases, and g) discharging the incinerator off-gases formed in step f) into the atmosphere.

30. A process as claimed in claim 29, in which the hydrogen sulphide-enriched absorbent separated in step a) is regenerated, the regenerated absorbent used again for further absorption of hydrogen sulphide and the hydrogen sulphide-rich gas mixture liberated in the regeneration is recycled to the Claus-type process.

31. A process as claimed in claim 29, in which the incineration is carried out in the presence of an incineration catalyst.

32. A process as claimed in claim 31, in which the incineration is carried out at a temperature in the range of from 1 500C to 4500C and in the presence of an incineration catalyst comprising as the catalytically active components bismuth in an amount in the range of from 0.6 to 10% by weight and copper in an amount in the range of from 0.5 to 5% by weight, calculated on the total weight of the catalyst.