GB2467930A

GB2467930A - An apparatus and method for forming sulphur from hydrogen sulphide

Info

Publication number: GB2467930A
Application number: GB0902828A
Authority: GB
Inventors: Ian Hibbitt
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2009-02-19
Filing date: 2009-02-19
Publication date: 2010-08-25
Anticipated expiration: 2029-02-19
Also published as: GB2467930B; GB0902828D0

Abstract

A method of forming sulphur by the partial oxidation of hydrogen sulphide (H2S) wherein hydrogen sulphide (H2S) gas and an oxidant gas are supplied to the inner region of a flame while a gas comprising sulphur dioxide (SO2) is supplied to an outer region of the flame. The apparatus comprises burner 2 is operated so as to establish a flame in a furnace 72 to which the oxidant gas, typically oxygen, and hydrogen sulphide gas are supplied via passageway 8 to the inner region of the flame and sulphur dioxide is supplied via passageway 18 to the outer region of the flame. The sulphur dioxide may be supplied from a Wellman-Lord plant. The resultant gas mixture comprising sulphur vapour is withdrawn from the furnace 72, which may form part of a Claus plant.

Description

A Method and Apparatus for the Partial Oxidation of Hydrogen Suiphide This invention relates to the partial oxidation (partial combustion) of hydrogen suiphide and in particular to a method of and apparatus for forming sulphur by partial oxidation of hydrogen sulphide.

Hydrogen suiphide containing gas streams (sometimes referred to as "acid gas streams") are typically formed in oil refineries and natural gas processing units. Such streams cannot be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide containing gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is combined with an oxidant gas such as air and is subjected to combustion in a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. Thus, the hydrogen sulphide is effectively partially oxidised. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed zaction between the residual hydrogen sulphide and the sulphur dioxide takes place. The sulphur vapour produced is condensed downstream of each stage.

The effluent gas (tailgas) from the most downstream of the sulphur extractions may be incinerated or subjected to further tieatment, eg by a scrubber. There are two different types of tailgas scrubber -reduction scrubbers and oxidation scrubbers. In a reduction scrubber, sulphur in the tailgas is converted to hydrogen sulphide in a reducing process and then the hydrogen sulphide is scrubbed from the gas stream and recycled to the feed of the Claus plant. The SCOT and Beavon processes are reduction scrubber processes.

In an oxidation scrubbing process the sulphur in the tailgas is oxidised to sulphur dioxide which is then scrubbed from the gas stream. The Weliman-Lord, Stauffer Aquaclaus, and the IFP-2 processes are oxidation scrubbing processes.

In the Wellman-Lord process tailgas from the Claus plant is burnt in an incinerator thereby oxidising all sulphur species to sulphur dioxide. After cooling and water removal the gas stream comprising sulphur dioxide is then reacted with aqueous sodium sulphite and sodium bisulphite which absorbs the sulphur dioxide to generate sodium bisulphite.

The resulting bisuiphite solution is heated to regenerate sulphur dioxide, water and sodium sulphite. The concentrated sulphur dioxide stream is recycled back to the Claus plant, typically entering the Claus process stream downstream of the main burner or at a catalytic stage.

Axially or longitudinally tired burners mounted on the back wall may be used in Claus furnaces. Such axially or longitudinally fired burners can be designed to provide average residence times comparable with those of cross -or tangentially -fired burners at a specified throughput, and may be preferred at higher levels of oxygen-enrichment.

The use of such an axially or longitudinally fired burner is disclosed in European patent application 0 315 225 A, in which there is a central pipe for oxygen, at least one second pipe for hydrogen sulphide containing feed gas which coaxially surrounds the central pipe, and an external coaxial pipe for air. The burner is used when the hydrogen sulphide feed gas contains at least 50/G by volume of carbon dioxide or hydrocarbons.

Temperatures in the range of 2000 to 3000°C are generated in the core of the burner flame, and a gas mixture having a temperature in the range of 1350 to 1650°C leaves the furnace.

Air may be used as an oxidant gas to support the combustion of hydrogen sulphide in the initial part of the Claus process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is, of course, present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of given size. This ceiling can be raised by using commercially pure oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide.

If commercially pure oxygen or oxygen-enriched air having a mole fraction of oxygen above 0.65 is used as an oxidant gas to support the combustion of the hydrogen suiphide there may be a relatively high risk of damage to the refractory lining of the furnace being due to the resulting high flame temperature. The degree of the risk will depend on the composition of the Claus feed gas. There are a number of proposals in the art to solve this problem.

For example, some Claus plants have two furnaces rather than one so as to limit the amount of combustion that is performed in each individual furnace, thereby limiting the temperature reached in each furnace. Another proposal is disclosed in EP 0 974 552 which describes a furnace having a burner which is designed to produce a flame having three different temperature zones. Thus, the burner has an inner group of gas outlets for supplying a hydrogen sulphide gas stream to an inner part of the flame and a group of outlets for supplying oxygen or oxygen-enriched air to that inner part of the flame. The burner is located in a port in a furnace with a gap between the burner and the internal wall of the port through which air flows and is supplied to an outer part of the flame. Between the inner part of the flame and the outer part of the flame there is an intermediate group of outlets for the supply of hydrogen suiphide rich gas and an intermediate group of outlets for the supply of oxygen or enriched air. The supplies of hydrogen sulphide rich gas and oxygen or oxygen-enriched air to each group of outlets and the supply of air to the outer region of the flame are each controlled by a respective flow control valve such that the overall mole ratio of combustibles to oxygen can be controlled so as to enable different local ratios of the reacting species to be created in different regions of the flame.

In that way, a hot innermost region can be maintained in the flame at a temperature in excess of 1400°C and a much lower temperature can be maintained at the periphery of the flame, with an intermediate temperature being obtained in the intermediate region between the inner region and the periphery. The high temperature of the inner region of the flame favours thermal disassociation of hydrogen suiphide and helps to consume any ammonia present in the hydrogen sulphide rich gas fed to that inner region. The lower temperature in the outer region of the flame helps to avoid damage to the refractory wall

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of the furnace. Typically, the rates of supply of the reactants are controlled such that the mole ratio of hydrogen sulphide to sulphur dioxide in the gas mixture leaving the furnace is approximately 2:1. Within the respective regions of the flame, however, the mole ratio of hydrogen sulphide and sulphur dioxide can vary significantly.

It is also known to upgrade existing Claus plants by fitting a specialised burner as described above in place of the original burner. Such retro-fits are generally aimed at increasing the throughput possible in the Claus plant by allowing the use of oxygen or oxygen-enriched air as the oxidant gas in place of atmospheric air whilst at the same time avoiding damage to the refractory liner of the furnace as described above.

There is a need to provide an improved process for the oxidation of hydrogen sulphide.

The present invention provides a method of forming sulphur by partial oxidation of hydrogen sulphide comprising: operating a burner so as to establish a flame in a furnace, supplying to an inner region of the flame an oxidant gas and a gas comprising hydrogen sulphide, supplying to an outer region of the flame a gas comprising sulphur dioxide, and withdrawing from the furnace a resultant gas mixture.

The invention also provides an apparatus comprising a furnace, a burner operable to establish a flame in the furnace and a source of sulphur dioxide, in which the burner includes an inner outlet or group of outlets for the supply of a gas comprising hydrogen sulphide to an inner region of the flame, an outlet or group of outlets for the supply of an oxidant gas to an inner region of the flame, and conduits for the supply of a gas comprising sulphur dioxide from the source of sulphur dioxide to an outer region of the flame.

In a further aspect the invention provides a method of oxidising hydrogen suiphide in a Claus plant in an oil refinery or a natural gas processing unit comprising a sour water stripper, an amine plant, a Claus plant and a Weliman-Lord plant in which the process comprises operating a burner so as to establish a flame in a furnace, supplying to an inner region of the flame oxygen or oxygen-enriched air together with gas comprising hydrogen sulphide generated by the sour water stripper, supplying to an intermediate region of the flame oxygen or oxygen-enriched air together with a gas comprising hydrogen suiphide generated by the amine gas plant, and supplying to an outer region of the flame a gas comprising sulphur dioxide generated by the Wellman-Lord plant, optionally in combination with air, and withdrawing from the furnace a resultant gas mixture comprising sulphur vapour, water vapour, sulphur dioxide, hydrogen and residual hydrogen sulphide.

In a further aspect, the invention provides an oil refinery or a natural gas processing unit comprising a sour water stripper, an amine plant, a Claus plant and a Wellman-Lord plant in which the Claus plant includes at least one furnace provided with a burner having a outlet or group of outlets for supplying hydrogen sulphide containing gas from the sour water stripper to an inner region of the flame, an outlet or group of outlets for supplying oxygen or oxygen-enriched air to the inner region of the flame, an outlet or group of outlets for supplying hydrogen sulphide containing gas from the amine plant to an intermediate region of the flame, an outlet or group of outlets for supplying oxygen or oxygen-enriched air to the intermediate region of the flame, and a conduit arranged to carry sulphur dioxide containing gas from the Wellman-Lord plant to the outer region of the flame, the apparatus optionally including a pump for the supply of air to the outer region of the flame in combination with the sulphur dioxide containing gas.

In the method of the invention a gas comprising sulphur dioxide is supplied to the outer region of the flame. That sulphur dioxide contributes to the sulphur dioxide content of the gas mixture leaving the furnace which ideally has a stoichiometry of sulphide dioxide to hydrogen sulphide of 1:2. At the same time it helps to shield the internal wall of the furnace from the hotter, inner region of that flame thereby allowing that inner region of

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the flame to be operated at a high temperature. For example, a relatively low temperature can be maintained in the outermost region even though a temperature in excess of 2000°C may be created in innermost region, and therefore risk of damage to any refractory lining of the furnace can be kept to acceptable levels. A high temperature, that is a temperature well in excess of 2000°C, is particularly advantageous because it facilitates destruction of any ammonia in the first combustible gas and creation of conditions which increase the proportion of the resulting sulphur vapour that is formed directly by thermal cracking of hydrogen sulphide rather than by the indirect route involving oxidation of some hydrogen sulphide or sulphur to sulphur dioxide and then reaction of the thus formed sulphur dioxide with residual hydrogen sulphide. Destruction of ammonia is desirable because this gas tends to affect adversely downstream processing of the effluent from the furnace in catalytic reactors in which hydrogen sulphide and sulphur dioxide react together to form further sulphur vapour, the ammonia acting to block the catalyst by formation of ammonium salts.

The various gases present in each region of the flame will of course tend to mix to some degree with the gases in the adjacent region or regions of the flame and therefore references herein to "inner", "outer" and "intermediate" regions of the flame should be understood accordingly.

Where the gas comprising sulphur dioxide is supplied without being combined with air and does not comprises any combustible substances, it will not take part in the combustion process, and the outer region of the flame may therefore encompass little or no combustion activity and the term "outer region of the flame" is to be construed accordingly.

Optionally, a gas comprising hydrogen sulphide and an oxidant gas are supplied to an intermediate region of the flame between the inner region of the flame and the outer region of the flame. In the apparatus of the invention the burner optionally has an outlet or group of outlets for the supply of gas containing hydrogen sulphide to an intermediate region of the flame located between the inner and outer regions of the flame and an outlet or group of outlets for the supply of an oxidant gas to the intermediate region of the flame. The outlets for the supply of gases to the inner part of the flame will typically be arranged in a central region of the mouth of the burner and the outlets for the supply of gases to the intermediate region of the flame will typically be located in a region of the mouth of the burner which surrounds that central region. The burner may comprise an outermost set of outlets for the supply of the gas comprising sulphur dioxide (and optionally air) or that gas may be supplied via a passage which is located between the burner and the internal wall of the furnace.

The gas comprising hydrogen sulphide supplied to the intermediate region of the flame may have the same composition as the gas comprising hydrogen sulphide supplied to the inner region of the flame. Alternatively, the gas comprising hydrogen sulphide supplied to the intermediate region of the flame may have a different composition to the gas comprising hydrogen sulphide supplied to the inner region of the flame. For example, the gas comprising hydrogen suiphide supplied to the inner region of the flame may consist substantially of gas produced by a sour water stripper, and the gas comprising hydrogen and sulphide supplied in the intermediate region of the flame may be provided by an amine plant.

In embodiments in which the flame has three or more regions, namely an innermost region, an outermost region, and one or more intermediate regions, it is possible to handle effectively a wider range of different rates of inflow of the hydrogen sulphide containing gas than if only two such regions are employed.

The oxidant gas supplied to the inner region of the flame may have the same composition as the oxidant gas supplied to the intermediate region of the flame. Alternatively, the oxidant gas supplied to the inner region of the flame may have a composition which is different to that of the oxidant gas supplied to the intermediate region of the flame.

Optionally, the oxidant gas supplied to the inner region of the flame and the oxidant gas supplied to the intermediate region of the flame, if present, are selected from oxygen, oxygen-enriched air, and air, and are preferably selected from oxygen and oxygen-enriched air. The oxidant gas supplied to the inner region of the flame and the oxidant gas supplied to the inner region of the flame, if present, optionally have a mole fraction of oxygen of at least 0.22.

The mole fraction of oxygen in the oxidant gas supplied to the inner region of the flame and the oxidant gas supplied to the intermediate region of the flame, if present, is typically in the range of 0.3 to 1.0 depending on the proportion of combustibles present in the other gas feeds. Care should be taken to avoid creating an excessive temperature at any location in any refractory employed to line the furnace. Modem commercially available refractories can typically withstand temperatures up 1650°C. Accordingly, in the method of the invention the furnace optionally comprises an internal wall of refractory material which contains the flame and the temperature of any part of the internal wall does not exceed 1650°C.

The oxygen enriched air or pure oxygen may be taken directly from an air separation plant. Depending on the purity of the oxygen product of the air separation plant, either or both of the oxidant gases supplied to the inner region and intermediate region, if present, of the flame may have a mole fraction of oxygen greater than 0.99. In general, however, particularly when handling sour water stripper gas or amine gas, or mixtures of the two, it is preferred to form either or both of the first oxidant gas and the second oxidant gas by mixing an oxygen product at the air separation plant with atmospheric air,.that is air which is neither enriched nor depleted of oxygen. Forming either or both of the oxidant gases in that way makes it possible to vary the mole fraction of oxygen during operation of the method and apparatus according to the invention. That ability to vary the mole fraction of oxygen adds to the flexibility of the method and apparatus according to the invention in effectively handling varying rates of supply of combustible gas. The gas comprising sulphur dioxide which is supplied to the outer region of the flame is optionally supplied in combination with air. The air may be mixed with the gas comprising sulphur dioxide before it is supplied to the outer region of the flame or alternatively the air and the gas comprising sulphur dioxide may be supplied to the outer

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region of the flame via separate conduits. In cases where the gas comprising sulphur dioxide is in combination with air, the air will preferably be present in a proportion of between 10 and 70%, more preferably between 30 and 60% of the total volume of the air and gas comprising sulphur dioxide.

The gas comprising sulphur dioxide comprises at least 80% by volume, optionally at least 90% by volume of sulphur dioxide.

The method and apparatus of the invention in one embodiment finds application in the refining of oil or the processing of natural gas. For example, the method of the invention may be applied to the oxidation of hydrogen suiphide in a Claus plant in an oil refinery or a natural gas processing unit. In that case, the furnace of the method and apparatus of the invention is part of a thermal stage of the Claus plant. The gas comprising sulphur dioxide which is supplied to the outer region of the flame may be obtained from any suitable source sulphur dioxide. In one embodiment the sulphur dioxide is generated by an oxidising scrubber plant, for example, a Weliman-Lord plant. The oxidising scrubber plant may be located in any part of the oil refinery or natural gas processing unit. In one embodiment the sulphur dioxide is generated in a Wellman-Lord plant arranged downstream of the Claus plant for treating the tailgas emitted from the Claus plant.

The Claus plant optionally has a single combustion stage. Where there are two or more combustion stages the Claus plant may include heat exchangers arranged between the combustion stages to cool the gas stream. Downstream of the combustion stage or stages the Claus plant will typically contain a heat exchanger in which the gas stream is cooled and condensed sulphur is recovered. The gas stream then passes typically to one or more catalytic stages in which the hydrogen sulphide is further oxidised to sulphur. The sulphur vapour is typically condensed in one or more heat exchangers and the remaining tailgas stream leaving the Claus plant typically contains 5% or less of the original sulphur contained in the gases fed into the Claus plant. The tailgas from the Claus plant is optionally fed to a scrubber for reducing the sulphur c9ntent of the tailgas to an environmentally acceptable level. The scrubber is optionally an oxidising scrubber such as a Wellman-Lord scrubber, as mentioned above.

The method and apparatus of the invention may be applied as a retro-fit to an existing Claus plant in order to improve the efficiency of that existing Claus plant. In particular, an increase in throughput of gases comprising hydrogen sulphide may be obtained. The application of the invention may be particularly beneficial to Claus plants having a single combustion stage because the improved shielding of the refractory wall of the furnace from the high temperatures of the inner part of the flame by the gas comprising sulphur dioxide supplied to the outer part of the flame thereby allows higher temperatures to be established in that inner part of the flame, thereby in turn making possible an increased consumption of hydrogen sulphide without increasing the size of the furnace.

The gas comprising sulphur dioxide may be supplied to the outer part of the flame in any suitable manner, for example, through an outer outlet or group of outlets in the burner or via a conduit or passage separate to the burner. Advantageously, the furnace has an inner wall and a port in the inner wall and the burner is arranged in the port such that there is a gap between the burner and the inner wall of the furnace. The gas comprising sulphur dioxide may be supplied to the flame via that gap. Alternatively, the burner may comprise an outermost outlet or group of outlets for the supply of the gas comprising sulphur dioxide to the outer part of the flame.

As mentioned above, the gas comprising sulphur dioxide may be supplied to the outer part of the flame in combination with air. Accordingly, the apparatus of the invention optionally comprises means such as a pump for supplying air to the outer region of the flame in combination with the gas comprising sulphur dioxide.

The furnace may be a right cylindrical furnace having a length to internal diameter ratio in the range of from 2:1 to 4:1.

Optionally, the burner is arranged so as to produce a tangential flame. Optionally, the flame extends generally longitudinally within the furnace. The furnace is optionally disposed with its longitudinal axis horizontal, and therefore the burner is optionally also disposed with its longitudinal axis horizontal. Such arrangements can help to keep down the risk of damage to any refractory lining employed in the furnace.

Mixing of the gas or gases comprising hydrogen sulphide and the oxidant gas or gases within the flame can be promoted by supplying the gases to the flame at different linear velocities and/or by supplying those gases along pathways which generate a swirling action within the flame.

The furnace will generally have an outlet or outlets through which the resultant gas mixture is withdrawn. The resultant gas mixture typically comprises sulphur vapour, water vapour, sulphur dioxide, and hydrogen suiphide.

In the method of the invention the mass flow rate of the gas comprising hydrogen sulphide supplied to the inner part of the flame, the oxidant gas supplied to the inner part of the flame, the gas comprising hydrogen sulphide supplied to the intermediate part of the flame, if present, the oxidant gas supplied to the intermediate part of the flame, if present, and the gas comprising sulphur dioxide supplied to the outer part of the flame are optionally each controllable independently of one another. Such an arrangement facilitates operation of the burner to handle variations in the total rate at which it is desired to feed gas or gases comprising hydrogen sulphide to the burner. The apparatus according to the invention therefore optionally includes a flow control valve for controlling each gas stream supplied to the burner.

In a typical refinery there is more than one source of combustible gas comprising hydrogen sulphide. The sources typically generate gas having different compositions.

Optionally, both the gas comprising hydrogen sulphide supplied to the inner part of the flame and the gas comprising hydrogen sulphide supplied to the intermediate part of the flame, if present, both contain at least 40% by volume of combustibles and at least 20% by volume of hydrogen suiphide, If there are two separate sources of combustible gas comprising hydrogen sulphide, one containing ammonia, the other not, then all the ammonia containing gas is preferably supplied to the inner region of the flame where a relatively high flame temperature can be maintained in order to destroy all the ammonia. For example, if one source of gas containing hydrogen sulphide is so-called "sour water stripper gas", which typically contains about 20 to 35% by volume of hydrogen sulphide and 30 to 45% by volume of ammonia, and another source of gas containing hydrogen suiphide is so-called "amine gas" which typically contains over 80% by volume of hydrogen sulphide, the gas comprising hydrogen suiphide supplied to the inner region of the flame optionally comprises a mixture of some of the amine gas but all of the sour water stripper gas, and the gas comprising hydrogen sulphide supplied to the intermediate region of the flame optionally comprises the remaining amine gas. Optionally, the composition of the mixture is varied with the total rate of flow of combustible gas comprising hydrogen sulphide to the flame, with the proportion of amine gas supplied to the inner region of the flame being increased if the said total rate of flow is reduced below a chosen value.

The method and apparatus according to the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic sectional side elevation of a burner arranged in a port of a furnace for use in the method and apparatus according to the invention; Figure 2 is a schematic end view of the mouth of the burner shown in Figure 1; Figure 3 is a schematic flow diagram illustrating apparatus for supplying gas comprising hydrogen suiphide and oxidant to the burner of Figures 1 and 2; Figure 4 is a schematic flow diagram of a Claus plant for treating an acid gas including hydrogen sulphide which may use the burner shown in Figures 1 and 2.

-12 -The drawings are not to scale.

Referring to Figures 1 and 2 of the drawings, a burner 2 is of generally cylindrical shape and has a proximal end 4 and a distal end (or mouth 6). The burner 2 has a central passageway 8 defined by an innermost tube 10 for flow of a gas stream comprising hydrogen sulphide to the inner part of the flame. The longitudinal axis of the burner 2 is coincident with the longitudinal axis of the tube 10. The central passageway 8 has a first outlet 12 at its distal end. A second tube 14 is coaxial with the first tube 10. The inner surface of the tube 14 makes a frictional engagement with the outer surface of the tube 10. (Alternatively the tubes 10 and 14 may be joined to one another by means of an internal flange or welded connection.) A third tube 16 is spaced from and is coaxial with the tube 14. Tubes 14 and 16 define a second, atmular, passageway 18 for a flow of gas comprising hydrogen sulphide to the intermediate part of the flame. The passageway 18 terminates at its distal end in an outlet 20. The tubes 14 and 16 terminate in the same plane as the tube 10.

An array of tubes 22 extends from beyond the proximal end of the tube 10 therethrough and defines passages 24 for the flow of an oxidant gas to the inner part of the flame. Each passageway 24 has an outlet 26. The tubes 22 terminate in the same plane as the tube 10.

The outlets 26 are typically disposed in a ring which is coaxial with the longitudinal axis of the burner 2.

A second array of tubes 28 is disposed in the passageway 18 defined by the tubes 14 and 16. Each tube 28 defines a passageway 30 for a second flow of oxidant gas terminating in respective outlet 32 for supply to the intermediate region of the flame. The tubes 28 each terminate in the same plane as the tube 10. The outlets 32 of the tubes 28 are arranged in a ring which is coaxial with the longitudinal axis of the burner 2. The respective tubes 22 and 28 may each be provided with a spider 34 to help support them when the burner is disposed with its longitudinal axis horizontal as shown in Figure 1. There is considerable flexibility in selecting the actual numbers of the tubes 22 and 28.

The construction of the burner 2 so as to enable to the respective flows of gas comprising hydrogen suiphide and oxidant gas mixtures to be supplied to it is relatively simple. The outer tube 16 is provided with a first port 36 for the flow of the combustible gas comprising hydrogen suiphide to the intermediate region of the flame. The proximal end of the outer tube 16 is formed with a flange 38 integral therewith or welded thereto. The flange 38 is bolted or otherwise secured to a similar flange 40 which is integral with or welded to the tube 14. If desired, a gasket or other sealing member (not shown) can be engaged between the flanges 40 and 38 so as to ensure a fluid-tight seal therebetween.

The flange 40 forms the distal end of a chamber 42 which receives the oxidant gas for the intermediate region of the flame and which has a port 44 enabling it to be placed in communication with a source of such oxidiant gas. The proximal ends of the tubes 28 are all received fluid-tight in complementary apertures through the flange 40. Thus, the tubes 26 communicate with the chamber 42. The chamber 42 has an outer wall 46, in which the port 44 is formed, which is provided at its distal end with a flange 48 which is fastened fluid-tight to the flange 40 and at its proximal end with a flange 50. The flange 50 is bolted or otherwise secured fluid-tight to a complementary flange 52 which is integral with or is welded to the proximal end of the tube 10. The flange 52 forms a proximal wall of the chamber 42. It also forms a distal wall of a further chamber 54 having a side wall 56 with a port 58 formed therein which enables the chamber 54 to be placed in communication with a source of the gas comprising hydrogen suiphide to be supplied to the inner region of the flame. The wall 56 of the chamber 54 has a first flange 60 at its distal end which is bolted or otherwise secured fluid-tight to the flange 52 and a second flange 62 at its proximal end which is bolted or otherwise secured fluid-tight to an end plate 64 which forms a dividing wall between the chamber 54 and a yet further chamber 66 for the oxidant gas supplied to the inner region of the flame and which receives fluid-tight in apertures formed therethrough the proximal ends of the tubes 22 so as to enable these tubes to receive a flow of the that oxidant gas. The chamber 66 is provided with a port 68 which is coaxial with the longitudinal axis of the burner 2 and is able to be placed in communication with the source of the oxidant gas to be supplied to the inner region of the flame.

As shown in Figure 1, the distal end 6 of the burner 2 extends into a port or quart 70 of a furnace 72 for the partial combustion of hydrogen sulphide. An annular gap 74 is defined between the distal end of the burner 2 and the interior wall of the furnace in the port 70.

A gas comprising sulphur dioxide is supplied to that gap 74. The gas comprising sulphur dioxide is drawn into the furnace and mixes at least partially into the outer part of the flame projecting from the mouth of the burner. The gas comprising sulphur dioxide acts to shield the internal wall of the furnace 70 from the high temperatures of the inner part of the flame. At the same time the sulphur dioxide acts as an oxidant which will in the downstream regions of the Claus plant oxidise hydrogen sulphide supplied to the inner and intermediate parts of the flame. The gas comprising sulphur dioxide can be obtained from any suitable source but is conveniently generated by a Wellman-Lord plant arranged downstream of the Claus plant for cleaning the tailgas from the Claus plant.

Conventionally, such gas comprising sulphur dioxide would have been returned to the Claus plant either just downstream of the combustion stages or just upstream of one of the catalytic stages.

When desired, the stream from the Wellman-Lord plant can be combined with air from a supply of compressed air prior to entry into the furnace. The oxygen in the air acts as an additional source of oxidant for the hydrogen sulphide to the burner 2. Alternatively, the sulphur dioxide containing gas from the Wellman-Lord plant can be used without combination with air.

If desired, the distal end of the outer tube 16 may be formed of a refractory metal. Other parts of the burner 2 may be formed of stainless steel.

In operation, the gas comprising hydrogen suiphide exiting the burner 2 from the outlet 12 becomes intimately mixed with oxidant gas that leaves through the outlets 26 to form an inner region of a flame. Similarly, the flow of gas comprising hydrogen sulphide leaving the burner 2 through the outlet 20 becomes intimately mixed with the flow of the oxidant gas which leaves the burner 2 through the outlets 32, thus forming an -15-intermediate region of the flame. An outer region of the flame is formed by intimate mixing of the gas comprising sulphur dioxide (and air, if present) passing through the gap 74 with the gas leaving the burner 2 through the outlet 20.

The arrangement for supplying the different gas flows to the burner 2 is shown in Figure 3. Referring to Figure 3, a first pipeline 80 for sour water stripper gas (which includes both hydrogen suiphide and ammonia) terminates in the port 58 of the burner 2. The first flow control valve 82 is disposed in the pipeline 80. A second pipeline 84 for amine gas (which predominantly comprises hydrogen sulphide) terminates in the port 36 of the burner 2 and has a second flow control valve 86 disposed therein. A third pipeline 88 communicating with a source (not shown) of first oxidant gas composed of air or oxygen-enriched air terminates in the port 68 of the burner 2. A third flow control valve 90 is located in the third pipeline 88. A fourth pipeline 92 communicating with a source (not shown) of oxidant gas composed of air or oxygen-enriched air terminates in the port 44 of the burner 2. A fourth flow control valve 94 is located in the fourth pipeline 92. A fifth pipeline 96 communicating with a Wellman-Lord plant (not shown) downstream of the Claus plant or other source of sulphur dioxide, terminates in an inlet 104 to a nozzle 106 which communicates with the annular gap 74 defined between the ports 70 and the burner 2. The pipeline 96 may also communicate with a blower (not shown) or other source of compressed air (neither enriched in nor depleted of oxygen). The pipeline 96 has a fifth flow control valve 98 disposed therein. In addition, a pipeline 100 extends through a region of the second pipeline 84 upstream of the second flow control valve 86 to a region of the first pipeline 80 downstream of the first flow control valve 82. A sixth flow control valve 102 is disposed in the pipe 100.

In operation, the flow control valves described above may be set to determine the overall mole ratio of combustibles:oxygen:sulphur dioxide supplied to the flame of the burner 2, so as to enable different local ratios of the reacting species to be created in different regions of the flame, so as to enable a hot innermost region to be maintained in the flame at a temperature in excess of 1400°C, so as to enable a much lower temperature to be maintained at the periphery of the flame, to create within a localised region of the flame conditions which favour thermal dissociation of hydrogen suiphide, and to ensure that all ammonia is destroyed. Typically, the rates of supply of the reactants are controlled such that the mole ratio of hydrogen sulphide to sulphur dioxide and the gas mixture leaving the furnace is approximately 2:1. Within the respective regions of the flame, however, the mole ratio of hydrogen sulphide to sulphur dioxide can vary significantly.

The kind of flame that is formed in operation of the burner is shown schematically in Figure 3 and is indicated therein by the reference numeral 110. The flame has three regions 112, 114 and 116. The inner stage 112 is a high intensity zone into which the gas from outlets 12 and 26 flows. In an example in which the combustible gas supplied to the inner region of the flow is composed of a mixture of sour water stripper gas and amine gas, the oxidant gas is supplied to the inner region of the flame at a rate that is sufficient to ensure the complete destruction of ammonia and any hydrocarbons in the first combustible gas and oxidation of more than one third of the hydrogen suiphide. A high temperature in the inner region 112 is thus ensured. The temperature can be controlled by the control valves 82, 90 and 102.

The intermediate region 114 of the flame 110 receives the oxidant gas from outlets 32 and all or part of gas comprising hydrogen suiphide from outlet 20. This stage 114 is typically operated oxygen-poor that is to say that the relative rates of supply of hydrogen sulphide and oxygen molecules to this stage are such that less than one third of this hydrogen sulphide is oxidised to sulphur dioxide. The paucity of oxygen in this region together with the heat radiated from the inner stage i 06 favour formation of sulphur vapour by thermal cracking of hydrogen sulphide. Since the thermal cracking of hydrogen sulphide proceeds endothermically, it provides a mechanism for moderating flame temperature and helps to prevent excessive temperatures being created in the outer region 116. Further, it can reduce the demand for nitrogen molecules to moderate the flame temperature, and thereby enables the oxidant gases to have higher mole fractions of oxygen than would otherwise be possible. The outer region 116 of the flame may receive some of the combustible gas from outlet 20 and any air which is supplied as oxidant gas to the pipeline 96 as well as the gas comprising sulphur dioxide from the Weilman-Lord plant. The rate of supply of any air is controlled so as to ensure that an excessive flame temperature is not created in the region 116. The total supply of oxidant and sulphur dioxide is controlled such that the desired ratio of H2S to SO2 is maintained after the waste heat reboiler.

The burner shown in Figures 1 and 2 of the drawings may be employed as the burner 600 shown in Figure 4. With reference to Figure 4, a combustible gas mixture which typically includes more than 40% by volume of hydrogen suiphide flows into the burner 600. Partial combustion of the hydrogen suiphide is supported by the supply of oxygen enriched air and atmospheric air to the burner 600. The burner 600 fires into a furnace 602. A gas mixture comprising hydrogen suiphide, sulphur dioxide, sulphur vapour, water vapour, nitrogen, carbon dioxide and hydrogen leaves the furnace 602 typically in the range of 1100°C to 1600°C. The effluent gas mixture passes through a waste heat boiler 604 in which its temperature is reduced to a little above the point at which sulphur vapour condenses. The mole ratio of hydrogen suiphide to sulphur dioxide in the effluent gas mixture is approximately 2 to I after some recombination of hydrogen and sulphur in the waste heat boiler 604. Downstream of the waste heat boiler 604 the effluent gas flows through a condenser 606 in which sulphur vapour is condensed. The resulting condensate is passed for storage. The residual gas mixture flows from the condenser 606 through successive catalytic Claus stages 608, 610 and 612. Each of the stages 608, 610 and 612, in accordance with the general practice in the art, comprises a train of units consisting, in sequence, of a reheater (not shown) to raise the temperature of the gas mixture to a temperature suitable for catalytic reaction between hydrogen sulphide and sulphur dioxide, a catalytic reactor (not shown) in which hydrogen sulphide reacts with sulphur dioxide to form sulphur vapour and water vapour, and a sulphur condenser (not shown).

If desired, depending on the environmental standards which the plant shown in Figure 14, one or more of the catalytic stages 608, 610 and 612 may be omitted.

The gas mixture leaving the downstream catalytic stage 612 is passed to a Welirnan-Lord plant in which it is subjected to incineration, scrubbing and regeneration of sulphur dioxide. The tailgas 615 from the Weliman-Lord plant 614 is vented to the atmosphere.

S

The stream 616 of gas comprising sulphur dioxide is returned to the gap between burner 600 and furnace 602, from where it enters the outer region of the flame in furnace 602.

Claims

Claims 1. A method of forming sulphur by partial oxidation of hydrogen sulphide comprising: operating a burner so as to establish a flame in a furnace, supplying to an inner region of the flame an oxidant gas and a gas comprising hydrogen suiphide, supplying to an outer region of the flame a gas comprising sulphur dioxide, and withdrawing from the furnace a resultant gas mixture.
2. A method as claimed in claim 1 in which a gas comprising hydrogen sulphide and an oxidant gas are supplied to an intermediate region of the flame between the inner region of the flame and the outer region of the flame.
3. A method as claimed in claim 1 or claim 2 in which the oxidant gas supplied to the inner region of the flame and the oxidant gas supplied to the intermediate region of the flame, if present, are selected from oxygen and oxygen-enriched air.
4. A method as claimed in any of claims 1 to 3 in which the gas comprising sulphur dioxide comprises sulphur dioxide and air.
5. A method as claimed in any of claims 1 to 3 in which the gas comprising sulphur dioxide comprises at least 80% by volume of sulphur dioxide.
6. A method as claimed in any of claims 1 to 5 in which the temperature in the inner region of the flame exceeds 2000°C.
7. A method as claimed in any of claims 1 to 6 in which the furnace comprises an internal wall of refractory material which contains the flame and the temperature of any part of the internal wall does not exceed 1650°C.

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8. A method as claimed in any of claims I to 7 in which the furnace is part of the thermal stage of a Claus plant in an oil refinery or natural gas processing unit.
9. A method as claimed in claim 8 which comprises generating the sulphur dioxide in a Weilman-Lord plant arranged downstream of the Claus plant for treating the tailgas emitted from the Claus plant.
10. A method as claimed in claim 8 or claim 9 in which the Claus plant has a single combustion stage.
11. An apparatus comprising a furnace, a burner operable to establish a flame in the furnace and a source of sulphur dioxide, in which the burner includes an inner outlet or group of outlets for the supply of a gas comprising hydrogen sulphide to an inner region of the flame, an outlet or group of outlets for the supply of an oxidant gas to an inner region of the flame, and conduits for the supply of a gas comprising the sulphur dioxide from the source of sulphur dioxide to an outer region of the flame.
12. An apparatus as claimed in claim 11 in which the burner has an outlet or group of outlets for the supply of a gas containing hydrogen sulphide to an intermediate region of the flame located between the inner and outer regions of the flame, and an outlet or group of outlets for the supply of an oxidant gas to the intermediate region of the flame.
13. An apparatus as claimed in claim II or claim 12 in whichthe furnace has an inner wall and a port in the inner wall and the burner is arranged in the port such that there is a gap between the burner and the inner wall of the furnace and the gas comprising sulphur dioxide is supplied to the flame via the gap.
14. An apparatus as claimed in any of claims 11 to 13 which comprises means for supplying air to the outer region of the flame in combination with the gas comprising sulphur dioxide.
15. An oil refinery or a natural gas processing unit comprising an apparatus as claimed in any of claims 11 to 14 in which the burner and furnace are part of a Claus plant and the source of sulphur dioxide is a Wellman-Lord plant arranged to treat the tailgas from the Claus plant. -22