PROCESS AND APPARATUS FOR RECOVERING SULPHUR FROM A GAS STREAM CONTAINING HYDROGEN SULPHIDE

Abstract: Oxygen or oxygen-enriched air is employed to support combustion in furnace (16) of part of the hydrogen sulphide content of a first feed gas stream. Sulphur is extracted from the resulting gas stream in a sulphur condenser (26). Catalyst Claus reaction between hydrogen sulphide and sulphur dioxide in the resulting sulphur vapour depleted gas stream takes place in a catalytic reactor (32). Sulphur is extracted in a further sulphur condenser (34). The resulting sulphur vapour depleted gas stream is passed into a catalytic reduction reactor (40) in which all the residual sulphur dioxide and any sulphur vapour are reduced to hydrogen sulphide. The resulting reduced gas mixture has water vapour extracted there from in a quench tower (52). The resulting water vapour depleted gas stream flows to a Claus plant for further treatment typically together with a second feed gas stream. Employing the catalytic Claus reactor (32) facilitates achievement of complete reduction of sulphur dioxide to hydrogen sulphide in the catalytic reactor (40) and therefore makes it possible to obtain high overall conversions of hydrogen sulphide to sulphur.

Description: PROCESS AND APPARATUS FOR RECOVERING SULPHUR FROM A GAS STREAM CONTAINING HYDROGEN SULPHIDE This invention relates to a method of and apparatus for treating a gas stream containing hydrogen sulphide. Gas streams comprising hydrogen sulphide are formed, for example, as waste gases in oil refinery or gas refinery operation. In view of their hydrogen sulphide content, these gas streams cannot be discharged to the atmosphere without first being treated so as to remove almost all the hydrogen sulphide. A standard method of treating such gas stream serves to recover sulphur there from is by the Claus process. Conventional Claus processes are described in the introductory paragraphs of EP-A-565 316. EP-A-565 316 also discloses a process in which in a first reactor a part of the hydrogen sulphide content of a feed stream comprising hydrogen sulphide is oxidised to sulphur dioxide, and thus formed sulphur dioxide is reacted with residual hydrogen sulphide to form sulphur vapour and water vapour. A partially reacted gas stream including sulphur vapour, water vapour, residual hydrogen sulphide and residual sulphur dioxide is withdrawn from the furnace. A sulphur condenser is employed to extract sulphur vapour from the partially treated gas stream so as to form a sulphur vapour depleted gas stream. At least part of the sulphur depleted gas stream is sent to a catalytic reactor in which its sulphur dioxide content is reduced by hydrogen to hydrogen sulphide. Water vapour is extracted from the resulting reduced gas stream. The water vapour-depleted gas stream is then preferably recycled to the furnace. A purge stream is taken from a chosen position in the above described cycle and is subjected to further treatment so as to render it fit for discharge to the environment. The purpose of the recycle is to obtain a very high effective conversion of hydrogen sulphide to sulphur vapour in the furnace and thereby facilitate the attainment of a total conversion efficiency which is sufficient to meet any prevailing environmental standard. By using pure oxygen (or air highly enriched in oxygen) the size of the initial furnace may be kept down. However, the advantages in size reduction of the initial furnace made possible by the use of pure oxygen (or oxygen highly enriched in air) as the oxidant are counteracted to some extent by the recycle of gas to the furnace. Although EP-A-0 565 316 further discloses that the recycle can be omitted, this is stated not to be preferred as it has an adverse effect on the effective percentage conversion of hydrogen sulphide to sulphur in the furnace. One solution to this problem suggested in EP-A-565 316 is to employ an amine separation unit to concentrate the recycle stream in hydrogen sulphide. Such amine separation units, however, tend to be particularly costly, even if only of a small size. It is an aim of the method according to the invention to provide an alternative solution to this problem which does not necessitate a recycle. According to the present invention there is provided a method of treating a feed gas stream comprising hydrogen sulphide, comprising the steps of: (a) in at least one first furnace oxidising to sulphur dioxide a part of the hydrogen sulphide content of at least one feed stream comprising hydrogen sulphide and reacting thus formed sulphur dioxide with residual hydrogen sulphide to form sulphur vapour and water vapour; (b) withdrawing from the first furnace a partially reacted gas stream including sulphur vapour, water vapour, residual hydrogen sulphide and residual sulphur dioxide; (c) extracting in a first sulphur condenser, sulphur vapour from the partially reacted gas stream so as to form a sulphur vapour depleted gas stream; (d) catalytically reacting hydrogen sulphide in the sulphur vapour depleted gas stream with sulphur dioxide in the sulphur vapour depleted gas stream to form further sulphur vapour and extracting the further sulphur vapour so as to form a further sulphur vapour depleted gas stream; (e) reducing with hydrogen to hydrogen sulphide the sulphur dioxide and any sulphur vapour content of the further sulphur vapour depleted gas stream so as to form a reduced gas stream comprising hydrogen sulphide; (f) extracting water vapour from the reduced gas stream so as to form a water vapour depleted gas stream; and (g) feeding the water vapour depleted gas stream to at least one Claus plant for recovering sulphur from hydrogen sulphide comprising at least one further furnace for the oxidation of hydrogen sulphide to sulphur dioxide and reaction of resulting sulphur dioxide with residual hydrogen sulphide, a first further sulphur condenser, and a plurality of further stages of catalytic reaction to form sulphur vapour, there being a second further sulphur condenser downstream of each stage of said catalytic reaction, and thereby extracting yet further sulphur vapour, wherein: a gas containing at least 40% by volume of oxygen molecules (and preferably at least 80% by volume thereof) is employed as oxidant in step (a), and in step (f) the catalytic reaction is a Claus reaction between hydrogen sulphide and sulphur dioxide or a selective oxidation of hydrogen sulphide to sulphur, or both reactions. The invention also provides apparatus for performing this method comprising: a) at least one furnace for oxidising to sulphur dioxide a part of the hydrogen sulphide content of at least one feed stream comprising hydrogen sulphide and for reacting thus formed sulphur dioxide with residual hydrogen sulphide; b) a first sulphur condenser for extracting sulphur vapour from effluent partially reacted gas from the furnace so as form a sulphur vapour depleted gas stream; c) a first catalytic Claus reactor for reacting hydrogen sulphide in the sulphur vapour depleted gas stream with sulphur dioxide in the sulphur vapour depleted gas stream so as to form further sulphur vapour; d) a second sulphur condenser for extracting the further sulphur vapour; e) a catalytic hydrogenation reactor for reducing to hydrogen sulphide the sulphur dioxide and any sulphur vapour content of the gas stream from which the further sulphur vapour has been extracted so as to form a reduced gas stream; f) a condenser for extracting water vapour from the reduced gas stream; and g) at least one Claus plant for recovering sulphur from hydrogen sulphide comprising at least one further furnace for the oxidation of hydrogen sulphide to sulphur dioxide and reaction of resulting sulphur dioxide with residual hydrogen sulphide, a first further sulphur condenser, and a plurality of further stages of catalytic reaction to form sulphur vapour dioxide, there being a second further sulphur condenser downstream of each further stage of catalytic reaction, wherein in said Claus plant each further stage of catalytic reaction is adapted to perform the Claus reaction between hydrogen sulphide and sulphur dioxide, or a selective oxidation of hydrogen sulphide to sulphur, or both. The method and apparatus according to the invention make it possible to achieve a high percentage conversion of hydrogen sulphide to sulphur upstream of the hydrogenation reactor while still obtaining a water vapour depleted gas stream sufficiently concentrated in hydrogen sulphide to enable it to be readily treatable in the Claus plant. Accordingly, if the Claus plant is an existing one, very high levels of uprating, typically at least 250%, and sometimes much higher can be achieved. The first catalytic Claus reactor plays a key role in making these results possible. First, it helps in gaining a high percentage conversion of hydrogen sulphide to sulphur upstream of the hydrogenation reactor. Second, substantially no hydrogen, which is typically formed in the initial furnace, is converted to water vapour in the catalytic Claus reactor. However, the sulphur dioxide concentration is decreased therein. These effects act synergistically to provide conditions in the catalytic hydrogenation reactor which drive essentially to completion the reduction of sulphur dioxide to hydrogen sulphide therein, thereby counteracting any tendency for sulphur vapour or sulphur dioxide to breakthrough the catalytic hydrogenator while making it possible to minimise the rate at which hydrogen needs to be supplied from an external source, or eliminating altogether the need for such an external supply. (Were such breakthrough to occur, there would be deposition of sulphur vapour intermediate the catalytic hydrogenator and the water condenser or in the condenser itself.) A further advantage of the method according to the invention is that it has such a heat balance that it is able to be a net explorer of high pressure, superheated, steam. This steam may, for example be expanded in a turbine which drives an electrical generator. Accordingly, electricity may be generated. Typically, the steam for export is raised in at least one waste heat boiler associated with the furnace of step (a) of the method according to the invention. Preferably, from 80% to 90% of the hydrogen sulphide is converted to sulphur vapour upstream of the hydrogenation reaction (step (e) ) in the method according to the invention. This facilitates the attainment of a high uprating of the Claus plant. By employing an oxidant that comprises at least 80%o and preferably at least 90%o by volume of oxygen, it is possible to create particularly high flame temperatures in the first furnace. As a result of such high flame temperatures there is considerable thermal dissociation of hydrogen sulphide to form hydrogen and sulphur vapour. Some of the hydrogen may be oxidised in the furnace to form water vapour, but the effluent gas from the furnace will still contain significant quantities of hydrogen. The hydrogen is further depleted by the reverse reaction with sulphur to form again hydrogen sulphide, this reverse reaction taking place as the effluent gas stream is cooled to a temperature at which the sulphur vapour is condensed. Nevertheless, a concentration of hydrogen of at least 15%o by volume (on a dry basis, i.e. excluding water vapour) can be readily achieved at the inlet to the first catalytic Claus reactor. Such high hydrogen concentrations make it possible to operate the catalytic hydrogenator without an external supply of hydrogen. The first furnace may have a single combustion stage or two combustion stages is series. If there are two combustion stages in series there is cooling between the stages. In addition there may, if desired, be an intermediate sulphur vapour condenser, and, if desired, an intermediate reheater downstream of the intermediate sulphur condenser. Thus the two stage combustion furnace may be operated with or without intermediate sulphur condensation. All these furnaces arrangements have their respective merits and drawbacks. Having two combustion stages offers the possibility of further enhancing the conversion of hydrogen sulphide to sulphur vapour but at the cost of a higher residual sulphur dioxide concentration and a lower hydrogen concentration, both effects increasing the possibility that an external supply of hydrogen will be required in the catalytic hydrogenator. Conversion of the hydrogen sulphide to sulphur vapour can be maximised in a two combustion stage embodiment if there is intermediate sulphur condensation. If the hydrogen level falls too low an external supply of hydrogen to the catalytic hydrogenator may be employed. (In any event, it is often desirable to have an external supply of hydrogen present so as to facilitate start up of the method and apparatus according to the invention.) Preferably, the flow rate of oxygen to the first furnace or the upstream stage of a two combustion stage first furnace is in the range of M to N where M = (0.8 a+b+0.16c) and N= (a+b+0.22c) where: a is the stoichiometric rate of supply of oxygen for the complete oxidation of any ammonia to nitrogen and water vapour in the said first feed gas stream; b is the stoichiometric rate of supply of oxygen for the complete combustion of any hydrocarbon(s) present in the said first feed gas stream; and c is the stoichiometric rate of oxygen supply required for the complete oxidation to sulphur dioxide and water vapour of hydrogen sulphide in the said first feed gas stream. Conventionally, in Claus processes, oxygen for reaction with hydrogen sulphide is supplied at a rate approximately equal to 0.33c. By operating with substantially less oxygen the hydrogen sulphide to sulphur dioxide ratio in the furnace is kept up thereby favouring the reduction of sulphur dioxide to sulphur vapour. Thus, in two combustion stage examples a mole ratio of hydrogen sulphide to sulphur dioxide preferably in the range of 4:1 to 10:1 , can be obtained at the inlet to the first catalytic Claus reactor, and in single combustion stage examples a substantially high mole ratio may be achieved. Such high ratios again help to convert sulphur dioxide to sulphur in the first catalytic Claus reactor. On the other hand, the lower the rate of oxygen supply, the lower the actual conversion of hydrogen sulphide to sulphur achieved in the first furnace and the lower the flame temperature in the first furnace. Preferably the rate of supply of oxygen to the downstream combustion stage of a two combustion stage first furnace is in the range P to Q, where P= 0.8 d+e+0.16f and Q= d+e+0.22f, and d is the stoichiometric rate of oxygen required for the total oxidation to nitrogen and water vapour of any ammonia entering the downstream furnace (preferably no ammonia enters the downstream furnace); e is the rate of oxygen supply required for the total combustion to carbon dioxide and water vapour of any hydrocarbons entering the downstream furnace (preferably all the hydrocarbons are oxidised in the upstream stage); and f is the stoichiometric rate of oxygen supply required for the complete combustion of hydrogen sulphide entering the downstream furnace. Any known catalyst of the reaction between hydrogen sulphide and sulphur dioxide may be employed in the first catalytic Claus reactor. The catalytic Claus reaction of step (d) is typically performed at a temperature in the range of 200°C to 400°C, with the sulphur depleted gas stream being reheated intermediate the further sulphur condensation step and the catalytic Claus reaction step (d). Lower temperatures favour higher conversion of hydrogen sulphide to sulphur. If desired, particularly if increased percentage conversion is required, more than one stage of catalytic reaction between hydrogen sulphide and sulphur dioxide may be performed in step (d) of the method according to the invention. Operation of the first catalytic Claus reaction step and the first furnace in accordance with the preferred aspects of the method according to the present invention make it possible to achieve a sulphur dioxide concentration (on a dry basis) in the further sulphur depleted gas stream of less than 1% by volume. The combination of a high in situ hydrogen concentration at the inlet to the catalytic hydrogenator with a low sulphur dioxide concentration facilitates complete reduction of residual sulphur dioxide to hydrogen sulphide in the catalytic hydrogenation step without the need to supply hydrogen from an external source to the catalytic hydrogenator. The external hydrogen may be supplied from a source of pure hydrogen, or in a mixture with another gas or gases, for example carbon monoxide, particularly from a reducing gas generator. Any known catalyst of the reaction between hydrogen and sulphur dioxide to form water vapour and hydrogen sulphide may be used in the catalytic hydrogenation stage. The inlet temperature to the catalytic hydrogenator is preferably in the range of 200°C to 400°C. The further sulphur depleted gas stream is preferably reheated intermediate the further sulphur condensation step and the catalytic hydrogenation step. If desired, the catalytic hydrogenation step may be performed with external cooling so as to limit the size of any temperature increase that takes place as a result of the exothermic reduction reaction. The external cooling is preferably performed by adding steam to the further sulphur vapour depleted gas stream. The water condensation step is preferably performed by direct contact with the water, the reduced gas stream being cooled by indirect heat exchange intermediate the catalytic reduction step and the water condensation step. Preferably the resulting water vapour depleted gas leaves the water condensation step as a gas saturated in water vapour at a temperature in the range of 30°C to 50°C. As a result, typically at least 85% of the water vapour present in the reduced gas stream is removed in the water condensation step. Preferably, the water vapour depleted gas stream contains at least about 40%o by volume of hydrogen sulphide. Such streams are generally readily treatable in conventional Claus plants that use air to support combustion. When the hydrogen sulphide concentration is in the order of 40%> by volume some oxygen-enrichment of air used to support combustion in the further furnace or furnaces may be used, or some of the water vapour depleted gas may be by-passed to a downstream region of the further furnace or furnaces. Thus the said plant in step (g) of the method according to the invention may be a conventional Claus plant, and hence the upstream units (a) to (f) of the apparatus according to the invention may be retrofitted to a subsisting Claus plant so as to uprate it, typically by at least 250%. Particularly, if the water vapour depleted gas stream has a hydrogen sulphide concentration of less than 40% by volume of hydrogen sulphide, the feed to the said Claus plant in step (g) of the method according to the invention may be supplied with a second feed stream containing more than 40% by volume of hydrogen sulphide. Alternatively, or in addition, an oxidant containing at least 80%> by volume of oxygen, preferably at least 90% by volume of oxygen, may be used to support combustion in the further furnace or furnaces. Otherwise, combustion may be supported by air unenriched in oxygen or oxygen-enriched air (or separate streams of air and oxygen or oxygen-enriched air) containing less than 80%> by volume of oxygen. Preferably two or three stages of catalytic reaction between hydrogen sulphide and sulphur dioxide are employed in the said Claus plant employed in step (g) depending on the overall conversion required. Preferably, the said Claus plant additionally includes at its downstream end a so- called "tail gas clean up unit" which may typically include, in series, a water condenser, a unit for the reduction of sulphur dioxide to hydrogen sulphide, and a unit for absorption of hydrogen sulphide from the tail gas. The absorbent is preferably an amine which is selective for hydrogen sulphide. If in the method according to the invention a plurality of Claus piants in parallel is employed, the Claus plants may share a common tail gas clean up plant. The furnace in step (a) of the method according to the invention is preferably operated at a pressure in the range of 1 to 2 bar absolute. The further furnace is preferably operated at a similar pressure. Preferably, in order to facilitate flow of the water vapour depleted gas stream into the further furnace, a flow of steam is employed so as to raise the pressure of the reduced gas stream. Preferably the steam is added to the reduced gas stream intermediate steps (e) and (f) of the method according to the invention. Other positions are, however, possible for the addition of the steam. One or more eductors may be used for this purpose. Using such eductors makes it unnecessary to employ a fan or other rotary device for feeding the water vapour depleted gas stream into the plant or plants employed in step (g) of the method according to the invention. Further, the introduction of the steam upstream of the water vapour extraction step (e) has the result that the total water vapour content of the water vapour depleted gas stream need not be increased and therefore has no detrimental effect of the operation of the downstream Claus plant or plants. If desired, particularly if the hydrogen content of the water vapour depleted gas stream is greater than, say, 10% by volume, the hydrogen may be separated from the water vapour depleted gas stream upstream of the further Claus plant. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a generalised schematic flow diagram of a plant 2 for recovering sulphur from a gas stream containing hydrogen sulphide comprising a set 4 of retrofitted units; a main Claus plant 6, and a tail gas clean up unit 8; Figure 2 is a schematic flow diagram illustrating a configuration of units 2 for use in the plant shown in Figure 1. Figure 3 is a schematic flow diagram illustrating a first alternative configuration of units 2 for use in the plant shown in Figure 1 ; Figure 4 is a schematic flow diagram illustrating a second alternative configuration of units 2 for use in the plant shown in Figure 1 ; Figure 5 is a schematic flow diagram illustrating a third alternative configuration of units 2 for use in the plant shown in Figure 1 ; Figure 6 is a schematic flow diagram of the main Claus plant 6 shown in Figure 1 ; and Figure 7 is a schematic flow diagram of the tail gas clean up unit 8 shown in Figure 1. The drawings are not to scale. Like parts in different Figures of the drawings are indicated by the same reference numerals. Referring to Figure 1 of the drawings, a sulphur recovery plant 2 comprises in sequence an upstream set of units 4 for performing steps (a) to (f) of the method according to the invention, a main Claus plant 6 for performing step (g) of the method according to the invention, and a tail gas clean-up plant 8 for cleaning the effluent gas from the Claus plant 6. Referring now to Figure 2 of the drawings, a hydrogen sulphide containing feed gas stream typically comprising hydrogen sulphide, carbon dioxide and water vapour, and sometimes additionally including ammonia and/or one or more hydrocarbons is fed from a pipeline 12 to a burner 14 which either fires axially into a thermal reactor in the form of a refractory-lined furnace 16, through one end wall 18 thereof, or fires tangentially through a side wall at a position close to the end wall 18, typically at right angles to the axis of the furnace 16. The feed gas typically contains at least 70% by volume of combustibles. If the feed gas stream is a waste stream of an oil refinery it may be an acid gas (sometimes referred to as "amine gas"), or a mixture of amine gas with sour water stripper gas. The hydrogen sulphide containing feed gas stream is supplied to the burner 14 typically at a temperature in the range of 0°C to 90°C, preferably 10°C to 60°C, and is typically not preheated upstream of the furnace 16. The burner 14 is supplied separately from a pipeline 20 with a stream of commercially pure oxygen or a stream of air highly enriched in oxygen. In either case, the mole fraction of oxygen in the gas that is supplied along the pipeline 20 is preferably at least 0.8. Indeed, the oxygen stream can typically contain at least 90%> by volume of oxygen and may be separated from air by, for example, pressure swing adsorption or by fractional distillation, the latter separation method being able to produce oxygen at a purity in excess of 99%. A purity in excess of 99%> is particularly preferred. By operation of the burner 14 a part of the hydrogen sulphide content of the first feed gas stream is burned in the furnace 16. The rate of flow of oxygen or the oxygen content of an oxygen-enriched air along the pipeline 20 is in the range of M to N, where M = (0.8 a+b+0.16c) and N = (a+b+0.22d), and where a is the stoichiometric flow of oxygen required for the complete oxidation to nitrogen and water vapour of any ammonia present in the feed, b is the stoichiometric flow rate of oxygen required for the complete oxidation to carbon dioxide and water vapour of any hydrocarbons present in the feed, and c is the stoichiometric flow rate of oxygen required for the complete oxidation to water vapour and sulphur dioxide of the hydrogen sulphide content of the first feed gas stream. In conventional terms, therefore the burner 14 operates with a relatively oxygen poor flame. Nonetheless, high flame temperatures, typically with a core temperature of over 2000°C, can be achieved without causing the outlet temperature of the furnace 16 to exceed 1600°C. In addition to the abovementioned reactions, there is also thermal dissociation of a part of the hydrogen sulphide into hydrogen and sulphur vapour and some thermal dissociation of ammonia into hydrogen and nitrogen. Employing a combustion supporting gas rich in oxygen facilitates thermal dissociation (also known as thermal cracking) of hydrogen sulphide and ammonia particularly if high temperature zone(s) at a temperature of, over, say 2000°C are created. Various other reactions may also take place in the furnace 16 such as the formation of carbon monoxide, carbon oxysulphide and carbon disulphide. In operating the burner 14 and the furnace 16, care should of course be taken to avoid damage to the refractory lining. The angle and position of entry of the burner 14 into the furnace 16 and the flame configuration are chosen so as to avoid such damage. The thermal dissociation of hydrogen sulphide has a cooling effect which can be taken into account in selecting the position and angle of entry of the burner 14. As a result of the reactions that take place in the furnace 16, an effluent gas stream typically comprising hydrogen sulphide, sulphur dioxide, water vapour, sulphur vapour, hydrogen, carbon dioxide, carbon monoxide, argon, nitrogen and traces of carbon oxysulphide leaves the furnace 16 through an outlet 22, typically at a temperature greater than 1000°C (and preferably at a temperature greater than 1400°C). At such temperatures, some of the components of the effluent gas stream are still reacting with one another so it is difficult to specify the precise composition of the gas mixture in the outlet 22. The gas stream passes from the outlet 22 directly into a waste heat boiler 24 or other form of heat exchanger in which it is cooled to a temperature typically in the range of 300°C to 500°C. During the passage of the gas stream through the waste heat boiler 24, there is a tendency for some of the hydrogen to re-associate with sulphur vapour to form hydrogen sulphide. Cooled effluent gas stream flows from the waste heat boiler 24 and sulphur vapour is extracted from it in a condenser 26. The condensed sulphur flows along a pipeline 28 to a sulphur pit (not shown) via a sulphur seal leg (not shown). Typically some 50 to 70% of the sulphur content of the incoming hydrogen sulphide is recovered in the sulphur condenser 26. The sulphur vapour depleted gas stream leaving the sulphur condenser is characterised by a high residual hydrogen content, the hydrogen mole fraction, on a dry basis, typically being over 20%> by volume. It is also characterised by a high hydrogen sulphide to sulphur dioxide mole ratio, typically at least 4:1. The sulphur vapour depleted gas stream flows through a reheater 30 in which it is reheated from its condensation temperature (typically in the order of 130°C) to a temperature in the order of 260°C by indirect heat exchange with hot gas, e.g. superheated steam. The thus reheated sulphur vapour depleted gas stream flows to a catalytic Claus reactor 32, which may employ any conventional catalyst of the Claus reaction between hydrogen sulphide and sulphur dioxide. Further Claus reaction therefore takes place in the catalytic reactor 32 between the hydrogen sulphide and the sulphur dioxide content of the sulphur vapour depleted gas stream to form further sulphur vapour and further water vapour. The hydrogen content of the sulphur depleted gas stream does not participate in any significant reaction in the reactor 32. The resulting further reacted gas stream passes out of the reactor 32 into a second sulphur condenser 34 in which the sulphur vapour is condensed at a temperature typically in the order of 135°C. The resulting condensed sulphur flows through a pipeline 36 to the sulphur pit (not shown) via a sulphur seal leg. The extent of the further reaction is sufficient to increase the recovery of sulphur vapour to over 70%) and to reduce the concentration of sulphur dioxide in the gas stream exiting the further sulphur condenser 34 to less than 1 % by volume (on a dry basis). The gas stream exiting the second sulphur condenser 34 flows through a reheater 38 in which it is reheated to a temperature of approximately 300°C by indirect heat exchange with hot gas or direct heat exchange with a reducing gas generator (not shown). The resultant heated gas stream passes to a catalytic reduction (hydrogenation) reactor 40 in which all the sulphur dioxide and residual traces of sulphur vapour are reduced by hydrogen to hydrogen sulphide over a suitable catalyst. The catalyst may, for example, include a mixed cobalt-molybdenum oxide. In addition to the reaction between sulphur dioxide and hydrogen to form hydrogen sulphide and water vapour and the reaction between hydrogen and any sulphur vapour to form hydrogen sulphide, other reactions can take place in the catalytic reduction reactor 40. In particular, any carbon monoxide present reacts with water vapour to form hydrogen and carbon dioxide. Further, at least 90%> but not all of any carbon oxysulphide present in the reheated further sulphur vapour depleted gas stream is hydrolysed to carbon dioxide and hydrogen sulphide. Similarly, any carbon disulphide present is hydrolysed to carbon dioxide and hydrogen sulphide. As previously mentioned, the gas stream entering the catalytic reduction reactor 40 contains a large concentration of hydrogen and a small concentration of sulphur dioxide. Accordingly, there is a very large stoichiometric excess of hydrogen with the result that the reaction therebetween goes to completion. It is important to ensure that this reaction does go to completion. Otherwise there will be a tendency for sulphur to deposit in downstream parts of the plant. It is preferred to have available a pipeline 42 for the addition of external hydrogen in the event of any temporary reduction of the hydrogen concentration in the gas mixture entering the reactor 40 to a level at which complete reduction of the sulphur dioxide might be jeopardised. The external hydrogen may be generated on site by, for example, partial oxidation of hydrocarbon, preferably using pure oxygen or oxygen-enriched air as the oxidant, or, in conjunction with carbon monoxide, by a reducing gas generator using air, oxygen-enriched air, or pure oxygen as the oxidant. If desire, the catalytic reduction reactor 40 may be provided with a cooling coil (not shown) in which a coolant, e.g. steam, may be passed in the event of there being an excessive generation of heat in the catalyst, or, alternatively, steam can be added directly. The reduced gas stream, now consisting essentially of hydrogen sulphide, hydrogen, water vapour, carbon dioxide, nitrogen and argon, leaves the reactor 40 and flows through a heat exchanger 46 in which it is cooled to a temperature in the range of 100°C to 200°C (e.g. 150°C) by indirect heat exchange of water and/or steam. The thus cooled gas stream flows through one or more eductors 48 in parallel with one another. In the eductors 48 the cooled, reduced, gas stream is mixed with superheated, pressurised steam supplied via a pipeline 50. Typically, the furnace 16 is operated at a pressure in the range of 1.2 to 2 bar. The Claus plant 6 shown in Figure 1 is operated at a similar pressure. The pressure of the reduced gas stream can be raised sufficiently by this addition of stream to ensure passage of all necessary gas to the Claus plant 6 by appropriate choice of the flow rate and supply pressure (and hence temperature) of the steam in the pipeline 50. Preferably, the steam is supplied at a pressure in the range of 10 to 50 bar and a corresponding temperature greater than 100°C but less than 265°C. The reduced gas stream, having been mixed with the steam is introduced into a desuperheating, direct contact, quench tower 52. In the quench tower 52, the gas stream flows upwardly and comes into contact with descending stream of water. The reduced gas stream is thus cooled and most (preferably more than 85%) of its water vapour content is condensed, the condensate entering the descending liquid stream. The condensate includes the steam added to the reduced gas mixtures in the eductors 48. The quench tower 52 preferably contains a random or structured packing (not shown) so as to facilitate mass transfer between the ascending vapour and descending liquid. As a result, a water vapour-depleted gas stream is formed. The water exiting the bottom of the quench tower 52 is recirculated by means of a pump 54 and is cooled in a cooler 56 upstream of being reintroduced into the top of the quench tower 52. Excess water is removed through an outlet 58 and is sent to a sour water stripper (not shown) in order to remove its hydrogen sulphide content. The resulting water vapour depleted gas stream, which passes out of the tower 52 through an outlet 60, typically contains in the order of 40% hydrogen sulphide and therefore makes a suitable feedstock for treatment in a conventional Claus plant. The water vapour depleted gas streams leaves the top of the quench tower 52 typically at a temperature in the range of 30°C to 50°C and is sent to the Claus plant 6 shown in Figure 6 of the accompanying drawings. Referring to Figure 6, the water vapour depleted gas stream is received by a burner 72 firing into a further refractory- lined furnace 70. The burner 72 may also receive a second feed gas stream which may be of the same composition as or of a different composition from that of the first feed gas stream. For maximum uprating of the Claus plant 6, however, all the feed gas to it comes from the quench tower 52. The tower 72 additionally receives a stream of air, oxygen-enriched air or oxygen through a pipeline 68. In an oil refinery, there are various different strategies for selecting the composition of the first and second feed gas streams. Generally in an oil refinery, there are one or more sources of amine gas, which typically contains more than 70%> by volume of hydrogen sulphide but is free of ammonia, and one or more sources of sour water stripper gas, which typically contains approximately equal proportions of water vapour, hydrogen sulphide and ammonia. One strategy is simply to mix the amine gas with the sour water stripper gas to obtain the same composition for both the first feed gas stream and the second feed gas stream. One problem that sometimes arises in Claus plant in that of effecting complete destruction of ammonia. If the ammonia is not completely destroyed it can poison or react with downstream Claus catalysts. Particularly if air unenriched in oxygen is employed to support combustion in the further furnace 70, it is desirable that a greater proportion of the ammonia to be destroyed finds its way to the first feed gas stream rather than the second feed gas stream. This is because the relatively high oxygen mole fraction of the gas used to support combustion in the furnace 16 facilitates the creation of high flame temperatures which favour destruction of ammonia. Accordingly, it is often most preferred that all the sour water stripper gas is used in forming the first feed gas stream. Typically, some of the amine gas is mixed with the sour water gas or supplied separately there from to the burner 14 that fires into the furnace 16. Any remainder of the amine gas is typically sent to the burner 72 as the second feed gas stream. The Claus plant shown in Figure 6 may be in essence a conventional Claus plant which is operated in essentially a conventional manner. The reactions that take place in the furnace 70 are analogous to those that take place in the furnace 16 shown in Figure 2 and need not be described further herein. A resulting gas stream having essentially the same components as the effluent gas stream from the furnace 16 shown in Figure 2 leaves the furnace 70 through an outlet 74. (Typically, if a lower flame temperature is attained in the furnace 70 than in the furnace 16, the gas mixture leaving through the outlet 74 will contain a lower mole fraction of hydrogen than the corresponding gas leaving the furnace 16.) The effluent gas passes from the outlet 74 to a waste heat boiler 76 or other heat exchanger in which it is cooled by heat exchange with steam or other coolant. The resultant cooled gas stream typically leaves the waste heat boiler 76 at a temperature in the range of 250°C to 400°C. The cooled effluent gas stream passes from the waste heat boiler 76 to a further sulphur condenser 78 in which the effluent gas further cooled to a temperature in the range of 120°C to 160°C and in which the sulphur vapour is condensed and is extracted via an outlet 80. The resulting liquid sulphur is typically passed to a sulphur pit (not shown) by a sulphur seal leg. One particularly important difference between the operation of the furnace 16, on the one hand, and of the furnace 70 on the other hand, is whereas at the outlet from the sulphur condenser 26 the mole ratio of hydrogen sulphide to sulphur dioxide in the sulphur depleted gas stream is typically at least 4:1 , the corresponding ratio at the outlet from the condenser 78 is in the order of 2:1. The sulphur vapour depleted gas stream flows from the further sulphur condenser 78 through three successive catalytic Claus stages 84, 86 and 88. Each of the stages 84, 86 and 88, in accordance with the general practice in the art, comprises a train of units comprising, in sequence, 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 (e.g. a temperature in the range of 200°C to 350°C), a catalytic reactor (not shown) in which hydrogen sulphide reacts with sulphur dioxide to form sulphur vapour and water vapour, and a yet further sulphur condenser (not shown). The gas mixture leaving the most downstream catalytic stage 86 may be subjected to any one of a number of known treatments for rendering Claus process effluent suitable for discharge to the atmosphere. For example, the gas mixture may flow to the tail gas clean up plant 8 shown in Figure 7 of the accompanying drawings. With reference to Figure 7, the gas mixture may pass to a reactor 90 in which the components present in the gas mixture are subjected to hydrolysis and hydrogenation. In the reactor 90, most of the residual carbon oxysulphide and carbon disulphide are hydrolysed over a catalyst, for example alumina impregnated with cobalt and molybdenum oxides to produce hydrogen sulphide and carbon dioxide. At the same time, residual elemental sulphur and sulphur dioxide are hydrogenated to hydrogen sulphide. The hydrolysis and hydrogenation typically take place at a temperature in the range of 300°C to 350°C. A resulting gas mixture comprising of hydrogen sulphide, nitrogen, carbon dioxide, water vapour and hydrogen leaves the reactor 90 and flows first to a water condensation unit 92 and then to a separate unit 94 in which hydrogen sulphide is separated by means of absorption in a selective absorbent such as methyldiethylamine. If desired, the hydrogen sulphide may be recycled to, for example, the furnace 70. The plant shown in Figure 1 is able to achieve more than 99.5%> and typically more than 99.7%o conversion of hydrogen sulphide to sulphur. By retrofitting the units 4 shown in Figure 2 to the Claus plant 6 shown in Figure 6, its capacity may be more than doubled without any loss of conversion, or even with a gain in conversion. Referring now to Figure 3 of the accompanying drawings there is shown a first modification to the arrangement shown in Figure 2. In this modification the furnace 16 is provided with a second refractory-lined combustion stage 300 downstream of the waste heat boiler 24. A lance 302 is employed to supply further pure oxygen (or, less preferably, oxygen-enriched air containing at least 80% by volume of oxygen) to the stage 300. The same reactions take place as in the first stage. The resulting gas exits the furnace 16 through another waste heat reboiler 304 and flows to the sulphur condenser 26. The rate of supplying oxygen is typically from 16 to 22% of the stoichiometric rate required for total combustion to sulphur dioxide and water vapour of the hydrogen sulphide present in the second stage 300. The percentage conversion achieved in the second stage 300 is less than that achieved in the first stage 16 because the operating temperature tends to be lower and because the oxygen also reacts with the hydrogen present in the gas entering the second stage 300. Nonetheless, the total conversion achieved in the plant shown in Figure 3 may typically be a few percent greater than in the corresponding plant shown in Figure 2. However, the hydrogen concentration in the gas stream entering the catalytic reduction reactor 40 tends to be less in the plant shown in Figure 3 than that shown in Figure 2, as a result of the aforementioned reaction between hydrogen and oxygen in the second stage 300, and therefore care needs to be taken to maintain an adequate mole ratio of hydrogen to sulphur dioxide in the reactor 40 so as to ensure complete reduction of the sulphur dioxide (any traces of sulphur vapour present) to hydrogen sulphide. Further, the hydrogen sulphide to sulphur dioxide mole ratio in the gas leaving the sulphur condenser 26 is typically substantially lower than in the arrangement of units 4 shown in Figure 2, typically being in the range of 4:1 to 5:1 , or higher. In Figure 4, rather than having a single furnace 16 with two combustion stages, a second combustion εtage is provided in a separate refractory lined furnace 400. Accordingly there is located in sequence, intermediate the sulphur condenser 26 and the reheater 30, (i) a further reheater 402 which raises the temperature of the sulphur-depleted gas stream to a temperature in the order of 500°C such that autogenous combustion of hydrogen sulphide can take place, (ii) the furnace 400 itself, the reheated gas stream being introduced via a burner 404 which fires into the furnace 400; and (iii) a further waste heat boiler 406 which has an inlet communicating with an outlet (not shown) from the furnace 400, and an outlet communicating with (iv) a further sulphur condenser 408 with an outlet pipeline 410 for condensed sulphur leading via sulphur seal leg to the sulphur pit (not shown). Oxygen-rich gas containing at least 80%, and preferably at least 90%o by volume of oxygen is fed to the lance 404 via a pipeline 412. The same reactions take place in the furnace 400 as in the furnace 16. Similarly to the arrangement shown in Figure 3, the rate of supplying oxygen to the lance 502 is typically from 16 to 22% of the stoichiometric rate required for total combustion to sulphur dioxide and water vapour of the hydrogen sulphide in the feed to the furnace 400. As a result of the condensation of the sulphur vapour in the sulphur condenser 26, there tends to be a greater conversion of hydrogen sulphide to sulphur in the furnace 400 than there is in the second stage 300 of the furnace 16 shown in Figure 3 of the accompanying drawings. Similarly to the plant shown in Figure 3, because of reaction between hydrogen and oxygen in the furnace 400, the hydrogen concentration in the gas stream entering the catalytic hydrogenation reactor 40 tends to be reduced in comparison with the plant shown in Figure 2, and therefore care again needs to be taken to maintain an adequate mole ratio of hydrogen sulphide to sulphur dioxide in the rector 40 so as to ensure complete reduction of the sulphur dioxide (and any traces of sulphur vapour present) to hydrogen sulphide. The further reacted gas stream flows from the furnace 400 to the further waste heat boiler 406 whose operation is similar to that of the waste heat boiler 24. The resulting cooled gas flows to the further sulphur condenser 408 whose operation is similar to that of the sulphur condenser 26. Condensed sulphur vapour flows to the sulphur seal pit (not shown) via the pipeline 410. The plant shown in Figure 5 of the accompanying drawings and its operation are essentially the same as that shown in Figure 4 with the exception that the reheater 402 is omitted. As a result a burner 502 is used instead of the lance 404. The burner 502 typically has its own ignition and flame detection systems. Although, referring again to Figure 1 , the series of units 2 has been described as being retrofitted to the Claus plant 4 and the tail gas clean up plant 6, the entire plant may be assembled at the same time. The method and apparatus according to the present invention is now illustrated by the following examples: Referring again to Figure 1 , a subsisting Claus plant 4 and associated tail gas clean up plant 6 treat a feed gas stream having the following composition. 71.78 mole % H 2 S 14.44 mole %> H 2 O 11.11 mole % NH 3 2.00 mole % C0 2 0.66 mole % C 2 H 6 This feed gas stream is formed by mixing together amine gas with sour water stripper gas in the ratio of two parts by volume of the former to one part by volume of the latter. The feed gas stream is supplied to the Claus plant 4 at a rate of 100 kmol/hr. This requires an air supply at a rate 202.73 kmol/hr. Accordingly, the furnace of the Claus plant has a volume sufficient to be able to receive a total of 302.73 kmol/hour. Three computer simulations (using a SULSIM 5 program) were performed in order to evaluate uprating of the Claus plant by passing the feed gas into each of the following: A) a set 2 of units as shown in Figure 2 of the accompanying drawings; B) a set 2 of units as shown in Figure 3 of the accompanying drawings; C) a plant according to Figure 4 of the drawings accompanying EP-A-565 316 (i.e. a plant similar to Figure 2 of the accompanying drawings but omitting the first catalytic Claus reactor and associated sulphur condenser). The simulations were performed assuming that the oxygen feed to the first furnace was 100%) pure. The results of the simulations are shown in Table 1 below in which all flow rates are in kmol/hr. Based on previous practical experience of the operation of oxygen- enhanced Claus furnaces, it is believed that the programme underestimates the percentage conversion achieved in such furnaces and therefore overestimates the requirement for external hydrogen in the catalytic hydrogenator. The results set out in Table 1 should therefore be taken as being confirmative of the operability of the examples of the method according to the invention rather than giving accurate results. Table 1 These results show that the two examples of the method according to the invention are able to achieve lower exit gas flow rates than the example of the method according to EP-A-565 316, thereby making possible a greater degree of uprating. Further, the exit gas in Example A has a substantial content of hydrogen which makes it a possible source of a hydrogen product were it to be separated. Such separation would substantially enhance the degree of uprating that could be achieved. Further, in example A no supply of external hydrogen is required. Example B is also particularly advantageous in that the exit gas flow rate is substantially less than that of example A. Based on the above results, the maximum feed rates of the acid gas to the retrofitted units 2 was calculated. The results of the calculations are set out below: A 300 kmol/hr B 545 kmol/hr C 275 kmol/hr Thus, the method A according to the invention makes possible a threefold uprating and the method B more than a fivefold uprating of the Claus plant 4. Even greater upratings are made possible if the Claus plant 4 is converted to operate in accordance with EP-A-0 237 217 in which its single combustion furnace is replaced with two combustion furnaces in series, in both of which oxygen is employed to support combustion. Now the maximum feed rates of acid gas become A 640 kmol/hr B 1030 kmol/hr C 600 kmol/hr A further advantage of employing the first catalytic Claus reactor is as follows. In operation, the composition of a feeds to a Claus plant can be subjected to a sudden surge in its concentration of hydrocarbons. As a result the sulphur dioxide mole fraction in the gas exiting the first furnace tends to fluctuate. Control systems which rely on real time measurements of sulphur dioxide concentrations (or hydrogen sulphide to sulphur dioxide ratios) compensate for the surge in hydrocarbon concentration by increasing the rate of flow of oxygen into the first furnace. The surge tends to decay from its peak more rapidly than the ability of the control system to respond to it. There is therefore a sudden increase in the sulphur dioxide concentration in the exit gas from the first furnace. In the absence of the first catalytic Claus reactor, the catalytic hydrogenator is exposed to this surge with the result that there is a heightened risk of breakthrough of sulphur dioxide and/or sulphur vapour from this reactor with deposition of sulphur in downstream parts of the plant. The first catalytic Claus reactor acts, however, as a buffer which is able substantially to reduce the sulphur dioxide concentration upstream of the catalytic hydrogenation reactor. Various changes and modifications to the plant show in the drawings may be made. For example, a considerate simplification of the plant 4 shown in Figure 3, may be achieved by employing a plurality of passes in the waste heat boiler 24. As a result the gas from the first pass can enter into a chamber (not shown) in which the lance 302 is positioned. This chamber thus forms the second stage 300 of the Claus furnace 16. Hot gas flows from the chamber through the second pass of the waste heat boiler 24 and downstream thereof proceeds directly to the sulphur condenser 26. As a result, the waste heat boiler 304 can be omitted altogether. In another modification, the eductor or eductors 48 can be omitted and the furnace 70 in the Claus plant 6 operated at a sufficiently lower pressure than the furnace 16 to obviate the need for a fan instead. A lower operating pressure can be created by reducing the total gas flow to the furnace 70 on retrofitting the units 4. This is particularly easy to do if the furnace 70 was before the retro-fit operated with air as an oxidant. If oxygen is substituted for at least some of the air, the flow rate of oxidant may be reduced by an amount necessary to give the desired lower operating pressure. In a yet further modification, the eductor or eductors 48 can be omitted from the arrangements shown in Figures 2 to 5 and redeployed intermediate the reactor 90 and the condenser 92 in the tail gas clean up plant shown in Figure 7. The examples above have shown how large upratings of a Claus plant can be achieved. It should be noted that these upratings can be achieved without loss of conversion efficiency. There are other examples of methods and apparatuses according to the invention which are particularly useful if the plant shown in Figure 1 does not have a tail gas clean up unit 8 and which include providing a second (or a second and a third stage) of catalytic Claus reaction in any of the arrangements shown in any one of Figures 2 to 5. Each additional stage of catalytic Claus reaction is followed by a sulphur condenser. There is also reheating of the gas stream downstream of each such sulphur condenser. In one example of this kind of plant the arrangement 4 may make use of parts of a second subsidiary Claus plant. For example, two subsisting plants in parallel, both employing three catalytic stages of Claus reaction, may be converted into single plant with the reheater associated with the most downstream of the catalytic Claus stage of one subsisting plant feeding a train of retrofitted intermediate stages comprising the catalytic hydrogenator 40, the heat exchanger 46 and the direct contact water condenser 52. The gas stream issuing from the outlet 60 of the direct contact water condenser 52 passes to the other subsisting Claus plant. A further improvement is to remove the catalyst from the most downstream of the catalytic reactors of what is to become the plant feeding the retrofitted stages and to replace the catalyst with a hydrogenation catalyst so that this stage now becomes a catalytic hydrogenation unit. This expedient therefore reduces the amount of new equipment that needs to be retrofitted. Various other modifications may typically need to be made including removal of any redundant unit, and, if desired, fitting of a second combustion stage and ancillary equipment at the front end of the modified plant. Typically most or all of the hydrogen sulphide will be sent to the upstream furnace of the modified plant. By such means it is possible to convert two plants each having a percentage conversion of, say, 97.5% to a single plant having a greater capacity than the combined capacity of the two plant and a percentage conversion of more than 99%>. Finally, a catalytic selective oxidation stage in which hydrogen sulphide is selectively reacted with oxygen to form sulphur vapour and water vapour over a selective oxidation catalyst can be substituted for or added to any of the catalytic Claus stages in the method and apparatus according to the present invention, particularly the most downstream catalytic Claus stage in the plant 6 shown in Figure 6 of the accompanying drawings. Selective oxidation catalysts are well known in the art. Such a selective oxidation stage may be particularly useful as the final catalytic stage in the plant 6 if the tail gas clean up unit 8 is omitted from the arrangement shown in Figure 1 because it helps to enhance the conversion of hydrogen sulphide to sulphur.

Claims: CLAIMS 1. A method of treating a feed gas stream comprising hydrogen sulphide, comprising the steps of: (a) in at least one first furnace oxidising to sulphur dioxide a part of the hydrogen sulphide content of at least one feed stream comprising hydrogen sulphide and reacting thus formed sulphur dioxide with residual hydrogen sulphide to form sulphur vapour and water vapour; (b) withdrawing from the first furnace a partially reacted gas stream including sulphur vapour, water vapour, residual hydrogen sulphide and residual sulphur dioxide; (c) extracting in a first sulphur condenser, sulphur vapour from the partially reacted gas stream so as to form a sulphur vapour depleted gas stream; (d) catalytically reacting hydrogen sulphide in the sulphur vapour depleted gas stream with sulphur dioxide in the sulphur vapour depleted gas stream to form further sulphur vapour and extracting the further sulphur in a second sulphur condenser so as to form a further sulphur vapour depleted gas stream; (e) reducing with hydrogen to hydrogen sulphide the sulphur dioxide and any sulphur vapour content of the further sulphur vapour depleted gas stream so as to form a reduced gas stream comprising hydrogen sulphide; (f) extracting water vapour from the reduced gas stream so as to form a water vapour depleted gas stream; and (g) feeding the water vapour depleted gas stream to at least one Claus plant for recovering sulphur from hydrogen sulphide comprising at least one further furnace for the oxidation of hydrogen sulphide to sulphur dioxide and reaction of resulting sulphur dioxide with residual hydrogen sulphide, a first further sulphur condenser, and a plurality of further stages of catalytic reaction to form sulphur vapour, there being a second further sulphur condenser downstream of each stage of said catalytic Claus reaction, and thereby extracting yet further sulphur vapour, wherein: a gas containing at least 40% by volume of oxygen molecules is employed as oxidant in step (a), and in step (f) the catalytic reaction is a Claus reaction between hydrogen sulphide and sulphur dioxide, or a selective oxidation of hydrogen sulphide to sulphur, or both. A method according to claim 1 , wherein from 80%o to 90% of the hydrogen sulphide is converted to sulphur vapour upstream of the hydrogenation reaction of step (e). A method according to claim 1 or claim 2, wherein the oxidant in step (a) comprises at least 90%> by volume of oxygen. A method according to any one of the preceding claims, wherein the concentration of hydrogen in the further sulphur vapour depleted gas stream is at least 15% on a dry basis. 5. A method according to any one of the preceding claims, wherein the first furnace has two combustion stages with cooling of gas flowing between the stages. 6. A method according to claim 5, wherein sulphur is extracted by condensation intermediate the two combustion stages. 7. A method according to claim 5 or claim 6, wherein the flow rate of oxygen to the upstream combustion stage of the first furnace is in the range of M to N where M = (0.8 a+b+0.16c) and N = (a+b+0.22c) where: a. is the stoichiometric rate of supply of oxygen for the complete oxidation of any ammonia to nitrogen and water vapour in the said first feed gas stream; b. is the stoichiometric rate of supply of oxygen for the complete combustion of any hydrocarbon(s) present in the said first feed gas stream; and c. is the stoichiometric rate of oxygen supply required for the complete oxidation to sulphure dioxide and water vapour of hydrogen sulphide in the said first feed gas stream. 8. A method according to any of claims 5 to 7, wherein the mole ratio of hydrogen sulphide to sulphur dioxide is in the range of 4: 1 to 10:1 at the inlet to the first catalytic Claus reactor. 9. A method according to any one of claims 5 to 8, wherein the rate of supply of oxygen to the downstream combustion stage of the first furnace is in the range P to Q, where P = 0.8 d+e+0.16f and Q= d+e+0.22f, and d is the stoichiometric rate of oxygen required for the total oxidation to nitrogen and water vapour of any ammonia entering the downstream combination stage. e. is the rate of oxygen supply required for the total combustion to carbon dioxide and water vapour of any hydrocarbons entering the downstream combustion stage. f. is the stoichiometric rate of oxygen supply required for the complete combustion of hydrogen sulphide entering the downstream combustion stage. 10. A method according to any one of claims 5 to 9, wherein the sulphur dioxide concentration (on a dry basis) in the sulphur depleted gas stream is less than 1 % by volume. 11. A method according to any one of the preceding claims wherein a second feed gas stream comprising hydrogen sulphide is introduced into the further furnace in addition to the water vapour depleted gas stream. 12. Apparatus for performing the method according to claim 1 comprising: (a) at least one furnace for oxidising to sulphur dioxide a part of the hydrogen sulphide content of at least one feed stream comprising hydrogen sulphide and for reacting thus formed sulphur dioxide with residual hydrogen sulphide; (b) a first sulphur condenser for extracting sulphur vapour from effluent partially reacted gas from the furnace so as form a sulphur vapour depleted gas stream; (c) a first catalytic Claus reactor for reacting hydrogen sulphide in the sulphur vapour depleted gas stream with sulphur dioxide in the sulphur vapour depleted gas stream so as to form further sulphur vapour; (d) a second sulphur condenser for extracting the further sulphur vapour; (e) a catalytic hydrogenation reactor for reducing to hydrogen sulphide the sulphur dioxide and any sulphur vapour content of the gas stream from which the further sulphur vapour has been extracted so as to form a reduced gas stream; (f) a condenser for extracting water vapour from the reduced gas stream; and (g) at least one Claus plant for recovering sulphur from hydrogen sulphide comprising at least one further furnace for the oxidation of hydrogen sulphide to sulphur dioxide and reaction of resulting sulphur dioxide with residual hydrogen sulphide, a first further sulphur condenser, and a plurality of further stages of catalytic reaction to form sulphur vapour, there being a second further sulphur condenser downstream of each further stage of catalytic reaction, wherein in said Claus plant each further stage of catalytic reaction is adapted to perform the Claus reaction between hydrogen sulphide and sulphide dioxide, or a selective oxidation of hydrogen sulphide to sulphur, or both.

Inventors: GRAVILLE STEPHEN RHYS, WATSON RICHARD WILLIAM, GRAVILLE, STEPHEN, RHYS, WATSON, RICHARD, WILLIAM
Applicants: BOC GROUP PLC, GRAVILLE STEPHEN RHYS, WATSON RICHARD WILLIAM, GRAVILLE, STEPHEN, RHYS, THE BOC GROUP PLC, WATSON, RICHARD, WILLIAM
Application year: 2001
Main classification: B01D53
IPC codes: B01D53, C01B17