MXPA01010057A - Treatment of combustible gas streams containing hydrogen sulphide - Google Patents

Abstract

A first combustible gas stream containing hydrogen sulphide is subjected to treatment in a first Claus plant including a first thermal Claus stage. Part of the hydrogen sulphide content of a second combustible gas stream containing hydrogen sulphide is burned in at least one further thermal Claus stage. The combustion is supported by oxygen-enriched air having an oxygen mole fraction of at least about 0.25 or by oxygen. Resulting sulphur dioxide reacts with residual hydrogen sulphide to form sulphide vapour which is condensed out of the effluent gas from the further thermal Claus stage to form a sulphur-depleted effluent gas stream. A first control signal is generated which is a function of the flow rate of the second gas. A second control signal which is a function of the hydrogen sulphide/sulphur dioxide mole ratio in the sulphur-depleted effluent stream is also generated. The control signals are employed in setting the rate at which the combustion-supporting gas is supplied to the second thermal Claus stage.

Description

TREATMENT OF COOKING GAS CURRENTS CONTAINING HIDROGEN SULFIDE.

BACKGROUND OF THE I NVENTION This invention relates to a method and apparatus for treating fuel gas streams containing hydrogen sulfide. Gas streams containing hydrogen sulfide (sometimes referred to as "acid gas streams") are typically formed in petroleum refineries and natural gas processing units. These currents should not be vented directly into the atmosphere since hydrogen sulfide is poisonous. A conventional method for treating the gas stream containing hydrogen sulfide (which, if desired, has been previously concentrated) is the Claus process. In this process a part of the hydrogen sulfide content of the gas stream is subjected to combustion in a thermal stage that takes the form of a furnace to form sulfur dioxide. The sulfur dioxide then reacts in the furnace with the residual hydrogen sulphide to form the sulfur vapor. The reaction between hydrogen sulfide and sulfur dioxide does not reach its completion. The effluent gas stream from the furnace is cooled and the sulfur is extracted, commonly by condensation, from the cooled effluent gas stream. The resulting gas stream, which still contains residual hydrogen sulphide and sulfur dioxide, passes through a series of steps in which the catalyzed reaction between the residual hydrogen sulfide and the residual sulfur dioxide takes place. The resulting sulfur vapor is extracted downstream of each stage. The effluent gas from most of the downstream of the sulfur extractions can be burned or subjected to further treatment, for example by SCOT or Beavon process, in order to form a gas stream that can be safely vented to the atmosphere . The air can be used to support the combustion of hydrogen sulfide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which of course is present in the air that supports combustion) flow through the process and therefore place a maximum limit on the speed at which which gas stream containing hydrogen sulfide can be treated in an oven of a certain size. This maximum limit can be raised using commercially produced oxygen or oxygen enriched air to support the combustion of hydrogen sulphide. In general, depending on the concentration of the gas stream containing hydrogen sulfide, the supply of commercial pure oxygen instead of air will result in the creation of excessive temperatures in the furnace which are prone to cause damage, particularly to the refractory lining from the oven. Various methods are known to increase the degree of air enrichment in oxygen without creating excessive temperatures. For example, U.S. Patent Application No. 2, 173, 780 A describes moderation of the temperature by introducing liquid water into the flame zone of the furnace. US Pat. No. 5,352,433 describes a particularly advantageous process in which the capacity or yield of a Claus process is increased by driving the combustion of hydrogen sulphide in two separate furnaces. Consequently, the general amount of heat generated by combustion is assigned between the two ovens in the need to employ an external or recycled temperature moderator. Therefore, a higher degree of classification can be achieved than with other methods. Typically, when the combustion of hydrogen sulphide takes place in two separate furnaces, it is possible to operate a process by reconversion of an additional furnace and the appropriate heat exchange equipment for an existing plant. It is an object of the present invention to provide a method and apparatus for recovering sulfur from gas streams containing hydrogen sulfide, which is flexible in operation, which can be easily controlled and which still offers at least some of the Advantages of operation with commercially pure oxygen or air enriched with oxygen.

BRIEF DESCRIPTION OF THE INVENTION According to the present invention there is provided a method of treating a plurality of fuel gas streams containing hydrogen sulfide, comprising the steps of: (a) operating to recover the sulfur from a first fuel gas stream that contains hydrogen sulfide, a first Claus plant having a sequence of stages including, sequentially, a first thermal stage Claus, a first sulfur condenser, and at least one sub-series of stages including a catalytic Claus stage and a second sulfur condenser current below it; (b) burning in at least one additional thermal Claus stage part of the hydrogen sulfide content of a second fuel gas stream; (c) supplying to the additional thermal stage Claus, a combustion support gas having a molar fraction of oxygen of at least 0.25 to withstand the combustion of the hydrogen sulfide therein, the gas that supports the combustion that is formed from a pure or impure oxygen stream separated from the air or from a mixture of an air stream with said stream of pure or impure oxygen; (d) removing an effluent gas stream containing hydrogen sulphide, sulfur dioxide, water vapor and sulfur vapor from the additional Claus thermal stage and removing the sulfur vapor from the effluent gas stream in an additional sulfur condenser to form an effluent gas stream devoid of sulfur; (e) mixing the stream of effluent gas devoid of sulfur with the first fuel gas undergoing treatment in the first Claus plant in a region of the same stream below the first stage Claus thermal and upstream of the start of the reaction Claus catalytic in said subsequence; (f) generating a first control signal that is a function of the flow velocity of the second fuel gas or at least one fuel component thereof within the additional thermal Claus stage; (g) generating a second control signal that is a function of the molar ratio of hydrogen sulfide / sulfur dioxide in the effluent gas stream devoid of sulfur; and (h) employing the control signals in setting the speed at which the combustion support gas is supplied to the additional thermal Claus stage. The invention also provides the plant for treating a plurality of fuel gas streams containing hydrogen sulfide, comprising: a) a first Claus plant for the recovery of sulfur from a first fuel gas stream containing hydrogen sulfide which has a sequence of steps including, in sequenced form, a first thermal stage Claus, a first sulfur condenser, and at least one sub-sequence of steps including a catalytic Claus reaction stage and a second sulfur condenser; b) at least one additional thermal Claus stage for combustion of part of the hydrogen sulfide content of a second fuel gas stream having hydrogen sulphide; c) at least one input to the additional thermal Claus stage for a gas that supports combustion having a molar fraction of oxygen of at least 0.25, the gas that supports combustion that is formed of a stream of pure or impure oxygen separated from the air or a mixture of an air stream with the same pure or impure oxygen stream; d) an output from the additional thermal Claus stage for an effluent gas stream containing hydrogen sulfide, sulfur dioxide, water vapor and sulfur vapor; e) an additional condenser for extracting sulfur vapor from the effluent gas stream, to form an effluent gas stream devoid of sulfur having an inlet communicating with the outlet from the additional thermal Claus stage; f) an outlet for the sulfur-free effluent gas stream that communicates with the region of the first Claus plant below the first thermal stage and upstream where the catalytic Claus reaction starts; g) means for generating a first control signal which is a function of the flow velocity of the second fuel gas stream or at least one of the fuel components thereof within said additional clause stage; h) means for generating a second control signal that is a function of the molar ratio of hydrogen sulfide to sulfur dioxide in the effluent gas devoid of sulfur; and i) responsible means for controlling the signals to establish the speed at which the gas supporting the combustion is supplied to the additional thermal Claus reaction stage. The method and plant according to the present invention offers a number of advantages. First, the production rate of the fuel gas containing hydrogen sulfide can be substantially higher than in a comparable plant in which the additional thermal Claus stage and the additional sulfur condenser are omitted. Second, the generation of the second control signal that is a function of the ratio of hydrogen sulfide to sulfur dioxide helps to achieve stable control of the method and apparatus according to the invention. Thirdly, the additional thermal Claus stage and the additional sulfur condenser can easily be reconverted to a previously established Claus plant or plants without the need to make substantial alterations to said plant and the process control equipment used therewith. In fact, previously existing plants can operate exactly as before the addition of the new equipment. Fourth, the additional thermal Claus stage can be used to supply one or more separate Claus plants with the effluent gas mixture lacking in sulfur. If the gas supporting the combustion is formed as a mixture of the first air stream and the second stream of pure or impure oxygen separated from the air, the first and second streams may be mixed in situ within the thermal Claus stage. Mixing does not need to be perfect. The molar fraction of oxygen in the gas that supports combustion is preferably at least 0.7. The supply of a gas that supports oxygen-rich combustion to an additional thermal stage Claus can tend to cause an excessive temperature, particularly, if the second fuel gas mixture containing hydrogen sulfide has a high mole fraction of hydrogen sulfide ( for example greater than about 0.7). Said tendency can be counteracted by supplying said additional thermal Claus with a moderating fluid. This moderator fluid can, for example, be liquid carbon dioxide, a recycle stream taken from the downstream of the additional sulfur condenser or the sulfur dioxide taken from a separate source. However, two additional thermal Claus steps in series are preferably used to limit the amount of combustion of the hydrogen sulfide that occurs at each individual stage. Preferably intermediate heat exchanger means exist between the two additional thermal Claus stages. Preferably, the intermediate heat exchanger means is a recovery boiler. If desired, an intermediate sulfur condenser downstream of the intermediate heat exchanger means may be employed although upstream of the two additional thermal Claus stages downstream. The second fuel gas stream may have the same composition as the first fuel gas stream containing hydrogen sulfide or may have a different composition therefrom. For example, the second fuel gas stream may have ammonia in addition to hydrogen sulfide, whereas the first fuel gas stream may be essentially free of ammonia. The use of a gas that supports combustion that has a mole fraction of at least about 0.7 in the additional thermal Claus stage (s) makes it possible to create a combustion regime therein with at least one flame zone with a high localized temperature particularly suitable for ammonia destruction. The effluent gas stream devoid of sulfur is preferably introduced into the first fuel gas mixture in a region of the first Claus plant downstream of the first sulfur condenser and upstream of any reheater that forms part of the first or only the subsequence of stages. Nevertheless, it is possible to conduct the mixing of the two gas streams in a different location. For example, the effluent gas stream devoid of sulfur can be mixed with the first fuel gas stream in a region just upstream of the first sulfur condenser. The means for generating the first control signal typically comprises a flow meter for measuring the flow velocity of the gas that supports combustion within the additional Claus stage operatively associated with a first valve controller. The means for generating the second control signal preferably comprises an analyzer, typically of the infrared type, which is capable of measuring the concentrations of hydrogen sulfide and sulfur dioxide, and means for calculating the molar ratio of hydrogen sulfide to dioxide sulfur, and means for comparing the calculated molar ratio of hydrogen sulphide to sulfur dioxide with a pre-set desired ratio. Any difference between them is used as the second control signal to adjust the flow of the gas that supports the combustion to the additional thermal stage Claus or stages. Preferably, the speed at which the combustion support gas is delivered to the additional thermal Claus stage (s) is controlled primarily by means of the generation of the first control signal. For this purpose at least a major or important part of the total flow of the gas supporting the combustion passes through at least a first flow control valve operatively associated with the means for generating the first control signal. If the gas that supports combustion is a pure or impure oxygen stream separated from the air, there may be a single main flow control valve. If, on the other hand, the gas supporting the combustion is formed from a mixture of an air stream with a pure or impure oxygen stream separated from the air, there may be a first main flow control valve located in a first conduit of the air stream and a second main flow control valve located in a second conduit of the air stream. If desired, the primary control can be improved by analyzing the second fuel gas stream. The second control signal provides precise tuning for the primary control of the flow rate of the gas that supports combustion for the additional thermal Claus stage (s). For this purpose, preferably there is at least one secondary (or compensating) control valve in parallel with the main control valve, the secondary control valve which is operatively associated with the means for generating the second control signal. Commonly, only a minor part of the gas that supports combustion flows through each secondary control valve. Any one of the different variants of this control strategy may be adopted depending on whether the gas supporting the combustion is supplied in an individual stream or a plurality of streams to the additional thermal Claus stage (s). If such a supply is in the form of an individual current, there may be an individual main control valve and a part of the current may bypass the main control valve and flow through the secondary control valve. Alternatively, if the gas supporting the combustion is supplied in a plurality of streams, each stream may have its own main control valve, and at least one of the streams may deviate from its associated main control valve by flowing through the stream. secondary control valve. Alternative control strategies can be used with the first and second control signals. For example, there may be an individual control valve and the control apparatus having a set point, the second control signal that is used to reset this point. In another alternative there may be a conduit to supply the air to the additional Claus stage and another conduit to supply the pure or impure oxygen, from it to a main flow control valve in each conduit and the means for generating the second control signal may be placed to obtain any main flow control valve. In general, it is preferred to use a second valve or compensator since difficulties may arise in the scope of fine control with minor adjustments to a relatively large individual control valve. Preferably, there is a similar arrangement of flow control valves for controlling the flow of air or air enriched with oxygen to the thermal stage Claus of the first Claus plant.

Thus, a flow control valve through which a smaller part of the total air passes or the oxygen enriched air flows into the first thermal stage Claus which preferably responds to signals generated by an analyzer having a detector or detectors located current below all the subsequences while another flow control valve through which the main flow of air or oxygen enriched air passes is preferably set according to the expected flow of the first fuel gas towards the first Claus plant and can be adjusted in response to any deviation detected from a specified mole fraction and / or hydrogen sulphide flow. Preferably, the method according to the invention includes the operation of at least one second Claus plant for the recovery of the sulfur from at least a third stream of fuel gas containing hydrogen sulfide having a sequence of steps including sequentially, a first thermal stage Claus, a first sulfur condenser, and at least one subsequence of stages including a catalytic reaction step Claus and a second sulfur condenser. During normal operation, only part of the sulfur-free effluent gas stream is mixed with the first fuel gas stream, the remainder of the sulfur-free effluent gas stream mixing with the third fuel gas stream in a region downstream of the first thermal stage Claus thereof and upstream of the start of the catalytic Claus reaction therein. An advantage of this arrangement is that the production of sulfur can continue when any of the first Claus plant, the second Claus plant and the additional thermal Claus stage is deactivated for routine maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS The method and plant according to the invention will now be described in exemplary form with reference to the accompanying drawings, by means of which Figures 1 to 3 are respectively schematic flow diagrams of a first, a second and a third example of the plant for treating a plurality of fuel gas streams containing hydrogen sulfide. The drawings are not to scale. For ease of illustration, several flow control valves and shutoff valves and other equipment have been omitted from the drawings. Similar parts shown in two or more of the drawings are indicated by the same reference number.

DETAILED DESCRIPTION In Figure 1 of the drawings, a first Claus 2 plant, a second Claus 4 plant and the additional equipment 6 for the recovery of sulfur from the gas stream containing hydrogen sulfide are illustrated. A first stream of fuel gas containing hydrogen sulfide is introduced into the first Claus plant through a pipeline 8. In an oil refinery, the source of the first fuel gas stream containing hydrogen sulfide can be a source of the fuel gas. called "amine gas" which typically contains more than 80% by volume of hydrogen sulphide (with most of the remainder being carbon dioxide) or a source of the so-called "sulfur water purge gas" which contains approximately 20 up to about 35% by volume of hydrogen sulfide and from about 30 to about 45% by volume of ammonia, with the remainder including water vapor and carbon dioxide. In another example the first fuel gas stream containing hydrogen sulfide is a mixture of sulphurous water trap gas and amine gas. The first fuel gas mixture containing hydrogen sulfide flows from the pipe 8 into a burner 10 which can be a mixed tip type ignition inside a furnace 12 which constitutes the thermal stage of the Claus plant. In order to withstand the combustion of the fuel components of the first gas mixture containing hydrogen sulfide, an air stream or air enriched with oxygen is fed through a pipe 16 to the burner 10. The relative flow velocities of the air or air enriched with oxygen and the first stream of fuel gas containing hydrogen sulfide are such that the burner 10 receives approximately 2 mols of hydrogen sulfide per each mole of oxygen. Accordingly, sufficient oxygen is supplied to support the combustion of about one third of the total flow rate of the hydrogen sulfide molecules in the burner 10. Sufficient additional oxygen molecules are also supplied to ensure total combustion of any ammonia or hydrocarbons present in the first fuel gas mixture containing hydrogen sulfide. The combustion of the hydrogen sulfide in the furnace 12 forms water vapor and sulfur oxide. The sulfur dioxide reacts within the furnace 12 with the residual hydrogen sulfide to form occasional steam and sulfur vapor. Also, other chemical reactions take place. For example, some thermal dissociation of hydrogen sulfide in hydrogen and sulfur also occurs. Various other reactions take place depending on the particular operating conditions in the furnace 12. For example, carbon monoxide (itself formed by thermal dissociation of carbon dioxide or by reaction of carbon dioxide with hydrogen sulfide) reacts with steam of sulfur to form carbon oxysulfide. Carbon disulfide can also be formed.

A gas mixture containing hydrogen sulphide, sulfur dioxide, sulfur vapor, water vapor, carbon dioxide, hydrogen and carbon monoxide and also including traces of carbon oxysulfide and carbon disulfide, flows out of the furnace. in a recovery boiler 14 in which it is typically cooled to a temperature in the range of about 250 ° C to about 350 ° C. The gas mixture thus cooled flows from the recovery boiler 14 into a sulfur condenser in which it is further cooled in common form to a temperature in the range of about 110 ° C to about 180 ° C. The sulfur condenser 18 also condenses at least part of the sulfur vapor in the gas mixture. The resulting condensate is passed into a sulfur closure cavity not shown. Because the Claus reaction between the hydrogen sulfide and the sulfur dioxide does not reach completion, the gas mixture leaving the condenser 18 contains appreciable proportions of sulfur dioxide and hydrogen sulfide. In order to extract the additional sulfide therefrom, the gas mixture is passed through a first subsequence 20 of steps which include the catalytic Claus reaction between the hydrogen sulfide and the sulfur dioxide and a second similar subsequence of stage that includes the catalytic reaction between hydrogen sulfide and sulfur dioxide. Subsequence 20 has, sequentially, a reheater in which the gas mixture is reheated to a temperature commonly in the range of about 200 ° C to about 250 ° C. From the superheater 24, the gas mixture passes through a first catalytic reactor Claus 26 in which the reaction between the hydrogen sulfide and the sulfur dioxide takes place on a catalyst, for example, activated alumina. As a result, additional sulfur vapor and water vapor are formed. The resulting gas mixture flows into the catalytic reactor 26 into another sulfur condenser 28 in which it is cooled to a temperature commonly in the range of about 1 10 ° C to about 150 ° C and the sulfur vapor formed in the condenser condenses. catalytic reactor. The resulting condensate is passed into the sulfur closure cavity (not shown). The sulfur-free gas mixture passes into the second sequence 22. The second sequence consists, sequentially, of an additional reboiler 30, an additional catalytic Claus reactor 32 and an additional sulfur condenser 34. The operation of these units is analogous to that of the respective units in the first sequence 20. The gas mixture leaving the additional sulfur condenser 34 may, depending on its concentration of residual sulfur compounds, be sent to an incinerator (not shown) and discharged into the atmosphere . Alternatively, the gas mixture can pass to a hydrolysis reactor (not shown) in which the present components of the gas mixture are subjected to hydrolysis and hydrogenation. In the hydrolysis reactor, the residual carbon oxysulfide and the carbon disulfide are hydrolyzed with steam to produce hydrogen sulfide on a catalyst, for example alumina impregnated with cobalt and molybdenum. Such catalysts are well known in the art. At the same time, residual elemental sulfur and sulfur dioxide are hydrogenated to form sulfur dioxide. Hydrolysis and hydrogenation take place on the aforementioned impregnated alumina catalyst at a temperature which is commonly in the range of about 300 to about 350 ° C. A resulting gas mixture consisting essentially of hydrogen sulphide, nitrogen, carbon dioxide, steam and hydrogen leaves the hydrolysis reactor and flows first to a water condensing unit (not shown) and then to a separate unit ( not shown) in which hydrogen sulfide is separated, for example, by chemical absorption. A suitable chemical absorbent is methyldiethylamine. If desired, the hydrogen sulfide thus reconverted can be recycled to the burner 10. A second fuel gas stream containing hydrogen sulfide is sent to the additional equipment 6. The second fuel gas stream can have the same composition or a different from the first fuel gas stream. If there are separate sources of amine gas and sulfur water purge gas, and, if air is used to withstand combustion (not enriched with oxygen) inside the furnace 12 of the first Claus 12 plant, it is generally preferred that the amine gas is sent to the first Claus 2 plant and the sulfurous water purge gas is sent to the additional equipment 6. The second stream of fuel gas containing hydrogen sulfide flows through a pipe 40 to a burner 42 that ignites axially or tangentially within of an oven 44 that constitutes a thermal Claus stage. A gas that supports combustion preferably has a molar fraction of oxygen of at least about 0.8 is supplied to the burner 42 through a pipe 45 to support combustion of the fuel components of the second fuel gas stream that It has hydrogen sulfide. The burner 42 can be of a mixed tip type. The combustion support gas is preferably commercially pure oxygen or air enriched with oxygen. The respective flow velocities of the gas streams to the burner are selected such that in operation the refractory lining (not shown) of the furnace 44 never reaches a temperature of about 1, 650 ° C or higher. Generally, the rate of supply of the combustion support gas is appreciably less than that required for the combustion of a third part of the hydrogen sulfide content of the second fuel gas stream. The reactions that take place inside the furnace 44 are essentially the same as those described above with respect to the furnace 12 of the first Claus 2 plant. The use of a gas that supports combustion having a mole fraction of oxygen of at least about 0.8, tends, however, to facilitate the thermal dissociation of hydrogen sulfide. Therefore, commonly, the proportion of hydrogen in the resulting gas is greater than that in the corresponding part of the first Claus plant 2. The effluent gas stream 44 containing hydrogen sulfide, sulfur dioxide, sulfur vapor , steam, hydrogen and carbon dioxide and typically nitrogen, carbon monoxide and traces of carbon oxysulfide and carbon disulfide exits the furnace 44 and passes into a recovery boiler 46 in which it is typically cooled to a temperature of the scale from around 500 ° C to around 600 ° C. The resulting cooled effluent gas stream flows into a second Claus furnace 48. The additional combustion support gas is supplied to the furnace 48 by means of a theory 50 which branches from the pipe 45. The combustion support gas enters the furnace 48 by means of lancets (not shown).

As a result, combustion of a portion of the hydrogen sulfide content of the effluent gas stream cooled from the furnace 44 takes place. Since the effluent gas stream commonly leaves the recovery boiler 46 at a relatively high temperature, the combustion of hydrogen sulfide occurs easily. The relative flow rates of hydrogen sulfide molecules and oxygen molecules within the furnaces 44 and 48 are arranged to be such that the molar ratio of hydrogen sulphide to sulfur dioxide in the effluent gas stream leaving the furnace 48 It is commonly on the scale of about 1.5: 1 to about 3: 1. The effluent gas mixture is cooled in an additional recovery boiler 52 typically to a temperature in the range of about 250 ° C to about 350 ° C. The mixture of cooled effluent gas containing the same species as the gas mixture leaving the recovery boiler 46 (although in different proportions) now flows from a sulfur condenser 54 which is further cooled to a temperature of about 10. ° C to approximately 150 ° C and in which at least part of the sulfur vapor is condensed. The resulting condensate is sent to the sulfur closure cavity (not shown). In the resultant sulfur-free effluent gas stream commonly divided into two subsidiary gas streams. A subsidiary gas stream is joined with the gas flow through the first Claus 2 plant in a region downstream of the condenser 18 but upstream of the reheater 24. The other part of the effluent gas stream lacking in sulfur passing out of the sulfur condenser 54 is used in a manner that will be described later. A third fuel gas stream containing hydrogen sulfide is passed into a second Claus 4 plant through a pipe 60. Air or oxygen enriched air is passed into the third Claus 4 plant through a 62 pipe. second floor Claus 4 comprises a burner 64 that ignites inside a furnace 66 constituting a thermal Claus stage. The effluent gas leaving the furnace 66 is cooled in the recovery boiler 68. The sulfur is condensed in a sulfur condenser 70 from the cooled effluent gas stream and the resulting condensate is passed into a sulfur closure cavity ( not shown). The sulfur-free gas mixture leaving the condenser 70 flows into a sequence of two subsequences 72 and 74 of the catalytic Claus stages. The upstream sequence 72 comprises, sequentially, a reheater 76, a first catal catalytic reactor 78 and a sulfur condenser 80. The second subsequence 74 similarly comprises a superheater 82, a Claus 84 catalytic reactor and a condenser of sulfur 86. The operation of the second Claus 4 plant is analogous to that of the first Claus 2 plant and will not be further described herein by only establishing that the other part of the effluent stream devoid of sulfur from the sulfur condenser 54 of the additional equipment 6 is introduced into the gas mixture passing through the Claus 4 plant downstream of the condenser 70 but upstream of the reheater 76. The provision of the additional equipment 6 allows the overall capacity of the two Claus plants to be increased . By using the additional equipment 6 a combustion support gas richer in oxygen than that used in the Claus 2 and 4 plants, it is possible to reduce the additional quantities of non-reactive gases, particularly nitrogen and carbon dioxide. Additional equipment 6 can commonly be reconverted for Claus 2 and 4 plants. By processing more feed through the additional equipment, and lower feeding through the thermal stages of the Claus 2 and 4 plants, the total feeding speed can be increased while maintaining a relatively unaltered flow through the catalytic stages of the Claus 2 and 4 plants. with the invention, control is exercised so that the addition of the sulfur-free effluent gas mixture containing hydrogen sulphide and sulfur dioxide from the additional equipment to the catalytic stages of the Claus 2 and 4 plants does not alter its operation . Figure 1 of the drawings illustrates a control scheme for the first floor Claus 2. An analyzer 90 is placed downstream of the condenser 34 and generates a control signal that is transmitted to a valve controller 92 that compares the actual molar ratio of the hydrogen sulphide to sulfur dioxide (or a function thereof) with a pre-established value of said ratio (or function of said ratio). There is a difference between the two values, a compensation valve 94 is restored to adjust the flow of air or air enriched with oxygen through the compensation line 96 to the pipe 16 to bring back the detected value of the sulfide ratio from hydrogen to sulfur dioxide to the pre-established value.

The compensation pipe 96 carries a small proportion of total flow of the oxygen enriched air to the pipe 16 of the Claus 2 plant. There is a main flow control valve 98 in parallel with the compensation valve 94. This valve can be fixed from according to the calculated flow of air or air enriched with oxygen completely required to oxidize any ammonia and hydrocarbons present in the first fuel gas stream and to oxidize a selected proportion of its content of hydrogen sulfide to sulfur dioxide and water vapor. A flow meter 100 that measures the flow velocity of the first fuel gas stream inside the first Claus 2 plant and generates a signal to a valve representative of the flow velocity that is transmitted to a valve controller to restore the control valve of main flow 38 in the event that the measured flow rate varies in relation to that specified. The control scheme can be based on a composition of the first fuel gas stream that is assumed from previous experiences or previous analyzes, or the composition can be determined by an analyzer on the current or analyzers (not shown) that generate a signal of auxiliary control. The control arrangement described above capable of achieving a stable operation of the first Claus 2 plant when the Claus 2 plant is operated without the addition of the sulfur-free effluent gas from the sulfur condenser 54 of the additional equipment 6.

When the additional equipment is operated 6. The main part of the oxygen or oxygen enriched air flows into the pipe 46 and passes through a main flow control valve 1 10. The flow velocity of the second gas stream The fuel quantity is measured by a flow meter 1 14 which transmits a signal representative of the flow velocity of the second fuel stream to a control valve 1 12. The valve controller 1 12 generates a first control signal that determines the position of the main flow control valve with the result that the supply rate of the oxygen or the oxygen enriched air to the additional equipment 6 is automatically adjusted from according to any variation in the scale at which the second fuel gas stream is supplied. An analyzer 4 is positioned to be able to measure the concentration of hydrogen sulfide to sulfur dioxide in the sulfur-free effluent gas stream immediately downstream of the sulfur condenser 54. The analyzer 104 transmits a signal to a valve controller 106 which is representative of the molar ratio of hydrogen sulfide to sulfur dioxide. The valve controller generates a second control signal to an "offset" flow control valve 108 in a line 109. The compensation flow control valve 108 is able to respond to the second control signal to make small adjustments at the rate of total flow of the oxygen or oxygen enriched air to the additional equipment 6 so that the molar ratio of the hydrogen sulfide to sulfur dioxide is maintained at a selected value in the effluent gas devoid of sulfur. As a result, the satisfactory operation of the catalytic Claus stages is maintained and the proportion of sulfur compounds in the final gas leaving the sulfur condenser 34 associated with the catalytic Claus reactor 32 does not exceed a specified maximum. If there is any deviation from the desired molar ratio, the analyzer will be able to detect this and adjust the compensation valve 94 associated with the first Claus 2 plant accordingly. In view of the relatively high concentrations of sulfur dioxide and hydrogen sulfide in the sulfur-free effluent gas stream leaving the sulfur condenser 54, in the absence of the analyzer 104 and the valve controller 106, it would be difficult to achieve the stable operation of the general plant. Commonly, additional flow control valves 120 and 122 are provided to allow the sulfur-free effluent gas stream leaving condenser 54 to be adequately provided between the first Claus 2 plant and the second Claus 4 plant. the second floor shown Claus 4 has associated with it the valve control equipment analogous to that associated with the Claus 2 plant. Various changes and modifications can be made to the plants and equipment shown in Figure 1 of the drawings. For example, an intermediate sulfur condenser (not shown) can be installed intermediate to the recovery boiler 46 and the second thermal stage 48 of the additional equipment 6 and the recovery boiler 46 operated to cool the gas mixture passing through. the same up to a lower temperature. In another example air and oxygen are fed separately to the additional equipment 6, the control signal from the analyzer can be used to control a compensation valve associated with either the air supply line or the oxygen supply line . Figure 2 illustrates another modification in which the second thermal stage 48 and its associated recovery boiler 52 are omitted from the additional equipment 6. Instead, part of the sulfur-free effluent gas stream leaving the sulfur condenser 54 is recycled by a pump 200 to the second fuel gas mixture containing hydrogen sulfide. The recycled gas stream modifies the temperature that would otherwise be created within the first Claus furnace 44 and thus allows a gas that supports combustion with a greater molecular fraction of oxygen than would otherwise be possible to be used. Again, the recovery boiler 46 is operated at a lower temperature than that in the equipment shown in Figure 1. In other aspects, the plants and equipment shown in Figure 2 are analogous to those shown in Figure 1. Referring now to Figure 3, a further modification is shown in which the second furnace Claus 48 and the recovery boiler 52 of the additional equipment 6 shown in Figure 1 are again omitted. In this case the moderation of the temperature in a first furnace 44 is achieved by direct injection of a fluid such as liquid water through a pipe 300 into the flame zone (not shown) inside the furnace 44.

Claims (10)

DIVACTION REVIEWS

1 . A method for treating a plurality of fuel gas streams containing hydrogen sulfide, comprising the steps of: (a) operating to recover the sulfur from a first fuel gas stream containing hydrogen sulfide, a first plant Claus having a sequence of steps including, sequentially, a first thermal stage Claus, a first sulfur condenser, and at least a subsequence of stages including a catalytic Claus stage and a second sulfur condenser current below the same; (b) burning in at least one additional thermal Claus stage part of the hydrogen sulfide content of a second fuel gas stream containing hydrogen sulfide; (c) supplying to the additional thermal stage Claus a combustion support gas having a mole fraction of oxygen of at least 0.25 to withstand the combustion of the hydrogen sulfide therein, the gas supporting the combustion that is formed of a stream of pure or impure oxygen separated from the air or from a mixture of an air stream with said stream of pure or impure oxygen; (d) removing an effluent gas stream containing hydrogen sulfide, sulfur dioxide, water vapor and sulfur vapor from the additional thermal Claus stage and removing the sulfur vapor from the effluent gas stream in a condenser additional sulfur to form an effluent gas stream devoid of sulfur; (e) mixing at least part of the stream of effluent gas devoid of sulfur with the first fuel gas supporting the treatment in the first Claus plant in a region of the same stream below the first stage Claus thermal and upstream of the start of the catalytic Claus reaction in said subsequence; (f) generating a first control signal that is a function of the flow velocity of the second fuel gas or at least one fuel component thereof within said additional thermal Claus stage; (g) generating a second control signal that is a function of the molar ratio of hydrogen sulfide / sulfur dioxide in the effluent gas stream devoid of sulfur; and (h) employing the control signals in setting the rate at which the combustion support gas is supplied to the additional thermal Claus stage.

2. A method according to claim 1, wherein the combustion support gas has a mole fraction of oxygen of about 0.7.

3. A method according to claim 1 or claim 2, wherein there is a single additional thermal stage Claus.

4. A method according to claim 4, wherein a temperature moderator fluid selected from liquid water, liquid carbon dioxide, sulfur dioxide and a recycle stream taken downstream from the additional sulfur condenser is supplied to the stage. Single additional thermal Claus. The method according to any one of the preceding claims, wherein the stream of effluent gas devoid of sulfur is introduced into the first fuel gas that supports the treatment in the first Claus plant in a region of the same stream downstream of the condenser of additional sulfur. The method according to one of the preceding claims, which additionally includes operating a second Claus plant for the recovery of sulfur from a third stream of fuel gas containing hydrogen sulfide having a sequence of steps including , sequentially, a first thermal stage Claus, a first sulfur condenser, and at least a subsequence of stages including a catalytic Claus reaction stage and a second sulfur condenser, and only part of the effluent gas stream no sulfur is mixed with the first fuel gas stream, the remainder of the sulfur-free effluent gas stream that is mixed with the third fuel gas stream undergoing the treatment in the second Claus plant in a region of the same stream downstream of the first thermal stage of the same and current stage above the start of the catalytic Claus reaction in it. Plant for treating a plurality of fuel gas streams containing hydrogen sulfide, comprising: a) a first Claus plant for the recovery of sulfur from a first fuel gas stream containing hydrogen sulfide having a sequence of steps including, in sequence, a first thermal stage Claus, a first sulfur condenser and at least a subsequence of stages including a catalytic reaction stage and a second sulfur condenser; b) at least one additional thermal Claus stage for combustion of part of the hydrogen sulfide content of a second fuel gas stream containing hydrogen sulphide; c) at least one input to the additional thermal Claus stage for a gas that supports combustion having a molar fraction of oxygen of at least 0.25, the gas that supports combustion that is formed from a stream of pure or impure oxygen separated from the air or a mixture of an air stream with the flow of pure or impure oxygen; d) an output from the additional thermal Claus stage for an effluent gas stream containing hydrogen sulfide, sulfur dioxide, water vapor and sulfur vapor; e) an additional condenser for extracting sulfur vapor from the effluent gas stream, to form an effluent gas stream devoid of sulfur having an inlet communicating with the outlet from the additional thermal Claus stage; f) an outlet for the sulfur-free effluent gas stream that communicates with a region of the first Claus plant downstream of the first thermal stage and upstream from where the catalytic Claus reaction starts; g) means for generating a first control signal that is a function of the flow velocity of the second fuel gas stream or at least one of the fuel components thereof in the additional thermal Claus stage; h) means for generating a second control signal that is a function of the molar ratio of hydrogen sulfide to sulfur dioxide through the effluent gas devoid of sulfur; and i) means responsive to the control signals to establish the rate at which the combustion support gas is delivered to the additional thermal Claus reaction stage. A plant according to claim 7, wherein the means for generating the first control signal comprises a flow meter for measuring the flow rate of the gas that supports combustion within the additional Claus stage operatively associated with a first controller valve. 9. A plant according to claim 7, wherein the means for generating the second control signal comprises an analyzer for measuring the concentrations of hydrogen sulfide and sulfur dioxide in the effluent gas devoid of sulfur, means for calculating from measured concentrations the molar ratio of hydrogen sulfide to sulfur dioxide, and means for preparing the calculated molar ratio of hydrogen sulfide to sulfur dioxide with a predetermined molar ratio. A plant according to any of claims 7 to 9, which additionally includes at least one main flow control valve through which a main part of the gas stream supporting the combustion is capable of flow and a compensation control valve through which a minor part of the gas stream supporting the combustion is able to flow, wherein the main flow control valve is operatively associated with the means for generating the first signal of control and the compensation flow control valve is operatively associated with the means for generating the second control signal. SUMMARY A first stream of fuel gas containing hydrogen sulphide is subjected to a treatment in a first Claus plant that includes a first thermal stage Claus. Part of the hydrogen sulfide content of a second fuel gas stream containing hydrogen sulfide is burned in at least one additional thermal Claus stage. The combustion is supported by air enriched with oxygen having a molar fraction of oxygen of at least about 0.25 or oxygen. The resulting sulfur dioxide reacts with the residual hydrogen sulphide to form sulfur vapor which is condensed out of the effluent gas from the additional thermal Claus stage to form an effluent gas stream devoid of sulfur. A first control signal is generated which is a function of the flow velocity of the second gas. A second control signal is also generated which is a function of the molar ratio of hydrogen sulphide / sulfur dioxide in the effluent stream lacking in sulfur. The control signals are used in setting the speed at which the combustion support gas is supplied to the second thermal stage Claus.