GB2187444A - Treatment of gas stream comprising H2S - Google Patents

Abstract

A hydrogen sulphide containing feed gas stream (typically including at least 70% by volume of hydrogen sulphide) is divided into a minor stream that flows through pipeline 4 and a major stream that flows through pipeline 6. The minor stream 6 is burnt in a first combustion region 10 to form sulphur dioxide and water vapour. The resulting gas mixture is cooled in a heat exchanger 20 and is typically employed in a second combustion region 26 in which a portion of the hydrogen sulphide content of the major stream is burnt to form sulphur dioxide and water vapour. Reaction between the sulphur dioxide and remaining hydrogen sulphide takes place in a thermal reaction region 30 to form sulphur vapour and water vapour. The proportions of sulphur burnt in the respective combustion regions 10 and 26 are chosen such that about one-third of the total hydrogen sulphide content of the feed gas stream is burnt to sulphur dioxide in these regions. Subsequent reaction between hydrogen sulphide and sulphur dioxide takes place in catalytic reaction regions 42 and 48 with sulphur vapour being extracted in sulphur condensers 38, 44 and 50. By using substantially pure oxygen to support combustion in the second combustion region, a relatively large flow rate of feed gas mixture may be handled in a plant of given size. <IMAGE>

Description

SPECIFICATION Treatment of gas streams This invention relatestothetreatmentofgas streams. In particular, it relates to the treatment of a gas stream comprising hydrogen sulphide.

Gas streams comprising hydrogen sulphide are typically produced as waste products or by-products from many industrial processes. For example, acid gas streams comprising carbon dioxide and hydrogen sulphide are typically produced during oil refinery operations in which sulphur is removed from crude oil. It is necessary to treat such hydrogen sulphide-containing streams before discharging them to the atmosphere so as to reduce or remove altogether their content of sulphur-containing gases.

One well known, widely practised process for treating a gas stream comprising hydrogen sulphide is the Claus process. This process is based on the reaction between hydrogen sulphide and sulphur dioxide to form sulphurvapourandwatervapourin accordance with the equation.

502+2H2S=2H20+3S Sulphur exists in the vapour phase in a number of different molecular species such as SSI S6 and S8 according to the temperature.

The first stage of the Claus process is to burn approximately a third ofthe hydrogen sulphide in the incoming gasstreamtoformsulphurdioxide and water vapour in accordance with the equation: 2H2S +302 = 2H20 + 2SO2 This combustion reaction takes place in a suitable urnace and normally air is used as the source of oxygen for the purposes of combustion. The furnace is designed such that reaction between the sulphur dioxide and hydrogen sulphide can start in the combustion zone and then continue downstream of the combustion zone.It is however a feature ofthe Claus reaction that at the temperature that is created by the combustion of hydrogen sulphide, it is not possible to convert more than about 75% of the remaining hydrogen sulphideto sulphur by reaction with sulphur dioxide, and typically between 50 to 70% of the hydrogen sulphide is so converted. It is however possible to achieve a higher percentage conversion in the presence of a catalyst at a reaction temperature in the order of 200 to 3500C by reacting the remaining hydrogen sulphide and sulphur dioxide. (Atsuch "catalytic" temperatures,thelower the temperature the higherthe percentage conversion that is achieved).Accordingly, after the gases pass out of the so-called thermal region ofthe furnace they are cooled to a temperature at which the sulphurthat is formed in t\'e furnace condenses. The sulphur is thus recovered. The gases are then reheated to atemperaturesuitableforthe performance of a catalysed reaction between hydrogen sulphide and sulphur dioxide, such temperaturetypicallybeing in the order of 2000C. A catalytic reaction is then carried out and typically about 60% of the remaining hydrogen sulphide is converted to sulphur.Nonetheless, it is still not possible to achieve 100% conversion as in practice conversions of more than 99.5% can be achieved onlyatatemperatureatwhichthesulphurvapour condenses and thereby substantially reduces the effectiveness of the catalyst. It is therefore typical to perform the catalytic oxidation of hydrogen sulphide with sulphur dioxide in more than onestagewith first condensation ofsulphurvapourandthen re-heating ofthe hydrogen sulphide bearing gas stream being carried out between each stage.

Various means may be employed to effect reheating ofthe gases prior to each catalytic stage.

Forexample, a smail partofthefeedgasmixturecan be diverted from upstream of the furnace and burnt in in-line burners completely to sulphurdioxide, there being typically one such burner upstream of each catalytic reactor. The hot, sulphur dioxide-containing gases are then mixed with the main gas stream upstream of each respective catalytic reactor so as to effect reheating.

Alternatively, a part of the main gas stream can be taken from, say, a waste heat boiler used to cool the main gas stream leaving the furnace and used in the same manner as the gas from the in-line burners.

Another alternative is to employ indirect heat exchange with, for example steam to effect reheating. Typically, after two or three such stages, sulphurformed in the most downstream stage is condensed outofthe gas stream which is then passed to a tail gas clean-up process of a known kind for handling relatively dilute hydrogen sulphide streams (for example the Scot, Beavon or Stretford process) orwhich is then incinerated.

Many variations on this basic Claus process are possible. Some ofthese alterations are summarised in the paper "SulfurCostsvarywith Process Selection" by H. Fischer, Hydrocarbon Processing, March 1979, ppl 25 to 129.

Recently, there has been a trend towards using crude oils of relatively high sulphur content and also a trend towards stricter environmental standards so far as the discharge to the atmosphere of sulphur-containing gases is concerned, thus requiring an increased number of hydrogen sulphide bearing streams to be treated and hence more treatment capacity for hydrogen sulphide containing gases. For example, where possible, it is desirable to increase the rate at which an exising Claus plant is ableto produce sulphur. In practice,the ability of such plants to handle an increased throughput of hydrogen sulphide containing gas is limited. It has been realised that in orderto supply the necessary oxygen for combustion, approximately 14volumes of air are required for each six volumes of hydrogen sulphide in the gas mixture.It has been proposed in for example a paper entitled "Oxygen Use in Claus SulphurPlants" by M.R. GrayandW.Y.Svrcek,1981 Gas Conditioning Conference, Oklahoma, 1981 and in a paper entitled "Modifications Jump Sulphur Recovery Plant Capacity", Oil and Gas Journal, August 20th 1984, pup1 08 to 112, that the capacity of existing Claus processes can be increased by substituting some commercially pure oxygen for air and thereby reducing the proportion of nitrogen in the gas mixture that flows through the process.In practice, however, in many plants, the amount of uprating that can be achieved by this method is limited as there is atendencyforthe reduced volume of nitrogen to lead to higher exittemperatures from the furnace that cannot be withstood by the waste heat boiler or heat exchanger associated with the furnace or by the refractory lining of the furnace.

Indeed, the more concentrated (in hydrogen sulphide) the gas stream, the less is the possibility for achieving any significant uprating, such possibility often becoming particularly limited for feed gas streams including 80% by volume or more of hydrogen sulphide. Another proposal for using pure oxygen in the Claus process is set out in US patent specification 3681 024 and its corresponding Canadian patent specification 854094. These patent specifications disclose burning one third of a hydrogen suiphide stream with oxygen of about 95% purity.Plant effluent from a one ortwo catalytic reactor unit is sent to a water scrubberto reduce the water content ofthe effluent and a sufficient amount ofthescrubberoff-gas is recycled to dilute the oxygen feed so that the furnace temperature is essentially equivalent to that obtained in operation with air.

This process is stated to have the advantage of enabling plant size to be reduced. However, existing plants constructed with the intention of using airto supportthecombustion of the hydrogen sulphide are not readily convertible to perform the process described in US patent specification 3681 024 and this process has not found commercial favour.

Moreover, the practice of recycling to the thermal reaction zone a gas mixture that has passed therethrough places a limitation on the amount by which the size of the furnace defining the thermal reaction zone can be reduced, particularly if the incoming hydrogen sulphide stream contains more than, say, 50% by volume of hydrogen sulphide. US patent specifications 3 331733 and 4552747 are other examples of proposals in which gas is recirculated in order to moderatethetemperature in the thermal reactor.

It is an aim ofthe present invention to provide an improved method and apparatus for recovering sulphurfrom a gas stream consisting of hydrogen sulphide or containing a relatively high proportion of hydrogensulphidewhicharecapableofminimising the volumes of "ballast" gas such as nitrogen that flowthrough the sulphur recovery process and which do not of necessity rely on recycling effluent gas to the inlet ofthe furnace.

According to the present invention there is provided a method of recovering sulphurfrom a feed gas stream comprising hydrogen sulphide, comprising dividing the feed gas stream into a major stream and a minorstream, burning in afirst combustion region at least 50% of the hydrogen sulphide content ofthe minor stream to form sulphur dioxide and water vapour, and then cooling the minor stream, burning a second combustion region less than one third ofthe hydrogen sulphide content ofthe major stream to form sulphur dioxide and water vapour, supporting the combustion of hydrogen sulphide in the major stream by supplying oxygen-rich gas (as hereinafter defined) to the second combustion region, reacting hydrogen sulphidewiththethus-formed sulphur dioxide in a thermal reaction region associated with said second combustion regiontoformsulphurvapourand watervapour, extracting such sulphurvapourfrom the resulting gas mixture, and reacting the residual hydrogen sulphide in the gas mixture with residual sulphurdioxidetoformfurthersulphurvapourand water vapour, and then extracting the further sulphurvapour, wherein the cooled minorstream is introduced into the second combustion region or the thermal region associated therewith (or both), and about one third of the total hydrogen sulphide content of the minor and major streams is burnt to form sulphur dioxide and watervapour.

Preferably, substantially all the hydrogen sulphide content of the minor stream is burnttoform sulphur dioxide and watervapour.

The invention also provides apparatus for recovering sulphurfrom a feed gas stream comprising hydrogen sulphide, including a first conduit for receiving a major portion of said feed gas stream; a second conduit for receiving a minor portion of said feed gas stream; a first combustion region having at least one first burner associated therewith for burning at least 50% of the hydrogen sulphide content of the minor stream to form sulphur dioxide and water vapour, said burner having an inlet communicating with said second conduit, and said first combustion region having an outlet communicating with an inlet of a heat exchange meansforcooling gas mixture from said first combustion region; a second combustion region having at least one second burner associated therewith for burning hydrogen sulphide to form water vapour and sulphur dioxide, said at least one second burner having an inlet in communication with said first conduit and an inlet communicating with a source of oxygen-rich gas (as hereinafter defined); a thermal reaction region in which, in operation, sulphur dioxide reacts with hydrogen sulphidetoform sulphurvapour and watervapour, said thermal reaction region communicating with an outletfrom said second combustion region; a condenser, downstream of said thermal reaction region, for extracting sulphurvapourfrom gas mixture exiting said thermal reaction region; at least one further reaction region downstream of said condenser, for conducting further reaction between hydrogen sulphide and sulphur dioxide to form furthersulphurvapourandwatervapour; afurther condenserfor extracting said further sulphur vapour, and means for introducing the cooled gas mixture exiting said heat exchange means into one or both ofthe second combustion region and said thermal reaction region, whereby, in operation, about one-third of the total hydrogen sulphide content ofthe said majorand minor portions isable to beburnttoformsulphurdioxideandwater vapour.

Bytheterm "oxygen-rich gas" as used herein, is meant a gaseous mixture containing at least 80% by volume of molecular oxygen. The oxygen-rich gas is preferably substantially pure oxygen. Alternatively, it may for example be oxygen-enriched air. By appropriately choosing the relative sizes of the minor and major streams, itis possible to ensurethat an excessive temperature is not generated in the second combustion region even in the event that the oxygen-rich gas is pure oxygen and the gas comprising hydrogen sulphide is relatively rich in hydrogen sulphide,that is contains morethan50% by volume of hydrogen sulphide (and typically more than 60% by volume of hydrogen sulphide),without the need to introduce any otherfluid into the second combustion region than the cooled minor stream and the oxygen-rich gas.Accordingly, in comparison with a conventional Claus process in which about one-third of the hydrogen sulphide stream is burnt to form sulphurdioxide in a single furnace, and air is employed to support combustion of the hydrogen sulphide, a relatively greaterthroughput of hydrogen sulphide may be achieved in the method according to the invention for a given size offurnace (incorporating the second combustion region and its associated thermal reaction region).

Typically when stoichiometric or near stoichiometric combustion takes place in the first combustion region up to 10% of the feed gas stream is employed to form the minor stream, and the balance of the feed gas stream to form the major stream.

Air or another gaseous mixture including molecular oxygen may be used to support the combustionoftheminorstream. ltisdesirableto prevent the formation ofsulphurtrioxide in the first combustion region. Accordingly, the amountof molecular oxygen supplied to the first combustion region is preferably in the range 90to 1 00% of that necessary for the complete combustion of the hydrogen sulphide content of the minor stream. It is also preferred that the gas exiting thefirst combustion region is, downstream of where it is cooled, introduced into the hot zone of the flame or one ortheflames in thesecond combustion region, whereby anytraces of sulphurtrioxide present in the gas may be destroyed.

Typically, in the event that air, oxygen-enriched air or pure oxygen is used to support combustion in the first combustion region, there may need to be additional cooling provided for such region so as to control the temperature at the inlet to heat exchange means downstream ofthe first combustion region.

Such cooling may be provided by introducing a moderator or quenchant into the first combustion region. The moderator or quenchant may for example be selected from steam, liquid water, nitrogen and carbon dioxide. If desired, the burner or burners employed in the first combustion means may each be provided with a jacket for the circulation of coolant, such as water. The use of such cooling jacket or jackets may be as an alternative or in addition to the use of a moderator.

All the cooled minor stream is typically introduced directly into the second combustion region.

Alternatively, a small portion or portions of this stream may be employed to provide reheat intermediate a sulphur condenser and a catalytic reaction region. Another alternative, which is preferred if pure oxygen or oxygen-enriched air is used to support combustion, is to return a portion of the cooled minor stream to the first combustion region as the moderator. In this event, preferably from 8 to 15% by volume of the feed gas stream is taken as the minor stream.

The method according to the invention may be performed on a plant built to custom forthis purpose. Itis however also possible to perform the method according to the invention on an existing plant for performing the Claus process with a need only for relatively minor modifications to the plant.

Thus, an existing Claus furnace can be employed to provide the second combustion region and its associated thermal region in the method according to the invention and in the event that water is used as a moderator in the first combustion region, a relatively small furnace, defining the first combustion region, and a relatively small heat exchanger can be retro-fitted to the Claus furnace.

This retro-fitting makes it possible to increase substantiallytheamountofsulphurproduced per unit time in an existing plant without loss of conversion efficiency. If the moderator is recycled gas, then an enhanced uprating of the Claus furnace is made possible, but a largerfirstcombustion region will be required.

The method according to the present invention will now be described bywayofexamplewith reference to the accompanying drawings, of which: Figure lisa schematic diagram illustrating one plant performing the method according to the invention, and Figure2 is a schematic diagram illustrating a modification to the plant shown in Figure 1.

Referring to Figure 1 ofthe drawings, a conduit 2 communicates with a source (not shown) of hydrogen sulphide-rich gas mixture. Typically, the hydrogen sulphide-rich mixture includes at least 70% by volume of hydrogen sulphide. It may also include one or more other gases such as carbon dioxide, nitrogen, water vapour and hydrocarbons.

The conduit 2 communicates with a first pipeline 4 fortheflowofa minorstream of hydrogen sulphide-rich gas and a second pipeline 6fortheflow of a major portion ofthe hydrogen sulphide-rich gas mixture. If desired, a blower (not shown) may be employed to assistthe flow ofthe minor stream into the pipeline 4. In operation, typically in the order of 5 to 10% ofthe gas mixtureflowingthroughthe conduit 2 is introduced into the pipeline 4 and the balance into the pipeline 6. The pipeline 4terminates in oneinlettoa burner8that,inoperation,firesintoa first combustion region 10 defined within a small furnace 12. The burner 8 has a first additional inlet 14 for air (or other oxygen-containing gas mixture) and a second additional inlet 1 6for liquid water (or other moderator). The furnace 10 has an outlet 18 communicating with one pass of a heat exchanger 20, in which in operation the gas mixture passing out of the furnace loins cooled. The resulting cooled gas mixture then passes along a pipeline 22 and is reunited with the major stream of hydrogen sulphide passing through the pipeline 6 at a region immediately upstream of its inlet into a second burner 24thatfires into a second or main furnace 28 defining a combustion region 26 therein.

Typically, in operation of the plant shown in the drawing, the rate at which air or oxygen is supplied to the burner 6 is sufficientforfrom 90 to 100% of the hydrogen sulphide content of the minor stream to be oxidised to sulphur dioxide in the combustion region 10. If desired, the oxygen pressure may be used to induce the flow of the minor stream into the pipeline 4. The rate at which liquid water or other moderator or quenchant is supplied to the combustion region l2throughtheinlet l6ofburner8 isdependentupon the maximum temperature that can be tolerated at the inlet to the heat exchanger 20.Typically, this maximum temperature may be in the order of 1250 C. The rate at which liquid water (or other moderator) is supplied to the inlet 16 is thus chosen inaccordancewiththerateatwhich hydrogen sulphide is supplied to the burner 8 and with the concentration of any other gases in the hydrogen sulphide stream entering the burner8 such thatthe aforesaid maximum temperature does not exceed 12500C or other chosen maximum temperature.

Downstream of its exitfrom the furnace 12 the minor stream is preferably cooled to a temperature in the order of 300 C, that is a temperature above the dew point of the various components ofthe mixture.

In the eventthat notall the hydrogen sulphide content of the minor stream is oxidised to sulphur dioxide, some of the residual hydrogen sulphidewill tend to react with the sulphur dioxide in the furnace 12. Any such sulphurvapourwill remain inthe vapour state during its passage through the heat exchanger 20.

The combustion region 26 into which the burner 24 fires is defined by a second or main furnace 28. The burner 24 is fitted at the upstream end of the furnace 28 and has an inlet 30 for oxygen-rich gas in addition to its inletforthe major stream of hydrogen sulphide (to which the cooled minor stream is returned from the heat exchanger 20) The oxygen-rich gas is preferably pure oxygen. The relative rates of supply of the hydrogen sulphide-containing gas stream and the oxygen stream to the burner 24 are such that, in total, the burners 8 and 24 achieve the necessary combustion ofthestoichiometricamountof hydrogen sulphide for complete conversion ofthe incoming hydrogen sulphide to sulphur.Since preferablysubstantiallyallthe minor stream of hydrogen sulphide-containing gas is burned in the burner 24, significantly less than one third ofthe hydrogen sulphide content of the major gas stream supplied to the burner 24 from the pipeline 6 is combusted in orderto achieve combustion of just one-third of the total content of hydrogen sulphide entering the conduit 2. The mixing of the major stream with the cooled gas stream from the heat exchanger 20 and the effect of the portion ofthe hydrogen sulphide that is not burnt, are capable of preventing an excessive temperature being created in the combustion region 26.The relative flow rates of the hydrogen sulphide-containing gas th rough the pipelines 4 and 6 are selected such that even in the event of the use of pure oxygen to support combustion of the hydrogen sulphide content ofthe major stream, an excessivetemperature is not created within the furnace 28. Within these confines, however, the proportion of the gas mixture entering the conduit 2 which is diverted to the pipeline 4for combustion in the burner 8 is preferably kept as small as possible.Typically, in the event thatthefeed gas mixture contains from 75 to 100% by volume of hydrogen sulphide; the proportion ofthefeed gas mixture that is diverted to the pipeline 4 is in the range 5 to 10% by volume, and at 90% hydrogen sulphide is in the order of 8.5% by volume.

The furnace 28 is in general substantially identical to a conventional Claus furnace. Accordingly, therefore, the furnace 28 has a suitable refractory lining (not shown) and a volume sufficientforthere to be an adequate thermal reaction zone in association with the combustion region 26. The reaction between hydrogen sulphide and sulphur dioxide is typically initiated in the combustion region 26 and continues in the thermal reaction region 30. If desired, the furnace 28 may be provided with baffles or means 32 in order to facilitate mixing of the gases within the thermal reaction region 30.The thermal reaction between hydrogen sulphide and sulphur dioxide is endothermic above about 600 C, so some temperature drop takes place in the thermal reaction region 30 where the temperature is typically in the range 1 3500C to 1450 C. The effluent gases are then cooled in a waste heat boiler or heat exchanger 36to a temperature, say, in the range 275 to 325#C.

The heat exchanger or waste heat boiler 36 has, as shown, two passes for the effluent gases from the furnace 28. A major portion of the effluent gases flows through both passes and is thus cooled to said temperature in the range 275 to 325 C. A minor portion of said gases flows through onlythefirst pass and leaves the waste heat boiler 36 at a higher temperature, in the range 590 to 600 C, and is used as is described below. The major portion of the effluent gases then enters a first sulphur condenser38 in which sulphurvapourformed by the reaction between sulphur dioxide and hydrogen sulphide is condensed out ofthe gas stream leaving thefurnace 28.This condensation is effected by cooling the gas stream to a temperature in the order of 140 C. The sulphur condensate is then passed to a sulphurseal pit 54. The gas mixture exiting from the condenser 38 typically comprises hydrogen sulphide, sulphur dioxide, water vapour, nitrogen (resulting, for example, from the supply of air to the burner8) and carbon dioxide together with traces of other gases.

This gas mixture is reheated at 40 to a temperature in the range 220 to 250 C, by being mixed with a first stream taken from said minor portion of the effluent gases. The reheated gas mixture is then passed through a first catalytic reactor 42 in which reaction takes place between residual hydrogen sulphide and sulphur dioxide to form further sulphurvapourand water vapour. This reaction takes places over a catalyst which is typically of a conventional kind,for example, an activated alumina.Since the catalytic reaction between hydrogen su Iphide and sulphur dioxide at these lowertemperatures is exothermic, there is a rise in the temperature in the first catalytic reactor42 and accordingly the gas mixture leaving this reactor 42 will typically have a temperature in the order of 300 to 350 C. If desired, the outlet temperature ofthe reactor 42 may be arranged to be higher, say in the range 350 to 400 C. Such a higher outlettemperature will tend to give improved hydrolysis of any carbon oxysulphide and carbon disulphide present in the gas mixture entering the reactor 42.

From the catalytic reactor 42, the gas mixture passes through a second sulphurcondenser44in which sulphur is condensed out of the gas mixture.

The resultant sulphur condensate is passed to the sulphurseal pit 54. Downstream ofthesulphur condenser 44, the gas mixture is reheated at 46 from a temperature of, say, 1 40'C to a temperature in the range, say, of 200 to 220 C by mixing with a second part of said minor portion of effl uent gases from the waste heat boiler 36, said temperature being typically slightly less than the inlet temperature to the first catalytic reactor 42.The gas stream then passes through a second catalytic reactor48where further reaction takes place between residual hydrogen sulphide and residual sulphurdioxideto form watervapour and sulphurvapourwith the evolution of heat such thatthetemperature ofthe gas mixture is typically raised in the order of 500C as it passes from the inlet to the outlet of the catalytic reactor 48. The catalyst employed in the second catalytic reactor 48 is typically the same as that employed in the first catalytic reactor 42.

After leaving the second catalytic reactor 48, the gas mixture passes through a third sulphur condenser 50 in which sulphur is condensed out of the gas stream. The sulphur condensate is passed to the sulphur seal pit 54. The gas stream leaves the third sulphurcondenser 50 as a tail gas stream ata temperature in the order of 1400C and then enters the tail gas clean-up plant 52. The tail gas clean-up plant 52 may be of a conventional kind.

Typically, the furnaces shown in the drawing are operated at a pressure a little above atmospheric pressure. For example, the pressure in the furnaces may be in the range 1.5 to 2 atmospheres absolute.

Typically, all the plant shown in Figure 1 ofthe drawings saveforthe burner 8, first furnace 10, heat exchanger 20 and associated pipelines may be an existing plant for recovering sulphurfrom a hydrogen sulphide containing gas stream by the Claus process. In normal operation of such plant, rather than supplying pure oxygen to support combustion of the hydrogen sulphide in the combustion region 26, air, unenriched in oxygen, is used for this purpose. Since about one-third of the hydrogen sulphide is burnt in such normal operation, approximately 14volumes of air, and hence 11 volumes of nitrogen, are employed for each 6volumes of hydrogen sulphide. Thus, a considerable part of the capacity of the plant is taken up in conveying nitrogen and not sulphur-containing gases.Substituting pure oxygen for the air, (and, if necessary, making modifications to the burner24) and adding the pipeline 4, burner8,furnace 12, heat exchanger 20 and pipeline 22 to the rest of the plant, makes it possibleforthe plant to be operated in accordance with the present invention while substantially reducing the mass flow rate of nitrogen through the plant. Moreover, since the total number of moles of water introduced into the burnerwill typically be a small fraction of the total number of moles of nitrogen contained in the combustion air in conventional operation of the plant, the plant may by operation in accordance with the invention be considerably uprated.

The method according to the invention is further illustrated bythefollowing example in which a plant similar to that shown in Figure 1 is used, but with reheat of the gas stream immediately upstream of each catalytic reactor being effected by indirect heat exchange rather than by mixing the gas stream with gas by-passed from an intermediate region of the waste heat boiler 36 (as shown in Figure 1).

A gas stream comprising 90% by volume of hydrogensulphideand 10%byvolumeofcarbon dioxide is treated at a rate of 100 Kmole per hour. A minor portion ofthe gas stream is passed at a rate of 8.5 Kmole per hour into a first combustion region and all of its hydrogen sulphide content is burntto form sulphur dioxide and water vapour. Pure oxygen is passed into the first combustion region ata rate of about 11.5 Kmole per hour in orderto support combustion ofthe hydrogen sulphide. In orderto maintain the maximum flame temperature at about 1 250"C, water is introduced in atomised state into the flame at a rate of 27 Kmole per hour.The combustion products comprising 80% by volume of water vapour, 18% by volume of sulphur dioxide and 2% by volume of carbon dioxide are passed at a rate of 43 K mole perhourthrough a heatexchangerto reduce theirtemperature to 300 C. The resulting cooled gas mixture is then mixed with the remainder of the gaseous mixture of hydrogen sulphide and carbon dioxide. This mixture is then passed into a second combustion region forming part of a furnace in which hydrogen sulphide is oxidised with oxygen to sulphur dioxide such that the resulting gas contains hydrogen sulphide and sulphur dioxide in the ratio of 2 to 1.In addition sulphur dioxide reacts in the furnace with hydrogen sulphide to form water vapour and sulphurvapour.

A resultant gas mixture comprising sulphur vapour, water vapour, hydrogen sulphide, sulphur dioxide, carbon dioxide, and small amounts of hydrogen, carbon monoxide and carbon oxysulphide (which are formed as a result of side reactions) leaves the furnace at a temperature of about 1 4230C and is reduced in temperature to about 31 60C in a waste heat boiler. The gas mixture leaving the waste heat boiler is passed through a sulphur condenser in which sulphur is condensed and the condensate is extracted from the gas mixture. After the extraction ofthe sulphurvapourthe gas mixture has the following composition by volume : sulphur dioxide 6.7%; hydrogen sulphide 12.75%; water vapour 70%; carbon dioxide 6.2%; hydrogen 3.2%; carbon monoxide 0.6%; carbon oxysulphide 0.55%.

There are typically two conventional stages of catalytic conversion to achieve further reaction between hydrogen sulphide and sulphur dioxide.

Upstream of the first such stage the gas mixture from the sulphur condenser is reheated to about 233"C. It leaves the first stage at a temperature of 343.5 C. After condensation and extraction of the thus formed sulphurvapour,the gas mixture is reheated to about215.5 C and is then passed through the second catalytic conversion stage, in which its temperature rises to about 261 C. The resultant gas mixture has sulphurvapour condensed and extracted therefrom and is then subjected to a conventional tail gas treatment to remove most of the hydrogen sulphide that remains in the gas mixture after the second catalytic stage.

A modification to the plant shown in Figure 1 is now described with reference to Figure 2. In this modification the first burner 8 is not supplied with water and hence the conduit is omitted. In its place a gas recycle is provided by means of a blower 60 disposed in a conduit 62 which terminates at its inlet in the conduit 22 downstream of the heat exchanger 20 and at its outlet in the conduit 4.In operation,the rate of recycle is chosen to maintain the flame temperature in the region 10 ata chosen value in the range 1200 to 1400 C. In one example ofthe use of the modified plant shown in Figure 2, a feed gas stream comprising 90% by volume of hydrogen sulphide and 10% by volume of sulphur dioxide is passed into the pipeline 2. 12% of the stream is diverted into the pipe 4.The hydrogen sulphide content ofthe gas mixture flowing through the pipe 4 is burnt to sulphur dioxide and watervapour in the combustion region 12 of the cooling of the resulting combustion products in the heat exchanger 20, a sufficient portion ofthe cooled gas mixture is recirculated to the combustion region 12 to maintain the temperature therein at a suitable level. The remainder of the cooled gas mixture is then mixed with the remainder of the feed gas stream in the conduit 6.

Afurther example of the use of the modified plant shown in Figure 2 is given below. In this example, reheat of the gas upstream of each catalytic reactor is effected by indirect heat exchange.

A gas stream comprising 90% by volume of hydrogen sulphide and 10% by volume of carbon dioxide is treated at a rate of 100 Kmole per hour. A minor portion ofthe gas stream is passed at a rate of 11.5 Kmole per hour into a first combustion region and all of its hydrogen sulphide content is burnt to form sulphur dioxide and water vapour. Pure oxygen is passed into the first combustion region ata rate of about 15.5 Kmole per hour in orderto support combustion ofthe hydrogen sulphide. In orderto maintain the maximum flame temperature at about 1250 C, a stream of moderating gas (whose formation is described below) is introduced into the flame at a rate of 71.5 Kmole per hour.The combustion products comprising 47.4% by volume of watervapour, 47.4% by volume of sulphur dioxide and 5.2% by volume of carbon dioxide are passed at a rate of 93.35 Kmole perhourthrough a heat exchangerto reduce their temperature to 300 C. The resulting cooled gas mixture is then divided into two parts. One part (71.5 Kmole/hr) is used as the moderating gas and is thus returned to the first combustion region. The other part (21.85 Kmole/hr) is mixed with the remainder (88.5 Kmole/hr) of the gaseous mixture of hydrogen sulphide and carbon dioxide.This mixture is then passed into a second combustion region forming part of a furnace in which hydrogen sulphide is oxidised with oxygen to sulphur dioxide such that the resulting gas contains hydrogen sulphide and sulphur dioxide in the ratio of 2to 1. In addition sulphur dioxide reacts in the furnace with hydrogen sulphide to form water vapourandsulphurvapour.

A resultant gas mixture comprising sulphur vapour, water vapour, hydrogen sulphide,sulphur dioxide, carbon dioxide, and small amounts of hydrogen, carbon monoxide and carbon oxysulphide (which are formed as a result of side reactions) leaves the furnace at a temperature of about 14230C and is reduced in temperature to about 31 60C in a waste heat boiler. The gas mixture leaving the waste heat boiler is passed through a sulphur condenser in which sulphur is condensed and the condensate is extracted from the gas mixture.After the extraction ofthe sulphurvapourthe gas mixture has the following composition by volume : sulphur dioxide 8.6%; hydrogen sulphide 16.65%; water vapour61.6%; carbon dioxide 7.95%; hydrogen 4.0%; carbon monoxide 0.67%; carbon oxysulphide 0.50%.

Therearetypicallytwoconventional stages of catalytic conversion to achieve further reaction between hydrogen sulphide and sulphur dioxide.

Upstream of the first such stage the gas mixture from the sulphur condenser is reheated to about 232 C. It leaves the first stage at a temperature of about369.50C.Aftercondensation and extraction of the thus formed sulphur vapour, the gas mixture is reheated to about2l5.5'C and is then passed through the second catalytic conversion stage, in which its temperature rises to about 275 C. The resultant gas mixture has sulphurvapourcondensed and extracted therefrom and is then subjected to a conventional tail gastreatmentto remove most of the hydrogen sulphide that remains in the gas mixture after the second catalytic stage.

Claims (24)

1. A method of recovering sulphurfrom a feed gas stream comprising hydrogen sulphide, comprising dividing the feed gas stream into a major stream and a minor stream, burning in a first combustion region at least 50% of the hydrogen sulphide content ofthe minorstream to form sulphurdioxide and watervapour, and then cooling the minor stream, burning in a second combustion region less than one-third ofthe hydrogen sulphide content ofthe major stream to form sulphur dioxide water vapour, supporting the combustion of hydrogen sulphide in the major stream by supplying oxygen-rich gas (as hereinbefore defined) to the second combustion region, reacting hydrogen sulphide with the thus formed sulphur dioxide in a thermal reaction region associated with said second combustionregiontoformsulphurvapourand water vapour, extracting said sulphurvapourfrom the resulting gas mixture, and reacting residual hydrogen sulphide in the gas mixture with residual sulphur dioxide to form furthersuiphurvapourand water vapour, and then extracting the further sulphur vapour, wherein the cooled minorstream is introduced into the second combustion region or the thermal region associated therewith (or both) and aboutone-thirdofthetotal hydrogensuiphide content of the minor and major streams is burntto form sulphur dioxide and watervapour.

2. A method as claimed in claim 1, in which substantially all hydrogen sulphide content of the minorstream is burnttoform sulphurdioxideand water vapour.

3. A method as claimed in claim 1 or claim 2, in which the oxygen-rich gas is pure oxygen.

4. A method as claimed in any one of the preceding claims, in which the feed gas stream contains more than 60% by volume of hydrogen sulphide.

5. A method as claimed in claim 4, in which the feed gas stream contains at least 70% volume of hydrogen sulphide.

6. A method as claimed in any one of claims 1 to 5, in which no otherfluid is introduced into the second combustion region than the cooled minor stream and the oxygen-rich gas.

7. A method as claimed in any one of the preceding claims, in which a gas or gas mixture containing molecularoxygen is used to support a combustion oftheminorstream,theamountof molecular oxygen being supplied to the first region being in the range of 90 to 1 00% of that necessary for the complete combustion of the hydrogen sulphide.

8. A method as claimed in claim 7, in which the gas exiting the first combustion region is, downstream of where it is cooled, introduced into the hotzoneofatleastoneflameinthesecond combustion region.

9. A method as claimed in claim 8 in which the said exiting gas is mixed with the said major stream upstream of its introduction into said hot zone.

10. A method as claimed in anyone ofclaims7to 9, additionally including the step of introducing a moderator or quenchant into the first combustion region so asto control thetemperature.

11. A method as claimed in claim 10, in which the moderator or quenchant is selected from steam, liquid water, nitrogen or carbon dioxide.

12. A method as claimed in any one of claims 7 to 11, in which upto 10% of the feed gas stream is em ployed to form the m inor strea m, and the balance ofthefeed gas stream to form the major stream.

13. A method as claimed in any one of claims 7 to 12, in which all the cooled minorstream is introduced directly into the second combustion region.

14. A method as claimed in claim 7, in which a portion ofthecooled minor stream is recycled to the first combustion region to moderate the temperature thereof.

15. Amethod as claimed in claim 14, in which from 8 to 15% by volume of the feed gas stream is used to form the minor stream, and the balance of the feed gas stream to form the major stream.

16. A method as claimed in any one of the preceding claims, in which said further sulphur vapour is formed over a catalyst of the reaction between hydrogen sulphide and sulphur dioxide.

17. A method of recovering sulphurfrom a feed gas stream comprising hydrogen sulphide, substantially as herein described with reference to Figure 1 the accompanying drawings or Figure 1 as modified in the manner shown in Figure 2.

18. Amethod of recovering sulphurfrom a feed gas stream as described in any one ofthe examples herein.

19. Apparatus for recovering sulphur from a feed gas stream comprising hydrogen sulphide, including afirstconduitforrecovering a major portion of said feed gas stream; a second conduit for receiving a minor portion of said feed gas stream; a first combustion region having at least first burner associated therewith for burning at least 50% of the hydrogen sulphide content of the minor stream to form sulphur dioxide and water vapour, said burner having an inlet communicating with said second conduit, and said first combustion region having an outletcommunicatingwith an inletofa heat exchange means for cooling gas mixture from said first combustion region; a second combustion region having at least one second burner associated therewith for burning hydrogen sulphide to form water vapour and sulphur dioxide, said at least one second burner having an inlet in communication with said first conduit and an inlet communicating with a source of oxygen-rich gas (as hereinbefore defined); a thermal reaction region in which, in operation, sulphur dioxide reacts with hydrogen sulphide to form sulphurvapourand watervapour, said thermal reaction region communicating with an outlet from said second combustion region; a condenser, downstream of said thermal reaction region, for extracting sulphurvapourfrom gas mixture exiting said thermal reaction region; at least one further reaction region downstream of said condenser, for conducting further reaction between hydrogensulphideandsulphurdioxidetoform furthersulphurvapourandwatervapour; afurther condenserforextracting said further sulphur vapour, and means for introducing the cooled gas mixture exiting said heater exchange means into one or both of the second combustion region and said thermal reaction region, whereby, in operation, about one-third ofthe total hydrogen sulphide content of the said major and minor proportions to able to be burntto form sulphur dioxide and water vapour.

20. Apparatus as claimed in claim 19, wherein said burner associated with the first combustion region has means for introducing a moderator or quenchanttherein, said moderator or quenchant being able to be selected from steam, liquid water, carbon dioxide and nitrogen.

21. Apparatus as claimed in claim 20, wherein the means for introducing the cooled gas mixture exiting said heat exchange means is able to introduce all the cooled gas mixture into at least one flame in the second combustion region.

22. Apparatus as claimed in claim 19, including means for recycling a part of the cooled gas stream to the first combustion region to moderatethe temperature therein.

23. Apparatus as claimed in any one of claims 19 to 22, wherein said at least one further reaction region includes a catalyst ofthe reaction between hydrogen sulphide and sulphur dioxide.

24. Apparatus for recovering sulphurfrom a feed gas stream comprising hydrogen sulphide, substantially as herein described with reference to Figure 1 of the accompanying drawings or Figure 1 as modified in the manner shown in Figure 2 ofthe accompanying drawings.