Abstract:
Sulphur is recovered from a first gas stream comprising hydrogen sulphide and at least 50% by volume of ammonia and from a second gas stream comprising hydrogen sulphide but essentially no ammonia, the first gas stream, the second gas stream, and combustion supporting gas comprising at least one stream of essentially pure oxygen or oxygen-enriched air are fed to a single combustion zone or a plurality of combustion zones in parallel with each other without premixing of combustible gas with oxygen or air, and creating in the or each combustion zone at least one region in which thermal cracking of ammonia takes place, and taking from the reactor an effluent gas stream including sulphur vapour, sulphur dioxide, and hydrogen sulphide, but essentially no residual ammonia.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to recovering sulphur from a gas stream comprising ammonia and hydrogen sulphide.  
         BACKGROUND OF THE INVENTION  
         [0002]    Waste gas streams comprising hydrogen sulphide and ammonia are frequently encountered in refineries. Because hydrogen sulphide and ammonia are poisonous gases such waste gas streams need to be appropriately treated before being discharged to the atmosphere. Although such gas streams can be employed as a feed stream to the Claus process, care has to be taken to ensure that all the ammonia is destroyed upstream of the catalytic stages of the process because residual ammonia tends to react with sulphur dioxide to form ammonium salts which will block or poison the catalyst. These problems tend to increase in severity with increasing ammonia concentration in the waste gas stream.  
           [0003]    As a result, particularly if the ammonia concentration in the waste gas stream is in excess of 30% by volume, the practice in the art is to employ two separate combustion zones at the front end of the Claus process. All the waste gas stream comprising ammonia and hydrogen sulphide is fed to an upstream combustion zone. The waste gas stream is typically mixed with one part of another waste gas stream comprising hydrogen sulphide but essentially no ammonia. The rest of the other waste gas stream is supplied to a downstream combustion zone. Accordingly any ammonia which is not destroyed in the upstream combustion zone will tend to be incinerated in the downstream combustion zone. Such processes are, for example, disclosed in WO-A-88/02350 and EP-A-0 325 286.  
           [0004]    EP-B-0 034 848 discloses destroying the ammonia content of a waste gas stream by supplying the gas stream to the outer of two concentric tubes forming part of a burner. A hydrogen sulphide stream free of ammonia is supplied to the inner concentric tubes. The two tubes debouch into a mixing chamber, of which the downstream end terminates in a combustion chamber. Although only a single combustion zone is nominally employed, difficulties arise in the fabrication and operation of the mixing chamber such the high temperatures created do not damage it. The reason for employing the mixing chamber is to ensure that the gases to be burned are thoroughly mixed with combustion—supporting air upstream of the combustion chamber. Intimate mixing is deemed to be necessary to ensure that all the ammonia is destroyed by combustion.  
         SUMMARY OF THE INVENTION  
         [0005]    We have discovered that the thermal dissociation of ammonia to nitrogen and hydrogen can play an important part in its destruction. Accordingly, provided an adequately high temperature region or regions within the flame zone can be created for the thermal cracking of ammonia it is not necessary either to employ two separate combustion or flame zones or, in the case of a single flame zone, to employ a discrete mixing chamber upstream thereof.  
           [0006]    According to the present invention there is provided a method of recovering sulphur from a first gas stream comprising hydrogen sulphide and at least 50% by volume of ammonia and from a second gas stream comprising hydrogen sulphide but essentially no ammonia, including feeding the first gas stream, the second gas stream, and combustion supporting gas comprising at least one stream of essentially pure oxygen or oxygen-enriched air to a single combustion zone or a plurality of combustion zones in parallel with each other within a reactor without premixing of combustible gas with oxygen or air, and creating in the or each combustion zone at least one region in which thermal cracking of ammonia takes place, and taking from the reactor an effluent gas stream including sulphur vapour, sulphur dioxide, and hydrogen sulphide, but essentially no residual ammonia.  
           [0007]    If desired a single burner or a plurality of burners may fire into the or each combustion zone.  
           [0008]    The method according to the invention is particularly suitable for use if the first gas stream contains at least 60% by volume of ammonia.  
           [0009]    Preferably, there is fed to the or each combustion zone in addition to the stream or streams of essentially pure oxygen or oxygen-enriched air a stream or streams of air. Such an arrangement facilitates the creation of a relatively hot thermal cracking zone or zones within the combustion zone without exceeding a maximum temperature for the effluent gas stream above which thermal damage is liable to be caused to the reactor even if the normal precaution of providing the reactor with an internal refractory lining is taken.  
           [0010]    Preferably, the or each combustion zone has at least three stages. In one arrangement of a burner for use in the method according to the invention to create a combustion zone having at least three stages, a first flow of the first gas stream is preferably supplied to the flame from a first region of the mouth of the burner; at least one second flow of a combustion supporting gas is caused to issue from the mouth of the burner and mix in the flame with the first gas stream; at least one third flow of the second gas stream is supplied to the flame from a second region of the mouth of the burner surrounding and spaced from the first region; at least one fourth flow of a combustion supporting gas is caused to issue from the mouth of the burner the and mix in the flame with the second gas stream, and at least one fifth flow of a combustion supporting gas of different composition from the second and fourth flows is caused to mix in the flame with the second gas stream. Burning the first and second gas streams in three stages, typically an innermost stage, an outermost stage, and an intermediate stage, makes it possible to achieve a relatively low temperature in the outermost stage in comparison with a temperature in excess of 2000°C. in the innermost stage. Such a high temperature in the innermost stage facilitates destruction of the ammonia in the first gas stream.  
           [0011]    Preferably the flame extends generally longitudinally within the furnace. The furnace is typically disposed with its longitudinal axis horizontal, and therefore the burner is typically also disposed with its longitudinal axis extending horizontally. Such arrangements can help to keep down the risk of damage to any refractory lining employed in the furnace.  
           [0012]    The second and fourth flows of combustion supporting gas preferably both have a mole fraction of at least 0.22 and may be oxygen-enriched air containing at least 50% by volume of oxygen or pure oxygen. The third oxidising gas is preferably atmospheric air neither enriched in nor depleted of oxygen, although enrichment up to 25 or 30% by volume of oxygen, or higher depending on the composition of the first and second gas streams, is generally acceptable.  
           [0013]    Mixing of the first gas stream with the first combustion supporting gas is preferably facilitated by directing at least some of the first combustion supporting gas along a path or paths which meet a path or paths followed by the first gas streams. Accordingly, the second outlet or at least some of the second group of outlets preferably each have an axis which extends at an angle to the axis of the first outlet or the axes of at least some of the second group of outlets. The angle is preferably in the range of 10 to 30°. Preferably, the flow of the first gas stream is axial and the flow of the first combustion supporting gas is at an angle to the axis of the burner. The combustible gas stream and the first gas stream may be supplied at the same velocity as one another or at different velocities.  
           [0014]    Alternatively, mixing of the first gas stream with the first combustion supporting gas can be facilitated by directing at least some of the first combustion supporting gas at a first linear velocity along a path or paths generally contiguous and generally parallel to a paths or paths followed by the first combustible gas at a second linear velocity, and one of the first and second linear velocities is from 25 to 150% (and preferably from 25 to 100%) greater than the other thereof. Mixing is facilitated because the differential velocity between the first combustion supporting gas and the first gas stream creates forces of shear therebetween. Preferably, it is the first linear velocity which is selected to be a greater of the two velocities. This arrangement facilitates design of the furnace to ensure that all the ammonia is destroyed in it. A further alternative or additional means for facilitating mixing of the first gas stream of the first oxidising gas is to impart a swirling motion to one or both of the streams. Devices which are able to impart swirl to such gas are well known.  
           [0015]    The natural curvarture of the flame tends to facilitate mixing of the forth flow of second oxidising gas with the third flow of the second gas stream. Nevertheless, it is preferred to arrange the supply of the said third and fourth flows so as further to facilitate mixing. Similar means to those described above with reference to the first and second flows can therefore be used.  
           [0016]    A particular advantage of the method according to the invention is that any oxides of nitrogen formed in the innermost or other oxygen rich stage of the flame will be reduced back to nitrogen with the result that the effluent gas stream is essentially free of oxides of nitrogen.  
       
    
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0017]    The method according to the invention will now be described by way of example with reference to the accompanying drawings, in which:  
         [0018]    [0018]FIG. 1 is a schematic sectional side elevation of a burner for use of a method according to the invention;  
         [0019]    [0019]FIG. 2 is a schematic end view of the mouth of the burner shown in FIG. 1; and  
         [0020]    [0020]FIG. 3 is a schematic flow diagram illustrating apparatus for supplying combustible gas and oxidising gas to the burner shown in FIGS. 1 and 2.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring to FIGS. 1 and 2 of the drawings, a burner  2  is of generally cylindrical shape and has a proximal end  4  and a distal end (or mouth)  6 . The burner  2  has a central passageway  8  defined by an innermost tube  10  for flow of a first combustible gas stream comprising hydrogen sulphide. The longitudinal axis of the burner  2  is coincident with the longitudinal axis of the tube  10 . The central passageway  8  has a first outlet  12  at its distal end. A second tube  14  is coaxial with the first tube  10 . The inner surface of the tube  14  makes a frictional engagement with the outer surface of the tube  10 . (Alternatively the tubes  10  and  14  may be joined to one another by means of an internal flange or welded connection.) A third tube  16  is spaced from and is coaxial with the tube  14 . Tubes  14  and  16  define a second, annular, passageway  18  for a second flow of combustible gas mixture comprising hydrogen sulphide terminating at its distal end in an outlet  20  for the second combustible gas mixture. The tubes  14  and  16  terminate in the same plane as the tube  10 .  
         [0022]    An array of tubes  22  extends from beyond the proximal end of the tube  10  therethrough and defines passages  24  for the flow of a first oxidising gas mixture. Each passageway  24  has an outlet  26 . (These outlets are termed “the second group of outlets” hereinabove.) The tubes  22  terminate in the same plane as the tube  10 . The outlets  26  are typically disposed in a ring which is coaxial with the longitudinal axis of the burner  2 .  
         [0023]    A second array of tubes  28  is disposed in the passageway  18  defined by the tubes  14  and  16 . Each tube  28  each defines a passageway  30  for a second flow of oxidising gas terminating in respective outlet  32 . The tubes  28  each terminate in the same plane as the tube  10 . The outlets  30  of the tubes  28  are arranged in a ring which is coaxial with the longitudinal axis of the burner  2 . The respective tubes  22  and  28  may each be provided with a spider  34  to help support them when the burner is disposed with its longitudinal axis horizontal as shown in FIG. 1. There is considerable flexibility in selecting the actual numbers of the tubes  22  and  28 .  
         [0024]    The construction of the burner  2  so as to enable to the respective flows of first and second gas streams and combustion-supporting gas to be supplied to it is relatively simple. The outer tube  16  is provided with a first port  36  for the flow of the second combustible gas mixture comprising hydrogen sulphide. The proximal end of the outer tube  16  is formed with a flange  38  integral therewith or welded thereto. The flange  38  is bolted or otherwise secured to a similar flange  40  which is integral with or welded to the tube  14 . If desired, a gasket or other sealing member (not shown) can be engaged between the flanges  40  and  38  so as to ensure a fluid-tight seal therebetween. The flange  40  forms the distal end of a chamber  42  which receives the second oxidising gas and which has a port  44  enabling it to be placed in communication with a source of such oxidising gas. The proximal ends of the tubes  28  are all received fluid-tight in complementary apertures through the flange  40 . Thus, the tubes  26  communicate with the chamber  42 . The chamber  42  has an outer wall  46 , in which the port  44  is formed, which is provided at its distal end with a flange  48  which is fastened fluid-tight to the flange  40  and at its proximal end with a flange  50 . The flange  50  is bolted or otherwise secured fluid-tight to a complementary flange  52  which is integral with or is welded to the proximal end of the tube  10 . The flange  52  forms a proximal wall of the chamber  42 . It also forms a distal wall of a further chamber  54  having a side wall  56  with a port  58  formed therein which enables the chamber  54  to be placed in communication with a source of the first combustible gas. The wall  56  of the chamber  54  has a first flange  60  at its distal end which is bolted or otherwise secured fluid-tight to the flange  52  and a second flange  62  at its proximal end which is bolted or otherwise secured fluid-tight to an end plate  64  which forms a dividing wall between the chamber  54  and a yet further chamber  66  for the first oxidising gas mixture and which receives fluid-tight in apertures formed therethrough the proximal ends of the tubes  22  so as to enable these tubes to receive a flow of the first oxidising gas mixture. The chamber  66  is provided with a port  68  which is coaxial with the longitudinal axis of the burner  2  and is able to be placed in communication with the source of the first oxidising gas mixture.  
         [0025]    As shown in FIG. 1, the distal end  6  of the burner  2  extends into a port or quart  70  of a furnace  72  for the partial combustion of hydrogen sulphide. An annular passage is defined between the distal end of the burner  2  and the port  70 . Air is supplied to this passage  74  as a third oxidising gas.  
         [0026]    If desired, the distal end of the outer tube  16  may be formed of a refractory metal. Other parts of the burner  2  may be formed of stainless steel.  
         [0027]    In operation, the first gas stream, which comprises a mixture of hydrogen sulphide, ammonia, carbon dioxide and water vapour containing at least 50% by volume of ammonia, exiting the burner  2  from the outlet  12  becomes intimately mixed with the first oxidising gas that leaves through the outlets  26  to form a first stage of a flame. Similarly, the flow of the second gas stream, which comprises a mixture of hydrogen sulphide, carbon dioxide and water vapour (but no ammonia), leaving the burner  2  through the outlet  20  becomes intimately mixed with the flow of the second oxidising gas which leaves the burner  2  through the outlets  32 , thus forming a second stage of the flame. A third stage of the flame is formed by intimate mixing of the air passing through the passage  74  with the second gas stream leaving the burner  2  through the outlet  20 .  
         [0028]    An arrangement for supplying different gas flows to the burner  2  is shown in FIG. 3. Referring to FIG. 3, a first pipeline  80  for sour water stripper gas (which includes both hydrogen sulphide and ammonia) terminates in the port  58  of the burner  2 . The first flow control valve  82  is disposed in the pipeline  80 . A second pipeline  84  for amine gas (which predominantly comprises hydrogen sulphide) terminates in the port  36  of the burner  2  and has a second flow control valve  86  disposed therein. A third pipeline  88  communicating with a source (not shown) of first oxidising gas composed of air or oxygen-enriched air terminates in the port  68  of the burner  2 . A third flow control valve  90  is located the third pipeline  88 . A fourth pipeline  92  communicating with a source (not shown) of second oxidising gas composed of air or oxygen-enriched air terminates in the port  44  of the burner  2 . A fourth flow control valve  94  is located in the fourth pipeline  92 . A fifth pipeline  96  communicating with a blower (not shown) or other source of compressed air (neither enriched in nor depleted of oxygen) terminates in an inlet  104  to a nozzle  106  which communicates with the annular passage  74  defined between the ports  70  and the burner  2 . The pipeline  96  has a fifth flow control valve  98  disposed therein. In addition, a pipeline  100  extends through a region of the second pipeline  84  upstream of the second flow control valve  86  to a region of the first pipeline  80  downstream of the first flow control valve  82 . A sixth flow control valve  102  is disposed in the pipe  100 .  
         [0029]    In operation, the flow control valves described above may be set to determine the overall mole ratio of combustibles to oxygen supplied to the flame of the burner  2 , so as to enable different local ratios of the reacting species to be created in different regions of the flame, so as to enable a hot innermost region to be maintained in the flame at a temperature typically in excess of 1700°C. in which zone both combustion and thermal reacting of ammonia takes place, so as to enable a much lower temperature to be maintained at the periphery of the flame, to create within a localised region of the flame conditions which favour thermal dissociation of hydrogen sulphide, and to ensure that all ammonia is destroyed. Typically, the rates of supply of the reactants are controlled such that the mole ratio of hydrogen sulphide to sulphur dioxide and the gas mixture leaving the furnace is approximately 2:1. Within the respective regions of the flame, however, the mole ratio of hydrogen sulphide to sulphur dioxide can vary significantly.  
         [0030]    The kind of flame that is formed in operation of the burner is shown schematically in FIG. 3 and is indicated therein by the reference numeral  110 . The flame has three stages  112 ,  114  and  116 . The innermost stage  112  is a high intensity zone into which the first oxidising gas and the first combustible gas flow. The first oxidising gas is supplied at a rate in excess of the stoichiometric rate that would be required for the oxidation of all the ammonia and one third of the hydrogen sulphide supplied to the innermost stage. In fact, as mentioned below, some of the ammonia is destroyed by thermal cracking to nitrogen and hydrogen. Hydrocarbons in the first combustible gas and oxidation of more than one third of the hydrogen sulphide. A high temperature in the innermost stage  112  is thus ensured. The temperature can be controlled by the control valves  82 ,  90  and  102 .  
         [0031]    The intermediate stage  114  of the flame  110  receives the second oxidising gas and part of the second gas stream. This stage  114  is typically operated oxygen-poor that is to say that the relative rates of supply of hydrogen sulphide and oxygen molecules to this stage are such that less than one third of this hydrogen sulphide is oxidised to sulphur dioxide. The paucity of oxygen in this region together with the heat radiated from the inner stage  106  favour formation of sulphur vapour by thermal cracking of hydrogen sulphide. Since the thermal cracking of hydrogen sulphide proceeds endothermically, it provides a mechanism for moderating flame temperature and helps to prevent excessive temperatures being created in the outermost stage  116 . Further, it can reduce the demand for nitrogen molecules to moderate the flame temperature, and thereby enables the first and second oxidising gases to have higher mole fractions of oxygen than would otherwise be possible. The temperature can be controlled in the stage  108  by the rate of flow of the first combustible gas and by its mole fraction of oxygen. The third or outermost stage  116  of the flame receives the rest of the second combustible gas and the air which is supplied as the third oxidising gas to the pipeline  96 . The rate of supply of air is controlled so as to ensure that an excessive flame temperature is not created in the stage on  116 . The total supply of oxidant is controlled such that the desired ratio of H 2 S to SO 2  is maintained after the waste heat reboiler (not shown).  
         [0032]    When the combined rates of supply of the first and second gas streams are at a specified maximum, typically the rates of supply of the oxidising gases are each at a maximum. If the total rate of supply of combustible gas falls, various control strategies are available to maintain suitable combustion conditions in the flame. Most simply, the control valves  94  and  98  may be reset to make a complementary reduction in the flow rate of the oxidising gases. In addition, means (not shown) may be provided in the apparatus shown in FIG. 3 for adjusting the mole fraction of oxygen in the first and second oxidising gases. For example, additional flow control valves (not shown) may be provided in pipes (not shown) which introduce commercially pure oxygen into the third and fourth pipelines  88  and  92 , respectively. Thus, the mole fraction of the oxygen in the first and second oxidising gases may be controlled.  
         [0033]    Referring again to FIGS. 1 and 2, it will be appreciated that all the gas streams leave the burner  2  substantially axially. Mixing between the first oxidising gas leaving the burner  2  through the outlets  26  and the first combustible gas leaving through the outlets  12  thereby takes place by virtue of shear between the respective gas streams. The degree of shear is enhanced by having a differential velocity between the first combustible gas and first oxidising gas. In one illustrative example, the velocity of the first oxidising gas leaving the burner  2  is 60 m/s and the velocity of the first combustible gas is 40 m/s.  
         [0034]    Any NO w  (oxides of nitrogen) formed in the innermost stage  114  or other region of the flame  110  is subsequently reduced again to nitrogen, particularly in the other stages of the flame which are reducing. Further, hydrogen formed by the thermal dissociation of ammonia tends to be oxidised to water vapour by reaction with oxygen in the flame  110 .