Abstract:
Novel sulfur recovery plants, and processes utilizing these plants are disclosed. These apparatuses eliminate the use of a condenser between the waste heat boiler and first Claus catalytic reactors, and also eliminate the use of reheaters in between Claus catalytic reactors.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention generally relates to methods and apparatus for recovering elemental sulfur from hydrogen sulfide-containing gas streams, and more particularly to such methods and apparatus that reduce the size and complexity of existing Claus plants. 
       BACKGROUND OF THE INVENTION  
       [0002]    Large quantities of H 2 S-containing gases are commonly produced in the natural gas and petroleum industry and concentrated by amine treating units and sour water stripping units. Claus sulfur recovery plants (“Claus plants”) are in widespread use to convert this environmentally hazardous H 2 S to useful elemental sulfur by oxidation according to the overall or net equation 
         [0000]      H 2 S+½O 2 →1/x S x +H 2 O   (1) 
         [0000]    wherein x=2, 6 or 8, depending on the particular conditions of temperature and pressure. The net production of elemental sulfur is usually accomplished as a series of process steps carried out according to a conventional plant flow scheme. A conventional Claus unit comprises a free flame combustion/reaction furnace stage and a catalytic stage. 
         [0003]    The free flame combustion step takes place by burning ⅓ of the H 2 S in burner according to the equation: 
         [0000]      H 2 S+3/2 O 2 →SO 2 +H 2 O   (2). 
         [0000]    Oxygen for the combustion stage is usually supplied by air from an air compressor or blower. The combustion stage is followed by the stages in which the “Claus reaction” takes place according to the equation 
         [0000]      2 H 2 S+SO 2 ⇄3/x S x +2 H 2 O   (3) 
         [0000]    wherein x=2, 6 or 8, depending on the particular conditions of temperature and pressure. 
         [0004]    The Claus reaction initially takes place in the reaction furnace immediately following the burner, and while the gases are at near-flame temperatures. After the gases exit the reaction furnace they are cooled in a waste heat boiler (WHB), usually with boiling water circulating in the waste heat boiler and being converted to medium to high-pressure steam. After cooling, the gases are cooled further in a sulfur condenser, in which boiling water is circulated to make low pressure steam. At this stage in the process about 50-70% of the incoming H 2 S will typically have been converted to elemental sulfur. The actual amount depends on such factors as inlet H 2 S concentration, flame temperature, residence time in the reaction furnace following the burner, and the presence and amount of other chemicals such as other combustibles or carbon dioxide. Condensed liquid sulfur product is usually recovered at this point in the process. 
         [0005]    A 70% level of conversion is insufficient by today&#39;s standards to allow the effluent from the Claus furnace to be emitted to the atmosphere or to make tail gas treatment economical at this point. An increase in the overall level of conversion is usually achieved by removing one of the reaction products from the mixture (e.g., by condensing and removing liquid elemental sulfur), and then allowing the remaining gases to continue reacting until equilibrium is reached (Equation 3). After the reaction furnace, the reacted gases are cooled in a WHB against boiling water. The gases can be cooled to allow condensation of sulfur in this WHB, or, more typically, the cooled gases from the WHB are further cooled in a separate sulfur condenser to facilitate condensation of the sulfur formed in the first reaction stage. 
         [0006]    In modified Claus plants, further recovery of sulfur is accomplished by taking the gases from the first condenser, reheating, and then passing the gases over a high surface area Claus catalyst in a packed bed reactor. The Claus reaction (Equation 3) takes place on the catalyst up to the equilibrium limit of the reaction. Some well-known Claus catalysts are bauxite, alumina and titania. The Claus catalytic reactors are normally operated in the gas phase to prevent condensed sulfur from plugging the pores of the catalyst. To enhance recovery of sulfur via the Claus reaction, the elemental sulfur is conventionally removed by condensation in a sulfur condenser which follows the catalytic reactor. Similar reheat, reaction and condensation steps are commonly repeated two to three times in order to maximize sulfur yield of the plant. Because of the equilibrium restraints inherent in the Claus reaction (Equation 3), adding more catalytic Claus reactors becomes ineffective beyond a total of three or four units, so other measures must be taken in order to further increase sulfur recovery beyond about 98 vol. % of the initial H 2 S and to complete the recovery of the remaining sulfur before the effluent is released to the atmosphere. 
         [0007]    The addition of equipment needed to improve recovery almost invariably decreases the capacity of the plant by adding resistance to flow from additional friction. Thus the addition of each reheater, catalytic Claus reactor, sulfur condenser and tail gas treatment unit is accompanied by a reduction in operating pressure. Moreover, as demand for sulfur recovery capacity grows in an existing facility, the flows of O 2 -containing gas and H 2 S-containing gas into the Claus plant will increase. This increase in flow causes an increase in pressure drop through the system approximated by the relationship 
         [0000]        DP   2   /DP   1 =( Q   2   /Q   1 ) 2    (4) 
         [0000]    where DP is pressure drop, Q is volumetric flow rate,  1  is the initial flow condition, and  2  is the new flow condition. In any given system, at a certain flow rate of H 2 S-containing gas the pressure drop due to friction from flow will exceed the available pressure drop through the unit. At that point, the unit is capacity constrained. Conventional Claus plants operate at low pressure, usually 20-30 psia at the front of the plant. In almost every case, a conventional sulfur recovery plant with a burner, reaction furnace, multiple reheat, catalytic Claus reactor, and condenser stages, and single tail gas treatment unit is limited to 5 to 15 psi of available pressure drop. Many existing Claus plants suffer from a severe constraint in capacity. 
         [0008]    Following LeChatelier&#39;s principle, the flame and reaction furnace section of the furnace should be operated at the highest temperature possible to drive the equilibrium conversion of sulfur. This temperature is usually regulated by the incoming reactant temperatures, by the concentration of H 2 S and other combustible gases, such as light hydrocarbons, and the presence of inerts in either the H 2 S-containing gas or in the air. It is assumed in Claus design that as the reaction mixture cools in the waste heat boiler following the reaction furnace, the mixture will be at or near equilibrium and the mixture will retain this composition by the rapid cooling in the waste heat boiler “quenching” the reaction. 
         [0009]    Another assumption is that the formation of sulfur in the reaction furnace/waste heat boiler will inhibit the formation of sulfur in subsequent catalytic stages according to LeChatelier&#39;s principle; that is, sulfur is a reaction product, so having sulfur in this stream will shift the reaction equilibrium the wrong direction if kept in the process stream. Therefore, the waste heat boiler is normally built with extra heat transfer capability to condense the bulk of the sulfur vapor formed, or a sulfur condenser after the waste heat exchanger is added. It is also typical to reduce the temperature of the gases from the condenser to get the maximum amount of sulfur out of the gas stream before proceeding to the next conversion stage. Simplification of the Claus process by removing pieces of equipment in the apparatus and process flow can be beneficial by reducing the cost of equipment and by decreasing the frictional resistance to flow thereby increasing unit capacity. 
       SUMMARY OF THE INVENTION  
       [0010]    In one embodiment of the present invention, a sulfur recovery plant consists of or consists essentially of: a) a burner; b) a reaction furnace; c) a waste heat boiler; d) a series of reactors in fluid flow communication with the waste heat boiler wherein there is a condenser in between each reactor and after the final reactor in the series of reactors; and e) a tail gas treatment zone. 
         [0011]    In another embodiment of the present invention, a sulfur recovery plant comprises, consists of, or consists essentially of: a) a burner; b) a reaction furnace; c) a waste heat boiler; d) a first reactor in fluid flow communication with the waste heat boiler and a series of subsequent reactors in fluid flow communication with the first reactor, wherein the series of subsequent reactors includes a final reactor, and wherein there is not a condenser between the waste heat boiler and the first reactor, and wherein a condenser precedes each reactor in the series of subsequent reactors, and there are no reheaters in between each condenser and reactor in each reactor in the subsequent series of reactors; and e) a final condenser between the final reactor and a tail gas treatment zone. 
         [0012]    In yet another embodiment of the present invention, a process for recovering elemental sulfur from a gas stream comprising hydrogen sulfide consists of or consists essentially of: a) passing a gas stream comprising hydrogen sulfide and an O 2 -containing gas through a burner, a reaction furnace and a waste heat boiler to yield a process gas stream comprising elemental sulfur, water, SO 2 , and any unreacted hydrogen sulfide; b) passing the process gas stream through a series of reactors in fluid flow communication with the waste heat boiler wherein the process gas stream passes through a condenser in between each reactor; c) passing the process gas stream through a final condenser after the last reactor in the series; and d) passing the process gas stream through a tail gas treatment zone. 
         [0013]    In yet another embodiment of the present invention, a process for recovering elemental sulfur from a gas stream comprising hydrogen sulfide, comprises, consists of, or consists essentially of: a) passing the gas stream through a burner, a reaction furnace and a waste heat boiler to yield a process gas stream comprising elemental sulfur, water, SO 2 , and any unreacted hydrogen sulfide; b) passing the process gas stream through a first reactor in fluid flow communication with the waste heat boiler wherein the process gas stream does not first pass through a condenser after leaving the waste heat boiler and before arriving at said first reactor; c) passing the process gas stream through a series of subsequent reactors in fluid flow communication with the first reactor, wherein the process gas stream passes through a condenser before passing through each reactor in the series of subsequent reactors, and wherein the process stream does not pass through a reheater in between each condenser and reactor in each of the series of subsequent reactors; d) passing the process gas stream through a final condenser after the last reactor in the series; and e) passing said process gas stream to a tail gas treatment zone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]      FIG. 1  is a schematic drawing of a conventional Claus sulfur recovery plant. 
           [0015]      FIG. 2  is a schematic drawing of a Claus sulfur recovery plant in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    As shown in  FIG. 1 , a Claus sulfur recovery plant  10  comprises a conventional burner  16  and reaction furnace  18  followed by a high temperature waste heat boiler (WHB)  20 . A stream comprising of either an acid gas (generally comprising of H 2 S and CO 2 ), or an acid gas and a sour water stripper gas (‘SWS gas’) (generally comprising of water vapor, H 2 S and NH 3 ) enters burner  16  via conduit  12 . Air for the combustion reaction taking place in burner  16  is supplied to it from an air compressor or blower via conduit  14 . Steam pressure in WHB  20  is generally in the range of from about 50 psig to about 600 psig. The temperature of WHB  20  is generally in the range of from about 500° F. to about 800° F. 
         [0017]    The stream exits WHB  20  via process gas outlet  22  and passes to first sulfur condenser  24 . First sulfur condenser  24  (along with sulfur condensers  36 ,  48 , and  60 ) has an outlet for steam (shown as ‘STM’ in the figure), an inlet for boiler feed water (shown as ‘BFW’ in the figure), and an outlet for liquid sulfur (shown as ‘Liq S’ in the figure). Steam pressure in sulfur condenser  24  (and also in sulfur condensers  36  and  48 ) is typically in the range of from about 40 to about 60 psig. The process gas that emerges from first sulfur condenser  24  passes to heater  28  for pre-heating via conduit  26  prior to entering a first Claus catalytic reactor  32  via conduit  30 . The temperature of conduit  30  can be adjusted between about 500° F. to about 550° F. to allow first Claus catalytic reactor  32  to attain temperatures above about 600° F. for COS and CS 2 , which are byproducts from burner/reaction furnace  16  and  18 , to be converted to H 2 S. Optionally, conduit  30  can be operated as cool as possible, generally from about 400° F. to about 450° F. to get maximum conversion in first Claus catalytic reactor  32 , following LeChatelier&#39;s principle (lower temperatures increase conversion). In this case, conduit  34  is kept at least 30° F. above the sulfur dew point by adjusting the temperature of conduit  30 . 
         [0018]    First Claus catalytic reactor  32  is followed by the second conventional sulfur condenser  36 , which the feed enters via conduit  34 . Heater  40  (via conduit  38 ) follows second condenser  36  and precedes second Claus catalytic reactor  44  (via conduit  42 ). Conduit  42  is typically heated at a temperature range from about 390° F. to about 450° F., depending on the sulfur dew point of conduit  46 . 
         [0019]    After reactor  44 , the feed passes into third sulfur condenser  48  via conduit  46 , followed by heater  52 , in which the feed enters via conduit  50 . Heater  52  precedes a third Claus catalytic reactor  56  via conduit  54 . Conduit  54  is typically heated at a temperature range from about 375° F. to about 425° F. depending on the sulfur dew point of conduit  46 . 
         [0020]    Following reactor  56  is fourth sulfur condenser  60 , which the feed enters via conduit  58 . The feed then enters a tail gas treatment zone or an incinerator  64  via conduit  62  for further treatment. Fourth sulfur condenser  60  can be operated in the same manner as condensers  24 ,  36 , and  48 , but it can also have lower pressure steam or heat pressurized water in order to keep the stream temperature leaving condenser  60  and passing through conduit  62  in the range of from about 250° F. to about 275° F. This reduces elemental sulfur passing to tail gas treatment zone  64 . Generally, the surface area of tubes located in condensers  24 ,  36 ,  48 , and  60  is designed to get the stream as cool as possible to take the maximum amount of sulfur vapor out of the streams leaving the condenser. 
         [0021]    As shown in  FIG. 2 , a modified Claus sulfur recovery plant  100  comprises a reaction furnace  108  followed by WHB  110 . A feed enters burner  106  via conduit  102 . Air is supplied via conduit  104 . The thermal stage of the Claus process in  FIG. 2  operates in generally the same manner as the thermal stage in  FIG. 1 . The surface area of WHB  110  is designed and steam pressure range is selected to keep the temperature of conduit  114  above the dew point of sulfur and below about 600° F. The actual operating temperature of conduit  114  is chosen to get maximum conversion in first Claus catalytic reactor  116  without going below the dew point of sulfur anywhere in the reactor. Optionally, the steam pressure in WHB  110  can be adjusted to compensate for changes in flow or amount of heat released in burner/reaction furnace  106 / 108  to keep the temperature in conduit  114  from going below the sulfur dew point or from getting above 600° F. While not wishing to be bound by theory, it is believed that the effect on not condensing the sulfur in WHB  110  or in a condenser after WHB  110  is minimal on overall unit efficiency. The equilibrium position of the reaction is determined by the reactants and their starting temperature, pressure, and composition and the final temperature and pressure of first Claus catalytic reactor  116 . By allowing the process gas stream to pass from WHB  110  to first Claus catalytic reactor  116 , the reaction can be continued to the higher conversion at a lower temperature without removal of any of the product elemental sulfur from reaction furnace  108 . 
         [0022]    The feed exits WHB  110  via process gas outlet  112  and, instead of passing to a condenser, passes to first Claus catalytic reactor  116  via conduit  114 . The process gas that emerges from first Claus catalytic reactor  116  passes to first sulfur condenser  120  via conduit  118 . First sulfur condenser  120  (along with sulfur condensers  128  and  136 ) has an outlet for steam and liquid sulfur, along with an inlet for BFW, as the condensers in  FIG. 1 . The surface area of condensers  120  and  128  is designed and the steam pressure range is selected to keep the temperatures of conduits  122  and  130  high enough to keep conduits  126  and  134 , respectively, above the dew point of sulfur. The temperature range of conduits  122  and  130  is typically from about 390° F. to about 450° F. The steam pressure range in condensers  120  and  128  is generally in the range of from about 22 psig to about 65 psig, depending on the temperatures of conduits  122  and  130 , on the final disposition of the steam generated, and on the flow rates of conduits  102  and  104 . 
         [0023]    The feed passes into second Claus catalytic reactor  124  via conduit  122 . After reactor  124 , the feed passes into second sulfur condenser  124  via conduit  126 . The feed then enters third Claus catalytic reactor  132  via conduit  130 . Following reactor  132  is third sulfur condenser  136 , which the feed enters via conduit  134 . Third sulfur condenser  136  operates in generally the same manner as condenser  60  in  FIG. 1 , above. The feed then enters a tail gas treatment zone or an incinerator  140  via conduit  138  for further treatment. 
       EXAMPLES 
       [0024]    The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill in the art to make and use the invention. These examples are not intended to limit the invention in any way. 
       Example 1 
     Conventional Claus Unit 
       [0025]    A computer model was used to simulate a sulfur recovery process in a conventional Claus unit. Two feed streams were used—an amine acid gas and a SWS acid gas. These feed compositions, temperatures and pressures are found in Table 1, below. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Feeds Used in Simulation 
               
             
          
           
               
                   
                 Feed 
                 Composition 
                 Temperature 
                 Pressure 
               
               
                   
                   
               
               
                   
                 Acid Gas 
                 93.47 mol. % H 2 S, 
                 110° F. 
                 26.7 psia 
               
               
                   
                   
                  6.02 mol. % CO 2   
               
               
                   
                 SWS Gas 
                 44.75 mol. % H 2 S, 
                 180° F. 
                 26.7 psia 
               
               
                   
                   
                 55.25 mol. % NH 3   
               
               
                   
                   
               
             
          
         
       
     
         [0026]    The feeds passed through the following components of a Claus unit in this order: an acid gas mixer, an acid gas preheater, a SWS gas preheater, an acid gas/SWS gas mixer, an air combuster, a reaction furnace, a waste heat boiler, a first condenser, a first reheater, a first reactor, a second condenser, a second reheater, a second reactor, a third condenser, a third reheater, a third reactor, and a fourth condenser. The total sulfur recovery was 97.1%. Operating parameters are shown in Table 2, below. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Operating Parameters for Example 1 
               
             
          
           
               
                 Component 
                 Temperature, ° F. 
                 Pressure, psia 
               
               
                   
               
             
          
           
               
                 Waste Heat Boiler Inlet 
                 2442.6 
                 24.4 
               
               
                 Waste Heat Boiler Gas Outlet 
                 600.0 
                 24.1 
               
               
                 Waste Heat Boiler Liquid Outlet 
                 600.0 
                 24.1 
               
               
                 First Condenser Inlet 
                 600.0 
                 24.1 
               
               
                 First Condenser Vapor Outlet 
                 350.0 
                 23.6 
               
               
                 First Condenser Liquid Outlet 
                 350.0 
                 23.6 
               
               
                 Second Condenser Inlet 
                 589.4 
                 22.7 
               
               
                 Second Condenser Vapor Outlet 
                 340.0 
                 22.2 
               
               
                 Second Condenser Liquid Outlet 
                 340.0 
                 22.2 
               
               
                 Third Condenser Inlet 
                 458.1 
                 21.3 
               
               
                 Third Condenser Vapor Outlet 
                 330.0 
                 20.8 
               
               
                 Third Condenser Liquid Outlet 
                 330.0 
                 20.8 
               
               
                 Fourth Condenser Inlet 
                 408.0 
                 19.9 
               
               
                 Fourth Condenser Vapor Outlet 
                 270.0 
                 19.4 
               
               
                 Fourth Condenser Liquid Outlet 
                 270.0 
                 19.4 
               
               
                   
               
             
          
         
       
     
       Example 2 
     Inventive 
       [0027]    A computer model was used to simulate a sulfur recovery process in a manner consistent with at least one embodiment of the present invention. Two feed streams were used—an acid gas and a SWS acid gas. These feed compositions, temperatures, and pressures were identical to those used in Example 1. Operating parameters of Example 2 are shown in Table 3, below. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Operating Parameters of Example 2 
               
             
          
           
               
                 Component 
                 Temperature, ° F. 
                 Pressure, psia 
               
               
                   
               
               
                 Waste Heat Boiler Inlet 
                 2442.5  
                 24.4 
               
               
                 Waste Heat Boiler Gas Outlet 
                 560.0 
                 24.1 
               
               
                 First Condenser Inlet 
                 662.4 
                 23.5 
               
               
                 First Condenser Vapor Outlet 
                 400.0 
                 23.0 
               
               
                 First Condenser Liquid Outlet 
                 400.0 (Simulated) 
                 23.0 
               
               
                   
                 275-310 Expected 
               
               
                 Second Condenser Inlet 
                 473.1 
                 22.4 
               
               
                 Second Condenser Vapor Outlet 
                 410.0 
                 21.9 
               
               
                 Second Condenser Liquid Outlet 
                 410.0 (Simulated) 
                 21.9 
               
               
                   
                 275-310 Expected 
               
               
                 Third Condenser Inlet 
                 419.3 
                 21.3 
               
               
                 Third Condenser Vapor Outlet 
                 270.0 
                 20.8 
               
               
                 Third Condenser Liquid Outlet 
                 270.0 
                 20.8 
               
               
                   
               
             
          
         
       
     
         [0028]    The feeds passed through the same unit of Example 1, with the exclusion of four components. The components excluded were: the condenser following the waste heat boiler and the first, second, and third reheaters. The first condenser in Table 3 above is located after the first catalytic reactor, not right after the waste heat boiler as in Example 1. The total sulfur recovery was 96.6%. 
         [0029]    Therefore, the removal of these four pieces of equipment had little to no effect on the total sulfur recovery.