Abstract:
A method for selectively oxidizing hydrogen sulfide to elemental sulfur is disclosed. The elemental sulfur can be effectively recovered from a gas mixture containing hydrogen sulfide in the presence of a multi-component catalyst. The multi-component catalyst includes an antimony-containing substance and a vanadium-and-magnesium-containing material. The antimony containing substance may be antimonous oxide (Sb 2 O 3 ) or antimony tetraoxide (α-Sb 2 O 4 ), and the vanadium and magnesium containing material may be magnesium pyrovanadate (Mg 3 V 2 O 8 ) or Mg 2 V 2 O 7 .

Description:
FIELD OF THE INVENTION 
     The present invention is related to a method for oxidizing hydrogen sulfide to elemental sulfur, and more particularly to a method for recovering elemental sulfur from a gas mixture containing hydrogen sulfide. 
     BACKGROUND OF THE INVENTION 
     In the petroleum refinery process for producing various fuel oil such as gasoline, diesel, kerosene, etc., the sulfur in the crude oil is removed as hydrogen sulfide gas by hydrodesulfurization process. The highly toxic hydrogen sulfide gas is then converted to elemental sulfur in sulfur-recovery plants or so-called Claus plants. During the last two decades, a great number of Claus tail-gas treating (TGT) processes have been developed to increase the total sulfur-recovery efficiency. Conventional Claus TGT processes involve a hydrogen sulfide absorption step, in which a tail gas containing unreacted hydrogen sulfide is introduced into an alkaline bath. Removing the last percentages of sulfur by means of these conventional Claus TGT processes is relatively expensive, both in terms of capital investment cost and energy consumption. 
     Recently, in order to avoid the shortcomings of these solution-absorption type Claus TGT processes, two dry types of TGT processes have been developed, that is, Mobil-direct-oxidation process developed by Mobil AG Company in Germany (Oil and Gas Journal, 86, p.63, 1988) and Super-Claus Process developed by Comprimo Company in Netherlands (Catalysis Today, 16, p263-271, 1993), both of which comprise a step of recovering elemental sulfur from Claus tail gas by selective oxidation of hydrogen sulfide in the presence of a catalyst. The catalyst used in Mobil-direct-oxidation process contains titanium dioxide (TiO 2 ). The catalyst used in Super-Claus Process is an active mixture of iron and chromium oxides deposited on an alpha-alumina support. These dry type Claus TGT processes are simple and economical; however, the chromium atom contained in the catalyst is a toxic substance. 
     In our experimental studies, it was found that vanadium and magnesium mixed catalyst can effectively oxidize hydrogen sulfide to elemental sulfur (U.S. Pat. No. 5,653,953 and Taiwan Patent Published No. 92615) However, as the content of magnesium in the mixed catalyst is increased, the yield of sulfur will be significantly reduced. 
     Therefore, a major object of the present application is to improve the defects encountered with the prior arts. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for oxidizing hydrogen sulfide to elemental sulfur. 
     Another object of the present invention is to provide a method for recovering elemental sulfur from a gas mixture containing hydrogen sulfide. 
     Another further object of the present invention is to provide a catalyst adapted to be used to oxidize hydrogen sulfide to elemental sulfur. 
     According to the present invention, the method includes a step of oxidizing hydrogen sulfide to elemental sulfur in the presence of a catalyst including an antimony-containing substance and a vanadium-and-magnesium-containing material. 
     In accordance with one aspect of the present invention, the antimony-containing substance is one selected from a group consisting of metallic antimony, antimonic oxide, antimonic sulfide, antimonic halogenide, antimonic carbide, antimonic hydroxide, antimonic hydride, antimonous oxychloride, antimonous sulfate, and antimonate. Preferably, the antimony-containing substance is antimonous oxide (Sb 2 O 3 ) or antimony tetroxide (α-Sb 2 O 4 ). 
     The vanadium-and-magnesium-containing material is one selected from a group consisting of metallic vanadium and magnesium, vanadium and magnesium oxides, vanadium and magnesium sulfides, vanadium and magnesium halogenides, and a mixture thereof. Preferably, the vanadium-and-magnesium-containing material is magnesium pyrovanadate (Mg 3 V 2 O 8 ) or Mg 2 V 2 O 7 . 
     The vanadium-and-magnesium-containing substance is deposited on a carrier in a form selected from a group consisting of monolith, particle, pellet, and porous carrier. The porous carrier is one selected from a group consisting of alumina, silica, aluminum-and-silicon-containing compound, zeolite, titanium oxide, and zirconium oxide. 
     Preferably, the molar ratio of antimony of the antimony-containing substance to vanadium and magnesium of the vanadium-and-magnesium-containing material is ranged from 0.01:1 to 100:1, preferably from 0.1:1 to 10:1. 
     The oxidizing reaction is performed at a temperature ranged between 50° C. and 400° C., preferable 100-350° C. 
     The oxidizing reaction is performed at a pressure ranged from 0.1 to 50 atm, preferably 1-10 atm. 
     The present invention will be further illustrated by the following examples. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more detailedly with reference to the following embodiments. It is to be noted that the following descriptions of the preferred embodiments of this invention are presented herein for the purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
     The present invention provides a method for oxidizing hydrogen sulfide to elemental sulfur, especially for recovering elemental sulfur from a gas mixture containing hydrogen sulfide. In order to understand the present invention in detail, some examples are illustrated as follows. 
     EXAMPLE 1 
     Mechanically Mixed Catalyst Comprising Antimony, Vanadium, and Magnesium 
     In this preferred embodiment, the catalyst comprising antimony, vanadium, and magnesium is prepared by mechanically mixing Mg 3 V 2 O 8  catalyst and α-Sb 2 O 4  catalyst. The preparing process is divided into three steps, A, B, and C below. 
     Step A: Preparation of Mg 3 V 2 O 8  Catalyst 
     (1) 12.82 g Mg(NO 3 ) 2 .6H 2 O is dissolved in 25 ml distilled water. 
     (2) 3.899 g NH 4 VO 3  is dissolved in 175 ml hot distilled water and is added to the solution of Step (1). 
     (3) 3.522 g citric acid is added to the obtained solution of Step (2). 
     (4) The obtained solution of Step (3) is heated to become a slurry and then dried at 100° C. for 16 hours. Thereafter, the resulting product is dried at 110° C. for 24 hours and then calcined at 300° C. for 16 hours. 
     (5) Finally, the product is calcined at 600° C. for 20 hours and then screened to obtain the particles of 20-40 mesh number. The resulting particles are analyzed by X-ray diffraction and identified as Mg 3 V 2 O 8 . 
     Step B: Preparation of α-Sb 2 O 4  Catalyst 
     Sb 2 O 3  is calcined at 500° C. for 20 hours and then screened to obtain the particles α-Sb 2 O 4  of 20-40 mesh number. 
     Step C: Mechanical Mixture of Mg 3 V 2 O 8  and α-Sb 2 O 4    
     1 g Mg 3 V 2 O 8  prepared from Step A and 1 g α-Sb 2 O 4  prepared from Step B (Mg 3 V 2 O 8 /(Mg 3 V 2 O 8 +Sb 2 O 4 )=0.5) are added to 50 ml pentane and is stirred at room temperature for 3 minutes. Thereafter, the mixture is evaporated under vacuum and is then dried at 80° C. for 12 hours to obtain the mixed catalyst comprising antimony, vanadium, and magnesium. Because the mixed catalyst has never been calcined, the vanadium-and-magnesium-containing compound and antimony-containing compound are physically mixed together in the mixed catalyst. 
     Selective Oxidation of Hydrogen Sulfide 
     0.5 g mixed catalyst of Step C in the form of 10-20 mesh number particles is packed into a tubular glass reactor having an inner diameter of 8 mm, an outer diameter of 10 mm, and a length of 80 cm. The reaction temperature is controlled by an electrical heater installed around the reactor. The catalyst bed is presulfurized at 250° C. for at least 8 hours by introducing a 9 vol % hydrogen sulfide gas mixture into the reactor until a gaseous reaction product leaving the reactor had a stable hydrogen sulfide concentration. After the presulfurization step, a gaseous feed stream consisting of 1 vol % hydrogen sulfide, 5 vol % oxygen, and 94 vol % nitrogen is then introduced into the reactor at 100 ml/min. The reaction product exiting the reactor is introduced into a gas-solid separator which is connected to an outlet of the reactor and maintained at 25° C., in which the reaction product is separated into a solid product and a gaseous product. The composition of the gaseous product is analyzed by gas chromatography. The reaction conditions and results are listed in Table 1, in which the conversion (%) is defined as the mole of hydrogen sulfide reacted per mole of hydrogen sulfide in the feed stream, and the selectivity is defined as the mole of elemental sulfur formed per mole of the reacted hydrogen sulfide. As the selectivity is lower than 100%, it means that there is sulfur dioxide appeared in the gas exiting the gas-solid separator. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Catalytic performance of mechanically mixed catalyst 
               
               
                 comprising antimony, vanadium, and magnesium 
               
             
          
           
               
                   
                 Temperature 
                 Conversion 
                 Selectivity 
                 Yield of sulfur 
               
               
                   
                 (° C.) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 200 
                 17.4 
                 100 
                 17.4 
               
               
                   
                 250 
                 49.2 
                 100 
                 49.2 
               
               
                   
                 300 
                 92.9 
                 99.8 
                 92.7 
               
               
                   
                 310 
                 97.6 
                 97.0 
                 94.6 
               
               
                   
                 320 
                 99.5 
                 94.2 
                 93.8 
               
               
                   
                 330 
                 99.8 
                 80.6 
                 80.4 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 2 
     Calcined Mixed Catalyst Comprising Antimony, Vanadium, and Magnesium 
     The mixed catalyst, comprising antimony, vanadium, and magnesium, used in this preferred embodiment is prepared by calcination of the mechanically mixed catalyst obtained from Step C of Example 1 at 600° C. for 144 hours. The reaction steps of selective oxidation of hydrogen sulfide are the same as those in Example 1. The reaction results are shown in Table 2. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Catalytic performance of calcined mixed catalyst comprising 
               
               
                 antimony, vanadium, and magnesium 
               
             
          
           
               
                   
                 Temperature 
                 Conversion 
                 Selectivity 
                 Yield of sulfur 
               
               
                   
                 (° C.) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 200 
                 46.9 
                 100 
                 46.9 
               
               
                   
                 250 
                 93.2 
                 100 
                 93.2 
               
               
                   
                 270 
                 98.9 
                 100 
                 98.9 
               
               
                   
                 280 
                 99.6 
                 99.0 
                 98.7 
               
               
                   
                 290 
                 99.9 
                 96.5 
                 96.3 
               
               
                   
                   
               
             
          
         
       
     
     It is found that the performance of the calcined mixed catalyst of this example is superior to that of Example 1. An excellent yield can be obtained under a lower temperature. The yield of elemental sulfur is up to 98.9% at 270° C. 
     Comparative Example 1 
     Mg 3 V 2 O 8  Catalyst 
     The preparation of the Mg 3 V 2 O 8  catalyst used in this comparative example is the same as that described in Step A of Example 1. The reaction steps of selective oxidation of hydrogen sulfide are the same as those in Example 1, except that the mixed catalyst comprising antimony, vanadium, and magnesium is replaced by Mg 3 V 2 O 8  catalyst. The reaction results are shown in Table 3. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Catalytic performance of Mg 3 V 2 O 8  catalyst 
               
             
          
           
               
                   
                 Temperature 
                 Conversion 
                 Selectivity 
                 Yield of sulfur 
               
               
                   
                 (° C.) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 200 
                 31.2 
                 100 
                 31.2 
               
               
                   
                 250 
                 78.7 
                 91.9 
                 72.3 
               
               
                   
                 260 
                 88.9 
                 83.3 
                 74.1 
               
               
                   
                 270 
                 93.9 
                 66.9 
                 62.8 
               
               
                   
                 280 
                 95.7 
                 40.7 
                 39.0 
               
               
                   
                   
               
             
          
         
       
     
     Comparative Example 2 
     α-Sb 2 O 4  Catalyst 
     The Preparation of the α-Sb 2 O 4  catalyst used in this comparative example is the same as that described in Step B of Example 1. The reaction steps of selective oxidation of hydrogen sulfide are the same as those in Example 1, except that the mixed catalyst comprising antimony, vanadium, and magnesium is replaced by α-Sb 2 O 4  catalyst. The reaction results are shown in Table 4. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Catalytic performance of α-Sb 2 O 4  catalyst 
               
             
          
           
               
                   
                 Temperature 
                 Conversion 
                 Selectivity 
                 Yield of sulfur 
               
               
                   
                 (° C.) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 200 
                 4 
                 100 
                 4 
               
               
                   
                 250 
                 14.5 
                 100 
                 14.5 
               
               
                   
                 300 
                 54.3 
                 100 
                 54.3 
               
               
                   
                 330 
                 72.9 
                 91.1 
                 66.4 
               
               
                   
                   
               
             
          
         
       
     
     From the data shown in Tables 1-4, the yield of the elemental sulfur (up to 98.9%), obtained after the selective oxidation of hydrogen catalyzed by the mixed catalyst comprising antimony, vanadium, and magnesium, is significantly superior to single α-Sb 2 O 4  catalyst or Mg 3 V 2 O 8  catalyst (the highest yield is only 74.1%). 
     Example 3 
     Mechanically Mixed Catalyst Comprising Antimony, Vanadium, and Magnesium 
     In this preferred embodiment, the catalyst comprising antimony, vanadium, and magnesium is prepared by mechanically mixing Mg 2 V 2 O 7  catalyst and α-Sb 2 O 4  catalyst. The preparing process is divided into three steps, A, B, and C. 
     Step A: Preparation of Mg 2 V 2 O 7  Catalyst 
     (1) 12.82 g Mg(NO 3 ) 2 .6H 2 O is dissolved in 25 ml deionized water. 
     (2) 5.849 g NH 4 VO 3  is dissolved in 175 ml hot water and the solution of Step (1) is added therein. 
     (3) 3.522 g citric acid is added to the solution of Step (2). 
     (4) The solution of Step (3) is evaporated to become a slurry and then dried at 100° C. for 16 hours. Thereafter, the product is dried at 110° C. for 24 hours and then calcined at 300° C. for 16 hours. 
     (5) Finally, the product is calcined at 600° C. for 15 hours and then screened to obtain the particles of 20-40 mesh number. The resulting particles are analyzed by X-ray diffraction and identified as Mg 2 V 2 O 7 . 
     Step B: Preparation of α-Sb 2 O 4  Catalyst 
     All steps are identical to those described in Step B of Example 1. 
     Step C: Mechanical Mixture of Mg 2 V 2 O 7  and α-Sb 2 O 4    
     1.4 g Mg 2 V 2 O 7  prepared from Step A and 0.47 g α-Sb 2 O 4  prepared from Step B (Mg 2 V 2 O 7 /(Mg 2 V 2 O 7 +Sb 2 O 4 )=0.75) are added to 50 ml pentane and stirred at room temperature for 3 minutes. Thereafter, it is evaporated under vacuum and then dried at 80° C. for 12 hours to obtain the mixed catalyst comprising antimony, vanadium, and magnesium. Because the mixed catalyst has never been calcined, the vanadium-and-magnesium-containing compound and antimony-containing compound are physically mixed together in the mixed catalyst. 
     Selective Oxidation of Hydrogen Sulfide 
     The reaction steps are the same as those of Example 1 and the results are shown in Table 5. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Catalytic performance of mechanically mixed catalyst 
               
               
                 comprising antimony, vanadium, and magnesium 
               
               
                 (Mg 2 V 2 O 7 /(Mg 2 V 2 O 7  + Sb 2 O 4 ) = 0.75) 
               
             
          
           
               
                   
                 Temperature 
                 Conversion 
                 Selectivity 
                 Yield of sulfur 
               
               
                   
                 (° C.) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 200 
                 26.6 
                 100 
                 26.6 
               
               
                   
                 250 
                 62.4 
                 100 
                 62.4 
               
               
                   
                 290 
                 92.2 
                 100 
                 92.2 
               
               
                   
                 300 
                 94.4 
                 97.8 
                 92.4 
               
               
                   
                 310 
                 96.1 
                 93.1 
                 89.4 
               
               
                   
                 320 
                 98.6 
                 82.1 
                 81.0 
               
               
                   
                   
               
             
          
         
       
     
     Comparative Example 3 
     Mg 2 V 2 O 7  Catalyst 
     The preparation of the Mg 2 V 2 O 7  catalyst used in this comparative Example 3 is the same as that described in Step A of Example 3. The reaction steps of selective oxidation of hydrogen sulfide are the same as those in Example 1, except that the mixed catalyst comprising antimony, vanadium and magnesium is replaced by Mg 2 V 2 O 7  catalyst. The reaction results are shown in Table 6. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Catalytic performance of Mg 2 V 2 O 7  catalyst 
               
             
          
           
               
                   
                 Temperature 
                 Conversion 
                 Selectivity 
                 Yield of sulfur 
               
               
                   
                 (° C.) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                   
                 200 
                 27.4 
                 100 
                 27.4 
               
               
                   
                 250 
                 79.6 
                 96.9 
                 77.1 
               
               
                   
                 260 
                 88.7 
                 88.7 
                 78.6 
               
               
                   
                 270 
                 93.9 
                 75.5 
                 70.9 
               
               
                   
                 280 
                 96.3 
                 55.1 
                 53.1 
               
               
                   
                   
               
             
          
         
       
     
     From all data shown in Tables 4-6, the yield of the elemental sulfur, obtained after the selective oxidation of hydrogen sulfide is catalyzed by the mixed catalyst (Mg 2 V 2 O 7 +Sb 2 O 4 ), is significantly superior to single α-Sb 2 O 4  or single Mg 2 V 2 O 7  catalyst. 
     In conclusion, the present invention provides a noval method for recovering elemental sulfur from a gas mixture containing hydrogen sulfide by selective oxidation of hydrogen sulfide in the presence of a catalyst comprising antimony, vanadium, and magnesium. The catalytic performance of the mixed catalyst according to the present invention is superior to that of single α-Sb 2 O 4 , Mg 3 V 2 O 8 , or Mg 2 V 2 O 7 . Not only does the mixed catalyst greatly increase the recovering yield of elemental sulfur, but it meets the requirement of environmental protection. 
     While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.