Abstract:
A composition, containing vanadium, phosphorus and a support is disclosed. A method of preparing such composition is also disclosed. The composition is employed in a process to remove a heavy metal from a gaseous feed stream which can optionally include a separate heavy metal adsorption stage.

Description:
The invention relates to a composition useful in the removal of heavy metals from a gaseous feed stream. In one aspect the invention relates to a method of preparing such composition. In yet another aspect the invention relates to a process for removing heavy metals from a gas stream using the inventive composition and, optionally, a second stage adsorption of the heavy metal. 
     BACKGROUND OF THE INVENTION 
     Heavy metals are released during the combustion process of many fossil fuels and/or waste materials. These heavy metals include, for example, arsenic, beryllium, lead, cadmium, chromium, nickel, zinc, mercury and barium. Most of these heavy metals are toxic to humans and animals. In particular, lead is thought to compromise the health and mental acuity of young children and fetuses. 
     Furthermore, there is every indication that the amount of mercury, and possibly of other heavy metals, now legally allowed to be released by those combusting various fossil fuels and/or waste materials, including coal burning powerplants, chemical plants and petroleum refineries, will be reduced by future legislation. While a variety of adsorbents are available for capture of heavy metals (in particular mercury), these adsorbents tend to have low capacities and are easily deactivated by other components in the gas stream, such as sulfur and nitrogen oxides. We have discovered a material that converts an elemental heavy metal to an oxidation state greater than zero, even in the presence of sulfur and nitrogen oxides. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an improved vanadium and phosphorous containing material which when used in the removal of heavy metal results in oxidation of the heavy metal to an oxidation state greater than zero, even in the presence of sulfur and nitrogen oxides. 
     A further object of this invention is to provide a method for making an improved vanadium and phosphorous containing material which when used in the removal of heavy metal results in oxidation of the heavy metal to an oxidation state greater than zero, even in the presence of sulfur and nitrogen oxides. 
     Another object of this invention is to provide an improved process for the removal of heavy metal from a heavy metal-containing gas which results in oxidation of the heavy metal to an oxidation state greater than zero, even in the presence of sulfur and nitrogen oxides, with an optional second stage for adsorption of oxidized heavy metal. 
     In accordance with a first embodiment of the invention, the inventive composition comprises vanadium, phosphorous and a support selected from the group consisting of: 1) amorphous silica-alumina; 2) a zeolite; 3) a material comprising meta-kaolin, alumina, and expanded perlite; 4) alumina; and 5) combinations thereof. 
     In accordance with a second embodiment of the invention, the inventive composition can be prepared by the method of: 
     a) incorporating a vanadium compound onto, into, or onto and into a support selected from the group consisting of: 1) amorphous silica-alumina; 2) a zeolite; 3) a material comprising meta-kaolin, alumina, and expanded perlite; 4) alumina; and 5) combinations thereof, in the presence of an oxidizing agent and a solvent, to thereby form a vanadium incorporated support; and 
     b) incorporating a phosphorous compound onto, into, or onto and into the vanadium incorporated support, to thereby form a phosphorous and vanadium incorporated support; and 
     c) calcining the phosphorous and vanadium incorporated support. 
     In accordance with a third embodiment of the invention, the inventive composition can be used in the removal of heavy metal from a gaseous feed stream comprising heavy metal by contacting, in a contacting zone, the gaseous feed stream with any of the inventive compositions of embodiments one or two above, with an optional second stage for adsorption of oxidized heavy metal. 
     Other objects and advantages of the invention will become apparent from the detailed description and the appended claims. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive composition comprises, consists of, or consists essentially of a support, phosphorous and vanadium. 
     The support is selected from the group consisting of: 1) amorphous silica-alumina; 2) a zeolite; 3) a material comprising, consisting of or consisting essentially of alumina, expanded perlite and meta-kaolin; 4) alumina; and 5) combinations thereof. As used in this disclosure, the term “Support” refers to a carrier for another catalytic component. However, by no means is a support necessarily an inert material; it is possible that a support can contribute to catalytic activity and selectivity. 
     The vanadium is present in the composition, on an elemental vanadium basis, in an amount in the range of from about 0.5 to about 50 weight %, preferably from about 1 to about 20 weight %, and most preferably from about 1.5 to about 15 weight %, based on the total weight of the composition. 
     The phosphorous is present in the composition, on an elemental phosphorous basis, in an amount in the range of from about 0.5 to about 50 weight %, preferably from about 1 to about 20 weight %, and most preferably from about 1.5 to about 15 weight %, based on the total weight of the composition. 
     In accordance with the second embodiment of the present invention, the inventive composition can be prepared by the method of, and a method is provided including: 
     a) incorporating a vanadium compound onto, into, or onto and into a support selected from the group consisting of: 1) amorphous silica-alumina; 2) a zeolite; 3) a material comprising, consisting of or consisting essentially of alumina, expanded perlite and meta-kaolin; 4) alumina; and 5) combinations thereof, in the presence of an oxidizing agent and a solvent, to thereby form a vanadium incorporated support; 
     b) incorporating a phosphorous compound onto, into, or onto and into the vanadium incorporated support, to thereby form a phosphorous and vanadium incorporated support; and 
     c) calcining the phosphorous and vanadium incorporated support. 
     The calcination temperature is preferably sufficient to volatilize and remove substantially all of the solvent, more preferably greater than about 125° C., and most preferably greater than about 150° C.; and is also preferably below about 400° C.; even more preferably below about 375° C.; and most preferably below about 350° C. 
     The composition is preferably heated, as described above, for a time period in the range of from about 0.1 hours to about 24 hours, and more preferably in the range of from about 1 hour to about 4 hours. 
     The vanadium compound can be any vanadium containing compound capable of incorporation onto and/or into a support. Preferably, the vanadium compound is selected from the group consisting of 1) ammonium metavanadate, 2) an alkali metavanadate of the formula MVO 3 , wherein M can be an alkali metal selected from Group IA, and combinations thereof; and 3) combinations of any two or more thereof. The most preferable vanadium compound is ammonium metavanadate. 
     The phosphorous compound can be any phosphorous containing compound capable of incorporation onto and/or into a support. Preferably, the phosphorous compound is selected from the group consisting of: 1) phosphoric acid; 2) phosphorous pentoxide (P 2 O 5 ); 3) an ammonium phosphate; 4) ammonium phosphite; and 5) combinations thereof. 
     The oxidizing agent can be any agent capable of oxidizing vanadium, and preferably is hydrogen peroxide or oxygen. The solvent is preferably an aqueous solution of oxalic acid. 
     Also, preferably the support comprises alumina, meta-kaolin, and expanded perlite, and is prepared by the method of: 
     1) adding expanded perlite to a mixture of alumina and water to thereby form a second mixture; 
     2) adding meta-kaolin to the second mixture to thereby form a third mixture; 
     3) adding a dispersant to the third mixture to thereby form a fourth mixture; and 
     4) calcining the fourth mixture to thereby form the support. 
     The calcining of step 4) preferably comprises heating the fourth mixture to a temperature in the range of from about 100° C. to about 200° C. for a first time period in the range of from about 0.5 hour to about 2 hours; and subsequently heating the fourth mixture to a temperature in the range of from about 500° C. to about 750° C. for a second time period in the range of from about 0.5 hour to about 2 hours. 
     In accordance with the third embodiment of the present invention, the inventive composition can be used in the removal of heavy metal from a gaseous feed stream comprising heavy metal by a process comprising, consisting of, or consisting essentially of contacting, in a contacting zone, under heavy metal removal conditions, the gaseous feed stream with any of the inventive compositions, and combinations thereof, of embodiments one through two above. A gaseous product stream is withdrawn from the contacting zone. The gaseous feed stream is typically a combustion gas; and is more typically a stack gas derived from the combustion of coal. The gaseous feed stream can also further comprise compounds selected from the group consisting of sulfur oxides, CO 2 , water, nitrogen oxides, HC1, and combinations of any two or more thereof. 
     The contacting of the gaseous feed stream with the inventive composition is preferably carried out at a temperature in the range of from about 100 to about 325° C., more preferably from about 110 to about 275° C., and most preferably from about 120 to about 225° C. 
     The heavy metal typically comprises a metal selected from the group consisting of arsenic, beryllium, lead, cadmium, chromium, nickel, zinc, mercury, barium, and combinations of any two or more thereof. The heavy metal most typically comprises mercury. 
     When the heavy metal is mercury, the mercury is typically present in the gaseous feed stream in an amount in the range of from about 0.1 to about 10,000 μm/ 3 , more typically in the range of from about 1 to about 800 μg/m 3  and most typically from about 3 to about 700 μg/m 3 . 
     The composition preferably converts at least a portion of the heavy metal in the gaseous feed stream to an elevated oxidation state. In the case of mercury, the composition preferably converts at least a portion of the mercury contained in the gaseous feed stream from a zero oxidation state to a +1 or a +2 oxidation state and also preferably removes mercury. “At least a portion”, as used in this paragraph, can mean at least 20 weight %, preferably at least 30 weight %, and more preferably at least 50 weight % mercury based on the total amount of mercury contained in the gaseous feed stream. 
     The gaseous product stream preferably contains less than about 20 weight %, more preferably less than about 10 weight %, and most preferably less than about 5 weight % of the mercury contained in the gaseous feed stream. 
     The gaseous product stream is optionally contacted with a separate adsorbent in an adsorption zone. The adsorbent can be any adsorbent capable of adsorbing a heavy metal. More preferably, the adsorbent comprises, consists of or consists essentially of a material selected from the group consisting of a zeolite, amorphous carbon, and combinations thereof. The amorphous carbon can be an activated carbon or an activated charcoal. A treated gaseous product stream is withdrawn from the adsorption zone and contains less than about 20 weight %, preferably less than about 10 weight %, and more preferably less than about 5 weight % of the heavy metal contained in the gaseous feed stream. 
     EXAMPLES 
     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. 
     Preparation of Support 
     The support is prepared from alumina, perlite, and metakaolin clay. The procedure involves mixing 254 grams of Vista Dispal alumina, 900 grams of de-ionized water, and 300 grams of expanded crushed perlite. To this slurry, 820 grams of ASP-600 metakaolin clay from Engelhard and 240 grams of Darvan 821A are added. The material is then heated to 150° C., held there for one hour, and heated to 600° C. for 8 to 10 hours. This material is ground to 10 to 20 mesh particles before the impregnation step. 
     Preparation of Sorbents 
     The preparation of the sorbents involves the addition of vanadium and phosphorus or just vanadium to the support above. To a solution of ammonium metavanadate (NH 4 VO 3 ) in 2 molar oxalic acid, a few drops of hydrogen peroxide (30 wt %) are added. (The red color of the solution suggests that vanadium is in +5 oxidation state). The vanadium solution is then impregnated onto the support by incipient wetness using one-third of the solution in three separate steps. Between impregnation steps, the vanadium-impregnated support is heated to 120° C. in a drying oven for one hour. After the three separate impregnation and drying steps, the sorbent is impregnated with either a solution of phosphoric acid (H 3 PO 4 ) in water (Sorbent A) or a solution of diammonium phosphate in water (Sorbent B). Sorbent A was then dried at 120° C. for one hour. Sorbent B was split into three different portions which were calcined for three hours at 300° C., 450° C. and 700° C., respectively. These sorbents contain 15 wt % V 2 O 5  and 5 wt % P 2 O 5 . For comparison purposes, a 15 wt % V 2 O 5  sorbent was prepared without phosphorus by eliminating the fourth impregnation step (Sorbent C). 
     Evaluation of Sorbents to Remove Mercury 
     The following procedure is used to test the ability of the sorbent to remove mercury from a gas stream. Mercury is added by passing a dry air stream at room temperature through a gas bottle containing elemental mercury. The mercury-containing stream is then passed through a sample tube containing approximately 0.4 grams of the sorbent to be tested at a gas hourly space velocity of 10,000 (approximately 40 to 50 ml/min). The tube is located in a furnace held at a temperature of around 150° C. The inlet and outlet elemental mercury concentrations are measured using a Jerome Mercury Analyzer. The efficiency of mercury removal is determined from the amount of mercury entering and leaving the solid sorbent, and is defined as the difference between the inlet and outlet mercury concentrations divided by the inlet concentration. 
     Sorbents A and C 
     Table 1 below summarizes the results obtained when passing mercury in dry air over various sorbents. Sorbents A and C were tested as prepared (i.e., 10 to 20 mesh) and ground and sieved to produce 20 to 40 mesh particles. The removal efficiency is determined as a function of mercury uptake; i.e., the cumulative amount of mercury already adsorbed on the sample in units of micrograms of mercury per gram of sorbent (μg/g). 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Mercury Removal Efficiency 
               
             
          
           
               
                   
                   
                   
                   
                 Mercury 
                   
               
               
                   
                   
                   
                   
                 Uptake 
                 Removal 
               
               
                 Sorbent 
                 Wt % V 
                 Wt % P 
                 Mesh Size 
                 (μg/g) 
                 Efficiency (%) 
               
               
                   
               
             
          
           
               
                 A 
                 15 
                 5 
                 10-20 
                 200 
                 95 
               
               
                   
                   
                   
                   
                 1000 
                 97 
               
               
                 A 
                 15 
                 5 
                 20-40 
                 200 
                 96 
               
               
                   
                   
                   
                   
                 1000 
                 99 
               
               
                   
                   
                   
                   
                 3000 
                 98 
               
               
                 C 
                 15 
                 — 
                 10-20 
                 200 
                 83 
               
               
                   
                   
                   
                   
                 1000 
                 95 
               
               
                   
                   
                   
                   
                 2000 
                 96 
               
               
                 C 
                 15 
                 — 
                 20-40 
                 200 
                 96 
               
               
                   
                   
                   
                   
                 1000 
                 99 
               
               
                   
                   
                   
                   
                 3000 
                 96 
               
               
                   
               
             
          
         
       
     
     The results in Table 1 indicate that the efficiency of mercury removal depends upon mercury uptake as well as other properties of the sorbent. For example, the 20 to 40 mesh particles appear to be more effective for mercury removal. In addition, the presence of phosphorus appears to have a positive effect on performance. 
     Sorbent B 
     Table 2 below summarizes results obtained when passing mercury in dry air over various Sorbent B portions calcined at differing temperatures. The removal efficiency is determined as a function of mercury uptake; i.e., the cumulative amount of mercury already adsorbed on the sample in units of micrograms of mercury per gram of sorbent (μg/g). 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Mercury Removal Efficiency 
               
             
          
           
               
                   
                   
                 Calcination 
                   
                   
               
               
                   
                   
                 Temperature 
                 Mercury Uptake 
                 Removal 
               
               
                 Wt % V 
                 Wt % P 
                 (° C.) 
                 (μg/g) 
                 Efficiency (%) 
               
               
                   
               
             
          
           
               
                 15 
                 5 
                 300 
                 162 
                 99.8 
               
               
                   
                   
                   
                 440 
                 100 
               
               
                   
                   
                   
                  455* 
                 97.8 
               
               
                   
                   
                   
                 487 
                 87.7 
               
               
                   
                   
                   
                 590 
                 97.6 
               
               
                   
                   
                   
                 964 
                 99.4 
               
               
                 15 
                 5 
                 450 
                  17 
                 98.3 
               
               
                   
                   
                   
                  46 
                 81.6 
               
               
                   
                   
                   
                 192 
                 30.7 
               
               
                 15 
                 5 
                 700 
                  19 
                 96.1 
               
               
                   
                   
                   
                  60 
                 95.8 
               
               
                   
                   
                   
                 442 
                 60 
               
               
                   
               
               
                 *Begin adding moisture to air for this run. 
               
             
          
         
       
     
     The results in Table 2 indicate that the efficiency of mercury removal depends upon mercury uptake as well as the calcination temperature. For example, the portion of Sorbent at 300° C. was most effective for mercury removal. 
     Reasonable variations, modifications and adaptations can be made within the scope of the disclosure and appended claims without departing from the scope of the present invention.