Abstract:
A process for removing NO x  oxides and SO x  oxides from gaseous streams contacts the gaseous stream with an aqueous solution of chloric acid. The process of the invention removes sulfur oxides and nitrogen oxides from waste gas streams without producing inorganic salts. In addition, the use of chloric acid provides improved oxidation of NO and SO 2 . The process can be operated to treat &#34;dirty&#34; gas streams containing particulates and compounds which may poison or contaminate NO x  removal systems employing catalysts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the treatment of waste gases generally and, more particularly, to a novel process for the removal of nitrogen oxides and sulfur oxides from such gases using chloric acid as an oxidizer. 
     2. Background Art 
     The amount of NO x  and SO x  in gases released, for example, from chemical operations, manufacturing facilities and power plants is limited by environmental air discharge permits issued by local, state, federal and regulatory agencies worldwide. Air discharge permits, in many locations in the United States, are being reviewed and revised to include all of the NO x  and SO x  gaseous species emitted. In many cases, the allowable NO x  gas discharge in a waste gas stream can be limited to concentrations as low as about 10 to 1.00 ppm in the output gas stream. 
     The NO x  or gaseous nitrogen oxides in waste gases are generally composed of a mixture of NO, NO 2 , N 2  O 3 , N 2  O 4 , and N 2  O 5 . As the gas phase generally contains water vapor, oxyacids such as HNO 2  and HNO 3  can also be present in the NO x  gas stream. In addition, there are processes which can generate significant amounts of N 2  O (nitrous oxide). Except for NO, nitric oxide, most of the other nitrogen oxides can be easily removed or scrubbed from the waste gas using suitable concentrations of alkaline solutions such as dilute NaOH, carbonates, or those containing aqueous sulfur-based reducing compounds such as sodium sulfide (Na 2  S). 
     The removal of NO x  by scrubbing is particularly a problem when the waste gas contains substantial quantities of NO in the NO x  gas stream. In this case, NO must be oxidized to NO 2  prior to removal by conventional scrubbing using water or alkaline chemical solutions. 
     The deleterious effects of sulfur oxides (SO x ) include the formation of acid rain (H 2  SO 4 ) by the reaction of sulfur oxides with atmospheric moisture. Acid rain has a pH of 2 or less and is responsible for acidifying streams and lakes. In addition, the presence of gaseous sulfur oxides may contribute to respiratory problems, to reduced atmospheric visibility, and to corrosion of materials. 
     Various oxidizing agents have been used in processes for removing NO x  and SO x  oxides from gaseous streams. Sodium chlorite and dilute solutions of chlorine dioxide (ClO 2 ) have been found to be very efficient in NO oxidation. Sodium hypochlorite, potassium permanganate, hydrogen peroxide, and other peroxyacid based oxidizers are significantly less efficient in NO oxidation. Ozone (O 3 ) is an effficient NO oxidation agent, but it is expensive to generate. 
     The use of chlorous acid for the removal of sulfur dioxide from combustion gases has been described in U.S. Pat. No. 4,968,501, issued Nov. 6, 1990 to J. A. Mason. The chlorous acid is formed by reacting a solution of sodium chlorite with an organic acid such as lactic, citric, malic, tartaric or glycolic acids. The chlorous acid is further reacted to form chlorine dioxide which reacts with the SO 2  present in the gas stream. Chloric acid is produced as an undesirable by-product during the formation of the chlorous acid. 
     A SO x  and NO x  -containing waste gas is oxidized with an oxidizing solution containing KClO 3  or NaClO 3  to oxidize NO to NO 2 , then the treated gas is scrubbed with an alkaline absorbent to absorb NO x  and SO x  ( Japanese patent no. 76-95977, issued Aug. 23, 1976 to T. Matsuda et al. Chem. Abstracts 87(4):28282p). 
     Japanese Patent Office, Patent Disclosure No. 74-113,598, published Apr. 2, 1976 by A. Kita, describes a process for removing nitrogen oxide from waste gases by contacting the gas with an aqueous solution containing an alkali metal or alkaline earth metal chlorate and an alkali metal or alkaline earth metal chloride. The NO is almost completely oxidized to NO 2 , after which the gas is treated with a conventional denitrating agent i.e., Na 2  SO 3  (Chem. Abstracts 85(24): 181722k). 
     Hydrogen sulfide in fluid streams is removed upon contact with a substantially neutral aqueous stream containing a polyvalent metal chelate. An oxidizing agent is used to oxidize elemental sulfur to soluble sulfur compounds with SO 2  being preferred. Other oxidizing agents which can be used include the alkali metal salts of inorganic oxidizing agents such as perchloric, chloric, hypochlorous and permanganic acids. (U.S. Pat. No. 4,451,442, issued May 29, 1984 to G. C. Jeffrey et al). 
     Y. Yamashita et al teach a process for the removal of NO x  from waste gases using an aqueous solution of ClO 2  (pH 5-8) in the first scrubber followed by a second scrubber containing Na 2  S. (Kyowa Chem. Indus. Japanese Patent Office, Patent Disclosure No. 1990-169012, Jun. 29, 1990). 
     Mixtures of alkali metal chlorates or alkaline earth metal chlorates with sulfuric acid have been used to remove NO x  and SO x  oxides from a waste gas (boiler flue gas, combustion gas). The gas is contacted with a solution containing Na or Ca chlorate, H 2  SO 4  and a ferrous salt to oxidize NO to NO 2 . The solution amy also contain HC1 or a chloride which produces HCl. (Sumitomo Chemical Co. Japan. Publn. 51-20772 Feb. 19, 1976). Similarly, a SO x  and NOx-contg. flue gas is oxidized with a solution containing K or Na chlorate and H 2  SO 4  to oxidize NO to NO 2 . The treated gas is then contacted with a red mud slurry obtained during alumina production. (Sumitomo Chemical Co. Japan. Publn. 53-113761, Oct. 4, 1978). However, the Applicants have found that the presence of sulfate ions in the chlorate-H 2  SO 4  mixture hinders or reduces the oxidation of NO gas resulting in lower NO gas removal efficiency. 
     There is need for a process of removing NO x  and SO x  oxides from gaseous streams which does not result in the formation of liquid effluents containing salts of inorganic acids. Further, there is need for an improved process of oxidizing NO and SO 2  present in gaseous waste streams. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a process for the removal of sulfur oxides and nitrogen oxides from waste gas streams without producing inorganic salts. 
     Another object of the invention is to provide such a process having improved oxidation of NO and SO 2 . 
     A further object of the invention is to provide a process that can be operated treating &#34;dirty&#34; gas streams containing particulates and compounds which may poison or contaminate NO x  removal systems employing catalysts. 
     These and other objects of the invention are accomplished in a process for removing NO x  oxides and SO x  oxides from gaseous streams comprising contacting the gaseous stream with an aqueous solution consisting essentially of chloric acid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Understanding of the present invention and the various aspects thereof will be facilitated by reference to the accompanying FIGURE, submitted for purposes of illustration only and not intended to limit the scope of the invention. 
     The FIGURE schematically illustrates a system for conducting the process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     The FIGURE illustrates a system for conducting the process of the present invention, generally indicated by the reference numeral 10, for purifying waste gases containing NO x  oxides and SO x  oxides. 
     System 10 includes oxidation scrubber 14, having oxidation scrubber surge tank 18 connected to the base thereof and having a recirculation pump 22 to recirculate liquid from oxidation scrubber surge tank 18 to the upper part of oxidation scrubber 14. 
     System 10 also includes an aqueous scrubber 30, having a surge tank 34 connected to the base thereof and having a recirculation pump 38 to recirculate liquid to the upper part of aqueous scrubber 30. A system blower 42, connected to aqueous scrubber 30, draws gas through system 10 and outputs purified gas therefrom. 
     In operation, waste gas containing, for example, nitrogen oxides is fed to the upper part of oxidation scrubber 14. An aqueous solution containing chloric acid is also fed to the upper part of oxidation scrubber 14 by means of circulating pump 22. If necessary, means (not shown) may be provided for cooling the waste gas stream to a suitable temperature. In oxidation scrubber 14 the waste gas containing nitrogen oxides contacts the aqueous solution containing chloric acid and nitrogen oxides such as NO are oxidized. In the lower part of the column., the liquid/gas mixture is separated and the liquid flows to oxidation scrubber surge tank 18. 
     Gas from the lower part of oxidation scrubber 14 is fed to the upper part of aqueous scrubber 30. The gas contacts an aqueous solution, containing, for example, water or aqueous solutions containing sodium sulfide and sodium hydroxide, to neutralize acidic components and to remove NO 2  and other nitrogen oxide species from the gas stream. The purified gas stream is separated from the liquid in the lower part of aqueous scrubber 30 and is removed from aqueous scrubber 30 by means of system blower 42. 
     Alternatively, scrubbers 14 and 30 can be individually operated in a counter-current mode with the waste gas entering the lower part of oxidation scrubber 14. Also alternatively, scrubbers 14 and 30 may be operated with the injection of single-pass liquid solutions. 
     The acidic solution recovered from oxidation scrubber 14 can be further processed for the recovery/recycle of the acids produced by the scrubbing process. Such processing may include distillation or other physical methods of separating and/or purifying the acids. Alternatively, the acidic solution recovered from oxidation scrubber 14 can be used to neutralize the effluent from aqueous scrubber 30. 
     The oxidation scrubber 14 may be operated at any suitable temperature, for example, at from about 0° C. to about 100° C., with a preferred temperature operating range of from about 5° C. to about 90° C., and more preferably from about 20° C. to about 80° C. Aqueous scrubber 30 may be operated at temperatures in these same ranges. Any suitable heating or cooling means may be used to maintain temperatures in oxidation scrubber 14 and aqueous scrubber 30. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The novel process of the invention employs as the oxidizing agent an aqueous solution of chloric acid, HClO 3 , which is substantially free of anionic and cationic impurities. Recently it has been discovered that highly pure chloric acid can be produced by the direct oxidation of an aqueous solution of hypochlorous acid having very low concentrations of chloride ions and dissolved chlorine and free of other anionic or cationic impurities. Suitable processes for the production of the chloric acid solutions used in the process of the invention are described in U.S. Pat. No. 5,089,095, issued Feb. 18, 1992 to D. W. Cawlfield et al. In one process an aqueous solution of hypochlorous acid is heated at a temperature in the range of 25° to 120° C. and a solution of chloric acid produced. Alternatively, an aqueous solution of hypochlorous acid is fed to the anode compartment of an electrolytic cell having an anode compartment, a cathode compartment, and an ion exchange membrane separating the anode compartment from the cathode compartment. The hypochlorous acid solution is electrolyzed in the anode compartment during a residence time of less than about 8 hours to produce a solution of chloric acid of high purity. Where desired, the chloric acid solution may be concentrated by heating the solution at a temperature above about 40° C. 
     In practising the process of the invention, gases containing NO x  oxides and/or SO x  oxides and mixtures thereof are contacted with the chloric acid solutions of high purity to oxidize any oxidizable species of NO x  oxides and/or SO x  oxides present in the gas. The concentration of the chloric acid is not critical and any suitable concentration may be used. For example, the chloric acid feed concentration range may be from about 0.05 wt. % to about 50 wt. %, with a preferred concentration range of from about 1.0 wt. % to about 40 wt. %, and a more preferred range of from about 5 wt. % to about 35 wt. %. 
     Where the gas to be purified contains NO x  oxides including NO, aqueous chloric acid solutions have been found to be efficient in oxidizing NO with an added feature that the reaction chemistry produces gaseous chlorine dioxide (ClO 2 ) as one of the by-products. The by-product gaseous C1O 2  further reacts to oxidize NO to NO x  and nitric acid. Thus the process provides both direct oxidation by chloric acid and secondary oxidation by the chlorine dioxide produced as a by-product of the reaction. 
     Without being limited hereby, it is believed that the reaction of chloric acid (HClO 3 ) with nitric oxide (NO) in the NO x  containing gas is represented by the following equation: 
     
         NO+2HClO.sub.3 →NO.sub.2 +2ClO.sub.2 +H.sub.2 O     (1) 
    
     In this reaction, NO is a reducing agent which reacts with the chloric acid to form ClO 2 . The net reactions of ClO 2  gas with nitric oxide (NO) and nitrogen dioxide (NO 2 ) are believed to be represented by the following equations: ##STR1## 
     Reaction (2) shows that one mole of ClO 2  is capable of oxidizing 2.5 moles of NO to NO 2  (molar reaction ratio of NO to C102 is 2.5:1). The NO 2  can be further oxidized to HNO 3  as in reaction 3, with one mole of ClO 2  being capable of oxidizing 5 moles of NO 2 . The actual net stoichiometry depends on the contact times and concentrations of the NO x  species and chloric acid in the scrubber, 
     The combination of reactions (1) and (2) is believed to be represented by the net reaction equation (5) as follows: 
     
         6NO+2HClO.sub.3 →2HCl+6NO.sub.2                     (5) 
    
     The reaction of equation (5) shows a 3:1 NO:HClO 3  molar reaction ratio in oxidizing NO to NO 2 . 
     Where the chloric acid oxidizer and its byproduct ClO 2  are totally reduced in the oxidation of NO and NO 2 , the net oxidation result, based on equations (1) and (4) is believed to be represented by equation (6) as follows: 
     
         13NO+6HClO.sub.3 +5H.sub.2 O →3NO.sub.2 +6HCl+10HNO.sub.3 (6) 
    
     This is a 2.27:1 NO:HClO 3  molar reaction ratio with some NO 2  residual in the byproduct gas. 
     In purifying SO x  oxide-containing gases in which SO 2  is the major component, the SO 2  oxidation chemistry is believed to be represented by the following equations: ##STR2## Net reaction equation (9) shows that there is a 3:1 SO.sub. 2:HClO 3  molar reaction ratio in the oxidation of SO 2  to H2SO 4 . 
     The oxidation scrubber liquor containing chloric acid maybe treated with a reducing agent to generate chlorine dioxide. The reducing agent is preferably one which does not introduce &#34;persistent anions&#34; into the solution. Suitable inorganic reducing agents are selected from the group of peroxides such as hydrogen peroxide and alkali metal peroxides such as sodium peroxide and potassium peroxide. Organic reducing agents are selected from the group of alcohols such as methanol, ethanol, propanol, isopropanol, and higher molecular weight linear and branched alcohols. Other suitable organic reducing agents are selected from the group of carbohydrate sugars such as glucose, sucrose, maltose, and others. 
     In an alternate embodiment, a mixture of chloric acid and an alkali metal chlorate may be used as the oxidizing agent. The mixtures of chloric acid and alkali metal chlorate are preferably produced in a process described in U.S. Pat. No. 5,084,148, issued Jan. 28, 1992 to J. J. Kaczur et al. An aqueous solution of chloric acid--alkali metal chlorate substantially free of anionic and cationic impurities is prepared electrolytically in an electrolytic cell having an anode compartment and a cathode compartment. At least one ion exchange compartment separates the anode compartment from the cathode compartment. The ion exchange compartment includes a flow channel for the aqueous alkali metal chlorate solution between two cation permeable ion exchange membranes. Suitable alkali metal chlorates include sodium chlorate, potassium chlorate and lithium chlorate, with sodium chlorate being preferred. The alkali metal chlorate solution fed to the ion exchange compartment may be prepared, for example, by dissolving a crystalline alkali metal chlorate, such as sodium chlorate, in pure or deionized water. Commercial sodium chlorate is suitable as it is generally substantially free of significant amounts of sulfate,-chloride, hypochlorite, and phosphate ions as anionic impurities. For example, the sulfate content of the chlorate feed solution is preferably less than about 1 wt %. When chloride ion is present in the sodium chlorate feed solution, chlorine dioxide may be spontaneously formed in the ion exchange compartments of the electrolytic cell. When present, chlorine dioxide migrates through the membrane into the cathode compartment. In the cathode compartment, chlorine dioxide can disproportionate into chloride and chlorate ions which are undesired impurities in, for example, the alkali metal hydroxide solution produced as the catholyte. The chloride ion content of the aqueous solution of sodium chlorate should be less than about 1.0 wt % and preferably less than about 0.1 wt %. Cationic impurities to be avoided include transition metal ions, such as those of iron, nickel, chromium and vanadium. 
     Aqueous sodium chlorate feed solutions which may be employed contain any suitable concentrations up to about saturation at the temperatures at which the solutions are fed to the ion exchange compartment of the electrolytic cell. Thus sodium chlorate solutions having a concentration in the range of from about 0.1% by weight to those saturated with sodium chlorate at temperatures in the range from about 0° C. to about 100° C., and preferably from about 15° C. to about 80° C., may be employed. These sodium chlorate solutions have a concentration in the range from about 0.1% by weight to about 60% by weight of NaCl0 3 , preferably from about 20% to about 55%, and more preferably from about 30% to about 50% by weight of NaClO 3 . 
     The anode compartment contains an anolyte, which initially can be an aqueous solution of, for example, sulfuric acid. The hydrogen ions generated in the anode compartment during electrolysis pass through the cation exchange membrane separating the anode compartment from the ion exchange compartment. As a hydrogen ion enters the sodium chlorate solution in the ion exchange compartment, a sodium ion is displaced and by electrical ion mass action passes through the cation membrane adjacent to the cathode compartment to maintain electrical neutrality. The sodium ion displacement results in the conred, sion of sodium chlorate to chloride acid over a wide range, for example, from about 1% to about 99.9%, preferably from about 5% to about 95%, and more preferably from about 15% to about 90%. 
     The sodium chlorate feed solution concentration, the residence time in the ion exchange compartment and the anode compartment, as well as the cell amperage are factors that affect the extent of the conversion of sodium chlorate to chloric acid. 
     The catholyte produced in the cathode compartment can be any suitable aqueous solution, including alkali metal chlorides, and acids such as hydrochloric, sulfuric, phosphoric, nitric, acetic or others. Preferably deionized or softened water or alkali metal hydroxide solution is used as the catholyte in the cathode compartment to produce an alkali metal hydroxide solution. 
     Following the oxidation of oxidizable species of NO x  oxides and SO x  oxides in the chloric acid solution, the treated gas is separated from a solution of chloric acid containing acidic components such as nitric acid and sulfuric acid. The treated gas is then fed to an aqueous scrubber. Suitable solutions for the aqueous scrubber include water, water soluble and water suspensions of sulfur-based reducing agents including alkali metal or alkaline earth sulfides and hydrosulfides with sodium sulfide, sodium hydrosulfide, and calcium sulfide being examples. Other sulfur-based reducing agents are sulfites, bisulfites, hydrosulfites and thiosulfates. The selection of the sulfur based reducing agent is generally based on chemical cost. 
     Alternate choices of aqueous scrubber liquors are alkali metal or alkaline earth metal hydroxides including sodium hydroxide and calcium hydroxide. Alkaline solutions of alkali metal and alkaline earth metal carbonates could also be used in the aqueous scrubber. These alkaline solutions can be readily prepared in electrolytic processes for producing chloric acid or mixtures of chloric acid and alkali metal chlorates. 
     Following aqueous scrubbing the purified gas, now substantially free of NO x  oxides and SO x  oxides, can be utilized or discharged without further treatment. 
     Chloric acid solutions or mixtures of chloric acid and alkali metal chlorates can also be used in the scrubbing/oxidizing of other oxidizable gases such as H 2  S present as an impurity in natural gas, as well as mercaptans and other sulfur-based gases found, for example, in air. Suitable oxidation or aqueous scrubbers include packed columns using plastic or ceramic Raschig rings, saddles, balls, and the like. Also suitable are scrubbers which employ packings in the form of porous metal or plastic pads. In addition, other scrubber designs such as spray compacting and Venturi-type scrubbers may also be employed. The plastics and metals used need to be resistant to chloric acid as well as nitric, sulfuric, and hydrochloric acids produced in the oxidation scrubber. Suitable resistant plastics are chlorine and fluorine based types such as polytetrafluorethylene, polyvinyl chloride, and polyvinylidene difluoride. Metals include the group of valve metals as well as high alloy type stainless steel such as the nickel-chromium based types such as Hastelloy C, Hastelloy 276, and the like. Scrubber selection can be determined by known engineering design methods based on the NO x  waste gas stream flowrate, NO x  concentration, and pressure drop requirements for the system. 
     All patent applications, patents and other documents cited herein are incorporated in their entirety by reference hereinto. 
     To further illustrate the invention the following examples are provided without any intention of being limited thereby. All parts and percentages are by weight unless otherwise specified. 
     EXAMPLE 
     A set of two 500 ml gas scrubber bottles with coarse glass frits were arranged in series flow. The first scrubber was filled with 500 ml of a pure chloric acid solution containing 5.0% by wt. of HClO 3 . The second scrubber was empty and employed as a collector to prevent liquid from entering the gas analyzer. An EXTREL® Corporation Questor Process Analyzer V 3.06 mass spectrometer was connected to the scrubbers. A gas containing 803 ppm of NO in argon, from a compressed gas cylinder, was metered into the first scrubber at a rate of about 500 ml/min. All solutions and gases were used at ambient temperature. The intensities of the various mass weight gaseous species detected from 12 to 100 atomic mass units were measured for the NO-containing gas before and after passing through the oxidizing scrubber containing chloric acid. After being scrubbed with the chloric acid solution, 94.5-95.4% of the NO had been removed from the gas. 
     COMPARATIVE EXAMPLE 
     The method of the Example was reproduced exactly with the exception that the first scrubber bottle was filled with 500 ml of a solution of 6.38 wt % sodium chlorate, NaClO 3 , in 10 wt % sulfuric acid, H 2  SO 4 . After being scrubbed with the sodium chlorate - sulfuric acid solution, 85.4-87.2% of the NO had been remove6 from the gas. 
     The Example illustrates the significantly improved oxidation of NO achieved by use of the process of the invention employing chloric acid over that of the prior art practise of using mixtures of sodium chlorate and sulfuric acid. This significant improvement is believed to be attributed to the absence of ions, such as sulfate ions, which appear to hinder the oxidation of NO.