Patent Publication Number: US-4929434-A

Title: Catalytic generation of chlorine dioxide

Description:
FIELD OF THE INVENTION 
     This invention relates to a process for producing chlorine dioxide gas. 
     DESCRIPTION OF THE PRIOR ART 
     Chlorine dioxide is used in pulp and paper mills to bleach paper. It is used in aqueous solution at concentrations of between 5 and 10 grams per liter. 
     The principal systems for production of the gas are the Mathieson, Solvay, R-2, R-3, and R-8 Processes. 
     All these processes reduce a chlorate, usually sodium chlorate, in a strong sulphuric acid medium. Generally speaking, the main difference between the processes is in the reducing agent used. In the Mathieson Process, the reducing agent is sulphur dioxide, in the Solvay and R-8 Processes, the reducing agent is methanol. In the R-2 and R-3 Processes, the reducing agent is mainly sodium chloride. 
     Copending U.S. Ser. No. 092,028, filed Sept. 1, 1987 discloses the use of sulphur as a reducing agent in the production of chlorine dioxide. Sulphur is used in the form of an aqueous sulphur solution containing stabilizers and dispersants. Fine sulphur particles seem to provide sufficient surface to accelerate the reduction of sodium chlorate beyond what was previously thought possible. 
     An essential part of all these processes is the reaction to produce chloric acid which, in turn, is reduced by the reducing agent to produce chlorine dioxide gas. The quantity of chlorine dioxide produced per unit time is a function of the reaction vessel size, essentially because of the limited solubility of chloric acid in sulphuric acid solutions containing sulphate ions. 
     In a typical case, where chlorine dioxide is manufactured by the sulphur dioxide reduction process, the following simplified equations should be noted: 
     
         6NaClO.sub.3 +4H.sub.2 SO.sub.4 →6HClO.sub.3 +2Na.sub.3 H(SO.sub.4).sub.2                                         ( 1) ##STR1## 
    
     In the case of the above sulphur reduction process, the overall reaction may be represented as: 
     
         6NaClO.sub.3 +3H.sub.2 SO.sub.4 +S→6ClO.sub.2 +2Na.sub.3 H(SO.sub.4).sub.2 +2H.sub.2 O                             (4) 
    
     The limiting factor for production of chlorine dioxide gas is the speed at which the reducing agent reacts with the chloric acid to produce chlorine dioxide. The chloric acid being reduced to chlorine dioxide will create space within the solution for more chloric acid to be formed via reaction 1. If reaction 2 could be accelerated, then it would follow that reaction 1 would produce the requisite chloric acid at a faster rate. 
     The same is also true for whatever process is used as may be exemplified by the following equations. ##STR2## 
     By accelerating production of chlorine dioxide in equations 5 and 6, chloric acid will be formed at a faster rate in equation 1. 
     It can therefore be seen that the reaction kinetics in equation 1 are dependent upon the rate of reduction of chloric acid in equations 2, 5 and 6. 
     It is known from Supplement 2, Part 1 of Mellor&#39;s Comprehensive Treatise on Inorganic and Theoretical Chemistry that acetic anhydride acts to liberate chlorine dioxide from sodium chlorite solutions at pH 4 and at room temperature. The release of chlorine dioxide from the sodium chlorite is increased thirty fold with as little as a 0.005 molar acetic anhydride addition. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a process of production of chlorine dioxide in which the rate of chlorine dioxide production is increased. 
     Accordingly the present invention is in a process for producing chlorine dioxide by a reducing a chlorate in sulphuric acid of at least 4N with a reducing agent and is the improvement comprising introducing into the process an oxygen containing organic compound. 
     In a preferred embodiment of the oxygen containing organic compound may be selected from ethanol, methanol, acetic acid, maleic acid, thioacetic acid and ethyl acetate. However, in a particularly preferred embodiment the oxygen containing organic compound is acetic anhydride. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Experiments were conducted to examine the catalytic effects of organic oxygen containing compounds in the production of chlorine dioxide by reduction of a chlorate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The experiment apparatus and results are shown in the accompanying drawings, in which: 
     FIG. 1 is a schematic view of the apparatus in which the experiments were conducted; 
     FIG. 2 is a graph relating the rate of chlorine dioxide production to time; 
     FIG. 3 is a graph relating the rate of chlorine dioxide production to the molarity of acetic anhydride used as a catalyst; and 
     FIG. 4 again relates the rate of chlorine dioxide production to time. 
    
    
     FIG. 1 shows an apparatus comprising a chlorine dioxide reactor 10 having an inlet 12 for dilution air. The inlet 12 has a branch pipe 14 to receive catalyst and reducing agent. The reactor 10 is equipped with a sealable top 16 and with a stirrer shaft 18. The stirrer shaft 18 can be rotated magnetically by a stirring motor 20 in a conventional manner. The reactor 10 is provided with a jacket 22 having an inlet 24 and an outlet 26. A temperature control medium, usually water, is passed through the jacket 22 and a heat exchanger 28 so that the reaction temperature can be controlled. 
     Gas produced in the chlorine dioxide reactor 10 can be drawn by a water operated eductor 30 through a first absorber 32 or a second absorber 34. Flow control through the absorbers 32 and 34 is by inlet valves 36 and outlet valves 38. The absorbers 32 and 34 are filled with a 10% potassium iodide solution to absorb chlorine dioxide. 
     Using the apparatus illustrated in FIG. 1 the following experiments were conducted. The reactants were introduced into the batch reactor 10 which contained sodium chloride in a 9N sulphuric acid solution. A suitable reducing agent, for example methanol or sulphur, was present and the rate of reaction was observed using as a catalyst certain oxygen containing organic compounds. 
     EXAMPLE 1 
     A batch of 9N H 2  SO 4  solution was prepared, then split into 5 110 milliliter samples. Into each sample 5 grams of NaClO 3  were added and each sample set aside. 
     Sample 1 was then heated to 60° C. in reactor 10 stirred with magnetic stirrer 18. At time zero (0), 0.8 cc of aqueous sulphur solution (70% w/w) was added, while outside air was drawn across the top of the reacting solution to sweep the ClO 2  from the reactor into two KI traps 32 and 34 mounted in parallel. 
     The valves 36 and 38 of traps 32 and 34 were arranged so that after a 5 minute time interval the non-use trap would be alternated into the product stream. Sample 1 was a blank run with no acetic anhydride (AA) in the reactor. Reactor temperature was maintained by circulating water at 60° C. through the jacket 22. 
     FIG. 2 shows the rate of ClO 2  production per unit time with a peak being developed at the 10 minute mark. Table 1 tables the results. 
     
                                           TABLE 1                                 
__________________________________________________________________________
TABULATED RESULTS                                                         
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EXP. NO. 24 EXP. NO. 25                                                   
                       EXP. NO. 26                                        
                                  EXP. NO. 27                             
                                             EXP. NO. 20                  
                                                        ELAPSED           
Cl02                                                                      
    Cl2                                                                   
       Cl02 Cl02                                                          
               Cl2                                                        
                  Cl02 Cl02                                               
                          Cl2                                             
                             Cl02 Cl02                                    
                                     Cl2                                  
                                        Cl02 Cl02                         
                                                Cl2                       
                                                   Cl02 TIME IN           
gms gms                                                                   
       gms/min                                                            
            gms                                                           
               gms                                                        
                  gms/min                                                 
                       gms                                                
                          gms                                             
                             gms/min                                      
                                  gms                                     
                                     gms                                  
                                        gms/min                           
                                             gms                          
                                                gas                       
                                                   gas/min                
                                                        MINUTES           
__________________________________________________________________________
CUMULATIVE  CUMULATIVE CUMULATIVE CUMULATIVE CUMULATIVE 0                 
                                                        ↓          
0.31                                                                      
    0.038                                                                 
       0.62 0.37                                                          
               0.002                                                      
                  0.074                                                   
                       0.5                                                
                          0.013                                           
                             0.1  0.6                                     
                                     0  0.12 0.72                         
                                                0.047                     
                                                   0.144                  
                                                        5                 
0.98                                                                      
    0.042                                                                 
       0.134                                                              
            1.06                                                          
               0.002                                                      
                  0.138                                                   
                       1.51                                               
                          0.013                                           
                              0.202                                       
                                  1.62                                    
                                     0  0.204                             
                                             1.82                         
                                                0.047                     
                                                   0.22 10                
1.53                                                                      
    0.049                                                                 
       0.11 1.75                                                          
               0.002                                                      
                  0.138                                                   
                       2.11                                               
                          0.015                                           
                             0.12 2.26                                    
                                     0  0.128                             
                                             2.42                         
                                                0.056                     
                                                   0.12 15                
 1.875                                                                    
    0.107                                                                 
       0.069                                                              
            2.26                                                          
               0.002                                                      
                  0.102                                                   
                       2.76                                               
                          0.084                                           
                             0.13 2.65                                    
                                     0.106                                
                                        0.078                             
                                             2.79                         
                                                0.063                     
                                                   0.074                  
                                                        20                
0 gms AA     0.018 gms AA                                                 
                       0.036 gms AA                                       
                                  0.072 gms AA                            
                                             0.108 gms AA                 
__________________________________________________________________________
                                  EXP. NO. 32                             
                                             EXP. NO. 34                  
                                                        ELAPSED           
                                  Cl02                                    
                                     Cl2                                  
                                        Cl02 Cl02                         
                                                Cl2                       
                                                   Cl02 TIME IN           
                                  gms                                     
                                     gms                                  
                                        gms/min                           
                                             gms                          
                                                gms                       
                                                   gms/min                
                                                        MINUTES           
__________________________________________________________________________
                                  CUMULATIVE CUMULATIVE 0                 
                                                        ↓          
                                  0.67                                    
                                     0.022                                
                                        0.134                             
                                             0.57                         
                                                0.13                      
                                                   0.114                  
                                                        5                 
                                  1.66                                    
                                     0.042                                
                                        0.198                             
                                             1.61                         
                                                0.13                      
                                                   0.208                  
                                                        10                
                                  2.32                                    
                                     0.058                                
                                        0.132                             
                                             2.34                         
                                                0.13                      
                                                   0.146                  
                                                        15                
                                  2.67                                    
                                     0.21                                 
                                        0.07 2.74                         
                                                0.10                      
                                                   0.080                  
                                                        20                
                                  0.049 gas AA                            
                                             0.08 gas AA                  
__________________________________________________________________________
 NOTE                                                                     
 All experiments were conducted over a period of 20 minutes with reactor  
 contents at 60° C. and containing:                                
 NaClO.sub.3  5 grams                                                     
 NaCl  0.25 grams                                                         
 H.sub.2 SO.sub.4  50 grams                                               
 H.sub.2 O  85 mils                                                       
 Sulphur Sol 0.8 cc.sup.s                                                 
 AA  Acetic Anhydride                                                     
 
    
     EXAMPLE 2 
     In this second and subsequent runs using the prepared acid solution, AA was added through a controlled glass dropper into the aqueous solution containing sulphuric acid and sodium chlorate. Within 5 seconds of the AA addition, the sulphur sol reducing agent was added and the stop watch was started. 
     FIG. 3 plots the results of these experiments, and shows that the reaction rate with no AA addition peaks at 0.134 gms of ClO 2  per minute. As the AA molarity is increased, the rate of ClO 2  production per unit time is increased accordingly, up to 0.22 gms per minute at an AA molarity of 0.0096. 
     EXAMPLE 3 
     Once again a 9N H 2  SO 4  /H 2  O solution was prepared as in the previous experiments listed as #24 to #28 inclusive and experiments #32 and #34. The preparation, volume, and operating temperature remained the same except for the catalyst, where ethyl alcohol was used. Experiments #29, 30, 31 and 33 are set out in Table 2. An improvement in Production rate did not become obvious until Exp. #23 where 0.114 gms of ethanol was used, or equivalent to 0.018 Molar. This is shown in FIG. 4. Because experiments 30 and 31  show negligible differences the curve for Experiment 31 was omitted. 
     
                                           TABLE 2                                 
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TABULATED RESULTS                                                         
EXP. NO. 29                                                               
           EXP. NO. 30                                                    
                      EXP. NO. 31                                         
                                 EXP. NO. 33                              
                                            ELAPSED                       
Cl02                                                                      
   Cl2                                                                    
      Cl02 Cl02                                                           
              Cl2                                                         
                 Cl02 Cl02                                                
                         Cl2                                              
                            Cl02 Cl02                                     
                                    Cl2                                   
                                       Cl02 TIME IN                       
gms                                                                       
   gms                                                                    
      gms/min                                                             
           gms                                                            
              gms                                                         
                 gms/min                                                  
                      gms                                                 
                         gms                                              
                            gms/min                                       
                                 gms                                      
                                    gms                                   
                                       gms/min                            
                                            MINUTES                       
__________________________________________________________________________
CUMULATIVE CUMULATIVE CUMULATIVE CUMULATIVE 0                             
                                            ↓                      
0.49                                                                      
   0.042                                                                  
      0.098                                                               
           0.37                                                           
              0.078                                                       
                 0.074                                                    
                      0.39                                                
                         0.066                                            
                            0.078                                         
                                 0.62                                     
                                    0.12                                  
                                       0.124                              
                                            5                             
1.29                                                                      
   0.142                                                                  
      0.16 1.15                                                           
              0.123                                                       
                 0.156                                                    
                      1.19                                                
                         0.108                                            
                            0.16 1.53                                     
                                    0.18                                  
                                       0.182                              
                                            10                            
1.85                                                                      
   0.19                                                                   
      0.112                                                               
           1.75                                                           
              0.132                                                       
                 0.12 1.77                                                
                         0.13                                             
                            0.116                                         
                                 2.09                                     
                                    0.22                                  
                                       0.112                              
                                            15                            
2.16                                                                      
   0.3                                                                    
      0.062                                                               
           2.09                                                           
              0.132                                                       
                 0.068                                                    
                      2.12                                                
                         0.20                                             
                            0.07 2.41                                     
                                    0.25                                  
                                       0.064                              
                                            20                            
0 gms EtOH 0.019 gms EtOH                                                 
                      0.038 gms EtOH                                      
                                 0.114 gms EtOH                           
__________________________________________________________________________
 NOTE                                                                     
 All experiments were conducted over a period of 20 minutes with reactor  
 contents at 60° C. and containing:                                
 NaCl0.sub.3  5 grams                                                     
 NaCl  0.25 grams                                                         
 H.sub.2 SO.sub.4  50 grams                                               
 H.sub.2 O  85 mils                                                       
 Sulphur Sol  0.8 cc.sup.s                                                
 EtOH  Ethyl Alcohol                                                      
 
    
     In addition to the above specific Examples various combinations of reducing agents and potential catalysts were tried. The following exemplifying the compounds used: 
     
         ______________________________________                                    
Reducing Agent      Catalyst                                              
______________________________________                                    
Sulphur Solution    Ethanol                                               
Sulphur Solution    Methanol                                              
Sulphur Solution    Acetic Acid                                           
Sulphur Solution    Acetic Anhydride                                      
Sulphur Solution    Maleic Acid                                           
Sulphur Solution    Thiocetic Acid                                        
Sulphur Solution    Ethyl Acetate                                         
Methanol            Ethanol                                               
Methanol            Acetic Anhydride                                      
______________________________________                                    
 
    
     All of the foregoing were tested with varying levels of success. It is correct to say that all catalysts demonstrated an increase in the kinetic reaction between oxidizing and reducing agent to produce chlorine dioxide gas. The effect on the catalyst is presently unknown as the reactor running times are of limited batch duration and the method of analysis complex. It may be postulated however, that the catalysts tested would eventually oxidize, until the affect of speeding up the chlorine dioxide production dropped to normal rates. 
     The above experimental work indicates that reaction rates are greatly increased using the process of the present invention. 
     The method of adding the catalyst may vary. In general, a system that will distribute the catalysts throughout the reactor solution quickly is preferred. This rapid mixing may be accomplished by pre-mixing the catalyst with the incoming sodium chlorate solution. If solid, crystal chlorate is added directly to the reactor, the hydrocarbon catalyst may be added to a circulating solution pump.