Abstract:
A system and method are disclosed for increasing the concentration of hypochlorous acid in a quantity of water. Acid is injected into chlorinated water to decrease the pH of the chlorinated water. By decreasing the pH, the hypochlorite/hypochlorous acid equilibrium of the chlorinated water is shifted to increase the concentration of hypochlorous acid on the treated water.

Description:
RELATED US APPLICATION DATA 
     This application claims priority from U.S. Provisional Application No. 60/261,923 filed Jan. 16, 2001, and U.S. Provisional Application No. 60/316,047 filed Aug. 30, 2001, both incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of disinfectants, and relates more specifically to a method and apparatus for producing hypochlorous acid solutions and maintaining hypochlorous acid concentrations by manipulating the pH of the solution. In particular the invention relates to the use of acids to produce or maintain hypochlorous acid levels in aqueous solutions. 
     2. Background of the Invention 
     Chlorination is known method for killing undesirable microorganisms. Chlorine may be provided in multiple forms including chlorine gas (Cl 2 ), sodium hypochlorite liquid, calcium hypochlorite powder or granules, or isocyurantes. Chlorine gas (Cl 2 ) is a relatively cheap and highly effective antimicrobial agent; however, it is also a highly toxic and corrosive gas. Hypochlorites such as NaOCl or Ca(OCl) 2  are a much safer alternative, but are considerably more expensive that gaseous chlorine. Finally, hypochlorite solutions (i.e., bleach) may also be utilized, however these are rarely used in large scale water treatment applications because they are bulky and expensive. Regardless of the chlorine source, hypochlorous acid (HOCl) and the hypochlorite ion (OCl − ) are the final desirable antimicrobial products. 
     One method of forming HOCl occurs when Cl 2  is dissolved in water. The reaction proceeds according to the following equation:
 
Cl 2 +H 2 OHOCl+H + +Cl −   (1)
 
     Another method for producing HOCl uses metal hypochlorites dissolved in water. The reaction proceeds according to the following equation: 
      NaOCl+H 2 ONaOH+HOCl  (2) 
     This method is generally utilized by common household hypochlorites and generates HOCl on a relatively small scale. 
     HOCl is a weak acid and will dissociate. In aqueous solution, HOCl and OCl −  are generally present in a pH dependent equilibrium:
 
HOClH + +OCl − pKa=7.53  (3)
 
     At low pH, HOCl is the predominant form, while at high pH, OCl −  predominates. The HOCl form is about 80 times more effective than OCl −  for killing microorganisms because HOCl crosses cell membranes easier than the hypochlorite ion. Accordingly, it would be desirable to control the pH of the chlorinated solution to increase the antimicrobial effectiveness of the chlorination process. 
     Processes and systems for adjusting the pH of a water stream are known in the art. U.S. Pat. No. 5,487,835 to Shane discloses a method and apparatus for controlling the pH of a water stream using carbon dioxide. Several methods currently are used to inject carbon dioxide into water. The most accepted method is to inject the carbon dioxide into the water by a direct gas feed through some type of diffusion system in a recarbonation basin; in effect, a bubbler. A mechanical mixer can be used in combination with this method for better efficiency. Another method for injecting carbon dioxide into water is to aspirate the carbon dioxide into a stream of water using a venturi type eductor. In this method, the carbon dioxide is injected into the stream of water and carried along with the stream of water to a grid system located in a basin or a pipeline. 
     Both the direct gas feed method and the venturi method of injecting carbon dioxide gas into water allow for the control of the pH and the stabilization of the treated water. However, it is difficult to control the efficiency of the carbon dioxide gas usage. Both of these processes require the use of a relatively large contact basin, a relatively long contact time or large amount of carrier water, all of which inherently are inefficient. 
     The formation of hypochlorous acid is also known in the art. For example, U.S. Pat. No. 4,017,592 to Penard et al. discloses a process for producing aqueous hypochlorous acid solution using a gas, liquid, solid phase system. U.S. Pat. No. 4,146,578 to Brennan et al. discloses a process for preparing hypochlorous acid wherein gaseous chloride is reacted with an aqueous solution of an alkali metal hydroxide in a finely divided mist at an elevated temperature to prevent condensation. U.S. Pat. No. 4,147,761 to Wojtowicz et al. discloses a hypochlorous acid process using a sweep reactor in which gaseous chloride is passed rapidly across the surface of an agitated aqueous solution of alkali metal hydroxide at a reduced temperature. 
     U.S. Pat. No. 4,190,638 to Hoekje et al. discloses the production of hypochlorous acid wherein a precipitate formed on a carbonating electrolytic cathode cell liquor is contacted in a fluidized bed with a mixture of gaseous chlorine and water vapor. The exit gas from the fluid vapor is absorbed in water. U.S. Pat. No. 4,504,456 to Yant et al. discloses a process and apparatus for forming hypochlorous acid by hydrolyzing chlorine with steam and water vapor and solid-gas stripping of the co-generated hydrochloric acid vapor. Hypochlorous acid vapor is dissolved into an aqueous product solution. 
     U.S. Pat. No. 5,037,627 to Melton et al. discloses a hypochlorous acid process by reacting an aqueous solution of an alkali metal hydroxide in droplet form with gaseous chlorine to produce hypochlorous acid vapor and solid alkali metal chloride particles. U.S. Pat. No. 5,322,677 to Shaffer et al. discloses a process for producing a concentrated aqueous hypochlorous acid solution by reacting droplets of an alkali metal hydroxide solution containing greater than 50 percent by weight of the alkali metal hydroxide with chlorine gas. 
     Despite the several known processes for producing hypochlorous acid, there remains a need for a quick, safe, and efficient process for producing hypochlorous acid solutions suitable for use as a disinfectant. 
     SUMMARY OF THE INVENTION 
     The present invention controls the hypochlorite/hypochlorous acid balance of a stream by mixing acid, preferably liquid acid, with a pressurized carrier stream which has been chlorinated by the addition of a chlorination agent. Preferably, all the streams are liquid streams. The introduction of the acid into the liquid stream reduces the pH of the liquid stream and increases the relative ratio of hypochlorous acid to hypochlorite of the liquid stream. The present invention discloses a novel hypochlorous acid forming process and system that are advantageous over other hypochlorous acid forming systems and processes because the pressurized streams of the present invention are readily manipulated to produce specific concentrations of hypochlorous acid thereby providing greater control over the reaction process. The hypochlorous acid solution formed using the present invention is suitable for use as a disinfectant in food processing, including but not limited to meat and poultry processing, treating wastewater, and treating drinking water. 
     In one embodiment, the system of the present invention takes a liquid acid and injects this acid into a chlorinated carrier water maintained at an elevated pressure. The lowering of the pH effectively converts the vast majority of the hypochlorite ions in the solution to its more highly effective hypochlorous acid form. 
     In yet another embodiment, the acid is a carbon dioxide-containing gas with a carrier liquid to form a carbon dioxide-water solution. This carbon dioxide-water solution is then injected into a liquid stream which has been chlorinated by the addition of a chlorination agent. The introduction of the carbon dioxide-water solution into the liquid stream causes the formation of carbonic acid therein, thereby reducing the pH of the liquid stream and increasing the relative ratio of hypochlorous acid to hypochlorite of the liquid stream. 
     The system of the present invention can take carbon dioxide gas at an elevated pressure and inject this gas into chlorinated carrier water, also at an elevated pressure. The chlorinated carrier water-carbon dioxide solution, still at an elevated pressure, then is injected into the water to be treated, which typically is at atmospheric pressure. As the pressurized carrier water-carbon dioxide solution is injected into the water to be treated, the carbon dioxide expands due to the lower pressure of the water to be treated and the excess carbon dioxide bursts forth out of solution as minute bubbles released into the main stream of water. The carbon dioxide bubbles mix rapidly with the main stream of water, thus reducing the pH of the main stream of water. In most cases, up to 95% of the chemical reaction between the main stream of water and the carbon dioxide gas is immediate, thus eliminating the need for a recarbonation basin or an extended contact area or contact time. The lowering of the pH also effectively converts a majority of the hypochlorite ion in the solution to a most highly effective hypochlorous acid form. 
     In another embodiment, the resulting hypochlorous acid solution of the present invention is greater than about 50%, more preferably about 77 to about 99 percent hypochlorous acid. 
     Accordingly, it is an object of the present invention to provide a solution feed system for injecting acid into a chlorinated water stream for reducing the pH of the water stream and increasing the concentration of hypochlorous acid therein. 
     It is yet a further object of the present invention to provide a solution feed system which can be operated continuously and eliminates the need for a batch treatment apparatus. 
     Another object of the present invention is to provide a solution feed system for controlling the pH of a chlorinated water stream which is efficient in operation, simple in construction and manufacture, and easy to operate. 
     It is an object of the present invention to provide a solution feed system for injecting carbon dioxide into a chlorinated water stream for controlling the pH of the water stream and increasing the concentration of hypochlorous acid therein. 
     Yet another object of the present invention is to provide a solution feed system in which recarbonation basins and/or extended contact areas or times are unnecessary. 
     A further object of the present invention is to provide a solution feed system in which elevated pressure carbon dioxide first is mixed with elevated pressure carrier water, and the elevated pressure carrier water-carbon dioxide solution then is injected into the water to be treated. 
     Still another object of the present invention is to provide a solution feed system in which carbon dioxide mixes almost immediately with the water to be treated, thus reducing the overall contact time needed between the carbon dioxide and the water to be treated. 
     These and other objects, features and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram of a solution feed system in accordance with a preferred embodiment of the present invention. 
         FIG. 2  is a flow diagram of a solution feed system in accordance with another preferred embodiment of the present invention. 
         FIG. 3  is a flow diagram of a solution feed system in accordance with another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a general schematic of the entire solution feed system is shown. In general, the solution feed system comprises a chlorination system  100 , and an acid injection system  200 . The various components of each of there systems are connected using standard piping. The hydrochlorous acid process of the present invention can be performed at ambient temperature or lower, i.e. about 25° C. or less. 
     As shown in  FIG. 1 , a stream of make up water AA is directed from a water source to chlorination system  100 . Stream AA is typically maintained at normal line pressures. Stream AA flows through shut-off valve  102 , and the total flow rate of make up water stream AA is controllable by, for example, the operation of a metering control valve  124  in response to signals from a Programmed Logic Controller (PLC)  104  which coordinates the overall system operation. A line  106  can split a portion of make up water stream AA providing greater control of the fluid volume in the chlorination system  100 . The remainder of make up water stream AA enters chlorination system  100  and is subjected to chlorination therein by the addition of a chlorinating agent. The chlorinating agent may be a chlorine gas, a solid hypochlorite salt (e.g., NaOCl or Ca(OCl) 2 ), or a liquid hypochlorite solution (i.e., a bleach). The chlorination agent serves to raise the concentration of chlorine in make up water stream AA in the hypochlorite ion (OCl − ), hypochlorous acid (HOCl), or a combination thereof. In one embodiment, the chlorination agent is not a metal chlorite including but not limited to NaOCl 2 . 
     Stream AA exits the chlorination system  100  as chlorinated stream BB through line  108  directed to a holding tank  110  through pumps  112  and  114  which increase the pressure of chlorinated water stream BB to at least about 50 psi. A small portion of chlorinated water stream BB can be diverted to a chlorine analyzer  116  from a point just downstream of gate valve  118  and check valve  120  via bypass stream  122 . Gate valve  118  and check valve  120  prevent back flow in the system. Chlorine analyzer  116  can sense the chlorine level (ppm) of chlorinated water stream BB and transmits a signal indicative of this level to PLC  104 . PLC  104  in turn generates a control signal operate metering control valve  124  to control the fraction of flow AA to maintain chlorinated water stream BB at a desired chlorine concentration. In one exemplary embodiment, the desired chlorine concentration is about 50 ppm or less. 
     A pH analyzer  126  can sense the pH of chlorinated water stream BB in holding tank  110  and communicates this information to PLC  104 . PLC  104  regulates booster pump  128  such that the volume of acid from acidifier systems  130  or  132  increases to maintain the pH acidified chlorinated water stream/hypochlorous acid stream CC in the range of about 4.3 to about 7 resulting in an increase in HOCl concentration compared to OCl −  concentration in holding tank  110  (i.e., the ratio of HOCl to OCl −  is greater than one). Hypochlorous acid stream CC preferably contains about 77 to about 99 percent hypochlorous acid at ambient temperature. 
     The added acid can be organic or inorganic. Suitable organic acids include formic acid, acetic acid, citirc acid, lactic acid, trifluoroacetic acid, oxalic acid, tartaric acid, fumaric acid, maleic acid, methanesulfonic acid, benzenesulfonic acid and p-toluenesulfonic acid. Suitable inorganic acids include, but are not limited to, hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid and phosphoric acid. The acid stream joins chlorinated stream BB via line  134  upstream of static mixers  136  and  138 . In one embodiment, hypochlorous acid stream CC is maintained at a pressure of at least 50 psi. It will be appreciated that the acids can be liquid or solid. 
     After acid injection, hypochlorous acid stream CC passes through the pair of static mixers  136  and  138  which are arranged in parallel and serve to evenly disperse the acid throughout the hypochlorous acid stream CC. Hypochlorous acid stream CC then optionally passes through a serpentine loop  140  which allows additional contact time for the injected acid to blend into hypochlorous acid stream CC. 
     Hypochlorous acid stream CC then enters holding tank  110  before injection into a target liquid stream DD via line  142 . Pump  144  moves stream DD out of line  142  optionally to a wash water line or a chiller. In one embodiment, stream DD is maintained at a pressure of at least about 50 psi. 
     The pH analyzer  126  is provided to sense the pH of target liquid stream DD downstream of the point at which the acidified chlorinated carrier water is injected and to provide a signal indicative of the sensed pH to PLC  104 . PLC  104  then adjusts the acid flow rate through pump  128  to control the amount of acid being introduced and thereby maintains the pH of target liquid stream DD at a desired setpoint for efficient chlorination as discussed above. 
     In another preferred embodiment of the present invention utilizing a carbon dioxide injection system, as shown in  FIG. 2 , the stream of make up water AA is directed from a water source to the chlorination system  200 . The description of the preferred embodiment of  FIG. 1  is not repeated here, as only the differences between the two embodiments are highlighted. It will be understood by those of skill in the art that similar elements of  FIGS. 1 and 2  perform the same or similar functions. 
     In this embodiment, a bypass line  150  diverts a portion of make up water stream AA around chlorination system  100  as a chlorination bypass stream EE. The remainder of make up water stream AA enters chlorination system  100  and is subjected to chlorination therein by the addition of a chlorinating agent. Stream AA exits the chlorination system  100  as chlorinated stream BB through line  108 . 
     Bypass stream EE and chlorination stream BB are both directed into a mixing chamber  152  wherein they are recombined. Mixing chamber  152  includes a level sensor  154  which generates a signal indicative of the water level therein. This signal is relayed to PLC  104  which in turn generates a control signal to control the operation of flow control valve  102  to maintain a desired liquid level in mixing tank  152 . Mixing tank  152  is sized to allow time for even mixing of the chlorinated subfraction of chlorination stream BB with bypass stream EE before allowing it to exit as mixed water stream FF. 
     Mixed water stream FF is directed from mixing chamber  152  through pumps  112  and  114 . A small portion of mixed water stream FF can be diverted to a sampling cell  156 , or directly to chlorine analyzer  116 . Chlorine analyzer  116 /Sampling cell  156  can sense the chlorine level (ppm) of mixed water stream FF and transmit a signal indicative of this level to PLC  104 . PLC  104  in turn generates a control signal operate metering control valve  102  to control the fraction of flow BB which passes through bypass line  150  to maintain mixed water stream FF at a desired chlorine concentration. 
     Mixed water stream FF next passes to acid injection system  200  being a carbon dioxide injection system  202  through gate valve  118  and check valve  120 . In its simplest form, carbon dioxide injection system  202  includes a regulated flow of pressurized CO 2  which is injected into mixed water stream FF at a pressure significantly higher than that of mixed water stream FF. In one preferred embodiment, the carbon dioxide gas is directed through an isolation ball valve  158  then a wye strainer  160 , then a pressure reduction valve  162 . After pressure reduction valve  162 , the carbon dioxide gas goes through a flow meter  164  and a CO 2  metering control valve  166  that is responsive to maintain the CO 2  flow at a desired rate as determined by PLC  104 . 
     In a preferred embodiment, mixed water stream FF is maintained at greater than or equal to 50 psi at the CO 2  injection point and the CO 2  is maintained at a minimum of 55-75 psi or at least approximately 10 psi greater than the pressure of mixed water stream FF. 
     Hypochlorous acid stream CC then passes through a full port ball valve  170  before injection into a target liquid stream DD via diffuser  172 . Diffuser  172  is designed to maintain system pressure, thus forcing the CO 2  gas to remain in solution in the hypochlorous acid stream CC. 
     In a preferred embodiment, the pressure of target liquid stream DD is less than that of the hypochlorous acid stream CC. Therefore, as the hypochlorous acid stream CC passes through the small holes in diffuser  172 , it is diffused into target liquid stream DD, thereby lowering the pH of target liquid stream DD and shifting the hyopchlorite/hypochlorous acid balance to form predominantly hypochlorous acid. 
     A pH sensor  174  can be provided to sense the pH of target liquid stream DD downstream of the point at which the hypochlorous acid stream CC is injected and to provide a signal indicative of the sensed pH to PLC  104 . PLC  104  then adjusts the CO 2  flow rate through CO 2  metering valve  166  to control the amount of carbonic acid being introduced and thereby maintains the pH of target liquid stream DD at a desired setpoint for efficient chlorination as discussed above. 
     As shown in  FIG. 3 , in an alternative embodiment, hypochlorous acid stream CC may also be added directly to target liquid stream DD immediately after injection of carbon dioxide gas. In this embodiment, the carbon dioxide gas will still be largely present in gaseous form because there are no static mixers or serpentine loop to provide additional mixing and time to allow the carbon dioxide to go into solution. A substantial portion of the carbon dioxide will still go into solution as carbonic acid in the target liquid stream as it flows to its end use. 
     As previously mentioned, in the treated water solution, HOCl and OCl −  are generally present in a pH dependent equilibrium:
 
HOClH + +OCl − pKa=7.53
 
     As shown in Table 1, at low pH, HOCl is the predominant form, while at high pH, OCl −  predominates: 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Percent HOCl 
               
             
          
           
               
                 pH\ 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Temp 
               
               
                 ° C. 
                 0 
                 5 
                 10 
                 15 
                 20 
                 25 
                 30 
               
               
                   
               
             
          
           
               
                  5.0 
                 99.85 
                 99.83 
                 99.80 
                 99.77 
                 99.74 
                 99.71 
                 99.68 
               
               
                 5.5 
                 99.53 
                 99.75 
                 99.36 
                 99.27 
                 99.18 
                 99.09 
                 99.01 
               
               
                 6.0 
                 98.53 
                 98.28 
                 98.01 
                 97.73 
                 97.45 
                 97.18 
                 96.92 
               
               
                 7.0 
                 87.05 
                 85.08 
                 83.11 
                 81.17 
                 79.23 
                 77.53 
                 75.90 
               
               
                 8.0 
                 40.19 
                 36.32 
                 32.98 
                 30.12 
                 27.62 
                 25.65 
                 23.95 
               
               
                 9.0 
                 6.30  
                 5.40  
                 4.69  
                 4.13  
                 3.68  
                 3.34  
                 3.05  
               
               
                 10.0 
                 0.67  
                 0.57  
                 0.49  
                 0.43  
                 0.38  
                 0.34  
                 0.31  
               
               
                 11.0 
                 0.067 
                 0.057 
                 0.049 
                 0.043 
                 0.038 
                 0.034 
                 0.031 
               
               
                   
               
             
          
         
       
     
     The HOCl is much more effective than OCl −  for killing microorganisms because HOCl is nonpolar and can cross the outer membrane of most microbes and bacteria. Therefore, it is desirable to control the pH of the treated water solution to between 4.3 and 7.0, and more preferable to between 6.0 and 6.2 in order to ensure almost complete (˜98%) conversion to the hypochlorous acid form and thereby increase the antimicrobial effectiveness of the chlorination of the target liquid stream. At a pH of about 4.3 or lower, chlorine gas evolves from the solution. Therefore, in one embodiment, the pH of the solution stream is greater than about 4.3 to about 7. 
     While the invention has been disclosed in its preferred forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims.