Abstract:
A method and apparatus for economically, reliably and efficiently controlling electrolytic processes by measuring the conductivity of the product of the electrolysis, and having the process terminate when a specified conductivity value is reached, that value corresponding to a desired pH and/or ORP value.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Electrolyzing a brine solution and passing it through an appropriate ion transfer membrane, so as to produce Hypochlorous Acid (HCIO) and/or Sodium Hydroxide (NaOH) solutions, has been know for decades. In the proper concentrations, these solutions are particularly effective and ecologically benign sanitizers and degreasers, respectively, and the efficient production of such solutions was the inspiration for the parent application of this application, noted above, the disclosure of which is included herein by reference. In order to obtain the proper concentration of these solutions, the electrolysis process must ordinarily be monitored by continually checking the pH and/or oxidation reduction potential (ORP), of the solutions, and the process must be stopped either manually in response to the pH and/or ORP reaching a target value, or else automatically by control systems that can properly interpret such readings and halt the process. 
         [0002]    Existing pH and ORP sensors, unfortunately, are expensive, vulnerable and finicky devices, requiring frequent cleaning and recalibration. Given the perils inherent in relying on a mis-reading or malfunctioning pH or ORP sensor, it is easily understood that it is desirable to come up with a simple and reliable method for monitoring and controlling this process, which is what this invention provides. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    It has been discovered that, as such an electrolysis process progresses, a chart showing the electrical conductivity of the produced solution changes in a largely linear manner plotted against the elapsed time of the process, right up until a critical deflection point is reached; at that point, the slope of that curve becomes significantly more acute, proceeding again in a largely linear manner from that point. Conductivity sensors are much simpler and less maintenance-intensive than pH and ORP sensors, and so controlling such electrolysis processes with conductivity sensors rather than pH or ORP sensors offers greater reliability and security to permit highly automated processes with infrequent need for outside intervention and maintenance. 
         [0004]    In addition, it happens that this critical deflection point occurs at approximately the optimal concentration of the solutions for sanitizing and degreasing purposes. This means that, for producing such solutions, the controlling conductivity sensors need not even be calibrated to specific values, or even capable of measuring specific values, but can control the processes simply by being able to identify this significant change in the rate of change of conductivity. 
         [0005]    Thus, controlling such electrolytic processes with the aid of conductivity sensors provides significant advantages of cost, simplicity and reliability. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0006]      FIG. 1  is a graph depicting the results of the curves of the change of pH ( 1 ), conductivity ( 2 ) and ORP, ( 3 ) over time in the anolyte chamber of a process electrolyzing a brine solution to produce a Hypochlorous Acid solution. 
           [0007]      FIG. 2  is a graph depicting curves of the change of pH ( 11 ), conductivity ( 12 ) and ORP ( 13 ) over time in the catholyte chamber of a process electrolyzing a brine solution to produce a Sodium Hydroxide solution. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    This invention is an improvement upon the methods and apparatuses for electrolyzing a brine solution and passing it through appropriate ion transfer membranes, so as to produce Hypochlorous Acid (HCIO) and/or Sodium Hydroxide (NaOH) solutions, as known in the current art and as described, by way of example, in U.S. patent application Ser. No. 13/008,152, by John Kuiphoff; the disclosure of that application is incorporated herein by reference. 
         [0009]    Some method for measuring the progress of such an electrolyzing process must be provided, so that the process can be arrested before the products become too acidic and/or too alkaline. In the current art, this is provided by measuring either the pH or the ORP of the product(s) or, more crudely, by simple timed control of the process; all three methods have their drawbacks. 
         [0010]    Measuring pH or ORP provides an accurate reading of the progress of the reaction; unfortunately, sensors that provide such readings are expensive, and susceptible to damage and going out of calibration, shortening their service intervals and further adding expense. It is possible to utilize pH or ORP sensors initially with a given apparatus in order to establish timing baselines, and subsequently simply control the process with a timer; unfortunately, fluctuations in water and salt purity and power supplies can affect the process in ways for which a timer cannot compensate, and over time the process will also inevitably be affected by deterioration of the efficacy of membranes and electrodes, again uncompensated for by a timer. 
         [0011]    It is clear that a means of controlling these processes that provides the precision of pH or ORP sensors while avoiding their drawbacks is needed, and that is precisely what is achieved by controlling such processes by measuring the conductivity of the products. 
         [0012]    As  FIGS. 1 and 2  show analogous and equally pronounced and readily-indentifiable conductivity curves, while reference will generally be made to the curves and figures of  FIG. 1 , it is understood that the disclosure applies similarly to the complementary process charted in  FIG. 2 . 
         [0013]    The slope of the anolyte conductivity curve  2  (in  FIG. 1 ) can be seen to reach a critical deflection point at a conductivity reading of approximately 700 μS where it intersects with the pH curve  1  at a pH reading of approximately 4.5, with an ORP of approximately 190 mV, after an elapsed reaction time of around 2250 seconds. It can also be seen that while the slope of the pH curve  1  is generally linear, there is a short, more acute slope to that curve centered on this intersection. This “kink” in the conductivity curve just happens to occur at, or very near, the optimum concentration of HCIO for use as sanitizer. Because of the distinct change in the slope of the conductivity curve at this critical point, it is alternatively possible to control this anolyte process with a sensor that simply measures when the rate of change in conductivity increases markedly, rather than specifying a particular conductivity value. 
         [0014]    The slope of the catholyte conductivity curve  12  (in  FIG. 2 ) does not display a similar kink when the pH reaches the optimum concentration of NaOH for use as a degreaser, which is again at around the 2250 seconds mark, at a conductivity reading of approximately 1250 μS, a pH of around 10.8 and an ORP reading near −130 mV. 
         [0015]    Experimentation has shown that, barring contamination of the process with foreign matter that materially alters the conductivity of the solution, the conductivity reading for a particular process in a particular apparatus is consistently predictable for a particular pH and/or ORP reading; in other words, for the purposes of monitoring and controlling such electrolytic processes, measuring conductivity is as accurate and reliable as measuring pH or ORP, and significantly more easily and economically achieved. 
         [0016]    While this invention is inspired by a need to improve brine electrolysis processes, it can readily be seen that it can be effectively applied to a wide variety of electrolytic processes, and should not be read as being limited to brine electrolysis.