Abstract:
An electrolytic cell includes an electrode in a chamber including an electrolyte and an electrical power supply arranged to supply electrical energy to the electrode. A current sensor detects electrical current in the cell. A reservoir of a metal salt solution is provided. A controller is arranged to supply metal salt solution from the reservoir to the chamber of the cell in dependence on the current detected at the sensor. This permits the conductivity of the electrolyte to be brought up to required levels quickly, independently of dynamic cell conditions. Previously, metal salt was added gradually to the electrolyte. This was time-consuming and produced inconsistent results.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of International Application No. PCT/GB2007/001211, filed Apr. 2, 2007, which claims the priority of United Kingdom Application No. 0607272.2, filed Apr. 11, 2006, the contents of both of which prior applications are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to apparatus for, and methods of, producing hydrogen peroxide by means of electrolytic cells, and especially to such apparatus arranged to produce hydrogen peroxide in, for example, an automatic dishwasher. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a conventional automatic dishwasher, detergents are employed to produce a wash liquid for use on dirty items placed in the dishwasher for washing. However, such detergents can decompose to produce pollutants when the wash liquid is released to the environment. 
         [0004]    It has been proposed to use a solution of hydrogen peroxide as a wash liquid in an automatic dishwasher. The advantage of hydrogen peroxide is that it decomposes into water and hydrogen only, which are harmless and pose no problem to the environment. However, hydrogen peroxide is unstable and is not capable of being stored for long periods without decomposing. Therefore, it has been proposed to generate hydrogen peroxide on-site electrolytically. 
         [0005]    A method and apparatus for generating hydrogen peroxide electrolytically is described in U.S. Pat. No. 6,767,447. Electrical energy is used to cause an electrochemical reaction, namely the electrolysis of water. By controlling the chemical reaction at the cathode of the cell, hydrogen peroxide is produced. 
         [0006]    A problem which may be encountered with the electrolysis of water is that, in order to produce good yields of hydrogen peroxide, the water must be electrically conductive. However, ordinary tap water, which would be the most convenient source of water, is not particularly conductive. In order to improve conductivity, a soluble metal salt may be introduced to the water. An advantageous choice of salt would be common salt—sodium chloride—owing to its availability. 
         [0007]    The introduction of sodium chloride to the electrolyte must be done in a controlled fashion in order to increase the conductivity of the electrolyte to the required level for efficient electrolytic reactions. However, gradual addition of sodium chloride to the electrolyte is time-consuming. The addition of a predetermined amount of salt solution to the electrolyte may not produce the required conductivity, due to variations in water quality, temperature and dynamic cell conditions. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, the invention provides apparatus for producing hydrogen peroxide comprising an electrolytic cell having an electrode in a chamber including an electrolyte, an electrical power supply arranged to supply electrical energy to the electrode, a current sensor arranged to detect electrical current in the cell, a reservoir of a metal salt solution and a controller arranged to supply metal salt solution to the chamber of the cell in dependence on the current detected at the sensor. 
         [0009]    The invention permits the conductivity of the electrolyte to be brought up to required levels quickly. Furthermore, the conductivity of the cell is controlled independently of conditions in the cell, such as changes in temperature and water quality. 
         [0010]    Advantageously, a pump is provided in order to circulate the electrolyte between the cell and a tank, with a mixer being employed in order to mix salt solution from the reservoir with the electrolyte prior to it being pumped into the cell. This arrangement provides a dynamic adjustment of the conductivity of the cell. 
         [0011]    Preferably, current in the cell is detected by a transducer. Data relating to the changes in conductivity of the cell may be recorded. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a schematic drawing of an electrolytic cell; 
           [0014]      FIG. 2  is a schematic diagram of a system constructed in accordance with the invention for controlling the conductivity of the cell of  FIG. 1 ; 
           [0015]      FIG. 3  is a graph showing change in current in the cell of  FIG. 1 ; 
           [0016]      FIG. 4  is a graph showing change in concentration of hydrogen peroxide in the cell of  FIG. 1 ; 
           [0017]      FIG. 5  is a graph showing a typical rate of decay of hydrogen peroxide; 
           [0018]      FIG. 6  is a graph showing  3  change in concentration of hydrogen peroxide in the cell of  FIG. 1  for different initial concentrations; 
           [0019]      FIG. 7  is a schematic diagram of a system for storing and replenishing hydrogen peroxide in the cell of  FIG. 1 ; and 
           [0020]      FIG. 8  is partly cut-away perspective view of the cabinet of an appliance incorporating the cell of  FIG. 1  and the systems of  FIGS. 2 and 7 . 
       
    
    
       [0021]    Like reference numerals refer to like parts throughout the specification. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  illustrates an electrolytic cell, indicated generally by the reference numeral  1 . This is purely a schematic drawing and none of the features are shown to scale. Only one cell  1  is illustrated in this drawing for clarity, but in practice a plurality of such cells would be connected in series. The cell  1  comprises two chambers  2 ,  3 , partitioned by an ion-exchange membrane  4  into an anode chamber  2  having an anode  5  and a cathode chamber  3  having a cathode  6 . The cell  1  also includes an air chamber  7 . The cathode  6  is a carbon cloth type of electrode that divides the cathode chamber  3  from the air chamber  7 . A pump  8  is provided in order to pump air from the surroundings into the air chamber  7 , so that it is held in the chamber  7  under pressure. The pressure causes the air to be forced into the carbon-cloth cathode  6 , so as to provide oxygen to the cathode. Oxygen is necessary to produce hydrogen peroxide in the cathode chamber  3 . 
         [0023]    The anode  5  and cathode  6  are connected to a power source in the form of DC power supply  9 . This may be derived from an electrical mains supply, suitably transformed and rectified to produce a DC supply  9  of an appropriate current and voltage. The electrolytic cell  1  is activated by applying electrical potential across the anode  5  and cathode  6  to force an internal chemical reaction between ions in the chambers  2 ,  3  and the electrodes. If the ions are positively charged cations they flow toward the cathode  6  and are reduced. If the ions are negatively charged anions they flow to the anode  5  and are oxidized. 
         [0024]    The chemical reaction at the cathode  6  can be simply expressed as follows: 
         [0000]      2H + +O 2 +2e − →H 2 O 2    
         [0025]    This reaction comprises the cathodic reduction of oxygen from the air chamber  7  by means of hydrogen ions and electrons which were generated in the anode chamber  2  and which migrate into the cathode chamber  3  via the membrane  4 . By controlling this reaction, hydrogen peroxide is produced in the cathode chamber  3 . 
         [0026]    The chambers  2 ,  3 , of the cell  1  have different respective electrolytes in them. The electrolyte associated with the cathode chamber  3  is called the catholyte  10 . The electrolyte associated with the anode chamber  2  is known as the anolyte  11 . 
         [0027]    In previous hydrogen-peroxide producing electrolytic cells, one electrolyte was used, common to both the anode chamber and the cathode chamber, and this was typically sodium chloride solution for convenience. However, in the electrochemical reaction at the anode, chlorine gas was produced which is highly poisonous and precludes such a cell being used in domestic applications, such as in an automatic dishwasher. Other electrolytes have been proposed, but these are generally more costly than sodium chloride solution and have to be replenished as the electrolyte is used up in the cathode chamber when producing hydrogen peroxide. 
         [0028]    This arrangement permits the user to select the most appropriate electrolyte for each of the electrodes, based on such considerations as user safety, cost, availability and efficacy. 
         [0029]    Before the electrolytic cell  1  is activated, the catholyte  10  comprises a solution of sodium chloride. This is a convenient choice of catholyte, particularly in the application of such a cell for an automatic dishwasher. Such dishwashers typically have a chamber in which dishwasher salt is stored. Conventionally, this salt has been used to maintain the efficiency of the water softener in the dishwasher. This store of salt and the supply of water from the local water mains provide a convenient source of sodium chloride solution. 
         [0030]    A suitable anolyte  11  is a solution of a metal sulphate salt, such as sodium sulphate. Softened tap water may also be employed as an anolyte. During the electrochemical reactions, only oxygen is produced at the anode  5 , which oxygen can be released harmlessly to atmosphere or reutilised in the cell. In the electrolytic generation of hydrogen peroxide, the anolyte  11  is not used up and therefore does not need to be replenished. 
         [0031]    The anode  5  is not consumed by the electrochemical reactions in the anode chamber. A suitable anode material is a mesh of titanium oxide coated with iridium oxide. Conventional electrolytic cells have had to employ anode materials that minimise the generation of chlorine gas, for example platinum. Platinum is costly, much more so than iridium oxide based anodes. With this apparatus, the anolyte is selected so that chlorine is not produced at the anode, permitting the use of these cheaper materials. A further advantage is that iridium oxide based anodes have a lower electrical resistance than those of platinum, which improves the rate of production of hydrogen peroxide. 
         [0032]    The ion exchange membrane  4  permits electrical contact between the electrolytes  10 ,  11  in the respective chambers  2 ,  3 , but does not permit the electrolytes to mix. 
         [0033]    The electrolytic cell  1  is very small, of the order of a few centimetres wide. Therefore, a typical cell can only produce small volumes of hydrogen peroxide. Greater yields can be achieved by employing a plurality of such cells connected in series. A further measure for improving yield is to circulate the catholyte  10  between the cell and a reservoir of catholyte held in a catholyte storage tank  12 , which is shown in  FIG. 2 . The catholyte storage tank  12  has a capacity of several litres, typically approximately five litres. In use, hydrogen peroxide is produced in the cathode chamber  3 , and this hydrogen peroxide is pumped, by means of catholyte pump  13 , into the catholyte tank  12 , to be replaced by more catholyte. As the catholyte is circulated between the cathode chamber  3  of the cell  1  and the catholyte tank  12 , the concentration of hydrogen peroxide in the catholyte gradually increases. By controlling the chemical reaction at the cathode  6 , and by recirculating the catholyte between the cell  1  and the storage tank  12 , a five-litre batch of hydrogen peroxide solution of a desired concentration can be produced. In typical domestic and light industrial applications, it has been found that a concentration of approximately 0.35% is adequate for most purposes. 
         [0034]    Similarly, there is provided an anolyte storage tank  14  for containing a reservoir of anolyte  11 . The anolyte tank  14  has a lower capacity, of typically approximately one litre. A pump  15  is employed to circulate anolyte  11  between the anode chamber  2  and the anolyte storage tank  14 , in order to supply a fresh batch of ions to the cell  1 . In the electrochemical reaction at the anode  5 , the anolyte  11  is not used up. The anolyte  11  only needs to be replenished by water from the main supply  16  occasionally, in order to replace anolyte that may have evaporated from the storage tank  14 . 
         [0035]    In order to increase the reaction speed of the cell  1 , and hence the production rate of hydrogen peroxide, an adequate current density is required at the cell. This current density needs to be stable in order to maintain a desired production rate. In accordance with the invention, a reservoir  17  of metal salt solution is provided and is controlled so as to dose the catholyte  10  with this salt solution in order to bring the conductivity level of the cell quickly to the desired level. In this embodiment, the metal salt solution held in the reservoir  17  is a solution of sodium chloride, that is to say brine. 
         [0036]    A current sensor  18  is arranged to detect electrical current passing through the cell  1 . This current is indicative of the conductivity of the cell  1 . In this embodiment, the current sensor  18  comprises a current transducer that produces a signal indicative of the current strength to a controller incorporating a processor  19 . The processor  19  periodically compares the signal with a predetermined current value and controls a pump  20  connected to the reservoir  17  of brine in dependence on this difference between measured and predetermined current values. If the current detected by the sensor  18  is below a predetermined value, the processor is arranged to activate the brine pump so as to dispense a volume of brine. Brine from the reservoir  17  enters a static mixer  21 , which forces the brine to mix with catholyte being pumped from the catholyte tank  12  into the electrolytic cell  1 . The combination of liquids exiting the static mixer  21  then enters the cell  1  and effects an immediate change in its conductivity. 
         [0037]      FIG. 3  is a graph showing the change in current detected at the transducer during a typical conductivity control process. The predetermined current level was set to  16  Amps. By increasing the concentration of metal ions in-line in the manner described above, the conductivity of the cell increases sharply, so that the cell reaches the required current level in a matter of minutes. By regular and periodic monitoring of the current and consequent control of the addition of brine solution, this level of current is maintained throughout the process of producing a batch of hydrogen peroxide, with relatively little deviation in current level. Thus, the conductivity of the cell  1  is changed dynamically, but is not influenced by the operating conditions of the cell itself, such as the air pressure at the cathode, variations in supply voltage, the temperature of the electrolytes and the quality of the water supply. 
         [0038]    The processor  19  may be arranged to record the conductivity data for monitoring of the system. Abnormal changes in conductivity may be indicative of a fault in the equipment and so the data can be employed to help alert a user or a technician to problems. 
         [0039]      FIG. 4  shows the rate of production of hydrogen peroxide by the cell  1  as the catholyte  10  is recirculated between the catholyte tank  12  and the cell. The rate of production is steady and the hydrogen peroxide comes up to the required concentration within two hours. 
         [0040]    Although this is a relatively rapid rate of controlled production of hydrogen peroxide, it may be slow for some applications. For example, in a domestic automatic dishwasher, it may be inconvenient for the user to have to wait two hours for sufficient hydrogen peroxide to be produced for the purposes of washing a load. The catholyte tank  12  is arranged to store a batch of hydrogen peroxide produced by the cell. However, hydrogen peroxide is known to decay over a period of a few days. Thus, if the batch of hydrogen peroxide is not used within this time, it will degrade and become unusable. 
         [0041]    The concentration of hydrogen peroxide stored in the catholyte tank  12  is topped up whenever a batch of hydrogen peroxide is required, by energising the electrolytic cell  1  and circulating the contents of the catholyte tank  12  through the cell, until the hydrogen peroxide thus produced reaches the required concentration. By starting the hydrogen peroxide-generating process with a catholyte that already contains hydrogen peroxide solution, a batch of hydrogen peroxide of a required concentration can be produced more quickly than by starting the process with fresh water as the catholyte. 
         [0042]    The processor  19  is arranged to monitor the length of time that the stored hydrogen peroxide has been held in the tank  12 .  FIG. 5  is a graph showing a typical decay rate for hydrogen peroxide. For example, if the hydrogen peroxide has been stored for five days, it will have decayed such that its concentration is reduced from approximately 0.4% to approximately 0.225%. When a batch of hydrogen peroxide is required, the processor  19  is arranged to activate the electrolytic cell  1  in order to bring the concentration back up to a predetermined level.  FIG. 6  is a graph showing the change in hydrogen peroxide concentration over time as the cell is activated at different initial concentrations. For example, if the initial concentration is 0.225%, it takes approximately one hour to get the concentration back up to 0.4%, which is significantly shorter than the two hours required to produce a fresh batch from pure water. This data can be used by the processor to determine by a simple algorithm how long it will take the cell  1  to refresh the hydrogen peroxide, so as to produce a batch of a predetermined concentration. 
         [0043]    This system is shown schematically in  FIG. 7 , as applied to an appliance in the form of a domestic dishwasher, such as that shown in  FIG. 8 . The dishwasher  22  comprises an insulated outer cabinet  23  containing a tub  24 , the front wall of which is pivotable about its bottom edge to provide a door  25  to give access to the tub. The dishes, other crockery, cutlery and utensils forming the load are placed in racks in the tub. One rack  26  is shown in  FIG. 8  in the upper portion of the tub  24 . Typically, another rack is provided in the lower portion of the tub  24 . Water is sprayed over the dishes from jets  27 ,  28  driven by a centrifugal pump which is in turn powered by an electric motor. The pump and motor are not visible in this drawing. The electrolytic cell  1 , the storage tanks  12 ,  14  and parts of the hydrogen peroxide-producing apparatus may be housed in convenient locations in the appliance, for example in a compartment under the cabinet or in partitions in the sidewalls. 
         [0044]    In operation, the door  25  is opened and the dishes etc forming the load of the machine are inserted into the rack. User-operable controls  29  are provided on a front panel  30  and are operated to start the washing operation. The machine fills with water and a heating element  31  is activated. When the water temperature is sufficient for the load to be adequately washed, the electric motor is operated and the pump drives hot water to the spray jets  27 ,  28  to start a pre-wash step. 
         [0045]    In  FIG. 7 , the tub of the dishwasher is indicated at  24  and is connected to the domestic main water supply  16 . For clarity, the controller incorporating the processor  19  is not shown in this drawing. The controller is arranged to control the pumps, valves and the electrolytic cell in the dishwasher. When the dishwasher  22  is activated, a flow diverter  32  directs water from a water softener  33  connected inline with the water mains  16  directly into the wash sump for the dishwasher. This initialises the pre-wash step, which is employed to rinse particles of food and other dirt from the items in the tub  24  to be washed. Whilst the pre-wash step is taking place, the peroxide solution which had been stored in the catholyte tank  12  is pumped through the electrolytic cell  1 , which is energised by means of the transformed and rectified power supply derived from the electric mains supply. Thus, the process of generating hydrogen peroxide does not start from scratch, but instead starts from the residual concentration remaining in the stored solution. This continues until a batch of hydrogen peroxide of the required concentration is produced, at which point the dishwasher is arranged to start its main wash step. 
         [0046]    During the main wash, a drain valve  34  connected to the catholyte tank  12  is activated, so that the batch of hydrogen peroxide contained therein is dispensed into the wash tub  24  of the dishwasher  22 . The hydrogen peroxide serves as a detergent for cleaning items in the tub  24 . It has been found that hydrogen peroxide is particularly suitable for washing items of glassware, which may become scratched and clouded by exposure to conventional detergents. A further advantage of hydrogen peroxide is that it decays into oxygen and water and so does not contribute to pollution when released into the environment. 
         [0047]    Whilst this main wash step happens, the catholyte tank  12  is replenished with fresh softened water from the water mains supply  16 . The process of hydrogen peroxide production is restarted afresh until a batch of hydrogen peroxide of the required concentration is produced. This batch is then stored in the catholyte tank  12  until the dishwasher is operated again, at which point the processor  19  determines the length of time that the peroxide has been in storage and thereby calculates the time that the cell  1  needs to run to replace the decayed hydrogen peroxide. This time may be communicated to a user of the dishwasher by means of, for example, a visual display on the control panel  30 . 
         [0048]    When the main wash has been completed, the tub  24  is emptied via a drain pump  35  and re-filled with fresh water, which is heated in order to rinse the load. After the rinse, the tub again drains. There may also be a drying step. Preferably, the drying step includes a short blast of high-speed airflow, to force residual water out of recesses in the dishes, such as the upturned bases of mugs. This may be followed by a period of slower-flowing air arranged to dry the dishes. The air may be heated. 
         [0049]    A further measure which may be implemented to reduce the time taken to refresh a batch of hydrogen peroxide involves reducing its rate of decay. By reducing the rate of decay, a batch of hydrogen peroxide stored for a long period will have a higher residual concentration than was achievable hitherto. This may be effected by controlling the pH of the catholyte. It has been found that a pH of less than 8.5 gives a batch of hydrogen peroxide with a slower decay rate. The graph of  FIG. 5  shows a typical decay for hydrogen peroxide at a pH of 8.11. It takes approximately five days for the concentration to reduce from approximately 0.4% to approximately 0.225%. However, if the pH is greater than 8.5, this rate of decay is much quicker. Typically, a batch of hydrogen peroxide produced at a pH of 8.64 at the cathode takes half a day to decay from a concentration of 0.4% to 0.225%. 
         [0050]    One way in which the pH of the catholyte may be controlled is by controlling the pH of the anolyte, since any change in pH of the catholyte produces a proportional change in the pH of the anolyte. The preferred value of the pH of the anolyte is between 1 and 2. This may be achieved by using a solution of sodium sulphate as the anolyte. Alternatively, softened water may be employed. 
         [0051]    During activation of the electrolytic cell  1 , the anolyte is not used up, but there will naturally be a small amount of loss due to, for example, evaporation. By replenishing the anolyte  11  with relatively small amounts of water compared with the overall volume of anolyte held in the tank, the consequent change in pH of the anolyte is kept to a minimum. In  FIG. 7  there is provided an anolyte tank valve  36 , arranged to divert water from the mains supply  16  to the anolyte tank  14 . If a larger amount of water is required, this is added in small amounts, to allow the pH of the anolyte  11  to stabilise between doses of water. Further stability in the pH of the anolyte  11  may be achieved by limiting exposure of the anolyte to air by, for example, making the anolyte tank  14  airtight, so that the anolyte does not need to be topped up so frequently. 
         [0052]    The invention has been described with reference to an automatic dishwasher employing hydrogen peroxide for dishwashing. However, the invention has a multitude of applications. For example, the invention may be employed for other cleaning operations, such as in floor cleaning appliances and particularly carpet cleaners. In a steam cleaner or steam iron incorporating the invention, the hydrogen peroxide would be heated, which is thought to increase its bleaching effect. 
         [0053]    Hydrogen peroxide has a sterilising effect, and so the invention could be employed both domestically and industrially to sterilise instruments, work surfaces, to treat injuries and infections and as a handwash dispenser. Further applications of the invention will be apparent to the skilled person.