Abstract:
An electrolytic cell comprising an electrolysis vessel for receiving a liquid electrolyte that fills the vessel to a predetermined level, electrodes for passing an electric current through the electrolyte, a vent for allowing gases produced by the electrolytic process to leave the vessel and an air inlet means located substantially adjacent to the predetermined level for directing air into the vessel at a predetermined rate.

Description:
PRIORITY CLAIM 
       [0001]    This Patent Application is the US National stage under U.S.C. 371 of PCT/GB2007/003591 filed Sep. 24, 2007, and designating the United States and claims priority to GB0618789.2, filed Sep. 23, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an electrolytic cell. In particular, it relates to an electrolytic cell for the production of Sodium Hypochlorite and a method of producing Sodium Hypochlorite. 
       BACKGROUND 
       [0003]    Sodium Hypochlorite (NaClO) is typically used as a disinfectant or as a “bleach”. It is also used to sanitise water at a water treatment works. In water treatment plants Sodium Hypochlorite is produced on site using a continuous process electrolytic cell. Sodium Hypochlorite can also be used to disinfect the water of swimming pools, which need to be treated regularly with disinfectants to ensure the water is of a suitable quality. Typically, Sodium Hypochlorite is purchased in solid crystalline form and used for disinfecting the water used in swimming pools. 
         [0004]    It is also common for water to be stored in storage tanks in food production factories and supermarkets, for example, so that it can be utilised in the event of an interruption in the mains water supply. It is advantageous to maintain a store of water because an interruption in the mains water supply may cause a break in production, which is costly. Water delivered to a factory or supermarket by the mains water supply can, at least in the United Kingdom, be assumed to be suitably clean for use in the production of food, for example. It is less common for the water stored in storage tanks to be treated prior to use, as it is assumed that the quality of the water has not decreased during storage. However, the water that is stored in the storage tanks may be standing for a substantial time and thus it is possible that the stored water may become unsuitable for use in certain production processes. 
       SUMMARY 
       [0005]    In one embodiment of this invention, Sodium Hypochlorite is produced on site so that stored water or swimming pool water can be disinfected as required. Prior to this invention, the type of electrolytic cell used at water treatment works was not suitable for the installations discussed above. This is primarily due to the large size of the cell and that hazardous gases (in particular Hydrogen) are produced as a by-product of the chemical reaction. The production of an explosive gas such as Hydrogen in a public building such as a swimming pool or in a factory can pose a substantial safety hazard. 
         [0006]    The explosive limits of a gas are the lowest and highest concentration in air at which the ignition of the gas will occur. Below the lower explosive limit the gas is of an insufficient concentration for ignition. Above the higher explosive limit the potentially explosive gas and air mixture is too rich and thus it cannot ignite. The smaller the electrolytic cell, the more confined the space is wherein the gases can circulate. Thus, in a confined space there is more chance of the concentration of the gas exceeding its lower explosive limit, and therefore being prone to ignition. 
         [0007]    According to a one aspect of the invention, an electrolytic cell comprises an electrolysis vessel for receiving a liquid electrolyte that fills the vessel to a predetermined level, electrodes for passing an electric current through the electrolyte, a vent for allowing gases produced by the electrolytic process to leave the vessel and an air inlet means located substantially adjacent to the predetermined level for directing air into the vessel at a predetermined rate. 
         [0008]    Accordingly, the electrolytic cell can be compact and is safe to use, in particular for the production of Sodium Hypochlorite. The incoming air dilutes any hazardous gases, such as Hydrogen, as close to the surface of the electrolyte as possible. Thus, this arrangement reduces the risk of the concentration of Hydrogen present in the cell exceeding the lower explosive limit during the production of Sodium Hypochlorite. 
         [0009]    Preferably the electrolytic cell of the invention is suitable for the production of Sodium Hypochlorite. Thus, the cell of the invention allows the relatively safe, small-scale production of Sodium Hypochlorite on site. 
         [0010]    Preferably the cell operates on a batch process. Thus, the cell is filled with electrolyte, electrolysis is performed, and then it is emptied and refilled with fresh electrolyte. In the case of the production of Sodium Hypochlorite the electrolyte is salt water, typically at approximately 3% concentration. 
         [0011]    Preferably the electrodes are concentric with each other. Preferably, the electrolytic cell includes a draught tube that surrounds the electrodes and promotes the circulation of electrolyte around the vessel. 
         [0012]    Preferably, the vent is located at the uppermost point of the electrolytic vessel. Preferably, the predetermined level is located adjacent, but below, an overflow means. The overflow means prevents the electrolytic vessel becoming over filled. 
         [0013]    Preferably, the overflow means and air inlet means share a conduit that is connected to the electrolytic vessel. Preferably the vessel includes an electrolyte inlet means. The electrolyte inlet means is adapted to fill the electrolytic vessel with fresh electrolyte. Thus, in the case of Sodium Hypochlorite production, brine is fed into the vessel through the inlet means. 
         [0014]    Preferably the vessel includes a product outlet means adapted to allow the processed electrolyte to leave the vessel. Preferably, the product outlet means is connected to a product tank, which stores the product of the electrolytic process until it is required. 
         [0015]    The overflow means may also be connected to the product tank. 
         [0016]    Preferably, air pressure generation means is connected to the product tank such that air is blown through the product tank and into the vessel through the air inlet means. Preferably the predetermined rate of the air directed into the vessel is chosen in accordance with the rate of production of hazardous gas. As such, the rate needs to be sufficiently high to effectively dilute the hazardous gas or gases for the duration of the electrolytic process. 
         [0017]    A second aspect of the invention comprises a method for the production of Sodium Hypochlorite using the electrolytic cell of the first aspect of the invention. The method can comprise the steps of;
       filling the electrolytic vessel with an electrolyte of brine water to a predetermined level;   applying a flow of air across the surface of the electrolyte;   performing electrolysis on the electrolyte for a predetermined period of time;   extracting the Sodium Hypochlorite produced by the electrolysis from the electrolytic cell.       
 
         [0022]    During this method, the chance of a build up of explosive amounts of Hydrogen gas is very low because the Hydrogen is diluted by the flow of air as soon as leaves the surface of the electrolyte. This method operates effectively in a “batch” mode of operation. 
         [0023]    There now follows by way of example only a detailed description of the present invention with reference to the accompanying drawings wherein; 
     
    
     
       BRIEF DESCRIPTION 
         [0024]      FIG. 1  shows a diagram view of an embodiment of the electrolytic cell of the invention; 
           [0025]      FIG. 2  shows an external view of the embodiment represented in  FIG. 1 ; and 
           [0026]      FIG. 3  shows an alternative embodiment of the invention as shown of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    An electrolytic cell  1  is shown in  FIG. 1  and is adapted to produce Sodium Hypochlorite by the electrolysis of brine. The brine is typically of a concentration of 3% salt. In one embodiment of the invention, the cell  1  operates in a “batch” mode of operation wherein it is filled with brine (fresh electrolyte), the electrolysis is begun and once a sufficient concentration of Sodium Hypochlorite has been generated, the cell is emptied of the products of the electrolysis (product electrolyte). 
         [0028]    The electrolytic cell  1  comprises an electrolysis vessel  2  having electrodes  3  disposed therein. The vessel  2  is adapted to receive an electrolyte  4  of brine that surrounds the electrodes  3 . The vessel  2  also includes a vent  5  and an air inlet means  6 . The air inlet means  6  is located adjacent to the surface level of the electrolyte  4 , which is represented by dashed line  7 . 
         [0029]    The vessel  2  comprises a tubular body  8  having a vent tube  10  that defines the vent at the uppermost point. A tapering conical part  11  separates the vent tube  10  from the tubular body  8 . In use, the vent tube  10  is connected to a waste gas system (not shown). The vessel  2  has a plurality of ports in the tubular body  2 . In one embodiment, the electrolyte inlet means  13  such as a fill port  13 , is connected to a means for filling the electrolytic vessel, piping for example, with fresh electrolyte  4 . A product outlet means  14 , a product port for example, is connected to a means for allowing the processed electrolyte to leave the vessel. An upper port  12  forms the air inlet means  6 ; the fill port  13  allows electrolyte  4  to be introduced into the vessel  2 ; and the product port  14  allows the product electrolyte to be extracted from the vessel  2 . The vessel  2  also includes a base plate  15  (shown in  FIGS. 2 and 3 ) that is secured to the tubular body part  8  by bolts  16 . The bolts  16  pass through the plate  15  and are secured to an outwardly turned flange  17  on the tubular body  2 . A sealing gasket  18  is located between the base plate  15  and the flange  17 . The base plate  15  has an aperture  20  therein that allow the passage of an electricity supply cable  21 . A gland (not shown) forms a seal between the cable  21  and the base plate  15 . The cable  15  is connected (not shown) appropriately to the electrodes  3 . 
         [0030]    The electrodes  3  comprise a tubular anode  22  and a tubular cathode  23 . The cathode  23  has a smaller diameter that the anode  22  and the cathode  23  is located concentrically within the anode  22 . The electrodes  3  are adapted to be completely submerged in the electrolyte  4 . 
         [0031]    A draught tube  24  comprising a tubular member is mounted within the vessel  2  and is affixed to the base plate  15  by a spacing member  25 . The spacing member  25  (represented by dashed lines) allows electrolyte  4  to flow from an area  27  outside of the draught tube  24  to an area  28  within the draught tube, as represented by arrow  26 . However, it will be appreciated that electrolyte  4  could flow in the other direction. The spacing member  25  comprises a cross-shaped member and the draught tube  24  is seated thereon such that its peripheral circular edge bridges the arms of the cross-shaped member and the gaps therebetween. Thus, electrolyte can flow between areas  27  and  28  via the gaps between the arms of the spacing member  25 . It will be appreciated that other arrangements of draught tube  24  will allow the flow of electrolyte around the tube  24 . For example, the draught tube  24  may be affixed directly to the base plate  15  and have a plurality of apertures located circumferentially around it adjacent the base plate  15 . In this embodiment (not shown), the spacing member  25  is not required as electrolyte  4  can flow through the apertures in the draught tube  24 . The draught tube  24  is also adapted to be completely submerged in the electrolyte  4 . 
         [0032]    The upper port  12  and the product port  14  are connected to a product tank  30  by conduits  31  and  32  respectively. The product tank  30  is adapted to store the product electrolyte when it has been drained from the vessel  2  and prior to its use as a disinfectant or the like. The product tank also receives an air inlet conduit  33 , which guides air under pressure into tank  30  and through conduit  31  and into the vessel  2  via the air inlet means  6 . The incoming air is placed under pressure by a fan  34 . It will be appreciated that other air pressure sources could be used, such as a compressor or pre-pressurised gas from a cylinder, for example. The product tank  30  also includes a product delivery outlet  35  to remove product from the product tank  30  for use. 
         [0033]    The vessel  2  also includes an overflow means. In this embodiment the overflow means  36  comprises the upper port  12  and thus the overflow means  36  and air inlet means share the port  12 . When the vessel  2  is filled with electrolyte  4  it is filled to the predetermined level  7  and the overflow means  36  is located adjacent to, but above, this predetermined level  7 . This ensures both that any increases above the predetermined level results in the excess electrolyte being drained into the product tank  30 , and also that the incoming air is directed over the surface of the electrolyte  4 . Thus, any hazardous gases liberated during electrolysis are diluted by the incoming air as soon as the gas leaves the electrolyte  4 . It will be appreciated that the air inlet means  6  and overflow means  36  may be coupled to the vessel at different ports. However, the present embodiment is advantageous as the flow of air through the product tank  30  and conduit  31  prevents the hazardous gas or gases entering the conduit  31  and product tank  30 . 
         [0034]    In use, a valve (not shown) is opened which allows electrolyte  14  to flow into the vessel  2  through port  13 . The vessel  2  is filled until it reaches the predetermined level  7 , where the electrodes  3  and the draught tube  24  are submerged and the upper port  12  is adjacent the surface of the electrolyte  14 . The fan  34  is switched on to deliver the flow of air across the electrolyte surface and electricity is applied to the electrodes  3 . The electrolysis of the brine causes the following reaction to occur; 
         [0000]      2NaCl+2H 2 O→Cl 2 +2NaOH+H 2    
         [0035]    The further side reaction also occurs thus producing Sodium Hypochlorite; 
         [0000]      Cl 2 +2NaOH→NaClO+NaCl+H 2 O 
         [0036]    Hydrogen gas is the predominantly produced gas and will rise to the surface of the electrolyte from the electrodes  3  through area  28  within the draught tube  24 . The rising gas promotes circulation of the electrolyte  4  in the direction of arrows  37  and  26 . This is advantageous as it ensures any un-reacted electrolyte in area  27 , for example, is urged nearer to the electrodes  3 . The flow of air introduced by the air inlet means  6  dilutes the Hydrogen gas as it leaves the surface of the electrolyte  4 . The flow rate of the inlet air is chosen to ensure any hazardous gases, such as Hydrogen, are sufficiently diluted to substantially reduce their hazardous effect. The diluted Hydrogen gas can then leave via vent  5  to a waste gas system (not shown). Once the electrolysis of the electrolyte has yielded a sufficient concentration of Sodium Hypochlorite the electricity supply to the electrodes is turned off. The fan  33  may also be turned off provided that the concentration of Hydrogen leaving the vent  5  is less than a predetermined level below the lower explosive limit. A valve in the product port (not shown) is then opened to allow the product electrolyte to leave the vessel  2  and enter the storage tank  30 . The Sodium Hypochlorite solution can then be extracted from the product tank  30  via outlet  35  when required. 
         [0037]    It will be appreciated that hazardous gases produced during electrolysis need not be diluted by air, and instead other gases such as Nitrogen may be used. Further, the gas acting as a diluting gas can be chosen in accordance with the particular hazardous gas produced by the given electrolytic reaction.