Abstract:
The present invention describes an apparatus for the production on site of a solution of sodium hypochlorite. It is based on the electrolysis of a dilute solution of sodium chloride and operates in flow through mode. Its main use is the potabilization of drinking water. Other uses include the production of sterilizing solutions for washing, wound disinfection, etc. It can be powered by a solar photovoltaic module (PVM), and it is portable in order to be easily transported in remote areas or emergency situations where chemicals or electricity are not available. The flow through operation is based on a gas lifting mechanism, the gas being hydrogen generated during the electrolysis of the sodium chloride solution. Therefore no moving parts are employed for pumping the solution through the electrolytic cell.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    A number of electrochemically generated oxidants may be used to purify water: for instance hydrogen peroxide and ozone, which are difficult and critical to produce with simple devices to be operated on site and in the field. Therefore the most traditional oxidizing agent in these circumstances is sodium hypochlorite solutions or chlorine in the form of solid compounds. It should be noted that this compounds are quite critical to storage and deliver, particularly in hot and remote areas. The alternative solution is to produce sodium hypochlorite on site. It can be easily obtained by simple electrolysis of a dilute solution of sodium chloride in water. 
         [0002]    The electrolysis process is well known. It is carried out by preparing a solution of sodium chloride in water, of proper concentration, and place it into an electrolysis cell composed of a container equipped with two electrodes, one anode and one cathode, through which is passed a dc electric current. 
         [0003]    The electrodes are made of a chemically inert metal, like Titanium coated with oxides of noble metals like Ruthenium, Titanium, Lanthanum, Iridium, etc. They have semiconductor properties and are excellent catalysts for chlorine generation. These electrodes are named DSA (dimensionally stable anodes) and the coatings are proprietary. 
         [0004]    In a cell of this type the anode and cathode are placed in the cell cavity without diaphragm or membranes separating them. In this way the electrochemical reaction products, chlorine at the anode and sodium hydroxide at the cathode, react instantly producing sodium hypochlorite. 
         [0005]    The current density on the electrodes can vary from 0.05 to 0.5 Amp/cm 2 . 
         [0006]    The current efficiency depends on temperature. (a high temperature should be avoided as it promotes the formation of undesired chlorates ClO 3 ), from pH of the solution and cell geometry. The energy conversion efficiency depends on the concentration of the saline solution, on the gap between the electrodes, and again from the geometry of the cell: all this parameters influence the conductivity of the solution and consequently the ohmic losses of the cell itself. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0007]      FIG. 1  illustrates a schematic diagram of the apparatus for performing the electrolysis and showing the gas lift mechanism layout. 
           [0008]      FIG. 2  illustrates the gas lifting action for the fluid transfer. 
           [0009]      FIG. 3  illustrates the layout of the apparatus as described in the working example. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    The present invention is related to an apparatus for the production in continuous mode (flow-through) of a sodium hypochlorite solution by means of the electrolysis of a dilute solution of sodium chloride in water. With reference to  FIG. 1  it is essentially composed by a saline solution reservoir  1 , hermetically closed by the screw-lid  2 , and connected to the vessel  3  by means of the two conduits  4  and  5 . The vessel  3  feeds, through the tube  6  and choke valve  13 , the solution to the electrolytic cell  7  and raiser tube  8 . The top end of tube  8  stays at a level higher by the amount h 2  with respect to the level  16  in the vessel  3 . 
         [0011]    The operation is as follows: 
         [0012]    1.—Electrolytic cell not in operation (electric current disconnected). 
         [0013]    The saline solution from reservoir  1  flows into the vessel  3  through the conduit  5  until the liquid level reaches the bottom end of conduit  4 . At this point the conduit  4  is closed by the liquid and air flow into reservoir  1  is inhibited, therefore blocking the flow of the solution through the conduit  5  into the vessel  3 . Therefore the level  16  on the vessel  3  is stabilized, being set by the position of the bottom end of the conduit  4  which acts as a liquid valve. By virtue of the hydraulic connection between vessel  3  and raiser tube  8  through the tube  6  and the electrolytic cell  7 , the liquid level in the raiser tube  8  is at the same level  16  as the liquid in the vessel  3  (see  FIG. 3 ). Therefore in this condition no liquid flow is possible from reservoir  1  into discharge tube  10 . The system is in a stable equilibrium condition (stand-by). 
         [0014]    2.—Electrolytic cell  7  in operation (electric current on). 
         [0015]    During operation hydrogen gas is evolved at the cathode  7   b.  As explained (Background of the Invention) chlorine gas evolved at the anode  7   a  reacts instantly with the sodium hydroxide, generated by secondary reaction of sodium ions with water, producing sodium hypochlorite. As a consequence there is a net evolution of hydrogen gas from the electrolytic cell  7 . This gas is forced upwards through the raiser tube  8 , it mixes with the liquid forming a series of bubbles  17 , divided by short liquid columns  18 , as shown in  FIG. 2 . The capacity of forming such a liquid-gas combination is due to the action of the surface tension of water and is conditioned by the inner diameter of the raising tube  8 . It is easy to understand that the liquid-gas mixture in tube  8  has a lower density compared to the density of the liquid alone. As a consequence a buoyant force is applied to said liquid-gas mixture which is pulled upwards trough the tube  8  and discharged into vessel  9 . From there the hypochlorite solution is discharged trough tube  10  to the storage container  11 . The buoyant force is supported by the hydrostatic pressure difference between vessel  3  and the electrolytic cell  7 . The liquid overflowing from tube  8  into vessel  9  produces a lowering of the liquid level  16  in the vessel  3 . This causes the bottom part  4   a  of the conduit  4  to open to air (note that the upper part of vessel  3  is open to air). This air is sucked into vessel  1  permitting some liquid to flow through the conduit  5  into vessel  3  until the bottom opening of conduit  4  is again closed by the liquid itself, inhibiting further liquid flow from vessel  1 . This cycle repeats continuously causing a net flow of saline solution from vessel  1 , through vessels  3 , electrolytic cell  7  and vessel  9 , to the tube  10  and storage  11 . The output flow contains the sodium hypochlorite solution generated in the cell  7 . The flow rate depends on the quantity of hydrogen generated, on the gas to gas+liquid volumes ratio ε(=gas /(gas+liquid)), the raiser tube  8  inner diameter and its length h 1 , and the raiser tube  8  extra length h 2  over the level  16 . From theoretical considerations, in order to have a regular and constant flow, it must be 0.2&lt;ε&lt;0.4. At this condition the flow is laminar. Incidentally at this value of ε an optimal balance is obtained between the hydrogen gas evolution rate and the liquid flow rate through the electrolytic cell  7 . It should be noted that for small units, like the one described in this application, the flow is laminar where the hydrogen bubbles act as plugs pushing the liquid upwards. This is illustrated schematically in  FIG. 2  where  18  are the liquid plugs and  17  are the gas bubbles. For large hypochlorite production rates the flow becomes turbulent with a chaotic bubble evolution. With this considerations in mind the raiser tube  8  inner diameter must be small enough in order the surface tension of the liquid is such as to generate alternate and regular liquid-gas plugs. 
         [0016]    A shallow vessel on which two electrodes  7   a  and  7   b  are fitted composes the electrolytic cell  7 . The electrodes are made of titanium coated with noble metals oxides like ruthenium, iridium, and platinum. They are in the form of flat parallel plates spaced a few millimetres, placed vertically and connected to two electric wires by means of watertight feed through connectors. The electric wires are connected to a polarity reversal circuit  14 , controlled by a timer. A constant current power supply  15  feeds the circuit  14 , and is powered, through the connection  19 , to an electricity source, grid line at 220 V ac, or dc source, any voltage. 
       A WORKING EXAMPLE  
       [0017]    As an example a unit will be described aimed to be portable and deliver a quantity of sodium hypochlorite with an equivalent chlorine amount of 2 to 3 g/hour. With reference to  FIG. 3  the unit consists essentially of a box (plastic or metal but preferably of plastic)  20 . Inside the box one or two vessels, as for this example,  3   a  and  3   b  are fitted as illustrated on  FIG. 3 . Into and on top of the two vessels two plastic containers, (could be large bottles of 5 to 10 L each),  1   a  and  1   b  can be inserted. Each bottle is equipped with a screw lid in the form of a plastic rod  4  and  5  on which two holes are drilled (conduits  4  and  5  in  FIG. 1 ), one for the air intake and one for the saline solution outflow. The rod is long enough to firmly hold the bottle in a vertical position, acting as a mechanical support for the bottles. The bottom of the two vessels  3   a  and  3   b  are interconnected with a plastic tube  6   a  and both connected with the tube  6  and through the choke valve  13  to the electrolytic cell  7  (see also  FIG. 1 ). The electrolytic cell  7  is connected to the hypochlorite solution collector  9  by means of the raiser tube  8 . The two vessels  3   a  and  3   b,  the vessel  9 , the electrolytic cell  7  and the power supply  14 ,  15  are all fitted in the box  20 . The upper part of the collector  9  is open to air for the exhaust  12  of the hydrogen. The electrolytic cell  7  is equipped with two flat plate electrodes  7   a  and  7   b  placed vertically and parallel to each other with a gap of 4 mm and a surface area of between 16 and 20 sq. centimetres. As already described the electrodes are made of noble metals oxides coated titanium. These dimensions give the best results in terms of conversion efficiency (approx. 90%). The raiser tube  8  has a length h 1  between 8 and 11 cm, preferably 9 cm, an inner diameter between 3 and 6 mm, preferably 4 mm, and the length h 2  between 1 and 3 cm, preferably 2 cm. The saline solution to be stored in the bottles  1   a  and  1   b  should have a salt concentration between 2 and 3%, preferably 2.5%. Based on these characteristics the apparatus was powered at a constant current of 2 Amp the equivalent chlorine produced was 2.45 g/hour. In another run at 3 Amp the chlorine production was 3.7 g/hour, in both cases with a conversion efficiency of 92%. The measured power input to the cell was 9 and 13 Watts respectively. The flow rate was adjusted with the choke valve  13  to 1.0 and 1.5 L/hour respectively. In another run the apparatus was powered with a photovoltaic module of 40 Watts in a clear summer day. During the 9 hours of useful solar radiation 22 g of equivalent chlorine were produced. 
         [0018]    This amount is sufficient to purify 14700 L of drinking water at a concentration of 1.5 mg/L of free chlorine. The PV module had an open circuit voltage of 24 V, and an MPP (maximum power point) voltage of 17.5 V at 2.3 Amp. The constant current power supply was designed to operate between 8 and 24 Volts. With this configuration the whole apparatus, including the PVM, weighted approximately 15 Kg, thus being easily transportable.