FIELD OF THE INVENTION
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The present invention relates to a method for production of sodium hypochlorite.
BACKGROUND OF THE INVENTION
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Sodium hypochlorite is widely used bleach. It is generally used for disinfection of drinking water/swimming pool water, treatment of industrial waste water, washing of textiles and stained or soiled fabrics, sterilization of food processing equipments and sterilization of packages.
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Sodium hypochlorite is produced on an industrial scale. However, commercial sodium hypochlorite tends to destabilise and its storage also involves safety hazards.
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Therefore, for some applications, sodium hypochlorite is generated at the point of use generally by electrolysis which is a safer process.
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Production of sodium hypochlorite by electrolysis is generally carried out by using dimensionally stable anodes (DSA).
US Pat. No. 3234110 (Beer et al ) describes one method of preparation of dimensionally stable anodes where a noble metal is coated over titanium substrate. Dimensionally stable composite anodes are preferred for their sustained activity and longer life. These anodes are prepared by applying micron sized coating of iridium oxide, ruthenium oxide or mixed oxides on a substrate of titanium. Dimensionally stable anodes initially have good current efficiency which remains reasonably constant over long period of time. However even their current efficiency rarely exceeds 80 %.
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It is a technical problem to retain high current efficiency over a longer period. Ions of calcium and magnesium which are often present as impurities in sodium chloride generally tend to deposit on the surface of the electrodes.
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Freshly activated platinum anodes have high current efficiency as compared to DSA. However, platinum electrodes lose their initial activity/current efficiency quickly and hence the electrodes become less current efficient as compared to DSA in the longer run. Platinum undergoes passivation due to formation of oxide film of about 10 A (Angstrom) which is believed to lower the exchange current density on the electrode. The film of oxide is formed during the oxidation of chloride ions to chlorine gas.
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We have now observed that the technical problem can be solved by cyclically reversing the polarity of the platinum electrodes at selective time intervals of 3 to 60 seconds. By doing so, the electrodes retain their initial high current efficiency for longer time. This leads to higher rate of production of sodium hypochlorite as compared to that achieved by using conventional dimensionally stable anodes.
SUMMARY OF THE INVENTION
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Disclosed is a method for electrolytic generation of sodium hypochlorite, the method including the steps of contacting an aqueous solution of sodium chloride with an electrolytic cell comprising at least one pair of electrodes having a surface of a noble metal through which current is passed, wherein polarity of the electrodes is cyclically reversed at a time interval of 3 to 60 seconds.
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The invention will now be explained in details.
DETAILED DESCRIPTION OF THE INVENTION
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During the process of electrolysis, in general, ions of calcium and magnesium which are present in water tend to deposit on the cathode thereby drastically reducing performance of the cell. It therefore becomes necessary to periodically switch the polarity of the electrodes so as to dissolve the film. One of the reasons for failure of any electrolytic cell for an application is the build up of an unwanted film of carbonates of magnesium and calcium over the surface of the cathode. These contaminants are invariably present in commercial sodium chloride. If this film is not periodically removed, the electrolytic cells lose operating efficiency and eventually fail. Various methods are employed to remove the film from the surface of the cathode which includes mechanical removal, acidic wash and intermittent air blast to dislodge the deposit and intermittently halting the electrolysis and flushing the cell to dislodge the film. These methods are expensive, time consuming, and are associated with corrosion problems. In recent times, it has been proposed to provide electrolytic cell having "self- cleaning ability" by periodic reversal of the polarity of the electrodes at an interval of 15 to 20 minutes.
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One of the major problems with reversal of polarity, in general, is that the active material which is present over the surface of the electrodes is stripped off during cathode phase which affects the usable life of electrodes.
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However, it has been observed that when the time at which polarity is reversed is maintained within specific limits, surprisingly the electrode not only lasts longer but retains its high current efficiency.
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In the method disclosed herein, an aqueous solution of sodium chloride is contacted with an electrolytic cell having at least one pair of electrodes having a surface of a noble metal through which current is passed, wherein the polarity of the electrodes is cyclically reversed at a time interval of 3 to 60 seconds.
In a preferred embodiment, platinum metal is used as the anode as well as cathode. It is preferred that polarity of the electrodes is reversed at the time interval of 10 to 50 seconds and more preferably 10 to 25 seconds. Reversal means that during the first half of the cycle time, one of the electrodes serves as anode and produces chlorine from an aqueous solution of sodium chloride, while the second electrode acts as cathode. During the second half of the cycle, the polarity is reversed. Now the second electrode serves as anode and produces sodium hypochlorite.
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Whichever electrode is in anodic phase, it electrochemically oxidizes chloride ions to chlorine, which in-turn reacts with water to produce hypochlorite ion. The electrochemical reactions can be represented as follows:
2 Cl → Cl2 + 2e-
Cl2 + H2O → OCI + 2H+ + Cl-
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Highest electrochemical activity and sustained high current efficiency is achieved by reversing the polarity within the most preferred range of 10 to 25 seconds.
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By optimizing the various operating conditions such as applied potential at the anode, concentration of sodium chloride solution and the flow rate of the electrolyte (sodium chloride), it is possible to further produce sodium hypochlorite at higher specific rate and higher current efficiency as compared to the conventional direct current (DC) mode of operation of the electrochemical cell and for a sustainably longer period of time.
Cell potential
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In a preferred method the cell potential of the cell is 2.45 V to 2.83 V, more preferably 2.45 V to 2.65 V and most preferably 2.50 to 2.55 V.
It is observed that the rate of production of sodium hypochlorite increases with the electrode potential but on the other hand the current efficiency decreases. The current efficiency is very high at the lower end of the cell potential. It is also observed that as the time interval at which the polarities are to be reversed is made shorter, the cell potential increases. The rate at which the platinum electrodes become passive bears an exponential relation with the applied potential. At the lower end of the potential, the current efficiency is very high
Concentration of sodium chloride
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The cell potential also bears a relationship with the concentration of sodium chloride. In a preferred method the concentration of the aqueous solution of sodium chloride is above 0.1 M, more preferably above 0.3 M and most preferably from 0.5 M to 3.5 M.
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As the concentration of sodium chloride increases, the quantity of sodium hypochlorite increases. Further preferably, as the concentration of sodium chloride increases, it is preferred to increase the time interval between two cycles. With an increase in the concentration of sodium chloride, in particularly, chloride ions, the current efficiency increases, however, over a period of time, the platinum electrode gets oxidised by water oxidation. The water oxidation reaction competes with the desired reaction of oxidation of chloride ions to chlorine. A part of the current gets utilised in the side reaction of water oxidation, therefore the current efficiency gradually decreases beyond the upper limit of the most preferred range, that is, beyond 3.5 M.
Temperature
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It is preferred that the method is carried out at temperature below 35°C, more preferably 25 to 30°C. With an increase in the temperature the hypochlorite ions tend to get converted into chloride ions.
Therefore, when the method is conducted below 35°C, the probability and extent of the competing side reaction can be minimised.
Distance between the anode and cathode
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It is preferred that the distance between an anode and its associated cathode is less than 5 mm, more preferably not greater than 2 mm. When the distance between the electrodes is greater than 5 mm, some of the applied potential will be lost on account of Ohmic drop. On the other hand, when the distance is 2 mm or less, then most of the applied potential gets utilised for Faradic reactions.
Flow rate of sodium chloride solution
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It is preferred that the flow rate of the sodium chloride solution, as it flows through the electrodes, is 25 to 125 ml/minute, more preferably 40 to 110 ml/minute. At this flow-rate, the current efficiency of the cell is about 50 %. When the flow-rate is significantly lower, the hydrogen gas which generates at the cathode resists the intended reaction by coating the platinum electrodes. Further increase in flow-rate has no practical effect on the efficiency because although there is more mass transfer, the turbulence of flowing electrolyte tends to reduce the efficiency.
Device
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In accordance with a second aspect is disclosed a device or equipment employing the method according to the first aspect.
Advantages and Applications
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The disclosed invention has several advantages over conventional technique which employs DSA. The cell produces sodium hypochlorite at higher rate and with higher current efficiency. It has "self cleaning ability", longevity and maintenance free operation. The use is not restricted to any particular type of cell design and any type of electrode configuration. It can be employed even in hand held, in-line and in-pool devices.
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The disclosed invention is applicable to in-situ production of sodium hypochlorite which has various applications such as treatment of drinking water/swimming pool water/industrial waste water, washing of textiles/stained or soil fabrics; sterilization of food processing equipments, sterilization of packages.
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Although it is preferred to use Platinum as the electrode material, it has longer life and seldom requires replacement. As the invention improves both production rate and current efficiency the size of the device can be reduced and energy can be saved.
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The invention will now be explained in further details with the help of non-limiting examples.
EXAMPLES
Example 1: Schematic diagram of an electrolytic cell for carrying out a preferred method
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An electrolytic cell was fabricated using transparent poly-(methyl methacrylate). It consisted of two blocks each having length 7cm, breadth 3 cm and thickness 0.6 cm. Each block had a groove of length 5 cm, breadth 1 cm and depth 0.1 cm. At the centre of each block was provided a smaller groove of dimension length and breadth 1 cm each and depth of 0.5 mm. The smaller groove was meant for fixing the platinum electrodes. A pair of platinum plate electrodes of size 1cm x 1cm and thickness 0.5 mm was made. Each platinum plate was fixed at the centre groove of the block. Two blocks were fixed by nuts and bolts in such a way that the two plates faced each other. The cell had one inlet and one outlet for flow of electrolyte solution. The inlet and the outlet were constructed by boring channels into the blocks.
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The setup for the experiment will now be described.
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A solution of sodium chloride of known initial concentration was circulated from a reservoir through the cell and back to the reservoir at a controlled flow rate using a peristaltic pump. The reservoir was a 200 ml capacity beaker, provided with a magnetic stirrer to maintain the solution at uniform concentration. The concentration of the generated sodium hypochlorite was measured at wavelength 292 nm using UV-Visible Spectrophotometer and also method of titration of Iodide. A signal generator was used to apply different form of potential signal across the electrodes. The output of the signal generator was connected to the input of an amplifier having variable gain from 0 to 150. The gain was fixed at 5. The amplified output signal was connected to one of the cell electrodes while the other electrode was grounded.
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The potential difference across the electrodes and the current flowing through the cell was recorded. The amplifier had the provision to measure independently the current (20mA=1V) as well as cell potential (100V=1V) in terms of potential. These two potential measuring probes were connected to a two-channel PC based oscilloscope. The current potential wave and cell potential wave were monitored continuously in the oscilloscope.
Some of the parameters were varied to find out their effect on production of sodium hypochlorite.
Example 2:
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The aim of this experiment was to determine the cumulative production of sodium hypochlorite versus time and the cumulative current efficiency versus time.
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The cell potential was maintained at 2.83 V, the flow rate of sodium chloride solution was maintained at 100 ml/minute, the switching time was fixed at 10 seconds and the concentration of sodium chloride was fixed at 0.5 M. The volume of sodium chloride was 100 ml.
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Performance of the cell is quantified by way of cumulative production of sodium hypochlorite per unit area of the anode,
P(
t)(mol.m
-2) and the cumulative current efficiency η(
t), both being the functions of time. These are obtained using the following equations:
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In the above equations, c(t) represents concentration of sodium hypochlorite in the reservoir at time t and Q(t) is the quantity of electric charge passed through the cell up to t. The terms V, A and F respectively represent the volume of the reservoir, the (geometric) surface area of the electrode and Faraday constant.
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The data is shown in table 1.
Table 1 | Time/seconds | Cumulative production of sodium hypochlorite | Current efficiency/% |
| Direct current | Switching | Direct current | Switching |
| 1800 | 0.9 | 4.0 | 76.0 | 95.2 |
| 3600 | 1.1 | 8.0 | 72.1 | 95.2 |
| 5400 | 1.3 | 10.4 | 63.0 | 91.6 |
| 7200 | 1.5 | 13.5 | 59.0 | 85.8 |
| 9000 | 1.6 | 15.5 | 55.0 | 82.0 |
| 10800 | 1.8 | 18.0 | 52.0 | 80.0 |
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The data in table 1 shows the comparison of cumulative production of sodium hypochlorite from 0.5 M sodium chloride solution in switching mode and in DC mode under the same operating conditions. The data clearly indicates that cumulative production of sodium hypochlorite is more when the polarities are switched as compared to operating the cell in DC mode. It can be seen that at the end of three hours, the quantity of sodium hypochlorite produced in switching mode is several orders more than that produced in the DC mode. Table 1 also shows cumulative current efficiency versus time and it is found that at the end of first hour of the experiment, the current efficiency is close to 95 % in switching mode against only about 75 % in the DC mode. The decrease in the current efficiency in both modes after an hour is due to reduction of sodium hypochlorite at the cathode as sodium hypochlorite is present in the solution.
Example 3:
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The aim of this experiment was to find out the effect of time taken to switch the polarity of electrodes on the cumulative production of sodium hypochlorite. Another aim of this experiment was to find out the effect of time taken to switch the polarity of electrodes on the current efficiency.
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The cell potential was maintained at 2.65 V, the flow rate of sodium chloride solution was maintained at 100 ml/minute, the concentration of sodium chloride was fixed at 0.5 M. The volume of sodium chloride was 100 ml. The experiment was conducted for 15 minutes.
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The data is shown in table 2.
Table 2 | Time/seconds taken to switch the polarity | Cumulative production of sodium hypochlorite | Current efficiency/% |
| 0.5 | 1.0 | 45.5 |
| 1.2 | 1.0 | 63.1 |
| 2.2 | 1.1 | 77.2 |
| 5.0 | 1.1 | 88.2 |
| 10.0 | 1.2 | 92.0 |
| 15.0 | 1.1 | 93.0 |
| 25.0 | 1.0 | 91.0 |
| 35.0 | 1.0 | 91.0 |
| 50.0 | 0.7 | 81.3 |
| 75.0 | 0.6 | 76.0 |
| 100.0 | 0.5 | 73.4 |
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The data in table 2 indicates that at the lower end of the switching time, the production of sodium hypochlorite is low and it increases with an increase in switching time until an optimum value is reached and later on it decreases with further increase in the switching time.
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The data also indicates that the current efficiency versus switching time also shows a similar trend. Optimum switching time for above conditions is found to be from 10 to 15 seconds.
Example 4:
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The aim of this experiment was to find out the effect of concentration of sodium chloride on the cumulative production of sodium hypochlorite. Another aim of this experiment was to find out the effect of concentration of sodium chloride on the current efficiency.
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The cell potential was maintained at 2.45 V, the flow rate of sodium chloride solution was maintained at 100 ml/minute. The volume of sodium chloride was 100 ml. The experiment was conducted for 15 minutes.
Table 3 | Switching time/seconds | Cumulative production of sodium hypochlorite in moles/m2 using 0.1 M NaCl | Cumulative production of sodium hypochlorite in moles/m2 using 0.25 M NaCl | Cumulative production of sodium hypochlorite In moles/m2 using 0.5 M NaCl |
| 0.5 | 0.18 | 0.38 | 0.53 |
| 1.2 | 0.21 | 0.39 | 0.56 |
| 2.5 | 0.18 | 0.37 | 0.57 |
| 5.0 | 0.16 | 0.36 | 0.58 |
| 10.0 | 0.12 | 0.36 | 0.66 |
| 15.0 | 0.09 | 0.36 | 0.65 |
| 25.0 | 0.07 | 0.31 | 0.66 |
| 35.0 | 0.04 | 0.26 | 0.67 |
| 50.0 | 0.04 | 0.23 | 0.64 |
| Switching time/seconds | Current efficiency/% at 0.1 M NaCl | Current efficiency/% at 0.25 M NaCl | Current efficiency/% at 0.5 M NaCl |
| 0.5 | 30.75 | 42.04 | 43.28 |
| 1.2 | 51.36 | 60.69 | 61.31 |
| 2.5 | 57.07 | 70.97 | 73.22 |
| 5.0 | 65.96 | 78.72 | 81.70 |
| 10.0 | 54.87 | 83.01 | 90.14 |
| 15.0 | 47.29 | 88.06 | 89.40 |
| 25.0 | 42.68 | 85.34 | 90.89 |
| 35.0 | 25.59 | 73.86 | 90.91 |
| 50.0 | 26.34 | 73.41 | 88.84 |
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The data in table 3 indicates that as the concentration of sodium chloride increases, the current efficiency and cumulative production of sodium hypochlorite increases.
Example 5:
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The aim of this experiment was to find out the effect of applied electrode potential on the cumulative production of sodium hypochlorite. Another aim of this experiment was to find out the effect of applied electrode potential on the current efficiency.
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The flow rate of sodium chloride solution was maintained at 100 ml/minute, the concentration of sodium chloride was fixed at 0.25 M. The volume of sodium chloride was 100 ml. The experiment was conducted for 15 minutes.
Table 4 | Switching time/ seconds | Cumulative production of sodium hypochlorite in moles/m2 at 2.45 V | Cumulative production of sodium hypochlorite in moles/m2 at 2.65 V | Cumulative production of sodium hypochlorite in moles/m2 at 2.83 V | Cumulative production of sodium hypochlorite in moles/m2 at 3.03 V |
| 0.5 | 0.37 | 0.58 | 0.77 | 0.93 |
| 1.2 | 0.38 | 0.65 | 0.83 | 0.94 |
| 2.5 | 0.37 | 0.67 | 0.83 | 0.87 |
| 5.0 | 0.37 | 0.63 | 0.67 | 0.69 |
| 10.0 | 0.35 | 0.57 | 0.52 | 0.55 |
| 15.0 | 0.35 | 0.48 | 0.42 | 0.49 |
| 25.0 | 0.31 | 0.37 | 0.32 | 0.45 |
| 35.0 | 0.26 | 0.33 | 0.28 | 0.41 |
| 50.0 | 0.23 | 0.25 | 0.26 | 0.41 |
| Switching time/ seconds | Current efficiency/% at 2.45 V | Current efficiency/% at 2.65 V | Current efficiency/% at 2.83 V | Current efficiency/% at 3.03 V |
| 0.5 | 42.04 | 43.2 | 44.91 | 40.67 |
| 1.2 | 60.69 | 62.39 | 58.13 | 53.29 |
| 2.5 | 70.97 | 76.46 | 75.95 | 60.16 |
| 5.0 | 78.72 | 83.08 | 72.87 | 54.92 |
| 10.0 | 83.01 | 86.71 | 65.14 | 50.78 |
| 15.0 | 89.06 | 81.61 | 59.15 | 50.60 |
| 25.0 | 85.34 | 73.12 | 48.97 | 47.19 |
| 35.0 | 73.86 | 72.10 | 45.64 | 44.77 |
| 50.0 | 73.41 | 59.33 | 44.68 | 43.33 |
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The data in table 4 indicates that as the electrode potential increases the production of sodium hypochlorite increases but the current efficiency decreases. The current efficiency is very high at low cell potential as compared to higher cell potential.
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The illustrated examples indicate that not only do the electrodes last longer but also they remain highly current efficient during most of their life time. The illustrated examples also make it possible to achieve high rate of production and high current efficiency compared to the conventional cells which use DSA.
