GB2445940A

GB2445940A - An electrochlorinator

Info

Publication number: GB2445940A
Application number: GB0701437A
Authority: GB
Inventors: Alexander Simpson; Andrew Hill; Paul Wilkins
Original assignee: ATRANOVA Ltd
Current assignee: ATRANOVA Ltd
Priority date: 2007-01-25
Filing date: 2007-01-25
Publication date: 2008-07-30
Anticipated expiration: 2027-01-25
Also published as: WO2008090367A1; GB0701437D0; GB2445940B

Abstract

An electro-chlorination system 1 is described, in which sodium chloride is subjected to electrolysis and where sodium chlorite is added to react with chlorine formed by the electrolysis, to produce chlorine dioxide. A porous barrier or membrane 51 ,53 is provided between anodes 59 and cathodes 63 of the electrolytic cell 47 in order to restrict flow of sodium hydroxide formed around cathodes 63 of the cell towards the anodes 59. As a result, the pH of the solution around the anodes 59 is kept low (around 2 or 3) and there is no need to add acid to the electrochlorinator 7 to facilitate the production of the chlorine dioxide.

Description

Application No (1B07() 4376 Dale.2 May 2007 The following terms are

registered trademarks and should he read as such wherever they occur in this docunient: Ebonex.

1 2445940 Electro-C hiorinator The present invention relates to a method and apparatus for electro-chiorination, as used for example in chlorinating, and thereby sterilising, water used in swimming pools.

The chlorination of water in swimming pools is well known. Typically, this is achieved by the addition of simple chlorine compounds, such as chlorine dioxide, which act as strong oxidising agents.

Previously, the applicant produced chlorine dioxide (dO2) via the electrolysis of an aqueous solution of sodium chloride (NaCI) and the addition of sodium chlorite (NaClO2) in the following manner: 1) An aqueous solution of sodium chloride (NaCI) was subjected to electrolysis and underwent the reaction described in (1), producing sodium hydroxide (NaOH), chlorine gas (Cl2) and hydrogen gas (H2).

2NaCl 2H20 -* 2NaOH+C12+H2 (1) 2) Sodium chlorite (NaClO2) was then introduced, and the evolved chlorine gas (Cl2) reacted with the sodium chlorite (NaClO2) to form chlorine dioxide (dO2) and sodium chloride (NaCI) via the reaction described in (2) 2NaClO2+C12 -* 2C102 +NaCl (2) The reaction described in (2) is in equilibrium, with the balance of the equilibrium determined by, amongst other factors, the acidity (pH) of the solution. For acidic solutions, the balance of the equilibrium of (2) is pushed to the right, favouring the production of chlorine dioxide (dO2), and was found to be optimised for a pH that is less than 4. Conversely, an alkaline solution favours the backward reaction, causing chlorine dioxide to revert back to chlorine. Under strongly alkaline conditions with a pH of 11 to 12, a further reaction was found to occur whereby the chlorine gas (Cl2) reacted with available sodium hydroxide (NaOH) to form sodium hypochiorite (NaOCl) via the reaction described in (3) 2NaOH+C12 -p NaOCI+NaCI+H20 (3) Reaction (3) pushes the equilibrium of reaction (2) to the left, favouring the reversion of chlorine dioxide (C 102) back into chlorine gas (Cl2), further generating sodium hypochlorite (NaOCI) via reaction (3) and so on in a feedback loop. It is therefore current standard practice to counter this by adding acid into the system, thereby acting to ensure the equilibrium of reaction (2) remained favourably balanced towards the right to produce the desired chlorine dioxide (dO2).

However, this requires the storage and handling of acid, which increases costs and can be hazardous, as accidental leakages can result in the uncontrolled liberation of poisonous chlorine gas.

The present invention therefore aims to provide an alternative electro-chiorinator and electro-chiorination process that alleviates the need to add acid to the system.

According to one aspect, the present invention provides an apparatus for use in an electro-chlorinator, the apparatus comprising: a housing having: i) a first inlet for receiving a chloride solution; ii) a second inlet for receiving a chlorite; iii) a first outlet for outputting chlorine dioxide; and iv) a second outlet for outputting a hydroxide; a flow channel extending between the first inlet and the first outlet; at least one barrier extending across the flow channel and at least partly along the flow path to define first and second sub-channels; one or more cathodes positioned within said first sub-channel and one or more anodes positioned within said second sub-channel for subjecting the chloride solution flowing along said flow channel to electrolysis; wherein said second inlet is positioned downstream of said one or more anodes and is operable to add said chlorite to react with chlorine formed around said one or more anodes to produce chlorine dioxide; wherein said second outlet is positioned downstream of said one or more cathodes and is operable to remove said hydroxide formed around said one or more cathodes; and wherein said at least one barrier is operable to restrict the flow of fluid from said first sub-channel to said second sub-channel.

In one embodiment the barrier is porous to hydrogen formed around said one or more cathodes, such that hydrogen can escape from the housing via said first outlet. The barrier prevents (or at least reduces) flow of high pH hydroxide from said first sub-channel to said second sub-channel.

Flow restricting screens may be provided both upstream and downstream of the electrolytic cell formed in the housing, in order to restrict the back flow of fluid through the housing and to evenly dissipate the liquids and evolved chlorine gas throughout the chamber.

In one embodiment, two barriers are provided in the flow channel to define three sub-channels, the outer two of which preferably house cathodes and the inner one of which house the anodes.

The electrodes can be formed of any suitable material, but Ebonex is preferably used due to its stability of operation in electrolytic cells.

The present invention also provides a method of producing chlorine dioxide comprising: passing a chloride based solution through an electrolytic cell; subjecting the chloride solution to electrolysis; providing a barrier between an anode and a cathode of said electrolytic cell to at least restrict the flow of hydroxide formed around said cathode towards chlorine formed around said anode; adding a chlorite to the chlorine formed around said anode for reacting with the chlorine to form chlorine dioxide; and outputting the formed chlorine dioxide.

These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the accompanying Figures in which: Figure 1 schematically illustrates an electro-chiorination process using an electro-chlorinator; Figure 2 is a three dimensional part cut away illustrating an electro-chlorinator embodying the present invention; Figure 3 is a cross-section of the electro-chlorinator shown in Figure 2.

Overview Figure 1 schematically illustrates an electro-chiorination system I that generates chlorine dioxide for addition to water to be treated. The system 1 includes an inlet pipe 3 for feeding a solution of sodium chloride (NaCl) through a valve 5 into the electro-chlorinator 7. Sodium chlorite (NaClO2) is also supplied into the electro-chlorinator 7 via another inlet pipe 9. As will be described in more detail below, the electro-chlorinator 7 includes a number of anodes and cathodes that subject the sodium chloride to electrolysis. This produces sodium hydroxide (NaOH), chlorine gas (Cl2) and hydrogen gas (H2) in accordance with reaction (1) discussed above. The sodium hydroxide and the hydrogen gas are formed at the cathodes and the resulting pH around the cathodes will be high (around 11-12).

The chlorine gas is formed at the anodes and the resulting pH will be low (around 2-3). As will be described in more detail below, porous barriers are provided between the cathodes and anodes in order to prevent the high pH mixture formed at the cathodes from raising the pH of the mixture around the anodes. The low pH mixture surrounding the anodes is pushed (by the continued addition of sodium chloride via the inlet 3) up into a tank at the top of the electro-chlorinator 7 where the sodium chlorite is added via the inlet pipe 9. As a result of the low pH mixture from the anodes, the equilibrium balance of reaction (2) is towards the right hand side, resulting in the formation of the desired chlorine dioxide.

The final solution containing the desired chlorine dioxide is passed from the tank at the top of the electro-chlorinator 7, via an outlet pipe 11, to a dosing tank 15.

This chlorine dioxide is then added to the water 17 to be treated. Undissolved hydrogen gas is vented separately from undissolved chlorine and chlorine dioxide gases.

Electro-chlorinator Figure 2 is a three dimensional part cut away view and Figure 3 is a cross-sectional view of the electro-chlorinator 7 used in this embodiment. As shown, the electro-chlorinalor 7 comprises a housing 21, of dimensions approximately 450 mm x 140 mm x 120 mm (height x width x depth) that is affixed, in use, to the ground by a base plate 23. The housing 21 has five main pipe connectors: an inlet 3 located at the side 25 of the housing 21 near the base plate 23, for receiving the solution of sodium chloride (NaCl); an inlet 9 located at the front 26 and near the top 27 of the housing for supplying the sodium chlorite into the tank 29 at the top of the housing 21; an outlet 11 for the final solution obtained from the tank 29 at the top 27 of the housing 21; and two drain outlets 31 and 33, located on opposing sides 25 and 35 of the housing 21 for removing the high pH mixture formed at the cathodes. The housing 21 also includes three vents 71, 72 and 79 at the top 27 of the housing 21. Vents 71 and 72 are for venting hydrogen gas and vent 79 is for venting chlorine and chlorine dioxide gasses.

The housing 21 defines a flow path for fluid to pass from the inlet pipe 3 to the outlet pipe 11. The inside of the housing 21 is divided, by two flow screens 41 and 43 made, in this embodiment, of Perspex , into a lower chamber 45, a middle electrolytic cell 47 and a tank 29. The two flow screens 41 and 43 have a number of 1mm holes to allow the passage of the solution from the lower chamber 45 through the electrolytic cell 47 and into the tank 29, while reducing the possibility of backflow. The top flow screen 43 also enables an even dissipation of the evolved chlorine gas ensuring that, once the sodium chlorite is added, an even distribution of chlorine dioxide results in the tank 29. The electrolytic cell 47 itself is divided into three parts by two 3mm thick porous polyethylene membranes 51 and 53 that extend from the lower flow screen 41 to the top 27 of the housing 21.

The electrolytic cell 47 is thus divided into a central "anode chamber" 57, which houses a number of anodes 59, and which is positioned between two "cathode chambers" 61 and 62, each of which houses a cathode 63.

In this embodiment, the membranes 51 and 53 are perforated with an array of holes of approximately 15 microns in diameter on average and which, while not preventing ions from crossing the membranes, act to restrict liquid flow and therefore substantially prevent mixing between the solutions in the anode chamber 57 and those in the cathode chambers 61 and 62, and between those in the cathode chambers 61 and 62 and those in the tank 29.

In this embodiment, six anodes 59 are provided in the anode chamber 57, each of which is formed from a tube of Ebonex (a substoichiometric oxide of titanium) having a 28mm diameter and.a 100mm length. The anodes 59 thus span from the front to the back of the housing 21 and provide a total anode area of approximately 530,000mm2. The anodes 59 are preferably constructed in accordance with the design described in US 6998031, which discloses how an electrical contact is made to the Ebonex tube by means of a titanium coil that runs along the inside of the tube so that it contacts the inside surface of the tube at a plurality of spaced apart points along its length. In this embodiment, each cathode chamber 61 and 62 includes a single plate cathode 63, again formed of Ebonex and having dimensions of approximately 100mm x 120mm. As shown, the cathodes 63 are aligned with their principal sides parallel to the adjacent sides 25 and 35 of the housing 21. The Ebonex used in the invention is preferably produced according to the method described in the applicant's co-pending British patent application GB 0618961.7, the contents of which are incorporated herein by reference.

The two drains 31 and 33, one for each of the two cathode chambers 61 and 62, are located towards the top of the corresponding cathode chamber 61 and 62 and allow for the removal of the sodium hydroxide mixture formed around the cathodes 63 as a result of the electrolysis as it is pushed upwards by the continued addition of sodium chloride through the inlet pipe 3.

The two hydrogen vents, 71 and 72, are located directly above the cathode chambers 61 and 62 respectively, and vent undissolved hydrogen gas evolved during the electrolysis. The vent 79 is located above the tank 29 and vents undissolved chlorine gas evolved during the electrolysis and also any undissolved chlorine dioxide gas formed as a result of reaction (2) above.

Unit operation A more detailed description will now be given of the way in which the electro-chlorination reactions take place in the electro-chlorinator 7 described above. In operation, the starting solution, in this embodiment a 5% aqueous solution of sodium chloride (NaCI), is pumped into the lower chamber 45 of the electro-chlorinator housing 21 via the inlet pipe 3, at a rate of fifteen litres per hour.

The solution passes through the holes in the lower flow screen 41 into the anode chamber 57 and cathode chambers 61 and 62 of the electrolytic cell 47, where it undergoes electrolysis in accordance with reaction (1), producing sodium hydroxide (NaOH) and hydrogen gas (H2) at the cathode in each cathode chamber 61 and 62, and chlorine gas (Cl2) at the anodes in the anode chamber 57.

The electrodes 59 and 63 are connected to a DC power supply (not shown) that applies a potential difference of approximately 20 volts across them, as a result of which a typical DC current of 10 amps flows through the cell for the electrolysis.

The solution formed in the cathode chambers 61 and 62 is alkaline (pH of 11 to 12), while the solution formed in the anode chamber 57 is acidic (pH of 2 to 3). As more sodium chloride is added via the inlet 3, the mixtures formed in the cathode chambers 61 and 62 are forced up the housing 21 and out the respective drains 31 and 33. Similarly, the mixture formed around the anodes 59 is forced up into the tank 29. The membranes 51 and 53 act to restrict liquid flow and therefore the high pH sodium hydroxide mixture formed around the cathodes 63 does not mix with the low pH chlorine mixture formed around the anodes 59. Therefore, as mentioned above, when the sodium chlorite is added into the tank 29, it reacts with the chlorine formed by the anodes 59 to form chlorine dioxide in accordance with the right hand side of the equilibrium reaction (2) given above. A controller (not shown) is provided to control the rate of addition of the sodium chlorite (NaClO2) into the tank 29 using a standard feedback control loop based on a measure of the pH at the outlet 11.

Aqueous chlorine dioxide is output from the tank 29, via the outlet pipe 11. Any undissolved hydrogen gas is vented off from the cathode chambers 61 and 62 via vents 71 and 72 respectively; any undissolved chlorine and chlorine dioxide gases are vented from the tank 29 via vent 79. In this embodiment, aqueous chlorine dioxide is supplied to the dosing tank 15 at a concentration of between and 500 ppm and is then fed into the water to be treated so that the resulting water contains chlorine dioxide at a concentration of about 0.6 to 1.5 ppm.

Modifications and Alternatives In the above embodiment, the starting solution used was a 5% aqueous solution of NaCI. In order to avoid the treated water tasting too salty, a weaker solution may be used if desired.

In the above embodiment, the starting solution used was an aqueous solution of sodium chloride (NaCI). Other alkali metal starting solutions such as aqueous potassium chloride (KCI) can be used, although NaCI is preferred as it is less expensive. Similarly other alkali metal chlorites may be used instead of sodium chlorite.

In the above embodiments, plate electrodes were used as cathodes and tubular electrodes were used as anodes. As those skilled in the art will appreciate, other shapes of electrodes can be used. Additionally, it is not essential to form the electrodes from Ebonex material. Other materials can be used, such as graphite or titanium.

In the above embodiment, two cathode chambers were provided. As those skilled in the art will appreciate, only a single cathode chamber may be provided in an alternative design.

In the above embodiments, a porous polyethylene membrane was used to separate the anode chamber from each cathode chamber. The use of such a porous membrane allowed the hydrogen gas formed around the cathodes to pass through the membrane to the outlet pipe 11, whilst restricting flow of sodium hydroxide from the cathode chambers to the anode chamber. As a result, the high pH mixture formed around the cathodes does not raise the pH of the mixture formed around the anodes and so the equilibrium equation defined in (2) above is biased to the right and hence to the generation of chlorine dioxide. Polyethylene was used for the membrane material as it is highly resistant to deterioration. As those skilled in the art will appreciate, other barriers may be used which achieve the same function, although non-ion-specific polymers such as polyethylene are preferred as they are neither anionic nor cationic and therefore merely restrict rather than prevent the passage of charged ions across the barrier.

In the above embodiment, a porous membrane of 3mm thickness was used. As those skilled in the art will appreciate, different thicknesses of membrane may be used, for example as thin as 1mm or as thick as 5mm.

Similarly, whereas the membranes in the above embodiment were perforated with an array of holes of approximately 15 microns in diameter on average, those skilled in the art will appreciate the smaller or larger holes may be used.

Preferably, the holes are between 10 microns and 100 microns, on average.

Those skilled in the art will also appreciate that the porous membrane need not comprise a perforated polymer sheet as in the above embodiment, but may instead comprise a mesh or woven structure, possibly made of two or more different materials.

In the above embodiment, flow screens having holes were provided across the flow channel upstream and downstream of the electrolytic cell. As those skilled in the art will appreciate, these flow screens are not essential. However, they are preferred as they help to evenly dissipate the flow of liquids up from the lower chamber 45 through to the anode chamber 57 and cathode chambers 61 and 62, and also of the evolved chlorine gas into the tank 29. They also reduce back flow of fluid which could allow sodium hydroxide solution to pass into the anode chamber and hence raise the pH in the anode chamber.

In the above embodiment, the flow screens 41 and 43 were made of Perspex .

As those skilled in the art will appreciate, the material used is largely immaterial, although if the chlorine dioxide solution formed is found to be particularly corrosive, a polymer blend such as Noryl may be used instead.

In the above embodiment, the cathodes were positioned on the outsides of the housing and the anodes were provided in a central area of the housing between the two cathode chambers. As those skilled in the art will appreciate, this is not essential. The anodes may be provided on the outside and the cathodes may be Jo provided in a central portion of the housing between the anodes. Further, it is not essential to have three chambers defined by two barriers. A single barrier may be provided that divides the flow channel into two parts, with one part being used as a cathode chamber and the other side being used as an anode chamber.

The above description has referred to "anodes" and "cathodes". Whilst these terms usually mean that the cathode electrodes are connected to a negative electric potential and the anode electrodes are connected to a positive electric potential, these terms should be given a broader interpretation in which the anode electrodes are connected, in use, to a higher electric potential than the cathode electrodes.

Claims

Claims 1. Apparatus for use in an electro-chlorinator, the apparatus

comprising: a housing having: i) a first inlet for receiving a chloride solution; ii) a second inlet for receiving a chlorite; iii) a first outlet for outputting chlorine dioxide; and iv) a second outlet for outputting an hydroxide solution; a flow channel extending between the first inlet and the first outlet and defining a flow path along which said chloride solution received at said first inlet can flow towards said first outlet; at least one barrier extending across the flow channel and at least partly along the flow path to define first and second sub-channels; one or more cathodes positioned within said first sub-channel and one or more anodes positioned within said second sub-channel for subjecting the chloride solution flowing along said flow channel to electrolysis; wherein said second inlet is positioned downstream of said one or more anodes and is operable to add said chlorite to react with chlorine formed around said one or more anodes to produce chlorine dioxide; wherein said second outlet is positioned downstream of said one or more cathodes and is operable to remove hydroxide solution formed around said one or more cathodes; and wherein said at least one barrier is operable to restrict the flow of fluid from said first sub-channel to said second sub-channel.
2. An apparatus according to claim 1, wherein said barrier is porous to hydrogen formed around said one or more cathodes.
3. An apparatus according to claim 2, wherein hydrogen gas formed around said one or more cathodes is operable, in use, to exit said housing via a third outlet positioned downstream of said cathode.
4. An apparatus according to any preceding claim, wherein said at least one barrier prevents flow of said hydroxide solution from said first sub-channel to said second sub-channel.
5. An apparatus according to any preceding claim, wherein said at least one barrier comprises a polymer material that is neither anionic nor cationic.
6. An apparatus according to any preceding claim, wherein said at least one barrier material is formed of polyethylene.
7. An apparatus according to any preceding claim, wherein said at least one barrier is perforated with an array of holes of diameter greater than 10 microns on average.
8. An apparatus according to any preceding claim, wherein said at least one barrier is perforated with an array of holes of diameter less than 100 microns on average.
9. An apparatus according to any preceding claim, wherein said at least one barrier is perforated with an array of holes of diameter approximately 15 microns on average.
10. An apparatus according to any preceding claim, wherein said at least one barrier has a thickness greater than 1mm.
11. An apparatus according to any preceding claim, wherein said at least one barrier has a thickness less than 5mm.
12. An apparatus according to claims I to 4, wherein said at least one barrier comprises a mesh structure.
13. An apparatus according to claim 12, wherein said at least one barrier comprises a mesh structure formed of two or more different materials.
14. An apparatus according to any preceding claim, further comprising a flow screen positioned adjacent said first inlet and operable to restrict flow of said chloride solution into said first and second sub-channels.
15. An apparatus according to claim 14, wherein said flow screen is operable to reduce flow of hydroxide solution from said one or more cathodes towards said first inlet.
16. An apparatus according to any preceding claim, comprising first and second barriers operable to divide said flow channel into three sub-channels.
17. An apparatus according to claim 16, comprising one or more anodes in one of said sub-channels and cathodes in the other two of said sub-channels.
18. An apparatus according to claim 16, wherein said anodes are provided in a central sub-channel and wherein said second inlet is operable to add said chlorite to said central sub-channel.
19. An apparatus according to any preceding claim, comprising a flow screen positioned across said flow channel and downstream of said one or more anodes and operable to define an output tank and wherein said second inlet is operable to supply said chlorite into said tank.
20. An apparatus according to any preceding claim, wherein said chloride is sodium chloride.
21. An apparatus according to any preceding claim, wherein said chlorite is sodium chlorite.
22. An apparatus according to any preceding claim, further comprising a power supply operable to supply electric potential to said at least one anode and said at least one cathode for said electrolysis.
23. An apparatus according to any preceding claim, wherein said housing is oriented, in use, such that said flow channel extends in a substantially vertical direction.
24. Apparatus for use in an electro-chiorinator, the apparatus comprising: a housing having: i) a first inlet for receiving a chloride solution; ii) a second inlet for receiving a chlorite solution; iii) a first outlet for outputting chlorine dioxide; and iv) a second outlet for outputting an hydroxide solution; a flow channel extending between the first inlet and the first outlet and defining a flow path along which said chloride solution received at said first inlet can flow towards said first outlet; an electrolytic cell positioned within said flow channel for subjecting said chloride solution to electrolysis; and at least one porous barrier positioned between electrodes of said electrolytic cell and operable to restrict flow of hydroxide solution formed, in use, around one electrode towards the other electrode.
25. A method of producing chlorine dioxide comprising: passing a chloride based solution through an electrolytic cell; subjecting the chloride solution to electrolysis; providing a barrier between an anode and a cathode of said electrolytic cell to restrict the flow of hydroxide formed around said cathode towards chlorine formed around said anode; adding a chlorite to the chlorine formed around said anode for reacting with the chlorine to form chlorine dioxide; and outputting the formed chlorine dioxide.
26. Apparatus for use in an electro-chlorinator, the apparatus comprising: a housing having: I) a first inlet for receiving a first solution; ii) a first outlet for outputting a first output; and iii) a second outlet for outputting a second output; a flow channel extending between the first inlet and the first outlet and defining a flow path along which said first solution received at said first inlet can flow towards said first outlet; at least one barrier extending at least partly along the flow path to define first and second sub-channels; a cathode positioned within said first sub-channel and one or more anodes positioned within said second sub-channel for subjecting the first solution flowing along said flow channel to electrolysis; wherein said second outlet is positioned downstream of said cathode and is operable to remove said second output formed around said cathode; and wherein said at least one barrier is operable to restrict the flow of fluid from said first sub-channel to said second sub-channel.
27. An apparatus comprising: a housing having: i) a first inlet for receiving a first solution; and ii) a first outlet for outputting a first output; a flow channel extending between the first inlet and the first outlet and defining a flow path along which said first solution received at said first inlet can flow towards said first outlet; and an Ebonex cathode positioned within said flow channel and one or more Ebonex anodes positioned within said second flow channel for subjecting the first solution flowing along said flow channel to electrolysis.
28. An apparatus according to claim 26 or 27, wherein said cathode is plate shaped and positioned within a first sub-channel of said flow channel so that the main face of the cathode lies parallel with said flow path.
29. An apparatus according to claim 26, 27 or 28, wherein at least one of said one or more anodes comprises a tubular anode positioned within a second sub-channel of said flow channel so that its longitudinal axis is substantially orthogonal to said flow path.
30. An apparatus for use in an electra-chlorinator substantially as described herein above with reference to or as shown in the accompanying figures.
31. A method of producing chlorine dioxide substantially as described herein above with reference to the accompanying figures.