FIELD OF THE INVENTION This invention relates to apparatus for the generation of chlorine dioxide by electrolysis which is used as a sterilizing agent in reservoirs of water such as in air conditioning and cooling systems. Of particular concern are legionnaires' disease carrying organisms which are best treated with chlorine dioxide and in flowing water such as potable water, recycled water and process water.

BACKGROUND OF THE INVENTION

There are many patent documents relating to chlorine dioxide generators. For example GB692763 discloses an electrolytic diaphragm cell which produces chlorine dioxide from an aqueous solution of a chlorite eg sodium chlorite in the presence of a water soluble sulphate. The anode may be of platinum and the cathode of lead and the anode chamber is provided with cooling means. In GB1044769 the initial solution subjected to electrolysis may comprise a chromate, a mineral acid or a soluble calcium salt.

US4542008 teaches a continuous process for electrolyzing an aqueous sodium chlorite solution to produce chlorine dioxide which dissolves to produce an aqueous solution of NaCl02 containing chlorine dioxide. This solution is passed through a photometric cell which measures and controls concentration of sodium chlorite ions which when electrolysed avoid the formation of by-products such as dark colored chlorine dioxide complex and sodium chlorate. US2004149571 employs a porous anode through which the aqueous feed solution passes to maximize the conversion of chlorite to chlorine dioxide. WO2008090367 discloses an electrolytic cell in which a porous barrier or membrane is provided between anodes and cathodes in order to restrict flow of sodium hydroxide formed around cathodes of the cell towards the anodes. As a result, the pH of the solution around the anodes is maintained low (around2 or 3) and there is no need to add acid to the electro-chlorinator ton facilitate the production of the chlorine dioxide. US2008308428 discloses a method for generating high concentration chlorine dioxide having purity over 90% using an electrolytic material prepared from NaCI and NaCI02 subject to the control of optimal operation parameters of current 80-110 Amp, concentration of sodium chloride to be 20-25% and sodium chlorite to be 5% minimum, operation temperature 55-65C and material feeding speed 30-50 ml/min. OBJECT OF THE INVENTION

However the present inventor has found that the prior art apparatus can be improved upon and it is therefore a ^" n object of the present invention to provide chlorine dioxide generation with improved efficiency or at least to provide an alternative to the prior art.

STATEMENT OF THE INVENTION

According to the present invention apparatus for producing chlorine dioxide comprises an electrolysis cell having an anode and a cathode which receives sodium chlorite solution as an anolyte flow and water as a catholyte flow, a separator which releases chlorine dioxide gas irom the anolyte flow which leaves the cell and a separator which releases hydrogen gas from the catholyte flow which leaves the cell.

Preferably the anode is formed from coated titanium mesh with an

electrocatalytic coating and the cathode is formed from plain titanium mesh.

Preferably the anode and cathode chambers are separated by an ion exchange membrane.

Preferably the gap between the anode and cathode is between 0 and 5mm. Preferably the shadow area current of the anode is between 1000 and 3500 amps/m2. Preferably the flow velocity of the anolyte solution over the anode is between 5 and 100m/min.

Preferably the cell is provided with channels carrying coolant to cool the anolyte and catholyte flows.

Preferably the cell is encased within metallic plates.

Preferably the cyclonic separator is a generally conical tube which receives the anolyte flow tangentially facilitating the release of chlorine dioxide gas through the top of the separator and allowing residual chlorine dioxide and sodium chlorite solution to fall to a reservoir in the bottom of the separator to be recirculated in the anolyte flow.

Preferably air is introduced into the centre of the separator to increase the release of chlorine dioxide gas.

Preferably the separator is constructed in three axial sections joined by pairs of flanges and sealed by gaskets. Preferably the hydrogen separator is an open pipe which receives the catholyte flow and allows the hydrogen gas to escape out of the top of the pipe and the residual catholyte to fall to the bottom of the separator whence it is recycled to the cell. BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is now described by way of example only with reference to the accompanying drawings in which:

Fig 1 is a flow diagram depicting various fluid flows between an electrolysis cell, a chlorine dioxide gas separator, a hydrogen gas separator and a storage tank.

Fig 2 is a cross section of the electrolysis cell of Fig 1

Fig 3 is a cross section of the chloride dioxide gas separator of Fig 1 and

Fig 4 is a cross section of the hydrogen gas separator of Fig 1.

DETAILED DESCRIPTION OF THE INVENTION

In Fig 1 , electrolysis cell 1 receives sodium chlorite flow 10 to the anode of cell 1 and water flow 11 to the cathode of cell 1. The reactions in cell 1 are complex but the net desirable reaction may be represented by the following equation:

NaCI02 + H20 + energy -> CI02 + NaOH + H2

Accordingly, anolyte outflow 5 is a chlorine dioxide solution and gas which enters separator 2 and the catholyte outflow 13 is a dilute sodium hydroxide solution with hydrogen gas which enters gas separator 3.

Separator 2 extracts chlorine dioxide gas 6 which flows to storage tank 4 and the residual solution of chlorine dioxide and sodium chlorite combines with anolyte input 10 or drains to waste 7. In gas separator 3, hydrogen 14 is vented to the atmosphere and the residual catholytel 1 is recirculated to cell 1 or drained to waste 12 and 15.

Fig 2 reveals the detailed construction of electrolysis cell 1. Cell casing 21 is cast in two sections 22 and 23 with an ion exchange membrane 24 separating them and sodium chlorite solution inlet 25 and water irilet 26 are provided. The sodium chlorite solution flows between anodically assigned electrode 27 and membrane 24 and the water flows between membrane 24 and cathodically assigned electrode 28 and anolyte outflow 29 and catholyte outflow 30 are provided. Electrodes 27 and 28 are positioned close to membrane 24 with an electrode gap 31 ranging between zero and 5mm. The preferred material for electrode 27 is coated titanium mesh and for electrode 28, plain titanium mesh. The mesh is built up from multiple layers of .5 to 1.5 mm mesh material to give an extended surface area.

DC power is applied to electrodes 27 and 28 by conductors 32 and 33 which are fastened to electrodes 27 and 28 via current distributors 34 and 35. The power loading is optimized to maximize chlorine dioxide production and to minimize pH build up in the anolyte chamber. The preferred shadow area anodic current density is less than 2000 amps/m2 although current densities may range up to 3,500 amps/m2.

Recirculation of anolyte solution is adjusted to maintain a flow velocity over electrode 27 of 20m per minute but flow rates may be in the range from 5 to 100m per minute. Membrane 24 is maintained in position by Viton rubber strip and plastic or rubber O ring material. Cell 1 is also provided with internal waterways 36 and 37 to allow cooling of anolyte 25 and catholite 26. External metallic plates 38 and 39 increase the strength of cell casing 21 and also assist in cooling.

Fig 3 reveals the detailed construction of separator 2. Anolyte flow 5 is introduced tangentially into the upper cylindrical section of separator 2 through inlet 43 generating centrifugal forces which release chlorine dioxide gas 45 upward through outlet 46 and allow the residual chlorine dioxide and sodium chlorite solution 56 to descend to reservoir 55, whence it flows through outlet 52 to be recirculated to cell 1. If the level in reservoir 53 reaches level 54, overflow 11 flows through outlet drain 50 to waste. Air 49 is introduced through pipe 48 to facilitate the release of chlorine dioxide gas 45. The casing of separator 2 is formed from a plastic tube 41 with a conical insert 42 which creates a cyclonic flow which enhances the release of chlorine dioxide gas 45.

Fig 4 reveals the detailed construction of hydrogen gas separator 3 formed from tube 61. Catholyte solution 67 enters tube 61 through inlet 62. Hydrogen gas 65 · rises upwards and exhausts from tube 61 into the atmosphere while sodium hydroxide solution 70 flows downwards and leaves outlet 69 to be recirculated to cell 1. Fresh make up water 71 enters tube 61 through inlet 72. Overflow 63 leaves tube 61 through outlet 64.

The apparatus of the present invention improves the efficiency of chlorine dioxide generation in two different aspects. First the efficiency of the electrolysis cell is improved so that more chlorine dioxide is produced from a given input. Second the chlorine dioxide gas and hydrogen gas separators with their attendant recirculation of anolyte and catholyte flows further increases the yield of chlorine dioxide and minimises waste.

TEST RESULTS A test was set up to compare a prior art cell to the cell described in the present invention. A 7.5% Sodium Chlorite Solution was pumped at a rate of 0.34 litres/hour into each cell in turn and a DC current of 10 Amps was applied. A recirculation flow of 4.57 litres/min was used and the conversion from Sodium Chlorite to Chlorine Dioxide was measured using a colorimeter. Over 3 tests, the conversion efficiency of the prior art cell was averaged at 67% and the

conversion efficiency of the Cell described in the present invention was averaged at 86%. VARIATIONS

It will be realized that the foregoing has been given by way of illustrative example only and that all other modifications and variations as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth. Throughout the description and claims of this specification the words "comprise" and variations of thai word such as

"comprises" and "comprising" are not intended to exclude other additives components integers or steps.

The reference to any prior art in this specification is not and should not be taken as an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge. It will be appreciated that the invention is not limited to the embodiments disclosed but is capable of numerous

rearrangements, modifications and substitutions without departing form the scope of the invention as set forth and defined by the following claims.