Title of Invention

"METHOD FOR THE ELECTROLYSIS OF AN AQUEOUS SOLUTION OF HYDROGEN CHLORIDE OR ALKALI METAL CHLORIDEIN AN ELECTROLYSIS CELL"

Abstract Method for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride in an electrolysis cell, consisting at least of an anode half-element and an anode, a cathode half-element and a gas diffusion electrode as the cathode, and a cation exchange membrane for separating the anode half-element and the cathode half -element, a gas containing oxygen being supplied to the cathode half-element and excess gas containing oxygen being discharged from the cathode half-element, characterized in that the excess gas containing oxygen and hydrogen discharged from the cathode half-element is subjected to catalytic oxidation of hydrogen, wherein the excess gas containing oxygen is sent through the catalytic converter at a temperature of from 120 to 300°C and is recycled and resupplied to the cathode half-cell.
Full Text Method for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride
The invention relates to a method for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride by means of a gas diffusion electrode as the cathode.The electrolysis of aqueous solutions of hydrogen chloride (hydrochloric acid) and aqueous alkali metal chloride solutions can be carried out electrolytically by using gas diffusion electrodes as oxygen-consuming cathodes. In this case, oxygen, air or oxygen-enriched air are fed in excess into the electrolysis cell. By using oxygen-consuming cathodes, the electrolysis voltage is reduced by approx. 30% compared with conventional hydrochloric acid or chloralkali electrolyses.A method for the electrolysis of hydrochloric acid is known, for example, from US-A 5 770 035. An anode space with a suitable anode, consisting for example of a substrate made of a titanium-palladium alloy, which is coated with a ruthenium, iridium and titanium mixed oxide, is filled with hydrochloric acid. The chlorine formed at the anode escapes from the anode space and is sent for suitable processing. The anode space is separated from the cathode space by a commercially available cation exchange membrane. On the cathode side, there is a gas diffusion electrode (oxygen-consuming cathode) adjoining the cation exchange membrane. The oxygen-consuming cathode in turn adjoins a current distributor. The oxygen supplied to the cathode space is reacted on the oxygen-consuming cathode.EP-A 1 067 215 discloses a method for the electrolysis of an aqueous alkali metal chloride solution by using an oxygen-consuming cathode. The electrolysis cell is composed of an anode half-element and a cathode half-element, which are separated from each other by a cation exchange membrane. The cathode half-element consists of an electrolyte space and a gas space, the electrolyte space being separated from the gas space by an oxygen-consuming cathode. The electrolyte space is filled with an alkali metal hydroxide solution. The gas space is supplied with oxygen. The anode space is filled with a solution containing alkali metal chloride.The formation of hydrogen on oxygen-consuming cathodes is in general prevented fully. Inter alia because of competition reactions, however, traces of hydrogen may even be formed if the oxygen supply is sufficient. The hydrogen is drawn off from the cathode space together with the excess oxygen. According to previous methods, the oxygen is released into the waste air after off-gas purification, since recycling entails the risk of concentrating the hydrogen to above the explosion limit of 4 vol.%.The oxygen supplied to the cathode half-element, or the supplied air which is optionally enriched with oxygen, will be referred to below as oxygen for the sake of simplicity.It is an object of the present invention for the oxygen used in excess to be recycled in an economically viable way. A method is to be provided in which the excess oxygen can be made available several times for the electrolysis process.The invention relates to a method for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride in an electrolysis cell, consisting at least of an anode half-element and an anode, a cathode half-element and a gas diffusion electrode as the cathode, and a cation exchange membrane for separating the anode half-element and the cathode half-element, a gas containing oxygen being supplied to the cathode half-element and excess gas containing oxygen being discharged from the cathode half-element, which is characterised in that the excess gas containing oxygen discharged from the cathode half-element is subjected to catalytic oxidation of hydrogen.The gas containing oxygen, which is fed in excess into the cathode half-element, is for example oxygen, air or oxygen-enriched air. With the aid of the catalytic oxidation of hydrogen, which is present in the excess gas containing oxygen when it leaves the cathode half-element, the level of hydrogen is reduced, in particular, to at most 2 vol.%. This makes it possible for the oxygen used in excess to be re-supplied to the cathode half-cell, without entailing the risk of enrichment up to the explosion limit of the oxygen/hydrogen mixture during repeated recycling.For the catalytic oxidation of hydrogen, in a preferred embodiment, the excess gas containing oxygen is sent through a catalytic converter, consisting at least of support body made of ceramic or metal, preferably ceramic, and a coating containing a catalytically active noble metal, for the oxidation of hydrogen.The support body of the catalytic converter is preferably monolithic. The monolithic support body of the catalytic converter preferably has a high specific surface. It is, however, also possible to use a bed of material as the support body, in order to achieve a high specific surface. The monolithic support body preferably has a multiplicity of channels, which may have any cross section. The monolithic support body may, for example, be constructed from essentially parallel channels which are not connected together transversely to the flow direction. It may, however, also have crosscurrent channels or consist of a foam, for example. The support body may, for example, be honeycombed.The wall thickness of the catalytic-converter channels should be small, in order to achieve a minimum flow resistance for a given cross section of the catalytic-converter unit. The minimum wall thickness for ceramic support bodies is approx. 0.3 mm. Thermal and mechanical damage to ceramic support bodies may take place if the wall thickness is less than this. A metal support body allows a smaller minimum wall thickness. It may be approx. 0.05 mm. The diameter of the channelsis, for example, 0,5 cm. Owing to the small wall thickness, the support body is heated rapidly and it is soon ready for operation when starting up the system.A ceramic support body is, for example, produced by extrusion. In order to produce 5 a metal support body, for example, metal foil is wound like corrugated cardboard consisting of two flat metal foils with corrugated metal foil lying in between.Preferred metals for the support body are, for example, titanium or stainless steel. A preferred ceramic support body consists, for example, of aluminium oxide.The support body is provided with a coating which contains at least one catalytically active noble metal, for example platinum or rhodium. The coating preferably contains platinum. The coating may also contain platinum and rhodium, for example in a weight ratio of 5:1. The amount of coating is, for example, from 1.4 15 to 1.8 mg/cm3. A larger amount of coating, approx. 2.7 mg/cm3, is also possible.At least one interlayer may also be applied between the support body and the coating, the essential purpose of this interlayer being to increase the specific surface of the catalytic converter. The interlayer consists of aluminium oxide, for example, so that 20 the specific surface is approx. 5000 times more than without an interlayer.Commercially available catalytic converters for exhaust-gas purification, for example, such as those used for petrol or diesel engines, or oxidation catalytic converters such as those used for diesel-driven lorries or buses (for example HJS Kat 25 2000 from HJS Fahrzeugtechnik GmbH Co., Germany) may be used as catalytic converters for the method according to the invention.Instead of a single catalytic converter, it is also possible to connect a plurality of catalytic converters in parallel, for example in order to oxidise gas streams with a high hydrogen content while at the same time keeping the pressure drop small.The excess gas containing oxygen is preferably sent through the catalytic converter at a temperature of from 120 to 300°C, particularly preferably from 150 to 180°C. This may be done either by heating the oxygen before it enters the catalytic converter, for example with the aid of a heat exchanger, or by heating the catalytic converter itself. A metal catalytic converter may, for example, be heated inductively. The catalytic converter may also be provided with a heating jacket in order to supply heat.The pressure drop when flowing through the catalytic converter is preferably less than 100 mbar, particularly preferably less than 10 mbar. Depending on the size of the catalytic converter selected, the flow rate of the oxygen may need to be limited if operation of the catalytic converter is to be ensured without a significant pressure drop. A pressure increase in the catalytic converter when increasing the flow rate is disadvantageous for the electrolysis method, in particular for the electrolysis of an aqueous solution of hydrogen chloride. For hydrochloric acid electrolysis, as is known from DE-A 10 138 215, the anode half-cell is kept at a higher pressure than the cathode half-cell. Owing to the higher pressure in the anode half-cell, the cation exchange membrane is pressed onto the gas diffusion electrode, which is in turn pressed onto the current distributor. If too high a pressure drop were to occur in the catalytic converter, therefore, the pressure in the cathode half-cell would increase and the gas diffusion electrode would then be pushed away from the current distributor. For the recycling of sizeable oxygen streams, connection of a plurality of catalytic converters in parallel or selection of a correspondingly large catalytic converter is therefore advantageous in order to avoid pressures which are too high in the catalytic converter. As an alternative, the pressure on the anode side would need to be raised simultaneously with the pressure drop on the cathode side, so as to maintain the pressure difference between the cathode space and the anode space.The excess gas containing oxygen is preferably re-supplied to the cathode half-element after the catalytic oxidation of hydrogen. In order to provide excess oxygen at all times, oxygen or air or oxygen-enriched air is also supplied to the cathode half-element. The freshly supplied oxygen is, for example, mixed with the purified oxygen stream before it enters the cathode half-element.With the method according to the invention, for example, it is possible for the approx. 70 to 100 m3/h of excess oxygen (oxygen off-gas stream) from an electrolyser with a chlorine production of approx. 10,000 t.p.a. to be purified without significant pressure build-up by using only one catalytic converter.The invention will be explained in more detail below with reference to Figure 1. Figure 1 shows a flow chart of an embodiment of the method according to the invention, as was used for test purposes in the following example.Examples
An oxygen off-gas stream 2 with a flow rate of from 2 to 6.5 m3/h, which is saturated with water vapour and is at a temperature of approx. 50° C, is discharged from the cathode space of a pilot electrolyser 1 having five elements, each with an area of 0.88 m2. The off-gas stream 2 is immersed approx. 30 cm deep in a container 3, which collects the water of reaction 4 flowing away through a separate outlet (the water of reaction, or cathode condensate, consists of very dilute hydrochloric acid, approx. 1 wt.% strength).Metered amounts of hydrogen from a hydrogen source 6 were supplied to the oxygen off-gas stream 2 leaving the container 3. A laboratory electrolysis cell for the electrolysis of water, in which hydrogen was cathodically formed, was used as the hydrogen source 6. The oxygen off-gas stream 2 with hydrogen added is sent through a stainless-steel pipe, which is externally heated by an electrical heating strip 5 with a heating power of 750 watts so that it is heated to approx. 150 to 180°C. The off-gas stream 2 is then sent through a catalytic converter 7, here a standard car catalytic converter of the type HJS VW 80/Coupe type 81 from HJS Fahrzeugtechnik GmbH Co., Germany.Both the pipe leading to the catalytic converter 7 and the catalytic converter 7 itself are thermally insulated. In the catalytic converter 7, any hydrogen gas present reacts with the oxygen off-gas 2 to form water. The catalytic converter 7 is arranged vertically, so that the oxygen off-gas stream 2 flows from the top downwards through the catalytic converter 7. If it is not present as water vapour so that it can leave the catalytic converter 7 in the form of a gas, the water formed in the catalytic converter 7 can therefore flow downwards out of the catalytic converter The oxygen off-gas 12 leaving the catalytic converter 7 is virtually hydrogen-free when the working temperature of at least 150 to 180°C is complied with, and it has a maximum hydrogen concentration of approx. 10 vol. ppmThe off-gas stream 12, from which the hydrogen has been removed, is mixed with fresh oxygen 11 and supplied to the cathode half-element of the electrolyser 1 by means of a nozzle 10, for example a Venturi nozzle.The following table reports the hydrogen concentrations (H.2 cone.) in vol. ppm before and after the catalytic oxidation in the catalytic converter 7, as well as the temperatures of the oxygen off-gas stream 2, 12 before and after the catalytic converter 7. The hydrogen concentration was measured by means of a catalytic measuring cell from Zellweger before the catalytic converter 7, and by a mobile gas chromatograph from Agilent after the catalytic converter 7. The hydrogen concentration before the catalytic converter 7 corresponds substantially to the amount of hydrogen which was supplied to the oxygen off-gas stream 2 by means of the hydrogen source 6. Also measured was the flow rate of the freshly supplied oxygen 11, which is mixed by means of a nozzle 10 with the oxygen off-gas stream 12, from which the hydrogen has been removed, before the oxygen gas stream enters the electrolyser 1.The experiments show the dependency of the hydrogen elimination on the temperature of the oxygen off-gas stream 2 before the catalytic converter. At a temperature of 198°C, the hydrogen concentration is reduced to less than 10 vol. ppm in spite of the high initial concentration.





WE CLAIM:
1. Method for the electrolysis of an aqueous solution of hydrogen chloride or alkali metal chloride in an electrolysis cell, consisting at least of an anode half-element and an anode, a cathode half-element and a gas diffusion electrode as the cathode, and a cation exchange membrane for separating the anode half-element and the cathode half-element, a gas containing oxygen being supplied to the cathode half-element and excess gas containing oxygen being discharged from the cathode half-element, characterized in that the excess gas containing oxygen and hydrogen discharged from the cathode half-element is subjected to catalytic oxidation of hydrogen, wherein the excess gas containing oxygen is sent through the catalytic converter at a temperature of from 120 to 300°C and is recycled and resupplied to the cathode half-cell.
2.Method as claimed in claim 1, wherein the excess gas containing oxygen is sent through a catalytic converter, consisting at least of support body made of ceramic or metal preferably ceramic, and a coating containing a catalytically active nobel metal, for the oxidation of hydrogen.
3.Method as claimed in claim 2, wherein the coatings contains platinum.
4.Method as claimed in claim 2, wherein the coatings contains platinum and rhodium.
5.Method as claimed in claims 2 to 4, wherein there is an interlayer, which contains aluminium oxide, between the support body and the coating.
6. Method as claimed in any one of claims 2 to 5, wherein the excess oxygen air or oxygen-enriched air is sent through the catalytic converter, at a temperature of from 120 to 300°C, preferably from 150 to 180°C.
7.Method as claimed in any one of claims 2 to 6, wherein the pressure drop when flowing through the catalytic converter is less than 100 mbar, preferably less than 10 mbar.

8..Method as claimed in any one of claims 2 to 7, wherein the excess oxygen, air or oxygen-enriched air is re-supplied to the cathode half-element after the catalytic oxidation of hydrogen.


Documents:

746-delnp-2006-Abstract-(11-03-2011).pdf

746-delnp-2006-abstract.pdf

746-delnp-2006-Claims-(11-03-2011).pdf

746-delnp-2006-claims.pdf

746-DELNP-2006-Correspondence Others-(23-03-2011).pdf

746-delnp-2006-Correspondence-Others-(11-03-2011).pdf

746-delnp-2006-correspondence-others-1.pdf

746-delnp-2006-correspondence-others.pdf

746-delnp-2006-description (complete).pdf

746-delnp-2006-Drawings-(11-03-2011).pdf

746-delnp-2006-drawings.pdf

746-delnp-2006-form-1.pdf

746-delnp-2006-form-18.pdf

746-delnp-2006-form-2.pdf

746-delnp-2006-Form-3-(11-03-2011).pdf

746-DELNP-2006-Form-3-(23-03-2011).pdf

746-delnp-2006-form-3.pdf

746-delnp-2006-form-5.pdf

746-delnp-2006-GPA-(11-03-2011).pdf

746-delnp-2006-gpa.pdf

746-delnp-2006-pct-210.pdf

746-delnp-2006-pct-304.pdf

746-DELNP-2006-Petition 137-(23-03-2011).pdf


Patent Number 249471
Indian Patent Application Number 746/DELNP/2006
PG Journal Number 43/2011
Publication Date 28-Oct-2011
Grant Date 21-Oct-2011
Date of Filing 14-Feb-2006
Name of Patentee BAYER MATERIALSCIENCE AG
Applicant Address 51368 LEVERKUSEN, GERMANY.
Inventors:
# Inventor's Name Inventor's Address
1 FRITZ GESTERMANN BERLINER STR. 83, 51377 LEVERKUSEN, GERMANY.
2 RICHARD MALCHOW MORSESTR. 15, 50769 KOLN, GERMANY.
3 WALTER HANSEN STRASSBURGER STR. 36, 51375 LEVERKUSEN, GERMANY.
4 GABY SENGSTOCK AM WEIHER 4-6, 51379 LEVERKUSEN, GERMANY.
PCT International Classification Number C25B 1/46
PCT International Application Number PCT/EP2004/009673
PCT International Filing date 2004-08-31
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 103 42 148.3 2003-09-12 Germany