EP1283281B1 - Process for the electrochemical production of chlorine from aqueous hydrochloric acid solutions - Google Patents

Description

The invention relates to a method for the electrochemical production of chlorine from aqueous solutions of hydrogen chloride in an electrolytic cell.

For example, aqueous solutions of hydrogen chloride (hydrochloric acids) are by-produced in the production of organic chlorine compounds by chlorination with elemental chlorine. Many of these organic chlorine compounds are intermediates for the large-scale production of plastics. The resulting aqueous solutions of hydrogen chloride must be recycled. The recovery preferably takes place in such a way that chlorine is again produced from the aqueous solutions of hydrogen chloride, which can then be used, for example, for further chlorinations.

The conversion to chlorine can be carried out, for example, by electrolysis of the aqueous solutions of hydrogen chloride on a gas diffusion cathode. A corresponding procedure is off US-A-5,770,035 and US-A-6149782 known. The electrolysis takes place according to US-A-5,770,035 in an electrolytic cell with an anode compartment, with a suitable anode, for example a noble metal-coated or -doped titanium electrode, which is filled with the aqueous solution of hydrogen chloride. The chlorine formed at the anode escapes from the anode compartment and is fed to a suitable treatment. The anode compartment is separated from a cathode compartment by a commercially available cation exchange membrane. On the cathode side, a gas diffusion electrode is located on the cation exchange membrane. Behind the gas diffusion electrode is a power distributor. In the cathode space usually an oxygen-containing gas or pure oxygen is introduced.

The anode compartment is maintained at a higher pressure than the cathode compartment. As a result, the cation exchange membrane on the gas diffusion cathode and these in turn pressed on the power distributor. The adjustment of the pressure can be done, for example, by a liquid immersion, through which the chlorine gas formed in the anode chamber is passed.

The end US-A-5,770,035 known method has the disadvantage that at high current densities, which in particular current densities greater than 4000 A / m ^{2 are} to be understood, a comparatively high amount of hydrogen is formed on the gas diffusion cathode. However, high current densities are necessary in the technical implementation of the method for economic reasons. In addition, at high current densities, a comparatively high voltage sets in, which requires high energy consumption.

US 6 149 782 A discloses a process for the electrochemical production of chlorine from aqueous solutions of hydrogen chloride in an electrolytic cell comprising an anode chamber and a cathode chamber. The anode compartment is separated from the cathode compartment by a cation exchange membrane, Nafion 324. The anode chamber contains an anode and the cathode chamber contains an oxygen-consuming cathode containing a rhodium compound catalyst. An aqueous solution of hydrogen chloride is introduced into the anode chamber, and oxygen is introduced into the cathode chamber. The partial pressure of the oxygen is 45-50 mbar, so the pressure in the cathode chamber is above 1.05 bar.

The object of the invention is to provide a method for the electrochemical production of chlorine from aqueous solutions of hydrogen chloride, wherein even when working with high current densities as little as possible hydrogen is formed and sets the lowest possible voltage.

The invention relates to a method according to claim 1 for the electrochemical production of chlorine from aqueous solutions of hydrogen chloride in an electrolytic cell, comprising at least one anode chamber and a cathode chamber, wherein the anode chamber is separated by a cation exchange membrane from the cathode chamber, the anode chamber an anode and the cathode chamber contains an oxygen-consuming cathode, and in the anode chamber, the aqueous solution of hydrogen chloride and in the cathode chamber, an oxygen-containing gas is introduced, wherein the absolute pressure in the cathode chamber is at least 1.05 bar.

By the inventively slightly increased pressure in the cathode chamber, the formation of hydrogen at the oxygen-consuming cathode is reduced and also achieved a lower electrolysis than in reaction under normal pressure, corresponding to the ambient pressure, in the cathode chamber. It is amazing and was not expected to have a comparatively small increase the pressure in the cathode chamber leads to a significant reduction of unwanted hydrogen evolution at the oxygen-consuming cathode and lower electrolysis voltages, which in turn is advantageous in terms of energy consumption.

As the oxygen-containing gas, for example, pure oxygen, a mixture of oxygen and inert gases, in particular nitrogen, or air can be used. Pure oxygen, in particular a purity of at least 99% by volume, is preferably used as the oxygen-containing gas.

The indication of the pressure in the cathode chamber are absolute values. The pressure in the cathode chamber is preferably 1.05 to 1.5 bar, particularly preferably 1.05 to 1.3 bar.

The adjustment of the pressure in the cathode chamber to the value according to the invention of at least 1.05 bar can be effected, for example, by accumulating the oxygen-containing gas supplied to the cathode chamber by means of a pressure-retaining device. A suitable pressure-holding device is, for example, a liquid compression, by which the cathode space is shut off. Throttling via valves is also a suitable method for adjusting the pressure in the cathode compartment.

In order to ensure adequate contact between cation exchange membrane and oxygen-consuming cathode, a pressure is set in the anode chamber, which is 200 to 500 mbar higher than the pressure in the cathode chamber.

The inventive method is operated at a current density of at least 5000 A / m ² .

The temperature of the supplied aqueous solution of hydrogen chloride is preferably from 30 to 80 ° C, particularly preferably from 50 to 70 ° C.

Preferably, the concentration of hydrochloric acid in the electrolyzer in carrying out the process according to the invention is 5 to 20 wt .-%, particularly preferably 10 to 15 wt .-%. The consumed hydrochloric acid in the electrolyzer can be supplemented by a hydrochloric acid supplied to the electrolyzer in the concentration range from 8 to 36% by weight.

The oxygen-containing gas is preferably supplied in an amount such that oxygen is present in excess relative to the theoretically required amount. Particularly preferred is a 1.2 to 1.5-fold excess of oxygen.

The process according to the invention is carried out in an electrochemical cell (electrolysis cell) whose anode chamber is separated from the cathode chamber by a cation exchange membrane, the cathode chamber containing an oxygen-consuming cathode.

The electrolytic cell used may, for example, comprise the following components: an anode in an anode chamber, a cation exchange membrane which is hydrostatically pressed onto an oxygen-consuming cathode (SVK), which in turn is supported on a cathode-side current distributor and thus electrically contacted, and a cathode-side gas space (cathode chamber) ,

The aqueous solution of hydrogen chloride is introduced into the anode chamber, the oxygen-containing gas in the cathode chamber.

The choice of the oxygen-consuming cathode is not critical. The known and partly commercially available oxygen-consuming cathodes can be used. Preferably, however, oxygen-consuming cathodes are used which contain a catalyst of the platinum group, preferably platinum or rhodium.

Examples of suitable cation exchange membranes are those made of perfluoroethylene, which contain sulfonic acid groups as active centers. Both single-layer membranes having sulfonic acid groups of equal equivalent weights on both sides and membranes having sulfonic acid groups of different equivalent weights on both sides are suitable. Also membranes with carboxyl groups on the cathode side are conceivable.

Suitable anodes are, for example, titanium anodes, in particular with an acid-resistant, chlorine-developing coating.

The cathode-side power distributor can consist, for example, of expanded titanium metal or noble metal-coated titanium.

A suitable electrolytic cell for carrying out the method according to the invention is shown schematically in FIG Fig. 1 shown.

The electrolysis cell 1 is divided by a cation exchange membrane 6 into a cathode chamber 2 with an oxygen-consuming cathode 5 and an anode chamber 3 with an anode 4 . The oxygen-consuming cathode 5 is located on the cathode side on the cation exchange membrane 6 . Behind the Sauerstoffverzehrkathode 5 is a power distributor. 7 Due to the higher pressure in the anode chamber 3 , the cation exchange membrane 6 is pressed onto the oxygen-consuming cathode 5 and this in turn onto the current distributor 7 . In this way, the Sauerstoffverzehrkathode 5 is sufficiently electrically contacted and supplied with power. The adjustment of the pressure in the cathode chamber 2 and anode chamber 3 takes place respectively via a pressure maintenance 8. Via an HCl inlet 12 , an aqueous solution of hydrogen chloride is introduced into the anode chamber 3 , wherein formed at the anode 4 chlorine, which flows through the pressure maintenance 8 and discharged via the Cl ₂ outlet 13 from the anode chamber 3 becomes. Via an O ₂ inlet 9 , oxygen-containing gas is introduced into the cathode chamber 2 , where it reacts with the oxygen-consuming cathode 5 to form water with protons, which diffuse out of the anode chamber 3 into the oxygen-consuming cathode 5 . The water formed is removed together with the excess oxygen-containing gas via the pressure maintenance 8 from the cathode chamber 2 , wherein the water formed via a H ₂ O outlet 11 and the oxygen-containing gas via an O ₂ outlet 10 is removed. It is also possible that the oxygen supply is from below and / or that the removal of water formed and oxygen-containing gas is carried out separately via a separate pressure maintenance.

In the following examples, the inventive method is further explained, the examples are not to be understood as limiting the general inventive concept.

Example 1 (comparative example )

The electrolysis was carried out in an electrolytic cell 1 divided into a cathode chamber 2 and an anode chamber 3 as shown in FIG Fig. 1 shown schematically and explained in more detail above. An activated titanium anode with a size of 10 cm × 10 cm was used as anode 4 . The anode chamber 3 was supplied with an aqueous solution of hydrogen chloride. The temperature of the aqueous solution of hydrogen chloride was 60 ° C, the concentration 12-15 wt .-%. In the cathode chamber 2 was located as Sauerstoffverzehrkathode 5, a gas diffusion electrode of the company E-TEK, type ELAT, which was directly on a power distributor 7 in the form of an activated expanded titanium metal. Cathode chamber 2 and anode chamber 3 were separated from a cation exchange membrane 6 from DuPont, type Nafion® 324. In the cathode chamber 2 pure oxygen having a content of greater than 99 vol .-% was introduced at a temperature of 20 ° C.

The electrolysis was at a pressure in the anode chamber 3 of 1 , 4 bar, abs. and a pressure in the cathode chamber 2 of 1 bar, abs., A voltage of 1.67 V and a current density of 6000 A / m ² operated. The excess oxygen-containing gas was removed from the cathode chamber 2 together with the water formed. The concentration of hydrogen in this gas was determined by gas chromatography. The hydrogen concentration was 700 ppm after an electrolysis time of 10 minutes, increased steadily during the electrolysis and was 1600 ppm after an electrolysis time of 3 hours.

Example 2 (comparative example )

An electrolysis of an aqueous solution of hydrogen chloride was carried out as described in Example 1, the pressure in the anode chamber 3, however, 1.15 bar, abs. amounted to. The hydrogen concentration was after 10 minutes Electrolysis time 700 ppm, steadily increased during the electrolysis and was after 3 hours at 1600 ppm.

Example 3

An electrolysis of an aqueous solution of hydrogen chloride was carried out, as described in Example 1, wherein the pressure in the cathode chamber 2, however, 1.06 bar, abs. was at a current density of 6000 A / m ^2, a voltage of 1.62 V was established. The hydrogen concentration was 300 ppm and remained constant over the electrolysis period of several days.

Example 4 (Comparative Example )

An electrolysis of an aqueous solution of hydrogen chloride was carried out as described in Example 1. The pressure in the anode chamber 3 was 1.4 bar, abs., The pressure in the cathode chamber 2 1 bar, abs., The voltage 1.82 V and the current density 7000 A / m ² . Already after an electrolysis time of 3 minutes, a hydrogen concentration of 8000 ppm was measured.

Example 5

It was carried out an electrolysis of an aqueous solution of hydrogen chloride, as described in Example 4, wherein the pressure in the cathode chamber 2, however, 1.12 bar, abs. and at the selected current density of 7000 A / m ² a voltage of 1.74 V was established. The hydrogen concentration was 600 ppm and remained constant over the entire electrolysis period of several days.

Claims (4)

1. Process for the electrochemical preparation of chlorine from aqueous solutions of hydrogen chloride in an electrolysis cell, comprising at least one anode chamber and at least one cathode chamber, the anode chamber being separated from the cathode chamber by a cation exchange membrane, the anode chamber containing an anode and the cathode chamber an oxygen-consuming cathode, and the aqueous solution of hydrogen chloride being passed into the anode chamber and an oxygen-containing gas into the cathode chamber, characterized in that the pressure in the cathode chamber is at least 1.05 bar, the pressure in the anode chamber being 200 to 500 mbar higher than the pressure in the cathode chamber, and the process being operated at a current density of at least 5 000 A/m².
2. Process according to Claim 1, characterized in that the pressure in the cathode chamber is 1.05 to 1.5 bar.
3. Process according to either of Claims 1 and 2, characterized in that the oxygen-consuming cathode used contains a catalyst of the platinum group, preferably platinum or rhodium.
4. Process according to any of Claims 1 to 3, characterized in that a cation exchange membrane comprising perfluoroethylene and preferably containing sulphonic acid groups as active centres is used.

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