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ODC Process

The oxgen depolarized cathode for HCl electrolysis

ODC Process


The anode chamber is fed with 14% (wt.) hydrochloric acid solution. Anodic oxidation takes place and chlorine is formed while hydrochloric acid is consumed. At the outlet of the anode chamber the depleted hydrochloric acid is recovered together with chlorine in the electrolyzer header. The generated chlorine is separated from the depleted aqueous hydrochloric acid solution and collected in the chlorine header. The hydrochloric acid outlet solution is then routed to the anolyte tank. A certain portion of water as well as H+ and Cl- ions are transferred through the membrane to the cathode compartment.

The cathode chamber is fed with oxygen. At the ODC the oxygen is cathodically reduced and reacts with H+ ions coming from the anode chamber, and water is subsequently formed.

The electrolyzer is operated with an overpressure of 200 mbarg on the anode side to ensure optimum contact between the membrane and the ODC. The pressure in the anode and cathode compartments is automatically controlled. In the event of overpressure in the anode chamber, wet chlorine gas is vented to a waste gas system. In addition, automatic cathode pressure control takes place and oxygen gas can be fed to a waste gas system in case of overpressure.

Anolyte circulation

The depleted hydrochloric acid leaving the anode chamber flows to the anolyte tank. From there it is pumped back to the electrolyzer; a small portion of this stream is sent back to the hydrochloric acid absorption unit. The concentration of the hydrochloric acid fed to the electrolyzer is automatically controlled by mixing 37% (wt.) hydrochloric acid from the hydrochloric acid absorption unit with the depleted circulating hydrochloric acid. Heat exchangers for cooling and heating ensure the correct temperature of the fed hydrochloric acid during all stages of operation.

Catholyte system

An optimized small portion of the oxygen gas leaving the electrolyzer cathode chamber is purged in order to avoid the accumulation of nitrogen and other trace pollutants in the oxygen cycle. The acidic condensate generated at the cathode can be sent to a waste water treatment system or recycled back to the anolyte system.


The purpose of this process is to condition the anhydrous hydrogen chloride gas. In line with the concept, either demineralized water or 12.5% (wt.) hydrochloric acid is used as an absorbent to produce 37% (wt.) hydrochloric acid at 30-35°C.

Anhydrous hydrogen chloride gas is guided through a falling-film column where the temperature is controlled by dissipating the absorption heat. The heat transfer within the absorber pipes is controlled by the thickness of the liquid film. Minimum penetration is ensured by constant liquid circulation within the 25 to 100% range. The raw acid of the absorber is therefore stored in a buffer tank and a partial flow is fed to the absorber by circulation pumps. As organic compounds and chlorine need to be taken into account here, a share of the circulated fluid is treated in a stripper.

The 37% (wt.) hydrochloric acid, which is generated in the sump, is continuously removed and stored after filtration, including e.g. active carbon filters. The stripper waste gas contains the majority of the organic compounds and chlorine. It then needs to be treated, for example in a scrubber.

Product treatment

Once the wet chlorine gas has been cooled and filtered, it is either fed directly to the consumer plant or dried and compressed, before either being directly routed back to the chlorine consumer to close the chlorine recycle loop, or liquefied for storage in tanks.

As the quality of the chlorine is very high, the chlorine can be sent directly to downstream consumers, i.e. no chlorine liquefaction and evaporation is required.