Hydrochloric acid cycles
Concentrated aqueous hydrochloric acid is fed to the electroIyzer within a temperature range of 65 - 70°C. The process has hydrochloric acid recycles for anolyte and catholyte. The two compartments receive aqueous acid feeds of different concentrations, the anode compartment approx. 23% (wt.) and the cathode approx. 21% (wt.). Chlorine and hydrogen are generated inside the cell. Depleted hydrochloric acid with a concentration of 17% (wt.) leaves both compartments of the cell. The anolyte acid flows through the anode chambers connected in parallel and the catholyte acid flows similarly through the cathode chambers. Any solid impurities are retained by filters, while the heat balance of the process is maintained using heat exchangers. A partial stream of the 17% (wt.) acid feedstock is diverted from the catholyte recycle, re-concentrated with hydrogen chloride to approx. 28% (wt.) in the absorption unit, and then fed back to both electrolyte cycles.
The absorption unit operates as an adiabatic process. Here the depleted acid produced by electrolysis flows countercurrent to the rising hydrogen chloride gas, thereby raising the acid concentration to approx. 28% (wt.). The heat liberated by the process is dissipated by the vaporization of water within the column. Due to the relatively low partial pressure of hydrogen chloride, the gas is almost completely absorbed by the dilute solution of hydrochloric acid so that nearly pure water vapor is released at the top of the absorption column. The water vapor is then condensed in a cooler and returned to the column. Any organic impurities present in the hydrogen chloride are normally expelled with the vapors at the top of the column. If greater quantities of impurities are present in the feed gas, it is advisable to have these impurities partially removed upstream of the absorption section using appropriate means, such as scrubbing or cooling and condensation. The concentrated hydrochloric acid solution may also be treated by adsorption using activated carbon.
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.
Hydrogen is a valuable by-product of the process and can be supplied to hydrogen consumers, such as hydrogenation plants, once it has been cooled and scrubbed with caustic soda.
Benefits of thyssenkrupp Uhde Chlorine Engineers‘s diaphragm electrolysis technology
- Careful use of resources considering ecological responsibility.
- Diaphragm Electrolysis produces H2 and has no need of O2 (compared to HCl-ODC).
- Significant interest to make business independent from HCl & Cl2 market prices and caustic market situation.
- Risks of Chlorine transport avoided.
- No costs related to hydrochloric acid neutralization or disposal.
- Wide & flexible plant operating window eases adaptations to the needs of related production processes.
- Electrolyzer design ensures optimal maintenance flexibility and eases potential capacity increases.
- High robustness and durability under corrosive conditions.
- Contribution of the collaboration with Covestro in terms of lab and test facilities.
- Proven and reliable technology.