Chlor-alkali electrolysis is an industrial process for the simultaneous production of chlorine, caustic soda, and hydrogen from table salt and water.
Chlor-alkali electrolysis is an industrial process for the simultaneous production of chlorine, caustic soda, and hydrogen from table salt and water. With European chlorine production totaling 7,289 kilotons in 2023, it is a cornerstone of the chemical industry. Germany dominates EU production; Dow Chemical’s Stade/Schkopau site alone accounts for 20% of European production.
Development began in the late 19th century with the diaphragm process (1885) and the amalgam process (1890s). In the 1970s, the membrane process was introduced in response to environmental concerns. The latest innovation is the oxygen-consumption cathode (SVK), which emerged around the turn of the millennium.
Significance for the hydrogen economy
As a byproduct, approximately 28 kg of high-purity hydrogen is produced per metric ton of chlorine. Given current global production levels, this amounts to a significant quantity of hydrogen. However, with the introduction of the oxygen-consumption cathode (SVK), the technology is now evolving in a direction where hydrogen is no longer produced as a byproduct.
Important Variations in the Procedure
- The membrane process is now standard and uses special ion-exchange membranes made of polytetrafluoroethylene (PTFE, also known as Teflon). It is the most environmentally friendly option and produces high-purity 35% sodium hydroxide solution.
- The amalgam process, which uses mercury as the cathode, also produces very pure products. However, it will be phased out worldwide by 2025 due to environmental risks.
- The older diaphragm method will also be phased out by 2025; it uses a porous partition, which, however, is often made of asbestos, a substance that is hazardous to health.
- The oxygen-consumption cathode could reduce power consumption from 3V to 2V per cell using a special RhxSy catalyst, but it no longer produces hydrogen as a byproduct
Energy Requirements and Environmental Considerations
Chlor-alkali electrolysis requires 454 kJ per mole in the form of electrical current—which is roughly equivalent to the energy needed to bring 5 liters of water from room temperature to a boil. This makes chlor-alkali electrolysis very energy-intensive. By comparison, water electrolysis requires 286 kJ/mol.
The SVK technology introduced in 2013 by Bayer MaterialScience (now Covestro) and ThyssenKrupp Uhde reduces energy consumption by 30 percent. If implemented nationwide in Germany, this could save 1 percent of the country’s electricity consumption—comparable to the annual consumption of the city of Cologne. In addition, CO2 emissions from electricity generation are reduced by one-third.
Future Prospects
With European chlorine production totaling 7,289 metric metric tons in 2023, chlor-alkali electrolysis remains one of the most important basic processes in the chemical industry, even though production declined by 11% compared to the previous year. The industry is currently facing a low capacity utilization rate of 62.2% (compared to 69.8% in 2022)—a consequence of high energy prices and weak demand.
At the same time, the industry is moving toward more energy-efficient and environmentally friendly technologies. Energy efficiency is increasingly becoming a priority, even if this sometimes comes at the expense of hydrogen production. Some electrolyzer manufacturers are now applying their expertise in chlor-alkali electrolysis to the development of alkaline water electrolyzers. Examples of this technology transfer can be found at Thyssenkrupp Nucera and Asahi Kasei.
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