Chlor-alkali electrolysis

At the heart of the modern chemical industry is chlor-alkali electrolysis, an indispensable process through which essential raw materials for various applications are obtained. This process technology is important not least due to the efficient production of sodium ions (Na+), chloride ions (Cl-) and hydroxide ions (OH-), which are indispensable as critical starting materials for the production of plastics, pharmaceuticals and in the textile industry.

In chlor-alkali electrolysis, an electrical DC voltage is applied to separate sodium chloride solution into elemental chlorine and sodium hydroxide; hydrogen is produced at the same time. Specialists attach particular importance to the functioning of these technically sophisticated electrolysis cells - specially designed to enable the transport of ions and at the same time prevent undesirable reactions between the products - because the efficiency and safety of the entire process depend to a large extent on precise control mechanisms and the stability of the membrane technologies used.

LiquiSonic^® measuring systems in chlor-alkali electrolysis

LiquiSonic^® measuring technology can be used advantageously in the various process stages of chlor-alkali electrolysis. The main customer benefit is the reduction in raw material and energy consumption and the increase in yield.

How does chlor-alkali electrolysis work?

Chlor-alkali electrolysis is an important technical process that is used to produce basic chemicals such as chlorine, hydrogen and caustic soda (sodium hydroxide). An aqueous solution of sodium chloride (salt) is used as the electrolyte. An electrical voltage is applied to the electrodes, which are made of special materials. This process oxidizes the chloride ions at the anode to chlorine, while water is reduced to hydrogen and hydroxide ions at the cathode. These hydroxide ions react with the sodium ions in the solution to form caustic soda. Chlor-alkali electrolysis is a very efficient process used in many industries as it is fast, reliable and cost-effective, and provides essential chemicals for various industrial applications

Using an electric current, the salt (NaCl) is broken down into chlorine (Cl₂), caustic soda (NaOH) and hydrogen (H₂).

What processes are used in chlor-alkali electrolysis?

Two main processes are used: the diaphragm process and the membrane process.

The same electrochemical reaction takes place in both processes: The NaCl flows into the anode compartment of the cell, where Cl₂ is deposited as chlorine gas. The solution then continues into the cathode chamber, where H₂ and NaOH are formed.

Diaphragm process explained:

In the diaphragm process, a porous diaphragm (separating wall) is used between the anode and cathode. It enables ion exchange, but prevents the mixing of chlorine and the sodium hydroxide solution. A salt solution is used as the electrolyte and chlorine is released at the anode, while hydrogen and sodium hydroxide are produced at the cathode. However, the quality of the sodium hydroxide is lower with this process than with other methods.

Membrane process explained:

This process uses a special ion-permeable membrane that blocks chlorine ions but allows sodium ions to pass through. This leads to the formation of chlorine at the anode and sodium hydroxide and hydrogen at the cathode.

The membrane and the diaphragm represent a high cost factor within both processes. The LiquiSonic^® measuring technology is used to precisely determine the concentration of the catholyte in order to identify and counteract any inefficiencies in the electrolyser. This ensures an optimum service life for the membrane.

Depending on the method used, the catholyte is either a NaOH solution (membrane method) or a NaOH-NaCl solution (diaphragm method). The concentration of the 3-component mixture is measured using a LiquiSonic^® 40 measuring system, in which the ultrasonic sensor is combined with a conductivity sensor.

Your advantage:

• Maximizing the efficiency of the electrolyser by continuously recording the concentrations in the process

• Energy savings and consumption optimization

• Reduction of time-consuming comparative analyses

• Increased service life of the membrane

Concentration of caustic soda

Chlor-alkali electrolysis is a process in which sodium chloride (common salt) is converted into chlorine, hydrogen and caustic soda (sodium hydroxide) under the influence of electrical energy. During this process, sodium ions (Na+) migrate to the cathode, which is negatively charged, and chloride ions (Cl-) migrate to the anode, which is positively charged. Oxidation of chloride ions takes place at the anode, releasing chlorine. At the cathode, water is reduced to hydrogen and hydroxide ions. These hydroxide ions react with the sodium ions to form caustic soda. There are different variants of this process, such as the amalgam process, in which a sodium amalgam is formed at the cathode, which is then further processed in a separate stage to form caustic soda, hydrogen and mercury. Regardless of the process used, the caustic soda obtained is often concentrated by evaporation in order to achieve a higher concentration.

Saleable caustic soda (NaOH) usually has a concentration of between 45 m% and 50 m%. As the NaOH taken from the electrolysis cells only has a concentration range between 12 m% and 33 m%, it is concentrated in multi-body evaporators.

If the solution contains NaCl as well as NaOH (diaphragm process), the excess salt in the caustic solution precipitates as crystals in the evaporator during evaporation. This achieves a NaOH concentration of between 45 m% and 50 m%.

The LiquiSonic^® measuring technology continuously determines the concentration of the caustic solution after the evaporator at all times. Subsequent dilution of the caustic soda solution to a customer-specific product concentration can also be monitored.

Your advantage:

• Continuous monitoring of the caustic soda concentration

• Reduction of energy costs during evaporation

Chlorine gas drying

The drying of chlorine gas is an essential step in the production of chlorine. This process involves the removal of moisture from the chlorine gas to make it suitable for industrial applications. Drying is carried out by physical methods such as cooling and condensing the gas or by using drying agents such as concentrated sulphuric acid or molecular sieves. These techniques ensure that the chlorine is in a pure and dry form. Although chlorine gas drying is a technically demanding process, it plays a crucial role in many industries as dried chlorine gas is used for a variety of applications, from water treatment to the manufacture of plastics and pharmaceuticals.

The chlorine gas produced in the anode area of the electrolyzer must be freed of its water content before it can be used again, as its corrosiveness increases with a moisture content of over 30 ppm. For drying, the chlorine gas is fed into absorption towers in which the water content in the chlorine gas is absorbed by highly concentrated sulphuric acid (80-99 m% H₂SO₄).

The effectiveness of this drying process significantly influences the productivity and quality of the gas. Reliable measurement of the H₂SO₄ concentration is therefore important. Compared to conductivity and density measurement, the LiquiSonic^® measuring system enables continuous and reliable monitoring of the H₂SO₄ concentration.

Your advantage:

• Elimination of time-consuming sampling

• Continuous monitoring of the H₂SO₄ concentration

• Clear signal for determining the concentration of H₂SO₄ between 80 m% and 100 m%

• Corrosion prevention through effective drying

Hydrochloric acid production

The chlorine gas produced at the anode of the electrolyser and the hydrogen supplied form the starting materials for the synthesis of hydrochloric acid. Both gases are fed into a burner where they react to form hydrogen chloride. The HCl gas formed then flows from the combustion chamber into the integrated isothermal falling film absorber. Here the gas is absorbed with the aid of water or weak acid, forming concentrated hydrochloric acid (37 m% HCl).

The hydrochloric acid concentration is continuously monitored using LiquiSonic^® measuring technology. This makes it possible to detect deviations from the target concentration and react accordingly.

Your advantage:

• Continuous monitoring of the hydrochloric acid concentration (20-40 m% HCl)

• Guarantee of a highly accurate target concentration