At the heart of the modern chemical industry is the chlor-alkali electrolysis, an indispensable process by which essential raw materials are obtained for various applications. This process technology gains significance not least through the efficient production of sodium ions (Na+), chloride ions (Cl−), and hydroxide ions (OH−), which are critical raw materials for the production of plastics, pharmaceuticals, and are indispensable in the textile industry.
By applying a direct electric voltage, a separating conversion of sodium chloride solution into elemental chlorine and sodium hydroxide is carried out in chlor-alkali electrolysis; at the same time, hydrogen is produced. The functionality 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 — is particularly valued in professional circles; because the efficiency and safety of the entire processdepend significantly on precise control mechanisms and the stability of the membrane technologies used.
LiquiSonic® Measuring systems in chlor-alkali electrolysis
The LiquiSonic® Measuring technology can be advantageously used in the various process stages of chlor-alkali electrolysis. The main customer benefit lies in the reduction of raw material and energy consumption as well as in the increase of yield.
LiquiSonic® System
LiquiSonic® is available in three system variants:
LiquiSonic® 20, LiquiSonic® 30 and LiquiSonic® 40.
LiquiSonic® 30 is a high-performance system consisting of a controller with connections for up to four sensors. The sensors can be used at different measuring points.
LiquiSonic® 20 is a variant with reduced functionality and connection for one sensor.
LiquiSonic® 40 enables the simultaneous determination of two concentrations in a mixture. For this, a second physical measurement variable is combined with the speed of sound. In chlor-alkali electrolysis processes, the LiquiSonic® 40 system usually contains a conductivity sensor as the second physical variable.
Measuring principle
The LiquiSonic® Measuring technology analyzes liquid parameters such as concentration or density, detects phase transitions, and serves reaction monitoring.
The measuring principle is based on determining the speed of sound in liquids. The distance (d) between the ultrasonic transmitter and receiver is constant by design, so the speed of sound (v) can be calculated by measuring the travel time (t) (v = d / t). Since the speed of sound depends on the concentration of the substance, there is a functional relationship through which the concentration can be calculated.
The speed of sound measurement is independent of the transparency of the liquid and impresses with high measurement accuracy, reproducibility, and stability. In addition to the speed of sound measurement, the LiquiSonic® sensor integrates a highly precise and fast temperature measurement for temperature compensation. For many applications, this offers significant advantages over conventional measurement methods.
sensor
The LiquiSonic® sensor continuously measures both the concentration and the temperature within the predefined range. The process data is updated every second.
The liquid-contacting sensor component is made of stainless steel or corrosion-resistant material such as Hastelloy C-2000, or coated with Halar or PFA.
Various additional functions integrated into the sensor, such as the flow monitor (Flow / Stop) or the wet-dry monitoring (full / empty pipeline), complement the process control.
The special LiquiSonic® high-performance technology ensures stable measurement results even in the presence of gas bubbles or strong signal attenuation by the process fluid.
Chloralkali electrolysis
How does a chloralkali electrolysis work?
Chloralkali electrolysis is an important technical process used for the production of basic chemicals such as chlorine, hydrogen, and caustic soda (sodium hydroxide). An aqueous solution of sodium chloride (salt) is used as the electrolyte. An electric voltage is applied to the electrodes, which are made of special materials. In this process, the chloride ions are oxidized to chlorine at the anode, while water is reduced to hydrogen and hydroxide ions at the cathode.Hydroxide ions react with sodium ions in the solution to form caustic soda. Chloralkali electrolysis is a very efficient process used in many industries because it is fast, reliable, and cost-effective, providing essential chemicals for various industrial applications.
With the help of electric current, the salt (NaCl) into chlorine (Cl2), Caustic Soda (NaOH) and hydrogen (H2) are decomposed.
What processes are there in chlor-alkali electrolysis?
Two main processes are used: the diaphragm process and the membrane process.
In both processes, the same electrochemical reaction occurs: The NaCl flows into the anode compartment of the cell, where Cl2 is deposited as chlorine gas. The solution then moves into the cathode compartment, where H2 and NaOH is formed.
Diaphragm process explained:
In the diaphragm process, a porous diaphragm (partition) is used between the anode and cathode. It allows ion exchange but prevents mixing of chlorine and 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 sodium hydroxide is lower in this process compared to other methods.
Membrane process explained:
This process uses a special ion-permeable membrane that blocks chlorine ions but allows sodium ions to pass. This results in the formation of chlorine at the anode and sodium hydroxide and hydrogen at the cathode.
The membrane and diaphragm represent a high cost factor in both processes. The LiquiSonic® Measuring technology is used for precise concentration determination of the catholyte, to identify and counteract any inefficiencies of the electrolyzer. This ensures optimal membrane lifespan.
Depending on the process used, the catholyte is either a NaOH solution (membrane process) or a NaOH-NaCl solution (diaphragm process). The concentration measurement of the 3-component mixture is carried out using a LiquiSonic® 40 measurement system implemented by combining an ultrasonic sensor with a conductivity sensor.
Your advantage:
- Maximization of the efficiency of the electrolyzer through continuous monitoring of concentrations in the process
- Energy savings and consumption optimization
- Reduction of complex comparative analyses
- Increase in membrane lifespan
Processing of end products
Caustic soda concentration
The chlor-alkali electrolysis is a process in which sodium chloride (table salt) is converted into chlorine, hydrogen, and caustic soda (sodium hydroxide) under the influence of electrical energy. During this process, Sodium ions (Na+) to the cathode, which is negatively charged, and Chloride ions (Cl-) to the anode, which is positively charged. At the anode, the oxidation of chloride ions takes place, releasing chlorine. At the cathode, water reduces to hydrogen and hydroxide ions. These hydroxide ions react with sodium ions to form caustic soda. There are various variations of this process, such as the amalgam process, in which a sodium amalgam is formed at the cathode, which is then further processed into caustic soda, hydrogen, and mercury in a separate stage. Regardless of the process used, theresulting caustic soda is often concentrated by evaporation to achieve a higher concentration.
Marketable Caustic Soda (NaOH) It usually has a concentration between 45 wt% and 50 wt%. Since the NaOH taken from the electrolysis cells only has a concentration range between 12 wt% and 33 wt%, it is concentrated in multi-effect evaporators.
Is next to NaOH also NaCl present in the solution (diaphragm process), the excess salt crystallizes out in the evaporator when the lye is evaporated. This achieves a NaOH concentration between 45 wt% and 50 wt%.
The LiquiSonic® Measuring technology continuously determines the concentration of the lye after the evaporator at any time. A subsequent dilution of the caustic soda to a customer-specific product concentration can also be monitored.
Your advantage:
- Continuous concentration monitoring of the caustic soda
- 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 removing moisture from the chlorine gas to make it suitable for industrial applications. The drying is done through physical methods such as cooling and condensing the gas or using drying agents like concentrated sulfuric acid or molecular sieves. These techniques ensure that the chlorine is in a pure and dry form. Although chlorine gas drying is a technicallydemanding process, it plays a crucial role in many industries because dried chlorine gas is used for a variety of applications, from water treatment to the production of plastics and pharmaceuticals.
The chlorine gas produced in the anode area of the electrolyzer must be freed from its water content before further use, as its corrosiveness increases with a moisture content above 30 ppm. For drying, the chlorine gas is directed into absorption towers, where the water content in the chlorine gas is absorbed by highly concentrated sulfuric acid (80-99 wt% H2SO4) is absorbed.
The effectiveness of this drying process significantly influences the productivity and quality of the gas. Therefore, reliable measurement of the H2SO4 -concentration is important. The LiquiSonic measurement system® enables continuous and safe monitoring of the H2SO4 -concentration compared to conductivity and density measurement.
Your advantage:
- Elimination of complex sampling
- Continuous monitoring of the H2SO4 -concentration
- Clear signal for concentration determination of H2SO4 between 80 wt% and 100 wt%
- Corrosion prevention through effective drying
Hydrochloric acid production
The chlorine gas produced at the anode of the electrolyzer and the supplied hydrogen form the starting materials for the synthesis of hydrochloric acid. For this purpose, both gases are fed into a burner and react there to form hydrogen chloride. Subsequently, the formed HCl gas flows from the combustion chamber into the integrated isothermal falling film absorber. Here, the gas is absorbed with the help of water or weak acid, resulting in concentrated hydrochloric acid (37 wt% HCl) is formed.
With the help of LiquiSonic® measurement technology, continuous monitoring of the hydrochloric acid concentration is carried out. This makes it possible to detect deviations from the target concentration and react accordingly.
Your advantage:
- Continuous concentration monitoring of hydrochloric acid (20-40 wt% HCl)
- Ensuring a highly accurate target concentration
Lösestation & Solere into igung
The starting product sodium chloride (NaCl) is obtained either by evaporating seawater, mining, or leaching salt deposits (caverns). The raw brine contains impurities and calcium or magnesium salts, which can clog the fine pores of the diaphragm or membrane during electrolysis and thus significantly reduce their lifespan. For this reason, these impurities are precipitated in agitator tanks (dissolution vessels) by adding caustic soda Sodium hydroxide aus. Nach der Fällung werden die Verunre into igungen mit Hilfe e into es Druckfilters abgetrennt.
The purity of the brine concentration is of particular importance for the subsequent electrolysis. The LiquiSonic® Measurement system ensures a highly precise determination of the brine concentration at any time. Installation is carried out in the dissolving station when using salts mined underground or at the transfer point from the brine supplier in cavern production.
Your advantage:
- Avoidance of quality drops in brine purification
- Increase in membrane lifespan
- Incoming goods inspection (in cavern production)
- Reduction of water or steam consumption (when dissolving the salt)
- Reduction of electrical energy
LiquiSonic® is an inline analysis system that determines the concentration or density of liquids or media directly in the process without delay. The device is based on the highly precise measurement of absolute sound velocity and process temperature, thus allowing the detection of phase differences.