Gas scrubber monitoring

Measurement accuracy: ± 0.05%

Measuring range: 0 - max %

Temperaturmessbereich: 0 - 120 °C

High resistance

In a variety of industrial processes, environmentally harmful, corrosive, or toxic gases are used. To protect humans and the environment, these are subject to strict regulations, especially regarding the cleaning criteria a gas scrubber must achieve.

In various (chemical) manufacturing processes, environmentally harmful or toxic gases are either starting materials or they arise as by-products.

Worldwide, gas scrubbers or wet scrubbers are used for the conversion of such gases to either eliminate the risk to humans and the environment in an emergency (emergency gas scrubbers) or to clean process gases of unwanted gas components for further processing (for example: Benfield process gas scrubbers).

In this process, the gases to be cleaned are converted into harmless components with a washing liquid (e.g., caustic soda), so that they no longer pose a risk to the environment or the process. These are mostly water and salts, which are easy to handle for further processing. Many gas scrubbers use caustic soda (NaOH) as the washing liquid. To achieve an optimal washing result, precise monitoring of caustic soda is necessary. LiquiSonic^® Measurement systems are ideally suited for this monitoring.

Challenge in measuring the concentration or density of washing liquids

Achieve financial goals

Avoid financial penalties
Utilize resources efficiently
Avoid expensive processes

Protect employees

Avoid unnecessary, dangerous process steps
Avoid accidents, risks
Design process safely

Protect the environment

Avoid environmental damage
Reduce pollution
Legal regulations

Ensuring the complete implementation of toxic components while efficiently using washing liquid often presents challenges for process engineers. The effectiveness of a gas scrubber depends on the precise dosage of the washing liquid (for example: caustic soda). Various measurement methods can be used to monitor the concentration, but they often provide only inadequate results. There are some commonly used methods for monitoring the washing liquid, which in practice, however,have major weaknesses:

Monitoring by means of pH value measurement

The lifespan of inline pH probes is severely limited due to the aggressive measurement conditions, causing regular maintenance effort. Additionally, the user faces the challenge of interpreting the pH result, as there is no selective concentration display for washing liquid and salts. Thus, there is a risk that despite the low concentration of caustic soda, the measured pH value gives the user a false sense of security. Furthermore, the varying concentration of the resulting salts remains with this measurement variant.unknown. However, this is essential, especially for isolating the salts.

Conductivity for washing liquid control

Inline conductivity measuring devices have a similar problem: The physical quantity (conductivity) is influenced both by the washing liquid itself and by the resulting salts. There is no way to consider both components separately. To accurately determine the concentrations, sampling and laboratory measurements, such as a time-consuming and costly titration, are usually necessary. Thus, the exact concentration determination of the washing liquid is only possible with great difficulty.

Determine the density and concentration of washing liquids inline accurately

To enable an exact determination of the concentration of the washing liquid and the salts, two measurement variants must be combined. Only by using measuring devices that combine sound velocity and conductivity can multi-component mixtures such as caustic soda and sodium chloride be analyzed safely and accurately.

The physical principles are ideally combined and the effect is utilized that sound velocity and conductivity react differently to a change in the concentration levels in the process liquid. Thus, both concentrations can be determined exactly and the washing process can be optimally adjusted.

Customer benefits in gas scrubber monitoring

In the determination of concentrations of washing liquids, it impresses LiquiSonic^® with its robust sensor design, which makes wear parts and maintenance unnecessary. The measuring system is configured as plug&play and impresses customers worldwide with its highly accurate measurement results and long process uptime.

By accurately determining the concentrations, underdosages are actively avoided and process disturbances can be responded to as quickly as possible. Through automatic and rapid replenishment of, for example, caustic soda, accidents, such as the release of chlorine gas, are prevented. The extensive diagnostic tools and data documentation are important tools for HSE management.

Through inline measurements with LiquiSonic^® are sampling and time-consuming laboratory measurements replaced and material costs reduced to a minimum.

LiquiSonic® in the gas scrubber process (example: phosgene gas scrubber)

In the production of many plastics, phosgene serves as a starting material. In case of an emergency, the excess gas is directed into an emergency gas scrubber, where it is neutralized with caustic soda, resulting in the salts sodium chloride (NaCl) and sodium carbonate (Na₂CO₃) are produced. To ensure all phosgene is absorbed, the concentration of caustic soda must be maintained in the range of maximum absorption.

For this purpose, the concentrations of caustic soda and salt compounds must be strictly monitored. If the concentration of caustic soda is too low, phosgene is no longer sufficiently absorbed, posing a critical safety incident. Re-dosing is necessary.

In the case of a high salt concentration in the solution, crystallization must be avoided. This requires precise inline measurement with real-time data.

Precise inline measurements as well as automatic re-dosing are easily achievable with LiquiSonic^® feasible.

Installation of the measuring technology

The LiquiSonic^® pipe and immersion sensors can be easily installed directly in the main line. An additional bypass is not necessary. A common installation site is in the circulation loop. The LiquiSonic^® Controller 40 is connected with the LiquiSonic^® sensor and the measuring unit for the second physical quantity (conductivity). The real-time measurement values can be transmitted to the process control system via various interfaces such as Profibus DP or Modbus TCP.