Gas scrubber monitoring

Monitor concentration of scrubbing liquid and salts simultaneously

Environmentally harmful, corrosive or toxic gases are used in a large number of industrial processes. To protect people and the environment, these are subject to strict regulations, in particular which purification criteria a gas scrubber must achieve.

In a wide variety of (chemical) manufacturing processes, environmentally harmful or toxic gases are either starting materials or they are produced as by-products.

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

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

Challenge of measuring the concentration or density of scrubbing liquids

Achieve financial objectives

  • Use resources efficiently
  • Avoid financial penalties
  • Avoid costly processes

Protect employees

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

Protecting the environment

  • Prevent environmental damage
  • Legal regulations
  • Reduce pollution

Ensuring complete conversion of toxic components while using the scrubbing liquid efficiently is often a challenge for process engineers. The effectiveness of a gas scrubber depends on the exact dosage of the scrubbing liquid (e.g. caustic soda). Various measurement methods can be used to monitor the concentration, but they often provide inadequate results. For the control of the scrubbing liquid there are some methods, often used, which, however, show great weaknesses in practice:

Control by pH measurement

The service life of inline pH probes is very limited due to the aggressive measuring conditions and leads to regular maintenance. In addition, the user faces the challenge of interpreting the pH result, as there is no selective concentration indication for the rinsing liquid and salts. Therefore, there is a risk that the measured pH value lulls the user into a sense of security even though the caustic concentration is too low. In addition, with this measurement variant the varying concentration of the salts formed is unknown. However, this is absolutely necessary, especially for the delimitation of the salts.

Conductivity for rinsing liquid control

Inline conductivity meters have a similar problem: the physical quantity (conductivity) is influenced by both the rinsing liquid itself and the salts formed. There is no possibility to consider both components separately. To determine the exact concentrations, it is usually necessary to take samples and perform laboratory measurements, e.g. a time-consuming and cost-intensive titration. Therefore, the exact determination of the concentration of the washing liquid is only possible with great difficulty.

Typical measurement methods in comparison

Precisely determine the density and concentration of scrubbing liquids inline

To enable an exact determination of the concentration of the scrubbing liquid and the salts, two measuring variants must be combined. Only when using measuring instruments that combine the sonic velocity and conductivity can multi-component mixtures such as caustic soda and sodium chloride be analyzed reliably and accurately.

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

Customer benefits in gas scrubber monitoring

For the concentration determination of scrubbing liquids, LiquiSonic® convinces with its robust sensor design, which makes wear parts and maintenance unnecessary. The measuring system is plug&play configured and convinces customers worldwide with its highly accurate measurement results combined with a long process lifetime.

Precise determination of concentrations actively prevents under-dosing and allows the fastest possible response to process disturbances. Accidents, such as the escape of chlorine gas, are prevented by the automatic and rapid dosing of sodium hydroxide solution, for example. The comprehensive diagnostic tools and data documentation are important tools for HSE management.

Inline measurements with LiquiSonic® replace sampling and time-consuming laboratory measurements and reduce material costs to a minimum.

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

In the production of many plastics, phosgene serves as a starting material. In the event of an emergency, the excess gas is fed into emergency scrubbers, where it is neutralized with sodium hydroxide solution to produce the salts sodium chloride (NaCl) and sodium carbonate (Na2CO3). In order for all the phosgene to be absorbed, the concentration of caustic soda must be kept within the range of greatest absorption.

To this end, it is imperative that the concentrations of sodium hydroxide and salt compounds be monitored. If the concentration of caustic soda is too low, the phosgene is no longer sufficiently absorbed and represents a safety-critical incident. A re-dosing is necessary.

In case of too 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 an automatic post-dosing can be easily implemented with LiquiSonic®.

Installation of the measurement technology

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

Typical measuring range:

  • Concentration range NaOH: 5 to 20 m%
  • Concentration range Na2CO3: 5 to 15 m%
  • Temperature range: 30 to 60 °C

Special gas scrubbers in detail

Benfield Process gas scrubber | MDEA Natural gas scrubber | Emergency vent scrubber | Phosgene gas scrubber


  • 18 May 2021 - Gas Scrubber Process Monitoring (Europe & America)
  • 20 May 2021 - Gas Scrubber Process Monitoring (Europe & Asia)

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Other areas of application for LiquiSonic®:

Flue gas scrubbers | Gas drying | Phosgene absorbers | Sulfur dioxide scrubbers | Ammonia plants | Synthesis gas plants | Natural gas plants | Chlorine gas scrubbers


High-precision measuring devices for monitoring the scrubbing liquid:

Download gas scrubber monitoring

Oil dry fractionation
Application report - Oil dry fractionation

Oil dry fractionation

Caustic Soda
Fact Sheet for Caustic Soda Applications

Caustic Soda

Application emergency vent scrubber
Concentration measurement with LiquiSonic® 40

Application emergency vent scrubber

Chemical Industry
The sonic velocity analyzer LiquiSonic® has a high variety of applications in the chemical and petrochemical industry.

Chemical Industry