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 the purification criteria that a gas scrubber must achieve.
In a wide variety of (chemical) manufacturing processes, environmentally harmful or toxic gases are either starting materials or are produced as by-products.
Gas scrubbers or wet 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 purify process gases from undesirable gas components for further processing (for example: Benfield process gas scrubber).
The gases to be cleaned are converted into harmless components using a scrubbing liquid (e.g. caustic soda) so that they no longer pose a risk to the environment or the process. These are usually water and salts, which are easy to handle for further processing. Many gas scrubbers use caustic soda (NaOH) as the scrubbing liquid. In order to achieve optimum scrubbing results, precise monitoring of caustic soda is necessary. LiquiSonic® measuring systems are ideally suited for this monitoring.
Ensuring the complete conversion of toxic components while using scrubbing liquid efficiently often poses problems for process engineers. The effectiveness of a gas scrubber depends on the exact dosage of the scrubbing liquid (for example: caustic soda). Various measuring methods can be used to monitor the concentration, but these often provide inadequate results. There are several frequently used methods for monitoring the scrubbing liquid, but these have major weaknesses in practice:
Monitoring by means of pH value measurement
The service life of inline pH probes is very limited due to the aggressive measuring conditions and requires regular maintenance. In addition, the user is faced with the challenge of interpreting the pH result, as there is no selective concentration display for washing liquid and salts. There is therefore a risk that the measured pH value will lull the user into a false sense of security despite the concentration of caustic soda being too low. In addition, the varying concentration of the resulting salts remains unknown with this type of measurement. However, this is absolutely essential, particularly for the purpose of circling out the salts.
Conductivity for scrubbing liquid control
Inline conductivity meters have a similar problem: the physical quantity (conductivity) is influenced both by the scrubbing liquid itself and by the salts formed. It is not possible to consider both components separately. Sampling and laboratory measurements, such as time-consuming and cost-intensive titration, are usually necessary to determine the exact concentration. It is therefore very difficult to determine the exact concentration of the washing liquid.
Precisely determine the density and concentration of washing liquids inline
In order to determine the exact concentration of the washing liquid and the salts, two measurement variants must be combined. Only by using measuring devices that combine 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 that the speed of sound and conductivity react differently to a change in the concentration levels in the process liquid is utilized. This allows both concentrations to be precisely determined and the scrubbing process to be optimally adjusted.
Customer benefits in gas scrubber monitoring
When determining the concentration of scrubbing liquids, LiquiSonic® impresses with its robust sensor design, which makes wearing parts and maintenance superfluous. The measuring system is configured plug&play and impresses customers worldwide with its highly accurate measurement results and long process service life.
Precise determination of concentrations actively prevents underdosing and enables the fastest possible response to process disruptions. Accidents, such as the escape of chlorine gas, are prevented by the automatic and rapid dosing of, for example, caustic soda. 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 scrubber process (example: phosgene gas scrubber)
Phosgene is used as a starting material in the production of many plastics. In the event of an emergency, the excess gas is fed into emergency gas scrubbers, where it is neutralized with caustic soda and the salts sodium chloride (NaCl) and sodium carbonate (Na2CO3) are produced. To ensure that all the phosgene is absorbed, the concentration of caustic soda must be kept within the range of maximum absorption.
For this purpose, it is essential to monitor the concentrations of caustic soda and salt compounds. If the concentration of caustic soda is too low, the phosgene is no longer sufficiently absorbed and represents a safety-critical incident. Additional dosing is necessary.
If the salt concentration in the solution is too high, crystallization must be avoided. This requires precise inline measurement with real-time data.
Precise inline measurements as well as automatic re-dosing can be easily implemented with LiquiSonic®.
Installation of the measurement technology
The LiquiSonic® pipe and immersion sensors can be easily installed directly in the main line. An additional bypass is not necessary. A standard installation point is in the circulation circuit. The LiquiSonic® Controller 40 is connected to the LiquiSonic® sensor and the measuring unit for the second physical variable (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 gas scrubber | Phosgene gas scrubber