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Phosgene Gas Scrubbing

Two different monomers are linked for the production of polyurethanes: polyisocyanates and polyols. This process is also referred to as polyaddition. Thereby two most significant monomers are polyether polyol and the toluene-2,4-diisocyanate (TDI). The production process is divided into 3 parts: 

  • preparation of polyether polyol
  • production of toluene diisocyanate
  • polyaddition. 

The TDI is produced from toluylendiamin (TDA) by phosgenation. In emergency cases, the unreacted phosgene must be absorbed, because the gas is highly toxic. In the chemical industry, it is only used in hermetically closed circuits. The neutralization of the gas to sodium chloride (NaCl) and sodium carbonate (Na2CO3) takes place with sodium hydroxide (NaOH).

Sonic velocity measurement with LiquiSonic® 40


In a gas scrubber, the scrubbing liquid absorbs certain gaseous, liquid or solid components from the gas stream. The circulating scrubbing liquid must be kept in the range of the maximum absorption. In order to ensure an effective absorption of the phosgene in the gas scrubber, the concentration of NaOH, NaCl and of Na2CO3 must be monitored. 

If the NaOH content is under limit, it must be redosed. If the content of the salt is too high, salt is removed from the circulation, since it would otherwise crystallize in the system. 

The LiquiSonic® analyzer allows the concentration measurement in 3-components-liquids. Due to the parallel detection of two physical quantities such as sonic velocity and conductivity, two concentration values can be determined simultaneously: the scrubbing liquid and the salts NaCl and Na2CO3. This allows a better response to concentration fluctuations in the srubbing liquid.


The LiquiSonic® immersion sensor can easily be installed directly into pipelines. A typical installation point, combined with a conductivity meter, are DN 80 pipelines from the scrubber to the regeneration tank. 

The LiquiSonic® controller 40 is connected to the LiquiSonic® immersion sensor and device for the second physical value. The controller displays NaOH concentration and salt content.

Typical measuring range:
NaOH concentration range: 5 to 20 wt%,
Na2CO3/NaCl concentration range: 5 to 30 wt%
temperature range: 30 to 60 °C

Customer value

The robust sensor construction and the optional spe-cial materials, like HC2000, promote long process life. 

LiquiSonic®enables a reduction of labor cost through the elimination of manual process steps: 

  • time saving: 1 h per day

The prevention of an over- and underdosing leads to the reduction of material input for the scrubbing liquid.