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On-line Monitoring of Caustic Scrubbers

AppliTek’s proprietary on-line analyzer system EZ-Caustic® was developed to monitor and control the scrubber performance by determination of all critical alkali process parameters. EZ-Caustic® samples automatically the outlet and monitors continuously levels of sodium hydroxide (NaOH), sodium carbonate (Na2CO3) and sodium sulphide (Na2S). The simultaneous measurement of all critical parameters allows a superior control of the scrubber’s operation and specific absorption rates, resulting in considerable savings and a fast return on investment (ROI) by:

  • Optimization of the NaOH process setpoint
  • Significant lowering of NaOH consumption
  • Significant decrease in the treatment load of the spent caustic

The analyzer system has been field-tested for more than 20 years in an ethylene plant, in which the process set point could be lowered from 3% towards typically 0.5%wt in the intermediate and weak caustic stages. For an estimate of the savings that could be achieved in your plant, contact us to calculate the projected payback time.

The EZ-Caustic® on-line analyzer system is typically installed in an AnaShell® double compartment protective SS316L shelter. An ex-proof sampling and preconditioning system specifically built for this application allows to automatically sample the outlet, compensating for the extreme sample conditions and the presence of any polymeric constituents.


The caustic wash tower or “wet scrubber” is a large-scale treatment unit that performs a continuous wash by spraying an incoming gas stream with a caustic absorption liquid. Caustic scrubbing is used in refineries to purify straight-run light hydrocarbons, middle distillates and feed streams to isomerization and polymerization units. Another important field of application is in olefin (ethylene) production where thermal cracking processes inevitably introduce a number of byproducts including acid gases hydrogen sulphide (H2S) and carbon dioxide (CO2) that need to be removed. In today’s competitive markets, cutting down on considerable consumption of a commodity chemical as sodium hydroxide is one way to enhance productivity and lower costs in the operation of caustic scrubbers.

Best Available Technology (BAT) for monitoring and control of caustic wash towers

On-line, automatic titration of all critical process parameters: NaOH, Na2CO3 and Na2S

Incorporated industrial panel PC with AppliTek controller software

Proprietary sampling-preconditioning system with fail-safe design, degassing and liquid recovery

Projected payback time: < 3 months (depending on current NaOH consumption)

Protection class (shelter): zone 2 | Group II | category 2 | Equipment Group IIC | Temp. Class T4 | Equipment Protection Level Gc conform ATEX

Analysis method

Discontinuous titration with self-finding inflection points using single pH electrode and double junction reference electrode

Measuring ranges

NaOH: 0.05 – 70 g/L

Na2CO3: 0.05 – 70 g/L

Na2S: 0.05 – 70 g/L

Analysis time

10 minutes

Response time

15 minutes (including settling and degassing)

Product type: Process analyzers

Measuring technology: Titration

Type of application process: Caustic scrubber, Wet scrubber control

Industries: Petrochemical, Refining