Engineering Evaluation Case: Caustic Scrubber pH Control Oscillates and the Tower Alternates Between Under-Dosing and Over-Dosing
A packed scrubber can have correctly selected packing and still show unstable removal performance because the liquid-control loop is unstable.
One common example is pH control.
If chemical dosing responds too slowly or too aggressively, the recirculating liquid can oscillate between:
- insufficient reagent;
- excessive reagent.
This changes both absorption performance and liquid chemistry.
Project Situation
Consider an acid-gas scrubber using NaOH.
The system includes:
- recirculation sump;
- pH probe;
- dosing pump;
- packed bed.
Operators observe a repeating pattern:
- outlet contaminant rises;
- caustic dosing increases;
- pH overshoots;
- dosing stops;
- pH falls again.
The bed pressure drop remains normal.
The first instinct may be to increase packing height.
The root problem may be the control loop.
pH Is Not Instantaneously Uniform
The pH probe measures one location.
Fresh caustic may enter elsewhere.
If mixing in the sump is poor, the probe may see a delayed response.
The controller continues dosing because it does not yet detect the chemical addition.
By the time the caustic reaches the probe, the system has already been overdosed.
Large Dead Volume Increases Delay
The loop includes:
- sump;
- piping;
- pump;
- distributor;
- packed bed.
There is a finite circulation time.
The process response to dosing may therefore be slower than the dosing pump itself.
Control tuning should reflect the real system dynamics.
Under-Dosing Reduces Absorption Driving Force
When pH falls too far, the chemical absorption capacity may decline.
Even with:
- excellent packing;
- full wetting;
the gas may not be removed effectively if the liquid chemistry is exhausted.
This creates an outlet spike without any hydraulic abnormality.
Over-Dosing Can Create Other Problems
Excess reagent increases chemical cost.
Depending on the process, it may also increase:
- salt formation;
- ionic concentration.
If another species is present, high pH may encourage unwanted reactions or precipitation.
Therefore, “more caustic” is not automatically safer.
The Probe Itself Can Be the Problem
A pH electrode can develop:
- coating;
- slow response;
- calibration drift.
Dirty or crystallizing scrubber liquid is particularly challenging.
A contaminated sensor can make a stable process appear chemically deficient.
Probe maintenance should therefore be included in troubleshooting.
Sampling Location Matters
If the sensor is installed in a poorly mixed corner of the sump, it may not represent the liquid being pumped to the distributor.
A more representative measurement location may reduce control error.
Chemical Addition Point Matters
Injecting concentrated reagent too close to the pH probe can produce the opposite problem:
- sensor sees high pH immediately;
- bulk liquid remains under-treated.
The controller stops dosing too early.
Good mixing is therefore required between chemical addition and control measurement.
Packing Cannot Fix Control Oscillation
Adding bed height may temporarily improve removal margin.
But the tower will still alternate between strong and weak chemical driving force.
A control problem should be corrected as a control problem.
What Data Helps Diagnose the Issue?
Trend together:
- pH;
- dosing-pump output;
- outlet contaminant;
- liquid circulation;
- sump level;
- liquid temperature.
If outlet concentration follows the pH cycle, the connection becomes visible.
Salt Precipitation Can Become a Secondary Symptom
Repeated local overdosing can create zones of high reagent concentration.
If the system contains ions that can precipitate at high pH, deposits may appear near the chemical injection point or inside the loop.
The unstable chemistry can therefore eventually create a fouling problem.
Engineering Takeaway
Packed-tower performance depends on stable liquid chemistry as well as packing geometry.
A poorly tuned pH loop can make a correctly designed absorber appear undersized.