Engineering Evaluation Case: A Water Decarbonator Re-Inhales Its Own CO₂-Rich Exhaust
A packed decarbonator removes dissolved CO₂ by contacting water with air.
The process relies on a low CO₂ partial pressure in the entering gas.
If the blower inlet draws air from near the tower exhaust, the system can recycle part of the CO₂ it just removed.
The packing may be perfectly sized while the driving force collapses.
Project Situation
Consider a packed water decarbonator with:
- forced or induced airflow;
- plastic random packing;
- outdoor air intake.
The tower achieves lower-than-expected CO₂ removal.
Mechanical inspection finds:
- clean packing;
- normal water flow;
- correct fan speed.
The air inlet is located near the CO₂-rich exhaust discharge.
Stripping Depends on Gas-Side CO₂ Concentration
Fresh ambient air provides a relatively low CO₂ concentration compared with the gas leaving the tower.
If outlet gas returns to the intake:
- inlet CO₂ increases;
- mass-transfer driving force decreases.
More Airflow Can Make Recirculation Worse
Increasing fan speed may enlarge the exhaust plume and strengthen local recirculation depending on:
- wind;
- stack geometry;
- intake position.
Therefore, more airflow does not automatically solve poor stripping.
Weather Can Change Performance
Wind direction and speed affect:
- plume dispersion;
- intake contamination.
A tower may perform well under one wind condition and poorly under another.
This creates an apparently random seasonal or daily efficiency problem.
Gas Sampling Can Prove the Mechanism
Measure CO₂ concentration at:
- ambient reference point;
- fan intake;
- tower exhaust.
If intake CO₂ rises when the tower operates, recirculation should be investigated.
Packing Height Cannot Fix the Boundary Condition
More packing helps only when a useful gas-liquid driving force exists.
If the stripping air already contains elevated CO₂, extra surface provides diminishing benefit.
Engineering Takeaway
A stripping tower can recycle the contaminant it just removed if its air intake and exhaust are poorly separated.