Engineering Evaluation Case: An Existing Water Scrubber Is Converted to Reactive Caustic Service Without Changing the Packed Tower
Plants often try to reuse existing equipment when process requirements change.
An existing water scrubber may be asked to handle a stronger acid-gas duty by changing the circulating liquid from water to NaOH solution.
The tower diameter and packing remain unchanged.
At first, this looks like a simple chemical upgrade.
In reality, changing the absorbent can change:
- chemistry;
- heat release;
- liquid properties;
- material exposure;
- salt formation.
The existing tower should be re-rated before the conversion.
Project Situation
Consider a packed scrubber originally designed for water washing.
The plant now needs greater removal of an acidic gas.
Instead of replacing the tower, it proposes:
“Keep the same packing and just add caustic.”
This may be practical.
But the original equipment was not necessarily designed for the new liquid chemistry.
Reactive Absorption Can Improve Mass Transfer
Adding caustic can increase the absorption driving force for suitable acidic contaminants.
The chemical reaction removes absorbed gas from the liquid phase.
This can substantially improve removal compared with water alone.
However, the tower may now be handling a more aggressive process.
Reaction Heat Must Be Considered
Water washing may have had limited heat release.
Reactive neutralization can be exothermic.
The liquid temperature may rise.
That can affect:
- absorption equilibrium;
- polymer temperature margin;
- evaporation.
A tower that ran cool with water should not automatically be assumed to remain cool with caustic.
Liquid Physical Properties May Change
A concentrated alkaline solution can differ from water in:
- density;
- viscosity;
- surface tension.
These changes can affect:
- distributor hydraulics;
- liquid holdup;
- pressure drop.
The original hydraulic design should therefore be rechecked using the new liquid properties.
Reaction Products Accumulate
The absorbed acid gas may form dissolved salts.
The previous water scrubber may have operated with little dissolved-solids accumulation.
Reactive service can require:
- blowdown;
- conductivity control;
- salt management.
Without these changes, scaling may develop.
Material Compatibility Must Be Rechecked
The existing packing may be chemically suitable for both water and caustic.
Other components may not be.
Review:
- pump;
- distributor;
- support;
- demister;
- gaskets;
- metallic fasteners;
- vessel lining.
The conversion changes the exposure of the complete system.
Existing Distributor May Behave Differently
If circulation rate or liquid viscosity changes, the original distributor may no longer provide the same flow pattern.
Its:
- pressure requirement;
- outlet balance;
- turndown
should be verified.
The Demister Can See Different Droplets
Reactive liquid can have different:
- density;
- surface tension;
- salt content.
Carryover characteristics may therefore change.
A demister that worked with clean water should still be checked under the new solution.
Process Control Becomes More Important
The water scrubber may have required only:
- pump control;
- makeup water.
Reactive service may also require:
- pH control;
- reagent dosing;
- blowdown.
The tower becomes part of a chemical process system rather than a simple washer.
The Original Bed Height May Still Be Adequate
Reactive absorption may reduce the required transfer height.
That does not mean this can be assumed.
The process should be recalculated using:
- inlet concentration;
- outlet target;
- gas flow;
- liquid chemistry.
The conversion may provide large performance improvement, but it should be demonstrated.
Do Not Forget Safety and Operating Procedures
Chemical storage and dosing introduce new:
- handling;
- maintenance;
- startup requirements.
These issues may sit outside the packing supplier's scope but are part of the retrofit decision.
Engineering Takeaway
Changing the scrubbing liquid can change the duty of every component even when no hardware is replaced.
A chemical upgrade should be treated as a process retrofit.