Engineering Evaluation Case: An Existing Spray Scrubber Is Converted to a Packed-Bed Scrubber Without Replacing the Shell
A plant may own an existing empty spray tower that provides acceptable gas cooling but insufficient contaminant removal.
Instead of buying a new vessel, the project team asks:
Can we install packing inside the existing spray scrubber and turn it into a packed-bed absorber?
This retrofit can be practical.
But the tower was originally designed around spray contacting—not around the mechanical and hydraulic requirements of a packed bed.
The conversion therefore affects much more than adding several cubic meters of packing.
Project Situation
Consider an existing vertical FRP or steel scrubber with:
- gas inlet near the lower section;
- spray headers;
- sump;
- top mist eliminator;
- no existing packing support.
The plant wants higher removal efficiency while keeping:
- the vessel shell;
- fan;
- pumps;
- most piping.
A packed section appears to provide additional gas-liquid contact without requiring a completely new scrubber.
The first question should be whether the existing tower can physically and hydraulically support the conversion.
The Shell Diameter May or May Not Be Suitable
The existing spray tower diameter was selected for spray operation.
Packed beds have their own hydraulic limits.
Before adding packing, check:
- actual gas flow;
- actual tower diameter;
- liquid circulation;
- proposed packing;
- expected pressure drop.
A tower that is comfortable as an open spray chamber may operate much closer to flooding once packing is installed.
The Fan Must Absorb the Additional Pressure Drop
An empty spray tower usually has relatively low internal resistance.
Adding:
- packing support;
- packing;
- distributor;
- hold-down;
- possibly a denser demister
increases total system pressure drop.
The existing fan should therefore be checked at the new operating point.
If the fan has little static-pressure margin, the conversion may reduce gas flow.
A stronger absorber is not useful if the ventilation system no longer captures the source properly.
Spray Nozzles Are Not Automatically a Proper Packing Distributor
The existing spray system may wet the open vessel adequately.
A packed bed requires reasonably uniform liquid introduction across the bed surface.
Spray nozzles can create:
- overlap zones;
- dry zones;
- wall wetting.
Whether the existing header can remain depends on:
- spray pattern;
- nozzle elevation;
- tower diameter;
- liquid flow;
- selected packing.
Sometimes the existing spray system can be adapted.
Sometimes a proper packing distributor is required.
A Packing Support Must Be Added
Loose packing cannot simply be dumped into the bottom of the scrubber.
The retrofit needs a support capable of carrying:
- dry packing;
- retained liquid;
- deposits;
- maintenance loads where applicable.
The support also needs enough open area for gas and liquid flow.
Its load must transfer safely into:
- existing support ring;
- vessel wall;
- new reinforced structure.
For FRP shells, the vessel designer should verify attachment loads.
Existing Gas Inlet Geometry May Be Poor for a Packed Bed
In an open spray chamber, gas can redistribute over a relatively large empty volume.
Once a support and bed are installed close above the inlet, a high-momentum side entry can send much more gas into one part of the bed.
This can create:
- local high velocity;
- uneven pressure drop;
- uneven mass transfer.
The inlet-to-support distance should therefore be reviewed.
A gas-distribution device may be needed if the available calming space is insufficient.
Vertical Space Is Usually the Main Retrofit Constraint
The existing shell already contains:
- sump;
- spray header;
- mist eliminator;
- access doors or manways.
The new packed section must fit between them.
The proposed bed height should not be decided only from available empty space.
It must also satisfy the process requirement.
If the process needs 2.5 m of packing but only 1.2 m can physically fit, the project may not achieve the expected improvement.
The Sump Level Must Stay Below the Packing
Adding a support at a low elevation can reduce the distance between:
- normal sump level;
- bottom of packing.
High liquid level or pump upset may then submerge the bed.
The retrofit should confirm:
- normal level;
- high-high level;
- support elevation.
Existing Mist Eliminator Duty May Increase
Improved gas-liquid contact can also produce more liquid entrainment.
If liquid circulation increases to support the new bed, the top demister may receive a different droplet load.
Therefore, the conversion should include a demister review rather than assuming the old separator remains suitable.
Material Compatibility May Change
A spray tower may originally use:
- water;
- dilute chemical.
The packed-bed retrofit may be paired with a stronger reagent to improve performance.
If so, existing:
- shell lining;
- nozzles;
- supports;
- mist eliminator
should be checked against the new chemistry.
Installation Access Matters
The new support and distributor must enter through existing access openings.
Large internals may require segmentation.
The retrofit design should therefore define:
- manway size;
- maximum segment dimensions;
- installation sequence.
Measure Performance Before the Retrofit
A useful baseline includes:
- gas flow;
- inlet contaminant;
- outlet contaminant;
- liquid flow;
- fan operating point;
- existing pressure drop.
Without a baseline, it becomes difficult to prove whether the packed conversion actually improved the system.
Engineering Takeaway
Converting a spray tower to a packed absorber can be a cost-effective retrofit, but it is not simply a matter of adding packing.
The vessel becomes a different hydraulic and mechanical system.