Engineering Evaluation Case: HCl Pickling-Line Scrubber Receives Iron-Containing Acid Droplets from the Process Bath
A pickling-line exhaust is often described simply as:
HCl fume.
The real gas entering the scrubber can be more complicated.
Mechanical agitation, bubbling, strip movement, and ventilation can entrain liquid droplets from the pickling bath.
Those droplets may contain:
- hydrochloric acid;
- dissolved iron chlorides;
- suspended scale.
The scrubber then handles both gas absorption and contaminated liquid aerosol.
Project Situation
Consider a steel pickling line using hydrochloric acid.
The exhaust hood sends fumes to a packed wet scrubber.
The client selects plastic packing because of HCl corrosion resistance.
After months of operation, the lower packing shows:
- brown or dark deposits;
- rising pressure drop;
- blocked distributor or support areas.
The plant assumes the acid is attacking the plastic.
The deposits may instead originate from process-bath carryover.
Gas-Phase HCl and Bath Droplets Are Different Loads
Gaseous HCl dissolves into the scrubbing liquid.
Entrained bath droplets physically carry:
- acid;
- dissolved iron;
- solids
into the scrubber.
The packing therefore sees a contamination load that is not represented by HCl concentration alone.
Iron Salts Can Accumulate in the Recirculation Loop
If the droplets are captured, their dissolved iron enters the sump.
With continued recycling, iron concentration can rise.
Changes in:
- pH;
- temperature;
- oxidation
may influence precipitation or deposit formation.
The liquid loop should therefore be monitored as a changing chemical system.
Scale Particles Can Produce Mechanical Fouling
Pickling operations can generate solid scale.
If fine particles enter the scrubber, they can accumulate on:
- packing;
- support grid;
- distributor openings.
An open packing geometry may tolerate this better than a very fine structure.
The First Question Should Be Where the Carryover Comes From
Possible sources include:
- excessive bath agitation;
- high local exhaust velocity;
- poor hood geometry;
- direct droplet entrainment.
Reducing liquid carryover upstream may be more effective than repeatedly cleaning the packing.
An Upstream Droplet Separator May Help
If large droplets are entering the scrubber, an upstream knock-out or suitable separator may reduce contamination.
The packed absorber can then focus more on gas-phase HCl.
The feasibility depends on:
- droplet size;
- available space;
- corrosion environment.
Circulating-Liquid Blowdown May Need Adjustment
Iron-containing liquid should not be recirculated indefinitely without considering accumulation.
The process may require controlled:
- blowdown;
- water makeup.
Otherwise, the scrubber becomes a concentration system for contaminants carried from the bath.
Material Selection Must Cover Both Acid and Salt Environment
PP or another polymer may remain suitable.
But metallic:
- fasteners;
- frames;
- pump parts
may see different corrosion conditions once iron salts and chlorides accumulate.
The complete material list should be reviewed.
Demister Fouling Can Follow
The top mist eliminator may also collect:
- acid droplets;
- solids.
If the lower tower begins entraining contaminated liquid, demister pressure drop can increase later.
The whole tower should be inspected rather than only the packing bed.
Use Deposit Analysis
A sample of the actual deposit can help distinguish between:
- corrosion product;
- iron salt;
- process scale;
- external dust.
This is often more useful than choosing a new packing from photographs alone.
Monitor the Pickling Process and Scrubber Together
Useful data include:
- bath acid concentration;
- iron concentration;
- exhaust rate;
- scrubber liquid iron content;
- pH;
- conductivity;
- pressure drop.
The scrubber problem may originate upstream in the pickling process.
Engineering Takeaway
A pickling scrubber does not necessarily receive pure HCl gas.
Bath droplet carryover can turn the tower into a combined absorber, droplet collector, and solids concentrator.