Packed Scrubber Foams After Maintenance: Can Cleaning Detergent Residue Be the Cause?

A packed scrubber may operate normally before shutdown, be thoroughly cleaned during maintenance, and then develop severe foam immediately after restart.

Operators naturally suspect:

• new packing;
• incorrect liquid flow;
• process contamination.

Sometimes the problem was introduced during cleaning.

Residual detergent or surface-active cleaning chemical can dramatically change the behavior of the circulating liquid.

Project Situation

Consider a wet packed scrubber shut down for maintenance.

The plant performs:

• detergent washing;
• chemical cleaning;
• water flushing.

The tower is visually clean.

After startup:

• sump foam increases;
• bed pressure drop becomes unstable;
• demister carryover appears;
• removal performance fluctuates.

Gas composition and production rate are similar to pre-shutdown operation.

A Small Residue Can Have a Large Surface Effect

Detergents are designed to alter surface behavior.

A relatively small amount remaining in:

• sump;
• distributor piping;
• packing;
• dead legs

can change liquid surface tension and stabilize bubbles.

The chemical quantity may be small compared with the total process-liquid inventory, but its hydraulic effect can still be significant.

Packing Provides an Ideal Foam-Generation Environment

The packed bed continuously creates:

• thin liquid films;
• gas-liquid interfaces;
• turbulence.

This is exactly the environment where a foaming liquid can generate large quantities of froth.

The tower may therefore show foam only after:

• fan starts;
• full gas flow is introduced.

A quiet sump test before startup may look completely normal.

Foam Consumes Void Space

Foam occupies much greater volume than the same mass of liquid.

Inside packing, this means:

• greater apparent liquid holdup;
• less free gas area;
• higher pressure drop.

The tower can behave as though it is overloaded even though gas and liquid flow are still within their normal values.

Demister Carryover Can Be a Secondary Symptom

Foam reaching the upper tower can collapse and generate droplets.

The mist eliminator then receives a much heavier liquid load.

Operators may conclude:

“The new demister is too small.”

But the real problem may be foam generated below it.

Cleaning Residue Can Hide in Dead Zones

A final rinse may appear clear at the main drain.

Cleaning liquid can remain in:

• distributor branches;
• pump bypasses;
• instrument lines;
• low pockets;
• dense packing sections.

Once circulation starts, these residues gradually enter the main loop.

The foaming problem can therefore begin minutes or hours after startup.

Conductivity Alone May Not Detect Detergent

Some cleaners strongly affect foaming at concentrations that do not create an obvious change in:

• pH;
• conductivity.

The absence of a dramatic chemistry change does not rule out surfactant contamination.

New Packing Can Be Wrongly Blamed

Maintenance shutdowns often include both:

• packing replacement;
• cleaning.

If foaming appears after restart, the new packing receives immediate suspicion.

Before replacing it again, compare:

• cleaning chemicals used;
• rinse procedure;
• sump condition;
• foam tendency of the liquid.

A Controlled Foam Test Can Help

Representative circulating liquid can be compared with:

• fresh process liquid;
• pre-maintenance sample if available.

A simple controlled laboratory comparison can show whether the restarted liquid has unusually persistent foam.

The plant should use an appropriate approved test method for its chemistry.

More Anti-Foam Is Not Automatically the Best Answer

Anti-foam can suppress symptoms.

It can also affect:

• packing wetting;
• mass transfer;
• downstream treatment.

If the cause is temporary detergent contamination, removing or diluting the residue may be preferable to creating a permanent anti-foam dependency.

Flushing Strategy Should Consider the Entire Liquid Path

A strong post-cleaning procedure should verify:

• packing rinse;
• distributor branches;
• sump;
• suction and return lines;
• drains.

The endpoint should be based on the cleaning chemistry and process cleanliness requirement rather than visual clarity alone.

Establish a Restart Baseline

During restart, trend:

• pressure drop;
• liquid flow;
• sump foam;
• demister differential pressure;
• outlet contaminant.

A foam-related event usually creates a recognizable relationship between these parameters.

Engineering Takeaway

Maintenance can introduce a new process fluid into the tower even when the purpose of maintenance was to remove contamination.