One Packed Scrubber Serves Different Batch Campaigns and Deposits Form During Chemical Changeover

Multi-product plants often reuse the same exhaust-treatment system for different production campaigns.

This can be economical.

It also creates a unique packed-scrubber problem:

the residue from the previous campaign may react with the chemistry of the next campaign.

Neither process alone may produce solids.

The changeover between them can.

Project Situation

Consider a chemical plant producing multiple products in campaigns.

The same packed scrubber alternates between exhaust containing:

• acidic contaminants during Campaign A;
• alkaline or different reactive contaminants during Campaign B.

The tower is drained or partially flushed between campaigns.

After several changeovers, operators begin finding:

• crystalline deposits;
• sludge;
• blocked distributor openings;
• unexpected pressure-drop increase.

The new campaign may be blamed.

The deposit may actually be a product of the transition.

Residual Liquid Never Disappears Completely by Gravity

After shutdown, liquid can remain in:

• packing films;
• distributor branches;
• support pockets;
• piping low points;
• demister drains.

A nominally “empty” scrubber can therefore still contain meaningful residual chemistry.

The Next Campaign Introduces a New Reaction Partner

When the next process starts, its absorbed chemicals mix with the remaining liquid from the previous campaign.

Possible results include:

• neutralization;
• precipitation;
• salt formation;
• heat release.

The bulk sump may eventually become homogeneous.

The first mixing often occurs locally inside the tower.

Local Concentration Can Be Extreme

Suppose a branch pipe contains concentrated alkaline residue.

An acidic wash from the next campaign enters.

Before full dilution occurs, the local chemical condition can be much more extreme than the eventual sump composition.

This is where:

• crystals;
• deposits;
• corrosion

can start.

Packing Retains Chemical Film

Random and structured packing are deliberately designed to retain and spread liquid films during operation.

After shutdown, some liquid continues to wet surfaces.

This is useful for mass transfer during operation.

It also means the packing can carry chemical residue from one campaign into the next.

Distributor Branches Are High-Risk Locations

Small branches and openings:

• drain slowly;
• can trap residual liquid;
• receive the first new liquid at restart.

If precipitate forms there, the next campaign starts with maldistribution.

The tower may then develop:

• dry areas;
• over-irrigated areas.

A Simple Sump Drain Is Not a Complete Changeover Procedure

Draining the main tank removes bulk inventory.

It does not necessarily remove liquid from:

• upper internals;
• dead legs.

The plant should identify the actual wetted inventory that remains after shutdown.

Flush Chemistry Must Be Compatible with Both Campaigns

A changeover rinse should not itself create:

• precipitate;
• dangerous reaction;
• incompatible waste.

The process team should define:

• rinse liquid;
• volume;
• sequence;
• endpoint.

The packing supplier can identify drainability and internal holdup but should not invent chemical neutralization procedures.

Packing Geometry Influences Cleanability

Very high-area or intricate packing may:

• retain more residual liquid;
• be harder to flush.

More open packing can be easier to clean between campaigns.

This can be a legitimate selection criterion in multi-product service even if the open packing has lower clean-state efficiency.

Material Compatibility Must Cover Every Campaign

A packing material acceptable for Campaign A may not be suitable for Campaign B.

The project should provide the full chemical envelope.

The same applies to:

• support;
• distributor;
• demister;
• gaskets.

Deposit Analysis Can Reveal Changeover Chemistry

If fouling appears only after repeated product switches, analyze the deposit.

Its composition may contain ions or compounds originating from both campaigns.

That is strong evidence that changeover—not either steady-state process—is the source.

Changeover Frequency Matters

A tower switching chemistry:

• once per year

faces a different maintenance burden from one switching:

• every two days.

The number of wet/dry and chemical-transition cycles should therefore be included in the application review.

Engineering Takeaway

In multi-product service, the most aggressive operating condition may occur between campaigns rather than during either campaign.