Engineering Evaluation Case: An Aqueous Scrubber Develops a Separate Organic Liquid Phase Inside the Recirculation Loop

A wet scrubber may be designed around one liquid phase:

water or an aqueous chemical solution.

The system becomes more complicated when an immiscible organic liquid enters the scrubber.

Instead of dissolving completely, the organic material can form:

• droplets;
• floating layer;
• dispersed emulsion;
• film on packing surfaces.

The tower is no longer operating as the original single-liquid-phase absorber.

Project Situation

Consider a water-based packed scrubber treating process exhaust.

A process upset or upstream carryover introduces an organic liquid that has limited water solubility.

Over time, operators observe:

• oily layer in the sump;
• unstable foam;
• altered packing wetting;
• irregular pump performance.

The gas-treatment target may also deteriorate.

The problem should not be treated only as “dirty water.”

A second liquid phase has appeared.

The Organic Phase Can Float or Sink

The relative density of the organic liquid determines whether it tends to:

• float above the aqueous phase;
• settle below it;
• remain dispersed.

This affects where contamination accumulates.

A pump suction located near one phase can suddenly circulate a liquid composition very different from the intended scrubbing solution.

Emulsions Can Hide the Phase Separation

Strong mixing in the pump and tower can break the organic liquid into small droplets.

The sump may appear cloudy rather than showing a clear two-layer interface.

This emulsion can circulate continuously through:

• distributor;
• packing;
• demister.

The tower then sees a two-liquid-phase system even though the original design assumed one.

Packing Wetting Can Change

An organic film can coat packing surfaces.

The aqueous scrubber liquid may then wet the surface differently.

Possible consequences include:

• reduced effective aqueous film area;
• channeling;
• changed mass-transfer performance.

This can happen without severe mechanical blockage.

Surface Tension and Foaming Can Change

Even a relatively small concentration of organic material may influence:

• surface tension;
• foam stability.

The tower can develop:

• increased liquid holdup;
• unstable differential pressure;
• greater droplet carryover.

The symptoms may resemble hydraulic overload.

The Organic Phase May Absorb the Target Gas Differently

In some cases, the organic liquid may absorb certain compounds more readily than water.

In others, it may simply interfere with the intended aqueous chemistry.

The process effect therefore depends on:

• organic identity;
• target contaminant;
• aqueous absorbent.

The supplier should not assume the additional phase is always harmful or helpful.

Material Compatibility Can Change Suddenly

The tower materials were selected for the aqueous system.

The new organic phase may attack:

• gasket;
• plastic packing;
• lining;
• seals

differently.

A PP scrubber that is excellent in acid-water service may not automatically tolerate every organic solvent.

The contamination should therefore be identified chemically.

Pump Performance Can Become Unstable

Two-phase liquid entering the pump can affect:

• suction;
• flow;
• cavitation behavior.

If floating organic material accumulates near the suction, the distributor may temporarily receive a high organic fraction.

That can create sudden tower-performance changes.

Phase Separation Outside the Tower May Be the Best Control

If the organic carryover is unavoidable, the liquid loop may need a method to remove the second phase.

Possible process concepts can include:

• settling;
• skimming;
• coalescence.

The final solution depends on:

• density difference;
• emulsion stability;
• safety.

This is a liquid-system problem, not a packing-only problem.

Mist Eliminator Loading Can Change

Organic-contaminated droplets may have different:

• density;
• surface tension.

They may also create deposits downstream.

The demister should be inspected if carryover increases after the second phase appears.

Identify the Upstream Source

Possible sources include:

• leaking heat exchanger;
• condenser carryover;
• process spill;
• upstream knockout failure.

If the source remains active, repeated cleaning of the packed bed will not provide a durable solution.

Deposit and Liquid Samples Are Valuable

Analyze:

• sump organic layer;
• packing film;
• upstream liquid.

This can reveal whether all three come from the same source.

Engineering Takeaway

An aqueous scrubber receiving immiscible organic liquid is no longer a simple water absorber.

The additional phase changes wetting, liquid properties, pumping, chemistry, and possibly material compatibility.