Engineering Evaluation Case: Hot Saturated Gas Condenses Inside a Packed Scrubber and Creates More Liquid Than the Distributor Supplies

Packed scrubber calculations often start with two obvious flow rates:

• gas entering the tower;
• scrubbing liquid entering through the distributor.

That approach can miss an important third source of liquid:

condensation generated inside the packed bed.

When hot, moisture-rich gas enters a cooler scrubber, part of the water vapor can condense as the gas temperature falls.

The packed bed may therefore handle substantially more liquid in its lower sections than the circulation pump alone suggests.

Project Situation

Consider a hot industrial exhaust entering a wet packed scrubber.

The gas contains:

• high water-vapor content;
• soluble contaminant;
• sensible heat.

Cool recirculating liquid enters from the top.

As gas travels upward:

• temperature decreases;
• water vapor condenses;
• condensate joins the descending liquid.

Operators later observe:

• higher-than-expected liquid flow from the tower bottom;
• increased pressure drop;
• occasional flooding near the lower bed.

The pump flow has not increased.

The tower itself is generating additional liquid.

Saturated Gas Can Condense Rapidly

If the inlet gas is already close to saturation, even a moderate reduction in temperature can generate substantial condensate.

This is especially important when gas enters after:

• combustion;
• hot wet processing;
• evaporation;
• quenching.

The gas-volume and liquid-load profiles can both change as cooling occurs.

Gas Volume Also Decreases

Condensation does not only increase liquid flow.

It removes water vapor from the gas phase.

The actual gas volume can therefore decrease through the tower.

That creates an unusual hydraulic profile:

• high gas volume near the inlet;
• increasing liquid flow downward;
• lower gas volume after cooling and condensation.

One single gas/liquid operating point may not represent the complete bed.

The Lower Bed May See the Highest Liquid Load

Circulating liquid already flows downward through the bed.

Condensate generated above joins that flow.

By the time the liquid reaches the lower packing, total downward flow may include:

• circulation liquid;
• condensed water;
• absorbed species.

The support plate and lower packing may therefore face the highest liquid drainage duty.

Flooding Can Begin Lower Than Expected

If the lower bed receives significantly more liquid than assumed, available void space decreases.

At the same time, the inlet gas may still have relatively high volumetric flow.

This combination can create local hydraulic loading or flooding near the bottom.

A tower-average calculation can hide that condition.

The Sump Balance Can Reveal Condensation

A useful diagnostic is the liquid balance.

Compare:

• recirculation flow;
• makeup water;
• blowdown;
• condensate returned from the tower;
• sump level trend.

If substantially more liquid appears than external additions can explain, gas-phase condensation may be contributing.

Condensation Changes Chemical Concentration

Condensed water dilutes the circulating liquid.

For a reactive scrubber, that can affect:

• reagent concentration;
• pH-control demand;
• blowdown.

A tower designed around one fixed liquid concentration may therefore behave differently under hot humid gas conditions.

Temperature Profile Should Be Measured

Useful measurements include:

• inlet gas temperature;
• intermediate temperature where possible;
• outlet gas temperature;
• liquid inlet and outlet temperatures.

These values help determine where most cooling and condensation occur.

Demister Duty May Change

Cooling gas can generate droplets through condensation.

If condensation continues near the upper tower or outlet duct, the mist eliminator may receive:

• scrubber-generated entrainment;
• newly condensed droplets.

The separator duty may therefore be larger than the distributor liquid rate implies.

Packing Selection Requires Drainage Capacity

A high-area packing may provide strong cooling and mass transfer.

But the application also requires:

• open liquid drainage;
• adequate flooding margin.

The highest surface area per cubic meter is not automatically the best choice when heavy condensation occurs inside the bed.

Engineering Takeaway

In hot saturated gas service, liquid flow is not always fixed at the distributor.

The packed tower can generate a major fraction of its own liquid load through condensation.