Engineering Evaluation Case: The Absorbent Becomes Much More Viscous in Winter but the Pump Still Shows the Same Flow Rate

Packed absorbers are often designed using liquid properties at one temperature.

In some services, viscosity changes strongly with temperature.

The pump may still deliver approximately the same measured flow.

The tower may still behave differently because the liquid no longer:

• distributes;
• drains;
• wets

in the same way.

Project Situation

Consider an absorber using a concentrated or organic scrubbing liquid.

During summer operation:

• liquid temperature is 35–40°C;
• pressure drop is stable;
• removal performance is acceptable.

During winter:

• stored liquid enters much colder;
• viscosity rises substantially.

The plant flow meter still reports the target circulation rate.

Yet operators see:

• higher bed differential pressure;
• poorer distribution;
• slower drainage.

Flow Rate Does Not Describe Liquid Rheology

Two liquids can have the same volumetric flow and completely different flow behavior.

Higher viscosity generally increases resistance in:

• piping;
• distributor holes;
• packing films.

The liquid may also drain more slowly from the bed.

Distributor Balance Can Change

A gravity distributor depends on liquid head.

When viscosity increases, small outlet passages require more pressure or head to pass the same flow.

If the distributor does not develop enough uniform head, flow can become uneven.

A pump-fed distributor may still achieve total flow while branch-to-branch balance deteriorates.

Liquid Holdup Can Increase

Viscous liquid moves more slowly over packing surfaces.

This can increase liquid retained inside the bed.

Greater holdup reduces available void space for gas.

Therefore:

• pressure drop increases;
• flooding margin decreases.

The packing has not changed.

The fluid has.

Pump Measurement Can Hide the Problem

The flow meter may be located in the main recirculation line.

It confirms total liquid flow.

It does not prove:

• equal distributor flow;
• correct film behavior;
• acceptable drainage.

The tower should therefore be evaluated from actual fluid properties at operating temperature.

Mass Transfer Can Change Too

Higher viscosity can thicken the liquid film and reduce internal mixing.

This can lower liquid-side mass-transfer performance.

Therefore, winter operation may suffer both:

• worse hydraulics;
• lower transfer efficiency.

Temperature Can Vary Through the Tower

The liquid may enter cold and warm as it absorbs heat.

Viscosity may therefore vary along the bed.

The most difficult hydraulic condition may occur near the liquid inlet rather than uniformly throughout the tower.

Reheating Can Be a Process Solution

If cold viscosity is the controlling problem, possible process approaches may include:

• liquid preheating;
• maintaining tank temperature;
• heat tracing.

Whether this is economical depends on the process.

Changing packing should not be the first response if the liquid property itself is outside the original design basis.

A Larger Packing May Improve Drainage but Reduce Efficiency

More open packing can reduce hydraulic sensitivity to viscous liquids.

But it may provide less surface area per unit volume.

This is a genuine trade-off.

The selection should consider the worst liquid viscosity, not only the normal warm condition.

What Data Is Needed?

Useful inputs include:

• viscosity versus temperature;
• density;
• surface tension where available;
• minimum liquid temperature;
• maximum liquid rate;
• gas flow;
• allowable pressure drop.

A single room-temperature viscosity value may be inadequate.

Cleaning and Startup Matter

Highly viscous liquids may also drain poorly during shutdown.

Residual material can remain on:

• packing;
• distributor;
• support.

If it solidifies or becomes even more viscous when cold, restart becomes more difficult.

Engineering Takeaway

The same pump flow does not mean the packed tower sees the same hydraulic condition.

Viscosity is part of the operating load.