Engineering Evaluation Case: An Existing Scrubber Needs a Higher-Efficiency Demister but There Is No Extra Vertical Space

Retrofit projects often begin with one apparently simple request:

“Replace the existing demister with a higher-efficiency unit.”

Inside a fixed tower, every millimeter of vertical space may already be occupied by:

• sump;
• packing support;
• packed bed;
• distributor;
• disengagement zone;
• demister.

A thicker separator can therefore take space away from another process function.

This becomes a tower-envelope optimization problem.

Project Situation

Consider an existing wet scrubber with:

• fixed shell height;
• fixed packed-bed support elevation;
• existing packing;
• thin conventional mist eliminator.

The plant has a carryover problem.

A new separator requires greater depth or more disengagement space.

There is no unused tower height.

The project team considers shortening the packing bed to make the new demister fit.

That decision should not be made mechanically.

Packing Height Has a Process Function

The existing bed depth contributes to:

• absorption;
• mass-transfer duty;
• removal efficiency.

Removing 300 mm or 500 mm of packing is not equivalent to trimming unused steel.

The process team should determine whether the bed has sufficient margin to lose that height.

Demister Depth Is Not the Only Required Space

A separator requires:

• its own physical thickness;
• support structure;
• drainage space;
• clearance above and below.

If the gas enters the demister immediately after leaving the packed bed with insufficient disengagement, droplet loading may be much higher.

Therefore, the required retrofit height is larger than the media thickness alone.

Moving the Distributor Downward Can Create Another Problem

One possible idea is to move:

• distributor;
• packing top

downward.

That may preserve demister clearance.

But it can reduce:

• packing height;
• space below the distributor;
• maintenance access.

The complete vertical stack should be reviewed.

A Thicker Demister Can Add Pressure Drop

Higher efficiency often comes with:

• denser media;
• greater depth;
• another separation stage.

The existing fan should therefore be checked for the added resistance.

A mechanically successful retrofit can still reduce plant airflow if the fan has no margin.

Carryover Root Cause Must Be Confirmed First

Before redesigning the tower, determine why the existing demister appears inadequate.

Possible causes include:

• gas flow increased;
• liquid circulation increased;
• distributor spraying upward;
• packing flooding;
• demister fouling;
• wall bypass;
• damaged mesh.

If the current separator is simply damaged, installing a much larger technology may be unnecessary.

Can the Existing Demister Area Be Improved Without More Depth?

Depending on the duty, possible engineering routes might include:

• different mesh construction;
• different vane geometry;
• improved wall sealing;
• corrected support.

The actual droplet size and gas velocity should determine whether a lower-profile solution is technically possible.

Can the Separator Move Outside the Main Shell?

In some retrofits, the tower shell does not offer enough volume.

The project may evaluate:

• enlarged top section;
• external separator vessel;
• downstream duct separator.

These options introduce:

• piping;
• support;
• drainage;
• pressure-drop considerations.

But they can preserve the process-critical packed-bed height.

Reducing Packing Height Requires Re-Rating

If shortening the bed remains the preferred solution, confirm the new process duty for:

• normal flow;
• maximum flow;
• inlet concentration;
• outlet target.

The new bed height should be approved by process engineering.

A supplier should not casually state:

“Remove one layer and it will be fine.”

Demister Drainage Must Fit the New Elevation

A new separator may collect more liquid.

The drainage path must allow that liquid to return without:

• falling directly into high-velocity gas;
• flooding the separator.

The retrofit should show the drainage route on the GA.

Manway Access May Decide the Technology

A larger or thicker separator may need more sections.

Confirm:

• access opening;
• maximum panel size;
• internal installation path.

The most efficient separator is useless if it cannot be installed through the existing vessel.

Structural Support May Need Modification

The existing support was designed for the old separator.

A new unit can have different:

• weight;
• depth;
• load distribution.

Existing clips or frames should be checked before fabrication.

Compare the Options as a System

A useful retrofit study can compare:

Option A: keep bed height, improve separator within existing envelope.Option B: reduce packing height and install larger separator.Option C: modify the tower top.Option D: add external downstream separation.

The correct choice depends on both process performance and project cost.

Engineering Takeaway

In a fixed-height scrubber, packing and mist elimination compete for the same physical space.

Improving one component should not silently reduce the performance margin of another.