Why a Mist Eliminator Can Improve Wet Electrostatic Precipitator Performance

Wet electrostatic precipitators, or WESPs, are used when gas cleaning requires removal of very fine:

• particles;
• aerosol;
• mist.

Because a WESP can remove much finer material than many conventional inertial separators, it may seem unnecessary to install a mist eliminator upstream.

In reality, the two devices can perform very different functions.

A conventional mist eliminator can remove:

• coarse droplets;
• bulk entrainment

before the gas reaches the WESP.

The WESP can then focus on the fine aerosol and particulate fraction for which electrostatic collection is especially valuable.

This is an example of separator staging by particle size and hydraulic duty, rather than using two devices to perform the same job.

A WESP Is Not Primarily a Bulk-Liquid Separator

A wet electrostatic precipitator charges particles or droplets and collects them on electrode surfaces.

It is particularly useful for fine material that is difficult to remove by ordinary inertial separation.

But a heavy stream of coarse liquid droplets can create unnecessary load inside the WESP.

The system may still collect them, but using the electrostatic stage for easily removable bulk liquid is often inefficient.

A simple upstream demister can reduce this burden.

Coarse Droplets Are Easier to Remove Mechanically

Large droplets have sufficient inertia to impact:

• vane surfaces;
• mesh.

They can therefore be removed with relatively simple low-energy separator technology.

The WESP can then receive gas containing a smaller mass of liquid but a higher fraction of difficult fine aerosol.

Each technology handles the particle range where it provides the greatest value.

This can improve overall system design.

Heavy Liquid Loading Can Disturb WESP Operation

A WESP already operates with wet collection surfaces.

Excessive bulk liquid entering the unit can increase:

• drainage demand;
• liquid distribution complexity.

Large droplets can also carry:

• dissolved solids;
• particulate contamination

onto internal surfaces.

Reducing bulk entrainment upstream can therefore help maintain a more controlled operating environment.

The objective is not to make the gas completely dry.

It is to remove unnecessary coarse liquid.

Solids Inside Droplets Can Increase Deposition

Wet scrubber exhaust may contain droplets carrying:

• salts;
• slurry;
• solids.

If these droplets enter the WESP, their nonvolatile content can accumulate on internal surfaces after water drains or evaporates.

An upstream fouling-tolerant vane separator can capture a large fraction of this coarse contaminated liquid before it reaches the electrostatic stage.

This can shift some of the maintenance burden to a simpler, more washable piece of equipment.

The Upstream Demister Must Not Create Excessive Pressure Drop

One reason WESPs are selected is that the overall pollution-control system may already contain significant resistance from:

• scrubbers;
• ducts;
• fans.

Adding an overly dense demister can consume unnecessary pressure budget.

A coarse pre-separator should therefore be designed for:

• sufficient bulk-liquid removal;
• low resistance.

There is little benefit in using fine high-pressure-drop mesh to remove the particle fraction that the WESP is intended to handle downstream.

The Demister Should Not Be Asked to Replace the WESP

The opposite design mistake is also possible.

A plant sees fine aerosol emissions and installs increasingly dense wire mesh upstream.

Pressure drop rises, but emissions remain.

If the remaining material is truly very fine aerosol, mechanical mesh may not be the correct final technology.

The WESP and demister should therefore have clearly defined responsibilities.

For example:

Demister

• bulk droplets;
• heavy liquid load.

WESP

• fine aerosol;
• fine particulate.

This division helps avoid overdesigning either stage.

Gas Distribution Between the Two Stages Matters

If the mist eliminator fouls unevenly, gas leaving it can become maldistributed.

The WESP downstream then receives:

• high flow in some regions;
• low flow in others.

This can reduce effective collection or create uneven internal loading.

The upstream separator should therefore maintain:

• reasonable open area;
• uniformity

throughout its operating cycle.

A severely fouled pre-separator can damage the performance of the high-efficiency stage it was intended to protect.

Wash Systems Should Be Coordinated

Both demister and WESP may use:

• washing;
• flushing.

Their wash cycles can interact.

If the upstream separator is heavily washed, temporary liquid loading to the WESP may increase.

Operating procedures should consider whether:

• both stages wash simultaneously;
• one stage should drain before another begins.

The ideal strategy depends on the system arrangement.

Condensation Can Change the Duty Between Stages

If gas cools between the mist eliminator and WESP, new droplets can form.

These droplets were not present when the upstream demister performed its separation.

The WESP may therefore receive higher liquid loading than expected.

Temperature profile should be included when evaluating staged separation.

The same principle applies to chemical aerosol formed downstream.

Material Compatibility Remains Important

The upstream separator may be exposed to aggressive:

• acidic;
• chloride-bearing;
• chemically contaminated liquid.

The material should be selected accordingly.

Because the pre-separator may require frequent washing, support and module design should also prioritize:

• maintainability;
• corrosion resistance.

A sacrificial low-cost separator that fails mechanically is not a good way to protect expensive downstream equipment.

Maintenance Strategy Can Improve

Using a washable pre-separator can sometimes reduce contamination reaching the WESP.

The demister can then be cleaned or replaced more easily than complex electrostatic internals.

This does not eliminate WESP maintenance.

It simply manages the contaminant load more intelligently.

Lifecycle maintenance should be part of the staging decision.

How to Know Whether a Pre-Demister Is Useful

Consider:

• inlet liquid loading to the WESP;
• droplet-size distribution;
• solids contained in droplets;
• WESP fouling history;
• available pressure drop.

If the inlet contains substantial coarse liquid that can be removed cheaply and reliably, an upstream mist eliminator may provide real value.

If almost all contamination is already fine aerosol with very low bulk-liquid loading, the benefit may be smaller.

What Should the Design Basis Include?

Useful information includes:

• gas flow;
• temperature profile;
• coarse droplet loading;
• fine aerosol concentration;
• solids content;
• allowable system pressure drop;
• wash strategy;
• WESP inlet requirements.

The two separator stages should be designed from one common process basis.

Final Engineering Perspective

A mist eliminator and WESP do not have to compete for the same separation duty.

When correctly staged, the mist eliminator removes easy, heavy, coarse liquid, while the WESP concentrates on difficult fine aerosol and particulate.

This can improve hydraulic stability, reduce downstream liquid burden, and simplify maintenance.

The value comes from assigning each technology the duty it handles most effectively.