How Scrubber Blowdown Rate Affects Mist Eliminator Scaling and Fouling

Mist eliminator fouling is often treated as a separator problem.

But in recirculating wet scrubbers, the root cause can begin much lower in the system:

the blowdown rate.

A scrubber continuously brings contaminants into the liquid.

If insufficient liquid is removed through blowdown, dissolved and suspended material accumulates.

The circulating liquor becomes more concentrated.

The same mist eliminator now receives droplets containing much more:

• salt;
• solids.

A separator that operated cleanly under the original chemistry can begin scaling rapidly even though gas flow and liquid circulation are unchanged.

Why Blowdown Exists

Recirculating scrubbers reuse process liquid.

Water may leave through:

• evaporation;
• outlet gas.

Many dissolved contaminants do not evaporate.

Without blowdown, they accumulate.

Removing part of the circulating liquid and replacing it with fresher water controls this concentration.

Evaporation Concentrates Nonvolatile Species

Hot gas can evaporate significant water.

If 100 units of water enter the system and part evaporates, salts remain behind.

The circulating liquid concentration rises.

Blowdown removes some of those nonvolatile species.

The balance between:

• evaporation;
• makeup;
• blowdown

determines long-term concentration.

The Mist Eliminator Sees the Concentrated Liquid

Entrained droplets originate from the circulation system.

If total dissolved solids double, each droplet carries more nonvolatile material.

After capture inside the demister, the water drains or evaporates.

More solid residue remains per unit of entrained liquid.

Fouling rate can therefore increase even if droplet mass loading does not.

Higher Concentration Can Approach Saturation

A salt that remains dissolved at low concentration may begin precipitating as concentration rises.

Thin liquid films inside a demister are particularly vulnerable because evaporation can continue locally.

The bulk sump can be below saturation while the separator experiences local crystallization.

Suspended Solids Also Accumulate

Blowdown can help remove:

• fine reaction solids;
• corrosion products.

If blowdown is reduced, suspended solids concentration can rise.

Wet mesh captures these particles effectively.

The demister becomes an unintended filter.

Why Increasing Wash Frequency May Not Solve the Root Cause

More frequent washing can remove deposits.

But if the circulation liquor remains highly concentrated, new deposits form quickly.

The plant enters a cycle:

clean → foul → clean → foul.

Improving blowdown control may reduce the source loading rather than only treating the separator surface.

Water Consumption Creates a Real Tradeoff

Higher blowdown requires:

• more makeup water;
• more wastewater handling.

Therefore, the correct answer is not simply:

“maximize blowdown.”

The process needs a concentration target that balances:

• chemistry;
• water use;
• waste treatment;
• demister reliability.

Conductivity Can Be a Useful Control Indicator

In many ionic scrubber systems, conductivity correlates roughly with dissolved solids.

A conductivity trend can therefore help monitor concentration buildup.

The exact relationship depends on chemistry.

Where appropriate, specific ion or total dissolved solids analysis provides a stronger basis.

Fouling History Can Reveal the Link

If demister cleaning intervals become shorter after:

• water-conservation changes;
• reduced blowdown,

the timeline is important.

Compare:

• liquid conductivity;
• blowdown rate;
• normalized demister DP.

A strong correlation can identify the process change driving separator fouling.

Blowdown Location Matters

The stream removed should represent the contaminated circulating liquid effectively.

A poorly placed withdrawal point may not remove settled solids or concentrated zones efficiently.

Scrubber liquid management is a process-design issue, not simply one valve-flow number.

More Concentrated Liquid Can Change Physical Properties

Increasing dissolved material may also change:

• density;
• viscosity;
• surface tension.

This can affect:

• drainage;
• droplet generation.

Therefore, concentration cycles influence the mist eliminator through both:

• deposit loading;
• fluid properties.

What Should Be Reviewed?

Useful data includes:

• makeup-water rate;
• blowdown rate;
• evaporation estimate;
• conductivity or TDS;
• key salt concentrations;
• suspended solids;
• demister DP history;
• cleaning interval.

This creates a direct link between scrubber water balance and separator reliability.

Final Engineering Perspective

Mist eliminator fouling is often the visible end point of a liquid-chemistry problem.

Blowdown controls how much nonvolatile material accumulates in a recirculating scrubber.

When concentration becomes excessive, each entrained droplet carries more fouling material to the separator.