How to Add an Online Wash System to a Fouling Mist Eliminator

A mist eliminator operating in dirty service can gradually accumulate salts, solids, polymer, or process deposits.

Frequent shutdown cleaning may restore performance, but the operating cycle becomes expensive.

Where the deposit can be removed by suitable liquid washing, an online or periodic wash system may extend service time.

The retrofit must do more than install spray nozzles.

It must ensure that the wash actually reaches the fouled surfaces and that the additional liquid can drain without creating new carryover.

Identify the Fouling Material

Not every deposit responds to washing.

Determine whether the contamination is:

• water-soluble;
• chemically soluble;
• loosely attached;
• sticky;
• hardened scale.

The cleaning liquid should be compatible with the separator material and process.

Identify Where Fouling Starts

Deposits may concentrate on:

• the gas-entry face;
• lower mesh layers;
• vane leading edges;
• drainage channels.

Spray coverage should target the real fouling zone.

Design Spray Coverage

A few nozzles can leave shadow areas.

The wash pattern should cover the active separator area sufficiently to prevent local buildup.

For large-diameter towers, nozzle arrangement and spray overlap become important.

Check Wash Pressure

Insufficient pressure may not penetrate dense mesh.

Excessive pressure can damage thin structures or generate fine droplets that increase carryover.

The wash system should deliver controlled cleaning rather than uncontrolled atomization.

Decide Whether Washing Occurs Online or Offline

Online washing introduces additional liquid while gas continues flowing.

This increases separator liquid load.

If the tower has limited margin, gas flow may need to be reduced during wash periods.

Offline washing is hydraulically easier but may not meet the plant's objective of avoiding shutdown.

Check Drainage Capacity

All wash liquid must leave the demister and tower.

A wash retrofit can fail if the separator drains slowly and becomes flooded.

Drainage should be sized for both captured process liquid and wash flow.

Protect the Packed Bed Below

Wash liquid may fall onto the packed bed or distributor.

Its chemistry and volume should not disturb the process.

If the wash fluid is different from process liquid, contamination effects should be considered.

Prevent Nozzle Plugging

A wash system operating in a dirty environment can itself foul.

Nozzles should be accessible, removable, or flushable where practical.

Review Material Compatibility

Cleaning chemicals can be more aggressive than normal process fluid.

Check compatibility of:

• demister media;
• support frame;
• spray piping;
• nozzles;
• tower lining.

Establish the Wash Sequence

Useful operating parameters include:

• wash duration;
• frequency;
• liquid rate;
• gas-rate limitation.

These should be based on fouling rate rather than arbitrary scheduling.

Monitor Pressure Drop

Demister differential pressure can provide a useful trigger for washing.

If pressure drop gradually rises and returns after washing, the system is addressing the actual deposit problem.

Do Not Hide a Process Problem

If fouling rate increases dramatically because upstream operation has changed, washing more frequently may only treat the symptom.

The root source should still be investigated.