How to Specify a Mist Eliminator for an HCl Wet Scrubber

Hydrogen chloride wet scrubbers are common applications for mist eliminators.

But specifying the separator by writing only:

“PP demister for HCl scrubber”

is not enough.

The mist eliminator must operate inside a system involving:

• corrosive gas;
• acidic liquid droplets;
• high humidity;
• possible salts;
• variable liquid loading.

The correct design therefore depends on both chemical compatibility and hydraulic duty.

A material that survives HCl may still be unsuitable mechanically.

A separator with excellent droplet capture may still foul or re-entrain if the hydraulic conditions are wrong.

First Define the Actual HCl Service

“HCl service” can describe very different operating conditions.

Important variables include:

• gas-phase HCl concentration;
• scrubber liquid composition;
• temperature;
• pressure;
• water content;
• contaminants.

A low-temperature dilute wet scrubber is not the same duty as a hot concentrated acidic process.

Material selection should therefore use the actual chemical and temperature conditions rather than the application name alone.

Where the Mist Comes From

In a packed HCl scrubber, mist may be generated by:

• gas-liquid interaction in the packing;
• spray headers;
• distributor splashing;
• foaming;
• packed-bed flooding.

The mist eliminator duty depends on how much liquid leaves these upstream stages and what droplet sizes are produced.

If the scrubber operates near flooding, the separator can receive far more liquid than under normal operation.

This should be considered before increasing mesh density to improve removal.

PP Is Common—but Not Universal

PP is widely used in corrosive scrubber systems because of its chemical resistance and cost effectiveness.

However, final suitability depends on:

• temperature;
• HCl concentration;
• mechanical design.

Plastic materials also have lower stiffness than stainless steel.

A large PP demister may require more careful support.

Long-term creep can become important at elevated temperature.

Therefore, “PP is corrosion-resistant” does not complete the engineering review.

PVDF May Be Considered for More Severe Conditions

PVDF provides strong chemical resistance in many aggressive environments.

It can be considered where PP does not provide sufficient temperature or chemical margin.

However, PVDF is generally more expensive.

The project should therefore justify the material from actual operating requirements.

Choosing PVDF automatically because it is “better” can add cost without engineering benefit.

Likewise, choosing PP automatically because it is cheaper can be risky if temperature is too high.

Stainless Steel Needs Chemistry Review

SS304 or SS316L may be suitable in some gas-treatment applications.

But chloride-containing acidic environments can be highly corrosive to stainless steels under certain conditions.

A generic assumption that SS316L is always suitable for HCl should be avoided.

Material selection should consider:

• liquid concentration;
• temperature;
• wetting;
• condensation.

The separator frame and support components need the same review as the active mesh.

Droplet Size Determines the Separation Challenge

If the scrubber mainly generates ordinary mechanically entrained droplets, wire mesh may provide effective removal.

If very fine acid aerosol is present, conventional mesh may not be sufficient.

This distinction is important.

A customer may call all visible or measured acid emission “HCl mist,” but the particle-size regime determines which separation mechanism is required.

The RFQ should therefore clarify whether the duty is:

• ordinary scrubber entrainment;
• very fine aerosol polishing.

Fouling From Salts Must Be Considered

HCl scrubbers can contain contaminants or neutralization products.

If alkaline chemicals are used elsewhere in the system, salt formation may occur.

Captured droplets can concentrate and leave deposits in the demister.

A very dense mesh may then plug.

Questions should include:

• Are solids present?
• Are salts formed?
• Is washing available?
• Has the existing demister fouled previously?

The best separator may be one that remains open and cleanable rather than the densest mesh available.

Liquid Loading Must Be Defined

A wire mesh demister can provide excellent droplet capture in clean service.

But high liquid loading can reduce drainage margin.

If the scrubber produces heavy entrainment, alternatives may include:

• more open mesh;
• vane separator;
• staged vane plus mesh.

The correct arrangement depends on whether the primary challenge is:

• liquid quantity;
• fine droplet size;
• both.

Gas Flow Must Be Converted to Actual Conditions

Scrubber RFQs often state flow in Nm³/h.

Mist eliminator face velocity requires actual flow at:

• operating temperature;
• pressure.

For an HCl scrubber, outlet gas can also be warm and humid.

Using the wrong gas-flow basis can lead to incorrect separator area and velocity.

The RFQ should therefore include both gas flow basis and operating conditions.

FRP Scrubbers Create Mechanical Interface Questions

Many HCl wet scrubbers use FRP towers.

The mist eliminator then has to integrate with:

• FRP support rings;
• plastic grids;
• lining geometry;
• manway access.

A stainless-steel support concept should not automatically be copied into an FRP vessel.

Loads need to be distributed appropriately.

Thermal expansion and chemical compatibility also matter.

What Should Be Included in an HCl Demister RFQ?

Useful information includes:

• actual gas flow;
• temperature;
• pressure;
• HCl concentration;
• liquid composition;
• tower ID;
• expected liquid loading;
• droplet data if known;
• fouling or salt formation;
• allowable pressure drop;
• preferred material;
• manway size;
• support arrangement.

This information allows the supplier to select a separator rather than merely quote a pad by diameter.

Performance Claims Should Be Defined

Avoid specifications such as:

“99% efficiency.”

Instead, where possible define:

• target droplet size;
• inlet loading;
• outlet carryover.

If these values are unknown, clearly label the design as preliminary screening.

This prevents unrealistic universal guarantees.

Final Engineering Perspective

An HCl scrubber mist eliminator must solve two problems simultaneously:

survive the chemical environment and separate the actual droplet load reliably.

Material selection, gas velocity, droplet size, liquid loading, fouling, supports, and FRP vessel geometry should therefore be evaluated together.

The application name “HCl scrubber” alone is not a separator specification.