Engineering Evaluation Case: HCl and NH₃ Are Scrubbed Separately but Form a White Plume After the Two Exhaust Streams Recombine

Two scrubbers can each meet their individual outlet targets and still create a new emissions problem downstream.

This can happen when an acidic exhaust and an ammonia-containing exhaust are treated separately but their cleaned gas streams later merge into a common header.

Residual HCl and NH₃ can react after the scrubbers and form fine ammonium-chloride-containing aerosol.

The plant then sees a white plume and assumes one of the scrubbers has failed.

The problem may actually be created after both scrubbers.

Project Situation

Consider two treatment trains.

Scrubber A handles HCl-containing exhaust.

Scrubber B handles NH₃-containing exhaust.

Each tower has:

• packing;
• recirculation liquid;
• demister.

Individual stack testing near each tower shows acceptable removal.

Farther downstream, however, the two gas streams combine before one common stack.

A visible white plume appears after the mixing point.

Residual Concentrations Can Still React

No industrial scrubber removes an infinite fraction of every contaminant.

Even low residual concentrations of:

• HCl;
• NH₃

may coexist after the two treated streams mix.

Their reaction can produce extremely fine particulate or aerosol.

Therefore, “both scrubbers pass separately” does not automatically mean the combined exhaust remains particle-free.

The New Aerosol Did Not Pass Through Either Demister

This is the critical diagnostic point.

The acid and ammonia may leave their respective towers mainly in gas form.

The solid or aerosol forms only after the gases mix.

Therefore, improving the existing tower demisters may have little effect because the particles do not yet exist at those locations.

Mixing Temperature and Humidity Matter

The downstream reaction and visible plume can be influenced by:

• concentration;
• temperature;
• humidity;
• mixing intensity.

A cold common duct may also promote condensation and make the plume more visible.

The plant should therefore identify exactly where the plume begins.

Sampling Location Matters

Sampling immediately after each scrubber answers one question.

Sampling after the common header answers another.

A proper investigation may compare:

• HCl after the acid scrubber;
• NH₃ after the ammonia scrubber;
• particulate or aerosol after mixing.

This separates individual scrubber performance from downstream chemistry.

Additional Packing Is Not Automatically the Best Solution

Adding packing could reduce the residual gas concentration from one stage.

But the required reduction should be established first.

Possible system solutions may include:

• improved removal in one or both scrubbers;
• changing mixing location;
• a downstream polishing stage.

The correct approach depends on the actual mass balance.

Common Exhaust Headers Should Be Part of Process Design

Scrubber battery limits should not stop at the outlet nozzle if downstream streams can chemically interact.

The complete ventilation system should consider:

• which exhausts combine;
• residual chemicals;
• gas temperature;
• condensate.

Engineering Takeaway

Two individually successful scrubbers can still generate a downstream emissions problem when their residual gases react after mixing.

Summary

When separately treated HCl and NH₃ exhaust streams recombine, residual gases can form fine ammonium-chloride-containing aerosol downstream. Determine where the plume forms, measure each residual gas separately, review humidity and temperature, and distinguish downstream reaction from demister carryover before modifying either packed bed.

URL:https://www.pxdaier.com/hcl-ammonia-scrubber-downstream-white-plume-case/

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P6-112

Engineering Evaluation Case: SO₃ Passes Through a Hot Gas Section and Becomes Sulfuric-Acid Mist Only After Cooling

A sulfur-containing gas stream can change physical form as it cools.

At one point in the system, the contaminant may exist mainly in the vapor phase.

Farther downstream, cooling can produce extremely fine sulfuric-acid aerosol.

This matters because equipment designed to absorb gas or remove coarse droplets may not control particles that form later.

Project Situation

Consider a hot process gas containing sulfur oxides.

The gas passes through treatment equipment while still relatively warm.

Initial measurements show acceptable gas removal.

After downstream cooling, however, operators observe:

• acid mist;
• white or bluish plume;
• corrosion.

The immediate conclusion is often that the packed scrubber or demister is inefficient.

The first question should instead be:

At what temperature and location does the acid aerosol actually form?

Vapor Can Cross an Acid Dew Point

As the gas cools, sulfuric-acid-containing vapor can reach conditions where condensation becomes favorable.

The resulting droplets may be extremely fine.

If condensation occurs after the existing mist eliminator, the separator never had an opportunity to capture them.

Cooling Location Controls the Separation Problem

Suppose gas remains hot through:

• absorber;
• conventional demister.

Then it enters a cold duct or heat exchanger.

Fine droplets may form only there.

Installing a denser upstream mesh pad may therefore add pressure drop while providing little improvement.

Fine Acid Mist Is Different from Coarse Carryover

Large scrubber droplets have enough inertia for conventional:

• mesh;
• vane separation.

Very fine acid aerosol may require different separation technology.

The project should establish the actual aerosol size range before selecting a separator.

Materials Can Fail Downstream First

Equipment upstream may operate above the acid-condensation region.

Downstream steel ductwork can be the first place exposed to concentrated acidic condensate.

This explains cases where:

• tower internals look acceptable;
• outlet duct or fan corrodes rapidly.

Material selection therefore needs the full temperature profile.

Operating Changes Can Move the Condensation Point

The location of acid condensation can shift when:

• gas temperature changes;
• water vapor changes;
• sulfur composition changes;
• ambient conditions change.

A system may operate for years and then develop corrosion after a heat-recovery modification.

Sampling Must Distinguish Species and Phase

A meaningful emissions investigation should distinguish:

• gaseous sulfur species;
• acid aerosol.

Otherwise, a reported “sulfur” measurement may be difficult to interpret against equipment performance.

Engineering Takeaway

Where an aerosol forms can be more important than where the gas contaminant originally enters the treatment system.

Summary

When SO₃- or sulfuric-acid-containing gas cools downstream, fine sulfuric-acid mist may form only after the packed scrubber or conventional demister. Review the gas temperature profile, acid dew-point behavior, aerosol formation location, downstream material exposure, droplet size, and polishing requirements before increasing packing or demister density.

URL:https://www.pxdaier.com/so3-sulfuric-acid-dew-point-mist-formation-case/

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P6-113

Engineering Evaluation Case: A Scrubber Has Enough Pump Capacity but Its Recirculation Tank Is Too Small to Stabilize the Process

Scrubber design often focuses on:

• tower diameter;
• packing;
• circulation rate.

The recirculation tank may receive far less attention.

If liquid inventory is too small, the entire chemical system can respond too quickly to every disturbance.

The result can be unstable:

• pH;
• temperature;
• concentration;
• pump suction.

Project Situation

Consider a compact packed scrubber with a very small sump.

The pump delivers the required nominal circulation rate.

However, operators observe:

• rapid pH swings;
• frequent reagent dosing;
• unstable level;
• temperature spikes during process peaks.

The tower packing appears normal.

Liquid Inventory Provides Buffer Capacity

A larger liquid inventory changes more slowly when a sudden contaminant load enters.

A very small inventory can be chemically consumed or heated quickly.

Therefore, the same contaminant spike produces a much larger change in:

• pH;
• reagent concentration;
• temperature.

High Circulation Rate Does Not Mean Large Inventory

A scrubber can circulate its entire sump volume many times per hour.

That does not provide the same process buffering as having sufficient stored liquid.

Pump flow and tank volume serve different functions.

Small Sumps Can Amplify Control Oscillation

Chemical dosing adds concentrated reagent into a limited liquid volume.

The pH changes rapidly.

The controller stops dosing.

The contaminant load then consumes the reagent quickly and pH falls again.

This produces repeated overshoot and undershoot.

Temperature Can Swing Too

Reactive absorption may release heat.

A larger liquid inventory absorbs transient heat with a smaller temperature increase.

A small sump can heat quickly during a peak event.

Higher temperature may then reduce absorption performance or material margin.

Level Stability Affects Pump Suction

Short process events can change liquid inventory through:

• condensation;
• evaporation;
• makeup;
• carryover.

If the sump is shallow, level changes can expose the pump suction to:

• vortexing;
• gas entrainment.

The result can be an apparently random loss of distributor flow.

Increasing Packing Height Does Not Add Enough Chemical Buffer

More packing adds contact surface.

It does not necessarily provide enough additional liquid inventory to stabilize a highly transient process.

The sump and circulation system should therefore be treated as part of absorber design.

Useful Design Inputs

The process team should consider:

• contaminant peak load;
• reagent consumption rate;
• desired response time;
• pump minimum submergence;
• temperature rise;
• makeup and blowdown.

Tank sizing should follow the required process response rather than only the physical space available under the tower.

Engineering Takeaway

The recirculation tank is not simply a container below the packed bed.

It is the chemical and thermal buffer of the scrubber.