Engineering Evaluation Case: Ammonia Scrubber Uses Sulfuric Acid but Ammonium Salt Crystals Begin Blocking the Packing

Acid scrubbing of ammonia can provide strong chemical absorption.

But the reaction creates a second engineering problem:

the absorbed ammonia does not disappear—it becomes dissolved salt in the circulating liquid.

As concentration increases, the tower can move from an absorption problem into a crystallization and fouling problem.

Project Situation

Consider a packed scrubber treating ammonia-containing exhaust.

The liquid circuit uses sulfuric-acid solution.

During early operation:

• ammonia removal is high;
• pressure drop is stable;
• packing remains clean.

After longer operation, the plant begins observing:

• crystalline deposits;
• blocked distributor openings;
• rising bed pressure drop;
• solids in the sump.

The packing is blamed first.

The more important question is whether the circulating liquor is approaching salt saturation.

Reaction Converts Gas Load into Liquid-Side Salt Inventory

Ammonia absorbed into acidic liquid forms ammonium-containing salts.

As the scrubber continues operating, those salts remain in the recirculation loop unless they are removed.

The system therefore continuously transfers ammonia mass from the gas phase into the liquid inventory.

If water evaporates or blowdown is insufficient, the dissolved salt concentration can rise.

Why Crystallization Often Appears Locally First

The average sump concentration may still appear acceptable.

Inside the tower, local conditions can differ because of:

• evaporation;
• temperature variation;
• pH gradients;
• localized high absorption.

Crystals may first appear at:

• distributor holes;
• packing contact points;
• support-grid surfaces.

These locations provide surfaces for nucleation.

The Smallest Openings Usually Fail First

A distributor designed for very uniform flow may contain many small openings.

That is excellent for clean liquid.

It can be problematic once crystals begin forming.

Partial blockage causes:

• lower flow through affected holes;
• more liquid through remaining holes;
• worsening maldistribution.

This creates a feedback loop.

Packing Geometry Changes the Run Length

A fine high-area packing may provide strong mass-transfer efficiency.

It also contains more:

• narrow passages;
• contact points;
• locations for crystal bridging.

A more open geometry may tolerate salt deposition longer.

But using more open packing does not solve the root cause if the liquid continues concentrating toward saturation.

Blowdown Is a Process-Control Variable

The tower needs a mass balance for the dissolved reaction product.

If salt enters the liquid through reaction faster than it leaves through blowdown or product withdrawal, concentration rises.

Therefore, packing selection should be coordinated with:

• blowdown rate;
• makeup water;
• acid dosing;
• evaporation.

The packing supplier should not pretend to solve this chemistry by changing one ring size.

Temperature Can Shift Solubility

Salt solubility can depend on temperature.

A scrubber that operates cleanly at one temperature may begin crystallizing after:

• seasonal cooling;
• heat-exchanger change;
• shutdown.

The process designer should check the expected concentration against the complete temperature range.

Shutdown Can Be More Dangerous Than Normal Operation

During shutdown, liquid may remain stagnant in:

• distributor branches;
• packing;
• low points.

Cooling or evaporation can then trigger crystallization.

When the tower restarts, these deposits may already restrict the liquid path.

A flushing or draining procedure can therefore be important.

What Should Be Measured?

Useful operating trends include:

• ammonium salt concentration where available;
• conductivity;
• pH;
• liquid density;
• temperature;
• blowdown;
• pressure drop.

These data can show whether the tower is moving toward a saturation problem before severe plugging occurs.

Cleaning Strategy Should Match the Salt

If the deposits are water soluble, controlled flushing may be effective.

If deposits contain other contaminants, cleaning may be more difficult.

The maintenance method should be defined before the tower becomes fully blocked.

Material Compatibility Still Matters

Sulfuric-acid and ammonium-salt service must still be checked against:

• packing;
• distributor;
• support;
• demister.

The best anti-fouling geometry is useless if the selected material cannot tolerate the process chemistry.

Engineering Takeaway

An ammonia acid scrubber is not only an absorption system.

It is also a salt-generation system.

The long-term design must manage both.