Tech Blog

Improvement of the digestion method for melamine concentrated ammonia solution

In the production of urea from CO₂, the stripping process often encounters operational challenges during melamine production. The by-product of melamine production, concentrated ammonia water, often causes bottlenecks in desorption, hydrolysis, steam consumption, and environmental emissions.

This article introduces a validated process improvement for the digestion of melamine concentrated ammonia water – implemented in a 500 ton/day urea plant – to reduce operating costs, decrease steam usage, reduce ammonia emissions, and increase urea production.

The Problem: Overloaded Stripping/Hydrolysis System

A 500 t/d urea plant (CO₂ stripping process) originally processed 10–15 m³/h of dilute ammonia water (NH₃ 6–8%, CO₂ 4–6%). After a melamine plant came on stream, the urea plant had to digest an additional 3.5 m³/h of concentrated ammonia water (NH₃ 12–14%, CO₂ 8–10% after mixing). The results:

Ammonia water feed	10–15 m³/h	18–22 m³/h
NH₃ concentration	6–8%	12–14%
CO₂ concentration	4–6%	8–10%
Stripper effluent NH₃	<5 ppm	150–250 ppm (exceeds limit)
Stripper steam consumption	4.5 t/h	7.5 t/h
Hydrolysis steam	1.0 t/h	1.5 t/h
Absorption tower tail gas NH₃	~1%	50–65% (severe pollution)

The stripping/hydrolysis system was severely overloaded. The reflux condenser could not condense all the stripped NH₃ and CO₂, sending large volumes of non‑condensable gas to the atmospheric absorption tower, which then released ammonia‑rich tail gas. Environmental compliance became impossible.

Key Improvement of the digestion method for melamine concentrated ammonia solution

The transformation will redirect melamine ammonia directly to the low-pressure ammonium methane condenser rather than completely desorbing it.

Process optimization

A 60% concentrated ammonia solution of melamine is directly fed to the low-pressure carbamate condenser to form high-concentration ammonium carbamate, which is then sent to the high-pressure urea synthesis circuit.
The remaining 40% continues to enter the desorption hydrolysis system.

Equipment upgrade

Add a second reflux condenser in series to improve condensation and reduce non-condensable gases.
Install an additional low-pressure ammonium methane condenser to handle the high heat release during the NH3- CO₂ reaction.

Water balance control

Maintain a water/carbon ratio (H₂O/CO₂) of 0.45-0.5 in high-pressure systems to maintain conversion efficiency.

Improvement results of the melamine concentrated ammonia digestion method

After the modification, the plant achieved dramatic improvements:

Key outcomes:

Steam saving: 2 t/h → annual saving of ~¥633,600 (≈ $87,000 at current rates)
Urea production increase: 2 t/day → annual extra revenue ~¥792,000 (≈ $109,000)
Total economic benefit: ~¥1.425 million/year (≈ $196,000)
Environmental benefit: Stripper effluent NH₃ dropped from 150–250 ppm to 30–50 ppm; tail gas ammonia cut dramatically.

Key Operating Points for Success

Maintain High Production Load

The modified process works best at high load. The water‑to‑carbon molar ratio in the synthesis solution should be kept at 0.45–0.5 (design 0.37). Higher load gives more operational flexibility.

Ensure High CO₂ Stripping Efficiency

If stripping efficiency drops (due to low HP steam pressure or equipment fouling), more unreacted NH₃ and CO₂ enter the low‑pressure loop, overloading the carbamate condensers. The modification cannot work effectively if stripping efficiency is poor.

Remove Reaction Heat from the Carbamate Condenser

The reaction of NH₃ and CO₂ to form carbamate is highly exothermic. To maintain stable operation:

Lower the temperature of the cooling water to the low‑pressure carbamate condenser.
Increase heat exchange area (the plant added a second condenser in series).
Target a carbamate solution concentration that improves water balance in the high‑pressure system.

Use High‑Concentration Melamine Ammonia Water

For best results, the melamine ammonia water should have NH₃ and CO₂ both above 18 wt%. Higher concentrations mean less water is introduced into the synthesis loop, helping maintain the water‑to‑carbon ratio below 0.5.

Optimize Reflux Condensation

To raise the concentration of the reflux liquid:

Lower the cooling water temperature to the reflux condensers (e.g., to ~40 °C with two condensers, or 35 °C with one).
Lower the outlet temperature of the first desorber (to 100–105 °C) by adjusting reflux flow or reducing stripping steam. Prioritize reducing excess steam rather than increasing reflux.

Why This Modification Is a Model for Integrated Urea‑Melamine Plants

Many chemical complexes produce both urea and melamine. The melamine process generates a concentrated NH₃‑CO₂‑H₂O stream that is often seen as a waste disposal problem. However, as this case shows, that stream can be valorized as a feedstock for urea synthesis.

Advantages of the approach:

Reduces environmental load – lower ammonia in wastewater and tail gas.
Lowers energy consumption – steam savings directly reduce CO₂ footprint.
Increases production – recovered ammonia and carbon dioxide translate into more urea.
Simple hardware changes – adding a few exchangers and re‑piping- are low‑cost compared to building new waste treatment facilities.

conclusion

The digestion of concentrated ammonia water derived from melamine in urea plants may not necessarily be a bottleneck. It alleviates overload during the adsorption-hydrolysis process, reduces steam use, decreases emissions, and increases urea production. It is strongly recommended that the urea melamine integrated plant adopt this mature and low-investment improvement.

Tech Blog

Improvement of the digestion method for melamine concentrated ammonia solution

The Problem: Overloaded Stripping/Hydrolysis System