The low-pressure gas phase quenching process for melamine powder production generates tail gas rich in ammonia (NH₃) and carbon dioxide (CO₂)—a challenge for environmental compliance and cost control if not properly treated. The integrated low-pressure melamine co-production with urea process, combining low-pressure melamine technology with the aqueous solution total-recycle urea process, offers a transformative solution.
By recovering melamine tail gas to synthesise urea (which is then reused as a raw material for melamine production), this process eliminates tail gas emission issues, reduces raw material consumption, and enhances economic and environmental sustainability. This article details the process principles, key workflows, technical parameters, and practical benefits, serving as a comprehensive guide for chemical industry professionals.
This integrated process is designed to address two critical pain points of standalone melamine production:
Tail Gas Waste: Traditional melamine plants struggle to efficiently recover NH₃ and CO₂ from tail gas, leading to environmental pollution and resource waste.
High Raw Material Costs: Melamine production relies on urea as a feedstock; recycling tail gas to produce urea creates a closed loop, reducing external urea procurement costs.
The process features a 150 kt/a aqueous solution total-recycle urea plant paired with two 60 kt/a low-pressure gas-phase quenching melamine plants. A key innovation: no fresh CO₂ is added to the urea system, eliminating the need for a CO₂ compressor, simplifying equipment, and reducing energy consumption.
Melamine Production: Urea decomposes to produce melamine, with NH₃ and CO₂ as byproducts:
6CO(NH2)2→C3N6H6+3CO2+6NH3
Urea Synthesis from Tail Gas: Recovered NH₃ and CO₂ react to form urea, which is reused for melamine production:
CO2+2NH3→CO(NH2)2+H2O
The integrated process consists of five key stages, optimised for tail gas absorption and closed-loop material reuse:
Melamine tail gas (split into low-pressure and medium-pressure streams) is fed into the urea system:
Low-pressure tail gas (0.4 MPa, 140℃): Directly enters the second-cycle first cooler to form a dimethyl solution, which is pressurised and sent to the medium-pressure absorption system.
Medium-pressure tail gas (2.0 MPa, 140℃): Compressed to 2.0 MPa, absorbed to form methyl solution, and pumped to the urea synthesis tower.
Key tail gas composition: 28.5% CO₂, 71.5% NH₃ (mass fraction), with a total mass flow of 14,980 kg/h per stream.
Liquid ammonia is pressurised (via ammonia booster pump) and heated to ~170℃ before entering the synthesis tower.
Methyl solution from the medium-pressure absorption tower is pressurised, heated to 180℃, and fed into the synthesis tower.
Heat required for urea formation (via ammonium carbamate dehydration) is supplied by the decomposition heat of ammonium carbamate in the preheater.
Critical equipment materials: The ammonium carbamate preheater uses corrosion-resistant 2522 stainless steel; the liquid ammonia preheater uses 316L stainless steel to withstand high-temperature corrosion.
The synthesis tower effluent (20 MPa, 188℃) is depressurised and fed into the first decomposition tower (1.8 MPa).
Free NH₃ is separated, and ammonium carbamate is partially decomposed; urine is heated to 158–160℃ in an external heater for further purification.
Decomposed gases are condensed and sent to the medium-pressure absorption tower for bubbling absorption, forming a recyclable methyl solution.
Medium-pressure decomposed liquid is depressurised to 0.4 MPa and fed into the second decomposition tower.
Residual ammonium carbamate is fully decomposed; gases (120℃, 0.4 MPa: 9.1% CO₂, 64.9% NH₃, 26.0% H₂O) are condensed in two-cycle coolers.
Tail gas is finally treated in a tail gas absorption tower before venting; concentrated urine is sent to a urine tank for further processing into molten urea.
Dimethyl solution from the second-cycle cooler is split into two streams for heat recovery (via flash heater and evaporator).
Combined materials are condensed in an external cooler and absorbed in the first absorption tower to form methyl solution (37.7% CO₂, 39.0% NH₃, 23.1% H₂O).
Methyl solution is pressurised to 22.0 MPa, heated to 180℃, and recycled to the synthesis tower; purified NH₃ (99.5% purity) is condensed and reused as process ammonia.
Compared to traditional aqueous solution total recycle urea processes, this integrated system features adjusted parameters to accommodate tail gas absorption:
| Parameter | Traditional Process | Integrated Process |
|---|---|---|
| NH₃/CO₂ Molar Ratio | 4.0 | 4.5 |
| H₂O/CO₂ Molar Ratio | 0.65 | 1.5 (optimizable to 1.0) |
| Synthesis Tower Conversion | 67% | 55% (up to 60% after optimization) |
| Urea Mass Fraction in Effluent | 30.5% | 22.4% |
Ammonia-Carbon Ratio Adjustment: Increasing the NH₃/CO₂ ratio to 4.5 compensates for reduced conversion caused by high H₂O/CO₂. Each 0.1 increase in NH₃/CO₂ boosts conversion by 1.0–1.5%.
Water-Carbon Ratio Control: Optimising H₂O/CO₂ to ~1.0 balances absorption efficiency and urea synthesis kinetics, minimising hydrolysis losses.
Load Flexibility: The system operates stably at 40–110% load, adapting to fluctuations in melamine production.
Reduced Urea Consumption: Theoretically, 1 ton of melamine requires 2.857 tons of urea. Recovered tail gas produces 1.476 tons of urea, cutting net urea consumption to 1.381 tons/ton melamine (1.4–1.5 tons in practical operation)—a 48% reduction.
Energy Savings: Eliminating the CO₂ compressor reduces power consumption by ~15–20% compared to standalone urea plants.
Zero Tail Gas Emission: NH₃ and CO₂ recovery efficiency exceeds 99%, eliminating pollutant emissions and meeting strict environmental standards.
Closed-Loop Material Use: Minimal waste generation, aligning with circular economy principles.
Simplified Equipment: No CO₂ compression system reduces maintenance costs and downtime risks.
Corrosion Resistance: High-grade stainless steel materials extend equipment lifespan, lowering maintenance expenses.
The low-pressure melamine co-producing urea process revolutionises melamine manufacturing by turning tail gas from a waste product into a valuable resource. Its closed-loop design reduces urea consumption by nearly 50%, eliminates environmental pollution, and enhances operational efficiency—addressing the industry’s dual challenges of cost and sustainability.
For chemical enterprises, this integrated process offers a win-win solution: improved profitability through resource reuse and compliance with stringent environmental standards. As the industry shifts toward circular economy practices, this technology will become a benchmark for sustainable melamine production, driving innovation in low-carbon chemical manufactu
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