📋 Case Study
Nitric Acid Absorption Tower Design for Tail-Gas Treatment
Incomplete absorption of NO and NO₂ due to slow liquid-phase oxidation kinetics and poor gas distribution
🏗️ Project Overview
Nitrogen fertilizer plant retrofit in Morocco to meet new NOₓ emission limits
🎯 Challenge
Incomplete absorption of NO and NO₂ due to slow liquid-phase oxidation kinetics and poor gas distribution
🔧 Design Approach
Detailed kinetic model of NO oxidation (2NO + O₂ → 2NO₂) and NO₂ hydrolysis (3NO₂ + H₂O → 2HNO₃ + NO); redesigned packing geometry and staged air injection
📐 Design Diagram
AI-generated project design illustration
📐 Key Calculations
NO Oxidation Half-Life
t_1/2 = ln(2)/(k * [O₂])
Result: 142 s @ 5% O₂
Dictated minimum residence time in oxidation zone
Liquid-Phase Mass Transfer Resistance
1/k_L = 1/k_L,film + 1/k_L,chem
Result: 0.83 s/m
Chemical enhancement factor (β) = 4.2 confirmed kinetic regime
Absorption Efficiency
η = 1 − exp(−K_Ga * H_E * Z / G)
Result: 89% → 99.2%
Met regulatory limit of <50 ppm NOₓ
📊 Results
NOₓ emissions reduced from 320 to 28 ppm, acid recovery increased by 17%, eliminated need for SCR downstream💡 Lessons Learned
- •Gas-phase oxidation must precede absorption—no effective 'direct absorption' of NO
- •Acid concentration critically impacts hydrolysis rate; optimal at 35–40 wt% HNO₃
- •Staged oxidant injection prevents localized over-oxidation and nitrous oxide formation
✅ Key Takeaways
- 1Gas-phase oxidation must precede absorption—no effective 'direct absorption' of NO
- 2Acid concentration critically impacts hydrolysis rate; optimal at 35–40 wt% HNO₃
- 3Staged oxidant injection prevents localized over-oxidation and nitrous oxide formation