Ammonia Formation over Steam Reforming Catalysts
-
Upload
gerard-b-hawkins -
Category
Technology
-
view
685 -
download
15
description
Transcript of Ammonia Formation over Steam Reforming Catalysts
Ammonia Formation over Steam Reforming Catalysts
Gerard B. Hawkins Managing Director
Ammonia Formation in Reformers
NH3 formation • Formed in primary from N2 in hydrocarbon
feed (if present) • Formed in secondary from N2 in burner air
Rule of thumb (Primary) • SOR: NH3 formation ~30% of equilibrium
value • EOR: NH3 formation ~10% of equilibrium
value Rule of thumb (Secondary)
• NH3 formation 30% - 70% of equilibrium value
Ammonia Formation in Reformers
NH3 formation • does NOT attain equilibrium • is kinetically limited
NH3 consequences • Environmental: NH3 in condensate • Environmental: Formation of MeNH2 -
odour • Solids: ammonium bicarbonate, NH4HCO3,
deposits in dead zones
Ammonia Formation in Reformers
Rule of thumb (Primary) • Assume NG feed with a few % N2 • NH3 in condensate 40 – 100 ppm
Rule of thumb (Secondary) • NH3 in condensate 300 - 500 ppm
N2 conversion is low Equilibrium position is far to the left
• N2 + 3 H2 2 NH3
Ammonia Formation - Theory
Rate = d[NH3]/dT = k.[N2].[H2]3 As H2 is in huge excess and for a given plant
relatively constant • Rate ~ k’.[N2]
Including the Arrhenius expression for rate constant • Rate = d[NH3]/dT = C. A. e–E/RT .[N2]
Where C, A = constants E = activation energy; R = gas
constant; T = temp in °K
Ammonia Formation - Theory
This means that NH3 formation is a function of • Nitrogen concentration • Hydrogen concentration
H2 has an effect but % variation in N2 likely to be higher for a given operation
Highest at tube exit • Steam:carbon ratio
Higher => less NH3 formation as [N2] is diluted
Ammonia Formation - Theory
This means that NH3 formation is a function of • Temperature: higher temperature drives
rate => production driven by tube exit conditions
• Active nickel (catalyst): activity reduces as catalyst ages (sinters). Smaller catalyst has more active Ni per volume
• Residence time: Longer contact time allows reaction to go closer to equilibrium
• Pressure: Higher P favours NH3 formation
Ammonia Formation - Practice
NH3 formation is • Sensitive to changes in process conditions • Relatively steady as most plants alter
conditions little • Influenced by catalyst type/age
Effect of Temperature and Pressure on NH3 Production
Ammonia Formation - Prediction
NH3 formation • (VULCAN Excel spreadsheet analysis)