Ammonia Formation in Reformers

1. Ammonia Formation over
Steam Reforming Catalysts
Gerard B. Hawkins
Managing Director

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. Effect of Temperature and Pressure on NH3 Production

10. Ammonia Formation - Prediction
 NH3 formation
• (VULCAN Excel spreadsheet analysis)