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)