Ammonia Synthesis Catalyst Chemistry and Operator Training

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Ammonia Synthesis Catalyst (VSG-A101)
Chemistry and Operator Training
by:
Gerard B. Hawkins
Managing Director, CEO

VSG-A101 Ammonia Synthesis Chemistry and Catalysis
Reaction stoichiometry and thermodynamics
Ammonia synthesis catalyst fundamentals
Molecular mechanism and kinetics
Activation and deactivation
Alternative catalyst technologies
Reaction stoichiometry and thermodynamics
NH3 synthesis reaction
N2 + 3 H2  2 NH3 DH700°K = - 52 kJ/mol
Equilibrium position favors NH3 synthesis at
• High pressure
• Low temperature
Pressure depends on capital and operating cost
Temperature depends on the balance of kinetics/equilibrium
P & T also depend on available catalyst activity

0
5
10
15
20
25
30
35
40
50 75 100 125 150
Pressure bara
NH3concentration%
380 C
400 C
420 C

VSG-A101 Ammonia synthesis catalyst fundamentals
VSG-A101 Ammonia Synthesis Catalyst - Formulation
Based on magnetite (Fe3O4) precursor
Defined form and crystal structure
Magnetite requires controlled reduction
Pre-reduction or in situ reduction
Oxygen is removed from the crystal lattice without shrinkage
Produces extremely porous metallic iron structure
Key in achieving a high activity catalyst
Promoters boost catalyst performance
VSG-A101 Ammonia Synthesis Catalyst - Requirements
High catalyst activity
Low sensitivity to catalyst poisons
High thermal resistance
Reasonable reduction time
High mechanical strength and abrasion resistance

VSG-A101 Ammonia Synthesis Catalyst - Production
Unique manufacturing process
Catalyst is not made via pelleting or extrusion
Components are mixed including promoters
Feed is melted in an electric arc furnace
Solidified melt is milled to give required shape and size distribution
Melt conditions are key to produce required Fe3O4 precursor phases and
morphology

Catalyst Type
Vulcan VSG-A101 Comp J Comp J Comp H Comp H
Fe 0.5 - 0.6 - 80 - 80
Fe3O4 67 - 69.5 95 12 - 13 94.5 ~10
Al2O3 2.2 - 2.6 2.5 3.3 2.1 2.8
CaO 1.4 1.8 2 2.5 1.5 2
K2O 0.6 - 0.8 0.8 1 0.6 0.8
SiO2 < 0.5 0.25 0.3 <0.2 <0.2
MgO - 0.25 0.3 - -
Co - - 1 1.3
Cl <0.0001 <0.001 <0.001 <0.001 <0.001
S <0.0001 <0.001 <0.001 <0.001 <0.001
VSG-A101 Ammonia Synthesis Catalyst - Incorporation of Promoters
Certain metal oxides promote activity and improve stability
Small and controlled amounts
Alumina and potash are the most important
Al2O3 is a ‘structural’ promoter
Restricts growth of iron crystallites during reduction and operation
Increases thermal stability
Alkali metals are ‘electronic promoters’
Greatly increase activity of the iron particles
K particularly effective
Other promoters include CaO, SiO2

Contaminants in the magnetite raw material
Must be considered and controlled during manufacture
Ensures optimum concentration of promoters
Ammonia Synthesis Catalyst - Effect of Promoters and Stabilizers
Al2O3 - stabilizes the internal surface
SiO2 - stabilizes activity in presence of oxygen compounds during normal
operation and reduction
K2O - increases intrinsic activity of Fe particles
CaO - protects the K promoter against neutralization and increases the stability
against poisoning by sulfur
Typical Operating Conditions
Temperature 360 - 530°C (680 – 986°F)
Pressure 100 - 600 bara
Space velocity 1000 - 5000 hr-1
Poisons limits
Oxygen and oxygen compounds normally 3 ppmv

Typical Converter Temperatures
Converter Heat Recovery
Heat recovered is that available from synthesis exotherm across synthesis beds
Catalyst bed temperatures usually similar
Bed 1 410 – 520°C ΔT = 110 °C
Bed 2 415 – 480°C ΔT = 65 °C
Bed 3 410 – 450°C ΔT = 40 °C
Total Bed ΔT = Converter ΔT = 215 °C

VSG-A101 Ammonia Synthesis Chemistry
Reaction Progress across Beds
VSG-A101 Ammonia Synthesis Catalyst - Effect of Size on Activity
Smaller pellets have higher activity
Reaction is subject to diffusion limitations
Film diffusion and pore diffusion effects
Thus, smaller means higher production (closer ATE) or lower catalyst volume
But higher pressure drop
use either axial-radial or radial flow beds to minimize
Basis of many converter internal retrofits

VSG-A101 Ammonia Synthesis Catalyst - Effect of Size on Activity
Ammonia Synthesis Catalyst - Catalyst Size Options
Size Grade Size
A 1.5 - 3.0 mm
B 3.0 - 4.5 mm
C 3.0 - 6.0 mm
D 6.0 - 10.0 mm
G 14.0 - 20.0 mm

Molecular mechanism and kinetics
Ammonia Synthesis Mechanism
Dissociative adsorption of H2
Dissociative adsorption of N2 - believed to be the Rate Determining Step (RDS)
Multi-step hydrogenation of adsorbed N2 Desorption of NH3

Ammonia Synthesis Kinetics
Detailed studies on intrinsic reaction rate
Influential work by Temkin et al.
Actual rate subject to diffusion limitations
Film diffusion and pore diffusion effects
Activation and deactivation
Catalyst Reduction
Depends on catalyst type
E.g. pre-reduced or oxide form
E.g. VSG-A101, Comp H, Comp J
Raise T in first bed to 350°C* (300°C #
), then slowly upwards @ 10°C/h
Control exit H2O level
Maintain downstream beds at 350°C* (300°C#
)
NH3 synthesis initiates as the catalyst reduces
Once H2O falls, heat 2nd
bed and repeat

VSG-A101 Deactivation – Background
Temperature
Re-crystallization of iron surface occurs slowly
Thermal sintering process
Minimize operating temperature
Commensurate with maintenance of conversion

VSG-A101 Deactivation - Common Poisons
Oxygenated species (H2O, CO, CO2)
Converted to H2O
Sintering/re-organization of catalyst surface
Temporary
Low level/short duration (days)
Permanent
High levels/weeks to months
Sulfur
Incorporation of Ca promoter enhances stability of catalyst
Arsenic, Antimony and Phosphorus
Chlorine
Formation of volatile metal chlorides
Lead to KCl formation and loss of K from catalyst
Physical foulants (Fe scale, etc)

Summary
Thermodynamics, kinetics and reaction mechanism considered
Features of ammonia synthesis catalysts
Activation and deactivation parameters discussed
Brief consideration of non-Fe based commercial catalyst

Ammonia Synthesis Catalyst Chemistry and Operator Training

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