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

NH

3 con

cent

ratio

n %

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