5. Ammonia Chemistry
Reaction : (Exothermic)
N2 + 3H2 <=> 2NH3 H(@ 700K) = - 52kJ/mol
Reaction is favored by high pressure and low
temperature
Pressure governed by capital and operating
cost
Temperature balance of kinetics/equilibrium
6. 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
7. Effect of Temperature Pressure on
Ammonia Equilibrium Concentration
0
5
10
15
20
25
30
35
40
50 75 100 125 150
Pressure bara
NH3concentration%
380 C
400 C
420 C
8. Ammonia Equilibrium Diagram
300 350 400 450 500 550 600 650
0
10
20
30
40
Equilibrium
Max Rate
Temperature °C
Ammoniacontent%
9. Effect of Catchpot Temperature on
Ammonia VLE
0
2
4
6
8
10
12
50 75 100 125 150
Pressure bara
NH3concentration%
0 C
minus 20 C
10. Catalyst Requirements
High catalyst activity
Low sensitivity to
catalyst poisons
High thermal
resistance
Reasonable
reduction time
High mechanical
strength and
abrasion resistance
11. Catalyst Formulation
The source of iron is magnetite, Fe3O4,
chosen for its crystal structure
During reduction, oxygen is removed from
the crystal lattice without shrinkage
This produces metallic iron which is
extremely porous
A significant factor in achieving a high
activity catalyst
12. Incorporation of Promoters
Small amounts of certain metal oxides
promote activity and improves stability
Alumina and potash are the most important
• They produce ‘doubly-promoted’ catalyst
• Alumina is a ‘structural’ promoter
• Restricts growth of iron crystallites during
reduction and operation
• Increases thermal stability of the catalyst
13. Incorporation of Promoters
Alkali metals are ‘electronic promoters’ and
greatly increase the activity of the iron
particles; potassium is the most cost
effective
Other promoters include calcium oxide, silica
& magnesia
Contaminants in the raw magnetite must also
be taken into account during manufacture to
ensure the optimum concentration of
promoters in the finished catalyst
14. Effect of Promoters and Stabilizers
Conventional Catalysts
AI2O3 - stabilizes the internal surface
SiO2 - stabilizes the activity in presence of oxygen
compounds during normal operation and
reduction.
K2O - increases the activity
- decreases the thermal stability and the
resistance against poisoning by oxygen
compounds
- minimizes the neutralization of K promoter
CaO - increases the stability against poisoning by
sulfur
15. Ammonia Synthesis - Catalyst
Parameters
Parameters as follows
Form Irregular particles
Production Method Melt, cool and grind
Size 1-3 mm
Magnetite % Balance %
Potash % 0.6-0.8 %
Calcium Oxide % 1.4-1.8 %
Alumina % 2.2-2.6 %
16. Ammonia Synthesis Catalyst
Production
Catalyst is unusual in that it is not made via
pelleting or extrusion
Unique manufacturing process
A mix is made of ingredients including
promoters
Feed is passed to an electric Arc furnace
Then milled to give correct shape distribution
17. Effect of Size on Activity
Particle Diameter (mm)
14121086420
RelativeActivity
120
100
80
60
40
0
20
18. Effect of Size on Activity
Smaller pellets = high activity
Therefore high production or small catalyst
volume
But pressure drop will rise
So must use either axial-radial or radial flow
beds to minimise pressure drop
Basis of many converter internal retrofits
19. Deactivation
Clean Gas
Thermal sintering
Contaminated Gas
Both Temporary and Permanent Poisons
• Oxygen induced sintering
• By water or CO and CO2
• Site blocking/Sintering
20. Uhde Converter Design
Uhde design a range of converters;
modern designs use radial flow with
inter-cooling & 'split converters' with
heat recovery between,
• Converter 1 : 2-bed, 1 interchanger
• Heat recovery (boiler)
• Converter 2 : 3rd bed
22. Gas inlet
Start up gas
Gas outlet
Second bed
First bed
Uhde Converter Design
Features of Krupp-Uhde
2-bed radial Ammonia Converters
• Easy withdrawal of the internal heat
exchanger without catalyst removal
• Comfortable access for catalyst
removal without removal of the
cartridge
• Access to all catalyst beds without
removal of intermediate heat
exchanger
• Reasonable transport dimensions
and weights even at high plant
capacities
23. Uhde Converter Design
Features of Krupp-Uhde
1-bed radial Ammonia
Converters
• One radial type catalyst bed
resulting in maximum
conversion rate, lower recycle
gas rate and low pressure drop
• Suitable large volumes of
catalyst with small grain size
• Simple and reliable design
• Comfortable access for catalyst
removal without removal of the
cartridge
• Reasonable transport
dimensions and weights even at
high plant capacities
Gas outlet
Gas inlet
Third bed
24. Ammonia Synthesis - Temperature
Profile
Equilibrium curve
% NH3
Heat exchanger type
Quench type
Temperature °C
500450400
0
5
10
15
20
27. Ammonia Synthesis - Performance
Monitoring
Monitor temperature profile
• Adjust accordingly to optimise production
Monitor pressure drop across converter
Monitor loop pressure
Monitor inert levels
• Helps identify upstream problems
28. Ammonia Synthesis - Problems
Ammonia Synthesis is a robust catalyst
• Delivers extremely long lives
• Performance is a function of converter and
catalyst
Must be aware of
• Effect of water
• Effect of CO and CO2
• Will poison the catalyst and therefore
reduce production