Ammonia Synthesis Purpose Reactions and chemistry Bed and system design Operating conditions Catalyst parameters Performance and monitoring Problems

1. Theory and Operation of
VSG-A101 Ammonia Synthesis
by:
Gerard B. Hawkins
Managing Director, CEO

2. Ammonia Synthesis
 Discuss Ammonia Synthesis
 Include
• Purpose
• Reactions and chemistry
• Bed and system design
• Operating conditions
• Catalyst parameters
• Performance and monitoring
• Problems

3. Natural
Gas
Steam
superheater
Air
Steam
30
bar
Steam
Steam
raising
350 C
200 C
Heat
Recovery
Steam
raising
Cooling
Cooling
Reboiler
CO
Cooling
Preheater
Heat
Recovery
Steam
Boiler
Process
Condensate
Quench
Quench
Liquid Ammonia
H
Hydrodesulphuriser Primary
Reformer
Secondary
Reformer
High
Temperature
Shift
Low
Temperature
Shift
Ammonia SynthesisMethanator
Carbon Dioxide
Purge Gas
Cooling
400 Co
390 Co
2
790 C
o
550 Co
1000 Co
o
420 Co
150 C
o
400 Co
470 C
o
o
220 C
o
290 Co
330 Co
2
CO Removal2
220 bar
Refrigeration
Condensate
Cooling
Ammonia
Catchpot
Simplified Flowsheet for a Typical Ammonia
Plant

4. Simplified Flowsheet for a Modern
Uhde Ammonia Plant

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

21. Uhde Converter Design

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

25. Typical Operating Conditions
Temperature (oC) 360-530
Pressure (bar) 100-600
Space velocity (hr-1) 1000-5000
Poisons oxygen and
oxygen
compounds normally
3ppm

26. Catalyst Reduction
Pre-reduced Oxidised
Max water in outlet gas
during reduction (ppm)
1000 3000
Formation of water
during reduction of 1te of
catalyst (kg)
25 280

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