Most modern ammonia processes are based on steam-reforming of natural gas or naphtha.
The 3 main technology suppliers are Uhde (Uhde/JM Partnership), Topsoe & KBR.
The process steps are very similar in all cases.
Other suppliers are Linde (LAC) & Ammonia Casale.
3. Introduction
Most modern ammonia processes are
based on steam-reforming of natural
gas or naphtha.
The 3 main technology suppliers are
Uhde (Uhde/JM Partnership), Topsoe
& KBR.
The process steps are very similar in all
cases.
Other suppliers are Linde (LAC) &
Ammonia Casale.
6. Ammonia Synthesis Loop
Synthesis reaction is equilibrium limited,
typically 15 – 20% NH3 at converter exit.
Therefore recycle in a ‘loop’ is required.
Multi-stage complex converters are
required to control bed temperatures.
Various designs are used depending on
contractor.
Liquid Ammonia is recovered by
refrigeration.
7. Simplified Flowsheet for a Typical Ammonia
Plant
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
8. Ammonia Plant Steam & Power
System
Waste Heat recovery is used to raise
HP steam, 100 – 120 bar
Steam is used to drive the main
compressors
• Process air
• Syn gas compression + circulator
• Refrigeration
Pass-out steam is used for process.
9. Ammonia Flowsheet Variations
1. Uhde
Top fired reformer
• Cold outlet manifold design
Secondary reformer with internal riser
H P loop (200 bar) with radial flow
converter
• 1 or 2 converters
Once-through synthesis section upstream
of main synthesis loop for very large
capacities (dual pressure Uhde process)
10. Ammonia Flowsheet Variations
2. KBR
Top-fired reformer
• With internal risers
Several synthesis loop options:
• Conventional 140 bar loop with 4bed
quench converter
• Higher pressure for large-scale plants
• Horizontal converter on modern plants.
• KAAP design – 100 bar loop with Ru/C
catalyst
Braun Purifier flowsheet
• Excess air with cryogenic ‘purifier’ to
remove excess N2 and inerts from MUG
12. Ammonia Flowsheet Variations
4. Linde LAC (Linde Ammonia
Concept)
Hydrogen plant + N2 addition from
air separation unit
Ammonia Casale synthesis loop
13. Ammonia Flowsheet Variations
5. ICI (JM)
AMV
• Large-scale process with excess air,
low pressure loop (80 – 110 bar)
LCA
• Small-scale plant based on GHR
technology
AMV / LCA technology is now part
of JM’s ‘background in ammonia’
14. 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
16. Uhde Dual-Pressure Process
C.W.Make up gas
from frontend
C.W.
Steam
Once
through
converter
Synthesis
Loop
Purge
NH3
NH3
NH3
1 2 3 R
C.W.Make up gas
from frontend
C.W.
Steam
Once
through
converter
Synthesis
Loop
Purge
NH3
NH3
NH3
1 2 3 R
17. Effect of Pressure on Ammonia
Equilibrium Concentration
0
10
20
30
40
50
60
50 75 100 125 150 175 200 225 250 275 300
NH3concentration%
Pressure bara
380 C
400 C
420 C
19. Effect of Catchpot Temperature on
Ammonia VLE
0.0
2.0
4.0
6.0
8.0
10.0
12.0
50 75 100 125 150 175 200 225 250 275 300
NH3concentration%
Pressure bara
0 C
minus 20 C
20. Synthesis Loop Principles:
Mass Balance
Overall Loop Mass Balance
• On a mass basis:
NH3 = MUG – Purge
• On a molar basis:
NH3 = (MUG – Purge) / 2
because 4 mol -> 2 mol in the NH3
reaction.
Converter balance, on a molar basis:
NH3 = Inlet gas – Outlet gas
21. Synthesis Loop Principles:
Mass Balance
Converter Molar balance:
NH3 = Circ Flow x (NH3out- NH3in)
1 + NH3out
NH3in is set by P & T of final
separator
+ position of MUG addition (before or
after separator).
22. Synthesis Loop Principles:
Effect of Purge
Circulating composition is the same
as the purge composition (like a
stirred-tank reactor).
Inerts (CH4 + Ar) build-up in loop.
Circulating gas H / N ratio is very
sensitive to MUG H / N ratio because
the reaction consumes gas in a 3 : 1
ratio.
23. Synthesis Loop Principles:
H2 : N2 ratio example
H / N = 3 : 1
MUG NH3 Purge
H2 3000 2700 300
N2 1000 900 100
H / N 3.0 3.0 3.0
H / N = 2.95 : 1
H2 2950 2700 250
N2 1000 900 100
H / N 2.95 3.0 2.50
24. Synthesis Loop Principles :
Inerts Balance
Inerts (CH4 + Ar) concentrate in the loop,
typically by a factor of about 10.
Note that some of the inerts (10 – 20% of
the total) dissolve in the product NH3.
A few loops with purified make-up gas
have a ‘self-purging loop’ where all the
inerts are removed in solution in the
product.
The NH3 content of the purge at the
flowmeter position is required to check the
loop mass balance.
25. Synthesis Loop Principles :
Effect of H2 Recovery
Most modern loops have H2 recovery.
2 systems are used, cryogenic or
membrane.
The overall effect is similar, typically 90%
H2 recovery at 90% purity.
Overall loop H2 conversion to NH3
increases from about 92% to 98%.
MUG H / N ratio changes from 3.0 to
approx. 2.85, and returns to 3.0 after H2
addition.
26. Synthesis Loop Principles :
Control of Catalyst Bed Temperatures
Multi-bed design :
2, 3, or 4 catalyst beds with
intermediate cooling.
27. Synthesis Loop Principles :
Converter Heat Balance
Older converter designs usually had an
interchanger after the final bed to contain
high temperatures within the converter.
Modern designs typically have no ‘overall’
interchanger because this gives better
heat recovery (heat available at a higher
temperature)
‘Split converter designs’ further increase
the heat recovery temperature.
28. 3 Bed Converter Example
450 C
1. Optimum Catalyst
Temperatures
410 C
520 C
415 C
480 C
410 C
29. 3 i/c design
‘Cold’ Converter
410 C
520 C
415 C
480 C
410 C
450 C
120 C
335 C
30. 2 i/c design
410 C
520 C
415 C
480 C
410 C
450 C
‘Hot’ Converter
235 C
31. 1 i/c design
410 C
520 C
415 C
480 C
410 C
450 C
‘Split’ Converter 305 C
32. Converter Heat Recovery Example
In all cases the amount of heat recovered
is the same, only the available
temperatures are different.
In all cases, the catalyst bed temperatures
are the same:
Bed 1 410 – 520 dT = 110
Bed 2 415 – 480 dT = 65
Bed 3 410 – 450 dT = 40
Total Bed dT = Converter dT = 215
33. Comparison of 74 & 35 Series
30
40
50
60
70
80
90
100
110
120
0 2 4 6 8 10 12 14
Time on line (years)
RelativeActivity
Severnside LCA
Standard Catalyst
34. Effect of Size on Activity
Particle Diameter (mm)
14121086420
RelativeActivity
120
100
80
60
40
0
20
35. Effect of Size on Activity
Smaller pellets = high activity
Therefore high production rate or
smaller catalyst volume
But pressure drop will rise
Either axial-radial or radial flow
beds are used to minimise
pressure drop
Radial flow is the basis of many
converter internal retrofits
36. Deactivation
Clean Gas
• Thermal sintering
Contaminated Gas
• Both Temporary and Permanent
Poisoning
• Oxygen induced sintering
• By water, CO and CO2
• Site blocking/Sintering
37. Typical Operating Conditions
Temperature (o
C) 360-520
Pressure (bar) 80-600
Space velocity (hr-1
)1000-5000
Poisons oxygen and oxygen
compounds
normally < 3ppm
39. Catalyst Reduction
Max water in outlet gas during
reduction (ppm)
Formation of water during
reduction of 1te of Catalyst (kg)
Pre-reduced Oxidized
1000 3000
25 280
42. Converter Designs
Objectives for modern designs are;
- low pressure drop with small catalyst
particles.
- high conversion per pass with high grade
heat recovery.
Principal types are designed by:
Uhde
Kellogg (KBR) - conventional, Braun,
KAAP
Topsoe
Ammonia Casale
JM (I C I)
43. Uhde
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.
45. M W Kellogg Converter Types
'Conventional' make-up gas and loop
layout, refrigeration to low temperature (-
25 C),
loop pressure typically 140 - 180 bar.
Converters:
4 bed quench ; conventional Kellogg
design.
Horizontal converter ;
• lower cost, low pressure drop, easier
installation
• 2 bed inter-cooled layout with small catalyst
48. KBR KAAP
Converter is made up of 4 beds
First bed uses magnetite catalyst
Ru can not be used since
temperature rise is too large
Lower beds use Ru catalyst
Ru catalyst has a carbon support
Catalyst developed by BP
• Very high activity even at low pressure
49. Braun Converter Types
Purifier Process gives pure make-up gas
- low levels of poisons; H2O, CO, CO2
- Low inerts; no purge from loop
Converters :
Basically 2-bed intercooled with each
catalyst bed in a separate vessel
Modern designs may use 3 converters
&/or radial flow
50. Haldor Topsøe S- Series
S-100 :Radial flow 2-bed quench
S-200 :Radial flow 2-bed inter cooled
S-250 : S-200, heat recovery, 2nd
converter with 1 radial flow bed
S-300 :Radial flow 3-bed inter cooled
52. Ammonia Casale
Ammonia Casale - 'axial-radial'
concept
- radial flow without a top cover on
the beds
- simpler mechanical design
No. of beds & type of inter-bed
cooling varies;
typically 3 bed, 2 interchanger.
53. ICI Types
Lozenge quench converter :
• single bed divided into 3 parts by quench
addition
• simple concept but suffered high pressure
drop
ICI AMV Process :
• Low pressure loop with H2 recovery at loop
pressure
• range of converters in use
• Terra: ICI 3-bed, 1 quench + 1 intercooler
axial flow
ICI LCA Process :
• Tube-cooled + adiabatic design.