Download - NH3 Syn Flowsheet
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Ammonia SynthesisFlowsheet
Operator TrainingBy
Gerard B. HawkinsManaging Director, CEO
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Introduction
Most modern ammonia processes arebased on steam-reforming of naturalgas or naphtha.
The 3 main technology suppliers areUhde (Uhde/JM Partnership), Topsoe& KBR.
The process steps are very similar in allcases.
Other suppliers are Linde (LAC) &Ammonia Casale.
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Simplified - NH3 PlantH2O
H/Cfeed
H/Cpurification
Removes
impur ities (S,Cl, metals)
Primaryreforming
Converts to
H2, CO, CO2 +
H2O + CH4
COShift
WGS
reaction
Secondaryreforming
Combustion +
Adiabatic Reforming+ Adds Nitrogen
Air
Ammonia
synthesis NH3
Converts N2 +
H2 => NH3
Syngas
compression
Purification
CO2 Removal
& Methanation
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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 arerequired to control bed temperatures.
Various designs are used depending on
contractor. Liquid Ammonia is recovered by
refrigeration.
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Simplif ied Flowsheet for a Typical Ammonia
Plant
NaturalGas
Steam
superheater
Air
Steam
30bar
Steam
Steam
raising
350 C200 C
HeatRecovery
Steam
raising
Cooling
Cooling
Reboiler
CO
Cooling
Preheater
Heat
Recovery
Steam
Boiler
ProcessCondensate
Quench
Quench
Liquid Ammonia
H
HydrodesulphuriserPrimaryReformer SecondaryReformer HighTemperature
Shift
LowTemperature
Shift
Ammonia Syn thesisMethanator
Carbon Dioxide
Purge Gas
Cooling
400 Co
390 Co
2
790 Co
550 Co
1000 Co
o
420 Co
150 Co
400 Co
470 Co
o
220 Co
290 Co
330 Co
2
CO Removal2
220 bar
Refrigeration
CondensateCooling
Ammonia
Catchpot
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Ammonia Plant Steam & Power
System Waste Heat recovery is used to raise
HP steam, 100 120 bar
Steam is used to drive the maincompressors Process air
Syn gas compression + circulator
Refrigeration
Pass-out steam is used for process.
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Ammonia Flowsheet Variations
1. Uhde
Top fired reformer Cold outlet manifold design
Secondary reformer with internal riser
H P loop (200 bar) with radial flowconverter 1 or 2 converters
Once-through synthesis section upstreamof main synthesis loop for very largecapacities (dual pressure Uhde process)
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Ammonia Flowsheet Variations
2. KBR Top-fired reformer
With internal risers
Several synthesis loop options:
Conventional 140 bar loop with 4bedquench converter
Higher pressure for large-scale plants
Horizontal converter on modern plants.
KAAP design 100 bar loop with Ru/Ccatalyst
Braun Purifier flowsheet Excess air with cryogenic purifier to
remove excess N2 and inerts from MUG
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Ammonia Flowsheet Variations
3. Topse Side-fired reformer
Radial flow converter
S-100 2 bed quench S-200 2 bed intercooled
S-250 = S-200 + boiler + 2nd converter
(1 bed) S-300 3 bed intercooled
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Ammonia Flowsheet Variations
4. Linde LAC (Linde Ammonia
Concept)
Hydrogen plant + N2 addition fromair separation unit
Ammonia Casale synthesis loop
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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 GHRtechnology
AMV / LCA technology is now partof JMs background in ammonia
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Ammonia Synthesis Mechanism
Dissociative adsorption of H2
Dissociative adsorption of N2 -
Believed to be the Rate DeterminingStep (RDS)
Multi-step hydrogenation of
adsorbed N2
Desorption of NH3
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Typical Uhde Synthesis Loop
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Uhde Dual-Pressure Process
C.W.Make up gas
from frontend
C.W.
Steam
Once
through
converter
SynthesisLoop
Purge
NH3NH3
NH3
1 2 3 R
C.W.Make up gas
from frontend
C.W.
Steam
Once
through
converter
SynthesisLoop
Purge
NH3NH3
NH3
1 2 3 R
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Effect of Pressure on Ammonia
Equilibrium Concentration
0
10
20
30
40
50
60
50 75 100 125 150 175 200 225 250 275 300NH3concentration%
Pressure bara
380 C
400 C
420 C
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Ammonia Equilibrium Diagram
300(572)
350(662)
400(752)
450(842)
500(932)
550(1022)
600(1112)
650(1202)
0
10
20
30
40
Equilibrium
Max Rate
Temperature C (F)
Ammoniac
ontent%
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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
NH
3concentratio
n%
Pressure bara
0 C
minus 20 C
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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
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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 orafter separator).
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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 verysensitive to MUG H / N ratio becausethe reaction consumes gas in a 3 : 1ratio.
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Synthesis Loop Principles:
H2 : N2 ratio example
H / N = 3 : 1
MUG NH3 Purge
H2 3000 2700 300N2 1000 900 100
H / N 3.0 3.0 3.0
H / N = 2.95 : 1
H2 2950 2700 250N2 1000 900 100
H / N 2.95 3.0 2.50
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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% ofthe total) dissolve in the product NH3.
A few loops with purified make-up gashave a self-purging loop where all theinerts are removed in solution in theproduct.
The NH3 content of the purge at theflowmeter position is required to check the
loop mass balance.
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Synthesis Loop Principles :
Effect of H2 Recovery
Most modern loops have H2 recovery.
2 systems are used, cryogenic ormembrane.
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 H2addition.
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Synthesis Loop Principles :
Control of Catalyst Bed Temperatures
Multi-bed design : 2, 3, or 4 catalyst beds with
intermediate cooling.
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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 betterheat recovery (heat available at a higher
temperature) Split converter designs further increase
the heat recovery temperature.
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3 Bed Converter Example
450 C
1. Optimum Catalyst
Temperatures
410 C
520 C
415 C
480 C
410 C
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3 i/c design
Cold Converter
410 C
520 C
415 C
480 C
410 C
450 C
120 C
335 C
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2 i/c design410 C
520 C
415 C
480 C
410 C
450 C
Hot Converter
235 C
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1 i/c design
410 C
520 C
415 C
480 C
410 C
450 C
Split Converter 305 C
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Converter Heat Recovery Example
In all cases the amount of heat recoveredis the same, only the availabletemperatures are different.
In all cases, the catalyst bed temperaturesare the same:
Bed 1 410 520 dT = 110
Bed 2 415 480 dT = 65Bed 3 410 450 dT = 40
Total Bed dT = Converter dT = 215
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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)
RelativeA
ctivit Severnside LCA
Standard Catalyst
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Effect of Size on Activity
Particle Diameter (mm)
14121086420
R
elativeAc
tivity
120
100
80
60
40
0
20
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Effect of Size on Activity
Smaller pellets = high activity
Therefore high production rate orsmaller catalyst volume
But pressure drop will rise
Either axial-radial or radial flowbeds are used to minimise
pressure drop
Radial flow is the basis of manyconverter internal retrofits
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Deactivation
Clean Gas
Thermal sintering
Contaminated Gas
Both Temporary and PermanentPoisoning
Oxygen induced sintering
By water, CO and CO2 Site blocking/Sintering
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Typical Operating Conditions
Temperature (oC) 360-520
Pressure (bar) 80-600
Space velocity (hr-1)1000-5000
Poisons oxygen and oxygen
compoundsnormally < 3ppm
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Catalyst Size
Grade Size
A 1.5-3.0 mm
B 3.0-4.5 mmC 3.0-6.0 mm
D / E 6.0-10.0 mm
G 14.0-20.0 mm
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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
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End
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Ammonia ConverterDesigns
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Converter Designs
Objectives for modern designs are;
- low pressure drop with small catalystparticles.
- high conversion per pass with high grade
heat recovery.Principal types are designed by:
Uhde
Kellogg (KBR) - conventional, Braun,
KAAPTopsoe
Ammonia Casale
JM (I C I)
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Uhde
Uhde design a range of converters:
Modern designs use radial flowwith inter-cooling & 'split
converters' with heat recoverybetween,
- Converter 1 : 2-bed, 1
interchanger
- Heat recovery (boiler)
- Converter 2 : 3rd bed.
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Uhde 3 bed
NH3 Converter
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M W Kellogg Converter Types
'Conventional' make-up gas and looplayout, refrigeration to low temperature (-25 C),
loop pressure typically 140 - 180 bar. Converters:
4 bed quench ; conventional Kelloggdesign.
Horizontal converter ;
lower cost, low pressure drop, easierinstallation
2 bed inter-cooled layout with small catalyst
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Kellogg Ammonia Quench ConverterOutlet
Inlet
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Kellogg Horizontal Converter
Bed 1Bed 2ABed 2B
Inlet
Outlet
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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
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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
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Haldor Topse S- Series
S-100 :Radial flow 2-bed quench
S-200 :Radial flow 2-bed inter cooled
S-250 : S-200, heat recovery, 2ndconverter with 1 radial flow bed
S-300 :Radial flow 3-bed inter cooled
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Topse S-200 ConverterInlet
Outlet
Cold
Bypass
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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-bedcooling varies;
typically 3 bed, 2 interchanger.
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ICI Types
Lozenge quench converter :
single bed divided into 3 parts by quenchaddition
simple concept but suffered high pressuredrop
ICI AMV Process :
Low pressure loop with H2 recovery at looppressure
range of converters in use Terra: ICI 3-bed, 1 quench + 1 intercooler
axial flow
ICI LCA Process :
Tube-cooled + adiabatic design.
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ICI Lozenge Quench Converter
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ICI Tube Cooled Converter
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ICI TCC Equilibrium Plot
300(572)
350(662)
400(752)
450(842)
500(932)
550(1022)
600(1112)
650(1202)
0
10
20
30
40
Equilibrium
Max Rate
Converter Profile
Temperature C (F)
Ammonia
content%