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Introduction to Ammonia Manufacture Ammonia is generally manufactured from natural gas using the steam reforming process. Other feedstocks and processes are used but these are not described here. There are several reaction stages and catalysts are key to the economic operation of modern ammonia production plants. Diagram 1 illustrates the chemistry of the ammonia process and the basic materials of the catalysts used. The first stage is purification where impurities, mainly sulphur compounds, are removed from the gas stream. Steam reforming is performed in two stages. In the primary stage, the endothermic reactions take place at pressures around 30 bar and temperatures of 800°C or higher. This is followed by an exothermic secondary reformer where air is added to the partially reformed gas stream. The carbon monoxide in the gas leaving the secondary reformer is converted to carbon dioxide in the shift reactors and then removed by scrubbing from the gas stream. Any residual carbon oxides are then converted back to methane by methanation before compression of the hydrogen and nitrogen to ammonia synthesis pressure. The final reaction stage is ammonia synthesis where the hydrogen and nitrogen combine to form ammonia. This reaction stage takes place at high pressure (100-350 bar) and is highly exothermic. Diagram 2 shows a simplified block diagram of a typical 1000 tonne/day ammonia plant including details of operating temperatures, catalyst volumes, KATALCO catalyst and gas compositions. The economics of ammonia production require careful energy management as illustrated by the flowsheet in Diagram 3 which shows many heat exchangers are necessary to optimised heat recovery as well as to generate the steam required for process purposes.
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Introduction to Ammonia Manufacture
Ammonia is generally manufactured from natural gas using the steam reforming process. Other feedstocks and processes are used but these are not described here. There are several reaction stages and catalysts are key to the economic operation of modern ammonia production plants. Diagram 1 illustrates the chemistry of the ammonia process and the basic materials of the catalysts used.
The first stage is purification where impurities, mainly sulphur compounds, are removed from the gas stream.
Steam reforming is performed in two stages. In the primary stage, the endothermic reactions take place at pressures around 30 bar and temperatures of 800°C or higher. This is followed by an exothermic secondary reformer where air is added to the partially reformed gas stream.
The carbon monoxide in the gas leaving the secondary reformer is converted to carbon dioxide in the shift reactors and then removed by scrubbing from the gas stream. Any residual carbon oxides are then converted back to methane by methanation before compression of the hydrogen and nitrogen to ammonia synthesis pressure.
The final reaction stage is ammonia synthesis where the hydrogen and nitrogen combine to form ammonia. This reaction stage takes place at high pressure (100-350 bar) and is highly exothermic.
Diagram 2 shows a simplified block diagram of a typical 1000 tonne/day ammonia plant including details of operating temperatures, catalyst volumes, KATALCO catalyst and gas compositions.
The economics of ammonia production require careful energy management as illustrated by the flowsheet in Diagram 3 which shows many heat exchangers are necessary to optimised heat recovery as well as to generate the steam required for process purposes.
CHEMISTRY OF AMMONIA PROCESS
HYDRODESULPHURISER(Sulphur Removal)
PRIMARY REFORMING(Steam Reforming)
SECONDARY REFORMING(Air Addition)
HIGH TEMP SHIFT(CO Conversion)
AMMONIA SYNTHESIS(Ammonia Formation)
METHANATORCO/CO Polishing
LOW TEMP SHIFT(CO Conversion)
RSH + H RH + H SHCI + NaAlO Al0OH + NaCLH S + ZnO ZnS + H OCatalyst: CoMo/NiMo
Modified Alumina Zinc Oxide
2 2
2
22
CO + H O CO + H
Catalyst: Iron/Chromium/Copper
2 2 2
CO + H O CO + H
Catalyst: Copper/Zinc/Aluminium
2 2 2 N + 3H 2NH
Catalyst: Fused Promoted Magnetite
2 32
2
CO Removal
K CO + H O + CO 2KHCO32 2 2 3
2KHCO K CO + H O + CO3 2 3 2 2
CO + 3H CH + H OCO +4H CH + 2H O
Catalyst: Nickel Oxide
2 2
2
4 2
4 2
Ammonia
Natural
Gas
CH + H O 3H + COCO + H O H + CO
Catalyst: Nickel Oxide
2
4 2
2 2
2 CH + H O 3H + CO2H + [O + N ] 2H O + N
Catalyst: Nickel Oxide
2
4 2
2 2
2
2 2
2
Diagram 1
Simplified block diagram of typical 1000 tonne/day ammonia plant
Purification Primary Reforming SecondaryReforming
High TemperatureCO Shift
Low TemperatureCO Shift
Methanation AmmoniaSynthesis
CORemoval
Exit Gas Composition
NCOCOCHN + ACO + CO2
2
4
2
2
CO
Rem
ova
l
57-4
18 M
54-8
25 M
71-5
45 M
83-3
60 M
11-4
25 M
35-4/8
65 M
41-6
10 M
59-3 10M
390 Co 790 Co 1000 Co
420 Co 220 Co 330 Co 470 Co
400 Co
550 Co
350 Co 200 Co 290 Co 400 Co
Steam AirNatural Gas Feed
Hydrogen
3
3
3
3 3 3 3 3 3
69.89.3
10.510.4
--
Volume %56.512.97.50.3
22.8-
Volume %60.3
3.015.6
0.320.8
-
Volume %61.40.3
17.80.2
20.3-
Volume %74.6
0.40.10.3
24.6-
Volume %74.2
--
0.825.0
5ppm
MUGVolume %
63.4--
9.524.9
<5ppmNH3 - 2.5
LoopVolume %
Ammonia
32-5 24 M
2
2
Diagram 2
indicating KATALCO catalyst types and volumes required
KATALCO KATALCO KATALCO KATALCO KATALCO KATALCO
KATALCO
KATALCO
KATALCO
Simplified flowsheet for typical ammonia plant
Natural Gas
Steamsuperheater
Air
Steam
30 bar
Steam
Steamraising
350 C200 C
Heat Recovery
Steamraising
Cooling
Cooling
Reboiler
CO
Cooling
Preheater
HeatRecovery
Steam
Boiler
Process Condensate
Quench
Quench
Liquid Ammonia
H
Hydrodesulphuriser Primary Reformer
SecondaryReformer
High Temperature
Shift
Low Temperature
Shift
Ammonia SynthesisCO Removal Methanator
Carbon DioxidePurge 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
2
220 bar
Refrigeration
CondensateCooling
Diagram 3
