CF Braun Ammonia Manufacturing Process
11 Process Steps
(1) Feed Desulfurization
(2) Primary Reforming
(3) Secondary Reforming
(4) Carbon Monoxide Shift Reaction Using High Temperature and Low Temperature
Catalysts
(5) Carbon dioxide Removal
(6) Methanation
(7) Molecular Sieve Drying
(8) Cryogenic Purification
(9) Compression
(10) Synthesis
(11) Ammonia Recovery
All reactions are catalytic. All reactions are exothermic except reforming which is
endothermic
1 Feed desulfurization
H2S present in the feed gas is adsorbed by zinc oxide in a reactor. Usually two reactors are
installed in a swing operation. One is active while the other is in a regeneration mode.
If the concentration in the feed gas is high, a MDEA absorption/Stripping system may have to be
used.
2 Primary Reforming Furnace
CH4 + H2O = CO +3H2
CH4 +2H2O = CO2 + 4H2
1100 F
485 psia
Endothermic, Catalytic
Conversion 30%
Convection heat is used to pre-heat feed, air and generate steam.
Reaction coil packed with catalyst heated by radiant heat.
3 Secondary Reforming
Same reactions as primary reforming
1600F
Slightly lower pressure than 485 psia
Catalytic
87% conversion
Reactor is cylindrical vessel filled with catalyst
Inject air directly to reactor to cause combustion with H2 and hydrocarbons.
Heat of combustion is more than sufficient for the endothermic reforming reaction resulting in a
product with a temperature much higher than the feed
Amount of air is controlled so that the H2/N2 mole ratio is about 2. Notice that the ratio is not 3
at this point.
4 Carbon Monoxide Shift Reaction Using High Temperature and Low Temperature
Catalysts
CO + H2O = CO2 + H2
Exothermic catalytic
High temperature reactor 800F 65% conversion.
Low Temperature reactor 400F almost 100% conversion.
High temperature increases rate of reaction
Low temperature increases equilibrium conversion
5 Carbon Dioxide Removal
Two stage Methyl diethanol amine (MDEA)
Absorption of CO2 by MDEA and Regeneration of MDEA by steam stripping dissolved
CO2
Produces pure CO2 byproduct
Most water is removed here by condensation
Absorption at 400 psia and stripping at atmospheric pressure
6 Methanation
Reverse of reforming reaction
Exothermic
Remove all residual CO and CO2.
CO poisons synthesis catalyst
CO2 freezes in Cryogenic section
CO + 3H2 = CH4 +H2O
CO2 + 4H2 = CH4 and 2H2O
600 F
7 Molecular Sieve Drying
Water is removed by a chiller followed by a knock-out drum.
Traces of water is removed by molecular sieve of desiccant to ppm range
8 Cryogenic Purification
This is the unique feature in the CF Braun Process
At this point the only components left are N2, H2 CH4 and traces of inert gases such as argon
which is present in the feed gas
The gas to the section has a H2/N2 ratio of 2.
A Cryogenic distillation column operating at -300 F produces an overhead consisting of H2 and
N2 in the ratio of 3, the stoichiometric ratio for NH3 formation and traces of Argon. All CH4 and
some N2 and argon, considered as waste gases are removed from the bottom of the column.
Even though cryogenic distillation operating at -300 F, through heat integration, no external
refrigerant is required.
The importance of the purification section is that it allows air to be introduced in the secondary
reformer in which the heat of combustion provides sufficient heat for the endothermic reaction.
Cryogenic distillation in this section removes any excess N2 introduced in the secondary
reformer.
9. Compression
Synthesis gas consisting H2 and N2 in the ratio of 3 and traces of argon is compressed to 2300
psia in a 4-stage compressor system with inter-cooling. Note that the synthesis reaction is
favored by high pressure.
10. Ammonia Synthesis
N2 +3H2 = NH3
Exothermic reaction
Equilibrium conversion is favored by high pressure and low temperature.
However, the rate of reaction always increases with temperature. Therefore, a temperature,
which balances equilibrium conversion with rate of reaction must be chosen. In the C F Braun
process 725 F and 2300 psia are the chosen conditions. It is also advantageous to use two
reactors in series with inter-cooling between reactors. The conversions for the two reactors are
about 15% and 11 % respectively.
The effluent from the last reactor is cooled and ammonia product is condensed. The unreacted
N2 and H2 and uncondensed NH3 and inert argon is recycled to the suction of the fourth stage
compressor. There is a bleed stream from the recycle to remove the inert gas argon.
11. Ammonia Recovery
Ammonia present in the bleed stream and pressure letdown drums is recovered by absorption and
stripping columns.