ammonia production

1. Pharos University
Petrochemical Department
Dr. Ahmed Saad Shehata
Alexandria Fertilizers Company
2019

2. Production of Ammonia
2
Lecture (4)
The chemistry of the process is simple; the reaction is:
The reaction is exothermic; the net heat of reaction is about
11,000 cal/g-mole at 18°C assuming NH3 is in the gaseous
state.
History of Ammonia Synthesis
formed
NH
of
kCal/kmol
11
2NH
3H
N 3
3
2
2 



3. Different Feedstocks for Ammonia Production
3
Lecture (4)
Feedstock for Ammonia Production with
Suitable Processing Methods
Natural gas Steam Reforming
Liquefied petroleum gas Steam Reforming
Naphtha Steam Reforming
Refinery gases Steam Reforming
Coke oven gas Steam Reforming
Heavy oil Partial oxidation
Hydrogen-rich off-gases Combined with other
process
Coal Gasification

4. Different Feedstocks for Ammonia Production
4
Lecture (4)
Steam reformation process of light hydrocarbon particularly
Natural Gas (NG) is the most efficient route for the
production of ammonia.
The other routes are the partial oxidation of heavy oils.
Coal as also been used to produce ammonia.
The following is an approximate comparison of the energy
consumption, cost of production and the capital cost of the
plants for three the feedstocks.
Natural gas Heavy oil Coal
Energy consumption 1.0 1.3 1.7
Investment cost 1.0 1.4 2.4
Production cost 1.0 1.2 1.7

5. Ammonia Production Process
5
Lecture (4)
Ammonia is produced basically from water, air and energy.
The energy source is usually hydrocarbon that provides
hydrogen for fixing the nitrogen. The other energy input
required is steam and power.
This can be through coal or petroleum products or purchased
power from a utility company.

6. Ammonia Production Process
6
Lecture (4)
The complete process of industrial ammonia production may
be subdivided into the following sections:
1. Synthesis gas production.
• Feedstock pretreatment and gas generation.
• Carbon monoxide conversion.
• Gas purification.
2. Compression.
3. Synthesis.

7. Ammonia Production Process
7
Lecture (4)
Block diagram of
ammonia production by
steam reforming of
natural gas

8. Ammonia Production Process
8
Lecture (4)
Three main types of process are currently used for synthesis gas
production:
1. Steam reforming of natural gas or other light hydrocarbons
(natural gas liquids, liquefied petroleum gas, naphtha).
2. Partial oxidation of heavy fuel oil or vacuum residue.
3. Coal gasification.

9. Ammonia Production Process
9
Lecture (4)
For the present time and near future, the steam
reforming concept based on natural gas is considered
to be the most dominating and best available
technique for production of ammonia, as the steam
reforming process accounts for over 80% of the
world’s ammonia production. So, in our course we will
study the steam reforming as a source for synthesis
gas production.

10. Synthesis gas production using steam reforming
10
Lecture (4)
Feedstock desulphurization
To remove the sulphur from the feedstock over a zinc oxide
catalyst bed, as sulphur is poison to the catalysis used in the
subsequent processed. The sulphur level is reduced to less than
0.1 ppm.
Natural gas is compressed to reformer pressure, and preheated.
Then, any remaining sulfur is removed to avoid poisoning of
catalysts. The sulfur removed by adsorption on activated carbon
at ambient temperature or by absorption by hot zinc oxide (290-
400°C) after the gas has been preheated.
ZnO + H2S → ZnS + H2O

11. Synthesis gas production using steam reforming
11
Lecture (4)
Feedstock desulphurization
Some natural gas may contain significant amounts of chlorides,
which can poison catalysts, particularly the low-temperature shift
catalyst.
If the feedstock contains nonreactive sulfur (RSH), hydro treating is
required.
The preheated gas or vaporized naphtha is mixed with a small
amount of hydrogen (recycled synthesis gas) and passed through a
“hydrotreater” containing a cobalt-molybdenum catalyst, which
converts sulfur compounds to H2S: the gas then goes to a sulfur-
removal catalyst (zinc oxide).
RSH + ½ H2 ↔ H2S + RH

12. Synthesis gas production using steam reforming
12
Lecture (4)
Primary Reforming
The gas from the desulphurizer is mixed with process steam,
usually coming from an extraction turbine, and steam gas
mixture is then heated further to 500-600°C in the convection
section before entering the primary reformer.
The amount of process steam is given to adjust steam to carbon-
molar ratio (S/C ratio), which should be around 3.0 for the
reforming processes.

13. Synthesis gas production using steam reforming
13
Lecture (4)
Primary Reforming
The optimum ratio depends on several factors:
1. Feedstock quality.
2. Puree gas recovery.
3. Primary reformer capacity.
4. Shift operation.
5. The plant steam balance.

14. Synthesis gas production using steam reforming
14
Lecture (4)
Primary Reforming
The primary reformer consists of a large number of high-nickel
chromium alloy tubes filled with nickel-containing reforming
catalyst in a big chamber (Radiant box) with burners to provide
heat.
The overall reaction is highly endothermic and additional heat is
provided by burning of gas in burners provided for the purpose,
to raise the temperature to 780-830°C at the reformer gas outlet
and 1100-1200°C at the top of the fire furnace (flue gas side).

15. Synthesis gas production using steam reforming
15
Lecture (4)
Primary Reforming
The heat for the primary reforming is supplied by burning natural
gas or other gaseous fuels, in the burners of a radiant box
containing catalyst filled tubes.
The composition of gas leaving the reformer is given by close
approach to the following chemical equilibrium:
CH4 + H2O ↔ CO + 3H2 DH = 49.2 kcal/mol
CO + H2O ↔ CO2 + H2 DH = -9.8 kcal/mol

16. Synthesis gas production using steam reforming
16
Lecture (4)
Primary Reforming
The flue gas leaving the radiant box has temperature in
excess of 900°C, after supplying the high level heat to the
reforming process. About 50-60% of fuel’s heat value is
directly used in the process itself. The heat content (waste
heat) of the flue-gas is recovered in the reformer convection
section, for various process and steam duties.
The fuel energy required in the conventional reforming
process is 40-50% of the process feed energy. The flue-gas
leaving the convection section at 100-200°C is one of the
main sources of emissions from the plant. These emissions
are mainly CO2, NOx, with small amounts of SO2 and CO.

17. 17
Lecture (4)
Primary reformer
Primary Reforming
Synthesis gas production using steam reforming

18. 18
Lecture (4)
Reformer firing box
Primary Reforming
Synthesis gas production using steam reforming

19. 19
Lecture (4)
Secondary Reforming
The gas leaving the primary reformer usually contains 5-15%
methane (dry basis). The object of the secondary reforming step
is to complete the conversion of methane to H2, CO, and CO2 and
to supply the required proportion of N2 for NH3 synthesis.
This is done by adding air in the amount required to give an N:H
atomic ratio of 1:3 in the synthesis gas after the shift conversion
step.
The oxygen accompanying the nitrogen in the air burns part of
the combustibles (H2, CO, and CH4) in the partially reformed gas,
thereby raising the temperature high enough or rapid completion
of the reforming.
Synthesis gas production using steam reforming

20. 20
Lecture (4)
Secondary Reforming
The process air is compressed to the reforming pressure and heated
further in the primary reformer convection section to about 500°C.
The process gas is mixed with the air in the mixing chamber of
secondary reformer then passed over a nickel catalyst.
The reformer outlet temperature is around 1000°C, and up to 99%
of the hydrocarbon feed (to primary reformer) is converted, giving a
residual; methane content of 0.2-0.3 (dry gas bases) in the process
gas leaving the secondary reformer.
The process gas is cooled to 350-400°C in a waste heat boiler or
waste heat boiler/superheater downstream from the secondary
reformer.
Synthesis gas production using steam reforming

21. 21
Lecture (4)
Primary
and
secondary
reformer
Secondary Reforming
Synthesis gas production using steam reforming

22. 22
Lecture (4)
Carbon Dioxide Conversion
Water-gas shift reaction:
The water-gas shift (WGS) reaction is used to convert carbon
monoxide (CO) to carbon dioxide (CO2) and hydrogen (H2)
through a reaction with water (H2O)
CO + H2O ↔ CO2 + H2 DH = -41 kJ/mol
The reaction is exothermic, which means the reaction
equilibrium shifts to the right and favors formation of the H2 and
CO2 products at lower temperatures. At higher temperatures, the
equilibrium shifts to the left, limiting complete conversion of CO
to H2.
Synthesis gas production using steam reforming

23. 23
Lecture (4)
Carbon Dioxide Conversion
Water-gas shift reaction:
The reaction is the basis for most of the industrial H2 produced in
the world from methane (CH4) in natural gas through steam-
methane reforming. Methane is first reformed to a mixture of
CO, CO2 and H2 in the presence of steam over a nickel catalyst. A
conventional water-gas shift reactor then uses a metallic catalyst
in a heterogeneous gas-phase reaction with CO and steam.
Synthesis gas production using steam reforming

24. 24
Lecture (4)
Carbon Dioxide Conversion
Water-gas shift reaction:
Although the equilibrium favors formation of products at lower
temperatures, the reaction kinetics are faster at elevated
temperatures. For this reason, the catalytic water-gas shift
reaction is initially carried out in a high-temperature shift (HTS)
reactor at 350-370°C. Conversion in the HTS reactor is limited by
the equilibrium composition at the high temperature. To achieve
higher conversions of CO to H2, the gas leaving the HTS reactor is
cooled to 200-220°C and passed through the LTS reactor.
Synthesis gas production using steam reforming

25. 25
Lecture (4)
Catalytic WGS process
Reformer HTS LTS H2 to
purification
Steam
Natural
gas
Carbon Dioxide Conversion
The reverse water-gas shift reaction is one of the available
methods for production of CO gas which important in Fischer-
Tropsch Process used to produce long chain hydrocarbons.
Synthesis gas production using steam reforming

26. 26
Lecture (4)
Carbon Dioxide Conversion
Carbon monoxide conversion process:
As ammonia synthesis needs only nitrogen and hydrogen, all
carbon oxides must be removed from the raw synthesis gas of
the gasification process. In the water gas shift reaction, the
carbon monoxide serves as reducing agent for water to yield
hydrogen and carbon dioxide. In this way not only is the carbon
monoxide converted to readily removable carbon dioxide but
also additional hydrogen is produced:
kJ/mol
1.2
4
ΔH
H
CO
O
H
CO 0
298
2
2
2 




Synthesis gas production using steam reforming

27. 27
Lecture (4)
Carbon Dioxide Conversion
Carbon monoxide conversion process:
As no volume change is associated with this reaction, it is practically
independent of pressure, but as an exothermic process, it is favored
by lower temperatures, which shift the equilibrium to the right-
hand side. Even with a low excess of steam in the gas, the
equilibrium concentrations of CO are low; for example. 0.2 vol % at
220°C and 0.12 vol% at 200°C.
The process is performed in steps, with intermediate heat removal
between the individual catalyst beds, in which the reaction runs
adiabatically. Recently, quasi-isothermal reactors have been
developed in which cooling tubes run though the catalyst layers.
Synthesis gas production using steam reforming

28. 28
Lecture (4)
Carbon Dioxide Conversion
Carbon monoxide conversion process:
The process gas from the secondary reformer contains 12-15%
CO (dry gas bases) and most of the CO is convened in the shift
section according to the reaction:
CO + H2O ↔ CO2 + H2
In the high temperature shift conversion (HTS), the gas is passed
through a bed of iron oxide/chromium oxide catalyst at around
400°C, where the CO content is reduced to about 3% (dry gas
bases), limited by the shift equilibrium at the actual operating
temperature. There is tendency to use copper containing catalyst
to increase conversion.
Synthesis gas production using steam reforming

29. 29
Lecture (4)
Carbon Dioxide Conversion
Carbon monoxide conversion process:
The gas from the HTS is cooled and passed through the low
temperature shift (LTS) converter.
The LTS is filled with a copper oxide/zinc oxide-based catalyst and
operates at about 200-220°C.
The residual CO content is important for the efficiency of the
process. Therefore, efficiency of shift step in obtaining the
highest shift conversion is very important.
Synthesis gas production using steam reforming

30. 30
Lecture (4)
HTS and LTS reactors
Synthesis gas production using steam reforming
Carbon Dioxide Conversion

31. 31
Lecture (4)
Gas Purification
A. Carbon dioxide removal:
The process gas from the low temperature shift converter contains
mainly H2, N2, CO2 (≈ 18%) and excess process steam. The gas is
cooled and most of the excess steam is condensed before it enters
the CO2 removal section. The condensate usually contains 1500-
2000 ppm of ammonia, 800-1200 ppm of methanol and minor
concentration of other chemicals. All these are stripped and in the
best practices the condensate is recycled. The heat released during
cooling/condensation is used for:
Regeneration of CO2 scrubbing solution.
Driving the absorption process.
Boiler water preheat.
Synthesis gas production using steam reforming