Describe all aspects of ammonia production

1. Chemical Process Technology
CHE-211
Asad U Khan (PhD)

2. Ammonia Manufacturing

3. Introduction:
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with
the formula NH₃. A stable binary hydride, ammonia is a colourless gas with a
distinctive pungent smell.
Formula: NH3
Molar mass: 17.031 g/mol
Boiling point: -33.34 °C
Density: 0.73 kg/m³
Melting point: -77.73 °C
Ammonia

4. Introduction:
• The Haber process, also called the Haber–Bosch process, is the main
industrial procedure for the production of ammonia
• The German chemists Fritz Haber and Carl Bosch developed it in the first
decade of the 20th century.
• The process converts atmospheric nitrogen (N2) to ammonia (NH3) by a
reaction with hydrogen (H2) using an iron metal catalyst under high
temperatures and pressures.
• This reaction is slightly exothermic, meaning that the reaction is favoured at
lower temperatures and higher pressures.
• It decreases entropy, complicating the process.
• Hydrogen is produced via steam reforming, followed by an iterative closed
cycle to react hydrogen with nitrogen to produce ammonia.
Ammonia Synthesis

5. Ammonia Synthesis
Hydrogen Production

6. Introduction:
The primary reaction is:
• Hydrogen required for ammonia synthesis is most often produced through gasification
of carbon-containing material, mostly natural gas, but other potential carbon sources
include coal, petroleum, peat, biomass, or waste.
• As of 2012, the global production of ammonia produced from natural gas using the
steam reforming process was 72%.
• Hydrogen can also be produced from water and electricity using electrolysis: at one
time, most of Europe's ammonia was produced from the Hydro plant at Vemork.
• Other possibilities include biological hydrogen production or photolysis, but at present,
steam reforming of natural gas is the most economical means of mass-producing
hydrogen.
Ammonia Synthesis

7. Introduction:
• The choice of catalyst is important for synthesizing ammonia.
• The most common catalyst consists catalyst particles consist of a core of magnetite
(FeO, ferrous oxide), surrounded by an outer shell of metallic iron
• In 2012, Hideo Hosono's group found that Ru-loaded calcium-aluminum oxide
C12A7:e− electride works well as a catalyst and pursued more efficient formation.
• This method is implemented in a small plant for ammonia synthesis in Japan.
• In 2019, Hosono's group found another catalyst, a novel perovskite oxynitride-
hydride BaCeO3−xNyHz, that works at lower temperature and without costly
ruthenium.
Ammonia Synthesis

8. Ammonia Synthesis
Hydrogen Production
• The major source of hydrogen is methane. Steam reforming extracts hydrogen from methane in a
high-temperature and pressure tube inside a reformer with a nickel catalyst. Other fossil fuel
sources include coal, heavy fuel oil and naphtha.
• Green hydrogen is produced without fossil fuels or carbon dioxide emissions from biomass, water
electrolysis and thermochemical (solar or another heat source) water splitting. However, these
hydrogen sources are not economically competitive with steam reforming.
Starting with a natural gas (CH4) feedstock, the steps are:
• Remove sulfur compounds from the feedstock, because sulfur deactivates the catalysts used in
subsequent steps. Sulfur removal requires catalytic hydrogenation to convert sulfur compounds in
the feedstocks to gaseous hydrogen sulfide (hydrodesulfurization, hydrotreating):
• Hydrogen sulfide is adsorbed and removed by passing it through beds of zinc oxide where it is
converted to solid zinc sulfide:

9. Ammonia Synthesis
Hydrogen Production
• Hydrogen sulfide is adsorbed and removed by passing it through beds of zinc oxide
where it is converted to solid zinc sulfide:
• Catalytic steam reforming of the sulfur-free feedstock forms hydrogen plus carbon
monoxide:
• Catalytic shift conversion converts the carbon monoxide to carbon dioxide and
more hydrogen:

10. Ammonia Synthesis
Hydrogen Production
• Carbon dioxide is removed either by absorption in aqueous ethanolamine
solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary
solid adsorption media.
• The final step in producing hydrogen is to use catalytic methanation to remove
residual carbon monoxide or carbon dioxide:

11. Ammonia Synthesis
Hydrogen Production
Reference: https://en.wikipedia.org/wiki/Haber_process
Figure: Illustrating inputs and outputs of steam reforming of natural gas, a process to
produce hydrogen

12. Ammonia Synthesis
Hydrogen Production
Steam Reforming Process
Equilibrium Conversion and
Conversion Rate Vs Temperature

13. Ammonia Synthesis
• The hydrogen is catalytically reacted with nitrogen (derived from process air) to form
anhydrous liquid ammonia.
• It is difficult and expensive, as lower temperatures result in slower reaction kinetics
(hence a slower reaction rate) and high pressure requires high-strength pressure
vessels that resist hydrogen embrittlement.
• Diatomic nitrogen is bound together by a triple bond, which makes it relatively inert.
Yield and efficiency are low, meaning that the ammonia must be separated and the
gases reprocessed for the reaction to proceed at an acceptable pace.
Ammonia Synthesis

14. Ammonia Synthesis
• The following reactions is known as the ammonia synthesis loop:
• The gases (nitrogen and hydrogen) are passed over four beds of catalyst, with cooling
between each pass to maintain a reasonable equilibrium constant. On each pass, only
about 15% conversion occurs, but unreacted gases are recycled, and eventually conversion
of 97% is achieved.
• Due to the nature of the (typically multi-promoted magnetite) catalyst used in the
ammonia synthesis reaction, only low levels of oxygen-containing (especially CO, CO2 and
H2O) compounds can be tolerated in the hydrogen/nitrogen mixture. Relatively pure
nitrogen can be obtained by air separation, but additional oxygen removal may be
required.
Ammonia Synthesis

15. Ammonia Synthesis
• Because of relatively low single pass conversion rates (typically less than 20%), a
large recycle stream is required. This can lead to the accumulation of inerts in the
gas.
• Nitrogen gas (N2) is unreactive because the atoms are held together by triple bonds.
The Haber process relies on catalysts that accelerate the scission of these bonds.
• Two opposing considerations are relevant: the equilibrium position and the reaction
rate.
• At room temperature, the equilibrium is in favor of ammonia, but the reaction
doesn't proceed at a detectable rate due to its high activation energy.
• Because the reaction is exothermic, the equilibrium constant decreases with
increasing temperature following Le Châtelier's principle. It becomes unity at around
150–200 °C (302–392 °F)
Ammonia Synthesis

16. Ammonia Synthesis
Pressure/temperature
The steam reforming, shift conversion, carbon dioxide removal, and methanation
steps each operate at absolute pressures of about 25 to 35 bar, while the
ammonia synthesis loop operates at temperatures of 300–500 °C (572–932 °F)
and pressures ranging from 60 to 180 bar depending upon the method used. The
resulting ammonia must then be separated from the residual hydrogen and
nitrogen at temperatures of −20 °C (−4 °F).

17. Heat of Reaction

20. Effect of Pressure and Temperature

21. Ammonia Synthesis
Ammonia Synthesis

22. Ammonia Synthesis
Ammonia Synthesis
A simplified flow diagram of Ammonia Synthesis starting from Natural Gas
Reference: https://en.wikipedia.org/wiki/Haber_process

23. Ammonia Synthesis
Ammonia Synthesis
Ammonia Converter

24. Ammonia Synthesis

25. This is a simplified flowsheet of the commercial ammonia plant
Ammonia Synthesis

26. Catalysts
Iron-based catalysts
• The iron catalyst is obtained from finely ground iron powder, which is usually obtained by
reduction of high-purity magnetite (Fe3O4/ FeO·Fe2O3; one-part FeO and one-part Fe2O3).
• The pulverized iron is oxidized to give magnetite (Fe3O4, ferrous oxide) particles of a specific
size. The magnetite particles are then partially reduced, removing some of the oxygen.
• The resulting catalyst particles consist of a core of magnetite, encased in a shell of magnetite
(FeO, ferrous oxide), which in turn is surrounded by an outer shell of metallic iron.
Ammonia Synthesis

27. Catalysts
Iron-based catalysts
• The catalyst maintains most of its bulk volume during the reduction, resulting in a highly
porous high-surface-area material, which enhances its catalytic effectiveness. Minor
components include calcium and aluminium oxides, which support the iron catalyst and help it
maintain its surface area. These oxides of Ca, Al, K, and Si are unreactive to reduction by
hydrogen.
Ammonia Synthesis

28. Catalysts
• Catalysts other than iron
• Many efforts have been made to improve the Haber–Bosch process. Many metals were tested
as catalysts. The requirement for suitability is the dissociative adsorption of nitrogen (i. e. the
nitrogen molecule must be split into nitrogen atoms upon adsorption).
• If the binding of the nitrogen is too strong, the catalyst is blocked and the catalytic ability is
reduced (self-poisoning).
• The elements in the periodic table to the left of the iron group show such strong bonds. Further,
the formation of surface nitrides makes, for example, chromium catalysts ineffective.
• Metals to the right of the iron group, in contrast, adsorb nitrogen too weakly for ammonia
synthesis. Haber initially used catalysts based on osmium and uranium. Uranium reacts to its
nitride during catalysis, while osmium oxide is rare.
• According to theoretical and practical studies, improvements over pure iron are limited. The
activity of iron catalysts is increased by the inclusion of cobalt.
Ammonia Synthesis

29. Catalysts
Ruthenium
Ruthenium forms highly active catalysts. Allowing milder operating pressures and temperatures,
Ru-based materials are referred to as second-generation catalysts. Such catalysts are prepared by
the decomposition of triruthenium dodecacarbonyl on graphite. A drawback of activated-carbon-
supported ruthenium-based catalysts is the methanation of the support in the presence of
hydrogen. Their activity is strongly dependent on the catalyst carrier and the promoters. A wide
range of substances can be used as carriers, including carbon, magnesium oxide, aluminium
oxide, zeolites, spinels, and boron nitride.
Ruthenium-activated carbon-based catalysts have been used industrially in the KBR Advanced
Ammonia Process (KAAP) since 1992. The carbon carrier is partially degraded to methane;
however, this can be mitigated by a special treatment of the carbon at 1500 °C, thus prolonging
the catalyst lifetime. In addition, the finely dispersed carbon poses a risk of explosion. For these
reasons and due to its low acidity, magnesium oxide has proven to be a good choice of carrier.
Carriers with acidic properties extract electrons from ruthenium, make it less reactive, and have
the undesirable effect of binding ammonia to the surface.
Ammonia Synthesis

30. Catalysts
Catalyst poisons
Catalyst poisons lower catalyst activity. They are usually impurities in the synthesis gas.
Permanent poisons cause irreversible loss of catalytic activity and, while temporary poisons
lower the activity while present. Sulfur compounds, phosphorus compounds, arsenic compounds,
and chlorine compounds are permanent poisons. Oxygenic compounds like water, carbon
monoxide, carbon dioxide, and oxygen are temporary poisons.
Although chemically inert components of the synthesis gas mixture such as noble gases or
methane are not strictly poisons, they accumulate through the recycling of the process gases and
thus lower the partial pressure of the reactants, which in turn slows conversion.
Ammonia Synthesis

35. Ammonia Synthesis
Ammonia Synthesis

36. Ammonia Synthesis
Ammonia Synthesis

37. Ammonia Synthesis
Ammonia Synthesis

38. Ammonia production from Natural Gas (methane)

39. KBR designed one of the first single-train, large-capacity ammonia plants

40. Modern ammonia plants designed by KBR