Lecture Slides by Prof. Imitaz Ali at Ghulam Ishaq Khan Institute for Engineering Sciences and Technology (GIKI).

1. Ammonia

2. Introduction
• Ammonia (NH3) is a compound of nitrogen
and hydrogen. It is a colorless gas with a
characteristic pungent smell of 'wet nappies'
− this is because urine can decompose to
ammonia.
Year Million tonnes of Ammonia
2005 148
2006 153
2007 159
2008 158
2009 158
2010 159

3. World consumption of Ammonia -
2011
China
33%
Russia
11%
India
9%
Pakistan
1%
America
11%
Others
35%

4. Uses
• Ammonia is major raw material for fertilizer
industries
• It is used during the manufacture of Nitro
compounds, Fertilizers e.g. urea, ammonium
sulfate, ammonium phosphate etc.
• It is also used in manufacture of Nitric acid,
Hydroxylamine, Hydrazine, Amines and
amides, and in many other organic
compounds

5. Uses
20% is used in
manufacturing:
• Plastics
• Synthetic fibres
• Explosives
• Dyes
• Pharmaceuticals
• Industrial refrigerant
• Industrial and domestic
cleaning agent

6. History
• Gaseous ammonia was first isolated by Joseph Priestley in 1774 and was
termed as "alkaline air". Claude Louis Berthollet ascertained its
composition in 1785.
• The Haber-Bosch process to produce ammonia from the nitrogen in the
air was developed by Fritz Haber and Carl Bosch in 1909 and patented in
1910.
Prior to the availability of cheap natural gas, hydrogen as a precursor to ammonia
production was produced via the electrolysis of water or using the chloralkali process.

7. Production of Ammonia (kt) in
Pakistan
0
20
40
60
80
100
120
140
160
2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

8. Process
• Ammonia is synthesized from nitrogen and hydrogen by the
following reaction:
N2 + 3H2 ⇌ 2NH3
• The best available source of nitrogen is from atmospheric air. The
hydrogen required can be produced from various feedstock but
currently it ias derived mostly from fossil fuels.
• Depending of the type of fossil fuel, two different methods are
mainly applied to produce hydrogen for ammonia production:
– steam reforming CnHm + nH2O  nCO + (n+ m/2) H2 H > 0
– partial oxidation CnHm + n/2 O2  nCO + m/2 H2 H < 0
• Steam reforming of light hydrocarbons is the most efficient route,
with about 77% of world ammonia capacity being based on natural
gas.

11. Steam reforming of NG
A) Synthesis gas
production
– Feedstock
pretreatment and gas
generation
– Carbon monoxide
conversion
– Gas purification
B) Compression
C) Synthesis

12. Hydro-desulfurization
Raw materials
• Air, natural gas (or other
hydrocarbons) and steam.
• Removal of sulfur from feedstock,
e.g. natural gas, involves these
chemical reactions:
R-CH2SH + H2  H2S + RCH3
H2S + ZnO  ZnS + H2O
• zinc oxide beds are typically not
regenerated but are replaced
with new absorbent once
exhausted

13. Primary reformer
• After the removal of sulfur compounds, the gas is mixed
with superheated steam and fed into a primary reformer.
• To achieve the required stoichiometric hydrogen/nitrogen
ratio for ammonia synthesis, the reforming reaction is split
into two sections.
• In the first section, the primary reformer, the reaction
proceeds to achieve a partial conversion only [in
conventional plants 65 % based on methane feed, leaving
around 14 mol % methane (dry basis) in the effluent gas].
• In the secondary reformer— the gas is mixed with a
controlled amount of air.
• The steam/carbon ratio used in modern commercial
primary reformers for natural gas is between 2.8 and 3.5.

14. Primary reformer
The burners primary reformers can be classified as top-fired, side-
fired, terraced-wall, or, less common, bottom-fired reformers

15. Primary reformer
• The mixture is heated to 770oC in the
presence of a nickel catalyst and
these reversible reactions occur:
CH4 + H2O ⇌ CO + 3H2 ∆H=+206kJmol−1
CnHm+ nH2O ⇌ nCO + (n+ m/2) H2
CO + H2O ⇌ CO2 + H2 ∆H=−41kJmol−1
Overall reaction:
CH4 + 2H2O ⇌ CO2 + 4H2 ∆H=+165kJmol−1
• The mixture that emerges is called
synthesis gas
• The overall reaction (CH4 and steam
to CO2 and H2) is highly endothermic.
• Therefore, maintaining a high
reaction temperature moves the
position of equilibrium to the right.

16. Primary reformer
• Primary reformer has been identified as bottle-neck in increasing
the capacity of existing natural-gas-based ammonia plants.
• This can be addressed by
– having pre-reforming, installed at the up-stream
• to transfer part of the conversion duty to the secondary reformer with
application of an superstoichiometric amount of air.

17. Secondary reformer
• Only 30-40% of the hydrocarbon is
reformed in the primary reformer
because of the equilibrium reactions.
The synthesis gas is cooled slightly to
735 oC, mixed with air and passed
into the secondary reformer. Highly
exothermic reactions happen such as:
CH4 + 2O2  2H2O + CO2 ∆H=−82kJmol−1
• With the energy released and further
heating a temperature of about
1000oC is reached and up to 99%
conversion to methane to hydrogen
achieved.
• Nitrogen (from the air) is used later in
the synthesis of ammonia.

18. Uhde combined autothermal reformer (CAR)
a) Sandwich type tubesheet; b) Enveloping tube;
c) Reformer tubes; d) Tubesheet; e) Refractory lining;
f) Water jacket
Alternate technologies
Kellogg reforming exchanger System (KRES)
exchanger reforming + noncatalytic partial
oxidation

19. Shift conversion
• Any remaining carbon monoxide in the gas
mixture is converted to carbon dioxide in the shift
section of the process:
CO + H2O ⇌ CO2 + H2 ∆H = −41 kJ mol−1
• The shift conversion happens in two stages:
–HTS | High temperature shift (iron oxide/chromium
oxide based catalyst at about 400 oC) that lowers the
CO content from 12-15% to about 3%.
–LTS | Low temperature shift (copper oxide/zinc oxide -
based catalyst at about 200-220 oC) which lowers the
CO content further to about 0.2-0.5%.

20. Purification
• CO2 containing synthesis gas is scrubbed under pressure with a solvent
capable of dissolving carbon dioxide in sufficient quantity and at sufficient
rate, usually in countercurrent in a column equipped with trays or packing
• The CO2-laden solvent is flashed, often in steps, to around atmospheric
pressure, and the spent scrubbing liquid is subsequently heated and
regenerated in a stripping column before being recycled to the pressurized
absorption column
• CO2 is removed by chemical or physical absorption.
• Chemical Absorption
– Tertiary amines methyldiethanolamine (MDEA) together with an activator. Approximately 80
percent of the ammonia plants use this process to aid in removing CO2
– The potassium carbonate with an activator, corrosion inhibitor
• Benfield LoHeat System (UOP)
• Catacarb Process
• Giammarco -Vetrocoke process
• LRS 10 of British Gas
• Exxon's Flexsorb process

22. Purification
• Physical solvent e.g. polyethylene glycol dimethyl ether,
polyethylene glycol methyl isopropyl ether, polypropylene
carbonate etc
– Selexol Process (UOP)
– Sepasolv MPE process (BASF)
– Fluor Solvent Process
• After bulk removal of the carbon oxides has been accomplished by
shift reaction and CO2 removal, the typical synthesis gas still
contains 0.2 – 0.5 vol % CO and 0.005 – 0.2 vol % CO2. These
compounds and any water present have to be removed down to a
very low ppm level, as all oxygen-containing substances are poisons
for the ammonia synthesis catalyst
• The gas mixture is further cooled to 40°C, water condenses out and
is removed.

23. Purification
Methanation
• Any remaining amounts of CO and CO2 must be removed before the NH3
synthesis stage as they would poison the catalyst. Methanation is the
simplest method to reduce the concentrations of the carbon oxides well
below 10 ppm and is widely used in steam reforming plants.
• The reaction is carried out over a supported nickel catalyst at a pressure
of 25 – 35 bar and a temperature of 250 – 350 °C.
CO + 3H2 ⇌ CH4 + H2O ∆H = −206 kJ mol−1
CO2 + 4H2 ⇌ CH4 + 2H2O ∆H = −165 kJ mol−1
• The emerging gas must be completely dry so water produced in these
reactions is removed by condensation. This is accomplished by passing the
makeup gas through molecular sieve adsorbers, which can be positioned
on the suction side or in an intermediate-pressure stage of the synthesis
gas compressor.

24. Effect of catalyst on reaction rate
• A catalyst has no effect on equilibrium. It speeds up
both forward and backward reactions equally.
• Provide an alternate pathway with lower activation
energy
• Increase rate of reaction

25. Effect of pressure on reaction rate
• Collision theory
– High pressure - More successful collisions
• Increase rate of reaction

26. Le Chatlier’s Principle
• If a chemical system at equilibrium experiences a change
in concentration, temperature, volume, or partial
pressure, then the equilibrium shifts to counteract the
imposed change and a new equilibrium is established.
N2 + 3 H2 ⇌ 2 NH3 + 92kJ
– Decreasing the temperature of a system in dynamic
equilibrium favours the exothermic reaction.
4 volumes ⇌ 2 volumes
– an increase in system pressure due to decreasing
volume causes the reaction to shift to the side with
the fewer moles of gas.

27. Compression
• Centrifugal compressor for make-up and
recycle gas compression of an ammonia plant
a) Air cooler; b) Separator; c) Silencer; d) Water cooler

28. Compression
Advantages over reciprocating compressors:
– Lower investment (single machines even for very
large capacities)
– Lower maintenance cost
– Less frequent shutdowns for preventive
maintenance
– High reliability (low failure rate)
Other compression duties in the plants are also performed by centrifugal
compressors.

29. Ammonia synthesis
Ammonia synthesis
• The synthesis of NH3 takes place on
an iron catalyst at pressures usually
in the range 10-25 MPa and
temperatures in the range 350-
550oC
N2 + 3H2 ⇌ 2NH3 ∆H = –92 kJ mol−1
• The conditions used are a
compromise between yield, speed
& energy demands.
Mittasch

30. Ammonia synthesis
• The synthesis reaction is limited by the unfavorable position of the
thermodynamic equilibrium, so that only partial conversion of the synthesis
gas (25 – 35 %) can be attained on its passage through the catalyst.
• Ammonia is separated from the unreacted gas by condensation, which
requires relatively low temperatures for reasonable efficiency.
• The unconverted gas is supplemented with fresh synthesis gas and recycled to
the converter.
• The concentration of the inert gases (methane and argon) in the synthesis
loop is controlled by withdrawing a small continuous purge gas stream.
• These basic features together with the properties of the synthesis catalyst and
mechanical restrictions govern the design of the ammonia synthesis converter
and the layout of the synthesis loop.

31. Ammonia synthesis
Schematic flow diagrams of typical ammonia synthesis loops
A) Synthesis loop for pure and dry make-up gas; B) Product recovery after recycle compression; C) Product recovery before
recycle compression (four-nozzle compressor design); D) Two stages of product condensation
a) Ammonia converter with heat exchangers; b) Ammonia recovery by chilling and condensation; c) Ammonia recovery by
condensation at ambient temperature; d) Synthesis gas compressor; e) Recycle compressor
Molecular sieve drying of
make-up gas has increasingly
been applied in order to realize
the energy-saving arrangement
of the synthesis loop
corresponding to Figure A.

32. Ammonia synthesis
a) Locus of
temperatures
resulting in
maximum
reaction rate
space velocity (per hour)  1/ contact time
reaction
rate v in
m3 NH3 /
(m3 cat
alyst · s)

33. Influence of the different variables on
the Synthesis Loop
• Pressure: increasing pressure will increase conversion
due to higher reaction rate and more favourable
ammonia equilibrium.
• Inlet temperature: there are two opposed effects as
increasing temperature enhances reaction rate but
decreases the adiabatic equilibrium concentration.
• Space velocity: increasing the space velocity normally
lowers the outlet ammonia concentration, but
increases total ammonia production.
• Inert level: increasing the inert level lowers the
reaction rate for kinetic and thermodynamic reasons

34. Ammonia converter
Commercial Ammonia Converters
• Commercial converters can be classified into two main
groups:
– Internally cooled with cooling tubes running through the
catalyst bed or with catalyst inside the tubes and the cooling
medium on the shell side. The cooling medium is mostly the
reactor feed gas, which can flow counter- or cocurrently to the
gas flow in the synthesis catalyst volume (tube-cooled
converters).
– The catalyst volume is divided into several beds in which the
reaction proceeds adiabatically. Between the individual catalyst
beds heat is removed by injection of colder synthesis gas
(quench converters) or by indirect cooling with synthesis gas or
via boiler feed water heating or raising steam (indirectly cooled
multi bed converter).

35. Ammonia converter
• Assignment
– Historical evolution of ammonia converter
technology.
• 15 hand written pages
• Deadline : 8 November 2013

36. Complete Ammonia Production Plants
Modern integrated single-train ammonia plant based on steam reforming of natural gas (Uhde process)
a) Sulfur removal; b) Primary reformer; c) Steam superheater; d) Secondary reformer; e) waste-heat boiler; f) Convection section; g) Forced draft fan; h) Induced
draft fan; i) Stack; k) HT and LT shift converters; l) Methanator; m) CO2 removal solvent boiler; n) Process condensate separator; o) CO2 absorber; p) Synthesis
gas compressor; q) Process air compressor; r) Ammonia converter; s) High-pressure ammonia separator; t) Ammonia and hydrogen recovery from purge and
flash gas

37. Complete Ammonia Production Plants
M.W. Kellogg's low energy process

38. Complete Ammonia Production Plants
Haldor Topsøe's low energy process
a) Desulfurization; b) Primary reformer; c) Secondary reformer; d) Shift conversion; e) CO2 removal; f) Methanation; g) Main compressor; h) Recycle compressor; i) Heat
recovery; j) Converter

39. Complete Ammonia Production Plants
The Braun purifier ammonia process
a) Sulfur removal; b) Primary reformer; c) Convection section; d) Secondary reformer; e) waste-heat boiler; f) Process air compressor; g) Gas turbine; h) High- and low-
temperature shift converters; i) CO2 removal solvent reboiler; k) CO2 absorber; l) CO2 desorber; m) CO2stripper; n) Methanator; o) Driers; p) Purifier heat exchanger; q)
Expansion turbine; r) Purifier column; s) Synthesis gas compressor; t) Synthesis converters; u) waste-heat boiler; v) High-pressure ammonia separator; w) Ammonia letdown
vessel; x) Ammonia recovery from purge gas

40. Complete Ammonia Production Plants
ICI AMV process
a) Desulfurization; b) Natural gas saturation; c) Process air compression; d1) Primary reformer; d2) Secondary Reformer; e) Boiler; f) High temperature shift; g) Low
temperature shift; h) Selexol CO2 removal; h1) CO2 absorber; h2) Regenerator; i) Single stage compression; j) Methanation; k) Cooling and drying; l) Circulator; m) Hydrogen
recovery; n) Ammonia converter; o) Refrigeration system

41. Complete Ammonia Production Plants
ICI LCA process (core unit)
PAC, purified air compressor; HDS, hydrodesulfurization; IP, intermediate pressure; LP, liquefied petroleum; BFW, boiler feed water

42. Complete Ammonia Production Plants
KBR KAAPplus Process
a) Air compressor; b) Sulfur removal; c) Process heater; d) Automatic reformer (ATR); e) Reforming exchanger (KRES); f) Condensate stripper; g) CO2 absorber; h) Methanator;
i) CO2 stripper; j) Dryer; k) Expander; l) Feed/effluent exchanger; m) Condenser; n) Rectifier column; o) Synthesis gas compressor; p) KAAP ammonia converter; q)
Refrigeration compressor; r) Refrigeration exchanger

43. Complete Ammonia Production Plants
Ammonia production based on heavy fuel oil (Linde flow scheme with Texaco gasification)
a) Air separation unit; b) Soot extraction; c) CO2 absorption; d) Methanol/H2O distillation; e) Stripper; f) Hot regenerator; g) Refrigerant; h) Dryer; i) Liquid N2 scrubber; j)
Syngas compressor; k) NH3 reactor