Ammonia and urea production

Manufacturing Process of
Ammonia & Urea

Manufacturing process of ammonia
• Introduction
• In large scale commercial ammonia production plants, the
feedstock which makes up the reactants are water, methane and
air.
• The hydrogen is formed by reacting natural gas and steam at
high temperatures and the nitrogen is supplied from the air.
(water and carbon dioxide are removed from the gas stream).
• Then ammonia is produced in a process known as the Haber
process, in which mixture of nitrogen and hydrogen passed over
an iron catalyst at high temperature and pressure to form the
ammonia.

• Ammonia Production
• In 1909 Fritz Haber established the conditions under which
nitrogen, N2(g), and hydrogen, H2(g), would combine using
• Medium temperature (~500oC)
• Very high pressure (~250 atmospheres, ~351kPa)
• A catalyst (a porous iron catalyst prepared by reducing
magnetite, Fe3O4).(Osmium is a much better catalyst for the
reaction but is very expensive)

• This process produces an ammonia, NH3(g), yield of
approximately 10-20%. The Haber synthesis was
developed into an industrial process by Carl Bosch.
• The reaction between nitrogen gas and hydrogen gas
to produce ammonia gas is exothermic, releasing
92.4kJ/mol of energy at 298K (25oC).
heat, pressure, catalyst
N2(g)+3H2(g) → 2NH3(g) heat= -92.4 kJ
mol-1

• Ammonia synthesis
• Ammonia is synthesized from hydrogen (from natural gas)
and nitrogen (from the air). Natural gas contains some
sulfurous compounds which damage the catalysts used in
this process. These are removed by reacting them with zinc
oxide, e.g.
• ZnO + H2S → ZnS + H2 O
• The methane from the natural gas is then converted to
hydrogen by steam reforming of methane.
CH4 + H2O → 3H2 + CO
CH4 + 2H2O → 4H2 + CO2
CO + H2O → H2 + CO2

• Air is mixed in with the gas stream to give a
hydrogen, nitrogen ratio of 3:1.
• Water, carbon monoxide and carbon dioxide (all of
which poison the iron catalyst used in the ammonia
synthesis) are removed.
• The carbon monoxide is converted to carbon dioxide
and the carbon dioxide removed for use in urea
production,
• CO + H2O→ CO2 + H2

• the gases cooled until the water becomes liquid
and can be easily removed.
• The nitrogen and hydrogen are then reacted at
high temperature and pressure using an iron
catalyst to form ammonia:
• N2 + 3H2→2NH3

• Manufacturing Steps
1. Desulphurization
• The catalyst used in the steam reforming process is highly sensitive to any
sulfur compounds, therefore these compounds need to be reduced to a
concentration of less than 0.15 mg/m3 feed gas.
• To achieve this, the feed gas is preheated up to 350 – 400 °C. Thereafter, the
sulfur compounds are hydrogenated to H2S, typically using a cobalt
molybdenum catalyst, and then finally adsorbed on zinc oxide. In this way, the
sulfur is removed to less than 0.1ppm in the gas feed
R-SH + H2 → H2 S + RH
H2S + ZnO → ZnS + H2 O
• The hydrogen required for the reaction is usually recycled from the synthesis
section of the plant.

Desulfurizer and catalysts

2.Primary reforming
• Gas from the desulfurizer is sent to the primary
reformer for steam reforming (ratio=steam/gas=3:1).
The primary reformer consists of a large number of
high-nickel chromium alloy tubes filled with nickel
reforming catalyst.
• Here superheated steam is fed into the reformer with
the methane. The gas mixture heated with natural gas
to 770oC in the presence of a nickel catalyst..

• 2. Primary reforming
• At this temperature the following equilibrium reactions are
driven, to converting the methane to hydrogen, carbon
dioxide and small quantities of carbon monoxide:
CH4 + H2O → 3H2 + CO
CH4 + 2H2O → 4H2 + CO2
CO + H2O → H2 + CO2
• This gaseous mixture is known as synthesis gas

3.Secondary reforming
• Only 30-40% of the hydrocarbon feed is reformed in
the primary reformer because of the chemical
equilibria at the actual operating conditions.
• The temperature must be raised to increase the
conversion. This is done in the secondary reformer by
internal combustion of part of the gas with the
process air, which also provides the essential nitrogen
for the final synthesis gas.

• 3. Secondary reforming
• The process air is compressed to the reforming pressure and heated
further in the primary reformer convection section to around 600 °C. The
process gas is mixed with the air in a burner and then passed over a
nickel catalyst in secondary reformer.
• The reformer outlet temperature is around 1,000 °C, and up to 99% of
the hydrocarbon feed (from the primary reformer) is converted, giving a
residual methane content of 0.2-0.3% (dry gas base) in the process gas
leaving the secondary reformer.
CO + H2O → CO2 + H2
O2 + 2CH4 → 2CO + 4H2
O2 + CH4 → CO2 + 2H2
2O2 + CH4 → 2H2O + CO2

• 3. Secondary reforming
• The process gas is cooled to 350-400 °C in a waste
heat steam boiler or boiler/super heater downstream
from the secondary reformer.
• As the catalyst that is used to form the ammonia is
pure iron, water, carbon dioxide and carbon
monoxide must be removed from the gas stream to
prevent oxidation of the iron. This is carried out in
the next three steps.

4. Shift conversion
• The process gas from the secondary reformer contains 12-
15% CO (dry gas base) and most of the CO is converted in
the shift section according to the reaction.
• CO + H2O → CO2 + H2 H298 = -41 kJ.mol-1
• In the High Temperature Shift (HTS) conversion, 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 base), limited by the shift equilibrium at the
actual operating temperature. There is a tendency to use
copper containing catalyst for increased conversion.

4.Shift conversion
• The gas from the HTS is cooled and passed through
the Low Temperature Shift (LTS) converter.
• This LTS converter is filled with a copper oxide/zinc
oxide-based catalyst and operates at about 200-
220 °C.
• The residual CO content in the converted gas is about
0.2-0.4% (dry gas base). A low residual CO content is
important for the efficiency of the process.

• Condensate: the process gas from the low temperature
shift converter contains mainly H2, N2, CO2 and the excess
process steam. The gas is cooled and most of the excess
steam is condensed before it enters the CO2 removal
system.
• This condensate normally contains 1,500-2,000 ppm of
ammonia 800-1,200 ppm of methanol. Minor amounts of
amines, formic acid and acetic acid could be present in the
condensate.
• All these components should be stripped from the
condensate and recycled in processes.

5. CO2 removal
• This process step removes the CO2 from the reaction gas.
The CO2 is removed in a physical absorption process.
• The solvents used in chemical absorption processes are
mainly aqueous amine solutions, e.g. mono ethanolamine
(MEA) activated methyl diethanolamine (MDEA) or
potassium carbonate solutions.
• Two typically used physical absorption solvents are glycol
dimethylethers and propylene carbonate which reduces the
contents down to about 50ppm.

6. Methanation
• The small amounts of CO and CO2, remaining in the synthesis gas, are
poisonous for the ammonia synthesis catalyst and must be removed by
conversion to CH4 in the methanator.
• CO + 3H2 -----> CH4 + H2O
• CO2+ 4H2 -------> CH4 + 2H2O
• The reactions take place at around 300 °C in a reactor filled with a nickel
containing catalyst. Methane is not involved in the synthesis reaction, but
the water must be removed before entering the converter.
• This is done firstly by cooling followed by condensation downstream of
the methanator.

7.Compression
• Modern ammonia plants use centrifugal compressors
to pressurize the synthesis gas to the required level
(100 – 250 bar, 350 – 550 °C) for ammonia synthesis.
• Molecular sieves are sometimes used after the first
compressor stage to remove the last traces of H2O,
CO and CO2 from the synthesis gas.
• These compressors are usually driven by steam
turbines, utilizing steam produced from the excess
process heat.

8.NH3 synthesis
• The synthesis of ammonia takes place on an iron
catalyst at pressure usually in the range of 100 – 250
bar and at temperatures of between 350 and 550 °C.
• N2 + 3H2 → NH3 H = -46 kJ/mol
• Only 10– 20 % of the synthesis gas is converted per
pass to ammonia, due to unfavorable equilibrium
conditions.

• The unreacted gas is recycled after removing the
ammonia formed and Fresh synthesis gas is
supplemented in the loop.
• As the exothermic synthesis reaction proceeds, there
is a reduction in volume and so a higher pressure and
lower temperature favors the reaction.
• Conventional reforming with methanation as the final
purification step produces a synthesis gas containing
unreacted gases and inerts (methane and argon).

• In order to prevent the accumulation of these inerts, a
continuous purge gas stream has to be applied. Purge
gas basically contains ammonia, nitrogen, hydrogen,
inerts and unreacted gases.
• The size of this purge stream controls the level of
inerts in the loop, keeping these to approximately 10
– 15 %.
• The purge gas is scrubbed with water to remove
ammonia, before then being used as fuel or before
being sent for hydrogen recovery.

Manufacturing process of Urea
• Urea is produced from NH3 & CO2 in two equilibrium reactions:
• The urea manufacturing process are designed to maximize these
reactions while inhibiting biuret formation:
• This reaction is undesirable, not only because it lowers the yield of
urea, but because biuret burns the leaves of plants. This means that
urea which contains high levels of biuret is unsuitable for use as a
fertilizer.

• Step 1 - Synthesis
• A mixture of compressed CO2 and NH3 at 240 bar is reacted to
form ammonium carbamate. This is an exothermic reaction, and
heat is recovered by a boiler which produces steam.
• The first reactor achieves 78% conversion of the CO2 to urea and
the liquid is then purified.
• The second reactor receives the gas from the first reactor and
recycle solution from the decomposition and concentration
sections. Conversion of CO2 to urea is approximately 60% at a
pressure of 50 bar in the second reactor.
• The solution is then purified in the same process as was used for
the liquid from the first reactor.

• Step 2 - Purification
• The major impurities in the mixture at this stage are
water from the urea production reaction and
unconsumed reactants (NH3, CO2 & ammonium
carbamate). The unconsumed reactants are removed
in three stages.
• Firstly, the pressure is reduced from 240 to 17 bar
and the solution is heated, which causes the
ammonium carbamate to decompose to NH3 & CO2

• Step 2 – Purification
• At the same time, some of the ammonia and carbon dioxide flash off.
• The pressure is then reduced to 2.0 bar and finally to -0.35 bar, with
more ammonia and carbon dioxide being lost at each stage.
• By the time the mixture is -0.35 bar a solution of urea dissolved in
water and free of other impurities.
• At each stage the unconsumed reactants are absorbed into the reactants
which is recycled to the secondary reactor. The excess ammonia is
purified and used as feedstock to the primary reactor.

• Step 3 - Concentration
• 75% of the urea solution is heated under vacuum,
which evaporates off some of the water, increasing
the urea concentration from 68% w/w to 80% w/w.
At this stage some urea crystals also form.
• The solution is then heated from 80 to 110oC to re-
dissolve these crystals prior to evaporation. In the
evaporation stage molten urea (99% w/w) is
produced at 140oC.
• The remaining 25% of the 68% w/w urea solution is
processed under vaccum at 135oC in a 2 series
evaporator arrangement.

• Step 4 - Granulation
• Urea is sold for fertilizer as 2 - 4 mm diameter
granules. These granules are formed by spraying
molten urea onto seed granules which are supported
on a bed of air.
• This occurs in a granulator which receives the seed
granules at one end and discharges enlarged granules
at the other as molten urea is sprayed through
nozzles. Dry, cool granules are classified using
screens.

• Step 4 - Granulation
• Oversized granules are crushed and combined with
undersized ones for use as seed.
• All dust and air from the granulator is removed by a
fan into a dust scrubber and discharges the air to the
atmosphere.
• The final product is cooled in air, weighed and
conveyed to bulk storage ready for sale.

Uses of ammonia
• About 80% or more of the NH3 produced is used
for fertilizing agricultural crops in the form of
aqua ammonia (an aqueous solution of ammonia),
(NH4)2SO4, (NH4)3PO4, NH4NO3 and (NH2)2CO.
• Some anhydrous liquid ammonia is also used
directly as a fertilizer.

Uses of Ammonia
• Fertilizer Industries, production of:
• Ammonium sulfate, (NH4)2SO4
• Ammonium phosphate, (NH4)3PO4
• Ammonium nitrate, NH4NO3
• Urea, (NH2)2CO.

Uses of Ammonia
• Chemical synthesis of
• HNO3, which is used in making explosives such as TNT
(2,4,6-trinitrotoluene), nitroglycerine etc.
• Sodium hydrogen carbonate (sodium bicarbonate), NaHCO3
• Na2CO3
• Hydrogen cyanide (hydrocyanic acid), HCN
• Hydrazine, N2H4 (used in rocket propulsion systems)

Uses of Ammonia
• Explosives
• Ammonium nitrate, NH4NO3
• Fibres & Plastic
• Nylon, -[(CH2)4-CO-NH-(CH2)6-NH-CO]-,and other
polyamides.
• Refrigeration
• Used for making ice, large scale refrigeration plants,
air-conditioning units in buildings and plants

Uses of Ammonia
• Pharmaceuticals
• Used in the manufacture of drugs such as
sulfonamide which inhibit the growth and
multiplication of bacteria, anti-malarials and
vitamins such as the B vitamins.
• Pulp & Paper
• Ammonium hydrogen sulfite, NH4HSO3, enables
some hardwoods to be used.

Uses of Ammonia
• Mining & Metallurgy
• Used in nitrating in steel.
• Used in zinc and nickel extraction
• Cleaning
• Ammonia in solution is used as a cleaning
agent such as in 'cloudy ammonia'

Uses of Urea
• As a component of fertilizer and animal feed,
providing a relatively cheap source of fixed nitrogen
to promote growth.
• As a raw material for the manufacture of plastics
specifically, urea-formaldehyde resin.
• As a raw material for the manufacture of various
glues (urea-formaldehyde or urea-melamine-
formaldehyde). The latter is waterproof and is used
for marine plywood.

Uses of Urea
• As an alternative to rock salt in the deicing of
roadways and runways. It does not promote metal
corrosion to the extent that salt does.
• As an additive ingredient in cigarettes, designed to
enhance flavor.
• Sometimes used as a browning agent in factory.
• As an ingredient in some hair conditioners, facial
cleansers, bath oils and lotions.

Uses of Urea
• As a clean burning fuel for motor vehicles and
stationary engines.
• Active ingredient for diesel engine exhaust
treatment, As a NOx-reducing reactant in diesel
exhaust.
• to make urea nitrate, a high explosive

Uses of Urea
• Used, along with salts, as a cloud seeding agent
to expedite the condensation of water in clouds,
producing precipitation.
• The ability of urea to form clathrates (also called
“loose compounds” ) was used in the past to
separate paraffins.

Ammonia and urea production

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