Report on the Ammonia/Urea Process

1. Introduction to Ammonia and Urea

Ammonia (NH₃) is a crucial chemical compound, serving as a foundational building block for various industries,
most notably agriculture. It's a colorless gas with a pungent smell, highly soluble in water, and easily liquefied under
pressure. The primary use of ammonia is in the production of fertilizers, accounting for approximately 80% of its
global consumption. Other applications include the production of plastics, explosives, pharmaceuticals, and
refrigerants.

**Urea (CO(NH₂)₂) **is the most widely used nitrogen fertilizer globally due to its high nitrogen content (46% N),
low cost, and ease of handling and application. It is a white, crystalline organic compound, highly soluble in water,
and non-toxic. Beyond agriculture, urea is used in animal feed supplements, resins (urea-formaldehyde), and as a de-
icing agent.

The production of urea is directly dependent on ammonia, as ammonia is a key reactant in its synthesis. Therefore,
ammonia and urea plants are often integrated, with the ammonia plant providing the necessary raw material for the
adjacent urea unit.

2. Ammonia Synthesis Process (Haber-Bosch Process)

The synthesis of ammonia primarily relies on the Haber-Bosch process, a highly energy-intensive but indispensable
industrial method that combines nitrogen from the air with hydrogen, usually derived from natural gas or other
hydrocarbons.

2.1. Feedstock Preparation

The first and most critical step is the production of a pure hydrogen-rich synthesis gas (syngas) mixture (H₂:N₂ in a
3:1 molar ratio).

 2.1.1. Natural Gas Desulfurization: Natural gas, the most common feedstock, contains sulfur compounds
(e.g., H₂S) that can poison catalysts used in subsequent steps. It is desulfurized by passing it over activated
carbon or zinc oxide beds to remove sulfur.
 2.1.2. Primary Reforming: Desulfurized natural gas is mixed with steam and fed to a primary reformer.
This is a large, fired furnace containing catalyst-filled tubes (nickel-based catalyst). The highly endothermic
reaction occurs at high temperatures (700−850∘C) and moderate pressures (30−40 bar), converting
hydrocarbons and steam into hydrogen, carbon monoxide, and carbon dioxide. CH4+H2O⇌CO+3H2
 2.1.3. Secondary Reforming: The effluent from the primary reformer enters a secondary reformer. Air
(source of nitrogen) is added, and the remaining methane reacts with oxygen in a catalytic combustion,
simultaneously providing the nitrogen required for ammonia synthesis. This is an exothermic reaction,
occurring at even higher temperatures (900−1100∘C). 2CH4+O2⇌2CO+4H2
 2.1.4. High and Low Temperature Shift Conversion (HTS & LTS): The gas mixture, now containing CO,
CO₂, H₂, N₂, and unreacted steam, is cooled and sent to shift converters. Here, carbon monoxide (CO) is
reacted with steam to produce more hydrogen and carbon dioxide (CO₂).
o High-Temperature Shift (HTS): Occurs at 350−450∘C using an iron-chromium catalyst. CO+H2
O⇌CO2+H2
 o Low-Temperature Shift (LTS): Follows HTS, occurring at 190−250∘C using a copper-zinc catalyst,
to reduce CO content further.
 2.1.5. Carbon Dioxide (CO₂) Removal: CO₂ is a catalyst poison for ammonia synthesis. It is removed using
physical or chemical absorption processes. Common methods include:
o MDEA (Methyl Diethanolamine) absorption: CO₂ reacts with the amine solution and is later
stripped off by heating, allowing the MDEA to be regenerated.
o Promoted Hot Potassium Carbonate (e.g., Benfield or Catacarb process): CO₂ is absorbed by a
potassium carbonate solution.
 2.1.6. Methanation: Trace amounts of CO and CO₂ (after CO₂ removal) remaining in the syngas can still
poison the ammonia synthesis catalyst. These are converted back to methane and water over a nickel catalyst
at 300−350∘C. CO+3H2⇌CH4+H2O CO2+4H2⇌CH4+2H2O This ensures the final syngas is free from
oxygen-containing compounds, leaving essentially a pure 3:1 H₂:N₂ mixture.

2.2. Syngas Compression

The purified syngas is compressed to high pressures, typically ranging from 150−250 bar (some modern plants go up
to 300 bar), before entering the synthesis loop. This high pressure is essential to drive the equilibrium reaction
towards ammonia formation.

2.3. Ammonia Synthesis Loop

 2.3.1. Ammonia Converter (Reactor): The compressed syngas is preheated and fed into the ammonia
converter. This is a large, multi-bed catalytic reactor containing iron-based catalyst (typically promoted with
K₂O, CaO, Al₂O₃, SiO₂). The reaction is exothermic and equilibrium-limited: N2(g)+3H2(g)⇌2NH3
(g)ΔH=−92.4kJ/mol The reaction occurs at temperatures between 350−550∘C. Due to equilibrium limitations,
only 15−30% of the reactants convert to ammonia in a single pass.
 2.3.2. Ammonia Recovery: The hot effluent from the converter is cooled, causing the ammonia to condense
into a liquid. Unreacted hydrogen and nitrogen remain as gas.
 2.3.3. Recycle: The unreacted H₂ and N₂ are separated from the liquid ammonia and recycled back to the
compressor inlet to maximize conversion efficiency. A small purge stream is taken to prevent the buildup of
inerts (like argon and methane) that enter with the air or natural gas.

2.4. Ammonia Storage

Liquid ammonia is typically stored under refrigeration at atmospheric pressure and −33∘C in large, insulated tanks, or
under pressure at ambient temperatures.

3. Urea Synthesis Process

Urea is synthesized from liquid ammonia and gaseous carbon dioxide (CO₂). This process typically occurs in two
main steps: the formation of ammonium carbamate, followed by its dehydration to urea.

3.1. CO₂ Compression

CO₂ from the ammonia plant's CO₂ removal unit is compressed to high pressure (typically 140−180 bar) for feeding
into the urea reactor.
3.2. Ammonia Pumping

Liquid ammonia from the ammonia plant is pumped to the required high pressure for the urea synthesis reactor.

3.3. Urea Reactor (Synthesis Section)

 3.3.1. Carbamate Formation: Liquid ammonia and gaseous CO₂ are fed into the high-pressure urea reactor
(140−180 bar, 180−200∘C). The exothermic reaction between ammonia and carbon dioxide forms ammonium
carbamate: 2NH3(g)+CO2(g)⇌NH2COONH4(l)ΔH=−159kJ/mol This reaction proceeds almost to
completion.
 3.3.2. Urea Formation: Ammonium carbamate then dehydrates endothermically to form urea and water:
NH2COONH4(l)⇌NH2CONH2(l)+H2O(l)ΔH=+15.5kJ/mol This reaction is equilibrium limited, with
conversion rates typically around 60−80% in the reactor. Unconverted ammonium carbamate and unreacted
ammonia and CO₂ remain.

3.4. High-Pressure (HP) and Low-Pressure (LP) Decomposition/Recovery

The effluent from the urea reactor contains urea, water, unconverted ammonium carbamate, and excess ammonia. To
recover unreacted species and maximize urea yield, the stream is sequentially depressurized and heated in a series of
decomposers (strippers) and condensers.

 3.4.1. High-Pressure Stripper: Most modern urea processes use a HP stripper (e.g., Stamicarbon's CO₂
stripping or Snamprogetti's ammonia stripping). Here, unreacted ammonia and CO₂ are stripped from the urea
solution by heating and/or passing inert gas or excess CO₂ through it. This significantly reduces the load on
downstream sections. The stripped gases are recycled to the reactor or an HP carbamate condenser.
 3.4.2. Low-Pressure Decomposers: The solution from the HP section is further depressurized and sent to LP
decomposers where remaining ammonium carbamate decomposes into ammonia and CO₂ at lower pressures.
These gases are condensed, and the resulting carbamate solution is recycled back to the reactor or earlier
stages. This typically involves several stages of pressure reduction and heating (e.g., 18−20 bar, then 2−4 bar).

3.5. Urea Concentration

The dilute urea solution (typically 70−75% urea) is then concentrated to high purity.

 Evaporation: Multi-stage evaporators are used to remove water, increasing the urea concentration to
96−99.8%. This process often occurs under vacuum to reduce the boiling point and prevent urea
decomposition.
 Crystallization (Alternative): In some processes, particularly for specialty urea products, crystallization
might be used to achieve higher purity.

3.6. Prilling / Granulation

The concentrated molten urea is then solidified into a marketable form.

 Prilling: Molten urea is sprayed from the top of a tall prilling tower, forming droplets that cool and solidify
into small, spherical prills as they fall against an upward flow of air. Prills are typically 1−4 mm in diameter.
 Granulation: More commonly used today, granulation (e.g., fluid bed granulation) produces larger,
stronger, and more uniform granules (2−5 mm). Molten urea is sprayed onto seed particles in a granulator,
building up layers until the desired size is achieved. Granules offer better handling, storage, and spreading
characteristics compared to prills.
4. Integration of Ammonia and Urea Plants

The co-location and integration of ammonia and urea plants offer significant advantages:

 Raw Material Synergy: The urea plant directly consumes the two main by-products/outputs of the ammonia
plant: ammonia (as liquid product) and carbon dioxide (as a byproduct from the CO₂ removal unit). This
eliminates the need for CO₂ purchase and transportation, improving overall efficiency and reducing costs.
 Energy Integration: Heat generated in exothermic steps of ammonia synthesis (e.g., synthesis converter,
methanation) can be utilized for heating requirements in the urea plant (e.g., evaporators, decomposers),
leading to energy savings.
 Reduced Transportation Costs: No need to transport ammonia and CO₂ between separate facilities.
 Simplified Logistics: Centralized management of utilities, maintenance, and personnel.
 Environmental Benefits: Efficient utilization of CO₂ as a feedstock rather than venting it, though some CO₂
may still be vented depending on the process.

5. Key Equipment and Technologies

Both ammonia and urea plants utilize specialized equipment and advanced technologies:

 Ammonia Plant:
o Reformers (Primary & Secondary): High-temperature furnaces with catalyst tubes.
o Shift Converters (HTS & LTS): Catalytic reactors for CO conversion.
o CO₂ Absorbers/Strippers: Towers with packing or trays for CO₂ removal (e.g., MDEA, Hot
Potassium Carbonate).
o Ammonia Converter: Large, multi-bed catalytic reactor for ammonia synthesis.
o Syngas Compressors: Multi-stage centrifugal compressors for high-pressure operation.
o Heat Exchangers: Extensive use for heat recovery and process stream cooling/heating.
 Urea Plant:
o Urea Reactor: High-pressure, high-temperature vessel for urea synthesis.
o HP/LP Strippers/Decomposers: Specialized heat exchangers/reactors for carbamate decomposition
and recovery.
o Carbamate Condensers: Recover ammonia and CO₂ from decomposition.
o Evaporators/Crystallizers: For concentrating the urea solution.
o Prilling Tower/Granulator: For solidifying molten urea.
o High-Pressure Pumps: For ammonia and carbamate solutions.

Major technology licensors for ammonia include Haldor Topsoe, KBR, Ammonia Casale, and thyssenkrupp Uhde.
For urea, leading licensors are Stamicarbon, Saipem (Snamprogetti), and thyssenkrupp Uhde.

6. Environmental and Safety Considerations

Both ammonia and urea production involve hazardous materials and significant energy consumption, necessitating
strict environmental and safety protocols.
  Ammonia: Highly toxic and corrosive. Leaks can lead to severe health hazards and environmental damage.
Proper ventilation, leak detection, and emergency response plans are crucial.
 CO₂ Emissions: While CO₂ is a feedstock for urea, the ammonia production process generates significant
CO₂ from reforming. Efforts are ongoing to reduce the carbon footprint, including carbon capture and
utilization (CCU) technologies.
 Wastewater: Process water from both plants requires treatment to remove ammonia, urea, and other
contaminants before discharge.
 Energy Efficiency: Optimizing energy consumption through advanced heat integration, process
improvements, and efficient machinery is a continuous focus due to the energy-intensive nature of both
processes.
 Catalyst Handling: Spent catalysts are often hazardous and require proper disposal or regeneration.
 High Pressure/Temperature Operations: Require robust equipment design, strict maintenance, and
comprehensive safety systems to prevent explosions or leaks.

7. Conclusion

The Ammonia/Urea process chain is a cornerstone of modern agriculture, enabling the large-scale production of
essential nitrogen fertilizers. The Haber-Bosch process for ammonia synthesis is a marvel of chemical engineering,
though highly energy-intensive. The subsequent urea synthesis effectively utilizes the ammonia and byproduct CO₂
to create a widely used and efficient fertilizer. The integration of these two processes on a single site offers
substantial economic and environmental benefits. Continuous innovation in catalyst technology, process optimization,
and energy efficiency remains vital to meet growing global food demands while addressing environmental concerns
and ensuring safe operations.