Ammonia Synthesis

Ammonia synthesis takes place on an iron catalyst at a pressure of 100-250 bar and temperature of 350-550oC according to the following reaction (IPTS/EC, 2007 p.43):

N2 + 3H2 ↔ 2NH3,   ΔΗο = - 46 kJ/mol

Only about 20-30% of the synthesis gas is converted to ammonia due to unfavorable equilibrium conditions. The ammonia produced is separated by cooling/condensation. After the ammonia formed is removed, the unreacted gas is recycled back to the converter and the reacted gas is compensated with new make-up synthesis gas.

As the exothermic reaction in the ammonia synthesis reactor proceeds with a decrease in volume, lower temperatures and higher pressures will favor the reaction. Controlling the catalyst temperature is important as the heat of reaction raises the temperature. One technique for temperature control is the subdivision of the catalyst into several layers. With this method, the temperature of the gases can decrease by adding cooled synthesis gas between the layers or indirectly by generating steam (IPTS/EC, 2007 p.43).   

Steam reforming with methanation as the final purification step results in a synthesis gas with high inerts content (unreacted methane and argon). The inert contents are controlled by taking out a purge gas stream from the loop. The size of the purge gas stream can regulate the inerts content to about 10-15%. The purge gas is scrubbed with water in order to remove the contained ammonia and then sent for use as a fuel in the primary reformer or for hydrogen recovery (IPTS/EC, 2007 p.43).

Compression

The synthesis gas needs to be compressed to high pressures, ranging from 100 to 250 bars, for ammonia synthesis. Modern plants employ centrifugal compressors which are usually driven by steam turbines that use the steam produced from excess process heat (IPTS/EC, 2007 p. 42). The energy requirements in a modern steam reforming plant are 40-50% above the thermodynamic minimum. Compression losses are responsible for more than half of the excess energy use (EFMA, 2000 p. 7).

The steam requirements in the turbines used for driving the synthesis gas compressor, the air compressors and the refrigeration compressors are about 3.9-6.5 GJ/t NH3 (Ecofys, 2013). With the use of more active ammonia synthesis catalysts, such as cobalt-promoted iron and ruthenium, lower pressures can be employed and the energy use can decrease (IPTS/EC, 2007 p.43).    

Ammonia SynthesisTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
Indirect Cooling

The investment cost for a 300,000 tonne per year reactor is 150 million yen, and the payback period is 1.5 years (1999 figures) (ECCJ, 1999, p. 149).

Commercial
Low Pressure Drop Synthesis Reactor in Combination with Smaller Particle Size Catalysts

Indian Flag For a 1 100 tpd plant, retrofit costs were US $ 1 million [1993 values] (FAI, 2013)

Conversion to radial flow or radial-axial flow converters require major retrofits in existing equipment, involving high costs and shut down times.

Commercial
Using Low-Pressure Ammonia Synthesis CatalystsCommercial
Waste Heat Recovery from Synthesis Gas Compressor

The investment cost is estimated at 60 million Yen, and the payback period is about 2 years [1999 values] (ECCJ, 1999 p.153). 

Commercial
Installation of a Purge Gas Hydrogen Recovery Unit

The investment cost of a hydrogen recovery unit (membranes or cryogenic separation) is estimated at about €10/GJ, while the operation and maintenance costs are estimated at €0.25/GJ (Nieuwlaar, 2001 p.21) 

Commercial
Synthesis Gas Molecular Sieve Dryer And Direct Synthesis Converter FeedCommercial
Improve The Ammonia Synthesis ConfigurationCommercial
Additional Radial Flow Ammonia Converter With High Pressure / Medium Pressure Boiler

Indian Flag Implementation costs for a 1 500 tpd plant are in the range of US $ 10 million [2007 figures] (FAI, 2013).

Commercial
Improving the Efficiency of Synthesis Gas Cooler

Indian Flag For a 1 500 tpd plant, required investment is in the range of US $1 million [2009 values]  (FAI, 2013).

Commercial
Recycling of Flashed Gases into SynthesisNot available. Commercial
Compressor Seal Leak-off Gas RecoveryNot available.Commercial

Ammonia Synthesis Reference Documents

Reference Document on Best Available Techniques for the Manufacture of Large Volume Inorganic Chemicals - Ammonia, Acids and Fertilisers

Prepared by the Institute for Prospective Technical Studies of European Commision, this document provides detalied information on Best Available Technologies applicable to Ammonia production – as well as on the production of Acids and Fertilizers.  

Page Number: 

42-43