Greener Industry - Ammonia

Haber-Bosch Synthesis

Ammonia Production:
Ammonia is produced continuously, as illustrated in this flow diagram. Move the mouse over the diagram and click to find further details of each stage.

Ammonia Synthesis

The synthesis gas is compressed to 100 - 250 atmospheres, heated to 350 - 550ºC and passed over an iron oxide catalyst with potassium hydroxide and alumina promoters.

Under the reactor conditions, the iron oxide (Fe₃O₄) is reduced to give iron particles with many small pores (8 nm in diameter). The alumina prevents the pores in the iron collapsing, which would reduce the surface area. Potassium hydroxide increases the activity of the iron catalyst by donating its outer electron to the iron, increasing its electron density and its ability to bond to the nitrogen.

Further details of this catalysed reaction can be found on the catalysis site

The synthesis reactor normally contains 2 - 4 catalyst beds, with heat exchangers or injections of cold process gas to remove heat between catalyst beds. This helps to ensure maximum conversion to ammonia. The ammonia produced is cooled and condensed, with un-reacted gases added back into the synthesis gas and recycled. By continuous recycling of un-reacted gas, yields of up to 98% ammonia are produced.

Conditions
According to the equation, the equilibrium mixture will contain more ammonia:

When the temperature is lower (the reaction is exothermic in the ammonia direction)
When pressure is higher (4 moles of reactant gas make 2 moles of product gas)

In practice, the equilibrium is run under conditions of moderate temperatures and pressure.

Low temperatures affect the equilibrium favourably, but the reaction would be too slow. Very high pressures, though favouring product creation, increase the costs of plant construction, and present a greater risk to plant workers.

With the conditions used, a yield of approximately 20 - 30 % is achieved from each pass over the catalyst.

Impact of Process
The production of ammonia is a relatively clean process, with the only emissions being carbon dioxide and oxides of nitrogen, both of which can be recovered or reduced to very low levels in modern plants.

graph: ammonia yield varies with conditions