Haber-Bosch Process: Ammonia Production Explained

Ammonia is one of the most produced chemicals in the world and the starting point for almost all nitrogen fertilisers, as well as nitric acid, explosives, plastics and fibres. Fritz Haber demonstrated the high pressure synthesis in the laboratory in 1909, and Carl Bosch at BASF turned it into an industrial process, with the first plant starting in 1913. The Haber-Bosch process is a classic example of chemical engineering: equilibrium, kinetics, catalysis, high pressure equipment, heat recovery and recycle all in one plant.

The chemistry

$$\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3} \qquad \Delta H \approx -92\ \text{kJ per mole of N}_2$$

The reaction is reversible, exothermic and reduces the number of gas moles from four to two. Le Chatelier’s principle therefore tells us:

• High pressure favours ammonia, because it shifts the equilibrium towards fewer gas moles.
• Low temperature favours ammonia at equilibrium, because the forward reaction releases heat.
• But at low temperature the reaction is far too slow, even with a catalyst.

The industrial compromise uses high pressure and moderate temperature with a very active catalyst. Even so, the gas leaving the converter typically contains only about 15 to 25 percent ammonia, which is why recycle is essential.

The catalyst

The catalyst is mainly iron, made from magnetite and promoted with small amounts of potassium oxide, aluminium oxide and calcium oxide. Promoters increase activity and stop the iron crystallites from sintering. The catalyst is poisoned by oxygen compounds (water, CO, CO₂) and sulphur, which is why the synthesis gas must be purified so thoroughly. Ruthenium based catalysts are also used in some modern plants because they are active at lower pressures.

Haber Bosch process steps in a natural gas plant

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1. Desulphurisation: sulphur compounds are removed from natural gas, because they poison the reforming and synthesis catalysts.
2. Primary reforming: methane reacts with steam over a nickel catalyst in externally fired tubes: CH₄ + H₂O ⇌ CO + 3H₂ (strongly endothermic).
3. Secondary reforming: air is added. Its oxygen burns part of the gas to complete the reforming, and its nitrogen supplies the N₂ needed for synthesis. The air rate is set to give a hydrogen to nitrogen ratio of about 3 to 1 in the final gas.
4. Shift conversion: CO reacts with steam to make more hydrogen, CO + H₂O ⇌ CO₂ + H₂, in high and low temperature stages.
5. CO₂ removal: CO₂ is absorbed in an amine or carbonate solution. The recovered CO₂ is often used to make urea from ammonia.
6. Methanation: the last traces of CO and CO₂ are converted back to methane, so no oxygen compounds reach the iron catalyst.
7. Compression: the synthesis gas is compressed, usually by steam turbine driven centrifugal compressors, to the loop pressure.
8. Synthesis loop: gas passes through catalyst beds in the converter. Heat from the exothermic reaction is recovered to raise steam. The gas is cooled and refrigerated so ammonia condenses and is separated as liquid. Unreacted gas is recompressed and recycled with fresh make-up gas.
9. Purge: inert gases, mainly methane and argon from the air, build up in the loop and are removed by a small purge stream; hydrogen is often recovered from it.

The loop is a textbook case of recycle and purge in a material balance, and the converter behaves close to a packed bed plug flow reactor.

Key equipment and engineering challenges

• High pressure equipment: converters, separators and piping are thick walled pressure vessels designed against hydrogen attack, which can embrittle ordinary steels at high temperature.
• Heat recovery: the reformer and converter heat is recovered in waste heat boilers and feed preheaters, using many heat exchanger types. Modern plants are highly heat integrated, a natural application of pinch analysis.
• Compression: large centrifugal compressors for synthesis gas, recycle and refrigeration.
• Safety: ammonia is toxic and flammable within limits, and hydrogen is highly flammable; plants rely on gas detection, relief systems and careful process instrumentation.

Products made from ammonia

Most ammonia is turned into fertilisers: urea (made by reacting ammonia with the CO₂ captured in the plant), ammonium nitrate, ammonium sulphate and ammonium phosphates. Ammonia is also oxidised to nitric acid, which leads to explosives and nylon intermediates, and it is used directly as a refrigerant and in flue gas treatment to reduce nitrogen oxides. It is increasingly studied as a carrier for hydrogen and as a carbon free fuel for ships and power plants.

Energy and environment

Ammonia production is energy intensive, and plants based on natural gas or coal emit large amounts of CO₂, mostly from making hydrogen. Efficiency gains over the decades have come from better reformers, catalysts, heat recovery and compressor drives. Current efforts include carbon capture from reforming (“blue” ammonia) and hydrogen from renewable powered electrolysis (“green” ammonia), which keeps the Haber-Bosch synthesis loop but removes the reforming steps.