About 140 million tons of ammonia
is made per year worldwide. It is used in the manufacture of explosives,
for dye making, for preparing medicines
and in the production of fertilizer. Ammonia
is made by first reacting natural gas mainly methane,
CH4, with steam to form hydrogen
in the primary reformer. In the secondary reformer
air is added to the gas stream. The carbon dioxide
is then removed by dissolving it in alkali.
In the converter the remaining hydrogen and
nitrogen from the air are combined to produce ammonia.
The uncombined gases recycle to the converter.
Ammonia is either stored or piped directly from the site.
This plant is situated close to other chemical plants where much of the ammonia
is used.
This is the primary reformer
and this is the secondary reformer. Carbon dioxide is absorbed from the gas stream
in these towers.
Nitrogen and hydrogen are combined here
in the converter.
Natural gas, which is about 90 percent methane, is piped from the North Sea
and fed to the plant through a pipeline.
The other raw materials are water and air.
Air is drawn in from the atmosphere. It is filtered
and compressed here. Natural gas contains sulfur compounds,
which are removed in this desulfuriser.
The sulfur free gas is piped to the primary reformer
where it reacts with the steam. The reaction
requires a nickel-based catalyst and this is heated by the natural gas
burners in the reformer.
The reaction in the reformer produces heat. Methane and steam react
to produce carbon monoxide and hydrogen. Some carbon monoxide reacts further
with the steam
to form carbon dioxide and more hydrogen. The stream of hydrogen and
oxides of carbon passes to the secondary reformer.
Heated air is fed to the stream of gas and this converts any remaining methane
into carbon dioxide.
Carbon dioxide would poison the catalyst used for conversion to ammonia.
The carbon dioxide is removed and sold for making fizzy drinks
and as a coolant in nuclear power stations.
The gas stream with hydrogen and nitrogen in the ratio 3:1 is fed to the ammonia converter.
The converter operates at a pressure of 200 atmospheres
and a temperature of about 400 degrees Celsius to convert nitrogen and
hydrogen to ammonia. The reaction is reversible
and uses an iron based catalyst. The conversion vessel
contains beds of this catalyst. The mixture of hydrogen and nitrogen in the
ratio 3:1
feeds in at the top. About 15 percent of the mixture
is converted to ammonia. The hot gas stream containing ammonia,
hydrogen and nitrogen is cooled to about -10 degrees Celsius
and compressed. Ammonia condenses out
as a liquid. The remaining nitrogen and hydrogen
are recycled back to the converter with fresh gas mixture.
This is the control room of the plant. Ammonia can be transported in tankers
as a liquid under pressure, or piped
to nearby plants.