You have likely produced ammonia in the laboratory from the thermal decomposition of ammonium chloride, or from the reaction of ammonium
and hydroxide ions. Have you ever wondered how ammonia is produced
on an industrial scale? In this lesson, we will learn about the Haber
process to produce ammonia, developed during World War I by Fritz Haber and Carl Bosch.
For their efforts and contributions to developing large-scale industrial processes, they were
awarded with Nobel Prizes in Chemistry. The Haber process is the industrial process
for the manufacture of ammonia from hydrogen and nitrogen.
Hydrogen is obtained from the reaction of methane and steam, producing carbon monoxide
as a byproduct. The hydrogen produced from this reaction also
reacts with oxygen from air, producing water, and leaving nitrogen behind.
Recall that air is 77% nitrogen. These gases are then compressed and delivered
to the reactor where ammonia is produced. These gases are then cooled down, and ammonia
is liquefied, ready to be tapped off. Unused hydrogen and nitrogen are recycled
back to the reactor. Let’s revisit the reaction again:
Note that this is a reversible reaction and that the forward reaction is exothermic.
The Haber process uses Le ChatelierÕs principle to maximise ammonia production while keeping
operating and production costs in mind. Le ChatelierÕs principle tells us that increasing
pressure will favour the side with fewer moles, for our case, the production of ammonia.
It would make sense to conduct the reaction at a very high pressure, but we must also
remember that it is expensive to build and operate a plant that can withstand such high
pressures. Therefore, a compromise pressure of 200atm
is used. Keeping Le Chatelier’s principle in mind,
what do you think is the best temperature conditions to maximise ammonia production?
Please pause the video to think about this, and resume once you are ready.
Since the forward reaction is exothermic, it would make sense to conduct this reaction
at a low temperature. The compromise temperature is 400-450¼C Ð
which is not exactly low, but not too high. If a low temperature were used, the rate of
reaction would be very slow, though the exothermic reaction would be favoured.
It would also actually take a long time for equilibrium to be reached, so a compromise
temperature of 400-450¼C ensures that the reaction proceeds with sufficient yield.
The yield of this process is 10-20% – remember from the introduction that unused gases are
recycled, so no reactants are wasted. As well, an iron catalyst is used in this
process. The presence of the catalyst does not affect
the position of equilibrium, but increases the rate at which equilibrium is reached.
In conclusion, the Haber process is the industrial process to produce ammonia from hydrogen and
nitrogen. It is conducted at 400-450¼C and at 200atm
in the presence of an iron catalyst. These are compromise conditions in order to
maximise ammonia production and economic profit.