Making Ammonia – the Haber process

A BRIEF SUMMARY OF THE HABER PROCESS – various sources, thanks…

Haber Tower
Haber Tower

habereq

 

ΔH=-92kJ/mol thus exothermic

The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.

Here’s the flowchartnh3

Notes on the conditions

The catalyst

The catalyst is actually slightly more complicated than pure iron. It has potassium hydroxide added to it as a promoter – a substance that increases its efficiency.

The pressure

The pressure varies from one manufacturing plant to another, but is always high. You can’t go far wrong in an exam quoting 200 atmospheres (200 x atmospheric pressure or 20MPa)

Recycling

At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.

The proportions of nitrogen and hydrogen

The mixture of nitrogen and hydrogen going into the reactor is in the ratio of 1 volume of nitrogen to 3 volumes of hydrogen.

Avogadro’s Law says that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. That means that the gases are going into the reactor in the ratio of 1 molecule of nitrogen to 3 of hydrogen.

That is the proportion demanded by the equation.

There is always a down-side to using anything other than the equation proportions. If you have an excess of one reactant there will be molecules passing through the reactor which can’t possibly react because there isn’t anything for them to react with. This wastes reactor space – particularly space on the surface of the catalyst.

TEMPERATURE

Equilibrium considerations

You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of ammonia in the equilibrium mixture.

The forward reaction (the production of ammonia) is exothermic.

According to Le Chatelier’s Principle, this will be favoured if you lower the temperature. In this case, the system will respond by moving the position of equilibrium to counteract this – in other words by producing more heat.

In case you’ve forgotten, Le Chat’s P  is stated as follows –

“The equilibrium position will respond to oppose a change in the reaction conditions”.

What this means in practice is:

If you remove a product, the equilibrium mixture changes to make more product. It tries to get back to the composition it had before the product was removed. You can carry on removing product until all the reactants have turned into product (quite useful!).

The reverse is also true. If you remove a reactant, the equilibrium changes to make more reactant (generally not useful).

In order to get as much ammonia as possible in the equilibrium mixture, you need as low a temperature as possible. However, 400 – 450°C isn’t a low temperature!

Rate considerations

The lower the temperature you use, the slower the reaction becomes. A manufacturer is trying to produce as much ammonia as possible per day. It makes no sense to try to achieve an equilibrium mixture which contains a very high proportion of ammonia if it takes several years for the reaction to reach that equilibrium.

You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.

The compromise

400 – 450°C is a compromise temperature producing a reasonably high proportion of ammonia in the equilibrium mixture (even if it is only 15%), but in a very short time.

Why do we need ammonia so much? 14 uses for ammonia here

 

 

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