C.U.EBONG

INDUSTRIAL MANUFACTURE OF AMMONIA GAS

Nitrogen is easily obtained from air by fractional distillation,(Air is almost 80% nitrogen).Hydrogen is obtained

from methane (natural gas) or naphtha.The hydrocarbon is reacted with steam.

Methane+  steam    carbon dioxide + hydrogen.

CH4(g)   +  2H2O(g)          CO2(g) +  4H2(g)

The raw materials are therefore; air - for nitrogen,methane and water - for hydrogen.

INDUSTRIAL CONDITIONS.

This reaction is reversible (shown by the arrow).The forward reaction (from left to right) is exothermic.

-------------------------(1)And the equilibrium constant for the reaction is defined as;

Keq =[NH3]2

[N2][H2]3

The industrial conditions are;

1) Temperature between 450°C and 500°C.

2) Pressure of 200 atm (200 atmospheres).   

3) Use an iron catalyst.   

Only about 15% of the reactants are converted into productsunder these conditions.Ammonia is cooled and

liquefied at the reaction pressure,and then removed as liquid ammonia. The remaining mix of nitrogen and

hydrogen gases (85%)are recycled and fed in at the reactant stage.The process operates continuously.

The amount of product (called the yield) that you get from areversible reaction depends on the conditions of the

reaction.The conditions include the temperature and pressure.

1. Increasing the temperature favours the endothermic reaction.

2. Increasing the pressure favours the smaller volume. 

3. Using a catalyst gives the equilibrium conditions more quickly.

When a reaction is reversible, it means that it can go both forwards and backwards.

The forward reaction is the one we want,in which reactants are converted into products.The backward reaction

is where the products become changed back into the original reactants.Reactions in both directions occur at the

same time.

If the forward reaction is exothermic then the backward reaction is endothermic.The amount of heat (energy)

transferred in the forward reaction is the same as the amount ofheat (energy) transferred in the backward

reaction.

1) Temperature between 450 °C and 500 °C.

The forward reaction (to form ammonia)is exothermic (it gives out heat). The backward reaction is

endothermic (it takes in heat).

N2(g) +  3H2(g)            2NH3(g)    ( + heat).

If we treat heat as a product, then removing heat (cooling the reaction down)will result in the equilibrium

mixturehaving more ammonia according toLe Chatelier's Principle.

Since we want ammonia from the Haber Process,why is the reaction conducted at 450°C ?Why don't we cool it

with ice, or at least let it run at room temperature?

The reaction rates, is dependent on the effect of temperature,we can see that all reactions go faster

when the temperature is raised.

In a reversible reaction like the Haber Process,raising the temperature will make the equilibrium mixture  

have more nitrogen and hydrogenbecause forming these from ammonia takes heat in.

If we cool the reaction down, theamount of ammonia in the equilibrium mixture will increase,but the rate at

which ammonia is formed will decrease(because the temperature is lower).

2. Pressure of 200 atm (200 atmospheres).

Why is such a high pressure used?

N2(g)   +   3H2(g)             2NH3(g)    ( + heat).

If we look at the reaction, the reactants and products are gases.One mole of any gas occupies a volume of

24,000 cm3.

On the left side of the equation, there is one mole of nitrogen, and three moles of hydrogen. The total is

four moles of reactant.

On the right side of the equation (the product),there are two moles of ammonia.

So, four moles of reactant give two moles of product.Since one mole of any gas takes up the same volume,

the volume of product is only half the volume of reactants.

Increasing the pressure (from Le Chatelier's Principle)makes the equilibrium mixture have more ammonia.

3) Use an iron catalyst.

We know from reaction rates that a catalyst can change the rate of a reaction.

With a reversible reaction, a catalyst which increases the ratewill increase the rate of both the forward and the

backwardreaction.

This is useful because the catalyst will cause the reaction mixtureto reach its equilibrium composition more

quickly. Note, the catalyst will not change the equilibrium composition.

In the Haber Process, the reactants can be pushed through the reaction vesselmore quickly in the presence of a

catalyst, while still having 15% conversion into ammonia.

The equilibrium mixture is then cooled,and ammonia is removed as a liquid product.

Without the catalyst, either the conversion to ammoniawould be less than 15% in the same time,

or the reactants would need more time in the reaction vessel.Without the catalyst, the process would take longer

and be more expensive.

The important thing to remember is thatany industrial process works in a way that gives

the maximum product for the minimum cost.The cost is not entirely dependant on thechemistry of the process

but also includes health and safety,energy, transport and the environment.