5.21C - Choosing industrial equilibrium conditions

Balancing output, supplies and cost

In an industrial reversible reaction, a compromise is a balanced choice between competing needs. A company wants a high yield of useful product, but it also wants the reaction to happen in an acceptable time and at an acceptable cost.

The word acceptable matters. The condition that gives the highest equilibrium yield may be too slow, too expensive, or too difficult to run safely on a large scale. The condition that gives the fastest rate may give too low a yield or may use too much energy.

In the Haber process, nitrogen and hydrogen react reversibly to form ammonia:

N2(g) + 3H2(g) ⇌ 2NH3(g)

Typical GCSE reference conditions are about 450 °C, about 200 atmospheres, and an iron catalyst. Industrial operating conditions vary between plants.

Raw materials and energy

The first part of the industrial decision is not the equilibrium itself. It is whether the raw materials and energy supplies are available and affordable on a large scale.

For the Haber process, nitrogen is obtained from air. Air is widely available, so nitrogen is a relatively cheap raw material. Hydrogen is usually obtained from natural gas, so its cost depends on fuel supply, processing and energy use.

These factors explain why industry cares about a condition even when it is chemically favourable. A very high pressure may increase the ammonia yield, but compressing gases to a very high pressure costs energy and requires strong, expensive equipment. A very low temperature may favour ammonia at equilibrium, but the reaction may take too long to be profitable.

Choosing temperature

The forward Haber reaction is exothermic. Lower temperature favours the formation of ammonia at equilibrium, so it gives a higher equilibrium yield.

That is only half of the industrial argument. At a lower temperature, particles have less energy and the reaction is slower. The plant may have to wait too long to produce enough ammonia. A very low temperature can therefore give a good equilibrium yield but a poor production rate.

Increasing the temperature makes the reaction faster, so ammonia is produced in a shorter time. However, because the forward reaction is exothermic, increasing temperature shifts the equilibrium away from ammonia and reduces the equilibrium yield. It also costs more energy to heat the gases.

A typical Haber process uses about 450 °C because it is a compromise:

• lower temperatures would give a higher equilibrium yield but too slow a rate
• higher temperatures would give a faster rate but a lower equilibrium yield and higher energy cost
• 450 °C gives an acceptable yield in an acceptable time

Pressure, catalyst and recycling

Pressure affects the Haber equilibrium because all the substances are gases. The equation has 4 moles of gas on the reactant side and 2 moles of gas on the product side:

N2(g) + 3H2(g) ⇌ 2NH3(g)

Increasing pressure favours the side with fewer moles of gas, so it favours ammonia. Higher pressure can also increase the rate because gas particles are closer together and collide more often. This helps produce an acceptable yield in an acceptable time.