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The Contact Process

Chemistry (Year 12) - Chemical Synthesis

Melanie Gamble

The contact process is the most common process used to produce sulphuric acid. This production method is a multistage reaction occurring across four stages:

Step 1: Sulphur is combusted to form sulphur dioxide:

Step 2: Sulphur dioxide reacts with oxygen to produce sulphur trioxide:

Step 3: Sulphur trioxide gas dissolves in sulfuric acid to form oleum:

Step 4: Oleum is mixed with water to produce the sulfuric acid:

It is important to note that Step 2 is a key step as it is a reversible reaction.

In Step 1, molten sulphur is sprayed as a mist into a reaction chamber and combusted with oxygen gas to produce sulphur dioxide gas.

This sulphur dioxide gas is then passed to the second and main reaction chamber. This marks the beginning of Step 2 where this gas reacts with oxygen gas to produce sulphur trioxide gas. Due to the reversibility of this stage the reaction conditions can be manipulated to maximise the rate and yield of the reaction.

From the equation we can identify 2 key things:

  • The reaction is exothermic (a negative change in enthalpy).

    • This means that a high temperature will increase the rate of reaction but decrease the yield, as the reverse reaction is favoured.

  • There are 3 moles of reactant gas particles for every 2 moles of product gas particles.

    • This means that a high pressure will increase both the rate and yield of reaction

As a result, a moderate temperature of 400 - 450°C is used to achieve a moderate reaction rate and yield, and a low pressure of 1 - 2 atm is used due to cost.

Note that a low pressure is used instead of a high pressure, as the increase in reaction rate and yield from the pressure increase does not justify the cost of operating at this higher pressure, in this case

A mesh vanadium oxide catalyst (with multiple layers, to create a large surface area) is also used to further increase the reaction rate. Once passed through the mesh any unreacted sulphur dioxide and oxygen gas are pumped back into the chamber to increase the overall reaction yield.

For Step 3, the sulphur trioxide gas is then reacted with concentrated sulphuric acid to produce oleum.

In Step 4, upon exiting the final reaction chamber, the oleum is finally reacted with water to form concentrated sulphuric acid.

The entire industrial contact process is summarised below:

A question that you might have asked yourself is why don’t we just directly convert sulphur trioxide to sulphuric acid by simply just adding water?

If you look at steps 3 and 4, it might make this question a little clearer:

Why can’t we simply just remove oleum from the equation entirely and remove a step?

The answer is that this direct conversion is avoided as this reaction pathway is highly exothermic and will turn sulphuric acid into a mist, making it difficult to collect.

Therefore, the sulphur trioxide is first converted to oleum and the oleum is then converted to sulphuric acid, as this method is much less exothermic and will not turn the sulfuric acid into a mist.

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