Conversion of Sulphur Trioxide to Sulphuric Acid

The contact process in chemistry refers to the conversion of sulphur trioxide to sulphuric acid. Sulphur dioxide and oxygen combine and form sulphur trioxide, which makes H₂SO₄ after contacting water.

The conversion of sulphur trioxide to sulphuric acid is the new method of industrially manufacturing sulphuric acid. It is otherwise known as the contact process. Sulphuric acid is made up of three elements: sulphur, air and water.

Invented by British merchant Peregrine Phillips, the contact process has now taken precedence over the other methods such as the chamber or lead-chamber processes. It is not only a more economical approach but also produces by-products such as sulphur trioxide and oleum.

Types of contact process

There are dedicated contact-process plants for the manufacturing process. The two ways of processing currently used have a far more modern approach than the processes used earlier. 

The first type has fewer complexities. The sulphur-burning contract plants make use of sulphur as the raw material. The process includes burning molten sulphur to obtain sulphur dioxide. Sulphur dioxide is then cooled and oxidised. 

The process is performed in the presence of pellets of porous siliceous material. The pellets are filled with vanadium pentoxide and a potassium compound. The process forms sulphur trioxide at mid-high temperatures on completion.

The other type of contact-process plant manufactures sulphur dioxide from a different kind of material. It uses low-grade sulphur-bearing material like pyrite.

The condensation process of gas is mandatory to remove the impurities and also to part water vapour findings, as it can be the source of acid dilution. The next step is drying the sulphur dioxide gas, along with concentrated sulphuric acid.

The process explained:

Importance of the conversion of sulphur trioxide to sulphuric acid 

Other than the contact process, there are several other ways to produce sulphuric acid. Parameters such as cost, effort, and purity of the sulphuric acid produced vary in each process. Since its invention, the contact process has become the most commonly used process. 

The contact process, used in the manufacturing of sulphuric acid, is divided into four steps: 

1. Extracting sulphur

2. Preparing sulphur dioxide

3. Conversion of sulphur dioxide to sulphur trioxide

4. Conversion of sulphur trioxide to sulphuric acid

Extraction of Sulphur

The most important element for sulphur dioxide gas production is pure sulphur. Among the many sources from where sulphur can be extracted, the recovery from oil and natural gas is the most important. The oil and natural gas are segregated from their organic and mineral portions to obtain sulphur. 

Apart from sulphur, several other elements can also be used to extract sulphur dioxide. The soil contains many metal ores in the form of sulphides. They are then roasted in order to extract their own oxide along with sulphur dioxide. This process is called metal refining. 

Production of sulphur dioxide

The easiest way of preparing sulphur dioxide is by burning pure sulphur. The process necessarily has to take place when there is access to abundant air. This leads to the product of sulphur dioxide. 

Sulphur dioxide is prepared by the following process:

The molten form of sulphur is pumped into the stationary atomiser, which leads to the formation of atomised sulphur.

In the next step, the atomised sulphur is applied to the hot furnace, where the preheated and dried air is also applied in the furnace. A sulphuric acid dehydrator is used to preheat and dry the air. 

One end of the hot furnace has a combination of molten sulphur and heated dry air. When the reaction occurs, the other end of the furnace produces sulphur dioxide.

Conversion process of sulphur dioxide to sulphur trioxide

Inside the lead chamber, the combination ratio of sulphur dioxide and oxygen is 1:1. Its temperature is set between 400 – 450 degrees Celsius, with 1 to 2 atm pressure. Vanadium pentoxide catalyst is used to catalyse the process. 

Conversion process of sulphur trioxide to sulphuric acid

There are two methods to convert sulphur trioxide to sulphuric acid. The first is by diluting sulphur trioxide in water. It will lead to the production of sulphuric acid. 

As simple as it seems, this is a highly exothermic process, hence is extremely dangerous. Adding sulphur trioxide in water, being an exothermic process, will lead the sulphuric acid to fume. The fumes also cause sulphur trioxide to not dissolve completely in water. Thus, considering its danger and partial dissolution, this process is not followed. 

The other method that is widely accepted and used is the dilution of sulphur trioxide in sulphuric acid. The process has a by-product called oleum. 

SO3 (g) + H2SO4 → H2S2O7 

Further usage of oleum can be made by diluting it in water and obtaining sulphuric acid in its concentrated form. 

H2S2O7 (1) + H2O(1) → 2H2SO4

Important chemical reactions in the conversion of sulphur trioxide to sulphuric acid

• Making of sulphur dioxide

Also called the first stage in the contact process, sulphur is burned in the presence of air in order to make sulphur dioxide:

sulphur + oxygen → sulphur dioxide

S(l) + O2(g) → SO2(g)

• This is an irreversible reaction. 

•  ‘I’ = liquid & ‘g’ = gas

• Making of sulphur trioxide

Here, sulphur dioxide is contacted with more oxygen to produce sulphur trioxide:

sulphur dioxide + oxygen ⇌ sulphur trioxide

2SO2(g) + O2(g) ⇌ 2SO3(g)

• This is a reversible reaction. 

• ‘g’ = gas

• Making sulphuric acid

The final process involves sulphur trioxide reacting with water to produce sulphuric acid:

H2O(l) + SO3(g) → H2SO4(aq)

• This is an irreversible reaction.

• ‘aq’ = aqueous/dissolved in water.

• The addition of sulphur trioxide directly to water is highly exothermic. 

• It may lead to the formation of sulphuric acid clouds. 

• Adding sulphur trioxide to highly concentrated sulphuric acid

To perform the process in a much safer manner, sulphur trioxide is added to sulphuric acid for obtaining a highly corrosive liquid – oleum. 

SO3 + H2SO4 → H2S2O7

Water is added to oleum, which forms sulphuric acid with a much less risky approach:

H2S2O7 + H2O → 2H2SO4

• Melting and atomisation

Production of sulphur dioxide using the hot furnace:

S(g) + O2 → SO2(g) + Δ

Conclusion

The conversion of sulphur trioxide to sulphuric acid is the new method of industrially manufacturing sulphuric acid. It is otherwise known as the contact process. Sulphuric acid is made up of three elements: sulphur, air and water.

Invented by the British merchant Peregrine Phillips, the contact process has now taken precedence over the other methods such as the chamber or lead-chamber processes. It is not only a more economical approach but also produces by-products such as sulphur trioxide and oleum.