Contact Process

What is the Contact Process?

Sulfuric acid is a very important chemical compound made of two hydrogen atoms, one sulfur atom, and four oxygen atoms. It has a high demand in the industrialization sector, that’s why it is known as the ‘King of acids.’

It is manufactured in the industry by the contact process.

So, what is the contact process?

The contact process is a modern method used by industries for manufacturing concentrated sulfuric acid.

During this process, sulfur dioxide and oxygen are passed over a hot catalyst (V2O5). They unite to form sulfur trioxide, which combines with water to produce sulfuric acid.

Contact Process Equation

                             2SO2(g) + O2(g) ⇌ 2SO3(g) 

                              SO3(g) + H2O(l) → H2SO4(l)

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                            Contact process diagram

Contact Process 

There are mainly five steps involved in the manufacture of sulfuric acid (H2SO4) by the contact process. They are:



Chemical equation


Production of sulfur dioxide from sulfur

Sulfur is melted and then combusted in O2 to form SO2.

S  + O2 → SO2


Purification of SO2

The removal of dual particles like the oxides of iron and arsenic in this step.


Catalytic oxidation of SO2 to SO3 is done at 450 °C

2SO2(g) + O2(g) ⇌ 2SO3(g)

The catalyst V2O5 is used for the oxidation of SO2 to SO3.


Conversion of SO3 to Oleum

SO3 + H2SO4 → H2S2O7


Dilution of H2S2O7 to H2SO

H2S2O7 +H2O → 2H2SO4

The reaction 2SO2(g) + O2(g) ⇌ 2SO3(g) is an equilibrium reaction, which means as SO3 forms, the reverse reaction may occur, i.e., the breakdown of SO3 back to its reactants.

This reaction is exothermic, which means heat is evolved. This means that the backward reaction would be favored at higher temperatures. That’s why the V2O5 catalyst takes 450 °C for its work. 

Since the gases need to be fast to reach the equilibrium stage so that they are in contact with the catalyst in the reactor, the high temperature (high rate of reactions) favors the quick forward reaction.

Contact Process Steps

Let’s understand the step-by-step manufacturing process of H2SO4.

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  1. Dusting Tower

The mixture of SO2 and air are the impurities that comprise pyrite dust and arsenious oxide. These dust particles may affect the catalyst’s efficiency, so we pass them through the dusting tower. This tower contains electric charges that attract the solid particles, which are later moved.

  1. Scrubbing Tower

The dusting tower cleaned the gas from the surface. However, to remove the remaining dust, it is fed into the water scrubber, where it frees from the dust particles completely.

Step 3: Drying tower

The cleaned wet gas is dried with a spray of concentrated sulfuric acid in the drying tower.

  1. Arsenic Purifier

At this stage, the gas is led into the arsenic purifier, which cleans all the traces of the arsenic oxide present in the gaseous mixture.

  1. Contact Tower or Oxidation Chamber

At this stage, the clean, dried gaseous mixture of sulfur dioxide and the air is passed through a tower loosely packed with Vanadium pentoxide (V2O5 ) or Platinum (Pt).

Here, the preheated mixture of sulfur dioxide and oxygen forms sulfur trioxide.

The reactions occur in the following manner:

2SO2(g) + O2(g) ⇌ 2SO3(g) + 45 KCal

2 volumes of reactants give 2 volumes of reactants.

The reaction occurs in the presence of a catalyst, i.e., V2O5 (contact process catalyst) at a temperature of 450 °C.

This reaction is the key chemical reaction occurring in the contact process. It is a reversible, exothermic reaction (a large amount of heat is released), accompanied by a decrease in the volume.

  1. Absorption Tower

At this stage, the absorption of sulfur trioxide in sulfuric acid takes place.

Sulfur trioxide is not dissolved directly in water as it releases a large amount of heat.

First, sulfur trioxide is cooled in the heat exchanger, and then it’s absorbed in sulfuric acid to give Oleum (H2S2O7).

                              SO3 + H2SO4 → H2S2O7        

  1. Dilution Tank 

Sulfuric acid is soluble in water in all proportions and dilution of pyrosulfuric acid (Oleum) is done by the addition of sulfuric acid to water.

                      H2S2O7 +H2O → 2H2SO4

So, we obtain the H2SO4 with the desired concentration.

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The contact process reaction occurred in this way to form the final product (H2SO4) of the desired concentration.

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So, these were the steps, we discussed one-by-one in the manufacturing procedure of H2SO4.

Applications of Sulfuric Acid

The manufacturing of sulfuric acid is done for the production of fertilizers like superphosphate of lime and ammonium sulfate.

Let's look at the major applications of sulfuric acid:

  1. Preparation of fertilizers ((NH)2SO4)

  2. Gasoline

  3. Automobile batteries

  4. Pharmaceuticals

  5. Paper, and sugar bleaching

  6. Water treatment

  7. Sulfonation agents

  8. Cellulose fibers

  9. Steel manufacturing

  10.  Coloring agents

  11. Amino and intermediates

  12. Regeneration and ion-exchange resins

Do you know?

  • The Global sulfuric acid market is expected to reach USD 13.4 bn by 2025.

FAQ (Frequently Asked Questions)

Q1: Why is the contact process called the Contact Process?

Ans: During the formation of SO3, a huge amount of heat evolves, so adding a high temperature increases the rate of reaction. This helps in quickening the forward reaction so that the hot gas, i.e., SO2 comes in contact with the catalyst bed. That’s why the contact process is called the contact process.

Q2: Why is the contact process preferred over the Lead-chamber Process?

Ans: Sulfur dioxide is a very valuable chemical compound obtained by the contact process. The advantage of this process is that it has replaced the traditional lead chamber. 

So, you can obtain Oleum and Sulfur trioxide besides the desired concentration of H2SO4.

The contact process is more economical as compared to the lead-chamber.

Q3: What happens when concentrated H2SO4 is added to CaF2?

Ans: When concentrated H2SO4 is added to CaF2, the reaction occurs in the following manner:

CaF2                +          H2SO4              →         HF            +       CaSO4

Calcium Fluoride   Sulfuric acid            Hydrogen Fluoride     Calcium Sulfate

So, the products are HF and CaSO4.

Q4: What is the meaning of Oleum?

Ans: Oleum is a Latin name, which means oil or fuming sulfuric acid. It is a term specifically used for pyrosulfuric acid.