Sulfuric acid manufacturing process | contact process

Sulfuric acid is a one of the most important chemical produced in the world. It is used in other chemical industries and have many uses in laboratories as a chemical compound.

Sulfuric acid characteristics| H2SO4

Sulfuric acid is a strong dibasic acid.

Releasing sulfuric acid to the environment may cause many harmful effects to humans, animals and also to the environment.

Oxidation number of sulfur atom in the sulfuric molecule is +6. Molecular shape of sulfuric molecule is tetrahedral.

sulfuric acid molecular structure

Sulfuric acid manufacturing processes

There are two ways to produce sulfuric acid in industry.

  1. Lead Chamber process
  2. Contact process

In this tutorial, contact process is discussed in detail.

Contact process of sulfuric acid manufacturing

Contact process is the most widely used method of sulfuric acid manufacturing. Sulfur dioxide is produced in many occasions.

Raw materials for sulfuric acid manufacturing process in contact process

  • Sulfur of sulfide compound - to produce SO2 gas
  • Air - to burn sulfur or sulfur compounds
  • Pre manufactured sulfuric acid - to dissolve produced sulfuric acid
  • Water

Taking sulfur as a by-product from petroleum industry

There are very strict environmental guidelines for sulfur content in the diesel and petrol, therefore sulfur is reduced to very low level in petroleum products. Therefore, sulfur is formed in the desulfurisation process of petroleum. This removed sulfur is available at very low cost. Therefore, many sulfuric acid manufacturing countries used that sulfur in their sulfur dioxide preparing process.

Process of catalysts in contact process

  • Pt pr V2O5 can be used.
  • Pt is more efficient. But Pt get poisoned rapidly in the presence of sulfur.
  • Hence we have to use V2O5. But V2O5 has low efficient. But it is cheap.

Sulfuric acid manufacturing process

Sulfuric acid manufacturing is a long process and catalysts, heating,high pressures, le chatelier's principle are applied during the complete process.

Step 1

Burning sulfur or sulfide compound

Sulfur or sulfide compound is burnt in air to produce sulfur dioxide gas (SO2). For sulfide compounds, ZnS, PbS, FeS are used.

burning sulphur in air

burning metal sulfides in air

Step 2

Conversion of sulfur dioxide to sulfur trioxide | SO2 to SO3

Furthermore SO2 is oxidized by oxygen. This reaction is a reversible one.
Conversion of sulfur dioxide to sulfur trioxide

Activation energy of this reaction is very large. Thus reaction does not happens spontaneously . Catalyst are applied in splendid high temperature. Sulfur dioxide is passed through several catalyst beds to convert SO3. Sulfur trioxide is removed from the last catalyst bed. It will shift the equilibrium towards right side of the reaction. This reaction is exothermic and rate of the reaction is small when catalyst is not exist.

Effect of temperature

In low temperatures, reaction rate is slow. But in higher temperatures , the reaction slows down because this reaction is exothermic. Hence we use about 4500C. This is the same situation we faced in haber process when manufacturing ammonia.

Effect of pressure

In forward reaction, number of molecules are decreased. Hence pressure is reduced. According to the le chatelier's principle, applying high pressure to this system will give successful product. But above conditions we have a good product rate and amount, we don't need more extra pressure. Cost is high when apply high pressures because affordable new equipments are required. Finally 1-3 atm pressure is good for converting SO2 to SO3.

Excess O2 will increase forward reaction rate.

Catalysts used in sulfur dioxide to sulfur trioxide

Platinum or vanadium oxide can be used as catalyst. Both have advantages and drawbacks.

advantages and drawbacks of platium and vanadium hydroxide as catalysts

  • Platinum (Pt) is more efficient than vanadium pentoxide (V2O5). But in the presence of sulfur platinum gets easily poisoned. But vanadium pentoxide does not get poisoned easily.
  • Pt is also very expensive compared to vanadium pentoxide.

Step 3

Reaction of sulfur trioxide and water

Reaction of sulfur trioxide and water is very passive one. Reason is, water vapor from the ystem reacts with SO3 easily and forms H2SO4 smog. That smog does not dissolve in water easily.
Therefore, SO3 is dissolved in a 98% - 99% pre manufactured sulfuric acid solution. Sulfur trioxide is more soluble in sulfuric acid and form fuming sulfuric acid.

Fuming sulfuric acid - H2S2O7

sulfur dioxide and sulfuric acid reaction

Fuming sulfuric acid is diluted with water to produce sulfuric acid

Fuming sulfuric acid and water water

contact process - sulfuric acid manufacturing tower cross section

Conditions used in contact process

  • vanadium pentoxide catalyst
  • a temperature of 5000C
  • excess air
  • 1 atm pressure

sulfuric acid production process
Fig 02: Sulfuric acid manufacturing process

Uses of sulfuric acid

  • Produce superphosphate fertilizer from apatite.
  • produce colorings
  • Produce HCl in the laboratory
  • to fill motor vehicle batteries

Environmental pollution due to sulfuric acid production

As other chemical industy, there is an environmental pollution in H2SO4 production.

In sulfuric acid production, sulfur dioxide and sulfur trioxide gases are formed. These gases are acidic. If they are leaked to the air, they will form sulfuric acid and cause to acid rains.

If sulfuric acid is leaked into a water stream, natural water becomes acidic and cause so many health problems.

450 - 500 0C temperature is used in the plant. Large heat will increase the temperature of environment.


What catalyst is used in contact process?

Platinum (Pt) or vanadium oxide (V2O5) is used as catalyst.