Chemical Engineering Spot: Sulfuric Acid (H2SO4) Manufacturing By Contact Process

Definition:

Sulfuric acid (H₂SO₄) is a strong acid. It’s a clear, colorless and odorless liquid. It’s more viscous than water. As an acid it reacts with almost all bases. About 98 % concentrated sulfuric acid is normally manufactured in industries.

Raw Materials Used for Sulfuric Acid (H₂SO₄) Production:

• Sulfur

• Air

• Water

Chemicals Used for Sulfuric Acid (H₂SO₄) Production:

• Vanadium Pentaoxide (V₂O₅): It’s used in the catalyst bed as a catalyst for converting Sulfur Dioxide (SO₂) to Sulfur Trioxide (SO₃)

• Sodium Carbonate (Na₂CO₃): It’s also called Soda Ash. It’s used in the stripper for absorbing unreacted Sulfur Dioxide (SO₂) and Sulfur Trioxide (SO₃) as they are very harmful to environment

Manufacturing Process of Sulfuric Acid (H₂SO₄):

Sulfuric acid (H₂SO₄) is manufactured by the following few steps:

Molten Sulfur Pit:

Granular sulfurs are dropped into molten sulfur pit. All the sulfurs are melted with the help of steam. The temperature of this pit is usually maintained at 150 -160°C.

Furnace:

Melted sulfur is pumped into the furnace and air is mixed with this sulfur in the right ratio. The temperature of furnace inlet is around 550 -650°C. The following reaction is performed into the furnace-

S (l) + O₂ (g) → SO₂ (g)

The temperature of furnace outlet is around 700 -800°C.

Waste Heat Boiler:

Then Sulfur Dioxide (SO₂) and unreacted air enter the waste heat boiler from where steam is generated. This steam is used to melt the granular sulfur. So the temperature of flue gas (SO₂+Air) from the outlet of boiler is around 240 -270°C.

Hot Gas Filter:

Into the hot gas filter, flue gas (SO₂+Air) is slightly preheated and it filters any impurities. The temperature in the outlet of hot gas filter is around 450°C.

Catalyst Bed:

Then the flue gas enters the catalyst bed where there are at least 4 beds with Vanadium Pentaoxide (V₂O₅) catalyst. The following reversible reaction is performed into every Catalyst Bed with the presence of Vanadium Pentaoxide (V₂O₅) catalyst-

2SO₂ (g) + O₂ (g) ↔ 2SO₃ (g)

The forward reaction is exothermic. After every bed the flue gas reduces it’s temperature with the help of heat exchanger. The final bed outlet temperature is around 300°C.

Absorber:

Then Sulfur Trioxide (SO₃) and trace amount Sulfur Dioxide (SO₂) enter the absorber through bottom side. And Sulfuric acid (H₂SO₄) enters the absorber through top side. The following reaction is performed into the absorber and Oleum (H₂S₂O₇)is produced. Oleum is also called Fuming Sulfuric Acid.

H₂SO₄ (l) + SO₃ (g) → H₂S₂O₇ (l)

For manufacturing Sulfuric acid (H₂SO₄), Sulfur Trioxide (SO₃) is not directly dissolved into water because it’s highly exothermic which is uncontrollable. Besides corrosive aerosol is produced from this reaction which is very difficult to separate. So the following reaction is not applied for the manufacturing of Sulfuric acid (H₂SO₄)

SO₃ (g) + H₂O (l) → H₂SO₄ (l)

Diluter:

Oleum (H₂S₂O₇) enters the diluter where water is mixed with it in the right rate for producing 98 % concentrated Sulfuric acid (H₂SO₄)

H₂S₂O₇ (l) + H₂O (l) → 2H₂SO₄ (l)

Uses of H₂SO₄:

Sulfuric acid(H₂SO₄) is widely used in the fertilizer factories, detergent factories, pharmaceutical industries, paint industries, for producing synthetic resins. Besides it’s mostly used for water treatment, battery production, paper sizing, aluminium reduction etc.

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