7.3 Contact Process

Introduction

Sulphuric acid is the most important industrial chemical, used for making a large number of compounds, e.g., fertilizer, plastic, explosives, paints, leather, tanning, textiles, metallurgy, etc. Sulphuric acid is used in almost all industries, which is why it is called the King of Chemicals.

The Contact Process is the industrial method for manufacturing sulphuric acid.

Definition

Contact Process: The Contact Process is the industrial method for manufacturing sulphuric acid. It is based on the catalytic oxidation of SO₂ to SO₃ in the presence of atmospheric oxygen.

In simple words: The Contact Process is how sulphuric acid is made in industry — by burning sulphur to make SO₂, then catalytically converting SO₂ to SO₃, and finally dissolving SO₃ in water to make H₂SO₄.

Main Idea

- Sources: Sulphur dioxide is produced by burning sulphur or roasting sulphide ores. - The key reaction is: 2SO_2(g) + O_2(g) ⇌ 2SO_3(g) + 45.2\ kcal - This reaction is reversible and exothermic. - Optimum conditions for the reaction are: 1. Concentration: A controlled amount of air is passed. 2. Temperature: The temperature of the reaction should be kept at 450^° C. 3. Pressure: Pressure is usually kept at 200\ kPa / 2\ atm. 4. Catalyst: Vanadium pentaoxide is an efficient catalyst.

Core Concept

Key point: The Contact Process uses a reversible and exothermic reaction. The optimum conditions are chosen to maximise the yield of SO₃ while keeping the reaction economically feasible.

Detailed Explanation

Sources of Sulphur Dioxide

Sulphur dioxide is produced by:

1. Burning sulphur: S_(s) + O_2(g) → SO_2(g)

2. Roasting sulphide ores.

The Key Reaction

The Contact Process is based on the catalytic oxidation of SO₂ to SO₃ in the presence of atmospheric oxygen.

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) + 45.2\ kcal

Key features:

• The reaction is reversible (indicated by the ⇌ symbol).
• The reaction is exothermic (releases 45.2\ kcal of heat).

Optimum Conditions for the Reaction

1. Concentration: A controlled amount of air is passed.

2. Temperature: The temperature of the reaction should be kept at 450^° C.

3. Pressure: Pressure is usually kept at 200\ kPa / 2\ atm.

4. Catalyst: Vanadium pentaoxide is an efficient catalyst.

Steps in the Contact Process

i. Burner:

Sulphur is burnt in a Sulphur burner, SO₂ gas produced and vapours of this gas are cooled by passing through a pipe cooler made up of iron.

S_(s) + O_2(g) → SO_2(g)

ii. Coke Filter:

The gases then pass through a coke filter which filters the traces of suspended matter.

iii. Heat Exchanger:

The purified gases then pass through the heat exchanger and are cooled to 25^° C.

iv. Catalytic Chambers (Converters):

From heat exchange, gases go to catalytic chambers called converters, where SO₃ is formed.

2SO_2(g) + O_2(g) V₂O₅→ SO_3(g) + Heat

v. Absorption Tower:

SO₃ is cooled and passed through an absorption tower. Steam, water, and dilute sulphuric acid run into the tower, and oleum (H₂S₂O₇) or fusing H₂SO₄ is obtained and stored in the oleum tower.

The oleum tower is made up of brick-lined steel packed with quartz and fed with 98% H₂SO₄.

H₂SO_4(l) + SO_3(g) → H₂S₂O_7(l) (Oleum)

vi. Dilution of Oleum:

By adding the appropriate amount of water, oleum is converted to concentrated H₂SO₄.

H₂S₂O_7(l) + H₂O_(l) → 2H₂SO_4(l)

Key point: The acid obtained from this method is extremely pure. Any concentration of H₂SO₄ can be obtained by this method.

Summary Table

Intermediate Level

Steps in the Contact Process:

1. Burner: Sulphur is burnt to produce SO₂. S + O₂ → SO₂

2. Coke Filter: Filters traces of suspended matter.

3. Heat Exchanger: Cools gases to 25^° C.

4. Catalytic Chamber: SO₂ is oxidized to SO₃ using V₂O₅. 2SO₂ + O₂ V₂O₅→ SO₃ + Heat

5. Absorption Tower: SO₃ is absorbed in H₂SO₄ to form oleum. H₂SO₄ + SO₃ → H₂S₂O₇

6. Dilution: Oleum is diluted with water to form concentrated H₂SO₄. H₂S₂O₇ + H₂O → 2H₂SO₄

Key point: The Contact Process produces extremely pure sulphuric acid, and any concentration can be obtained by this method.

Important Points

• - Sulphuric acid is called the King of Chemicals.
• - The Contact Process is the industrial method for manufacturing sulphuric acid.
• - Sources of SO₂: Burning sulphur or roasting sulphide ores.
• - Key reaction:
• 2SO₂ + O₂ ⇌ 2SO₃ + 45.2\ kcal
• - The reaction is reversible and exothermic.
• - Optimum conditions:
• 1. Concentration: Controlled amount of air
• 2. Temperature: 450^° C
• 3. Pressure: 200\ kPa / 2\ atm
• 4. Catalyst: Vanadium pentaoxide (V₂O₅)
• - Steps: Burner → Coke filter → Heat exchanger → Catalytic chamber → Absorption tower → Dilution
• - Oleum formation: H₂SO₄ + SO₃ → H₂S₂O₇
• - Dilution of oleum: H₂S₂O₇ + H₂O → 2H₂SO₄
• - The acid obtained is extremely pure.
• - Any concentration of H₂SO₄ can be obtained.

Remember

• - Sulphuric acid = King of Chemicals.
• - Contact Process = industrial method for H₂SO₄.
• - Sources of SO₂: Burning sulphur or roasting sulphide ores.
• - Key reaction: 2SO₂ + O₂ ⇌ 2SO₃ + 45.2\ kcal.
• - Reversible and exothermic.
• - Optimum conditions:
• - Temperature: 450^° C
• - Pressure: 200\ kPa / 2\ atm
• - Catalyst: V₂O₅
• - Steps: Burner → Coke filter → Heat exchanger → Catalytic chamber → Absorption tower → Dilution.
• - Oleum: H₂S₂O₇.
• - Dilution: H₂S₂O₇ + H₂O → 2H₂SO₄.
• - Acid is extremely pure.

Common Mistakes

Quick Revision

Contact Process for the Manufacture of Sulphuric Acid:

Sulphuric acid is a most important industrial chemical used for making a large number of compounds e.g. fertilizer, plastic, explosives, paints, leather, tanning, textiles, metallurgy etc. Sulphuric acid is used in almost all industries which is why it is called the King of Chemicals.

Sources: This process is based on the catalytic oxidation of SO₂ and SO₃ in the presence of atmospheric oxygen. Sulphur dioxide is produced by burning sulphur or roasting sulphide ores.

• 2SO_2(g) + O_2(g) ⇌ 2SO_3(g) + 45.2\ kcal

This reaction is reversible and exothermic. Therefore the optimum conditions for the reaction are:

• Concentration: A controlled amount of air is passed.
• Temperature: The temperature of the reaction should be kept at 450^° C.
• Pressure: Pressure is usually kept at 200\ kPa / 2\ atm.
• Catalyst: Vanadium pentaoxide is an efficient catalyst.

Steps:

i) Burner: Sulphur is burnt in a Sulphur burner, SO₂ gas produced and vapours of this gas are cooled by passing through a pipe cooler made up of iron.

• S_(s) + O_2(g) → SO_2(g)

ii) The gases then pass through a coke filter which filters the traces of suspended matter.

iii) The purified gases then pass through the heat exchanger and are cooled to 25^° C.

iv) From heat exchange gases go to catalytic chambers called converters. Where SO₃ is formed.

• 2SO_2(g) + O_2(g) V₂O₅→ SO_3(g) + Heat

SO₃ is cooled and passed through an absorption tower, steam, water and dil. sulphuric acid runs into the tower and oleum H₂S₂O₇ or fusing H₂SO₄ is obtained and stored in the oleum tower. The oleum tower is made up of brick-lined steel packed with quartz and fed with 98\%\ H₂SO₄.

• H₂SO_4(l) + SO_3(g) → H₂S₂O_7(l)\ (Oleum)

By adding the appropriate amount of water oleum to concentrated H₂SO₄:

• H₂S₂O_7(l) + H₂O_(l) → 2H₂SO_4(l)

The acid obtained from this method is extremely pure. Any concentration of H₂SO₄ can be obtained in this method.

60-Second Revision

• Sulphuric acid = King of Chemicals.
• Contact Process = industrial method for H₂SO₄.
• Sources of SO₂: Burning sulphur or roasting sulphide ores.
• Key reaction: 2SO₂ + O₂ ⇌ 2SO₃ + 45.2\ kcal.
• Reversible and exothermic.
• Optimum conditions:
• Temperature: 450^° C
• Pressure: 200\ kPa / 2\ atm
• Catalyst: V₂O₅
• Steps: Burner → Coke filter → Heat exchanger → Catalytic chamber → Absorption tower → Dilution.
• Oleum: H₂S₂O₇.
• Dilution: H₂S₂O₇ + H₂O → 2H₂SO₄.
• Acid is extremely pure.
• Any concentration can be obtained.
• ⚠️ Warning: SO₃ is not directly dissolved in water — it is absorbed in H₂SO₄ to form oleum, which is then diluted with water.