10.1 Manufacture of Ethanol

Introduction

Ethanol (also called ethyl alcohol) is one of the most important organic compounds. It is widely used as a fuel, a solvent, a disinfectant, and in the manufacture of many industrial chemicals.

Ethanol can be manufactured by two main methods:

• Fermentation of glucose
• Catalytic hydration of ethene

Each method has its own advantages and disadvantages. This section explains both methods and compares them.

Definition

Ethanol (CH₃CH₂OH): Ethanol is an alcohol with the molecular formula C₂H₅OH. It is the alcohol found in alcoholic beverages and is widely used as a fuel, solvent, and disinfectant.

In simple words: Ethanol is an alcohol that can be made by fermenting sugar or by reacting ethene with steam.

Main Idea

There are two methods for manufacturing ethanol:

• Fermentation — glucose from plants is converted to ethanol and carbon dioxide by yeast enzymes in the absence of oxygen.
• Catalytic hydration of ethene — ethene from crude oil is reacted with steam at high temperature and pressure in the presence of an acid catalyst.

Each method has different raw materials, conditions, and advantages.

Core Concept

Key point: Fermentation uses renewable resources and is a slow batch process; hydration uses non-renewable resources and is a fast continuous process.

Detailed Explanation

1. Making Ethanol by Fermentation

What happens:

When aqueous glucose is mixed with yeast and left for several days at 25-35°C in the absence of oxygen, enzymes present in yeast convert the glucose into ethanol and carbon dioxide.

Equation:

C₆H₁₂O₆(aq) → 2CH₃CH₂OH(aq) + 2CO₂(g)

Key features:

• The fermentation process produces 10-15% ethanol.
• The yeast is killed beyond this concentration.
• The ethanol is separated from the mixture by fractional distillation, which gives 96% pure alcohol — also called rectified spirit.
• The remaining 4% of water cannot be removed by fractional distillation.
• Rectified spirit is converted into 100% pure ethanol by using a dehydrating agent such as concentrated H₂SO₄.

Advantages of fermentation:

• Uses renewable resources (glucose from plants)
• Uses low energy levels

Key point: Fermentation uses yeast enzymes to convert glucose into ethanol and carbon dioxide. It produces 10-15% ethanol, which is then distilled to 96% purity.

2. Making Ethanol by the Hydration of Ethene

What happens:

Ethanol is produced when steam is added to ethene at 300°C and 6000 kPa / 6 atm in the presence of phosphoric acid as a catalyst.

Equation:

CH₂ = CH₂(g) + H₂O(g) H₃PO₄→ CH₃CH₂OH(g)

Key features:

• Only 4% of ethene is converted into ethanol.
• The ethanol produced is condensed into liquid and unreacted ethene is recycled in the process.
• By repeating these steps, 96% ethanol is produced.
• This method uses non-renewable resources and high energy levels.

Key point: Hydration of ethene uses steam and a phosphoric acid catalyst to produce ethanol at high temperature and pressure. Unreacted ethene is recycled.

Comparison of Hydration and Fermentation

The source provides the following comparison table:

Key point: Hydration is fast, continuous, and produces pure ethanol but uses non-renewable resources and high energy. Fermentation is slow, batch, and produces impure ethanol but uses renewable resources and less energy.

Summary Table

Intermediate Level

Fermentation:

• Raw material: Glucose (from plants)
• Source: Renewable
• Process: Batch
• Temperature: 25-35°C
• Catalyst: Enzymes (from yeast)
• Product: 10-15% ethanol → distilled to 96%
• Energy: Moderate

Equation: C₆H₁₂O₆(aq) → 2CH₃CH₂OH(aq) + 2CO₂(g)

Hydration of Ethene:

• Raw material: Ethene (from crude oil)
• Source: Non-renewable
• Process: Continuous
• Temperature: 300°C
• Pressure: 6000 kPa / 6 atm
• Catalyst: Phosphoric acid
• Product: Pure ethanol
• Energy: High

Equation: CH₂ = CH₂(g) + H₂O(g) H₃PO₄→ CH₃CH₂OH(g)

Key point: Fermentation uses renewable resources and is a slow batch process; hydration uses non-renewable resources and is a fast continuous process.

Solved Example

Example: Write the Equation for the Fermentation of Glucose

Question: Write the balanced chemical equation for the fermentation of glucose to ethanol.

Solution:

C₆H₁₂O₆(aq) → 2CH₃CH₂OH(aq) + 2CO₂(g)

Final Answer: C₆H₁₂O₆(aq) → 2CH₃CH₂OH(aq) + 2CO₂(g)

Example: Write the Equation for the Hydration of Ethene

Question: Write the balanced chemical equation for the hydration of ethene to ethanol.

Solution:

CH₂ = CH₂(g) + H₂O(g) H₃PO₄→ CH₃CH₂OH(g)

Final Answer: CH₂ = CH₂(g) + H₂O(g) H₃PO₄→ CH₃CH₂OH(g)

Important Points

• - Ethanol can be manufactured by fermentation or catalytic hydration.
• - Fermentation:
• - Glucose + yeast → ethanol + carbon dioxide
• - Temperature: 25-35°C
• - No oxygen needed
• - Produces 10-15% ethanol
• - Distilled to 96% pure alcohol (rectified spirit)
• - Uses renewable resources (plants)
• - Slow batch process
• - Equation: C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂
• - Hydration of Ethene:
• - Ethene + steam → ethanol
• - Temperature: 300°C
• - Pressure: 6000 kPa / 6 atm
• - Catalyst: Phosphoric acid (H₃PO₄)
• - Produces pure ethanol
• - Uses non-renewable resources (crude oil)
• - Fast continuous process
• - Equation: CH₂=CH₂ + H₂O → CH₃CH₂OH
• - 96% pure alcohol is also called rectified spirit.
• - 100% pure ethanol is obtained by using a dehydrating agent such as concentrated H₂SO₄.

Remember

• - Two methods: fermentation and hydration.
• - Fermentation: glucose + yeast → ethanol + CO₂.
• - Temperature: 25-35°C.
• - Produces 10-15% ethanol.
• - Distilled to 96% (rectified spirit).
• - Uses renewable resources.
• - Slow batch process.
• - Hydration: ethene + steam → ethanol.
• - Temperature: 300°C; Pressure: 6000 kPa / 6 atm.
• - Catalyst: Phosphoric acid.
• - Produces pure ethanol.
• - Uses non-renewable resources.
• - Fast continuous process.
• - 96% pure alcohol = rectified spirit.
• - 100% pure ethanol = rectified spirit + dehydrating agent.

Common Mistakes

Quick Revision

Manufacture of Ethanol:

1. Making ethanol by fermentation: When aqueous glucose is mixed with yeast and left for several days at 25-35°C in the absence of oxygen. Enzymes present in yeast convert the glucose into ethanol and carbon dioxide. This process is called fermentation.

• C₆H₁₂O₆(aq) → 2CH₃CH₂OH(aq) + 2CO₂(g)

The fermentation process produces 10-15% ethanol. The yeast is killed beyond this concentration. The ethanol is separated from the mixture by fractional distillation, which gives 96% pure alcohol also called rectified spirit. The remaining 4% of water cannot be removed by fractional distillation. Rectified spirit is converted into 100% pure ethanol by using a dehydrating agent such as concentrated H₂SO₄. This method uses renewable resources and low energy levels.

• 2. Making ethanol by the hydration of ethene:
• Ethanol is produced when steam is added to ethene at 300°C and 6000 kPa / 6 atm. in the presence of phosphoric acid as a catalyst.

• CH₂ = CH₂(g) + H₂O(g) H₃PO₄→ CH₃CH₂OH(g)

Only 4% of ethene is converted into ethanol. The ethanol produced is condensed into liquid and unreacted ethene is recycled in the process. By repeating these steps 96% ethanol is produced. This method uses non-renewable resources and high energy levels.

Table 10.1: Comparing hydration and fermentation:

60-Second Revision

• Two methods: fermentation and hydration.
• Fermentation: glucose + yeast → ethanol + CO₂.
• Temperature: 25-35°C.
• Produces 10-15% ethanol.
• Distilled to 96% (rectified spirit).
• Uses renewable resources.
• Slow batch process.
• Hydration: ethene + steam → ethanol.
• Temperature: 300°C; Pressure: 6000 kPa / 6 atm.
• Catalyst: Phosphoric acid.
• Produces pure ethanol.
• Uses non-renewable resources.
• Fast continuous process.
• 96% pure alcohol = rectified spirit.
• 100% pure ethanol = rectified spirit + dehydrating agent.
• ⚠️ Warning: fermentation produces impure ethanol (10-15%) which must be distilled to 96% — the remaining 4% water can only be removed by a dehydrating agent.