The manufacture of ethanol by fermentation is an anaerobic process carried out by yeast (typically Saccharomyces cerevisiae). The process involves the breakdown of glucose in aqueous solution into ethanol and carbon dioxide.

Process:

• Glucose (C₆H₁₂O₆) in aqueous solution is supplied to a fermentation vessel.
• Yeast is added to the glucose solution.
• The vessel is kept at a temperature between 25-35°C, which is optimal for yeast activity.
• Oxygen is excluded from the vessel to ensure anaerobic conditions.
• The yeast enzymes catalyze the breakdown of glucose.

Reactants and Products:

• Reactant: Aqueous glucose (C₆H₁₂O₆)
• Products: Ethanol (C₂H₅OH) and Carbon Dioxide (CO₂)
• The chemical equation for the reaction is: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Role of Yeast:

• Yeast contains enzymes that catalyze the fermentation process.
• These enzymes break down glucose into pyruvate.
• Pyruvate is then converted to ethanol and carbon dioxide through a series of enzymatic reactions.
• The carbon dioxide produced is released from the solution, causing the solution to bubble.

Fermentation vs. Catalytic Addition of Steam: Environmental Impact Comparison

Fermentation:

• Positive: Generally considered more environmentally friendly due to the use of renewable resources (starch/sugars). Lower carbon footprint compared to fossil fuel-derived ethene. Potential for waste product utilization (e.g., animal feed).
• Negative: The energy required for distillation to obtain pure ethanol can be significant. Land use for growing crops (e.g., maize, sugarcane) can lead to deforestation and habitat loss. Greenhouse gas emissions from agricultural practices (e.g., fertilizer use).

Catalytic Addition of Steam:

• Positive: High efficiency in converting ethene to ethanol minimizes waste. Can be integrated with existing petrochemical infrastructure.
• Negative: Relies on non-renewable fossil fuels (ethene). High energy consumption due to high temperatures and pressures. Potential for greenhouse gas emissions associated with fossil fuel extraction and refining. Catalyst production and disposal can have environmental impacts.

Comparison:

Conclusion: While catalytic addition of steam offers higher efficiency, fermentation is generally considered more sustainable due to its reliance on renewable resources. However, both processes have environmental impacts that need to be considered and mitigated.

Here's a comparison of the catalytic hydration and fermentation methods for ethanol production, outlining three advantages and disadvantages of each:

Catalytic Hydration (Advantages)

• Higher Yield: Catalytic hydration generally produces a higher yield of ethanol per unit mass of starting material compared to fermentation.
• Continuous Process: The catalytic hydration process can be carried out continuously, allowing for a more efficient and consistent production rate.
• Less Temperature Sensitivity: While still requiring high temperatures, the catalytic hydration process is less sensitive to minor temperature fluctuations than fermentation.

Catalytic Hydration (Disadvantages)

• High Energy Consumption: The high temperature and pressure requirements of the catalytic hydration process lead to significant energy consumption.
• Expensive Equipment: The specialized equipment needed to withstand high temperatures and pressures is expensive to install and maintain.
• Potential for By-products: Side reactions can occur, leading to the formation of unwanted by-products, reducing the overall yield of ethanol.

Fermentation (Advantages)

• Lower Energy Consumption: Fermentation occurs at relatively low temperatures (25-35°C), resulting in significantly lower energy consumption.
• Relatively Simple Process: The fermentation process is relatively simple to carry out and does not require complex or expensive equipment.
• Renewable Resource: Glucose, the feedstock for fermentation, can be derived from renewable resources like starch-rich crops.

Fermentation (Disadvantages)

• Lower Yield: Fermentation typically produces a lower yield of ethanol per unit mass of starting material compared to catalytic hydration.
• Batch Process: Fermentation is typically carried out in batches, which can lead to inconsistent production rates.
• Sensitivity to Conditions: Fermentation is sensitive to variations in temperature, pH, and the presence of contaminants.