How CO₂ is Recovered and Purified from Ethanol Fermentation

Ethanol production generates more than just fuel. During fermentation, yeast converts sugars into ethanol and carbon dioxide (CO₂). Instead of releasing this gas into the atmosphere, ethanol plants can capture it, purify it and convert it into liquid CO₂ for commercial use.

Recovered CO₂ is used in carbonated beverages, food processing, dry ice manufacturing, water treatment, MIG welding, and various industrial applications. However, its suitability for these uses depends heavily on effective purification, moisture removal, and strict quality control.

In this guide, we explain how CO₂ is recovered from ethanol fermentation, the key purification stages, and how quality is maintained for different industrial and food-related applications.

Why Fermentation CO₂ is Ideal for Recovery

During fermentation, yeast converts glucose into ethanol and carbon dioxide:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

The process produces approximately 0.96 kg of CO₂ for every kilogram of ethanol, based on the theoretical reaction. Actual recovery depends on fermentation efficiency, operating conditions and collection parameters.

Fermentation gas is particularly suitable for CO₂ recovery because it contains a high CO₂ concentration (>98%) compared to fossil-fuel combustion off-gases. However, raw fermentation gas also carries moisture, alcohol vapors, aldehydes, and trace volatile organic compounds (VOCs) that must be systematically extracted before commercial distribution.

Recovering this gas allows ethanol producers to convert a by-product into a valuable resource while supporting circular, zero-waste operations.

The CO₂ Recovery and Purification Process

The exact equipment and operating conditions vary between plants, but a typical CO₂ recovery system involves the following stages.

1. CO₂ Collection from Fermenters

During fermentation, yeast releases CO₂ continuously. The gas is captured directly from fermentation tanks through a dedicated piping system and directed to the recovery unit.

Maintaining positive pressure prevents ambient air ingress, keeping nitrogen and oxygen levels minimal.

2. Washing and Cooling

The collected gas is passed through a water scrubber where counter-current water washes out entrained particles and water-soluble compounds like ethanol vapor. 

Cooling the gas condenses excess moisture, and recovered ethanol streams are piped back to the distillation section to maximize ethanol yields.

3. Compression

Once washed, the CO₂ gas enters multi-stage compressors to increase pressure to operational levels required for filtration and cryogenic liquefaction (18 - 22 bar). Oil-free compressors or food-grade lubrication systems are utilized to prevent secondary hydrocarbon contamination.

Equipment selection, lubrication arrangements and maintenance practices are especially important when producing CO₂ intended for food and beverage applications.

4. Removal of Organic and Sulphur Compounds

Raw fermentation CO₂ can contain trace contaminants such as aldehydes, volatile organic compounds and sulphur compounds. Even at low concentrations, certain impurities can affect the gas's odour, taste or suitability for its intended application.

To eliminate these, the gas stream passes through:

• Activated carbon: Adsorbs selected organic compounds and odour-causing contaminants.
• Sulphur-removal media: Removes targeted sulphur compounds where required.
• Additional treatment systems: Address specific contaminants depending on the raw gas composition and product specification.

The treatment system must be selected according to the impurities present. Monitoring and timely replacement or regeneration of purification media help maintain consistent performance.

5. Drying and Dehydration

Moisture must be reduced before the gas enters the cold sections of the recovery system. Excess water can freeze during cooling and liquefaction, potentially obstructing equipment and affecting reliable operation.

CO₂ is typically passed through twin-tower desiccant dryers packed with molecular sieves, reducing the moisture dew point below -50°C (≤ 20 ppm H2O).

6. Liquefaction

Once purified and dried, the compressed CO₂ is cooled using a refrigeration system until it condenses into clear liquid CO₂ under suitable pressure and temperature conditions (-20°C to -25°C).

Liquefaction shrinks the chemical volume significantly, allowing efficient bulk transport.

7. Stripping and Final Distillation

Liquid CO₂ may contain dissolved non-condensable gases, including oxygen, nitrogen and hydrogen. A stripping or distillation column separates these non-condensable gases from the liquid phase, driving final purity to 99.9% or higher.

8. Storage and Logistics

After purification, liquid CO₂ is transferred to suitable insulated pressure vessels designed to maintain the required storage conditions.

Storage tanks, transfer lines, valves and loading equipment must be maintained and handled properly to prevent contamination. Documented cleaning procedures, equipment inspections and controlled loading practices help preserve product quality during delivery.

Food-Grade vs. Industrial-Grade CO₂

Both food-grade and industrial-grade CO₂ can be produced through recovery and purification, but their specifications depend on the intended application.

Parameter	Food and Beverage CO₂	Industrial CO₂
Purity Level	99.9% or higher	99.0% - 99.5%
Primary Concern	Taste, odor neutrality, human safety	Process performance, oxidation prevention
Compliance Standard	IS 307 / IS 16422 / FSSAI Guidelines	Industrial buyer specifications
Impurity Thresholds	Strict limits on benzene, sulphur compounds, VOCs, moisture	Less stringent; depends on operational use
Quality Testing	In-line Gas Chromatography (GC) & batch recording	Routine physical and purity checks
Common Uses	Beverage carbonation, MAP packaging, dry ice	Shielding gas welding, pH neutralization, chemical synthesis

Important: A 99.9% overall purity metric alone does not make CO₂ food-grade. Specific trace contaminants, food safety standards (such as FSSAI certification), dedicated handling equipment, and documented batch testing must all be verified.

Quality Control in CO₂ Recovery

Producing reliable liquid CO₂ requires continuous analytical oversight across every manufacturing shift:

• Gas Chromatography (GC): Detects trace hydrocarbon and volatile impurities at parts-per-million (ppm) and parts-per-billion (ppb) levels.
• Moisture Dew Point Analysis: Ensures moisture levels remain safely below freezing limits.
• Oxygen & Non-Condensable Testing: Monitors for air leaks in fermentation collection lines.
• Sensory Assessment: Confirms liquid samples produce odorless, taste-neutral gas upon vaporization.

• Benefits of CO₂ Recovery from Ethanol Plants

• CO₂ recovery offers several potential benefits for ethanol producers and their customers.
• Enhanced Resource Efficiency: Captures a natural biological byproduct and repurposes it into commercial materials.
• Diversified Revenue Streams: Generates valuable secondary revenue alongside bioethanol, maize oil, and DDGS co-products.
• Resilient Local Supply Chains: Serves regional beverage bottlers, pharmaceutical units, and industrial fabricators with steady, local liquid CO₂ deliveries.
• Lower Carbon Footprint: Replaces fossil-fuel combustion-derived CO₂ with plant-derived, biogenic carbon sources.

Reliable CO₂ Supply from Ethanol Production

Producing high-quality CO₂ requires more than capturing fermentation gas. Each stage, from collection and purification to liquefaction, storage and testing, contributes to the final product's consistency and suitability.

At Edhas Biofuel, our advanced grain-based bio-refinery in Gujarat integrates modern CO₂ recovery and purification systems. We deliver certified, high-purity liquid CO₂ tailored to strict industrial and food-grade specifications.