Concentration and Recovery of Bioethanol (Fermented Ethanol, Butanol)

Bioethanol is being developed as an alternative energy source to gasoline, aimed at reducing our dependency on petroleum resources and combating global warming. Additionally, bioethanol presents the advantage of utilizing surplus agricultural crops, which can also contribute to increasing agricultural income.

However, the traditional production methods for bioethanol, which is generated through the fermentation of plants, result in a product that contains water and other impurities. To achieve high concentrations, azeotropic distillation is used, which requires a significant amount of energy.

Pervaporation is a method that employs separation membranes to dehydrate efficiently while saving energy. By exposing the silicone membrane module’s supply side (the inner diameter side) to the mixed liquid, and either pulling a vacuum on the permeate side (the outer diameter side) or flowing sweep gas, alcohol molecules that pass through the membrane are vaporized. These vapors are then cooled and condensed using a chiller for recovery. The separation mechanism, known as the solution-diffusion model, produces selectivity based on the differences in dissolution and diffusion speeds as the molecules dissolve in and diffuse through the membrane. The substances targeted for separation are volatile organic compounds, making this a method known as pervaporation, which has been gaining attention as an alternative to distillation in recent years. This separation technique does not require large installations like distillation and is effective in separating azeotropic mixtures, offering significant advantages over other separation methods.

For example, passing a 10% ethanol aqueous solution through the membrane can result in the permeate side separating and concentrating into a 60% ethanol aqueous solution. Furthermore, fermented butanol has been gaining attention as a renewable fuel in recent years. One of the challenges with butanol fermentation is its strong cytotoxicity to cells. When the concentration exceeds 1wt%, it inhibits the production. A strategy to mitigate this is to use hollow fiber membranes to continuously separate and recover butanol from the fermentation broth, significantly enhancing butanol productivity. This is known as extractive fermentation.