(728f) Energy-Efficient Bio-Ethanol Recovery

The use of bio-ethanol as a fuel additive is among the most successful innovations in alleviating global dependency on fossil fuels, replacing a portion of automotive gasoline with a renewable alternative. Gasoline containing ethanol is also cleaner burning than standard gasoline blends.

However, during fermentation, ethanol concentrations of over 10%wt are poisonous to the micro-organisms used to convert biomass into ethanol and thus, inhibit bio-ethanol production. Therefore, the bioreactor product is, by necessity, fairly dilute. As a result, a great deal of water must be removed from the reactor product, as fuel standards require extremely low water content (<1%v/v).

Unfortunately, there is a high energy requirement associated with obtaining pure ethanol from a mixture which is predominantly comprised of water. Distillation is currently the most commonly used method of separation. However, it is energy-intensive in this situation due to the high specific heat and enthalpy of vaporization of water. As a consequence, the process utilizes a large amount of energy to vaporize water, which exits the column mainly as a bottoms product. Separation by distillation is further inhibited by a water-ethanol azeotrope which forms at 95%wt ethanol, which is well below the minimum concentration accepted for usage as a fuel additive. Further separation is required downstream from the distillation column using other separation methods, such as membrane separation or ternary distillation using an entrainer.

The extra cost of this problematic and energy-intensive separation acts as a barrier to the widespread adoption of high-ethanol content gasoline blends, making it economically viable in most countries only when oil prices are highly elevated.

In a previous presentation made at the 2011 Aiche Spring Meeting, (Hadjitheodorou et al. 2011), the idea was put forward that obtaining pure ethanol is not a necessary step in the overall process, since the final product is a blend of gasoline and ethanol.

In this work, this idea is addressed in detail, and a flow-sheet is developed which, instead of aiming to obtain pure ethanol which is then blended into gasoline at a later stage, gasoline is blended into the water/ethanol mixture and then the gasoline/ethanol blend is removed from the resulting mixture.

This results in a less energy-intensive process which takes advantage of the hydrophobicity of the organic compounds found in gasoline in order to obtain a non-pure ethanol-gasoline blend with acceptable water content. In this work, the separation of a gasoline and ethanol blend from a ternary mixture of water, isooctane and ethanol has been shown to be less energy intensive than a binary distillation process to obtain the same recovery of ethanol.

This separation can be achieved using solvent extraction, since the mixture of gasoline, ethanol and water exhibits Liquid-Liquid-Equilibrium (LLE) behavior, splitting into two liquid phases, one comprised predominantly of water and the other containing an ethanol and gasoline blend. This means that significant separation can be achieved at ambient temperature and with minimal energy inputs. The product streams do then require further purification using alternative separation methods, but the energy requirements of these are minimized since much of the separation has already been performed.

Further, the ideal work of separation is lessened when the final product is not pure, but is instead a mixture. It can easily be proven thermodynamically that the ideal work of separation is significantly less for obtaining mixtures of desired products rather than pure components.

In this presentation a flow-sheet is presented with energy requirements substantially lower than those associated with ethanol purification through conventional distillation.