Researchers at Imperial College London have developed a membrane-based extraction system for the production of biofuels which uses less than 25% of the energy of current processes and produces ten times more biofuel with more than 99.5% purity.

The open-access findings are published in the RSC journal Energy & Environmental Science.

Widespread use of biofuels is inhibited by the significant energy burden of recovering fuel products from aqueous fermentation systems. Here, we describe a membrane-based extraction (perstraction) system for the recovery of fuel-grade biobutanol from fermentation broths which can extract n-butanol with high purity (499.5%) while using less than 25% of the energy of current technology options.

This is achieved by combining a spray-coated thin-film composite membrane with 2-ethyl-1-hexanol as an extractant. The membrane successfully protects the micro-organisms from the extractant, which, although ideal in other respects, is a metabolic inhibitor. In contrast to water, the extractant does not form a heterogeneous azeotrope with n-butanol, and the overall energy consumption of for n-butanol production is 3.9 MJ kg^-1, substantially less than other recovery processes (17.0–29.4 MJ kg^-1).

By (a) extracting n-butanol from the fermentation broth without a phase change, (b) breaking the heterogeneous azeotrope relationship (less energy consumption for distillation), and (c) utilizing a small volume ratio of extractant:fermentation broth (1 : 100, v/v), the need for high energy intensity processes such as pervaporation, gas stripping or liquid–liquid extraction is avoided. The application of this perstraction system to continuous production of a range of higher alcohols is explored and shown to be highly favourable.

—Kim et al.

Because they are expensive, biofuels are usually combined with petrol or diesel to make them ‘go further’. Our new technology could help to drive down the cost of biofuels so that they eventually replace fossil fuels in transport and aviation—a much happier situation for the environment and one we are all working towards.

—Professor Andrew Livingston of Imperial’s Department of Chemical Engineering

Biofuels can be produced by fermenting waste biomass and recovering the fuel from the fermented solution using an extractant. However, both biofuels and extractants are toxic to the necessary microorganisms in the solution, so can inhibit production and increase energy consumption.

In their labs at Imperial, researchers investigated the performance of several thin-film composite membranes and settled on one which can block the transport of extractant and water, allowing only the biofuel to travel through. They found that this protected the microorganisms and enabled continuous production, resulting in a ten-fold increase in productivity compared with conventional techniques.

They tested the membrane with three different extractant solvents to further fine-tune the best operating conditions. They found that the 2-ethyl-1-hexanol extractant exhibited a five times faster recovery rate, which reduced the energy consumption of the process to less than one quarter of conventional recovery systems.

By combining our ultra-thin membrane with a highly efficient extraction liquid we can significantly reduce the cost of producing biofuels, which is an important step to driving down their overall price.

—Co-author Dr Ji Hoon Kim, also of the Department of Chemical Engineering

As well as being cleaner for the environment, biofuels are able to overcome issues with capacity and energy storage associated with other renewable technologies like batteries, which have so far prevented them being successfully adapted for use in aviation and long-distance transport.

Another benefit of low-ethanol biofuels is that the majority of conventional combustion engines are already equipped to use this type of fuel. However, the challenge remains to make them financially appealing for businesses to adopt on a large scale.

The Livingston Group now aims to complete a pilot- or large-scale study of their work to further validate the findings, and to refine their process even more by doing membrane modulation and operating continuous recovery with immobilised micro-organisms.

This project was funded by bp International Centre for Advanced Materials (bp ICAM).

Resources

• Ji Hoon Kim, Marcus Cook, Ludmila Peeva, Jet Yeo, Leslie W. Bolton, Young Moo Lee and Andrew G. Livingston (2020) “Low energy intensity production of fuel-grade bio-butanol enabled by membrane-based extraction” Energy & Environmental Science doi: 10.1039/D0EE02927K