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Isobutanol synthesis and extraction process could reduce energy compared to alternatives

By Scott Jenkins |

Bio-based isobutanol is attractive as a renewable blendstock for gasoline because it has higher energy density than bioethanol, and does not impose vehicle-range penalties as a fuel additive, but thus far, processes to synthesize and isolate isobutanol have required a sizable fraction of the energy embodied in the fuel. Now, a collaborative team of researchers from Worcester Polytechnic Institute (WPI; Worcester, Mass.; www.wpi.edu) and the Massachusetts Institute of Technology (MIT; Cambridge, Mass.; www.mit.edu) have devised a process for making and extracting isobutanol that could cut energy requirements by as much as a factor of five.

The biosynthesis of isobutanol is carried out by a genetically engineered microbe isolated from rock samples at a geological carbon dioxide reservoir. The MIT group, led by Kris Prather, Janelle Thompson and Jason Boock, introduced genes for isobutanol biosynthesis into the microbe (Bacillus megaterium), which can survive in high-pressure CO2 environments. The ability to survive in these conditions is critical to the extraction process, which relies on supercritical CO2.

WPI chemical engineer Mike Timko led the team that developed the extraction technique. If the supercritical-CO2 -tolerant microbe is coupled to an aqueous fermentation process, the CO2 can be used as an extraction solvent. “Supercritical CO2 allows us to preferentially extract isobutanol from the fermentation step with more favorable thermodynamics than conventional isobutanol processes,” he says. “Then, by partially reducing the CO2 pressure, we can separate product at low energies.

In earlier research on isobutanol processes, product yields have been low, and energy costs high, because isobutanol is toxic to microbes at only 2 wt.%, and methods to extract it, such as gas stripping, have consumed too much energy. The WPI-MIT project has calculated that it is possible to generate and extract isobutanol using its system at 2 MJ/kg, Timko says.

The team has completed proof-of-principle studies for both the biosynthesis and the extraction process, and is now working on a complete integrated process and a better understanding of microbial growth. Timko is also thinking about how to apply the extraction technique to other mid-polar compounds.

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