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The smaller the bubbles, the better

27/01/2012

Microbubbles buoy promise of algal biofuels

Oil industry to adopt first; hopes for Chinese lakes

Adam Duckett

ALGAE could become an economical source of biofuels, thanks to an energy-efficient harvesting technique involving microbubbles developed by chemical engineers at the UK’s University of Sheffield.

Algae are an attractive source of biofuels as they produce oils relatively efficiently and don’t take up valuable land space to compete with food production (as is the case with say maize-based biofuels). However, the inherent disadvantage with algae is the cost-effective removal of water that’s required to process the biofuel.

Will Zimmerman, a professor at Sheffield’s department of chemical and processing engineering, has already demonstrated that his bubble technology can make it easier for biofuel producers to grow algae (for which he won IChemE’s Moulton Medal in 2009), and has now shown that the same system also improves harvesting.

Zimmerman’s system produces microbubbles using a fluidic oscillator. This switches continuous airflow between outlets, creating pulses of microbubbles that travel up through a floatation cell of suspended algae, collecting the cells as they go and forming a thick crust at the top of the tank. The smaller the bubbles, the more efficient the separation. Chemical flocculants and coagulants added to the water improve matters further, making the algal particles hydrophobic and attracted to the bubbles. As more bubbles rise they compress the crust from beneath, forcing water from the sludge, concentrating the algae.

Zimmerman’s ‘microflotation’ technique uses as much as 1000 times less energy to produce microbubbles than existing flotation systems. The system avoids using high pressures to form bubbles as is the case with more conventional methods such as the dissolved air flotation (DAF) technique used by industry to purify water and concentrate fine minerals.

The bubbles rise gently in laminar, rather than turbulent flow, so avoid producing random eddies that re-disperse separated flocs, requiring more energy to separate them again.

Zimmerman has a long-running partnership with Tata Steel’s production site in Scunthorpe, which is providing flue gas to feed the algae. He says in the longterm Tata Steel hopes to convert its waste carbon dioxide into profitable biofuels.

Zimmerman is in talks with industry and expects the oil sector to adopt his technique first. He is in talks with a company that will license the technology for oil-water separations of produced water from oil drilling.

“The development work is straightforward,” he says. “We have solved the problem on the science side. It now relies on industry to have the confidence to take it up.”

Zimmerman is also in talks with the office of China’s Prime Minister about using the technique to remediate the country’s eutrophied lakes of algae, providing a cheap and clean form of drinking water. It’s estimated that China will need to spend US$40b/y on infrastructure for the next ten years to meet its demand for clean water.

Biotechnology and Bioengineering doi: 10.1002/bit.24449