Microbubbles take first prize

Exhaust gases such as CO2, which are a product of fuel combustion would be removed from the air and used to help grow other sources of fuel – biofuels. It all seems like a Utopian dream but who knows, we may be closer to achieving it than you may think, thanks to something as simple as a process for creating microbubbles.

In fact, it is an energy efficient method of producing <50 μm microbubbles which has earned its developers the Institution of Chemical Engineers' Moulton Medal for the best paper published by the Institution's journals in 2009 [Zimmerman et al., Food and Bioproducts Processing (2009) 87, 3, 215]. The team which is based at the University of Sheffield has created an airlift loop bioreactor incorporating a fluidic oscillator to oscillate liquids at varying frequencies ranging between 1 Hz and 100 Hz. This process generates microbubbles at the optimum size for CO2 absorption and subsequent delivery to an organism or process.

Creating such small bubbles is harder than you may think, and has taken a long time to achieve. In fact, one of the main problems normally encountered is that bubbles are limited to a minimum of 1-2 mm in diameter, even if the pore size from which they are created is very small.

Will Zimmerman of the University of Sheffield explains how they have overcome this problem to create tiny bubbles. “We use a fluidic oscillator to split the flow into two streams, but when one is on, the other is off. Then it switches, all by itself. The effect of this on the bank of holes or pores which is in contact with liquid downstream is that compact, strong pulses of gas arrive, and carry on straight-through, like shooting bullets out of a gun. Our ‘bullets’ do not stop at the pore, like conventional bubbles, which grow slowly larger, and therefore don't waste energy with friction as the bubble grows. So we get much smaller bubbles and pay less ‘energy’ cost to make them than the bigger bubbles we would have got with the same flow rate. Since smaller is better anyway for exchanging gases in bioreactors, we win twice.”

So how does this all link up with fuels and CO2 emissions? Algae have the potential to become biofuels if they can be grown at a fast enough rate and in sufficient density for scientists to refine out a lipid/oil from the algal mass which is then refined further into a biodiesel. However, algae need to sequester CO2 from water for growth and so their success as a biofuel depends on how rapidly they can take up the gas and how quickly and easily the gas can be delivered to them.

This is where the new method for creating microbubbles comes in. Bubbles can be used to sequester CO2dissolved in water and these microbubbles are the optimum size for algae to take up CO2 efficiently and at low energy cost, allowing algae to grow rapidly and at high densities. The O2 produced by algae as a product can also be removed by the microbubbles, enabling further rapid growth at a rate and density which is threefold that of previous methods.

An obvious source of CO2 is a power plant or other industrial processes, so if the gas can be sequestered from industry and converted into microbubbles, then the benefits not only include removal of a pollutant from air but also the production of more fuel in the form of an algal biomass.

Speaking recently on the BBC's Radio 4, Zimmerman explained that the airlift loop bioreactor has already gone beyond experiments in his laboratory and is currently undergoing trials in a steel company. The exhaust gases from the plant are dissolved in water and then delivered to algae for growth. As laboratory experiments indicate a threefold increase in growth rate with one hour of daily CO2 dosing, it is expected that the 8-10 hours of dosing which are being used should have a significant impact.

The trials do not end here; the bioreactor is also being tested for its applications in water purification and waste water aeration by water suppliers in the region. Estimates are that there will be an 18-80% reduction in energy requirement for wastewater aeration and a 90-95% reduction in energy requirement for water purification.

Zimmerman told Materials Today that he has recently been contacted by several other companies from all over the world since receiving the medal, all interested in carrying out trials on site.

Of course, we all know that this process for creating microbubbles is far from being the answer to all our environmental problems but if the many industrial trials that are being carried out prove successful, it could have a real impact on many processes and also on the future of energy.