Bursting with energy

The air-lift loop bioreactor produces micro-bubbles to reduce the energy usage in biochemical processes.

A team of engineers led by Will Zimmerman, professor of Biochemical Dynamical Systems at Sheffield University, has created a bioreactor that could lead to a more energy-efficient way to produce renewable fuels.

The manufacture of biofuels currently requires large amounts of power. When the process uses too much energy, it becomes uneconomic. The new bioreactor consumes less energy than conventional designs and could prove to be vital for the production of alternative fuels.

At the heart of the so-called air-lift loop bioreactor is a patented system that Zimmerman has developed to create micron-sized bubbles of gas that are much smaller than the conventional 1-3mm bubbles produced by existing steady flow-through systems.

The micro-bubbles are created through a process called fluidic oscillation – a technique that allows the bubbles to become smaller when the apertures from which they emerge are made smaller.

Using this approach, the researchers have created 20 micron-sized bubbles from a diffuser with 20 micron-sized pores and adapted the air-lift loop bioreactor with the micro-porous diffuser/fluidic oscillator system.

Aside from biofuel production, Zimmerman said that there are many applications where the deployment of such micro-bubble generation systems could be particularly useful, such as transferring materials to and from a gas, suspending particles or droplets and separating particles or droplets by flotation.

He added that these are central to about one third of processing in biochemical production and that the energy-efficient production of micro-bubbles could dramatically reduce the energy use for all of these processes.

To exploit the benefits of the new technology, Zimmerman’s team is currently engaged in feasibility studies and field trials with several UK companies.

Yorkshire Water, for instance, is using the parts of the bioreactor that produce micro-bubbles to treat wastewater. So far, pilot trials have shown that the technology can reduce the energy consumption of wastewater aeration systems by at least 20 per cent, and possibly as much as 80 per cent, over conventional systems.

Prof Martin Tillotson from Yorkshire Water said: ’Many of our processes use forced air in order to treat water and wastewater streams and, given the huge volumes, it is very costly in electricity and carbon terms. This technology offers the potential to produce a step-change in energy performance.’

To further demonstrate the effectiveness of the micro-bubble generator, the air-lift loop bioreactor will be trialled at a Corus steel plant, where it will be used to treat exhaust gas by dissolving CO2 in water to grow micro-algae that are rich in oils that could be used to create biofuels.

Zimmerman said: ’Our lab trials have shown a 30 per cent increase in growth rate with only a modest dosing rate [one hour a day], not only because the CO2 dissolves faster but also because the micro-bubbles remove the oxygen produced, which is toxic to the algae, and because the bubbles keep the algae in suspension and well-mixed.

The steel plant exhaust gas trials will have eight to 10 hours of dosing a day and will exhibit the feasibility of using micro-bubbles to sequester the CO2.

The team also has projects in the wings that will use the micro-bubble generation technique to speed the offshore processing of oil-water mixtures produced from the wells. Zimmerman said that separating the water lowers the pumping costs of the oil to shore.

’The potential for large energy savings with our micro-bubble generation approach is huge, particularly in commodity chemical production for gas dissolution and stripping, where energy savings could enhance profitability,’ he explained. ’There are many routes to becoming green, and reducing energy consumption with the same or better performance must be the most painless.’

The key facts to take away from this article

» The new bioreactor uses less energy than conventional designs
» The bioreactor creates 20 micron bubbles through fluidic oscillation
» Bubbles can be used for mixing, separation and solubilising processes
» Applications include biochemicals and exhaust gas treatment