Algae is being researched as a source for renewable fuels and plastics.By Ed Stiles, College of Engineering
Most of us don't intentionally cultivate algae – that green slime that grows in stagnant ponds, and, if we're unlucky, our swimming pools.
But these single-celled plants are more useful than you might think – they can help in creating renewable fuels and plastics.
Several companies are doing research on algae-to-biofuels technologies, and a team of agricultural and biosystems engineering students at The University of Arizona also studied the process for a senior design project this year.
Supercritical CO2 Extraction
The students focused on using supercritical CO2 as an alternative to harsher solvents for extracting oils from algae.
They grew Botryococcus braunii in a photo-bioreactor, pumped the algae into a pressure vessel and then used supercritical CO2 to extract the lipids. The oil floated to the top and the dead plant material sank to the bottom.
“We got the oil to separate, but the problem is that we had too much water content and we ended up with an oil slick that would have required another component to complete the separation,” said Sean Henry, an agriculture and biosystems engineering student. Unfortunately, that would have required some kind of solvent, just what the students were trying to avoid.
But their experiment did succeed as a proof-of-concept. The next step is to refine the process.
Supercritical CO2 is becoming more popular as a solvent in many areas because of its low toxicity and light touch on the environment. In fact, if you've had a cup of decaffeinated coffee lately, it's very likely that supercritical CO2 was used to extract the caffeine from the beans, said Darren Haskett, an agriculture and biosystems engineering student.
“Supercritical” means that the CO2 is kept in liquid form by pressurizing it at temperatures where it normally would exist as a gas. Supercritical CO2 behaves like a gas in that it can permeate the algae cell membrane, but it's actually in liquid form. Once the CO2 gets into the cell, it creates enough pressure to burst the cell wall, releasing the oil, which the cell has produced to allow it to float on the surface of the water where it's growing.
Grappling With Lots of Issues
Although the project focused on using supercritical CO2 to extract oil from algae, the students had quite a few auxiliary issues to grapple with, said Edward Moreno, the third member of the team.
While it seems that, given a chance, algae will grow anywhere, the students found Botryococcus braunii to be particularly finicky when it came to optimal conditions. The algae grow best at 73 to 75 degrees Fahrenheit but the students did manage to grow it inside the bioreactor chamber in a greenhouse where the temperature fluctuated between 73 and 90 degrees. The colder the water, the more oil the algae produces – up to 50 percent of its weight – but the warmer the water is, the faster the algae grows. “It's a balancing act,” Henry said.
Some researchers theorize that algae similar to Botryococcus braunii might have produced most of the world's massive oil deposits; it took a lot of heat, pressure and time to do so. “We're just trying to take the place of gravity and eons of time to speed up the process and extract the oil in an environmentally friendly way,” Henry said.