Researchers develop one-step algae harvesting, FAME conversion

Researchers from the University of Texas at Austin have developed a proof-of-concept design to harvest, dewater and convert the dried algae to biodiesel in a one-step process using resins.

In a paper titled, “Use of Anion Exchange Resins for One-Step Processing of Algae from Harvest to Biofuel,” University of Texas at Austin scientists Jessica Jones, Cheng-Han Lee, James Wang and Martin Poenie note that to circumvent the cost-prohibitive obtainment of algae oil due to the pumping and processing of large volumes of dilute algal suspensions, they used Amberlite anion exchange resins to simplify the process, which has the potential to greatly reduce processing costs.

In their design, the researchers packed a column with the beaded, fixed-charge Amberlite anion exchange resins and poured dilute algae water through. The algae bind with the resins and, at the point of saturation, the water coming out of the bottom of the column changed from clear to green-colored. Then, the researchers eluted the algae off the resins with a mix of methanol and sulfuric acid, essentially regenerating the resins for reuse.

“In this study, we show that anion exchange resins such as Amberlite can concentrate and dewater algae (i.e., harvest algae) and then be eluted with 5 percent sulfuric acid/methanol reagent,” the researchers note in the paper. “The eluted algae appear to dissolve in the sulfuric acid reagent and esterified fatty acids are converted to FAMEs (biodiesel).”

One of the researchers, Martin Poenie, told Biodiesel Magazine they achieved 40 percent dry weight conversion to biodiesel, “which is very good,” he said. “And then we used a hydrophobic resin to pull the biodiesel out. We don’t think that’s a commercial way to do it, but we were just showing that there are ways to pull the biodiesel out of the sulfuric acid and methanol mix. And then you can recycle the sulfuric acid and methanol.”

Poenie said the current generation of resins is limited in this work because they hold a little bit of water, “and that’s not good,” he said, adding this is why acid and not base catalysis was used in the proof-of-concept. “With this particular batch of resins, it doesn’t work well with base because the resin itself can bind with base and there is some water that’s held by the resins. So in this particular incarnation, acid works better.” The researchers have another paper coming out soon, specifically on the use of resins for harvesting. “Those resins can be used in the same capacity and, in that case, the base catalysis might work much better,” Poenie said.  

The fixed-charge resins used in the proof-of-concept design work, but Poenie said they’re not the best for the job because, in addition to holding water, they don’t have a very high binding capacity. “We’ve come out with two resins that have much better binding capacities, and just changing the pH causes the algae to be released,” Poenie said. “We’re not talking about drastic changes [but it] causes the algae to be released, so it’s particularly good for harvesting. We haven’t tested them for one-step biodiesel, but it’s likely they’ll be good for that as well.”  

Poenie also said more resin design work needs to be done. “We don’t think beads are the best form,” he said. “They’re porous beads, so because of the porosity they hold water. We envision belt harvesting using resin-based belts. We have spent a good bit of research on design and synthesis of resins, a lot of that was trying to get resins that don’t bind nonspecifically, that don’t foul, and yet have very high binding capacity. The resin in this one did not have a high binding capacity, but we’ve developed resins that can bind up to 150 milligrams dry weight algae per gram of resin. We elute at quite high concentrations, 50 grams per liter.” At 100 grams per liter, the algae is paste-like.

The beauty of this, Poenie said, is that you could devise a process using resins where you don’t need to pump water to harvest the algae because the resins just suck them up. “And then it allows you to elute the algae into a very concentrated form rather easily,” he said, “so this seems to be an inexpensive and clean way to harvest algae.”