Impinging jet mixers are applied to bio-crude harvesting from algae

[USA] Microalgae have been investigated as a source of “bio-crude” — fatty acid molecules (lipids) that can be refined into renewable transportation fuels — but harvesting lipids from the algae cells economically remains the most challenging and energy-intensive step in the process.

A common method for harvesting lipids involves drying the algae, then grinding the cells and dissolving the lipids with a blend of strong solvents. Now, researchers from the University of Utah (Salt Lake City; www.utah.edu) have developed a new harvesting method that eliminates the need for drying and grinding, potentially allowing a lower-energy harvesting process.

The new method adapts confined impinging jet mixers (CIJMs) for the first time to the task of stretching and breaking the algae cell walls and extracting cell contents. Inside a CIJM, jets of microalgae slurry collide with jets of organic solvent (hexanes) at high flowrates (1,200 mL/min), such that extreme local turbulence is created. The turbulence generates shear forces capable of rupturing algae cell walls (diagram). The lipids that are liberated in this way are dissolved by the solvent for further processing.

In adapting the CIJMs to microalgae harvesting, the research team determined the appropriate size of the reactor where the slurry and solvent are mixed, explains Swomitra Mohanty, University of Utah chemical engineering professor and research co-leader. “Also, we had to characterize the correct flowrates and determine the solvent-to-algae biomass ratio to create a working process,” he says.

“Our research is really looking for a pathway to energy parity in algae biofuels — that is, we want to establish a method of extracting more energy from the algae than is needed to harvest it,” Mohanty says. “The CIJMs enable a process that can use algae slurry directly, rather than requiring energy to remove the water and grind the biomass, and this allows significant energy savings.”

The CIJM extraction process has been demonstrated at the bench scale, and the team is exploring industrial partnerships for scaling up the technology.

 

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