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Researchers at the University of have developed an unusually rapid method to deliver cost-effective algal biocrude in large quantities using a specially-designed jet mixer. The new kind of jet mixer extracts lipids from algae with much less energy than the older extraction method, a key discovery that now puts this form of energy closer to becoming a viable, cost-effective alternative fuel. The new mixer is fast, too, extracting lipids in seconds.

The team’s results were published in an open-access paper in a new peer-reviewed journal, Chemical Engineering Science X.

Even as electrified vehicles penetrate the short distance transportation market, high energy density transportation fuels remain essential to long distance transportation. However, scalable high energy density alternatives to fossil fuels remain challenging. Yet, microbial fuels show promise. For example, unlike first generation biofuels that use food crops as fuel sources (e.g., corn), microorganisms (e.g., bacteria, fungi, and algae) may be grown on non-arable land and with saline water, wastewater or/and produced water from mineral and petroleum extraction. Unlike second generation biofuels, which use lignocellulose biomass and suffer from complicated harvest steps, microorganisms have simpler cell structures, produce more lipids per harvestable area, and have shorter growing cycle of microorganisms of at most 7–14 days. These features make microorganisms an interesting candidate as a biofuel source. Technological and economic barriers to industrial scale up remain, with microorganisms (e.g., algae) harvesting ranking among the main challenges.

… Where mass transfer limits the rate at which lipids transfer, confined impinging jet mixers (CIJMs) show promise. These devices drive two or more turbulent jets coaxially into a confined mixing chamber. Although microscale devices, they do not suffer from the slow laminar mixing of microfluidics, because rapid turbulent energy dissipation promotes microscale mixing to accelerate molecular scale processes. Due to the high inlet flow rate and relatively small mixing chamber, the residence time within CIJMs remains small yet the flow structure ensures that feed streams mingle intimately. Furthermore, CIJMs have been used in continuous processing of nanoprecipitation, nanomedicines, and nanoparticle production at industrially relevant scales and rates.

Here we critically evaluate the potential of CIJMs for lipid extraction. … Lipid yield does not vary significantly with the concentration of algae feedstock in the tested algae concentration range, implying that algae culture may be used directly as feedstock to CIJMs without intervening dewatering steps. Algal biocrude obtained from CIJMs converts successfully into biodiesel, and cascades of CIJMs increase the net lipid production. CIJMs provide fast lipid extraction, suggesting compelling opportunities to use CIJMs for extraction generally.

—Tseng et al.

The key piece here is trying to get energy parity. We’re not there yet, but this is a really important step toward accomplishing it. We have removed a significant development barrier to make algal biofuel production more efficient and smarter. Our method puts us much closer to creating biofuels energy parity than we were before.

—Leonard Pease, co-author

Now, in order to extract the oil-rich lipids from the algae, scientists have to pull the water from the algae first, leaving either a slurry or dry powder of the biomass—the most energy-intensive part of the process. That residue is then mixed with a solvent where the lipids are separated from the biomass. What’s left is a precursor, the biocrude, used to produce algae-based biofuel.

That fuel is then mixed with diesel fuel to power long-haul trucks, tractors and other large diesel-powered machinery. But because it requires so much energy to extract the water from the plants at the beginning of the process, turning algae into biofuel has thus far not been a practical, efficient or economical process.

The Utah team created a new mixing extractor, a reactor that shoots jets of solvent at jets of algae, creating a localized turbulence in which the lipids “jump” a short distance into the stream of solvent. The solvent then is taken out and can be recycled to be used again in the process.

Our designs ensure you don’t have to expend all that energy in drying the algae and are much more rapid than competing technologies.

—University of Utah chemical engineering assistant professor Swomitra “Bobby” Mohanty, co-author

This technology could also be applied beyond algae and include a variety of microorganisms such as bacteria, fungi, or any microbial-derived oil, says Mohanty.

In 2017, about 5% of total primary energy use in the United States came from biomass, according to the US Department of Energy. Other forms of biomass include burning wood for electricity, ethanol that is made from crops such as corn and sugar cane, and food and yard waste in garbage that is converted to biogas.

The benefit of algae is that it can be grown in ponds, raceways or custom-designed bioreactors and then harvested to produce an abundance of fuel. Growing algae in such mass quantities also could positively affect the atmosphere by reducing the amount of carbon dioxide in the air.

Resources

• Yen-Hsun Tseng, Swomitra K. Mohanty, John D. McLennan, Leonard F. Pease (2019) “Algal lipid extraction using confined impinging jet mixers,” Chemical Engineering Science: X Volume 1, 100002 doi: 10.1016/j.cesx.2018.100002