Water under subcritical conditions in a continuous-flow reactor (flash hydrolysis) has proved to be an efficient and environmentally friendly method for hydrolyzing proteins from microalgae biomass in a very short residence time (few seconds). In this study, flash hydrolysis experiments were conducted at three different temperatures (240, 280, and 320°C) and three residence times (6, 9, and 12 s) to understand the kinetics of the hydrolysis of algae proteins to water-soluble peptides and arginine. Laboratory-grown protein-rich Scenedesmus sp. with an average composition of 54% proteins, 17% lipids, and 23% carbohydrates was used as the feedstock. After flash hydrolysis, both liquid and solid products were collected, and the contents of soluble peptides and arginine in the liquid fraction and of remaining proteinaceous material in the solids were analyzed. For all experiments above 240°C at all residence times, the yield of soluble peptides was in the range of 57−67% of the algae protein, whereas the maximum arginine yield (81.51%) was achieved at 320°C and a residence time of 6 s. The protein solubilization to soluble peptides fitted second-order reaction kinetics, whereas for arginine, the process was zeroth-order; the activation energies were calculated to be 43.0 and 34.1 kJ/mol, respectively. The results of this study suggest that flash hydrolysis can be an environmentally benign method for hydrolyzing proteins from microalgae to produce valuable coproducts such as arginine as a free amino acid and water-soluble peptides along with lipid-rich solids (biofuel intermediate) as a feedstock for biofuel production.

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The cost‐effective production of liquid biofuels from microalgae is limited by several factors such as recovery of the lipid fractions as well as nutrients management. Flash hydrolysis, a rapid hydrothermal process, has been successfully applied to fractionate the microalgal biomass into solid biofuels intermediates while recovering a large amount of the nutrients in the aqueous phase (hydrolyzate) in a continuous flow reactor. The aim of the work is to enhance the quality of a high‐ash containing marine algae Nannochloropsis gaditana as biofuel feedstock while recycling nutrients directly for algae cultivation. Characterization of products demonstrated an increase in extractable lipids from 33.5 to 65.5 wt % (dry basis) while retaining the same fatty acid methyl ester profile, in addition to diminution of more than 70 wt % of ash compared to raw microalgae. Moreover, the hydrolyzate was directly used to grow a microalga of the same genus. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1494–1502, 2017

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Generation of coproducts from nutrients is purported to improve the sustainability of algae-derived transportation fuels by minimizing life cycle impacts and improving economic sustainability. Although algae cultivation produces lipids that is upgraded to drop-in transportation fuel products, life cycle assessment and techno-economic analysis have shown that without coproducts, energy/economic returns are diminishing regardless of processing methods. This study utilizes a combined flash hydrolysis (FH), hydrothermal liquefaction (HTL), and coproduct conversion technology (atmospheric precipitation/AP; hydrothermal mineralization/HTM) to conserve the most recyclable nutrients for coproduct marketability. Six biofuel pathways were developed and compared in terms of “well-to-pump” energy, life cycle greenhouse gas (LC-GHG) emissions, and economic profitability: renewable diesel II (RDII), renewable gasoline (RG), and hydroprocessed renewable jet (HRJ) fuel, each were modeled for AP and HTM coproduct conversion. A functional unit of 1 MJ usable energy was employed. RG showed a promising energy-return-on-investment (EROI) due to multiple coproducts. All models demonstrated favorable EROI (EROI > 1). LC-GHG emissions tie in with EROI such that RG produced the least emissions. HRJ-HTM was determined to be the most profitable model with a profitability index (PI) of 0.75. Sensitivity analyses revealed that dewatering affects EROI and PI significantly. To achieve break-even, gasoline must sell at $4.10/gal, diesel at $5.64/gal, and jet fuel at $3.43/gal.