Date of Award

Winter 2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil/Environmental Engineering

Committee Director

Sandeep Kumar

Committee Member

Gary Schafran

Committee Member

Mujde Erten-Unal

Committee Member

James W. Lee

Abstract

Microalgae have shown much higher growth rates and productivity when compared to conventional agricultural crops, aquatic plants and tree species, requiring much less land area than other biodiesel feedstock. To harness that potential the hydrothermal liquefaction of algae biomass was studied and a new process called "Flash Hydrolysis" was developed to use water under subcritical conditions, this process capitalizes on the difference in reaction kinetics of algae polymeric components and fractionates proteins in liquid phase in seconds of residence time.

The main objectives for this study are: Analyze the effect of temperature in FH process to maximize the extraction of protein from the microalgae biomass and its recovery as soluble peptides and free amino acids (Chapter 2). To obtain enough experimental data to fit in a mathematical model for a kinetics study of protein and arginine solubilization via Flash Hydrolysis (rates constant k, reaction order and activation energy Ea) and characterize both liquid and solid products collected after Flash Hydrolysis (Chapter 3). Evaluate the possibility of recycle the extracted nutrients in the aqueous phase products after FH to grow more algae and close the loop in a continuous production system (Chapter 4).

In Chapter 2 all the experiments were conducted using flocculated Scenedesmus sp. cultivated in the laboratory using photobioreactors. The effect of temperature and residence time on protein hydrolysis to water-soluble fractions (algal hydrolyzate) and yield of lipid-rich solids (biofuels intermediate) was studied using a lab-scale continuous flow reactor. More than 60 wt% of the total nitrogen content (dry basis) in Scenedesmus sp. was extracted within 10 s of residence time above 240 °C. The ion chromatography and NMR spectra of the algal hydrolyzate showed that the extracted proteins were present both as free amino acids and peptides. The carbon content of biofuels intermediate increased up to 66 wt% making it lipid- and energy-dense feedstock suitable for biofuels production. The scanning electron microscope image of biofuels intermediate indicated that the solids were globular and smaller in size as compared to the untreated microalgae.

In Chapter 3 a new set of Flash Hydrolysis experiments were conducted at two different temperatures (240 and 280°C) and three residence times (6, 9 and 12 seconds) to understand the kinetics of algae proteins hydrolysis to water-soluble peptides and arginine. The proteins-rich microalgae Scenedesmus sp. with an average composition of 55% proteins, 18% lipids, and 20% carbohydrates was used as feedstock. After Flash Hydrolysis both liquid and solid products were collected and the soluble peptides and arginine contents were analyzed in the liquid fraction, as well as the content of remaining proteinaceous material in the solids. The experiments at 280 °C and 9 s residence time was the optimum process conditions for soluble-peptides yield (63.7%) whereas the maximum arginine yield (54.4%) was achieved at 280 °C and 12 s of residence time.

The protein solubilization to soluble peptides fitted second order reaction kinetics, while for arginine was zeroth order and the activation energy was calculated to be 40.7 and 53.6 KJ/mol, respectively. The results of the study suggest that the Flash Hydrolysis can be an environmentally benign method to hydrolyze proteins from microalgae for producing valuable co-products such as arginine and water soluble peptides along with lipid-rich solids (biofuels intermediate) as a feedstock for biofuels production.

Flash hydrolysis (FH) of microalgae biomass is a promising conversion and extraction method capable of solubilize more than 60% of the protein and recover it in the hydrolyzate (aqueous phase) as organic nitrogen (mix of ammonia, amino acids and soluble peptides). In a similar way almost 100% of the organic phosphorus from the microalgae biomass is recovered as soluble phosphates.

In Chapter 4 the evaluation and potential use of the hydrolyzate obtained after FH as a source of nutrients for continuous microalgae production was demonstrated in laboratory conditions. The hydrolyzate contains a combination of nitrogen species that include soluble ammonia and peptides. This two organic nitrogen species are available for the algae to use as nitrogen source an support its growth in levels comparable to those obtained pure culture media (AM-14). Besides nitrogen, soluble phosphorus is recovered after Flash hydrolysis and its present mostly as orthophosphate. The amount of P supplemented by the hydrolyzate was taken by the algae almost completely in a similar way as it could metabolize the P provided by the culture media. The recycle of nitrogen and phosphorus would reduce the initial requirement of those nutrients in the balanced media used to grow algae and will reflect in a reduction in the total production cost.

DOI

10.25777/senz-tz18

ISBN

9781321556858

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