Date of Award

January 2020

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

First Advisor

Wayne S. Seames

Abstract

The interest in microalgae as a plausible alternative to crop oils as a raw material in the form of triglycerides and free fatty acids (FA oils) for renewable fuels and chemicals is increasing and it is a widespread research topic at the lab scale. Microalgae contain a higher lipid content on a dry weight basis compared to oilseeds such as soybeans. Additionally, the growth and cultivation cycle of microalgae is 15 days, in comparison to soybeans where the cycle occurs once or twice annually. Despite these advantages, to date it has been uneconomical to produce microalgae oils in a world-scale facility due to limitations in cultivating microalgae at commercial scales and the inefficiency and high costs to extract the lipids.

Extensive research has been done to identify ideal microalgae strains in order to increase lipids production, biomass growth rate and density, and to minimize nutrient consumption, environmental impacts, invasive biologicals and other external factors. Of the hundreds of different strains of microalgae commercially available, a strain which has proven to yield a high lipids content is Chlorella Vulgaris which is also one of the fastest growing microalgae strains. Additionally, this strain of microalgae has been found to be amenable to heterotrophic adaptation.

Recent developments suggest that the use of heterotrophic microalgae may be economically feasible for large-scale oil production. Traditional autotrophic microalgae cultivation at the industrial scale is challenging because either numerous photo bioreactors or large open ponds are required to disperse photons throughout the feedstock for efficient photosynthesis but present a challenge because of the considerable economic investment to procure the large quantity necessary for commercial scale FA oil production. Recent research has explored the potential of transforming autotrophic microalgae to heterotrophic microalgae, negating the light dependence of the studied strains and thus relieving this key scale-up constraint. The transition to heterotrophic halts the photosynthesis process, but requires an organic carbon source to provide energy, as the heterotrophic strain of microalgae is unable to assimilate carbon dioxide as an energy source via photosynthesis. The transition from autotrophic to heterotrophic has been shown to increase the FA oil content of the microalgae by replacing the chlorophyll cells produced during photosynthesis with additional lipids.

This thesis presents three studies. Each addresses a different challenge related to the commercial feasibility of fatty acid-based oil extraction from microalgae. First, a comparative scoping study was performed analyzing the feasibility of an industrial scale process plant for the growth and extraction of oil from microalgae from autotrophic and heterotrophic subspecies of the same microalgae strain. Processes were developed at the preliminary design level using heterotrophic subspecies and autotrophic subspecies of Chlorella Vulgaris. AACE Class 4 cost estimates and economic analyses were performed. This study concludes that processes based on heterotrophic microalgae are more likely to reach economic feasibility than processes using autotrophic microalgae. However, a few barriers still remain to achieve free market economic viability.

The second study provides thermal carbon analysis, as well as ultimate analysis to showcase the differences between the autotrophic and heterotrophic strains of Chlorella Vulgaris grown in-house at the University of North Dakota, and an additional autotrophic strain of Chlorella Vulgaris obtained from the University of Leeds. Both analyses indicate an increased lipids content in the heterotrophic microalgae when directly compared to autotrophic microalgae.

Finally, a study was performed of the most attractive of the various techniques previously reported for optimization of microalgae lipid extraction using an autotrophic version of Chlorella Vulgaris. The best method was then applied to a heterotrophic version of the same microalgae strain for comparison. The factors which were able to be optimized were: 1) the effect of three different solvents: methanol, ethanol, and hexane; 2) the effect of a mechanical pre-treatment of ball mill with a variety of grinding speeds; 3) the effect of various microalgae to solvent ratios; 4) the effect on extraction when the process is facilitated by microwave; 5) the effect on extraction when the process is facilitated by sonication; 6) the effect on extraction when the process is facilitated by temperature; and 7) the effect of in-situ transesterification on extraction efficiency.

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