Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Warren D. Seider

Abstract

Biodiesel, derived from renewable feedstocks like algae, has the potential to replace traditional, petroleum-based fuels — providing a carbon-neutral, sustainable transportation fuel. However, with plummeting oil prices, alternative fuels have become less competitive. Thus, process modeling and optimization are needed to reduce costs. Extensive modeling has been done for the conversion of algae and plant lipids to biofuels, but the upstream operations remain poorly understood. We partnered with other organizations to create an overall techno-economic model for a commercial-scale algae-to-biodiesel venture, using software packages like ASPEN PLUS, the ASPEN Process Economic Analyzer, gPROMS, and AIMMS. The two most important findings from this model were that: (1) cultivation represented 90% of the total capital expense because of the massive fields required to grow the algae, and (2) extraction of the oil from algae had highly variable cost estimates, which spanned three orders of magnitude. The low photosynthetic efficiency of the algae was the major limiting factor in terms of algae growth. Therefore an exergy analysis was undertaken to rigorously calculate the efficiency (3.9%) and determine what could be done to improve it. Overall, the algae cell’s absorption of sunlight was the largest loss of exergy, and therefore the most crucial factor in decreasing capital expenditures for this venture. Regarding the extraction of the oils, supercritical carbon dioxide is a green, non-toxic solvent that can be used to extract and convert algae-oils to biodiesel in a single step, eliminating the need for pre- or post-processing of the oil or biodiesel product. The statistical associating fluid theory equations-of-state in ASPEN PLUS (PC-SAFT) and gProms (SAFT-γ Mie) were used to perform the fluid-phase equilibria calculations because of their improved robustness and higher accuracy for long-chain hydrocarbons when compared with cubic equations-of-state. A multi-phase reactor model was formulated to account for the effects of changing phase equilibria on reaction conversions. While further research is required to obtain cost estimates, preliminary results for this system show that it is possible to achieve high oil-to-biodiesel conversions at much lower pressures than previous anticipated.

Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS