Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Robert H. Davis

Second Advisor

Christine M. Hrenya

Third Advisor

Mark P. Stoykovich

Fourth Advisor

Matthew C. Posewitz

Fifth Advisor

Matthew W. Liberatore

Abstract

In the future, increased demand and rising costs are expected for energy and liquid fuels; meanwhile, global climate change is attributed to carbon-dioxide emissions from fossil-fuel combustion. These challenges may be addressed by lignocellulosic and algal biofuels. Lignocellulosic biomass may be converted to bioethanol via sequential pretreatment, enzymatic saccharification and fermentation, while algal biomass may be converted to biodiesel or possibly gasoline. For lignocellulosic bioethanol, two predominant costs are associated with the biomass feedstock and the enzyme preparation, and these costs are due, in part, to slow reaction rates as well as poor conversion during the enzymatic saccharification step. Algal biofuels are faced with challenges in large-scale production, harvesting and extraction. In particular, harvesting is a significant portion of the total production costs due to the microscopic and dilute nature of autotrophically-grown algae. To address poor conversion rates during cellulose saccharification, a battery of experiments was designed to attribute the main inhibition mechanisms as reductions in (1) enzyme activity and (2) substrate reactivity, or (3) an accumulation of inhibitory products in solution. Inhibition mechanisms due to product sugars and reduced substrate reactivity were found to be dominant. Conversion was increased via product-sugar removal via ultrafiltration. To address algal harvesting costs, primary concentration using sedimentation via flocculation or via inclined sedimentation was studied. Additive-free flocculation and sedimentation of algae using naturally-available Mg2+ present in brackish-water was evaluated in an effort to reduce flocculant costs. An optimum pH was observed at which rapid settling and yet minimal precipitation occurred. Using inclined sedimentation, high-aspect ratio settlers inclined at shallow angles were used to achieve rapid algal clarification, at bulk-average velocities up to 500 times the terminal velocity of single cells. Sludge flow at a particular angle was found to be dependent on sludge mass and adhesion. Negatively-charged, Alconox-treated glass achieved sludge flow at angles as low as 10° from horizontal. Alternatively, algae harvest and concentration was demonstrated by a two-step process consisting of a long clarification and deposition step followed by a short sludge resuspension and recovery step. Using this technique, a 0.7 g/L algal suspension was concentrated to 59 g/L.

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