Date of Award

12-2014

Degree Name

MS in Engineering - Biochemical Engineering

Department

Biomedical and General Engineering

Advisor

Yarrow Nelson

Abstract

ABSTRACT

Photobioreactor Design for Improved Energy Efficiency of Microalgae Production

Alexander Burns

The objective of this research was to investigate a new photobioreactor (PBR) design for microalgae production that retains the typical advantages of existing tubular PBRs while reducing power consumption by providing simultaneous culture circulation and gas exchange with airlift alone and no centrifugal recirculating pump. Traditional tubular PBR designs feature a compressed air supply and a centrifugal pump for culture circulation and gas exchange. Circulation and gas exchange in a closed-system PBR is necessary to keep the algae suspended and to provide sufficient mass transfer (mainly for the exchange of oxygen and carbon dioxide). In a traditional tubular PBR sparged air keeps the culture well mixed and strips out excess dissolved oxygen in an airlift-column unit, while the centrifugal pump circulates the culture in the tubular stage and decreases the amount of air bubbles traveling into this stage; where most of the photosynthesis occurs. The PBR design proposed herein does away with the usual centrifugal pump. The air blower performs both gas exchange in the airlift columns and system-wide circulation. This builds on a previous tubular PBR design that provides circulation and gas exchange by airlift alone, which was patented by Cathcart in 2011. However, the Cathcart patent does not provide data on mixing, gas exchange, energy consumption, flow regime or biomass productivity. The new design described here builds on the Cathcart design, but includes several unique design features, such as larger diffuser columns which provide airlift-induced flow for a series of vertical PBR tubes. To perform a power consumption v analysis, a pilot-scale prototype of the new PBR design was built and operated. The prototype PBR consisted of two airlift columns attached to 9 m of vertical serpentine tubing connected to the top and bottom by standard 90-degree PVC elbows in a U-bend fashion to each column to make a total working volume of 235 L. The airlift columns were about 1.5 m tall and 30.5 cm ID, while the serpentine tubes were about 0.9 m tall and 7.6 cm ID to make a total of five vertical tubes for every airlift column. Data collected for this prototype design suggest an average overall areal productivity (OAP) of 111 g m-2 d-1 (g biomass m-2 total land area with empty space day-1), an average illuminated surface productivity (ISP) of 14.3 g m-2 d-1 (g biomass m-2 reactor photo-stage day-1), an average volumetric productivity (VP) of 0.55 g L-1 d-1 (g biomass L-1 reactor working volume day-1), a specific power input in the range of 330 to 360 W m-3 (W power needed for culture circulation and gas exchange m-3 reactor working volume) and a specific biomass productivity (SBP) in the range of 17.6 to 19.1 mg kJ-1 (mg biomass kJ-1 energy needed for culture circulation and gas exchange) with Chlorella vulgaris as the model algae. The biomass productivity per energy input (SBP) of the new PBR design appears to be higher than that of similar designs currently described in the literature. Elimination of the centrifugal pump in a tubular PBR design is a concept worth further study for potential energy savings.

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