Optimization of vertical photobioreactors

This study focuses on optimizing the design of a photobioreactor to increase carbon dioxide sequestration by phototrophic microalgae. It is known that a significant amount of carbon dioxide emissions can be attributed to energy production processes and is expected to increase as global energy needs increase. The carbon dioxide emissions can be reduced by feeding carbon dioxide in the flue gas from power generation to an algal cultivation system. To increase carbon dioxide sequestration, it is imperative that an economical and robust photobioreactor that is capable of operating in any weather condition and location be designed. The focus of this study was to optimize vertical photobioreactors that are known to have a higher aerial productivity (higher biomass per unit area) and hence more efficient in sequestering carbon dioxide than the horizontal reactors.Horizontal tubular photobioreactor and vertical bubble column type units differ substantially in many ways; particularly with respect to the surface-to-volume ratio, the amount of gas in dispersion, the gas-liquid mass transfer characteristics, hydrodynamics, and internal irradiance levels. The effect of superficial velocity, temperature, carbon dioxide concentration, light irradiance, illuminated surface-to-volume ratio, pH, and gas holdup, and reactor geometry (diameter and height) were investigated with the algal specie Chlorella vulgaris in vertical bubble column reactors. Experiments were conducted on vertical photobioreactors of 2, 3, 4, 5, 6, 12, and 18 inch diameters of various heights (5, 8, and 10 feet) by changing the parameters that affect the algal density. In addition to the vertical bubble column configuration, algal densities in split and co-annular reactor designs were also investigated and found to be 1.334 and 1.842 g/L respectively. At the conditions studied, the 3 inch diameter vertical bubble column reactor produced the highest algal density (5.21 g/L) and the other reactors with 2, 4, 5, 6, 12 and 18 inch diameters reached densities of 0.938, 3.611, 3.296, 2.417, 1.665, 0.919 g/L respectively. The results show, that an increase in the ratio of the illuminated surface area to liquid volume in the reactor proportionately increases the biomass yields in vertical bubble column reactors and has the most effect on the final biomass yield in a vertical bubble column reactor. It was observed that increasing the reactor height from 5 feet to 10 feet had no effect on the biomass yields. Increasing the growth temperature from 20 to 35°C increased the biomass yields (20-27%) of Chlorella vulgaris. Increasing the CO2 concentration up to 9% in the air / CO2 flow increased the biomass yields and provided asufficient level of pCO2 while maintaining the pH level required for algal growth in the reactor geometries tested. This thesis stands out from others in presenting the data for larger operating volumes and geometries of vertical reactors (bubble column, split and co-annular). The growth data from a reactor with an 18 inch diameter and a height of 10 feet amounting to a reactor volume of 500 liters is among the first known publicly disclosed data reported for vertical bubble column reactors