(139d) Effect of Hydrodynamics on Microalgal Growth in Externally-Illuminated Tubular Bubble-Column Photobioreactors

This study focuses on how the kinetics of microalgal growth in a bubble-column photobioreactor are affected by radial recirculation. We employ an experimental setup to show how the growth dynamics change over time for two distinct flow patterns in a bubble column photobioreactor: one that only allows for axial air flow and another that allows for adequate radial algal movement. Our experiments have been performed using extremophillic microalga Chlorella sorokiniana for 6-8 days in a 5 L externally lit bubble-column photobioreactor, with various CO₂ concentrations, ranging from atmospheric condition (0.04% v/v) through 1%, 2%, 3%, 4% to 5% concentrations. External illumination was kept at 10500 lux throughout the course of the experiment.

Our investigation demonstrates that the well-circulated radial flow pattern inside a bubble column cylindrical photobioreactor encourages higher microalgal yield compared to its counterpart at lower CO₂ concentrations (atmospheric to 2% condition). Both nitrogen-limited and non-nitrogen-limited instances were used to verify our assertions, and we discovered that both cases showed similar findings. The microalgal biomass content was found to be 4.62 and 5.40 g/L after 186 hours in radially-mixed nitrogen limited and non-nitrogen limited instances, respectively, as opposed to 4.13 and 4.87 g/L in its counterparts. However, at higher CO₂ concentrations, the algal media's pH, in the radially well-mixed configuration, declines rapidly to below 7.5, which reduces the algal productivity. The failure of the radial bubbles at higher algal density to drive them across the reactor radii to the periphery of the externally-lit reactor, as well as the lower pH, may be responsible for the lower average algal concentration in the radially well-mixed scenario at higher CO₂ concentrations.

As a result, we advocate greater radial mixing at lower CO₂ levels (<3% CO­₂). Conversely, axial mixing is preferred for greater algal output at higher CO₂ input concentrations.

Keywords: Bioprocessing, Bioenergy, Microalgae, Green Energy