pH influence on oxygen mass transfer coefficient in a bubble column. Individual characterization of kL and a

Design, Commissioning, and Performance Assessment of a Lab-Scale Bubble Column Reactor for Photosynthetic Biogas Upgrading with Spirulina platensis

The two-step bubble column-photobioreactor photosynthetic biogas upgrading system can enable simultaneous production of biomethane and value-added products from microalgae. However, due to the influence of a large number of variables, including downstream processes and the presence of microalgae, no unanimity has been reached regarding the performance of bubble column reactors in photosynthetic biogas upgrading. To investigate this further, the present work documents in detail, the design and commissioning of a lab-scale bubble column reactor capable of treating up to 16.3 L/h of biogas while being scalable. The performance of the bubble column was assessed at a pH of 9.35 with different algal densities of Spirulina platensis at 20 °C in the presence of light (3–5 klux or 40.5–67.5 μmol m^–2 s^–1). A liquid/gas flow (L/G) ratio of 0.5 allowed consistent CO₂ removal of over 98% irrespective of the algal density or its photosynthetic activity. For lower concentrations of algae, the volumetric O₂ concentration in the upgraded biomethane varied between 0.05 and 0.52%, thus providing grid quality biomethane. However, for higher algal concentrations, increased oxygen content in the upgraded biomethane due to both enhanced O₂ stripping and the photosynthetic activity of the microalgae as well as clogging and foaming posed severe operational challenges.

Numerical Simulation of Mass-Transfer Characteristics of a Bubble Rising in Yield Stress Fluids

The mass-transfer characteristics of bubbles rising in yield stress fluids was investigated numerically using volume-of-fluid and user-defined function methods in this study. The CO₂ concentration profiles inside the liquid phase near the bubble equator at different bubble diameters, yield stresses, consistency coefficients, and flow indices were observed. The results revealed that the rate of mass transfer decreased with the increase of yield stress, consistency coefficient, and flow index of the liquid phase and the decrease of bubble diameter. Moreover, two empirical correlations for the drag coefficients and Sherwood numbers were developed by introducing one correction factor X for C _D correlation and another correction factor f _c for Sherwood numbers for correcting the influence of yield stress behavior on bubble motion and mass transfer, respectively. The predictions of the two correlations were compared with the simulated data, and a satisfactory agreement was found.

Characterization of bubble column photobioreactors for shear-sensitive microalgae culture

The shear-sensitive marine algal dinoflagellate Karlodinium veneficum was grown in a cylindrical bubble column photobioreactor with an internal diameter of 0.044 m. Initial liquid height varied from 0.5 to 1.75 m, superficial gas velocities from 0.0014 to 0.0057 ms^-1, and nozzle diameter from 1 to 2.5 mm. Computational fluid dynamics was used to characterize the flow hydrodynamics and energy dissipation rates. Experimental gas holdup and volumetric mass transfer coefficient strongly depended on the liquid height and correlated well with the Froude number. Energy dissipation near the head space (ED_top) was one order of magnitude higher than the average energy dissipation in the whole reactor (ED_whole), and the value in the sparger zone (ED_spar) was one order of magnitude higher than ED_top. Cultures of K. veneficum were limited by CO₂ transfer at low ED_whole and severely stressed above a critical value of ED_whole.