Effects of bubble cutting dynamic behaviors on microalgal growth in bubble column photobioreactor with a novel aeration device

Abstract

Introduction: Carbon sequestration by microalgae is an effective approach for achieving carbon neutrality owing to its high carbon capture efficiency and environmental friendliness. To improve microalgae CO₂ fixation efficiency, various methods to enhance CO₂ transfer at the gas-liquid interface have resulted in high energy consumption. Methods: In this study, a novel aeration device with bubble cutting slices was installed in a photobioreactor for CO₂ supply, which could precisely separate bubbles into sizes on the way to rising after departure, achieving CO₂ transfer enhancement without extra energy consumption. Subsequently, the bubble cutting dynamic behaviors in the photobioreactor were studied, and the effects of thickness, hydrophilicity, and arrangement of cutting slices on microalgal growth were analyzed. Results: It was found that bubble cutting caused the maximum dry weight and biomass productivity of microalgae to improve by 6.99% and 33.33%, respectively, compared with those of the bioreactor without cutting units, owing to a 27.97% and 46.88% decrease in bubble size and rising velocity, respectively, and an 84.55% prolongation of bubble residence time. Discussion: Parallel cut slices with a thickness and spacing of less than 3 mm successfully cut the bubbles. The hydrophobic slice surface prevented daughter bubble departure and prolonged the bubble residence time, impeding microalgae growth owing to bubble coalescence with subsequent bubbles. The optimal cutting slice parameters and culture conditions for microalgal growth were 1 mm slice thickness, less than 1 mm slice spacing, 5% inlet CO₂ concentration, and 70 mL/min gas flow rate.

Keywords: aeration device; bubble cutting; chlorella pyrenoidosa; hydrophilicity; microalgae carbon sequestration; visualization.

FIGURE 1

Effects of bubble cutting dynamic behaviors on microalgal growth in bubble column photobioreactor with a novel aeration device (A) Schematic of the experimental setup (B) Picture of the experimental setup.

FIGURE 2

Effects of bubble behaviors on microalgal growth with cutting units in the bioreactor (cutting units:δ = 1 mm, M = 0, gg) (A) Bubble deformation process during bubble cutting (B) Distribution of bubble diameter in the height direction of bioreactor (C) Growth curve of Chlorella pyrenoidosa (D) Variation of pH during algal growth.

FIGURE 3

Effects of cutting units thickness on bubble behaviors and microalgal growth (cutting units: M = 0, gg) (A) Distribution of bubble diameter in the height direction of bioreactor (B) Growth curve of Chlorella pyrenoidosa (C) Variation of pH during algal growth.

FIGURE 4

Effects of cutting units hydrophilicity on bubble behaviors and microalgal growth (cutting units:δ = 1 mm, M = 0) (A) Distribution of bubble diameter in the height direction of the bioreactor (B) Growth curve of Chlorella pyrenoidosa (C) Variation of pH during algal growth.

FIGURE 5

Effects of cutting units arrangement on bubble behaviors and microalgal growth (cutting units:δ = 0.15 mm, gg) (A) Distribution of bubble diameter in the height direction of the bioreactor (B) Growth curve of Chlorella pyrenoidosa (C) Variation of pH during algal growth.

FIGURE 6

Performance of the optimal aeration device in microalgae photobioreactor (cutting units: δ = 1 mm, M = 0 mm, gg) (A) Variation of growth curve with different inlet CO₂ concentrations (B) Variation of pH with different inlet CO₂ concentrations (C) Variation of growth curve with different gas flow rates (D) Variation of pH with different gas flow rates.