Low dissolution of CO2 in closed tubular photobioreactors results in insufficient carbon source supply, and often limits microalgae growth. An optimized ZIF8-SE medium containing zeolitic imidazolate framework-8 (ZIF-8) nanoparticles was developed for strengthening CO2 mass transfer in the photobioreactor, which improved CO2 dissolution to increase microalgal biomass yield under 15% CO2. The CO2 mass transfer coefficient markedly increased from 1.82 to 3.85, leading to an increased dissolved inorganic carbon concentration. Meanwhile, microalgae biomass accumulation with 0.01 mmol/L ZIF-8 nanoparticles was 25.5% higher than that without ZIF-8 nanoparticles. Furthermore, the average cell size decreased from 2.61 to 2.38 mu m and the cell fractal dimension increased from 1.38 to 1.56 when ZIF-8 nanoparticle concentration increased from 0 to 0.2 mmol/L, indicating that potential toxicity might occurred with excess ZIF-8 nanoparticles. Reutilization of ZIF-8 nano particles in the recycled culture medium improved microalgal growth by 23%, which was similar to the promotion of fresh ZIF8-SE medium. This new finding provides a new strategy to improve CO2 mass transfer in the horizontal tubular photobioreactor, which makes it a feasible platform for CO2 capture and utilization.

Computational fluid dynamics were used to analyze the flash light effect and CO2 bubble behavior in an optimized flow field generated by double paddlewheels in a flat plate photoreactor to enhance microalgal biomass productivity. The increased D/w ratio significantly enhanced the average turbulent kinetic energy and flash cycle frequency. However, the effects became weak when the D/w ratio was over 0.67. Appliance of double paddlewheels increased flash cycle frequency from 0.035 to 0.121 Hz and increased light time ratio from 8.3% to 31.5%. Meanwhile, the bubble dynamic behavior was characterized using population balance model. The average bubble size reduced by 24.4% and the bubble rising velocity reduced by 10.6%, which facilitated CO2 mixing and mass transfer in microalgal solution. Therefore, biomass accumulation of microalgae Chlorella in the photoreactor with double paddlewheels increased by 62.3% under 15% CO2.

Poor light utilization efficiency and large occupied area of traditional raceway pond photobioreactors result in low areal microalgal biomass yield in industrial applications. In this study, a pond-tubular hybrid photobioreactor (PTH-PBR) comprising raceway ponds and horizontal tubes was developed to strengthen flash light effect and improve areal microalgal biomass yield. The highest flash cycle frequency (0.63 Hz) of microalgae cells along flow pathway was obtained in the raceway pond of PTH-PBR when shaded area percentage was 20% and ratio of adjacent tube interval to tube diameter was 1, which enhanced microalgal biomass yield by 31.2% than traditional raceway pond. Meanwhile, intracellular chlorophyll content increased by 33.6% and PSII maximum quantum yield (Fv/Fm) increased by 8.1% due to decreased photoinhibition stress. The areal microalgal biomass yield of PTH-PBR was 54.7% higher than that of traditional raceway pond without horizontal tubes.

In order to enhance the CO2 dissolution rate and prolong gas-liquid contact time to improve microalgal growth, a spiral-ascending CO2 dissolver was developed to enhance CO2 mass transfer in a horizontal tubular photobioreactor. A spiral-ascending flow pattern of CO2 bubbles was formed with a helical baffle and central hollow tube in the CO2 dissolver, which markedly extended flow trajectory of CO2 bubbles and intensified the mixing effect. This novel CO2 dissolver with the helical baffle shortened the bubble generation time by 30.6% and decreased the bubble generation diameter by 23.4%, thereby enhancing the mass transfer coefficient by 69.2%. The bubble retention time dramatically increased by 190.2%, which prolonged CO2 gas-liquid contact time to enhance dissolved CO2 concentration. The liquid phase mixing time decreased by 15.8% to give a uniform status in microalgal suspension, allowing for sufficient nutrient supply for photosynthesis during microalgal circulation in horizontal tubes. This led to an increased microalgal biomass accumulation by 40.8% with 15% CO2 in the horizontal tubular photobioreactor.

Spermidine enhanced resistance of Chlorella to high levels of CO2 and light intensity.

To reduce bubble diameter and enhance mass transfer, a novel jet-aerated tangential swirling-flow plate photobioreactor was developed that improves the growth rate of microalgae. In this system, the circulating microalgal solution enters a jet aerator that takes up 15% CO2 by vacuum suction and then injects into a plate photobioreactor through four centrally symmetric nozzles. Each jetflow is tangent to a tangential circle, driving vertical vortex movement of the surrounding microalgal solution, which markedly reduced the bubble diameter and enhanced mass transfer. The mass transfer coefficient was enhanced by decreasing the nozzle number (n) and increasing the ratio of tangential circle diameter to plate photobioreactor equivalent diameter (d/D). The average bubble diameter decreased by 80.2% to 0.37 mm and the mass transfer coefficient increased 4.6 times to 48.9 h-1 when n was 4 and d/D was 0.34. Finally, the optimized system increased the biomass dry weight of microalgae by 49.4%.

A CO2 microbubbles dissolver (CMD) was developed to facilitate dissolving inorganic carbon and strengthening mass transfer in a horizontal tubular photo-bioreactor system (HTPBRS), which enhanced microalgae biomass productivity with flue gas containing 15% CO2. The influence of pump power on the bubble formation and mixing effect was found to be more obvious than that of gas flow rate. Ceramic shell aerator was more favorable for reducing bubble diameter and enhancing mass transfer than traditional rubber strip aerator. Bubble formation time decreased by 53.4% and mixing time decreased by 68.9% in response to the increased pump power. When the base area ratio of ceramic shell aerator to dissolver in the HTPBRS increased, bubble formation time decreased by 19.6% and mass transfer coefficient increased by 80.9%. The biomass yield of microalgae Chlorella PY-ZU1 with ceramic shell aerator was 30% higher than that with rubber strip aerator in the HTPBRS.

To reduce the diffusion resistance of CO2 and enhance its permeability in mixed matrix membranes (MMMs), lantern Zn/Co-ZIF nanoparticles were loaded in-situ into semi-interpenetrating cross-linked poly(ethylene oxide) (XLPEO) membranes to provide efficient channels for CO2 permeability. TEM and SEM results showed that the Zn/Co-ZIF nanoparticles (diameter 490-550 nm) treated at 145 degrees C possessed a lantern structure with hollow cores (diameter 380-450 nm) and thin shells (thickness 20-85 nm). XRD and BET results indicated that the nanocrystal structure and porosity of the Zn/Co-ZIF remained intact after etching at 145 degrees C. Peaks shifts in the infrared spectra confirmed that the Zn/Co-ZIF nanoparticles were well integrated with the XLPEO membrane through hydrogen bonds. Therefore, doping 15 wt% lantern Zn/Co-ZIF nanoparticles into the membrane dramatically increased CO2 permeability by 72% to 761.9 +/- 3.0 barrers, while the CO2/N-2 selectivity was above the upper bound established by Robeson in 2008.

Double paddlewheels were proposed to generate cycle flow for increasing horizontal fluid velocity between dark and light zones in a flat plate photo-bioreactor, which strengthened the mass transfer and the mixing effect to improve microalgal growth with 15% CO2. Numerical fluid dynamics were used to simulate the cycle flow field with double paddlewheels. The local flow field measured with particle image velocimetry fitted well with the numerical simulation results. The horizontal fluid velocity in the photo-bioreactor was markedly increased from 5.8x10(-5) m/s to 0.45 m/s with the rotation of double paddlewheels, resulting in a decreased dark/light cycle period. Therefore, bubble formation time and diameter reduced by 24.4% and 27.4%, respectively. Meanwhile, solution mixing time reduced by 31.3% and mass transfer coefficient increased by 41.2%. The biomass yield of microalgae Nannochloropsis Oceanic increased by 127.1% with double paddlewheels under 15% CO2 condition.

A novel serial lantern-shaped draft tube (LDT) that generates vortices is proposed to increase radial velocity between dark and light regions for improving CO2 fixation with microalgae in a gas-lift circumflux column (GCC) photobioreactor. Clockwise vortices are generated in the downflow outerloop of the GCC photobioreactor with LDT. Radial velocity was improved from 1.50 to 4.35x10(-2) m/s, thereby decreased liquid cycle period between dark and light regions by 1.9 times. Mixing time decreased by 21%, and mass transfer coefficient increased by 26% with LDT. Liquid radial velocity in the downflow outerloop and mass transfer coefficient in the GCC photobioreactor both first increased and then decreased when single-lantern height was increased. Peak CO2 fixation rate increased from 0.62 to 0.87 g/L/d, microalgal biomass yield increased by 50%. Removal efficiencies of pollutants (chemical oxygen demand, ammonium, tilmicosin, and ethinylestradiol) in wastewater were 62-90% with microalgae growth in GCC photobioreactor with LDT.

Computational fluid dynamics were employed to simulate microalgal cells movement with enhanced flash-light effects in a gaslift loop-current column photobioreactor (GLCP) with serial lantern-shaped draft tube (LDT). Clockwise and anticlockwise vortexes were formed in outer down-flow region of GLCP with LDT. The radial velocity, axial velocity, and turbulent kinetic energy of microalgal solution appeared periodical change around the lanterns. The average radial velocity showed a sixfold improvement from 0.003 m/s to 0.021 m/s, and average turbulent kinetic energy was enhanced by 18.2% from 22.5 x 10(-4)m(2)/s(2) to 26.6 x 10(-4)m(2)/s(2), thus increasing light/dark cycle frequency by 54%. The light/dark cycle frequency increased first and then decreased with an increase of individual lantern height. The increased lantern number promoted the light/dark cycle frequency and light time ratio. Microalgal biomass yield in the GLCP with LDT was improved by 30%, and CO2 fixation peak rate was promoted by 35%.