Enhancement Of CO₂ Transfer And Carbon Fixation By Microalgae In The Airlift Photobioreactors

Microalgae are considered as the optimal medium for biological CO₂ fixation and bio-fuels production,due to their significant CO₂ capture capacity and high lipid content.Gaseous carbon dioxide can provide an essential carbon source during the photosynthesis of microalgae.CO₂ rich gases are generally aerated into the microalgal photobioreactor?PBR?by gas distributors forming rising bubble flows.CO₂ molecules in the bubbles pass through the gas-liquid phase interface and dissolve into the suspension for microalgae cells photosynthesis.Mixing,dissolution and transportation characteristics of carbon dioxide have significant impacts on the growth of microalgae.However,the fast bubble rising velocities cannot provide sufficient gas-liquid contact time for CO₂ dissolution and transportation.Consequently,significant CO₂ molecules are not able to be transferred to culture medium,resulting in the escape of CO₂ and low carbon fixation efficiency.Based on the technology of carbon bio-fixation by microalgae,the objective of this work is to enhance CO₂ transfer from gas phase to liquid phase and hance improve the microalgae growth and CO₂ fixation.To fufill this purpose,firstly,a gas distributor was proposed,and the effect of aeration condition on the CO₂ bubble dynamic behaviors,the dissolution and mixing characteristics of carbon dioxide and the performances of microalgal growth and carbon bio-fixation have been investigated.Then,optimizing the gas-liquid flow field as the mean of enchancing CO₂ transfer,flow and mass transfer characteristics with different gas distribution modes were obtained by CFD method.Finally,two inverted arc trough internalss were introduced into airlift PBRs which can enhance the transfer of CO₂ from gas phase to culture medium by means of prolonging gas-liquid contact time and increasing the partial pressure of CO₂ within microalgae suspension.The bubble behaviors,the CO₂ dissolution characteristics and the performances of microalgal growth and carbon dioxide fixation have been investigated.The main conclusions are as follows.?1?By optimizing the structure of the gas distributor,the bubble detachment diameter was decreased by 55.6%,and the specific surface area of gas-liquid was greatly increased.The rising velocity of bubble was reduced with decreasing orifice diameter and spacing,thereby increasing the retention time of CO₂ in the microalgal suspension.As the parameters of gas distributor are optimized,the volumn mass transfer coefficient was improved by 143%,the mixing time was reduced by 24%,the concentration of microalgae was increased by 18.8%,and the carbon dioxide fixation rate was increased by 23.2%.Under the optimal operational conditions for microalgae growth of 15%?v/v?in the inlet CO₂ concentration and 0.1 vvm in the gas flow rate,the maximum biomass concentration and CO₂ fixation efficiency reached 2.62 g·L^-1 and1.040 g·L^-1·d^-1.?2?In the air-lift column photobioreactors,the axial liquid velocity,gas holdup and volumetric mass transfer coefficient of differernt gas distribution modes decreased with the increase of height,presenting a parabolic distribution.The circumferential gas distribution had the highest gas-liquid mass transfer ability for the largest volume average gas holdup.The variation coefficient of gas holdup and volume mass transfer coefficient of circumferential gas distribution were the smallest,and the values decreased by 45.3%and 35.8%,respectively compared with the central distribution and symmetrical distribution.The difference of microalgae growth distribution and inorganic carbon source distribution with differernt gas distribution modes were mainly in the lag phase of microalgae.When circumferential gas distribution was used,the culture period of microalgae was shortened by 2 days and maximum CO₂ fixation rate improved by 34.2%compared with the central distribution.?3?The airlift photobioreactors embedded with nverted arc trough internals were constructed.When unperforated inverted arc trough?UIAT?internals was introduced to a flat-plate photobioreactor,the amplitude of bubble trajectory was increased,the wavelength of bubble trajectory was decreased,and the average contact time between the CO₂ bubble and the liquid phase was prlonged by 89%.When perforated inverted arc trough?PIAT?internals was introduced to a flat-plate photobioreactor,gas-liquid contact time was prolonged to 256 s and the CO₂ partial pressure inside the PIAT internals was between 13.5 kPa and 15.5 kPa during the CO₂ fixation process,moreover,the periodic aeration effect could provide suspension mixing.These can enhance the transfer of CO₂ from gas phase to culture medium.And thus the dissolved CO₂concentration in the UIAT-PBR and PIAT-PBR was 24 mg·L^-1 and 37 mg L^-1respectively higher than that in the control PBR without internals after 200min of 15%CO₂ was bubbled in.Consequently,the elevated CO₂ transfer contributed to 12.6%and20.9%increment in biomass concentration(3.12 g L^-1 and 3.35g L^-1)and 31.8%and26.2%increment in CO₂ fixation rate in log phase(38.2 mg·L^-1·h^-1 and 36.6 mg L^-1 h^-1)respectively relative to that obtained in the PBR without internals.