| Microalgae are widely applied in the fields of energy, food, feed, environment as well as pharmaceuticals industries. Photobioreactors (PBRs) are the key devices for microalgae cultivation. It is urgent to optimize and scale up PBRs, especially with the upsurge of microalgae energy research. So far, there have been a lot of researches on PBRs, but these works almost rely on empirical approach, and thus traditional experimental method which needs heavy workload were mainly used to optimize specific type of PBR. Meanwhile, the results of these reasearches almost cannot be broadly applicable to other types of PBRs. Recently, the Computational Fluid Dynamics (CFD) technology is applied to the optimization of PBR. However, these researches only focus on theoretical computational results, few of which are validated by using microalgae cultivation experiments. Different parameters are used to scale up PBRs, and it is still not clear which of these parameters are most efficient when scaling up different PBRs. Therefore, there is not a systemic, reliable and universal method for optimization and scale up of PBRs as for it is not known which parameters of mixing and light regime characteristic are most important and sensitive to the performance of PBR. Additionally, the pilot-scale experiment for the technology of heterotrophy-dilution-photoinduction sequential cultivation invented in our laboratory is in progress. As the key devices for microalgae photoinduction, the PBR is urgently required to be investigated.Accordingly, an approach which integrates the CFD method and photoinduction (part of photoautotrophic cultivation) of Chlorella was used. The universal key parameters for affecting performance of PBR were determined through sensitive analysis method. Then pilot-scale system for microalgae photoinduction was designed and optimized based on key parameters. The pilot research of heterotrophy-dilution-photoinduction sequential cultivation was carried out.(1) Optimization and establishment of light attenuation model of Chlorella pyrenoidosa and investigation of the effect of pigment content on light attenuation were implemented. Lambert-Beer light attenuation model is suitable only for low density of microalgae, and the Hyperbolic light attenuation model suit to high microalgae concentration. The Cornet light attenuation is applicable to wide range of microalgae concentration. M-Hyperbolic model established by us fits the light attenuation process better in the condition of low microalgae concentrations less than2.68g/L. Additionally, at the same cell concentration, the degree of light attenuation increased with the increase of pigment content of microalgal cell. However, the effect of pigment content on light attenuation is little when the cell concentration is high.(2) The effects of incident light intensity, microalgal cell concentration and air flow rate on parameters of mixing and light regime characteristic in flat plate PBR was investigated, and theoretical analysis of different formulas for scaling up PBR was executed. With the increase of microalgae concentration, specific irradiation rate (q;) and volume averaged light intensity (Iavv) decrease exponentially; as the incident light intensity increase, q; and Iavv increase linearly; as the air flow rate increases, turbulence energy (TE) and turbulence energy dissipation (TED) of microalgae culture show linear increasing trend, the radial velocity of fluid (Ur) show increasing trend of power function, the mixing time (tm) decrease exponentially, the light dark cycle time of microalgal cell decreases quickly and the time averaged light intensity (Iav’) of cell and the fraction of the light time in light dark cycle (LF) are almost not change. The microalgae concentration in15L PBR can only be respectively0.26times and0.36times of that in3L PBR with the same incident light intensity. When the PBR is scaling up based on formulas of equivalence of qi and Iav\incident light intensity for15L PBR is respectively3.7fold and2.81fold of that in3L PBR with the same cell concentration. The unit volume power of aeration in15L PBR approximately1times,2times and5.7times of that in3L PBR when formulas of equal radial mixing time (Tr), tm and LDCT are employed.(3) The correlation analysis of photoinduction results of microalgae (the death rate of microalgal cell, the increasing rate of chlorophyll and protein content) with parameters of mixing and light regime characteristic in indoor3L PBR, outdoor70L open tank culture system and50L bubble basin was executed. The results show that Iavv, lav’as well as q; are the key parameters to photoinduction of microalgae. The irradiance level of microalgal culture is the main factor influencing the performance of system at the conditions of the same incident light intensity but different light path of PBRs. The mixing is the key factor for performance of PBR when PBRs have the same incident light intensities and same light paths. The good results are obtained when formulas of equal Iavv and q; are used, and the bad results appeared when formulas of equal Trand tm are employed to scale up the flat plate PBRs.(4) The novel inner structures and relevant15L PBRs are deigned based on enhancing the microalgal culture along the light irradiance direction. The correlation analysis of photoautotrophic results of Chlorella pyrenoidosa with mixing and light regime characteristic parameters indicated that the velocity of fluid along the light attenuation direction (Ul) and LDCT of microalgal cell are the main sensitive parameters for affecting the performance of PBR. The validation of sensitive parameters was executed in three type of15L flat plate PBRs, i.e. Bubble, Split and Airlift. It is the first time to determine the performance of these three types of flat plate PBR:the Split PBR is better than Airlift PBR; and the Bubble PBR is the worst. The maximum cell concentration and specific growth rate of microalgae in type-a PBR are increased respectively92.5%and28.0%than that in Bubble PBR (the control), the maximum cell concentration and specific growth rate of microalgae in type-b PBR are increased respectively33.8%and25.6%than the control, and the maximum cell concentration and specific growth rate of microalgae in type-c PBR are increased respectively45.2%and29.8%than those in the control.(5) Optimization of the large scale PBR based on sensitive parameters and the results validated by microalgae culture experiment were carried out. The results of microalgae photoinduction and photoautotrophic culture in1000L flat plate PBRs and300L column PBRs with different inner structures, middle raceway ponds with different operation conditions and large circular ponds with different type of impellers all show that UL and LDCT of microalgae significantly affect the performance of PBR. Not only the reliability of sensitive parameters is validated, but also it is demonstrated that UL and LDCT have the universality for different types of PBRs.(6) A Serie of large scale systems for microalgae cultivation were designed based on sensitive parameters. And the pilot study of heterotrophy-dilution-photoinduction sequential cultivation was carried out. The growth rate of Chlorella pyrenoidosa in6000L fermenter can still achieved about3.0g/L/h at late stage of the heterotrophic culture. This growth rate of microalgal cell is comparative with that in50L fermenter. Temperature remarkably affects efficiency of Chlorella pyrenoidosa photoinduction, and the river water can replace tap water for Chlorella pyrenoidosa photoinduction. The chlorophyll and protein content of microalgal cell can reach over20mg/g and50%respectively after10～20h photoinduction in large scale microalgal culture system. The efficiency of photoinduction in these large scale systems is almost the same with that in small scale systems such as50L bubble basins.In thispresent study, our results can not only provide basis for optimization and scale up of PBRs, but also load good foundation for industrialization of heterotrophy-dilution-photoinduction sequential cultivation of Chlorella pyrenoidosa. |
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