Optimal Design For Gas Sparger Geometry And Inner-structure Of Microalgae Cylindrical Photobioreactor Based On The CFD Technology

Microalgae is a kind of autotrophic plant which can effectively utilize light energy,carbon dioxide and water.It is widely used in renewable energy manufacturing,food industry,pharmaceutical industry,environmental testing and purification.Photobioreactor,which is the key equipment for microalgae cultivation,plays an important role in large-scale cultivation of microalgae.Among the many types of photobioreactors,the bubble-column and airlift types are very attractive because of their simple design,operational flexibility,and energy saving.In this paper,the gas distributor structure,the draft tube diameter,the top and sparger clearance and the operating parameter(i.e.,the superficial gas velocity)on the mixing efficiency and the illumination performance of the microalgae photobioreactor are studied based on the computation fluid dynamics(CFD)technology.To guide the design of the structure and the operation parameter with the purpose of improving the efficiency of the microalgae cultivation.Under the same superficial gas velocity and light conditions,it is found that a more uniform air inlet arrangement of the gas distributor leads to a more uniform gas distribution in the reactor and a better fluid mixing performance.The uniformity of the pore distribution facilitates the circulation of the microalgae cells between the light and dark zones.As a result the light/dark cycle of the reactor is smaller and the time-averaged light intensity is greater.In the bubble column,the motion of the microalgae is more random without fixed flow pattern,while in the airlift reactor,a uniform flow pattern is generated by the draft tube,which enables the culture to circulate between the riser and downcomer.Therefore,the mixing in the airlift reactor is more effective to prevent the occurrence of light inhibition and light deficiency.Within the variation range of our study,it is found that increasing the top clearance and the sparger clearance can improve the liquid flow velocity in the reactor,the light dark cycle frequency and the time-averaged light intensity under the same superficial gas velocity and light conditions.The external light conditions and critical light intensity of the microalgae should be considered when selecting the diameter of the draft tube to ensure that the downcomer of the reactor is completely in the light zone,while the riser contains both the light and dark zones,which is beneficial to improve the light dark cycle frequency and the time-averaged light intensity of the microalgae cells in the reactor.Under the same light conditions,increasing the superficial gas velocity is helpful to improve the mixing performance of the reactor,but at the same time,the power input and the air consumption of the reactor are also increased.