Local gas holdup and bubble dynamics investigation during microalgae culturing in a split airlift photobioreactor

Abstract

To make the process of microalgae cultivation for bioenergy, and wastewater and flue gas treatment economically viable, it is important to completely understand the gas–liquid interaction inside photobioreactors in real microalgae cultures completely. Due to limitations of the conventional measurement techniques in the literature, only the overall parameters (such as overall gas holdup and interfacial area) have been studied mostly in air–water systems in the literature. Thus, the variation of local parameters such as local gas holdup and bubble dynamics properties like bubble passage frequency, bubble chord length and velocity, and interfacial area in real culturing systems remains unclear. In this study, these properties were studied at different axial locations inside a split- airlift photobioreactor at superficial gas velocities of 1.0, 2.0, and 2.8cm/s while culturing microalgae Scenedesmus sp. The viscosity of the medium was seen to increase with the optical density of the culture, while the surface tension remained the same throughout the experiment. While, as expected, an increase in superficial gas velocity increased the bubble passage frequency, gas holdup, and interfacial area, the effect of optical density was observed to be the opposite. Over the duration of the experiment, bubble passage frequency in the riser decreased by 62% at 1.0cm/s, 34% at 2.0cm/s, and 25% at 2.8cm/s; the gas holdup in the riser at velocities of 1.0 and 2.0cm/s decreased by 33%, whereas, at 2.8cm/s, it first reached a maximum and then decreased sharply by 29%; the interfacial area in the riser decreased steadily at velocities 1.0 and 2.0cm/s by 40% overall, while that at 2.8cm/s first remained constant at an average of 1.24 and then dropped by 60% at the end of the experiment. While no significant axial variation in the bubble properties was observed in the riser, an axial variation in these properties was observed in the downcomer due to a decrease in the number of bubbles descending through the downcomer. The bubble chord length and bubble velocity distributions became wider at higher superficial gas velocities and higher optical densities. New correlations were developed, accounting for the change in optical density as well as superficial gas velocity, to predict the gas holdup in the riser and at different axial locations in the downcomer.