Biochemical Engineering Journal 87 (2014) 25–32 Contents lists available at ScienceDirect Biochemical Engineering Journal jo ur nal home page: www.elsevier.com/locate/bej Regular Article Improvement of mixing time, mass transfer, and power consumption in an external loop airlift photobioreactor for microalgae cultures Ali Pirouzi a , Mohsen Nosrati a,∗ , Seyed Abbas Shojaosadati a , Saeed Shakhesi b a Biotechnology Group, Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran b Engineering Research Institute, P.O. Box 13445-754, Tehran, Iran a r t i c l e i n f o Article history: Received 16 November 2013 Received in revised form 16 March 2014 Accepted 22 March 2014 Available online 31 March 2014 Keywords: Airlift bioreactors Mixing Gas hold-up Mass transfer Power consumption Counter-current flow a b s t r a c t Longer mixing times and higher power consumption are common problems in the design of photobiore- actors. In this study, a vertical triangular external airlift loop photobioreactor was designed, constructed and operated for microalgae production studies. Gas feeding was performed by two spargers: one at the bottom of the hypotenuse (downcomer) and another at the bottom of the vertical side (riser). This configuration provided more effective countercurrent liquid–gas flow in the hypotenuse. The mass transfer coefficient, gas hold-up, mixing time, circulation time, dimensionless mixing time, bubble size, and volumetric power consumption were measured and optimized using response surface methodol- ogy. Investigations were carried out on the performance of the riser (the vertical side), downcomer (the hypotenuse), and separator. The countercurrent flow in the hypotenuse provided sufficient contact between gas and liquid phases, and increased mixing and mass transfer rates, in contrast to the results of previous studies. The promising results of this geometry were shorter mixing time and a significant decrease in volumetric power consumption in comparison with other configurations for photobioreac- tors. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Hydrodynamics and transport phenomena are crucial and complicated aspects of sparged photobioreactor cultivation of microalgae [1–3]. The level of complexity increases strongly in response to interactions between the fluid phases, biomass, and nutrients generally found in photobioreactors [4]. The growth rate of most microalgae is rapid [5–11], thus, photosynthesis in microalgae demands high CO 2 absorption and high O 2 release. Consequently, the rate of CO 2 absorption and O 2 desorption are usually limiting factors inhibiting the overall growth rate of the biomass [2,12–14]. Photosynthesis depends on sufficient mass transfer between the three phases involved; liquid culture, sus- pended solid biomass, and sparged gas, which is the main carbon source required for growth [13,15–17]. Superficial fluid velocity, gas hold-up, photobioreactor geometry, and mixing time influence overall mass transfer [2,10], which in turn affects biomass function and process productivity [10,16]. ∗ Corresponding author. Tel.: +98 21 82884372; fax: +98 21 82883381. E-mail address: mnosrati20@modares.ac.ir (M. Nosrati). Microalgae are sensitive to stress [18–21] and harsh mechanical stirring may discourage growth [16,22–26]. Photobioreactors have been designed and tested to address this limitation, the majority of which are gas-lift photobioreactors [3,13,17,27]. Both the intensity and history of illumination influence the growth rate of microalgae [2,3]. Loop photobioreactors may yield better performance in this area over flat-panel or bubble column air-lift photobioreactors. External loop airlift bioreactors show good performance at dif- ferent gas velocities, good compatibility with sensitive organisms, are easy to maintain, low cost, and have low energy consump- tion [5,28]. They can accommodate multiphase and heterogeneous gas-liquid systems, allow once to scale-up, and provide good sterilization, making them promising candidates for microalgae cultivation [13,16,17,29]. The present study designed, built, and operated a specially shaped external loop airlift photobioreactor. The mass transfer coefficient, gas hold-up, mixing time, and circulation time were studied and correlated for the riser (vertical side), downcomer (hypotenuse), and separator. The main variables in this configu- ration were superficial gas velocities sparged from the bottom of the vertical side (V gs2 ) and the hypotenuse (V gs1 ). The results of the experimental study were compared with other configurations for bioreactors. Better results were found in all aspects, especially http://dx.doi.org/10.1016/j.bej.2014.03.012 1369-703X/© 2014 Elsevier B.V. All rights reserved.