Elsevier

Bioresource Technology

Volume 275, March 2019, Pages 1-9
Bioresource Technology

Characterization of bubble column photobioreactors for shear-sensitive microalgae culture

https://doi.org/10.1016/j.biortech.2018.12.009Get rights and content

Highlights

Abstract

The shear-sensitive marine algal dinoflagellate Karlodinium veneficum was grown in a cylindrical bubble column photobioreactor with an internal diameter of 0.044 m. Initial liquid height varied from 0.5 to 1.75 m, superficial gas velocities from 0.0014 to 0.0057 ms−1, and nozzle diameter from 1 to 2.5 mm. Computational fluid dynamics was used to characterize the flow hydrodynamics and energy dissipation rates. Experimental gas holdup and volumetric mass transfer coefficient strongly depended on the liquid height and correlated well with the Froude number. Energy dissipation near the head space (EDtop) was one order of magnitude higher than the average energy dissipation in the whole reactor (EDwhole), and the value in the sparger zone (EDspar) was one order of magnitude higher than EDtop. Cultures of K. veneficum were limited by CO2 transfer at low EDwhole and severely stressed above a critical value of EDwhole.

Introduction

Bubble columns are widely used as multiphase reactors because of their simple construction and operation. They are used in chemical, metallurgical and biochemical processes (Ferreira et al., 2013), providing a competitive alternative in two- and three-phase processes with mass and heat transfer limitations, where efficient interphase contacting is needed (Nedeltchev et al., 2014). A bubble column is also a popular configuration for closed photobioreactors that has received considerable attention over the last decade (Pegallapati and Nirmalakhandan, 2012, Bitog et al., 2014, Manjrekar et al., 2017, López-Rosales et al., 2017). The bubble column provides a number of advantages over other photobioreactor configurations such as simplicity in design and construction with no moving parts, ease of operation, small floor space requirements, excellent heat transfer characteristics and temperature control, and suitable interphase mass transfer at low energy input. Nonetheless, due to a lack of knowledge on the complex bubble-liquid hydrodynamics and its influence on transport process, cell growth, and scale up, bubble column photobioreactors are still not well understood. Hydrodynamics in bubble columns are determined by gas sparging at the bottom section. Gas injection is essential for microalgae culture processes, driving the flow and mixing, and indeed mass transfer and light penetration and distribution. However, bubble formation at the sparger, its upward movement, and rupture at the culture surface can produce significant shear forces in small regions that can potentially induce cell damage. Bubble-associated cell damage has been studied extensively in animal cells. Since the early works, before 1990, bubble rupture at the culture surface is believed to be responsible for most of the cell deaths (Hu et al., 2011, Liu et al., 2014, Zhu et al., 2008). However, studies with microalgae are more limited. In an early study, Suzuki et al. (1995) found a linear relationship between specific rate of cell death of Dunaliella tertiolecta and the inverse of culture height. This relationship implies that cell damage is solely due to bubble rupture. Contreras et al. (1998) found a relationship between growth rate of Phaeodactylum tricornutum and the presence of bubbles in an airlift photobioreactor, whereby decreasing growth rate was related to increasing shear rate. García-Camacho et al. (2000) observed a positive linear relationship between death rate and culture height for Porphyridium cruentum. The author suggested that cells attached to rising bubbles were carried to the culture surface and damaged during bubble rupture. The critical role of cell-bubble attachment in cell death due to bubble rupture was also demonstrated by Meier et al. (1999). Barbosa et al. (2003) found the most significant microalga cell damage in the region near the sparger due to bubble formation, and subsequently determined a critical gas entrance velocity that induced good mixing without harming cells. Recently, López-Rosales et al. (2017) investigated the growth of the shear-sensitive dinoflagellate microalgae Karlodinium veneficum in a bubble column. According with their studies, bubble break-up at the gas-liquid interphase was much more detrimental for cells than bubble formation at the sparger.

Since complex hydrodynamics of sparged bioreactors prevent the use of simple correlations relating cell response to a specific condition in a bioreactor (Hu et al., 2011, López-Rosales et al., 2017), in parallel with the aforementioned experimental studies, attempts of theoretical or computational studies have been reported to obtain insights into the sparging-associated cell damage. Traditionally, different hydrodynamic parameters have been used to correlate cell damage in photobioreactors, including shear rate or shear stress (Silva et al., 1987, Gudin and Chaumont, 1991, Contreras et al., 1998), micro-eddy length scale using Kolmogorov’s theory (Contreras et al., 1998) and energy dissipation rates (ED) (Contreras et al., 1998) are the most widely used. Nonetheless, although different hypotheses have been proposed, little is known about sparging-associated mechanisms of microalgae cell damage at a fundamental level (López-Rosales et al., 2015, López-Rosales et al., 2017).

This paper utilizes computational fluid dynamics (CFD)-assisted characterization of a bubble column photobioreactor to interpret the shear-sensitivity responses of the marine microalga K. veneficum. Different parameters influencing microalga growth (gas holdup, liquid velocity, energy dissipation and mass transfer) were studied and correlated with dimensionless Froude number. ED in different zones of the PBR were found to be dependent on each other. ED has been previously used to characterize cell damage in different reactors and laboratory equipment (Contreras et al., 1998, Mollet et al., 2004). However, in this paper, we have integrated the energy dissipation rate field in the photobioreactor into an existing growth model that has been successfully extrapolated to other scaled-up sparged photobioreactors. To the authors’ knowledge, this methodology is used here for the first time in dinoflagellate cultures, and will be of great help in the scaling-up process of aerated cultures of shear-sensitive microorganisms.

Section snippets

Database

The environment to which the cells are subjected inside a bubble column photobioreactor is governed mainly by the system design (including culture height, internal column diameter, and sparger geometry) and working parameters (gas flow rate). These variables govern gas velocity at the sparger, gas holdup, bubble diameter, interfacial area and, ultimately, mass transfer, but these variables are also responsible for different mechanisms of cell damage. In this work a database of a genetic

Results and discussion

The development of efficient CO2 supply and oxygen removal systems is one of the most challenging topics in algal processes, particularly in closed photobioreactors. Despite the efforts made over the years (Ferreira et al., 1998, Carvalho et al., 2006), air or CO2-enriched air sparging remains the most popular and convenient method of supplying CO2 and removing oxygen from microalgae culture in large-scale photobioreactors. Therefore, Q is a crucial parameter to determine the environment for

Conclusions

In this study, CFD was used to simulate hydrodynamics in a bubble column photobioreactor. Distribution of gas holdup, liquid velocity and specific energy dissipation rate were obtained for different culture height, gas velocities and nozzle diameter. Gas holdup, energy dissipation and mass transfer correlated well with Froude number. This work presents the first study to substantiate that K. veneficum growth was limited by CO2 transfer depending on the threshold of energy dissipation and a

Acknowledgements

This research was funded by the Spanish Ministry of Economy and Competitiveness (grant CTQ2014-55888-C3-02) and the European Regional Development Fund Program.

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