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Bioprocess and Biosystems Engineering

, Volume 41, Issue 1, pp 31–45 | Cite as

Computational fluid dynamics (CFD) analysis of airlift bioreactor: effect of draft tube configurations on hydrodynamics, cell suspension, and shear rate

  • Sanjay B. PawarEmail author
Research Paper

Abstract

The biomass productivity of microalgae cells mainly depends on the hydrodynamics of airlift bioreactor (ABR). Thus, the hydrodynamics of concentric tube ABR was initially studied using two-phase three-dimensional CFD simulations with the Eulerian–Lagrangian approach. The performance of ABR (17 L) was examined for different configurations of the draft tube using various drag models such as Grace, Ishii–Zuber, and Schiller–Naumann. The gas holdups in the riser and the downcomer were well predicted using E–L approach. This work was further extended to study the dispersion of microalgae cells in the ABR using three-phase CFD simulations. In this model (combined E–E and E–L), the solid phase (microalgae cells) was dispersed into the continuous liquid phase (water), while the gas phase (air bubbles) was modeled as a particle transport fluid. The effect of non-drag forces such as virtual mass and lift forces was also considered. Flow regimes were explained on the basis of the relative gas holdup distribution in the riser and the downcomer. The microalgae cells were found in suspension for the superficial gas velocities of 0.02–0.04 m s−1 experiencing an average shear of 23.52–44.56 s−1 which is far below the critical limit of cell damage.

Keywords

Microalgae cultivation CFD modeling Three-phase flow Airlift photobioreactor Discrete phase model 

List of symbols

Ad

Cross-sectional area of downcomer (m2)

Ar

Cross-sectional area of the riser (m2)

Ccd

Interphase momentum transfer coefficient for the interphase drag force

CD

Drag coefficient

CL

Lift coefficient

Cwl

Wall lubrication coefficient

D

Diameter of the reactor (m)

db

Bubble diameter (m)

Eo

Eotvos number

FB

Buoyancy force acting on the particle (N)

FD

Drag force acting on particle (N)

FP

Pressure gradient force (N)

FVM

Virtual (or added) mass force (N)

g

Gravity vector (m s−2)

k

Turbulent energy (m2 s−2)

M

Interfacial momentum transfer term between gas–liquid phases

Mo

Morton number

mb

Mass of single bubble (kg)

P

Pressure (Pa)

Pk

Turbulence production due to viscous forces (kg m−1s−3)

Pkb

Turbulence production due to buoyancy effect (kg m−1s−3)

Pεb

Turbulence dissipation due to buoyancy effect (kg m−1s−3)

Re

Reynold’s Number

Reω

Vorticity Reynolds’s Number

t

Time (s)

U

Averaged liquid-phase velocity (m s−1)

Ub

Bubble velocity (m s−1)

Uc

Continuous phase velocity (m s−1)

Up

Particle velocity (m s−1)

Ut

Bubble terminal velocity (m s−1)

V

Velocity (m s−1)

Vg

Superficial gas velocity (m s−1)

Vld

Liquid circulation velocity (m s−1)

Subscript

b

Bubble phase

c

Continuous phase

p

Particle (solid) phase

eff

Effective

g

Gas phase

l

Liquid phase

lam

Laminar

t

Turbulent

OV

Overall

r, R

Riser

d, D

Downcomer

Greek letters

µ

Viscosity of liquid phase (kg m−1s−1)

ρ

Density of phase (kg m−3)

ε

Turbulent energy dissipation (m2 s−3)

σ

Surface tension (N m−1)

σρ

Turbulent Schmidt number

α

Gas holdup

Ω

Rotation vector

Notes

Acknowledgements

The author is very grateful to the Department of Science and Technology, New Delhi for their financial support for this research work under the scheme of DST Inspire Faculty Award (IFA13-ENG63). The author is also very thankful to the Director, CSIR–NEERI Nagpur for providing enough infrastructure facilities to carry out this research.

Compliance with ethical standards

Conflict of interest

The author declares that there is no conflict of interest.

Supplementary material

449_2017_1841_MOESM1_ESM.tif (1.3 mb)
Fig. 1 s. Meshing of the geometry (for example: ALC3 and ALC5). (TIFF 1335 kb)
449_2017_1841_MOESM2_ESM.tif (587 kb)
Fig. 2 s. Mesh independency results for ALC3 geometry, A) V g = 0.01 m s−1, B) V g = 0.03 m s−1. (TIFF 587 kb)
449_2017_1841_MOESM3_ESM.tif (356 kb)
Fig. 3 s. Effect of different virtual mass force coefficients on overall gas holdup, riser gas holdup, and liquid velocity in ALC2 geometry at V g = 0.02 and 0.04 m s−1. (TIFF 356 kb)
449_2017_1841_MOESM4_ESM.tif (276 kb)
Fig. 4 s. Effect of consideration of lift force (LF) and wall lubrication (WL) force on overall gas holdup, riser gas holdup and liquid velocity in ALC2 and ALC3 geometries at V g = 0.02 and 0.04 m s−1, respectively. (TIFF 275 kb)
449_2017_1841_MOESM5_ESM.tif (4.7 mb)
Fig. 5 s. Prediction of hydrodynamics parameters under three-phase simulations of ABR (ALC3 geometry) for superficial gas velocity of 0.04 m s−1 a) microalgae velocity profile; b) volumetric distribution of microalgae cells; c) path followed by algae cells shown by streamlines; d) microalgae shear rate profile; and e) air bubble velocity profile and distribution of air bubbles. (TIFF 4860 kb)

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Copyright information

© Springer-Verlag GmbH Germany 2017

Authors and Affiliations

  1. 1.Environmental Biotechnology and Genomics Division, DST Inspire FacultyCSIR-National Environmental Engineering Research Institute (NEERI)NagpurIndia

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