On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation

https://doi.org/10.1016/j.fbp.2009.03.006Get rights and content

Abstract

Microbubble generation by a novel fluidic oscillator driven approach is analyzed, with a view to identifying the key design elements and their differences from standard approaches to airlift loop bioreactor design. The microbubble generation mechanism has been shown to achieve high mass transfer rates by the decrease of the bubble diameter, by hydrodynamic stabilization that avoids coalescence increasing the bubble diameter, and by longer residence times offsetting slower convection. The fluidic oscillator approach also decreases the friction losses in pipe networks and in nozzles/diffusers due to boundary layer disruption, so there is actually an energetic consumption savings in using this approach over steady flow. These dual advantages make the microbubble generation approach a promising component of a novel airlift loop bioreactor whose design is presented here. The equipment, control system for flow and temperature, and the optimization of the nozzle bank for the gas distribution system are presented.

Introduction

Airlift reactors are perceived to have performance advantages over bubble columns and stirred tank bioreactors for many applications, biorenewables production in particular. Where the product is a commodity biochemical or biofuel, energy efficiency is the primary concern. There are multiple objectives for the optimization of energy efficiency, however. The hydrodynamics of stirring is an important consideration, as are the phase transfer of nutrient influx and the efflux of inhibitor products and byproducts. Finally, the metabolism of cells or microbes engaged in the biochemical production are a major constraining factor – mass transfer from the bulk liquid to the bioculture must be maintained. There are two important reasons to use airlift loop bioreactors (ALB) that arise from the airlift effects: flotation and flocculation. Small bubbles attached to particles or droplets significantly lower the density of the aggregate. Grammatika and Zimmerman (2001) describe these generalized flotation effects. Such aggregates are susceptible to floc formation. Typically, microbes or cells that sediment out of the suspension accumulate in stagnation zones at the bottom expire.

Given the importance of energy usage in the operation of ALBs, it is surprising that the sparging system, which is the central power consumption feature of the ALB, has not received more attention. Jones (2007) gives a good review of the major features of ALB, including the conventional types of sparger design. Chisti and Moo-Young (1987) classify the spargers used in the ALB as dynamic and static. Dynamic spargers use injection through nozzles to disperse the gas introduced. Static spargers are typically less reliant on the momentum of the jet, and the gas is introduced typically through a perforated plate (see Deshpande and Zimmerman, 2005a, Deshpande and Zimmerman, 2005b) or less commonly, through a porous baffle (Heijnen and Van’t Tiet, 1984). This study was motivated by the development of a novel microbubble generation technique based on fluidic oscillation diverting jets used in sparging (Zimmerman et al., 2008).

The paper is organized as follows. In Section 2, the microbubble generation mechanism by fluidic oscillation is discussed, leading to design criteria for sparging systems and nozzle banks that achieve high energy efficiency. In Section 3, aspects of the design of ALBs that are influenced by the incorporation of the microbubble generator by fluidic oscillation, the design itself, and simulation of an such an ALB are presented. In Section 4, a summary is given and conclusions are drawn.

Section snippets

Microbubble generation

The concept for the fluidic oscillator driven microbubble generation mechanism (Zimmerman et al., 2008) stems from the systems biology objective of using oscillatory nutrient feed streams in a conventional fermentor or chemostat to investigate the kinetics of metabolic pathways. Zimmerman (2005) demonstrated from the simulation of the glycolytic pathway in yeast that oscillating the glucose feed stream could create information-rich time series responses in extracellular metabolic production,

Design aspects of an airlift loop bioreactor

In the previous section, the design aspects of a microbubble generator component of an airlift loop bioreactor were discussed, demonstrating that the usual design trade-off between friction losses with small apertures and distributor channels and performance gains in transfer efficiency with small bubbles can be “triangulated” with the fluidic oscillator principle, with the oscillatory flow resulting in less friction loss while still generating small bubbles. This argument poses the advantage

Discussion and conclusions

This paper presents the many advantages of a microbubble generation mechanism actuated by fluidic oscillation – potential low energy consumption, high heat, mass, and momentum transfer rates, flotation and flocculation potential – and argued for its inclusion in a standard design of a draught tube internal loop airlift bioreactor. The expected impact on the canonical design for such an ALB is discussed, particularly with regard to the expected nearly monodisperse, non-coalescent bubbly flow

Acknowledgements

WZ would like to thank support from the EPSRC for the development of the acoustic actuated fluidic oscillator (Grant No. GR/S67845) and for financial support from the Food Processing Faraday Partnership, Yorkshire Water, Sheffield University Enterprises Ltd., the European Union Objective 1 Fund administered through the University of Sheffield, and Wastetoethanol Ltd. WZ would like to thank the Royal Academy of Engineering/Leverhulme Trust Senior Research Fellow programme. The authors wish to

References (25)

  • J.J. Heijnen et al.

    Mass transfer, mxing and heat transfer phenomena in low viscosity bubble column reactors

    Chemical Engineering Journal and the Biochemical Engineering Journal

    (1984)
  • B.N. Hewakandamby

    A numerical study of heat transfer performance of oscillatory impinging jets

    Journal of Heat & Mass Transfer

    (2008)
  • Cited by (117)

    • Transient effects and the role of wetting in microbubble generation

      2023, Current Opinion in Colloid and Interface Science
    View all citing articles on Scopus
    View full text