Air lift

Manuscript

On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation.
William B. Zimmerman1, Buddhika N. Hewakandamby2,Václav Tesa 3, H.C. Hemaka Bandulasena1, Olumuyiwa A. Omotowa1
1 Department of Chemical and Process Engineering; University of Sheffield, Sheffield S10 2TN 2 Department of Chemical and Environmental Engineering, UniversityPark, Nottingham, NG7 2RD 3 Institute of Thermomechanics of the Academy of Sciences of the Czech Republic v.v.i., 182 00 Prague, Czech Republic.

Abstract
M icrobubble 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 generationmechanism 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 thereis 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 eq uipment, control system for flow and temperature, and the optimization of the nozzle bank for the gas distribution system are presented.

1 §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 obj ectives for the optimization of energy efficiency, however. The hydrodynamics of stirring is animportant consideration, as are the phase transfer of nutrient influx and the efflux of inhibitor products and byproducts. F inally, the metabolism of cells or microbes engaged in the biochemical production are a maj constraining factor – or mass transfer from the bulk liq to the bioculture must be maintained. There are two uid important reasons to use airlift loop bioreactors ( ALB)that arise from theairlift effects: flotation and flocculation. Small bubbles attached to particles or droplets significantly lower the density of the aggregrate. 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 ex pire. Giventhe 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. J ones ( 2007)gives a good review of the maj features of ALB, or including the conventional types of sparger design. Chisti and M ooYoung ( 1987)classify the spargers used in the ALB as dynamic and static.Dynamic spargers use inj ection through nozzles to disperse the gas introduced. Static spargers are typically less reliant on the et, see momentum of the j and the gas is introduced typically through a perforated plate ( Heij Deshpande and Zimmerman, 2005a,b)or less commonly, through a porous baffle ( nen t . and Van’ Riet, 1984) This study was motivated by the development of a novel microbubble ueets Zimmerman generation techniq based on fluidic oscillation diverting j used in sparging ( et al. 2008) .

The paper is organized as follows. In § the microbubble generation mechanism by fluidic 2, oscillation is discussed, leading to design criteria for sparging systems and nozzle banks that achieve high energy efficiency. In § aspects of the design of ALBs that are influenced by 3, the...