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Bioprocess Engineering

, Volume 15, Issue 5, pp 279–288 | Cite as

Saccharomyces cerevisiae fermentations in a pilot scale airlift bioreactor: Comparison of air sparger configurations

  • D. J. Pollard
  • P. Ayazi Shamlou
  • M. D. Lilly
  • A. P. Ison
Originals

Abstract

Hydrodynamic and oxygen transfer comparisons were made between two ring sparger locations, draft tube and annulus, in a concentric pilot scale airlift reactor with a baker's yeast suspension. Sectional hydrodynamic measurements were made and a mobile DOT probe was used to characterise the oxygen transfer performance through the individual sections of the reactor.

The hydrodynamic performance of the reactor was improved by using a draft tube ring sparger rather than the annulus ring sparger. This was due to the influence of the ratio of the cross sectional area of the downcomer and riser (AD/AR) in conjunction with the effect of liquid velocity and a parameter,C0, describing the distribution of the liquid velocity and gas holdup across the riser on the bubble coalescence rates.

The mixing performance of the reactor was dominated by the frequency of the passage of the broth through the end sections of the reactor. An optimum liquid height above the draft tube, for liquid mixing was demonstrated, above which no further improvement in mixing occurred. The liquid velocity and degree of gas entrainment showed little dependency on top section size for both sparger configurations.

Extreme dissolved oxygen heterogeneity was demonstrated around the vessel with both sparger configurations and was shown to be detrimental to the oxygen uptake rate of the baker's yeast. Dissolved oxygen tensions below 1% air saturation occurred along the length of the riser and then rose in the downcomer. The greater oxygen transfer rate in the downcomer than in the riser was caused by the combined effects of a larger slip velocity in the downcomer which enhancedkLa and gas residence time, high downcomer gas holdup, and the change in bubble size distribution between the riser and downcomer. The position of greatest oxygen transfer rate in the downcomer was shown to be affected by the reactor from the influence on downcomer liquid linear velocity.

Keywords

Riser Oxygen Transfer Liquid Velocity Draft Tube Dissolve Oxygen Tension 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of symbols

AD

cross sectional area of the downcomer (m2)

AR

cross sectional area of the riser (m2)

C

saturation concentration of disolved oxygen (mol. m−3)

C0

distribution parameter, dependent on the radial position of the gas holdup and superficial liquid velocity across the column (-)

CL

dissolved oxygen concentration (mol. m−3)

dB

Sauter bubble diameter (m)

DOT

dissolved oxygen tension (% air saturation)

H

Henry's law constant for water (Pa. mol fr−1)

HD

height of the liquid dispersion (m)

HDT

height of the top of the draft tube from the vessel base (m)

kL

liquid side mass transfer coefficient (ms−1)

kLa

volumetric mass transfer coefficient (s−1)

MW

molecular weight of water (kg.m−3)

NA

rate of oxygen transfer to the liquid phase (mol m−3 liquid. s−1)

OUR

oxygen uptake rate (mmol.l−1. h−1)

P

hydrostatic pressure (Pa)

PIN

pressure at the point of gas inlet to the vessel (Pa)

QGO

volumetric flowrate of gas through the sparger (m3. s−1)

R

universal gas constant (mol. kg−1. s.−1)

T

absolute temperature (K)

TEX

absolute temperature of exit gas stream (K)

TIN

absolute temperature of exit gas stream (K)

tc

liquid circulation time (s)

tm

mean mixing time to achieve 90% homogeneity (s)

VL

liquid volume (m3)

ULD

mean liquid linear velocity in the downcomer (ms−1)

ULR

mean liquid linear velocity in the riser (ms−1)

Usg

superficial gas velocity based on the riser (ms−1)

x

vertical displacement from the base of the vessel (m)

yEX

mean oxygen mole fraction of exit gas stream (mol fr)

yIN

mean oxygen mole fraction of inlet gas stream (mol fr)

yO2

oxygen composition of gas (mol fr)

ρw

density of water (kg·m−3)

η

solubility in fermentation medium compared with that in water at same conditions (-)

ε

gas holdup (-)

εD

means downcomer gas holdup (-)

ε0

mean overall gas holdup (-)

εR

mean riser gas holdup (-)

εTS

mean top section gas holdup (-)

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Copyright information

© Springer-Verlag 1996

Authors and Affiliations

  • D. J. Pollard
    • 1
  • P. Ayazi Shamlou
    • 1
  • M. D. Lilly
    • 1
  • A. P. Ison
    • 1
  1. 1.The Advanced Centre for Biochemical Engineering Department of Chemical and Biochemical EngineeringUniversity College LondonLondonU.K.

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