Dj. Pollard et al., SACCHAROMYCES-CEREVISIAE FERMENTATIONS IN A PILOT-SCALE AIRLIFT BIOREACTOR - COMPARISON OF AIR SPARGER CONFIGURATIONS, Bioprocess engineering, 15(5), 1996, pp. 279-288
Hydrodynamic and oxygen transfer comparisons were made between two rin
g sparger locations, draft tube and annulus, in a concentric pilot sca
le airlift reactor with a baker's yeast suspension. Sectional hydrodyn
amic measurements were made and a mobile DOT probe was used to charact
erise the oxygen transfer performance through the individual sections
of the reactor. The hydrodynamic performance of the reactor was improv
ed by using a draft tube ring sparger rather than the annulus ring spa
rger. This was due to the influence of the ratio of the cross sectiona
l area of the downcomer and riser (A(D)/A(R)) in conjunction with the
effect of liquid velocity and a parameter, C-0, describing the distrib
ution of the liquid velocity and gas holdup across the riser on the bu
bble coalescence rates. The mixing performance of the reactor was domi
nated by the frequency of the passage of the broth through the end sec
tions of the reactor. An optimum liquid height above the draft tube fo
r liquid mixing was demonstrated, above which no further improvement i
n mixing occurred. The liquid velocity and degree of gas entrainment s
howed little dependency on top section size for both sparger configura
tions. Extreme dissolved oxygen heterogeneity was demonstrated around
the vessel with both sparger configurations and was shown to be detrim
ental to the oxygen uptake rate of the baker's yeast. Dissolved oxygen
tensions below 1% air saturation occurred along the length of the ris
er 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 enhanced k(L)a and gas
residence lime, high downcomer gas holdup, and the change in bubble si
ze distribution between the riser and downcomer. The position of great
est oxygen transfer rate in the downcomer was shown to be affected by
the reactor from the influence on downcomer liquid linear velocity.