SACCHAROMYCES-CEREVISIAE FERMENTATIONS IN A PILOT-SCALE AIRLIFT BIOREACTOR - COMPARISON OF AIR SPARGER CONFIGURATIONS

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.