K. Fujie et al., DETERMINATION OF VOLUMETRIC OXYGEN-TRANSFER COEFFICIENT BY OFF-GAS ANALYSIS, Journal of fermentation and bioengineering, 77(5), 1994, pp. 522-527
In various aerobic bioreactors including activated sludge aeration tan
ks, the volumetric mass transfer coefficient K(L)a is frequently used
as an estimate of the rate of oxygen dissolution into the liquid phase
. The K(L)a measurement in such-bioreactors is widely applied with the
aid of sodium sulfite (Na2SO3) as an oxygen-consuming substance used
to maintain low dissolved oxygen concentration. In the present study,
the effect of the addition of Na2SO3 on K(L)a, determined by an off-ga
s analysis, was investigated specifically from the viewpoint of variat
ions in the size of air bubbles and the enhancement factor associated
with the change in sulfite concentration. Experiments were conducted i
n a draft-tube bubble column, using a zirconia electrode oxygen analyz
er for measurement of the O2 mole fraction in the exhaust gas and a du
al electrical resistivity probe for measurement of the bubble size. It
was found that the increase in the specific gas-liquid interfacial ar
ea, resulting from bubble size reduction effected by Na2SO3 functionin
g as an electrolyte, is more pronounced than the enhancement of the ab
sorption rate through the interface. The upper limit of Na2SO3 concent
ration for sustaining physical absorption, in the absence of any catal
yst, ranges from 30 to 70 mol/m3, while that for preventing the averag
e bubble size from decreasing is about 15 mol/m3. Furthermore, to secu
re a reliable K(L)a measurement, the K(L)a value should not exceed 50
h-1 for the liquid depth of 3 m even when the limiting conditions are
not exceeded. The off-gas analysis proposed in this study for K(L)a de
termination is expected to be extremely useful provided that the above
conditions are fulfilled, since it only requires moderate addition of
the sulfite as the oxygen-consuming substance and will not interrupt
the reactor operation as long as oxygen uptake occurs in the system.