Comparative study on the effects of n-dodecane addition on oxygen transfer in stirred bioreactors for simulated, bacterial and yeasts broths

The efficiency of oxygen transfer into the fermentation broths could be enhanced by adding in broths oxygen-vectors, such as hydrocarbons or fluorocarbons, without increasing the energy consumption for mixing or aeration. The experimental results obtained for simulated, Propionibacterium shermanii and Saccharomyces cerevisiae broths indicated the considerable increase of k(L)a and (k(L)a)(v)/(k(L)a)(0) by adding n-dodecane, but the magnitude of this effect must be correlated with broths and biomass characteristics, especially with the cells affinity for oxygen-vector droplets. Thus, due to the higher affinity of yeasts cells for hydrocarbon droplets, the increase of oxygen mass transfer rate was lower than that recorded for simulated or bacterial broths. By means of the experimental data, some mathematical correlations describing the influences of the main parameters (apparent viscosity, concentration of biomass, specific power input, superficial air velocity) on k(L)a in presence of n-dodecane have been proposed for each considered fermentation systems. These equations have the general expression of k(L)a = alpha(P-a/V)(beta)v(s)(gamma)eta(delta)(a) or k(L)a = alpha(P-a/V)beta v(s)(gamma)C(x)(delta), the coefficients (beta, gamma and delta values being dependent on n-dodecane concentration, and offer a good agreement with the experiment, the average deviations being between +/- 9.6 and +/- 10.8%. (c) 2006 Elsevier B.V. All rights reserved.