Differing findings on the volumetric mass transfer coefficients k(L)a in CMC solutions in bubble column bioreactors have been reported in the literature. Therefore, oxygen mass transfer was studied again in CMC solutions in a 14-cm-i.d. x 270-cm-height bubble column using different spargers. The k(L)a values were determined along with the dispersion coefficients by fitting the prediction of the axial dispersed plug model with the experimental oxygen concentration profiles in the liquid phase. Surprisingly, the obtained liquid phase dispersion coefficients for CMC solution are higher than one would expect from correlations. The k(L)a data depend largely on the flow regime. In general, they are lower than those reported in the literature. The data for developing slug and established slug flow are dependent on the gas velocity and the effective viscosity of the solution and can br correlated by a simple correlation. This correlation describes k(L)a values measured on fermentation broth of Penicillium chrysogenum with striking agreement.

The applicability of axial dispersion model (ADM) for the measurement of the gas-liquid mass transfer coefficient (kLa) in bubble columns is discussed. It is shown that the misinterpretation of the concentration jump near the column inlet can lead to wrong conclusions regarding the mass transfer rates. It is further illustrated that some reported kLa dependencies on the superficial liquid velocity and the axial distance are resulted from the use of an incorrect model and can be rectified by an application of ADM. Finally, it is shown that ADM can be effectively used to calculate kLa for: (a) highly viscous liquids, (b) SCOPEThe axial dispersion model (ADM) has been extensively used to analyze the gas-liquid mass transfer characteristics of bubble columns. Recently, Alvarez-Cuenca and coworkers (AlvarezCuenca et al., 1980(AlvarezCuenca et al., , 1980aNerenberg, 1981), and Herbrechtsmeier et al. (1981) have concluded that ADM is not applicable under certain operating conditions. Owing to the growing interest in this mass transfer phenomenon, it seems timely: first, to clarify the contradictory findings; and second, to point out once again that ADM is well suited to determine the kLa data in bubble columns from the measured concentration profiles. This is achieved:(I) By showing that the evaluation of kLa is not strongly affected by the liquid-phase dispersion coefficient. Nevertheless, dispersion must not be neglected.(2) By critically analyzing the work of Alvarez-Cuenca and Nerenberg (1981), and showing that under physically reasonable assumptions, their data can be evaluated with the help of ADM.(3) By demonstrating that the use of incorrect model leads to dependencies of kLa on the liquid velocity and the dispersion height, which are not observed if a more appropriate model, i.e., the ADM, is applied.(4) By presenting new and updated measurements for oxygen mass transfer in various bubble columns under different set of operating conditions. CONCLUSIONS AND SIGNIFICANCEThe applicability of axial dispersion model (ADM) is tested over a wide range of operating conditions. Following conclusions can be drawn from the results:When applying ADM, it should be remembered that the dispersion is a macroscopic phenomenon. Therefore, dispersed flow may not be fully developed at the system boundaries; for instance, in the close vicinity of the gas sparger.In partially backmixed reactors, one must be aware of the concentration jump at the reactor inlet. This jump is a result of backmixing and must not be interpreted as an enhanced mass transfer rate.Dependencies of kLa on the liquid velocity and the column height are an outcome of the application of wrong model, PFM.If an appropriate model, ADM, is used, such dependencies disappear.For kLa determinations in bubble column reactors, measurements of steady-state oxygen profiles are strongly recommended. Evaluation of such profiles on the basis of the ADM gives consistent and conclusive kLa data, which can be correlated by simple relations, and in addition, are independent o...

Gaslliquid mass transfer has been studied in airjwater fluidized beds of 0.05-8 mm glass spheres in a 0.14 m diameter reactor. The volumetric mass transfer coefficients kLa were independent of bed height, and, for particle diameters up to 1 mm, decreased linearly with solids concentration. Low solids loadings as well as large diameter particles significantly increased kL and a , respectively, as compared to the two-phase system. (s-') (m * s-1) AlChE Journal (Vol. 31, No. 2)with 11% mean deviation. At low gas velocities and low solids loadings, specifically, the solids significantly increased gas-liquid mass transfer, presumably, by increasing the liquidside mass transfer coefficients k~, Further studies are needed to access the effects of particle shape and density and different liquid properties.4s < (0.58 -0.70 (m/4

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Nguyen-tien et al.9) have reported on oxygen transfer measurements in TPFBusing water and air as liquid and gas phase, respectively. A reactor of0.14m diameter by 2.65 m height was employed. As fluidized particles, glass spheres were used the diameter of which ranged from 0.05 to 8 mm. used a well-established method1>2) to reliably determine volumetric mass transfer coefficients by measuring the oxygen steady-state axial concentration profiles along the reactor. Evaluation of such profiles was madeby using models which account for axial mixing such as, for instance, the axial dispersion model and the equivalent but more flexible back flow cell model. Hence, matching model predictions to the experimental profiles by applying Marquardt's optimization procedure yields both the volumetric mass