Applicability of axial dispersion model to analyze mass transfer measurements in bubble columns

Abstract

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...

Enhancement of Gas Holdup and Oxygen Transfer in Three-Phase Circulating Fluidized-Bed Bioreactors with Swirling Flow

“…The volumetric oxygen transfer coefficient, k L a, was estimated by fitting the analytic solution of Eq. (1) to the concentration profiles of dissolved oxygen in the axial direction of the column (Deckwer et al, 1983;Schumpe et al, 1989;Kang et al, 1991;Son et al, 2008b). The error bound for the determination of k L a value was within the range of 3-5%.…”

“…A lowpass filter was used in taking the signals to avoid the noise as well as the small disturbance owing to solid. To obtain the volumetric oxygen transfer coefficient in the riser, the steady-state axial dispersion model (Deckwer et al, 1983;Schumpe et al, 1989;Kang et al, 1991;Yang et al, 1999) was employed. Based on this model, the oxygen balance in the liquid phase around a differential volumetric section of the bed yields:…”

“…Typical concentration profiles of dissolved oxygen in the liquid phase in the riser can be seen in Figure 7, which are well fitted to the axial dispersion model (Deckwer et al, 1983). The value of volumetric oxygen transfer coefficient (k L a) was obtained by fitting the analytical solution of Eq.…”

Mass transfer studies in shallow bubble column reactors

“…Additional verification of the experimental results is performed by comparing the experimental result in the current study to literature data under similar air-water systems [19,[21][22][23][24][25][26][27][28]. Since there is limited literature data available for low L/D ratio conditions, only experimental results obtained at L/D ratio of 7.2 are used for comparison.…”

Measurement of Volumetric Mass Transfer Coefficient in Bubble Columns

“…In the "standard dynamic method" (the most commonly used technique) [4,10,11,13,18], the change of oxygen concentration is created by sudden interchange between two inlet gases with different oxygen concentration (in most cases, interchange between nitrogen and air is used) while the gas flow rate is kept unchanged. The drawback of the method lies in the fact that the description of the mixing in the phases can't be simplified by ideal mixing because it can bring significant errors.…”