Oxygen mass transfer coefficient

KLa is the volumetric oxygen mass transfer coefficient, owing to the oxygen transfer from the gas phase or air, c, the surface of the cells, cx or to the transfer of oxygen dissolved in water to the surface of the cells. 

The importance of a correct evaluation of kLa(03) or kla 02) was confirmed in a study on the simulation of (semi-)batch ozonation of phenol (Gurol and Singer, 1983). It was shown that a close match between the measured and the calculated data was only obtained when kLa(02) was measured as a function of the residual phenol concentration. The oxygen mass transfer coefficient was observed to change from kLa(02) = 0.049 s 1 at c(M) = 50 mg IF1 phenol to kLa(02) = 0.021 s- at c(M) = 5.0 mg L 1 phenol. 

The most common and appropriate methods used to determine the mass transfer coefficient and the problems inherent in each are presented in the following sections. The methods are discussed from a practical viewpoint for the direct determination of the ozone mass transfer coefficient. However, it may be impractical, even impossible to use ozone as the transferred species, because of fast reactions which cause mass transfer enhancement etc. Then the oxygen mass transfer coefficient can be used to indirectly determine the ozone mass transfer coefficient. The procedure is described below and special aspects of oxygen mass transfer experiments are referred to in the following sections whenever necessary or of general importance. 

A variety of problems encountered with the measurement of oxygen mass transfer coefficients by using the nonsteady state method are well known and well understood. Libra (1993) gives a comprehensive discussion based on oxygen mass transfer measurements. The most important problems are found in the following table. 

Furthermore, it is almost impossible to use ozone for fc, -measurements when organic substances are present that are (easily) oxidized by molecular ozone. Mass transfer enhancement will occur during such measurements, so that the mass transfer coefficient based on only the physical process cannot be determined. In this case, the oxygen mass transfer coefficient kLa 02) should be determined to assess the mass transfer rate without reaction. The enhanced mass transfer due to reaction should be considered separately, because it is not only dependent on the parameters listed above in equation 3-10, but also dependent on the concentration of the reactants. 

One would think a solution to increase the productivity would be to use oxygen enrichment, not just during the choke-out period but also during the remainder of the fermentation in order to sustain the productivity. This does not work because of broth viscosity and gas holdup problems. In highly mycelial systems, a 15% increase in cell mass doubles the viscosity. The volumetric oxygen mass transfer coefficient and the bubble rise velocity—... 

Utilizing this relationship, the lumped oxygen mass transfer coefficient can be estimated as... 

JClA oxygen mass transfer coefficient, mmol O, L h (atm Oj) ... 

Fig. 7.7 presents two of the associated input variables, the batch time and the overall oxygen mass transfer coefficient, that gave rise to the Pareto domain. The input space corresponding to the best 5% is located at nearly the same position as the one that was obtained for the two-objective optimization, except that it has been shghtly inflated. Akin to the two-objective optimization problem, it was found that the initial concentrations of both the initial substrate and initial biomass are very close to their maximum values (50 g/L and 1.0 UOD/mL, respectively). 

Fig. 7.11 presents the graph of the overall mass transfer coefficient Kid) as a function of the batch time ts) associated with the results of Fig. 7.10. The distribution of the input space is significantly different from the results that were obtained with NFM (Fig. 7.7) where a significantly larger batch time is required and slightly higher oxygen mass transfer coefficient. The other two variables So and Xq) remained nearly identical, at their upper bounds.

Table 14.4 Volumetric oxygen mass transfer coefficients (kta [h" ]) in a Wave bioreactor at different rocking rates and rocking angles...

The calculation of the reactor volume requires the oxygen mass transfer coefficient and the interfacial area to be known. This in turn necessitates knowledge of the stirrer speed. It was shown in Sec. 14.3.g, however, how the choice of the stirrer speed depends on the reactor dimensions and geometry, so that the design has an iterative character a reactor diameter is chosen first then the stirrer speed is derived from this and finally the reactor volume required to achieve the desired conversion is calculated the resulting reactor diameter is compared with the initially chosen value. 

Hickman, A. D. (1988b). The measurement of oxygen mass transfer coefficients using a simple novel technique, Proc. 3rd Bioreactor Project Research Symposium, NEL, East Kilbride, Glasgow, Scotland, May 4. 

A two-chamber reactor is used to measure the oxygen mass transfer coefficient through Nafion 117, where the membrane area is 3.5 cm, the bottle liquid volume is 320 mL, and the dissolved oxygen saturation concentration at ambient temperature (25 C) and pressure is 8.1 mg/L. Using the data below, calculate the mass transfer coefficient. 

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