Bioreactors mass transfer coefficients

The above correlation is valid for a bioreactor size of less than 3000 litres and a gassed power per unit volume of 0.5-10 kW. For non-coalescing (non-sticky) air-electrolyte dispersion, the exponent of the gassed power per unit volume in the correlation of mass transfer coefficient changes slightly. The empirical correlation with defined coefficients may come from the experimental data with a well-defined bioreactor with a working volume of less than 5000 litres and a gassed power per unit volume of 0.5-10 kW. The defined correlation is ... 

In the absence of viable cells in the bioreactor, an effective mass transfer coefficient can be obtained from... 

Transport in Capillary and Vortex Flow Bioreactors 5.1.2.2.1 Mass Transfer Coefficients... 

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

Understanding the effect of reactor diameter on the volumetric mass transfer coefficient is critical to successful scale up. In studies of a three-phase fluidized bed bioreactor using soft polyurethane particles, Karamanev et al. (1992) found that for a classical fluidized bed bioreactor, kxa could either increase or decrease with a change in reactor diameter, depending on solids holdup, but for a draft tube fluidized bed bioreactor, kxa always increased with increased reactor diameter. 

Response time. In the literature, response time is usually specified as the time taken for the electrode to reach > 90% of the output. Typical response times are around 30 sec. A fast response time is critical when one is measuring transient phenomena such as oxygen respiration rates in tissue or suspended cells and dynamic measurements of the volumetric mass transfer coefficient in bioreactors. 

In a bioreactor, one is interested in the transfer per unit of volume of reactor, called Kia or the volumetric mass-transfer coefficient, a is the interfacial surface area per unit of volume of liquid. In a perfectly mixed tank, C has identical values at any point and C depends on the conditions in the gas phase at the outlet of the reactor. Several authors [60] consider that a better estimate of the driving force is given by the logarithmic mean concentration difference between the entry and the exit of gas. 

Y. Kawase, B. Halard, M. Moo-Young, Liquid-phase mass transfer coefficients in bioreactors, Biotech. Bioeng. 39 (1992) 1133— 1140. 

If 02 consumption were indeed zero order for a particular plant species, then it would appear that any phytoproduction process involving that species would require only that a minimum dissolved 02 concentration be maintained any concentration increase beyond that would be irrelevant. In the case of tobacco cells, any concentration greater than 15 % of air saturation would yield the same metabolic rate and, presumably, the same productivity of all metabolites. If, on the other hand, consumption is first order in the concentration range achievable in a practical bioreactor (equivalently, if Kf is comparable to working concentrations), then its concentration is an important control parameter in the bioreactor. However, Kobayashi et al. studied berberine production by suspended and immobilized cells of Thalictrum minus [50]. They assert that 02 uptake is a zero-order process but observed that berberine production depended on 02 availability. They controlled that availability by adjusting the speed of shaking of the culture flasks, thus varying the mass transfer coefficient for absorption of 02. 

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

A cost analysis of an extractive membrane bioreactor (EMB) for wastewater treatment has been reported by Freitas dos Santos and Lo Biundo [6.24]. The EMB studied was similar with those reported in Chapter 4. Calculations were carried out for a feed flowrate of 1 m h of wastewater polluted with dichloromethane at a concentration of 1 g l A minimum pollutant removal rate of 99 % and 8000 h of operation per year were considered. As expected, the analysis indicated that the costs are strongly dependent on the pollutant flow entering the bioreactor to be transformed. Two key parameters, namely the total membrane area required and the external mass transfer coefficient, were studied. The results show that the costs and membrane area decrease significantly as the mass transfer coefficient increases from 0.5 x 10 to 2.0 x 10 m-s (these values are typical for large units, while laboratory measured values harbor around 5x10 m-s [624]). Using a mass transfer coefficient of 1.0 x 10 m s the authors calculated the costs and the membrane area required for different wastewater flowrates. These results are shown in Fig. 6.3. 

Specific activity of biocatalyst molar concentration of substrate B (alternatively coefficient in Eq. 5.3) initial molar concentration of substrate B coefficient in Eq. 5.3 concentration of biocatalyst time of a cycle of reactor operation enzyme activity initial enzyme activity molar concentration of enzyme species Eij volumetric activity of enzyme species Ey enzyme volumetric activity initial enzyme volumetric activity bioreactor feed flow-rate total flow-rate to downstream operations initial feed flow-rate to bioreactor i number of half-lives of biocatalyst use film volumetric mass transfer coefficient for substrate Michaelis-Menten constant catalytic rate constant first-order inactivation rate constant transition rate constants... 

In the following sections, a few examples of the effect of antifoam agents on the properties of cultivation mediiun and foam are considered. Most of the authors evaluated the effect of antifoam agents on the voliunetric mass transfer coefficients in bioreactors. 

The area ratio effects on the liquid-phase mass transfer coefficient are more difficult to predict. Area ratio effects are usually studied by keeping the bioreactor volume equal, which requires the effective bioreactor height to be adjusted. As the height is increased, the interfacial solute gas concentration increases as well, which decreases the gas solubility and, in turn, the liquid-phase mass transfer coefficient. In addition, an increase in the area ratio decreases the liquid circulation rate, which increases gas holdup, but may decrease surface renewal. The greater height also raises the pressure drop and power consumption, which increases surface renewal and the liquid-phase mass transfer coefficient. The extent of these effects is dependent on the operational scale and power level, and it is hard to predict which will dominate. 

Gas-liquid mass transfer coefficients follow the same gas holdup trends. As shown in Figure 8.9, the gas-Uquid mass transfer coefficient increases monotoni-cally with riser superficial gas velocity. The correlations by Chisti etal. (1988b) (as cited by Murchuk and Gluz (1999) and Popovic and Robinson (1984) were developed using external-loop airlift bioreactors, while the others used the draught tube internal-loop airlift bioreactor. The DT-ILALR has much better performance than the ELALR. It is unfortunate to note that gas-liquid mass transfer correlations are much fewer in number than their gas holdup counterpart. 

TABLE 8.2 Gas-Liquid Mass Transfer Coefficient Correlations for Airlift Bioreactors... 

Liquid properties influence the behavior of the bubble interlace and, consequently, have a strong effect on both the liquid-phase mass transfer coefficient and the inter-facial area. Research using organic liquids, which would be very useful for bioreactor and gas-liquid mass transfer optimization, is almost nonexistent for fixed bed reactors. 

Gogate, P.R., and Pandit, A.B. (1999b), Survey of measurement techniques for gas-liquid mass transfer coefficient in bioreactors, Biochemical Engineering Journal, 4(1) 7-15. 

Whitton, M.J., and Nienow, A.W. (1993), Scale-up correlations fr gas hold-up and mass transfer coefficients in stirred tank reactors in Proceedings of the 3rd International Conference on Bioreactor and Bioprocess Fluid Dynamics, 135-149. 

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