Fermenter design oxygen transfer rate

To estimate the design parameters for oxygen uptake in a fermenter, you can use the correlations presented in the previous sections, which can be applicable to a wide range of gas-liquid systems in addition to the air-water system. However, the calculation procedure is lengthy and the predicted value from those correlations can vary widely. Sometimes, you may be unable to find suitable correlations which will be applicable to your type and size of ferment ers. In such cases, you can measure the oxygen-transfer rate yourself or use correlations based on those experiments. 

Since the oxygen is sparingly soluble gas, the overall mass-transfer coefficient KL is equal to the individual mass-transfer coefficient KL. Our objective in fermenter design is to maximize the oxygen transfer rate with the minimum power consumption necessary to agitate the fluid, and also minimum air flow rate. To maximize the oxygen absorption rate, we have to maximize KL, a, C - CL. However, the concentration difference is quite limited for us to control because the value of C L is limited by its very low maximum solubility. Therefore, the main parameters of interest in design are the mass-transfer coefficient and the mterfacial area. 

Gas-liquid mass transfer is commonly modeled in terms of a gas film (between the bulk gas and interface) and a liquid film (between the interface and bulk liquid). Hindrance to mass transfer causes soluble gas (e.g., O2) concentrations to decrease across these films. The highest mass transfer resistance usually exists in the liquid film therefore, it controls the overall oxygen transfer rate (OTR). In aerobic fermentation, an effective fermenter design achieves an efficient OTR through intimate gas-liquid contact. OTR is described in terms of oxygen concentration and characteristics of the gas-liquid interface, as follows ... 

Gas-liquid mass transfer plays a very important role in aerobic fermentation. The rate of oxygen transfer from the sparged air to the microbial cells suspended in the broth or the rate of transfer of carbon dioxide (produced by respiration) from the cells to the air often controls the rate of aerobic fermentation. Thus, a correct knowledge of such gas-liquid mass transfer is required when designing and/or operating an aerobic fermentor. 

From the mass transfer point of view, airlift fermenters should be designed and operated in such a way that carryover of air from the riser into the downcomer is kept as low as possible. Gas in the downcomer liquid contributes little to oxygen transfer. It reduces the effective density difference between the contents of the riser and downcomer, however, which reduces the liquid circulation rate and also impairs the mixing performance of the fermenter. In fermentations where even a momentary lack of oxygen can very seriously affect productivity, some air is essential to maintain aerobic conditions and sustain fermentation in the downcomer. 

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