Fermenter design aerobic

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 ... 

A suitable means of scale-up for aerobic processes is to measure the dissolved oxygen level that is adequate in small equipment and to adjust conditions in the plant until this level of dissolved oxygen is reached. However, some antibiotic fermentations and the production of fodder yeast from hydrocarbon substrates have very severe requirements, and designers are hard-pressed to supply enough oxygen. 

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. 

The most traditional application of the fermentor is in batch mode. In anaerobic fermentations the reactor looks like a normal batch reactor, since gas-liquid contact is not an important design consideration. Depending on the reaction, the microbes may or may not be considered as a separate phase. For aerobic fermentations, oxygen is bubbled through the media, and mass transfer between phases becomes one of the major design factors. 

Generally, the fermentation process involves the addition of a specific culture of microorganisms to a sterilized liquid substrate or broth in a tank (submerged fermentation), addition of air if aerobic, in a well-designed gas-liquid contactor. The fermentation process is then carried out to grow microorganisms and to produce the required chemicals. Table 11-1 lists examples of the processes used by fermentation. 

As an illustration of the complexity of a bioreactor design, consider the critical need of an adequate oxygen supply in aerobic fermentations. In order to prevent irreversible cell damage, oxygen must be supplied continuously to... 

The focus of this entry is limited to the design of aerobic fermenters, i.e., stirred tank and concentric tube airlift fermenters, which are commonly utilized in the bioprocessing industries. Design principles and the basic calculations are described with a couple of industrial examples. Readers with a limited background in mixing technology are referred to the gas-liquid contactor entry of this encyclopedia for a more comprehensive understanding of fluid flow and mass transfer characteristics in stirred tank reactors and bubble columns. 

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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