Aerobic fermentors

In chemical engineering, the terms transfer of heat, mass, and momentum are referred to as the transport phenomena. The heating or cooling of fluids is a case of heat transfer, a good example of mass transfer being the transfer of oxygen from air into the culture media in an aerobic fermentor. When a fluid flows through a conduit, its pressure drops because of friction due to transfer of momentum, as shown later. 

It is for this reason that the gas-phase resistance can be neglected for oxygen transfer in aerobic fermentors. 

As most biochemical reactions occur in the liquid phase, bioreactors usually handle liquids. Processes in bioreactors often also involve a gas phase, as in cases of aerobic fermentors. Some bioreactors must handle particles, such as immobilized enzymes or cells, either suspended or fixed in a liquid phase. With regard to mass transfer, microbial or biological cells may be regarded as minute particles. 

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. 

Thus, when deahng with gas transfer in aerobic fermentors, it is important to consider only the resistance at the gas-liquid interface, usually at the surface of gas bubbles. As the solubihty of oxygen in water is relatively low (cf. Section 6.2 and Table 6.1), we can neglect the gas-phase resistance when dealing with oxygen absorption into the aqueous media, and consider only the liquid film mass transfer coefficient Aj and the volumetric coefficient k a, which are practically equal to and K a, respectively. Although carbon dioxide is considerably more soluble in water than oxygen, we can also consider that the liquid film resistance will control the rate of carbon dioxide desorption from the aqueous media. 

Standard correlations for in an aerated stirred tank and the bubble column were provided in Chapter 7. However, such correlations were obtained under simplified conditions and may not be applicable to real fermentors without modifications. Various factors that are not taken into account in those standard correlations may influence the k a values in aerobic fermentors used in practice. 

Carbon dioxide produced in an aerobic fermentor should be desorbed from the broth into the exit gas. Figure 12.2 [11] shows, as an example, variations with time ofthe dissolved CO2 and oxygen concentrations in the broth, CO2 partial pressure in the exit gas, and the cell concentration during batch culture of a bacterium in a stirred fermentor. It can be seen that CO2 levels in the broth and in the exit gas increase, while the dissolved oxygen (DO) concentration in the broth decreases. 

Two geometrically similar stirred tanks with flat-blade turbine impellers of the following dimensions are to be operated at 30 °C as pilot-scale and production-scale aerobic fermentors. 

Temperature Ihe temperature in a bioreactor is an important parameter in any bioprocess, because all microorganisms and enzymes have an optimal temperature at which they function most efficiently. For example, optimal temperature for cell growth is 37 °C for Escherichia coli and 30 °C for Saccharomyces sp, respectively. Although there are many types of devices for temperature measurements, metal-resistance thermometers or thermistor thermometers are used most often for bioprocess instrumentation. The data of temperature is sufficiently reliable and mainly used for the temperature control of bioreactors and for the estimation of the heat generation in a large-scale aerobic fermentor such as in yeast production or in industrial beer fermentation. 

Microorganisms in liquids and gases can be removed by microfiltration hence, air supplied to aerobic fermentors can be sterilized in this way. Membrane filters are often used for the sterilization of liquids, such as culture media for fermentation (especially for tissue culture), and also for the removal of microorganisms from various fermentation products, the heating of which should be avoided. 

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