Agitated fermenter design

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. 

Several simple theories of interaction at the gas-liquid interface are pertinent to the design of pneumatically and mechanically agitated fermenters, and are now discussed. 

Since no agitator, gear box or crane rail is needed, less structural steel is used and cheaper fermenter design results. 

Popovic, M., and Robinson, C.W. (1984), Estimation of some important design parameters for non-Newtonian liquids in pneumatically-agitated fermenters, in Proceedings of the 34th Canadian Chemical Engineering Conference, Quebec City, Canada, 258-263. 

A few variations on the standard fermenter have been attempted, but none has become popular. An obsolete design in which the fermenter was rotated to aerate the medium is shown in Fig. 24-5. Performance was unsatisfactory, and the units were turned on end, with spargers and agitation added. One of the largest fermenters used for antibiotics is a horizontal cylinder with several agitators, as in Fig. 24-6. Multiple agitator motors and shafts have also been used with vertical cylindiic vessels. 

Figure 5.9 Design of a generalized microbial cell fermentation vessel (a) and an animal cell bioreactor (b). Animal cell bioreactors display several structural differences compared with microbial fermentation vessels. Note in particular (i) the use of a marine-type impeller (some animal cell bioreactors-air lift fermenters-are devoid of impellers and use sparging of air-gas as the only means of media agitation) (ii) the absence of baffles (iii) curved internal surfaces at the bioreactor base. These modifications aim to minimize damage to the fragile animal cells during culture. Note that various additional bioreactor configurations are also commercially available. Reprinted with permission from Proteins Biochemistry and Biotechnology (2002), J. Wiley Sons...

In this chapter, we study various correlations for gas-liquid mass transfer, interfacial area, bubble size, gas hold-up, agitation power consumption, and volumetric mass-transfer coefficient, which are vital tools for the design and operation of fermenter systems. Criteria for the scale-up and shear sensitive mixing are also presented. First of all, let s review basic mass-transfer concepts important in understanding gas-liquid mass transfer in a fermentation system. 

Eqs. (9.30) and (9.31) were confirmed by Calderbank and Jones (1961), for mass transfer to and from dispersions of low-densitv solid particles in agitated liquids which were designed to simulate mass transfer to microorganisms in fermenters. 

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