Bubble column fermentors

The main operating factor of a bubble column fermentor is the superficial gas velocity Uq over the column cross section, which should be kept equal in scaling up. As mentioned in connection with Equation 7.41, k a in bubble columns <60 cm in diameter will increase with the column diameter D to the power of 0.17. 

Estimate the liquid-phase volumetric coefficient of oxygen transfer for a bubble column fermentor, 0.8 m in diameter 9.0 m in height (clear liquid), containing the same liquid as in Problem 12.2. The superficial gas velocity is 150 m h . ... 

The main operating factor of a bubble column fermentor is the superficial gas velocity UG over the column cross-section, which should be kept equal when scaling-up. As mentioned in connection with Equation 7.41, kLa in bubble columns less than 60 cm in diameter will increase with the column diameter D to the power of 0.17. As this trend levels off with larger columns, however, it is recommended that kLa values estimated for a 60 cm column are used. If heat transfer is a problem, then heat transfer coils within the column, or even an external heat exchanger, may become necessary when operating a large, industrial bubble column-type fermentor. Scale-up of an internal loop airlift-type fermentor can be achieved in the same way as for bubble column-type fermentors, and for external loop airlifts see Section 7.7. 

The 1980 s and the early 1990 s have seen the blossoming development of the biotechnology field. Three-phase fluidized bed bioreactors have become an essential element in the commercialization of processes to yield products and treat wastewater via biological mechanisms. Fluidized bed bioreactors have been applied in the areas of wastewater treatment, discussed previously, fermentation, and cell culture. The large scale application of three-phase fluidized bed or slurry bubble column fermen-tors are represented by ethanol production in a 10,000 liter fermentor (Samejima et al., 1984), penicillin production in a 200 liter fermentor (Endo et al., 1986), and the production of monoclonal antibodies in a 1,000 liter slurry bubble column bioreactor (Birch et al., 1985). Fan (1989) provides a complete review of biological applications of three-phase fluidized beds up to 1989. Part II of this chapter covers the recent developments in three-phase fluidized bed bioreactor technology. 

Two major types of fermentors are widely used in industry. The stirred tank, with or without aeration (e.g., air sparging) is most widely used for aerobic and anaerobic fermentations, respectively. The bubble column (tower fermentor) and its modifications, such as airlifts, are used only for aerobic fermentations, especially of a large scale. The important operating variables of the sparged (aerated)... 

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. 

Fermentation broths are suspensions of microbial cells in a culture media. Although we need not consider the enhancement factor E for respiration reactions (as noted above), the physical presence per se of microbial cells in the broth will affect the k a values in bubbling-type fermentors. The rates of oxygen absorption into aqueous suspensions of sterilized yeast cells were measured in (i) an unaerated stirred tank with a known free gas-liquid interfacial area (ii) a bubble column and (iii) an aerated stirred tank [6]. Data acquired with scheme (i) showed that the A l values were only minimally affected by the presence of cells, whereas for schemes (ii) and (iii), the gas holdup and k a values were decreased somewhat with increasing cell concentrations, because of smaller a due to increased bubble sizes. 

Aerated stirred tanks, bubble columns, and airlifts are usually used for aerobic fermentations. One criterion of scaling-up aerated stirred tank fermentor is k a, approximate values of which can be estimated by Equation 7.36a or b. For the turbulent range, a general correlation for k a in aerated stirred fermentors is of the following type [3] ... 

Pons, A., C. G. Dussap, and J. B. Gros, Xanthan Batch Fermentations Compared Performances of a Bubble Column and a Stirred Tank Fermentor, Bioprocess Eng., 5, 107, (1990). 

Coils or other internals may be inserted into the bubble column to promote heat transfer. In addition, the columns may be sectionalized by a baffle system or perforated plates to inhibit liquid phase backmixing or bubble coalescence. Simplicity of operation, lack of moving parts, ease of removal of heat and low operating costs are major advantages of bubble columns as reactors or fermentors. 

Recently, Lubbert and Larson (1990) analyzed the local mixing behavior in bubble columns and in air-lift fermentors utilizing a heat pulse-probe experiment. They considered the spreading of an originally narrowly distributed clump along the principle flow direction. Based on theory, the width of the spreading cloud (i.e. standard deviation) can be described by ... 

Y ou are studying dispersion in a small air-lift fermentor. This ferment or is 1.6 m tall, with a 10-cm diameter. Air and pure water are fed into the bottom at superficial velocities of 11 and 0.78 cm/sec under these conditions the gas bubbles occupy 45% of the column volume. You continuously add 15 cm /min of 1-M NaCl solution near the top of the column. You find by conductance that the salt concentration halfway down the column is 2.32-10 M. What is the dispersion coefficient Answer 54 cm /sec. 

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