Stirred-tank fermentors

Reaction times of fermentation range from a few hours to several days. Batch processes are common, but continuous stirred tanks also are used either singly or in stages. A continuous stirred tank reactor (CSTR) also is called a chemostat. Figure 8.4 is a schematic of a fermentor with representative dimensions from the literature. 

Cell Growth in Batch Fermentors and Continuous Stirred-Tank Fermentors (CSTF) S3... 

Stirred-tank reactors can be used for continuous fermentation, because cells can grow in this type of fermentors without their being added to the feed medium. In contrast, if a plug flow reactor is used for continuous fermentation, then it is necessary to add the cells continuously in the feed medium, but this makes the operation more difficult. 

There are two different ways of operating continuous stirred-tank fermentors (CSTFs), namely chemostat and turbidostal. In the chemostat, the flow rate of the... 

Gas-liquid mass transfer in fermentors is discussed in detail in Section 12.4. In dealing with in gas-sparged stirred tanks, it is more rational to separate and a, because both are affected by different factors. It is possible to measure a by using either a light scattering technique [9] or a chemical method [4]. Ihe average bubble size can be estimated by Equation 7.26 from measured values of a and the gas holdup e. Correlations for have been obtained in this way [10, 11], but in order to use them it is necessary that a and d are known. 

Yeast cells are cultivated under aerobic conditions at 30 C in an aerated stirred-tank fermentor where air (21 vol% of oxygen) is supplied at a superficial gas velocity of 50 mh . The overall volumetric coefficient for oxygen transfer based on the liquid phase concentration is 80 h T... 

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)... 

The rates of heat transfer between the fermentation broth and the heat-transfer fluid (such as steam or cooling water flowing through the external jacket or the coil) can be estimated from the data provided in Chapter 5. For example, the film coefficient of heat transfer to or from the broth contained in a jacketed or coiled stirred-tank fermentor can be estimated using Equation 5.13. In the case of non-Newtonian liquids, the apparent viscosity, as defined by Equation 2.6, should be used. 

A fermentation broth contained in a batch-operated stirred-tank fermentor, 2.4m in inside diameter D, is equipped with a paddle-type stirrer of diameter (L) of 0.8 m that rotates at a speed Af = 4s -. The broth temperature is maintained at 30 °C with cooling water at 15°C, which flows through a stainless steel helical coil that has a 50 mm outside diameter and is 5 mm thick. The maximum rate of heat evolution by biochemical reactions, plus dissipation of mechanical energy input by the stirrer, is 51000 kcal h , although the rate varies with time. The physical properties of the broth at 30 °C were density p = 1000 kg m " , viscosity p = 0.013 Pa s, specific heat Cp = 0.90 kcal kg °C , and thermal conductivity K = 0.49 kcal h m °C = 0.000136 kcals m °C . ... 

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. 

The values of k a for CO, desorption in a stirred-tank fermentor, calculated from the experimental data on physically dissolved CO, concentration (obtained by the above-mentioned method) and the CO2 partial pressure in the gas phase, agreed well with the k a values estimated from the k a for O, absorption in the same fermentor, but corrected for any differences in the liquid-phase diffusivities [11]. Perfect mixing in the liquid phase can be assumed when calculating the mean driving potential. In the case of large industrial fermentors, it can practically be assumed that the CO, partial pressure in the exit gas is in equilibrium with the concentration of CO, that is physically dissolved in the broth. The assumption of either a plug flow or perfect mixing in the gas phase does not have any major effect... 

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] ... 

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. 

Suppose that a well-mixed stirred tank is being used as a fed-batch fermentor at a constant feed rate F (m h ), substrate concentration in the feed C j (kg m ), and at a dilution rate D equal to the specific cell growth rate p. Ihe cell concentration Cjj (kgrn ) and the substrate concentration (kgm ) in the fermentor do not... 

As mentioned in Section 7.2.1, a well-mixed stirred-tank reactor, when used continuously, is termed a continuous stirred-tank reactor (CSTR). Similarly, a well-mixed stirred-tank fermentor used continuously is termed a continuous stirred-tank fermentor (CSTF). If cell death is neglected, the cell balance for a CSTF is given as... 

Estimate the liquid-phase volumetric coefficient of oxygen transfer for a stirred-tank fermentor with a diameter of 1.8 m, containing a viscous non-Newtonian broth, with consistency index K = 0.39, flow behavior index n = 0.74,... 

For an animal cell culture, satisfactory results were obtained with a pilot fermentor, 0.3 m in diameter, with a liquid height of 0.3 m (clear liquid), at a rotational impeller speed N of 1.0 s (impeller diameter 0.1 m) and an air rate (30 °C) of 0.02 m min. The density and viscosity of the broth are 1020kg rn and 0.002 Pa s, respectively. The value can be correlated by Equation 7.36b. When k a is used as the scale-up criterion, and the allowable impeller tip speed is 0.5 m s , estimate the maximum diameter of a geometrically similar stirred tank. 

It may also be economical to remove the inhibitory product directly from the ongoing fermentation by extraction, membranes, or sorption. The use of sorption with simultaneous fermentation and separation for succinic acid has not been investigated. Separation has been used to enhance other organic acid fermentations through in situ separation or separation from a recycled side stream. Solid sorbents have been added directly to batch fermentations (18,19). Seevarantnam et al. (20) tested a sorbent in the solvent phase to enhance recovery of lactic acid from free cell batch culture. A sorption column was also used to remove lactate from a recycled side stream in a free-cell continuously stirred tank reactor (21). Continuous sorption for in situ separation in a biparticle fermentor was successful in enhancing the production of lactic acid (16,22). Recovery in this system was tested with hot water (16). 

The term fermentation is used to describe the biological transformation of chemicals. In its most generic application, a fermentor may be batch, continuous-stirred tank (chemostat), or continuous plug flow (immobilized cell). Most industrial fermentors are batch. Several configurations exist for these batch reactors to facilitate aeration. These include sparged tanks, horizontal fermentors, and biological towers. 

---