Stirred-tank Fermenter

Fig. 10.5. Continuous stirred tank fermenter, experimental setup with instrumentations and controllers, effluent.

Two continuous stirred-tank fermenters are arranged in series such that the effluent of one forms the feed stream of the other. The first fermenter has a working volume of 100 1 and the other has a working volume of 50 1. The volumetric flowrate through the fermenters is 18 h-1 and the substrate concentration in the fresh feed is 5 g/1. If the microbial growth follows Monod kinetics with //, = 0.25 h-1, Ks = 0.12 g/1, and the yield coefficient is 0.42, calculate the substrate and biomass concentrations in the effluent from the second vessel. What would happen if the flow were from the 50 1 fermenter to the 100 1 fermenter ... 

The generalised material balances for a well-stirred tank fermentation can be represented as ... 

An aerated stirred-tank fermenter equipped with a standard Rushton turbine of the following dimensions contains a liquid with density p = 1010kgm and viscosity n = 9.8 X 10 Pa s. The tank diameter D is 0.90 m, liquid depth Hl = 0.90 m, impeller diameter d = 0.30 m. The oxygen diffusivity in the liquid Dl is 2.10 X 10 5 cm- s T Estimate the stirrer power required and the volumetric mass transfer coefficient of oxygen (use Equation 7.36b), when air is supplied from the tank bottom at a rate of 0.60 m min at a rotational stirrer speed of 120 rpm, that is 2.0 s T... 

The shape of the performance curve for a continuous stirred-tank fermenter is dependent on the kinetic behaviour of the micro-organism used. In the case where the specific growth rate is described by the Monod kinetic equation, then the productivity versus dilution rate curve is given by equation 5.137 and has the general shape shown by the curve in Fig. 5.58. However, if the specific growth rate follows substrate inhibition kinetics and equation 5.65 is applicable then, at steady state, equation 5.131 becomes ... 

Fig. 5.59. Performance curves for a continuous stirred-tank fermenter with substrate...

The more generalised case of several continuous stirred-tank fermenters in series may be analysed by considering N such fermenter vessels, each of volume V and with fresh feed introduced to the first tank at a volumetric flowrate F (see Fig. 5.63). Because no streams enter or leave intermediately, the flowrates between stages and of the final product will also be F. 

Note that this expression does not involve the feed to the first fermenter, which may well operate under sterile feed conditions. In that case, and provided that the tanks are of equal size, the washout condition is analogous to the case of the single stirred-tank fermenter, that is ... 

The performance of a set of fermenters in series may be improved by the inclusion of a recycle stream, as in the case of a single stirred-tank fermenter. The situation for N vessels each of volume V may be analysed in a similar manner to the case for no recycle. 

Fig. 5.64. Stirred-tank fermenters in series with recycle of biomass over the ith fermenter gives ...

It may be seen that equation 5.186 reduces to equation 5.163 when N= 1 that is, it agrees with the expression derived for the critical dilution rate for a single stirred-tank fermenter with recycle of biomass. 

In contrast to the batch fermentation based methods of determining kinetic constants, the use of a continuous fermenter (Fig. 3.71) requires more experiments to be performed, but the analysis tends to be more straightforward. In essence, the experimental method involves setting up a continuous stirred-tank fermenter to grow the micro-organisms on a sterile feed of the required substrate. The feed flowrate is adjusted to the desired value which, of course, must produce a dilution rate below the critical value for washout, and the system is allowed to reach steady state. Careful measurements of the microbial density X, the substrate concentration S, and the flowrate F are made when a steady state has been achieved, and the operation is then repeated at a series of suitable dilution rates. 

It has been shown (equation 3.127) that the material balance for substrate across a continuous stirred-tank fermenter gives ... 

The steady-stale substrate and biomass concentrations for a continuous stirred-tank fermenter operated at various dilution rates are given below. Given that the fresh feed concentration is 700 mg/l, calculate the values of the Monod constants and Ks the yield coefficient, Y and the endogenous respiration coefficient kd. 

Fig. 6.4 Schematic diagram of (a) batch stirred-tank fermenter and plug-flow fermenter...

Microbial populations can be maintained in a state of exponential growth over a long period of time by using a system of continuous culture. Figure 6.7 shows the block diagram for a continuous stirred-tank fermenter (CSTF). The growth chamber is connected to a... 

Fig. 6.7 Schematic diagram of continuous stirred-tank fermenter (CSTF)...

Many alternative fermenters have been proposed and tested. These fermenters were designed to improve either the disadvantages of the stirred tank fermenter-high power consumption and shear damage, or to meet a specific requirement of a certain fermentation process, such as better aeration, effective heat removal, cell separation or retention, immobilization of cells, the reduction of equipment and operating costs for inexpensive bulk products, and unusually large designs. 

Let s examine the stability of recombinant cells in the continuous stirred-tank fermenter. The material balance for the plasmid-carrying cells around a CSTF yields... 

Stirred tank reactor systems can also operate in a continuous mode. In this configuration, fresh medium is continually supplied to the reactor and the desired products are continuously removed in the course of production. A continuous system is referred to as a chemostat when the flow rate is set to a constant value. It is further known as a turbidostat when the flow rate is set to maintain a constant turbidity or cellular concentration.f Continuous reactor systems are commonly abbreviated as CSTR or CSTF and they refer to continuous stirred tank reactor and continuous stirred tank fermenter, respectively. 

A stirred tank fermenter consists of a centrally mounted agitation system inside a cylindrical vessel. Typically, the agitation system is composed of either multiple radial flow impellers (see Fig. 4A) or a combination of radial and axial flow impellers as shown in Fig. 4B. A gas sparger is located below the radial gas dispersing impeller. The role of the bottom radial impeller... 

Fig. 4 Stirred tank fermenters (A) with multiple radial impellers and (B) with axial and radial impellers.

Fig. 5 Draft-tube-impeller stirred tank fermenter. (View this art in color at www.dekker.com.)...

In the case of stirred tank fermenters, heat that must be dissipated includes not only that generated by microbial metabolic activity but also that evolved from agitation power (i.e., 2500Btu/shp-hr) and expansion of sparged gas. This can lead to scale-up problems because vessel volume is proportional to vessel diameter cubed, while heat transfer area is proportional only to vessel diameter squared. 

For high-viscosity applications, lack of turbulence adjacent to the bubble, and the general difficulty in pumping around the draft tube (riser), prevent the use of simple pneumatically agitated airlift fermenters. Stirred tank fermenters are typically used for media with a viscosity exceeding 100 cP. As noted before, an... 

This, the first of two fermenter design examples, deals with a stirred tank fermenter. Nondimensional parameters that are applied in this example are defined as follows. 

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