CSTFs

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

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

The characteristics of the CSTF, when operated as a chemostat, are discussed in Chapter 12. 

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... 

If a CSTF is considered (Fig. 5.56), which has a volume V, volumetric feed flow rate F, with influent substrate and biomass concentrations S0 and X0 respectively, then suppose that the substrate and biomass concentrations in the fermenter are 5 and X. A material balance can be established over the fermenter in the same manner as for the batch fermenter. This is ... 

Then, substituting for S using equation 5.133 so that for a CSTF at steady state utilising sterile feed, the biomass concentration is ... 

The specific production rate of biomass by the CSTF is the product of the biomass concentration and the volumetric flowrate of feed divided by the volume of the fermenter. 

The biomass concentration prevailing in the CSTF at the condition for maximum productivity may be obtained by substitution of the expression for >op into equation 5.135. On simplification this gives ... 

It is reasonable to assume that, since a high productivity is being considered or sought, then the feed substrate concentration would be high, and in particular S0 Ks so that the expression for maximum productivity of a CSTF reduces to ... 

The ratio of the CSTF productivity to that of the batch fermenter is obtained by dividing equation 5.143 by 5.145 to give ... 

Another design criterion for a CSTF is the critical dilution rate above which biomass would be removed from the fermenter at a rate faster than it could regenerate itself and ultimately lead to the absence of biomass in the reactor. This condition is referred to as washout. Since there is no biomass present, washout is characterised by the substrate concentration in the reactor becoming equal to that in the feed solution. Rewriting equation 5.132 as ... 

The quantity-----2— wni always be less than unity, and Z)crjt for a CSTF will always... 

Flo. 5.61. CSTF with settler-thickener and recycle of biomass... 

The denominator in equation 5.157 is clearly less than unity, provided the separator has any concentrating ability (i.e. works at all). As in the case for the expression linking D and n (equation 5.131) for a simple CSTF, it is independent of the microbial kinetics. If R - 0 or 4= 1 then equation 5.157 reduces to equation 5.131, corresponding to a CSTF at steady-state with no recycle. The effect of the recycle is to allow the fermenter to be operated at higher dilution rates than would otherwise be possible. 

This confirms the result anticipated from equation 5.157 that the dilution rate for washout will be greater than that for the simple CSTF. 

The last term in equation 5.245 represents the dilution of active component /, by the expansion of the biomass. Esener et al.m also present a two-compartment model which takes this effect into account and they emphasise the need to devise the theory so that it can be tested by experiment. In their model they identify a K compartment of the biomass which comprised the RNA and other small cellular molecules. The other compartment contained the larger genetic material, enzymes, and structural material. The model assumes that the substrate is absorbed by the cell to produce, in the first instance, K material, and thence it is transformed into G material. Additionally, the G material can be reconverted to K material, a feature intended to account for the maintenance requirement of the micro-organism. A series of material balances for the cellular components during growth in a CSTF produced the following differential equations ... 

Concentration of A Arrhenius constants Arrhenius constant Constant in equation 5.82 Surface area per unit volume Parameter in equation 5.218 Cross-sectional area Concentration of B Stoichiometric constants Parameter in equation 5.218 Concentration of gas in liquid phase Saturation concentration of gas in liquid Concentration of G-mass Concentration of D-mass Dilution rate DamkOhler number Critical dilution rate for wash-out Effective diffusion coefficient Dilution rate for maximum biomass production Dilution rate for CSTF 1 Dilution rate for CSTF 2 Activation energy Enzyme concentration Concentration of active enzyme Active enzyme concentration at time t Initial active enzyme concentration Concentration of inactive enzyme Total enzyme concentration Concentration of enzyme-substrate complex with substance A... 

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)...

The material balance for the microorganisms in a CSTF (Figure 6.7) can be written as... 

For the steady-state operation of a CSTF, the change of cell concentration with time is equal to zero (dCx/dt = 0) since the microorganisms in the vessel grow just fast enough to replace those lost through the outlet stream, and Eq. (6.27) becomes... 

Figure 6.8 shows the l/rx versus Cx curve. The shaded rectangular area in the figure is equal to the residence time in a CSTF when the inlet stream is sterile. This graphical illustration of the residence time can aid us in comparing the effectiveness of fermenter systems. The shorter the residence time in reaching a certain cell concentration, the more effective the fermenter. The optimum operation of fermenters based on this graphical illustration is discussed in the next section. 

In this section, we set a material balance for the microbial concentration and derive various equations for the CSTF. The same equations can be also obtained by setting material balances for the substrate concentration and product concentration. 

The equality of the specific growth rate and the dilution rate of the steady-state CSTF shown in Eq. (6.30) is helpful in studying the effects of various components of the medium on the specific growth rate. By measuring the steady-state substrate concentration at various flow rates, various kinetic models can be tested and the value of the kinetic parameters can be estimated. By rearranging Eq. (6.30), a linear relationship can be obtained as follows ... 

However, the limitation of this approach to determine the kinetic parameters is in the difficulty of running a CSTF. For batch runs, we can even use shaker flasks to make multiple runs with many different conditions at the same time. The batch run in a stirred fermenter is not difficult to carry out, either. Since there is no input and output connections except the air supply and the length of a run is short, the danger of contamination of the fermenter is not serious. 

For CSTF runs, we need to have nutrient and product reservoirs which are connected to the fermenter aseptically. The rate of input and output stream needs to be controlled precisely. Sometimes, the control of the outlet flowrate can be difficult due to the foaming or plugging by large cell aggregates. Since the length of the run should last several days or even weeks to reach a steady state and also to vary the dilution rates, there is always a high risk for the fermenter to be contaminated. Frequently, it is difficult to reach a steady state because of the cell s mutation and adaptation to new environment. 

Furthermore, since most large-scale fermentations are carried out in batch mode, the kinetic parameters determined by the chemostat study should be able to predict the growth in a batch fermenter. However, due to the significantly different environments of batch and continuous fermenters, the kinetic model developed from the CSTF runs may fail to predict the growth behavior of a batch fermenter. Nevertheless, the verification of a kinetic model and the evaluation of kinetic parameters by running chemostat is the most reliable method because of its constant medium environment. 

The operating condition for the maximum productivity of the CSTF can be estimated graphically by using l/rx versus Cx curve. The maximum productivity can be attained when the residence time is the minimum. Since the residence time is equal to the area of the rectangle of width Cx and height l/rx on the l/rx versus Cx curve, it is the minimum when the 1 / rx is the minimum, as shown in Figure 6.11. 

It would be interesting to derive the equations for the cell concentration and residence time at this maximum cell productivity. The cell productivity for a steady-state CSTF with sterile feed is... 

The most productive fermenter system is a CSTF operated at the cell concentration at which value of 1 /rx is minimum, as shown in Figure 6.12(a), because it requires the smallest residence time. 

If the final cell concentration to be reached is in the stationary phase, the batch fermenter is a better choice than the CSTF because the residence time required for the batch as shown in Figure 6.12(b) is smaller than that for the CSTF. 

---