Continuous bioreactor, typical

Figure 9.16A shows a typical operational configuration of a continuous bioreactor culture. In Figure 9.16B, the cell concentration data of a real continuous culture, operated at three different dilution rates, are presented. The rectangles indicate the different steady states observed, whereas the arrows show the moment of change in dilution rate. Furthermore, a guide line has been included in Figure 9.16B to help interpretation of the cell concentration profile. 

Doran outlines several of the key parameters involved in the scale-up of a biological process from laboratory-scale shake flasks to production-scale bioreactors. In the first stage of studies a bench-top bioreactor, typically 1-2 L, is used to determine the oxygen requirements of the cells, their shear sensitivity, foaming characteristics, and any limitations that the reactor imposes on the organism. The results of these early studies enable decisions regarding operation in the batch, fed-batch, or continuous mode. A pilot-scale... 

Batch cultivation is perhaps the simplest way to operate a fermentor or bioreactor. It is easy to scale up, easy to operate, quick to turn around, and reliable for scale-up. Batch sizes of 15,000 L have been reported for animal cell cultivation [2], and vessels of over 100,000 L for fermentation are also available. Continuous processes can be classified into cell retention and non-cell retention. The devices typically used for cell retention are spin filters, hollow fibers, and decanters. Large-scale operation of continuous processes can reach up to 2,000 L of bioreactor volume. Typically, the process is operated at 1-2 bioreactor volumes... 

Fermentation is typically conducted in dilute suspension culture. The low concentration in such systems limits reaction efficiency, and the presence of particulate and colloidal solids poses problems for product recovery and purification. By circulating the fermentation broth through an ultrafiltration system, it is possible to recover product continuously as they are generated while minimizing loss of enzyme or cells and keeping product concentration in the bioreactor below the self-inhibition level for the biocatalyst. This process is referred to as perfusion. As the ultrafiltration unit is part of the production process, the entire system is often considered a membrane reactor. 

Figure 9.19 shows typical cell concentrations reached in the main industrial bioreactors and a comparison of these values with those found in microbial fermentations. As can be observed, batch and fed-batch cultivations attain dry biomass values comparable to those of continuous cultures of microorganisms, so that mass and heat transfer capacities are not limited for these operation modes. However, high cell density cultivation in heterogeneous bioreactors, such as hollow-fiber devices, reaches dry biomass values similar to the maxima observed in microbial cultures. 

Sterilized and introduced to a bioreactor or fermenter, is typically equipped with agitators, baffles, air spargers, and sensing devices for the control of the operating conditions. A pure strain of microorganisms is introduced into the vessel. The number of cells multiplies exponentially after a certain period of lag time and reaches a maximum cell concentration as the medium is depleted. The fermentation is then stopped and the contents are pumped out for the product recovery and purification. This process is operated either by batch or continuous mode. 

While the Monod equation is an oversimplification of the complicated mechanism of cell growth, it often adequately describes fermentation kinetics. The Monod kinetic parameters can be determined by making a series of ideal continuous stirred-tank bioreactors, which will be discussed later. Table 19.6 shows the typical values of the Monod s kinetic parameters when glucose is a limiting substrate. 

The above discussion may involve both control and dynamics of steady state (single and multiple), unsteady state (transition between two states) and unstable operations of the bioreactor resulting from typical non-linear response of biological systems. In spite of the fact that the stability analysis of non-linear systems is quite advanced, experimental confirmation of multiple steady states and instabilities lags behind (for a review of theory and experiments see (45)). An excellentexample of experimental demonstration of unstable operation of a continuous reactor is in (46). 

The Verax system comprises a bioreactor (fluidization tube), a control system (for pH, oxygen, medium flow rates), gas and heat exchanger, and medium supply and harvest vessels. The system is run continuously for long periods (typically over 100 days). In the authors laboratory it produced 15 x 10 cells/ hire and 540 mg mAb/litre/day (compared to 166 in the fixed bed described above, 25.5 in a stirred reactor, and 18.5 mg in an airlift fermenter). Protocols for its operation come with the equipment and versions have been published (41). In summary it is probably the most productive system available giving the cells a very high specific production rate but does require some skill to operate to its maximum potential... 

Continuous flow processes sometimes use a plug flow bioreactor in which there is little mixing of fluid elements in the direction of flow (Fig. 25a). This type of flow is typically achieved in long tubes and channels. The composition of the broth does not change with time at a fixed... 

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