Modes of Fermentor Operation

Batch operation of fermentors is much more common than continuous operation, although theories for continuous operation are well established (as will be indicated later in this section). The reasons for this are as follows  

1) The operating conditions of batch fermentors can be more easily adjusted to required specific conditions, as the fermentation proceeds. 

2) The capacities of batch fermentors are usually large enough, especially in cases where the products are expensive. 

3) In the case of batch operation, damages by the so-called contamination - that is, the entry of unwanted organisms into the systems, with resultant spoilage of the products - is limited to the particular batch that was contaminated. 

In the fed-batch operation of fermentors (which is also commonly practiced), the feed is added either continuously or intermittently to the fermentor, without any product withdrawal, the aim being to avoid any excessive fluctuations of oxygen demand, substrate consumption, and other variable operating conditions. 

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In a third paper Scragg et al. (625) compared three different modes of bioreactor operation batch, fed-batch, and draw-fill. The experiments were conducted using airlift fermentors of 7 and 30 liters working volume. Batch and fed-batch cultures were also performed in shake flasks. It appeared that serpentine production in batch cultures in shake flasks was highest. Batch cultures performed in airlift bioreactors showed a lower productivity. Fed-batch cultures showed a lower productivity compared with batch cultures, both in shake flasks and in airlift reactors. When the draw-fill method was used, serpentine production was negligible. 

Bioreactors that use enzymes but not microbial cells could be regarded as fermentors in the broadest sense. Although their modes of operation are similar to those of microbial fermentors, fed-batch operation is seldom practiced for enzyme reactors. The basic equations for batch and continuous reactors for... 

Typically, the SSF process is carried out in a CSTR reactor in batch mode. Under these reaction conditions, the fermentation product, ethanol, exerts its effect not only on microbes but also on saccharification. To overcome this problem, and to improve the efficiency of ethanol production from cellulose, the continuous removal of end-product during ethanol production would have advantages. With this type of process application, the SSF process can be operated in a fed-batch mode. Fed-batch operation is similar to continuous operation except the fermentation broth is retained in the fermentor at all times whereas the solid substrate is continuously fed into the fermentor [73]. Another method is to continuously remove ethanol during the SSF process (see Sect. 2.1.3). 

Suppose that a well-mixed, stirred tank is being used as a fed-batch fermentor at a constant feed rate F (m3 h-1), substrate concentration in the feed Csi (kg m-3), and at a dilution rate D equal to the specific cell growth rate /i. The cell concentration Cx (kgm 3) and the substrate concentration Cs (kgm-3) in the fermentor do not vary with time. This is not a steady-state process, as aforementioned. Then, by switching the mode of operation, part of the broth in the tank is continuously... 

This exercise involves not only novel configuration but, more essentially, also a novel mode of operation of a fermentor. 

The alternative to batch mode operation is continuous operation. In the continuous mode there is a continuous flow of medium into the fermentor and of product stream out of the fermentor. Continuous bioprocesses often use homogenously mixed whole cell suspensions. However, immobilised cell or enzyme processes generally operate in continuous plug flow reactors, without mixing (see Figure 2.1, packed-bed reactors). 

This bioreactor mode refers to a tank fermenter operated semi-continuously. The rate of the feed flow, F0, may be variable, and there is no outlet flow rate from the fermentor. As a consequence of feeding the reactor volume will change with respect to time. 

Chen and Lee [24] studied lactic acid production from dilute acid pretreated a-cellulose and switchgrass by L. delbruckii NRRL-B445 in the presence of a fungal cellulase in a fermentor extractor employing a microporous hollow fiber membrane (MHF). This reactor system was operated in a fed-batch mode with continuous removal of lactic acid by in situ extraction. A tertiary amine (alamine... 

In this bioprocess, specific kinetics can be expressed in terms of the concentration of the limiting substrate ghicose(Ghi ). Therefore, the material balances for cell mass and limiting substrate are sufficient to describe kinetics of the fermentor en operated in fed-batch mode. There are... 

The fed-batch mode starts the same as batch operation. However, concentrated medium is at some time continuously or intermittently added into the fermentor until the desired working volume is reached. The volume of the medium inside the fermentor changes in the fed-batch mode due to the frequent additions of substrate and nutrients. This is done to keep the substrate concentration low to minimize substrate inhibition to microorganisms. Higher product concentration and yield can be obtained in fed-batch compared to batch operation. Both batch and fed-batch fermentations are run until completion and no products are taken out from the fermentor until the end of fermentation. 

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