Fermentors batch operation

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 . ... 

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 ... 

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. 

In a fed-batch culture (semi-batch culture, see Figure 7.1b), a fresh medium that contains a substrate but no cells is fed to the fermentor, without product removal. The fed-batch operation has special importance in biotechnology, as it is the most... 

The characteristics of the fed-batch culture, shown by Equations 12.21 and 12.23, make it possible to keep the concentrations of the substrate and/or the cell at the desired values. For example, after a batch culture, a feed medium that contains the substrate at a high concentration can be fed, either continuously or intermittently, to the fermentor under a fed-batch operation. The values of the dilution rate and the substrate concentration in the feed medium can be determined using Equation 12.23. Thus, by using the fed-batch operation, the yield and/or productivity can be greatly improved in a variety of areas of biotechnology by controlling the concentrations of substrate and cell. Some examples of where the fed-batch operation can be effectively used are as follows. 

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). 

Figure 5. Highly instrumented pilot fermentor for fed-batch operations.

Bosetti et al [4.29] have reported the oxidation of naphthalene by Pseudomonas fliioroscens to optically pure cA-1,2-dihydroxy-1,2-dihydronaphtalene. They reported that the MBR, after 25 h of reaction, had a production rate three times that of a batch reactor. Recently Miyano et al [4.30] reported improvement in the rate of vitamin B12 production by Propionibacterium freudenreichi using a hollow-fiber MBR. The strain used produced also propionic and acetic acid, which inhibit its activity. The authors compared the MBR with a batch reactor using a co-culture. For the batch reactor application, a second strain Ralstodia. Eutropha), which was able to metabolize partially the toxic propionic acid, was introduced into the fermentor. The experimental results of Miyano et al [4.30] showed that the MBR was much more efficient than the co-culture batch operation. 

The N. rustica hairy roots were successfully grown in a packed-bed fermentor, yielding biomass densities (DW) of 10 g/liter (90). The growth rate was comparable with a cell suspension culture. The fermentor was operated as a batch fermentor for 11 days, after which is was run as a continuous culture. Nicotine was isolated from the medium during the continuous operation. The alkaloid production rate was estimated to be 1.54 mg/liter/day during this phase. To improve the release of alkaloids by the hairy roots, a continuous removal of the alkaloids from the medium with XAD-4 as an adsorbent was tested. Hairy roots cultured in flasks did not produce more alkaloids in the presence of sachets with XAD-4. However, the amount of alkaloid released into the medium increased in other words, the accumulation ratio was affected by the adsorbent. 

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. 

Commercial-scale operations are conducted in batch, fed-batch, or continuous culture systems. Fermentation vessels include the conventional baffled aerated tank, with or without impeller agitation, and the ak-lift tower fermentors in which ak is sparged into an annular space between the... 

C. uti/is yeast is produced by either fed-batch or continuous processes. Aerated-agitated fermentors range up to 300 m total capacity and ate operated in the same manner as described for S. cerevisiae (2,5). C. utilis is capable of metabolizing both hexose and pentose sugars. Consequendy, papermiU wastes such as sulfite waste Hquot that contain these sugars often ate used as substrates. 

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). 

True. Batch cultures give lower overall outputs than continuous cultures, as they suffer from non-productive down-time (the time taken to empty, clean, re-sterilise and re-fill the fermentor). After inoculation, considerable time can be taken for biomass to build up to a level where substrates are effectively utilised. Continuous cultures do not suffer such drawbacks once they are in operation. 

At the start of the batch fermentor operation, the broth must be heated to the fermentation temperature, which is usually in the range of 30-37 °C, by passing steam or warm water through the coil or the outer jacket. 

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

Thus, if D is made equal to fi, the cell concentration would not vary with time. For a given substrate concentration in the fermentor C, fi is given by the Monod equation (i.e.. Equation 4.6). Alternatively, we can adjust the substrate concentration Cj for a given value of fi. Practical operation usually starts as batch culture and, when an appropriate cell concentration is reached, the operation is switched to a fed-batch culture. 

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... 

Commercial fermentations are conducted in large bioreactors which are usually referred to as fermentors and arc designed tor operation in batch, fed-baldi. or continuous ferine illation modes. The batch and fed-batch procedures are used for most commercial antibiotic fermentations. 

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