Fermentation processes, modes operation

Most fermentation processes are operated in either batch or fed-batch mode. The fed-batch mode has been introduced ia an increasing number of fermentation processes. This is because theoretical analysis as well as e q)erimental data of many fermentation processes have revealed that controlled feeding during a fermentation... 

Fermentation processes can be conducted in batch, fed-batch or continuous modes. However, many fermentation processes are operated in batch or fed-batch modes due to the advantages of these schemes compared to continuous operation. These advantages include easier set-up and operation, less complication in process control, less susceptibility to microbial contamination, and high final product titer and yields. On the other hand, continuous bioreactors have advantages such as consistent quality of product, high productivity and flexibility in system investigation and analysis. 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

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. 

The ability of Monod s empirical relation to fit kinetic data for biochemical reactions has its foundations in generalizations of two phenomena frequently observed for fermentation processes (1) nature places a cap on the quantity of microorganism that can be achieved during the exponential phase of growth in a bioreactor operating in a batch mode and (2) as the concentration of the limiting substrate approaches zero, the rate laws for biochemical reactions approach pseudo-first-order behavior with respect to that substrate. The cap indicated on the cell growth rate has been associated with the natural limit on the maximum rate at which replication of DNA can be achieved. 

Decisions as to whether a chemostat or a batch reactor is preferable for use in a particular application depend on a number of factors. However, an important consideration is whether or not the enhancement in productivity associated with the use of a continuous fermentation process is sufficient to offset the difficulties associated with this mode of operation the need for continuously feeding sterile supplies of the growth medium and (if appropriate) either air or oxygen, the potential for other modes of contamination of the fermentation broth, the potential for mutation of the microorganism, and so on. As an introduction to these considerations, let us consider the relative productivities of... 

If, for example, one is dealing with a microbial growth process in a CSTR, the Xj = 0, and one may apply Equ. 2.5a for the rate of growth and thereby obtain the well-known relations for the equilibrium state. This is the basis for the CSTR, the so-called chemostat" or realstat mode of operation of a continuous fermentation process (see Sect. 6.1). 

The main tank fermentation process is carried out as a fed batch process, since this mode of operation is ideal for many microbial processes. The feed substrate is in most cases the carbon source, which is added continuously during the entire batch. This allows the process engineer to control the growth rate of the organism and thus to keep the culture at its optimal conditions for product... 

Given the diversity of bioconversion/fermentation processes, different reactor configurations and modes of operation have been developed, so as to provide the more adequate... 

The fermentation process and downstream processing are usually unlinked. The fermentation broth is harvested and stored, and the recovery is carried out later. In rare cases, when the product is not stable, purification is carried out immediately after fermentation. The construct of the expression plasmid also influences the purification strategy. It is important to take this fact into account when an efficient and sustainable process must be designed. The expression plasmid determines whether a protein is secreted into the culture medium, into the periplasm, or stays in the cytoplasm. It also plays a pivotal role in the half-life of a protein. Although it may sound odd, the N-terminal amino acid is responsible for the half-life of a protein in the cell [26,27]. The most important features determining the functionality of an expression system are listed in Table 2. The expression system, together with the mode of operation, produces a characteristic product concentration profile with time (Fig. 3). 

A full set of bioreactors with pH and temperature controllers are shown in Figure 1.3. The complete set of a 25 litre fermenter with all the accessory controlling units creates a good opportunity to control suitable production of biochemical products with variation of process parameters. Pumping fresh nutrients and operating in batch, fed batch and continuous mode are easy and suitable for producing fine chemicals, amino acids, and even antibiotics. 

A chemical reactor is an apparatus of any geometric configuration in which a chemical reaction takes place. Depending on the mode of operation, process conditions, and properties of the reaction mixture, reactors can differ from each other significantly. An apparatus for the continuous catalytic synthesis of ammonia from hydrogen and nitrogen, operated at 720 K and 300 bar is completely different from a batch fermenter for the manufacture of ethanol from starch operated at 300 K and 1 bar. The mode of operation, process conditions, and physicochemical properties of the reaction mixture will be decisive in the selection of the shape and size of the reactor. 

This mode is seldom used except for extremely slow processes such as fermentation or for very small reactors where the surface area for heat transfer is large enough to maintain the reactor thermostatted at the temperature of the surroundings. In fact, we seldom want to operate a reactor isothermally, because we want to optimize the temperature and temperature profile in the reactor to optimize the rate and selectivity, and this is most efficiently achieved... 

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