Continuous Stirred-Tank Fermentor

Stirred-tank reactors can be used for continuous fermentation, because the cells can grow in this type of fermentor without their being added to the feed medium. In contrast, if a plug-flow reactor is used for continuous fermentation, then it is necessary to add the cells continuously to the feed medium, but this makes the operation more difficult. 

There are two different ways of operating a continuous stirred-tank fermentor, namely chemostat and turbidostat. In the chemostat, the flow rate of the feed medium and the liquid volume in the fermentor are kept constant. The rate of cell growth will then adjusts itself to the substrate concentration, which depends on the feed rate and substrate consumption by the growing cells. In the turbidostat the liquid volume in the fermentor and the liquid turbidity, which varies with the cell concentration, are kept constant by adjusting the liquid flow rate. Whereas, turbidostat operation requires a device to monitor the cell concentration (e.g., an optical sensor) and a control system for the flow rate, chemostat is much simpler to operate and hence is far more commonly used for continuous fermentation. The characteristics of the continuous stirred-tank fermentor (CSTF), when operated as a chemostat, is discussed in Chapter 12. 

1 Escherichia coli grows with a doubling time of 0.5 h in the exponential growth 

How much time would be required to grow the cell culture from 0.1 kg-dry cell m-3 to 10 kg-dry cell m-3  

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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... 

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... 

Continuous-Stirred Tank Fermentor (Without Micro-organism Death) (Fig. 3.18)... 

This part includes modeling (46,53) for the continuous-stirred tank fermen-tor with/without ethanol-selective membranes. It includes modeling for each of the different feedstocks and each of the different strains of micro-organisms suggested. The biokinetic models and parameters obtained from the literature should be used in the modeling of the continuous-stirred tank fermentor proposed in this part. The membrane flux equations and parameters obtained from the literature should also be used for the membrane continuous-stirred tank fermentor. 

Reaction times of fermentation range from a few hours to several days. Batch processes are common, but continuous stirred tanks also are used either singly or in stages. A continuous stirred tank reactor (CSTR) also is called a chemostat. Figure 8.4 is a schematic of a fermentor with representative dimensions from the literature. 

It may also be economical to remove the inhibitory product directly from the ongoing fermentation by extraction, membranes, or sorption. The use of sorption with simultaneous fermentation and separation for succinic acid has not been investigated. Separation has been used to enhance other organic acid fermentations through in situ separation or separation from a recycled side stream. Solid sorbents have been added directly to batch fermentations (18,19). Seevarantnam et al. (20) tested a sorbent in the solvent phase to enhance recovery of lactic acid from free cell batch culture. A sorption column was also used to remove lactate from a recycled side stream in a free-cell continuously stirred tank reactor (21). Continuous sorption for in situ separation in a biparticle fermentor was successful in enhancing the production of lactic acid (16,22). Recovery in this system was tested with hot water (16). 

The term fermentation is used to describe the biological transformation of chemicals. In its most generic application, a fermentor may be batch, continuous-stirred tank (chemostat), or continuous plug flow (immobilized cell). Most industrial fermentors are batch. Several configurations exist for these batch reactors to facilitate aeration. These include sparged tanks, horizontal fermentors, and biological towers. 

O Brien and Craig (1996) used commercially available polydimethyl-siloxane membrane for EtOH production in a continuous fermentation/membrane PV system. The PV module contained 0.1 m of a commercially available polydimethylsilox-ane membrane and consistently produced permeate of 20%-23% (w/w) EtOH while maintaining a level of 4%-6% EtOH in a stirred-tank fermentor. PV flux and EtOH selectivities were 0.31-0.79 1/m h and 1.8-6.5 respectively. 

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... 

For example, different fermentation schemes have been developed for the production of ethanol. Conventional batch, continuous, cell recycle and immobilized cell processes, as well as membrane, extraction and vacuum processes, which selectively remove ethanol from the fermentation medium as it is formed, were compared on identical bases using a consistent model for yeast metabolism (Maiorella et al., 1984). The continuous flow stirred tank reactor (CSTR) with cell recycle, tower and plug flow reactors all showed similar cost savings of about 10% compared to batch fermentation. Cell recycle increases cell density inside the fermentor, which is important in reducing fermentation cost. 

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