Continuous fermentation stirred tank fermenters

Ueno, Y., Haruta, S., Ishii, M., and Igarashi, Y. (2001). Changes in product formation and bacterial community by dilution rate on carbohydrate fermentation by methanogenic microflora in continuous flow stirred tank reactor. Appl. Microbiol. Biotechnol. 57, 65-73. 

Kim, S. H., Han, S. K., and Shin, H. S. 2005. Performance comparison of a continuous-flow stirred-tank reactor and an anaerobic sequencing batch reactor for fermentative hydrogen production depending on substrate concentration. Water Sci. Technol., 52 (10-11), 23-29. 

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. 

Fig. 10.5. Continuous stirred tank fermenter, experimental setup with instrumentations and controllers, effluent.

Stirred tank models have been widely used in pharmaceutical research. They form the basis of the compartmental models of traditional and physiological pharmacokinetics and have also been used to describe drug bioconversion in the liver [1,2], drug absorption from the gastrointestinal tract [3], and the production of recombinant proteins in continuous flow fermenters [4], In this book, a more detailed development of stirred tank models can be found in Chapter 3, in which pharmacokinetic models are discussed by Dr. James Gallo. The conceptual and mathematical simplicity of stirred tank models ensures their continued use in pharmacokinetics and in other systems of pharmaceutical interest in which spatially uniform concentrations exist or can be assumed. 

Two continuous stirred-tank fermenters are arranged in series such that the effluent of one forms the feed stream of the other. The first fermenter has a working volume of 100 1 and the other has a working volume of 50 1. The volumetric flowrate through the fermenters is 18 h-1 and the substrate concentration in the fresh feed is 5 g/1. If the microbial growth follows Monod kinetics with //, = 0.25 h-1, Ks = 0.12 g/1, and the yield coefficient is 0.42, calculate the substrate and biomass concentrations in the effluent from the second vessel. What would happen if the flow were from the 50 1 fermenter to the 100 1 fermenter ... 

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. 

Let us consider an ideal continuously stirred tank reactor with constant broth volume. The mass balance equation for substrate as a carbon source (Eq. 27), biomass (Eq. 28) and oxygen in the fermentation broth (Eq. 29) can be given for the liquid phase, as follows [65,66] ... 

Many reviews and several books [61,62] have appeared on the theoretical and experimental aspects of the continuous, stirred tank reactor - the so-called chemostat. Properties of the chemostat are not discussed here. The concentrations of the reagents and products can not be calculated by the algebraic equations obtained for steady-state conditions, when ji = D (the left-hand sides of Eqs. 27-29 are equal to zero), because of the double-substrate-limitation model (Eq. 26) used. These values were obtained from the time course of the concentrations obtained by simulation of the fermentation. It was assumed that the dispersed organic phase remains in the reactor and the dispersed phase holdup does not change during the process. The inlet liquid phase does not contain either organic phase or biomass. 

Stirred-tank reactors can be used for continuous fermentation, because cells can grow in this type of fermentors 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 in the feed medium, but this makes the operation more difficult. 

The shape of the performance curve for a continuous stirred-tank fermenter is dependent on the kinetic behaviour of the micro-organism used. In the case where the specific growth rate is described by the Monod kinetic equation, then the productivity versus dilution rate curve is given by equation 5.137 and has the general shape shown by the curve in Fig. 5.58. However, if the specific growth rate follows substrate inhibition kinetics and equation 5.65 is applicable then, at steady state, equation 5.131 becomes ... 

Fig. 5.59. Performance curves for a continuous stirred-tank fermenter with substrate...

It is frequently desirable, particularly in the field of waste-water treatment, to operate a continuous fermenter at high dilution rates. With a simple stirred-tank this has two effects—one is that the substrate concentration in the effluent will rise, and the other is that such a system in practice tends to be unstable. One solution to this problem is to use a fermenter with a larger working volume, but an alternative strategy is to devise a method to retain the biomass in the fermenter whilst allowing the spent feed to pass out. There are several methods by which this may be achieved (see Fig. 5.60), and the net effect is the same in each case, but the analysis might... 

The more generalised case of several continuous stirred-tank fermenters in series may be analysed by considering N such fermenter vessels, each of volume V and with fresh feed introduced to the first tank at a volumetric flowrate F (see Fig. 5.63). Because no streams enter or leave intermediately, the flowrates between stages and of the final product will also be F. 

In contrast to the batch fermentation based methods of determining kinetic constants, the use of a continuous fermenter (Fig. 3.71) requires more experiments to be performed, but the analysis tends to be more straightforward. In essence, the experimental method involves setting up a continuous stirred-tank fermenter to grow the micro-organisms on a sterile feed of the required substrate. The feed flowrate is adjusted to the desired value which, of course, must produce a dilution rate below the critical value for washout, and the system is allowed to reach steady state. Careful measurements of the microbial density X, the substrate concentration S, and the flowrate F are made when a steady state has been achieved, and the operation is then repeated at a series of suitable dilution rates. 

It has been shown (equation 3.127) that the material balance for substrate across a continuous stirred-tank fermenter gives ... 

The steady-stale substrate and biomass concentrations for a continuous stirred-tank fermenter operated at various dilution rates are given below. Given that the fresh feed concentration is 700 mg/l, calculate the values of the Monod constants and Ks the yield coefficient, Y and the endogenous respiration coefficient kd. 

Microbial populations can be maintained in a state of exponential growth over a long period of time by using a system of continuous culture. Figure 6.7 shows the block diagram for a continuous stirred-tank fermenter (CSTF). The growth chamber is connected to a... 

Fig. 6.7 Schematic diagram of continuous stirred-tank fermenter (CSTF)...

Let s examine the stability of recombinant cells in the continuous stirred-tank fermenter. The material balance for the plasmid-carrying cells around a CSTF yields... 

The production of substances that preserve the food from contamination or from oxidation is another important field of membrane bioreactor. For example, the production of high amounts of propionic acid, commonly used as antifungal substance, was carried out by a continuous stirred-tank reactor associated with ultrafiltration cell recycle and a nanofiltration membrane [51] or the production of gluconic acid by the use of glucose oxidase in a bioreactor using P E S membranes [52]. Lactic acid is widely used as an acidulant, flavor additive, and preservative in the food, pharmaceutical, leather, and textile industries. As an intermediate product in mammalian metabolism, L( +) lactic acid is more important in the food industry than the D(—) isomer. The performance of an improved fermentation system, that is, a membrane cell-recycle bioreactors MCRB was studied [53, 54], the maximum productivity of 31.5 g/Lh was recorded, 10 times greater than the counterpart of the batch-fed fermentation [54]. 

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. 

When maximum concentration of enzyme is attained in the fermenter, the broth may be filtered, the solid washed thoroughly with water, and the enzyme-containing cell-mass used directly in isomerization reactions. Takasaki and Kamibayashi described a heat-treatment system that was applied to a Streptomyces fermentation-broth before filtration, and that fixed the isomerase within the cell structures by inactivating lytic enzymes that would otherwise cause the isomerase enzyme to be leached out of the cell when added to substrate in the isomerization reactors.44 The cell-fixed isomerase can be used in stirred-tank or fixed-bed reactors through which the D-glucose solution to be isomerized may be passed continuously while the enzyme that is fixed within the cell material is retained in the reactor. 

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