Continuous fermentation steady state

J J Explain what is meant by a steady state for a continuous fermentation. 

Despite the advantages of continuous cultures, the technique has found little application in the fermentation industry. A multi-stage system is the most common continuous fermentation and has been used in the fermentation of glutamic add. The start-up of a multi-stage continuous system proceeds as follows. Initially, batch fermentation is commenced in each vessel. Fresh medium is introduced in the first vessel, and the outflow from this proceeds into the next vessel. The overall flow rate is then adjusted so that the substrate is completely consumed in the last vessel, and the intended product accumulated. The concentration of cells, products and substrate will then reach a steady state. The optimum number of vessels and rate of medium input can be calculated from simple batch experiments. 

There is an interior optimum. For this particular numerical example, it occurs when 40% of the reactor volume is in the initial CSTR and 60% is in the downstream PFR. The model reaction is chemically unrealistic but illustrates behavior that can arise with real reactions. An excellent process for the bulk polymerization of styrene consists of a CSTR followed by a tubular post-reactor. The model reaction also demonstrates a phenomenon known as washout which is important in continuous cell culture. If kt is too small, a steady-state reaction cannot be sustained even with initial spiking of component B. A continuous fermentation process will have a maximum flow rate beyond which the initial inoculum of cells will be washed out of the system. At lower flow rates, the cells reproduce fast enough to achieve and hold a steady state. 

The term chemostat refers to a tank fermentation which is operated continuously. This bioreactor mode of operation normally involves sterile feed (Xo=0), constant volume and steady state conditions, meaning that dV/dt=0, d(VSd/ dt=0, d VX1)/dt=0. 

A continuous fermenter with sterile feed is referred to as a chemostat. For constant volume operation, the inlet volumetric flow rate is equal to that at the output. With this model chemostat start-up, resultant steady state behaviour and cell washout phenomena are easily investigated by simulation. 

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. 

Calculate the steady-state substrate and biomass concentrations in a continuous fermenter which has an operating volume of 25 1 when the sterile feed stream contains limiting substrate at 2000 mg/1 and enters the vessel at 8 1/h. The values of Ks and ftm are 10.5 mg/l and 0.45 h " respectively, and the yield coefficient may be taken to be 0.48. 

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 ... 

Two continuous stirred-tank fermenters are connected in series, the first having an operational volume of 100 1 and that of the second being 50 1. The feed to the first fermenter is sterile and contains 5000 mg/1 of substrate, being delivered to the fermenter at 18 1/h. If the microbial growth can be described by the Monod kinetic model with /x, = 0.25 h l and Ks = 120 mg/l, calculate the steady-state substrate concentration in the second vessel. What would happen if the flow were from the 50 I fermenter to the 100 1 fermenter ... 

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. 

In reality, the PFF fermenter is hard to be found. However, the packed-bed fermenter and multi-staged fermenter can be approximated as PFF. Even though the steady-state PFF is operated in a continuous mode, the cell concentration of an ideal batch fermenter after time t will be the same as that of a steady-state PFF at the longitudinal location where the residence time i is equal to t (Figure 6.4). Therefore, the following analysis applies for both the ideal batch fermenter and steady-state PFF. 

If all cells are recycled back into the fermenter, the cell concentration will increase continuously with time and a steady state will never be reached. Therefore, to operate a CSTF with recycling in a steady-state mode, we need to have a bleeding stream, as shown in Figure 6.19. The material balance for cells in the fermenter with a cell recycling unit is... 

Fig. 7.9 The effect of dilution rate (D) on the fraction of plasmid-carrying cells during steady-state operation of CSTF. The initial value of f was calculated by assuming that the number of generations required for the step inoculation and initial batch and unsteady-state continuous fermentation was 20.

Several special terms are used to describe traditional reaction engineering concepts. Examples include yield coefficients for the generally fermentation environment-dependent stoichiometric coefficients, metabolic network for reaction network, substrate for feed, metabolite for secreted bioreaction products, biomass for cells, broth for the fermenter medium, aeration rate for the rate of air addition, vvm for volumetric airflow rate per broth volume, OUR for 02 uptake rate per broth volume, and CER for C02 evolution rate per broth volume. For continuous fermentation, dilution rate stands for feed or effluent rate (equal at steady state), washout for a condition where the feed rate exceeds the cell growth rate, resulting in washout of cells from the reactor. Section 7 discusses a simple model of a CSTR reactor (called a chemostat) using empirical kinetics. 

The sodium sulfite feed technique has been proposed for measuring the absorption rate in large tanks (fermenters) [228]. 1 x 10 mol/1 of COSO4 as catalyst was added to the test liquid and during aeration a 0.1-0.3 M sodium sulfite solution was continuously fed in. It is thus a steady-state physical absorption in a materia] system, whose coalescence condition corresponds to that of pure water, as long as the salt concentration does not exceed 0.5 g/1. 

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