Fed-batch cycle

Monitoring of large-scale fed-batch manufacture of baker s yeast was also possible with the electronic nose [33]. The cultivation took place in a 200-m3 bubble-column reactor. The monitoring procedure is complicated by the large phase variation and circulation times in the bioreactor. On the 200-m3 scale, ethanol and biomass were predicted but with lower accuracy than in the laboratory (10%). The data was compensated for increasing reactor liquid volume and aeration rate during the fed-batch cycle, simply by including these variables in the inputs to the ANN. 

Irrespective of the method employed, this procedure need only be carried out once at the beginning of the first batch cycle. Once product of the correct CSTR composition is produced, a portion may be used to seed further fed-batch cycles, and the sequence is self-sustaining as a result. 

Observe that the concentrations start at the CSTR seed concentration, and slowly approach the CSTR concentration associated with t str = 0.65h. It appears that a fed-batch reaction time of approximately 3.0 h is sufficient to approximate the steady-state CSTR concentration. Once this concentration has been achieved, a fraction of the product volume may be spared and used for seeding of subsequent fed-batch cycles. 

The total internal resistance can also be obtained from a power curve from the notion that at maximum power the internal resistance is equal to the external resistance. Consequently, the internal resistance can be calculated with J int = P/I -Recording a polarization curve in a BES needs to be performed with some care. Researchers have recorded curves with multiple fed-batch cycles and one external resistor per cycle, during one fed-batch cycle with multiple resistors, and during continuous-mode operation [70]. Therefore, it is advisable that before comparing results, the experimental conditions should also be taken into account This was excellently shown by Winfield et al., who analyzed the effect of scan rate, biofilm maturity and feedstock concentration on the polarization curve [71, 72]. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

At the beginning of the batch cycle, both the reactor liquid and the jacket water are at 203°F. At this point in lime, catalyst is added to the reactor and a reaction occurs which generates heat at a constant rate of 15,300 Btu/min. At this same moment in time, makeup cooling water at 68°F is fed into the jacket at a constant 832 lb ,/min flow rale. 

The bioreactor operation mode is normally defined at the outset of process configuration. Insect cells have been cultured in almost all known cultivation modes batch [10], repeated-batch [70], perfusion [71-74], fed-batch [75, 76], semi-continuous [77,78] and continuous [79]. In spite of this multitude of different strategies, the batch or, eventually, fed-batch mode is normally preferred due to the lytic infection cycle of the baculovirus. 

In many enzyme fermentations, the limiting component, usually the C-source, has to be added semi-continuously to keep its concentration at a predetermined, usually low, value. This measure makes it possible either to influence selectivity between different pathways or to uncouple predominantly cell growth during the first phase of the fermentation from predominantly product (i.e., enzyme) formation in the later stages of the fermentation cycle. Often, protein formation is induced by adding an inducer (see Chapter 4). During the fed-batch phase, the broth volume increases. Either the broth is harvested when the maximum volume is reached, or broth is withdrawn from time to time. The product is present in high concentrations. 

A semi batch reactor comes in two flavors. The first is when some material is fed to the reactor during the batch cycle. There is an initial charge of some of the reactants, but the rest of the reactants or catalysts are continuously fed into the reactor during the cycle. This is called a fed-batch reactor. Many batch polymerization reactors operate in the fed-batch mode, with monomer fed into the reactor during the batch cycle. The fed-batch reactor has the inherent advantage that the concentration of the limiting reactant (or catalyst) can be kept low enough to prevent runaway reactions. 

One important issue of the fed-batch operation is the variable volume of material in the reactor and its effect on heat transfer area. If jacket cooling is used, the heat transfer area covered by the liquid in the reactor will be proportional to the volume of the liquid at any point in time. However, if the reaction liquid is circulated through an external heat exchanger, the full heat transfer area is available throughout the batch cycle. 

Once the vessel is full, the fresh feed is cut off. The setpoint of the second controller is raised to 340 K at this point in time, which is about 130 min into batch. The batch cycle is stopped when the conversion reaches 98%. This takes about 353 min. Figure 4.17 gives a Matlab program for this fed-batch system. 

Fig.3 Optimised fed-batch data showing (a) viable cell(Xv) concentration and (b) antibody (MAb) concentration for cell cycle-arrest at 126h and two other cell cycle-arrest times at 78h and 96h. Simulation results are shown by lines.

The model-based dynamic optimisation results that were obtained from a fixed feed composition, same initial condition as the batch culture, and a feeding interval of 6-12 h, suggested an optimal cell cycle-arrest time at 126 h and supplementation with feed from 48 h onwards. The results of three different fed-batch cultures with identical supplementation strategies but various cell cycle-arrest times are shown in Fig.3. The viable cell concentration, Xy, was closely predicted up to about 80 h. However, after lOOh, Xv decreased significantly in all three cultures. The predicted MAb concentration was in accordance with the experimental results with only a slight under-prediction around 80-100 h. Both model predictions and experimental results indicated a small difference in MAb yield when the cultures were arrested at different times. The optimised fed-batch experiments involved a total of 9 shake flask cultures so the... 

K. Uchiyama and S. Shioya, 1999. Modeling and optimization of a-amylase production in a recombinant yeast fed-batch culture taking account of the cell cycle population distribution. Journal of Biotechnology, 71, 133-141. 

The Mitsubishi Chemical Company has described a process for the commercial production of L-aspartate using an cx-amino-zr-butyric acid resistant mutant of B. flavum [11]. The enzyme is moderately thermal resistant, allowing the process to be run at 45°C. The process is run using immobilized cells in a fed batch system in which the biocatalyst is recycled [4]. An initial problem was the conversion of fumarate to malic acid by an intracellular fumarase activity, which led to low l-aspartic acid yields during the first cycle. This problem was circumvented by preheating the biocatalyst for 1 hour at 45°C, which completely destroyed the fumarase activity [4,11]. Recently, the aspartase gene from B. flavum has been cloned [28] and has presumably been used to improve the efficiency of this process. 

Where the product is secreted, it should form the major proteinaceous component of the harvest broth. The best time to harvest wiU depend on the dynamics of protein expression, secretion and stability, but in general it is beneficial to seek an appropriate harvesting window in the production cycle where product expression and accumulation are maximized but degradation is Hmited. For example, in batch or fed-batch processes, peak production of the recombinant protein often occurs in the exponential phase, and begins to de-cHne at the point where the cell mass and total protein content of the culture reach... 

For structure 2, three distinct reaction intervals are needed, two of which are fed-batch operations. The cycle is initiated with a constant a interval, followed by a nonconstant a interval, and completed with a period of standard batch reaction. This is given in Figure 7.5(b) by the path ADBC. 

For Cd = 0.3 mol/L, the recommended operating policy is structure 2. This structure is different compared to structure 1 in that three reaction periods are required. The batch cycle begins with a fed-batch period of constant a in accordance with the equilibrium CSTR concentration, given by point D. The cycle is then brought into a period of varying a, where the sidestream addition is observed to increase sharply to a maximum value of approximately 1.356 s and then completed with a standard batch period lasting approximately 9.24 s the total batch cycle time for this structure is thus roughly 15 s. 

The feeding rate policies for the fed-batch reactors may then be found from the critical DSR a policies, whereas the batch cycle times are found from the equivalent reactor residence times under continuous operation. 

The fed-batch and batch cultivation systems share the same cleaning and sterilization process in which the bioreactor operation is stopped and the bioreactor is emptied. This stoppage creates considerable costs and operational downtime. The repeated or cyclic system, which can be applied to both batch and fed-batch cultivation systems, may be installed in order to maximize the productivity. The cyclic cultivation system does not enter the cleaning and sterilization process, but rather empties a portion of the bioreactor while preserving part of the batch for the next cycle. Another method to increase productivity is cell retention techniques such as fluidized beds, membranes, or external separators. These options allow multiple cycles without cleaning and sterilization, which is initiated only if it is deemed that mutation risks exceed tolerable levels (Bellgardt, 2000b). 

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