Continuous fermenters

Candida utilis is grown on sulfite waste Hquor in Western Europe and North America, on sugar cane molasses in Cuba and Taiwan and on ceUulose acid hydrolysates in Eastern Europe and the former Soviet Union. C. ///i/if utilizes hexoses, pentoses, and many organic acids. Sulfite Hquor from hardwoods contains 2—3% fermentable sugars of which 20% are hexoses and 80% pentoses in softwood Hquors the proportions are reversed. The SO2 must be stripped out to allow yeast growth, which is carried out in large, highly-aerated fermentors. Eor continuous fermentations, carried out at pH 4 and 30°C, the dilution rate is 0.27—0.30 (34). 

In the early 1970s open fermentors and the continuous fermenting systems were found to be obsolete. The batch process was going to survive, and many new fermentor constmctions appeared. The cylindroconical fermentor seemed to be the preferred solution for both a single- and a combi-vessel fermentation system, ie, fermentation and 1 agering in the same vessel (Fig. 11). 

Mass balances for one vessel in a series of continuous fermenters... 

In continuous fermentation, maximum productivity equals total productivity since the preparation time and the time in lag phase of growth are small relative to the total fermentation time. 

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

Studies show that the production of 1kg dry biomass requires 2.0 kg sugar, 0.7 kg oxygen, 0.1 kg ammonia, with the liberation of 12,300 k Joules heat. A typical continuous fermentation operates at a dilution rate (D) = 0.2 h 1, with sugar concentration of 3% (w/v) in the incoming medium. With a fermentor of 50 m3 capacity and 90% utilisation of carbohydrate [ie 0.3% (w/v) sugar in the outgoing medium] what would be ... 

Compare a) the minimum OTR and b) heat evolution rate of a continuous fermentation system based on n-alkanes operating at a dilution rate of 0.2 h 1 and a biomass concentration of 13.5 kg m 3, to a similar system based on carbohydrate. You may need to look back a few pages to get the relevant information concerning carbohydrate utilisation (Section 4.7). 

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. 

Table 8.1. Continuous fermentation of dual substrate (glucose, xylose) in ICR at 36 °C...

A high glucose concentration of 150 g l 1 was used in continuous fermentation with immobilised S. cerevisiae the obtained data for sugar consumption and ethanol production with retention time are shown in Figure 8.13. As the retention time gradually increased the glucose concentration chopped, while the ethanol concentration profile showed an increase. The maximum ethanol concentration of 47 g l 1 was obtained with a retention time of 7 hours. The yield of ethanol production was approximately 38% compared with batch data, where only an 8% improvement was achieved. 

Najafpour, G., Organic Acids for Biomass by Continuous Fermentation , Resour. Consent. 17, 187 (1987). (Riley, et al, 1996) (Senthuran, etal, 1997). 

In fermentation for the production of acetic acid, ethyl alcohol is used in an aerobic process. In an ethanol oxidation process, the biocatalyst Acetobacter aceti was used to convert ethanol to acetic acid under aerobic conditions. A continuous fermentation for vinegar production was proposed for utilisation of non-viable A. aceti immobilised on the surface of alginate beads. 

The experiment is accomplished with a 2 litre B. Braun fermenter biostat (Germany) equipped with DO and pH meters. Temperature and level controllers are very sensitive, with highly accurate response from the sensors installed in the vessel. Figure 10.7 shows a perfect continuous fermentation set up used in photosynthetic production. A small modification of... 

All of the above processes are operated as batch fermentations, in which a volume of sterile medium in a vessel is inoculated. The broth is fermented for a defined period. The tank is then emptied and the products are separated to obtain the antibiotic. The vessel is then recharged for batch operation with medium and the sequence repeated, as often as required. Continuous fermentation is not common practice in the antibiotics industry. The antibiotic concentration will rarely exceed 20gT 1 and may be as low as 0.5g-l 1. 

Technology for very large (400,000-gallon) continuous fermenters was developed and is being practiced in the United Kingdom. This development pushes biochemical engineering to hmits not yet explored in the United States. 

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. 

Fed-batch fermentation process is a production technique between batch and continuous fermentation. A proper medium feed rate is required to add sequentially into the fermentor during the process and the product is harvested at the end of fermentation just like a batch type. 

However, it should be noted that there is a small percentage of the total time in which productivity rate is near its maximum. It is sometimes possible to maintain very high rates of products for a long time with continuous fermentation. Although it can get much more productivity from the fermentor, enhancement over batch fermentation in terms of the total volume of fermentor is not high because equipment needs to be sterilized to support the continuous tank. 

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