Fermentation oxygen transfer products

The production of PHA by fermentation processes has been intensively studied over the last 30 years. During production of PHA in the fermenter, mixing of gaseous and liquid phase and biomass is important which can be achieved by means of aeration and agitation. For aerobic fermentation, oxygen transfer is a key variable and is a function of aeration and agitation (Castilho et al., 2009). Therefore, it is necessary to establish optimum combination of aeration and agitation for maximum production of PHA. 

Herbst, H., A. Schumpe, and W. Deckwer (1992). Xanthan production in stirred tank fermenters oxygen transfer and scale up, Chem. Eng. Technol, 15, 425-434. 

Fermentation biomass productivities usually range from 2 to 5 g/(l h). This represents an oxygen demand in the range of 1.5 to 4 g 0/(l h). In a 500-m fermenter, this means achievement of a volumetric oxygen transfer coefficient in the range of 250 to 400 h"f Such oxygen-transfer capabihties can be achieved with aeration rates of the order of 0.5 (volume of air at STPA ohime of broth) and... 

The common indices of the physical environment are temperature, pressure, shaft power input, impeller speed, foam level, gas flow rate, liquid feed rates, broth viscosity, turbidity, pH, oxidation-reduction potential, dissolved oxygen, and exit gas concentrations. A wide variety of chemical assays can be performed product concentration, nutrient concentration, and product precursor concentration are important. Indices of respiration were mentioned with regard to oxygen transfer and are particularly useful in tracking fermentation behavior. Computer control schemes for fermentation can focus on high productiv-... 

A strain of Azotobacter vinelandii was cultured in a 15 m3 stirred fermenter for the production of alginate. Under current conditions the mass transfer coefficient, kLa, is 0.18 s. Oxygen solubility in the fermentation broth is approximately 8 X 10 3 kgm-3.9 The specific oxygen uptake rate is 12.5 mmol g 1 h. What is the maximum cell density in the broth If copper sulphate is accidentally added to the fermentation broth, which may reduce the oxygen uptake rate to 3 mmol g 1 h 1 and inhibit the microbial cell growth, what would be the maximum cell density in this condition ... 

Fed-batch fermentations with P. oleovorans have been carried out using octa-nol and octanoate as substrate [53]. To ensure high oxygen transfer rates pure oxygen was used. With octanoate as substrate 41.8 g 1 1 biomass with a cellular PHA content of 37 % and a productivity of 0.34 g l"1 were reached. Higher biomass concentrations could not be achieved due to accumulation of the toxic octanoate. 

Viscosity The viscosity of a broth is monitored by a rotational viscometer in some fermentation processes that use filamentous bacteria such as in antibiotics production where the viscosity of the culture broth increases as fermentation progresses. Increase in the viscosity of a culture broth results in a decrease in the oxygen transfer rate and the increase of power consumption. 

Submerged fermentations are mostly operated in batch processes but can also be run continuously in certain cases (continuous fermentation). Batch fermentations may last up to 10 days. Following the fermentation the flavour raw material is extracted from the fermentation broth. In industrial fermentations typically cell counts of 10-30 g/1 are obtained. For a profitable cost/efficiency relation a product yield of 20-30 g/1 has to be achieved. Aerobe fermentations require oxygen transfer rates to the fermentation broth of about 100 mmol/1 per hour. Depending on the viscosity of the media 0.75-2.5 KW stirring power has to be applied for each m of fermentation broth. 

From the mass transfer point of view, airlift fermenters should be designed and operated in such a way that carryover of air from the riser into the downcomer is kept as low as possible. Gas in the downcomer liquid contributes little to oxygen transfer. It reduces the effective density difference between the contents of the riser and downcomer, however, which reduces the liquid circulation rate and also impairs the mixing performance of the fermenter. In fermentations where even a momentary lack of oxygen can very seriously affect productivity, some air is essential to maintain aerobic conditions and sustain fermentation in the downcomer. 

The highest KlA reported by Nicholson et al. [39] was 320 mmol O2L" h (atm 02) . At an oxygen pressure of 0.23 atm, a growth rate of 0.04 h and with 40 g L cell mass, the maximum protein productivity supported by this rate of oxygen transfer is 0.152 g (g cells) h This is a significantly higher protein productivity than has been reported, and indicates that, under ideal conditions, a fermentation vessel can deliver sufficient oxygen to the cells. 

It is proposed to increase the solids concentration in the system to 40 g//, which will effectively double the productivity of the fermentation tank itself The oxygen transfer activity of the mixer is lowered, due to the increased viscosity, to 4 mol of oxygen per MJ. 

The DO concentration control along the fermentations was initially accomplished manually. However, the difficulty of that control resulted in great variations in the DO concentration value during the fermentations. That variation was not desired, as the objective was to evaluate the influence of the amount of oxygen transferred and also of its concentration in the medium during the biosurfactant production. This fact justified the implementation of a control system with more efficient equipment connected to a PLC. The difference in the DO profile during fermentation with the manual-controlled system and with the use of the new system can be observed in the Fig. 3. It was noticed that the... 

The batch xylitol fermentations conducted in the bench-scale fermentors were more typical of studies on inhibitor toxicities at high cell densities. It is possible that the xylitol production phase could also be considered at the microscale (100- to 200- xl volume), as this is a microaerophilic process in which low oxygen transfer rates are desirable. While this may not provide an accurate measure of the maximum volumetric productivity for a particular hydrolysate, it may be useful for investigating fundamental toxicological effects and inhibitor synergies on xylitol production. 

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