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• Rooth Adajet

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Examples and problems: 1. Consider the scale-up of a fermenter producing E-coli from nominal volume of 10L to 10m3 with aspect ratio of 3. Impeller diameter is 30% of tank diameter. Assume geometric similarity. If agitator speed is 500 rpm with 3 rushton-turbine impellers, determine dimensions of the large fermenter and agitator speed for constant; a. Po/VL b. Tip speed c. NRe d. Decide which operating condition is best suitable for e-coli and for a CHO cell line e. Compare mixing speed if lab-scale is changed to 1 impeller-system with 500 rpm vs. 3 impellersystem. Determine speed of a 3-impeller system. 2. A fermentation broth with viscosity 10-2 Pa.s and density 1000 kg/m3 is agitated in a 2.7m3 working volume using rushton turbine impellers with diameter 0.5m and stirrer speed of 1 s-1. a. Estimate mixing time. b. If speed is increased by 30%, what is the new mixing time? 3. To ensure turbulent conditions and minimum mixing time during agitation with a turbine impeller, the Reynolds number must be at least 104. a. A stirred laboratory-scale fermenter with a turbine impeller 5 cm in diameter. If the density of broth being stirred is close to that of water with 3.33cp, what is the minimum impeller speed necessary if adequate mixing is to be maintained? b. The mixing system is scaled up so the tank and impeller are 15 times the diameter of the laboratory equipment. The stirrer in the large vessel is operated so that the stirrer tip speed (tip speed = pi. N.Di) is the same as in the laboratory apparatus. How does scale-up affect the maximum viscosity allowable for maintenance of turbulent mixing conditions? 4. A cylindrical bioreactor diameter 3m has 4 baffles. A rushthon turbine mounted in the vessel has a diameter 1/3 of tank diameter and is operated at a speed of 90 rpm. The density of the fluid is similar to water. The reactor is used to culture an anaerobic organism in which broth initially can be assumed Newtonian but as cells grow, viscosity increases. Compare power requirements when viscosity is; a. Equal to water b. 100x greater than water c. 104x greater than water 5. Laboratory-scale fermenters are usually mixed using small stirrer motors between 100-500W. Once such motor is used to drive a 10-cm turbine impeller in a small reactor containing fluid with the properties of water. The stirrer speed is 900rpm. Estimate power requirements for this process if the actual stirrerdrive is a 300-W motor. a. How do you explain the difference between the amount of electrical power consumed by motor and power delivered to the fluid?

6. A cylindrical stirred bioreactor with diameter and height of liquid to 2m has a rushton turbine 1/3 of tank diameter. The fermentation broth is a Newtonian fluid with viscosity 4cp. a. If the specific power consumption must not exceed 1.5kW/m3, determine the maximum allowable stirrer speed. What is the mixing time under these conditions?