生物反应工程教学课件-Assignment-II.pdfVIP

Problems for Chapter 6 6.1 Scale-up of mixing system 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 is operated at 800 rpm. If the density of broth being stirred is close to that of water, what is the upper limit for viscosity of the suspension 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 (N D ) is the same as the laboratory apparatus. How does scale-up affect the maximum i i viscosity allowable for maintenance of turbulent mixing conditions? 6.2 Effect of viscosity on power consumption A cylindrical bioreactor of diameter 3 m has four baffles. A Rushton turbine mounted in the reactor has a diameter one third the tank diameter and in operated at a speed of 90 rpm. The density of the fluid is approximately 1 g cm3. The reactor is used to culture an anaerobic organism that does not require gas sparging. The broth can be assumed Newtonian. As the cells grow, the viscosity of the broth increases. (a) Compare power requirements when the viscosity is: (i) approximately that of water; (ii) 100 times greater than water; 4 (iii) 10 times greater than water. (b) When the viscosity is 1000 times greater than water, estimate the power required to achieve turbulence. 6.3 Electrical power required for mixing Laboratory-scale fermenters are usually mixed using small stirrers with electrical motors rated between 100 and 500 W. One such motor is used to drive a 7 cm diameter turbine impeller is a small reactor containing fluid with the properties