Agitation and Aeration Questions

• Effect of surfactants in aerated bioreactors: Adding small amounts of detergents (surfactants) changes bubble behavior. Why can oxygen transfer rates increase in such systems?
• Gas hold-up measurement in aerated vessels: If Zf is the height of the aerated (gassy) liquid and Zl is the height of the non-aerated clear liquid, what is the expression for the average fractional gas hold-up H?
• Preventing surface vortex formation: Which measures effectively prevent vortexing in stirred tank reactors during bioprocess agitation?
• Mixing hardware — In fluid mixing terminology, a typical household ceiling fan would be best described as which kind of impeller? (Consider the primary flow direction it generates in air or liquid.)
• Interphase mass transfer — Which of the following is NOT one of the three best-known classical theories for gas–liquid interphase mass transfer?
• Gas–liquid equilibrium — Henry’s law relates which pair of oxygen variables in bioprocessing at constant temperature?
• Mixing Reynolds number — The transitional (sometimes called transient) flow range for impeller Reynolds number Re_i in stirred tanks corresponds to which interval?
• Aeration in stirred tanks — Why does a flooded impeller often lead to poor oxygen transfer rates (low k_La)?
• Impeller construction — A (radial) turbine impeller is typically built with flat blades having which configuration relative to the hub and disk?
• Measuring mixing power — Which instrument(s) can be used to assess power consumption in a stirred vessel?
• Path of oxygen to cells — The rate-limiting step for oxygen moving from a gas bubble to a cell in a bioreactor is typically diffusion across which zone?
• Preventing swirl and vortex — In agitated tanks, which design choices are effective at suppressing swirling and vortex formation?
• Gas holdup in fermenters — The volumetric gas fraction (gas hold-up) depends mainly on which operating parameter(s)?
• Stirred-Tank Reactor (STR) Cooling — A coolant is pumped through an internal cooling coil to remove heat. Which statement about improving the rate of cooling is NOT correct?
• Mixing Reynolds Number in STRs — With liquid density and viscosity constant, how does the impeller Reynolds number depend on the impeller diameter (Dtank fixed)?
• Oxygen Transfer Sizing — A microbial population has μm = 0.2 h^-1, KO = 0.2 mg O2/L, YX/O2 = 0.5 mg dry weight per mg O2, and a critical dissolved oxygen Ccrit = 0.8 mg/L. For a required cell concentration X = 1000 mg/L and saturation concentration C* = 5.8 mg/L, the minimum required kLa must be greater than:
• Radial-Flow Impellers in Bioreactors — Which statements correctly describe their hydrodynamics and typical application?
• Standard Geometric Ratio — In many STR designs, what is the typical ratio of impeller diameter to tank diameter (Da : Dt)?
• Cylindrical Reactor Geometry — If the liquid height is HL and the tank diameter is Dt, which expression correctly gives the liquid volume VL (assume a right circular cylinder)?
• Impeller Selection — Which impeller type is commonly used across a wide range of viscosities in mixing and bioprocess applications?