Mixing

Question. Select the CORRECT statements regarding mixing and turbulent eddies in bioreactors:

(a) Turbulent flow is characterized by a wide spectrum of eddy sizes. (b) Large eddies extract kinetic energy directly from the impeller and transfer it down to smaller eddies. (c) Viscous dissipation of kinetic energy into heat occurs primarily at the smallest eddy scale (Kolmogorov scale). (d) High shear in the immediate vicinity of impeller blades can cause shear damage to filamentous fungi and animal cells.

Solution.

1. (a) is the defining feature of turbulence used above — eddies range from impeller/tank scale down to the Kolmogorov microscale — correct.
2. (b) describes the energy cascade exactly: the impeller feeds kinetic energy into the largest eddies first, and it passes down the size spectrum from there — correct.
3. (c) matches step 3 of the cascade: viscous forces only dominate at the smallest (Kolmogorov) scale, which is where dissipation into heat actually happens, not at the large scale — correct.
4. (d) matches the shear-damage mechanism above: small, energetic eddies near the impeller blades generate the highest local shear, damaging rigid fungal hyphae and wall-less animal cells — correct.

Answer: All four statements — (a), (b), (c), and (d) — are correct.

Question. A fermentation of Aspergillus niger produces a broth with viscosity exceeding $100,000\ \text{cP}$ ($100\ \text{Pa} \cdot \text{s}$). Which impeller configuration best maintains bulk liquid motion and prevents stagnant zones near the wall: (a) single Rushton turbine at the bottom, (b) helical ribbon impeller, (c) high-speed marine propeller, or (d) a vibrating disc sparger?

Solution.

1. Filamentous fungi like A. niger grow as mycelial networks or pellets, producing highly viscous, shear-thinning (pseudoplastic) broth — this is the extreme end of the viscosity ladder.
2. At $100,000\ \text{cP}$, flow around any impeller is in the laminar (creeping-flow) regime — turbulent-style impellers (Rushton, propeller) cannot propagate motion past a small cavern around the blade, so most of the tank, especially near the wall, stays stagnant. This rules out (a) and (c).
3. A sparger distributes gas; it does not drive bulk liquid circulation, so (d) is the wrong category of equipment for this question.
4. Helical ribbon spans the full height and diameter of the vessel and stays close to the wall along its entire helix, mechanically dragging the whole cross-section top-to-bottom — the standard impeller for broths past the viscosity where even anchor's bulk turnover is sufficient.

Answer: (b) Helical ribbon impeller.