The Workhorse of Biotechnology
The stirred-tank bioreactor is the most widely used vessel for industrial fermentation, from brewing beer to manufacturing monoclonal antibodies. Its conceptually simple design — a cylindrical vessel with mechanical agitation, gas sparging, and temperature control — conceals complex fluid dynamics that critically affect cell growth, product formation, and process economics.
Geometry and Scaling Rules
Standard bioreactor geometry follows well-established rules of thumb: impeller diameter d/D ≈ 1/3, liquid height H/D ≈ 2-3 for microbial processes, four equally spaced baffles of width D/10, and impeller clearance of D/3 from the bottom. These proportions ensure efficient mixing and gas dispersion. Deviating from standard geometry requires careful computational fluid dynamics analysis.
Power and Mixing
Power input determines mixing intensity, oxygen transfer, and heat generation. The dimensionless power number Np relates power to impeller speed and diameter. In turbulent flow (Re > 10,000), Np is constant — about 5.5 for Rushton turbines and 1.7 for pitched-blade turbines. Mixing time scales as the inverse cube root of specific power input, meaning doubling the mixing rate requires an eightfold increase in power.
Shear Considerations
While vigorous mixing benefits oxygen transfer and nutrient distribution, excessive shear stress damages cells. Mammalian cells are 100-1000× more shear-sensitive than bacteria. The maximum shear rate occurs at the impeller tip and scales with tip speed v = πNd. Keeping tip speed below 1.5 m/s for mammalian cells and below 7 m/s for bacteria is a common design guideline.