Question

Microcarrier beads 120 μm in diameter are used to culture recombinant CHO cells for production of growth hormone. It is proposed to use a 6-cm turbine impeller to mix the culture in a 3.5-1itre stirred tank. Air and carbon dioxide are supplied by flow through the reactor headspace. The microcarrier suspension has a density of approximately 1010 kg m^-3 and a viscosity of 1.3 x 10^-3 Pa s. Estimate the kinematic viscosity.

(a) 1.30 × 10^-6 m^2s^-1

(b) 1.29 × 10^-6 m^2s^-1

(c) 1.50 × 10^-6 m^2s^-1

(d) 1.49 × 10^-6 m^2s^-1

This question was posed to me by my school principal while I was bunking the class.

The query is from Role of Shear in Stirred Fermenters in portion Fluid Flow and Mixing of Bioprocess Engineering

Answer 1

Right option is (b) 1.29 × 10^-6 m^2s^-1

Easiest explanation: Damage due to eddies is avoided if the \(K \ddot{o}\) 1mogorov scale remains greater than 2/3-1/2 the diameter of the beads. Let us determine the stirrer speed required to create eddies with size, λ = 2/3 (120 μm) = 80 μm = 8 x 10^-5 rn. The stirrer power producing eddies of this dimension can be estimated:

Kinematic viscosity, v = \(\frac{\mu}{\rho} = \frac{1.3×10^{-3} kg \,m^{-1} s^{-1}}{1010 \,kgm^{-3}}\) = 1.29 × 10^-6 m^2s^-1.