INGENIEUR
Figure 3 : Experiments showing the suspension of Cytodex microcarriers in determining energy input for suspension ( photo courtesy of Raja Shazrin Shah )
Figure 4 ( a ): Mixing experiments with model fluid in dished-base tank
Figure 4 ( b ): An axial impeller in a dishedbase vessel interpenetration of components , allowing for reactants contact , mass transfer , and good heat distribution to eliminate hot spots or dead zones in the vessel .
When two or more phases exist , the multiple phases have to be well dispersed in each other . In bioprocess , cells or microcarrier beads are solid particles which have to be suspended in the liquid medium . If air is sparged for aerobic systems , the gas has to be dispersed throughout the liquid medium to ensure every cell receives adequate oxygen supply . It has been established that the oxygen mass transfer coefficient in a bioreactor is directly related to the energy imparted into the system .
The bioreactor energy input is a function of impeller speed , diameter and geometry , and liquid density . It is not measured from the power consumed by the motor , but is measured as the energy resulting after friction losses at the shaft bearing . The impeller Power Number is a dimensionless group given by : -
P = N p ρ n 3 D 5
Where P = Power measure after the shaft bearing ,
Watts
N p
= Power Number ρ = Density of liquid , kg / m 3 n 3 = Agitator speed measured in revolutions / second D 5 = Impeller diameters in meters
In animal cell culture , there is a concern about cell damage if the power per unit mass is too high . Studies using microcarrier beads have shown that even in conventional reactors with standard impellers , sufficiently low specific power can be achieved to provide a safe environment for energetically sensitive systems . 7 , 8
Conclusion
As the most commonly employed device for reactors and bioreactors , the stirred vessel has been thoroughly researched over a broad range of conditions encompassing liquid , solid and gas properties , particle size , concentrations , and the overall rheology . While biological components are almost neutrally buoyant and easily suspended in liquids , care is taken to ensure cells integrity during agitation . In the competitive biopharmaceutical processing industry , the stirred vessel has evolved into disposable alternatives with cubical geometry compared to the traditional cylindrical shape . The key components are still the vessel , motor and impeller ; and one state-of-the art Covid-19 vaccine facility has featured the classic axial flow impeller geometry rotating in a cubic vessel 6 which emphasises the reliability and adaptability of the time-tested stirred vessel configuration .
36 VOL 86 APRIL-JUNE 2021