Lecture 6 - Gas Liquid Mass Transfer Flashcards
Derive the equation for transfer of A through a gas boundary layer
Draw diagram
slide 3-9
Factors that affect O2 demand
Cell species
Culture growth
Carbon source in media
Time (batch culture)
Draw a diagram of mass transfer and outline the 8 major steps involved
And comment on the magnitude of their resistances
i) transfer from bubble interior to GL interface
ii) movement across G-L interface
iii) diffusion through stagnant liquid film around bubble
iv) transport through bulk liquid
v) diffusion through stagnant liquid film around cells
vi) Movement across liquid cell interface
vii) diffusion through cell clump or solid particle to the individual cell
viii) transport through cytoplasm to reaction site
How would you go about measuring KLa
It is difficult so measure in situ
Oxygen balance method
- steady state mass balance on gas inlet and outlet
- most reliable method, allows determination from a single point measurement
- can be applied to fermenters during normal operations
- but needs very accurate oxygen measurements
Dynamic method - based on an unsteady state mass balance on O2
- low cost of equipment required
Outline the steps used to calculate kla
slide 21-26
What is the equation of the transfer of A through a gas boundary layer
Nag = Kga(Cag - Cagi)
Kg = gas phase mass transfer coefficient a = interfacial area Cagi = concentration at phase boundary Cag = concentration in bulk
What is the equation for the transfer of A through the liquid boundary layer
Nal = Kla(Cali - Cal)
Kl = liquid phase mass transfer coefficient a = interfacial area Cali = concentration at phase boundary Cal = concentration in bulk
How do Cagi and Cali relate?
Under what conditions
At equilibrium (steady state) Cagi = mCali
Where m is the distribution coefficient
Define the terms in Na = Kla(C*al - Cal)
with respect to oxygen uptake in cells
Na = Rate of O2 transfer per unit volume kl = the liquid phase transfer coefficient a = interfacial area C*al = O2 in equilibrium with gas phase; solubility of O2 in broth Cal = O2 in broth
Define the terms in Q = qx
Q = oxygen uptake in whole broth x = cell concentration q = specific oxygen uptake rate (oxygen uptake rate per cell)
How would you eliminate oxygen limitations
dissolved O2 > Ccrit
What is Kla used for
Used to characterise the O2 mass transfer capability of the reactor
What is the relationship between the size of Kla and the ability for the reactor to deliver O2 to he cells
If Kla is small the ability for the reactor to deliver O2 to cells is limited
what is the equation for the maximum cell concentration that can be supported by the reactors mass transfer ability
Xmax = KlaCal*/qo
What is the equation for the mimimum kla required to maintain Cal > Ccrit
Klacrit = qX/Cal* - Ccrit
