Bio301 Overview of Topics
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Transcript of Bio301 Overview of Topics
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Bio301 Overview of TopicsIntro
Bioprocessing – Biotechnology: Make money from bioprocesses
Inputs are of lower value than outputs (products)
Computer based learning activities (CBLA) are onhttp://sphinx.murdoch.edu.au/units/extern/BIO301/teach/index.htm
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Lecture overview L1-3
Lecture 1: Intro, study guide, what is a bioreactor,Learning by interacting
Lecture 2: What is diffusion, how can we predict the behaviour of a randomly moving molecule? moving dots, entropy, driving force, equilibrium, rate of diffusion, first order kinetics, kLa
Lecture 3: oxygen transfer rate, kLa value. Graphical method of determining the kLa. Mathematical (2 point) determination of kLACalculation and prediction of oxygen transfer as function of DO. Oxygen transfer efficiency. Bacterial OUR, DO. steady state
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Molecular diffusion relies on random movement resulting in uniform distribution of molecules
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Oxygen Transfer Rate (OTR)Overview
Diffusion, how does it work, how can we predict it?
Diffusion is random …. and yet predictable.A simple model simulation can show that although the diffusion movement is random, it can be precisely predicted for large number of molecules (e.g. Fick’s law of diffusion)
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Oxygen Transfer Rate (OTR)(diffusion, convection)
LowOTR High
OTR
Transfer by diffusion is extremely slowand depends on surface area
Wind
Oxygen transfer by convection(turbulences) is more efficient
Air In
••
•
••• • ••
• • Bioreactors combine maximum convectionwith maximum diffusion
• Course bubbles cause more convection,fine bubbles more diffusion
How soluble is oxygen?11
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The net transfer of oxygen from
•gas phase to solution reaches a dynamic equilibrium
•O2 input = O2 output
•equilibrium results in defined saturation concentration (cs).
•The saturation concentration is also the oxygen solubility
•How soluble is oxygen?
Oxygen solubility (cS)
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Oxygen is not very polar poorly soluble in water.
Oxygen Solubility is described by Henry’s Law which applies to all gases
p = k*cSp = partial pressure of oxygenk = constant depending on gas type, solution
and temperaturecS = concentration of oxygen dissolved in water
• Meaning: The amount of oxygen which dissolves in water is proportional to the amount of oxygen molecules present per volume of the gas phase.
• Partial pressure ~ number of O2 molecules per volume of gasincreases with O2 concentration in gas
increases with total gas pressure
How to calculate partial pressure? (refer to CBLA)
Oxygen solubility (cS)
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Examples of using the proportionality between partial pressure of oxygen in the atmosphere and the saturation concentration cS:
p = k*cSp = partial pressure of oxygenk = constant depending on gas type, solution
and temperaturecS = concentration of oxygen dissolved in water
• If the reactor is operated under 2 times atmospheric pressure (200kPa instead of 100 kPa air pressure), the new saturation concentration will be abou 16 mg/L instead of 8 mg/L.
• If air (partial pressure = 0.21* 100 kPa) is replaced by pure oxygen atmosphere (partial pressure 100kPa) the oxygen saturation concentration is about 40 mg/L (more precise 8*100/21) instead of 8 mg/L.
How to calculate partial pressure? (refer to CBLA)
Oxygen solubility (cS)
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Effect of temperature
Oxygen solubility decreases with increasing temperature.
Overall: oxygen is poorly soluble (8mg/: at room temp.)
More important than solubility is oxygen supply rate (oxygen transfer rate OTR).
0
5
10
15
0 20 40 60
cs =468
(31.6 + T)
Oxy
gen
Sat
ura
tio
n
Co
nce
ntr
atio
n c
s (
mg
/L)
Temperature (°C)
Oxygen solubility (cS)
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Oxygen Transfer Rate (OTR)(gradient, driving force)
Question: What is the driving force for oxygen dissolution?
OTR At oxygen saturation concentration (cs): dynamic equilibrium exists between oxygen transferred from the air to water and vice versa. No driving force
Answer: The difference between cS and the actual dissolved oxygen concentration (cL) is the driving force. OTR is proportional to the that difference. Thus:
OTR ~ (cS – cL)
Need to determine the proportionality factor
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1. Deoxygenation (N2, sulfite + Co catalyst)2. Aeration and monitoring dissolved oxygen concentration (D.O. or cL) as function of time
3. OTR = slope of the aeration curve (mg/L.h or ppm/h)
Significance of OTR: critical to know and to control forall aerobic bioreactors
0 5 10
8 Air On
cL (
pp
m)
Time (min)
OTR – depends on DO (cL)
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OTR = kLa (cs - cL)
Mg/L/h h-1 mg/L
4. Observation: OTR decreases over time (and with incr. cL)
5. OTR is not a good measure of aeration capacity of a bioreactor
6. OTR is highest at cL = zero (Standard OTR)
7. OTR is zero at oxygen saturation concentrations (cs)
8. OTR is negatively correlated to cL
9. OTR is correlated to the saturation deficit (cs - cL),
which is the driving force for oxygen transfer
9. The factor of correlation is the volumetric mass transfer
coefficient kLa
OTR – depends on DO (cL)
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First: steep step in oxygen (top layer saturated, next layer oxygen free)
Then: buildup of a gradient of many layers.
Each layer is only slightly different from the next Transfer from layer to layer has little driving force.
Gradient build-up inhibits fast diffusion
OTR –Significance of gradient
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10. OTR is not a useful parameter for the assessment of the aeration capacity of a bioreactor. This is because it is
dependent on the oxygen concentration (cL)
11. The kLa value is a suitable parameter as it divides OTR by saturation deficit:
12. kLa = the key parameter oxygen transfer capacity. How to determine it?
OTR(cs - cL)
kLa =
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Lec 2 summary:
Oxygen is poorly soluble depending mainly on •partial pressure in headspace•Temperature
OTR is driven and proportional to driving force (cS-cL)
kLa is the proportionality factor (first order kinetics)
kLa describes the performance of a bioreactor to provideOxygen to microbes
Next lecture: quantify kLA
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Lec 3 outlook:
•Aeration curve•Quantify OTR at a given point of an aeration curve•Quick estimate of kLa•Graphical determination of kLa•Mathematical determination of kLa•Run computer simulation to obtain data•Oxygen transfer efficiency (OTE)•OTR proportional to cs-cL•OTR inverse proportional to cL
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OTR – Quick estimate of kLAExample: determine OTR at 6 mg/L
OTR is the slope of the tangent for each oxygen concentration
OTR = ∆ cL/ ∆ t = 5 mg/L/ 4.5 min = 1.1 mg/L/min = 66 mg/L/h
0 5 10
8 Air OncL
(m
g/L
)
Time (min)
6
4.5 min
5 mg/L
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kLa = OTR
= 66 ppm / h
(cs-cL)
= 3.3 h-1
(8 ppm – 6 ppm)
Q: Problem with this method?
A: based on one single OTR slope measurement and unreliable to obtain from real data.
OTR – quick estimate of kLA
Time
DO
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1. Monitor aeration curve
2. Determine graphically the OTR at various oxygen concentrations (cL)
8
2
4
6
0 5 10
3. Tabulate OTR and corresponding cL values
cL (
pp
m)
0.53.04.06.08.0
Time (min)
7.55.04.02.00.0
306050250
cL (mg/L) Cs - cL (mg/L) OTR (mg/L/h)
At 6 ppm: OTR = 25 mg/L/h
At 4 ppm: OTR = 50 mg/L/hAt 3 ppm: OTR = 60 mg/L/h
At 0.5 ppm: OTR = 30 ppm/h
OTR – Graphical determination of kLa
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0
50
100
0 2 4 6 8
4. Plot OTR values as a function of cs - cL.O
TR
(m
g/L
/h) Standard OTR
cs
cs- cL (mg/L)
5. A linear correlation exists between kLa and the saturation deficit (cs - cL) which is the driving force of the reaction.
6. The slope of the plot OTR versus cs - cL is the kLa value.
7. The standard OTR (max OTR) can be read from the intercept with the cs line. (Standard OTR = 100 ppm/h)
kLa = = 12 h-170 mg/L/h
6 mg/L
OTR – Graphical determination of kLa
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Mathematical Determination of kLa1. OTR is a change of cL over time, thus = dcL/dt
Integration gives2. kLa = dcL/dt
(cs- cL)
( )cs - co3. kLa = ln cs - ci
ti - to
( )8 - 3 ppm kLa = ln 8 - 6 ppm
10.5 - 6.1 min
= 0.21 min-1 = 12.5 h-1= ln 2.5
4.4 min
•
•D
isso
lved
Oxy
gen
Con
cent
ratio
n (m
g/L)
ci = 6
co = 3
to ti Time (min)
cs
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4. This method should be carried out for 3 to 4 different intervals. By aver
5. Once the kLa is known it allows to calculate the OTR at any given oxygen concentration:
OTR = kLa (cs - cL)
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Factors Affecting the Oxygen Transfer Coefficient kLa
kLa consists of:
• kL = resistance or thickness of boundary film• a = surface area
Bubble BulkLiquid
Cell
[Oxy
gen
]
Distance
Main boundary layer = steepest gradient→ rate controlling, driving force 29
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Effect of Fluid Composition on OTR
The transfer across this boundary layer increases with:
1) ↓ thickness of the film, thus ↑ degree of shearing (turbulence)
2) ↑ surface area
3) ↓ surface tension
4) ↓ viscosity (best in pure water)
5) ↓ salinity
6) ↓ concentration of chemicals or particles
7) detergents?
8) ↑ emulsifiers, oils, “oxygen vectors”
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Oxygen Transfer Efficiency (OTE)
OTE =oxygen transferred
oxygen supplied
Significance of OTE: economical, evaporation
Calculation of OTE (%):
% OTE =oxygen transferred (mol/L.h)
oxygen supplied (mol/L.h)X 100
Why do students find this type calculation difficult?Units are disregarded. Molecular weights are misused.
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Oxygen Transfer Efficiency (OTE)
A bioreactor ( 3 m3) is aerated with 200 L/min airflow. If the OTR is constant (100 mg/L/h) determine the %OTE.
1. Convert the airflow into an oxygen flow in mmol/L/h
200 L air /min = 12000 L air/h
= 2520 L O2/h
= 102.9 mol O2/h
= 34.3 mmol O2/L.h
(x 21%)
(÷ 24.5 L/mol)
(÷ 3000 L)
2. OTR
(x 60)
100 mg/L.h = 3.1 mmol O2/L.h (÷ 32 g/mol)
% OTE =3.1 (mmol/L.h)
34.3 (mol/L.h)X 100
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Oxygen Transfer Efficiency (OTE)
OTE is dependent upon the cL in the same way than OTR
OTE decreases with increasing airflow(more oxygen is wasted)
% O
TE
5
10
Airflow
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Engineering Parameters Influencing OTR
Increase depth vessel
Decrease bubble size
Increase air flow rate
Increase stirring rate
Deeper vessel bubbles rise a long way ↑ OTR, OTE but more pressure required ↑ $$
Larger surface area ↑ OTR, OTEsmaller bubbles rise slower more gas hold up ↑ OTR, OTE
↑ Number of bubbles ↑ OTR but ↓ OTE
↑ turbulence ↓ thickness of boundary layer ↑ OTR, OTE
↓ Bubble size ↑ OTR, OTE 34
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Rate is proportional to concentration First order kinetics
Slope = kLa
max OTR
OT
R (
mg
/L.h
)
Dissolved oxygen [mg/L]
OTR = kLa (O2 saturation (cS) – O2 concentration (cL))
(first order kinetics)
Aeration Curve
Time
Dis
solv
ed O
xyg
enAir on(cs)
OTR – from aeration curve to kLa summary
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During aeration of oxygen free water, the dissolved oxygen increases in a characteristic way
OTR – Aeration curve from CBLA
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Can the relationship between rate and DO be expressed mathematically?
•Highest Rate at lowest dissolved oxygen concentration
•Rate of zero when DO reaches saturation concentration
OTR – aeration curve from CBLA
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