aeration and agitation
TRANSCRIPT
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Course outline9.1. Introduction9.2. Basic Mass-Transfer Concepts 9.3. Correlation for Mass-Transfer Coefficient9.4. Measurement of Interfacial Area9.5. Correlations for a and D329.6. Gas Hold-Up9.7. Power Consumption9.8. Determination of Oxygen-Absorption Rate9.9. Correlation for kLa9.10. Scale-Up9.11. Shear-Sensitive Mixing
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Typical Bioprocessing Stock Culture Raw Materials
Shake Flasks
Seed Fermenter
Medium Formulation
Sterilization
Fermenter
Recovery
Purification Products
Air
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Aeration and Agitation Important factor in a fermenters Provision for adequate mixing of its contents
Mixing in fermentation
to disperse the air bubbles to suspend the cells
to enhance heat and mass transfer in the medium
All relate to Gas-liquid mass transfer
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Definition of FermentationEarly
The production of alcohol and lactic acidsGeneral
Anaerobic microbial conversion processesIndustrial
All microbial conversion processes including aerobic cultivations.
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Introduction Aeration Aeration refers to the process of introducing air
to increase oxygen concentration in liquids Aeration may be performed by bubbling air
through the liquid, spraying the liquid into the air or agitation of the liquid to increase surface absorption
Gas-liquid mass transfer in bioreactors
http://www.foodprocessing-technology.com/glossary/aeration.html
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IntroductionBackground The aerobic fermentation: the primary method of product formation very few anaerobic fermentation (lactic acid bacteria) Supplying oxygen to aerobic cells: a significant challenge The problem: oxygen is poorly soluble in water The solubility of oxygen in pure water is 8 mg/L at 4oC (sucrose is soluble to 600 g/L) The solubility of oxygen decreases as with increasing temperature and concentration of solutes
in the solution
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IntroductionBackground The factors affect oxygen transfer How fermentation systems can be designed to maximize dissolved oxygen concentration in
bioreactors The supply of oxygen the rate limiting step in an aerobic fermentation
Satisfy oxygen demands constitute a large proportion of the operating and capital
cost of a industrial scale fermentation system
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Introduction Gas exchange and mass transfer (Crueger and Crueger, 1990)
The most critical factors in the operation of a large scale fermenter is the provision of adequate gas exchange.
Oxygen is the most important gaseous substrate for microbial metabolism
Carbon dioxide is the most important gaseous metabolic product.
When oxygen is required as a microbial substrate, it is frequently a limiting factor in fermentation.
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Because of its low solubility, only 0.3 mM 02, equivalent to 9 ppm, dissolves in one liter of water at 20 in an air/water mixture℃
Due to the influence of the culture ingredients, the maximal oxygen content is actually lower than it would be in pure water.
The solubility of gases follows Henry's Law in the gas pressure range over which fermenters are operated.
Introduction Gas exchange and mass transfer (Crueger and Crueger, 1990)
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Henry's Law Describes the solubility of O2 in nutrient
solution in relation to the O2 partial pressure in the gas phase
C* is the oxygen saturation concentration of the nutrient solution, Po is the partial pressure of the gas in the gas phase and H is Henry's constant, which is specific for the gas and the liquid phase
Aeration with air 9 mg O2/L dissolves in water, with pure oxygen 43 mg O2/L.
HP
*C o
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The oxygen transfer process Step 1 - Diffusion through the bubble to the gas-liquid interfa
ce Step 2 - Diffusion across the gas-liquid interface Step 3 - Diffusion through the bubble boundary layer Step 4 - Movement through the bulk liquid by forced convecti
on and diffusion Step 5-9: Movement through the floc
Step 5 - movement through the boundary layer surrounding the microbial slime
Step 6 - entry into the slimeStep 7 - movement through the slimeStep 8 - movement across the cell membraneStep 9 - reaction
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The oxygen transfer process
Step 1 Diffusion through the bubble to the gas-liquid
interface Gas molecules move quickly they are evenly distributed throughout the
bubble.
O2
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The oxygen transfer process Step 2 - Diffusion across the gas-liquid interface This step will be very rapid if the concentration of
oxygen in the bubble high. High oxygen concentrations in the bubble (as measured in terms of partial pressure) will push the oxygen molecules across the interface, into the boundary layer.
If the medium is rich in CO2 , then the carbon dioxide will be pushed into the bubble.
The bubble contains a low concentration of oxygen, then the rate of oxygen transfer out of the bubble will be slow or even zero
O2
CO2
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Step 3- Diffusion through the bubble boundary layer The movement of solutes through the boundary layer is
slow. Solutes move through the liquid by diffusion. The movement of the molecule will be driven by the
concentration gradient across the boundary layer. Factors affect the rate of diffusion of oxygen through the
boundary layer, including : temperature concentration of oxygen in the bulk liquid saturation concentration of oxygen in the liquid concentration of oxygen in the bubble size of the molecule and viscosity of the medium
The oxygen transfer process
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Step 4 Movement through the bulk liquid by forced convection and diffusion
The rate of movement of an oxygen molecule through the bulk liquid is dependent on
the degree of mixing (relative to the volume of the reactor)
viscosity of the medium
O2
The oxygen transfer process
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Step 5-9: Movement through the floc complete the journey of the oxygen moleculeStep 5 - movement through the boundary layer surroundi
ng the microbial slime. Step 6 - entry into the slimeStep 7 - movement through the slimeStep 8 - movement across the cell membraneStep 9 - reaction Steps 5 and 7 are slow processes.
The oxygen transfer process
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If only suspended cells are involved the level of mixing in the bulk liquid is
sufficiently highThen the rate limiting step in the oxygen transfer
process is the movement of the oxygen molecules
through the bubble boundary layer. (Step 3)
The oxygen transfer process
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OTRCCakN LLA )*(
NA = Volume-dependent mass transfer(mMO2/Lh)kL = Transfer coefficient at the phase boundarya = Specific exchange surfacekLa = Volumetric oxygen transfer coefficient (h-1) C* = Saturation value of the dissolved gas in the phase boundaryCL = Concentration of the dissolved gas (mM/L)OTR = O2 Transfer Rate (mM O2/Lh)
The oxygen transfer processStep 3 The interphase oxygen transfer equation
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The transfer of massFick’s Law of diffusion
D : the diffusivity (the movement of mass)成分 A在成分 B之擴散係數
dzdCDJ
Am A
ABA
]/[/length
volumemassDArea
timemassAB
])([2
timelengthDAB
The oxygen transfer process
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Molecular Diffusion in Liquids
When the concentration of a component varies from one point to another
the component has a tendency to flow in the direction that will reduce the local differences in concentration
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Diffusivity The kinetic theory of liquids is much less advanced
than that of gases The correlation for diffusivities in liquids is not as
reliable as that for gases The Wilke-Chang correlation (for dilute solutions
of nonelectrolytes)
(9.4)
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Diffusivity Othmer and Thakar correlation (the solvent is wate
r)
Example 9.1 : Estimate the diffusivity for oxygen in water at 25°C. Compare the predictions from the Wilke-Chang and Othmer-Thakar correlations with the experimental value of 2.5×10−9 m2/s (Perry and Chilton, p. 3-225, 1973). Convert the experimental value to that corresponding to a temperature of 40°C.
(9.5)
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Mass-Transfer Coefficient (kL & kG)
where •CS is the dissolved concentration of the solute in
the bulk liquid •k is the mass transfer coefficient for the solute
through the boundary layer •A is the total interfacial area and •Cs* is the concentration of the solute in the
boundary layer.
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Mass-Transfer Coefficient (kL & kG)
Since the amount of solute transferred from the gas phase to the interface must equal that from the interface to the liquid phase,
NG =NL (9.8) Substitution of Eq. (9.6) and Eq. (9.7) into Eq. (9.8)
gives
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Mass-Transfer Coefficient (kL & kG)
It is hard to determine the mass-transfer coefficient
Because the interfacial concentrations, CLi or CG
i cannot be measure To define the overall mass-transfer coefficient a
s follows :
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Mass-Transfer Coefficient (kL & kG)
For sparingly soluble gases, the slope of the equilibrium curve is very steep
M is much greater than 1 and from Eq. (9.14)KL ≈kL (9.15)
Similarly, for the gas-phase mass-transfer coefficient,
KG ≈kG (9.16)
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Mechanism of Mass Transfer The two-film theory (雙膜理論 ) The penetration theory (滲透理論 ) The surface renewal theory (表面更新理論 ) Read textbook p. 9-8 ~ 9-9 All these theories require knowledge of one unknow
n parameter, the effective film thickness Zf, the exposure time te, or the fractional rate of surface renewal s. Little is known about these properties, so as theories, all three are incomplete.
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Correlation for Mass-Transfer Coefficient Mass-transfer coefficient is a function of physical
properties and vessel geometry Because of the complexity of hydrodynamics in
multiphase mixing, it is difficult, if not impossible, to derive a
useful correlation based on a purely theoretical basis It is common to obtain an empirical correlation for
the mass-transfer coefficient by fitting experimental data. The correlations are usually expressed by dimensionless groups since they are dimensionally consistent and also useful for scale-up processes.
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Sauter-mean diameter D32
D32 can be calculated from measured drop-size distribution from the following relationship
Example 9.3 Determine appropriate dimensionless parameters that can relate the mass transfer coefficient by applying the Buckingham-Pi theorem.
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Dimensionless Number for Mass Transfer Correlations
32total mass transfer diffusive mass transfer
LSh
AB
k DND
= =
/momentum diffusivity mass diffusivity
c cSc
AB
ND
= =
332
2
gravitation force viscous force
cGr
c
D gN
= =
(9.21)
(9.22)
(9.23)
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Correlations for Mass Transfer Coefficients (Calderbank & Moo-Young, 1961)
3/13/131.0 GrScSh NNN
1/3 1/32.0 0.31Sh Sc GrN N N= +
(9.26)
(9.28)
For small bubbles (D < 2.5 mm)
For large bubbles (D > 2.5 mm)
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Correlations for interfacial area
Gas Sparging with No Mechanical Agitation
Gas Sparging with Mechanical Agitation
0.10.5 321.13
2
1 3
CC LC
L
gDg DaD H
1/ 20.4 0.2
0 0.6 1.44 scm
t
VP vaV