Lecture 4 - Physical Bioprocesses 2
Transcript of Lecture 4 - Physical Bioprocesses 2
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Physical Processes in Bioreactors 1
Master of Sciencein Chemical Engineering
Course in
University of San CarlosDepartment of Chemical Engineering
Nasipit, Talamban, Cebu CityTelefax: +63323446783
Email: [email protected]
Bioprocess Technology
Physical Processes in Bioreactors
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Physical Processes in Bioreactors 2
Contents of the Lecture
Fluid flow and mixing
Heat transfer
Mass transfer
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Physical Processes in Bioreactors 3
Heat Transfer in Bioreactors
In situ sterilization of liquid medium Temperature control during reactor operation
Heat-transfer configuration for bioreactors
a) Jacketed vessel
b) External coil
c) Internal helical coild) Internal baffle-type coil
e) External heat exchanger
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Physical Processes in Bioreactors 4
Heat-transfer configurations for
bioreactors
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Physical Processes in Bioreactors 5
Temperature changes for control of fermentation
temperature using cooling water
Te m
p e r a t
u r e
Distance from cold-fluid inlet
TF
Tci
Tco
Fermenter temperature
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Physical Processes in Bioreactors 6
Heat Transfer Mechanisms
RECALL, Conduction
Convection Radiation
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Physical Processes in Bioreactors 7
Heat Transfer Between Fluids
ThThw
Tcw
Tc
Hot fluidCold fluid
(1) (2) (3)
(1) ( )H h h hwh A T T =
(2)
( )
( )hw cw
H
T T
L
=
(3) ( )H c cw ch A T T =
Resistances: 1h
h
R
h A
=
1c
c
Rh A
=
w
LR
=
Where:
h = individual heat transfer coefficient.
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Physical Processes in Bioreactors 8
Overall Heat Transfer Coefficient
( )H h cUA T T = (a)
1T h w cR R R R
UA= = + +
But,
1 1 1
h c
LUA h A A h A
= + +
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Physical Processes in Bioreactors 9
Fouling Factors
1 1 1 1 1fh h c fc
LU h h h h
= + + + +
Fouling factor of the hot-fluid sideFouling factor of the cold-fluid side
Design equations for heat-transfer systems
Equation (a) and energy balance equations
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Physical Processes in Bioreactors 10
Estimation of Heat Transfer
Coefficients
= wb
L
D
GrfNu
,,Pr,Re,
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Physical Processes in Bioreactors 11
Heat Transfer in Aerobic
Fermentations
rate of oxygen consumption by cells
Maximum cell concentration supported by heat transfer system
rxnH
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Physical Processes in Bioreactors 12
Heat Transfer in Aerobic
Fermentations
9Heat transfer is severe when: Fluid is viscous
Turbulence & high U are difficult toachieve
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Physical Processes in Bioreactors 13
Mass Transfer in Bioreactors
9Conduction (role in bioprocessing) Scale of mixing
Solid-phase reaction Interphase transfer
9Convection
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Physical Processes in Bioreactors 14
Diffusion TheoryCA1
CA2
JA
x
dx
dC
Da
N
JA
AB
A
A ==
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Physical Processes in Bioreactors 15
Mass Transfer Across A Phase Boundary
=
propertiesfluid
geometry,ics,hydrodynamfk
Rate of transfer
AA
CkaN =
Resistance to mass transfer
kaRm1
=
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Physical Processes in Bioreactors 16
Mass Transfer Across A Phase
Boundary
Liquid-solid Liquid-liquid
Gas-liquid
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Physical Processes in Bioreactors 17
Liquid-solid Mass Transfer
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Physical Processes in Bioreactors 18
Gas-liquid Mass Transfer
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Physical Processes in Bioreactors 19
Oxygen Transfer
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Physical Processes in Bioreactors 20
Steps for Oxygen Transfer from gas bubble to cell
i. Transfer from the interior of the bubble to the gas-liquidinterface
ii. Movement across the gas-liquid interfaceiii. Diffusion through the relatively stagnant liquid film
surrounding the bubble
iv. Transport through the bulk liquid
v. Diffusion through the relatively stagnant liquid filmsurrounding the cells
vi. Movement across the liquid-cell interface
vii. If the cells are in a floc, clump or solid particle,diffusion through the solid to the individual cell
viii. Transport through the cytoplasm to the site of reaction
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Physical Processes in Bioreactors 21
Analysis of G-L Mass Transfer in Bioreactors
i. Transfer through the bulk gas phase in the bubble is relatively fastii. The gas-liquid interface itself contributes negligible resistance
iii. The liquid film around the bubbles is a major resistance to oxygen transfer
iv. In a well-mixed fermenter, concentration gradients in the bulk liquid are
minimized & mass-transfer resistance in this region is small. However, rapidmixing can be difficult to achieve in viscous fermentation broths; if this is the
case, oxygen-transfer resistance in the bulk liquid may be important.
v. Because single cells are much smaller than gas bubbles, the liquid film
surrounding each cell is much thinner than that around the bubbles and its
effect on mass transfer can generally be neglected. On the other hand, if thecells form large clumps, liquid-film resistance can be significant.
vi. Resistance at the cell-liquid interface is generally neglected.
vii. When the cells are in clumps, intraparticle resistance is likely to be significant
as oxygen has to diffuse through the solid pellet to reach the interior cells.
The magnitude of this resistance depends on the size of the clumps.
viii. Intracellular oxygen-transfer resistance is negligible because of the small
distances involved.
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Physical Processes in Bioreactors 22
Oxygen demand during cultivation of yeast cells
In the culture medium the O2 solubility is
CO2* = 1.1 mmol/l = 35.2 mg / 1 (ppm),
(a)
Which in case of air saturation reduces to
CO2* = 35.2 x 0.209 = 7.36 ppm
(b)
The typical O2 demand of the yeast fermentation is in the following range
Q O2 = 0.3 g O2 / (g Dm h) (c)
Hence, for biomass concentration of 20 g Dm/l the oxygen demand is
Q O2 = 0.3 x 20 = 6 g O2 / h
(d)This implies that the number of complete saturations of the fermentation broth amounts to
n-Sat = 6 / 0.00736 = 816 h-1
(e)
This is not an easy task, particularly, if higher biomass concentrations are desired which are usual inpractice (for instance 60 g DM/l).
Critical oxygen concentration
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Physical Processes in Bioreactors 23
Critical oxygen concentration
To avoid limitations of cell growth and product formation by lack of oxygen it is necessary that a certain
oxygen concentration is maintained in the culture media.
At next, an estimation is done to find out the conditions under which the biochemical reactions involved in
cell growth are the limiting steps. Usually, the highest oxygen consumption rates are observed during cell
growth. The largest mass transfer rates for oxygen (mol/cm3.s) are obtained for CL = 0, hence
max = kL a CL (1
On the other hand, the maximum oxygen consumption during cell growth is given by
rmax
= max X . 1/YO2 (2)
with the yield coefficient
YO2 = generated biomass / consumed moled O2 (3)
It is understood that the main resistance for oxygen utilization is predominantly limited by microbialmetabolism if
max >> rmax (4)
or
kLa CL* >> 1/YO2 max X , (5)
respectively.
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Physical Processes in Bioreactors 24
In this case there is no mass transfer resistance for oxygen and the entire growth process is controlled
by the biochemical reactions in the cell machinery. However, if CL < CL, crit
( = Ko2 ), oxygen can be the limiting substrate and hence the oxygen input into the culture media is thelimiting step of the process (Fig. 9).
With the assumption CL* >> CL it follows
X/'ak*CY1
X/'akKY
maxLLO
maxOO
2
L22
It can be anticipated as a guideline that no mass transfer limitation for oxygen exists at the gas/liquid
interface if eq. (38) appliesC
L> 3
2OK ( = CL, crit )
From the biochemical viewpoint this implies that always sufficient oxygen is available to except the
electron pairs which pass through the respiratory chain. To estimate the critical oxygen concentration
which leads to growth limitation by lack of oxygen a kinetic law of Monod type is assumed to apply
for oxygen, i.e.,
r = max
x2
2OLO
L
Y
1
CK
C
+
Then the stationary oxygen balance is given by
OTR = OUR
Oxygen transfer rate oxygen uptake rate
kL
a (CL* - C
L) =
maxx
22
OLO
L
Y
1
CK
C
+
(6)
(7)
CL = CL* (8)
(9)
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Physical Processes in Bioreactors 25
Typical values of the critical O2
concentration
(at sufficient supply with other substrates)
CL* (at air saturation) = 0.23 mmol/l
Microorganisms T, C CL,
Crit mmol/l
Azetobacter vinlandii 30 0.03
E. coli 37.8 0.008
15 0.003Pseudomonas denitroficans 30 0.009
Yeast 34.8 0.004620 0.0037
P. chrysogenum 24 0.022
30 0.009It can be assumed
as a rule of thumb that0.003 < C
L, crit < 0.05 mmol/l
which corresponds to an air saturation of 1 to 25%
Fig.9
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Physical Processes in Bioreactors 26
Scale-up
The performance of a bioreactor depends on process specific data (stoichiometric coefficients,
thermodynamic and kinetic data) and transport parameters (hydrodynamics, heat and mass
transfer properties).
Specific data are scale independent and the transport parameters may be drastically dependenton the reactor size. For instance, the oxygen solubility (CL*) is a thermodynamic quantity
which depends only on the medium composition and the temperature while the actual oxygen
concentration in the culture medium is a complex function of the biochemical kinetics and the
scale dependent transport parameters (kLa, , etc.).
Phenotypical appearance of an organism is determined by both its genotype and the living
conditions in the culture.
Kossen has proposed to select production strains in laboratory scale at conditions which prevailin large reactors. Instead of looking for a baseball player in a soccer field, one should do scale
down of reactors for proper screening of microbes.
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Physical Processes in Bioreactors 27
The different methods for scale-up can be summarized as follows:
(1) Fundamental methods (modeling of the process on the basis of the balances for mass, heat
and momentum which requires detailed knowledge of all transport effects and their
interaction)
(2) Semifundamental methods (solution of simplified balance equations)
(3) Dimensional analysis (is only of limited value as not always applicable)
(4) Rules of thumbs, know how(5) Trial and error
In todays fermentation industry only methods (4) and (5) are established. The often used rules
of thumbs imply: Do the scale-up in such a way that the culture conditions (environment)
and some typical engineering parameters (P/V, , kLa, Re, ) are kept constant.
Scale-up criteria used in industry
% of industry Criterium used
30% P/V constant30% kLa constant
20% utip constant*
20% Po2 constant
*utip = stirrer tip velocity
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Physical Processes in Bioreactors 28
Examples for a successful scale-up on the basis of different criteria are given in Fig. 10.
The intrinsic know how is the choice of the correct criteria for the specific process.
Different scale-up criteria and their consequencesScale-up from a 10 litre reactor to 10 m3 which is a linear scale factor of 10.
1 0.22 2.15 21.5
102 1 10 102
0.1 0.1 1 10
10-4 10-2 0.1 1
103
105
102
0.1
P/V
N(-1)
utipRe
Value in 10 m3 scale
P/V N(-1) ND Re
PCriterium:
Constancy of
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Some criteria cannot be fulfilled in practice. An example is the requirement for equal mixing times in
small and large scale. For many stirrer systems we have the empirical relation
300D
p5
3
=
Considering this, the equality of mixing times ( = idem) implies that
l2S2)
D
V/P()
D
V/P( =
2
S
l
)D
D
For a linear scale factor of 10 (0.01 to 10) and (P/V)S
= 2 kW/m3 it follows
(P/V)l= 200 kW/m3
Such a high energy input in a bioreactor is nearly impossible and would be extremely costly.
(P/V)l = (P/V)S (
(10)
(11)
(12)