gas transfer rates - marmara Üniversitesimimoza.marmara.edu.tr/~zehra.can/enve401/10. gas...
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Gas transfer rates
If either phase concentration can not be predicted
by Henry's law then there will be a transfer of
mass across the interface until equilibrium is
reached.
The mechanisms and rate expressions for this
transfer process have been conceptualized in a
variety of ways so that quantitative descriptions
are possible.
One common conceptualization method is the film
theory.
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Film Theory Developed by Nernst (1904)
• The simplest conceptualization of the gas-
liquid transfer process
• Near the interface, there exists a stagnant film.
• This stagnant film is hypothetical since we
really don't know the details of the velocity
profile near the interface.
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• In this film, bulk liquid is not moving.
Therefore, transport is governed essentially
by molecular diffusion.
• Fick's law describes flux through the film.
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Molecular diffusion
A “random” and spontaneous mixing process that moves the
“solute” from regions of high concentration to regions of low
concentration. Molecular diffusion is due to concentration gradients
and the random motion of molecules.
calm environment
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Molecular diffusion
Rates of simple one-dimensional microscale mass transfer
processes in homogeneous fluids or solids are expressed in
terms of Fick’s first law of diffusion.
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According to “Fick’s first law”, the flux of component "c" across a
chemical front per unit time is directly proportional to the size of
the spatial gradient in concentration and a constant of
proportionality (the diffusion constant – D )
Area that is perpendicular
to the flow.
Substance moves due to
concentration differences.
(cm2/sec) specific to solute
Molecular diffusion
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Therefore, flux through the film is
CJ D
X
Typical units for flux are
2
mol
cm sec 2
mg
cm secor
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If the thickness of the stagnant film is given by
dn then the gradient can be approximated by:
C C Cb i
X n
d
Cb and Ci are concentrations in the bulk and at
the interface, respectively.
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• Assume there are no reactions in the stagnant
film.
• Therefore, no accumulation in the film.
• Assume that D = constant
At steady-state:
(C C )b iJ Dn
d
What is the
value of Ci?
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• Ci is calculated by assuming that equilibrium is
attained instantly at the interface.
• Nernst’s model assumes that the other phase
doesn't have a "film".
CgCi
Hc
(if the film side is liquid and the
opposite side is the gas phase).
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Two film model
• Nernst’s model assumes that the other phase
doesn't have a "film".
• This theory is the most widely accepted
model.
• It makes it easier to visualize the concepts.
• It is usually applied for gas transfer by bubble
formation in liquid.
• Therefore, we use the Lewis-Whitman (1923)
two-film model as described below.
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Two film model
• It assumes that there are two stagnant layers
of film at the interface.
• The mass transfer across the films is by
molecular diffusion.
• The bulks of gas and liquid are homogeneous
with respect to solute.
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Assuming that the mass transfer is in the X direction, in
other words from bulk gas phase to bulk liquid phase.
uniform
concentration
and pressure
because of
turbulence
uniform
concentration
because of
turbulence
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• Since the solute is diffusing from the gas
phase into the liquid, there must be a
concentration gradient in the direction of
mass-transfer within each phase.
• The bulk gas concentration Cg falls to Cgi at
the interface.
• Similarly, in the liquid phase, the
concentration falls from Cli at the interface to
Cl in the main body of liquid.
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• The bulk gas concentrations Cg and Cl are
clearly not the equilibrium values since
otherwise the mass transfer would not occur.
the interface.
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Now, the flux from the bulk gas to the interface is
where
Jg = kg(Cgi-Cg)
Jl = kl(Cl -Cli)
and the flux from the interface to the bulk liquid is
klDl n
d
kg
Dg
nd
kl or kg = mass transfer coefficient for either the liquid
or gas phase as (dimensions = Length/time).
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The same assumptions apply to the two films as apply
in the single Nernst film model.
So at steady state, the flux from the bulk gas to the
interface is equal to the the flux from the interface to
the bulk liquid.
Therefore,
kg(Cgi-Cg) = kl(Cl -Cli)
(C C )gi g
(C C )l
kl = kg li
Jg = Jl
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• The difficulty is finding interface concentrations,
Cgi or Cli.
• We cannot directly measure the interface
concentrations.
• We want to be able to relate flux to bulk air and
water concentrations.
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• Therefore, overall mass transfer coefficients
are defined.
• These coefficients are based on the
difference between the bulk concentration in
one phase and the concentration that would
be in equilibrium with the bulk concentration
in the other phase.
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0
0,002
0,004
0,006
0,008
0,01
0,012
0,014
0,016
0 1 2 3 4 5 6
Cg
Cl
g i c l iC H C
Cl*
Cg*
Cli Cl
Cgi
Cg
Cg
Cl
Equilibrium line A
B
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In the previous graph, point A represents the two
bulk phase concentrations and point B represents
the concentrations at the interface.
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Define:
J
J
*K C Cl ll
*K C Cg g g
Kl = overall mass transfer coefficient based on
liquid-phase concentration.
Kg = overall mass transfer coefficient based on
gas-phase concentration.
Cli is replaced with Cl*
Cgi is replaced with Cg*
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Kg,l have dimensions of Length/t.
Cl* = liquid phase concentration that would be in
equilibrium with the bulk gas
concentration.
= Cg/Hc (typical dimensions are moles/m3).
Cg* = gas phase concentration that would be in
equilibrium with the bulk liquid
concentration.
= HcCl (typical dimensions are moles/m3).
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Expand the liquid-phase overall flux equation to
include the interface liquid concentration.
*J K C C C Cll l li li
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Then substitute
CgiCli Hc
and Cg*ClHc
to get:
C Cggi
J K C Cl lil Hc
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In the steady-state, fluxes through all films must
be equal. Let all these fluxes be equal to J.
C Cl liJ
kl
and
C Cgi gJ
kg
Jg = kg(Cgi-Cg) Jl = kl(Cl -Cli)
On an individual film basis:
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Since all J’s are equal:
J JJ Kl k H kl c g
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This can be arranged to give:
1 1
k gl kl
1
K Hc
Hc : Contaminant specific
kl and kg : Environment specific, such as mixing
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A similar manipulation starting with the overall flux
equation based on gas phase concentration will give:
H 1c
k k
1
g l gK
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These last two equations can be viewed as
"resistance" expressions where 1/Kg or 1/Kl
represent total resistance to mass transfer based
on gas or liquid phase concentration,
respectively.
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In fact, the total resistance to transfer is made up of
three series resistances:
i) liquid film,
ii) interface
iii) gas film.
However, we assume instant equilibrium at the
interface so there is no transfer limitation here. It
should be noted that model selection (penetration,
surface renewal or film) does not influence the
outcome of this analysis.
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The Two Special Cases - Single film control
• It is possible that one of the films exhibits
relatively high resistance and therefore
dominates the overall resistance to transfer.
• This, of course, depends on the relative
magnitudes of kl, kg and Hc.
• So the solubility of the gas and the
hydrodynamic conditions which establish the
film thickness determine if a film controls.
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For highly soluble gases such as NH3 and SO2
(Hc is low)
1 1
k H kl c g
1
Kl g
1
H kc
Hcl gK k
Mass transfer is controlled by gas film resistance.
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In general, slightly soluble gases such as N2 and
O2 (Hc is high)
1 1
k gl kl
1
K Hc
1 1
k H kl c g
1
Kl
1
kl or llK k
Mass transfer is controlled by liquid film resistance.