CENG 371 Environmental Control - kexhu.people.ust.hkkexhu.people.ust.hk/ceng371/371-00-2.pdf · m =...
Transcript of CENG 371 Environmental Control - kexhu.people.ust.hkkexhu.people.ust.hk/ceng371/371-00-2.pdf · m =...
CENG 4710 Environmental Control
39
Air Stripping
a mass transfer process
passing air through water
useful for removing low concentration (<200 mg/L)
volatile organic compounds (VOCs)
using packed towers, tray towers, spray systems,
diffused aeration, or mechanical aeration
the reverse of absorption
• 1 m = 3.28 ft
• 1 ft = 0.3049 m
• 1 US gallon = 3.785 liters
• 1 UK gallon = 4.546 liters
• 1 US gallon per minute (GPM) = 0.2271 m3/h
• 1 m3/h = 4.403 GPM
• Density of water at 0oC = 1000 kg/m3 = 62.4 lb/ft3
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Since the concentration in air stripping is low, Henry’s
law can be applied to describe the equilibrium between
the gas and liquid phases, i.e. A H C '
where A is the concentration in air, C is the
concentration in water, and H’ is the dimensionless
Henry’s law constant .
If the stripping tower is assumed ideal, the effluent air is
in equilibrium with the inlet water, i.e. A H Cout in '
Furthermore, if the influent air contains no contaminant
(Ain = 0) and the effluent water is free of contaminant
(Cout = 0, 100% efficiency), the mass balance equation
is Q C Q H Cw in A in ( ' )
Q Q Hw A '
or H Q QA w' ( / ) 1
The expression R=H’(QA/ QW) is called the stripping
factor.
R > 1 stripping
R = 1 equilibrium
R < 1 absorption
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Stripping Theory
The transfer of a volatile organic
compound from water to air
follows the two-film theory
covering mass transfer from:
bulk liquid to liquid film
liquid film to air film
air film to bulk film
An overall mass transfer coefficient, KLa (s-1) can be
used to describe the transfer rate of contaminant from
water to air. For design purpose, KLa should be
determined experimentally. However, for dilute
solutions, Sherwood and Holloway equation may be
used: 5.01
305
L
n
LLD
LDaK
where
DL = liquid diffusion coefficient (m2/s),
L = liquid mass loading rate (kg/m2 s)
= viscosity of water (0.001002 Pa s at 20oC)
= density of water (998.2 kg/m3 at 20 oC)
, n = constants from Table 9-1 (p. 450)
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DL may be estimated using the Wilke-Chang method:
DT
VsL
m
5 06 10 7
0 6
./ )
. (cm2
where
T = temperature (K)
= viscosity of water (centipoises, cP)
Vm = molar volume of contaminant (cm3/mol)
(Table 3-4, p. 97)
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Design equation
Consider a section of the stripping Tower with a cross-
sectional surface area B, and a differential thickness dZ,
the mass transferred per unit volume of the tower is
)m (kmol/s 3
dZ
dC
B
Q
BdZ
dCQM ww
where Qw = liquid flow rate (m3/s)
C = contaminant concentration (kmol/ m3)
B = surface area (m2)
Z = depth in column (m)
DC/dZ = concentration gradient (kmol/ m4)
This mass transfer should be the same as that
transferred across the air/water interface:
eqL CCaKM
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where Ceq = concentration in water in equilibrium
with the air at a specified point.
C - Ceq = the degree to which the system is out
of equilibrium
Since the concentration is low in air stripping,
Ceq = A/H’
where A = concentration in air (kmol/ m3)
H’ = dimensionless Henry’s constant
Hence,
eqLw CCaK
dZ
dC
B
Q
The liquid flow rate (Qw) can be replaced by the liquid
molar loading rate (L) (kmol/s m2):
w
w
M
L
B
Q
where Mw = molar density of water
= 1000 (kg/m3)/18 (kg/kmol)
= 55.6 kmol/ m3 = 3.47 lb mol/ft3
So,
eqLw CC
dC
aKM
LdZ
eqLw
CCaKdZ
dC
M
L
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The required column height is
in
out
CC
eqLw CC
dC
aKM
LZ
The first term is independent of C, and is called the
height of a transfer unit (HTU):
aBK
Q
aKM
LHTU
L
w
Lw
The integration part is dimensionless, which is
designated as the number of transfer unit (NTU):
inout
CC
eqCC
dCNTU
Hence, Z = HTU x NTU
Ceq may be determined from the mass balance from the
bottom of the column up to the differential section:
and A = Ceq H’
if Ain = 0, outweqA CCQHCQ -'
R
CC
HQ
CCQC out
A
outweq
'
where R = stripping factor = H’(QA /Qw).
outwinA CCQAAQ
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R
RCC
R
R
CCR
CCRd
R
R
CCRC
RdC
R
CCC
dCNTU
outin
CC
out
out
CC
out
CC
out
inout
inout
inout
1)1)(/(ln
1
)1(
])1[(
1
This equation can only be used if the inlet air has no
contaminant (Ain =0).
Example 9-2 Preliminary design of air stripping column.
A ground water supply has been contaminated with
ethylbenzene. The maximum level of ethylbenzene in
the ground water is 1 mg/L and this must be reduced to
35 g/L using an air stripping column.
KLa = 0.016 s-1 Qw = 7.13 L/s T = 20 oC
H = 6.44 x 10-3 atm m3 /gmol
Select: Column diameter = 0.61 m
Air-to-water ratio (QA/Qw) = 20
Determine: Liquid loading rate (L)
Stripping factor (R)
HTU, NTU, height of packing in column
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Solution: H’= H/RT = 6.44×10-3 /(8.205×10-5×293.2)
= 0.27
1. Liquid loading rate:
Cross-sectional area of column = R2
= (0.61/2) 2 = 0.292 m2
mass rate = 1.0 kg/L x 7.13 L/s = 7.13 kg/s
mass loading = 7.13/0.292 = 24.4 kg/(s m2)
L = (24.4 kg/(s m2))(1000 g/kg)(1/18 mol/g)
= 1360 mol/s m2
2. Stripping factor
R=H’(QA/Qw) = 0.27x20 = 5.4
3. Height of transfer unit:
m 53.1016.055600
1360
aKM
LHTU
Lw
4. Number of transfer units:
units transfer3.88
5.4
1)14.5)(35/1000(ln
14.5
4.5
1)1)(/(ln
1-
R
RCC
R
RNTU outin
5. Height of packing in column
Z = NTU x HTU = (3.88)(1.53) = 5.93 m = 17.7 ft
Take a 10% safety factor, the column length used
should be
Z’ = 17.7 1.1 = 19.45 ft
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Design Consideration
Stripping tower: diameter = 0.5 - 3 m
height = 1 - 15 m
QA/QW > 5
R = 2 - 10 or higher
Flooding: as the air flow in a tower is increased, it will
ultimately hold back the free downward flow of
water.
Channelling: this occurs when water flows down the
tower wall rather than through the packing, use
distribution plates at every 5 diameters to avoid this.
Pressure drop: to avoid flooding, this should be
200 - 400 N/m2 m packing height
(= 0.25 - 0.5 inch H2O/ft)
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Example 9-3 Use the data in example 9-2 to determine
the pressure drop through the tower and examine the
impact on effluent quality of varying the air-to-water
ratio (A/W) and the column height.
Solution