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


40


41

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


42

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)


43


44

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)


45


46

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


47

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


48

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


49

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


50

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


51

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)


52


53

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


54

ceng 371 environmental control - kexhu.people.ust.hkkexhu.people.ust.hk/ceng371/371-00-2.pdf · m =...

Documents