Encyclopedia of Environmental Science and Engineering, Volume I and II

1224 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

The design of absorbers involves the estimation of

column diameter, height, and pressure drop. The column

diameter is fixed by the contaminated gas flow rate. The

determination of the height of the two-phase contacting zone

involves an estimation of the mass transfer coefficients, the

alternating use of equilibrium concentration relationship,

and the law of mass conservation. For nonisothermal or adi-

abatic operating conditions, the law of energy conservation

needs to be considered. There are many types of equipment

and configurations for absorbers or scrubbers. For example,

McCarthy (1980) discussed the scrubber types and selec-

tion criteria.

For packed towers the interfacial area, a, differs from

the packing surface area, aT, because the packing is not

always completely wetted. A fraction of the surface may

not be active in mass transfer. Also, stagnant pockets will

be less effective than flowing streams. The correlation of

Onda et al. (1968) may be used to estimate the value of

interfacial area.

a

a

L

a

L a

g

c L

L

t

L

1

75

1

(^12)
2
(^52)
1
1 4 5
 


exp
.

...
s
s m r























 rr sL ta

























.2

Where sc represents the critical surface tension above

which the packing can’t be wetted. The values of sc for

various packing materials are shown in Table 4 (Onda

et al., 1967)

L : the superficial liquid mass

flow rate

PL, mL : density and viscosity of the

liquid, respectively

s : surface tension of the liquid,

dynes/cm.

This equation correlates results with a maximum error of

20% except in the case of Pall rings where it is conserva-

tive. The reason is probably that the interfacial and wetted

area are different for Pall rings whose shape forces a frac-

tion of the liquid phase to be dispersed in small droplets

that are not accounted for in the values of a. Values of a

Pall rings are underestimated about 50% according to

Charpentier (1976).

The liquid phase mass transfer coefficient can be esti-

mated using Mohunta’s equation (1969).

k a

g

a

g

g

L

L

t L

L

L

L t

L

 25 10^4

(^662111) 3 3
2 4
r
m
r
m
m
r











..
L a










.

.

25

5





m

r

L

L LD

within a range of 20%.

The range of variables and physical properties of

Mohunta’s equation are:

Variables Range

L 0.1–42 kg/m^2 sec

G 0.015–1.22 kg/m^2 sec

mL 0.7–1.5 CP

mL/rLDL 140–1030

d 0.6–5 cm

D (column) 6–50 cm

where G : superficial gas mass flow rate

DL : liquid diffusivity

d : packing diameter.

For the gas phase mass transfer coefficient, Laurent and

Charpentier (1974) derived the correlation

k P

G

C

M

a d

D

g

t

G

G G

 

 

( ).

..
1 7

3 5

Gd

mG

m

r

















where P : total pressure, atm

M : gas molecular weight

DG : solute gas diffusively

C  2.3 for d ,  1.5 cm

C  5.23 for d  1.5 cm.

Tray type towers have also been used successfully.

Bubble cap plates correlations have been proposed by

Andrew (1961)

k u S D

k u S D

a u

g g

l l











7

11

0 7

1 4 1 2 1 2

1 4 1 2 1 2

1 2

/ / /

/ / /

/

cm/sec

cm/sec

. SS5 6/

TABLE 4

Critical surface tension of packing materials

Material sc dynes/cm

Carbon 56

Ceramic 61

Glass 73

Paraffin 20

Polyethylene 33

Polyvinylchloride 40

Steel 75

C022_001_r03.indd 1224 11/18/2005 2:32:59 PM