Encyclopedia of Environmental Science and Engineering, Volume I and II

VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1225

in which:

S  effective liquid area on plate, cm^2

u  superficial gas velocity, cm/sec

a  interfacial area/unit area of plate.

A conservative estimate for the liquid phase coefficient in a

sieve plate may be obtained from the Equation of Claderbank

and Moo–Young (1961)

kl0 31. ( ) ( )gv d v 1 1 3/ l/ cm/sec 1 2 3/

where vl is the kinematic liquid viscosity. For CO 2 and water

at ordinary temperature, kl ≈ 0.01 cm/sec. Typical sieve plate

Ni values are about 10−4 g mole/cm^2 -sec-atm.

“Each tray of Figure 15 has a series of drawn orifices fitted

with a cage and cap. The orifice has a flared entrance. This

reduces the dry pressure drop allowing a greater percentage of

the work expended to be utilized for scrubbing. The floating

cap maintains the scrubbing efficiency even with variations in

gas flow as wide as 40–110% of capacity.

Gas enters the vessel flowing upward through the valve

trays. Liquid is introduced on the top tray and flows across

each tray over a weir and then to a sealed downcomer to the

next lower tray. A level is maintained over each tray by the

weir. The upward flowing gas is given a horizontal component

by the cap causing atomization of the liquid on the tray. The

froth formed consisting of a myriad of small droplets traps the

particles and absorbs or reacts with acid or alkaline vapors.

The liquid agitation on the tray surface prevents buildup. This

has been demonstrated by the many successful applications in

the Petrochemical Industry involving tarry solids and liquids.

Each tray has a 1½ W.G. pressure drop. A typical instal-

lation with four (4) trays would require less than 8′′ W.G.

pressure loss.”

An excellent review of gas phase absorption may be

found in the work of Danckwerts (1970). The molecular dif-

fusivities in the vapor phase Dr, and in the liquid D 1 , may

be found from existing correlations, for example see Bird

et al. (1960). Unlike the solid in adsorption the liquid sol-

vent in absorption usually leaves the system where it can be

regenerated. Hence a steady state plug flow analysis in either

phase in terms of overall coefficients is possible

LdC GdC K a C C dz

K a C C dz

il ig g ig ig

il il

  

 

( )

( ).

*

*

1

The required tower height is then given by either of the fol-

lowing integral relationships:

Z

G

K a

dC

C C

L

K a

dc

g C C

g

g g

C

C il

il il

C

go

ge le







∫ * 1 ∫ 0 *

in which: a is the surface area of contact per unit volume

of bed; L and G are per superficial mass velocities. Further

discussion on the subject may be found in the work of

Cooper and Alley (1994).

PROPERTIES OF ABSORBENTS

Henry’s law constants for CO, CO 2 , NO and H 2 S, are pre-

sented in Table 5. Lower values of H such as those for H 2 S

correspond to higher solubility values. Table 6 contains spe-

cific wt. fraction absorbed at equilibrium vs. gas partial pres-

sure for both ammonia and SO 2 in water; Table 7 has similar

material for HCl.

Highly soluble materials have absorption rates which are

controlled by diffusion through the gas phase (see Table 8).

REACTION

Processes exist for catalytically removing gaseous pollutants

by either forming harmless products or products more ame-

nable to recovery. The behavior of most catalytic reactions

requires a more substantial analysis than homogeneous sys-

tems because of the presence of at least two phases. One of the

FIGURE 15 Flexitrary Scrubber (Courtesy of Koch

Engineering).

C022_001_r03.indd 1225 11/18/2005 2:33:02 PM