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Raschig ring -- most popular type.
Berl saddle -- ecient but costly.
Pall rings -- good liquid distribution.
Te llerette -- very low unit weight.
Intalox saddle -- ecient but expensive.
FIGURE 16.3
Various packing used in packed tower scrubbers. (Adapted from AIHA,
Air Pollution
Manual: Control Equipment, Part II
, American Industrial Hygiene Association, Detroit, MI, 1968.)
16.3.2 e
quilibrium
s
olubility
and
h
enry
'
s
l
aW
Under certain conditions, Henry's law can express the relationship between the gas phase concen-
tration and the liquid phase concentration of the contaminant at equilibrium. Henry's law states that
for dilute solutions, where the components do not interact, the resulting partial pressure (
p
) of a
component
A
in equilibrium with other components in a solution can be expressed as
p
=
Hx
A
(16.1)
where
p
= Partial pressure of contaminant in gas phase at equilibrium.
H
= Henry's law constant.
x
A
= Mole fraction of contaminant or concentration of
A
in liquid phase.
Equation 16.1 is the equation of a straight line where the slope (
m
) is equal to
H
. Henry's law can
be used to predict solubility only if the equilibrium line is straight—that is, when the solute concen-
trations are very dilute. In air pollution control applications this is usually the case. For example,
an exhaust stream that contains a 1000-ppm SO
2
concentration corresponds to a mole fraction of
SO
2
in the gas phase of only 0.001. Another restriction on using Henry's law is that it does not hold
true for gases that react or dissociate upon dissolution. If this happens, the gas no longer exists as
a simple molecule. For example, scrubbing HF or HCl gases with water causes both compounds to
dissociate in solution. In these cases, the equilibrium lines are curved rather than straight. Data on
systems that exhibit curved equilibrium lines must be obtained from experiments.
The units of Henry's law constants are atm/mole fraction. The smaller the Henry's law constant,
the more soluble the gaseous compound is in the liquid. The following example from USEPA (1981)
illustrates how to develop an equilibrium diagram from solubility data.
■
EXAMPLE 16.1
Problem:
Given the following data for the solubility of SO
2
in pure water at 303°K (30°C) and 101.3
kPa (760 mmHg), plot the equilibrium diagram and determine if Henry's law applies.
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