AQUAMAG in SO2 Scrubbers

1
AQUAMAG in SO2 Scrubbers
Low SO2
The operation of an SO2 scrubber is fairly simple
in principle High concentrations of SO2
in, Dilute or No SO2 out.
High SO2
2
AQUAMAG in SO2 Scrubbers
Low SO2
Henrys Law governs the diffusion of SO2(g) to
SO2(aq). The concentration of SO2(aq) is not
exactly proportional to the partial pressure of
SO2(g) due to ionization as follows SO2(g) H2O
? SO2(aq) H2SO3 H2SO3 ? H HSO3-
High SO2
3
AQUAMAG in SO2 Scrubbers
Low SO2
As a result of this ionization SO2 H2O ? H
HSO3- when fresh water is used as the scrubbing
liquid, it enters at a near neutral pH and leaves
acidic. With high enough SO2 concentrations, pH
lt 2
High SO2
4
AQUAMAG in SO2 Scrubbers
Low SO2
Sometimes caustic soda is used to improve SO2
removal efficiency and neutralize the acid water
leaving the scrubber
High SO2
5
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
NaOH ? Na OH-
The neutralization of the proton formed during
ionization enhances the diffusion process via
Henrys Law.
High SO2
6
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
NaOH ? Na OH-
However, if pH is not carefully controlled,
excess free hydroxyl ions can pull a proton off
of the bisulfite ion leaving sulfite SO3-2
High SO2
7
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
NaOH ? Na OH-
This has the effect of doubling caustic demand
with no net increase in SO2 removal or removal
efficiency.
High SO2
8
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
NaOH ? Na OH-
Moreover, if there are Ca ions present, as is
common in fresh water supplies, the SO3-2 can
lead to unwanted scale or sludge.
High SO2
9
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
Mg(OH)2 ? MgOH OH- ? Mg2 2OH-
NaOH ? Na OH-
BUT, when AQUAMAG is substituted for caustic
soda
High SO2
10
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
Mg(OH)2 ? MgOH OH- ? Mg2 2OH-
pH variations across the scrubber are not nearly
as severe.
High SO2
11
AQUAMAG in SO2 Scrubbers
Low SO2
SO2(g) ? H2SO3 ? H HSO3-
Mg(OH)2 ? MgOH OH- ? Mg2 2OH-
And because of Aquamags relative insolubility,
there are no excess free hydroxyl ions
converting bisulfite to sulfite
High SO2
12
Slowly dissolving magnesium hydroxide particles
release OH- to react directly with H, but are
not available to excessively raise pH or react
with HSO3-
Slowly DissolvingMg(OH)2 Particle Surface