Page 290 - Air Pollution Control Engineering

05_chap_wang.qxd 05/05/2004 3:51 pm Page 269
Wet and Dry Scrubbing 269

packed tower wet scrubber (Fig. 1b) using a suitable packing material (such as Q-
PAC from Lantec Products, or an equivalent packing from another manufacturer)
and a typical scrubbing liquor (such as 0.1% caustic and 0.3% sodium hypochlo-
rite), with pH control to 9.0–9.5 and ORP control to 550–600 mV at 80°F and
atmospheric pressure.
2. Described here are the chemical reactions involved in scrubbing hydrogen sulfide
(single stage) and the wet scrubber performance. The wet scrubber described in part
1 has been proven capable of a removal efficiency of 99.9% of hydrogen sulfide from
a contaminated airstream at various airflow rates, superficial gas velocities, liquid flux
rates, tower diameters, and HTU values (Tables 20 and 21, Fig. 15). Different scrub-
bing liquors can be used in hydrogen sulfide control. It is important to realize that
whatever scrubbing liquor is chosen, the chemistry of a hydrogen sulfide scrubber is
essentially two step. First, the hydrogen sulfide becomes soluble in the presence of
caustic and is then oxidized by an oxidizing agent such as hydrogen peroxide, chlorine,
or potassium permanganate. The following reactions are for a single-stage scrub-
bing system using 0.1% caustic and 0.3% sodium hypochlorite to control hydrogen
sulfide emissions:
H S + 2 NaOH → Na S + 2 H O
2 2 2
NaOCl + H O → HOCl + NaOH
2
3 HOCl + Na S → Na SO + 3 HCl
2 2 3
HOCl + Na SO → Na SO + HCl
2 3 2 4
HCl + NaOH → NaCl + H O
2
A single-stage scrubbing system as described in this example will therefore always
need to be overdosed with oxidizing agent. Additionally, sodium hypochlorite
decomposes slowly in storage, which represents additional long-term costs to a
municipality or other industry controlling hydrogen sulfide with a wet scrubber
system.
Sodium hypochlorite may also be consumed if other VOCs or SVOCs are present in
the airborne emissions from a sewage- or water-treatment plant. The presence of such
compounds can be difficult to predict, as these compounds will occur because of the
materials to be processed, time of year, and other factors. If such compounds are present
in the scrubber system, then the discharge Na S will need to be treated and disposed
2
of by chemical precipitation. For instance,
3 Na S + 2 FeCl → Fe S + 6 NaCl
2 3 2 3
where the sodium sulfide is the soluble pollutant, ferric chloride is the soluble pre-
cipitation agent and the ferric sulfide (fool’s gold) is the insoluble precipitate pro-
duced in the aforementioned chemical reaction. Several other precipitation reactions
using different chemicals for control of Na S have been reported by Wang et al. (39).
2
Although other oxidizing agents may be used, as previously mentioned, sodium
hypochlorite remains the predominant choice of oxidizing chemical in H S odor con-
2
trol scrubbing systems in North America. This is so because it is far less expensive
and less dangerous than hydrogen peroxide, it is more active than potassium perman-
ganate (which will also stain purple everything it touches), and it does not have to be
stored in pressurized containers as does chlorine gas.