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258 Lawrence K. Wang et al.

5. Oxygenated hydrocarbons: methanol, phenols, ethylene glycol, and so forth.
6. Inorganic gases: hydrogen sulfide, hydrogen chloride, sulfur dioxide, nitrogen
oxide, nitrogen dioxide, and so forth.
7. Metals: mercury, lead, cadmium, arsenic, zinc, and so forth.
8. Polynuclear aromatics: naphthalene, benzo(a)pyrene, anthracene, chrysene, poly-
chlorinated biphenyls (PCBs), and so forth.
9. Pesticides, herbicides: chlordane, lindane, parathion, and so forth.
10. Other (miscellaneous): asbestos, cyanides, radionuclides, and so forth.

The appendix A lists compounds classified as hazardous by the US Environmental
Protection Agency (US EPA) when present in an air emission stream (33). Original
equipment suppliers (OEMs) of commercial absorber (scrubber) systems in North
America are available refs. 31 and 34–36.

Example 18
Removal of sulfur dioxide from an air emission stream by wet scrubbing is presented in
this example in order to assess the suitability of various packings and materials.
Contacting efficiencies and pressure drop of various packings were studied under identi-
cal controlled conditions in a packed tower wet scrubber shown in Fig. 1b.
Q-PAC, 3.5 in. Tri-Packs, 2K Tellerettes, and 50-mm Pall Rings were tested in a counter-
current packed scrubber for removal of SO from an air emission stream. The SO system
2 2
has long been used by environmental engineers for comparison of packings because it
allows for precise, reproducible measurement of operating parameters and mass transfer
rates not affected by changes in the weather. The efficiency of mass transfer depends on
the ability of the packing to create more gas–liquid contacting surface, so the results of this
test are a good predictor of the relative performance of the tested packings in an acid gas
or similar scrubber.
The test apparatus (36,37) consists of a vertical countercurrent scrubber with a cross-
2
sectional area of 6.0 ft packed with the media being tested to a depth of 3.0 ft. The scrubber
is equipped with a variable-speed fan and pump drives allowing an engineer to adjust both
the gas flow and the liquid loading of the scrubber. The air was spiked with SO fed from
2
a cylinder under its own vapor pressure. The injection point was 15 duct diameters
upstream from the scrubber inlet to ensure adequate mixing. The regulator on the SO
2
cylinder was adjusted manually to give an inlet concentration in the range of 80–120 ppmv
(parts per million by volume) at each airflow rate. Inlet and outlet SO concentrations were
2
measured simultaneously using Interscan electrochemical analyzers.
The air emission stream was scrubbed with 2% sodium bicarbonate liquor. An automated
chemical feed system added caustic to maintain a constant pH of 9.15 ± 0.05 throughout the
test. The airstream and liquid flow rates were used in the ranges typically encountered in a
2
wet scrubber operation. The gas loading was varied from 500 to 3000 lb/h-ft corresponding
2
to superficial gas velocities of 110–670 fpm. The liquid loading ranged from 5 to 8 gpm/ft .
The test results are summarized in Table 19 and Figs. 8–14. Gas–liquid contacting effi-
ciency is quantified in terms of the height of transfer unit, or HTU. (This is the depth of
packing required to reduce the SO concentration to approximately 37% of its initial
2
value.) Discuss the following:
1. The suitability of packing materials evaluated under this optimization project for SO
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removal.
2. Chemical reactions involved in SO scrubbing.
2