Page 298 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 277

The hardness in the groundwater contains mainly calcium bicarbonate, magnesium
bicarbonate, magnesium sulfate, and calcium sulfate, which are to be removed. CO
2
in the flue gas is reused as a chemical for hardness removal from the groundwater.
Lime (calcium hydroxide or calcium oxide) and soda ash (sodium carbonate) are addi-
tional chemicals required for groundwater treatment as well as carbon dioxide gas
stripping. The following are chemical reactions for the combined flue gas (air emis-
sion stream) and groundwater treatment in the combined wet scrubbing and stripping
process system:

Contaminated flue gas → air + CO
2
Contaminated groundwater → H O + VOCs + Ca(HCO ) + Mg(HCO )
2 3 2 3 2
CO + Ca(OH) → CaCO (precipitate) + H O
2 2 3 2
Ca(HCO ) + Ca(OH) → 2CaCO (precipitate) + 2H O
3 2 2 3 2
Mg(HCO ) + Ca(OH) → CaCO (precipitate) + MgCO + 2H O
3 2 2 3 3 2
MgCO + Ca(OH) → Mg(OH) (precipitate) + CaCO (precipitate)
3 2 2 3
MgSO + Ca(OH) → Mg(OH) (precipitate) + CaSO
4 2 2 4
CaSO + Na CO → CaCO (precipitate) + Na SO
4 2 3 3 2 4
Air effluent → air + VOCs (to be removed by gas phase GAC)
Purified air → ambient environment
Purified groundwater (H O) → industrial water supply
2
The precipitates produced from the above chemical reactions occurred in the com-
bined wet scrubbing and stripping process and must be further removed by one of the
following water–solid separation processes (49,57), before the purified groundwater
can be reused as an industrial water supply: (1) dissolved air flotation and filtration,
(2) sedimentation and filtration, or (3) ultrafiltration or microfiltration.
The air effluent from the combined wet scrubbing and stripping process will contain
air and stripped VOCs. Before the air effluent can be discharged into ambient envi-
ronment, it must be further purified by gas-phase granular activated carbon (GAC) or
an equivalent air pollution process.

More research on simultaneous air and water pollution by a combined wet scrubbing
and stripping process system should be conducted aiming at water recycle, greenhouse
gas reduction, and resource recovery (i.e., CO is a useful chemical for pH control,
2
hardness precipitation, protein precipitation).

Example 23
What is the sound engineering solution to a described process situation? A chemical com-
pany in southern Louisiana manufactures 100,000 tons per day of herbicide and was faced
with a potentially costly dilemma. The plant needed to treat the plant’s output but required
a Cl stripper with a capacity of 75 gpm to do so. This represented a 50% increase in the
2
capacity of the existing Cl stripper or the need to build (1) a second stripper, (2) a new strip-
2
per, or (3) find a packing that would allow for the 50% increase in capacity in the existing
stripper. If possible, solution 3 is the most economical choice. This means that the plant
would need to find a packing that had a substantially lower pressure drop compared to the