Page 220 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 199

Carbon dioxide gas can be effectively controlled with a wet scrubber. Unfortunately,
initial capital costs as well as subsequent operating and maintenance costs of such a wet
scrubber limit the use of such scrubbers.
The wet absorption of particulate matter (PM) from a gas stream involves the use of
specially designed particulate scrubbers. AVenturi scrubber captures PM by impingement
and agglomeration of the PM with liquid droplets.
1.3. Dry Scrubbing or Dry Absorption

Two principle methods of dry absorption systems are currently being used in
industry: dry–dry absorption and semidry absorption. A dry–dry system injects a
powdered alkali absorption agent into the polluted gas stream. The semidry method
injects concentrated slurry into the polluted gas stream and then removes the liquid
by evaporation, leaving the active, dry alkali absorption agent. Both methods remove
any alkali agent not consumed or other solid wastes with an electrostatic precipita-
tor or a fabric filter. All dry scrubbers contain a chemical injection zone followed by a
reaction zone where the pollutant in the gas being treated reacts with the dry alkali. The
process is completed with the removal of residual PM by an electrostatic precipitator or
a fabric filter.

2. WET SCRUBBERS
2.1. Wet Absorbents or Solvents
2.1.1. Absorption of Gaseous Pollutants
As previously discussed, absorption is either physical or chemical. Physical absorption
occurs when the pollutant compound dissolves into the solvent (absorbent). If there is a
subsequent reaction between the pollutant and the solvent or chemicals present in the
solvent, then the absorption is said to be chemical. Commonly used liquid absorbents are
most often water or water-based solutions. Less commonly encountered, but nevertheless
significant, are wet scrubbing systems using mineral oil or nonvolatile hydrocarbon oils
as the absorbing liquid.
It is important to note here that when a pollutant is physically absorbed in a wet
scrubber, no destruction of the pollutant species has occurred. The pollutant has simply
moved from the gas phase into the liquid phase. In a subsequent chemical reaction, the
pollutant may be neutralized or otherwise altered but still not destroyed. As a result, a
wet scrubber often produces a liquid stream that must be treated to achieve final
destruction of the given pollutant. Such secondary treatment requires an additional cost
that needs to be included when considering the economics of a wet scrubber project.
The pH of the scrubbing liquor is often an important process parameter. Low-pH
liquor is required for ammonia scrubbing, neutral or high pH is needed for acid gas
scrubbing. When scrubbing trace organics, liquor with alkaline pH is often used as the
absorbent. Common alkali liquors used in scrubbing acid gases and CO are lime and
2
caustic solutions.
Sodium-based salts are always preferable to calcium or other group II (periodic table)
metal salts for adjusting liquor chemistry. This is because almost all sodium compounds
are soluble, whereas deposits from hard water (Ca 2+ and Mg 2+ salts) are often observed