Encyclopedia of Environmental Science and Engineering, Volume I and II

1238 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

with sulphuric acid produces the gaseous pollutants associ-

ated with this process, most notably hydrogen fluoride, and

silicon tetrafluoride. Both of these gases can be scrubbed in

aqueous medium, but each one causes a severe handling prob-

lem, after scrubbing. The HF is highly corrosive, and the SiF 4

combines with water to form a gelatinous mass which can

clog an improperly designed scrubber.

Another constraint is the extremely low effluent concentra-

tions allowed by law, about 0.02 mg/ft^3. Since each unit in the

process stream is a source, there are at least 250,000 ft^3 /min

which much be scrubbed down from 200–500 mg/ft^3. This is a

reasonable requirement, considering the toxicity of the pollut-

ants, but it is still a difficult objective to attain.

Teller claims to solve all these problems at once with a

cross flow scrubber and a high water flow rate. The device is

a baffled tank with wetted baffles so that any solids which do

not immediately dissolve will just drop to the bottom after

colliding and not clog the air ducts. The large water flow rate

keeps the fluorine concentration low, so that the same effi-

ciency (4.8 transfer units) typical of a countercurrent flow

scrubber can be maintained.

Cost factors cause the designer to look very hard at the

size of this system and its pumping costs. It is obvious that a

scrubber which has a greater than necessary number of trans-

fer units will be un-economical. Evan with a tight design on

both size and pressure drop, the enormous volumes of gas

can still make this system undesirable. A way must be found

to decrease the volume.

The best way to do this is by taking advantage of the

fact that not all the units produce pollutants at the same

INLET

FLUE GAS

CONTAINING

NOx

GAS PHASE

OXIDATION

NO

NO

NO

YES

YES

YES

ARE

NITROGEN

COMPOUNDS

REDUCED

ARE

NITROGEN

COMPOUNDS

REDUCED

OX IDATION-ABSORPTION-REDUCTION

OXIDATION-ABSORPTION

ABSORPTION-REDUCTION

ABSORPTION-OXIDATION

FIGURE 20 Classification system for wet NOx removal processes (Fawcett et al., 1977).

TABLE 17

Flue gas treating applications in the US and Japan

Process type For Units Per cent of total

1 Selective catalytic reduction NOx 70 63.7

2 Selective reduction Exxon Thermal DeNOx NOx 25 22.7

3 Shell/UOP selective NOx and SO 2 2 1.8

4 Shell/UOP selective reduction-adsorption NOx and SO 2 2 1.8

5 Wet oxidation-adsorption reduction NOx and SO 2 9 8.2

6 Wet adsorption-reduction NOx and SO 2 4 3.6

110 100

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