BLEACH PLANT SCRUBBER


Chlorine dioxide and chlorine gases can be found in the vents from bleaching stages and
from chlorine dioxide generating plants. Under the ever increasing stringent
environmental regulations, it is necessary to scrub these vent gases in order to remove
both the chlorine dioxide and chlorine to prevent or reduce emissions to the atmosphere.

The design of scrubbers for removal of these gases is site specific. The location of the
sources and the ability to combine various vents prior to scrubbing determines the
number and location of scrubbers required.

Chlorine is easily scrubbed from these vent gases utilizing an alkaline scrubbing liquor.
Typically, kraft mill weak wash or "E" stage effluents are sufficiently alkaline to absorb the
chlorine through the formation of hypochlorite. A large recirculation of scrubbing liquor
around the scrubber will insure that some hypochlorite is present which will act as a
catalyst in chlorine absorption and reaction.

When chlorine dioxide is also present in the vent gases, additional scrubbing
requirements must be maintained. Chlorine dioxide is soluble in cold water, and
frequently a cold water scrubber is placed in series prior to the chlorine scrubber. The
low solubility of chlorine dioxide in water requires that a large volume of water be utilized
in a scrubber of this type. However, if the scrubbing liquor provides a chemical or organic
component with which the chlorine dioxide can react, more efficient removal of the
chlorine dioxide, can be achieved. With proper selection of the scrubbing medium, both
chlorine and chlorine dioxide can be removed in the same unit.

When sufficient organic residuals are present in weak wash or "E" stage effluent, both
chlorine dioxide and chlorine can be scrubbed from the vent gases. Also, when sodium
sulfide is present, the chlorine dioxide will be reacted. Thus, weak wash, or white liquor,
can become a suitable scrubbing medium.

The scrubbers usually are packed towers constructed of FRP with plastic packing. The
packing selected must be suitable for the scrubbing liquor to be used. When "E" stage
effluents are used, it is necessary to select a packing which will not be plugged by pulp
fiber that may be in the liquor. When a scrubbing liquor containing organics of sulfides
are not available, chlorine dioxide can sometimes be destroyed through the addition of
sulfur dioxide in the gas stream ahead of the scrubber. Reaction products are then
removed in the alkaline scrubber. Prior to design or specification of a bleach plant scrubber, the
source locations, the chlorine dioxide and chlorine contents of the sources, and the
final emission levels permitted must be determined. Only then can a proper design be
developed.

Waste Chlorine Gas Recovery Scrubber
DL-3975

Waste Chlorine Gas Recovery Scrubber
Typical Arrangement
DL-3976