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1) General
The primary function of a
scrubber is to contact chlorine with a scrubbing fluid.
Chlorine scrubbing systems have been installed to abate
a wide range of process streams. Depending on the size
and on the main objective of the scrubber, there are
various options to select from when basic design choices
are made. Examples of streams being scrubbed are continuous
process vents, system emergency relief, and vents from
rooms storing chlorine containing vessels. The selection
of a continuous system over a batch or emergency unit
will impact the method used for transporting the fluids
and the method for bringing them in contact. Each type
of vent poses different demands on equipment. For emergency
chlorine spill control the conservative design system
would assume the chlorine to be wet. A review of the
location for the scrubbing system should consider the
safety of adjacent areas.
The use of redundant instrumentation,
increased safety factors in the design, and stand-by
electrical power supply should be considered. Reliability
takes precedence over economics. Below sections 2) through
8) give guidance to the designer relative to the selection
of system components.
2) Materials of Construction
In the selection process
for the materials to be used, the engineer must consider
the operating parameters in which the scrubbing system
will be expected to perform not only under normal process
conditions but also during process upsets. Different
concentrations and temperatures of hypochlorite and
caustic can affect the corrosiveness of these solutions.
Therefore, the particular conditions will impart the
selection of the appropriate materials of construction.
Each installation may have a different application which
must be studied to obtain a safe, economic, and efficient
system. The use of improper materials of construction
can lead to premature failure of piping and/or the scrubber.
As reference and without
the intention to exclude others, a common set of conditions
found in the operation of a scrubber system show the
demanding environment in which the materials must perform.
These conditions are:
| Component |
Concentration |
Temperature |
| Wet
chlorine gas |
50
to 99 % |
180
to 200oF (82 to 93oC) |
| Caustic
solution |
10
to 50 % |
Ambient |
| Hypochlorite |
5
to 15 % |
70
to 200oF (21 to 93oC) |
3) Contactors
Table 7.2 represents various
types of contacting apparatus presently being used in
scrubbing systems throughout the industry. Comments
are given to provide some of the positive and negative
points of each system. Selection of the contactor must
be dependent on specific need or application.
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CHLORINE CONTACTORS
(TABLE 7.2) |
| Type |
Descriptions |
Properties |
Conditions |
| Sparger |
This
device is made by submerging a distribution pipe
below the surface of the neutralizing liquid and
allowing the stream to be scrubbed to bubble through. |
1.
Very simple, passive and commonly used.
2. Usually batch operation
3. Very economical to build
4. Uses system pressure for contacting |
1.
High D P
2. Misting may be a problem
3. Performance is typically predicted by experience
or test |
| Spray
Tower |
This
device is usually of counter flow design with
multiple layers of nozzles with overlapping spray
patterns of neutralizing solution. |
1.
Low D P
2. Simple design
3. Most economical tower design |
1.
Limited contacting stages
2. Nozzle pluggage possible
3. Performance is typically predicted by experience
or test |
| Packed
Tower |
This
device is usually of counter flow design with
bottom gas entry into open area under packed section.
Liquid enters at top of packed section in liquid
distribution trays or pipes, liquid flows down
through the packed section creating multi stage
contacting. |
1.
High contacting surface - multi-stage contacting
2. Moderate D P
3. Performance readily predicted
4. operates over a wide range of gas flow |
1.
Pluggage of packing possible
2. Complete wetting of packing is necessary for
proper tower performance |
| Tray
Tower |
This
device is usually of counter flow design with
gas entering at the bottom of the column with
scrubbing liquid entering on the top tray. Gas
bubbles through the liquid on each tray creating
a multistage neutralization column |
1.
Moderate to high D
P
2. Multi-stage contacting
3. Performance readily predicted |
1.
Tray pluggage possible
2. Most expensive tower to build
3. Trays must be filled for proper performance |
| Venturi
scrubber |
This
device uses a high pressure liquid stream to create
as vacuum using an ejector. As the high pressure
liquid passes through the throat of the venturi
intimate contacting occurs. |
1.
Eliminates the need for gas mover
2. Pre-engineered units are available |
1.
Number of transfer units limited in a single stage
2. Efficiency determined only by testing
3. Less effective for dilute chlorine stream |
To be continued.
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