Venturi scrubber

2.  What are air pollution control devices?
 Wet scrubbers
 Venturi scrubber
 Construction and Working
 Advantages and Disadvantages.

3.  Since the industrial revolution, air pollution has been
on a steady rise and to reduce it, certain efforts have
been undertaken. Various control devices at differe
nt levels of industry are in effect. Control devices
either eliminate or reduce the extent of pollutant rel
eased into the atmosphere. Control devices
 The following items are commonly used as pollution c
ontrol devices by industry or transportation devices. T
hey can either destroy contaminants or remove the
m from an exhaust stream before it is emitted into the
atmosphere.

4. Mechanical collectors (dust cyclones, multicyclones):
Electrostatic precipitators An electrostatic precipitator (ESP), or
electrostatic air cleaner is a particulate collection device that removes
particles from a flowing gas (such as air) using the force of an induced
electrostatic charge. Electrostatic precipitators are highly efficient
filtration devices that minimally impede the flow of gases through the
device, and can easily remove fine particulates such as dust and smoke
from the air stream.
Baghouses:
Designed to handle heavy dust loads, a dust collector consists of a
blower, dust filter, a filter-cleaning system, and a dust receptacle or dust
removal system (distinguished from air cleaners which utilize disposable
filters to remove the dust).
Particulate scrubbers:
Wet scrubber is a form of pollution control technology. The term
describes a variety of devices that use pollutants from a furnace flue gas
or from other gas streams. In a wet scrubber, the polluted gas stream is
brought into contact with the scrubbing liquid, by spraying it with the
liquid, by forcing it through a pool of liquid, or by some other contact
method, so as to remove the pollutants.

5.  They are broadly classified as
 1)Dry Scrubbers.
 2) Wet Scrubbers.
 Following are certain wet scrubbers:
 Baffle spray scrubber
 Cyclonic spray scrubber
 Ejector venturi scrubber
 Mechanically aided scrubber
 Spray tower.

6. The term wet scrubber describes a
variety of devices that remove
pollutants from a furnace flue gases or
from other gas streams. In a wet
scrubber, the polluted gas stream is
brought into contact with the scrubbing
liquid, by spraying it with the liquid, by
forcing it through a pool of liquid, or by
some other contact method, so as to
remove the pollutants.

7.  Venturi Scrubber is a type of air pollution
control devices.
 It falls under the category of wet scrubbers.
 About 35 years ago, Johnstone (1949) and
other researchers found that they could
effectively use the venturi configuration to
remove particles from gas streams.

8.  A venturi scrubber consists of three sections: a
converging section, a throat section, and a
diverging section.
 The inlet of polluted gases is through the
converging section of the scrubber.
 Whereas, water containing the washed away
suspended particles and dissolved gases finds an
outlet at the diverging section.

9. Figure 1

10.  The inlet gas stream enters the converging section and, as
the area decreases, gas velocity increases (in accordance
with the Bernoulli Equation). Liquid is introduced either at
the throat or at the entrance to the converging section.
 The inlet gas, forced to move at extremely high velocities in
the small throat section, shears the liquid from its walls,
producing an enormous number of very tiny droplets.
 Particle and gas removal occur in the throat section as the
inlet gas stream mixes with the fog of tiny liquid droplets.
The inlet stream then exits through the diverging section,
where it is forced to slow down.
 Venturis can be used to collect both particulate and gaseous
pollutants, but they are more effective in removing particles
than gaseous pollutants.

11. Liquid can be injected at the converging
section or at the throat. Figure 2 shows
liquid injected at the converging
section. Thus, the liquid coats the venturi
throat making it very effective for
handling hot, dry inlet gas that
contains dust. Otherwise, the dust would
have a tendency to cake on or abrade a
dry throat. These venturis are sometimes
referred to as having a wetted approach.

12. Figure 2

13. Figure 3 shows liquid injected at the
venturi throat. Since it is sprayed at or
just before the throat, it does not actually
coat the throat surface. These throats are
susceptible to solids buildup when the
throat is dry. They are also susceptible to
abrasion by dust particles. These venturis
are best used when the inlet stream is cool
and moist. These venturis are referred to
as having a non-wetted approach.

14. Figure 3

15. Venturis with round throats (Figures
2 and 3) can handle inlet flows as
large as 88,000 m³/h (40,000 cfm)
(Brady and Legatski 1977). At inlet
flow rates greater than this, achieving
uniform liquid distribution is difficult,
unless additional weirs or baffles are
used. To handle large inlet flows,
scrubbers designed with long, narrow,
rectangular throats (Figure 4) have
been used.

16. Figure 4

17. Water sprays help prevent solids buildup. The principal
atomization of the liquid occurs at the rods, where the
high-velocity gas moving through spacings creates the small
droplets necessary for fine particle collection. These rods
must be made of abrasion-resistant material due to the high
velocities present.
All venturi scrubbers require an entrainment separator
because the high velocity of gas through the scrubber will
have a tendency to entrain the droplets with the outlet
clean gas stream.
Cyclonic, mesh-pad, and blade separators are all used to
remove liquid droplets from the flue gas and return the
liquid to the scrubber water. Cyclonic separators, the most
popular for use with venturi scrubbers, are connected to
the venturi vessel by a flooded elbow (Figure 5). The liquid
reduces abrasion of the elbow as the outlet gas flows at
high velocities from the venturi into the separator.

18. Figure 5

19.  Small space requirements: Scrubbers reduce the temperature
and volume of the unsaturated exhaust stream. Therefore,
vessel sizes, including fans and ducts downstream, are smaller
than those of other control devices. Smaller sizes result in lower
capital costs and more flexibility in site location of the
scrubber.
 No secondary dust sources: Once particulate matter is
collected, it cannot escape from hoppers or during transport.
 Handles high-temperature, high-humidity gas streams: No
temperature limits or condensation problems can occur as in
baghouses or ESPs.
 Minimal fire and explosion hazards: Various dry dusts are
flammable. Using water eliminates the possibility of explosions.
 Ability to collect both gases and particulate matter.

20.  Corrosion problems: Water and dissolved pollutants
can form highly corrosive acid solutions. Proper
construction materials are very important. Also, wet-
dry interface areas can result in corrosion.
 High power requirements: High collection
efficiencies for particulate matter are attainable only
at high pressure drops, resulting in high operating
costs.
 Water-disposal problems: Settling ponds or sludge
clarifiers may be needed to meet waste-water
regulations.
 Difficult product recovery: Dewatering and drying of
scrubber sludge make recovery of any dust for reuse
very expensive and difficult.