AIR POLLUTION CONTROL DEVICES:

VENTURI SCRUBBER

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

WHAT ARE AIR POLLUTION CONTROL DEVICES?

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 differ-ent levels of industry are in effect. Control devices either eliminate or reduce the extent of pollutant re-leased into the atmosphere. Control devices

The following items are commonly used as pollution control devices by industry or transportation devices. They can either destroy contaminants or remove them from an exhaust stream before it is emitted into the atmosphere.

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.

TYPES OF SCRUBBERS 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.

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.

WHAT IS VENTURI SCRUBBER? 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.

CONSTRUCTION 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.

Figure 1

WORKING 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.

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.

Figure 2

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.

Figure 3

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.

Figure 4

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.

Figure 5

ADVANTAGES OF VENTURI SCRUBBER

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.

DISADVANTAGES OF VENTURI SCRUBBER

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.