Wet scrubber

The term wet scrubber describes a variety of devices that remove 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.
Wet scrubbers capture relatively small dust particles with the wet scrubber's large liquid droplets. In most wet scrubbing systems, droplets produced are generally larger than 50 micrometres. As a point of reference, human hair ranges in diameter from 50 to 100 micrometres. The size distribution of particles to be collected is source specific.
For example, particles produced by mechanical means tend to be large ; whereas, particles produced from combustion or a chemical reaction will have a substantial portion of small and submicrometre particles.
The most critical sized particles are those in the 0.1 to 0.5 micrometres range because they are the most difficult for wet scrubbers to collect.

Design

The design of wet scrubbers or any air pollution control device depends on the industrial process conditions and the nature of the air pollutants involved. Inlet gas characteristics and dust properties are of primary importance. Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. The versatility of wet scrubbers allow them to be built in numerous configurations, all designed to provide good contact between the liquid and polluted gas stream.
Wet scrubbers remove dust particles by capturing them in liquid droplets. The droplets are then collected, the liquid dissolving or absorbing the pollutant gases. Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by means of another device referred to as a mist eliminator or entrainment separator. Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant.
A wet scrubber's ability to collect small particles is often directly proportional to the power input into the scrubber. Low energy devices such as spray towers are used to collect particles larger than 5 micrometers. To obtain high efficiency removal of 1 micrometer particles generally requires high-energy devices such as venturi scrubbers or augmented devices such as condensation scrubbers. Additionally, a properly designed and operated entrainment separator or mist eliminator is important to achieve high removal efficiencies. The greater the number of liquid droplets that are not captured by the mist eliminator, the higher the potential emission levels.
Wet scrubbers that remove gaseous pollutants are referred to as absorbers. Good gas-to-liquid contact is essential to obtain high removal efficiencies in absorbers. Various wet-scrubber designs are used to remove gaseous pollutants, with the packed tower and the plate tower being the most common.
If the gas stream contains both particulate matter and gases, wet scrubbers are generally the only single air pollution control device that can remove both pollutants. Wet scrubbers can achieve high removal efficiencies for either particles or gases and, in some instances, can achieve a high removal efficiency for both pollutants in the same system. However, in many cases, the best operating conditions for particles collection are the poorest for gas removal.
In general, obtaining high simultaneous gas and particulate removal efficiencies requires that one of them be easily collected, or by the use of a scrubbing reagent such as lime or sodium hydroxide.
The "cleaned" gases are normally passed through a mist eliminator to remove water droplets from the gas stream. The dirty water from the scrubber system is either cleaned and discharged or recycled to the scrubber. Dust is removed from the scrubber in a clarification unit or a drag chain tank. In both systems solid material settles on the bottom of the tank. A drag chain conveyor system removes the sludge and deposits in into a dumpster or stockpile.

Droplet production

Droplets are produced by several methods:

Injecting liquid at high pressure through specially designed nozzles
Aspirating the particle-laden gas stream through a liquid pool
Submerging a whirling rotor in a liquid pool.

These droplets collect particles by using one or more of several collection mechanisms such as impaction, direct interception, diffusion, electrostatic attraction, condensation, centrifugal force and gravity. However, impaction and diffusion are the main ones.

Impaction

In a wet scrubbing system, dust particles will tend to follow the streamlines of the exhaust stream. However, when liquid droplets are introduced into the exhaust stream, particles cannot always follow these streamlines as they diverge around the droplet. The particle's mass causes it to break away from the streamlines and impact or hit the droplet.
Impaction increases as the diameter of the particle increases and as the relative velocity between the particle and droplets increases. As particles get larger they are less likely to follow the gas streamlines around droplets. Also, as particles move faster relative to the liquid droplet, there is a greater chance that the particle will hit a droplet. Impaction is the predominant collection mechanism for scrubbers having gas stream velocities greater than 0.3 m/s .
Most scrubbers operate with gas stream velocities well above 0.3 m/s. Therefore, at these velocities, particles having diameters greater than 1.0 μm are collected by this mechanism. Impaction also increases as the size of the liquid droplet decreases because the presence of more droplets within the vessel increases the probability that particles will impact on the droplets.

Diffusion

Very small particles experience random movement in an exhaust stream. These particles are so tiny that they are bumped by gas molecules as they move in the exhaust stream. This bumping, or bombardment, causes them to first move one way and then another in a random manner, or to diffuse, through the gas. This irregular motion can cause the particles to collide with a droplet and be collected. Because of this, diffusion is the primary collection mechanism in wet scrubbers for particles smaller than 0.1 μm.
The rate of diffusion depends on the following:

The relative velocity between the particle and droplet
The particle diameter
The liquid-droplet diameter.

For both impaction and diffusion, collection efficiency increases with an increase in relative velocity and a decrease in liquid-droplet size.
However, collection by diffusion increases as particle size decreases. This mechanism enables certain scrubbers to effectively remove the very tiny particles.
In the particle size range of approximately 0.1 to 1.0 μm, neither of these two collection mechanisms dominates. This relationship is illustrated in Figure 3.

Other collection mechanisms

In recent years, some scrubber manufacturers have utilized other collection mechanisms such as electrostatic attraction and condensation to enhance particle collection without increasing power consumption.
In electrostatic attraction, particles are captured by first inducing a charge on them. Then, the charged particles are either attracted to each other, forming larger, easier-to-collect particles, or they are collected on a surface.
Condensation of water vapor on particles promotes collection by adding mass to the particles. Other mechanisms such as gravity, centrifugal force, and direct interception slightly affect particle collection.

Advantages and disadvantages

For particulate control, wet scrubbers are evaluated against fabric filters and electrostatic precipitators. Some advantages of wet scrubbers over these devices are as follows:

Wet scrubbers have the ability to handle high temperatures and moisture.
In wet scrubbers, flue gases are cooled, resulting in smaller overall size of equipment.
Wet scrubbers can remove both gases and particulate matter.
Wet scrubbers can neutralize corrosive gases

Some disadvantages of wet scrubbers include corrosion, the need for entrainment separation or mist removal to obtain high efficiencies and the need for treatment or reuse of spent liquid.
Wet scrubbers have been used in a variety of industries such as acid plants, fertilizer plants, steel mills, asphalt plants, and large power plants.

Advantages

Disadvantages

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.

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 pollution problems: ash ponds, settling ponds or sludge clarifiers may be needed to meet wastewater regulations.

Difficult product recovery: Dewatering and drying of scrubber sludge make recovery of any dust for reuse very expensive and difficult.