The term wet scrubber describes a variety of devices that remove pollutant

Pollutant

A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

s from a furnace

Furnace

A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...

 flue gas

Flue gas

Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...

 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.

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 (if particles are present) are of primary importance. Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbers remove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases by dissolving or absorbing them into the liquid.

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

Entrainment (engineering)

Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. For example:* The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke....

 eliminator or entrainment

Entrainment (engineering)

Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. For example:* The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke....

 separator (these terms are interchangeable). Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant.

There are numerous configurations of scrubbers and scrubbing systems, all designed to provide good contact between the liquid and polluted gas stream.

Figures 1 and 2 show two examples of wet scrubber designs, including their mist eliminators.
Figure 1 is a venturi scrubber

Venturi scrubber

A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...

 design. The mist eliminator for a venturi scrubber is often a separate device called a cyclonic separator.
Figure 2 has a tower design where the mist eliminator is built into the top of the structure. Various tower designs exist.

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 (or less) 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

Liquid-to-gas ratio

An important parameter in wet scrubbing systems is the rate of liquid flow. It is common in wet scrubber terminology to express the liquid flow as a function of the gas flow rate that is being treated...

 is essential to obtain high removal efficiencies in absorbers. A number of wet scrubber designs are used to remove gaseous pollutants, with the packed tower

Packed bed

In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing...

 and the plate

Theoretical plate

A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Such equilibrium stages may also be referred to as an equilibrium stage, ideal stage or a theoretical...

 tower being the most common.

If the gas stream contains both particle 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 (i.e., that the gases are very soluble in the liquid or that the particles are large and readily captured) or by the use of a scrubbing reagent such as lime

Lime (mineral)

Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...

 or sodium hydroxide.

Advantages and disadvantages

For particulate control, wet scrubbers (also referred to as wet collectors) are evaluated against fabric filters and electrostatic precipitator

Electrostatic precipitator

An electrostatic precipitator , or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas using the force of an induced electrostatic charge...

s (ESPs). Some advantages of wet scrubbers over these devices are as follows:

• Wet scrubbers have the ability to handle high temperature
  Temperature
  Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
  
  s and moisture
  Moisture
  Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...
  
  .
• 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
  Corrosion
  Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
  
   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.

Table 1 summarizes these advantages and disadvantages. Wet scrubbers have been used in a variety of industries such as acid plants, fertilizer

Fertilizer

Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...

 plants, steel mill

Steel mill

A steel mill or steelworks is an industrial plant for the manufacture of steel.Steel is an alloy of iron and carbon. It is produced in a two-stage process. First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or...

s, asphalt

Asphalt

Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

 plants, and large power plants.

Table 1. Relative advantages and disadvantages of wet scrubbers compared to other control devices
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-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.

Wet scrubber systems

Wet scrubber systems generally consist of the following components:

• Ductwork and fan
  Fan (mechanical)
  A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...
  
   system
• A saturation chamber (optional)
• Scrubbing vessel
• Entrainment separator or mist eliminator
• Pumping (and possible recycle system)
• Spent scrubbing liquid treatment and/or reuse system
• An exhaust stack
  Stack
  -Mathematics:* Stack , general category-theoretical concept to formalise "pull-back" operations in geometry and algebra* Algebraic stack, a generalisation of scheme and algebraic space in algebraic geometry; a specific type of the above-Computers:...
  
  

A typical wet scrubbing process can be described as follows:

• Hot flue gas from a furnace enters a saturator (if present) where gases are cooled and humidified prior to entering the scrubbing area. The saturator removes a small percentage of the particulate matter present in the flue gas.
• Next, the gas enters a venturi scrubber
  Venturi scrubber
  A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...
  
   where approximately half of the gases are removed. Venturi scrubbers have a minimum particle removal efficiency of 95%.
• The gas flows through a second scrubber, a packed bed absorber
  Packed bed
  In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing...
  
  , where the rest of the gases (and particulate matter) are collected.
• An entrainment separator or mist eliminator removes any liquid droplets that may have become entrained in the flue gas.
• A recirculation pump moves some of the spent scrubbing liquid back to the venturi scrubber
  Venturi scrubber
  A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...
  
   where it is recycled and the remainder is sent to a treatment system
  Wastewater Treatment
  Wastewater treatment may refer to:* Sewage treatment* Industrial wastewater treatment...
  
  .
• Treated scrubbing liquid is recycled back to the saturator and the packed bed absorber
  Packed bed
  In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing...
  
  .
• Fans
  Fan (mechanical)
  A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...
  
   and ductwork move the flue gas stream through the system and eventually out the stack
  Stack
  -Mathematics:* Stack , general category-theoretical concept to formalise "pull-back" operations in geometry and algebra* Algebraic stack, a generalisation of scheme and algebraic space in algebraic geometry; a specific type of the above-Computers:...
  
  .

Categorization of wet scrubbers

Since wet scrubbers vary greatly in complexity and method of operation, devising categories into which all of them neatly fit is extremely difficult. Scrubbers for particle collection are usually categorized by the gas-side pressure drop

Pressure drop

Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

 of the system. Gas-side pressure drop refers to the pressure difference, or pressure drop

Pressure drop

Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

, that occurs as the exhaust gas is pushed or pulled through the scrubber, disregarding the pressure that would be used for pumping or spraying the liquid into the scrubber.

Scrubbers may be classified by pressure drop as follows:

• Low-energy scrubbers have pressure drops of less than 12.7 cm (5 in) of water.
• Medium-energy scrubbers have pressure drops between 12.7 and 38.1 cm (5 and 15 in) of water.
• High-energy scrubbers have pressure drops greater than 38.1 cm (15 in) of water.

However, most scrubbers operate over a wide range of pressure drop

Pressure drop

Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

s, depending on their specific application, thereby making this type of categorization difficult.

Another way to classify wet scrubbers is by their use - to primarily collect either particulates or gaseous pollutants. Again, this distinction is not always clear since scrubbers can often be used to remove both types of pollutants.

Wet scrubbers can also be categorized by the manner in which the gas and liquid phases are brought into contact. Scrubbers are designed to use power, or energy, from the gas stream or the liquid stream, or some other method to bring the pollutant gas stream into contact with the liquid. These categories are given in Table 2.

Table 2. Categories of wet collectors by energy source used for contact
Wet collector	Energy source used for gas-liquid contact
Gas-phase contacting Liquid-phase contacting Wet film Combination Liquid phase and gas phase Mechanically aided	Gas stream Liquid stream Liquid and gas streams Energy source: Liquid and gas streams Mechanically driven rotor

Material of construction and design

Corrosion can be a prime problem associated with chemical industry scrubbing systems. Fibre-reinforced plastic

Fibre-reinforced plastic

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually fibreglass, carbon, or aramid, while the polymer is usually an epoxy, vinylester or polyester thermosetting plastic...

and dual laminates are often used as most dependable materials of construction.

External links

• The Encyclopedia Of Filters - Dust Collection An overview of the science of dust collection systems, including Wet Scrubbers.