This type of technology is a part of the group of air pollution

Air pollution

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

 controls collectively referred to as wet scrubber

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...

s.

An ejector or venturi scrubber

Scrubber

Scrubber systems are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. Traditionally, the term "scrubber" has referred to pollution control devices that use liquid to wash unwanted pollutants from a gas stream...

is an industrial pollution control device, usually installed on the exhaust flue gas stack

Flue gas stack

A flue-gas stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is combusted in an industrial furnace, a power...

s of large 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...

s, but may also be used on any number of other air exhaust

Exhaust gas

Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...

 systems.
To this end, an ejector venturi scrubber (as well as the spray tower

Spray tower

Spray towers or spray chambers are a form of pollution control technology. They consist of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed...

) uses a preformed spray, the difference is that only a single nozzle

Nozzle

A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....

 is used instead of many nozzles. This nozzle operates at higher pressures and higher injection rates than those in most spray chambers

Spray tower

Spray towers or spray chambers are a form of pollution control technology. They consist of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed...

. The high-pressure spray nozzle (up to 689 kPa

Pascal (unit)

The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 or 100 psig) is aimed at the throat section of a venturi

Venturi effect

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

 constriction.

The ejector venturi is unique among available scrubbing systems since it can move the process gas without the aid of a blower or fan

Fan (implement)

A hand-held fan is an implement used to induce an airflow for the purpose of cooling or refreshing oneself. Any broad, flat surface waved back-and-forth will create a small airflow and therefore can be considered a rudimentary fan...

. The liquid spray coming from the nozzle creates a partial vacuum in the side duct of the scrubber. This has the same effect as the water aspirator used in high school chemistry labs to pull a small vacuum for filtering precipitated materials (due to the Bernoulli effect). This partial vacuum can be used to move the process gas through the venturi as well as through the facility's process system. In the case of explosive or extremely corrosive atmospheres, the elimination of a fan in the system can avoid many potential problems.

The energy for the formation of scrubbing droplets comes from the injected liquid. The high pressure sprays passing through the venturi throat form numerous fine liquid droplets that provide turbulent mixing between the gas and liquid phases. Very high liquid-injection rates are used to provide the gas-moving capability and higher collection efficiencies. As with other types of venturis, a means of separating entrained liquid from the gas stream must be installed. 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....

 separators are commonly used to remove remaining small droplets.

Particle collection

Ejector venturis are effective in removing particles larger than 1.0 µm in diameter. These scrubbers are not used on submicrometer-sized particles unless the particles are condensable [Gilbert, 1977]. Particle collection occurs primarily by impaction as the exhaust gas (from the process) passes through the spray.

The turbulence that occurs in the throat area also causes the particles to contact the wet droplets and be collected. Particle collection efficiency increases with an increase in nozzle pressure and/or an increase in the liquid-to-gas ratio

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...

. Increases in either of these two operating parameters will also result in an increase in 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...

 for a given system. Therefore, an increase in pressure drop also increases particle collection efficiency. Ejector venturis operate at higher L/G ratios than most other particulate scrubbers (i.e., 7 to 13 l/m³ compared to 0.4-2.7 l/m³ for most other designs).

Gas collection

Ejector venturis have a short gas-liquid contact time because the exhaust gas velocities through the vessel are very high. This short contact time limits the absorption efficiency of the system. Although ejector venturis are not used primarily for gas removal, they can be effective if the gas is very soluble or if a very reactive scrubbing reagent is used. In these instances, removal efficiencies of as high as 95% can be achieved [Gilbert, 1977].

Maintenance problems

Ejector venturis are subject to abrasion problems in the high-velocity areas - nozzle and throat. Both must be constructed of wear-resistant materials because of the high liquid injection rates and nozzle pressures. Maintaining the pump that recirculates liquid is also very important. In addition, the high gas velocities necessitate the use of entrainment separators to prevent excessive liquid carryover. The separators should be easily accessible or removable so that they can be cleaned if plugging occurs.

Summary

Because of their open design and the fact that they do not require a fan, ejector venturis are capable of handling a wide range of corrosive and/or sticky particles. However, they are not very effective in removing submicrometer particles. They have an advantage in being able to handle small, medium and large exhaust flows. They can be used singly or in multiple stages of two or more in series, depending on the specific application.

Multiple-stage systems have been used where extremely high collection efficiency of particles or gaseous pollutants was necessary. Multiple-stage systems provide increased gas-liquid contact time, thus increasing absorption efficiency.

Table 1 lists the operating parameters for ejector venturis.

Table 1. Operating characteristics of ejector venturis
Pollutant	Pressure drop (Δp)	Liquid-to-gas ratio (L/G)	Liquid-inlet pressure (pL)	Removal efficiency	Applications
Gases	1.3–13 cm of water	7-13 l/m3	100-830 kPa	95% for very soluble gases	Pulp and paper industry Chemical process industry Food industry Metals processing industry
Particles	0.5-5 in of water	50-100 gal/1,000 ft3	15-120 psig	1 µm diameter