Venturi scrubber
This type of technology is a part of the group of air pollution controls collectively referred to as wet scrubbers.
In the late 1940s, H.F. Johnstone[1], William Jones,[2] and other researchers found that they could effectively use the venturi configuration to remove particles from gas streams.
The inlet gas, forced to move at extremely high velocities in the small throat section, turbulently mixes with the liquid, producing an enormous number of very tiny droplets.
This method allows particulates in the stream that may be prone to caking onto surfaces to be washed away and reduces the mechanical abrasion of particles hitting the throat at high speed.
Wetting of the throat can be achieved with a spray directed at the walls or with a weir encircling the converging section which water flows over.
This method can be used only at liquid injection source as the high velocity gas will shear droplets from the walls.
Gas flows through the annular opening and atomizes liquid that is sprayed onto the plunger or swirled in from the top.
[4] 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.
[3] The rectangular-throat venturi is often built to be adjustable by introducing moving plates or flaps into the throat, as shown in Figure 6.
By placing a number of pipes parallel to each other, a series of longitudinal venturi openings can be created as shown in Figure 7.
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.
The ejector venturi is unique among available scrubbing systems since it can move the process gas without the aid of a fan or blower.
The liquid spray coming from the nozzle creates a partial vacuum in the side duct of the scrubber.
In the case of easily-clogging material, explosive gasses, or extremely corrosive atmospheres, the elimination of a fan in the system can avoid many potential problems.
The atomized liquid provides an enormous number of tiny droplets for the dust particles to impact on.
These liquid droplets incorporating the particles must be removed from the scrubber outlet stream, generally by cyclonic separators.
At these pressure drops, the gas velocity in the throat section is usually between 30 and 120 m/s (100 to 400 ft/s), or approximately 270 mph at the high end.
[5] Particle collection occurs primarily by impaction as the exhaust gas (from the process) passes through the spray.
[6] Ejector venturis have a short gas-liquid contact time because the exhaust gas velocities through the vessel are very high.
To reduce abrasion here, the elbow at the bottom of the scrubber (leading into the separator) can be flooded (i.e. filled with a pool of scrubbing liquid).
A common technique to help reduce abrasion is to use a precleaner (i.e., quench sprays or cyclone) to remove the larger and more damaging particles.
In addition, the high gas velocities necessitate the use of entrainment separators to prevent excessive liquid carryover.
[3] 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.
