Venturi scrubber and its process description. Essay

A venturi scrubber has a convergingdiverging flow channel. In this type of system the crosssectional area of the channel decreases along the length of the channel (converging section), initially up to the narrowest area, known as throat. Then after starts increasing (diverging section) as shown in Figure 6.27 which presents a typical crosssection of a Venturi scrubber. The narrowest area is referred to as the throat.
Figure 6.27 Typical crosssection of a venturi scrubber
Dth = Diameter of throat
Lth = Length of throat
= Dth or 3Dth
DC = Diameter of converging section / cone
Dd = Diameter of diverging cone / section
Dc = Dd = 3Dth
Lc = Length of convergent cone / section
= 2.5 Dth
Ld = Length of divergent cone
= 7.5 Dth
LT = Total length of the venturi scrubber
= Lc + Lth + Ld
= Angle of convergent cone / section with the horizontal
= Angle of the divergent cone / section with the horizontal
The Throat Length
For a properly designed scrubber the throat length is important. It should be sufficiently long so that most of the particulate collection occurs in the throat itself. If the throat is too short, few particles will be collected in the throat, so that most of the potential collection efficiency of the device will be lost. It would be preferable to employ a larger diameter, but longer throat, which would achieve the same collection efficiency with less pressure drop. However a throat which is longer than necessary will cause an unnecessarily large pressure drop along it.
The Removal Mechanisms
The mechanisms affecting the collection of particulates in the venturi scrubber are inertial impaction, diffusion, electrostatic phenomenon and condensation and agglomeration. The predominant of all these is the inertial impaction.
The Process Description
In the converging section, the decrease in area causes the waste gas velocity, and hence the turbulence, to increase. The gas velocity is accelerated to 60120 m/s at the throat. The scrubbing liquid, usually water, is injected into the scrubber slightly upstream of the throat or directly into the throat section through low pressure spray nozzles. The scrubbing liquid is atomized by the turbulence in the throat, improving gasliquid contact. The droplets accelerate in the throat section and due to the velocity difference between the particles and the droplets the particles are impacted against the slowmoving droplets. These droplets scrub the gas of its particles as they accelerate to the gas peed This acceleration of the droplets is not likely to be complete at the end of the throat, so that particle collection continues to some extent into the diverging section of the venture. The gasliquid mixture then decelerates as it moves through the diverging section, causing additional particledroplet impacts and agglomeration (grow into a mass) of the droplets. The liquid droplets are then separated from the gas stream in an entrainment section, usually consisting of a cyclonic separator and mist eliminator as shown in Figure 6.28.