AIR POLLUTION CONTROL EQUIPMENT SELECTION GUIDE - CHAPTER 19 pdf

chapter 19Venturi scrubbers Device type Venturi scrubbers are wet scrubbers that use a change in gas velocity to shear liquid streams usually water into tiny target droplets into which p

chapter 19

Venturi scrubbers

Device type

Venturi scrubbers are wet scrubbers that use a change in gas velocity to shear liquid streams (usually water) into tiny target droplets into which particulate and soluble gases are transferred They are considered as a workhorse of the available air pollution control technologies given their low capital cost, reli-ability, and effectiveness on a variety of applications They tend to use more energy than alternative designs particularly on applications treating over 50,000 acfm of gases Venturi scrubbers are used where the collected product can be handled wet They are often used on processes, such as calciners and dryers, wherein the blowdown from the scrubber can be returned to a wet portion of the process They can also handle the heavy dust loadings, which can occur from these sources Venturi scrubbers can ingest dust loadings of over 30 grs/dscf if designed correctly Figure 19.1 shows a rectangular throat Venturi scrubber, a workhorse of the wet scrubbing industry

Typical applications

Venturi scrubbers are best used to remove particulate 0.6 µm aerodynamic diameter and larger where the gas flow is from 1 to 500,000 acfm if the particles are 10 µm and larger, and from 1 to 50,000 acfm if the particles are 0.6 µm and larger They have been successfully used, however, to remove submicron particulate at pressure drops of up to about 60 inches water column

If the gas stream has primarily submicron particulate (say from a hazardous waste incinerator), a condensing wet scrubbing system, or a wet electrostatic precipitator, or similar lower energy input system might be used instead

There are literally hundreds of applications, however, in which the par-ticulate is 1 to 20 µm diameter where the Venturi scrubber provides excellent results The result is that thousands of Venturi scrubbers are in daily use throughout the world

Rectangular throat Venturis are commonly used on product dryers and calciners where there is a wet stage Mineral lime kilns and lime sludge kilns (such as in the recausticizing section of a Kraft pulp mill) often use Venturi scrubbers Agricultural product rotary dryers are often equipped with pri-mary product collecting cyclones, which are followed by Venturi scrubbers Grinding milling (wet), mulling, and other operations that generate dust often use Venturi scrubbers for particulate control Venturis on mineral lime kilns usually operate a 10 to 16 inch water column pressure drop and units

on lime sludge kilns are designed to run at 22 to 26 inches water column and sometimes higher if the lime mud being burned is high in sodium Boilers such as those firing bagasse or bark are often equipped with Venturi scrubbers The boiler usually incorporates a primary knockout zone and cyclone collector followed by a medium energy Venturi (approximately

10 to 15 inches water column)

Some metallurgic furnaces are equipped with higher energy Venturi scrubbers because the particles generated are smaller

Annular Venturi scrubbers are used when the gas volume exceeds about 25,000 acfm The reason for this is that designers like to maintain a throat width of 4 to 6 inches maximum Sometimes a rectangular throat of this size would be too long to suit the gas inlet The throat is therefore wrapped around to form the annular type These designs are often seen on waste burning boilers, larger kilns and calciners, and large capacity dryers Figure 19.2 shows an annular Venturi scrubber designed and built by TREMA in Europe Note the ring-shaped liquid header at the top and the throat positioner at the bottom

Eductor Venturi scrubbers are used where the designer wants to elimi-nate the use of a fan and is willing to use more liquid at higher pressure instead These conditions might prevail where space is limited, the source

Figure 19.1 Venturi scrubber (Bionomic Industries Inc.).

may be explosive (a fan wheel spark could cause a problem) or where the application requires simplicity Eductors are used on tank vent systems, on tools used in the manufacture of semiconductor products, on odor control systems where fan noise may be an issue, and on emergency gas control systems (such as for chlorine control)

Reverse jet scrubbers are used on these same applications with primary focus on applications where the total energy input is an issue They use a lower static pressure fan but a higher pressure pump, but have a lower total energy input in many cases

Operating principles

Venturi scrubbers all operate by creating a dispersion of closely packed target droplets into which the contaminant particulate is impacted The droplet dispersion may be created by a high differential velocity between the scrubbing liquid and the gas resulting in a droplet-forming shearing effect Other designs use pump hydraulic pressure and spray nozzles to generate the droplets The overall intent is to impact the smaller particle into the larger droplet, which is more easily separated from the carrying gas stream inertially

Once the particulate is impacted into the droplet, the droplet is separated from the gas stream using centrifugal force or interception on a waveform (chevron), baffle, or similar device

Primary mechanisms used

Impaction is the primary collection mechanism in Venturi scrubbers (see Chapter 1) Interception and diffusion also come into play particularly at

Figure 19.2 Annular Venturi scrubber (Trema Verfahrenstechnik GmbH)

pressure drops above 10 to 15 inches water column where the smaller droplet size and droplet proximity enhance such capture mechanisms

For gas absorption, diffusion is considered to be the primary method of capture Venturi scrubbers can sometimes achieve 0.5 to 1.0 transfer units, although the residence time in the Venturi throat is very short (typically milliseconds)

Design basics

Typical Venturi scrubber types are:

1 Rectangular throat designs, both fixed throat and adjustable

2 Annular type designs wherein the throat zone is an annular gap This gap can be adjusted by moving the center body plumb-bob up and down to vary the open area and, therefore, the pressure drop

3 Eductor Venturis wherein the momentum of pressurized liquid in-troduced into the device both provides mass transfer and provides motive force to the gas

4 Reverse jet designs wherein the liquid is injected countercurrent to the gas flow These designs force the particle into a nearly head-on collision with the liquid spray to enhance the application of the spray energy

5 Collision type designs split the gas streams and impacts them nearly head-on to enhance momentum transfer from gas to particle

6 Some Venturi scrubbers are made from parallel tubes or pipes as in the multi-Venturi (see below) These pipes may be oriented horizon-tally, vertically or on an inclined angle The scrubbing liquid is usu-ally sprayed on the tubes or pipes The slots formed between the pipes for the Venturi shape

Gas inlet velocities for all of these designs are generally the same as the ductwork conveying velocities, that is, 45 to 60 ft/sec The Venturi section outlet duct is usually sized for a similar velocity to reduce pressure losses through velocity changes

The liquid rate for gas velocity atomized Venturis (using fans) is 5 to 30 gpm/1000 acfm treated with 5 to 10 gallons/1000 acfm being common The liquid-to-gas ratio is increased as the inlet dust loading is increased Liquid pressures are under 15 psig with 5 to 10 psig being common Hydraulically pressurized (spray nozzle type) Venturi scrubbers may use lower liquid rates; however, it is the dust loading that truly dictates the liquid rate The greater the particulate loading, the higher the liquid rate Lime kilns, with inlet dust loadings of over 20 grs/dscf, may use 15 to 20 gallons/1000 acfm, whereas a dryer equipped with a product recovery cyclone may use only 4

to 8 gallons/1000 acfm Figure 19.3 shows the manner in which the L/G increases with increasing dust loading

Various researchers have derived equations based on fluid mechanics to predict the pressure drop of a Venturi scrubber Formulas by Howard Hes-keth were presented in the book Wet Scrubbers (Technomic/CRC Publishers) and in other publications Seymour Calvert, Shui-Chow Yung, and others produced useful equations that also predict the pressure drop Venturi scrub-ber vendors use these predictions (often with some modification to suit their particular designs) to size the Venturi throat zone It is therefore suggested that vendors be relied on to make Venturi throat parameter selections Suspended solids contents of 6 to 8% and higher are not uncommon, although many units operate at 2 to 4% suspended solids This is significantly higher than many other wet scrubber designs (such as tray scrubbers) Designs using nozzles are typically limited to approximately 2 to 4% sus-pended solids; otherwise, nozzle plugging can occur

Eductor type Venturi designs operate at much higher liquid rates and pres-sures because the liquid is also being used to create a draft These units run at

20 to 50 gallons/1000 acfm with header pressures of 30 to 60 psig being common Reverse jet designs have liquid rates in the range between the gas veloc-ity atomized designs and the eductors The liquid rate can be 50 to 100 gallons/1000 acfm or as low as 3 to 4 gallons/1000 acfm, depending on the dust loading and application

Throat velocities vary from 70 to 90 ft/sec to over 400 ft/sec in high energy designs

Cyclonic separator vertical velocities range from 8 ft/sec to 10 to 12 ft/sec on larger systems (separators over 9 to 10 ft diameter)

The removal efficiency of a Venturi scrubber is a function of its pressure drop Vendors have developed pressure drop versus efficiency curves as shown in Figure 19.4 Knowing the aerodynamic diameter of the particle (as

Figure 19.3 Liquid to gas ratio (L/G) vs loading

INLET PARTICULATE LOADING vs.

LIQUID/GAS RATIO (L/G)

Particulate Loading, grs/dscf

0 0.5 1 3 5 10 20

20 18 16 14 12 10 8 6 4

determined by a cascade impactor), the designer can select the pressure drop

at which the Venturi must operate Often, removal guarantees can only be provided based on a known particle size distribution

Let’s look at various Venturi scrubber designs

A rendering in cut-away of an annular Venturi is shown in Figure 19.5 The gas inlet in this sketch is at the top and the gas outlet is at the lower left The conical device in the cutaway portion is the plumb-bob It defines the annular gap between itself and the tapered vessel wall The slope or pitch angle of the plumb-bob allows the throat area to be adjusted as the plumb-bob moves up (to increase pressure drop) or down (to decrease pres-sure drop) The actuation is usually accomplished by mounting the plumb-bob on a pipe resulting in what looks like an umbrella The pipe extends down to the base of the Venturi and terminates outside the vessel Moving this pipe or shaft up or down moves the plumb-bob A packed seal is incorporated surrounding the shaft to prevent leakage These throats can be automated by using an electric or pneumatic jackscrew positioner to move the pipe based on pressure drop or draft signal

Figure 19.4 Composite fractional efficiency curve (From Schifftner, K and Hesketh, H., Wet Scrubbers, 2nd ed., Technomic Publishers, Lancaster, PA, 1996.)

Eductors, shown above, operate by administering a jet of liquid (usually water) into the throat zone in the direction of gas travel An energy exchange occurs between the liquid and gas The high velocity and therefore kinetic energy of the liquid is exchanged with the surrounding gas, accelerating the gas In part, the gas is also entrapped between droplet arrays and is pulled

Figure 19.5 Annular Venturi (Bionomic Industries Inc.).

Figure 19.6 Eductor type Venturi (Bionomic Industries Inc.).

C C

through the unit The diverging section helps to enhance the effect by allow-ing the droplets to slow down and achieve greater energy transfer

Eductors can actually produce a draft at the eductor inlet without the use of an external gas moving device (such as a fan) They are therefore often used where a rotating device such as a fan would not be compatible with the process, or space does not allow its installation They are often used for small gas flows such as ventilating tanks or collecting dopant gases from semiconductor manufacturing The mechanical efficiency is quite low, how-ever, so they are not commonly used on high volume (over 5000 acfm) without a supplemental fan

The Dyna-Wave scrubber (Figure 19.7) improves impaction by spraying the scrubbing liquid countercurrent into the gas stream The velocity of the liquid is directed into the gas stream so the differential velocity is much higher than in a conventional Venturi scrubber This allows less gas side pressure drop to be used and can save horsepower by shifting the energy input duty from the low efficiency fan to the higher efficiency pump

A froth is created where the liquid reaches zero velocity and then turns

180 degrees and moves concurrent with the gas The particulate in the gas stream is impacted directly into this froth zone and is removed Dyna-Wave scrubbers have been used on a large number of particulate scrubbing appli-cations The resulting concurrent discharge of the liquid limits, to some extent, their gas absorption capability In those cases, they are used in stages

or are combined with absorbers such as packed towers

Figure 19.7 Reverse jet or Dyna Wave Venturi (Monsanto Enviro-Chem Systems, Inc.).

Dirty Gas In

Clean Gas

Pump

Make-up

The collision scrubber shown in Figure 19.8 was developed by Seymour Calvert and has been used to collect submicron fumes from hazardous waste incinerators and other difficult applications In this case, the inlet gas stream

is split into two equal streams, is turned 90 degrees and is impacted head

on As in the Dyna-Wave, the goal is to maximize the differential in speed between the particle carried by the gas and the liquid These type Venturis can also be made to be adjustable through the use of a movable T section mounted where the two throats converge

The multi-Venturi shown in Figure 19.9 uses closely spaced rods or pipes that create long Venturi slots It is known that an excessive throat width in a Venturi scrubber can result in a loss in efficiency For that reason, and others, multiple Venturis are used The throat width is reduced to a group of narrow slots Although the total open throat area is nearly the same as in a conventional Venturi, the throat width is but a fraction of its conventional cousin The wetted surface of the multi-Venturi is also greater Some say that the increased wetted surface improves particulate removal

It does increase the cost, however, particularly if exotic alloys are used in its construction

For all of the designs, a separating device is used after the Venturi to remove the droplets that are now carrying the collected particles and absorbed gases A cyclonic separator as shown in Figure 19.10 is a very common application Centrifugal force is used to spin the liquid droplets from the gas stream Sometimes a packed tower or mesh pad type separator

Figure 19.8 Collision scrubber (Monsanto Enviro-Chem Systems, Inc.).

follows the Venturi (this is common for eductors, which may precede or be followed by packed towers for enhanced gas absorption)

Crossflow type droplet eliminators as shown in Figure 19.11 are also used These use waveform type droplet eliminators (chevrons) that provide

a surface upon which the droplets impact, accumulate, and drain If the

Figure 19.9 Multi-Venturi ( Hosokawa Mikropul).

Figure 19.10 Cyclonic separator (Bionomic Industries Inc.).

Scrubbing Liquid Header Gas Outlet

Clean Gas Outlet

Demist Section

Slurry Outlet Pro-Demist Baffles

Multi-Ventri

Rod Deck

Dirty Gas Inlet

Stack

Gas inlet