Typical Venturi scrubber types are as follows:
Gas inlet velocities for all these designs are generally the same as the ductwork conveying velocities, that is, 45-60 ft/s. 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 5-30 gpm/1000 acfm treated, with 5-10 gallons/1000 acfm being common. The liquid-to-gas (L/G) ratio is increased as the inlet dust loading is increased. Liquid pressures are under 15 pounds per square inch (psig), with 5-10 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 15-20 gallons/1000 acfm, whereas a dryer equipped with a product recovery cyclone may use only 4-8 gallons/1000 acfm. Figure 19.3 shows the way 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 Hesketh 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 scrubber vendors use these predictions (often with some modification to suit their 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%-8% and higher are not uncommon, although many units operate at 2%-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%-4% suspended solids, otherwise nozzle plugging can occur.
Liquid-to-gas (L/G) ratio versus loading.
Eductor type Venturi designs operate at much higher liquid rates and pressures because the liquid is also being used to create a draft. These units run at 20-50 gallons/1000 acfm, with header pressures of 30-60 psig being common.
Reverse jet designs have liquid rates in the range between the gas velocity atomized designs and the eductors. The liquid rate can be 50-100 gallons/1000 acfm or as low as 3-4 gallons/1000 acfm, depending on the dust loading and application.
Throat velocities vary from 70 to 90 ft/s to over 400 ft/s in high-energy designs.
Cyclonic separator vertical velocities range from 8 ft/s to 10 to 12 ft/s on larger systems (separators over 9-10 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 determined by a cascade impactor), the designer can select the pressure drop at which the Venturi must operate. Often, removal guarantees can be provided based on only a known particle size distribution.
Let us look at various Venturi scrubber designs. A rendering in cutaway of an annular Venturi is shown in Figure 19.5. The gas inlet in this sketch is
Composite fractional efficiency curve. (From Schifftner, K. and Hesketh, H., Wet Scrubbers, 2nd ed., Technomic Publishers, Lancaster, PA, 1996.)
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 pressure drop). The actuation is usually accomplished by mounting the plumb bob on a pipe, resulting in what looks like an
Annular Venturi. (Bionomic Industries, Inc.)
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.
Eductors, shown in Figure 19.6, 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 through the unit. The diverging section helps enhance the effect by allowing the droplets to slow down and achieve greater energy transfer.
Eductors can 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 where space does not allow its installation. They are often used for small gas flows such as ventilating tanks or collecting dopant gases from
Eductor type Venturi. (Bionomic Industries, Inc.)
semiconductor manufacturing. The mechanical efficiency is quite low, however, so they are not commonly used on high volume applications (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
Reverse jet or Dyna-Wave Venturi. (Monsanto Enviro-Chem Systems, Inc.)
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° 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 many particulate scrubbing applications. 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.
Collision scrubber. (Monsanto Enviro-Chem Systems, Inc.)
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, turned 90°, and 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 Venturis can also be made to be adjustable using 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 the designs, a separating device is used after the Venturi to remove the droplets that are now carrying the collected particles and absorbed gases.
Multi-Venturi (BACT Process Systems, Inc.)
Multi Venturi (BACT Engineering).
A cyclonic separator as shown in Figure 19.10 is a quite common application. Centrifugal force is used to spin the liquid droplets from the gas stream. Sometimes a packed tower or mesh pad type separator 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 Venturi operates at over 35 inches water column, many vendors like to use crossflow droplet eliminators rather than cyclonic designs because the former offers greater small droplet removal than the latter. Without proper droplet control, the liquid could be entrained to the stack testing equipment and the particulate those droplets contain be counted as emissions. Droplet separation is critical.
In this configuration, the multi-venturi is located to the right and the cross- flow separator is located to the left.
The crossflow separator typically uses modules configured in parallel such as described in Figure 19.11. The droplets impact on the chevron vanes and then drain downward out of the gas stream. Cleaning sprays are often used to wash the vane surface.
When a multiple-venturi design is in crossflow orientation, the droplet eliminator is integrated with the venturi stage. This results in a compact arrangement that may be useful in applications where headroom is a factor of the installation.
Cyclonic separator. (Bionomic Industries, Inc.)
There are literally thousands of Venturi scrubbers in operation worldwide. General tricks of the trade include sending the scrubbing liquid to an elevation above the Venturi and letting the liquid drain from the bottom of the header into the Venturi if high solids loadings must be handled. Adjustable throats are of great benefit in setting the scrubber pressure drop and tuning the scrubber to the source. These adjustable throats are sometimes automated with a feedback loop to a differential pressure or draft controller that
Crossflow droplet eliminator. (Munters Corp.)
allows the pressure drop to follow a process setpoint or an emissions permit parameter.
If the gas stream contains abrasive particles, wear plates are often used in the upper section (approach section), in the throat, and in the elbow area where the gases turn 90° to enter the separator. These elbows may also be
designed to be flooded with water, that is, the flooded elbow uses the water surface as an abrasion-resistant barrier. The conventional elbow is called a sweep elbow because it sweeps the gases toward the separator.
So-called horizontal Venturi scrubbers are usually inclined on an angle to allow liquid drainage. The gas and liquid tend to take a downward arc trajectory that limits performance, so horizontal Venturis are rare.
Separators are sometimes mounted on top of open surface decant tanks on applications where the collected product may float (such as bark char, bagasse fines, carbon black, etc.). Other units are operated "water-once-through" to flush high dust loadings to a remotely mounted clarifier.
Other systems use a product recovery liquid cyclone in the scrubber recirculation loop. These are sometimes used on foundry cupola or precious metals recovery applications. The underflow from the cyclone is sent to product recovery and the clears go to the Venturi headers.
If the recycle liquid contains solids (such as limestone), the liquid distributor to the Venturi is often mounted above the injection point so that the solids are flushed out of the bottom of the header, thereby reducing buildup and plugging. For clean liquids, the headers often discharge from the top so that the header is always full, and the liquid is evenly distributed.
The simple configuration and reliability of the Venturi scrubber makes it a true air pollution control workhorse.