AIR POLLUTION CONTROL EQUIPMENT SELECTION GUIDE - CHAPTER 18 doc

© 2002 by CRC Press LLC chapter 18 Vane type scrubbers Device type Vane type scrubbers are wet scrubbers that use one or more stationary vanes through which or within which the contaminant gas streams mix with scrubbing liquid. There are many innovative designs within this category. They are used to remove particulate in the 5 µ m and larger size range and provide moderate gas absorption capability. These scrubbers are considered to be low to medium energy input devices and find themselves in use where the particulate loading is under 4 to 5 grs/dscf and the particle size is 10 µ m or above. There are a number of very interesting and efficient vane type scrubbers currently being provided by vendors worldwide. Typical applications Vane type scrubbers are often found in use on rotary dryers, grinders, mullers, and similar devices producing relatively large particulate. At higher pressure drops (above 10–15 inches water column), cage type units have been used on non-ferrous metals remelt furnaces to remove residual metallic dusts, etc. There are hundreds of vane type scrubbers in daily use. Some, in recent years, have been followed by wet electrostatic precipitators or other devices for enhanced capture of submicron sized particulate. If the gas stream contains sticky particulate, certain vane designs can rapidly plug. Care must be taken to fully and constantly wet all surfaces and this can be difficult to accomplish. Given that they require centrifugal action, the gas speed in this type scrubber is of importance. Turndown ratios of approximately 25% are common, below which point some reduction in efficiency may occur. Operating principles Vane type scrubbers basically use a multiplicity of Venturi scrubber sec- tions to accelerate the gas stream causing liquid that is dispersed on the © 2002 by CRC Press LLC vanes to shear into tiny target droplets. In addition, the designs use centrifugal force to throw the gas stream toward (in most cases) the vessel wall. In some designs, the vanes direct the dispersion of droplets inward into a high droplet density cloud that is configured to maximize impaction and interception. Primary mechanisms used Centrifugal and impaction forces are most commonly applied in vane type scrubbers. The vanes may be configured in a near horizontal plane (with vanes oriented much like a gas turbine blade) or as a vertical cage of vanes similar in appearance to a squirrel cage blower impeller. Other designs use various vane combinations but share these primary separation mechanisms. The vane blades are typically close together forming a multiplicity of Venturis and are angled to deflect the gas stream in a way that increases its rotational motion either outward toward the vessel wall, or inward into a confined spray zone. These blade groups are often sprayed with the scrubbing solution or the liquid is allowed to cascade onto the vane surface. Depending on the orientation of the vane group, the liquid may produce a froth somewhat like a fluidized bed scrubber. A significant difference between the contact zones is that the froth in a vane type scrubber usually proceeds from the vane area and is thrown outward against the vessel wall. In fluidized bed scrubbers, the froth or fluidized zone descends back directly into the gas path and helps to create and maintain the froth zone. Figure 18.1 Mikrovane scrubber (Hokosawa Mikropul). Clean Gas Outlet Scrubbing Liquid Inlet Dirty Gas Inlet Slurry Outlet Scrubbing Vanes Spray Wetted Fan Wheel © 2002 by CRC Press LLC Figure 18.1 shows a vane type scrubber wherein the vane group is horizontal. The gas usually enters tangential below this vane group to impart a cyclonic motion and provide some centrifugal separation as in a cyclone collector. The gases then move vertically into the vane group area where the angle of attack of the vanes helps to impart greater centrifugal force. The reduction in open area given the existence of the vanes tends to accelerate the gas speed. When scrubbing liquid is administered to this zone, impaction, and shearing forces are applied as well. The spinning action of the gas tends to throw the liquid/gas mixture towards the vessel wall where the distance between the droplets created and the gas is reduced. This action helps to increase particulate capture. A popular and clever vertical cage type vane scrubber is made by Ento- leter, Inc. (Hamden, CT). Figure 18.2 shows this vane cage as viewed from above. The gas stream enters tangentially and centrifugal force throws the larger particulate to the vessel wall where it impacts and is flushed down to the sump. The gases then follow a decreasing radius until they reach the vane cage. The slots in the vane cage function as a multiplicity of Venturi scrubber throats. When scrubbing liquid is injected into this swirling stream, the tangential motion tends to spin the liquid at an angle back outward into the path of the gas stream. This action increases the relative velocity between the gas and liquid and improves impaction and separation. A spray cloud is formed in the vane cage zone as diagrammed in Figure 18.3. You can see that the spinning action tries to throw the liquid outward but the gas is being directed inward by the vanes. A droplet cloud is thus formed and these droplets serve as targets for particulate capture much as in a Venturi scrubber. Vane type scrubbers are often found on rotary and tray type dryers such as found in the grain drying industry. Their compact size makes them attrac- tive for roof mounting if space is a problem. As seen in Figure 18.4, a pair of vane type scrubbers are seen roof mounted with the exhaust stacks mounted directly on top of the scrubber’s gas outlet. Figure 18.2 Centrifield scrubber (Entoleter, Inc.). FEED PIPE © 2002 by CRC Press LLC When these type scrubbers have a relatively short gas/liquid contact time their gas absorption can suffer. To improve gas absorption, a vane type scrubber can often be combined with a gas absorber such as a packed tower as seen in Figure 18.5. A ring and cone trap is used to separate the two stages. The cone can be seen just below the packing section. This type design can also be used to separate 10 µ m particulate in the lower stage followed by gas absorption in the upper stage where both contaminants are present at the same time. The following figure shows a vane or centrifugal type scrubber in fabrication. The turbine-like vanes can clearly be seen. Note, also, the central sleeve from which the vanes radiate. Many vane type scrubbers use a center disc or sleeve to act as a vortex finder that stabilizes the rotating gas pattern. When the vane is used as a droplet eliminator on larger units, this center sleeve often does double duty as an access manway to permit passage above or below the vane deck. Figure 18.3 Centrifield cloud zone (Entoleter, Inc.). Figure 18.4 Vane type scrubber installation (Entoleter, Inc.). FEED PIPE © 2002 by CRC Press LLC Design basics Vane type scrubbers can operate at pressure drops of less than 3 to 4 inches water column to over 35 inches water column for the vertical cage type designs. They generally make good use of the scrubbing liquid and operate at low liquid to gas ratios of 2 to 10 gallons/1000 acfm treated. Higher L/G ratios are used where the dust loading exceeds approximately 3 to 5 grs/dscf. Figure 18.5 Vane scrubber plus packed bed (Entoleter, Inc.). Figure 18.6 Vane type scrubber in fabrication (Trema Verfahrenstechnik GmbH). © 2002 by CRC Press LLC The vertical vessel velocities are similar to other cyclonic type wet scrubbers, that is, about 8 to 12 ft/sec vertical velocity. Gas inlet speeds range from 35 to 55 ft/sec, but these speeds are often increased once inside the vessel to provide first stage cyclonic separation. Gas outlet speeds are 40 to 55 ft/sec if no stack is used and approximately 30 to 40 ft/sec if a stack is in place. These scrubbers usually require a disengaging space above the vane area because the gas stream is spinning and the droplets require some time to separate. This disengaging zone varies by manufacturer but is typically 1 / 2 to 1 vessel diameter above the vane area. More disengaging zone is needed where horizontally oriented vanes are used because the gas flow tends to take an upward angular spiral rather than a more flat spin as in the vane cage type. Operating suggestions Vane type scrubbers encompass a number of proprietary designs. It is therefore best to consult with the vendor regarding each specific application. If the gas stream contains over 20% submicron particulate, vane type scrubbers may not be able to meet current air emissions regulations. The scrubber vendor should be able to predict the scrubber performance from a particle size analysis. When in doubt, perform or acquire an aerodynamic diameter particle size analysis for your application and submit it to the scrubber vendor for review. Given sufficiently large particulate, a vane type scrubber can provide economical performance versus medium to low energy competition such as Venturi scrubbers. Savings can accrue from reduced scrubbing liquid requirements and a more compact installation. Given the extensive use of internals in these designs, the capital cost may be higher than competitive designs because more material and fabrication labor time may be needed. Given the generally low L/G ratios at which these designs operate, liquid distribution is critical. If spray distributors are used, strainers are recommended to reduce nozzle plugging. Upon internal inspection, care should be taken to observe the spray impact patterns and make adjustments to the nozzle spray patterns or angles to provide complete liquid coverage. Telltale patterns can usually be easily seen on the vanes. Because the vanes are inside the vessel, they can be attacked by corrosion from both sides. When selecting materials of construction, one should take into account that the vanes should have sufficient thickness for double-sided corrosion. Too often, the vanes are thin and localized attack can shorten their effective life. If the application is corrosive, remember that any vanes inside can be attacked from both sides; therefore, your corrosion allowance should be doubled for interior components. If the scrubber uses a lower stage primary cyclonic knock out section with central drain fitting, make certain that the scrubber is equipped with vortex breakers to stop the liquid from spinning so that the liquid may drain smoothly. . grain drying industry. Their compact size makes them attrac- tive for roof mounting if space is a problem. As seen in Figure 18. 4, a pair of vane type scrubbers are seen roof mounted with the. gas absorption in the upper stage where both contaminants are present at the same time. The following figure shows a vane or centrifugal type scrubber in fabrication. The turbine-like vanes. capture. A popular and clever vertical cage type vane scrubber is made by Ento- leter, Inc. (Hamden, CT). Figure 18. 2 shows this vane cage as viewed from above. The gas stream enters tangentially