Scrubber, wet

A wet scrubber is a simple method to clean exhaust air or exhaust gas and ranove toxic or smeUing compounds. In the flue gas scrubber, the gas gets in close contact with fine water drops in a cocurrent or counter current flow. This method is more effective when the water drop size gets smaller and the total surface between water or washing fluid and the gas gets larger. The water or washing fluid is recirculated normally in order to save water and reduce the amount of waste water. 

A wet scrubber can also be combined with other flue gas-cleaning methods. Gases can be first cleaned by a washer, then treated by a thermal reactor, and then treated again by a wet scrubber. 

The following components can be removed from coal gas (1) water-soluble substances will be dissolved, (2) dust will be precipitated, (3) chemicals that can be hydrolyzed are decomposed, and (4) steam will be condensed. The result is decontamination, detoxification, dust removal, and dehumidification of the coal gas, as well as removal of many odiferous constituents of the gas. 

When hot raw gas is to be cooled directly or if it has already been cooled in a waste-heat recovery system, it will normally be treated in a wet scrubber. Wet scrubbers are devices that utilize gas/ liquid contacting to cool the gas stream, condense high-boiling hydrocarbons, dissolve some constituents, and separate particles from gas streams. There are many different wet scrubber designs, but all utilize similar mechanisms. 

In wet scrubbers droplets are either directly sprayed into the incoming dusty gas or the gas is allowed to shear a somce of liquid into droplets, which achieves the same effect. Due to their inertia, the particles impact on the droplets and are incorporated in them. The particle-containing droplets have a larger diameter than the dust particles, which allows them to be separated more easily from the gas stream in inertial t3rpe separators such as cyclones, settling chambers or knock-out vessels. 

Wet scrubbers can have a high efficiency for small particle sizes, especially the venturi t3rpe scrubbers, where the flow is subjected to strong acceleration—a condition that encourages efficient coalescence between particles and droplets. In wet scrubbers, the pressure drop remains constant (except in some relatively rare cases where calcium or other deposits may accumulate in the throat area of the scrubber). Degradation of the separation medium is normally not a problem. However, most scrubbers recycle most of the scrubbing water and the efficiency of the scrubber will suffer if the solids or other 

Centrifugal and cyclonic type separators are the subjects of this book. Compared with the other methods, their advantages are  

These come in many shapes and forms and may be used for removal of both 

Venturi scrubber, (Fig. 5.7) the gas is accelerated by passage through a narrow throat into which water sprays are injected. As a result of the high relative velocity of the gas and water droplets and the high turbulence in the exit of the throat, efficiency is high, but the pressure drop is also considerable giving rise to relatively high operational costs. Residual water spray is removed by a cyclone separator (Fig. 5.7). 

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Table 13. Particle Size Collection Capabilities of Various Wet Scrubbers ...

Sodium Sulfide Wet Scrubber for Oxides of Nitrogen (NO ) Mbsorption, Technical Service Bulletin, PPG Industries, New Martinsville, W. Va., 1995. Chemtox Database, on DIALOG File 337, Dialog Information Services, Palo Alto, Calif., last updated June 6, 1993. 

Vacuum-shelf dryers require auxiliary stream jets or other vacuum-producing devices, intercondensers for vapor removal, and occasionally wet scrubbers or (heated) bag-type dust collectors. 

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Vapors (from drying) are removed at the feed end of the dtyer to the atmosphere through a natural-draft stack and settling chamber or wet scrubber. When employed in simple drying operations with 3.5 X 10 to 10 X 10 Pa steam, draft is controlled by a damper to admit only sufficient outside air to sweep moisture from the cylinder, discharging the air at 340 to 365 K and 80 to 90 percent saturation. In this way, shell gas velocities and dusting are minimized. When used for solvent recovery or other processes requiring a sealed system, sweep gas is recirculated throu a scrubber-gas cooler and blower. 

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

Wet Scrubbers Scrubbers have not been widely used for the collection of purely liquid particulate, probably because they are generally more complex and expensive than impaction devices of the types previously discussed. Further, scrubbers are no more efficient than the former devices for the same energy consumption. However,... 

Ejfect of Gas Saturation in Scrubbing If hot unsaturated gas is introduced into a wet scrubber, spray particles will evaporate to cool and saturate the gas. The evaporating liquid molecules moving away from the target droplets will repel particles which might collide with them. This results in the forces of diffusiophoresis opposing particle... 

Gas pressure drop, cm of water across wet scrubber collection device... 

Kingman and Sheppard (Air Pollut. Control Assoc. Prepr. 75-30.3) have developed an ioniziug wet scrubber in which the charged mist particles are collected in a grounded, irrigated cross-flow bed of TeUerette packing. Particles smaller than 1 [Lm have been collected... 

Sintering Smoke, particulates (dust), SO2, NO Proper hooding, cyclones, wet scrubbers, baghouse, or precipitator... 

Acid gas control efficiency not as high as with wet scrubber... 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or diy. In wet scrubber systems, the absorption tower uses a hme-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Diy scrubbers can be grouped into three catagories (1) spray diyers (2) circulating spray diyers and (3) dry injection. Each of these systems yields a diy product that can be captured with a fabric filter baghouse downstream and... 

A fluid-bed incinerator uses hot sand as a heat reservoir for dewatering the sludge and combusting the organics. The turbulence created By the incoming air and the sand suspension requires the effluent gases to be treated in a wet scrubber prior to final discharge. The ash is removed from the scrubber water by a cyclone separator. The scrubber water is normally returned to the treatment process and diluted with the total plant effluent. The ash is normally buried. 

Water collection and treatment ponds associated with wet scrubbers... 

Dry aerosols, or particulate matter, differ so much from the carrying gas stream that their removal should present no major difficulties. The aerosol is different physically, chemically, and electrically. It has vastly different inertial properties than the carrying gas stream and can be subjected to an electric charge. It may be soluble in a specific liquid. With such a variety of removal mechanisms that can be applied, it is not surprising that particulate matter, such as mineral dust, can be removed by a filter, wet scrubber, or electrostatic precipitator with equally satisfactory results. 

Mist eliminators are widely used in air polluhon control systems to prevent free moisture from entering the atmosphere. Usually, such mist eliminators are found downstream from wet scrubbers. The recovered mist is returned to the liquid system, resulhng in lowered liquid makeup requirements. 

Absorption of pollutant gases is accomplished by using a selective liquid in a wet scrubber, packed tower, or bubble tower. Pollutant gases commonly controlled by absorption include sulfur dioxide, hydrogen sulfide, hydrogen chloride, chlorine, ammonia, oxides of nitrogen, and low-boiling hydrocarbons. 

Calcined limestone or lime reacts with sulfur oxides, which are then removed by wet scrubbers. 

List the advantages and disadvantages of using a baghouse, wet scrubber, or LSI tor particulate collection from an asphalt plant drying kiln. The gases are at 250 X and contain 450 mg m of rock dust in the 0.1-10 /rm size range. Gas flow is 2000 min. Consider initial and operation cost, space requirement, ultimate disposal, etc,... 

Figure 30-lE includes a hot ESP for fly ash collection prior to a catalytic NO , unit. Having a hot ESP dictates the use of a conventional wet scrubber and perhaps the need for a second particulate matter control device at the end of the system. Fly ash and scrubber sludge would be separate byproducts, but sludge could be contaminated with NH4 from the catalytic NO process. 

Air pollution control systems using wet scrubbers will remove some water-soluble gases, but the removal of particulate matter is the primary concern for a control system. The air pollution control system, therefore, is usually a single device such as a wet scrubber, small-diameter multiple cyclones, fabric filters, or ESPs. The multicyclones are the least expensive system and the ESPs the most expensive. 

Calcium oxide (lime) Rotary kilns, vertical and shaft kilns, fluidized bed furnaces Particulate matter Cyclones plus secondary collectors (baghouse, ESP, wet scrubbers, granular bed filters, wet cyclones)... 

Sodium carbonate (soda ash) Solvay (ammonia-soda) Particulate matter Wet scrubbers... 

Wet scrubber for formaldehyde and methanol only afterburner for organic vent gases Venturi scrubber followed by cyclone separator and packed countercurrent scrubber... 

Treatment of slimes for economic recovery of silver, gold, selenium, tellurium, and other trace elements requires fusion and oxidation in a furnace. The furnace gases are exhausted through a wet scrubber followed by an ESP to recover the metals. 

Refractory metals Zirconium Hafnium Titanium Kroll process, chlorination, and magnesium reduction Chlorine, chlorides, SiCli Wet scrubbers... 

Sinter machine discharge Particulate matter Multiple cyclones, baghouse, or low-energy wet scrubber... 

Blast furnace exhaust gases Particulate matter Multiple cyclone plus wet scrubber or wet ESP, two-stage wet scrubber... 

Cupolas Particulate matter Baghouses, wet scrubbers, and ESPs... 

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