Scrubber

Wet collectors, also called scrubbers, refer to a type of separating device in which a liquid is used to aid the collection of dusts, fumes, and mists in a 

One or more of the following mechanisms are employed in the separating action of the wet scrubbers. 

Wetting—of particle to help agglomerate and prevent re-entrainment. 

Diffusion—dust particles deposited on the liquid droplets. Predominant for the submicron and particles up to about 5 A. 

Applied Process Design for Chemical and Petrochemical Plants 

Condensation—will cause particle size to grow if gas cooled below its dew-point. 

The fluid from reliefs, sometimes two-phase flow, must first go to a knockout system, where the liquids and vapors are separated. Liquids are subsequently collected and the vapors may or may not be vented. If the vapors are nontoxic and nonflammable, they may be vented unless some regulation prohibits this type of discharge. 

If the vapors are toxic, a flare (described previously) or a scrubber system may be required. Scrubber systems can be packed columns, plate columns, or venturi-type systems. Details of scrubber designs are covered by Treybal.11 

A simple condenser is another possible alternative for treating exiting vapors. This alternative is particularly attractive if the vapors have a relatively high boiling point and if the recovered condensate is valuable. This alternative should always be evaluated because it is simple and usually less expensive and because it minimizes the volume of material that may need additional post-treatment. The design of condenser systems is covered by Kern.12 

This is briefly discussed by Parry141 and DIERSl31/Grosselm. 

Guidelines for Pressure Relief and Effluent Handling Systems , CCPS/AIChE, 1998, ISBN 0-8169-00476-6 

HE Huckins, Selection and Design of Systems to Handle the Effluent from Emergency Relief Systems , International Symposium on Runaway Reactions and Pressure Relief Design, 710-741, AlChE, 1995, ISBN 0-8169-0676-9 

H G Fisher et al., Emergency Relief System Design Using DIERS Technology , Chapter V, DIERS/AIChE, 1992, ISBN 0-8169-0568-1 

j OF Parry, Relief Systems Handbook , IChemE, Rugby, 1994, ISBN0 85295 

Estimations of phosgene emissions have been made for a large hypothetical plant producing 200 Mlb (90718 tons) per year [2088b]. The total phosgene emission of about 

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Scrubbers. Scrubbers are designed to contact a liquid with the particle-laden gas and entrain the particles with the liquid. They offer the obvious advantage that they can be used to remove gaseous as well as particulate pollutants. The gas stream may need to be cooled before entering the scrubber. Some of the more common types of scrubbers are shown in Fig. 11.2. 

A considerable reduction in particle size separation can be achieved at the expense of increased pressure drop using a Venturi scrubber (see Fig. 11.2c). 

Figure 11.2 Various types of scrubbers can be used to treat air pollution from solid particles. (Reproduced with permission from Stenhouse, Pollution Control in Teja, Chemical Engineering and the Environment, Blackwell Scientific Publications, Oxford, U.K., 1981.)...

When the main object of the absorption is to remove impurities these columns are often referred to as scrubbers. ... 

Flue gas handling Flue gas scrubber Flue-gas scrubbers Flue-gas scrubbing FlueUite Fluent... 

Screen printing inks Screen-process inks Screens Screw dislocation theory Scrip set Scrubbers... 

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

VS = vent gas scrubber EC = ethylbenzene recovery column DC = diethylbenzene recovery column. 

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. 

Dried product is collected in either cyclones or baghouses depending upon the product-particle size. When primary coUection is carried out in cyclones, secondary collection in a baghouse or scrubber is usually necessary in order to comply with environmental regulations. A rotary valve is used to provide an airlock at the discharge point. Screws are utihzed to combine product from multiple cyclones or large bag-houses. If required, a portion of the dried product is separated from the main stream and returned to the feed system for use as backmix. 

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. 

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