Scrubbers Venturi

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). 

Venturi scrubbers are similar in that they need high velocity to achieve small droplets. They are primarily employed for mist and dust collection and are discussed further in the mist-collection portion of this section. 

Pressure drop in a venturi scrubber is controlled by throat velocity. While some venturis have fixed throats, marw are designed with variable louvers to change throat dimensions and control performance for changes in gas flow. Pressure-drop equations have been developed by Calvert (R-13, R-14, R-15), Boll [Ind Eng Chem Fundam, 12, 40 (1973)], and Hesketh [J. Air Pollut Control Assoc, 24, 939 (1974)]. Hollands and Goel [Ind Eng Chem Fundam, 14, 16 (1975)] have developed a generalized pressure-drop equation. 

The Hesketh equation is empirical and is based upon a regression analysis of data from a number of industrial venturi scrubbers ... 

All venturi scrubbers must be followed by an entrainment collector for the liquid spray. These collectors are usually centrifugal and will have an additional pressure drop of several centimeters of water, which must be added to that of the venturi itself. 

The various venturi-scrubber models embody a variety of assumptions and approximations. The solutions of the equations for particulate collection must in general be determined numerically, although... 

Primarily because of the lack of adequate experimental techniques (particularly, the produc tion of appropriate monodisperse aerosols), there has been no comprehensive experimental test of any of the venturi-scrubber models over wide ranges of design and operating variables. The models for other types of scrubbers appear to be essentially untested. 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

The principal advantage of self-induced spray scrubbers is the efim-ination of a pump for recirculation of the scrubbing liquid. However, the designs for high-energy service are somewhat more complex and less flexible than those for venturi scrubbers. 

In Venturi scrubbers the gas is the motive fluid. This equipment is of simple design and is able to handle slurries and large volumes of gas, but the gas pressure drop may be high. When the reaction is slow, further holdup in a spray chamber is necessary. 

Lime kiln Particulates (dust), H2S Venturi scrubbers... 

Preconditioning for Particulates Heavy particulate loading of the inlet gas with dust, grease, oils, or other aerosols can be very dam-aging to the pore structure of the filter bed, resulting in an eventual pressure-drop increase. Oils and heavy metals that are deposited on the filter bed can be poisonous to the microorganisms that live within the biofilm. Particulate APC equipment such as fabric filters and venturi scrubbers are generally adequate for this level of particulate removal. 

Increasing either the gas velocity or the liquid droplet velocity in a scrubber will increase the efficiency because of the greater number of collisions per unit time. The ultimate scrubber in this respect is the venturi scrubber, which operates at extremely high gas and liquid velocities with a very high pressure drop across the venturi throat. Figure 29-8 illustrates a commercial venturi scrubber unit... 

A widely used type of scrubber for mists and droplets is the venturi scrubber. It has been used for the collection of sulfuric acid and phosphoric add mists with very high effidency. The scrubbing contact is made at the... 

Sonic agglomerators, which have been used experimentally for sulfuric acid mists and as mist eliminators. Commercial development is not projected at this time because the energy requirements are considerably greater than those for venturi scrubbers of similar capacity. 

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

Coke oven charging Hydrocarbons, carbon, coal dust Aspiration systems to draw pollutants into oven, venturi scrubbers... 

Coke oven discharging (pushing) Hydrocarbons, coke dust Hoods to fans and venturi scrubbers, low-energy scrubbers followed by ESPs (may use water spray at oven outlet)... 

The electric arc furnace process accounted for about 25% of the 1982 U.S. steelmaking capacity (14). Most of the raw material used for the process is steel scrap. Pollutants generated by the electric furnace process are primarily particulate matter and CO. The furnaces are hooded, and the gas stream containing the particulate matter is collected, cooled, and passed to a bag-house for cleaning. Venturi scrubbers and ESPs are used as control devices at some mills. Charging and tapping emissions are also collected by hoods and ducted to the particulate matter control device. 

Chargine and tapping Particulate matter Baghouse or venturi scrubber... 

Furnace waste gases Particulate matter (7-30 kg per metric ton of steel) ESP or venturi scrubber... 

Gases or vapours that are water soluble or miscible or that are only soluble or highly reactive in other agents Absorption with multiple surface contact by atomizing liquid with spray nozzle or jet impaction Crabtree ozone analyser or midget venturi scrubber Water, acid, or alkali 5-25 60-100 Venturi scrubber satisfactory if dust is present Atomizer absorber will plug... 

Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 g. or less) is lower than that of a venturi scrubber. Spray towers can handle larger gas flows with minimal pressure drop and are therefore often used as precoolers. Because wet scrubbers may contribute to corrosion, removal of water from the effluent gas of the scrubbers may be necessary. 

Baghouses are preferred over venturi scrubbers for controlling particulate matter emissions from loading and pushing operations because of the higher removal efficiencies. ESPs are effective for final tar removal from coke oven gas. Stack air emissions should be monitored continuously for particulate matter. Alternatively, opacity measurements of stack gases can suffice. Fugitive emissions should be monitored annually for VOCs. 

When the pollutant loading is exeeptionally high or consists of relatively large particles (> 2 /tm), venturi scrubbers or spray chambers may be used to reduce the load on the ESP. Much larger particles (> 10 /tm) are controlled with mechanical collectors such as cyclones. Gas conditioning equipment to reduce both inlet concentration and gas temperature is occasionally used as part of the original design of wet ESPs (AWMA, 1992 Flynn, 1999). 

The power consumed to operate a wet electrostatic precipitator is much less than that required by most other methods of control. There are four areas in which power is consumed (1) electrostatic power, (2) fan power, (3) insulator heating power, and (4) pump power. The total electrostatic power input required for operation is 0.8 to 1.0 kW/1,000 ft of collection area. A comparable piece of equipment is a venturi scrubber with 50-in.wg pressure drop. The power required for this installation would be 6 to 7 kW/1,000 cfm. This would mean that approximately seven times the power would be needed to achieve the same amount of cleaning with a venturi scrubber as opposed to using a precipitator. 

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