Venturi scrubber-cyclone

SUMMARY OF AVAILABLE ENERGY CONSUMPTIONS FOR VENTURI SCRUBBER/CYCLONE COMPARISON ... 

For the venturi scrubber/cyclone comparison, evaluation of available energy consumptions revealed that the 55.1 kPa (8 psi) pressure drop of system A implied additional operating and capital costs which by far exceed its apparent cost savings over system B. This dramatic revelation proceeded from only a very few straightforward calculations. 

Figure 13.3.6 is a photograph of a skid-mounted venturi-scrubber/cyclone separator system. Spray water injected (see far left-hand side in the photograph) is captured by the rather short cylindrical-bodied cyclone separator located immediately downstream of the scrubber. We note that the cyclone has a scroll-type rectangular inlet and that the downstream blower makes this a pull-through type system. Scrubber water collected by the cyclone reports to a receiving tank located below the cyclone. Prom here, it is pumped back up through the scrubber s spray nozzles. 

Fig. 13.3.6. A skid-mounted venturi scrubber/cyclone ensemble by Fisher-Klosterman, Inc.

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. 

Normal superphosphate Sip4, HF Venturi or cyclonic scrubber... 

Diammonium phosphate Gaseous F, NH, Venturi or cyclonic scrubber with 30% phosphoric acid... 

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

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). 

A venturi scrubber is a venturi-shaped air passage with water introduced just ahead of or into the venturi throat. The liquid-gas contact is at a maximum in the venturi throat. The relative velocity between gas and liquid aerosol droplets is high, with the gas velocities in the range of 50-100 m/s. The particles are conditioned in the throat, and condensation is the important collection mechanism. After the particles in the gas have been deposited on droplets, a comparatively simple device such as a cyclone collector can be used to collect the wetted dust. 

Methods of dust removal depend mainly on the particle size of the dust and the temperature and moisture content of the gas. The methods used are broadly divided into dry methods and wet methods. The dry methods involve the use of gravity and baffle chambers, cyclones, filters, and electrostatic precipitators, while the wet methods involve the use of spray towers and venturi scrubbers. In principle, wet cleaning is preferred to dry cleaning because of the excessive wear associated with and the difficulty in handling the fine dusty material removed in the dry methods. The wet methods, however, must be followed by such operations as filtration, drying of filter cakes, and recycling of water. 

Figure 1.69 shows a venturi scrubber in which water is injected at the throat and the separation is then carried out in a cyclone separator. 

For example, Venturi scrubber, ESP s, cyclones, baghouses. 2At 5% of investment. 

Almost all particle-separating devices can be converted into wet scrubbers by adding liquid spraying systems. Three types of commonly used scrubbers are the spray chamber, cyclonic scrubber, and venturi scrubber. Figure 7.18 shows a simple spray chamber in which water is sprayed through a series of nozzles into a settling chamber. The dust-laden gas is fed from the bottom of the chamber and exits from the upper portion of the chamber. 

The collection of particles is achieved in a countercurrent flow between the water droplets and the particulates. In a cyclonic scrubber, water is injected into the cyclone chamber from sprayers located along the central axis, as shown in Fig. 7.19. The water droplets capture particles mainly in the cross-flow motion and are thrown to the wall by centrifugal force, forming a layer of slurry flow moving downward to the outlet at the bottom of the cyclone. Another type of scrubber employs a venturi, as shown in Fig. 7.20. The velocity of the gas-solid suspension flow is accelerated to a maximum value at the venturi throat. The inlet of the water spray is located just before the venturi throat so that the maximum difference in velocity between droplets and particles is obtained to achieve higher collection efficiency by inertial impaction. A venturi scrubber is usually operated with a particle collector such as a settling chamber or cyclone for slurry collection. 

Figure 1. Typical Collection Efficiency Curves for Various Types of Collectors A - high Throughput Cyclone B High Efficiency Cyclone C - Dry Electrostatic Precipitator D w Spray Tower E -Venturi Scrubber...

Kraft pulp mills Digesters batch and continuous Multiple-effect evaporators Recovery furnace Weak and strong black-liquor oxidation Smelt tanks Lime kiln Mercaptans, methanol (odors) H2S, other odors H2S, mercaptans, organic sulfides, and disulfides h2s Particulates (mist or dust) Particulates (dust), H2S Condensers and use of lime kiln, boiler, or furnaces as afterburners Caustic scrubbing and thermal oxidation of noncondensables Proper combustion controls for fluctuating load and unrestricted primary and secondary air flow to furnace and dry-bottom electrostatic precipitator noncontact evaporator Packed tower and cyclone Demisters, venturi, packed tower, or impingement-type scrubbers Venturi scrubbers... 

A review of Table 8 and Fig. 3-2 indicates that large-diameter particles can be removed with low-energy devices such as settling chambers, cyclones, and spray chambers. Submicron particles must be removed with high-energy units such as bag filters, electrostatic precipitators, and venturi scrubbers. Intermediate particles can be removed with impingement separators or low-energy wet collectors. Obviously, other equipment performance characteristics as noted in Table 8 will also have their influence on the final equipment... 

Any surface that was not wet would form a mud, causing frequent cleaning of the collector interior. In order to have a scrubber operating efficiently, the velocity in the scrubber has to be such as to drive the dust particles into the water. Venturi scrubbers have efficiencies in the range of 90-95% compared to dry cyclones in the range of 75-85%. 

Figure 20.6. Wet scrubbers, (a) Wet cyclone scrubber, (b) Spray scrubber, (c) Venturi scrubber, (d) Orifice scrubber.

Gasifier. 2. Primary cyclone. 3. Secondary cyclone. 4. Venturi scrubber. 5. Water scrubber. 6. Water scrubber. 7, Gas tank.)... 

A schematic flowsheet of the WCF is shown in Figure 3. Solidiflca-tion takes place in the 4-ft-diameter by 13-ft-high fluidized-bed calciner. Most of the plant equipment is devoted to cleaning up the entrained radioactive particles in the calciner off-gas. The off-gas passes into a cyclone to remove most of the solids, a quench tank to cool the gas, a venturi scrubber-separator-demister, a silica gel adsorber for the removal of voltaile ruthenium, another cyclone, and Anally a high-efficiency HEPA filter to remove submicron particles before discharging to the 250-ft plant stack. 

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