Scrubbers: Wet Cyclonic, Packed Tower, Impingement, and Venturi

Scrubbers are the most commonly used technique for separating particulates from gas streams. The field of scrubber technology is very mature, with liter­ally hundreds of thousands of scrubbers in operation worldwide, some in op­eration for more than 50 years.

The basic principle of wet collectors is to wet the contaminant particles in order to remove them from the gas stream. There is a wide range in scrubber design, cost, and performance.44“52 Because of this wide variability, scrubbers must be carefully matched to specific applications.

Many investigators have attempted to define the parameters which relate cleaning efficiency to scrubber design and operating conditions. The conclu­sion from their investigations is that scrubber efficiency depends on energy in­put per unit of gas flow, whether the energy is supplied to the air or the water (contact power theory). This conclusion applies only to well-designed equip­ment when the energy is expended in the gas-liquid contacting process. As a result, equivalent power requirements give comparable collection efficiencies for all different equipment manufacturers.

TABLE 13.7 Standard Parameters for HEPA Filters Face dimensions (in.) Depth, less gasket (in.) Design airflow capacity at clean filter resistance 1.0 in. WG (scfrn) 25 4 mm WG (mJ/h) 24×24 11 ] 1000 1607 24 X 24 51 500 803 12 x 12 125 200 8 x 8 5| 50 80 8X8 25 40

The principal requirements of an effective wet scrubber are these;

1. Bring liquid and particulates into intimate contact to promote high collection efficiency.

2. Prevent dust buildup and plugging at the wet inlet interface.

3. Provide a suitable method to prevent water carryover in the cleaned exhaust gas.

4. Provide an acceptable disposal system for the collected dust.

The three mechanisms by which dust particles are wetted and collected are impingement, diffusion, and condensation. The mechanism of impinge­ment is making dust particles impinge and adhere to water droplets or a water film. Diffusion is the mechanism by which the smaller particles are collected by the larger particles. Condensation takes place if a liquid spray causes the gas to pass through its dew point, with the dust particles acting as condensa­tion nuclei. This results in an increase in effective size of particles and simpli­fies their subsequent collection by mechanical means. This is important in cases where a high collection efficiency is required for relatively low concen­trations of very fine dusts.

Wet scrubbers find a wide range of applications because of the following advantages:

1. Moisture-laden gases or sticky, adhesive particles can be handled without plugging.

2. Hot gases can be handled easily and economically. The hot gas is cooled at the collector inlet so that the collector can be sized to handle the smaller cooled gas volume.

3. Fire or explosion hazards are eliminated.

4. Collected material can be disposed of dust-free.

These are disadvantages of wet scrubbers:

1. Scrubbers often convert an air pollution problem into a water pollution problem. If regulations require 100% recycling, the scrubber water may require flocculants or neutralizing chemicals or both.

2. Corrosion can be a serious problem requiring high maintenance or special materials of construction.

3. Very high power consumptions are required to collect very fine particles.

4. Extensive freezing protection is required for the whole scrubber system in cold climates.

5. Loss of plume buoyancy can affect ground-level concentrations,

6. High fan maintenance costs.

7. High noise level.

Although there are numerous scrubber designs, typical characteristics of four of the more common types are listed in Table 13.8. Table 13.9 presents data on the features, operating principles and problem areas with these com­mon scrubbers. A brief description of each scrubber type follows.

A wet cyclonic scrubber is a cyclone collector with centrally located, coarse water sprays. These water sprays are usually directed radially outward,

HH:’ TABLE 13.8 Typical Characteristics of Gas-Cleaning Equipment

Approximate collection efficiency (wt%)

Зпіггн.’ЛІ Staling Крагагогч chamber Baffled Chamber

20 0.1-0.4 10 10 10 5-І. S 0.2 20 1-5 10 [0 20-60 5~ 5 5 2-200 10-20 1.0 10 40-80 5-75 0.2-1.0

High — Throughput Cyclone High­ Efficiency Cyclone

0.2-20 10-20 10 10-40 80-95 50-12.5 0.8-2.0 0.2-20 10-20 10 20-80 90-95 50-150 0.8-2.0

Wet Wet cvclovtic. 5crubbcr*i

Typical Dust Air Loading velocity I (g/m3) (m/s) mi

Pressure Power Loss requirement (mm WG) fkW per mJ/s)

Equipment

Large: space require­ment, collects coarse dust oalv. ptecicancr. P >tl I’tuJ * D L’ m Pi l’vojt .ul’ea-, coarse dust: sanly. precleaner. Good efficiency at 2u—r0 mictons, c’ic ip, vi> irrer — il •lit construction Good efficiency at 5­10 microns, cheap, any material of con­struction. higher pressure drop. Most common type, utilizes centrifugal forces, low water

Prirnari? y used foi gases* i/spors, and mist removal; pack­ing may plug with high dust loads.

Packed tower 0.2-20 1-1.5 N/A N/A N/A 40-100 0.7-1.5

Fabric Mechanical Filters shaking

0.2-20 0.005- 99 99.9 0.025

Impingements 0.2-20 15-25 10 20-80 90-95 100-150 1.7-2.5	Widely USCr. i for average to coarse dust, good for hot gas, low to medium energy require­ments. Require high fan speed, imbalance and corrosion prob­lems, good for fume collection. Suitable fur high temperature or cor rosiovr, efficiency sensitive to varia­tions in dust, gas, or process. Suitable tor high temperature or cor­rosion* reeiuram — menr of fine dust is eliminated. 1 lit, ir t e? lKi nc c< n it l i be ap Ai J t nip ra uire max. ot, H>0 ‘L.. caution with stickv dusts and lugh — moisture it
0.2-20 100 80-90 99 99 750-1000 13-17
Venturi
Electrostatic Dry precipitator*
0.2-2. 1-2.5 80-99 80-99 +99 5-20 0.4-1.2
Wet	0.2-2 1-2.5 80-99 90-99
+99 5-20 +99.9 50-125
0.8-2.0

Handies more diffi cult dusts, cai: obtain good bag life. " More compact design, new applica­tions on metallurgi­cal fume, cleaning air must be dry.

0.2-20 0.005- 99 99.9 +99.9 50-125 0.8-2.0

0.025

Pulse ie:

0.2-20 0.01-0.1 99 99.9 +99.9 100-200 1-2.5

			Approximate collection efficiency (wt%)
Equipment Type		Typical Dust Loading (g/mJ)	Air Velocity 1 1-5 5-10 (m/s) micron microns microns	Pressure Loss (mm WG)	Power Requirement (kW per m»/s)	Remarks
	Cartridge	0.02-0.2	1.3 99.9 +99.9 +99.9	.50-150	0.8-2.5	Newer develop roent, very attractive for recirculaiion oi plant air, can be used with limestone pTecoanngtor sup" plv air.
HEPA	Throw-away Pleated Cartridge	0.02	!.3 +99.9 +99.9 +49.9	25-50	0.4-0.8	Highest efficiency, used in the nuclear industry for filter­ing radioactive: dusts.

Snicira: Goodfellow.!1

And their prime purpose is to cool the gas and prevent reentrainment. The dust collected on the walls is slurried and carried away by centrifugal force.

A packed-tower scrubber consists of a tower filled with packing to pro­vide surface area for the liquid—gas contacting process. The scrubbing liquid is introduced at the top of the packing and trickles down through it. For a coun­tercurrent design, the contaminated gas passes up through the wetted packing, while the liquid flows downward. As the gas stream rises, it loses some of its contaminant. The clean liquid being introduced at the top allows the remain­ing contaminant to be absorbed more easily. A suitably designed mist elimina­tor is critical for a successful installation.

An impingement scrubber is designed to make the dust particles impinge and adhere to water droplets. Gas is introduced at the bottom and passes up through water-covered perforated trays, where dust is removed by the scrubbing liquid.

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.

The collection efficiency of wet scrubbers is dependent on parameters such as the size and quantity of liquid droplets, the liquid/gas ratio, high wa­ter-to-particle relative velocity, wettability of dust, particle density, gas viscos­ity, etc. For any specific application, the design procedure is to review operating data available from the technical literature or from manufacturers for similar applications. If data are not available, it may be necessary to per form pilot scale tests, which can be used for scale-up purposes.

The key to a successful scrubber installation is the proper selection of a given scrubber method for a given application. Specific industrial applications are covered in subsequent volumes of the Design Guidebook.

Type	Features	Operating principles	Problem areas
Wt’t cyclon ic	Individually controlled	Gas introduced at bot­	Spray or onf’ct wear.
	Spray nozzles.	Tom.	Pattern mctkctive-
	Simple design.	Tangential inlet.	Ness.
	Low energy require­	Helical gas-flow pat­	Plugged orifice.
	Ments.	Tern.	Flow is critical-reduced,
	No baffles or impinge­	Radially injected spray	Or increased flow
	Ment surfaces.	Pattern, follows heli­	Causes functional
	Minimum entrainment.	Cal gas-flow pattern.	Problems, poor turn­
	Easy to service.		Down or turnup. Shell wear, high dust concentration.
Countei’flow	Low energy require­	Vertical gas flow.	Particulates blind bed.
Packed	Ments.	Gas introduced at bot­	Plugged drain.
Tower	Easy maintenance.	Tom.	Limited volume.
	Good gas absorption.	Gas passes through bed	Corrosion, unless
	Large contact area.	Of irrigated packing.	Proper material is
	Can be fabricated from	Gas and liquid contact	Selected.
	Variety of materials.	In lower bed.
	Inexpensive to operate.	Dry uppei; bed, mist eliminator, or demis­ter pad.
Impingement	Low to medium energy	Gas introduced at bot­	Plate flooding because
	Requirements.	Tom.	Of excessive gas vol­
	Underspray system pre­	Flow passes through	Ume.
	Vents wet-dry inter­	Water zone consist­	Bypassing seal pots
	Face problems.	Ing of spent liquor	Because of high pres­
	Nonplugging weir.	And water from noz­	Sure drop.
	High efficiency-to-	Zles under plates.	Plugged trays*
	Horsepower ratio.	Trays can be flooded to	Gases not sufficiently
	Can be used as heat-	Varying depths,	Saturated.
	Transfer unit for cool­	Depending on heat-	Spray failure, poor cov­
	Ing gases.	Transfer needs.	Erage.
	Variable number of	Gas flows through per­	Improper weir setting.
	Trays.	Forated plates. Gas impinges on tabs, passes through water layer. Small drops help encap­sulate dust, simplify removal. Spent liquor exists at bottom.	Incorrect water flow, improper pressure. Missing deflector plates on multiple-tray unit.
Venturi	Submicron panicles can	Gases enter at top.	High or low gas and
	Be removed effi­	Accelerated gas stream	Water flows.
	Ciently.	Shatters scrubbing	Venturi throat and
	Free-flow liquid entry.	Liquor, forming fine	Cyclone erode and
	Low maintenance	Drops that wet and	Plug.
	Needs.	Agglomerate particles. Scrubbing liquor and particles are separated by cyclonic action.	Plugged drains. Inadequate coverage of inlet by scrubbing liquor.

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