AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 22 pps

Design and Application of Wet Scrubbers 22.1 INTRODUCTION In wet scrubbing, an atomized liquid, usually water, is used to capture particulatedust or to increase the size of aerosols.. Cy

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Design and Application

of Wet Scrubbers

22.1 INTRODUCTION

In wet scrubbing, an atomized liquid, usually water, is used to capture particulatedust or to increase the size of aerosols Increasing size facilitates separation of theparticulate from the carrier gas Wet scrubbing can effectively remove fine particles

in the range from 0.1 µm to 20 µm The particles may be caught first by the liquid,

or first on the scrubber structure, and then washed off by the liquid Because mostconventional scrubbers depend upon some form of inertial collection of particulates

as the primary mechanism of capture, scrubbers when used in a conventional wayhave a limited capacity for controlling fine particulates Unfortunately inertial forcesbecome insignificantly small as particle size decreases, and collection efficiencydecreases rapidly as particle size decreases As a result, it becomes necessary togreatly increase the energy input to a wet scrubber to significantly improve theefficiency of collection of fine particles Even with great energy inputs, wet scrubbercollection efficiencies are not high with particles less than 1.0 µm in size

Wet scrubbers have some unique characteristics useful for fine particulate trol Since the captured particles are trapped in a liquid, re-entrainment is avoided,and the trapped particles can be easily removed from the collection device Wetscrubbers can be used with high-temperature gases where cooling of the gas isacceptable and also with potentially explosive gases Scrubbers are relatively inex-pensive when removal of fine particulates is not critical Also, scrubbers are operatedmore easily than other sophisticated types of particulate removal equipment.Wet scrubbers can be employed for the dual purpose of absorbing gaseouspollutants while removing particulates Both horizontal and vertical spray towershave been used extensively to control gaseous emissions when particulates arepresent Cyclonic spray towers may provide slightly better particulate collection aswell as higher mass transfer coefficients and more transfer units per tower than otherdesigns Although there is theoretically no limit to the number of transfer units thatcan be built into a vertical countercurrent packed tower or plate column, if it is madetall enough, there are definite limits to the number of transfer units that can bedesigned into a single vertical spray tower As tower height and gas velocities areincreased, more spray particles are entrained upward from lower levels, resulting in

con-a loss of true countercurrency Achievcon-able limits hcon-ave not been clecon-arly defined inthe literature, but some experimental results have been provided.1 There have beenreports of 5.8 transfer units in a single vertical spray tower and 3.5 transfer units inhorizontal spray chambers Researchers have attained 7 transfer units in a singlecommercial cyclonic spray tower Theoretical discussion and a design equation for22

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The disadvantages of wet scrubbers include the necessity of reheating cooledscrubber effluents for discharge up a stack Furthermore, the water solutions mayfreeze in winter and become corrosive at other times In some cases, the resultantliquid sludge discharge may have to be treated for disposal It should be noted alsothat operating costs can become excessive due to the high energy requirements toachieve high collection efficiencies for removal of fine particulates.

22.2 COLLECTION MECHANISMS AND EFFICIENCY

In wet scrubbers, collection mechanisms such as inertial impaction, direct tion, Brownian diffusion, and gravity settling apply in the collection process Mostwet scrubbers will use a combination of these mechanisms, therefore, it is difficult

intercep-to classify a scrubber as predominately using one particular type of collectionmechanism However, inertial impaction and direct interception play major roles inmost wet scrubbers Thus, in order to capture finer particles efficiently, greater energymust be expended on the gas This energy may be expended primarily in the gaspressure drop or in atomization of large quantities of water Efficiency of collectionmay be unexpectedly enhanced in a wet scrubber through methods that cause particlegrowth Particle growth can be brought about by vapor condensation, high turbu-lence, or thermal forces in the confines of the narrow passages in the scrubberstructure Condensation, the most common growth mechanism, occurs when a hotgas is cooled or compressed The condensation will occur preferentially on existingparticles rather than producing new nuclei Thus, the dust particles will grow largerand will be more easily collected When hydrophobic dust particles must be col-lected, there is evidence that the addition of small quantities of nonfoaming surfac-tants may enhance collection The older literature is contradictory on this point, butcareful experiments by Hesketh2 and others indicate enhancement can definitelyoccur

22.3 COLLECTION MECHANISMS AND PARTICLE SIZE

When a gas stream containing particulates flows around a small object such as awater droplet or a sheet of water, the inertia of the particles causes them to move

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toward the object where some of them will be collected This phenomenon is known

as inertial impaction, which customarily describes the effects of small-scale changes

in flow direction Because inertial impaction is effective on particles as small as afew tenths of a micrometer in diameter, it is the most important collection mechanismfor wet scrubbers Since this mechanism depends upon the inertia of the particles,both their size and density are important in determining the efficiency with whichthey will be collected All important particle properties may be lumped into oneparameter, the aerodynamic impaction diameter which can be calculated from theactual particle diameter by the following relationship:

C′ = Cunningham’s correction factor

By a fortunate circumstance, most methods for measuring particle size determinethe aerodynamic impaction diameter The Cunningham correction factor is given bythe following formulas:

dp

1 0 0 16.

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Knowing the value of the mean free path of molecules at a given temperatureand pressure, the mean free path at other conditions can be calculated fromEquation 22.5:

(22.5)

where

λo = 0.0653 µm for air at 23°C and 1.0 atm

µo, To, Po= viscosity, temperature, and pressure, respectively, at the same

condi-tions for which λo is known

22.4 SELECTION AND DESIGN OF SCRUBBERS

Calvert3 and co-workers have prepared an extensive report of wet scrubbers fromboth theoretical considerations and literature data In considering the types of scrub-bers to use for a particular application, the designer must have in mind the requiredcollection efficiency for a particular size emission The data of Table 22.1 can beused as a rough guide for initial consideration of adequacy of different devices

22.5 DEVICES FOR WET SCRUBBING

The following material is a compilation of facts and figures for typical wet scrubbers

Table 22.2 serves as a guideline to the general operational characteristics of varioustypes of devices Following Table 22.2 are Figures 22.1 through 22.6 which areschematics illustrating the six type of scrubbers listed in the table The scrubbersdepicted in Figures 22.1 and 22.2 are the same as Figures 11.2 and 11.3 shown in

λ λ µµ

Pp

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Chapter 11, absorption for HAP and VOC control This emphasizes the fact thatthese devices may be used to collect both particles, primarily by inertial impaction,and to absorb gases, usually with a solvent that also promotes chemical reaction

22.6 THE SEMRAU PRINCIPLE AND

In order to capture fine particles, greater energy must be expended on the gas Thereare two ways to do this:

1 Increase the gas pressure

2 Atomize large quantities of water

Efficiency of collection may be unexpectedly enhanced in a wet scrubber throughmethods that will cause particle size growth Particle growth can be brought about by:

• Lower temperatures that cause vapor condensation

• Increased flow rates that increase turbulence

• Thermal gradients in the narrow passages of the scrubber which increasesdiffusion of particles into the liquid

Condensation is the most common growth mechanism The hot gasses are cooled

by the lower temperatures resulting from contact with the scrubbing liquid Thegasses may also be compressed in the narrow passages of the scrubber, which wouldtend to enhance condensation The condensation occurs on the existing particlesrather than the new nuclei Thus the dust particles will grow larger and will be moreeasily collected

TABLE 22.2

Characteristics of Wet Scrubbers

Spray Tower

Cyclonic Spray Tower

Self-Induced Sprays

Plate

Venturi Jet

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In general the efficiency of a wet scrubber is directly related to the energyexpended to produce the gas–liquid contact The more energy expended, the greaterthe turbulence in the contacting process and the higher the efficiency of collection.Semrau7,8 defined the contacting power as the energy dissipated per unit volume ofgas treated Contacting power should be determined from the friction loss acrossthe wetted portion of the scrubber Pressure losses due to the gas stream kineticenergy should not be included However, energy provided by the mechanical devicesalong with the energy provided by the gas and liquid are part of the contactingpower Semrau treated scrubber efficiency by relating the number of transfer units

FIGURE 22.1 Spray tower (same as Figure 11.2 for absorption):

• Countercurrent vertical tower

• Droplets sufficiently large so that the settling velocity is greater than the upward gas velocity

• Droplet size controlled to optimize particle contact and to provide easy droplet separation

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FIGURE 22.2 Cyclonic spray tower (same as Figure 11.3 for absorption):

• Gas is introduced tangentially which increases the forces of collision and relative velocity of the droplets and gas stream

• Well designed cyclonic spray towers greatly increase the collection of particles smaller than 10 µ m when compared to simple countercurrent spray towers

• Droplets produced by spray nozzles

• Droplets collected by centrifugal force

• Two types: (1) spinning motion imparted by tangential entry, (2) spinning motion produced by fixed vanes

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FIGURE 22.3 Self-induced spray tower:

• Air impinges on a liquid surface then on a series of baffles

• Particles are initially captured in pool of water by direct interception or inertial impingement

• Some water is atomized into spray droplets which aids collection

• A final change in gas direction, or by baffles, serves as an entrainment separator

• The water circulation rate is low, and water is primarily required to replace evaporation losses

• Droplets are formed by breaking through a sheet of liquid or by impinging on a pool of water

• Droplets collected by gravity attraction

Fan

Inlet

Liquid level

Baffle Outlet

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The following plot from Semrau, Figure 22.7, is a performance curve for aventuri scrubber collecting a metallurgical fume.8Table 22.3 reports values of theparameters that could be used in Semrau’s equation for the number of transfer units.7

22.7 A MODEL FOR COUNTER-CURRENT

SPRAY SCRUBBERS

Drops are formed by atomizer nozzles and then sprayed into the gas stream In thecounter-current tower, drops settle vertically against the rising gas stream which iscarrying the particles Atomization provides a wide variety of droplet size It iscustomary to take the Sauter mean drop diameter equivalent to the volume/surfacearea ratio and defined by the following equation to represent all the droplets

(22.8)

FIGURE 22.4(a) Impingement plate scrubber

dV

QQ

d g L L

L

L L

L G

µ

σ ρ

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where

dd = Sauter mean droplet diameter, µm

ρL = Density of the liquid, gm/cm3

σL = Liquid surface tension, dyne/cm

µL = Liquid viscosity, poise

Vg = Superficial gas velocity, cm/s

QL = Volumetric liquid flow rate, M3/s

QG = Volumetric gas flow rate, M3/s

Inertial impaction is depicted in Figure 22.8, primary capture mechanism Inthis case of inertial impaction, a particle is carried along by the gas stream Approach-ing the collecting body which is a water droplet in the case of a spray scrubber, theparticles tend to follow the streamlines However, for many particles, their inertiawill result in the particle separating from the gas stream and striking the waterdroplet The result is for the water droplet to collect the particle The separationnumber in Figure22.8, NSi, is the same as the inertial impaction parameter, Kp,defined by the following equation:

FIGURE 22.4(b) Several plate types.

• Plates of all types used — sieve plates, slot plates, valve trays, and bubble cap trays

• One modification, the turbulent contact absorber, uses a layer of fluidized plastic spheres

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µg = gas viscosity, poise

Figure 22.9 exhibits single-droplet target efficiency for ribbons, spheres, and

cylinders The curves apply for conditions in which Stokes’ Law holds for the motion

of the particle Equation 22.10 is an approximate equation for a single-droplet target

efficiency, ηd

(22.10)

FIGURE 22.5 Venturi scrubber:

• Gas accelerated at throat

• Atomized water droplets added at throat as a spray or jet, collect particles

• Can be combined with a cyclonic collector to disengage water droplets from air

KK

=+

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The principles stated above can be used to derive a model for the design of a spray

tower This model can be used along with particle distribution to determine the

overall penetration or efficiency of a spray tower making use of the grade efficiency

calculation method The basic assumptions for the model are

1 Droplets are of uniform size and immediately reach the terminal velocity

2 Droplets do not channel down to the scrubber walls

Figure 22.10 depicts the relative velocities of a water droplet and a particle In this

case we can define the following parameters:

Vdt = droplet terminal velocity, cm/s

Vpt = particle terminal velocity, cm/s

Vg = superficial gas velocity, cm/s

FIGURE 22.6 Venturi jet scrubber:

• Used for fume scrubbing

• High pressure water atomized from a jet nozzle into a throat of a Venturi which

induces the flow of the gas to be scrubbed

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FIGURE 22.7 Performance curve for a Venturi scrubber collecting a metallurgical fume.

(Used with permission from Semrau, K T., J Air Poll Contr Technol Assoc., 13(12), 587,

Lime Kiln Dust and Fume

cyclonic spray

Orifice and pipeline

2.97 2.70

0.362 0.362 Black liquor recovery furnace fume

Cold scrubbing water humid gases

Hot fume solution for scrubbing (humid gases)

Hot black liquor for scrubbing (dry gases)

Venturi and cyclonic spray Venturi, pipeline, and cyclonic spray Venturi evaporator

1.75 0.740 0.522

0.620 0.861 0.861

Source: Used with permission from Semrau, K T., J Air Poll Contr Technol Assoc., 10(3), 200, 1960.

9695908070605040

4 3

2

1 0.9 0.8 0.7 0.6 0.5 0.4

o

o o

o o o

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Vg = QG/A

QG = Gas flow rate in m3/s

A = cross sectional area of tower, m2

It is assumed that the particle travels with velocity, Vp, equal to the gas velocity,i.e.,

Vp = Vg

A mass balance is made next, based on Figure 22.11, mass balance in towercross-section The mass balance:

FIGURE 22.8 Primary capture mechanism.

FIGURE 22.9 Single-droplet target efficiency for ribbons, spheres, and cylinders.

Cylinder Sphere Ribbon

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