Chapter 5

Design and Operation of Wet Dust Scrubbers

Contents

1. Introduction 107
2. Some Examples of Industrial Applications of Wet Dust Scrubbers. 108
2.1 Applications in Steel Industry . . 109
2.2 Applications in Foundry Industry. 112
2.3 Applications in Chemical Industry 113
2.4 Treatment of Contaminated Liquid 114
3. Fundamentals of Wet Dust Scrubbing 115
3.1 Dust Particle Collection by Liquid Drops 116
3.1.1 Basic Information on Dust Particle Collection 116
3.1.2 Particle Collection by High-Velocity Drop Movement 118
3.1.3 Particle Collection by High-Velocity Gas and Dust Particle Movement. 119
3.2 Generation of Interfacial Area between Gas and Liquid 121
3.2.1 Generation of Liquid Films 122
3.2.2 Generation of Liquid Jets 123
3.2.3 Generation of Drops . . . 124
3.2.4 Generation of Bubbles. . . 126
4. Design Calculations for Wet Dust Scrubbers 126
4.1 Calculation of Dust Collection Efficiency. . 126
4.2 Design Calculations for Column Scrubbers. 128
4.3 Design Calculations for Jet Scrubbers . . . 134
4.4 Design Calculations for Vortex Scrubbers . 135
4.5 Design Calculations for Rotating Disc Scrubbers 137
4.6 Design Calculations for Venturi Scrubbers . . . 139
5. Comparison and Selection of Wet Dust Scrubbers. 143
6. List of Symbols. 146
7. References . . . 147

1. Introduction

In dust scrubbers the dust particles are removed from a gas stream by a
dispersed liquid. The difference in the design of various scrubbers is primarily
due to the mechanism by which the liquid is dispersed.
Large-scale dust scrubbing was introduced at the end of the last century in
steel industry. In 1892, G. Zschocke was granted a patent for a wetted grid
scrubber. Although this scrubber was of a very crude design it was the most
H. Brauer et al., Air Pollution Control Equipment
© Springer-Verlag Berlin, Heidelberg 1981
108 Chapter 5: Design and Operation of Wet Dust Scrubbers

effective dust removal apparatus available at that time. But only a' few years
later an even more effective dust scrubber was introduced in steel industry: the
disintegrator which is actually a dust removing machine. Both types of dust
removal equipment, namely scrubbing apparatus and scrubbing machines, have
been improved and new types have been developed in the following decades. In
the Federal Republic of Germany, about a hundred different designs for
scrubbing equipment are presently offered on the market. The same is probably
true of other highly industrialized countries. Most of the available designs fall
into one of the following five groups:
a) Column scrubber,
b) jet scrubber,
c) vortex scrubber,
d) Venturi scrubber,
e) scrubbing machines.
The dust removal process in a scrubber involves several steps:
a) Gas and dust conditioning, e.g. reduction of temperature and saturation with
water vapor,
b) dispersion of liquid, for instance by a gas stream, by pressure nozzles, and by
rotating discs,
c) collection of dust particles by liquid lamella or drops.
d) Removal of dust laden liquid from gas stream.
The liquid demand for scrubbing is of the order of 1 to 31 per m 3 of gas.
Wet dust scrubbers have found a wide field of application in various industries,
especially in steel, foundry, and chemical industry. Some examples will be
discussed in the next chapter.

2. Some Examples for Industrial Applications

of Wet Dust Scrubbers
Wet dust scrubbers may be applied in all industries whenever dust pollutes air
or gas streams. Examples for the application of wet dust scrubbers have therefore
to be taken only from a few industries that are of special inportance to the
industrial and technological development of a country. It is for this reason that
examples have been selected from steel and foundry industry and from chemical
industry.
Steel industry is characterized by very big units for the production of a single
product, namely steel. The consequence of a single product process is of course
the production of a large mass of pollutant at a constant rate. Large equipment
for pollution control is characteristic of steel industry.
Quite different is the situation in chemical industry. An extremely great
number of products and just as great a number of pollutants are characteristic
properties of chemical industry. The consequence is a multitude of relatively
small and medium size, pollution control plants. Some of these installations .are
designed for the treatment of effluents from several production plants.
2. Some Examples for Industrial Applications of Wet Dust Scrubbers \09

2.1 Applications in Steel Industry

A blast-furnace gas cleaning plant is schematically described in Fig. 1
(Bischoff KG, Essen) [1]. The production capacity of the blast furnace is of the
order of 5,000 to 5,500 tons of hot metal per day. The diameter of the hearth is
11 meters. The volumetric flow rate of the gas changes between 370,000 and
450,000m 3 /h at temperatures between 200 and 500°C. The dust load of the
furnace gas varies between 15 and 30g/m~ *. From the gas exhaust the furnace
gas is transported to a first dust separation unit. It consists of a dry system in
which the coarse dust fraction is removed. In the second stage, consisting of two
wet scrubbers, the fine dust is removed from the gas. The purified gas leaves the
plant at a temperature of about 60°C with a dust residue of less than 5 mg/m~.
The contaminated water is withdrawn from the wet dust scrubbers and taken to
a mechanical waste water treatment plant. The purified water is recycled while
the sludge is withdrawn from the sedimentation tanks and appropriately dis-
posed of.
The dry dust separation unit may be replaced by a wet scrubber, for instance
a Venturi scrubber, in which the gas is precooled and the coarse dust fractions
are removed. Fig. 2 shows a photograph of such a unit [2]. The use of a simple
but effective wet dust scrubber in a first cleaning stage proves to be advan-
tageous, when a wet electrical precipitator is installed in the second dust
removal stage.

raw gas flowratr; 370000mi!/h

gas tempr;rature 200·C
dust content 15g/m~
gas compressor
blost furnace
,
gas torch

purified
gas

blast
furnace

warm water
tank

Fig. 1. Blast furnace gas purification plant using wet dust scrubbers

* The index n in m~ denotes standard conditions for the gas; pressure 1 bar and
temperature 20 °e.
110 Chapter 5: Design and Operation of Wet Dust Scrubbers

Fig. 2. Venturi scrubber system for precooling,

saturation, and prescrubbing of blast furnace gas

fresh
r
I fresh Fig. 3. Purification of waste gases
water supply water supply
contaminated from oxygen converters by means
water
of wet scrubbers

In a second example, dedusting of waste gases from oxygen converters will be

discussed. Fig. 3 schematically depicts the necessary installations for dust
removal from waste gases of two oxygen converters operated in parallel [1]. The
smelting capacity of each converter is 180 tons per charge. The dust load of the
2. Some Examples for Industrial Applications of Wet Dust Scrubbers III

gas is of the order of 30 g/m~. After leaving the converters, the gas streams pass
through a boiler where the gas temperature is reduced to about 1,000 0c, and
from there to a first Venturi scrubber in which the gas temperature is further
reduced to about 70 to 80°C, the Gas saturated with water vapor, about 90 %of
the dust being removed from the gas stream. The velocity of the gas in the
throat of the Venturi is about 50 m/s and the pressure drop is of the order of
2,000 N/m2. The dust laden water is seperated from the gas streams in elbow
separators. The removal of the remaining fine dust takes place in high efficiency
Venturi scrubbers. In these the throat velocity is of the order of 90 to 100m/s,
while the pressure drop amounts to about 6,000 to 7,000 N/m2. The residual
dust content of the gas streams is 90 to 110mg/m~. The contaminated water
from the elbow separators and the Venturi scrub.bers in the second stage is
carried to a waste water treatment plant and recycled. The sludge is disposed of.
The dust removal from waste gases strongly depends on the pressure drop in
the Venturi scrubbers. In Fig. 4 is plotted the dust content of the purified gas
streams after treatment in Venturi scrubbers against the pressure drop for waste
gases from Thomas converters, oxygen converters, and blast furnaces. The data
included in this diagram have been obtained from industrial units [1]. The costs
involved in dust scrubbing of waste gases from a two-oxygen converter plant
may be obtained from Fig. 5 [1]. An increase in steel production from 100,000

8, 300
"(>
Thomas
~.... 200 conve:rter
=>
Q.
.!:; 100 oxyg~n

~<..<...<..<:u converter

o '--::,.....-J'-----'_--L-==-~-L~..:...L_....J Fig. 4. Dust content of the purified gas

2.10 3 4 6 8 10 12 14 16./03 stream after passing through a Venturi
pressure drop fJp [Nlm 2] scrubber as a function of pressure drop

2.0

"0
~
0
E_ 1.5
~~
-II)
II)
1.0
-5-0

Fig. 5. Costs for wet dust scrubbing as

a function of steel production in thou-
o10~0:-_-~'5:-:C0---:2~00-:----:2-!:'50:----"'300-':-:--3--l50 sand tons per month in a two-con-
steel productIon lOOOtlmonth verter unit
112 Chapter 5: Design and Operation of Wet Dust Scrubbers

to stack in case
of accident

fl'===:JLI~rp gas
clean

Fig. 6. Schematic picture of an electric arc furnace and plant for dust removal from waste
gas

to 350,000 tons per month results in a reduction of the scrubbing costs to about
60 %. In 1962, the scrubbing costs for a steel production of about 100,000 tons
per month amounted to about 1.5 DM per ton of steel produced.
In a third example, dust extraction from the waste gases of a ferro-alloy
submerged arc furnace will be discussed. Fig. 6 gives a schematic description of
a closed 24 MW FeMn-furnace and waste gas cleaning plant [3]. After leaving
the arc furnace, the waste gases pass security channels and installations to reach
a column scrubber 4 in which the gas is saturated with water vapor; the
temperature is reduced to favorable operating conditions in the second cleaning
stage in which the coarse dust fractions are extracted. The second cleaning stage
consists of a Venturi scrubber 5 in which the fine dust is removed from the gas.
The contaminated water is separated from the gas stream in mist collector 6.
The purified gas stream is transported by a suction blower 7 for further use to
another plant. A certain fraction of the clean gas is recirculated through pipe 9
to the Venturi scrubber 5. The contaminated water from column scrubber 4 and
Venturi scrubber 5 is collected in collection tanks 10 and 11 from where it is
taken for further treatment to the waste water purification plant. The purified
water is recirculated.

2.2 Applications in Foundry Industry

For many gas cleaning purposes in the foundry industry, the vortex scrubber
has found a wide field of application. Fig. 7 gives a perspective view of a typical
vortes scrubber and a photograph of two of these scrubbers arranged in parallel
in a foundry [3].
2. Some Examples for Industrial Applications of Wet Dust Scrubbers 113

Fig. 7. Vortex scrubbers installed for dust removal from waste gases of a foundry

dried
product
contaminated
water Fig. 8. Wet dust scrubbing of recycled
inert gas in a spray drying unit

2.3 Applications in Chemical Industry

Dust removal from a gas stream that passes through a spray dryer in a wet
scrubber is a typical example of the application of wet scrubbers in chemical
industry [4, 5]. Fig. 8 schematically describes a spray dryer with gas cleaning
installations. The powder to be dried is used for paint production. Safety
considerations demand for an inert gas as heat carrying medium that was to be
recirculated. After separation from the inert gas stream, the powder is with-
drawn from the spray dryer and from the cyclone. A very fine powder fraction of
1,02 g/m 3 remaines in the gas stream which leaves the cyclone and therefore has
to be removed in a wet scrubber. The mean diameter of the powder entering the
wet scrubber with the inert gas is 211m, the density 1,400kg/m 3 . In the wet dust
114 Chapter 5: Design and Operation of Wet Dust Scrubbers

aerosol.
separatIon
in a two-stage
rotating
disc scrubber

N OH v.entilator 3
a V = 20 000 m '/h
fresh water feed .tJp=200mm H2 0
~N=19kW

~~H[Ji:
~gas '-V

Fig. 9. Wet dust scrubbing of waste gases in a chemical factory

scrubber the powder content in the gas has to be removed to less than 30mg/m 3 .
At the same time, the temperature of the inert gas is reduced from 80 to 30°C.
The wet scrubber consists of two stages. Dust separation is accomplished in the
first stage and cooling of the inert gas in the second.
In Fig. 9 is schematically described another example of wet dust scrubber
application in chemical industry, namely a multipollutant treatment plant quite
typical of conditions prevailing in chemical industry. The pollutants are very fine
dust particles from a paint production plant, namely sulfuric acid mist and acidic
gases like Hel and S02' In the purification process, solid and gaseous pollutants are
removed from a waste air stream of 20,000 m 3/h. Because production conditions
in the plants vary, a purification system for fluctuating pollutant concentrations
had to be designed. The wet scrubbing system worked out consists of a two-
stage rotating disc scrubber and a two-stage jet scrubber. Dust removal takes
place primarily in the scrubbing machine while absorption of the gaseous
pollutants is chiefly accomplished in the jet scrubbers. The scrubbing machines
are fed with water and the jet scrubbers with sodium hydroxide solution. The
contaminated liquids must be treated separately.

2.4 Treatment of Contaminated Liquid

The examples for dust removal from gas streams by means of wet scrubbers
discussed so far reveal that the application of wet scrubbers convert the air
pollution problem into a water pollution problem. From a technical and an
economical point of view this conversion of the problem is acceptable only if the
volumetric flow rate of the contaminated water is extremely small compared to
3. Fundamentals of Wet Dust Scrubbing 115

purified gas
high pressure

t"a"
feed
filter

raw as

fresh,_-I--1'-..........
water

Fig. 10. Schematic representation of a

water sludge pump plant for the treatment of contam inated
tank water

the gas flow rate. Furthermore, treatment of the water must be accomplished by
a simple process.
Figure 10 schematically illustrates a contaminated water treatment plant
[6]. The water is withdrawn from a combined gas cooler and pre-scrubber and
from a venturi end-scrubber. The contaminated water is fed to a sedimenta-
tion tank where it is partially freed from the solid particles, The mechanically
treated water passes through a neutralization system and is pumped back to the
pre-scrubber and the end-scrubber. The sludge that is withdrawn from the
sedimentation tank is pumped to the filter presses where as much water as
possible is separated from the solid and refed to the sedimentation tank. The
filter cake obtained from the filter presses is transported for further treatment to
an in cinerary unit or is deposited,

3. Fundamentals of Wet Dust Scrubbing

Wet dust scrubbing is a process in which dust particles are transferred from a
gas stream a liquid, This transfer process strongly depends on the size of the
interfacial area between gas and liquid and on the relative motion between the
two fluid phases and between the dust particle and liquid, The generation of
interfacial area, relative velocities between phases, and gas-liquid separation will
be discussed in the following sections of this chapter. First of all, it is however
necessary to give a general description of dust particle collection by liquid
drops.
116 Chapter 5: Design and Operation of Wet Dust Scrubbers

3.1 Dust Particle Collection by Liquid Drops

Dust particle collection is markedly affected by the relative motion between
particles and drops. After giving fundamentals in the first section, the following
two sections are devoted to situations observed in high efficiency scrubbers.

3.1.1 Basic Information on Dust Particle Collection

The collision of dust particles with liquid drops is generally studied for the
case described in Fig. 11. The following assumptions are made:

a) gas and dust particles have the same velocity,

b) gas and drops have the same direction of velocity,
c) there is a relative velocity between gas and drop,
d) the drop has a spherical shape.

In Fig. 11 a is indicated the movement of the gas and the dust particles by
streamlines and trajectories [7,8]. Due to inertia forces dust particles approach-
ing the drop wiIl not follow but cross the streamlines of the gas and impinge on
the drop. The possibility for dust particles to cross the streamlines of the gas will
increase with

a) increasing inertia force of the dust particles, and

b) decreasing radius of curvature of the gas streamline (see Fig. 11 b).

According to generally accepted theoretical considerations, all those parti-

cles approaching the drop inside an area with diameter do will impinge on
the drop as indicated in Fig. 11 c. The dust particles will either accumulate on
the surface of the drop in the case of poor wettability of the dust (Fig. 11 d) or
penetrate the drop in the case of good wettability (Fig. 11 e). The dust particles
impinging on the drop surface wiIl move toward the rear stagnation point and
accumulate there. Those dust particles that hit the drop close to the forward
stagnation point will however stay there because the tangential velocity in the

~ J
r-do----r
'"
CO>
:§
I
I
J
E I

h
I
t>
CO>
.':::
'"

6
V,
a b c d e
Fig. 11. Dust particle collection by a liquid drop in the simplest type of flow field
3. Fundamentals of Wet Dust Scrubbing 117

interface of the drop tends toward zero when the forward stagnation point is
approached.
The collection efficiency of wet dust scrubbers has been considered for some
time to greatly depend on wettability. Experimental evidence available however
proves that all dust particles that hit the surface of the drop will either
penetrate the drop or adhere to the surface. This process is independent of
interfacial tension. Wettability is therefore not an important property of the dust
particle-liquid system,
The diameter ratio dold l is called impingement factor:

(1)

This factor can vary between 0 and 1. It has been shown to be a function of the
inertia number t/I, that is defined by:

(2)

Sometimes, this dimensionless group is also called Stokes' number. The symbols
used have the following meaning: wr = relative velocity between dust particle
and drop, Pp = density of particle, d p = drop diameter, 1J g = dynamic viscosity of
gas, and d l = drop diameter.
Fig. 12 describes the dependence of the impingement factor on the inertia
number (Schuch and Laffler [8J). The parameter Ref is the Reynolds number:

(3)

In this definition, Pg is the density of the gas. Fig. 12 indicates that the
probability for impingement will increase with increasing relative velocity wf'

90'
1.0

II /'
v f- - V --
/'

IY
5 0.8
U
.E V
<:
I! if~
0.6
CIJ
E
~ 0.4
·sCl.
§ 0.2 L /
V l,-y
10' 10° 10'
wr ep dff
inertia number '" = 8 d
1 119 ,

Fig. 12. Impingement factor tpj plotted against the inertia number with Reynolds number
Ref as a parameter
118 Chapter 5: Design and Operation of Wet Dust Scrubbers

dust particle density Pp and diameter dp' due to the inertia of the particles. The
chances for impingement will however diminish when gas viscosity IJ g and drop
diameter d 1 increase, because in this case the frictional forces will dominate and
the gas carries the dust particles away.
The impingement factor given in Fig. 12 is of qualitative value only. Real
conditions for the movement of gas, dust particles, and drops will be quite
different from the assumed ones.
In high-efficiency wet scrubbers two cases of gas/dust-particle/drop-movement
dominate:
a) high velocity drop movement normal to low velocity gas and dust
particle movement (drops approach dust particles),
b) high velocity gas and dust particle movement co current to low velocity drop
movement (dust particles approach drops).
In both cases, the impingement factor ({Jj is much higher than that given in
Fig. 12.

3.1.2 Particle Collection by High-Velocity Drop Movement

This case of particle collection occurs for instance in scrubbing machines like
rotating disc scrubbers. The generation of relatively small drops is achieved by
means of rotating discs. The movement of the drops takes place in a horizontal
plane while gas and dust particle motion occurs in the vertical direction. This
type of gas/dust-particle/drop-motion is sketched in Figs. 13a-c. The character-
istic feature is a high velocity drop movement normal to a low velocity gas/dust-
particle-flow field. The drops generated by rotating discs are in general very
small and will consequently retain their spherical shape even at high velocity.
The drop passes through a swarm of particles. The drop velocity is Wd and the
particle velocity is wp' In a first approximation, the drop velocity may be
assumed to be independent of size while the dust particle velocity is generally a
function of the particle diameter. The drop velocity vector Wd is large in
comparison with dust particle velocity vector wp.
Gas streamlines and particle trajectories in the vicinity of a drop are sketched
in Fig. 13 b. Because the gas movement occurs normal to the drop movement,
close to the surface of the drop, the streamlines of the gas are strongly diverted.
This diversion is achieved by a rather small radius of curvature. As a con-
sequence, particle collection is far more efficient than in the case of parallel
motion of drop and dust particles depicted in Fig. 11.
When large drops are produced by the rotating discs, the drops may lose their
spherical shape and assume ellipsoidal shape as described in Fig. 13 c. The shape
changing process is due to frictional forces. It must be expected that collection
efficiency of ellipsoidal drops is much lower than for spherical drops, as may be
deduced from the gas streamlines and particle trajectories. The ratio of the
active dust particle collection surface to the geometrical surface is much smaller
for an ellipsoidal drop than for a number of smaller spherical drops having the
same volume as the ellipsoidal drop. Therefore, in the case under investigation,
the generation of large drops should be avoided by all means.
3. Fundamentals of Wet Dust Scrubbing 119

• SwaIm 01 d~, s' particles

•
c::'::1 • w~ • T
~
Cj---r;----'---"-----
•
Wd

r T

] 56 78
I I I I
I I I I
I 1'1

T+ I

Fig. 13. Dust particle collection by a liquid drop moving with high velocity in a normal
direction through a gas/dust particle flow field

Extremely big drops will be split into several smaller ones. But this disinte-
gration process takes time and one must expect that most of the big drops have
passed through the dust laden gas stream before disintegration becomes effec-
tive.

3.1.3 Particle Collection by High-Velocity Gas and Dust Particle Movement

The interaction of dust particles with drops described in this section is typical
of the situation occurring in the throat of a Venturi scrubber which is the most
effective wet dust scrubber. Fig. 14a shows that drops, dust particles and gas
move cocurrently at widely different velocities. In this case, rather big drops are
propelled into the gas stream at normal direction. The drop trajectory is
changing from a direction normal to the gas stream to a direction parallel to the
gas stream. The final section of the large-size drop movement is sketched in Fig.
14a.
The action of frictional forces, due to the high velocity gas stream, will enforce
disintegration of the big drop into several smaller ones that assume and retain
120 Chapter 5: Design and Operation of Wet Dust Scrubbers

•
• swarm 01

.
dust particles
•
•

• •
•
• •

•
a

secondary drop

torus

parachu te
lamella
el l ipsoidal
drop

Fig. 14. Dust particle collection by liquid elements moving with very low velocity
cocurrently with a high velocity gas/dust particle stream

spherical shape. Intermediate steps of the disintegration process are illustrated

in Figs. 14b and c. This process involves the following steps:
a) Deformation of spherical drops into ellipsoidal ones,
b) further deformation into the parachute lamella
c) disintegration of the lamellae into liquid filaments and drops,
d) disintegration of liquid filaments into drops.
The energy required for the deformation and disintegration process is pro-
vided by the high velocity gas stream.
Gas flow and particle movement around an ellipsoidal drop is sketched in
Fig. 14b. Because of the small radius of curvature of the streamlines close to the
rear of the ellipsoidal drop, the collection efficiency is quite high. This is
contrary to the situation described by Fig. 13c in which an ellipsoidal drop
moves normal to the gas stream. This stresses the point that the directions of the
drop and gas/dust particle movement as well as the shape of the drops are of
fundamental importance to particle collection.
As has been mentioned, the ellipsoidal drop is just an intermediate state of
the disintegration process. There is experimental evidence available that the
shape of ellipsoidal drops is unstable and that the drops perform pulsating
movements as illustrated in Figs.15a-c. Streamlines are indicated by arrows.
Depending on the shape of the drop, there are vortices on the forward or rear
part of the drop. Particle trajectories are not included in Fig. 15 in order not to
obscure the field of streamlines.
3. Fundamentals of Wet Dust Scrubbing 121

b a c
Fig. 15. Pulsating motion of ellipsoidal drops and gas movement with vortex formation

With progressing drop deformation another important intermediate state is

reached in which the liquid is spread out in a parachute-like lamella. In this
state, the surface of the drop available for particle collection has attained its
maximum. The active particle collection surface is the inner surface of the
parachute-like lamella, Close to this area, the gas streamlines are reversed while
the dust particles remain almost uneffected by this movement and impinge on
the surface of the lamellae.
The parachute is not only characterized by the thin liquid lamella but also by
the liquid torus at the rim of the lamella and at the breaking points of the torus
by small drops. On account of inertial forces, the liquid torus will break away
from the lamella and disintegrate into small drops. At the rim of the remaining
lamella, a new torus will build up, break away and disintegrate into secondary drops.
This process repeats itself until the originally large drop is split up into small
drops containing the dust particles.
Dust particle collection is most effective in the intermediate states of ellip-
soidal drops and parachute-like lamellae. The secondary drops are quite in-
effective dust particle collectors, because these drops move with almost the same
velocity as the dust particles.

3.2 Generation of Interfacial Area between Gas and Liquid

The interfacial area between gas and liquid is a potential collector of dust
particles. Whether it is an active particle collector depends on its size and
distribution in the dust laden gas stream, and on the relative motion of dust
particles and interface. The generation of the gas-liquid interface is under all
circumstances closely related to its distribution in the available space.
The interfacial area is generally offered to the dust laden gas stream by the
surface of
122 Chapter 5: Design and Operation of Wet Dust Scrubbers

a) liquid films,
b) liquid jets,
c) drops, and
d) bubbles.
The generation of the interface is therefore closely related to the generation of
liquid films, jets, drops, and bubbles. These processes and the effectiveness in
relation to dust particle collection will be discussed in the next sections.

3.2.1 Generation of Liquid Films

Liquid films are generated by spreading the liquid over the surface of solid
elements like various packing materials and tube bundles.
Packing elements of practical importance are Raschig rings and spherical
particles. Raschig rings are hollow cyclinders with an outer diameter being
equal to the height. Fig. 16 shows a random packing of Raschig rings. The
distribution of liquid over the surface of Raschig rings is indicated in Fig. 17. In
general, there is a parallel motion of liquid and gas in a wetted packing. The
direction of gas flow is primarily parallel to the surface of the liquid film , only
slight interruptions occurring when gas and liquid move from one Raschig ring
to the other. Gas flow normal to liquid flow is scarcely observed. Gas and liquid
may move either counter- or cocurrentiy through the packed column. In the

Fig. 16. Photograph of a ran-

dom packing of Raschig rings

liquid film

Fig. 17. Schematic representation of gas

gas flaw and and liquid flow in a random packing of
dust particles Raschig rings
3. Fundamentals of Wet Dust Scrubbing 123

case of co current flow, the flow direction may be either upwards or downwards.
Wetted packed columns do not offer very effective conditions for dust scrubbing
although the distribution of the interfacial area in the volume of the packing is
quite satisfactory. The pressure drop across the wetted packing is however
relatively low.
Dust collection efficiency of a wetted packed column is improved when it is
operated under flooding conditions. In this case, liquid downflow is hindered by
the upflow of the gas. Intensive mixing of both phases within the packing takes
place. At the same time, a gas-liquid layer is being built up on top of the
packing through which the gas is bubbling. The size of the bubbles is very large
compared with the diameter of the dust particles and the relative velocities
between dust and liquid interface is very small. This is the reason why dust
particle collection in wetted packed columns is in most cases less effective than
might be expected.
A further improvement of collection efficiency should be obtainable with a
bundle of closely packed parallel tubes arranged on top of the random packing
of spheres or other elements as shown in Fig. 18a. Gas and liquid move in the
same direction. Gas and liquid movement in a tube is depicted in Fig. 18b. The
bundle is made of tubes with a diameter between 2 to 4 mm. In such narrow
tubes, surface tension enforces the generation of individual bubbles of cylindrical
shape. As the bubbles are forced through the tubes by the pressure difference,
the relative velocity between gas and liquid may be quite appreciable. The
relative velocity and the small diameter bubbles are favorable to dust collection
efficiency.

3.2.2 Generation of Liquid Jets

In jet scrubbers liquid jets are used for the generation of the interfacial area.
In Fig. 19 it is indicated that the jet emerges from a pressure nozzle. After a
certain length, the jet breaks up into an aggregate of drops with a wide drop
diameter distribution. The gas moves parallel to the jet. During the jet break-up
process, intensive mixing of gas and liquid drops occurs. Further downstream,
the jet of gas/liquid mixture impinges on the surface of the liquid reservoir
disintegrating part of this liquid too. Because of the small relative velocity
between dust particles and liquid surface, the collection efficiency of this system
is only slightly higher than that of wetted packed columns without tube bundle

gas/liquid layer
0
tube bundle

gas bubble
0
liquid
t t t t t tube
gas flow and
dust particles Fig. 18. Arrangement of random pack-
a b ing and tube bundle
124 Chapter 5: Design and Operation of Wet Dust Scrubbers

pressure nozzle

liquid core of Jet

dust laden gas stream

Fig. 19. Break-up of a liquid jet

vor tex chamber

entrance of dust
laden gas stream

liquid surface Fig. 20. Dispersion of liquid by a

gas stream in a vortex chamber

elements. The water jet pumping effect avoids pressure drop in the gas stream.
The distribution of the interfacial area in the jet column is still satisfactory.

3.2.3 Generation of Drops

Generation of liquid drops from a given volumetric flow rate of liquid is
achieved either by dominating frictional forces or inertial forces.
Liquid dispersion by domination of frictional forces is accomplished by one of
two processes that are of practical importance. In the first case, dispersion is
primarily performed by directing the dust laden gas stream of high velocity
parallel to the liquid surface as indicated in Fig. 20. The liquid drops are
separated from the bulk liquid by the feed gas flowing with high velocity parallel
to the liquid surface. The gas and the liquid drops pass through a vortex
chamber. A change of the general flow direction in this chamber produces the
necessary relative motion of dust particles and drops for an efficient dust
collection process. After leaving the vortex chamber, separation of dust laden
drops and purified gas takes place. The drops generated are relatively large,
depending on the gas velocity. In industrial applications, the allowable pressure
drop sets limits to the size of the drops and consequently to the collection
efficiency.
3. Fundamentals of Wet Dust Scrubbing 125

In the second case of liquid dispersion by dominating frictional forces, large

drops are preproduced and transferred to a very high velocity gas/dust particle
stream where further disintegration of the large drops into very small ones takes
place. This process occurs in a Venturi scrubber. The particle collection is
described in detail in Fig. 14 (Section 3.1.3). The introduction of the large drops
is accomplished by pressure nozzles arranged in the throat of the Venturi
scrubber where the gas/dust particle stream has the highest velocity, favorable
for a very efficient dust particle collection. In Venturi scrubbers, the distribution
of the interfacial area in the available volume is very good. The volume of a
Venturi scrubber is therefore very small.
Drop generation by dominating inertial forces is chiefly accomplished by
rotating discs. The liquid is fed in the axis and forms a thin film on the surface of
the rotating disc. Around the circumference of the disc, a torus of liquid builds
up from which further disintegration takes place. In Fig. 21 few photographs are
reproduced which show various mechanisms of liquid atomization by rotating
discs. The diameter of the drops produced depends on the thickness of the liquid
film at the outer edge and therefore on the rotational speed of the disc. The drop
diameter decreases with increasing number of revolutions. The dust particle
collection process is described in Fig. 13 (Section 3.1.2). The distribution of
drops by means of a rotating disc occurs generally in a horizontal layer of
very small thickness.

Fig. 21. Photograph of liquid dispersion

by a rotating disc at various rotational
speeds
126 Chapter 5: Design and Operation of Wet Dust Scrubbers

As has been pointed out in Section 3.1.2 and 3.1.3 dust particle collection
proves to be extremely efficient in systems with a very high relative velocity
between drops and dust particles. Such systems are realized in rotating disc and
Venturi scrubbers.
3.2.4 Generation of Bubbles
Instead of dispersing a small volume of liquid in a large volume of gas,
situations may occur in which the dispersion of a small volume of dust laden gas
in a large volume of liquid is desirable. In general, this system proves to be
rather ineffective because of the extremely low relative velocity between gas and
dust particles within a bubble. Such a low-efficiency system will not be further
discussed.

4. Design Calculations for Wet Dust Scrubbers

The wet dust scrubbers available may be classified into one of the five groups
which have already been mentioned. These include column scrubbers. jet scrubbers.
vortex scrubbers, Venturi scrubbers, and scrubbing machines.
From each one of these groups a typical example will be selected for which
design calculations are presented. Because of the rather complicated physical
phenomena related to wet dust scrubbing, equipment design still considerably
depends on practical experience rather than on theory.
The most important design parameters are dust collection efficiency and
pressure drop. Collection efficiency is determined by a procedure that is same
for all wet scrubbers. This procedure will be dealt with in Section 4.1. In the
following sections the individual scrubbers will be discussed.

4.1 Calculation of Dust Collection Efficiency

The dust collection efficiency is defined by:

(4)

Mpl and Mp2 [kg/m3] are dust concentrations measured in mass [kg] of dust
per unit volume [m 3] of the gas stream at the entrance and exit of the dust
scrubber. The collection efficiency of a wet dust scrubber can be determined by
the following equation:

cP = L (,1 RcpF)k' (5)

k~l

In Fig. 22 the dust residue R and the fractional collection efficiency CPF are
plotted against the dust diameter d p • The diameter range is subdivided in to n
fractions. For fraction k = 4, the mean value (CPF)k and (,1 R)k are indicated.
4. Design Calculations for Wet Dust Scrubbers 127

Fig. 22. Determination of dust

(flF'k -- collection efficiency of wet scrub-
O~~~--~~~~~~~~~ bers by means of dust residue curve
k=1 2 3 , 5 R and fractional collection ef-
diameter dp of dust particles ficiency curve ({JF

The residue R provides information on the size distribution of the dust

particles contained in the feed gas. This curve must be determined for each dust.
A characteristic property of the residue curve is the particle diameter d p (R50)' at
which R = 50 %, that is where 50 %of the mass of dust has a smaller or a greater
diameter. Very fine dusts have a very small value for d p (R50)'
The fractional collection efficiency provides information on the dust fraction k
which is separated from the gas stream in the scrubber. The fractional collection
efficiency curve must be determined for each scrubber, but in general for one
dust only. This dust will be called the test dust, its diameter is designated dpt •
The fractional collection efficiency ({JFt for the test dust is therefore given as a
function of dpt • The question then arises how this curve for ({JFt can be used for
the determination of the collection efficiency ({JF in a practical situation in which
a different dust with a diameter d p has to be separated in a scrubber for which the
tests have been carried out.
The application of the test dust curve ({JFt to an industrial dust is based on the
assumption that the fractional separation efficiency of two dusts are identical
when the settling velocities are the same. Assuming the applicability of Stokes'
law, the settling velocity wst for the test dust is given by:

(6)

where g is the gravitational acceleration, 1'f g the viscosity, Pg the density of the
gas, which has been used in the tests, and dpt the test dust diameter. The gas
density is generally negligible compared with the particle density. Therefore, the
following equation may be applied to the determination of the settling velocity
of the test dust:

(7)

For a real dust the corresponding equation reads:

(8)
128 Chapter 5: Design and Operation of Wet Dust Scrubbers

dp dp1 dp
dust particle diameter dpt and dp
Fig. 23. Conversion of test dust fractional collection efficiency qJFt into the efficiency qJF
for a real dust

With the basic assumption

(9)

and the second assumption that 1Jgt~1Jg, one obtains:

(10)

This is the fundamental equation for the conversion of the test fractional
collection efficiency curve into the real fractional collection efficiency curve. This
conversion is carried out in two steps:
1st step: ({JFt = ({JF = const.
2nd step: convert dpt«({JFt) into dp«({JF) by means of Eq. (9)
The results of two conversion processes are illustrated by Fig. 23:
1st case: Pp>Ppt: for ({JFt=({JF=const. one obtains
d p< dpt' i.e. the collection efficiency increases.
2nd case: Pp<Ppt: for ({JFt=({JF=const. one obtains
d p> d pt . i.e. the collection efficiency decreases.
Curves for the fractional collection efficiency ({JFt for a selected test dust will
be presented for each wet dust scrubber discussed in detail in the following
sections.

4.2 Design Calculations for Column Scrubbers

Various types of wet dust column scrubbers are currently used in industry.
Some of these scrubbers are schematically illustrated in Fig. 24.
The simplest type is the spray nozzle scrubber shown in Fig. 24a. It consists of
an almost empty cylindrical vessel in which are arranged one or more horizontal
nozzles which disperse the feed water. The interaction of drops and dust
4. Design Calculations for Wet Dust Scrubbers 129

,=;~
contaminated
water
~ contaminated
water
contaminated
w at er
spray nozzle ColUITIII packed column
countercurrent flow packed column
of gas and liquid cocurrent flow
of gas and liquid
a b
c
Fig. 24. Various types of column scrubbers

particles is described in Fig. 11 (Section 3.11). The collection efficiency of this

scrubber is relatively poor because dust particles and drops move in vertical
direction with a rather small relative velocity. Collection of dust particles with a
diameter below 10 I!m is unsatisfactory. On the other hand, the pressure drop is
extremely small so that the spray nozzle column is suited for precooling and
prescrubbing purposes. The pressure drop is of the order of 50 to 100 N/m2. The
spray nozzle scrubber is insensitive to dirt and foaming.
In Figs. 24b and c packed column scrubbers with either countercurrent or
cocurrent flow of gas and liquid are schematically described. The possible
gas/liquid interaction has been discussed in Section 3.21 (see Fig. 17). In
packed columns the liquid is dispersed in such a way that liquid films flow over
the surface of the packing elements. Gas flow is primarily parallel to liquid film
flow.
Countercurrent flow of gas and liquid is limited by the flooding velocity of the
gas. Flooding occurs when the gas starts to reverse liquid flow. In this case, a
bubble layer commences to build up on top of the packing. The pressure drop
increases strongly with velocity. This state of operation should be avoided if the
scrubber has not been prepared to operate under such conditions. Consequently, a
wetted packed column scrubber with countercurrent gas/liquid flow is generally
operated at a rather low gas velocity. Collection efficiency and pressure drop are
fairly low, although somewhat higher than for spray nozzle columns because of
the relatively great height of the packing. The mean gas velocity in the empty
column is of the order of 1 to 2 m/s; the pressure drop amounts to about
100 N/m2 for a packing height of 1 m. 50% dust collection is achieved at a dust
particle diameter of about 1.5 to 2 I!m.
l30 Chapter 5: Design and Operation of Wet Dust Scrubbers

Fig. 25. Collection efficiency of packed

column scrubbers with co- and coun-
tercurrent gasjliquid flow as a function
height of wetted packing of the height of the packing

The collection efficiency of packed column scrubbers can be improved by

increasing the gas velocity substantially. This can be accomplished only with
cocurrent gasjliquid flow because flooding has to be avoided. A packed column
operating under co current flow conditions is illustrated in Fig. 24c. The col-
lection efficiency for co- and countercurrent gasjliquid flow conditions is quali-
tatively described in Fig. 25 as a function of packing height. With co current high
velocity of the gas stream, dust collection is far more effective than with
countercurrent low velocity of the gas stream at the same height of the packing.
On the other hand, the pressure drop is much higher under high velocity
co current than under low velocity countercurrent flow conditions.
Both types of packed columns are very sensitive to clogging, as the dirt will
settle down in the interstices of the packing, and to foam formation. Dust
properties with respect to foaming must be investigated very carefully before a
suitable scrubber is selected.
Fig. 25 indicates that it is advisable to operate countercurrent flow columns
under flooding conditions and co current flow columns with a relatively small
height of packing. A column operated under such conditions has been depicted
by Holzer [4, 5]. A schematic description is given in Fig. 26. The liquid is
sprayed by means of nozzles against the bottom layer of the packing. The gas
stream carries the liquid through the packing. As the column is operated under
flooding conditions, a gasjliquid layer builds up on top of the packing. The
height of the gas/liquid bubble layer is of the order of 50 to 100 mm; it is kept
under control by a water withdrawal pipe with a conical opening.
In Fig. 26 the fractional collection efficiency ({JFI' as determined by Holzer, is
plotted as a function of the particle diameter of the test dust, dpl' for three values
of the height h of packing. According to these results, it is not advisable to
increase the height h of packing beyond 100 to 200 mm. This proves that this
system is operated at exceptionally low height of packing.
The dust properties and test conditions are listed in the table included in
Fig. 26. d pt(R 50) denotes the diameter of the test dust in the feed gas at a residue
t

of R t = 50 % and d pt ('PFt 50 ) the diameter of the test dust for which the fractional
collection efficiency is ({JFt = 50 %. The test dust was a very fine dust of which
50 % by mass had a diameter smaller than 2.7 /lm. The corresponding diameter
at a wetted packing height of h = 200 mm is d pt ('PFt 5 0) = 1.05 /lm. The test results
4. Design Calculations for Wet Dust Scrubbers 131

100
-
fIl
;:::-
& f2 ~
.7 ~f2/ ",<:i

---- ---
~f-f.
jll/ :
Q5
.J? 1.0
1/ I
VV
dp t(rpFt 5O )

1.5 2.0 2.5 3.0 3.5

b test dust diameter dpt [Ilm ] Fig. 26. Packed column
test dust properties test conditions scrubber for cocurrent flow
dust
material
e
pI IldpffR, SO) w. I
v/ ,I
M. ,
[kg/m 3] [I'm] [ mh] [mf(m}h)] [g/mJ] [mm]
,I
d. s operation with shallow pack-
quartz 2600 I 2.7
ing of spherical particles;
1.05 , 3.0 I 1 I 10 geometry and fractional
collection efficiency

indicate that the mean diameter at <{JFt = 50 % can scarcely be reduced below
Ijlm.
The tests were carried out with a mean gas velocity Wg = 1.05 m/s and a
volumetric liquid flow density of VI = 3.0 m 3 /(m 2 h). The liquid/gas flow rate
ratio was 0.81 water/m 3 gas. The mass of dust in the raw gas was 1 g per m 3 of
gas. The diameter of the spherical particles used as packing material was d ps
=10mm.
The packed column scrubber described is quite insensitive to fouling because
the gas velocity applied in the flooding range is so high that the spherical
particles undergo motions which ensure a self-cleaning effect. Foaming is of
course a danger to the operation of any packed column.
The column scrubber described in Fig. 26 is operated under flooding con-
ditions, i.e. at a gas velocity which is slightly higher than the flooding velocity
designated by wg • o. The flooding velocity is calculated in two successive steps. In
the first step, the single-phase pressure drop LI PI is determined, and in the
second step the related flooding velocity wg• o [9]:

LlPI = [50wtO.16 +2.5 . 1O- 6wt1.9l- I (11)

gp1h
132 Chapter 5: Design and Operation of Wet Dust Scrubbers

wt is the dimensionless liquid velocity in the packed column under flooding

conditions, defined by:

w (V-.l )1/31-e
w*=-----'!' -- (12)
I - d f g2 e'

where g=acceleration, PI=liquid density, h=height of packing, wl=mean

liquid velocity under flooding conditions, VI = kinematic viscosity of the
liquid. e = porosity of dry packing. and d f = fluid dynamic diameter of the flow
passage in the packing. defined by:

e
(13)
d f =1 3/2 '
-+-(1-e)
de dp

where de is the column diameter and d p the particle diameter defined by

(14)

vp denotes volume and Ap the geometrical surface area of the considered

particles. The mean gas and mean liquid velocities are calculated by the
following equations:

_ Vg
(15)
w = d~n/4'
g

_ VI
(16)
wl = d~n/4'

where Vg and VI are the volumetric flow rates of the gas and liquid.
In the second step. the pressure drop LI PI is related to the flooding velocity
wg, o. This is accomplished by means of equations in which the frictional factor 1/1
is related to the Reynolds number Reg,o [10]. These dimensionless numbers are
defined as follows:

(17)

(18)

rt g denotes the dynamic gas viscosity and e the porosity of the dry packing
which is defined by;

(19)
4. Design Calculations for Wet Dust Scrubbers 133

v c is the volume of the column filled with the packing and V p the volume of the
packing elements.
Frictional factor laws are given for packings of Raschig rings, saddles and
spheres. For Raschig ring packings, the following equation applies:

(20)

In this equation d p is to be replaced by d pr :

(21)

with

(22)

(23)

(24)

tested ranges of application:

10 1 ::;;Re g • o ::;;7.10 3
1.6::;; djd a ::;; 42
0.56::;; e ::;; 0.97.

In these equations d a is the outer and d i the inner diameter of the Raschig
nngs.
For packings of Berl and Intalox saddles the following relation holds:

137 3.85
1jJ=~+Reo.l' (25)
g.o g,o

tested range of application:

Packings of spherical particles are expressed by

160 3.1
1jJ=~+ReO,l' (26)
g,o g,o

tested range of application:

134 Chapter 5: Design and Operation of Wet Dust Scrubbers

Iterative methods have to be applied to the determination of the flooding

velocity wg, o using Eqs, (20), (25) and (26).

4.3 Design Calculations for Jet Scrubbers

The jet scrubber is a very simple multipurpose apparatus used for the
simultaneous removal of gaseous and solid pollutants from waste gases [4, 5].
The jet scrubber is therefore a simultaneous absorber and deduster. Fig. 27 a
schematically describes a two-stage jet scrubber and Fig. 27b gives geometrical
details of the jet column proper.
For purely dedusting operations, a one-stage jet scrubber will suffice. The
second stage improves dust collection only marginally. The application of two-
stage scrubbers is only justified in the case of absorption.
The jet scrubber is actually a water jet pump. The liquid jet emerges from the
pressure nozzle and penetrates the throat of the column with a mean velocity of
about wlc = 20 to 35 m/s. The pressure in the nozzle is of the order of 6 to 8 bar.
The feed gas is sucked by the water jet into the inlet chamber of the scrubber

~. con verger
d "tJUthroot
purified
gos
==I>
;g gaslliquid
r
waler lank
with U5m 3 ~ mixing
-L---.J'--J;.:;5);;m;;=.;;4 of wa ler ..r
a per stage _ 70 diffuser

---- -
IV-::
b,/ Ka
-f, I
-
IV
L V:/~PI(Vjc,.so}
Q5 1.0 1.5 2.0 25 3.0
c test dust diam eter dp,[j.lmJ

test dust properlies lest conditions

dust Qpt dp tlRt 50) Wgc /YIp 1 number

[kglm 3] [pm] curve Emls1 [ glm 3] ofstages
material Fig. 27. Jet scrubber; geome-
quartz 2600 2.7 a II 1 1
try and fractional collection
quartz 2600 2.7 b 11 1 2 efficiency
4. Design Calculations for Wet Dust Scrubbers 135

where distribution and change of flow direction take place. In the con verger the
gas is accelerated to the throat velocity of WgC = 10 to 20 m/s. The high relative
velocity between gas and liquid jet, which is of the order of 10 to 20 mis,
supports the liquid jet break-up and the mixing of liquid and gas in the diffusor.
The mixing process is of fundamental importance to the dust collection and
absorption process. Generation and break-up of liquid jets has been discussed in
some detail in section 3.2.2 (Fig. 19). The energy required for the dispersion of
the liquid is supplied by the kinetic energy of the liquid jet. Consequently, the
dispersion process does not depend on the gas flow rate so that the scrubbing
efficiency is more or less the same for low and high flow rates of the gas.
The dispersed liquid and the gas impinge on the surface of the liquid in the
storage tank. Impingement supports dust collection and absorption. Separation
of gas and liquid takes place in the tank. Most of the polluted water is
recirculated, only a small amount of it is withdrawn and replaced by fresh water.
The liquid/gas ratio in the jet column scrubber is of the order of 5 to 20l/m 3 gas
and is therefore higher than in all other types of wet dust scrubbers.
The fractional collection efficiency of a single- and two-stage jet column
scrubber is illustrated in Fig. 27 c. Curve a gives the experimental results for a
single-stage unit and curve b for a two-stage unit. As has already been
mentioned, the second stage only slightly improves collection efficiency.
Test dust properties and test conditions are summarized in the table included
in Fig. 27. The test dust properties are the same as those of the dust used for
testing the column scrubber described in Fig. 26.
According to the curves given for the fractional collection efficiency the dust
particle diameter dpt «1'Ft50)' for which ({JFt = 50 %, is 0.9 11m in the case of a single-
stage and 0.75 11m in the case of a two-stage unit. These data prove that a jet
scrubber removes an appreciable amount of dust with a diameter smaller than
111m.
The application of a jet scrubber offers several advantages when
a) dust removal is accompanied by the removal of gaseous pollutants,
b) gas flow rate changes due to varying process conditions,
c) no or very little pressure drop is available for waste gas treatment.
A great disadvantage of this scrubber is its sensitivity to foaming. According
to Holzer [4] the curves describing the fractional collection efficiency may be
safely applied to cases with gas flow rates up to 10,000 or 15,000 m 3 /h.

4.4 Design Calculations for Vortex Scrubbers

Many different designs of vortex scrubbers are available. In all cases, dust
scrubbing chiefly occurs in a vortex space. A typical vortex space is discussed in
Section 3.2.3 (Fig. 20). A vortex scrubber is schematically depicted in Fig. 28a.
After the feed gas has entered the entrance chamber, it impinges on the
surface of the water, thereby dispersing some of the water, and moves with the
drops through the vortex passage. Dust collection by the water starts with the
dispersion process and is completed in the vortex passage. Behind the vortex
passage the separation process sets in. Large drops fall out of the gas stream and
136 Chapter 5: Design and Operation of Wet Dust Scrubbers

return directly to the water pool. Small drops are separated from the gas stream
in the drop separator.
The mean gas velocity in the vortex passage is about 10 to 30 m/s. The
pressure drop L1 p amounts to 1.500 to 3,000 N/m 2 . The liquid/gas ratio is
estimated to be of the order of 1 to 31/m 3 gas.
Liquid dispersion, i.e. dust scrubbing efficiency of the vortex scrubber, mark-
edly depends on the gas flow rate because the energy required for the water
dispersion is supplied by the gas stream. Due to changing process conditions,
the gas flow rate may either increase or decrease. By an increased gas flow rate,
the level of the water in the entrance chamber may be pressed down so far that
liquid dispersion-and consequently dust scrubbing-will be reduced. On the
other hand, a decrease in the gas flow rate will also result in a diminution of the
water dispersion and dust scrubbing efficiency. The range of gas flow rate for
effective scrubbing service is rather narrow. The application of vortex scrubbers
is suggested especially in such cases in which the gas flow rate can be kept
constant.
In Fig. 28b the fractional collection efficiency ({J F1' as determined by Holzer [4, 5],
is plotted against the test dust diameter d p1 ' Test dust properties and test con-
ditions are summarized in the table included in Fig. 28. The tests have been

drap
seperator

vorlex
passage _

wa ter
tank

100
/'
I
----
--I.
j
/:
:/ .dpl ( 'PF",SOJ
./
0.5 1.0 1.5 2.0 2.5 3.0 3.5
b test dust diameter d pl [ pm]

I
lesl dusl properlies test conditions
dust
moteriol
Il p l
[kg/m3] [11m]
~I dpl(RI50) IIg
[m3/h]
I II Mp ' Ap
[g/m3] [N/m2]
quartz I 2600 I 2.7 2500 I 1 I 2200

Fig. 28. Vortex scrubber; geometry and fractional collection efficiency

4. Design Calculations for Wet Dust Scrubbers 137

carried out at a volumetric flow rate of the gas Vg= 2,500 m 3 /h and a dust
concentration in the raw gas Mpl = 1 g/m 3 gas. The dust studied is the same as
that used for the testing of column and jet scrubbers. A fractional collection
efficiency of 50% is obtained for a dust particle diameter of dpt «PFt50) =0.77 11m.
The collection efficiency of a vortex scrubber is slightly higher than that of the
single-stage jet scrubber.
The advantages of the vortex scrubber are simple design, insensitivity to
fouling, and low maintenance. It should be born in mind though that because of
the water circulation system the vortex scrubber is very sensitive to foaming.

4.5 Design Calculations for Rotating Disc Scrubbers

There are available many different designs for scrubbing machines. The
simplest type of a scrubbing machine is the rotating disc scrubber. Dispersion of
the liquid is accomplished by rotating discs. The dispersion process is described
in Section 3.2.3 and the dust collection process in Section 3.1.2 by means of
Fig. 13. According to the conditions generated in the rotating disc scrubber, this
type of scrubber must be a very efficient dust collector.
The geometry of a rotating disc scrubber is illustrated in Fig. 29a. The
scrubber consists of a cylindrical vessel in which two rotating discs are arranged.
These discs generate and distribute drops in horizontal planes through which
the feed gas flows. The drops collect dust particles on their way from the disc to
the wall of the vessel on which the impinging drops coalesce and form a liquid
film that flows downwards under the action of gravity. In this way, the collected
dust is withdrawn from the scrubber in a very short time, thereby avoiding the
danger of foam formation.
The feed gas enters the scrubber via a tangential inlet at the bottom of the
column. On account of the induced rotational movement of the gas/dust mixture
on a screw-like path to the top of the column, centrifugal forces assist in the
process of separating the dust from the gas. The purified gas leaves the scrubber
at the top after passing a mist-separator, in which very fine drops are withdrawn
from the gas. The mean axial gas velocity for test conditions is Wge = 1.1 m/s.
Under such conditions, the pressure drop is in general less than 1,000 N/m2. The
dimensions of the rotating disc column tested by Holzer [4, 5] are given in
Fig. 29 a. Because of the rotating discs it is scarcely possible to either increase or
decrease the diameter of the column. The diameter of the tested column was de
= 900 mm, the overall height of the column he = 3.33 de = 3,000 mm and the
diameter of the disc d r = 450 mm; hence, dr/d e= 0.5. The rotational speed of the
outer edge of the disc is about 60 to 70 m/s corresponding to about 2,500 rpm of
the disc with d r = 450 mm. The amount of water dispersed by the discs is 1I/m 3
of gas. As the volumetric flow rate of the gas was 2,500 m 3 /h, about 2.5 m 3 of
water had to be dispersed during the tests per hour. The water leaves the
scrubber at the bottom and is directed to a tank from where part of it is
recirculated. Another part of the contaminated water is withdrawn for further
treatment and replaced by fresh water.
Fractional collection efficiencies, as determined by Holzer [4, 5], are given in
Fig. 29b for two values of the liquid/gas ratio VIN g [l/m 3 ] and for a single and
138 Chapter 5: Design and Operation of Wet Dust Scrubbers

demisler

100r---'---::::::::=-:C::==:f~-"-----'
c:: .......
~ ~ 80 r----,1--~~+-----+_----+_--~
~~
8 ~ ro r---T-~~~+-----+_----+_--~
- :....
.,g
g .g
~
~' r-~r-~----+-----+_----+_--~
g~
~ Q;

1.5 2.0 2.5

b test dus t diameter dpt [ IJ m]

test dust properties test condit ions

dust dpl(R,SOJ
~pl ~,d~ ~2/~
~'
Wgc
curve film J [11m J [g m 3J
material lkglm3J fJl.m) fmlsJ
quartz 2600 2.7 a 1.1 0.96 -- 1
quartz 2600 2.7 b 1.1 0.1.8 0.1.8 1
quartz 2600 2.7 c 1.1 1.56 -- 1

Fig. 29. Rotating disc scrubber; geometry and fractional collection efficiency

for two rotating discs. The liquid flow rate is the same for curves a and b; for
curve c it is dispersed by two discs. The collection efficiency of a two-stage
arrangement is slightly higher than that of a single-stage arrangement. An
increase in the liquid flow rate by about 60 % results in a reduction of the
particle diameter at ({JFt= 50 % from 0.55 to OJ /lm.
As the liquid dispersion is independent of the gas flow rate, the rotating disc
scrubber is well suited for dust removal from gas streams with variable flow
rate. The dust concentration Mpl of the feed gas may increase to about
300 g/m3 . The dust concentration of the water may be up to 200 gi l. Because of
the very short residence time of the contaminated water in the scrubber, there is
no danger of fouling and scrubbing. The rotating disc scrubber is an extremely
efficient dust remover comparable with the Venturi scrubber. It is sometimes
pointed out that rotating elements are disadvantageous to a dust scrubber.
However, experiences of more than eight years have proved that reliability and
availability of rotating disc scrubbers are just as good as those of other
scrubbers.
4. Design Calculations for Wet Dust Scrubbers 139

4.6 Design Calculations for Venturi Scrubbers

The Venturi scrubber is the most widely used wet scrubber. (For a schematic
representation see Fig. 30a.) The Venturi consists of a converger, a throat and a
diffusor. It is named after the Italian physicist G.B. Venturi (1746-1822) who
first studied the effect of constricted channels on the flow of fluids. The Venturi
tube was invented in 1886 by the American engineer Clemens Herschel for the
purpose of increasing the fluid velocity, thereby causing a decrease in pressure.
The Venturi tube has found application in many different fields of engineering.
The feed gas enters the Venturi scrubber at the converger and leaves the
scrubber at the diffusor. In the converger the gas is accelerated so that it reaches
the highest velocity in the throat. Typical throat velocities are in the range of Wgc
= 50 to 150 m/s. Here, the gas is contacted with the water which is forced
through small holes in the wall of the throat; the holes have a diameter of about
2 mm. The water pressure is of the order of 2 to 3 bar. Proper working of a
Venturi scrubber strongly depends on the water distribution in the throat. Each
element of the cross sectional area of the throat must be supplied with the same
amount of water. If the water pressure is too low. the water will not reach the

raw gas

~ I +_--water
12 holes, ; 2mm

'tY dilfu50r

~ "'17°
I

purified gas to
drop separator
a
100
a...,.--;: -r- -- -----
.~~ 80
vI.!..... d..j cj / / .....b
~ ~ 60
89."-
-1/'/
~ ~ .0
.~.~ 20 IY
e~ If L,/d (~Ft50)
pt
....... 00 a25 as a75 to
b test dust diameter dot [p.m]

test dust properties test conditions

dust 9p , 4'(R,50) curve Wgc ~/I' Mpl 6p,
material [kglm3] {p.m] [mls] [ilm~J [glm3] [Nlm 2j
paraffin 900 0.4 a 87. 3 3 2.7 12500
paraffin 900 0.• b 87.3 U 1.26 5000
paraffin 900 0.4 c 87.3 3 2.7 7500
sillitin 2600 1.4 d 87.3 3 7.8 7500

Fig. 30. Venturi scrubber; geometry and fractional collection efficiency

140 Chapter 5: Design and Operation of Wet Dust Scrubbers

center of the throat. Some of the gas remains untreated so that the collection
efficiency sharply decreases.
The energy required for dispersion is provided by the high velocity gas
stream. Some details of the liquid dispersion process are described in Sec-
tion 3.1.3 (Fig. 14). According to Mayinger and Neumann [11], the dust col-
lection process takes place in the throat of the Venturi. Others however, for
instance Dau [12], assume that the dust collection of the drops occurs both in
the throat and in the diffusor of the Venturi scrubber. The lenght of the diffusor
therefore should be an important parameter of collection efficiency.
The quality of the water dispersion depends on the velocity of the feed gas
stream. Therefore, the collection efficiency is determined by the gas velocity. A
decrease in the gas velocity will lead to a diminution of the collection efficiency.
Practical experience has proved that it is very difficult to effectively distribute
the water over the throat cross sectional area, when the throat diameter is
greater than about 100 or 120 mm. Therefore, at high volumetric gas flow rates,
Venturi scrubbers with rectangular cross sections are applied. The width is about
100 or 120 mm. Fig. 31 shows such a Venturi scrubber. In order to adjust a
Venturi scrubber to variable gas flow rates, the cross sectional area is altered by
means of movable plates in the throat.
Due to high collection efficiency and simplicity of design many experimental
and theoretical investigations have been carried out with the aim to determine
pressure drop Ll Pv and collection efficiency ({Jv ; only a few of these studies will be
mentioned [12, 13, 14, 15]. All equations established for pressure drop are of the
same type. Based on the equation by Giintheroth [15] the following equation
has been developed by Lemann [14]:

(27)

Fig. 31. Photograph of a venturi scrubber with a rectangular cross section; design CEAG
4. Design Calculations for Wet Dust Scrubbers 141

In this equation 'v is the friction factor for the Venturi scrubber:
(28)

with

kl =0.786+0.878 .10- 2 Wgc' (29)

VI [ljh] is the volumetric flow rate of the water, Vg [m 3 jh] the volumetric
flow rate of the gas and WgC [m/s] the gas velocity in the throat.
An interesting theoretical analysis of dust collection in a Venturi scrubber has
been reported by Dau [12]. Some of the results of this analysis obtained by
numerical methods will be discussed by means of Fig. 32. These results have
been obtained for a dust with a uniform particle diameter d p = 1 [11m] and
density Pp = 2,600 kg/m 3 . In a field with coordinates for pressure drop A Pv'
liquid/gas ratio VIN g and throat velocity WgC as parameter, curves for a constant
value of the collection efficiency <{Jv = 99 % are given for three values of the
throat diameter de' The relation

is considered for four cases the results of which are summarized in Table 1.
According to further results obtained by Dau, the collection efficiency increases
with pressure drop and liquid/gas ratio. With a ratio VIN g';:;;; 2, the maximum for
<(Jv is obtained. For values VIN g> 2, the collection efficiency decreases, especially
for a particle diameter d p < 111m. Dau found satisfactory agreement between

1
1
1
1
1
1
f.! 1 1
~ 1 ~'?I
<::J, ~I
f::! &1
1"'1 \~
1I 'O\:)~
1/ / -'
'I / 0 (fI/;" -'
'I / 0 /
3 -'
1.10 / /.; -/0 rn/ s _
,../ "!c-_---
o o 2
Fig. 32. Collection efficiency diagram for
water/air ratio Vt /Vg [I/m 3] Venturi scrubbers after Dau
142 Chapter 5: Design and Operation of Wet Dust Scrubbers

Table 1. Relation between collection efficiency ifJy and some important parameters for 4
cases

Case 1

ifJy V/Vg de Ll py Wge

[%] [1/m 3 ] [mm] [N/m 2] [m/s]

99 1 50 7,600 100
99 1 100 3,700 76
99 1 500 2,400 65

ifJv and liquid/gas ratio VIN g are kept constant: With increasing throat diameter, Ll Pv
decreases. Consequently, the throat velocity has to be reduced.

Case 2

ifJv Llpv de VIN Wge

[%] [N/m2] [mm] [l/m~] [m/s]

99 6,000 50 1.60 79
99 6,000 100 0.73 103
99 6,000 500 0.58 112

ifJ v and Ll Pv are kept constant: With increasing throat diameter de' the gas/liquid ratio
VIN g has to be reduced while the throat velocity has to be increased.

Case 3

ifJy Wge de Ll Pv VIN

[%] [m/s] [mm] [N/m2] [l/m j ]

99 80 50 6,100 1.57
99 80 100 3,900 0.95
99 80 500 3,300 0.76

ifJy and Wge are kept constant: With increasing throat diameter, pressure Llpy will decrease
and liquid/gas ratio VIN g has to be reduced.

Case 4

ifJv de WgC Ll Pv VIN

[%] [mm] [m/s] [N/m 2] [l/m j ]

99 100 120 10,000 0.77

99 100 100 7,600 1.00
99 100 80 6,100 1.57

ifJv and de are kept constant: \yit~ decreasing throat velocity Wge , pressure drop Llpy will
decrease and liquid/gas ratio YI/V g has to be increased.
5. Comparison and Selection of Wet Scrubbers 143

numerical results and some experimental results published by Wicke [16J and
Calvert, Lundgren and Mehta [17]. The success achieved by Dau should
encourage research workers in the field to carryon with the mathematical
treatment of the problem based on even more insight into the collection process.
In general, it seems to be advisable to determine the collection efficiency of a
Venturi scrubber on the basis of the fractional collection efficiency ({JF which has
experimentally been found for a test dust. Holzer [4, 5J has established for
various experimental conditions ({JFt curves which are illustrated in Fig. 30b.
These curves are in qualitative agreement with the theoretical results obtained
by Dau.
After passing the Venturi scrubber the drops have to be removed from the gas.
Because of the relatively large drops it is possible to use a cyclone for drop
removal.
Venturi scrubbers are the most efficient and the smallest wet dust scrubbers.
They are insensitive to fouling and foaming because the residence time of the
dust laden drops in the scrubber is extremely short. On the other hand, pressure
drop and noise emission are very high. Although the Venturi is a small-size
scrubber, one has to take into account the nescessity of equipment for drop
removal the size of which may be several times that of the Venturi.

5. Comparison and Selection of Wet Dust Scrubbers

For a comparison and selection of wet dust scrubbers for definite purposes,
Holzer [4, 5] compiled the important properties of scrubbers (Fig. 33). The cut
diameter of the dust particle is the diameter of that dust fraction which will be

Column Jet Vortex Rotating disk Venturi

Type of scrubber scrubber scr ubber scrubber scrubber scrubber

schematic
drawing
. :1-
f 1 2
Q;;1t;;:;;;;;;;m;;

cut diameter
dpt('PFfSOJ[jlm} 07-1. 5 0.8-0.9 0.6- 09 0 1- 05 0.05-0.2
relative velocity
gas/liquid lm/sl 10- 25 8-20 25-70
pressure drop
102[N/m2) 2-25 15-28 '-10 30-200
wa ter/air ratio
V;/Vq {/lrn31 O.OS-S S- 20 · 1-3" os- s
specific energy
demand kWh/ /O()()".,JJ 0.2-IS '2-3 1-2 2- 6 1.5-6
• per slage

Fig. 33. Compilation of properties of wet dust scrubbers

144 Chapter 5: Design and Operation of Wet Dust Scrubbers

1'\ ~ K '5
•
15

"'"
~"
~
~ 20 ~
~
~~~ ft
17

to"1-~
~/~
:9(" 12
~~
.:;,,,~
Pt
~ ~1
\' ~
"'-'\ 11()\
~
3 1\ Drl

N 5 \--2
['..
~

2
'"
161
~ 2 2
cut diameter d p t('I'F/50) fJlm]

Fig. 34. Relation between specific energy demand and cut diameter for wet dust
scrubbers

test conditions

type of air flow pressure water/air

Nr. symbol rate drop ratio
scrubber
{m 3/hl l()2lN/m' ; {I/m 3]
1 wet cyclon l> 450 19 Q2 - 0.8
2 column scrubber - 250 5 - 20 0.5 - I
3 vortex scrubber
• 3000 20 -
4 jet scrubber - 500 (+2) 3 - 10
5 column scrubber - 1000 5 - 18 Q5 - 1
5 vortex scrubber 0 2500 15 - 22 -
7 Ip.ltenturi scrubber
8 scrubbingmoriline
•
0
2000
17500
15 -20
10 2-3
-
9 Ip~lventuri scrubber - 3000 15 -40 -
-•
10 impinger 1000 18 0.5
II IP!kntimpinger 1000 22 - 37 Q5 - I
12 wet cyclonelte V 500 18 0.6 -1
13 lfi\tenturi scrubber ... 3500 20 -
-
" mixed type
15 scrubbing machin
0
•
1000
300
20
8 -
16 scrubbing machine - 500 18 - 70 Q5 - 2
17 scrubbing madline ~ 2000 (.15) 0.1 - 0.4
18 mixed type - 500 20-50 0.5
19 mixed type ~ 2000 (.15) Q5 -0.8
20 scrubbing machine - 2800 8 -10 · I -2
21 hll.ent. scrubber - 1500 25-100 0.8
22 h,;y.nt.scrubber - 500 40-100 1-3 l)/p. -low pressure
21hp.-high pressure
5. Comparison and Selection of Wet Scrubbers 145

Table 3. Dust and scrubber properties as a basis for the selection of equipment

Dust properties:
1. size and size distribution, residue curve,
2. dust particle density,
3. dust concentration in feed gas,
4. formation of dust agglomerates,
5. indication to plugging,
6. chemical reaction intensity,
7. toxicity,
8. smell intensity,
9. explosiveness,
10. optical intensity,
11. wettability,
12. foaming,
13. cost.
Equipment properties:
1. collection efficiency,
2. reliability,
3. availability,
4. flexibility to operational variations,
5. pressure drop,
6. energy demand,
7. sensitivity to corrosion, erosion, and foaming,
8. construction materials,
9. size,
10. area demand,
11. waste disposal,
12. installation and operation costs.

removed from the gas stream by 50 %. The cut diameter decreases from the
column scrubber in the direction of the Venturi scrubber. The increased col-
lection efficiency is due to the increased relative velocity between liquid and gas
(see Section 3.1). A rise in the relative velocity results in an increase of the
pressure drop. The water/air ratio is given in a further line with the exception of
the jet scrubber; for all scrubbers this ratio ranges from 1 to 311m 3 .
Next to the cut diameter d pt ('Ppt 5 0)' which characterizes the collection ef-
ficiency, the specific energy demand is the most important property of wet dust
scrubbers (given in the last line of Fig. 33). Fig. 33 proves that the collection
efficiency can generally be improved only by increasing the energy input.
The relation between cut diameter and energy demand is more clearly
demonstrated by Fig. 34. Further information are compiled in Table 2. All data
obtained from experiments with 22 scrubbers are arranged such that a lower
limiting curve can be drawn. This means that there is a minimum demand of
specific energy for each value of the cut diameter. Furthermore, Fig. 34 proves
that it is not always advisable to increase the energy input of a certain type of
scrubber in order to obtain a small cut diameter. This applies, for instance, to
146 Chapter 5: Design and Operation of Wet Dust Scrubbers

scrubbers 1, 4 and 5. In these cases, it is advisable to select another type of

scrubber. Of course, one should always prefer, if possible those scrubbers that
are arranged close to the lower limiting curve.
A rational selection of wet scrubbers should be based on defined properties of
the dust and the equipment. These properties are listed in Table 3.

6. List of Symbols
[m 2] Geometrical surface area of particles
em] Outer diameter of Raschig rings
em] Inner diameter of Raschig rings
em] Column diameter
em] Drop diameter
em] Dust particle diameter
em] Test dust particle diameter
em] Test dust particle diameter at test dust residue R t = 50 %
em] Test dust particle diameter at test dust fractional col-
lection efficiency ({JFt = 50 %
g [mjs2] Gravitational acceleration
h em] Height of packing
Mpl [kgjm 3 ] Dust concentration in raw gas
Mp2 [kgjm 3 ] Dust concentration in purified gas
Vc [m 3 ] Volume of column
yp [m 3 ] Volume of particles in packing
yg [m 3 js] Volumetric flow rate of gas
VI [m3/s] Volumetric flow rate ofliquid
Wg [m/s] Mean gas velocity
wg,o [m/s] Mean gas velocity under flooding conditions
WI [mjs] Mean liquid velocity
wr [m/s] Relative velocity
Ws [m/s] Settling velocity
w st [m/s] Settling velocity of test dust
Llpl [N/m2] Single-phase pressure drop in packed column
Llpv [Njm 2 ] Pressure drop in Venturi scrubber
1'/g [kgj(ms)] Dynamic viscosity of gas
1'/1 [kgj(ms)] Dynamic viscosity of liquid
Vg [m 2 js] Kinematic viscosity of gas
VI [m 2 /s] Kinematic viscosity of liquid
Pg [kgjm 3] Density of gas
PI [kgjm 3 ] Density of liquid
Pp [kgjm 3 ] Density of dust particle
Re r Reynolds' number (Eq. (3))
Reg, 0 Reynolds' number of gas at flooding conditions (Eq. (18))
wf Dimensionless mean liquid velocity (Eq. (12))
8 Porosity of packing (Eq. (19))
7. References 147

Porosity of Raschig ring (Eq. (23))

Frictional factor for venturi scrubber (Eq. (27))
Collection efficiency (Eq. (5))
Fractional collection efficiency
Fractional collection efficiency for test dust
Impingement factor (1)
Inertia parameter (Eq. (2))
Frictional factor (Eq. (17))

7. References
[IJ E. Weber, W. Brocke: Apparate und Verfahren der industriellen Gasreinigung.
Band 1: Feststoffabscheidung; R. Oldenbourg Verlag, Miinchen-Wien, 1973. - [2J
1.H. Eisner: Wasser Luft Betrieb 14, 233 (1970). - [3J Ceagfilter and Entstaubungs-
technik GmbH Report: Neue ProblemlOsungen fUr Luftreinigungstechnik und Gas-
reinigung, 1978. - [4J K. Holzer: Chem.-Ing.-Techn. 51, 200 (1979). - [5J K. Holzer:
Staub-Reinh. Luft 34, 360 (1974). - [6J VDI-Richtlinie 3679: NaBarbeitende Abschei-
der. - [7J G. Schuch, F. Laffler: Verfahrenstechnik 12, 302 (1978). - [8J G. Schuch, F.
Laffler: Chem.-Ing.-Techn. 51, 301 (1979). - [9J W. Reichelt: Stramungstechnische
Untersuchungen an mit Raschigringen gefUllten Fiillkarperrohren und -saulen, Disserta-
tion, Techn. Univers. Clausthal, 1970. - [10J H. Brauer: Grundlagen der Einphasen-
und Mehrphasenstramungen; Verlag Sauerlander, Aarau u. Frankfurt/M. 1971. -
[l1J F. Mayinger, M. Neumann: Chem.-Ing.-Techn. 49, 433 (1977). - [12J G. Dau:
Germ. Chern. Eng. 2, 61 (1979). - [13J H.E. Hesketh: 1. Air Poll. Control Assoc. 24, 939
(1974). - [14J M. Lemann: EinfluB von Kondensationsvorgangen in mit Wasserdampf
gesattigten Staub-Luftgemischen auf die Staubabscheidung in einem Venturiwascher;
Dissertation ETH-Ziirich, 1977. - [15J H. Giintheroth: Fortschrittber. VDI-Zeitschr.,
Reihe 3, Nr. 13, 1966. - [16J M. Wicke: Fortschrittber. VDI-Zeitschr., Reihe 3, Nr. 3,
1970. - [17J S. Calvert, D. Lundgren, D.S. Mehta: 1. Air Poll. Assoc. 22,529 (1972).