Part II Wastegas Engineering
9 Control of Primary Particles
9.1 Wall Collection Devices
The first three types of control devices we
consider--gravity settlers,cyclone separators,and
electrostatic precipitators--all function by driving
the particles to a solid wall,where they adhere to
each other to form agglomerates that can be
removed from the collection device and disposed
of,
9.1.1 Gravity Settlers
It is an old,unsophisticated device that must
be cleaned manually at regular intervals,
But it is simple to construct,requires little
maintenance,and has some use in industries
treating very dirty gases,e.g.,some smelters
and metallurgical processes,
Gravity settlers have little practical
industrial use because they are ineffective
for small particles,
At even modest velocities and common radii,
the centrifugal forces acting on particles can
be two orders of magnitude larger than the
gravity forces,For this reason centrifugal
particle separators are much more useful
than gravity settlers,
There are many other variants on the centrifugal
collector idea,but none approaches the cyclone in
breadth of application,These devices are simple
and almost maintenance-free,Because any
medium-sized welding shop can make one,the big
suppliers of pollution control equipment,who
have test data on the effects of small changes in
the internal geometry,have been unwilling to
make these data public,
When it is used to separate solids from liquids it is
generally called a hydroclone,
If gravity settlers and centrifugal separators
are devices that drive particles against a
solid wail,and if neither can function
effectively (at an industrial scale) for
particles below about 5 μm in diameter,
then for wall collection devices to work on
smaller particles,they must exert forces that
are more powerful than gravity or
centrifugal force,The electrostatic
precipitator (ESP) is like a gravity settler or
centrifugal separator,but electrostatic force
drives the particles to the wall,
The basic idea of all ESPs is to give the particles
an electrostatic charge and then put them in an
electrostatic field that drives them to a collecting
wall,This is an inherently two-step process,In one
type of ESP,called a two-stage precipitator,
charging and collecting are carried out in separate
parts of the ESR,This type,widely used in
building air conditioners,is sometimes called an
electronic air filter,However,for most industrial
applications the two separate steps are carried out
simultaneously in the same part of the ESP,
Figure 9.2 shows in simplified form a wire-and-
plate ESP with two plates,The gas passes between
the plates,which are electrically grounded (i.e.,
voltage = 0),Between the plates are rows of wires,
held at a voltage of typically -40 000 volts,The
power is obtained by transforming ordinary
alternating current to a high voltage and then
rectifying it through some kind of solid-state
rectifier,
On the plates the particles lose their charge and
adhere to each other and the plate,forming a
"cake."
Solid cakes are removed by rapping the
plates at regular time intervals with a
mechanical or electromagnetic rapper
that strikes a vertical or horizontal
blow on the edge of the plate,
Some of the cake is always re-entrained,
thereby lowering the wastegas flowrate
whereas enhancing the removal
efficiency of the system,
If the collected particles are liquid,
e.g.,sulfuric acid mist,they run down
the plate and drip off,
9.2 DIVIDING COLLECTION DEVICES
Filters and scrubbers divide the flow into
smaller parts where they can collect the
particles,
Two types of filters,surface filters and
depth filters,are commonly used in air
pollution control,
9.2.1 Surface Filters
The filter is a membrane (sheet steel,cloth,
wire mesh,or filter paper) with holes
smaller than the dimensions of the particles
to be retained,
Although industrial air filters rarely have holes
smaller than the smallest particles captured,they
often act as if they did,The reason is that,as fine
particles are caught on the sides of the holes of a
filter,they tend to bridge over the holes and make
them smaller,Thus as the amount of collected
particles increases,the cake of collected material
becomes the filter,and the filter medium (usually
a cloth) that originally served as a filter to collect
the cake now serves only to support the cake,and
no longer as a filter,
This cake of collected particles will have
average pore sizes smaller than the diameter
of the particles in the oncoming gas stream,
and thus will act as a sieve for them,The
particles collect on the front surface of the
growing cake,For that reason this is called
a surface filter,
One may visualize this situation with a
screen having holes 0.75 in,(1,91 cm) in
diameter,We could collect a layer of Ping-
Pong balls easily on this screen,Once we
had such a layer,we could then collect
cherries,which,by themselves,could pass
through the holes in the screen but cannot
pass through the spaces between the Ping-
Pong balls,Once we have a layer of cherries,
we could put on a layer of peas,then of rice,
then of sand,
The two most widely used designs of
industrial surface filters are shown in Figs,
9.13 and 9.14,Because the enclosing sheet
metal structure in both figures is normally
the size and roughly the shape of a house,
this type of gas filter is generally called a
baghouse,The design in Fig,9.13,most
often called a shake-deflate filter,consists
of a large number of cylindrical cloth bags
that are closed at the top like a giant
stocking,toe upward,
? Fig,9-4 Typical industrial baghouse of the shake-deflate design
These are hung from a support,Their lower ends
slip over and are clamped onto cylindrical sleeves
that project upward from a plate at the bottom,
The dirty gas flows into the space below this plate
and up inside the bags,The gas flows outward
through the bags,leaving its solids behind,The
clean gas then flows into the space outside the
bags and is ducted to the exhaust stack or to some
further processing,
For the baghouse in Fig,9.13 there must be
some way of removing the cake of particles
that accumulates on the filters,Normally
this is not done during gas-cleaning
operations,instead the baghouse is taken
out of the gas stream for cleaning,When the
gas flow has been switched off,the bags are
shaken by the support to loosen the
collected cake,
A weak flow of gas in the reverse direction
may also be added to help dislodge the cake,
thus deflating the bags,
Often metal rings are sewn into filter bags at
regular intervals so that the bag will only
partly collapse when the flow is reversed,
Typically,for a major continuous source
like a power plant,about five baghouses
will be used in parallel,with four operating
as gas cleaners during the time that the
other one is being shaken and cleaned,Each
baghouse might operate for two hours and
then be cleaned for 10 minutes; at all times
one baghouse would be out of service for
cleaning or waiting to be put back into
service,
Fig,9-14 Typical industrial baghouse of
the pulse-jet design
The other widely used baghouse design,
called a pulse-jet filter,is shown in Fig,
9.14,In it the flow during filtration is
inward through the bags,which are similar
to the bags in Fig,9.13 except their ends
open at the top,The bags are supported by
internal wire cages to prevent their collapse,
Just after the cleaning the control efficiency
will be less than just before the next
cleaning,but the average efficiency meets
the legal control requirements,
9,2.2 Depth Filters
Another class of filters,widely used for air
pollution control,does not form a coherent
cake on the surface,but instead collects
particles throughout the entire filter body,
The examples are the filters on filter-tipped
cigarettes and the lint filters on many home
furnaces,
Such filters are often used where the
particles to be caught are fine drops of
liquids that are only moderately viscous,
The most widespread air pollution control
use of depth filters is in the collection of
very fine liquid drops,sulfuric acid mist,
produced in sulfuric acid plants,
One brand uses the trade name Demister,
This kind of device is also used for cleaning
the air of industrial clean rooms or hospital
surgical suites and in personal protection
dust masks,The filters are thrown away
when they have collected enough particles
that their pressure drop begins to increase,
9.2.4 Scrubbers for Particulate Control
Scrubbers effectively divide the flow of
particle-laden gas by sending many small
drops through it,
A complete scrubber has several parts,as
sketched in Fig,9.20,
? Fig,Component parts of a scrubber installation
If possible,the engineer should try to save
money by finding a place where the
contaminated water stream can be recycled
inside the plant without first removing the
solids,
Obviously,if there is no good way to deal
with the contaminated water stream,then
the scrubber has merely changed an air
pollution problem into a water pollution
problem,
9.3 Choosing the Collectors
In choosing a primary particle collection
device one must consider the size of the
particles to be collected,the required
collection efficiency,the size of the gas
flow,the allowed time between cleanings,
and details of the nature of the particles,
The following rules of thumb may be
helpful,
1,Small or occasional flows can be treated by
throwaway devices,e.g.,cigarette and
motor oil filters,in which the collected
particles remain in the device,Large and
steady flows require collection devices that
operate continuously or semicontinously,
and from which the collected particles can
be removed continuously or
semicontinuously,
2,Sticky particles (e.g.,tars) must be collected either
on throwaway devices or into a liquid,as in a
scrubber or cyclone,filter,or wet ESP whose
collecting surfaces are continually coated with a
film of flowing liquid,
3,Particles that adhere well to each other but not to
solid surfaces are easy to collect,Those that do the
reverse often need special surfaces,e.g.,Teflon-
coated fibers in filters that release collected
particles well during cleaning,
4,Electrical properties of the particles are of
paramount importance in ESPs,and they are often
significant in other control devices where friction-
induced electro-static charges on the particles can
aid or hinder collection,
5,For nonsticky particles larger than about 5 μm,a
cyclone separator is probably the only device to
consider,
6,For particles much smaller than 5 μm one
normally considers ESPs,filters,and scrubbers,
Each of these can collect particles as small as a
fraction of a micron,
7,For large flows the pumping cost makes scrubbers
very expensive; other devices are chosen if
possible,
8,Corrosion resistance and acid dew point must
always be considered,
9.4 SUMMARY
1,Gravity settling chambers,cyclones,and
ESPs work by driving the particles to a
solid wall where they form agglomerates
that can be collected,These three devices
have similar design equations,
2,Filters and scrubbers divide the flow,They
have different design equations from
wall collection devices and from each other,
3,Both surface and depth filters are used for
particle collection,Surface filters are used
to collect most of the particles in a heavily
laden gas stream,Depth filters are mostly
used for the final cleanup of air or gas that
must be very clean or for fine liquid drops,
which coalesce on them and then drop off,
4,To collect small particles,a scrubber must
have a very large relative velocity between
the gas being cleaned and the liquid drops,
For this reason co-flow scrubbers are most
often used,The venturi scrubber is the most
widely used type of co-flow scrubber,
10 Control of Volatile Organic Compounds
10.1 Introduction
Volatile organic compounds (VOCs) are liquids or
solids that contain organic carbon (carbon bonded
to carbon,hydrogen,nitrogen,or sulfur,but not
carbonate carbon as in CaCO3 nor carbide carbon
as in CaC2 or CO or CO2),which vaporize at
significant rates,VOCs are probably the second-
most widespread and diverse class of emissions
after particulates,
VOCs are a large family of compounds
some (e.g.,benzene) are toxic and
carcinogenic,The principal concern with
VOCs is that they participate in the "smog"
reaction and also in the formation of
secondary particles in the atmosphere,
These latter are mostly in the fine particle
size range,Some VOCs are powerful
infrared absorbers and thus contribute to the
problem of global warning,
10.2 VOCs
VOCs are those organic liquids or solids whose
room temperature vapor pressures are greater than
about 0.01 psia (0.0007 atm) and whose
atmospheric boiling points are up to about 500。 F
(260。 C),which means most of these organic
compounds with less than about 12 carbon atoms,
A lighted cigarette produces a gaseous mixture of
high-boiling organic compounds; when this
mixture is cooled on leaving the cigarette it forms
a smoke of fine particulate droplets,
In common usage it would often be grouped
with the hydrocarbons,
Hydrocarbons are only slightly soluble in
water,so we can normally separate liquid
HCs from liquid water by simple phase
separation and decantation,However,the
water left behind often contains enough
dissolved hydrocarbon that it cannot be
discharged to the sewer or natural body of
water without additional treatment,
Polar VOCs,which almost all contain an
oxygen or nitrogen atom in addition to
carbons and hydrogens (alcohols,ethers,
aldehydes and ketones,carboxylic acids,
esters,amines,nitriles) are much more
soluble in water,This difference in
solubilities makes the polar VOCs easier to
remove from a gas stream by scrubbing
with water,but harder to remove from water
once they dissolve in it,
10.3 CONTROL BY CONCENTRATION AND
RECOVERY
10.3.1 Adsorption
Adsorption means the attachment of molecules to
the surface of a solid,In contrast,absorption
means the dissolution of molecules within a
collecting medium,which may be liquid or solid,
Generally,absorbed materials are dissolved into
the absorbent,like sugar dissolved in water,
whereas adsorbed materials are attached onto the
surface of a material,like dust on a wall,
Absorption mostly occurs into liquids,adsorption
mostly onto solids,
Industrial face masks of activated carbon
are worn by workers exposed to solvents,as
in paint spraying or solvent cleaning,The
worker's lungs suck the air in through thin
beds of activated carbon,contained in
replaceable cartridges on the face mask,
When the activated carbon is loaded (i.e.,
the solvent begins to come through into the
worker's breathing space) the cartridge of
activated carbon is discarded and a fresh
one installed,
10.3.2 Absorption (Scrubbing)
If we can find a liquid solvent in which the VOC
is soluble and in which the remainder of the
contaminated gas stream is insoluble,then we can
use absorption to remove and concentrate the
VOC for recovery and re-use,or destruction,The
standard chemical engineering method of
removing any component from a gas stream--
absorption and stripping--is sketched in Fig,10.15,
If we can find a liquid solvent in which the
gaseous component we wish to selectively remove
is much more soluble than are the other
components in the gas stream,the procedure is
quite straightforward,
The feed gas enters the absorber,which is a
vertical column in which the gas passes upward
and the liquid solvent passes downward,Normally,
bubble caps,sieve trays,or packing is used in the
interior of the column to promote good
countercurrent contact between the solvent and the
gas,The stripped solvent enters the top of the
column and flows countercurrent to the gas,
The loaded solvent,which now contains most of
the component we are removing from the gas,
passes to the stripper,which normally is operated
at a higher temperature and/or a lower pressure
than the absorber,
? Fig,Three plausible arrangements for scrubbing a
gas with a liquid,(a)bubbler;(b)spray
chamber;(c)packed
In Fig,10.15 the separated component is
shown leaving as a gas for use,sale,or
destruction,In some cases it is condensed
and leaves as a liquid,The stripped or lean
solvent is sent back to the absorber column,
Very large absorption-stripping systems
often use tray columns,but the small ones
used in most air pollution control
applications use internal packings,
1,It must afford reasonable solubility for the material to be
removed,and,if this material is to be recovered at
reasonable purity,it must not dissolve and thus carry along
any of the other components of the gas stream,
2,In the absorber,the gas being treated will come to
equilibrium with the stripped solvent,The vapor pressure
of the solvent,at absorber temperature,must be low
enough that if the cleaned gas is to be discharged to the
atmosphere,the emission of solvent is small enough to be
permissible,Some solvent is lost this way; the cost of
replacing it must be acceptable,If the solvent is water this
is not a problem (unless we need the gas to be dry for its
next use),but for other solvents this can be a problem,
3,At the higher temperature (or lower pressure) of the
stripping column,the absorbed material must come out of
solution easily,and the vapor pressure of the solvent must
be low enough that it does not contaminate the recovered
VOC,If the solvent vapor pressure in the stripper is too
large,one may replace the stripper by a standard
distillation column (combination stripper and rectifier) to
recover the transferred material at adequate purity,
4,The solvent must be stable at the conditions in the
absorber and stripper,and be usable for a considerable
time before replacement,
5,The solvent molecular weight should be as low as possible,
to maximize its ability to absorb,This requirement
conflicts with the low solvent vapor pressure requirement,
so that a compromise must be made,
10.4 Biological Oxidation (Biofiltration)
As discussed above,the ultimate fate of
VOCs is to be oxidized to CO2 and H20,
either in our engines or furnaces,or
incinerators,or in the environment,
Microorganisms can also oxidize the VOCs
contained in gas or air streams,
The typical biofilter consists of the
equivalent of a swimming pool,with a set
of gas distributor pipes at the bottom,
covered with several feet of soil or compost
or loam or other carriers in which the
microorganisms live,The contaminated gas
enters through the distributor pipes and
flows slowly up through soil,allowing time
for the VOC to dissolve in the water
contained in the soil,and then to be
oxidized by the microorganisms that live
there,
Typically these devices have soil depths of 3 to 4 ft,
void volumes of 50%,upward gas velocities of
0.005 to 0.5 ft/s,and gas residence times of 15 to
60 s,They work much better with polar VOCs,
which are fairly soluble in water (see Sec,10.2)
than with HCs whose solubility is much less,The
microorganisms must be kept moist,protected
from conditions that could injure them,and in
some cases given nutrients,Because of the long
time the gases must spend in them,these devices
are much larger
In spite of these drawbacks,there are some
applications for which they are economical,and
for which they are used industrially,
10.5 SUMMARY
1,VOCs are emitted from a wide variety of sources and have
a wide variety of individual components,each with its own
properties,We use VOCs mostly as petroleum-based fuels
and solvents,The majority of our VOC emissions come
from fuel and solvent usage,transportation,and storage,
2,The control alternatives are prevention,concentration and
recovery,or oxidation,
3,Some of these control options can also be used for non-
VOC emissions,e.g.,incineration for odor control of H2S,
adsorption for SO2 or mercury vapor,and leakage control
for any process source,
11 Control of Sulfur Oxides
The control of particulates and VOCs is
mostly accomplished by physical processes
(cyclones,ESPs,filters,leakage control,
vapor capture,condensation),
Some particles and VOCs are chemically
changed into harmless materials by
combustion,
Control of sulfur oxides and nitrogen
oxides is largely chemical rather than
physical,
Sulfur and nitrogen oxides are ubiquitous
pollutants,which have many sources,SO2,
SO3,and NO2 are strong respiratory irritants
that can cause health damage at high
concentrations,
These gases also form secondary particles in
the atmosphere,contributing to our PM10
and PM2.5 problems and impairing visibility,
They are the principal causes of acid rain,
11.1 The Elementary Oxidation-Reduction
Chemistry Of Sulfur And Nitrogen
Both sulfur and nitrogen in the elemental
state are relatively inert and harmless to
humans,Both are needed for life; all
animals require some N and S in their
bodies,However,the oxides of sulfur and
nitrogen are widely recognized air
pollutants,The reduced products also are,in
some cases,air pollutants,
Both hydrogen sulfide and ammonia are
very strong-smelling substances,gaseous at
room temperature (-60。 C and -33。 C
boiling points,respectively),and toxic in
high concentrations,(High concentrations
due to accidental releases often cause
fatalities,These occur in the production and
use of ammonia as a fertilizer and
refrigerant and in the production and
processing of "sour" gas and oil,which
contain hydrogen sulfide.),
In the atmosphere NO2 and SO3 react with water
to form nitric and sulfuric acids,which then react
with ammonia or any other available cation to
form particles of ammonium nitrate or sulfate or
some other nitrate or sulfate,These particles,
generally in the 0.1 to 1-μm size range,are very
efficient light-scatterers,They are significant
contributors to urban PMl0 and PM2.5 problems,
They are the principal causes of acid deposition
and of visibility impairment,NO and NO2 also
play a significant role in the formation of O3,
11.1 Dry systems
The solids handling and wet sludge
handling and disposal difficulties that are
integral to wet throwaway processes
induced engineers to develop dry
throwaway processes that would have fewer
corrosion and scaling difficulties and would
produce a waste product much easier to
handle and dispose of,All of these systems
inject dry alkaline particles into the gas
stream,where they react with the gas to
remove SO2,
The SO2-containing particles are then captured in
the particle collection device that the plant must
have to collect fly ash (most often a baghouse,
sometimes an ESP),If successful,this approach
eliminates the problems with disposal of wet
scrubber sludge and all the difficulties involved
with the wet limestone process,It increases the
volume of dry solids to be disposed of,but that is
considered a less difficult problem,The flow
diagrams for such systems are sketched in Fig,
11.7,
The first two call for the injection of powdered limestone or
lime into the boiler,In the high-temperature part of the
furnace the limestone would convert to lime,so that either
way the active reagent would be CaO,The desired reaction
is
CaO + SO2 ---> CaSO3
CaSO3 would then oxidize to CaSO4,In principle this
should work,but most tests have shown that to get high
SO2 collection efficiencies one must put a large excess of
lime or limestone into the system,thus increasing reagent
costs,increasing the load on the particle collector,and
increasing the volume of solid wastes to be disposed of,
However,if one uses more reactive (and much more
expensive) NaHCO3 or Na2CO3,the collection efficiency
is much better,mostly because of the much higher
chemical reactivity of these sodium salts,
Mass transfer between gases and solids is
much less well understood than that
between gases and liquids,so that the
design of these devices is much more
heavily dependent on test and empiricism
than is the design of systems like that in Fig,
10.15,
11.2 Regenerative systems
Table 11.4 shows an entirely different category of
systems,In these some kind of absorbent or
adsorbent is used to capture SO2 from the flue gas,
Then in some separate device or set of devices the
adsorbent or absorbent is regenerated to produce a
flow of relatively pure SO2 or H2SO4,
Recently,work has begun on regenerative
processes that will simultaneously capture both
SO2 and NOx,These systems have not yet
advanced to commercial scale,but they may have
a major role in future air pollution control,
11.3 SUMMARY
1,SO2 emissions from human activities are
mostly due to the combustion of sulfur-
containing fossil fuels and the smelting of
metal sulfide ores,
2,The overall control strategy for SO2
emissions is to convert the sulfur to
CaSO4.2H2O
and return it to the ground in some kind of
landfill,or use it to make wallboard,
3,For liquid or gaseous fuels containing reduced sulfur,the
most common approach
is to use catalytic processes to convert the contained sulfur
to H2S,remove that by
scrubbing the gas with a weakly alkaline solution,convert
the H2S to elemental
sulfur by the Claus process,and either sell that sulfur for
sulfuric acid production or place it in a landfill,
4,For metal sulfide ore smelting,which produces waste gases
with 4 percent or more SO2,the common approach is to
convert that SO2 to sulfuric acid,
5,For coal (or high-sulfur oil) used in a large power
plant,the most common approach is to burn the
coal and then treat the plant's exhaust gas
(typically containing about 0.1 percent SO2) with
limestone or lime in a forced-oxidation wet
scrubber or a spray dryer,to convert SO2 to
CaSO4, 2H20,which will then go to a landfill or a
wallboard plant,
6,Other alternatives are being explored,some in
large-scale demonstrations,They may replace
those just listed in the future,
