Adsorption part 1

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Adsorption(part 1)
Instructor Prof. Moo Been Chang Date 2008/10/08
Graduate Institute of Environmental
EngineeringNational Central University
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Outline

• What is the adsorption?
• Adsorption Isotherms
• Adsorbent Material
• The Application of Activated Carbon
• -- VOCs control
• -- Dioxins (PCDD/Fs) control

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What is the adsorption?
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Adsorption
Definition the gases, liquids or dissolved
substances (adsorbate) on the surface of solids
(adsorbent)

• 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, adsorbed materials are attached onto
  the
• surface of a material, like dust on a wall.
  Absorption
• mostly occurs into liquids, adsorption mostly
  onto
• solids.

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Adsorption Requirements

• adsorbent must have large surface areas,
• adsorbent must have internal micropores and
  macropores (eg., activated carbon zeolite),
• selective adsorption (moisture must be avoided),
• need good contact time for successful separation,
• pre-treatment to lower gas composition required,
• no mal-distribution of flow in the bed,
• easy regeneration of bed should be possible,
• continuous operation requires multiple beds in
  tandem.

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Category of Adsorption

• Physisorption
• (Physical or van der Waals adsorptions)
• weak bonding of gas molecules to the solid
• exothermic ( 0.1 kcal/mole)
• no physical or chemical changes
• reversible
• multilayer adsorption
• does not accompany catalysis

• Chemisorption
• chemical bonding by reaction
• exothermic (gt 10 kcal/mole)
• adsorbent changes characteristics
• irreversible
• monolayer in most case
• definitely catalyzed

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Adsorption Mechanism
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Temperature Effect on Adsorption
Q Why adsorption capacity decreases when gas
temperature is increased?
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Thermodynamic Analysis
According to 2nd law of thermodynamic ?G ?H -
T?S ?G Gibbs free energy ?H enthaplpy ?S
entropy
Since the adsorption is a spontaneous reaction,
so we can say that the entropy and Gibbs free
energy are below 0.(?Slt0 ) (?G lt0 ) .
So the enthaplpy (?H) must be negative. (?Hlt0)
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Thermodynamic Analysis
The adsorption is an exothermic reaction.
The higher temperature the lower
efficiency of adsorption
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Adsorption Isotherms
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Adsorption Isotherms
Data relating adsorbed concentration (g/g of bed
weight) to equilibrium gas phase concentration
(g/ml of stream) is given in terms of adsorption
isotherms.
Wads f (P,T)

• Three common types of isotherms
• Langmuir
• Freundlich
• BET

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Favorable and Unfavorable Adsorption
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Langmuir Isotherm
The earliest model of gas adsorption suggested by
Langmuir (1916). The classical Langmuir model is
limited to monolayer adsorption. It is assumed
that gas molecules striking the surface have a
given probability of adsorption. Molecules
already adsorbed similarly have a given
probability of desorption. At equilibrium, equal
numbers of molecules desorb and adsorb at any
time. The probabilities are related to the
strength of the interaction between the adsorbent
surface and the adsorbate gas.
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Langmuir Isotherm (contd)
Rate of adsorption,
Rate of desorption,
At equilibrium,
where, Wads the mass of gas adsorbed at
pressure P Wmax the mass of gas which covers
the entire adsorbing surface with a monolayer P
the partial pressure of interest in the gas
phase ? coverage C a constant for the
gas/solid combination ka/kd ka the
adsorption rate coefficient kd the desorption
rate coefficient.
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Langmuir Isotherm (contd)
Some physisorption and most chemisoption
processes follow this isotherm. It is the one
with the best theoretical basis, which assumes
that adsorption is limited to one monolayer on
the surface.
One can obtain the two constants by linearization
of the isotherm
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Langmuir Isotherm (contd)
It is particularly suited to represent binary and
ternary systems.
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Freundlich Isotherm
The Fruendlich isotherm model is valid for
heterogeneous surfaces, monolayer coverage.
Common for most adsorption work since it fits
almost all data. It is empirical in nature,
although some theoretical foundations do exit.
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Freundlich Isotherm
The expression Wads KF P 1/n
(KF and n are experimentally determined
parameters)

• When n 1, the reaction is linear and called
  partitioning.
• When n gt 1, the reaction is said to be
  favorable as the incremental change in amount
  sorbed decreases with increasing concentrations.
• While n lt 1 is called unfavorable because the
  reverse is true.
• Most natural adsorbents exhibit either linear or
  favorable adsorption.
• The Langmuir and Fruendlich models for n lt 1 are
  concave downwards, so both models can be
  calibrated to similar data..

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Freundlich Isotherm (contd)
lnWads lnKF 1/n lnP
Wads KF P 1/n
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Freundlich Isotherm Parameters
Available for a wide variety of organic vapors on
various activated carbon types
Wads KF P 1/n
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Brunauer-Emmett-Teller (BET) Isotherm

• Brunauer, Emmett and Teller (BET) developed
  several models for gas adsorption on solids which
  have become the effective standard for surface
  area measurements.
• BET isotherm is valid for multiple layers on
  homogeneous surfaces.

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Brunauer-Emmett-Teller (BET) Isotherm

• The assumptions underlying the simplest BET
  isotherm are
• Gas adsorbs on a flat, uniform surface of the
  solid with a uniform heat of adsorption due to
  van der Waals forces between the gas and the
  solid.
• There is no lateral interaction between the
  adsorbed molecules.
• After the surface has become partially covered by
  adsorbed gas molecules, additional gas can adsorb
  either on the remaining free surface or on top of
  the already adsorbed layer. The adsorption of
  the second and subsequent layers occurs with a
  heat of adsorption equal to the heat of
  liquefaction of the gas.

multi-layers adsorption
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BET Isotherm (contd)
Work for almost any type of data on the
adsorption of gases on solids. It describes
every type of isotherm including the linear, and
Langmuir isotherms. The theoretical basis is
sound.
For single component the equation is,
for n ? ?
for finite n
Note that n is the number of adsorbed monolayers,
and x P/P0. Where, P is the actual partial
pressure of gas in the stream and P0 is the vapor
pressure of the pure gas.
Note The BET simplifies to the Langmuir when
relative pressure x lt 0.01 and C gt100 (Valsaraj
et al., 1992).
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BET Isotherm (contd)
To obtain the parameters in the BET equation, one
needs to linearize the equation
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Empirical Equations for Adsorption
(1). Correlation using a logarithmic series
expansion such as
Note that a, b and c are constants specific to a
typical compound.
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The most common isotherm models Dastgheib and
Rockstraw, 2002
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Adsorbent Material
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Adsorbent Material

• Silica Gel
• Molecular Sieves (zeolite)

• Activated Carbon
• Activated Alumina

Polar and Non-polar adsorbents
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H2O
H2O
H2O
O
OH
OH
OH
OH
heating
hydrophobic
hydrophilic
Physical property might be changed as overheated.
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Physical Properties of Adsorbents
Source Cooper and Alley (2002)
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Activated carbon from various sources
Source Cooper and Alley (2002)
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Adsorbent Material

• Activated Carbon

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Adsorbent Material

• Activated Carbon
• The most common adsorbent which apply to
  various works and utilizes to deodorization,
  decolor ,remove various toxic substances and so
  on.
• As the research show that about 280,000 tons
  activated carbons are consumed each year in the
  world.

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Adsorbent Material

• Activated carbons have unique porous structures,
  large
• specific surface area and porosity, and
  various surface
• functional groups.

• These physical and chemical properties make
  activated
• carbons the most commonly employed adsorbents
  for
• removal of VOCs from gaseous and liquid
  phases.

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Adsorbent Material
The adsorption capacities and kinetics of
activated carbons depends on their surface
microstructure, including (a)specific surface
area, (b)pore volume, pore size distribution and
(c)various surface function groups
(a) Specific Surface Area
Generally speaking, the higher surface area can
have higher adsorption capacity.
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Adsorbent Material
(b) Porosity
The activated carbon have higher porous
structure. Some researches indicate the
surfaces of the pores of 1 g AC is equal 8
tennis courts.
According to the IUPAC(International Union of
Pure and Applied Chemistry, 1972) define the
diameter of the pores.
(1)macropore diameter lt2nm
(2)mesoropore diameter 250 nm
(3)micropore diameter gt50 nm
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Adsorbent Material

• According to the research indicates that the
  diameter of air pollutants
• are on the range of 0.40.85 nm in general,
  so the proportion of the
• micropores are more important for those .
  Stenzel (1993)

• The dioxin compounds are larger than those
  contaminants.
• The diameter of dioxin is about 0.351.37nm.

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Adsorbent Material

• (3) Surface Functional Groups.

• Generally speaking, activated carbons are
  non-polar
• adsorbents which have higher affinity to
  non-polar organic matters.

• The surface functional groups can affect the
  characteristics of
• adsorption, especially oxygen groups.

The oxygen groups polar

• Most oxygen groups can react with H2O molecular
  and reduce
• the adsorption capacities. (When H2O molecules
  exist. )

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Adsorbent Material
The figure of oxygen groups
The research indicated that the oxygen groups
could hinder the adsorption of the non-polar
organic maters (i.e CCl4 ) . Ishizaki (1988)
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Pore Structure of Activated Carbon
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Pore Structure (contd)
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Indicator
(1) Molasses Number
Decolorizing Index
(2)Methylene Blue Number
The adsorption indicator of aryl organic matters
(3)Phenol Number
Because of phenol molecules have higher
solubility and often exist in the environmental
pollution. Thus phenol number is an important
indicator for adsorption ability.
(4)Alky Benzene Sulphonate,ABS
The adsorption indicator of large molecular
(5) Iodine Number
Iodine number B.E.T surface area
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Adsorbent Material
According to the difference forms, we can
separate five styles
(1) Powder Activated Carbon( PAC)
PACs have large external surface area and short
diffusion path. The velocity of adsorption are
most fast.
(2) Granular Activated Carbon( GAC)
The surface area of GACs are smaller than PACs,
but the GACs have many advantages such as to fill
easily, .to regenerate easily, have lower
pressure drop and so on.
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Adsorbent Material
(3)Spherical or Cylindrical Activated Carbon)

• Spherical or Cylindrical Activated Carbon
  usually have
• higher mechanistic intensity.

(4) Activated Carbon Fiber, (ACF)

• ACFs have higher surface area than PACs and have
  lower pressure
• drop than GACs .But ACFs also have higher
  price than others.

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Adsorbent Material
(5) Impregnated Activated Carbon (IAC)

• To put activated carbon into specific chemical
  solution and
• make these chemical substances to fix on the
  surface of
• activated carbon.

• Activated carbon also can coating specific metals
  as a catalyst.

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The influence of activated carbon adsorption
The characteristic of adsorbent

• Specific surface area
• Surface Functional Groups
• Porosity

The characteristic of adsorbate

• The molecular size of adsorbate
• The polar of adsorbate
• The concentration of adsorbate

The factor of environment

• Temperature
• Moisture

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The influence of Temperature
The adsorption of naphthol in different
temperature
The adsorption is an exothermic reaction.
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The influence of humidity

• Water vapor is one of the major impurities in
  atmospheric air and it is, in many
• cases, present in most air pollution control
  problems. The fact that the effluent
• gas streams to be treated in adsorption
  processes present virtually always
• water vapor may result in a very inefficient
  performance of the activated
• carbon in pollutants removal.

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The influence of humidity
Isotherm for toluene trichloroethylene and
water vapor (individual)
Amount of trichloroethylene adsorbed as a
function of relative humidity
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A way to reduce water
For instance,a hazardous waste incinerator in
Turkey. (Environ. Sci. Technol. 2004,38,
1201-1207)
Description of Activated carbon unit.
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A way to reduce water

• Flue gases leaving the wet scrubbers at 60-65 C
  are generally
• supersaturated with water vapor (about 22-24
  (v/v) at normal
• conditions, corresponding to relative
  humidities (RH) between
• 110 and 130).

• Therefore, a condensation process is required
  before the AC.

• High humidity can negatively affect the
  adsorption of a carbon
• bed by filling up the pores in the carbon
  particles with condensed
• water.

• Condensation occurs in the condensation chamber,
  which is
• combined with the AC unit, by a sudden
  decrease in the velocity of
• the flue gas and contact with the cold
  metallic surfaces.

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A way to reduce water
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The Application of Activated Carbon
Volatile organic compounds (VOCs) control
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VOCs Control

• Volatile organic compounds (VOCs) are one of the
  most common
• pollutants emitted by the chemical process
  industries, which
• include most solvents such as thinner,
  degreasers, cleaners,
• lubricants, and liquid fuels.

• VOCs are present in many types of waste gases and
  are often
• removed by adsorption and activated carbon
  (AC) is commonly
• used as adsorbents of gases and vapors because
  of their
• developed surface area and large pore volumes

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VOCs Control

• Several techniques for VOCs have been
  investigated such
• as thermal incineration, catalytic oxidation,
  condensation,
• absorption, bio-filtration, adsorption, and
  membrane separation

• The main APCD giving rise to recovery and
  recycling of
• VOC are the following

• phase transfer technologies adsorption and
  absorption.
• VOC concentration technologies condensation,
  cryocondensation
• 
  
• 
  and membrane processes.

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VOCs Control

• combustion processes incineration and catalytic
  oxidation.

• chemical or photochemical oxidation technologies.
• biotechnologies biotrickling filter,
  bioscrubber, and biofilter.

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VOCs Control
Table 1 presents some characteristics of the main
APCD, as applied to the VOC elimination
(Hurashima and Chang, 2000 Degreve et al.,
2001Wang et al., 2001)
Figure 1 Application limits (flow rateVOC
concentration) of different APCD, based on
references of Crocker and Schnelle, 1998Juteau,
1997, and Devinny et al., 1998.
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Figure 1. Application limits
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The Application of Activated Carbon
PCDD/Fs control
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PCDD (Polychlorinated Dibenzo-p-dioxins) and PCDF
(Polychlorinated Dibenzofurans)
Dioxins Control
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Inrodution

• PCDD and PCDF are commonly known as dioxin which
  has been listed as one of the persistent organic
  pollutants (POPs).
• The PCDD/Fs originate mainly from waste
  incineration processes including municipal waste
  incinerators (MWIs), and industrial waste
  incinerators (IWIs) and medical waste
  incinerators.
• In addition to the waste incineration process,
  major anthropogenic sources for PCDD/Fs emission
  include industrial process such as chemical
  manufacturing and metal smelting processes
  including electric arc furnaces (EAFs) and sinter
  plants.

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(TEFs) for PCDD/F and PCB congeners
4Cl
5Cl
2,3,7,8 TeCDD
6Cl
7Cl
PCB-126
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The hurt for humanFor instance, the present in
UkraineVictor Yushchenko

• Blood and tissue registered concentrations of
  dioxin 1,000 times above normal levels

Before (Feb, 2004)
Present (2005)
2,3,7,8-TCDD causes effects on the skin
(chloracne) and may cause cancer in people.
"We had not seen anything like that for the past
100 years, I believe it would be appropriate to
compare this to the fall of the Soviet Union or
the fall of the Berlin wall.
Dr. Michael Zimpfer, the head doctor of the
Rudolfinerhaus clinic, reveals Yushchenko's blood
test results to the international media.
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The mechanism of dioxins and furans formation

• Formation in incinerator/furnace
• Each of the factors is presented inTable 1
• Formation of PCDD/Fs from precursor compounds
• Cl and a phenolic precursor, which combine to
  form a chlorinated precursor, followed by
  oxidation fo chlorinated precursors (catalyzed by
  a copper catalyst such as CuCl2 ).
• (a) 2HCl1/2 O2 H2O
  Cl2
• (b) phenol Cl2
  chlorophenol (dioxin precursor)
• (c) 2-chlorophenol1/2O2
  dioxin Cl2
• De novo synthesis
• De novo synthesis promotes the formation of
  PCDD/Fs in the combustive oxidation of carbon
  particulates catalyzed by a transition metal in
  the presence of chlorine.

CuCl2
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The factors affecting PCDD/F formation during
combustion process (Mackay, 2002)
Table 1
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The Regulations

• The emission abatement of toxic chemicals, such
  as
• polychlorinated dioxins and furans (PCDD/F)
  as well as
• polychlorinated benzenes and phenols,
  polycyclic aromatic
• hydrocarbons (PAHs), and some heavy metals,
  is growing in
• importance because of general environmental
  and health
• concern and is reflected in more stringent
  emission
• standards for these components.

• PCDD/F emission standards in Taiwan summarized in
  Table2.

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PCDD/F emission standards in Taiwan
Table2
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Dioxins (PCDD/Fs) control
Effective PCDD/Fs control methods
Applying activated carbon to adsorb PCDD/Fs

• As an end-of-pipe technique, the removal of
  PCDD/Fs in flue gases is
• necessary to reduce the emissions of PCDD/Fs to
  environment .
• A number of equipments have been tested such as
  ESP, scrubber, bag filter,
• adsorbent injection, and combination of these
  under different operating
• conditions.
• The combination of a scrubber, a bag filter
  coupled with activated carbon
• injection has been found to be a most
  effective technique for PCDD/F
• emission control.

76
Techniques available for PCDD/Fs emission control
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Two different methods of contacting gas with
carbon adsorbent have been used.

• Carbon injection process

• Fixed-Bed Activated Carbon Filter

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Dioxins (PCDD/Fs) control

• Carbon injection process

Principal
Carbon injection is a process that involves the
injection of powdered activated carbon or a
mixture of dry powdered lime and carbon into a
combustion gas somewhere in the air pollution
control train.
The carbon is collected in a dry particulate
control device such as fabric filter.
Along with particulate from the combustion
process, the carbon forms a cake on the fabric
filter bags that gives additional PCDD/Fs
removal, acting as a carbon bed, until the carbon
and particulate cake is removed from the filter
bag surface.
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Dioxins (PCDD/Fs) control
According to the method of activated carbon
addition, the entrained-flow process, activated
carbon is injected before the bag filter and
carried by flue gas to the filter where it builds
up a carbon layer which removes PCDD/F from the
flowing gas.
Thus, a AC adsorber integrated with a fabric
filter has the potential to replace conventional
APCDs.
Whereas PCDD/F abatement in MSWIs is mostly
achieved through entrained-phase adsorption upon
pulverized activated carbon (Donghoon et al.,
1999Everaert et al., 2003),
Source
K. Everaert, J. Baeyens , Environ. Sci. Technol.
2003, 37, 1219-1224
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Removal efficiencies of PCDD/Fs by air pollution
control devices in municipal solid waste
incinerators
Kim Sam-Cwan, Jeon Sung Hwan , Chemosphere
43(2001)773-776
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As shown in Table 2, six of the nine MSW
incineration facilities were equipped with EP and
WS to control dust and acidic gases. Four of the
nine facilities were of the newly installed MSW
incineration facilities have adopted SNCR-SDA/BF
or SDA/BF-SCR followed by the rapid cooling
system as a combination of APCDs to control the
PCDD/Fs emission.
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Table 3 shown the removal efficiencies of PCDD/Fs
by EP were in the range of -113 to 95.
When the AC was injected in front of it PCDD/Fs
wre removed to about 68 to 95, but PCDD/Fs were
synthesized to about -44 to -113 when AC was
not injected.
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Table 4 shown that three incinerators equipped
with SDA/BF had as high as 99 PCDD/Fs removal
efficiencies when the mixed lime and AC was
sprayed into the SDA.
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• Fixed-Bed Activated Carbon Filter

Principal
In fixed-bed mode of operation the gas flows
through a bed of solid adsorbent. Sufficient
adsorbent is provided so that the operation can
continue for a long time, from several hours to
1yr or more, before the bed becomes saturated.
Although this technique is relatively complex
towards process engineering, it is able to
achieve the highest separation efficiency.
Source
K. Everaert, J. Baeyens Waste Management 2004,
24 ,3742
AYKAN KARADEIR ,Environ. Sci. Technol. 2004, 38,
1201-1207
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Fixed-Bed Adsorption System
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Breakthrough Curve
Total mass adsorbed
0.5C0
0.05C0
t5
t50
Mass adsorbed at t5
TPR gt 0.7 LUB lt 0.3 are recommended for Vapor
Recovery Systems (by Rood).
Q What do TPR, q, LUB mean?
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Fixed-Bed Activated Carbon Filter
Description

• A fixed bed is used on a continuous basis and is
  disposed off when the
• carbon bed is nearly saturated, i.e. prior to
  breakthrough of the
• pollutant in the effluent.

• For granular materials (gtgt300 mm), combustion and
  explosion hazards
• are negligible.

• The advantages of the fixed bed adsorbers
  include
• the cross- or countercurrent operation with
  beds of sufficient depth/thickness
• guarantees very high PCDD/F removal
  efficiencies (?99.5, against90 98
• in entrained-phase pulverized systems.

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Description

• With negligible combustion and explosion hazards,
  safety precautions can be
• limited to a temperature monitoring.
  Pulverized carbon processes are more
• liable to explosion and combustion, therefore
  temperature measurements need
• to be complemented by e.g. CO-monitoring in
  storage silos

• The disadvantage of fixed bed adsorbers relate to
  the low gas velocity used
• (thus increasing the cross-sectional area of
  the adsorber), the deep beds
• used to avoid breakthrough (thus operating at
  high pressure drop) and the
• possible clogging by residual flue gas dust.

• To minimize pressure drop in fixed beds, granular
  or pelletized adsorbent
• is used the particle size of granules is
  typically about 14 mm.

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Comparison
Each method has some advantages and drawbacks.
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Application
A hazardous waste incinerator in Turkey
FIGURE 1. Flow diagram of post-combustion units
with associated temperature profiles and
retention times in the plant.
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Application
Figures 2 and 3 illustrate the distribution of 17
TEF-valued PCDD/F congeners and PCDD/F homologues
before and after the AC unit.
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Results and Discussion

• Dioxin distribution between solid and vapor
  phases in the flue gas is related to the vapor
  pressure.
• AC can effectively remove gas-phase dioxins and
  its ineffective in removing particle-bound
  dioxins.
• The average gas/particle ratio at 65oC in a flue
  gas was calculated as 50 for lower chlorinated
  PCDD/Fs, while it was about 0.1 for highly
  chlorinated ones.
• There was a predominance of PCDD/F on the solid
  in the
• same unit after the AC, highly
  volatile congeners were adsorbed comparatively
  more strongly than lower volatile ones.

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Results and Discussion

• Recent study by Chang et al. agrees that higher
  removal efficiencies for lower chlorinated PCDD/F
  congeners (with higher vapor pressures )by carbon
  adsorption.
• Figures 4 and 5 show that as the chlorination
  level of dioxin congeners and homologues
  increases, the removal efficiency of PCDD/F by
  carbon adsorption decreases.
• The flue gas pass through ESP and wet scrubbers
  before the AC, the very fine particles could not
  be removed by these pollution control stages.
• As Chang et al states, the removal efficiency of
  common APCES for particles with small diameters
  (especially with diameters of 0.11µm)is
  relatively low.
• The ultrafine particles escaping the wet
  scrubbers, on the other hand, most of the
  volatile and semi-volatile pollutants including
  dioxins were adsorbed on such small particles due
  to the high surface area/volume ratios.

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FIGURE 4. Average removal efficiencies of
TEF-valued PCDD/F congeners.
FIGURE 5. Average removal efficiencies of PCDD/F
homologues.
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Results and Discussion

• Effect of Flue Gas Composition on AC Removal of
• PCDD/Fs.
• These include organic products of incomplete
  combustion (PICs), volatile metallic compounds,
  acid gases and moisture.
• PCDD/Fs removal efficiencies showed negative
  correlations with SO2(R20.51), NOx(R20.65),
• ,HCl(R20.64).
• SO2 and NOx concentrations were reduced by5060
  and 510, respectively , through AC.
• sulfuric acid , hydrogen chloride.
• decrease AC adsorptive capacity.

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Results and Discussion

• Because of these acidic effects, the AC material
  is getting wet.
• SO2and HCl adsorbed on the activated carbon
  and they can combine with flue gas moisture
  easily.
• Since the RH of the flue gas is high (7080) in
  the AC unit at IZAYDAS, the decrease in the
  removal efficiency could be attributed to the
  increase of SO2 and HCl concentrations.

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Conclusion

• Both methods are widely used on municipal waste
  combustors, hazardous waste incinerators, and
  electric steel plants.
• Both methods have higher efficiencies to control
  PCDD/Fs emission.
• Newly methods will be introduced on next chapter.
• (ACFC and AC moving bed)

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Advantages

• In addition to remove PCDD/Fs, AC also can remove
  VOCs, PAHs, PCBs, and heavy metals (especially
  Hg), etc, in flue gas at the same time.
• The cost of AC which used to control air
  pollution are more cheap than SCR.
• To use AC is more energy saving than SCR.
• The AC adsorption technologies has been employed
  in Taiwan, because of the removal efficiency can
  reach more than 90.

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END
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