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Adsorption (part 1). Instructor: Prof. Moo Been Chang Date: 2008/10/08. Graduate Institute of Environmental Engineering National Central University. Outline. What is the adsorption ? Adsorption Isotherms Adsorbent Material The Application of Activated Carbon -- VOCs control

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Adsorption(part 1)

Instructor: Prof. Moo Been Chang

Date: 2008/10/08

Graduate Institute of Environmental EngineeringNational Central University


  • What is the adsorption?

  • Adsorption Isotherms

  • Adsorbent Material

  • The Application of Activated Carbon

    --VOCs control

    -- Dioxins (PCDD/Fs) control

What is the 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


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.

Category of Adsorption


(Physical or van der Waals adsorptions):

weak bonding of gas molecules to the solid

exothermic (~ 0.1 kcal/mole)

no physical or chemical changes


multilayer adsorption

does not accompany catalysis


chemical bonding by reaction

exothermic (> 10 kcal/mole)

adsorbent changes characteristics


monolayer in most case

definitely catalyzed

Adsorption Mechanism

Temperature Effect on Adsorption

Q: Why adsorption capacity decreases when gas temperature is increased?

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.(△S<0 ) (△G <0 ) .

So the enthaplpy (△H) must be negative. (△H<0)

Thermodynamic Analysis

The adsorption is an exothermic reaction.

The higher temperature the lower efficiency ofadsorption

Adsorption Isotherms

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

Favorable and Unfavorable Adsorption

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.




Langmuir Isotherm (cont’d)

Rate of adsorption,

Rate of desorption,

At equilibrium,


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.


S = 1/Wmax



J = 1/CWmax




Langmuir Isotherm (cont’d)

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:

Langmuir Isotherm (cont’d)

It is particularly suited to represent binary and ternary systems.

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.

n > 1

n = 1


n < 1


Freundlich Isotherm

The expression: Wads = KFP1/n

(KF and n are experimentally determined parameters)

  • When n = 1, the reaction is linear and called “partitioning”.

  • When n > 1, the reaction is said to be “favorable” as the incremental change in amount sorbed decreases with increasing concentrations.

  • While n < 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 < 1 are concave downwards, so both models can be calibrated to similar data..

n > 1

n = 1



ln Wads


n < 1

ln KF


ln P

Freundlich Isotherm (cont’d)

lnWads = lnKF + 1/n lnP

Wads = KFP1/n

Freundlich Isotherm Parameters

Available for a wide variety of organic vapors on various activated carbon types

Wads = KFP1/n

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.

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



BET Isotherm (cont’d)

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< 0.01 and C >100 (Valsaraj et al., 1992).

S = (C-1)/CWmax

J = 1/CWmax


BET Isotherm (cont’d)

To obtain the parameters in the BET equation, one needs to linearize the equation:

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.

The most common isotherm models [Dastgheib and Rockstraw, 2002]

Adsorbent Material

Adsorbent Material

  • Silica Gel

  • Molecular Sieves (zeolite)

  • Activated Carbon

  • Activated Alumina

Polar and Non-polar adsorbents

Source: Air Pollution Engineering Manual., 1992

Source: Air Pollution Engineering Manual., 1992

Source: Air Pollution Engineering Manual., 1992

Source: Air Pollution Engineering Manual., 1992












Physical property might be changed as overheated.

Physical Properties of Adsorbents

Source: Cooper and Alley (2002)

Activated carbon from various sources

Source: Cooper and Alley (2002)

Adsorbent Material

Activated Carbon

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.

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.

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.

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 <2nm

(2)mesoropore : diameter 2~50 nm

(3)micropore : diameter >50 nm

Adsorbent Material

  • According to the research indicates that the diameter of air pollutants

    are on the range of 0.4~0.85 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 0.35~1.37nm.

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. )

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)

Internal porosity

Macropores (accessible to solvent and solute)

Pore Structure of Activated Carbon

Pore Structure (cont’d)


(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

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.

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.

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.

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

The influence of Temperature

The adsorption of naphthol in different temperature

The adsorption is an exothermic reaction.

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.

Q: How can we reduce the impact of humidity?

The influence of humidity

Isotherm for toluene & trichloroethylene

and water vapor (individual)

Amount of trichloroethylene adsorbed

as a function of relative humidity

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.

A way to reduce water

  • Flue gases leaving the wet scrubbers at 60-65 °C are generally

  • supersaturated with watervapor (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


  • 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.

A way to reduce water

The Application of Activated Carbon

Volatile organic compounds (VOCs) control

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

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.

VOCs Control

  • combustion processes: incineration and catalytic oxidation.

  • chemical or photochemical oxidation technologies.

  • biotechnologies: biotrickling filter, bioscrubber, and biofilter.

VOCs Control

Table 1 presents some characteristics of the main APCD, as applied to the VOC elimination (Hurashima and Chang, 2000 ;Degr`eve et al., 2001;Wang et al., 2001)

Figure 1 Application limits (flow rate—VOC concentration) of different APCD, based on references of Crocker and Schnelle, 1998;Juteau, 1997, and Devinny et al., 1998.

Figure 1. Application limits

The Application of Activated Carbon

PCDD/Fs control

PCDD (Polychlorinated Dibenzo-p-dioxins)and PCDF(Polychlorinated Dibenzofurans)

Dioxins Control


  • 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.

(TEFs) for PCDD/F and PCB congeners



2,3,7,8 TeCDD




The hurt for human:For instance, the present in UkraineVictor Yushchenko

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

Present (2005)

Before (Feb, 2004)

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.

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) 2HCl+1/2 O2 H2O+ Cl2

    (b) phenol +Cl2 chlorophenol (dioxin precursor)

    (c) 2-chlorophenol+1/2O2 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.


The factors affecting PCDD/F formation during combustion process (Mackay, 2002)

Table 1

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.

PCDD/F emission standards in Taiwan



Emission sources













10 ton





300 ton/day









Municipal/Industrail Waste



over 4




Incinerator (MWI/IWI)








below 4













Electric Arc Furnace (EAF)













Sinter plant



new facility







Waelz plant




new facility

















ther facilities

new facility









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.

Techniques available for PCDD/Fs emission control

Two different methods of contacting gas with carbon adsorbent have been used.

  • Carbon injection process

  • Fixed-Bed Activated Carbon Filter

Dioxins (PCDD/Fs) control

  • Carbon injection process


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.

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., 1999;Everaert et al., 2003),


K. Everaert*, J. Baeyens , Environ. Sci. Technol. 2003, 37, 1219-1224

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

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.

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.

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.

  • Fixed-Bed Activated Carbon Filter


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.


K. Everaert*, J. Baeyens Waste Management 2004, 24 ,37–42

AYKANKARADEIR ,Environ. Sci. Technol. 2004, 38, 1201-1207

Fixed-Bed Adsorption System


Throughput ratio:

Adsorption capacity:

Length of Unused Bed:

Breakthrough Curve

Total mass adsorbed





Mass adsorbed at t5

TPR > 0.7 & LUB < 0.3 are recommended for Vapor Recovery Systems (by Rood).

Q: What do TPR, q, LUB mean?

Fixed-Bed Activated Carbon Filter


  • 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 (>>300 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.


  • 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 1–4 mm.


Each method has some advantages and drawbacks.


A hazardous waste incinerator in Turkey

FIGURE 1. Flow diagram of post-combustion units with associated temperature profiles and retention times in the plant.


Figures 2 and 3 illustrate the distribution of 17 TEF-valued PCDD/F congeners and PCDD/F homologues before and after the AC unit.

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 it’s 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.

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 levelof dioxin congeners and homologuesincreases, 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 0.1~1µm)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.

FIGURE 4. Average removal efficiencies of TEF-valued PCDD/F congeners.

FIGURE 5. Average removal efficiencies of PCDD/F homologues.

Results and Discussion

Effect of Flue Gas Composition on AC Removal of


  • These include organic products of incomplete combustion (PICs),volatile metallic compounds, acid gases and moisture.

  • PCDD/Fs removal efficiencies showed negative correlations with SO2(R2=0.51), NOx(R2=0.65),


  • SO2and NOx concentrations were reduced by50~60% and 5~10%, respectively , through AC.

    sulfuric acid , hydrogen chloride.

    decrease AC adsorptive capacity.

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 (70~80%) in the AC unit at IZAYDAS, the decrease in the removal efficiency could be attributed to the increase of SO2 and HCl concentrations.


  • 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)


  • 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%.


Thank you for your attention

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