activated carbon ullman

Carbon, 5. Activated Carbon

KLAUS-DIRK HENNING, CarboTech Aktivkohlen GmbH, Essen, Germany

HARTMUT VON KIENLE, (formerly Degussa AG, Hanau), Heusenstamm, Germany

1. General Aspects . . . . . . . . . . . . . . . . . . 12. Carbonaceous Adsorbents . . . . . . . . . . 22.1. Types of Carbonaceous Adsorbents . 22.2. Chemical Properties. . . . . . . . . . . . . 52.3. Mechanical Properties . . . . . . . . . . . 52.4. Adsorption Properties . . . . . . . . . . . 62.5. Quality Control . . . . . . . . . . . . . . . . 73. Production . . . . . . . . . . . . . . . . . . . . . . 93.1. General Aspects . . . . . . . . . . . . . . . . 93.2. Raw Materials . . . . . . . . . . . . . . . . . 103.3. Activating Furnaces . . . . . . . . . . . . . 113.4. Methods of Activation . . . . . . . . . . . 133.4.1. Chemical Activation . . . . . . . . . . . . . 133.4.2. Gas Activation. . . . . . . . . . . . . . . . . . 133.5. Granular and Pelletized Carbons . . 143.6. Carbon Molecular Sieves . . . . . . . . . 153.7. Further Treatment . . . . . . . . . . . . . . 15

3.8. Impregnation . . . . . . . . . . . . . . . . . . 164. Applications . . . . . . . . . . . . . . . . . . . . 164.1. Gas-Phase Applications . . . . . . . . . . 164.1.1. Solvent Recovery . . . . . . . . . . . . . . . 174.1.2. Process-Gas and Air Purification . . . . 184.1.3. Gas Separation . . . . . . . . . . . . . . . . . 194.1.4. Gasoline Vapor Adsorption . . . . . . . . 204.1.5. Flue Gas Cleaning . . . . . . . . . . . . . . . 204.2. Liquid-Phase Applications . . . . . . . . 214.2.1. Water Treatment . . . . . . . . . . . . . . . . 214.2.2. Micellaneous Liquid-Phase

Applications . . . . . . . . . . . . . . . . . . . 224.3. Impregnated Activated Carbon . . . . 234.4. Catalysts and Catalyst Supports . . . 245. Regeneration and Reactivation . . . . . 246. Economic Aspects . . . . . . . . . . . . . . . 25

1. General Aspects

Definition. Activated carbon [7440-44-0] isthe collective name for carbonaceous adsor-bents which are defined as follows [1]: Activat-ed carbons are nonhazardous, processed carbo-naceous materials having a porous structure anda large internal surface area. They can adsorb awide variety of substances, that is, they are ableto attract molecules to their internal surface andtherefore act as adsorbents. The pore volume ofactivated carbons is generally greater than0.2 mL/g. The internal surface area is generallygreater than 400 m2/g. The width of the poresranges from 0.3 to several thousand nanometers.

All activated carbons [2–6] are characterizedby a ramified pore system (Fig. 1) in whichpores of various sizes, such as mesopores (d¼ 2–50 nm), micropores (d ¼ 0.8–2.0 nm)and submicropores (d � 0.8 nm) branch offfrom macropores (d � 50 nm).

X-ray investigations show that the carbon ismainly in the form of very small crystalliteswith a graphite structure. However, the typical

graphite arrangement of the carbon platelets oneabove the other is absent. The crystalline regionsare only 0.7–1.1 nm thick and 2.0–2.5 nm indiameter, a considerably smaller size than isobserved in graphite (see Table 1).

This means that in each crystallite there areusually only three or four layers of carbon atomswith about 20–30 carbon hexagons in eachlayer. The spaces between the crystallites arefilled with amorphous carbon which is bondedin three dimensions with other atoms, especiallyoxygen. The irregular arrangement of the car-bon atoms is broken up by numerous cracks andfissures (pores), which are often idealized ascylindrical in shape. The large number of veryfine pores (micropores and submicropores)gives activated carbon a large inner surface,which is the basis of its remarkable adsorptionproperties.

A common feature of these adsorbents is anamorphous structure with a high carbon contentand a hydrophobic surface properties. Activatedcarbon is made on commercial scale fromcarbon-containing raw materials (wood, peat,

� 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.n05_n04

coconut shells, lignite, hard coal) by chemicalactivation or gas activation. Activated carbon iscommercially available in shaped (cylindricalpellets), granular, or powdered form.

History. The medical use of charcoal wasdescribed as early as 1550 B.C. in an ancientEgyptian papyrus and later by HIPPOCRATES andPLINY the ELDER. The products described at thattime and later in the 1700s were of varyingeffectiveness and included wood, blood, andanimal charcoals.

The decolorization of sugar solutions bybone black was first carried out commerciallyin England in 1811. Bone black, however, con-sists mainly of calcium phosphate and containsonly a small amount of carbon. Strictly speak-ing, it is not a carbon. Methods of obtainingdecolorizing charcoals from plant materialswere first set out in British patents in 1856–1863. The first industrially manufactured acti-vated carbons in the proper sense were Eponitdecolorizing carbons, which have been pro-duced since 1909 according to a patent ofR. VON OSTREJKO [7] by heating wood charcoalwith steam and carbon dioxide in a furnacespecifically designed for the purpose.

In 1911 in the Netherlands, Norit NV begancommercial activation of peat by using steam.The chemical activation of sawdust with zinc

chloride to produce Carboraffin was first de-scribed in a patent of the Austrian Associationfor Chemical and Metallurgical Production [8].This process was first operated in Aussig, CzechRepublic, and in 1915 by Bayer. In the UnitedStates during World War I, the activation ofcoconut charcoal for gas masks was developed.The many and varied applications of activatedcarbon are amply illustrated by the existence ofover 1500manufacturing patentsworldwide [3].

During the first decades of this century,activated carbon was used mainly for the puri-fication of products of the chemical, pharma-ceutical, and food industries; purification ofdrinking water was also an important applica-tion from the outset. It is increasingly used forthe prevention of environmental pollution andfor meeting the constantly increasing demandsfor purity of natural and synthetic products.

2. Carbonaceous Adsorbents

2.1. Types of CarbonaceousAdsorbents

Due to the wide variation in the properties ofactivated carbon there is no comprehensivenomenclature or standardization. The productgroup can be characterized by appearance, poreradius distribution, or by typical applications.

Classification by appearance:

* Powdered activated carbon (PAC)* Granular activated carbon (GAC)* Cylindrical pellets

Figure 1. Schematic model of activated carbon

Table 1. Typical dimensions of crystal regions

La, nm Lb, nm c/2, nm

Natural graphite 210 94 0.351Activated carbon 2.0–2.5 0.7–1.1 0.35–0.37

2 Carbon, 5. Activated Carbon

* Spherical pellets* Activated carbon fibers (ACF)* Activated coke

Classification by pore radius distribution:

* Activated coke* Activated carbon

fine-poremedium-porewide-pore

* Carbon molecular sieves (CMS)

Classification by field of application:

* Carbon molecular sieve (CMS)* Decolorizing carbon* Water-treatment carbon* Catalyst carbon* Drinking-water carbon* Solvent-recovery carbon

Carbonaceous adsorbents are usually char-acterized by appearance as powder, granules, orformed shapes such as cylindrical or sphericalpellets. Grouping only according to end use isnot meaningful because the requirements are sovaried. Also no useful classification is possibleon the basis of raw materials or productionmethods. In practice these products are oftencalled, for example, powdered decolorizing car-bon, granular carbon for water treatment, orpelletized catalyst carbon.

In industrial practice, carbonaceous adsor-bents are often classified by their pore radiusdistributions into activated carbon, activatedcoke, and carbon molecular sieves [9]. Someinformation on characteristic data and applica-tions of commercial activated cokes and carbonmolecular sieves are given in Tables 2, 3, and 4.

Figure 2 shows the schematic pore diameterdistribution of activated carbon, activated coke,and carbon molecular sieves. Carbon molecularsieve clearly exhibits narrower pores than acti-vated carbon and activated coke. The pore dia-meters are matched to the molecular sizes of thegases to be adsorbed.

Activated carbon typically exhibits porevolumes of well above 25 cm3/100 g and porediameters of less than 2 nm. The specific inner T

able

2.Gas-phase

applications

andtypicaldata

ofcarbon-based

adsorbents

Adsorbent

Activated

carbon,fine-pore

Activated

carbon,medium-pore

Activated

carbon,wide-pore

Activated

coke

Carbonmolecular

sieves

Typical

applications

intake

airandexhaust

aircleanup,

odor

control,

adsorption

oflow-boiling

hydrocarbons

solventrecovery,adsorption

ofmedium-boiling

hydrocarbons

adsorption

andrecovery

ofhigh-boiling

hydrocarbons

dioxin

andfuranadsorption,

SO2andNOxremoval

N2andO2recovery

from

air,CH4from

biogas

Com

pacted

density,akg/m

3400–500

350–450

300–400

500–600

620

Apparentdensity,akg/m

3800

700

600

900

ca.1000

Truedensity,kg/m

32100–2200

2100–2200

2100–2200

1900

2100

Porevolumeforpore

size

d<

20nm

mL/g

0.5–0.7

0.4–0.6

0.3–0.5

0.05–0.1

0.2

d>

20nm

mL/g

0.3–0.5

0.5–0.7

0.5–1.1

0.2–0.3

>0.3

Specificsurfacearea,m

2/g

1000–1200

1200–1400

1000–1500

<400

<100

Specificheat

capacity,J/kgK

850

850

850

850

850

aFor

powderedactivatedcarbon,only

applicable

withlimitations.

Carbon, 5. Activated Carbon 3

surface area of commercial activated carbonsranges between 500 m2/g and 1500 m2/g. Dueto its hydrophobic character, activated carbon isparticularly suited to the adsorption of nonpolarorganic substances, which has led to a broadrange of applications in air pollution control andwater treatment. In addition, catalytic reactionsoccur on activated carbon surfaces. For thisreason activated carbon is used commerciallyas catalyst or catalyst support.

Activated coke is manufactured from ligniteor hard coal and has typical pore volumes of upto 25 cm3/100 g and specific surface areas of upto 400 m2/g. Depending on the source material

Tab

le3.

Liquid-phaseapplications

andtypicaldata

ofcarbon-based

adsorbents

Adsorbent

Activated

carbon,fine-pore

Activated

carbon,medium-pore

Activated

carbon,wide-pore

Activated

coke

Typical

applications

dechlorination,removal

ofmicropollutants,gold

recovery,decaffeination

potableandwastewater

purification

decolorization,wastewater

purification

deozonisation,

removal

ofmanganese

andiron

Com

pacted

density,akg/m

3400–500

350–450

300–400

500–600

Apparentdensity,*kg/m

3800

700

600

900

Truedensity,kg/m

32100–2200

2100–2200

2100–2200

1900

Porevolumeforpore

size

d<

20nm

mL/g

0.5–0.7

0.4–0.6

0.3–0.5

0.05–0.1

d>

20nm

mL/g

0.3–0.5

0.5–0.7

0.5–1.1

0.2–0.3

Specificsurfacearea,m

2/g

800–1200

800–1300

800–1400

<400

Specificheat

capacity,J/kgK

850

850

850

850

aFor

powderedactivatedcarbon,only

applicable

withlimitations.

Table 4. Applications and characteristic data of activated carbon incatalysis

Adsorbent Activated carbon

fine-pore medium- andwide-pore

Applications catalyst forchemicalreactions

catalyst supportfor impregnation

Compacted density,a

kg/m3400–500 300–450

Apparent density,a

kg/m3800 600–700

True density, kg/m3 2100–2200 2100–2200Pore volume for poresize d < 20 nm mL/g

0.5–0.7 0.3–0.6

Pore volume for poresize d > 20 nm mL/g

0.3–0.5 0.5–1.1

Specific surface area,m2/g

1000–1200 1000–1500

Specific heat capacity,J/kgK

850 850

aFor powdered activated carbon, only applicable withlimitations.

Figure 2. Schematic pore diameter distributions of carbo-naceous adsorbents


and the manufacturing process, adsorptive orcatalytic characteristics may predominate. Ac-tivated coke is used for removing SO2 anddioxins from waste and flue gases.

Carbon molecular sieves have microporediameters that range from 0.50 to 1.00 nm withpore volumes of up to 20 cm3/100 g. The sepa-ration effect of carbon molecular sieves (CMS)is based on differing rates of diffusion into thepore system. For this purpose the pore diametersof the CMS are matched carefully to thediameters of the molecules to be separated.Thus, a kinetic separation effect (sieve effect)is obtained. Larger molecules are adsorbed at asubstantially lower rate in spite of higher equi-librium loads. For example, oxygen is adsorbedten times faster than nitrogen although its mo-lecular diameter is only slightly smaller.

2.2. Chemical Properties

Activated carbon contains not only carbon, butalso small amounts of oxygen, nitrogen, sulfurand hydrogen, which are chemically bonded inthe form of various functional groups, such ascarbonyl, carboxyl, phenol, lactone, quinone,and ether groups [6, 10, 11]. These surfaceoxides are sometimes derived from the rawmaterial or they can be formed during or afterthe activation process by the action of air orwater vapor. They usually have acidic character(seldom basic), and they give to the activatedcarbon the character of a solid acid or base.Under suitable conditions, surface sulfides andcarbon–chlorine compounds can be formed.These surface chemical properties play a sig-nificant role in adsorption and catalysis.

All the raw materials used for the productionof activated carbons contain mineral compo-nents which become concentrated during theactivation process.Moreover, the inorganic che-micals used in chemical activation are oftenonly incompletely removed. The ash content ofmany products is reduced by water or acidwashing. Hence, commercial products containfrom a few tenths of a percent up to 20% ash.The main constituents of the ash are salts of thealkali and alkaline earth metals, mostly carbo-nates and phosphates, together with silica, iron,and aluminum oxides.

2.3. Mechanical Properties

Performance characteristics for activatedcarbon are generally expressed in relation toits mass. If figures on a volume basis areneeded, bulk density or tapped density figuresare also needed to carry out the conversioncalculation.

The bulk density is very much dependent onthe filling technique, the geometry of the vesselused, and the grain size of the material. There-fore, a defined tapping or shaking process isusually included in the case of the higher tappeddensities [1].

Another important feature of activated car-bon is its grain size distribution. This determinesthe resistance of a layer of activated carbon tothe flow of a liquid or gas (Fig. 3).

The fineness of grinding of a powderedcarbon affects the filtration properties.

For filling a high adsorption tower or for anyother static loading, the granules or pelletsmust be resistant to crushing. For transporta-tion procedures such as pneumatic delivery, theabrasion resistance is very important. Caremust also be taken that the thermal and chemi-cal resistance properties are adequate to with-stand any severe temperature variations oraggressive environments such as oxidizingatmospheres.

Figure 3. Pressure drop of molded activated carbon withdifferent particle diameter (20 �C, 1-m layer, dense packing)


2.4. Adsorption Properties

The adsorption properties of activated car-bon [2–6, 12–16] depend principally on its innersurface area, which in commercial products is500–1500 m2/g. To make use of the inner sur-face which is provided by the walls of the pores,the accessibility of this surface is important, thatis, the pore size and the pore size distribution.This is determined by various methods, mostlyfrom nitrogen adsorption isotherms, and is re-presented as integral or differential distributioncurves (Fig. 4).

The term “adsorption“ refers to the accumu-lation of gaseous or dissolved components onthe surface layer of a solid (the adsorbent) [9,11–15]. On their surface activated carbons have“active sites“ where the binding forces betweenthe neighboring carbon atoms are not fullysaturated and adsorption of foreign moleculestakes place. Depending on the intensity of inter-actions between adsorbent and the componentto be adsorbed, an adsorption enthalpy of5–65 kJ/mol is released. With increasing tem-perature the quantity of the matter adsorbed inthe equilibrium state decreases; in gas phase theinfluence of temperature on adsorption capacityis greater than in liquid phase. The reverse of theadsorption process is called desorption.

The adsorption mechanisms are classified aschemisorption and physisorption. Physisorptionis reversible and involves only physical interac-tion forces (van der Waals forces). Chemisorp-tion is characterized by higher interaction ener-gies which result in a chemical modification ofthe adsorbed component.

The adsorption capacity (loading) of anadsorbent for a given component is normally

represented as a function of the component’sconcentration c in the gas (or liquid) for theequilibrium condition at constant temperature,known as the adsorption isotherm x ¼ f(c)T.

There are a variety of approaches derivedfrom different model assumptions for the quan-titative description of adsorption isotherms (seealso ! Adsorption, Chap. 4, ! Air). TheLangmuir isotherm is based on the assumptionof ideal monolayer adsorption. The BET iso-therm additionally takes into accountmultilayeradsorption. However, their underlying assump-tion of a homogeneous surface structure is notapplicable to adsorbents whose structure ischaracterized by active sites with widely differ-ing bonding energies. In such cases, the empiri-cal Freundlich isotherm is often useful. Accord-ing to the Freundlich isotherm, the logarithmicadsorbent loading increases linearly with thepartial pressure of the component to be adsorbedin the carrier gas.

However, commercial adsorbents do nothave a smooth surface but are highly poroussolids with a very irregular and rugged innersurface. This fact is taken into account by thepotential theory which forms the basis of theDubinin isotherm. It describes the logarithm ofthe adsorpt volume V, which is determined viathe density of the liquid phase of the componentto be adsorbed as a function of the relativesaturation (log p/ps)

n. According to [12, 13],this equation is applicable to some wide-poreactivated carbons with an exponent of n ¼ 1,while an exponent of n ¼ 2 results for a varietyof activated carbons with organic vapors andcertain carbon molecular sieves.

At adsorption temperatures below the criticaltemperature of the component to be adsorbed,the adsorbent pores may fill up with liquidadsorpt. This phenomenon is known as capillarycondensation and enhances the adsorption ca-pacity of the adsorbent. Assuming cylindricalpores, capillary condensation can be quantita-tively described with the aid of the Kelvinequation, the degree of pore filling beinginversely proportional to the pore radius.

In the liquid phase, the empirical Freundlichisotherm is a very helpful tool, as theposition and gradient of this isotherm allowsconclusions to be drawn regarding carbonproperties in practical applications. Often,specific substances like phenol, iodine,

Figure 4. Pore sizedistributionofdifferent activatedcarbons—— Gas-phase activated carbon (narrow pores); . . . . .

Liquid-phase activated carbon (wide pores)


alkylbenzenesulfonate, or methylene blue areused for characterizing the adsorptive propertiesof activated carbons.

For commercial adsorption processes notonly the equilibrium value, but also the rate atwhich it is achieved (adsorption kinetics) is ofdecisive importance. The adsorption kineticsare determined by the following series of indi-vidual steps:

* Transfer of molecules to the external surfaceof the adsorbent

* Boundary layer film diffusion* Diffusion into the particle* Actual adsorption step

Adsorption kinetics in the liquid phase aremuch slower than in gas phase; therefore ad-sorber design is different for liquid and gasphase applications (see Table 5).

2.5. Quality Control

Methods of quality control are of great impor-tance for both technical and commercial prac-tice. Some tests are used in production control tocheck the basic properties of final or intermedi-ate products. Special requirements are placed onacceptance tests, since they may be used indisputes between buyer and supplier. The buyerwishes to be certain of the material’s suitabilityfor his purpose and uses tests in which theconditions simulate as closely as possible hisoperational conditions. For example, activecarbon for sugar refineries is tested for itsdecolorizing efficiency on a molasses solutionto determine the adsorption properties but alsoside effects due to the ash content and pH value.A gas-mask carbon is tested dynamically for itsservice time with the substances that are to beremoved in practice [1, 4].

Physical and Mechanical Tests [1]. Bulkdensity (DIN-ISO 787 11, ASTM D2854) isdefined as the mass of a unit volume of thesample in air, including both the pore systemand the voids between the particles. It is ex-pressed in kg/m3 on a dry basis. The bulk densityof activated carbon depends on the shape, size,and density of the individual particles. Bulkdensity data are useful for the estimation oftank or packing volume.

The particle density, otherwise known as Hgdensity, is defined as the mass of a unit volumeof the carbon particle, including its pore system,normally expressed in g/cm3. The particle den-sity is an important characteristic of granularcarbon that is used for the determination of bedporosity or void fraction. This in turn, is neces-sary for the determination of numerous otherproperties. Under the conditions of this method,pores r � 7 mm or more in radius are filled andtherefore do not contribute to the density of theparticle.

The absolute or helium density is defined asthe mass of a unit volume of the solid carbonskeleton that is inaccessible to He, normallyexpressed in g/cm3.

The pressure drop gives information aboutthe resistance to flow of a gas through a pelletedor granular carbon layer. The pressure drop overa packed bed is adequately defined by a modi-fied form of the semi-empirical Ergun equationas a function of the shape and size of theparticles and the temperature, pressure, andsuperficial velocity of the gas. It is expressedin pascals per meter of carbon bed length. Air ispassed through an activated carbon layer ofgiven length, and the pressure drop is measuredas a function of gas velocity. The resistance tothe flow of liquids or gases has a practicalsignificance, since it is related to the filtrationresistance of a carbon layer. The relationshipbetween flow rate and pressure drop is shown inFigure 3.

Particle size is an important property thatinfluences the flow characteristics, adsorptionkinetics and catalytic behavior of granular acti-vated carbon layers. The grain size distributionof granular activated carbons (ASTMD2862) isdetermined by using standard sieves and motor-driven sieving apparatus. All common methodsprovide formechanical separationwith standardsieves, the aperture of which is expressed in

Table 5.Typical design data of adsorbers for adsorptivegas andwaterpurification

Parameter Gas purification Water purification

Carbon particle size, mm 3–5 0.5–2.5Depth of adsorbent bed, m 0.5–1.5 2–15Mass transfer zone, m 0.05–0.3 0.5–5Superficial velocity, cm/s 10–50 0.03–0.4 a

Residence time, s 1–15 1800–7200 b

a1–15 m/h.b0.5–2 h.


millimeters or mesh. Determination of particlesize by sieving is not applicable to extrudedactivated carbons. The fineness of powderedcarbons can be determined by elutriation or bylaser-beam scattering. Equipment such as theCoulter counter or sedimentation tests lead toincorrect results because the individual particlesof activated carbon vary in electrical conductiv-ity and density.

Mechanical strength is an important factor inmost technical applications of granular activat-ed carbon. Mechanical strength tests are modi-fied to conformwith the technical requirements:it differs for active carbon for gas masks, inwhich the granules suffer attrition, from that forsolvent recovery, for which coherence of thegranules is critical. The determination ofmechanical strength simulates the resistance toabrasion or attrition under practical conditions.Depending on various practical requirements,there are many different test methods usingvibration, impact, rotary motion, or motion asin a fluidized bed. Avariety of tests are availablefor the evaluation of the mechanical strength ofgranular activated carbon. In these tests thechange in particle size distribution or theamount of fines produced is determined. Anoverview of the wide range of hardness tests,which test different aspects of the mechanicalstrength and hence cannot be related to oneanother mathematically is given in thefollowing:

Ball-mill hardness: The activated carbon is abraded for a giventime in a horizontal cylinder with steelor ceramic balls under prescribedconditions

Abrasion strength: The activated carbon is abraded byan iron rod in a horizontal rotatingcylindrical sieve of given dimensionsfor a prescribed time

Impact hardness: The activated carbon particles arebroken by dropping a weight onto asample under controlled conditions

Ball-pan hardness(ASTM D 3802):

The activated carbon is shaken for a giventime in a pan together with agiven number of steel balls of knowndiameter

Crushing strength: The pressure required to crush a granuleof activated carbon (not applicable forbroken granules)

Impact hardness(fluidized bed):

The activated carbon is pneumaticallyagitated for a standard time in a verticalcylinder, the top of which is equippedwith an impact plate.

Attrition andabrasion resistance(ASTM D4058):

The activated carbon is rotated for aperiod of time in a cylindrical drumhaving a single baffle

Stirring abrasion(AWWA B604):

The activated carbon is abraded bya T-shaped stirrer in a specialabrasion unit

Ro-Tap abrasion(AWWA B604):

The activated carbon is shaken withsteel balls in the testing pan ofa Ro-Tap sieve machine

Chemical and Physicochemical Tests.Moisture content (ASTMD2867) is determinedby heating the sample in air in an oven atconstant temperature to constant weight (3 h at150 �C). The oven-drying method is used whenwater is the only volatile material present in theactivated carbon.A xylene distillationmethod isused when the carbon is known or suspected tobe heat-sensitive or to contain water-insolublevolatile compounds as well as water.

Ash content (ASTM D2866) is determinedby ignition of the sample to constant weight in amuffle furnace (air circulation) at 650 � 25 �C.Frequently only the water-soluble or acid-solu-ble part of the ash is determined. In cases wherecertain cations or anions have an adverse effecton the products, they can be determined by firstextracting with hydrochloric acid, nitric acid, orwater in a prescribed way, and then analyzingthe extract. Arsenic, cyanide, and sulfide mustbe determined on the activated carbon itself.

Volatile matter is determined by heating thesample at 900 �C for 7 min. The percentage ofvolatilematter is calculated from theweight lossof the sample, corrected for moisture content.

Ignition temperature (ASTM D3466) is de-termined by exposing a sample of carbon to aheated air stream, the temperature of which isslowly increased until the carbon ignites. Thetemperature of the carbon bed and of the airentering the bed are recorded, and ignition isdefined as the point at which the carbon tem-perature suddenly rises above the temperature ofthe air entering the bed. The test provides a basisfor comparing the ignition characteristics ofdifferent carbons, or the change in ignitioncharacteristics of the same carbon after a periodof service.

The self-ignition test (IMDG Code,Class 4.2) determines whether a sample ofchemically activated carbon ignites at 140 �Cunder specified conditions. Steam-activatedcarbon is considered to be non-self-ignitable.


Activated carbon bearing inorganic andchemically active groups on its surface mayalter the pH of liquids to which it is added. Apredictive standard test giving, a good approxi-mation of actual conditions has been devised.The acid or alkali content is determined byboiling an aqueous suspension and measuringthe pH of the filtered or decanted aqueousextract. The pH of the extract is defined as thepH value of the activated carbon.

Adsorption Measurements. The most com-monmethod ofmeasuring adsorption propertiesof activated carbon is by the determination ofthe BET surface, e.g., according toDIN 66 131.However, the numerical value thus obtained hasonly a limited practical significance, because ina practical situation, the molecules adsorbed areusually to large to reach the inner surface of thevery small pores due to the large size of theirmolecules, whereas determination of the nitro-gen isotherm [17] or assessment of porosity byusingmercury vapor [18] gives deep penetrationinto the pore structure. It is easier to estimate theporosity bymeasuring the amount of benzene orcyclohexane vapor adsorbed by activated car-bon at 20 �C and at a range of partial pressures.The difference between two measured values ofthe isotherm represents a certain pore volume,and limiting values of pore sizes can be relatedto this. An example of the single-pointmethod isthe determination of the carbon tetrachlorideretentivity according to ASTM [19].

Carbons for gas masks are characterized bythe breakthrough times (holding times or ser-vice times), which are determined by usingcertain test substances such as chloropicrin.

For many practical applications involvinggas-phase adsorption, the total adsorptioncapacity is of minor interest in comparison withthe adsorption capacity after regeneration. Insuch cases, the “working capacity” after severalcycles of adsorption and desorption is quoted.

3. Production

3.1. General Aspects

Nearly all carbon-containing materials can beused for the manufacture of activated carbon;e.g., wood, nut shells, fruit stones, peat,

charcoal, brown coal, lignite, bituminous coal,mineral oil products, and some waste materials.Cellulose and organic polymers are used for themanufacture of fiber and spherical activatedcarbon [2–6]. These starting materials varyconsiderably with regard to the extent to whichthey can be activated; e.g., calcined petroleumcoke or high-temperature coke from coal aredifficultmaterials, whilewood charcoal is easilyactivated. In addition, the purity of the activatedcarbon produced as well as its pore size distri-bution is very much dependent on the startingmaterial.

There are two principal methods of activa-tion, i.e., that which uses chemicals and thatwhich uses gases. Chemical activation is basedon the dehydrating action of certain substances,e.g., phosphoric acid [7664-38-2] or zinc chlo-ride [7646-85-7], mostly on uncarbonized start-ing materials such as sawdust or peat. A tem-perature of 400–1000 �C is usually used. Afterremoval of the chemicals, e.g., by extraction, theporous and active carbon structure of the rawmaterial remains. Cellulose fibers or wovenmaterials produce products having the samephysical form, but made of activated carbon.Polyacrylonitrile fibers are also used.

Gas activation entails the use of gases con-taining combined oxygen, such as steam orcarbon dioxide. At temperatures of 800–1000 �C, some of the carbonaceous startingmaterial is decomposed, producing numerousextremely fine pores or cracks. The inner sur-face area of the carbon determines its degree ofactivation. The yield is, therefore, dependent onthe degree of activation. A high degree of acti-vation is associated with a low yield, and thiscan be between 20 and 60%.

Production of activated carbon can be carriedout in rotary kilns, multiple hearth furnaces, orfurnaces of the vertical-shaft or fluidized-bedtype, each type being suitable for a particularparticle size of starting material. At the presenttime, internally heated rotary kilns are mostcommonly used, since these are suitable forproduction of activated carbon of a large rangeof particle sizes from powder and granularmaterial up to cylindrical pellets. The activationof coarse materials such as softwood and beechwood charcoal is carried out in a shaft furnace.The product is usually ground to a powder foruse as a decolorizing agent. Fluidized-bed


furnaces are suitable for granules and cylindri-cal pellets.

Important factors that determine the adsorp-tion properties of an activated carbon are thepore volume, the pore size distribution, and thetype of functional groups on the surface (surfaceoxides). The large inner surface area is mainlydue to the micropores. Consequently, particularattention is paid to these during manufacture.Pore volume and pore size are affected by thetype of startingmaterial and the heating process.Thus, gas activation of coconut shell charcoalalways gives a high proportion of fine pores,while the same process with softwood charcoalyields a product with open pores. Chemicalactivation produces carbons with extremelyhigh proportions of meso- and micropores.

3.2. Raw Materials

Wood, sawdust, peat, straw, and other cellulose-containing materials are usually only treated bychemical activation.

The direct gas activation of uncarbonizedproducts is possible, although usuallywith theserawmaterials, a carbonization process (possiblyat low temperature) is added. This has theadvantage that the partly carbonized intermedi-ate product can be screened, giving a standardparticle size. Norit NV (Netherlands) carries outgas activation of peat without producing anypartially carbonized intermediate material. In afurther development of the classical procedureof OSTREJKO [7], activated carbon is produceddirectly from uncarbonized carbonaceous ma-terial by heating to 840–900 �C in an atmo-sphere of combustion products that containshydrocarbons but no oxygen [8].

Raw materials very suitable for gas activa-tion are wood charcoal, nut shell charcoal, andcoke from brown coal or peat. Wood charcoal[7440-44-0] is obtained by carbonization ofpieces of wood from beech, spruce, or pine inlarge-capacity batch retorts or continuous verti-cal retorts (! Charcoal). Carbonized coconutshells are used by many producers in Europe,Japan, and the Far East.

Bituminous coals are suitable for gas activa-tion to varying extents. A useful criterion is theircarbon, oxygen, and hydrogen contents. Thediagram in Fig. 5 gives an approximate guide

to these values [20]. The high-carbon materialsgraphite and anthracite are very difficult toactivate. Bituminous coals with high oxygenand hydrogen contents can have troublesomesticking and swelling properties. In these cases,a preoxidation is usually carried out before gasactivation [21, 22]. This is done with air oroxygen-containing gases at temperatures be-tween 150 and 350 �C. This causes 5–30%oxygen to be taken up. The optimum tempera-ture is 220–250 �C. This procedure is carriedout as long as necessary to cause completedisappearance of the swelling and sticking prop-erties; as much as 5 h may be required.

Bituminous coal may be more easily activat-ed after addition of mineral acids such as phos-phoric acid. For this purpose, the rawmaterial isfinely ground and mixed with a few percent ofacid before being formed into shapes. Thisprocess, which was developed by Carborun-dum [23], is operated in the United States byCeca. It uses a combination of chemical and gasactivation. In some types of bituminous coal, thequality of the activated product is adverselyaffected by the high ash content; however, asmall ash content that includes potassium com-pounds, for example, can have a catalytic effectand, when the material is preoxidized, can leadto intensive gasification of the coal. In thesecases the ash content of the raw material can be

Figure 5. Characterization of different raw materials


reduced by grinding and flotation. The preox-idation can be carried out on the powder or aftergranulation. Some companies which activatebituminous coal: the Calgon Carbon Corp.(United States), Norit (United States), and theCarboTech Aktivkohlen GmbH (Germany).

Brown coals and the related lignites offer aninexpensive and readily activated starting mate-rial for the production of activated carbons.Their relatively high ash and sulfur contents aredetrimental. Therefore, methods have been de-veloped for removing ash from brown coalbefore coking, e.g., by treatment with an oil–water mixture. The ash goes into the aqueousphase, while the coal remains in the oil phase.By this means the ash content can be reduced by80–90%. Lignite is activated in high tonnagesby Norit in the United States according to aprocess developed by ICI United States [24].

Many publications and patents describe theproduction of activated carbon from mineral oilproducts. Petroleum sludges, fly ash, old tires,domestic refuse, and sewage sludge have alsobeen proposed as raw materials for activatedcarbon, but none of these products have attainedcommercial importance [25, 26]

3.3. Activating Furnaces

Shaft furnaces originally consisted of simplevertical chambers with smooth walls made ofrefractory bricks. Heating is external. As theprocess was developed, the mixing of the feedmaterials was improved and, consequently, sowas the reaction with the activating gas. Thiswas done by installing ceramic attachments inthe form of gratings or replaceable louverswhich can control the direction and velocity ofthe gas stream within the furnace. The reactiongases (hydrogen and carbon monoxide) can beremoved at various levels. The temperature ofthe furnace, usually 5–8 m high, can be con-trolled by means of a number of burners orafterburners (Fig. 6) [27]. Shaft furnaces canbe used for the reactivation of exhausted acti-vated carbon [28].

Rotary kilns are the most commonly usedactivating furnaces. Due to the length of the kilnand the high temperatures necessary for gasactivation, direct heating is the only feasiblemethod when the materials of construction of

the kiln are considered. To be able to control gascomposition and temperature throughout theentire length of the kiln, several burners andgas supply lines are distributed along andaround the kiln casing, e.g., in the AmericanNorit furnace. Figure 7 shows an arrangementincluding lifters to give improved mixing of thefeed material. By means of a variable steaminjection rate, the water vapor content and,therefore, the activation rate can be furthercontrolled.

Multiple-hearth furnaces (Fig. 8) with rotat-ing arms and stationary floors on each stage areused by several firms such as Calgon in theUnited States and Belgium. This type of furnaceis also operated for the purpose of reactivation,e.g., by Windhoek Municipality (Republic ofSouth-West Africa).

Fluidized-bed furnaces offer the advantageof extremely intensive heat and mass transfer.This means not only that the activating gases arequickly brought into contact with the raw mate-rial, but also that the waste gases are just asquickly removed. Furnaces have been devel-oped which are operated continuously (Fig. 9)

Figure 6. Shaft furnace


and in which several fluidized beds are run inseries. The activating gases may be introducedinto the spaces between the fluidized layers andalso into the circulatory system. Thus, it is

possible to achieve a preoxidation in the firststage with oxygen-containing gas and then tocarry out the actual activation with oxygen-freegases in a second stage. The activation in thefluidized bed is so intensive that usually onlyfragile products with poor resistance to abrasion

Figure 7. Rotary kiln for steam-activation processa) Steam; b) Gas; c) Air; d) Burner; e) Brick lining; f) Lifters

Figure 8. Multiple hearth furnacea) Raw material silo; b) Inlet; c) Burner; d) Off-gas suction;e) Outlet for activated carbon; f) Dust collector; g) Off-gasstack

Figure 9. Fluidized-bed furnacea) Raw material silo; b) Inlet; c) Combustion chamber(indirect heating); d) Burner; e) Gas distribution plate;f) Outlet for activated carbon; g) Heat exchanger


are obtained, which are processed to givedecolorizing carbon in powder form. By main-taining certain conditions, in particular bykeeping thewater vapor content of the gas below0.6 kg/m3 and by maintaining a neutral orslightly reducing atmosphere, it is possible toproduce abrasion-resistant granules.

3.4. Methods of Activation

3.4.1. Chemical Activation

Zinc Chloride Process. In the classical zincchloride process for the chemical activation ofcarbonaceous materials, 0.4–5.0 parts of zincchloride as a concentrated solution are mixedwith 1 part peat or sawdust. The mixture is thendried and heated to 600–700 �C in a rotary kiln.The product is washed with acid and water, andthe zinc salts are recovered. In some cases,chemical activation is followed by steam acti-vation to obtain additional fine pores. In spite ofthe efficiency and simplicity of the process, it isin decline because of the problems of environ-mental contamination with zinc compounds.

Phosphoric acid [7664-38-2] can be used totreat either uncarbonized or carbonized rawmaterials, and the process is operated by Ceca(France), Hooker (Mexico), and Norit (UnitedKingdom andUnited States). Finely ground rawmaterial such as sawdust is mixed with a phos-phoric acid solution, forming a pulp. This isdried and heated to 400–600 �C in a furnacesuch as a rotary kiln. The phosphoric acid is thenextracted, sometimes after neutralizing it to givephosphate salts, and the material is dried, givingan activated carbon which usually has finerpores than the zinc chloride product. Activationwith a combination of phosphoric acid andsteam is also possible.

As with zinc chloride activation, a highlyactive decolorizing carbon is obtained by a rapidprocess in high yield and at a relatively lowreaction temperature. However, the cost ofrecovering the activating chemicals is high.Activation by phosphoric acid has becomemorepopular and there is no doubt that improvedmethods of phosphoric acid recovery have con-tributed to this. These innovations have hardlybeen reported in the literature; the know-how isnot divulged by the producers.

Other Chemicals. In the literature, manychemicals have been proposed for the activationof carbonaceous rawmaterials, but none of themattained industrial importance.

3.4.2. Gas Activation

In gas activation, carbonaceous material is trea-ted at elevated temperatures with suitable gases,the most common being steam, carbon dioxide,and mixtures thereof. Experiments using graph-ite have established the reaction velocities:steam has been shown to be 8 times as reactiveas carbon dioxide. Both gases behave as mildoxidizing agents at 800–1000 �C, there beingseveral simultaneous reactions:

H2OþC ! COþH2 DH ¼ þ117 kJ

2H2OþC ! CO2þ2H2 DH ¼ þ75 kJ

CO2þC ! 2 CO DH ¼ þ159 kJ

Due to the endothermic character of thesereactions, the carbon particles must be broughtinto intimate contact with the activating gas.This must be hotter than the required reactiontemperature; otherwise, the necessary heatenergy will not be provided, or only withdifficulty. Below 800 �C, the reaction velocityis so seriously reduced that the activation pro-cess ceases for all practical purposes. A usefulimprovement to the heat supply can beobtained by combustion of gases producedduring activation:

COþ0:5 O2 ! CO2 DH ¼ �285 kJ

H2þ0:5 O2!H2O DH ¼ �238 kJ

Modern furnace construction takes advan-tage of this fact by introducing oxygen and air atsuitable points, which at the same time has theeffect of regenerating the activating gases. Thus,the best way of dealing with the carbon monox-ide and hydrogen that are produced is by burn-ing them off in the reactor itself. This is neces-sary for the additional reason that these gasesreduce the velocity of activation, carbon mon-oxide to a noticeable extent and hydrogen verymarkedly. The reaction mechanism for the gasactivation of carbon with steam or carbon diox-ide is characterized by an initial adsorption of


these gases with subsequent oxidation of thecarbon surface as the rate-determining step:

CþCO2 ! CðOÞþCO

CðOÞ ! CO

where C(O) signifies surface oxide.The retarding action of the carbon monoxide

and hydrogen can be attributed to the formationof C(CO) andC(H) surface complexes, the latterin particular being much more stable than theC(O) surface complex. Thus, the active siteswhich could adsorb oxygen are blocked byhydrogen.

Oxygen or air are unsuitable as activatinggases. In amixturewith steam or inert gas, smallamounts of oxygen lead to activated materialwith very large pores. Oxygen reacts with car-bon about 100 times as fast as carbon dioxide.This reaction velocity is even further increasedby potassium salts, so that potassium-containingraw materials react so vigorously when oxygenis present in the gas that an uncontrolled com-bustion takes place without producing activa-tion. The chemical condition of the carbonsurface, especially the presence of larger orsmaller amounts of carbonyl and carboxylgroups, can determine the adsorption propertiesof the activated carbon and, very importantly, itsproperties as a catalyst.

Acidic surface oxides are formed by heatingactivated carbon in air or oxygen for a shortperiod below the ignition temperature. If carbonis first heated to 1000 �C and then allowed toreact with air at room temperature, basic surfaceoxide groups are formed, although the quantityof these is at most much less than the quantity ofacidic groups.

It has long been known that the gas activationof carbonaceous materials is accelerated bysmall amounts of various compounds, e.g., saltsof alkali and alkaline earth metals, almost allchlorides, sulfates, acetates, and carbonates, aswell as most acids and hydroxides. The mostimportant catalysts used industrially are causticpotash and potassium carbonate. Amounts be-tween 0.1% and 5% are used. The activationaccelerators can be used in solid form mixedwith the finely powdered carbonaceous sub-stances or added as solutions, sometimes fol-lowed by molding into shapes and low-temper-ature carbonization.

If bituminous coal is activated with additionof alkali metal salts, the gas mixtures containingcarbon dioxide must be used for the activationprocess in preference to pure steam.

In addition to these accelerators, the patentliterature also refers to compounds of iron,manganese, and aluminum.

The theory of catalytically accelerated acti-vation is described in [29] and [30].

3.5. Granular and Pelletized Carbons

For a number of applications in liquid and gaspurification, the activated carbon must be pro-vided in the form of grains or granules; there-fore, the rawmaterial used to produce it must besimilarly shaped, e.g., wood charcoal or coconutshell charcoal. Alternatively, the raw material,such as coal, can be pulverized, briquetted byusing a binder, and finally carbonized. Beforeactivation, the material is broken down to therequired particle size. For applications requiringa carbon with a high mechanical strength, it isoften an advantage to use carbonwhich has beenspecially preshaped, i.e., pelletized. In somecases, the hardness is dependent not only onthe raw material and binder, but also on thedegree of activation and porosity. Highly acti-vated carbons have a high pore volume and,therefore, low density and strength. Similarly,products with open pores are not as hard as thosewith fine pores.

Carbons in pellet form are produced fromfinely powdered raw material as follows. Thepowder is first mixed with a binder in a heatedpaste mixer to give a flowable mass. This mate-rial is then extruded to form strands � 1 mm incross section, ormay be formed discontinuouslyin a cylinder press (Fig. 10). These strands,sometimes after drying, are then broken intoshort lengths, the length of each piece beingapproximately equal to its diameter. These arethen either chemically activated or carbonizedat 400–500 �C and finally gas-activated. Suit-able binders are coal tar, wood tar, lignosulfonicacids, or mixtures of phenols and aldehydes ortheir condensation products. Bases such as caus-tic soda or chalk neutralize the acidic groups ofthe tar and improve gas activation.

A simplified flow sheet of the CarboTechprocess is shown in Figure 11. The specially


de-ashed hard coal feed is finely ground, andpartially oxidized with air. Then the coal dust ismixed with a binder. The binder is added toproduce a plastic coal/bindermixture that can beused to form shaped extrudates of the desireddiameter. The extrudates are then carbonized toactivated coke in a rotary kiln at 900–1000 �C.In a final activation step, the carbon skeleton ofthe activated coke is partially gasified by steamactivation in a multistage fluidized-bed furnace.

The production of pelletized activated car-bon from material which is already in an acti-vated state, such as powdered decolorizing car-bon, is at the present time of little commercialimportance. In the beverage and food industries,products of this type have found increasedapplication due to their dust-free nature. Tomake the fullest use of the available adsorptioncapacity of agglomerates of this type in an

aqueous medium, it is necessary to disintegratethem.

There are a number of patents describing theproduction of microporous carbon spheres frompitch. The process involves several stages: melt-ing, dispersing, oxidizing with air to render thematerial insoluble, and finally, activation bysteam.

3.6. Carbon Molecular Sieves

Carbon molecular sieves for air separation areproduced from coal, coconut shells, or resins.For example, bituminous coal is ground to a finedust and oxidized in air at a temperature belowthe ignition temperature. The oxidized coal ismixedwith a binder and shaped into pellets withdiameter of 2.5 mm. The pellets are carbonizedin a special rotary kiln.

The final step is treatment with hydrocarbonunder cracking conditions to deposit carbonexactly on pore openings [31]. The pore diame-ter is then smaller than that of the initial materi-al, and the diameter of the “bottle necks“ are onthe same range as those of nitrogen and oxygenmolecules. Oxygen molecules can penetratemuch quicker than the nitrogen molecule intothe pores. Therefore, the most of the nitrogen isrecovered, while almost all of the oxygen isadsorbed.

3.7. Further Treatment

Many firms produce low-ash varieties of acti-vated carbon by removal of various impurities

Figure 10. Flow sheet for production of pelletized activatedcarbona) Crusher; b) Mill; c) Kneader; d) Extrusion; e) Drying;f) Carbonization; g) Activation; h) Screening; i) Packing

Figure 11. Production steps of formed activated carbon


by washing with water or acids such as hydro-chloric or nitric acid. If the activated carbon is tobe used for the production of fine chemicals orpharmaceutical preparations, a particularlythorough washing procedure is required, andactivated carbons for use as catalysts or catalystsupport require similar treatment. Basic consti-tuents and accelerators such as caustic potashmay be neutralized by acid or washed out withwater. Active carbons with fine pores, made bysteam activation at 800 �C, can be further acti-vated by air at 500–600 �C in the presence ofalkali, giving improved decolorizing ability.However, this two-stage activation process hasno known practical importance. The most im-portant result of a postoxidation of activatedcarbon by nitrous gases, in particular nitrogendioxide, is the formation of additional surfaceoxides rather than any further activation. Theopposite effect, i.e., the removal of chemicallybound oxygen, is possible by treating activatedcarbonwith hydrogen at 200–500 �C. Loweringof sulfur content can be achievedby the action ofsteam and hydrogen. In the literature amethod isdescribed of producing activated carbon with alow iron content. This involves converting theiron into volatile compounds by treating thecarbon while it is still hot with suitable gasesor vapors such as halogens, halogen com-pounds, or carbon monoxide.

3.8. Impregnation

For cost-effective removal of certain impuritiescontained in gases (e.g., hydrogen sulfide, mer-cury, and ammonia), the adsorption capacityand removal ratemust be substantially increasedby impregnation of the activated carbon withsuitable chemicals. When these chemicals aredeposited on the internal surface of the activatedcarbon, the removal mechanism also changes.The impurities are no longer removed by ad-sorption but by chemisorption [32].

For the manufacture of impregnated activat-ed carbon, an activated carbon of suitable quali-ty for the particular application is impregnatedwith solutions of salts or other chemicals which,after drying or other aftertreatment steps,remain on the internal surface of the activatedcarbon. As well as soaking impregnation, sprayimpregnation can be used. In that case the

activated carbon is sprayed in a rotary kiln orin a fluidized bed under defined conditions. Theimpregnated wet activated carbon must be driedin, for example, a rotary kiln or fluidized-beddrier. After the drying step, most impregnatedactivated carbons can be used industrially. Insome applications the impregnating agents arepresent in the form of hydroxides, carbonates,chromates, or nitrates and must be subjected tothermal aftertreatment at higher temperatures(150–400 �C) to decompose the anions.Depending on the application, various activatedcarbons (pellets, granules, powders) are impreg-nated with suitable organic or inorganic chemi-cals. Homogeneous distribution of the impreg-nating agents on the internal surface of anactivated carbon is important. Furthermore,blocking of the micropores and macroporesmust be avoided so that the impregnating agentremains accessible for the reactants.

4. Applications

Since the range of applications for carbona-ceous adsorbents in the gas and liquid phasesis very broad only a small selection can be dealtwith here. The major applications are in watertreatment, gas purification, food processing,gold recovery, and solvent recovery (Fig. 12).

4.1. Gas-Phase Applications

The majority of gas- and vapor-phase applica-tions of activated carbon are in process gaspurification, air purification, catalysis, flue gas

Figure 12. Gas- and liquid-phase applications of carbona-ceous adsorbents


purification, solvent recovery, and automotiveemission control, and personal protection(Fig. 13).

4.1.1. Solvent Recovery

Activated carbon can be used advantageouslyfor the removal of organic vapors from gases, itsperformance being good even at very low partialpressures. This may be deduced from the linearadsorption isotherm plotted in Figure 14. Forexample, a pelletized activated carbon is able totake up ca. 18% of its mass of toluene from acurrent of air that contains only 0.11 g of tolu-ene perm3. This represents an enrichment factorof over 400 000 : 1. This ability to concentrate asubstance is particularly useful for producingextremely pure gases, as well as for protectingthe environment [2, 33–37].

When activated carbon is used for the recov-ery of solvents, which usually occurs at con-centrations between 1 and 20 g/m3, efficienciesof > 90% are sought. The cost of regeneration,whether by steam, hot gas, or electrical heating,is usually small in comparison to the value of the

recovered solvent. The charge of activated car-bon retains its effectiveness for a long time if theregeneration is carried out at a sufficiently hightemperature. However, fine material which isformed by attrition and thermal or chemicalstress must be removed and replaced from timeto time. A survey of some areas of application ofsolvent recovery is given in Table 6.

In industry, the solvent recovery is carriedout in vertical or horizontal adsorbers, and byarranging these in parallel a continuous opera-tion is possible by changing over from one toanother (Fig. 15) [9]. The gas flow is usually inan upward direction, whereas the desorptionwith hot steam is in the opposite direction. InFigure 16 a typical temperature–time graph fora cycle is given. Usually, fixed-bed adsorbershave a bed thickness of 0.8–2 m. A more recentdevelopment is the use of fluidized-bed reactorswith finely granulated activated carbon [37].Another development is the use of activatedcarbon fibers. Both methods are, however,rarely used.

Practical solvent recovery systems use gasflow rates of 0.2–0.5 m/s. Lower flow rateswould lead to better utilization of the adsorption

Figure 13. Gas-phase applications of carbonaceousadsorbents Figure 14. Linear adsorption isotherm for toluene

Table 6. Solvent recovery

Industry Typical solvents

Plastic film and foil ether, acetone, methyl ethyl ketone, alcohols, methylene chloride, tetrahydrofuran, cyclohexanonePrinting toluene, petroleum spirit, trichloroethene, n-hexaneMetal degreasing trichloroethane, trichloroethene, tetrachloroetheneRubber petroleum spirit, benzene, tolueneViscose and rayon carbon disulfideDry cleaning tetrachloroethene, fluorochloro hydrocarbonsSynthetic leather and fibers alcohol, acetone, hexane, toluene, esters, dimethylformamideAdhesives petroleum spirit, hexane, toluene


capacity of the carbon, but there is a danger thatthe heat of adsorption is not carried away, andtherefore overheating and even ignition of thecarbon charge can occur. Typical operating datafor solvent recovery plants and design rangesare given in the following:

Air velocity 0.2–0.4 m/sAir temperature 20–40 �CBed height 0.8–1.5 mSteam velocity 0.1–0.2 m/s

Time cycle per adsorberAdsorption 2–6 hDrying (hot air) 0.2–0.5 hCooling (cold air) 0.2–0.5 hSolvent concentration 1–10 g/cm3

Solvent adsorbed per cycle 10–25 wt%Steam/solvent ratio (2–5):1Energy 50–600 kWh/t solventCooling water 30–100 m3/t solventActivated carbon 0.5–1 kg/t solvent

Recovery units have at least two, but moreusually three or four adsorbers which pass suc-cessively through the stages of the operationcycle. While adsorption takes place in one ormore of them, desorption, drying, and coolingare carried out in the others.

4.1.2. Process-Gas and Air Purification

Many gas purification processes use activatedcarbon, e.g., for production of pure gases in thechemical industry, in protection against poisongas, in air conditioning, for removal of oil fromcompressed air, and in purification of waste air.Small traces of unwanted gases or vapors areadsorbed onto activated carbon which is oftenimproved by being specially impregnated, inwhich case regeneration with recovery of theextracted materials is impossible. Activatedcarbon is also used to remove resin-formingand other hydrocarbons from gases before pass-ing them over sensitive molecular sieves orcatalysts [2, 9, 31–36].

Small amounts of hydrogen sulfide can beconverted to elemental sulfur in the presence ofoxygen by means of activated carbon which hasbeen impregnated with potassium iodide [36].This reaction is used in the viscose industry forpurification of waste air. In this case, it iscombined with carbon disulfide recovery andis known as the Sulfosorbon process [36, 38].The Sulfren process uses sulfur dioxide as anoxidizing agent.

Sulfur compounds such as carbon disulfide,carbonyl sulfide, and organic thiols are removedfrom moist gases containing excess oxygen athigh temperature on alkaline carbon. This is theDesorex process. The adsorbate is hydrolyzedand oxidized to sulfate, which can be washedout.

In air-conditioning installations, activatedcarbon is used for the purification of air drawnin from outside, e.g., in airports, near chemicalplants, or for environmentally controlled roomsof hospitals or museums. By the use of activatedcarbon, the amount of necessary cold outside air

Figure 15. Flow sheet of a solvent recovery unita) a1) Adsorber 1; a2) Adsorber 2; b) Exhaust air; c) Blast;d) Desorption; e) Condenser; f) Cooler; g) Separator

Figure 16. Temperature diagram for the carbon bed of asolvent recovery unita) Upper part; b) Middle; c) Lower part


can be reduced. Because of the large quantitiesof air to be handled for such large spaces, a smallresistance to flow is required, and therefore, thinlayers of carbon are used. Often exchangeablecartridge filters are used, and complete filterelements made of specially shaped activatedcarbon bodies are available. Composite materi-als have been produced, in which powderedactivated carbon is bonded onto polyurethanefoam or some other suitable carrier. For theremoval of some substances, impregnated acti-vated carbons like those in industrial respiratorsare used.

In nuclear power installations, activated car-bon impregnatedwith iodine compounds is usedto remove radioactive iodine compounds fromthe air exhausted to the atmosphere. This takesplace by isotope exchange. In some countriesimpregnation with the base tetraethylenedia-mine (TEDA) is used for the same purpose; inthis case the iodine compounds are removed bysalt formation. Activated carbon is also used inoff-gas delay beds. Deep beds of activated car-bon with very fine pores adsorb radioactivegases such as krypton and xenon long enoughfor the isotopes to decay to safe levels of radio-activity, after which they can be released into theatmosphere.

Since World War I, activated carbon filtershave been used by the armed forces for respira-tors, and this use has since been extended toprotection against hazardous gases in industry.For both applications, virtually complete re-moval of impurities is necessary, and therefore,only fine-grained activated carbon can be usedso as to make a filter as compact as possible.Table 7 shows how the service time depends onthe grain size. For respirators, impregnation ofactivated carbon with chromium and coppersalts is a well-proven technology; due to thetoxicity of chromium compounds the latest im-pregnations are based on copper and molybde-num salts. These substances have a strong oxi-dizing action and high reactivity toward chlo-rine, hydrogen cyanide, and their derivatives.

Other substances used for impregnation arecaustic potash (acidic gases), zinc salts (ammo-nia and hydrogen sulfide), and iodine com-pounds (mercury vapor).

Activated carbon and impregnated activatedcarbon is also used for many odor emissioncontrol applications: wood chip drying (pinene,terpene), plastic processing (styrene, benzene,etc.), home application (kitchen hood, refriger-ator). Apart from physisorption, chemisorptionon impregnated activated carbon (Section 4.3)can be applied, especially for H2S and mercap-tan removal.

In cigarette filters and in attachments fortobacco pipes, activated carbon is used to reducethe nicotine and tar content of the smoke.

4.1.3. Gas Separation

In 1960 the first pressure-swing adsorption(PSA) plants for gas drying, gas purificationand gas separation were built. All PSA process-es have in common that adsorption is operated ata higher and desorption at a lower total pressure.In the majority of processes the adsorptionpressure is markedly higher than atmosphericpressure. For desorption the pressure is eitherreduced to atmospheric pressure or vacuum isapplied to lower the pressure below atmosphericpressure. In some processes, desorption isboosted by a flushing cycle. The pressure-swingadsorption cycle comprises the followingsteps:

* Adsorption at higher pressure* Desorption by pressure reduction* Flushing with product gas* Pressure build-up with raw gas or product gas

to adsorption pressure

Intervals between these individual stepsare quite short (30 s to a few minutes) anddepend on the separation process and plantdesign.

Several types of product gases can beobtained during the absorption or desorptionstep, see Table 8 [31, 39].

For gas separation, carbon molecular sievesor activated carbons with extremely fine poresand molecular sieve properties are used. From amixture of hydrogen, carbon monoxide, and

Table 7. Effect of grain size on the service time of a respirator fittedwith an activated carbon filter (test gas: chloropicrin in moist air)

Diameter of carbon granules mm 0.8 1.2 1.6 2.5 4.0Service time min 75 60 35 18 5


methane, it is possible to produce highly purehydrogen by adsorption of CO and CH4 underpressure (1.5–4.0 MPa). If pure hydrogen isthen passed through in the opposite directionat lower pressure (0.2–0.5 MPa), the adsorbedsubstances are again desorbed. Since the hold-ing time of an adsorber which operates in thisPSA mode is in the range of 30 s to a fewminutes, a large number of adsorbers (8–16)are used in rotation, so that pressure changes canbe minimized between the adsorbers and com-pression energy can be saved. The method canbe used to separate oxygen and nitrogen, as wellas for production of synthetic natural gas frombiogas (biologically produced methane).

4.1.4. Gasoline Vapor Adsorption

Gasoline vapors evaporate during manufacture,distribution, refueling, and running of cars andenter the environment. Vapor recovery units areinstalled at tank farms and distribution terminalsof refineries. These waste air streams, saturatedwith organic vapors, are often cleaned by com-bined processes [40]:

* Absorption and pressure-swing adsorption* Membrane permeation and pressure-swing

adsorption* Condensation and adsorption

To avoid pollution of the environment bygasoline vapor frommotor vehicles, installationof an activated carbon filter in the ventilation

port of the gasoline tank is required in theUnitedStates, the EU, and Japan. When a car is leftstanding in the hot sun, for example, the gaso-line that evaporates from the tank is adsorbed bythe activated carbon and then desorbed againwhen the vehicle runs and fresh air for theengine is drawn through the carbon cartridge.

4.1.5. Flue Gas Cleaning

For SO2 and NOx removal from flue gases frompower plants and waste incineration plantsmany activated carbon/activated coke processeshave been developed. The adsorption capacityof activated carbon and activated coke for sulfurdioxide from flue gas is only a few percent byweight. Therefore, processes are based on theability of activated coke to oxidize sulfur diox-ide in the presence of oxygen, forming sulfuricacid.

2 SO2þO2þ2 H2O ! 2 H2SO4

The spent activated coke can be regeneratedthermally at 400–500 �C in a desorber; thecarbon of the activated carbon skeleton is usedas a reactant.

2 H2SO4þC ! 2 SO2þCO2þ2 H2O

This carbon consumption results in a largerinner surface area and higher catalytic activity,converting the activated coke to activated car-bon. The SO2-rich gas can be processed toelemental sulfur or sulfuric acid.

The catalytic properties of activated carbonare used for catalytic NO reduction by additionof gaseous ammonia.

4 NOþ4 NH3þO2 ! 4 N2þ6 H2O

Activated coke processes for simultaneousSO2 and NOx removal are the Mitsui MiningProcess, the Sumitomo Heavy Process, and theUhde/Bergbau-Forschung/Mitsui Process [41].In the Sulfacid process, which is designed forSO2 removal, regeneration by water extractioncontinuously yields dilute sulfuric acid [36].

In waste incineration plants, powdered acti-vated carbon is injected into the flue gas streamthrough an atomizer. The loaded adsorbent iscollected in a fabric filter; further pollutantremoval takes place in the filter cake depositedon the fabric [42].

Table 8. Selected application fileds for pressure-swing processes forgas separation

Separation problem Adsorbent b Productionphase a

Gas drying Al2O3, SiO2, ZMS AHydrogen from coke-ovenor reformer gas

CMS, ZMS, Al2O3 A

Helium from diving gases CMS, ZMS, Al2O3 ANitrogen from air CMS A

ZMS DOxygen from air CMS D

ZMS AMethane from biogas CMS ACarbon dioxide from exhaust gas CMS, ZMS D

aA ¼ adsorption phase, D ¼ desorption phase.bAl2O3 ¼ aluminium oxide; CMS ¼ carbon molecular sieves;SiO2 ¼ silica; ZMS ¼ zeolitic molecular sieves.


4.2. Liquid-Phase Applications

Liquid-phase applications are estimated to ac-count for over two-thirds of world activated car-bon consumption; both granular and powderedactivated carbons are in use [2, 4, 5, 34, 44].

There are many processes available for treat-ment of liquids and solutions with activatedcarbon which have found wide industrial appli-cation. In the batch contact unit operation,powdered activated carbon is added to the liquidbeing treated, either directly or (more usually)as a previously prepared suspension. The tem-perature is raised to reduce the viscosity andhence also the diffusion time. Equilibrium isnormally reached after 15–30 min, after whichthe mixture is filtered, usually after adding afiltration aid such as diatomite.

In continuous-layer filtration, the liquid ispumped through a prepared bed of powderedcarbon, which is usually made more permeableby the addition of diatomite. Due to the shortcontact time, the purification process is usuallyincomplete, and because of the limited amountof carbon present in the bed, the operating life isshort. The method is used for liquids with onlylow levels of impurities or for purification ofworking liquids which can be pumped so as tobypass the filter, e.g., in electroplating, in swim-ming baths, or in dry cleaning machines.

Percolation through granular carbon is usedparticularly in the field of water purification anddecolorization. This continuous process has theadvantage that large charges of carbon can beemployed, particularly when several filters arearranged in series. Depending on the concentra-tion and viscosity of the solution, the contacttime in the percolation process can range from10 min to more than 1 h. In some applicationscatalytic side reactions such as oxidation mayoccur, or changes of pH can be brought about bythe ash content of the carbon.

Activated carbon and activated coke are of-ten used in combination with other filter mate-rials in multilayer filters for wastewater anddrinking water treatment [45].

4.2.1. Water Treatment

Water treatment (35% of world consumption)can be divided into drinking water, industrialand municipal wastewater, and groundwater.

In drinking water, activated carbon is used toremove unpleasant odors and tastes and reducethe concentration of compounds constituting ahealth hazard (pesticides, chlorinated hydrocar-bons, etc.).

Powdered activated carbon is often used tosolve temporary pollution problems and is addedto the water as a slurry at the same time or justbefore adding of flocculant. After a suitablecontact time, the powdered activated carbon isremoved with the flocculant by sedimentation.

When granular activated carbon is used topurify drinking water, after a rapid gravity sandfiltration (removal of suspended solids) thepercolation process is used almost exclusively,and in addition to the adsorptive purification, thecatalytic decomposition of the oxidizing agentschlorine, chlorine dioxide, and ozone plays animportant role. The filtration velocities arebetween 5 and 20 m/h, which for a layer depthof 2–4 m corresponds to a contact time of6–48 min. The backwashing velocities are40–50 m/h. After a service life of 6–24 months,the spent carbon is reactivated in a on-sitereactivation plant or in the supplier’s reactiva-tion plant.

Figure 17 compares the different treatmentsteps of drinking water with powdered (PAC)and granular activated carbon (GAC).

Figure 17. Drinking-water treatment with powdered andgranular activated carbon


The complex composition of industrial was-tewaters and dumpsite leachates mostlyrequires a combination of various purificationmethods to obtain the required purity economi-cally. A combination of activated carbon andbiological purification is suitable inmany cases.Powdered activated carbon (PAC) is added toaerobic or anaerobic biological treatment plantsto adsorb toxic contaminants and stabilize thebiological activity. There is then no impairmentof the biological activity, and the impurity-load-ed carbon is removed along with the microor-ganisms and usually incinerated. PAC is alsoused independently in treatment plants [46].Granular activated carbon filter beds are oftenused as a tertiary treatment after conventionalsecondary biological treatment. Industrial was-tewaters may exhibit several times higherconcentrations of organic substances than drink-ing water. Even though these differences inconcentration are accommodated in plantdesign (contact time 0.5–4 h, filtration veloci-ties of 2–6 m/h), the adsorption lifetime of theactivated carbon filters is reduced from severalmonths to few days. Accordingly, cost-effectiveuse of activated carbon adsorbers only becamepossible with the development of processes foractivated carbon regeneration in the 1980s.Dump-site leachates are often purified by bio-logical treatment followed by a filtration ormembrane permeation process and an activatedcarbon step. The main purpose of adsorption onactivated carbon is removal of organic halogencompounds.

4.2.2. Micellaneous Liquid-PhaseApplications

Both powdered and granular activated carbonmade from a variety of rawmaterials are used infood and beverage processing. Wood-basedchemically activated carbons are preferable forthe removal of large color bodies and other highmolecular mass impurities. Peat- and coal-based steam-activated carbons are used fordecolorization and removal of unpleasant tastes,odors, and other low and medium molecularmass impurities. Microporous coconut-shellactivated carbons are less efficient in decolori-zation. Powdered grades are used in batch pro-cesses which already require a filtration stage

and where the dosage needs to be varied accord-ing to different process conditions. The pow-dered activated carbon is mixed with the liquidto be purified. When the impurities have beenadsorbed, the carbon is removed from the solu-tion by filtration or sedimentation, and is thendischarged. Granular activated carbons are usedwhere high volumes of liquids of a consistentquality or grade are continuously processed inlarge quantities. Fixed-bed and moving bedadsorbers are used, and the spent activatedcarbon can be reactivated on site or by thecarbon supplier.

Some products which are purified with acti-vated carbon follow:

Alcoholic beverages BeerWineVodkaWhite rumWhisky

Soft drinks Dechlorination and removalof taste and organicmatter from water

Decolorisation of sugarand fruit juices

Sugar and sweeteners Cane sugarBeet sugarPolysaccharidesGlucose, lactose, maltose,

fructose corn syrupsXylitolAspartame

Decaffeinated coffee Caffeine removal byrecirculating CO2 or hotwater and adsorptionon activated carbon

Edible oils Coconut oilPalm oilFish oilSoybean oilOlive oil

Flavorings Yeast extractMonosodium glutamateHydrolysed vegetableProteins

Chemicals and pharmaceuticalproducts

Paraffins

WaxesPhosphoric acidSodium hydroxideCitric acidGelatinPectinQuinineInsulinAntibioticsSulfonamides

Activated carbon processes have been devel-oped gold recovery from low-grade ores. In the


carbon-in-pulp process (CIP) a suspension ofthe ore is treated with cyanide to produce goldcyanide, which is then adsorbed onto granularactivated carbon (! Gold, Gold Alloys, andGold Compounds, Section 4.3. By using multi-stage countercurrent adsorption, the gold cya-nide complex is concentrated on the activatedcarbon. The carbon is then separated by me-chanical sieving and subjected to a elutionprocess under slight pressure. Gold is recoveredfrom the activated carbon by elution followed byelectrowinning. After thermal reactivation thecarbon can be re-used in the process. Activatedcoconutcarbonorextrudedgranularcarbon,withhigh attrition and abrasion resistance, are used.

One of the oldest uses is in medicine for theadsorption of harmful bacteria and their meta-bolic products in the gastrointestinal tract. Forblood dialysis treatment in cases of kidney andliver diseases or poisoning, granular activatedcarbon with a semipermeable coating is used.

4.3. Impregnated Activated Carbon

Impregnated activated carbon is predominantlyused in the following applications:

* Gas purification* Civil and military gas protection* Catalysis

For these applications the manufacturers of-fer various qualities of impregnated activatedcarbon. Table 9 lists frequently used impregnat-ed activated carbons. A given impregnatingagent is frequently used for various purificationtasks [32].

Potassium iodide promotes the action ofactivated carbon as an oxidation catalyst andthus allows catalytic oxidation of hydrogensulfide to sulfur or of phosphine to phosphoricacid. The same impregnation technique is usedto extract radioactive methyl iodide and othergaseous compounds arising in nuclear installa-tions. Hydrogen sulfide and formaldehyde canbe oxidized to nonhazardous substances withthe aid of activated carbon impregnated withmanganese dioxide; at high temperatures theformaldehyde is oxidized not just to formic acid,but directly through to carbon dioxide. Impreg-nation with iron salts and conversion to iron(III)oxide enables the removal of divalent sulfurcompounds from gas mixtures low in oxygen.About 1.5 times the stoichiometric amount of

Table 9. Commercial grades of impregnated activated carbon [32]

Impregnation

Chemicals Quantity, wt% Activated carbona Examples for applications

Sulfuric acid 2–25 F 1–4 mm Ø ammonia, amine, mercuryPhosphoric acid 10–30 F 1–4 mm Ø ammonia, aminePotassium carbonate 10–20 F 1–4 mm Ø acid gases (HCl, HF, SO2, H2S, NO2),

carbon disulfideIron oxide 10 F 1–4 mm Ø H2S, thiols, COSPotassium iodide 1–5 F 1–4 mm Ø H2S, PH3, Hg, AsH3, radioactive gases/radioactive

methyl iodideTriethylenediamine 2–5 F 1–2 mm Ø radioactive gases/radioactive methyl iodide

G 6–16 meshSulfur 10–20 F 1–4 mm Ø, G mercuryPotassium permanganate 5 F 3 þ 4 mm Ø H2S from oxygen-lacking gasesManganese IV oxide G 6–16 mesh aldehydeSilver 0.1–3 F 3 þ 4 mm Ø F: phosphine, arsine

0.05–0.4 G 8–30 mesh G: domestic drinking water filters(oligodynamic effect)

Zinc oxide 10 F 1–4 mm Ø hydrogen cyanideChromium–copper–silver molybdenum salts 10–20 F 0.8–3 mm Ø civil and military gas protection

G 12–30 mesh phosgene, chlorine, arsineG 6–16 mesh chloropicrin, sarin, and other nerve gases

Mercury (II) chloride 10–15 F 3 þ 4 mm Ø vinyl chloride synthesis, vinyl fluoride synthesisZinc acetate 15–25 f 3 þ 4 mm Ø vinyl acetate synthesisNoble metals 0.5–1.0 F, G, P organic synthesis, hydrogenation(palladium, platinum) 0.5 G 2–5 mm purification of terephthalic acid

aF ¼ pelletized activated carbon, G ¼ granulated activated carbon, P ¼ powdered activated carbon, Ø ¼ pellet diameter.


oxygen is required to oxidize these com-pounds [36]. Activated carbon treated with ele-mental sulfur is used to eliminatemercury vaporfromnatural gas, hydrogen, andothergases [43].Silver-impregnated activated carbon is used forpurification of drinking water due to its oligo-dynamic effect.

4.4. Catalysts and Catalyst Supports

In addition to its adsorption properties, activatedcarbon has catalytic properties which allowcommercial use as catalyst and catalyst support.The catalytic action of activated carbon is due tothe crystalline structure of the skeleton, whichconsists of a mixture of amorphous and graphit-ic carbon. On the rims of the layers, there aremany unsaturated edges and ridges which act aslattice vacancies. On the internal activatedcarbon surface, there are surface oxides, whichhave been identified as carbonyl groups,lactone groups, phenolic hydroxyl groups, andcarboxyl groups [6, 11]. These surface oxidescan participate in redox reactions, and they are areason for the effectiveness of activated carboncatalysts in oxidation reactions. The type andquantity of surface oxides play a part in suchoxidation reactions as the conversion of sulfurdioxide to sulfur trioxide or when activatedcarbon is incorporated into dry cells in conjunc-tion with manganese dioxide for the depolari-zation process. Activated carbon serves as acatalyst in the synthesis of phosgene from car-bon monoxide and chlorine and in the synthesisof sulfuryl chloride from sulfur dioxide andchlorine. The catalytic activity of the carbon isassociated with the p electrons in a similarmanner as graphite crystals catalyze the trimer-ization of cyanogen chloride to cyanuric chlo-ride. In each case, weakening of the doublebonds in the reactants occurs. In addition, theindustrial process in which hydrogen chloride isremoved from chloroethane in the presence ofactivated carbon presumably has a similarmechanism.

By incorporation of heteroatoms the surfaceof activated carbon can be modified to achievevery high activity for redox reactions such as thedecomposition of chloramines and the oxidationof H2S or SO2. When these catalytic/adsorptivecarbons are used for removal of sulfur com-

pounds, the end product is sulfuric acid, whichcan be washed out with water.

Some types of activated carbon are used ascatalyst support for industrial syntheses of vinylacetate and vinyl chloride. Activated carbon isparticularly suitable as catalyst support because,in contrast to oxidic catalyst supports, it has noLewis acidity and therefore does not lead tounwanted polymerization of the vinyl mono-mers. Vinyl acetate is produced by vinylation ofacetic acid with acetylene in a heterogeneouscatalytic gas-phase reaction in presence of azinc acetate/activated carbon catalyst at160–240 �C. Vinyl chloride can be producedby direct catalytic addition of hydrogen chlorideto ethyne at 120–150 �C in the presence of anactivated carbon impregnated with about 10%of mercury(II) chloride is used as catalyst.

Catalysts for various purposes are obtainedby depositing noble metals on activated carbon.Awell-known example is palladium-coated ac-tivated carbon powder [7440-05-3], a typicalhydrogenation catalyst for use in suspension.

In the Merox process for oxidation ofmercaptans in mineral oil, activated carbonsimpregnated with cobalt phthalocyanine[3317-67-7] are used.

5. Regeneration and Reactivation

The use of activated carbon for gas and watercleaning in industries and environmental pro-tection is particularly economical if suitableprocesses for the regeneration of spent activatedcarbon are available. Generally for the regener-ation of the spent carbon pressure swing, ther-mal desorption, extraction, or thermal reactiva-tion processes are used (Table 10) [2–5, 44].

If the adsorbate is bound reversibly onto thesurface of the adsorbent the activation energyEdes is required for desorption only. For lowvalues of Edes (10–30 kJ/mol), as in gas separa-tion processes, desorption can be achieved bylowering the pressure in a PSA process.

For adsorptive removal and recovery of sol-vent, a higher energy of desorption (30–60 kJ/mol) is needed for desorption. Usually, super-heated steam or hot inert gas is passed throughthe carbon bed, and this also removes thedesorbed substances from the intergranularspaces.


Apart from thermal desorption, extractionwith solvents can be used to remove adsorbedsubstances from used carbons. For example, theelemental sulfur produced in the Sulfosorbonprocess can be extracted by carbon disulfide.In the literature solvents such as dimethylfor-mamide, acids, and alkali solutions aredescribed [2, 3].

The success of activated carbon in drinking-water and wastewater treatment has been due tothe development of regenerable activated car-bon types and the development of effectiveprocesses for the regeneration of the spent car-bon. Spent activated carbon used in water treat-ment usually contains many different kinds oforganic substances. A complete thermal desorp-tion of all adsorbed substances is not possible.The decomposition temperatures of highmolecular mass compounds are below the de-sorption temperature. Thus a residual load con-sisting of pyrolysis products and carbon depos-its remains on the internal surface of the acti-vated carbon. These spent activated carbons arethermally regenerated at 800–850 �C in a pro-cess that is similar to the production of activatedcarbon and hence more properly called reacti-vation. Independent of the reactor type used, thespent activated carbon passes through four mainsteps during the temperature rise to 850 �C:

� 100 �C water removal100–350 �C desorption of low-boiling organic compounds350–800 �C thermal cracking of high molecular

mass compounds800–850 �C gasification of the pyrolysis product and

carbon deposits by H2O and CO2

Investigations have shown that carbonaceousdeposits formed by thermal desorption treat-ment of the residual load show a higher reactiv-ity due to their disturbed structure than thecarbon surface of the activated carbon. Under

such conditions, the loaded activated carbon canbe selectively treated with the water gas shiftreaction at ca. 800 �C, which restores the origi-nal surface structure and adsorption perfor-mance of the activated carbon. Drying, desorp-tion, thermal cracking, and gasification can beperformed in a single industrial reactor. Alltypes of furnaces used for activation of activatedcarbon can be used for reactivation. Fluidized-bed reactors, rotary kilns, and multiple-hearthreactors are widely used.

Some methods for regenerating powderedcarbons have been proposed in the past, butthese have not yet attained widespread practicalapplication. To date, powdered activated carbonhas only been reactivated in a few cases.

6. Economic Aspects

World consumption of activated carbon wasestimated at 450 000 t in 1998, and capacityutilization of the activation plants at 70–80%.Percentage consumption (1998) by region wasestimated as United States 37%, Europe 32%,Japan 18%, rest of the world 13%. Worlddemand was forecast to rise by 2–3% per an-num. Areas of potential growth are flue gaspurification in Europe and Japan, water treat-ment in the United States, Europe, and Japan,and automotive emission control.

The regeneration of spent granular activatedcarbon (Europe: 50 000–60 000 t/a) is a grow-ing market in all areas, but increased regenera-tion will lead in a fall in the rate of growth indemand for virgin material.

Worldwide there are more than 100 activatedcarbon producers, but the ten largest producersaccount for about 70% of world capacity.About 45% of production is powdered activatedcarbon and about 55% is granular and extruded

Table 10. Regeneration of spent activated carbon

Process Basic concept Main applications

Pressure-swingadsorption

adsorption at higher pressure,desorption at low pressure

hydrogen purification, air separation (oxygen/nitrogen),methane enrichment from biogas, gasoline vapor recovery

Thermal desorption steam desorption at 100–150 �C, inert gasdesorption at 150–250 �C

solvent recovery, (toluene, alcohols, hydrocarbons, etc.)

Extraction elution of organics by solvents or alkali phenols by alkaline extraction, sulfur extraction with CS2Thermal reactivation selective gasification of carbonized residues and

carbon deposits by steam at 800–900�Cspent activated carbon from water treatmentand other applications


activated carbon (pellets). The largest produ-cers, divided by country, are given in thefollowing:

AmericaUSA Anticarb

Barnebey and Sutcliffe Corp.Calgon Carbon Corp.Norit America Inc.Westvaco Corp.

Brasil Industrias Quimicas CarbomafraMexico Clarimex SA de CV

Nobrac Mexicana SA de CVEuropeBelgium Chemviron CarbonFrance Ceca

PicaGermany A.U.G.

CarboTech Aktivkohlen GmbHNetherlands NoritUK Norit UKAsiaChina Datong Yuanghua Activated Carbon Plant

Ningxia Huahui Activated Carbon Co.Ningxia Longde Activated Carbon plantShanxi Detong Minerals BureauHuai Yu Shan Activated Carbon GroupTianjin Anfull Chemical Co. Ltd.Zhejiang Shuichang Activated Carbon Co.

India Indian Dyestuff IndustriesIndo German Carbon Ltd.

Indonesia PT. IkaindoJapan Futamura Chemical Industries

Kuraray ChemicalsMitsubishi ChemicalsSankyo SangyoTakeda Chemical Industries

Malaysia Century Chemical WorksPasific Activated Carbon

Philippines Cenapro ChemicalDavao Central ChemicalPhilippine Activated CarbonPhileppines Japan Activated Carbon

Sri Lanka Bieco-Link CarbonHaycarbTajit

Taiwan China Activated Carbon IndustriesTaiwan Active Carbon IndustriesCarbokarn Thailand

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activated carbon ullman

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