adsorption with granular activated carbon (gac) - · pdf filecontacted with granular activated...

PIERO M. ARMENANTENJIT

Adsorption withGranular Activated Carbon

(GAC)


Adsorption Processes UtilizingGranular Activated Carbon (GAC)

for Wastewater TreatmentIn all these processes the wastewater iscontacted with granular activated carbon (GAC)typically in a semi-batch or continuous operation.Processes that utilize this type of carbon include:

• Fixed-bed or expanded-bed adsorption

• Moving-bed adsorption

• Fluidized-bed adsorption


Fixed-Bed and Expanded-BedAdsorption SystemsWastewater

in

Wastewaterout

Wastewaterout

Wastewaterin

Fixed-Bed Expanded-Bed


Modes of Operation of Fixed-Bed andExpanded-Bed Systems

• Downflow → Fixed-bed

• Upflow

- Expanded-bed (if the wastewater velocityexpands the bed by about 10% or higher)

- Fixed-bed


Downflow Fixed-Bed Adsorbers• This is the most common type of adsorption

column for wastewater treatment

• These columns must be provided with a system forthe removal of spent carbon and the addition offresh or regenerated carbon

• Because or their construction and operationdownflow fixed-bed adsorbers also acts as depthfilters for particles that can be contained in thewastewater

• Therefore this adsorption column must also beprovided with facilities for backwashing (includingair scouring, if necessary)


Typical GAC Contactor

After Metcalf and Eddy, Wastewater Engineering, 1991, p. 316


Characteristics of Commercial AdsorbersHeight of packing 3 - 9 m (10 - 30 ft)

Particle size 8 - 40 mesh

Hydraulic loading 1.4 - 6.8 L/m2 s (2 -10 gpm/ft2)

Residence time 10 - 60 min (typically 20 -30 min)

Typical carbon requirements

- pretreatment - tertiary treatment

(in g carbon/m3 wastewater)60 - 20025 - 50

Operating pressure < 20 KPa/m of bedAfter Sundstrom and Klei, Wastewater Treatment, 1979, p. 270 and

Metcalf and Eddy, Wastewater Engineering, 1991, p. 753


Properties of Commercially AvailableCarbons

After Eckenfelder, Industrial Water Pollution Control, 1989, p. 268


Downflow Fixed-Bed Adsorbers inSeries

Wastewater in


Downflow Fixed-Bed Adsorbers inParallel

Wastewater in


Granular Activated Carbon Process Flow Diagram

After Corbitt, R. A. 1990, The Standard Handbook of Environmental Engineering,p. 6.199

Results of AdsorptionTests on Typical

Industrial Wastewaters


Upflow Expanded-Bed Adsorbers• In general, most upflow adsorbing columns

operate in the expanded-bed mode

• Expanded-bed adsorbers are used when thewastewater fed to the column contains asignificant fraction of suspended particles

• Since the bed is always expanded the columndoes not act as a filter for the suspendedparticles


Upflow Fixed-Bed Adsorbers inSeries

Wastewater in


Moving-Bed AdsorptionCarbon in

Carbon out

Wastewaterout

Wastewaterin


Fluidized-Bed Adsorption

Wastewaterout

Wastewaterin


Adsorption Beds as Filter Beds andHeavy Metal Adsorbers

• GAC beds are sometimes used as deep-bedfilters (as well as adsorbers)

• the capital cost associated with dual purposeadsorber-filter beds is typically lower thanseparate beds

• the regeneration of the adsorptive capacity ofthe bed should be followed by the removal ofsolids via bed fluidization

• most heavy metals are also adsorbed onactivated carbon beds


Biological Reactions in Adsorption BedsThe presence of organic material in a typical activatedcarbon bed coupled with the presence ofmicroorganisms in the wastewater makes the bed anideal breeding ground. This can have both negativeand positive effects:

• the microorganisms may contribute to thebreakdown of pollutants adsorbed on the bed, thusimproving its performance

• the presence of excessive organic material and thetypical lack of oxygen may result in anaerobicgrowth (associate with the potential for odorgeneration) and the plugging of the bed due toexcessive growth


Most Important Design Factors inFixed-Bed Adsorption Systems

• Particle size

• Diameter of column

• Flow rate of incoming wastewater (orresidence time)

• Height of adsorption bed

• Pressure drop

• Time required to achieve breakthrough

• (Time of exhaustion)


Analysis of Fixed-BedAdsorption Systems


Steps Involved in AdsorptionAs the wastewater moves through a fixed bed ofcarbon the pollutant to be adsorbed will move fromthe wastewater to the carbon bed. Several steps areinvolved in the overall adsorption process of asingle molecule of pollutant:

• Mass transfer step. Mass transfer from the bulkof the wastewater to the surface of the carbonparticle through the boundary layer around theparticle

• Diffusion step. Internal diffusion through a pore

• Adsorption step. Adsorption on to the surface ofthe particle


Relative Magnitude of the RatesInvolved in Adsorption Process

In most wastewater treatment applications theoverall adsorption process is dominated by masstransfer, especially intraparticle mass transfer. Aqualitative ranking of the magnitude of theresistances is:

• External interparticle mass transfer step →slow to not-so-slow, depending on the operation

• Intraparticle diffusion step → typically slow

• Adsorption step → typically fast


External InterparticleMass Transfer Film

IntraparticleDiffusion

Adsorption

LiquidBulk

Carbon Particle

Pore


Effect of Carbon Particle Size onPressure Drop and Intraparticle Mass

TransferThe size of the activated carbon particle has anopposite impact on the pressure drop across thebed and the intraparticle diffusion resistance (andhence on the overall adsorption process)


Effect of Carbon Particle Size onPressure Drop

The effect of particle size, Dp, on pressure drop,∆P, can be determined recalling equations suchas the Ergun Equation for pressure drop ingranular media:

( )∆PL

Du

p pL s= − +

−

1501 175

13

2

Re.ε ε

ερ

or the Fair-Hatch equation (laminar flow):

( )∆P k

LD

up

s= −36

1 2

3 2µ εε


Effect of Carbon Particle Size onPressure Drop

The pressure drop in a carbon bed is inverselyproportional to the particle size. In particular it is:

∆PDp

∝ 1

for turbulent flow through granular media, and:

∆PDp

∝ 12

for laminar flow in granular media.


Effect of Carbon Particle Size onIntraparticle Mass Transfer

As a first approximation the effect of particle size onthe intraparticle mass transfer of pollutant can beestimated as follows:pollutant transferred inside the particle

bed volume

pollutant transferred inside the particlebed volume

≈− ≈

≈− ≈ −−

∝

D dCd r

AV

D Cr D

D CD D

D

p

p

p p p

p

∆∆

6 00

12

12


Effect of Carbon Particle Size onPressure Drop and Diffusion Resistance

Larger carbonparticle size

↓

Smaller carbonparticle size

↓Smaller pressure

drop↓

Larger pressure drop

↓Smaller mass of

pollutant diffusinginside the particle

Larger mass ofpollutant diffusinginside the particle


Size of Activated Carbon Particles Usedin Fixed-Bed Adsorption

• Typically carbon particle sizes between 0.4 and2.5 mm are used in fixed bed adsorptionapplications

• This size range results from a practicalcompromise between limiting the pressuredrops on one hand and providing adequatesurface area and promote mass transfer forpollutant adsorption on the other

• Larger sizes also minimize losses duringcarbon handling and packed bed operation


Adsorption Zone and Adsorption Wave• In fixed bed adsorption, at any given time the

bed can be divided into three approximatezones, i.e., the saturated zone(containing carbon nearlysaturated with the pollutants),followed by the adsorptionzone (were adsorption actuallytakes place), followed by azone in which the carbon contains little or noadsorbed pollutant

• The size and location of the three zones withinthe bed change with time

AdsorptionZone

SaturatedZone


Adsorption Zone and Adsorption Wave(continued)

• As the wastewater enters the bed it firstencounters the saturated zone in which thecarbon is already nearly saturated with thepollutant (this is not true for fresh completely“clean” beds just being put on line, of course).Practically no adsorption occurs in thesaturated zone

• As more wastewater travels through the bedthe saturated zone expands progressivelythrough the bed, eventually including itcompletely



• Pollutant adsorption occurs nearly exclusivelyover a portion of the bed called the adsorptionzone, downstream of the saturated zone

• The concentration of pollutant in the carbonvaries from near saturation (at the beginning ofthe adsorption zone) to near zero (toward theend of the adsorption zone). Conversely, thepollutant concentration in the wastewater incontact (at that time) with the carbon changesfrom nearly full load (at the beginning of theadsorption zone) to nearly zero (at the end)



• At any given time the portion of the beddownstream of the adsorption zone containsvery little adsorbed pollutant since thewastewater it is in contact with has alreadybeen nearly completely depleted of thepollutant(s)

• As time goes by a greater portion of the bedbecomes saturated with the pollutant and theadsorption zone moves downstream formingan adsorption wave


Breakpoint and Breakthrough Curve• Eventually the forward part of the adsorption

wave reaches the end of the bed. When thishappens the bed begins to release wastewaterhaving a concentration of pollutant higher thanthe desired value (typically 5-10% of theinfluent concentration). This point is called thebreakpoint

• The corresponding curve of pollutantconcentration in the effluent vs. time is calledthe breakthrough curve


Exhaustion Point and Bed Saturation• Past the breakpoint the pollutant concentration

in the effluent rises rapidly (i.e., thebreakthrough curve is typically steep), until itreaches an arbitrarily defined exhaustion pointwhere the column approaches saturation

• If more wastewater is passed through the bedthe entire carbon content of the bed becomessaturated. Then, the wastewater leaving thebed has, for all practical purposes, the sameconcentration of pollutant as the incomingwastewater


Breakpoint and Breakthrough Curve

Cumulative Wastewater Volume

C

AdsorptionZone

C CC C

Co Co Co Co

B E

Breakpoint

BreakthroughCurve

Exhaustion Point


Concentration of Adsorbate in theCarbon Along a Fixed-Bed Column

Position Along Fixed-Bed Column

q

AdsorptionZone

C CC C

Co Co Co Co

B E

1 2 3 4

1 2 3 4


Evolution of Concentration Profiles in in the Wastewater Leavingthe Column and in the Carbon Bed as a Function of Time

Cumulative Wastewater Volume

C

Breakpoint

BreakthroughCurve

Exhaustion Point

t1 t2 t3t4

Position Along Fixed-Bed Column

q

t1 t2 t3 t4


Progressive Saturation of Adsorber inColumn as a Function of Time

Position Along the Column

q q q q

tF t tB tE


Typical Breakthrough Curves

After Eckenfelder, Industrial Water Pollution Control, 1989, p. 272


Shapes of Breakthrough Curves

V

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

C/C

o

VB VE

Short Depth of Adsorption Zone

V

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

C/C

o

VB VE

Large Depth of Adsorption Zone


Analysis of Fixed-Bed AdsorptionColumns

The primary objectives of such an analysis are:

• determination of the total (maximum) columnadsorption capacity;

• determination of the depth of the adsorptionzone and the shape of the breakthrough curve;

• determination of the breakpoint, including thevolume of wastewater that can be treatedbefore the breakpoint is reached, the time atwhich this happens, and the degree of columnsaturation at breakpoint.


Total Column Adsorption CapacityIf the adsorption equilibrium curve is known thenby knowing the volume of the column and its voidfraction, one can calculate the total cumulativevolume of wastewater, Vmax, that could be treatedif the column became completely saturated:

( )V SLqC

SLqCs

So

osapp

So

omax = − =1 ε ρ ρ

where: S = column cross sectional areaL = height of packingε = void fractionqSo = g(Co) = value of q in equilibrium with Co

ρs and ρs app = real and apparent density of solid


Approaches to the Design ofAdsorption Columns

Many different approaches are available. In general,partial differential equations can be written,incorporating the different mass transfer andadsorption mechanisms. Typically these models arecomplex and require numerical solutions.

Other models rely on experimental data and the use ofsimpler models to interpret them so as to producesatisfactory designs. These models can be used tosize the column by determining the depth of theadsorption zone, the shape of the breakthrough curve,the time at breakpoint and the pollutant removed atbreakpoint.


Models Examined HereThree models will be examined here in somedetail:

• Simplified Method for Estimation of Fixed-BedAdsorption Performance

• Design of Adsorption Columns Based onCapacity of Adsorption Zone (Mass TransferModel)

• Design of Adsorption Columns Based on BedDepth Service Time (BDST) Method (SurfaceReaction Model)


Simplified Method for the Estimationof Fixed-Bed Adsorption Performance

Simplifying assumptions:

• the pollutant concentration in the effluentwastewater from the column increases linearlywith time until it reaches the breakpoint value,CB

• at breakpoint the average concentration ofpollutant in the bed is only a fraction, ζ , of thesaturation value (typically 50%)

• the wastewater flow rate to the column isconstant and equal to Q



From a mass balance for the pollutant atbreakpoint it is:

M q B q B Qt CC

Qt CB so B oB

B o= = = −

≅ζ

2

where:M = cumulative mass of pollutant adsorbed atbreakpointB = mass of carbon in bed = ρs app S L, and:

q K Cso F on= 1



Hence, the time required to reach breakthroughis:

tq B

Q CC

K C B

Q CCB

B

oB

F on

oB

=−

=−

2 2

1ζ

The cumulative volume of wastewater, VB, treatedat breakpoint is given by:

V Q tB B=


Design of Adsorption Columns Basedon Capacity of Adsorption Zone

(Mass Transfer Model)This model assumes that the adsorption zone hasa constant shape that moves down the columnwith time and that the process is controlled by themass transfer around the carbon pellets.


DefinitionsL = height of packed bedtF = time required for the adsorption zone to form

tL = time required for the fully formed adsorptionzone to move down the length of the column, L,until the effluent concentration is equal to CE

tE = time required for the effluent concentration toreach the exhaustion point, CE

tA = time, required for the adsorption zone tomove its own height down the column


Time Required to Exhaust the BedThe time, tE, required for the effluent concentration toreach the exhaustion point, CE, is equal to the sum of:

• the time, tF, required for the adsorption zone toform

• the time, tL, required for the fully formedadsorption zone to move down the length of thecolumn, L, until the effluent concentration is equalto CE

Then, it is:t t tE F L= +

i.e.: t t tL E F= −


Time Required to Exhaust the BedThe time, tE, required for the effluentconcentration to reach the exhaustion point, CE, isalso equal to:

t VQ

V SQ SE

E E= =

where:

VE = cumulative wastewater volume passedthrough the column during the time interval0 - tE

Q = wastewater flow rate


Time Required for Adsorption Zoneto Move Its Own Height

The time, tA, required for the adsorption zone tomove its own height down the column is:

t V VQA

E B≅ −

where:

VB = cumulative wastewater volume passedthrough the column during the time interval0 - tB

tB = time required for the effluent concentration toreach the breakpoint, CB


Adsorption Wave VelocityThe velocity, uA, of the adsorption zone (equal tothe wave velocity, uw) is given by:

u Lt

Lt t

u LtW

L E FA

A

A

= =−

= =

which can be rearranged to give:

LL

tt

tt t

A A

L

A

E F

= =−

where:

L = height of column

LA = height of adsorption zone


Fractional Pollutant Removed in theAdsorption Zone

If additional wastewater is passed through thecolumn after the effluent concentration hasreached the breakpoint, CB, more pollutant will beremoved until the effluent concentration becomesequal to CE. This extra amount of pollutant is:

( )m C C dVoV

V

B

E= −∫This amount is only a fraction of that which couldbe removed if the adsorption zone was not at theend of the column, in which case it would be:

( )m C V Vs o E B= −



The fractional pollutant removal capacity at theend of the column, f, is then:

( )( )f

mm

C C dV

C V Vs

oV

V

o E B

B

E

= =−

−∫

The fractional pollutant removal capacity, f, canbe determined experimentally by monitoring theeffluent concentration (after the breakpoint) as afunction of the effluent cumulative volume.



fmm

CC

dV VV Vs o

B

E B

= = −

−

−

⌠⌡ 1

0

1

0 0.2 0.4 0.6 0.8 1(V-VB)/(VE-VB)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

C/C

o

C /CB o

C /CE o

ExperimentalPoints


Pollutant Removed in the AdsorptionZone: Large f Value (f → 1)

CE/Co

CB/Co

ExperimentalPoints

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

(V-VB)/(VE-VB)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1C

/Co

f = Gray Area


Pollutant Removed in the AdsorptionZone: Large f Value (f→ 1)

• If f→ 1, then the adsorption zone at the end of thecolumn already contains a significant amount of theadsorbate and has the potential for adsorbing verylittle extra pollutant.

• This implies that, in general, the wastewatersaturates the carbon “layer by layer” and that thetransition zone from the fully saturated carbon zoneto the unsaturated zone is short. This typicallyoccurs when the wastewater velocity is “low”.

• This also means that the adsorption zone is quiteshort and the time required for its formation is closeto zero.


Pollutant Removed in the AdsorptionZone: Small f Value (f→ 0)

0 0.2 0.4 0.6 0.8 1

(V-VB)/(VE-VB)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1C

/Co

f = Gray Area

ExperimentalPoints

CB/Co

CE/Co


Pollutant Removed in the AdsorptionZone: Small f Value (f→ 0)

• If f→ 0, then the adsorption zone at the end of thecolumn contains little adsorbate in the carbon. Hence ithas the potential for adsorbing nearly as much as asimilar zone made of fresh carbon.

• This implies that, in general, the wastewater does notsaturate the carbon “layer by layer,” but that thetransition zone from the fully saturated carbon zone tothe unsaturated zone is gradual. This typically occurswhen the wastewater velocity is “high”.

• This also means that the adsorption zone is quite deepand the time required for its formation is “large” andnearly equal to that needed for the adsorption zone tomove a distance equal to its depth.


Time Required for the Formation ofthe Adsorption Zone

• As the wastewater first enters a column containing fresh(or regenerated) activated carbon the adsorption zone isnot established. The time required for the formation ofthe adsorption zone can be estimated from:

( )t f tF A= −1

• When f→ 1, the length of the adsorption zone is quitesmall and the time for the formation of the adsorptionzone, tF, is nearly zero (“layer-by-layer” buildup);

• When f→ 0, the length of the adsorption zone is quiteextended and the time for the formation of theadsorption zone, tF, is significant.


Height of Adsorption ZoneRecalling that it is:

LL

tt

tt t

A A

L

A

E F

= =−

it is possible now to eliminate tF from theexpression of the LA/L ratio to get:

( )LL

tt

tt f t

A A

L

A

E A

= = − −1

which can also be expressed in terms of thecorresponding cumulative volumes as:

( )( )LL

V VV f V V

A E B

E E B

= −− − −1


Height of Adsorption ZoneThe equation:

( )( ) ( )LL

V VV f V V

VS

VS

VS

fVS

VS

A E B

E E B

E B

E E B'= −

− − −=

−

− − −

1 1

can be use to experimentally determine the heightof the adsorption zone, LA, from experimental labdata (since only the ratios Vi/S are important).The experiments should be conducted for thesame value of the ratio Q/S. Then the value of LAcan be determined provided the height of the testcolumn (L') is known.


Interpretation of Experimental Data• A common way to collect information on the

performance of an adsorption column and toscale up the results consists insetting up a column as high asthat eventually to be used inthe full scale application, butof much smaller diameter

• Data are then collected bydetermining the time atbreakpoint, tB, at differentpoints (ports) along thecolumn

Port

Port


Experimental Determination ofthe f Value

The f value can be determined experimentally byobtaining C vs. V data as the adsorption zonepasses through the sampling port. Since VB andVE can also be determined with this approachthen f can be calculated from its definition:

( )( )f

mm

C C dV

C V Vs

oV

V

o E B

B

E

= =−

−∫


Degree of Column Saturation atBreakpoint

The degree of column saturation at breakpoint, ξ,is defined as the fraction of column saturation atbreakpoint, i.e.:

ξ = Amount of pollutant in column at breakpointMaximum theoretical amount of pollutant in column

The value of ξ is within the range 0 - 1, and isproportional to the degree of utilization of theactivated carbon packing at breakpoint, i.e.,before the column must be shut down for carbonreplacement or regeneration.


Degree of Column Saturation atBreakpoint (continued)

SaturatedZone

C C

Co Co

B o

Column atbreakpoint

Saturated column

AdsorptionZone

( ) ( ) ( ) ( )( )

( ) ( )

ξρ ε ρ ε

ρ ερ ρ

ρ

= =− − + − −

−=

=− + −

MM

S L L q SL f qSL q

S L L q SL f qSL q

S

A s So A s So

s So

A s app So A s app So

s app So

1 1 11

1


Degree of Column Saturation atBreakpoint (continued)

By simplification one can obtain the followingequation for ξ:

( ) ( )ξ = =

− + −MM

L L L fLS

A A 1

i.e.:

ξ = = −MM

L f LLS

A

If f and LA have been determined ξ can becalculated for any column of length L.


Calculation of Cumulative PollutantRemoval at Breakpoint

Once ξ is known it is possible to determine thecumulative amount of pollutant, M, removed bythe time the breakpoint is reached (and thecolumn must be taken off line):

M M q SL K C SLS So sapp F o sappn= = =ξ ξ ρ ξ ρ1

where Co is the pollutant concentration in theincoming wastewater.


Calculation of Breakpoint TimeThe breakpoint time, tB, can be calculated as:

tB = cumulative amount of pollutant adsorbed at breakpointrate of pollutant fed to column

i.e.:

tM

QCBo

=

at which time the column must be taken off line.

The cumulative volume of wastewater, VB, treatedat breakpoint is given by:

V Q tB B=


Alternative Determination of theAdsorption Zone Profile

An alternative method to determine the profile andlength of the adsorption zone (and hence the fvalue and LA) is to model the adsorption zone as ifit were an independent column operating atsteady state and having a continuous input ofboth solid activated carbon and wastewater.

This approach is equivalent to assume that theframe of reference moves with the adsorptionwave.


Alternative Determination of theAdsorption Zone Profile

Co

CE

CB

C=0

CE

CB qB

qE

L LALA

dZ

Q

Qq

qso

qE

qB

q=0


Alternative Determination of theAdsorption Zone Profile: Mass Balance

• In performing a mass balance for such a continuouscolumn the assumption is made that the incomingand outgoing streams are nearly in equilibrium (ifthey were really in equilibrium the column will beinfinitely high).

• This implies that the incoming wastewater (pollutantconcentration: Co) encounters a carbon which ispractically saturated with the pollutant at thatconcentration (qSo).

• Similarly the wastewater leaving the column containspractically no pollutant, and so does the freshincoming carbon.



A material balance around the entire continuouscolumn gives:

( ) ( )Q C Q qo q So− ≅ −0 0

where:Q = wastewater flow rate

QP = carbon flow rate

Co = pollutant concentration in incomingwastewater

qSo = pollutant concentration in spent carbon



Similarly, a material balance between the bottomof the column and a generic section gives:

( ) ( )Q C Q qq− ≅ −0 0

Hence, the operating line (mass balance) for thecontinuous column is:

Cq

Cq

QQ

o

So

q= =

Remark: the points (CE, qE) and (CB, qB) must lieon the operating line.



q (g solute/g carbon)

C (g

/L)

Isotherm

Operating Line

Qq/Q

qB qE qSo

CB

CE

Co

CC*

q


Alternative Determination of theAdsorption Zone Profile: Mass Balancein a Differential Section of the Column

Assuming that external (or external-internal) masstransfer dominates the overall adsorptionphenomenon a differential balance for thepollutant in an infinitesimally thin layer of thecolumn yields:

Rate of pollutant removal from the wastewater = Rate of pollutant transfer to the carbon


Alternative Determination of the AdsorptionZone Profile: Mass Balance in a Differential

Section of the ColumnIn mathematical terms the previous equation can bewritten as:

( )QS

dC u dC K a C C dzs L= = − *

KL = mass transfer coefficient between the bulk of thewastewater and the surface of the carbon (m/s)

a = external surface area of carbon particles per unitbed volume

C* = concentration of pollutant in the wastewater thatwould be in equilibrium with the adsorbedconcentration, q


Alternative Determination of theAdsorption Zone Profile: Integration

of Mass Balance EquationIntegration of the previous equation yields:

zu

K adC

C CC C Cs

L C

C

B EB

= −⌠⌡ ≤ ≤'

' '*for

In addition it is:

Lu

K adC

C CAs

L C

C

B

E

= −⌠⌡

'' '*


Alternative Determination of theAdsorption Zone Profile: Calculation of f

From the previous equations it is:

zL

V VV V

dCC C

dCC C

z L V V VA

B

E B

C

C

C

C A B EB

B

E= −

− =−

⌠⌡

−⌠⌡

< < < <

'' '*

'' '*

with and0

From this equation and recalling that:

fmm

CC

dV VV Vs o

B

E B

= = −

−

−

⌠⌡ 1

0

1

the value of f can be calculated knowing only CB,CE and the adsorption isotherm.


Design of Adsorption Columns Basedon Bed Depth Service Time (BDST)Method (Surface Reaction Model)

This method is based on the used of experimentaldata from columns of different depths (or a singlecolumn with ports) and the determination of thebreakthrough curves as a function of time. Thesedata provide information on the service time foreach bed-depth (from which the name).Equations can be used to analyze these data toproduce design relationships.

This method is an extension of a surface reactionmodel derived by a number of investigators.


Breakpoint Time vs. Column LengthThe breakpoint time for an adsorption column is:

( )t t t tB F L A= + −

where:

tF = time required for the adsorption zone to form

tL = time required for the adsorption zone to traveldown the column

tA = time required for the adsorption zone to travelits own height

(compare this with definition of tE)


Breakpoint Time vs. Column Length(continued)

Recalling that uW is the velocity of the adsorptionwave (and hence of the adsorption zone, uA) theprevious equation becomes:

t t Lu

tB FW

A= + −

Recalling that ( )t f tF A= −1 then it is:

( )t f tL

utB A

WA= − + −1



By rearranging the previous equation one finds:

tu

Lf LuB

W

A

W

= −1

which implies that the breakpoint time increaseslinearly with the length of the column, L.

This also means that a plot of tB vs. L (for exampleusing an experimental column in which L is theheight of the different ports) should give astraight line.



The rate of advancement of the saturated zone inthe column is equal to the wave velocity, uW. As aresult, each layer of upstream of the adsorptionzone will become progressively saturated as theadsorption zone moves. Hence a mass balancefor the pollutant gives:

QC q Suo so s app W= ρ

pollutant removed from wastewater = pollutant adsorbed



The previous equation provides us with anexpression for uW:

uQS

CqW

o

s app so

= ρ

which can be used in the equation for tB to give:

tq

u CL

q fu C

LBs app so

S o

s app so

S oA= −

ρ ρ

since uS = Q/S.



Finally, experimental data from test columns canbe interpreted using this equation:

tq

u CL

qu C

f LBapp so

S o

app so

S oA= −

ρ ρ

t LB = −α βA plot of the breakpoint times, tB’s at differentcolumn heights, L’s, should give a straight line.

Remark: The parameter (ρs app qso) is more reliablyobtained from experiments than equilibrium data.


Bohart and Adams EquationUsing a completely different approach Bohart andAdams (1920) proposed an equation very similarto the previous one:

tq

u CL

k CCCB

s app so

S o r o

o

B

= − −

ρ 11ln

which can also be used to interpret theexperimental data from test columns.


Interpretation of Experimental Datafrom Test Columns of Different Lengths

0 10 20 30 40 50t (days)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

C/C

o Port 11.5 m

Port 23 m

Port 34.5 m

tB tE

Port 1

Port 2

1.5 m

Port 3



Example of experimental data:

Height ofColumn (m)

tB(days)

tE(days)

1.5 6.3 18.5

3 18.1 31.5

4.5 26.3 41.1

Port 1

Port 2

1.5 m

Port 3



0 1 2 3 4 5

Column Height, L, (m)

-10

0

10

20

30

40

50

t B o

r t E

(day

s)

BreakpointExhaustion Point

Pollutant Concentration in Wastewater at Breakpoint = 10% Co Pollutant Concentration in Wastewater at Exhaustion Point = 90% Co

(ρs app qso)/Co us

Approximate Height of Adsorption Zone


Design Method for BDST Method• Obtain experimental tB vs. L data at breakpoint

for columns of different depths. Possibly (butnot necessarily) use same flow velocity (uS) inthe experimental and in the full scale column;

• Interpret the data using the expression fortB=f(L) or the Bohart and Adams equation andobtain equation parameters (ρs app qso andintercept);

• Determine service time (tB) for the a full scalecolumn of a fixed depth (or the column depthfor a given time).


Determination of Moving-BedAdsorption Performance

The same equations that were used above for thealternative determination of the adsorption zoneprofile can be used also for sizing anddetermining the pollutant removal efficiency of acontinuous moving bed adsorber in which carbonis continuously added to the column.


Monitoring of Adsorption Columns toDetect Breakpoint

By monitoring the pollutant concentration in thewastewater at different sampling ports along thecolumn the progression of the adsorption wavecan be monitored and the breakpoint anticipated


Regeneration of Carbon BedsDepending on the type of pollutant adsorbed onthe bed spent activated carbon beds can beregenerated using two different methods, i.e.:

• chemical regeneration

• thermal regeneration

Thermal regeneration is by far the most commonmethod.


Chemical Regeneration• Chemical regeneration relies on the use of

chemicals to produce dissolution or oxidationof the adsorbate

• Chemical regeneration agents that have beenused include:

- NaOH - NaOCl

- HCl - H2O2

- CCl4• Chemical regeneration is only partially

effective and is not as commonly used asthermal regeneration


Thermal Regeneration• Thermal regeneration consists of processing

the spent carbon at high temperature tovolatilize and/or burn the adsorbed materials.

• Thermal regeneration typically involves someloss of carbon (typically between 5 and 10%)and some changes in the adsorptioncharacteristics of the carbon itself (mainlycaused by the reactivation process producinglarger pores than those present in the originalmaterial).

• Thermal regeneration can be conducted onsite or, more often, off-site.


Thermal Regeneration• Thermal regeneration is typically conducted in

a furnace (e.g., rotary hearth, fluidized bed).

• In those cases in which only or mainly volatilecompounds have been adsorbed on the bedsteam stripping can be effectively used attemperature high enough to producevolatilization of the adsorbate, but low enoughto minimize degradation of the activatedcarbon. The stripped volatile organics canthen be condensed separately.

• If non-volatile compounds are also presentthey will stay in the carbon and accumulate.


Thermal RegenerationThermal regeneration typically includes thefollowing steps:

• Drying at 100 to 150 oC (212 to 300 oF) toremove the water

• Volatilization of the light organic compoundsat 150 to 315 oC (300 to 600 oF)

• Pyrolysis of the heavier organic compounds attemperatures as high as 800 oC (1500 oF)

• Re-activation of the carbon at temperatures ashigh as 1040 oC (1900 oF) in the presence of anoxidizer (steam, carbon monoxide, or oxygen)


Additional Information and Examples on AdsorptionAdditional information and examples can be foundin the following references:

• Corbitt, R. A. 1990, The Standard Handbook ofEnvironmental Engineering, McGraw-Hill, NewYork, pp. 5.140-5.146; 6.195-6.202.

• Droste, R. L., 1997, Theory and Practice of Waterand Wastewater Treatment, John Wiley & Sons,New York, pp. 484-507.

• Eckenfelder, W. W., Jr., 1989, Industrial WaterPollution Control, McGraw-Hill, New York, pp.263-284.


Additional Information and Examples on Adsorption• Freeman, H. M. (ed.), 1989, Standard Handbook

of Hazardous Waste Treatment and Disposal,McGraw-Hill, New York, pp. 6.3-6.21.

• LaGrega, M. D., Buckingham, P. L., Evans, J. C.,1994, Hazardous Waste Management, McGraw-Hill, New York, pp. 476-486.

• Metcalf & Eddy, 1991, Wastewater Engineering:Treatment, Disposal, and Reuse, McGraw-Hill,New York, pp. 314-324; 751-755.

• Sundstrom, D. W. and Klei, H. E., 1979,Wastewater Treatment, Prentice Hall, EnglewoodCliffs, NJ, pp. 241-273.


Additional Information and Examples on Adsorption

• Treybal, R. E., 1968, Mass Transfer Operation,McGraw-Hill, New York, pp. 490-568.

• Weber, W. J., Jr., 1972, Physicochemical Processfor Water Quality Control, Wiley-Interscience,John Wiley & Sons, New York, pp. 199-259.

• Wentz, C. W., 1995, Hazardous WasteManagement, Second Edition, McGraw-Hill, NewYork, pp. 201-202.

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