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Activated Carbon Process

DOOIL KIM

PH.D.

CENTER FOR ENVIRONMENTAL TECHNOLOGY RESEARCH

KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY

DECEMBER 1st, 2010

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Activated Carbon

Types of Activated Carbon

- PAC (Powdered Activated Carbon)

- GAC (Granular Activated Carbon)

- FAC (Fibrous Activated Carbon)

Target of adsorption process

- Natural organic matter (NOM)

- Taste and odor (MIB, Geosmin)

- Synthetic organic compounds (e.g., pesticides)

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Activated Carbon

Macro-pores (above 50 nm diameter)

Meso-pores (2-50 nm diameter)

Micro-pores (under 2 nm diameter)

Pellet type

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Adsorption Isotherm (Experimental Method)

Same initial adsorbate concentration

with different amount of adsorbentm1 m2 m3

C0 C0 C0

C0= initial adsorbate concentration (mg/L)

Ce,i= equilibrium adsorbate concentration (mg/L)

mi= mass of adsorbent (g AC)

Cc= adsorbent concentration (g AC/L)

Calculate

c

ie

ie

C

CCq

,0

,

!

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Adsorption Isotherm

Constant temperature equilibrium relationship between solid phase

concentration (qe)and liquid phase concentration (Ce)

Log Ce

Log qe

Equilibrium solutionconcentration (mg/L)

Adsorbate per unit of

adsorbent (mg / g AC)

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Langmuir Isotherm

bC1

bCqq xma

e

!

Assumption

- Physisorption of monolayer

- Every adsorption site is equivalent

- One adsorbate molecule adsorbed per site

- No interaction between adsorbed adsorbate

LangmuirIsotherm equation

Fractional coverage

xma

e

q

q

availablesitesadsorptionofNumber

occupiedsitesadsorptionofNumber!!U

Mono-layer

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Langmuir Isotherm

)1( UU

! CK

dt

da

Rate of adsorption

Rate of desorption

At equilibrium

UU

dKdt

d!

UU dea KCK ! )1(

eda

eda

ead

ea

CKK

CKK

CKK

CK

)/(1

)/(

!

!U

e

xa

qq!U

d

a

KKb !

bC1

bCqq xae

! Theoretical formula

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Freundlich Isotherm

n

ee CKq/1!

Empirical adsorption model for solid-liquid system

- most widely used for PAC adsorption isotherms

Linearized form

ee Cn

Kq log1

loglog !

Related to the strength of adsorption

(Smaller 1/n = stronger adsorption)

Related primarily to the capacity of adsorption(Bigger K = bigger capacity)

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Various Isotherm

? A1)X-c(c1X)-(1

Xc

V

V

m !!U

Isotherm Linearized form

Langmuir

Feundlich

BET

bC

bCqq ae

!

1

n

ee CKq/1!

BET: Isotherm for multi-layer adsorption (Langmuir model applies adsorption in each layer)

Vm = Volume corresponding to monolayer

X= ratio of adsorbate gas pressure to vapor pressure of pure liquidc= constant

xaxaeq

1

bCq

1

q

1!

ee Clogn

1Klogqlog !

cV

X1c

cV

1

X-1V

X !

Stephen Brunauer, Paul Hugh Emmett, Edward Teller

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PAC Application

Advantage

- Low capital cost- Added either powder or slurry

- Easy to respond to water quality change

Disadvantage

- High operating cost when high PAC doses are required for long time

- Inability to regenerate

- Low TOC removal

- Increased difficulty of sludge disposal

- Difficulty of complete removal of PAC particles from water

Factors needed to be considered

- Types of carbon (Surface area, pore size distribution etc.)

- Degree of mixing to provide good contact between PAC and contaminants

- Contact time for maximum use of carbon

- Interference by treatment chemicals

- Water composition (pH, divalent ion, etc.) - Removal of PAC

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Adsorption Kinetics

Assumptions

- Concentration gradients exist only in radial direction.- Surface diffusion is more greater than pore diffusion.

- Surface diffusion is described by Ficks law.

- Activated carbon surface is homogeneous.

- No reaction on surface.

r

q + q

qr

q

r

qs

3L

mg/gQ

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Mass balance on PAC

rr4q 2 (T( vv

Mass accumulation = mass in mass out

- Mass accumulation =

- Mass in =

- Mass out =

tr

qDr4 s

2 ((

(T vvv

tr

qq

Drr4 s2

((

((

(T v

vv

tDr

qr

r4rr4q s

22 (((

((

T(T( v

!vv

r

r

qr

r

D

t

q

2

2

s

(((

(

(

(

!

Next page

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tr

qqDrr4t

r

qDr4 s

2

s

2 ((

(((T(((T !

Mass balance on PAC

tr

qqDrrr2r4t

r

qDr4 s

22

s

2 ((

((((T(

(

(T

!

tDrqrr2

rqrr2

rqr

rqr4r

rqDr4 s22s2 (((((((((((((T(((T

!

rr4q 2 (T(

tDr

qrr2

r

qr4 s

2 (((

(((

(T

!

tDr

qrr4

s

2

((

(

(

(T

!

r

r

qr

r

D

t

q

2

2

s

(

((

(

(

(

!

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Mass balance on PAC

xx

xx!

xx

r

qr

rrt

q 22

s

Governing equation

0)r(q !

Initial condition

t= 0 and 0 ereR

Boundary conditions

General solutions

sqq ! tu 0 and r = R

0dr

dq! tu 0 and r = 0

dd

ddv! g

!

R

0

t

0s

R

Di

i

s1

1i

tR

Di

d)(qe)1(DirdR

risin)r(qr

R

risine

Rr

2)t,r(q

2s22

2s22

PPTTT PTT

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GAC Filtration

MTZ :

MassTransfer

Zone

Influent

0 C0Saturated zone

Ce = C0,

q = (qe)0

Fresh GAC zone

Ce = 0,

q=

0

MTZ decreases as

Smaller carbon size

Higher temperature

Larger adsorbate diffusion coeff.

Greater strength of adsorption of

adsorbate

Breakthrough Point

Active

Adsorption

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rateFlow

columnofolumetimecontactbedEmpty !

GAC Filtration

EBCT

BedV

TreatedVolumeBV!

Bed Volume (BV)

Breakpoint concentration CB

Maximum acceptable concentration

Critical depth LCritical

Critical depth that leads to immediate appearance of an effluent

Concentration equal to CB when column start-up

Minimum EBCT

A/Q

LEBCT Critical

min

!

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GAC Filter Mass Balance : Liquid Phase

Area = A

x

x

QL,C0

QL

C

C+C

xAC (I( vvv

Mass accumulation = mass in mass out + mass transferred

- Mass accumulation =

- Mass in =

- Mass out =

- Mass transferred =next page

tCQtx

CEA

LL((

(

(I vvvv

tCCQtxCCEA

LL ((((((I vvvvvv

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txACCaK iL ((I

Mass transferred =

Activated

Carbon

C

Ci

qs

Bulk solution

Overall mass balance

txACCaKtCQtx

CEAxCA iLLL ((I((((

((I(I( !

GAC Filter Mass Balance

Control volume

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iLL

2

2

L CCaKx

CU

x

CE

t

C

xx

xx

!xx

I

Liquid side governing equation

Initial condition

Boundary conditions

0tat0xC !!

0x

L00x

xd

dCECC

!

!!

0xd

dC

0x

!!

x= 0

x= L

GAC Filter Mass Balance

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r

q + q

qr

q

r

qs

GAC Filter Mass Balance : Solid Phase

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Mass accumulation = mass in mass out

- Mass accumulation =

- Mass in =

- Mass out=

tr

qDr2 s ((

(T vvv

tr

qq

Drr2 s ((

((

(T v

vv

tDr

qr2rr2q s ((

((T(T( v

!vv

r

r

qr

r

D

t

q s

(((

(

((

!

GAC Filter Mass Balance

rr2q (T( vv

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xx

xx!

xx

r

qr

rrt

q s

Governing equation

0)r(q !

Initial condition

t= 0 and 0 ereR

Boundary conditions

Ciis unknown. Therefore, we need additional equation.

sqq ! tu 0 and r = R

0dr

dq! tu 0 and r = 0

GAC Filter Mass Balance

n/1

is CKq v!

Rr

ssiL

dr

dqCCaK

!

!