29 - adsorption design

ADSORPTION DESIGN

PARALLEL BED ADSORPTION PROCESS

ADSORBING BED

REGENERATING BED

CONTAMINATEDFLUID

CLEAN FLUID

REGENERATIONFLUID

CONCENTRATION PROFILES

PROFILE CHANGES IN THE BED AS A FUNCTION OF TIME AND POSITION (SEE FIG. 12.3-1)


FLUID CONTAMINANT CONCENTRATION AT THE BED DISCHARGE IS INITIALLY NEAR THE LOW LEVEL EQUILIBRIUM VALUE, BASED ON REGENERATED BED CONCENTRATIONS.

BOUNDARY LAYER IN THE BED (MASS TRANSFER ZONE IN FIGURE) RANGES FROM A HIGH EQUILIBRIUM VALUE AT POINTS WHERE THE BED IS SATURATED TO LOW VALUES WHERE THE BED IS AT REGENERATION CONCENTRATIONS


WHEN THE DISCHARGE CONCENTRATION EXCEEDS AN ACCEPTABLE LIMIT, c/co = cb/co, THEN THE BEDS ARE SWITCHED

IF BED BECAME COMPLETELY SATURATED, THEN THE EXIT CONCENTRATION WOULD EQUAL THE INLET CONCENTRATION, c/co = 1.0

BREAKTHROUGH POINT

CONCENTRATION PROFILE MAY BE PLOTTED AS EITHER c/co VS. BED LENGTH OR c/co VS. TIME– PLOTTED AGAINST TIME, THE AREA

BEHIND THE CURVE REPRESENTS THE BED CAPACITY

– MAXIMUM CAPACITY OCCURS AT A TIME GREATER THAN THAT REQUIED TO SATURATE THE BED

).( 131210 0

dtc

ctt

USEFUL CAPACITY

OCCURS WHEN THE DISCHARGE CONCENTRATION REACHES THE BREAKTHROUGH VALUE:

).( 231210 0

bt

t dtc

ct

UNUSED BEDUNUSED BED

LENGTH OF UNUSED BED DEPENDS UPON THE SHAPE OF THE BREAKTHROUGH CURVE

.BREAKTHROUGH TIME CAN BE CALCULATED IN TERMS OF THE LENGTH OF UNUSED BED

NARROW PROFILE WIDE PROFILE

LENGTH OF UNUSED BEDLENGTH OF UNUSED BED

BED LENGTH USED AT BREAK BED LENGTH USED AT BREAK POINT:POINT:

).(

)&.(

5312

1

433121

0 0

BUNBT

t

u

t

uUNBT

t

uB

HHH

dtc

ct

t

tHH

t

tH

b

COLUMN CAPACITYCOLUMN CAPACITY

CAPACITY OF ADSORBENT COLUMN– CALCULATION OF BED CAPACITY IS

BASED ON EXPERIMENTAL ADSORPTION DATA.

– FEED IS EXPRESSED AS A FLUX, WITH c0 IN mol/vol, u0 IN VELOCITY:

AA MWcuF 00

LOADING AT SATURATIONLOADING AT SATURATION

tt IS DETERMINED USING EXPERIMENTAL DATA AND (12.3-1)

.CAPACITY OF THE BED, Wsat, IS THEN CALCULATED USING THE BULK DENSITY OF THE ADSORBENT, ρbulk

LENGTHBEDLL

tFW

bulk

tAsat

BREAKTHROUGH LOADINGBREAKTHROUGH LOADING

AT BREAKTHROUGH THE AT BREAKTHROUGH THE LOADING IS WLOADING IS Wbb::

FRACTION LOADING IS:FRACTION LOADING IS:

LENGTHBEDLL

tFW

bulk

bAb

sat

bLOADING W

Wfraction

TYPICAL EXPERIMENTAL TYPICAL EXPERIMENTAL DATEDATE

Serpa, A. L.; Schneider, I. A. H.; Rubio, J., Adsorption onto Fluidized Powdered Activated Carbon Flocs-PACF Environ. Sci. Technol.; (Article); 2005; 39(3); 885-888

DESIGN FACTORSDESIGN FACTORS

TEMPERATURE EFFECTS DURING PILOT OPERATIONS– ADSORPTION IS EXOTHERMIC AND

SOLUBILITY IS AFFECTED BY TEMPERATURE

SCALE-UP IS PRACTICAL WHEN A LARGE CROSS-SECTION BED IS USED SO HEAT TRANSFER TO TO THE WALL REFLECTS FULL-SCALE CONDITIONS

ADSORPTION RATE EQUATIONS

DIFFERENTIAL MASS BALANCE RATE OF ACCUMULATION IN THE

PARTICLES AND FLUID IN VOIDS EQUALS CHANGE BETWEEN INPUT & OUTPUT FLOWS:

L

uo, co

dL

1 - 1 -

uo, c uo, c + dc

RATE OF MASS TRANSFERRATE OF MASS TRANSFER

FOR DIFFERENTIAL SECTION FOR DIFFERENTIAL SECTION WHERE WHERE εε = BED EXTERNAL VOID = BED EXTERNAL VOID FRACTIONFRACTION::

L

cu

t

W

t

cOR

dccucut

WdL

t

cdL

p

p

0

00

1

1

)(

OVERALL COEFFICIENTOVERALL COEFFICIENT

USING OVERALL VOLUMETRIC USING OVERALL VOLUMETRIC COEFICIENT WITH INTERNAL & COEFICIENT WITH INTERNAL & EXTERNAL DIFFUSION:EXTERNAL DIFFUSION:

eff

p

EXTcINTcEXTcc

cp

D

D

kkkK

SPHERESFOR

WONBASEDISc

ccaKt

W

10

1111

1

,,,

:

*

*)(

SOLUTIONS TO EQUATIONSSOLUTIONS TO EQUATIONS

DEPEND UPON TYPE OF DEPEND UPON TYPE OF ISOTHERMISOTHERM

USING DIMENSIONLESS TIME USING DIMENSIONLESS TIME

NUMBER OF TRANSFER UNITSNUMBER OF TRANSFER UNITS

01

1

0

000

u

L

WL

uL

cuT

satTp

T

0u

aLKN Tc

IRREVERSIBLE ADSORPTION

TRANSFER RATE IS BASED ON TRANSFER RATE IS BASED ON FLUID CONCENTRATIONFLUID CONCENTRATION

0 3-3NTU( -1)

1.0

0

c/co

Internal poreDiffusion Control

External FilmControl

Equal Resistance

Nu

aLK

c

cbedtheofendtheAt

u

aLK

c

cIntegrated

acKL

cu

c

c

c

00

00

0

ln:

ln:

IRREVERSIBLE ADSORPTION

IN TERMS OF DIMENSIONLESS TIME:IN TERMS OF DIMENSIONLESS TIME:

NUMBER OF TRANSFER UNITS NUMBER OF TRANSFER UNITS EXPRESSED AS INTERNAL & EXTERNALEXPRESSED AS INTERNAL & EXTERNAL

110

Nc

cln

00

10111

uD

aLDN

u

aLkN

NNN p

TeTextcext

ext

int,

int

CLASSIFICATION OF CYCLESCLASSIFICATION OF CYCLES

ACCORDING TO THE METHOD OF REGENERATION.

MOST SYSTEMS USE A COMBINATION OF: TEMPERATURE SWING ARE USED TO

REGENERATE BED BY RAISING VAPOR PRESSURE OF CONTAMINANT BY PREHEATING SWEEP GAS.– PRESSURE SWING VAPORIZES CONTAMINANT BY

PRESSURE REDUCTION– STRIPPING SETS UP A CONTAMINANT LEAN (LOW

PARTIAL PRESSURE) CONDITION WHICH RESULTS IN DIFFUSION FROM SOLID TO SWEEP GAS

– DISPLACEMENT CONTACTS SOLID AND REMOVES CONTAMINANT TO A SWEEP LIQUID.

29 - adsorption design

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