29 - adsorption design
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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)
CONCENTRATION PROFILES
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
CONCENTRATION PROFILES
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:
).(
)&.(
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1
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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
,,,
:
*
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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.
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