1. Adsorption
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2. Definition Physical vs Chemical
The concentration of gases, liquids or dissolved substances (adsorbate) on the surface of solids (adsorbent)
Physical vs Chemical
Physical Adsorption (van der Waals adsorption):
weak bonding of gas molecules to the solid;
exothermic (~ 0.1 Kcal/mole);
reversible
Chemisorption:
chemical bonding by reaction;
exothermic (10 Kcal/mole);
irreversible
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3. Properties of Activated Carbon
Sorbent Materials
Activated Carbon
Activated Alumina
Silica Gel
Molecular Sieves (zeolite)
Polar and Non-polar adsorbents
Properties of Activated Carbon
Bulk Density
22-34 lb/ft3
Heat Capacity
BTU/lboF
Pore Volume
cm3/g
Surface Area
m2/g
Average Pore Diameter
15-25 Å
Regeneration Temperature (Steaming) oC
Maximum Allowable Temperature
150 oC
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4. Properties of Activated Alumina Properties of Molecular Sieves
Bulk Density
Granules
38-42 lb/ft3
Pellets
54-58 lb/ft3
Specific Heat
BTU/lboF
Pore Volume
cm3/g
Surface Area
m2/g
Average Pore Diameter
18-48 Å
Regeneration Temperature (Steaming) oC
Maximum Allowable Temperature
500 oC
Properties of Molecular Sieves
Anhydrous Sodium Aluminosilicate
Anhydrous Calcium Aluminosilicate
Anhydrous Aluminosilicate
Type 4A 5A
13X
Density in bulk (lb/ft3) 44 38
Specific Heat (BTU/lboF)
0.19 -
Effective diameter of pores (Å) 4 5 13
Regeneration Temperature (oC)
Maximum Allowable Temperature (oC)
600
Crystalline zeolite
Uniform pores to selectively separate compounds by size & shape
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5. Properties of Silica Gel
Bulk Density
44-56 lb/ft3
Heat Capacity
BTU/lboF
Pore Volume
0.37 cm3/g
Surface Area
750 m2/g
Average Pore Diameter
22 Å
Regeneration Temperature oC
Maximum Allowable Temperature
400 oC
Q: What if it is heated over 250 oC?
H2O
H2O
H2O OH OH OH O OH
heating
hydrophobic
hydrophilic
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6. Adsorption Mechanism
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7. Langmuir Isotherm
Adsorption Isotherm: the mass of adsorbate per unit mass of adsorbent at equilibrium & at a given temperature
Rate of adsorption
(f: fraction of surface
area covered)
Rate of desorption f
At equilibrium
1-f
Mono-layer coverage
( m: mass of adsorbate adsorbed
per unit mass of adsorbent)
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8. Langmuir Isotherm
( p: partial pressure of the adsorbate) m p
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9. Aerosol & Particulate Research Lab
For liquid-solid
a = mass of adsorbed solute required to saturate completely a unit mass of adsorbent
K = experimental constant
Ce = equilibrium concentration
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Aerosol & Particulate Research Lab

10. Freundlich Isotherm
For liquid-solid
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11. Effects of Humidity Amount of trichloroethylene adsorbed
as a function of relative humidity
Isotherm for toluene & trichloroethylene
and water vapor (individual)
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12. Fixed-Bed Adsorption System
Isotherm!!!
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Aerosol & Particulate Research Lab

13. Aerosol & Particulate Research Lab
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14. Aerosol & Particulate Research Lab
Regeneration
Q: In addition to steam, what else can we use?
Q: Typically only 30 ~ 40% of the equilibrium isotherm is used. Why is that?
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Theodore & Buonicore, 1988
Aerosol & Particulate Research Lab

15. Aerosol & Particulate Research Lab
Q: How will you select the regeneration time?
A well-designed system has steam consumption in the range of 1 to 4 lb of steam/lb of recovered solvent or 0.2 to 0.4 lb of steam/lb of carbon
In a continuous operation, a minimum of 2 adsorption units is required.
Q: Three-units? Any advantage?
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Aerosol & Particulate Research Lab

16. Aerosol & Particulate Research Lab
Rotary Bed System
Mycock et al., 1995
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Aerosol & Particulate Research Lab

17. Aerosol & Particulate Research Lab
Fluidized-Bed System
Q: Benefits?
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18. Aerosol & Particulate Research Lab
Pressure Drop
P: pressure drop (lb/ft2)
D: bed depth (ft)
e : void fraction
G’: gas mass flux (lb/ft2-hr)
mg: gas viscosity (lb/ft-hr)
dp: carbon particle diameter (ft)
Typical operating range:
< 20 in H2O; 20 < V < 100 ft/min
==> determine the
Maximum Adsorbent Bed Depth
Q: Why?
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Aerosol & Particulate Research Lab

19. Aerosol & Particulate Research Lab
Union Carbide Empirical Equation
P: bed pressure drop, in H2O
V: gas velocity, ~ ft/min
D: bed depth, ~5-50 inches
dp: 4X6 mesh sized carbon
Minimum Adsorbent Bed Depth
Need to be at least longer than the MTZ
CB: breakthrough capacity %
CS: saturation capacity %
XS: degree of saturation in the MTZ (usually 50%)
D: bed depth
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Aerosol & Particulate Research Lab

20. Other Systems: Nongenerable
Canister adsorber
Thin-bed adsorber
Q: What need to be known to start the design of an adsorption bed system?
Mycock et al., 1995
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Aerosol & Particulate Research Lab

21. Aerosol & Particulate Research Lab
Exercise I
An exhaust stream contains 1880 ppm of n-pentane at 95 oF. The flow rate to be treated is 5500 acfm. Carbon capacity is 3.5 lb n-pentane/100 lb AC. Carbon density is 30 lb/ft3. 2-bed system: 1 hr for adsorption and the other hr for regeneration.
Q: Mass flow rate of n-pentane? Volume of carbon bed? Flow velocity? Steam requirement? Pressure drop?
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Aerosol & Particulate Research Lab

22. Aerosol & Particulate Research Lab
Exercise II
Conditions: 10,000 acfm of 77 oF at 1 atm containing 2000 ppm toluene (MW = 92) to be treated. 95% removal efficiency by 4X10 mesh carbon expected (density of carbon = 30 lb/ft3)
Q: how many lb/hr of toluene to be removed?
Q: If regeneration at 212 oF, what’s the working capacity?
Q: Design an adsorption system with max DP of 8 inH2O, 4 hr cycle, two beds.
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Aerosol & Particulate Research Lab

23. Aerosol & Particulate Research Lab
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24. Aerosol & Particulate Research Lab
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Aerosol & Particulate Research Lab

25. Aerosol & Particulate Research Lab
Quick Reflection
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Aerosol & Particulate Research Lab