Post on 31-Jan-2018
Adsorption
SHR Chapter 15
1 Adsorption.key - April 9, 2014
Example: DehumidificationAdsorption cycle:
• water adsorbs onto adsorbent (activated carbon)
• adsorbent acts like a “sponge” with an absorbent front that saturates the bed
• stop before “breakthrough” when the bed is saturated
Regeneration cycle:
• cycle onto fresh bed for adsorption
• blow-down (drop pressure) to regenerate.
Pressure-swing adsorption
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AdsorbentsActivated Carbon • often made from biomass oxidation (char)
Molecular sieve carbon • very small pore size • used to separate air ‣N2: 3.0 x 4.1Å, O2: 2.8 x 4.0Å
‣cryogenic distillation is another common technique...
Molecular sieve zeolites • uniform pore size • aluminosilicate minerals: ‣Zeolite 3A: K12[(AlO2)12(SiO2)12], dp=3Å
‣Zeolite 13X: Na86[(AlO2)86(SiO2)86], dp=10Å
Silica gel • affinity for polar compounds • in lots of items you buy (silica pouches) to reduce
humidity
SHR §15.1.1
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Properties of AdsorbentsKey quantity: specific area, Sg (surface area per unit mass).
Adsorbent Nature d(Å) ϵ ρ
(g/cmS
(m
Hat 25ºC and
4.6 mmHg, wt% (dry basis)
Activated Alumina
Hydrophilic, amorphous
10- 75
0.5 1.25 320 7
Silica gel (small pore)
amorphous 22- 26
0.47 1.09 750- 850
11
Silica gel (large pore)
100-150
0.71 0.62 300- 350
-
Activated Carbon (small pore)
Hydrophobic, amorphous
10- 25
0.4- 0.6
0.5-0.9 400- 1200
1
Activated Carbon (large pore)
>30 - 0.6-0.8 200- 600
-
Molecular-sieve carbon
Hydrophobic, nonpolar
2-10 - 0.98 400- 1200
-
Molecular-sieve zeolites
Polar-hydrophilic, crystaline
3-10 0.2- 0.5
1.4 600- 700
20-25
SHR §15.1.1, Table 15.2
Specific pore
volume: Vp =
✏p⇢p
=
porosity
density
activated carbon
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Equilibrium
No good theory exists for getting K-values for adsorption. • each adsorbate-absorbent pair needs to be treated individually - typically
using experiments.
Equilibrium is a function of: • concentration (liquids) or partial pressure (gases)
• solute loading on the absorbent (moles adsorbate per unit mass of adsorbent)
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Isotherms - Pure GasesTypically, data are taken over a range of adsorbate concentrations at
constant temperature to obtain an “adsorption isotherm”
NH3 on charcoal
SHR §15.2.1
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Isotherm Models - Pure Gases
q = kp1/nFreundlich isotherm
(empirical model)
Linear isotherm (empirical model)
q = kp “Henry’s law” - applicable for low loadings.
dq
dt= kap(1� ✓)� kd✓ At equilibrium, dq/dt = 0 so ✓ =
Kp
1 +Kp✓ ⌘ q
qmθ fraction of occupied sites qm maximum loading (complete surface coverage)
ka adsorption rate constant
kd desorption rate constant
K ka/kd (reaction equilibrium constant)
Langmuir isotherm q =Kqmp
1 +Kp
Assume adsorption/desorption is a first-order process:
q - equilibrium loading (amount adsorbed per unit mass of adsorbent)
k, n are empirical parameters to fit data.
K, qm are unknown parameters.
0 200 400 600 800 1000 120040
60
80
100
120
140
p (psia)
q (c
m3 /g
)
dataFreundlichLangmuir
SHR Example 15.4
Note: for liquids, concentrations rather than pressures are commonly used.
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Other ways of looking at equilibrium
Isotherms
Isobars Isosteres
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Heat of AdsorptionClausius-Clapeyron equation (note mistake in SHR equations (15-17)
and (15-18))
ΔHvap = 4600 cal/mol
Adsorption is an exothermic process. What does Le Chatelier’s principle imply about the
dependency of loading on temperature?
NH3 on charcoal
d ln p
d (1/T)=
��Hads
R
ΔHads varies by sorbent
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Effect of Adsorbent
Typically, experiments are required for each adsorbent/solute pair
Pure propane vapor
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RegenerationPressure-swing absorption Temperature-swing absorption
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