Adsorption

SHR Chapter 15

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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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