Adsorption processes

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Adsorption processes
Recommended reading R. Treybal, Mass-Transfer
operations, Chapter 11 Diran Basmadjian, Little
adsorption book
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Adsorption processes
Adsorption occurs whenever a solid surface is
exposed to a gas or a liquid it is defined as
the enrichment of material or increase of
density of the fluid in the vicinity of an
interface
(Rouquerol et al)
Adsorbent (porous structure)
Adsorbate
Adsorbent (flat surface)
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Adsorption processes
Adsorption occurs whenever a solid surface is
exposed to a gas or a liquid it is defined as
the enrichment of material or increase of
density of the fluid in the vicinity of an
interface
(Rouquerol et al)
4
Adsorption processes
Adsorption
Physical (physisorption) - van der Waals
interactions (result in attractive forces
between adsorbent and adsorbate molecules)
- Adsorbed molecules maintain their
identity - Multilayers - No activation
energy - Always reversible
Chemical (chemisorption) - Chemical bonds
between adsorbate and adsorbent formed -
Adsorbed molecules loose their identity
- Monolayers - Often activation energy
required - Can be irreversible
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Adsorption processes
Mechanisms of selectivity
one component exhibits stronger interaction
with adsorbent and therefore is selectively
removed from the mixture
one component is not able to fit in the pores
and therefore is excluded from adsorption
one component exhibits higher transport
diffusion and is selectively removed from the
mixture
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Adsorption processes Applications
Purifications - Removal of organics from
vent gases - SO2 from vent gases - H2O from
air, methane, N2 - Removal of solvent, odours
from air - NOx from N2 - Organics from water
solution - Water from organic solution -
Decolourization
Separations - N2/O2 - Acetone from vent
stream - C2H4 from vent - Normal paraffins/ Iso
praffins - CO, CH4, CO2, N2, Ar from
hydrogen - Normal paraffins from Iso
paraffins - Normal paraffins from olefins
gas phase
liquid phase
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Adsorbents
At the heart of adsorption processes is the
interaction of adsorbate molecules with the
surface of adsorbent - Therefore adsorbent
materials are usually materials with
extensive porous structure
Other requirements Selectivity High
capacity Chemical and thermal stability
Low solubility in the carrier solvent Hardness
and mechanical strenght Low cost
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Adsorbents Characterization
1) Crystalline/amorphous 2) Hydrophobic/Hydrophil
ic 3) Surface area (100-1000m2/g) 4) Pore
size rlt2nm microporours 2nmltrlt
50nm mesoporous rgt50nm macroporous 5)
Pore shape slits, channels, cavities, cages,
shapeless - often modelled as cylindrical
channels
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Adsorbents Examples
Silica gels - granular porous form of
silica - amorphous - hydrophilic -
700-800m2/g - water removal
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Adsorbents Examples
Activated Carbon - partial oxidation of coal -
amorphous - hydrophobic - 400-1200m2/g -
organic trace removals - air filters
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Adsorbents Examples
Zeolites - porous crystalline minerals -
hydrophilic - 600-700m2/g - highly structured
porous space - N2 removal from air
Mordenite
ZSM-5
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Adsorbents Thermodynamic considerations
(partial pressure, or concentration of i)
T, P
(amount adsorbed per gram of porous material)
- Unlike for vapour-liquid equilibria, there is
no particularly good thermodynamic theory to
predict this partitioning - It is necessary to
obtain experimental data for a particular
adsorbent/adsorbate pair at specified
conditions - This information is usually
presented in the form of adsorption
isotherms, where we plot amount adsorbed per
mass of adsorbent as a function of partial
pressure or concentration of the adsorbent in
the bulk phase
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Adsorbtion isotherms
I. a) Monolayer adsorption b) Microporous
materials II. Multilayer adsorption III.
Unfavourable interaction with
adsorbent IV. Porous materials V. IV with
unfavourable interaction with substrate VI.
Distinct layering transitions
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Adsorbtion isotherms Effect of temperature
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Adsorbtion isotherms Effect of pressure
P1
P2ltP1
ni
yi
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Adsorption Design considerations
Adsorbent regeneration (desorption)
Adsorption phase
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Temperature swing (thermal swing) adsorption
Step 1 The feed fluid at p1 and T1 is passed
through adsorbent. Equilibrium loading n1 is
reached. Step 2 The temperature is raised to T2
and the equilibrium partial pressure raises to
p2, causing desorption of the component from
the adsorbent Step 3 Purge stream is passed of
the sample removing the component from the gas
phase Step 4 Cooling returns us back to T1
line with some remaining loading n2
1
2
3
4
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Temperature swing (thermal swing) adsorption
Applications 1) Drying 2) Sweetening (H2S,
SO2 removal from natural gas, H2) 3) NOx
removal 4) HCl removal from Cl2
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Pressure swing adsorption
Step 1 The feed fluid at yp1/P1 and T1 is
passed through adsorbent. Equilibrium loading
n1 is reached. Step 2Total pressure
reduced (blowdown step). y2p1/P2gty1 Step 3
Purge stream is passed of the sample removing
the component from the gas phase Step
4Repressurization tp P1
1
2
3
4
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Pressure swing adsorption
Applications 1) H2 purification 2) Air
separation 3) Iso/normal separations
1
2
3
4
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Adsorption Design considerations
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Adsorption Design considerations
- stage-wise operations - continuous
operations steady state unsteady state
- Stage operations Single stage operation
(limited to treatment of dilute liquid
solutions) Adsorption of iodine onto carbon
from brines Collection of insulin from dilute
solutions
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Adsorption Design considerations
- Stage operations
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Simple isotherm equations Langmuir
Assumptions - Single layer - Interaction
between molecules in the layer are negligible
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Simple isotherm equations Langmuir
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Adsorption Conventions for the
isotherm (equilibrium line)
q kg of adsorbate/ kg of pure adsorbent
Y
(flip)
q
Y kg of adsorbate/kg carrier gas
(More like in absorption studies)
(Langmuir equation)
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Adsorption Stage operations
Adsorbent S, q0
kg/s
Gas or liquid feed G,
Y0
G, Y1
kg/s
1
Weight, or mole ratios Y y/(1-y) kg
adsorbate/ kg carrier gas qx/(1-x) kg
adsorbate/kg adsorbent
S, q1
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Adsorption Stage operations
Adsorbent S, q0
kg/s
Gas or liquid feed G,
Y0
G, Y1
kg/s
1
Weight, or mole ratios Y y/(1-y) kg
adsorbate/ kg carrier gas qx/(1-x) kg
adsorbate/kg adsorbent
S, q1
Design objectives Adsorption - the amount S
of adsorbent required
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Adsorption Stage operations
Adsorbent S, q0
Gas or liquid feed G,
Y0
G, Y1
1
Y y/(1-y) kg adsorbate/ kg carrier
gas qx/(1-x) kg adsorbate/kg adsorbent
S, q1
Operating line
Equilibrium line (Langmuir, Freundlich)
Y
q0, Y0
-S/G
q1, Y1
q
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Adsorption Stage operations
Adsorbent S, q0
Gas or liquid feed G,
Y0
G, Y1
1
Y y/(1-y) kg adsorbate/ kg carrier
gas qx/(1-x) kg adsorbate/kg adsorbent
S, q1
Operating line
Equilibrium line (Langmuir, Freundlich)
Y
q0, Y0
-S/G
q1, Y1
q
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Adsorption Stage operations
Adsorbent S, q0
Gas or liquid feed G,
Y0
G, Y1
1
Y y/(1-y) kg adsorbate/ kg carrier
gas qx/(1-x) kg adsorbate/kg adsorbent
S, q1
Operating line
Equilibrium line (Langmuir, Freundlich)
Y
q0, Y0
-S/G
Y1 the amount of S required S Y1 you can
achieve If adsorption equation is available an
analytical solution is possible
q1, Y1
q
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Adsorption Stage operations
Adsorbent S1, q0
Adsorbent S2, q0
Gas or liquid feed G,
Y0
G, Y1
G, Y2
1
2
S1, q1
S2, q2
Operating line
Y
q0, Y0
-S1/G
q0, Y1
q1, Y1
q2, Y2
q
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Adsorption Stage operations
Adsorbent S1, q0
Adsorbent S2, q0
Gas or liquid feed G,
Y0
G, Y1
G, Y2
1
2
S1, q1
S2, q2
Operating line
Y
q0, Y0
-S1/G
Objective to minimize S1 S2
q0, Y1
q1, Y1
-S2/G
q2, Y2
q
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Adsorption Stage operations
Gas or liquid feed G,
Y0
G, Y1
G, Y2
G, YN
1
2
S, q1
S, qN1
S, q2
Design objectives Adsorption - the amount S
of adsorbent required - number of stages
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Adsorption Stage operations
Gas or liquid feed G,
Y0
G, Y1
G, Y2
G, YN
1
2
S, q1
S, qN1
S, q2
Operating line
Y
q1, Y0
S/G
qN1, YN
q
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Adsorption Stage operations
Gas or liquid feed G,
Y0
G, Y1
G, Y2
G, YN
1
2
S, q1
S, qN1
S, q2
Operating line
Y
Minimum adsorbent requirement
q1, Y0
Smin/G
Design analogy with stage-wise absorption
qN1, YN
q
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Counter current stage-wise adsorption example
G, Y0
G, YN
G, Y2
G, Y(N-1)
G, Y1
N
1
2
LN, XN
L, X1
L, X2
L, X(N1)
L3, X3
Continuous contact (steady state) Fluidized bed
adsorber (Purasiv)
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Adsorption Design considerations
- Stage operations
Example A water solution containing a valuable
solute is coloured by small amounts of impurity
9.6impurity/kg solution. The impurity is to be
removed by adsorbing on carbon. In a series of
laboratory tests the following equilibrium
relation has been established q impurity/kg
carbon Yimpurity/kg solution (Mass
ratios) 200 0.7 400 1.7 600 4.0 800 6.
3 1000 8.6 It is desired to remove 90 of
impurity from solution. Calculate the amount of
adsorbent you need in a single stage operation
for a 1000kg of solution.
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Adsorption
Continuous contact (steady state) Hypersorber -
adsorbent particles fall through the rising
stream of gas - analogy to absorption in
packed columns
Design objectives - the length of contact zone
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Adsorption continuous contact
Ga,Ya
Sa,qa
Z
Integration leads to
Gb,Yb
Sb,qb
S
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Adsorption continuous contact
Ga,Ya
Sa,qa
Z
Complete analogy to absorption in packed towers
Gb,Yb
Sb,qb
S
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Adsorption Design considerations
- Continuous contact Steady state
Example The adsorption process is designed to
dry air at 27C, atmospheric pressure from an
initial absolute humidity of 0.005kg water/kg
dry air to a final humidity of 0.0001kg
water/kg dry air. A countercurrent isothermal
adsorber will be used. The adsorbent is silica
gel with mass flow rate of 500kg/h/m2 the water
free air mass flow rate is 1000kg dry
air/hr/m2. The following mass transfer
coefficients have been derived
kgH20/h/m3 kgH20/h/m3 The equilibrium
line is a straight line under conditions of
interest. Estimate the height of the adsorber.
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Adsorption Design considerations
- Continuous contact Unsteady state
Fixed bed
Gas (liquid) is send through a system containing
fixed (not moving) adsorbent. A component is
removed from passing mixture until saturation of
the bed is reached. The process is then stopped
and the system regenerated
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Fixed bed considerations
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Fixed bed considerations
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Fixed bed considerations
saturated section of the bed no more adsorption
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Fixed bed considerations
saturated section of the bed no more adsorption
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Fixed bed considerations
Ideal concentration profiles
q
q
Y
q
q
0
z (Bed length)
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Fixed bed considerations
Ideal concentration profiles
q
q
0
z (Bed length)
Length of unused bed (LUB)
Saturated equilibrium section/Length of
equilibrium section (LES)
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Fixed bed considerations
Ideal breakthrough curve
1
0
Time
t
breakthrough curve
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Fixed bed considerations
Realistic concentration profiles
q
q
0
z (Bed length)
Mass transfer zone (MZT)
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Fixed bed considerations
Realistic breakthrough curve
1
0
time
tb
tb break point
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Fixed bed considerations
Realistic breakthrough curve
0
time
tb
Area total length of the bed
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Fixed bed considerations
Realistic breakthrough curve
1
0
time
tb
Area Length of unused bed
1
1
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Fixed bed considerations Design
Fixed bed
G kg/m2s, Y0
G kg/m2s, Y
A m2
Density of the bed kg/m3
Design objectives Adsorption - operation
time before bed becomes saturated - amount of
adsorbent required (minimum bed) Desorption -
time required for complete regeneration -
amount of purge required for regeneration
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Fixed bed considerations Design
1. Ideal concentration profiles
q
q
0
2. Type I adsorption isotherms
q
Langmuir equation
Y
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Fixed bed considerations Design
Mass balance considerations
q
Consider particular time t At this time, z
length of the bed is saturated
V
q
0
z (t)
kg
Amount introduced up to time t (with the gas
stream) Amount retained up to position z

kg
Propagation velocity
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Fixed bed considerations Design (Adsorption)
Mass balance considerations
q
Consider particular time t At this time, z
length of the bed is saturated
V
q
0
z (t)
Bed weight
Weight of gas treated
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Fixed bed considerations Design (Adsorption)
Bed weight
Weight of gas treated
q
q
Y
- breakthrough time
- minimum bed requirement (per kg of gas carrier)
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Fixed bed considerations Design (Desorption)
q
0
z
movement of the concentration profile in the bed
1
0
time
tb
tb break point
concentration of the purge gas as monitored
at the exit point of the bed
61
Fixed bed considerations Design (Desorption)
0
z
q
Y
- desorption time
kg purge/ kg bed
- minimum amount of purge