Adsorption Processes - Unit Operations - Lecture Slides, Slides of Engineering Chemistry

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Adsorption processes

Recommended reading: R. Treybal, Mass-Transfer operations,Chapter 11Diran Basmadjian, Little adsorption book

Adsorption processes

Adsorption

occurs whenever a solid surface is exposed to

a gas or a liquid: it is defined as the enrichment of material orincrease of density of the fluid in the vicinity of an interface

(Rouquerol et al)

Adsorbent(porous structure)

Adsorbate

Adsorbent(‘flat’ surface)

Adsorption processes

Adsorption

Physical (physisorption)

• van der Waals interactions

(result in attractive forces betweenadsorbent 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

Adsorption processes

Mechanisms of selectivity

• one component exhibits stronger

interaction with adsorbent and thereforeis 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

Adsorbents At the heart of adsorption processes is the interaction ofadsorbate 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

Adsorbents: Characterization 1) Crystalline/amorphous2) Hydrophobic/Hydrophilic3) Surface area (100-1000m2/g)4) Pore size

r<2nm:

microporours

2nm<r<50nm:

mesoporous

r>50nm:

macroporous

5. Pore shape: slits, channels, cavities, cages, shapeless

• often modelled as cylindrical channels

Adsorbents: ExamplesActivated Carbon

• partial oxidation of coal
  • amorphous- hydrophobic- 400-1200m2/g- organic trace removals- air filters

Adsorbents: ExamplesZeolites

• porous crystalline minerals
  • hydrophilic- 600-700m2/g- highly structured porous

space

• N2 removal from air

Mordenite

ZSM-

Adsorbtion isotherms

III

II

I

IV

V

VI

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

Adsorbtion isotherms: Effect of temperature

Adsorption: Design considerations

Adsorptionphase

Adsorbentregeneration(desorption)

Temperature swing (thermal swing)adsorption

Step 1

: The feed fluid at p

and T1 is passed throughadsorbent. Equilibrium loadingn1 is reached. Step 2

: The temperature is raised

to T2 and the equilibriumpartial pressure raises to p2,causing desorption of thecomponent from the adsorbent Step 3

: Purge stream is passed

of the sample removing thecomponent from the gas phase Step 4

: Cooling returns us

back to T1 line with someremaining loading n

Pressure swing adsorption

Step 1

: The feed fluid at y=p1/P

and T1 is passed throughadsorbent. Equilibrium loadingn1 is reached. Step 2

:Total pressure reduced

(blowdown step). y2=p1/P2>y1 Step 3

: Purge stream is passed

of the sample removing thecomponent from the gas phase Step 4

:Repressurization tp P

Applications:1) H2 purification2) Air separation3) Iso/normal separations

Pressure swing adsorption