Adsorption Flashcards
(34 cards)
Define Selective Adsorption
The separation of components in a fluid mixture by the transfer of one or more components to the internal surface of a porous solid where they are held by intermolecular forces
Define Desorption
The reverse process which the adsorbates are removed for the solid surface (adsorbent is regenerated)
What three mechanisms does selective separation depend on
Differences in adsorption equilibria
differences in rates of adsorption
molecular size
What is thermodynamic equilibrium
Where the rate of adsorption is equal to the rate of desorption, this is a dynamic process
What are the separation factors concerned with the nature of the adsorbent surface
Polar - non - polar
Hydrophillic or hobic
ect
The amount of adsorbate which is adsorbed onto an adsorbent is always reduced with
Increased temperature
Reduced partial pressure (conc)
What are the cyclic process called that are based on temperature and pressure
Thermal swing adsorption
Pressure swing adsorption
Outline the differences between physisorption and chemisoprtion
Physisorption
- Low heat of adsorption
- non specific
- mono or multilayer
- no dissociation of adsorbed species
- only significant at low temperatures
- rapid, non active, reversible
- no electron transfer
Chemisorption
- High heat of adsorption
- highly specific
- mono layer only
- may involve dissociation of adsorbed species
- possible over wide range of temperatures
- activated, irreversible, may be slow
- electron transfer leads to chemical bonds
Define a zeolite
Literally means boiling stone
Zeolites are crystalline structures composed of alumino silicate cages and channels connecting the cavities
Because zeolites are crystals they have a precise pore size
What are some important adsorption properties required
- High contaminant adsorption capacity - most obvious and crucial to adsorb lots of contaminant
- Selectivity and fast kinetics - the faster the adsorption kinetics the less amount of adsorbent will be required to adsorb a given volume
- Tolerance to feed impurities - resistance to adverse chemicals extends the life of adsorbents
What are the three distinct mechanisms of adsorption
Kinetic separation - rates of adsorption
Steric separation - derives from molecular sieving properties
Equilibrium separation - most processes operate through equilibrium adsorption
What are the advantages of regeneration by pressure swing (2)
- Good for weakly adsorbed species
- rapid cycling
for gases only
What are the disadvantages of regeneration by pressure swing (3)
- Very low pressure may be required for strong adsorption
- Mechanical energy is more expensive to provide than thermal energy
- Desorbate is recovered at low purity
Draw a simple two bed PSA and explain how nitrogen may be separated from oxygen
slide 6 Lecture 2
1 - Two columns, one adsorbing, one being regenerated
2 - N2 is adsorbed 3 - 4 times as strongly as oxygen so nearly pure O2 can be produced
3 - The adsorption time is short as there is a lot of nitrogen in air
4 - the holdup of the gas in the bed is significant
5 - after adsorption the bed is depressurised which removes mos of the gas hold up
6 - the depressurisation of column 1 is commonly known as blow down
7 - The bed is purged at 1 atm with part of the product gas (oxygen)
8 - the bed is then pressurised with oxygen
9 - This allows all the N2 to be desorbed allowing more room for more
10 - column 2 goes through a similar sequence of events
How has the pressure swing regeneration been improved
Pressure vacuum swing
can produce oxygen at purities of 90-94%
What is used as a desorbing purge fluid
Steam, also provides heat for raising bed temperature which leads to further adsorption
When might distillation be necessary in TSA
If the adsorbate needs to be recovered from the steam stream
what are the advantages of thermal swing (2)
Good for strongly adsorbed species
Desporbate recovered at high concentration
for gases only
What are the disadvantages of thermal swing (4)
- thermal ageing of adsorbent
- heat loss leads to thermal inefficiency
- long cycle times means inefficient use of adsorbent
- High latent heat to be input for liquids
What is regeneration by displacement
Displacing an adsorbate with a molecule with a higher affinity for the adsorbent
What are the advantages of regeneration by displacement
- good for strongly adsorbed species
- avoids risk of cracking reactions
- avoids thermal ageing of adsorbent
What are the disadvantages of regeneration by displacement
- Product separation and recovery is required
- Choice of desorbent fluid is crucial in terms of its recovery use
Advantages of regeneration by purge gas stripping
- Carried out at essentially constant pressure
Disadvantages of regeneration by purge gas stripping
- only for weakly adsorbed species
- cannot be used when recovery of adsorbate is required
