L5 - Porous materials (adsorbents) for air treatment and gas separation

Question 1

What are the three mechanisms of adsorptive separation of gases and vapors?

Answer 1

1. Exclusion based on differences in the interaction of the adsorbate with the surface of the porous material.
2. Exclusion based on kinetic effects due to differences in size or shape.
3. Exclusion based solely on the size or shape of the adsorbate.

Question 2

What are the steps in the Pressure Swing Adsorption (PSA) process?

Answer 2

Adsorption, Depressurization, Purging, Repressurization.

Question 3

What is the purpose of depressurization in Pressure Swing Adsorption (PSA)?

Answer 3

Depressurization is the first regeneration (desorption) step where the pressure drops, releasing the adsorbed gas molecules and discharging the target gas from the adsorber vessel.

Question 4

What is the purpose of the purging step in Pressure Swing Adsorption (PSA)?

Answer 4

During purging, the adsorbent bed is purged with the target gas to fully regenerate the adsorbent for the next cycle.

Question 5

What happens during the repressurization step in Pressure Swing Adsorption (PSA)?

Answer 5

The adsorber is pressurized with a feed gas mixture or the target gas until the conditions for adsorption are restored.

Question 6

Why can’t fine powders be used directly in PSA?

Answer 6

Fine powders can clog and impede gas flow, making the PSA process inefficient and potentially causing blockages.

Question 7

What are the two processes used to shape powders in PSA?

Answer 7

Granulation and extrudation.

Question 8

What is done to microporous powder before granulation or extrudation in PSA?

Answer 8

The microporous powder is mixed with a binder or set of binders, typically clays or polymers.

Question 9

What is Pressure-swing adsorption (PSA) often used for?

Answer 9

PSA is often used for H2 purification; from
steam methane reforming off gas
(SMOG) and refinery off gas (ROG)

Question 10

What principle does Pressure Swing Adsorption technology for H2
purification rely on?

Answer 10

PSA relies on the principle that H2
adsorbs very weakly to adsorbents due to its weak interaction.

Question 11

What is a drawback of PSA in terms of H2 recovery?

Answer 11

Recovery is moderate as some H2
is lost in the purge step.

Question 12

Why is activated carbon placed before zeolite in the adsorbent bed for H2 purification?

Answer 12

Activated carbon is placed first because H2O and CO2 can be recovered more easily from the carbon adsorbent.

Question 13

What is the purpose of Zeolite 5A columns in H2 purification with PSA?

Answer 13

Zeolite 5A columns capture mainly CH4, CO, and N2, allowing purified H2 to pass through.

Question 14

What is the role of activated carbon columns in H2 purification using PSA?

Answer 14

Activated carbon columns are used to remove strongly adsorbing CO2 and H2O and are placed first in the bed.

Question 15

What types of adsorbent columns are used in PSA for H2 purification?

Answer 15

Activated carbon and zeolite sorbent columns.

Question 16

What type of adsorbents are used for O2
​production in PSA?

Answer 16

Zeolite adsorbents.

Question 17

Which gas adsorbs more strongly to zeolite adsorbents, N2 or O2? And why?

Answer 17

Nitrogen N2 adsorbs more strongly to zeolite than oxygen (O2) due to its stronger interaction with the electric field gradient created by cations in the zeolite. This interaction is enhanced by nitrogen’s quadrupole moment, making it more easily adsorbed than oxygen.

Question 18

What type of adsorbents are used for N2 purification from air?

Answer 18

Carbon molecular sieves (CMSs) are used.

Question 19

How does O2 adsorption compare to N2 adsorption on carbon molecular sieves?

Answer 19

The adsorption of O2 is stronger than that of N2, opposite to the behavior with zeolites.

Question 20

How is selectivity for N2 enhanced in CMSs during air purification?

Answer 20

Selectivity is enhanced by diffusion. O2 stays longer in the adsorbent bed because it can enter smaller pores in the carbon molecular sieve, while N2 passes through more quickly, resulting in an N2-rich output stream.

Question 21

What methods can be used to upgrade raw biogas?

Answer 21

Pressure Swing Adsorption (PSA), membrane processes, cryogenics, scrubbing with water, or scrubbing with amines.

Question 22

What is raw biogas composed of after biomass fermentation?

Answer 22

About 60% CH4 and 40% CO2

Question 23

What is the required CH4 content for vehicle-fuel-standard biogas?

Answer 23

95–96% CH4

Question 24

Why is biogas considered a sustainable vehicle fuel?

Answer 24

Biogas is sustainable, especially when produced from biowaste or specific crops, but it requires a proper Life Cycle Assessment.

Question 25

What are the two types of emissions related to air pollution?

Answer 25

Particulate emissions and gaseous emissions.

Question 26

What are the two types of emission-reduction techniques?

Answer 26

Particulate-purification techniques and gas purification techniques.

Question 27

Besides emissions, what other hazards can be present in air pollution?

Answer 27

Airborne biological hazards/pathogens.

Question 28

What is the efficiency of High Efficiency Particulate Air (HEPA) filters?

Answer 28

HEPA filters remove 99.97% of particulate matter smaller than 0.3 microns.

Question 29

What processes are used for adsorptive drying of gas?

Answer 29

Pressure Swing Adsorption (PSA) or Temperature Swing Adsorption (TSA).

Question 30

What types of adsorbents are typically used for gas drying?

Answer 30

Hydrophilic adsorbents such as silica, alumina, or zeolites.