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Multicomponent Distillation > Adsorption > Flashcards

Adsorption Flashcards

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1
Q

What is adsorption?

A

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid (adsorbent), forming a molecular or atomic film (adsorbate)

Adsorption differs from absorption, where a substance is taken up throughout the bulk of a material.

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2
Q

What form do adsorbents usually take in commercial processes?

A

Adsorbents are usually in the form of small particles in a fixed bed.

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3
Q

What happens when the adsorption bed is almost saturated?

A

The flow in this bed is stopped, and the bed is regenerated thermally or by other methods, leading to desorption.

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4
Q

What is desorption?

A

Desorption is a phenomenon whereby a substance is released from or through a surface.

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5
Q

What are the criteria for the use of adsorption?

A
  • Key relative volatility < 1.5 (including azeotropes)
  • Product of interest is a high-boiler at low concentration (< 20 wt%)
  • Undesirable high-boiler must be removed
  • Overlapping boiling ranges of components
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6
Q

In what situations is adsorption preferred over distillation?

A
  • Cryogenic conditions or high temperatures (> 523 K)
  • Vacuum pressures (< 2 KPa) or high pressures (> 5 MPa)
  • Undesirable side reactions during distillation
  • Low throughputs (< a few tons per day)
  • Corrosion and precipitation issues
  • Explosive conditions
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7
Q

What is the structure of adsorbent particles?

A

Adsorbent particles have a very porous structure with many fine pores and pore volumes up t

0.1 to 12 mmo 50% of the total particle volume.

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8
Q

What is the rate of mass transfer of the adsorbate onto the adsorbent based on?

A

The diffusion of adsorbate from the bulk solution through the stagnant film surrounding the particle external surface.

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9
Q

What are examples of historical applications of adsorption?

A
  • Water treatment with charred wood
  • Decolourizing sugar solutions with bone char
  • Air purification with wood charcoal in hospitals
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10
Q

What are the major applications of gas-phase adsorption?

A
  • Removal of water from hydrocarbon gases
  • Removal of sulfur compounds from natural gas
  • Removal of solvents from air and other gases
  • Removal of odors from air
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11
Q

What factors influence the effectiveness of adsorbents?

A
  • Selective concentration of components
  • Reversibility of adsorption
  • High delta loading (change in weight of adsorbate per unit weight of adsorbent)
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12
Q

What are the types of pore sizes in adsorbents?

A
  • Macropores: d > 50 nm
  • Mesopores: 2 < d < 50 nm
  • Micropores: d < 2 nm
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13
Q

What is activated carbon?

A

Microcrystalline material made by thermal decomposition of wood, vegetable shells, coal, etc. with surface areas from 300 to 2500 m²/g.

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14
Q

What is the Freundlich Isotherm?

A

q = KC ^1/n

An empirical isotherm where K is the Freundlich capacity parameter and 1/n is the Freundlich intensity parameter, often used for liquid adsorption.

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15
Q

What is the Langmuir Isotherm?

A

An isotherm derived under the assumption that there are a fixed number of active sites available for adsorption, only a monolayer is formed, and adsorption is reversible.

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16
Q

What is the significance of ΔH in adsorption?

A

ΔH of adsorption is always negative, indicating that adsorption is an exothermic process.

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17
Q

What effect does temperature have on adsorption?

A

Usually, as temperature increases, the amount adsorbed decreases.

Adsorption is exothermic so increase in temp does not favour adsorption.

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18
Q

What is Henry’s Law in the context of adsorption?

A

q = KC

A linear isotherm where K is a constant determined experimentally, used to approximate data in the dilute region.

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19
Q

What is the Ideal Adsorbed Solution Theory (IAST)?

A

A theory that allows the single-solute behavior represented by the Freundlich adsorption isotherm to predict multicomponent adsorption.

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20
Q

What is the linear driving force (LDF) model?

A

A model used to describe the kinetics of adsorption, where the change in adsorbate concentration is related to the total mass uptake by the adsorbent.

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21
Q

What is a common challenge in moving adsorbents?

A

It is extremely difficult to move adsorbents from one place to another without damaging either particles or equipment.

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22
Q

What does IAST stand for in multicomponent adsorption?

A

Ideal Adsorbed Solution Theory

IAST is used to describe the equilibrium concentration in a multicomponent system.

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23
Q

What is the Freundlich capacity parameter represented as?

A

K_i

It is part of the Freundlich isotherm equation used in adsorption studies.

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24
Q

What are the three basic strategies for loading and desorbing an adsorbate?

A
  • Temperature-Swing Adsorption (TSA)
  • Pressure-Swing Adsorption (PSA)
  • Concentration-Swing Adsorption (CSA)

Displacement-Purge Adsorption (DPA) is also included in the context of liquid separations.

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25
True or False: Most adsorption processes operate in batch mode.
False ## Footnote Most adsorption processes operate in semi-batch mode.
26
What complication is associated with heat effects in adsorption processes?
Heat is released during adsorption and required during desorption ## Footnote This heat can raise the temperature of the adsorbent and limit maximum adsorption.
27
What is the typical cycle time for Pressure-Swing Adsorption (PSA)?
A few minutes ## Footnote This rapid cycle time makes PSA attractive for bulk-gas separations.
28
Fill in the blank: The delta loading in Temperature-Swing Adsorption (TSA) is calculated as _______.
X_1 - X_2
29
What is the role of the purge in Temperature-Swing Adsorption (TSA)?
To provide heat and dilute the concentration of adsorbate ## Footnote A heated inert stream can also help in desorption.
30
What type of adsorbate concentration reduction is used in Pressure-Swing Adsorption (PSA)?
PSA uses pressure reduction to lower the adsorbate’s partial pressure and drive desorption. Inert purge is generally not used — desorption occurs via pressure swing alone. ## Footnote This helps facilitate desorption during the regeneration step.
31
What is a major limitation of Pressure-Swing Adsorption (PSA)?
Only a fraction of the adsorbate-free stream is recovered ## Footnote This limits PSA applications to scenarios where only one pure product is needed.
32
What is the primary application of Displacement-Purge Adsorption (DPA)?
Liquid-feed bulk separation ## Footnote DPA uses a gas/liquid that adsorbs similarly to the adsorbate.
33
What is the cycle time for Inert Purge Process?
Only a few minutes ## Footnote This process can achieve a maximum delta loading equal to X_1.
34
Why is adsorption always exothermic?
As absorbate is adsorbed onto surface, its movement is restricted. This restricted loading results in a decrease in entropy. Energy is released.
35
What is an adsorption isotherm?
Isothermal relationship for the equilibrium between concentration of adsorbate in external fluid and concentration of absorbate in adsorbent.
36
Describe 7 circumstances when adsorption might be considered the best option to perform a separation of a given multicomponent mixture of chemical species.
- Low concentration of component to be removed in mixture. - The component to be removed has a high boiling point. - The component to be removed has a strong and specific interaction with a known adsorbent whereas other components do not. - The adsorbent can be cleaned of the component removed upon it to recover where required. - Process is high added value to pay for the adsorbent - Component may be explosive if heated - Component may be corrosive to metals.
37
Why is adsorption suitable when the component to be removed is at low concentration in the mixture?
Adsorption is highly effective for removing trace-level components due to high surface affinity and selectivity.
38
Why is adsorption preferred when the component to be removed has a high boiling point?
Adsorption avoids the need for heating, which makes it more efficient than distillation for high-boiling compounds.
39
Why choose adsorption if the target component strongly interacts with a specific adsorbent?
Adsorption provides high selectivity when the target binds strongly to an adsorbent while others do not.
40
Why is reusability of the adsorbent a benefit in adsorption processes?
The adsorbent can be regenerated and reused, improving cost-efficiency and sustainability.
41
When is adsorption justified despite higher costs?
When the process has high added value, making the cost of adsorbents acceptable.
42
Why use adsorption if the component is explosive when heated?
Adsorption operates at low temperatures, avoiding the risk of explosion from heating.
43
Why is adsorption advantageous for removing corrosive components?
Adsorption prevents corrosion damage by avoiding direct contact between corrosive components and metal equipment.
44
What factors influence the process batch time?
- Temperature of process - If mixture is stirred (improves mass transfer). - Pore size relative to molecules X. - Porosity of adsorbent. - Size of adsorbent granules.
45
What 7 characteristics are important when choosing an adsorbent?
1. Cheap material. 2. Cheap to manufacture. 3. Chemically stable. 4. Thermal and mechanical stability. 5. Selective adsorption to desired adsorbent. 6. Low heat of adsorption. 7. Large delta loading.
46
What is hysteresis?
When the system's response differs during adsorption and desorption. As pressure increases, the amount adsorbed follows one path (adsorption), and as pressure decreases, it follows a different path (desorption).
47
What does a Type I adsorption isotherm represent?
Concave curve that rises sharply at low pressure and plateaus early.
48
What is happening physically in a Type I isotherm?
Rapid monolayer adsorption on microporous surfaces; once sites are filled, no more adsorption occurs.
49
Describe a Type I isotherm.
Indicates strong adsorbent–adsorbate interaction, typical of microporous solids. Langmuir-type behaviour.
50
What is the shape of a Type II adsorption isotherm?
Sigmoidal (S-shaped), with a clear inflection point.
51
What is happening physically in a Type II isotherm?
Initial monolayer adsorption followed by multilayer adsorption after the surface is saturated.
52
Describe a Type II isotherm.
Found in non-porous or macroporous solids. Indicates multilayer formation and a wide range of pore sizes.
53
What is the shape of a Type III adsorption isotherm?
Convex upward curve with no plateau.
54
What is happening physically in a Type III isotherm?
Weak interaction between adsorbent and adsorbate; significant adsorption only occurs at high pressure due to adsorbate–adsorbate interactions.
55
Describe a Type III isotherm.
Uncommon; suggests clustering of adsorbate and poor adsorption at low pressure.
56
What is the shape of a Type IV adsorption isotherm?
Similar to Type II with an inflection point, but includes a hysteresis loop.
57
What is happening physically in a Type IV isotherm?
Multilayer adsorption followed by capillary condensation in mesopores; hysteresis arises due to delayed desorption.
58
Describe a Type IV isotherm.
Indicates mesoporous structure (2–50 nm pores) and capillary effects.
59
What is the shape of a Type V adsorption isotherm?
Convex upward curve with a hysteresis loop.
60
What is happening physically in a Type V isotherm?
Weak surface interaction at first, followed by capillary condensation at high pressure.
61
Describe a Type V isotherm.
Like Type III at first, but shows pore condensation; rare and seen in specific mesoporous materials.
62
What is the shape of a Type VI adsorption isotherm?
Stepwise curve with sharp vertical jumps between flat sections.
63
What is happening physically in a Type VI isotherm?
Layer-by-layer adsorption on a highly uniform surface; each step corresponds to a new adsorbed layer.
64
Describe a Type VI isotherm.
Indicates strong, ordered surface–adsorbate interactions and typically occurs near melting point of the adsorbate gas.

Multicomponent Distillation (8 decks)

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