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Materials Chemistry for Environmental Appications > L3 - Membrane Fundamentals > Flashcards

L3 - Membrane Fundamentals Flashcards

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1
Q

What is the primary function of a membrane?

A

It acts as a thin interface that regulates the permeation of chemical species, allowing selective passage.

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2
Q

What is the difference between membranes and filters?

A

Filters are used to separate particles larger than 10 microns, while membranes allow selective passage based on solubility and mobility.

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3
Q

What is the solution-diffusion model?

A

A mechanism for dense membranes where permeation depends on solubility and concentration gradients, following Fick’s Law of Diffusion.

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4
Q

What does the pore flow model describe?

A

It describes molecular filtration in microporous membranes where separation occurs through molecular filtration, driven by pressure differences and described by Darcy’s Law. These membranes generally offer higher flux than diffusion models.

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5
Q

What is an isotropic membrane?

A

A membrane with a uniform structure, which can be dense or microporous.

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6
Q

What is an anisotropic membrane?

A

A membrane with a thin functional layer on a porous support, enhancing transport rates by reducing thickness.

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7
Q

What are charged membranes used for?

A

Ion exchange; they separate ions based on charge, repelling ions with a charge similar to the membrane.

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8
Q

Name common membrane processing technique.

A

Solution casting, phase separation, or interfacial composite membrane formation.

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9
Q

What are nano-enabled membranes?

A

Membranes containing nanoparticles like graphene or carbon nanotubes, enhancing ion exchange or catalytic functions.

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10
Q

List some methods for characterizing membrane pore size.

A

Gas adsorption,
mercury porosimetry,
thermoporosimetry,
microscopy (SEM, AFM)
NMR.

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11
Q

What is water flux measurement?

A

The flow of water through a membrane, calculated by measuring flux and pressure gradients.

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12
Q

List three industrial applications of membranes.

A

Gas separation, water purification, and air filtration.

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13
Q

What is a polymer membrane?

A

Membranes made from rubbery or glassy polymers, which show different permeability based on molecular structure.

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14
Q

Describe the unique feature of graphene membranes.

A

They have an atomic thickness and can be tailored with nanopores for specific separations, such as gas or ion filtration.

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15
Q

What are carbon-based membranes, and what are they used for?

A

Carbon nanotube or carbon molecular sieve membranes, used for selective gas permeability in applications like air purification.

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16
Q

What tests are used to assess membrane mechanical properties?

A

Tensile strength and flexural strength tests.

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17
Q

How is membrane functionality characterized?

A

Through rejection capacity, molecular weight cut-off (MWCO), and adsorption capacity.

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18
Q

What are the three types of diffusion through a membrane?

A

Knudsen diffusion, molecular diffusion, and surface/solution diffusion.

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19
Q

Define Knudsen diffusion.

A

A diffusion mechanism in long, narrow pores (< 50 nm), where molecules collide more with the pore walls than with each other.

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20
Q

What is molecular diffusion in membranes?

A

Molecular diffusion in membranes is when molecules move through pores due to differences in concentration on each side of the membrane.

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21
Q

What is surface diffusion?

A

Occurs when permeants adsorb along the pore walls due to high affinity for the membrane surface.

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22
Q

How is permeability different from permeance in membranes?

A

Permeance = Depends on thickness + material
Permeability = Only depends on the material

Permeance measures how easily a substance moves through a membrane, including the effect of thickness.

Permeability describes the material’s natural ability to let substances pass, without thickness being a factor.
Formula: Permeability = Permeance × Thickness (by multiplying with thickness it gets removed since it is already in permeance as flow/thickness)

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23
Q

Why are membranes advantageous in industrial applications?

A

They avoid phase changes, are modular and scalable, use resources efficiently, and can recycle by-products.

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24
Q

List some key industrial applications of membranes.

A

Gas separation, water purification, air filtration, and removal of CO₂, H₂S, and VOCs from gas streams.

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25
Q

How do polymer membranes function based on their structure?

A

Rubbery polymers allow larger molecules to permeate, while glassy polymers are rigid, favoring small molecules.

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26
Q

What unique property does graphene bring to membranes?

A

Atomic thickness with tunable nanopores, suitable for applications like gas or ion filtration.

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27
Q

What are carbon-based membranes and their primary types?

A

Carbon molecular sieve membranes, CNT membranes, and graphene membranes, used for selective gas separation.

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28
Q

How is pore size characterized by the Barrett-Joyner-Halenda method?

A

Through nitrogen gas adsorption, suitable for mesoporous samples up to 100 nm in dry conditions.

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29
Q

Explain the concept of bubble point in membrane characterization.

A

A measure of pore diameter, calculated by the pressure required for gas to bubble through a wetted membrane.

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30
Q

What does molecular weight cut-off (MWCO) indicate in membrane characterization?

A

The molecular weight at which 90% of macromolecular solutes are rejected by the membrane.

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31
Q

How do conventional air purifiers use membranes?

A

They filter pollutants within a specific size range, typically failing to remove VOCs and some viruses.

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32
Q

What are two common applications for graphene membranes?

A

Water purification and selective gas separation.

33
Q

What is the Barrett-Joyner-Halenda (BJH) method used for in membrane characterization?

A

The Barrett-Joyner-Halenda (BJH) method measures pore volume and pore size distribution, particularly in mesoporous materials, using nitrogen gas adsorption up to a relative pressure of 1. It assumes cylindrical pores and works best for dry samples with pore sizes up to 100 nm.

34
Q

What is microfiltration (MF) in membrane technology?

A

Microfiltration (MF) has a pore size of 0.1–10 microns and removes large particles like suspended solids, bacteria, and some fats.

35
Q

What are the typical applications of microfiltration (MF)?

A

Microfiltration is used in wastewater treatment, dairy processing, and as a pre-treatment in water purification.

36
Q

What is ultrafiltration (UF) in membrane technology?

A

Ultrafiltration (UF) has a pore size of 0.01–0.1 microns, filtering out smaller particles like viruses, proteins, and fine colloidal materials.

37
Q

What are the typical applications of ultrafiltration (UF)?

A

Ultrafiltration is used in food and beverage processing, pharmaceuticals, and water treatment to remove pathogens.

38
Q

What is nanofiltration (NF) in membrane technology?

A

Nanofiltration (NF) has a pore size of 1–10 nanometers, removing most organic molecules, certain salts, and smaller contaminants.

39
Q

What are the typical applications of nanofiltration (NF)?

A

Nanofiltration is used for softening water, removing certain dissolved salts, and treating wastewater to remove trace organic contaminants.

40
Q

What is reverse osmosis (RO) in membrane technology?

A

Reverse osmosis (RO) has an extremely small pore size (~0.1 nanometers) that filters out ions, dissolved salts, and small organic molecules, producing very pure water.

41
Q

What are the typical applications of reverse osmosis (RO)?

A

Reverse osmosis is used in desalination, industrial water purification, and producing drinking water in areas with limited fresh water sources.

42
Q

What is the structure and function of an isotropic microporous membrane?

A

Homogeneous
structure, similar to
filter but with
significantly smaller
pores (0.01-10
microns in diameter).
Separates solutes by
molecular size via size
exclusion.

43
Q

What is the structure and function of an electrically charged membrane?

A

Can be dense or
microporous and
functions as ion
exchange membranes.
Excludes ions with
same charge as the
membrane. Efficiency
affected by charge on
concentration of ions

44
Q

What is the structure and function of a nonporous dense membrane?

A

Dense films where
permeants are
transported by diffusion,
driven by pressure, conc.
or electrical potential
gradient. Separation of
similar sized permeants
occur if they differ in the
solubility in the membrane
(pervaporation, RO etc).

45
Q

What is screen filtration, and how does it work?

A

Screen filtration uses a thin, porous barrier with uniform holes to block particles larger than the pore size. It captures particles on the surface of the filter and is effective for filtering larger particles.

46
Q

What are common applications of screen filtration?

A

Screen filtration is commonly used in water purification, industrial process filtration, and pre-filtration for other systems to remove large particles.

47
Q

What is depth filtration, and how does it work?

A

Depth filtration uses a thick, porous medium that traps particles throughout the filter material, not just on the surface. It captures a wide range of particle sizes and allows higher dirt-holding capacity.

48
Q

What are the advantages of depth filtration?

A

Depth filtration has a high dirt-holding capacity, captures particles of varying sizes, and provides a longer lifespan in filtration processes due to capturing particles within the material.

49
Q

In which applications is depth filtration commonly used?

A

Depth filtration is used in applications like drinking water treatment, oil and gas processing, and pharmaceutical manufacturing to remove fine particles and contaminants.

50
Q

What are the four main particle capture mechanisms in depth microfiltration?

A

The four main mechanisms are sieving, electrostatic adsorption, inertial impaction, and Brownian diffusion.

51
Q

How does Brownian diffusion contribute to particle capture in depth microfiltration?

A

Brownian diffusion captures very small particles that move randomly due to molecular motion, leading them to collide with the filter fibers.

52
Q

What is inertial impaction in depth microfiltration?

A

Inertial impaction occurs when heavier particles deviate from the fluid streamlines due to their inertia and collide with the filter fibers, capturing larger particles.

53
Q

How does electrostatic adsorption work in depth microfiltration?

A

Electrostatic adsorption captures particles by attracting them to oppositely charged fibers in the filter, effective for particles smaller than the pore size.

54
Q

What is the sieving mechanism in depth microfiltration?

A

Sieving blocks particles that are larger than the filter pores, using a size-exclusion method similar to screen filtration.

55
Q

Why is molecular weight cut-off (MWCO) considered a useful tool for characterizing ultrafiltration (UF) membranes?

A

MWCO is useful because it defines the size limit at which a membrane will reject 90% of molecules, helping determine the membrane’s effectiveness in filtering specific sizes, making it essential for characterizing UF membranes.

56
Q

Explain why membrane systems offer greater efficiency in raw material usage and by-product recycling compared to conventional separation methods.

A

Compared to conventional separation methods like distillation or chemical treatments; Membrane systems use less energy and chemicals, and they allow easier recovery and reuse of by-products, making them more efficient and sustainable.

57
Q

List some current applications of membrane technology in gas separation.

A
  1. Removal of nitrogen or oxygen from air
  2. Removal of volatile organic components (VOCs) from exhaust streams
  3. Removal of CO₂ from natural gas
  4. Separation of hydrogen from gases like nitrogen and methane
  5. Recovery of hydrogen from ammonia plant product streams
  6. Recovery of hydrogen in oil refinery processes
  7. Separation of methane from biogas
  8. Enrichment of oxygen from air for medical or metallurgical use
  9. Removal of H₂S from natural gas
58
Q

What are the main characteristics of glassy polymers in membrane technology?

A

Glassy polymers are hard and rigid, with tightly packed molecular chains that limit space for larger molecules. This structure allows smaller molecules to pass through more easily.

59
Q

Why do glassy polymers favor the permeation of smaller molecules?

A

Glassy polymers have a rigid and tightly packed structure, which restricts space, making it easier for small molecules to pass through while blocking larger ones.

60
Q

What are the main characteristics of rubbery polymers in membrane technology?

A

Rubbery polymers are soft and flexible, with mobile molecular chains that create space for larger molecules to dissolve into and pass through the membrane.

61
Q

Why do rubbery polymers favor the permeation of larger molecules?

A

The flexible structure of rubbery polymers allows larger molecules to dissolve into the membrane and move through it more easily than smaller molecules.

62
Q

What is a template-synthesized membrane with carbon nanotubes?

A

A membrane made by adding carbon materials into tiny pores of a pre-made structure, like anodized alumina, to create carbon nanotubes. The template guides the shape and size of the nanotubes.

63
Q

What is a dense-array outer-wall membrane with carbon nanotubes?

A

A membrane where carbon nanotubes are arranged vertically in a dense layer, leaving small gaps between the tubes. These spaces allow for filtration, using the outer walls of the carbon nanotubes.

64
Q

How are open-ended carbon nanotube membranes made?

A

Carbon nanotubes are grown vertically and covered with a solid material (like a polymer or ceramic). Then, plasma treatment opens the tips of the tubes, allowing substances to pass through the carbon nanotubes directly.

65
Q

What is a mixed-matrix membrane with carbon nanotubes?

A

A membrane where carbon nanotubes are mixed into a polymer material, with the nanotubes acting as “fillers.” This gives the membrane added strength or specific filtering properties.

66
Q

List the six main techniques used in conventional air purification as shown in the image.

A

electrostatic precipitation, adsorption, filtration, plasma cleaning technology, ultraviolet sterilization, and photocatalysis.

67
Q

What role does photocatalysis play in air purification?

A

helps remove inorganic gases and microbes from the air.

68
Q

What contaminants does ultraviolet sterilization target in air purification?

A

microbes in the air.

69
Q

What is plasma cleaning technology used for in air purification?

A

removes volatile organic compounds, bacteria, viruses, and some suspended particles from the air.

70
Q

What types of contaminants does filtration remove in air purification?

A

bacteria and suspended particles from the air.

71
Q

Which air contaminants does adsorption target in air purification?

A

volatile organic compounds

72
Q

What is the purpose of an electrostatic precipitator in air purification?

A

removes suspended particles and microbes from the air by using an electric charge to capture particles.

73
Q

What is a graphene membrane?

A

A graphene membrane is an advanced membrane made from a single layer of carbon atoms arranged in a hexagonal lattice, with exceptional filtration and separation properties.

74
Q

What are the key properties of graphene membranes?

A

Graphene membranes are extremely thin (atomic thickness), highly strong, and can have selective nanopores, allowing high permeability and selective filtering.

75
Q

Why is the atomic thickness of graphene membranes beneficial?

A

The atomic thickness allows high permeability, enabling fast flow of water or gases through the membrane.

76
Q

How does the mechanical strength of graphene membranes contribute to their function?

A

The high mechanical strength makes graphene membranes durable and resistant to damage, even though they are only one atom thick.

77
Q

What is selective pore size in graphene membranes, and why is it important?

A

Selective pore size allows graphene membranes to let specific molecules pass while blocking others, making them effective for precise filtration and separation.

78
Q

What are common applications of graphene membranes?

A

Common applications include water purification (desalination), gas separation (like hydrogen separation), and biomedical uses (drug delivery and ion filtration).

79
Q

How do graphene membranes work for filtration?

A

Graphene membranes filter substances based on size exclusion (blocking larger particles) and chemical interactions, allowing only certain molecules to pass.

Materials Chemistry for Environmental Appications (8 decks)

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