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Chem Eng - Y1.S2 > Principles - Membrane Separation > Flashcards

Principles - Membrane Separation Flashcards

1
Q

What’s perm-selectivity?

A

Separating components by rejecting constituents selectively.

Membranes
are more permeable to those constituents passing through it (permeate) than those which are rejected by it (retentate)

Permeate = effluent
Retentate = what remains in reactor.
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2
Q

What’s permeation flux?

A

Permeate flow specific to membrane surface area at constant temperature, pressure, electric field

Measured: L3·L-2·T-1 (e.g., m3·m- 2·d-1)

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3
Q

What does MBR represent?

A

Membrane bioreactor

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4
Q

What does the concentrate and permeate from an MBR (membrane bioreactor) contain?

A

Permeate: Ss and So

Concentrate: Ss, So and Xb

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5
Q

When will substances ‘A’ and ‘B’ mix?

A

Chemical separation requires energy (heat, mechanical)

A + B substances will mix when the free enthalpy of the mixture (DGmix) is lower than the sum of the DG of the substances.

Minimum separation energy (Wmin) required is equal to or larger than the free enthalpy of mixing:

Wₘᵢₙ > ΔG ₘᵢₓ = ΔHₘᵢₓ - TΔSₘᵢₓ

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6
Q

What is considered on selection of membranes?

A 
Membranes vary widely in their structure, function and operation:
– Solid or liquid
– Homogeneous or heterogeneous 
– 0.1 μm –100’s mm thick
– Isotropic or anisotropic

As well as the conditions under which they perform:
- Ambient temperature
– By physical means (no chemical alteration) – Properties can be tailored for use
– Technically simpler and energy efficient

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7
Q

How can membranes be defined by pore size?

A
  • Membranes can be defined according to the type of separation duty, providing an indication of the pore size.

Daltons (Da) – are equivalent to the mass of the smallest molecule that the membrane is capable of rejecting* (1 Da = the mass of a hydrogen atom).

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8
Q

What’s MF, UF and NF?

A

Microfiltration

Ultrafiltration

Nanofiltration

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9
Q

What’s RO?

A

Reverse osmosis

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10
Q

What can MF, UF and NF be used for?

A

Microfiltration - can reject particulate matter and retain bacteria

Ultrafiltration - can also reject viruses

Nanofiltration - is more selective than reverse osmosis (RO), rejecting bulk organic matter and micropollutants,

Reverse osmosis (RO) can also reject singly-charged (i.e. monovalent) ions (Na+) and (Cl-).

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11
Q

What is the permeate and retentate?

A

Permeate = effluent

Retentate = what remains in reactor

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12
Q

What’s a membrane?

A

A material which allows some physical, chemical, or biological components to pass more readily through it than others.

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13
Q

How is specific mass flux calculated?

A

Specific mass flux = velocity * concentration

j = v*C

[Intensive form of flux:
dj/dx = (Q/v)*Ss

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14
Q

How is total mass flux calculated?

A

Total mass flux = flow rate * concentration

J = C*Q

[Intensive form;
J,Ss = Q*Ss

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15
Q

What factors influence the performance of a membrane?

A

Chemical propensities

Electrical charge of membrane material

Chemical composition of membrane

Driving force applied (e.g. pressure)

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16
Q

What are advs and disadvs of membrane processes?

A

Adv:

  • Continuous rather than batch
  • Low energy consumption
  • Easy to scale up
  • Ambient temperature control (of food, drugs, microbes etc.)

Disadv:

  • Concentration polarisation
  • Fouling
  • Low membrane lifetimes
  • Immature technology
  • Low selectivity in some cases
  • Low flux can be too costly
  • Scale up is linear
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17
Q

What’s selectivity?

A

The measure of the ability of the membrane to separate out different components.

The selectivity is usually expressed using the retention

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18
Q

How is retention calculated?

A

R = (Cf - Cp) / Cf

= 1 - Cp / Cf

Therefore if a membrane completely rejects a solute, Cp = 0 and then R = 1 (perfect separation).

If the membrane does nothing and passes all solute through into the permeate, Cp = Cf (then R = 0)

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19
Q

What are typical membrane materials?

A

Initially, cellulosic materials were mainly used.

More recently, polyamide, polysulphone, polycarbonate and other advanced polymers used.

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20
Q

What are asymmetric membranes?

A

Membranes with a thin, dense top layer (0.5 um), and a porous sublayer.

The top layer / skin determines transport rate.
The porous layer acts only as a support.

The permeation rate is inversely proportional to the thickness of the actual barrier layer

Asymmetric membranes show a higher permeation rate (flux) than (homogeneous) symmetric membranes of a comparable thickness.

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21
Q

What is micro-filtration (MF) used for?

A

To remove bacterial cells and spores

Pharmaceutical production
Water treatment
Fermentation broths

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22
Q

What is ultrafiltration (UF) used for?

A

Removal of macromolecules e.g. proteins and carbohydrates

  • food products
  • pulp and paper industry
  • pharmaceutical production
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23
Q

What’s reverse osmosis (RO) used for?

A

To remove salts.

Used to produce drinking water from brackish or sea
water and production of high grade water supplies for industry or the laboratory

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24
Q

How can membranes be distinguished?

A

For being porous, non-porous or carried membranes.

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25
What does MWCO stand for?
Molecular weight cut off It refers to the lowest molecular weight solute (in Da) of which 90% is retained by the membrane.
26
What’s molecular weight cut off?
The lowest molecular weight solute (in Da) of which 90% is retained by the membrane. Molecules with mass near the MWCO will diffuse across the membrane slower than molecules significantly smaller than the MWCO.
27
What’s sintering?
A method of forming a membrane: Compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction. It is only suitable for micro-filtration membranes and has generally low porosity.
28
What’s stretching (membrane tech)?
Formation of a membrane, where mechanical stress is applied and small ruptures occur in a partially crystalline polymeric material. This produces pores, useful for ultrafiltration.
29
What is track etching?
A method of making a membrane, where high energy particle radiation is applied perpendicular to polymer films. This damages the polymer and creates tracks. It is then immersed in an acid bath and the polymer is etched away along these tracks, making cylindrical pores.
30
What’s phase inversion?
A method of making membranes where a polymer is changed from liquid to solid. Suitable for RO.
31
What’s microfiltration and it’s characteristics?
A pressure driven membrane process. The solvent is the continuous phase. Salute concentration is quite low. Pore sizes of MF membranes range from 10 to 0.05 μm The process suitable for retaining suspensions and emulsions. Typically used for microbes
32
What’s the microfiltration membrane structure based on?
Particle size of solute Molecular size of solute Chemical properties Pore size Pore distribution / porosity
33
How are synthetic polymers divided into 2 classes?
Hydrophobic and hydrophilic The angle the drop makes with the surface indicates whether the surface is hydrophilic or hydrophobic. Ø > 90, indicates a hydrophobic surface. Ø < 90 indicates a hydrophilic surface.
34
What’s Darcy’s law?
J = k P Where: J is flux P is the pressure drop K is permeation coefficient k - the permeation coefficient, which includes (lumps) factors, such as, resistances, viscosity, porosity, pore size and pore size distribution.
35
What’s the Hagen-Poiseuille relationship?
Relationship for straight capillaries J = (Ęr^2 / 8 mu t ) * P/X Where: J - flux (m3 m-2 s-1) Ę - porosity (%) r - pore radius (m) X - membrane thickness (m) mu - dynamic viscosity (kg m-1 s-1) t- tortuosity factor P- pressure drop (N m-2)
36
What’s porosity?
The fraction of void space in the material, where the void may contain, for example, air or water. It is defined by the ratio: Ę = Vv / Vt Where: Vv - volume of void space Vt - total or bulk volume of material, including the solid and void volumes.
37
What’s the tortuosity factor?
A measure of how bendy a path is. If straight, t is close to one. The greater the value (above 1), the bendier the path.
38
What are nodular structures?
Structures that resemble an assembly of spherical particles.
39
What’s the Kozeny-Carman equation (used for nodular structures)?
J = (Ę^3r^2 / 45 mu (1 - Ę)^2) * P/X Where: J - flux (m3 m-2 s-1) Ę - porosity (%) r - pore radius (m) X - membrane thickness (m) mu - dynamic viscosity (kg m-1 s-1) P- pressure drop (N m-2)
40
What’s the main issue with microfiltration?
Flux decline. This is caused by either concentration polarisation or fouling. To reduce fouling the surface of an MF membrane is swept clear using cross-flow.
41
What are characteristics of ultrafiltration?
Ultrafiltration lies between nano and microfiltration. | 0.5 nm to 0.1 um
42
What’s the MWCO (molecular weight cut off) of ultrafiltration?
2000 Da
43
How is UF membrane flux calculated?
J = h(D) * ln (Cw/Cf) Where: Cw/Cf - polarisation modulus h(D) is overall mass transport coefficient D/l (D is diffusion coefficient and l is film thickness)
44
What happens if there’s an increase in pressure u dear pressure independent conditions (UF)?
If an increase in pressure occurs under pressure independent conditions, this produces a temporary increase in flux which brings more solute to the gel-layer and increases its thickness → reducing the flux to the ini al level.
45
What does the concentration-polarisation model show?
As the concentration at the membrane increases, the solute eventually precipitates on the membrane surface. This layer of precipitated solute is known as the gel- layer with a concentration CG
46
What are the properties of UF?
UF membranes have an asymmetric structure with a denser top layer (smaller pore size and lower surface porosity); They have a much higher hydrodynamic resistance; UF usually operates at a much higher pressure than MF does; The top layer is usually less than 1 μm.
47
How is the chemical potential of component ‘i’ in a mixture defined?
It is defined in thermodynamics as partial molar free energy – a form of potential energy that can be absorbed or released during a chemical reaction or phase transition. dG = the sum of chemical potentials of species * change of particle numbers.
48
When does osmotic pressure arise?
When two solutions of different concentration (chemical potential, mu) are separated by a semipermeable membrane. The solvent molecules in the dilute phase have a higher (more negative) chemical potential than those in the concentrated phase. Mu causes the flow and the flow continues until osmotic equilibrium is reached.
49
How is osmotic pressure (pi) calculated?
Pi = icRT ``` Where: I - Van ‘t Hoff factor c - Molar concentration of salt ions R - gas constant T - temperature (K) ```
50
What are properties of RO?
The main application of reverse osmosis is too separate out salt from water. The solute molecules are much smaller than for UF – Denser membranes are needed to separate them – A much higher hydraulic resistance is involved – Much higher transmembrane pressures are needed Most have an asymmetric structure.
51
What’s dip-coating?
The simplest method for producing a composite membrane. Once the sub layer is produced, the thin dense layer is made by dip coating: 1. support is immersed in a bath of the coating (polymer, pre- polymer or monomer in solvent). 2. The membrane is removed 3. the solvent evaporates 4. This leaves a thin layer on the support
52
What’s in-situ polymerisation (membranes)?
When the top layer of a composite membrane is polymerised in top of the support
53
What’s interfacial polymerisation?
A method of forming composite membranes where two extremely reactive monomers are brought together at the interface between two immiscible solvents.
54
How are the fundamental properties of MF characterised?
Materials used: poly (PE, PP, PC etc) Main applications: particle separation (yeast and viruses) Membrane structures: symmetric or asymmetric. Pore size: 0.1-10 um. Driving force: pressure below 2 bar
55
How do you determine membrane flux using Darcy’s law?
The volume flow through a MF membrane is directly proportional to the difference between transmembrane pressure change and osmotic pressure change across the membrane.
56
How do you determine the chemical potential across a membrane?
It can be calculated as a function of molar chemical potential, chemical activity volume, pressure and temperature.
57
What is used to calculate the osmotic pressure of seawater?
The Van’t Hoff formula (= pi)
58
What causes membrane flux decline?
– Membrane resistance – Concentration polarisation and gel layer formation – Pore blocking by solute and/or biofilm formation – Solute adsorption – Changes in feed viscosity These phenomena introduce additional resistances on the feed side to the transport across the membrane.
59
What is gel resistance?
Resistance caused by concentration polarisation is known as R.cp. However, conc’ of accumulated solute molecules may become so high that the gel layer formed on the membrane results in minimal flux across the membrane. This gel has the resistance, Rg.
60
What materials cause fouling?
``` Foulants include: – polysaccharides – proteins – minerals – microbial cells ``` Foulants form a cake on the membrane surface, or clog the membrane’s pores.
61
How can fouling be reduced?
– Pre-treatment of feed solution – Selection of appropriate membrane properties – Selection of effective module design – Period cleaning the membrane
62
How can a feed solution be pretreated?
– heat treatment – addition of chelating agents (e.g. EDTA) – chlorination – pre-microfiltration or pre-ultrafiltration - pH adjustment
63
How is pH used to reduce fouling?
The pH where overall charge on a protein is zero is known as the isoelectric point. Without a net charge, the protein is less likely to foul the membrane.
64
What membrane properties affect fouling characteristics.
Charge Roughness Pore size distribution Hydrophobicity Proteins typically adsorb at hydrophilic surfaces more easily. Surfactants can be used to change hydrophobicity. Negatively charged colloids would be filtered best with a -ve charged membrane.
65
How does hydrophobicity affect membrane properties? | Regards to fouling
Membrane fouling is determined by hydrophilicity/hydrophobicity and membrane chemistry. Hydrophilic membranes - the surface adsorbs water, and so reduces the organic matter binding. They show resistance to fouling by hydrophobic substances (so have higher separation eff) Hydrophobic membranes show resistance to fouling by hydrophilic substances.
66
What are the different physical and chemical membrane cleaning methods?
Physical: Backflushing Relaxation Chemical: Using a base - caustic soda/citric/oxalic Acid - hydrochloric, sulphuric etc Oxidant e.g. hydrogen peroxide or hypochlorite. Both:
67
What happens in membrane backflushing (to clean)?
– feed pressure is released – a small volume of permeate is forced back through the membrane – This removes foulants stuck on the surface or in the pores
68
What happens in the back-flushing of RO/NF membranes?
Concentrated salt solution use instead of air. High osmotic pressure on the feed side causes permeation flow to change direction - direct osmosis (DO) - as the slug moves along the membrane, thereby removing scaling; High salt concentration also kills bacteria growing e.g., in biofilm
69
What are the (3) types of membrane cleaning?
Chemical Mechanical Physical
70
What’s a module (membranes)
The smallest unit into which the membrane area is packed. Several module designs are possible based on two types of membrane configuration: • Flat membranes – Plate and frame – Spiral wound • Tubular membranes – Tubular – Capillary – Hollow fibre
71
What are the 2 main configurations of membrane modules?
Flat membranes Tubular membranes
72
What are the properties of plate and frame (flat) membrane modules?
Two membranes are sandwiched together Feed sides facing out Packing density is 100 – 400 m2 m-3 Area/module=19 m2
73
What are the properties of spiral wound (flat) modules?
Similar idea, but wrapped around a central collection pipe The feed flows axially through the cylindrical module, parallel to the pipe in turbulence promoting mesh separators The permeate flows into the central pipe Compact density – 300 – 1000 m2 m-3 These systems are highly prone to fouling; Area/module=5 m2
74
What are the main properties of tubular modules?
Tubular membranes are usually ceramic in nature. The packing density is low (< 300 m2 m-3) The membranes are generally brittle and expensive Very resistant to chemical cleaning and can be autoclaved if necessary to kill bacteria. Membrane lifetime is also much greater with ceramics than for polymers. Advantageous to have a turbulent flow regime (Re>10000) e.g., with high solids Consists of a number of tubes mounted on a porous support – The feed is passed down the tubes – Permeate passes through the porous support into the module housing Can be capillary modules (small) or hollow fibre modules (even smaller) also
75
What are the arrangements of membranes in plant design?
Feed and bleed Multiple stage feed and bleed Continuous single-pass membrane plants
76
What’s concentration polarisation?
When solvent passes through a membrane, the solutes remain behind/are retained by the membrane. The build up of molecules on the membrane surface results in the diffusion of molecules away from the membrane surface.
77
What’s the permeate and what’s the retentate?
Permeate - low concentrate stream Retentate - concentrated stream
78
What chemical agents are used to clean i) proteins, ii) salts/water scale, iii) bacteria/yeast, iv) oils/fats from membranes?
i) alkali ii) acids iii) hydrogen peroxide iv) surfactants

Chem Eng - Y1.S2 (10 decks)

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