Principles - Membrane Separation Flashcards
What’s perm-selectivity?
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.
What’s permeation flux?
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)
What does MBR represent?
Membrane bioreactor
What does the concentrate and permeate from an MBR (membrane bioreactor) contain?
Permeate: Ss and So
Concentrate: Ss, So and Xb
When will substances ‘A’ and ‘B’ mix?
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
What is considered on selection of membranes?
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
How can membranes be defined by pore size?
- 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).
What’s MF, UF and NF?
Microfiltration
Ultrafiltration
Nanofiltration
What’s RO?
Reverse osmosis
What can MF, UF and NF be used for?
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-).
What is the permeate and retentate?
Permeate = effluent
Retentate = what remains in reactor
What’s a membrane?
A material which allows some physical, chemical, or biological components to pass more readily through it than others.
How is specific mass flux calculated?
Specific mass flux = velocity * concentration
j = v*C
[Intensive form of flux:
dj/dx = (Q/v)*Ss
How is total mass flux calculated?
Total mass flux = flow rate * concentration
J = C*Q
[Intensive form;
J,Ss = Q*Ss
What factors influence the performance of a membrane?
Chemical propensities
Electrical charge of membrane material
Chemical composition of membrane
Driving force applied (e.g. pressure)
What are advs and disadvs of membrane processes?
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
What’s selectivity?
The measure of the ability of the membrane to separate out different components.
The selectivity is usually expressed using the retention
How is retention calculated?
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)
What are typical membrane materials?
Initially, cellulosic materials were mainly used.
More recently, polyamide, polysulphone, polycarbonate and other advanced polymers used.
What are asymmetric membranes?
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.
What is micro-filtration (MF) used for?
To remove bacterial cells and spores
Pharmaceutical production
Water treatment
Fermentation broths
What is ultrafiltration (UF) used for?
Removal of macromolecules e.g. proteins and carbohydrates
- food products
- pulp and paper industry
- pharmaceutical production
What’s reverse osmosis (RO) used for?
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
How can membranes be distinguished?
For being porous, non-porous or carried membranes.
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.
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.
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.
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.
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.
What’s phase inversion?
A method of making membranes where a polymer is changed from liquid to solid.
Suitable for RO.
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