RoNz 424302 Massöverföring och Separationsteknik Mass transfer and separation technology Massöverföring och separationsteknik (”MÖF-ST”) 404302, 7 sp 15. Membrane separations Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory tel. 3223 ; [email protected]RoNz 424302 Massöverföring och Separationsteknik mars 2015 15.1 Overview, Membrane units Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo 2/34
Transcript
RoNz
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ik Mass transfer and separation technologyMassöverföring och separationsteknik (”MÖF-ST”) 404302, 7 sp
15. Membrane separations
Ron ZevenhovenÅbo Akademi University
Thermal and Flow Engineering Laboratorytel. 3223 ; [email protected]
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15.1 Overview, Membrane units
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Membrane separations
Membrane separations involve the separation of a feed (stream) into components using a semi-permeable barrier through which the components move with different velocity.
The products are referred to as permeate and retentate; a so-called sweep stream can be used to remove the permeate.
The feed, permeate and retentate are usually gases or liquids but may be solids as well.
Typically for membrane separations 1) retentate and permeate are miscible; 2) the ”separating agent” is a semi-permeable barrier; and 3) sharp separations are difficult to achieve otherwise.
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Industrial application examples
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Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo
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Example: Natural gas separation
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Membrane materials The semi-permeable
barrier is either– A thin, non-porous
polymer film, or– A porous polymer,
ceramic or metal, or– A liquid or gas
Natural polymers may be wood, cotton, rubber
Many suitable synthetic organic polymers have been developed since ~ 1930
During the process the membrane should not disintegrate (dissolve, break, ...)
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Permeance Important for a membrane is
– High permeance for a given species to be separated– A high ratio of permeance for the species to be separated
– Permeance PM can be compared to a mass transfer coefficient: for a membrane with thickness ℓM (m) and driving force Δc (mol/m3, kg/m3) or Δp (Pa/m), it can be defined for a certain transport rateNi of species i per m2 area (or ”flux”) as
where PMi is the permeability for species i, and PMi is the permeance
The unit barrer is widely used for permeability, where1 barrer = 3.348·10-19 kmol· m/(m2· s· Pa)
force drivingforce driving i,M
M
i,Mi P
PN
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Membrane types Membranes can be dense (= non-porous) or (micro-) porous In dense membranes, pores are < a few Å (1Å = 0.1 nm), so that
most species must dissolve and diffuse through the material. (In crystalline materials diffusion can be difficult and proceeds primarily through amorphous regions.)
In micro-porous membranes, pores of 0.001 – 10 µm are large compared to molecule size. (Low selectivity for small molecules.)
Technical membranes are usually composites of thin, dense film on a supporting, much thicker micro-porous material → permeance for a species can be high even if permeability is low.
Polymer membrane types
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Membrane modules Important membrane shapes
for use are flat sheets, tubes, fibres, or (ceramic) monoliths
Common membrane units are available a compact modules that support the membrane and guide input / output flows
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15.2 Transport in membranes
10Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo
RoNzmars 2015 Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo
Transport in membranes /1
Mass transfer across membranes is determined by (for pore sizes large to non-existing)– convection, which can almost always be described by Hagen-Poiseuille
laminar tube flow, corrected for void fraction (voidage)– gas or liquid diffusion in pores, based on Fick’s Law and corrected for the
effect of voidage and tortuosity (both leading to collisions of diffusing species with the pore wall), for gases Knudsen diffusion
– solid diffusion, where the gas or liquid component is absorbed, diffuses through the membran and desorbs at the other end.
Transport in membranes: (a) Bulk flow (convection), (b) Pore diffusion, (c) Restricteddiffusion, (d) Solution diffusion.a-c: Porous membrane; d: Dense membrane)
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Transport in membranes /2 For dense membranes, the so-called solution-diffusion model
describes the concentration profile across a membrane by assuming phase equilibrium at the fluid-membrane surfaces
For gas separations, external mass transfer resistances can be neglected compared to membrane diffusion resistance
Concentration / partial pressure profiles for transport through membranes:(a) Porous membrane, liquids(b) Dense membrane, liquids(a) Porous membrane, gases(b) Dense membrane, gases
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Transport in membranes /3
The most important flow patterns are based on mixing (which may be limited in practice), co-current flow, counter flow or cross flow
For fast permeation rates the cross-flow model may be appliedFor example → with pressures pP and pF, transfer of n moles of species A, membrane area AM(m2) for permeability PMA:
θ = ”cut” fraction of feed
RA
FA
x
x FAPAMA
AMM pxpyP
dnyA
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15.3 Membrane separation processes
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Dialysis Consider a liquid feed at pressure p1,
containing solutes A and B and insoluble, dispersed matter (colloidal) in a solvent; at the other side of the membrane pressure is p2 and a sweep flow removes permeate
Solutes A and B are transported →through the membrane by dialysis
Dialysis (from Greek δια = ”through” and λυρις= ”to loosen”)
If p1= p2, solvent can pass the membrane ← by osmosis (p1 > p2 stop or will prevent osmosis)
Colloidal matter cannot pass the membrane
Important applications:Reverse osmosisElectrodialysis / electro-osmosisThermal osmosis
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Electrodialysis
Different products are collected in different compartments
A DC current is applied using two chemically neutral electrodes
Electrodialysis has been usedsince ~1900 to improve dialysisrates in electrolyte solutions
Currently, electro-dialysis is used for separating (aqueous) electrolyte solutions intoconcentrate (”brine”) and diluate (”desalted water”)
Ion-selective membranesare used: anion-selective membranes carry a positive charge and permit passage of negative ions (anions), vice versafor cation-selective membranes
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Reverse osmosis
Osmosis (from Greek oσμως = ”push”) implies to transfer of solvent througha membrane more permeable for solvent than for solute.
The osmotic pressure π can be estimated by π = RT· c/M, with solute (dissolved salt) concentration c and average molar mass M for the solute (dissolved salt ions).
Important application: de-salination of sea water or brackish water
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Gas permeation Gas permeation allows for
separating light species (< 50 g/mol) from small amounts of more heavy species, at high feed gas pressure
Permeate-side pressure is much lower, typically 1 atm
Usually dense membranes, sometimes micro-porous
Examples:– H2 removal from methane– O2 enrichment in air– CO2 removal– Natural gas purification– Removal of pollutants from air
Picture: SH06
Important alternative for cryogenic distillation
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Pervaporation
Pictures: SH06
In pervaporation the phase state on either side of the membrane is different
For a liquid feed mixture with solute species A and B, keeping the pressure p2below the vapour pressure for species A (may be near vacuum) increases the selectivity for A
Inside the membrane, a liquid zone and a vapour zone can be distinguished, and swelling may occur
Evaporation of A has a heat effect: operate adiabatically or with heat transfer
Examples:– Separation of water from alcohols, esters,
ketones, other organics– Cyclohexane – benzene separation
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Ultrafitration, microfiltration /1 Ultrafiltration, microfiltration are (like reverse osmosis) pressure driven Micro-porous membranes allow water and small molecules to pass,
