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Membrane technologies: Overview on concepts and recent trends
Mathias Ulbricht
Lehrstuhl für Technische Chemie II Zentrum für Wasser- und Umweltforschung (ZWU)
Center for Nanointegration Duisburg-Essen (CENIDE) Universität Duisburg-Essen, 45117 Essen, Germany
www.uni-due.de/tech2chem
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membrane technologies
potential absolute barriers for water purification
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
membrane module
permeate feed
Membrane technologies
dead-end mode
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
membrane module
retentate feed
permeate
Flux: J = permeate volume or mass or mole / membrane area * time
Rejection: R = cfeed – cpermeate / cfeed
Membrane technologies
Ji = K * ∆Xi / ∆x
gradient (driving force)
mobility through membrane
cross-flow mode
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes ― Transport mechanisms
Porous vs. non-porous barrier
Porous membranes separate by filtration / size exclusion (as function of flow through pores)
Non-porous membranes separate by solution in and/or diffusion through
the membrane material
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membrane technologies
Classification of membranes
Isotropic membranes
Anisotropic membranes
R. Baker, 2004.
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes ― Separation mechanisms
Porous vs. non-porous barrier
Porous membranes separate by filtration / size exclusion (as function of flow through pores)
separation of heterogeneous mixtures or colloidal systems, based on size and/or particle-surface interactions
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Micro- and Ultrafiltration
Porous barrier — Selectivity
Surface („screen“) filtration Depth filtration
Ultrafiltration (dp = 50 – 2 nm) Nanofiltration (dp < 2 nm) Microfiltration
Microfiltration (dp > 50 nm)
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes ― Separation mechanisms
Porous vs. non-porous barrier
Porous membranes separate by filtration / size exclusion (as function of flow through pores)
Non-porous membranes separate by solution in and/or diffusion through
the membrane material
separation of homogeneous mixtures, based on molecular properties and interactions
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes ― Transport models
Non-porous barrier — Solution-diffusion model
Transport as function of concentration vs. pressure gradients
Sea water with ~3,5% salt
∆ π ~ 30 bar
Reverse osmosis
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Reverse Osmosis
Desalination performance of a good quality RO membrane (SW-30) as function of different parameters.
Separation
From solution-diffusion model:
Water flux: Ji = A (∆p – ∆π)
Salt flux: Jj = B (cj,o – cj,p)
Salt rejection: R = [1 – ρi B / A (∆p – ∆π)] * 100%
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membrane technologies
Flux ranges and pressures in pressure-driven membrane processes
Membrane process Pressure range (bar)
Permeance range (Lm-2h-1bar-1)
Microfiltration 0.1 – 2 >50
Ultrafiltration 1 – 5 10 – 50
Nanofiltration 5 – 30 1 – 15
Reverse osmosis 10 – 100 0.05 – 2
Alternative driving forces / membrane processes
- electrical potential difference electrodialysis
- temperature difference vapor pressure difference membrane distillation
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Electrodialysis
Charged barrier
R. Baker, 2004.
Selectivity based on electrostatic („Donnan“) exclusion from the membrane
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Process
R. Baker, 2004.
Electrodialysis
Electrical potential as driving force; efficient use of driving force and scale-up of capacity by numbering up (membranes in parallel)
R. Baker, 2004.
Electrodialysis
Process efficiency
~ 1 kWh / m3
(only electrical energy)
>> 10 kWh / m3
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ion transport through membrane equivalent to electrical current (power consumption)
for same product purity (low ion content) higher power consumption at higher feed concentration
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Thermally driven membrane processes
R. Baker, 2004.
Membrane distillation
membrane contactor temperature gradient causes concentration gradient coupling of heat and mass flux (within the membrane)
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Thermally driven membrane processes
Membrane distillation
membrane contactor
= efficient alternative to passive solar distillation: much higher mass transfer rate at same driving force due to much shorter distance (membrane thickness ~ 200 µm)
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Membrane distillation
Process
R. Baker, 2004.
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Process
Flow scheme and performance data of a MD process for production of pure water from salt solutions: Counter flow of hot feed and cool distillate, water flux is almost constant up to very high salt concentrations (Rippberger, 1988).
Membrane distillation
Water desalination
NOTE: feed must not be heated to 100°C (@ normal pressure) to achieve relatively high flux use water with „waste heat“ couple with solar energy optimize mass / heat transfer (http://www.memsys.eu/technology.html)
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Advantages of membrane technologies - unique separation principle – perm-selective barrier - low energy consumption - mild conditions - no additives (chemicals) - continuous processes easy - scale up (or scale down) easy
Organisation / integration of more complex processes - hybrid separation processes - controlled release systems, sensor systems, membrane reactors, biohybrid organs …
Limitations - often low selectivity or flux („trade-off“) of available membranes - concentation polarization - membrane fouling - up-scaling more or less linear
Membrane technologies
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Membranes ― Polymeric materials
Dominating membrane types and materials for water purification
Reverse osmosis / Nanofiltration non-porous / microporous barrier, mainly solution/diffusion
- anisotropic cellulose acetate by phase separation - thin-film composite polyamide by interfacial polymerization on polysulfone (from phase separation) Ultrafiltration / Microfiltration porous barrier, sieving & viscous flow
- organic polymers (polysulfone, polyvinylidenefluoride) by phase separation - ceramic by sol-gel method or phase separation with „polymeric binder“, both followed by thermal treatment * because of the focus onto water desalination not further covered here.
*
Electrodialysis nonporous charged barrier, electromigration
- ion-exchange polymers (e.g. polystyrene-based or Nafion)
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Membranes ― Polymeric materials
Semisynthetic or synthetic polymers as materials for dialysis and UF membranes
**
F
F H
H
n Polyvinylidenefluoride
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Membranes ― Preparation
… from organic polymers
Methods for processing a film of a polymer solution into a polymer membrane (porous or nonporous) = phase separation (PS):
- precipitation in a non-solvent (typically water) non-solvent induced: NIPS
- precipitation by absorption of non-solvent (water) from the vapor phase vapour induced: VIPS
- solvent evaporation evaporation induced: EIPS
- precipitation by cooling thermally induced: TIPS
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Membranes ― Preparation
NIPS of polymer solutions
polymer solution
casting knife
coagulation bath
post-treatment: rinsing, annealing, drying, etc.
polymer membrane (roll)
1)
2)
3)
4)
„Proto-membrane“ influence of: - residence time - conditions (temperature, relative humidity, …) - composition/properties of polymer solution
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Membranes ― Preparation
NIPS of polymer solutions
Variables to control membrane characteristics
Characteristics of the casting solution polymer solvent, polymer concentration (viscosity)
Solvent / nonsolvent system miscibility, affinity (e.g., difference in Hildebrand parameter)
Additives co-solvents, “pore-forming agents”, non-solvents, cross-linker
Characteristics of coagulation bath temperature, solvent, surfactant
Exposure time and condition of proto-membrane before precipitation temperature, solvent volatility, absorption of non-solvent (water), ...
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Membranes ― Preparation
Phase separation of polymer solutions
Polymer concentration profile within the casted film (is a function of time!) due to solvent evaporation or solvent exchange with non-solvent, both across the phase boundary (top side). R. Baker, 2004.
„skin“ formation:
integrally anisotropic membranes
essential for RO, NF, UF! (Loeb & Sourirajan, 1961)
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Membranes ― Preparation
Polymer membranes from phase separation - NIPS
0102030405060708090
100
1000 10000 100000 1000000
Rej
ectio
n, R
(%)
Molar mass, M (g/mol)
however: ● relatively broad pore size distribution
moderate size selectivity ● relatively low porosity
limited flux
thin porous separation layer low resistance / high flux selectivity by size exclusion
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Membranes ― Preparation
Polymer membranes from phase separation - NIPS
Microfiltration membrane (PES), flat-sheet
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Membranes ― Manufacturing
(NIPS)
hollow-fiber membranes
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Membranes ― Manufacturing
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes
Performance characteristics for RO membranes with seawater at 56 bar and 25°C.
Reverse osmosis - Separation performance
free hydroxyl
no hydroxyl
R. Baker, 2004.
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Membranes ― Preparation
Polymer composite membrane via interfacial reaction Carbonic acid chloride solution
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes ― Manufacturing
Polymer composite membrane via interfacial reaction
Machine for manufacturing of composite membranes via interfacial polycondensation.
Reverse osmosis membrane (Composite: Polyamide on PESf)
price: < 10 EUR/m2
Lehrstuhl für Technische Chemie II Universität Duisburg-Essen
Membranes
Polymer composite membrane via interfacial reaction
Polyamid
50 nm
pore diameters (PALS):
„aggregate“ 0.7 … 0.8 nm
„network“ 0.3 … 0.4 nm
ultrathin selective barrier
< 50 nm V. Freger, 2002, 2006.
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Membranes
Chlorine resistance of RO membranes
in Advanced membrane technology and applications (N. N. Li et al., Eds.), Wiley, 2008.
V. T. Do, et al., Env.. Sci. Techn. 2012, 46, 13184.
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Reverse osmosis – State-of-the-art
Reverse osmosis membranes – aromatic polyamide
Thin barrier layer of RO composite membrane.
Trade-off between permeability and selectivity for state-of-the-art polyamide composite RO membranes.
?
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permeability, selectivity, robustness,
material cost, scalability,
compatibility with existing manufacturing
infrastructure
M. M. Pendergast, E. M.V. Hoek, Energy Environ. Sci., 2011, 4, 1946.
Advanced membranes by nanotechnology
Membrane modules
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… for flat-shet membranes: Spiral-wound
R. Baker, 2004.
Membrane modules
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… for flat-shet membranes: Spiral-wound
GE, Zenon.
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Membrane modules
R. Baker, 2004.
… for hollow-fiber / capillary membranes
also for water ultrafiltration two engineering options: outside-in vs. inside-out
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Membrane modules
GE, Zenon.
… for hollow-fiber membranes
Flexible hollow-fibers, for out-to-in UF in combination with aeration (control of fouling) main application: MBR
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Membrane process ― Polarisation phenomena
p
Mass balance for solute flux through membrane:
Jv ci – Di dci/dx = Jv ci,p
(ci,o – ci,p) / (ci,b – ci,p) =
(1 / Eo – 1) / (1 / E - 1) =
exp (Jv δ / Di) E … enrichment factor
E = ci,p / ci,b
E0 = ci,p / ci,0 (in absence of boundary layer) E / E0 … concentration polarization modulus
Film model
R. Baker, 2004.
consequences: - reduced flux - reduced rejection - increased fouling tendency
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© M. Elimelech, Yale Univ., USA.
influence of membrane-solute interactions effects of membrane surface chemistry (incl. charge)
Membrane process ― Fouling
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Membrane process ― Fouling
Types of membrane fouling (= loss of membrane performance) ► Scaling – inorganic salts (CaCO3, CaSO4, MeSiOx, BaSO4, SrSO4, CaF, …) ► Silt – colloids (determined via SDI; ASTM standard = MF throughput) ► Organic fouling – (macro)molecules (proteins, lipids, polysaccharides, …) ► Biofouling – bacteria and released material (cells, EPS biofilm, …)
Strategies to reduce fouling problems
◄ Feed pre-treatment ◄ Module design (cross-flow, enforced mixing) ◄ Control of concentration polarisation (critical flux sustainable flux) ◄ Membrane cleaning (mechanical /e.g. back-washing/, chemical) ◄ Membrane surface modification
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Exemplary industrial membrane technologies
Water desalination … relative cost
Reverse osmosis is the most cost-efficient
technology for sea water desalination
(0.4 to 0.8 EUR/m3) Sea water
Brackish water
Distillation
Salt concentration (g/L)
R
elat
ive
cost
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Exemplary industrial membrane technologies
Large scale sea water desalination with reverse osmosis
M. Elimelech, W. A. Philip, Science 2011, 333, 712.
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Exemplary industrial membrane technologies
Sea water desalination with reverse osmosis
reduced energy consumption: - higher-permeability membranes - energy recovery devices
- more efficient pumps very large scale RO < 0.5 EUR / m3
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Exemplary industrial membrane technologies
Seawater desalination ― very large scale RO
Veolia, 2006.
Ashkelon plant (Israel, since late 2005): production capacity of drinking water: 320,000 m3/day product salt concentration: 30 mg/L – water price 0.5 EUR / m3
Overall process: 1. Intake 2. Pre-treatment 3. Desalination 4. Post-treatment
Pre-treatment is crucial for efficiency and sustainability of the desalination step: rapidly increasing use of MF / UF for pre-treatment
drastically reduced electrical energy
consumption: < 2 kWh / m3
Feed waters and targets for purification
L. F. Greenlee, et al. Water Research 2009, 43, 2317.
boronic acid: neutral at pH 7!
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RO water desalination: Treatment schemes
Intake Screen Floccu- lation
Multi media filtration
Cartridge filtration
Simplified process scheme of conventional pre-treatment
Disinfection
Coagulation / flocculation agents
pH adjustment Antiscaling agents
agents
Dechlorination
RO
Post-treatment Adjustment of salinity (pH, hardness / taste, pipe corrosion) Disinfection
CO32- / HCO3
- Ca2+
Ultrafiltration
Alternative
RO
much less flocculation agent
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RO water desalination
L. F. Greenlee, et al. Water Research 2009, 43, 2317.
Plant and running costs
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Water–Energy Nexus
Water Energy
Energy for water (e.g., thermal desalination, SWRO)
Water for energy (e.g., oil extraction, cooling water)
alternative/renewable energy: - solar or solar-driven desalination
- wind-driven desalination
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Osmosis
Maximum energy that is theoretically extractable from reversible mixing of water with saline solutions from five sources and height needed to produce the same energy from falling water.
Water for energy
B. E. Logan, M. Elimelech, Nature 2012, 488, 313.
reduce power consumption of desalination develop sustainable power generation
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Osmosis
FO … forward osmosis (separation) PRO … pressure-retarded osmosis (energy conversion)
T.Y. Cath, A.E. Childress, M. Elimelech, J. Membr. Sci. 2006.
exemplaric draw solutions
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Forward osmosis
J.R. McCutcheona, R.L. McGinnisb, M. Elimelech, Desalination 2005.
projected consumption of electrical energy:
< 0.25 kWh/m3
Adapted membrane development necessary! Prozess scheme for FO water desalination
Water desalination
Membrane fouling!
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Forward osmosis
Bag for water purfication: containing nutrients and minerals, forming draw solution
Soft drink from contaminated water
T.Y. Cath, A.E. Childress, M. Elimelech, J. Membr. Sci. 2006.
Water purification
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Forward osmosis
Potential other applications
C. Klaysom et al., Chem. Soc. Rev. 2013, 42, 6959.
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Forward osmosis
Hybrid FO-RO process for water augmentation
C. Klaysom et al., Chem. Soc. Rev. 2013, 42, 6959.
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Pressure retarded osmosis
Sustainable power generation
Economical for > 5 W /m2 membrane area
with RO membranes: 0.1 W/m2
Adapted membrane development necessary!
recently lab scale: ~10 W/m2
Membrane fouling!
B. E. Logan, M. Elimelech, Nature 2012, 488, 313.
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Reverse electrodialysis
B. E. Logan, M. Elimelech, Nature 2012, 488, 313.
Sustainable power generation
Optimized membranes and stack architecture!
projected ~4 W/m2
selective ion transport through membrane equivalent to electrical current (power generation)
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Water–Energy Nexus
Water Energy
Energy for water (e.g., thermal desalination, SWRO)
Water for energy (e.g., oil extraction, cooling water)
energy-efficient alternative (hybrid) processes: - forward osmosis
energy from water treatment (salinity gradients): - pressure retarded osmosis
- reverse electrodialysis
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Waste water recycling: Options for integrated membrane technologies
efficient and safe removal of: - micropollutants - viruses - bacteria
e.g.: NewWater, Singapore
M. A. Shannon et al., Nature 2008, 452, 301.
Advanced membrane systems
another option: osmotic power generation from mixing of brine with feed
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Conclusions
Membrane technologies
- unique separation principle, with large diversity of concepts (combinations of barriers, driving forces, …)
- industrially established unit operations for water desalination, purification and recycling
- significant potential for process intensification, by integration into established separation schemes, but also by combination of different membrane processes
- critical role of materials and process engineering for further development
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