Membrane technologies: Overview on concepts and … · Membrane technologies: Overview on concepts and recent trends ... both followed by thermal treatment ... hollow-fiber membranes

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


membrane module

permeate feed


dead-end mode


membrane module

retentate feed

permeate

Flux: J = permeate volume or mass or mole / membrane area * time

Rejection: R = cfeed – cpermeate / cfeed


Ji = K * ∆Xi / ∆x

gradient (driving force)

mobility through membrane

cross-flow mode


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



Classification of membranes

Isotropic membranes

Anisotropic membranes

R. Baker, 2004.


Membranes ― Separation mechanisms



separation of heterogeneous mixtures or colloidal systems, based on size and/or particle-surface interactions


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)


Membranes ― Separation mechanisms



Non-porous membranes separate by solution in and/or diffusion through

the membrane material

separation of homogeneous mixtures, based on molecular properties and interactions


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


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%



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


Electrodialysis

Charged barrier

R. Baker, 2004.

Selectivity based on electrostatic („Donnan“) exclusion from the membrane


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


ion transport through membrane equivalent to electrical current (power consumption)

for same product purity (low ion content) higher power consumption at higher feed concentration


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)


Thermally driven membrane processes


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)



Process

R. Baker, 2004.


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).


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)


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



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)


Membranes ― Polymeric materials

Semisynthetic or synthetic polymers as materials for dialysis and UF membranes

**

F

F H

H

n Polyvinylidenefluoride


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



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



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), ...



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)



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



Polymer membranes from phase separation - NIPS

Microfiltration membrane (PES), flat-sheet


Membranes ― Manufacturing

(NIPS)

hollow-fiber membranes




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.



Polymer composite membrane via interfacial reaction Carbonic acid chloride solution



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


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.


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.


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.

?


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


… for flat-shet membranes: Spiral-wound

R. Baker, 2004.

Membrane modules


… for flat-shet membranes: Spiral-wound

GE, Zenon.


Membrane modules

R. Baker, 2004.

… for hollow-fiber / capillary membranes

also for water ultrafiltration two engineering options: outside-in vs. inside-out


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


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


© M. Elimelech, Yale Univ., USA.

influence of membrane-solute interactions effects of membrane surface chemistry (incl. charge)

Membrane process ― Fouling


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


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



Large scale sea water desalination with reverse osmosis

M. Elimelech, W. A. Philip, Science 2011, 333, 712.



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



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!


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


RO water desalination

L. F. Greenlee, et al. Water Research 2009, 43, 2317.

Plant and running costs



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


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


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


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!


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


Forward osmosis

Potential other applications

C. Klaysom et al., Chem. Soc. Rev. 2013, 42, 6959.


Forward osmosis

Hybrid FO-RO process for water augmentation

C. Klaysom et al., Chem. Soc. Rev. 2013, 42, 6959.


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!



Reverse electrodialysis


Sustainable power generation

Optimized membranes and stack architecture!

projected ~4 W/m2

selective ion transport through membrane equivalent to electrical current (power generation)


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


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


Conclusions


- 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

Documents

Membrane technologies: Overview on concepts and … · Membrane technologies: Overview on concepts and recent trends ... both followed by thermal treatment ... hollow-fiber membranes