Pressure Driven Processes

- Pressure-driven membrane processes:

Membrane Separations

Mid-size organic substances,multiple charged ions

MF0.11-0.3 MPa

RO0.5-1.5 MPa

NF0.5-1.5 MPa

UF0.15-0.5 MPaP=

Bacteria, parasites, particles

Low molecular substances, single charged ionsHigh molecular substances, viruses

Microfiltration (MF)

- Pressure-driven membrane processes:

Ultrafiltration (UF)

Nanofiltration (NF)

Reverse Osmosis (RO)


- Rejects particles between 0.2 and 10 m.- Pore size between 0.05 and 10 m (0.45 m).

Microfiltration (MF)- First commercially developed (1930).- Very popular and used (sharing with RO).

- Bacteria, fragmented cells or colloids.- MW 300000.- Driving force: pressure difference.- Low operation pressure: 0.2 to 3.5 bar.- Typical flux: > 200 L m-2 h-1.


Microfiltration (MF)- Separation given by size exclusion mechanism.

- Hydrophobic membranes.- Occurrence of adsorptioneffects between themembrane surface and therejected species.- Important fouling and pore plugging.- Dead-end filtration and cleaning.

- Flux: Hagen-Poiseuille or Kozeny-Carman.


Repels waterAbsorbs oilFouls with free oilsLower fluxDifficult to clean

Repels oilsAbsorbs waterNot fouled by free oilsHigher fluxEasier to clean

- Also ceramic, basically -Al2O3, because oftheir easy cleaning, long lifetime and availablesterilisation

Microfiltration (MF)- Polymeric membranes made of a wide variety ofpolymers or polymers blends:PTFE, PVDF, PP, PS, CA, CN, CTA, PE, PC,PEST, PI, Nylon ...

CA, CN o CTA are preferred due to theirhydrophilicity preventing fouling.


Microfiltration (MF)- Applications:Bioengineering, food industry, wastewatertreatment. Examples:

High organic loading water treatment.

Fermentation product separation.

Cutting oil emulsion treatment.

Recovering of precipitated metals.

Juice, wine or beer clarification.


- Pore size between 1 and 50 nm.

Ultrafiltration (UF)- Microporous membranes (?).

- Rejects particles from 15 to 2000 .- Polymers, proteins and colloids.- Molecular weight from 5000 to 5106 Daltons*.- Driving force: pressure difference.- Moderate operating pressure: 1 to 10 bar.

* 1 Dalton 1.6610-24 g

- Typical flux: 5-200 L m-2 h-1.


Ultrafiltration (UF)- Characteristic parameter: Molecular Weight Cut-OffMWCO is the minimum MW the rejection being of90%*.

Relation between MWCO and the pore size for UFmembranes.

Pore DiameterMWCO (Daltons) P nm

1000000 0.1 100 1000500000 0.02 20 200100000 0.01 10 10050000 0.04 4 4010000 0.0025 2.5 255000 0.0015 1.5 15

* To design, the membrane MWCO must be taken about the half of lower MW speciesto be retained.


Ultrafiltration (UF)- Polymeric membranes made of a wide variety ofpolymers or polymers blends:CA, PS, PES, PAN, PVDF, PI

CA: low fouling, high flux.

Easy cleaning, long lifetime and availablesterilisation.

- Also ceramics, mainly made of and -Al2O3.

PS: chemically stable.PES: available sterilisation.PI: solvent resistant, only tubular.


Ultrafiltration (UF)- Separation mostly given by size exclusion.- In low pore size membrane, some solution-diffusion phenomena are present (typical forRO).

- Significant fouling and pore plugging.

- Cross-flow filtration and cleaning.- Any configuration.

- Performance affected by pressure, temperature,stirring, concentration and ionic environment.


Ultrafiltration (UF)- Applications:Food industry, wastewater treatment.Also potentially useful paper pulpingor textile industry. Some Examples:

COD reduction in wastewater.

Metal finishing water treatment. Treatment of cutting-oil emulsion.


Treatment of black-liquor from paper pulping. Protein recovery from blood plasma. Egg white concentration. Serum recovery from milk.

UF UF UF

Feed

Concentrate

Permeate

Ultrafiltration (UF)- Serum recovery from cheese production. The cheese production is abiochemical process followed by precipitation (of the solid cheese). Theremaining solution contains, in addition to the water, the most part of the initiallactose, proteins, vitamines and minerals.

Continuous UF process for recovering of lactoserum.


Nanofiltration (NF)- Recent definition. Separation range between UFand RO.

- Efficiently rejects, due to electrostatic interactions,charged particles, mainly multivalent ions.- Rejects non charged molecules (MW > 200 g/mol)by molecular sieving.

- Typical pore size 2 nm.

- Driving force: pressure difference.- Moderate Pressure: 15 bar.- Typical flux: 20-80 L m-2 h-1.


- Polymeric membranes (CA, PA, PVA):- Configuration of spiral-wound module.- Applications: Water pretreatment, food industry,recovery of metals.

Removal of coloured organic compounds and humicacids (precursors of the trihalometanes). Water softening (removal of multivalent ions).

- Still looking for applications. Potentially whenUF does not give sufficient rejection and RO is noteconomical.

Nanofiltration (NF)Membrane Separations

- In 1996 there were 150 plants all around the worldfor drinking water by means of NF, with an overallcapacity of 600000 m3/day.(compare with 3000000 m3/day capacity of plantsbased on RO)

- For instance, it must be noted that NF is thetechnology selected to obtain drinking water inFlorida (USA).

Nanofiltration (NF)Membrane Separations

- Operating pressure between 10-25 (brackish) and40-80 bar (seawater).

Reverse Osmosis (RO) [Hyperfiltration]- Developed in the 50s.- Very popular and used.- Ions and low molecular weight species (MW < 200).- Pore size < 1 nm.

- Driving force: pressure difference.- Water molecules freely pass (0.2 nm).

- Average flux: 5-40 L m-2 h-1.


Reverse Osmosis (RO)Membrane Separations

Reverse Osmosis (RO)- Why does water cross the membrane ?

( ) ( )iiioii clnTRPvT ++=)2(

w)1(

w =)2(

w)2()2(

w)1(

w)1()1(

w alnTRPvalnTRPv +=+

( ) )1(w)2(w)2()1(w alnTRalnTRPPv =pi==pi Paln

v

TRw

w

TRc pi


(1) (2)

membrane

Definitionif csalts


- The water freely flows through the membranedue to the pressure difference corrected by the pi.

( )pi== PA AQJ

m

ww

Jw: Solvent volume flux (m3/sm2)Qw: Solvent volume flowrate (m3/s)Am: Membrane area (m2)A: Permeability (m3/sm2Pa)*P: Hydraulic pressure difference (Pa)pi: Osmotic pressure difference (Pa)

* A = AoKt Kc Ke t: temperature; c: compaction; e: fouling


- Salt flux is due to both diffusion and convectivetransport.

( ) mwpmpws CJMCCB CJJ +==

Js: Solute mass flux (mol/sm2)Bi: Permeability (m/s)Cm: Solute concentration on the membrane surface (mol/m3)Cp: Solute concentration in the permeate (mol/m3)M: Distribution constant (~ 0.005)

( )( ) m

pm

w

sp CMPA

CCBJJ

C +pi

==


- Thus a relation between the local rejection,Rl, and the permeate flux (or applied pressure)can be found.

( ) ( ) wl J1

M1B

M11

R1

+

=

m

p

m

pml C

C1

CCC

R =

=

( ) ( ) ( )pi+= PA 1M1 BM1 1R1l


- Flow limited by concentration polarisation.

- Membranes mostly made of CA or PA.

- Fouling problems and cleaning. Pretreatment.

- Configuration in spiral-wound or hollow fiber.

- Typical conversion between 10 and 30%.

- Rejection up to 99% (sometimes higher).


- Concentration polarisation

0/x

bulk

cb

cm

cp

cJ

dxdc

D

pcJ

boundarylayer

DJ

cc

ccln

pb

pm =

=

DJ

expcc

cc

pb

pm

D: Diffusion coeficient (m2/s)

( )

+

=

kJ

expR1R

kJ

exp

c

c

llb

m

/: boundary layer thickness (m)k=D//: mass transfer coeficient (m/s)


- Applications: Drinking water, food industry,wastewater treatment. Examples:

Desalting of process water.

Production of ultrapure water for laboratories orelectronic industry.

Desalination of brackish or seawater.

Concentration wastewater in paper pulping. Concentration of juices, milk or sugar solutions. Concentration of coffee, te or soups. Concentration of aminoacids (and otherpharmaceutical substances).


ED5%

RO35%

EVM60%

Desalination technologies (1996)- Seawater desalination:

30-40% marketVery competitive processModule cascade in series and parallel

Comparison between the energetic cost forseawater desalination.

Process Energy (kWh/m3)Multiple distillation 15.5

Reverse Osmosis 9RO with energy recovery 6.5

Electrodialysis 12


Permeate

Feed

UFpretreatment

Concentrate

: RO module

1st pressure stage

2nd pressure stage

Water production plant based on RO.


Pressure Driven Processes

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