part (i) introduction to membrane technology in drinking & industrial water production

Part (I)

Introduction to Membrane Technology in Drinking &

Industrial Water Production

What is a membrane?

A membrane is...

? ? ?

...a physical barrier (no necessarily solid) that gives, or at least helps, the separation of the components in a mixture.

Membrane Separations

The sorting demon...


- Membrane processes are not based in thermodynamic equilibrium but based in the different transport rate of each species through the membrane.

- The membrane market is still growing. In the 1986-96 decade, the sales related to membrane products and systems doubled.

- In 1998, these sales were over 5000 million €.


Advantages

Energy savings. The energy consumption is very low as there is no phase change.

Low temperature operation. Almost all processes proceed at room temperature, thus they can deal with compounds that are not resistant at high temperatures.

Water reuse. When applied to recover water, they avoid the transport of large water volumes and permit the reduction of the Chemical Oxygen Demand (COD) loading in sewage plants.

Recovery. Both the concentrate and the permeate could be recovered to use.


Advantages

Compact operation. Which permits to save space .

Easy scale-up. Because usually they are designed in modules, which can be easily connected.

Automatic operation. The most of the membrane plants are managed by expert systems.

Tailored systems. In many cases, the membranes and systems can be specifically designed according the problem.


Disadvantages

High cost. Membranes (and associated systems) are costly, but for low selective separations.

Lack of selectivity. In many cases, the separation factors are still insufficient.

Low fluxes. The permeat flowrate available are still too low for some applications.

Sensitive to chemical attack. Many materials can be damaged by acids, oxidants or organic solvents.

Lack of mechanical resistance. Many materials do not withstand abrasion, vibrations, high temperatures or pressures.


- The membrane operations more widely used are those based in applying a pressure difference between both sides of the membrane.

• Microfiltration (MF).

• Ultrafiltration (UF).

• Nanofiltration (NF).

• Reverse osmosis (RO).

- Although similar in appearance, the involved mechanisms in the separation can be very very different.


Micro Filtration (MF)(10-0.1m)Bacteria, suspended particles

Ultrafiltration (UF)(0.05-0.005m) Colloids, macromolecules

Nanofiltration (NF)5e-3-5.e-4 mSugars, dyes, divalent salts

Reverse Osmosis (RO)(1.e-4-1e-5 m)Monovalent salts, ionic metals

Water

Micro Filtration (MF)(10-0.1m)Bacteria, suspended particles

Ultrafiltration (UF)(0.05-0.005m) Colloids, macromolecules

Nanofiltration (NF)5e-3-5.e-4 mSugars, dyes, divalent salts

Reverse Osmosis (RO)(1.e-4-1e-5 m)Monovalent salts, ionic metals

Water

Hemoglobin(7 nm)

0.1 1 10 100 1000 10000

Pore diameter (nm)

Cells,bacteria

and polymers

Virus andproteinsVitamins

and sugars

Salts andlow molecular

weight compounds

H2O(0,2 nm)

Na+

(0,4 nm)

Glucose(1 nm)

Influenza Virus (100 nm)

PseudomonasDiminuta(280 nm)

Staphylococcus(1000 nm)

Starch(10000 nm)

Microfiltration

Ultrafiltration

Nanofiltration

Reverse Osmosis

Emulsionsand colloids

Name of the membrane process in function of the particle size.


More examples.


... and more.


- There are other separation operations where a membrane is the responsible of the la selective separation of the compounds:

• Dialysis.• Electrodialysis (ED).• Pervaporation.

• Gas permeation (GP).

• Liquid membranes.

- In others, the membrane is not directly responsible for the separation but it actively participates:

• Membrane extraction.• Membrane distillation.• Osmotic distillation.


Type of filtration.

Dead-end Cross-flow


Simple scheme of a membrane module.

MembraneFeed Retentate(Concentrate)

Permeate(Filtrate)

CA,r, CB,r

CA,p, CB,p

CA,f, CB,f


- Synthetic membranes are solid barriers that allow preferentially to pass specific compounds due to some driving force.

(Very) Simple scheme for some mechanisms of selective separation on a porous membrane.

+

+

+

+

+ ++


- The separation ability of a synthetic material depends on its physical, chemical properties.

• Pore size and structure

• Design

• Chemical characteristics

• Electrical charge


- The membranes can be roughly divided in two main groups: porous and non porous.

- Porous membranes give separation due to...

• size• shape• charge

...of the species.- Non porous membranes give separation due to...

• selective adsorption• diffusion

...of the species.


Main parameters.

- Rejection, R, if there is just one component (RO)

f,A

p,A

f,A

p,Af,A

C

C1100

C

CC100 (%)R

- Separation factor - Enrichment factor

B

A

B,fA,f

B,pA,pA,B /CC

/CCα

A,f

A,pA C

C


for two or more component

Main parameters.

- In RO, often we use the Recovery (Y)

Qp: Permeate flowrate (m3/s)

Qf: Feed flowrate (m3/s)

100Q

Q(%)Y

f

p


Main parameters.

- Passive transport in membranes. The permeate flux is proportional to a given driving force (some difference in a property).

(X) ForcerivingD )A( onstantC (J)Flux

Driving forces:

Pressure (total o partial) Concentration Electric Potential


Main parameters.Membrane processes and driving force.

ProcessFeedphase

Permeatephase

DrivingForce

Microfiltration L L ΔP

Ultrafiltration L L ΔP

Nanofiltration L L ΔP

Reverse Osmosis L L ΔP

Dialysis L L Δc

Electrodialysis L L ΔΕ

Pervaporation L G ΔP

Gas Permeation G G ΔP


Main parameters.

- Permeate flux.

dP

8r

AQJ

2

mw

w

Jw: Solvent flux (m3/s·m2)

Qw: Solvent flowrate (m3/s)

Am: Membrane area (m2) r: Pore radius (m)

d: Membrane thickness (m)

: Viscosity (Pa ·s)

P: Hydraulic pressure difference (Pa)

: Tortuosity

In MF and UF, porous membrane model is assumed, where the a stream freely flows through the pore. Then, the transport law follows the Hagen-Poiseuille equation.

: Porosity


Main parameters.

- The above model is good for cylindrical pores. However, if the membrane is rather formed by a aggregated particles, then the Kozeny-Carman relation works much better.

JW: Solvent flux (m3/s·m2)QW: Solvent flowrate (m3/s)S: Particle surface area (m2/m3)

K: Kozeny-Carman constant d: Membrane thickness (m): Viscosity (Pa ·s)

dP

1SK

AQJ

22

3

mw

w

Am: Membrane area (m2)


- In the operations governed by the pressure, a phenomenon called concentration polarisation appears, which must be carefully controlled. This is due to the solute accumulation neighbouring the membrane surface.

FeedPolarisation layer

membrane

membrane

Permeate

Permeate

Formation of the polarisation layer.


- Concentration polarisation.

(It is not fouling!!!)


- Fouling: Irreversible reduction of the flux throughout the time.

• Pore size reduction by irreversible adsorption of compounds.

• Pore plugging.

• Formation of a gel layer over the membrane surface (cake).


- Membrane can be classified in several ways, but always there are arbitrary classifications.

• Structure: symmetric, asymmetric

• Configuration: flat, tubular, hollow fiber

• Material: organic, inorganic

• Surface charge: positive, negative, neutral

• ...and even other divisions and subdivisions


- Structure:

• Symmetric. Also called homogeneous. A cross section shows a uniform porous structure.

• Asymmetric. In a cross section, one can see two different structures, a thin dense layer and below a porous support layer.

- Integral: the layers are continuous. - Composites: the active layer (thickness 0.1-0.5 μm) is supported over a highly porous layer (50-150 μm), sometimes both layers are of different materials.


Symmetric UF membrane of 0.45 m made of cellulose acetate (Millipore).


Symmetric ceramic membrane of 0.2 m made of alumina (Al2O3) (AnoporeTM).

Surface Cross section


Asymmetric ceramic membrane made of -Al2O3 (Membralox).


UF integral asymmetric membrane made of polypropylene.


RO composite membranes.

Cellulose acetate

Polyamide


- Configuration and modules

• Configuration: geometric form given to the synthetic membranes.

• Module: name of the devices supporting one or several membranes (housing).

The module seals and isolates the different streams. The geometry and specific fluid movement through the confined space characterises each module. The type of flux, the transport mechanism and the membrane surface phenomena depend on the module design.


- Configuration:

• Flat.

- The active layer is a flat.

- Synthesised as a continuous layer.

- Later, one can select a desired geometry (rectangle, circle,...) to be placed in the module.

- Used in two kind of modules: plate-and-frame and spiral wound.

- High surface area/volume ratio.


Plate-and-Frame Membrane System.


Consists of layers of membranes separated by corrugated structural sheets, alternating layers with feed material flowing in and retentate flowing out in one direction, while permeate flows out in the other direction.

Spiral-wound module.


- Configuration:

• Tubular.

- It is like a tube.

- Usually the active layer is inside.

- The permeate crosses the membrane layer to the outside (this is, the feed flows inside).

- Low surface are/volume ratio.

- Several lengths and diameter (>10 mm).

- Modules grouping one or various membranes.


Different types of tubular modules.


Hollow fiber module.


Hank of polyamide hollow fiber for RO (DuPont).


Cross section of hollow fiber (Monsanto). Comparison with a clip.


Hollow fiber cross section of polyamide for RO (DuPont).


Hollow fiber made of polysulfone ( 1 mm) for UF (detail).


Hollow fiber cross section of 1 mm (Monsanto).


Hollow fiber surface of polypropylene (Celgard).


Hollow fiber ceramic membranes (CEPAration).


- Comparison between modular configurations.

Module

Parameter Tubular Spiral-wound Hollow fiber

Specific surface area (m2/m3) 300 1000 15000Inside diameter or spread (mm) 20-50 4-20 0.5-2Flux (L/m2 day) 300-1000 300-1000 30-100Production (m3/m3 per module & day) 100-1000 300-1000 450-1500Space velocity (cm/s) 100-500 25-50 0.5Pressure loss (bar) 2-3 1-2 0.3Pretreatment Simple Medium HighPlugging Small Medium ElevatedReplacement Easy Difficult ImpossibleCleaning:

MechanicalChemical

PossiblePossible

Not possiblePossible

Not possiblePossible


- Comparison between modular configurations.

Modular configurations and processes.

Module

Operation Tubular Spiral-wound Hollow fiber

Reverse Osmosis A VA VA

Ultrafiltration VA A NA

Microfiltration VA NA NA

Pervaporation A VA VA

Gas Permeation NA VA VA

VA = Very appropriate; A = Appropriate; NA = Not appropriate


- Material:

• Organic.

- Made of polymers or polymer blends.

- Low cost.

- Problems with their mechanical, chemical resistance.

Temperature

pH, Solvents

Pressure


Polypropylene with 0.2 m pores (Accurel).


Polytetrafluoroetylene with 0.2 m pores.


Membrane Technology

• Dialysis

- Applied since the 70’s.

- Low industrial interest.

- Ions & species of low MW (<100 Da).

- Ionic Membranes (just like ED).

- Driving Force: concentration gradient.

- Slow and low selective.

• Dialysis

- NaOH recovery in textile effluents, alcohol removal from beer, salts removal (pharmaceutical industry).

- Artificial kidney.

Membrane Technology

GS

MF

UFRO

HD

EDPV

• Dialysis

Membrane and module markets

Looks not very important...?.

Membrane Technology

• Electrodialysis (ED)

- Ion Separations.

- Ionic Membranes (non porous).

- Driving Force: gradient in electrical potential.

- Flat configuration.

- Hundreds of anionic and cationic membranes placed alternatively.

- Orthogonal electrical field.

- First applications back at 30’s.

- Potential: 1-2 V.

Membrane Technology


Membrane Technology


- Ionic Membranes (non porous).

- Based on polystyrene or polypropylene with sulfonic and quaternary amine groups.

- ED with reverse polarization (EDR).

- ED at high temperature (60ºC).

- Thickness: 0.15-0.6 mm.

- ED with electrolysis.

Membrane Technology


- Required membrane area

j+

VC c+

in

VC c+

out

0dczVdAj Cm Mass balance (in equivalents)

Charge flow

mdAdI

iFj

combining

i

FzccVi

FzccVNANA outinoutinC

mT

η: global electrical efficiency (~0.5 commercial equipment)

F: Faraday constant (96500 C/eq) N: number cells in the equipment

i: electric current density (A/m2)Am: membrane surface (m2)

j: cation flow (eq/m2 s)

z: cation charge (eq/mol)

Membrane Technology


- Then the required energy, E (J), is

tRINtIUN E C2

C

as

then

FzccV

AiI outinCm

UC: potential gradient in a cell (V)

RC: total resistance in a cell ()

tRFzcV

N E C

2C

C

2C R

FzcVN P ó

P: required Power (J/s)

Membrane Technology


Where, the required specific energy, (J/m3), is

C

2

CC

RFzc

VtVN

E E

La cell resistance can be estimated from a model based on series of resistances where the resistances to transport are considered through two membranes and the compartments concentrate and diluted.

Membrane Technology

• Electrodialysis (ED)- How to determine operational i?

t

Ficcz

DtFi DMD

M

Cation Transport

tt

cczFDi

M

DMD

If cDM+ = 0

tt

czFDi

M

Dlim

Usually: i = 0.8ilim t: transport numberD: diffusion coefficient

Cationic Membrane

Limit boundary

cCM+

cC+

cDM+

cD+

- +

concentrate diluted

Membrane Technology

• Electrodialysis (ED)- Intensity Evolution versus applied potential

i (A/m2)

ilim

Ohmic zone

U (V)

Resistance rise

Ionic water splitting

Membrane Technology


- Fields of application: Water desalination.

- Competing to RO.

- Economically more interesting at very high or very salt concentrations.

- Other fields of application: Food Industry.

Treatment of heavy metal polluted water.

Membrane Technology


- Examples:

Production of drinking water from salty water.

Water softening. Nitrate removal. Lactose demineralization. Acid removal in fruit juice. Tartrate removal from wines. Heavy metal recovery. Production of chlorine and sodium hydroxide.

Membrane Technology


electrolytic Cell for the production of chlorine and sodium hydroxide with cationic membrane.

Membrane Technology


Electrolytic cell for the production of sulfuric acid and sodium hydroxide with bipolar membrane.

Membrane Technology

Membrane Technology


Hydrogen fuel cell with a cationic membrane.

Cathode

Anode

Global

H2 2H++ 2e-

O2 + 4e- + 4H+ 2H2O

2H2 + O2 2H2O

• Pervaporation

- Discovered 1917.

- Only operation with phase change.

- Non-Porous Membranes.

- Mechanism solution-diffusion.

- Driving force: difference in partial pressure.

-Vacuum (<40 mm Hg), dilution (inert gas, N2) or temperature difference.

Membrane Technology

• Pervaporation

Condenser

Pervaporat. module

Feed

Retentate

Vacuum pump

Permeate condensate

Heater

General Pervaporation system.

Membrane Technology

• Pervaporation

- Industrial applications.

- Alternative to distillation when thermodynamic limitations.

Low energy costs. Low investment costs. Better selectivity, without thermodynamic limitations. Clean and closed operation. No process wastes. Compact and scalable units.

Membrane Technology

• Pervaporation

- Drawbacks:

- Organic substances dehydratation.

Scarce Membrane market. Low permeate flows. Limited applications:

- Recovery of volatile compounds at low concentrations.

- Separation of azeotropic mixtures.

Membrane Technology

• Pervaporation.

- Do not mistake with a distillation where a membrane is just separating phases.

- Three steps mechanisms:

Selective absorption on the membrane. Dissolution at the membrane. Diffusion through the membrane.

Membrane Technology

• Pervaporation

- The membrane is active in this process.

- The permeability coefficient (P) of a compound depends on the solubility (S) and the diffusivity (D), in the polymeric phase, of the crossing compound

Pi = Si (ci, cj)· Di (ci, cj)

- Simplificated transport equation:

pioiii

ii pypx

dP

J

Ji: flux of component i d: membrane thickness xi: molarfraction in liquid i: activity coefficientpi

o: vapour pressure yi: molar fraction at permeate pp: pressure at permeate side

Membrane Technology

• Pervaporation

- Main membrane parameters:

- Separation factors - Enrichment factors

B

A

B,fA,f

B,pA,pA,B /CC

/CCα

A,f

A,pA C

C

Membrane Technology

• Pervaporation

Pervaporation process of an ethanol/water mixture with a PVA membrane.

0.00.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0

azeotrope

pseudoazeotropeEh

tan

ol a

t p

erm

eate

(v

apou

r)

Ethanol at feed (liquid)

Phase equilibria

pervaporation

0.0

Membrane Technology

Intermediat tank

Condenser

Pervap. unit

Feed

Ethanol >90% w/w

Permeate

Boiler

Water

Distillation column

Ethanol >99.95% w/w

Ethanol 20-80% w/w

Ethanol 15% w/w

• Pervaporation

Combination of distillation and pervaporation for the production of pure ethanol.

Plant for production of ethanol from sugar (Bethéniville, France).

Membrane Technology

• Pervaporation

Methanol Alil alcohol Ethyl Acetate Tricloretilene Ethanol Furfurol Buthyl acetate Tetrachloretane n-Propanol Methylfurfurol Diethyl ether Tretrahydrofurane Isopropanol Diethilenglicol Diisopropyl ether Aniline n-Buthanol Acetone Dipropyl ether Benzene t-Buthanol Buthanone Ethyl propyl ether Toluene 2-Penthanol Cyclohexanone Chloroform Xylene Hexanol Methyl ethyl Ketone Methyl Chloride Ethylen diamine Cyclohexanol Metil isobuthyl Ketone Chlorethylene Ethanol amine Isoamilic Alcohol Caprolactame Dichloro ethylene Diethyl amine

Organic solvents to apply pervaporation.

Dehydration of organic solvents.

Hydrophilic membranes: PVA, PAN...

Membrane Technology

• Pervaporation

- Organic compounds recovery.

For volatile compounds. Economically competitive. Hydrophobic membranes: PDMS and derivatives.

- Azeotrope breaking of organic compounds.

Studied at lab scale. Low selectivity.

Membrane Technology

• Pervaporation

Lab scale separations reported.

Mixture Membrane Selectivity Ethylbenzene/xylene Polyethylene Not available

p-xilene/o-xilene Polyethylene Not available m-xilene/p-xilene Polypropilene m-Xylene

Dichlor ethane/trichlor ethane

Poliamide/polyeth Dichlorethane

Benzene/cyclohexane Polyimide Benzene Acetone/cyclohexane Polyimide Acetone

Membrane Technology

• Pervaporation

Hybrid process: extractive distillation and pervaporation for the production of pure benzene and cyclohexane .

Pure cyclohexane

Pervaporation unit

Pure benzene

Feed

Solvent

Column

1

Column

2

Membrane Technology

• Gas permeation

- Since 50’s.

- Membranes: porous and no porous.

- Several possible mechanisms for gas transport:

Knudsen Flow.

- The last two are selective.

Solution-diffusion.

X Viscous Flow.

Membrane Technology

• Gas Permeation

- Knudsen Flow (porous membranes). When the porous diameter is on the range of the average free space of the molecule (kinetic theory for gases).

ik

i PTR

Dd

J

MTR8

r32

Dk

i

j

j

i

M

M

JJ

Transport equation

Knudsen diffusivity

: porosity d: membrane thickness: tortuosity R: gas constant

T: temperature P: transmembrane P

r: porus radi

M: MW

Enrichment

Membrane Technology

• Gas permeation

-Solution-diffusion (non-porous membranes).

j

i

j

i

j

iij S

SDD

PP

The selectivity is referred to the separation factors of the compounds to be separated

Pi = Si· Di

There are “slow” and “fast gases” for a determined membrane.

Membrane Technology

• Gas permeation

- Driving force: partial pressure gradient.

- Working pressure: up to 100 bar.

- Non-porous polymeric membranes:

PDMS, CA, PS, PES i PI

- Ceramic Membranes (small pores for Knudsen).

- Metallic membranes (Pd and Ag alloys).

Membrane Technology

• Gas permeation

- Asymmetric membranes.

- Thin polymer on a structural porous material.

- Preferred configuration Hollow Fiber or Spiral, others like flat or tubular also possible.

- Applied in petrochemistry.

Purification of H2, CO2, CH4 and gaseous hydrocarbons of difficult distillation.

Nitrogen purification.

Membrane Technology

• Gas permeation

- Some examples:

Enrichment, recovery and dehydration of N2.

H2 recovery in residual flows of proceses, purge o natural gas.

Adjust of the ratio H2/CO synthesis gas.

Acid gas removal (CO2, H2S) from natural gas.

Helium recovery from natural gas and other sources.

VOC removal from process flow.

Membrane Technology

Residuals gases

Gas permeation

to fuel-gas

Hydrogen

Recycle of n-C4

Unitat de isomerització

n-Butane

Isobutane

Recycle

H2 (96%)

• Gas permeation

Hydrogen recovery in a butane isomeration plant.

A typical PRISM® Separator (Airproducts)

Membrane Technology

• Liquid Membranes

- A liquid barrier between to phases.

- Not yet industrial uses.

- Driving force: chemical potential, concentration.

Emulsion (ELM).

- Two configurations:

Supported Liquid Membranes (SLM).

Membrane Technology


Possible configuration for LM.

Emulsion liquid Mem.

Organic liquid + surfactant (membrane)

Receiving phase

Aqueous phase

Porous Support

Organic liquid impregnated into the pores

SLM

Membrane Technology


- Advantages:

High flows due to the transport velocity in liquids.

- Drawbacks:

Selective separations due to the presence of specific reagents.

Pumping effect (against the gradient) due to the carrier equilibrium.

Small quantities of solvent lets to the application of expensive solvents.

Low stability of emulsions in ELM.

Leaching out of organic phase from the pores of a SLM .

Membrane Technology


Facilitated Transport in Liquid Membrane.

Liquid Membrane

M + B MB

B: selective carrierM: selectively separated

M M

B

MBN

P

O

Ag+

O

O

O

O

O

O diphenyl-18-crown-6

O

O

O

O

O

O

Ag+

Membrane Technology

• Liquid Membranes- ELM: low practical interest

- SLM: lab scale and few applications restricted high added value compounds.- Hydrophobic Membranes (PE, PP ...).- Hollow fibers.

- Potential applications:

Selective removal and concentration of cations in solution.

Selective separation of gases.

Recovery of acid or basic compounds.

Organic compound separation in complex mixtures.

Membrane Technology

• Other Techniques

- Membrane distillation.

A hydrophobic membrane separates two aqueous phases.

The volatile compounds cross the membrane and condensate.

The hydrophobic membrane avoids the aqueous phases to get into the membrane. The driving force in the temperature gradient.

Membrane Technology

• Other techniques

- Membrane distillation.

Driven by the phase equilibrium in both sides of the membrane.

The membrane acts just like a physical barrier.

Some applications:

Water demineralization. Inorganic acid or salt concentration.

Ethanol extraction at the fermentation.

Membrane Technology

• Other techniques- Osmotic distillation.

Similar to membrane distillation. Both phases at the same temperature.

The osmotic pressure is risen by adding appropriate compounds to the receiving phase.

The partial pressure gradient due to the osmotic pressure is the driving force.

Attractive to the food industry provided it maintains the temperature.

Alcohol removal from wine and beer. Fruit juice enrichment.

Membrane Technology

• Other techniques

- Membrane extraction.

The membrane acts as a barrier to separate immiscible phases.

It has to assure immiscibility between phases.

Hollow Fiber membranes have high area.

It makes possible to avoid the separation at decanting of the phases at the end.

Lab scale research.

Membrane Technology

part (i) introduction to membrane technology in drinking & industrial water production

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