23. membrane technology - nptelnptel.ac.in/courses/103107088/module23/lecture1/lecture1.pdf ·...

Membrane Technology & Separation

Processes

(Electrodialysis & Concentration

Polarization)

Module- 23

Lec- 23

Dr. Shishir Sinha

Dept. of Chemical Engineering

IIT Roorkee

Mixing vs Separation

Two substances a and b will mix or separate depending on free enthalpy of mixing(ΔGm

)

ΔGm

= ΔHm

– TΔSm

where ΔHm

is the enhalpy of mixing and ΔSm

is the entropy of mixing.

ΔGm

< 0:Spontaneous mixing ;

ΔGm

> 0:Spontaneous separation

In most cases and always when A and B are gases the mixing occurs spontaneously and

minimum amount of energy, Wmin

= ΔGm

The actual energy requirement for the separation will bemany times greater than Wmin

The actual energy requirement depends on the type of separation processes

Membrane Separations

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 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.

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

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.

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.

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

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

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

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

Water

• Microfiltration (MF).

• Ultrafil

• Nanofil

• Reverse

- Althoug

different.

Name of

tration (UF)

ltration (NF)

e osmosis (R

gh similar in

.

f the membr

).

).

RO).

n appearanc

rane proces

e, the involv

s as functio

ved mechan

n of the par

nisms in the

rticle size.

separation ccan be veryy very

- There a

separatio

• Dialysi

• Liquid

- In other

in:

• Membr

• Membr

• Osmoti

Membra

are other sep

on of the com

s.

membranes.

rs, the memb

rane extractio

rane distillati

c distillation

ane Separat

paration ope

mpounds:

• Gas perm

. • Pervapor

brane is not

on.

ion.

n.

ions

erations whe

meation (GP)

ration.

directly resp

Type

re a membra

). • Ele

ponsible for

of filtration

ane is the re

ectrodialysis

the separati

n.

esponsible o

s (ED).

on but it act

of the la sele

tively partici

ective

ipates

Membra

Membra

- Synthet

to some d

ane Separat

ane Separat

tic membran

driving force

ions

Simp

ions

nes are solid

e.

ple scheme o

barriers that

of a membr

t allow prefe

rane module

erentially to

e.

pass specificc compound

ds due

(Very) Si

Membra

- The sep

• Pore siz

• Design

• Chemic

• Electric

Membra

- The me

imple schem

ane Separat

paration abili

ze and struct

cal character

cal charge

ane Separat

embranes can

me for some

ions

ity of a synth

ture

ristics

ions

n be roughly

e mechanism

hetic materia

y divided in t

ms of selectiv

al depends o

two main gro

ve separatio

on its physica

oups: porous

on on a por

al, chemical

s and non po

ous membr

l properties.

orous.

rane.

- 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)

100Q

Q(%)Y

f

p

Qp: Permeate flowrate (m

3

/s)

Qf: Feed flowrate (m

3

/s)


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.

Process Feed phase Permeate phase Driving Force

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.

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.

dP

8r

AQJ

2

mw

w

Jw: Solvent flux (m

3

/s·m2

) Qw: Solvent flowrate (m

3

/s) Am

: Membrane area (m2

)

d: Membrane thickness (m) : Viscosity (Pa ·s) P: Hydraulic pressure difference (Pa)

r: Pore radius (m) : Porosity : Tortuosity


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.

dP

1SK

AQJ

22

3

mw

w

JW

: Solvent flux (m3

/s·m2

) QW

: Solvent flowrate (m3

/s)

S: Particle surface area (m2

/m3

) K: Kozeny-Carman constant

Am

: Membrane area (m2

) d: Membrane thickness (m) : Viscosity (Pa ·s)


- In the

appears,

the memb

Membra

- Concen

(It is not

Membra

operations g

which must

brane surfac

ane Separat

ntration polar

fouling!!!)

ane Separat

governed by

be carefully

ce.

For

ions

risation.

ions

y the pressur

y controlled.

rmation of t

re, a phenom

This is due

the polarisa

menon calle

to the solute

ation layer.

ed concentra

e accumulat

ation polaris

tion neighbo

sation

ouring

- Fouling

• Pore siz

• Pore plu

• Formati

Membra

g: Irreversibl

ze reduction

ugging.

ion of a gel l

ane Separat

le reduction

by irreversi

layer over th

ions

of the flux th

ble adsorptio

he membrane

hroughout th

on of compo

e surface (ca

he time.

ounds.

ake).

- 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.


Symmetr

Membra

Symmetr

Membra

ric UF memb

ane Separat

ric ceramic m

ane Separat

brane of 0.45

ions

Surfa

membrane of

ions

5 m made o

ce

f 0.2 m ma

of cellulose

ade of alumin

acetate (Mil

Cross sectio

na (Al2O

3) (A

llipore).

on

AnoporeTM

)

).

Asymme

Membra

Membra

etric ceramic

ane Separat

U

ane Separat

c membrane

ions

UF integral as

ions

made of -A

symmetric m

Al2O

3 (Memb

membrane m

bralox).

made of polyp

propylene.

Membra

- Configu

• Configu

• Module

The mod

through t

and the m

Membra

- Configu

• Flat.

ane Separat

uration and m

uration: geom

e: name of th

dule seals an

the confined

membrane su

ane Separat

uration:

ions

modules

metric form

he devices su

nd isolates th

d space chara

urface pheno

ions

RO comp

given to the

upporting on

he different

acterises eac

omena depen

posite membr

e synthetic m

ne or several

streams. The

h module. T

nd on the mo

ranes.

membranes.

membranes

e geometry

The type of f

odule design

s (housing).

and specific

flux, the tran

n.

c fluid move

nsport mecha

ement

anism

- The act

- Synthes

- Later, o

- Used in

- High su

Membra

Consists

with feed

out in the

Membra

tive layer is a

sised as a co

one can selec

n two kind of

urface area/v

ane Separat

of layers o

d material fl

e other direc

ane Separat

a flat.

ntinuous lay

ct a desired g

f modules: p

volume ratio.

ions

Pla

of membrane

lowing in an

tion.

ions

yer.

geometry (re

plate-and-fra

.

ate-and-Fram

es separated

nd retentate

ectangle, circ

ame and spira

me Membran

d by corruga

flowing out

cle,...) to be

al wound.

ne System.

ated structur

t in one dire

placed in th

ral sheets, a

ection, while

e module.

alternating l

e permeate f

layers

flows

Membra

Membra

ane Separat

ane Separat

ions

ions

Spiral-w

Spiral-w

wound modu

wound modu

ule.

ule.

- Configu

• Tubular

- It is like

- Usually

- The per

- Low su

- Several

- Module

Membra

Membra

uration:

r.

e a tube.

y the active l

rmeate cross

urface are/vo

l lengths and

es grouping o

ane Separat

ane Separat

layer is insid

ses the memb

lume ratio.

d diameter (>

one or variou

ions

D

ions

de.

brane layer t

>10 mm).

us membran

Different type

to the outside

nes.

es of tubular

e (this is, the

r modules.

e feed flows inside).

Membra

Cross sec

Membra

ane Separat

ction of hollo

ane Separat

ions

ow fiber (M

ions

Hollow

onsanto). Co

w fiber modu

omparison w

ule.

with a clip.

Hollow f

Membra

Hollow f

Membra

fiber cross se

ane Separat

fiber made o

ane Separat

ection of pol

ions

f polysulfon

ions

lyamide for

ne ( 1 mm

RO (DuPon

m) for UF (d

nt).

detail).

Hollow f

Membra

Membra

fiber cross se

ane Separat

ane Separat

ection of

ions

Hollow f

ions

1 mm (Mo

fiber surface

onsanto).

of polyproppylene (Celg

gard).

- Comparison between modular configurations.

Module

Parameter Tubular Spiral-wound Hollow fiber

Specific surface area (m2/m3) 300 1000 15000

Inside diameter or spread (mm) 20-50 4-20 0.5-2

Flux (L/m2 day) 300-1000 300-1000 30-100

Production (m3/m3 per module & day) 100-1000 300-1000 450-1500

Space velocity (cm/s) 100-500 25-50 0.5

Pressure loss (bar) 2-3 1-2 0.3

Pretreatment Simple Medium High

Plugging Small Medium Elevated

Replacement Easy Difficult Impossible

Cleaning:

Mechanical

Chemical

Possible

Possible

Not possible

Possible

Not possible

Possible


- 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

Membra

Membra

Membra

ane Separat

ane Separat

ane Technol

ions

Polypr

ions

Polyt

ogy

ropylene wit

tetrafluoroet

th 0.2 m po

tylene with 0

ores (Accure

0.2 m pores

el).

s.

• Dialysi

- Applied

- Low ind

- Ions &

- Ionic M

- Driving

- S

Membra

• Dialysi

- Artificia

- NaOH r

industry)

s

d since the 7

dustrial inter

species of lo

Membranes (j

g Force: conc

low and low

ane Technol

s

al kidney.

recovery in t

).

0’s.

rest.

ow MW (<

just like ED)

centration gr

w selective.

ogy

textile efflue

100 Da).

).

radient.

ents, alcoholl removal froom beer, saltts removal (p

pharmaceutiical

Membra

• Dialysi

Looks no

Membran

Membra

• Electro

- First ap

- Ion Sep

- Ionic M

ane Technol

s

ot very impo

ne and modu

ane Technol

dialysis (ED

pplications ba

parations.

Membranes (n

ogy

ortant...?.

ule markets

ogy

D)

ack at 30’s.

non porous)

HD

.

GS PV

RO

V ED

MF

UFF

- Driving

- Potentia

- Flat con

- Hundre

- Orthogo

Membra

• Electro

Membra

• Electro

g Force: grad

al: 1-2 V.

nfiguration.

eds of anioni

onal electric

ane Technol

dialysis (ED

ane Techno

dialysis (ED

dient in elect

c and cation

al field.

ogy

D)

logy

D)

trical potenti

nic membran

ial.

nes placed altternatively.

Membra

• Electro

- Ionic M

- Based o

- Thickne

- ED with

- ED at h

- ED with

Membra

ane Technol

dialysis (ED

Membranes (n

on polystyren

ess: 0.15-0.6

h reverse po

high tempera

h electrolysi

ane Technol

ogy

D)

non porous)

ne or polypr

6 mm.

larization (E

ature (60ºC).

is.

ogy

.

ropylene with

EDR).

h sulfonic annd quaternarry amine grooups.

• Electrodialysis (ED)

- Required membrane area

Mass balance (in equivalents)

0m Cj dA V z dc

Vc

C out

j

Vc

c in

Charge flow i: electric current density (A/m2)

Am

: membrane surface (m2)

m

j F dIi

dA

combining

C in out in out

T m

N V c c z F V c c z FA N A

i i

η: global electrical efficiency (~0.5 commercial equipment) j: cation flow (eq/m2 s)

F: Faraday constant (96500 C/eq)

N: number cells in the equipment

z: cation charge (eq/mol)

Membrane Technology


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

2C CE N U I t N I R t U

C: potential gradient in a cell (V)

RC: total resistance in a cell ()

as

C in out

m

V c c z FI i A

then

2

CC

VE N R t

c z F

2

CC

Vó P N R

c z F

P: required Power (J/s)

Membrane Technology


Where, the required specific energy, (J/m3

), is

2

C CC

EE V R

N V

c z F

t

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.

Membra

• Electro

- How to

Cation

iMtF

Di

F

If cDM

+

=

lim

Di

ane Technol

dialysis (ED

determine o

Transport

zD

Dc c

z D DM

M

c c

t t

= 0

z D

M

F c

t t

ogy

D)

operational i

iDMct

F

M

i?

Usually:

t: transpo

D: diffus

Membra

• Electro

- Intensit

Membra

• Electro

- Fields o

W

- Compet

- Econom

- Other fi

F

T

i = 0.8ilim

ort number

ion coefficie

ane Technol

dialysis (ED

ty Evolution

ane Technol

dialysis (ED

of applicatio

Water desalin

ting to RO.

mically more

fields of appl

ood Industry

Treatment of

ent

ogy

D)

versus appl

ogy

D)

n:

nation.

e interesting

lication:

y.

heavy metal

ied potential

at very high

l polluted wa

l

h or very salt

ater.

t concentrati

ions.

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


elect

Membra

• Electro

trolytic Cell

ane Technol

dialysis (ED

l for the pro

ogy

D)

oduction of

m

chlorine an

membrane.

nd sodium hhydroxide wwith cationicc

Electr

Membra

• Electro

rolytic cell f

ane Technol

dialysis (ED

for the prod

ogy

D)

Hydroge

duction of su

m

en fuel cell

ulfuric acid

membrane.

with a catio

and sodium

onic membr

m hydroxide

rane.

e with bipollar

23. membrane technology - nptelnptel.ac.in/courses/103107088/module23/lecture1/lecture1.pdf ·...

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