Separation and Classification of Nano Particles PDF

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Chapter 8. Separation and Classification of Nanoparticles



8.1 Introduction

- Separation = recovery = collection

- Classification

Separation Mechanisms

- Sedimentation*: Settling chamber, centrifuge

- Inertial deposition: Cyclone*, scrubber, inertial impactor

- Brownian diffusion: Diffusion batteries

- Migration of charged particle in an electric field :

Electrostatic precipitator, dynamic mobility analyzer

- Thermophoresis: Thermal precipitator (thermopositor)

- Filters: particle collection by the combined mechanism.

* Generally not suitable for nanoparticle collection but used for precollector



Collection efficiency

- Fraction of particles fed in collected (deposited) on the interior wall of the collector…

* Fractional (grade) efficiency

- based on number of particles

- based on mass of particles

cf. f(d p ) vs. n(d p )

* Total efficiency

( ))(

)()(

)(

)()(

p feed

p product p feed

p p feed

p p product p p feed

p N d n

d nd n

dd d n

dd d ndd d nd G

−=

−≡

( ) )()()(

,

,,

p feed m

p product m p feed m pM

d nd nd nd G −≡

∫ ∞

=0

)()( p p pT dd d nd G E



8.2 Separation by Mechanical Forces

(1) Gravitational settler

- For laminar or plug flow

By analysis of particle trajectory

- For turbulent Flow

Considering the particle trajectory in differential length analysis

* Cut size (diameter): d p,50

: particle diameter at G(d p

)=0.5

TurbulenceLaminarsublayer

G

pd 0

1.0

0.5

50, pd

G

pd

0

1.0

*

pd

( ) H U

Ld U yd G P t

p

)(=∗≡

( ))

−−=

−−=∴ Q

d U A

UH

Ld U

d G pT C pT

p

)(

exp1exp1



(2) Inertial Separator

* Particle trajectory from similitude analysis and thus for G(d p )

where L: characteristic length of the separator

U: characteristic velocity of the particle in the separator

where and

* For given inertial separator

Efficiency

Cut diameter

* Power requirement

where

- Similar similitude analysis gives

where

Ld St f d G p p /Re,,=

L

U d St

p P

µ

ρ

18

2

≡µ

ρ UL f ≡Re

pQ∆≡( )µ ρ υ ,,, f L f p =∆

( )Re f Eu =

22υ ρ f

p Eu

∆≡

( )Re,St f d G p =

( ) ( )ReRe,5.0 15050 f St St f =→= For given Re



Cyclone (hydrocyclone)

- Grade efficiency of practical cyclone

Based on fluid tangential velocity profile

where ,

Flow patterns in cyclones

)exp(1)(M

p p d d G Ψ−−=

1

1

+= mM

3.0

14.0

283)67.01(1

−−=

T

Dm c

const r U m

f =

2/

318

)1(2

M

c

c p

D

mC KQ

+=Ψ

µ

ρ

where Dc(m); d p (cm); (g/cm3

) ; T(K); (g/c m s); Q(m3

/s) ρ µ

K: dimensionless geometric

parameter



- From both theoretical and actual analysis for given cyclone and

For wide range of Re,

~ constant

~ constant

* Standard Cyclone Design – determination of dimension “ Stairmand design rule”

- High efficiency Stairmand cyclone: St 50=1.4x10-4 and Eu=320

High flowrate Stairmand cyclone: St 50

=6x10-3 and Eu=46

≡

D

U d St

p P

µ

ρ

18

2

50 → Q Dd p p ρ µ /3

50, ∝

∆≡2

2U P Eu

f ρ 42

DQ p ∝∆→

0.750.750.3750.8750.5751.54.0Stairmand, High

flowrate

0.50.50.20.50.3751.54.0Stairmand, High

efficiency

D jab L D sh H Cyclone type



Impactor

- Separation by impact on the surface perpendicular to the flow

- From numerical and/or experimental analysis

- St 50: also almost independent of Re and further independent of geometry…

*For 500 < Re < 3000 and S/D j> 1.5

For circular nozzle, St 50 = 0.22 For rectangular nozzle, St 50 = 0.53

2/1

5050

18

=∴

U

DSt d

p

p

ρ

µ



- To collect nanoparticles, D↓ ↓ , U ↓ ↓ and C c↑ ↑

Vacuum operation with supersonic velocity is required…

“hypersonic impactor”

* Cascade impactor

- Overlapping of efficiency curve of one stage with neighboring plate: avoided - Measurement of particle size distribution

- Used for classification of particles

* Andersen impactor



Venturi Scrubbers

- Collection of particles by use of water spray

- Scavenging of particles by water droplets

- Formation of slurry droplets by condensational growth

of particles in humid air

* Grade efficiency

Calvert(1984)

where W : water feed rate (m3 /s)

G, U g : gas flow rate (m3

/s) and gas velocityd d :droplet diameter (m)

f: empirical parameter encountering mode other than

impaction, usually =0.5

⋅−= )2(55

1

exp1)( f St F

d U

G

W d G

g

d l g

pµ

ρ



* Characteristics of venturi scrubber

- High efficient for particles smaller than 2 um

- The only choice for sticky, flammable or highly corrosive particles

- High gas velocity(~120 m/s)→ smaller-size equipment made of less corrosion-

resistant materials

- Liquid-to-gas volumetric flow rate ratio = 0.001~0.003



8.3 Separation by Filters

(1) Introduction

natural polymers

polysaccharides polypeptides

rubbers

synthetic polymers

thermoplastics

rubbery polymers

soluble linear

insoluble crosslinked

ion-containing polymers polysiloxanes

polyphosphazenes

glasses

ceramics

metals

polymers

Organic Inorganic - Organic Inorganic

Filter and membrane materials

extruded dense films (silicone films)

extruded and stretched dense film (teflon and olefin microfilters)

cast or extruded films with phase inversion step

(cellulose acetate ultrafilters)

nuclear-particle track etched (polycarbonate

microfilters)

electrochemical deposition (homoporous

alumina microfilters)

sol-gel (ceramic

ultrafilters) compression or sintering

(metal and glass filters

and frits)

extruded (alumina

microfilter monoliths)

wet-lay (many paper

filters) dry-lay (spunbonded

olefins)

wound (glass filament

cartridges)

woven (polymeric and/or

metal filter meshes)

Films Particles Fibers

Formation Techniques



chemical composition

hydrophobicity -hydrophilicity

surface charges

solute-membrane affinity

surface texture

pore size distribution

pore shape

pore morphology gradient

through membrane thickness

solvent flow (hydraulic

permeability)

solute or particle rejection

(sieving coefficient)

solute diffusion

Surface properties Pore size characteristics Transport properties

Characteristics of filter and membranes

* Filter rating

- Speed: how fast you can process a specified volume of fluid.

-Q/A ratio

- Collection efficiency

- Pressure drop: power requirement

- Stability: life, depending on chemical and mechanical strength

* Asymmetric membrane



(2) Gas filtration

Filter materials – cellulose (wood), glass, plastic fibers

* High-temperature filters - metal. graphite, quartz, ceramic

Air filters - depth filters

- Filter Types

- Low solid loading ~mg/m3

e.g. air-conditioning filters

-

* HEPA (high efficiency particulate air) filter

- used in glove box, clean rooms, nuclear fuel industry

-

Fibrous filters Membrane(porous) filters Capillary filters

Pa p smU 100010~,/5.125.0~ −∆−

Pa p smU 200~,/1.0~ ∆



* Collection mechanisms of the fibrous filters

- Diffusion :

- Inertial impaction :

- Interception :

- Gravity:

- Electrostatic attraction :

* Grade efficiency of air filters

where Single fiber efficiency

d f : fiber diameter

Kuwabara number

: solid fraction(1- ), : void fraction

, T, U 0: mean free path, viscosity, temperature, and approaching

velocity of the gas

m3.0<m13.0 −

mµ 101−

m10>mm 501.0 −

−−=

f

f

p d

t E d G

π

α 4exp1)(

9/1

104

0

4511

44.1

−=

f

f d U

kT

Ku E

µ

λ α

443

2ln 2α α α −+−−= Ku

α ε

λ ,

ε



Particle diameter of minimum efficiency

Filter efficiency for individual mechanism and combined mechanisms.

9/2

0

2

min,1

885.0

−

=U

d kT Kud

f

pµ

λ

α



Bag (fabric) filters - surface filters

- Filter media : cylindrical bag type

- L/D ratio ~ 20, D~ 120-150mm

- High solid loading ~ g/m3

* Particle collection mechanisms

- Firstly, collection on individual fibers

- Secondly, filtration by particle cake

* Collection Efficiency

where W : Dust mass per unit bag surface area, Areal density, kg/m2,

c : Inlet dust loading, kg/m3

t : Operation time since last cleaning

V : Gas-to-cloth ratio,

: Cake penetration decay rate

)exp(1 W d G p α −−=

cVt W =

QV ≡

α



* Permeation rate and pressure drop

where Rm: resistance of filter media, reciprocal of permeance

Rc: resistance of filter cake,

K: function of the properties of dust

- Constant-pressure operation: permeation rate decrease

* Regeneration (cleaning) of filters

- shaker (vibrator), reverse flow, pulse jet

- use of cleaning ring

)(

)(

t R R

t pV

C m +∆=

( ) KcVt t Rc =



(3) Liquid filtration See http://www.membranes.nist.gov/ACSchapter/pellePAGE.html

* Classification of liquid filtration

RO, GS, dialysis width < 0.35 nm Ultrapore

RO, GS, dialysis width < 0.7 nm Ultramicropore

RO, NF 0.7 nm < width < 2 nm Supermicropore

NF width < 2 nm Micropore

UF, NF 2 nm < width < 50 nm Mesopore

UF, MF, and filtration width > 50 nm Macropore Pore Characteristics

(UF - ultrafiltration, MF -

microfiltration, NF - nanofiltration, RO

- reverse osmosis. GS - gas and

vapor separation)



Table .Table .Table .Table . Comparison of pressure-driven liquid (aqueous) phase membrane processes

* Permeation rate and pressure drop across filter membrane

where : osmotic pressure

- Constant- pressure operation

- Constant-flow rate operation

* Clean-up by back-flushing

( ))(t R R

pV

cm +

∆Π−∆=

Π



* Equipments

- Plate-and-frame

- Spiral wounded

- Hollow fiber



8.4 Separation by Nonequilibrium Gas

(1) Thermal precipitators

- Collection efficiency for particles having

- Used in lab-scale particle collection for electron microscopes

- Volumetric flow rate ~ 4-5cm3 /min

- ΔT=50-200K with 1000-10000K/cm

* Wire-and-plate form

- Used for dust collection for British mines

- 0.25mm Nichrome wire

- Temperature gradient: 8000K/cm- Gas flow rate: 7.2cm3 /min

1105 =−⟨ md p µ



(2) Electrostatic precipitator

Particle Charging - Corona Discharge

For a cylindrical (wire-in-tube) ESP

As V ↑ , air → electrical breakdown near the wire

Two zones in corona discharge

- Active zone → active electrical breakdown

"Electron avalanche" - blue glow

- Passive zone → particle charging

Single-zone ESP

Electron avalanche



* Positive corona vs. negative corona

*Diffusion charging vs. field charging

*Two-zone ESP

-More stable than positive corona

-Needs electron absorbing gas(SO 2 ,

O 2 , H

2 O)

-Produces O 3

as byproduct

-Suitable for industrial applications

Suitable for domestic

application

Negative corona Positive corona



Collection Efficiency

where : electrical migration velocity

Ac: cross sectional area of the ESP

P: Perimeter of the ESP wall (P=A/L)

−=

−=−=

Q

d AU

Q

d PLU

n

nd G

pe pe

in

out p

)(exp1

)(exp11)(

p

ce

d

qEC U

πµ 3=



Particles suitable for ESP collection

Electrical resistivity of particles ←

e.g. Fly ash : 106 ~ 1011

Carbon black : 10-5

- If : fast charge transfer to electrode→ reentrainment of particles → G↓

- If : slow charge transfer (charge: longer stay)→ reverse corona → G↓

∴ Optimum for ESP:

* ESP vs. fabric filter system

l iiRV ρ ==

m⋅Ω

m⋅Ω

m⋅Ω⟨ 210 ρ

m⋅Ω×⟩ 8102 ρ

mm ⋅Ω⟨⟨⋅Ω 861010 ρ

ρ

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