Fundamentals of

Particle Formation

Dr. Frank Ernst ernst@ptl.mavt.ethz.ch

phone: 044 632 25 10

Office hour:

Thursdays after the lecture

Department of Mechanical and Process Engineering

ETH Zurich, www.ptl.ethz.ch

Verbrennung und chemisch reaktive Prozesse in der Energie- und Materialtechnik

TiCl4

TiCl4

TiCl4 H2

H2 H2 O2

O2 O2

TiO2

TiO2

TiO2 TiO2

H2O H2O H2O

HCl

HCl

2

Revisited flames from a materials perspective

Examples of flame-made materials

Key processes of particle formation in flames

Lecture outline

3

Flames revisited

Combustion:

Rapid chemical reaction between fuel and oxidant

releasing heat

CH4 + 2O2 CO2 + 2H2O + energy

4

Flames revisited

Combustion:

Rapid chemical reaction between fuel and oxidant

releasing heat

CH4 + 2O2 CO2 + 2H2O + energy

Flame:

Luminous zone formed by combustion of fuel and

oxidant

5

Flames revisited

Basic flame types:

Diffusion (non-premixed)

Premixed Fuel / oxidiser mixing

Fluid motion Laminar Turbulent Laminar Turbulent

Examples

Configuration

Flat flame Spark ignition

engine

Candle Pulverised coal

combustor

6

Combustion is a chemical reaction

A + B C + D - heat

HC +

Air

or

O2

CO2/CO + H2O + NOx + C +HC

CO2 +C2H2 + C2H4 +…….

C(s) + CO + H2O

utilize heat for high temperature supply (e.g. cement production)

utilize chemical reaction and heat for material production

- heat

7

Flames revisited

Contacting

fuel/oxygen

&

chemical

reaction

Chemical reactions kinetics & thermodynamics

Fluid flow Laminar & turbulent

Temperature

Combustion science:

• Focus on energy and gaseous species formation

• Particle and material formation focus often environmental eg. Soot

• Flames are also useful to make many interesting and commercial materials

Reaction A b E

OH + H2 => H + H2O 2.14E+08 1.52 3449

O + OH => O2 + H 2.02E+14 -0.4 0

O + H2 => OH + H 5.06E+04 2.67 6290

H + O2( + M) => HO2( + M) 4.52E+13 0 0

H + O2( + N2) => HO2( + N2) 4.52E+13 0 0

H + O2( + H2) => HO2( + H2) 4.52E+13 0 0

H + O2( + H2O) => HO2( + H2O) 4.52E+13 0 0

OH + HO2 => H2O + O2 2.13E+28 -4.827 3500

H + HO2 => OH + OH 1.50E+14 0 1000

H + HO2 => H2 + O2 8.45E+11 0.65 1241

H + HO2 => O + H2O 3.01E+13 0 1721

O + HO2 => O2 + OH 3.25E+13 0 0

OH + OH => O + H2O 3.57E+04 2.4 -2112

H + H + M => H2 + M 1.00E+18 -1 0

H + H + H2 => H2 + H2 9.20E+16 -0.6 0

H + H + H2O => H2 + H2O 6.00E+19 -1.25 0

H + OH + M => H2O + M 2.21E+22 -2 0

H + O + M => OH + M 4.71E+18 -1 0

O + O + M => O2 + M 1.89E+13 0 -1788

HO2 + HO2 => H2O2 + O2 4.20E+14 0 11982

OH + OH( + M) => H2O2( + M) 1.24E+14 -0.37 0

H2O2 + H => HO2 + H2 1.98E+06 2 2435

H2O2 + H => OH + H2O 3.07E+13 0 4217

H2O2 + O => OH + HO2 9.55E+06 2 3970

H2O2 + OH => H2O + HO2 2.40E+00 4.042 -2162

8

Materials produced in flames

1. Gas Production:

HCl, C2H2, O2, N2, SO2 (H2SO4), P2O5 (H3PO4)

2. Solid Manufacture and Processing

(Welding; Cutting with C2H2/O2 torches etc.)

Particles (carbon black, TiO2, SiO2, Al2O3)

Films or Coatings (light-guides, ceramics: thermal sprays)

9

Why materials?

Materials are used in every aspect of society

Particulate materials (eg. Powders) are used in large

volumes and for many different applications

Some examples…

10

Established

Product Particles

Carbon black

Titania

Fumed Silica

Volume

t/y

8 M

2 M

0.2 M

Ind.Process

(dominant)

Spray Flame

Vapor Flame

Vapor Flame

Use

(exemplary)

Inks, tires

Paints

Toothpaste, flowaid

PROMISING: Catalysts, Sensors, Photonics, Biomaterials

S.E. Pratsinis, "Flame Aerosol Synthesis of Ceramic Powders" Prog. Energy Combust. Sci. 24, 197-219 (1998).

Solids manufacture – in flames

11

Carbon black

12

13

Voll, M.; Kleinschmitt, P. Carbon Black. In Ullman's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co., 2002.

Furnace Separation Product

oil

gas water

air

heat

recovery baghouse

cyclone

Tail gas

(energy recovery) Exhaust air

Fluffy carbon

black Pellet carbon

black

pelletizer

dryer

storage

Furnace black process

14

Furnace

Process:

Carbon Black

by Spray

combustion

15

Silica (SiO2)

16

Hydrogen

Oxygen (air)

Silicon tetrachloride

HCl absorption

Fumed silica

vaporizer

mixing

burner

cooling

Filters

and/or

cyclone

de-acidification

hopper

Ettlinger, M. “Pyrogenic silica”. In Ullman's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co., 2000.

Manufacture of Fumed Silica

17

Transmit light over very long

distance

Requirements:

high purity

controlled refractive index

Deposition of Films or Coatings - Optical Fibers

18

Titania (TiO2)

19

Titania Pigments (TiO2)

Chloride Process

20

Why flames to make particles?

The flame is the reactor!

high mobility of reactants and particles

highly reactive environment

high heat and mass transfer

Rapid synthesis. O(10) to O(100) milliseconds

Easily up-scaled from gram to tonne scale

For material synthesis we want to increase amount of

particles formed and control their properties

21

Particle formation

Product particles are a solid material with sizes up to microns (mm)

Fixed chemical composition

Eg. TiO2, SiO2, C, Al2O3

Growth proceeds by joining together of small molecular-scale units

molecules particles

22

Particle formation & growth

Chemical reactions kinetics & thermodynamics

Fluid flow Laminar & turbulent

Temperature

Source of molecular building unit “monomer”

Environment supporting particle growth

eg. TiCl4 + 2O2 TiO2 + Cl2

monomer

TiO2

TiO2 TiO2

TiO2

TiO2

TiO2

23

Particle formation & growth – key steps

Chemical reaction

Chemical Reaction:

Gas-phase chemical reactions are source of

monomer species.

TiCl4 + O2 TiO2 + 2Cl2

SiCl4 + O2 SiO2 + 2Cl2

C2H2 + kO2 2(1-k)C + 2kCO + H2

24

Particle formation & growth – key steps

Chemical reaction

Chemical Reaction:

Gas-phase chemical reactions are source

of monomer species.

TiCl4 + O2 TiO2 + 2Cl2

SiCl4 + O2 SiO2 + 2Cl2

C2H2 + kO2 2(1-k)C + 2kCO + H2

dCkC

dt

First-order consumption of precursor

eg. C is concentration of TiCl4

then

expoC C kt

where Co is initial precursor

concentration

Pratsinis, S. E. Flame aerosol synthesis of ceramic powders. Progress in Energy and Combustion Science 24, 197-219 (1998).

25

Particle formation & growth – key steps

Chemical reaction Nucleation

Nucleation:

Formation of initial clusters

1. Homogenous nucleation: equilibrium between condensation and evaporation

2. Coagulation: If monomer is thermodynamically stable the monomer molecule

is the critical cluster

- This is generally the case for metal oxide systems

TiO2

TiO2 TiO2

TiO2 TiO2

TiO2

cluster

TiO2 TiO2

evaporation condensation

26

Particle formation & growth – key steps

Chemical reaction Nucleation

Coagulation:

Collisions between clusters, monomers

and other particles leads to larger

particles

Coagulation

1

28 Bk Tu

m

u

Average velocity (u) of

molecules and small particles

kB = Boltzmann constant

m = Particle mass

27

Particle formation & growth – key steps

Chemical reaction Nucleation

Coagulation:

Collisions between clusters, monomers

and other particles leads to larger

particles

Coagulation

Velocity Collision

Coalescence via

rearrangement

u

Average velocity (u) of

molecules and small particles

kB = Boltzmann constant

m = Particle mass

1

28 Bk Tu

m

28

Coagulation

Collisions and coalescence of particles gives fewer but

larger particles

Dynamics of particle growth related to

Velocity of particles (Temperature, mass)

Probability of collision (Concentration, size)

Velocity Collision Coalescence via

rearrangement

29

Monodisperse = all particles have the same size

Assume here that all particles stay same size despite

collisions (strong but useful assumption!)

We only consider the total number concentration

(# particles per unit volume)

Idealized case – Monodisperse coagulation

30

Rate of change of number concentration

2

1 1

1,

2

dNv v N

dtb

N = # particles per volume

b= Collision frequency function (rate of collisions per particle per volume)

v = particle volume

1 3 1 3

1 11 3 1 3

1 1

2 81 1

3 3

B Bk T k Tv v

v vb

m m

2

2

dNN

dt

b

1

12

oo

NN N t

b

where

then

integration

Idealized case – Monodisperse coagulation

monodisperse

same sized

(continuous regime)

31

More correctly, for true coagulation the number of

particles decreases and the average particle size

increases

We consider the number concentration of each particle

size (# particles per unit volume)

Coagulation

32

Coagulation

1

,2

i j i j

k i j

v v n nb

1

,k i k i

i

v v n nb

1

1, ,

2

ki j i j k i k i

k i j i

dnv v n n v v n n

dtb b

Then net rate of change in particle concentration is:

Loss of particles of size k by collision with other particles:

Birth of particles of size k = i+j is:

(½ factor corrects for double counting)

Consider nk to be the number concentration of particles of size k.

polydisperse

33

1

1, ,

2

ki j i j k i k i

k i j i

dnv v n n v v n n

dtb b

nk

size (k)

nk

size (k)

nk

size (k)

nk

size (k)

t = 0

Increasing time (t)

Coagulation

34

Motion in Gases

Air molecules are in continuous

motion

They collide among themselves

because of their translational energy

The translational energy is

proportional to the air temperature

35

Gas mean free path

The average distance traveled by

gas molecules is called the mean

free path (l) :

2

u

ml

where

1

28 Bk Tu

m

m = dynamic viscosity

= density

36

Properties of Air

Hinds, W. C., Aerosol technology: properties, behavior, and measurement of airborne particles, Wiley, New York (1999).

37

Coagulation

For atmospheric T and P, l is typically O(100 nm)

The ratio of the mean free path to the particle size (dp) is the Knudsen

number (Kn):

Kn >> 1: Free Molecular Regime

Kn ~ 1: Transition Regime

Kn << 1: Continuum Regime

Particles smaller than the mean free path (ie. Kn > 1) are in the Free-

molecular regime

u Gas molecule

Particle

38

Coagulation in the Free Molecular Regime

Collision frequency function (b) for free-molecular regime1

However, for particle growth in the free-molecular regime we can directly

estimate the average diameter by2:

1 2 1 2

1 62

1 3 1 3

,

63 1 1

4

Bi j i j

p i j

k Tv v

v vb

1 .Wooldridge, M. S. Gas-phase combustion synthesis of particles. Progress in Energy and Combustion Science 24, 63-87 (1998). 2. Pratsinis, S. E. Flame aerosol synthesis of ceramic powders. Progress in Energy and Combustion Science 24, 197-219 (1998).

1 2

5 2 5 2 610 B Pp po

p p

k T C M td d

Size as function of time but no information on distribution

39

Particle formation & growth – key steps

Chemical reaction Nucleation Aggregation Coagulation

Aggregation:

Occurs when particles are too

large or T is too low for coalescent

rearrangement

Collisions leads to particle “chains”

1. Aggregates (“Hard agglomerates”): Particles strongly joined together

2. Agglomerates (“Soft agglomerates”): Weak forces. Particles easily separated

40

Particle formation & growth – key steps

Chemical reaction

Source of

monomer

species

Nucleation

Formation of clusters

Aggregation

Spherical particles form

via particle-particle

collisions

Collisions between

spherical particles form

chains

Coagulation

TiO2 TiCl4 + 2O2 TiO2 + Cl2

Decreasing number concentration

Increasing size and mass

41

TiCl4

TiCl4

TiCl4 H2

H2 H2 O2

O2 O2

TiO2

TiO2

TiO2 TiO2

H2O

H2O H2O

HCl

HCl

Chemical reaction

Nucleation

Aggregation

Coagulation

T (K)

25

00

20

00

15

00

10

00

50

0 Particle formation & growth – flames

42

Revisited flames from a materials perspective

Why materials are interesting. Examples of flame-made materials

Key processes of particle formation in flames

Chemical reaction, nucleation, coagulation, aggregation

Coagulation: monodisperse and polydisperse

Coagulation: free-molecular and continuous regime

Further reading

Pratsinis, S. E. Flame aerosol synthesis of ceramic powders. Progress in Energy and Combustion

Science 24, 197-219 (1998).

Wooldridge, M. S. Gas-phase combustion synthesis of particles. Progress in Energy and Combustion

Science 24, 63-87 (1998).

Hinds, W. C., Aerosol technology: properties, behavior, and measurement of airborne particles. Wiley,

New York (1999).

Friedlander, S.K. Smoke, Dust and Haze: Fundamentals of aerosol dynamics. Oxford Press, New York

(2000).

Lecture summary