fundamentals of particle formation - eth z · particles formed and control their properties . 21...
TRANSCRIPT
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Fundamentals of
Particle Formation
Dr. Frank Ernst [email protected]
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
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Revisited flames from a materials perspective
Examples of flame-made materials
Key processes of particle formation in flames
Lecture outline
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Flames revisited
Combustion:
Rapid chemical reaction between fuel and oxidant
releasing heat
CH4 + 2O2 CO2 + 2H2O + energy
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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
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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
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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
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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
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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)
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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…
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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
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Carbon black
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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
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Furnace
Process:
Carbon Black
by Spray
combustion
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Silica (SiO2)
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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
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Transmit light over very long
distance
Requirements:
high purity
controlled refractive index
Deposition of Films or Coatings - Optical Fibers
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Titania (TiO2)
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Titania Pigments (TiO2)
Chloride Process
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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
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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
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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
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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
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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).
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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Properties of Air
Hinds, W. C., Aerosol technology: properties, behavior, and measurement of airborne particles, Wiley, New York (1999).
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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
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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
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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
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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
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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
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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