Evaporation of Solution Droplets in Low Pressures, for Nanopowder Production by Spray Pyrolysis

August 2004

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Outline

Introduction Objective Experimental set-up Future Work Theoretical Model Timetable

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Introduction: Spray Pyrolysis

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Crust Formation

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Zirconia Production

Zirconium nitrate

ZrO(NO3)2.xH2O ZrO2 + NO2+H2O Decomposition temperature: 270 0C

Zirconium chloride

ZrOCl2.8H2O ZrO2 + HCL + H2O Decomposition temperature: 380 0C

Zirconium acetate

Zr(CH3COO)4 +H2O ZrO2 + CO2 + HCL

Decomposition temperature: 320 0CMUSSL

Modeling Nanopowder Production Nanopowder production in the atmospheric pressure occurs

in the Transition Regime: Kn~1

Actual caseP=101 kPad=100 nmKn=1.8

Modeled caseP=0.05 kPad=200,000 nmKn=1.8d

Kn2

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Evaporation in low pressures

Continuum assumption is no longer valid when the pressure is relatively low.

For low density gases in equilibrium the kinetic theory applies.

Nanopowder production occurs in the transition regime and in this region the Boltzmann equation should be solved for the velocity distribution.

Evaporation data of solution droplets for low pressures is very sparse in the literature.

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Objectives

Experimental investigation on the effect of operating conditions (Chamber P, T, φ, and droplet D and Cin) on the morphology of nanopowders of ZrO2.

Experimental investigation on the single droplet evaporation in low pressures.

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Important Issues

Chamber heating in low pressures

Adequate chamber height

Uniform droplet generation

Accurate imaging

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Droplet Evaporation Characteristics

Evaporation Time Terminal Velocity Evaporation Length

Water Methanol Pentane Water Methanol Pentane Water Methanol Pentane

30 micron

0.35 s 0.056 s 0.011 s 0.021 m/s

0.017 m/s 0.013 m/s 0.73 cm 0.095 cm 0.014 cm

200 micron

8 s 1.38 s 0.28 s 0.93 m/s

0.74 m/s 0.59 m/s 700 cm 102 cm 18 cm

300 micron

13 s 2.22 s 0.47 s 1.95 m/s

1.6 m/s 1.33 m/s 2500 cm

350 cm 60 cm

400 micron

17 s 2.95 s 0.63 s 3.15 m/s

2.6 m/s 2.3 s 5300 cm

7670 cm 150 cm

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Evaporation of Pentane Droplets: Effect of Pressure

Pressure (kPa)

Kn ReD U terminal Evaporation Time (symmetric)

Evaporation Time

Convective

Evaporation Time

Kinetic theory

101 0.0009 4.5 0.59 0.51 0.28 -

53 0.0017 2.38 0.59 0.44 0.27 -

34 0.0021 1.5 0.59 0.40 0.27 -

10 0.0093 0.44 0.59 0.34 0.27 -

4 0.0232 0.18 0.6 0.31 0.26 -

2.5 0.0374 0.11 0.61 0.29 0.26 -

1 0.0928 0.048 0.64 0.28 0.25 -

0.65 0.14 0.032 0.65 0.27 0.25 -

0.13 0.69 0.007 0.76 0.25 0.24 -

0.07 1.32 0.004 0.8 0.24 0.24 0.000045

T of ambient=400 K, T of droplet=300 K, Humidity=0, Droplet initial diameter= 200 μm

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Experimental Set-up

Camera

Droplet generator

View ports

Laser Sourcephotodiode

Data acquisition system

Support frame

To the vacuum pump

Grooved plate

Powders

Light

Heaters

thermocouples

Liquid and power feedthrous

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Vacuum System

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Chamber Accessories

• Thermocouple feedthroughs• Power feedthroughs• Liquid feedthroughs• Signal feedthroughs• Pressure gauge• Discharge Valve

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Droplet Generator Requirements

• Repeatable droplet generation (equal size)

• Capable to operate in hot and low pressure environments

• Easy to operate

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Droplet Generator

Piezoelectric droplet generator

Pneumatic droplet generator

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Pneumatic droplet generator

Air flow rate Air pressure Pulse width Liquid level Liquid properties Orifice size

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Droplet Generator Operation

t=10 x 10-4

t=25 x 10-4

t=40 x 10-4

t=55 x 10-4

t=70 x 10-4

t=85 x 10-4

t=100 x 10-4

t=115 x 10-4

t=130 x 10-4

• Single Droplet Generation

• Multiple Droplet Generation: A droplet with several satellites

• Difficult to produce, but relatively repeatable

• Droplets wander during their fall. To reduce droplet drift, a glass tube will be used around the flow path.

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Droplet Generator Operation

t=0

t=15 x 10-4

t=30 x 10-4

t=45 x 10-4

t=60 x 10-4

t=75 x 10-4

t=90 x 10-4

t=105 x 10-4

t=120 x 10-4

t=135 x 10-4

• Stream of droplets: Smaller droplets are produced, but not repeatable

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Data Acquisition System

IEEE 488 GPIB Interface Temperature module Non-conditioning module SCXI 1000 Chassis LabView software:

Temperature measurement Pulse generation Trigger system Pressure recording

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Trigger System

Photodiode: a semiconductor sensorLight Source: Laser

Laser

DAQ

Camera

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Heating Elements

Four 1800 Watts Convective Heaters Maximum Surface Temperature: 325 0C

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Imaging

• FASTCAM-Ultima 1024 model 16K16000 fps

• One camera will be moved to take several images at different locations

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Future Work

2

XRD TEST Reflection of x-ray beams from parallel atomic planes Identifying crystalline phases Crystallite size

TEM or SEM TEST Examine microstructure Identifying Hollow or dense particles

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Theoretical Model

R

rs

RA

4~8

222 sin)1(41 ss

Inviscid free stream of gas outside its wake and flowing over the droplet

Gas-phase viscous boundary layer and near wake.

Core region within the droplet, that is rotational but nearly shear free and can be approximated as a Hill’s spherical vortex.

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Gas Phase Analysis

02

2

3

3

d

fdf

d

fd

1)(0)0( d

df

d

df

1)(0)0( d

df

d

df

B

ff

)0()0(

• Boundary Layer Equations of Momentum, Energy and Mass is applied to the boundary layer around the droplet.

• For the stagnation point and the shoulder region (θ=π/2), where the pressure gradient is zero and the flow locally behaves like a flat-plate flow, local similarity is believed to be a very good approximation

)0()( Afv s

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Heat Transfer in the Droplet

lll T

CTT

)1(2

2

ll

ll

l

qT

atiii

TTatii

Tati

,1)(

,0)(

0,0)(

2

3

0

2

3

02)(

R

R

d

d

R

RCC

p

S

lS

lc

L

B

TTfk

Tq

)(

8

Re)]0([

2

1

With a certain coordinate transformation, the large Peclet number problem can be cast as a one-dimensional, unsteady

problem (Tong and Sirignano ).

In axisymmetric form of the energy equation, and in a large Peclet number situation, heat and mass transport within the droplet involve a strong convective transfer along the streamline with conduction primarily normal to the stream surface

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Concentration Equation in the Droplet

lll Y

CLe

Y

Le

Y)

1(

2

2

ll

ll

ll

fY

atiii

YLe

Yatii

YYati

,1)(

,0)(

,0)( 0

)1(8

Re)]0([,

2/1,

mslll

S

mlm Y

DDfkY

f

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Algorithm

• At any given time instant with known droplet surface temperature Ts and solvent phase species mass fraction Yls, ,the gas phase species mass fractions at the droplet surface Ygs

can be obtained by means of Raoult’s and Clausius-Clapyron laws.

• Therefore, boundary conditions of the gas phase equation will be determined.

• From the solution of the gas phase, the boundary conditions of the liquid phase will be

determined.

• Enegy and concentration equations will be solved. The new droplet surface temperature and the new liquid phase mass fractions at the droplet surface are used for the gas phase solution for the next time step.

• When the surface concentration reaches the critical super saturation (CSS), precipitation starts from the surface of the droplet

• If at this moment, the concentration of the droplet center is higher than the equilibrium saturation (ES) of the solution, a solid particle will form, otherwise, the particle will be hollow.

• This new model predicts that the dried particle will have two not necessarily spherical pores on account of the fluid circulation within the droplets

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