Fluidized Bubbling Bed Reactor Model For Silane Pyrolysis In Solar Grade Silicon Production

Yue Huang1, Palghat . A. Ramachandran1, Milorad. P. Dudukovic1, Milind S. Kulkarni2

1 Chemical Reaction Engineering Laboratory (CREL), Department of Energy, Environmental & Chemical Engineering, Campus Box 1198, Washington University in St. Louis, St. Louis, MO 631302 MEMC Electronic Materials, Inc., 501 Pearl Drive, St. Peters, MO 63376

Solar Energy

clean, green, renewable: environmentally friendly tremendous source: sunlight intensity on the earth 1000 W/m2

At some time in the future (50 years or more) fossil fuels will be depleted and humans will have to turn to other energy sources and solar cells will be a big part of generating electricity.

Why Solar Cell Needs Silicon

Semiconductor material in over 95% of all the solar cells produced

worldwide : Silicon

Demand of Solar Grade Silicon [1]

[1] Block et al., Silicon for the Chemical Industry V, 2000

Availability and demand of solar grade (SG)Silicon (Worldwide)

Market development as a function ofprice of modules

Challenge: develop a low cost SG-Si production route

Wp= Watt Peak, which is the Direct Current Watts output of a Solar Module as measured under an Industry standardized Light Test

Price for a 6KW module: 40K USDLife time: 15~20 yrs

Current processes for Silicon

Chlorosilane

s + hydrogen

or silane

cooled bell jar

high temperature Si rods

Siemens (Komatsu) process Fluidized Bed Reactor (FBR) process

Si particles

SiH4+H2

Si seeds

Product

Heater

High energy consumption (1100 C, 800~850 oC)Discontinuity of the processLong duration of the process

High cost: 50~60 $/kg

Lower energy consumption (600~650 oC)Continuous operation

Low cost: <15 $/kg

Objective of research

ONLY MEMC Inc. commercialized FBR process,

because

very expensive and time consuming scale-up

complex reaction mechanism

lack of engineering model for large-scale

reactors

OBJECTIVE

Pathways

SiH4 Si Vapor

Growing Large Si Particles

Si nuclei

Si clusters

(1)

(3)

(2) (6) (5) (4)

(8)(7)

(1)CVD growth on large particles(2)CVD growth on fines(3) Homogeneous silane decomposition (4) Homogeneous nucleation(5) Molecular bombardment of fines(6) Diffusion to growing large particles(7) Coagulation and coalescence of fines(8) Scavenging by large particles on fines

SiH4

Growing Seed Si Particles

Si Fines

(1)(2)

(3)

(1)CVD growth on large particles(2) Homogeneous silane decomposition (3) Scavenging by large particles on fines

Our ModelModel in literatures*

* Caussat et al., 1995Pina et al., 2006White et al. 2006

Model Scheme

Mass&heatexchange

Gas enters buuble phase

Gas in bubble phase

Plug flow

Emulsion gas

Well mixedMass&heatexchange

Gas enters emulsion phase

Feed gas

Gas leaving reactor, from bubble phase

Gas leaving reactor, from emulsion phase

Large Si ParticlesWell mix

edEmulsion

gasWell mixed Mass

exchange

Massexchange

Massexchange

Mass&heatexchange

Massexchange

Mass&heatexchange

Feeding of large Si particles

Discharge of large Si particles

SiH4 + H2

Bubble phase

Emulsion phase

Emulsionphase

Bubblephase

Pathways

),,(/2

2 TCCfsmkmolr HsnHTHT ),,(/2

3 TCCfsmkmolr HsnHDHD

2*5.0

3

3

4exp

2/ Si

A

si

cAHN C

RT

NrmNsmkmolr

(1) & (2): CVD growth on large particles and fines (3): Homogeneous silane decomposition

(4): Homogeneous nucleation

02

2/ SiSi

sisDF CC

M

RTsmkmolr

0,2 2

/ SiSiP

SsiABDL CC

d

Dsmkmolr

20

30, 2

1/1 MsmrCC

(5): Molecular bombardment of fines (6): Diffusion to growing large particles

(7): Coagulation and coalescence of fines

6/16/1

2/5 6

4

32 v

kT

si

where

13/ Msmkgr sisca

mfp

mfmf duPe

2

132 3/2

D

udPe mfSdp,

Fnpd

kTD

,3

(8): Scavenging by large particles on fines.

where

Bubble Phase: Plug Flow bHDbbHTbFnbbsneSnSnbbeb

bSnbG rraCCKdz

Cud,,,,,,

,,

bbSnbH RT

PCC ,,2

bHNbbDFbFnbbHDbbsieSiSibbebbSibG rrarCCK

dz

Cud,,,,,,,

,,

AbHNbbeFnbbebbbbbbG NrMMKM

dz

Mud,,0,0,

2,0

,0,

2

1

SiH4 mass balance

H2 mass balance

Si vapor mass balance

0th moment of fines

1st moment of fines *,,1,1,,3/2

,1,bAbHNbbeFnbbebbbb

bbG vNrMMKMdz

Mud

2*,,2,2,

2,1,3/5

,2, 2 bAbHNbbeFnbbebbbbbbbbbG vNrMMKMM

dz

Mud

bHTbFnrbbHDrbbebbebb

gbbGbp raHrHTTH

dz

dT

RT

PuC ,,,

0,,

2nd moment of fines

Energy balance

Emulsion Phase: Stirring Tank

SiH4 mass balance

H2 mass balance

Si vapor mass balance

0th moment of fines

eHDeHTeFneHTeSd

H

eSnbSnlf

Snbbei

beSnouteineSnine

i

rrara

dzCCH

KV

VCqCq

V

lf

,,,,,

0 ,,,,,,,,

11

e

H

lfeSneH RT

PdzH

CC

lf

0

,,2

1

eDFeFneDLeSdeHN

eHD

H

eSibSilf

Sibbei

beSiouteineSiine

i

rarar

rdzCCH

KV

VCqCq

V

lf

,,,,,

,0 ,,,,,,,,

11

eee

AeHN

H

eblf

Fnbbei

beouteineine

i

MM

NrdzMMH

KV

VMqMq

V

lf

,02

,0

,0 ,0,0,,0,,,0,

2

1

11

Emulsion Phase: Stirring Tank

1st moment of fines

2nd moment of fines

Energy balance

eee

eAeHN

H

eblf

Fnbbei

beouteineine

i

MM

vNrdzMMH

KV

VMqMq

V

lf

,1,3/2

*,0 ,1,1,,1,,,1,

11

eeeee

eAeHN

H

eblf

Fnbbei

beouteineine

i

MMM

vNrdzMMH

KV

VMqMq

V

lf

,22

,1,3/5

2*,0 ,2,2,,2,,,2,

2

11

0

)()(1

ˆ11

,,,,,

0

,1,,,,1,,,,,,,

eHDeHTeFneHTeSdr

eddi

deww

i

wH

eblf

bbei

b

eeouteineineineSiSipi

eeouteineineineepi

rraraH

TThV

ATTh

V

AdzTT

HH

V

V

TMqTMqCV

TCqTCqCV

lf

Pathways

Rate of Various Pathways (kg/hr)

SiH4Si Vapor

Growing Large Si Particles

Si Fines

(1):71.88

(3): 12.02

(2):0.08

(6):6.15(5)

4.72

(4)1.14

(8): 5.18

Emulsion Phase

SiH4Si Vapor

Si Fines

(3): 10.89

(2):0.16 (5)

9.81

(4)1.08

Bubble Phase

Example

Reaction or transfer control?

Silane concentration, Csn, kmol/m3

0.000 0.001 0.002 0.003 0.004

Hei

ght,

m

0

1

2

3

4

Conversion, %

0 20 40 60 80 100

Csn in bubbles

Csn in emulsion phase

Conversion

Bubble size, m

0.0 0.2 0.4 0.6 0.8 1.0H

eigh

t, m

0

1

2

3

4

Mass transfer coefficient, 1/s

0.1 1 10 100 1000

Bubble sizeMass transfer coeff.

Unreacted silane: mainly in bubbles Bubble size strongly affects interphase exchange

Bed Temperature

Temperature, K

750 800 850 900 950 1000

Hei

ght,

m

0

1

2

3

4

-40-20020

If T , conversion & fines

There is an optimal T

profile to maximize the

productivity

Silane Concentration

If Csn , fines If Csn , productivity but

cost of raw materials

Bed Height

If H , conversion

If H , productivity

but equipment

investment & energy

consumption

Conclusions

AcknowledgementThe financial support provided by

A phenomenological model was developed;

Mechanism of the process was investigated;

Enhancement of interphase exchange is the key to im

prove the reactor performance;

This study provides a good basis for optimization of o

perating conditions and for scale-up of reactor.