1. 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 63130
2 MEMC Electronic Materials, Inc., 501 Pearl Drive, St. Peters, MO 63376

2. 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.

3. Why Solar Cell Needs Silicon
Semiconductor material in over 95% of all the solar cells produced worldwide : Silicon

4. Demand of Solar Grade Silicon [1]
Availability and demand of solar grade (SG)
Silicon (Worldwide)
Market development as a function of
price of modules
Wp= Watt Peak, which is the Direct Current Watts output of a Solar Module as measured under an Industry standardized Light Test
Challenge: develop a low cost SG-Si production route
Price for a 6KW module: 40K USD
Life time: 15~20 yrs
[1] Block et al., Silicon for the Chemical Industry V, 2000

5. Current processes for Silicon
Siemens (Komatsu) process
Fluidized Bed Reactor (FBR) process
Si particles
SiH4+H2
Si seeds
Product
Heater
Chlorosilanes + hydrogen or silane
cooled bell jar
high temperature Si rods
High energy consumption (1100 C, 800~850 oC)
Discontinuity of the process
Long duration of the process
Lower energy consumption (600~650 oC)
Continuous operation
High cost: 50~60 $/kg
Low cost: <15 $/kg

6. Objective of research OBJECTIVE
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

7. Growing Large Si Particles Growing Seed Si Particles
Pathways
Model in literatures*
Our Model
SiH4
Si Vapor
Growing Large Si Particles
Si nuclei
Si clusters
(1)
(3)
(2)
(6)
(5)
(4)
(8)
(7)
SiH4
Growing Seed Si Particles
Si Fines
(1)
(2)
(3)
CVD growth on large particles
CVD growth on fines
Homogeneous silane decomposition
Homogeneous nucleation
Molecular bombardment of fines
Diffusion to growing large particles
Coagulation and coalescence of fines
Scavenging by large particles on fines
CVD growth on large particles
Homogeneous silane decomposition
Scavenging by large particles on fines
* Caussat et al., 1995
Pina et al., 2006
White et al. 2006

8. Model Scheme Bubble phase Emulsion gas Emulsion phase Well mixed
Feeding of large Si particles
Mass&heat
exchange
Gas enters buuble phase
Gas in bubble phase
Plug flow
Emulsion gas
Well mixed
Gas enters emulsion phase
Feed gas
Gas leaving reactor, from bubble phase
Gas leaving reactor, from emulsion phase
Large Si Particles
Mass
Bubble phase
Emulsion phase
Discharge of large Si particles
SiH4 + H2
Emulsion
phase
Bubble
phase

9. Pathways (1) & (2): CVD growth on large particles and 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
where
where

10. Bubble Phase: Plug Flow
SiH4 mass balance
H2 mass balance
Si vapor mass balance
0th moment of fines
1st moment of fines
2nd moment of fines
Energy balance

11. Emulsion Phase: Stirring Tank
SiH4 mass balance
H2 mass balance
Si vapor mass balance
0th moment of fines

12. Emulsion Phase: Stirring Tank
1st moment of fines
2nd moment of fines
Energy balance

13. Growing Large Si Particles
Pathways
Example
(3): 10.89
SiH4
Si 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
SiH4
Si Vapor
(2):
0.16
(5)
9.81
(4)
1.08
Si Fines
Bubble Phase
Emulsion Phase
Rate of Various Pathways (kg/hr)

14. Reaction or transfer control?
Unreacted silane: mainly in bubbles
Bubble size strongly affects interphase exchange

15. Bed Temperature If T  , conversion  & fines 
There is an optimal T profile to maximize the productivity

16. Silane Concentration If Csn  , fines 
If Csn  , productivity  but cost of raw materials 

17. Bed Height If H  , conversion 
If H  , productivity  but equipment investment  & energy consumption 

18. Conclusions A phenomenological model was developed;
Mechanism of the process was investigated;
Enhancement of interphase exchange is the key to improve the reactor performance;
This study provides a good basis for optimization of operating conditions and for scale-up of reactor.
Acknowledgement
The financial support provided by