Fluidized Bubbling Bed Reactor Model
For Silane Pyrolysis In Solar Grade
Silicon Production
Yue Huang1, Palghat . A. Ramachandran1, Milorad. P.
Dudukovic1, Milind S. Kulkarni2
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
1
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
10000
Availability
Shortage
700
600
8000
6000
4000
15
500
400
10
300
200
5
2000
100
0
0
1995 1997 2000
0
1980
2005 2010 2015
1985
1990
1995
2000
2005
2010
Market Development, MWp / y
20
12000
Price, $ / Wp
Solar Grade Silicon, ton/y
Demand of Solar Grade Silicon
[1]
Year
Year
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
[1] Block et al., Silicon for the Chemical Industry V, 2000
Price for a 6KW module: 40K USD
Life time: 15~20 yrs
Current processes for Silicon
Siemens (Komatsu) process
Fluidized Bed Reactor (FBR) process
Si seeds
cooled bell jar
Heater
Si particles
high temperature Si rods
Chlorosila
nes +
hydrogen
or silane
Product
SiH4+H2
o
High energy consumption (1100 C, 800~850 C)
Discontinuity of the process
Long duration of the process
High cost: 50~60 $/kg
o
Lower energy consumption (600~650 C)
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
Our Model
Model in literatures*
SiH4
SiH4
(2)
(1)
Si Fines
Growing Seed
Si Particles
(3)
(3)
Si Vapor
(6)
(2)
(5) (4)
(1)
Growing Large
Si Particles
(8)
Si nuclei
(7)
Si clusters
(1) CVD growth on large particles
(1) CVD growth on large particles
(2) CVD growth on fines
(2) Homogeneous silane decomposition
(3) Scavenging by large particles on fines (3) Homogeneous silane decomposition
(4) Homogeneous nucleation
(5) Molecular bombardment of fines
* Caussat et al., 1995
(6) Diffusion to growing large particles
Pina et al., 2006
(7) Coagulation and coalescence of fines
White et al. 2006
(8) Scavenging by large particles on fines
Model Scheme
Feeding of
large Si
particles
Bubble phase
Gas leaving reactor, from
bubble phase
Emulsion
gas
Mass&heat
exchange Well mixed
Emulsion phase
Discharge
of large Si
particles
Mass
exchange
Mass&heat
Gas in
bubble phase exchange
Mass
exchange
Mass&heat
exchange
Mass
exchange
Emulsion
gas
Mass&heat Well mixed
exchange
Mass
exchange
Plug flow
SiH4 + H2
Emulsion Bubble
phase
phase
Gas leaving reactor,
from emulsion phase
Gas enters
buuble phase
Gas enters
emulsion phase
Feed
gas
Large Si
Particles
Well
mixed
Pathways
(1) & (2): CVD growth on large particles and fines

(3): Homogeneous silane decomposition



rHD kmol/ m3  s  f HD (Csn , CH2 , T )
rHT kmol/ m2  s  f HT (Csn , CH2 , T )
(4): Homogeneous nucleation
rHN
0.5


 4r * N A 

2

m


2
c
 exp 

km ol/ m  s   N A

C


Si

 si     
3RT 


3

(5): Molecular bombardment of fines


rDF km ol/ m  s   s
2

RT
0
 C Si  C Si
2M si
(7): Coagulation and coalescence of fines




rDL kmol/ m2  s 
2DAB,siS
dP
0

.

rsca kg / m3 s  si M1
 1/ 6
v


 CSi  CSi
(8): Scavenging by large particles on fines


1
2
rCC ,0 1 / m 3  s   M 0
2
1/ 6
 6kT
5/ 2  3 
where   2 
 
 4    si
(6): Diffusion to growing large particles
where
  2Pe2 / 3  u mf
Pe 
d p,Sd u mf
D
31   mf

2d p  mf
D
kT
3d p , Fn
Bubble Phase: Plug Flow
SiH4 mass balance
H2 mass balance
d uG ,b C Sn ,b 
dz
C H 2,b  C Sn ,b 
Si vapor mass balance
0th moment of fines
d uG ,b C Si ,b 
dz
d uG ,b M 0,b 
dz
1st moment of fines
d uG ,b M 1,b 
2nd moment of fines
dz
d uG ,b M 2,b 
dz
Energy balance
  b K be b,Sn C Sn ,e  C sn ,b    b a Fn,b rHT ,b   b rHD ,b
C p,b uG,b
P
RTb
  b K be b,Si C Si ,e  C si ,b    b rHD ,b   b a Fn,b rDF ,b   b rHN ,b
1
2
   b  b M 0,b   b K be b, Fn M 0,e  M 0,b    b rHN ,b N A
2
  b b M 2 / 3,b   b K be b, Fn M 1,e  M 1,b    b rHN ,b N A vb
*
 2   b b M 5 / 3,b   b  b M 1,b   b K be b, Fn M 2,e  M 2,b    b rHN ,b N A vb
2
P0 dTb
  b H be b Te  Tb    b  H r rHD ,b   b  H r aFn,b rHT ,b
Tb Rg dz
*2
Emulsion Phase: Stirring Tank
SiH4 mass balance
1
qe,inC Sn,e,in  qe,out C Sn,e   Vb K be b,Sn 1
Vi
Vi
H lf
 C
H lf
0
Sn ,b
 C Sn ,e dz
 a Sd ,e rHT ,e  a Fn,e rHT ,e  rHD ,e
H2 mass balance
C H 2,e  C Sn ,e 
1
H lf

H lf
0
Pdz
RTe
Vb
1
1




q
C

q
C

K
Si vapor mass balance
e ,in Si ,e ,in
e ,out Si ,e
be b , Si
Vi
Vi
H lf
 C
H lf
0
Si ,b
 C Si ,e dz  rHD ,e
 rHN ,e  a Sd ,e rDL ,e  a Fn,e rDF ,e
0th moment of fines
1
qe,in M 0,e,in  qe,out M 0,e   Vb K be b,Fn 1
Vi
Vi
H lf

1
 e M 0 , e 2  M 0 , e
2
 M
H lf
0
0 ,b
 M 0,e dz  rHN ,e N A
Emulsion Phase: Stirring Tank
1st moment of fines
1
qe,in M 1,e,in  qe,out M 1,e   Vb K be b,Fn 1
Vi
Vi
H lf
 M
H lf
0
1,b
 M 1,e dz  rHN ,e N A ve
*
  e M 2 / 3,e  M 1,e
2nd moment of fines
1
qe,in M 2,e,in  qe,out M 2,e   Vb K be b,Fn 1
Vi
Vi
H lf
 M
H lf
0
2 ,b
 M 2,e dz  rHN ,e N A ve
*2
 2   e M 5 / 3,e   e M 1,e  M 2,e
2
Energy balance
1
C p ,e qe,in
Vi
 C T
V
1
 b H be b
Vi
H lf
e ,in
e ,in
 qe,out
 C T   V1 Cˆ
e
e
p , Si
 Si qe,in M 1,e,inTe,in  qe,out M 1,eTe 
i
Aw
Ad


T

T
dz

h
(
T

T
)

hd (Td  Te )
w
w
e
0 b e
V
V
H lf
i
  H r a Sd ,e rHT ,e  a Fn,e rHT ,e  rHD ,e   0
i
Pathways
Example
(3): 10.89
SiH4
(2):
0.16
Si Vapor
(5)
9.81
(4)
1.08
Si Fines
Bubble Phase
(3): 12.02
SiH4
(1):
71.88
(6):
6.15 (5)
4.72
(2):
0.08
Growing Large
Si Particles
Si Vapor
(8): 5.18
Si Fines
Emulsion Phase
Rate of Various Pathways (kg/hr)
(4)
1.14
Reaction or transfer control?
Mass transfer coefficient, 1/s
Conversion, %
0
20
40
60
80
0.1
100
1
4
100
1000
Bubble size
Mass transfer coeff.
Csn in bubbles
Csn in emulsion phase
3
10
4
3
Height, m
Height, m
Conversion
2
2
1
1
0
0.000
0
0.001
0.002
0.003
0.004
0.0
Silane concentration, Csn, kmol/m3
0.2
0.4
0.6
Bubble size, m
 Unreacted silane: mainly in bubbles
 Bubble size strongly affects interphase exchange
0.8
1.0
Bed Temperature
14
Selectivity to fines, %
12
4
-40
-20
0
20
Height, m
3
2
10
8
6
4
2
0
1
-40
-20
0
20
Temperature Profile
86.0
800
850
900
950
1000
Temperature, K
 If T  , conversion  & fines 
 There is an optimal T profile
to maximize the productivity
Productivity, kg/hr
0
750
85.5
85.0
84.5
84.0
-40
-20
0
Temperature Profiles
20
100
20
98
18
Selectivity to fines, %
Silane conversion, %
Silane Concentration
96
94
92
90
8
10
12
14
16
18
20
Productivity, kg/hr
0.72
120
0.70
100
0.68
60
0.66
8
10
12
14
16
18
Molar fraction of silane in feed, %
20
22
Specific productivity, kg Si/ kg silane
0.74
80
12
8
Molar fraction of silane in feed, %
140
14
10
22
160
16
10
12
14
16
18
20
22
Molar fraction of silane in feed, %
 If Csn  , fines 
 If Csn  , productivity 
but cost of raw materials 
Bed Height
100
15
Selectivity to fines, %
Silane conversion, %
99
98
97
96
95
94
14
13
12
11
10
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Bed height, m
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Bed height, m
86
 If H  , conversion 
Productivity, kg/hr
85
84
 If H  , productivity 
83
but equipment investment 
82
& energy consumption 
81
80
3.0
3.5
4.0
4.5
Bed height, m
5.0
5.5
6.0
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