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

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] 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

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

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

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

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

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

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

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

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

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)

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

Bed Temperature 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 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