P.A.Ramachandran M.P.Dudukovic Reactor Models for Silane Pyrolysis in Fluid Bed Reactors: A Brief Review of Past Work P.A.Ramachandran M.P.Dudukovic Chemical Reaction Engineering Laboratory Chemical Engineering Department Washington University in St Louis MO 63130. Email: rama@wuche.wustl.edu 12/31/2018

Outline Review progress in the phenomenological models for Silane pyrolysis Backmixed Reactor Model Two phase fluid bed model Two phase with grid zone Effect of operating and design parameters 12/31/2018

Advantages of Fluid Bed reactors Traditionally Siemens reactor is used where deposition occurs over hot rods. This is both capital and operating cost intensive. Fluid beds have a number of advantages. Continuous operation Higher production rates Lower energy consumption. More Uniformity in Product However, Design of FBR is complex (Role of CREL). 12/31/2018

Reaction Chemistry SiH4(g)Si(s) + 2H2(s) Overall Reaction: SiH4Si + 2H2 Homogeneous Decomposition RHD=2x 1013 * exp(-26000/T) CVD growth (Heterogenoues reaction) RHT=2.79 x108 e(-19530/T) More complex Langmuir type of models have also been proposed*. SiH4(g)Si(s) + 2H2(s) 12/31/2018

Two Step Kinetics SiH2 is postulated as an intermediate. Formed SiH2 then adosrbs on the Surface. 12/31/2018

Reaction Pathways of Silane Pyrolysis in Fluid Bed Reactor Homogeneous nucleation Growth of nuclei to produce fines Capture of fines by seeds Heterogenous nucleation on existing particles. Si nuclei Si Clusturs Si Vap SiH4 Growing Seed Si Particles dPL>100mm dPF<2mm 12/31/2018

Schematic of pathways and Modeling Approaches Nucleation Agglomeration Coagulation Gas Adsorption of gas Surface Reaction Molecular Modeling and Plausible Chemical Reaction Mechanisms Aerosol Dynamic Modeling, Method of Moments, Particle Distribution Coalescence and Breakup Particle Size Distribution Individual Particle Growth Rate Diffusion Reactor Scale Modeling Grid Region Bubble Phase Emulsion Phase CFD Modeling 12/31/2018 Reactor Hydrodynamics

Backmixed Reactor Model Reactor models as a backmixed reactor. Provides a simple framework to identify the effect of various pathways. Lower bound on the fines formed since the whole reactor is full of seeds which capture the fines. Upper bound for silane conversion. (production rate) since there is no by-passing of the gas phase 12/31/2018

Backmixed Reactor: Model Equations Mass Balance on Silane: Mass Balance on Silicon Vapor Energy Balance Growth Rate of Large Seed Particles 12/31/2018 Lai S., M. P. Dudukovic and P. A. Ramachandran, Chemical Vapor deposition and Homogeneous Nucleation in Fluidized Bed Reactor, Chem. Eng. Sci. 41(4), 633, (1986).

Backmixed Reactor: Particle size distribution model Population Balance For Fines Mass Balance Over Fines 12/31/2018

Sample Results for Backmixed Reactor Model Melu=0.0477 kg/h Si nuclei Si Clusturs Si Vap SiH4 Growing Seed Si Particles VIrHD 0.488(kg/h) ATLrDL 0.364(kg/h) ATFrDL 0.035(kg/h) msca 0.111(kg/h) Elutriation vIrHN=3.97x10-5 kg/sec ATFrDF= 0.124kg/sec) Ff=1.4% ATLrHT=2.824 kg/h 12/31/2018

Two phase model Reactor is assumed to consist of an emulsion and a bubble (slug) phase. Heterogeneous reaction occurs in the emulsion phase. Fine formation occurs in both phases but to a large extent in the slug phase. Upper bound on the fines formation. Lower bound on reactor productivity. 12/31/2018

Main features in Fluidized Bed Reactors Modeling Approach Main features in Fluidized Bed Reactors Two Phase Model 12/31/2018

Importance of Grid Zone In beds of large particles, jets are observed near the distributor (the grid zone) Gas phase flows in plug flow here The grid zone exists up to a ‘’jet penetration height” which is calculated from empirical correlations 12/31/2018

Grid Region Effects: Equations Spout Bubble Phase Emulsion Phase 12/31/2018

Results for Fluid bed reactor model with Grid zone Effect of enhanced contacting Efficiency Near Distributor for Slow and Fast Reactions Comparison of Two Phase Model Results with Experimental Data 12/31/2018

Results for Fluid bed reactor model with Grid zone Effect of Silane Inlet Concentration on Performance of Reactor (Operating Conditions) 12/31/2018

Reduction of fines: current model predictions Exchange coefficient plays a significant role. Larger the exchange coefficients lower the fines. Improve grid to emulsion mass transfer Reduce holdup of the bubble phase Particle temperature should be larger than the bubble phase temperature. 12/31/2018

Summary Previous Work of CREL on Silane Pyrolysis is Reviewed Literature over last 20 years identified Needs to be reviewed carefully 12/31/2018

Some Recent Studies Mechanism Nucleation Growth Girshick S. L., M. T. Swihart, S.-M. Suh, M. R. Mahajan, and S. Nijhawan, Numerical Modeling of Gas-Phase Nucleation and Particle Growth during Chemical Vapor Deposition of Silicon, Journal of The Electrochemical Society, 147 (6) 2303-2311 (2000). Mirko Peglowa, Jitendra Kumar, Gerald Warnecke, Stefan Heinrich, Lothar Mörl, A new technique to determine rate constants for growth and agglomeration with size- and time-dependent nuclei formation, Chem. Engg. Sci. 61, 282 – 292 (2006). Silicon Deposition From Silane or Disilane in a Fluidized Bed-Part-I : Experimental Study, Caussat B., Hemati M. and J. P. Couderc, Chem. Eng. Sci. 50(22), 3615 (1995). Silicon Deposition From Silane or Disilane in a Fluidized Bed-Part-II : Theoriticle Analysis and Modeling, Caussat B., Hemati M. and J. P. Couderc, Chem. Eng. Sci. 50(22), 3622 (1995). Tajero-Ezpeleta MP, Buchholz S and Mleczko L, Optimization of Reaction Conditions in a Fluidized Bed for Silane Pyrolysis, Can. Jou. of Chem. Engg., 82(3), 520, 2004. Ruvalcaba JRR, Caussat B, Hemati, M, Influence of Principle Operating Parameters on Chemical Vapor Deposition of Silicon from Silane in a Fluidized bed to Limit Agglomeration Problems, Can. Jou. of Chem. Engg., 87(5), 955-963 (2000). Nan Xie1, Francine Battaglia and Rodney O Fox Combust. Simulations of multiphase reactive flows in fluidized beds using in situ adaptive tabulation, Combustion Theory Modelling, 8 195–209 (2004). O’Brien T. J., Madhava Syamlal, Chris Guenther, Computational Fluid dynamic Simulation of Chemically Reactive Fluidized Bed Processes, 3rd International Conference on CFD in the Minerals and Process Industries, CSIRO, Melbourne, Australia, 10-12 December 2003. Experiments and Modeling Process Design CFD Modeling Issues 12/31/2018