(3ec) Solar Grade Silicon Production in a Fluidized Bed Reactor

Solar
Grade Silicon Production in a Fluidized Bed Reactor

 Juan Du

Carnegie
Mellon University, Pittsburgh, Pennsylvania, USA

Poly-silicon, the feedstock for semiconductor and photovoltaic
industries, has witnessed an explosive demand due to the expansion of the photovoltaic
(PV) industry and limited recovery of rejected silicon from the semiconductor
industry. Siemens process is the classic method to produce electronic grade
silicon, the price of which for semiconductor industry is not as critical as in
the solar industry. Fluidized bed reactors have excellent heat and mass
transfer characteristics and can be utilized for solar grade silicon production
to overcome the energy waste problem in Siemens process. The energy consumption
is reduced because the decomposition operates at a lower temperature and
cooling devices are not required. Moreover fluidized beds have higher
throughput rate and operate continuously to reduce further capital and operating
cost. FBR process has been commercialized by several companies and promises to
deliver solar grade silicon at reasonable and stable price.

We present a multi-scale modeling approach for solar
grade silicon production in fluidized bed reactor by thermal decomposition of Silane. Silane decomposes to form
hydrogen and silicon. The seed particles grow due to the heterogeneous chemical
vapor deposition and scavenge of silicon powder produced in homogeneous gas
phase reactor. The dynamics of gas and particle phases are modeled in
computational fluid dynamics (CFD) model and reaction model. The CFD module
shows the hydrodynamics. The volume fraction of solid phase is imported into
the reaction model. The reaction model describes the emulsion phase and bubble
phase of FBR. The mass exchange and heat transfer between two phases are
developed by establishing mass and heat balance for each phase. The output of
the reaction module, i.e. total deposition rate, is passed to population
balance which describes particle growth process. A novel discretization scheme
is presented to obtain discretized version of the population balance. The
particle size distribution calculated by discretized population balance is used
as input of CFD module. The complex interplay between gas phase and particle
phase is captured by integrating those modules. The multi-scale model is
validated by experimental data provided by pilot plants in industry.Based
on the multi-scale model, an inventory control system is designed to control
particle size distribution and hence to regulate the average size of the
product by manipulating the product withdrawal rate and seed addition rate.