Reverse Circulation Fluidized Bed Reactor for Polysilicon
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Solution Overview
Problem
Conventional fluidized bed reactors for polysilicon production face issues with silicon dust formation, silicon deposition on walls and zone dividers, and high hydrogen content in the product, leading to inefficient processes and additional dehydrogenation steps.
Innovation Solution
A reverse flow fluidized bed reactor design with a pre-reaction heating gas flow that lifts particles upward to encounter the reaction gas at the top of the reaction zone, minimizing dust formation and deposition, and incorporating a dehydrogenation zone for continuous hydrogen removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If reaction gas is heated by seed particles to high temperature for silicon deposition, then silicon deposition rate is improved, but silicon dust formation increases due to spontaneous decomposition of reaction gas molecules
Solution Approach 1:
The patent inverts the conventional particle flow direction by introducing a downward gas flow in the reaction zone that counteracts the upward particle circulation. This reverse flow approach allows particles to be heated effectively while preventing reaction gas from overheating and decomposing spontaneously, thereby reducing silicon dust formation while maintaining deposition rate
Solution Approach 2:
The patent creates different flow conditions in different zones: upward particle circulation in the heating zone for effective heating, and downward gas flow in the reaction zone for controlled deposition. This localized quality differentiation allows high temperature heating without causing spontaneous decomposition and dust formation in the reaction zone
2Temperature
If external heaters are used to heat pre-reaction heating zone walls for particle heating, then particle heating is improved, but silicon deposition on walls and zone dividers occurs
Solution Approach 1:
The patent extracts the heating function from the wall structure by using internal particle-to-particle heat transfer and gas-phase heating within the particle bed. The walls are no longer the primary heating surface, eliminating the temperature gradient that causes silicon deposition on walls while maintaining effective particle heating through the fluidized bed mechanism
Solution Approach 2:
The silicon particles themselves serve as the heating medium through their mutual thermal interaction and circulation. The hot particles transfer heat to cooler particles during circulation, creating self-sustaining heat distribution without requiring heated walls, thereby preventing wall deposition while maintaining particle temperature
3Productivity
If conventional FBR reactor design is used for continuous polysilicon production, then productivity is improved, but process complexity increases due to need for separate dehydrogenation step
Solution Approach 1:
The patent merges the dehydrogenation function with the existing pre-reaction heating zone by introducing a downward gas flow that creates a dehydrogenation environment within the heating zone. This integration eliminates the need for a separate dehydrogenation reactor or process step, reducing equipment complexity while maintaining continuous production capability
Solution Approach 2:
The pre-reaction heating zone is given multiple functions: it continues to heat particles for deposition while simultaneously serving as a dehydrogenation zone through the downward gas flow. This multi-functionality reduces the number of separate process steps and equipment needed, simplifying the overall system while maintaining continuous polysilicon production
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution reduces silicon dust and wall deposition, eliminates the need for a separate dehydrogenation step, and produces polysilicon with lower hydrogen content, enhancing process efficiency and product quality.
Implementation Method 1
A pre-reaction fluidizing gas that does not contain silicon, typically hydrogen gas, is introduced through a pre-reaction gas port 112 into the pre-reaction heating zone 104, where it fluidizes the silicon particles
Implementation Method 2
where they are heated by a heater 108 to a temperature above the silicon deposition temperature
Implementation Method 3
the reaction gas, which is introduced into the reaction zone 106 through a reaction gas port 114. Due to the high temperature of the silicon particles, the reaction gas is decomposed upon contact with the particles, and silicon is deposited onto the particles, causing them to grow
Implementation Method 4
They are then carried upward by the reaction gas, and those particles that do not receive sufficient silicon to fall out of the reactor through the exit port 118 are carried over the vertical separator 102
Implementation Method 5
at least one vertical separator 102 that creates within the reactor a pre-reaction heating zone 104, a reaction zone 106
Data Source
AI summary
A reverse fluidized bed reactor (FBR) is separated by a thermally insulating vertical divider into a pre-reaction heating zone, a reaction zone, and a dehydrogenation zone. The dehydrogenation zone can be distinct, or the heating zone can serve as the dehydrogenation zone. Particles of polysilicon circulate upward through the heating zone and into the top of the reaction zone, where deposition of silicon occurs, and the grown particles slowly settle until they reenter the bottom of the pre-reaction heating zone. Dust formation, wall deposition, and hydrogen content in the product silicon particles are thereby minimized.