What Is a Fluidized Bed?

A fluidized bed, referred to as a fluidized bed, is a reactor that uses a gas or a liquid to pass through a granular solid layer to make solid particles in a suspended motion state, and performs a gas-solid reaction process or a liquid-solid reaction process. Including bulk fluidized bed, poly fluidized bed (bubble bed), ebullated bed, turbulent bed and gas transport bed.

A fluidized bed, referred to as a fluidized bed, is a reactor that uses a gas or a liquid to pass through a granular solid layer to make solid particles in a suspended motion state, and performs a gas-solid reaction process or a liquid-solid reaction process. When used in gas-solid systems, it is also called ebullating bed.

When the flow velocity reaches a certain limit, the particles in the bed begin to fluidize just when the bed can be supported by the fluid. The linear velocity of the fluid at this time is called the critical fluidization velocity _mf .

1. Bulk fluidized bed: For liquid-solid systems, there is not much difference between the density of liquid and particles, so _{mf is} generally small. When the flow rate is further increased, the bed expands uniformly and the fluctuation is small, and the particles are distributed uniformly in the bed. Called a bulk fluidized bed.

2. Polymerized fluidized bed (bubble bed): For gas-solid systems, bubbles usually appear after the gas velocity exceeds the critical fluidization velocity. The higher the gas velocity, the more violent the disturbance caused by the bubbles and the frequent fluctuation of the bed. This type of fluidized bed is called a poly fluidized bed or bubble bed.

3. Ebullating bed: The part below the bed surface in a fluidized bed is called a dense phase bed, and the part above the bed surface is called a dilute phase bed. The dense phase bed is shaped like water boiling, so it is called an ebullated bed.

4. Turbulent bed: As the gas velocity increases, the degree of turbulence in the fluidized bed also increases. Therefore, some people call the situation called turbulent bed.

5. Gas transport bed: When the gas velocity exceeds the particle carrying speed, the particles will be carried away by the air flow and become the gas transport bed.

There are two distinct phases in a polymerized fluidized bed: a gas phase (also called a dilute phase) that is mainly a gas; a milk phase (also called a granular phase or a dense phase) composed of particles and inter-particle gas, in which bubbling Both fluidized beds and turbulent fluidized beds are polyfluidized. The scattered fluidized bed refers to a fluidized bed in which particles are more uniformly distributed in the bed. Generally, it is more common in liquid-solid fluidized beds, and it also appears in high-density gas fluidized beds ^[1]

In recent years, people's understanding of fluidized beds has not been deepened, and the research on mathematical models is still under development. Here we will mainly introduce two models, two-phase models and bubbling bed models ^[3]

The companies that use this method to produce granular polysilicon include: Norwegian Renewable Energy Company (REC), German Wacker Company, HemLock, and MEMC.
REC Norway is the only company in the world whose business runs through the entire solar energy industry chain and is the world's largest producer of solar-grade polysilicon. The company uses silane gas as a raw material and uses a fluidized-bed reactor closed-loop process to decompose granular polycrystalline silicon without generating by-products and waste. This unique patented technology makes REC unique in the global solar industry. REC is also actively working on the development of a new fluidized bed reactor technology (FBR), which allows polysilicon to be deposited in a fluidized bed reactor, rather than in a traditional pyrolysis deposition furnace or a Siemens reactor. The earth reduces plant investment and production energy consumption. In the past few years, REC has conducted trial production of this technology. In 2006, it is planned to build a new solar polysilicon factory using this technology. It is expected to reach the production capacity in 2008 with a capacity of 6,500 tons. In addition, REC is actively developing Fluidized bed polysilicon deposition technology (expected for trial production in 2008) and modified Siemens-reactor technology.
The German company WACKER has developed a new production process for granulated polysilicon fluidization reactor technology. The process is based on fluidized bed technology (with trichlorosilane as the feedstock) and has undergone industrial-scale production trials in two experimental reactors. WACKER recently invested approximately 200 million euros to establish a new facility in Burghausen, Germany. The annual production capacity of the ultra-pure solar polysilicon plant is 2500t. In addition to other expansion measures, the commissioning of the new plant will enable WACKER to reach an annual production capacity of 9,000t in 2008 and eventually reach a production capacity of 11,500t in 2010.

Hemlock Corporation of the United States will open an experimental granular silicon production line to reduce the cost of silicon. Helmlock Corporation plans to increase its production capacity to 19 000 tons in 2010. MEMC plans to have a production capacity of about 7,000 tons by the end of 2010.