Fluidized bed

When small-particle powders are placed in a container and gas or liquid is passed through a flow straightener such as a perforated plate at the bottom, the particles remain stationary (a so-called fixed layer) while the flow rate is low, but when the flow rate exceeds a certain level, the flow resistance applied to the particles becomes equal to gravity, and the powders and granules become easily fluid, just like boiling liquid. This phenomenon is called fluidization, and the layer in this state is called a fluidized layer, also called a fluidized bed. In a fluidized bed, the particles in the container are almost uniformly mixed, there is good contact between the particles and the fluid, the temperature can be easily controlled, and a large amount of powder and granules can be continuously processed with simple equipment, and some of it can be withdrawn or fed. However, there are disadvantages to this method, such as the particles being carried away by the fluid and being worn away.

The Winkler Gasifier (1926), which was used to produce synthetic gas from powdered coal, marked the beginning of industrial applications of fluidized beds, and they were then used in fluid catalytic cracking (FCC process, 1942), which cracks diesel to produce gasoline. Since then, fluidized beds have been widely used in a variety of gas-solid reactions, as well as physical operations such as drying, adsorption, and heat exchange, continuous powder supply, and particle separation equipment. Multi-stage fluidized beds, in which fluidized beds are stacked, have also been developed to compensate for the short contact time between fluids and particles.

[Otake Denyu]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

粒径の小さい粉粒体を容器に入れ、その底部の多孔板のような整流器を経てガスあるいは液体を流すと、流速が小さい間は粒子が静止したいわゆる固定層のままであるが、ある流速以上になると、粒子に加わる流動抵抗と重力とが等しくなって、粉粒体はちょうど沸騰した液体のように容易に流動できる状態になる。この現象が流動化fluidizationであって、この状態の層が流動層であり、流動床(しょう)ともいう。流動層では、容器内の粒子はほぼ均一に混合され、粒子と流体との接触がよく、温度調節が容易で、簡単な装置で多量の粉粒体を連続的に処理し、その一部を抜き出したり、供給したりすることができる。しかし、粒子が流体に伴って持ち去られたり、摩耗したりする欠点がある。

　粉炭からの合成ガス製造に利用されたウィンクラーガス発生炉（1926）が、流動層の工業的応用の始まりであり、ついで軽油を分解してガソリンを製造する流動接触分解（FCC法、1942）に利用された。それ以降、各種の気‐固系反応のほかに、乾燥、吸着、熱交換などの物理操作や粉体の連続供給、粒子の分離装置などに広く利用されている。流体と粒子との短い接触時間を補うために流動層を積み重ねた多段流動層も開発されている。

［大竹伝雄］

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