Fluidized bed

A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding vessel) is forced to behave as a fluid; usually by the forced introduction of pressurised fluid, often a gas through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids; such as the ability to free-flow under gravity, or to be pumped using fluid type technologies. It reduces the density of the medium; without affecting its elemental nature. The resulting phenomenon is called fluidization. Fluidized beds are used for several purposes, such as fluidized bed reactors (types of chemical reactors), fluid catalytic cracking, fluidized bed combustion, fluidized bed biofilter (used for biological treatment of polluted waters) or applying a coating on solid items. Precautions must be taken against producing explosive vapours by combination with this process.

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Typically, pressurized gas or liquid enters the fluidized bed vessel through numerous holes in a distributor plate at the bottom af the fluidized bed. The fluid flows upward through the bed, causing the solid particles to be suspended. This process can be made to take place at elevated temperature.

A fluidized bed is a bed of solid particles with a stream of air or gas passing upward through the particles at a rate great enough to set them in motion. An expanded bed is formed when the gas or airflow rate increases and particles move apart. A few visibly vibrate and move about in restricted regions. At still higher velocities of airflow, all the particles become suspended. At this point, the frictional force between a particle and air balances the weight of the particles, the vertical component of the compressive force between adjacent particles disappears, and the pressure drop through any section of the bed approximates the weight of air and particles in that section. The bed is referred to as an incipiently fluidized bed or a bed at minimum fluidization. With an increase in airflow rates beyond minimum fluidization, large instabilities with bubbling and channeling of air create different types of beds.

A slugging bed is a fluid bed in which air bubbles occupy entire cross sections of the vessel and divide the bed into layers. A boiling bed is a fluid bed in which the air or gas bubbles are approximately the same size as the solid particles. A channeling bed is a fluid bed in which the air (or gas) forms channels in the bed through which most of the air passes. A spouting bed is a fluid bed in which the air forms a single opening through which some particles flow and fall to the outside. At higher airflow rates, agitation becomes more violent and the movement of solids becomes more vigorous. Additionally, the bed does not expand much beyond its volume at minimum fluidization. Such a bed is called an aggregative or bubbling fluidized bed.