Fluidized bed

For other articles, see Fluidized bed (disambiguation).

Oldest power station utilizing circular fluidized bed technology, in Lünen, Germany

A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding vessel) is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid 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.

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, heat or mass transfer or interface modification, such as applying a coating onto solid items.

Properties of fluidized beds

A fluidized bed consists of fluid-solid mixture that exhibits fluid-like properties. As such, the upper surface of the bed is relatively horizontal, which is analogous to hydrostatic behavior. The bed can be considered to be an inhomogeneous mixture of fluid and solid that can be represented by a single bulk density.

Furthermore, an object with a higher density than the bed will sink, whereas an object with a lower density than the bed will float, thus the bed can be considered to exhibit the fluid behavior expected of Archimedes' principle. As the "density" of the (actually the solid volume fraction of the suspension) of the bed can be altered by changing the fluid fraction, objects with different densities comparative to the bed can, by altering either the fluid or solid fraction, be caused to sink or float.

In fluidized beds, the contact of the solid particles with the fluidization medium (a gas or a liquid) is greatly enhanced when compared to packed beds. This behavior in fluidized combustion beds enables good thermal transport inside the system and good heat transfer between the bed and its container Similarly to the good heat transfer, which enables thermal uniformity analogous to that of a well mixed gas, the bed can have a significant heat-capacity whilst maintaining a homogeneous temperature field.

Application

Fluidized beds are used as a technical process which has the ability to promote high levels of contact between gasses and solids. In a fluidized bed a characteristic set of basic properties can be utilised, indispensable to modern process and chemical engineering, these properties include:

• Extremely high surface area contact between fluid and solid per unit bed volume
• High relative velocities between the fluid and the dispersed solid phase.
• High levels of intermixing of the particulate phase.
• Frequent particle-particle and particle-wall collisions.

History

In 1922 von Winkler designed a reactor that for the first time utilized a coal gasification process. Further application of the fluidized bed included the catalytic cracking of mineral oils in the 1940s. During this time theoretical and experimental research improved the design of the fluidized bed. In the 1960s VAW-Lippewerk in Lönen implemented the first industrial bed for the combustion of coal and later for the calcination of aluminium hydroxide.

Fluidized bed types

Bed types can be coarsely classified by their flow behavior, including:

• Stationary or bubbling beds, where the fluidization of the solids is relatively stationary, with some fine particles being entrained.
• Circulating beds, where the fluidization suspends the particle bed, due to a larger kinetic energy of the fluid. As such the surface of the bed is less smooth and larger particles can be entrained from the bed than for stationary beds. These particles can be classified by a cyclone separator and separated from or returned to the bed, based upon particle cut size.

Flow behavior

Several flow regimes are generally used to describe bed flow, these include:

• slugging bed: A bed in which air bubbles occupy entire cross sections of the vessel and divide the bed into layers.
• boiling bed: A fluid bed in which the air or gas bubbles are approximately the same size as the solid particles.
• channeling bed: A bed in which the air (or gas) forms channels in the bed through which most of the air passes.
• spouting bed: 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.

Bed design

A diagram of a fluidized bed

Basic model

When the packed bed has a fluid passed over it, the pressure drop of the fluid is approximately proportional to the fluid's superficial velocity. In order to transition from a packed bed to a fluidized condition,the gas velocity is continually raised. For a free-standing bed there will exists point, known as the minimum or incipient fluidisation point, whereby the bed's mass is suspended directly by the flow of the fluid stream. The corresponding fluid velocity, known as the "minimum fluidization velocity", $u_{mf}$.

Beyond the minimum fluidization velocity ($u\geq u_{mf}$), the bed material will be suspended by the gas-stream and further increases in the velocity will have a reduced effect on the pressure, owing to sufficient percolation of the gas flow. Thus the pressure drop from for $u>u_{mf}$ is relatively constant.

At the base of the vessel the apparent pressure drop multiplied by the cross-section area of the bed can be equated to the force of the weight of the solid particles (less the buoyancy of the solid in the fluid).

$\Delta p_{w}=H_{w}(1-\epsilon _{w})(\rho _{s}-\rho _{f})g$

Geldart Groupings

Dr. Geldart Schüttgüter proposed the grouping of powders in to four so-called "Geldart Groups". The groups are defined by their locations on a diagram of solid-fluid density difference and particle size. Design methods for fluidized beds can be tailored based upon the particle's Geldart grouping. Group A For this group the particle size is between 20 and 100 um, and the particle density is typically 1400kg/m³. Prior to the initiation of a bubbling bed phase, beds from these particles will expand by a factor of 2 to 3 at incipient fluidization, due to a decreased bulk density. Most powder-catalyzed beds utilize this group.

Group B The particle size lies between 40 and 500 um and the particle density between 1400 and 4500 kg/m³. Bubbling typically forms directly at incipient fluidization.

Group C This group contains extremely fine and subsequently the most cohesive particles. With a size of 20 to 30 um, these particles fluidize under very difficult to achieve conditions, and may require the application of an external force, such as mechanical agitation.

Group D The particles in this region are above 600 um and typically have high particle densities. Fluidization of this group requires very high fluid energies and is typically associated with high levels of abrasion.

Distributor

Typically, pressurized gas or liquid enters the fluidized bed vessel through numerous holes via a plate known as a distributor plate, located at the bottom of the fluidized bed. The fluid flows upward through the bed, causing the solid particles to be suspended. If the inlet fluid is disabled the bed may settle or pack onto the plate.

External links

• GBM&Co. Spray Drying concept with Fluidized Bed - Interactive animation

• Video: Liquid-Solid Fluidized Bed