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A fluidized bed is a physical phenomenon that occurs when a
solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural r ...
particulate substance (usually present in a holding vessel) is under the right conditions so that it behaves like a
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
. The usual way to achieve a fluidize bed is to pump pressurized fluid into the particles. The resulting medium then has many properties and characteristics of normal fluids, such as the ability to free-flow under gravity, or to be pumped using fluid technologies. The resulting phenomenon is called
fluidization Fluidization (or fluidisation) is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the ...
. Fluidized beds are used for several purposes, such as
fluidized bed reactor A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a solid granular material (usually a catalyst ...
s (types of
chemical reactor A chemical reactor is an enclosed volume in which a chemical reaction takes place. In chemical engineering, it is generally understood to be a process vessel used to carry out a chemical reaction, which is one of the classic unit operations in chem ...
s), solids separation,
fluid catalytic cracking Fluid Catalytic Cracking (FCC) is the conversion process used in petroleum refineries to convert the high-boiling point, high-molecular weight hydrocarbon fractions of petroleum (crude oils) into gasoline, olefinic gases, and other petroleum prod ...
,
fluidized bed combustion Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels. In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which ...
, heat or mass transfer or interface modification, such as applying a
coating A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powde ...
onto solid items. This technique is also becoming more common in
aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
for the production of shellfish in integrated multi-trophic aquaculture systems.


Properties

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 a heterogeneous 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 Archimedes' principle (also spelled Archimedes's principle) states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimedes ...
. As the "density", (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 fluidised beds, the contact of the solid particles with the fluidisation medium (a gas or a liquid) is greatly enhanced when compared to
packed bed In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structur ...
s. This behavior in fluidised 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 gases and solids. In a fluidised 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. Taking an example from the food processing industry: Fluidized beds are used to accelerate freezing in some individually quick frozen (IQF) tunnel freezers. These Fluidized bed tunnels are typically used on small food products like peas, shrimp or sliced vegetables, and may use
cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...
or
vapor-compression refrigeration Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings an ...
. The fluid used in Fluidized beds may also contain a fluid of catalytic type; that's why it is also used to catalyse the chemical reaction and also to improve the rate of reaction. Fluidized beds are also used for efficient bulk drying of materials. Fluidized bed technology in dryers increases efficiency by allowing for the entire surface of the drying material to be suspended and therefore exposed to the air. This process can also be combined with heating or cooling, if necessary, according to the specifications of the application.


History

In 1922, Fritz Winkler made the first industrial application of fluidization in a reactor for a
coal gasification Coal gasification is the process of producing syngas—a mixture consisting primarily of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4), and water vapour (H2O)—from coal and water, air and/or oxygen. Historically, coal ...
process. In 1942, the first circulating fluid bed was built for
catalytic cracking Fluid Catalytic Cracking (FCC) is the conversion process used in petroleum refineries to convert the high-boiling point, high-molecular weight hydrocarbon fractions of petroleum (crude oils) into gasoline, olefinic gases, and other petroleum pro ...
of
mineral oil Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils. The name 'mineral oil' by itself is imprecise, ...
s, with fluidisation technology applied to metallurgical processing (roasting
arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemen ...
) in the late 1940s. During this time theoretical and experimental research improved the design of the fluidised bed. In the 1960s VAW-Lippewerk in Lünen, Germany 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 Particulate fluidized bed is the classical approach where the gas at low velocities is used and fluidization of the solids is relatively stationary, with some fine particles being entrained. *In Bubbling fluidized (also called Aggregative fluidized bed), the velocities of the fluid is high, thus forming of two separate phases – continuous phase (dense or emulsion phase) and a discontinuous phase (lean or bubble phase). * Circulating fluidized beds (CFB), where gases are at a higher velocity sufficient to suspend 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. Entrained particles are recirculated via an external loop back into the reactor bed. Depending on the process, the particles may be classified by a cyclone separator and separated from or returned to the bed, based upon particle cut size. * Vibratory fluidized beds are similar to stationary beds, but add a mechanical vibration to further excite the particles for increased entrainment. * Transport or flash reactor (FR): At velocities higher than CFB, particles approach the velocity of the gas. Slip velocity between gas and solid is significantly reduced at the cost of less homogeneous heat distribution. * Annular fluidized bed (AFB): A large nozzle at the center of a bubble bed introduces gas as high velocity achieving the rapid mixing zone above the surrounding bed comparable to that found in the external loop of a CFB. * Mechanically Fluidized Reactor (MFR): A mechanical stirrer is used to mobilize particles and achieve properties similar to that a well-mixed fluidized bed. It does not require fluidization gas. *Narrow fluidized beds (NFB): For this case, the ratio between the tube and the grain diameters is equal or less than around 10. The dynamics of the bed is then different from the other types of fluidized beds because of strong confinement effects, and the presence of granular plugs, consisting of regions with high concentrations in solids alternating with low solids concentrations, is common.


Bed design


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 Superficial velocity (or superficial flow velocity), in engineering of multiphase flows and flows in porous media, is a hypothetical (artificial) flow velocity calculated as if the given phase or fluid were the only one flowing or present in a give ...
. In order to transition from a packed bed to a fluidised condition, the gas velocity is continually raised. For a free-standing bed there will exist a 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 fluidisation velocity", u_. Beyond the minimum fluidisation velocity ( u \geq u_ ), 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 Percolation (from Latin ''percolare'', "to filter" or "trickle through"), in physics, chemistry and materials science, refers to the movement and filtering of fluids through porous materials. It is described by Darcy's law. Broader applicatio ...
of the gas flow. Thus the pressure drop for u > u_ 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_ = H_ (1- \epsilon_) (\rho_ - \rho_) g = _ g / A \rho_ - \rho_) / \rho_ where: \Delta p_ is the bed pressure drop H_ is the bed height \epsilon_ is the bed voidage, i.e. the fraction of the bed volume that is occupied by the voids (the fluid spaces between the particles) \rho_ is the apparent density of bed particles \rho_ is the density of the fluidising fluid g is the acceleration due to gravity M_ is the total mass of solids in the bed A is the cross-sectional area of the bed


Geldart Groupings

In 1973, Professor D. Geldart 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 fluidised beds can be tailored based upon the particle's Geldart grouping: Group A For this group the particle size is between 20 and 100 µm, and the particle density is typically less than 1.4g/cm3. Prior to the initiation of a bubbling bed phase, beds from these particles will expand by a factor of 2 to 3 at incipient fluidisation, due to a decreased bulk density. Most powder-catalyzed beds utilize this group. Group B The particle size lies between 40 and 500 µm and the particle density between 1.4-4g/cm3. Bubbling typically forms directly at incipient fluidisation. Group C This group contains extremely fine and consequently the most cohesive particles. With a size of 20 to 30 µm, these particles fluidise 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 µm and typically have high particle densities. Fluidisation of this group requires very high fluid energies and is typically associated with high levels of abrasion. Drying grains and peas, roasting coffee beans, gasifying coals, and some roasting metal ores are such solids, and they are usually processed in shallow beds or in the spouting mode.


Distributor

Typically, pressurized gas or liquid enters the fluidised bed vessel through numerous holes via a plate known as a distributor plate, located at the bottom of the fluidised 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, pack onto the plate or trickle down through the plate. Many industrial beds use a sparger distributor instead of a distributor plate. The fluid is then distributed through a series of perforated tubes.


See also

*
Cyclonic separation Cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydrocyclone is used; while from gas, a gas cyclo ...
– A method of separating gases and particulate matter *
Fluidization Fluidization (or fluidisation) is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the ...
– Principles and theory of fluidization *
Fluidized bed combustion Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels. In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which ...
– Application of fluidized beds to combustion *
Fluidized bed reactor A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a solid granular material (usually a catalyst ...
– Application of fluidized beds to reactive chemical processes *
Fluidized bed concentrator A fluidized bed concentrator (FBC) is an industrial process for the treatment of exhaust air. The system uses a bed of activated carbon beads to adsorb volatile organic compounds (VOCs) from the exhaust gas. Evolving from the previous fixed-bed an ...
– Application of fluidized beds to remove VOCs/HAPs from industrial exhaust *
Unit operation In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomerizat ...
– Other Engineering unit operations *
Chemical looping combustion Chemical looping combustion (CLC) is a technological process typically employing a dual fluidized bed system. CLC operated with an interconnected moving bed with a fluidized bed system, has also been employed as a technology process. In CLC, a met ...
– Dual fluidized bed application


References


External links


Video: Liquid-Solid Fluidized Bed



US DOE NETL Fact Sheet
{{Authority control Chemical equipment Combustion engineering Fluidization