Fluidized bed reactor
Overview
 
A fluidized bed reactor is a type of reactor
Chemical reactor
In chemical engineering, chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction...

 device that can be used to carry out a variety of multiphase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

 chemical reactions. In this type of reactor, a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization
Fluidization
Fluidization is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state...

, imparts many important advantages to the FBR.
Encyclopedia
A fluidized bed reactor is a type of reactor
Chemical reactor
In chemical engineering, chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction...

 device that can be used to carry out a variety of multiphase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

 chemical reactions. In this type of reactor, a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization
Fluidization
Fluidization is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state...

, imparts many important advantages to the FBR. As a result, the fluidized bed reactor is now used in many industrial applications.

Basic principles

The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed reactor is typically supported by a porous plate, known as a distributor. The fluid is then forced through the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid passes through the voids in the material. This is known as a packed bed
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 structured packing...

 reactor. As the fluid velocity is increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the weight of the solid material. This stage is known as incipient fluidization and occurs at this minimum fluidization velocity. Once this minimum velocity is surpassed, the contents of the reactor bed begin to expand and swirl around much like an agitated tank or boiling pot of water. The reactor is now a fluidized bed. Depending on the operating conditions and properties of solid phase various flow regimes can be observed in this reactor.

History and current uses

Fluidized bed reactors are a relatively new tool in the chemical engineering field. The first fluidized bed gas generator was developed by Fritz Winkler in Germany in the 1920s. One of the first United States fluidized bed reactors used in the petroleum industry was the Catalytic Cracking Unit, created in Baton Rouge, LA in 1942 by the Standard Oil Company of New Jersey (now ExxonMobil
ExxonMobil
Exxon Mobil Corporation or ExxonMobil, is an American multinational oil and gas corporation. It is a direct descendant of John D. Rockefeller's Standard Oil company, and was formed on November 30, 1999, by the merger of Exxon and Mobil. Its headquarters are in Irving, Texas...

). This FBR and the many to follow were developed for the oil and petrochemical industries. Here catalysts were used to reduce petroleum to simpler compounds through a process known as cracking
Cracking (chemistry)
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...

. The invention of this technology made it possible to significantly increase the production of various fuels in the United States. In the late 1980's, the work of Gordana V. Novakovic, Robert S. Langer, V.A. Shiva Ayyadurai and others began the use of fluidized bed reactors in biological sciences for understanding and visualizing the fluid dynamics of blood deheparinization.

Today fluidized bed reactors are still used to produce gasoline and other fuels, along with many other chemicals. Many industrially produced polymers are made using FBR technology, such as rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

, vinyl chloride
Vinyl chloride
Vinyl chloride is the organochloride with the formula H2C:CHCl. It is also called vinyl chloride monomer, VCM or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride . At ambient pressure and temperature, vinyl chloride...

, polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

, and styrenes. Various utilities also use FBR’s for coal gasification
Coal gasification
Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide , hydrogen , carbon dioxide and water vapour –from coal...

, nuclear power plants, and water and waste treatment settings. Used in these applications, fluidized bed reactors allow for a cleaner, more efficient process than previous standard reactor technologies.

Advantages

The increase in fluidized bed reactor use in today’s industrial world is largely due to the inherent advantages of the technology.
  • Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material, fluidized beds do not experience poor mixing as in packed beds. This complete mixing allows for a uniform product that can often be hard to achieve in other reactor designs. The elimination of radial and axial concentration gradients also allows for better fluid-solid contact, which is essential for reaction efficiency and quality.
  • Uniform Temperature Gradients: Many chemical reactions require the addition or removal of heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a fluidized situation such as an FBR. In other reactor types, these local temperature differences, especially hotspots, can result in product degradation. Thus FBRs are well suited to exothermic
    Exothermic
    In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

     reactions. Researchers have also learned that the bed-to-surface heat transfer
    Heat transfer
    Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

     coefficients for FBRs are high.
  • Ability to Operate Reactor in Continuous State: The fluidized bed nature of these reactors allows for the ability to continuously withdraw product and introduce new reactants into the reaction vessel. Operating at a continuous process
    Continuous production
    Continuous production is a method used to manufacture, produce, or process materials without interruption. Continuous production is called a continuous process or a continuous flow process because the materials, either dry bulk or fluids that are being processed are continuously in motion,...

     state allows manufacturers to produce their various products more efficiently due to the removal of startup conditions in batch processes
    Batch reactor
    The Batch reactor is the generic term for a type of vessel widely used in the process industries. Its name is something of a misnomer since vessels of this type are used for a variety of process operations such as solids dissolution, product mixing, chemical reactions, batch distillation,...

    .

Disadvantages

As in any design, the fluidized bed reactor does have it draw-backs, which any reactor designer must take into consideration.
  • Increased Reactor Vessel Size: Because of the expansion of the bed materials in the reactor, a larger vessel is often required than that for a packed bed reactor. This larger vessel means that more must be spent on initial capital costs.
  • Pumping Requirements and Pressure Drop: The requirement for the fluid to suspend the solid material necessitates that a higher fluid velocity is attained in the reactor. In order to achieve this, more pumping power and thus higher energy costs are needed. In addition, the pressure drop
    Pressure drop
    Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

     associated with deep beds also requires additional pumping power.
  • Particle Entrainment: The high gas velocities present in this style of reactor often result in fine particles becoming entrained
    Entrainment (engineering)
    Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. For example:* The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke....

     in the fluid. These captured particles are then carried out of the reactor with the fluid, where they must be separated. This can be a very difficult and expensive problem to address depending on the design and function of the reactor. This may often continue to be a problem even with other entrainment reducing technologies.
  • Lack of Current Understanding: Current understanding of the actual behavior of the materials in a fluidized bed is rather limited. It is very difficult to predict and calculate the complex mass and heat flows within the bed. Due to this lack of understanding, a pilot plant
    Pilot plant
    A pilot plant is a small chemical processing system which is operated to generate information about the behavior of the system for use in design of larger facilities....

     for new processes is required. Even with pilot plants, the scale-up can be very difficult and may not reflect what was experienced in the pilot trial.
  • Erosion
    Erosion
    Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

     of Internal Components:
    The fluid-like behavior of the fine solid particles within the bed eventually results in the wear of the reactor vessel. This can require expensive maintenance and upkeep for the reaction vessel and pipes.
  • Pressure Loss Scenarios: If fluidization pressure is suddenly lost, the surface area of the bed may be suddenly reduced, this can either be an inconvenience (e.g. making bed restart difficult), or may have more serious implications, such as runaway reactions (e.g. for exothermic reactions in which heat transfer is suddenly restricted).

Current research and trends

Due to the advantages of fluidized bed reactors, a large amount of research is devoted to this technology. Most current research aims to quantify and explain the behavior of the phase interactions in the bed. Specific research topics include particle size distributions, various transfer coefficients, phase interactions, velocity and pressure effects, and computer modeling. The aim of this research is to produce more accurate models of the inner movements and phenomena of the bed. This will enable chemical engineers to design better, more efficient reactors that may effectively deal with the current disadvantages of the technology and expand the range of FBR use.

See also

  • Fluidized bed combustion
    Fluidized bed combustion
    Fluidized bed combustion is a combustion technology used in power plants. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective...

  • Chemical looping combustion
    Chemical looping combustion
    Chemical looping combustion typically employs a dual fluidized bed system where a metal oxide is employed as a bed material providing the oxygen for combustion in the fuel reactor...

  • Chemical reactor
    Chemical reactor
    In chemical engineering, chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction...

  • Chemical engineering
    Chemical engineering
    Chemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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