Fluidized Bed Reactor
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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 In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
(gas or liquid) is passed through a solid
granular material A granular material is a conglomeration of discrete solid, macroscopic scale, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when granulation, grains collide). T ...
(usually a
catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
) at high enough speeds to suspend the solid and cause it to behave as though it were a fluid. This process, known as
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 ...
, imparts many important advantages to an FBR. As a result, FBRs are used for many industrial applications.


Basic principles

The solid substrate material (the catalytic material upon which chemical species react) in the fluidized bed reactor is typically supported by a porous plate, known as a distributor.Howard, J. R. (1989). ''Fluidized Bed Technology: Principles and Applications.'' New York, NY: Adam Higler. 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 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.Tavoulareas, S. (1991.) Fluidized-Bed Combustion Technology. **Annual Reviews Inc.** 16, 25-27. 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 Standard Oil Company was a corporate trust in the petroleum industry that existed from 1882 to 1911. The origins of the trust lay in the operations of the Standard Oil Company (Ohio), which had been founded in 1870 by John D. Rockefeller. The ...
of New Jersey (now
ExxonMobil Exxon Mobil Corporation ( ) is an American multinational List of oil exploration and production companies, oil and gas corporation headquartered in Spring, Texas, a suburb of Houston. Founded as the Successors of Standard Oil, largest direct s ...
). This FBR and the many to follow were developed for the oil and petrochemical industries. Here
catalysts Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
were used to reduce petroleum to simpler compounds through a process known as cracking. The invention of this technology made it possible to significantly increase the production of various fuels in the United States. Today, fluidized bed reactors are still used to produce gasoline and other fuels, along with many other chemicals. Many industrially produced
polymers A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...
are made using FBR technology, such as
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Types of polyisoprene ...
,
vinyl chloride Vinyl chloride is an organochloride with the formula H2C =CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. It is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC). Vinyl chloride is a ...
,
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...
, styrenes, and
polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer Propene, propylene. Polypropylene belongs to the group of polyolefin ...
. Various utilities also use FBRs for
coal gasification In industrial chemistry, coal gasification is the process of producing syngas—a mixture consisting primarily of carbon monoxide (CO), hydrogen (), carbon dioxide (), methane (), and water vapour ()—from coal and water, air and/or oxygen. H ...
, 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.Trambouze, P., & Euzen, J. (2004). ''Chemical Reactors: From Design to Operation.'' (R. Bononno, Trans.). Paris: Editions Technip. *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, improving 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 fluidized beds. In other reactor types, these local temperature differences, especially hotspots, can result in product degradation. Thus FBRs are well suited to
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
reactions. Researchers have also learned that the bed-to-surface
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
coefficients for FBRs are high. *
Continuous production Continuous production is a flow production 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 bu ...
: The fluidized bed nature of these reactors allows them to continuously withdraw product and introduce new reactants into the reaction vessel, eliminating the startup conditions in batch processes, and thus improving efficiency.


Disadvantages

As in any design, the fluidized bed reactor does have its 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 (often abbreviated as "dP" or "ΔP") is defined as the difference in total pressure between two points of a fluid carrying network. A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as i ...
associated with deep beds also requires additional pumping power. * Particle entrainment: The high fluid velocities present in this style of reactor often result in fine particles becoming entrained 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 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 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.Arastoopour, H. (Ed.). (1998). ''Fluidization and Fluid Particle Systems: Recent Research and Development.'' New York, NY: American Institute of Chemical Engineers. The aim of this research is to produce more accurate models of the inner movements and phenomena of the bed.{{Cite journal, last1=Abbasi, first1=Mohammad Reza, last2=Shamiri, first2=Ahmad, last3=Hussain, first3=M.A., title=Dynamic modeling and Molecular Weight Distribution of ethylene copolymerization in an industrial gas-phase Fluidized-Bed Reactor, journal=Advanced Powder Technology, volume=27, issue=4, pages=1526–1538, doi=10.1016/j.apt.2016.05.014, year=2016, url=https://discovery.ucl.ac.uk/id/eprint/10114278/ 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

*
Chemical engineering Chemical engineering is an engineering field which deals with the study of the operation and design of chemical plants as well as methods of improving production. Chemical engineers develop economical commercial processes to convert raw materials ...
* Chemical looping combustion *
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 che ...
* Fluidized bed combustion * Siemens process


References

Chemical reactors Industrial processes Fluidization