Fluidized bed reactors (FBR) are catalytic reactors in which the catalyst is fluidized within the reactor.
Fluidized bed reactors are heterogeneous catalytic reactors in which the mass of catalyst is fluidized. This allows for extensive mixing in all directions. A result of the mixing is excellent temperature stability and increased mass-transfer and reaction rates.
Fluidized bed reactors are capable of handling large amounts of feed and catalyst. Pictured below is an FBR used for treating aniline-and nitrobenzene-contaminated wastewater.
The movie below shows the operation of a fluidized bed reactor. Before the reactor is started the catalyst pellets lie on a grate at the bottom of the reactor. Reactants are pumped into the reactor through a distributor continuously, causing the bed to become fluidized. The bed's behavior after initial fluidization depends on the state of the reactant. If it is a liquid the bed expands uniformly with increased upward flow of the reactant. This is called homogenous fluidization. If the reactant is a gas the bed will be non-uniform because the gas forms bubbles in the bed, resulting in aggregative fluidization. Sometimes these bubbles in coarse materials can grow larger than two-thirds of the bed's diameter, which can cause slugging. Slugging can result in variable pressures, vibrations in the bed, and heat transfer reductions. Increasing the velocity of the gas leads to a turbulent regime, as shown below. In the fast fluidization regime the bed surface starts to disappear. Increasing the gas velocity further results in pneumatic transport, in which the bed is completely removed and the particles are uniformly spaced in the fluid. During this process the reactants react due to the presence of the catalyst pellets, forming products that are removed continuously.
Fluidized bed reactors are generally very large. They must be designed so that the fluid flowrate is sufficient to suspend the catalyst particles. The particles typically range in size from 10 - 300 microns.
When designing a fluidized bed reactor, the catalyst life must also be taken into account. Most fluidized bed reactors, such as the one shown here have a separate compartment to regenerate the catalyst.
Fluidized bed reactors are commonly used in catalytic cracking processes. They are also used in the oxidation of naphthalene to phtalic anhydride, roasting of sulfide ores, coking of petroleum residues, and the calcination of limestone. They are often used when there is a need for large amounts of heat input or output, or when closely controlled temperatures are required.
The fluidized bed reactors below are used in NASA's Jet Propulsion Laboratory for the removal of perchlorate and chlorinated solvent from groundwater. The system can remove perchlorate from up to 350 gallons of groundwater per minute.
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