Fluidized Beds with Recirculating Solids

Fluidized beds with recirculating solids developed primarily as reactors for the catalytic cracking of oil to gasoline. Today their uses are much broader. Prior to the early 1940s, catalytic cracking of oil was performed in fixed-bed reactors. After that time, fluidized-bed reactors have been developed for this application. Today, fluidized beds with recirculating zeolite-based catalysts are used for catalytic cracking of oil. 

Schematic of fluidized-bed reactor with recirculating solids. [From Reactor Technology by B. L. Tarmy, Kirk-Othmer Encyclopedia of Chemical Technology, vol. 19, 3rd ed., Wiley (1982). Reprinted by permission of John Wiley and Sons, Inc., copyright 1982.] 

In the first step the hydrocarbon is oxidized by lattice oxygen from the oxide-containing catalyst to create surface oxygen vacancies in the oxide. The second step involves filling the vacancies by oxidization of the reduced oxide surface with 

Dupont has recently commercialized the reaction of n-butane to maleic anhydride using a vanadium phosphorus oxide (VPO) catalyst in a recirculating solid reactor system. The overall reaction is  

A proposed reaction pathway over the VPO catalyst is as shown below  

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The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keaims (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below. 

Many chemical processes recirculate solids. Catalytic systems recirculate catalyst in a reaction/regeneration cycle. First the catalyst is used to supply heat or a reactant to the process it is then transferred to a separate vessel to regenerate the catalyst, and then it is returned to the reactor. Circulating fluidized bed combustors recirculate fuel and ash around a loop to burn the fuel completely. A system with a cyclone collecting entrained solids above a fluidized bed and returning the solids to the bed via the cyclone dipleg is also a recirculating solid system. All of these recirculation systems employ standpipes. 

This concept was first called a recirculating fluidized bed by Yang and Kearins (1974). Several other names have also been used to describe the same concept the fluid-lift solids recirculator (Buchanan and Wilson, 1965), the spouted fluid bed with a draft tube (Y ang and Keaims, 1983 Hadzismajlovic et al., 1992), the internally circulating fluidized bed (Milne et al., 1992 Lee and Kim, 1992) or simply a circulating fluidized... 

Figure 23.2(b) shows a fast-fluidized-bed reactor, together with external equipment, such as cyclones, for separation of fluid and solid particles carried out of the reactor, and subsequent recirculation to the reactor. In a fast-fluidized bed, the fluidization velocity... 

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

A theoretical model (Beck and Bauer, 1989), based on ethanol inhibition alone as the limiting factor in gas-solid fluidized bed fermenters run with recirculating inert gas, suggested that the potential of this technique has not been fully explored. Hayes (1998) suggested significant improvements to the model and provided experimental confirmation of its validity. 

The two-phase (gas-solid) continuous stirred-tank reactors are represented by laboratory reactors as, for instance, one-pass differential reactors, reactors with forced recirculation, one-pellet reactors, etc. The industrial applications are the fluidized beds.2 Table V presents a list of experimental studies along with a very brief description of each system studied. 

Several reactors are presently used for studying gas-solid reactions. These reactors should, in principle, be useful for studying gas-liquid-solid catalytic reactions. The reactors are the ball-mill reactor (Fig. 5-10), a fluidized-bed reactor with an agitator (Fig. 5-11), a stirred reactor with catalyst impregnated on the reactor walls or placed in an annular basket (Fig. 5-12), a reactor with catalyst placed in a stationary cylindrical basket (Fig. 5-13), an internal recirculation reactor (Fig. 5-14), microreactors (Fig. 5-16), a single-pellet pulse reactor (Fig. 5-17), and a chromatographic-column pulse reactor (Fig. 5-18). The key features of these reactors are listed in Tables 5-3 through 5-9. The pertinent references for these reactors are listed at the end of the chapter. 

The expansion of the bed height is determined by bubbles behavior and bed temperature. The residence time of gases in the fluidized bed is merely estimated with the gas flow rate and the cross sectional area of the reactor bed. The char is recirculated in each compartment to adjust solids residence times. A small fraction of char, found from the eluiriation model [18], is allowed to escape to the above stage. 

Fig. 7.75 is the photograph of the turbulently moving surface of a fluidized bed above which the atomizing nozzle for binder liquid is located. Other designs use atomization nozzles that are submerged in the bed to achieve more intimate contact between the liquid and the solids. Solids, fresh feed and recycle, are typically fed into the bed below its surface to allow collisions with other particles, adhesion, and agglomeration before they may be entrained in the off-gas leaving the apparatus. The fines are then removed from the gas, collected, and once more recirculated to the back-mixed fluidized bed. 

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