Entrained solids fluidized reactor

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

The fluidized bed reactor is a vertical steel vessel to which TDF is fed through a side port. A fluidized bed of TDF is maintained with hot air. The abrasive action of the fluidized particles erode the char from the TDF, reducing the tire material to small pieces. As the TDF decomposes, ash and char are swept out of the reactor with the fluidizing air. The biggest disadvantages of a fluidized bed system are the need to remove entrained solids from the vapors, and the need to maintain the hot, fluidizing gas. 

The mass flow rate Gs of entrained solids per unit area leaving the fluidized-bed reactor is the sum of contributions from the entrainable particle size fractions (ut < u)... 

Entrainment Most fluidized bed reactors employ one or more cyclone, either inside the freeboard region at the top of the vessel or located externally, to capture entrained solids that are then returned continuously to the base of the fluidized bed via a standpipe and a mechanical (e.g., slide) valve or aerated nonmechanical valve (see Ref. ° for details of solid return systems). A flapper gate, acting as a check valve, is commonly employed to prevent backflow of gas up the standpipe. While cyclones are by far the most popular, other gas-solid separators like impingement separators, electrostatic precipitators, filters, and scrubbers are sometimes provided, especially as second- or third stage separators. 

In U.S. plants hydrofluorination is carried out in two stirred fluidized-bed reactors in series, with counterflow of solids and gases. The bed to which UO2 is fed and from which exhaust gases are discharged runs at 300°C, partially converts UO2 to UF4, and reduces the HF content of the effluent gases to around 15 percent. The bed to which anhydrous HF and the partially converted UO2 are fed runs at 500°C and converts more than 95 percent of the UO2 to UF4. To prevent caking of the fluidized beds, it has been found necessary to provide each reactor with a vertical-shaft, slow-speed stirrer to scrape the reactor walls. Production rates around 700 to 900 kg/h are obtained in 0.75-m-diameter reactors. Effluent gases are filtered to remove entrained solids, cooled to condense aqueous HF, and scrubbed to remove the last traces of HF. 

Stream, and the amount of solids in the reactor can only be maintained by recycling the entrained solids (circulating fluidized bed [Contractor 1999], e = 0.9-0.98). Beyond a certain fluid velocity, the system enters the realm of pneumatic conveying, in which the solid particles are carried out of the reactor without residence time, and the pressure drop increases noticeably due to acceleration of the particles. 

Multiphase reaction engineering has developed into a very active field of research, with international symposia held at frequent intervals. We have only considered a few common reactor types currently in use. A few others of potential importance in relatively large volume intermediates production are the gas-liquid-solid fluidized beds, liquid entrained reactors, and rotating drum reactors. 

Luss and Amundson (1968) have studied the dynamics of catalytic fluidized beds. The system is a good example of a stiff set of differential equations. Catalytic fluidized beds are utilized for a variety of reactions such as oxidation of naphthalene and ethylene and the production of alkyl chlorides. A batch fluidization reactor is usually built as a cylindrical shell with a support for the catalyst bed. The reactants enter from the bottom through a cone and cause the catalyst particles to be fluidized in the reactor. The reactants leave through a cyclone in which the entrained solids are separated and returned to the bed. 

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. 

There are a large number of processes where solid catalysts are employed. The process streams are either gaseous or liquid, or a combination of these. The catalysts usually have a high porosity. Especially in the petroleum and petrochemical industries one encounters catalytic reactors of a great variety. Well known reactor types are fixed bed, moving bed, fluidized bed, entrained bed, slurry reactor and three phase packed column. The choice between these reactor types is mostly based on the relative requirements for mass transfer, degree of conversion and heat transfer. Heterogeneous catalysis is a well developed science, with its own specific literature. In this chapter some technical acpects will be presented, particularly those related to scale-up. 

In FBMRs, the membranes are inserted inside the fluidized catalyst bed, serving as a product extractor or a reactant distributor. Figure 7.1 shows a typical FBMR structure for selective removal of a product (hydrogen) [4,5]. Pd-membrane tubes are placed vertically in the FBMR.The reactant gas is fed through the gas distribution plate at the bottom of the reactor to fluidize the fine particulate catalysts. Entrained solids are separated from the reaction product gas stream by internal cyclone separator and then returned to the reactor catalyst bed. 

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