Circulating fluidized beds solids circulation

In the circulating fluidized bed, solids are removed from the top and recycled to the bottom, to be carried up again by the flowing fluid, thus forming a co-up system. Generalized fluidization predicts a decrease in voidage, as shown in the constant-n chart. The high solids concentration,... 

Senior RC, Brereton CHM. Modelling of circulating fluidized bed solids flow and distribution. Chem Eng Sci 47 281-296, 1992. 

The design of CFB reactors is substantially more complicated than that of turbulent fluidized beds because of the external circulation loop. Together with multiple fluidized beds, solids must flow through standpipes, slide valves (for control), cones. 

System E consists of an isothermal PFR with a bypass stream In a fluidized bed, solid catalyst is circulated within the bed however, the fluid moves through the bed essentially in plug flow. Maldistribution of fluid, due to the formation of bubbles and voids, and channeling, may cause some fluid to bypass the catalyst. This model is used in catalytic fluidized bed applications or in any other application where the solid does not react. A portion of the reactants forms an emulsion phase with... 

Fluidized-bed process incinerators have been used mostly in the petroleum and paper industries, and for processing nuclear wastes, spent cook liquor, wood chips, and sewage sludge disposal. Wastes in any physical state can be applied to a fluidized-bed process incinerator. Au.xiliary equipment includes a fuel burner system, an air supply system, and feed systems for liquid and solid wastes. The two basic bed design modes, bubbling bed and circulating bed, are distinguished by the e.xtent to which solids are entrained from the bed into the gas stream. 

A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

Fig. 4. Effect of solid circulation rate on CO2 removal in a fast fluidized-bed reactor...

The reactivities of pure NaHCOa solid. Sorb NHR, NHR5, and NX30 sorbents were examined in a fast fluidized bed reactor. The CO2 removal of the pure NaHCOa solid increased from 3 % to 25 % when the variables were altered. Removal increased as gas velocity was decreased, as the carbonation temperature was decreased, or as the solid circulation rate was increased. The CO2 removal of Sorb NHR and NHR5 was initially maintained at 100 % for a short period of time but quickly dropped to a 10 to 20 % removal. However, the Sorb NX30 sorbent showed fast kinetics in the fast fluidized reactor, capturing all of the 10 % of the CO2 in the flue gas within 3 seconds in the fast fluidized reactor. 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Horio s scaling law derivation was based on the requirement that two similar circulating fluidized beds have equal values of voidage distribution, dimensionless core radius, gas splitting to core and annulus, solid splitting to core and annulus, and cluster voidage. The CAFM equations were then examined to determine how these requirements could be met. 

The scaling law proposed by Horio for circulating fluidized beds can be shown to be equivalent to the simplified set of parameters. Horio also discussed reductions to his list of scaling parameters in which the solid/gas density ratio is omitted. He claimed that this reduced scaling law... 

Figure 54. Solid fraction profile comparison between pressurized circulating fluidized bed combustor and one-half size scale model based on simplified scaling law. (Glicksman et al., 1995.)...

Beaude, F., and Louge, M., Similarity of Radial Profiles of Solid Volume Fraction in a Circulating Fluidized Bed, Fluidization VIII, pp. 245-253 (1995)... 

Dou, S., Herb, B., Tuzla, K., and Chen, J. C., Dynamic Variation of Solid Concentration and Heat Transfer Coefficient at Wall of Circulating Fluidized Bed, Fluidization VII, 793-801 (1993)... 

Horio, M., Morshita, K., Tachibana, O., and Murata, M., Solid Distribution and Movement in Circulating Fluidized Beds, Circ. Fluid. Bed Tech. II, 147— 154 (1988)... 

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... 

---