Fluidized beds circulating solids

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... 

The book is arranged in two parts Part I deals with basic relationships and phenomena, including particle size and properties, collision mechanics of solids, momentum transfer and charge transfer, heat and mass transfer, basic equations, and intrinsic phenomena in gas-solid flows. Part II discusses the characteristics of selected gas-solid flow systems such as gas-solid separators, hopper and standpipe flows, dense-phase fluidized beds, circulating fluidized beds, pneumatic conveying systems, and heat and mass transfer in fluidization systems. 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

Bader, R Findlay, J. and Knowlton, T.M. (1988), Gas-solid flow patterns in a 30.5 cm diameter circulating fluidized bed, Circulating Fluidized Bed Technology , Vol. II, P. Basu and J.F. Large, eds, Pergamon Press, New York. 

A very complex solids flow pattern will result when solid obstacles exist in the fluidized bed. The solids recirculation pattern in a cylindrical bed with a single sphere was presented by Lin, Chen, and Chao (1985), and in a 2-D bed with a single and multiple cylinders by Ai (1991). It was demonstrated that large obstacles would not only affect the local solids velocity, but also the global solids circulation patterns. 

One of the more interesting methods for continuous countercurrent ion exchange is the use of fluidized bed techniques for continuous circulation of the resin. Figure 1.6 shows the Dorrco Hydro-softener. In the fluidized bed, a solid phase is suspended in a liquid or gas. Consequently, the solid behaves like a fluid and can be pumped, gravity fed, and handled very much like a liquid. The fluidized resin moves down through the softener on the right and is then picked up by a brine-carrier fluid and transferred to the regenerator on the left. 

Belin F, Flynn TJ. Circulating fluidized bed boiler solids system with in-fumace particle separator. Proceedings of the 1991 Fluidized Bed Combustion Conference, ASME, 1991, pp 287 294. 

Figure 4.10.9 Forms of gas-solid reactors with regard to agitation of solid (a) beginning of expansion of the bed (gas velocity = minimum fluidization velocity u ,f) (b) formation of solid-free gas bubbles (c) formation of gas slugs (d) turbulent impinging fluidized bed and (e) expanded circulating fluidized bed with solid recycle system. Adapted from ErtI, Knoezinger, and Weitkamp (1997).

The particular characteristics of three-phase fluidized bed reactors have been covered in several recent reviews by Ostergaard [1], Wild [2], Epstein [3], Baker [4] and Muroyama and Fan [5]. Epstein [3] distinguished in particular the difference between three-phase fluidized beds and slurry reactors. In slurry reactors the size of the solid particles is normally smaller than 0.1 mm while in three-phase fluidization the particle diameter is bigger than 0.2 mm. The volumetric solid fraction is another significant difference, being 10% or below for slurry reactors and between 20-40% in three-phase fluidized bed units. In three-phase fluidized beds the particles are supported by the liquid and/or the gas while in slurry reactors the solid particles are suspended by the momentum transferred from the gas bubbles to the liquid and from the liquid to the solids. In slurry reactors the solid particles are normally carried into and out of the unit by the liquid stream. In three-phase fluidized beds the solids are not transported out of the unit by the liquid stream, they are fed and withdrawn independently of the liquid stream [3]. Epstein [3] introduced an interesting classification for three-phase reactors and particularly proposed four modes of operation for three-phase fluidized beds Mode I Cocurrent upflow circulation of gas and liquid with the liquid as a continuous phase. Mode II cocurrent upflow circulation of gas and liquid with the gas as a continuous phase. Mode III Countercurrent circulation of gas (upflow) and liquid (downflow) with the liquid as the continuous phase. Mode IV Countercurrent circulation of gas (upflow) and liquid (downflow) with the gas as the continuous phase. 

Solsvik and Jakobsen [140] performed a set of one-dimensional two-fluid model simulations in order to elucidate whether such simple models can be suitable for further simulations of two interconnected fluidized bed reactor units with a dynamic solid flux transferred between these reactor units which collectively is denoting a circulating fluidized bed. Dynamic solid circulation between two fluidized bed units that operate at different conditions (e.g., temperatures and feed compositions) is an inherent requirement for the novel SE-SMR technology operated in fluidized bed reactors. A less computational demanding one-dimensional model to study the performance of interconnected reactor units will be an important contribution to the progress of the commercialization of circulating fluidized bed reactors intended for the SE-SMR technology. 

At high ratios of fluidiziug velocity to minimum fluidizing velocity, tremendous solids circulation from top to bottom of the bed assures rapid mixing of the solids. For aU practical purposes, beds with L/D ratios of from 4 to 0.1 can be considered to be completely mixed continuous-reaction vessels insofar as the sohds are concerned. 

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. 

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... 

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