Slugging fluidized bed

DiFelice, R., Rapagna, S., and Foscolo, P. U., Dynamic Similarity Rules Validity Check for Bubbling and Slugging Fluidized Beds, Powder Technol., 71 281 (1992a)... 

Fluidized Bed Tests. These tests have direct relevance to all applications where particles are subjected to conditions of fluidization. Some authors believe that these tests can also to some extent simulate the stress of pneumatic transport. Coppingeretal. (1992) found at least a good correlation with the attrition resistance in dense-phase pneumatic conveying when they tested various powders in a slugging fluidized bed. 

Baeyens, J. and Geldart, D., An investigation into slugging fluidized beds, Chem. Eng. Set., 29 (1974) 255-265. 

A regime map of Fo versus the solid volume fraction, ap, for various gas-solid flows was presented by Hunt (1989), as shown in Fig. 4.3. Hunt (1989) suggested that except when Fo > 1 and ap > 0.1, use of the pseudocontinuum model is inappropriate. Thus, from Fig. 4.3, it can be seen that the pseudocontinuum model is applicable to packed beds, incipient fluidized beds, and granular flows, whereas it is not applicable to pneumatic transport flows, dilute suspensions, bubbling beds, and slugging fluidized beds [Glicksman and Decker, 1982 Hunt, 1989]. 

Figure ] 3.3-3 Pressure drop diagram for fluctuating and slugging fluidized beds, from Kunii and Levenspiel [3]). 

Figure 4.6 Schematic of two-phase model representation of bubbling or slugging fluidized-bed reactor.

Hovmand S. Davidson JF. Chemical conversion in a slugging fluidized bed. Transactions. Institution of Chemical Engineers 1968 46 190-203. 

Kehoe PWK, Davidson JF. The fluctuation of surface height in freely slugging fluidized beds. AIChE Symp Ser 69(128) 41-48, 1973a. 

Andersson S, Johnsson F, Leckner B. Fluidization regimes in non-slugging fluidized beds. Proceedings of the 10th International Conference on Fluidized Bed Combustion. San Francisco, 1989, pp 239-247. 

Di Felice et al. (1992a) investigated the validity of the full set of scaling laws for bubbling and slugging fluidized beds. They used an experimental facility that permitted the pressurization of different diameter test sections to match the scaling parameters. Minimum fluidization measurements, video measurements of bed expansion, and pressure fluctuation data were used to compare the similarity of five different bed eonfigurations. Three of the beds were scaled properly. 

Farrell (1996) experimentally evaluated the importance of the solid-to-gas density ratio (Ps/pf) for scaling the hydrodynamics of bubbling and slugging fluidized beds. Two bed materials, polyethylene plastic (pg = 918kg/m ) and a dolomite/limestone sorbent mixture (ps = 2670 kg/m ), were used to create a mismatch in the density ratio. The size of the particles was chosen so that the remaining simplified scaling parameters were matched. The internal angle of friction was similar between the two materials. 

DiFelice R, Rapagna S, Foscolo PU. Dynamic similarity rules validity check for bubbling and slugging fluidized beds. Powder Technol 71 281, 1992a. 

Figure 35 Pressure fluctuation signal from a slugging fluidized bed and their cross-correlation function. (Roy et al., 1990.) At = 0.12 X 0.12m = 1.6 m silica sand bed, dp = 240(a) Original signal from pressure probes (upper row 1.5 m...

The best known model for slugging fluidized beds is that of Hovmand and Davidson (45,46). This is a variant on the Orcutt model (17,18) (see also Table 2) which assumes plug flow of gas in the dense phase. The interface mass transfer is again composed of two parts, a throughflow component and a diffusional component, with the dimensionless mass transfer coefficient given approximately by... 

An alternative model for slugging fluidized bed reactors was proposed by Raghuraman and Potter (48). This Is an extension of their three-phase (bubbles, cloud-wake phase, emulsion phase) countercurrent backmlxlng model (21) which accounts for gas downflow In the dense-phase, said to occur for slugging beds when U > 2.5 U f Agreement with experimental results Is said (48,49) to be somewhat better for this model than for the Hovmand-... 

Hovmand, S. and J.F. Davidson, Chemical Conversion in a Slugging Fluidized Bed, Trans. Instn. Chem. Engrs. 46 (1968) 190-203. 

Raghuraman, J. and O.E. Potter. Countercurrent Backmixing Model for Slugging Fluidized-Bed Reactors. A.I.Ch.E. Journal 24 (1978) 698-704. 

This extension of the model has the aim of facilitating a detailed dynamic investigation of the slugging fluidized bed reactor whether of the catalytic type or of the type exhibiting reaction between gas and solid as in mineral roasting and fluidized bed combustion. The extended model reported here seeks to describe the cyclic variations which occur due to slugging in a steady-state catalytic system. 

Although the studies completed and proposed have direct application only to slugging fluidized bed reactors there will be insights given into the behaviour of freely bubbling beds. 

Figure 1. Slugging fluidized bed. (A) well-mixed wake region (B) piston-flow region.

A model has been formulated which follows the cyclical behaviour of the slugging fluidized bed and adequately incorporates mixing of the solids. The results for the steady-state agree with a previously formulated time-averaged model. The new mechanistic model provides a point of departure for stability studies and for the analysis of fluidized bed combustion and roasting of ores. 

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