Circulating fluidized beds voidage

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. 

Louge, M., Lischer, J., and Chang, H., Measurements of Voidage Near the wall of a Circulating Fluidized bed Riser, Powder Tech., 62 269-276 (1990)... 

Silva, E. M. V., Ferreira, M. C., and Freire, J. T., Mean Voidage Measurements and Fluid Dynamics Analysis in a Circulating Fluidized Bed with a Spouted Bed Type Solids F eeding System, Proc. of2nd European Thermal-Sciences and 14th UIT National Heat Transfer Conference, Rome (1996)... 

Wang, N., Li, Y., Zheng, X., and Kwauk, M., Voidage Profiling for Fast Fluidization, First Intern. Conf. Circulating Fluidized Beds, Halifax, Canada (1985)... 

Reh, L. and Li, J., Measurement of voidage in fluidized beds by optical probe, in "Circulating Fluidized Beds Technology III" (P. Basu, M. Horio, M. Hasatani, Eds.), p. 105. Pergamon Press, Oxford, UK (1991). 

As opposed to the relatively uniform bed structure in dense-phase fluidization, the radial and axial distributions of voidage, particle velocity, and gas velocity in the circulating fluidized bed are very nonuniform (see Chapter 10) as a result the profile for the heat transfer coefficient in the circulating fluidized bed is nonuniform. 

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,... 

Li, J., Tung, Y., and Kwauk, M. Axial voidage profiles of fast fluidized beds, in Circulating Fluidized Bed Technology II (P. Basu and J. F. Large, eds.), p. 193. Pergamon Press, 1988. 

Nakajima, M., Harada, M., Asai, M., Yamazaki, R., and Jimbo, G. Bubble fraction and voidage in an emulsion phase in the transition to a turbulent fluidized bed, in Circulating Fluidized Bed Technology III (P. Basu, M. Horio and M. Hasatani, eds.), pp. 79-84. Pergamon Press, 1991. 

Wang ZL, Yao JZ, Liu SJ, Li HZ, Kwauk M. Studies on the voidage distribution in an internally circulating fluidized bed (in Chinese). In Proceedings 6th National Conf. on Fluidization, Wuhan, China, Oetober 1993, pp 150-154. 

Zhu JX, Salah M, Zhou YM. Radial and axial voidage distributions in circulating fluidized bed with ring-type internals. J Chem Eng Jpn 30(5) 928-937, 1997. 

Louge M, Lischer J, Chang H. Measurements of voidage near the wall of a circulating fluidized bed riser. Powder Tech 62.269-276, 1990. 

The voidage is the fraction of volume occupied by the gas in a gas-solid suspension. Voidage is a key variable in circulating fluidized bed risers. Several different voi-dages are in common usage ... 

Reh L, Li J. Measurement of voidage in fluidized beds by optical probes. In Basu P, Horio M, Hasatani M, eds. Circulating Fluidized Bed Technology III. Oxford Pergamon Press, 1991, pp 163-170. 

Zhou J, Graee JR, Qin S, Brereton CHM, Lim CJ, Zhu J. Voidage profiles in a circulating fluidized bed of square cross-seetion. Chem Eng Sci 49 3217-3226, 1994. 

A frequently encountered but underreported disruption of an otherwise relatively homogeneous liquid-fluidized bed occurs when nonuniformity of the liquid velocity profile above the distributor gives rise to bulk circulation of the solids. This effect can also manifest itself as a convective instability associated with a uniform distributor having an insufficient pressure drop compared to that of the fluidized bed (Medlin et al., 1974 Medlin and Jackson, 1975 Agarwal et al., 1980). Most commonly it occurs with upflow of solids at the center of the column and downflow near the walls (e.g., Latif and Richardson, 1972). The net effect, usually unnoticed and therefore underreported, is that the bed expansion or overall voidage is smaller than in the absence of the bulk circulation, i.e., smaller than would be predicted by, e.g., Eq. (32), for the same superficial velocity (Epstein et al., 1981). 

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