Abstract
The Richardson and Zaki (1954, Sedimentation and fluidization. Trans. Inst. Chem. Eng. 32, pp. 35–53.) equation has been used extensively to investigate the expansion profiles of homogeneous gas-fluidized beds. The experimental value of the parameter n appearing in the equation indicates how significantly interparticle forces affect the expansion of these beds, revealing the relative importance of these forces with respect to the fluid dynamic ones. In this work, we modeled the stable expansion of gas-fluidized beds of different diameter, accounting for enduring contacts among particles and wall effects. We solved the model numerically to obtain the bed expansion profiles, back-calculating from them the values of the parameter n. For all the cases considered, we observed that the values of n are higher than those obtained by purely fluid dynamic correlations, such as those advanced by Richardson and Zaki, and Rowe (1987, A convenient empirical equation for estimation of the Richardson and Zaki exponent. Chem. Eng. Sci. 42, pp. 2795.). This effect was more pronounced in beds of smaller diameter. To validate our model, we carried out fluidization and defluidization experiments, analyzing the results by means of the Richardson and Zaki equation. We obtained a reasonable agreement between numerical and experimental findings; this suggests that enduring contacts among particles, which are manifestations of cohesiveness, affect homogeneous bed expansion. This effect is amplified by wall friction.
Highlights
We study the role of wall friction in uniform gas-fluidized beds
Foscolo and Gibilaro (1984) derived a stability criterion based on fluid dynamic arguments, resorting to the stability theory of Wallis (1969), to show that stability depends on the relative magnitude of the kinematic and dynamic waves that propagate in the bed (Mazzei et al, 2006; Mazzei, 2008)
Mutsers and Reitema (1977) further argued that, if the stability of gas-fluidized beds is due to fluid dynamic forces, the voidage at minimum bubbling should depend on the ratio g=μ2g ; if the stability is due to a network of interparticle contacts, it should depend on g=μg
Summary
We study the role of wall friction in uniform gas-fluidized beds. We conduct fluidization/defluidization experiments to validate our theoretical results. He proposed a predictive criterion for stability based on fluid dynamic considerations He did not support the idea that stability is a result of the dependence of the drag force on the void fraction gradient, as Foscolo and Gibilaro (1984) maintained; he showed instead that stability can arise from random fluctuations in the particle velocity. Reitema (1973) and Mutsers and Reitema (1977), adopted the stability criterion of Wallis (1969) to show that the stable behavior observed in Group A powders may be attributed to the cohesive forces among particles Their experiments show that when a uniform gas-fluidized bed is tilted over a horizontal axis, it remains stable until a critical tilting angle is reached, and at this angle the bed surface suddenly shears off. They concluded that the stability of gas-fluidized particles is due to the presence of particle–particle contact forces
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