The Capacitance Effect on the Transfer of Gas or Heat Between a Bubble and the Continuous Phase in a Gas-Solid Fluidized Bed

Toei, Ryozo; Matsuno, Ryuichi; Hotta, Hiroki; Oichi, Morihiro; Fujine, Yukio

doi:10.1252/jcej.5.273

Cited by 17 publications

(2 citation statements)

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“…Thus, with the integration of our high-temperature data with other results-at room temperature-in the literature (Hartman et al, 2001;Toei et al, 1972), Figure 6, the following empirical correlation between φ and U mf is obtained: mf is obtained: (15) to calculate φ in a predictive mode leads to heat transfer coeffi cients within ±50%, see Figure 7 (a). These data were also used to evaluate the models proposed by Toei et al (1972) and Kunii and Levenspiel (1991); comparisons are shown in Figures 7 (b) and (c). For these calculations, the particle volume fraction in the bubble, γ P.B , was chosen as 0.001 (at the lower end of the suggested range 0.001 < γ P.B < 0.01).…”

Section: Resultssupporting

confidence: 59%

“…For these calculations, the particle volume fraction in the bubble, γ P.B , was chosen as 0.001 (at the lower end of the suggested range 0.001 < γ P.B < 0.01). Calculations showed that the model by Toei et al (1972) underestimates the heat transfer coeffi cient at the room temperature, but overestimates the heat transfer coeffi cient at the high temperature. Kunii and Levenspiel's (1991) model appears to be satisfactory at room temperature; however, it overestimates the heat transfer coeffi cient at higher temperatures.…”

Section: Resultsmentioning

confidence: 99%

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Heat Transfer to an Isolated Bubble Rising in a High‐Temperature Incipiently Fluidized Bed

Agarwal

2004

Can J Chem Eng

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Section: Resultssupporting

confidence: 59%

Section: Resultsmentioning

confidence: 99%

Heat Transfer to an Isolated Bubble Rising in a High‐Temperature Incipiently Fluidized Bed

Agarwal

2004

Can J Chem Eng

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Axial distribution of reactivity inside a fluid‐bed contactor

1976

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The axial distribution of reactivity inside a fluid-bed contactor is studied for the hydrogenation of ethylene. The reactivity is large near the surface of the dense phase. The significant effect of the dilute phase, or the free board region, is verified experimentally, and the contact efficiency in the transition zone and in the dilute phase is obtained. The contact efficiency was approximated by T~ = 1 -0.75(c,/~,d~)O.~. The temperature rise in the dilute phase is measured in the small reactor. The transition zone between the dense and dilute phases is almost isothermal, but it is found that temperature increases in the dilute phase. Toei et al., 1972). Some of the models originate from basic studies on the behavior of bubbles inside the bed, and these are referred to as physical bubble models by Rowe (1972) who compared them with arbitrary two-phase models. A comparison of these models shows that the main differences between these models are related to whether or not some fraction of the catalyst is in direct contact with the bubble gas, the extent of axial mixing in each phase, and the axial change in bubble size. SHINTARO FURUSAKI TATSUJIRecently, Miyauchi (1974 a, b) proposed the concept of successive contact, taking account of catalyst particles suspended above the dense bed. This effect is important because of the good contact in the suspending zone (called the dilute phase). According to this theory, the reaction proceeds first in the dense phase in poor contact with the catalyst, then in the dilute phase in good contact. According to the theory, Miyauchi and Furusaki (1974) calculated conversions using the HHU or the contact efficiency. Experimental data previously obtained (Lewis et al., 1959;Van Swaay and Zuiderweg, 1972) were compared successfully with the calculation. The importance of the catalyst suspended in the dilute phase in controlling the selectivity was also discussed. This paper presents experimental verification of the successive contact by the hydrogenation of ethylene on MS-Ni catalyst. This proof is essential in pursuing the analysis of fluid-bed contactors and is examined in detail. If the efficiency in the dilute phase is much higher than that in the dense phase as shown above, control of the reactivity in the dilute phase by the use of fines is considered useful in determining the optimal operation of fluid-bed contactors. This seems one of the reasons of utilizing fine particles in a fluid bed. This point is briefly discussed. CONCLUSIONS AND SIGNIFICANCEIn studying the operation of a fluid-bed reactor by the concept of the successive contact, it is important to study the axial distribution of reactivity inside the bed. This is done by moving the axial position of the nozzle, through which one of the reactants, CzH4, is introduced into the fluid bed. By this method, the reactivity in the dilute phase is proved large, especially for the case of large reaction rates. The transition zone between the dense and dilute phases is found to be quite effective. The effective reactivity in t...

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Heat transfer between an isolated bubble and the dense phase in a fluidized bed

Hartman

Chen

et al. 2001

Can J Chem Eng

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xchange of heat between a rising bubble and the dense phase is a component of the overall gas-particle heat transfer behaviour in E a bubbling fluidized bed. However, surprisingly few measurements and models have been reported on this subject. This is, perhaps, because reactant conversion in fluidized beds is calculated assuming that the bed is isothermal. This is expected to be the case for modelling solid-catalyzed gas-phase reactions where the bubbles represent a dead volume in terms of reaction; the bypassing of reactant in bubbles reduces the conversion per unit bed volume. In fluidized bed combustion, however, the bubble phase may represent a more favourable reaction volume for the combustion of volatile gases released from coal (Agarwal, 1986; Hayhurst and Tucker, 1990; Hesketh and Davidson, 1991). For modelling such systems, an accurate description of the heat exchange between the bubble and dense phases is vital (Srinivasan et al., 1998). In fluidized bed drying systems, the drying media in the bubble phase acts partly as the heat carrier (Chen et al., 1999); modelling such systems also requires accurate heat transfer coefficient between the bubble and dense phases. Kunii and Levenspiel (1991) obtained a model for the heat transfer coefficient between the bubble and dense phases by applying the analogy between heat and mass transfer to their well-established model for mass exchange. However, the large difference in thermal capacities of gas and solids in the fluidized bed implies that the mechanisms for heat and mass transfer need not be analogous. To explain the measurements on gas-particle heat transfer in a fluidized bed, Kunii and Levenspiel (1991) introduced an additional term representing heat exchange between bubble gas and the very small fraction of solids, which may be present within the bubble. The heat transfer coefficient between the particles and the bubble gas was calculated using the Ranz and Marshall correlation (1 952). The volumetric fraction (of the bubble) occupied by solid particles in the bubble was taken, based on measurements in a two-dimensional bed, as yp,B = (1 -2) x 1 0-3 It can be established quite readily that the contribution of the additional term to the total heat transfer far outweighs that from the terms obtained through the application of the heathass transfer analogy.Toei et al. (1 972), comparing the relative contributions of various heat transfer mechanisms to the overall heat transfer coefficient, concluded that the effect of the particles falling through the bubble was the dominant heat transfer mechanism. The contact time between the particles and the bubble gas was taken as the free-fall time of a particle through the bubble. Again, the Ranz and Marshall equation was recommended for the calculation of the heat transfer coefficient between the particles and the bubble gas.*Author to whom correspondence may be addressed: E-mail address: paganVal@ uwyo.edu Heat transfer between the bubble and dense phases of a bubbling fluidued bed plays a very important role in t...

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The Capacitance Effect on the Transfer of Gas or Heat Between a Bubble and the Continuous Phase in a Gas-Solid Fluidized Bed

Cited by 17 publications

References 1 publication

Heat Transfer to an Isolated Bubble Rising in a High‐Temperature Incipiently Fluidized Bed

Heat Transfer to an Isolated Bubble Rising in a High‐Temperature Incipiently Fluidized Bed

Axial distribution of reactivity inside a fluid‐bed contactor

Heat transfer between an isolated bubble and the dense phase in a fluidized bed

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