Teeter beds

Operation of a fluid bed depends on both height and diameter. Slugging properties have been studied extensively by Davidson and co-workers (H17, K8, K9, S21). Generally the ratio Lf/Dx is much larger than unity for small-scale reactors (C2). For solids particles in a teeter bed, this ratio will lie well into the slugging region. However, the fluidity in fluid bed is quite different from that in a teeter bed, as explained in Section II,A. 

The lateral distribution of gas bubbles induces bulk recirculation of the emulsion phase. For teeter beds, the flow pattern of solid particles has been studied by Werther (W7, W8), Bui ess and Calderbank (B17), Oki and Shirai (03) and Whiteheade/a/. (W12), in experiments carried out with alumina particles, quartz sand, and petroleum coke under conditions of dp s 83 /Ltm and (f/, - Umd/Umf — 14. The mode of bulk circulation was centrally descending and peripherally ascending for If/Dr 1. while the ascending flow moved toward the center with increasing Lf/Dx. Werther (W8) showed that this circulation flow is caused entirely by bubbles which carry solid particles upward in their wakes. 

In contrast to the teeter bed, less work has been carried out on the bulk flow pattern in fluid beds (Lll, M29). Measurements at the relatively high gas velocities of practical interest (C/f > 10 cm/sec, Uf/Umt 1) show that the rate of circulation exceeds that of solid particles conveyed by the bubble wake, and results from the buoyant force induced between the centrally ascending bubble-rich phase and the peripherally descending emulsion phase of low bubble content (see Fig. 2). The behavior greatly resembles that in bubble columns, as will be discussed quantitatively in a later section (H9, K15, M31, P3, P6, T23, U3, Y2, Y22, and Kato and Morooka, unpublished data). For a bubble column, Shyu and Miyauchi... 

Currently available data for the flow properties of the fluidized catalyst bed are fragmentary, since the local motion of the emulsion phase is diflicult to measure experimentally. Therefore, it is useful to clarify the flow properties of the bed in terms of our knowledge of bubble columns. First, the fluid-dynamic properties of the bubble columns will be explained then, the available data will be adapted to apply to fluid catalyst beds. The reader will be able to picture an emulsion phase of carefully prepared catalyst particles operating in intense turbulence for fluidized beds under conditions of practical interest. This turbulence distinguishes the flow properties of fluid catalyst beds from those of widely studied teeter beds. 

Fluidized catalytic reactions have been industrially operated in the fluid bed conditions, but most of the research has been carried out for the teeter bed. Several studies of fluidized catalytic reaction are listed in Table VI, which are of interest in considering transport phenomena in fluidized catalyst beds. 

Most of the models for fluidized catalytic reactions have not devoted enough attention to flow properties of FCBs. The important features of the FCB are as follows (a) Bubbles grow much more slowly than in teeter beds (b) A circulating flow exists, centrally upward and peripherally downward (c) Above the dense phase are the transition region and the dilute phase, where particle density decreases gradually with height. 

Galvin KP, Pratten S, Nguyen-Tran-Lam G. Differential settling in a teeter bed separator. Presented at Third World Congress on Particle Technology. Brighton, England I Chem E, 1998, Paper 228, pp 1-11. 

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