Heat transfer, fluidized beds vertical tubes

Figure 1738. Heat transfer coefficient in fluidized beds [Wender and Cooper, AIChE J. 4, 15 (1958)]. (a) Heat transfer at immersed vertical tubes. All groups are dimensionless except kgICgpg, which is sqft/hr. The constant CR is given in terms of the fractional distance from the center of the vessel by CR = 1 + 3.175(r/R) — 3.188(r/R)2. (b) Heat transfer at the wall of a vessel. Lh is bed depth, Ot is vessel diameter.

Dow, W.M. and Jakob, M., Heat transfer between a vertical tube and fluidized bed air mixture, Chem. Eng. Prog., 47,637,1951. 

Heat Transfer Heat-exchange surfaces have been used to provide means of removing or adding heat to fluidized beds. Usually, these surfaces are provided in the form of vertical tubes manifolded at top and bottom or in trombone shape manifolded exterior to the vessel. 

Heat Transfer Heat-exchange surfaces have been used to provide the means of removing or adding heat to fluidized beds. Usually, these surfaces are provided in the form of vertical or horizontal tubes manifolded at the tops and bottom or in a trombone shape manifolded exterior to the vessel. Horizontal tubes are extremely common as heat-transfer tubes. In any such installation, adequate provision must be made for abrasion of the exchanger surface by the bed. The prediction of the heat-transfer coefficient for fluidized beds is covered in Secs. 5 and 11. 

In a bubbling fluidized bed the coefficient of heat transfer between bed and immersed surfaces (vertical bed walls or tubes) can be considered to be made up of three components which are approximately additive (Botterill, 1975). 

It should be noted that in this approach, only the convective contribution to heat transfer is considered. Chen (1976) tested five different correlations, (Vreedenberg, 1958 Wender et al., 1958 Miller et al., 1951), for vertical tubes against a uniform set of experimental data obtained by Ozkaynak (1974). Figure 8 shows a comparison of the correlations and the experimental data, for a fluidized bed of 240 pm diameter glass spheres at room temperature. It is seen that there is very little agreement between the various correlations or between correlation and data. The uncertainty is of the order of 100%. 

General Characteristics. Energy addition or extraction from fast fluidized beds are commonly accomplished through vertical heat transfer surfaces in the form of membrane walls or submerged vertical tubes. Horizontal tubes or tube bundles are almost never used due to concern with... 

The interaction of parametric effects of solid mass flux and axial location is illustrated by the data of Dou et al. (1991), shown in Fig. 19. These authors measured the heat transfer coefficient on the surface of a vertical tube suspended within the fast fluidized bed at different elevations. The data of Fig. 19 show that for a given size particle, at a given superficial gas velocity, the heat transfer coefficient consistently decreases with elevation along the bed for any given solid mass flux Gs. At a given elevation position, the heat transfer coefficient consistently increases with increasing solid mass flux at the highest elevation of 6.5 m, where hydrodynamic conditions are most likely to be fully developed, it is seen that the heat transfer coefficient increases by approximately 50% as Gv increased from 30 to 50 kg/rrfs. 

Lu, H., Yang, L. D Bao, Y. L., and Chen, L. Z. The Radial Distribution of Heat Transfer Coefficient Between Suspensions and Immersed Vertical Tube in Circulating Fluidized Bed (Chinese), The Proceeding of 5th National Conference on Fluidization, pp. 164-167 (1990). 

The presence of vertical membrane elements or modules also helps to prevent bubble coalescence, thus favoring heat and mass transfer in the reactor. Their spacing should be sufficiently small so that the maximum number of the membrane tubes or modules may be provided in the reaction zone and large enough that no blockage or bridging of the fluidized bed occurs. 

H.A. Vreedenberg, Heat transfer between a fluidized bed and a horizontal tube, Chem.Eng. Sci. 9, 52-60 (1958) Vertical tubes, Chem. 

Good heat transfer is one of the most attractive features of the fluidized bed. From the standpoint of its use as a chemical reactor, the most important mode of heat transfer is that from a fluidizing bed to a bank of tubes (with a circulating fluid) immersed within it. The value of the heat transfer coefficient will depend on whether the tube bank is vertical or horizontal. A number of correlations are available for predicting these and other modes of heat transfer in a fluidized bed, and good reviews of these correlations can be found in Botterill (1966), Zabrodsky (1966), Muchi et al. (1984), and Kunii and Levenspiel (1991). Most of them are restricted to relatively narrow ranges of variables. Two useful correlations are listed in Table 12.6. It is important to note that reactions such as the chlorination of methane (Doraiswamy et al., 1972) are entirely heat transfer controlled. The rate of heat removal and design of reactor internals become the crucial considerations in such cases. 

Recent studies have made it easier to design reactors with vertical tubular inserts. This arises from the observation (Gunn and Hilal, 1994, 1996, 1997) that the heat transfer coefficients for these systems are almost equal to those for the corresponding open fluidized beds of the same diameter operating with the same particles. Hence, correlations for the latter (which are readily available) can be used for vertical inserts without significant loss of accuracy. Vertical inserts have an additional advantage over horizontal inserts. In horizontal inserts there is accumulation of particles on top of the tubes and depletion of particles at the bottom, a situation that does not exist in the vertical orientation. 

Fluidized beds equipped with internal heaters or immersed tubes transfer heat indirectly to the drying material. Horizontal tube bundles (Figure 8.15) are used extensively compared to vertical type. Tube pitch is an important design parameter. Fluidizing gas stream fluidizes the material and carries over the evaporated moisture. As a result, total sensible heat of gas and thus quantity of gas required are reduced. Immersed tubes or internally heated FBDs are used to dry smaller-size or fine powders. This is because the heat transfer coefficient decreases with increasing particle size. Instead of tubes, vertical plates are also used as immersed heaters. 

Vreedenberg, H.A., Heat transfer between fluidized beds and vertically inserted tubes, J. Appl. Chem., 2, 26,1952. 

Borodulya VA, Ganzha VL, Podberezsky AI, Upadhyay SN, Saxena SE. High pressure heat transfer investigations for fluidized beds of large particles and immersed vertical tube bundles, Int J of Heat and Mass Transfer 26 1577-584, 1983. 

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