Circulating fluidized beds heat transfer coefficient

Measurements of heat transfer in circulating fluidized beds require use of very small heat transfer probes, in order to reduce the interference to the flow field. The dimensions of the heat transfer surface may significantly affect the heat transfer coefficient at any radial position in the riser. All the treatment of circulating fluidized bed heat transfer described is based on a small dimension for the heat transfer surface. The heat transfer coefficient decreases asymptotically with an increase in the vertical dimension of the heat transfer surface [Bi et al., 1990]. It can be stated that the large dimensions of the heat transfer surface... 

Heat Transfer, Viswanathan et al, [76] measured wall-to-bed heat transfer in a jacketed fluidized bed containing by air, water and quartz particles (0,928 mm and 0,649 mm). Steam was circulated through the jacket. The amount of condensate collected and the sensible heat gained by the liquid phase were used to assess the heat transferred to the fluidized bed. The wall-to-bed heat transfer coefficient, h, and its dependence upon the ratio of air to liquid flow are shown in Figure 8, It was found that h... 

To provide the pr equisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrod3aiamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve... 

Another parametric effect is the apparent dependence of the heat transfer coefficient on the physical size of the heat transfer surface. Figure 24, from Burki et al. (1993), graphically illustrates this parametric effect by showing that the effective heat transfer coefficient can vary by several hundred percent with different vertical lengths of the heat transfer surface, for circulating fluidized beds of approximately the same particle diameter and suspension density. This size effect significantly contributed to confusion in the technical community since experimental measurements by inves-... 

Dou, S., Herb, B., Tuzla, K., and Chen, J. C., Heat Transfer Coefficients for Tubes Submerged in Circulating Fluidized Bed, Experimental Heat Transfer, 4 343-353 (1991)... 

The mechanism of heat transfer in circulating fluidized beds is described in this section. Effects of the operating variables on the local and overall heat transfer coefficients are discussed. 

To understand the radiative heat transfer in a circulating fluidized bed, the bed can be regarded as a pseudogray body. The radiative heat transfer coefficient is [Wu et al., 1989]... 

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. 

Figure 12.17. Effect of gas velocity on the radial distribution of the heat transfer coefficient in a circulating fluidized bed (from Bi et at., 1989).

Derive the axial profile of the particle convective heat transfer coefficient in a circulating fluidized bed of fine particles using the information given in 10.4.1. It can be assumed in the derivation that particles in the bed are all in a cluster form. 

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). 

Mahalingam, M and Ajit, Kumar Kolar. "Experimental Correlation for Average Heat Transfer Coefficient at the Wall of a Circulating Fluidized Bed, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 390-395. Somerset, Pennsylvania (1993). 

In this chapter, emphasis will be given to heat transfer in fast fluidized beds between suspension and immersed surfaces to demonstrate how heat transfer depends on gas velocity, solids circulation rate, gas/solid properties, and temperature, as well as on the geometry and size of the heat transfer surfaces. Both radial and axial profiles of heat transfer coefficients are presented to reveal the relations between hydrodynamic features and heat transfer behavior. For the design of commercial equipment, the influence of the length of heat transfer surface and the variation of heat transfer coefficient along the surface will be discussed. These will be followed by a description of current mechanistic models and methods for enhancing heat transfer on large heat transfer surfaces in fast fluidized beds. Heat and mass transfer between gas and solids in fast fluidized beds will then be briefly discussed. 

FIGURE 13.15 Comparison between experimentally determined and calculated (correlation Eq. 13.4.1) heat transfer coefficients in circulating fluidized bed at ambient temperature (from Wirth [71]). 

TABLE 13.6 Influence of Surface Location and Orientation on Bed-to-Surface Heat Transfer Coefficient in a Circulating Fluidized Combustor (from Grace [86])... 

The influence of surface location and orientation on the bed-to-surface heat transfer coefficient in circulating fluidized bed combustors is summarized in Table 13.6. The geometric construction of the combustor and the heat transfer surface is shown in Fig. 13.17. Besides the location and orientation, differences in local heat transfer can also be found on the heat transfer surface/tube. For example, the upper part of the horizontal tube shows the smallest value for the heat transfer coefficient in dense-phase fluidized beds due to less frequent bubble impacts and the presence of relatively low-velocity particles. 

The heat transfer behavior in a spouted bed is different from that in the dense-phase or circulating fluidized bed system due to the inherent differences in their flow structures. The gas-to-particle heat transfer coefficient in the annulus region is usually an order of magnitude higher than that in the central spout region. The bed-to-surface heat transfer coefficient reaches a maximum at the spout-annulus interface and also increases with the particle diameter. 

The thorough mixing of the solid leads to effective gas-solid heat exchange with an excellent heat-transfer characteristic and hence a uniform temperature distribution in the reaction space. Heat-transfer coefficients are typically 100-400 kJm h K and for small particles can be as high as 800 kJm h K. For fine particles and at high reaction rates, circulating fluidized-bed reactors with separation and recycling of the soUd are particularly suitable. 

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. 

Dou S, Herb B, Tuzla K, Chen JC. Heat transfer coefficients for tubes submerged in circulating fluidized bed. Experimental Heat Transfer 4 343-353, 1991. 

---