Heat transfer in dense-phase fluidized beds

3 Heat Transfer in Dense-Phase Fluidized Beds 

As noted, most of the heat transfer models and correlations for gas-solid fluidization systems were originally developed for dense-phase fluidized beds (see Chapter 9). In the following, the behavior of heat transfer coefficients between the suspension (or bed) and the particle, between the suspension (or bed) and the gas, and between the suspension (or bed) and the wall or heat transfer surface are discussed. 

---

TABLE 13.3 Heat Transfer in Dense-Phase Fluidized Beds... 

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. 

For catalytic reactions, particles used in fluidized bed processes are usually in the range of 40 to 100 pm in mean diameter. Similarly, particle-to-gas heat transfer coefficients in dense phase fluidized beds can be estimated by (Kunii and Levenspiel, 1991) ... 

Development of a mechanistic model is essential to quantification of the heat transfer phenomena in a fluidized system. Most models that are originally developed for dense-phase fluidized systems are also applicable to other fluidization systems. Figure 12.2 provides basic heat transfer characteristics in dense-phase fluidization systems that must be taken into account by a mechanistic model. The figure shows the variation of heat transfer coefficient with the gas velocity. It is seen that at a low gas velocity where the bed is in a fixed bed state, the heat transfer coefficient is low with increasing gas velocity, it increases sharply to a maximum value and then decreases. This increasing and decreasing behavior is a result of interplay between the particle convective and gas convective heat transfer which can be explained by mechanistic models given in 12.2.2, 12.2.3, and 12.2.4. 

The preceding model successfully explains the role played by the particles in the heat transfer processes occurring in the dense-phase fluidized bed at voidage a < 0.7. But it predicts very large values when the contact time of particles with the heating surface decreases because the nonuniformity of the solids concentration near the wall is not taken into account in this model. 

For the bed-to-surface heat transfer in a dense-phase fluidized bed, the particle circulation induced by bubble motion plays an important role. This can be seen in a study of heat transfer properties around a single bubble rising in a gas-solid suspension conducted... 

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. 

Since the solid particles in the spouted bed are well mixed, their average temperature in different parts of the annulus can be considered to be the same, just as in the case of a fluidized bed. The maximum value of the heat transfer coefficient in the h-U plot is also similar to that in a dense-phase fluidized bed [Mathur and Epstein, 1974]. 

The book is arranged in two parts Part I deals with basic relationships and phenomena, including particle size and properties, collision mechanics of solids, momentum transfer and charge transfer, heat and mass transfer, basic equations, and intrinsic phenomena in gas-solid flows. Part II discusses the characteristics of selected gas-solid flow systems such as gas-solid separators, hopper and standpipe flows, dense-phase fluidized beds, circulating fluidized beds, pneumatic conveying systems, and heat and mass transfer in fluidization systems. 

The particle-to-gas heat transfer coefficient in dense-phase fluidization systems can be determined from correlation Eq. 13.3.1 [2] given in Table 13.3. The correlation indicates that the values of particle-to-gas heat transfer coefficient in a dense-phase fluidized bed lie between those for fixed bed with large isometric particles (with a factor of 1.8 in the second term [49]) and those for the single-particle heat transfer coefficient (with a factor of 0.6 in the second term of the equation). 

Another deep-bed spiral-activated solids-transport device is shown by Fig. ll-60e. The flights cany a heat-transfer medium as well as the jacket. A unique feature of this device which is purported to increase heat-transfer capability in a given equipment space and cost is the dense-phase fluidization of the deep bed that promotes agitation and moisture removal on drying operations. 

It is important to do the balance over the whole of the dense phase when this is, by assumption, uniform, failure to do this led to an erroneous result in an earlier paper of ours, Heat transfer in fluidized and moving beds (Proc. Symp. on Interactions between Fluids and Particles. 1962. pp 176-182. The error was ultimately corrected in 1990 see Manners makyth Modellers, Chem Eng Sci, 46 1535-44 (1991) and in Trans IChemE, 68 165-174 (1991). 

The heat transfer behavior in a spouted bed (see 9.8) is different from that in dense-phase and circulating fluidized bed systems as a result of the inherent differences in their flow structures. The spouted bed is represented by a flow structure that can be characterized by two regions the annulus and the central spouting region (see Chapter 9). The heat transfers in these two regions are usually modeled separately. For the central spouting region, the correlation of Rowe and Claxton (1965) can be used for Repf > 1,000... 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

---