Fluidized beds convective heat transfer

A. M. Xavier and J. F. Davidson, Heat Transfer in Fluidized Beds Convective Heat Transfer in Fluidized Beds, in Fluidization, 2d ed., Davidson, Clift, and Harrison eds, London Academic Press, 1985. 

Fundamental models correctly predict that for Group A particles, the conductive heat transfer is much greater than the convective heat transfer. For Group B and D particles, the gas convective heat transfer predominates as the particle surface area decreases. Figure 11 demonstrates how heat transfer varies with pressure and velocity for the different types of particles (23). As superficial velocity increases, there is a sudden jump in the heat-transfer coefficient as gas velocity exceeds and the bed becomes fluidized. 

Designing a model fluidized bed which simulates the hydrodynamics of a commercial bed requires accounting for all of the mechanical forces in the system. In some instances, convective heat transfer can also be scaled but, at present, proper scaling relationships for chemical reactions or hydromechanical effects, such as particle attrition or the rate of tube erosion, have not been established. 

In general, gas-to-particle or particle-to-gas heat transfer is not limiting in fluidized beds (Botterill, 1986). Therefore, bed-to-surface heat transfer coefficients are generally limiting, and are of most interest. The overall heat transfer coefficient (h) can be viewed as the sum of the particle convective heat transfer coefficient (h ), the gas convective heat transfer coefficient (h ), and the radiant heat transfer coefficient (hr). 

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

The data of Fig. 20 also point out an interesting phenomenon—while the heat transfer coefficients at bed wall and bed centerline both correlate with suspension density, their correlations are quantitatively different. This strongly suggests that the cross-sectional solid concentration is an important, but not primary parameter. Dou et al. speculated that the difference may be attributed to variations in the local solid concentration across the diameter of the fast fluidized bed. They show that when the cross-sectional averaged density is modified by an empirical radial distribution to obtain local suspension densities, the heat transfer coefficient indeed than correlates as a single function with local suspension density. This is shown in Fig. 21 where the two sets of data for different radial positions now correlate as a single function with local mixture density. The conclusion is That the convective heat transfer coefficient for surfaces in a fast fluidized bed is determined primarily by the local two-phase mixture density (solid concentration) at the location of that surface, for any given type of particle. The early observed parametric effects of elevation, gas velocity, solid mass flux, and radial position are all secondary to this primary functional dependence. 

The simplest correlations are of the form shown by Eq. (15), in attempts to recognize the strong influence of solid concentration (i.e., suspension density) on the convective heat transfer coefficient. Some examples of this type of correlation, for heat transfer at vertical wall of fast fluidized beds are ... 

Ebert, T., Glicksman, L., and Lints, M., Determination of Particle and Gas Convective Heat Transfer Components in Circulating Fluidized Bed, Chem. Eng. Sci., 48 2179-2188 (1993)... 

Han, G. Y., Experimental Study of Radiative and Particle Convective Heat Transfer in Fast Fluidized Beds, Ph.D. Dissertation, Lehigh University (1992)... 

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. 

A unique feature of the dense-phase fluidized bed is the existence of a maximum convective heat transfer coefficient /zmax when the radiative heat transfer is negligible. This feature is distinct for fluidized beds with small particles. For beds with coarse particles, the heat transfer coefficient is relatively insensitive to the gas flow rate once the maximum value is reached. 

In circulating fluidized beds, the clusters move randomly. Some clusters are swept from the surface, while others stay on the surface. Thus, the heat transfer between the surface and clusters occurs via unsteady heat conduction with a variable contact time. This part of heat transfer due to cluster movement represents the main part of particle convective heat transfer. Heat transfer is also due to gas flow which covers the surface (or a part of surface). This part of heat transfer corresponds to the gas convective component. 

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. 

Little is known about the fluid wall heat transfer in the case of gas -liquid flow in a fixed-bed reactor. Some research on this subject, however, has been carried out for the specific case of cocurrent downflow over a fixed-bed reactor. This is summarized in Chap. 6. Some work on the slurry-wall heat-transfer rate for a three-phase fluidized bed has also been reported. The heat-transfer rate is characterized by the convective heat-transfer coefficient between the slurry and the reactor wall. Some correlations for the heat-transfer coefficient in a three-phase slurry reactor are discussed in Chap. 9. 

The gas fluidized-bed reactor is the most efficient approach to pyrolysis. In this reactor the waste plastic is suspended around the heating medium and snbjected to pyrolysis by means of immersed heating tubes and gas-solid convective heat transfer. At present the only difficulty with this reactor is the problem of its structure. Fluidized-bed pyrolyzers have been designed for pyrolysis of waste tyre mbber in Taiwan and in Hangzon. A schematic apparatus of a fluidized-bed pyrolyzer is shown in Fignre 27.2. 

Moreover, very few parameterizations are reported on the wall- and fluid-granular material convective thermal heat transfer coefficients. For introductory studies, the work of Natarajan and Hunt [55], Gunn [25], Kuibe and Broughton [40], Kuipers et al [41] and Patil et al [59] might be consulted. To enable validation and reliable predictions of non-isothermal non-adiabatic reactive granular flows the thermal conductivity and the convective heat transfer coefficients have to be determined with sufficient accuracy. For certain processes this may be an important task for future research in the field of granular flows in fluidized beds. 

In most dense gas-solid fluidization systems, particle circulation (e.g., induced by the bubbles) is the primary cause of particle convective heat transfer. The heat transfer rate is high when there is an extensive solids exchange between the in-bed region and the region near the heat transfer surface. In light of the relative importance of the particle convective component, more discussion is focused on particle convective heat transfer in the following sections in the context of the heat transfer models and corresponding correlations. 

In liquid-solid fluidized beds, the presence of solids increases the turbulence in the system and provides additional surface renewal through the thermal boundary layer at the wall. Early studies have indicated that the heat transfer by particle convective mechanism is insignificant and that the convective heat transfer due to turbulent eddies is the principal... 

The liquid to be dried is sprayed into the fluidized bed it coats the inert particle snrfaces. The coated layer dries as a result of combined convective heat transfer from hot air and contact heat transfer due to sensible heat of the particles. When the thin layer is dry, it becomes brittle, cracks, and is peeled off due to attrition by particle-particle and particle-wall collisions. As a result, a fine powder is formed and is carried over by the exhaust gas to be collected and separated in suitable gas-cleaning devices such as cyclones or bag filters. 

The convective heat transfer coefficient between gas and solid in the fluidized bed can be estimated from the correlation... 

Fluidized bed furnaces utilize intense gas convection heat transfer and physical bombardment of solid heat receiver surfaces with millions of rapidly vibrating hot solid particles. The furnaces take several forms. 

Explain what is meant by particle convective heat transfer in a fluidized bed. In which Geldart group is particle convective heat transfer dominant ... 

Convective Heat Transfer. Most researchers concur that for bubbling fluidized beds, the gaseous convection is significantly enhanced by the presence of sohd particles (Ozkaynak and Chen, 1980 Kunii and Levenspiel, 1991). Models often have focused on the dense/particle phase contribution, or on the total convective contribution,... 

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