Suspension-to-Surface Heat Transfer

A mechanistic account of suspension-to-surface heat transfer is necessary to quantify the heat transfer behavior accurately and to assess the form of dependency of dimensionless groups in the correlations. In the following, the modes and regimes of suspension-to-surface heat transfer along with the three mechanistic models accounting for this heat transfer behavior are described. 

Figure 12.7. Conceptual representation of the film-penetration model for suspension-to-surface heat transfer (from Yoshida et at., 1969).

Wu RL, Grace JR, Lim CJ, Brereton CMH. Suspension-to-surface heat transfer in a circulating fluidized bed combustor. AIChE J 35 1685-1691, 1989. 

Need for very tall vessel small scale CFB processes are therefore seldom viable Substantial backmixing of solid particles Internals (e.g., baffles, heat transfer surfaces) not viable because of wear/attrition Wall wastage sometimes a serious problem Suspension-to-surface heat transfer less favorable than for low-velocity fluidization Lateral gradients can be considerable Losses of particles due to entrainment. 

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... 

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 more common approach is to treat the particle-gas suspension as an equivalent gray surface parallel to the heat transfer surface. Equation (11) would than be used with Ftaken as unity. Grace (1986) suggests that the emissivity of the particle-gas suspension can be approximated as,... 

The governing heat transfer modes in gas-solid flow systems include gas-particle heat transfer, particle-particle heat transfer, and suspension-surface heat transfer by conduction, convection, and/or radiation. The basic heat and mass transfer modes of a single particle in a gas medium are introduced in Chapter 4. This chapter deals with the modeling approaches in describing the heat and mass transfer processes in gas-solid flows. In multiparticle systems, as in the fluidization systems with spherical or nearly spherical particles, the conductive heat transfer due to particle collisions is usually negligible. Hence, this chapter is mainly concerned with the heat and mass transfer from suspension to the wall, from suspension to an immersed surface, and from gas to solids for multiparticle systems. The heat and mass transfer mechanisms due to particle convection and gas convection are illustrated. In addition, heat transfer due to radiation is discussed. 

The model based on the concept of pure limiting film resistance involves the steady-state concept of the heat transfer process and omits the essential unsteady nature of the heat transfer phenomena observed in many gas-solid suspension systems. To take into account the unsteady heat transfer behavior and particle convection in fluidized beds, a surface renewal model can be used. The model accounts for the film resistance adjacent to the heat transfer... 

The suspension-to-wall surface heat transfer mechanism in a circulating fluidized bed (see Chapter 10) comprises various modes, including conduction due to particle clusters on the surface or particles falling along the walls, thermal radiation, and convection due to... 

The bed-to-wall heat transfer coefficient depends mainly on the suspension density in the bed. For specified operating parameters, what needs to be considered is therefore only the transfer between the suspension and the heat transfer surface, although the actual mechanism involves various heat transfer processes. 

Bulk Suspension Temperature The heat transfer coefficient increases with bulk temperature (e.g., see Wu et al., 1989 Basu, 1990 Golriz and Sunden, 1994a,b), due to both the higher gas thermal conductivity and the increased radiation. At temperatures > 500° C, especially for relatively dilute beds (suspension densities <15 kg/m ), where radiation tends to be the dominant mode of transfer, the increase in heat transfer with temperature can be very significant. Figure 29 shows the local heat transfer coefficient along a 1.5 m long heat transfer surface for two bulk temperatures (Wu et al., 1989). The 30 0% in-... 

Polymerization in Hquid monomer was pioneered by RexaH Dmg and Chemical and Phillips Petroleum (United States). In the RexaH process, Hquid propylene is polymerized in a stirred reactor to form a polymer slurry. This suspension is transferred to a cyclone to separate the polymer from gaseous monomer under atmospheric pressure. The gaseous monomer is then compressed, condensed, and recycled to the polymerizer (123). In the Phillips process, polymerization occurs in loop reactors, increasing the ratio of available heat-transfer surface to reactor volume (124). In both of these processes, high catalyst residues necessitate post-reactor treatment of the polymer. 

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liqiiids to obtain uniformity, a speed of 3.50 or 420 r/min should be stipulated. For dispersion of dry particles in hquids or for rapid initial mixing of hquid reactants in a vessel, an 11.50- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth below the hquid surface if the submergence of floating hquids or particiilate solids is other wise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

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