Suspension-surface heat transfer

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. 

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).

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 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... 

Particulate fouling may be defined es the accumulation of particles from liquid or gaseous suspensions onto heat transfer surfaces. Particulate fouling occurs in a wide range of situations. In liquid streams the best known example is probably that of corrosion products In boiler waters and reactor coolants. In... 

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. 

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. 

Pafticulate fouling. Particles held in suspension in the flow stream will deposit out on the heat-transfer surface in areas of sufficiently lower velocity. 

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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