Heat transfer in fluidized beds

Fluidized beds are widely used to achieve either chemical reactions or physical processing that require interfacial contact between gas and particles. Heat transfer is important in many of these applications, either to obtain energy transfer between the solid and gas phases or to obtain energy transfer between the two-phase mixture and a heating/cooling medium. The latter case is particularly important for fluidized bed reactors which require heat addition or extraction in order to achieve thermal control with heats of reaction. 

Heat transfer between gas and particle phases tend to be efficient due to the large volumetric concentration of interface surface. Hence this topic is rarely of significant concern and will not be dealt with in this chapter. Most of the chapter concerns heat transfer between the two-phase medium and submerged surfaces. This is the most pertinent engineering problem since heat addition or extraction from the fluidized or conveyed mixture is commonly achieved by use of heat exchangers integral to the vessel wall or submerged in the particle/gas medium. 

As discussed in other chapters of this book, two-phase flows of gas and particles occur with different flow regimes. The mechanisms for heat transfer and the resulting heat transfer coefficients are strongly affected by the different flow characteristics, resulting in different design correlations and predictive models for each flow regime. This chapter will deal with the two most often encountered flow regimes  

Correlation of fluid bed to external surface heat transfer coefficients. Wender and Cooper [1958]. From Zenz and Othmer [I960]. 

It follows from the above correlations that the heat transfer coefficient between a fluidized bed and a surface is high, often in the range of 0.232 to 0.697 kj/m s K. 

Finally, the temperature difference between the gas and the catalyst surface has to be calculated. The following correlation has been proposed for air  

Correction factor for nonaxially located internal heat transfer surface. Based on data of Vreedenberg [1960], from Zenz and Othmer [I960]. 

The transfer of heat between fluidized solids, gas and internal surfaces of equipment is very good. This makes for uniform temperatures and ease of control of bed temperature. 

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Chen, J. C., Chen, K. L., Analysis of Simultaneous Radiative and Conductive Heat Transfer in Fluidized Beds, Chem. Eng. Commun., 9 255-271 (1981)... 

Wood, R. T., Staub, F. W., Canada, G. S., and McLaughlin, M. H., Two-Phase Flow and Heat Transfer in Fluidized Beds, Technical Report, RP 525-1, General Electric Co., Schenectady, NY (1978)... 

Molerus, O and WlRTH, K.E. Heat Transfer in Fluidized Beds. (Chapman and Hall, 1997). 

Zabrodsky, S. S. Hydrodynamics and Heat Transfer in Fluidized Beds (The M.I.T. Press, 1966). 

FRANTZ, J. F. Chem. Eng. Prog. 57 (1961) 35. FLuid-to-particle heat transfer in fluidized beds. 

The use of the Plank/Nagaoka model can be illustrated with a simple example. Consider a fluidized bed 0.75 m wide and 5 m long which is used to freeze peas 8 mm in diameter at a rate of 6000kgh k Assume that the peas enter the bed at 12°C, have a freezing temperature of -2°C and that the fluidizing air enters the bed at -35°C and at a velocity such that the heat transfer coefficient (see Heat transfer in fluidized bed freezers, below) is 170Wm K k What is the necessary bed depth ... 

Sheen, S. and Whitney, L.F., Modelling heat transfer in fluidized beds of large particles and its applications in the freezing of large food items, J. Food Eng., 12 (1990) 249-265. 

Froessling, N. (1938). The Evaporation of Falling Drops. Gerlands Beitr. Geophys., 52,170. Gel Perin, N. I. and Einstein, V. G. (1971). Heat Transfer in Fluidized Beds. In Fluidization. Ed. 

Xavier, A. M. and Davidson, J. F. (1985). Heat Transfer in Fluidized Beds Convective Heat... 

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