Mass transfer in fluidized beds

Beek, W. J., Mass Transfer in Fluidized Beds, in Fluidization by J. F. Davidson and D. Harrison, Academic Press, New York, 1971. 

Agarwal, P. K. and la Nauze, R. D. Chem. Eng. Res. Des. 67 (1989) 457. Transfer processes local to the coal particle. A review of drying, devolatilization and mass transfer in fluidized bed combustion. 

S. Heinrich, L. Mori, G. Kruger, Modelling and experimental verification of the heat and mass transfer in fluidized bed spray-granulation, Proceedings, 2nd European Congress of Chemical Engineering ECCE-2, Montpellier, October 5-7,1999, No. 67, vol. 

Martin, H. Heat and mass transfer in fluidized beds. International Chemical Engineering, 22, No. 1 (1982). 

Descriptive Behavior of the Kurui-Levenspiel Bubbling Bed Model Mass Transfer in Fluidized Beds Reaction in a Fluidized Bed... 

Knto et al.11 and Davidson and Harrison 4 have reviewed available correlations for both heat and mass transfer coefficients in gas-solid fluidized beds. Some investigated have proposed dial the same correlation can be applied 1 3 for both fized and fluidized beds by proper choice of the fluid velocity teres. Further disen salons of mass transfer in fluidized beds can he found in Davidsou and Harrison " aed Kunii and Lavenspiel, 5 Because of the difficulty of operating small laboratory fluidized beds under conditions comparable to those of a full-scale unit, the applicability of many of theae studies may he quesiioanble. 

Table 12.2 is a summary of useful correlations for jp and jfj that have been proposed by various investigators. The indicated correlations are quite consistent with one another in the regions where they overlap. Although the mass transfer coefficients determined from these correlations are not particularly large, they lead to high mass transfer rates when they are multiplied by the external surface area of the bed. For more detailed treatments of mass transfer in fluidized beds, see the text of Kunii and Levenspiel (83) and the review by Beek (84). 

Heat and mass transfer in fluidized beds have been discussed in Refs. [6,138-141], The latter reviewed the most important correlations and proposed Equations T9.11 and T9.12 of Table 4.9 for the calculation of heat and mass transfer coefficients, respectively. Further information for fluidized bed drying can be found in Ref. [142]. 

The particle-gas mass transfer in a gas-solid fluidized bed has been an essential subject of investigation since the invention of the fluidized bed technology. Historically, there have been two approaches in modeling the rate of mass transfer in fluidized bed reactors. One approach, called the homogeneous bed approach, considers the fluidized bed reactor to behave like a fixed bed and correlates the fluidized bed mass transfer coefiicient in a manner similar to that in a fixed bed based on a plug-flow model. The other approach, called the bubbling bed approach, considers the fluidized bed to consist of two phases, a bubble and an emulsion, and the gas interchange between the two phases constitutes the rate of mass transfer. The objective of this chapter is to review the two approaches. 

Wakabayashi T, Kunii D. Contribution of solids dispersed in bubbles to mass transfer in fluidized bed. J Chem Eng (Japan) 4 226-230, 1971. 

By injection of a tracer gas into the flow (continuous or batchwise form) and by measuring its concentration and velocity one obtains information on the residence time distribution (RTD), mixing or mass transfer. Examples of applications of gas tracers to fluidized beds are presented in Table 9. Figure 27 presents a model of gas flow in the fluidized bed and an example of recorded data. Gas tracking is also a useful way to measure mass transfer in fluidized beds, as reported by Ebert et al. (1993) and de Kok et al. (1986). 

The early correlations of low flux particle-liquid mass transfer in fluidized beds were patterned after those for fixed beds, in which, for spheres in the absence of free convection. 

Beek WI. Mass transfer in fluidized beds. In Davidson JF, Harrison D, eds. Fluidization. London Academic Press, 1971, pp 431 70. 

Heat and mass transfer in fluidized beds have been discussed in Refs. [6,137 140]. The latter reviewed the most important eorrelations and proposed Equation... 

Predicting the mass transfer in fluidized beds under higher pressure is difficult. Obviously, the minimum fluidization velocity will be lower, but this has no great effect on the mass transfer rate. What is more important is that the bubbles become smaller, and the effective mass transfer rate goes up. Predictions based on the the well known empirical relations will therefore be conservative. 

For reviews on mass transfer in fluidized beds, see e.g., Kunii and Levenspiel (1969), Van Swaaij (1978, 1985), and Davidson, Qift and Harrison (1985). 

The fact remains, however, that the study of gas-solid heat or mass transfer in fluidized beds is largely an academic matter, because the transfer rate is quite rapid. The principal transfer problem is the exchange of matter or thermal energy between the bubble and the emulsion phases. 

---