doi:10.1016/j.ces.2006.09.017
Copyright © 2006 Elsevier Ltd All rights reserved.
Drying in fluidized beds with immersed heating elements
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H. Groenewold1, a, 
and E. Tsotsas
, a,
aLehrstuhl für Thermische Verfahrenstechnik, Otto-von-Guericke-Universität, Universitätsplatz 2, D-39106 Magdeburg, Germany
Available online 12 September 2006.
Abstract
The drying of six different fractions of γ-Al2O3 with particle diameters between 50 and 
has been investigated in a batchwise operated, lab-scale fluidized bed equipped with a cylindrical, vertically or horizontally placed heater. Resulting drying curves and heater-to-bed heat transfer coefficients are compared with various model versions, which are derived by combination of well established modules for fluidized bed drying, indirect heat transfer and intraparticle kinetics. In this way, good agreement with the experimental results is obtained for large particles. For small particles, a strong dependence of heat transfer on solid moisture content is observed, in the sense of an enhancement in comparison to the behaviour of the dry product. The part of this effect due to phase change (latent heat sink) in the interior of drying particles could be identified and analysed by extension and modification of existing model modules. This analysis points out limitations of such modules and still remaining modelling challenges. For all types of product, a very significant intensification of the drying process by immersed heating elements is demonstrated.
Keywords: Drying; Fluidization; Heat transfer; Particles; Phase change; Porous media
Fig. 1. Desorption equilibria of water on selected particle fractions.
Fig. 3. Partially normalized single-particle drying curve for product G1150; Comparison of experimental data (full points) with calculations by the volume averaging method (broken line).
Fig. 4. Flowsheet of experimental set-up.
Fig. 5. Fluidized bed drying curves of three selected fractions without (full symbols) and with (empty symbols) indirect heating (further parameters in Table 10).
Fig. 6. Wall-to-bed heat transfer coefficients during the drying of all used fractions (selected experiments after Table 10).
Fig. 7. Influence of moisture content on wall-to-bed heat transfer coefficient (three selected fractions, data correspond to those of Fig. 6).
Fig. 8. Comparison of measured heat transfer coefficients with the model of Martin (model version A, Table 1).
Fig. 9. Transformation of data and calculations from Fig. 8 by means of Eq. (19) in order to isolate the direct influence of latent sink on heat transfer.
Fig. 10. Same data as in Fig. 7, Fig. 8 and Fig. 9, though now treated with model version G, which accounts for the cooling effect and prolongs empirically the wall–particle contact time in the model of Martin (Table 1).
Fig. 11. Absolute values corresponding to data and calculations from Fig. 10; while model version G brings 
closer to unity (Fig. 10), it collaterally decreases the absolute value of αw-bed for fine particles (here: fraction NG100).
Fig. 12. Drying curves measured with indirect heating for all fractions according to Table 10, in comparison with model versions A, D, and G (Table 1).
Fig. 13. The same data as in Fig. 12, compared with the “short-cut” model versions C and J (Table 1).
Table 1.
Summary of model modules and the total of 10 investigated model versions
Table 2.
Main equations of drying module; combined indices mean “from-to”, e.g., 
is the mass flow rate from the particles to the suspension gas (evaporation flow rate)
Table 3.
Transfer coefficients in the drying module
Table 4.
Fluidization parameters
Table 5.
Indirect heat transfer module after Martin
Table 6.
Indirect heat transfer module after Kunii and Levenspiel
Eqs. (A64)–(A66): own approximations of data from Kunii and Levenspiel (1991); LH (in m): distance of the middle of the heater from the distributor; Eqs. (A57), (A58) from Table 5 still apply.
Table 7.
Used materials
Table 8.
Modified normalized drying curves (mndc) for single particles of the used products
Table 9.
Overview of experimental program
o: without indirect heating, v: vertical heater, h: horizontal heater, ndc: run to the purpose of derivation of single-particle kinetics; Re0 and εdry are calculated values.
Table 10.
Main parameters of experiments used in Figs. 5 and subsequent
o: without indirect heating, v: vertical heater.
Table 11.
Performance of all model versions in predicting the heat transfer coefficient αw-bed for vertically placed heater
means perfect agreement with the measurements.
Table 12.
As Table 11; however, for horizontally placed heater
Table 13.
Performance of all model versions in predicting drying rates
, in fluidized beds with or without indirect heating; 
means perfect agreement with the measurements.
Corresponding author. Tel.: +49 391 67 18784; fax: +49 391 67 11160.
1 Now with: AMMAG GmbH, Dahlienstrasse 11, A-4623 Gunskirchen, Austria.
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