Description Of Activation Process
DESCRIPTION OF ACTIVATION PROCESS
USING RANDOM PORE MODEL
B. Buczek, P. Zabierowski
University of Mining and Metallurgy, Faculty of Fuels and Energy,
al. Mickiewicza 30, 30-059 Cracow Poland
The random pore model was compared with experimental results. A good agreement for the model and experiments was found. The model appears to be suitable for predicting time - conversion and conversion-surface correlations in the kinetically controlled region (973 K). The direct consequence of these results is to validate the use of the random pore model as apart of mathematical model of the reactor for activation process.
Random Pore Model
Bhatia and Perlmutter [1] and independently Gavalas [2] have proposed a random pore model for which it is not necessary to assume an idealised geometrical structure of porous solid. Pores can differ in sizes and shapes and their geometrical structure changes during the reaction. The only simplification is that there are no closed pores in the solid.
Experimental
The predictions of the model were compared with the experimental data obtained in our Department [3] on the steam activation of cylindrical pellets prepared of coal (70% wt.) and tar (30% wt.). Prior to the reaction the particles 1.4 mm in diameter and 5 mm in height have been carbonised in nitrogen stream at 1223 K. The reaction was carried out in a specially constructed reactor with one pellet layer to simulate reaction conditions of single particle and eliminate some perturbations which occur in fluidized-bed reactors. The steady temperature of 973 K maintained in the reactor was lower that the temperature of carbonisation. The reaction was performed at different periods of time (t). As a result, sample active carbons of different conversions (X) were obtained.
Results and Discussion
For the parameters of the model, functions X = f(t) was calculated. The results of calculations compared with experimental points are plotted in Fig. 1.
Fig.1. Conversion-reaction time curve for char-steam reaction at 973K model o experimental
Calculations at higher temperatures, 1013 and 1073 K, were also performed [4]. In this temperature region diffusion slows down the reaction rate. The reaction performed at higher temperatures shows a marked influence on diffusive resistance. Therefore the kinetic model predicts too high conversions in comparison with the experimental ones.
Detailed experimental investigations have revealed radial heterogeneity of the porous structure, the character of the surface, and burn-off of material within carbon particles[5]. A change of the properties throughout the particles is experimental proof that activation process occurred in a region characterised by significant resistance to heat and mass transfer.
References
1. Bhatia S. K., Perlmutter D. D., AIChEJ, 26, 379 (1980)
2. Gavalas G. R., AIChEJ, 26, 577 (1980)
3. Dziubalski R., Ph. D. thesis, University of Mining and Metallurgy, Cracow 1979
4. Buczek B., Dziewiński J., Dziubalski R., Korta A., Koks, Smoła, Gaz, 31, 8 (1986)
5. BuczekB., in Adsorption and its Application in Industry and Environmental Protection, ed. A. Dąbrowski, Amsterdam 1999, Vol. 120A, pp. 507.
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