engleski

Population balance analysis of the crystallization of zeolite A from clear aluminosilicate solutions

Heating of clear aluminosilicate solutions (10Na2O 0.3Al2O3 SiO2 120 H2O) at 40, 50 and 60o C, causes a spontaneous formation (precipitation) of the solid aluminosilicate phase(s) (at the early stage of the reaction) followed by a spontaneous transformation of the amorphous phase to zeolite A. Both, the rates of crystal growth and the crystallization of zeolite A increases with the increasing crystallization temperature in accordance with the Arrhenius law. However, while Lm = 0 at tc = 0 (Lm is the size of the largest crystals at any crystallization time tc) for the crystallization at 40o C, Lm = 0 at tc = tg(0) > 0 for the crystallization at 50o C and 60o C, respectively. An analysis of the influence of the heating of the reaction mixture from the ambient temperature (Ta = 23o C) to the reaction temperature (TR = 40, 50 or 60o C) on the crystal growth rate have shown that this influence is negligible, and thus that the heating of the reaction mixture from T(0) to TR does not cause the observed “ delaying” of the crystal growth. Hence, it is really to assume that the observed “ delaying” of crystal growth is connected with the “ delaying” of nucleation. To prove or disapprove this assumption, the rates on nucleation were calculated by Zhdanov’ s method, using the measured growth kinetics and crystal size distributions of the crystalline end products. Analysis of the kinetics of nucleation undoubtedly showed that nucleation does not start at the zero tome of the crystallization process, but at any time tc = tn > 0. This means that the nuclei are formed in the matrix of the amorphous phase during its spontaneous precipitation. Since nuclei can start to grow after their releasing from the dissolved part of the precursor (precipitated amorphous aluminosilicate), one can assume that “ delaying” of the growth process in our experiments may be connected with the time needed for (a) formation of gel particles, (b) formation of nuclei in the gel particles, and (c) release of the nuclei from the gel particles during their dissolution. To prove or disapprove this assumption, the obtained data were simulated/analyzed by population balance methodology. Almost perfect correlation between measured and calculated kinetics of nucleation, crystal growth, crystallization (expressed as the mass of formed zeolite A) as well as the changes in the concentrations of silicon and aluminium in the liquid phase, firmly indicates that the crystallization of zeolite A from clear aluminosilicate solution takes place by a surface controlled growth of nuclei formed in the amorphous precursor during its precipitation, and released from the amorphous matrix during its dissolution.

clear solution; population balance analyse

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