network of undamaged pores within a spherical particle. The differential equations are solved according to the numerical procedure of Haynes, Jr. (1975). The model is intended to simulate more highly skewed chromatographic response curves than the two-zone model recently proposed by Chou and Hegedus (1978).Pulse chromatographic measurements in fresh NaMgA zeolites using i-butane as a tracer which only enters the macropores yield response curves with relatively low skew, which are readily fitted by a system of equations accounting for diffusion in a monodisperse pore system. For hydrothermally treated zeolites effective diffusivities and Henry constants are considerably lower and the skew higher than in fresh samples.Curve fitting with the model proposed below is quite satisfactory, and the characteristic parameters determined are approximately constant at the flowrates studied. Application of the model should not be restricted to hydrothermally treated zeolites, but may be extended to catalysts or adsorbents with macroporous structures damaged by other forms of ageing.
SCOPEEvaluating parameters by the method of moments is a rapid and widely used procedure since the work of Kubin (1965). However, the moments method makes use of integrated functions of a response curve, and is therefore not model discriminatory, since various forms of a response curve can lead to the same lowerarder moments. It is thus advisable to calculate sample response curves in the time domain with the parameter values determined by the moments method.Our estimates show that for highly skewed pulse chromatographic response curves with hydrothermally treated zeolites satisfactory agreement could be expected neither with the common macropore model nor with the Chou and Hegedus (1978) two-zone model for spent catalysts, which assumes blockage at the outer shell of the pellet. The purpose of this work is to present and test a model for calculating highly skewed response curves on the basis of effective parameters determined from the first and second experimental moments.It will be asumed that the monodisperse macroporous system disintegrates into a bidisperse system in the course of ageing.The undamaged pores act as transport pores and are assigned the Henry constant measured in the fresh sample. The damaged pores are distributed along the transport pores in the same manner as the micropores in the model of Ruckenstein et al (1971). Contrary to the common interpretation of the bidisperse model, the transport pores have the higher adsorption capacity. The remaining model parameters are determined by curve fitting.
CONCLUSIONS AND SIGNIFICANCEPulse response curves are measured in fresh NaMgA-zeolitic adsorbents; with the Henry constants and diffusion coefficients determined for i-butane, which does not enter the micropores, the response curves are calculated awurately in the time domain wing a monodisperse pore model. Hydrothermally treated samples show a decrease in Henry constant and diffusivity as well as a large increase in skew. Using the ...
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