Effective diffusivity by the gas chromatography technique: Analysis and application to measurements of diffusion of various hydrocarbons in zeolite NaY

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Abstract

The gas chromatography technique was applied to measurements of diffusion of n-butane in zeolite NaY. The linear chromatography theory failed to explain these results quantitatively, and a significant system nonlinearity was demonstrated. This nonlinearity is likely associated with non-Fickian diffusion. Order of magnitude estimates of the diffusivity could still be obtained, however. Over the range of temperatures 105 to 240°C, the n-butane diffusivities were in the range 10−8 to 10−6 cm2/s. Similar results were obtained with n-hexane and calculated diffusivities were about an order of magnitude smaller than the corresponding n-butane values. In contrast, limited experiments with cyclohexane, 2, 2-dimethylbutane, and trans-decalin were entirely consistent with the linear chromatography theory. At the measurement temperatures, the intracrystalline diffusion was too rapid to be detected in any of these systems. Attempts to operate at lower temperatures where diffusion might be significant were frustrated by extensive peak broadening and concomitant loss of detector response.

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