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A new appraisal of mass transfer processes in zeolites by transient methods
Author(s) -
Authelin JeanRené,
Schweich Daniel,
Villermaux Jacques
Publication year - 1988
Publication title -
chemical engineering and technology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.403
H-Index - 81
eISSN - 1521-4125
pISSN - 0930-7516
DOI - 10.1002/ceat.270110155
Subject(s) - chemistry , methanol , argon , adsorption , mass transfer , dimethyl ether , analytical chemistry (journal) , inert gas , diffusion , helium , crystallite , mass transfer coefficient , thermodynamics , chromatography , organic chemistry , crystallography , physics
Transient diffusion of inert gases into Zeolon 900 H, a zeolite of the mordenite type, was studied by the chromatographic method. Experiments were performed with argon and helium either as tracer or as carrier gas. The diffusion of argon was also studied in a carrier gas containing methanol at a low temperature (75°C, no reaction) and at higher temperatures (up to 130°C) when some dehydration of methanol to dimethyl ether takes place. The adsorption isotherm of argon is linear. In the absence of methanol, its penetration into microcrystals of zeolite is equally well represented either by an effective diffusion coefficient D ′ μ = 7.4 × 10 −8 exp(−2873/ T )m 2 s −1 or by a mass transfer coefficient at the crystallite surface k eμ = 1.9 × 10 −4 exp(−1324/ T ) ms −1 . A small amount of methanol in the carrier gas considerably slows down the transfer of argon (by a factor of 40 for D ′ μ and 550 for k eμ ) showing the “blocking” effect of adsorbed methanol. In addition, transient adsorption can no longer be accounted for by a single transfer time. The size distribution of microcrystals, determined from Scanning Electron Microscope photographs leads to a Transfer Time Distribution which account well for experimental breakthrough curves without any parameter fitting. This interpretation is supported by experiments under reaction conditions where transport of argon within crystallites increases faster with temperature than in the absence of reaction due to the transformation of methanol into dimethyl ether, which is less adsorbed and thus provides lesser hindrance to argon diffusion.