Synthesis, characterization and modeling of zeolitic membranes for gas separation

• Autor(a) principal: Azenha, Ivo Samuel Caniceiro
• Data de Publicação: 2015
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/15272
• topic: Engenharia química; Separação de membranas; Zeólitos

The interest on inorganic membranes has been growing significantly due to their remarkable properties like mechanical, thermal and chemical stabilities, and the ability to perform selective separations based on the kinetic diameter and surface affinity, acting as molecular sieves. The main objectives of this work were: (i) the application of the Maxwell-Stefan (MS) formalism to gas permeation through zeolite-type membranes in order to explore its potential as purely predictive tool; (ii) the development of new MS thermodynamic factors for the Dubinin-Astakhov and Dubinin-Radushkevich isotherms, since they are fundamental in the field of microporous materials; (iii) modeling water permeation through zeolites 13X and 4A membranes after accurate analysis of the influence of isotherm data and effective diffusivity upon permeation; (iv) synthesis and characterization of AM-3 (Aveiro Manchester number 3) titanosilicate membranes for gas separation. Regarding membranes preparation, they were synthesized hydrothermally up to three consecutive times on tubular α-alumina supports through a secondary growth technique. Through X-ray diffraction and scanning electron microscopy it was possible to conclude that after three depositions the AM-3 becomes clearly evident. The influence of pH was also studied and it was verified that a pure phase of AM-3 was not obtained. The dynamic characterization of the membrane disclosed the existence of macro and/or meso-defects, since helium permeances decreased with increasing temperature. The consecutive heating and cooling cycles were able to enhance gas permeance values. Concerning modeling, the MS thermodynamic factors of Dubinin-Astakhov and Dubinin-Radushkevich isotherms were derived for the first time. These factors were successfully validated using published data for methane and ethane on a silicalite-1 membrane. The results showed they are simultaneously able to correlate pure gas permeation and are capable of predicting binary mixture separation, which is a much more important achievement in this field. It was also disclosed that the methane and ethane diffusivities are essentially independent of the fractional occupancy. The water permeation through zeolites 13X and 4A membranes was predicted using MS approach. The influence of temperature and solid loading upon the effective diffusivities was previously modeled, being possible to observe very distinct and non-linear behaviors for both materials. The results evidenced higher fluxes for zeolite 13X, due to its higher pore size. Moreover the flux decreases with increasing temperature, being the opposite verified for the zeolite 4A membrane.