Mass Transport in Thin Supported Silica Membranes

In this thesis multi-component mass transport in thin supported amorphous silica membranes is discussed. These membranes are micro-porous, with pore diameters smaller than 4Å and show high fluxes for small molecules (such as hydrogen) combined with high selectivities for these molecules with respect to larger ones. The chemical and thermal stability of silica membranes are favourable compared to those of organic membranes. These properties make silica membranes interesting candidates for separation of permanent gases in chemically and thermally aggressive environments. The field of membrane science is briefly introduced, with emphasis on the relevance of inorganic membranes for the chemical process industry. Several aspects of supported amorphous micro-porous silica layers are discussed, followed by the project objectives and thesis outline. A concise discussion on the most relevant characteristics of irreversible processes is presented, leading to three well-known macroscopic theories for the description of mass transport behaviour of multi-component mixtures; these include the classical (Onsager) theory of irreversible processes (the phenomenological equations), the Maxwell-Stefan theory and the generalised law of Fick. The interrelation of these theories is discussed. Definitions for the appropriate driving force (diffusion vector), fluxes and different diffusion coefficients are given. Transport mechanisms occurring in macro-and meso-porous media are briefly discussed. The presence of the porous medium is accounted for by introducing two structure parameters, K 0 and B 0 , which can also be expressed in terms of a flow average pore size d p , and the ratio of the porosity ε and tortuousity τ. The structure parameters are independent of temperature and properties of the permeating gas. For two different α-alumina support materials (AKP-30 and AKP-15) the flow average pore sizes are 0.09 µm and 0.16 µm, respectively. Both materials have a tortuousity of approximately τ = 3.2. The transport resistance of a γ-alumina layer is negligible viii compared to that of an α-alumina support. For counter diffusion of H 2 and N 2 , at 30ºC and atmospheric pressure, calculations using the Dusty Gas Model are in fair agreement with experiment. External gas-membrane resistance is negligible for the range of conditions covered by the experiments. The macroscopic transport theories, and an entirely microscopic theory, are applied to describe the transport of a mixture of mobile species in a micro-porous material obeying ideal Langmuir sorption behaviour. If a unary system, i.e., a single mobile species, is considered the chemical diffusion coefficient is independent of concentration. In this …