(210b) Models and Experiments on the Preparation of Silicon Carbide Microporous Membranes

Inorganic membranes have been attracting attention for industrial separations, because they work at high temperatures, where polymeric membranes are not capable of functioning [1]. As a result, in recent years there has been a considerable effort for producing high temperature ceramic (alumina, silica, and zeolite) and metal (Pd, Ag, and their alloys) membranes, which exhibit improved properties. They have, so far, all proven unstable, however, for high temperature applications, particularly in the presence of steam. A new type of membrane, which promises to overcome such difficulties, is made of SiC [2], a promising material, with high fracture toughness, good thermal shock resistance, and capable of withstanding high temperatures and corrosive environments. In our previous study we reported using chemical vapor infiltration (CVI) or chemical vapor deposition (CVD) techniques to produce SiC membranes [2]. As a precursor we utilized (C₃H₇)₃SiH (tri-isopropylsilane or TPS) to form SiC on porous alumina substrates. The membranes were characterized for their permeance, selectivity, and hydrothermal stability. Depending on the preparation conditions, the He permeance ranged from 8.06x10e-8 mol/Pa•Sec•m² to 1.72x10e-6 mol/Pa•Sec•m² with a He/N₂ selectivity from 4 to larger than 100. In this paper we report results of ongoing investigations on the preparation of microporous SiC membranes using CVD/CVI of TPS. For fabricating the membranes we utilize porous SiC disk and tubular supports [3]. Using SiC substrates to prepare the microporous membranes minimizes the difficulties that may arise due to the potential mismatch in the thermal expansion coefficients between the membrane layer and its' support. The focus of our work has been on understanding the CVI/CVD process, and the effect of the various parameters that determine deposition and the characteristics of the resulting membranes. A fundamental understanding of the various physicochemical processes that take place during formation of the SiC film is of vital importance in order to determine and control the final characteristics of the resulting membranes. Our work combines theory and experiments in order to study the various phenomena that take place during membrane formation. We have developed a comprehensive model of the membrane fabrication process which accounts for the reaction, fluid mechanics, and heat and mass transfer processes. The model is solved using the finite-element methods. It predicts teh membrane characteristics (e.g., permeances, porosities), as a function of deposition time and dimension.

Reference

[1] Sanchez, J.; Tsotsis, T. T.; 2002. Catalytic membranes and membrane reactors. Wiley VCH.

[2] Ciora, R.J.; Fayyaz, B.; Liu, P.K.T.; Suwanmethanond, V.; Mallada, R., Sahimi, M.; Tsotsis, T.T.; Chem. Eng. Sci. 2004, 59, 4957-4965.

[3] Varaporn Suwanmethanond, V., Goo, E., Liu, P.K.T., Johnston, G., Sahimi, M., Tsotsis, T.T., Ind. Eng. Chem. Res. 2000, 39, 3264-3271.