| As the chemical industries strive to produce higher quality chemicals and specialty chemicals the limits of current separation processes are tested. One of the major options being explored for meeting these increased separation demands is the use of membranes for separations. Many types of materials, both polymeric and inorganic, are being considered for use as membranes. Historically, only polymeric membranes have been used but recently the development of inorganic membranes, such as porous ceramics, has provided an alternative to the polymeric membranes. Another material that has characteristics desirable in a membrane is porous silicon.; A procedure for production of a porous monocrystalline silicon membrane and the evaluation of its ability to perform gas-gas separations is presented. Characterization of the pore size distribution of porous silicon membranes was done by the BET-BJH isothermal nitrogen adsorption technique. This analysis demonstrated the ability to control the pore size distribution and cutoff in the 3 to 20 nm range. The permeation rates of helium, methane, nitrogen and carbon dioxide at membrane pressure drops up to 7 MPa were evaluated for several membranes formed at different etching conditions. The permeability coefficients for all tests remained constant, indicating that gas flow through the membrane was by one of the molecular flow mechanisms (Knudsen or slip). Additionally, the experimental data was compared to the results predicted by a computer program incorporating the pore size distributions for each of the samples and Knudsen and slip type flow. As a test of the membranes ability to perform separations, two gas mixtures, a helium-carbon dioxide mixture and a methane-nitrogen mixture, were separated by each of the samples with an average separation efficiency of 50 percent of the ideal separation possible by pure molecular flow in pores. Finally, the results of the extension of the porous monocrystalline silicon membrane production procedure to the production of a porous polycrystalline silicon membrane is presented. |
