Glass membranes for purification of aggressive gases Part II. Adsorption measurements and diffusion coefficient estimations

摘要

In the article Part I of this work durability and separation properties for several types of glass membranes in aggressive gas environment have been evaluated. A surface modified glass membrane (modified with (heptadecafluoro-1,1,2,2-tetrahydrodecyl) dimethyl chlorosilane, Pf-C10), proved to be the best choice with respect to stability, permeability and selectivity. For a better understanding of the gas separation taking place according to the governing mechanism, selective surface flow, the sorption and diffusion coefficients were investigated more closely. This is reported in the current paper for the gases Cl-2, HCl, R22 (CHF2Cl), He, H-2, N-2, CO, O-2, Xe, SF6 and CO2. Temperature and pressure range focused on were 1-4 bar and 30-45 degrees C, respectively, as these ranges were judged to be most interesting with respect to possible changes in the transport through the Pt-C10 surface modified glass membrane. The degree of selective surface flow, SSE relative to the Knudsen flow is also discussed in this work. When plotting the sorption coefficient versus degree of SSE, there seems to be two distinct patterns: The Cl-2, HCl and R22 (all containing chlorine and have high critical temperatures) are described by exceptionally high sorption coefficients, while the other gases are best described by an exponential fit. The heat of adsorption for Cl-2 and HCl was found to be comparable to the heat of condensation at the same pressure. It was assumed that the adsorption of these gases corresponds to the proposed "sliding liquid layer" flow described in the literature, while the exponential behaviour of the other gases correspond to the "site to site hopping" or 2D gas flow. (c) 2005 Elsevier B.V. All rights reserved.