(253b) Mixture Permeability, Solubility, and Diffusivity in Vapor Selective Polymers

Membrane separation technology has
recently emerged as a potential alternative technique to remove higher
hydrocarbons (C₃₊) from natural gas¹. For economic reasons, membranes for this application should be organic vapor selective materials such as poly(dimethylsiloxane) (PDMS) or ultra-high free volume polymers such as poly(1-trimethylsilyl-1-propyne) (PTMSP). These polymers, often called solubility selective polymers, sieve penetrant molecules based strongly on relative penetrant solubility in the polymer.

Traditionally, membrane separation
performance is estimated from pure gas permeation measurements. However, this
approach is not accurate in certain cases, especially in glassy polymers. For
example, the presence of CO₂ in cellulose acetate induces membrane
plasticization and decreases the CO₂/CH₄ selectivity as
the CO₂ partial pressure increases². In some cases, the selectivity of a vapor over a permanent gas in a mixture is actually higher than that estimated from pure gas measurements, which is attributed to the high solubility of vapor in the polymer, which partially blocks the permeation pathway of the smaller, less soluble permanent gas³.

To accurately estimate the membrane
separation performance, the mixed gas studies are required. This paper presents
the n-C₄H₁₀/CH₄mixed gas permeability, solubility, and diffusivity in vapor selective
polymers at various temperatures from -20^oC to 50^oC. The
dilation isotherms are also reported to complement the mixture sorption data.

In rubbery PDMS, the presence of
larger, more condensable n-C₄H₁₀
considerably improves the CH₄ solubility, permeability, and
diffusivity in the polymer. On the other hand, the n-C₄H₁₀
sorption and transport properties in mixtures are unaffected by the presence of
CH₄ and are similar to those observed under pure gas conditions. The
overall n-C₄H₁₀/CH₄ permeability selectivity
in mixtures increases as n-C₄H₁₀ activity
increases and as temperature decreases, and the selectivity is lower than that
estimated from pure gas measurements. The overall selectivity in PDMS is mainly
governed by solubility selectivity. This report represents the first combined
presentation of gas mixture permeability, solubility, and diffusivity in PDMS,
which is the most widely used vapor separation polymer.

Another polymer whose properties
will be discussed is poly(1-trimethylsilyl-1-propyne) (PTMSP), an ultra-high
free volume, solubility selective, glassy polymer. These studies help illustrate
the fundamental mechanism of competitive permeation, sorption, and diffusion in
glassy polymers.

(1)        Baker, R. W. Membrane Technology and Applications; McGraw-Hill: New York, 2000.

(2)        Lee,
S. Y.; Minhas, B. S. AICHE Symposium Series 1970, 84,
93-101.

(3)        Pinnau,
I.;  Casillas, C. G.;  Morisato, A.; Freeman, B. D. Journal of Polymer
Science: Part B: Polymer Physics 1996, 34, 2613-2621.