gas separation

gas permeability

free volume

Trends in polymer science

Polymeric membranes for the separation ofsupercritical gases are typically glassy and achieve high selectivity by a size-sieving mechanism. This property permits some polymers to be more than 100 times more permeable to hydrogen (kinetic diameter = 2.89A) than to methane (kinetic diameter = 3.8A), despite the fact that hydrogen solubility is markedly lower than methane solubility. The permeation properties of glassy polymers for the separation of supercritical gases follow an immutable trade-off relationship : polymers that are more permeable tend to be less selective and vice versa. In contrast, new gas. separation applications, such as removal of higher hydrocarbons from natural gas for dew-point and heating-value control, removal of higher hydrocarbons from hydrogen streams, and separation of valuable or toxic organic vapors from air or nitrogen streams, generally require membranes that are more permeable to the larger, more condensable components in a mixture (i.e. vapors) than to smaller supercritical gases such as hydrogen or nitrogen. Membrane materials that are highly permeable to the larger components in a mixture must derive high selectivity from the relative difference in solubility of the components in the membrane matrix. That is, polymers with little size-sieving ability are useful for such separations. Rubbery polymers, such as polydimethylsiloxane, and ultra-high free volume, 'superglassy'polymers, such as poly(1-trimethylsilyl-1-propyne), are examples of two different classes of solubility-selective polymers for these new applications. These polymers are more permeable to large molecules than to smaller, supercritical gases. Interestingly, polymers exhibiting high permeability to organic vapors also exhibit high selectivity for these components in mixtures with supercritical gases. This behavior leads to the compelling conclusion that well-designed materials for these separations will have both high permeability and high selectivity.

Separation of gases using solubility-selective polymers