Separation of Hydrocarbon Isomers by Thermal Diffusion

Abstract

RECENT work on the transport properties of gaseous hydrocarbons¹ has suggested that highly flexible molecules undergo ‘wrestling’ intermolecular collisions, involving less persistence of forward velocity than collisions between other molecules. Since a straight-chain paraffin molecule is always more flexible than a branched chain, this effect gives rise to differences in the transport phenomena shown by isomeric hydrocarbons. Such differences might be expected to enable separation of a mixture of isomers by thermal diffusion. One view of the mechanism of separation by thermal diffusion is that those molecules which move faster when leaving the hot zone push the slower molecules towards the cold zone². It is, in any event, clear that thermal diffusion must depend, not only on the mass and diameter of the molecules, but also on the nature of the inter-molecular collisions. If a mixture of an n-paraffin with a branched-chain isomer is subjected to thermal diffusion, the former, which is the more flexible, will leave the hot zone with lower forward velocity, and would thus be expected to concentrate in the cold zone.