Unmixing gases with sound

Thermoacoustic processes use an oscillating pressure field to create a thermal gradient, or vice versa. (See the article in Physics Today, July 1995, page 22 .) In a new twist, thermoacoustics is being used to separate the constituents of a binary gas mixture by mass. At Los Alamos National Laboratory, Greg Swift and his colleagues coupled speakers to a long pipe connecting two reservoirs and filled the system with a mixture of two gases. During the pressure maximum of an acoustic cycle, the heavier gas diffused to the viscous boundary layer near the pipe’s wall, while the lighter gas was carried along the pipe. In the second half-cycle, during the pressure minimum, the heavier gas was drawn out of the boundary layer and transported in the opposite direction along the pipe. The first separation experiments were done in Swift’s lab by Phil Spoor in 1998 with helium-xenon and helium-argon mixtures. Now, Drew Geller and Swift have built off-the-shelf speaker-pipe modules that, when hooked together, use a traveling acoustic wave to achieve arbitrary degrees of purity in the separation process. For example, with five 1/4-wavelength modules, they enriched a natural mixture of neon isotopes (9.2% ²²Ne, 90.8% ²⁰Ne) to 9.7% ²²Ne in one of the reservoirs. A concentration of 90% ²²Ne would call for 148 modules; 99% needs 232 modules. (D. A. Geller, G. W. Swift, J. Acoust. Soc. Am. 115, in press.)