Thermal Diffusion of Gases

When a gas mixture is subjected to a temperature gradient, diffusion occurs establishing a separation, 9. 

11 columns used to separate argon isotopes (Mound Laboratories USDOE, or one of its commercial heirs, Isotec Corp. or Cambridge Isotopes Inc.). The fractionations at different points in the cascade, and the feed, waste, and production rates are indicated (notice that 38 Ar concentrates in the middle of the cascade, which in this example is operated without a stripping section). Thermal diffusion dissipates large amounts of power and is primarily used for laboratory scale separations. Results illustrating the versatility of the method are given in Table 8.2. 

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The accuracy of this technique is to within 0.3%. Both tungsten and platinum wires are used with diameters of below 7 i im, more typically 4 ixm (for low-pressure gas measurements). Small diameters reduce errors introduced by assuming the wire is a line source (infinitely thin). The effect becomes significant for gases and increases with decreasing pressure. The thermal diffusivity of gases is inversely proportional to the pressure and thus thermal waves may extend to the cell wall. For these cases, a steady state hot-wire technique is used for which the design equation is ... 

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