Thermal diffusion Clusius-Dickel column

Thermal diffusion, also known as the Ludwig-Soret effect [1, 2], is the occurrence of mass transport driven by a temperature gradient in a multicomponent system. While the effect has been known since the last century, the investigation of the Ludwig-Soret effect in polymeric systems dates back to only the middle of this century, where Debye and Bueche employed a Clusius-Dickel thermogravi-tational column for polymer fractionation [3]. Langhammer [4] and recently Ecenarro [5, 6] utilized the same experimental technique, in which separation results from the interplay between thermal diffusion and convection. This results in a rather complicated experimental situation, which has been analyzed in detail by Tyrrell [7]. 

Thermal diffusion column. Thermal diffusion remained a scientific curiosity until 1938, when Clusius and Dickel [C5] developed their thermal diffusion column, which made possible useful separations in simple equipment. In the Qusius-Dickel column the mixture to be separated is... 

The basic construction of a horizontal thermal diffusion cell is sketched in Figure 19.14(a). When gases are to be separated, the distance between the plates can be several mm for liquids it is a fraction of a mm. The separation effects of thermal diffusion and convection currents are superimposed in the equipment of Figure 19.14(b), which is called a thermogravitational or Clusius-Dickel column after the inventors in 1938. A commercially available column used for analytical purposes is in Figure 19.14(c). Several such columns in series are needed for a high degree of separation. 

Convective flow is used in both the thermogravitational (Clusius-Dickel) column and in electrodecantation. In the thermogravitational system, one wall of the channel is heated or, alternatively, a hot wire is placed along the axis of the channel. The fluid at the cold surface then tends to sink relative to that at the hot surface. Simultaneously, thermal diffusion (Section 8.8) causes different levels of enrichment in the hot and cold regions of the channel. The enriched solutes then move up and down the channel at a rate depending upon their distribution between hot and cold regions. In binary... 

The Clusius-Dickel column is shown schematically in Figure 2. A wire is mounted at the axis of a cylinder. The wire is heated electrically and the outer wall is cooled. This sets up a radial thermal gradient which leads to a thermal diffusion separation in the x direction. As a result of the radial temperature gradient, a convection current is established in the gas, which causes the gas adjacent to the hot wire to move up the tube with respect to the gas near the cold wall. The countercurrent flow leads to a multiplication of the elementary separation factor. For gas consisting of elastic spheres, the light molecules will then concentrate at the top of the column, while the heavy molecules concentrate at the bottom. The transport theory of the column has been developed in detail (3, iS, 18) and will not be presented here. In a later section we shall discuss the general aspects of the multiplication of elementary separation processes by countercurrent flow. 

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