Thermogravitational separation

The perpendicular flow methods have a number of important similarities and differences [6]. Most significantly, both thermogravitational separation and electrodecantation are limited to the separation of only two fractions (i.e., peak capacity equals two). These methods are thus restricted to preparative applications. Field-flow fractionation is the only method in the /r(+)cd class designed specifically for multicomponent analytical separations. We describe this method in greater detail below. 

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. 

In some cases, natural convective flow plays an integral role in separations. For example, thermogravitational (TG) columns rely on a combination of thermal convective flow and relative (selective) displacement by thermal diffusion. 

A few less common perpendicular flow techniques are not used to generate multicomponent separations, although they are inherently capable of doing so. These techniques include the thermogravitational column [21] and electrodecantation [22]. Here the extended path serves to enhance the selectivity generated by the field and to allow the collection of two enriched or separated fractions at the two extremes of the flow coordinate. 

Figure 19.14. Construction and performance of thermal diffusion columns, (a) Basic constraction of a thermal diffusion cell, (b) Action in a thermogravitational column, (c) A commercial column with 10 takeoff points at 6 in. intervals the mean dia of the annulus is 16 mm, width 0.3mm, volume 22.5 mL (Jones and Brown, I960), (d) Concentration gradients in the separation of cis and trails isomers of 1,2-dimethylcyclohexane (Jones and Brown, I960), (e) Tenuinal compositions as a fimction of charge composition of mixtures of cetane and cumene lime 48 hr, 50°C not wall, 29°C cold wall (Jones and Brown, I960).

Platten, J.K., Bou-Ah, M.M., Dutrieux, J.F. (2003). Enhanced molecular separation in inclined thermogravitational columns. Journal of Physical Chemistry B, 1(17, 11763-11767. 

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