Separations by Thermal Diffusion

2 Soret Coefficients for Aqueous Polyethylene Glycol Solutions 

The flow of a solute in a nonisothermal solution is caused by the solute concentration gradient and by the temperature gradient 

The separation of polymers due to thermal diffusion may be quite large. For example, the thermal diffusion ratio for dilute solutions of polystyrene in tetrahydrofuran is around 0.6 K1. This indicates that the change of polystyrene concentration per degree is 60%. The type of solvent and polymer pair may have a considerable effect on both the thermal diffusion ratio and the thermal diffusion coefficient. 

The segmental model predicts the DT independent of the polymer molecular weight, and is given by 

Schimpf and Semenov (2004) developed the following model for the thermal diffusion coefficient for dilute solutions of flexible polymers 

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The foremnner of the modern methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76-4]) to remove the resinous components (51). Further modifications and preferences led to the development of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by electrolytic methods (61), and by repeated solvent fractionations (62,63). 

TABLE 19.10, Examples of Separations by Thermal Diffusion (a) Hydrocarbon Isomers... 

We have observed that the direct separation of components in static systems is commonly carried out with both electrical and sedimentation fields. By contrast, we rarely hear of direct (Sc) separations using thermal diffusion. At present, thermal diffusion is nearly always employed with flow in such a manner that the flow multiplies the separability [46]. It is reasonable to inquire if an opportunity has been overlooked in the neglect of static separations by thermal diffusion. In obtaining an answer to this question, we develop an approach that can be used for judging the potential effectiveness of other uncommon gradients and fields in direct Sc separations as well. 

Example 7.2 Separation by thermal diffusion Consider two vessels connected by a thermally insulated conduit. The system is filled with a solution of hydrogen and nitrogen. The hydrogen mass fraction is wu = 0.2. Estimate the difference between the mass fraction of the components in the two vessels at stationary state when one of the vessels is at 200 K and the other at 370 K. 

Table 14.24 Examples of isotope separation by thermal diffusion...

Sections 0 and 0 demonstrate that almost any desired isotope can be separated by thermal diffusion or electromagnetic methods on a small scale. For large-scale separation, enormous amounts of material must be processed and energy consumption, cascade design, and other such factors become important. Tables 51.3 and 51.4 list theoretical upper limits of single-stage separation factors for H/D, and for several different separa-... 

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