Thermal diffusion separation performance

Figure 19.14. Construction and performance of thermal diffusion columns, (a) Basic construction of a thermal diffusion cell, (b) Action in a thermogravitationai column, (c) A commercial column with 10 takeoff points at 6 in. intervals the mean dia of the annulus is 16 mm, width 0.3 mm, volume 22.5 mL (Jones and Brown, 1960). (d) Concentration gradients in the separation of cis and trans isomers of 1,2-dimethylcyclohexane (Jones and Brown, 1960). (e) Terminal compositions as a function of charge composition of mixtures of cetane and cumene time 48 hr, 50°C hot wall, 29°C cold wall (Jones and Brown, 1960).

The separation performance of membranes with nonporous barriers is - because of the transport via solution-diffusion (cf. Section 2.2) - predominantly influenced by the polymer material itself. Therefore, the material selection is directly related to the intrinsic (bulk) properties of the polymer, but - as for porous membranes - filmforming properties, mechanical and thermal stability form the basis of applicability (cf. Section 2.3.2.1). The following characteristics should be considered ... 

Thermal diffusion of petroleum samples is carried out in the annular space defined by two coaxial cylinders whose surfaces are separated by distances of approximately 0.2 mm. These surfaces are maintained at different temperatures. Figure 3.15 demonstrates a laboratory setup34 used to practice this technique. Separation is performed by filling the annular space with the sample, then allowing the system to equilibrate for a period up to several days. In one configuration, the cylinder diameters are about 0.5 in., the annular spacing is about 0.0115 in., and the vertical length is 6 ft. The sample is injected at the center position and the product samples are taken off at a number of sample points, frequently ten of these, on the vertical axis after an appropriate time (3 to 10 days in practice) for the diffusional separation to take place. The samples, a few milliliters or less in size, are then analyzed by modem instrumental methods. 

Their separation performance was characterized by two parameters. F is In ypjyp, the overall separation between top and bottom when equilibrium is attained at total reflux. 0 is a parameter that was inferred from the rate at which product composition at total reflux approached equilibrium. The theory of the time dependent separation performance of a thermal diffusion column developed by Cohen [C6] and others shows that 0 is given by... 

The maximum separative capacity, A , and the power consumed per unit separative capacity, G/ max, given in the last two columns of Table 14.25 have been calculated from Abelson s parameters Y and 0 to permit comparison with the other processes for enriching uranium treated in this chapter. Because the thermal diffusion column operates with constant reflux ratio, its steady-state separation performance as an enricher is given by Eq. (14.237), expressed here in the form... 

Theoretical prediction of the constants Cj and Cs of the UF thermal diffusion column would be very difficult because of the great difference in properties of UF between the liquid at the cold wall and the dense gas at the hot wall. For other gases at pressures around atmospheric, at temperature differences between hot and cold walls small enough so that separation performance can be characterized by gas properties at a mean temperature, closed expressions can be given for the separation parameters Ci and C5. Quantities involved are... 

Equations for thermal diffusion column. Equations for the separation performance of a thermal diffusion colunm can be derived in somewhat similar fashion to the countercunent gas centrifuge of Sec. 5.5. The results will be summarized for the simplest case to treat theoretically, that of an aimular column in which the spacing d between the heated and cooled... 

Smaller components move into the faster flows farther away from the accumulation wall and therefore elute before large molecules. This normal elution mode is reverse to that observed in SEC. The separation can be performed with different force fields. In thermal FEE, the separation is based on thermal diffusion between temperature difference of top and bottom wall. In case of electric FEE, the separation is driven by charge differences and electrophoretic mobility of the molecules applying an electrical field. Another way to separate components depending on their density properties is sedimentation FEE, where a circular channel rotates and generates a gravitational force field. 

Thermal uniformity in the cold zone was found to be from 0.01 to 0.02 °C, and that in the hot zone was found to be better than + 0.5 °C vertically and + 0.1 °C horizontally. Thermal gradients near the solid-liquid interface were achieved in excess of 30 °C cm " in the crystal region and up to 20 °C cm" in the melt. The growth of crystals was performed in a sealed transparent silica ampoule, which has two rooms for As source and GaAs polycrystalline, respectively, separated by a quartz diffusion barrier. For details of the growth process the reader is referred to Ref. 43. In this experiment the As source temperature T. was systematically reduced by 2 °C at 3 h intervals from 620 °C to 614 °C. 

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