Thermal gradient column

Figure 11. Effect of pressure on the top product yield during thermal gradient column fractionation of glyceride mixtures.

Figure 12. Dynamic glyceride composition profile in thermal gradient column during fractionation of glyceride mixtures.

The value for the heat of fusion of PPS, extrapolated to a hypothetical 100% crystalline state, is not agreed upon in the literature. Reported values range from approximately 80 J/g (19 cal/g) (36,96,101) to 146 J/g (35 cal/g) (102), with one intermediate value of 105 J/g (25 cal/g) (20). The lower value, 80 J/g, was originally measured by thermal analysis and then correlated with a measure of crystallinity deterrnined by x-ray diffraction (36). The value of 146 J/g was deterrnined independendy on uniaxiaHy oriented PPS film samples by thermal analysis, density measurement via density-gradient column, and the use of a calculated density for 100% crystalline PPS to arrive at a heat of fusion for 100% crystalline PPS (102). The value of 105 J/g was obtained by measuring the heats of fusion of weU-characterized linear oligomers of PPS and extrapolation to infinite molecular weight. 

MW separation factor Pore size Column i.d. Interstitial void size Thermal gradient... 

Cryofocusing traps are often used to interface purge and trap concentrators to gas chromatographs with capillary columns. The enhanced performance characteristics of the design provide a significant improvement over previous systems. The use of a sophisticated cyrotrap with a thermal gradient ensures that the sample will be trapped and injected with high efficiency. 

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 chromatographic oven for SFC should meet the same requirements as a conventional GC oven. The oven is usually designed such that thermal gradients between any two points in the oven area where a column is placed are less than 0.1 °C. 

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. 

Figure 24 Chondrite-normalized abundances of REEs in a wall-rock harzburgite from Lherz (dotted lines— whole-rock analyses), compared with numerical experiments of ID porous melt flow, after Bodinier et al. (1990). The harzburgite samples were collected at 25-65 cm from an amphibole-pyroxenite dike. In contrast with the 0-25 cm wall-rock adjacent to the dike, they are devoid of amphibole but contain minute amounts of apatite (Woodland et al., 1996). The strong REE fractionation observed in these samples is explained by chromatographic fractionation due to diffusional exchange of the elements between peridotite minerals and advective interstitial melt (Navon and Stolper, 1987 Vasseur et al, 1991). The results are shown in (a) for variable t t ratio, where t is the duration of the infiltration process and t the time it takes for the melt to percolate throughout the percolation column (Navon and Stolper, 1987). This parameter is proportional to the average melt/rock ratio in the percolation column. In (b), the results are shown for constant f/fc but variable proportion of clinopyroxene at the scale of the studied peridotite slices (<5 cm). All model parameters may be found in Bodinier et al. (1990). As discussed in the text, this model was criticized by Nielson and Wilshire (1993). An improved version taking into account the gradual solidiflcation of melt down the wall-rock thermal gradient and the isotopic variations was recently proposed by Bodinier et al. (2003).

Dehydration, or more generally, devolatilization of the oceanic crust is a process that combines continuous and discontinuous reactions in a variety of heterogeneous bulk compositions. In addition, within a vertical column—the sedimentary, mafic, and serpentinized peridotite layers— each experience a significant thermal gradient. The result is a continuous, but not constant, production of a fluid or melt, with the rate of mobile phase production generally decreasing with depth. Peaks in the volatile flux result from significant discontinuous reactions. However, despite the continuous fluid flux, trace elements may not necessarily be released continuously. 

Temperatures and thermal gradients need to be scalable. Larger columns tend to have larger radial thermal gradients than smaller columns. Ensuring that the fluid entering the column is at the oven temperature helps minimize thermal differences between column diameters. 

Figure 8 Thermal gradient supercritical fluid fractionation column.

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