Paraffins, chromatographic separation

Figure 6.2 Chromatographic separation of rt-paraffin from non-rt-paraffin.

The usefulness of retention data from gas chromatography can be enhanced by reporting standardized times or retention indices (RI), which involves expressing retention in terms of a ratio of the retention time (RT) of an analyte to the RT of a standard. Retention scaling based on the Kovats (1965) method requires the chromatographic separation of a homologous series of normal paraffins, esters, and others, producing an index that is the ratio of the RT of an analyte minus the RT of a less retentive standard to the RT difference between... 

Dispersion forces cannot be explained by the magnetic analogy nor by conventional electrostatics. They are weak forces that exist even in mon-oatomic gases that are symmetrical and nonpolar. It is believed that at any particular instant this symmetry is somewhat distorted due to the motion (and position) of the electrons of a given atom, which produces a momentary polarity. This momentary polarity can attract and be attracted by a similar polarity in a neighboring atom or molecule in such a way as to produce a net attraction. As we have already seen, such inductions depend on the polarizability of the molecule or atom. Dispersion forces will always be possible between molecules, but they are the only forces between nonpolar hydrocarbons such as the alkanes. For this reason alkanes are often chosen as the ideal molecules for study or for use as standards. An example of a chromatographic separation in which the only forces are dispersion forces would be the GC separation of alkanes on squalane, a branched paraffin. 

Figure 4. Chromatographic separation of mono-methylparaffin from N-Paraffin depleted Kerosene using silicalite adsorbent, SO/SO n-C6/cyclohexane desorbent, and iso-octane pre-pulse...

The process was demonstrated in a simulated continuous counter-current chromatographic separation pilot plant. Both the primary method of operation and the pre-pulse technique were demonstrated, with the pre-pulse technique showing improved recoveiy. Using commercial n-paraffin depleted kerosene (Molex Raffinate) feedstock we routinely demonstrated the ability to achieve better than 90% mono-methyl and normal paraffin purity with greater than 70% recovery of mono-methyl paraffins. 

Waldi [181] has separated the esters on silica gel G, using carbon tetrachloride-chloroform (95 + 5). Kauemann et al. [96] have used either tetraUn-hexane (25 + 75) or (50 + 50) for adsorption TLC or paraffin/methyl ethyl ketone-acetonitrile (70 + 30) as reversed phase partition system. All the esters from acetate to stearate yield thus a series of spots like a string of beads. Acetic acid is also a suitable mobile phase, as in PC [125]. Unsaturated fatty acid esters can be differentiated from the saturated esters only with difficulty in this way success is more likely when an adsorption chromatographic separation is carried out... 

In the early 1970s, Union Oil developed and patented a chromatographic system based on the principle of a simulated moving bed (SMB) [6-8]. A schematic of a SMB unit is shown in Figure 1.4. Streams of the mobile phase (the desorbent ) and of the feed to separate are continuously injected into the column while streams of the less retained (the raffinate ) and the more retained components (the extract ) are continuously withdrawn, all at constant flow rates. The rotary valves switch periodically the positions in the columns where these streams enter or exit. The operation of SMB imits is discussed in detail in Chapter 17. Manufacturing facilities have been built and are operated for the fractionation of various petroleiun distillates, for example, the selective separation of p-xylene, o-xylene and ethylbenzene from the C7-C8 aromatic fraction of light petroleum reformates, the separation of olefins from paraffins in feed mixtures of hydrocarbons having 10 to 14... 

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