Fractional destraction

Termed Fractional Destraction, the method fractionates residuum according to the solubility of its constituent components in a supercritical fluid. The novel aspect of the approach is the incorporation of a system to promote reflux of less-soluble components onto a packed bed. 

Figure 1. Sectional view of fractional destraction unit.

REPLICATE FRACTIONAL DESTRACTIONS OF TOLUENE OVERHEAD USING CYCLOHEXANE... 

Figure 5. Comparison of three replicate fractional destractions (reflux mode) of toluene overhead using cyclohexane.

TABLE II. Characterization of Materials Produced From the Cyclohexane Fractional Destraction of the Toluene Overhead Sample in Table I. 

In Figure 6 the GPC traces for the four fractions and for the residue from each of the three cyclohexane fractional destractions are overlaid. The trend to higher molecular weight distributions as the fractionation proceeded is evident, as well as the reproducibility of the fractionation process. Additional characterization of similar samples produced in the FDU is the subject of another paper (10). 

A two-step destraction procedure was developed to maximize the amount o T102 residuum brought overhead. In the first step, toluene is used in a manner similar to conventional supercritical extraction to produce a nearly ash-free material for subsequent fractionation. This first step is called the non-reflux mode because the column of the FDV is maintained at the same temperature as the extraction section. In the second step, called the reflux mode, the column and finger are heated to a higher temperature than the extraction section, which causes the density of the fluid to decrease as it travels up the column and thus promotes reflux of the less-soluble components. These two steps are described below. 

Figure 3 illustrates the difference between operation of the FDU in the non-reflux and the reflux modes with cyclohexane. This figure depicts the results in terms of the overhead concentration of residuum. The reflux mode data represent one of three replicate fractionations that were performed on the T102 toluene destraction overhead. Each point represents a 30-minute sample collection period. Owing to the limited quantity of toluene overhead produced, no non-reflux mode experiments were conducted using this material. From the earlier development work, however, several non-reflux mode experiments were performed on the T102 residuum sample from Run 242. The non-reflux mode data in Figure 3 were derived from one of these experiments and adjusted for comparability to the data from the reflux mode. The adjustment compensates for separator inefficiency and for residuum insoluble in supercritical... 

Other methods of fractionation with supercritical fluids are conceivable. One such possiblility could involve manipulation of the pressure during the destraction or upon subsequent separation of the fluid and residuum. The relative merits of such possibilities remain to be explored. Successful development of such technology will result in the ability to fractionate and characterize material currently intractable by conventional methods. 

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