Paraffins vapor-phase extraction

In both the liquid- and vapor-phase extraction processes, the kerosene feed is typically prefractionated to narrow the feed to the desired four-carbon number range (either Cjo-Cu or C11-C14). This heartcut is hydrotreated to remove the majority of kerosene contaminants that may compromise the performance of life of the adsorbent or subsequent quality of the LAB or LAS properties. In some process flow schemes, the fractionation into the discrete n-paraffin cuts may be deferred until after the extraction process. 

The liquid-phase extraction process operates at 177°C and 24.6 kg/cm (g), whereas the vapor-phase extraction process operates at 310-350°C and 2.2 kg/cm (a). Most importantly, the n-paraffin product from either process can surpass 99% purity and attain paraffin recoveries exceeding 95%. 

The linear paraffins used for LAB production are obtained by liquid- or vapor-phase extraction from kerosene followed by fractionation to obtain the C10-C13 cut or narrower C-chain... 

The solvent is regenerated in a second column, where the aromatics are reextracted by a paraffinic hydrocarbon, preferably the same as the reflux compound, namely butane, usually at temperature of about 50°C and under sufficient pressure to ensure that the butane does not pass into the vapor phase. The aromatics are then recovered by simple fractionation of the paraffinic extract While the reextraction solvent is directly recycled, they undergo prior distillation before their final separation, and the distillate, essentially consisting of paraffins and benzene, is returned with the feedstock to the initial extraction stage. Small fractions ofDMSO (1 per cent of throughput) dissolved in the hydrocarbon phases are recovered by water washing, and concentrated under vacuum. 

For separations not now possible by other methods In distillation, where the vapor phase is created from the liquid by addition of heat, the vapor and liquid are necessarily composed of the same substances and are therefore chemically very similar. The separations produced then depend upon the vapor pressures of the substances. In liquid extraction, in contrast, the major constituents of the two phases are chemically very different, and this makes separations according to chemical type possible. For example, aromatic and paraffinic hydrocarbons... 

When aromatics are to be extracted a second, high-temperature hydrogenation step is needed. This operates in the vapor phase and converts most of the olefins to paraffins and removes the sulfur compounds. Aromatics remain unchanged. 

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