Strip purification scheme, solvent

Dual-Solvent Fractional Extraction As discussed in Commercial Process Schemes, under Introduction and Overview, fractional extraction often may be viewed as combining product purification with product recovery by adding a washing section to the stripping section of a standard extraction process. In the stripping section, the mass transfer we focus on is the transfer of the product solute from the wash solvent into the extraction solvent. If we assume dilute conditions and use shortcut calculations for illustration, the extraction factor is given by... 

In the Hoechst process, for example, hexane is used as the diluent (108,109). Hexane, ethylene, alpha-olefin, catalyst components, and hydrogen are continuously fed into a stirred reactor for polymerization. The slurry is then transferred into a smaller reactor for post-polymerization, after which the total charge is separated by a centrifuge into a liquid stream (which is returned to the initial reactor) and solid polymer. The wet polymer is steam-stripped from the solvent, dried, and pelletized. The stripped hexane is purified and recycled. Although stirred tanks are most common, loops can also be used in this fashion. In some schemes, a portion of the recycle diluent from the centrifuge is returned to the reactor, and a portion is fed to recycle purification for wax removal. This step removes some of the lowest molecular weight pol5mier, which dissolves in the diluent. 

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