may be added to the process equipment at either an end or to one of the center stages of the equipment liquid liquid extractions are typically run by any of three routes mixer settler systems liquid liquid extraction tower systems or centrifugal contacters the particular equipment used in this process may be any of several types of mixer settlers any of several types of column configurations or any of several types of centrifugal contacters the separation of the two phases may be conventional gravity settling or by any of several enhanced separators such as enhanced gravity settlers or centrifugal separators in selecting the solvents to be used in any of these systems it is necessary for the solvent pair to have specific properties that allow the separation to be made for commercial interest it is necessary that several additional properties be obtained in the solvent pair thus while some general guidelines do exist for the selection of the solvents in a system involving crude pcnb containing hcb and other impurities these guidelines are at best difficult to apply because these particular compounds have very similar structural properties it is thus quite unexpected that this purification can be accomplished at all with a liquid liquid solvent extraction system in which one of the solvents is a polar solvent and the other is a nonpolar solvent the.