Abstract
A novel variant of continuous relative and absolute counter-current distribution (CCD), called forced-flow multi-phase liquid extraction (FFMLE), is introduced as a new separation method. This method is a special case of liquid—liquid extraction requiring three immiscible liquid phases and movement of inert gas bubbles through the three liquid phases as the fourth constituent of the multi-phase system. On using three immiscible liquid phases, two basic situations prevail: two phases may be stationary and one mobile, or one stationary and two mobile. Among the five basic possibilities, four belong to relative CCD in the ascending and descending modes. The extraction columns are filled with the immiscible stationary phase or phases and mobile phase or phases are forced through the stationary phase or phases by means of pumps. For the fifth possibility, the two mobile phases move in opposite directions as very small droplets. This method constitutes absolute CCD, and results in exhaustive and rapid extraction. The efficiency of FFMLE is achieved not just because the mobile phases move in opposite directions, but is also a result of three other effects that are discussed. The different types of relative and absolute CCD methods may also be varied by connecting two or more columns together. A typical three-phase system can be obtained on mixing n-hexane, acetonitrile and water as the basic ternary system, with one of diethyl ether, dichloromethane, toluene or chloroform as the fourth solvent (auxiliary solvent). A virtual solvent strength ( S v) value is introduced for characterization of the three-phase liquid system. The solvent and phase ratio of some three-phase systems are given for different S v values and selectivity. The method can be used for the rapid purification of various substance classes occurring as complex matrices. Experiments carried out with a laboratory extraction column (3.5 1) show that the separation of ca. 10 g of raw extract into two or more fractions requires less than 1 h.
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