Continuous extractors solvent heavier than water

Contimious liquid extraction techniques are used when the sample volume is large, the distribution constant is small, or the rate of extraction is slow. The efficiency of extraction depends on many factors including the viscosity of the phases, the magnitude of the distribution constant, the relative phase volumes, the interfacial surface area, and the relative velocity of the phases. Numerous continuous extractors using llghter-than-water and heavier-than-water solvents vee been described [3,2 7,42,73,74]. Generally, either the ligi Pr or heavier density... 

Figure 8.6 Apparatus used for saeple preparation involving solvent extraction. A, heavier-than-water continuous liquid-liquid extractor B, pressurized Soxhlet extractor for use with supercritical fluids C, Kudema-Danlsh evaporative concentrator 0, autonated evaporative concentrator.

Continuous Liquid-Liquid Extractor. 1 L sample capacity, suitable for use with heavier than water solvents. 

Refening to Figure 10-4, the solvent is placed in the flask. A, and heated. The vapor rises to B and then up to C, where it is condensed. The Figure 10-4. A continuous solvent-liquid cannot get back in the flask because of the seal at B and runs into heavier-than-water extractor. 

Methods of Liquid—Liquid Extraction. Most methods described for LLE are the batch type (12, 13). This is surprising since continuous extraction has an obvious advantage over serial extraction because larger sample volumes are extracted. Kahn and Wayman (25) and Goldberg et al. (26) have described continuous LLE systems for lighter and heavier than water extractions, respectively. Such extractors are used with multiple chambers and internal solvent recycling. Kahn and Way-man successfully recovered chlorinated hydrocarbon pesticides with 96-100% efficiency in a 3-chamber system with petroleum ether (25). An average residence time of 45 minutes per chamber at a 1 1 solvent to water phase ratio was used on a 20-liter sample of less than 400 ppb concentration. 

In addition to solid-liquid collection techniques, in situ sampling methods were developed that contemporarily performed analyte collection and extraction. These systems utilize either sequential or continuous LLE. An example is provided by the aqueous phase liquid extractor (APLE) of Clement and coworkers [22] designed to sample surface water near dump sites where PCDDs and PCDFs were present at level of pg/L. The APLE was capable of extracting up to 200 L water in a single batch process. A spray-bar on the top dispersed a heavier-than-water solvent (methylene chloride) as a fine spray across the surface of the water sample, pushed into the system by a submersible pump. Efficiency in extraction is ensmed by continuous recirculation of the solvent. Devices like the one described avoid the problem of transporting to the laboratory large volumes of sample, but they often remain cumbersome and difficult to be transported. 

The ratio of the amount extracted to the amount remaining is called the distribution ratio. There are many situations in which the distribution ratio is low, which would require large amounts of solvent if separatory funnels were used exclusively. This can be very expensive in both chemical and labor costs, and the additional cost of solvent disposal is now often prohibitive. Continuous extractors, in which a small volume of solvent is used to extract a portion of the compound, then evaporated, condensed, and used again, are an ideal solution. This process can be repeated for days if necessary and at the end, there is only a small volume of solvent to remove and dispose of Continuous extractors that involve solvents both heavier and lighter than water are discussed in Chapter 10. A widely used apparatus for continuously extracting components from solids is the batch extractor developed by Soxhiet. The sample is placed in a porous paper thimble and then placed in a horizontal tube with a closed bottom. The extraction solvent is dripped onto the top of the solid, percolates through it, and siphons off after a short time, the process is repeated as often as necessary. Soxhiet extraction is covered in Chapter 10. 

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