Continuous extractors solvent lighter than water

Continuous extractors using solvents lighter than water follow the same principles as described in the above section, except that the apparatus must be modified to handle the less dense extraction solvent. Figure 10-7, p. Ill, shows several types of apparatus Refer to the right apparams. The sample, usually dissolved in water, is placed in the reservoir at A to fill it to less than half full. The extraction solvent, usually diethyl ether, is placed in flask B. Flask B is heated by a heating mantle with the temperature controlled by a variable transformer. When the solvent is heated, it evaporates and goes up through the apparatus to the condenser... 

Figure 10-8. Microscale continuous extractor for solvents lighter than water. (Courtesy - Lab Glass Inc., Vineland, NJ )...

Figure 10-7. Continuous extractors using solvents lighter than water.

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. 

This extractor continuously cycles the sample steam distillate through an immiscible lighter-than-water solvent such as hexane. Routinely, large volumes of sample (4000 mL) can be distilled/extracted in a 5 mL volume of solvent. Emulsions and interferences that are commonly observed in an extraction are reduced or eliminated entirely. Normally, species that can be steam distilled can also be gas-chromatographed. This results in a cleaner extract and reduces the build-up of residue in the injector of the gas chromatograph. A prediction of whether or not a particular compound will steam strip from water can be made on the basis of its relative volatiUty to water ... 

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. 

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