Extraction with solvents, continuous technique

With solid materials (such as foods) the initial step in sample preparation involves Soxhlet extraction. With this technique a solid sample is placed in a permeable thimble above a flask of solvent but below a cold water condenser (Figure 8.2). The extraction solvent is boiled, condenses and runs down into the part of the apparatus containing the sample. As the container fills, the sample is extracted with solvent. The apparatus is designed such that as the solvent level reaches a certain height it siphons back into the flask at the bottom along with any extracted residue. The process is then continuously repeated often for many hours. The end result is a... 

Applications Simultaneous steam distillation-solvent extraction has been in use for many years [163]. Steam distillation combined with continuous liquid extraction is an efficient technique for the removal and isolation of volatile compounds in various matrices (environmental,... 

Reliable mechanistic conclusions require high intrazeolite yields that account for the majority of the substrate mass balance. This can be a challenge because of the small-scale reactions often conducted for mechanistic studies. In addition, rapid removal of the products from the zeolite, and/or low conversions to decrease residence time, is occasionally necessary because of the sensitivity of the reaction products to the zeolite environment.44,45 Intrazeolite products are generally recovered by extractive techniques from either the intact zeolite, or from a mixture formed after mild digestion of the zeolite. Polar solvents such as tetrahydrofuran or acetonitrile coupled with a continuous extraction technique is in particular an effective means to remove polar products with an affinity for the interior of the zeolite.44 Zeolite digestion with mineral acids, in order to liberate the products, must be conducted with care in order to prevent acid catalyzed product decomposition or reaction.46,47... 

Separation is carried out using various techniques, depending on the nature of the compounds. For nonionic species the solubilities of the compounds are generally low and similar, and it has been necessary to use either fractionation by continuous extraction with low boiling solvents (36) or thin-layer chromatography (58, 60). The first method has been used with air-sensitive compounds, whereas the second has been applied only to air-stable substances. In both cases, it is possible to separate only limited amounts of compounds, whose characterization is, therefore, carried out using particular techniques such as mass spectroscopy (58-60). 

As reported in a lot of reviews, extraction with supercritical solvents has very promising commercial potential. Until now the commercialization has been restricted mainly to batchwise extraction of solids with carbon dioxide (e.g. decaffeination of coffee and tea, extraction of hops). Although laboratory and pilot-plant experiments have indicated very good economics for continuous extraction of liquids with carbon dioxide and other gases, so far this technique has been applied industrially only for the production of 2-butanol by Idemitsu Petrochemical Corp. in Japan. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

SFE and PLE are continuous extraction techniques that have enhanced mass transfer rates due to an increase of the concentration gradient between the phases, whereas Soxhlet is a batch technique, though it can be considered a continuous technique because freshly distilled solvent is contacted with the solid matrix with each cycle. 

Extraction of dissolved hydrocarbons from sea water Most of the methods used are based on the liquid—liquid extraction of lipids by a solvent various solvents such as CH2CI2, CHCI3, CQ4, nCeHn, nCsHi2, petroleum ether and ethyl acetate have been tested for the recovery and employed (Table I). Recovery of a synthetic mixture of saturated hydrocarbons by extraction with petroleum ether and ethyl acetate is about 97% (Jeffrey et al., 1964). Blumer (1970) demonstrated that the extraction of dissolved lipids at pH 2 using pentane as the solvent was quantitative after four extraction steps. Parker et al. (1972), using a technique for the continuous extraction from sea water with hexane, found in control experi-... 

When a product is very soluble in water, it is often difficult to extract using the techniques described in Sections 12.4 12.7 because of an unfavorable distribution coefficient. In this case, you need to extract the aqueous solution numerous times with fresh batches of an immiscible organic solvent to remove the desired product from water. A less labor-intensive technique involves the use of a continuous liquid-liquid extraction apparatus. One type of extractor, used with solvents that are less dense than water, is shown in Figure 12.15. Diethyl ether is usually the solvent of choice. 

---