Extraction, advantages single-solvent

When the products are partially or totally miscible in the ionic phase, separation is much more complicated (Table 5.3-2, cases c-e). One advantageous option can be to perform the reaction in one single phase, thus avoiding diffusional limitation, and to separate the products in a further step by extraction. Such technology has already been demonstrated for aqueous biphasic systems. This is the case for the palladium-catalyzed telomerization of butadiene with water, developed by Kuraray, which uses a sulfolane/water mixture as the solvent [17]. The products are soluble in water, which is also the nucleophile. The high-boiling by-products are extracted with a solvent (such as hexane) that is immiscible in the polar phase. This method... 

A single SFE/ESE instrument may perform (i) pressurised C02 (SFE), (ii) pressurised C02/modifier and (iii) pressurised modifier (i.e. ASE /ESE , organic solvent) extractions. The division between SFE and ASE /ESE blurs when high percentages of modifier are used. Each method has its own unique advantages and applications. ESE is a viable method to conduct matrix/analyte extraction provided a solvent with good solvating power for the analyte is selected. Sample clean-up is necessary for certain matrix/analyte combinations. In some circumstances studied [498], SFE may offer a better choice since recoveries are comparable but the clean-up step is not necessary. 

We do not consider on-site, mobile incinerators for the following reason. Mobile incinerators are currently licensed to burn only PCB contaminated liquids, but EPA officials anticipate final approval to bum PCB contaminated solids by 1986 [ 4 ]. Hence, we cannot, concentrate the extract any further if a mobile incinerator were to be used. Tbe mobile incinerators appear to have a capacity of about 3.6 x 10 kg/day [5 ]. To incinerate the concentrated extract produced by solvent extraction process, in excess of 2700 days of continuous operation of a mobile incinerator would be required. To complete the incineration in a single working season (see below), many mobile incinerators would be required. The cost of a mobile incinerator has been estimated to be about 2 million dollars [6 ]. Applying the criteria of completing the cleanup in one season (this point will be discussed later), it appears as though mobile incinerators are unlikely to be economically advantageous. Hence, we eliminate this alternative. 

A single solvent-solvent extraction step seldom satisfies the purification requirements for a GC determination multiple extractions are often needed for a more complete sample purification. However, such multiple extractions and transfers of a sample from one medium to another may result in further uncontrolled sample losses. Selective fractionations are used with advantage wherever some unique features of the analyzed molecules exist among the most typical approaches, researchers have extensively used ion-pairing extractions of various ionic samples and the digitonin or Girard separations of steroidal compounds. 

Solvent extraction of adsorbents is usually relatively efficient and reproducible. Large volumes of solvent may be necessary to extract the sample from some adsorbents. The greater the volume of solvent, the more dilute will be the analytes. Consequently, the extract must be reduced in volume (see below). The advantage of solvent extraction is that only a single aliquot of the final reduced extract is consumed in the analytical procedure. The disadvantage is that only 0.1% to about 0.5% of the sample can be analysed at a time. This, in turn, means that large sample volumes must be collected in order to get sufficient material to detect and quantify. 

A continuous nitrate/fluoride process has been proposed to take advantage of the ready solubility of columbite ore in a mixture of nitric and hydrofluoric acids, and thus allow the feed solution to be prepared in a cheap and simple manner. The feed contains 45 g/1. of niobium, about 25 g/1. of tantalum and is 8N in hydrofluoric acid and 3N in nitric acid. It is extracted in six stages with three volumes of solvent, together with an additional stage for acid equilibration . A half volume aqueous strip solution is also 8N in hydrofluoric acid and 3N in nitric acid. About 99 per cent of the niobium is extracted into the solvent and is then backwashed in a second extractor with three volumes of 0-5N hydrofluoric acid. The solvent finally passes to a single-stage backwash extractor for removal of tantalum by sodium carbonate solution. 

The major emphasis in the development and optimization of contactors has been to increase throughput while maintaining efficiency, and to increase efficiency without reduction in throughput [1,5], Many types of contactors are available for achieving mass transfer, each with its own particular advantage so that there is no single contactor that is best for all solvent extraction processes, either technically or economically. A classification chart of various contactor types is given in Fig. 7.8 [1]. 

SPEs offer distinctive advantages over conventional liquid-liquid extractions. They are relatively fast, require small sample size, eliminate emulsification problems, provide the possibility of performing both cleanup and preconcentration of the extract in one analytical step, and offer high precision. Another great advantage of SPEs over liquid-liquid extractions is solvent savings. Unlike liquid-liquid extractions that often require hundreds of milliliters for single or multiple extractions, SPEs require only a few milliliters of solvents for analyte extraction and cleanup. 

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