Solvent extraction continuous countercurrent

Sedimentation is also used for other purposes. For example, relative motion of particles and Hquid iacreases the mass-transfer coefficient. This motion is particularly useful ia solvent extraction ia immiscible Hquid—Hquid systems (see Extraction, liquid-liquid). An important commercial use of sedimentation is ia continuous countercurrent washing, where a series of continuous thickeners is used ia a countercurrent mode ia conjunction with reslurrying to remove mother liquor or to wash soluble substances from the soHds. Most appHcations of sedimentation are, however, ia straight sohd—Hquid separation. 

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

General trends are focused on reduced-solvent extractions or adsorption-based methods — enviromnentaUy friendly solvents for both solid and liquid samples. In recent decades, advanced techniques like supercritical fluid extraction (SFE), ° pressurized liquid extraction (PLE)," microwave-assisted extraction (MAE), ultrasound-assisted extraction, countercurrent continued extraction (www.niroinc.com), solid... 

P. Rivalier, J. Duhamet, C. Moreau, and R. Durand, Development of a continuous catalytic heterogeneous column reactor with simultaneous extraction of an intermediate product by an organic solvent circulating in countercurrent manner with the aqueous phase, Catal. Today, 24 (1995) 165-171. 

Prepared cottonseed meats containing 35 per cent of extractable oil are fed to a continuous countercurrent extractor of the intermittent drainage type using hexane as the solvent. The extractor consists of ten sections and the section efficiency is 50 per cent. The entrainment, assumed constant, is 1 kg solution/kg solids. What will be the oil concentration in the outflowing solvent if the extractable oil content in the meats is to be reduced by 0.5 per cent by mass ... 

An important feature in the construction of continuous countercurrent extraction towers is the design of the nozzles through which the solvent and oil feed enter. Nozzles should be designed to distribute the stream evenly over the cross-sectional area of the tower in order to minimize channeling and to make maximum use of the packing near the inlet. Consideration should also be given to the use of intercoolers to enforce a temperature gradient. 

Figure 14.3. Representation of countercurrent extraction batteries, (a) A battery of mixers and settlers (or separators), (b) Schematic of a three-stage countercurrent battery, (c) Simulation of the performance of a three-stage continuous countercurrent extraction battery with a series of batch extractions in separatory funnels which are designated by circles on the sketch. The numbers in the circles are those of the stages. Constant amounts of feed F and solvent S are mixed at the indicated points. As the number of operations is increased horizontally, the terminal compositions Et and R3 approach asymptotically the values obtained in continuous countercurrent extraction (Treybal, 1963, p. 360).

In the early years of plutonium scrap processing operations, the CAW stream was routed to trenches(1 ) specially excavated in Hanford soil. Batch recovery of americium was started in 1965. Later (1970-1976), a continuous countercurrent solvent extraction process employing DBBP (dibutylbutyl phosphonate) as the extractant was operated to recover, at least partially, plutonium and americium values from the CAW stream. Aqueous waste from the DBBP extraction process, still containing some plutonium and americium, was blended with other Plutonium Reclamation Facility (PRF) wastes, made alkaline, and routed to underground tanks for storage. 

For recovery, the broth is clarified by means of rotary vacuum filters. The penicillin, being acidic, is extracted from the aqueous phase into a solvent, such as methyl isobutyl ketone or amyl acetate, at a pH of 2.5 by means of a continuous countercurrent extractor, such as a Podbielniak. The penicillin is then reextracted with an aqueous alkaline solution or a buffer at a pH of 6.5-7.0. About 90 percent recovery yield is typical at this step. The... 

Although Peligot observed in 1842 that uranyl nitrate is soluble in ether, it was not until materials of high purity were needed for nuclear reactors that extensive applications and developments, both industrial and analytical, were made. The literature on applications of liquid-liquid extraction (solvent extraction) is extensive for details of the various procedures the reader is referred to the original papers and to compilations. " This chapter examines separations involving distribution of a solute between two immiscible phases and chemical equilibria of significance to the distribution ratio. Batch, countercurrent, and continuous liquid-liquid extractions are described in turn, followed by consideration of the factors governing the distribution ratio and finally by some illustrative applications. 

Mechanical expression of rice bran yields less oil, 10-12%, than solvent extraction, 16-18%. Rice bran is treated with steam and dried prior to pressure expression. Prepressing is usually carried out at 70 kg/cm followed by oil expulsion at 105-316 kg/cm (9). As a result of the low yield of oil from mechanical extraction, residual oil in the bran is recovered with hexane. Hexane extraction can be performed by batch or a continuous operation. Continuous operation uses countercurrent flow to improve mass transfer. Solvent extraction at high temperatures results in higher crude oil yield, but the crude oil is of lower quality. A new oil-extraction process, which involves premolding of rice bran at 14% moismre content and <40°C followed by hexane extraction at <15°C, was reported to yield a light-colored crude oil with no wax (9). 

Coconut Flour. Coconut flour is produced from the shredded kernel, dried in a continuous countercurrent drier, and subsequently extracted with solvent to remove the residual oil. The white meal produced contains 25% protein and 65% carbohydrate, as well as various minerals and vitamins (62). This coconut product... 

Cold pilot-plant tests are presently in progress using 2-inch diameter pulsed plate columns in continuous countercurrent flow. Synthetic Zr-Al high-level waste loaded with 0.2 g/L Ce(III) is being used as feed and 20 volume % DHDECMP in 2 1 decalin-DIPB as extractant. Cerium is being used as an americium simulant. To date, three complete cycles have been run including a solvent recycle wash with Na2C03. No major problems have been encountered. 

When the distribution ratio is not highly favorable, it is still possible to obtain a quantitative and selective separation through the use of a countercurrent liquid extraction approach. Although such approaches are no longer practical, having largely been supplanted by instrumental techniques such as preparative HPLC and continuous solvent extraction, countercurrent separations are conceptually useful. These approaches can be applied to preliminary separation of complex mixtures or in the isolation of compounds that do not perform well in LC because of undesirable interaction with the stationary phase (irreversible adsorption, denaturation, etc.). For these reasons, most applications of countercurrent separations involve the isolation of natural or biochemical products from plant or animal extracts. As will be described below, countercurrent extractions form the theoretical basis for LLE cartridges. 

In some cases, especially with multiple solutes and complex phase equilibria, it may be useful to perform laboratory batch experiments to simulate a continuous, countercurrent, multistage process. These experiments can be used to test/verify calculation results and determine the correct distribution of components. For additional information, see Treybal, Chap. 9 in Liquid Extraction, 2d ed. (McGraw-Hill, 1963), pp. 359-393, and Baird and Lo, Chap. 17.1 in Handbook of Solvent E raction (Wiley, 1983 Krieger, 1991). 

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