Solvent extraction, mass transport

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

A growing-drop method has been reported [53] for measuring interfacial liquid-liquid reactions, in which mass transport to the growing drop was considered to be well-defined and calculable. This approach was applied to study the kinetics of the solvent extraction of cupric ions by complexing ligands. 

A significant advance was made in this field by Watarai and Freiser [58], who developed a high-speed automatic system for solvent extraction kinetic studies. The extraction vessel was a 200 mL Morton flask fitted with a high speed stirrer (0-20,000 rpm) and a teflon phase separator. The mass transport rates generated with this approach were considered to be sufficiently high to effectively outrun the kinetics of the chemical processes of interest. With the aid of the separator, the bulk organic phase was cleanly separated from a fine dispersion of the two phases in the flask, circulated through a spectrophotometric flow cell, and returned to the reaction vessel. 

Table 10.4 gives critical data for the most common solvents used in high-pressure extraction. Table 10.5 illustrates the favorable mass transport properties that can be achieved in the supercritical area owing to a low viscosity and a high diffusivity, compared with the liquid phase. 

We report on a number of on-line chemical procedures which were developed for the study of short-lived fission products and products from heavy-ion interactions. These techniques combine gas-jet recoil-transport systems with I) multistage solvent extraction methods using high-speed centrifuges for rapid phase separation and II) thermochromatographic columns. The formation of volatile species between recoil atoms and reactive gases is another alternative. We have also coupled a gas-jet transport system to a mass separator equipped with a hollow cathode- or a high temperature ion source. Typical applications of these methods for studies of short-lived nuclides are described. 

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

Kubisova, L ., Sabolova, E., Schlosser, ., Martak, J. and Kertesz, R. (2004) Mass-transfer in membrane-based solvent extraction and stripping of 5-methyl-2-pyrazinecarboxylic acid and co-transport of sulphuric add in HF contactors. Desalination, 163, 27. 

The copper electrowinning process requires concentrated solutions to improve mass transport and increase the solution conductivity. The pregnant leach solutions from leaching are too dilute and too impure for the direct production of high-purity cathodes. Electrowinning from these solutions would give impure, dendritic deposits. Solvent extraction provides the means for producing pure,... 

Handbook of Solvent Extraction, Lo, Baird, and Hanson, eds, (Wiley, 1983 Krieger, 1991)] recommends the following equation from Lad-dha and iSegaleesan [Transport Phenomena in Liquid Extraction (McGraw-Hill, 1978), p. 233] to estimate the overall volumetric mass-transfer coefficient ... 

Hollow fiber contactors use membranes to separate two phases and transport is due to diffusion, chemical reaction, or chemical potential rather than pressure. The main examples of hollow fiber contactors are found in dialysis, gas adsorption/deadsorption, and solvent extraction. Use of hydrophilic and hydrophobic fiber materials controls the wetting of the pores. Typically, the phase that has higher mass transfer is allowed to wet the pores in order to minimize overall mass transfer resistance. 

In facilitated transport, unlike solvent extraction and other equihbrium stage wise processes, the overall mass transfer rate is not governed by the usual equihbrium considerations alone. Instead, the solute transport process is controlled by a combination of the diffusion rate and the complexation reaction rate and in case of coupled transport the solute can be transported against its concentration gradient thus opening up the possibilities of separation from even very dilute solute solutions. 

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