Extraction processes solvent selection

Selectivity. It will be shown later (Chap. 4) that for purposes of obtaining a successful extraction process, the selectivity of a solvent is a more important index of its usefulness than the simple distribution coefficient of the consolute substance. As will be shown, selectivity of B for C is defined as follows ... 

Hollow fibers have recently received attention with the objective of performing the simultaneous sorption and desorption of the target metal. Hollow chitosan fibers were prepared and the system was used for the recovery of chromate anions. The hollow fibers were immersed in the chromate solution while an extractant was flowed through the lumen of the fiber. Chromate anions adsorbed on the fiber were re-extracted by the solvent extractant. The hollow fiber acts simultaneously as a physical barrier that can make the extraction process more selective. (Vincent, 2000, 2001)... 

Another reactive separation processes studied for ethyl lactate production is the catalytic extractive reaction (Figure 20.4.7). In this case, the esterification is performed in a biphasic liquid solvent system composed by a reactive polar liquid phase which contains the esterification constituents lactic acid, eflianol and catalyst, and an extractive organic solvent selective of the ester. Therefore, ethyl lactate should preferably be dissolved in the extractive organic phase shifting, in this way, the reaction equilibrium to ester formation. The immiscible extractive solvent is an aromatic or other solvent like toluene, benzene or diethyl ether, among others. Nevertheless, it has also been used an immiscible solvent based on fatty acid methyl ester, but in this case, the procedure represents a method to produce an organic biosolvent and not just ethyl lactate as solvent. 

Extraction and Extractive Distillation. The choice of an extraction or extractive distillation solvent depends upon its boiling point, polarity, thermal stabiUty, selectivity, aromatics capacity, and upon the feed aromatic content (see Extraction). Capacity, defined as the quantity of material that is extracted from the feed by a given quantity of solvent, must be balanced against selectivity, defined as the degree to which the solvent extracts the aromatics in the feed in preference to paraffins and other materials. Most high capacity solvents have low selectivity. The ultimate choice of solvent is deterrnined by economics. The most important extraction processes use either sulfolane or glycols as the polar extraction solvent. 

Selectivity for a single metal of a group is the basis of a solvent extraction process for the recovery of copper (qv) from low concentration ore leach solutions containing high levels of iron (qv) and other interfering metals (16). 

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

Removal of diluent by an extraction process To obtain the final stable macroporous structure, the liquid organic diluents and the linear polymer are removed from the crosslinked structure by extraction with a good solvent for the inert diluents and particularly for the linear polymer. Toluene or methylene chloride are usually preferred for the removal of linear polystyrene from the divinylbenzene crosslinked macroporous polystyrene particles [125,128]. The extraction is carried out within a Soxhelet apparatus at the boiling point of the selected solvent over a period usually more than 24 h. 

We are not aware of any previous studies of the removal of plutonium or americium from (NH )2ZrF6-NHltF-NH N03 solutions. For ready plant-scale application, precipitation, sorption on inorganic materials, or batch solvent extraction processes may all be satisfactory. An inexpensive inorganic material with great selectivity and capacity for sorbing actinides, and with suitable hydraulic properties, would be especially attractive. 

The polarity index is a measure of the polarity of the solvent, which is often the most important factor in the solvent choice for the particular application. In extraction processes, the tenet that like dissolves like (and conversely, opposites do not attract ) is the primary consideration in choosing the solvent for extraction, partitioning, and/or analytical conditions. For example, hexane often provides a selective extraction for nonpolar analytes, and toluene may provide more selectivity for aromatic analytes. 

MIBK is a valuable industrial solvent used primarily in the paint and coating industry and in metallurgical extraction processes. It is also used as a precursor in the production of specialty chemicals such as pesticides, rubber anti-oxidants as well as antibiotics and pharmaceuticals (1). Historically, MIBK has been produced commercially from acetone and hydrogen feedstock in three stages. First, acetone is dimerized to produce diacetone alcohol (DAA). Second, DAA is dehydrated to produce MO and water. Finally, the carbon-carbon double bond of MO is selectively hydrogenated to produce MIBK. These consecutive reactions are outlined in equations (1-3). 

As discussed already, crowns may be involved in solvent extraction processes in which an inorganic reagent is transferred (sometimes selectively) from one phase (often water) into an immiscible organic phase the extraction involves ion-pair formation between the cationic crown complex and the counter ion (Blasius Janzen, 1981). 

In ODS, sulfur compounds present in fuels are oxidized to more polar sulfones / sulfoxides to facilitate their removal by solvent extraction or adsorption. Various oxidation systems have been reported in the literature for this transformation. Among these oxidants like hydrogen peroxide (H2O2) and carboxylic acid as catalyst3"5. For the chemical industry, it becomes more and more important to develop cleaner technologies. Solvent extraction processes are used to separate sulfones / sulfoxides from oxidized fuels. These processes required suitable and selective solvents for separation of oxidized sulfur compounds from petroleum feedstocks. 

Solvent extraction is a major industrial technique. The usual objective is to selectively remove one or more solutes from a complex mixture. Selectivity usually depends on strong specific solvent-solute interactions or on the formation of complexes between ions and ligands. Thus solvent extraction systems are likely to include a number of chemical reactions and to exhibit large deviations from ideality. The design of liquid extraction processes may require many kinds of data. References (31, 32, 55, 61, 81, and 118) are concerned specifically with solvent extraction. 

In all extraction processes, the important feature is the selective nature of the solvent, in that the separation of compounds is based on differences in solubilities, rather than differences in volatilities as in distillation. In recent years, it has become possible to use computerised techniques to aid in the choice of a solvent with the required selectivity and to design appropriate molecular structures. 

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