Solvent extraction candidates

Toluene is to be removed from a wastewater stream. The flowrate of the waste stream is 10 kg/s and its inlet composition of toluene is 5(X) ppmw. It is desired to reduced the toluene composition in water to 20 ppmw. Three external MSAs are considered air (S2) for stripping, activated carbon (S2) for adsorption, and a solvent extractant (S3). The data for the candidate MSAs are given in Table 3.6. The equilibrium data for the transfer of the pollutant from the waste stream to the yth MSA is given by... 

The solvent extracts can be cleaned up by traditional column chromatography or by solid-phase extraction cartridges. This is a common cleanup method that is widely used in biological, clinical, and environmental sample preparation. More details are presented in Chapter 2. Some examples include the cleanup of pesticide residues and chlorinated hydrocarbons, the separation of nitrogen compounds from hydrocarbons, the separation of aromatic compounds from an aliphatic-aromatic mixture, and similar applications for use with fats, oils, and waxes. This approach provides efficient cleanup of steroids, esters, ketones, glycerides, alkaloids, and carbohydrates as well. Cations, anions, metals, and inorganic compounds are also candidates for this method [7],... 

Phase-equilibrium calculations were discussed for vapor-liquid equilibria (VLB), liquid-liquid equilibria (LLE), and solid-solid equilibria (SSE). Results from VLE calculations often take the form of K-factors and relative volatilities, especially when thermodynamic calculations serve as intermediate steps in computer-aided process-design programs. In those situations, K-factors are routinely provided to subprograms that size distillation columns and gas-liquid absorbers. Similarly, the distribution coefficients computed for LLE serve as bases for sizing solvent-extraction columns moreover, liquid-liquid distribution coefficients may be helpful in screening candidate solvents for use in an extraction. 

Although non-ionic surfactants would appear to be unlikely candidates as complexing agents for metal ions, the interaction of some polyoxyethylene glycols with metal ions has recently attracted interest [90,91]. The reaction of non-cyclic polyoxyethylene derivatives with alkali and alkaline earth metals has been studied by means of solvent extraction of their thiocyanates or iodides. Polyoxyethylene dodecyl ethers with more than 7 ethylene oxide units were able to bind potassium ion in the water phase and to transfer the complexed salt to the organic phase the extracting power of Ci2Eg was about one sixth of that of a crown ether [92]. Some results are shown in Fig. 11.13. 

Gas chromatography (GC) involves the analysis of volatile organic compounds, that is, materials that exist in the vapor phase, at least at the typical GC operating temperatures between 40 and 300°C. Because aroma compounds must, by their very nature, leave the food matrix and travel through the air to be perceived, they are generally excellent candidates for analysis by GC. Although many of these compounds may be solvent extracted, distilled, or otherwise isolated from the food matrix, it is frequently preferable to take advantage of their volatility and rely instead on techniques of headspace analysis. 

The uranium ore concentrate or yellowcake is not a very pure substance. But, the dissolution, filtering, solvent extraction, and evaporation processes in the production of uranyl nitrate result in a reasonably pure uranium compound such as specified by the American Society for Testing and Materials standards. Thus, a chemically pure form of natural uranium is available as uranyl nitrate or UO3 prior to introduction of safeguards. Because of the high neutron-capture cross section for " N, uranyl nitrate is not a likely candidate as a fuel for a natural uranium reactor. However, pure UO3 could potentially be used as a ready source for fuel in a specially designed plutonium production reactor thus raising a potential diversion concern. 

As a starting point for identifying candidate solvents, all compounds having boiling points below that of any component in the mixture to be separated should be eliminated. This is necessary to yield the correct residue curve map for extractive distillation, but this process implicitly rules out other forms of homogeneous azeotropic distillation. In fact, compounds which boil as much as 50°C or more above the mixture have been recommended (68) in order to minimize the likelihood of azeotrope formation. On the other hand, the solvent should not bod so high that excessive temperatures are required in the solvent recovery column. 

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

TABLE 13-22 Comparison of Candidate Solvents for Methanol/Acetone Extractive Distillation... 

Baron Cagniard de la Tour [244] made the first reported observation of the occurrence of a supercritical phase in 1822. Tables 3.12 and 3.13 summarise some of the main useful features of SCFs. Several properties of SCFs make them ideal candidates as solvents for industrial extraction processes [245,246],... 

Figure 3.2 Drugs from natural sources different molecules can be isolated from the leaves, stems, and roots. From each of these sources, extracts conducted with solvents with different polarities will yield different natural products. This complex extraction system ensures the identification of all possible candidate molecules from a plant source.

Several methods have been proposed to guide the solvent replacement process for the many applications described in the chemical literature. These efforts attempt to build an organized framework for this process and provide a substantial improvement over previously ad hoc or trial-and-error approaches to solvent replacement. Joback outlines a methodology for the selection of replacement solvents for various processes such as extraction or cleaning (Joback, 1994). There are basically four steps to this process identify constraints on important solvent properties, compile data for all properties, rank solvents satisfying the target constraints, and evaluate top solvent candidates using simulation. 

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