Inefficient extraction

The yields are low because of the great solubility of the compound in water, which makes extraction inefficient. 

Complete dehydration before extraction is not recommended. A small amount of water is often useful when a low-polarity solvent mixture is the extractant. On the other hand, excess water may make extraction inefficient. Using the right extractant is of great importance and depends largely on which carotenoids are sought. Using the hexane/acetone mixture as the extractant is advantageous because the same pair of solvents is used later for crystallization. 

This set of data displays some important differences between the as-prepared and annealed samples. First, the extraction of charges from the annealed layers is faster and far more efficient. For example, half of the initially photogenerated carriers are swept out of the device at 0 V within 1 ms and non-geminate recombination is almost absent at short-circuit conditions (Qtot is essentially independent of fd). When raising the bias close to Voc, extraction is slowed down and the carrier density in the sample declines as a result of non-geminate recombination. For the as-prepared layer, non-geminate recombination is seen for both short-circuit and open-circuit conditions, rendering extraction inefficient for both bias conditions. Also, extraction is considerably slower and recombination is more efficient in these samples. 

This direct filter teclnrique is very powerfiil [, 59] in extracting highly excited states, since only the propagation of a wavepacket is required. However, it is inefficient when there are closely-lying eigenvalues T... 

Note 1. The sulfoxide has a very good solubility in water. Extraction with diethyl ether is inefficient. 

An extensive survey has been carried out by McKervey and coworkers [7], who prepared the carbo-alkoxymethyl ethers of p-tert-h x y calix[4]arene, p-/< r/-butyl calix[6]arene, p-tert-bu y calix[8]arene, ca-lix[4]arene, calix[6Jarene, and calix[8]arene, and measured their abilities to extract cations from the aqueous phase into the nonaqueous phase. They concluded the following general aspects for the phase-transfer experiments (1) the calix[4]arene compounds show the greatest selectivity for Na (2) phase-transfer of Li is inefficient with all of the compounds (3) the calix[6]arene compounds show less affinity for Na than for K, with plateau selectivity for Rb" and Cs (4) the calix[8]ar-ene compounds are the least efficient of the cyclic oligomers, showing low levels of transport and low discrimination for all five cations (5) the calix[6]arene... 

Despite its widespread application [31,32], the kinetic resolution has two major drawbacks (i) the maximum theoretical yield is 50% owing to the consumption of only one enantiomer, (ii) the separation of the product and the remaining starting material may be laborious. The separation is usually carried out by chromatography, which is inefficient on a large scale, and several alternative methods have been developed (Figure 6.2). For example, when a cyclic anhydride is the acyl donor in an esterification reaction, the water-soluble monoester monoacid is separable by extraction with an aqueous alkaline solution [33,34]. Also, fiuorous phase separation techniques have been combined with enzymatic kinetic resolutions [35]. To overcome the 50% yield limitation, one of the enantiomers may, in some cases, be racemized and resubmitted to the resolution procedure. 

A sum-over-states expression for the coefficient A for the expansion of the diagonal components faaaa was derived by Bishop and De Kee [20] and calculations were reported for the atoms H and He. However, the usual approach to calculate dispersion coefficients for many-electron systems by means of ab initio response methods is still to extract these coefficients from a polynomial fit to pointwise calculated frequency-dependent hyperpolarizabiiities. Despite the inefficiency and the numerical difficulties of such an approach [16,21], no ab initio implementation has yet been reported for analytic dispersion coefficients for frequency-dependent second hyperpolarizabiiities which is applicable to many-electron systems. 

These opposing tendencies may defeat the purpose of the fractional precipitation process. The fractional precipitation of crystalline polymers such as nitrocellulose, cellulose acetate, high-melting polyamides, and polyvinylidene chloride consequently is notoriously inefficient, unless conditions are so chosen as to avoid the separation of the polymer in semicrystalline form. Intermediate fractions removed in the course of fractional precipitation may even exceed in molecular weight those removed earlier. Separation by fractional extraction should be more appropriate for crystalline polymers inasmuch as both equilibrium solubility and rate of solution favor dissolution of the components of lowest molecular weight remaining in the sample. 

Probably the most common distillation method used as a form of sample preparation for chromatographic analysis is steam distillation [31,32]. Solvent extraction and gas phase stripping methods are generally inefficient procedures for isolating polar, acidic, or basic compounds in an aqueous matrix due to the low efficiency of water immiscible solvents for the extraction of these compounds and their low volatility and high water affinity which results in a very slow transfer to the gas phase using... 

Bruland et al. [122] have shown that seawater samples collected by a variety of clean sampling techniques yielded consistent results for copper, cadmium, zinc, and nickel, which implies that representative uncontaminated samples were obtained. A dithiocarbamate extraction method coupled with atomic absorption spectrometry and flameless graphite furnace electrothermal atomisation is described which is essentially 100% quantitative for each of the four metals studied, has lower blanks and detection Emits, and yields better precision than previously published techniques. A more precise and accurate determination of these metals in seawater at their natural ng/1 concentration levels is therefore possible. Samples analysed by this procedure and by concentration on Chelex 100 showed similar results for cadmium and zinc. Both copper and nickel appeared to be inefficiently removed from seawater by Chelex 100. Comparison of the organic extraction results with other pertinent investigations showed excellent agreement. 

Iron Sponge Also called Dry box. An obsolete process for removing hydrogen sulfide from gas streams by reaction with iron oxide monohydrate. The ferric sulfide that is formed is periodically re-oxidized to regenerate ferric oxide and elemental sulfur. When this process becomes inefficient because of pore-blockage, the sulfur is either oxidized to sulfur dioxide for conversion to sulfuric acid, or is extracted with carbon disulfide. 

Air sparging Injects air below the water table and captures it above the water table to extract volatile contaminants and promote biodegradation Can be inefficient in low-permeability zones and complex geologic settings Typically limited to depths less than 30 ft Multicomponent mixtures can adversely affect extractability... 

Steam-enhanced extraction Injects steam above and/or below water table to promote volatilization of contaminants Can be inefficient in low permeability zones or complex geologic settings... 

The chemical and physical aspects of crud can dilfer for each separate operation and will vary in inorganic composition, organic content, color, and density. The composition of many cruds appears to have in common such constituents as Si, Al, Fe, P, SO4, particles of gypsum, clay, and other fine particles together with the solvent. Often there is a direct relation between the feed liquor and the crud compositions, indicating possible aqueous carryover as well as inefficient clarification before solvent extraction [33]. Various researchers have reported on the formation of crud and its characterization in their circuits [42-45]. 

Thus far in the discussion of industrial practice, we have referred to stages of extraction without defining a stage. Clearly these stages could be scaled-up versions of separatory funnels, but this is inefficient, because it implies batch rather than continuous operation. Industry prefers continuous operation because it is generally simpler to control automatically, and because it makes better use of labor. 

Comparison with the earlier data for equilibrium conditions shows that an extra stage is needed. In spite of this, the final raffinate is significantly higher than was achieved at 100% extraction efficiency. The influence of the inefficiency is clearly greater the more dilute the solutions. 

Figure 8.11 shows how the equilibrium curve shrinks in the presence of inefficiencies. In multicomponent systems where there is mutual interference in extraction by several components, the efficency shrinkage comes on top of the other reductions in the equilibrium curve, and for this reason there is stress in such systems on achieving high efficiency. 

Feed Equilibrium Inefficient extract Extract Feed... 

Fig. 7. The four silver layers of a 1 T FFC magnet. The picture shows also a detail of the inner wall of the smallest, most internal layer. The thickness of the cuts is 0.16 mm. At 400 A, the hottest spot power density dissipation reaches 1.1 W/mm. One of the most difficult engineering challenges is to cool these hot spots of the innermost layer efficiently enough to extract all the heat. If the cooling is inefficient, the layer can be seriously damaged or even destroyed.

When autohydrolysed aspen was treated with cellulase enzyme, poor results were obtained. Based on a ton of dry wood, only about 300 lb of fermentable sugars were obtained, and the fermentation was very inefficient with low yields of ethanol. These results compare poorly with those shown in Figure 5 when autohy-drolysed-caustic extracted aspen was treated with cellulase enzyme, and then fermented. It would appear from these results that the caustic extraction step for lignin removal would be necessary if the enzymatic hydrolysis process were to be adopted and advisable when acid hydrolysis is used. 

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