Stage Solvent

In both cases, the entire method consists of four stages solvent extraction and partition, cleanup by gel permeation chromatography (GPC) and/or mini silica gel column chromatography and gas chromatography (GC) determination. Except for the central GPC, several variations occur at each stage depending on the kind of sample material and the residues to be analyzed. The variations can be combined with each other in a variety of ways according to the requirements. 

The problem with solvents is not so much their use, but the inefficiencies associated with their recovery and reuse. High volatility, whilst being an extremely useful property, leads to solvent losses to the environment. If a process consists of a reaction stage and a purification stage, solvents may be used and lost at each stage, as shown schematically in Figure 1.20a. Real chemical processes may include several separation steps, with further opportunities for solvent loss. 

Figure 12. Variation of fraction extraction with time for the second stage of two-stage solvent extraction of coal first stage at 250°C. for 8 hours not shown...

Trtic, T.M., Vladisavljevic, G.T. and Comor, J.J. (2001) Single-stage and 2-stage solvent-extraction of Tl(III) in hollow-fiber contactors under recirculation mode of operation. Separation Science and Technology, 36, 295. 

Where selective H2S removal is required, together with deep C02 removal, two absorption and regeneration columns may be required essentially in a two-stage process, as illustrated in Figure 5.25.195 H2S is selectively removed in the first column by a lean solvent that has been deeply stripped with steam, while C02 is removed, from the now H2S-free gas, in the second absorber. The second-stage solvent can be regenerated with air or nitrogen if very deep C02 removal is required.189... 

A second organic phase formed when the thorium concentration in first-stage solvent extraction feed was as high as 1.5 M. 

M. Bengtsson and G. Johansson, Preconcentration and Matrix Isolation of Heavy Metals through a Two-Stage Solvent Extraction in a Flow System. Anal. Chim. Acta, 158 (1984) 147. 

Fig. 6.2-1 Graphical presentation of single stage solvent extraction...

The wet cellulose acetate membranes prepared for reverse osmosis purposes can be used for gas separation when they are dried. The water in the cellulose acetate membrane cannot be evaporated in air, however, since the asymmetric structure of the membrane will collapse. Instead, the multi-stage solvent exchange and evaporation method is applied. In this method, a water-miscible solvent such as ethanol first replaces the water in the membrane. Then, a second volatile solvent such as hexane replaces the first solvent. The second solvent is subsequently air-evaporated to obtain a dry membrane [13,14]. The reason for replacing water with hexane is to reduce the capillary force inside the pore so that it will not collapse during the drying process. 

The bottoms of the extractive column has a very small impurity of acetone (0.01 mol%), so it is essentially a binary mixture of methanol and DMSO. The bottoms stream is fed to the 17-stage solvent-recovery column on Stage 8. The pressure is 1 atm, and the reflux ratio is 0.5. The distillate product is high-purity (99.95 mol%) methanol. The bottoms is recycled back to the extractive column as high-purity (99.99 mol%) DMSO solvent. 

Film formation. During the film formation stage solvents are emitted. The emission rate is directly proportional to the VOC concentrations in the product and inversely proportional to the film thickness (first order of kinetics). When the film has completely formed, the emission is controlled by diffusion processes, the emission rate is now inversely proportional to the square of the film thickness. 

Yield. To a large extent, the properties of a raffinate of fixed V.I. extracted from a given oil are essentially the same irrespective of the extraction processes (number of stages, solvent/oil ratio, solvent) used to obtain it S7g). For example, the oil for which equilibrium data are given in Fig. 11.9 gave raffinate products whose gravity and viscosity are shown as a... 

Consider a two-stage solvent extraction of the benzoic acid with the... 

J. Wang, E.H. Hansen, Flow injection on-line two-stage solvent extraction preconcentration coupled with Et-AAS for determination of bismuth in biological and environmental samples. Anal. Lett. 33 (2000)... 

The wet cellulose acetate membranes prepared in the foregoing step for re verse osmosis should be dried to be used for gas separation. The water in the membrane cannot be evaporated in air the asymmeuic porous structure of the membrane is destroyed during the evaporation step. Instead, the following multiple-stage solvent exchange method can be applied. 

Separation to any extent, however, can be achieved, so long as their distribution coefficients are different, by the techniques of fractional extraction. The simplest flowsheet for this is shown in Fig. 10.33. Here solutes B and C, which constitute the feed, are introduced into a countercurrent cascade where partly miscible solvents A and D flow countercurrently. At the feed stage, both solutes distribute between the solvents, with solute B favoring solvent A, solute C favoring solvent D. In the section to the left of the feed stage, solvent A preferentially extracts the B from D, and D leaves this section with a solute content rich in C. In the section to the right of the feed stage, solvent D preferentially extracts the C from A, and A leaves with a solute content rich in B. 

Shabani, M. B., and Masuda, A. (1991). Sample introduction by on-line two-stage solvent extraction and back-extraction to eliminate matrix interference and to enhance sensitivity in the determination of rare-earth elements with inductively coupled plasma mass spectrometry. Ana/. Chem. 63(19), 2099-2104. 

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