Single-contact extraction

Fig. 6.5. Single-contact extraction with solvent recovery.

Fig. 6.10. Single-contact extraction and solvent recovery on Janecke coordinates.

Distribution Diagram. Although computations for single-contact extraction can be carried out on distribution-diagram coordinates, it is inconvenient because a trial-and-error solution is required hence description of this method is omitted. 

This process is an extension of single-contact extraction, wherein the raffinate from the first stage is extracted with fresh solvent of the same composition in successive stages, as the flowsheet of Fig. 6.I80 indicates, the concentration of C in the raffinate thus being further reduced. Different quantities of solvent may be used in the various stages. 

Single-contact Extraction. These calculations were first described by Hunter (10). Refer to Fig. 7.1. On tetrahedral diagrams of this sort, the geometrical rules applicable to mixtures on ternary triangular diagrams apply. Consequently, if feed solution F, a solution of components A and By is extracted with solvent S, a solution of C and D, the point M representing the mixture as a whole is on the straight line FS, such that... 

One hundred pounds of a solution containing 20% acetic acid, 80% water are to be extracted with methyl isobutyl ketone as solvent, at 25 C. Equilibrium data of Sherwood, Evans, and Longcor are available in [Ind. Eng. Chem. 31, 1144 (1939)]. a. For single-contact extraction,... 

C = CM/CAo, where product is shown in the lower phase and Sc = CAu/CAo, where product partitions to the upper phase. When single-stage extraction does not give sufficient recovery, repeated extraction can be carried out in a chain or cascade of contacting and separation units. 

A single-stage extraction using the same total volume of solvent achieves only 92% extraction, and the extract concentration is only 0.23, vs. nearly 0.25 for the cross-flow extraction. The use of four cross-flow extraction stages is clearly preferable to a single extraction. Equally, of course, the use of more than four extraction stages, each with a proportionately smaller volume, would improve the performance. In the limit, one would seek a differential contacting process similar to the Soxhlet extractor employed for extraction from solid phases, but such a contactor has not found use in solvent extraction. 

From an operational viewpoint, a single-contact removal of americium is most desirable. To avoid secondary problems caused by the production of magnesium (by Equations 2 and 3) in excess of its solubility in plutonium, americium removals are limited to about 85% per extraction stage. Multiple-stage extractions consequently are used when 85% or greater removal of americium is required. 

Table I gives the values of the extraction factor (a) required for americium removals ranging from 80 to 99%. These required values of a are given for the three modes of extraction (single-contact, two stage crosscurrent, and two stage countercurrent).

Consider a single stage extraction process, Eig. 2A, where a feed liquid (i.e., water) containing a solute (i.e., acetone) contacts an extraction solvent (i.e., chloroform). In this... 

Other important features in assessing industrial extractions are whether one or more extraction stages are to be used. This will depend in part on the partition coefficient of the system of interest, and on the flow pattern of the solvent and extracted phase (the raffinate) to be used, if more than one extraction stage is employed. Single-stage extraction requires a mixer to bring about intimate contact between the two phases, followed by a settler which allows phase separation and a means for independent removal of the two phases (Fig. 10.7). 

Magic angle spinning NMR spectra with variable cross polarization contact times were obtained on the intact, non-extracted sediments. The time-dependent spectra reveal subtle differences in organic carbon with depth differences not observed in single contact experiments. Dlpolar-dephased spectra of these same sediments indicate the presence of substantial amounts of substituted aromatic/olefinic carbons which are rapidly altered with depth. 

The fraction extracted therefore becomes greater, the greater the ratio E/F of solvent to feed, but an infinite amount of solvent is needed for complete extraction in a single contact. 

This is an extension of single-stage extraction wherein the raffinate is successively contacted with fresh solvent, and may be done continuously or in batches. Figure 7.9 is a schematic diagram for a three-stage crosscurrent extraction process. A single final raffinate results, and the extracts can be combined to provide a composited extract, as shown. As many stages as necessary can be used. 

Single contact. This involves the use of a single stage, where solution to be separated and extracting solvent are contacted once and extract and raffinate phases separated. Operation may be batch wise or continuous. The distillation analog is flash vaporization or equilibrium distillation. 

Cocurrent multiple contact. This may be batch or continuous and is an extension of the single contact wherein the raffinate is contacted repeatedly with fresh extracting solvent. In the limit represented by an infinite number of stages, this becomes the same as differential extraction. 

Solvent Recovery. Only a single raflBnate, that from the last stage n, is treated for solvent recovery. The extracts from all stages are ordinarily combined and the mixture then treated for solvent recovery, although individual extracts may be treated separately if so desired. Refer to Fig. 6.20. The flowsheet and construction are the same for single contact except that a combined extract E is prepared ... 

Purity of Products. The maximum purity of A in the raffinate will be given by an operation in which the nth tie line, corresponding to the last stage, passes through S when extended. This necessarily requires n to be infinity. The absolute maximum purity of C in the solvent-stripped extract will correspond to the case for single contact (tangency of solvent-removal line to the solubility curve for the combined extracts). Since E ordinarily falls within the two-liquid-phase area, this cannot usually be realized, however. 

Solvent Recovery. Since but a single raflSnate and a single extract are the products of this type of operation, solvent-recovery calculations are identical with those of the single-contact operation described previously. The maximum purity of C in the finished extract will accordingly result if the solvent-removal line EiSe is tangent to the binodal curve. 

Equations (6.121) and (6.111) used alternately will permit calculation of the weights of the various extracts and raffinates. Solvent recovery is identical with that for single contact, and if pure B is the solvent, Xo = Xfl , and lines SF and SRn are vertical since S is at infinity. 

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