Solvent single stage

After a single-stage liquid-liquid contact the phase remaining from the feed sohition (raffinate) can be contacted with another quantity of fresh extraction solvent. This crosscurrent extraction scVieme (Fig. 15-4) is an excellent laboratoiy procedure because the extrac t and raffinate phases can be analyzed after each stage to generate equihbrium data. Also, the feasibility of solute removal to lowTevels can be demonstrated. 

Petroleum products may be treated with various solvents for the removal by selective solubility of undesirable constituents or for the recovery of by-products. The solvent and solute must be separated to yield the desired product and to recover the solvent for reuse. The solvents normally boil at a lower temperature than the products from which they are to be removed and so are generally distilled off as overhead products. The pipe stills used for this service may be single-stage or multi-stage units, depending on the service involved. Some solvents can be removed by the use of steam heated stills. In other cases, the high temperature required necessitates the use of fired heaters and vacuum towers. 

Fig. 7.11. Sequence of single stages of extraction with fresh solvent.

Equilibrium data must be obtained for material balance showing raffinate and extracted phases. A simple separation funnel for single-stage extraction using amyl acetate as organic solvent is shown in Figure 7.13. 

An infinitely large volume of solvent is needed to achieve complete extraction of a solute in a single stage. This necessitates application of multistage extraction. It allows essentially complete recovery deploying a limited volume of the solvent. In the countercurrent proc-... 

Evaporation separates a volatile solvent from a solid. Single-stage evaporators tend to be used only when the capacity needed is small. For larger capacity, it is more usual to employ multistage systems that recover and reuse the latent heat of the vaporized material. Three different arrangements for a three-stage evaporator are illustrated in Figure 10.21. 

We, at IIP carried out certain oxidation studies and report the results on oxidation of sulfur compounds present in gas oil using carboxylic acid / H2O2 system. Extent of sulfur removal in gas oil after oxidative desulfurization was monitored by sulfur analysis. Oxidation and solvent extraction reduced sulfur in gas oil from 1.5% to 0.20% in single stage batch extractor. 

In the single-stage batch process illustrated in Figure 13.1, the solvent and solution are mixed together and then allowed to separate into the two phases—the extract E containing the required solute in the added solvent and the raffinate R, the weaker solution with some associated solvent. With this simple arrangement mixing and separation occur in the same vessel. 

Figure 13.10. Single-stage process with solvent recovery...

All freeze separation processes depend on the formation of pure solvent crystals from solution, as described for eutectic systems in Section 15.2.1. which allows single-stage operation. Solid-solution systems, requiring multistage-operation, are not usually economic. Several types of freeze crystallisation processes may be designated according to the kind of refrigeration system used as follows . 

Returning to the example shown in Table 8.1, it is intuitively obvious that the low degree of extraction achieved when using a phase ratio 0 = 0.1 could be improved if the raffinate could be reextracted with fresh solvent, and that if this reextraction were to be carried out at the same phase ratio, the second extract would show a similar purity. The first and second extracts could then be combined to give overall a better recovery than could be achieved in a single stage. 

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. 

The volume of solvent needed for a particular duty depends on the system adopted for the extraction in general, a single-stage extraction requires more solvent phase than a cross-flow cascade, which in turn needs more than a countercurrent cascade. 

In this chapter, we have seen the way in which laboratory studies of solvent extraction are adapted to industrial use. Starting from a batch extraction, it was shown how both recovery and product purity could be markedly influenced by the volumetric phase ratio, and how it was impossible to achieve both high recovery and high purity in a single stage. Cross-flow or repeated extraction was then evaluated, and it was shown how this could improve... 

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