Raffinate reflux

A potential fourth type of fractional extraction operation involves the use of reflux at both ends of a dual-solvent process, i.e., reflux to the raffinate end of the process (the stripping section) as well as reflux to the extract end of the process (the washing section). The authors are not aware of a commercial apphcation of this kind however, Scheibel [Chem. Eng. Prog., 62(9), pp. 76-81 (1966)] discusses such a process scheme in light of several potential flow sheets. In the special case of single-solvent fractional extraction with extract reflux, Skelland [Ind. Eng. Chem., 53(10), pp. 799-800 (1961)] has pointed out that addition of raffinate reflux is not effective from a strictly thermodynamic point of view as it cannot reduce the required number of theoretical stages in this special case. 

Another result of the simulation is that a molar boil-up ratio of 0.180 is required in the stripper to achieve the bottoms mass purity of500 ppm n-heptane considering only the hydrocarbons (solvent-free basis). This boil-up ratio corresponds to a reboiler duty of 3695 kW, or roughly 6700 kg/h of 12-bar gauge steam, and results in 12,914 kg/h of extract reflux for an extractor reflux-to-feed ratio of 0.323. Compositions and rates of the extract, raffinate, reflux, and stripper bottoms streams are given in Table 15-12. 

Extractors with reflux at one or both ends of the column (or series of mixing vessels) may be used to enhance the purity of the products. An extractor column with both extract and raffinate reflux is shown in Figure 11.1. In this configuration, the feed is sent to an intermediate stage, and the extractor performs as a distillation column, separating two components in the feed. In the column section above the feed, the raffinate phase is stripped of the extract component (the solute), and in the lower section the extract phase is enriched in the extract component. 

Show that the degrees of freedom for a liquid-liquid extraction column with two feeds and raffinate reflux is 3C + 2N + 13. 

The significance of line segments and the general methodology apply to extraction columns with or without extract and/or raffinate reflux. Raffinate reflux, in fact, is seldom employed and frequently results in greater, not lower stage requirements, as discussed in Chapter 11. 

A feed stream containing 35 wt% acetone in water is to be extracted at 25°C in a countercurrent column with extract and raffinate reflux to give a raffinate containing 12% acetone and an extract containing 55% acetone. Pure 1,1,2,-trichloroethane, which is to be the solvent, is removed in the solvent separator, leaving solvent-free product. Raffinate reflux is saturated. Determine... 

The extraction cascade in Pig. 11.6 has both extract and raffinate reflux. Raffinate reflux is not processed through the solvent recovery unit since additional solvent would have to be added in any case. It is necessary, however, to remove solvent from extract reflux. 

The use of raffinate reflux has been judged to be of little, if any, benefit by Wehner and Skelland. Since only the original raffinate plus extra solvent is refluxed, the amount of material, not the compositions, is affected. 

Figure 11,6. Solvent extraction with extract and raffinate reflux.

The extraction process shown beiow is conducted in a multiple-feed countercurrent unit without extract or raffinate reflux. Feed F is composed of solvent and solute and is an extract phase feed. Feed F" is composed of unextracted raffinate and solute and is a raffinate phase feed. 

RaflSnate Reflux Only. Operation with raffinate reflux only is analogous to the use of a stripping section in a distillation column (7, 14, 18), Fig. 6.63. Since solvent removal from the final extract produces the richest... 

Illustration 10. One hundred pounds per hour of a feed solution containing 25% ethanol (C), 75% water (A), is to be reduced to 2% ethanol (saturated) with ethyl ether (B) as solvent. Twice the minimum raffinate reflux ratio is to be used, and the reflux +1 is to be saturated. The temperature is to be 25 "C. Calculate the number of stages and amount of solvent required. 

For minimum raffinate reflux, tie lines to the left of F are extended. Tie line DO intersects line En iPp at the lowest value, Nw = —1.8. 

Let us illustrate how we can solve for ideal stages in an extraction system. The process that we will consider (see Figure 13-13) will be for a countercurrent multiple contact system with both extract and raffinate reflux. 

An extractor column with both extract and raffinate reflux is shown in Figure 11.1. In this configuration the feed is sent to an intermediate stage in the column. 

Refluxing part of the raffinate, on the other hand, does not increase the removal of the extract component from the raffinate since maximum removal is achieved with pure solvent. The raffinate reflux, which amounts to recirculation of an equilibrium phase back to the same equilibrium stage, does not in principle change the product composition. In practice, the raffinate reflux may help in achieving a better approach to phase equilibrium due to premixing with the solvent. The raffinate reflux could, that is, improve the stage efficiency. 

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