Extraction - Principles of Chemical Separations with Environmental Applications - page 152

The spent solvent is often recovered (usually by distillation) and reused in the extraction
process.
Extraction can be done in a single stage, countercurrent cascades, and cross-flow cas-
cades. Extraction columns may have distinct stages or packing, and many have highly
specialized pieces of equipment to facilitate mixing.
Accounting for efficiencies in extraction columns is much more difficult than for distil-
lation columns, and is not considered here.
All extraction systems are partially miscible to some extent, but when partial miscibility
is very low, the system may be treated as completely immiscible and McCabe-Thiele
analysis is appropriate.
Leaching is a solid-liquid extraction and is sometimes treated similarly to partially
miscible liquid-liquid extraction. The general term “extraction” is usually limited to the
liquid-liquid type.
Strengths
- Can be done at low temperatures to protect unstable molecules
- Can be energy efficient
- Can separate components with azeotropes or low relative volatilities
Weaknesses
- Some equipment can be very complicated and/or expensive
- Added complexities of a mass-separating agent (solvent): recovery, recycle, storage,
etc.
- More difficult to model and scale-up vapor/liquid than distillation.

5.13 Questions

5.1 In Figure 5.20, V 0 and O 0 are not on the solubility envelope while O 1 , O 2 , O 3 , V 1 ,
V 2 , and V 3 are. Why?
5.2 In Figure 5.20, which stage would be best for addition of pure solvent? Why?
5.3 Why isn't a cocurrent cascade used in solvent extraction?
5.4 Acetic-acid extraction examples in this chapter used both isopropyl ether and
1-butanol. Which is a better solvent? Why?

5.14 Problems

5.1 Using the lever-arm rule, derive equations for O F .

5.2 Show that the lever-arm rule is valid for one inlet and two outlet streams for a contact
stage.
5.3 For acetic-acid extraction using isopropyl ether, redo the calculation using a composi-
tion of 95 wt% isopropyl ether and 5 wt% water. This composition is easier to obtain
in the distillation step.

/ F and V /