LIQUID–LIQUID EXTRACTION

Liquid–liquid extraction is used to separate a component from a liquid mixture by treatment with a solvent in which the desired component is preferentially soluble. .

A high percentage extraction of product must be obtained 

It must be concentrated in a smaller volume of solvent.

Check the solubility of the product using a wide range of solvents, in a small scale before attempting   large scale extractions.

Polar liquids mix with each other and dissolve salts and other polar solids. The solvents for nonpolar compounds are liquids of low or nil polarity.

The dielectric constant is a measure of the degree of polarization of a compound.

If this value is known it is then possible to predict whether a compound will be polar or nonpolar, with a high value indicating a highly polar compound.

The final choice of solvent will be influenced by the distribution or partition coefficient K where,

 K = Concentration of solute in extract

     Concentration of solute in raffinate

 Raffinate- a liquid from which impurities have been removed by solvent extraction

 A relatively high K value shows good separation of the aqueous and solvent phases possibility of a single-stage extraction system. 

Eg., A value of 50 indicates that the extraction can be easy and single-stage whereas a value of 0.1 shows that the extraction will be difficult and that a multistage process will be necessary.

In a number of systems the value of K is low and concurrent or countercurrent multistage systems have to be utilized. 

The cocurrent system and countercurrent system is shown below

Cocurrent- There are n mixer/separator vessels in line and the raffinate goes from vessel 1 to vessel n. Fresh solvent is added to each stage, the feed and extracting solvent pass through the cascade in the same direction. Extract is recovered from each stage. Although a relatively large amount of solvent is used, a high degree of extraction is achieved.

A countercurrent system also has a number of mixer/separators connected. The extracted raffinate passes from stage 1 to stage n while the product-enriched solvent is flowing from stage n to stage 1. The feed and extracting solvent pass through the stages in opposite directions.

The most efficient system for solvent utilization is countercurrent operation, showing a considerable advantage over batch and co-current systems. If done under centrifugal force it is found to be most productive. 

The Podbielniak centrifugal extractor is a common example.

 Penicillin G is an antibiotic which is recovered from fermentation broths by centrifugal countercurrent solvent extraction.

The solvents are expensive and their disposal is environmentally sensitive. Ther should be recovered after the process.Thus the success of a process depend on efficient solvent recovery and reuse.

Different types of  liquid -liquid extraction

1) Solvent recovery-distillation

2) Two phase aqueous extraction

3) Supercritical fluid extraction

(contd.)