Overview

• Introduction

• Important aspects in design and

operation of the extraction processes.

• Selectivity and Distribution coefficient

• Extraction techniques

• Extraction apparatus and equipment

Definition

Liquid-Liquid extraction is a mass

transfer operation in which a liquid

solution (the feed) is contacted with an

immiscible or nearly immiscible liquid

(solvent).

.

Diluent A and a solute B which are miscible.

.

The objective is to recover the solute

A + B

S

A + eB

S + B

Raffinate

Extract

Two streams result from this contact: the extract,

which is the solvent rich solution containing the

desired extracted solute, and the raffinate, the

residual feed solution containing little solute.

Extraction

Partition between two phases

Liquid/solid,

gas/liquid,

liquid/liquid

Partition based on relative solubility in the

two solvents

Two techniques:

Macroscale, using a separatory funnel

Microscale, using pipets and centrifuge tubes

Extraction Process

(in macroscale)

• Mixture dissolved in

solvent 1 & placed in

separatory funnel

• Immiscible solvent 2

added, stoppered &

agitated

• Mixture components

distribute according to

solubility

• Bottom layer drained

through stopcock to

separate phases

Microscale Extraction

• Mixture dissolved in

solvent 1 in a

centrifuge tube

• Immiscible solvent 2

added & agitated

• Bottom layer

removed with

Pasteur pipet &

transferred to clean

tube

The following need to be carefully evaluated

when optimizing the design and operation of

the extraction processes.

•Solvent selection

•Operating Conditions (pH,T,P & residence time)

•Mode of Operation

•Extractor Type

•Design Criteria

Selection of solvent influenced by

• -its physico-chemical properties (allowing an easyrecovery of the solute or of the solvent),

• -a negligible solubility of the solvent in the diluent(post-extraction processes have to be as cheap aspossible),

• -physical characteristics offering acceptabledispersion and separation times of the post-contactphases (viscosity, interfacial tension, densitydifference compared to the feed),

• -favorable properties (Mass transfer kinetics –equilibrium after contact less than a few minutes - ;Economy – cheap and available solvent - ; Safety ofuse – low toxicity, low flammability, low volatility,low corrosion in comparison with usualconstruction materials -),

But particularly a property called selectivity

Selectivity can be defined as the ability of the solvent to pick up the desired component in the feed as compared to other components. The desired properties of solvents are a high distribution coefficient, good selectivity towards solute and little or no miscibility with feed solution. Also, the solvent should be easily recoverable for recycle.

β =/ (mass fraction A in E) β > 1

/ (mass fraction A in R)

↓

This ratio is called the As for the second ratio, it is

distribution coefficient of always greater than 1 (there

the solute between the is more diluent in the raffinate

extract and the raffinate than in the extract

phases, m

The higher the m, the higher β and thus the more selective the solvent.

Consequently, a sufficient condition for a solvent to be selective is: m greater than 1

Distribution ratio

In solvent extraction, a distribution ratio is often quoted

as a measure of how well-extracted a species is.

Partition or Distribution Coefficient

KD or D = [𝐀]𝟏

[𝐀]𝟐 =

𝐬𝐨𝐥𝐮𝐛𝐢𝐥𝐢𝐭𝐲 𝐨𝐟 𝐀 𝐢𝐧 𝐚 𝐬𝐨𝐥𝐯𝐞𝐧𝐭 𝟏

𝐬𝐨𝐥𝐮𝐛𝐢𝐥𝐢𝐭𝐲 𝐨𝐟 𝐀 𝐢𝐧 𝐬𝐨𝐥𝐯𝐞𝐧𝐭 𝟐 =

𝐦𝐚𝐬𝐬 𝐀/𝐯𝐨𝐥𝐮𝐦𝐞 𝟏

(𝐦𝐚𝐬𝐬 𝐀/𝐯𝐨𝐥𝐮𝐦𝐞)𝟐 =

(𝐦𝐚𝐬𝐬 𝐀)𝟏

𝐦𝐚𝐬𝐬 𝐀 𝟐 ●

𝐯𝐨𝐥𝐮𝐦𝐞 𝟏

𝐯𝐨𝐥𝐮𝐦𝐞 𝟐

For typical extractions:

solvent 1 = organic, solvent 2 = water

Example: solubilities of adipic acid @ 15°C water (1.5 / 100 ml) ; ether (0.6g / 100 ml)

KD = 𝟎.𝟔𝒈

𝟏.𝟓𝒈 ●

𝟏𝟎𝟎 𝒎𝒍

𝟏𝟎𝟎 𝒎𝒍 = 0.40

Solvent Distribution

Coefficient @

20C

Miscibility with

water

wt% @ 20C

n-Butanol 1.6 >10

Ethyl Acetate 0.9 10

MIBK 0.7 2.0

Toluene 0.06 0.05

n-Hexane 0.01 0.015

No solvent offers all possible favorable conditions. Thus, a

compromise has to be found between all the constraints.

Solvents for Acetic Acid Extraction

Four extraction techniques

Single

stage

Cross-

current

Counter-

current

Counter-

current

with reflux

To design an extraction apparatus, it is necessary to:

- determine the number of ideal stages

- determine the phases flowrates, as well as the

solute distribution between the phases

- choose the most adapted apparatus

- study the hydrodynamics of the apparatus

- determine the size and the configuration of the

apparatus

EquipmentMixer-Settler

Centrifugal Extractor

Static / Agitated Columns

References

1. http://iweb.tntech.edu/chem311dc/

LabPDF/Extraction.pdf

2. Paul Ashall 2007. Liquid-liquid extraction

principle

3. Nadine LE BOLAY, Gilbert CASAMATTA.

Liquid extraction