Liquid Liquid Extraction

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

. .

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.

S Extract

A+B

S+B A + eB Raffinate

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 •Mode of Operation •Extractor Type •Design Criteria

(pH,T,P & residence time)

• • •

•

Selection of solvent influenced by -its physico-chemical properties (allowing an easy recovery of the solute or of the solvent), -a negligible solubility of the solvent in the diluent (post-extraction processes have to be as cheap as possible), -physical characteristics offering acceptable dispersion and separation times of the post-contact phases (viscosity, interfacial tension, density difference compared to the feed), -favorable properties (Mass transfer kinetics – equilibrium after contact less than a few minutes - ; Economy – cheap and available solvent - ; Safety of use – low toxicity, low flammability, low volatility, low corrosion in comparison with usual construction 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= This ratio is called the distribution coefficient of the solute between the extract and the raffinate phases, m

/ (mass fraction A in E) β>1 / (mass fraction A in R) ↓ As for the second ratio, it is always greater than 1 (there is more diluent in the raffinate than in the extract

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

Solvents for Acetic Acid Extraction Solvent

n-Butanol Ethyl Acetate MIBK Toluene n-Hexane

Distribution Coefficient @ 20C 1.6 0.9 0.7 0.06 0.01

Miscibility with water wt% @ 20C >10 10 2.0 0.05 0.015

No solvent offers all possible favorable conditions. Thus, a compromise has to be found between all the constraints.

Four extraction techniques Countercurrent with reflux

Single stage Crosscurrent

Countercurrent

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

Equipment Mixer-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