Liquid-Liquid ExtractionLecture 23

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26 Nov 2012

Overview

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• Liquid-Liquid Extraction• (solvent extraction)

• Pioneered during 1940’s (uranium purification)• Alternative to distillation, absorption/stripping• Energy savings• Sometimes easier separation• Lower temperatures

• Usually two distinct phases formed

• Usual purpose, to either purify the• Raffinate, or• Solute

Liquid-Liquid Extraction

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• Separation accomplished by chemical differences

• Usually in two phase• - light phase• - heavy phase

• Usually coupled with another separation technique

let:

xassociatedwithmequilibriuiny

phaseextractinsolutefractionmassy

phaseraffinateinsolutefractionmassx

*

Extract

Solvent

Feed

Raffinate

[a+b]

[b] (+ a & s)

[s + a] (+b)

[s]

a = soluteb = diluents = solvent

Separator could be:column w/ stages or packingcolumn with moving internalssingle stage mixer/settlerequilibrium stage(s)

Extractor

Example Industrial Processes

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Seader & Henley (2006)

Typical LL Extraction

Process

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Seader & Henley (2006)

Equipment Examples

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Seader & Henley (2006)

Treybal (1980)

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8

9

10

Spray Columns:

Seader & Henley (2006)

11

Seader & Henley (2006)

Packed-bed Column

Treybal (1980)

Light liquid - dispersed phase

12Treybal (1980)

Sieve-tray Extraction Column: light phase dispersed

13Seader & Henley (2006)

Oldshue-Rushton (Mixco Lightnin CMContactor)

column Scheibel column

14Seader & Henley (2006)

Podbielniak Extractor

15Treybal (1980)

Equipment

16Seader & Henley (2006)

Equipment Examples

17Seader & Henley (2006)

Equilateral Triangular Diagrams

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[s] [b]

[a]

Rearrange:

mixturecombinedkgM

mixtureextractkgE

mixtureraffinatekgR

][

][

][

Overall material balance:

][kgMER Component material balance (on a):

MxEyRx MER

RM

ME

xx

xy

E

R

[s] [b]

Lever principle:

MR

EM

m

e

E

R

Equilateral Triangular Diagrams

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[s] [b]

[a]

[s] [b]

[a]

Type I Type II

Examples:• water (b), ethylene glycol (a), furfural (s)• water (b), acetone (a), chloroform (s)

Example:• n-heptane (b), methylcyclohexane (a), aniline (s)

Distribution Curves

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[s] [b]

[a]

[s] [b]

[a]

Type I

Type II

Rx

Ey

Distribution Curves

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[s] [b]

[a]

[s] [b]

[a]

Type I

Type II

Rx

Ey

Rx

Ey

Distribution Curves

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[s] [b]

[a]

[s] [b]

[a]

Type I

Type II

Rx

Ey

Rx

Ey

Effect of Temperature (and Pressure)

23Treybal (1980)

Effect of Temperature (and Pressure)

24Treybal (1980)

Choice of Solvent

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• Selectivity separation factor

• Distribution Coefficient better if

• Insolubility of Solvent better if less soluble in R phase

• Solvent Recoverability should be easy to separate solvent from E and R

• Density large density differences between the two phases is desired

• Interfacial Tension would like large for easier coalescence of dispersed phase

• Others:• solvent stable, inert, nontoxic, nonflammable, low cost• low viscosity• low vapor pressure• low freezing point

raffinateb

a

extractb

a

xx

yy

1;1 better

1K

Mixer – Settler (single stage extraction)

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ExtractSolvent

Feed Raffinate

mixer settlerNewSolvent

solventrecovery

solventrecovery

PurifiedRaffinate

PurifiedExtract

Recycled Solvent

Black Box: Rx

ExtractSolvent

Feed Raffinate

1stage

Fx

Sy Ey

F R

S E

Material balance:

ERSF M

[s] [b]

[a]

Mixer – Settler (single stage extraction)

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[s] [b]

[a]

Component material balance (on a in feeds): MxSyFx MSF

SF

SyFxx SFM

Component material balance (on a in products): MxEyRx MER

RE

RM

xy

xxME

given: SyFx SF ,,, find: REyxMx ERM ,,,,,

Mixer – Settler (single stage extraction)

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[s] [b]

[a]

Minimum Solvent (rate):

Maximum Solvent (rate):

SD

DF

yx

xx

DS

FD

F

S

min

SK

KF

yx

xx

KS

FK

F

S

max

Cross-Current (multi-stage extraction)

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[s] [b]

[a]

Final Extract

Feed FinalRaffinateFx

F1x

Solvent

Stage1

Sy

1y

1R

S

1E

2x

Solvent

Stage2

Sy

2y

2R

S

2E

3x

Solvent

Stage 3

Sy

3y

3R

S

3E

Final Extract: 321 EEE

321

332211

EEE

EyEyEyyFE

Continuous Multistage Countercurrent Extraction

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[s] [b]

[a]

1x

Extract Solvent

Feed Raffinate1Fx

1y 2y

F1R

1E2E

SyS

2 2x

3y

2R

3E2Nx

1Ny

2NR

1NEN-1 1Nx

Ny

1NR

NEN

NR

Nx

Total MB: MRESF N 1

SF

SyFxx SFM

Total MB on a:

If known (specified), thenflowrates can befound.

Nxy &1

NRE &1

Continuous Multistage Countercurrent Extraction

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[s] [b]

[a]

1x

Extract Solvent

Feed Raffinate1Fx

1y 2y

F1R

1E2E

SyS

2 2x

3y

2R

3E2Nx

1Ny

2NR

1NEN-1 1Nx

Ny

1NR

NEN

NR

Nx

Total MB: 1EFSRN MB from feed to N-1 stage: 11 EFER NN

Operating Point: 1EFR

R

Continuous Multistage Countercurrent Extraction

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[s] [b]

[a]

1x

Extract Solvent

Feed Raffinate1Fx

1y 2y

F1R

1E2E

SyS

2 2x

3y

2R

3E2Nx

1Ny

2NR

1NEN-1 1Nx

Ny

1NR

NEN

NR

Nx

Now step off to find number of equilibrium stages:

R

Questions?

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