Unit OperationsLecture 23

15 Nov 2010

1

Overview•Liquid-Liquid ExtractionLiquid Liquid Extraction

•(solvent extraction)

•Pioneered d ring 1940’s ( rani m p rification)•Pioneered during 1940’s (uranium purification)•Alternative to distillation, absorption/stripping

•Energy savings•Sometimes easier separation•Lower temperatures

•Usually two distinct phases formedUsually two distinct phases formed

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

2

Liquid-Liquid Extraction• Separation accomplished by Extract Feed Sepa at o acco p s ed by

chemical differences• Usually in two phase

• - light phaseh h

Feed[a+b][s + a] (+b)

a = solute • - heavy phase• Usually coupled with another

separation technique

a = soluteb = diluents = solvent

Solvent Raffinate[b] (+ a & s)[s]

Separator could be:column w/ stages or packingcolumn with moving internalssingle stage mixer/settler

let: phaseraffinateinsolutefractionmassx

single stage mixer/settlerequilibrium stage(s)

xassociatedwithmequilibriuinyphaseextractinsolutefractionmassy

pfff

*

3

Example Industrial Processes

4

Seader & Henley (2006)

Typical LL Extraction

Process

5

Seader & Henley (2006)

Equipment ExamplesExamples

Treybal (1980)

6

Seader & Henley (2006)

Spray Columns:

7Seader & Henley (2006)

Seader & Henley (2006)Packed-bed Column

Light liquid - dispersed phase

8Treybal (1980)

Light liquid dispersed phase

Sieve-tray Extraction Column: l h h d dlight phase dispersed

9Treybal (1980)

Oldshue-Rushton (MixcoLightnin CMContactor) column

Scheibel column

10Seader & Henley (2006)

11Seader & Henley (2006)

Podbielniak Extractor

12Treybal (1980)

Equipment

13Seader & Henley (2006)

Equipment Examples

14Seader & Henley (2006)

Equilateral Triangular Diagrams[a] mixtureraffinatekgR ][[a]

mixturecombinedkgMmixtureextractkgE

mixtureraffinatekgR

][][][

[s] [b]

Overall material balance:][kgMER

Component material balance (on a):[s] [b]

Rearrange:

p ( )MxEyRx MER

RM

ME

xxxy

ER

RM xxE

Lever arm rule:

[s] [b]MREM

me

ER

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Equilateral Triangular Diagrams[a] [a][a] [a]

[s] [b] [s] [b][s] [b] [s] [b]

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)

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Distribution Curves[a][a]

Ey

[s] [b][s] [b]

[a]

Type I Rx

[s] [b]

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Type II

Distribution Curves[a][a]

Ey

[s] [b][s] [b]

[a]

Type I Rx

EyEy

[s] [b]

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Type II Rx

Distribution Curves[a][a]

Ey

[s] [b][s] [b]

[a]

Type I Rx

EyEy

[s] [b]

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Type II Rx

Effect of Temperature (and Pressure)

20Treybal (1980)

Effect of Temperature (and Pressure)

21Treybal (1980)

Choice of Solventay

•Selectivity separation factor

raffinateb

a

extractb

xx

y

1;1 better

•Distribution Coefficient better if

•Insolubility of Solvent better if less soluble in R phase

1Ky p

•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

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Mixer – Settler (single stage extraction)Purified

E t tSolvent

Feed Raffinate

i ttlN l t

solventrecovery

PurifiedRaffinate

PurifiedExtractSolvent mixer settlerNew

Solventsolventrecovery

Extract

Recycled Solvent

Black Box: RxFeed Raffinate

1FxF R[ ]Black Box:

ExtractSolvent stage

Sy EyS E

Material balance:

[a]

ERSF M

[ ] [b]

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[s] [b]

Mixer – Settler (single stage extraction)given: SyFx SF ,,, find: REyxMx ERM ,,,,,

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

g ySF yERM ,,,,,

p S

SFSyFxx SF

M

[ ]

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

RM xxME [a]

RE xyE

[ ] [b]

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[s] [b]

Mixer – Settler (single stage extraction)Minimum Solvent (rate):

[a]

Minimum Solvent (rate):

SD

DF

yxxx

DSFD

FS

min

[s] [b]Maximum Solvent (rate):

KF xxFKS max

SK

KF

yxKSF max

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Cross-Current (multi-stage extraction)Final ExtractyE yE yE Final Extract

Feed FinalRaffinateFx

F1x

Stage1

1y

1R1E

2xStage

2

2y

2R2E

3xStage

3

3y

3R3E

1

SolventSyS

2

SolventSyS

3

SolventSyS

[a]

Final Extract: 321 EEE

332211 EyEyEyyFE

[b]

321 EEEyFE

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[s] [b]

Continuous Multistage Countercurrent Extraction

1x

Extract Solvent

Feed Raffinate1FxF

1R

2E S2 2x

2R

3E2Nx2NR

1NEN-1 1Nx

1NR

NEN

NRNx

Solvent1y 2y

1E Sy3y 1Ny Ny

Total MB: MRESF N 1[a]

SFSyFxx SF

M

Total MB on a:

If known (specified), thenflowrates can befound.

Nxy &1

NRE &1

[b]

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[s] [b]

Continuous Multistage Countercurrent Extraction

1x

Extract Solvent

Feed Raffinate1FxF

1R

2E S2 2x

2R

3E2Nx2NR

1NEN-1 1Nx

1NR

NEN

NRNx

Solvent1y 2y

1E Sy3y 1Ny Ny

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

[a]Operating Point: 1EFR

[b]R

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[s] [b]

Continuous Multistage Countercurrent Extraction

1x

Extract Solvent

Feed Raffinate1FxF

1R

2E S2 2x

2R

3E2Nx2NR

1NEN-1 1Nx

1NR

NEN

NRNx

Solvent1y 2y

1E Sy3y 1Ny Ny

Now step off to find number of equilibrium [a]stages:

[b]R

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[s] [b]

Questions?

30