Perry Hambook

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15-1

Section 15

Liquid-Liquid Extraction and OtherLiquid-Liquid Operations and Equipment*

Timothy C. Frank, Ph.D. Research Scientist and Sr. Technical Leader, The Dow Chemi-cal Company; Member, American Institute of Chemical Engineers (Section Editor, Introduction

and Overview, Thermodynamic Basis for Liquid-Liquid Extraction, Solvent Screening Methods,Liquid-Liquid Dispersion Fundamentals, Process Fundamentals and Basic Calculation Meth-ods, Dual-Solvent Fractional Extraction, Extractor Selection, Packed Columns, Agitated Extrac-

tion Columns, Mixer-Settler Equipment, Centrifugal Extractors, Process Control Considerations,Liquid-Liquid Phase Separation Equipment, Emerging Developments)

Lise Dahuron, Ph.D. Sr. Research Specialist, The Dow Chemical Company (Liquid Den-sity, Viscosity, and Interfacial Tension; Liquid-Liquid Dispersion Fundamentals; Liquid-LiquidPhase Separation Equipment; Membrane-Based Processes)

Bruce S. Holden, M.S. Process Research Leader, The Dow Chemical Company; Member,American Institute of Chemical Engineers [Process Fundamentals and Basic Calculation Meth-ods, Calculation Procedures, Computer-Aided Calculations (Simulations), Single-Solvent Frac-

tional Extraction with Extract Reflux, Liquid-Liquid Phase Separation Equipment]

William D. Prince, M.S. Process Engineering Associate, The Dow Chemical Company;Member, American Institute of Chemical Engineers (Extractor Selection, Agitated ExtractionColumns, Mixer-Settler Equipment)

A. Frank Seibert, Ph.D., P.E. Technical Manager, Separations Research Program, TheUniversity of Texas at Austin; Member, American Institute of Chemical Engineers (Liquid-

Liquid Dispersion Fundamentals, Process Fundamentals and Basic Calculation Methods,Hydrodynamics of Column Extractors, Static Extraction Columns, Process Control Considera-

tions, Membrane-Based Processes)

Loren C. Wilson, B.S. Sr. Research Specialist, The Dow Chemical Company (Liquid Den-sity, Viscosity, and Interfacial Tension; Phase Diagrams; Liquid-Liquid Equilibrium Experi-mental Methods; Data Correlation Equations; Table of Selected Partition Ratio Data)

*Certain portions of this section are drawn from the work of Lanny A. Robbins and Roger W. Cusack, authors of Sec. 15 in the 7th edition. The input from numer-ous expert reviewers also is gratefully acknowledged.

INTRODUCTION AND OVERVIEWHistorical Perspective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-6Uses for Liquid-Liquid Extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-7Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-10

Desirable Solvent Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-11Commercial Process Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-13

Standard Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-13Fractional Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-13

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Dissociative Extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-15pH-Swing Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-16Reaction-Enhanced Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-16Extractive Reaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-16Temperature-Swing Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-17Reversed Micellar Extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-18Aqueous Two-Phase Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-18Hybrid Extraction Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-18Liquid-Solid Extraction (Leaching) . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-19Liquid-Liquid Partitioning of Fine Solids . . . . . . . . . . . . . . . . . . . . . . 15-19

Supercritical Fluid Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-19Key Considerations in the Design of an Extraction Operation . . . . . . . 15-20Laboratory Practices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-21

THERMODYNAMIC BASIS FOR LIQUID-LIQUID EXTRACTION

Activity Coefficients and the Partition Ratio. . . . . . . . . . . . . . . . . . . . . . 15-22Extraction Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-22Separation Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-23Minimum and Maximum Solvent-to-Feed Ratios. . . . . . . . . . . . . . . . 15-23Temperature Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-23Salting-out and Salting-in Effects for Nonionic Solutes . . . . . . . . . . . 15-24Effect of pH for Ionizable Organic Solutes. . . . . . . . . . . . . . . . . . . . . 15-24

Phase Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-25Liquid-Liquid Equilibrium Experimental Methods. . . . . . . . . . . . . . . . 15-27Data Correlation Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-27

Tie Line Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-27Thermodynamic Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-28Data Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-28

Table of Selected Partition Ratio Data . . . . . . . . . . . . . . . . . . . . . . . . . . 15-32Phase Equilibrium Data Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-32Recommended Model Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-32

SOLVENT SCREENING METHODS

Use of Activity Coefficients and Related Data . . . . . . . . . . . . . . . . . . . . 15-32Robbins Chart of Solute-Solvent Interactions . . . . . . . . . . . . . . . . . . . . 15-32Activity Coefficient Prediction Methods . . . . . . . . . . . . . . . . . . . . . . . . . 15-33Methods Used to Assess Liquid-Liquid Miscibility . . . . . . . . . . . . . . . . 15-34Computer-Aided Molecular Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-38High-Throughput Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . 15-39

LIQUID DENSITY, VISCOSITY, AND INTERFACIAL TENSION

Density and Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-39Interfacial Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-39

LIQUID-LIQUID DISPERSION FUNDAMENTALS

Holdup, Sauter Mean Diameter, and Interfacial Area . . . . . . . . . . . . . . 15-41Factors Affecting Which Phase Is Dispersed . . . . . . . . . . . . . . . . . . . . . 15-41Size of Dispersed Drops. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-42Stability of Liquid-Liquid Dispersions . . . . . . . . . . . . . . . . . . . . . . . . . . 15-43Effect of Solid-Surface Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-43Marangoni Instabilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-43

PROCESS FUNDAMENTALS AND

BASIC CALCULATION METHODS

Theoretical (Equilibrium) Stage Calculations . . . . . . . . . . . . . . . . . . . . . 15-44McCabe-Thiele Type of Graphical Method . . . . . . . . . . . . . . . . . . . . 15-45Kremser-Souders-Brown Theoretical Stage Equation . . . . . . . . . . . . 15-45Stage Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-46

Rate-Based Calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-47Solute Diffusion and Mass-Transfer Coefficients . . . . . . . . . . . . . . . . 15-47Mass-Transfer Rate and Overall Mass-Transfer Coefficients . . . . . . . 15-47Mass-Transfer Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-48

Extraction Factor and General Performance Trends . . . . . . . . . . . . . . . 15-49Potential for Solute Purification Using Standard Extraction . . . . . . . . . 15-50

CALCULATION PROCEDURES

Shortcut Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-51Example 1: Shortcut Calculation, Case A . . . . . . . . . . . . . . . . . . . . . . 15-52

Example 2: Shortcut Calculation, Case B . . . . . . . . . . . . . . . . . . . . . . 15-52Example 3: Number of Transfer Units . . . . . . . . . . . . . . . . . . . . . . . . 15-53

Computer-Aided Calculations (Simulations). . . . . . . . . . . . . . . . . . . . . . 15-53Example 4: Extraction of Phenol from Wastewater . . . . . . . . . . . . . . 15-54

Fractional Extraction Calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-55Dual-Solvent Fractional Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-55Single-Solvent Fractional Extraction with Extract Reflux . . . . . . . . . 15-56Example 5: Simplified Sulfolane ProcessExtraction

of Toluene from n-Heptane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-56

LIQUID-LIQUID EXTRACTION EQUIPMENT

Extractor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-58Hydrodynamics of Column Extractors . . . . . . . . . . . . . . . . . . . . . . . . . . 15-59

Flooding Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-59Accounting for Axial Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-60Liquid Distributors and Dispersers . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-63

Static Extraction Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-63Common Features and Design Concepts . . . . . . . . . . . . . . . . . . . . . . 15-63Spray Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-69Packed Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-70Sieve Tray Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-74Baffle Tray Columns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-78

Agitated Extraction Columns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-79Rotating-Impeller Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-79Reciprocating-Plate Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-83Rotating-Disk Contactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-84Pulsed-Liquid Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-85

Raining-Bucket Contactor (a Horizontal Column) . . . . . . . . . . . . . . . 15-85Mixer-Settler Equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-86Mass-Transfer Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-86Miniplant Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-87Liquid-Liquid Mixer Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-87Scale-up Criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-88Specialized Mixer-Settler Equipment . . . . . . . . . . . . . . . . . . . . . . . . . 15-89Suspended-Fiber Contactor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-90

Centrifugal Extractors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-91Single-Stage Centrifugal Extractors. . . . . . . . . . . . . . . . . . . . . . . . . . . 15-91Centrifugal Extractors Designed for

Multistage Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-92

PROCESS CONTROL CONSIDERATIONS

Steady-State Process Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-93Sieve Tray Column Interface Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-94Controlled-Cycling Mode of Operation. . . . . . . . . . . . . . . . . . . . . . . . . . 15-94

LIQUID-LIQUID PHASE SEPARATION EQUIPMENT

Overall Process Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-96Feed Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-96Gravity Decanters (Settlers). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-97

Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-97Vented Decanters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-98Decanters with Coalescing Internals. . . . . . . . . . . . . . . . . . . . . . . . . . 15-99Sizing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-99

Other Types of Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-101Coalescers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-101Centrifuges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-101Hydrocyclones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-101Ultrafiltration Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-102Electrotreaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-102

EMERGING DEVELOPMENTS

Membrane-Based Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-103Polymer Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-103Liquid Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-104

Electrically Enhanced Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-104Phase Transition Extraction and Tunable Solvents . . . . . . . . . . . . . . . . . 15-105Ionic Liquids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-105

15-2 LIQUID-LIQUID EXTRACTION AND OTHER LIQUID-LIQUID OPERATIONS AND EQUIPMENT


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LIQUID-LIQUID EXTRACTION AND OTHER LIQUID-LIQUID OPERATIONS AND EQUIPMENT 15-3

a Interfacial area per unit m2/m3 ft2/ft3

volume

ap Specific packing surface area m2

/m3

ft2

/ft3

(area per unit volume)aw Specific wall surface area m

2/m3 ft2/ft3

(area per unit volume)bij NRTL model regression K K

parameter (see Table 15-10)A Envelope-style downcomer m2 ft2

areaA Area between settled layers m2 ft2

in a decanterAcol Column cross-sectional area m

2 ft2

Adow Area for flow through m2 ft2

a downcorner (orupcomer)

Ai,j/RT van Laar binary interaction Dimensionless Dimensionlessparameter

Ao Cross-sectional area of a m2 in2

single hole

C Concentration (mass or kg

m

3

or lb/ft

3

ormol per unit volume) kgmolm3 lbmolft3

or gmolLCA

i Concentration of component kgm3 or lb/ft3 orA at the interface kgmolm3 lbmolft3

or gmolLC* Concentration at equilibrium kgm3 or lb/ft3 or

kgmolm3 lbmolft3

or gmolLCD Drag coefficient Dimensionless DimensionlessCo Initial concentration kgm

3 or lb/ft3

kgmolm3 or lbmolft3

or gmolLCt Concentration at timet kgm

3 or lb/ft3

kgmolm3 or lbmolft3

or gmolLd Drop diameter m indC Critical packing dimension m indi Diameter of an individual drop m indm Characteristic diameter of m in

media in a packed beddo Orifice or nozzle diameter m indp Sauter mean drop diameter m ind32 Sauter mean drop diameter m inDcol Column diameter m in or ftDeq Equivalent diameter giving m in

the same areaDh Equivalent hydraulic diameter m inDi Distribution ratio for a given

chemical species includingall its forms (unspecified units)

Di Impeller diameter or m in or ftcharacteristic mixerdiameter

Dsm Static mixer diameter m in or ftDt Tank diameter m ftD Molecular diffusion coefficient m2/s cm2/s

(diffusivity)DAB Mutual diffusion coefficient m

2/s cm2/sfor componentsA andB

E Mass or mass flow rate of kg or kg/s lb or lb/hextract phase

E Solvent mass or mass flow rate(in the extract phase)

E Axial mixing coefficient m2/s cm2/s(eddy diffusivity)

EC Extraction factor for case C Dimensionless Dimensionless

[Eq. (15-98)]

Ei Extraction factor for Dimensionless DimensionlesscomponentiEs Stripping section extraction Dimensionless Dimensionless

factorEw Washing section extraction Dimensionless Dimensionless

factorfda Fractional downcomer area Dimensionless Dimensionless

in Eq. (15-160)fha Fractional hole area in Dimensionless Dimensionless

Eq. (15-159)F Mass or mass flow rate of kg or kg/s lb or lb/h

feed phaseF Force N lbfF Feed mass or mass flow rate kg or kg/s lb or lb/h

(feed solvent only)FR Solute reduction factor (ratio of Dimensionless Dimensionless

inlet to outlet concentrations)g Gravitat ional acceleration 9.807 m/s2 32.17 ft/s2

Gij NRTL model parameter Dimensionless Dimensionlessh Height of coalesced layer at m ina sieve tray

h Head loss due to frictional flow m inh Height of dispersion band in m in

batch decanterhiE Excess enthalpy Jgmol Btulbmol

of mixing or calgmolH Dimensionless group defined Dimensionless Dimensionless

by Eq. (15-123)H Dimension of envelope-style m in or ft

downcomer (Fig. 15-39)H Steady-state dispersion band m in

height in a continuously feddecanter

HDU Height of a dispersion unit m inHe Height of a transfer unit due m in

to resistance in extract phaseHETS Height equivalent to a m in

theoretical stageHor Height of an overall m in

mass-tranfer unit based onraffinate phase

Hr Height of a transfer unit due m into resistance in raffinate phase

I Ionic strength in Eq. (15-26)k Individual mass-transfer m/s or cm/s ft/h

coefficientk Mass-transfer coefficient

(unspecified units)km Membrane-side mass-transfer m/s or cm/s ft/h

coefficientko Overall mass-transfer m/s or cm/s ft/h

coefficientkc Continuous-phase m/s or cm/s ft/h

mass-transfer coefficientkd Dispersed-phase mass-transfer m/s or cm/s ft/h

coefficientks Setschenow constant Lgmol Lgmolks Shell-side mass-transfer m/s or cm/s ft/h

coefficientkt Tube-side mass-transfer m/s or cm/s ft/h

coefficientK Partition ratio (unspecified units)Ks Stripping section partition Mass ratio/ Mass ratio/

ratio (in Bancroft coordinates) mass ratio mass ratio

Nomenclature

A given symbol may represent more than one property. The appropriate meaning should be apparent from the context. The equations given in Sec. 15 reflect theuse of the SI or cgs system of units and not ft-lb-s units, unless otherwise noted in the text. The gravitational conversion factor gc needed to use ft-lb-s units is notincluded in the equations.

U.S. Customary U.S. CustomarySymbol Definition SI units System units Symbol Definition SI units System units


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Re Reynolds number: for pipe Dimensionless Dimensionlessflow, Vd; for an impeller,mDi

2m; for drops, Vsodpc

c; for flow in a packed-bedcoalescer, Vdmc; for flowthrough an orifice, Vododd

ReStokes c gd3p18c2 Dimensionless DimensionlessS Mass or mass flow rate of kg or kg/s lb or lb/h

solvent phaseS Dimension of envelope-style m ft

downcomer (Fig. 15-39)S Solvent mass or mass flow kg or kg/s lb or lb/h

rate (extraction solvent only)Ss Mass flow rate of extraction kg/s lb/h

solvent within strippingsection

Sw Mass flow rate of extraction kg/s lb/hsolvent within washing section

Si,j Separation power for Dimensionless Dimensionlessseparating componenti fromcomponentj [defined byEq. (15-105)]

Stip Impeller tip speed m/s ft/stb Batch mixing time s or h min or h

T Temperature (absolute) K Rut Stokes law terminal or m/s or cm/s ft/s or ft/min

settling velocity of a droput Unhindered sett ling velocity m/s or cm/s ft/s or ft/min

of a single dropv Molar volume m3kgmol or ft3lbmol

cm3gmolV Liquid velocity (or m/s ft/s or ft/min

volumetric flow perunit area)

V Volume m3 ft3 or galVcf Continuous-phase m/s ft/s or ft/min

flooding velocityVcflow Cross-flow velocity of m/s ft/s or ft/min

continuous phase atsieve tray

Vdf Dispersed-phase m/s ft/s or ft/minflooding velocity

Vdrop Average velocity of a m/s ft/s or ft/mindispersed dropVic Interstitial velocity of m/s ft/s or ft/min

continuous phaseVo,max Maximum velocity through m/s ft/s or ft/min

an orifice or nozzleVs Slip velocity m/s ft/s or ft/minVso Slip velocity at low m/s ft/s or ft/min

dispersed-phase flow rateVsm Static mixer superficial liquid m/s ft/s or ft/min

velocity (entrance velocity)W Mass or mass flow rate of kg or kg/s lb or lb/h

wash solvent phaseWs Mass flow rate of wash solvent kg/s lb/h

within stripping sectionWw Mass flow rate of wash solvent kg/s lb/h

within washing sectionWe Weber number: for an Dimensionless Dimensionless

impeller, c2

Di3; for flowthrough an orifice or nozzle,

Vo2dod; for a static mixer,

V2smDsmcx Mole fraction solute in feed Mole fraction Mole fraction

or raffinateX Concentration of solute in feed

or raffinate (unspecified units)X Mass fraction solute in feed Mass fractions Mass fractions

or raffinateX Mass solute/mass feed Mass ratios Mass ratios

solvent in feed or raffinateXf

B Pseudoconcentration of Mass ratios Mass ratiossolute in feed for case B[Eq. (15-95)]

Kw Washing section partition ratio Mass ratio/ Mass ratio/(in Bancroft coordinates) mass ratio mass ratio

K Partition ratio, mass ratio basis Mass ratio/ Mass ratio/(Bancroft coordinates) mass ratio mass ratio

K Partition ratio, mass fraction Mass fraction/ Mass fraction/basis mass fraction mass fraction

Ko Partition ratio, mole Mole fraction/ Mole fraction/ fraction basis mole fraction mole fraction

Kvol Partition ratio (volumetric Ratio of kg/m3 Ratio of lb/ft3

concentration basis) or kgmolm3 or lbmolft3

or gmolLL Downcomer (or m in or ft

upcomer) lengthLfp Length of flow path in m in or ft

Eq. (15-161)m Local slope of equilibrium line

(unspecified concentrationunits)

m Local slope of equil ibrium line Mass ratio/ Mass ratio/(in Bancroft coordinates) mass ratio mass ratio

mdc Local slope of equilibrium linefor dispersed-phaseconcentration plotted versuscontinuous-phase

concentrationmer Local slope of equilibrium

line for extract-phaseconcentration plottedversus raffinate-phaseconcentration

mvol Local slope of equilibrium Ratio of kg/m3 Ratio of lb/ft3 orline (volumetric or kgmolm3 lbmolft3

concentration basis) or gmolL unitsM Mass or mass flow rate kg or kg/s lb or lb/hMW Molecular weight kgkgmol or lblbmol

ggmolN Number of theoretical stages Dimensionless DimensionlessNA Flux of component A (mass (kg or kgmol)/ (lb or lbmol)

or mol/area/unit time) (m2s) (ft2s)Nholes Number of holes Dimensionless DimensionlessNor Number of overall Dimensionless Dimensionless

mass-transfer units based

on the raffinate phaseNs Number of theoretical stages Dimensionless Dimensionlessin stripping section

Nw Number of theoretical stages Dimensionless Dimensionlessin washing section

P Pressure bar or Pa atm or lbf/in2

P Dimensionless group defined Dimensionless Dimensionlessby Eq. (15-122)

P Power W or kW HP or ftlbf/hPe Pclet number Vb/E, Dimensionless Dimensionless

whereVis liquidvelocity, E is axial mixingcoefficient, andb is acharacteristic equipmentdimension

Pi,extract Purity of solutei in wt % wt %extract (in wt %)

Pi,feed Purity of solutei in feed wt % wt %

(in wt %)Po Power number P(m3Di

5) Dimensionless DimensionlessPdow Pressure drop for flow bar or Pa atm or lbf/in

2

through a downcomer(or upcomer)

Po Orifice pressure drop bar or Pa atm or lbf/in2

q MOSCED induction Dimensionless Dimensionlessparameter

Q Volumetric flow rate m3/s ft3/minR Universal gas constant 8.31 JK 1.99 BtuR

kgmol lbmolR Mass or mass flow rate of kg or kg/s lb or lb/h

raffinate phaseRA Rate of mass-transfer (moles kgmols lbmolh

per unit time)

Nomenclature(Continued)



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LIQUID-LIQUID EXTRACTION AND OTHER LIQUID-LIQUID OPERATIONS AND EQUIPMENT 15-5

Nomenclature(Concluded)


XfC Pseudoconcentration of Mass ratios Mass ratios

solute in feed for case C[Eq. (15-97)]

Xi,extract Concentration of solutei Mass fraction Mass fractionin extract

Xi,feed Concentration of solutei Mass fraction Mass fractionin feedXij Concentration of component Mass fraction Mass fraction

i in the phase richest injy Mole fraction solute in Mole fraction Mole fraction

solvent or extractY Concentration of solute in

the solvent or extract(unspecified units)

Y Mass fraction solute Mass fraction Mass fractionin solvent or extract

Y Mass solute/mass extraction Mass ratio Mass ratiosolvent in solvent orextract

YsB Pseudoconcentration of Mass ratio Mass ratio

solute in solvent for case B[Eq. (15-96)]

z Dimension or direction of m in or ftmass transfer

z Sieve tray spacing m in or ftz Point representing feed

composition on a tie linezi Number of electronic Dimensionless Dimensionless

charges on an ionZt Total height of extractor m ft

Greek Symbols

MOSCED hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2

acidity parameter Solvatochromic hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2

acidity parameteri,j Separation factor for solutei Dimensionless Dimensionless

with respect to soluteji,j NRTL model parameter Dimensionless Dimensionless MOSCED hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2

basicity parameter

Solvatochromic hydrogen-bond (J/cm3

)1/2

(cal/cm3

)1/2

basicity parameteri,j Activity coefficient ofi Dimensionless Dimensionless

dissolved inj Activity coefficient at Dimensionless Dimensionless

infinite dilutionCi Activity coefficient, Dimensionless Dimensionless

combinatorial part ofUNIFAC

iI Activity coefficient of Dimensionless Dimensionless

componenti in phase Ii

R Activity coefficient, residual Dimensionless Dimensionlesspart of UNIFAC

Void fraction Dimensionless Dimensionless Fractional open area of a Dimensionless Dimensionless

perforated plate Solvatochromic polarizability (J/cm3)1/2 (cal/cm3)1/2

parameterd

Hansen nonpolar (dispersion) (J/cm3)1/2 (cal/cm3)1/2

solubility parameterh Hansen solubility parameter (J/cm

3)1/2 (cal/cm3)1/2

for hydrogen bondingp Hansen polar solubility (J/cm

3)1/2 (cal/cm3)1/2

parameter

Greek Symbols

i Solubility parameter for (J/cm3)1/2 (cal/cm3)1/2

componenti

Solubility parameter for mixture (J/cm3)1/2 (cal/cm3)1/2

Tortuosity factor defined by Dimensionless Dimensionless

Eq. (15-147) Residence time for total liquid s s or mini Fraction of solutei extracted Dimensionless Dimensionless

from feed MOSCED dispersion parameter (J/cm3)1/2 (cal/cm3)1/2

m Membrane thickness mm in Liquid viscosity Pas cPi

I Chemical potential of J/gmol Btu/lbmolcomponenti in phase I

m Mixture mean viscosity Pas cPdefined in Eq. (15-180)

w Reference viscosity (of water) Pas cP1 MOSCED asymmetry factor Dimensionless Dimensionlessbatch Efficiency of a batch Dimensionless Dimensionless

experiment [Eq. (15-175)]continuous Efficiency of a continuous Dimensionless Dimensionless

process [Eq. (15-176)]m Murphree stage efficiency Dimensionless Dimensionless

md Murphree stage efficiency Dimensionless Dimensionlessbased on dispersed phaseo Overall s tage efficiency Dimensionless Dimensionless Solvatochromic polarity (J/cm3)1/2 (cal/cm3)1/2

parameter Osmotic pressure gradient bar or Pa atm or lbf/in

2

Liquid density kg/m3 lb/ft3

m Mixture mean density defined kg/m3 lb/ft3

in Eq. (15-178) Interfacial tension N/m dyn/cm MOSCED polarity parameter (J/cm3)1/2 (cal/cm3)1/2

i,j NRTL model parameter Dimensionless Dimensionless Volume fraction Dimensionless Dimensionlessd Volume fraction of dispersed Dimensionless Dimensionless

phase (holdup)d,feed Volume fraction of dispersed Dimensionless Dimensionless

phase in feedo Initial dispersed-phase holdup Dimensionless Dimensionless

in feed to a decanter Volume fraction of voids Dimensionless Dimensionlessin a packed bed

Factor governing use of Eqs. Dimensionless Dimensionless(15-148) and (15-149)

Parameter in Eq. (15-41) Dimensionless Dimensionlessindicating which phase islikely to be dispersed

Impeller speed Rotations/s Rotations/min

Additional Subscripts

c Continuous phased Dispersed phasee Extract phasef Feed phase or flooding condition (when combined with d orc)i Componentij ComponentjH Heavy liquid

L Light liquidmax Maximum valuemin Minimum valueo Orifice or nozzler Raffinate phases Solvent


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GENERAL REFERENCES: Wankat, Separation Process Engineering, 2d ed.(Prentice-Hall, 2006); Seader and Henley, Separation Process Principles, 2d ed.(Wiley, 2006); Seibert, Extraction and Leaching, Chap. 14 in Chemical ProcessEquipment: Selection and Design, 2d ed., Couper et al., eds. (Elsevier, 2005);Aguilar and Cortina, Solvent Extraction and Liquid Membranes: Fundamentalsand Applications in New Materials (Dekker, 2005); Glatz and Parker, EnrichingLiquid-Liquid Extraction, Chem. Eng. Magazine, 111(11), pp. 4448 (2004); Sol-vent Extraction Principles and Practice, 2d ed., Rydberg et al., eds. (Dekker, 2004);Ion Exchange and Solvent Extraction, vol. 17, Marcus and SenGupta, eds. (Dekker,2004), and earlier volumes in the series; Leng and Calabrese, Immiscible Liquid-

Liquid Systems, Chap. 12 in Handbook of Industrial Mixing: Science and Practice,Paul, Atiemo-Obeng, and Kresta, eds. (Wiley, 2004); Cheremisinoff, Industrial Sol-vents Handbook, 2d ed. (Dekker, 2003); Van Brunt and Kanel, Extraction withReaction, Chap. 3 in Reactive Separation Processes, Kulprathipanja, ed. (Taylor &Francis, 2002); Mueller et al., Liquid-Liquid Extraction in Ullmanns Encyclope-dia of Industrial Chemistry, 6th ed. (VCH, 2002); Benitez, Principles and ModernApplications of Mass Transfer Operations(Wiley, 2002); Wypych, Handbook of Sol-vents (Chemtec, 2001); Flick, Industrial Solvents Handbook, 5th ed. (Noyes,1998); Robbins, Liquid-Liquid Extraction, Sec. 1.9 in Handbook of SeparationTechniques for Chemical Engineers, 3d ed., Schweitzer, ed. (McGraw-Hill, 1997);Lo, Commercial Liquid-Liquid Extraction Equipment, Sec. 1.10 in Handbook ofSeparation Techniques for Chemical Engineers, 3d ed., Schweitzer, ed. (McGraw-Hill, 1997); Humphrey and Keller, Extraction, Chap. 3 in Separation ProcessTechnology (McGraw-Hill, 1997), pp. 113151; Cusack and Glatz, Apply Liquid-Liquid Extraction to Todays Problems, Chem. Eng. Magazine, 103(7), pp. 94103(1996); Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley,1994); Zaslavsky,Aqueous Two-Phase Partitioning (Dekker, 1994); Strigle, Liquid-Liquid Extraction, Chap. 11 in Packed Tower Design and Applications, 2d ed.

(Gulf, 1994); Schgerl, Solvent Extraction in Biotechnology (Springer-Verlag,1994); Schgerl, Liquid-Liquid Extraction (Small Molecules), Chap. 21 inBiotechnology, 2d ed., vol. 3, Stephanopoulos, ed. (VCH, 1993); Kelley and Hat-ton, Protein Purification by Liquid-Liquid Extraction, Chap. 22 in Biotechnol-ogy, 2d ed., vol. 3, Stephanopoulos, ed. (VCH, 1993); Lo and Baird, Extraction,

Liquid-Liquid, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.,vol. 10, Kroschwitz and Howe-Grant, eds. (Wiley, 1993), pp. 125180; Science andPractice of Liquid-Liquid Extraction, vol. 1, Phase Equilibria; Mass Transfer andInterfacial Phenomena; Extractor Hydrodynamics, Selection, and Design, and vol.2, Process Chemistry and Extraction Operations in the Hydrometallurgical,Nuclear, Pharmaceutical, and Food Industries, Thornton, ed. (Oxford, 1992);Cusack, Fremeaux, and Glatz, A Fresh Look at Liquid-Liquid Extraction, pt. 1,Extraction Systems, Chem. Eng. Magazine, 98(2), pp. 6667 (1991); Cusack andFremeauz, pt. 2, Inside the Extractor, Chem. Eng. Magazine, 98(3), pp. 132138(1991); Cusack and Karr, pt. 3, Extractor Design and Specification, Chem. Eng.

Magazine,98(4), pp. 112120 (1991); Methods in Enzymology, vol. 182, Guide toProtein Purification, Deutscher, ed. (Academic, 1990); Wankat, EquilibriumStaged Separations (Prentice Hall, 1988); Blumberg, Liquid-Liquid Extraction(Academic, 1988); Skelland and Tedder, ExtractionOrganic Chemicals Process-ing, Chap. 7 in Handbook of Separation Process Technology, Rousseau, ed. (Wiley,1987); Chapman, ExtractionMetals Processing, Chap. 8 in Handbook of Sepa-ration Process Technology, Rousseau, ed. (Wiley, 1987); Novak, Matous, and Pick,Liquid-LiquidEquilibria, Studiesin ModernThermodynamics Series,vol. 7 (Else-vier, 1987); Bailes et al., Extraction, Liquid-Liquid in Encyclopedia of ChemicalProcessing and Design, vol. 21, McKetta and Cunningham, eds. (Dekker, 1984),pp. 19166; Handbook of Solvent Extraction, Lo, Baird, and Hanson, eds. (Wiley,1983; Krieger, 1991); Sorenson and Arlt,Liquid-Liquid Equilibrium Data Collec-tion, DECHEMA, Binary Systems, vol. V, pt. 1, 1979, Ternary Systems,vol. V, pt.2, 1980, Ternary and Quaternary Systems,vol. 5, pt. 3, 1980, Macedo and Ras-mussen,Suppl.1, vol. V, pt.4, 1987; Wisniak andTamir,Liquid-LiquidEquilibriumand Extraction, a Literature Source Book, vols. I and II (Elsevier, 19801981),Suppl. 1 (1985); Treybal, Mass Transfer Operations, 3d ed. (McGraw-Hill, 1980);King, Separation Processes, 2d ed. (McGraw-Hill, 1980); Laddha and Degaleesan,

Transport Phenomena in Liquid Extraction (McGraw-Hill, 1978); Brian, StagedCascades in ChemicalProcessing (Prentice-Hall, 1972);Pratt,CountercurrentSep-aration Processes (Elsevier, 1967); Treybal, Liquid Extractor Performance,Chem. Eng. Prog., 62(9), pp. 6775 (1966); Treybal, Liquid Extraction, 2d ed.(McGraw-Hill, 1963);Alders,Liquid-Liquid Extraction, 2d ed. (Elsevier, 1959).

INTRODUCTION AND OVERVIEW

Liquid-liquid extraction is a process for separating the components ofa liquid (the feed) by contact with a second liquid phase (the solvent).The process takes advantage of differences in the chemical proper-ties of the feed components, such as differences in polarity andhydrophobic/hydrophilic character, to separate them. Stated moreprecisely, the transfer of components from one phase to the other is

driven by a deviation from thermodynamic equilibrium, and theequilibrium state depends on the nature of the interactions betweenthe feed components and the solvent phase. The potential for sepa-rating the feed components is determined by differences in theseinteractions.

A liquid-liquid extraction process produces a solvent-rich streamcalled the extract that contains a portion of the feed and an extracted-feed stream called the raffinate. A commercial process almost alwaysincludes two or more auxiliary operations in addition to the extractionoperation itself. These extra operations are needed to treat the extractand raffinate streams for the purposes of isolating a desired product,recovering the solvent for recycle to the extractor, and purgingunwanted components from the process. A typical process includestwo or more distillation operations in addition to extraction.

Liquid-liquid extraction is used to recover desired componentsfrom a crude liquid mixture or to remove unwanted contaminants. In

developing a process, the project team must decide what solvent orsolvent mixture to use, how to recover solvent from the extract, andhow to remove solvent residues from the raffinate. The team mustalso decide what temperature or range of temperatures should beused for the extraction, what process scheme to employ among manypossibilities, and what type of equipment to use for liquid-liquid con-tacting and phase separation. The variety of commercial equipmentoptions is large and includes stirred tanks and decanters, specializedmixer-settlers, a wide variety of agitated and nonagitated extractioncolumns or towers, and various types of centrifuges.

Because of the availability of hundreds of commercial solvents andextractants, as well as a wide variety of established process schemesand equipment options, liquid-liquid extraction is a versatile technol-ogy with a wide range of commercial applications. It is utilized in the

processing of numerous commodity and specialty chemicals includingmetals and nuclear fuel (hydrometallurgy), petrochemicals, coal and

wood-derived chemicals, and complex organics such as pharmaceuti-cals and agricultural chemicals. Liquid-liquid extraction also is animportant operation in industrial wastewater treatment, food process-ing, and the recovery of biomolecules from fermentation broth.

HISTORICAL PERSPECTIVE

The art of solvent extraction has been practiced in one form oranother since ancient times. It appears that prior to the 19th centurysolvent extraction was primarily used to isolate desired componentssuch as perfumes and dyes from plant solids and other natural sources[Aftalion, A History of the International Chemical Industry (Univ.Penn. Press, 1991); and Taylor, A History of Industrial Chemistry(Abelard-Schuman, 1957)]. However, several early applicationsinvolving liquid-liquid contacting are described by Blass, Liebel, andHaeberl [Solvent ExtractionA Historical Review, InternationalSolvent Extraction Conf. (ISEC) 96 Proceedings (Univ. of Mel-bourne, 1996)], including the removal of pigment from oil by using

water as the solvent.The modern practice of liquid-liquid extraction has its roots in the

middle to late 19th century when extraction became an important lab-oratory technique. The partition ratio concept describing how a solutepartitions between two liquid phases at equilibrium was introduced byBerthelot and Jungfleisch [Ann. Chim. Phys.,4, p. 26 (1872)] and fur-ther defined by Nernst [Z. Phys. Chemie,8, p. 110 (1891)]. At aboutthe same time, Gibbs published his theory of phase equilibrium (1876and 1878). These and other advances were accompanied by a growingchemical industry. An early countercurrent extraction process utiliz-ing ethyl acetate solvent was patented by Goering in 1883 as a methodfor recovering acetic acid from pyroligneous acid produced bypyrolysis of wood [Othmer, p. xiv in Handbook of Solvent Extraction(Wiley, 1983; Krieger, 1991)], and Pfleiderer patented a stirred extrac-tion column in 1898 [Blass, Liebl, and Haeberl, ISEC 96 Proceedings(Univ. of Melbourne, 1996)].

15-6


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With the emergence of the chemical engineering profession in the1890s and early 20th century, additional attention was given to processfundamentals and development of a more quantitative basis forprocess design. Many of the advances made in the study of distillationand absorption were readily adapted to liquid-liquid extraction, owingto its similarity as another diffusion-based operation. Examplesinclude application of mass-transfer coefficients [Lewis, Ind. Eng.Chem., 8(9), pp. 825833 (1916); and Lewis and Whitman, Ind. Eng.Chem., 16(12), pp. 12151220 (1924)], the use of graphical stagewise

design methods [McCabe and Thiele, Ind. Eng. Chem., 17(6), pp.605611 (1925); Evans, Ind. Eng. Chem., 26(8), pp. 860864 (1934);and Thiele, Ind. Eng. Chem., 27(4), pp. 392396 (1935)], the use oftheoretical-stage calculations [Kremser, National Petroleum News,22(21), pp. 4349 (1930); and Souders and Brown, Ind. Eng. Chem.24(5), pp. 519522 (1932)], and the transfer unit concept introducedin the late 1930s by Colburn and others [Colburn, Ind. Eng. Chem.,33(4), pp. 459467 (1941)]. Additional background is given byHampe, Hartland, and Slater [Chap. 2 in Liquid-Liquid ExtractionEquipment, Godfrey and Slater, eds. (Wiley, 1994)].

The number of commercial applications continued to grow, and bythe 1930s liquid-liquid extraction had replaced various chemical treat-ment methods for refining mineral oil and coal tar products [Varter-essian and Fenske, Ind. Eng. Chem., 28(8), pp. 928933 (1936)]. It

was also used to recover acetic acid from waste liquors generated inthe production of cellulose acetate, and in various nitration and sul-

fonation processes [Hunter and Nash, The Industrial Chemist,9(102104), pp. 245248, 263266, 313316 (1933)]. The article byHunter and Nash also describes early mixer-settler equipment, mixing

jets, and various extraction columns including the spray column, baf-fle tray column, sieve tray column, and a packed column filled withRaschig rings or coke breeze, the material left behind when coke isburned.

Much of the liquid-liquid extraction technology in practice todaywas first introduced to industry during a period of vigorous innovationand growth of the chemical industry as a whole from about 1920 to1970. The advances of this period include development of fractionalextraction schemes including work described by Cornish et al., [Ind.Eng. Chem., 26(4), pp. 397406 (1934)] and by Thiele [Ind. Eng.Chem., 27(4), pp. 392396 (1935)]. A well-known commercial exam-ple involving the use of extract reflux is the Udex process for separat-ing aromatic compounds from hydrocarbon mixtures using diethylene

glycol, a process developed jointly by The Dow Chemical Companyand Universal Oil Products in the 1940s. This period also saw theintroduction of many new equipment designs including specializedmixer-settler equipment, mechanically agitated extraction columns,and centrifugal extractors as well as a great increase in the availabilityof different types of industrial solvents. A variety of alcohols, ketones,esters, and chlorinated hydrocarbons became available in large quan-tities beginning in the 1930s, as petroleum refiners and chemicalcompanies found ways to manufacture them inexpensively using thebyproducts of petroleum refining operations or natural gas. Later, anumber of specialty solvents were introduced including sulfolane(tetrahydrothiophene-1,1-dioxane) and NMP (N-methyl-2-pyrrolidi-none) for improved extraction of aromatics from hydrocarbons.Specialized extractants also were developed including numerousorganophosphorous extractants used to recover or purify metals dis-solved in aqueous solutions.

The ready availability of numerous solvents and extractants, com-bined with the tremendous growth of the chemical industry, drove thedevelopment and implementation of many new industrial applica-tions. Handbooks of chemical process technology provide a glimpse ofsome of these [Riegels Handbook of Industrial Chemistry, 10th ed.,Kent, ed. (Springer, 2003); Chemical Processing Handbook, McKetta,ed. (Dekker, 1993); and Austin, Shreves Chemical Process Industries,5th ed. (McGraw-Hill, 1984)], but many remain proprietary and arenot widely known. The better-known examples include the separationof aromatics from aliphatics, as mentioned above, extraction of phe-nolic compounds from coal tars and liquors, recovery of -caprolactamfor production of polyamide-6 (nylon-6), recovery of hydrogen perox-ide from oxidized anthraquinone solution, plus many processes involv-ing the washing of crude organic streams with alkaline or acidic

solutions and water, and the detoxification of industrial wastewaterprior to biotreatment using steam-strippable organic solvents. Thepharmaceutical and specialty chemicals industry also began using liq-uid-liquid extraction in the production of new synthetic drug com-pounds and other complex organics. In these processes, ofteninvolving multiple batch reaction steps, liquid-liquid extraction gener-ally is used for recovery of intermediates or crude products prior tofinal isolation of a pure product by crystallization. In the inorganicchemical industry, extraction processes were developed for purifica-

tion of phosphoric acid, purification of copper by removal of arsenicimpurities, and recovery of uranium from phosphate-rock leach solu-tions, among other applications. Extraction processes also were devel-oped for bioprocessing applications, including the recovery of citricacid from broth using trialkylamine extractants, the use of amylacetate to recover antibiotics from fermentation broth, and the use of

water-soluble polymers in aqueous two-phase extraction for purifica-tion of proteins.

The use of supercritical or near-supercritical fluids for extraction, asubject area normally set apart from discussions of liquid-liquidextraction, has received a great deal of attention in the R&D commu-nity since the 1970s. Some processes were developed many yearsbefore then; e.g., the propane deasphalting process used to refinelubricating oils uses propane at near-supercritical conditions, and thistechnology dates back to the 1930s [McHugh and Krukonis, Super-critical Fluid Processing, 2d ed. (Butterworth-Heinemann, 1993)]. In

more recent years the use of supercritical fluids has found a numberof commercial applications displacing earlier liquid-liquid extractionmethods, particularly for recovery of high-value products meant forhuman consumption including decaffeinated coffee, flavor compo-nents from citrus oils, and vitamins from natural sources.

Significant progress continues to be made toward improving extrac-tion technology, including the introduction of new methods to esti-mate solvent properties and screen candidate solvents and solventblends, new methods for overall process conceptualization and opti-mization, and new methods for equipment design. Progress also isbeing made by applying the technology developed for a particularapplication in one industry to improve another application in anotherindustry. For example, much can be learned by comparing equipmentand practices used in organic chemical production with those used inthe inorganic chemical industry (and vice versa), or by comparingpractices used in commodity chemical processing with those used in

the specialty chemicals industry. And new concepts offering potentialfor significant improvements continue to be described in the litera-ture. (See Emerging Developments.)

USES FOR LIQUID-LIQUID EXTRACTION

For many separation applications, the use of liquid-liquid extraction isan alternative to the various distillation schemes described in Sec. 13,Distillation. In many of these cases, a distillation process is more eco-nomical largely because the extraction process requires extra opera-tions to process the extract and raffinate streams, and these operationsusually involve the use of distillation anyway. However, in certain casesthe use of liquid-liquid extraction is more cost-effective than using dis-tillation alone because it can be implemented with smaller equipmentand/or lower energy consumption. In these cases, differences in chem-ical or molecular interactions between feed components and the sol-

vent provide a more effective means of accomplishing the desiredseparation compared to differences in component volatilities.For example, liquid-liquid extraction may be preferred when the

relative volatility of key components is less than 1.3 or so, such that anunusually tall distillation tower is required or the design involves highreflux ratios and high energy consumption. In certain cases, the distil-lation option may involve addition of a solvent (extractive distillation)or an entrainer (azeotropic distillation) to enhance the relative volatil-ity. Even in these cases, a liquid-liquid extraction process may offeradvantages in terms of higher selectivity or lower solvent usage andlower energy consumption, depending upon the application. Extrac-tion may be preferred when the distillation option requires operationat pressures less than about 70 mbar (about 50 mmHg) and an unusu-ally large-diameter distillation tower is required, or when most of the

INTRODUCTION AND OVERVIEW 15-7


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feed must be taken overhead to isolate a desired bottoms product.Extraction may also be attractive when distillation requires use ofhigh-pressure steam for the reboiler or refrigeration for overheadscondensation [Null, Chem. Eng. Prog., 76(8), pp. 4249 (August1980)], or when the desired product is temperature-sensitive andextraction can provide a gentler separation process.

Of course, liquid-liquid extraction also may be a useful option whenthe components of interest simply cannot be separated by using distil-lation methods. An example is the use of liquid-liquid extraction

employing a steam-strippable solvent to remove nonstrippable, low-volatility contaminants from wastewater [Robbins, Chem. Eng. Prog.,76(10), pp. 5861 (1980)]. The same process scheme often provides acost-effective alternative to direct distillation or stripping of volatileimpurities when the relative volatility of the impurity with respect to

water is less than about 10 [Robbins, U.S. Patent 4,236,973 (1980);Hwang, Keller, and Olson, Ind. Eng. Chem. Res.,31, pp. 17531759(1992); and Frank et al., Ind. Eng. Chem. Res.,46(11), pp. 37743786(2007)].

Liquid-liquid extraction also can be an attractive alternative to sepa-ration methods, other than distillation, e.g., as an alternative to crystal-lization from solution to remove dissolved salts from a crude organicfeed, since extraction of the salt content into water eliminates the needto filter solids from the mother liquor, often a difficult or expensiveoperation. Extraction also may compete with process-scale chromatog-raphy, an example being the recovery of hydroxytyrosol (3,4-dihydroxy-

phenylethanol), an antioxidant food additive, from olive-processingwastewaters [Guzman et al., U.S. Patent 6,849,770 (2005)].

The attractiveness of liquid-liquid extraction for a given applicationcompared to alternative separation technologies often depends uponthe concentration of solute in the feed. The recovery of acetic acidfrom aqueous solutions is a well-known example [Brown, Chem. Eng.Prog., 59(10), pp. 6568 (1963)]. In this case, extraction generally ismore economical than distillation when handling dilute to moderatelyconcentrated feeds, while distillation is more economical at higherconcentrations. In the treatment of water to remove trace amounts oforganics, when the concentration of impurities in the feed is greaterthan about 20 to 50 ppm, liquid-liquid extraction may be more eco-nomical than adsorption of the impurities by using carbon beds,because the latter may require frequent and costly replacement of theadsorbent [Robbins, Chem. Eng. Prog.,76(10), pp. 5861 (1980)]. Atlower concentrations of impurities, adsorption may be the more eco-

nomical option because the usable lifetime of the carbon bed islonger.Examples of cost-effective liquid-liquid extraction processes utiliz-

ing relatively low-boiling solvents include the recovery of acetic acidfrom aqueous solutions using ethyl ether or ethyl acetate [King, Chap.18.5 in Handbook of Solvent Extraction, Lo, Baird, and Hanson, eds.(Wiley, 1983, Krieger, 1991)] and the recovery of phenolic compoundsfrom water by using methyl isobutyl ketone [Greminger et al., Ind.Eng. Chem. Process Des. Dev., 21(1), pp. 5154 (1982)]. In theseprocesses, the solvent is recovered from the extract by distillation, anddissolved solvent is removed from the raffinate by steam stripping(Fig. 15-1). The solvent circulates through the process in a closedloop.

One of the largest applications of liquid-liquid extraction in termsof total worldwide production volume involves the extraction of aro-matic compounds from hydrocarbon mixtures in petrochemical oper-

ations using high-boiling polar solvents. A number of processes havebeen developed to recover benzene, toluene, and xylene (BTX) asfeedstock for chemical manufacturing or to refine motor oils. Thisgeneral technology is described in detail in Single-Solvent FractionalExtraction with Extract Reflux under Calculation Procedures. Atypical flow diagram is shown in Fig. 15-2. Liquid-liquid extractionalso may be used to upgrade used motor oil; an extraction processemploying a relatively light polar solvent such as N,N-dimethylform-amide or acetonitrile has been developed to remove polynuclear aro-matic and sulfur-containing contaminants [Sherman, Hershberger,and Taylor, U.S. Patent 6,320,090 (2001)]. An alternative process uti-lizes a blend of methyl ethyl ketone + 2-propanol and small amountsof aqueous KOH [Rincn, Caizares, and Garca, Ind. Eng. Chem.Res.,44(20), pp. 78547859 (2005)].

Extraction also is used to remove CO2, H2S, and other acidic contam-

inants from liquefied petroleum gases (LPGs) generated during opera-tion of fluid catalytic crackers and cokers in petroleum refineries, andfrom liquefied natural gas (LNG). The acid gases are extracted from theliquefied hydrocarbons (primarily C1 to C3) by reversible reaction with

various amine extractants. Typical amines are methyldiethanolamine(MDEA), diethanolamine (DEA), and monoethanolamine (MEA). In atypical process (Fig. 15-3), the treated hydrocarbon liquid (the raffi-nate) is washed with water to remove residual amine, and the loadedamine solution (the extract) is regenerated in a stripping tower for recy-cle back to the extractor [Nielsen et al., Hydrocarbon Proc., 76, pp.4959 (1997)]. The technology is similar to that used to scrub CO2 andH2S from gas streams [Oyenekan and Rochelle, Ind. Eng. Chem. Res.,45(8), pp. 24652472 (2006); and Jassim and Rochelle, Ind. Eng. Chem.Res., 45(8), pp. 24572464 (2006)], except that the process involves liq-uid-liquid contacting instead of gas-liquid contacting. Because of this, acommon stripper often is used to regenerate solvent from a variety of

gas absorbers and liquid-liquid extractors operated within a typicalrefinery. In certain applications, organic acids such as formic acid arepresent in low concentrations in the hydrocarbon feed. These contami-nants will react with the amine extractant to form heat-stable aminesalts that accumulate in the solvent loop over time, requiring periodicpurging or regeneration of the solvent solution [Price and Burns,Hydrocarbon Proc.,74, pp. 140141 (1995)]. The amine-based extrac-tion process is an alternative to washing with caustic or the use of solidadsorbents.

A typical extraction process used in hydrometallurgical applicationsis outlined in Fig. 15-4. This technology involves transferring thedesired element from the ore leachate liquor, an aqueous acid, into anorganic solvent phase containing specialty extractants that form acomplex with the metal ion. The organic phase is later contacted withan aqueous solution at a different pH and temperature to regeneratethe solvent and transfer the metal into a clean solution from which it

can be recovered by electrolysis or another method [Cox, Chap. 1 inScience and Practice of Liquid-Liquid Extraction, vol. 2, Thornton,ed. (Oxford, 1992)]. Another process technology utilizes metals com-plexed with various organophosphorus compounds as recyclablehomogeneous catalysts; liquid-liquid extraction is used to transfer themetal complex between the reaction phase and a separate liquid phaseafter reaction. Different ligands having different polarities are chosento facilitate the use of various extraction and recycle schemes [Kanelet al., U.S. Patents 6,294,700 (2001) and 6,303,829 (2001)].

Another category of useful liquid-liquid extraction applicationsinvolves the recovery of antibiotics and other complex organics fromfermentation broth by using a variety of oxygenated organic solventssuch as acetates and ketones. Although some of these products areunstable at the required extraction conditions (particularly if pH must


FIG. 15-1 Typical process for extraction of acetic acid from water.


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Extract

Raffinate to WaterWash Column

E

XTR

Solvent

RecoveredSolvent

Reflux

Reformate (Feed)

S

TRIPPE

RProduct

DIST

SimulatedProcess(Example 5)

FIG. 15-2 Flow sheet of a simplified aromatic extraction process (see Example 5).

Extract

Raffinate

E

XTR

DIST

To Acid GasDisposal

Recycle Solvent

Sour

Feed

Washwater

To Amine Recovery or Disposal

Sweetened Hydrocarbon

FIG. 15-3 Typical process for extracting acid gases from LPG or LNG.


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be low for favorable partitioning), short-contact-time centrifugalextractors may be used to minimize exposure. Centrifugal extractorsalso help overcome problems associated with formation of emulsionsbetween solvent and broth. In a number of applications, the wholebroth can be processed without prior removal of solids, a practice thatcan significantly reduce costs. For detailed information, see The His-tory of Penicillin Production, Elder, ed., Chemical EngineeringProgress Symposium Series No. 100, vol. 66, pp. 3742 (1970); Queenerand Swartz, Penicillins: Biosynthetic and Semisynthetic, in SecondaryProducts of Metabolism, Economic Microbiology, vol. 3, Rose, ed. (Aca-demic, 1979); and Chaung et al.,J. Chinese Inst. Chem. Eng., 20(3), pp.

155161 (1989). Another well-known commercial application of liquid-liquid extraction in bioprocessing is the Baniel process for the recoveryof citric acid from fermentation broth with tertiary amine extractants[Baniel, Blumberg, and Hadju, U.S. Patent 4,275,234 (1980)]. This typeof process is discussed in Reaction-Enhanced Extraction under Com-mercial Process Schemes.

DEFINITIONS

Extraction terms defined by the International Union of Pure andApplied Chemistry (IUPAC) generally are recommended. [See Rice,Irving, and Leonard, Pure Appl. Chem. (IUPAC), 65(11), pp.26732396 (1993); and J. Inczdy, Pure Appl. Chem. (IUPAC), 66(12),pp. 25012512 (1994).] Liquid-liquid extraction is a process for sep-arating components dissolved in a liquid feed by contact with a secondliquid phase. Solvent extraction is a broader term that describes a

process for separating the components of any matrix by contact with aliquid, and it includes liquid-solid extraction (leaching) as well as liquid-liquid extraction. The feed to a liquid-liquid extraction process is thesolution that contains the components to be separated. The major liquidcomponent (or components) in the feed can be referred to as the feedsolvent or the carrier solvent. Minor components in solution oftenare referred to as solutes. The extraction solvent is the immiscible orpartially miscible liquid added to the process to create a second liquidphase for the purpose of extracting one or more solutes from the feed.It is also called the separating agent and may be a mixture of severalindividual solvents (a mixed solvent or a solvent blend). The extrac-tion solvent also may be a liquid comprised of an extractant dissolvedin a liquid diluent. In this case, the extractant species is primarilyresponsible for extraction of solute due to a relatively strong attractive

interaction with the desired solute, forming a reversible adduct or mol-ecular complex. The diluent itself does not contribute significantly tothe extraction of solute and in this respect is not the same as a trueextraction solvent. A modifiermay be added to the diluent to increasethe solubility of the extractant or otherwise enhance the effectiveness ofthe extractant. The phase leaving a liquid-liquid contactor rich in extrac-tion solvent is called the extract. The raffinate is the liquid phase leftfrom the feed after it is contacted by the extract phase. The word raffi-

nate originally referred to a refined product; however, common usagehas extended its meaning to describe the feed phase after extraction

whether that phase is a product or not.

Industrial liquid-liquid extraction most often involves processingtwo immiscible or partially miscible liquids in the form of a disper-sion of droplets of one liquid (the dispersed phase) suspended inthe other liquid (the continuous phase). The dispersion will exhibita distribution of drop diameters di often characterized by the volumeto surface area average diameter or Sauter mean drop diameter.The term emulsion generally refers to a liquid-liquid dispersion witha dispersed-phase mean drop diameter on the order of 1 m or less.

The tension that exists between two liquid phases is called theinterfacial tension. It is a measure of the energy or work required toincrease the surface area of the liquid-liquid interface, and it affectsthe size of dispersed drops. Its value, in units of force per unit lengthor energy per unit area, reflects the compatibility of the two liquids.Systems that have low compatibility (low mutual solubility) exhibithigh interfacial tension. Such a system tends to form relatively largedispersed drops and low interfacial area to minimize contact between

the phases. Systems that are more compatible (with higher mutual sol-ubility) exhibit lower interfacial tension and more easily form smalldispersed droplets.

A theoretical or equilibrium stage is a device or combination ofdevices that accomplishes the effect of intimately mixing two liquidphases until equilibrium concentrations are reached, then physicallyseparating the two phases into clear layers. The partition ratio K iscommonly defined for a given solute as the solute concentration in theextract phase divided by that in the raffinate phase after equilibrium isattained in a single stage of contacting. A variety of concentration unitsare used, so it is important to determine how partition ratios have beendefined in the literature for a given application. The term partitionratio is preferred, but it also is referred to as the distribution con-stant, distribution coefficient, or the Kvalue. It is a measure of the


Stripping (Back Extraction)

Solvent Extraction

Ore

Acid LeachingDepletedLeachate

AqueousLeachate

LeanOrganic

LoadedOrganic

Impurities

AqueousScrubLiquor

Impurity Removal

Winning

DepletedAqueous

LoadedAqueous

Metal

FIG. 15-4 Example process scheme used in hydrometallurgical applications. [Taken from Cox, Chap. 1 inScience and Practice of Liquid-Liquid Extraction, vol. 2, Thornton, ed. (Oxford, 1992), with permission.Copyright 1992 Oxford University Press.]


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thermodynamic potential of a solvent for extracting a given solute andcan be a strong function of composition and temperature. In somecases, the partition ratio transitions from a value less than unity to a

value greater than unity as a function of solute concentration. A systemof this type is called a solutrope [Smith, Ind. Eng. Chem., 42(6), pp.12061209 (1950)]. The term distribution ratio, designated by Di, isused in analytical chemistry to describe the distribution of a speciesthat undergoes chemical reaction or dissociation, in terms of the totalconcentration of analyte in one phase over that in the other, regardless

of its chemical form.The extraction factor E is a process variable that characterizes the

capacity of the extract phase to carry solute relative to the feed phase.Its value largely determines the number of theoretical stages requiredto transfer solute from the feed to the extract. The extraction factor isanalogous to the stripping factor in distillation and is the ratio of theslope of the equilibrium line to the slope of the operating line in aMcCabe-Thiele type of stagewise graphical calculation. For a stan-dard extraction process with straight equilibrium and operating lines,E is constant and equal to the partition ratio for the solute of interesttimes the ratio of the solvent flow rate to the feed flow rate. The sep-aration factor ai,j measures the relative enrichment of solute i inthe extract phase, compared to solutej, after one theoretical stageof extraction. It is equal to the ratio of Kvalues for componentsi andjand is used to characterize the selectivitya solvent has for a givensolute.

A standard extraction process is one in which the primary pur-pose is to transfer solute from the feed phase into the extract phase ina manner analogous to stripping in distillation. Fractional extractionrefers to a process in which two or more solutes present in the feed aresharply separated from each other, one fraction leaving the extractorin the extract and the other in the raffinate. Cross-current or cross-flow extraction (Fig. 15-5) is a series of discrete stages in which theraffinate R from one extraction stage is contacted with additional freshsolvent S in a subsequent stage. Countercurrent extraction (Fig.15-6) is an extraction scheme in which the extraction solvent entersthe stage or end of the extraction farthest from where the feed Fenters, and the two phases pass each other in countercurrent fashion.The objective is to transfer one or more components from the feedsolution F into the extract E. Compared to cross-current operation,countercurrent operation generally allows operation with less solvent.

When a staged contactor is used, the two phases are mixed with

droplets of one phase suspended in the other, but the phases are sep-arated before leaving each stage. A countercurrent cascade is aprocess utilizing multiple staged contactors with countercurrent flowof solvent and feed streams from stage to stage. When a differentialcontactoris used, one of the phases can remain dispersed as dropsthroughout the contactor as the phases pass each other in countercur-rent fashion. The dispersed phase is then allowed to coalesce at theend of the device before being discharged. For these types ofprocesses, mass-transfer units (or the related mass-transfer coef-ficients) often are used instead of theoretical stages to characterizeseparation performance. For a given phase, mass-transfer units are

defined as the integral of the differential change in solute concentra-tion divided by the deviation from equilibrium, between the limits ofinlet and outlet solute concentrations. A single transfer unit repre-sents the change in solute concentration equal to that achieved by asingle theoretical stage when the extraction factor is equal to 1.0. Itdiffers from a theoretical stage at other values of the extraction factor.

The term flooding generally refers to excessive breakthrough orentrainment of one liquid phase into the discharge stream of the other.The flooding characteristics of an extractor limit its hydraulic capacity.Flooding can be caused by excessive flow rates within the equipment,byphase inversion due to accumulation and coalescence of disperseddroplets, or by formation of stable dispersions or emulsions due to thepresence of surface-active impurities or excessive agitation. The floodpoint typically refers to the specific total volumetric throughput in(m3/h)/m2 or gpm/ft2 of cross-sectional area (or the equivalent phasevelocityin m/s or ft/s) at which flooding begins.

DESIRABLE SOLVENT PROPERTIES

Common industrial solvents generally are single-functionality organicsolvents such as ketones, esters, alcohols, linear or branched aliphatichydrocarbons, aromatic hydrocarbons, and so on; or water, which may

be acidic or basic or mixed with water-soluble organic solvents. Morecomplex solvents are sometimes used to obtain specific propertiesneeded for a given application. These include compounds with multi-ple functional groups such as diols or triols, glycol ethers, and alkanolamines as well as heterocyclic compounds such as pine-derived sol-

vents (terpenes), sulfolane (tetrahydrothiophene-1,1-dioxane), andNMP (N-methyl-2-pyrrolidinone). Solvent properties have been sum-marized in a number of handbooks and databases including those byCheremisinoff, Industrial Solvents Handbook, 2d ed. (Dekker, 2003);

Wypych, Handbook of Solvents (ChemTech, 2001); Wypych, SolventsDatabase, CD-ROM (ChemTec, 2001); Yaws, Thermodynamic andPhysical Property Data, 2d ed. (Gulf, 1998); and Flick, Industrial Sol-

vents Handbook, 5th ed. (Noyes, 1998). Solvents are sometimesblended to obtain specific properties, another approach to achieving amultifunctional solvent with properties tailored for a given applica-tion. Examples are discussed by Escudero, Cabezas, and Coca [Chem.

Eng. Comm., 173, pp. 135146 (1999)] and by Delden et al. [Chem.Eng. Technol., 29(10), pp. 12211226 (2006)]. As discussed earlier, asolvent also may be a liquid containing a dissolved extractant species,the extractant chosen because it forms a specific attractive interaction

with the desired solute.In terms of desirable properties, no single solvent or solvent blend

can be best in every respect. The choice of solvent often is a compro-mise, and the relative weighting given to the various considerationsdepends on the given situation. Assessments should take into accountlong-term sustainability and overall cost of ownership. Normally, thefactors considered in choosing a solvent include the following.

1. Loading capacity. This property refers to the maximum con-centration of solute the extract phase can hold before two liquidphases can no longer coexist or solute precipitates as a separate phase.


S1

F

E1

S2

R1

E2

S3

R2

E3

R3

FIG. 15-5 Cross-current extraction.

S

F E1 or E

Feed Stage

R1 E2

Raffinate Stage

R2 E3

Ror R3

FIG. 15-6 Standard countercurrent extraction.


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If a specialized extractant is used, loading capacity may be determinedby the point at which all the extractant in solution is completely occu-pied by solute and extractant solubility limits capacity. If loadingcapacity is low, a high solvent-to-feed ratio may be needed even if thepartition ratio is high.

2. Partition ratio Ki = Yi/Xi. Partition ratios on the order of Ki = 10or higher are desired for an economical process because they allowoperation with minimal amounts of solvent (more specifically, with aminimal solvent-to-feed ratio) and production of higher solute con-

centrations in the extractunless the solute concentration in the feedalready is high and a limitation in the solvents loading capacity deter-mines the required solvent-to-feed ratio. Since high partition ratiosgenerally allow for low solvent use, smaller and less costly extractionequipment may be used and costs for solvent recovery and recycle arelower. In principle, partition ratios less than Ki = 1.0 may be accom-modated by using a high solvent-to-feed ratio, but usually at muchhigher cost.

3. Solute selectivity. In certain applications, it is important notonly to recover a desired solute from the feed, but also to separate itfrom other solutes present in the feed and thereby achieve a degree ofsolute purification. The selectivity of a given solvent for solutei com-pared to solutej is characterized by the separation factor i,j = Ki/Kj.

Values must be greater than i,j = 1.0 to achieve an increase in solutepurity (on a solvent-free basis). When solvent blends are used in a com-mercial process, often it is because the blend provides higher selectiv-

ity, and often at the expense of a somewhat lower partition ratio. Thedegree of purification that can be achieved also depends on theextraction scheme chosen for the process, the amount of extractionsolvent, and the number of stages employed.

4. Mutual solubility. Low liquid-liquid mutual solubility betweenfeed and solvent phases is desirable because it reduces the separationrequirements for removing solvents from the extract and raffinatestreams. Low solubility of extraction solvent in the raffinate phaseoften results in high relative volatility for stripping the residual solventin a raffinate stripper, allowing low-cost desolventizing of the raffinate[Hwang, Keller, and Olson, Ind. Eng. Chem. Res., 31(7), pp.17531759 (1992)]. Low solubility of feed solvent in the extract phasereduces separation requirements for recovering solvent for recycleand producing a purified product solute. In some cases, if the solubil-ity of feed solvent in the extract is high, more than one distillationoperation will be required to separate the extract phase. If mutual sol-

ubility is nil (as for aliphatic hydrocarbons dissolved in water), theneed for stripping or another treatment method may be avoided aslong as efficient liquid-liquid phase separation can be accomplished

without entrainment of solvent droplets into the raffinate. However,very low mutual solubility normally is achieved at the expense of alower partition ratio for extracting the desired solutebecause a sol-

vent that has very little compatibility with the feed solvent is not likelyto be a good extractant for something that is dissolved in the feed sol-

ventand therefore has some compatibility. Mutual solubility alsolimits the solvent-to-feed ratios that can be used, since a point can bereached where the solvent stream is so large it dissolves the entirefeed stream, or the solvent stream is so small it is dissolved by thefeed, and these can be real limitations for systems with high mutualsolubility.

5. Stability. The solvent should have little tendency to react withthe product solute and form unwanted by-products, causing a loss in

yield. Also it should not react with feed components or degrade toundesirable contaminants that cause development of undesirableodors or color over time, or cause difficulty achieving desired productpurity, or accumulate in the process because they are difficult to purge.

6. Density difference. As a general rule, a difference in densitybetween solvent and feed phases on the order of 0.1 to 0.3 g/mL ispreferred. A value that is too low makes for poor or slow liquid-liquidphase separation and may require use of a centrifuge. A value that istoo high makes it difficult to build high dispersed-droplet populationdensity for good mass transfer; i.e., it is difficult to mix the two phasestogether and maintain high holdup of the dispersed phase within theextractorbut this depends on the viscosity of the continuous phase.

7. Viscosity. Low viscosity is preferred since higher viscositygenerally increases mass-transfer resistance and liquid-liquid phase

separation difficulty. Sometimes an extraction process is operated atan elevated temperature where viscosity is significantly lower for bet-ter mass-transfer performance, even when this results in a lower par-tition ratio. Low viscosity at ambient temperatures also facilitatestransfer of solvent from storage to processing equipment.

8. Interfacial tension. Preferred values for interfacial tensionbetween the feed phase and the extraction solvent phase generally arein the range of 5 to 25 dyn/cm(1 dyn/cm is equivalent to 103 N/m).Systems with lower values easily emulsify. For systems with higher

values, dispersed droplets tend to coalesce easily, resulting in lowinterfacial area and poor mass-transfer performance unless mechani-cal agitation is used.

9. Recoverability. The economical recovery of solvent from theextract and raffinate is critical to commercial success. Solvent physicalproperties should facilitate low-cost options for solvent recovery, recy-cle, and storage. For example, the use of relatively low-boiling organicsolvents with low heats of vaporization generally allows cost-effectiveuse of distillation and stripping for solvent recovery. Solvent proper-ties also should enable low-cost methods for purging impurities fromthe overall process (lights and/or heavies) that may accumulate overtime. One of the challenges often encountered in utilizing a high-boil-ing solvent or extractant involves accumulation of heavy impurities inthe solvent phase and difficulty in removing them from the process.Another consideration is the ease with which solvent residues can bereduced to low levels in final extract or raffinate products, particularly

for food-grade products and pharmaceuticals.10. Freezing point. Solvents that are liquids at all anticipated

ambient temperatures are desirable since they avoid the need forfreeze protection and/or thawing of frozen solvent prior to use. Some-times an antifreeze compound such as water or an aliphatic hydro-carbon can be added to the solvent, or the solvent is supplied as amixture of related compounds instead of a single pure componenttosuppress the freezing point.

11. Safety. Solvents with low potential for fire and reactive chem-istry hazards are preferred as inherently safe solvents. In all cases, sol-

vents must be used with a full awareness of potential hazards and in amanner consistent with measures needed to avoid hazards. For infor-mation on the safe use of solvents and their potential hazards, see Sec.23, Safety and Handling of Hazardous Materials. Also see Crowl andLouvar, Chemical Process Safety: Fundamentals with Applications(Prentice-Hall, 2001); Yaws, Handbook of Chemical Compound Data

for Process Safety (Elsevier, 1997); Lees, Loss Prevention in theProcess Industries (Butterworth, 1996); and Brethericks Handbook ofReactive Chemical Hazards, 6th ed., Urben and Pitt, eds. (Butter-

worth-Heinemann, 1999).12. Industrial hygiene. Solvents with low mammalian toxicity and

good warning properties are desired. Low toxicity and low dermalabsorption rate reduce the potential for injury through acute expo-sure. A thorough review of the medical literature must be conductedto ascertain chronic toxicity issues. Measures needed to avoid unsafeexposures must be incorporated into process designs and imple-mented in operating procedures. See Goetsch, Occupational Safetyand Health for Technologists, Engineers, and Managers (Prentice-Hall, 2004).

13. Environmental requirements. The solvent must have physi-cal or chemical properties that allow effective control of emissionsfrom vents and other discharge streams. Preferred properties

include low aquatic toxicity and low potential for fugitive emissionsfrom leaks or spills. It also is desirable for a solvent to have low pho-toreactivity in the atmosphere and be biodegradable so it does notpersist in the environment. Efficient technologies for capturing sol-

vent vapors from vents and condensing them for recycle includeactivated carbon adsorption with steam regeneration [Smallwood,Solvent Recovery Handbook (McGraw-Hill, 1993), pp. 714] and

vacuum-swing adsorption [Pezolt et al., Environmental Prog., 16(1),pp. 1619 (1997)]. The optimization of a process to increase the effi-ciency of solvent utilization is a key aspect of waste minimization andreduction of environmental impact. An opportunity may exist toreduce solvent use through application of countercurrent processingand other chemical engineering principles aimed at improving pro-cessing efficiencies. For a discussion of environmental issues in



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process design, see Allen and Shonnard, Green Engineering: Envi-ronmentally Conscious Design of Chemical Processes (Prentice-Hall, 2002)]. Also see Sec. 22, Waste Management.

14. Multiple uses. It is desirable to use as the extraction solvent amaterial that can serve a number of purposes in the manufacturingplant. This avoids the cost of storing and handling multiple solvents. Itmay be possible to use a single solvent for a number of differentextraction processes practiced in the same facility, either in differentequipment operated at the same time or by using the same equipment

in a series of product campaigns. In other cases, the solvent used forextraction may be one of the raw materials for a reaction carried out inthe same facility, or a solvent used in another operation such as a crys-tallization.

15. Materials of construction. It is desirable for a solvent to allowthe use of common, relatively inexpensive materials of construction atmoderate temperatures and pressures. Material compatability andpotential for corrosion are discussed in Sec. 25, Materials of Con-struction.

16. Availability and cost. The solvent should be readily availableat a reasonable cost. Considerations include the initial fill cost, theinvestment costs associated with maintaining a solvent inventory inthe plant (particularly when expensive extractants are used), as well asthe cost of makeup solvent.

COMMERCIAL PROCESS SCHEMES

For the purpose of illustrating process concepts, liquid-liquid extrac-tion schemes typically practiced in industry may be categorized into anumber of general types, as discussed below.

Standard Extraction Also called simple extraction or single-solvent extraction, standard extraction is by far the most widely prac-ticed type of extraction operation. It can be practiced usingsingle-stage or multistage processing, cross-current or countercurrentflow of solvent, and batch-wise or continuous operation. Figure 15-6illustrates the contacting stages and liquid streams associated with atypical multistage, countercurrent scheme. Standard extraction isanalogous to stripping in distillation because the process involvestransferring or stripping components from the feed phase intoanother phase. Note that the feed (F) enters the process where theextract stream (E) leaves the process, analogous to feeding the top ofa stripping tower. And the raffinate (R) leaves where the extraction

solvent (S) enters. Standard extraction is used to remove contaminantsfrom a crude liquid feed (product purification) or to recover valuablecomponents from the feed (product recovery). Applications caninvolve very dilute feeds, such as when purifying a liquid product ordetoxifying a wastewater stream, or concentrated feeds, such as whenrecovering a crude product from a reaction mixture. In either case,standard extraction can be used to transfer a high fraction of solutefrom the feed phase into the extract. Note, however, that transfer ofthe desired solute or solutes may be accompanied by transfer ofunwanted solutes. Because of this, standard extraction normally can-not achieve satisfactory solute purity in the extract stream unless theseparation factor for the desired solute with respect to unwantedsolutes is at least i,j = Ki/Kj = 20 and usually much higher. Thisdepends on the crude feed purity and the product purity specification.(See Potential for Solute Purification Using Standard Extractionunder Process Fundamentals and Basic Calculation Methods.)

Fractional Extraction Fractional extraction combines soluterecovery with cosolute rejection. In principle, the process can achievehigh solute recovery and high solute purity even when the solute sep-aration factor is fairly low, as low as i,j = 4 or so (see Dual-SolventFractional Extraction under Calculation Procedures). Dual-solventfractional extraction utilizes an extraction solvent (S) and a wash sol-

vent (W) and includes a stripping section at the raffinate end of theprocess (for product-solute recovery) and a washing section at theextract end of the process (for cosolute rejection and product purifi-cation) (Fig. 15-7). The feed enters the process at an intermediatestage located between the extract and raffinate ends. In this respect,the process is analogous to a middle-fed fr

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