in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. 0-07-154222-1
The material in this eBook also appears in the print version of this title: 0-07-151138-5.
All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. Where such designations appear in this book, they have been printed with initial caps.
McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. For more information, please contact George Hoare, Special Sales, at [email protected] or (212) 904-4069.
TERMS OF USE
This is a copyrighted work and The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and its licensors reserve all rights in and to the work. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw-Hill’s prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms.
THE WORK IS PROVIDED “AS IS.” McGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THE WORK, INCLUDING ANY INFORMATION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill and its licensors do not warrant or guarantee that the functions contained in the work will meet your requirements or that its operation will be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccuracy, error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill has no responsibility for the content of any information accessed through the work. Under no circumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise.
15-1
Liquid-Liquid Extraction and Other
Liquid-Liquid Operations and Equipment*
Timothy C. Frank, Ph.D. Research Scientist and Sr. Technical Leader, The Dow Chemi-cal Company; Member, American Institute of ChemiChemi-cal 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 ConsideraExtrac-tions, Liquid-Liquid Phase Separation Equipment, Emerging Developments)
Lise Dahuron, Ph.D. Sr. Research Specialist, The Dow Chemical Company (Liquid Den-sity, ViscoDen-sity, and Interfacial Tension; Liquid-Liquid Dispersion Fundamentals; Liquid-Liquid Phase 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 Extraction Columns, Mixer-Settler Equipment)
A. Frank Seibert, Ph.D., P.E. Technical Manager, Separations Research Program, The University 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, ViscoDen-sity, 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 OVERVIEW
Historical Perspective . . . 15-6 Uses for Liquid-Liquid Extraction. . . 15-7 Definitions . . . 15-10
Desirable Solvent Properties . . . 15-11 Commercial Process Schemes . . . 15-13 Standard Extraction . . . 15-13 Fractional Extraction . . . 15-13
Dissociative Extraction . . . 15-15 pH-Swing Extraction . . . 15-16 Reaction-Enhanced Extraction . . . 15-16 Extractive Reaction. . . 15-16 Temperature-Swing Extraction . . . 15-17 Reversed Micellar Extraction. . . 15-18 Aqueous Two-Phase Extraction . . . 15-18 Hybrid Extraction Processes . . . 15-18 Liquid-Solid Extraction (Leaching) . . . 15-19 Liquid-Liquid Partitioning of Fine Solids . . . 15-19 Supercritical Fluid Extraction . . . 15-19 Key Considerations in the Design of an Extraction Operation . . . 15-20 Laboratory Practices. . . 15-21 THERMODYNAMIC BASIS FOR LIQUID-LIQUID EXTRACTION Activity Coefficients and the Partition Ratio . . . 15-22
Extraction Factor . . . 15-22 Separation Factor . . . 15-23 Minimum and Maximum Solvent-to-Feed Ratios. . . 15-23 Temperature Effect . . . 15-23 Salting-out and Salting-in Effects for Nonionic Solutes . . . 15-24 Effect of pH for Ionizable Organic Solutes. . . 15-24 Phase Diagrams . . . 15-25 Liquid-Liquid Equilibrium Experimental Methods . . . 15-27 Data Correlation Equations . . . 15-27 Tie Line Correlations . . . 15-27 Thermodynamic Models. . . 15-28 Data Quality . . . 15-28 Table of Selected Partition Ratio Data . . . 15-32 Phase Equilibrium Data Sources . . . 15-32 Recommended Model Systems . . . 15-32
SOLVENT SCREENING METHODS
Use of Activity Coefficients and Related Data . . . 15-32 Robbins’ Chart of Solute-Solvent Interactions . . . 15-32 Activity Coefficient Prediction Methods . . . 15-33 Methods Used to Assess Liquid-Liquid Miscibility . . . 15-34 Computer-Aided Molecular Design . . . 15-38 High-Throughput Experimental Methods . . . 15-39
LIQUID DENSITY, VISCOSITY, AND INTERFACIAL TENSION Density and Viscosity . . . 15-39 Interfacial Tension . . . 15-39
LIQUID-LIQUID DISPERSION FUNDAMENTALS Holdup, Sauter Mean Diameter, and Interfacial Area . . . 15-41 Factors Affecting Which Phase Is Dispersed . . . 15-41 Size of Dispersed Drops. . . 15-42 Stability of Liquid-Liquid Dispersions . . . 15-43 Effect of Solid-Surface Wettability . . . 15-43 Marangoni Instabilities . . . 15-43
PROCESS FUNDAMENTALS AND BASIC CALCULATION METHODS
Theoretical (Equilibrium) Stage Calculations . . . 15-44 McCabe-Thiele Type of Graphical Method . . . 15-45 Kremser-Souders-Brown Theoretical Stage Equation . . . 15-45 Stage Efficiency . . . 15-46 Rate-Based Calculations. . . 15-47 Solute Diffusion and Mass-Transfer Coefficients . . . 15-47 Mass-Transfer Rate and Overall Mass-Transfer Coefficients . . . 15-47 Mass-Transfer Units . . . 15-48 Extraction Factor and General Performance Trends . . . 15-49 Potential for Solute Purification Using Standard Extraction . . . 15-50
CALCULATION PROCEDURES
Shortcut Calculations . . . 15-51 Example 1: Shortcut Calculation, Case A . . . 15-52
Example 2: Shortcut Calculation, Case B . . . 15-52 Example 3: Number of Transfer Units . . . 15-53 Computer-Aided Calculations (Simulations). . . 15-53 Example 4: Extraction of Phenol from Wastewater . . . 15-54 Fractional Extraction Calculations. . . 15-55 Dual-Solvent Fractional Extraction . . . 15-55 Single-Solvent Fractional Extraction with Extract Reflux . . . 15-56 Example 5: Simplified Sulfolane Process—Extraction
of Toluene from n-Heptane . . . 15-56 LIQUID-LIQUID EXTRACTION EQUIPMENT
Extractor Selection . . . 15-58 Hydrodynamics of Column Extractors . . . 15-59 Flooding Phenomena . . . 15-59 Accounting for Axial Mixing . . . 15-60 Liquid Distributors and Dispersers . . . 15-63 Static Extraction Columns . . . 15-63 Common Features and Design Concepts . . . 15-63 Spray Columns . . . 15-69 Packed Columns . . . 15-70 Sieve Tray Columns . . . 15-74 Baffle Tray Columns . . . 15-78 Agitated Extraction Columns . . . 15-79 Rotating-Impeller Columns . . . 15-79 Reciprocating-Plate Columns . . . 15-83 Rotating-Disk Contactor . . . 15-84 Pulsed-Liquid Columns . . . 15-85 Raining-Bucket Contactor (a Horizontal Column) . . . 15-85 Mixer-Settler Equipment . . . 15-86 Mass-Transfer Models . . . 15-86 Miniplant Tests . . . 15-87 Liquid-Liquid Mixer Design . . . 15-87 Scale-up Criteria . . . 15-88 Specialized Mixer-Settler Equipment . . . 15-89 Suspended-Fiber Contactor. . . 15-90 Centrifugal Extractors . . . 15-91 Single-Stage Centrifugal Extractors. . . 15-91 Centrifugal Extractors Designed for
Multistage Performance . . . 15-92 PROCESS CONTROL CONSIDERATIONS
Steady-State Process Control . . . 15-93 Sieve Tray Column Interface Control . . . 15-94 Controlled-Cycling Mode of Operation. . . 15-94
LIQUID-LIQUID PHASE SEPARATION EQUIPMENT Overall Process Considerations . . . 15-96 Feed Characteristics . . . 15-96 Gravity Decanters (Settlers). . . 15-97 Design Considerations . . . 15-97 Vented Decanters . . . 15-98 Decanters with Coalescing Internals . . . 15-99 Sizing Methods . . . 15-99 Other Types of Separators . . . 15-101 Coalescers . . . 15-101 Centrifuges . . . 15-101 Hydrocyclones . . . 15-101 Ultrafiltration Membranes . . . 15-102 Electrotreaters . . . 15-102 EMERGING DEVELOPMENTS Membrane-Based Processes . . . 15-103 Polymer Membranes . . . 15-103 Liquid Membranes . . . 15-104 Electrically Enhanced Extraction . . . 15-104 Phase Transition Extraction and Tunable Solvents . . . 15-105 Ionic Liquids . . . 15-105
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 m2/m3 ft2/ft3
(area per unit volume)
bij NRTL model regression K K
parameter (see Table 15-10)
A Envelope-style downcomer m2 ft2
area
A Area between settled layers m2 ft2
in a decanter
Acol Column cross-sectional area m2 ft2
Adow Area for flow through m2 ft2
a downcorner (or upcomer)
Ai,j/RT van Laar binary interaction Dimensionless Dimensionless
parameter
Ao Cross-sectional area of a m2 in2
single hole
C Concentration (mass or kg m3or lb/ft3or
mol per unit volume) kgmol m3 lbmol ft3
or gmol L
CAi Concentration of component kg m3or lb/ft3or
A at the interface kgmol m3 lbmol ft3
or gmol L
C* Concentration at equilibrium kg m3or lb/ft3or
kgmol m3 lbmol ft3
or gmol L
CD Drag coefficient Dimensionless Dimensionless Co Initial concentration kg m3or lb/ft3 kgmol m3 or lbmol ft3 or gmol L Ct Concentration at time t kg m3or lb/ft3 kgmol m3 or lbmol ft3 or gmol L d Drop diameter m in
dC Critical packing dimension m in
di Diameter of an individual drop m in dm Characteristic diameter of m in
media in a packed bed
do Orifice or nozzle diameter m in
dp Sauter mean drop diameter m in
d32 Sauter mean drop diameter m in
Dcol Column diameter m in or ft
Deq Equivalent diameter giving m in the same area
Dh Equivalent hydraulic diameter m in
Di Distribution ratio for a given
chemical species including all its forms (unspecified units)
Di Impeller diameter or m in or ft
characteristic mixer diameter
Dsm Static mixer diameter m in or ft
Dt Tank diameter m ft
D Molecular diffusion coefficient m2/s cm2/s
(diffusivity)
DAB Mutual diffusion coefficient m2/s cm2/s
for components A and B
E Mass or mass flow rate of kg or kg/s lb or lb/h extract 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 Dimensionless
component i
Es Stripping section extraction Dimensionless Dimensionless
factor
Ew Washing section extraction Dimensionless Dimensionless
factor
fda 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 phase
F Force N lbf
F′ 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 Gravitational acceleration 9.807 m/s2 32.17 ft/s2 Gij NRTL model parameter Dimensionless Dimensionless h Height of coalesced layer at m in
a sieve tray
h Head loss due to frictional flow m in
h Height of dispersion band in m in batch decanter
hiE Excess enthalpy J gmol Btu lbmol
of mixing or cal gmol
H 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 fed
decanter
HDU Height of a dispersion unit m in
He Height of a transfer unit due m in
to resistance in extract phase
HETS Height equivalent to a m in
theoretical stage
Hor Height of an overall m in
mass-tranfer unit based on raffinate phase
Hr Height of a transfer unit due m in
to resistance in raffinate phase
I Ionic strength in Eq. (15-26)
k Individual mass-transfer m/s or cm/s ft/h coefficient k Mass-transfer coefficient (unspecified units) km Membrane-side mass-transfer m/s or cm/s ft/h coefficient ko Overall mass-transfer m/s or cm/s ft/h coefficient kc Continuous-phase m/s or cm/s ft/h mass-transfer coefficient kd Dispersed-phase mass-transfer m/s or cm/s ft/h coefficient
ks Setschenow constant L gmol L gmol
ks Shell-side mass-transfer m/s or cm/s ft/h
coefficient
kt Tube-side mass-transfer m/s or cm/s ft/h
coefficient
K Partition ratio (unspecified units)
K′s 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 the use of the SI or cgs system of units and not ft-lb-s units, unless otherwise noted in the text. The gravitational conversion factor gcneeded to use ft-lb-s units is not
included in the equations.
U.S. Customary U.S. Customary
Re Reynolds number: for pipe Dimensionless Dimensionless flow, Vdρ µ; for an impeller,
ρmωDi2 µm; for drops, Vsodpρc
µc; for flow in a packed-bed
coalescer, Vdmρc µ; for flow
through an orifice, Vodoρd µd
ReStokes ρc∆ρgd3p 18µc2 Dimensionless Dimensionless
S Mass or mass flow rate of kg or kg/s lb or lb/h solvent phase
S 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)
S′s Mass flow rate of extraction kg/s lb/h
solvent within stripping section
S′w Mass flow rate of extraction kg/s lb/h
solvent within washing section
Si,j Separation power for Dimensionless Dimensionless
separating component i from component j [defined by Eq. (15-105)]
Stip Impeller tip speed m/s ft/s
tb Batch mixing time s or h min or h
T Temperature (absolute) K °R
ut Stokes’ law terminal or m/s or cm/s ft/s or ft/min
settling velocity of a drop
ut∞ Unhindered settling velocity m/s or cm/s ft/s or ft/min
of a single drop
v Molar volume m3 kgmol or ft3 lbmol
cm3 gmol
V Liquid velocity (or m/s ft/s or ft/min volumetric flow per
unit area)
V Volume m3 ft3or gal
Vcf Continuous-phase m/s ft/s or ft/min
flooding velocity
Vcflow Cross-flow velocity of m/s ft/s or ft/min
continuous phase at sieve tray
Vdf Dispersed-phase m/s ft/s or ft/min
flooding velocity
Vdrop Average velocity of a m/s ft/s or ft/min
dispersed drop
Vic Interstitial velocity of m/s ft/s or ft/min
continuous phase
Vo,max Maximum velocity through m/s ft/s or ft/min
an orifice or nozzle
Vs Slip velocity m/s ft/s or ft/min
Vso Slip velocity at low m/s ft/s or ft/min
dispersed-phase flow rate
Vsm 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 phase
W′s Mass flow rate of wash solvent kg/s lb/h
within stripping section
W′w Mass flow rate of wash solvent kg/s lb/h
within washing section
We Weber number: for an Dimensionless Dimensionless impeller, ρcω2Di3 σ; for flow
through an orifice or nozzle,
Vo2doρd σ; for a static mixer, V2smDsmρ
c σ
x Mole fraction solute in feed Mole fraction Mole fraction or raffinate
X Concentration of solute in feed or raffinate (unspecified units)
X″ Mass fraction solute in feed Mass fractions Mass fractions or raffinate
X′ Mass solute/mass feed Mass ratios Mass ratios solvent in feed or raffinate
XfB Pseudoconcentration of Mass ratios Mass ratios
solute in feed for case B [Eq. (15-95)]
K′w 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 kgmol m3 or lbmol ft3
or gmol L
L Downcomer (or m in or ft
upcomer) length
Lfp Length of flow path in m in or ft
Eq. (15-161)
m Local slope of equilibrium line (unspecified concentration units)
m′ Local slope of equilibrium line Mass ratio/ Mass ratio/ (in Bancroft coordinates) mass ratio mass ratio
mdc Local slope of equilibrium line
for dispersed-phase concentration plotted versus continuous-phase concentration
mer Local slope of equilibrium
line for extract-phase concentration plotted versus raffinate-phase concentration
mvol Local slope of equilibrium Ratio of kg/m3 Ratio of lb/ft3or
line (volumetric or kgmol m3 lbmol ft3
concentration basis) or gmol L units
M Mass or mass flow rate kg or kg/s lb or lb/h MW Molecular weight kg kgmol or lb lbmol
g gmol
N Number of theoretical stages Dimensionless Dimensionless
NA Flux of component A (mass (kg or kgmol)/ (lb or lbmol)
or mol/area/unit time) (m2⋅s) (ft2⋅s)
Nholes Number of holes Dimensionless Dimensionless
Nor Number of overall Dimensionless Dimensionless
mass-transfer units based on the raffinate phase
Ns Number of theoretical stages Dimensionless Dimensionless
in stripping section
Nw Number of theoretical stages Dimensionless Dimensionless
in washing section
P Pressure bar or Pa atm or lbf/in2
P Dimensionless group defined Dimensionless Dimensionless by Eq. (15-122)
P Power W or kW HP or ft⋅lbf/h
Pe Péclet number Vb/E, Dimensionless Dimensionless where V is liquid
velocity, E is axial mixing coefficient, and b is a characteristic equipment dimension
Pi,extract Purity of solute i in wt % wt %
extract (in wt %)
Pi,feed Purity of solute i in feed wt % wt %
(in wt %)
Po Power number P (ρmω3Di5) Dimensionless Dimensionless
∆Pdow Pressure drop for flow bar or Pa atm or lbf/in2
through a downcomer (or upcomer)
∆Po Orifice pressure drop bar or Pa atm or lbf/in2 q MOSCED induction Dimensionless Dimensionless
parameter
Q Volumetric flow rate m3/s ft3/min R Universal gas constant 8.31 J⋅K 1.99 Btu⋅°R
kgmol lbmol
R Mass or mass flow rate of kg or kg/s lb or lb/h raffinate phase
RA Rate of mass-transfer (moles kgmol s lbmol h
per unit time) Nomenclature(Continued)
U.S. Customary U.S. Customary
Nomenclature(Concluded)
U.S. Customary U.S. Customary
Symbol Definition SI units System units Symbol Definition SI units System units
XfC Pseudoconcentration of Mass ratios Mass ratios
solute in feed for case C [Eq. (15-97)]
Xi,extract Concentration of solute i Mass fraction Mass fraction
in extract
Xi,feed Concentration of solute i Mass fraction Mass fraction
in feed
Xij Concentration of component Mass fraction Mass fraction i in the phase richest in j
y Mole fraction solute in Mole fraction Mole fraction solvent or extract
Y Concentration of solute in the solvent or extract (unspecified units)
Y″ Mass fraction solute Mass fraction Mass fraction in solvent or extract
Y′ Mass solute/mass extraction Mass ratio Mass ratio solvent in solvent or
extract
YsB Pseudoconcentration of Mass ratio Mass ratio
solute in solvent for case B [Eq. (15-96)]
z Dimension or direction of m in or ft mass transfer
z Sieve tray spacing m in or ft
z Point representing feed composition on a tie line
zi Number of electronic Dimensionless Dimensionless
charges on an ion
Zt Total height of extractor m ft
Greek Symbols
α MOSCED hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2
acidity parameter
α Solvatochromic hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2
acidity parameter
αi,j Separation factor for solute i Dimensionless Dimensionless
with respect to solute j
αi,j NRTL model parameter Dimensionless Dimensionless
β MOSCED hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2
basicity parameter
β Solvatochromic hydrogen-bond (J/cm3)1/2 (cal/cm3)1/2
basicity parameter
γi,j Activity coefficient of i Dimensionless Dimensionless
dissolved in j
γ∞ Activity coefficient at Dimensionless Dimensionless
infinite dilution γC
i Activity coefficient, Dimensionless Dimensionless
combinatorial part of UNIFAC
γiI Activity coefficient of Dimensionless Dimensionless
component i in phase I
γiR Activity coefficient, residual Dimensionless Dimensionless
part of UNIFAC
ε Void fraction Dimensionless Dimensionless ε Fractional open area of a Dimensionless Dimensionless
perforated plate
δ Solvatochromic polarizability (J/cm3)1/2 (cal/cm3)1/2
parameter
δd Hansen nonpolar (dispersion) (J/cm3)1/2 (cal/cm3)1/2
solubility parameter
δh Hansen solubility parameter (J/cm3)1/2 (cal/cm3)1/2
for hydrogen bonding
δp Hansen polar solubility (J/cm3)1/2 (cal/cm3)1/2
parameter
Greek Symbols
δi Solubility parameter for (J/cm3)1/2 (cal/cm3)1/2
component i δ
⎯ Solubility parameter for mixture (J/cm3)1/2 (cal/cm3)1/2
ζ Tortuosity factor defined by Dimensionless Dimensionless Eq. (15-147)
θ Residence time for total liquid s s or min θi Fraction of solute i extracted Dimensionless Dimensionless
from feed
λ MOSCED dispersion parameter (J/cm3)1/2 (cal/cm3)1/2
λm Membrane thickness mm in
µ Liquid viscosity Pa⋅s cP
µiI Chemical potential of J/gmol Btu/lbmol
component i in phase I
µm Mixture mean viscosity Pa⋅s cP
defined in Eq. (15-180)
µw Reference viscosity (of water) Pa⋅s cP
ξ1 MOSCED asymmetry factor Dimensionless Dimensionless
ξbatch Efficiency of a batch Dimensionless Dimensionless
experiment [Eq. (15-175)]
ξcontinuous Efficiency of a continuous Dimensionless Dimensionless
process [Eq. (15-176)]
ξm Murphree stage efficiency Dimensionless Dimensionless
ξmd Murphree stage efficiency Dimensionless Dimensionless
based on dispersed phase
ξo Overall stage efficiency Dimensionless Dimensionless
π Solvatochromic polarity (J/cm3)1/2 (cal/cm3)1/2
parameter
∆π Osmotic pressure gradient bar or Pa atm or lbf/in2
ρ Liquid density kg/m3 lb/ft3
ρm Mixture mean density defined kg/m3 lb/ft3
in Eq. (15-178)
σ Interfacial tension N/m dyn/cm
τ MOSCED polarity parameter (J/cm3)1/2 (cal/cm3)1/2
τi,j NRTL model parameter Dimensionless Dimensionless
φ Volume fraction Dimensionless Dimensionless φd Volume fraction of dispersed Dimensionless Dimensionless
phase (holdup)
φd,feed Volume fraction of dispersed Dimensionless Dimensionless
phase in feed
φo Initial dispersed-phase holdup Dimensionless Dimensionless
in feed to a decanter
ϕ Volume fraction of voids Dimensionless Dimensionless in a packed bed
Φ Factor governing use of Eqs. Dimensionless Dimensionless (15-148) and (15-149)
χ Parameter in Eq. (15-41) Dimensionless Dimensionless indicating which phase is
likely to be dispersed
ω Impeller speed Rotations/s Rotations/min
Additional Subscripts
c Continuous phase
d Dispersed phase
e Extract phase
f Feed phase or flooding condition (when combined with d or c)
i Component i
j Component j
H Heavy liquid
L Light liquid max Maximum value min Minimum value
o Orifice or nozzle
r Raffinate phase
(Wiley, 2006); Seibert, “Extraction and Leaching,” Chap. 14 in Chemical Process
Equipment: Selection and Design, 2d ed., Couper et al., eds. (Elsevier, 2005);
Aguilar and Cortina, Solvent Extraction and Liquid Membranes: Fundamentals
and Applications in New Materials (Dekker, 2005); Glatz and Parker, “Enriching
Liquid-Liquid Extraction,” Chem. Eng. Magazine, 111(11), pp. 44–48 (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 with
Reaction,” Chap. 3 in Reactive Separation Processes, Kulprathipanja, ed. (Taylor & Francis, 2002); Mueller et al., “Liquid-Liquid Extraction” in Ullmann’s
Encyclope-dia of Industrial Chemistry, 6th ed. (VCH, 2002); Benitez, Principles and Modern Applications of Mass Transfer Operations (Wiley, 2002); Wypych, Handbook of Sol-vents (Chemtec, 2001); Flick, Industrial SolSol-vents Handbook, 5th ed. (Noyes,
1998); Robbins, “Liquid-Liquid Extraction,” Sec. 1.9 in Handbook of Separation
Techniques for Chemical Engineers, 3d ed., Schweitzer, ed. (McGraw-Hill, 1997);
Lo, “Commercial Liquid-Liquid Extraction Equipment,” Sec. 1.10 in Handbook of
Separation Techniques for Chemical Engineers, 3d ed., Schweitzer, ed.
(McGraw-Hill, 1997); Humphrey and Keller, “Extraction,” Chap. 3 in Separation Process
Technology (McGraw-Hill, 1997), pp. 113–151; Cusack and Glatz, “Apply
Liquid-Liquid Extraction to Today’s Problems,” Chem. Eng. Magazine, 103(7), pp. 94–103 (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); Schügerl, Solvent Extraction in Biotechnology (Springer-Verlag, 1994); Schügerl, “Liquid-Liquid Extraction (Small Molecules),” Chap. 21 in
Biotechnology, 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,
Practice of Liquid-Liquid Extraction, vol. 1, Phase Equilibria; Mass Transfer and Interfacial 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. 66–67 (1991); Cusack and Fremeauz, pt. 2, “Inside the Extractor,” Chem. Eng. Magazine, 98(3), pp. 132–138 (1991); Cusack and Karr, pt. 3, “Extractor Design and Specification,” Chem. Eng.
Magazine, 98(4), pp. 112–120 (1991); Methods in Enzymology, vol. 182, Guide to Protein Purification, Deutscher, ed. (Academic, 1990); Wankat, Equilibrium Staged Separations (Prentice Hall, 1988); Blumberg, Liquid-Liquid Extraction
(Academic, 1988); Skelland and Tedder, “Extraction—Organic Chemicals Process-ing,” Chap. 7 in Handbook of Separation Process Technology, Rousseau, ed. (Wiley, 1987); Chapman, “Extraction—Metals Processing,” Chap. 8 in Handbook of
Sepa-ration Process Technology, Rousseau, ed. (Wiley, 1987); Novak, Matous, and Pick, Liquid-Liquid Equilibria, Studies in Modern Thermodynamics Series, vol. 7
(Else-vier, 1987); Bailes et al., “Extraction, Liquid-Liquid” in Encyclopedia of Chemical
Processing and Design, vol. 21, McKetta and Cunningham, eds. (Dekker, 1984),
pp. 19–166; 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 and Tamir, Liquid-Liquid Equilibrium
and Extraction, a Literature Source Book, vols. I and II (Elsevier, 1980–1981),
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, Staged Cascades in Chemical Processing (Prentice-Hall, 1972); Pratt, Countercurrent Sep-aration Processes (Elsevier, 1967); Treybal, “Liquid Extractor Performance,” Chem. Eng. Prog., 62(9), pp. 67–75 (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 of a 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 and hydrophobic/hydrophilic character, to separate them. Stated more precisely, the transfer of components from one phase to the other is driven by a deviation from thermodynamic equilibrium, and the equilibrium state depends on the nature of the interactions between the feed components and the solvent phase. The potential for sepa-rating the feed components is determined by differences in these interactions.
A liquid-liquid extraction process produces a solvent-rich stream called the extract that contains a portion of the feed and an extracted-feed stream called the raffinate. A commercial process almost always includes two or more auxiliary operations in addition to the extraction operation itself. These extra operations are needed to treat the extract and raffinate streams for the purposes of isolating a desired product, recovering the solvent for recycle to the extractor, and purging unwanted components from the process. A typical process includes two or more distillation operations in addition to extraction.
Liquid-liquid extraction is used to recover desired components from a crude liquid mixture or to remove unwanted contaminants. In developing a process, the project team must decide what solvent or solvent mixture to use, how to recover solvent from the extract, and how to remove solvent residues from the raffinate. The team must also decide what temperature or range of temperatures should be used for the extraction, what process scheme to employ among many possibilities, and what type of equipment to use for liquid-liquid con-tacting and phase separation. The variety of commercial equipment options is large and includes stirred tanks and decanters, specialized mixer-settlers, a wide variety of agitated and nonagitated extraction columns or towers, and various types of centrifuges.
Because of the availability of hundreds of commercial solvents and extractants, as well as a wide variety of established process schemes and 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 including metals and nuclear fuel (hydrometallurgy), petrochemicals, coal and wood-derived chemicals, and complex organics such as pharmaceuti-cals and agricultural chemipharmaceuti-cals. Liquid-liquid extraction also is an important 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 or another since ancient times. It appears that prior to the 19th century solvent extraction was primarily used to isolate desired components such 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 applications involving liquid-liquid contacting are described by Blass, Liebel, and Haeberl [“Solvent Extraction—A Historical Review,” International Solvent 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 solute partitions between two liquid phases at equilibrium was introduced by Berthelot and Jungfleisch [Ann. Chim. Phys., 4, p. 26 (1872)] and fur-ther defined by Nernst [Z. Phys. Chemie, 8, p. 110 (1891)]. At about the same time, Gibbs published his theory of phase equilibrium (1876 and 1878). These and other advances were accompanied by a growing chemical industry. An early countercurrent extraction process utiliz-ing ethyl acetate solvent was patented by Goerutiliz-ing in 1883 as a method for recovering acetic acid from “pyroligneous acid” produced by pyrolysis 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)].
With the emergence of the chemical engineering profession in the 1890s and early 20th century, additional attention was given to process fundamentals and development of a more quantitative basis for process design. Many of the advances made in the study of distillation and absorption were readily adapted to liquid-liquid extraction, owing to its similarity as another diffusion-based operation. Examples include application of mass-transfer coefficients [Lewis, Ind. Eng. Chem., 8(9), pp. 825–833 (1916); and Lewis and Whitman, Ind. Eng. Chem., 16(12), pp. 1215–1220 (1924)], the use of graphical stagewise design methods [McCabe and Thiele, Ind. Eng. Chem., 17(6), pp. 605–611 (1925); Evans, Ind. Eng. Chem., 26(8), pp. 860–864 (1934); and Thiele, Ind. Eng. Chem., 27(4), pp. 392–396 (1935)], the use of theoretical-stage calculations [Kremser, National Petroleum News, 22(21), pp. 43–49 (1930); and Souders and Brown, Ind. Eng. Chem. 24(5), pp. 519–522 (1932)], and the transfer unit concept introduced in the late 1930s by Colburn and others [Colburn, Ind. Eng. Chem., 33(4), pp. 459–467 (1941)]. Additional background is given by Hampe, Hartland, and Slater [Chap. 2 in Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley, 1994)].
The number of commercial applications continued to grow, and by the 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. 928–933 (1936)]. It was also used to recover acetic acid from waste liquors generated in the production of cellulose acetate, and in various nitration and sul-fonation processes [Hunter and Nash, The Industrial Chemist, 9(102–104), pp. 245–248, 263–266, 313–316 (1933)]. The article by Hunter 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 with Raschig rings or coke breeze, the material left behind when coke is burned.
Much of the liquid-liquid extraction technology in practice today was first introduced to industry during a period of vigorous innovation and growth of the chemical industry as a whole from about 1920 to 1970. The advances of this period include development of fractional extraction schemes including work described by Cornish et al., [Ind. Eng. Chem., 26(4), pp. 397–406 (1934)] and by Thiele [Ind. Eng. Chem., 27(4), pp. 392–396 (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 usseparat-ing diethylene glycol, a process developed jointly by The Dow Chemical Company and Universal Oil Products in the 1940s. This period also saw the introduction of many new equipment designs including specialized mixer-settler equipment, mechanically agitated extraction columns, and centrifugal extractors as well as a great increase in the availability of 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 chemical companies found ways to manufacture them inexpensively using the byproducts of petroleum refining operations or natural gas. Later, a number 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 numerous organophosphorous 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 the development and implementation of many new industrial applica-tions. Handbooks of chemical process technology provide a glimpse of some of these [Riegel’s Handbook of Industrial Chemistry, 10th ed., Kent, ed. (Springer, 2003); Chemical Processing Handbook, McKetta, ed. (Dekker, 1993); and Austin, Shreve’s Chemical Process Industries, 5th ed. (McGraw-Hill, 1984)], but many remain proprietary and are not widely known. The better-known examples include the separation of aromatics from aliphatics, as mentioned above, extraction of phe-nolic compounds from coal tars and liquors, recovery of ε-caprolactam for production of polyamide-6 (nylon-6), recovery of hydrogen perox-ide from oxidized anthraquinone solution, plus many processes involv-ing the washinvolv-ing of crude organic streams with alkaline or acidic
solutions and water, and the detoxification of industrial wastewater prior to biotreatment using steam-strippable organic solvents. The pharmaceutical 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, often involving multiple batch reaction steps, liquid-liquid extraction gener-ally is used for recovery of intermediates or crude products prior to final isolation of a pure product by crystallization. In the inorganic chemical industry, extraction processes were developed for purifica-tion of phosphoric acid, purificapurifica-tion of copper by removal of arsenic impurities, 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 citric acid from broth using trialkylamine extractants, the use of amyl acetate 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, a subject area normally set apart from discussions of liquid-liquid extraction, has received a great deal of attention in the R&D commu-nity since the 1970s. Some processes were developed many years before then; e.g., the propane deasphalting process used to refine lubricating oils uses propane at near-supercritical conditions, and this technology 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 number of commercial applications displacing earlier liquid-liquid extraction methods, particularly for recovery of high-value products meant for human 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 introducextrac-tion of new methods to esti-mate solvent properties and screen candidate solvents and solvent blends, new methods for overall process conceptualization and opti-mization, and new methods for equipment design. Progress also is being made by applying the technology developed for a particular application in one industry to improve another application in another industry. For example, much can be learned by comparing equipment and practices used in organic chemical production with those used in the inorganic chemical industry (and vice versa), or by comparing practices used in commodity chemical processing with those used in the specialty chemicals industry. And new concepts offering potential for 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 is an 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 operaopera-tions usually involve the use of distillation anyway. However, in certain cases the use of liquid-liquid extraction is more cost-effective than using dis-tillation alone because it can be implemented with smaller equipment and/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 desired separation 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 an unusually tall distillation tower is required or the design involves high reflux ratios and high energy consumption. In certain cases, the distil-lation option may involve addition of a solvent (extractive distildistil-lation) or an entrainer (azeotropic distillation) to enhance the relative volatil-ity. Even in these cases, a liquid-liquid extraction process may offer advantages in terms of higher selectivity or lower solvent usage and lower energy consumption, depending upon the application. Extrac-tion may be preferred when the distillaExtrac-tion opExtrac-tion requires operaExtrac-tion at pressures less than about 70 mbar (about 50 mmHg) and an unusu-ally large-diameter distillation tower is required, or when most of the
feed must be taken overhead to isolate a desired bottoms product. Extraction may also be attractive when distillation requires use of high-pressure steam for the reboiler or refrigeration for overheads condensation [Null, Chem. Eng. Prog., 76(8), pp. 42–49 (August 1980)], or when the desired product is temperature-sensitive and extraction can provide a gentler separation process.
Of course, liquid-liquid extraction also may be a useful option when the 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. 58–61 (1980)]. The same process scheme often provides a cost-effective alternative to direct distillation or stripping of volatile impurities 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. 1753–1759 (1992); and Frank et al., Ind. Eng. Chem. Res., 46(11), pp. 3774–3786 (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 organic feed, since extraction of the salt content into water eliminates the need to filter solids from the mother liquor, often a difficult or expensive operation. 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-processing wastewaters [Guzman et al., U.S. Patent 6,849,770 (2005)].
The attractiveness of liquid-liquid extraction for a given application compared to alternative separation technologies often depends upon the concentration of solute in the feed. The recovery of acetic acid from aqueous solutions is a well-known example [Brown, Chem. Eng. Prog., 59(10), pp. 65–68 (1963)]. In this case, extraction generally is more economical than distillation when handling dilute to moderately concentrated feeds, while distillation is more economical at higher concentrations. In the treatment of water to remove trace amounts of organics, when the concentration of impurities in the feed is greater than 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 the adsorbent [Robbins, Chem. Eng. Prog., 76(10), pp. 58–61 (1980)]. At lower concentrations of impurities, adsorption may be the more eco-nomical option because the usable lifetime of the carbon bed is longer.
Examples of cost-effective liquid-liquid extraction processes utiliz-ing relatively low-boilutiliz-ing solvents include the recovery of acetic acid from 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 compounds from water by using methyl isobutyl ketone [Greminger et al., Ind. Eng. Chem. Process Des. Dev., 21(1), pp. 51–54 (1982)]. In these processes, the solvent is recovered from the extract by distillation, and dissolved solvent is removed from the raffinate by steam stripping (Fig. 15-1). The solvent circulates through the process in a closed loop.
One of the largest applications of liquid-liquid extraction in terms of 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 have been developed to recover benzene, toluene, and xylene (BTX) as feedstock for chemical manufacturing or to refine motor oils. This general technology is described in detail in “Single-Solvent Fractional Extraction with Extract Reflux” under “Calculation Procedures.” A typical flow diagram is shown in Fig. 15-2. Liquid-liquid extraction also may be used to upgrade used motor oil; an extraction process employing 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 amounts of aqueous KOH [Rincón, Cañizares, and García, Ind. Eng. Chem. Res., 44(20), pp. 7854–7859 (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, and from liquefied natural gas (LNG). The acid gases are extracted from the liquefied hydrocarbons (primarily C1to C3) by reversible reaction with various amine extractants. Typical amines are methyldiethanolamine (MDEA), diethanolamine (DEA), and monoethanolamine (MEA). In a typical process (Fig. 15-3), the treated hydrocarbon liquid (the raffi-nate) is washed with water to remove residual amine, and the loaded amine solution (the extract) is regenerated in a stripping tower for recy-cle back to the extractor [Nielsen et al., Hydrocarbon Proc., 76, pp. 49–59 (1997)]. The technology is similar to that used to scrub CO2and H2S from gas streams [Oyenekan and Rochelle, Ind. Eng. Chem. Res., 45(8), pp. 2465–2472 (2006); and Jassim and Rochelle, Ind. Eng. Chem. Res., 45(8), pp. 2457–2464 (2006)], except that the process involves liq-uid-liquid contacting instead of gas-liquid contacting. Because of this, a common stripper often is used to regenerate solvent from a variety of gas absorbers and liquid-liquid extractors operated within a typical refinery. In certain applications, organic acids such as formic acid are present in low concentrations in the hydrocarbon feed. These contami-nants will react with the amine extractant to form heat-stable amine salts that accumulate in the solvent loop over time, requiring periodic purging or regeneration of the solvent solution [Price and Burns, Hydrocarbon Proc., 74, pp. 140–141 (1995)]. The amine-based extrac-tion process is an alternative to washing with caustic or the use of solid adsorbents.
A typical extraction process used in hydrometallurgical applications is outlined in Fig. 15-4. This technology involves transferring the desired element from the ore leachate liquor, an aqueous acid, into an organic solvent phase containing specialty extractants that form a complex with the metal ion. The organic phase is later contacted with an aqueous solution at a different pH and temperature to regenerate the solvent and transfer the metal into a clean solution from which it can be recovered by electrolysis or another method [Cox, Chap. 1 in Science and Practice of Liquid-Liquid Extraction, vol. 2, Thornton, ed. (Oxford, 1992)]. Another process technology utilizes metals com-plexed with various organophosphorus compounds as recyclable homogeneous catalysts; liquid-liquid extraction is used to transfer the metal complex between the reaction phase and a separate liquid phase after reaction. Different ligands having different polarities are chosen to facilitate the use of various extraction and recycle schemes [Kanel et al., U.S. Patents 6,294,700 (2001) and 6,303,829 (2001)].
Another category of useful liquid-liquid extraction applications involves the recovery of antibiotics and other complex organics from fermentation broth by using a variety of oxygenated organic solvents such as acetates and ketones. Although some of these products are unstable at the required extraction conditions (particularly if pH must FIG. 15-1 Typical process for extraction of acetic acid from water.
Extract Raffinate to Water Wash Column E X T R Solvent Recovered Solvent Reflux Reformate (Feed) S T R I P P E R Product D I S T Simulated Process (Example 5)
FIG. 15-2 Flow sheet of a simplified aromatic extraction process (see Example 5).
Extract Raffinate E X T R D I S T To Acid Gas Disposal Recycle Solvent Sour Feed Washwater
To Amine Recovery or Disposal Sweetened Hydrocarbon
be low for favorable partitioning), short-contact-time centrifugal extractors may be used to minimize exposure. Centrifugal extractors also help overcome problems associated with formation of emulsions between solvent and broth. In a number of applications, the whole broth can be processed without prior removal of solids, a practice that can significantly reduce costs. For detailed information, see “The His-tory of Penicillin Production,” Elder, ed., Chemical Engineering Progress Symposium Series No. 100, vol. 66, pp. 37–42 (1970); Queener and Swartz, “Penicillins: Biosynthetic and Semisynthetic,” in Secondary Products of Metabolism, Economic Microbiology, vol. 3, Rose, ed. (Aca-demic, 1979); and Chaung et al., J. Chinese Inst. Chem. Eng., 20(3), pp. 155–161 (1989). Another well-known commercial application of liquid-liquid extraction in bioprocessing is the Baniel process for the recovery of citric acid from fermentation broth with tertiary amine extractants [Baniel, Blumberg, and Hadju, U.S. Patent 4,275,234 (1980)]. This type of process is discussed in “Reaction-Enhanced Extraction” under “Com-mercial Process Schemes.”
DEFINITIONS
Extraction terms defined by the International Union of Pure and Applied Chemistry (IUPAC) generally are recommended. [See Rice, Irving, and Leonard, Pure Appl. Chem. (IUPAC), 65(11), pp. 2673–2396 (1993); and J. Inczédy, Pure Appl. Chem. (IUPAC), 66(12), pp. 2501–2512 (1994).] Liquid-liquid extraction is a process for sep-arating components dissolved in a liquid feed by contact with a second liquid phase. Solvent extraction is a broader term that describes a process for separating the components of any matrix by contact with a liquid, and it includes solid extraction (leaching) as well as liquid-liquid extraction. The feed to a liquid-liquid-liquid-liquid extraction process is the solution that contains the components to be separated. The major liquid component (or components) in the feed can be referred to as the feed solvent or the carrier solvent. Minor components in solution often are referred to as solutes. The extraction solvent is the immiscible or partially miscible liquid added to the process to create a second liquid phase 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 several individual solvents (a mixed solvent or a solvent blend). The extrac-tion solvent also may be a liquid comprised of an extractant dissolved in a liquid diluent. In this case, the extractant species is primarily responsible 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 to the extraction of solute and in this respect is not the same as a true extraction solvent. A modifier may be added to the diluent to increase the solubility of the extractant or otherwise enhance the effectiveness of the extractant. The phase leaving a liquid-liquid contactor rich in extrac-tion solvent is called the extract. The raffinate is the liquid phase left from the feed after it is contacted by the extract phase. The word raffi-nate originally referred to a “refined product”; however, common usage has 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 processing two immiscible or partially miscible liquids in the form of a disper-sion of droplets of one liquid (the dispersed phase) suspended in the other liquid (the continuous phase). The dispersion will exhibit a distribution of drop diameters dioften characterized by the volume
to surface area average diameter or Sauter mean drop diameter. The term emulsion generally refers to a liquid-liquid dispersion with a dispersed-phase mean drop diameter on the order of 1 µm or less.
The tension that exists between two liquid phases is called the interfacial tension. It is a measure of the energy or work required to increase the surface area of the liquid-liquid interface, and it affects the size of dispersed drops. Its value, in units of force per unit length or energy per unit area, reflects the compatibility of the two liquids. Systems that have low compatibility (low mutual solubility) exhibit high interfacial tension. Such a system tends to form relatively large dispersed 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 small dispersed droplets.
A theoretical or equilibrium stage is a device or combination of devices that accomplishes the effect of intimately mixing two liquid phases until equilibrium concentrations are reached, then physically separating the two phases into clear layers. The partition ratio K is commonly defined for a given solute as the solute concentration in the extract phase divided by that in the raffinate phase after equilibrium is attained in a single stage of contacting. A variety of concentration units are used, so it is important to determine how partition ratios have been defined in the literature for a given application. The term partition ratio is preferred, but it also is referred to as the distribution con-stant, distribution coefficient, or the K value. It is a measure of the Stripping (Back Extraction)
Solvent Extraction Ore Acid Leaching Depleted Leachate Aqueous Leachate Lean Organic Loaded Organic Impurities Aqueous Scrub Liquor Impurity Removal Winning Depleted Aqueous Loaded Aqueous Metal
FIG. 15-4 Example process scheme used in hydrometallurgical applications. [Taken from Cox, Chap. 1 in Science and Practice of Liquid-Liquid Extraction, vol. 2, Thornton, ed. (Oxford, 1992), with permission.
thermodynamic potential of a solvent for extracting a given solute and can be a strong function of composition and temperature. In some cases, the partition ratio transitions from a value less than unity to a value greater than unity as a function of solute concentration. A system of this type is called a solutrope [Smith, Ind. Eng. Chem., 42(6), pp. 1206–1209 (1950)]. The term distribution ratio, designated by Di, is
used in analytical chemistry to describe the distribution of a species that undergoes chemical reaction or dissociation, in terms of the total concentration 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 required to transfer solute from the feed to the extract. The extraction factor is analogous to the stripping factor in distillation and is the ratio of the slope of the equilibrium line to the slope of the operating line in a McCabe-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 interest times the ratio of the solvent flow rate to the feed flow rate. The sep-aration factor ai,jmeasures the relative enrichment of solute i in
the extract phase, compared to solute j, after one theoretical stage of extraction. It is equal to the ratio of K values for components i and j and is used to characterize the selectivity a solvent has for a given solute.
A standard extraction process is one in which the primary pur-pose is to transfer solute from the feed phase into the extract phase in a manner analogous to stripping in distillation. Fractional extraction refers to a process in which two or more solutes present in the feed are sharply separated from each other, one fraction leaving the extractor in 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 the raffinate R from one extraction stage is contacted with additional fresh solvent S in a subsequent stage. Countercurrent extraction (Fig. 15-6) is an extraction scheme in which the extraction solvent enters the stage or end of the extraction farthest from where the feed F enters, and the two phases pass each other in countercurrent fashion. The objective is to transfer one or more components from the feed solution 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 a process utilizing multiple staged contactors with countercurrent flow of solvent and feed streams from stage to stage. When a differential contactor is used, one of the phases can remain dispersed as drops throughout the contactor as the phases pass each other in countercur-rent fashion. The dispersed phase is then allowed to coalesce at the end of the device before being discharged. For these types of processes, mass-transfer units (or the related mass-transfer coef-ficients) often are used instead of theoretical stages to characterize separation performance. For a given phase, mass-transfer units are
defined as the integral of the differential change in solute concentra-tion divided by the deviaconcentra-tion from equilibrium, between the limits of inlet and outlet solute concentrations. A single transfer unit repre-sents the change in solute concentration equal to that achieved by a single theoretical stage when the extraction factor is equal to 1.0. It differs from a theoretical stage at other values of the extraction factor. The term flooding generally refers to excessive breakthrough or entrainment 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, by phase inversion due to accumulation and coalescence of dispersed droplets, or by formation of stable dispersions or emulsions due to the presence of surface-active impurities or excessive agitation. The flood point typically refers to the specific total volumetric throughput in (m3/h)/m2or gpm/ft2of cross-sectional area (or the equivalent phase velocity in m/s or ft/s) at which flooding begins.
DESIRABLE SOLVENT PROPERTIES
Common industrial solvents generally are single-functionality organic solvents such as ketones, esters, alcohols, linear or branched aliphatic hydrocarbons, aromatic hydrocarbons, and so on; or water, which may be acidic or basic or mixed with water-soluble organic solvents. More complex solvents are sometimes used to obtain specific properties needed for a given application. These include compounds with multi-ple functional groups such as diols or triols, glycol ethers, and alkanol amines as well as heterocyclic compounds such as pine-derived sol-vents (terpenes), sulfolane (tetrahydrothiophene-1,1-dioxane), and NMP (N-methyl-2-pyrrolidinone). Solvent properties have been sum-marized in a number of handbooks and databases including those by Cheremisinoff, Industrial Solvents Handbook, 2d ed. (Dekker, 2003); Wypych, Handbook of Solvents (ChemTech, 2001); Wypych, Solvents Database, CD-ROM (ChemTec, 2001); Yaws, Thermodynamic and Physical Property Data, 2d ed. (Gulf, 1998); and Flick, Industrial Sol-vents Handbook, 5th ed. (Noyes, 1998). SolSol-vents are sometimes blended to obtain specific properties, another approach to achieving a multifunctional solvent with properties tailored for a given applica-tion. Examples are discussed by Escudero, Cabezas, and Coca [Chem. Eng. Comm., 173, pp. 135–146 (1999)] and by Delden et al. [Chem. Eng. Technol., 29(10), pp. 1221–1226 (2006)]. As discussed earlier, a solvent 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 considerations depends on the given situation. Assessments should take into account long-term sustainability and overall cost of ownership. Normally, the factors 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 liquid phases 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 E1or E Feed Stage R1 E2 Raffinate Stage R2 E3 RorR3