Table Of ContentSu perc r it ical FI u i d
Extraction
Principles and Practice
2ND EDITION
Mark A. McHugh
Department of Chemical Engineering
Johns Hopkins University
Val J. Krukonis
Phasex Corporation
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McHugh, Mark A.
Supercritical fluid extraction : principles and practice / Mark A.
McHugh, Val J. Krukonis. - 2nd ed.
p. cm. - (Butterworth-Heinemann series in chemical
engineering)
Includes index.
ISBN 0-7506-9244-8
1. Supercritical fluid extraction. I. Krukonis, Val J.
11. Title. 111. Series.
TF'156.E8M39 1993
660' ,284248-4~20 93-31797
CIP
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BUTTERWORTH-HEINEMANN SERIES IN
CHEMICAL ENGINEERING
SERIES EDITOR
HOWARD BRENNER
Massachusetts Institute of Technology
ADVISORY EDITORS
ANDREAS ACRIVOS E. BRUCE NAUMAN
The City College of CUNY Rensselear Polytechnic Institute
JAMES BAILEY J.R.A. PEARSON
California Institute of Technology Schlumberger Cambridge Research
MANFRED MORARI ROBERT K. PRUD’HOMME
California Institute of Technology Princeton University
SERIES TITLES
Bubble Wake Dynamics in Liquids and Liquid-Solid Suspensions Liang-Shih Fan and Katswni
Tsuchiya
Chemical Process Equipment: Selection and Design Stanley M. Walas
Chemical Process Structures and Information Flows Richard S. H. Mah
Computational Methods for Process Simulations W. Fred Ramirez
Constitutive Equations for Polymer Melts and Solutions Ronald G. Larson
Fluidization Engineering, 2nd ed. Daizo Kunii and Octave Levenspiel
Fundamental Process Control David M. Prett and Carlos E. Garcia
Gas-Liquid-Solid Fluidization Engineering Liang-Shih Fan
Gas-Separation by Adsorption Processes Ralph T. Yang
Granular Filtration of Aerosols and Hydrosols Chi Tien
Handbook of Industrial Crystallization Allan S. Myerson
Heterogeneous Reactor Design Hong H. Lee
Interfacial Transport Processes and Rheology David A. Edwards, Howard Brenner, and Darsh T.
Wasan
Introductory Systems Analysis for Process Engineers E. Bruce Nauman
Laminar Flow and Convective Transport Processes L. Gary Leal
Microhydrodynamics: Principles and Selected Applications Sangtae Kim and Seppo 1. Karrila
Modeling With Differential Equations in Chemical Engineering Stanley M. Walas
Molecular Thermodynamics of Nonideal Fluids Lloyd L. Lee
Phase Equilibria in Chemical Engineering Stanley M. Walas
Physicochemical Hydrodynamics: An Introduction Ronald F. Probstein
Porous Media: Geometry and Transports Pierre M. Adler
Slurry Flow: Principles and Practice Clifton A. Shook and Michael C. Roco
Transport Processes in Chemically Reacting Flow Systems Daniel E. Rosner
Viscous Flows: The Practical Use of Theory Stuart W. Churchill
REPRINT TITLES
Advanced Process Control W. Harmon Ray
Applied Statistical Mechanics Thomas M. Reed and Keith E. Gubbins
Elementary Chemical Reactor Analysis Rutherford Ark
Kinetics of Chemical Processes Michel Boudart
Reaction Kinetics for Chemical Engineers Stanley M. Walas
Con tents
1. Introduction 1
2. Historical Perspective 17
3. Phase Diagrams for Supercritical Fluid-Solute Mixtures 27
Phase Diagrams for Binary Mixtures 31
Solid-Supercritical Fluid Phase Diagrams 45
Polymer-Supercritical Fluid Phase Diagrams 61
Phase Diagrams for Ternary Mixtures 71
4. Experimental Techniques in High-pressure Studies 85
Dynamic Methods for Measuring Solubilities in Supercritical
Fluids 85
Static Methods for Measuring Solubilities in Supercritical Fluids 91
Methods for Determining Phase Border Cunies 94
5. Thermodynamic Modeling of Supercritical Fluid-Solute Phase
Behavior 99
Intermolecular Forces 99
Solubility Parameters 105
Vapor-Liquid Calculations 110
Liquid-Liquid-Vapor Calculations 114
Polymer-Supercritical Fluid Calculations 120
Solid-Supercritical Fluid Calculations 127
6. Process Operations 135
7. Early Industrial Applications 145
Propane Deasphalting 145
Solexol Process 150
ROSE Process 153
8. Supercritical Fluid Process Development Studies 157
Activated Carbon Regeneration 158
Separation of Organic-Water Solutions 170
Breaking an Azeotrope 182
9. Polymers and Monomers Processing 189
High-pressure Polyethylene Polymerization 189
Polymer Fractionation Processes 192
Polyethylene Fractionation 198
V
vi Contents
Fractionation of Ethylene-Based Copolymers 205
Polysiloxane and Polysiloxane/PMMA Copolymer
Fractionation 217
Fractionation of Polymer Binders for Solid Propellants 250
Oligomer Extraction from Polymers 257
Supercritical Fluid Chromatography Analysis of Polystyrene 258
Polymer Fiber Spinning 260
Fractionation of Various Polymers 262
Polymer-Organic Solvent Phase Separation 280
Monomer Purification 285
10. Processing Pharmaceuticals, Natural Products, Specialty Chemicals, and
Waste Streams 293
Coffee Decaffeination 294
Edible Oils Extraction 299
Extraction of Chemotherapeutic Agents 304
Isomer Separations 307
Treatment of Waste Streams 309
11. Chemical Reactions in Supercritical Fluids 311
Enzyme Reactions 311
High-Temperature Reactions 321
Heterogeneous Catalysis 322
Viscosity Effects 325
Reaction/Separation Schemes 326
Enhanced Reaction Rates and Selectivities 328
12. Special Applications 333
Supercritical Fluid Nucleation 333
GAS Antisolvent Recrystallization 342
Swelling of Polymers 357
Formation of Porous Polymers 360
13. Epilogue 367
References 373
Appendix A. Patent Reviews 395
1. The Zosel Patent 397
2. Petroleum and other Fossil Fuels
Separations 399
3. Coffee Decaffeination 417
4. Extraction of other Vegetable and Animal
Materials 427
5. Polymers Processing 441
6. Separation of Organic-Water Solutions 444
7. Materials Processing 449
8. Miscellaneous 453
Appendix B . Calculating Binary, Vapor-Liquid Equilibria using the Peng-
Robinson Equation of State 463
Contents vii
Calculating Ternary, Vapor-Liquid Equilibria using the Peng-
Robinson Equation of State 470
Calculating Ternary, Vapor-Liquid-Liquid Equilibria using the
Peng-Robinson Equation of State 478
Calculating Binary, Solid-SCF or Solid-Liquid-Gas Equilibria
using the Peng-Robinson Equation of State 487
Calculating Binary, Vapor-Liquid Equilibria using the Sanchez-
Lacombe Equation of State 492
Calculating Polymer-Solvent-Solvent, Vapor-Liquid Equilibria
using the Sanchez-Lacombe Equation of State 497
Calculating Polymer-Solvent-Solvent, Vapor-Liquid-Liquid
Equilibria using the Sanchez-Lacombe Equation of
State 500
Index 503
I
Introduction
In the first edition we were both moderately quiet about the hype and hoopla,
and the myth, magic, and mystery of supercritical fluids (SCFs) that was
epidemic during the late 1970s and early 1980s, especially in articles in such
widely subscribed technical and business journals as Chemical & Engineering
News, Business Week, Chemical Week, and Chemical Engineering (table 1.1).
Based upon SCF information provided to journalists by companies in the field,
the article titles summarize the events of that time.
The trade literature touted SCF technology as a panacea for the
chemicals, petroleum, and foods industries; it was going to solve all sorts of
problems in energy reduction. waste treatment, chemicals separation, and
biotechnology. Quotes from some of these articles attest to the unbridled and,
in our opinion, unwarranted enthusiasm for this new technology.
Ideal solvents . . .
It’s an exciting technology, and it is just about to take off.
They [SCFs] have a tremendous capacity to dissolve.
This technology will become a significant alternative to distillation.
Critical-fluid extraction is a process . . . that has been studied before only in
the laboratory.
We’re on the threshold of a new technology.
SCF extraction is nothing less than a new unit operation.
The technology for scaling up is not that well known.
The mystique of the term supercritical alone commands attention.
. . . they are putting this magic gas-liquid to work . . .
The magic of supercritical fluids . . . .
Unfortunately, the term “magic” was pervasive throughout the articles.
Most of the large-scale processes that were promised in the articles did not
come to fruition during the 1978-1984 period, and supercritical fluids lost
favor. Perhaps it is no wonder that an article appeared in Chemical Engineering
(February, 1985) with a title that could have been an ominous portent:
“Supercritical Fluids: Still Seeking Acceptance.” During this time Europe was
very quietly building supercritical fluid extraction plants capable of processing
1
2 Introduction
Table 1.1 Articles on SCF Technology Appearing in the Trade Literature
Title Magazine
Supercritical Fluids Try for CPI Chemical Engineering, 1979
Applications
Critical Fluids Aim for a Broad Industrial Chemical Week, 1980
Role
Gas Solvents: About to Blast Off Business Week, 1981
Supercritical Fluids Offer Improved Chemical & Engineering News, 1981
Separations
The Promise for Supercritical Fluids Chemical Week, 1983
Separation Technology: Keep an Eye on Chemical Process, 1985
Supercritical Extraction
scores of millions of lbiyr material. A plant for decaffeinating coffee beans was
built in Bremen, Germany, and several plants for extracting hops and spices
were built in Germany, France, and the United Kingdom. It is apparent that
some of the statements above were grossly overstated, particularly those that
related to the “fact” that supercritical fluid extraction had been studied only in
the laboratory due to lack of scale-up information.
We didn’t include information from trade literature from the 1978-1984
period in the first edition. So why have we chosen to discuss it in the second
edition? By mid-1985 when we had completed the penultimate draft of the first
edition the media was rather quiet. Some large-scale developments in the
United States were becoming known (but now with no hoopla) within the
supercritical fluids community. Supercritical fluids were, therefore, no longer a
laboratory curiosity; no more “promises” and no more “blast-offs” were seen
in the trade literature from 1985 to early 1986. So we thought the supercritical
fluids community had matured.
Unfortunately, in about 1987 the hype of supercritical fluids was renewed,
this time in concert with another subject that was being discussed with
impassioned fervor: cholesterol. Low cholesterol or no cholesterol became a
prominent marketing slogan in 1988. Cholesterol-free corn oil and margarine,
to name just two, appeared on the supermarket shelves. The irony is that these
vegetable oils never had any cholesterol to begin with, but “cholesterol-free”
was a powerful consumer attractant. Table 1.2 bears witness to the merging of
supercritical fluids and cholesterol issues in another swirl of activity reminiscent
of the early 1980s, when SCF technology was touted for energy reduction and
other applications.
The SCF-cholesterol articles listed in table 1.2 also have broad claims.
You can have a tasty, juicy steak without fat and cholesterol-just as soon as
scientists perfect a process they call supercritical fluid extraction.
. . . basing their hope on a process called supercritical fluid extraction.
Introduction 3
Table 1.2 Articles on SCF Technology Applied to Cholesterol Applications Appearing
in the Trade Literature
Title Magazine
Process for Removing Cholesterol from Chemical Reporter, 1989
Milk Seen as Commercially Viable
High Tech Lowers Dairy Cholesterol Food Development, 1989
Scientists Are Working to Remove Fat Illinois Agricultural News, 1989
and Cholesterol from Meat
Dairies Skim Cholesterol from Milk Wall Street Journal, October, 1988
In Search of a Cholesterol Solution Farm Forum, 1990
. . . supercritical fluid extraction could have a world-wide impact on the
dairy industry.
. . . the [SCF] process may have other benefits . . . it can help reduce the
unhealthy saturated fats in butter that are still present after cholesterol has
been removed.
. . . egg yolks are fed down a column, C02 removes 7545% cholesterol
while retaining flavor . . .
Some of these statements and the attendant hopes are lamentable. It
might be advantageous to extract the cholesterol from a juicy steak without
altering any of its other properties, such as taste, for example, but it won’t
happen because carbon dioxide cannot penetrate the dense protein-lipoprotein
structure to extract the inextricably entwined cholesterol; it’s not because the
scientists won’t “perfect” the process they call supercritical fluid extraction. On
the other hand, some food scientists have finally recognized that fact and are
using carbon dioxide to extract cholesterol and fat from meat products that
have first been freeze dried. This process makes the fat and cholesterol
amenable to extraction, but even here selective extraction of cholesterol is not
possible. The processing of liquid egg yolks with SCF solvents to remove the
cholesterol is also another difficult proposition based on scientific facts. Merely
passing egg yolks downward in a column with supercritical carbon dioxide is
inadequate. The solvent will not selectively extract a significant amount of
cholesterol without drying the egg yolk or without also removing a significant
part of the lipids. This is because a large solvent-to-feed ratio is required. Even
presaturating the carbon dioxide with water does not remove the difficulty of
selectively extracting cholesterol from liquid egg yolks. If the egg yolk is dried
by spray or freeze drying, it is possible to remove cholesterol and lipids from
the egg. And concerning the statement above about “extracting saturated fats
from butter after removing the cholesterol” since butter contains over 50%
saturated fatty acids, we wonder what will be the result if the “unhealthy
saturated fats in butter” are removed along with the cholesterol.
4 Introduction
Because of all the ill-advised and ill-fated developments of the early 1980s,
and because of the sometimes desperate attempts of various research groups to
garner publicity on their applications of supercritical fluids to develop low
cholesterol milk, meat, and liquid eggs, supercritical fluids are still today
frequently met with skepticism whenever they are suggested as a solution to a
real problem even in spite of general knowledge of the large-scale industrial
successes in coffee, tea, spices, and hops. We have chosen to discuss these less
than lustrous periods in the progression of supercritical fluid technology to
highlight the exaggerated claims that were later found to be merely that,
exaggerations. Supercritical fluids may have been misapplied in the past, but
that is no reason to dismiss them in the future. We reiterate throughout the
book that each application must be evaluated on a case-by-case basis to
determine the potential viability of supercritical fluids. In later chapters, we
discuss some of the technical developments mentioned in the above articles,
and we point out why some of the developments were ill-fated.
Several large-scale plants are in operation throughout the United States
and Europe. Events in Europe were very different from events in the United
States. Plants in Germany, France, and the United Kingdom were commis-
sioned for coffee, tea, hops, spices, and flavor extraction, while in the United
States the difficulties of scaleup and the magic properties of SCF were being
discussed. However, in mid-1985 Pfizer began operation of a hops extraction
plant in Sydney, Nebraska, stated to be the largest carbon dioxide extraction
plant in the world (Cookson, 1987); Kraft General Foods (Maxwell House
Coffee Division) started construction on a coffee decaffeination plant in
Houston, Texas, and we use information about that plant to corroborate that
supercritical fluid extraction is no longer a laboratory curiosity.
Figure 1.1 shows part of the coffee decaffeination plant at a very early
stage of construction. A vessel 7 ft diameter x 70 ft fall (roughly scaled from
other parts of the photograph, namely the personnel at the top of the yet
unfinished structural steel), is being brought into position by a crane; the
crane, quite interestingly, is the world’s largest. Figure 1.2 is a diagram of the
Maxwell House coffee decaffeination process described in a Kraft General
Foods coffee patent (US 4,996,317, Katz et al., 1990). We consider the process
an engineering marvel that combines technical, economic, and environmental
aspects of an advanced technology that is applied to a commodity material
(a quite inexpensive commodity, we point out, with wholesale coffee beans
selling at about $1 per pound).
In the process depicted in figure 1.2, coffee beans charged to the extractor,
10, via line (18) are decaffeinated with supercritical carbon dioxide. The
caffeine-laden C02 stream leaving the extractor is passed into the bottom of a
water-wash column where the caffeine is removed from the COz. (Chapter 8
describes the thermodynamic equilibrium situation for the water-wash col-
umn.) The caffeine-free C02 is recycled to the coffee bean column, and
decaffeinated coffee beans leave the extractor via line 20. Let’s now follow the
water stream leaving the bottom of the C02 scrubbing tower. The caffeine-
