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Extraction of Natural Products
Using Near-Critical Solvents
Extraction of Natural Products
Using Near-Critical Solvents
Edited by
M.B. KING and T.R. BOTT
School of Chemical Engineering
University of Birmingham
Published by
Blackie Academic & Professional, an imprint of Chapman & Hall,
Wester Cleddens Road,
Bishopbriggs, Glasgow G64 2NZ
First edition 1993
© Springer Science+Business Media Dordrecht 1993
Originally published by Chapman & Hall in 1993
Softcover reprint of the hardcover 1st edition 1993
Typeset in 10/12 pt Times New Roman by Pure Tech Corporation, Pondicherry,
India
ISBN 978-94-010-4947-4
ISBN 978-94-011-2138-5 (eBook)
DOI 10.1007/978-94-011-2138-5
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Preface
The aim of this book is to present the current state of the art of extracting
natural products with near-critical solvents and to view the possibilities of
further extensions of the technique. Relevant background theory is given but
does not dominate the book. Carbon dioxide is the near-critical solvent used
in most recent applications and inevitably receives prominence. In addition to
general descriptions and reviews, the book contains three chapters by industrial practitioners who describe in detail the operation of their processes and
discuss the market for their products. Sections on the design of the pressure
vessels and pumps required in these processes and on the acquisition of the
data required for design are included. The costing of the processes is also
discussed.
There is good scope for combining a near-critical extraction step with other
process steps in which the properties of near-critical solvents are utilised,
for example as a reaction or crystallisation medium and a chapter is devoted
to these important aspects.
It is hoped that the work will be found to contain a great deal of specific
information of use to those already familiar with this field. However the style
of presentation and content is such that it will also be useful as an introduction.
In particular it will be helpful to those wondering if this form of separation
method has anything to offer for them, whether they are engineers, chemists
or managers in industry, or in academic or research institutions.
M.B. King
T.R. Bott
Acknowledgements
We wish to thank the many people including postgraduate students, postdoctorate fellows. visitors
and colleagues who have worked with us on near-critical fluids. We also wish to thank Distillers
MG Limited for their supply of carbon dioxide and for their friendly support and encouragement.
If it had not been for the above contributions. it would not have been possible for us to have
conceived this book and we dedicate it gratefully to all the above helpers and co-workers.
Contents
1 Introduction
1
M.B. KING and T.R. BOTT
Compressed and liquefied gases as solvents: the commercial applications
The scope of the book
Range of solvent conditions regarded as 'near-critical'
Range of available solvents
Range of components present in natural products: typical phase behaviour with
near-critical carbon dioxide and similar near-critical solvents
1.5. 1 Classification of phase behaviour in systems containing a near-critical
component
1.6
Role of solvent density
1.7
Possible and actual process layouts
1.8
Advantages in use of near-critical solvents: future prospects
Appendix: Some historical notes
References
1.1
1.2
1.3
1.4
1.5
2 Food legislation and the scope for increased use of
near-critical fluid extraction operations in the food,
flavouring and pharmaceutical industries
I
3
4
5
10
10
19
23
26
28
31
34
N. SANDERS
2.1
2.2
Introduction
Solvent extraction of foodstuffs and flavourings: legal restrictions on solvents
used and residual solvent levels
2.3
Widening the choice of extraction solvent: compressed and liquefied gases
as solvents: economic and other problems in their use
2.4
Use of carbon dioxide for dense gas extractions: 'rule of thumb' solubility rules
2.5
Actual and proposed applications of extractions using dense C02: tabular review
of the literature
2.5.1 Future applications
References
3 Other uses for near-critical solvents: chemical reaction
and recrystallisation in near-critical solvents
34
34
37
40
43
45
48
50
M.H.M. CARALP, A.A. CLIFFORD and S.E. COLEBY
3.1
Chemical reaction in near-critical solvents
3.1.1 Optimising physical properties and phase behaviour
3.1.2 Supercritical fluids in the critical region as reaction media
3.1.3 Supercritical fluids as continuum solvents
3.1.4 Transition state theory and supercritical fluids
3.2
Recrystallisation in near-critical solvents
3.2.1 Recrystallisation by pressure reduction
3.2.2 Recrystallisation by other methods
References
50
51
61
67
69
76
76
79
80
viii
CONTENTS
4 Commercial scale extraction of alpha acids and hop
oils with compressed CO2
84
D.S. GARDNER
4.1
Introduction, composition and brewing value of hops
4.1.1 Composition
4.1.2 Varietal differences
4.2
Convenience of hop extracts
4.2.1 Organic solvents originally used for preparing hop extracts
4.3
Advantages of compressed CO2 over conventional organic solvents for
the extraction of hops: extraction plant using this solvent
4.3 .1 The advent of C02 extraction plant
4.3 .2 Extraction plant utilising liquid CO 2
4.3.3 Supercritical CO 2 extraction
4.3.4 Relative merits of liquid and supercritical C02 as extraction solvents
for hops
4.4
Conclusions
References
5 Commercial scale decatTeination of coffee and tea using
supercritical CO2
84
84
87
87
89
89
91
91
95
97
99
100
101
E. LACK and H. SEIDLITZ
5.1
5.2
Introduction: the extent of coffee and tea production worldwide and the need
for decaffeination
5.1.1 Processing of raw coffee and tea: caffeine levels and the effects
of caffeine
Present-day demand for decaffeinated products and trends in the market
5.2.1 Decaffeinated coffee consumption in the USA
101
105
105
5.2.2
106
The decaffeinated coffee market in Europe
101
5.3
Brief description of the currently used processes for decaffeination and
their history
5.3.1 Examples of processes for decaffeinating coffee using organic
solvents
5.3.2 Water decaffeination of coffee beans
5.3.3 Decaffeination processes which use compressed CO2 as solvent
5.3.4 Tea decaffeination
5.4
Decaffeination with compressed CO 2
5.4.1 Advantages of supercritical CO 2 as a decaffeination solvent
5.4.2 General description of the basic COz-based coffee decaffeination
processes
5.4.3 Proposed modifications to the basic coffee extraction schemes
5.4.4 Process for the continuous extraction of green coffee beans with
compressed C02
5.4.5 Decaffeination of tea with supercritical CO 2
5.5
The patent literature for decaffeination processes
5.6
Comparison of economic aspects of the COz-based and ethylacetate-based
decaffeination processes
5.7
Technical aspects of plant design
5.7.1 CO 2 recovery system
5.7.2 Caffeine recovery systems
5.7.3 Extractor vessel and internals
5.7.4 Plant safety and control
5.8
Conclusions
References
107
109
110
111
111
111
112
112
119
121
122
123
125
129
129
132
132
136
137
138
CONTENTS
6 Extraction of flavours and fragrances with compressed CO 2
ix
140
D.A.MOYLER
6.1
6.2
Introduction
The properties of CO 2 as an extraction solvent
6.2.1 'Naturalness'
6.2.2 Selectivity
6.2.3 Use of entrainers to enhance solubilities in CO 2
6.2.4 Stability of extract and the role of lipids
6.3
The raw materials
6.3.1 Origins
6.3.2 Crop to crop variations
6.3.3 Storage and pretreatment prior to extraction
6.4
Equipment
6.4.1 Laboratory scale equipment
6.4.2 Equipment for extraction with liquid CO 2 on a commercial scale
6.4.3 Commercial extraction with supercritical CO2
6.5
Commercial use of C02 extracts since 1982
6.6
Conclusions
Appendix: COz-extracted flavour and fragrance ingredients
References
7 Physico-chemical data required for the design of
near-critical fluid extraction process
140
141
142
142
147
147
148
148
148
149
149
149
lSI
153
156
157
158
178
184
M.B. KING and O. CATCHPOLE
7.1
7.2
The need for physico-chemical data
Phase equilibria
7.2. 1 Types of phase behaviour of components typically present in natural
products: typical solubilities
7.2.2 Use of equations of state and other methods for collating phase
equilibrium data for systems with a near-critical component (the solvent)
7.2.3 Other correlation techniques: the Chrastil-Stahl correlation
7.2.4 Experimental determination of phase equilibria in systems containing a
near-critical component
7.3
Mass transfer rate parameters
7.3.1 Modelling mass transfer processes
7.3.2 Diffusion coefficients
Appendix: The fluid mass balance equations
References
8 Design and operation of the pressure vessels used in
near-critical extraction processes
184
185
185
186
206
207
209
209
220
225
228
232
R. EGGERS
8.1
8.2
8.3
Introduction
.C1assification of pressure vessels
8.2.1 Influence of process type on pressure vessel requirements
8.2.2 Classification of pressure vessels according to method of construction
8.2.3 Relative merits of multilayer and thick solid-walled vessels
8.2.4 Examples of solid-walled and multilayer extraction vessels
Vessel design
8.3.1 Process engineering criteria: sizing the vessels
8.3.2 Construction
8.3 .3 Closure systems
8.3.4 Inner fittings and baskets
232
232
232
234
237
240
241
241
244
247
252
x
CONTENTS
8.4
Operation of the pressure vessels in a near-critical extraction plant
8.4.1 Columns
8.4.2 Operation of the extraction vessels
8.4.3 Pressure release and CO 2 recovery
8.5
Design and development of equipment for the continuous extraction of solids
References
9 Pumps and compressors for super critical extraction
254
254
255
255
257
259
261
G. VETTER
9.1
9.2
Introduction
Process requirements
9.2.1 Continuous extraction processes
9.2.2 Discontinuous extraction processes
9.3
Pumps for liquids
9.3.1 Pump characteristics and selection of the best type of pump
9.3.2 Reciprocating pumps
9.3.3 Centrifugal pumps
9.4
Compressors for gas recovery
References
10 Estimation of separation cost
261
262
263
264
265
265
271
290
296
297
299
M.B. KING, O.J. CATCHPOLE and T.R. BOTT
10.1
10.2
10.3
Introduction
Energy and economic assessment of near-critical extraction processes
Extraction with a marginally subcritical solvent
10.3.1 Process layout
10.3.2 Energy consumption
10.3.3 Mass transfer considerations
10.3.4 Capital versus energy costs
10.4 Extraction with supercritical solvent
10.4.1 Separation by pressure reduction followed by recompression
10.4.2 Separation by temperature change
10.5 Energy and other costs in some typical cases
10.6 Example of preliminary costing
10.6.1 Energy consumption and heat exchanger duties
10.6.2 Installed capital costs of the compressor, extraction towers and
product separator
10.6.3 Estimation of separation cost
10.6.4 Discussion
References
Index
299
301
301
301
303
305
305
307
308
309
309
310
311
312
317
317
319
321
Contributors
Dr T.R. Bott
Dr M.H.M. Caralp
Dr O. Catchpole
Dr A.A. Clifford
Dr S.E. Coleby
Professor R. Eggers
Dr D.S. Gardner
Dr M.B. King
Dr E. Lack
Mr D. Moyler
Mr N. Sanders
Dr H. Seidlitz
Professor Dipl.-Ing. G. Vetter
School of Chemical Engineering, University of Birmingham, Birmingham B 15 2TT,
UK
School of Chemistry, University of Leeds,
Leeds LS2 9JT, UK
Industrial Research Limited, Lower Hutt,
New Zealand
School of Chemistry, University of Leeds,
Leeds LS2 9JT, UK
School of Chemistry, University of Leeds,
Leeds LS2 9JT, UK
Technische Universitat Hamburg-Harburg,
Verfahrenstechnik II, Eissendorfer Strasse
38, Postfach 901403, 2100 Hamburg 90,
Germany
English Hops Ltd, Paddock Wood, Tonbridge, Kent
School of Chemical Engineering, University of Birmingham, Birmingham B 15 2TT,
UK
Schoeller-Bleckmann Gesselschaft mbH,
Verfahrenstechnik, Haupstrasse 2, A2630
Ternitz, Austria
Universal Flavors Ltd, Bilton Road,
Bletchley, Milton Keynes MKI IHP, UK
Reading Scientific Services Ltd, The Lord
Zuckerman Research Centre, Reading
University, Whiteknights, Reading RG6
2LA, UK
Schoeller-Bleckmann Gesselschaft mbH,
Verfahrenstechnik, Haupstrasse 2, A2630
Ternitz, Austria
Lehrstuhl fur Apparatetechnik und Chemiemaschinenbau, Universitat ErlangenNurnberg, Couerstrasse 4, 8520 Erlangen,
Germany
1 Introduction
M.B. KING and T.R. BOTT
1.1 Compressed and liquefied gases as solvents: the commercial
applications
If the pressure is raised sufficiently, many substances which are gaseous at
ambient pressure either liquefy or begin to behave like liquids in that they
exert appreciable solvent power, even for solutes of low volatility. For
example, at temperatures up to 31.06°C (the critical temperature) carbon
dioxide can be liquefied by raising the pressure (Figure 2.1) and this liquid
can be used to dissolve natural oils and quite a wide range of non-polar or
slightly polar materials. Many of these are natural products and several commercial processes are based on this solubility behaviour. For example liquid
CO2 has been used commercially as a solvent for obtaining hop extracts since
1980 (chapter 4). Liquid propane has also been used for extracting natural
products: at one time there were about five commercial plants in use for
extracting natural oils by the propane-based' Solexol' process. (see appendix
to this chapter). Propane has the disadvantage of being a fire-hazard, but it is
a more powerful solvent than carbon dioxide and the pressures required when
using it as a solvent are usually lower. Although not an example of the
extraction of a natural product (the natural products considered in this book
are of recent vegetable origin), it is relevant to remember the existence of the
propane-based process for de-asphalting petroleum. This has been widely used
since the 1930s.
The physical properties of the liquefied gaseous solvents in the applications
mentioned above can deviate substantially from those of normal liquid solvents, due to the higher solvent reduced temperatures at which the extractions
are carried out. These typically range from 1.0 down to 0.9 or slightly below,
whereas the solvent reduced temperatures in normal liquid extraction operations do not normally exceed about 0.7. One consequence of this is that the
isothermal compressibilities are higher. At a reduced temperature of 0.95 the
compressibility is about 10 times as great as that for a liquid at the normal
boiling point. At higher reduced temperatures the isothermal compressibility
at the saturation point rises rapidly towards its theoretically infinite value at
the critical point. This fact can cause complications when these liquids are
pumped (see section 9.3.1). Viscosities and diffusivities also differ from those
in normal liquids and, as the reduced temperature rises towards unity,
2
NEAR-CRITICAL SOL VENTS
approach those in supercritical fluids. The above effects start to become
significant when the solvent reduced temperature rises above about 0.9 and
liquids at reduced temperatures exceeding this value are described as 'nearcritical' liquids in this monograph.
Although, at temperatures above their critical temperatures, gases do not
liquefy on raising the pressure, they can still exhibit liquid-like solvent properties if the pressure is sufficiently high for the density to approach a liquid-like
value. The dissolving power of the solvent is then strongly density dependent
and can be varied by changing the pressure.
This ability of compressed gases to display solvent powers akin to liquids
is termed the 'gas extraction' effect. The existence of this effect has been
known for over 100 years (a brief historical survey is included as an appendix
to this chapter). However it was not until about 1978 that a commercially
viable extraction process based entirely on the gas extraction effect came into
operation. The process, developed by HAG.AG in Germany, was for the
decaffeination of coffee using compressed supercritical carbon dioxide and is
described in chapter 5. Today this process and variants of it have been applied
also to the decaffeination of tea and to the extraction of the bitter components
from hops (chapters 4 and 8).
For reasons discussed in section 1.3, extractions with supercritical solvents
(i.e. solvents whose reduced temperatures and pressures exceed unity under
the extraction conditions) are not normally carried out at very high reduced
temperatures and it is convenient to classify extractions of this type together
with extractions with marginally subcritical solvents as 'near-critical' extraction operations. The two types of operation are closely related and many of
the convenient properties of supercritical solvents are shared by marginally
subcritical ones also.
In each case the solvent can be chosen to be gaseous under ambient conditions. If this is done, nearly all the solvent will automatically be expelled from
the product when the latter comes to ambient pressure. This is a distinct
advantage over normal liquid extraction processes, much of the energy requirement for which can lie in the separation of the product from the solvent
(a specific example of this is given in section 5.6). This is particularly true
where the product is to be used for human consumption since residual
solvent levels then have to meet the stringent food standards described in
section 2.2.
A second advantage in the use of near-critical rather than normal liquid
solvents is that this extends the range of solvents which can be used at
near-ambient temperatures. (Other advantages are discussed in section 1.8.)
Compressed carbon dioxide, for example, can be used. This solvent is of
particular interest to the food industry. It has the advantages of being nonflammable and also of producing no toxicity problems in the final product. Its
critical temperature (31.06°C) is close to ambient with the result that heatsensitive materials can be subjected to near-critical CO 2 extraction without
INTRODUCTION
3
decomposition. Both marginally subcritical and supercritical CO2 are used in
present-day commercial processes. Typical operating temperatures and pressures are about goC and 60 bar in the subcritical case though pressures of
several hundred bar are required when this solvent is used for supercritical
extraction. Higher solubilities can be obtained in the supercritical case, though
this can be at the cost of selectivity and in some cases degradation may occur
at the higher temperatures involved (chapter 6).
As outlined above, several commercial processes for extracting natural products with near-critical solvents already exist. Although the use of the technique is not as yet widespread, it is slowly increasing.
Within the last year or two additional new plants have been commissioned
for extracting hops (in Germany, UK, Australia and the United States), for
decaffeination (in Germany, the United States and Spain) and for extracting
flavour essences and natural pigments (in France, the UK, Japan and elsewhere). Virtually all the above operations are carried out on a batch or semibatch scale and the product is used for human consumption. The solvent used
in the above cases is compressed carbon dioxide. In addition, continuous
carbon dioxide based processes are now in the pilot plant stage of development
for refining and fractionating seed oils and for fractionating milk fat [1].
In most of the above cases the carbon dioxide based process is in direct
competition with earlier extraction processes in which organic solvents such
as hexane, methylene chloride or ethyl acetate have been used. There is
increasing resistance to the use of synthetic chemical solvents in the food
industry, a fact which should increasingly favour the use of the car based
process in the above applications.
The scope for further applications of near-critical solvents in the food flavouring and pharmaceutical industries is discussed in the next chapter.
1.2 The scope of the book
The purpose of this monograph is to point out the extent to which extraction
processes with near-critical solvents (usually compressed CO2) already exist,
to describe these and the advantages which they have to offer in some
detail, and to show how processes of this type can be designed and costed.
Relevant background theory and data acquisition are discussed and likely
areas for further applications of this technique are assessed. Other ways
in which near-critical solvents may prove useful in future are described in
chapter 3.
Fairly detailed information is given about the design and construction of the
necessary hardware such as pumps and pressure vessels and the way in which
these units integrate into the plant.
It is hoped that the work will be found to contain specific information of
value to those already familiar with this field. However the style of presentation
4
NEAR-CRITICAL SOL VENTS
and content is such that it should also be useful as an introduction. In particular
it is hoped that it may be helpful to those wondering whether this form of
separation method has anything to offer for them.
1.3 Range of solvent conditions regarded as 'near-critical'
In order to examine a little more closely the range of solvent conditions in
which we are interested, it is convenient to refer to the reduced pressure versus
reduced density diagram for carbon dioxide shown in Figure 1.1. Although the
figure is based on the properties of carbon dioxide, the dependence of reduced
density on reduced temperature and pressure is qualitatively correct for other
non-polar and slightly polar solvents also.
Key
Process
Solvent
AD
Supercritlcnl Hop ex trnc tion plnn t I (hnpter 8)
(02
sIX!
Supercri ticnl De - (nffeinntion plnnt I (hnpter 5)
(02
(~
Subcriticnl Hop extrnction plnnt I (hnpter 4)
nnd nlso Subcriticnl Extrnction of nnturnl oils
from herbs I (hnpter 6)
oQP
U.K (onl Sonrd Pilot Plnnt for extrncting
products of thermolysis
Rnnge of conditions in propnne de-nsphnlting
column
(02
Mixture
Isee text)
(3 Ha
10
...
Q..
5
Q..
QI
L-
:::J
'"'"
QI
L-
a.
...
'0
QI
:::J
'0
QI
a:
I
I
,,~-------------------
I
o·1 ~-"----'-----'----I_-L....I_---1_.........J
o
1·0
3·0
Reduced density pip e
Figure 1.1 Range over which 'near-critical' extraction operations have been reported.
INTRODUCTION
5
The range of solvent conditions regarded as 'near-critical' for the purpose of
this book is shown on the diagram but is essentially arbitrary. It includes both
marginally subcritical liquid solvents and supercritical solvents (i.e. solvents
with reduced temperature and pressure exceeding unity). Extraction conditions
in several commercial processes are entered on the diagram. The area covered
in the supercritical region is shown shaded and is similar to that originally
proposed by Williams [2] as being of most interest for supercritical extraction
operations. The isotherms in this region are comparatively flat resulting in a
considerable increase in density (and hence of dissolving power) with comparatively small increase in pressure. The hatched area does not extend to solvent
reduced temperatures (Tr) substantially in excess of about 1.3. One factor
leading to this upper limit is the inconveniently high pressures which must be
applied at higher values of Tr to achieve densities sufficiently great for the gas
extraction effect for low volatility solutes to be appreciable. However, there
may well be other good reasons, such as thermal degradation of the product,
for keeping the extraction temperature as low as possible in specific cases.
The marginally subcritical part of the near-critical region is shown with
slanted hatching. A lower limit (here taken to be about 0.9) to the reduced
temperature in this area is provided by the gradual onset of normal liquid
properties as Tr is lowered below unity. As discussed above and as may be
seen from Figure 1.1, the liquid phase remains appreciably compressible at
reduced temperatures down to 0.9 and a little below. A consequence of this
appreciable compressibility is that the solvent power of near-critical liquids is
appreciably pressure dependent.
The solvent conditions for those near-critical extraction processes for which
data are available are marked on Figure 1.1. With the exception of the Coal Board
(now British Coal) pilot plant, all of these are commercial processes, though not
all are for the extraction of natural products. The solvent used in all cases except
the de-asphalting column and the Coal Board process is compressed carbon
dioxide, in either the liquid or the supercritical state. The solvent used in the Coal
Board process, work on which has now ceased, was a mixture containing aromatic
and fully hydrogenated aromatic compounds with a pseudo-critical pressure of 31
bar and a pseudo-critical temperature of 713 K [3]. It is seen that the solvent
reduced densities in the above cases fall in ·the approximate range 1.3 to 2.1 :
solvent reduced temperatures are in the selected range 0.9 to 1.3.
1.4 Range of available solvents
As outlined in the previous section, the desired extraction temperature determines the approximate range of solvent critical temperatures (and hence solvents) which can be used. Natural products usually contain thermally labile
materials, which restrict the extraction temperatures that can be used to a
limited range around ambient. For reasons given in section 2.3 carbon dioxide
6
NEAR-CRITICAL SOLVENTS
is, of the various solvents complying with the above criteria, the one which is
almost universally used for extracting natural products. It certainly has good
health and safety credentials and is cheap. It has the disadvantage of having
a comparatively high critical pressure (Table 2.5), which raises the pressures
which are required when using it as an extractant above those for other
solvents. It also has an inconveniently high triple-point pressure (5 bar) and a
rather low dissolving power for many of the interesting compounds present in
natural products, particularly when these have polar characteristics or have
very long chain lengths. In many ways carbon dioxide can be regarded as a
typical non-polar solvent. Solubilities in this solvent are a decreasing function
both of the molecular weight of the solute and of its 'polarity' (in particular
the extent to which solute/solute hydrogen bonding occurs). An extensive
study of the solvent properties of liquid CO2 with a very wide range of solutes
has been carried out by Francis [4] .
Volatile hydrocarbons, such as pentane, are completely miscible with liquid
COz (Figure 1.2) and, as discussed in section 1.5, become completely miscible
with supercritical CO2 at pressures exceeding the system critical pressure at
the temperature of extraction. The same applies to other alkanes up to n-dodecane and also to aldehydes, ketones, esters and alcohols. About half the
compounds studied by Francis, including limonene, thiophene, pyridine, formic, acetic and caprylic acids and common laboratory solvents such as benzene, carbon disulphide, carbon tetrachloride, chlorofonn and ether were
completely miscible with liquid CO2 •
100
80
....
c:J
-'=I
CII
....
60
~
iii
iii
CII
....
40
I
CL.
curve at 2S O( "
I
20
_
00
10
20
30
40
50
60
_ __ '-'
70
IDe'"
... <point
ctS'Ve
80 90
100
Mole per cent carbo n dioxide
Figure 1.2 Pressure/comJXlsition diagrams for carbon dioxide/pentane (a Class A system) at temperatures above and below the critical temperature (31.1 0c) of carbon dioxide. (Data from ref. (70).)
However citric acid, which with its three carboxyl groups and single OH
group is capable of fonning very strong hydrogen bond links, is virtually
INTRODUCTION
7
insoluble as are other fruit acids. Water is· only slightly soluble (Figure 1.3c)
and the solubility of glycerol is about half that of water on a weight basis.
Because of their high molecular weight (about 800), the natural oils are also
only very sparingly soluble in liquid CO2 (Figure 1.4). Essential oils such as
thymol (molecular weight 150) are more soluble than the natural oils as are
the free fatty acids. The solubilities of many of the components typically
extracted from natural products using liquid carbon dioxide are quite low
(typically in the range 0.1 to 1.0 weight %). Higher solubilities may be
obtained by operating with supercritical CO2 though at the expense of using
higher temperatures. Tabular summaries of some recent solubility determinations are included in chapter 2 and the appendix to chapter 6.
The solubilities of many of the compounds of interest could in principle be
enhanced using a suitable entrainer or co-solvent. The entrainer would normally have a volatility intermediate between that of the solvent and the component (or components) to be extracted and it should have a good solubility
in (or in some cases be completely miscible with) the solvent.
Quite spectacular solubility enhancement can sometimes be obtained in this
way . For example Wong and Johnston [5] obtained a hundred-fold enhancement of the solubility of cholesterol in compressed CO2 using 9% methanol
as entrainer and Kumik and Reid [6] have reported an even greater effect on
the solubility of hydroquinone using 2% tributyl phosphate as entrainer. They
also found that the solubility of phenanthrene increased by factors of between
2 and 3 in the presence of naphthalene. Larson and King [7] found that a
ten-fold increase occurred in the solubility of a typical steroid in compressed
CO2 at 350 bar and 40°C when 5% methanol was added to the solvent, though
an amino acid which they tested remained virtually insoluble. Kim et al. [8]
found that the solubility of benzoic acid in CO2 at 35°C and 200 bar is approximately doubled when 8 wt% S02 is added to the solvent. There are many other
examples of this type.
The largest enhancements occur when there is a specific interaction between
the entrainer and the solute, such as hydrogen bonding or strong dipole-dipole
interactions. In the absence of such effects, solubility enhancements may occur
if the entrainer produces a significant increase in the density of the solvent
phase. A theoretical description of C02 /entrainerisolute phase behaviour has
been given by Dobbs et al. [9].
A second class of 'entrainers', which can only briefly be mentioned here,
are surfactant molecules which can form reverse micelles within a non-polar
solvent, such as compressed CO2 • Highly hydrophilic compounds can be
solubilised within the reverse micelle structure. The use of reverse micelles to
solubilise polar components from an aqueous phase using near-critical solvents has been described in a number of papers [10-12].
The authors are not aware of any entrainer-based processes which are in use
commercially at the present time. However Peter and Brunner [13], who were
among the first workers to point out the potential importance of entrainers,
8
NEAR-CRITICAL SOLVENTS
(a)
t.
-..,
p • • thrtt
pl'la.u
pr.uu-r.
8/
7
t'--C
\
P"
,"
I
~/
0 ~----------1~~-~--~-~-~-----~
-Holl fract ion of carbon d loxide-
1
_ {HOle fract lon} _
of water. 10'
(c)
(b) 240
0· 4
0·3
20"1:
200
160
120
0·1
240
IS·,
2S·'
Prnsure
bar
I
80
40
0·2
<>
a
'S.,
lO ' (
t
Pressure
bar
I
200
160
120
80
40
. . 2S ",
0
002 0 -03 0 ·04
- Mole fraction CO, 0·01
0
0 ·996
0·998
1·0
- Hole frac t lon of CO 2 _
Figure 1.3 Phase behaviour of carbon dioxide/water system at temperatures between the critical
hydrate temperature and the upper critical solution temperature. (a) Typical pressure/composition diagram for carbon dioxide/water (a Class 82 system) at temperatures below the critical
temperature of carbon dioxide but above the critical hydrate formation temperature. Data for arms
8 and C are shown in (b) and (c) respectively. (b) Solubility of liquid CO2 in water as a function of temperature and pressure (arm C in (a». (c) Solubility of water in liquid COz as a function
of temperature and pressure (arm 8 in (a». (d) The three phase pressure curve compared with the
vapour pressure curve of carbon dioxide showing the critical locus CsU (i.e. locus of points such
as C' on (e) where vapour properties merge with those of solvent-rich liquid). (Data reference [75].)
(e) Detail of the isothermal pressure/composition diagram at 25°C (on left) and at temperature
between Tc and Tu (on right). Subscripts I and 2 denote water-rich and C02-rich phase. Critical
point C' is shown as blocked-in circle. (Data reference for (b) and (c) is [811.)
9
INTRODUCTION
(d )
7_
73
72
7.
p
(ba,)
I
70
69
I
I
,
68
I
I
I
I
I
I
67
I
I
I
I
66
I
I
:
I
65
I
TC Tiol
62-
27
26
28
--,·e
29
30
3'
32
(e)
64 5
c'
L,
+L2
74.5
P(bar )
74
0995
0996
0997
0998
01199
- - m ole fra ct.on c~
•0
995
996
997
998
999
- - -mole fra c t lon~ -
did suggest a process. This utilised the enhanced solubility of the glycerides
of oleic acid in compressed CO2 which could be achieved with an acetone
en trainer. They also proposed a rather similar process for decaffeinating coffee, which is described in section 5.4.3.3.
An unfortunate aspect to the use of entrainers is that in some cases their
inclusion detracts from the good health and safety characteristics of pure
carbon dioxide.