Chapter 2 Supercritical fluid extraction and its application in the food industry DAVID STEYTLER, School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ 2.1 INTRODUCTION Solvent extraction is one of the oldest methods of separation known and certainly dates back to prehistory. The science of solvent extraction has evolved accordingly over a long period of time and much progress has been made in the understanding of solvation and the properties of liquid mixtures used in extraction processes. The associated literature on phase behaviour is certainly extensive and, although representation of highly non-ideal mixtures is still problematic, many theoretical models have been successfully developed (Fredenslund, 1975; Hildebrand and Scott, 1950: Prausnitz et aL., 1986). Extensive databanks of pure component properties have grown to support such models in order to predict solvent performance in process applications. Today, even with the introduction of new separation technologies, solvent extraction remains one of the most widespread techniques operating on an industrial scale. Hannay and Hogarth's(1879)early observations of the dissolution of solutes in supercritical fluid (SCF) media introduced the possibility of a new solvent medium. However, it is only in recent years(since 1960) that commercial process applications of supercritical fluid extraction have been extensively examined In the last decade many advances have been made in researching SCF extraction both in terms of fundamental aspects and commercial applications. In particular the high degree of selectivity and control over solubilities afforded by pressure(and temperature) ariation has led to the introduction of many novel SCF extraction and fractionation processes. Of all possible gases, the benign properties(non-toxic, non-flammable)and accessible critical temperature of CO2 have ensured its predominance as a safe SCF solvent for the food industry The essential features of a modern solvent extraction process(using a liquid or SCF solvent medium) are illustrated schematically in Fig. 2. 1. The material to be extracted
Chapter 2 Supercritical fluid extraction and its application in the food industry DAVID STEYTLER, School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ 2.1 INTRODUCTION Solvent extraction is one of the oldest methods of separation known and certainly dates back to prehistory. The science of solvent extraction has evolved accordingly over a long period of time and much progress has been made in the understanding of solvation and the properties of liquid mixtures used in extraction processes. The associated literature on phase behaviour is certainly extensive and, although representation of highly non-ideal mixtures is still problematic, many theoretical models have been successfully developed (Fredenslund, 1975; Hildebrand and Scott, 1950; Prausnitz et al., 1986). Extensive databanks of pure component properties have grown to support such models in order to predict solvent performance in process applications. Today, even with the introduction of new separation technologies, solvent extraction remains one of the most widespread techniques operating on an industrial scale. Hannay and Hogarth’s (1879) early observations of the dissolution of solutes in supercritical fluid (SCF) media introduced the possibility of a new solvent medium. However, it is only in recent years (since 1960) that commercial process applications of supercritical fluid extraction have been extensively examined. In the last decade many advances have been made in researching SCF extraction both in terms of fundamental aspects and commercial applications. In particular the high degree of selectivity and control over solubilities afforded by pressure (and temperature) variation has led to the introduction of many novel SCF extraction and fractionation processes. Of all possible gases, the benign properties (non-toxic, non-flammable) and accessible critical temperature of C02 have ensured its predominance as a safe SCF solvent for the food industry. The essential features of a modern solvent extraction process (using a liquid or SCF solvent medium) are illustrated schematically in Fig. 2.1. The material to be extracted is
8 D. Stevtle Extractor Separato :s(g) M S(|)i Fig. 2.1. Schematic representation of a solvent extraction process (S= solvent, M=materia xtracted, E= extract). placed in an extraction vessel (extractor)into which solvent is introduced under conditions(temperature, flow rate etc. )which optimise the dissolution of the desired components. The solvent stream is then passed to a separation vessel(separator)where conditions are set to selectively separate the solvent from the extracted components. The solvent is then condensed and recycled through the system. In conventional liquid extraction, solvents of low volatility are employed with vapour pressures less than one atmosphere. In the course of the extraction process the solvent exists as a liquid during the extraction stage and a gas when it is removed from the extract by distillation. variations in pressure are small and do not significantly exceed the vapour pressure of the liquid at the extraction temperature. Although temperature variation gives some control over solubility, selective removal of components from a mixture is largely determined by the chemical nature of the solvent. Progressive fractionation can therefore only be achieved by a fortuitous response to temperature or by systematically changing the solvent or the composition of a mixed-solvent system The initial aims of this chapter are to establish the basic principles involved in SO traction. Selected applications are later reviewed with reference to the underlying fun damental properties that serve to differentiate the behaviour of scfs from conventional d solvents 2.2 THE SUPERCRITICAL FLUID STATE The P-T phase diagram for CO2 showing all four physical states(solid, liquid, gas and SCF)is shown in Fig. 2.2. Below the freezing point solid CO2 (dry ice) exists which melts on heating when the thermal energy of the molecules overcomes the lattice energy The integrity of the liquid state so formed is maintained by relatively weak attractive intermolecular forces(van der Waals). The formation of a supercritical fluid state above the critical temperature(Te=3105C)can be viewed as an analogous process in which the thermal energy of the molecules overcomes all attractive interactions maintaining the liquid state. Like a gas, the SCF state formed then occupies all available volume. Strictly the SCF state exists above both the critical temperature and pressure(T>Tc: P>Pc)
18 D. Steytler Extractor Separator Condenser Fig 2 1 Schematic representation ot a solvent extraction process (S = solvent, M = material extracted, E = extract) placed in an extraction vessel (extractor) into which solvent is introduced under conditions (temperature, flow rate etc.) which optimise the dissolution of the desired components. The solvent stream is then passed to a separation vessel (separator) where conditions are set to selectively separate the solvent from the extracted components. The solvent is then condensed and recycled through the system. In conventional liquid extraction, solvents of low volatility are employed with vapour pressures less than one atmosphere. In the course of the extraction process the solvent exists as a liquid during the extraction stage and a gas when it is removed from the extract by distillation. Variations in pressure are small and do not significantly exceed the vapour pressure of the liquid at the extraction temperature. Although temperature variation gives some control over solubility, selective removal of components from a mixture is largely determined by the chemical nature of the solvent. Progressive fractionation can therefore only be achieved by a fortuitous response to temperature or by systematically changing the solvent or the composition of a mixed-solvent system. The initial aims of this chapter are to establish the basic principles involved in SCF extraction. Selected applications are later reviewed with reference to the underlying fundamental properties that serve to differentiate the behaviour of SCFs from conventional liquid solvents. 2.2 THE SUPERCRITICAL FLUID STATE The P-T phase diagram for C02 showing all four physical states (solid, liquid, gas and SCF) is shown in Fig. 2.2. Below the freezing point solid COz ('dry ice') exists which melts on heating when the thermal energy of the molecules overcomes the lattice energy. The integrity of the liquid state so formed is maintained by relatively weak attractive intermolecular forces (van der Waals). The formation of a supercritical fluid state above the critical temperature (T, = 3 1 .OS"C) can be viewed as an analogous process in which the thermal energy of the molecules overcomes all attractive interactions maintaining the liquid state. Like a gas, the SCF state formed then occupies all available volume. Strictly the SCF state exists above both the critical temperature and pressure (T> T,; P > P,)
Supercritical fluid extraction 19 SOLID LIQUID P Tc 50 Fig. 2.2. Pressure-temperature phase diagram for COz showing isochores(g cm") though the latter condition is often relaxed in the technical literature. A substance above its critical temperature therefore behaves like a gas and always occupies all available volume as a single phase. However, unlike a gas, a Scf cannot be condensed to a d-gas state by application of pressure. Similarly when the critical pressure is exceeded it is possible to go from a SCF state to a compressed liquid condition by cooling, but a single-phase filling all available volume is always maintained It should be appreciated that there are no phase boundaries delineating the SCF state and therefore no sharp changes in physical properties occur on entering this region Transition to the SCf state from a gas or liquid is thus an 'invisible process. However, it a coexisting liquid-gas mixture is heated at constant volume along the vapour pressure curve, the density of the liquid phase decreases while that of the gas phase increases,ul at the critical point they become equal and the meniscus between them disappears. As this point is approached density fluctuations of microscopic dimensions give rise to a distinctive light-scattering phenomenon known as 'critical opal Although the supercritical state offers a greater range of density, which in turn provides greater control over solubilities, the liquid state of compressed gases is often mployed in extraction processes, particularly for separation of thermolabile components at low temperatures. In order to avoid restrictive and confusing nomenclature, it is convenient to use the term near-critical liquid(NCL)to distinguish the state of compressed gas just below T from a'normal'liquid at NTP, for which T< Te. The term near critical fluid(NCF) will be used in this chapter to represent both SCF and NCL states of compressed-gas solvents Many liquids commonly employed as solvents enter an SCF state on heating, but for most purposes the critical temperatures are too high to permit their use as SCF solvents
Supercritical fluid extraction 19 I 600 - 1.1 11.0 - 400 - - - k3 e a -50 0 Tc 50 100 T (“C) Fig. 2.2. Pressure-temperature phase diagram for CO, showing isochores (g ~rn-~). though the latter condition is often relaxed in the technical literature. A substance above its critical temperature therefore behaves like a gas and always occupies all available volume as a single phase. However, unlike a gas, a SCF cannot be condensed to a coexisting liquid-gas state by application of pressure. Similarly when the critical pressure is exceeded it is possible to go from a SCF state to a compressed liquid condition by cooling, but a single-phase filling all available volume is always maintained. It should be appreciated that there are no phase boundaries delineating the SCF state and therefore no sharp changes in physical properties occur on entering this region. Transition to the SCF state from a gas or liquid is thus an ‘invisible’ process. However, if a coexisting liquid-gas mixture is heated at constant volume along the vapour pressure curve, the density of the liquid phase decreases while that of the gas phase increases, until at the critical point they become equal and the meniscus between them disappears. As this point is approached density fluctuations of microscopic dimensions give rise to a distinctive light-scattering phenomenon known as ‘critical opalescence’. Although the supercritical state offers a greater range of density, which in turn provides greater control over solubilities, the liquid state of compressed gases is often employed in extraction processes, particularly for separation of thermolabile components at low temperatures. In order to avoid restrictive and confusing nomenclature, it is convenient to use the term ‘near-critical liquid’ (NCL) to distinguish the state of a compressed gas just below T, from a ‘normal’ liquid at NTP, for which T < T,. The term ‘near critical fluid’ (NCF) will be used in this chapter to represent both SCF and NCL states of compressed-gas solvents. Many liquids commonly employed as solvents enter an SCF state on heating, but for most purposes the critical temperatures are too high to permit their use as SCF solvents
20 D Steytler (e.g. T for hexane is 234 C). All substances with accessible critical temperatures are gases at NTP and representative examples for use in extraction processes are shown in Table 2. 1. Being non-toxic, non-flammable, and chemically inert, CO2 has obvious actical advantages over other potential gases for use in large-scale extraction processes under pressure Table 2.1. Potential gases for near-critical fluid extraction Formula Pc (bar) Carbon dioxide 31.1 Nitrous oxide 36.4 71.5 Ammonia Ethane C2H6 322 48.2 pane Ethylene CH Freon 13 CCIF 289 38.7 2.2.1 Physical properties of NCF CO2 Density Isochores, representing constant density, are shown in Fig. 2. 2 for CO 2 in the NCL, gas and SCF regions of the P-T phase diagram. In the NCL phase, densities are typical normal liquid solvents(900-1100 kg m-)and isothermal compressibility is relatively w. In contrast the SCF state includes a wide range of densities ranging from gas-like values at low pressure(< 100 kg m)to 'liquid-like' values at elevated pressure. The region near the critical point is particularly interesting as it represents the region of highest compressibility The capability of a solvent to solvate and dissolve a particular solute is directly related to the number of solvent molecules per unit volume. This is because the overall solvation energy is determined by the sum of the solute-solvent interactions occurring primarily within the first solvation shell. Density is therefore a key parameter in determining the effect of temperature and pressure on solubilities in NCF extraction. Indeed, solubility isotherms often exhibit a steep rise with pressure just above the critical point of the solvent where density is rapidly increasing with pressure. The ability to control solubilities through pressure is one of the main features that distinguish NCFs from liquid solvents. Moreover, the potential for differential control of solubilities in multicomponent systems (Johnston er al., 1987)can enable novel fractionation processes that would be ssible using conventional liquid extraction processes stematic assessment of the representation of density, and other thermodynamic properties, of Co, by various theoretical models has been made by IUPAC (Angus et al 1976). This comprehensive treatise provides procedures based on equations of state which
20 D. Steytler (e.g. Tc for hexane is 234°C). All substances with accessible critical temperatures are gases at NTP and representative examples for use in extraction processes are shown in Table 2.1. Being non-toxic, non-flammable, and chemically inert, C02 has obvious practical advantages over other potential gases for use in large-scale extraction processes under pressure. Table 2.1. Potential gases for near-critical fluid extraction Name Formula TC PC (“C) (bar) Carbon dioxide co2 31.1 73.8 Nitrous oxide N20 36.4 71.5 Ammonia NH3 132.4 111.3 Ethane C2H6 32.2 48.2 Prop an e C3H8 96.6 41.9 Freon I3 CClF3 28.9 38.7 Ethylene C2H4 9.2 49.7 2.2.1 Physical properties of NCF COz Density Isochores, representing constant density, are shown in Fig. 2.2 for COz in the NCL, gas and SCF regions of the P-T phase diagram. In the NCL phase, densities are typical of normal liquid solvents (900-1 100 kg m-3) and isothermal compressibility is relatively low. In contrast the SCF state includes a wide range of densities ranging from ‘gas-like’ values at low pressure (< 100 kg m-3) to ‘liquid-like’ values at elevated pressure. The region near the critical point is particularly interesting as it represents the region of highest compressibility. The capability of a solvent to solvate and dissolve a particular solute is directly related to the number of solvent molecules per unit volume. This is because the overall solvation energy is determined by the sum of the solute-solvent interactions occurring primarily within the first solvation shell. Density is therefore a key parameter in determining the effect of temperature and pressure on solubilities in NCF extraction. Indeed, solubility isotherms often exhibit a steep rise with pressure just above the critical point of the solvent where density is rapidly increasing with pressure. The ability to control solubilities through pressure is one of the main features that distinguish NCFs from liquid solvents. Moreover, the potential for differential control of solubilities in multicomponent systems (Johnston et al., 1987) can enable novel fractionation processes that would be impossible using conventional liquid extraction processes. A systematic assessment of the representation of density, and other thermodynamic properties, of C02 by various theoretical models has been made by IUPAC (Angus et al., 1976). This comprehensive treatise provides procedures based on equations of state which
Supereritical fluid extraction 21 best reproduce available experimental data. An extensive tabulation of properties covering a wide range of pressure and temperature is also included Substances that are near their critical temperature at NTP all comprise small, weakly nteracting molecules. These characteristics give rise to a high degree of molecular mobility which give NCFs a lower viscosity and higher diffusivity than liquid solvents The state of a supercritical fluid approaches that of a gas at low pressure and that of a low-viscosity liquid at elevated pressure. However, to achieve reasonable levels of solubility, the density of the NCF must be modest (> 400 kg m ) When transport properties with liquid solvents, it is therefore more meaningful to use examples of comparable density. Viscosity isotherms for NCF CO2 are shown in Fig. 2.3, which includes a density isochore (770 kg m-3) representing typical conditions employed in NCF extraction processes. It is interesting that, at this constant density, the viscosity is not strongly affected by temperature or pressure and has a value of approximately 600 uP This is to be compared with a typical petrochemical solvent such as hexane, for which n= 3000 uP at NTP. The smooth transition in physical properties between liquid and SCF states is illustrated by the density isochore which shows no sharp change on passing between the two states. The low viscosity of NCFs and their mixtures can provide some important benefits in extraction proce LIQUID 1000 Fig. 2.3. Viscosity isotherms for NCF CO?(the dashed line represents a fluid density of 770kgm-3)
1500 1000 v 5 F 500 0 T V) 30 = 20 60 LIQUID 100 -- SCF _______- - ---- ____----- - JJ150 ___--- GAssi I I I I 0 P, 200 400 600 800
(1) In leaching processes it enables effective percolation of the solvent through packed beds and rapid penetration of the internal pore structure of individual particles comprising the bed (2) When a liquid is being extracted the ncf solvent will often dissolve in the coexisting liquid phase and lower its viscosity. This is particularly beneficial when extracting highly viscous liquids which can present mass transfer problems due to poor solvent contact. Indeed NCFs are sometimes used to thin highly viscous naterials to facilitate their transport in extraction processes, e.g. the injection propane into crude lecithin in the NCF separation of phospholipids(Peter, 1987: ee Section 2.6.3) (3) It facilitates transfer of the solvent and reduces the dimensions of pipework equired in extraction plants. This is particularly important with high-pressure equipment where costs are strongly linked with scale Diffusion In low-viscosity media, translational diffusion is enhanced and diffusion coefficients NCFs are therefore significantly higher than in liquid solvents. Self-diffusion isotherms for CO2 are shown in Fig. 2.4, which also shows an isochore with a typical NCF extraction density of 0.68 g cm -(680 kg m -). Again it is found that at constant density the diffusion coefficient is not greatly affected by temperature or pressure and there is no abrupt change on passing between the NCl and SCF states. It is informative to compare 250 Fig. 2.4. The self-diffusion coefficient of NCF CO2(the dashed line represents a fluid density of
250 2oo 150 - u) s 0 1 7 n 50 0 - GAS T (“C) 100 - 75 - 50 c -___ ________ ____ LIQUID I Ij I 25 I
Supercritical fluid extraction 23 the value of the diffusion coefficient under these conditions(D=4x10-8m2s-)with hexane at NTP(D=4x10-9m2s-). These values are fairly representative and it is generally observed that self-diffusion coefficients for NCFs under typical extraction con ditions are about an order of magnitude greater than in liquid solvents. Diffusion coeffi cients of solutes in NCFs are generally enhanced to a similar extent( Section 2.3.3) Volatility(vapour pressure) In conventional extraction processes liquid solvents are recovered by distillation at elevated temperature(and/ or reduced pressure)in which valuable volatile components of he extract can be lost. Near-critical fluids are highly volatile and can be completely removed and recycled at low temperatures during an extraction process. This has important implications for improving the quality of extracts, since (1) Highly volatile components in the extract are retained. This is of particular gnificance in the extraction of flavours and fragrances ( 2) The extract is not subjected to thermal or chemical degradation(e.g. oxidation)at the elevated temperatures employed in distillation 3) The high volatility ensures'complete'removal of solvent residues. Any legislative restrictions regarding residual solvent levels are thereby avoided Chemical properties Of all NCFs, CO2 is the safest medium for use in solvent extraction as it provides a non flammable, non-oxidative environment. CO2 does, however, undergo chemical reactions with water which often need to be considered when extracting food materials. One familiar set of reactions is the dissolution of CO2 in water to produce carbonic acid The carbonic acid then dissociates and lowers the ph of the aqueous phase in contact ith CO2 H,CO2+Ho H3O*+ HCO3 HCO +HO H20+C The pH of water is therefore primarily determined by the partial pressure of Coz with which it is in contact. Water in contact with atmospheric CO2 has a pH of approximately 5.7 at 20C and 3. 8 when contacted with pure CO2 at the same pressure. With increasing pressure the pH falls further, so that when in contact with liquid CO at 100 bar the pH is about 3. This represents a fairly typical acidity for water in an NCF CO2 extraction ince K2>K3 the hydrogen ion concentration is primarily determined by the dissociation of carbonic acid. If the increased acidity is problematic it is possible to suppress the dissociation and buffer the coexisting aqueous phase by addition of
Supercritical fluid extraction 23 the value of the diffusion coefficient under these conditions (D = 4 x lo-* m2 s-l) with hexane at NTP (D = 4 x lo-' m2 s-'). These values are fairly representative and it is generally observed that self-diffusion coefficients for NCFs under typical extraction conditions are about an order of magnitude greater than in liquid solvents. Diffusion coefficients of solutes in NCFs are generally enhanced to a similar extent (Section 2.3.3). Volatility (vapour pressure) In conventional extraction processes liquid solvents are recovered by distillation at elevated temperature (and/or reduced pressure) in which valuable volatile components of the extract can be lost. Near-critical fluids are highly volatile and can be completely removed and recycled at low temperatures during an extraction process. This has important implications for improving the quality of extracts, since: (1) (2) (3) Highly volatile components in the extract are retained. This is of particular significance in the extraction of flavours and fragrances. The extract is not subjected to thermal or chemical degradation (e.g. oxidation) at the elevated temperatures employed in distillation. The high volatility ensures 'complete' removal of solvent residues. Any legislative restrictions regarding residual solvent levels are thereby avoided. Chemical properties Of all NCFs, C02 is the safest medium for use in solvent extraction as it provides a nonflammable, non-oxidative environment. C02 does, however, undergo chemical reactions with water which often need to be considered when extracting food materials. One familiar set of reactions is the dissolution of C02 in water to produce carbonic acid: (2.1) The carbonic acid then dissociates and lowers the pH of the aqueous phase in contact with C02: K, C02+H20 4 H2C03 K* H2C03 + H20 e H30+ + HCOT (2.2) HCOC + H20 e H30+ + C0:- K3 (2.3) The pH of water is therefore primarily determined by the partial pressure of C02 with which it is in contact. Water in contact with atmospheric C02 has a pH of approximately 5.7 at 20°C and 3.8 when contacted with pure C02 at the same pressure. With increasing pressure the pH falls further, so that when in contact with liquid C02 at 100 bar the pH is about 3. This represents a fairly typical acidity for water in an NCF C02 extraction process. Since K2 S K3 the hydrogen ion concentration is primarily determined by the initial dissociation of carbonic acid. If the increased acidity is problematic it is possible to suppress the dissociation and buffer the coexisting aqueous phase by addition of
Steytler bicarbonate anion( Lovell, 1988). The pressure of Co 2 could, however, be used to control the pH of water in a unique fashion since no chemical residues(of acids or bases)remain The potential applications of this technique have not been widely explored A less familiar reaction of CO, with water is the formation of a solid hydrate below about CO2+6H,O=CO2 6H,O (2.4) This restricts the use of NCF CO in the extraction of aqueous systems to temperat up to 10C higher than the freezing point of water. (Note: This depends on the type concentration of the solute. At modest levels CO? is non-toxic and so represents a completely safe NCF solvent fo food applications with no legislative restrictions governing its use. The only possible, but unlikely, physiological hazard involves asphyxiation by displacement of air following a considerable leak in a confined area The combined effects of high hydrostatic pressure and low acidity in water-containing systems can be beneficially employed to prevent food spoilage by destroying bacteria Kamihira et aL., 1987: Taniguchi, 1987a). Rapid decompression of dissolved gas is sometimes used to expand and disrupt the cell structure of natural materials and could also be used as a means of sterilisation. Although SCF CO2 can be an effective apol medium for enzyme reactions(van Eijs et aL., 1988; Steytler et aL., 1991), it has also been used to selectively inactivate enzymes(Taniguchi, 1987b; Weber, 1980). In practice these chniques could be applied either in situ, during an extraction process, or as a separate unit operation 2. 3 PROPERTIES OF NCF SOLUTIONS 2.3.1 Solubilities in NCFs There has been much confusion in some of the literature concerning the solvent properties of NCF CO2. An impression is often given that NCFs are universal solvents which can be to extract virtually any component of a mixture by selecting a suitable set of conditions of temperature and pressure. Statements to the effect that NCFs are 'good solvents, implying that solute loadings are high, are also prevalent and highly misleading. Before examining the solvent properties of NCFs in detail, it is worth stating a few basic principles (1) To be'supercritical'intermolecular attractive interactions must be relatively weak compared with thermal energy. This necessitates an absence of all polar inter actions, such as hydrogen bonding, and defines a medium of low dielectric onstant. All NCFs are therefore essentially apolar solvents (2) The absence of strong attractive interactions between molecules means that solvation energies are generally low and solubilities in NCFs are thus often much lower than in liquid solvents
24 D. Steytler bicarbonate anion (Lovell, 1988). The pressure of C02 could, however, be used to control the pH of water in a unique fashion since no chemical residues (of acids or bases) remain. The potential applications of this technique have not been widely explored. A less familiar reaction of C02 with water is the formation of a solid hydrate below about 10°C: C02 + 6H2O = C02.6H20 (2.4) (g) (1) (SI This restricts the use of NCF CO, in the extraction of aqueous systems to temperatures up to 10°C higher than the freezing point of water. (Note: This depends on the type and concentration of the solute.) Biochemical properties At modest levels C02 is non-toxic and so represents a completely safe NCF solvent for food applications with no legislative restrictions governing its use. The only possible, but unlikely, physiological hazard involves asphyxiation by displacement of air following a considerable leak in a confined area. The combined effects of high hydrostatic pressure and low acidity in water-containing systems can be beneficially employed to prevent food spoilage by destroying bacteria (Kamihira et al., 1987; Taniguchi, 1987a). Rapid decompression of dissolved gas is sometimes used to expand and disrupt the cell structure of natural materials and could also be used as a means of sterilisation. Although SCF C02 can be an effective apolar medium for enzyme reactions (van Eijs et al., 1988; Steytler et al., 1991), it has also been used to selectively inactivate enzymes (Taniguchi, 1987b; Weber, 1980). In practice these techniques could be applied either in situ, during an extraction process, or as a separate unit operation. 2.3 PROPERTIES OF NCF SOLUTIONS 2.3.1 Solubilities in NCFs There has been much confusion in some of the literature concerning the solvent properties of NCF C02. An impression is often given that NCFs are universal solvents which can be ‘tuned’ to extract virtually any component of a mixture by selecting a suitable set of conditions of temperature and pressure. Statements to the effect that NCFs are ‘good’ solvents, implying that solute loadings are high, are also prevalent and highly misleading. Before examining the solvent properties of NCFs in detail, it is worth stating a few basic principles: (1) To be ‘supercritical’ intermolecular attractive interactions must be relatively weak compared with thermal energy. This necessitates an absence of all polar interactions, such as hydrogen bonding, and defines a medium of low dielectric constant. All NCFs are therefore essentially apolar solvents. The absence of strong attractive interactions between molecules means that solvation energies are generally low and solubilities in NCFs are thus often much lower than in liquid solvents. (2)
Supercritical fluid extraction 25 (3) NCFs can be highly discriminating and frequently offer a greater selectivity than liquid solvents. Any attempt at increasing solubilities by changing conditions or injecting entrainers(see Section 2.3. 2) usually serves to reduce selectivity Although some selectivity is sacrificed it is often preferable to operate at high pressures (and temperatures) to obtain sufficient solubility to make a process viable. The conditions cited for a specific process are often arrived at from an optimisation of these opposing effects of selectivity and solubility. However, the electivity that is exploited in extraction processes is sometimes an intrinsic roperty of the NCF solvent and is not always dramatically changed by the onditions (e.g. in the selective extraction of triglycerides from phospholipids General princip Effect of molecular structure Any pragmatic assessment of a solvent extraction process must examine what type of molecules are soluble and to what extent. With nCFs the molecular structure of the solute is of major importance as small changes in molecular weight and functional groups can affect solubility to a greater extent than with liquid solvents. In fact the viability of many simple separation processes using NCFs can be realised without recourse to extensive solubility data covering a wide range of conditions. Francis (1954)has pains takingly measured the solubilities of 261 substances in liquid CO2, and this pioneering work still acts as a useful guide to the relative solubilities of different classes of com- pounds in NCF CO. Dandge et al. (1985) have used this and other data to correlate the solubilities of different classes of chemical compounds with molecular structure. Some of the broad principles emerging from this work are given below (1)Solubility is reduced by increasing polarity. A good illustration of this is to be found in the relative solubilities of ethanol and ethylene glycol in liquid CO2. Whereas the former is completely miscible(M), increasing the overall polarity by introducing a second hydroxyl group reduces the solubility to 0. 2%. Miscibility with liquid CO2 can recovered by methy lation of the Oh groups, which reduces the polarity of the molecule ethanol dimethyl ether (02%) (2) Solubility declines with increasing molecular weight and for any homologous series the solubility decreases rapidly beyond a given carbon number. This effect is illustrated in Fig. 2.5, which also serves to demonstrate the effect of polarity on solubility since the more polar alcohol has a lower carbon number'cut off than the parent alkane
Supercritical fluid extraction 25 NCFs can be highly discriminating and frequently offer a greater selectivity than liquid solvents. Any attempt at increasing solubilities by changing conditions or injecting entrainers (see Section 2.3.2) usually serves to reduce selectivity. Although some selectivity is sacrificed it is often preferable to operate at high pressures (and temperatures) to obtain sufficient solubility to make a process viable. The conditions cited for a specific process are often arrived at from an optimisation of these opposing effects of selectivity and solubility. However, the selectivity that is exploited in extraction processes is sometimes an intrinsic property of the NCF solvent and is not always dramatically changed by the conditions (e.g. in the selective extraction of triglycerides from phospholipids; Section 2.6.2). (3) General principles Effect of molecular structure Any pragmatic assessment of a solvent extraction process must examine what type of molecules are soluble and to what extent. With NCFs the molecular structure of the solute is of major importance as small changes in molecular weight and functional groups can affect solubility to a greater extent than with liquid solvents. In fact the viability of many simple separation processes using NCFs can be realised without recourse to extensive solubility data covering a wide range of conditions. Francis (1954) has painstakingly measured the solubilities of 261 substances in liquid C02, and this pioneering work still acts as a useful guide to the relative solubilities of different classes of compounds in NCF C02. Dandge et al. (1985) have used this and other data to correlate the solubilities of different classes of chemical compounds with molecular structure. Some of the broad principles emerging from this work are given below: (1) Solubility is reduced by increasing polarity. A good illustration of this is to be found in the relative solubilities of ethanol and ethylene glycol in liquid C02. Whereas the former is completely miscible (M), increasing the overall polarity by introducing a second hydroxyl group reduces the solubility to 0.2%. Miscibility with liquid C02 can be recovered by methylation of the OH groups, which reduces the polarity of the molecule. HO 4 Ho # OH CH30 - OCH3 ethylene glycol ethylene glycol dimethyl ether ethanol (M) (0.2%) (M) (2) Solubility declines with increasing molecular weight and for any homologous series the solubility decreases rapidly beyond a given carbon number. This effect is illustrated in Fig. 2.5, which also serves to demonstrate the effect of polarity on solubility since the more polar alcohol has a lower carbon number 'cut off' than the parent alkane
C,H2n+1OH Carbon number(c) Fig. 2.5. Effect of carbon number (Cn)on the solubility of n-alkanes and alcohols in liquid CO2. 3)Branching increases solubility, Thus 2, 6, 10, 14 tetramethyl pentadecane is miscible with liquid CO2, whereas n-nonadecane is less than 3%( soluble) 2,6,1 c. (4)Unsaturation increases solubility as illustrated by I-octadecene, which is about ree times more soluble than its saturated homologue, n-octadecane 1-Octadecane (10%)
20 15 - c - 3 .- b 10 c% .- - D = - 5 0 - I I I - \ \ \ I \ CnH2nt2 Il,l, \, 20 - I I \ \ CnH2n+10H 0 5 10 15 h
