(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