Supercritical Fluid Extraction. Ali Ahmadpour Chemical Eng. Dept. Ferdowsi University of Mashhad

Transcription

1 Supercritical Fluid Extraction Ali Ahmadpour Chemical Eng. Dept. Ferdowsi University of Mashhad

2 Contents Introduction SCF state (physical & chemical properties) Properties of NCF solutions (solubilities, EOSs, diffusivities) NCF efficiencies (extraction & separation stages) Equipment & techniques Applications 2

3 References Supercritical Fluid Processing of Food & Biomaterials, S.S.H. Rizvi, Supercritical Fluid Extraction, M. McHugh, Supercritical Fluid Extraction, L.T. Taylor, Natural extract using supercritical carbon dioxide, M. Mukhopadhyay,

4 Introduction First experimental work: 1879 by Hannay & Hogarth Commercial application: 1960 Advantage: High selectivity & solubility by changing P & T 4

5 What is Supercritical Fluid Extraction? Supercritical Fluid Extraction is the process of extracting components such as oils and resins from solids utilizing the special properties of CO 2, or other solvents. When these solvents are cooled and compressed, they have the density of a liquid and the dispersion properties of a gas. This is then able to penetrate the product and dissolve the oils or other components wanted. When the pressure is reduced, the solvent will then either evaporate harmlessly into the atmosphere, or be recovered and recompressed. The end results leaves only the concentrated extract. 5

6 SCF process 6

7 SCF extraction characteristics SCF extraction may be done on solids or liquids. The solvent is a gas at conditions of T and P at which the gas will not condense into a liquid phase. The gas has a density almost that of a liquid,but it is not a liquid. The solubility of solutes in a SCF approaches the solubility in a liquid. The low viscosity and near zero surface tension of the gas at the supercritical conditions are unique properties, which are superior to liquid solvents as an extracting agents. 7

8 Cont. Among different gases, CO 2 is selected as a safe SCF solvent for food industry because of: Non-toxic Non-flammable Low critical temp. Low critical pressure Low cost Good solubility 8

9 The Supercritical State A pure component is considered to be in a supercritical state if its temperature and pressure are higher than the critical values (T c and P c, respectively). At critical conditions for P and T, there is no sudden change of component properties. 9

10 The phase diagram of a single substance 10

11 P-T phase diagram of CO 2 11

12 Phase diagram of SCF CO 2 12

13 Phase diagram of a pure material and thermodynamic state of various separation processes 13

14 Disappearance of meniscus at the critical point 14

15 Phase transition of CO 2 to SCF Separate phases of carbon dioxide. The meniscus is easily observed. With an increase in temperature, the meniscus begins to diminish. 15

16 Cont. Increasing the temperature causes the gas and liquid densities to become more similar. The meniscus is less easily observed but still evident. Once the critical temperature and pressure have been reached the two distinct phases of liquid and gas are no longer visible. One homogenous phase called the "supercritical fluid" phase occurs which shows properties of both liquids and gases. 16

17 Near critical fluid (NCF) The term near -critical liquid (NCL) used to distinguish the state of a compressed gas just blow Tc from a normal liquid at NTP, for which T<Tc. The term near critical fluid (NCF) will be used to represent both SCF and NCL state of compressed-gas solvents. 17

18 Physico-chemical properties of SCFs Above the critical temp., the pure gaseous component cannot be liquefied regardless of the pressure applied. In the supercritical environment only one phase exists. The fluid, is neither a gas nor a liquid and is best described as intermediate to the two extremes. This phase retains solvent power approximating liquids as well as the transport properties common to gases. A comparison of typical values for density, viscosity and diffusivity of gases, liquids, and SCFs is presented in Table 1. 18

19 Table 1. Comparison of physical and transport properties of gases, liquids, and SCFs. Property Density (kg/m 3 ) Viscosity (cp) Diffusivity (mm 2 /s) Gas SCF Liquid

20 Advantages & Disadvantages of SFE Advantages Dissolving power of the SCF is controlled by pressure and/or temperature SCF is easily recoverable from the extract due to its volatility Non-toxic solvents leave no harmful residue High boiling components are extracted at relatively low temperatures Separations not possible by more traditional processes can sometimes be effected Thermally labile compounds can be extracted with minimal damage as low temperatures can be employed by the extraction Disadvantages Elevated pressure required Compression of solvent requires elaborate recycling measures to reduce energy costs High capital investment for equipment 20

21 Molecular Basis of SFE 21

22 SCF Process (fractionation system( 22

23 NCF CO 2 plant 23

24 Solvents of supercritical fluid extraction The choice of the SFE solvent is similar to the regular extraction. Principle considerations are the followings: Good solving property Inert to the product Easy separation from the product Cheap Low P C because of economic reasons 24

25 Comparison of physical properties of air, water, mercury (at 298K, 1bar) and SCF CO 2 Density Viscosity Kinematics viscosity 25

26 Useful SCFs with critical parameters Fluid T C (K) P C (bar) Carbon dioxide Ethane Ethylene Propane Propylene Trifluoromethane Chlorotrifluoromethane Trichlorofluoromethane Ammonia Water Cyclo-hexane n-pentane Toluene

27 SCF solvents Organic solvents are usually explosive so a SFE unit working with them should be explosion proof and this fact makes the investment more expensive. The organic solvents are mainly used in petrolchemistry. CFC-s are very good solvents in SFE due to their high density, but the industrial use of chloro-fluoro hydrocarbons are restricted because of their effect on the ozonosphere. CO 2 is the most widely used fluid in SFE. Beside CO 2, water is the other increasingly applied solvent. One of the unique properties of water is that, above its critical point (374 C, 218 atm), it becomes an excellent solvent for organic compounds and a very poor solvent for inorganic salts. This property gives the chance for using the same solvent to extract the inorganic and the organic component respectively. 27

28 Physical properties of NCF CO 2 Density: In NCL phase, CO 2 densities are typical of normal liquid solvent ( kg/m 3 ). The SCF state of CO 2 includes a wide range of densities from gaslike values at low P (<100kg/m 3 ( to liquid-like values at elevated pressure. The region near the critical point has the highest compressibility. The solubility is directly related to the number of solvent molecules per unit volume. Therefore, density is the key parameter in determining the effect of T & P on solubilities. Above the critical point, solubilities have steep rise with P at constant T. Therefore, ability of controlling solubilities with P is one of the main features that distinguish NCFs from liquid solvents. 28

29 Cont. Viscosity: NCFs have high degree of molecular mobility (low viscosity) and higher diffusivity than liquid solvents. To reach reasonable solubility, the density of NCF must be modest (>400kg/m 3 ). NCFs-CO 2 usually have low viscosities (600μP) when the density is around 770kg/m 3. Low viscosity of NCF provide several benefits in extraction processes: In leaching, it enables effective percolation of solvent through packed bed and rapid penetration to the internal pore structure of particles. In extracting liquid, the NCF solvent dissolve in the liquid phase and lower its viscosity. With high viscous liquid, part of mass transfer problem could be solved. It facilitates solvent transfer and reduces pipeline dimensions in extraction plants. 29

30 Cont. Diffusion: In low viscosity media, diffusion is enhanced and diffusion coefficients in NCFs are significantly higher than in liquid solvents (about 10 times). At constant density, the diffusion coefficient is not greatly affected by T or P. 30

31 Cont. Volatility (vapor pressure): NCFs are highly volatile and can be completely removed and recycled at low T. This has important implications for improving the quality of extracts, since: Highly volatile components in the extract are retained (flavors and fragrances) The extract is not subjected to thermal or chemical degradation at high T. High volatility ensures complete removal of solvent residues. 31

32 Chemical properties of NCF CO 2 Although, CO 2 is the safest medium in extraction, it undergo chemical reactions with water and need to be considered when extracting food materials. One reaction is the dissolution of CO 2 that gives carbonic acid. The acid is then dissociates and lowers the ph of the aqueous phase. If acidity is problematic, it is possible to add bicarbonate anion. Also, the pressure of CO 2 can be used to control the ph of water. Another reaction of CO 2 with water is the formation of solid hydrate below about 10 C. This restricts the use of NCF CO 2 in the extraction of aqueous systems to temps. Up to 10 C higher than the freezing point of water. 32

33 Properties of NCF solutions Solubilities: Intermolecular attractive interactions must be weak. Therefore, all NCFs are essentially non-polar solvents. NCFs offer greater selectivity than liquid solvents. Any attempt at increasing solubilities by changing conditions or adding entrainers usually reduce selectivity. Therefore, opposing effects of selectivity and solubility should be optimized. 33

34 Cont. General principles about solubilities: Effect of molecular structure Effect of temp. and pressure Effect of entrainers 34

35 Cont.. Effect of molecular structure In NCFs, the molecular structure of the solute is very important as small changes in MW and functional groups can affect solubility to a greater extent than with liquid solvents. Solubility is reduced by increasing polarity. Branching increases solubility. Unsaturation increases solubility. Aromaticity decreases solubility. 35

36 Cont.. Effect of temp. and pressure For liquid solvents, the pressure has very little effect on solubility. In NCFs, the effect of pressure is related to the solute-solvent interactions which depends on solvent density. At very high pressures, the solubilities decrease with increasing pressure. The solubility increases with increasing temp. at constant density. 36

37 Cont.. Effect of entrainers A liquid cosolvent (or entrainer) is sometimes added to NCFs to improve solubility level of polar or high MW substances. Entrainers are liquid solvents (e.g. ethanol, acetone, ethyl acetate, ( that completely miscible with the NCF and added at low levels (<10%). Although, they improve solubility, they reduce selectivity and introduce further operations for their removal. 37

38 Properties of NCF solutions Diffusion coefficients: Diffusion coefficients for solutes in NCFs are significantly higher than in liquid solvents. Above the density of about 500 kg/m 3, the solute diffusion coefficients are of order 10-8 m 2 /s which is about an order of magnitude greater than that of liquid solvents. 38

39 Cont. EOSs for solubility prediction: Among all theoretical methods, the solution phase equilibrium using EOSs are most widely applied to predict the solubilities in NCFs. The most familiar EOS is van der Waals. One limiting factor that restricts application of EOS models for food materials is the lack of available data for the fundamental properties of pure components. Another problem is the ambiguity of mixing rules and adjusted parameters in equations. 39

40 Relationship between (a) sublimation of a pure solid & (b) dissolution in an NCF Solvent: 1 Solute: 2 Pure solid: ' NCF: '' x ' = mole fraction of solid in solid phase y '' = mole fraction of solid in NCF 40

41 Cont. At equilibrium: T'=T''=T P'=P''=P f i '=f i '' (for all i) Assume that NCF doesn t dissolve in the solid: f 2 '=f 2 '' For ideal gas: f i = y i P In general: f i = Ф i y i P For pure solid phase (S) at T & P: f 2 ' (T,P)= Ф 2S P S 2 (T) At high pressure (poynting correction): S S F P V2 f 2 (T,P) 2P2 (T) exp dp S P 2 RT V 2 : molar volume of pure solid 2 41

42 Cont. Assume incompressible solid: f (T,P) 2 F S 2 P S 2 V 2[P P (T) exp RT (T)] Therefore, the fugacity of the pure solid phase at the system T and P can be obtained from sublimation pressure and molar volume data. S 2 For phase equilibrium f 2 '=f 2 '' F y 2 V 2[P P (T) exp RT (T)] F 2y P S S S 2 P P S 2 (T) F P F S 2 2 V 2[P P exp RT S 2 (T)] (1) 42

43 Cont. Since: Ф 2 S =1 (ideal) 1 P V RT ln F 2 dp RT 0 n 2 P T,P,n2 (2) Solution of eqn. (2) requires an EOS: P f (T,V,x ) (3) i Van der Waals: P RT a 2 V b V 43

44 Efficiency of NCF extraction The efficiency of solvent extraction process is judged by: Rate of process operation Energy consumption The rate-limiting step is most frequently the rate of mass transfer at the extraction stage. This mass transfer resistance requires high solvent circulation rates and results in high recompression costs which may make the process unattractive. 44

45 Cont. The energy input is determined by: The conc. of solute in the NCF solvent leaving the extraction vessel (determines the No. of cycles to complete the extraction) The differential conditions of P & T between the extraction and separation vessels (determine the energy consumption per cycle) 45

46 Extraction stage In leaching, the rate of solute removal for any solvent depends on: The amount of solute in the particle Its distribution within the matrix The particle size and shape The geometry of the porous network The solubility is a prime factor for determination of solvent effectiveness in an extraction process. When solubility is low, it can also determine the extraction mechanism. 46

47 Extraction mechanism There are two different mechanisms in extraction: The free diffusion model The shrinking core model In SCF processes, the solubilities are often low and shrinking core model exist. In liquid solvent extraction, the common free diffusion happens. 47

48 Extraction of solute from a slab Shrinking core model Free diffusion model 48

49 Free diffusion model (Carmen-Haul) It happens when: S>>C It is usually expressed as: S>>C/φ The fractional extraction of solute from a infinite slab of halfthickness (L) is : M M t 1 n 1 S: solubility C: concentration of solute in the matrix (solute/pore volume) φ: fraction of pore space (porosity) M t & M : mass of solute extracted at time a: volume ratio of solvent to slab q n : positive non-zero roots of tanq n = -a q n 2a 1 a 2 1 a a q 2 n exp Deff q 2 L 2 n t D eff D b : bulk diffusivity τ: tortuosity D t b 49

50 Shrinking core model It happens when: S<< C/φ The fractional extraction of solute from a infinite slab of halfthickness (L) is: M M t 2DSt L 2 C 50

51 Parameters in the shrinking core model Solubility: The most important feature of the model is that the rate of extraction is determined not only by diffusion coefficient but also by the solubility of the solute. When the solubility is low, rate of extraction can be very low. Therefore, it is good for the controlled drug delivery. Diffusivity: Usually higher diffusivity in SCF gives rapid extraction. This is true when the free diffusion model applies. Low solubility in SCFs often switch the mechanism to shrinking core in which the enhanced diffusion is offset by the solubility. 51

52 Cont. Adsorption: The extent of extraction is determined by adsorption energy and solvation energy. In leaching. The adsorption/desorption of solute can be accounted by a linear isotherm. Here, the diffusion coefficient is replaced by the modified one inversely related to the adsorption coefficient. D eff K D 1 ads ; K ads Conc. Conc. of of adsorbed solute freely diffusing solute When K ads >1 Low extraction rate 52

53 Cont. In fixed-bed extraction, in the absence of adsorption, maximum conc. of solute in the solvent (C max ) is given by the solubility. This determines the minimum amount of solvent required. C max S 1 The implications for extraction efficiency are twofold: K ads More solvent is required for complete extraction (more operation cycles). A greater pressure is required as the level of solute in the SCF is low. Both factors increase the energy consumption of the process. 53

54 Cont. Role of water: In the extraction of plant materials it is found that the addition of water is essential to achieve a good extraction rate. Water does not usually increase solubilities in SCF (different from entrainers). Water affects the rate through rehydrating and swelling the internal cellular structure of died plant. It has two opposing effects: Increasing the particle size will increase the diffusion distance. Expansion of the internal structure will shorten the diffusion path by opening channels. 54

55 Cont. In some cases, water play a crucial role in determining not only the rate but also the mechanism of the extraction. For example, dry tea contains 3% w/w caffeine. Its solubility in SCF CO 2 is low (S<<c/φ) Shrinking core model The solubility of caffeine in water is significantly higher than in CO 2 Free diffusion model 55

56 Separation stage Since the solubility of solutes in SCFs decline with decreasing pressure, the separation stage operates at low pressure High energy consumption One solution is to partition the solute to a coexisting solid (AC) or liquid phase in the separation vessel (at the same pressure as extractor). This is useful when the extract presents at low level (contaminants). Another new idea for separation stage is crystallization. Pressure variation has rapid response and can be used to control the crystal size. 56

57 Equipments and experimental techniques in NCF Extraction and Fractionation 57

58 Major problems in SCF extraction Two of the major problems of SCF extraction are: Channeling of fluid flow through the bed of solids Entrainment of the non-extractable component by the SCF. The contact time is related to the solubility of the solute in the SCF and the rate of flow of the fluid through the bed of solids. A large quantity of solute is extracted within a reasonable length of time. SCF penetration into the interior of a solid is rapid, but solute diffusion from the solid into the SCF may be slow and may contribute to the prolonged contact time needed for extraction. 58

59 Extraction SCF extraction is done in a single-stage contractor with or without recycling of the fluid. When recycling is used, the process involves pressure reduction in a separator in which the solid is separate by gravity and then the gas compress back to the supercritical conditions and recycled. Temperature reduction may also be used to drop the solute and the solvent is reheated for recycling without the need for recompression. 59

60 Pilot plants with recirculation Designed for testing application on a relatively small scale (<10kg). 60

61 Small pilot plant with total loss of CO 2 With ml scale. It is not necessary to recover and recirculate CO 2 61

62 Fractionation In order to fractionate mixtures it is more efficient (in terms of time) to employ equipment specifically designed for this purpose. In cascade configuration, the CO 2 stream passes to the first separation vessel where the condition P1,T1 are set to precipitate the first fraction of least soluble components. The output stream from the first separator is then passed to a second vessel at lower pressure P2 where the second fraction precipitates. 62

63 Cascades of separation vessels P1>P2>P3 Or T1<T2<T3 63

64 Zosel s hot finger fractionation column Increasing temp. results in a drop in solubility. 64

65 Industrial applications Supercritical extraction offers many advantages such as high purity, low residual solvent content, and environment protection. It has also some disadvantages such as high capital cost, high pressure and low solubilities. Several applications have been fully developed and commercialized. 65

66 SCF applications Extraction was the first commercial use, in the extraction of hops and the decaffeination of coffee. Several research papers have been produced on a wide range of natural products, including high value pharmaceutical precursors. Advantages: speed due to rapid diffusion, less pollution in the working and general environment, less solvent residues in products, less solvent disposal costs. Fractionation of liquid mixtures can be achieved by countercurrent extraction and this can be improved by imposing a temperature gradient on the column which causes refluxing to occur. It is largely applied to natural products such as essential oils and lipid products and can be used to concentrate substances prior to chromatography. Advantages: countercurrent extraction with reflux can be carried out in one unit, less pollution, no solvent residues in products, no solvent disposal costs. Chromatography can be applied to high value products and chiral separations. Efficient simulated bed units are available. Advantages: narrower peaks and more efficient separations due to rapid diffusion, no solvent residues in products, less solvent disposal costs. 66

67 Cont. Chemical Reactions are being researched with some in production. Advantages: product control and ease of product separation, more rapid reaction in diffusion-controlled, heterogeneous and enzyme reactions due to rapid diffusion, less pollution in the working and general environment, less solvent disposal costs. Metals Processing, including extraction and separation and clean-up, using complexing agents in the fluid. Advantages: speed due to rapid diffusion, efficient separation processes, less pollution in the working and general environment, less solvent disposal costs. Impregnation and Dyeing of polymers and synthetic fibres is established and the dyeing of cotton is being researched. Advantages: considerable reduction in water pollution from dyeing. Particle Formation in the micron range with a narrow size distribution can be carried out, with the option of coating particles. Advantages: less solvent residues in products, degradation by heating during milling avoided. 67

68 Applications in the food industries Decaffeination of coffee and tea Extraction of essential oils (vegetable and fish oils) Extraction of flavors and fragrances from natural resources Extraction of aroma and ingredients from spices and red peppers Extraction of fat from food products Extraction of vitamin additives Production of cholesterol-free egg powder De-fat potato chips 68

69 The main advantages 1) The extraction and separation can be carried out at low temperature in an inert environment thereby avoiding thermal damage and chemical degradation. 2) The extract has improved solubility in formulations. 3) The high vapour pressure of CO 2 enables it to be removed without losses. 4) Undesirable component are not extracted 69

70 Large Scale Systems Natural Products Rose oil residue Essential oil extraction Flavors and fragrances Nicotine extraction Natural pigment extraction Pharmaceuticals Synthetic drug production Separation of isomers Ethical drug purification Enzyme catalyzed reactions Residual solvent removal Drug micro particle crystallization Foods Hops extraction Decaffeination Cholesterol from butter Fatty acids from barley Seed oil extraction 70

71 Pictures of SCF extractors 71

72 Supercritical water Water has obvious attractions as a solvent for clean chemistry. Both nearcritical and supercritical water (SCH 2 O) have increased acidity, reduced density and lower polarity, greatly extending the possible range of chemistry which could be carried out in water. SCH 2 O can be applied most effectively for organic synthesis leading to useful products. As water is heated towards its critical point (T c =374 C, P c =218 atm.), it undergoes a transformation considerably more dramatic than that of most other substances. It changes from the polar liquid to an almost non-polar fluid. The change occurs over a relatively wide temperature range; even at 200 C, the density drops to 0.8 g/ml and, at Tc, the fluid becomes miscible both with organics and with gases. Diffusivity increases and the acidity is enhanced more than would be expected purely on the basis of higher temperatures. A major research effort has been focused on the total oxidation of toxic organics and hazardous wastes in SCH 2 O, "incineration without a smokestack". The process is highly effective but there can be serious problems of corrosion associated with large scale waste destruction. 72