1 Application of Supercritical Fluids Review Yoshiaki Fukushima Abstract Many advantages of supercritical fluids come Supercritical water is expected to be useful in from their interesting or unusual properties which waste treatment. Although they show high liquid solvents and gas carriers do not possess. solubility solutes and molecular catalyses, solvent Such properties and possible applications of molecules under supercritical conditions gently supercritical fluids are reviewed. As these fluids solvate solute molecules and have little influence never condense at above their critical on the activities of the solutes and catalysts. This temperatures, supercritical drying is useful to property would be attributed to the local density prepare dry-gel. The solubility and other fluctuations around each molecule due to high important parameters as a solvent can be adjusted molecular mobility. The fluctuations in the continuously. Supercritical fluids show supercritical fluids would produce heterogeneity advantages as solvents for extraction, coating or that would provide novel chemical reactions with chemical reactions thanks to these properties. molecular catalyses, heterogenous solid catalyses, Supercritical water shows a high organic matter enzymes or solid adsorbents. solubility and a strong hydrolyzing ability. Supercritical fluid, Supercritical water, Solubility, Solvation, Waste treatment, Keywords Coating, Organic reaction applications development reached the initial peak 1. Introduction during the period from the second half of the 1960s There has been rising concern in recent years over to the 1970s followed by the secondary peak about supercritical fluids for organic waste treatment and 15 years later. The initial peak was for the other applications. The discovery of the presence of separation and extraction technique as represented 1) critical point dates back to 1822. The temperature by the extraction of caffeine from coffee beans. and pressure beyond the critical point are in the Since the practical application in Germany in 1978, 2) supercritical state. As indicated in Table 1, critical a number of practical plants have been in operation. points are often in high pressure. Therefore, a high- The secondary peak concerns decomposition of pressure test facility (high temperature and high organic (waste) substances, which started when the pressure depending on the case) is required for a U.S. Department of Defense began studies on using systematic study of fluids in the near- or supercritical water as a means for completely supercritical state. This might have been the main decomposing chemical weapons. And the tertiary reason why studies in this field were comparatively peak is about to come with mounting interest in the few until recently. possible development of new material program. Research on supercritical fluids and the R&D Review of Toyota CRDL Vol. 35 No. 1 2 the force to make free thermal movement. At high 2. What are supercritical fluids? temperature, however, the force of particles to move Supercritical fluids are described as “fluids in the about freely exceeds that of the inter-molecular force temperature and pressure state of over the critical if the inter-molecular distance is minimized under point” as shown in Fig. 1. This definition, however, pressure. The supercritical state is thus reached is not sufficient and not providing a good where no condensed phase is produced. explanation for understanding them. When the pressure rises, the gas density becomes For persons handling them, they are not different higher, however. The inter-molecular distance then from “high-pressure gas.” They are not becomes shorter, making it impossible to disregard distinguishable from gas because they are in a non- the effects of inter-molecular action. In other words, condensed state, which disables handling without when viewed from the molecules inside or from the using a sealed container. In other words, they may microscopic standpoint, they are in a state close to be considered to be gas that does not transform to liquid. If the molecular movement can be directly the liquid phase with discontinuous change in the observed, it is assumed to be as violent as in the density when the pressure rises under a constant gaseous state. When observed in a still photograph, temperature. Substances change their phase it may resemble that of the liquid. This is the reason depending on the balance of forces between the why “supercritical is the intermediate state between force to form the liquid or solid phase by gas and liquid.” condensation under inter-molecular attraction and 3. Characteristics of supercritical fluids Table 1 Critical points of typical solvents.2) “Supercritical fluids in the intermediate state between gas and liquid have high diffusivity Critical Critical Critical comparable with a gas and high solubility as a Temperature Pressure Density Compounds (°C) (atm) (g/ml) liquid.” This is the popular description of supercritical fluids. This expression may mislead us Carbon Dioxide 31.3 72.9 0.448 to expect that supercritical fluids are wonderful Ammonia 132.4 112.5 0.235 Water 374.15 218.3 0.315 Critical Temperature Nitrous Oxide 36.5 71.7 0.45 31.06˚C Xenon 16.6 57.6 0.118 AAAAAAA AAAAAAAAAAAAAA Krypton -63.8 54.3 0.091 AAAAAAAAAAAAAA Methane -82.1 45.8 0.2 35 AAAAAAAAAAAAAA line Melting AAAAAAAAAAAAAA Ethane 32.28 48.1 0.203 30 AAAAAAAAAAAAAA AAAAAAAAAAAAAA Ethylene 9.21 49.7 0.218 25 AAAAAAAAAAAAASupercriticalA Propane 96.67 41.9 0.217 AAAAAAAAAAAAAFluid A 20 AAAAAAAAAAAAAA Pentane 196.6 33.3 0.232 AAAAAAAAAAAAAA Solid Methanol 240.5 78.9 0.272 15 AAAAAAAAAAAAAA Pressure/MPa AAAAAAAAAAAAAA Liquid Ethanol 243.0 63.0 0.276 10 AAAAAAAAAAAAAA AAAAAAA Critical Pressure Isopropanol 235.3 47.0 0.273 7.3MPa 5 Critical Isobutanol 275.0 42.4 0.272 Boiling lineGas Point 0 Chlorotrifluoromethane 28.0 38.7 0.579 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature/˚C Sublimation Monofluoromethane 44.6 58.0 0.3 Cycrohexanol 356.0 38.0 0.273 Fig. 1 Phase (Pressure-Temperature) diagram for CO2. R&D Review of Toyota CRDL Vol. 35 No. 1 3 solvents that make two mutually opposed properties be easily removed after completing the reaction by compatible. As shown in Fig. 2, solubility rises as returning it to the atmospheric pressure. the density becomes higher under pressure, In addition to these general characteristics, in the simultaneously delaying diffusion, however. In case of supercritical water, properties specific to the addition, it is a fact that solubility is lower than water expand the sphere of possible application, liquid in a simple system, while the diffusion is which will be described in detail in Section 5. slower than gas. 4. Application as extractant Therefore, general descriptions of characteristics of supercritical fluids are limited. Their 4. 1 Basic principle characteristics in use differ by each field of Solubility of supercritical fluids to various solutes application to be stated later. They may be listed as is not necessarily high compared with liquid. The follows: characteristics of using the supercritical fluids for (1) Generate no liquid (condensed phase) under extractants are shown in Fig. 2, and their solubility pressure change or on solid surfaces. can be continually changed to a large degree. (2) Able to control density (solubility), polarity, Moreover, since they do not form condensed phase, viscosity and other properties of the fluids they can infiltrate into very fine gaps of solids under continuously and over a wide range. high pressure. These properties enable them to solve (3) Can change the property sharply with very a target substance efficiently from solids to be little change in pressure as shown in Fig. 3, deposited outside. particularly in the vicinity of the critical temperature Large change in the solubility is the main reason and pressure.3) This suggests that largely different why supercritical fluids are used for extractants. To state of property can be simultaneously realized near realize efficient extraction, increase of solubility the critical point with the addition of solutes, itself is desirous, too. One of the effective means to disturbances on the solid surfaces and fluctuations of do this is to add a component for increasing the the fluids. solubility. This component is called an entrainer or (4) Molecular association by inter-molecular modifier. Generally, several percent of a component actions or the formation of solvation structure of that reacts strongly with the solute and which is solute molecules is local and short-lived, hence not soluble to the basic solvent is added. Such an inhibiting reactions. additive not only increases solubility, it also (5) Gaseous substance under ordinary temperature improves the separation characteristics by and atmospheric pressure such as carbon dioxide can controlling the solubility of the solvent. A lot of the Fig. 2 Diffusivity CO2 and solubility of naphtalene in 3) CO2 at 40˚C. Fig. 3 Pressure-Density curves for water. R&D Review of Toyota CRDL Vol. 35 No. 1 4 research and development done on separation and absorbents and other separation method.5) A method extraction concern this entrainer (additive). Still of setting the extraction condition on multiple stages finer separation will become possible by combining was also added.12) 5) the adsorptive phenomenon. Around 1990, there were concentrated patent 4. 2 Applications examples applications concerning the removal of organic Measurement of solubility of substance to the components, the molding binders, from ceramic supercritical fluids was conducted about 50 years injection moldings. These applications searched for after the discovery of the above-mentioned extract conditions13) without deforming the moldings supercritical state. It was recognized that they have at a temperature below the softening point of the dissolving power close to that of the liquid. resin. Many of the applications examined extraction Research has been fully conducted since the under high pressure of about 35 MPa at which the extraction of caffeine from coffee beans was density of carbon dioxide, which is the dissolving 6) patented in 1964.