Application of supercritical fluids

Application of Supercritical Fluids
Abstract
Many advantages of supercritical fluids come Supercritical water is expected to be useful in from their interesting or unusual properties which 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 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 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 solubility and a strong hydrolyzing ability.
Supercritical fluid, Supercritical water, Solubility, Solvation, Waste treatment, 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 critical point dates back to 1822.1) 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, supercritical state. As indicated in Table 1,2) 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 possible development of new material program.
R&D Review of Toyota CRDL Vol. 35 No. 1 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.
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 condensation under inter-molecular attraction and 3. Characteristics of supercritical fluids
“Supercritical fluids in the intermediate state between gas and liquid have high diffusivity comparable with a gas and high solubility as a liquid.” This is the popular description of supercritical fluids. This expression may mislead us to expect that supercritical fluids are wonderful -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Phase (Pressure-Temperature) diagram for CO2.
R&D Review of Toyota CRDL Vol. 35 No. 1 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.
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 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 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,
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 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 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 Diffusivity CO2 and solubility of naphtalene inCO2 at 40˚C.
R&D Review of Toyota CRDL Vol. 35 No. 1 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 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 patented in 1964.6) Since the practical application in power, becomes saturated, while the temperature Germany in 1978, commercial plants for extracting was near the comparatively low critical temperature.
hop extract, perfume, tobacco nicotine, spices or However, as stated earlier, since the dissolving power of supercritical fluids is not as strong as that Operation of those plants requires high pressure of organic solvent liquids, they are not suitable for (130°C, 35 MPa for processing coffee beans or removing binders having high molecular weight but extracting perfume), thus limiting the application to for removing paraffin oil and other substance of low foods and other high-value added products. A trial conducted circa 1990 indicated that a perfume In recent years, the number of cases directly extraction plant7) with an annual production of about associated with the composition of material15) with 800 tons required an equipment cost of some $3 the aim of improving the characteristics by removing million and the processing cost of $1.10 (personnel the component of low molecular weight from the cost of $0.43 included) per kilogram while a organic polymer has increased remarkably. Also decaffein plant for coffee beans with an annual there are a number of applications reflecting the production of about 200,000 tons8) required an rising concern of environmental problems. Reports equipment cost of $34.5 million and a processing have been made on the examination of hazardous cost of $0.35 (personnel cost of $0.19 included) per substances (organic chlorine, organic mercury, PCB, kilogram. Reduction of production cost through insecticide, herbicide, etc.) from soil,16) waste17) or continuous processing, etc. is yet to be developed.
animal or plant tissues,18) and their removal As thus far stated, research and development conditions. In most cases, these cases provide the concerning separation and extraction have basic extraction data using carbon dioxide as the continued. In Japan, application was made for about base solvent and alcohol and/or water as the 80 patents during the 25 years from 1970 to 1995.
Most of the applications, however, state the novelty Separation and extraction remain the central field of the subject substance with only few stating of application for supercritical fluids, where the development beyond the content described in the basic data are most richly accumulated. Application basic principle stated in the preceding subsection.
as a cleaning solvent in semiconductor manu- These applications related to foods are characterized facturing19) is as an extension of this technology.
by the removal of grease and oil9) from grain, cholesterol10) from eggs, residual agricultural 5. Decomposition of organic using supercritical
chemicals11) and other unnecessary or hazardous substance from foods. Also separation and 5. 1 Specificity of supercritical water
extraction of neutral fat and phospholipid other fine Water is the most commonly used solvent. It is a separation12) were attempted. In these separations, a most specific fluid having interesting properties.
combination was made with the above-mentioned Because of its high critical points for temperature R&D Review of Toyota CRDL Vol. 35 No. 1 and pressure, attention has recently been given to its that water under critical temperature dissolves inorganics easily and dissolves organics when over The outline of the characteristics of water as a the critical temperature where inorganic salt is reactive solvent can be understood with an ionic product and a dielectric constant. The ionic product Since supercritical water requires high temperature becomes the measure for interaction with hydrolysis and pressure, commercial plants could not and the dielectric constant with solutes.20) As Fig. 4
immediately be constructed. Nippon Shokubai shows, water retains a comparatively high ionic helped to ease this condition by using a catalyst.
product under pressure.3) On the other hand, its This company carried on systematic studies on the dielectric constant decreases remarkably near the matter and applied for about 30 patents during the critical point3) as shown in Fig. 5. That is,
period from 1989 to 1995. They mainly examined supercritical water is a non-polar solvent of low catalysts and reported that titanium and zircon dielectric constant. It has the capability to dissolve oxides,22) gold, palladium and other precious organic compounds, simultaneously retaining active metals23) or iron or copper ion additives24) were state of high hydrolysis performance.
effective. On the other hand, Osaka Gas applied for 5. 2 Application to decomposition of organic
an almost identical number of patents in their compounds
research for possibilities under critical temperature.
Patents applications for using water in the They searched for a method of supplementing subcritical (under critical temperature) to super- incomplete decomposition reaction with precision critical state as a solvent increased in 1989 and rose filtration,25) biological and other processings.26) In recent years, organic decomposition action Moder, a venture company related to the chemical using supercritical water itself that contains no weapon decomposition project of the U.S.
oxidizer has attracted attention. Nishikawa Rubber Departments of Energy and Defense applied for and Company and Kochi University conducted joint received a basic patent around 1989.21) Here, efficient oxidative degradation is realized using the devulcanization and oil development of vulcanized properties of the oxidizing agent (oxygen), which is rubber. They found that subcritical water with soluble to the critical water with ease, in addition to sulfuric absorbents added is effective for the above-mentioned properties of supercritical devulcanization and oil development27) and that water. Furthermore, the company made it possible to separate inorganics by adroitly using the property R&D Review of Toyota CRDL Vol. 35 No. 1 supercritical water added with zinc oxide28) or 6. 2 Particulate manufacturing and coating
hydrochloric acid29) is effective in further promoting technology - Use of extraction and
devulcanization and oil development. In addition, crystallization growth by changing
Japan is the center of positive basic studies on the solubility
reactions attributable to the solubility and high ion It is possible to change the solubility of product of supercritical water particularly from the supercritical fluids sharply and continuously with viewpoint of reaction rate control. Using the pressure (density) of additives. Solubility drops supercritical water realizes a cellulose decomposition sharply when the high pressure in a container in rate of 102 to 104 compared with conventional acid which supercritical fluids are stored is discharged catalyst-based decomposition.30) Based on the results rapidly. During the process, solute is extracted and of these basic studies, research has begun on the inorganic or organic particulates are grown. This decomposition of biomass31) and polymers32) or method is called Rapid Expansion of Supercritical improvement of coal or heavy oil quality.33) Solution (RESS) by which growth of silica, alumina, Development of devices for downsizing the reaction zirconium and other inorganic oxides36) and section by a using fast reaction rate is expected medicines, polymers and other organic substances37) along with operating cost reduction.
are verified. Particulates grown here are often non- crystal, but when made to react under supercritical 6. Recent studies
water of high temperature, this method can form iron 6. 1 Manufacture of inorganic porous bodies
oxides, zirconia, titania (anatase), ceria and other and use of supercritical drying
crystal particulates.38) Moreover, a single spraying Supercritical drying is known as a method for enables a microcapsule with the effect of the applying supercritical fluids. This method removes chemical content shielded with poly-lactic acid, solvent by using the characteristics that make it possible to change the density continuously from a The spraying mechanism based on rapid expansion density close to liquid to one close to gas without resembles coating equipment. The above-mentioned phase change. This method was used for preparing composition, however, sprays only particulates and samples for observation with an electron microscope it is not possible to expect a uniform adhesion of the from a biosubstance with a high water content. It is particulates on the substrate. Union Carbide of the possible to prevent contraction caused by the solid U.S. began research on a method to substitute the particles that condense when the water or other coating solvent and the compressed air in the air solvent dries by vaporization. Matsushita Electric spray with supercritical carbon dioxide from the mid Works applied this property for manufacturing 1980s. The company still continues its research and inorganic oxide porous bodies.34) In water, alcohol applied for a number of patents concerning the basic and other solvents, clay mineral and inorganic principle,40) paints and the solvent feeding method particulates synthesized with sol gel reaction to form during that period. To stably solve organic a suspension or sol where these particulates are polymers, which are the basic components of paint, highly dispersed. Supercritical drying is effective and realize about the same finish as ordinary for drying the suspension or sol while retaining the coatings, the company adds ordinary organic suspended state as much as possible. By using this solvents as well. It closely examines the process of drying method, an inorganic porous solid is formed.
solution and extraction under the ternary system.41) Spaces where the solvent molecules existed remain Ideally, the polymer is solved in mixed supercritical as voids. They are useful as heat insulators, fluids of organic solvent and carbon dioxide in a adsorbents or catalyst supports. In particular, a high-pressure container as Fig. 6E shows.
combination of sol gel reaction and supercritical Immediately after the spraying, the carbon dioxide drying enables transparent inorganic porous solids first vaporizes, leaving the organic solvent and (aerogel)35) having density of under 0.01g/cc to be polymer behind. After attaching itself to the base material, the organic solvent vaporizes, forming the R&D Review of Toyota CRDL Vol. 35 No. 1 polymer film just as ordinary paints do. The controlled with ease for precision reaction including principle may seem simple, but there are a number stereochemistry. However, the catalyst and reaction of problems, which include the sharp drop of the molecules in the solution are often inactive as they temperature during expansion, retention of viscosity are always surrounded by a number of solvent suitable to the coating and optimal composition, molecules. Strong solvation to such solvents and processing of pigments and other additives, the reaction molecules often has a negative effect on the spraying of high-pressure fluids of over 10 MPa, and acceleration of reaction. The solvent least affects so forth. Beside these, there is a long way to go reaction in the gas phase when a solid catalyst is before practical application can be achieved, used. In this case, reaction is rapid because the including reduction of hazardous organic solvents, catalyst and reaction molecules are in naked state.
development of devices and operating costs. The Control of reaction is difficult because of the technology is quite interesting and important.
heterogeneity of the catalytic structure, however.
6. 3 Characteristics as synthesized organic
Furthermore, molecular catalysts cannot be applied solvents - Specific intermolecular action
In the synthesis of substances, solvents are used to The ideal catalytic reaction is a high-speed accelerate or control the reaction. In this case, the selective reaction provided with priority given to solvent is first required to be able to solve the both reactions in the gas and liquid phases. To reactive material, catalyst or other additives. The realize this, it is necessary to have a reaction system characteristics when supercritical fluids are used for to make molecular catalyst soluble and suppress the reaction solvent are best described by Ikariya deactivation caused by the catalyst to a minimum.
This is the reason why we are paying attention to “To effect molecular catalytic reaction in a supercritical fluids having intermediate properties solution, solvents are used to form a uniform system with catalyst and reaction molecules for, and Based on this point, they succeeded in accelerating effective mass transfer. To do this, molecular the reaction by using supercritical carbon dioxide as catalytic reaction in the liquid phase can be the solvent in a reaction to form formic acid useful as a chemical material from carbon dioxide and hydrogen by using Ru-complex as the catalyst.43) Moreover, it is found that reaction is further accelerated by adding dimethylamine that is soluble in supercritical carbon dioxide. This extracts formic acid as amine salt insoluble in supercritical carbon dioxide. “The intermediate property between liquid and gas” mentioned here, that is, the “property that makes molecules soluble but not inactive” is attributable to the characteristics (4) of the aforementioned supercritical fluids. Section 2 stated
that supercritical fluids can be handled as “gas of high density.” From the viewpoint of the reaction solvent, however, it is more understandable if we think of it as liquid of intensive molecular motion.
Enzyme reaction is normally effected in an aqueous solution. Here too, supercritical fluids are being used as an interesting solvent for the very same reasons as for molecular catalysts.44) In this case, it is necessary to give consideration to the Fundamentals of coating with supercritical fluids inactivation of enzymes and other restrictions caused R&D Review of Toyota CRDL Vol. 35 No. 1 by reaction between enzyme protein and carbon ; Inada, S. : Examined Patent Pub. H07-16373 dioxide. The characteristics of supercritical fluids are utilized for accelerating high reaction, making ; Yoshida, O., et al. : Examined Patent Pub. water-insoluble molecules react, forming organic polymers of uniform molecular weight, etc.
10) Hasegawa, M., et al. : Unexamined Patent Pub. 7. Conclusion
11) King, J. W., et al. : J. Chromatogr. Sci., 31-1 (1993), 1
Supercritical fluids have a number of interesting ; Ahronson, N., Lehotey, S. J. and Ibrahim, M. A. : and useful characteristics. Above all, the viewpoints J. Agric. Food Chem., 42-12(1994), 2817
of Noyori et al. in the preceding section are 12) Montanari, L., et al. : J. Food Sci., 61-6(1996), 1230
considered to be the most important and interesting.
13) Nakajima, N., et al. : Examined Patent Pub. The characteristics of supercritical fluids are that while they form a uniform solution system as a ; Ishihara, M., et al. : Unexamined Patent Pub. whole, they simultaneously realize a heterogeneous state where the surrounding conditions of the ; Mizuno, S. : Unexamined Patent Pub. S63-11577 molecules are in constant change with violent fluctuations. Because of such fluctuations, ; Ishihara, M., et al. : Unexamined Patent Pub. supercritical fluids are a solvent that can realize advantages of both liquid and gas simultaneously.
; Masuoka, H., et al. : Examined Patent Pub. One of the interesting fields for future research and development would be to make positive studies 14) Chartier, T. M., Ferrat, M. and Baumard, J. F. : and use of the fluctuations including solid surfaces, "Supercritical debinding of injection molded adsorption or heterogeneous catalytic reaction.
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