Supercritical Fluid Engineering Science

Chapter 1

Current State of Supercritical Fluid Science and Technology 1

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Erdogan Kiran and Joan F. Brennecke Downloaded by SUNY DOWNSTATE MEDICAL CTR on April 24, 2016 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch001

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Department of Chemical Engineering, University of Maine, Orono, ME 04469 Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556

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This chapter is an overview of the current state of the supercritical fluids science and technology. It gives the new directions in research and applications which involve both physical and chemical transformations. Among the new trends are the greater research and use of binary and multicomponent fluids to optimize and facilitate these transformations. Applications have expanded beyond extractions and separations to include reactions, polymer processing, pharmaceuticals, food processing and environmental remediation.

Interest in supercritical fluids and their applications is continuing with an expanding scope and sophistication. This monograph contains selected papers from the Symposium on Supercritical Fluids at the Annual AIChE meeting held in Los Angeles, California, in November 1991 and purposely reflects the diversity and expanding scope of supercritical fluids research. The basic philosophy of utilization is centered around the fact that the properties of supercritical fluids can be varied from gas-like to liquid-like values by simply adjusting the pressure. These fluids are therefore very attractive as tunable process solvents or reaction media. Even though in earlier years most of the attention was on single processing fluids such as carbon dioxide and extractions as the primary mode of application, in recent years emphasis has been shifting to binary and multi-component fluids and processes with a greater degree of complexity which may include either physical or chemical transformations. As a result, recent research is focused more on the generation and prediction of fundamental property values needed for design, and understanding of the molecular interactions and physicochemical processes in such complex systems. In addition to a broadening of activities related to

0097-6156/93A)514-0001$06.00/0 © 1993 American Chemical Society

Kiran and Brennecke; Supercritical Fluid Engineering Science ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Downloaded by SUNY DOWNSTATE MEDICAL CTR on April 24, 2016 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch001

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SUPERCRITICAL FLUID ENGINEERING SCIENCE

applications involving physical transformations, a marked increase is noted in explorations related to chemical transformations. Among these, oxidation reactions in supercritical water for waste destruction has been most visible. Organic substances and oxygen are soluble in supercritical water which permits oxidation to occur in a single phase. Inorganic salts are only sparingly soluble in supercritical water and can be separated from the reaction medium. Currently, supercritical water oxidation is at an advanced stage of evaluation for treatment of industrial wastes, and NASA is considering the technology for implementation in long duration extraterrestrial human exploration missions. It is being considered as an advanced life support technology for processing of solid waste and reclamation of water in space stations. The use of binary and multicomponent fluids is driven by a desire to either manipulate the critical temperature of the mixture, or to introduce polar or nonpolar features to regulate interactions of the fluid with a specific compound. Binary mixtures of carbon dioxide with polar compounds such as alcohols and with non-polar compounds such as alkanes have therefore been of special interest. In binary mixtures, critical temperatures assume values between the critical temperatures of the components. The critical pressures, however, often take values higher than the critical pressures of the pure components. For example, addition of about 7 mole % ethanol to carbon dioxide results in a mixture with a critical temperature of about 52 °C, but a critical pressure of 97 bar, compared to 31.1 °C and 73.8 bar for carbon dioxide and 240.9 °C and 61.4 bar for ethanol. Such a mixture offers the desirable features of having polar character in the fluid at a much lower temperature than would be possible by using pure alcohols. Ability to adjust and lower the operational temperature becomes important for many applications involving thermally labile compounds such as biomaterials. Similar intermediate critical temperatures with a modest increase in the critical pressures are achieved also in binary mixtures of carbon dioxide with alkanes which would have desirable features of alkanes at lower operational temperatures. The following overview is divided into four sections: (1) measurement and prediction of the phase behavior, (2) determination of transport properties, (3) understanding and using the local structure of fluid solutions, and (4) applications. The emphasis is on the use of supercritical fluid mixtures and their application for both physical and chemical processes.

Measurement and Prediction of Phase Behavior Since there is a growing interest in the use of binary and multicomponent supercritical fluid mixtures to tailor separations, modify physical properties and influence reactions, there is a corresponding interest in the phase behavior of these mixtures. Experimentally determined critical data are however not available for all binary mixtures of interest, and the available data for some systems are limited only to a narrow concentration range. To fill this gap, there is continual research activity on the phase behavior and critical properties of binary mixtures. The first



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State of Supercritical Fluid Science and Technology

two chapters of the present book indeed deal with binary mixtures of carbon dioxide with alcohols, and alkanes. To ensure operation in the supercritical region of binary solvent mixtures of defined compositions, knowledge of the gas-liquid critical point is essential. Such information, in addition to being of great value in designing binary process fluids, are equally important to operations in which carbon dioxide may be used to separate the other component from more complex mixtures. Carbon dioxide can, for example, be used to remove alcohols from aqueous solutions. If pressure and temperature conditions are not above the critical temperature and pressure for the mixture, unlike pure fluids, binary fluid mixtures may display multiphase equilibrium behavior such as the liquid-liquid-vapor (llg) phase behavior. Multiphase equilibria of binary and ternary mixtures composed of both polar and non-polar species are important especially with respect to separability of solutes between the liquid phases. Two chapters in the book (Chapters 4 and 5) are concerned with multiphase equilibria encountered in mixtures of carbon dioxide with alcohols or hydrocarbons, and in mixtures of propane with triglycerides. The latter is of significance in processing of natural products such as edible oils. Another chapter (Chapter 7) discusses the four-phase, liquid-liquid-liquid-gas equilibrium observed for ternary mixtures of carbon dioxide, water, and 2butoxyethanol at conditions near the critical point of carbon dioxide. These mixture are important for understanding process applications involving microemulsions. Estimation of the solubility of a substance in supercritical fluids or in mixtures containing a component at supercritical conditions is key to the evaluation of many applications of supercritical fluids. Research on predictive procedures are continuing. For these predictions, the physical properties of the solvent and the solute and an equation of state are required. Simpler forms of equations of state such as the ideal gas, truncated virial equation of state, and the basic form of the cubic equation of state, the van der Waals equation, are not very effective in describing fluids at high pressures. Modified cubic equation of states such as the Redlich-Kwong or the Peng-Robinson equations of state which incorporate temperature dependent attractive terms are found to be more successful and are, therefore, used most frequently. These equations of state are used to describe not only single fluids but multicomponent fluid mixtures as well. For mixtures, the success of the equation depends on the use of proper mixing rules and assignment of interaction parameters. Interaction parameters are normally determined from experimental data. Recently, a group contribution method has been developed to allow estimation of the interaction parameter for some systems in the absence of experimental data (Chapter 6). Even with modified cubic equations, for systems containing polar components, predictions become poorer. One of the more recent approaches to model associating systems is the use of the SAFT ( Statistical Associating Fluid Theory) equation of state. The use of various equations of state are illustrated in Chapter 2 for carbon dioxide-alcohol systems, and in Chapter 7 for carbon dioxide -water- surfactant mixtures, and in Chapter 6 for estimation of solubility of hydrocarbons and cholesterol in supercritical carbon


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dioxide. Solubility of cholesterol is, of course, of significance in applications related to food industry.


Determination and Correlation of Transport Properties Even though phase equilibria and solubility information is essential for effective use of supercritical fluids, for many applications such as extractions, reactions, or impregnation processes, information on transport properties such as diffusivity, viscosity, and thermal conductivity become equally important. These questions are particularly important for processing of materials such as coal, biomass, or polymers. A well known advantage of supercritical fluids compared to ordinary liquid solvents is that diffusion coefficients in supercritical fluids are higher than in liquid, leading to more favorable mass transfer rates. The available data is, however, very limited even for self diffusion coefficients. Nonetheless, recent research is expanding the data base for binary diffusion coefficients and exploring the influence on solute diffusivities the addition of a second component to the process fluid. Chapter 8 provides an account of the limitations of the existing methods such as the hydrodynamic Wilke-Chang correlation, BatchinskiHildebrand free-volume theories, or the dense gas Enskog relationships based on the hard-sphere theory and its modifications. Data are reported for the diffusivity of selected organic compounds (benzoic acid, phenanthrene and acridine) in carbon dioxide and in carbon dioxide/ methanol and carbon dioxide/acetone mixed solvents. The results show that both the increased local density of the solvent around the solute in the pure solvent case and the preferential attraction of the cosolvent (acetone or methanol) around the solute in the mixed solvent systems significantly influence the diffusion of the solute. These transport measurements corroborate some of the investigations of local intermolecular interactions discussed in the next section. At present understanding and modeling of the diffusivity in mixed solvents is severely limited by the lack of available volumetric and viscosity data on mixed solvents. Experimental data on the viscosity of supercritical fluids and solutions are needed however, not only for modeling diffusivities and prediction of mass transfer rates , but also for (1) modeling and prediction of viscosities themselves, (2) proper design of process equipment such as pumps, mixers, and heat exchangers, and (3) for establishment of improved processing conditions. Polymer solutions constitute an important case and data is needed over a wide range of concentrations since supercritical fluids can be used in fairly diverse polymer applications— dissolution and fractionation at one end, and lowering the viscosity of polymer melts for easier processing at the other. Recent work on the viscosity of normal alkanes and polymer solutions has shown that density is an excellent scaling factor and viscosities can be described as an exponential function of density. The exponential dependence on density points to greater applicability of the Doolittle type free volume theories in describing viscosity at high pressures. Chapter 9 provides an account of various methods of


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correlation of viscosity and presents data for solutions of polystyrene in supercritical η-butane. Investigation of pressure or density dependence of viscosity of polymer solutions can provide information on the change of "goodness" of the solvent and solvent/polymer interactions. The concept of Theta Pressure or Theta Density as opposed to conventional Theta Temperature introduced in that chapter has significant theoretical and practical implications for polymer solutions.

Investigation and Modeling of Molecular Interactions and Local Fluid Structure A considerable amount of recent research is directed to develop an improved understanding of local fluid structure and the molecular interactions between solutes and other components in supercritical fluid mixtures. Theoretical studies based on molecular dynamic simulations, Monte Carlo calculations, and integral equation theories, as well as experimental studies using absorbance and fluorescence spectroscopy point to local density enhancement of the solvent around the solute in near critical fluids. Energetic and entropie contributions to this enhancement are discussed in Chapter 11. Description of local fluid structure in fluid mixtures containing a co-solvent, and systems involving polar solutes has been of particular interest. This is because with addition of cosolvents, solute interactions with the solvent mixture can be varied dramatically. Two main questions arise regarding cosolvent addition to supercritical fluids: (1) what is the nature and the mechanism of the increased solubility of solutes, and (2) how does pressure, and temperature and the nature of the supercritical solvent influence chemical association, especially of common co-solvents that can self-associate through hydrogen bonding? Chapter 12 shows that much of the solute solubility enhancement is due simply to the increase in local and bulk density when the cosolvent is added. In addition,flourescentprobes which are sensitive to the local solvent environment are being used to study the nature of solvent and co-solvent interactions. The reactions of the probe 7-azaindole in methanol/carbon dioxide fluid mixtures reveal that the structure of the hydrogen bonding environment between the solute and the cosolvent is significantly weaker than what one observes in liquids (Chapter 17). This is also true for the self-association of alcohols in supercritical carbon dioxide and ethane, which is significantly less than in liquid organic solutions. However, the results based on molecular dynamic simulations and infrared spectroscopy demonstrate that the cluster sizes in a supercritical fluid depend on the nature of the solvent and there is a distribution of cluster sizes (Chapters 13 and 14). For example, at similar operational conditions, methanolmethanol aggregation is found to be greater in ethane than in carbon dioxide whereas solvent-methanol aggregation is observed to be greater in carbon dioxide than in ethane. These observations are of significance in understanding the function of alcohols as modifiers in enhancing solubility of polar solutes in non-polar supercritical fluids.




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Generally, the solubility enhancement when a cosolvent is added to a supercritical fluid is determined by tedious solubility measurements. Chapter 18 in this book presents a fast way, using supercritical fluid chromatography, that requires very small amounts of modifier to determine the cosolvent effect, thus yielding information on the strength of solute/cosolvent interactions. Some of the same methods, i.e., integral equation theory calculations, used to describe the interaction between species in supercritical fluid mixtures can be used to analyze the structure of these mixtures in the vicinity of a solid surface (Chapter 15). These studies are aimed at providing an understanding of adsorptiondesorption equilibria between a solid surface and a fluid media. Such analyses are important for many processes involving adsorption from a supercritical fluids solution or desorption into a supercritical fluid solution. Examples include regeneration of sorbents with supercritical fluids, supercritical fluid chromatography and supercritical fluid extraction of contaminated solids. The fact that the local densities and local compositions around solutes or solid surfaces in supercritical fluids are different than in bulk have consequences not only for solubility, or adsorption-desorption processes, but also for reactions in supercritical fluids. Information on molecular interactions and solvent effects on reactions is of great significance. A review chapter in the book (Chapter 16) provides an overview of a broad range of spectroscopic investigations of reactions in supercritical fluids. While the solvent can act in a variety of manners to affect reaction rates including (1) increased reactant solubilities and reduced mass transfer resistances, (2) facilitated separation of products from the reaction medium, (3) catalyst life extention through minimization of fouling and deactivation, (4) pressure effect on the rate constant, (5) changes in selectivities, and (6) increased diffusion rates, there is growing realization of the importance of the effect of local densities and local compositions. Understanding the local molecular phenomena may be the key to future developments for better description of reaction rates in supercritical fluids.

Applications As already stated, the application areas of supercritical fluids are expanding very rapidly. A number of chapters in the book have been devoted to specific application areas. These are related to pharmaceuticals, polymers, chromatography, extractions, coal and petroleum processing, and environmental remediation. They involve both physical and chemical transformations and the use of multicomponent supercritical fluid mixtures. An interesting example of pharmaceutical applications is the use of supercritical fluids in production of controlled release drugs. Chapter 19 describes codissolution of a biocompatible polymer such as poly(D,L-lactic acid) and a pharmaceutical compound (such as lovastatin) in supercritical carbon dioxide followed by rapid expansion to form polymer-drug microspheres of controllable



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size. Carbon dioxide, being non-toxic and a gas at normal temperatures and pressures, is ideal for such applications. Use of supercritical fluids in polymer processing encompasses a wide range of applications from polymerization to fractionation, or impregnations and morphological modifications. Chapter 21 describes use of supercritical fluids in forming microcellular foams (polymer aerogels) which display small pore sizes and low densities. Formation of microporous structures from polymerization of methacrylate based co-monomers in supercritical Freon-22 is described. In this particular application, using supercritical fluids as the polymerization medium helps form the polymer network which can then be supercritically dried in the same vessel without the need to exchange the solvent. Perhaps one of the most successful application areas of supercritical fluids is Supercritical Fluid Chromatography. It has become a widely used analytical technique for separation and analysis. As already indicated above, the technique is also used to generate fundamental information on molecular interactions and thermodynamics of solvent-solute interactions. It is used alone or coupled with supercritical fluid extraction or with post characterization techniques such as IR or MS. Various modes of operation include pressure, temperature, or density programming, or solvent gradient methods in which a second component is added to the eluent fluid. Proper selection of the pressure, temperature, density or solvent gradients with time are important for improved separations and optimization. Chapter 22 describes the recent methodology of optimization. Extractions and reactions using supercritical fluids still constitute the major mode of operation in many application areas. Extractions are carried out either to isolate a desired compound of higher value, or remove undesirable components from a raw material to obtain a product with improved properties. Extraction of caffeine from coffee is a well known example. Removal of impurities from polymers, cleaning of electronic parts, binder removal from ceramics, separation of buckminsterfullerenes, and environmental remediation are among more specific applications. Chapter 28 describes an interesting application in which isotropic petroleum pitch, a waste material produced from crude oil refining, has been extracted with supercritical toluene to produce a valuable product, mesophase pitch, which can be used to make high performance carbon-fibers. Chapter 30 illustrates a different use for supercritical toluene. It has been used as a reaction medium to carry out cracking of cis-polyisoprene while suppressing the formation of polycondensates. Such reactions have significance in disposing of used tires, a major source of solid waste. Supercritical toluene has in the past been used to conduct cracking reactions of coal for liquefaction. With respect to coal extraction, recent research is exploring effectiveness of polar solvents. Chapter 29 describes the kinetics of extraction of coal with t-butanol. Supercritical water is being evaluated for removal of N , S, and Ο containing organic compounds from coal to produce cleaner burning fuels (Chapter 26). At present, an application area of much activity is the environmental remediation and removal of toxic contaminants from soils and industrial waste using supercritical fluids. These also involve either extractions or reactions.




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Several specific applications are covered in the book. Chapter 23 is related to extraction of organics from contaminated soils with near-critical carbon dioxide. Model studies have been conducted using soil, activated carbon and alumina loaded with known amounts of naphthalene or phenol. Slurrying the soil with water is introduced to facilitate the handling and processing of solids at high pressures. Presence of water affects system dynamics and the solutes are distributed in three phases, i.e., carbon dioxide, water and the solid. How the differences in solidsolute and solvent-solute interactions affect the extractability of the organic solutes into the critical fluid phase are discussed in terms of the different behaviors displayed by phenol and naphthalene. For total treatment of wastewaters and sludges, reactive remediation using supercritical water oxidation is rapidly expanding. Complete miscibility of oxygen and organic compounds in supercritical water creates a single-fluid phase and favorable reaction environments. In supercritical water greater than 99.99% destruction of many of the EPA priority pollutants are achievable with short residence times. However, salts which are soluble in subcritical water become insoluble at supercritical conditions. Therefore, one of the current practical issues related to implementation of the technology is the removal of solids from the process. Chapter 24 discusses the kinetics of oxidation of organic compounds in supercritical water, and Chapter 28 reports on model studies on the engineering aspects of removal of sodium chloride, sodium sulfate, sodium nitrate from supercritical oxidation process streams. Reactions of some model compound in supercritical water are discussed in Chapter 26.

This review is by no means inclusive of all the developments and applications related to supercritical fluids. Those that have been covered are the ones that we discussed during the AIChE symposium in Los Angeles, in November 1991. The breadth and the growth in the field can be further appreciated through examination of some of the recent monographs and conference proceedings. The most recen ones are (1) Supercritical Fluid Technology. Theoretical and Applied Approaches in Analytical Chemistry; Bright, F.V.; McNally, M. E. P., Eds; ACS Symposium Series 488, American Chemical Society: Washington, DC; 1992. (2) Supercritical Fluid Technology. Reviews in Modern Theory and Applications; Bruno, T.J.; Ely, J. F., Eds.; CRC Press: Boston, MA; 1991. (3) Abstracts of the 4th International Symposium on Supercritical Fluid Chromatography and Extraction, Cincinnati, Ohio, May 20-22, 1992. (4) Proceedings of the 2nd International Symposium on Supercritical Fluids, Boston, Massachusetts, 20-22 May,

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The Journal of Supercritical Fluids routinely provides accounts of the most recent research. RECEIVED July 23, 1992


Supercritical Fluid Engineering Science

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