Supercritical Fluids

This introductory chapter is intended to acquaint the reader with the unusual properties of supercritical fluids, and with the ways these properties are exploited for a variety of applications in the chemical process industry. The presentation is closely tied to the program of this Advanced Study Institute (ASI), and points to chapters to follow in various subject areas. The behavior of thermodynamic and transport properties near a critical point is described, with water as an example. The structure of the supercritical fluid is discussed. The unusual solvent properties of supercritical fluids are explained within the framework of binary fluid phase diagrams, including a solid solute. Tunable solvent properties and environmental compatibility make supercritical fluids desirable agents in the chemical process industry. This ASI will focus on their role as extractants of food and other products, as carriers in chromatography, and as media for chemical reactions and for materials processing; moreover, virtually all aspects of polymer processing may involve the use of supercritical solvents. In this chapter, the basic knowledge and terminology required for an understanding of the chapters to follow will be introduced at an elementary level. For more advanced treatments, see [1], [2].

Fluid solvents, especially in the critical and supercritical regions, are of increasing interest for many fields. They are used as reaction or processing media with continously variable density and dielectric permittivity, high solvent power, low viscosities, and high diffusion coefficients. Examples of application fields include geology and mineralogy (e.g., hydrothermal synthesis), industrial high-pressure processes (e.g., the polymer industry), the oil and natural gas industries (e.g., tertiary oil recovery), and modern separation techniques, such as supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). Supercritical fluids are promising solvents for chemical reactions (such as waste destruction by supercritical water oxidation (SCWO)), for the deposition of small-sized solid particles (by rapid expansion of supercritical solutions (RESS), or from gas-saturated solutions (PGSS)), and for the dyeing of textile fibers, etc. These and many other applications are discussed in the present chapter and other contributions in this volume.

Ternary mixtures cante used as model systems for some important applications in super- and near-critical fluid technology, which increasingly find their way into laboratories and industry. In contrary to gasses, which, in general, have very poor solubility properties, near- and supercritical gases can be used as solvents for various substances, operating at moderate temperatures. At present, some distinct separation processes already make use of the advantages that supercritical fluid technology has to offer. Supercritical fluid extraction (SFE) is based on the fact that, near the critical point of the solvent, its properties, like the density, and with it, the ability for selectively dissolving non-volatile (e.g. organic) substances, change rapidly with only slight variations of pressure. This can form the basis for many powerful separation processes, but also makes efficient process control necessary, which is a challenge for engineers. In addition, the phase behavior of the mixture of substances involved has to te known very accurately. This often caused problems in the past, since phase behavior in the critical region was considered to te too complicated to control Today, research has provided much more insight into the phase behavior of mixtures in the critical region and former difficulties have partly become opportunities for the development of new, more efficient separation and production processes.

The thermodynamic behavior of fluids near critical points is drastically different from the critical behavior implied by classical equations of state. This difference is caused by long-range fluctuations of the order parameter associated with the critical phase transition. In one-component fluids near the vapor-liquid critical point the order parameter may be identified with the density or in “incompressible” liquid mixtures near the consolute point with the concentration. To account for the effects of the critical fluctuations in practice, a crossover theory has been developed to bridge the gap between nonclassical critical behavior asymptotically close to the critical point and classical behavior further away from the critical point. We shall demonstrate how this theory can be used to incorporate the effects of critical fluctuations into classical cubic equations of state like the van der Waals equation. Furthermore, we shall show how the crossover theory can be applied to represent the thermodynamic properties of one-component fluids as well as phase-equilibria properties of liquid mixtures including closed solubility loops. We shall also consider crossover critical phenomena in complex fluids, such as solutions of electrolytes and polymer solutions. When the structure of a complex fluid is characterized by a nanoscopic or mesoscopic length scale which is comparable to the size of the critical fluctuations, a specific sharp and even nonmonotonic crossover from classical behavior to asymptotic critical behavior is observed. In polymer solutions the crossover temperature corresponds to a state where the correlation length is equal to the radius of gyration of the polymer molecules. A similarity between crossover critical phenomena in polymer solutions and in some ionic systems is also discussed.

One of the most promising applications of supercritical fluids is in materials processing [1]. Interest is driven by the possibility of making highly pure materials, with desirable and controllable properties, under mild operating conditions, and with minimal downstream processing. There are many routes to the formation of solid phases from a supercritical medium, but they all involve one of two fundamental mechanisms of phase separation, nucleation or spieodal decomposition. A clear understanding of these two basic processes is necessary for the engineering design of processes involving the formation of solid phases from a supercritical medium. This article reviews the fundamentals of nucleation and of spinodal decomposition.

Polymers are long chain molecules that have become an indispensable part of the modem day living. They form the basis for materials of choice that are customized for a wide range of applications from baby diapers to medical devices, to computer boards. A variety of techniques are used in the synthesis, modification and processing of polymeric materials for a given end-use application.

Already in the eighteenth century it was realized that the capillary effect of fluids must arise from attractive forces between the constituents of matter, the molecules. This realization led to the idea that examination of the capillary effects could tell something about the attractive forces and possibly also about the molecules. Also modem physicists are interested in the explanation of the capillary phenomena in terms of intermolecular forces. TMs chapter highlights some of the applications of the square gradient theory of van der Waals [1] in modeling the behaviour of fluids near interfaces. For a more extensive discussion of this theory we refer to Rowlinson and Widom [2].

The unique density dependence of fluid properties makes supercritical fluids attractive as solvents for colloids including microemulsions, emulsions, and latexes, as discussed in recent reviews [1–4]. The first generation of research involving colloids in supercritical fluids addressed water-in-alkane microemulsions, for fluids such as ethaneaed propane[2, 5]. The effect of pressure on the droplet size, interdroplet interactions[2] and partitioning of the surfactant between phases was determined experimentally [5] and with a lattice fluid self-consistent field theory[6]. The theory was also used to understand how grafted chains provide steric stabilization of emulsionsaed latexes.

One of the key success factors in today’s polymers industry is the economy of scale. High pressure production units with a capacity up to 500,000 tonnes of polymer a year are being built. It goes without saying that thermodynamic optimization of such processes is a must, minor improvements will lead to significant cost savings. However, the thermodynamic models used in these industrial process optimizations are mostly of a semi-empirical nature. There are at least three reasons for this. In the first place, most of the process streams consist of at least 5 constituents (one of them being a (co)polymer with its intrinsic polydispersity). Secondly, almost all separation steps have to be carried out at an elevated pressure. And thirdly, polymer solutions are usually highly viscous. All transport phenomena and the settlement of thermodynamic equilibria are thus affected by this viscosity.

Polymerization is the process of converting moeomer(s) to long chain molecules. It is a basic process to produce materials with “microstructural” features. The microstractural consequences of polymerization are reflected in the molecular weight, molecular weight distribution, chain end groups, repeat unit orientation and chain regularity (as in tacticity), monomer sequence distributions (as in copolymers), branching, or crosslinking. The chain microstructure influences the ultimate properties of polymers that find ever increasing use in our everyday life.

Supercritical fluids (SCFs) offer a number of advantages for the synthesis and processing of polymeric materials. Early work in the field focused on the high-pressure polymerization of ethylene or exploited the adjustable physicochemical properties of SCFs, including density and dielectric constant, for the fractioeation, extraction and impregnation of polymers.¹ Recently, the field has expanded to include polymer synthesis and modification by reactive processing in the presence of CO₂. With few exceptions these are heterogeneous processes that are facilitated by enhanced transport within the SCF-swollen polymer and are sensitive to the partitioning of solutes between the fluid and polymer phases. Examples include heterogeneous dispersion polymerizations in CO₂ ², condensation reactions in dilated melts³, and the preparation of composite materials by conducting chemical reactions directly within solid polymers swollen by CO₂ ^4.

Controlled drag delivery systems received considerable attention in the last years and they are providing in general a more controlled rate of uptake of the drag by the body. In this way their therapeutic action is prolonged without increasing the dosage. The common way of controlling the release is by incorporating the drug in a polymeric carrier: the active pharmaceutical is released in the affected site by way of diffusion or surface erosion.

Dense, supercritical high pressure fluids find rapidly increasing interest in science and industry. Experimental research at the Institutes for Physical Chemistiy of Göttingen and Karlsruhe Universities has accompanied this development since the 1950’s. With am adequate selection of examples, a historical perspective will be attempted which is personal and cannot be representative of the development in the field in general. Supercritical water is the most important fluid. My own interest was initiated in 1953 by c. W. Correns of Mineralogy in Göttmgen, who slowed me that quarte and alkali halides dissolve in dense steam. TMs stimulated our investigation of electrolytic dissociation within a wide range of high temperatures and pressures along with other thermophysical properties of aqueous systems and related fluids.

The Interest in understanding chemical phenomena in aqueous solutions at elevated temperatures and pressures has grown significantly during the last decade[1–9] Practical applications include hydrothermal oxidation of organic wastes, hydrothermal growth of crystals, spraying of ceramics, and hydrothermal synthesis reactions, e.g., the commercial hydrolysis of chlorobenzene to produce phenol and dibenzofuran. Because water at high temperatures is highly compressible, small changes in temperature and pressure lead to large changes in the density and the dielectric constant which produce large variations in ion solvation and acid-base equilibria. Fundamental chemical properties, which are well-known in aqueous chemistry at 298 K, are much less available for supercritical water (SCW) (T _c = 647.13 K, p _c = 0.322 g/cm^3, P _c = 220.55 bar) solutions. Examples of such properties include ion solvation and acid-base equilibria, which play a central role in solvent effects on chemical reaction rate and equilibrium constants, phase equilibria, and corrosion. In this article these properties are discussed on the basis of in-situ spectroscopic measurement and computer simulation of ion solvation and chemical equilibria. The structure of water about ions is also discussed elsewhere in this book [10].

Most biochemical and Industrial processes occur in solution, making It crucial to be able to understand solvatlon effects on solubility and chemical reactivity [1, 2]. Numerous recent developments have resulted In an Increased Interest in supercritical water (SCW) and aqueous solutions [3–5]. In particular, supercritical water Is generating Interest as an environmentally-benign solvent for a variety of chemical processes and technological applications Including selective -synthesis [6], coal conversion [7], deuteration of simple organic compounds [8], and conversion of organic waste to light feedstock [9], where water participates as a solvent, a catalyst, and a reactant. Perhaps the most promising application In this area Is the destructive oxidation of biochemical and pharmacological hazardous wastes, known as the supercritical water oxidation (SCWO) process [10-14].

Supercritical fluid (SCF) solvents are unique in that their densities can be varied continuously from gas-like to liquid-like values simply by varying the thermodynamic conditions. Because many of a fluid’s solvating properties are strongly dependent on the fluid density, such large changes in density can have dramatic effects on solute reactivity [1,2]. For example, at low pressures supercritical water supports homolytic, free radical reactions, whereas at higher pressures, heterolytic, ionic reactions dominate [3,4]. Thus, thermodynamic control of SCF solvent densities promises to enable us to control reaction outcome and selectively produce desired products.

In 1966 John Connolly of Standard Oil Co. published remarkable data on hydrocarbon solubilities in water at high temperatures and pressures [2]: he observed that, in some regions of the phase diagram, hydrocarbons (e.g., benzene, heptane) and water are miscible in all proportions. Rapid development of experimental techniques made Connolly’s work possible and speculations began about the consequences of his observations. For example, in 1970 Gerhard Schneider suggested the extension of wet air oxidation to higher temperatures for disposal of organic materials [3]. In the mid 1970’s Sanjay Amln, a student working with Robert Reid and Michael Modell at Massachusetts Institute of Technology (MIT), studied decomposition of organic compounds in hot water and found that the Intractable tars that formed below the critical temperature of water, disappeared above It. Research and development on supercritical water oxidation (SCWO) for disposal of organic waste materials began soon after [4].

As is shown by working groups all over the world, supercritical water oxidation is a promising technology for the complete oxidation of aqueous hazardous waste from different sources, such as aqueous waste from the pharmaceutical and chemical industries [1, 2]. The function of the reaction medium supercritical water during oxidation, howver, is not well understood.

The processing of agrimaterials utilizing sub- and supercritical fluids Is one of the more challenging and time-honored applications of the technology. Agrimaterials can by definition include food, natural products, and nutraceuticals; however It Is probably raw agricultural materials that encompass and present so many undefined variables to the technologist attempting to use critical fluid processing. Agrimaterials are “nature” In Its most raw form, and the application of critical fluids for unit operations must be able to respond to a wide variation in substrate moisture content, molecular composition, and physical or seasonal morphology. In these cases, a unit operation like supercritical fluid extraction (SFE) will not be conducted under Idealized conditions, i. e, those that are used to measure solute solubility in a laboratory environment. Such complications however have not limited the application of critical fluid technology to agrimaterials, since some of the most often cited and commercially-successful uses of the technology occur in this area.

Different cell and tissue structures of various biological materials as well as their moisture content play an important role in the extraction of lipids with supercritical carbon dioxide (SC-CO₂). All biological matter is made up of water, proteins, lipids, carbohydrates and inorganic salts. Hundreds of components are organized within the cell structure where they interact with each other in various ways. When such a complicated system is placed in a high-pressure environment with the objective of extracting specific components using a supercritical solvent, the complexity of the system increases dramatically. Our understanding of the component interactions during supercritical fluid extraction is quite limited. Thus, structure of the starting material and location of the components of interest need to be examined to have a better understanding of the component interactions and how they affect extraction kinetics.

Mathematical modeling of the extraction of natural materials is an activity of increasing importance due to the economic potentials it offers. A fundamentally sound and sufficiently detailed mathematical model may be used to project and extend the scope of the available experimental findings to obtain a better understanding of the systems and the phenomena involved for the design, scale-up and operation of the related equipment, and the complex systems having such equipment. Almost all mathematical models describing operations involving complex phenomenon have several simplifying assumptions attached to a basic physical model, assumed to best describe the actual phenomena. What is required of the model is to predict the available experimental data accurately and precisely. It is the best if non of the model parameters are system dependent, that is they are all calculated from theoretical principles, or are evaluated from data obtained from completely independent experimental systems. The strength of the models are measured by the number of system types they can accurately predict, and the spans of their ranges of applicability, within acceptable limits of accuracy and precision.

Separation processes with supercritical gases, Supercritical Fluid Extraction (SFE) or Gas Extraction (GE) processes, is a group of separation processes which applies supercritical fluids as separating agents in the same way as for instance, liquid solvents are used in separation processes like liquid-liquid extraction or absorption. For gas extraction, the solvent is a supercritical component or a supercritical mixture of components [1].