Supercritical Water: A Green Solvent: Properties and Uses

By Yizhak Marcus

Discover the many new and emerging applications of supercritical water as a green solvent

Drawing from thousands of original research articles, this book reviews and summarizes what is currently known about the properties and uses of supercritical water. In particular, it focuses on new and emerging applications of supercritical water as a green solvent, including the catalytic conversion of biomass into fuels and the oxidation of hazardous materials.

Supercritical Water begins with an introduction that defines supercritical fluids in general. It then defines supercritical water in particular, using the saturation curve to illustrate its relationship to regular water. Following this introduction, the book:

• Describes the bulk macroscopic properties of supercritical water, using equations of state to explain temperature-pressure-density relationships
• Examines supercritical water's molecular properties, setting forth the latest experimental data as well as computer simulations that shed new light on structure and dynamics
• Explores the solubilities of gases, organic substances, salts, and ions in supercritical water in terms of the relevant phase equilibria
• Sets forth the practical uses of supercritical water at both small scales and full industrial scales

Throughout the book, the author uses tables for at-a-glance reviews of key information. Summaries at the end of each chapter reinforce core principles, and references to original research and reviews serve as a gateway and guide to the extensive literature in the field.

Supercritical Water is written for students and professionals in physical chemistry, chemistry of water, chemical engineering, and organic chemistry, interested in exploring the applications and properties of supercritical water.

• Language: English
• Publisher: Wiley
• Release date: May 15, 2012
• ISBN: 9781118310274

Book preview

Preface

As of the summer 2011, there were more than 3000 topics dealing in detail with supercritical water (SCW) in the SciFinder literature search instrument of the American Chemical Society. However, there were more than 14,000 entries outlining this concept. In the 1980s some 100 papers and in the 1990s some 900 papers on supercritical water were published, while at present there are already more than 2000 papers. As it is impossible to compile all the published information in a book, an attempt has been made to include the maximum possible important properties and uses of supercritical water. Factual information is given in numerous tables along with suggested references for more details on the subject. Where appropriate, the reader is referred to several reviews relevant to the topics included in this book.

Prior to 1980, only a few dozen papers dealt with SCW, considering SCW mainly within the broad subject of high-pressure steam in the context of electric power generation. The papers dealt principally with the heat transfer in SCW, mineral solubilities in it, and corrosion by it. E. U. Franck, a pioneer in the study of supercritical fluids, however, published in 1968 a review (Endeavour, 22, 55) that highlighted some of the properties of this fluid and its possible uses. The properties contrasted with those of water vapor and of the liquid water at ambient condition. They included the complete miscibility of SCW with nonpolar fluids, the very high mobility of ions from electrolytes dissolved in SCW, and the water itself acquiring appreciable electrical conductivity. Knowledge of the chemical behavior of high-temperature water, in the pure state and when serving as a solvent, led to the understanding of the structural features of SCW and of hydration phenomena in it. The properties of geochemically important hydrothermal solutions could also be explained and possible technical applications were suggested.

The properties of dense steam or compressed hot water below the critical point and solutions in such media can be of interest, as these are able to act as green solvents. In the present book, the so-called near-critical water is however only cursorily dealt with, as it is mainly devoted to the properties and uses of supercritical water. SCW in itself can also be deemed to be a green solvent, that is, environment-friendly.

Having dealt for many years with liquids and solutions, the author's interest in SCW was raised by the proposal he received in the late 1990s from the INTAS agency for his participation in an international collaboration on this subject. During the period of 3 years of the project that involved three groups from Russia, one from Greece, one from Germany, and the present author, experimental and theoretical studies of supercritical aqueous solutions as a medium for new environmentally friendly and energy efficient technologies of pollution control were carried out. As a further result of this collaboration, one of the participants, A. Kalinichev, was invited together with the present author by R. Ludwig, the editor of the book Water: From Hydrogen Bonding to Dynamics and Structure, to write a chapter on SCW for it. This provided the initial impetus to the writing of the present book, seeing that none existed so far that summarized the state of the art and in view of the increasing interest in the subject as reflected by the increasing number of published papers.

The book is divided into five chapters. Chapter 1 introduces supercritical fluids in terms of the phase diagrams of the fluids and their critical points. A brief description of a variety of supercritical fluids that have been used as solvents is given. Attention is then turned toward the water substance, in its gaseous state (water vapor) and ordinary liquid water and their properties. As water is heated toward the critical point, near-critical water is reached, and a short discussion of this state of water (that has found some applications as a green solvent) is presented. Chapter 2 deals with the macroscopic measurable properties. Foremost of these are the temperature–pressure–density or volume (PVT) relationships described by means of equations of state. Other important thermophysical properties of SCW are the heat capacity and the enthalpy and entropy. The electrical and optical properties include the static dielectric constant, the light refraction, and the electrical conductivity of neat SCW. The transport properties involve the viscosity, the self-diffusion, and the thermal conductivity. The ionic dissociation of SCW is then discussed, and finally the properties of SCW relevant to the solubility of solutes in it are briefly described. Chapter 3 deals with the structure and dynamics as inferred from experimental data and computer simulations. Diffraction of X-rays and in particular of neutrons provides information on the molecular structure of SCW. Computer simulations provide information on both the structure (by the Monte Carlo method) and the dynamics (the molecular dynamics method). Spectroscopic studies, involving infrared light absorption, Raman light scattering, nuclear magnetic resonance, and dielectric relaxation, complement the aforementioned studies. It is shown that SCW has appreciable hydrogen bonding between its molecules and the extent of this is explored. Finally, the dynamics of the water molecules in SCW and the lifetime of various configurations in it are discussed. Chapter 4 describes the solubilities of gases, organic substances, salts, and ions in SCW in terms of the relevant phase equilibria. The interactions that take place between the water molecules and the solutes of the various categories are presented. In particular, for ions and salts, the properties of such solutions are dealt with. In case of ions, their association on the one hand and their hydration on the other determine these properties. Finally, Chapter 5 includes the current practical uses, whether on a modest or on a full industrial scale. Conversion of biomass to fuel, gaseous or liquid, is one such use. SCW oxidation (SCWO) of pollutants and hazardous materials is another important use, the problems associated with which have not so far been completely resolved. Some other uses include organic synthesis, where SCW is both a reaction medium and a reactant, nanoparticle production of inorganic substances (mainly oxides), and as a neutron moderator in nuclear power reactors and at the same time as the coolant, providing the fluid for turbine operation. Geochemistry is another field where SCW plays a role, because deep strata in the earth's crust provide the high temperature and pressure to convert any water derived from hydrous minerals to SCW. This is then evolved in thermal vents, carrying along some minerals dissolved in it. Finally, some of the corrosion problems met with in applications of SCW are briefly dealt with.

A vast amount of information is available on SCW and solutions therein; this book however provides those numerical data in tables that help the reader to appreciate the quantitative aspects of SCW and its properties. Some other tables include annotated examples of the uses of SCW, but on the whole, it is possible only to point out what various authors have studied, to summarize it, and as necessary to comment on this. This book includes references to which the readers interested in having in-depth knowledge of the topics may refer. It is hoped that the book will help understand the concept of supercritical water, its properties, and uses.

Yizhak Marcus

Jerusalem, 2011

List of Acronyms and Symbols

Acronyms

Symbols

Subscripts and Superscripts

Chapter 1

Introduction

1.1 Phase Diagrams of Single Fluids

Substances appear in Nature in several states of aggregation: crystalline solids, amorphous solids, glasses, liquids, and gases. The latter two states are collectively termed fluids because they are subject to flow under moderate stresses (forces). A phase is a portion of space in which all the properties are homogeneous, that is, they do not depend on the precise location in the phase (except at its boundaries). Depending on the external conditions (thermodynamic states) of temperature and pressure, a substance may exist at several states of aggregation at the same time, each of which is represented as a different phase. These phases may be at equilibrium with each other, and the phase diagram, in terms of the external conditions of the temperature and pressure, represent which phases are at equilibrium with each other. These phase equilibria have several important features, governed by the phase rule of Gibbs. This rule states that number of degrees of freedom (Df) in a system is equal to 2 plus the number of components (Co) minus the number of phases that exist at equilibrium (Ph):

(1.1) equation

The degrees of freedom of interest in the present context are the external conditions that can be independently chosen: the pressure P, the temperature T, and the composition of mixtures. When the latter are expressed as the mole fractions of the components: x1, x2, . . ., xN for an N-component mixture, there are N − 1 independent composition variables. A component is a substance that can be added independently: water is an example of a component and a salt, such as NaCl, is another example, but each of their constituent ions (H+ or H3O+, OH−, Na+, Cl−) is not. Only neutral combinations of the ions can be considered as components, since they can be actually handled.

For a single substance Co = 1 and up to three phases can exist at equilibrium, in which case Df = 0, there remain no freely determinable external conditions (no degrees of freedom): the temperature and pressure are fixed. This invariant point (involving a solid, a liquid, and their vapor) is called the triple point of the substance. In the case of water: ice, liquid water, and water vapor are at equilibrium at 0.01°C (T = 273.16 K) and P = 0.61166 kPa [1].

Two phases of a single component at equilibrium permit according to the phase rule (1.1) a single degree of freedom: either the temperature is variable and the pressure is then fixed or for a variable pressure the temperature is fixed. This simple function determines a line in the two-dimensional phase diagram. For the water substance, at temperatures below freezing several phases of ice exist, depending on the pressure, and a pair of which can exist at equilibrium along the lines of the relevant phase diagrams. Ice may sublime to form water vapor, and again the two phases may exist at equilibrium along the sublimation line of the phase diagram of water. These aspects of the phase diagram of water are outside the scope of this book. The phase diagram of water is shown in Fig. 1.1, the phase boundaries being the coexistence lines of two phases at equilibrium. The phase diagram of water at very low temperatures and very high pressures is complicated by the existence of several ice phases of different densities and structures (not shown in Fig. 1.1), but these are of no concern in the present context.

Figure 1.1 The phase diagram of water: point corresponds to the triple point (ttr = 0.01°C, Ptr = 0.611 kPa), point to the melting point (tm = 0°C, Pm = 101.3 kPa), point to the normal boiling point (tb = 100°C, Pb = 101.3 kPa), and point to the critical (tc = 374°C, Pc = 22.1 MPa). The line between and is the saturation line, P = pσ. Various compressed ices exist above point that are of no concern in the present context.

When attention is drawn toward fluid phases, vapor–liquid equilibria (VLE) constitute a very important subject. Again, in such two-phase systems the feature of the phase diagram of water is a line, called the saturation line for VLE, and is designated by the subscript (σ). This line for water is shown in Fig. 1.1, extending from the triple point up to the critical point (see below). The fact that the normal boiling point of water at ambient pressure (1 atm = 1.01325 bar = 101.325 kPa) happens to correspond to 100°C (373.15 K), is incidental in this respect. Wagner and Pruss reported the IAPWS 1995 formulation for the thermodynamic properties of ordinary water substance [1] that appears still to be the last word on the subject. The expression for the saturation vapor pressure pσ(T) takes the following form:

(1.2)

equation

where Pc is the critical pressure and τ = 1 − T/Tc, with Tc being the critical temperature. The values of Pc, Tc, and the coefficients ai (i = 1–6) are shown in Table 1.1.

Table 1.1 Parameters for the Saturation Vapor Pressure of Water, Eq. (1.2)

Numerical values of pσ at several temperatures are shown in Table 1.5 along with data for other properties of liquid water discussed in Section 4.

A supercritical fluid (SCF) consists of a single phase and since Co = Ph = 1 it has two degrees of freedom according to Eq. (1.1). Its temperature and the pressure can be chosen at will, provided they are larger than the critical values (see below).

1.2 The Critical Point

As the temperature and pressure of a fluid increase, a point is reached where the two phases, the liquid and the vapor, coalesce into a single phase. The density of the liquid diminishes along the saturation line whereas that of the vapor increases as its pressure increases, until finally they become equal at the critical point. This is characterized by a critical temperature Tc, a critical pressure Pc, and a critical density ρc. A liquid that is confined in a vessel in a gravitational field has a free surface with respect to its vapor, hence, being denser, lies below it. However, at the critical point the meniscus that is characteristic of this surface disappears. Slightly below Tc the fluid becomes opalescent because of the fine dispersion of minute droplets of the liquid in the vapor of almost the same density. At temperatures and pressures above the critical ones, T > Tc and P > Pc, the substance exists as a single clear phase, the supercritical fluid. According to the phase rule, Eq. (1.1), the supercritical fluid has two degrees of freedom, and the temperature and pressure can be chosen at will. These two external variables determine the properties of the supercritical fluid, such as its density, heat capacity, viscosity, relative permittivity, among many others, as are dealt with for the supercritical water (SCW) substance in Chapter 2. The critical temperature of water is Tc = 647.096 K = 373.946°C and its critical pressure is Pc = 22.064 MPa = 21.78 atm (cf. Tables 1.1 and 1.3). The critical density of water is ρc = 322 kg m−3 and its critical molar volume is Vc = 56.0 × 10−6 m³ mol−1, the latter two quantities being known to no better than ±1%.

For many purposes, and especially for the description of the properties of supercritical fluids, it is expedient to use the reduced variables: the reduced temperature, Tr = T/Tc, the reduced pressure Pr = P/Pc, the reduced density ρr = ρ/ρc, or the reduced molar volume Vr = V/Vc. In the vicinity of the critical point there are some general relationships that depend on the deviation of the temperature from the critical point: τ = 1 − Tr. As the critical point is approached from below (τ > 0) the enthalpy of vaporization tends to zero and the heat capacity of the system tends to infinity according to the proportionality relationship

(1.3) equation

The dependence of the density difference between the liquid and vapor phases on τ is according to Eq. (1.4):

(1.4) equation

The compressibility of either phase depends on the temperature as

(1.5) equation

At τ = 0 (at the critical point) the reduced pressure depends on the reduced density as

(1.6) equation

These proportionalities are described by means of the critical indices, the commonly accepted theoretical values for them being α ≈ 0, β = ½, γ = 1, and δ = 3, assumed to be universal. However, for specific substances the values of these indices differ from the universal ones, as is the case for water (Section 5).

The densities of the liquid and vapor phases of a substance approach each other as the critical point is approached. The mean specific volume along the saturation curve is a linear function of the temperature, a nearly perfect experimental fact called the law of rectilinear diameters:

(1.7) equation

The value of the temperature coefficient b is generally small and negative, the a and b values for water are presented in Section 5.

1.3 Supercritical Fluids as Solvents

Supercritical fluids have been proposed as solvents for many uses, both in the laboratory and industrially. Their properties as solvents are, therefore, of interest. The following comparison with gases and liquids (Table 1.2) are illuminating in this respect.

Table 1.2 Comparison of the Property Ranges of Supercritical Fluids with those of Gases and Liquids at Ambient Conditions.

Supercritical fluids have an advantage over ordinary gaseous fluids for many applications in having much higher densities, and an advantage over ordinary liquids in having lower viscosities and considerable higher diffusivities. SCFs are well integrated in the modern tendency toward green solvents for reactions and separations that are ecologically friendly. They can be employed as reaction media, for extraction, separation, and purification, and for drug formulations, among other uses. In addition to being green, they are tunable, so that their properties can be varied at will. The solvent power of supercritical fluids can be fine-tuned by adjustment of the temperature and pressure, hence of the density. This gives them some advantage over common solvents used at ambient conditions, although the solvent power of the latter can be tuned by mixing with cosolvents, as can also be done for SCFs, of course.

Table 1.3 reports the critical temperature, Tc, the critical pressure, Pc, and the critical density, ρc, of substances of interest in the present context.

Table 1.3 The Critical Points of Some Substances, Together with their Critical Densities, ρc

The general mode of the application of SCFs is by dissolution of the reactants or the materials to be separated in them, carrying out the reaction and separation, and then recovering the products by either one of two techniques. One is by rapid diminution of the pressure, allowing the rapid expansion of the supercritical solvent (RESS technique) and eventual formation of the gaseous solvent (for recovery) leaving liquid or solid products behind. The other is the addition of an antisolvent that diminishes the solubility of the product in the SCF.

Following are some examples of the uses of SCFs as solvents.

Synthesis: Diels–Alders reactions in supercritical water; polymerization of methyl methacrylate in supercritical difluoromethane; phase transfer catalysis by tetraheptylammonium bromide in supercritical carbon dioxide (SCD) modified with acetone.

Pyrolysis: Conversion of biomass to fuel; complete