Turk / Türk Particle Formation with Supercritical Fluids

Challenges and Limitations
1. Auflage 2014
ISBN: 978-0-444-59443-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 6, 152 Seiten

Reihe: Supercritical Fluid Science and Technology

Particle formation with supercritical fluids is a promising alternative to conventional precipitation processes as it allows the reduction of particle size and control of morphology and particle size distribution without degradation or contamination of the product. The book comprehensively examines the current status of research and development and provides perspectives and insights on promising future directions.The introduction to high pressure and high temperature phase equilibria and nucleation phenomena provides the basic principles of the underlying physical and chemical phenomena, allowing the reader an understanding of the relationship between process conditions and particle characteristics.Bridging the gap between theory and application, the book imparts the scientific and engineering fundamentals for innovative particle formation processes. The interdisciplinary 'modus operandi' will encourage cooperation between scientists and researchers from different but complementary disciplines. - Focuses on the general principles of particle formation in supercritical fluids - Considers high pressure and high temperature phase equilibria, fluid dynamics and nucleation theory - Discusses the underlying physical and chemical phenomena needed to understand the different applications, pointing out the relationship between process conditions and product properties

Michael T?rk received his Dipl.-Ing. degree in Chemical Engineering from the Universit„t Karlsruhe (Technische Hochschule). In 1993 M. T?rk completed his PhD thesis in the field of thermodynamic properties and intermolecular interactions of binary gaseous mixtures. In his professorial dissertation of 2001 a theory was proposed allowing understanding of the relationship between process conditions and the properties of organic particles produced by supercritical fluid based processes. His current research activities are focused on the use of supercritical fluids (mainly CO2 and H2O) as media to prepare organic, inorganic and metallic materials by physical transformation or chemical reaction and the development of new, energy-efficient and environmentally-friendly strategies to create novel products with extraordinary performance for pharmaceutical, energy and biomedical applications. So far, M. T?rk has authored 125 articles (including 13 book chapters); has graduated 20 PhD students (10 as supervisor and 10 as co-advisor) and more than 50 Diploma / Master and 15 Bachelor students.

Autoren/Hrsg.

Turk, Michael

Türk, Michael

Inhaltsverzeichnis

1;Front
Cover;1
2;SUPERCRITICAL FLUID SCIENCE AND TECHNOLOGY;3
3;Particle Formation
with Supercritical Fluids;4
4;Copyright;5
5;Contents;6
6;Foreword;10
7;Preface;12
8;Chapter 1 - Introduction;14
8.1;1.1 SOME CONVENTIONS;14
8.2;1.2 SOLID COMPOUNDS OF INTEREST;15
8.3;1.3 SCOPE OF THE BOOK;16
8.4;1.4 MOTIVATION;17
8.5;REFERENCES;23
9;Chapter 2 - Fundamentals;26
9.1;2.1 PURE SUPERCRITICAL FLUIDS;26
9.2;2.2 MIXTURES CONSISTING OF AN SCF AND A LOW VOLATILE SUBSTANCE;30
9.3;2.3 USEFUL EQUATIONS OF STATE;32
9.4;2.4 MODELING BINARY SYSTEMS;37
9.5;2.5 MODELING TERNARY SYSTEMS;46
9.6;2.6 APPARATUS/EXPERIMENTAL TECHNIQUES FOR MEASURING (HIGH PRESSURE) PHASE EQUILIBRIA DATA;48
9.7;2.7 SELECTED PARTICLE CHARACTERIZATION METHODS;51
9.8;REFERENCES;54
10;Chapter 3 - Basics of Particle Formation Processes;58
10.1;3.1 FLUID DYNAMICS, MASS, ENERGY, AND MOMENTUM BALANCES;58
10.2;3.2 SUPERSATURATION;60
10.3;3.3 ENERGETICS OF NUCLEUS FORMATION;61
10.4;3.4 KINETICS OF PHASE TRANSITION;64
10.5;3.5 PARTICLE FORMATION AND GROWTH;65
10.6;REFERENCES;67
11;Chapter 4 - Formation of Organic Particles Using a Supercritical Fluid as Solvent;70
11.1;4.1 RAPID EXPANSION OF SUPERCRITICAL SOLUTION;70
11.2;4.2 MODIFICATION OF THE RESS PROCESS;74
11.3;4.3 CONDITIONS FOR SUCCESSFUL PARTICLE FORMATION;83
11.4;REFERENCES;86
12;Chapter 5 - Formation of Organic Particles Using a Supercritical Fluid as Antisolvent;90
12.1;5.1 GAS ANTISOLVENT;90
12.2;5.2 MODIFICATIONS OF THE GAS PROCESS;93
12.3;5.3 CONDITIONS FOR SUCCESSFUL PARTICLE FORMATION—RESULTS AND APPLICATIONS;95
12.4;REFERENCES;97
13;Chapter 6 - Formation of Organic Particles Using a Supercritical Fluid as Solute;100
13.1;6.1 PARTICLES FROM GAS SATURATED SOLUTIONS;100
13.2;6.2 MODIFICATION OF THE PGSS PROCESS;102
13.3;6.3 CONDITIONS FOR SUCCESSFUL PARTICLE FORMATION—TYPICAL RESULTS;106
13.4;REFERENCES;108
14;Chapter 7 - Formation of Inorganic Particles Using a Supercritical Fluid as Reaction Media;110
14.1;7.1 SUPERCRITICAL FLUID REACTIVE DEPOSITION;110
14.2;7.2 HYDROTHERMAL SYNTHESIS IN SUPERCRITICAL WATER;112
14.3;7.3 CONDITIONS FOR SUCCESSFUL PARTICLE FORMATION;114
14.4;REFERENCES;120
15;Chapter 8 - State of the Art Modeling of Particle Formation in Supercritical Fluids;124
15.1;8.1 RAPID EXPANSION OF A SUPERCRITICAL SOLUTION PROCESS;125
15.2;8.2 GAS ANTISOLVENT PROCESS;127
15.3;8.3 PARTICLES FROM GAS-SATURATED SOLUTIONS PROCESS;130
15.4;8.4 SUPERCRITICAL FLUID REACTIVE DEPOSITION PROCESS;133
15.5;8.5 HTS PROCESS;135
15.6;REFERENCES;137
16;Chapter 9 - Perspectives in Future Trends and Research Needs;140
17;List of Most Important Symbols;144
18;Index;148

Leseproben

Chapter 2

Fundamentals

Michael Türk

Abstract

Chapter 2 contains the concise description of the basic fundamentals pertaining to supercritical fluids and thermodynamics of binary and ternary mixtures with an emphasis on those systems that involve substances of interest for the various particle formation processes. Density-based models as well as equations of state and their use in modeling and description of binary and ternary vapor–liquid equilibria and the solubility of substances with low volatility are presented. High-pressure techniques as well as particle characterization techniques are also briefly reviewed.

Keywords

Phase behavior; complex mixtures; Equation of State

Our human life takes place at so-called ambient pressure (0.1 MPa) and temperature (around 298 K). In opposite thereto, a large number of technical applications are carried out at elevated pressures and temperatures, e.g., coffee, hops and flavors extraction, or chemical production processes such as the ammonia synthesis. Under such process conditions, it is often not practical to distinguish between a liquid and a gas phase. This holds especially for the supercritical region that is characterized by a temperature and pressure above the fluid's critical values (e.g. in case of CO2, 7.4 MPa and 304 K). In this region, only a single homogenous phase exists due to the absence of phase boundaries. Therefore, the expression “fluid” is used for all aggregate states that are not solid.

2.1. Pure Supercritical Fluids

A simplified schematic p–T projection of the phase diagram for a pure substance is shown in Figure 2.1. The three solid lines divide the diagram into the gaseous, liquid, and solid state. At the triple point (TTP), the three phases coexist while along the lines, the two phases—gas–solid, liquid–solid, and liquid–gas—are in equilibrium with each other. The liquid–gas equilibrium pS = pS(T) line ends at the critical point (CP); above the critical temperature TC and the corresponding critical pressure pC, the pure substance is in its supercritical state and is described as a supercritical fluid (SCF). Table 2.1 lists some common SCFs and their corresponding critical data and the temperature at the triple point. Due to its low critical temperature, CO2 is the most common SCF for processes with heat-sensitive substances. At ambient pressure, CO2 is in the gaseous state that enables the formation of a solvent-free product without any thermal or mechanical postprocessing. Furthermore, it is nontoxic, nonflammable, and inexpensive.

Compared to CO2, H2O is characterized by its high critical data but also by extraordinary physical–chemical properties. The unique properties of near- and supercritical H2O are the basis for new innovative technologies, in which supercritical H2O acts as a solvent or reactant or catalyst. The solvent behavior of H2O is controlled by its dielectric permittivity that decreases with increasing temperature and increases with increasing density. Therefore, H2O changes its character from a solvent for salt at ambient conditions to a solvent for nonionic species at supercritical conditions. More details about the multitude of applications for near- and supercritical H2O can be found in various articles (e.g., Refs [2–4]).

FIGURE 2.1 Schematic pressure–temperature diagram for a pure substance (left) and density–pressure plot for pure CO2 (right) [5].

TABLE 2.1

Critical Data and Triple Point Temperature for Some Selected Solvents [1]

CHF3 CO2 C2H6 H2O

299.0 304.2 305.4 647.3

4.80 7.38 4.88 22.1

7.52 10.6 6.81 17.4

118.15 216.55 90.07 273.15

The right-hand side of Figure 2.1 shows the density–pressure plot for pure CO2. It shows the liquid–gas saturation curve and the density at a subcritical temperature of 290 K and in the supercritical region at the two temperatures of 305 and 333 K. In this figure, l = g denotes the two-phase region in which both a liquid and a gaseous phase exist. CP stands for the CP of the fluid at which the distinction between a gaseous and a liquid phase disappears. It is obvious that above the fluid's CP, a low-density gas can be compressed into a dense fluid without a phase transition. In the region 1 < T/TC < 1.1, the SCF is highly compressible and minor changes in the temperature or pressure result in large changes in the fluid's density and hence, its solvent power. The comparison between the subcritical (290 K) and supercritical (305 K) isotherm clarifies descriptively that—e.g., at 15 MPa—the density of an SCF (ca 836 kg/m3) is very similar to that of a liquid (880 kg/m3).

TABLE 2.2

Orders of Magnitude for Density and Transport Properties in the Gaseous, SCF, and Liquid State [1]

Density Viscosity Diffusivity Kinematic viscosity

kg/m3 Pa s cm2/s cm2/s

1 10-5 10-1 10-5

5 × 102 10-4–10-5 10-3–10-4 10-7–10-8

103 10-3 10-6 1

Thus, the extraordinary properties of SCFs are mainly governed by the pressure and temperature dependence of the density. Especially close to the fluid's CP, small changes in either pressure or temperature lead to a dramatic change in the fluid's solvent power. This fact is crucial for SCF-based particle formation processes such as rapid expansion of supercritical solutions (RESS) (Section 4.1) since a nonvolatile solute can be dissolved at high pressure and subsequently recovered by expansion. In case of a very rapid expansion, very high supersaturations can be attained, leading to small and uniform particles.

Table 2.2 shows that SCFs are characterized by liquid-like densities and mass transfer properties (viscosities and diffusivities) lying between those of gases and liquids. The viscosity of a gas is typically less than one order of magnitude lower than the viscosity of SCFs, but its density is at least two orders of magnitude higher. The ratio of these two quantities, the kinematic viscosity, is very low in the supercritical region. These specific characteristics are important for various applications in materials processing because natural convection effects are inversely proportional to the square of the kinematic viscosity.

Similar to the density, the transport properties such as the viscosity of SCFs vary with changes in pressure and temperature. It is depicted in Figure 2.2 that the viscosity of CO2 increases at 313 K nearly to factor 6 as the pressure increases from 0.1 to 30 MPa. In opposite thereto, the kinematic viscosity decreases by two orders of magnitude as the pressure increases from 0.1 to 7 MPa. The viscosity of sc-CO2 is significantly lower than that of classical liquid solvents (e.g. H2O, 890 µPa/s) resulting in lower pressure drops in pipelines and packed beds columns.

The optimization of the current and the development of new SCF-based processes require the knowledge of reliable T, p, and ? (incl. TC, pC, and ?C) data of pure substances and fluid mixtures under subcritical and supercritical conditions. Usually a static equilibrium apparatus, equipped with a view-cell, can be used for the determination of the critical state of pure substances and binary or ternary mixtures by observation of the critical opalescence (Figure 2.3). With carefully carried out decrease (50 mK/h) of the fluid's temperature from the supercritical to the subcritical state caused that the appearance of the fluid changes from colorless to yellowish and reddish tones. Exactly at the CP, the fluid is completely opaque (black). A further temperature reduction to the subcritical state caused that the fluid reaches its colorless appearance again. If the density of the fluid is carefully adjusted, the liquid-vapor interface occurs exactly in the middle of the equilibrium view cell. In this case, the critical density of the fluid can be determined from the known cell volume and the mass of the test fluid. More details about the apparatus and the experimental procedure, especially the determination of the critical data, can be found elsewhere [5,7,8].

FIGURE 2.2 Variation of the viscosity of CO2 with pressure at 313 K [6].

FIGURE 2.3 Observation of the critical opalescence of difluoromethane (CH2F2). The deviation from the critical temperature (351.23 K) and pressure (5.78 MPa) is given below the pictures [7].

2.2. Mixtures Consisting of an SCF and a Low Volatile Substance

For the design of SCF-based particle formation processes, namely RESS and particles from gas-saturated solution (PGSS), binary mixtures consisting of (1) an SCF and (2) a low volatile substance are of...