Books
in black and white
Main menu
Home About us Share a book
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
Previous << 1 .. 2 3 4 < 5 > 6 7 8 9 10 11 .. 101 >> Next

1.1 Process Parameters............................246
1.2 Equipment Considerations......................253
1.3 Operating Costs...............................259
2.0 COST OF OWNERSHIP FOR SCF CLEANING
SYSTEMS ..........................................260
2.1 Operating Costs...............................261
2.2 Net Cash Flow...................................262
2.3 Cumulative Net Cash Flow........................262
3.0 RELATIVE COST OF OWNERSHIP FOR SCF VERSUS
OTHER CLEANING SYSTEM...............................263
4.0 CONCLUSIONS.........................................265
5.0 ACKNOWLEDGMENT......................................265
11 A Practical Guide to Supercritical Fluid
Cleaning.......................................267
Robert J. Purtell
1.0 INTRODUCTION..............................267
2.0 SOLUBILITY: THEORETICAL AND EMPIRICAL.....269
3.0 TRANSPORT ................................272
4.0 REACTOR DESIGN............................275
5.0 PARTS DRYING..............................277
6.0 SUMMARY ..................................278
ACKNOWLEDGMENTS.................................279
REFERENCES ......................................279
Index............................................281
The Supercritical State
Gurusamy Manivannan and Samuel P, Sawan
1.0 INTRODUCTION
A hundred and twenty-five years ago, the behavior of highly compressed gases was a major research field that interested many famous chemists of the day, including even Mendeleev.[11 In 1869, the existence of an additional state of matter became known when Andrews discovered the critical phenomenon and supercritical state. The values reported by him for the critical point of carbon dioxide are in close agreement to the presently accepted values. Later in 1879, the phenomenon of supercritical fluid (SCF) solubility was studied by Hannay and Hogarth.^ They found that gases could be good solvents under supercritical conditions and the dissolving power of a SCF is highly pressure-dependent. There was substantial controversy about the initial finding of the pressure-dependence of solubility in a supercritical fluid after it was first reported at the Royal Society Meeting by Hannay and Hogarth. Small changes in pressure continuously alter the density of these fluids from gas-like to liquid-like, thereby allowing their solubility power to be adjusted over wide ranges.
Supercritical fluid technology has been widely used in extraction and purification processes in the food and pharmaceuticals industryPM5J and for techniques such as supercritical fluid chromatography.^'^ Recently, there has been a significant increase in interest of the use of sub- as well as supercritical (SC) carbon dioxide as a substitute for chlorofluorocarbons (CFCs) for a variety of specific and specialized applications^1 ^ in which the choices of environmentally acceptable alternatives are quite limited.
Today most chemists are familiar with the critical point of a substance as defined by the critical temperature and pressure, Tc and Pc respectively, but often their knowledge goes no further. The behavior of gases close to the critical point is so far removed from ideal that, the topic is usually ignored. Hence, many scientists are unaware that near-critical fluids display unusual and intriguing properties which can lead to new chemistry with exciting applications.
A substance is said to be in the gaseous state when heated to temperatures beyond its critical point. However, the physical properties of a substance near the critical point are intermediate between those of normal gases and liquids, and it is appropriate to consider such supercritical fluid as a fourth state of matter. For applications such as cleaning, extraction and chromatographic purposes, supercritical fluid often has more desirable transport properties than a liquid and orders of magnitude better solvent properties than a gas. Typical physical properties of a gas, a liquid, and a supercritical fluid are compared in Table 1. The data show the order of magnitude and one can note that the viscosity of a supercritical fluid is generally comparable to that of a gas while its diffusivity lies between that of a gas and a liquid.
Table 1. Physical Properties of a Gas, Liquid, and Supercritical Fluid
Density Diffusivity Dynamic Viscosity
g/ml cm2/sec g/cm sec
Gas 1 x 10-3 1 x lo-1 1 x lo-4
Liquid 1.0 5 x 10-6 1 x lo-2
Supercritical Fluid 3 x lo-1 1 x IQ*3 1 x icH
2.0 PHYSICAL TRANSFORMATIONS OF PURE
SUBSTANCES
The simplest applications of thermodynamics to chemically significant systems involve the phase transitions that pure substances undergo. The phase of a substance is a form of matter that is uniform throughout in chemical composition and physical state. The word phase comes from the Greek word for appearance. Thus, we speak of the solid, liquid, and gas phases of a substance, and of different solid phases distinguished by their crystal structures (such as white and black phosphorus). A phase transition, spontaneous conversion of one phase to another, occurs at a characteristic temperature for a given pressure. Thus, at 1 atm, ice is the stable phase of water below 0C, but above 0C the liquid is more stable. The difference indicates that, below 0C, the chemical potential of ice is lower than that of liquid water, //(solid) < //(liquid) (Fig. 1), and that above 0C, //(liquid) < //(solid). The transition temperature is the temperature at which the chemical potentials coincide and //(solid) = //(liquid).
Previous << 1 .. 2 3 4 < 5 > 6 7 8 9 10 11 .. 101 >> Next